Tuning Hydrogen Evolution Activity via Composition-Engineered MoS₂-WS₂ Mixed Crystals

We report growth of mixed crystal of transition metal dichalcogenides (TMDs) Mo1-xWxS2 (0 ≤ 𝑥 ≤ 1) thin films using simple and scalable solution-based method, i.e. spin coating for enhanced hydrogen evolution reaction (HER). For this purpose, a mixture of single source Mo/W precursors (with different Mo/W ratios) solution was spin coated onto a planer substrate, followed by moderate temperature sulfurization in a reducing environment. The composition of the mixed crystal was precisely tuned by varying Mo:W precursor ratio. The uniformity and surface coverage of the obtained thin films was confirmed by optical microscopy, AFM and SEM. High resolution AFM image revealed a nanoflower type nanostructure which has potentially higher active edge sites. Raman, XPS and UV-Vis spectroscopy verified successful formation of mixed crystal films and revealed systematic shift in the structural and electronic signature with changing composition. Electrochemical study revealed a strong influence of the composition of the mixed crystal on the catalytic performance. For instance, mixed crystal Mo0.5W0.5S2 showed superior HER activity compared to MoS2 and WS2 in its pure form, with overpotential of 229 mV at 10 mA/cm 2 , Tafel slope 69 mV/dec and charge transfer resistance of 75 ohm. These enhancements in the catalytic activities are attributed to optimization of electronic band structure of the alloy and defect induced activation sites. Overall, this work shows the potential of solution-based growth of mixed crystal as an economical, scalable approach for enhance hydrogen evolution reaction.