Mingsen Deng

Surface Polarization Matters: Enhancing the Hydrogen-Evolution Reaction by Shrinking Pt Shells in Pt–Pd–Graphene Stack Structures†

Surface charge state plays an important role in tuning the catalytic performance of nanocrystals in various reactions. Herein, we report a synthetic approach to unique Pt-Pd-graphene stack structures with controllable Pt shell thickness. These unique hybrid structures allow us to correlate the Pt thickness with performance in the hydrogen-evolution reaction (HER). The HER activity increases with a decrease in the Pt thickness, which is well explained by surface polarization mechanism as suggested by first-principles simulations. In this hybrid system, the difference in work functions of Pt and Pd results in surface polarization on the Pt surface, tuning its charge state for hydrogen reduction. Meanwhile, the supporting graphene provides two-dimensional channels for efficient charge transport, improving the HER activities. This work opens up possibilities of reducing Pt usage while achieving high HER performance.

Integration of an Inorganic Semiconductor with a Metal–Organic Framework: A Platform for Enhanced Gaseous Photocatalytic Reactions

Ultrafast spectroscopy demonstrates that charge transfer can occur between photoexcited inorganic semiconductors and metal-organic frameworks (MOFs), supplying long-lifetime electrons for the reduction of gas molecules adsorbed on the MOF. As a proof of concept, a unique method is developed for synthesizing Cu3 (BTC)2 @TiO2 core-shell structures with macroporous semiconductor shells that allow gas molecules to be captured in the cores.

Designing p‐Type Semiconductor–Metal Hybrid Structures for Improved Photocatalysis

A practical strategy is proposed to facilitate the migration of holes in semiconductor (the low rate of which limits photocatalytic efficiency) by taking advantage of the Schottky barrier between p-type semiconductor and metal. A high work function is found to serve as an important selection rule for building such desirable Schottky junction between semiconductor surface facets and metal. The intrinsic charge spatial distribution has to be taken into account when selecting the facets, as it results in accumulation of photoexcited electrons and holes on certain semiconductor facets. Importantly, the facets have a high work function, the same characteristic required for the formation of Schottky junction in a p-type semiconductor-metal hybrid structure. As a result, the semiconductor crystals in the hybrid design may be better enclosed by single facets with high work function, so as to synergize the two effects: Schottky barrier versus charge spatial separation.

A Unique Semiconductor–Metal–Graphene Stack Design to Harness Charge Flow for Photocatalysis

A novel semiconductor-metal-graphene stack design, which reduces interfacial defect density as well as provides channels for charge transport, has been demonstrated to harness the charge flow for efficient electron-hole separation. As a direct outcome, the designed hybrid structures exhibit significantly improved performance in photocatalytic hydrogen production from water.

Unraveling the formation mechanism of graphitic nitrogen-doping in thermally treated graphene with ammonia.

Nitrogen-doped graphene (N-graphene) has attractive properties that has been widely studied over the years. However, its possible formation process still remains unclear. Here, we propose a highly feasible formation mechanism of the graphitic-N doing in thermally treated graphene with ammonia by performing ab initio molecular dynamic simulations at experimental conditions. Results show that among the commonly native point defects in graphene, only the single vacancy 5–9 and divacancy 555–777 have the desirable electronic structures to trap N-containing groups and to mediate the subsequent dehydrogenation processes. The local structure of the defective graphene in combining with the thermodynamic and kinetic effect plays a crucial role in dominating the complex atomic rearrangement to form graphitic-N which heals the corresponding defect perfectly. The importance of the symmetry, the localized force field, the interaction of multiple trapped N-containing groups, as well as the catalytic effect of the temporarily formed bridge-N are emphasized, and the predicted doping configuration agrees well with the experimental observation. Hence, the revealed mechanism will be helpful for realizing the targeted synthesis of N-graphene with reduced defects and desired properties.

Activation of specific sites on cubic nanocrystals: a new pathway for controlled epitaxial growth towards catalytic applications

A method has been developed for controlled epitaxial growth on cubic nanocrystals by selectively activating their surface via etching. For example, it produces Pd concave nanocubes via seeding growth on their corners and edges, and in turn, formulates highly active sites for catalysis. This method offers a better capability of preventing atomic addition on undesired locations and maintaining particle size in the seeding process, as compared with the previous technique. With the particle size well maintained, the products fully exhibit superior electrocatalytic performance enabled by active sites and high-index facets in formic acid oxidation. Another contribution of this work is to enable the growth of a noble metal with high catalytic activities on another type of cheaper metal, which greatly reduces the usage of expensive materials while retaining high catalytic activity. In this article, we have demonstrated the deposition of a very limited amount of Pt (only 3.3 wt%) on Pd nanocrystals towards high electrocatalytic activities in an oxygen reduction reaction. Preliminary studies demonstrate that the synthetic strategy can be also applied to the controllable deposition of a different material on the faces of a nanocrystal by simply altering surface conditions.

Two-Dimensional Near Ultraviolet (2DNUV) Spectroscopic Probe of Structural-Dependent Exciton Dynamics in a Protein

Understanding the exciton dynamics in biological systems is crucial for the manipulation of their function. We present a combined quantum mechanics (QM) and molecular dynamics (MD) simulation study that demonstrates how coherent two-dimensional near-ultraviolet (2DNUV) spectra can be used to probe the exciton dynamics in a mini-protein, Trp-cage. The 2DNUV signals originate from aromatic transitions that are significantly affected by the couplings between residues, which determine exciton transport and energy relaxation. The temporal evolution of 2DNUV features captures important protein structural information, including geometric details and peptide orientations.

Probing flexible conformations in molecular junctions by inelastic electron tunneling spectroscopy

The probe of flexible molecular conformation is crucial for the electric application of molecular systems. We have developed a theoretical procedure to analyze the couplings of molecular local vibrations with the electron transportation process, which enables us to evaluate the structural fingerprints of some vibrational modes in the inelastic electron tunneling spectroscopy (IETS). Based on a model molecule of Bis-(4-mercaptophenyl)-ether with a flexible center angle, we have revealed and validated a simple mathematical relationship between IETS signals and molecular angles. Our results might open a route to quantitatively measure key geometrical parameters of molecular junctions, which helps to achieve precise control of molecular devices.

A comparative theoretical study on core-hole excitation spectra of azafullerene and its derivatives

The core-hole excitation spectra-near-edge x-ray absorption spectroscopy (NEXAFS), x-ray emission spectroscopy (XES), and x-ray photoelectron spectroscopy (XPS) shake-up satellites have been simulated at the level of density functional theory for the azafullerene C59N and its derivatives (C59N)(+), C59HN, (C59N)2, and C59N-C60, in which the XPS shake-up satellites were simulated using our developed equivalent core hole Kohn-Sham (ECH-KS) density functional theory approach [B. Gao, Z. Wu, and Y. Luo, J. Chem. Phys. 128, 234704 (2008)] which aims for the study of XPS shake-up satellites of large-scale molecules. Our calculated spectra are generally in good agreement with available experimental results that validates the use of the ECH-KS method in the present work. The nitrogen K-edge NEXAFS, XES, and XPS shake-up satellites spectra in general can be used as fingerprints to distinguish the azafullerene C59N and its different derivatives. Meanwhile, different carbon K-edge spectra could also provide detailed information of (local) electronic structures of different molecules. In particular, a peak (at around 284.5 eV) in the carbon K-edge NEXAFS spectrum of the heterodimer C59N-C60 is confirmed to be related to the electron transfer from the C59N part to the C60 part in this charge-transfer complex.

C2N-supported single metal ion catalysts for HCOOH dehydrogenation

High catalytic performance of a single-atom transition metal ion (TMx+) anchored on the two-dimensional (2D) C2N lattice is predicted for HCOOH dehydrogenation. Considering the Co2+, Cu2+ and Ni2+ non-noble metal ions supported by C2N, we use density functional theory to demonstrate dehydrogenation energy barriers as low as those for pure Pt and Pd catalysts. The high catalytic performance is ascribed to the reaction occurring through a dual-active center composed of TMx+ and a nearby N atom of C2N. Specifically, C2N–Co2+ in the low spin state (S = 1/2) greatly promotes HCOOH dehydrogenation by decreasing the barrier of the rate-determining step to only 0.30 eV, mainly due to the strong ability of TMx+ to extract charges from HCOOH and C2N. The obtained mechanistic insights help the rational design of single-atom based transition metal ion catalysts supported by 2D materials.