Ultrafast Carrier Dynamics Of Photocatalytic Metal-Organic Framework And Semiconductor Ultrathin Nanosheet Systems

Metal-organic frameworks(MOFs),a burgeoning class of porous coordination polymers,have been exploited to optimize the desired performance in photocatalysis due to their unique properties such as high specific surface areas,abundant catalytic sites,and outstanding thermal stability.Nevertheless,the performance of MOFs in photocatalysis is usually far from satisfactory.Furthermore,the carrier-transfer processes within the MOF-based photocatalysts still remain ambiguous.Towards gaining further insights into the involved mechanism,we conducted a systematic study on several MOF-based photocatalytic nanosystems,by means of femtosecond-resolved transient absorption spectroscopy.First of all,we studied the size effect of metal co-catalyst particles on the photo-activity and carrier dynamics in a typical hybrid system formed by MOFs and metal single atoms.Second,we found that,in a hybrid system comprising metal nanoparticles(NPs)and MOFs,in which MOFs play a role as electron donors,the location of metal NPs relative to MOFs matters in photocatalytic performance and the internal carrier dynamics.Third,we revealed the location effect in a prototypical photocatalytic hybrid system of MOF materials interfaced with semiconductor NPs.Fourth,we synthesized a high-efficiency catalyst by combining MOFs with conductive polymers,and then revealed the carrier-transfer mechanism within the MOF-polymer hybrid system.These researches provide theoretical guidance for the rational design of MOF-based photocatalysts in the future.In addition,we conducted several researches on photocatalytic nanosystems based on ultrathin semiconductor nanosheets.First,we studied the ultrafast carrier dynamics of photocatalytic ultrathin ?-CoOOH nanosheets.Second,we releaved the carrier-transfer mechanism of a photocatalytic hybrid nanosystem integrating CdS nanorods and 1T-MoS2 nanosheets.The current studies not only enable a deeper understanding on the carrier-transfer mechanism for semiconductor nanomaterials,but also provide a unique perspective for the development of novel photocatalysts.The main parts in this dissertation are as followed:(?)Ultrafast spectroscopy and dynamics studies on photocatalytic nanosystems based on MOFs(a)Ultrafast carrier dynamics of a photocatalytic hybrid system consisting of a typical kind of MOF and single Pt atomsIn this work,single Pt atoms were successfully loaded into a MOF material(i.e.,Al-TCPP)for the first time,denoted Al-TCPP-0.1Pt.A better utilization of Pt atoms in Al-TCPP-0.1Pt further remarkably enhanced the photocatalytic efficiency.Mechanism information gleaned from ultrafast transient absorption spectroscopy has elucidated that the photogenerated electrons can efficiently transfer from MOFs to the single Pt atoms.Such rapid electron transfer substantially suppressed the undesirable electron-hole recombination and resulted in an improved photocatalytic performance.(b)Ultrafast spectroscopy studies on the location effect in a photocatalytic hybrid system formed by MOFs and Pt nanoparticlesUniform Pt NPs were deliberately encapsulated inside or supported on a typical kind of MOF material(i.e.,UiO-66-NH2)to afford Pt@UiO-66-NH2 and Pt/UiO-66-NH2,respectively.The only difference between these two composites is the Pt location.Compared to Pt/UiO-66-NH2,Pt@UiO-66-NH2 exhibits around 5 times higher photocatalytic efficiency in hydrogen production by splitting of water,highlighting the Pt location relative to MOF matters in photocatalytic process.Observations by photoluminescence spectroscopy and ultrafast transient absorption spectroscopy revealed the underlying electron-transfer mechanism and further verified that a more efficient charge separation was achieved in the Pt@UiO-66-NH2 case.(c)Location effect revealed by transient absorption spectroscopy in a prototypical photocatalytic hybrid system of MOF materials interfaced with TiO2 NPsIt is interesting to see if the location effect observed in metal-MOF system also manifests in semiconductor-MOF system,in which MOFs usually play as electron acceptors.To this end,we performed a set of femtosecond time-resolved transient absorption measurements on a typical semiconductor-MOF system with TiO2 NPs being incorporated inside or supported onto Cu3(BTC)2,forming the two composites denoted TiO2@Cu3(BTC)2 and TiO2/Cu3(BTC)2,respectively.We find that the interface states formed between TiO2 and Cu3(BTC)2 can act as an effective relay for electron transfer,whose efficiency rests on the relative location of the two components.Such a subtle location effect is responsible for the higher electron-transfer efficiency and the better performance in photocatalytic CO2 reduction,in the TiO2/Cu3(BTC)2 case.(d)Ultrafast carrier dynamics and the involved mechanism within a photocatalytic system comprising MOFs and conductive polymersWe successfully synthesized a high-efficiency photocatalyst,Al-PMOF-Cu-PPY,by successively introducing organic ligands(i.e.,a porphyrin-based complex containing bivalent copper ions)and typical conductive polymers(i.e.,polypyrrole)to a representative MOF material(i.e.,Al-PMOF).By using ultrafast transient absorption spectroscopy,we tracked in real time the electron transfer dynamics and unveiled the underlying mechanism.In pristine Al-PMOF,photo-induced electrons were trapped within a long-lived intermediate state(r>>4 ns).The organic ligands containing bivalent copper ions can act as an effective relay and electrons could transfer to polypyrrole via this relay,thereby further enhanced the photocatalytic performance.(?)Ultrafast spectroscopy studies on photocatalytic nanosystems based on ultrathin semiconductor nanosheets(a)Ultrafast carrier dynamics of photocatalytic ultrathin ?-CoOOH nanosheetsUltrathin ?-CoOOH ultrathin nanosheets were successfully synthesized through an ultrasonic method.The as-prepared ultrathin ?-CoOOH nanosheets turned out to be highly efficient in photocatalytic hydrogen production by water splitting.Femtosecond transient absorption measurements,together with first-principle calculations,revealed that the disastrous electron-hole recombination in the ultrathin ?-CoOOH nanosheets was substantially suppressed,thereby activating the photocatalysis.(b)Ultrafast carrier dynamics of a photocatalytic hybrid nanosystem integrating CdS nanorods and 1T-MoS2 nanosheetsBy fabricating CdS nanorods with metallic-phased 1T-MoS2 nanosheets,we successfully synthesized an optimized photocatalyst,1T-MoS2@CdS.As illustrated by ultrafast transient absorption and photoluminescence spectroscopy,the photoexcited electons of CdS could rapidly transfer to 1T-MoS2 and then the suppression of the undesirable carrier recombination was achieved,thereby the photocatalytic performance was optimized.