Photocatalytic Water Splitting for Hydrogen Production with Novel M2YbSbO7 (M = In, Gd, Y) by Using Visible Light Photoenergy

Luan, Jingfei; Pei, Donghua

doi:https://doi.org/10.1155/2012/890865

International Journal of Photoenergy

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Abstract Introduction Experimental Results and Discussion Conclusion Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2012 | Article ID 890865 | https://doi.org/10.1155/2012/890865

Photocatalytic Water Splitting for Hydrogen Production with Novel M₂YbSbO₇ (M = In, Gd, Y) by Using Visible Light Photoenergy

Jingfei Luan¹and Donghua Pei¹

Academic Editor: Vincenzo Augugliaro

Received31 Oct 2011

Accepted19 Dec 2011

Published23 Feb 2012

Abstract

Novel photocatalysts M₂YbSbO₇ (, Gd, Y) were synthesized by solid state reaction method for the first time. A comparative study on the structural and photocatalytic properties of M2YbSbO7 M₂YbSbO₇ (, Gd, Y) was reported. The results showed that In₂YbSbO₇, Gd₂YbSbO₇, and Y₂YbSbO₇ crystallized with the pyrochlore-type structure, cubic crystal system and space group Fd3m. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water with In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ as the photocatalyst under visible light irradiation. ( nm). Moreover, under visible light irradiation ( nm), H₂ and O₂ were also evolved by using In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ as catalyst from CH₃OH/H₂O and AgNO₃/H₂O solutions respectively. The In₂YbSbO₇ photocatalyst showed the highest activity compared with Gd₂YbSbO₇ or Y₂YbSbO₇. At the same time, The Y₂YbSbO₇ photocatalyst showed higher activity compared with Gd₂YbSbO₇. The photocatalytic activities were further improved under visible light irradiation with In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ being loaded by Pt, NiO, or RuO₂. The effect of Pt was better than that of NiO or RuO₂ for improving the photocatalytic activity of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇.

1. Introduction

Since water splitting catalyzed by TiO₂ was discovered in 1972 [1], photocatalysis has attracted far-ranging attention from both academic and industrial organizations [2–6]. In particular, water splitting by the photocatalytic approach has been considered as a highly promising process to obtain clean and renewable H₂ source [5–13]. Presently, TiO₂ was known as the most common photocatalyst for water splitting. However, TiO₂ could only split water under ultraviolet light irradiation which only occupy less than 5% of the whole sunlight. Moreover, ultraviolet light only occupied 4% of sunlight, which was a restrained factor for photocatalysis technology with TiO₂ as catalyst. Therefore, some efficient catalysts which could generate electron-hole pairs under visible light irradiation should be developed because visible light occupied 43% of sunlight.

Fortunately, A₂B₂O₇ compounds were often considered to own better photocatalytic properties under visible light irradiation [14, 15]. In our previous work [14], we had found that Bi₂GaVO₇ crystallized with the tetragonal crystal system and could split pure water into hydrogen under ultraviolet light irradiation and seemed to have potential for improvement of photocatalytic activity by modification of its structure. According to above analysis, we could assume that the substitution of Bi³⁺ by In³⁺, Gd³⁺ or Y³⁺, and the substitution of Ga³⁺ by Yb³⁺, and the substitution of V³⁺ by Sb⁵⁺ in Bi₂GaVO₇ might increase carriers concentration. As a result, a change and improvement of the electrical transportation and photophysical properties could be found in the novel In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ compound which might own advanced photocatalytic properties.

In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ had never been produced and the data about their structural and photophysical properties such as space group and lattice constants had not been found previously. In addition, the photocatalytic properties of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ have not been studied by other investigators. The molecular composition of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was very similar with other A₂B₂O₇ compounds. Thus the resemblance suggested that In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ might possess photocatalytic properties under visible light irradiation, which was similar with those other members in A₂B₂O₇ family. In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ also seemed to have potential for improvement of photocatalytic activity by modification of its structure because it had been proved that a slight modification of a semiconductor structure would result in a remarkable change in photocatalytic properties [16].

In this paper, newly synthesized semiconductor compound In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was used as photocatalyst for split water into hydrogen under visible light irradiation. The structural, photophysical, and photocatalytic properties of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ were investigated in detail

2. Experimental

The novel photocatalysts were synthesized by a solid-state reaction method. Yb₂O₃, In₂O₃, Gd₂O₃, Y₂O₃, and Sb₂O₅ with purity of 99.99% (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) were utilized as starting materials. All powders were dried at 200°C for 4 h before synthesis. In order to synthesize In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇, the precursors were stoichiometrically mixed, then pressed into small columns and put into an alumina crucible (Shenyang Crucible Co., Ltd., China). Finally, calcination was carried out at 1320°C for 65 h in an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., China). The crystal structure of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was analyzed by the powder X-ray diffraction method (D/MAX-RB, Rigaku Corporation, Japan) with CuKα radiation (λ = 1.54056). The data were collected at 295 K with a step-scan procedure in the range of –100°. The step interval was 0.02° and the time per step was 1.2 s. The chemical composition of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was determined by scanning electron microscope-X-ray energy dispersion spectrum (SEM-EDS, LEO 1530VP, LEO Corporation, Germany) and X-ray fluorescence spectrometer (XFS, ARL-9800, ARL Corporation, Switzerland). The optical absorption of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was analyzed with a UV-visible spectrophotometer (Lambda 40, Perkin-Elmer Corporation, USA). The surface area of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was measured by the Brunauer-Emmett-Teller (BET) method (MS-21, Quantachrome Instruments Corporation, USA) with N₂ adsorption at liquid nitrogen temperature.

The photocatalytic water splitting was conducted under visible light irradiation in a gas closed circulation system with an inner-irradiation-type reactor (quartz cell). A light source (300 W Xe arc lamp, Beijing Dongsheng Glass Light Source Factory, China) with the incident photon flux of 0.056176 umol cm⁻² s⁻¹ was focused through a shutter window and a 420 nm cutoff filter onto the window face of the cell. The gases evolved were determined with a TCD gas chromatograph (6890 N, Agilent Technologies, USA), which was connected to the gas closed circulation system. Before reaction, the closed gas circulation system and the reaction cell were degassed until O₂ and N₂ could not be detected. Then about 35 Torr of argon was charged into the system. H₂ evolution reaction was carried out in CH₃OH/H₂O solution (50 mL CH₃OH, 300 mL H₂O) with Pt-, NiO-, or RuO₂-loaded powder (1.0 g) as the catalyst which was suspended in CH₃OH/H₂O solution under stirring.

For H₂ evolution reaction, Pt, NiO, or RuO₂ which was loaded on the surface of the catalysts was prepared. Pt was loaded on the catalyst surface by an in situ photodeposition method by using aqueous H₂PtCl₆ solution (Shanghai Chemical Reagent Research Institute, China) as the Pt source. NiO or RuO₂ which was loaded on the surface of the catalysts were prepared by the impregnation method by using Ni(NO₃)₂ or RuCl₃ solution (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China), separately.

3. Results and Discussion

3.1. Characterization

The mean particle size was 98 nm for Y₂YbSbO₇ and 35 nm for In₂YbSbO₇. Y₂YbSbO₇, In₂YbSbO₇, and Gd₂YbSbO₇ were nanosized particles and irregular shapes. It could be seen from the results that the average particle size of In₂YbSbO₇ was smaller than that of Gd₂YbSbO₇ or Y₂YbSbO₇. SEM-EDS spectrum which was taken from the prepared In₂YbSbO₇ displayed the presence of indium, ytterbium, antimony, and oxygen. Similarly, SEM-EDS spectrum that was taken from the prepared Gd₂YbSbO₇ also indicated the presence of gadolinium, ytterbium, antimony, and oxygen. SEM-EDS spectrum that was taken from the prepared Y₂YbSbO₇ also indicated the presence of yttrium, ytterbium, antimony, and oxygen. Other elements could not be identified from Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇.

Figure 1 shows the X-ray powder diffraction patterns of In₂YbSbO₇, Gd₂YbSbO₇, and Y₂YbSbO₇. It could be seen from Figure 1 that Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ was a single phase. According to the Rietveld analysis, Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ owned the pyrochlore-type structure and a cubic crystal system which had a space group Fd3m. The lattice parameter for In₂YbSbO₇ was 10.340277 Å. The lattice parameter for Gd₂YbSbO₇ was 10.639527 Å and that for Y₂YbSbO₇ was 10.499778 Å. Moreover, the XRD results showed that 2 theta angles of each reflection of In₂YbSbO₇ changed with In³⁺ being substituted by Y³⁺ or Gd³⁺. The lattice parameter α increased from α = 10.340277 Å for In₂YbSbO₇ to α = 10.499778 Å for Y₂YbSbO₇, and from α = 10.499778 Å for Y₂YbSbO₇ to α = 10.639527 Å for Gd₂YbSbO₇, which indicated a increase in lattice parameter of the photocatalyst with increase of the M ionic radii (In³⁺ (0.92 Å) < Y³⁺ (1.019 Å) < Gd³⁺ (1.053 Å)).

Figure 2 represents the absorption spectra of Y₂YbSbO₇, In₂YbSbO₇, and Gd₂YbSbO₇. Compared with well-known photocatalyst TiO₂ whose absorption edge was only 380 nm, the absorption edge of In₂YbSbO₇ was found to be at 525 nm, that of Gd₂YbSbO₇ was found to be at 502 nm, and that of Y₂YbSbO₇ was found to be at 419 nm, which belonged to the visible region of the spectrum. Clearly, the obvious absorption (defined hereby as 1-transmission) did not result from reflection and scattering. Consequently, the apparent absorbance at subbandgap wavelengths (520 to 800 nm for In₂YbSbO₇, 530 to 800 nm for Gd₂YbSbO₇, and 428 to 800 nm for Y₂YbSbO₇) was higher than zero.

For a crystalline semiconductor, the optical absorption near the band edge followed the equation: [17, 18]. Here, , , , and were proportional constant, absorption coefficient, bandgap, and light frequency, respectively. Within this equation, determined the character of the transition in a semiconductor. and could be calculated by the following steps: (i) plotting versus by assuming an approximate value of , (ii) deducing the value of according to the slope in this graph and (iii) refining the value of by plotting versus hν and extrapolating the plot to . According to this method, the bandgap of In₂YbSbO₇ was estimated to be 2.361 eV. The bandgap of Gd₂YbSbO₇ was 2.469 eV and that of Y₂YbSbO₇ was 2.521 eV.

3.2. Photocatalytic Activity of Y₂YbSbO₇, In₂YbSbO₇, and Gd₂YbSbO₇

Generally speaking, the semiconductor photocatalysis started from the direct absorption of suprabandgap photons and the generation of electron-hole pairs in the semiconductor particles. Subsequently, the diffusion of the charge carriers to the surface of the semiconductor particle was followed. Under visible light irradiation, we measured H₂ and O₂ evolution rate by using In₂YbSbO₇, Gd₂YbSbO₇, and Y₂YbSbO₇ as photocatalysts from CH₃OH/H₂O and AgNO₃/H₂O solutions, respectively. Wavelengths dependence of the photocatalytic activity under light irradiation from full arc up to was measured by using different cutoff filters.

Figure 3 shows the photocatalytic H₂ evolution from aqueous methanol solution with Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ as catalyst under visible light irradiation (, 0.5 g 0.1 wt% Pt-loaded powder sample, 50 mL methanol solution, 200 mL pure water). It could be found from Figure 3 that under visible light irradiation, the rate of H₂ evolution in the first 28 h with In₂YbSbO₇ as catalyst was 5.264 μmol h⁻¹ g⁻¹, that with Y₂YbSbO₇ as catalyst was 3.800 μmol h⁻¹ g⁻¹, that with Gd₂YbSbO₇ as catalyst was 3.257 μmol h⁻¹ g⁻¹, indicating that the photocatalytic activity of the In₂YbSbO₇ photocatalyst was much higher than that of Y₂YbSbO₇ or Gd₂YbSbO₇. The quantum yield for hydrogen evolution at 420 nm with Gd₂YbSbO₇ as catalyst was 0.0795%, and that with Y₂YbSbO₇ as catalyst was 0.0928%, and that with In₂YbSbO₇ as catalyst was 0.1285% under visible light irradiation. Furthermore, the Y₂YbSbO₇ photocatalyst showed slightly higher photocatalytic activity than the Gd₂YbSbO₇ photocatalyst. This also proved that the conduction band level of Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ was more negative than the reduction potential of H₂O for forming H₂. Such results were in good agreement with the optical absorption property of Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ (see Figure 2). The rate of H₂ evolution also increased with increasing illumination time. The photocatalytic activity of the In₂YbSbO₇ photocatalyst increased by about 150% than that of the Gd₂YbSbO₇ photocatalyst.

Figure 4 shows the photocatalytic O₂ evolution from AgNO₃ solution with Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ as catalyst under visible light irradiation (, 0.5 g photocatalyst, 1 mmol AgNO₃, 270 mL pure water). It could be found from Figure 4 that under visible light irradiation, the rate of O₂ evolution in the first 28 h with In₂YbSbO₇ as catalyst was 18.186 μmol h⁻¹ g⁻¹, that with Y₂YbSbO₇ as catalyst was 8.628 μmol h⁻¹ g⁻¹, and that with Gd₂YbSbO₇ as catalyst was 5.404 μmol h⁻¹ g⁻¹, indicating that the valence band level of In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ was more positive than the oxidation potential of H₂O for forming O₂. The formation rate of O₂ increased with decreasing the M ionic radii within M₂YbSbO₇ (M = In, Gd, Y), In³⁺ (0.92 Å) < Y³⁺ (1.019 Å) < Gd³⁺ (1.053 Å). The quantum yield for oxygen evolution at 420 nm with Gd₂YbSbO₇ as catalyst was 0.2639%, that with Y₂YbSbO₇ as catalyst was 0.4214%, and that with In₂YbSbO₇ as catalyst was 0.8881% under visible light irradiation.

Figure 5 shows the photocatalytic H₂ evolution from aqueous methanol solution with Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ as catalyst under visible light irradiation (390 nm cut-off filter, 0.5 g 0.1 wt% Pt-loaded powder sample, 50 mL CH₃OH, 200 mL pure water). It could be found from Figure 5 that under visible light irradiation, the rate of H₂ evolution in the first 28 h with In₂YbSbO₇ as catalyst was 14.464 μmol h⁻¹ g⁻¹, that with Y₂YbSbO₇ as catalyst was 10.614 μmol h⁻¹ g⁻¹, and that with Gd₂YbSbO₇ as catalyst was 7.900 μmol h⁻¹ g⁻¹, indicating that the effect of wavelength dependence on the photocatalytic activity was very important. The quantum yield for hydrogen evolution at 420 nm with Gd₂YbSbO₇ as catalyst was 0.1929%, that with Y₂YbSbO₇ as catalyst was 0.2592%, and that with In₂YbSbO₇ as catalyst was 0.3532% under visible light irradiation (390 nm cut-off filter). Figure 6 shows the photocatalytic H₂ evolution from aqueous methanol solution with Y₂YbSbO₇, In₂YbSbO₇, or Gd₂YbSbO₇ as catalyst under visible light irradiation (no cut-off filter, 0.5 g 0.1 wt% Pt-loaded powder sample, 50 mL CH₃OH, 200 mL pure water). It could be found from Figure 6 that under visible light irradiation without using any filters, the rate of H₂ evolution in the first 28 h with In₂YbSbO₇ as catalyst was 32.321 μmol h⁻¹ g⁻¹, that with Y₂YbSbO₇ as catalyst was 24.693 μmol h⁻¹ g⁻¹, and that with Gd₂YbSbO₇ as catalyst was 22.864 μmol h⁻¹ g⁻¹, indicating that In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ showed not only high photocatalytic activity under full arc irradiation but also an activity under visible light irradiation. The quantum yield for hydrogen evolution at 420 nm with Gd₂YbSbO₇ as catalyst was 0.5583%, that with Y₂YbSbO₇ as catalyst was 0.6030%, and that with In₂YbSbO₇ as catalyst was 0.7892% under visible light irradiation without using any filters. The photocatalytic activity decreased with increasing incident wavelength . As to In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇, the turnover number—the ratio of total amount of gas evolved to catalyst—exceeded 0.09381 for In₂YbSbO₇, 0.06220 for Y₂YbSbO₇, and 0.06578 for Gd₂YbSbO₇, respectively, after 28 h of reaction time under visible light irradiation (). The turnover number which was in terms of reacted electrons relative to the amount of In₂YbSbO₇ reached 1 at 60 h reaction time. As to Y₂YbSbO₇, the turnover number exceeded 1 after 75 h reaction time. As to Gd₂YbSbO₇, the turnover number exceeded 1 after 72 h reaction time. Under the condition of full arc irradiation, after 28 h of reaction time, the turnover number exceeded 0.576 as to In₂YbSbO₇, and the turnover number exceeded 0.404 as to Y₂YbSbO₇, and the turnover number exceeded 0.462 as to Gd₂YbSbO₇. The above results were enough to prove that the reaction occurred catalytically. The reaction stopped when the light was turned off in this experiment, showing the obvious light response.

It was known that the TiO₂ photocatalyst had very high photocatalytic activity under ultraviolet light irradiation. By contrast, the photocatalytic activity was not obtained with Pt/TiO₂ as catalyst under visible light irradiation , while an obvious photocatalytic activity was observed with In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ as catalyst, showing that the In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ could respond to visible light irradiation. The formation rate of H₂ evolution with In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ as catalyst was much larger that with TiO₂ as catalyst under visible light irradiation. This indicated that the photocatalytic activity of In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ for decomposing CH₃OH/H₂O solution was higher than that of TiO₂. The structure of In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ after photocatalytic reaction was also checked by using X-ray diffraction method and no change in their structures were observed during this reaction, which indicated that the H₂ evolution was resulted from the photocatalytic reaction of H₂O. SEM-EDS results also confirmed that the chemical composition of In₂YbSbO₇, Y₂YbSbO₇, or Gd₂YbSbO₇ did not change after reaction.

Figure 7 shows effect of Pt, NiO, and RuO₂ cocatalysts on the photoactivity of In₂YbSbO₇ under visible light irradiation (, 0.5 g powder sample, 50 mL methanol solution, 200 mL pure water). In principle, the photoinduced electrons preferentially enriched on the surface of cocatalyst particles and the recombination of the photoinduced electrons with the photoinduced holes were therefore markedly suppressed. It could be found from Figure 7 that in the first 28 h under visible light irradiation, the rate of H₂ evolution was estimated to be 9.471 μmol h⁻¹ g⁻¹ with 0.2 wt%-Pt/In₂YbSbO₇ as catalyst, and that was estimated to be 5.886 μmol h⁻¹ g⁻¹ with 1.0 wt%-NiO/In₂YbSbO₇ as catalyst, and that was estimated to be 5.371 μmol h⁻¹ g⁻¹ with 1.0 wt%-RuO₂/In₂YbSbO₇ as catalyst, indicating that the photocatalytic activities could be further improved under visible light irradiation with In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ being loaded by Pt, NiO, or RuO₂. The quantum yield for hydrogen evolution at 420 nm with 0.2 wt%-Pt/In₂YbSbO₇ as catalyst was 0.2313%, that with 1.0 wt%-NiO/In₂YbSbO₇ as catalyst was 0.1437%, and that with 1.0 wt%-RuO₂/In₂YbSbO₇ as catalyst was 0.1312% under visible light irradiation (). The effect of Pt was better than that of NiO or RuO₂ for improving the photocatalytic activity of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇.

It was known that the process for photocatalysis of semiconductors was the direct absorption of photon by bandgap of the materials and generated electron-hole pairs in the semiconductor particles, and the excitation of an electron from the valence band to the conduction band was initiated by light absorption with energy equal to or greater than the bandgap of the semiconductor. Upon excitation of photon the separated electron and hole could follow surface of solid. This suggested that the the narrow bandgap was easier to excite an electron from the valence band to the conduction band. If the conduction band potential level of the semiconductor was more negative than that of H₂ evolution, and the valence band potential level was more positive than that of O₂ evolution, decomposition of water can occur even without applying electric power [1]. Based on above analysis, the photon absorption of In₂YbSbO₇ was much easier than that of the Gd₂YbSbO₇ or Y₂YbSbO₇, which led to higher photocatalytic activity of In₂YbSbO₇.

The specific surface area of In₂YbSbO₇ was measured to be 1.98 m²/g which was about 3.7% of the surface area of the TiO₂ photocatalyst (53.8 m²/g), and the surface area of Gd₂YbSbO₇ was measured to be 1.32 m²/g which was only about 2.5% of the surface area of TiO₂, and the specific surface area of Y₂YbSbO₇ was measured to be 1.70 m²/g which was only about 3.2% of the surface area of TiO₂. It indicated much higher potential efficiency of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇. Although the surface area of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was smaller than that of TiO₂, In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ showed higher photocatalytic activity for H₂ evolution under visible light irradiation, which indicated that the high photocatalytic activity of the In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ was not due to big surface area but due to the narrow bandgap. It was obvious that further increase in photocatalytic activity might be prospected from increasing the surface area of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇. Since an efficient photocatalytic reaction process occurred on the photocatalyst surface, the increase of the surface area for the photocatalysts might result in the increase of their photocatalytic activity.

4. Conclusion

In the present work we prepared single phase of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ by solid-state reaction method and investigated the structural, optical, and photocatalytic properties of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇. Rietveld structure refinement revealed that In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m. The lattice parameter for In₂YbSbO₇ was 10.340277 Å. The lattice parameter for Gd₂YbSbO₇ was 10.639527 Å, and that for Y₂YbSbO₇ was 10.499778 Å. The bandgap of In₂YbSbO₇ was estimated to be 2.361 eV. The bandgap of Gd₂YbSbO₇ was 2.469 eV and that of Y₂YbSbO₇ was 2.521 eV. In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ showed optical absorption in the visible light region, indicating the photocatalysts had the ability to respond to the wavelength of visible light region. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water with In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ as the photocatalyst under visible light irradiation. (Wavelength > 420 nm). Moreover, under visible light irradiation (), H₂ and O₂ were also evolved by using In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ as catalyst from CH₃OH/H₂O and AgNO₃/H₂O solutions, respectively. The In₂YbSbO₇ photocatalyst showed the highest activity compared with Gd₂YbSbO₇ or Y₂YbSbO₇. At the same time, the Y₂YbSbO₇ photocatalyst showed higher activity compared with Gd₂YbSbO₇. The photocatalytic activities were further improved under visible light irradiation with In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ being loaded by Pt, NiO, or RuO₂. The effect of Pt was better than that of NiO or RuO₂ for improving the photocatalytic activity of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇. In addition, the synthesis of In₂YbSbO₇, Gd₂YbSbO₇, or Y₂YbSbO₇ offered some useful insights for the design of new photocatalysts for the photocatalytic H₂ and O₂ evolution.

Acknowledgments

This paper was supported by a Grant from China-Israel Joint Research Program in Water Technology and Renewable Energy (no. 5). This paper was supported by a Grant from New Technology and New Methodology of Pollution Prevention Program from Environmental Protection Department of Jiangsu Province of China during 2010 and 2012 (no. 201001). This paper was supported by a Grant from The Fourth Technological Development Scheming (Industry) Program of Suzhou City of China from 2010 (SYG201006). This paper was supported by the National Natural Science Foundation of China (no. 20877040). This paper was supported by a Grant from the Technological Supporting Foundation of Jiangsu Province (no. BE2009144). This paper was also supported by a Grant from the Fundamental Research Funds for the Central Universities.

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Copyright © 2012 Jingfei Luan and Donghua Pei. 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.

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