Visible light induced enhanced photocatalytic degradation of organic pollutants in aqueous media using Ag doped hollow TiO₂ nanospheres

Abstract

The silver doped hollow TiO₂ (Ag-h-TiO₂) nanoparticles were synthesized by a sacrificial core (AgBr) method. The Ag doping and the core removal was done simultaneously during the dissolution of the core in (NH₄)OH solution. The mean particle size of synthesized Ag-h-TiO₂ nanoparticles is 17.76 ± 2.85 nm with a wall thickness of ∼2.5 nm. The hollow structured nanoparticles have 103.2 m² g⁻¹ more specific surface area compared to solid TiO₂ nanoparticles. The suitability of these synthesized hollow nanoparticles as photocatalyst were tested for the photocatalytic degradation of three important different classes of organic compounds such as nitrobenzene (NB), metronidazole (MTZ) antibiotic, and methylene blue dye (MBD) in aqueous solution under irradiation of visible light. Photodegradation studies show there is a significant enhancement of the degradation efficiency of the TiO₂ after the hollow structure formation and silver doping. The recycling tests of the catalysts show only ∼10% decrease in efficiency for NB and MTZ degradation after sixth cycle of reuse. The light emission capacity in terms of quantum yield (QY) is enhanced by 18.7% for Ag-h-TiO₂ than that of pure TiO₂ nanoparticles.

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1 Introduction

Metal oxide semiconductor nanoparticles play an important role in different areas of science and engineering such as photo catalysis,^1–3 solar cells,^4–6 Li-ion batteries,^7,8 piezoelectric devices,^9,10 fuel cells,¹¹ sensors,^12,13 and so on. The photo catalytic degradation of organic pollutants is considered as an advanced environmental remediation process in recent years over conventional processes such as membrane filtration, adsorption. Among several metal oxide semiconductor nanoparticles TiO₂ is a promising photocatalyst because of its relatively higher efficiency, low toxicity, long term stability, low cost, photoinduced strong oxidation activity properties.^14–16 However, the application of pure TiO₂ as a photocatalyst is limited in visible light because of its high band gap (3.03 eV for rutile and 3.18 eV for anatase). Because of this reason, the development of visible light induced TiO₂ photocatalyst with higher efficiency is highly essential to make the industrial process more feasible and economic. Since the reduction of band gap enhance the photocatalytic activity of a photocatalyst under visible light, significant efforts have been made till now to dope different elements into the TiO₂ host lattices such as, Fe,^17,18 Mn,¹⁹ Mg,²⁰ Cr,²¹ Co,²² Sn,²³ Sm,²⁴ Nd,²⁵ and so on. In addition to the band gap of a photocatalyst, the surface area is also extremely important to enhance the activity of a catalyst. In this regard, TiO₂ with different morphologies such as nanowires,²⁶ nanotubes,²⁷ nanorods²⁸ have been studied. More advanced material like hollow nanoparticles along with doping have attracted considerable interests because of its larger surface area along with some important properties, such as low density, good surface permeability and high light-harvesting efficiencies,^29–32 The TiO₂ photocatalysts are not only used for the remediation of contaminated water but also useful in air purification,³³ cathodic catalyst for low-cost fuel cell.³⁴ Some studies are also available on hollow TiO₂ nanoparticles doped with different elements such as Nd,³⁵ N and Ce,³⁶ Sn,³⁷ Bi,³⁸ in all these studies the C was used as the core and it was removed by calcination. The other template such as polystyrene was used for Sm³⁺ doped hollow TiO₂ structure.³⁹ Sulfur doped hollow sphere TiO₂ was also synthesized by our group, where S was doped during the removal of the sacrificial S core.⁴⁰ Considering catalytic properties of TiO₂, incorporation of noble metal like Ag to hollow TiO₂ structure is important because of its unique characteristics in resonant collective oscillations of the conduction electrons by electromagnetic radiation and the localized surface plasmon resonance (SPR).^41,42 Since nanoscale silver has good antibacterial property,^43,44 Ag doped TiO₂ nanoparticles also show similar property.^45–47 Silver–TiO₂ composite hollow nanoparticles was reported by template free method⁴⁸ and PSA latex templated method⁴⁹ as a visible light active photocatalyst. Recently, Ag and AgCl doped TiO₂ hollow nanoparticles were synthesized using polystyrene particles as template for getting a visible light active photocatalyst.⁵⁰

It has been noted from most of the literature that the doped hollow structure was formed after removing the core, and the doping was achieved in the presence of some extra precursor. However, a single step method for the core removal and doping could make the process simple.

As mentioned before, a significant fraction of research activities is going on environmental remediation of organic pollutants using TiO₂ nanoparticles. The main advantage of this process over the other conventional separation processes is toxic pollutants can be converted to useful or nontoxic compounds. Different classes of organic compounds such as aromatics (benzo compounds, phenolic compounds, naphthalene, trinitrotoluene, etc.), pharmaceutical products (antibiotics, antipyretic etc.), dyes (synthetic dyes, reactive dyes, azo dyes, basic cationic dyes etc.), some volatile organic compounds (formaldehyde, methylene chloride, ethylene glycol, etc.) can be degraded using this catalysts. The benzo compounds such as nitrobenzene are highly toxic organic compound mainly used for the production of aniline, paper and pulp, pesticides, dyes, explosives, cosmetics, pharmaceuticals, and so on.^51–53 The long term exposure of nitrobenzene to the environment, even at low concentration, causes risks to human, such as liver or kidney damage, lung irritation, increase heart rate, skin problem, vomiting, etc. Therefore, removal of nitrobenzene from the environment is a major concern. Degradation of nitrobenzene in effluent water is difficult by conventional chemical method because of the nitro group which has strong electron withdrawing property and inhibits its oxidation, or by biological method because of its toxic and mutagenic effect on the biological systems.^54,55 Apart from aromatic compounds, the presence of pharmaceutical products for instance antibiotics in the environment, even at low concentrations, causes the growth of antibiotic-resistant bacteria^56,57 and creates microbial population,^58,59 which may cause of ineffectiveness of the present forms of treatment and major epidemics. Metronidazole (MTZ) is one such type of antibiotic which is primarily used for the infectious diseases caused by anaerobic bacteria and protozoa. Complete removal of metronidazole from the environment by conventional method is difficult because of its low degradability and high solubility in water. Dye contamination is another critical environmental problem and addressed by several researchers till now. The sources of synthetic dyes in wastewater are from different industries such as, textile, dye and dye intermediates, paper and pulp, printing, colour photography, petroleum industries, and so on.^60,61 Continuous discharge of dye-bearing effluents from these industries into natural stream and rivers poses severe environmental problems as toxic to useful microorganisms, aquatic life, and human beings. So, suitable and efficient techniques are highly essential for the treatment of these industrial effluents.

In this study Ag-h-TiO₂ nanoparticles were synthesized through a sacrificial core technique using AgBr as the core. The core was removed under a mild condition by dissolving in ammonium hydroxide solution, and the silver doping was also achieved during the dissolution of the core without any addition extra dopant precursor. After Ag doping, pure anatase phase TiO₂ particles were obtained even after heating at 450 °C. The hollow structure nanoparticles have higher surface area (198.3 m² g⁻¹) compared to solid TiO₂ nanoparticles (95.1 m² g⁻¹) and hollow structures reported by other researchers.^62–66 To test the photocatalytic activity of the developed nanoparticles towards three important organic compounds such as nitrobenzene (NB), metronidazole (MTZ), and methylene blue dye (MBD) degradation studies were conducted with low catalyst dose and high initial MTZ concentration compared to the studies reported till now. The obtained TiO₂ particles are having low band gap, anatase phase, high surface area, good crystallinity, and efficient visible light induced photocatalytic property to degrade important organic compounds such as nitrobenzene, antibiotic, and dye.

2 Materials and methods

2.1 Materials

Reagent-grade silver nitrate (99.9%) and potassium bromide (99.3%) were purchased from Rankem. Anionic surfactant sodium dodecyl benzene sulphonate (SDBS) (technical grade, Cat no. 28995-7) and titanium(IV) butoxide (Ti[O(CH₂)₃CH₃]₄ or TOBT, 97%) were obtained from Sigma Aldrich. Ammonium hydroxide (25%), nitric acid (69% GR), cyclohexane, and methylene blue dye were bought from Merck. Nitrobenzene was provided by Nice Chemicals Pvt. Ltd. Metronidazole was supplied by J B Chemicals & Pharmaceuticals Ltd. All the chemicals were used as it was received without any further purification. Ultrapure water of 18.2 MΩ cm resistivity and pH 6.4–6.5 was used for all the experiments.

2.2 Methods

A sol–gel method was followed for the synthesis of silver doped hollow TiO₂ nanoparticles using AgBr as the sacrificial core (Scheme 1). The AgBr nanoparticles were synthesized in aqueous SDBS media according to our previous reported study.⁶⁷ The concentrations of both AgNO₃ and KBr were maintained at 0.1 mM. After the formation of AgBr NPs the suspension was sonicated using a bath sonicator for 20 min, TBOT was then added slowly under constant stirring condition for uniform coating on the core surface. After 5 minutes of addition of TBOT, HNO₃ was added to maintain the pH of ∼2 and kept for 35 min under stirring condition. The AgBr/TiO₂ core/shell nanoparticles were then separated by centrifugation at 25 000 rpm for 20 min and washed thrice with ethanol and water mixture (2 : 1, v/v). Then the particles were dipped in ammonium hydroxide solution (2.5 mM) for 48 h for the dissolution of AgBr core. The particles were separated and calcined at 450 °C for 2 h to improve the crystallinity. For quantum yield (QY) calculation of Ag-h-TiO₂ NPs, phenol was used as a reference (QY = 0.14).

Scheme 1 Schematic presentation for the formation of Ag-h-TiO₂ nanoparticles.

2.3 Particles characterization

The particle size was measured initially by dynamic light scattering (DLS) using a Malvern Zeta Size analyzer, (Nano ZS). The crystallinity of synthesized particles was characterized using powder X-ray diffraction (XRD) (Philips, PW 1830 HT) with scanning rate of 0.01° s⁻¹ in the 2θ range from 20° to 70°. The size and shape of the particles were observed under a field emission scanning electron microscope (FE-SEM) (FEI, Nova NanoSEM NPE212) and transmission electron microscope (TEM) (FEI, Tecnai S-twin). The elemental composition of the sample was analyzed by energy-dispersive X-ray spectroscopy (EDX) (Oxford Instruments, model X-sight) attached to the HR-TEM (JEOL, JEM 2100). The light absorbance and luminescence properties were also characterized by UV-vis-NIR Spectroscopy (Shimadzu, UV-3600) and fluorescence spectroscopy (Hitachi, F-7000) respectively. Fourier transform infrared spectroscopy (FT-IR) was carried out using an FT-IR (Thermo Fisher Scientific, Nicolet iS10). The chemical composition of the samples and the valence states of various elements were analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Inc., PHI 5000 Versa Probe II.). The surface area of the nanoparticles was measured by Brunauer–Emmett–Teller (BET) technique (Quantachrome, USA).

2.4 Photocatalytic degradation

The photocatalytic activity of the synthesized catalyst was tested by the degradation of NB, MTZ antibiotic, and MBD in aqueous solutions. For nitrobenzene degradation the initial concentration of nitrobenzene and catalyst dose were maintained 61.5 mg L⁻¹ and 0.05 g L⁻¹. For MTZ degradation the initial concentration of the MTZ solution was maintained to 15 mg L⁻¹ for all experiments and the catalyst concentration was 0.5 g L⁻¹. The degradation studies were conducted in a photoreactor equipped with a 125 W high pressure mercury vapour lamp (λ = 435.8 nm, light intensity 27 000 LUX, light energy 56 ± 2 W m⁻²) placed about 12 cm away from the solution. The solutions were stirred continuously using a magnetic stirrer. Prior to each test, the lamp was turned on 10 min before in order to get a maximum light intensity. The samples were magnetically stirred for 20 min in the dark to allow physical adsorption equilibrium of NB, MTZ, and MBD on the catalyst surface. Then the suspension was exposed to visible light irradiation under constant stirring condition. The sample was then taken out and centrifuged to remove the nanoparticles after each 20 min interval for MTZ and 30 min interval for NB and MBD degradation. Then the concentration of the NB, MTZ, and MBD solution were analyzed using a UV-Vis-NIR Spectrophotometer at its maximum absorbance wavelength of λ = 267, 320, and 661 nm for NB, MTZ, and MBD, respectively. The cyclic degradation test was conducted to check the reusability of the photocatalyst. After each cycle of degradation the catalyst was separated by centrifugation and was used for the next cycle of degradation without any pre-treatment. The intermediate and final degraded products during nitrobenzene degradation were analyzed by liquid chromatography-mass (LC-MS) (Flexar SQ 300 MS, Perkin Elmer). The sample was extracted with cyclohexane and 70% water/30% acetonitrile (v/v) mobile phase was used for the analysis.

3 Results and discussion

3.1 Synthesis of hollow TiO₂ particles

The hollow particles were synthesized by the templated sacrificial core method using AgBr as a core. In aqueous media the surface charge of AgBr nanoparticles is −15 to −25 mV. Addition of anionic surfactant (SDBS) makes the surface charge of AgBr more negative (−47.8 mV). The surface charge of TiO₂ nanoparticles is positive, so, it is expected that the TiO₂ will favourably coat on the AgBr core surface because of the electrostatic attraction. When ammonium hydroxide solution was added to the particle suspension, AgBr would try to dissolve in ammonium hydroxide through the formation of soluble complex according to eqn (1). During the dissolution of AgBr, there is a chance of doping of Ag on TiO₂ shell.

AgBr (s) + 2NH₃ (aq) → Ag(NH₃)₂⁺ (aq) + Br⁻ (1)

3.2 Characterization of particles by UV-Vis spectroscopy

UV-Vis spectroscopic study was done to investigate the light absorption property of the nanoparticles, and the results are presented in Fig. 1. The figure shows that the sharp peak of pure AgBr is present in the UV region (237 nm), and it disappears after the coating of external shell layer of TiO₂ on AgBr. Pure TiO₂ is having major absorption below 350 nm wavelength, and there is no absorption in the visible region, this is because of its high energy band gap (3.2 eV). However, the UV-Vis spectra of Ag-h-TiO₂ nanoparticles show a significant absorption in 300–650 nm wavelength range. The shifting of absorbance in the visible range is attributed to the fact of lowering of the band gap of the obtained nanoparticles because of doping of Ag. In addition, noteworthy to mention that, the absorption intensity of Ag-h-TiO₂ is higher compared to that of pure solid TiO₂ in both UV as well as in visible regions. The increase in absorbance is also attributed to the fact of porous hollow structure and formation of shell layer by the deposition of small particles. The band gap energy was calculated using the equation E_g = 1239.8/λ (ref. 68) to support the above fact, where E_g is the band gap (eV) and λ is the wave length (nm) of the absorption edges in the UV-Vis spectrum. It has been found that the calculated band gaps are 2.69 and 3.29 eV for AgBr and TiO₂, which are in good agreement with the reported values of 2.7 eV (ref. 69) and 3.28 eV (ref. 70) respectively, whereas, 2.48 and 2.25 eV for AgBr/TiO₂ and Ag-h-TiO₂ nanoparticles respectively. Since the band gap of the TiO₂ is reduced after the formation of doped hollow structure, it is expected to be useful as good photocatalyst under the visible light irradiation, dye sensitized solar cells, or other electrochemical applications.

Fig. 1 UV-Vis spectra of as synthesized AgBr, TiO₂, AgBr/TiO₂, and Ag-h-TiO₂ nanoparticles.

3.3 Characterization of particles by XRD

The crystallographic phases of the nanoparticles were identified by XRD technique. The diffraction patterns of all particles are presented in Fig. 2. For pure AgBr, the XRD pattern displays peaks at 2θ value of 26.6, 30.9, 44.3, 55.2, 64.5, and 73.3° corresponding to the planes (111), (200), (220), (222), (400), and (420) of cubic structure (JCPDS card no. 79-0149). For pure TiO₂, the anatase peaks were obtained at 2θ value of 25.1, 36.5 and 62.3° corresponding to the planes (101), (103), and (204) (JCPDS card no. 71-1169). The peaks rutile phase were obtained at 2θ values of 27.3, 54.1, and 68.6° corresponding to the planes (110), (211), and (301) (JCPDS card no. 76-0319). For AgBr/TiO₂ core/shell nanoparticles the XRD pattern shows the peaks of both AgBr and TiO₂, but the peak intensity of AgBr is reduced and that of TiO₂ becomes more dominant mainly because of TiO₂ coating. For Ag-h-TiO₂ NPs the TiO₂ anatase peaks (2θ) were obtained at 25.6, 37.4, 48.6, and 63.2° corresponding to the planes of (101), (103), (200), and (204) (JCPDS card no. 75-1537) with the peaks of elemental silver at 2θ values of 44.3 and 64.7, corresponding to the planes (200) and (220) (JCPDS card no. 03-0931) respectively. Additionally, another important observation is the rutile peak at 2θ = 27.3° is absent in the hollow structure, while compare with pure TiO₂ NPs. The XRD analysis also shows that the anatase TiO₂ peak (2θ) of (101) plane shifts from 25.1° to 25.6° after the formation of hollow structure, which attributed to the silver doping to TiO₂ lattices.

Fig. 2 The X-ray diffraction patterns of AgBr, TiO₂, AgBr/TiO₂ core/shell, and Ag-h-TiO₂ nanoparticles.

3.4 Characterization of particles by TEM and FE-SEM

The Fig. 3a shows the TEM image of AgBr/TiO₂ core/shell nanoparticles before expose to ammonium hydroxide solution. The contrast difference between the core and shell in the image indicates the formation of core/shell structure. The particles are spherical in shape, and the size distribution is plotted in Fig. 3c which indicates the mean particle size of 17.86 ± 2.46 nm and the shell thickness is ∼2.5 nm. The TEM image of AgBr/TiO₂ core/shell particles after dissolution in ammonium hydroxide solution is presented in Fig. 3b. The particle size distribution is plotted in Fig. 3d which shows mean particle size of 17.76 ± 2.85 nm, which is almost same as it was before the dissolution. However, the average particle size obtained by DLS measurement (S1, ESI†) was comparatively larger (20.79 nm) than that obtained by TEM measurement, this might be because of the adsorbed surfactant layer and hydrated water molecules.⁶⁷ The FE-SEM image (inset Fig. 3b and S2, ESI†) clearly shows some of the broken hollow spheres. The TEM image of single contrast indicates the removal of core. So, the TEM and FE-SEM images confirm the formation of hollow TiO₂ particles after treatment with ammonia solution. The high resolution TEM image of Ag-h-TiO₂ nanoparticles (Fig. 4a) shows lattice fringes spacing of 0.353 nm corresponding to the (101) plane for anatase phase TiO₂ (JCPDS card no. 75-1537) previously confirmed by XRD. But there is no lattice fringe of rutile TiO₂ or Ag, which confirms 100% anatase phase of TiO₂ and no surface deposition of Ag. The Fig. 4b shows the selected area electron diffraction (SAED) pattern of the hollow particle by focusing the electron beam on a single particle. The ring like diffraction pattern is indexed to (101) and (200) planes of TiO₂ together with (200) diffraction of silver, which confirms the presence of silver inside the crystal structure of TiO₂ nanoparticles. The EDX in Fig. 4c further confirms the presence of silver, titanium, and oxygen atoms. Some extra peaks of Cu and C are mainly from TEM grids.

Fig. 3 TEM images of (a) AgBr/TiO₂ core/shell, and (b) Ag-h-TiO₂ hollow nanoparticles, inset shows FE-SEM image of a hollow broken particle (more hollow particles are shown in Fig. S2†). Particle size distributions of (c) AgBr/TiO₂ core/shell, and (d) Ag-h-TiO₂ nanoparticles.

Fig. 4 (a) HR-TEM image (b) SAED pattern, and (c) EDX pattern of Ag-h-TiO₂ nanoparticles.

3.5 Characterization of particles by XPS

For the further confirmation of the surface composition and the chemical states of Ag-h-TiO₂ nanoparticles, XPS analysis was carried out. The high resolution XPS spectra of Ag (3d), Ti (2p) and O (1s) are presented in Fig. 5. The XPS peak at 462.9 eV corresponds to Ti 2p_1/2, and 457.2 eV corresponds to Ti 2p_3/2 in the Ti 2p region (Fig. 5a). The slitting between Ti 2p_1/2 and Ti 2p_3/2 is 5.9 eV which indicates the normal state of Ti⁴⁺ in the mesoporous anatase TiO₂.⁷¹ The XPS peak (Fig. 5b) at ∼530 eV corresponds to the lattice oxygen of TiO₂ crystal and peaks at 532.1 and 531.7 eV correspond to the adsorbed oxygen on TiO₂ surface. Fig. 5c shows the Ag 3d_3/2 and Ag 3d_5/2 peaks at the binding energy of 366.5 and 372.6 eV, respectively with a slitting of the 3d doublet of 6.1 eV, which suggest the zero valence silver instead of Ag⁺ ions within the material.⁷² So, the possible location of dopant is attributed as the surface or in the interstitial sites of the host material. The TEM or FE-SEM images confirm that there is no surface deposition of the silver on the particles. So, there is a fair possibility of the silver doping in the interstitial sites of the host material.

Fig. 5 High resolution XPS narrow scan spectra of (a) Ti(2p), (b) O (1s), and (c) Ag (3d) for Ag-h-TiO₂ nanoparticles.

3.6 Characterization of particles by BET analysis

The specific surface area of the pure and hollow doped TiO₂ nanoparticles was measured by BET technique. Before the adsorption–desorption study the sample was degasified for 1 h at 150 °C temperature. The BET surface area of solid TiO₂ and Ag-h-TiO₂ nanoparticles are 95.1 and 198.3 m² g⁻¹ respectively. The Ag-h-TiO₂ nanoparticles are having 103.2 m² g⁻¹ more surface area compare to that of solid TiO₂. The obtained surface area of hollow structure TiO₂ is higher compared to the literature values of 38.9–123 m² g⁻¹ (ref. 62–66) This might be because of the core removal technique and the final particle size. In most of the literature the core was removed by calcinations but in our case the core is removed by dissolution at room temperature. The high surface area hollow structure TiO₂ is expected to be useful for the catalysis or other applications.

3.7 Photoluminescence activity of the synthesized nanoparticles

The photoluminescence (PL) properties of the synthesized nanoparticles were also checked by the fluorescence spectroscopy in terms of light emission. The light emission data was recorded between 500–600 nm wavelengths while excitation at 270 nm. The emission spectra of AgBr, TiO₂, AgBr/TiO₂, and Ag-h-TiO₂ NPs are presented in Fig. 6. All the particles show emission in the visible region of wavelength ∼543.8 nm. The intensities of emission are in the following order: AgBr > Ag-h-TiO₂ > AgBr/TiO₂ > TiO₂. The pure TiO₂ have very low intensity compared to pure AgBr and the intensity increases after the formation of Ag-h-TiO₂ NPs. The quantum yield (QY) was calculated for these materials from UV and PL data (eqn (SI1), ESI†) using phenol as the standard material (QY = 0.14). The QYs of TiO₂, AgBr, AgBr/TiO₂, and Ag-h-TiO₂ NPs are 0.29, 0.498, 0.36, and 0.477 respectively. So, the quantum yield of TiO₂ is enhanced by 18.7% after the formation of hollow structure and Ag doping. From these results it can be pointed out that the Ag-h-TiO₂ nanoparticles has enhanced light emission property and QY compared to that of pure TiO₂ nanoparticles. So, the light emission properties along with the catalytic properties could be utilized for better photocatalyst or dye sensitized solar cells applications.

Fig. 6 Light emission spectra of AgBr, TiO₂, AgBr/TiO₂, and Ag-h-TiO₂ NPs after excitation at 270 nm wavelength.

3.8 Photo catalytic applications of synthesized nanoparticles

The photocatalytic activities of the synthesized Ag-h-TiO₂ nanoparticles were first tested for the degradation of NB under the visible light irradiation. The degradation studies using pure and Ag doped solid TiO₂ (Ag–TiO₂) were also employed as the photocatalyst for the comparison purpose. The results of NB degradation in visible light are shown in Fig. 7. It has been found that the degradation process reaches a plateau level after 3.5 h of light exposure time. The results show that at the plateau level, the degradation efficiencies of NB are 58.46, 89.61, and 95.5% in the presence of TiO₂, Ag–TiO₂, and Ag-h-TiO₂ NPs respectively. The results clearly indicate that the maximum degradation of the NB was obtained in the presence of the Ag-h-TiO₂ NPs. The degradation results were also fitted with different kinetic models, and it was found that the first order kinetics fits well for all the materials. The Fig. 7b presents the reaction kinetics of NB degradation. The rate constants (k) of the photocatalytic degradation reactions obtained from the experimental results are 0.0045, 0.0109, and 0.016 min⁻¹ for TiO₂, Ag–TiO₂, and Ag-h-TiO₂ NPs respectively. The rate constant of Ag-h-TiO₂ photocatalytic process is about 3.6 and 1.5 times than that of pure TiO₂ and Ag–TiO₂ photocatalytic processes respectively. A comparison at 50% degradation times for NB shows 136.5, 41.5, and 27 min for TiO₂, Ag–TiO₂, and Ag-h-TiO₂ nanoparticles, respectively. These results reveal that Ag-h-TiO₂ nanoparticles have higher photocatalytic activity compared to that of pure TiO₂ and Ag–TiO₂ NPs. This is attributed to the fact that pure TiO₂ is having low yield of ˙OH radicals because of recombination of the charge carriers during photodegradation, which helps to oxidize the organic compound as stated in our previous study.⁷³ In Ag doped TiO₂ NPs the Ag dopant accepts the photoinduced electrons and holes, which in turn act as electron/hole traps and prevents the recombination of the charge carriers. In Ag-h-TiO₂ NPs, addition to Ag doping it is having more surface area (198.3 m² g⁻¹ as mentioned in the BET analysis) compared to solid TiO₂ (95.1 m² g⁻¹), which is useful for better catalytic processes. Additionally, as mention in the XPS section, the presence of adsorbed oxygen on the TiO₂ surface allows the H⁺ hydroxylation to form –Ti(OH)–O–Ti–(OH)– in the presence of water. During the photodegradation, this compound may help to generate the photogenerated hole (h⁺_VB) and finally turn to ˙OH free radicals, which is beneficial for the photocatalysis.⁷⁴ While comparing the degradation efficiency and PL intensity, it can be seen that Ag-h-TiO₂ is having high degradation efficiency as well as PL intensity compared to that of pure TiO₂. In general, high PL intensity is due to the high recombination of electrons and holes. Our results can be attributed in the following way. In the presence of doping, the reduction in recombination continues up to the certain limit of doping percentage (optimum doping) and then again recombination starts increasing with the increasing doping %.^75,76 From the EDX study it is seen that 9.02% doping is achieved, which is higher compared to optimum doping percentage (1%) for TiO₂,⁷³ we believe because of that the PL intensity is high. At the same time, as the surface area of hollow particles is higher than solid TiO₂, the degradation efficiency is more because of more adsorption of dye molecules. The reusability of Ag-h-TiO₂ NPs was tested by recycling the catalyst under the same condition, and the results are plotted in Fig. 7c. The results show there is a ∼10.83% decrease (95.5 to 84.67%) in degradation efficiency after sixth cycles. The turn over number (TON) of the photocatalyst is calculated as 0.467, 0.716, and 0.763 mol mol⁻¹ for the first cycle of the TiO₂, Ag–TiO₂, Ag-h-TiO₂, respectively. From the reported literatures (Table S1, ESI†) it is has been found that more than 90% degradation efficiency of nitrobenzene was achieved either in the presence high catalyst dose, low organic concentration, in the presence of UV light irradiation, or in presence of some additives and at specific pH. In this regard, our catalytic process is more effective in normal visible light and in the absence of any additives than the already reported literatures.

Fig. 7 (a) Photodegradation kinetics of nitrobenzene under visible light of high pressure mercury vapour lamp using TiO₂, Ag–TiO₂ and Ag-h-TiO₂ NPs. (b) Fitting of first order kinetics with the experimental data. (c) Recycling test of Ag-h-TiO₂ for the degradation of nitrobenzene solution.

Further, to explore the degradation pathway of NB in the presence of TiO₂, Ag–TiO₂, and Ag-h-TiO₂ photocatalysts, LC-MS study was conducted to identify different photo degraded products. Based on the LC-MS results probable degradation path is proposed in Fig. 8. During the photodegradation, the ˙OH radicals attack the nitrobenzene and subsequently produce some aromatic compounds, such as benzene, phenol, catechol, resorcinol, and hydroquinone. The hydroquinone again forms benzoquinone through the dehydrogenation. Further degradation of the aromatic compounds happens through the ring cleavage and some degraded product, such as propionic acid, n-butanol, di-ethyl ether, furan, orthoformic acid, propanol, and acetic acid are formed.

Fig. 8 Probable pathway for the degradation of nitrobenzene under visible light in the presence of TiO₂, Ag–TiO₂, and Ag-h-TiO₂.

Since the Ag-h-TiO₂ nanoparticles show good photocatalytic degradation against the NB, the same catalyst was also used to study the photodegradation efficiency on other two classes of organic compounds such as metronidazole antibiotic and methylene blue dye under the visible light irradiation. The degradation results (Fig. 9a) show that at the plateau level the degradation of MTZ is 96.55% in the presence of Ag-h-TiO₂ NPs, while pure TiO₂ and Ag–TiO₂ degraded 80.78 and 94.39% of MTZ.⁷³ The rate constant of the photocatalytic degradation reactions obtained from the experimental results is 0.0269 min⁻¹ for Ag-h-TiO₂ NPs. The results of reusability test of Ag-h-TiO₂ NPs (Fig. 9b) show there is a ∼10.76% decrease (96.55 to 85.79%) in degradation efficiency after sixth cycles. It can be noted that the difference between hollow and solid TiO₂ is not significant (∼2%), in spite of significantly high surface area for hollow particles, which can be attributed to the low initial MTZ concentration. So, the initial concentration of MTZ was increased to 30 mg L⁻¹ and it has been found that after 2 h the Ag–TiO₂ and Ag-h-TiO₂ NPs degraded 88.5 and 94.77% MTZ (Fig. 9c) with rate constants 0.019 and 0.024 min⁻¹ (Fig. 9d) for Ag–TiO₂ and Ag-h-TiO₂ NPs respectively. These results indicate that, when the concentration of organic molecules is high hollow particles show better efficiency. Our approach of MTZ degradation is noble compared to the reported conventional method as well as photocatalytic degradation approach as discussed in the ESI (Table S2†). The surface chemistry of Ag-h-TiO₂ photocatalyst was studied before and after degradation of MTZ solution by FT-IR spectroscopy. The FT-IR results (Fig. S3, ESI†) show there is no significant peak change between the virgin Ag-h-TiO₂ and recycled Ag-h-TiO₂ after degradation. The broad absorption peak at 3373 and 2359 cm⁻¹ are associated with the asymmetric and symmetric stretching vibrations of the Ti–OH group. The peaks in the range 400–1000 cm⁻¹ are the contribution of anatase phase Ti–O. So, these results strongly suggest that the degradation of MTZ solution is due to the photocatalytic degradation instead of any chemisorption or physisorption of MTZ on the catalyst surface.

Fig. 9 (a) Photodegradation kinetics and fitting of first order kinetics with the experimental data of MTZ degradation under visible light of high pressure mercury vapour lamp using Ag-h-TiO₂ NPs with 15 mg L⁻¹ of initial MTZ concentration. (b) Recycling test of Ag-h-TiO₂ for the degradation of MTZ solution. (c) Photodegradation kinetics and (d) fitting of first order kinetics with the experimental data of MTZ using Ag–TiO₂ and Ag-h-TiO₂ NPs with 30 mg L⁻¹ initial concentration of MTZ.

The MBD degradation was carried out under the visible light with 25 mg L⁻¹ initial concentration of dye and the catalyst concentration of 0.25 g L⁻¹. The degradation results are presented in Fig. S4, ESI.† The dye degradation efficiency of Ag-h-TiO₂ is 96.77% after 3 h exposure time. The rate constant of photocatalytic degradation of MBD under the visible lamp using Ag-h-TiO₂ NPs is 0.0216 min⁻¹ (Fig. S4c, ESI†). Similar to previous study Ag-h-TiO₂ sample was further selected for cyclic degradation test under the same condition and found there is a decrease in efficiency of ∼10.47% (96.77 to 86.3%) after sixth cycle of photodegradation (Fig. S4d, ESI†).

For each type of organic compound degradation, two sets of reference studies, (i) organic solution + catalyst in dark and (ii) only organic solution under visible light irradiation were carried out. The first was to test only adsorption capacity. Since the catalyst concentration was low, there was no significant change found because of adsorption. So we confirmed that the concentration change was only because of degradation.

4 Conclusions

The hollow TiO₂ nanoparticles were synthesized by a sacrificial core method using AgBr as sacrificial core. During the removal of core, Ag was doped in the interstitial sites of the host TiO₂. The obtained Ag-h-TiO₂ nanoparticles are purely in anatase phase and good crystalline in nature. The mean particle size is 17.76 ± 2.85 nm with wall thickness of ∼2.5 nm. The increase of surface area for hollow nanoparticles is 103.2 m² g⁻¹ more compared to that of solid TiO₂ nanoparticles. Apart from the surface area, the quantum yield of Ag-h-TiO₂ nanoparticles also increases to 18.7% compared to that of pure solid TiO₂ nanoparticles. The nanoparticles were used for the photo degradation of NB, MTZ antibiotic and MBD. The maximum NB degradation was obtained 95.5% under visible light irradiation for 3.5 h. The metronidazole degradation efficiency was found to be 96.55 and 94.77% under the irradiation of visible light for the initial MTZ concentration of 15 and 30 mg L⁻¹ with catalyst dose of 0.5 g L⁻¹. The Ag-h-TiO₂ NPs show only 10.47% decrease in degradation efficiency even after sixth cycle of reuse. The synthesized Ag-h-TiO₂ nanoparticles may also be useful for dye sensitized solar cells and other electrochemical applications.

Acknowledgements

The financial support from the Council of Scientific and Industrial Research (CSIR), Grant no. 22(0527)/10/EMR-II, for this project is gratefully acknowledged. S.S.B. thanks CSIR, India, for a Senior Research Fellowship to pursue this work. Authors are grateful to Indian Institute of Technology, Kharagpur, India, for providing their XPS facility. Authors also acknowledge the Saha Institute of Nuclear Physics, Indian Association for the Cultivation of Science, Kolkata, India, for giving the opportunity to access their TEM facility.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03421c

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