Hydrogen evolution on Au(111) catalyzed by rhodium nanoislands

Do Rh-Hydride Phases Contribute to the Catalytic Activity of Rh Catalysts under Reductive Conditions?

Rh-hydride phases were believed to be key causes of the exceptional catalytic ability of Rh catalysts under H2 reductive conditions. Here, we utilize the large-scale machine-learning-based global optimization to explore millions of Rh bulk, surface, and nanoparticle structures in contact with H2, which rules out the presence of subsurface/interstitial H in Rh and Rh-hydride phases as thermodynamically stable phases under ambient conditions. Instead, an exceptional Rh-H affinity is identified for surface Rh atoms in Rh nanoparticles that can accommodate a high concentration of adsorbed H, with the surface Rh to H ratio reaching ∼2.5, featuring stable six-H-coordinated Rh, [RhH6]. Such [RhH6] species forming at edged Rh sites are found to be the key intermediates in the electrochemical hydrogen evolution reaction (HER) on Rh. Guided by theory, our synthesized Rh concave nanocubes with a high density of edged Rh sites achieve a Tafel slope of 28.4 mV dec-1 and a low overpotential of 36.1 mV at jECSA = 1 mA cm-2, which outperforms commercial Pt/C and other morphologies of Rh catalysts. Our results clarify the active phase in Rh-H nanosystems and guide the catalyst design by precise morphology control of nanocatalysts.

Adsorption and solar light activity of noble metal adatoms (Au and Zn) on Fe(111) surface: a first-principles study

Noble metals such as gold (Au), zinc (Zn), and iron (Fe) are highly significant in both fundamental and technological contexts owing to their applications in optoelectronics, optical coatings, transparent coatings, photodetectors, light-emitting devices, photovoltaics, nanotechnology, batteries, and thermal barrier coatings. This study presents a comprehensive investigation of the optoelectronic properties of Fe(111) and Au, Zn/Fe(111) materials using density functional theory (DFT) first-principles method with a focus on both materials' spin orientations. The optoelectronic properties were obtained employing the generalized gradient approximation (GGA) and the full-potential linearized augmented plane wave (FP-LAPW) approach, integrating the exchange-correlation function with the Hubbard potential U for improved accuracy. The arrangement of Fe(111) and Au, Zn/Fe(111) materials was found to lack an energy gap, indicating a metallic behavior in both the spin-up state and the spin-down state. The optical properties of Fe(111) and Au, Zn/Fe(111) materials, including their absorption coefficient, reflectivity, energy-loss function, refractive index, extinction coefficient, and optical conductivity, were thoroughly examined for both spin channels in the spectral region from 0.0 eV to 14 eV. The calculations revealed significant spin-dependent effects in the optical properties of the materials. Furthermore, this study explored the properties of the electronic bonding between several species in Fe(111) and Au, Zn/Fe(111) materials by examining the density distribution mapping of charge within the crystal symmetries.

Atomic Structures of Single-Layer Nanoislands of Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au Supported on Au(111) from Density Functional Theory Calculations

We have used density functional theory calculations to study the atomic structure of single-layer nanoislands of metal M (M=Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au) supported on M(111) and Au(111) surfaces. Nanoislands of Cu, Pd, Ag, Pt, and Au have planar structures on Au(111), while nanoislands of Ni, Rh, and Ir are nonplanar. The calculations also show that nanoislands of Cu, Pd, Pt, and Au on Au(111) with a diameter below 3 nm can have one of several atomic structures. Two of these structures have atoms at the edges of the nanoislands located near bridge sites on Au(111), and the other structures have atoms at the edges and center of the nanoislands located near bridge sites. The relative stability of these atomic structures depends on the size and nature of the Au-supported nanoparticles. Our findings provided computational support for the work of Liao and Ya [J. Phys. Chem. C. 121 (2017) 19218-19225] reporting the formation of two phases of Pt nanoislands on Au(111). These findings also reveal the rich and complex atomic structures of small single-layer metal nanoislands supported on metal surfaces.

Simulation of Metal-Supported Metal-Nanoislands: A Comparison of DFT Methods

We have evaluated various density functional theory (DFT) methods to simulate geometric, energetic, electronic, and hydrogen adsorption properties of metal-nanoparticles supported on metal surfaces. We used Pt and Pd nanoislands on Au(111) as model systems. The evaluated DFT methods include GGA (PW91, PBE, RPBE, revPBE, and PBESol), GGA with van der Waals (vdW) corrected (PBE-D3), GGA with optimized vdW functionals (revPBE-vdW), meta-GGA (SCAN and MS2), and the machine learning-based method BEEF-vdW. The results show that the various DFT methods yield similar geometric and electronic properties for Pt (or Pd) nanoislands on Au(111). The DFT methods also produce similar relative energetics for small Pt (or Pd) clusters with different conformations on Au(111). The results show that a triatomic cluster of Pt on Au(111) is more stable with a linear conformation. In contrast, a triatomic cluster of Pd is more stable with a triangular conformation. For clusters with four or more atoms, Pt and Pd clusters on Au(111) prefer non-linear conformation. We found that the various DFT methods yield different results only for the adsorption energy of hydrogen.

Hydrogen Adsorption on Au-Supported Pt and Pd Nanoislands: A Computational Study of Hydrogen Coverage Effects

We have studied the dissociative adsorption of hydrogen under high coverage conditions of adsorbed hydrogen on Pd and Pt nanoislands supported on Au(111) using Density Functional Theory calculations. The results reveal that for Pd/Au(111), the free energy of hydrogen adsorption ΔG is close to 0 kJ/mol when the coverage of adsorbed hydrogen is near 1 ML, where the available catalytic sites are located at the edges of the Pd nanoislands. In the case of Pt/Au(111), ΔG ≈ 0 kJ/mol under a broad range of hydrogen coverage conditions, from 1 ML to 3 ML, depending on the size of the Pt nanoislands. This is the case because the available catalytic sites are located at both the steps and terraces of Pt nanoislands. These findings indicate that Au surfaces with Pd or Pt nanoislands offer catalytic sites with ΔG ≈ 0 for hydrogen reactions, one key factor for an ideal electrocatalyst for hydrogen reactions.

A nano AuTiO2-x composite with electrochemical characteristics of under-potential deposition of H (H-UPD) as a highly efficient electrocatalyst for hydrogen evolution

An obvious H-UPD for a nano AuTiO2-x composite has been found for the first time in terms of the electrochemical characteristics of the Au composite. The electronic effect between Au and TiO2 and the oxygen vacancy defect would change the adsorption energy of H and HER activity. The HER activity of the AuTiO2-x electrode is 6.44 times that of the Au electrode.

Semimetallic molybdenum disulfide ultrathin nanosheets as an efficient electrocatalyst for hydrogen evolution

Molybdenum disulfide (MoS₂) ultrathin nanosheets, as a well-known inorganic two dimensional (2D) material with a graphene-like structure, has attracted tremendous attention due to its unique microscopic and macroscopic properties brought by the confinement of charge and heat transfer upon the basal plane. However, as the prototype Mott-insulator, its relatively low conductivity and carrier concentration still greatly hamper its wide applications. Here, we developed a novel intralayer vanadium-doping strategy to produce semimetallic vanadium-doped MoS₂ (VMS) ultrathin nanosheets with less than five S-(V, Mo)-S atomic layers, as a new inorganic 2D material. By incorporation of intralayer vanadium atoms, fine regulation of intrinsic electrical properties within the pristine MoS₂ structure has been successfully realized, achieving semimetallic MoS₂-based 2D materials with tunable conductivity and higher carrier concentration for the first time. Benefiting from the enhanced in-plane conductivity, the improved carrier concentration as well as the shortened electron transfer paths, the semimetal-like VMS nanosheet have enhanced catalytic activity with an overpotential of 0.13 V and a smaller Tafel slope, exhibiting enhanced catalytic performance compared with that of a pure MoS₂ system. The intralayer doping in the 2D structure opens a new avenue in building highly efficient catalysts through the regulation of their intrinsic electrical properties, and also gives a new perspective for enlarging the design space of 2D materials.