Microstructural and optical studies of CuO thin films prepared by chemical ageing of copper substrate in alkaline ammonia solution Abdul Hai Alami a,⇑ , Anis Allagui a , Hussain Alawadhi b a Sustainable and Renewable Energy Engineering, University of Sharjah, PO Box 27272, Sharjah, United Arab Emirates b Department of Applied Physics, University of Sharjah, PO Box 27272, Sharjah, United Arab Emirates article info Article history: Received 12 June 2014 Received in revised form 30 July 2014 Accepted 31 July 2014 Available online 8 August 2014 Keywords: Chemical conversion coating Cupric oxide Optical spectroscopy Band gap abstract In this paper, we report on the time-dependent development of cupric oxide films by chemical modifica- tion of copper substrates submerged horizontally in a room temperature 75 mmol/L ammonia solution at pH 11, over a period of nine days. Morphological and structural characterization of the oxide-substrate tandems were carried out by SEM–EDX and XRD, while the relative directional spectral absorptivity and reflectivity were determined by Vis–NIR spectrometry. The ageing process, controlling both the color and morphological structure of the predominately amorphous-CuO/Cu, has positively contributed to the enhancement of spectral absorptivity, while band gap values evolve from 1.29 to 1.39 eV for exposure times from 36 to 168 h. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Copper is a material of choice in many applications because of its high thermal and electrical conductivities, in addition to its non- toxicity, natural abundance and relatively inexpensive cost. Its oxi- des are amongst the first known p-type direct band gap semicon- ductor materials, although n-type conductivity was also reported [1], and have found numerous applications in the fabrication of photovoltaic devices [2], electrochromics [3], catalytic and photo- catalytic reactions [4–7], magnetic storage and gas sensing materi- als [8]. The yellowish cuprous oxide (Cu 2 O) is highly transparent and usually absorbs at wavelengths shorter than 600 nm, whilst the brownish black cupric oxide (CuO) absorbs strongly throughout the whole visible spectral range. The direct optical band gap for copper oxides depends mainly on the crystalline phase and stoichi- ometry [9,10]. Depending on the fabrication method, the reported band gap energies vary between 2.1 and 2.6 eV for Cu 2 O [11–14] and between 1.3 and 2.1 eV for CuO [11,12,15]. The preparation of CuO x deposits has been carried out by many physical and chemical techniques such as chemical vapor deposi- tion [16], electrochemical deposition [14,17], thermal oxidation [18,19], reactive sputtering [13], plasma evaporation [12], and molecular beam epitaxy [20]. However, the interest was mainly focused on crystalline structures. Crystalline materials have well-defined properties, and as such received more channeled efforts to develop proper manufacturing techniques when com- pared to amorphous materials. The latter have disordered bulk structures with coarse surfaces because of the large structural defects density. The disorder in amorphous materials structures does not necessarily diminish their applicability, as has been shown, for example, with amorphous iron oxide for the electrocat- alytic oxidation of cyclohexane by Srivastava et al. [21], and by Suslick et al. [22] for the Fischer–Tropsch hydrogenation of carbon monoxide and for hydrogenolysis and dehydrogenation of satu- rated hydrocarbons. Another example is that the luminescence efficiency is found to be higher for amorphous silicon because of the natural structural disorder [23]. The band gap energy of bulk amorphous Si (1.6 eV) is also larger than that of bulk crystalline phase. On the other hand, apart from the ordered atomic self- arrangement specific for the above-mentioned techniques, extra steps and expensive chemicals are usually required to obtain pure CuO phase, such as oxygen flow or heat treatment [18], making them more demanding for industrial applications. In this work, we investigate the ageing effect on the surface microstructural and optical properties of a bulk copper substrate submerged in a room temperature alkaline solution of ammonia, manifesting in the simple and low-cost formation of amorphous copper (II) oxide thin films. The morphological and structural characterization of the resulting layer were carried out by scanning electron microscope equipped with energy dispersive X-ray detector and X-ray diffraction. Spectral measurements were conducted in the visible and near-IR light ranges to examine the change in absorptivity and reflectivity of the resulting composite http://dx.doi.org/10.1016/j.jallcom.2014.07.221 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +971 56 160 5355. E-mail address: aalalami@sharjah.ac.ae (A.H. Alami). Journal of Alloys and Compounds 617 (2014) 542–546 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom