Synthesis of PtSe₂ Nanoparticles and the Se Vacancy Defect Formation (PtSe_x) Towards Hydrogen Evolution Reaction

The hydrogen evolution reaction (HER) has been a topic of interest. Like other gas evolution reactions, the HER needs an overpotential, and catalyst engineering aims to maximize the process’s effectiveness. The best catalysts for the HER are the Pt catalysts, but due to their high cost and rarity are not viable candidates.¹ New studies show a lot of promising catalysts for the HER, among them are the transition metal dichalcogenides (TMDCs), which can be obtained as a 2D nanostructure, and a lot of effort is being made to improve these materials. The PtSe₂, is a TMDC that shows high potential due to its high catalytic and conductivity properties.²Many paths exist to induce defects on the surface of materials³. Our focus is on a new method of synthesis of PtSe₂ nanoparticles through the selenization of previously synthesized Pt nanoparticles. The selenization process was performed in a closed alumina reactor with an excess of Se in the presence of Pt nanoparticles. At higher temperatures, Se becomes liquid and the selenization reaction occurs. The excess of Se was removed by solubilization in trioctylphosphine (TOP) and centrifugation. First, Pt nanoparticles were synthesized using 90 mg of Pt(acac)₂, 10 mL of oleylamine, and 1 mL of oleic acid in a three-neck round bottom flask with a condenser attached to a Schlenk line. The system is heated to 80 °C under vacuum and constant stirring for complete precursor solubilization. Then, under N₂atmosphere, the temperature was raised to 250°C for 60 min for precursor decomposition, particle nucleation, and growth. The products are isolated by centrifugation, washed with acetone, and dispersed in hexane. Excess Se is added to the mixture of Pt nanoparticles and heated to 400 °C for 120 min. To remove the Se byproduct, TOP was added to the final reaction product, kept under stirring for 2 hours, and washed with hexane several times followed by centrifugation. To induce defects into PtSe₂ nanoparticles, the PtSe₂was dispersed in 1 mL of hexane and 1.6 mL of butyl-lithium hexane solution was added under magnetic stirring, followed by 10 min in ultrasonic bath. The dispersion was washed with hexane. To see the changes in the structure of the PtSe₂ the material was characterized by High-Resolution Transmission Electron Microscopy (HRTEM) and EDS chemical analysis. The images of the PtSe₂ revealed few-layered ordered flakes (~5.1 Å between layers) with high crystallinity. After the lithium treatment, the defects were visible in the flakes that look like broken structures with small regions of atomic ordering. Energy Dispersive X-ray Spectroscopy (EDS) showed that the Pt/Se ratio decreased from 2 for ordered PtSe₂ to 1.5 after intercalation. EDS indicated that Se vacancies are induced in the structure (PtSe_x), which was corroborated by X-ray Photoelectron Spectroscopy (XPS) results. The new PtSe_x structure presented huge shifts in Raman Spectroscopy of 40 cm^-1, and E_g/A_1gpeak intensity inversion and a new peak appeared at a lower energy of spectrum due to the defects. After confirming the defects, we made preliminary catalytic tests, resulting in an overpotential of 145 mV (measured at 10 mA cm^-2) for the PtSe_x nanoparticles. We hope that future improvement in PtSe_x defect engineering and catalyst optimization in the electrode could lead to an even better HER performance.<br/>The authors would like to acknowledge FAPESP (Grants 2018/05159-1, 2021/03321-9) and CNPq (409787/2021-3).<br/>1. Eftekhari, A. <i>et. al.</i> Int. J. Hydrog. Energy. 2017, 42, 16, 1053-11077.<br/>2. Ghorai, A. <i>et al.</i> Dalton Trans. 2016, 45, 14979-14987.<br/>3. Ping, X. <i>et al.</i> Nano Lett. 2021, 21, 9, 3857–3863