Photoluminescence, photocurrent response and photocatalytic activity of hydrothermally derived nanocrystalline ZnO with native point defects

Nanocrystalline zinc oxide (ZnO) samples are synthesized through a facile hydrothermal route at different temperatures. Sample synthesized at 80 °C (sample code HT80) has average crystallite size of 32 ± 2 nm and the morphology is not unform with the presence of rods, wires and discs. Sample prepared at 160 °C (sample code HT160) has rod-like morphology with an average crystallite size of 130 ± 5 nm. X-ray photoelectron spectra reveal the presence of hydroxyl groups bonded to Zn²⁺, oxygen vacancies and adsorbed oxygen species in both the samples. Atomic percentage of the bonded/adsorbed hydroxides and oxygen containing species is higher in HT80. Band gaps values are lower in comparison with bulk ZnO due to band bending. Photoluminescence spectra has a UV peak at ∼381 nm (3.25 eV) and a broad emission band in the visible region centred at ∼550 nm (2.25 eV) respectively corresponding to the band edge emission and defect mediated emission. The emission profile of both samples is similar indicating the presence of similar type of native point defects viz., Zn vacancies, Zn interstitials and oxygen vacancies in different charge states. Sample HT160 has more intense emission due to the better photonic absorption. The photocurrent response under wide light irradiation is also larger for HT160 with a sensitivity of 37 due to the better photonic absorption and larger crystallite size. Wavelength dependent photocurrent response for this sample shows highest sensitivity at 380 nm with a sensitivity of 152 corresponding to the band-to-band excitation. A weak photoresponse for visible light irradiation (∼600 nm) due to the desorption of adsorbed O₂ or hydroxide species caused by the direct photoexcitation of the captured electron to the conduction band is also noted. Photocatalytic efficiency of both the samples are comparable with rate constants 3.1 × 10⁻² min⁻¹ and 3.7 × 10⁻² min⁻¹ respectively for HT80 and HT160. The reusability is much better for sample HT160 which is attribute to larger average crystallite size and uniform morphology.