Researches On Preparation And Photocatalytic Properties Of Novel Cu₂ZnSn S₄ Materials

Researches on solar energy conversion have received increasing attention due to the energy demand and environmental problems. Cu(In Ga)Se2(CIGS) has been widely investigated as one of sustainable and high-efficiency solar absorber materials. However,the scarcity and the high cost of In and Ga elements in CIGS have limited its wide applications. An alternative material is thus imperative to replace CIGS. CZTS, a p-type semiconductor, has potential applications in solar energy conversion fields due to its optimal bandgap(1.4-1.6 e V), high absorption coefficient(> 104cm-1), environmental friendliness, natural abundance, and it shows good photocatalytic properties. However, it is difficulty to control the preparation of CZTS materials due to the quarternary components, and low-cost synthesis is also a large challenge by solution-based methods.Hence, researches on the chemical preparation and photocatalytic properties of CZTS are keys of applications in successful conversion of solar-chemical energies.Nowadays, CZTS materials can be prepared by many methods. For the preparation of CZTS nanocrystals, hot-injection is the most common method. However, many costly additives(surfactants and long-chain molecules), complex operations and high reaction temperature of above 250 ℃ are required during the process. In view of this, low-cost and convenient methods, such as solvothermal and hydrothermal methods, are adopted to synthesize CZTS nanopaticles in this paper. In ethanediamine-solvothermal system,ethanediamine not only severs as the strong chelating agent and solvent, but plays a significant role in nucleation of CZTS nanoparticles. In the ammonia-assisted hydrothermal process, ammonia solution plays an important role in crystal growth. The thiacetamide acts as a sulfur source and a reducing agent. It is found that the morphologies and phases of CZTS nanocrystals can be controlled without the usage of surfactants. Two kinds of CZTS nanoparticles are deposited into films by a doctor blade method, and they show weak photocatalytic activities during the measurements of photoelectrochemical performance.In order to improve the photocatalytic properties of CZTS materials, an effort is made in nanoscale. For the first time hard-template method(nanocasting) is applied to prepare CZTS nanorods with a kesterite crystal structure and small sizes. Low-cost and stable methanol-based metal chlorides-thiourea solution acts as sol precursor solution and SBA-15 serves as the template. The sol undergoes alcoholysis and polycondensation reactions and forms a gel in the template channels. The gel decomposes and crystallizes into CZTS compound after annealing.The resulting pure kesterite CZTS nanorods possess a diameter of 6.8 nm and lengths of 20-100 nm after the removal of template.CZTS film is deposited from nanorods using a drop-coating method, which shows better property of photocatalytic water splitting than the above-mentioned CZTS nanoparticles.It is attributed to the enhanced separation of photogenerated electron-hole pairs by one-dimensional structure.Although the photocatalytic property of CZTS nanorods is better than that of CZTS nanoparticles, the films prepared by “ink” coating methods(doctor blade and drop-coating) show strong dark current in photoelectrochemical reactions, which results in many holes, cracks, and large boundary densities on film surfaces, as well as the photocorrosion of CZTS materials. Therefore, CZTS thin films have been required depositing on the substrates directly via simple and low-cost solution-based methods(solovthermal and spin-coating). After comparision, CZTS thin film with smooth,compact, and crackless features prepared by solution-based spin-coating method possesses better photoelectrocatalytic performance. To overcome the photocorrosion of CZTS materials, CZTS thin film has been modified by Cd S and Ti O2 layers to prepare a photoelectrode with a structure of CZTS/Cd S/Ti O2. It not only shows outstanding photoelectrocatalytic properties, also exhibits remarkable stablity in long term photoelectrochemical reaction for hydrogen production.In this paper, we successfully prepare CZTS materials with different morphologies by preparing different precusor solutions and using diverse methods, and investigate the corresponding photocatalytic properties, and the relationships among the properties,structures, and preparation processes of CZTS thin films are also researched. These results are significant experimental foundations and theoretical guidances in photocatalytic water splitting of CZTS materials.