**8. Optical properties of cuprate**

Cuprates represent a very interesting class of semiconductor and superconductor materials, widely known for their important technological applications in the field of display devices, optical smart windows, electrochromic devices (ECDs), and gas sensors [66, 67]. These materials of which optical properties vary with the reduction and/ or the oxidation by the injection of ions and electrons which modify their electronic structure are responsible for the change of its visual qualities.

So far, most of the research on Ln2CuO4 and Ln2–xMxCuO4 (Ln = lanthanides, M = Ca2+, Sr2+, and Ba2+) have focused on superconductivity, but few studies in the literature on the preparation and optical properties of Ln2CuO4 and Ln2–xMx-CuO4 materials in the UV–VIS–NIR region are reported [68]. Doped and undoped Ln2CuO4 can be used as photocathodes for the photo-electrochemical decomposition of water, in which the Cu2+ ion provides the small bandgap of this material while the incorporated lanthanum ion provides the energy-level adjustment [69]. According to several studies [46], the La2CuO4 electrode acts as a photocathode for the photoelectrochemical decomposition of water and presents a photo-current of 0.5 mA/ cm<sup>2</sup> . For effective photocatalysts, the bandgap must be large enough to support the 1.23 eV dissociation energy of water. On the other hand, the bandgap should be less than 2.1 eV [70], which would allow the materials to capture and absorb most of the solar energy. For example, La2CuO4 shows broad absorption in the UV–visible region [200–800 nm] with an energy band of about 1.24 eV [71, 72] attributed to O2– → Cu2+ charge transfer. For Ln2CuO4 (Ln=Pr, Nd, Sm, and Gd), the gap energy is 0.79 eV; 1.06 eV; 1.20 eV, and 1.36 eV, respectively [73]. Chyi-Ching et al. [74] related the observed optical bandgap to the rare-earth ionic radius and show that the bandgap decreases with increasing ionic radii. The optical properties of lanthanum copper

oxide-doped La2–xMxCuO4–δ (M = Ca, Sr, and Ba) have been the subject of some articles [71, 75, 76]. They *consider* the effect of the grain size on the optical properties of the prepared samples. Therefore, they used soft chemical synthesis methods, such as sol–gel, combustion and co-precipitation [10, 11, 77]. We can see that the optical bandgap decreases with increasing Ca2+ and Ba2+ levels but evolves in the same direction in the case of Sr2+ and undoped La2CuO4, which was 1.88 eV [78]. This value is different from the value quoted by other authors [79].

Throughout the UV–VIS–NIR region, the La2–xCaxCuO4 cuprates have a wide absorption band and a bandgap that increases linearly with the doping level at 0 ≤ x ≤ 0.12; which will respond effectively to its use as a photocatalyst [62, 69].
