Review on modified N–TiO₂ for green energy applications under UV/visible light: selected results and reaction mechanisms

Abstract

Titanium dioxide photocatalyst has witnessed considerable interest for use in water and air cleanup owing to its fascinating properties like non-toxicity, ease of preparation, favorable band edge positions, water insolubility, multifaceted electronic properties, surface acid–base properties, and super hydrophilicity. Although it possesses much functionality, large bandgap and massive charge carrier recombination limits its wide utility under natural solar light. These drawbacks were overcome through nitrogen doping in titania matrix (N–TiO₂), which alters the surface-bulk structure for visible light absorption with high quantum efficiency. In this review, we highlight the recent progress of N–TiO₂ towards pollutant degradation, hydrogen evolution and its use in organic synthesis under ambient conditions. The preparation of N–TiO₂ via different methods (physical and chemical methods) with diverse morphologies, nature of chemical dopants, induced defects and fundamental reaction parameters governing efficient photoinduced reactions are explored in this review. Further improvements in the photoefficiency of N–TiO₂ were achieved through co-doping with foreign ions, heterostructuring with other semiconductors, metal deposition and the tuning of N–TiO₂ with reactive exposed facets. The resultant improvement from each of the modification is discussed in the light of charge carrier generation–separation–transfer–recombination dynamics together with pollutant adsorption and their reactions with reactive oxygenated species in the liquid or gaseous regime. This review attempts to give an overview of the research highlights concerned with N–TiO₂. Although it is impossible to cover all of the research articles in the literature, several milestones in the pathway towards highly applicable N–TiO₂ are explored. It is hoped that this review article will trigger further research in synthesizing N–TiO₂ with multifunctional features to enhance its capacity for green energy applications.

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L. Gomathi Devi

Professor L. Gomathi Devi received her MSc degree (1981) from the University of Mysore and obtained her PhD in Chemistry from the Indian Institute of Science (1988), Bangalore, India. She is currently working as a senior professor in the Physical Chemistry section, Department of Chemistry, Bangalore University. Her research interest spans a broad spectrum of energy, environmental and material chemistry problems. She has authored more than 60 research articles in international peer reviewed journals and has authored several review articles and physical chemistry text books.

R. Kavitha

R. Kavitha has obtained her MSc degree in Physical Chemistry (2010) from the Department of Chemistry, Bangalore University. She is presently pursuing her PhD degree under the supervision of Professor L. Gomathi Devi. Her research work focuses on the preparation and characterization of nonmetal doped titania nanomaterials for energy and environmental applications.

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1 Introduction

Titania has shown great promise in several green applications, such as wastewater purification, catalyst in organic compound synthesis, gas sensors, photovoltaics and hydrogen generation, which can be ascribed to its favourable electronic band structure, biochemical compatibility, strong oxidizing power, non-toxicity and long-term stability against photochemical corrosion.^1–10 However, TiO₂ exhibits low photochemical quantum yield due to its relatively high recombination rate of photogenerated electron–hole pairs. In addition, the large bandgap of TiO₂ absorbs only UV light, which accounts for merely 5% of solar photons, resulting in very low quantum yield in light to energy conversion. The abovementioned challenges serve as the impetus to engineer an environmentally benign and adsorptive-solar photocatalytic material by modifying the surface-electronic structure of TiO₂ while retaining its advantageous catalytic properties.^11–13 Numerous efforts, including crystal shape engineering, doping, co-doping with foreign ions, noble metal surface deposition and sensitization by inorganic complexes or organic dyes, hydrogen plasma reduction of TiO₂, surface complexation, defect creation and multi-component heterostructuring, are frequently reported to display very high photocatalytic efficiency.^14–30 Among the various approaches, nonmetal doping, is one of the promising techniques. In particular, nitrogen incorporation (N–TiO₂) with different chemical entities into the titania lattice or on its surface is accepted to be beneficial for the improvement of photoefficiency of titania under UV/visible light.

The previously published review articles dealt with several fundamental aspects like synthesis, physical/chemical properties, role of nitrogen dopant, wavelength-dependent photoactivity and the synergism of electronically modified N–TiO₂ with other materials that are aimed at enhancing TiO₂ applications.^11,13 Recently, our research group reported a comprehensive overview on nonmetal-doped TiO₂, which focused on several factors like surface modification by noble metal deposition, co-doping with other foreign ions, organic dye sensitization and heterostructuring with other semiconductors, in addition to highlighting the correlations with photocatalytic reaction mechanisms.¹¹ To date, reviews dedicated solely to N–TiO₂ are rare in the literature; therefore, to fill this vacuum, we have discussed selected results of N–TiO₂ that are mainly concerned with the preparation, structural modifications, morphology and synergistic effects of co-doping with other elements for the enhancement of photocatalytic activity to trigger research interest in this field.

2 Influence of preparative methods on the photocatalytic activity of N–TiO₂

TiOHN_x (x = 10 min is N₂ plasma discharge time in min) obtained by the reduction–nitridation method via nonthermal plasma treatment was more favorable for methylene blue (MB) degradation under visible light compared to the samples prepared in other conditions like in the absence of N₂ (TiOH) and H₂ plasma treatment (TiON_x).³¹ The above experimental condition promoted N_s doping in TiOHN_x, which narrowed the bandgap efficiently when compared to simple nitridation treatment. Under visible light, electron is excited from N_s to the conduction band (CB), which then initiates superoxide radical production. The induced oxygen vacancies created by nitrogen dopants serve as trap sites for electrons at lower concentration, and they also facilitate recombination of electrons with the holes in N_s at high concentration.^32,33 TiOHN_x was found to be more stable in photocatalytic reactions compared to TiON₁₀ for three consecutive runs, as the oxygen vacancies created by H₂ plasma lead to the incorporation of nitrogen dopants deep into the TiO₂ lattice (Table 1). When the lattice nitrogen on surface layer is oxidized by holes in the first run,³⁴ a protective passivation layer is formed. This inhibits the further oxidation of nitrogen in TiO₂ present in the deep layers, leading to stable lattice nitrogen content in subsequent cycles. In the case of TiON₁₀, nitrogen is doped only into the surface layer, which can be easily oxidized by holes.³¹ The TiO_2−xN_x prepared by annealing TiO₂ under NH₃ flow decomposed gaseous 2-propanol at a faster rate under UV light compared to visible light. Note that UV light excites electrons from both valence band (VB) and dopant energy level, while visible light excites electrons only from the dopant level.³⁵ The quantum yield for the degradation reaction decreased as the nitrogen content is increased in N–TiO₂ (Fig. 1). During the process of annealing, oxygen sites were partially replaced with nitrogen atoms, which enhances the Ti³⁺ states, and oxygen vacancies below CB edge (0.75–1.18 eV) at higher dopant concentration, which promotes the recombination process.³⁶ The N–TiO₂ (N/Ti = 0.025 atomic ratio) prepared by sol–gel reverse micelle method with disodium ethylenediaminetetraacetate (Na₂EDTA) as the nitrogen source showed remarkable activity for methyl orange (MO) degradation under visible light, which is attributed to the synergistic effects of nitrogen content, high crystallinity and large surface area of the sample.³⁷ These above properties enable the catalyst to show higher extent of adsorption of oxygen/molecular water to react with photogenerated charge carriers to form oxygenated free radicals.³⁷ The N–TiO₂ calcined in atmosphere (200–250 °C, 0.5 h) displayed superior activity for MB decomposition under visible light compared to the sample sintered in N₂ atmosphere (Fig. 2).³⁸ In the former case, the number of active sites increases with the removal of organic residues on the surface, while in the latter case the number of such active sites will be lower as the organic residues cannot be oxidised. Note that the zeta potential of N–TiO₂ was higher than TiO₂, indicating that nitrogen doping and sintering induced more charges on the surface of these nanoparticles. Computational and ATIR results indicated that nitrogen doping occurs as a complexation between the central titanium metal ion and nitrogen atom.³⁸ The preparation of N–TiO₂ via the precipitation of [(TiO(C₂O₄)₂)²⁻] in aqueous ammonia under alkaline conditions at low temperature, followed by calcination at 400 °C, exhibited better MO degradation under UV/visible light.³⁹ This was attributed to the Bronsted acid sites created from covalently bonded dicarboxyl groups, which enhanced the capacity of N–TiO₂ for MO adsorption (Fig. 3). The adsorption process in dark conditions revealed that N–TiO₂ releases H⁺ ions when mixed with MO solution, reducing the solution pH and causes the catalyst surface to become more acidic.³⁹ The N–TiO₂ obtained by wet method using TiCl₄ and NH₄Cl as nitrogen source showed good performance for 4-nitrophenol degradation under UV/visible light compared to N–TiO₂ catalyst prepared using TiOSO₄ (NaOH) via wet and hydrothermal methods.⁴⁰ The high photoactivity was due to the optimum anatase–rutile ratio,^41–44 which allowed the vectorial transfer of charge carriers and retarded their recombination. Furthermore, bandgap narrowing,^45,46 formation of oxygen vacancies and color centers also contributed to the activity in visible light.⁴⁷ Contrarily, the presence of rutile phase was detrimental towards the degradation of MB and 4-cholrophenol (4-CP) using N–TiO₂ calcined at > 500 °C, which was prepared by sol–gel method using 1,3-diaminopropane as nitrogen source.⁴⁸ The VB of nitrogen-doped rutile TiO₂ was lowered by 0.4 eV on the insertion of N 2p levels, which are lower in energy than pure rutile VB (0.05 eV). In this study, interstitial nitrogen served as mid bandgap states to render visible light response and also promoted anatase to rutile phase transition (ART).⁴⁹ This suggests that titanium and nitrogen source, along with the preparative methods, play a vital role in altering the electronic properties of N–TiO₂.

Table 1 Stability of lattice nitrogen in TiOHN_x and TiON₁₀ photocatalysts for its use in three repetitive cycles and the comparison of their activities^a

Sample	Fresh catalyst/at%	First reuse/at%	Second reuse/at%	Third reuse/at%
a Reprinted with permission from ref. 31, Copyright (2011) Elsevier Publications.
TiOHNx	1.4	1.28	1.26	1.26
TiON10	0.32	0.24	0.20	0.18

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Fig. 1 Quantum efficiency versus nitrogen dopant content (x) in TiO₂ for the decomposition of gaseous 2-propanol under UV and visible light irradiation. Reprinted with permission from ref. 35, Copyrights (2003) American Chemical Society.

Fig. 2 C/C₀ versus temperature plot for N–TiO₂ sintered at different temperatures (for 30 min) either in air or in N₂ atmosphere. Photocatalytic activity was measured after 160 min of irradiation by a Xe arc lamp with wavelengths >400 nm. Reprinted with permission from ref. 38, Copyrights (2008) American Chemical Society.

Fig. 3 Mechanism of Bronsted acid site formations on the surface of N–TiO₂ calcined at 400 °C (a schematic illustration). Reprinted with permission from ref. 39, Copyrights (2008) American Chemical Society.

The N–TiO₂ thin films prepared by laser ablation method under N₂ gas atmosphere of 200 Pa showed activity for MB degradation under UV/visible light. The reaction rate decreased for samples prepared at N₂ pressure >200 Pa due to ART under low N₂ pressure.⁵⁰ XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy) provided information for a plausible mechanism for N–TiO₂ by laser ablation. It was found that crystal structure transition from rutile to anatase occurs in the pressure range of 27 to 67 Pa. Moreover, it was observed that the nitrogen incorporation is easier near the critical transition point due to crystal instability. Furthermore, the molecular nitrogen becomes monoatomic and splits more easily under low N₂ pressure compared to higher ones.⁵¹ In addition, N-TiO₂ thin film turned yellow for the substrate at temperature >500 °C, suggesting that thermal energy originating from higher substrate temperatures is required for nitrogen doping, which otherwise does not occur in gas phase between the target and substrate.⁵⁰ The nitrogen doping in the form of O_x–Ti–N_y and –(NO) obtained under O₂/Ar plasma was active for isopropyl alcohol decomposition under UV/visible light, while nitrogen substitution in the form of –(NO₂) under N₂/O₂/Ar plasma was less active compared to bare TiO₂.⁵² The N_i–TiO₂ (interstitially doped nitrogen) thin films synthesized by combinatorial atmospheric pressure chemical vapour deposition showed better MB and stearic acid degradation under UV and white light, respectively, than N_s–TiO₂ due to instantaneous charge carrier recombination processes in N_s–TiO₂.⁵³ The N–TiO₂ films fabricated by annealing TiO₂ films in gaseous NH₃ showed lower quantum yields for gaseous propanol decomposition and high decomposition rates for aqueous ethylamine ions (CH₃CH₂NH₃⁺) as positively charged cationic ethylamine ions could adsorb efficiently on the negatively charged N–TiO₂ surface.⁵⁴ The zeta potential evaluation of N–TiO₂ revealed that surface is negatively charged with an increase in the concentration of nitrogen dopant (Fig. 4). However, quantum yield values under visible light were much lower compared to studies under UV light, although the number of absorbed photons were identical under both excitation sources. This suggests that the holes produced by visible light were localized at isolated levels between VB and CB and were unable to diffuse to the upper edge of VB due to inefficient mixing of N 2p levels with O 2p orbital.⁵⁴

Fig. 4 Zeta potential versus pH value for the thin films, open square: pure TiO₂, closed circle: N–TiO₂ (annealed in NH₃ at 600 °C). Reprinted with permission from ref. 54, Copyrights (2004) Royal Society of Chemistry.

The N–TiO₂ prepared by the heat treatment of nanotubic titanic acid (NTA) in ammonia solution and gaseous ammonia flow possessed better activity for organic pollutant degradation under visible light.^55,56 This was attributed to the single electron trapped oxygen vacancy (SETOV), electron trapping sites, interstitial nitrogen doping (N_i) and also the formation –NO–Ti hot centers as shown in the following equations.⁵⁷

N˙ + O₂ → O₂⁻ + N (3)

On illumination, electron transfer takes place from VB to CB, and then they are trapped by SETOV, which prevents instant recombination. Moreover, nitrogen atom in Ti–O–N in interstitial site transfers the electron cloud towards oxygen atom and gains high chemical valence by entrapping another electron from SETOV. This interstitial nitrogen atom with an excess electron is unstable and immediately detraps the electron to oxygen (Fig. 5).⁵⁸ However, undoped TiO₂ did not show visible light photocatalytic activity despite SETOV formation due to the absence of nitrogen dopant.⁵⁶

Fig. 5 Schematic illustrations of the role of nitrogen dopant in preventing photogenerated charge carrier recombination (a) for N–TiO₂ with enhanced visible light absorption and photocatalytic activity (b) for novel TiO₂ with enhanced visible light absorption without photocatalytic activity. Reprinted with permission from ref. 59, Copyrights (2004) Elsevier Publications.

The N–TiO₂ obtained by mixing TiO₂ with 0.5 M guanidine carbonate as nitrogen source exhibited better activity for 2-propanol decomposition in visible light than the N–TiO₂ samples prepared using urea and guanidine hydrochloride.⁵⁹ The N–TiO₂ synthesized at 160 °C by hydrothermal treatment showed superior MO degradation under visible light compared to N–TiO₂ samples prepared at 120 °C by the same method.⁶⁰ The visible light activity of N–TiO₂ increases with the decrease in pH (3–10) in the preparative process as the nitrogen concentration strongly depends on pH of the reaction medium.⁶⁰ The electron spin resonance spectra (ESR) revealed that anatase N–TiO₂ with N_b˙ centers was active for formic acid degradation, while rutile N–TiO₂ with paramagnetic Ti³⁺ centers had very low activity.^61,62

3 N–TiO₂ with hierarchical morphologies

To resolve the problem of low quantum efficiency, bulk diffusion and effective surface charge transfer, along with maximum light harvestation, are the important key factors. It is believed that complex hierarchical morphologies would offer new opportunities in integrating several fascinating properties like increased surface area, high porosity, low bulk density, enhanced photonic efficiency and dispersity in reaction solution together with structural and catalytic stability with a high extent of charge carrier separation.^63,64 These properties originate from geometrical structure, good mechanical strength, multiple scattering, and better access of reactants to the catalyst together with high light harvesting efficiencies.⁶⁵

The amorphous N–TiO₂ nanotubes exhibited activity for rhodamine B (RhB) and MB degradation under visible light compared to crystalline N–TiO₂ and DP25, indicating that high nitrogen content, high surface area and large pore volume played a crucial role in promoting the activity.^66,67 The loss of nitrogen content,⁶⁷ destruction of tubular structure, reduced specific surface area and oxygen vacancy formation became detrimental after calcining the catalyst >350 °C.⁶⁶ The CO₂ was reduced to HCOOH, HCHO and CH₃OH effectively on N–TiO₂ nanotube under visible light compared to unmodified TiO₂ nanotube.⁶⁸ The energy state of substituted nitrogen (N_s) was found on VB edge, while the energy state of N_i appeared in the forbidden TiO₂ bandgap, thus altering its electronic structure.^69,70 In addition, increased surface area favored more CO₂ adsorption on the catalyst surface.⁶⁸ Note that the oxidation of CO failed with N–TiO₂ nanotube, but promoted the reduction of resazurin to resofurin under visible light, which was a kinetically favored reaction.⁷¹ The high degree of recombination induced by N 2p state hampered the activity under UV light, as oxygen reduction is a four-electron reaction with tedious kinetics and high overpotential.⁷¹ The N–TiO₂ nanotubes synthesized via solvothermal treatment method heated at 120 °C for 5 h using protonated titanate nanotube in an NH₄Cl–ethanol–water solution, followed by calcination at 450 °C, showed a better performance for MB degradation under visible light compared to protonated titanate nanotube.⁷² The presence of chloride ion in the synthesis played a key role in transition from titanate to N-TiO₂ at low temperature. This fact is revealed by the use of various nitrogen sources like NH₃, (NH₄)₂CO₃, NH₂CONH₂ and NH₄NO₃ where the conversion of titanate to N-TiO₂ was inefficient.⁷² The N–TiO₂ mesosponge fabricated by the transformation of TiO₂ nanotube arrays in ethanolic NH₃ exhibited high photocurrent densities and MO degradation under UV light, which was attributed to the excellent degree of crystallinity and the efficient utilization of UV light by N–TiO₂ as a result of localized N 2p level above TiO₂ VB, which narrowed the bandgap and efficiently promoted charge carrier separation.⁷³ The stable and reusable N–TiO₂ nanotube array films fabricated by electrochemical anodization, followed by wet immersion in aqueous NH₃, and then calcination at 450 °C for 2 h decomposed MO efficiently under UV/visible light.⁷⁴ The XRD patterns revealed that bare TiO₂ was composed of anatase and rutile phases, while nitrogen doping via O–Ti–N surface state suppressed ART process owing to the strong crystal distortion force.^75,76 In addition, the presence of porous nanotube structure and nitrogen dopant inhibited the migration and rearrangement of titanium and oxygen atoms to form rutile phase during the calcination process.^77,78 The electrochemical impedance spectroscopy (EIS) spectra showed a smaller impedance arc radius for N–TiO₂ under UV illumination, demonstrating that N–TiO₂ had a greater charge carrier separation efficiency and fast charge transfer than pure TiO₂ at solid–liquid interface.⁷⁴

The N–TiO₂ hollow sphere was more active for bisphenol A (BPA) degradation and mineralization under blue, green and yellow lights compared to undoped TiO₂ hollow spheres and Hombikat UV100 (Table 2).⁷⁹ The degradation rate monotonically increased with the increase in pH, and the rate was highest at pH ∼ 9.6 (pK_a of BPA ∼1), which can be attributed to the increased hydroxyl radical concentration. The high surface area, high porosity and low bulk density enhanced the photonic efficiency and dispersity in reaction solution.⁷⁹ In another study, activity of N–TiO₂ hollow spheres synthesized by co-precipitation and template elimination method and spherical N–TiO₂ showed similar performance for dichlorophenol (DCP) degradation, indicating that photocatalytic reaction does not occur on the inner surface of the hollow sphere but occurs only on the outer surface of the hollow sphere as sufficient light cannot reach the inner surface.⁸⁰ The photoanode consisting of multilayer films of Ti_0.91O_2−xN_x nanosheets showed an increase in photocurrent compared to undoped nanosheets under visible light irradiation, which was mainly attributed to the photoexcitation of oxygen vacancy related states induced by nitrogen doping.⁸¹ These nanosheets were prepared by solid state proton exchange of Cs_0.68Ti_1.83O₄ with H_0.68Ti_1.83O₄, which was followed by subsequent exfoliation process.⁸² The nitrogen doping was carried out using solid state reaction step with gaseous NH₃. The interlayer galleries of layered Cs_0.68Ti_1.83O₄ precursor provided excellent channels for nitrogen diffusion over layered titanate particles.⁸³ The N–TiO₂ was obtained from these titanates via ion exchange process in HCl solution (1 mol dm⁻³), followed by a subsequent exfoliation process in tetrabutylammonium (TBAH) solution (Fig. 6).⁸¹ The mesoporous N–TiO₂ prepared by exfoliation–reassembling strategy with ethylamine as the nitrogen source and a delaminating agent for layered titanate showed exceptional activity for MO and 4-aminobenzoic acid degradation under visible light compared with P25, pristine protonic titanate and N–TiO₂ fabricated from template-free route.⁸⁴ This high activity is ascribed to high porosity, enhanced visible light absorption, increased surface area and efficient charge carrier separation. The mesopore size of 5 nm was large enough for easy access for organic molecules to the catalyst surface. Due to structural similarity, both TiO₆ octahedra and titanate nanosheets were modified by nitrogen species,^85,86 which introduced localized states in both TiO₂ and titanate nanosheets, enabling visible light absorption and bandgap narrowing.^87,88 The charge carrier transfer was improved due to the presence of straddling gap in energy potentials between TiO₂ and titanate nanosheets. The MO degradation was hardly suppressed with the addition of tertiary butanol and was completely inhibited in the presence of ethylenediaminetetraacetate (EDTA), suggesting the dominant role of holes and negligible contribution from the sensitization process. Transmission electron microscope (TEM) images and XRD patterns revealed that the random reassembly between titanate nanosheets and TiO₂ gave rise to its porous structure.⁸⁴ The enhancement in hydrogen evolution under visible light irradiation with 10 vol% methanol by fibrous hierarchical meso-macroporous N–TiO₂ was attributed to its porous network with uniform nitrogen dispersion throughout the catalyst.⁸⁹ The hierarchical and fibrous structure allows channelization of electrons for effective surface charge transfer process (Fig. 7). The presence of macroporous channel within the catalyst enhances the efficiency of photoabsorption and improves the mass transfer process within the macro-mesoporous frame. The macrochannel acts as a light transferring path, which introduces incident photon flux into the inner surface, and this light activated surface area increases as the process absorption, reflection and scattering takes place within the material.⁹⁰ With an increase in calcination temperature to >400 °C, the destruction of mesoporosity and loss of nitrogen content leads to poor activity.⁸⁹ The N–TiO₂ with morphologies of nano-octohedra, nanoparticles and nanosphericals were successfully synthesized through the nitrification of TiO₂ in the presence of NH₃ (700 °C, 30 min) as depicted below:⁹¹

TiO₂ + NH₃ → N–TiO₂ (4)

Table 2 Pseudo-first-order rate constant (k′) and regression coefficient (R²) values for various photocatalysts under different light sources for the photocatalytic degradation of BPA^a,b

Sample	Blue LED		Green LED		Yellow LED
	k′ (h^−1)	R^2	k′ (h^−1)	R^2	k′ (h^−1)	R^2
a Within the initial 3 h of reaction.b Reprinted with permission from ref. 79, Copyrights (2010) Elsevier Publications.
TiO2 powder (pH 6.0)	0.238	0.996	0.034	0.931	0.034	0.932
TiO2 hollow sphere (pH 6.0)	0.353	0.977	0.048	0.981	0.040	0.978
N–TiO2 hollow sphere (pH 6.0)	1.131^a	0.989	0.148	0.976	0.076	0.969
N–TiO2 hollow sphere (pH 3.0)	0.507^a	0.971	—	—	—	—
N–TiO2 hollow sphere (pH 10)	1.342^a	0.971	—	—	—	—

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Fig. 6 Schematic procedure for the preparation of nitrogen-doped Ti_1.91O_2−xN_x nanosheets starting from Cs_0.68Ti_1.83O₄. TBA⁺: tetrabutylammonium ion. Reprinted with permission from ref. 81, Copyrights (2009) Royal Society of Chemistry.

Fig. 7 Illustration of the mechanism of hydrogen production from fibrous hierarchical meso-macroporous N–TiO₂ under visible light irradiation. Reprinted with permission from ref. 89, Copyrights (2013) Elsevier Publications.

The N–TiO₂ nano-octahedra exhibited the best activity for MO degradation compared to other N–TiO₂ structures under UV/visible light irradiation, attributed to the impurity state above the VB by nitrogen dopant, which also improved the TiO₂ conductivity and inhibited recombination rates. In addition, well exposed {101} facet on crystallized surface serves as possible reservoirs for electrons to facilitate the production of superoxide radicals.⁹¹ The N–TiO₂ with rice grain-like structure prepared by the electrospinning method, followed by annealing at 450 °C for 1 h, showed superior MB degradation and hydrogen evolution compared to TiO₂ rice grains, which was attributed to the nitrogen dopant with rice grain nanostructure.⁹² The rutile N–TiO₂ with V-shaped nanorods synthesized by hydrothermal method using HCl and TiN showed an enhanced visible light absorption and a red shift in bandgap absorption.⁹³ The separation of nitrogen ions from TiN provided a route for self-doping in the formation of nanorods with HCl. The chemical reactions involved in the transformation from TiN to TiO₂ are represented as follows:⁹⁴

TiN → Ti³⁺ + N³⁻ (5)

Ti³⁺ + H₂O → Ti(OH)₂ + H⁺ (6)

Ti(OH)₂ + O₂ → Ti(IV)oxo species (7)

Ti(IV)oxo species + N species → N–TiO₂ (8)

The Ti(IV)oxo species is an intermediate between TiO²⁺ and TiO₂, consisting of partly dehydrated polymeric Ti(IV) hydroxide.^95,96 The Ti(OH)₂ was derived from the precursor solution (Ti³⁺N³⁻) and O₂ from autoclave or the reaction solution. There were two types of V-shaped nanorods with inner angles of 114.4° and 134.9° with the same coherent boundary on the {101} plane. Oriented attachment (OA) and Ostwald ripening (OR) processes lead to the formation of straight and V-shaped N–TiO₂ nanorods (Fig. 8). The small V-shaped nanocrystal is formed via OA mechanism by attaching two similar grains, in which two {101} high energy planes join together and are eliminated, and it finally follows the OR coarsening process. The formation of V-shaped nanocrystal is essentially a facet-mediated aggregation and there is sequential elimination of high energy surfaces owing to the significant contribution of surface energy to the total free energy in a nanoscale system.⁹³ The V-shaped structures are of particular interest because the sudden break down in lattice periodicity at the junction offers a good lateral confinement; thus, they can enhance the excitonic optical response.⁹⁷ The dual structure of rutile N–TiO₂ nanoflower film aggregates sitting on top of the anatase layer showed superior activity for RhB degradation under visible light compared to the activity of titania nanorod film and titania derived from the P25 slurry.⁹⁸ The activity was ascribed to its unique morphology of top layer, which possessed a high specific surface area of 77.8 m² g⁻¹.⁹⁸ These N–TiO₂ nanoflower films were fabricated by oxidizing metallic Ti plates with H₂O₂-containing hexamethylene tetraamine (HMT) and concentrated nitric acid at 353 K for 72 h. The low-temperature growth of flower like rutile TiO₂ on the top layer occurred only after 60 h when the reaction solution contained almost no hydrogen peroxide or Ti(IV) ions.⁹⁸ In thermodynamics, two factors were believed to contribute to the low-temperature growth of rutile nanoflower, i.e., additives like HMT and nitric acid, and the porous hydrated titania layer formed in the first 60 h of the reaction. At this stage, a porous hydrated amorphous titania layer was deposited onto the titanium substrates, on which a top layer of rutile nanoflower precipitated due to a supersaturated Ti(IV) solution. With prolonged reaction time of up to 72 h, small rutile nanorods precipitated from the solution, which aligned to form bundles and eventually fused through an OA mechanism to achieve a larger nanorod. The large nanorods self-assembled to a flower-like morphology with the assistance of structure-directing agents like NH₃ and NO₃⁻, which were produced by the decomposition of HMT:^99,100

(C₆H₂)₆N₄ + 6H₂O + H⁺ → 6HCHO⁻ + 4NH⁺₄ (9)

Fig. 8 The mechanism of formation of straight and V-shaped N–TiO₂ nanorods starting from TiN. Reprinted with permission from ref. 93, Copyrights (2012) Royal Chemical Society.

These ions also helped in the synchronal incorporation of nitrogen into the titania matrix; however, a red shift for the light response of the titania nanoflower film was not observed.⁹⁸

4 Co-doping

There exist unfavorable factors such as oxygen vacancy and Ti³⁺ defects due to charge imbalance induced by nitrogen, which may serve as recombination centers. However, co-doping maintains the charge balance through charge compensation, creates new electronic levels, delays recombination of charge carriers and further shifts the absorption edge to visible region. The probability of charge carriers to reach catalyst surface before recombination is higher for co-doped TiO₂ compared to single ion-doped and undoped counterparts. The activity of co-doped titania is largely determined by the chemical nature of the dopants and the bonds between them.^101–104

4.1 Incorporation of nitrogen into TiO₂ with other nonmetal dopants

Nitrogen and boron codoped TiO₂ (N–B–TiO₂) with 10 mol% dopant exhibited high activity for 4-CP degradation under UV/visible light compared to single-doped titania.¹⁰⁵ XPS and XRD gave evidence for the coexistence of interstitial nitrogen, [NOB] species in bulk and NO_x species, and B₂O₃ on TiO₂ surface when the boron content is high. The interstitial B³⁺ ion was sp²-hybridized with lattice oxygen forming a planar structure while lattice oxygen atoms linked with one boron and two titanium atoms to form Ti–O–B–O–Ti structure,^106–109 which shifted the bandgap absorption to the visible region. Although the [NOB]˙ species do not directly contribute to the visible light response, trapped CB electrons will generate the [NOB⁻] diamagnetic center, which inhibits the recombination reaction pathways.¹¹⁰ Under UV light, electrons are excited from both TiO₂ VB and [NOB]˙ species to TiO₂ CB, which in turn are transferred to B₂O₅ CB.^111,112 The simultaneous hole transfer from VB to surface state energy level of NO_x species leads to the direct oxidization of surface adsorbed 4-CP.¹¹³ On the other hand, the electron could transfer from dopant-induced electronic state to CB under visible light illumination. Moreover, the electrons residing at B₂O₃ CB are trapped by adsorbed oxygens to generate superoxide radicals.¹¹⁴ The N–B–TiO₂ (red anatase TiO₂) prepared by heating anatase TiO₂ microspheres (with a predoped interstitial boron shell) between 580 and 620 °C in a gaseous NH₃ atmosphere with a flux of 50 mL min⁻¹ for 60 min exhibited high photoelectrochemical water splitting activity under visible light.¹¹⁵ The UV/visible absorption spectra of red anatase TiO₂ shows an extended absorption edge up to 700 nm, covering the full visible light spectrum due to a bandgap gradient, which varies from 1.94 eV on the surface to 3.22 eV in the core by gradually elevating the VB maximum (Fig. 9). Raman studies showed an additional active mode in the range of 450 to 500 cm⁻¹ for red TiO₂, attributed to the substitutional nitrogen dopant in the TiO₂ bulk, which breaks down the Raman selection rules to generate a new active mode by lowering the geometric symmetries of TiO₂.¹¹⁶ This new doping approach by a predoped interstitial boron gradient improved the solubility of substitutional nitrogen in TiO₂ bulk without introducing the Ti³⁺ impurity level. The interstitial boron dopant effectively weakened the surrounding Ti–O bonds to facilitate easier nitrogen substitution and increased the chemical stability of codoped TiO₂. Moreover, the extra electrons from boron dopant compensated for the charge difference between the lattice O²⁻ and substitutional N³⁻ ion to maintain charge neutrality.¹¹⁶

Fig. 9 Schematic of the band structure of boron and nitrogen-doped red TiO₂ depicting band gap gradient. Reprinted with permission from ref. 115, Copyrights (2012) Royal Chemical Society.

The nitrogen and carbon codoped TiO₂ (N–C–TiO₂) showed high activity for MB decomposition under visible light due to the synergistic effect between carbon and nitrogen co-doping.¹¹⁷ The degradation process followed two pathways: (i) reactive CB electrons reduce surface adsorbed oxygen to its radical anion, which transforms into H₂O₂ and hydroxyl radical; (ii) reactive holes oxidize MB to its radical cation through hydroxyl radical produced by the oxidation of water. During the course of interfacial electron transfer, adsorbed carbonaceous species gets excited and injects electrons into TiO₂ CB, thereby increasing the kinetics of dioxygen reduction. The nitrogen dopant creates an intra bandgap state close to VB and also shifts the position of flat band potential to a higher level than undoped TiO₂, thus increases the interfacial electron transfer kinetics. The ion–dipole interaction between TBA⁺ (carbon source) and water molecules hinders the TBA diffusion into the TiO₂ bulk, indicating that carbon atoms are difficult to weave into TiO₂. This leads to TBA segregation on the TiO₂ surface inhibiting the crystal growth. Moreover, the adsorption experiments indicated that surface carbonaceous species degraded more number of MB molecules and transferred them to residual vacancies through surface diffusion, which is faster than free diffusion in solution, thus enhancing its degradation rate.¹¹⁷ The hydrothermal synthesis of N–C–TiO₂ nanotubes exhibited 2.3 times higher sonocatalytic activity for RhB degradation, which was ascribed to various factors like co-doping, high surface area, lower bandgap energy and the creation of surface vacancies as a result of Ti³⁺ formation.¹¹⁸ XPS results indicated that interstitial nitrogen atoms were bonded to the oxygen atoms via Ti–O–N and Ti–N–O and substituted nitrogen atoms at oxygen sites via N–Ti–O bond, and the carbon dopant via Ti–C bond and as complex carbonate species on the catalyst surface. Sonoluminescence technique produced light flash with an average photon energy of 6 eV. Under these conditions, both surface adsorbed carbonaceous species and RhB serve as sonosentizers injecting electrons into TiO₂ CB,^119–121 resulting in the reduction of Ti⁴⁺ to Ti³⁺ state, which is localized at 0.75–1.18 eV below TiO₂ CB. These reactive sites facilitate oxygen adsorption on the TiO₂ surface.^118–123 In addition, water molecules are also decomposed under acoustic cavitation to release active hydroxyl and hydrogen radical species, contributing to the overall performance.¹¹⁸

N–C–TiO₂ + ))) → e⁻ + h⁺, ))) = ultrasonic irradiation (10)

Ti⁴⁺ + e⁻ → Ti³⁺ (11)

O_2(ads) + e⁻ → O₂˙⁻ (12)

Ti⁴⁺ + e⁻ → Ti³⁺ (13)

O_2(ads) + e⁻ → O₂˙⁻ (14)

H₂O + h⁺ → OH˙ + H⁺ (15)

O₂˙⁻ + H⁺ → HOO˙ (16)

2HOO˙ + H₂O → H₂O₂ + OH˙ (17)

H₂O₂ + ))) → 2OH˙ (18)

H₂O + N–C–TiO₂ + ))) → OH˙ + H⁺ (19)

The N–F–TiO₂ under optimized conditions (catalyst dosage = 600 mg L⁻¹, ratio of Cr(VI)/Benzoic acid (BA) = 5 and pH = 4) initiated a simultaneous redox reaction: the reduction of Cr(VI) ions and the oxidation of BA under visible light.¹²⁴ In this binary system, coupled oxidation of BA consumes holes or hydroxyl radicals, while Cr(VI) ions serve as electron acceptors. The electrons in states below N–F–TiO₂ CB have a weaker reduction ability, which can preferably reduce strong oxidizing agents like Cr(VI), and therefore enhance the target reduction reaction without competition from other ineffective reduction reactions.¹²⁴ The mesoporous N–F–TiO₂ performed better for acid orange (AO7) degradation under visible light due to large surface area and surface acidity, which promoted the increased adsorption of organic pollutants on catalyst surface.¹²⁵ In addition, fluorine doping suppressed the anatase to rutile phase transformation process, enhanced the formation of oxygen vacancies and facilitated the generation of highly oxidative free hydroxyl radicals in solution.^126,127 The N–F–TiO₂ nanobelts prepared by a solvothermal method employing amorphous titania microspheres showed pronounced activity for MO degradation under UV/visible light, which is ascribed to the following factors:¹²⁸ (i) enhanced visible light absorption and scattering;^129–131 (ii) rapid transport of diffusion-free electron along the longest direction;^132,133 (iii) high degree of crystallinity with minimum grain boundaries;^129–131 (iv) smaller pore size and larger surface area. The fluorine doping leads to the formation of several new active sites and generates more number of hydroxyl radicals. The photocatalytic test measurement via the new testing method of scotch tape-covered N–F–TiO₂ nanobelt further enhanced its activity as more photons can be captured by the nanobelts to simulate charge carrier generation compared to conventional methods. The BET results revealed the presence of large amounts of wormhole like mesopores which act as prisons and it is not easy for the photons to escape from these prisons, leading to the collision of photons with nanobelts for the exchange of energy.¹²⁸

N–Si–TiO₂ hollow microspheres obtained by facile aerosol flow method showed good performance for salicylic acid degradation under visible light due to the cooperative effects of nitrogen dopant and Ti–O–Si bond.¹³⁴ The aerosol-assisted flow synthesis process was initiated by homogeneous aqueous solution containing soluble precursors when the droplets passing through tubular furnace carried by an air stream reduced the temperature of the droplets from the outer to interior layer.¹⁰⁹ The solvent evaporated quickly and precursor ions assembled in the interior layer of droplets via a diffusion process (Fig. 10). Simultaneous generation of NH₃ and HF from the decomposition of NH₄F played a vital role in the formation of hollow structured particles. Further treatment with HF solution led to the formation of microspheres with macroscopic pores, which increased the specific surface area of the catalyst.¹³⁴ The N–P–TiO₂ (0.05 P to Ti ratio) showed a remarkable visible light activity for MB and RhB decomposition rather than N–TiO₂ and N–S–TiO₂.¹³⁵ The nitrogen doping resulted in the formation of O–Ti–N linkage, while the dopant phosphorous existed either in the pentavalent oxidation state (P⁵⁺) or as a PO₄³⁻ group coordinated to TiO₂. The oxygen vacancies formed due to this coordination promoted electron and hole transfer to the surface, thereby reducing the charge recombination.^136,56

Fig. 10 Illustration of various multistage process involved in the formation of N–Si–TiO₂ porous hollow microsphere. Reprinted with permission from ref. 134, Copyrights (2012) Elsevier Publications.

The cooperative effects of visible light response and high charge carrier lifetime contributed to the excellent performance of mesoporous N–S–TiO₂ (B) nanobelts (thiourea to Ti molar ratio [R] = 3) for potassium ethyl xanthate degradation under visible light.¹³⁷ With a further increase in the R value to 4, the activity decreased as the dopants sites acted as recombination centers.¹³⁷ The mixed phase (62% anatase + 38% rutile) N–S–TiO₂ modified by thiourea exhibited superior activity for MB degradation under visible light.¹³⁸ The Fourier transform infrared spectroscopy (FTIR) and XPS studies showed the formation of a Ti⁴⁺–thiourea complex, S⁶⁺ doping and the existence of nitrogen as lattice (N–Ti–N) and interstitial (Ti–N–O) species in the heterojunction. The isolated S 3p, N 2p and π* N–O states between VB and CB narrowed the bandgap, thereby rendering visible light response.^139,140 The increased surface acidity of TiO₂ due to the electron-withdrawing inductive effect of sulphate ions and effective charge transfer in biphasic anatase–rutile coupled system increased the activity.^141,49 In another study, N–S–TiO₂–montmorillonite was synthesized by impregnating doped titania sol into the interlayers of montmorillonite, followed by calcination at 350 °C, showed a better performance for acid red decomposition under UV light than undoped TiO₂–montmorillonite, which is attributed to its large surface area, pore volume, crystallinity and the high number of hydroxyl radical species on the surface.¹⁴² Furthermore, electron-deficient nitrogen atom in O–Ti–N states trapped the CB electron and prevented the charge carrier recombination.¹⁴³ The N–I–TiO₂ having 1 : 5 Ti : N/I molar ratio showed simultaneous reduction of Cr(VI) and BA oxidation under visible light irradiation similar to N–F–TiO₂ as reported earlier.¹⁴⁴ The electron paramagnetic resonance (EPR) data suggests that N_b˙ species narrows the bandgap and facilitates the formation of Ti³⁺ surface ion, which plays a key role in capturing gaseous oxygen and reduction–oxidation process. The light absorption ability of co-doped titania was proportional to the concentration of N_b˙ species insertion, and it was found deeper in TiO₂ lattice with lower E_g values. The doping of I⁵⁺ promotes the formation of photoinduced surface Ti³⁺ electrons, which react with adsorbed Cr(VI) and O₂ to form radicals such as O₂˙⁻, HO₂˙⁻and OH˙. In addition, EPR experiments showed the stabilization of oxygen trapped holes TiO⁴⁺–O˙⁻. This was created by the reaction of anatase holes with lattice oxygen O²⁻ and its concentration was inversely correlated with the oxidation rates.¹⁴⁴ The hierarchical macro-/meso-porous N–TiO₂, N–S–TiO₂ and N–F–TiO₂ prepared by hydrothermal method without calcination did not show any visible light activity for RhB degradation, since the non-metal ions existed as surface impurities before calcinations. However, this thermally calcined N–TiO₂, N–S–TiO₂ and N–F–TiO₂ were active for RhB degradation under the same experimental conditions.¹⁴⁵ Thermal calcination effectively increases nitrogen implantation into TiO₂ lattice and causes a red shift in optical absorption. However, sulphur and fluoride ions resided mainly on the surface. The visible light activity was enhanced when the catalyst was stored for six months in desiccators or by stirring in water for 1 h, which facilitates the electrostatic adsorption of water molecules with surface hydroxyl groups on the catalyst surface. However, water-treated N–TiO₂ and N–S–TiO₂ were not more active than TiO₂ calcined at 400 °C due to the presence of nitrogen and sulphur residues on the surface, which disturbed the bonding of water and subsequent water-mediated adsorption switching process. Interestingly, water-treated N–F–TiO₂ showed a high activity despite the presence of nitrogen and fluoride species on the surface, which can induce the formation of electrostatic adsorption with RhB.^146,147

4.2 Incorporation of nitrogen along with other metal dopants into titania lattice

The co-doping of nitrogen with other metals ions improves the photoactivity of titania, in which metal ions with changing valence serve as carrier reactive trapping sites and passivate the defect bands produced by monodoping.^148–150 In general, metal ion dopants form energy level below CB edge and anionic dopants of nonmetals generate energy levels immediately above the TiO₂ VB, effectively narrowing the bandgap, which enables the utilization of a large fraction of the solar spectrum.¹⁵⁰

4.2.1 Incorporation of nitrogen along with 3d block metal ions into titania lattice. The co-doping of nitrogen and vanadium in TiO₂ lattice (N–V–TiO₂) showed activity for RhB degradation under visible light, and the activity is attributed to optimal dopant concentration with enhanced light absorption ability and efficient charge carrier separation.¹⁵¹ In addition, N–V–TiO₂ with small crystallite size with a good degree of dispersion provided high surface to volume ratio, thereby increasing the contact area between active sites and reactants. Due to the strong V–N covalent bond, V 3d states lowers the N 2p energy levels, bringing them closer to VB, and thereby enhances the mixing of N 2p and O 2p states, which produces favorable conditions for trapping holes.¹⁵² On the contrary, the position of V 3d states are shifted to low energy regions, therefore diminishing any possible overlap between V 3d and Ti 3d due to the bonding interaction between the V 3d and N 2p orbitals. The presence of V⁵⁺ ions in the form of V₂O₅ (E_g = 2 eV) produces a space charge layer at the TiO₂ interface due to the difference in electrochemical potential,¹⁵³ which acts as driving force for CB electron to be injected instantaneously into the V⁵⁺ species.¹⁵⁴ The codoping of Mn and nitrogen (Mn–N–TiO₂) stabilized the anatase phase and also performed better RhB degradation under visible light,¹⁵⁵ while doping with only Mn was reported to promote rutile formation even at moderate temperatures.^156–162 In the case of Fe–N–TiO₂, the synergy of a higher degree of crystallization and pronounced mesoporosity with narrow pore-size distribution resulted in phenol and MO degradation under visible light.^163,164 The high degree of crystallization and small crystallite size facilitated rapid electron transfer from bulk to surface effectively inhibited carrier recombination,³² while ordered mesoporous channels facilitated the diffusion and adsorption of reactant molecules for efficient degradation process.^164,165 The iron atoms distributed on TiO₂ surface in the form of Fe₂O₃ suppressed the recombination and increased the quantum efficiency.¹⁶⁶ The N–Zn–TiO₂ (1 atom% Zn) displayed excellent activity for MB degradation under visible light due to the oxygen vacancies induced by zinc doping and the formation of N 2p states above VB edge.¹⁶⁷ The co-doping of zinc into N–TiO₂ shifted the bandgap absorption to the lower wavelength region due to the formation of oxygen vacancies and the Ti³⁺ state in TiO₂.^168,169 The PL spectra indicated that zinc doping suppressed the free exciton recombination emission due to electron trapping by oxygen vacancies and these trapped electrons further recombined with holes to increase the low energy PL intensity. However, nitrogen dopant can hold back the recombination of trapped electron in oxygen vacancy in TiO₂ with holes by N 2p states above VB maximum. It has been stated that the surface oxygen vacancy interacts strongly with exoteric oxygen molecules to form O₂ (efficient electron scavenger), which traps electrons to form O⁻ crucial for oxidation process.^169–171

4.2.2 Incorporation of nitrogen along with 4d block metal ions into titania lattice. The photoactivity of N–Y–TiO₂ towards MB degradation in visible light was ascribed to charge trapping and photocatalytic active centers induced by yttrium doping.¹⁷² However, the high content of yttrium dopant served as recombination centers thus reducing the activity. The ionic radius of Y³⁺ (0.089 nm), which is higher than Ti⁴⁺ (0.068 nm), expanded the TiO₂ lattice on Y³⁺ substitution.¹⁷² The N–Zr–TiO₂ synthesized via modified sol–gel route using supercritical CO₂ decomposed MB under visible light, which was ascribed to several factors:¹⁷³ (i) high surface area with mesoporous structure favoring adsorption of reactant molecules; (ii) reduction in crystallite size, which provided higher interfacial area and access to active sites; and (iii) defects induced by nitrogen dopant captured more electrons and reduced the recombination.^174–177 The nitrogen-doped Ti_1−xZr_xO_x solid solutions obtained by hydrothermal method exhibited a much higher activity for acid red 88 and benzene degradation under visible light irradiation compared to N–TiO₂, which was attributed to the synergistic effect of nitrogen and zirconium dual-modification.¹⁷⁸ XPS and DRS studies revealed that nitrogen was mainly doped on the surface layer of catalyst and induced a surface state close to VB. On the other hand, lattice Ti⁴⁺ ions were substituted by Zr⁴⁺ ions, resulting in the elevation of CB thus facilitating electron transfer and separation of charge carriers.¹⁷⁸ In N–TiO₂ solid solutions, electrons are excited from surface state to CB and subsequently transferred to adsorbed surface species. In comparison to N–TiO₂, the elevation of CB in N–Ti_1−xZr_xO₂ could not utilize the visible light efficiently; however, it had the strong potential required to reduce oxygen leading to the formation of active oxygen radicals (Fig. 11).¹⁷⁸ The N–Nb–TiO₂ (25 mol% Nb) prepared by sol–gel process, followed by nitridation under flowing ammonia, exhibited a 6-fold increase in MB degradation under visible light compared to other Nb doped samples (<25 mol%).¹⁷⁹ The energy of N 2p levels are close to the hydroxyl radical potential [E(H₂O/˙OH) = 2.37 eV] such that the photoinduced holes can react with surface hydroxyl species to form hydroxyl radicals.^180–182 However, the O 2p levels are low enough in energy for the photoinduced holes to oxidize either H₂O or surface OH⁻ to ˙OH. The XPS results revealed that mole percentage of substituted nitrogen in the TiO₂ was a linear function of the mole percentage of niobium. The incorporation of Nb⁵⁺ in the sol–gel reaction led to the charge compensation as depicted below:¹⁸³

Fig. 11 Schematic illustration of band structure of TiO₂ and nitrogen-doped Ti_1−xZr_xO₂ solid solution as photocatalysts for the production of various free radicals. Reprinted with permission from ref. 178, Copyrights (2013) Elsevier Publications.

The free electrons formed facilitated the reduction of Ti⁴⁺ to Ti³⁺ ions, which acted as recombination centers. At low mole percentages (1–5%), a stronger Ti³⁺ EPR signal and lesser substitutional nitrogen were found, while the opposite trend was observed at high mole percentages of niobium (10–30%).¹⁷⁹ The N–Nb–TiO₂ nanotube arrays showed an enhanced photoelectrochemical activity for water splitting under UV/visible light.¹⁸⁴ The XPS results suggested that nitrogen content increased because of niobium predoping, which reduced the formation energy of co-doped systems compared to monodoped TiO₂.^185,186 The ratio between β-N and niobium was 0.46, suggesting that the positive charge induced by niobium was partially compensated by nitrogen at the surface. In addition, Nb reduced charge recombination by the presence of continuum like the hybridized states from N 2p to Nb 4d orbital above the VB instead of the localized states as observed in N–TiO₂.¹⁸⁵

The narrow bandgap of N–Mo–TiO₂ was effective in RhB degradation under visible light.¹⁸⁷ The electronic structure and optical properties calculated using polarized density functional theory revealed the continuum states of hybridized N 2p and O 2p states at the VB edge and Ti 3d and Mo 4d states at the CB lower energy edge. Moreover, localized states vanished completely from the forbidden bandgap, which prolonged the lifetimes of charge carriers.^188,189 The N–Mo–TiO₂ synthesized through the hydrolysis precipitation method combined with sonication post-treatment was positive for phenol degradation under visible light, which was attributed to small crystallite size, intense visible light absorption and narrow bandgap.¹⁹⁰ On co-doping with Mo⁶⁺ and N⁻, the electrons migrated from N 2p level to CB and Mo⁶⁺ dopant level leading to the generation of more charge carrier density for participation in photocatalytic reactions. XPS results revealed that anionic N⁻ substitutes oxygen atoms and coexist as β-N and γ-N, while Mo⁶⁺ substitutes Ti⁴⁺ lattice sites.¹⁹⁰ The N–Cd–TiO₂ was better for MO degradation under visible light, which was attributed to the modification of TiO₂ structure as doping leads to crystal structure distortion, crystallinity reduction and an increased specific surface area that promoted the effective adsorption of organics on the catalyst surface.¹⁹¹ Crystal surface defects can inhibit electron–hole recombination on catalyst surface. Cadmium was present as CdCO₃ due to a gradual expulsion from Ti–O framework to titania surface, and it hindered the crystal growth during thermal treatment. The first-principles calculations indicated that the energy gap of N–Cd–TiO₂ became narrow and local internal fields induced by co-doping enabled photoexcited electron–hole pair separation more efficiently.¹⁹¹

4.2.3 Incorporation of nitrogen along with poor metal ions into titania lattice. The photoactivity of various catalysts for 4-CP degradation under UV/visible light was as follows: N–Sn–TiO₂ > N–TiO₂ > Sn–TiO₂ > TiO₂, attributed to the enhanced visible light absorption and the inhibition of charge carrier recombination.¹⁹² XRD studies revealed that there was no unit cell expansion for N–Sn–TiO₂ despite the large ionic radius of nitrogen compared to oxygen, suggesting that nitrogen was distributed on titania surface rather than doping into the bulk. The Sn⁴⁺ substitutes Ti⁴⁺ lattice sites, which forms an energy level below the CB edge, while nitrogen on the surface generates energy states that are situated above the VB edge. During photocatalysis, electrons are excited from VB to Sn⁴⁺ energy level and from surface states energy level to CB. Moreover, CB electrons get trapped at Sn⁴⁺ energy level located at 0.4 eV below the CB.¹⁹³ Since the energy level of O₂/O₂⁻ is lower than Sn⁴⁺ and CB level, the excited electron are captured directly by surface adsorbed O₂ molecules to form reactive oxygen species. In contrast, the holes in the VB and surface states oxidize hydroxyl groups to hydroxyl radical or can directly oxidize 4-CP molecules.¹⁹² Similarly, N–Bi–TiO₂ was active for acid orange (AO7) degradation under UV/visible light.¹⁹⁴ On visible light irradiation, electrons were excited from VB or nitrogen impurity states to titania CB, which were later captured by Bi⁴⁺/Bi³⁺ state. The density of oxygen vacancies induced by nitrogen dopant was decreased upon bismuth doping, which thus passivates the recombination centers and increases the photocatalytic activity.¹⁹⁵ The N–In–TiO₂ (In = 7%) exhibited high activity for 4-CP degradation under visible light irradiation than N–TiO₂ and TiO₂.¹⁹⁶ XPS and diffused reflectance spectroscopy (DRS) results revealed unique chemical species such as N–O and O–In–Cl_x (x = 1 or 2) on the surface whose surface state energy levels were located close to VB and CB, respectively. The electrons were excited from both energy levels of N–O species to TiO₂ CB and from VB to O–In–Cl_x (x = 1 or 2) on the surface, leading to an enhanced visible light absorption and charge carrier separation. Moreover, CB electrons may have fallen into O–In–Cl_x energy levels and the VB holes migrated to energy levels of N–O species. The 4-CP molecules adsorbed in the surface active sites were immediately oxidized by holes, resulting in complete decomposition.¹⁹⁶

4.2.4 Incorporation of nitrogen along with 5d metal ions into titania lattice. Mesoporous N–W–TiO₂ (W = 1.5%) showed a high tendency for RhB degradation under visible light due to the synergism in nitrogen and tungsten co-doping.¹⁹⁷ Upon illumination, electrons were promoted from VB to tungsten impurity level, which was then captured by adsorbed oxygen giving rise to superoxide radicals, while holes reacted with surface adsorbed water to give hydroxyl radicals. Finally, these active oxygen species attacked dye radical cations and decomposed them.¹⁹⁷ The N–W–TiO₂ synthesized by microemulsion method, followed by a NH₃ treatment (400 °C), showed selective oxidation of toluene and styrene under sunlight and the activity was attributed to the lack of tungsten-rich patches/zones on the surface, a decrease in bandgap and also the favorable surface properties for hydroxyl radical formation.¹⁹⁸ The chemical entity of nitrogen species in N–W–TiO₂ existed as (CN)ⁿ⁻ species in bulk and as (NH_x)ⁿ⁺ on surface. The increase in calcination temperature >450 °C resulted in a detrimental effect since tungsten was excluded from TiO₂ lattice, resulting in a large heterogeneity of the nitrogen species.¹⁹⁹ The superior activity of N–W–TiO₂ (1% W) with twist-like helical structure for phenol degradation under UV/visible was ascribed to following reasons:²⁰⁰ (i) high specific surface area, which allows for the efficient diffusion of electrons, vacancies and adsorbed molecules to catalyst surface, which leads to photoreactions;²⁰¹ (ii) stable mesoporous architecture and 3D connected pore system improved the molecular transport of reactants and products; (iii) W⁶⁺ trapped the CB electron to prevent formation of recombination centers; (iv) existence of Ti³⁺ and tungsten dopant created a new band below CB that served as reactive electron-trapping sites; and (v) highly crystalline anatase phase facilitated the transfer of vacancies from bulk to surface. The twist-like helical structure was formed due to the use of acetic acid in the synthesis, where the bidentate acetate ligand replaced chloride ions and bonded to metal ions in TiCl₄ precursor. This modified the polymeric structure at the molecular level to promote the emergence of 3D type polymeric structure, which were later transferred to the helical structure with regular ventages by the decomposition of acetate ligands in the gel. Acetic acid inhibits the hydrolysis condensation process, which prolongs the gelation time and prevents the formation of irregular TiO₂ particles.²⁰² The N–W–TiO₂ (3 wt% W) nanoparticles fabricated by combining sol–gel and mechanical alloying method showed activity for MB degradation, while increasing the tungsten content to 10 wt% favored sulfosalicylic acid degradation under visible light.²⁰³ The ‘F’ center induced by oxygen vacancies and donor–acceptor level induced by co-dopants served as carrier traps that contributed to high photocatalytic activity. XRD results showed the presence of anatase, rutile, srilankite and brookite phases, where srilankite phase was formed only in the ball milling process. During mechanical activation, the powders were repeatedly welded, fractured and re-welded. Note that mechanical alloying increases the contact area between reactant powder particles as a result of reduced particle size and allows fresh surfaces to come into contact repeatedly. As a consequence, phase transformations that normally require high temperature proceeds favorably at low temperatures. A large number of defects, such as vacancies and dislocations, could be induced into TiO₂, which can significantly accelerate the atom diffusion process to induce nitrogen incorporation.

2(NH₂)₂CO → NH₃ + NH₂CONHCONH₂ (22)

3(NH₂)₂CO → 3NH₃ + C₃H₃N₃O₃ (23)

The NH₃ adsorbed on surface and interface released active [N] and [H] under high pressure, which diffuse faster through defects created by mechanical alloying. [H] reacts with lattice oxygen to produce new oxygen vacancy, while active [N] occupies the oxygen vacancy position or serves as an interstitial atom via O–Ti–N bond.²⁰³ The N–W–TiO₂ showed exceptional potential for partial oxidation of styrene under visible light due to the combined effect of co-doping. The theoretical calculations revealed that titanium cation vacant sites (V_Ti) and surface wolframyl (W=O) species compensated for the extra charge of the W⁶⁺ and N anions in N–W–TiO₂.²⁰⁴ The incorporation of nitrogen at substitutional positions with the concomitant presence of W=O species introduced localized states within the bandgap. The nitrogen impurity incorporates preferentially into the first coordination sphere of tungsten cation by ∼0.43 eV with respect to the equivalent positions of the first coordination sphere of the titanium cation. The stability diagrams for nitrogen incorporation in an undoped and tungsten-doped anatase {101} surface show that the implantation energy of substitutional nitrogen decreases with increasing tungsten content. On the contrary, interstitial nitrogen is destabilized by the presence of tungsten impurity (Fig. 12). The nitrogen doping allowed the absorption of visible light as electrons in VB were excited into these mid gap states and later to CB. The placement of W=O π levels above VB partially overlapped with N 2p levels, which facilitated the electrons to excite from this mid gap state. As a result, a weaker W–O and highly specific reactive surface oxygen radical was formed. The W=O entity was regenerated at the surface by the presence of oxidative atmosphere, which was a thermodynamically favorable process with ΔE = 1.3 eV.²⁰⁴ The visible light activity of N–W–TiO₂ electrode sensitized with Fe–chlorophyllin (Fe³⁺–Por) towards MO and phenol degradation was found to be dependent on the concentration of sensitizer.²⁰⁵ The activity was maximum at 2 mM Fe³⁺–Por and decreased at higher concentrations since the electrons were inclined to be consumed on TiO₂ surface. Adsorption of Fe³⁺–Por at high concentrations hinder the surface activity and light absorption capacity of TiO₂, resulting in a lower photoelectrocatalytic (PEC) activity (Fig. 13).²⁰⁶ In addition, 3d orbital of tungsten lowered the CB level and 2p orbital of nitrogen shifted the VB upwards.

TiO₂ + hv → TiO₂ (e_CB⁻h⁺) (24)

Fe³⁺–Por + e_CB⁻ → Fe²⁺–Por (25)

TiO₂–Por + hv → TiO₂ (e⁻)–Por⁺ (26)

TiO₂ (e⁻)–Por⁺ + O₂ → [TiO₂–Por⁺] + O⁻₂˙ (27)

O⁻₂˙ + organic → degradation products (28)

Ti–OH + h⁺ → Ti–OH⁺ (29)

Ti–OH˙⁻ + organic → degradation products (30)

Fig. 12 Implantation energies (in eV) as a function of the oxygen chemical potential μ^′_O (bottom axis) or O₂ partial pressure (top axis) at a fixed temperature (T = 600 °C) for interstitial and substitutional nitrogen dopant in W-doped and undoped (101) anatase surface models. Reprinted with permission from ref. 204, Copyrights (2012) American Chemical Society.

Fig. 13 The illustration of the band structure of TiO₂, N–TiO₂ and N–W–TiO₂–Fe–Chl excited under visible illumination showing photosensitization by Fe³⁺–porphyrin. Reprinted with permission from ref. 205, Copyrights (2012) Elsevier Publications.

4.2.5 Incorporation of nitrogen along with f block metal ions into titania lattice. The advantages of the lanthanides (Ln) as doping agents for TiO₂ can be outlined as follows: (i) the complexing ability of Ln provides an effective mode for the adsorption of organic pollutants on TiO₂ surface; (ii) effective trapping of CB electrons by Ln when they are confined to the TiO₂ surface; and (iii) the electronic structure of lanthanide ions, which could lead to different optical properties and the formation of labile oxygen vacancies with relatively high mobility.^207–209 The lanthanide ions with special 4f electron configurations are known for their ability to form complexes with various Lewis bases like acids, amines, aldehydes, alcohols, and thiols. This interaction can increase the extent of adsorption of organic pollutants on the catalyst surface and is beneficial for improving the photocatalytic activity.²¹⁰

The N–Ce–TiO₂ was exceptional for MO and reactive red dye X-3B degradation under visible light due to the combined effects of nitrogen and cerium dopant in the titania lattice.^211,212 The nitrogen doping narrowed the bandgap, whereas cerium with varied valence and special 4f level served as an electron trap.^213–215

Ce⁴⁺ + e⁻ → Ce³⁺ (31)

Ce³⁺ + O₂ → O⁻₂˙ + Ce⁴⁺ (32)

h⁺ + H₂O → ˙OH + H⁺ (33)

O₂⁻ + 2H⁺ → 2˙OH (34)

The existence of Ce⁴⁺/Ce³⁺ pairs created a charge imbalance, which resulted in the adsorption of a greater number of hydroxide ions on the surface and produced large number of hydroxyl radicals.²¹⁶ In addition, cerium doping also increased TiO₂ surface area by decreasing their crystallite size. The XRD and thermogravimetric-differential scanning calorimetry (TG-DSC) analysis showed that cerium dopant inhibited the phase transformation to rutile as Ce⁴⁺ ion with a larger ionic radius cannot substitute Ti⁴⁺ ion; therefore, it surrounded the anatase crystalline forming Ti–O–Ce bonds at CeO₂–TiO₂ interface.²¹⁷ At this interface, Ti⁴⁺ was expected to substitute for Ce⁴⁺ in CeO₂ lattice to form octahedral Ti sites. This interaction between tetrahedral Ti and octahedral Ti inhibited the anatase to rutile phase transformation.^218,219 Similar results were also found for N–Pr–TiO₂ towards BPA decomposition under visible light.^220,221 In contrast, N–Nd–TiO₂ showed poor photocatalytic activity for malachite green degradation compared to undoped and individually doped TiO₂.²²²

The Sm–N–TiO₂ with 1.5 mol% samarium calcined at 400 °C exhibited high activity for salicylic acid decomposition under visible light owing to its appropriate crystallite size, high surface area, and good adsorptive capacity due to the formation of Lewis acid–base complexes between Sm–N–TiO₂ and salicylic acid (Fig. 14).²²³ In addition, the recombination process was effectively inhibited by the samarium dopant. The optimal dopant concentration of samarium was found to be 1.5 mol% as the charge carriers are efficiently separated only when the thickness of space charge region is equal to light penetration depth.^224,225 The dopant inside the matrix can suppress or enhance the recombination depending on its concentration and the position of its energy level. The charge carrier trapping at low dopant concentration may not be obvious but at high concentration they may serve as recombination centers. Thus, optimal dopant concentration is necessary to prolong the lifetime of charge carriers. Beyond the optimum dopant concentration, the rate of recombination starts to dominate the reaction in accordance with the following equation:^226,227

K_RRα exp(−2R/a₀) (35)

where K_RR is the rate of recombination, R is the distance separating the electron and hole pairs, and a₀ is the hydrogenic radius of the wave function for the charge carrier.^228,229 As a consequence, the rate of recombination increases exponentially with the dopant concentration. This is because the average distance between the trap sites decreases with increasing number of dopant atoms. On the other hand, the thickness of space charge layer is influenced by the dopant concentration according to the following equation:where W is the thickness of space charge layer, ε and ε₀ are the static dielectric constants of the semiconductor and vacuum respectively, V_s is the surface potential, N_d is the number of dopant donor atoms and e is the electronic charge. The equation above clearly shows that W decreases as the dopant concentration increases. In addition, penetration depth (l) of the light into the solid is given by l = 1/a, where a is the light absorption coefficient at a given wavelength. When the value of W approximates that of l, all the photons absorbed by the solid catalyst generates electron–hole pairs that are efficiently separated. Consequently, the existence of optimum value of N_d, for which a space charge region whose potential is not less than 0.2 eV and whose thickness is more or less equal to light penetration depth, can be understood by the equations above.^224,225 The charge carriers generated beyond the space charge region are not under the influence of electric/potential field and hence recombine rapidly. At high dopant concentrations, the dopant level itself can act as recombination sites for the charge carriers evidently by decreasing the photocatalytic activity. The N–Eu–TiO₂ particles with spheroidal shape synthesized through modified sol–gel method exhibited high photocatalytic activity for the degradation of brilliant red X-3B under visible light illumination, ascribed to the high crystallinity and bandgap narrowing.²³⁰ The Eu 4f level is shown to trap the CB electron to activate superoxide radical formation.^231,232 The N–Eu mesoporous TiO₂ microspheres with yolk–shell structure (TiO₂@void@TiO₂) obtained by facile one-pot hydrothermal method at low temperature (180 °C, 8 h) showed better activity for RhB and MO degradation under visible light illumination due to the synergistic effects induced by unique yolk–shell structure with high specific surface area and large pore volume.²³³ The scanning electron microscopy (SEM) and TEM techniques revealed that the formation of yolk–shell structure followed the OR mechanism.^234–237 XRD and XPS studies suggested that Eu³⁺ ions were distributed over titania surface since it is difficult to incorporate them into TiO₂ matrix at low temperature (180 °C) due to its larger radius (0.112 nm) compared to Ti⁴⁺ ion (0.064 nm). The abundant mesoporous structure was formed by CO₂ gas bubbles released during the hydrothermal decomposition of urea. Upon excitation, electrons located in localized nitrogen states were stimulated to CB and they were trapped quickly by Eu 4f level, followed by a reaction with molecular oxygen.^238,239 The photogenerated holes formed in localized nitrogen states reacted with hydroxyl groups and H₂O molecule to produce hydroxyl radicals, which degraded the pollutant molecules.²⁴⁰

TiO₂ (e⁻) + Eu³⁺ → TiO₂ + Eu²⁺ (37)

Eu²⁺ + O₂ → Eu³⁺ + O₂˙⁻ (38)

Eu²⁺ + O₂ + H⁺ → Eu³⁺ + H₂O₂ (39)

H₂O₂ + O₂˙⁻ → OH˙ + OH⁻ + O₂ (40)

TiO₂ (h⁺) + H₂O (or H₂O₂ or O₂˙⁻) →→ Products (41)

Fig. 14 Formation of Lewis acid–base complexes between Sm–N–TiO₂ and salicylic acid and its photocatalytic process under visible light irradiation. Reprinted with permission from ref. 223, Copyrights (2012) Elsevier Publications.

N–Gd–TiO₂ prepared by hydrothermal method showed high photocatalytic activity for RhB degradation under visible light illumination.²⁴¹ The activity was attributed to the inhibition of particle growth, which resulted in larger surface areas and surface active sites.²⁴² In addition, Gd³⁺ ion on the surface acts as sensitizer and absorbs light energy and transfers it to TiO₂, thereby enhancing its activity.²⁴³

4.3 Photocatalytic activity of tridoped TiO₂

The tridoped C–N–S–TiO₂ hollow microspheres prepared by fluoride-induced self-transformation using L-cysteine as carbon, nitrogen and sulphur source showed superior activity for brilliant red X-3B degradation under visible light illumination.²⁴⁴ This enhancement in the activity was due to the following reasons: (i) the ability of photocatalyst to harvest photons from visible light; (ii) the enrichment of dye adsorption on catalyst surface; and (iii) the effective charge carrier separation by two phases of the same semiconductor-undoped and doped TiO₂.^245–247 In another study, C–N–S–tridoped TiO₂ nanorods prepared by hydrothermal method with 1 : 25 molar ratio of L-cysteine to TiCl₄ showed a high removal efficiency for gaseous NO under simulated solar light irradiation.^248–251 The XRD study revealed an enhancement in the anatase fraction with an increase in the amount of L-cysteine at the expense of rutile. During the synthesis of undoped TiO₂, large amount of H⁺ ions produced by TiCl₄ hydrolysis increased the solution acidity, leading to the production of rutile phase. In the presence of L-cysteine, –NH₂, –COOH and –SH groups coordinated with the Ti atoms to bind L-cysteine to neighbouring protonated octahedrons sharing one edge. When another TiO₆ octahedra complex attacks the two octahedra sharing one edge with L-cysteine, only a spinal chain of octahedrons forms because of the steric hindrance given by L-cysteine linked tightly to the terminal Ti ions which inhibit rutile formation.²⁴⁸ The C–N–S–TiO₂ (molar percent of cysteine to TiO₂ [R] is 4.2) was active for phenol degradation under simulated sun light illumination, which was attributed to the large number of surface hydroxyl groups and efficient charge carrier separation.²⁵² XRD results showed only the anatase phase for C–N–S–TiO₂ and triphasic for undoped TiO₂ (85% anatase, 10% rutile and 5% brookite) (Table 3). The XPS studies revealed that S⁶⁺ substituted the Ti⁴⁺ ion, and nitrogen occurred at both substitutional (N–O–Ti) and interstitial (O–Ti–N) position and carbon existed as a mixed layer of carbonate on TiO₂ surface. The C–N–S–TiO₂ (R = 4.2) showed a stronger surface photovoltage signal (SPS) at 345 nm because of an electronic transition from O²⁻ anti bonding orbital to the lowest empty Ti⁴⁺ orbital;²⁵³ the stronger the SPS signal, the higher the charge carrier separation as SPS signal arises from electron–hole pair generation followed by separation under in-built electric field (also called space-charge layer). In positive electric field, SPS signal increases markedly due to the same direction of added outer and in-built electric field and the signal becomes broad, which is ascribed to the trap-to-band transitions from surface states, while SPS response decreases for negative electric field, which is characteristic of n-type semiconductors.²⁵⁴ The highly transparent N–C–F–TiO₂ films prepared by a simple layer-by-layer dip-coating method using TiO₂ sol and NH₄F methanol solution as precursors without additional pore inducing agent showed an enhanced activity for stearic acid degradation under visible light.²⁵⁵ The high performance originated from the combined effects of tridoping and high surface area of the photocatalyst.²⁵⁵ The doped carbon on the surface acts as sensitizers, which could be excited to inject electrons into TiO₂ CB.²⁵⁶ The doped nitrogen atoms improve the visible light absorption, while doped fluorine atoms facilitated the formation of oxygen vacancies, which are important active species for initiating a photocatalytic reaction.^257,258 Initially, a layer of TiO₂ was coated onto the glass substrate, followed by a second layer of NH₄F methanol solution, and a third coating of TiO₂ sol. The first TiO₂ coating prevents fluoride ions from reacting with the glass substrate. Subsequent coatings of NH₄F solution and TiO₂ sol prevent immediate precipitation of the reactants. The ensuing heat treatment at 500 °C results in a reaction between NH₄F and TiO₂ sol to form N–C–F–TiO₂. The gaseous byproducts (NH₃ and HF) induce high surface roughness in TiO₂ coating and generates nonirradiated superhydrophilic surface. This superhydrophilicity is mainly due to the high roughness and highly accessible pores in N–C–F–TiO₂ films that reduce diffusion resistance within the film structure and subsequently allows a better penetration of water through the void.²⁵⁹ The contact angle measurements of the films were 2.3°–3.1° in the absence of illumination, and they increased slowly in the dark (<1.8° in 30 days), which could be applied for fabricating self-cleaning surfaces.²⁵⁵ The contact angles were tested without light illumination and kept in the dark for 1–30 days.

Table 3 Phase composition, grain size, surface area and micropore volume values of P25–TiO₂, undoped TiO₂ and C–N–S–TiO₂ (R = 4.2) photocatalyst^a

Sample	Phase composition (%)			Grain size (nm)	Surface area (m^2 g^−1)	Micropore volume (cm^3 g^−1)
	Anatase	Rutile	Brookite
a Reprinted with permission from ref. 252, Copyrights (2012) Elsevier Publications.
P25–TiO2	82	18	0	23	58.2	—
Undoped TiO2	85	10	5	6.94	111	0.140
C–N–S–TiO2 (R = 4.2)	100	0	0	5.23	194	0.230

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The N–S–Fe–TiO₂ synthesized through simple one step sol–gel reactions in the presence of ammonium ferrous sulphate showed visible light activity for phenol degradation.²⁶⁰ The dopants Fe³⁺, S⁴⁺ and S⁶⁺ ions were substituted at Ti⁴⁺ lattice site and nitrogen coexisted as both substitutional (O–Ti–N) and interstitial (Ti–O–N) positions. The mechanism of photocatalytic process is shown in the following equations.²⁶⁰

F–N–S–TiO₂ + hv → h_VB⁺ + e_CB⁻ (42)

Fe³⁺ + e_CB⁻ → Fe²⁺ (43)

Fe²⁺ + O_2(ads) → Fe³⁺ + O₂⁻ (44)

Fe²⁺ + O_2(ads) → Fe³⁺ + O₂⁻ (45)

O₂⁻ + 2H⁺ + e_CB⁻ → OH˙ + OH⁻ (46)

h_VB⁺ + OH⁻ → OH˙ (47)

OH˙ + C₂H₅OH → … → CO₂ + H₂O (48)

The high SPS response of Fe–N–S–TiO₂ was due to the improvement in crystallinity, which made semiconductor electronic band structure perfect, which lead to a decrease in surface defects and promoted the charge separation and transfer process.²⁶⁰

The ellipsoidal N–F–W–TiO₂ particles around 20 nm in length and 10 nm in width showed 98% RhB degradation and 94% carbon removal under visible light.²⁶¹ The doping of fluorine induces the formation of Ti³⁺ to a higher extent and the dopant W⁶⁺ ion forms energy level below CB that serves as electron trapping sites to reduce recombination. On the other hand, nitrogen doping created an acceptor level, which facilitates visible light absorption to produce more charge carriers.^262,263

W⁶⁺ + e⁻ (TiO₂)_CB → W⁵⁺ (49)

W⁵⁺ + h⁺ (TiO₂)_VB → W⁶⁺ (50)

In another study, MO degradation by N–S–Gd–TiO₂ (Gd = 0.6 at wt%) was observed under visible light and the activity is ascribed to the following reasons:²⁶⁴ (i) an increase in charge carrier lifetime by Gd³⁺ ions using 4f level for trapping the electrons and the results were substantiated by fluorescence lifetime studies, and (ii) all dopants introduced new electronic states in bandgap, which enhanced visible light absorption.^265–267 The PL studies revealed that PL signals were largely raised from the defect level and that the peak maximum around 430 nm was due to an increased electron density at the oxygen site as a result of nitrogen and sulphur substitution in TiO₂ lattice, both of which are less electronegative than oxygen.²⁶⁴

The N–F–Ta–TiO₂ with 1% molar ratio of Ta to Ti using NH₃ and TaF₅ as precursors showed a degree of efficiency for RhB and phenol degradation under visible light due to synergistic effects such as increased crystallite surface area, increased hydroxyl radical generation and high visible light absorption.²⁶⁸ EPR and XPS studies revealed that N–Ta interaction induces charge compensation to form fully occupied continuum like the N 2p–Ta 5d hybridized states in VB edge, which promotes visible light absorption and improves charge carrier separation (Fig. 15). The fluorine dopant facilitated nitrogen incorporation, which promoted the formation of N 2p–Ta 5d hybridized states. The coexistence of tantalum and nitrogen narrows the bandgap by a charge transfer from the positively charged tantalum to the negatively charged nitrogen, leading to the formation of stable N–Ta chemical bond. The N_n (nitrogen atom next to the tantalum atom) and N_f (nitrogen atom without an adjacent tantalum atom) existed in the tridoped TiO₂ and the holes produced in N 2p band from N_n had high oxidation power than N 2p band of N–TiO₂.²⁶⁹ The N–C–Ce–TiO₂ mesoporous membrane synthesized via a weakly alkaline sol–gel route using P123 template and calcined at 450 °C showed potentials for MO degradation under visible light.²⁷⁰ The cerium ion facilitated successful doping of nitrogen and carbon atoms, which lowered the bandgap to 2.14 eV and improved the visible light activity. The cerium doping strengthened the Ti–O bond and inhibited the transformation to rutile and rapid growth of the crystals, thereby preventing the collapse of assembled pores during the calcination process. The filtration experiments of the composite membrane presented a low cut-off molecular weight of 3300 Da and pure water flux of 4.05 Lm⁻² h⁻¹ per bar. This suggests that such photocatalytic membranes have great implications for environmental applications like wastewater treatment because of their ability to decompose dissolved organic contaminants while simultaneously removing pollutants.²⁷⁰

Fig. 15 Comparison and modification of band structure for N–TiO₂ and tridoped N–F–Ta–TiO₂. Reprinted with permission from ref. 268, Copyrights (2012) Elsevier Publications.

The N–Yb–P–TiO₂ with optimal 5 wt% phosphorous dopant exhibited the highest photoactivity for MB degradation under UV/visible light due to the cooperative effects of tridoping.²⁷¹ The doped phosphorous inhibited crystal growth and recombination of charge carriers by serving as electron trap centers and enhanced the photon absorption efficiency.²⁷² In addition, substituted P⁵⁺ ion induced a charge imbalance, which was compensated by additional surface hydroxyl groups and by the decrease of oxygen vacancies. The Yb³⁺ ions acted as electron scavengers, and then released them to surface adsorbed O₂ to form superoxide radicals. Moreover, tridoping increased the number of surface hydroxyl groups and decreased the point of zero charge of TiO₂ from 6.58 to 3.29, contributing to the higher degree of adsorption and dispersion of MB molecules on the surface. The increased mesopore sizes were beneficial to mass transfer because they allow the rapid diffusion of reactants and products. During photocatalytic reactions, the apparent rate constant indicated that ytterbium and phosphorous doping enhanced the activity, while N–TiO₂ lowered its activity compared to undoped TiO₂ (Table 4). On the contrary the activity of N–P–TiO₂ was higher than N–TiO₂ but lower than P–TiO₂ indicating the absence in synergetic effect for N–P co-doped catalyst.²⁷¹ The N–P–Mo–TiO₂ showed efficient activity for MB and sulfosalicylic acid degradation under visible light than Mo–TiO₂ and P–N–codoped TiO₂, which was attributed to an efficient reduction in bandgap.²⁷³ The phosphorous and nitrogen dopants introduced acceptor levels above VB while the Mo donor levels were found below the CB edge. XRD results showed MoO₃ phase formation at higher Mo content (0.72), which was beneficial for improving the photoactivity of N–P–Mo–TiO₂.²⁷³

Table 4 First order kinetic apparent rate constants and relative coefficients for the degradation of MB over various photocatalysts under visible and UV light irradiation^a

Photocatalyst	Kapp under visible light irradiation (min^−1)	R^2	Kapp under UV irradiation (min^−1)	R^2
a Reprinted with permission from ref. 271, Copyrights (2013) Elsevier Publications.
TiO2	9.2 × 1 0^−3	0.995	1.06 × 10^−3	0.986
Yb–TiO2	1.09 × 10^−2	0.990	1.44 × 10^−2	0.992
N–TiO2	7.35 × 10^−3	0.996	5.42 × 10^−4	0.992
P–TiO2	2.95 × 10^−2	0.992	2.90 × 10^−3	0.999
N–Yb–TiO2	1.92 × 10^−2	0.996	1.44 × 10^−3	0.999
N–P–TiO2	2.52 × 10^−2	0.991	2.22 × 10^−3	0.999
N–P–Yb–TiO2 (1 wt% PO4^3−)	1.56 × 10^−2	0.990	1.59 × 10^−3	0.999
N–P–Yb–TiO2 (5 wt% PO4^3−)	4.66 × 10^−2	0.996	4.15 × 10^−3	0.990
N–P–Yb–TiO2 (10 wt% PO4^3−)	1.81 × 10^−2	0.982	2.58 × 10^−3	0.992

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5 N–TiO₂ coupled with other semiconductors

The coupling of two semiconductors with suitable band edge potentials lead to the construction of heterojunction interface between the semiconductors with electric-field-assisted charge carrier transport from one semiconductor to the other via interfaces.^274,275 The geometry of particles, surface texture, and particle size play a significant role in interparticle electron transfer.^276,277 Appropriate placement of an individual semiconductor and an optimal thickness of the covering semiconductor are crucial for efficient charge separation. In addition, abundant charge carriers will be available at the surface/interface of the two semiconductors for redox reactions and will also extend the energy range of photoexcitation.^278,279

The N–TiO₂ obtained by direct amination of DP25 and triethanol amine as nitrogen source showed visible light activity for MO degradation.²⁸⁰ XPS and wavelength dispersive spectroscopy spectra (WDS) spectra proved that nitrogen atoms substituted titanium (y = 27) and oxygen (x = 0.36) lattice sites with 21% nitrogen dopant concentration in the Ti–O–N–O linkage (nitrogen substituted for titanium) and Ti–N–Ti–O/Ti–N–Ti–N linkage (nitrogen substituted for oxygen). In addition, Ti–O–N–N linkages also originated from nitrogen substitution for both titanium and oxygen.²⁸¹ Thus, the formation of new heterostructure (TiO₂)_m/(Ti_1−y−mO_2−x−2mN_x+y) having both wide and narrow bandgaps improved the visible light absorption and charge carrier separation as a result of vectorial transfer of electron and hole from narrow band (TiO₂)_m–(Ti_1−y−mO_2−x−2mN_x+y) to wide bandgap TiO₂.²⁸⁰ The 1D N–TiO₂ nanorods with anatase–brookite heterostructure (82.4% anatase and 17.6% brookite) showed high activity for MO and 4-CP degradation compared to TiO₂ nanoparticle without 1D structure under UV/visible light.^282,283 This architecture was designed by one-pot solvothermal method (200–220 °C, 48 h) using hydrazine hydrate and TiO₂ colloids as starting materials. During the alkaline thermal process, titania nanoparticles are dissolved in solution and recrystallized to give TiO₂ embryos. When the N₂H₄·H₂O/TiO₂ colloids concentration ratio was 16.8, N–TiO₂ had a smaller nanorod content, since the growth of TiO₂ embryos was retarded by the increase in the number of N₂H₄ molecules per unit volume concentration of TiO₂ colloids. At the same time, if the N₂H₄·H₂O/TiO₂ colloids ratio was below 2.8, ellipsoidal nanoparticles were observed. Under visible light, electrons from localized N 2p states in both anatase and brookite phases were excited to individual CB leaving holes in localized states. Moreover, band edge positions of anatase and brookite facilitate the interfacial migration of electrons from brookite to anatase. The energy barrier between anatase and brookite suppresses back electron transfer, which functions like a one-way valve inhibiting the recombination rate.^284–288 The same scenario occurs under UV light except that electrons in VB are excited to CB (Fig. 16).²⁸² N–P25TiO₂/amorphous Al₂O₃ composites fabricated via one-step solution combustion method exhibited enhanced activity for MO and MB degradation mediated by hydroxyl and superoxide radicals.²⁸⁹ This was ascribed to the cooperative effects of amorphous A1₂O₃, and the nitrogen doping in TiO₂ and large surface area.²⁸⁹ When visible light was irradiated, electron was either transferred from the nitrogen impurity state to CB of Al 3d level or from direct excitation from VB to Al 3d level (Fig. 17). The distance between aluminium and oxygen atoms in combustion-synthesized amorphous Al₂O₃ samples became shorter and energy bandgap became smaller, which favored electron transitions.²⁹⁰ Amorphous Al₂O₃ has a high electron transfer ability from P25 as it contains more defect sites than crystalline A1₂O₃.²⁸⁹

Fig. 16 Electron migration process in one-dimensional nanorod along with nitrogen-doped anatase and brookite structures: (path a) under visible light irradiation; (path b) under UV light illumination. Reprinted with permission from ref. 282, Copyrights (2012) Royal Society of Chemistry.

Fig. 17 Pictorial representation of the possible photodegradation mechanisms of MO over N–TiO₂/Al₂O₃ composite under visible light irradiation. Reprinted with permission from ref. 289, Copyrights (2012) Elsevier Publications.

The pore wall structure and surface properties of mesoporous N–TiO₂/ZrO₂ resulted in fast ethylene decomposition in air under visible light.²⁹¹ The introduction of ZrO₂ stabilized the porous structure by rendering large surface to volume ratio, inhibited anatase to rutile transition and further stabilized nitrogen in the N–Ti–O structure. The preservation of interconnected porous nanonetworks facilitate the transport of small molecules through interior space and favored visible light harvesting through multiple scattering within the solid framework.²⁹² The performance of N–TiO₂/V₂O₅ (0.5 wt% V₂O₅) was appreciable for gaseous toluene degradation under visible light compared to N–TiO₂, SiO₂/V₂O₅ and V₂O₅.²⁹³ Since the band edge potential of V₂O₅ (E_CB⁰ = 0.48 eV) was lower than that of N–TiO₂ (E_CB⁰ = −0.19 eV) CB level, electron transfer from N–TiO₂ to V₂O₅ was thermodynamically favorable, which facilitated the partial reduction of V⁵⁺ to V⁴⁺, the electron from which was then detrapped to the adsorbed oxygen to promote charge separation.^294–297

V⁵⁺ (V₂O₅) + e⁻ → V⁴⁺ (51)

V⁴⁺ + O₂ → V⁵⁺ (V₂O₅) + O₂⁻˙ (52)

The mesoporous SiOC/20 wt% N–TiO₂ (65 wt% anatase and 35 wt% rutile) ceramic composites prepared by the incorporation of N–TiO₂ powders into the vinyl-functionalized polysiloxane polymer, followed by pyrolysis (700 °C, 2 h in argon atmosphere), enhanced the MB degradation rate under UV/vis illumination.²⁹⁸ This was attributed to changes in electronic structure, synergistic effects of higher surface area and high mesoporosity that promoted the pollutant adsorption on catalyst surface. XRD results showed that pyrolysis temperature strongly influenced the phase stability as pure TiO₂ was stable till 800 °C, and the aggregation of dopants in TiO₂ retarded the phase transformation from anatase to rutile at this temperature. On increasing the pyrolysis temperature to 900 °C, the anatase phase disappeared completely with the formation of new Ti₄O₇ phase and a small amount of rutile. Above 900 °C, rutile peaks vanished completely, and Ti₄O₇ and Ti₂O₃ phases started to dominate. In addition, microporous pure SiOC gets was transformed to mesoporous composite due to a structural rearrangement of polymer chains on the addition of N–TiO₂. The intensity of UV/visible absorption spectra was high for SiOC/20 wt% N–TiO₂ owing to the presence of amorphous SiOC matrix incorporation to crystalline oxide powders, which increased the electric charge of the oxide and resulted in the modification of electron–hole formation process.²⁹⁹ The hollow N–Co–TiO₂/SiO₂ microspheres showed an enhanced RhB degradation under visible light irradiation compared to N–TiO₂/SiO₂, TiO₂/SiO₂ and Co–TiO₂/SiO₂.³⁰⁰ Cobalt served to broaden the visible light absorption.

The N–TiO₂/ZnO nanotube arrays composite with well-aligned surface structure showed gaseous NO_x decomposition under UV/visible light, which was superior to ZnO nanorods, ZnO nanotube, N–TiO₂, commercial TiO₂ and ZnO nanorod arrays/N–TiO₂ composite.³⁰¹ The heterojunction structure promoted vectorial charge transfer from one semiconductor to another with thermodynamically favorable band edge position, leading to an increased interfacial charge transfer and catalytic efficiency (Fig. 18). The peculiar tubular nanostructure provided large surface area for NO_x adsorption and the photons were perfectly trapped and absorbed by both inner and outer walls of the nanotubes.³⁰¹ The N–TiO₂/Bi₂₀TiO₃₂ (10% Bi) increased the rate of 2,4-DCP degradation under visible light due to the combined effects of visible light absorption, low charge carrier recombination and improved crystallinity accompanied by a reduction in intrinsic surface defects.^302,303 XRD and XPS results showed the presence of Bi₂₀TiO₃₂ on titania surface, ascribed to the abundant bismuth ions and interactions between surface titanium and bismuth ions during calcination. Apparently, Bi₂₀TiO₃₂ has a small bandgap of 2.09 eV, where VB is composed of hybridized energy levels which are largely dispersed involving Bi 6s and O 2p orbitals, which increased the charge carrier mobility.^304,305 The ability of Bi–O bond to donate electrons helps in transferring the electrons to surface adsorbed species, which eliminates the recombination pathways. The excited electrons from nitrogen impurity band are transferred to titania and Bi₂₀TiO₃₂ CB under visible light. Moreover, these excited electrons can also be captured by doped Bi⁴⁺/Bi³⁺ ions with energy levels below titania CB and subsequently transferred to adsorbed oxygen to produce superoxide radicals and VB holes, which directly oxidizes hydroxyl anion. Finally, Bi₂₀TiO₃₂ also absorbs visible light generating charge carriers for further redox reactions.³⁰²

Fig. 18 Schematic illustration of charge carrier separation and band structure of ZnO/N–TiO₂ semiconductor heterojunction. Reprinted with permission from ref. 301, Copyrights (2012) Elsevier Publications.

The high adsorption of SMZ on N–P25–TiO₂/activated carbon (AC) resulted in its faster degradation under solar light.³⁰⁶ This composite had an anatase–rutile mixture that increased the efficiency of charge-carrier separation by transferring electrons from anatase to rutile CB^307,308 and also from rutile CB to anatase trapping sites (Fig. 19).³⁰⁹ In addition, AC support reduced the agglomeration of supported N–TiO₂ nanoparticles, whereas doped nitrogen created localized states within the bandgap, which served as trap sites for holes.^310,311 The effect of ultrasonication disruption on the physical stability revealed that N–P25–TiO₂ (20 + 15)/AC exhibited a greater resistance to titania dislodgement due to a stronger chemical bonding between N–P25 and N–TiO₂ (sol).³⁰⁶ The N–TiO₂/AC exhibited high adsorption capacity and degradation rates for BPA over a range of excitation sources and demonstrated high potential for reuse compared to unsupported photocatalyst and binary mixture of titania and AC.³¹² Furthermore, the calcination of the composite under air removed organic residues to permit greater exposure of titania surface to receive incident light. The creation of interfacial energy between surface AC and N–TiO₂ resulted in an anti-calcination effect, significantly restraining rutile growth.³¹³ The carbon-coated N–TiO₂ mesostructure prepared by one-step solvothermal method with chitosan as carbon and nitrogen source showed high efficiency in the visible light degradation of MB compared to only carbon coated TiO₂, which was attributed to bandgap narrowing and the heterojunction formed between the carbon and N–TiO₂.³¹⁴ The nitrogen doping level above the VB shifted the absorption to visible light region³¹⁵ and converted some Ti⁴⁺ to Ti³⁺, which then formed a donor energy level below the CB, enhancing its light absorption. The carbon species acted as surface sensitizers and transported the electrons to TiO₂ CB, in addition to enhancing the MB adsorption on the catalyst surface due to the existence of acidic surface groups.³¹⁴ The N–TiO₂ nanoparticles decorated on graphene sheets [N–TiO₂/GB] was active for MO degradation under visible light (Fig. 20).³¹⁶ The graphene in composite extends visible light absorption as a result of a chemical binding via Ti–O–C bond between TiO₂ and graphene. The excellent conductivity and two-dimensional planar structures facilitated rapid transport and the separation of charge carriers.^317,318 The N–S–TiO₂/graphene oxide (5% GO) composite showed an enhanced activity for MO degradation under UV light irradiation.³¹⁹ The high activity was ascribed to a large surface area, oxygen-containing functional groups and large aromatic domains that were inclined to be bound by conjugated MO molecules via π–π stacking. In the composite, excited electrons were transferred from CB to GO via percolation mechanism that accelerates the interfacial electron transfer process.³²⁰ In addition, the dopants increased the number of electrons and holes to form reactive species, and the number of oxygen vacancies induced for charge compensation was reduced by co-doping.³¹⁹ The N–TiO₂/8 wt% graphite-like carbon nitride (g-C₃N₄) showed a significantly enhanced activity for RhB decomposition under fluorescence light irradiation due to dye adsorption and the sensitizing role of g-C₃N₄.³²¹ SPS revealed a small SPS signal for N–TiO₂/g-C₃N₄ (8 wt%), indicating a minimum band bending difference and an accumulation of static electrons on the surface.

Fig. 19 Schematic illustration of the proposed photocatalytic mechanisms of anatase and rutile N–TiO₂ supported on activated carbon. Reprinted with permission from ref. 306, Copyrights (2012) Elsevier Publications.

Fig. 20 Proposed mechanism for the enhanced visible light photocatalytic activity of graphene/N–TiO₂ composite photocatalysts. Reprinted with permission from ref. 316, Copyrights (2012) Elsevier Publications.

A dual phase material (DP160) comprising hydrated titanates (H₂Ti₃O₇·xH₂O) and anatase TiO₂ showed phenol degradation under visible light attributed to the excitation via inter band states, surface sensitization, improved adsorptive properties of aromatic compounds due to the presence of N-carbonaceous species layer containing Ti–N bonds (e.g. –O–Ti–NH⁺–R–NH⁺–Ti–O–) and heterojunction-like structure promoting directional interfacial charge transfer.³²²

A lepidocrocite-type titanate K_0.81Ti_1.73Li_0.27O₄ coupled with N–TiO₂ composite showed excellent photocatalytic activity for the decomposition of NO_x gas under UV/visible light.³²³ This activity was attributed to the bridging structure formed between nano- and plate-like particles together with the increased specific surface area, which provided better accessibility for the target molecules. Moreover, sandwich-like structure increased the dispersion of N–TiO₂, thereby inhibiting their agglomeration. The recombination of charge carriers was effectively depressed due to the different bandgap structures of K_0.81Ti_1.73Li_0.27O₄ and N–TiO₂.³²³ The N–TiO₂/CaAl₂O₄: (Eu, Nd) luminescent composite prepared by a soft planetary ball milling at 200 rpm for 20 min exhibited potential activity for gaseous NO degradation under UV light and the persistent activity continued even after the light source was turned off for 3 h, which was attributed to the depression of agglomerated N–TiO₂ nanoparticles and visible light absorption by N–TiO₂ via long afterglow emitted by CaAl₂O₄: (Eu, Nd) even in the dark.³²⁴ On irradiating CaAl₂O₄: (Eu, Nd), electrons and holes were produced in Eu²⁺ ions while Nd³⁺ ions captured some free holes moving into the VB. When the excitation source was cut off, some holes captured by the Nd³⁺ ions acting as traps were thermally released to the excited state of Eu²⁺, accompanied by the emission of light. This emission was a symmetrical band at 440 nm, a strong blue emission, which was attributed to the typical 4f⁶ 5d¹–4f⁷ transition of Eu²⁺.³²⁵ The N–TiO₂ with nitrogen impurity band above VB induced a second bandgap about 2.34 eV, which was lower than the emitted blue luminescent energy.⁴⁵ Hence N–TiO₂ was excited once the light was turned off. When the composite sample was prepared by planetary ball milling at 200 rpm, the extent of degradation of de NO_x increased with an increase in the ball-milling speed due to homogeneous mixing. However, the de NO_x degradation decreased at ball-milling speed >200 rpm due to an increase in lattice strain and defect, which promoted the recombination of charge carriers. The optimum loading was found to be 40% N–TiO₂, leading to a better dispersion of N–TiO₂ on the CaAl₂O₄: (Eu, Nd) surface, which is a phosphor material.³²⁴ Similarly, N–TiO₂/Sr₄Al₁₄O₂₅: (Eu, Dy) was active for acetaldehyde removal in the dark compared to N–TiO₂/CaAl₂O₄: (Eu, Nd).³²⁴

6 Metal deposition on N–TiO₂ (M/N–TiO₂)

The deposition of various metals like Pt, Ag, Au, Pd, Ni, Rh and Cu on semiconductor surface enhances photocatalytic activity by modifying semiconductor surface properties, discharging photogenerated electrons across the interface and providing a redox pathway with low overpotentials to inhibit the recombination of charge carriers.^326–330 The formation of a space-charge layer at the semiconductor–metal interface facilitates charge separation under bandgap excitation. It is known that the surface plasmons created by the noble metal deposits on the catalyst surface couple with the electric fields of the incident radiations to enhance the separation of photogenerated charge carriers and assist in the charge carrier transfer process across the catalyst/liquid interface.^331,332

The photoactivity of various catalysts towards MO and MB degradation under visible light shows the following order: Ag/N–TiO₂ hollow spheres > Ag-hollow spheres > Ag–N/P25 > Ag–P25 > N/hollow sphere > N/P25.³³³ This was ascribed to the bandgap narrowing by nitrogen doping and the presence of silver clusters, which served as an electron sink thereby improved the charge separation.³³⁴ The embedded silver nanoparticles between N–TiO₂ pillars degraded RhB under visible light. The induced localized surface plasmon absorption edge of silver particles in the visible region and its role as an electron sink favored efficient photocatalysis.³³⁵ The N–TiO₂ pillar was synthesized by reactive dc sputtering to produce TiN porous film followed by a simple oxidation process at elevated temperature in oxygen or air, while the silver nanoparticles were embedded between N–TiO₂ pillars by the photoreduction of Ag⁺ ions in aqueous solution under visible light. The bandgap of N–TiO₂ was tuned by controlling the oxidation temperature, oxygen concentration and oxidation time. In air at 900 °C, maximum oxidation was achieved in 5 h, corresponding to the blue shift from 530 (at 1 h oxidation) to 500 nm (after 5 h). In contrast, in pure oxygen at 900 °C, the blue shift to 500 nm was completed within 1 h and no further change took place upon increasing duration (up to 10 h), suggesting that the replacement of nitrogen by oxygen is a gradual process and saturation level is achieved after 5 h.³³⁵ The Ag/N–TiO₂ prepared via hydrothermal process in silver–ammonia solutions showed better visible light activity for RhB degradation.³³⁶ XPS results suggested that Ag loading on TiO₂ surface significantly restrained the removal of nitrogen dopant from the bulk lattice during the hydrothermal treatment (130 °C, 3 h) and its probable removal gradually decreased with an increase in Ag content (Fig. 21). This stabilization of nitrogen dopant was attributed to the electron transfer from Ag 5s to N 2p orbital. Since the energy of N 2p (N²⁻) is lower than Ag 5s orbital, the electron transfers from Ag 5s to N 2p orbital to make a completely filled (N³⁻) electronic configuration. The fluorescence spectra revealed that the activity increases gradually with increasing Ag content initially and decreases after reaching the optimal Ag/Ti molar ratio of 0.92 mol%, suggesting that optimum content of Ag will serve as electron traps, while superfluous Ag will be detrimental.³³⁶ The Ag/N–TiO₂ hollow nanorod arrays synthesized on glass substrate by one-pot liquid phase deposition method using ZnO nanorod arrays as template exhibited the highest photocatalytic activity for MB degradation under UV and visible light illumination with 0.03 and 0.07 M AgNO₃ concentrations, respectively.³³⁷ This enhanced activity was attributed to the synergistic effects of Ag loading, nitrogen doping and the multiphase structure of anatase–rutile.³³⁷ Raman results suggested that AgNO₃ additive in the precursor solution promoted anatase to rutile phase transition. XPS studies showed an Ag 3d peak around 367.8 eV with a negative shift compared to bulk Ag (368.2 eV) owing to electron transfer from TiO₂ to metallic Ag deposit.^338–340 Under visible light, electrons were excited from VB and nitrogen impurity energy levels to TiO₂ CB, which were then trapped by Ag deposits, favoring interfacial charge transfer process.³⁴¹ The Ag/N–C–TiO₂ showed an enhanced activity for DCP degradation under visible light and inhibited luminescent activity of Vibrio fisheri owing to TiO₂-induced oxidative stress, which led to genotoxicity and cytotoxicity in the microbial organism.^342,343

Fig. 21 Effect of the amount of Ag loading on the stability of implanted nitrogen in TiO₂ after hydrothermal treatment. Reprinted with permission from ref. 336, Copyrights (2013) Elsevier publications.

The Pt/N–TiO₂ exhibited enhanced oxidation kinetics for acetic acid, toluene and acetaldehyde. Compared to N–TiO₂, Cu- and Fe-loaded N–TiO₂ showed high rates for acetic acid and formic acid photooxidation, respectively.³⁴⁴ The extremely high rate of formic acid oxidation over Pt/N–TiO₂ was due to the combined effect of photocatalysis and thermal catalysis (in the dark) at room temperature, facilitated by the presence of nanosized Pt (1–2 nm) on the surface. The ESR studies confirmed that the presence of O⁻ and O₃⁻ stabilized on the catalyst surface were responsible for this dark thermal catalysis. The Fenton-like reactions were found to be operative with Fe- and Cu-loaded N–TiO₂ leading to high degradation rates.³⁴⁴ Fe/N–TiO₂ and Pt/N–TiO₂ improved the visible light induced activity for the decomposition of de NO_x, whereas Pt/N–TiO₂ exhibited activity several times higher than that of N–TiO₂.³⁴⁵ The activity was related to the chemiluminescence intensity generated by singlet oxygen (¹O₂). The rate of degradation of de NO_x (gaseous NO_x pollutant) decreased with an increase in chemiluminescence emission intensity, indicating that singlet oxygen generation was detrimental for de NO_x degradation. The photoexcitation of titania led to the formation of superoxide radicals, which are a doublet with one unpaired electron (−41.4 kJ mol⁻¹). The holes may trap one electron from a superoxide radical to produce a singlet oxygen or a triplet oxygen. The singlet oxygen has a pair of electrons in one orbital and a second equal-energy empty orbital. Metastable singlet oxygen possesses more energy (94.7 kJ mol⁻¹) than ground-state triplet oxygen (0 kJ mol⁻¹).³⁴⁶ From the energy level, it is observed that superoxide radical is produced easily and quickly, while singlet oxygen is formed slowly and requires extended energy (Fig. 22). The formation of singlet oxygen competes with the formation of superoxide and hydroxyl radicals:³⁴⁵

˙O₂⁻ + h_VB⁺ → ¹O₂ (53)

˙O₂⁻ + h_VB⁺ → ³O₂ (54)

Fig. 22 Schematic illustration of the photoinduced charge transformation on N–TiO₂ with metallic loading. The formation of singlet oxygen competes with the formation of superoxide and hydroxyl radicals. Reprinted with permission from ref. 345, Copyrights (2008) American Chemical Society.

The simultaneous surface and bulk modification achieved through Pt or Au deposition and N–F-codoping was favorable for downhill reaction (formic acid oxidation) and uphill reaction (H₂ production).³⁴⁷ The NH₄F doping guarantees that the most active TiO₂ phase is stabilized at high calcination temperatures, ensuring high crystallinity and good photoinduced charge carrier production, whereas Pt or Au deposition on the catalyst surface increased the charge carrier separation. However, Pt was a better co-catalyst than Au for photocatalytic activity, which was related to their work function, i.e. energy required to promote an electron from the Fermi level to vacuum. The greater the difference between the metal work function and the semiconductor is, the higher will be the height of Schottky barrier generated by the band alignment at the metal semiconductor heterojunction.³⁴⁸ The efficiency of photogenerated electron trapping by the metal and subsequent transfer to oxygen molecules adsorbed on the catalyst is consequently increased.³⁴⁷ The high photoactivity of Pd/N–TiO₂ (0.6% Pd) for eosin yellow decomposition under visible light arise from the synergistic effects of palladium deposition and nitrogen doping.³⁴⁹ The palladium on TiO₂ surface creates a Schottky junction between the metal and the semiconductor, which acts as a sink for photogenerated electrons.³⁴⁹ The noble metal loading (Pt, Au and Ag) on mesoporous N–TiO₂ improved the CO₂ photoreduction to methane by water under visible light. XPS results indicated that the dopant nitrogen existed as a molecularly chemisorbed nitrogen species (N₂) or nitroxide species (e.g. NO and NO₂), which induced visible light activity.³⁵⁰ The efficiency of the co-catalyst followed the order of Pt > Au > Ag with an optimum loading of 0.2 wt%, 0.02 wt% and 0.1 wt%, respectively. This optimum metal island traps more number of electrons, whereas an excess amount would serve as recombination centers and reduce the light absorption capability of the catalyst. The Pt has a higher work function of 5.65 eV than Au (5.1 eV) and Ag (4.26 eV);³⁵¹ thus, electrons can transfer more efficiently to Pt. Simultaneously, the adsorbed water molecules react with holes to form H⁺, which in turn reacts with electrons detrapped by Pt to produce H˙ radicals that subsequently reduce CO₂ to hydrocarbons.^352–354 The optimum nitridation temperature to achieve the best activity was 525 °C, and the optimum amount of nitrogen was 0.84% on the basis of lattice oxygen atoms. A larger amount of nitrogen would result in defect sites and nonstoichiometry in the material. The blank test performed with 0.2 wt% Pt/TiO₂ in the presence of water vapor, and in the absence of CO₂ under UV illumination in N₂ atmosphere revealed that methane could be produced from the catalyst associated with carbon residues.³⁵⁰ The N–TiO₂ (B) nanofibers decorated with Pt nanoparticles showed the highest hydrogen generation rates of 700 μmol h⁻¹ and 2250 μmol h⁻¹ under UV-A (365 nm) and UV-B (312 nm) light irradiation compared to Pd-decorated N–TiO₂ nanofibers, which was attributed to the direct electron transition from p-states to the empty states of Pt nanoparticle that are more energetically favored compared to the conventional transition involving VB or impurity level to CB, followed by a subsequent transfer to the metal.³⁵⁵ The higher activity under UV-B exposure was due to the presence of a sufficient amount of high energy photons to induce the generation of electron–hole pairs. The N–TiO₂ (A) nanofibers were synthesized by the calcination of H_2−xNa_xTi_yO_2y−1 nanofibers at 600 °C in ammonia gas flow for 15 h, while N–TiO₂ (B) was synthesized from H_2−xNa_xTi_yO_2y−1 calcined in air at 600 °C for 12 h to form TiO₂ anatase nanofibers, followed by a second calcination step in ammonia gas at same temperature for 3 h.³⁵⁵ In Pt/N–TiO₂, Pt has a smaller particle size and a high dispersion value on catalyst surface compared to Pd.³⁵⁵ The amine-functionalized, silicate sol–gel supported, gold-deposited N–P25 (APS/Au–N–P25) showed an enhanced photocatalytic oxidation for CO and reduction of Hg(II) ions under visible light, attributed to the synergistic effects of gold nanoparticles (with sub 5 nm size) deposited on N–P25.^356–360 When Au and NP25 are connected electrically, electron migration from TiO₂ to Au occurs until the two Fermi levels are aligned. Hence, the metal surface acquires the excess negative charges and semiconductor becomes positively charged. The Au acts as an electron sink, which increases the interfacial charge transfer process and minimizes the recombination rates. The presence of –NH₂ group in amine-functionalized silicate sol–gel with nitrogen having nonbonding electrons allows the interaction of Hg(II) ions with the APS/Au–N–P25 surface.^361,362 This results in the increased adsorption and preconcentration of Hg(II) ions on the surface. The Au transfers electrons instantaneously to reduce Hg(II) to Hg and the holes in the VB oxidize the oxalic acid which acts as a sacrificial electron donor. The optimization of Au was found to be 2 and 4 wt% for the reduction of Hg(II) ions and CO oxidation, respectively. The excess Au loading decreased the activity due to the following reasons: (i) absorption and scattering of incident light by excess Au;^363,364 (ii) the shielding of incident photon at N–P25 by Au; and (iii) the negative effect on photoreduction due to the oxidation of Au by holes or surface hydroxyl radicals.³⁵⁶

The mesoporous Cu/N–TiO₂ showed good degrading ability for gaseous xylene under UV/visible light, which was ascribed to its mesoporous structure.^365,366 The catalytic activity decreased for Cu loadings higher than 0.6 mol% as the active sites on the catalyst were covered by excess Cu with a simultaneous increase of recombination kinetics.³⁶⁷ The bottles coated with Cu/N–TiO₂ films and annealed at 600 °C degraded MB more rapidly than uncoated bottles in field trials, while N–TiO₂ coating did not show any change as the holes produced by visible light irradiation in mid gap level (induced by doping) did not have sufficient redox potential to oxidize MB.³⁶⁸ The Cu-doped TiO₂ showed E. coli and Enterococcus faecalis inactivation due to the increased visible light absorption and anti-microbial properties of copper atoms on exposed surfaces. However, the doping with nitrogen atoms (Cu/N–TiO₂) reduced the degree of inactivation as nitrogen atom in lattice acted as recombination centers. The Cu/N–TiO₂ accelerated bacterial inactivation when coated on glass beads, but not on internal surface of glass bottles, indicating that reactive species produced at Cu–N–TiO₂ surface are short-lived and can only diffuse short distances (in the order of μm) and that bacterial inactivation by such reactive species may thus be transport limited. The H₂O₂ and superoxide radical are important reactive species that might plausibly have a mean diffusion distance on this length scale.³⁶⁹ Bacterial cells may also adhere to catalytic surface, thereby magnifying the effects of short-lived radical species.³⁷⁰ Note that the formation of singlet oxygen, a less oxidative reactive oxidation species, was reported to be responsible for bacterial decontamination.³⁷¹ The loading of copper (0.5 wt%) on N–TiO₂ was found to be beneficial for gaseous acetaldehyde and bactericidal activity under visible light, while loading with Pt, Zn and La did not exhibit any effect, suggesting that Cu serves as electron trap sites and extends the carrier lifetime.^367,372

7 Hydrogen evolution using N–TiO₂

The solar energy available in nature is 3 × 10²⁴ J per year, which is 10 000 times higher than the present energy need.³⁷³ This provides a wide scope for photocatalytic reduction of water to generate clean and green fuel, i.e., hydrogen (H₂), as an energy carrier with a high calorific value 122 kJ g⁻¹ (2.75 times greater than hydrocarbon fuels).³⁷⁴ The recent findings reveal that N–TiO₂ could be a better photocatalyst for H₂ production.

N–TiO₂ prepared by heating urea and TiO₂ at 350–700 °C in air favored hydrogen evolution under UV/visible light in the presence of Na₂SO₃ as a positive hole scavenger.³⁷³ XPS studies showed both molecularly chemisorbed N₂ and substituted nitrogen contributed to visible light response. The calcination temperature of the catalyst removed chemisorbed nitrogen, which had the least influence on photoactivity. The high activity of the sample calcined at 600 °C resulted from phase transformation based on the CB edge position. The flat band potential of rutile coincides exactly with NHE potential (H⁺–H₂ level), whereas anatase is shifted cathodically by ∼200 mV, suggesting that the driving force for water reduction is very small for rutile.³⁷⁵ It is also evident that anatase–rutile mixture enhances electron–hole separation due to the operation of surface charge layer and space charge layer between the anatase and thin rutile layer. This combined space-charge layer acts cooperatively towards the migration of holes from anatase to rutile and to the surface.³⁷³ The platinized N–B–TiO₂ (B = 1.5 at%) calcined at 350 °C showed remarkable photocatalytic activity for hydrogen production under visible light compared to N–TiO₂, attributed to synergistic effects of nitrogen and boron doping.³⁷⁶ The nitrogen doping extends the visible light absorption via N–Ti–O bond while boron plays multiple roles: (i) B³⁺ substitutes Ti⁴⁺ lattice sites and serves as shallow traps for carriers;³⁷⁷ (ii) boron eliminates oxygen vacancies induced by nitrogen doping and enhances surface hydroxyl groups on its surface;³⁷⁸ and (iii) boron distributed on the catalyst surface forms space charge layer to separate electron–hole pairs.³⁷⁹ The N–Ce–TiO₂ (0.6% molar ratio) calcined at 500 °C exhibited the highest hydrogen evolution under visible light irradiation, ascribed to synergistic effects of co-doping.³⁸⁰ XRD results indicated that as the ionic radii of Ce³⁺ and Ce⁴⁺ are larger than Ti⁴⁺, and they lead to lattice distortion and expansion where some strain energy is accumulated in the crystal, reducing the phase transition from anatase to rutile.³⁸⁰

The hydrogen evolution with Pd co-catalyst and methanol as sacrificial reagent for various photocatalysts followed the order of N–In–TiO₂ > In–TiO₂ > N–TiO₂ > TiO₂.³⁸¹ The indium doping at cationic site decreased the bandgap by mixing of 5s and 5p orbitals with Ti 3d orbital without generating any trap sites, making the electrons available at CB for the reduction of protons to hydrogen.³⁸² In addition, impregnated Pd facilitated interfacial electrons transfer and reduced the overpotential for hydrogen evolution by accumulating the electrons on Pd deposits.^383–385 The co-catalyst NiO supported on N–TiO₂ showed high hydrogen generation from aqueous methanol solution under sunlight-type irradiation, which was ascribed to electron trapping nature of NiO.³⁸⁶ These trapped electrons in NiO initiate H⁺ reduction formed by water splitting to liberate hydrogen.³⁸⁷ The nitrogen doping decreased the bandgap and increased the number of electrons in the presence of NiO. During hydrogen evolution reaction, oxygen and nitrogen were not liberated, suggesting that titanium oxynitride synthesized by trioctyl amine treatment of TiO₂ was stable under reaction conditions.³⁸⁶ In another study, Pd served as better co-catalyst compared to NiO on reduced N–TiO₂ for hydrogen evolution under visible light.³⁸⁸ The induced Ti³⁺ state produces anion vacancies in the lattice, which introduces additional level below CB. These levels overlap with TiO₂ CB, which decrease the bandgap with increase in its concentration.³⁸⁸ The Pt (1 wt%) loaded N–S–TiO₂/V₂O₅ exhibited the highest activity for hydrogen evolution (296.6 μmol h⁻¹) under visible light, as CB electrons are trapped by Pt to reduce H⁺ ions and the holes oxidize methanol to form CO₂ and H₂.^389–392

2H₂O → 2H⁺ + 2OH⁻ (56)

2e⁻_CB + 2H⁺ → H₂ (57)

The CB and VB of N–S–TiO₂ lie above the energy band of V₂O₅. Thus, electrons accumulate in N–S–TiO₂ CB and holes in V₂O₅VB effectively separating charge carriers at the interface.^393–396 However, Pt-deposited N–TiO₂ thin film showed poor hydrogen gas evolution under visible light compared to UV light due to the presence of oxygen defects, which trapped the electrons necessary to initiate hydrogen production.⁵⁰ The nanocomposite of graphene oxide with N–TiO₂ exhibited enhanced photocatalytic efficiency for hydrogen evolution under UV/visible light, which was attributed to nitrogen doping and incorporation of graphene oxide.³⁹⁷ The nitrogen doping and formation of Ti–C and Ti–O–C bonds along with the existence of Ti³⁺ red shifted the bandgap of TiO₂. The Fermi level in graphene is −4.42 eV³⁹⁸ is close to CB energy (−4.21 eV) of TiO₂. Therefore, the CB electrons are injected into the Fermi level orbital in graphene, while the holes are trapped by N–TiO₂. At the same time, graphene can withdraw and shuttle electrons as electron transporting bridges and electron sinks because of its ultrahigh electron mobility (>1000 cm² V⁻¹ s⁻¹), and the plate-like monolayer textural nature with a large specific surface area affords quick electron transfer resulting in charge carrier separation. The electrons from graphene are transferred to H⁺ in aqueous methanol to afford hydrogen generation, and the holes irreversibly oxidize the sacrificial reagents instead of water.³⁹⁷

8 Reactivity of N–TiO₂ with exposed facets

Engineering TiO₂ material with exposed crystal facets is currently a hot area in photocatalytic reactions. Both theoretical and experimental studies have revealed that {001} surface of anatase TiO₂ nanosheets is much more reactive than the thermodynamically stable {101} surface.³⁹⁹ Based on thermodynamics, {101} facet with low surface energy remains the most stable, accompanied by diminishing {001} and {110} facets, during crystallization. The average surface energy of {001}, {100} and {101} facets are 0.90, 0.53 and 0.44 J m⁻², respectively.⁴⁰⁰ Owing to the low atomic coordination numbers of exposed atoms and the wide bond angle of Ti–O–Ti, the anatase TiO₂ {001} facet is theoretically considered to be more reactive than the {101} facet in heterogeneous reactions.

Doping with nonmetals is reported to shift the bandgap response of {001} facets of anatase titania towards the visible region. N–TiO₂ with an exposed {001} facet (∼67%) prepared by solvothermal synthesis of TiN in a HNO₃–HF ethanolic solution exhibited a higher activity for hydrogen evolution compared to N–TiO₂ microcrystallite with exposed {001} facet due to the larger surface area of the former compared to the latter. In addition, ethanol used in the synthesis process served as a capping agent, which hindered the growth of the single crystal anatase TiO₂ because of its specific bonding with the TiO₂ surface via the Ti–O–C bond.^400b The solvothermal treatment of TiN in acidic NaBF₄ resulted in N–TiO₂ with a dominant {001} facet, which exhibited high activity for MB degradation and excellent photoelectrochemical properties under visible light. The presence of weak Ti–B–F surface structure facilitated easy dissociation and exchange of surface hydroxyl groups to form hydrogen peroxide and peroxide radicals.^401,402 The photocurrent (400 μA cm⁻²) response of N–TiO₂ with a dominant {001} facet is 3.3 times higher than N–P25, indicating efficient charge separation and transportation of electrons from TiO₂ surface to counter electrode via an external circuit under visible light illumination.^401a Hydrogenated N–H–F–TiO₂ with {001} facet exhibited high activity for RhB degradation under visible light due to the formation of energy belts and disorder structures.⁴⁰³ The differently coordinated Ti³⁺ and oxygen vacancies like O_vac (F_e), O_vac (N_e) and Ti³⁺ (F_e), Ti³⁺ (N_e), Ti³⁺ (H), and Ti³⁺ (H′) (e refers to escape, for example, O_vac (F_e) refers to oxygen vacancy induced by the escape of F⁻ ions) narrowed the bandgap and significantly enhanced light absorption to facilitate charge carrier migration (Fig. 23).⁴⁰³ The N–Mo–TiO₂ sheets with dominant {001} facets obtained by a hydrothermal process using TiN, MoO₃, HF and HNO₃ showed activity for MB and methyl violet degradation under visible light.¹⁸⁹ The XPS spectra confirmed the nitrogen at interstitial site or O–Ti–N structure and Mo⁶⁺ substitution in TiO₂. The Mo doping elevated the CB edge and expanded the TiO₂ bandgap, raising the reduction potential energy and improving the H⁺ reduction ability. The removal of fluorine from the catalyst surface reduced its activity, although the adsorption of the pollutant on the catalyst was enhanced.¹⁸⁹ The N–Ni–TiO₂ with exposed {001} facets prepared by a two-step hydrothermal reaction using H₂Ti₃O₇ and HF (shape controlling agent) showed enhanced activity for MB degradation under UV/visible light. This can be attributed to its suitable bandgap (2.0 eV) and band edge positions, which matched the redox potential of water.⁴⁰⁴ The dopants, nitrogen and nickel, shifted VB edge up significantly but left CB unchanged. The n-type (Ni) and p-type (N) with unequal charge states decreased the density of recombination centers and increased the migration efficiency of charge carriers. For the synthesis process, H₂Ti₃O₇ nanotubes were obtained by treating TiO₂ with NaOH. At this stage, few Ti–O–Ti bonds were broken, followed by the formation of Ti–O–Na bonds, which on washing with acid and water generated new Ti–O–H bonds to produce metastable H₂Ti₃O₇ that were further transformed into TiO₂ nanotubes as shown below.⁴⁰⁴

TiO₂ + NaOH → Na₂Ti₃O₇ (61)

Na₂Ti₃O₇ + HCl → H₂Ti₃O₇ (62)

H₂Ti₃O₇ → 3TiO₂ + H₂O (63)

Fig. 23 The schematic illustration of band structure, disorder structure and defect energy belt of N–H–F–TiO₂ and its visible light photocatalytic activity. Reprinted with permission from ref. 403, Copyrights (2012) Elsevier publications.

The hierarchical N–TiO₂ hollow sphere-in-sphere microstructure comprised of nanothorns with exposed anatase {101} facets exhibited potential activity for MB and orange II degradation under visible light. This was ascribed to the synergistic effects of large surface area, hierarchical structure, and crystalline {101} facets on the surface and inner sphere, which allowed multiple reflections of light with effective usage of incident photons.⁴⁰⁵ Water was found to play a major role for hollowing process, which promoted the crystallization and dissolution processes. When the molar ratio of H₂O/Ti is 0.3, formation of nanothorns on the surface of spheres occurs, while heavily aggregated TiO₂ solid microspheres with nanoflakes are formed without water. The hollow structure improves the specific surface area dispersion ability and light harvesting ability of the photocatalyst. However, hollow spheres containing {101} facets without sphere-in-sphere structures exhibited low activity.^405–408

The MB degradation was achieved at a faster rate under visible light on the surface of N–C–TiO₂ nanosheets with exposed {001} facet compared to their microsheets and nanoparticles counterparts.⁴⁰⁹ The fluorine ions preferentially adsorbed on {001} facets hindering crystal growth along {001} axis but facilitated crystalline growth in both {100} and {101} directions with lower surface energy.⁴¹⁰ Moreover, the ethanol served as a capping agent by adsorbing on TiO₂, which hindered the aggregation of nanoparticles and promoted nanocrystals formation.^411,412 The carbon and nitrogen co-doping intensified the absorption in the visible region and facilitated the exposure of a large amount of reactive facets.⁴⁰⁹ The N–F–TiO₂ platelets with dominant {001} facets prepared via solid state phase pathway, i.e. by nitriding TiOF₂ precursor in NH₃ gas flow demonstrated excellent performance for water oxidation under visible light.⁴¹³ The presence of fluorine dopant facilitates nitrogen doping into the crystal by maintaining charge balance and stabilizes the structure.⁴¹⁴ When TiOF₂ was treated with NH₃ gas flow, the latter facilitated the collapse of TiO₂ boxes into platelets and the simultaneous incorporation of nitrogen atoms into the TiO₂ lattice. Moreover, a negligible amount of O₂ was detected with N–TiO₂ because water oxidation reaction requires the removal of four protons and four electrons and the formation of an oxygen–oxygen double bond.⁴¹³ The N–S–TiO₂ nanosheets with exposed {001} facets prepared by a simple mixing-calcination method using the hydrothermally prepared TiO₂ nanosheets showed an enhanced degradation of 4-CP under visible light irradiation, which is attributed to the synergetic effects as follows:⁴¹⁵ (i) the red-shift of absorption edges and intense absorption in the visible light region, and (ii) the exposure of highly reactive {001} facets of nanosheets. The former effect indicates more photogenerated electrons and holes participating in the photocatalytic reactions. The latter effect is due to 2D nanosheets with exposed {001} facets, which adsorbs pollutant molecules better due to their higher reactivity. Under visible light irradiation, electrons can transfer from VB to impurity states easily, and the electrons in the impurity states move to CB after a secondary excitation. The intermediate states offer “extra steps” for the absorption of low energy photons via the excitation of electrons at the top of VB to the intermediate bands, where they can be excited again to the CB bottom level, resulting in an effective photoresponse. The photocatalytic experiments indicated that pure TiO₂ nanosheets show visible light activity for 4-CP degradation on irradiation. The substrate–surface complexation formed by a phenolate linkage reaction between the ligand and Ti(IV) site on the TiO₂ surface as shown below.

RTi–OH + OH–Ph → RTi–O–Ph + H₂O (64)

This complexation extends the absorption of titania to visible light region through ligand to titanium charge transfer.⁴¹⁵ The N–S–TiO₂ nanocrystals with high percentage of {001} facets synthesized via solvothermal process, followed by calcination with thiourea at 300 °C, demonstrated superior MB degradation and photocatalytic inactivation of E. coli bacteria under visible light.⁴¹⁶ This superior activity is ascribed to the reduced crystal size with {001} facets, which largely increased their specific surface area (70 m² g⁻¹) and the contact efficiency with contaminants.⁴¹⁶ The N–F–Cr–TiO₂ microspheres were prepared by hydrothermal method by treating the thermally sprayed TiN/Ti coating with HF aqueous solution and chromium powder at 180 °C for 5–24 h.⁴¹⁷ In the absence of chromium powders, the interaction of TiN/Ti with HF resulted in an interconnected, truncated bipyramids, while high energy {001} facets were formed in the presence of chromium microspheres. The increase of surface vacancy due to chromium doping increases the surface energy, contributing to the selective erosion of the truncated bipyramids to a drum-like morphology. In addition to the incorporation of nitrogen, fluorine and chromium led to a significantly enhanced solar absorption and reduced the bandgap remarkably.⁴¹⁷

9 Application of N–TiO₂ in organic reactions and organic synthesis

Since various intermediates are formed during titania photocatalysis, degradation reactions can be tuned at specific intervals so as to obtain the required organic compounds, which otherwise demand harsh conditions or multiple steps for their preparation under organic protocols. Thus, most of the organic compounds can be obtained in a single step under laboratory conditions.^418,419 N–TiO₂ displayed a reductive cleavage of azoxybenzene into their corresponding amines or 2-phenylindazoles with methanol in the presence of N₂ atmosphere under UV/solar light illumination.⁴²⁰ The amines (aniline) were obtained in pure methanol, while 2-phenylindazoles was formed in aqueous methanol (20% water + 80% methanol). The product yield decreased with an increase in carbon chain length of alcohol, highlighting the importance of solvents in tuning the reaction rate and corresponding yields. On illumination electron–hole pairs are generated on TiO₂, in which CB electron is captured by oxygen, retarding amine formation via reduction in the aqueous methanol system. The hole oxidizes methanol to generate hydroxymethyl radical and hydrogen radical that react with azobenzene to form a hydroxymethyl adduct, which later dehydrates and cyclizes to indazole or can be cleaved by protons to form amines (Fig. 24).⁴²¹ The as-prepared tubular TiO₂ immersed in 1 M acetic acid was effective in the ring opening reaction of an epoxide and Friedel–Crafts reaction.⁴²² The protonation of oxygen atom in styrene oxide from Bronsted acidic site, followed by the attack of nucleophilic part of methanol onto this carbon center, lead to the ring opening reaction (Fig. 25). Friedel–Crafts reaction is initiated by the activation of CO group in methyl vinyl ketone through protonation, followed by an attack of π electrons in indole upon vinyl group, suggesting that the catalyst exhibited a prominent acidic nature.⁴²²

Fig. 24 Possible reaction pathways for the reductive cleavage of azoxybenzene into their corresponding amines. Reprinted with permission from ref. 420, Copyrights (2012) Elsevier publications.

Fig. 25 Schematic representation of (a) ring opening reaction of epoxide by alcohol (b) Friedel–Crafts reaction of indole. Reprinted with permission from ref. 422, Copyrights (2011) American Chemical Society.

The activity of various catalysts towards one-pot synthesis of quinaldines from nitrobenzene in ethanol under both UV and visible light followed the order of N–TiO₂ > Pt–TiO₂ > Au–TiO₂ > Ag–TiO₂ > bare TiO₂.⁴²³ The photoirradiation of alcohol solution of different nitroarenes with N–TiO₂ showed that the electron releasing group at the para position inhibits the condensation of amino group with aldehyde. In the case of 3,5-dimethylnitrobenzene, cyclization reaction was hindered due to steric effect, which decreased the product yield (66%) compared to 3-nitrobenzene (80%). The photoinduced dehalogenation resulted in lower yields in the case of 4-chloro and 4-fluronitrobenzenes (Table 5). Contrarily, the addition of KI and NaI increased the product yield due to the hole scavenging effect of the iodide ion.⁴²³ In another study, N–TiO₂ promoted rapid and selective quinaldine production from an ethanolic solution of aniline and with other substituted anilines under UV/visible light.⁴²⁴ The initial step involved the oxidation of alcohol to aldehyde via VB holes from N–TiO₂. The oxidized product of ethanol condensed with aniline to produce an imine (Schiff base), which on further condensation and cyclization yielded corresponding quinaldine. There was no significant effect on the product yields with other substituents except for halo anilines. In the case of 4-chloro and 4-fluoroanilines, the yields of halo-substituted quinaldines were very low due to a photoinduced dehalogenation before the reactions. N–TiO₂ with poor crystallinity prepared at low temperature using NH₄Cl as the nitrogen source showed a partially enhanced oxidative transformation of 4-methoxybenzyl alcohol to p-anisaldehyde under simulated solar light.⁴²⁵ The high reactivity was due to the synergistic effect of anatase–rutile phases,⁴²⁶ which reduced the recombination rate. In addition, the nitrogen in bulk improved the reaction rate and its presence on the catalyst surface reduced mineralization sites, which increased the selectivity. Though the thermal treatments yielded an improvement in crystallinity and activity (in terms of initial disappearance rate) were highly detrimental for selectivity and exploitation of UV light. This is due to the primary influence of particles sintering and higher crystallinity leading to high activity and low selectivity.⁴²⁵ These types of photoinduced chemical transformations, leading to the formation of industrially important compounds, are encouraging to further explore applications of TiO₂ in organic synthesis.⁴²⁴

Table 5 Photocatalytic synthesis of various substituted quinaldines using N–TiO₂^a,b

Reactant	Product yield (%)	Byproduct	Conversion
a All reactions were performed with a 25 mM alcoholic solution of the reactant containing 50 mg of N–TiO2 suspension, intensity (I) = 1.381 × 10^−6 einstein L^−1 s^−1, irradiation time = 5 h.b Reprinted with permission from ref. 423, Copyrights (2012) Royal Society of Chemistry.
Nitrobenzene	Quinaldine (70)	29	99
3-Nitrobenzene	2,7-Dimethylquinoline (80)	18	98
4-Nitrobenzene	2,6-Dimethylquinoline (75)	23	98
4-Methoxynitrobenzene	6-Methoxy-2-methylquinoline (70)	26	96
3-Methoxynitrobenzene	7-Methoxy-2-methylquinoline (72)	22	94
3,5-Dimethoxylnitrobenzene	2,5,7-Trimethylquinoline (66)	19	85
4-Chloronitrobenzene	6-Chloro-2-methylquinoline (36)	64	99
4-Fluoronitrobenzene	6-Fluoro-2-methylquinoline (20)	79	99

---

10 Conclusion

The superiority of N–TiO₂ is demonstrated by its low cost and simple, controllable synthesis, high surface hydrophilicity and photoinduced charge carrier transfer from bulk to surface under visible light. Nitrogen has similar ionic size and posses low ionization energy to that of oxygen, which effectively controls both chemical and electronic state along with the surface structure of TiO₂. Because of the comparable ionic size, induced lattice distortion will not be significant; hence a large number of recombination centers are not generated. In addition, nitrogen as a dopant does not segregate or lead to the formation of impure products like dopant oxide. Therefore, the interaction of nitrogen dopant within titania matrix and its homogeneous distribution within the host lattice remains an engaging research topic for researchers.

The following general conclusions can be drawn based from the available literature:

(i) The doping of nitrogen ion at substitutional or interstitial lattice position shifts the bandgap of titania to visible region, allowing it to operate under solar light.

(ii) Na₂EDTA, gaseous NH₃, NH₄OH, NH₄Cl, NH₄NO₃, (NH₄)₂CO₃, NH₂CONH₂, C₂H₅NH₂, N(C₄H₉)₄–OH, guanidine, HMT and 1,3-diaminopropane are the most used nitrogen precursors.

(iii) The annealing atmosphere (oxidizing/reducing/inert) changes the dopant diffusion properties into the bulk of TiO₂ lattice, affecting the relative chemical stability.

(iv) Depending on the experimental conditions the possible nitrogen states in TiO₂ lattice are O–Ti–N, Ti–O–N, Ti–N–O, N–Ti–O, N–Ti–N, –(NO), –(NO₂), N_b˙.

(v) Nitrogen doping is always accompanied by the creation of defects like Ti³⁺ ion or oxygen vacancies, which can either suppress or enhance recombination pathways, depending on their distribution in the lattice.

(vi) N–TiO₂ with diverse morphologies like nanotubes, nanoflowers, hollow spheres, nanosheets, fibre, nano-octahedral, rice grain, V-shape were more beneficial compared to regular nanoparticles due to their unique structures.

(vii) Various modifications like co-doping with metal/nonmetal ions, coupling with other semiconductors or surface metal deposition influences the electronic band structure and induces exceptional stability to N–TiO₂. These modifications impart multiple charge transfer pathways to hinder recombination process.

(viii) The stabilization of {001} facets by the nitrogen dopant is remarkable since such situation is hard to achieve with other metal ions.

(ix) The applications of N–TiO₂ for hydrogen evolution and in organic synthesis are still at the initial stages and much more research work is needed to correlate material properties with the above probe reactions.

Acknowledgements

The authors acknowledge University Grants Commission (UGC) and Department of Science and Technology (DST-IDP & DST-SERC), Government of India for their financial supports.

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