POTENTIOSTATIC ELECTRODEPOSITION OF CUPROUS OXIDE THIN FILMS

Current-potential scam were used to investigate the electrodeposition of cuprous oxide thin films in an acetate bath. YJe found that a narrow potential domain, from 0 V vs SCE to -300 mV vs SCE, is available for the potentiostatic electrodeposition of cuprous oxide thin films and extension of this domain towards more cathodic potentials will result in theco-deposition of copper. These results were further verified by the X-ray diffraction measurements on the thin films formed by the electrodeposition a t various electrode potentials. Optical transmission studies revealed that electrodeposited cuprous oxide is a direct band gap semiconductor of 2.0 eV.


INTRODUCTION
Electrodeposition is a promising .techniquethat can be used to prepare thin semiconductor films for applications in semiconductor devices.Cuprous oxide is a n inexpensive and non -toxic semiconductor with potential use i n low-cost solar energy converting devices.5-8 Many reports have been recently published on the ele.ctrical, optical and photoelectrochemical properties of electrodeposited cuprous oxide thin films prepared under the gdavanostatic and potentiostatic c o n ~T i t i o n .s .~-~~~-~~ Although the physical properties of the electrodeposited cuprous oxide films were reported, there have been no reprts on the electrochemical aspects of the electrodeposition of cuprous oxide thin films.Indeed, such a study 1s very important for further improvement of the physical properties ofthe films, a s the semiconductinp,properties ofthe films can be controlled easily by incorporatingimpurity atoms during the electrodeposition process.There is particularly the possibility of having better control over the conductivity of cuprous ordde t h n films using the electrodeposition method.
In this investigation we have carried out a study on the electrochemical aspects of the electrodeposition of cuprous oxide thin films in an acetate bath, by the method of cyclic voltammetry.We investigated the potential domain where the electrodeposition of cuprous oxide is possible and its dependence on the electrolytic concentration a n d th.e temperature of the electrolytic bath.Electrodeposited films tiere investigated by electrochemical, X-ray diffraction (XRD) and optical absorption methods.

METHODS AND MATERIALS
The electrochemical set up used in this investigation is shown in Fig. 1, where the electrochemical cell is kept in a water bath and its temperature is controlled automatically.The counter electrode was a platinum electrode and the reference electrode was a saturated calomel electrode (SCE).Electrolytic solutions were prepared with distilled water and reagent grade chemicals were employed without de-oxygenation.We employed a platinum wire as the working electrode and obtained the voltammetric curves in a bath containing 0.1 M sohum acetate and various concentrations ofcupric acetate solutions.A homemade potentiostat and an X-Y recorder were used to obtain the voltammetric curves and the bath solution was stirred continuously using a magnetic stirrer.Thin films were electrodeposited on indium doped tin oxide (ITO) coated glass substrates, in order to characterize the films by optical transmission and XRD methods.Prior to the frlm deposition substrates were cleaned with a detergent and a dilute aqueous solution of hydrochloric acid, and subsequently rinsed in distilled water.Films were eledrodeposited on IT0 substrates at various potentials, in the range of 0 V vs SCE to -0.6 V vs SCE, using a bath containing 0.1 M sodium acetate and 1.2 x 10-'M cupric acetate, at a temperature of 40°C.XRD measurements were taken with a SHIMADZU (model XD-Dl) X-ray difiactrometer and optical transmittance measurements were taken with a HITACHI U-2000 spectrophotometer.

RESULTS
The currentpotential curves obtained with a platinum wire as the working electrode are shown in Fig. 2. Curve (a) in Fig. 2 was obtained with a 0.1 M sodium acetate solution while curves (b) t o (g) were obtained in solutions containing 0.1 M sodium acetate with the cupric acetate concentrations of 0 .0 5 ~1 0 -~M , O.lx 10-2M, 0 . 2 ~1 0 .~ M, 0 .4 ~ M, 0 .8 ~1 0 -~ M and 1 .6 ~1 0 -~ M, respectively.For all the curves the scan rate was 10 mV/s and the scans in the cathodic direction were started from their respective rest potentials.It is evident in Fig. 2 that a well defined cathodic wave is developed in the potential range of about 0 Vvs SCE to -300 mVvs SCE due to the presence of cupric ions in the solution, and this wave is developed to a cathodic peak around the potential 0 V vs SCE a t higher cupric acetate concentrations.At the potentials more cathodic to -300 mV vs SCE, increase in the current with the potential is also observed.Also, we observed that the electrodeposition of films occur at any potential value, starting from 0 V vs SCE to more cathodic potentials, as we could see them visually by examining the electrode surface.
In the study of the cyclic voltammetric curves we observed two distinct potential regions where the scans in the reversed direction resulted in two different results.Namely, if the cathodic scan was initiated from the rest potential an.d the potential where it is reversed is in the potential range of 0 V vs SCE to -300 mV vs SCE then only two anodic peaks can be observed in the reversed scans.This result is shown in Fig. 3(a) where the cupric acetate concentrationis 1 .0 ~ 10-2M: On the other hand, ifthe potential where the reverse scan starts to go beyond the potential value of -300 mV vs SCE, a n additional anodic peak (AI) is observed a t -150 mVvs SCE.As examples, two cyclic potential scans are shown in Fig. 3(b) and Fig. 3(b), where the scans were obtained in a cupric acetate concentration of 1.0 x 1 W 2 M. In Fig. 3(b), the potenkial where the reversed scan started is about-550 mV vs SCE, while in Fig. 3(c), this potential is about 700 mV vs SCE.It is clearly seen in Fig. 3(b) and Fig. 3(c), in addition to the anodic peaks A11 and AIII an additional anodic peak (AI) is present a t the potential around -100 mV vs SCE, as compared to Fig. 3(a).Poten'tial (mV vs SCE) We stumed the temperature dependence on the currentpotential curves in the temperature range of 30°C to 80°C.Except the general increase of the current with the temperature, we did not observe any significant departure of the qualitative behavior of the current-potential curves, as compared to the curves shown in Figures 2 and 3.
The XRD spectra of the films deposited on IT0 substrates a t the potentials -300 mV vs SCE, -500mV vs SCE and -700 mV vs SCE are shown in Fig. 4. The depositing potential for the film shown in Fig. 4 (a) was selected so that it lies in the potential range of 0 V vs SCE to -300 mV vs SCE, the potential range where a plateau is observed in Fig. 2. For the films shown in Fig. 4(b) and Fig. 4(c), the depositing potentials were cathodic to -300 mV vs SCE.I t is clearly seen in Fig. 4(a) that for the film deposited at potential -100 mV vs SCE, XRD peaks correspond to cuprous oxide and the IT0 substrate are evident.However, for the films deposited at the potentials-500 mVvs SCE and -700 mV vs SCE, additional peaks that correspond to Cu can be observed.
In general, the films grown on I T 0 substrates were uniform and adhered well to the substrate.The film thickness was estimated by measuring the total charge passed during the film deposition.The long wavelength transmittance spectrum of a film of thickness 2.41m, deposited at -100 mV vs SCE on I T 0 substrate is shown in Fig. 5(a).Fig. 5(b) shows the determination ofthe band gap ofthe film using the data in Fig. 5(a), from the plot ofthe equation o ah^)^ = A (hv -EP), where a is the absorption coefficient, Eg is the band gap and Ais a constant.The above equation applies for direct band gap materials and for the optical transmission near the band edge.

DISCUSSION
It is clearly seen that the electrodeposition of films on the electrode occurs in the entire cathodic potential range we studied here, as it is m d e s t e d in Fig. 2 by the increase in the cathodic current due to the presence of cupric ions in the bath solution.The plateau in Fig. :! corresponds to a particular reaction which produces a certain film on the electrode and the reaction products can be identified easily from the results shown in Figures 3 and 4. As shown in Fig. 3, the additional anodic peak (AI) present a t the potential -150 mV vs SCE can be seen only if the reversed scan started a t potentials cathodic to -300 mV vs SCE.
Therefore, the origin of this anodic peak must be due to the oxidation of reduced species in the cathodic potentials beyond -300 mV vs SCE.This ano&c peak has beenidentified previously as due to the oxidation of Cu to Cu,O and therefore Cu is deposited a t more cathodic potentials than -300 mV vs SCE.L2aL3 The other anodic peaks (AII) and (MII) are identified as due to the oxidation of Cu20 to CuO and CU(OH),.'~I~~Thus the potentiostatic electrodeposition of cuprous oxide is possible in the potential range between 0 V vs SCE and -300 mV vs SCE.If the depositing potential becomes more negative than -300 mV vs SCE, Cu is codeposited.This is clearly indicated by the hysteresis seen in the I-V curves in Fig. 3.This agrees very well with the XRD results shown in Fig. 3, where additional Cu peaks are evident if the depositing potential exceeds -300 mV vs SCE.This finding is very important in the electrodeposition of cuprous oxide films because it clearly indicates that electrodeposition of cuprous oxide thin films is possible only within a narrow potential domain of 0 V vs SCE to -300 V vs SCE.Further, our observations reveal that this domain does not depend on the temperature and the concentration of the bath solution.
The electrode reaction which governs the electrodeposition of Cu,O is

2.3
Energy (eV) The standard potential for this reaction is 0.203 V vs NHE and the estimated value of the reaction potential in an acetate bath of 0 .4 ~1 0 -~ M cupric ion concentration i s about -0.1 V v s SCE.This i s well within t h e potential values we found here for the electrodeposition of cuprous oxide films.We measured the thicknesses of the films directly and compared them with the c d r t l d ~+ d .
+J?lCkcps,c, b j k.rlnwi.cgt h ~ tsLtnlc h a r s ys,w.d.during the film.deposition.In the calculation we assumed that 2 electrons are consumed for depositing one Cu,O molecule and the densi ty of films is the bulk density ofsingle crystals.A very good agreement was obtained between the two methods.Particularly, our result ind~cates that by monitoring the current we can control the film thickness accurately, a.s the films are always uniform.The optical absorption of films deposited on tin oxide coated substrate clearly shows that the electrodeposited cuprous oxide is a semiconductor having a chrect band gap of 2.0 eV which is in very good agreement with the previously reported results.
In conclusion, we have found that the potentiostatic eletrodeposition of cuprous oxide thin films can be carried out in an acetate bath in a potential range of 0 Vvs SCE to -300 mVvs SCE.Also, we could observe that the extension of the depositing potential towards more cathochc potentials from -300mV vs SCE resulted in the co-deposition of copper.Further, i t was observed that electrodeposited cuprous oxide is a semiconductor of a direct band gap of 2.0 eV.

Figure 1 :
Figure 1: Diagram of the experimental arrangement used for the electrodeposition of cuprous oxide films.

Figure 5 :
Figure 5: (a) Long wavelength transmission spectrum and (b) (ahvIP vs energy (El for the cuprous oxide film deposited at -100 mV vs SCE.