Density Functional Kinetic Monte Carlo Simulation of Water–Gas Shift Reaction on Cu/ZnO

We describe a density functional theory based kinetic Monte Carlo study of the water–gas shift (WGS) reaction catalyzed by Cu nanoparticles supported on a ZnO surface. DFT calculations were performed to obtain the energetics of the relevant atomistic processes. Subsequently, the DFT results were employed as an intrinsic database in kinetic Monte Carlo simulations that account for the spatial distribution, fluctuations, and evolution of chemical species under steady-state conditions. Our simulations show that, in agreement with experiments, the H₂ and CO₂ production rates strongly depend on the size and structure of the Cu nanoparticles, which are modeled by single-layer nano islands in the present work. The WGS activity varies linearly with the total number of edge sites of Cu nano islands. In addition, examination of different elementary processes has suggested competition between the carboxyl and the redox mechanisms, both of which contribute significantly to the WGS reactivity. Our results have also indicated that both edge sites and terrace sites are active and contribute to the observed H₂ and CO₂ productivity

Directed Assembly of Nanodiamond Nitrogen-Vacancy Centers on a Chemically Modified Patterned Surface

Nitrogen-vacancy (NV) centers in nanodiamond (ND) particles are an attractive material for photonic, quantum information, and biological sensing technologies due to their optical propertiesbright single photon emission and long spin coherence time. To harness these features in practical devices, it is essential to realize efficient methods to assemble and pattern NDs at the micro-/nanoscale. In this work, we report the large scale patterned assembly of NDs on a Au surface by creating hydrophobic and hydrophilic regions using self-assembled monolayer (SAM). Hydrophobic regions are created using a methyl (−CH₃) terminated SAM of octadecanethiol molecules. Evaporating a water droplet suspension of NDs on the SAM patterned surface assembles the NDs in the bare Au, hydrophilic regions. Using this procedure, we successfully produced a ND structures in the shape of dots, lines, and rectangles. Subsequent photoluminescence imaging of the patterned NDs confirmed the presence of optically active NV centers. Experimental evidence in conjunction with computational analysis indicates that the surface wettability of the SAM modified Au surface plays a dominant role in the assembly of NDs as compared to van der Waals and other substrate–ND interactions

Site-Dependent Activity of Atomic Ti Catalysts in Al-Based Hydrogen Storage Materials

Doping catalytically inactive materials with dispersed atoms of an active species is a promising route toward realizing ultradilute binary catalyst systems. Beyond catalysis, strategically placed metal atoms can accelerate a wide range of solid-state reactions, particularly in hydrogen storage processes. Here we analyze the role of atomic Ti catalysts in the hydrogenation of Al-based hydrogen storage materials. We show that Ti atoms near the Al surface activate gas-phase H₂, a key step toward hydrogenation. By controlling the placement of Ti, we have found that the overall reaction, comprising H₂ dissociation and H spillover onto the Al surface, is governed by a pronounced trade-off between lowering of the H₂ dissociation barrier and trapping of the products near the active site, with a sharp maximum in the overall activity for Ti in the subsurface layer. Our findings demonstrate the importance of controlling the placement of the active species in optimizing the activity of dilute binary systems

Electrochemical Characteristics and Li⁺ Ion Intercalation Kinetics of Dual-Phase Li₄Ti₅O₁₂/Li₂TiO₃ Composite in the Voltage Range 0–3 V

Li₄Ti₅O₁₂, Li₂TiO₃, and dual-phase Li₄Ti₅O₁₂/Li₂TiO₃ composite were prepared by sol–gel method with average particle size of 1, 0.3, and 0.4 μm, respectively. Though Li₂TiO₃ is electrochemically inactive, the rate capability of Li₄Ti₅O₁₂/Li₂TiO₃ is comparable to that of Li₄Ti₅O₁₂ at different current rates. Li₄Ti₅O₁₂/Li₂TiO₃ also shows a good rate performance of 90 mA h g^–1 at a high rate of 10 C in the voltage range 1–3 V, attributable to increased interfaces in the composite. While Li₄Ti₅O₁₂ delivers a capacity retention of 88.6% at 0.2 C over 50 cycles, Li₄Ti₅O₁₂/Li₂TiO₃ exhibits no capacity fading at 0.2 C (40 cycles) and a capacity retention of 98.45% at 0.5 C (50 cycles). This highly stable cycling performance is attributed to the contribution of Li₂TiO₃ in preventing the undesirable reaction of Li₄Ti₅O₁₂ with the electrolyte during cycling. Cyclic voltammetric curves of Li₄Ti₅O₁₂/Li₂TiO₃ in the 0–3 V range exhibit two anodic peaks at 1.51 and 0.7–0.0 V, indicating two modes of lithium intercalation into the lattice sites of active material. Owing to enhanced intercalation/deintercalation kinetics in 0–3 V, the composite electrode delivers a superior rate performance of 203 mAh/g at 2.85 C and 140 mAh/g at 5.7 C with good reversible capacity retention over 100 cycles

Surface Defects: Possible Source of Room Temperature Ferromagnetism in Co-Doped ZnO Nanorods

Contradicting results about the origin of room temperature ferromagnetism (RTFM) from measurements on different forms of transition metal (TM)-doped ZnO nanostructured materials lead to strong debates on whether RTFM could be an intrinsic property to TM-doped ZnO or not. Through careful synthesis and extensive characterizations, we have excluded the extrinsic contaminations as the cause of RTFM. Our experimental study confirms that defects such as oxygen vacancies lie on surface of nanorods and are likely a source of RTFM. X-ray absorption and emission spectroscopy (XAS and XES) suggest that the doped Co ions, primarily in the divalent state, replace the Zn ions inside the tetrahedral without introducing Co clustering or Zn-related defects. Band gap narrowing upon Co doping is observed in both optical reflectance and O K-edge XAS/XES and is in agreement with the presence of oxygen vacancies and strong sp–d hybridization. Furthermore, such a trend can be nicely reproduced in GGA+U band structure calculations. Calculations also suggest that these oxygen vacancies are likely to congregate at low-energy (101) and (100) surfaces, instead of inside the bulk. Our findings highlight the importance of using the nanocrystalline surfaces to enhance the impurity concentrations and stabilize the ferromagnetism without post-sample annealing in an oxygen-deficient environment