Nanostructured p-TiO₂/n-GaN heterostructure as a potential photoelectrode for efficient charge separation

This work proposes p-TiO₂/n-GaN as a new semiconductor heterostructure, which holds great promise as a photoelectrode material. To fabricate p-TiO₂/n-GaN heterostructures, wurtzite GaN is grown in two different morphologies by molecular beam epitaxy, and a TiO₂ overlayer is formed by atomic layer deposition. The XRD and Raman experiments confirm the anatase crystal structure of TiO₂ and SEM shows the conformal coating of TiO₂ on GaN. The system with GaN nanowall network morphology (TiO₂/GaN NWN, calls as TGN) showed better photoelectrochemical response with a photocurrent density of ∼0.65 mA cm^-2 and an IPCE of 17% compared to 0.24 mA cm^-2 and 6% of the planar heterostructure (TiO₂/GaN epilayer, called TGE), at an applied bias of 1.24 V and at an incident power of only 13 mW cm^-2. Chronoamperometric analysis show that electrodes are highly stable. The p-n diagram derived by using the data from cathodoluminescence (CL) and valence band (VB) spectra showed higher barrier height for the TGN structure due to interfacial band bending, and thus, favoring photogenerated charge separation. The evolution of hydrogen in the form of bubbles is visually evident at an applied bias of -0.56V for TGN. The enhanced photocurrent density, cathodic shift in the onset potential and the stability show the superiority of TGN to TGE in charge separation as well as in the photoelectrochemical performance.