Easy synthesis of titania–tungsten trioxide nanocomposite films by anodising method for solar water splitting

Abstract

Titania–tungsten trioxide (W-TiO₂) films were in situ fabricated by the electrochemical anodising of titanium in a non-aqueous electrolyte solution containing sodium tungstate as the tungsten source. Glycerol was used as a green solvent. The morphology, structure and composition of synthesised films were characterised by SEM, X-ray diffraction, energy dispersive X-ray spectroscopy and UV–visible respectively. The results indicated that sodium tungstate concentration in anodising solution significantly influenced the morphology of the surface, structure and photocatalytic activity of these films. So to access an optimum morphology, a proper concentration of sodium tungstate is required. Photocatalytic activity of samples was evaluated by testing the solar water splitting. The W1-TiO₂ sample exhibited better photocatalytic activity than the bare TiO₂ film and other W-TiO₂ samples.

Keywords

Coating Film Nanoporous Nanotube Solar water splitting Tungsten

Introduction

An effective way to solve the increasing global energy crisis and pollution problems is to develop a renewable and environmentally friendly energy.¹ Hydrogen has emerged as a potential energy carrier in various low greenhouse gas energy applications due to its harmless and renewability to the environment. Fossil fuels and biomasses are some of the essential sources of hydrogen; however, their utilisation for energy production is not only capital intensive but also involves the emission of carbon dioxide that contributes to the adversities of the greenhouse effect and global warming.^2,3 Currently, the majority of hydrogen demands are sourced from steam reforming of natural gas; however, this process does not mitigate dependence on fossil fuels and generates a large amount of carbon dioxide. Alternatively, the generation of hydrogen from water is a more sustainable approach that provides a means through which renewable sources such as solar energy can be harnessed effectively, and most importantly, this process is carbon neutral, thus preventing environmental pollution. Harvesting of solar energy is still a challenging venture, and the constraints associated to this process necessitate improved methods and advanced technologies for production.³ The ability of semiconductor materials to utilise photons of light for the generation of electrons and holes has attracted a great deal of attention and has found potential application in different fields.^4–13 Many scientists believe that the use of photocatalytic water splitting system is one of the most promising technologies for producing hydrogen gas to secure a clean and sustainable supply of energy. Photocatalysis is defined as the chemical reaction induced by photoirradiation in the presence of a catalyst or, more specifically, a photocatalyst. Such material will facilitate chemical reactions without being consumed or transformed. The basic working principle of photocatalysis is simple. First, irradiation of light with energy greater than the band gap of photocatalyst, separating the vacant conduction band (CB) and filled valence band (VB), excites an electron in VB into CB to result in the formation of an electron–hole pair. These electron and hole reduce and oxidise respectively chemical species on the surface of photocatalyst, unless they recombine to give no net chemical reaction. The original structure of photocatalyst remains unchanged if an equal number of electron and hole is consumed for chemical reaction and/or recombination.¹⁴ In a photocatalytic water splitting reaction, photocatalyst plays a crucial role. Until now, TiO₂ has been a widely used photocatalyst for photocatalytic water splitting because it is stable, non-corrosive, environmentally friendly, abundant and cost effective. More importantly, its energy levels are appropriate to initiate the water splitting reaction.¹⁵ In other words, the CB of TiO₂ is more negative than the reduction energy level of water (E_H+/H2 = 0 V), while the VB is more positive than the oxidation energy level of water (E_O2/H2O = +1.23 V). Despite the many advantages of TiO₂, its photocatalytic water splitting efficiency under solar energy is still quite low, mainly due the wide band gap of TiO₂ limits its application within the ultraviolet region. In order to increase the utilisation efficiency of TiO₂ on solar energy, researchers do much work from decreasing the band gap of TiO₂ and suppressing the recombination of electrons and holes.¹⁶ At present, the modification methods mainly include surface sensitisation, noble metal deposition, metal ion doping and non-metal ion doping and so on.^17–21 Tungsten trioxide doped TiO₂ has generated considerable interests as a photocatalyst showing optical response in the visible region. Different methods were used to prepare W doped TiO₂, but most of these methods were multistage, time consuming, expensive with weak reproducibility and, in some cases, dangerous for the environment. Therefore, a simple, facile and inexpensive method of synthesising W doped TiO₂ is necessary.

Herein, a facile, convenient and low cost method was used to synthesise W-TiO₂ films by anodising of titanium substrate in a non-aqueous bath containing various amounts of sodium tungstate as the tungsten source. We used glycerol as a green solvent in the present study. W-TiO₂ films with different amounts of tungsten were obtained by controlling the concentration of sodium tungstate in anodising electrolyte. The influence of tungsten trioxide concentration on the hydrogen evolution of fabricated films has been systematically investigated. The morphology and structure were characterised by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Optical properties were investigated by UV–visible (vis) diffuse reflectance spectra.

Experimental

All the used chemicals were of analytical grade without further purifying before experiment and solutions were prepared with distilled water.

W-TiO₂ films were synthesised by anodising of titanium in a mixture electrolyte, which was mixing glycerol and NH₄F, followed by the dissolution different concentrations of sodium tungstate. Before anodising, a piece of titanium sheet (99.99% purity, 1 mm thick) was cut into desired dimension, and the titanium electrodes were first mechanically polished with different emery type abrasive papers (with the following grades: 80, 240, 800, 1200 and 2400), rinsed in a bath of distilled water and then chemically etched by immersing in a mixture of HF and HNO₃ acids for 30 s. The ratio of components HF/HNO₃/H₂O in the mixture was 1:4:5 in volume. The last step of pretreatment was rinsing with distilled water. The anodising experiments were carried out using a two-electrode system with titanium foil as anode and platinum foil as cathode respectively. A controlled DC power source (ADAK, PS405) supplied the required constant voltage. Anodising was carried out in mentioned solutions under a constant voltage of 60 V for 6 h at room temperature. After anodising at constant potential, the as formed samples were annealed in the air at 400°C for 2 h (1°C min^− 1) to obtain crystalline film. In the present work, we compared the photocatalytic performance of bare TiO₂ and W-TiO₂ films with varied weight loading percentage of 0.3, 0.5, 0.8 and 1.0% of tungsten referred to as W1-TiO₂, W2-TiO₂, W3-TiO₂ and W4-TiO₂ respectively. Table 1 summarises the experimental conditions for five different samples. A schematic of the pretreatment method of titanium and process of producing W-TiO₂ films on titanium is shown in Fig. 1.

Table 1

Experimental parameters for fabrication of different samples

Samples	Anodising solution	Anodising condition	Annealing condition
TiO₂	98 mL glycerol+2 mL H₂O+0.13M NH₄F	60 V for 6 h at room temperature	400 °C for 2 h (1 °C min^− 1)
W1-TiO₂	98 mL glycerol+2 mL H₂O+0.13M NH₄F+0.6 mM Na₂WO₄.2H₂O	60 V for 6 h at room temperature	400 °C for 2 h (1 °C min^− 1)
W2-TiO₂	98 mL glycerol+2 mL H₂O+0.13M NH₄F+1.2 mM Na₂WO₄.2H₂O	60 V for 6 h at room temperature	400 °C for 2 h (1 °C min^− 1)
W3-TiO₂	98 mL glycerol+2 mL H₂O+0.13M NH₄F+1.8 mM Na₂WO₄.2H₂O	60 V for 6 h at room temperature	400 °C for 2 h (1 °C min^− 1)
W4-TiO₂	98 mL glycerol+2 mL H₂O+0.13M NH₄F+2.4 mM Na₂WO₄.2H₂O	60 V for 6 h at room temperature	400 °C for 2 h (1 °C min^− 1)

Schematic presentation of pretreatment method of titanium sheets and process of producing W-TiO₂ films on titanium foils

The surface morphology of all samples was characterised by field emission scanning electron microscopy (Hitachi S-4160, Japan), and the elemental composition was estimated by energy dispersive X-ray spectroscopy (EDX). The crystalline phases were identified by XRD with Equinox 3000 diffractometer (Inel, France). Diffraction patterns were recorded in the 2θ range from 20 to 80^o at room temperature. The optical absorption of the samples was determined using a diffuse reflectance UV–vis spectrophotometer (JASCO V-570).

Photocatalytic water splitting to generate hydrogen was carried out in a three-branch quartz cell. A 200 W xenon lamp was used as the light source. The luminous intensity of the xenon lamp was 100 mW cm^− 2. The bare TiO₂ and W-TiO₂ samples was fixed in the cell containing 1.0M NaOH (pH 13.6) solution facing the lamp. Hydrogen evolution was measured for 360 min, and H₂ gas was collected using the water displacement technique. A Pt coil served as the cathode. The cathode was inserted into a burette where the hydrogen was collected via electrolyte displacement. The volume of hydrogen was measured by directly reading the variation of the electrolyte level in the burette for various times.

Results and discussion

Figure 2 shows top view field emission scanning electron microscopy images of different samples, which clearly shows formation of various films on the surface of titanium. In Fig. 2a and b, different samples displayed nanotube arrays wherein their surface was open. In Fig. 2a, highly ordered and vertically aligned nanotubes were formed on the titanium substrate. The average pore diameter of these nanotubes as calculated from SEM images is 90–100 nm and wall thickness is 20–25 nm. In Fig. 2b, a combined structure of nanotubes and nanohoneycombs was formed. It can be seen that nanoparticles (40–70 nm) are distributed on the surface of the titania nanostructures. The internal diameter of the tubes is around 50–90 nm. In Fig. 2c, a combined structure of nanotubes and nanoporous was formed. It seems on the surface of the nanotubes, a porous structure is emerging. In Fig. 2d and e, no nanostructure formed. Compact films, without porosity, were formed on the surface of titanium. It can be said that when sodium tungstate concentration in anodising solution increased to 1.8 mM, surface is covered with a compact layer and thereafter not formed regular nanostructures, implying that an appropriate concentration of sodium tungstate is important for the structure of nanotube or nanoporous films.

Top view images (SEM) of bare TiO₂ and W-TiO₂ samples with different magnification (insets: cross-sectional images of corresponding samples) (a bare TiO₂; b W1-TiO₂; c W2-TiO₂; d W3-TiO₂; e W4-TiO₂)

Cross-sectional views of different samples show that with increasing sodium tungstate concentration in anodising solution, the length of the films will increase. In Fig. 2a1, it can be seen that the nanotubes are parallel aligned and are very regular. The tubes are well separated, and their length is in the range of 2–2.5 μm. The regular structure can be seen in Fig. 2b1. Compared to Fig. 2a1, the length of formed structure has increased and is in the range of 3–3.5 μm. In Fig. 2c1, the arrangement of the structure is reduced but their length is increased and is in the range of 5–6 μm. In Fig. 2d and e, that compact films were formed on the surface of titanium; again, the film thickness has increased. To explain this phenomenon, it can be said that the growth of nanotube is accompanied by three stages: the formation of barrier layer of TiO₂, oxide growth and dissolution and the balance of growth and chemical dissolution.^22,23 The process of chemical dissolution is related to the concentration of fluorine ions (F⁻). The formation of TiO₂ nanotube films can be expressed using the following equations

(i) Oxidation of the metal tjat releases Ti⁴⁺ ions and electrons (1)

(ii) Combination of Ti⁴⁺ ions with water (2)

(iii) Fluorine ions attack to the oxide layer (3) Moreover, in electrolyte containing tungsten ions, the following reaction could be carried out

(iv) Coordination reaction between tungsten ions and fluorine ions (4) Equation (4) is the coordination reaction between W⁶⁺ and F⁻. When sodium tungstate was added to anodising solution, the coordination of W⁶⁺ and F⁻ decreased the concentration of free fluorine ions (F⁻). Therefore, the chemical dissolution was suppressed with the increase in sodium tungstate concentration, leading to the formation of a combined structure of nanotubes and nanoporous when the concentration of sodium tungstate in electrolyte was 1.2 mM or a compact film was formed when the concentration of sodium tungstate in electrolyte is 1.8 mM. It is expected that by reducing or stopping chemical dissolution, film thickness increases. It can be said that at concentrations >1.8 mM of sodium tungstate, there is no free fluorine ion in anodising solution and the chemical dissolution stops. So, the thickness of titania–tungsten trioxide films regularly increases.

The XRD patterns of the bare TiO₂ and W1-TiO₂ samples are shown in Fig. 3. Diffractions that are attributable to anatase TiO₂ are clearly observed in the annealed sample. The Ti peaks were due to the titanium substrate. The XRD pattern of the W1-TiO₂ show the diffraction peaks of both WO₃ and TiO₂, and two diffraction peaks of WO₃ are found in this sample. The presence of tungsten on the films was also confirmed through EDX spectrum obtained from W1-TiO₂ sample (Fig. 3). It was seen that the films mainly consisted of Ti, O and W. The analysis revealed that the surface presents similar composition with presence of Ti as main energy E = 4.5 kV and 4.9 kV, W at E = 1.7 kV and O at E = 0.5 kV, which confirms the presence of the tungsten in the films. The occurrence of traces of contaminants such as carbon and nitrogen from precursors is also observed. The presence of the C species is favorable to the absorption of carbon from the organic glycerol.²¹

a X-ray diffraction patterns of bare TiO₂ and W1-TiO₂; b EDX spectrum of W1-TiO₂

The optical properties such as band gap energy of samples were studied. Figure 4 shows the optical band gap energy of bare TiO₂ nanotubes and W1-TiO₂ films annealed at 400°C respectively. The value of band gap of samples was estimated using the following equation²¹

Plots of (αhυ)^1/2 versus hυ employed to calculate band gap value of a bare TiO₂ sample and b W1-TiO₂ sample

(5) where E_g is the band gap energy of the material, h is Planck's constant, υ is the frequency of vibration, hυ is the incident photon energy, A is a proportional constant and α is the absorption coefficient per unit length (cm^− 1). Based on this equation, the band gap E_g of samples was obtained by extrapolating the linear portion of [(αhυ)^1/2 versus hυ] plot to α → 0. The results obtained have shown that the band gap became narrower and red shifted for W-TiO₂ films. The band gap energies decreased progressively from ∼3.23 eV for bare TiO₂ to ∼2.78 eV for W1-TiO₂ film, according to the equation. These results indicate that tungsten trioxide dopant in the TiO₂ films decreases the band gap.

The primary purpose of this work is investigating the activity of TiO₂ modified with different amounts of tungsten trioxide through water splitting over these samples. Figure 5 shows the amount of hydrogen generated as a function of time using bare TiO₂ and W-TiO₂ samples under Xe light illumination. Control experiments indicated that no appreciable hydrogen production was detected in the absence of either light irradiation or photocatalyst, suggesting that hydrogen was produced by photocatalytic reactions on samples. In all samples, hydrogen evolves steadily over extended periods of time. It can be found that W1-TiO₂ sample performed the highest photocatalytic activity under light irradiation. The total amount of H₂ evolved on the W1-TiO₂ sample was 49 mL cm^− 2 after 6 h, which is ∼1.32 times higher than that on the sample W2-TiO₂ (37 mL cm^− 2), 4.45 times higher than that on the sample W3-TiO₂ (11 mL cm^− 2), 5.44 times higher than that on the sample W4-TiO₂ (9 mL cm^− 2) and 24.5 times higher than that on the bare TiO₂ sample (2 mL cm^− 2).

a H₂ production by water splitting over different samples under irradiation, and b H₂ evolution for different samples as function of running times

The higher photocatalytic activity of W-TiO₂ samples can be attributed to the combined effect of several factors: the tungsten trioxide dopant in the TiO₂ film, the one dimensional nanostructure and the increased light harvesting ability (capability to absorb visible light and higher light absorption, compared to bare TiO₂ sample). According to literature, it can be said that tungsten trioxide dopant can induce more effective visible light harvesting, which is caused by photoexcitation of the extrinsic absorption band of the catalyst. The extrinsic absorption comes from the photoionisation of original or newly formed defects and the excitation of surface states. Furthermore, the extrinsic absorption can create oxygen vacancies to enhance the photocatalyst ability of the W-TiO₂ samples.²⁴ Such absorption requires less energy to activate. Thus, W-TiO₂ probably generates more free charge carriers to induce surface chemical reactions than pristine TiO₂ under visible light irradiation. Therefore, the photocatalytic activity of W-TiO₂ samples is larger than that of undoped TiO₂ sample in our study. However, the unique one-dimensional nanostructure of the nanotubes also contributes to its high photocatalytic ability. The one-dimensional morphology allows for electrons move axially along the length of the nanotubes providing direct and faster electron transport to the back to the contact, while photogenerated holes are separated and collected over relatively short distances in an orthogonal direction. Nanotubes are believed to have exceptional electron transport properties and have been considered as alternatives to nanoparticles. It has been reported that nanotubes can reduce intercrystalline contacts between grain boundaries and its stretched grown structure to the specific directionality makes a slightly favorable contribution to the electron transport and the significant improvement of electron lifetime by the degraded charge recombination through experiencing the less frequent trapping/detrapping events.^17,24,25 For this reason, photocatalytic activities of nanotubes (W1-TiO₂ and W2-TiO₂) are greater than those of compact samples (W3-TiO₂ and W4-TiO₂). In addition, the nanotube architecture has a large internal surface area and can be easily filled with liquid, thus enabling intimate contact with electrolyte. In other words, the surface area of the nanotubes is higher than the nanoporous, and the surface area of nanotubes and nanoporous is higher than that of the compact films. Therefore, we can expect such behaviour in photocatalytic activity of samples.

One important feature of the catalyst to reduce the cost of the process is its ability to be reused. The recyclability of W-TiO₂ samples was tested during six runs of photocatalytic reaction under Xe light illumination. Figure 5b shows the hydrogen production of W-TiO₂ samples as a function of sequence of reaction time, and the hydrogen production volume has no obvious decay after six continuous running, which indicates that the these samples are a relatively stable photocatalyst and can keep the activity for a period of time.

Conclusions

The major conclusions from the present work are as follows.

Bare TiO₂ and W-TiO₂ films were successfully synthesised using a facile anodising method.

Glycerol was used as a green solvent.

Sodium tungstate was used as the tungsten source in anodising process.

The sodium tungstate concentration in anodising solution plays an important role in the morphology and thickness of these films.

By changing sodium tungstate concentration, nanotubes, nanoporous, nanohoneycombs and compact films are created on the titanium surface.

The XRD and EDX analyses showed presence of tungsten on the W-TiO₂ films.

UV–visible spectroscopy data show that the band gap decrease with increasing sodium tungstate from ∼3.23 eV for bare TiO₂ to ∼2.78 eV for W1-TiO₂.

The photocatalytic testing of these films through solar water splitting revealed that tungsten trioxide dopant in the TiO₂ film can significantly improve the H₂ production amount of these materials, so the total amount of H₂ evolved on the W1-TiO₂ sample was ∼24.5 times higher than that on the bare TiO₂ sample.

References

Xue

, Sun

, Wang

, Yan

and Zhao

: ‘Effect of NH₄F concentration and controlled-charge consumption on the photocatalytic hydrogen generation of TiO₂ nanotube arrays’, Electrochim. Acta, 2015, 155, 312–320. doi: 10.1016/j.electacta.2014.12.134

Muradov

N. Z.

and Veziroglu

T. N.

: ‘Green path from fossil-based to hydrogen economy: an overview of carbon-neutral technologies’, Int. J. Hydrogen Energy, 2008, 33, 6804–6839. doi: 10.1016/j.ijhydene.2008.08.054

Parayil

S. K.

, Kibombo

H. S.

, Wu

C. M.

, Peng

, Baltrusaitis

and Koodali

R. T.

: ‘Enhanced photocatalytic water splitting activity of carbon-modified TiO₂ composite materials synthesized by a green synthetic approach’, Int. J. Hydrogen Energy, 2012, 37, 8257–8267. doi: 10.1016/j.ijhydene.2012.02.067

Momeni

M. M.

and Hosseini

M. G.

: ‘Different TiO₂ nanotubes for back illuminated dye sensitized solar cell: fabrication, characterization and electrochemical impedance properties of DSSCs’, J. Mater. Sci. Mater. Electron., 2014, 25, 5027–5034. doi: 10.1007/s10854-014-2267-6

Yang

, Wang

, Yang

, Li

, Zhang

and Zhao

: ‘Energy storage ability and anti-corrosion properties of Bi-doped TiO₂ nanotube arrays’, Electrochim. Acta, 2015, 169, 227–232. doi: 10.1016/j.electacta.2015.04.076

Hosseini

M. G.

and Momeni

M. M.

: ‘Platinum nanoparticle-decorated TiO₂ nanotube arrays as new highly active and non-poisoning catalyst for photoelectrochemical oxidation of galactose’, Appl. Catal. A, 2012, 427A, 35–42. doi: 10.1016/j.apcata.2012.03.027

, Wang

, Ma

, Xiao

, Yuan

and Zhang

: ‘Honeycombed TiO₂ nanotube arrays with top-porous/bottom-tubular structures for enhanced photocatalytic activity’, Ceram. Int., 2015, 41, 2527–2532. doi: 10.1016/j.ceramint.2014.10.075

Hosseini

M. G.

, Momeni

M. M.

and Faraji

: ‘Fabrication of Au nanoparticle /TiO₂ nanotubes electrodes using electrochemical methods and their application for electrocatalytic oxidation of hydroquinone’, Electroanalysis, 2011, 23, 1654–1662. doi: 10.1002/elan.201000656

Kanta

A. F.

, Poelman

and Decroly

: ‘Electrochemical characterisation of TiO₂ nanotube array photoanodes for dye-sensitized solar cell application’, Sol. Energy Mater. Sol. Cells, 2015, 133, 76–81. doi: 10.1016/j.solmat.2014.10.029

10.

, Shen

Y. G.

, Alajmi

, Yang

S. Y.

, Sun

J. M.

and Zhang

H. M.

: ‘Sol–gel preparation and properties of Ag-TiO₂ films on surface roughened Ti-6Al-4V alloy’, Mater. Sci. Technol., 2015, 31, 501–505. doi: 10.1179/1743284714Y.0000000650

11.

Torkian

, Amini

M. M.

and Amereh

: ‘Sol–gel synthesised silver doped TiO₂ nanoparticles supported on NaX zeolite for photocatalytic applications’, Mater. Sci. Technol., 2013, 28, 111–116.

12.

Yang

R. C.

, Zhang

Z. H.

, Ren

Y. M.

, Zhang

, Chen

Z. M.

and Xu

M. D.

: ‘Green synthesis of bi-component copper oxide composites and enhanced photocatalytic performance’, Mater. Sci. Technol., 2015, 31, 25–30. doi: 10.1179/1743284714Y.0000000586

13.

S. H.

and Liu

Z. F.

: ‘Enhanced photocatalytic activity of core shell SnO₂/ZnO photocatalysts’, Mater. Sci. Technol., 2013, 28, 234–237.

14.

Liao

C. H.

, Huang

C. W.

and Wu

J. C. S.

: ‘Hydrogen production from semiconductor-based photocatalysis via water splitting’, Catalysts, 2012, 2, 490–516. doi: 10.3390/catal2040490

15.

Kudo

and Miseki

: ‘Heterogeneous photocatalyst materials for water splitting’, Chem. Soc. Rev., 2009, 38, 253–278. doi: 10.1039/B800489G

16.

, Wang

, Kong

, Jia

and Wang

: ‘Synthesis and characterization of carbon modified TiO₂ nanotube and photocatalytic activity on methylene blue under sunlight’, Appl. Surf. Sci., 2015, 344, 176–180. doi: 10.1016/j.apsusc.2015.03.085

17.

Momeni

M. M.

, Ghayeb

and Davarzadeh

: ‘Single-step electrochemical anodization for synthesis of hierarchical WO₃-TiO₂ nanotube arrays on titanium foil as a good photoanode for water splitting with visible light’, J. Electroanal. Chem., 2015, 739, 149–155. doi: 10.1016/j.jelechem.2014.12.030

18.

Gim

, Seong

, Choi

Y. W.

and Choi

: ‘RuO₂-doping into high-aspect-ratio anodic TiO₂ nanotubes by electrochemical potential shock for water oxidation’, Electrochem. Commun., 2015, 52, 37–40. doi: 10.1016/j.elecom.2015.01.004

19.

Momeni

M. M.

and Ghayeb

: ‘Photoelectrochemical water splitting on chromium-doped titanium dioxide nanotube photoanodes prepared by single-step anodizing’, J. Alloys Compd, 2015, 637, 393–400. doi: 10.1016/j.jallcom.2015.02.137

20.

Hua

, Dai

, Bai

, Ye

, Gu

and Huang

: ‘A facile one-step electrochemical strategy of doping iron, nitrogen, and fluorine into titania nanotube arrays with enhanced visible light photoactivity’, J. Hazard. Mater., 2015, 293, 112–121. doi: 10.1016/j.jhazmat.2015.03.049

21.

Momeni

M. M.

, Ghayeb

and Davarzadeh

: ‘Electrochemical construction of different titania-tungsten trioxide nanotubular composite and their photocatalytic activity for pollutant degradation: a recyclable photocatalysts’, J. Mater. Sci. Mater. Electron., 2015, 26, 1560–1567. doi: 10.1007/s10854-014-2575-x

22.

Zhao

J. L.

, Wang

X. H.

, Chen

R. Z.

and Li

L. T.

: ‘Fabrication of titanium oxide nanotube arrays by anodic oxidation’, Solid State Commun., 2005, 134, 705–710. doi: 10.1016/j.ssc.2005.02.028

23.

Sun

, Li

, Wang

C. L.

, Li

S. F.

, Chen

H. B.

and Lin

C. J.

: ‘An electrochemical strategy of doping Fe³⁺ in to TiO₂ nanotube array films for enhancement in photocatalytic activity’, Sol. Energy Mater. Sol. Cells, 2009, 93, 1875–1880. doi: 10.1016/j.solmat.2009.07.001

24.

Zhu

, Tao

and Dong

: ‘Preparation and photoelectrochemical activity of Cr-doped TiO₂ nanorods with nanocavities’, J. Phys. Chem. C, 2010, 114C, 2873–2879. doi: 10.1021/jp9085987

25.

Iliev

, Tomova

, Rakovsky

, Eliyas

and Li Puma

: ‘Enhancement of photocatalytic oxidation of oxalic acid by gold modified WO₃/TiO₂ photocatalysts under UV and visible light irradiation’, J. Mol. Catal. A, 2010, 327A, 51–57. doi: 10.1016/j.molcata.2010.05.012