Xiaoxi Huang

Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis

The applications of copper (Cu) and Cu-based nanoparticles, which are based on the earth-abundant and inexpensive copper metal, have generated a great deal of interest in recent years, especially in the field of catalysis. The possible modification of the chemical and physical properties of these nanoparticles using different synthetic strategies and conditions and/or via postsynthetic chemical treatments has been largely responsible for the rapid growth of interest in these nanomaterials and their applications in catalysis. In addition, the design and development of novel support and/or multimetallic systems (e.g., alloys, etc.) has also made significant contributions to the field. In this comprehensive review, we report different synthetic approaches to Cu and Cu-based nanoparticles (metallic copper, copper oxides, and hybrid copper nanostructures) and copper nanoparticles immobilized into or supported on various support materials (SiO2, magnetic support materials, etc.), along with their applications in catalysis. The synthesis part discusses numerous preparative protocols for Cu and Cu-based nanoparticles, whereas the application sections describe their utility as catalysts, including electrocatalysis, photocatalysis, and gas-phase catalysis. We believe this critical appraisal will provide necessary background information to further advance the applications of Cu-based nanostructured materials in catalysis.

N-, O-, and S-Tridoped Nanoporous Carbons as Selective Catalysts for Oxygen Reduction and Alcohol Oxidation Reactions

Replacing rare and expensive metal catalysts with inexpensive and earth-abundant ones is currently among the major goals of sustainable chemistry. Herein we report the synthesis of N-, O-, and S-tridoped, polypyrrole-derived nanoporous carbons (NOSCs) that can serve as metal-free, selective electrocatalysts and catalysts for oxygen reduction reaction (ORR) and alcohol oxidation reaction (AOR), respectively. The NOSCs are synthesized via polymerization of pyrrole using (NH4)2S2O8 as oxidant and colloidal silica nanoparticles as templates, followed by carbonization of the resulting S-containing polypyrrole/silica composite materials and then removal of the silica templates. The NOSCs exhibit good catalytic activity toward ORR with low onset potential and low Tafel slope, along with different electron-transfer numbers, or in other words, different ratios H2O/H2O2 as products, depending on the relative amount of colloidal silica used as templates. The NOSCs also effectively catalyze AOR at relatively low temperature, giving good conversions and high selectivity.

Electrospinning Synthesis of Bimetallic Nickel–Iron Oxide/Carbon Composite Nanofibers for Efficient Water Oxidation Electrocatalysis

The development of earth‐abundant water oxidation electrocatalysts with high activity and durability is very important for many renewable energy conversion/storage processes. Herein, we report a facile synthetic method for the preparation of amorphous nickel–iron oxide/carbon composite nanofibers with high electrocatalytic activity and stability for the oxygen evolution reaction (OER). This method involves two main steps: (i) the electrospinning synthesis of Ni‐ and Fe‐embedded polyvinylpyrrolidone (PVP) polymer nanofibers as the precursor and (ii) the thermal conversion of this precursor in air at 250 °C into nickel–iron oxide/carbon composite nanofibers. Moreover, we show that the as‐obtained composite material exhibits a comparable catalytic activity and a superior catalytic stability to IrOx/C and RuOx, which are state‐of‐the‐art noble‐metal‐based water oxidation electrocatalysts. In particular, the obtained amorphous nickel–iron oxide/carbon composite nanofibers with an optimal Ni/Fe molar ratio of 1:2 afford a small overpotential of 310 mV at a current density of 10 mA cm−2, show high catalytic stability for >15 h, and give >90 % Faradaic yield toward the OER. The efficient catalytic activity of the material can be attributed to its overall conducive structural features for the OER, mainly including the amorphous phase structure of nickel–iron oxide, tunable Ni/Fe atomic ratio, and strongly coupled interaction between nickel–iron oxide and nanocarbon.

Yeast Cells-Derived Hollow Core/Shell Heteroatom-Doped Carbon Microparticles for Sustainable Electrocatalysis

The use of renewable resources to make various synthetic materials is increasing in order to meet some of our sustainability challenges. Yeast is one of the most common household ingredients, which is cheap and easy to reproduce. Herein we report that yeast cells can be thermally transformed into hollow, core-shell heteroatom-doped carbon microparticles that can effectively electrocatalyze the oxygen reduction and hydrazine oxidation reactions, reactions that are highly pertinent to fuel cells or renewable energy applications. We also show that yeast cell walls, which can easily be separated from the cells, can produce carbon materials with electrocatalytic activity for both reactions, albeit with lower activity compared with the ones obtained from intact yeast cells. The results reveal that the intracellular components of the yeast cells such as proteins, phospholipids, DNAs and RNAs are indirectly responsible for the latters higher electrocatalytic activity, by providing it with more heteroatom dopants. The synthetic method we report here can serve as a general route for the synthesis of (electro)catalysts using microorganisms as raw materials.

Three-dimensional ultrathin In2O3 nanosheets with morphology-enhanced activity for amine sensing

Ultrathin materials have a wide range of applications in catalysis and sensing owing to their very large surface to volume ratio and great amount of exposed active sites. Herein, we report the synthesis of three dimensional (3D) In2O3 materials with a high surface area composed of ultrathin nanosheets, ca. 2 nm, using indium glycerolate as the precursor. The structural evolution process of the indium glycerolate precursor was monitored by thermogravimetric analysis, infrared spectroscopy and transmission electron microscopy. The resulting In2O3 nanosheets show excellent amine sensing performance at room temperature because ultrathin nanosheets offer a large amount of active sites on the surface and the 3D structure adds an additional advantage of avoiding aggregation and facilitating the diffusion of the target gas. In addition, the gas sensing mechanism is also proposed in this study.

From ionic liquid-modified cellulose nanowhiskers to highly active metal-free nanostructured carbon catalysts for the hydrazine oxidation reaction

Ionic liquid (or [C4mim][CH3SO3])-modified cellulose nanowhiskers (CNWs) are synthesized and successfully used as precursors to make heteroatom (N and S)-doped nanostructured carbon catalysts. The catalysts can efficiently electrocatalyze the hydrazine oxidation reaction (HOR) with an onset potential close to the reactions thermodynamic value, or with a value better than those obtained for other related materials. The synthesis of these metal-free carbon electrocatalysts generally involves only a few, relatively less demanding synthetic steps. Based on relevant control experiments, the outstanding catalytic activity of the materials is attributed to the heteroatom dopants and defect sites in the materials, which form during carbonization due to the [C4mim][CH3SO3] placed around the CNWs. However, it is not necessarily the density of heteroatom dopant species introduced into the nanostructured carbon materials by the ILs that directly affect the electrocatalytic activity of these materials; it is rather the specific type of dopant-associated chemical moiety and vacancy site created in the materials, which are the main factors positively affecting the electrocatalytic activity of the materials toward the reaction. The surface areas of the materials play a relatively lesser role in affecting the electrocatalytic properties of the materials toward the HOR as well.

Copper‐Decorated Microsized Nanoporous Titanium Dioxide Photocatalysts for Carbon Dioxide Reduction by Water

A series of metallic copper (Cu)‐decorated microsized, nanoporous titanium dioxide (TiO2) materials with different loadings of Cu were synthesized by in situ hydrolysis of Sn2+‐grafted titanium glycolate microspheres in the presence of Cu2+ ions. The resulting materials showed good photocatalytic activity for CO2 reduction with water. In particular, the material prepared with an optimal loading of Cu (≈0.4 wt %) exhibited the highest photocatalytic activity for the reduction of CO2 to hydrocarbon fuel (CH4), with an efficiency that was 21‐fold higher than that of the most commonly studied and utilized commercial photocatalyst, Degussa P25 TiO2, under similar reaction conditions.

N- and O-doped mesoporous carbons derived from rice grains: efficient metal-free electrocatalysts for hydrazine oxidation

Nitrogen and oxygen co-doped mesoporous carbons that can serve as metal-free electrocatalysts are synthesized via a novel synthetic route using milled rice as a precursor and colloidal silica as a template. The materials efficiently electrocatalyze the hydrazine oxidation reaction with only a small onset potential, while giving a high peak current density and showing good long-term stability.

Glutathione-triggered release of model drug molecules from mesoporous silica nanoparticles via a non-redox process

Model drug-loaded mesoporous silica nanoparticles (MSNs) that are responsive to the pH rather than the redox changes normally related to glutathione (GSH) are prepared using surfactant-free MSNs as precursor. The nanoparticles are successfully shown to serve as GSH-triggered release vehicles for the guest molecules. Unlike the generally known GSH-based redox-triggered drug release action, a non-redox, ionic process associated with GSH and GSH-induced pH change enables the MSNs to release their guest molecules.

Hybrid Materials and Nanocomposites as Multifunctional Biomaterials

This review article provides an overview of hybrid and nanocomposite materials used as biomaterials in nanomedicine, focusing on applications in controlled drug delivery, tissue engineering, biosensors and theranostic systems. Special emphasis is placed on the importance of tuning the properties of nanocomposites, which can be achieved by choosing appropriate synthetic methods and seeking synergy among different types of materials, particularly exploiting their nanoscale nature. The challenges in fabrication for the nanocomposites are highlighted by classifying them as those comprising solely inorganic phases (inorganic/inorganic hybrids), organic phases (organic/organic hybrids) and both types of phases (organic/inorganic hybrids). A variety of examples are given for applications from the recent literature, from which one may infer that significant developments for effective use of hybrid materials require a delicate balance among structure, biocompatibility, and stability.