Preparation and Photocatalytic Characterization of(SrTiO3)1-x(SrTaO2N)xSolid Solution

LI Rui-Pu LUO Wen-Jun LI Zhao-Sheng＊,,2,3 YU Tao,3 ZOU Zhi-Gang＊,,3

(1Ecomaterials and Renewable Energy Research Center(ERERC),Department of Physics,Nanjing University,Nanjing 21009)

(2Department of Materials Science and Engineering,Nanjing University,Nanjing 210093)

(3National Laboratory of Solid State Microstructures,Nanjing University,Nanjing 210093)

Preparation and Photocatalytic Characterization of(SrTiO3)1-x(SrTaO2N)xSolid Solution

LI Rui-Pu1LUO Wen-Jun1LI Zhao-Sheng＊,1,2,3YU Tao1,3ZOU Zhi-Gang＊,1,3

(1Ecomaterials and Renewable Energy Research Center(ERERC),Department of Physics,Nanjing University,Nanjing21009)

(2Department of Materials Science and Engineering,Nanjing University,Nanjing210093)

(3National Laboratory of Solid State Microstructures,Nanjing University,Nanjing210093)

N-doped SrTiO3and(SrTiO3)1-x(SrTaO2N)xwere prepared by a thermal ammonolysis method.The samples were characterized by XRD,UV-Vis spectroscopy,XPS spectroscopy and low temperature nitrogen adsorptiondesorption.The unit cell edge length of(SrTiO3)1-x(SrTaO2N)xincreases linearly and the band gaps reduce from 3.2 to 2.14 eV as x increases from 0 to 0.40.The photocatalytic activities of the samples were investigated in an aqueous suspension system.Aqueous methanol solution(50 mL CH3OH+220 mL H2O)for H2evolution and aqueous silver nitrate solution(270 mL,0.01 mol·L-1)for O2evolution were used as sacrificial reagents.The samples show photocatalytic activities under visible light(λ＞420 nm)irradiation.

solid state method;solid solution;visible light;water splitting

Water splitting using a heterogeneous photocatalyst is an attractive solution to supply clean and recyclable hydrogen energy.Many photocatalysts suitable for water splitting,for example SrTiO3(band gap=3.2 eV),have been developed,however,most of the photocatalysts proposed to date consist of metal oxides and work only in the ultraviolet region[1-2].UV light energy accounts for only about 4%of the total energy of the sunlight on the earth,while visible light makes up about 43%.At present,there is a lack of suitable materials with sufficiently norrow band gap,an appropriate band position for water splitting,and thestability.In order to utilize more solar energy,therefore, many approaches have been tried to explore visiblelight-active photocatalysts[3],such as doping ions, forming new valence bandsin wide band gap semiconductors,and making solid solutions between wide and narrow band gap semiconductors.

Since it was reported in 2001 that TiO2showed visible light activity by N doping[4],anion-doped oxide oroxynitride photocatalysts have been explored intensively[5-12].Recently,we reported that(SrTiO3)1-x(LaTiO2N)xhad visible light activities[13].Among them, ATaO2N(A=Ca,Sr,Ba)and N-doped SrTiO3exhibits visible-light activities[13-14].The lattice parameters of SrTiO3are similar to those of SrTaO2N.Therefore, it is possible to obtain visible-light driven(SrTiO3)1-x(SrTaO2N)xphotocatalysts by adjusting the x values.

1 Experimental

1.1 Preparation of photocatalysts

The N-doped SrTiO3and(SrTiO3)1-x(SrTaO2N)x(x= 0.1～0.4)solid solutions were synthesized by thermal ammonolysis from the oxide precursors.The oxide precursors were obtained at high temperatures by a solid-state reaction method using SrCO3(99%),TiO2(98%)and Ta2O5(99.99%)as starting materials.The well-ground mixtures of SrCO3,TiO2and Ta2O5in a stoichiometric ratio were first preheated at 1 100℃for 10 h and then reground and finally calcined in air at 1 400℃for 12 h in an alumina crucible.Four grams of the mixed powder was put in a zirconia pot(80 mL inner volume)with 40 g zirconia balls of 1 mm in diameter and was ball milled using a high energy planetary mill (Pulverisette-7,Fritsch,Germany)at 800 r·min-1for 2 h at room temperature.The oxide precursors were nitrided at 930℃ for 10 h in a tube furnace with NH3flow of 200 mL·min-1.

1.2 Characterization of photocatalysts

The crystal structures of the samples were determined using an X-ray diffracto-meter(Rigaku,UltimaⅢ)with Cu Kα radiation source(λ=0.154 18 nm）and with the following experimental comditions:40 kV voltage,40 mA current,10°·min-1scan rate and the 0.05°step width over a 2θ range from 20°～80°.X-ray photoelectron spectroscopy(XPS)with Al Kα(hν=1486.6 eV)X-rays(Thermo ESCALAB 250,U.S.)evaluated the amount and state of each element in the powders.The peak position of each element was corrected by using C1s(284.8 eV).The diffuse reflectance spectra were measured with a UV-Vis spectrophotometer(UV-2550, Shimadzu).The apparent specific surface area(BET) was obtained by the Brunauer-Emmet-Teller(BET) method.

1.3 Photocatalytic activity evaluation

The photocatalytic reactions were carried out under visible light illumination emitted from a Xe lamp(CERMAX LX 300,USA,300 W)with a 420 nm cutoff filter(λ＞420 nm).The intensity of the lamp on the reactor was 130 mW·cm-2.The photoreactor was home-made and the volume was450 mL.A photocatalyst(0.2 g)was suspended in the reactor with a magnetic stirrer.Photocatalytic H2evolution from an aqueous methanol solution(50 mL CH3OH+220 mL H2O),and O2evolution from an aqueous silver nitrate solution(270 mL,0.01 mol·L-1)were conducted in a closed gas circulation system.A Pt cocatalyst was loaded by a photodeposition method from an aqueous solution of H2PtCl6.The evolved gases were analyzed using a thermal conductivity detector gas chromatograph(Shimadzu,GC-8A,3 m×3 mm stainless steel column with molecular sieve as the packing material 5A was used and the detector temperature was 250℃.Carrier gas was Ar with a flow rate of 10 mL·min-1.).

2 Results and discussion

2.1 Crystal structures and UV-Vis diffuse reflect-

ance spectra

Fig.1 shows the XRD patterns of N-doped SrTiO3and(SrTiO3)1-x(SrTaO2N)xpowder samples at room temperature.Perovskite-type structures were obtained after calcining in NH3at high temperature.The structure of SrTiO3consists of a framework of TiO6octahedra,the sites between the octahedra being filled with Sr atoms.The ion radius of N3-(0.146 nm)is larger than that of O2-(0.140 nm),and Ta5+(0.064 nm)is larger than Ti4+(0.060 nm),so the diffraction peaks shift to lower angles with the increase of N and Ta in solid solution.The samples exhibit broadening of the crystalline peaks and worse crystallization with increasing of N concentrations in the(SrTiO3)1-x(SrTaO2N)xsolid solutions.

In orderto check whetherN have been incorporated successfully into the SrTiO3and(SrTiO3)1-x(SrTaO2N)xlattice,the valence states of N and Ta were investigated by the XPS measurement.Fig.2 shows that the N1s orbital has a binding energy of 395.5 eV in the samples in agreement with literature results[15].Table 1 lists the doping concentrations of N and Ta analyzed by XPS.It shows that nitrogen is easier to be doped into SrTiO3with increases in Ta content in (SrTiO3)1-x(SrTaO2N)x.

Fig.1 XRD patterns of(a)N-doped SrTiO3,(b)x=0.1, (c)x=0.2,(d)x=0.3,(e)x=0.4 after calcination at 930℃for 10 h in NH3

Table 1 Doping concentration of N and Ta analyzed by XPS

Fig.2 XPS spectrum of N1s and Ta4p3/2of (SrTiO3)0.6(SrTaO2N)0.4

The diffuse reflectance spectra of the solid solution are shown in Fig.3.The band structures of(SrTiO3)1-x(SrTaO2N)xsolid solutions are expected to be tuned by adjusting the compositions of the samples.There are two absorption bands,one at 400～600 nm and the other at more than 600 nm in the absorption spectra.For the absorption band of 400～600 nm,the hybridization of N2p and O2p makes the valance band continuous, therefore the absorption edge shifts to longer wavelengths with increasing N content in the sample.Duringhigh energy milling,SrTiO3powdersare subjected to severe mechanical deformation by the collisions with milling balls and pot.Consequently, plastic deformation at high strain rates occurs within the powders and much oxygen defects form.The absorption band of larger than 600 nm originates from the oxygen defects in the solid solution.

Fig.3 Diffuse reflectance spectra of(a)N-doped SrTiO3, and(SrTiO3)1-x(SrTaO2N)x,(b)x=0.1;(c)x=0.2; (d)x=0.3;(e)x=0.4 after calcination at 930℃for 10 h in NH3

2.2 Photocatalytic activity

Fig.4 shows the O2production rate of(SrTiO3)1-x(SrTaO2N)xphotocatalysts in the presence of AgNO3as sacrificial reagents,wherein the surface area of each sample is also given.It can be noticed from Fig.4 that O2production rate increases when x increases from 0 to 0.2.However,the evolution rate of oxygen decreases when x＞0.2.This is the result of the competition among the oxidizing power,particle size and visible-light absorption.The rates of hydrogen evolution in the presenceofmethanolassacrificialreagentsare negligible.It could come from the Ti3+as the centers of electron-hole recombination.

Fig.4 Production rates of O2(●)and the surface area (□)of(SrTiO3)1-x(SrTaO2N)xphotocatalysts

3 Conclusion

The visible light activities for water decomposition were obtained by modifying the band gaps.The hybridization of N2p and O2p makes the valance band continuous,as a result,the absorption edge shifts to longerwavelengthswith increasingofN in the (SrTiO3)1-x(SrTaO2N)xsolid solution.The highest visiblelight photocatalytic activity for oxygen evolution is 6.5 μmol·h-1·g-1over(SrTiO3)0.8(SrTaO2N)0.2sample. The rates of hydrogen evolution are negligible.

Acknowledgements:Financial support from the National Natural Science Foundation of China(No 20528302),the Science and Technology Research Program of the Ministry of Education (MOE)of China(No.307012)and the National Basic Research Program of China(973 Program,2007CB613301,2007CB613305) is gratefully acknowledged.

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(SrTiO3)1-x(SrTaO2N)x固溶体的制备及光催化性能研究

李瑞璞1罗文俊1李朝升＊,1,2,3于 涛1,3邹志刚＊,1,2,3

(1南京大学物理系环境材料与再生能源研究中心，南京 210093）
(2南京大学材料系，南京 210093)
(3南京大学固体微结构国家重点实验室，南京 210093）

通过高温固相反应合成了N掺杂的SrTiO3和(SrTiO3)1-x(SrTaO2N)x固溶体，对其进行了X射线衍射，紫外可见吸收光谱，X射线光电子能谱分析和比表面积的表征。随x由0增大至0.4，固溶体带隙变窄，由3.2 eV降至2.3 eV，吸收光谱由紫外光区扩展到可见光区。在甲醇溶液(50 mL CH3OH+220 mL H2O)中进行了光催化分解水产生氢气的反应，在硝酸银溶液(270 mL，0.01 mol·L-1)中进行了光催化分解水产生氧气的反应，在可见光(λ＞420 nm)照射下，实现了可见光响应的光催化分解水。

固相法；固溶体；可见光；分解水

O643.3

A

1001-4861(2010)01-0149-04

2009-07-19。收修改稿日期：2009-11-21。

李瑞璞，男，26岁，在读硕士研究生；研究方向：光催化材料及其应用。

国家自然科学基金(No.20773064 and 50732004)资助项目。

＊通讯联系人。E-mail：zgzou@nju.edu.cn