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Hydrogen generation via photoelectrocatalytic water splitting using a tungsten trioxide catalyst under visible light irradiation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 8 e9 1 5
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Hydrogen generation via photoelectrocatalytic water splitting using a tungsten trioxide catalyst under visible light irradiation Wei Zhao, Zhiyong Wang, Xiaoyan Shen, Jinlai Li, Chunbao Xu, Zhongxue Gan* State Key Laboratory of Coal-based Low-carbon Energy, ENN Group Co., Ltd., Langfang, Hebei 065001, PR China
article info
abstract
Article history:
In this paper, a novel approach for photoelectron-catalytic hydrogen production under
Received 17 December 2010
visible light irradiation using tungsten oxide as photocatalyst is studied. A tungsten oxide
Received in revised form
thin film with an average thickness of 50 mm was successfully fabricated on a tungsten foil
22 March 2011
substrate. The film is dense, uniform and robust, which provides benefit in applications
Accepted 24 March 2011
such as catalysis and photoelectrocatalytic water splitting. Hydrogen production experi-
Available online 10 December 2011
ments were carried out both indoor (with a 300 W Xe lamp as the light source) and outdoor. The tests confirmed that the tungsten oxide catalyst showed high photocatalytic perfor-
Keywords:
mance toward water splitting under natural light irradiation. A solar-to-hydrogen effi-
Photoelectrocatalytic
ciency of 3.76% was achieved in this system adding oxalic acid as a sacrificial agent under
Photocatalyst
natural sunlight irradiation.
Water splitting
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Hydrogen evolution
reserved.
Tungsten trioxide
1.
Introduction
The increasing energy demand and growing environmental concerns are the main driving forces for exploring clean and renewable energy sources, to replace the currently used fossil fuels in human society. Among multiple choices, solar energy attracted more and more attention in recent years since it is by far the most abundant clean energy source on this planet, and people employ photovoltaic devices to convert solar radiation directly into electricity [1e5]. Another route of using solar energy is to split water into molecular hydrogen and oxygen, by utilizing solar power and appropriate catalysts via a process named photoelectrolysis, or photoelectrocatalytic water splitting [6e22]. The energy stored in hydrogen generated during the day time can be converted to electricity via
fuel cells during the night time, thus providing a solution to power balancing and efficient energy storage. Irradiation of semiconductor/solution interphases with light in appropriate wavelength ranges leads to water splitting. Normally the photoelectrocatalytic cell consists of an n-type semiconductor as a photoanode, as well as a metal cathode. Under light illumination, electron-hole pairs are generated within the photoanode, and they further move to the external surface. The holes oxidize water molecules to oxygen, while the electrons transport to the cathode via external circuit and reduce protons to hydrogen. This process on a TiO2 catalyst in a photoelectrocatalytic cell has been demonstrated first by Honda and Fujishima in the 1970s [23]. Since then, the library of photoelectrode materials was enriched greatly, including TiO2, WO3, Fe2O3, TaON, BiVO4 etc.
* Corresponding author. Tel.: þ86 316 259 6900; fax: þ86 316 259 6907. E-mail address: [email protected] (Z. Gan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.161
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 8 e9 1 5
[24e28]. Titania dioxide is an excellent photocatalyst since it is cheap, efficient, and stable to chemical and photo erosion. However, the activity of TiO2 falls into ultraviolet (UV) light irradiation range due to its wide band-gap (3.0e3.2 eV), thus limiting its photocatalytic applications in the natural light environment. To decrease the band-gaps of semi-conducting photocatalysts, these materials were later modified by doping with exotic elements, such as carbon, nitrogen, hydrogen, indium and iron [29e33]. Another choice is to utilize semiconductors with shorter band-gaps such as WO3 (2.8 eV) and Fe2O3 (2.0e2.2 eV). Although a-Fe2O3 allows visible light adsorption up to 550e600 nm, the large recombination loss of electron-hole pairs during light irradiation significantly lowers its photoelectrocatalytic efficiency for hydrogen evolution via water splitting [34]. Growing interests in tungsten oxides have sparked extensive research due to their semiconducting and highly porous properties in the last decade. It has also been noticed that tungsten oxides can be potentially useful for electrochemical [35], gas sensing [36], especially photocatalytic [37e40] applications. Cross and Parkin [41] have found that tungsten oxide film exhibits photocatalytic activity for decomposition of stearic acid when exposed to lights of certain wavelengths. It has been confirmed by Luo and Hepel [42] that tungsten oxide film has a greater photocatalytic activity than that of nanoparticulate TiO2 film. WO3 can also be used as a photoelectrode for the hydrogen generation from photocatalytic water splitting under ultraviolet or visible light [43e47]. Therefore we focused on WO3 in this paper primarily due to the excellent chemical/ photochemical stability and good photocatalytic property it possesses. WO3 could be easily synthesized via a facile process. Many people have reported their work on fabricating nanostructured tungsten oxides, i.e. nanowires [48e50], nanorods [51e55], and nanofibers [56,57]. Some of their work has been involved with the fabrication of tungsten oxide nanostructures on tungsten metal substrates [48e50]. For example, Wang et al. [58] have developed a single step route for growing tungsten oxide nanobelt arrays by heating a tungsten sheet with an alternating current power supply in a vacuum-based chamber. Our strategy is to fabricate a photoanode composed of tungsten oxide thin film supported on a metallic tungsten foil. This innovative technique generated a dense, uniform and robust thin film coating on the tungsten substrate, and scaled up the dimension scales to several centimeters. We adopted the photoelectrocatalytic cell design since the applied external potential on the photoanode facilitated the electron-hole pair separation process, thus improving the photoelectrocatalytic water splitting efficiency. The addition of sacrificial agents such as oxalic acid significantly increased both photocurrent and hydrogen evolution rate of the photocatalytic system.
paper, followed by ultrasonic cleaning in de-ionized water, ethanol and acetone in sequence. The foils were finally dried under flowing nitrogen at room temperature. To prepare tungsten oxide thin film, the tungsten foils were calcined at 800 C for 10 min in oxygen atmosphere at 1 atm in a tube furnace equipped with a quartz tube and a gas flow meter. The temperature ramping rate was set at 10 C min1, and the oxygen flowing rate was kept constant at 60 cm3 min1. After calcination, the sample was cooled down to room temperature naturally.
2.2.
Material and methods
2.1.
Preparation of tungsten oxide catalyst
Tungsten foils with the thickness of 0.1 mm were purchased from Beijing Cuibolin Non-Ferrous Technology Developing Co., Ltd. The as-received foils were polished with 600 mesh sand
Materials characterization
The morphology of surface and cross-section of the tungsten oxide thin film was observed with a Hitachi S-4300 electron microscope (SEM). The crystallinity of the prepared sample determined by powder X-ray diffraction (XRD) using a diffractometer with Cu Ka line radiation (Rigaku D/max 2500) at 40 kV and 40 mA. The XRD patterns were collected at 2q angles of 20e80 at a scan rate of 1 min1. Optical absorbance measurements were carried out by diffuse reflectance spectroscopy (DRS) using a UVevis spectrophotometer (Hitachi U-3010 with a 150 mm integrating sphere), in a wavelength range of 350e700 nm. X-ray photoelectron spectroscopy (XPS) in version ESCA Lab 220I-XL (VG Scientific) was used to characterize the chemical state and electronic state of the elements that exist in the prepare tungsten oxide film. A survey scan provided information of elemental species while high-resolution scans revealed specific electron states of different elements.
2.3.
Photoelectrochemical measurement
Photocurrent density was measured on an electrochemical station (CorrTest CS Electrochemical Workstation) in a conventional three-electrode configuration, with the tungsten oxide film as the photoanode, a platinum foil as the counter electrode and an Ag/AgCl electrode as the reference electrode. The electrolyte solution is 0.5 M H2SO4 dissolved in de-ionized water. A 300 W Xe lamp (Perfect Light, model number PLS-SXE300) equipped with a UV filter (l 420 nm) was used as the light source. The incident light intensity was fixed at 0.1 W cm2 for all measurements. Hydrogen evolution experiment was conducted on a closed gas circulation system. The gas evolved was determined with TCD gas chromatograph (Agilent 6820) equipped with carbon molecular sieve column (TDX-01). Outdoor hydrogen evolution test was carried out in a sealed quartz reactor under natural light irradiation between 11 am and 2 pm at Langfang (Hebei Province, China; 116.74 E, 39.60 N). The sunlight irradiation density is 0.06 W cm2.
3. 2.
909
Results and discussion
3.1. Morphological characterization of tungsten oxide film catalyst We scaled up the photocatalyst preparation by adopting tungsten foils with larger areas. The dimension of the assynthesized sample was 4.0 8.0 cm2, with a thickness of
910
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0.208 0.006 mm. The gray piece was tungsten metal foil, as received from the provider (Fig. 1, left). The yellowish green piece was the oxidized foil (Fig. 1, right). The as-prepared film was dense, uniform and robust. No crack could be observed with naked eye. The morphology of the tungsten oxide thin film was observed under SEM. The top surface (Fig. 2A) of the film consisted of agglomerated nanoparticles, with diameters in the range of 90e160 nm. The neck sizes between adjacent nanoparticles varied, which indicated a random coarsening effect among different areas. The tungsten oxide thin film contacted closely with the tungsten substrate, as revealed from the cross-section image of the sample (Fig. 2B). After calcination, tungsten oxide films were generated on both sides of the tungsten foil. One side of the tungsten oxide films was intentionally removed for a better electronic contact before SEM measurement. In addition, exfoliation of tungsten oxide film was observed in some areas, which might be caused by the applied forces during sample preparation (Fig. 2C). The thickness of the tungsten oxide thin film was ca. 56 mm, as measured directly from Fig. 2C. Interestingly, high-resolution images showed bundles of pillar-shaped structures inside the tungsten oxide thin film (Fig. 2D), although it was not reflected on the top-view image (Fig. 2A). These rods with diameters of ca. 510 nm formed a more continuous framework compared with the pure aggregated nanoparticle films, thus facilitating electron transfer along the direction from electrolyteeelectrode interface to current collector.
3.2.
Analysis of crystalline structure of the catalyst
To identify the crystalline structure of the as-synthesized tungsten oxide film, XRD pattern was measured for the sample oxidized at 800 C. As shown in Fig. 3, the existence of a pure tungsten trioxide phase with an orthorhombic structure (PDF#20-1324) was confirmed, as revealed by the perfect match of the patterns from the sample and the standard (red bars). Weak and broad peaks from the above XRD pattern may be the result of the low film thickness and/or small nanoparticle size. The particle size of tungsten trioxide was ca. 31 nm, as estimated from the Scherrer equation [59,60]. This value is significantly less than the measured particle size on SEM (Fig. 2A), which suggested the agglomeration of primary particles.
3.3.
UVevis spectrum
To efficiently utilize the solar spectrum, a photocatalyst material should absorb as much light in the visible range as possible. The absorption spectrum of tungsten oxide film sample in the visible-light regime was displayed in Fig. 4. The band edge of the absorption spectra for tungsten oxide was approximated to 490 nm, which correspond to the bandgap energy of ca. 2.54 eV. This value is consistent with the previously reported characteristic values measured for tungsten trioxide materials, which range from 2.4 to 2.8 eV [61].
3.4.
XPS analysis
XPS spectra were presented in Fig. 5. A low-resolution scan was performed to collect the information of elemental composition on sample surface (Fig. 5A). The C 1s peak centered at 284.8 eV and the deconvolution analysis of this peak revealed CeO and C]O bonding, which suggested the existence of organic hydrocarbons (Fig. 5B). The O 1s line positioned at 530.5 eV is due to oxides networks (Fig. 5C). Deconvolution results exhibited another peak centered at 532.2 eV, which was attributed either to oxygen dissolved in the metal or to surface absorbed oxygen [62]. The tungsten signal (Fig. 5D) split into two peaks centered at 35.7 eV and 37.8 eV, corresponding to a doublet composed of 4f5/2 and 4f7/2, which indicated a W6þ oxidation state from WO3. This result was consistent with the XRD analysis.
3.5. Photoelectrocatalytic response under visible light irradiation Fig. 6 shows the photocurrent and dark current plots of the assynthesized tungsten oxide film in a 0.5 M H2SO4 aqueous solution. A potential ranging from 0 V to 1.5 V was applied to the tungsten oxide film photoanode with reference to an Ag/AgCl reference electrode. A significant difference in terms of photocurrent density was observed between the dark and illuminated signal, which reflected a good response of the sample toward visible light irradiation. The saturated photocurrent density was found to be 0.85 mA cm2. Small current peaks below 0.2 V were observed for the sample scanned both in dark and light conditions, which was caused by the electrochromic redox response of HxWO3/WO3 [63].
3.6. Effect of applied bias on photoelectrocatalytic performance
Fig. 1 e Photograph of tungsten foil (left) and tungsten oxide film formed after high-temperature oxidation (right).
To find the onset point of the applied bias at which the tungsten oxide thin film started to display a significant hydrogen evolution behavior, a series of hydrogen evolution experiments were carried out at different voltages. During all the experiments, a Xe lamp was used as a light source and its light irradiation intensity was set as 0.1 W cm2. The effective area of the photocatalyst film exposed to light was fixed at 9 cm2 with the assistance of a mask. Within a period of 4 h in each test, accumulated hydrogen yield was recorded and plotted versus reaction time (Fig. 7A). To attain a more straightforward result for the comparison of hydrogen generation ability of the catalyst at different voltages, hydrogen evolution rates were
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Fig. 2 e SEM images of tungsten oxide thin films supported on tungsten foil: (A) top view (B) tilted sample (C) cross-section, the measurement is for the tungsten oxide thin film (D) high-resolution observation of C.
calculated from the slope of each line derived from leastsquare linear fitting technique. The highest value of the hydrogen evolution rate was 0.13 mmol h1 which was occurred at a bias of 1.2 V. Researchers have reached consensus that the solar-to-hydrogen efficiency was adopted as a more universal parameter to compare different catalysts or same catalyst in different reaction conditions during a photocatalytic hydrogen evolution experiment. Although a couple of theories and formula were proposed [24,29,64,65], the authors believed that the efficiency calculated directly from the amount of hydrogen evolved, the electric power consumed and the light energy fed into the photocatalytic system was a more suitable logic to better describe the phototo-hydrogen efficiency.
3.7.
Stability evaluation of the catalyst
Fig. 3 e XRD pattern of as-synthesized tungsten oxide thin film.
Fig. 4 e Diffuse reflectance UVevis absorption spectra (DRS) of as-synthesized tungsten oxide film.
To evaluate the stability of the tungsten oxide film photocatalyst, hydrogen evolution experiment was performed in a prolonged time and cycled fashion (Fig. 8). The photoelectrocatalytic decomposition of water was carried out in a Pyrex glass reactor which was irradiated at room temperature with visible light from a 300 W Xe lamp. A closed gas circulation and evacuation system was used to avoid contamination from air. The generated gas was directly transferred from the reaction cell into a gas chromatograph to analyze the hydrogen content. Within 6 h in each cycle, the plot of accumulated hydrogen yielded vs. reaction time exhibited a straight line, which indicated a stable water
912
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Fig. 5 e XPS surface spectra of as-synthesized tungsten oxide thin film: (A) survey scan (B) C 1s peak (C) O 1s peak (D) W 4f peak.
splitting process. The hydrogen production rates in the 1st cycle and 7th cycle were 0.087 and 0.143 mmol h1 respectively, as calculated from the slops of the corresponding lines. The hydrogen yield rates increased with prolonged cycling tests. Hydrogen production performance of the catalyst stabilized after the first cycle, after which the hydrogen evolution rate stabilized at 0.126 0.012 mmol h1.
3.8.
Fig. 6 e CurrenteVoltage characteristics of tungsten oxide film in 0.5 M H2SO4 aqueous solution under dark and 0.1 W cmL2 Xe light irradiation. The scan rate was 5 mV sL1.
3.9.
Role of additives on hydrogen evolution
Addition of organic species into the sulfuric acid solution significantly improved hydrogen evolution reaction. Compounds such as methanol, formic acid and oxalic acid were utilized. As seen in the accumulated hydrogen yield curves (Fig. 9A), all tests showed straight lines, suggesting stable reaction behavior during the experiment period. To demonstrate the hydrogen evolution ability of the catalyst in different organic solutions more clearly, the original data were converted to hydrogen evolution rate by calculating the slope of each line. The hydrogen evolution rate values were also plotted accordingly in a rod-shape table (Fig. 9B). From this rod-shape table, one could observe that all systems showed hydrogen production rates higher than the value from pure sulfuric acid solution, especially for organic acids. The oxalic acid system exhibited the highest hydrogen production rate, which was 8.4 times as the sulfuric acid solution. We propose that the enhanced activity is due to the sacrifice agent oxidation process with excited holes happened on anode which leads to efficient charge separation and effectively suppresses the recombination between electrons and holes [66,67].
Hydrogen evolution under natural sunlight
Hydrogen evolution ability of the tungsten oxide film via water splitting was also evaluated under a natural sunshine
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913
Fig. 7 e Hydrogen evolution tests at different applied voltages using a Xe lamp as a light source.
condition (Fig. 10). All experiment conditions were kept consistent except that the exposed catalyst film was positioned normal to the sunlight as accurate as possible by frequently adjusting the reactor. The sun irradiation intensity was to be 0.06 W cm2, as measured using a silicon solar cell photometer. The experimental result was demonstrated by
Fig. 8 e Hydrogen evolution test with Xe lamp as light source. The experiment was repeated for many cycles to evaluate the stability of the photocatalyst. Fresh 0.5 M H2SO4 solution was replaced after each cycle. The diagram here is a plot of accumulated hydrogen yield vs. reaction time.
Fig. 9 e Hydrogen evolution experiments for the tungsten oxide photoanode in aqueous solutions containing 0.5 M H2SO4 and sacrificial agents: methanol, formic acid and oxalic acid. The accumulated hydrogen evolution amounts were monitored with reaction duration (A), and the hydrogen evolution rates for different compounds were normalized with the rate for pure sulfuric acid set as 1 (B).
plotting accumulated amount (mmol) of generated hydrogen gas versus reaction time (h). Calculated from the slope of the fitted curve, the hydrogen evolution rate using 0.5 M H2SO4 solution was 0.131 mmol h1 under sunlight irradiation during the outdoor test (Fig. 10, circles). The hydrogen evolution rate from the indoor test using a Xe lamp as a light source was 0.126 mmol h1. Therefore, it was found that the hydrogen evolution rate increased slightly after the catalyst system was exposed to sunlight irradiation. This value is higher than that obtained in an indoor test. One reason the authors believed that could explain this discrepancy was that the light sources to initiate the water splitting reaction were different: Xe-lamp indoor but natural sunshine outdoor. The UV cut filter removed most light irradiation with the wavelengths less than 420 nm. On the contrary, the stronger UV light in the natural sunshine, especially during summer, contributed significantly to the electron-hole pair generation process, which further increased the solar-to-hydrogen efficiency. The other reason is the increased temperature caused by continuous irradiation
914
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0.68 mmol h1, which was about 5.2 times as the experiment conducted in 0.5 M H2SO4 solution. Based on the discussion in Section 3.6, we adopted the following formula for this calculation of solar-to-hydrogen efficiency h¼
Fig. 10 e Outdoor photoelectrocatalytic hydrogen evolution test under natural sun irradiation. The solutions used were 0.5 M H2SO4 (circles) and a mixture of 0.5 M H2SO4 and 8 wt % oxalic acid (solid spheres).
of sunlight during the test. The dependence of the reaction rate on temperature is formulated by Arrhenius equation k ¼ k0 exp
Ea RT
where k is the reaction rate of the chemical reaction, k0 is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, and T is the absolute temperature. According to the Arrhenius equation, an increase of temperature can result in an increase of the rate of reaction. An increased temperature can also lead to further dissociation of water þ
H2O ¼ H þ OH .Kw where Kw is the ionic product of concentration of Hþ and OH, Kw ¼ cHþ * cOH. Kw is a constant which increases with the increase of temperature. In our system, concentration of Hþ was roughly constant (z0.5 mol/L). Since Kw increases with the increase of temperature, the concentration of OH increases with the increase of temperature, too. An increase in OH concentration could be another factor which contributes to the increased hydrogen evolution rate. In order to test the effect of temperature on hydrogen evolution rate, a series of experiments at different temperatures were carried on. As is shown in Table 1, the hydrogen yield rate showed a clear trend of increasing with the increase of temperature, which verified the postulation very well. The hydrogen evolution ability of the tungsten oxide film was also tested in a 0.5 M H2SO4 solution containing 8 wt% oxalic acid (Fig. 10, solid spheres) with an applied bias of 1.2 V. The average hydrogen evolution rate was calculated to be
Table 1 e Hydrogen yield rates in 0.5 M H2SO4 solution at different temperatures. Temperature ( C) Hydrogen yield rate (mmol/h)
20 0.108
35 0.162
50 0.201
65 0.257
DG$RH2 I$V Pr $A
Here h is the solar-to-hydrogen efficiency after eliminating the contribution from the electricity. DG is the Gibbs free energy of the water splitting reaction. RH2 is the rate of hydrogen evolution reaction, as calculated from the plot of accumulated hydrogen yield versus reaction time. I is the electric current during the reaction as monitored by the electrochemical workstation. V is the bias applied across the photocathode and the anode in a two-electrode configuration. Pr is the light irradiation intensity per unit area, either from a Xe lamp or the natural sunshine. A is the efficient area of the catalyst exposed normally to the light irradiation. Since other parameters were fixed during the test, the solar-tohydrogen conversion efficiency thus obtained was 3.76% as calculated from the formula proposed above. Close examination of the trend of the hydrogen evolution from the oxalic acid solution showed a slope increase, which suggested a raise of hydrogen evolution rate. Based on the above-mentioned discussion, we can arrive at a conclusion that the raise of hydrogen evolution rate could also be attributed to the increased temperature of the reactor caused by the continuous irradiation of sunlight during the test.
4.
Conclusions
A tungsten trioxide thin film was prepared by hightemperature oxidation method in an oxygen atmosphere. The as-synthesized sample showed a bundle-of-pillar structure with a nanometer dimension, thus rendered high photocatalytic water splitting capability to the film. The excellent photoresponse of the catalyst was confirmed by photocurrent measurements and hydrogen evolution experiments. After adding oxalic acid as a sacrificial agent, a solar-to-hydrogen efficiency of 3.76% was achieved in this catalyst system under natural sunlight irradiation.
Acknowledgements This work was supported by grant from ENN Research & Development Co. Ltd.. The authors also thank Dr. Hongwei Ji for the assistance of sample characterization.
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Hydrogen generation via photoelectrocatalytic water splitting using a tungsten trioxide catalyst under visible light irradiation Wei Zhao, Zhiyong Wang, Xiaoyan Shen, Jinlai Li, Chunbao Xu, Zhongxue Gan* State Key Laboratory of Coal-based Low-carbon Energy, ENN Group Co., Ltd., Langfang, Hebei 065001, PR China
article info
abstract
Article history:
In this paper, a novel approach for photoelectron-catalytic hydrogen production under
Received 17 December 2010
visible light irradiation using tungsten oxide as photocatalyst is studied. A tungsten oxide
Received in revised form
thin film with an average thickness of 50 mm was successfully fabricated on a tungsten foil
22 March 2011
substrate. The film is dense, uniform and robust, which provides benefit in applications
Accepted 24 March 2011
such as catalysis and photoelectrocatalytic water splitting. Hydrogen production experi-
Available online 10 December 2011
ments were carried out both indoor (with a 300 W Xe lamp as the light source) and outdoor. The tests confirmed that the tungsten oxide catalyst showed high photocatalytic perfor-
Keywords:
mance toward water splitting under natural light irradiation. A solar-to-hydrogen effi-
Photoelectrocatalytic
ciency of 3.76% was achieved in this system adding oxalic acid as a sacrificial agent under
Photocatalyst
natural sunlight irradiation.
Water splitting
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Hydrogen evolution
reserved.
Tungsten trioxide
1.
Introduction
The increasing energy demand and growing environmental concerns are the main driving forces for exploring clean and renewable energy sources, to replace the currently used fossil fuels in human society. Among multiple choices, solar energy attracted more and more attention in recent years since it is by far the most abundant clean energy source on this planet, and people employ photovoltaic devices to convert solar radiation directly into electricity [1e5]. Another route of using solar energy is to split water into molecular hydrogen and oxygen, by utilizing solar power and appropriate catalysts via a process named photoelectrolysis, or photoelectrocatalytic water splitting [6e22]. The energy stored in hydrogen generated during the day time can be converted to electricity via
fuel cells during the night time, thus providing a solution to power balancing and efficient energy storage. Irradiation of semiconductor/solution interphases with light in appropriate wavelength ranges leads to water splitting. Normally the photoelectrocatalytic cell consists of an n-type semiconductor as a photoanode, as well as a metal cathode. Under light illumination, electron-hole pairs are generated within the photoanode, and they further move to the external surface. The holes oxidize water molecules to oxygen, while the electrons transport to the cathode via external circuit and reduce protons to hydrogen. This process on a TiO2 catalyst in a photoelectrocatalytic cell has been demonstrated first by Honda and Fujishima in the 1970s [23]. Since then, the library of photoelectrode materials was enriched greatly, including TiO2, WO3, Fe2O3, TaON, BiVO4 etc.
* Corresponding author. Tel.: þ86 316 259 6900; fax: þ86 316 259 6907. E-mail address: [email protected] (Z. Gan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.161
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[24e28]. Titania dioxide is an excellent photocatalyst since it is cheap, efficient, and stable to chemical and photo erosion. However, the activity of TiO2 falls into ultraviolet (UV) light irradiation range due to its wide band-gap (3.0e3.2 eV), thus limiting its photocatalytic applications in the natural light environment. To decrease the band-gaps of semi-conducting photocatalysts, these materials were later modified by doping with exotic elements, such as carbon, nitrogen, hydrogen, indium and iron [29e33]. Another choice is to utilize semiconductors with shorter band-gaps such as WO3 (2.8 eV) and Fe2O3 (2.0e2.2 eV). Although a-Fe2O3 allows visible light adsorption up to 550e600 nm, the large recombination loss of electron-hole pairs during light irradiation significantly lowers its photoelectrocatalytic efficiency for hydrogen evolution via water splitting [34]. Growing interests in tungsten oxides have sparked extensive research due to their semiconducting and highly porous properties in the last decade. It has also been noticed that tungsten oxides can be potentially useful for electrochemical [35], gas sensing [36], especially photocatalytic [37e40] applications. Cross and Parkin [41] have found that tungsten oxide film exhibits photocatalytic activity for decomposition of stearic acid when exposed to lights of certain wavelengths. It has been confirmed by Luo and Hepel [42] that tungsten oxide film has a greater photocatalytic activity than that of nanoparticulate TiO2 film. WO3 can also be used as a photoelectrode for the hydrogen generation from photocatalytic water splitting under ultraviolet or visible light [43e47]. Therefore we focused on WO3 in this paper primarily due to the excellent chemical/ photochemical stability and good photocatalytic property it possesses. WO3 could be easily synthesized via a facile process. Many people have reported their work on fabricating nanostructured tungsten oxides, i.e. nanowires [48e50], nanorods [51e55], and nanofibers [56,57]. Some of their work has been involved with the fabrication of tungsten oxide nanostructures on tungsten metal substrates [48e50]. For example, Wang et al. [58] have developed a single step route for growing tungsten oxide nanobelt arrays by heating a tungsten sheet with an alternating current power supply in a vacuum-based chamber. Our strategy is to fabricate a photoanode composed of tungsten oxide thin film supported on a metallic tungsten foil. This innovative technique generated a dense, uniform and robust thin film coating on the tungsten substrate, and scaled up the dimension scales to several centimeters. We adopted the photoelectrocatalytic cell design since the applied external potential on the photoanode facilitated the electron-hole pair separation process, thus improving the photoelectrocatalytic water splitting efficiency. The addition of sacrificial agents such as oxalic acid significantly increased both photocurrent and hydrogen evolution rate of the photocatalytic system.
paper, followed by ultrasonic cleaning in de-ionized water, ethanol and acetone in sequence. The foils were finally dried under flowing nitrogen at room temperature. To prepare tungsten oxide thin film, the tungsten foils were calcined at 800 C for 10 min in oxygen atmosphere at 1 atm in a tube furnace equipped with a quartz tube and a gas flow meter. The temperature ramping rate was set at 10 C min1, and the oxygen flowing rate was kept constant at 60 cm3 min1. After calcination, the sample was cooled down to room temperature naturally.
2.2.
Material and methods
2.1.
Preparation of tungsten oxide catalyst
Tungsten foils with the thickness of 0.1 mm were purchased from Beijing Cuibolin Non-Ferrous Technology Developing Co., Ltd. The as-received foils were polished with 600 mesh sand
Materials characterization
The morphology of surface and cross-section of the tungsten oxide thin film was observed with a Hitachi S-4300 electron microscope (SEM). The crystallinity of the prepared sample determined by powder X-ray diffraction (XRD) using a diffractometer with Cu Ka line radiation (Rigaku D/max 2500) at 40 kV and 40 mA. The XRD patterns were collected at 2q angles of 20e80 at a scan rate of 1 min1. Optical absorbance measurements were carried out by diffuse reflectance spectroscopy (DRS) using a UVevis spectrophotometer (Hitachi U-3010 with a 150 mm integrating sphere), in a wavelength range of 350e700 nm. X-ray photoelectron spectroscopy (XPS) in version ESCA Lab 220I-XL (VG Scientific) was used to characterize the chemical state and electronic state of the elements that exist in the prepare tungsten oxide film. A survey scan provided information of elemental species while high-resolution scans revealed specific electron states of different elements.
2.3.
Photoelectrochemical measurement
Photocurrent density was measured on an electrochemical station (CorrTest CS Electrochemical Workstation) in a conventional three-electrode configuration, with the tungsten oxide film as the photoanode, a platinum foil as the counter electrode and an Ag/AgCl electrode as the reference electrode. The electrolyte solution is 0.5 M H2SO4 dissolved in de-ionized water. A 300 W Xe lamp (Perfect Light, model number PLS-SXE300) equipped with a UV filter (l 420 nm) was used as the light source. The incident light intensity was fixed at 0.1 W cm2 for all measurements. Hydrogen evolution experiment was conducted on a closed gas circulation system. The gas evolved was determined with TCD gas chromatograph (Agilent 6820) equipped with carbon molecular sieve column (TDX-01). Outdoor hydrogen evolution test was carried out in a sealed quartz reactor under natural light irradiation between 11 am and 2 pm at Langfang (Hebei Province, China; 116.74 E, 39.60 N). The sunlight irradiation density is 0.06 W cm2.
3. 2.
909
Results and discussion
3.1. Morphological characterization of tungsten oxide film catalyst We scaled up the photocatalyst preparation by adopting tungsten foils with larger areas. The dimension of the assynthesized sample was 4.0 8.0 cm2, with a thickness of
910
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0.208 0.006 mm. The gray piece was tungsten metal foil, as received from the provider (Fig. 1, left). The yellowish green piece was the oxidized foil (Fig. 1, right). The as-prepared film was dense, uniform and robust. No crack could be observed with naked eye. The morphology of the tungsten oxide thin film was observed under SEM. The top surface (Fig. 2A) of the film consisted of agglomerated nanoparticles, with diameters in the range of 90e160 nm. The neck sizes between adjacent nanoparticles varied, which indicated a random coarsening effect among different areas. The tungsten oxide thin film contacted closely with the tungsten substrate, as revealed from the cross-section image of the sample (Fig. 2B). After calcination, tungsten oxide films were generated on both sides of the tungsten foil. One side of the tungsten oxide films was intentionally removed for a better electronic contact before SEM measurement. In addition, exfoliation of tungsten oxide film was observed in some areas, which might be caused by the applied forces during sample preparation (Fig. 2C). The thickness of the tungsten oxide thin film was ca. 56 mm, as measured directly from Fig. 2C. Interestingly, high-resolution images showed bundles of pillar-shaped structures inside the tungsten oxide thin film (Fig. 2D), although it was not reflected on the top-view image (Fig. 2A). These rods with diameters of ca. 510 nm formed a more continuous framework compared with the pure aggregated nanoparticle films, thus facilitating electron transfer along the direction from electrolyteeelectrode interface to current collector.
3.2.
Analysis of crystalline structure of the catalyst
To identify the crystalline structure of the as-synthesized tungsten oxide film, XRD pattern was measured for the sample oxidized at 800 C. As shown in Fig. 3, the existence of a pure tungsten trioxide phase with an orthorhombic structure (PDF#20-1324) was confirmed, as revealed by the perfect match of the patterns from the sample and the standard (red bars). Weak and broad peaks from the above XRD pattern may be the result of the low film thickness and/or small nanoparticle size. The particle size of tungsten trioxide was ca. 31 nm, as estimated from the Scherrer equation [59,60]. This value is significantly less than the measured particle size on SEM (Fig. 2A), which suggested the agglomeration of primary particles.
3.3.
UVevis spectrum
To efficiently utilize the solar spectrum, a photocatalyst material should absorb as much light in the visible range as possible. The absorption spectrum of tungsten oxide film sample in the visible-light regime was displayed in Fig. 4. The band edge of the absorption spectra for tungsten oxide was approximated to 490 nm, which correspond to the bandgap energy of ca. 2.54 eV. This value is consistent with the previously reported characteristic values measured for tungsten trioxide materials, which range from 2.4 to 2.8 eV [61].
3.4.
XPS analysis
XPS spectra were presented in Fig. 5. A low-resolution scan was performed to collect the information of elemental composition on sample surface (Fig. 5A). The C 1s peak centered at 284.8 eV and the deconvolution analysis of this peak revealed CeO and C]O bonding, which suggested the existence of organic hydrocarbons (Fig. 5B). The O 1s line positioned at 530.5 eV is due to oxides networks (Fig. 5C). Deconvolution results exhibited another peak centered at 532.2 eV, which was attributed either to oxygen dissolved in the metal or to surface absorbed oxygen [62]. The tungsten signal (Fig. 5D) split into two peaks centered at 35.7 eV and 37.8 eV, corresponding to a doublet composed of 4f5/2 and 4f7/2, which indicated a W6þ oxidation state from WO3. This result was consistent with the XRD analysis.
3.5. Photoelectrocatalytic response under visible light irradiation Fig. 6 shows the photocurrent and dark current plots of the assynthesized tungsten oxide film in a 0.5 M H2SO4 aqueous solution. A potential ranging from 0 V to 1.5 V was applied to the tungsten oxide film photoanode with reference to an Ag/AgCl reference electrode. A significant difference in terms of photocurrent density was observed between the dark and illuminated signal, which reflected a good response of the sample toward visible light irradiation. The saturated photocurrent density was found to be 0.85 mA cm2. Small current peaks below 0.2 V were observed for the sample scanned both in dark and light conditions, which was caused by the electrochromic redox response of HxWO3/WO3 [63].
3.6. Effect of applied bias on photoelectrocatalytic performance
Fig. 1 e Photograph of tungsten foil (left) and tungsten oxide film formed after high-temperature oxidation (right).
To find the onset point of the applied bias at which the tungsten oxide thin film started to display a significant hydrogen evolution behavior, a series of hydrogen evolution experiments were carried out at different voltages. During all the experiments, a Xe lamp was used as a light source and its light irradiation intensity was set as 0.1 W cm2. The effective area of the photocatalyst film exposed to light was fixed at 9 cm2 with the assistance of a mask. Within a period of 4 h in each test, accumulated hydrogen yield was recorded and plotted versus reaction time (Fig. 7A). To attain a more straightforward result for the comparison of hydrogen generation ability of the catalyst at different voltages, hydrogen evolution rates were
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911
Fig. 2 e SEM images of tungsten oxide thin films supported on tungsten foil: (A) top view (B) tilted sample (C) cross-section, the measurement is for the tungsten oxide thin film (D) high-resolution observation of C.
calculated from the slope of each line derived from leastsquare linear fitting technique. The highest value of the hydrogen evolution rate was 0.13 mmol h1 which was occurred at a bias of 1.2 V. Researchers have reached consensus that the solar-to-hydrogen efficiency was adopted as a more universal parameter to compare different catalysts or same catalyst in different reaction conditions during a photocatalytic hydrogen evolution experiment. Although a couple of theories and formula were proposed [24,29,64,65], the authors believed that the efficiency calculated directly from the amount of hydrogen evolved, the electric power consumed and the light energy fed into the photocatalytic system was a more suitable logic to better describe the phototo-hydrogen efficiency.
3.7.
Stability evaluation of the catalyst
Fig. 3 e XRD pattern of as-synthesized tungsten oxide thin film.
Fig. 4 e Diffuse reflectance UVevis absorption spectra (DRS) of as-synthesized tungsten oxide film.
To evaluate the stability of the tungsten oxide film photocatalyst, hydrogen evolution experiment was performed in a prolonged time and cycled fashion (Fig. 8). The photoelectrocatalytic decomposition of water was carried out in a Pyrex glass reactor which was irradiated at room temperature with visible light from a 300 W Xe lamp. A closed gas circulation and evacuation system was used to avoid contamination from air. The generated gas was directly transferred from the reaction cell into a gas chromatograph to analyze the hydrogen content. Within 6 h in each cycle, the plot of accumulated hydrogen yielded vs. reaction time exhibited a straight line, which indicated a stable water
912
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Fig. 5 e XPS surface spectra of as-synthesized tungsten oxide thin film: (A) survey scan (B) C 1s peak (C) O 1s peak (D) W 4f peak.
splitting process. The hydrogen production rates in the 1st cycle and 7th cycle were 0.087 and 0.143 mmol h1 respectively, as calculated from the slops of the corresponding lines. The hydrogen yield rates increased with prolonged cycling tests. Hydrogen production performance of the catalyst stabilized after the first cycle, after which the hydrogen evolution rate stabilized at 0.126 0.012 mmol h1.
3.8.
Fig. 6 e CurrenteVoltage characteristics of tungsten oxide film in 0.5 M H2SO4 aqueous solution under dark and 0.1 W cmL2 Xe light irradiation. The scan rate was 5 mV sL1.
3.9.
Role of additives on hydrogen evolution
Addition of organic species into the sulfuric acid solution significantly improved hydrogen evolution reaction. Compounds such as methanol, formic acid and oxalic acid were utilized. As seen in the accumulated hydrogen yield curves (Fig. 9A), all tests showed straight lines, suggesting stable reaction behavior during the experiment period. To demonstrate the hydrogen evolution ability of the catalyst in different organic solutions more clearly, the original data were converted to hydrogen evolution rate by calculating the slope of each line. The hydrogen evolution rate values were also plotted accordingly in a rod-shape table (Fig. 9B). From this rod-shape table, one could observe that all systems showed hydrogen production rates higher than the value from pure sulfuric acid solution, especially for organic acids. The oxalic acid system exhibited the highest hydrogen production rate, which was 8.4 times as the sulfuric acid solution. We propose that the enhanced activity is due to the sacrifice agent oxidation process with excited holes happened on anode which leads to efficient charge separation and effectively suppresses the recombination between electrons and holes [66,67].
Hydrogen evolution under natural sunlight
Hydrogen evolution ability of the tungsten oxide film via water splitting was also evaluated under a natural sunshine
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913
Fig. 7 e Hydrogen evolution tests at different applied voltages using a Xe lamp as a light source.
condition (Fig. 10). All experiment conditions were kept consistent except that the exposed catalyst film was positioned normal to the sunlight as accurate as possible by frequently adjusting the reactor. The sun irradiation intensity was to be 0.06 W cm2, as measured using a silicon solar cell photometer. The experimental result was demonstrated by
Fig. 8 e Hydrogen evolution test with Xe lamp as light source. The experiment was repeated for many cycles to evaluate the stability of the photocatalyst. Fresh 0.5 M H2SO4 solution was replaced after each cycle. The diagram here is a plot of accumulated hydrogen yield vs. reaction time.
Fig. 9 e Hydrogen evolution experiments for the tungsten oxide photoanode in aqueous solutions containing 0.5 M H2SO4 and sacrificial agents: methanol, formic acid and oxalic acid. The accumulated hydrogen evolution amounts were monitored with reaction duration (A), and the hydrogen evolution rates for different compounds were normalized with the rate for pure sulfuric acid set as 1 (B).
plotting accumulated amount (mmol) of generated hydrogen gas versus reaction time (h). Calculated from the slope of the fitted curve, the hydrogen evolution rate using 0.5 M H2SO4 solution was 0.131 mmol h1 under sunlight irradiation during the outdoor test (Fig. 10, circles). The hydrogen evolution rate from the indoor test using a Xe lamp as a light source was 0.126 mmol h1. Therefore, it was found that the hydrogen evolution rate increased slightly after the catalyst system was exposed to sunlight irradiation. This value is higher than that obtained in an indoor test. One reason the authors believed that could explain this discrepancy was that the light sources to initiate the water splitting reaction were different: Xe-lamp indoor but natural sunshine outdoor. The UV cut filter removed most light irradiation with the wavelengths less than 420 nm. On the contrary, the stronger UV light in the natural sunshine, especially during summer, contributed significantly to the electron-hole pair generation process, which further increased the solar-to-hydrogen efficiency. The other reason is the increased temperature caused by continuous irradiation
914
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0.68 mmol h1, which was about 5.2 times as the experiment conducted in 0.5 M H2SO4 solution. Based on the discussion in Section 3.6, we adopted the following formula for this calculation of solar-to-hydrogen efficiency h¼
Fig. 10 e Outdoor photoelectrocatalytic hydrogen evolution test under natural sun irradiation. The solutions used were 0.5 M H2SO4 (circles) and a mixture of 0.5 M H2SO4 and 8 wt % oxalic acid (solid spheres).
of sunlight during the test. The dependence of the reaction rate on temperature is formulated by Arrhenius equation k ¼ k0 exp
Ea RT
where k is the reaction rate of the chemical reaction, k0 is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, and T is the absolute temperature. According to the Arrhenius equation, an increase of temperature can result in an increase of the rate of reaction. An increased temperature can also lead to further dissociation of water þ
H2O ¼ H þ OH .Kw where Kw is the ionic product of concentration of Hþ and OH, Kw ¼ cHþ * cOH. Kw is a constant which increases with the increase of temperature. In our system, concentration of Hþ was roughly constant (z0.5 mol/L). Since Kw increases with the increase of temperature, the concentration of OH increases with the increase of temperature, too. An increase in OH concentration could be another factor which contributes to the increased hydrogen evolution rate. In order to test the effect of temperature on hydrogen evolution rate, a series of experiments at different temperatures were carried on. As is shown in Table 1, the hydrogen yield rate showed a clear trend of increasing with the increase of temperature, which verified the postulation very well. The hydrogen evolution ability of the tungsten oxide film was also tested in a 0.5 M H2SO4 solution containing 8 wt% oxalic acid (Fig. 10, solid spheres) with an applied bias of 1.2 V. The average hydrogen evolution rate was calculated to be
Table 1 e Hydrogen yield rates in 0.5 M H2SO4 solution at different temperatures. Temperature ( C) Hydrogen yield rate (mmol/h)
20 0.108
35 0.162
50 0.201
65 0.257
DG$RH2 I$V Pr $A
Here h is the solar-to-hydrogen efficiency after eliminating the contribution from the electricity. DG is the Gibbs free energy of the water splitting reaction. RH2 is the rate of hydrogen evolution reaction, as calculated from the plot of accumulated hydrogen yield versus reaction time. I is the electric current during the reaction as monitored by the electrochemical workstation. V is the bias applied across the photocathode and the anode in a two-electrode configuration. Pr is the light irradiation intensity per unit area, either from a Xe lamp or the natural sunshine. A is the efficient area of the catalyst exposed normally to the light irradiation. Since other parameters were fixed during the test, the solar-tohydrogen conversion efficiency thus obtained was 3.76% as calculated from the formula proposed above. Close examination of the trend of the hydrogen evolution from the oxalic acid solution showed a slope increase, which suggested a raise of hydrogen evolution rate. Based on the above-mentioned discussion, we can arrive at a conclusion that the raise of hydrogen evolution rate could also be attributed to the increased temperature of the reactor caused by the continuous irradiation of sunlight during the test.
4.
Conclusions
A tungsten trioxide thin film was prepared by hightemperature oxidation method in an oxygen atmosphere. The as-synthesized sample showed a bundle-of-pillar structure with a nanometer dimension, thus rendered high photocatalytic water splitting capability to the film. The excellent photoresponse of the catalyst was confirmed by photocurrent measurements and hydrogen evolution experiments. After adding oxalic acid as a sacrificial agent, a solar-to-hydrogen efficiency of 3.76% was achieved in this catalyst system under natural sunlight irradiation.
Acknowledgements This work was supported by grant from ENN Research & Development Co. Ltd.. The authors also thank Dr. Hongwei Ji for the assistance of sample characterization.
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