| About 80% of the total global primary energy consumption depends on fossil fuels such as oil,coal,and natural gas,emitting greenhouse gases into the atmosphere.Securing alternative energy sources for fossil fuel is recognized as an imminent global issue for realizing sustainable development.Solar energy is viewed as a promising opportunity since it is by far the most abundant,sustainable,carbon-neutral,and inexpensive energy source.Photoelectrochemical（PEC）hydrogen（H2）generation is an efficient and environment friendly method for production of hydrogen with the help ofsolar energy,which can solve one of the great challenges of utilizing renewable energy.PEC H2generation is the process of solar-to-hydrogen conversion using a photoelectrode,usually an ordinary semiconductor material such as Ti O2,Bi VO4,Fe2O3,Zn O,WO3,or Cd S.However,most metal oxides’intrinsically wide-bandgap properties limit their absorption range within the UV part of the solar spectrum（only 5%of the total solar irradiation can be exploited）.Colloidal quantum dots（CQDs）are often used as effective sensitizers of metal oxides for PEC hydrogen production systems due to their wide absorption range,adjustable band gap,multiple exciton generation effect and high absorption coefficient.Up to now,most high-performance PEC devices used for H2generation have widely used Pb,Cd-based QDs.Although QDs have excellent efficiency and stable performance,their high toxicity is the main disadvantage of commercial scale applications.In order to solve this problem,many environment-friendly Ag,Cu-based QDs have been developed.Among them,I-III-VI silver indium sulfide（Ag-In-S/Se）QDs are considered as promising candidates for solar H2production due to their narrow band gap,wide solar absorption and high photoluminescence quantum yield.In general,pure core QDs are prone to exhibiting a large number of surface defects and present a surface very sensitive to the external environment,such as moisture and illumination,that degrades the optical properties,photo-stability and limit their practical applications.In recent years,this problem has been solved by introducing a core/shell structure,that is,coating a solid inorganic shell around the core QD（usually Zn S,Zn Se）.Therefore,this dissertation mainly focuses on Ag In S2/Zn S and Ag In Se2/Zn Se QDs with core-shell structure,using alloying,doping and other means to control the energy band structure and charge carrier dynamics of the core-shell structure QDs,and study their optical and photoelectric properties.The main research contents are as follows:1.Environment-friendly colloidal core/shell quantum dots（QDs）with controllable optoelectronic characteristics are promising building blocks for future commercial solar technologies.Herein,we synergistically tailor the electronic band structure and charge carrier extraction of eco-friendly Ag In S2（AIS）/Zn S core/shell QDs via Mn-alloying and Cu-doping in the core and shell,respectively.It is demonstrated that the Mn-alloying in AIS core can broaden the band gap to facilitate delocalization of photogenerated electrons into the shell and further incorporation of Cu in the Zn S shell enables the creation of Cu-related states that capture the photogenerated holes from core,thus leading to charge carrier recombination and accelerated transfer of photogenerated electrons in the core/shell QDs.As-prepared Mn-AIS/Zn S@Cu QDs were assembled as light harvesters in a photoelectrochemical（PEC）device for light-driven hydrogen evolution,delivering a maximum photocurrent density of～6.4 m A cm-2with superior device stability under standard one sun irradiation(AM 1.5G,100 m W cm-2).Compared with the hydrogen production rate of AIS/Zn S quantum dot photoelectrodes without structural regulation,the optimized quantum dot photoelectrode significantly increased its hydrogen production rate(83.4μmol cm-2h-1).Our findings highlight that simultaneously engineering the band alignment and charge carrier dynamics of“green”core/shell QDs endow the feasibility to design future high-efficiency and durable solar hydrogen production systems.2.From the conclusion in Chapter 3,it can be concluded that reasonable design of element doping in colloidal environmentally friendly core/shell QDs can synergistically adjust their electronic band structure and carrier dynamics for more"green"and efficient solar energy conversion systems.Herein,we have conducted simultaneous Cu doping in both the core and shell regions of environment-benign Ag In Se（AISe）/Zn Se core/shell QDs to realize high-efficiency solar-driven photoelectrochemical（PEC）hydrogen evolution.It is verified that Cu doping in AISe core enables an upward shift in the position of the band edge relative to the Zn Se shell,which promoted the electron delocalization and extended the lifetime of exciton.Simultaneously,Cu doping in the Zn Se shell further results in the trapping of photoinduced holes from AISe core,leading to a decelerated recombination of carriers.The prepared Cu-AISe/Zn Se:Cu QDs with optimized optoelectronic properties have been successfully employed to fabricate QDs-PEC devices,delivering a maximum photocurrent density of 9.1 m A cm-2under standard illumination(AM 1.5G 100 m W cm-2).Therefore,compared to Chapter 3,the hydrogen evolution efficiency of quantum dot photoanodes has also been enhanced(88.1μmol cm-2h-1).Our findings indicate that synchronous elemental doping in eco-friendly core/shell QDs is a promising strategy to achieve future high-performance solar-to-hydrogen conversion systems.3.Currently,a large amount of research is mainly focused on single electrode photocatalytic systems.However,as compared to the single-electrode system,the two-electrode system allows capture of lower energy photons and thus a larger part of the solar spectrum,which can potentially lead to a higher solar-to-fuel conversion efficiency.Furthermore,the reduction and oxidation catalytic centers in the two-electrode system are spatially separated,which not only minimizes the undesirable back-reaction but also separates the photosynthetic products.Therefore,this chapter utilizes the optimized band structure of QDs in Chapters 3 and 4,and uses Cu-AISe/Zn Se:Cu QDs to sensitize n-type semiconductor Ti O2as photoanode,while using Mn-AIS Zn S@Cu QDs modified p-type semiconductor Cu2O as a photocathode,and then spontaneously decomposes water to produce H2and O2under simulated sunlight.Among them,due to the significant difference in the conduction band position between Mn-AIS Zn S@Cu QDs and Cu2O photocathode,the photoexcited electrons are easily transferred from the photocathode to the quantum dot,resulting in a reduction reaction to produce H2.Due to the higher conduction band position of Cu-AISe/Zn Se:Cu QDs compared to Ti O2photoanodes,photo generated holes can easily transfer from the conduction band of Ti O2to QDs,and then migrate to the electrolyte for oxidation reaction to produce O2.The STH efficiency of the QDs two electrode system under unbiased voltage reached 0.41%,and two hours of continuous illumination produced approximately 692 nmol of H2and 355 nmol of O2.This development is a remarkable step toward the demonstration of a complete QD-based artificial photosynthetic system that is efficient,durable,and cost-effective. |
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