BRIEF COMMUNICATIONPreparation and photoelectric properties of mesoporous ZnO filmsMing Ming Wu •Yue Shen •Feng Gu •Yi An Xie •Jian Cheng Zhang •Lin Jun WangReceived:24June 2009/Accepted:21October 2009/Published online:6November 2009ÓSpringer Science+Business Media,LLC 2009Abstract 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 21IntroductionIt 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 semiconductorwith 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.WangSchool of Materials Science and Engineering,Shanghai University,Shanghai 200072,Chinae-mail:yueshen@;yueshen126@J Sol-Gel Sci Technol (2010)53:470–474DOI 10.1007/s10971-009-2099-72ExperimentalThe Pluronic P123triblock co polymer(EO20PO70EO20) with a molar weight of5800was kindly donated by BASF. All other chemicals were of analytical grade and used as received.M-ZnOfilms were prepared by doping process and sol–gel method via the following procedure:1.6ml concentrated HCl was slowly added to0.17ml tetrabutyl titanate(TBOT, [98%purity)and2.085g zinc acetate dihydrate(Zn(Ac)2, [99%purity)at room temperature under vigorous stirring. Separately,0.75g P123wasfirst dissolved in8.3ml 1-butanol([99%purity),then added to the HCl/TBOT/ Zn(Ac)2solution.At last,2ml acetylacetone(AcAc)was added.This solution was subsequently aged with stirring at room temperature for6h.The molar ratio of P123/1-buta-nol/Zn(Ac)2/TBOT//AcAc was0.013:9:0.95:0.05:2:2.M-ZnOfilms were prepared by spin coating the freshsolution onto Indium tin oxides(ITO)substrate at900rpm for10s and3,300rpm for20s.The as-synthesizedfilms were aged at40°C for1days and then annealed at120°C for5h at vacuum.The thickfilms were prepared by repeating the above steps for5times.Thefilms were sub-sequently calcined at a rate of1K min-1to350°C for5h. ITO glasses had been eroded to form plan electrodes before the spin coating process,and cleaned successively in de-ionized water,acetone and ethanol,for10min each.For ease of comparison,we prepared mesoporous TiO2(M-TiO2)thin films using the same process.The molar ratio of P123/ 1-butanol/TBOT/HCl/AcAc was0.013:9:1:2:2.The thick-ness of thinfilms is about100nm.Thefilms were characterized by(X-ray diffractometer, RigakuD/MAX-2550,Tokyo)with Cu K a radiation (k=1.54056A˚),operated at40kV and200mA.The small angle scanning range was from0.5°to3°with a scanning rate of0.25°min-1.Transmission electron microscopic(TEM)images of M-ZnO thinfilms were obtained using Japan JEM-2010F microscope operating at an acceleration voltage of200kV.A JASCO V570spec-trophotometer was used to measure the optical spectra of the thinfilms.PL spectra were measured at room temper-ature with a spectrometer(Horiba Jobin Yvon HR800) using the excitation source of the325nm line of a He-Cd laser.Current-voltage measurements were carried out by semiconductor characterization system(Keithley4200, America)with a tungsten lamp(250W).All measurements were performed at room temperature in air.3Results and discussionsSAXRD and HRTEM are two typical ways to investigate the order properties of mesoporous materials.The SAXRD patterns of M-ZnO and M-TiO2thinfilms are shown in Fig.1,and illustrate characteristic peaks at2h=0.62°and 0.75°,respectively,suggesting that the M-ZnO and M-TiO2thinfilms exhibited mesoporous structure.The diameter/d value,determined as distance between meso walls,is around14.23and11.76nm calculated from the2h values of the characteristic peaks by the Bragg equation. Further structural characterization of M-ZnO was per-formed using HRTEM and is shown in Fig.2.Image(a) presents a honeycomb-like porous structure and the pore size is conforming to the results of SAXRD.In image(b), there are obvious lattice fringes,which indicate thefilms have nano-crystalline phase structure.Figure3shows the wide angle X-ray diffraction patterns of M-ZnO thickfilms(on ITO substrates)and ITO sub-strates,respectively.It can be seen that M-ZnO thickfilms exhibit hexagonal wurtzite structure and ternary ZnTiO3 phases,together.Yet no peaks corresponding to titanium and/or titanium oxide were detected.The crystal lattice constants of M-ZnO calculated from the wide-angle X-ray diffraction are a=3.243A˚and c=5.190A˚,which are close to the card JCPDS No.36-1451,a=3.250A˚and c=5.207A˚.The differences result from the introduction of Ti ion in ZnO,because the Ti4?radius(0.68A˚)is smaller than that of Zn2?(0.74A˚).The slight change of lattice parameters of M-ZnO confirms that the Ti ions have been incorporated into the ZnO lattice.Ultraviolet-visible(UV/vis)transmittance spectra of M-ZnO and M-TiO2thinfilms were measured in Fig.4. Compared to the M-TiO2thinfilm,the fundamental transmittance edge of the M-ZnO thinfilm shows a blue shift from350to300nm.The inset graph is the absorbance spectra of M-ZnO and M-TiO2thinfilms.It illustrates that the absorption rate of M-ZnO is greater than that of M-TiO2in the visible range,suggesting the highzinc Fig.1SAXRD patterns of(a)M-ZnO and(b)M-TiO2content M-ZnO composite material can increase the light-harvesting capability as photoelectrode film.The plot of (a h m )1/2versus h m of M-ZnO and M-TiO 2films is shown in Fig.5,where a is the absorption coefficient,h m is the photon energy.Following the well-known Tauc function:(a h m )1/2µ(h m -Eg)and extrapolating the linear portion to (a h m )1/2=0,the optical-gap energy (Eg)can be deter-mined.It could be found that the Eg of M-ZnO and M-TiO 2were 3.25and 3.37eV,respectively.Figure 6shows the room-temperature PL spectra of M-ZnO thick films as a function of the excitation power density.The five excitation power intensities are 2,20,50,100,and 200mW,respectively.The spot radius is 1l m.Dominant emission peaks of M-ZnO centered at 380nm,corresponding to 3.26eV,are ascribed to direct electron-hole recombination which should be equal to the M-ZnO band gap.It is worth noting that there is asignificantFig.2TEM images of a M-ZnO (50,0009)and b M-ZnO (200,0009)Fig.3The wide angle X-ray diffraction of M-ZnO thick films and ITOsubstratesFig.4Transmittance spectra of (a )M-ZnO and (b )M-TiO 2(inset:UV/vis absorbance spectra of (a )M-ZnO and (b )M-TiO 2)Fig.5Energy gap (Eg)of (a )M-ZnO and (b )M-TiO 2increase of PL intensity of M-ZnO thick films at 380nm as compared to the visible bands emission with excitation powers increasing from 2to 200mW.This result is con-sistent to literature [27]and can be expected to be caused by the doping process due to enhanced emission efficiency from free exciton emission [27].For M-ZnO films,Ti atoms occupy Zn atom sites in the lattice of ZnO.When incident UV light excite the carriers in the films,the photocarriers may escape more easily from Ti ions than from Zn ions,which leads to the quick diffusion of excitons and increased exciton concentration in the M-ZnO films.Current-voltage properties of M-ZnO and M-TiO 2thin films were tested in dark and under irradiation for 5s with a tungsten lamp (250W,height to the film was 15cm).As shown in Fig.7b,photoconductivity of the M-TiO 2thin film was 6.023910-10S and dark conductivity was 1.070910-9S at bias voltage of 1V,photoconductivitydecreased about 1.8times under irradiation compared with that in the dark.However,under the same irradiation condition,it was interesting to find that the M-ZnO thin film exhibits a positive photovoltaic effect.Photoconduc-tivity of the M-ZnO thin film reached 9.718910-7S while dark conductivity was 3.256910-7S,photocon-ductivity increased about 3times as shown in Fig.7a.Although TiO 2was widely used in DSSC,it has a low electron transfer rate and high combination rate of the pair of excited electrons [8,9],which induced a negative pho-tovoltaic effect itself.While ZnO has very high electron mobility,which is about 155cm 2V -1s -1[28],ZnO materials can improve the electronic transfer rate and hinder the dark current generation [13,14].Furthermore,it contains some intrinsic defects,which can act as capture centers of photoelectrons and thereby stop the recombina-tion of photoelectrons and photo-holes.This may improve the energy conversion efficiency of M-ZnO in DSSC.4ConclusionIn conclusion,M-ZnO films doped with Ti 4?were pre-pared by sol–gel and spin coating method.Eg of M-ZnO is 3.25eV,which is smaller than that of bulk ZnO.M-ZnO films exhibit hexagonal wurtzite structure and ternary ZnTiO 3phases.The PL intensity of M-ZnO centered at 380nm is increased obviously with the excitation power,which is expected to be caused by enhanced emission efficiency from free exciton emission.Current-voltage properties of M-ZnO films display a positive photovoltaic effect and indicate 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