Self-Powered Photoresponse Of SnS_x/TiO₂ Heterojunctions

SnS and SnS₂ in sulfur-tin compounds are typical two-dimensional（2D）layered semiconductors with excellent photoelectric properties and good compatibility with other materials that can make good use of visible light benefited from their narrow band gap that has attracted much attention in solar cell,photocatalysis,photodetection and other fields.Due to its excellent physical and chemical properties,TiO₂ is widely used in photoelectric devices.The 2D/1D hybrid nanostructure heterojunction photodetectors can be constructed by combining 2D SnS,SnS₂,and 1D TiO₂ nanorods.Highly ordered TiO₂ nanorod arrays（TiO₂ NRs）can transport carriers directionally,reducing carrier recombination probability.And the large specific surface area of 2D SnS_x（SnS and SnS₂）is used to provide more interfaces for photogenerated carrier separation.The interface energy level structure and interface state of heterojunction photodetectors affect the separation and transmission of photocarriers,and the photocurrent and response speed of devices need to be further improved.Therefore,the performance of photodetectors can be improved by adjusting the interface contact,reducing the interface state,and improving the carrier transport capacity.In this paper,the chemical bath method is used to deposit 2D SnS_x nanosheets on 1D TiO₂ NRs as the substrate to prepare SnS_x/TiO₂ heterojunction photodetectors.The performance of SnS_x/TiO₂ heterojunction photodetectors can be improved by changing the deposition conditions（reactant concentration,growth time,solvent,etc.）to control the morphology,crystal structure and energy band structure of SnS_x.The main research contents and conclusions are as follows:（1）The specific surface area of TiO₂ NRs is relatively large,and the surface suspension bond will absorb hydroxyl,water molecules,and other substances,on the one hand,it will affect the surface deposition of other materials;on the other hand,in the composite heterostructure formed,these surface defects will affect the separation and transmission of photogenerated carriers at the interface.The surface treatment of TiO₂ NRs with the acid solution can reduce the surface state and improve the deposition uniformity and adhesion of SnS_x.The light current of SnS_x/TiO₂heterojunction devices treated with acid solution has been significantly improved,and the devices have response current to UV light without external power applied.（2）The band structure of SnS_x nanosheets film affects the response current value of SnS_x/TiO₂ heterojunction devices.The concentration changes of the precursor reactants Sn and S change the Sn⁴⁺and Sn²⁺ratio of SnS_x nanosheets,resulting in the changes of SnS_x Fermi level position,optical band gap,and carrier concentration.The energy band changes of SnS_x/TiO₂ nanocomposites affect the bending degree of energy band,the barrier/well of conduction band or valence band,and the recombination and separation of photogenerated carriers at the interface.The SnS_x/TiO₂ heterojunction device exhibits UV spectrum selectivity without bias applied,positive and negative current values for different wavelengths,and realizes binary optical response characteristics of wavelength regulation.（3）The binary optical response of SnS_x/TiO₂ heterojunctions is related to the carrier recombination at the heterojunction interface.In order to further improve the binary optical response of the device,isopropanol was selected as the solvent to control the deposition rate of Sn and S atoms and improve the interface contact of the heterojunction.By controlling the synthesis time of SnS_x,the phase ratio of SnS and SnS₂ in SnS_x is changed,and the band structure,interface transfer resistance,and carrier transport are regulated.Under the 0 V bias applied,the peak value of forward current is 375 nm and the peak value of negative current is 400 nm.The heterojunction with 60 min growth time has the maximum optical responsivity and detection rate,which is less than 3 ms.