Guozhen Zhang

Combining photocatalytic hydrogen generation and capsule storage in graphene based sandwich structures

The challenge of safe hydrogen storage has limited the practical application of solar-driven photocatalytic water splitting. It is hard to isolate hydrogen from oxygen products during water splitting to avoid unwanted reverse reaction or explosion. Here we propose a multi-layer structure where a carbon nitride is sandwiched between two graphene sheets modified by different functional groups. First-principles simulations demonstrate that such a system can harvest light and deliver photo-generated holes to the outer graphene-based sheets for water splitting and proton generation. Driven by electrostatic attraction, protons penetrate through graphene to react with electrons on the inner carbon nitride to generate hydrogen molecule. The produced hydrogen is completely isolated and stored with a high-density level within the sandwich, as no molecules could migrate through graphene. The ability of integrating photocatalytic hydrogen generation and safe capsule storage has made the sandwich system an exciting candidate for realistic solar and hydrogen energy utilization.

Optical Excitation in Donor–Pt–Acceptor Complexes: Role of the Structure

The optical properties of the Pt complexes in the form of donor-metal-acceptor (D-M-A) were studied at the first-principles level. Calculated results show that for the frontier molecular orbitals (MOs) of a D-M-A structure the energies of unoccupied frontier MO can be mainly determined by the interaction between M and A, whereas the M-A and M-D interactions both determine the energies of occupied frontier MO. By developing a straightforward transition dipole decomposition method, we found that not only the local excitations in D but also those in A can significantly contribute to the charge-transfer (CT) excitation. Furthermore, the calculations also demonstrate that by tuning the dihedral angle between D and A the transition probability can be precisely controlled so as to broaden the spectrum region of photoabsorption. For the D-M-A molecule with a delocalized π system in A, the CT excitation barely affects the electronic structures of metal, suggesting that the oxidation state of the metal can be kept during the excitation. These understandings for the optical properties of the D-M-A molecule would be useful for the design of dye-sensitized solar cells, photocatalysis, and luminescence systems.

Two-Dimensional Linear Dichroism Spectroscopy for Identifying Protein Orientation and Secondary Structure Composition

Quantitative measurements of protein orientation and secondary structure composition are of great importance for protein biotechnology applications and disease treatments, and yet, they are technically challenging for a spectroscopic study. On the basis of quantum mechanics/molecular mechanics simulations, we demonstrate that two-dimensional (2D) linear dichroism spectroscopy is capable of probing the direction of α-helix motifs in proteins. Compared to the conventional linear dichroism (LD) spectra, 2D spectra double the measurable range of orientation of secondary structures. In addition, by calculating the ratio of transverse ππ* signals to longitudinal ππ* signals in 2D spectra, we can achieve quantitative measurement of the fraction of α-helix content in a protein.

Fluorescent-Cavity Host: An Efficient Probe to Study Supramolecular Recognition Mechanisms

Using fluorometry to study the interactions between guests and host cavities is often challenging, especially for hosts with small cavities because the fluorophore may not be close to the binding site. Therefore, it is critical to overcome this hurdle to broaden the applicability of fluorometry in supramolecular chemistry. Herein, we designed a fluorescent-cavity host (H1) by conjugating the binding site of a pillar[5]arene cavity and studied its host-guest recognition mechanism in the cavity. Distinct fluorescent responses of H1 were observed for cyano homologues: the fluorescence was enhanced for succinonitrile but quenched for malononitrile. Such an unusual phenomenon with such subtle difference in guest structure was attributed to the different host-guest interactions induced by the subtle difference of guest locations within the H1 cavity. Our results indicate that developing fluorescent-cavity hosts as probes will provide a powerful and insightful way to explore the exquisite detail of host-guest recognition, self-assembly, and molecular machinery.

Self-Adaptive Switch Enabling Complete Charge Separation in Molecular-Based Optoelectronic Conversion

Achieving high charge recombination probability has been the major challenge for the practical utilization of molecule-based solar harvesting. Molecular switches were introduced to stabilize the charge separation state in donor-acceptor systems, but it is difficult to seamlessly incorporate the ON/OFF switching actions into the optoelectronic conversion cycle. Here we present a self-adaptive system in which the donor and acceptor are bridged by a switchable moiety that enables a complete charge separation repeatedly. Calculations are presented for a platinum(II) terpyridyl complex with an azobenzene bridge. The charge transfer induced by light extracts electrons from the azobenzene group, automatically triggering a trans → cis isomerization. The resulting conformation suppresses charge recombination. Energized charges are trapped in the acceptor, ready for charge collection by electrodes. The bridge then goes through inverse isomerization to restore the conjugation and conductance. This self-adaptive design provides a novel way to improve the performance of optoelectronic conversion and realize practical solar-harvesting applications in organic molecular systems.

Insight into Electronic and Structural Reorganizations for Defect-Induced VO2 Metal–Insulator Transition

An oxygen vacancy defect in monoclinic VO2 has been shown to modulate the metal-insulator transition (MIT) at room temperature. However, as the electronic and structural reorganizations occur simultaneously, the origin of MIT is still unclear. Here we performed first-principles calculations to examine electronic variations separately from structural reorganizations during MIT. It was found that the oxygen defect induces electronic reorganization by creating polarized 3d orbitial electrons, while structure reorganization makes the conduction band edge states available for occupation. The conduction band states thus hold polarized charges that delocalize over space, bestowing metallic property on the originally insulated VO2. A linear relationship for the number of polarized electrons and the defect concentration is revealed, which would lead to cost-effective control of VO2 MIT behavior by defect engineering.

Material descriptors for photocatalyst/catalyst design

Rational design of high‐performance photocatalysts/catalysts is crucial for sustainable development. To achieve this goal, a comprehensive understanding and precise description of structure–performance relationships of photocatalysts/catalysts are highly desirable. While photocatalysis/catalysis involves complex systems and processes, approximate descriptors have been proposed for sorting out simple pictures of complicated structure–performance relationships concerned. In this review, some important descriptors involved in photocatalyst/catalyst design including work function, dipole moment, d‐band center, and Fermi softness are reviewed first with special attention being paid to their working mechanisms and applications. Then strategies of tuning photocatalytic/catalytic performance on the basis of these descriptors are outlined. Finally, challenges and opportunities for photocatalyst/catalyst design based on descriptor control are discussed.

Energy Materials Design for Steering Charge Kinetics

Charge kinetics is a critical factor that determines working efficiencies of energy materials in their various applications. It is governed by electronic structures of the materials of interest and can be fine-tuned via purposeful adjustment of electronic structures. Recent advances in the development of energy materials with desirable electronic structures to steering charge kinetics toward specific applications are highlighted here. Two key strategies are presented: one is through the tuning of energy states and the other is to control spatial distributions of charges. Each strategy is described by several different schemes. Finally, the challenges and perspectives in designing energy materials with fine control of charge kinetics are discussed.