Template-Free Synthesis of Zn_1–<i>x</i>Cd_<i>x</i>S Nanocrystals with Tunable Band Structure for Efficient Water Splitting and Reduction of Nitroaromatics in Water

A facile, one-pot, solvothermal synthesis of nanocrystals (NCs) of Zn_1–<i>x</i>Cd_<i>x</i>S (<i>x</i> = 0.1 (<b>S1</b>)–0.9 (<b>S9</b>)) solid solutions has been successfully carried out using 4,4′-dipyridyldisulfide (DPDS = (C₅H₄N)₂S₂)) as a new temperature-dependent <i>in situ</i> source of S^2– ions. Powder XRD patterns of the samples revealed gradual phase transformation from cubic to hexagonal upon increasing the Cd content (<i>x</i>) in the solid solutions Zn_1–<i>x</i>Cd_<i>x</i>S (0 ≤ <i>x</i> ≤ 1). FESEM analyses showed almost spherical morphology of the solid solutions, <b>S2</b>, <b>S5</b>, and <b>S9</b>. HR-TEM analyses of <b>S3</b> and <b>S9</b> 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 Zn_1–<i>x</i>Cd_<i>x</i>S 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 Zn_0.7Cd_0.3S 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 Zn_1–<i>x</i>Cd_<i>x</i>S 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 Zn_1–<i>x</i>Cd_<i>x</i>S 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 Zn_1–<i>x</i>Cd_<i>x</i>S solid solutions

Reversible Alkaline Hydrogen Evolution and Oxidation Reactions Using Ni–Mo Catalysts Supported on Carbon

Unitized regenerative fuel cells based on hydroxide exchange membranes are attractive for long duration energy storage. This mode of operation depends on the ability to catalyze hydrogen evolution and oxidation reversibly, and ideally using nonprecious catalyst materials. Here we report the synthesis of Ni–Mo catalyst composites supported on oxidized Vulcan carbon (Ni–Mo/oC) and demonstrate their performance for reversible hydrogen evolution and oxidation. For the hydrogen evolution reaction, we observed mass-specific activities exceeding 80 mA/mg at 100 mV overpotential, and additional measurements using hydroxide exchange membrane electrode assemblies yielded full cell voltages that were only ~100 mV larger for Ni–Mo/oC cathodes compared to Pt–Ru/C at current densities exceeding 1 A/cm2. For hydrogen oxidation, Ni–Mo/oC films required <50 mV overpotential to achieve half the maximum anodic current density, but activity was limited by internal mass transfer and oxidative instability. Nonetheless, estimates of the mass-specific exchange current for Ni–Mo/oC from micropolarization measurements showed its hydrogen evolution/oxidation activity is within 1 order of magnitude of commercial Pt/C. Density functional theory calculations helped shed light on the high activity of Ni–Mo composites, where the addition of Mo leads to surface sites with weaker H-binding energies than pure Ni. These calculations further suggest that increasing the Mo content in the subsurface of the catalyst would result in still higher activity, but oxidative instability remains a significant impediment to high performance for hydrogen oxidation