Stabilizing an ultrathin MoS layer during electrocatalytic hydrogen evolution with a crystalline SnO underlayer.

Amorphous MoS has been investigated abundantly as a catalyst for hydrogen evolution. Not only its performance but also its chemical stability in acidic conditions have been reported widely. However, its adhesion has not been studied systematically in the electrochemical context. The use of MoS as a lubricant is not auspicious for this purpose. In this work, we start with a macroporous anodic alumina template as a model support, add an underlayer of SnO to provide electrical conduction and adhesion, then provide the catalytically active, amorphous MoS material by atomic layer deposition (ALD). The composition, morphology, and crystalline or amorphous character of all layers are confirmed by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, grazing incidence X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. The electrocatalytic water reduction performance of the macroporous AAO/SnO/MoS electrodes, quantified by voltammetry, steady-state chronoamperometry and electrochemical impedance spectroscopy, is improved by annealing the SnO layer prior to MoS deposition. Varying the geometric parameters of the electrode composite yields an optimized performance of 10 mA cm at 0.22 V overpotential, with a catalyst loading of 0.16 mg cm. The electrode's stability is contingent on SnO crystallinity. Amorphous SnO allows for a gradual dewetting of the originally continuous MoS layer over wide areas. In stark contrast to this, crystalline SnO maintains the continuity of MoS until at least 0.3 V overpotential.