Rupali Nagar and Abha Mahajan
In this work, the growth of cupric oxide (CuO), a p-type semiconductor with narrow band gap range (1.3 to 2.1 eV) is studied. CuO is a diverse material with its uses ranging from solar cells, heterogeneous catalysis, field emission devices, lithium-ion electrodes, sensing devices, cathode catalyst in fuel cells, etc. Here, copper sulphate pentahydrate (CuSO4??5H2O), sodium hydroxide (NaOH) and polyethylene glycol 2000 (PEG) have been used as-received. Appropriate amounts of precursors were stirred magnetically and reduced using NaOH. Thereafter, the samples were filtered, washed, and dried. Various morphologies ranging from flakes to slender wires were obtained as shown in Figure 1. The flaky microstructure is attributed to presence of PEG that acts as a stabilizer, dispersant while mild heat treatment (60-80 °C) of precursors leads to slender wire morphology. The ratio of copper precursor (CP) and PEG solutions influences the morphology. While molarity ratio of CP:PEG=1 yields flaky feather-like morphology, CP:PEG=50 results in stacked flakes. When PEG is present in abundance, it maintains layered morphology of copper hydroxide, an intermediate product, and its strong steric repulsion inhibits stacking of hydroxide morphology. Upon heating, the hydroxide reduces to flaky CuO structures. For low PEG concentrations, the Cu nuclei first agglomerate. On simultaneous heat treatment, PEG degrades, mechanical strength of polymer network wanes leading to stacked growth. Further, if NaOH is added in one shot along with heating, slender wire morphology is obtained. The combined effects of lower PEG concentration and temperature lead to weaker steric repulsion and mechanical strength of polymer chain resulting in slender morphology of CuO wires. All the morphologies support a high surface area and can serve as potential sensors due to availability of large number of adsorption sites. The reported technique has very simple implementation, is cost-effective and yet yields diverse CuO morphologies with promising sensing capabilities.
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