Highly (001)-textured p-type WSe2 Thin Films as Efficient Large-Area Photocathodes for Solar Hydrogen Evolution

Abstract

Highly (001)-textured, photoactive WSe2 thin films have been prepared by an amorphous solid-liquid-crystalline solid process promoted by palladium. By increasing the thickness of the Pd promoter film (≥10 nm) the structure and texture of the WSe2 films can be improved significantly. However, these as-crystallized WSe2 films are only weakly photoactive in a 0.5 М H2SO4 electrolyte under AM 1.5 solar irradiation which we attribute to an inefficient photogenerated charge transfer across the WSe2/electrolyte interface via the prevailing van der Waals planes of the WSe2 crystallites. In this work photochemically deposited platinum on the p-type WSe2 photocathode is used for an efficient electron transfer thus inducing the hydrogen evolution reaction. Upon illuminating the WSe2 photocathodes in a Pt-ion containing electrolyte, the photogenerated electrons reduce Pt+ to Pt leading to the precipitation of Pt islands, preferentially at edge steps of the WSe2, i.e. at the grain boundaries of the WSe2 crystallites. The increasing amount of Pt islands at the grain boundaries linearly enhances the photocurrent density up to 2.5 mA cm−2 at 0 VRHE in sulfuric acid, the highest reported value up to now for WSe2 thin films.

Efficient Charge Transfer at a Homogeneously Distributed (NH4)2Mo3S13 / WSe2 Heterojunction for Solar Hydrogen Evolution

“…The AM 1.5-spectrum integrated photocurrent density of the ATM/WSe 2 photocathode from the IPCE curve is 3.1 mA cm -2 , which has been shown in our previous report 25 showed a good stability for about 2 h at a constant photocurrent density at about 1.5 mA cm -2 for polycrystalline WSe 2 thin films coated with a Pt/Cu catalyst in sulfuric acid. In our case, the stability of the ATM/WSe 2 (200 nm thick) photoelectrode was tested in 0.5 M H 2 SO 4 (0 pH) under AM 1.5 illumination.…”

Efficient charge transfer at a homogeneously distributed (NH4)2Mo3S13/WSe2 heterojunction for solar hydrogen evolution

“…21 As the vdW basal planes are catalytically inert, nanoparticles are of interest for increasing the number of active sites at the edges of the hexagonal planes. 25 Recently, it was reported that ammonium thiomolybdate (NH 4 ) 2 Mo 3 S 13 nanoclusters showed an even better performance in the HER in comparison to MoS 2 nanoparticles. 22 This catalyst is characterized by a high number of catalytically active [S 2 ] 2sites.…”

Efficient Charge Transfer at a Homogeneously Distributed (NH4)2Mo3S13 / WSe2 Heterojunction for Solar Hydrogen Evolution

“…To achieve an efficient hydrogen evolution, the number of catalytically active sites must be increased. This can be done by Pt deposition, 19,25 which activates the grain boundaries or by chemical/electronic activation of the (00.1) vdW surfaces. The Mo 3 S 13 clusters in ATM exhibit a higher number of S-active sites compared to MoS 2 nanoparticles.…”

“…Even, when evaporating a thin Pt film as a HER catalyst homogeneously onto the WSe 2 van der Waals planes, no significant photocurrent can be observed. However, when depositing photochemically Pt nanoislands preferentially at the crystallite edges, 25 the transfer from the edge sites of WSe 2 crystallites to the semiconductor-electrolyte interface can be significantly improved. This clearly shows that Pt acts as a HER catalyst only at the edges of (00.1)oriented WSe 2 crystallites.…”

“…39 Based on an old model of Hecht 40 for the carrier collection efficiency in AgCl single crystals, Abeyweera et al 39 derived a simple analytical formula for the carrier collection efficiency of thick films where a carrier generation depth profile exists which can be expressed as: compared to the diffusion length derived from time-resolved microwave conductivity measurements (TRMC), reported recently. 25 This can be explained by the anisotropy of WSe 2 : while in a PEC experiment, the transport along the c-axis of the (0001)-oriented WSe 2 crystallites (i.e. perpendicular to the van der Waals planes) governs the transport, in a TRMC measurement the diffusion length in the van der Waals planes is measured, which is much higher (see for instance 14 ).…”