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Preparation and photoelectric properties of mesoporous ZnO ?lms

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 ?lms doped with Ti 4?(M-ZnO)have been prepared by doping process and sol–gel method.The ?lms 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 ?lms con?rms that M-ZnO has hexagonal wurtzite structure and ternary ZnTiO 3phases.Ultraviolet–visible transmittance spectra,absorbance spectra and energy gaps of the ?lms 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 ef?ciency.M-ZnO thin ?lms display a positive photovoltaic effect compared to mesoporous TiO 2(M-TiO 2)?lms.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 speci?c 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 ef?-ciency of over 11%has been achieved with ?lms 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 insuf?cient internal surface areas,which limits their energy conversion ef?ciency at a relatively low level,for example,1.5–2.4%for ZnO nanocrystalline ?lms [15–17],0.5–1.5%for ZnO nanowire ?lms [18–20],2.7–3.5%for uniform ZnO aggregate ?lms [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 ?lm 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 dif?culty 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 ?lms through doping process and sol–gel method.Photoelectric proper-ties of M-ZnO ?lms were studied and compared with M-TiO 2?lms,which can get the highest solar-to-electric conversion ef?ciency.

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@https://www.360docs.net/doc/1d11619044.html,;yueshen126@https://www.360docs.net/doc/1d11619044.html,

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

2Experimental

The 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-ZnO?lms 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 P123was?rst 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-ZnO?lms were prepared by spin coating the fresh

solution onto Indium tin oxides(ITO)substrate at900rpm for10s and3,300rpm for20s.The as-synthesized?lms were aged at40°C for1days and then annealed at120°C for5h at vacuum.The thick?lms were prepared by repeating the above steps for5times.The?lms 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 ?lms using the same process.The molar ratio of P123/ 1-butanol/TBOT/HCl/AcAc was0.013:9:1:2:2.The thick-ness of thin?lms is about100nm.

The?lms 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 thin?lms 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 thin?lms.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 discussions

SAXRD and HRTEM are two typical ways to investigate the order properties of mesoporous materials.The SAXRD patterns of M-ZnO and M-TiO2thin?lms are shown in Fig.1,and illustrate characteristic peaks at2h=0.62°and 0.75°,respectively,suggesting that the M-ZnO and M-TiO2thin?lms 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 the?lms have nano-crystalline phase structure.

Figure3shows the wide angle X-ray diffraction patterns of M-ZnO thick?lms(on ITO substrates)and ITO sub-strates,respectively.It can be seen that M-ZnO thick?lms 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 con?rms that the Ti ions have been incorporated into the ZnO lattice.

Ultraviolet-visible(UV/vis)transmittance spectra of M-ZnO and M-TiO2thin?lms were measured in Fig.4. Compared to the M-TiO2thin?lm,the fundamental transmittance edge of the M-ZnO thin?lm shows a blue shift from350to300nm.The inset graph is the absorbance spectra of M-ZnO and M-TiO2thin?lms.It illustrates that the absorption rate of M-ZnO is greater than that of M-TiO2in the visible range,suggesting the high

zinc Fig.1SAXRD patterns of(a)M-ZnO and(b)M-TiO2

content M-ZnO composite material can increase the light-harvesting capability as photoelectrode ?lm.The plot of (a h m )1/2versus h m of M-ZnO and M-TiO 2?lms is shown in Fig.5,where a is the absorption coef?cient,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 ?lms as a function of the excitation power density.The ?ve 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 a

signi?cant

Fig.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 ?lms and ITO

substrates

Fig.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 2

increase of PL intensity of M-ZnO thick ?lms 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 ef?ciency from free exciton emission [27].For M-ZnO ?lms,Ti atoms occupy Zn atom sites in the lattice of ZnO.When incident UV light excite the carriers in the ?lms,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 ?lms.Current-voltage properties of M-ZnO and M-TiO 2thin ?lms were tested in dark and under irradiation for 5s with a tungsten lamp (250W,height to the ?lm was 15cm).As shown in Fig.7b,photoconductivity of the M-TiO 2thin ?lm was 6.023910-10S and dark conductivity was 1.070910-9S at bias voltage of 1V,photoconductivity

decreased about 1.8times under irradiation compared with that in the dark.However,under the same irradiation condition,it was interesting to ?nd that the M-ZnO thin ?lm exhibits a positive photovoltaic effect.Photoconduc-tivity of the M-ZnO thin ?lm 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 ef?ciency of M-ZnO in DSSC.

4Conclusion

In conclusion,M-ZnO ?lms 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 ?lms 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 ef?ciency from free exciton emission.Current-voltage properties of M-ZnO ?lms display a positive photovoltaic effect and indicate the promising applications in DSSC.

Acknowledgments The work was supported by Innovation Pro-gram of Shanghai Municipal Education Commission (08YZ08),AM and other Research Foundation of Shanghai City Committee of Sci-ence and Technology (0852*******,0752nm016,07JC14058)and Shanghai Leading Academic Disciplines (S30107).We thank Dr.Qiang Li,Bo Lu and Jian Huang for their assistance in the measurement at Shanghai University.

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