Template-free synthesis of Zn1–xCdxS nanocrystals with tunable band structure for efficient water splitting and reduction of nitroaromatics in water

Abstract

A facile, one-pot, solvothermal synthesis of nanocrystals (NCs) of Zn1–xCdxS (x = 0.1 (S1)–0.9 (S9)) solid solutions has been successfully carried out using 4,4′-dipyridyldisulfide (DPDS = (C5H4N)2S2)) as a new temperature-dependent in situ source of S2– ions. Powder XRD patterns of the samples revealed gradual phase transformation from cubic to hexagonal upon increasing the Cd content (x) in the solid solutions Zn1–xCdxS (0 ≤ x ≤ 1). FESEM analyses showed almost spherical morphology of the solid solutions, S2, S5, and S9. HR-TEM analyses of S3 and S9 unveiled the presence of small nanocrystals (NCs) of size 7 and 15 nm, respectively, and highlights the discontinuity in the pattern of lattice fringes. Optical measurements revealed that Zn1–xCdxS solid solutions exhibit precisely tunable band structure with varying the concentration of Cd content. Furthermore, visible-light-assisted photocatalytic investigation revealed very good activity of the Zn0.7Cd0.3S solid solution for water splitting with H2 generation rate of 750 μmol h–1 g–1. Interestingly, for the first time, the water splitting activity of the Zn1–xCdxS NCs has been applied for efficient reduction of nitroaromatic pollutants in water by utilizing water as a source of hydrogen. Remarkably, various substituted nitroaromatics containing both electron-donating and -withdrawing groups as well as dinitroaromatics can be efficiently reduced to their corresponding amines in high yield and selectivity. Also, the photocatalyst can be recycled and reused for several cycles without significant loss of the activity. The plausible mechanism for the reduction of nitroaromatics in water by Zn1–xCdxS solid solution has also been studied. Herein we demonstrate a unique approach wherein water acts as a source of reducing agent for the visible-light-assisted photocatalytic reduction of nitroaromatic pollutants in water by Zn1–xCdxS solid solutions.