We have demonstrated hydrothermal synthesis of rectangular pillar-like CuO nanostructures at low temperature (~60°C) by selective growth on top of NiO porous structures film deposited using chemical bath deposition method at room temperature using indium tin oxide (ITO) coated glass plate as a substrate. The growth of CuO not only filled the NiO porous structures but also formed the big nanopillars/nanowalls on top of NiO surface. These nanopillars could have significant use in nanoelectronics devices or can also be used as p-type conducting wires. The present study is limited to the surface morphology studies of the thin nanostructured layers of NiO/CuO composite materials. Structural, morphological, and absorption measurement of the CuO/NiO heterojunction were studied using state-of-the-art techniques like X-ray diffraction (XRD), transmission electron microscopy (SEM), atomic force microscopy (AFM), and UV spectroscopy. The CuO nanopillars/nanowalls have the structure in order of (5？±？1.0)？μm × (2.0？±？0.3) μm; this will help to provide efficient charge transport in between the different semiconducting layers. The energy band gap of NiO and CuO was also calculated based on UV measurements and discussed. 1. Introduction In the modern society, environmental and energy resource concerns have been increasing, and because of that, greater stress has been placed on development of renewable energy resources especially on solar energy based photovoltaic cells, whose economic feasibility relies on efficient collection, retention, and utilization of photons. Over the past decade, research on solar cell has become one of the hot topics within science and engineering [1–3]. The need for higher solar cell efficiencies at lower cost has become apparent, and at the same time synthetic control of nanostructures using top-down/bottom-up approaches has improved such that the high performance electronic devices are becoming possible [4, 5]. Inorganic nanostructures [6] with tailored geometry over their organic counterparts are expected to play significant roles for the next-generation nanoscale electronic, optoelectronic, electrochemical, and electromechanical devices [7–10]. Copper oxides (CuO and Cu2O) are p-type semiconductor oxides, suitable materials for high efficiency solar cells due to their band gap of 1.3 and 2.0？eV, respectively, which are close to the ideal energy gap for solar cells, and well matched with the solar spectrum. CuO has been intensively studied for photovoltaic/sensing devices due to its rich family of nanostructures and promising
References
[1]
A. I. Hochbaum and P. Yang, “Semiconductor nanowires for energy conversion,” Chemical Reviews, vol. 110, no. 1, pp. 527–546, 2010.
[2]
M. Law, J. Goldberger, and P. Yang, “Semiconductor nanowires and nanotubes,” Annual Review of Materials Research, vol. 34, no. 1, pp. 83–122, 2004.
[3]
Z. Fan, D. J. Ruebusch, A. A. Rathore et al., “Challenges and prospects of nanopillar-based solar cells,” Nano Research, vol. 2, no. 11, pp. 829–843, 2009.
[4]
L. Cao, J. Park, P. Fan, B. Clemens, and M. L. Brongersma, “Resonant germanium nanoantenna photodetectors,” Nano Letters, vol. 10, no. 4, pp. 1229–1233, 2010.
[5]
J. Xiang, W. Lu, Y. Hu, Y. Wu, H. Yan, and C. M. Lieber, “Ge/Si nanowire heterostructures as high-performance field-effect transistors,” Nature, vol. 441, no. 7092, pp. 489–493, 2006.
[6]
W. Shi, S. Song, and H. Zhang, “Hydrothermal synthetic strategies of inorganic semiconducting nanostructures,” Chemical Society Reviews, vol. 42, no. 13, pp. 5714–5743, 2013.
[7]
C. N. R. Rao and A. Govindaraj, “Synthesis of inorganic nanotubes,” Advanced Materials, vol. 21, no. 42, pp. 4208–4233, 2009.
[8]
J. Yu, F. Wang, Y. Wang, H. Gao, J. Li, and K. Wu, “Interfacial reaction growth approach to preparing patterned nanomaterials and beyond,” Chemical Society Reviews, vol. 39, no. 5, pp. 1513–1525, 2010.
[9]
P. Yang, R. Yan, and M. Fardy, “Semiconductor nanowire: whats next?” Nano Letters, vol. 10, no. 5, pp. 1529–1536, 2010.
[10]
J. Jie, W. Zhang, I. Bello, C. Lee, and S. Lee, “One-dimensional II-VI nanostructures: synthesis, properties and optoelectronic applications,” Nano Today, vol. 5, no. 4, pp. 313–336, 2010.
[11]
M. Faisal, S. B. Khan, M. M. Rahman, A. Jamal, and A. Umar, “Ethanol chemi-sensor: Evaluation of structural, optical and sensing properties of CuO nanosheets,” Materials Letters, vol. 65, no. 9, pp. 1400–1403, 2011.
[12]
X. Jiang, W. Huang, H. Li, and X. Zheng, “Catalytic properties of CuO/Ce0.2Ti0.8O2 and CuO/Ce0.5Ti0.5O2 in the NO + CO reaction,” Energy Fuels, vol. 24, pp. 261–266, 2009.
[13]
D. Li, Y. H. Leung, A. B. Djuri？i？ et al., “CuO nanostructures prepared by a chemical method,” Journal of Crystal Growth, vol. 282, no. 1-2, pp. 105–111, 2005.
[14]
G. Gao, H. Wu, M. Chen, L. Zhang, B. Yu, and L. Xiang, “Synthesis of size- and shape-controlled CuO assemblies,” Journal of the Electrochemical Society, vol. 158, no. 3, pp. K69–K73, 2011.
[15]
L. C. Olsen, R. C. Bohara, and M. W. Urie, “Explanation for low‐efficiency Cu2O Schottky‐barrier solar cells,” Applied Physics Letters, vol. 34, no. 1, pp. 47–50, 1979.
[16]
J. Herion, E. A. Niekisch, and G. Scharl, “Investigation of metal oxide/cuprous oxide heterojunction solar cells,” Solar Energy Materials, vol. 4, no. 1, pp. 101–112, 1980.
[17]
M. Izaki, K. Mizuno, T. Shinagawa, M. Inaba, and A. Tasaka, “Photochemical construction of photovoltaic device composed of p-copper(I) oxide and n-zinc oxide,” Journal of the Electrochemical Society, vol. 153, no. 9, pp. C668–C672, 2006.
[18]
M. Izaki, T. Shinagawa, K. Mizuno, Y. Ida, M. Inaba, and A. Tasaka, “Electrochemically constructed p-Cu2O/n-ZnO heterojunction diode for photovoltaic device,” Journal of Physics D: Applied Physics, vol. 40, no. 11, article 010, pp. 3326–3329, 2007.
[19]
H. Huang, J. Tian, W. K. Zhang et al., “Electrochromic properties of porous NiO thin film as a counter electrode for NiO/WO3 complementary electrochromic window,” Electrochimica Acta, vol. 56, no. 11, pp. 4281–4286, 2011.
[20]
U. M. Patil, R. R. Salunkhe, K. V. Gurav, and C. D. Lokhande, “Chemically deposited nanocrystalline NiO thin films for supercapacitor application,” Applied Surface Science, vol. 255, no. 5, pp. 2603–2607, 2008.
[21]
G. Cai, J. Tu, J. Zhang et al., “An efficient route to a porous NiO/reduced graphene oxide hybrid film with highly improved electrochromic properties,” Nanoscale, vol. 4, no. 18, pp. 5724–5730, 2012.
[22]
X. H. Xia, J. P. Tu, J. Zhang, X. L. Wang, W. K. Zhang, and H. Huang, “Morphology effect on the electrochromic and electrochemical performances of NiO thin films,” Electrochimica Acta, vol. 53, no. 18, pp. 5721–5724, 2008.
[23]
M. A. Dar, Q. Ahsanulhaq, Y. S. Kim, J. M. Sohn, W. B. Kim, and H. S. Shin, “Versatile synthesis of rectangular shaped nanobat-like CuO nanostructures by hydrothermal method; structural properties and growth mechanism,” Applied Surface Science, vol. 255, no. 12, pp. 6279–6284, 2009.
[24]
Y. Xu, D. Chen, and X. Jiao, “Fabrication of CuO pricky microspheres with tunable size by a simple solution route,” Journal of Physical Chemistry B, vol. 109, no. 28, pp. 13561–13566, 2005.
[25]
H. Xu, W. Wang, W. Zhu, L. Zhou, and M. Ruan, “Hierarchical-oriented attachment: from one-dimensional Cu(OH)2 nanowires to two-dimensional CuO nanoleaves,” Crystal Growth and Design, vol. 7, no. 12, pp. 2720–2724, 2007.
[26]
H. Yu, J. Yu, S. Liu, and S. Mann, “Template-free hydrothermal synthesis of CuO/Cu2O composite hollow microspheres,” Chemistry of Materials, vol. 19, no. 17, pp. 4327–4334, 2007.
