GaP–ZnS Multilayer Films: Visible-Light Photoelectrodes by Interface Engineering

Abstract

In the field of solar water splitting, searching for and modifying bulk compositions have been the conventional approaches to enhancing visible-light activity. In this work, manipulation of heterointerfaces in ZnS−GaP multilayer films is demonstrated as a successful alternative approach to achieving visible-light-active photoelectrodes. The photocurrent measured under visible light increases with the increasing number of interfaces for ZnS−GaP multilayer films with the same total thickness, indicating it to be a predominantly interface-driven effect. The activity extends to long wavelengths (650 nm), much longer than those expected for pure ZnS and also longer than those previously reported for GaP. Density functional theory calculations of ZnS−GaP multilayers predict the presence of electronic states associated with atoms at the interfaces between ZnS and GaP that are different from those found within the layers away from the interfaces; these states, formed due to unique bonding environments found at the interfaces, lead to a lowering of the band gap and hence the observed visible-light activity. The presence of these electronic states attributed to the interfaces is confirmed by depth-resolved X-ray photoelectron spectroscopy. Thus, we show that interface engineering is a promising route for overcoming common deficiencies of individual bulk materials caused by both wide band gaps and indirect band gaps and hence enhancing visible-light absorption and photoelectrochemical performance.

ZnS-GaP Solid Solution Thin Films with Enhanced Visible-Light Photocurrent

“…The solid solution was achieved by sequential deposition of ZnS and GaP layers using the growth parameters summarized in Table . The number of pulses applied to each target are the smallest possible that can give a ratio of ZnS:GaP consistent with our previous work (i.e., 3 and 2 for ZnS and GaP, respectively), to ensure sub-monolayer deposition and hence intimate mixing of ZnS and GaP. For comparison, a multilayered film with 10 ZnS-GaP interfaces, multilayered ZnS-GaP (Z/G-ML), was also deposited.…”

“…With this overarching concept, the authors demonstrated ZnS-GaP multilayered thin films, where intentionally induced bonding between ZnS and GaP layers remarkably improved photoactivity and its effect scaled commensurately with the number of ZnS-GaP interfaces . A follow-up report showed that interdiffusion at the ZnS-GaP interfaces plays a significant role .…”

“…For comparison, a multilayered film with 10 ZnS-GaP interfaces, multilayered ZnS-GaP (Z/G-ML), was also deposited. The total number of laser pulses used for the film deposition was the same as for Z/G-SS, although this results in a lower total film thickness compared to the Z/G-ML film (Table ); fabrication details for the Z/G-ML can be found elsewhere …”

“…Many materials are capable of producing only low photocurrents under (simulated) sunlight due to their narrow visible-light absorption range associated with their large band gaps. , Recently, we demonstrated a nonoxide thin film photoelectrode system that can have photoactivity under visible light extending to long wavelengths. − Our photoelectrode was based on the ZnS-GaP system, motivated by density functional theory (DFT) calculations that found the incorporation of GaP into ZnS significantly reduces the band gap . From the crystal structure point of view, forming ZnS-GaP mixtures is not difficult; both ZnS and GaP crystallize with zinc blende structures with less than 1% lattice mismatch and thus a solid solution with ZnS and GaP as the end compounds is energetically feasible. − This means that even though both ZnS and GaP themselves have band gaps that are not ideal for solar conversion, we can engineer heterostructures and solid solutions of these two parent semiconductors that possess low direct band gaps and hence intrinsically high photoactivity under visible light. ,,− …”

Energy landscapes of perfect and defective solids: from structure prediction to ion conduction

“…1) suggests that for (ZnS) 1-x (GaP) x compositions where x ≤ 0.75, the band gap is direct and we predict that compositions with x = 0.25, 0.5 and 0.75 are promising photocatalysts for the production of hydrogen from water under solar radiation due to their band gaps and the positions of the bands relative to vacuum. The potential for achieving visible-light activity with the ZnS-GaP system due to control of local bonding environments has been confirmed in experimental work [99].…”