Preparation of nanoporous BiVO4/TiO2/Ti film through electrodeposition for photoelectrochemical water splitting.

A nanoporous BiVO4/TiO2/Ti film was successfully fabricated by electrodepositing a nanoporous BiOI film on nanoporous TiO2 arrays followed by annealing at 450°C for 2 h. The electrodeposition of BiOI film was carried out at different times (10, 30, 100, 500 and 1000 s) in Bi(NO3)3 and KI solution. The morphological, crystallographic and photoelectrochemical properties of the prepared BiVO4/TiO2/Ti heterojunction film were examined by using different characterization techniques. UV-vis spectrum absorption studies confirmed an increase in absorption intensities with increasing electrodeposition time, and the band gap of BiVO4/TiO2/Ti film is lower than that of TiO2/Ti. The photocatalytic efficiency of BiVO4/TiO2/Ti heterojunction film was higher compared to that of the TiO2/Ti film owing to the longer transient decay time for BiVO4/TiO2/Ti film (3.2 s) than that of TiO2/Ti film (0.95 s) in our experiment. The BiVO4/TiO2/Ti heterojunction film prepared by electrodeposition for 1000 s followed by annealing showed a high photocurrent density of 0.3363 mA cm-2 at 0.6 V versus saturated calomel electrode. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO4/TiO2/Ti film (1000 s) under irradiation.

Introduction

In the past few decades, photoelectrochemical (PEC) catalyn class="Chemical">tic pan> class="Chemical">water splitting by using nanostructured semiconductors has been an effective way of producing hydrogen and oxygen [1,2]. Many types of photoelectrode materials have been reported, including BiVO4, CuO and Fe3O4 [3-6]. Among these materials, BiVO4 has attracted significant attention because of its high photocatalytic activity in oxygen evolution under an external bias. However, the conduction band (CB) level is low and it is unsuitable for O2 formation. The high-energy electrons excited by light usually relax in the bottom of the CB quickly owing to its weak charge-carrier separation [7,8].

Recently, there are two types of modificationpan> methods to further improve the photocatalypan> class="Chemical">tic ability of the semiconductor photocatalyst of BiVO4. One is doping modification, such as doped metal [9], non-metal [10] and semiconductor [11]. The second method is morphology modification by changing the crystal structure, morphology and specific surface area of BiVO4 [12,13]. The heterogeneous catalytic materials combining with BiVO4 are the most promising structures owing to their uniqueness and excellent coupling ability. There are many types of heterogeneous materials including WO3/BiVO4, g-C3N4/BiVO4 and TiO2/BiVO4 [14,15].

Among these heterogeneous materials, n class="Chemical">BiVO4/pan> class="Chemical">TiO2 remains one of the best photocatalytic materials because of the low cost of TiO2 and the faster electron transfer properties. Moreover, the TiO2 membrane has great potential in developing highly efficient water treatment and reuse systems, such as the decomposition of organic pollutants [16]. Concerning BiVO4/TiO2, it has been extensively reported for photo-oxidative degradation of pollutants, such as rhodamine [17,18]. In addition, most of the BiVO4/TiO2 electrodes are prepared through sol–gel, metal organic decomposition, chemical thermal deposition and spinning [17,19-21]. An efficient charge transfer between BiVO4 and TiO2 has already been reported [22]. Nanoporous TiO2/Ti arrays have been successfully prepared by anodic oxidation and the electrocatalytic properties have also been extensively analysed [23]. However, few studies that put porous BiVO4 film onto nanoporous TiO2/Ti arrays through electrodeposition of BiOI film followed by a sintering method have been conducted.

Therefore, the present work adds useful informationpan> onpan> heterogenpan>eous materials. pan> class="Chemical">BiVO4/TiO2/Ti photoanodes were fabricated by electrodepositing BiOI film on nanoporous TiO2 arrays followed by annealing at 450°C. PEC characterization of BiVO4/TiO2/Ti heterojunction material with different BiVO4 thicknesses was first performed in 0.2 M Na2SO4 electrolyte under an Xe lamp irradiation using CHI660E, which will provide new insights on further works in the field of BiVO4-based heterogeneous materials.

Experimental procedure

Preparation of nanoporous TiO2/Ti film

Commercially pure n class="Chemical">titanium plates (0.5 mm thick, purity greater thanpan> 99.5%) were first degreased inpan> pan> class="Chemical">acetone, mechanically polished, and finally, chemically polished at 25°C in a solution consisting of H2O : HNO3 : HF = 6 : 3 : 1 (vol%) for 30 s. The pretreated titanium plates were anodically oxidized at 25°C in 1 wt% hydrofluoric acid at 20 V for 15 min to produce nanoporous TiO2/Ti arrays. Then the nanoporous TiO2 arrays were washed three times with de-ionized water and dried for the next step usage. The prepared nanoporous TiO2 arrays were sintered in air at 450°C for 2 h to prepare the nanoporous TiO2/Ti film.

Preparation of BiVO4/TiO2/Ti heterojunction photoanodes

The n class="Chemical">BiOI film was successfully prepared by electrodeposition [24,25]. The 0.04 M Bi(NO3)3 solution was prepared by dissolving Bi(NO3)3·5H2O in 50 ml 0.4 M KI solution after its pH was adjusted to 2.0 by adding HNO3. Then the solution was mixed with 20 ml absolute ethanol (100%) containing 0.23 M p-benzoquinone and was vigorously stirred for 15 min. A typical three-electrode cell was used for electrodeposition. Nanoporous TiO2/Ti film, saturated calomel electrode (SCE) and Pt silk were used as the working electrode, reference electrode and counter electrode, respectively. CHI660E (Shanghai Chenhua Device Company, China) was used for electrodeposition and subsequent PEC studies. Electrodeposition was performed potentiostatically at −0.15 V versus SCE at room temperature with varying deposition times (10, 30, 100, 500 and 1000 s). Then the 0.2 M vanadyl acetylacetonate (VO (acac)2) dissolved in dimethyl sulfoxide was drop-cast onto the working electrode with a volume of 1 ml followed by heating in a muffle furnace at 450°C for 2 h in air. After that, the excess V2O5 on the BiVO4/TiO2/Ti photoelectrodes was removed in 1 M NaOH solution for 1–2 h with stirring [24]. The resulting heterojunction BiVO4/TiO2/Ti photoelectrodes were then rinsed with de-ionized water and dried at room temperature.

Characterization

The surface morphologies of heterojunctionpan> pan> class="Chemical">BiVO4/TiO2/Ti photoanodes were observed by Hitachi S-4700 field emission scanning electron microscopy (FESEM) after spraying the conducting layer with platinum. The bulk composition was investigated by energy-dispersive X-ray spectroscopy. The phases present in the coatings were characterized by a small angle diffractometric study carried out on a Riga KuD/max 2550PC X-ray automatic diffractometer. The optical performance of the as-prepared materials was evaluated by using a UV–vis Lambda 750S in a wavelength ranging from 300 to 800 nm.

The PEC performance was evaluated in a three-electrode electrochemical cell with a quartz window to allow illuminationpan>. The workinpan>g electrodes were the sinpan>tered nanpan>oporous pan> class="Chemical">TiO2/Ti film and BiVO4/TiO2/Ti heterojunction film. SCE and Pt silk were used as the reference electrode and counter electrode, respectively. All the working electrodes were characterized in 0.2 M Na2SO4 by CHI660E. Linear sweep voltammetry (LSV) was measured at a scanning rate of 0.01 V s−1. Electrochemical impedance spectroscopy (EIS) was carried out under an open circuit voltage with frequencies ranging from 105 to 10−2 Hz with an AC voltage amplitude of 5 mV. The potentials in the I–V curves and in the PEC performance experiments were also controlled by CHI660E. A 150 W Xe lamp (Beijing Trust Tech Co. Ltd) was used to provide the visible light. EIS was used to explore the conductivity of the as-compared electrodes in dark and illumination environments in 0.2 M Na2SO4 solution.

Results and discussion

The crystal structures of sintered nanoporous n class="Chemical">TiO2/pan> class="Chemical">Ti film and BiVO4/TiO2/Ti heterojunction films prepared with different electrodeposition times were characterized by X-ray diffraction (XRD) and are shown in figure 1. In figure 1a, the diffraction peaks at 2θ of 29.6°, 31.6°, 45.3° and 51.3° can be indexed to BiOI (JCPDS no. 10–0445). The as-anodized nanoporous TiO2/Ti films are of amorphous state due to the broad peak and only the peaks of the titanium substrate are present in the diffractogram [26]. After sintering at 450°C for 2 h, peaks at approximately 25° appear corresponding to anatase phase [26]. After modifying by BiVO4 layer, new peaks appear at 2θ of 18.6°, 18.9°, 28.7°, 34.4°, 35.2°, 42.3°, 46.4° and 58.3°, which correspond to (101), (011), (-121), (200), (002), (051), (002), (202) and (321) of BiVO4. The observed diffraction peaks are in conformity with monoclinic scheelite structure (JCPDS 14-0688). The XRD patterns confirm the full conversion of BiOI to BiVO4 after annealing at 450°C. From the XRD pattern, it was clarified that BiVO4 could be successfully modified on nanoporous TiO2/Ti films.

Figure 1.

XRD patterns: (a) TiO2/Ti before sintering and BiOI/TiO2/Ti with 1000 s electrodeposition time; (b) sintered nanoporous TiO2/Ti film and BiVO4/TiO2/Ti heterojunction films prepared with different electrodeposition times.

XRD patterns: (a) n class="Chemical">TiO2/n class="Chemical">Ti before sintering and BiOI/TiO2/Ti with 1000 s electrodeposition time; (b) sintered nanoporous TiO2/Ti film and BiVO4/TiO2/Ti heterojunction films prepared with different electrodeposition times.

The morphology and nanostructures of n class="Chemical">BiVO4/pan> class="Chemical">TiO2/Ti heterojunction photoanodes were characterized using FESEM. Here, the morphology of TiO2/Ti film was not shown, because its morphology could be easily seen from the morphology of BiVO4/TiO2/Ti heterojunction photoanodes obtained with 10 s electrodeposition time. From the top view of the anode with 10 s electrodeposition time (figure 2a), it is found that a small amount of BiVO4 existed in the interstice between the TiO2 nanotubes and on the top of TiO2 nanoporous wall. This is confirmed by the cross-section image in the inset picture of figure 2a. The TiO2 nanotube arrays are formed with regular and orderly structure. The ratio of pore length to diameter was approximately 2 : 1. When the electrodeposition time increased to 30 s, BiVO4 covered the interstices between the TiO2 walls and left the TiO2 pores open. Some of the TiO2 nanopores were covered by BiVO4 when the electrodeposition time increased to 100 s (figure 2c). BiVO4 covered the TiO2 nanopores and particles of approximately 200 nm grew on the surface when the electrodeposition time increased to 500 s (figure 2d). When the electrodeposition time increased to 1000 s, the morphology of the obtained photoanode was nanoporous and the thickness of BiVO4 was approximately 1 µm. Compared with figure 2d,f, the particle size of BiVO4 is almost 100 nm. This phenomenon is different from the conventional electrodeposition procedure in which the particle size usually increases with electrodeposition time [27].

Figure 2.

SEM images of photoelectrodes prepared with different electrodeposition times: (a) surface image of electrode with 10 s electrodeposition time; inset picture is the cross-section morphology prepared by mechanical fracturing; (b) surface image of electrode with 30 s electrodeposition time; (c) surface image of electrode with 100 s electrodeposition time; (d) surface image of electrode with 500 s electrodeposition time; (e) surface image of electrode with 1000 s electrodeposition time; (f) cross-section of sample (e) with mechanical fracturing.

The UV–vis spectra of n class="Chemical">TiO2/pan> class="Chemical">Ti film and BiVO4/TiO2/Ti heterojunction films with different amounts (denoted by the electrodeposition time) of BiVO4 are shown in figure 3a. The absorption edge of the annealed TiO2/Ti film is approximately 375 nm. After being coupled with BiVO4, the absorption in the visible light region between 400 and 500 nm of BiVO4/TiO2/Ti heterojunction film increases with the increase in the amount of BiVO4, and the absorption edge shifts to 450 nm for the BiVO4/TiO2/Ti film prepared with 1000 s electrodeposition time. The band gap of a semiconductor can be calculated by employing the following equation [28]: where a is the absorption coefficient, v is the light frequency and Eg is the band gap of a semiconductor. From the curve of (ahv)2 versus hv shown in figure 3b, the energy of the band gap of TiO2/Ti film is 3.20 eV, which is the same as the result reported before [29]. The band gap energies of the BiVO4/TiO2/Ti film with 10, 30, 100, 500 and 1000 s electrodeposition times are calculated as 2.98, 2.83, 2.62, 2.58 and 2.53 eV, respectively. All BiVO4/TiO2/Ti films exhibited visible-light-driven absorption characteristics. Moreover, the nanoporous BiVO4 film in the BiVO4/TiO2/Ti film has slightly shifted on the absorption edge to the larger wavelength within the visible-light range owing to the effect of additional BiVO4.

Figure 3.

(a) UV–vis absorption spectra and (b) Tauc plot of nanoporous TiO2/Ti film and BiVO4/TiO2/Ti heterojunction films prepared with different electrodeposition times.

Figure 4 shows the current–potenn class="Chemical">tial plots of nanpan>oporous pan> class="Chemical">TiO2/Ti film and nanoporous BiVO4/TiO2/Ti heterojunction photoanodes under a 150 W Xe lamp illumination. The photocurrent densities of nanoporous TiO2/Ti film in the dark and under illumination were 0.02634 mA cm−2 and 0.0308 mA cm−2 at 0.6 V (versus SCE), respectively. The BiVO4/TiO2/Ti prepared with 10 s electrodeposition time under illumination exhibited a higher photocurrent density of 0.1583 mA cm−2 at 0.6 V (versus SCE) and showed a 403% higher photoactivity compared with bare TiO2/Ti film. Moreover, the photocurrent density of BiVO4/TiO2/Ti heterojunction photoanode increased with the increase in electrodeposition time in our experiment. When the electrodeposition time increased to 1000 s, the photocurrent density increased to 0.3363 mA cm−2 at 0.6 V (versus SCE). These results clearly indicate that the modification of nanoporous TiO2/Ti film with BiVO4 effectively reduces the recombination of electrons and holes generated in the nanoporous BiVO4/TiO2/Ti film due to the formation of the heterojunction and excellent electron transport between TiO2 film and the titanium substrate [30]. When BiVO4 layers were coated on the TiO2 surface, the light absorption range and intensity of BiVO4/TiO2/Ti films were improved and the electrons of BiVO4 film could easily transfer to the nanoporous TiO2, resulting in a high photocurrent density.

Figure 4.

LSV plots of nanoporous TiO2/Ti film and nanoporous BiVO4/TiO2/Ti films prepared with different electrodeposition times in 0.2 M Na2SO4 solution under 150 W Xe lamp illumination.

n class="Chemical">LSV plots of nanoporous n class="Chemical">TiO2/Ti film and nanoporous BiVO4/TiO2/Ti films prepared with different electrodeposition times in 0.2 M Na2SO4 solution under 150 W Xe lamp illumination.

The photocurrent response of n class="Chemical">photoanodes inpan> the electrolyte directly correlates with the genpan>eration and transfer of the photo-excited charge carriers in the photocatalytic process [31]. The photocurrent responses of TiO2/Ti film and BiVO4/TiO2/Ti heterojunction films prepared with different electrodeposition times were investigated to enhance the charge separation in 0.2 M Na2SO4 electrolyte at 0.6 V bias versus SCE. Both TiO2 and BiVO4 absorbed the photons and generated electron-hole pairs under the simulated sunlight illumination [32]. As shown in figure 5, it is clear that the photocurrent abruptly increased and decreased when the light source was switched on and off. The photoanodes of BiVO4/TiO2/Ti heterojunction films present obviously enhanced the photocurrent response compared with those of bare TiO2/Ti film. A photocurrent spike is clearly obtained in sudden illumination due to capacitive charging of the interface, and the spike decays because of recombination of the charge carriers associated with holes getting trapped at the surface [33]. When the simulated sunlight was turned on, the photocurrent density was a little higher than that 10 s later, which indicated the poor recombination abilities of photogenerated electrons with the holes in the TiO2 modified with BiVO4 electrodes [34]. It is obvious that the BiVO4/TiO2/Ti heterojunction films with 1000 s electrodeposition time represent the highest photocurrent density compared to that of other photoanodes, which can be ascribed to the high nanoporous surface of BiVO4/TiO2/Ti heterojunction photoanodes and their excellent charge separation and transport properties. Thus, it can also be confirmed that the separation of electron-hole pairs was derived from the heterojunction [35].

Figure 5.

(a) Transient photocurrent responses of TiO2/Ti film and BiVO4/TiO2/Ti heterojunction film prepared with different electrodeposition times under 150 W Xe lamp illumination in 0.2 M Na2SO4 solution at 0.6 V versus SCE. (b) Transient decay times of TiO2/Ti film and BiVO4/TiO2/Ti heterojunction film.

(a) Transient photocurrent responses of n class="Chemical">TiO2/pan> class="Chemical">Ti film and BiVO4/TiO2/Ti heterojunction film prepared with different electrodeposition times under 150 W Xe lamp illumination in 0.2 M Na2SO4 solution at 0.6 V versus SCE. (b) Transient decay times of TiO2/Ti film and BiVO4/TiO2/Ti heterojunction film.

The transient decay n class="Chemical">time canpan> be anpan>alysed by a logarithmic plot of parameter D, usinpan>g the followinpan>g equation [33,36]: where I is the current at time t, I the stabilized current and I is the current spike. The transient decay time can be defined as the time at which ln D = −1 [37]. Figure 5b displays the logarithmic plots of parameter D of the BiVO4/TiO2/Ti heterojunction film prepared with 10 s and TiO2/Ti photoelectrodes. The transient decay time for BiVO4/TiO2/Ti (3.2 s) is longer than that of TiO2/Ti (0.95 s), indicating a lower charge carrier recombination rate in BiVO4/TiO2/Ti heterojunction photoanodes, leading to an enhanced charge separation efficiency and prolonging the hole lifetimes. The BiVO4/TiO2/Ti electrodes prepared with increasing electrodeposition time maintained an improving PEC performance.

To evaluate the kinen class="Chemical">tics of the charge tranpan>sfer process of the pan> class="Chemical">TiO2/Ti and BiVO4/TiO2/Ti photoelectrodes, EIS tests were carried out at 0.2 V versus SCE under a simulated solar light illumination. Figure 6a displays the Nyquist diagrams in the frequency range of 0.01 Hz to 100 kHz. In the plot, symbols indicate the experimental results and the inset picture is the magnified view of the Nyquist diagram of BiVO4/TiO2/Ti heterogeneous photoanodes. The arc in the Nyquist plot indicates the charge transfer kinetics on the working electrode. Obviously, the BiVO4/TiO2/Ti photoelectrodes present a lower charge transfer resistance, suggesting that the BiVO4/TiO2/Ti heterojunction facilitates charge transfer and separation. The simulated EIS results were obtained from the fitting procedures according to the ZSimpWin software, and the equivalent Randles circuit is shown in figure 6b. In the equivalent Randles circuit, Rs is the solution resistance, Qcpe is the constant phase element for the electrolyte/electrode interface and R is the charge transfer resistance across the interface of electrode/electrolyte. The arcs in the Nyquist plot are related to the charge transfer at the interface of the photoelectrode/electrolyte. The fitted values of R were 3183, 4373 and 322 000 Ω cm−2 for BiVO4/TiO2/Ti (1000 s), BiVO4/TiO2/Ti (100 s) and TiO2/Ti electrodes, respectively. The efficient charge transfer at the interface between photoelectrode and electrolyte hinders the charge recombination and induces the facile charge transport of electrons through the films. Thus, the bare TiO2/Ti film has a very low efficiency of charge transfer and shows the highest R value. The lowest R value for BiVO4/TiO2/Ti (1000 s) indicates that the charge transfer characteristics of BiVO4/TiO2/Ti heterojunction are good. Therefore, the modification of TiO2/Ti film with nanoporous BiVO4 by forming the heterojunction could improve the charge transfer and photocatalytic ability of photoanodes.

Figure 6.

(a) EIS spectra of TiO2/Ti film and BiVO4/TiO2/Ti heterojunction film prepared with different electrodeposition times under 150 W Xe lamp illumination in 0.2 M Na2SO4 solution at 0.6 V versus SCE. (b) Equivalent circuit for photoanodes.

(a) EIS spectra of n class="Chemical">TiO2/n class="Chemical">Ti film and BiVO4/TiO2/Ti heterojunction film prepared with different electrodeposition times under 150 W Xe lamp illumination in 0.2 M Na2SO4 solution at 0.6 V versus SCE. (b) Equivalent circuit for photoanodes.

The photogenerated electrons can move to the CB of n class="Chemical">TiO2 from the CB of pan> class="Chemical">BiVO4 easily owing to the type II heterojunction. The excited electrons in TiO2 were facilely transported by the conductive Ti and directed to the Pt counter electrode via the external circuit (shown in figure 7). Therefore, the photogenerated electrons were scavenged by hydrogen ions on the Pt foil, while the photogenerated holes oxidized the water on the surface of the BiVO4/TiO2/Ti. The significantly enhanced PEC performance is attributed to the nanoporous structure, which improved the charge transport [38] and collection efficiency as well as the excellent contact between the TiO2 and BiVO4 with a large interface area facilitating the charge separation. Overall, the BiVO4/TiO2/Ti heterojunction film offered a remarkable photoconversion efficiency.

Figure 7.

Schematic of energy bands and charge transfers at BiVO4/TiO2/Ti film.

Conclusion

A new type of nanoporous n class="Chemical">BiVO4/pan> class="Chemical">TiO2/Ti heterojunction photoanode was designed and fabricated by electrodepositing BiOI onto a TiO2/Ti nanoporous film followed by sintering at 450°C in vanadium (IV) oxy acetylacetonate solution for 2 h. A significant change was observed in the PEC properties of the BiVO4/TiO2/Ti heterojunction film by varying the electrodeposition time. The film electrodeposited for 1000 s showed a high photocurrent density of 0.3363 mA cm−2 at 0.6 V versus SCE. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO4/TiO2/Ti heterojunction film electrodeposited for 1000 s under irradiation. Our results demonstrate that the nanoporous heterojunction BiVO4/TiO2/Ti photoanode is an effective design for improving the PEC performance owing to the excellent transport and separation efficiency. It will open a new opportunity for BiVO4/TiO2/Ti heterojunction photoelectrodes for water splitting by using solar energy.