Mechanism Study On The Physical Properties Influence Of Transition Metal Manganese Elements On The Environmentally Friendly Colloidal Quantum Dots And Their Photoelectrochemical Applications

With the continuous development of human technological life,the consumption of energy and the harm caused to the ecological environment have gradually become a dilemma faced by human beings.The human demand for energy and the environmental problems caused by the consumption of traditional fossil energy are in conflict.Therefore,more and more scientists are dedicated to develop and explore new renewable energy sources.One of the most promising ways is the Photoelectrochemical cells(PEC)hydrogen generation,which converts solar energy directly into hydrogen with high energy density,clean and efficient,and easy to store and transport.The PEC system generates electron-hole pairs through the semiconductor material of the photoelectric anode after being excited by light,and the electrons are transferred to the counter electrode under bias voltage to split water to generate hydrogen.The selection of suitable semiconductor anode materials is crucial to improve the photoelectric conversion efficiency/performance.Colloidal semiconductor quantum dots(CQDs)are nanomaterials with size/morphology/composition-dependent optical properties,excellent light absorption coefficients and mature synthesis techniques,which can be used as good sensitizers to improve the light absorption capacity and enhance the efficiency of optoelectronic devices.However,most of the high-efficiency quantum dot-based PEC devices currently used contain heavy elements such as Pb and Cd.Considering the harm to human health caused by potential large-scale commercial applications in the future,the development of efficient and environmentally friendly CQDs sensitizers are imminent.However,current environmentally friendly colloidal CQDs such as Cu In S,Cu In Se are seriously hampering the development of high performance and stable quantum dot optoelectronic devices due to their inherent surface defects/traps resulting in poor chemical/optical stability and other disadvantages.Conventional optimization strategies such as wrapping wide band semiconductor resulting in type I band alignment are not conducive to the effective separation of photogenerated carriers.In response to these problems,this thesis takes Cu In S?CuInZnS,and Cu In Se semiconductor QDs as the main materials,and introduces the transition metal Mn from doping to alloying to carry out a systematic study the changes of the crystal structure,optical properties,and optoelectronic properties after the introduction of transition metal Mn.The main research contents are as follows:(1)In Chapter 3,an environmentally friendly Mn-doped CuInZnS(Mn:CIZS)QDs was synthesized and realized high-efficiency photoelectrochemical hydrogen generation.UV-vis absorption and photoluminescence(PL)spectrum characterizations indicate that Mn is doped in the CIZS QDs host and exhibits the characteristic spin-flip emission of Mn,introducing the Mn intermediate energy level.The prepared QDs were deposited into the mesoporous Ti O₂ thin film electrode by electrophoretic deposition(EPD)method to construct the photoanode.The cross-sectional SEM image of the QDs sensitized photoanode clearly shows that the QDs are uniformly dispersed throughout the film electrode.Combined with the UV-vis absorption and ultraviolet photoelectron(UPS)energy spectrum of Mn:CIZS QDs,it is proved that the energy band alignment of the heterojunction composed of QDs and Ti O₂ is beneficial to the separation and transport of electron-hole pairs.Finally,the Mn:CIZS QDs-sensitized photoanode was assembled into a PEC system,which was tested under standard one sunlight illumination(AM 1.5 G,100 m W cm^-2).The saturated photocurrent of the Mn:CIZS QDs-based photoanode reaches?3.8 m A cm^-2.In contrast,the saturated photocurrent of the pristine CIZS QDs-based photoanode is only?2.0 m A cm^-2.In addition,the stability test also found that the stability of Mn:CIZS QDs was much better than that of the pristine CIZS QDs.(2)Experimental results from the first work show that the intermediate energy levels introduced by Mn doping not only increase the saturated photocurrent density of the pristine material-based PEC devices,but also substantially improve the stability of the device.The optical properties of Mn-doped core-shell structured QDs and their application in PEC hydrogen production were explored in this work.An environmentally friendly Mn-doped CuInSe/ZnSe core-shell structured QDs(Mn:CISe/ZnSe)was synthesized using epitaxial solution growth method.In addition,this chapter 4 analyzes the effect of different concentrations of Mn doping on the optoelectronic properties,and then varies the[Cu]/[In]atomic ratio in the optimal content of Mn doping to investigate the changes of optical properties caused by the component changes.Optical analysis showed that unlike the Mn-doping that results in almost unchanged structural and optical properties in the case of core QDs,the Mn-doping in the core/shell QD system significantly alters the optical characteristics.This is because the typical Mn²⁺-related ~4T₁-~6A₁ transition energy(?2.1 e V)is much larger than the bandgap energy of our Mn:CISe-2(?1.84 e V)([Cu]/[In]=1/2)QDs,the introduced Mn-energy levels could stagger with that of the Mn:CISe-2 QDs and hence forbid the exciton-to-dopant energy transfer.Nevertheless,charge transfer from CISe-2core QDs to the Mn dopant states(i.e.~4T₁ and ~6A₁)is still possible.The introduced Mn²⁺intermediate state can also act as an electron storage center,shielding the trapping of electrons by surface defects,which is beneficial to the charge carrier transfer and separation of Mn:CISe-2/ZnSe core/shell QDs.It is found that Mn doping can indeed improve the performance of CISe/ZnSe core/shell QDs-based PEC under standard one sunlight illumination condition(AM 1.5G,100 m W cm^-2).The optimized Mn:CISe-2/ZnSe core-shell QDS have saturated photocurrent of?6.0 m A cm^-2,which is much higher than the pristine CISe/ZnSe core/shell QDs(?1.7 m A cm^-2),and also have good device stability.(3)In the second work mentioned above,the performance of the Mn alloyed CISe/ZnSe core-shell quantum dot sensitized photoanode was also explored and evaluated,with improved performance compared to the pristine CISe/ZnSe,but not as good as the doping.Alloying is not the optimal case for this system.However,the lattice mismatch between sphalerite CIS and Mn S is only 1.5%,and the lattice mismatch between sphalerite CIS and ZnS is only 2.0%,which provides theoretical feasibility for the synthesis of Mn alloying Cu In S/ZnS(CIS/ZnS).Therefore,CIS/ZnS is chosen as the main material in Chapter 5,and synthesized CIS/ZnS core/shell structure QDs with Mn alloying by epitaxy solution growth method and applied them in PEC hydrogen generation.Subsequently,the performance test shows that the performance of the Mn-alloyed CIS/ZnS core/shell QDs-based PEC is significantly improved compared with the pristine CIS/ZnS core-shell core/shell QDs under standard one sunlight intensity(AM 1.5G,100 m W cm^-2),The device exhibits saturated photocurrent up to?5.7 m A cm^-2 and good device operation stability.