Optical properties of La2CuO4 and La2 − xCaxCuO4 crystallites in UV–vis–NIR region synthesized by sol–gel process

La2CuO4 and La2 − xCaxCuO4 crystallites were prepared via a simple sol–gel process. The as-prepared La2CuO4 and La2 − xCaxCuO4 crystallites were characterized by X-ray diffraction, transmission electron microscope and UV–vis–NIR spectra. Results show that the grain size of La2CuO4 crystallites increases with the increase of heat treatment temperature from 600 °C to 800 °C. Optical properties show that La2CuO4 crystallites have broad absorption both in the UV–vis region and in the NIR region. The band gap of the as-prepared crystallites decreases from 1.367 eV to 1.284 eV with the increase of calcination temperature from 600 °C to 800 °C. In the series of La2 − xCaxCuO4 compounds (x = 0.05, 0.08, 0.10, 0.12, 0.15 and 0.20), all of the samples exhibit an orthogonal crystal structure and the solubility limit of Ca2 + in La2CuO4 is within the range of x = 0.12–0.15. In the whole UV–vis–NIR region, La2 − xCaxCuO4 crystallites exhibit a broad absorption and the corresponding band gap first increases and then decreases with increasing of Ca2 + content.

► The optical band gap can be tuned by adjusting the grain size and Ca2 + content.
► La2CuO4 crystallites exhibit a broad absorption band both in the UV–vis region and in the NIR region.
► The band gap increases from 1.284 eV to 1.319 eV with the decrease of heat treatment temperature.
► In the whole UV–vis–NIR region, the La2 − xCaxCuO4 crystallites displayed a broad absorption.
► The band gap of La2 − xCaxCuO4 increases linearly with doping level when 0 ≤ x ≤ 0.12.