Xin-Hao Li

Metal nanoparticles at mesoporous N-doped carbons and carbon nitrides: functional Mott-Schottky heterojunctions for catalysis

Porous carbons and porous carbon nitrides are well known support materials. Some of these materials are, however, not only a geometric construct for immobilization, enabling mass transport at the same time, but contribute due to their extended electronic structure to a potential catalytic event as such. When appropriate band schemes and electron reactivity are chosen, immobilized metal nanoparticles can exhibit a highly enhanced chemical reactivity. This is due to electronic interaction and electron transfer between the metal and semiconductor, as introduced by Mott and Schottky for planar metal-semiconductor interfaces. A rational choice of mesoporous semiconductor and metal particle allows to create a new generation of catalysts and catalytic schemes with unparalleled performances. This tutorial review highlights the latest development in the synthesis and applications of mesoporous N-doped carbon and carbon nitride supported metal nanoparticles, and concentrates on the catalytic effect of the charge transfer between the metal nanoparticles and semiconductive components.

Metal-Free Activation of Dioxygen by Graphene/g-C3N4 Nanocomposites: Functional Dyads for Selective Oxidation of Saturated Hydrocarbons

Graphene sheet/polymeric carbon nitride nanocomposite (GSCN) functions as a metal-free catalyst to activate O(2) for the selective oxidation of secondary C-H bonds of cyclohexane. By fine-tuning the weight ratio of graphene and carbon nitride components, GSCN offers good conversion and high selectivity to corresponding ketones. Besides its high stability, this catalyst also exhibits high chemoselectivity for secondary C-H bonds of various saturated alkanes and, therefore, should be useful in overcoming challenges confronted by metal-mediated catalysis.

Surface and Interface Engineering of Electrode Materials for Lithium-Ion Batteries

Lithium-ion batteries are regarded as promising energy storage devices for next-generation electric and hybrid electric vehicles. In order to meet the demands of electric vehicles, considerable efforts have been devoted to the development of advanced electrode materials for lithium-ion batteries with high energy and power densities. Although significant progress has been recently made in the development of novel electrode materials, some critical issues comprising low electronic conductivity, low ionic diffusion efficiency, and large structural variation have to be addressed before the practical application of these materials. Surface and interface engineering is essential to improve the electrochemical performance of electrode materials for lithium-ion batteries. This article reviews the recent progress in surface and interface engineering of electrode materials including the increase in contact interface by decreasing the particle size or introducing porous or hierarchical structures and surface modification or functionalization by metal nanoparticles, metal oxides, carbon materials, polymers, and other ionic and electronic conductive species.

Synthesis of Monolayer‐Patched Graphene from Glucose

The extraordinary electronic and mechanical properties of graphene have stimulated intense research on developing simple methods for the large-scale synthesis of graphene. High-quality large-area graphene films prepared by the chemical vapor deposition of various carbon-containing molecules on arbitrary substrates could meet the requirements of large-area electronic applications. For the industrial production of conductive graphene powder on the ton scale, 11–13] the chemical exfoliation of graphite minerals still remains the main manufacturing path. On the other hand, the exclusive two-dimensional polymerization of graphene-like structures from simple monomers still presents a challenge for carbon chemists. Further fine-tuning of the Fermi level of graphene by doping offers a way to control the electronic structure of carbonaceous materials and is of major interest for their application in electronics, electrodes, and catalysis. The electronic properties of doped graphene are strongly linked to the dopant concentration, which is only poorly controlled by current methods. It is therefore highly challenging but desirable to develop effective approaches for fabricating graphene that is cheap yet of high quality (e.g. high surface area, high conductivity, doping level, and uniform morphology) in a controlled manner. Herein we report a simple yet versatile approach for the synthesis of two-dimensional (2D) carbon materials ranging from free-standing monolayers to oligolayered graphene by the calcination of glucose, a most abundant, sustainable compound. In this synthesis only dicyandiamide (DCDA) was added for the temporary in situ formation of layered graphic carbon nitride (g-C3N4), which serves as a sacrificial template. This approach is also facile for gradually tuning the concentration of the nitrogen dopant in a broader range without disturbing the morphology of graphene. In a typical synthesis, the two-step heating of a mixture of DCDA and glucose under a protective flow of N2 directly resulted in freestanding graphene with a yield of 28–60% (calculated based on added carbon from glucose). The overall formation process is depicted in Figure 1: Thermal condensation of DCDA creates a layered carbon

Polycondensation of Boron‐ and Nitrogen‐Codoped Holey Graphene Monoliths from Molecules: Carbocatalysts for Selective Oxidation

A simple but powerful chemical process--the copolymerization of biomass (glucose) and boric acid as templated by dicyandiamide (see picture)--was used to fabricate high-quality doped graphene monoliths with through-plane nanopores. The holey graphene monoliths had a high surface area and showed excellent performance as metal-free carbocatalysts for selective oxidation.

Facilitating room-temperature Suzuki coupling reaction with light: Mott-Schottky photocatalyst for C-C-coupling

The Suzuki coupling reaction is one of the most practiced classes of catalytic C-C bond formation. The development of new means of activating molecules and bonds over old catalysts for C-C bond formation is a fundamental objective for chemists. Here, we report the room-temperature C-C bond formation over heterogeneous Pd catalysts by light-mediated catalyst activation. We employ stimulated electron transfer at the metal-semiconductor interface from optically active mesoporous carbon nitride nanorods to Pd nanoparticles. This photocatalytic pathway is highly efficient for coupling aryl halides with various coupling partners with high activity and selectivity under photo irradiation and very mild conditions.

Activating Cobalt Nanoparticles via the Mott–Schottky Effect in Nitrogen-Rich Carbon Shells for Base-Free Aerobic Oxidation of Alcohols to Esters

Heterogeneous catalysts of inexpensive and reusable transition-metal are attractive alternatives to homogeneous catalysts; the relatively low activity of transition-metal nanoparticles has become the main hurdle for their practical applications. Here, the de novo design of a Mott-Schottky-type heterogeneous catalyst is reported to boost the activity of a transition-metal nanocatalyst through electron transfer at the metal/nitrogen-doped carbon interface. The Mott-Schottky catalyst of nitrogen-rich carbon-coated cobalt nanoparticles (Co@NC) was prepared through direct polycondensation of simple organic molecules and inorganic metal salts in the presence of g-C3N4 powder. The Co@NC with controllable nitrogen content and thus tunable Fermi energy and catalytic activity exhibited a high turnover frequency (TOF) value (8.12 mol methyl benzoate mol-1 Co h-1) for the direct, base-free, aerobic oxidation of benzyl alcohols to methyl benzoate; this TOF is 30-fold higher than those of the state-of-the-art transition-metal-based nanocatalysts reported in the literature. The presented efficient Mott-Schottky catalyst can trigger the synthesis of a series of alkyl esters and even diesters in high yields.

A Green Chemistry of Graphene: Photochemical Reduction towards Monolayer Graphene Sheets and the Role of Water Adlayers

Clean sheets: Stable aqueous dispersions of graphene sheets (GSs) are obtained by exposing graphene oxide to irradiation with light at room temperature, without using any chemical additives. The photochemical reduction method is sustainable and scalable, repairs a majority of defects in the graphene layers, and can be used to fine-tune surface functional groups. Interestingly, the aqueous GS dispersions are stable without any added surfactant. The existence of a water layer that is strongly bound to GS is evidenced.

Room-temperature transfer hydrogenation and fast separation of unsaturated compounds over heterogeneous catalysts in an aqueous solution of formic acid

The facile conversion of olefins and unsaturated biomass to saturated compounds is achieved over heterogeneous catalysts composed of noble metal nanoparticles and carbon nitride. Reactions could proceed smoothly at room temperature in water using formic acid as the hydrogen source. The reusability of such a hybrid catalyst is high due to the strong Mott–Schottky effect between the metal nanoparticles and the carbon nitride support. The fast and automatic separation of the as-formed saturated hydrocarbons from water combined with the mild reaction conditions and the excellent reusability of catalysts make the catalytic process a highly “green” path for hydrogenation of unsaturated compounds and biofuel upgrading.

In situ catalytic growth of large-area multilayered graphene/MoS2 heterostructures

Stacking various two-dimensional atomic crystals on top of each other is a feasible approach to create unique multilayered heterostructures with desired properties. Herein for the first time, we present a controlled preparation of large-area graphene/MoS2 heterostructures via a simple heating procedure on Mo-oleate complex coated sodium sulfate under N2 atmosphere. Through a direct in situ catalytic reaction, graphene layer has been uniformly grown on the MoS2 film formed by the reaction of Mo species with S pecies, which is from the carbothermal reduction of sodium sulfate. Due to the excellent graphene “painting” on MoS2 atomic layers, the significantly shortened lithium ion diffusion distance and the markedly enhanced electronic conductivity, these multilayered graphene/MoS2 heterostructures exhibit high specific capacity, unprecedented rate performance and outstanding cycling stability, especially at a high current density, when used as an anode material for lithium batteries. This work provides a simple but efficient route for the controlled fabrication of large-area multilayered graphene/metal sulfide heterostructures with promising applications in battery manufacture, electronics or catalysis.