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Femtosecond Transient Absorption Spectroscopy Of Inorganic Semiconductor Photocatalytic Nanosystems

Posted on:2018-01-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z W ChenFull Text:PDF
GTID:1311330542474333Subject:Physical chemistry
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This dissertation reports on a systematic study of ultrafast dynamics and involved mechanisms in inorganic photocatalytic nanosystems by means of femtosecond time-resolved transient absorption spectroscopy,towards providing fundamental guidance for relevant applications.This dissertation includes two parts:(1)Ultrafast spectroscopy and dynamics studies on unitary inorganic photocatalytic nanosystems:(a)The hole transfer dynamics at the interface of methanol/g-C3N4;(b)The trap-state dynamics of ZnIn2S4 ultrathin nanosheets.(2)Ultrafast spectroscopy and dynamics studies on binary photocatalytic nanosystems:(a)Ultrafast carrier transfer dynamics at the interface of BiOCl/g-C3N4;(b)Ultrafast electron transfer dynamics at the interface of Gaphene/TiO2.(1)Ultrafast dynamics of unitary inorganic photocatalytic nanosystems(a)The hole transfer dynamics at the interface of methanol/g-C3N4We report on a first experimental scrutiny of the photoexcited hole dynamics in a prototypical system in which the hole-scavenging methanol molecules are chemisorbed on the semiconductor g-C3N4 substrate.A set of well-designed comparison and control experiments by means of femtosecond time-resolved transient absorption(fs-TA)spectroscopy were conducted.Firstly,we identified the elusive reverse hole transfer(RHT)process,which was found to occur on a timescale of a few hundred picoseconds.Secondly,the critical role of interfacially chemisorbed methoxy as the dominant species responsible for hole scavenging was confirmed by a control experiment using protonated g-C3N4 as the substrate.Thirdly,the impact of electron-phonon coupling on hole transfer dynamics was verified by the use of different interband photoexcitation scenarios.We revealed for the first time that the RHT rate is the key factor governing the hole-scavenging ability of different hole scavengers,thereby unveiling the fundamental mechanism behind the well-known empirical practice that methanol is routinely adopted as one of the most popular hole scavenger in realistic semiconductor-based photoelectrochemical applications.(b)The trap-state dynamics of ZnIn2S4 ultrathin nanosheetsIt is highly desirable to gain ambiguous but plausible correlation between the defect structures and photocatalytic activity:Taking the typical direct-bandgap semiconductor of hexagonal ZnIn2S4 with a layered structure as a model,by ultrafast transient absorption spectroscopy analysis,we found that although the rates of electron transfer have no change,the higher zinc vacancy concentrations allow for?1.7-fold increase in average recovery lifetime,thereby promoting the carrier separation and improving the photocatalytic performance.(2)Ultrafast spectroscopy and dynamics studies on binary photocatalytic nanosystems(a)Ultrafast carrier transfer dynamics at the interface of BiOCl/g-C3N4There exists an awkward ambiguity about the transfer direction of photogenerated charge carriers in p-n nanosystems.In view of this confusing situation,it is apparently imperative to pinpoint the ambiguous transfer direction of photogenerated charge carriers in the BiOCl(p)/g-C3N4(n)system.Careful analysis of our TA results on the BiOCl(p)/g-C3N4(n)system(in reference to those on its bare components)demonstrates that the photogenerated electrons are transferred from the CB of BiOCl to that of g-C3N4,not on the contrary.Moreover,such an electron transfer process was found to occur on a time scale of a few picoseconds.(b)Ultrafast electron transfer dynamics at the interface of Graphene/TiO2We demonstrate by ultrafast transient absorption spectroscopy the relationship between the average lifetime of the photoexcited electrons in TiO2 and the graphene growth time.In the early stage of graphene formation,the density of band-gap defect states in TiO2 increases gradually and dominates the charge transfer.The further growth of graphene adlayers on the TiO2 surface only slightly modifies the density of defect states in TiO2,and hence the graphene-mediated channel becomes predominant.
Keywords/Search Tags:femtosecond pump-probe, ultrafast transient absorption spectroscopy, inorganic semiconductor nano-photocatalysis, hole transfer dynamics, ultrafast charge carrier dynamics
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