Enhanced photoelectrocatalytic water oxidation using CoPi modified GaN/MWCNTs composite photoanodes

Abstract

GaN is promising material for photoelectrochemical (PEC) water oxidation owing to its adjustable band gap and suitable band edge locations, which straddle the H+/H2 and O2/H2O redox conditions. However, due to its wide band gap and unavoidable surface defects, the PEC conversion efficiency of GaN is insufficient to be used for practical applications. To improve the PEC efficiency of GaN, we report fabrication of a cobalt phosphate (CoPi)-modified GaN/multiwall carbon nanotube (MWCNT) composite photoanode through combination of metal organic chemical vapor deposition, dip and dry methods, and electrodeposition. Here, GaN photoanodes with different MWCNT surface coverages were obtained by cost effective ‘dip and dry’ method. Different deposition cycles were performed to prepare a series of samples, which were evaluated for their photoelectrochemical water oxidation. The highest photocurrent density (Jph) of 2.23 mA/cm2 at (0 V vs. Pt electrode) was achieved in a sample with moderate incorporation of MWCNTs, formed by through three cycles of the dip and dry coating (i.e., GaN/MWCNT 3T). We attribute this result to the MWCNTs facilitating light sensitization. However, excessive MWCNT loading produced a shielding effect, which lowered the photocurrent density, and an undesirable two-plateau photocurrent behavior indicating sluggish reaction kinetics. The maximum photocurrent density Jph of 2.81 mA/cm2, (vs. Pt electrode) with solar to hydrogen conversion efficiency 0.91% was achieved for GaN/MWCNTs 3T/CoPi ternary composite photoanode, which was approximately 2.51 and 1.26 times as high as that of bare GaN and GaN/MWCNTs 3T photoanodes.