No abstract

Single-atom catalysts provide an effective approach to reduce the amount of precious metals meanwhile maintain their catalytic activity. However, the sluggish activity of the catalysts for alkaline water dissociation has hampered advances in highly efficient hydrogen production. Herein, we develop a single-atom platinum immobilized NiO/Ni heterostructure (PtSA-NiO/Ni) as an alkaline hydrogen evolution catalyst. It is found that Pt single atom coupled with NiO/Ni heterostructure enables the tunable binding abilities of hydroxyl ions (OH*) and hydrogen (H*), which efficiently tailors the water dissociation energy and promotes the H* conversion for accelerating alkaline hydrogen evolution reaction. A further enhancement is achieved by constructing PtSA-NiO/Ni nanosheets on Ag nanowires to form a hierarchical three-dimensional morphology. Consequently, the fabricated PtSA-NiO/Ni catalyst displays high alkaline hydrogen evolution performances with a quite high mass activity of 20.6 A mg−1 for Pt at the overpotential of 100 mV, significantly outperforming the reported catalysts.

We report a study of Zn(2+) by Cd(2+) cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (i.e., fraction of Cd(2+) ions originally in solution, Cdsol, that is incorporated in the ZnSe NC) increases with temperature and depends also on the Cdsol/ZnSe ratio. Interestingly, the reaction temperature can be used as a sensitive parameter to tailor both the composition and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150 °C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temperatures (200 and 220 °C) produce (Zn1-xCdx)Se gradient alloy NCs, with increasingly smoother gradients as the temperature increases, until homogeneous alloy NCs are obtained at T ≥ 240 °C. Remarkably, sequential heating (150 °C followed by 220 °C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn1-xCdx)Se gradient alloy NCs. Thermal treatment at 250 °C converts the ZnSe/CdSe core/shell HNCs into (Zn1-xCdx)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSe-CdSe NCs.

Luminescent CuInS2 nanocrystals are prepared by partial cation exchange in Cu2‐xS nanocrystals (NCs) suspended in toluene using a MeOH solution of In(NO3)3 in the molar ratio of In/Cu of ≈1.

Recently, electrochemical deposition is arising as a promising technique to synthesize supported nanoparticles which are important for many applications including electrocatalysis 1 and electroanalysis. 2 By means of electrochemical deposition, nanoparticles grow direclty from the substrate without the need of further sample preparation. Moreover, the technique is surfactant-free and cost-effective and allows to tune the nature of the nanoclusters by changing electrolyte composition and deposition parameters. 3 Several groups have electrodeposited metallic nanoparticles on various substrates such as glassy carbon, 4,5 highly oriented pyrolitic graphite, 6,7 indium-doped tin oxide, 8,9 carbon nanotubes, 10 and others. 11 However, in order to investigate the size-dependent properties of small nanoparticles and to use them as electrocatalysts or in sensing devices, distributions with low size dispersions need to be obtained. This still represents a challenge in the field of electrochemical deposition. In order to overcome this issue and improve nanoscale electrodeposition, it becomes extremely important to understand nanocluster nucleation and growth.Since many decades plenty of theoretical and experimental literature is available reviewing this topic, 3,12À14 with the most widely referred model being the one developed by Scharifker and Hills, 15 which has been slightly reformulated several times. 16À18 Conventionally, electrochemical nucleation and growth studies have been addressed indirectly by measuring the currentÀtime transients in potentiostatic experiments and correlating them to models that take into account the random nature of nucleation and the coupled growth of hemispherical nuclei under diffusion limitations. According to these models, an electrochemically formed nucleus grows by direct reduction of ions onto its surface if this action is thermodynamically favorable (i.e., if the Gibbs free energy is reduced by the addition of more atoms to the cluster), whereas it will dissolve if this condition is not fulfilled. Therefore, it has been normally considered that when a constant overpotential is applied, nuclei whose radii are bigger than a critical radius (r c ) are formed progressively in the surface and grow by direct attachment. The growth of each of them affects both the concentration of active species and the overpotential distribution in the cluster vicinity creating zones of reduced concentration and overpotential and thus reduced nucleation rate. Then, if multiple clusters are considered, their local zones of reduced nucleation rate spread and gradually overlap. This is a very complex problem which is frequently solved by approximating the areas of reduced nucleation rate by overlapping planar diffusion zones in which nucleation is fully arrested. 15À18 It is then foreseen that for progressive nucleation the number of growing nuclei can be expressed by an exponential law:where N 0 is the total number of active sites and A is the nucleation rate constant. Under these conditions, average particl...