Fabrication Of TiO_x-Si And SnO₂-Si Photoelectrode And Their Photoelectrocatalytic Performance

Photocatalytic technology has the advantages of environmental friendliness,no secondary pollution,room temperature depth reaction and direct use of sunlight as the light source to drive the reaction.As a result,photocatalytic technology has gradually become an ideal technology for environmental pollution control and clean energy production.The most common photocatalysts could absorb less than 5%of solar light because of their wide bandgap such as TiO₂.The band gap of silicon is 1.12 eV,can absorb relatively wide spectral light with wavelength expanded to 1100 nm,taking more than 65%in the solar spectrum.In addition,silicon is abundant in the earth's crust.Therefore the application of silicon to photocatalysis has a good prospect.However,silicon is not stable in aqueous solution or moist air,silica or silicates will generate on the surface.This oxidation layer will block the photogenerated electrons and holes migrate to solution.TiO_x-Si and SnO₂-Si photoelectrodes were obtained by depositing TiO_x thin films and Sn O₂ films on the surface of silicon to prevent the contact between silicon and water or oxygen.Compared with the traditional protective layer,TiO_x film has a relatively negative overpotential for oxygen evolution,a wide range of light absorption,and the ability to form the carrier migration pattern of the Z-system with Si substrate.SnO₂ film has high electron mobility,strong chemical stability,and relatively positive overpotential for oxygen evolution.The specific research contents are as follows:?1?TiO_x with gradient change of oxygen stoichiometry ratio was designed as a protection layer and fabricated via a chemical vapour deposition process.The SEM results showed that the Si substrate was covered uniformly.The results of TEM and XRD indicate that the crystal structure of TiO_x is anatase and well crystallized.We also acquired the atomic profile of the TiO_x by XPS deep analysis.According to the Atomic%of the Ti 2p and O 1s in the surface of TiO_x film,the amount of O 1s was approximately twice as many compared to Ti 2p,and the proportion of O 1s and Ti 2p was gradually to 0.6 and tend to be stable with the increasing of the etching depth,which was owing to the generation of Ti³⁺and Ti²⁺.The DRS results illustrate that TiO_x cover layer extends the absorption to the visible light range.TiO_x film can effectively protect the Si substrate,and 1min is the ideal deposition time according to the photocurrent density.The onset-potential is-0.7 V,which is favorable for the water splitting.TiO_x-Si photoelectrode has a 3.25 folds of photocurrent density compared to that of stoichiometric TiO₂-Si at 0 V?SCE?under simulated sunlight.Open-circuit photovoltage measurement shows that the TiO_x-Si photoelectrode exhibited?0.6 V more negative potential in the light than in the dark,which is more negative than the sum of Si substrate and individual TiO_x,which proved that the migration of the photogenerated carriers is similar to Z-scheme system without redox mediator.This system can enhance the photogenerated-charge separation and redox ability of TiO_x-Si.?2?SnO₂ was designed as a protection layer and fabricated via a chemical vapour deposition process.The SnO₂-Si photoelectrode was characterized by TEM,SEM,XRD,XPS and DRS,and the performance of photocatalytic degradation of phenol under visible light was studied.The XRD results show that the crystallinity of SnO₂ film fabricated at 400°C is relatively high in samples obtained under different temperature conditions.The SnO₂-Si photoelectrode fabricated using 0.10 g precursor has the biggist photocurrent density,and the onset-potential of water oxidation was very positive,which could avoid the side reaction while be used to degrade organic pollutant.The SnO₂-Si photoelectrode can remain stable in the neutral,acidic and alkaline electrolyte solution,which proves that the photoelectrode has stable photochemical properties.The phenol?10 mg/L?can be fully degrated at 1.8 V bias under visible light after 3 h.All the reaults above show that the SnO₂ layer has the advantages of strong chemical stability,high optical transparency and relatively positive onset-potential of ^waterI n^{o x}cⁱo^dn^ac^tl^iousⁿi.on,two kinds of protective coatings for Si are studied in this paper.One is the TiO_x layer,which can promote carrier separation by orm the carrier migration pattern of theZ-system with Si substrate.The other one is the SnO₂ layer,whose positive onset-potential of water oxidation could avoid the side reaction while be used to degrade organic pollutant.