Tuning Complex Shapes in Platinum Nanoparticles: From Cubic Dendrites to Fivefold Stars

The Central Role of Nitrogen Atoms in a Zeolitic Imidazolate Framework-Derived Catalyst for Cathodic Hydrogen Evolution

Platinum usually offers the most effective active center for hydrogen evolution reaction (HER), because of the optimal trade-off between the adsorption and desorption of hydrogeN atoms (H*) on Pt atoms. Herein, we report an unusual result regarding the active center of a HER catalyst, which was synthesized by electrodepositing traces of Pt nanoparticles (NPs) into a porous nitrogen-rich dodecahedron matrix derived from zeolitic imidazolate framework ZIF-8. With an ultra-low Pt loading of 2.76 μg cm^-2 , the N-Pt-bonded catalyst can produce a current density of 117 mA cm^-2 for the HER in 1.0 m H₂ SO₄ at an overpotential of 50 mV, whereas the commercial Pt/C (300 μg cm^-2 Pt) can only reach 50 mA cm^-2 under the same conditions. Cyclic voltammetry demonstrates that both the H* adsorption and the Pt oxidation are not allowed to occur on this catalyst, due to a full surface coverage of the trace Pt NPs by imidazole. The results from the specially designed experiments indicate that the imidazole N atoms may act as proton anchor-sites for the HER due to their electron donor nature. Density functional theory calculations also support a catalytic HER mechanism centered at the Pt-supported N active center, which needs a Gibbs free energy of H* absorption (ΔG_H* ) significantly smaller than the absolute value of ΔG_H* on the Pt(111) surface. We hope that the results of this study will encourage the research on novel N-centered catalysts for the HER.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants

There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

Multi-generation overgrowth induced synthesis of three-dimensional highly branched palladium tetrapods and their electrocatalytic activity for formic acid oxidation

Highly branched noble metal nanostructures are highly attractive for catalytic applications owing to their specific physical and chemical properties. In this work, three-dimensional highly branched palladium tetrapods (Pd-THBTs) have been constructed in the presence of polyvinylpyrrolidone (PVP) through one-step hydrothermal reduction of ethylenediamine-tetramethylene phosphonate-palladium(II) (EDTMP-Pd(II)) by formaldehyde. The morphology and structure of the Pd-THBTs were fully characterized and the growth mechanism was explored and discussed based on the experimental observation. The concave Pd tetrahedra grew into highly branched Pd tetrapods consisting of four nanothorn-like branches with tetrahedral dimensions through interesting multi-generation nanocrystal overgrowth. The electrocatalytic activities of the as-synthesized Pd-THBTs toward formic acid oxidation were also studied by cyclic voltammetry and chronoamperometry. The Pd-THBTs showed higher catalytic activity and stability for formic acid oxidation than the commercial Pd black.