Doped MoS2 Polymorph for an Improved Hydrogen Evolution Reaction

Green hydrogen produced from solar energy could be oneof the solutionsto the growing energy shortage as non-renewable energy sources arephased out. However, the current catalyst materials used for photocatalyticwater splitting (PWS) cannot compete with other renewable technologieswhen it comes to efficiency and production cost. Transition-metaldichalcogenides, such as molybdenum disulfides (MoS2),have previously proven to have electronic and optical properties thatcould tackle these challenges. In this work, optical properties, thed-band center, and Gibbs free energy are calculated for seven MoS2 polymorphs using first-principles calculations and densityfunctional theory (DFT) to show that they could be suitable as photocatalystsfor PWS. Out of the seven, the two polymorphs 3H(a) and 2R(1) were shown to have d-band center values closest to the optimalvalue, while the Gibbs free energy for all seven polymorphs was within5% of each other. In a previous study, we found that 3H(b) had the highest electron mobility among all seven polymorphs andan optimal bandgap for photocatalytic reactions. The 3H(b) polymorphs were therefore selected for further study. An in-depthanalysis of the enhancement of the electronic properties and the Gibbsfree energy through substitutional doping with Al, Co, N, and Ni wascarried out. For the very first time, substitutional doping of MoS2 was attempted. We found that replacing one Mo atom with Al,Co, I, N, and Ni lowered the Gibbs free energy by a factor of 10,which would increase the hydrogen evolution reaction of the catalyst.Our study further shows that 3H(b) with one S atom replacedwith Al, Co, I, N, or Ni is dynamically and mechanically stable, whilefor 3H(b), replacing one Mo atom with Al and Ni makes thestructure stable. Based on the low Gibbs free energy, stability, andelectronic bandgap 3H(b), MoS2 doped with Al forone Mo atom emerges as a promising candidate for photocatalytic watersplitting.