The Performance Of Zn Doped TiO₂ Films In Photo-electrochemical Splitting Of Water

Photo-electrochemical splitting of water is one of efficient methods to convert solar energy into sustainable and environmental friendly H2.Titanium dioxide?TiO₂?has been extensively investigated as a photoanode for photo-electrochemical water splitting because of its favorable band-edge positions,superior photocorrosion resistance and chemical stability,and low cost.Elemental doping is one of the effective approaches to modify its performances with the dopants being metallic or nonmetallic elements.Among these elements,transition elements,such as Fe,Cr,Co,V,have been extensively investigated for they can effectively improve the photo activity of TiO₂ and increase the lifetime of photo-induced electrons and holes,and alsothey can adjust the band edge structures of TiO₂.As a result,the water splitting performance will be improved.In this work,the influence of transition Zn element doping on the performances of TiO₂ films in photo-electrochemical splitting of water was studied.The main experimental results are as follows:1.Zn-doped TiO₂ films were prepared via magnetron co-sputtering.During the preparation,quartz and FTO glasses,as the substrates,were ultrasonically cleaned in acetone,anhydrous ethanol and deionized water in sequence.Metallic Ti and ceramic AZO were used as targets with the working atmosphere being mixture gas of high-purity Ar and O₂.The Zn concentration was controlled to be 0-8%,by adjusting the power of two targets and packaging Ti sheet on the AZO target.2.The effects of various annealing processes were investigated.Measurements of photo-electrochemical splitting of water were conducted on a three-electrode system on an electrochemical workstation.The experimental results showed that the Zn-TiO₂ films prepared by rapid thermal annealing at 600? exhibited a higher saturated photocurrent density and carrier density,in comparison with the films which was annealed in a PECVD equipment at 600?.These results indicated that the surface recombination significantly reduced by rapid thermal annealing.3.The influence of Zn concentration was also studied by rapid thermal annealing at 600?.Raman spectroscopy and X-ray diffraction?XRD?analysis demonstrated that all the films were anatase TiO₂.The measurements of photo-electrochemical splitting of water showed that the carrier density decreased in the beginning,and then increased with increasing Zn concentration.Zn dopant also caused negative shifts of both the flat band potential and the onset potential of redox reaction of the films in the light.The results showed that the saturated photocurrent density of the Zn-doped TiO₂ films at 3.5%Zn was the highest,indicating the best water splitting performance.4.The influence of annealing temperature was studied.It was found that the valence band edge moved up along with the annealing temperature increasing,indicating the p-type doping characteristics.But the films remained n-type on the basis of Mott-Schottky plots.X-ray photoelectron spectrum?XPS?confirmed that Zn existed as Zn2+ ions.It was also found that,annealed at 600? displayed the highest saturated photocurrent density and carrier density.5.The effect of sputtering bias voltages was studied.The films were deposited at bias voltages of 0 V,50 V,100 V,and 150 V,respectively.It is found that 100 V was the optimized bias voltage for the deposition,resulting in the highest photocurrent density and carrier density.In summary,Zn was a p-type substitutional impurity for TiO₂ films.The Zn-doped TiO₂ films deposited at 100 V sputtering bias voltage with 3.5 at.%Zn concentration followed by rapid thermal annealing at 600? showed the best performance of photo-electrochemical splitting of water.