Ž .Electrochemistry Communications 2 2000 765–768www.elsevier.nlrlocaterelecom

Selective electrodeposition of ZnO onto Cu O2

Jaeyoung Lee a, Yongsug Tak b,)

a Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germanyb Department of Chemical Engineering, Inha UniÕersity, Inchon 402-751, South Korea

Received 1 August 2000; received in revised form 29 August 2000; accepted 29 August 2000

Abstract

Ž . Ž . Ž .The co-deposition of cuprous oxide Cu O and zinc oxide ZnO on indium tin oxide ITO substrate was executed by two different2

electrochemical methods and the formation mechanism of ZnO onto Cu O was investigated by ex-situ SEM, XRD, and XPS. The single2

galvanostatic electrodeposition step in a mixed nitrate electrolyte offered a useful method in preparing ZnO onto triangular Cu O islands2

formed. On the other hand, hexagonal shaped ZnO phase was electrodeposited on ITO substrate as well as on Cu O islands when two2

steps of the galvanostatic and potentiostatic process were applied. q 2000 Published by Elsevier Science S.A.

Keywords: Co-deposition; Cu O; ZnO2

1. Introduction

w x w xMetal oxides such as Cu O 1–5 , Y O 6 , ZnO2 2 3w x w x7–10 , and PbO 11 have been prepared by electrochem-2

Ž .ical deposition due to the following reasons; 1 thicknessand morphology of films can be precisely controlled by

Ž .electrochemical parameters, 2 relatively uniform filmsŽ .can be formed on substrates of complex shapes, 3 higher

Ž .deposition rates can be easily obtained, and 4 the equip-ment required is inexpensive due to the non requirement ofeither high vacuum or high reaction temperatures.

Cu O is a non stoichiometric p-type semiconductor2

with a band gap of 2.0 eV that was recently suggested as aphotocatalytic material for water splitting by visible lightw x12,13 . Cu O is also a potential material for the fabrica-2

w xtion of low-cost solar cells 14,15 , while its thermoelectricand piezoelectric responses are being exploited as a sensor

w xmaterial 16 . ZnO is a n-type semiconductor with a bandgap of 3.2 eV, which can be used for applications in

w xsensors, solar cells, and optoelectronic devices 17–19 .We have reported in our previous study that Cu O2

Ž .111 was epitaxally grown below pH 5 by applying a lowŽ 2 . w xcathodic current density y0.5 mArcm 3 and that

) Corresponding author. Tel.: q82-32-860-7471; fax: q82-32-866-0587.

Ž .E-mail address: ystak@inha.ac.kr Y. Tak .

compact and uniform cuprous oxide films were preparedw xby current modulation methods 4 . In the presence of

Ž .dissolved O in Zn NO solution, the initial behaviour2 3 2

of ZnO formation and the subsequent film growth mecha-nism, investigated by using an in-situ electrochemical

Ž .quartz crystal microbalance EQCM and ex-situ FTIR,w x w xwere also discussed 10 . Of late, Nasser et al. 20 demon-

strated the preparation of different composition of copper-zinc oxide films to detect various gases by the spraypyrolysis technique, which is useful method for the deposi-tion of solid solution thin films.

In this paper, we report the application of the twodistinct electrodeposition techniques to synthesize nanos-tructures of zinc oxide onto copper oxide and study thegrowth mechanism by using an ex-situ XPS, XRD, andSEM. We believe that this study can serve as reference forthe co-deposition of two metal oxide by electrochemicalmethods.

2. Experimental section

The experiments were performed in a conventionalthree-compartment electrochemical cell. ZnO and Cu O2

were cathodically formed on ITO deposited quartz crystalŽ .approximately 15 VrI, Seiko Instruments Inc. . Thegeometric area of the electrode during electrodepositionwas 0.196 cm2.

1388-2481r00r$ - see front matter q 2000 Published by Elsevier Science S.A.Ž .PII: S1388-2481 00 00120-X

( )J. Lee, Y. TakrElectrochemistry Communications 2 2000 765–768766

Fig. 1. SEM images of Cu O and ZnO prepared by applying two step2Ž . Ž . Ž .methods; a Cu O-ZnO image =3,000 on ITO and b is blow up of2

Ž . Ž .a =13,000 ; I: Cu O; II: ZnO.2

Prior to the electrodeposition, the ITO cathode wasthermally treated at 500 8C for 3 h to reduce the resistanceby removing the organic contaminants on the surface, andwas subjected to acetone cleaning in an ultrasonic bathbefore subsequently rinsing in ultrapure water. A platinumplate was used as a counter electrode and saturated calomel

Ž .electrode SCE was employed as the reference electrode.Ž . Ž . Ž .Cu NO Sigma Aldrich, 99.999% and Zn NO3 2 3 2

Ž .Sigma Aldrich, 99.999% were dissolved in ultrapureŽ .water Millipore, 18.2 MV cm . All experiments were

performed at 65 8C in stirred solution. To investigate theeffect of dissolved oxygen, the electrolyte was bubbled

Ž .with O 99.999% for 1 h through the electrochemical cell2

before each experiments.Two different experimental methods have been applied

to investigate the formation mechanism of the ZnOrCu O2

nanostructures on ITO electrode. Method 1 is a two se-quence process, i.e. galvanostatic and potentiostatic experi-ments, for the preparation of nanocomposites; at first

Ž .Cu O was prepared in 5 mM Cu NO solution by2 3 2

applying y0.5 mArcm2 for 2 min after which the work-ing electrode was transferred to the second cell containing

Ž .0.1 M Zn NO solution. Then ZnO was deposited both3 2

on Cu O and on substrate ITO under potentiostatic condi-2

Ž .tions by applying y0.721 V vs. SCE for 5 min. Method2 is a single step galvanostatic process performed in a

Ž . Ž .mixed solution of 5 mM Cu NO and 0.1 M Zn NO3 2 3 2

by applying y0.5 mArcm2.All currents and potentials used for electrochemical

preparation of ZnO and Cu O were performed with a2Ž .potentiostat & galvanostat EG&G, PAR 273A . The mor-

phology and The crystallinity and the surface morphologyŽof the deposited material were investigated by XRD Philips

. Ž .DY616 and SEM Hitachi S-4200 . X-ray PhotoelectronŽ .Spectroscopy XPS measurements were performed using

a SPECS GMBH, EA200 system using MgKa radiation.

3. Results and discussion

When two different deposition methods are carried out,morphologies of the surface are observed by SEM whoseimages are shown in Fig. 1 and Fig. 3. In the two step

Ž .experiments, following two processes reaction 1 and 2occur and produce OHy ions, which enhance the pH of thesurface by reaction 3 in the weak acidic solution. As

w xdiscussed in our earlier work 3,4,10 , pH enhancement onthe cathode due to reductions of nitrate ion and dissolvedoxygen leads to Cu O and ZnO formation.2

O q2H Oq4e™4OHy, E0 s0.401 V 1Ž .2 2

NOyqH Oq2 e™NOyq2OHy, E0 s0.01 V 2Ž .3 2 2

ITOq)qOHy™ ITO-OHy 3Ž .ad

where ) denotes a vacant surface site, subscript ‘ad’indicates species adsorbed on the surface and E0 is thestandard hydrogen electrode potential.

Ž . Ž .In Fig. 1 a and b , the large tri-angular crystal visibleŽ . w xI corresponding to Cu O 3,4 on the substrate is de-2

Ž 2 .posited by applying a constant current y0.5 mArcm inthe copper nitrate solution by reaction 4a and 5a. In thefollowing potentiostatic deposition stage, ZnO crystals are

Fig. 2. XRD data of ZnO and Cu O deposited by applying method 1.2

( )J. Lee, Y. TakrElectrochemistry Communications 2 2000 765–768 767

Fig. 3. The morphology of ZnO onto Cu O with different electrolysis2

time at 1, 2, and 5min, respectively in single step deposition technique,Ž . Ž . Ž .magnified a =60,000, b =50,000, and c =35,000.

found onto both ITO and Cu O islands which have previ-2Žously formed in the first galvanostatic mode reaction 4b,

.5b, and 5c . This results from a sufficient pH increase ofITO substrates to form zinc hydroxide.

The Cu O and ZnO are cathodically obtained in the2

same way discussed above, i.e. deposition current of y0.5mArcm2 and y0.721 V for Cu O and ZnO, respectively.2

XRD data, Fig. 2 shows that several different phase resultduring electrolysis and as already presented in SEM analy-sis, these are identified as zinc oxide, copper oxide, andITO substrate.

Ž . Ž .However, Fig. 3 a – c shows that Cu O nucleates on2

isolated sites and the size of Cu O increases with electrol-2

ysis time, i.e., the ca. 500 nm Cu O crystal at 1min grew2

into 750 nm islands in 5min. We obtain the same triangu-lar shape of copper oxide crystal each experiments by

ex-situ SEM measurement. In parallel, ZnO ultrathin filmsis only formed on Cu O islands surface in single step2

Ž .method. Fig. 3 c shows that ZnO bulk film perfectlycovers the Cu O crystal.2

Cuq2 qe™CuqqOHy™Cu OH s 4aŽ . Ž . Ž .ad

Zn2qq2OHy™Zn OH s 4bŽ . Ž . Ž .2ad

2Cu OH s ™Cu OqH O 5aŽ . Ž . Ž .2 2

Zn OH s ™ZnOqH O 5bŽ . Ž . Ž .2 2

Cu O-2OHy qZn2q™Zn OH s -Cu OŽ . Ž .22 ad 2

™ZnO-Cu OqH O 5cŽ .2 2

Ž .where s means chemical precipitation.In Fig. 4, XRD data present that copper oxide peak

become dominant with electrolysis time by applying thecritical current density, y0.5 mArcm2 to form only cop-per oxide not to prepare copper metal and also severalphases of zinc oxide deposited onto Cu O are obtained. By2

comparing Fig. 4a with Fig. 4c, XRD peak intensity ofITO relatively reduce by growth of the cuprous oxide, thatis, Cu O islands onto ITO become larger.2

In a single step deposition method, the reduction ofdissolved oxygen induced to the adsorption of OHy on thesurface and relatively slight increase of surface pH. There-fore copper hydroxide gets preferentially precipitated onthe cathode and Cu O is uniquely formed at the beginning2

of electrolysis because cuprous ion gets transferred to thecathode and reacts with hydroxide ion faster than the zincion. After forming a minimum density of Cu O nuclei, the2

zinc hydroxides get precipitated with appropriate pH for

Fig. 4. XRD data of ZnO onto Cu O under single step. Electrolysis time;2Ž . Ž . Ž .a 1 min, b 2 min, and c 5 min.

( )J. Lee, Y. TakrElectrochemistry Communications 2 2000 765–768768

Ž . Ž .Fig. 5. XPS data of ZnO a and Cu O b spectra for a ITO surface;22 Ž .electrolysis at y0.5 mArcm for 5 min in 5 mM Cu NO and 0.1 M3 2

Ž . Ž .Zn NO same as Fig. 3 c .3 2

ZnO formation. ZnO crystals, however, are not formed onITO substrate presumably because the surface pH of ITOsubstrate is not sufficient to form zinc hydroxide, andOHy ion is better adsorbed on the cuprous oxide crystals.

w x Ž .As already shown in our earlier work 10 , Zn OH takes2

longer time to attain optimal condition to be precipitatedand only then ZnO is deposited on the ITO surface bydehydration. This showed that Zn2q required more adsorp-tion of OHy, i.e. ZnO could be formed on higher pHsurface. Post-deposition behavior may also be explained bydifferent band-gap energy between galvanostaticallyformed Cu O and ITO substrate, which is a heavily doped2

n-type semiconductor with a bandgap between 3.5 and 4.3eV.

Method 2, namely the constant current single mode, isused for making the p-n electrode, ZnO onto Cu O crystal2

when ITO cathode is used. In Fig. 5, the XPS spectrumrecorded on the ITO substrate shows the formation of ZnOŽ . Ž .a on Cu O b by electrodeposition. The depositing2

current density, y0.5 mArcm2 is constantly applied.Principally, the value of peak range of Zn and Cu metal instandard XPS data has almost same range as copper oxide

w xand zinc oxide, but in our previous studies 3,10 , wedidn’t observe the zinc and copper metal by applying

2 Ž .y0.5 mArcm and y0.721V vs. SCE . However, bothseveral phases of Cu metal and copper oxide peak wereobserved when relatively higher current density, y5

2 w xmArcm was applied 4 . So, the peak at 1021.7 eV inXPS spectrum can be assigned to the 2p atomic state of3r2

zinc oxide. In parallel, the peak located at lower E , 952.5b

eV may be attributed to the 2p oxidized states of1r2

copper. These results show that the composite of Cu O2

and ZnO is deposited by single galvanostatic experiment.

4. Conclusions

ZnO and Cu O were deposited by electrochemical reac-2Ž .tion. The different results obtained by two step method 1

Ž .and executed by single step method 2 were explained asfollows: In method 1 using two metal solutions, ZnOcrystals were potentiostatically deposited on ITO substrateas well as on Cu O formed under first galvanostatic mode2

Ž .in Cu NO weak acidic solution. ZnO thin films, how-3 2

ever, was only deposited on Cu O islands at critical2Ž 2 .current density y0.5 mArcm in mixed single solutions

and ZnO completely covered triangular Cu O islands.2

These results of deposition at selective regions suggest thatthe nanoscale codeposition of metal oxides of solar cellscan be electrochemically prepared.

Acknowledgements

This work was supported under 1999 Inha UniversityResearch Support Program. J. Lee gratefully acknowledgesMax-Planck-Gesellschaft for the fellowship.

References

w x1 T.D. Golden, M.G. Shumsky, Y. Zhou, R.A. VanderWerf, R.A. VanŽ .Leeuwen, J.A. Switzer, Chem. Mater. 8 1996 2499.

w x2 E.W. Bohannan, L.Y. Huang, F.S. Miller, M.G. Shumsky, J.A.Ž .Switzer, Langmuir 15 1999 813.

w x Ž .3 J. Lee, Y. Tak, Electrochem. Solid-State Lett. 2 1999 559.w x Ž .4 J. Lee, Y. Tak, Electrochem. Solid-State Lett. 3 2000 69.w x Ž .5 D. Lu, K. Tanaka, J. Electrochem. Soc. 143 1996 2105.w x Ž .6 J. Lee, Y. Tak, J. Ind. Eng. Chem. 5 1999 139.w x Ž .7 M. Izaki, T. Omi, Appl. Phys. Lett. 68 1996 2439.w x Ž .8 M. Izaki, J. Electrochem. Soc. 146 1999 4517.w x Ž .9 M. Izaki, J. Katayama, J. Electrochem. Soc. 147 2000 210.

w x Ž .10 J. Lee, Y. Tak, J. Ind. Eng. Chem. 5 1999 87.w x Ž .11 J. Lee, H. Varela, S. Uhm, Y. Tak, Electrochem. Commun. 2 2000

646.w x12 M. Hara, T. Konto, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka,

Ž .J.N. Kondo, K. Domen, Chem. Commun. 1998 2185.w x13 P.E. de Jongh, D. Vanmaekelbergh, J.J. Kelly, Chem. Commun.

Ž .1999 1069.w x14 A.O. Musa, T. Akomolafe, M.J. Carter, Solar Energy Mater. Solar

Ž .Cells 51 1998 305.w x15 K. Santra, P. Chatterjee, S.P. Sen Gupta, Solar Energy Mater. Solar

Ž .Cells 57 1999 345.w x Ž .16 V. Chang, H. Rojas, J. Jorge, Sens. Actuators 375–378 1993 37.w x Ž .17 H. Nanto, T. Minami, S.J. Takata, Appl. Phys. 60 1986 482.w x Ž .18 A.V. Nurmikko, R. L Gunshor, Solid State Commun. 92 1994 113.w x Ž .19 J.S.C.J. Prentice, Phys. D: Appl. Phys. 32 1999 2146.w x20 S.A. Nasser, H.H. Afify, S.A. El-Hakim, M.K. Zayed, Thin Solid

Ž .Films 315 1998 327.