We describe a simple and efficient methodology for the aqueous synthesis of stable, uniform, and size tunable Au@Ag core–shell nanoparticles (NPs) that are stabilized by citrate ions. The synthetic route is based on the stepwise Ag reduction on preformed Au NPs. The final size of the core–shell NPs and therefore their optical properties can be modulated at least from 30 to 110 nm by either tuning the Ag shell thickness or changing the size of the Au core. The optical properties of the Au@Ag core–shell NPs resemble those of pure Ag NPs of similar sizes, which was confirmed by means of Mie extinction calculations. We additionally evaluated the surface-enhanced raman scattering (SERS) enhancing properties of Au@Ag core–shell NP colloids with three different laser lines (532, 633, and 785 nm). Importantly, such core–shell NPs also exhibit a higher SERS efficiency than Ag NPs of similar size under near-infrared excitation. The results obtained here serve as a basis to select Au@Ag core–shell NPs of specific size and composition with maximum SERS efficiency at their respective excitation wavelengths for SERS-based analytical and bioimaging applications.

Ni and NiSn supported on zirconia (ZrO₂) and on indium (In)-incorporated zirconia (InZrO₂) catalysts were prepared by a wet chemical reduction route and tested for hydrogenation of CO₂ to methanol in a fixed-bed isothermal flow reactor at 250 °C. The mono-metallic Ni (5%Ni/ZrO₂) catalysts showed a very high selectivity for methane (99%) during CO₂ hydrogenation. Introduction of Sn to this material with the following formulation 5Ni5Sn/ZrO₂ (5% Ni-5% Sn/ZrO₂) showed the rate of methanol formation to be 0.0417 μmol/(g_cat·s) with 54% selectivity. Furthermore, the combination NiSn supported on InZrO₂ (5Ni5Sn/10InZrO₂) exhibited a rate of methanol formation 10 times higher than that on 5Ni/ZrO₂ (0.1043 μmol/(g_cat·s)) with 99% selectivity for methanol. All of these catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy, CO₂-temperature-programmed desorption, and density functional theory (DFT) studies. Addition of Sn to Ni catalysts resulted in the formation of a NiSn alloy. The NiSn alloy particle size was kept in the range of 10–15 nm, which was evidenced by HRTEM study. DFT analysis was carried out to identify the surface composition as well as the structural location of each element on the surface in three compositions investigated, namely, Ni₂₈Sn₂₇, Ni₁₈Sn₃₇, and Ni₃₇Sn₁₈ bimetallic nanoclusters, and results were in agreement with the STEM and electron energy-loss spectroscopy results. Also, the introduction of “Sn” and “In” helped improve the reducibility of Ni oxide and the basic strength of catalysts. Considerable details of the catalytic and structural properties of the Ni, NiSn, and NiSnIn catalyst systems were elucidated. These observations were decisive for achieving a highly efficient formation rate of methanol via CO₂ by the H₂ reduction process with high methanol selectivity.