Photoelectric Response Of TiO₂ Nanorod Arrays Based Heterojunctions

TiO₂ is a typical n-type semiconductor material with a wide bandgap of 3.0-3.2 eV.Due to its excellent physical and chemical properties,TiO₂ is widely used in photocatalysis,photodetectors and other fields.Highly ordered TiO₂ nanorod arrays?TiO₂ NRs?have the advantage of large specific surface area and carrier-directed transport.TiO₂ NRs are usually synthesized by the low-temperature chemical methods.It is easy to produce a large number of defect states in the preparation process,which causes the carrier recombination and hinders the carrier transport and separation.Therefore,it is particularly important to reduce the TiO₂NRs defect states and improve the ability of the carrier transport and separation.The main research contents and results are as follows:?1?TiO₂ NRs were prepared by hydrothermal method and NiO nanoflakes were prepared on the surface of TiO₂ NRs by chemical bath deposition?CBD?to form NiO/TiO₂ NRs heterojunctions.The devices based on NiO/TiO₂ NRs heterojunctions exhibited good rectification characteristics,fast response speed and high photocurrent.The ultraviolet?UV?self-powered devices were realized.It was found that the growth time of NiO nanoflakes affected the UV photoresponse performance of the heterojunctions.The heterojunctions with NiO nanoflakes prepared for 8 min reaction time exhibited higher rectification ratio and larger photocurrent,which was attributed to the better interfacial contact of NiO/TiO₂ NRs.In addition,the heterojunction devices were prepared using Ag and Au metals with different work functions as top electrodes.The devices with Au electrode show high photocurrent under UV light illumination without applied bias,which is attributed to the better match between work function of Au and the Fermi level of NiO nanoflakes and the facilitated collection of photogenerated hole carriers.?2?The crystal quality and orientation of TiO₂ NRs will affect the photoelectric response of NiO/TiO₂ NRs heterojunctions.The deposition of TiO_x thin films on the FTO substrate improves the crystal quality,orientation and photoelectric properties of TiO₂ NRs,and then enhances the self-powered UV photoelectric response properties of NiO/TiO₂ NRs heterojunctions.The TiO_x thin films can improve the crystal quality,orientation and density of TiO₂ NRs,reduce the surface defect states of nanorods and the thickness of surface depletion layer,increase the carrier concentration and thus improve the photocurrent,photoresponsivity,detectivity and response speed of the heterojunctions at zero bias.The high orientation of TiO₂ NRs are favorable for the orientation transport of photogenerated carriers.The density of vertical TiO₂ NRs increases the junction area of NiO/TiO₂ NRs,reduces the recombination of photogenerated carriers and promotes the separation of photogenerated carriers.For the heterojunctions with TiO_x thin film,the photoresponsivity reaches 5.66 mW/cm² without the applied bias.?3?The two-dimensional MoS₂ nanoflakes have high carrier mobility and are widely used in the fields of photocatalysis and optoelectronic devices.MoS₂ nanoflakes were grown on FTO and TiO₂ NRs substrates by hydrothermal method in the different initial reactant concentrations.The effects of substrates and initial reactant concentration on the nucleation and growth of MoS₂ were investigated.When the concentration of initial reactants is low,MoS₂ nanoflakes partially covers the FTO substrate and TiO₂ NRs substrates.As the reactants increase,MoS₂ nanoflakes can self-assemble stacked into nanoflowers.The high concentration of reactants can provide more Mo and S sources,promote the nucleation and growth of MoS₂.MoS₂ nanoflakes tend to grow on the surface and lateral side of TiO₂ NRs.The?101?crystal plane of TiO₂ NRs favors the nucleation and growth of MoS₂ for the smaller lattice mismatch.The MoS₂/TiO₂ NRs composites exhibit the photoresponse performance under the UV and red light.The heterojunctions with 4 mM Mo source show the enhanced photoresponse performance,which is attributed to the formation of the heterojunctions and the good transport properties of MoS₂ nanoflakes.