Modification Of Tio₂ Nanowire Arrays And Its Application In Photoelectrochemical Photovoltaic Cells

With the depletion of fossil energy and the global concern about the greenhouse effect,it is urgent to develop clean renewable energy which can replace the traditional energy.The efficient use of solar energy is an important means to solve the energy crisis and environmental pollution.Photoelectrochemical photovoltaic cells based on semiconductor nanomaterials have attracted much attention,mainly including dye-sensitized solar cells（DSSC）and photoelectrochemical ultraviolet detectors（PEC UVPDs）.Photoanode of Photoelectrochemical photovoltaic cells plays an important role in devices.In recent years,the application of one-dimensional titanium dioxide nanostructured materials in DSSC and PEC UVPDs has attracted much attention.Unique morphology and excellent properties improve the photovoltaic properties of DSSC and PEC UVPDs.Titanium dioxide nanowire arrays are prominent in one-dimensional nanostructured titanium dioxide materials.However,low light scattering,electron recombination at the photo-anode/electrolyte interface and low electron transfer rate of titanium dioxide still hamper the practical application prospects of titanium dioxide nanowire arrays.In view of the above constraints,focusing on how to improve the photoelectric conversion efficiency of photoelectrochemical photovoltaic cells,we modified the nanowire arrays of titanium dioxide by different methods.The specific contents are as follows:Ultralong TiO₂ NWAs were prepared by hydrothermal method.Different morphologies of NWAs could be obtained by changing the ratio of deionized water to anhydrous ethanol and reaction time.Among them,the NWAs with a diameter of about 12 nm and a length of 40μm have the largest aspect ratio.Cl^?ion can be preferentially adsorbed on the（110）crystal plane of TiO₂ crystal to accelerate the growth of TiO₂ NWAs along the[001]direction,thus forming TiO₂ NWAs with high aspect ratio.DSSC and PEC UVPDs were fabricated with optimized titanium nanowire arrays.The photoelectric conversion efficiency（η）of DSSC reached 7.46%,which has the potential of practical application.The photoresponsivity（Rλ）of PEC UVPD at 365 nm reached 168 mA W^?1,and the response speed was fast.By etching method,the surface area of NWAs can be further increased,and the porosity and dye adsorption of NWAs can also be increased.Theηof DSSC based on reasonably etched TiO₂ NWAs can reach 8.28%.The Rλof the PEC UVPD at 365nm reaches 255 mA W^?1.Compared with the unetched ones,the rise time（τ_r）increases and the decay time（τ_d）decreases.TiO₂@MgO core-shell heterostructured NWAs were prepared by the hydrothermal method.MgO coating has no effect on the crystal structure and morphology of TiO₂ NWAs.DSSC based on MgO@TiO₂ core-shell NWAs show that theηof DSSC sharplyd ecreases because the conduction band of MgO is higher than the LUMO of N719 dye,which leads to a significant decrease in electron injection efficiency.For PEC UVPDs,when the coating time of MgO is short,the photocurrent density increases due to the inhibition of photogenerated electrons recombination at the photo-anode/electrolyte interface.When the coating time of MgO is too long,the reaction between photogenerated holes and I^?of electrolyte leads to the accumulation of holes,increasing the probability of direct recombination of photogenerated electrons with photogenerated holes and decreases the photocurrent density.The Rλof PEC UVPDs based on MgO@TiO₂ core-shell heterostructured NWAs reaches 362 mA W^?1 at 365 nm.Theτ_r andτ_d are 13 and 12 ms,respectively.Sr-doped free TiO₂@SrTiO₃ core-shell heterostructured NWAs were prepared by the hydrothermal method.Sr-doping has no effect on the crystal structure and morphology of the NWAs.However,too long coating time will lead to Sr doping into the lattice of the TiO₂ NWAs.DSSC and PEC UVPDs were fabricated with the optimized Sr-doped free TiO₂@SrTiO₃ core-shell heterostructured NWAs.It was found that when the coating time of SrTiO₃ was short,the suppression of photogenerated electrons recombination caused by type-Ⅱenergy band structure could effectively improve the performance of DSSC and PEC UVPD.However,for long coating time,the lattice defects and oxygen vacancies caused by Sr doping could capture photogenerated electrons,which leads to the loss of photogenerated electrons.Theηof DSSC based on Sr-doped free TiO₂@SrTiO₃ core-shell heterostructured NWAs reaches 9.07%,which has the potential of practical application.The on/off ratio of PEC UVPD is 28065,and the peak value of Rλat 360 nm is 358 mA W^?1,theτ_r andτ_d are 13 and 11 ms,respectively.Sn-doped TiO₂ NWAs were prepared by the hydrothermal method.Sn doping does not affect the cross-sectional shape and size of single nanowire,but excessive Sn doping can shorten the length of NWAs.Theηof DSSC based on the optimal Sn-doped TiO₂ NWAsreached 8.75%,which has potential for practical application.The on/off ratio of PEC UVPD is 15482,and the Rλat 360 nm is 362 mA W^?1,theτ_r andτ_d are 13 and 10 ms,respectively.Sn-doping leads to the increase of electron transport speed,which is the main reason for the improving performance of PEC photovoltaic cells based on Sn-doped TiO₂ NWAs.