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International Journal of Photoenergy
Volume 2008, Article ID 173943, 7 pages
http://dx.doi.org/10.1155/2008/173943
Research Article

Photocatalytic Hydrogen or Oxygen Evolution from Water over S- or N-Doped under Visible Light

Department of Material Science, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Fukuoka 804-8550, Kitakyushu-shi, Japan

Received 20 March 2007; Accepted 22 May 2007

Academic Editor: M. Sabry A. Abdel-Mottaleb

Copyright © 2008 Kazumoto Nishijima et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

S- or N-doping of powder having an anatase or rutile phase extended the photocatalytic activity for water oxidation and reduction under UV light and visible light irradiation. For the reduction of water, anatase-doped showed higher level of activity than that of doped having a rutile phase using ethanol as an electron donor. Furthermore, the activity level of S-doped for hydrogen evolution was higher than that of N-doped photocatalysts under visible light. Photocatalytic oxidation of water on doped having a rutile phase proceeded with fairly high efficiency when ions were used as electron acceptors compared to that on doped having an anatase phase. In addition, water splitting under visible light irradiation was achieved by construction of a Z-scheme photocatalysis system employing the doped having anatase and rutile phases for and evolution and the redox couple as an electron relay.

1. Introduction

Photocatalytic reactions on the surfaces of particles have been attracting much attention because of their possible application to the conversion of solar energy into chemical energy (water splitting) [1, 2] and to water purification from pollutants [35]. is chemically stable and nontoxic and it has a high oxidation power under photoirradiation. However, a high oxidation power of is only shown under UV light, which is a great disadvantage for practical application. Therefore, the development of a photocatalyst that shows a high level of activity under visible light is needed.

N, S, or C anion-doped photocatalysts having an anatase phase that shows a relatively high level of activity under visible light irradiation have been reported [69]. Sakthivel and Kisch succeeded in preparing photocatalysts containing some carbon species that show photocatalytic activity under visible light [10]. We have also reported the preparation of S cation-doped having anatase and rutile phases and its photocatalytic activity for oxidation of organic compounds [11, 12]. The absorbance of S cation-doped in the visible region is larger than that of N, C, or S anion-doped , and the photocatalytic activity level of S cation-doped for oxidation of organic compounds is relatively high under visible light compared to that of N, C, or S anion-doped . It should be noted that S atoms were incorporated into the lattice of as cations and were thought to be replaced by Ti ions in the case of S cation-doped .

Among the chemical reactions suitable for energy conversion is the decomposition of water, because the change in Gibbs energy is large. Hence, there have been many attempts to achieve photodecomposition of water using semiconductor particles such as [13], [14], and [15].

Abe et al. developed a water-splitting system by combining two photocatalytic systems including oxidation of water and reduction of water systems under visible light [16]. However, the efficiency of each system (hydrogen-evolving and oxygen-evolving systems) is not sufficient for application to a practical system. The aim of our study was to establish effective oxygen- and hydrogen-evolving systems. In this paper, we report the photocatalytic activities of S-doped or N-doped for reduction of water into molecular hydrogen and oxidation of water into molecular oxygen under visible light using ethanol and ions as electron donors and electron acceptors, respectively.

2. Experimental

2.1. Materials and Instruments

Various kinds of powders having anatase and rutile crystal structures were obtained from the Catalysis Society of Japan (TIO-3), Ishihara Sangyo (ST-01), TAYCA Corporation (MT-150A), and Toho Titaniumu Co. (NS-51). The contents of anatase and the surface areas of these powders were as follows: ST-01: 100%, 320.5/g; MT-150A: 0%, 81.5/g; TIO-3: 0%, 48.1/g; NS-51: 1.0%, 6.5/g. Ethanol, , thiourea, and urea were obtained from Wako Pure Chemical Industry. Other chemicals were obtained from commercial sources as guaranteed reagents and were used without further purification. The crystal structures of powders were determined from X-ray diffraction (XRD) patterns measured with an X-ray diffractometer (Philips, X'Pert-MRD) with a Cu target K-ray (). The relative surface areas of the powders were determined by adsorption/desorption isotherm analysis (BET method). The measurements were performed using Micromeritics FlowSorb II 2300. The absorption and diffuse reflection spectra were measured using a Shimadzu UV-2500PC spectrophotometer. X-ray photoelectron spectra (XPS) of the powders were measured using a JEOL JPS90SX photoelectron spectrometer with an Mg K source (1253.6?eV). The shift of binding energy due to relative surface charging was corrected using the C 1s level at 284.6?eV as an internal standard. The XPS peaks were assumed to have Gaussian line shapes and were resolved into components by a nonlinear least-squares procedure after proper subtraction of the baseline.

2.2. Preparation of S-or N-Doped Powders

S- or N-doped powders were synthesized by previously reported methods [11, 12, 1719].

S- or N-doped powders having a rutile or anatase phase were prepared as follows. Thiourea (for S doping) or urea (for N doping) was mixed with various kinds of powders having an anatase or rutile phase in an agate mortar. The mixed powder was packed in an alumina crucible and calcined at under air atmosphere for 3 hours. After calcination, the powder was washed with distilled water. The resulting samples were yellow in color and were found to have a homogenous anatase or rutile phase from XRD analysis. For powder calcined at a temperature higher than , no absorbance in the visible region was observed. The surface areas of S-doped and N-doped powders having an anatase phase (ST-01) calcined at were 120/g and 90/g, respectively. The surface areas of S-doped powders having a rutile phase (MT-150A, TIO-3, and NS-51) calcined at were 52.5/g, 32.2/g, and 5.4/g, respectively. The surface areas of N-doped powders having a rutile phase (TIO-3 and NS-51) calcined at were 26.3/g and 4.9/g, respectively. The relative surface area decreased with increase in the calcination temperature.

2.3. Preparation of Pt-Loaded Photocatalysts for Hydrogen Evolution

Pt-loaded powders were prepared by a photochemical deposition method. The doped photocatalyst was stirred in a 5% ethanol aqueous solution containing and irradiated by a 500?W high-pressure Hg lamp (2?mW/) for 12 hours. Photoreduction of took place, and highly dispersed Pt metal particles were deposited on the surface of the photocatalyst. After filtrating and washing by deionized water, the powder was dried at for 2 hours under reduced pressure to remove the ethanol adsorbed on the surface of the photocatalyst.

2.4. Photocatalytic Reaction

The photocatalytic reactions were carried out using a closed gas-circulating system (Figure 1). The reaction under a wide range of incident light including UV and visible light was performed using a quartz glass-made outer irradiation type reactor. Irradiation to the aqueous suspension of a catalyst was carried out from outside the reactor using a 500?W xenon lamp (USHIO Co. Ltd., SXUI-501XQ) for evolution and evolution. To limit the irradiation wavelength, the light beam was passed through a UV-35, L-39, or L-42 filter (Kenko Co.) to cut off wavelengths shorter than 350, 390, or 420?nm, respectively. The irradiance of irradiation light was adjusted to 340?mW/. The photocatalyst powder (0.3?g) was suspended in distilled water using a magnetic stirrer. S- or N-doped having anatase phase (ST-01) was used for hydrogen evolution, and S- or N-doped having rutile phase (NS-51) was used for oxygen evolution. The required amount of sacrificial reagent ethanol for hydrogen evolution and for oxygen evolution was added to the suspension. The pH of the solution containing ions was adjusted to 2.4 using 1?M HCl aqueous solution in order to prevent formation of .

173943.fig.001
Figure 1: Schematic diagram of the closed gas circulating system.

In the case of water splitting in the two-step redox system, a 500?W high-pressure mercury lamp (USHIO Co. Ltd., SX-U1501HQ) was used as a light source. The Hg lamp has emission line spectrum with peaks at 314, 365, 405, and 436?nm. To limit the irradiation wavelength, the light beam was passed through a UV-35 or L-39 filter (Kenko Co.) to cut off wavelengths shorter than 350 or 390?nm, respectively. A mixture (0.3?g) of S-doped powder having anatase phase (ST-01) and S-doped powder rutile phase (NS-51) which weight ratio was was suspended in distilled water. The required amount of solute, such as NaI, was added to the suspension. ions were used as a redox couple [16]. The pH of the solutions was adjusted to 9 using 1?M NaOH aqueous solution in order to prevent generation of ions [16].

Finally, the suspension was thoroughly degassed and then argon gas (35?Torr) was introduced into the system and the system was exposed to irradiation. Evolved or gases were analyzed by online gas chromatography (TCD, molecular sieve 5?A).

3. Results and Discussions

3.1. Physical Properties of S-Doped and N-Doped

Diffuse reflectance spectra for the obtained S-doped and N-doped are shown in Figure 2. All doped powders showed absorption in the visible region. Photoabsorption in the visible region of the doped having an anatase phase was stronger than that of the doped having a rutile phase. The having a rutile phase was hardly doped by S atoms or N atoms because the rutile phase has a smaller surface area and more stable crystal structure than those of the anatase phase. The absorption band edge of S-doped shifted to a longer wavelength than that of N-doped . The band gap energies of S-doped and N-doped can be estimated from the tangent lines in the plot of the square root of the Kubelka-Munk functions against the photon energy [20], as shown in the insert to Figure 2. The band gap energies were estimated to be 2.28?eV (S-doped having an anatase phase), 2.58?eV (N-doped having an anatase phase), 2.34?eV (S-doped having a rutile phase), and 2.94?eV (N-doped having a rutile phase).

fig2
Figure 2: Diffuse reflectance spectra for the obtained S-doped and N-doped having an anatase or rutile phase. (a) having an anatase phase (ST-01), (b) having a rutile phase (NS-51).

The chemical states of S atoms or N atoms incorporated into were studied by measuring the XPS spectra of the S-doped or N-doped . Figure 3(a) shows the spectrum of S-doped . As shown in Figure 3(a), a strong peak around 168?eV was observed. This peak is thought to consist of several oxidation states of S atoms such as and states [11, 12]. Figure 3(b) shows the N 1s spectrum of N-doped . Two peaks around 396?eV and 399?eV were observed. The peak around 396?eV was assigned to N atoms forming Ti–N bonds, and the peak around 399?eV was assigned to molecules adsorbed on the surface [17].

fig3
Figure 3: XPS spectra of S-doped and N-doped . (a) S-doped , (b) N-doped .
3.2. Selective Water Oxidation over Doped Photocatalysts in an Aqueous Solution Containing Ions

We investigated photocatalytic reduction of water into or oxidation of water into over various semiconductor photocatalysts in an aqueous solution containing an electron donor or an electron acceptor. In most cases, the reactions readily terminated when the concentration of the product in the solution reached a certain level because of the backward reactions. However, we found that efficient and selective evolution proceeded over photocatalysts having a rutile phase in the aqueous solution () containing ions as electron acceptors with the following reactions [21]:

The optimized concentration of ions was 2?mmol/L. The rate of oxygen evolution was lowered by raising the concentrations of ions owing to a filter effect of the solution at a high concentration [21]. Excess ions decreased rate of evolved oxygen because aqueous solution has absorption in visible light region. Figure 4 shows the amounts of evolved over S- or N-doped having rutile phase (NS-51) and N-doped having anatase phase (ST-01) from water containing ions as electron acceptors. The photocatalytic activity level of S-doped for water oxidation was higher than that of N-doped . However, the amounts of evolved on S- or N-doped did not agree with concentration of ions. The backward reaction, oxidation of ions with holes, was thought to proceed. S-doped showed photoinduced oxidation of water under visible light irradiation at wavelengths longer than 390?nm. Photocatalytic oxidation of water on doped having a rutile phase proceeded with a fairly high efficiency compared to that on doped having an anatase phase as shown in Figure 4. This result suggested that the activity for the evolution of from H2O is dependent on the particle size of doped powders. On anatase particles with a large surface area, decomposition of pollutants in air and water proceeds efficiently [4, 5]. In these reactions, the large surface area is especially important, because the concentrations of the pollutants are usually very low. On the other hand, for splitting water, which is an important reaction to convert light energy into chemical energy, rutile particles with a small surface area are efficient [2123]. In this case, a band bending should be developed in each particle to oxidize water, and hence large particles are advantageous.

173943.fig.004
Figure 4: Amount of evolved from water containing ions as electron acceptors on S-doped having rutile phase (NS-51), N-doped having rutile phase (NS-51), and N-doped having anatase phase (ST-01).
3.3. Selective Water Reduction over Doped Photocatalysts Deposited with Pt in an Aqueous Solution Containing Ethanol

evolution from water containing ethanol as a sacrificial reagent proceeded over platinized having an anatase phase under UV and visible light irradiation. To improve the rate of evolution on doped , Pt was deposited on the surface. The optimized amount of Pt deposited on the surface of was 0.5?wt%. Deposition of an excess amount of Pt (1.0?wt%) decreased photocatalytic activity because aggregated Pt particles on the surface prevented the photocatalyst from absorbing incident light or because surface active sites were covered with Pt particles. The reduction of water containing ethanol is as follows:

Figure 5 shows the amounts of evolved from water containing ethanol over S- or N-doped having anatase phase (ST-01) and S-doped having rutile phase (NS-51) under UV light irradiation. The amount of evolved on doped was smaller than that on pure because impurity level was formed by doping treatment. However, under visible light irradiation at wavelengths longer than 420?nm, the amount of evolved on doped was higher than that on pure as shown in Figure 5(b). S-doped showed a higher level of activity than that of N-doped for water reduction because S-doped showed higher photoabsorption than that of N-doped in the visible region as shown in Figure 2(a). Photocatalytic reduction of water on doped having an anatase phase proceeded with a fairly high efficiency compared to that on doped having a rutile phase as shown in Figure 5. This result suggested that the activity for the evolution of from O is dependent on the surface area of doped powder. Potential of the conduction band of anatase phase is at a more negative position than that of rutile phase. Therefore, the excited electrons easily transfer ions on the surface of having an anatase phase.

fig5
Figure 5: Amount of evolved from water containing ethanol as an electron donor on S- or N-doped (anatase phase: ST-01, rutile phase: NS-51) (a) under UV light irradiation at wavelengths longer than 350?nm or (b) under visible light irradiation at wavelengths longer than 420?nm.
3.4. Water Splitting over Photocatalysts in an Shuttle Redox-Mediated System

Photocatalytic water splitting into and was observed in a two-step redox system using an iodate/iodide redox mediator. In this system, we used Pt-loaded S-doped having an anatase phase (ST-01) for evolution and S-doped having a rutile phase (NS-51) for evolution under incident light including UV and visible light. Figure 6 shows the mechanism of the two-step redox system. On having an anatase phase, water is reduced to by photoexcited electrons and the iodide ions are oxidized to iodate ions by holes. The iodate ions are reduced to generate iodide ions by photoexcited electrons at the same time as water is oxidized to by holes on having a rutile phase. The optimized concentration of ions was 20?mmol/L. The oxidation products of ions were and . An excess concentration of ions resulted in generation. The accumulation of caused a light loss due to the strong absorption of around 350?nm, resulting in a lower efficiency of the photocatalytic reaction [16]. Reactions over a rutile phase are as follows: On the other hand, reactions over an anatase phase are as follows:

173943.fig.006
Figure 6: Mechanism for water splitting on with an redox-mediator system.

Amounts of evolved and in the two-step redox system under incident light including UV and visible light are shown in Figure 7. Under UV light irradiation without a cutoff filter and UV light irradiation at wavelengths longer 350?nm, both and evolved. However, the ratio of amount of evolved to amount of evolved was not . molecules generated from water oxidation were thought to adsorb on the surface [24]. Under visible light irradiation at wavelengths longer than 390?nm, no evolution was observed because the number of holes is not sufficient for generation under low intensity of incident light. Under these circumstances, the intermediates generated by water oxidation were thought to be reduced by photoexcited electrons.

173943.fig.007
Figure 7: Amounts of evolved and from water containing redox on S-doped powders having an anatase phase (ST-01) and a rutile phase (NS-51) under UV or visible light irradiation. (a) under UV light irradiation without a cutoff filter, (b) under UV light irradiation at wavelengths longer than 350?nm, (c) under visible light irradiation at wavelengths longer than 390?nm.

4. Conclusion

evolution from water containing ethanol as an electron donor was performed on N- or S-doped under visible light irradiation. An anatase powder showed a higher level of activity than that of a rutile powder. Oxidation of water containing ions as electron acceptors proceeded on doped under UV and visible light irradiation. Water splitting was performed in an redox system on having an anatase phase for evolution and having a rutile phase for evolution. Under UV irradiation, both and were evolved, though the ratio of amount of evolved to amount of evolved was not .

Acknowledgments

This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Science, and Technology (MEXT), Japan, and Nissan Science Foundation.

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