Study On The Effect Of Manganese On Colloidal Quantum Dots For Photoelectrochemical Hydrogen Evolution

The overexploitation and overuse of fossil energy(coal,oil and natural gas)have caused global warming and environmental pollution,posing a threat to the sustainable development of human society.To address the growing demand for clean energy,efficiently converting solar energy to hydrogen fuel is considered the most promising strategy.Photoelectrochemical(PEC)process uses solar energy to produce hydrogen directly from water in an efficient,low-consumption,and clean method.To date,a variety of metal oxide semiconductors have been developed as photoelectrodes,but none of them meet the requirements for commercialization(STH efficiency>10%and long-term stability>1000 h).The main reason is that the wide band gap of metal oxide semiconductors limit light absorption range.To address these issues,metal cadmium sulfide colloidal quantum dots(CdX QDs,X=S,Se,Te)have efficient visible-light harvesting,controllable synthesis process and widely tunable optoelectronic properties,which thus serves as light-absorbing materials to sensitize photoelectrodes.However,the bare colloidal QDs have a high specific surface area and surface defects,which act as non-radiative recombination centers for excitons and affect the optoelectronic properties of QDs.Typically,the construction of heterogeneous shells with wide band gaps on the bare QDs can effectively passivate the surface defects.Unfortunately,the band structure of heterostructured QDs is not efficient for charge separation/transfer,and the lattice mismatch of the core/shell materials would cause lattice strain and interfacial defects.It has been demonstrated that incorporation of Mn2+ dopants into QDs can modulate intrinsic optoelectronic properties and exciton recombination processes.However,due to the extremely small size of QDs,the local environment of Mn2+dopants in QDs is difficult to be clearly characterized,and therefore the mechanism of modulating the optoelectronic properties of metal cadmium chalcogenide colloidal QDs using Mn2+ dopants is yet to be investigated.To address these issues,in this thesis,cadmium selenide(CdSe)QDs and cadmium selenide/cadmium sulfide(CdSe/CdS)core-shell QDs were used as the host materials,in which different concentrations of Mn2+ dopants were introduced,respectively.The effects of Mn2+ dopants on the morphological size,crystal structure,chemical state,optical properties and optoelectronic properties of the QDs were systematically investigated,and the modulation mechanism of Mn2 +dopants on the optoelectronic properties of the QDs was investigated.The main results are as follows:(1)CdSe QDs sensitized photoanodes with different Mn2+ concentrations were designed and synthesized for efficient sunlight-driven photoelectrochemical hydrogen production.Combining spectroscopic and electrochemical characterization,it is found that Mn2+ dopants are doped on the surface of CdSe,so that the morphology and structure of the QDs remain unchanged.Mn2+ dopants significantly change the behavior of excitons by introducing the intragap state in host QDs,which exhibits increased Stokes shifts and long-lived excited states.Furthermore,the Mn2+ intragap state can mitigate photogenerated electrons being annihilated by surface defects and promote more photogenerated electrons to be injected into the TiO2 film.Therefore,the Mn0.03-CdSe QDs sensitized photoanodes exhibit a low photogenerated charge transfer resistance.Under standard one sun illumination(AM 1.5G,100 mW cm-2),the optimized Mn0.03CdSe QDs sensitized photoanodes exhibit significantly higher saturation photocurrent density(～7.2mA cm-2)and long-term photochemical stability compared to the CdSe QDs(～6.5 mA cm-2).In contrast,excess Mn2+ dopants cause a poor photoelectrochemical performance(～5.3 mA cm-2)and inefficient photogenerated electron transfer in QDs based photoanodes.(2)Mn2+dopants improve the charge transfer in QDs and cause their photoanodes to exhibit significantly higher saturation photocurrent density and reliable photoelectrochemical stability.However,the strategy of surface doping of Mn2+ is prone to enhance the magnetic coupling between adjacent Mn2+.In addition,the defects(derived from unstable ligands or suspended bonds)on the surface of bare QDs not only act as non-radiative recombination centers to suppress the luminescence,but also make QDs sensitive to surroundings.Therefore,CdSe/Mnx-CdS QDs are prepared via a two-step method for photoelectrochemical hydrogen generation.The construction of CdS shells significantly passivates the surface defects of QDs and prevent the photogenerated electrons from being annihilated.By combining the electron paramagnetic resonance spectroscopy(EPR)and X-ray absorption fine structure(XAFS)spectroscopy indicate the construction of shell plays a role in forcing Mn2+into the lattice of CdS.The magnetic coupling between adjacent Mn2+is effectively suppressed by controlling the concentration of Mn2+in CdS shells.In addition,Mn2+dopants introduce the intragap states in host QDs.It not only reduces the offset between the conductive band of core/shell materials(CdSe and CdS),but also suppress the non-radiative recombination of photogenerated electrons.More photogenerated electrons can thus transferred from type Ⅰ QDs to the TiO2 film.This efficient separation/transfer process of photogenerated charges enables the CdSe/Mn0.03CdS QDs sensitized photoanode to achieve a saturation photocurrent density of～18.7 mA cm-2 under standard illumination,which is almost twice higher than that of undoped CdSe/CdS QDs(～9.6 mA cm-2).The CdSe/Mn0.03-CdS QDs sensitized photoanode also exhibits the long-time stability of photoelectrochemical hydrogen production.In contrast,excessive doping leads to aggregated Mn2+on the surface of QDs,which acts as a structural defect,increasing the non-radiative recombination of photogenerated electrons.Thus,the saturation photocurrent density of CdSe/Mn0.06-CdS QDs sensitized photoanode decreases to～15.0 mA cm-2.These results demonstrate that using Mn2+to modulate the separation/transfer process of photogenerated charges in heterostructured QDs has a great potential to realize an efficient photoelectrochemical hydrogen production system.