Decoration of vertically aligned TiO₂ nanotube arrays with WO₃ particles for hydrogen fuel production

Abstract

WO₃ decorated photoelectrodes of titanium nanotube arrays (W-oxide TNTAs) were synthesized via a two-step process, namely, electrochemical oxidation of titanium foil and electrodeposition of W-oxide for various interval times of 1, 2, 3, 5, and 20 min to improve the photoelectrochemical performance and the amount of hydrogen generated. The synthesized photoelectrodes were characterized by various characterization techniques. The presence of tungsten in the modified TNTAs was confirmed using energy dispersive X-ray spectroscopy (EDX). Field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscope (HRTEM) proved the deposition of W-oxide as small particles staked up on the surface of the tubes at lower deposition time whereas longer times produced large and aggregate particles to mostly cover the surface of TiO₂ nanotubes. Additionally, the incorporation of WO₃ resulted in a shift of the absorption edge toward visible light as confirmed by UV-Vis diffuse reflectance spectroscopy and a decrease in the estimated band gap energy values hence, modified TNTAs facilitated a more efficient utilization of solar light for water splitting. From the photoelectrochemical measurement data, the optimal photoelectrode produced after 2 min of deposition time improved the photo conversion efficiency and the hydrogen generation by 30% compared to that of the pure TNTA.

Keywords

titanium dioxide nanotube arrays / potentiostaticanodization / electrodeposition method / tungsten oxide / photoelectrochemical water splitting

Introduction

In recent years, the huge increase in world population and industrial activities has led to accelerated energy consumption and unabated release of hazardous pollutants, which lead to pollution-related diseases and global warming [¹–⁴]. These aroused our awareness of the urgency of initiating an alternative renewable energy sources that would deliver the least environmental negative impacts. Solar energy is the most attractive source of renewable energy available in abundance and the amount of global energy needs is only a small fraction of the solar energy, where the energy that the Earth receives from the Sun is about 10000 times more than the global population currently consumes [⁵,⁶]. Thus, global efforts are made to develop new high-performance solar energy conversion systems.

Solar energy can be converted into either electrical energy (via photovoltaic cells) or chemical energy in the form of hydrogen (via photoelectrochemical (PEC) water splitting), that make solar energy more competitive [⁶]. Hydrogen which can be efficiently used to produce electric power using fuel cell, is a raw material for numerous applications and can be directly driven to an internal combustion engine to provide a clean and affordable energy supply. For these reasons it can minimize the dependence on oil, reduce environmental pollution, and produce only water upon oxidation which, in turn, has no hazardous exhaust of greenhouse gases [²,⁶–⁹]. As a result, PEC water splitting process is considered as an ideal method for renewable energy production and storage

The key component of a PEC water splitting system is the efficient semiconductor photocatalyst. Oxide semiconductors, in general, and TiO₂-based materials in particular, are the photocatalysts of interest and have been studied in water splitting for various reasons, for instance, its wide range of pH stability, non-toxicity, environmentally friendliness, and good photoactivity in the presence of UV light [⁹–¹²]. The electronic structure, specifically the band gap, is the key functional property of semiconductor photocatalysts and has a critical impact on the energy conversion efficiency. Only photons of energy equal to and larger than the band gap may be absorbed and used for conversion of solar energy. The part of the solar energy spectrum available for conversion by TiO₂ involves the photons with energy higher than 3.0 eV and is only a small part of the entire solar spectrum. To this end, extensive efforts are directed toward reduction of the band gap of TiO₂ in order to shift the optical response from the UV toward the visible spectrum, since the bulk of the terrestrial solar energy is in the visible spectrum through modification with various materials, including Cr [¹³–¹⁵], Fe [¹⁶,¹⁷]‏, Co [¹⁸]‏, Pd [¹⁹]‏, Cu [²⁰]‏, Ag [²¹]‏, Au [²²]‏, ZnO [²³]‏, WO₃ [²⁴]‏, and reduced grapheme [²⁵]‏. Ge et al. [²⁶]‏ have presented an important review on various modification methods for TiO₂ nanotube in order to enhance the visible light absorption and suppress the recombination of photogenerated electron/hole pairs for photo/photoelectro-catalytic water splitting. One strategy is the development of multicomponent nanocrystalline semiconductor photoanodes which consist of several semiconductors that vary in their band gaps. These systems can serve two purposes: the first purpose is to allow the extension of the photoresponse of a TiO₂ wide-band gap semiconductor by coupling it with a narrow band gap semiconductor so as to utilize the advantage of the entire solar spectrum. The second purpose is to slow down or inhibit the charge-carrier recombination with the enhanced charge separation [⁹,²⁷]. Compared to other 1D TiO₂ nanostructured arrays, TiO₂ nanotube arrays are excellent photoanode materials that have been used because they are easily synthesized, inexpensive, and recyclable [⁹]. With these objectives in mind, we have prepared TiO₂ nanotube arrays decorated with various amounts of tungsten oxide and are examined as photoanodes in PEC water splitting system.

Tungsten oxide (WO₃) is an n-type semiconductor with a 2.5 - 2.8 eV band gap and its conduction band is located at a more positive value compared to H⁺/H₂ level, therefore, the spontaneous generation of H₂ by the photogenerated electrons in WO₃ is not possible [²⁸–³²]. The WO₃ valence band and the conduction band are lower than those of TiO₂ so, by illumination, the photogenerated electrons transfer from TiO₂ conduction band to WO₃ conduction band and the photogenerated holes move from valence band of WO₃ toward TiO₂ valence band, consequently accumulation of holes in TiO₂ and electrons in WO₃. For that reason, the synthesis of TiO₂-WO₃ hybrid materials is subjected to many researches due to its improved charge carrier separation and the photoelectrochemical performance of the photoelectrodes are boosted [³³–⁴¹].

Paramasivam et al. have prepared TiO₂-WO₃ nanotube arrays via anodizing various TiW alloys (0.2 at % W (Ti 0.2W) and 9 at % W (Ti 9W)) [⁴²]. They have presented the influence of the amount of WO₃ on the photocatalytic decomposition of Rhodamine B. The Ti 0.2 W alloy shows the highest efficiency due to the improved hole-transfer to the electrolyte, while at a high WO₃ content causes charge trapping effect.

Nazari et al. have prepared TiO₂-WO₃ nanotube arrays through anodizing Ti substrate in tungstate containing electrolytes (1, 5, and 10 g/L) to produce TW 1, TW 5, and TW 10 for the degradation of 4-chlorophenol [⁴³]. They have attributed the improvement of the photocatalytic activity of TiO₂-WO₃ to the formation of a more acidic surface that enables the absorption of hydroxyl groups and, therefore, promoting the photocatalytic performance in addition to the suppression of charge carrier recombination. They have concluded that there is an optimum concentration of WO₃ that produces the maximum photocatalytic activity.

Momeni et al. [²⁴] have prepared hierarchical WO₃-TiO₂ nanotube arrays by single-step electrochemical anodization of titanium foil in the presence of tungsten salt. These electrodes have been utilized in PECs for water splitting under visible light. They have presented that the incorporation of WO₃ decreases the TiO₂ band gap and the photocurrent of pure TiO₂ nanotube array is increased from ~0.81 mA/cm² at 0.70 V vs. Ag/AgCl to more than 1.61 mA/cm² in the presence of the appropriate amount of WO₃ in the composite. Momeniand Ghayeb has synthesized TiO₂-WO₃ nanotubular composite electrodes via one-step electrochemical anodizationin presence of various concentrations of sodium tungstate (0.6, 1.2, and 1.8 mM) [⁴⁴]. The degradation of methylene blue (MB) has been used to evaluate the photocatalytic activity of the obtained samples. They have concluded that the concentration of tungsten salt has a significantly influence on the morphology and photocatalytic activity of TiO₂-WO₃ films fabricated. A small concentration of tungsten salt produces a nanotubular film, and with increasing the dopant concentration up to 1.2 mM and 1.8 mM, the samples become nanoporous and compact, respectively. The nanotube array produced using 0.6 mM exhibits a better photocatalytic activity than other samples under visible light. This enhancement is due to the one-dimensional morphology which provides direct and faster electron transport, the generation of more free charge carriers, and the higher surface area of TiO₂-WO₃ nanotubes compared to the nanoporous and compact films.

Zhong et al. have synthesized TiO₂-WO₃ nanotube films using Ti plate or Ti mesh as a substrate in the anodic oxidation process and then electrodeposited WO₃ to produce WTP and WTM, respectively [⁴⁵]. They have used these arrays to evaluate the photocatalytic degradation of methylene blue (MB) under visible light after 120 min. It is 37% for WTP and 72% for WTM. This is resulted from the porous characteristic of WTM, which can facilitate the flowing of the pollutant and the higher light utilization rate.

This work focuses on the preparation of nanostructured modified TiO₂ arrays in order to reduce the electron-hole recombination, sensitization toward the visible light, and enhance the PEC performance. From this viewpoint, various amounts of tungsten oxide for improving the photoconversion efficiency of TiO₂ based photoelectrodes have been electrodeposited through different duration times (1, 2, 3, 5, and 20 min) using the same bath which has been used for the modification of TiO₂ nanorod arrays [⁴⁶]. From those works, it can be concluded that the morphology of the array, nanowires or nanotubes has a pronounced effect where pure nanotube array produces higher photoconversion efficiency as compared to pure nanorod array. This may be attributed to its higher surface area, the highly aligned ordered tubular structure of TNTA, and the fact that the anatase phase is catalytically more active than the rutile phase. Additionally, whatever the morphology of TiO₂ array is, the characterization results indicate that there is an optimum amount of WO₃ to improve the photoelectrochemical activity of TiO₂-WO₃ mixed oxide photoelectrodes for water oxidation. Besides, a further increase in the WO₃ amount may shield the role and the active sites of TiO₂. Moreover, WO₃ can act as recombination centers for the charge carriers. Therefore, the electrode photocurrent and photoconversion values have decreased. In this study, the optimum sample produced after 2 min of deposition time has a photoconversion efficiency of 30% higher relative to unmodified TiO₂ electrode.

Experimental

Materials

The titanium foil was provided by Sigma Aldrich, the ammonium fluoride and hydrogen peroxide (30%) were purchased from Alpha Chemica^TM, the glycerol was supplied by Oxford Laboratory, the sodium tungstate was supplied by Lobachemie, and the ethylene glycol was obtained from S D Fine-Chem Limited. All chemicals were used without further purification.

Method

The anodic oxidation combined with electrodeposition of W-oxide was employed for the synthesis of vertically aligned TiO₂ nanotube arrays (TNTAs) modified with W-oxide as illustrated in Fig. 1. Briefly, TNTAs were prepared by anodizing Ti substrate in 0.25 M NH₄F, 10% (wt) H₂O, and glycerol at 30 V for 4 h. After anodization, the nanotube arrays were annealed in air at 400°C for 2 h and denoted as T. To deposit WO₃ on the obtained nanotubes, a three electrode cell was used, Pt as the counter electrode, Ag/AgCl as the reference electrode, and the synthesized TNTA as the working electrode. The WO₃ electrodeposition was conducted as presented in Ref. [⁴⁷]: an aqueous solution of 25 mM Na₂WO₄ and 0.075% H₂O₂ electrolyte, at constant potential –450 mV for various periods (1, 2, 3, 5, 20 min). Finally, a series of modified TiO₂ nanotube arrays were produced and designated as TW x min, where “x” indicated the tungsten oxide deposition time.

Characterization techniques

Morphological characterization and elemental analysis of unmodified and modified TNTAs were conducted using a Quanta 250 FEG field-emission scanning electron microscope (FESEM) and a high resolution transmission electron microscope (HRTEM, JEM2100, Jeol, Japan) with an energy dispersive X-ray spectroscope (EDX) attachment. The phase identification of arrays was examined by X-ray diffraction (XRD, PANalyticalX’Pert PRO diffractometer operated at 40 mA and 45 kV, using Cu Ka radiation, Netherland). The UV-Vis diffuse reflectance spectra were measured on a 3600 Shimadzu UV-Vis-NIR spectrophotometer (Japan). The photoelectrochemical measurement was performed in a three-electrode configuration using Ag/AgCl reference electrode, Pt counter electrode, and TNTA or TW x min as working electrode as presented in Fig. 2. The electrodes with an area of 1 cm² were immersed inside a 1 M KOH containing 10% (wt) of ethylene glycol. A Volta Laboratory PGZ100 potentiostat was used to measure the current density versus the Ag/AgCl reference electrode at a scan rate of 20 mV/s in the dark and under the illumination of a Newport solar simulator equipped with a 150 W Ozone-free Xenon lamp and an AM 1.5 filter. The light intensity was maintained at 110 mW/cm² as calibrated by a solar power meter and multimeter (DI-LOG, SL102).

Results and discussion

Figure 3 shows field emission scanning electron microscopy (FESEM) images of the as-prepared TNTAs and after decoration with WO₃ particles. Figure 3(a) reveals the surface morphology of TiO₂ after anodization with highly ordered vertically aligned nanotube structure with an outer diameter of about 85-110 nm, a wall thickness of ca. 20 nm, and a length of approximately 800 nm. Figure 3(b) displays the FESEM images of TiO₂ nanotube arrays modified with tungsten oxide obtained after 2 min of electrodeposition and designed as TW 2 min. It indicates that W-oxide particles cover some of the TiO₂ nanotubes and the tubular structure of TiO₂ is well maintained with the same outer and inner diameters. The morphology of TW 5 min is changed dramatically with the appearance of large and aggregate particles of W-oxide on the top of TiO₂ nanotubes, as clearly revealed at different magnifications in Fig. 3(c). Furthermore, it is noticed that this array still retain the tubular structure of TiO₂.

To prove the existence of W-element, the chemical composition was detected by using EDX attached in FESEM. Table 1 lists the EDX data of TiO₂ nanotube array (TNTA) and TiO₂ nanotube array modified with tungsten oxide synthesized after 2 min deposition time (TW 2 min). It reveals that pure TiO₂ array contains only Ti and O and TW 2 min consists of W as well as Ti and O. This is an evidence for incorporation of tungsten in the modified TNTAs.

Detailed information concerning the morphology of TNTA and TW x min is delivered by HRTEM. The samples for HRTEM analysis was prepared by sonication the arrays in ethanol to remove the nanotubes from Ti substrate. Then this dispersing was dropped on a copper grid for measurements. Figure 4(a) depicts the tubular structure of pure TiO₂. Figure 4(b) displays the image of TW 2 min and proves the deposition of WO₃ as small particles. After a longer deposition time to produce TW 5 min, WO₃ particles form clusters as presented in Fig. 4(c). To determine further elemental composition analysis for the samples, the energy dispersive X-ray analyzer (EDX) fitted to HRTEM was used. Figures 4(d), 4(e), and 4(f) present the EDX spectra of TNTA, TW 2 min, and TW 5 min, respectively. It is seen that pure TNTA contains only Ti and O. Figure 4(d), and TW 2 min consists of W as well as Ti and O (Fig. 4(e)). Additionally, the intensity of the peaks related to W element in TW 5 min sample proves that a higher amount of WO₃ is deposited, as displayed in Fig. 4(f).

The as-prepared TiO₂ nanotube array was amorphous, and therefore, the calcination step was performed to attain the crystalline structure [⁴²,⁴⁸]. Figure 5(b) presents the XRD pattern of pure TNTA with the characteristic peaks of anatase TiO₂. The labeled peaks at 2q 25.2° and 48.1° are attributed to (101) and (200) diffractions, respectively. These results agree with those from Refs. [⁴⁹–⁵²], and Ti substrate peaks can be observed in the XRD profile. Ref. [⁴⁹] presented the deposition of monoclinic structures for WO₃ by using the same electrodeposition bath as it was confirmed from XRD analysis. However, as can be observed from Fig. 5(c), at 2 min deposition time, the produced sample shows the characteristic peaks of TiO₂ and there are no peaks of crystalline WO₃. This may be attributed to the amorphous structure of W-oxide particles or the low concentration of the tungsten oxide in the array. This is in line with Refs. [⁵³,⁵⁴], where any diffraction peaks of tungsten oxide in XRD patterns of TiO₂-WO₃ could neither be observed.

To investigate the optical properties of TNTA and W-oxide TNTA electrodes, the diffuse reflectance spectroscopy was employed. Figure 6 presents the UV-Vis diffuse reflectance spectra (DRS) of TNTA and TW 2 min. It is clearly seen that the absorption edge of W-oxide TNTA electrode is shifted toward longer wavelength in comparison to that of the unmodified TNTA electrode, reflecting that the band gap value of the obtained W-oxide TNTA is decreased. Furthermore, the red shift of the absorption edge indicates that the modified TNTA electrode can facilitate a more efficient utilization of solar light for photoelectrochemical water splitting. Nazari et al. have observed a shift in the absorption edge of the TiO₂-WO₃ composite nanotube arrays relative to pure TiO₂ nanotube array [⁴³].

The band gap energy of TNTA and TW 2 min electrodes have been estimated using the Kubelka-Munk function (F(R)) based on the diffuse reflectance (R) [⁵⁵–⁵⁸].

(1)F(R)={\displaystyle \frac{{(\mathrm{1}-R)}^{\mathrm{2}}}{\mathrm{2}R}}.

Figure 7 plots the [F(R)·hn]^1/2 versus the energy of the light (eV) and the calculated band gap energy of pure TNTA and TW 2 min is 3.25 and 3.0 eV, respectively. This confirms the fact that decoration of TNTA with W-oxide decreases its band gap.

Figure 8 displays the photocurrent density versus applied potential of pure TNTA in comparison with W-oxide TNTAs prepared at different deposition times (1, 2, 3, 5, 20 min) under solar simulator illumination. The photocurrent increases with increasing deposition time and attains a maximum at 2 min of deposition time after which photocurrent begins to decrease with increasing time. Inset Fig. 8 proves that the current of all electrodes under dark conditions is very small and can be neglected. Owing to the fact that the conduction and valance band of WO₃ are lower than that of TiO₂, the electrons transfer from the conduction band of TiO₂ to the WO₃ conduction band and the holes move from the WO₃ valance band to the TiO₂ valence band, which causes an efficient charge separation [¹,⁴²,⁴⁶,⁵³,⁵⁹], as shown in Fig. 9. Accordingly, the photocurrent and the photocatalytic activity are enhanced and reach maximum at 2 min, which provides an in-between coverage for TiO₂. However, at a longer deposition time of up to 3, 5 and 20 min, W-oxide particles grow more and aggregate together and cover up too much TiO₂ nanotubes. For this reason, WO₃ particles begin to act as recombination centers for the charge carriers and the photocurrent dramatically decreases. The above results indicate that there is an appropriate amount of W-oxide to enhance the photochemical properties of the TiO₂ nanotube array. Additionally, Nazari et al. have observed that the TiO₂-WO₃ nanotube array contains an intermediate amount of WO₃ and has the highest photocatalytic reaction rate [⁴³]. Similar trend of photoactivity has been observed by Bai et al. [⁵³]. They have concluded that the presence of high concentration of WO₃ in TiO₂-WO₃ mixed oxide samples increases the surface trapping centers and the recombination, and subsequently decreases the photocatalytic activity.

The photoconversion efficiency (h) is derived from photocurrent plots for all the prepared electrodes according to Eq. (2) [⁴⁹,⁶⁰].

(2)\eta ={{\displaystyle J}}_{\mathrm{P}}[({{\displaystyle E}}_{\mathrm{r}\mathrm{e}\mathrm{v}}^{\mathrm{0}}-\left|{{\displaystyle E}}_{\mathrm{a}\mathrm{p}\mathrm{p}}\right|)/{{\displaystyle I}}_{\mathrm{0}}]\times \mathrm{100}\%,

where J_P is photocurrent density (mA/cm²), {{\displaystyle E}}_{rev}^{\mathrm{0}} is standard reversible potential for water splitting (1.23 V), and E_app is applied potential (V); E_app = E_meas – E_aoc, E_meas is working electrode potential and E_aoc is the working electrode potential in open circuit condition; and I₀ is the power density of incident light. Figure 10 displays an enhancement in photoconversion efficiency of TNTAs through modification with W-oxide until optimum deposition time (2 min). After that, the photoconversion efficiency decreases. The maximum photoconversion efficiency of TiO₂ arrays modified with tungsten oxide is 0.52%, which is 30% higher than that of the pure TiO₂ nanotube array.

Figure 11 illustrates the amount of hydrogen gas generated, which is measured using the solution replaced method, for all samples. Pure TNTA produces about 15 mmol/(h·cm²) and as W-oxide particles increase as the collected hydrogen gas increases until 2 min of deposition time, afterwards with a further increase in deposition time causes a decrease in the amount of hydrogen produced. The highest hydrogen rate (19 mmol/(h·cm²)) is produced from TW 2 min and thus agrees well with the photocurrent and the photoconversion efficiency results.

Conclusions

TiO₂ nanotube arrays with different contents of WO₃ were synthesized through anodic oxidation of Ti substrate followed by electrodeposition of WO₃ for different times (1, 2, 3, 5, and 20 min). By characterization using FESEM, HRTEM, EDX, XRD, UV-Vis diffuse reflectance, and photoelectrochemical measurements, the following conclusions are made from the results obtained.

A lower deposition time leads to a partial coverage of TNTA with very small WO₃ particles on the nanotube surface whereas at longer times these particles aggregate together and the TiO₂ surface becomes mostly covered by WO₃.

EDX analysis reveals that the modified TNTA films contain Ti, W, and O elements.

The diffuse reflectance spectra and the estimated band gap values demonstrate that the incorporation of WO₃ leads to the red shift of the absorption edge and narrowing in the band gap, which indicates that the modified TNTA electrode can facilitate a more efficient utilization of solar energy for photoelectrochemical water splitting.

There is an optimal amount of WO₃ to improve the photoelectrochemical activity of TiO₂-WO₃ photoelectrodes for water oxidation and a further increase in the WO₃ particles may shield the role of TiO₂ and in this case WO₃ may act as recombination centers for the charge carriers, and as a result, the electrode performance efficiency decreases.

The improvement in the photocurrent and photoconversion efficiency is ascribed to the efficient separation of photogenerated charge carriers as a result of the modification effect by WO₃.

The morphology of TiO₂ has a pronounced influence on the photocatalytic acivity as it is clearly observed by comparing this work with the previous one.

The optimum sample has a photoconversion efficiency of 30% higher relative to that of the pure TiO₂ electrode.

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