Improved activity and stability of ZnIn2S4 for H2 production under visible light through cerium UiO-66

Abstract

Indium zinc sulfide (ZnIn₂S₄) has been reported to be highly photoactive for H₂ production with Na₂S and Na₂SO₃ as sacrificial reagents, but it still suffers from serious corrosion. In this work, we report an efficient composite of hexagonal ZnIn₂S₄ microspheres (ZIS) and cerium UiO-66 metal–organic framework (MOF) octahedra, both of which harvest light at wavelengths up to 550 nm. Under a 420 nm LED lamp and in a Na₂S/Na₂SO₃ aqueous solution, the amount of H₂ evolved at 2 h (nH₂h) was negligible for the MOF. After 18, 27, 37, and 47% MOF loading, however, ZIS had an increased n_H2h by factors of approximately 4.8, 5.6, 4.2, and 3.2, respectively. This trend for the n_H2h coincided with that for the efficiency of charge separation, inferred by emission spectroscopy. A five-repeat test for H₂ production showed that ZIS partially decomposed into metallic indium, while MOF/ZIS was much more stable than ZIS. Moreover, for proton reduction or for water photo-oxidation on an electrode, MOF/ZIS was also more active than ZIS, followed by the MOF. According to the measured band parameters, a possible mechanism is proposed, including the electron transfer from ZIS to Ce-U66, and the hole transfer from Ce-U66 to ZIS, which improves the efficiency of charge separation. It is worth noting that 30% Ce-MOF/ZIS is approximately 1.4 times more active than 30% Zr-MOF/ZIS.