Highly Ordered1-D ZnO Nanopencil Array For Photoelectrochemical (PEC) Water Splitting

Photoelectrochemical (PEC) water splitting is one of the promising technologiesfor environmental friendly and renewable hydrogen production. Zinc oxide (ZnO) hasbeen extensively investigated as a photoanode for PEC water splitting because it has avariety of advantages such as extensive sources, different methods for facilepreparation, low cost and non-toxicity. However, ZnO-based PEC material has twomajor problems:(i) no light absporption in the visible region due to its large band-gap(~3.37eV), and ii) fast recombination of photo-generated carriers due to the shortdiffusion paths of charge carriers. As a result, ZnO could hardly make an efficient useof solar energy and the photocatalytic efficiency of ZnO is low.Compared to ZnO films comprised of nanoparticles, hierarchical structures ofZnO offer direct electrical pathways for photo-generated electrons, leading to highelectron transport rate, which in turn suppress the recombination of electron-hole pairs.Compared to ZnO nanorod array (ZnO NR), highly ordered1-D ZnO nanopencilarray (ZnO NP) has an increased surface area and enhanced electron conductivity forimproved photocurrent harvest efficiencies. Here, the modification of electronic andstructural properties of ZnO by changing the morphology or loading noble metals(e.g., Au NPs) were used to increase the absorption of visible light, which couldfurther enhance the efficiency of solar energy conversion.This paper describes the synthesis of1-D ordered ZnO nanopencil array by a3-step aqueous chemical method. The as-prepared ZnO NP was used as photoanode inPEC water splitting. The photocurrent of ZnO NP is significantly higher compared toZnO NR. The ZnO NP reaches a photocurrent as twice as ZnO NR does. Furthermore,the ZnO NP exhibits an incident-photon-to-current-conversion efficiency (IPCE) of19.0%at a wavelength of375nm. The overall solar to hydrogen (STH) efficiency ofZnO NP is calculated to be~0.1%at a lower bias voltage, which is three times higherthan that of ZnO NR.The plasmonic Au-ZnO NP was synthetized by a photoreduction method, and theparticle size of Au is15nm. As Au particles attached on ZnO NP nanostructure, theAu-ZnO NP shows improved absorption in visible light region and the UV-Visabsorption spectra of Au-ZnO show an absorption peak at520nm, which was causedby the surface plasmon resonance of the Au nanoparticles. The novel Au-ZnO NP shows an enhanced photocurrent than both ZnO NP and ZnO NR. The Au-ZnO NPcould reach a photocurrent of1.5mA/cm2at1.0V versus Ag/AgCl (pH=6.8) undersimulated sunlight illumination (AM1.5G,100mW/cm2). Besides, The Au-ZnO NPcould reach a photocurrent of0.15mA/cm2at0.8V versus Ag/AgCl under visiblelight illumination (>420nm), while the photocurrent of ZnO NP is almost0. The PECperformance proves that the deposition of Au nanoparticles could enhance thelight-harvesting efficiency of ZnO NP in visible region, resulting in a superiorphotocatalytic activity.