Stabilizing an ultrathin MoS₂ layer during electrocatalytic hydrogen evolution with a crystalline SnO₂ underlayer

Amorphous MoS₂ 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 MoS₂ 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 SnO₂ to provide electrical conduction and adhesion, then provide the catalytically active, amorphous MoS₂ 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/SnO₂/MoS₂ electrodes, quantified by voltammetry, steady-state chronoamperometry and electrochemical impedance spectroscopy, is improved by annealing the SnO₂ layer prior to MoS₂ deposition. Varying the geometric parameters of the electrode composite yields an optimized performance of 10 mA cm^-2 at 0.22 V overpotential, with a catalyst loading of 0.16 mg cm^-2. The electrode's stability is contingent on SnO₂ crystallinity. Amorphous SnO₂ allows for a gradual dewetting of the originally continuous MoS₂ layer over wide areas. In stark contrast to this, crystalline SnO₂ maintains the continuity of MoS₂ until at least 0.3 V overpotential.