One step solvothermal synthesis of ultra-fine N-doped TiO₂ with enhanced visible light catalytic properties

Abstract

Herein, we report the one-step solvothermal synthesis of a highly crystalline nitrogen doped TiO₂ (N-TiO₂) comprised of ultra fine particles (∼4 nm) with N-methyl-2-pyrrolidone (NMP) as solvent and doping agent. The method yields a highly crystalline, high surface area N-TiO₂ of pure anatase nature and is attributed to the use of organic solvents alone. The N-TiO₂ exhibited higher visible light (>400 nm) catalytic activity (∼50% increase) in the photodegradation of rhodamine B (RhB) compared to that of the commercial P25. Annealed N-TiO₂ (aN-TiO₂) showed a ∼70% increase in the visible light degradation of RhB and nearly 100% increase in transient photocurrent density to that of P25. The improvement in the visible light photoactivity of N-TiO₂ and aN-TiO₂ is assigned mainly to the improved visible light response because of the narrowed the band gap by introducing intra-band electronic states and reduced charge transfer resistance and the comparatively better performance of aN-TiO₂ is assigned to the additional Ti³⁺ defect states caused by oxygen vacancies formed during annealing.

---

1 Introduction

Titanium dioxide (TiO₂) is an extensively investigated semiconductor in various photocatalytic applications such as hydrogen fuel generation,^1–3 photodegradation of dyes^4–7 etc. due to its low cost, non-toxicity and corrosion resistance. However, the effective utilization of solar light by TiO₂ is hampered by its large band gap (∼3.2 eV) due to which its activity is restricted to the UV part of the solar spectrum which accounts for only ∼4% of the solar energy. Many strategies have been adopted to broaden the photo response of TiO₂, such as metal doping,^8–10 non-metal doping,^11–13 electron beam (EB) modification¹⁴ and defect engineering^15,16 so that the more abundant visible light solar spectrum is utilized. Among them, the non-metal doping of TiO₂ is a popular approach due to its comparatively simpler nature. Asahi et al.¹¹ has showed that the doping of non-metal such as N, S, P and C into TiO₂ extend the visible light activity by decreasing the band gap. Among these, N and C doping has been widely investigated due to their compatible ionic sizes to oxygen as compared to that of S and P.¹²

It is widely accepted that doping of TiO₂ by N improves the visible light activity.^11,17,18 Typically N-doping of TiO₂ has been achieved by utilizing ammonia, urea, triethylamine, NH₄OH and hexamethylenetetramine, guanidine hydrochloride, triethylamine etc.^19–24 as external dopants, plasma treatment using N₂ or N₂–H₂ gas mixtures,²⁵ mechanochemical method,²⁶ sol–gel and hydrothermal/solvothermal methods etc. Solvents such as isopropyl alcohol (IPA),²⁷ N-methyl-2-pyrrolidone (NMP)¹⁹ has been used as doping agent for C and N-doping, respectively by subjecting already prepared TiO₂ to water based ambient condition sol (WACS) method.¹⁹ Kaewgun et al.¹⁹ has reported a N-doped P25 using NMP by solvent-based ambient condition sol method (SACS). The method has utilized the P25 as a starting material which leads to the formation of N-doped polymorphic titania.

Here we report, a single step procedure to the synthesis N-doped TiO₂ (N-TiO₂) from its precursor by solvothermal method using NMP as solvent and dopant. Since the N-TiO₂ is formed from its precursor, it is possible to achieve molecular level N-doping. While previous reports¹⁹ have obtained polymorphic titania and our method yields ultra fine sized (∼4 nm) highly crystalline N-TiO₂ with pure anatase phase and very high surface area of 191 m² g⁻¹. The obtained N-TiO₂ was annealed at 400 °C under inert atmosphere and the annealed product of N-TiO₂ was referred to as aN-TiO₂. It is interesting to note that the aN-TiO₂ showed a narrowed band gap of 3.10 eV as compared to the 3.15 and 3.3 eV of N-TiO₂ and P25, respectively. The aN-TiO₂ exhibited the enhanced photodegradation activity for rhodamine B (RhB) and transient photocurrent density under visible light compared to that of the P25 and attributed to the N-doping which induces intra-band electronic states. Photoelectrochemical experiments showed that aN-TiO₂ has comparable or lower charge transfer resistance to that of P25 and is probably due to the defect states. Thus, the method provides a one-step procedure for synthesizing highly crystalline, ultra small sized anatase N-doped TiO₂ with higher surface area and improved visible light activity. Annealed N-TiO₂ shows enhanced visible light PD activity and photocurrent than N-TiO₂ and all this without the use of any external doping agents.

2 Experimental method

2.1 Materials

Aeroxide (P25) (21 nm), RhB and titanium isopropoxide were purchased from Sigma Aldrich India Co. Ltd. Sodium sulfate (Na₂SO₄), acetyl acetone and Triton-X were purchased from Merck, India Ltd. NMP and IPA were obtained from Spectrochem India Ltd. Distilled water was used for all studies. All the solvents and reagents used in this study were of analytical grade.

2.2 Preparation of aN-TiO₂

Typical synthesis involved with the addition of 3 mL titanium isopropoxide and 25 mL of NMP–IPA (20 : 5) mixture in an autoclave under stirring. Then the autoclave was heated to 180 °C for 20 h. After the reaction, the reaction mixture was allowed to cool to room temperature. Then 50 mL of water was added to it and then stirred for 1 h. The yellowish white precipitate was centrifuged and washed plenty of time with water and finally washed with IPA and dried at 75 °C overnight. To study the effect of doping, temperature and time has been varied and the composites are labelled as N-TiO₂ (T–t), where T refers to temperature while t refers to duration of reaction in hours. Initially the photocatalytic activities were tested and the N-TiO₂ (180–20) was found to be a best photocatalyst and it was called as N-TiO₂ for further discussion. This N-TiO₂ was annealed at 400 °C for 2 h under inert atmosphere in order to remove any surface adsorbed N-species, and was named as aN-TiO₂.

2.3 Photocatalytic degradation (PD) of dyes

A stock solution of 0.015 mg mL⁻¹ of RhB was used in the studies. Typically, 17 mg of the photocatalyst was dispersed in 25 mL of the RhB stock solution. The mixture was bath sonicated for 2 min and then the suspension was stirred for 60 minutes under dark conditions for attaining adsorption equilibrium. Under ambient conditions, the suspensions were irradiated with visible light (>400 nm) using 84 W light sources. For determining PD, photocatalyst was removed by centrifugation prior to recording UV-vis spectra and the concentration of RhB was determined from the absorbance value at 554 nm.

2.4 Recycling studies

Photocatalytic recycling studies were carried out using aN-TiO₂ for the photodegradation of RhB. Solar simulator with a lamp source of 150 W xenon lamp was used as the light source. Typically 4.5 mg of the catalyst was added to 20 mL of 7.8 μM RhB solution. The suspension was sonicated for 10 min in a bath sonicator and then kept for photodegradation. Complete decolouration of RhB was taken as the end point of each cycle. Time taken for the complete decolouration for the first cycle was considered 100% and consecutive cycle's efficiencies were calculated based on the time taken for complete decolouration with respect to the first cycle. No washing was done in between the cycles.

2.5 Photoelectrochemical studies

Titanium (Ti) metal of 0.25 mm thickness was cleaned by ultrasonicating in water and isopropanol sequentially. To the 70 mg of the catalyst, 0.8 mL of acetylacetone and 1.2 mL of Triton-X were added and probe sonicated for 1 h. The prepared suspension was spin coated onto the cleaned Ti electrodes. The prepared electrodes were annealed at 400 °C for 1 h. Photoelectrochemical characterizations were carried out using a conventional three electrode configuration. Ag/AgCl, platinum and the coated Ti electrodes were used as reference, counter and working electrodes respectively. Aqueous Na₂SO₄ solution (0.2 M) was used as an electrolyte. Electrochemical impedance spectra (EIS) measurements were performed over a frequency range of 10 kHz to 0.1 Hz to with AC amplitude of 10 mV. 500 W halogen lamp was used for visible light irradiation during photocurrent measurement. The light intensity on the electrode was set to be 100 mW cm⁻². Transient photocurrent measurements were done using chrono amperometric methods at 0.5 V vs. Ag/AgCl. Aqueous Na₂SO₄ solution (0.2 M) was used as an electrolyte for electrochemical impedance spectral (EIS) studies.

2.6 Characterization

Fourier transform infra-red (FTIR) spectra were recorded in a Perkin Elmer spectrum100 FTIR spectrophotometer. X-ray diffraction (XRD) analyses were carried out on a Bruker D8 discover small angle X-ray diffractometer. The Cu Kα radiation (λ = 1.54 Å) was used as an excitation source. Absorption spectra in the solutions were recorded using CARY 100 Bio UV-visible spectrophotometer. Solid state diffuse reflectance spectra (DRS) were recorded out in Shimadzu 2600 instrument. Raman spectroscopic measurements were carried out in Renishaw inVia Raman microscope with the excitation laser wavelength of 532 nm. Scanning electron microscope (SEM) images were taken using Hitachi SU6600 variable pressure FESEM. High resolution transmission electron microscopy (HRTEM) analyses were done using FEI, TECNAI S twin microscope with an accelerating voltage of 300 kV. Brunauer–Emmett–Teller (BET) surface area (SA) analyses were made in Tristar II micromeritics. Photoelectrochemical characterizations were carried out using Autolab PGstat320N from Metrohm. X-ray photoelectron spectroscopy (XPS) analyses were carried out using Kratos analytical axis ultra-X-ray photoelectron spectrometer with the excitation source of Al Kα. Particle size analysis and zeta potential (ZP) was carried out using Mavern zeta sizer nanoseries instrument. Particle size and ZP measurements were done at neutral pH.

3 Results and discussions

FTIR spectroscopy is an important tool to characterize the N-doping of TiO₂ and the spectra of N-TiO₂, aN-TiO₂ and P25 (for comparison) are shown in Fig. 1A. The difference in N functional group intensity can be distinguished from FTIR spectra. The nitrogen incorporation was evident from the peaks at 1060 and 1398 cm⁻¹ which correspond to the Ti–N bond vibration and the surface adsorbed NO₃⁻ groups respectively,^29,30 which were present in both N-TiO₂ and aN-TiO₂. The intensity of the peak at 1398 cm⁻¹ in aN-TiO₂ was decreased and attributed to the partial removal of surface adsorbed NO₃⁻ groups during annealing. Fig. 1B shows the Raman spectral features of the prepared N-TiO₂ and P25. Anatase phase of TiO₂ has six active Raman modes at 144 (E_g), 197 (E_g), 399 (B_1g), 513 (A_1g), 519 (B_1g) and 639 (E_g) cm⁻¹,^21,31 in our case there was a small shift in the 144 (E_g) to 146 cm⁻¹ compared to that of the P25. The blue shift in E_g may be possibly ascribed to the surface pressure and phonon confinement effect due to the ultra-small size of N-TiO₂ ³² as evidenced by the TEM and the XRD. The peaks in the range of 250 to 350 cm⁻¹ might have arisen from the first-order traverse acoustic mode of Ti–N bond.³³ Since the E_g peak at 197 cm⁻¹ was very weak, it was observed as a small tail near the E_g peak at 146 cm⁻¹. Thus the Raman spectra, confirms the anatase nature of N-TiO₂ and suggest the N-doping in TiO₂. The XRD patterns of both N-TiO₂ and aN-TiO₂ (Fig. 1C) exhibited sharp peaks indicating the crystalline nature of N-TiO₂. The peaks at 25.8°, 38.5°, 48.1°, 54.8° and 63.2° correspond to the anatase phase (JCPDS no. 71-1168) of TiO₂ and the absence of peaks of rutile nature indicated that both the N-TiO₂ and aN-TiO₂ were purely anatase in nature and the sharpness of aN-TiO₂ peaks suggest higher crystallinity of aN-TiO₂. The crystallite size of N-TiO₂ and aN-TiO₂ were calculated from XRD using Debye–Scherrer formula and found to be ∼4 and 9 nm, respectively. No new diffraction peaks were found due to the presence of N.²² The ultra-fine size of N-TiO₂ was ascribed to the solvothermal method which utilizes organic solvents only. The previous reports^19,28 of N doped TiO₂ from NMP based on SACS method yielded polymorphic N-doped titania and this was due to the difference in the synthetic procedure followed, in the reports WACS treated P25 was further undergone SACS treatment while in this work N-TiO₂ was prepared in a single step from titanium isopropoxide and NMP : IPA mixture.

Fig. 1 (A) FTIR spectra (B) Raman spectra and (C) XRD pattern of (a) P25; (b) N-TiO₂ and (c) aN-TiO₂.

The HRTEM images of N-TiO₂ and aN-TiO₂ are given in Fig. 2. The lattice spacing of 0.34 nm in Fig. 2B confirms the anatase form of TiO₂. The TEM images clearly showed that the particle sizes are 3–5 nm for N-TiO₂ while it was ranging from 9 to 12 nm for aN-TiO₂ (Fig. 2C) which is agreeing with the sizes calculated from XRD results. The size increase in aN-TiO₂ is due to annealing.

Fig. 2 (A & B) HRTEM image of N-TiO₂ composed of ultra-fine nanoparticles and the magnified image of the portion in square box, respectively; (C & D) HRTEM image and SAED pattern of aN-TiO₂ respectively, the TEM image clearly shows the increased particle size in aN-TiO₂ and SAED pattern shows the anatase nature of aN-TiO₂. The 0.34 nm shown in the figure spacing corresponds to (101) facet of anatase TiO₂.

Fig. 3 shows the XPS data corresponding to the wide spectra, deconvoluted N1s and Ti2p peaks of N-TiO₂ and aN-TiO₂. Wide spectrum clearly showed the presence of Ti, O, N and C elements in N-TiO₂ and aN-TiO₂. N-Doping was evidenced by the presence of N1s binding energy levels (398–406 eV). The reduced intensity of N1s peak in the wide scan of aN-TiO₂ was due to the partial removal of surface adsorbed NO₃⁻ molecules. Apart from surface adsorbed N-groups, interstitial and substitutional doped N can be identified from deconvoluted spectra of XPS. Deconvolution of N1s spectra of N-TiO₂ (Fig. 3) showed three peaks at 399.6, 400.3 and 404 eV while that of the aN-TiO₂ showed two peaks at 399 and 400.3 eV.^34,35 The absence of peak at 404 eV in aN-TiO₂ was complementing the results obtained from the FTIR. Asymmetrical nature of the O1s peaks of N-TiO₂ and aN-TiO₂ as compared to that of the P25 (Fig. S2†) suggested the influence of N atom in the O1s environment other than the Ti atom.^21,36,37 As expected, the binding energies of Ti2p has been shifted to 458.9 (Ti2p_3/2) and 464.6 eV (Ti2p_1/2) as compared to that of the Ti2p peaks [459.5 (Ti2p_3/2) and 465.2 eV (Ti2p_1/2)] of P25 (Fig. S2†).³⁸ The quantification of weight% of N-doping was done for N-TiO₂'s prepared under different conditions and found to be 0.64, 0.94, 0.98 and 0.97% for N-TiO₂ (140–20), N-TiO₂ (180–20), N-TiO₂ (200–20) and N-TiO₂ (180–30), respectively. The weight% of N decreased from 0.94% in N-TiO₂ to 0.52% in aN-TiO₂ due to the removal of surface adsorbed N-species. Incorporation of N can lead to increase in the electron density on Ti atom due to the comparatively lower electronegativity of N to that of oxygen, thus influencing the binding energies of nearby Ti and O atoms.

Fig. 3 XPS spectra of N-TiO₂ and aN-TiO₂ corresponding to wide, deconvoluted N1s peaks and Ti2p peaks. The N1s peak in aN-TiO₂ is confirming the partial removal of nitrogen sources.

Reduced binding energies of Ti2p peaks of N-TiO₂ as compared to P25 (Fig. S2†) indicated that N incorporation and assist the formation of Ti³⁺ defect states. Moreover the peak at 462.7 eV corresponds to the Ti2p_1/2 peak of Ti³⁺ defect state in N-TiO₂ ¹⁴ while that peak was diminished in aN-TiO₂. This can be due to lower detection limit of XPS since the extent of N functional groups has been reduced during annealing. The presence of Ti³⁺ defect states in aN-TiO₂ can still be identified from the asymmetrical Ti2p peaks. It was well reported^12,39–41 that N doping can induce the formation of Ti³⁺ defect states and oxygen vacancies below the conduction band of TiO₂. Livraghi et al.⁴¹ has reported the enhancement in the visible light absorbance of N-doped TiO₂ due to the formation of Ti³⁺ defect states and oxygen vacancies. Nitrogen doping^21,42 can induce the formation of oxygen vacancies and therefore Ti³⁺ defect states both are known to improve the conductivity as well as the reducing the band gap.

DRS spectra and the corresponding Tauc plots were obtained for the solid N-TiO₂, aN-TiO₂ and P25 are given in Fig. 4A & B respectively. It was evident from the DRS spectra (Fig. 4) that both N-TiO₂ and aN-TiO₂ showed improved visible light absorption than that of P25. Reports show that N doping of TiO₂ creates intra-band states that are close to the valence band edges, which induces visible light absorption.⁴³ Band gaps obtained from Tauc plots for N-TiO₂ and aN-TiO₂ were 3.15 and 3.1 eV, respectively, lower compared to that of the P25 (3.3 eV). The slight band gap reduction in N-TiO₂ and aN-TiO₂ were attributed to the introduction of intra-band states in the band gap of TiO₂. Due to annealing, changes were observed with regard to N-doping, such as the removal of surface adsorbed NO₃⁻ groups and partial conversion of interstitial N doping into substitutional N doping as supported by FTIR and XPS analyses. Both interstitial N and substitutional N, can cause the reduction in band gap.³⁹ In addition, an improvement in the visible light absorptivity was observed for aN-TiO₂ to that of N-TiO₂, and is attributed to the formation of Ti³⁺ defects by the formation oxygen vacancies during annealing as supported by XPS. Valentin et al.³⁹ and Livraghi et al.⁴¹ reported the formation of Ti³⁺ defects and oxygen vacancies during N-doping especially at high temperature reductive condition.

Fig. 4 (A) DRS spectra and (B) Tauc plots of (a) P25; (b) N-TiO₂ and (c) aN-TiO₂.

Table 1 shows that the BET specific SA values are in the order of N-TiO₂ > aN-TiO₂ > P25. The higher SA of N-TiO₂ (~4 times that of the P25) was attributed to the preparation method which utilizes organic solvent alone, resulting in ultra-fine TiO₂ particles. The SA of aN-TiO₂ decreased to around half the value of that of N-TiO₂ and is assigned to the annealing process which increased the particle size. Furthermore, N-doping and particle size distribution were analyzed using zeta-potential (ZP) and particle size measurements. The increase in the hydrodynamic volume of aN-TiO₂ (1704 nm) as compared to N-TiO₂ (803 nm) suggested that annealing increases the particle size. The ZP values of the samples show that for N-TiO₂ (−12.9 mV) has the highest negative value compared to that of aN-TiO₂ (0.7 mV) and P25 (14 mV) which agree with previous reports that have demonstrated. The ZP of N-doped TiO₂ is comparatively more negative due to the acidic properties of N-doped surface and due to the lower electronegativity of N compared to that of O in N-TiO₂.⁴⁴ The relatively lower ZP of aN-TiO₂ is attributed to the reduced surface groups due to the increased particle size and to the partial removal of surface adsorbed N-groups during annealing. The result indicates the N-doping of TiO₂ to form N-TiO₂. Thus, the characterization techniques confirm the doping of N in TiO₂ in a single step method using NMP solvent.

Table 1 BET surface area and zeta-potential, particle size of photocatalysts

Photocatalysts	BET surface area (m^2 g^−1)	Zeta-potential (mV)	Particle size (nm)
N-TiO2	191	−12.9	803
aN-TiO2	78	0.7	1704
P25	49	14	1876

---

Visible light photodegradation (PD) of RhB (Fig. 5) was conducted for the different N-TiO₂ samples prepared by varying the temperature and time and based on the performance of the various N-TiO₂, the best was reported for the temperature of 180 °C and 20 h sample. Further increase in temperature and time did not improve the catalytic activity and therefore, we chose this condition for annealing the N-TiO₂. The visible light catalytic properties of the N-TiO₂, aN-TiO₂ and P25 were investigated using the PD of RhB (Fig. 6). The adsorption of RhB for 2 h and the UV-vis absorption spectra of RhB during the visible light photodegradation are given in Fig. S3.† Adsorption in the dark in the first one hour was ∼5% for both N-TiO₂ and aN-TiO₂ and no further adsorption was observed during the next one hour. The order of PD activity of the catalysts is as follow aN-TiO₂ > N-TiO₂ > P25 and their PD% are 80, 72 and 50%, respectively and evidently follow their visible light absorption capability. Higher photoactivity of aN-TiO₂ as compared to N-TiO₂ is due to the improved visible light absorptivity induced by Ti³⁺ and oxygen vacancies. Even though BET SA of N-TiO₂ (191 m² g⁻¹) is higher than aN-TiO₂ (78 m² g⁻¹), the formation of Ti³⁺ and oxygen vacancies favour improved visible light absorption and hence higher photoactivity, which suggests that the PD is mainly due to photocatalysis mechanism rather than dye-sensitized mechanism.

Fig. 5 Effect of various N-TiO₂ on the visible light photodegradation of RhB.

Fig. 6 Visible and UV light PD of RhB a by (a) no catalyst; (b) P25; (c) N-TiO₂ and (d) aN-TiO₂.

In order to further ascertain the visible light activity of aN-TiO₂, the PD reactions of RhB were conducted in UV light and the results (Fig. 6) show that the UV light activity follows the order P25 > aN-TiO₂ > N-TiO₂ with 84, 65 and 57% PD, respectively. The results clearly show that the activity has reversed its trend in the UV light confirming that the improved activities of aN-TiO₂ and N-TiO₂ in the visible light are due to their improved visible light absorption. The relatively higher activity of P25 under UV light is assigned to its better charge separation due to the rutile and anatase phase interface alignment.⁴³

Recycling studies of aN-TiO₂ were done using the PD of RhB and the results are given in Fig. S4.† The result shows that there was an initial decrease from (100–76%) in the photodegradation and after that the decrease in PD efficiency observed was very minimal (∼1–2%) for each cycle and after 6 cycles the PD was 68%. It should be noted that the photocatalyst was not washed in between the cycles. Therefore, the initial decrease may be due to the blocking of surface active sites by products/reactants. Further decrease in PD after second cycles were negligible which indicates the stability of the N-doped TiO₂.

To further confirm the visible light absorption of N-TiO₂ and aN-TiO₂, we have conducted the transient photocurrent response experiments, which involves no dyes, under visible light (>400 nm) (Fig. 7) and observed that the photocurrent densities are higher for N-TiO₂ and aN-TiO₂ (1.2 μA cm⁻² and 1.4 μA cm⁻², respectively) compared to that of P25 (0.6 μA cm⁻²). The result confirms the visible light responsiveness of both N-TiO₂ and aN-TiO₂ as evidenced by DRS spectra. The photocurrent density was fairly stable throughout the experiment indicating the stability of both N-TiO₂ and aN-TiO₂ under visible photoirradiation. The shape of the transient photocurrent response gives information of the N-doped surface states. The response of N-TiO₂ shows an initial steep rise of photocurrent density immediately upon switching-on the light, and thereafter a rapid exponential decay is observed, followed by a cathodic current “overshoot” when the light is switched off. This overshoot is a typical fingerprint of intense surface recombination processes due to the defects caused by doping.^45–47 The overshoot observed in case of N-TiO₂ may possibly due to the surface defect states caused by N-centered states while such an overshoot is not observed in aN-TiO₂ and this interesting observation may be attributed to the removal of surface –NO₃ groups, which agrees with our XPS data. This overshoot is not observed in P25 either. The result of photocurrent measurements confirm the visible light activity of aN-TiO₂ and N-TiO₂ and thus the N-doping of TiO₂ by the single step method using the solvent, NMP.

Fig. 7 Transient photocurrent density of (a) P25; (b) N-TiO₂ and (c) aN-TiO₂ at 0.5 V vs. Ag/AgCl.

Electrochemical kinetics can be used to explain the charge transfer resistance across the interfaces. Nyquist plots of EIS analysis offer the idea of charge transfer resistance. Lower the charge transfer resistance smaller will be the radius of semicircle in the plot. Nyquist plots (Fig. 8) show that the semicircle radius is smaller for N-TiO₂ to that of the P25, indicating reduced charge transfer resistance in N-TiO₂ and is attributed to the defect states introduced by N-doping.

Fig. 8 Nyquist plots of (a) P25 and (b) N-TiO₂ in a 0.2 M Na₂SO₄ aqueous electrolyte.

4 Conclusions

We have demonstrated successfully a one-step solvothermal synthesis of N-doped TiO₂ of ultrafine size (∼4 nm), high crystallinity and high SA using NMP as solvent and dopant. Both the N-TiO₂ and its annealed sample, aN-TiO₂, showed improved visible light absorption and reduced band gap due to introduction of the intra-band states by N doping. Both N-TiO₂ and aN-TiO₂ exhibited enhanced visible light photocatalytic activity for the PD of RhB to that of P25. The transient photocurrent of ​N-TiO₂ and aN-TiO₂ under visible light increased ~2 fold to that of P25. Improved visible light absorbance and reduced charge transfer resistance contributed to the enhanced photoactivity and photocurrent of both N-TiO₂ and aN-TiO₂. The better visible light response, photocatalytic activity of aN-TiO₂ than that of N-TiO₂ is attributed to the surface defects such as Ti³⁺ and oxygen vacancies generation associated with N-doping.

References

1. A. Fujishima and K. Honda, Nature, 1972, 238, 37.
2. B. O'Regan and M. Grätzel, Nature, 1991, 335, 737.
3. Y. Ma, X. Wang, Y. Jia, X. Chen, H. Han and C. Li, Chem. Rev., 2014, 114, 9987.
4. J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo and D. W. Bahnemann, Chem. Rev., 2014, 114, 9919.
5. A. L. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735.
6. M. M. Ali and K. Y. Sandhya, Sol. Energy Mater. Sol. Cells, 2016, 144, 748.
7. M. M. Ali and K. Y. Sandhya, Carbon, 2014, 70, 249.
8. X. Chen, P. Y. Yu and S. S. Mao, Science, 2011, 331, 746.
9. M. U. Khan, M. Al-shahry and W. B. Ingler Jr, Science, 2002, 297, 2243.
10. J. Yu, G. Dai, Q. Xiang and M. Jaroniec, J. Mater. Chem., 2011, 21, 1049.
11. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269.
12. R. Asahi, T. Morikawa, H. Irie and T. Ohwaki, Chem. Rev., 2014, 114, 9824.
13. Y. Lin, Z. Jiang, C. Zhu, X. Hu, X. Zhang, H. Zhu, J. Fan and S. H. Lin, J. Mater. Chem. A, 2013, 1, 4516.
14. M. M. Khan, S. A. Ansari, D. Pradhan, M. O. Ansari, D. H. Han, J. Lee and M. H. Cho, J. Mater. Chem. A, 2014, 2, 637.
15. S. Kalathil, M. M. Khan, S. A. Ansari, J. Lee and M. H. Cho, Nanoscale, 2013, 5, 6323.
16. S. Wendt, P. T. Sprunger, E. Lira, G. K. H. Madsen, Z. Li, J. Ø. Hansen, J. Matthiesen, A. B. Rasmussen, E. Lægsgaard, B. Hammer and F. Besenbacher, Science, 2008, 320, 1755.
17. Z. Jiang, D. Liu, D. Jiang, W. Wei, K. Qian, M. Chen and J. Xie, Dalton Trans., 2014, 43, 13792.
18. Y. L. Pang and A. Z. Abdullah, Chem. Eng. J., 2013, 214, 129.
19. S. Kaewgun and C. A. Nolph, Catal. Lett., 2008, 123, 173.
20. A. V. Emeline, V. N. Kuznetsov, V. K. Rybchuk and N. Serpone, Int. J. Photoenergy, 2008, 258394.
21. G. Yang, Z. Jiang, H. Shi, T. Xiao and Z. Yan, J. Mater. Chem., 2010, 20, 5301.
22. Q. Xiang, J. Yu, W. Wang and M. Jaroniec, Chem. Commun., 2011, 47, 6906.
23. Y. Nosaka, M. Matsushita, J. Nishino and A. Y. Nosaka, Sci. Technol. Adv. Mater., 2005, 6, 143.
24. J. L. Gole, J. D. Stout, C. Burda, Y. B. Lou and X. B. Chen, J. Phys. Chem. B, 2004, 108, 1230.
25. K. Matsubara, M. Danno, M. Inoue, Y. Honda and T. Abe, Chem. Eng. J., 2012, 181, 754.
26. S. Yin, Q. W. Zhang, F. Saito and T. Sato, Chem. Lett., 2003, 32, 358.
27. Y. Park, W. Kim, H. Park, T. Tachikawa, T. Majima and W. Choi, Appl. Catal., B, 2009, 91, 355.
28. S. Kaewgun, D. Mckinney, J. White, A. Smith, M. Tinker, J. Ziska and B. I. Lee, J. Photochem. Photobiol., A, 2009, 202, 154.
29. D. Huang, S. Liao, S. Quan, L. Liu, Z. He, J. Wan and W. Zhou, J. Mater. Res., 2007, 22, 2389.
30. J. Geng, D. Yang, J. Zhu, D. Chen and Z. Jiang, Mater. Res. Bull., 2009, 44, 146.
31. H. Kamisaka, T. Adachi and K. Yamashita, J. Chem. Phys., 2005, 123, 084704.
32. W. J. Ong, L. Tan, S. P. Chai, S. T. Yong and A. R. Mohamed, Nano Res., 2014, 7, 1528.
33. H. Du, Yi. Xie, C. Xia, W. Wang and F. Tian, New J. Chem., 2014, 38, 1284.
34. W. Balcerski, S. Y. Ryu and M. R. Hoffmann, J. Phys. Chem. C, 2007, 111, 15357.
35. D. H. Wang, L. Jia, X. L. Wu, L. Q. Lu and A. W. Xu, Nanoscale, 2012, 4, 576.
36. Y. Cong, J. Zhang, F. Chen and M. Anpo, J. Phys. Chem. C, 2007, 111, 6976.
37. S. K. Joung, T. Amemiya, M. Murabayashi and K. Itoh, Appl. Catal., A, 2006, 312, 20.
38. J. Wang, W. Zhu, Y. Zhang and S. Liu, J. Phys. Chem. C, 2007, 111, 1010.
39. C. Di Valentin, E. Finazzi, G. Pacchioni, A. Selloni, S. Livraghi, M. C. Paganini and E. Giamello, Chem. Phys., 2007, 339, 44.
40. H. Sun, Y. Bai, W. Jin and N. Xu, Sol. Energy Mater. Sol. Cells, 2008, 92, 76.
41. S. Livraghi, M. C. Paganini, E. Giamello, A. Selloni, C. Di Valentin and G. Pacchioni, J. Am. Chem. Soc., 2006, 128, 15666.
42. A. K. Rumaiz, J. C. Woicik, E. Cockayne, H. Y. Lin, G. H. Jaffari and S. I. Shah, Appl. Phys. Lett., 2009, 95, 262111.
43. D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. A. Shevlin, A. J. Logsdail, S. M. Woodley, C. R. A. Catlow, M. J. Powell, R. G. Palgrave, I. P. Parkin, G. W. Watson, T. W. Keal, P. Sherwood, A. Walsh and A. A. Sokol, Nat. Mater., 2013, 12, 798.
44. M. Miyauchi, A. Ikezawa, H. Tobimatsu, H. Irie and K. Hashimoto, Phys. Chem. Chem. Phys., 2004, 6, 865.
45. Y. Zhao, X. Qiu and C. Burda, Chem. Mater., 2008, 20, 2629.
46. L. Pan, J. J. Zou, X. Y. Liu, X. J. Liu, S. Wang, X. Zhang and L. Wang, Ind. Eng. Chem. Res., 2012, 51, 12782.
47. R. Beranek and H. Kisch, Electrochem. Commun., 2007, 9, 761.

---

Footnote

† Electronic supplementary information (ESI) available: Details of digital images, SEM image, XPS and UV-vis DRS data. See DOI: 10.1039/c6ra09525a

---