Controllable Synthesis of Vapor-Liquid-Solid Grown GaN Nanowires for Photoelectrochemical Water Splitting Applications

Abstract

Vapor-liquid-solid (VLS) grown single crystalline GaN nanowire (NW) arrays of controllable aspect ratio and density were employed as the working photoanode for photoelectrochemical water splitting (PEC-WS) applications. The NW density of GaN NWs was controlled by tuning the different VLS growth parameters, while the aspect ratio was depending on the growth time. We observed a significant increase of the photocurrent with denser GaN NWs due to the enhanced light trapping effects; however, the PEC-WS process stability was degraded with time. Further increase of the photocurrent by about 34% at 1 Volt and a better time-dependent PEC-WS stability were achieved by increasing the aspect ratio of the GaN NWs due to the enhanced surface-to-volume ratio of the photoanode. The applied bias photon-to-current efficiency of the GaN NWs revealed a great shift of the optimal potential to lower values with enhanced NW lengths, indicating an interesting modification in the photoanode performance with increasing the aspect ratio of the GaN NWs. 10-13 Compared to thin films or nano-sized powders, onedimensional (1D) GaN NWs with high aspect ratio may be considered good candidates to enhance the photocatalytic activity due to the remarkable improvement of the effective surface-to-volume ratio. 11,13The transportation of photo-generated carriers may also be enhanced in the GaN NWs due to the reduced traveled distance of the carriers to the electrolyte. 13 Furthermore, GaN NWs showed high chemical stability against the harsh PEC experimental conditions in the presence of corrosive acidic electrolytes.11,14 Consequently, using GaN NWs as a photoanode for the production of clean and renewable hydrogen energy by PEC-WS is worth studying.Recently, considerable efforts have been made in the enhancement of the photocatalytic properties of GaN NWs through the bandgap engineering, metal decoration or the deposition of InGaN to form heterostructure photoanodes. In this regard, heavily Si-doped GaN NWs were reported to enhance the photocurrent density in the PEC-WS due to the GaN surface band bending and its variation with Si doping. 12It was also found that the decoration of GaN NWs with Rh/Cr 2 O 3 core/shell nanoparticles will promote the hydrogen evolution during the PEC-WS process.13 Additionally, the PEC-WS performance was enhanced by employing a double-band InGaN/GaN core/shell NW photoanode due to the enhanced light absorption. 15 In this study, we introduce a simple way to enhance the PEC-WS performance and the hydrogen evolution through the control of the NW density and aspect ratio of undoped GaN NWs without any metal decoration or complex structures. To the best of our knowledge, density-and aspect ratio-dependent PEC-WS of GaN NWs have rarely been reported. 10Owing to the large degree of randomness in the vapor-liquid-solid (VLS) growth process, the controllable synthesis and reproducibility of VLS grown GaN NWs is essential to meet the prerequisites of PEC-WS applications. In this context, several approaches have been applied to ...

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