Studies On Synthesis And Photocatalytic Properties Of Cu₂ZnSnS₄ Nanomaterials

With the depletion of fossil energy,solar energy has received more and more attention as source of new renewable energy.Inorganic semiconductor nanomaterials have been favored for converting solar energy into renewable energy by researchers.At present,a variety of semiconductor nanomaterials have been used for renewable energy.Among these semiconductor nanomaterials,Cu₂ZnSnS₄?CZTS?has gained extensive attention and research because it has optimum band gap for solar spectrum,high light absorption coefficient,good catalytic ability,as well as the elements are abundant and environmentally friendly.And,it has been used in thin film solar cells,dye-sensitized solar cells and photocatalysis.Fujishima A and Honda K from University of Tokyo realized the photocatalytic hydrogen production reaction on single-crystal TiO₂ electrodes for the first time in 1972,so the probability of directly transforming solar energy to hydrogen energy become true.Since then,the development of research on photocatalytic hydrogen production using semiconductor photocatalysis has been triggered.Although some star materials such as TiO₂ and CdS have achieved high photocatalytic hydrogen production efficiency so far,the utilization efficiency of sunlight is low because the most of them only use ultraviolet light and a small amount of visible light.In contrast,CZTS can make full use of the light due to the small band gap.In addition,the constituent elements are abundant in the earth's crust.At present,some progress has been made in the hydrogen production reaction using CZTS semiconductor as a photocatalyst.Although CZTS can effectively use solar energy,the efficiency of photocatalytic hydrogen production is still low,mainly because of the following issues:?1?The multielement and non-stoichiometry nature of CZTS make it self has large amount of lattice and/or surface defects in CZTS.These defects often cause fast recombination of photogenerated carriers in bulk and surface of CZTS,which caused bad effect on photocatalytic hydrogen production performance of CZTS.?2?The intrinsic CZTS is a direct bandgap semiconductor with a narrow band gap of only about 1.5 eV.Under the illumination condition,the excited electrons will migrate to the conduction band.However,they may re-enter the valence band in a short time and recombine with the photogenerated holes in the valence band.After recombination of photogenerated carriers,the energy will emitted in the form of light energy or heat energy.And,this parts of photogenerated carriers has little effect on the improvement of photocatalytic hydrogen production performance.?3?The effective specific surface area as well as the surface composition of CZTS prepared by different methods are quite different,that may affect the photocatalytic hydrogen production performance of CZTS.In connection with these issues,we firstly synthesized different CZTS nanomaterials with different properties and morphologies using different methods and investigated their photocatalytic hydrogen production performance.We found that the photocatalytic hydrogen production performance of CZTS is quite different because of the structure differences?including crystallinity,crystal structure and active surface?of CZTS nanomaterials prepared by different methods.However,the photocatalytic hydrogen production performance of pristine CZTS nanomaterials is quite poor.Next,we synthesized Na doped CZTS to reduce the bulk defect in CZTS for reducing the recombination of photogenerated carriers in bulk CZTS as well as the photocatalytic hydrogen production performance of CZTS.In the study,we found that the introduction of Na had little effect on the optical absorption properties of CZTS nanomaterials.However,the introduction of Na could significantly increase the concentration of photogenerated carriers and prolong their lifetime.At last,the introduction of Na can improve the the photocatalytic hydrogen production performance of CZTS to some extent.Although the Na doped CZTS show enhanced photocatalytic hydrogen production performance froming 90?mol/g·h to165?mol/g·h,the photocatalytic hydrogen production performance of CZTS is still far low for practical use.In another word,the recombination of photogenerated carriers in bulk CZTS is not the main issue.In the end,we constructed p-n type CZTS decorated CdS nanorods heterojunction,creating a built-in electric field at the interface between CdS and CZTS.This built-in electric field enables the hydrophilic CZTS-CdS nanocomposites following advantages:1.Facilitating photo-induced carriers separation and migration between CdS and CZTS,increasing the number of effective photogenerated electron-hole?e^--h⁺?pairs.2.This type II band alignment between CdS and CZTS could promote the transfer of photogenerated electrons from the CZTS conduction band to CdS conduction band,meanwhile the the transfer of photogenerated holes from the CdS valence band to CZTS valence band.3.Reducing defects at the interface between CZTS and CdS,thus reducing the recombination probability of photogenerated carriers to a large extent.4.It prolongs the lifetime of photogenerated carriers,thus significantly improving the utilization of photogenerated carriers.CZTS-CdS?150?,namely prepared from 150 mg CdS nanorods,possess the highest hydrogen production rate of 11 mmol/g·h.