Manipulating The Physical Properties Of Environmentally Friendly Ag-In-Se Colloidal Quantum Dots For High-efficiency Photoelectrochemical Cells

Photoelectrochemical（PEC）hydrogen（H₂）production,which can directly use solar energy to generate clean fuel（H₂）from water,is regarded as one of the key technologies to solve the future energy crisis.Due to the wide band gap（～3.2 eV）of the TiO₂photoanode belonging to typical PEC cells,it can only utilize the ultraviolet（UV）part of sunlight（accounts for 5%solar photons）,thus limiting the further application of PEC H₂production.Based on the tunable size,shape,and composition of colloidal semiconductor quantum dots（QDs）,QD-sensitized PEC cells have the advantages of enhanced light absorption,abundant surface binding sites,and tunable energy band positions,which effectively improve the overall performance of the PEC H₂ production system.However,the components of the well-researched QD-based PEC cells often contain highly toxic elements（such as Cd,Pb,Hg,etc.）,which will cause serious harm to the ecological environment and human health,and also limit the further commercial application of this technology.AgInSe（AISe）QDs have a narrow band gap（～1.24 eV）,which enables the absorption reach to the near-infrared region（NIR）.In addition,AISe QDs exhibit good biocompatibility and optical/electrical properties,showing the potential as a sensitizer for efficient PEC cells.However,the surface of bare AISe QDs has a high density of defects/trap states,which leads to the fast recombination of electrons on the surface and the degradation of optical/electrical properties and stability.To deal with these problems,this thesis is based on Ag-In-Se QDs carrying out a series control of physical properties（including:alloying,core-shell structure growth,shell doping）,and finally obtained the QDs with prolonged electroluminescence（PL）lifetime,better electron dynamics and photophysical/chemical stability.Moreover,an electrode structure with high electron transport efficiency was constructed by introducing graphene oxide（GO）nanosheet material,which possessing good compatibility and conductivity,into the traditional TiO₂ photoanode.The optimized QDs and photoanodes were combined to systematically study the PEC performance and optimization mechanism of these environmentally friendly QD-based photoanode.The main research contents are as follows:1.NIR environment-friendly Zn-Ag-In-Se alloy QDs were synthesized through hot injection method by introducing Zninto Ag-In-Se QDs.Optical characterizations demonstrate that Zn-Ag-In-Se QDs possess NIR absorption spectra,tunable optical properties depended on Zncontent.In addition,a series of TiO₂-GO photoanodes with different GO contents were obtained by doping minute amount of GO nanosheets in the TiO₂ paste.The analysis of the diffuse reflectance spectrum（DRS）and cross-sectional morphology of TiO₂-GO photoanode exhibit good transparency and uniform structure when the GO concentration is appropriate.Then,ZnAgInSe（ZAISe）QDs with the longest PL lifetime were selected as sensitizers to deposite on the TiO₂-GO photoanode by electrophoretic deposition（EPD）for the working electrode of PEC hydrogen production system.The morphology analysis of the sensitized film and the elemental surface scanning analysis of the film section reveal a close contact structure formed between ZAISe QDs,TiO₂,and GO,which is conducive to the transfer of photogenerated electrons from the QDs to the photoanode.The DRS and ultraviolet electron spectroscopy（UPS）of the sensitized electrode demonstrate that the absorption extends to NIR range after ZAISe QDs sensitizing,and formation of Ⅱ band alignment between ZAISe QDs and TiO₂ is conducive to electron transfer.Finally,though the PEC performance test and mechanism analysis,it is found that the GO-doped electrode have better electron transport efficiency,improved the photocurrent density and stability compared to TiO₂ photoanode.The sensitized photoanode with GO concentration of 0.015 wt%reveals the best PEC performance,which exhibits an excellent saturated photocurrent density（～6.7 mA/cm~2）,a 2-hour H₂ production of～80.2μmol/cm~2,and a Faradaic efficiency of 50%under 1standard irradiation（AM 1.5G,100 mW/cm~2）.2.Environmentally friendly AISe/ZnSe core/shell QDs with pyramid structures were successfully synthesized by a two-step method.The shell thickness of the core/shell QDs can be controlled by adjusting the injection amount of the Zn/Se precursor during the shell growth.The size of the AISe core and the thickness of the ZnSe shell were roughly calculated by the ideal tetrahedron model.Optical characterization AISe/ZnSe core/shell QDs prove the tuable optical properties by controlling the ZnSe thickness,and the PL lifetime of QDs is greatly prolonged after the growth of ZnSe shell.Though UPS analysis,it is found that although AISe/ZnSe QDs have a type Ⅰ core/shell structure,a type Ⅱ band alignment is formed between the QDs and TiO₂,which is benefit to electron transfer.When the thickness of ZnSe shell is appropriate,electrons can rapidly transfer from core/shell QDs to TiO₂.Taking advantage of the small lattice mismatch between ZnSe and AISe,the AISe/ZnSe core/shell QD-sensitized TiO₂-GO photoanodes exhibit good PEC performance and stability.And ZnSe also shows small lattice mismatch with ZnS,hence these sensitized photoanodes can be optimized by multiple deposition of ZnS passivation layer.The final best-performing photoanode reveals a saturated photocurrent density as high as～7.5 mA/cm~2 under 1 standard irradiation（AM 1.5G,100 mW/cm~2）.The H₂ production after 2 hours continuous illumination is～96.0μmol/cm~2 and the Faradaic efficiency is 56%.3.The Cu-doping AISe/ZnSe:Cu core/shell quantum dots were successfully prepared by injecting the Cu precursor solution during the growth of the ZnSe shell though hot injection method.From the morphology and size analysis of QDs,it can be seen that the doping of Cu will not change the morphology and size of the original I-type AISe/ZnSe core/shell QDs.Crystal phase characterization and elemental analysis can prove that Cu is doped into the lattice of ZnSe,and the Cu dopant exists in the form of Cu⁺.Through optical characterization,it can be found that the absorption spectrum and PL emission of these QDs will red-shift with the increase of Cu doping concentration.AISe/ZnSe:Cu core/shell QDs also exhibit prolonged PL lifetime,demonstrating that Cu doped in the ZnSe shell traps photogenerated holes,thereby promoting electron-hole separation in type Ⅰ core/shell QDs.Finally,these Cu-doping core/shell QDs were deposited on TiO₂-GO photoanode by EPD and assembled into PEC cell for H₂production.Though doping Cu in the ZnSe shell,AISe/ZnSe:Cu QDs can prevent the fast recombination caused by type Ⅰ core/shell structure,which confine both the electrons and holes in the core region.Moreover AISe/ZnSe:Cu also retain the advantages of type Ⅰ core/shell QDs such as excellent stability.The AISe/ZnSe:Cu core/shell QD-sensitized TiO₂-GO photoanode shows a outstanding saturation photocurrent density（～6.5 mA/cm~2）under 1 standard irradiation and excellent stability（～70%of initial value）.Compared with AISe/ZnSe quantum dots,the H₂ production and Faradaic efficiency of the best preformance photoanode also reveal a significant improvement.After 2 hours operating,the H₂ production is～135.9μmol/cm~2,and the Faradaic efficiency can reach 68%.