Alice Hu

Atomic Configuration of Point Defect Clusters in Ion-Irradiated Silicon Carbide

Silicon Carbide (SiC) is a promising cladding material for accident-tolerant fuel in light water reactors due to its excellent resistance to chemical attacks at high temperatures, which can prevent severe accident-induced environmental disasters. Although it has been known for decades that radiation-induced swelling at low temperatures is driven by the formation of black spot defects with sizes smaller than 2 nm in irradiated SiC, the structure of these defect clusters and the mechanism of lattice expansion have not been clarified and remain as one of the most important scientific issues in nuclear materials research. Here we report the atomic configuration of defect clusters using Cs-corrected transmission electron microscopy and molecular dynamics to determine the mechanism of these defects to radiation swelling. This study also provides compelling evidence that irradiation-induced point defect clusters are vacancy-rich clusters and lattice expansion results from the homogenous distribution of unrecovered interstitials in the material.

Atomic scale Pt decoration promises oxygen reduction properties of Co@Pd nanocatalysts in alkaline electrolytes for 310k redox cycles

Nanocatalysts (NCs) with Co core–Pd shell structures and surface decoration of atomic scale Pt clusters (namely Co@Pd–Pt) are synthesized by using a self-aligned wet chemical reduction method in carbon nanotube supports. The Co@Pd–Pt contains ∼2.48 at% Pt metal. It shows a 30.2-fold mass activity (2056.3 mA mg−1) of the Pt metal as compared to that of commercial Pt catalysts (67.1 mA mg−1) at 0.85 volt (vs. RHE) and shows an exceptional stability of retained current density of ∼100% vs. the initial ones in an accelerated degradation test (ADT) for over 310k cycles in an alkaline electrolyte. The results of structural characterization and electrochemical analyses reveal that the high current density with substantial stability in the ORR is attributed to a strong electronic coupling and interface lattice that extract electrons from Co and Pd atoms in the presence of atomic Pt clusters in the Pd shell. A worth noticing finding is that such exceptional electrochemical performances are developed in a novel composition window in which Pt atoms are mostly positioned in defect sites of the Pd–Co interface in the shell region.

Few-Layer PdSe2 Sheets: Promising Thermoelectric Materials Driven by High Valley Convergence

Herein, we report a comprehensive study on the structural and electronic properties of bulk, monolayer, and multilayer PdSe2 sheets. First, we present a benchmark study on the structural properties of bulk PdSe2 by using 13 commonly used density functional theory (DFT) functionals. Unexpectedly, the most commonly used van der Waals (vdW)-correction methods, including DFT-D2, optB88, and vdW-DF2, fail to provide accurate predictions of lattice parameters compared to experimental data (relative error > 15%). On the other hand, the PBE-TS series functionals provide significantly improved prediction with a relative error of <2%. Unlike hexagonal two-dimensional materials like graphene, transition metal dichalcogenides, and h-BN, the conduction band minimum of monolayer PdSe2 is not located along the high symmetry lines in the first Brillouin zone; this highlights the importance of the structure–property relationship in the pentagonal lattice. Interestingly, high valley convergence is found in the conduction and valence bands in monolayer, bilayer, and trilayer PdSe2 sheets, suggesting promising application in thermoelectric cooling.

Crystal shape controlled H2 storage rate in nanoporous carbon composite with ultra-fine Pt nanoparticle

This study demonstrates that the hydrogen storage rate (HSR) of nanoporous carbon supported platinum nanocatalysts (NC) is determined by their heterojunction and geometric configurations. The present NC is synthesized in an average particle size of ~1.5 nm by incipient wetness impregnation of Pt4+ at carbon support followed by annealing in H2 ambient at 102–105 °C. Among the steps in hydrogen storage, decomposition of H2 molecule into 2 H atoms on Pt NC surface is the deciding factor in HSR that is controlled by the thickness of Pt NC. For the best condition, HSR of Pt NC in 1~2 atomic layers thick (4.7 μg/g min) is 2.6 times faster than that (1.3 μg/g min) of Pt NC with higher than 3 atomic layers thick.

Pt3 clusters-decorated Co@Pd and Ni@Pd model core–shell catalyst design for the oxygen reduction reaction: a DFT study

The high cost of oxygen reduction reaction (ORR) catalysts is a critical hurdle to fuel cell commercialization. In an effort to develop low-cost ORR catalysts with low noble-metal loading but high activity and stability, we designed model catalysts of small Pt3 clusters-decorated core–shell structures (e.g. Co or Ni as the core, Pd as the shell) by tuning the shell thickness, calculated by density functional theory (DFT). We found that incorporation of Pt3 into the bimetallic surface is more stable than adsorption of Pt3 on the surface. Additionally, the magnitude of atomic oxygen adsorption of our model catalysts depends on their adsorption sites and Pd thickness. Small Pt3-decorated Co@Pd and Ni@Pd model core–shell catalysts with exact monolayers of Pd are expected to exhibit high ORR activity based on their moderate atomic oxygen adsorption energies on various stable active sites, −0.57 to −0.93 and −0.53 to −0.96 eV, and moderate O2 dissociation reaction barriers, 0.74 to 0.87 and 0.59 to 0.72 eV. The d-band centers (ed) of selected model catalysts were computed; the outcomes reveal that the ed values of the optimum model catalysts are close to that of pure Pt(111) and fall around the top of the volcano plot, further demonstrating that high ORR activity should be expected.

Exceptional Optical Absorption of Buckled Arsenene Covering a Broad Spectral Range by Molecular Doping

Using density functional theory calculations, we demonstrate that the electronic and optical properties of a buckled arsenene monolayer can be tuned by molecular doping. Effective p-type doping of arsenene can be realized by adsorption of tetracyanoethylene and tetracyanoquinodimethane (TCNQ) molecules, while n-doped arsenene can be obtained by adsorption of tetrathiafulvalene molecules. Moreover, owing to the charge redistribution, a dipole moment is formed between each organic molecule and arsenene, and this dipole moment can significantly tune the work function of arsenene to values within a wide range of 3.99–5.57 eV. Adsorption of TCNQ molecules on pristine arsenene can significantly improve the latter’s optical absorption in a broad (visible to near-infrared) spectral range. According to the AM 1.5 solar spectrum, two-fold enhancement is attained in the efficiency of solar-energy utilization, which can lead to great opportunities for the use of TCNQ–arsenene in renewable energy. Our work clearly demonstrates the key role of molecular doping in the application of arsenene in electronic and optoelectronic components, renewable energy, and laser protection.

Programming ORR Activity of Ni/NiOx@Pd Electrocatalysts via Controlling Depth of Surface-Decorated Atomic Pt Clusters

Carbon nanotube supported ternary metallic nanocatalysts (NCs) comprising Nicore–Pdshell structure and Pt atomic scale clusters in shell (namely, Ni@Pd/Pt) are synthesized by using wet chemical reduction method with reaction time control. Effects of Pt4+ adsorption time and Pt/Pd composition ratios on atomic structure with respect to electrochemical performances of experimental NCs are systematically investigated. By cross-referencing results of high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption, density functional theoretical calculations, and electrochemical analysis, we demonstrate that oxygen reduction reaction (ORR) activity is dominated by depth and distribution of Pt clusters in a Ni@Pd/Pt NC. For the optimum case (Pt4+ adsorption time = 2 h), specific activity of Ni@Pd/Pt is 0.732 mA cm–2 in ORR. Such a value is 2.8-fold higher as compared to that of commercial J.M.-Pt/C at 0.85 V (vs reversible hydrogen electrode). Such improvement is attributed to the protection of defect sites from oxide reaction in the presence of Pt clusters in NC surface. When adsorption time is 10 s, Pt clusters tends to adsorb in the Ni@Pd surface. A substantially increased galvanic replacement between Pt4+ ion and Pd/Ni metal is found to result in the formation of Ni@Pd shell with Pt cluster in the interface when adsorption time is 24 h. Both structures increase the surface defect density and delocalize charge density around Pt clusters, thereby suppressing the ORR activity of Ni@Pd/Pt NCs.

Rapid crystal growth of bimetallic PdPt nanocrystals with surface atomic Pt cluster decoration provides promising oxygen reduction activity

Carbon nanotube-supported bimetallic Pd nanoparticles (NPs) with atomic Pt cluster surface decoration (the mole ratio of Pt/Pd is 0.1) are synthesized by a wet chemical method with sodium borohydride as the reducing agent. The core component is controlled by metal ion reduction sequences. The mass activity for the oxygen reduction reaction (ORR) over Pt metal of the presented PdPt NPs is improved by 29.4 times as compared to that of commercial Pt catalysts. Such an enhancement is accounted by an expanded Pt–Pd bond length and heteroatomic intermixing-induced electron relocation.