Controlling Cu2ZnSnS4 photocatalytic ability through alterations in sulfur availability

• Synthesis of Cu2ZnSnS4 nanocrystals in a one-pot solvothermal process was tuned by changing free- and ligand-bound-S.
• The photocatalysis of methyl viologen (MV) from MV2+ to MV+ state by CZTS was used as the probe to test the photovoltaic nature.
• Increased long-range crystal order resulted from slow-release ligand-bound-S leads to an enhanced photocatalytic response.
• Optical band gap tuning was achieved by controlling S-release of the ligand-bound-S.
• XAFS is another powerful tool in addition to other synchrotron spectroscopy techniques for energy materials.

Cu2ZnSnS4 (CZTS) nanocrystals (NCs) were made via a one-pot solvothermal method with various amounts of available free-sulfur and a fixed amount of sulfur bound to 2-mercapto-5-n-propylpyrimidine (MPP). Varying the sulfur availability yields CZTS NCs of different stoichiometry, from which five distinct samples were analyzed for consistency both microscopically and macroscopically. As revealed by X-ray absorption fine structure investigation, samples fabricated in the presence of decreased free-sulfur showed decreased CZTS character, with sporadic compositions and no long term order; however, when fabricated in the presence of no free-sulfur, sulfur from the degraded MPP was found incorporated into the CZTS structure. These NCs showed improved long-term order over standard synthetic procedure. The catalysis of methyl viologen (MV) from MV2+ to MV+ state by CZTS under light irradiation was used as the probe to test the photovoltaic nature. The photocatalysis was enhanced in the films made from NCs fabricated without available free-sulfur. This enhancement is consistent with the measured band gaps, with more ordered NCs showing a band gap that better matches the most intense regions of the solar spectrum.

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