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Preparation and photoelectric properties of mesoporous ZnO films

Ming Ming Wu •Yue Shen •Feng Gu •

Yi An Xie •Jian Cheng Zhang •Lin Jun Wang

Received:24June 2009/Accepted:21October 2009/Published online:6November 2009ÓSpringer Science+Business Media,LLC 2009

Abstract Mesoporous ZnO films doped with Ti 4?(M-ZnO)have been prepared by doping process and sol–gel method.The films have mesoporous structures and consist of nano-crystalline phase,as evidenced from small angle X-ray diffraction and high resolution transmission electron microscopy.The wide angle X-ray diffraction of M-ZnO films confirms that M-ZnO has hexagonal wurtzite structure and ternary ZnTiO 3phases.Ultraviolet–visible transmittance spectra,absorbance spectra and energy gaps of the films were measured.The Eg of M-ZnO is 3.25eV.Photoluminescence intensity of M-ZnO centered at 380nm increases obviously with the excitation power,which is due to the doping process and enhanced emission efficiency.M-ZnO thin films display a positive photovoltaic effect compared to mesoporous TiO 2(M-TiO 2)films.Keywords Photoelectric properties ÁMesoporous ÁZnO ÁTiO 2

1Introduction

It has been recently shown that semiconducting mesoporous metal oxides,e.g.,SnO 2[1,2]or TiO 2[3],with large specific surface areas and uniform pore widths show interesting properties which are superior to non porous samples of the same metal oxides.Zinc oxide (ZnO)is attracting tremendous research interest due to its vast spectrum properties and applications.ZnO is an n-type direct band-gap semiconductor

with E g =3.37eV and an exciton-binding energy of 60meV.It has been applied for light-emitting diodes [4–6],lasers [7],photovoltaic solar cells [8],UV-photodetectors [9]and sensors [10].Particularly,it has attracted great attention in Dye-sensitized solar cells (DSSC).

To date,the highest solar-to-electric conversion effi-ciency of over 11%has been achieved with films that consist of mesoporous TiO 2nanocrystallites sensitized by ruthe-nium-based dyes [11].Besides the optical properties similar to TiO 2,ZnO has other advantages such as higher light absorbance below 400nm than TiO 2[12],improved elec-tronic transfer rate and hindered dark current generation [13,14].Nevertheless,ZnO nanostructure electrodes seem to have insufficient internal surface areas,which limits their energy conversion efficiency at a relatively low level,for example,1.5–2.4%for ZnO nanocrystalline films [15–17],0.5–1.5%for ZnO nanowire films [18–20],2.7–3.5%for uniform ZnO aggregate films [21,22]and 5.4%for poly-disperse ZnO aggregates [8].

In spite of a great deal of effort to successfully synthesize mesoporous ZnO powders successfully [23,24],however,many barriers still exist due to the intrinsic properties of zinc versus silicon.To the best of our knowledge,there were few reports about ordered mesoporous ZnO thin film prepared by wet chemical method.The main hurdles in the synthesis of well-ordered mesoporous ZnO are the high reactivity of Zn ion precursors toward hydrolysis [25]and difficulty for Zn to form the three-dimensional network structure of Zn-O as compared to Si and Ti [26].

In this work,we report a highly reproducible synthetic method to produce thermally stable M-ZnO films through doping process and sol–gel method.Photoelectric proper-ties of M-ZnO films were studied and compared with M-TiO 2films,which can get the highest solar-to-electric conversion efficiency.

M.M.Wu ÁY.Shen (&)ÁF.Gu ÁY.A.Xie ÁJ.C.Zhang ÁL.J.Wang

School of Materials Science and Engineering,Shanghai University,Shanghai 200072,China

e-mail:yueshen@;yueshen126@

J Sol-Gel Sci Technol (2010)53:470–474DOI 10.1007/s10971-009-2099-7