Trans. Nonferrous Met. Soc. China 26(2016) 2390-2396

Photoelectrocatalytic reduction of CO₂ into formic acid using WO_3-x/TiO₂ film as novel photoanode

Ya-hui YANG¹, Ren-rui XIE¹, Hang LI¹, Can-jun LIU², Wen-hua LIU², Fa-qi ZHAN²

1. College of Resources and Environment, Hunan Agricultural University, Changsha 410128, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

Received 11 June 2016; accepted 1 September 2016

Abstract: A novel WO_3-x/TiO₂ film as photoanode was synthesized for photoelectrocatalytic (PEC) reduction of CO₂ into formic acid (HCOOH). The films prepared by doctor blade method were characterized with X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The existence of oxygen vacancies in the WO_3-xwas confirmed with an X-ray photoelectron spectroscopy (XPS), and the accurate oxygen index was determined by a modified potentiometric titrimetry method. After 3 h of photoelectrocatalytic reduction, the formic acid yield of the WO_3-x/TiO₂ film is 872 nmol/cm², which is 1.83 times that of the WO₃/TiO₂ film. The results of PEC performance demonstrate that the introduction of WO_3-xnanoparticles can improve the charge transfer performance so as to enhance the performance of PEC reduction of CO₂ into formic acid.

Key words: photoelectrocatalytic reduction; CO₂; formic acid; WO_3-x; TiO₂; film photoanode

1 Introduction

The global warming and depletion of fossil fuels are two major problems that mankind is facing today [1-3]. Photoelectrocatalytic (PEC) or photocatalytic (PC) reduction of CO₂ to fuel is considered an ideal and practical solution to the problems, as it utilizes solar energy and H₂O for the reduction of CO₂ in a similar way to natural photosynthesis without pollutants [4-6]. And the fuel as product can be used in the internal combustion engine or as chemical feedstock.

An ideal photocatalyst for CO₂ reduction should be environmentally friendly. More importantly, it should have a suitable band structure for reducing CO₂ and oxidizing water simultaneously [7,8]. However, the efficiency of reducing CO₂has been limited by the recombination of the photogenerated charge carriers. Recently, a photoanode-driven PEC system for conversion of CO₂ to fuels has attracted much attention because it can supply the additional energy to reduce CO₂ and oxidize H₂O with decreased charge recombination [1,9]. Among various photoanode materials, TiO₂ is widely used for CO₂ photoreduction due to its low toxicity, inexpensive and photocorrosion resistance [5, 10]. To improve the photocatalytic activity of TiO₂ based photoanode, many research efforts have been done such as doping with metals and mixing with other metal oxides [11-16].

Tungsten oxide (WO₃) with a band gap of about 2.7 eV has been widely utilized to couple with TiO₂[17,18]. This is mainly because WO₃ can serve as a coupling agent with TiO₂ to facilitate charge separation and mobility. Similar strategy can be applied in the TiO₂ based dye-sensitized solar cells (DSSCs) [19]. More interestingly, non-stoichiometric tungsten oxide with oxygen deficiency (WO_3-x) has a higher electron mobility (10-20 cm²·V^-1·s^-1) [20,21] and can be widely used as electron-selective materials in solar cells [22,23]. Considering the unsatisfactory efficiency of photoelectrocatalytic CO₂ reduction by the TiO₂ photoanode, combining TiO₂ with WO_3-x may provide a strategy to improve the seperation of photo-generated electrons and holes. To the best of our knowledge, no such constructed photoanode has been used to photoelectrocatalytic reduction of CO₂.

In this work, we fabricated a novel WO_3-x/TiO₂ film photoanode by the doctor blading method at low temperature. The introduction of WO_3-x improved the rate of electron transfer and suppressed recombination synergistically, resulting in an improvement in the performance of PEC reducing CO₂ into formic acid. In order to better understand the improved reason, the PEC performance has been investigated by photocurrent and electrochemical impedance measurements.

2 Experimental

2.1 Materials

TiO₂ nanoparticles (Degussa P25, Germany), ammonium metatungstate ((NH₄)₆H₂W₁₂O₄·xH₂O, 99.99%, Aladdin), polyvinylpyrrolidone (PVP, 99.99%, Aladdin), polyethylene glycol (PEG1000, 99.99%, Aladdin) were used as-received. The fluorine-doped SnO₂ glass (FTO) as the substrate of film was purchased from NSG Corporation.

2.2 Synthesis of WO_3-xnanoparticles

The WO₃ nanoparticles were prepared by a solution method according to our previous work [24]. 2 g of PVP was dissolved in 15 mL of deionized (DI) water. (NH₄)₆H₂W₁₂O₄·xH₂O (1.478 g) was suspended in 10 mL of DI water and then dropped into the above PVP solution with continual stirring. After 30 min of ultrasonic treating and 2 h of stirring, 2.000 g of PEG1000 was added in it with continual stirring for additional 4 h. The as-prepared precursor was completely dried at 80 °C, followed by sintering at 600 °C for 1 h.

The WO_3-x nanoparticles were prepared by a hydrogen reduction method. In a typical experiment, 1 g of WO₃ nanoparticles was calcined in reducing gas (V(Ar):V(H₂)=95:5) at the flow rate of 0.2 L/min and 400 °C for 2 h.

2.3 Preparation of TiO₂/WO_3-xfilms

The TiO₂/WO_3-x film was prepared by using a doctor-blade technique. In detail, 0.285 g P25, 0.015 g WO_3-xnanoparticles, 1.585 mL ethyl alcohol and 0.415 mL dilute HCl solution (pH≈4) were added to an agate jar. The TiO₂/WO_3-x paste can be obtained by ball milling for 2 h. The prepared paste was coated on the FTO substrate by a doctor-blade coater and then dried at 60 °C for 60 min. Finally, the as-prepared film was baked at 120 °C for 15 min.

2.4 Characterization

The crystalline phase of the samples was characterized with an X-ray diffractometer (XRD, D/Max2250, Rigaku Corporation, Japan) at a scanning speed of 8 (°)/min. The surface morphology and microstructure of the sample were investigated with a field emission scanning electron microscope (FESEM, NanoSEM 230) coupled with energy dispersive X-ray spectroscopy (EDS) and a transmission electron microscope (TEM, TECNAI G2 F20, FEI). EDS was used to analyse the elements of as-prepared sample. The chemical composition of the sample was analyzed with an X-ray photoelectron spectroscope (XPS, K-Alpha 1063, Thermo Fisher Scientific). The absorbance of photoanodes were performed through a diffuse reflectance ultraviolet and visible spectrophotometer (UV-Vis Pgeneral TU-1901).

2.5 Determination of oxygen index

The oxygen index was determinated by a modified method of potentiometric titrimetry [25-27]. In details, 0.200 g WO_3-x powder was put in a 250 mL conical flask. 5 mL of 0.2 mol/L K₃[Fe(CN)₆] and 15 mL of 1 g/L KOH solution were added. The mixture was heated in an oven at 70 °C for 15 min to dissolve WO_3-x powder, and then naturally cooled to room temperature. After that, 10 mL of concentrated hydrochloric acid, 10 mL of DI water and 20 mL of 10 g/L KI solution were injected. The potentiometric titrimetry was carried out in a two electrode configuration in which Pt plate and Ag/AgCl/satd. KCl were used as work electrode and counter/reference electrodes, respectively, and the above solution was used as electrolyte. Under continuous stirring, it was titrated with a standard solution of Na₂S₂O₃ (0.05 mol/L). The titration was terminated when an obvious potential change happens, and the consumption of Na₂S₂O₃is V₁. In addition, a coefficient K=G₂/G₁ was also gained, where G₁ is the mass of sample before annealing, and G₂ is the mass of sample after annealing at 400 °C for 30 min in air. Finally, the oxygen index (3-x) of non-stoichiometric tungsten oxides was calculated based on the following equation:

          (1)

and the oxygen index in this experiment is 2.65.

2.6 Photoelectrochemical measurements

The photoelectrochemical experiments were carried out using an electrochemical analyzer (Zennium, Zahner, Germany) with a three electrode quartz cell that includes a work electrode, a platinum or copper electrode and an Ag/AgCl/satd. KCl reference electrode. A 500 W Xe lamp adjusted to 100 mW/cm² (CHF-XM35, Beijing Trusttech Co. Ltd.) was used as the light source. For photoelectrochemical measurements, the electrodes were immersed in 0.2 mol/L Na₂SO₄ solution and the scanning rate of cyclic voltammetry is 20 mV/s. The electrochemical impedance spectra were measured at the potential of 0.8 V (vs Ag/AgCl) with a 10 mV AC voltage perturbation and the range of frequency is 10000 to 0.1 Hz. For the experiment of reducing CO₂, the reactor with a Nafion membrane has two compartments for water oxidation (anodic) and CO₂ reduction (cathodic), respectively. In the cell for water oxidation, the as-prepared film, Ag/AgCl and 0.2 mol/L Na₂SO₄solution were used as photoanode, reference electrode and electrolyte, respectively. In the compartment for CO₂ reduction, the copper electrode and 0.5 mol/L KHCO₃solution were used as cathode and electrolyte, respectively. A constant potential of 1.2 V (vs Ag/AgCl) was applied for CO₂ reduction. The CO₂ gas was injected during the whole test process. The formic acid in the electrolyte was analyzed by headspace method using a gas chromatograph-mass spectrometer (ICS 2000, Dionex, USA).

3 Result and discussion

3.1 Morphology and structure

The morphology of WO_3-x nanoparticle was investigated using FESEM. Figure 1(a) shows the morphology of WO_3-x nanoparticles. As shown in Fig. 1(a), all the WO_3-x nanoparticles exhibit the morphology of spherical particles with a diameter of 30-80 nm. The XRD pattern of WO_3-x nanoparticles is shown in Fig. 1(b). The diffraction peaks of WO_3-x nanoparticles at 23.0°, 23.5°, 24.3°, 26.5°, 28.7°, 33.1°, 33.8°, 35.4°, 41.6°, 47.0°, 48.1°, 49.9° and 55.7° are in accordance with (002), (020), (200), (120), (112), (022), (202), (122), (222), (004), (040), (232) and (402) planes of tungsten oxide, which can be indexed to WO₃ (JCPDS No. 89-7796).

Fig. 1 SEM image (a) and XRD pattern (b) of WO_3-x nanoparticles

Fig. 2 TEM images of WO₃ (a, b) and WO_3-x (c, d) nanoparticles

In order to better observe the particle size and crystal structure of WO₃ and WO_3-x nanoparticles, the TEM images of WO₃ and WO_3-x nanoparticles are shown in Fig. 2. Figures 2(a) and (b) show the low resolution and high resolution TEM images of WO₃ nanoparticles, respectively. As shown in Fig. 2(a), the size of WO₃ nanoparticles is 30-80 nm. The observed lattice fringes of 0.36 nm in Fig. 2(b) corresponds to the (002) plane of monoclinic WO₃ (JCPDS No. 89-7796). The TEM images of WO_3-x nanoparticles are shown in Figs. 2(c) and (d). It can be seen from Fig. 2(c) that the particle size of WO_3-x nanoparticle is similar to that of WO₃ nanoparticle. The interplanar spacing is 0.36 nm, which is consistent with the (200) plane of monoclinic WO₃ (JCPDS No. 89-7796). The TEM results indicate that there is almost no change in the particle size and crystal structure of WO₃ nanoparticles after H₂-treatment.

Figure 3(a) shows the FESEM image of WO_3-x/TiO₂ films. As shown in Fig. 3(a), the WO_3-x/TiO₂ film is composed of uniform nanoparticles with a thickness of 1 μm. The local composition of WO_3-x/TiO₂film was analyzed with an EDS spectrometer. The EDS result (Fig. 3(b)) indicates the existence of O, W and Ti.

Fig. 3 FESEM image (a) and EDS pattern (b) of WO_3-x/TiO₂ film

The crystallographic structure and phase purity of TiO₂, WO₃/TiO₂, and WO_3-x/TiO₂ films were examined by XRD analysis and the results are shown in Fig. 4. The XRD patterns of the three films present the same peaks, corresponding to rutile TiO₂ (JCPDS No. 73-2224) and FTO substrate (SnO₂, JCPDS No. 71-0652), and no other obvious peaks can be observed in all patterns, which are likely due to low WO₃ and WO_3-x content.

Fig. 4 XRD patterns of TiO₂, WO₃/TiO₂, and WO_3-x/TiO₂films

3.2 Chemical composition of WO_3-x

To confirm the existence of oxygen vacancies, the chemical states of O and W in the WO_3-x nanoparticles were studied by XPS. The XPS spectra have been fitted by the Gaussian-Lorentzian function. The O 1s and W 4f XPS spectra of WO_3-x are shown in Fig. 5. In Fig. 5, the characteristic peaks at 530.5 and 531.2 eV are attributed to O^2- and O^-, respectively [28,29]. For the W 4f XPS spectrum, the separated peaks centered at the binding energies of 34.8 and 36.9 eV correspond to the typical binding energies of W⁵⁺ [30]. This confirms that oxygen vacancies and W⁵⁺ are created after H₂-treatment.

Fig. 5 XPS spectra of WO_3-x

3.3 Optical properties

The light absorption spectra of all films are presented in Fig. 6. The pure TiO₂ film exhibits an absorption edge of ~400 nm. With the introduction of WO₃ and oxygen vacancies (WO_3-x), a high absorbance in the visible region is obtained.

Fig. 6 UV-Vis spectra of TiO₂, WO₃/TiO₂, and WO_3-x/TiO₂films

3.4 Photoelectrochemical performance

The PEC CO₂ reduction experiments were carried out under irradiation of a 500 W Xe lamp with a power density of 100 mW/cm², and the formic acid in solution was analyzed with a gas chromatograph-mass spectrometer (GC-MS). After 3 h of photoelectrocatalytic reduction, the production yields of HCOOH for TiO₂, WO₃/TiO₂ and WO_3-x/TiO₂ films were recorded as shown in Fig. 7. Compared with the pure TiO₂ film (335 nmol/cm²), the WO₃/TiO₂ film exhibits an enhanced production yield of HCOOH (475 nmol/cm²), which is because the improved separation of photon-generated carrier by the introduction of WO₃[19,31]. Particularly, the production yield of WO_3-x/TiO₂ film is 872 nmol/cm², which is 1.83 times that of WO₃/TiO₂ film. This indicates that the WO_3-x/TiO₂ film has a good ability for the PEC CO₂ reduction.

Fig. 7 Formic acid yields for TiO₂, WO₃/TiO₂ and WO_3-x/TiO₂ film after 3 h of photoelectrocatalytic reduction

To better understand the enhanced performance of PEC CO₂ reduction for WO_3-x/TiO₂ film, TiO₂, WO₃/TiO₂ and WO_3-x/TiO₂ films were used as photoanodes in a typical PEC cell, and their PEC performance was investigated in 0.2 mol/L Na₂SO₄and 0.5 mol/L NaHCO₃solution. Figure 8(a) shows the photocurrent density plots of TiO₂, WO₃/TiO₂ and WO_3-x/TiO₂ films. The WO₃/TiO₂film exhibits a higher photocurrent density than the pure TiO₂ electrode. Importantly, the photocurrent density of WO_3-x/TiO₂ film is larger than that of the WO₃/TiO₂film. It agrees with the performance of PEC CO₂ reduction for the three films. Figure 8(b) shows the electrochemical impedance spectroscopy (EIS) of TiO₂, WO₃/TiO₂ and WO_3-x/TiO₂ films at the applied potential of 0.8 V (vs Ag/AgCl) under illumination conditions. The EIS analysis is a very useful tool to study the electron transport property of film photoelectrode. In EIS Nyquist plot, a smaller circular radius means lower charge transfer resistance in the electrode [32-34]. As shown in Fig. 8(b), the EIS Nyquist plot of WO_3-x/TiO₂ film exhibits the smallest circular radius among the three films, suggesting the lowest charge transfer resistance in WO_3-x/TiO₂ film. This may be a probable explanation why the WO_3-x/TiO₂ film exhibits a better performance of PEC CO₂ reduction.

Fig. 8 Photocurrent density (a) and EIS (b) plots of TiO₂, WO₃/TiO₂ and WO_3-x/TiO₂ films

4 Conclusions

1) A novel WO_3-x/TiO₂ photoanode was fabricated by a hydrogen reduction method and low temperature doctor blade technique.

2) After 3 h of photoelectrocatalytic reduction, the formic acid yield of the WO_3-x/TiO₂ film is 872 nmol/cm², which is 1.83 times that of the WO₃/TiO₂ film.

3) The introduction of WO_3-x nanoparticles reduces the charge transfer resistance, resulting in an improved performance of PEC CO₂ reduction for the WO_3-x/TiO₂ film.

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新型WO_3-x/TiO₂薄膜光阳极光电催化还原CO₂制备甲酸

杨亚辉¹，解人瑞¹，黎 航¹，刘灿军²，刘文华²，占发琦²

1. 湖南农业大学 资源与环境学院，长沙 410128；

2. 中南大学 化学化工学院，长沙 410083

摘  要：采用刮涂法制备一种新型光阳极WO_3-x/TiO₂薄膜，并对其进行光电催化还原CO₂制备甲酸。运用X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)对光阳极薄膜进行表征。通过XPS确认WO_3-x中存在氧空位，并通过电位滴定法精确测定WO_3-x中的氧指数。光电催化还原CO₂ 3 h后，WO_3-x/TiO₂薄膜光阳极的甲酸产量为872 nmol/cm²，是WO₃/TiO₂薄膜光阳极的1.83倍。光电化学测试表明，由于氧空位的存在提高材料电荷传输性能，从而提高光电催化还原CO₂活性，故WO_3-x/TiO₂薄膜光阳极相对WO₃/TiO₂具有更好的光电催化还原活性。

关键词：光电催化还原；CO₂；甲酸；WO_3-x；TiO₂；薄膜光阳极

(Edited by Sai-qian YUAN)

Foundation item: Project (21471054) supported by the National Natural Science Foundation of China

Corresponding author: Ya-hui YANG; Tel: +86-731-84617670; E-mail: yangyahui2002@sina.com

DOI: 10.1016/S1003-6326(16)64376-5