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) 3 (Ln = La, Pr, and Nd). The Ln(OH) 3 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) 3 enriched with surface oxygen vacancies has been examined in a 1 M Na 2 S solution under illumination conditions. La(OH) 3 exhibited a highly stable and saturated current density of ∼26 mA/cm 2 at 0.8 V (vs Ag/AgCl). Similarly, the hydroxides of Pr and Nd demonstrated current densities of 18 and 14 mA/cm 2 , 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) 3 . The stability of Ln(OH) 3 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) 3 exhibited excellent stability by demonstrating their phase purity after photoelectrochemical water splitting. We propose Ln(OH) 3 as highly stable photoelectrochemical water-splitting catalysts in highly concentrated sulfide-based electrolytes and anticipate Ln(OH) 3 systems to be explored in a major scale for the production of H 2 as an ecofriendly process.

The electrochemical stability of MOFs in aqueous medium is most essential for MOFs based electrocatalysts for hydrogen production via water splitting. Since most MOFs suffer from instability issues in aqueous systems, there is enormous demand for electrochemically stable MOFs catalysts. Herein, we have developed a simple postsynthesis surface modification protocol for La (1,3,5-BTC) (H2O)6 metal-organic frameworks (LaBTC MOFs) using Mercaptopropionic acid (MPA), to attain electrochemical stability in aqueous mediums. The MPA treated LaBTC MOFs exhibited better stability than the bare LaBTC. Further, to facilitate light harvesting properties of LaBTC MOFs, Au nanoparticles (NPs) and CdSe quantum dots (QDs) are functionalized on LaBTC. The sensitization of LaBTC with Au NPs and CdSe QDs enhances the light harvesting properties of LaBTC in the visible region of solar spectrum. Using as a photoanode, the electrode generates the current density of ∼80 mA/cm(2) at 0.8 V (vs Ag/AgCl) during photoelectrochemical water splitting. The heterostructured LaBTC photoanode demonstrates the long-term stability for the period of 10 h. The electrode post-mortem analysis confirms the conversion of CdSe QDs into single crystalline 2D-CdS nanosheets. The present investigation reveals that CdS nanosheets together with SPR Au NPs improve the photoelectrochemical water splitting activity and stability of LaBTC MOFs.