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

Authors: Jingfei Luan; Donghua Pei
Journal: International Journal of Photoenergy
 (2012)

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## Abstract

Novel photocatalysts M2YbSbO7 (M=In, 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 (M=In, 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. (wavelength>420 nm). Moreover, under visible light irradiation (λ>420 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.


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## Body

## 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 2θ=10–100°. 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 fluxIo 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.Figure1 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 1
X-ray powder diffraction patterns of In2YbSbO7, Gd2YbSbO7, or Y2YbSbO7 prepared by a solid-state reaction method at 1320°C.Figure2 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.Figure 2
The diffuse reflectance absorption spectra of In2YbSbO7, Gd2YbSbO7, and Y2YbSbO7.For a crystalline semiconductor, the optical absorption near the band edge followed the equation:αhν=A(hν-Eg)n [17, 18]. Here, A, α, Eg, and ν were proportional constant, absorption coefficient, bandgap, and light frequency, respectively. Within this equation, n determined the character of the transition in a semiconductor. Eg and n could be calculated by the following steps: (i) plotting ln(αhν) versus ln(hν-Eg) by assuming an approximate value of Eg, (ii) deducing the value of n according to the slope in this graph and (iii) refining the value of Eg by plotting (αhν)1/n versus hν and extrapolating the plot to (αhν)1/n=0. 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 (λ) dependence of the photocatalytic activity under light irradiation from full arc up to λ=420nm was measured by using different cutoff filters.Figure3 shows the photocatalytic H2 evolution from aqueous methanol solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 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 3
Photocatalytic H2 evolution from aqueous methanol solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 0.5 g 0.1 wt% Pt-loaded powder sample, 50 mL methanol solution, 200 mL pure water). Light source: 300 W Xe lamp.Figure4 shows the photocatalytic O2 evolution from AgNO3 solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 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 4
Photocatalytic O2 evolution from AgNO3 solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 0.5 g photocatalyst, 1 mmol AgNO3, 270 mL pure water). Light source: 300 W Xe lamp.Figure5 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 (λ) 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 λ. 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 (λ>420nm). 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.Figure 5
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). Light source: 300 W Xe lamp.Figure 6
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). Light source: 300 W Xe lamp.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 (λ>420nm), 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.Figure7 shows effect of Pt, NiO, and RuO2 cocatalysts on the photoactivity of In2YbSbO7 under visible light irradiation (λ>420nm, 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 (λ>420nm). The effect of Pt was better than that of NiO or RuO2 for improving the photocatalytic activity of In2YbSbO7, Gd2YbSbO7, or Y2YbSbO7.Figure 7
Effect of Pt, NiO, and RuO2 co-catalysts on the photoactivity of In2YbSbO7 under visible light irradiation (λ>420nm, 0.5 g powder sample, 50 mL methanol solution, 200 mL pure water). Light source: 300 W Xe lamp.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.


## 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.Figure1 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 1
X-ray powder diffraction patterns of In2YbSbO7, Gd2YbSbO7, or Y2YbSbO7 prepared by a solid-state reaction method at 1320°C.Figure2 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.Figure 2
The diffuse reflectance absorption spectra of In2YbSbO7, Gd2YbSbO7, and Y2YbSbO7.For a crystalline semiconductor, the optical absorption near the band edge followed the equation:αhν=A(hν-Eg)n [17, 18]. Here, A, α, Eg, and ν were proportional constant, absorption coefficient, bandgap, and light frequency, respectively. Within this equation, n determined the character of the transition in a semiconductor. Eg and n could be calculated by the following steps: (i) plotting ln(αhν) versus ln(hν-Eg) by assuming an approximate value of Eg, (ii) deducing the value of n according to the slope in this graph and (iii) refining the value of Eg by plotting (αhν)1/n versus hν and extrapolating the plot to (αhν)1/n=0. 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 (λ) dependence of the photocatalytic activity under light irradiation from full arc up to λ=420nm was measured by using different cutoff filters.Figure3 shows the photocatalytic H2 evolution from aqueous methanol solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 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 3
Photocatalytic H2 evolution from aqueous methanol solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 0.5 g 0.1 wt% Pt-loaded powder sample, 50 mL methanol solution, 200 mL pure water). Light source: 300 W Xe lamp.Figure4 shows the photocatalytic O2 evolution from AgNO3 solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 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 4
Photocatalytic O2 evolution from AgNO3 solution with Y2YbSbO7, In2YbSbO7, or Gd2YbSbO7 as catalyst under visible light irradiation (λ>420nm, 0.5 g photocatalyst, 1 mmol AgNO3, 270 mL pure water). Light source: 300 W Xe lamp.Figure5 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 (λ) 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 λ. 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 (λ>420nm). 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.Figure 5
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). Light source: 300 W Xe lamp.Figure 6
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). Light source: 300 W Xe lamp.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 (λ>420nm), 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.Figure7 shows effect of Pt, NiO, and RuO2 cocatalysts on the photoactivity of In2YbSbO7 under visible light irradiation (λ>420nm, 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 (λ>420nm). The effect of Pt was better than that of NiO or RuO2 for improving the photocatalytic activity of In2YbSbO7, Gd2YbSbO7, or Y2YbSbO7.Figure 7
Effect of Pt, NiO, and RuO2 co-catalysts on the photoactivity of In2YbSbO7 under visible light irradiation (λ>420nm, 0.5 g powder sample, 50 mL methanol solution, 200 mL pure water). Light source: 300 W Xe lamp.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 (λ>420nm), 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.



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*Source: 890865-2012-02-23.xml*