Xiyu Li

Oxide Defect Engineering Enables to Couple Solar Energy into Oxygen Activation

Modern development of chemical manufacturing requires a substantial reduction in energy consumption and catalyst cost. Sunlight-driven chemical transformation by metal oxides holds great promise for this goal; however, it remains a grand challenge to efficiently couple solar energy into many catalytic reactions. Here we report that defect engineering on oxide catalyst can serve as a versatile approach to bridge light harvesting with surface reactions by ensuring species chemisorption. The chemisorption not only spatially enables the transfer of photoexcited electrons to reaction species, but also alters the form of active species to lower the photon energy requirement for reactions. In a proof of concept, oxygen molecules are activated into superoxide radicals on defect-rich tungsten oxide through visible-near-infrared illumination to trigger organic aerobic couplings of amines to corresponding imines. The excellent efficiency and durability for such a highly important process in chemical transformation can otherwise be virtually impossible to attain by counterpart materials.

Toward Enhanced Photocatalytic Oxygen Evolution: Synergetic Utilization of Plasmonic Effect and Schottky Junction via Interfacing Facet Selection

DOI: 10.1002/adma.201501200 participate in oxidation reaction. As long as a semiconductor with appropriate bandgap (i.e., wide bandgap) is selected, the redox abilities of electrons or holes can be maintained as high as those in wide-bandgap semiconductors despite the use of incident visible light with relatively low energy. Unfortunately, the reported photocatalytic effi ciencies purely offered by the plasmonic hot carrier injection effect in the absence of semiconductor photoexcitation are negligible in contrast to those by semiconductor photoexcitation. [ 11,19 ] The major reason for this limitation is the lack of a driving force to steer the migration of injected electrons or holes to semiconductor surface for reduction or oxidation reactions. The low charge migration rates and uncertain charge diffusion directions make the charge carriers randomly walk in the semiconductor, so only a small portion of plasmonic hot carriers can arrive at the catalytic sites. We have thus decided to develop a new approach to better harness the utilization of plasmonic hot carriers. Thus far, use of a Schottky junction has been recognized as the most wellestablished strategy for steering the fl ow of the carriers that are photogenerated in semiconductor. It is well known that metal (especially for nonplasmonic metal, Pt and Pd) can serve as an sink for the photogenerated electrons or holes when forming a Schottky junction with n-type or p-type semiconductor, respectively (Figure S1, Supporting Information). [ 20,21 ] The formed Schottky barrier can inhibit the backfl ow of electrons or holes from metal to semiconductor. As a result, the charge “pump” role of the Schottky junction ensures the effi cient unidirectional transfer of charge carriers across the interface of metal– semiconductor (M–S) junction. Naturally we consider the possibility whether this Schottky-junction effect may be extended to the utilization of plasmonic hot carriers in photocatalysis through guiding their migration directions. However, this idea can be hardly accomplished by a single M–S junction between plasmonic metal and semiconductor. When a plasmonic metal is used for both the Schottky junction and hot carrier injection, the injection of plasmonic hot carriers would follow an opposite direction to the carrier fl ow driven by the Schottky junction (Figure S1, Supporting Information). [ 11,14,15,18,22 ] This severe competition dramatically reduces the effi ciency of carrier trapping on metal and e–h separation, particularly when metal and semiconductor are both excited under full-spectrum irradiation. In this communication, we report a new design for synergizing the plasmonic effect with the Schottky junction. The core concept of this work is to separate the Schottky junction from the plasmonic hot carrier injection by building two M–S interfaces based on the selection of semiconductor facets and metals. The functions of these two interfaces are synergized by Photocatalytic water splitting represents a highly important approach to addressing current energy and environmental demands. Photocatalysis requires effi cient separation of photo generated electron–hole (e–h) pairs in semiconductor to undergo redox reactions. [ 1 ] The reduction and oxidation capabilities of photogenerated electrons and holes in a semiconductor are determined by the positions of conduction band (CB) and valence band (VB) edges, respectively. Only when the CB edge lies at a higher position (more negative) than the redox potential of reduction half reaction, and meanwhile, the VB edge is at a lower position (more positive) than the potential of oxidation half reaction, can an overall photocatalytic reaction take place. [ 2 ] Thus wide-bandgap semiconductors with higher CB and lower VB edges generally show higher redox abilities as well as more promising photocatalytic performance in comparison with narrow-bandgap ones. However, semiconductors with wide bandgaps can only absorb light in the UV region which accounts for ≈5% of solar spectrum, thereby limiting their solar energy conversion effi ciency for practical applications. [ 3 ] For this reason, the relationship between absorption of long-wavelength light and high redox abilities of charge carriers is essentially an irreconcilable contradiction for bare semiconductor photocatalysts. Most recently, integration of surface plasmon into photocatalysis has been widely explored by composing hybrid structures between noble metals and semiconductors, which may potentially circumvent this situation. [ 4–10 ] As demonstrated by many research groups, [ 11–18 ] the metal with surface plasmon (e.g., Ag and Au) that directly contacts a semiconductor can be excited under visible light illumination to generate and inject hot carriers into the semiconductor. Specifi cally, hot electrons may fl ow into the CB of n-type semiconductor [ 13 ] and in the case of p-type semiconductor, instead hot holes are injected into the VB of semiconductor (Figure S1, Supporting Information). [ 16 ]

A New Cubic Phase for a NaYF4 Host Matrix Offering High Upconversion Luminescence Efficiency

A NaYF4 host matrix with a new cubic phase is fabricated to offer high upconversion luminescence efficiency. The new cubic phase is formed through a hexagonal-to-cubic phase transition by shining intense near-infrared light on lanthanide-doped hexagonal NaYF4 materials.

Magnetic and charge ordering in nanosized manganites

Perovskite manganites exhibit a wide range of functional properties, such as colossal magneto-resistance, magnetocaloric effect, multiferroic property, and some interesting physical phenomena including spin, charge, and orbital ordering. Recent advances in science and technology associated with perovskite oxides have resulted in the feature sizes of microelectronic devices down-scaling into nanoscale dimensions. The nanoscale perovskite manganites display novel magnetic and electronic properties that are different from their bulk and film counterparts. Understanding the size effects of perovskite manganites at the nanoscale is of importance not only for the fundamental scientific research but also for developing next generation of electronic and magnetic nanodevices. In this paper, the current understanding and the fundamental issues related to the size effects on the magnetic properties and charge ordering in manganites are reviewed, which covers lattice structure, magnetic and electronic properties in both ferromagnetic and antiferromagnetic based manganites. In addition to review the literatures, this article identifies the promising avenues for the future research in this area.

Design of Efficient Catalysts with Double Transition Metal Atoms on C2N Layer

Heterogeneous catalysis often involves molecular adsorptions to charged catalyst site and reactions triggered by catalyst charges. Here we use first-principles simulations to design oxygen reduction reaction (ORR) catalyst based on double transition metal (TM) atoms stably supported by 2D crystal C2N. It not only holds characters of low cost and high durability but also effectively accumulates surface polarization charges on TMs and later deliveries to adsorbed O2 molecule. The Co-Co, Ni-Ni, and Cu-Cu catalysts exhibit high adsorption energies and extremely low dissociation barriers for O2, as compared with their single-atom counterparts. Co-Co on C2N presents less than half the value of the reaction barrier of bulk Pt catalysts in the ORR rate-determining steps. These catalytic improvements are well explained by the dependences of charge polarization on various systems, which opens up a new strategy for optimizing TM catalytic performance with the least metal atoms on porous low-dimensional materials.

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.

Graphitic carbon nitride supported single-atom catalysts for efficient oxygen evolution reaction

Based on DFT calculations, we propose a TM@CN hybrid structure, in which the single-atom transition metal (TM = Pt, Pd, Co, Ni, Cu) is supported by graphitic carbon nitride (g-CN), as a promising high-performance OER catalyst. Our work reveals the importance of local TM coordination in catalysts for the OER, which would lead to a new class of low-cost, durable and efficient OER catalysts.

Defective Tungsten Oxide Hydrate Nanosheets for Boosting Aerobic Coupling of Amines: Synergistic Catalysis by Oxygen Vacancies and Brønsted Acid Sites

Adsorption and activation of molecules on a surface holds the key to heterogeneous catalysis toward aerobic oxidative reactions. To achieve high catalytic activities, a catalyst surface should be rationally tailored to interact with both organic substrates and oxygen molecules. Here, a facile bottom-up approach to defective tungsten oxide hydrate (WO3 ·H2 O) nanosheets that contain both surface defects and lattice water is reported. The defective WO3 ·H2 O nanosheets exhibit excellent catalytic activity for aerobic coupling of amines to imines. The investigation indicates that the oxygen vacancies derived from surface defects supply coordinatively unsaturated sites to adsorb and activate oxygen molecules, producing superoxide radicals. More importantly, the Brønsted acid sites from lattice water can contribute to enhancing the adsorption and activation of alkaline amine molecules. The synergistic effect of oxygen vacancies and Brønsted acid sites eventually boosts the catalytic activity, which achieves a kinetic rate constant of 0.455 h-1 and a turnover frequency of 0.85 h-1 at 2 h, with the activation energy reduced to ≈35 kJ mol-1 . This work provides a different angle for metal oxide catalyst design by maneuvering subtle structural features, and highlights the importance of synergistic effects to heterogeneous catalysts.

Suppressing Electron–Phonon Coupling through Laser-Induced Phase Transition

Using first-principle calculations, we introduced a strategy of laser-induced phase transition that suppress electron-phonon couplings in crystal lattice. We explained unusual irreversible phase transitions in previous experiments on MoTe2 and NaYF4 crystals. Laser irradiations produced local heats in 2H-MoTe2 and Hex NaYF4, driving atom reorganizations toward new lattices. The reorganization with effective electron-phonon couplings continues with spontaneously generated heats, whereas a 1T-MoTe2 and a metastable cubic NaYF4 phases were kept because of suppressed vibrational relaxations. Long time laser treatments create phases with weak electron-phonon couplings. Such irreversible transitions guarantee complete conversions, opening a new door to selective material modifications.