Role of Surface Oxygen Vacancies and Lanthanide Contraction Phenomenon of Ln(OH) (Ln = La, Pr, and Nd) in Sulfide-Mediated Photoelectrochemical Water Splitting.

Herein, we report the role of surface oxygen vacancies and lanthanide contraction phenomenon on HS anion adsorption and desorption in the sulfide-mediated photoelectrochemical water splitting of Ln(OH) (Ln = La, Pr, and Nd). The Ln(OH) were synthesized via a solvothermal route using ethylenediamine as the solvent. The surface defects are characterized by Raman, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and high-resolution transmission electron microscopy analyses. The photoelectrochemical water-splitting behavior of Ln(OH) enriched with surface oxygen vacancies has been examined in a 1 M NaS solution under illumination conditions. La(OH) exhibited a highly stable and saturated current density of ∼26 mA/cm at 0.8 V (vs Ag/AgCl). Similarly, the hydroxides of Pr and Nd demonstrated current densities of 18 and 14 mA/cm, respectively, at 0.8 V (vs Ag/AgCl). A reduction trend in the saturated current densities from La to Nd indicates the lanthanide contraction phenomenon, where the basicity decreases in the same order. The results also demonstrate that the surface adsorption of the HS anion in the active sites of the surface oxygen vacancies played a vital role in enhancing the photoelectrochemical water-splitting behavior of Ln(OH). The stability of Ln(OH) was examined after 4 h of chronoamperometry studies at 0.8 V (vs Ag/AgCl) and analyzed using X-ray diffraction, Fourier transform infrared, Raman, and EPR and XPS analyses. The results show that the Ln(OH) exhibited excellent stability by demonstrating their phase purity after photoelectrochemical water splitting. We propose Ln(OH) as highly stable photoelectrochemical water-splitting catalysts in highly concentrated sulfide-based electrolytes and anticipate Ln(OH) systems to be explored in a major scale for the production of H as an ecofriendly process.