Controllable Synthesis Of Quaternary Cu-Zn-Sn-S Nanocrystals And Their Applications In Photocatalytic Hydrogen Evolution

Multinary copper-based chalcogenide semiconductor nanocrystals have exhibited potential applications in the photocatalytic hydrogen evolution due to their tunable crystal structures,morphologies,abundant species and adjustable light absorption band gap.This thesis presented the controllable synthesis of quaternary Cu-Zn-Sn-S（CZTS）nanocrystals and the effects of the reaction condictions including surfactant types,reaction temperature and precursor dosage on the compositions,crystal types and morphologies of the products.Moreover,the photoelectrochemical test and photocatalytic hydrogen evolution of the samples with different crystal strucutres and morphologies were also studied.The detailed results were summarized as follows:First of all,ternary Cu-Sn-S nanocrystals with petrukite phase were synthesized by using a simple one-pot method,and quaternary Cu-Zn-Sn-S nanocrystals with wurtzite-kesterite polytypism were also synthesized by incorporation of Zn ions into the ternary couterparts,which indicated that the formation of the quaternary Cu-Zn-Sn-S nanocrystals were dominated by the partial cation-diffusion mechanism.Based on this result,a series of quaternary Cu-Zn-Sn-S nanocrystals with different crystal structures and morphologies were synthesized by adjusting the types of surface ligands,the reaction temperature and the Cu precursors dosage.The phase transition from kesterite phase to hexagonal wurtzite phase was realized by changing the reaction temperature and surfactant ratios.Moreover,hat-shaped and bullet-shaped Cu-Zn-Sn-S nanocrystals were obtained at relatively high temperature（>260°C）,and the high-resolution transmission electron microscopy（HRTEM）results indicated that the exposed facets were mainly（002）facet.Furthermore,the crystal structure of the quaternary Cu-Zn-Sn-S nanocrystals could be effectively tailored by tuning the Cu precursors dosage,which changed from Sn S at low dosage to wurtzite phase at intermediate dosage,and finally the kesterite phase at high dosage.This further confirmed the formation was dominated by partial cation diffusion reaction.The aforementioned results provided the basis for the controllable synthesis of multinary copper-based chalcogenide nanocrystals.Next,the photoelectric response and photocatalytic hydrogen evolution of a series of quaternary Cu-Zn-Sn-S nanocrystals with different crystal phases,morphologies and components were studied,and the results indicated that the hexagonal wurtzite Cu-Zn-Sn-S nanocrystals with hat-shape or bullet-shape exhibited better photocatalytic activities,and the photocatalytic hydrogen evolution rate could reach up to 830μmol g^-1h^-1.Based on the results from the UV-Vis diffuse reflection spectra,photoelectric response and impedance test,the better photocatalytic activities could be attributed to low optical band gap and resistance as well as the higher photoelectric reponse,which was in favor of the extending absorption region and the separation of photo-indued electron-hole pairs.By comparing the photocatalytic hydrogen evolution of the Cu-Zn-Sn-S nanocrystals synthesized in the presence of different Cu precursor dosage,the sample obtained at 1 mmol Cu dosage exhibited better photocatalytic activities,and the crystal structure of the sample in this case was wurtzite phase.Based on the result,the photocatalytic performance was further optimized,which exhibited better photocatalytic activities after 4 times test,indicating better stability of the samples.This work layed the foundation for further optimization of photocatalytic performanc of multinary copper-based chalcogenide nanocrystals.