Chemical bath deposition of thin films of CuO ...

16

Int. J. Nanomanufacturing, Vol. 5, Nos. 1/2, 2010

Chemical bath deposition of thin films of CuO nanorods and their characterisation Nillohit Mukherjee and Anup Mondal* Department of Chemistry, Bengal Engineering and Science University, Shibpur, P.O: Botanic Garden, Howrah-711 103, West Bengal, India Fax: (+91)33-2668-2916 E-mail: [email protected] E-mail: [email protected] *Corresponding author

Utpal Madhu School of Materials Science and Engineering, Bengal Engineering and Science University, Shibpur, P.O: Botanic Garden, Howrah-711 103, West Bengal, India E-mail: [email protected] Abstract: CuO is a p-type semiconducting material with room temperature band gap energy nearly at 1.4 eV for the bulk systems. Here, we present a simple one-pot chemical bath deposition of CuO thin films with nanorod-like crystallites and excellent uniformity of the surface, from an alkaline solution of Cu2+ ions. Structural characterisations of the films were carried out by studying XRD patterns and FESEM, EDX and AFM analyses. Broad FWHM of the XRD peaks indicates the formation of nanocrystals. Nanorod-like grain growth along c-axis is evident from the FESEM image. Average diameter of the nanorods was found to be ~40 nm. Optical absorption spectrum (UV-VIS) of the films showed a blue shift in the band gap energy to ~2.0 eV, due to quantum confinement effect showed by the nanocrystals. Various electrical measurements were also carried out. Keywords: chemical deposition; thin films; semiconducting material; nanorods; structural characterisations; electrical properties. Reference to this paper should be made as follows: Mukherjee, N., Mondal, A. and Madhu, U. (2010) ‘Chemical bath deposition of thin films of CuO nanorods and their characterisation’, Int. J. Nanomanufacturing, Vol. 5, Nos. 1/2, pp.16–24. Biographical notes: Nillohit Mukherjee received his MSc in Applied Chemistry from Bengal Engineering and Science University, Shibpur, India in 2003. Since then, he is working as a Research Fellow of the Department of Chemistry of the same university. He had already submitted his PhD thesis in the field of development and characterisation of thin film semiconducting materials. Presently, he is working as a CSIR Senior Research Fellow.

Copyright © 2010 Inderscience Enterprises Ltd.

Chemical bath deposition of thin films of CuO nanorods

17

Anup Mondal received his BSc (Chemistry), MSc (Inorganic) and PhD (Science, 1985) from Indian Institute of Technology, Kharagpur. He was a Postdoctoral Fellow at the Institute of Energy Conversion, University of Delaware, USA, during 1991–1992. He is currently a Professor of the Department of Chemistry, Bengal Engineering and Science University, Shibpur, India. His areas of interests are thin film semiconductors, solar cells and gas sensors. Utpal Madhu obtained his BE Mechanical Engineering from Regional Engineering College, Surat (NIT SURAT), India and MTech in Materials Engineering from Bengal Engineering and Science University, Shibpur, India. Currently, he is pursuing his Doctoral research on thin film solar cell at Bengal Engineering and Science University, Shibpur.

1

Introduction

CuO is a p-type semiconducting material with room temperature band gap energy of about 1.4 eV. The material is of interest due to its non-toxicity and applications in the fields like solar cells (Herion et al., 1980; Rakhshni, 1986; Tanakai et al., 1990; Yoona et al., 2000) (due to its high solar absorptance and low thermal emittance), gas sensors (Vasiliev et al., 1998; Bae and Choi, 1999), Li cells (Morales et al., 2005) and in high Tc superconductors (Desai et al., 2000), etc. There are several techniques like sol-gel, sputtering (Nakamura et al., 1990), chemical vapour deposition (Maruyama, 1998), spray pyrolysis (Morales et al., 2005), spin coating (Bae and Choi, 1999), etc., for CuO thin film deposition. Among these, the chemical methods were least studied. In our present work, we report a simple one-pot chemical bath deposition of CuO thin films from an alkaline solution of Cu2+ ions.

2

Experimental results and discussion

A properly cleaned transparent conducting oxide (TCO) coated glass substrate was dipped into a 100 ml beaker containing 95 ml of 0.01 M Cu(NO3)2 solutions. Then 1.5 ml of NH3 and after two minutes, 1.5 ml of NaBH4 solution was added to it. The temperature of the solution was gradually raised to 60°C. The NaBH4 solution was prepared by dissolving 0.3783 gm of solid NaBH4 in 30 ml NH3 (25%) and 70 ml of double distilled water, to make a 0.1 M solution. All the reagents were of A.R quality. The deposition starts after about 15 minutes from the addition of the NaBH4 solution. Another 1 ml of NaBH4 solution was added after five minutes from the start of film deposition and the same was repeated five minutes later. The reaction was carried out under continuous stirring. After about an hour, saturation in thickness was observed. The temperature was maintained around 60°C, below which, the rate of the reaction became very slow. At still higher temperature, the rate was fast, leading to poor film adherence of the film on the substrate.

18

N. Mukherjee et al. The chemical reactions that may be involved in this process can be cited as: Cu(NO3 ) 2 → Cu ++ + 2NO3− Cu ++ + 4NH3 → Cu(NH3 ) 4++ Cu(NH3 ) 4++ + NO3− + BH 4− + 2OH − = CuO + NO 2− + BO 2− + 4NH 3 + 3H 2 ↑

2.1 Study of film growth rate The nature of the film growth was studied under three different conditions, i.e.: a

by varying the amount of NaBH4 solution, while maintaining a constant volume of NH3 (1.5 ml) (Figure 1)

b

by varying the amount of NH3 at a fixed volume of NaBH4 solution (3.5 ml) (Figure 2)

c

a study of time vs. growth rate, at fixed amount of NH3 and NaBH4 solution (Figure 3).

The first two cases were investigated for a deposition time of one hour. The thickness of the films were measured gravimetrically, using a ‘AB54-S’ Mettler balance with an accuracy of 0.1 mg. Figure 1 represents the change in CuO film thickness on TCO glass with increasing amount of NaBH4 solution. It is evident from the plot that a maximum film thickness of about 2.7 × 10–4 cm was obtained on the addition of 3.5 ml of NaBH4 solution, for the one hour deposition. On keeping the amount of NaBH4 solution constant at 3.5 ml but varying the amount of NH3, the maximum thickness of the film of about 2.7 × 10–4 cm was obtained after one hour for 1.5 ml of NH3 (Figure 2). Figure 1

Plot of thickness vs. amount of NaBH4 solution added

Note: Time of deposition = one hour at 60°C

Chemical bath deposition of thin films of CuO nanorods Figure 2

19

Plot of thickness vs. amount of NH3 solution added

Note: Time of deposition = one hour at 60°C

In Figure 3, a typical plot of time vs. thickness of the deposited CuO films on TCO glass at 60°C has been represented, keeping both the amount of ammonia and NaBH4 solution fixed. It was found that the thickness of the deposited CuO film from the 60°C bath saturates after about 60 minutes. The growth rate was about 0.026 × 10–4 cm/min. Figure 3

Plot of time vs. thickness at 3.5 ml NaBH4 solution and 1.5 ml of NH3 at 60°C

2.2 X-ray diffraction pattern analysis The X-ray diffraction (XRD) pattern of the as-deposited powdered CuO (scraped off from the film after 30 minutes of deposition) was obtained by using a ‘SEIFERT 3000 P’ parallel beam X-ray diffractometer with the ‘Bragg-Brentano’ goniometer geometry is shown in Figure 4. The pattern revealed the presence of cubic phase of CuO, mainly with (002) and (111) diffraction planes (JCPDS reference no. 05-0661). Broad full width at

20

N. Mukherjee et al.

half maximum (FWHM) values for the major peaks is indicative of the nanocrystalline growth. Small diffraction from (110), (020), (113) and (022) planes of cubic CuO were also observed. Figure 4

XRD patterns of the powered and as-deposited CuO obtained after 30 minutes of depositions at 60°C

2.3 FESEM image analysis Figure 5 represents the FESEM image of a CuO film (taken by a ‘JEOL’ field emission scanning electron microscope of model no. ‘JEM 6700 F’) deposited on TCO-coated glass at 60°C. Nanorod-like grain growth with c-axis orientation is evident from this image. Average diameter of the nanorods was found to be around 40 nm. Figure 5

FESEM images of the CuO thin film deposited for 30 minutes at 60°C on TCO substrate

Note: Inset: high magnification image.


21

2.4 UV-VIS spectral analysis The UV-VIS spectra (in absorbance vs. eV mode) of the CuO films deposited at 60°C on TCO-coated glass was taken by a ‘JASCO V-530’ UV-VIS spectrophotometer and is shown in Figure 6. According to Amekura et al. (2006), the basis of optical transition for CuO and its interpretation is still in confusion. Some people have designated the absorption edge at ~1.4 eV to an indirect transition of semiconductors (Pena et al., 2006), whereas the majority opinion is the Mott-insulator type charge transfer transition (Ito et al., 1998). Here, we have observed a sharp rise in absorption spectrum from nearly 2.0 eV, which corresponds to the band gap energy for the CuO nanocrystals. The observed band gap energy value (~2.0 eV) is significantly higher than the value 1.4 eV (Amekura et al., 2006) for the bulk systems. This blue shift in the band gap energy might be due to the quantum confinement effect exerted by the CuO nanocrystals. Figure 6

(αhν)2 vs. hν plot for the CuO thin films deposited at 60°C for 30 minutes

2.5 DC I-V measurement DC I-V measurement of the SnO2:F (TCO)/CuO heterostructure was carried out in a voltage range of –1.0 V to +1.0 V under a frequency of 1 kHz, and shown in Figure 7. The electrical contact to the CuO films was made with Au, while the SnO2:F itself acted as the other electrical contact. It was found that the p-type CuO films formed Schottky junction with the n-type SnO2:F of the TCO-coated glass. The onset of Zener effect was also found to occur at nearly –0.75 V.

22 Figure 7

N. Mukherjee et al. DC I-V plot for the SnO2:F /CuO heterostructure

2.6 Capacitance-frequency and capacitance-voltage measurements The capacitance-frequency (C-ω) and capacitance-voltage (C-V) measurements for the CuO films were carried out using an ‘Agilent 4284 A precision LCR meter’. The contact material on the top of the CuO film was Au, while the bottom contact layer was TCO. The thickness of the CuO films was 1.85 × 10–4 cm. The C-ω measurements were carried out under three different DC bias levels and the output is shown in Figure 8. At a particular frequency, the film was found to have higher capacitance value under higher level of DC bias, due to the fact that the capacitance of a thin film increases under increased bias. The change in capacitance with frequency follows the typical exponential decay path. Figure 8

C-ω plot for CuO film deposited on TCO-coated glass


23

The non-linear nature of the C-V plot for the CuO films taken under 1 kHz frequency (Figure 9) also proves the fact that p-type CuO makes a rectifying barrier with the n-type SnO2 of the TCO coated glass. Figure 9

C-V plot for CuO film deposited on TCO coated glass

3

Conclusions

1

A simple chemical technique has been developed to deposit thin films of CuO.

2

The X-ray data revealed that the CuO films deposited at 60°C was of cubic phase and highly oriented along (111) and (002) plane.

3

Nanorod-like grain growth with c-axis orientation is evident from FESEM image. Average diameter of the nanorods was found to be around 40 nm.

4

UV-VIS spectrum showed a blue shifted band gap energy of about 2.0 eV.

5

I-V and C-V measurements showed that the p-CuO films formed a p-n junction with the n-SnO2 of the TCO.

Acknowledgements The authors would like to acknowledge Mr. Nepal Rakshit of BESU, Shibpur, Mr. Krishanu Sarkar and Mr. Debraj Chandra, IACS, Kolkata, for their technical assistance. NM is indebted to CSIR, India (8/3(0051)/2008-EMR-I) for financial assistance.

24

N. Mukherjee et al.

References Amekura, H., Umeda, N., Takeda, Y. and Kishimoto, N. (2006) ‘Optical transitions of Cu2O nanocrystals in SiO2 fabricated by ion implantation and two-step annealing’, Applied Physics Letters, Vol. 89, No. 22, pp.223120. Bae, H.Y. and Choi, G.M. (1999) ‘Electrical and reducing gas sensing properties of ZnO and ZnO-CuO thin films fabricated by spin coating method’, Sensors and Actuators B, Vol. 55, No. 1, pp.47–54. Desai, N.V., Shivagan, D.D., Ekal, L.A. and Pawar, S.H. (2000) ‘Role of room-temperature electrochemical oxidation in Pb-doped Bi-Sr-Ca-CuO superconducting thin films’, Materials Chemistry and Physics, Vol. 65, No. 1, pp.68–73. Herion, J., Niekish, E.A. and Scharl, G. (1980) ‘Investigation of metal oxide/cuprous oxide heterojunction solar cells’, Solar Energy Materials, Vol. 4, No. 1, pp.101–112. Ito, T., Yamaguchi, H., Masumi, T. and Adachi, S. (1998) ‘Optical properties of CuO studied by spectroscopic ellipsometry’, Journal of the Physical Society of Japan, Vol. 67, pp.3304–3309. Maruyama, T. (1998) ‘Copper oxide thin films prepared by chemical vapor deposition from copper dipivaloylmethanate’, Solar Energy Materials & Solar Cells, Vol. 56, No. 1, pp.85–92. Morales, J., Sanchez, L., Martyn, F., Ramos-Barrado, J.R. and Sanchez, M. (2005) ‘Use of low-temperature nanostructured CuO thin films deposited by spray-pyrolysis in lithium cells’, Thin Solid Films, Vol. 474, Nos. 1–2, pp.133–140. Nakamura, Y., Yoshioka, H., Miyayama, M. and Yanagida, H. (1990) ‘Selective CO gas sensing mechanism with CuO/ZnO heterocontact’, Journal of the Electrochemical Society, Vol. 137, No. 3, pp.940–943. Pena, O., Rodriguez-Fernandez, L., Cheang-Wong, J.C., Santiago, P., Crespo-Sosa, A., Munoz, E. and Oliver, A. (2006) ‘Characterization of nanocluster formation in Cu-implanted silica: influence of the annealing atmosphere and the ion fluence’, Journal of Non-Crystalline Solids, Vol. 352, No. 4, pp.349–354. Rakhshni, A.E. (1986) ‘Preparation, characteristics and photovoltaic properties of cuprous oxide – a review’, Solid State Electronics, Vol. 29, No. 1, pp.7–17. Tanakai, T., Ibarak, T., Saito, I., Sindo, H. and Fujnaka, M. (1990) ‘Voltaic effect of the humidity of (Zn, Cu)O thin films deposited by R.F. sputtering’, Thin Solid Films, Vol. 189, No. 1, pp.9–17. Vasiliev, R.B., Rumyantseva, M.N., Yakovlev, N.V. and Gaskov, A.M. (1998) ‘CuO/SnO2 thin film heterostructures as chemical sensors to H2S’, Sensors and Actuators B, Vol. 50, No. 3, pp.186–193. Yoona, K.H., Choia, U.W.J. and Kang., D.H. (2000) ‘Photoelectrochemical properties of copper oxide thin films coated on an n-Si substrate’, Thin Solid Films, Vol. 372, Nos. 1–2, pp.250–256.