Practical one-step solvothermal synthesis of photoactive lanthanum(III)hydroxide-metaborate composite nanosheets with suitable band gap for optoelectronics

Rare earth compounds, including lanthanide hydroxides, have been greatly interested for their promising uses, such as in optoelectronics or photonics, due to their unique electronic and optical transitions properties based on their 4f electron shells. In this work, for the first time new transparent flower-like lanthanum(III)hydroxide (La(OH)3) with borate (LaB3O6) composite nanosheets having a semiconducting band gap suitable for optoelectronics were synthesized by a very simple one-step solvothermal method from a mixture of pure lanthanum oxide and boron without using water or any acidic solvent or surfactant. The practical synthesis was made for two different solvothermal time durations. The synthesized nanocomposites were investigated by XRD, FIB-SEM imaging, EDS, FTIR, UV-vis absorbance, Raman, Ellipsometry, and TG-DTA analyses. The pure precursors lanthanum oxide (La2O3) and boron converted to La(OH)3 and LaB3O6. EDS elemental mapping showed the homogeneous distribution of elements in the nanocomposites. A strong deep-UV absorbance band was detected. The very low reflectance and excellent visible transmittance (exceeding 86% for the sample with a longer solvothermal process time) reaching infrared region were achieved. The direct and indirect band gap energies were estimated by the Tauc plot method for both of the composite samples. Adding boron reduced the literature given direct band gap value (around 4.5-5.5 eV) of pure La(OH)3. DTA and TGA results revealed the steps of mass losses, including decomposition of La(OH)3. FTIR analyses confirmed the presence of the formed compounds. All results showed that the La(OH)3+LaB3O6 nanocomposites can be suitably used in related applications, including electro-optic, photonic, solar cell, catalysts etc.