Preparation And Characterization Of WO₃ Nanostructured Photoelectrodes And Their Photoelectrochemical Properties

Low utilization efficiency of solar energy, complicated fabrication of high performance anode materials, and unclear complicated fabrication of high performance anode materials make hydrogen evolution from water splitting using photoelectrochemical (PEC) cells difficult. In this paper, we demonstrated PEC cells using WO3 anodes because of their visible light response and excellent electronic transport property. The PEC cells based on WO3 was built and optimized by fabrication of ordered nanostructured anode materials and investigation on their photoelectrochemical properties. The photoelectrochemical properties of ordered nanostructured WO3 were enhanced by doping non-metallic element, expanding its potential application in solar energy. The study provides a basis and reference for building a stable and high performance semiconductor electrode photoelectrochemical system.Firstly, nanocrystalline WO3 films were synthesized on various substrates by the polymeric precursor method using ammonium metatungstate as the precursor and PEG 1000 as the structure directing agent. The effect of contents of PEG, anneal temperature, pH value of precursor sol and substrates on the crystal structure and crystalline formation of WO3 was investigated. The results show that the WO3 films were crystallized by sintering at over 400?, and the films prepared on fluorine-tin oxide glass substrates were distorted cubic in crystalline phase. However, a monoclinic crystal was formed by coating films on graphite and quartz glass substrates. Photoelectrochemical activity was evaluated under visible light irradiation. The WO3 electrode calcined at 450?exhibited a photocurrent density of up to 2.7 mA/cm2, the higher photo conversion efficiency is 0.81% and donor carrier density ND =2.44×1022 cm-3 in 0.5 M H2SO4 electrolyte. The photoanode was stable up to 120 min with continuous gas evolution.The polymeric precursor method for preparation of WO3 nanocrystalline films was improved by adding citric acid. The effect of citric acid on precursor sol, microstructure and photoelectrochemical properties of WO3 films was investigated. The stability of precursor is proven by the chelating ability of citric acid with W6+. With the increase of content of citric acid in the precursor solution, the WO3 film has a larger particle size and a rougher surface, resulting in enhancement of light absorption. When the mole ratio of citric acid and ammonium metatungstate is 2.0, incident photon-to-current conversion efficiencies improved 14% at 350 nm and the photocurrent increased by 25%.Self-organized WO3 nanoporous photoelectrodes were prepared by anodization of different tungsten foils in a variety concentration of NH4F/(NH4)2SO4 electrolytes at preselected voltages. The morphology, crystal structure, formation mechanism and photoelectrochemical response were investigated. Optimal reaction conditions for preparation are as follows,50 V for oxidation voltage,0.5 wt.%for concentration of NH4F electrolytes. The self-organized nanoporous WO3 was formed through the formation of oxide layer and nanoporou, stationary growth and dissolution of nanoporous. The nanoporous electrode calcined at 450?for 3 h was identified a monoclinic WO3 structure and fine preferential orientation of (002), (020) and (200) planes. Photoelectrochemical measurements of illustrative films show incident photon-to-current conversion efficiencies 89.5% at 340 nm and 22.1% at 400 nm. The photocurrent is 5.85 mA/cm2 and photo conversion efficiency is 1.93%, which is 5.42 and 5.41 times of that on the compact electrode. Moreover, the photocurrents of nanoporous electrodes obtained by anodization of three different tungsten foils are 5.85,5.81 and 5.82 mA/cm2, indicating an excellent photoelectrochemical activity.Furthermore, to improve photoresponse to visible light, we have successfully fabricated N-doped nanoporous WO3 photoelectrode by annealing in NH3/N2. The crystal structure, composition and morphology of pure and nitrogen doped WO3 were compared. The results indicate that nitrogen can be doped successfully into WO3 nanoporous photoelectrodes by controlling annealing temperature. It is apparent that N-doped sample has a bandgap of 2.42 eV and extended its optical response to above 550 nm. In comparison to undoped WO3, the N-doped samples show a significant enhancement in photoresponse with the photocurrent increase of 15%.Finally, a study and comparison of the structure and photoelectrocatalytic activity of nanocrystalline and nanoporous WO3 photoelectrode was carried out. The photoelectrochemical kinetics of two types of WO3 electrode were also investigated by transient photocurrent spectroscopy and intensity modulated photocurrent spectroscopy. Due to the large surface area interfacial heterojunction area, the first order reaction kinetics constant of the nanoporous electrode is an order of magnitude higher than that of the nanocrystalline WO3 electrode in hole transfer process.