Low-Temperature Electrodeposition of Cu2O Thin Films: Modulation of

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Low-Temperature Electrodeposition of Cu2O Thin Films: Modulation of Micro-Nanostructure by Modifying the Applied Potential and Electrolytic Bath pH S. Bijani,*,† L. Martı´nez,† M. Gaba´s,† E. A. Dalchiele,‡ and J.-R. Ramos-Barrado† Departamento de Fı´sica Aplicada I, Laboratorio de Materiales y Superficies, Campus de Teatinos, UniVersidad de Ma´laga, 29071 Ma´laga, Spain, and Instituto de Fı´sica, Facultad de Ingenierı´a, Herrera y Reissing 565, C.C. 30, 1100 MonteVideo, Uruguay ReceiVed: June 25, 2009; ReVised Manuscript ReceiVed: September 24, 2009

Copper(I) oxide (Cu2O) films were cathodically electrodeposited on titanium substrates. The influence of several electrodeposition parameters such as applied potential, pH, and bath temperature on phase composition, degree of crystallinity, grain size, and orientation were carefully examined using X-ray diffraction and scanning electron microscopy. Two different surface morphologies with different preferential crystal orientations are found at low temperature (30 °C) and pH 9 as a function of the applied potential. At the same temperature, highly crystalline pure Cu2O films are found at pH 12, which indicates that the crystallinity depends on the bath pH. A possible deposition mechanism is proposed, and we report on the influence of the applied potential on the crystalline structure of the deposited material. 1. Introduction Cuprous oxide (Cu2O) is a well-known p-type semiconductor, which has been the subject of research for some time.1 It is an attractive material for device applications because of its abundant availability, nontoxicity, and low production cost. Diverse fields of application have been found such as solar energy cells,2,3 photoelectrochemical cells,4,5 photocatalysts,6 gas sensors,7 and negative electrodes for Li ion batteries.8–10 Thin films of cuprous oxide have been deposited by various techniques such as thermal oxidation,11 chemical vapor deposition,12 electrodeposition,13 spray pyrolysis,14 chemical oxidation,15 anodic oxidation,16 and sputtering.17 Among the various deposition techniques available, electrodeposition is a versatile and low-cost technique for preparing thin films of semiconductor oxides over conductive substrates. Its interest arises from the versatility of the technique, which has the following salient features: low processing temperature, simple control of deposition thickness, high purity of deposits, and low instrumentation cost. Recently, our group reported on the electrochemical performance of Cu2O thin films prepared by electrodeposition starting from cupric lactate alkaline solutions.18,19 These electrodes exhibited a good electrochemical response compared to lithium electrodes. However, a detailed study of the structural and morphological characteristics of the deposited films was not presented in these works. We must take into account that size, morphology, degree of agglomeration, and specific surface area of the Cu2O particles in the electrodes play a crucial role in the performance of lithium ion batteries, and all of these parameters are strongly dependent on the films electrodeposition conditions. It is thus reasonable to expect that detailed investigation of the synthesis of these Cu2O films should enrich our understanding of its fundamental properties and further extend its scope of application. Switzer et al. and other authors13,20–25 studied the * To whom correspondence should be addressed. Telephone: +34952137654. Fax: +34-952132382. E-mail: [email protected]. † Universidad de Ma´laga. ‡ Instituto de Fı´sica, Facultad de Ingenierı´a.

influence of electrochemical variables on deposition in the potentiostatic mode of Cu2O films using a cupric lactate bath, but an in-depth study of the films deposited at low temperature (30 °C) and bath pH 9 has not yet been undertaken. This paper reports on the influence of the applied potential, temperature, and pH bath on electrodeposited cuprous oxide thin films. In particular, the studies have focused on phase composition, crystalline orientation, and structure and surface morphology. 2. Experimental Section Electrodeposition was performed cathodically on titanium substrates at two different temperatures (30° and 60 °C), using a solution consisting of 0.4 M cupric sulfate and 3 M lactic acid (85%). An adequate amount of sodium hydroxide was added in order to adjust the bath pH values to either 9 or 12. The substrates were used as cathodes (or working electrodes WE) in a deposition cell. This consists of a platinum sheet (anode) which acts as a counter electrode (CE) and a commercial saturated calomel electrode (SCE) as the reference electrode (RE). The RE assembly consisted of a Pyrex glass tube with a capillary opening at one end, filled with a saturated solution of KCl. The electrodeposition cell was made from a Pyrex glass beaker of 110 mL disolution volume, closed with a gastight lid made from Teflon. Several openings in the lid were used to insert electrode holders, a thermometer, reference electrode assembly, and inlet and outlet terminals for gas purging. The cell was mounted on a heater-stirrer unit. Electrodeposition was carried out in the potentiostatic mode at different applied potential values, from -150 mV to -800 mV, with respect to the SCE. Deposition parameters were controlled and measured simultaneously by using an AMEL 2053 potenciostat-galvanostat apparatus. The deposition area was about 1 cm × 1 cm. For all of the samples, the charge passed during electrodeposition was about 0.75 C/cm2. The electrolytic solutions were deaerated prior to the experiment by purging with nitrogen. Before the electrochemical experiment, the electrode surface was polished with emery paper, washed with detergent for 15 min in an ultrasonic bath, and finally

10.1021/jp905952a CCC: $40.75  2009 American Chemical Society Published on Web 10/20/2009

Low-Temperature Electrodeposition of Cu2O Thin Films

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Cu2+ + 2e- f Cu

Figure 1. (a) Linear sweep voltammogram for Cu (II) lactate reduction at 30 °C and pH 9. (b) Current density-time curve of a Cu2O thin film deposited at an applied potential of -200 mV under the above conditions.

etched in HF (24% vol) for approximately 20 s and rinsed with doubly distilled water immediately before immersion in the electrolytic bath. To determine the phase and crystalline structure of the deposited films, X-ray diffraction profiles from as-deposited films were recorded at room temperature on a Siemens D5000 diffractometer, using Cu KR radiation operating at 40 kV and 40 mA in a step scan mode with a step size of 0.02° 2θ (normally between 10° and 90°) and a counting time of 2 s per step. The surface morphology of the electrodeposited films was examined in a JEOL JSM-5410 SEM. 3. Results and Discussion 3.1. Electrochemical Study. To determine the electrochemical processes that take place in the cathodic reduction of cupric lactate at a bath temperature of 30 °C and pH 9, a linear sweep voltammogram was carried out scanning cathodically between -150 and -1100 mV at a scan rate of 10 mV/s. This sweep, shown in Figure 1a, reflects two cathodic peaks placed around the potential values of -350 and -700 mV versus SCE. The specific reduction reactions the system undergoes at these potentials were unknown a priori. However, the determination of the crystalline composition of thin films synthesized at these potentials using XRD indicates that the cathodic peak observed at -350 mV corresponds to the reduction of Cu2+ to Cu+ and the subsequent formation of Cu2O, and the peak at about -700 mV is due to the reduction of Cu2+ to Cu. Thus, the proposed reactions for the cathodic reduction of cupric lactate are

2Cu2+ + 2e- + H2O f Cu2O + 2H+

(1)

(2)

where reactions 1 and 2 are specified in Figure 1a as process A and B, respectively. The mechanism that describes the electrodeposition of Cu2O by reduction of a basic aqueous solution of cupric lactate, according to the aforementioned reaction 1, has already been reported in the literature.9,13,26 Figure 1b shows the current density versus time curve of the Cu2O thin film deposited at -200 mV versus SCE, 30 °C, and pH 9. A sharp rise in the current density is observed in the first moments of the process as a result of double-layer charging, posterior nucleation, growth, and coalescence mechanisms of the deposited grains. After this initial time, a decrease in current density is recorded because the deposited material Cu2O is a semiconductor. 3.2. Structural Determination by X-ray Diffraction (XRD). Influence of Applied Potential. The effect of the applied potential on the crystal phase composition of the films deposited at a temperature of 30 °C and pH bath of 9 was investigated by XRD. Figure 2a illustrates the X-ray diffraction profiles of the films deposited using applied potential values versus SCE between -150 and -800 mV. The diffraction peaks obtained from the JCPDS card (78-2076) corresponding to a polycrystalline pure Cu2O powder pattern are also shown in Figure 2b. Apart from the reflections corresponding to the titanium substrate, the diffraction peaks corresponding to synthesized films between -150 and -600 mV are ascribed to pure Cu2O, while the formation of no other crystalline phase is observed. At -800 mV, only diffraction peaks corresponding to pure metallic Cu are seen. To determine the preferred orientation of the Cu2O films, the intensity ratio I(111)/I(200) for the two most intense peaks is plotted as a function of the applied potential, and it is shown in Figure 3 using a layout taken from ref 13. The relative intensity ratio I(111)/I(200) value corresponding to a polycrystalline pure Cu2O sample without preferential orientation is indicated by a

Figure 2. (a) XRD patterns for Cu2O films electrochemically deposited under different applied potentials at 30 °C and pH 9: Cu2O (b), metallic Cu (1), and Ti substrate (*). (b) Reflections of Cu2O according to the JCPS card (78-2076).

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Figure 3. Plot of relative intensity I (111)/I (200) of X-ray diffraction peaks as a function of applied potential at 30 °C and pH 9 during film deposition. Figure 6. XRD patterns for Cu2O films electrochemically deposited at -575 mV, with a bath temperature of 30 °C and bath pH 9 (below) and pH 12 (above). Ti peaks are marked with an asterisk.

Figure 4. Crystallite sizes of Cu2O thin films grown at different electrodeposition potentials at 30 °C and pH 9, estimated from the fwhm of the principal diffraction peaks: (111) and (200) as indicated.

Figure 7. Plot of relative intensity I(111)/I(200) of X-ray diffraction peaks as a function of applied potential; T ) 60 °C and pH 9.

Figure 5. XRD patterns for Cu2O films electrochemically deposited at -400 mV, with a bath temperature of 30 °C and bath pH 9 (below) and pH 12 (above). Ti peaks are marked with an asterisk.

solid horizontal line, and it has a value of 2.88. Above this value, polycrystalline Cu2O thin films are formed in a [111] preferred orientation, whereas if this ratio is smaller than 2.88, the samples reveal a [100] texture. Thus, pure Cu2O films deposited between -150 and -400 mV displayed a [100] preferential orientation, while films deposited between -500 and -600 mV displayed a slight [111] preferential orientation. The dimensions of the crystallites were estimated from the fwhm of the principal diffraction peaks, (111) and (200), using the Scherrer formula.27 When the term “crystallite size” is used,

we refer to the dimensions of the coherent diffracting domain. This formula is applicable to samples where lattice strain is absent. Electrodeposited thin films can possess some strains, which could also contribute to peak widening, thereby affecting the estimation of the crystallite size. Therefore, the size of the crystalline domains determined from the XRD peak widths is used only as a comparative parameter among samples. The crystallite sizes are presented in Figure 4. A marked decrease in crystallinity can be observed as the electrodeposition potential becomes more cathodic. It is known that the use of high current densities for the electrodeposition of materials almost invariably leads to fine-grained polycrystalline growth.20 When comparing these results with those shown in Figure 3, it is also seen that the change in the crystal size as a function of the applied potential corresponds to a change in the preferred orientation, i.e., crystal sizes in the [100] oriented films are larger. Influence of Bath pH. The effect of bath pH on the preferred orientation and grain size of Cu2O films was investigated by selecting the applied potential at a fixed value and using bath pH values of 9 and 12. The bath temperature was maintained at 30 °C. Figure 5 shows the XRD patterns of the Cu2O samples electrodeposited at -400 mV for both pH values. A change in the preferential orientation as a function of pH is observed. Thus, for films grown at -400 mV, those deposited at a bath pH of 12 show the [111] preferred orientation, while those deposited

Low-Temperature Electrodeposition of Cu2O Thin Films

Figure 8. Plot of crystallite size values obtained from the fwhm corresponding to the X-ray diffraction peak (111) of Cu2O films electrodeposited at 30 and 60 °C as a function of the applied potential.

at a bath pH of 9 grow with the [100] crystal orientation. Some authors have observed that depositions carried out at pH values between 8 and 9 and at temperatures above 30 °C produce Cu2O films with a preferential orientation in the [100] direction, whereas films deposited at pH > 9 have a preferential orientation in the [111] direction.22 Figure 6 shows the effect of bath pH in samples electrodeposited at -575 mV, which reflects a significant increase in the degree of preferential orientation in the [111] direction when increasing the bath pH from 9 to 12.

J. Phys. Chem. C, Vol. 113, No. 45, 2009 19485 On the other hand, for both potentials, the signal-to-noise ratio is much improved with a pH increase, which represents an increase in the films crystallinity. Influence of Bath Temperature. An interesting aspect of the cathodic electrodeposition of Cu2O is the observation of how the electrodeposition bath temperature influences composition, preferred orientation, grain size, and surface morphology of the deposited materials. Thus, in order to compare with the results obtained for the films deposited at 30 °C, different samples were electrodeposited at a bath temperature of 60 °C and pH 9. The XRD results (not shown here) reveal that the Cu2O/Cu composite film is detected around -500 mV versus SCE, and a pure metallic Cu film is obtained when deposition is carried out at -600 mV. However, at the latter potential and 30 °C, pure Cu2O films are obtained. Therefore, the potential window for the formation of pure Cu2O is narrower when the bath temperature is fixed at 60 °C. To determine the preferred orientation of thin films deposited at 60 °C, the plot of the relative intensity I(111)/I(200) of the X-ray diffraction peaks as a function of the applied potential is shown in Figure 7. While increasing the applied potential toward more negative values, a dramatic change in the preferred orientation from [100] to [111] takes place before -400 mV.13 This effect is more pronounced for films deposited at 60 °C than for those deposited at 30 °C. It should be noted that at -400 mV, the films deposited at 60 °C show a preferential orientation in the [111] direction, while those synthesized at 30 °C are oriented in the [100] direction. Therefore, we may conclude that regardless of the temperature of the electrolytic

Figure 9. SEM micrographs of Cu2O films deposited at different applied potentials; T ) 30 °C and pH 9.

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Figure 12. SEM micrographs of Cu2O films deposited at different bath temperatures; applied potential and bath pH are -300 mV and 9, respectively.

Figure 10. Cross-sectional view of SEM micrographs of Cu2O films deposited at different applied potentials; T ) 30 °C and pH 9.

Figure 13. SEM micrographs of Cu2O films deposited at different bath temperatures; applied potential and bath pH are -600 mV and 9, respectively.

Figure 11. SEM micrographs of Cu2O films deposited at different bath pH values; applied potential and bath temperature are -400 mV and 30 °C, respectively.

bath, a change in the preferential orientation of the Cu2O deposited grains at pH 9 is observed when the applied potential is changed. A comparative plot of the crystallite size values, obtained from the fwhm of the Cu2O (111) diffraction peaks using Scherrer formula, for typical films deposited at 30 and 60 °C is shown in Figure 8. The films electrodeposited at 30 °C show crystallite size values that decrease as the deposition potential is made more and more cathodic. On the other hand, the

crystallite size values exhibited by the films electrodeposited at 60 °C are greater than that presented by the films grown at 30 °C, along the same electrodeposition potential range. The films grown at 60 °C, after a high and anomalous crystallite size value at -300 mV, exhibit a decrease in crystallinity as the potential is made more cathodic, exactly as it happens when bath temperature is 30 °C. 3.3. Morphological Study by Scanning Electron Microscopy (SEM). Influence of Applied Potential. The morphological observations made by SEM of the films deposited at 30 °C, pH 9, and different applied potentials are shown in Figure 9. The surface of the films deposited at -150, -200, and -400 mV (Figure 9a-c, respectively) are formed by regular, well-faceted, polyhedral crystallites. There are appreciable differences in grain size because it decreases as the applied potential becomes more cathodic. Thus, at -150 mV, the length of the largest polyhedral edges tends to be approximately 800 nm, whereas films deposited at -400 mV give a grain size of less than 200 nm. The morphology of the Cu2O particles on the films changes dramatically when increasing the applied overpotential. Panels

Low-Temperature Electrodeposition of Cu2O Thin Films d-f of Figure 9 show the SEM micrographs of films deposited at -500, -575, and -600 mV, respectively. These films exhibited a granular morphology, and the grains agglomerate with a cauliflower-like appearance, which has not been seen so far in the bibliography. Irrespective of the applied potential, it should be noted that the grains of these films were on the order of tens of nanometers. These observations imply, on the one hand, that each of the two preferred orientations detected corresponds to a different surface morphology, and on the other hand, that the grains with the [100] orientation are larger than the [111] oriented grains. These results indicate that the applied potential has a very significant influence on the morphology and grain size of Cu2O films deposited under specified conditions. In panels a and b of Figure 10, SEM cross-sectional images of films deposited at -400 and -500 mV are shown, also displaying differences in the compactness of the films as a function of the applied potential. Porous deposits are obtained in samples synthesized at potentials equal to or more negative than -500 mV, i.e., with [111] oriented Cu2O grains, while a much more compact film is observed with an applied potential of -400 mV or less cathodic. Influence of Bath pH. SEM images reveal that the estimated grain size in films synthesized at 30 °C is strongly pH dependent as shown in Figure 11. This result has already been corroborated by the XRD results. The increase in bath pH causes an enlargement of grain size as it is when working at higher temperatures.13 In general, no differences in the surface morphology are observed as a function of the applied potential for films deposited in a bath of pH 12, always retaining compact granular polyhedral forms with an irregular distribution of grains and/or inhomogeneous crystals. No samples were obtained with aporous cauliflower morphology, despite being prepared with high overpotentials. Influence of Bath Temperature. SEM images of the Cu2O thin film surfaces deposited at both bath temperatures studied and with two different applied potentials are shown in Figures 12 and 13. In the case of the films deposited at -300 mV (Figure 12), the same compact surface morphology is observed for both temperatures, although there is an appreciable increase in grain size for the samples electrodeposited at 60 °C. In the case of the films deposited at -600 mV (Figure 13), considerable differences are obvious in the surface morphology of both samples. The previously mentioned nanometric-sized grain, with a “cauliflower-like” porous granular morphology, is observed at 30 °C, while the films deposited at 60 °C maintain the compact granular morphology with larger polyhedral grains. 4. Conclusions A potentiostatic cathodic deposition of Cu2O thin films has been carried out on titanium substrates under various deposition conditions. The microstructural parameters for pure Cu2O films were evaluated and found to be strongly dependent on the deposition conditions. For a low deposition temperature, 30 °C, and pH value of 9, when the potential is between -150 and -400 mV versus SCE, we see the formation of regular, wellfaceted, polyhedral crystallites. By contrast, the films deposited at more negative potentials (from -500 to -600 mV) exhibited a granular morphology with grains showing a cauliflower-like appearance. A change of the preferential orientation from the

J. Phys. Chem. C, Vol. 113, No. 45, 2009 19487 [100] to [111] direction is observed as a function of the applied potential. Preferred orientation is also temperature and pH bath dependent. The crystallinity of the films is strongly dependent on the bath pH value. The films deposited at pH 12 always have a compact morphology. In the same way, the morphology of the films deposited at 60 °C and pH 9 does not change with the applied potential. Acknowledgment. This paper is supported by the FEDER funds of the European Commission for Scientific Infrastructure, managed in collaboration with the Spanish Ministry of Science and Technology. The Spanish Ministry of Innovation and Science, with the research project reference Project MAT200204477-C02-01, TEC2007-60996, and Project Consolider Ingenio 2010 “FUNCOAT”, and the Junta de Andalucı´a (Regional Andalusian Government) through the research group FQM-192 have also contributed. E.A.D. thanks CSIC-UDELAR and PEDECIBA-FISICA Uruguay for the suport received. References and Notes (1) Rakhshani, A. E.; Al-Jassar, A. A.; Varghese, J. Thin Solid Films 1987, 148, 191. (2) Mittiga, A.; Salza, E.; Sarto, F.; Tucci, M.; Vasanthi, R. Appl. Phys. Lett. 2006, 88, 163502. (3) Georgieva, V.; Ristov, M. Sol. Energy Mater. Sol. Cells 2002, 73, 67. (4) Siripala, W.; Ivanovskaya, A.; Jaramillo, T. F.; Bareck, S.-H.; McFarland, E. W. Sol. Energy Mater. Sol. Cells 2003, 77, 229. (5) Mahalingam, T.; Chitra, J-S.P.; Chu, J. P.; Moon, M.; Kwon, H. J.; Kim, Y. D. J. Mater. Sci.: Mater. Electron. 2006, 17, 519. (6) Hara, M.; Kondo, T.; Domen, K. Chem. Commun. 1998, 3, 357. (7) Shishiyanu, S. T.; Shishiyanu, T. S.; Lupan, O. I. Sens. Actuators, B 2006, 113, 468. (8) Grugeon, S.; Laruelle, S.; Herrera-Urbina, R.; Dupont, L.; Poizot, P.; Tarascon, J.-M. J. Electrochem. Soc. 2001, 148, A285. (9) Laik, B.; Poizot, P.; Tarascon, J.-M. J. Electrochem. Soc. 2002, 149, A251. (10) Fu, L. J.; Gao, J.; Zhang, T.; Cao, Q.; Yang, L. C.; Wu, Y. P.; Holze, R.; Wu, H. Q. J. Power Sources 2007, 174, 1197. (11) Gong, Y. S.; Lee, C.; Yang, C. K. J. Appl. Phys. 1995, 77, 5422. (12) Maruyama, T. Jpn. J. Appl. Phys. 1998, 37, 4099. (13) Zhou, Y.; Switzer, Y. A. Scr. Mater. 1998, 38 (11), 1731. (14) Ottosson, M.; Carlsson, J. O. Surf. Coat. Technol. 1996, 78, 263. (15) Ray, S. C. Sol. Energy Mater. Sol. Cells 2001, 68, 307. (16) Ashworth, V.; Fairurst, D. J. Electrochem. Soc. 1997, 124, 506. (17) Ghosh, S.; Avasthi, D. K.; Shah, P.; Ganesan, V.; Gupta, A.; Sarandi, D.; Bhatacharya, R.; Assmann, W. Vacuum 2000, 57, 377. (18) Morales, J.; Sa´nchez, L.; Bijani, S.; Martı´nez, L.; Gaba´s, M.; Ramos-Barrado, J. R. Electrochem. Solid-State Lett. 2005, 8, A159. (19) Bijani, S.; Gaba´s, M.; Martı´nez, L.; Ramos-Barrado, J. R.; Morales, J.; Sa´nchez, L. Thin Solid Films 2007, 515, 5505. (20) Rakhshani, A. E.; Varghese, J. J. Mat. Sci. 1998, 23, 3847. (21) Chatterjee, A. P.; Mukhopadhyay, A. K.; Chakraborty, A. K.; Sasmal, R. N.; Lahiri, S. K. Mater. Lett. 1991, 11 (10,11,12), 358. (22) Mahalingam, T.; Chitra, J. S. P.; Chu, J. P.; Sebastian, P. J. Mater. Lett. 2004, 58, 1802. (23) Daltin, A.-L.; Addad, A.; Chopart, J.-P. J. Cryst. Growth 2005, 282, 414. (24) Mizuno, K.; Izaki, M.; Murase, K.; Shinagawa, T.; Chigane, M.; Inaba, M.; Tasaka, A.; Awahura, Y. J. Electrochem. Soc. 2005, 152, C179. (25) Li, X.; Tao, F.; Jiang, Y.; Xu, Z. J. Colloid Interface Sci. 2007, 308, 460. (26) Bijani, S. Electrodeposicio´n y Caracterizacio´n de la´minas de Cu2O. Aplicacio´n como electrodos de baterı´as de Io´n-Litio; Servicio de Publicaciones de la Universidad de Ma´laga: Ma´laga, Spain, 2008; ISBN 978-849747-503-7. (27) Cullity, B. D. Elements of X-ray Diffraction; 2nd ed.; AddisonWesley: Reading, MA, 1978.

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