Solar water splitting is a sustainable and in principle environmentally sound approach to solve the high energy demand of mankind using renewable energies[1], because of the high energy density of hydrogen and no CO₂ emission when used as a fuel. To provide hydrogen in large quantities, nontoxic, earth abundant and cheap catalysts are in demand to replace costly platinum for reducing the overpotential in the hydrogen evolving reaction (HER) process[2].

In our study, molybdenum sulfide layers of different crystallinity and morphology have been prepared by reactive magnetron sputtering and tested as HER catalyst. Best performance was obtained starting from an amorphous MoS_x layer deposited at room temperature on FTO (η₁₀=180mV at pH 0 sulfuric acid electrolyte), which in Raman spectroscopy shows similar vibrations as the highly efficient HER catalyst, (NH₄)₂Mo₃S₁₃[3]. However, during electrochemical measurement, production of hydrogen was proved along with the release of H₂S especially during the first 20min of CV (activation step). Afterwards, only hydrogen was evolved from the electrode. A structural change of the material from amorphous MoS_x to MoS₂ layered structure happens during CV was proved by Raman measurement with the disappearance of terminal and bridging [S₂]^2- units and the emergence of the characteristic vibration mode A_1g (out of plane S bonding) from layer structured MoS₂. Mo atoms at the edges of formed MoS₂ nano-islands are oxidized to Mo(VI) after removal from the electrolyte. From the ratio of Mo (IV) to Mo(VI) in the XPS spectra, the size of MoS₂ nano-islands could evaluated. The high number of this Mo atoms at the edges explains the good performance of the HER electrode. For stability test, room temperature sputtered MoS_x electrode was measured under CV conditions for 10h exhibiting an increase in overpotential of 46 mV.

1.            Lewis, N.S. and D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization. Proc Natl Acad Sci U S A, 2006. 103(43): p. 15729-35.

2.            McCrory, C.C., et al., Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J Am Chem Soc, 2015. 137(13): p. 4347-57.

3.            Kibsgaard, J., T.F. Jaramillo, and F. Besenbacher, Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate Mo3S13 (2-) clusters. Nature Chemistry, 2014. 6(3): p. 248-253.