Xiaoxin Zou

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.

Coupling Mo2C with Nitrogen-Rich Nanocarbon Leads to Efficient Hydrogen-Evolution Electrocatalytic Sites

In our efforts to obtain electrocatalysts with improved activity for water splitting, meticulous design and synthesis of the active sites of the electrocatalysts and deciphering how exactly they catalyze the reaction are vitally necessary. Herein, we report a one-step facile synthesis of a novel precious-metal-free hydrogen-evolution nanoelectrocatalyst, dubbed Mo2 C@NC that is composed of ultrasmall molybdenum carbide (Mo2 C) nanoparticles embedded within nitrogen-rich carbon (NC) nanolayers. The Mo2 C@NC hybrid nanoelectrocatalyst shows remarkable catalytic activity, has great durability, and gives about 100 % Faradaic yield toward the hydrogen-evolution reaction (HER) over a wide pH range (pH 0-14). Theoretical calculations show that the Mo2 C and N dopants in the material synergistically co-activate adjacent C atoms on the carbon nanolayers, creating superactive nonmetallic catalytic sites for HER that are more active than those in the constituents.

Efficient noble metal-free (electro)catalysis of water and alcohol oxidations by zinc-cobalt layered double hydroxide.

Replacing rare and expensive noble metal catalysts with inexpensive and earth-abundant ones for various renewable energy-related chemical processes as well as for production of high value chemicals is one of the major goals of sustainable chemistry. Herein we show that a bimetallic Zn-Co layered double hydroxide (Zn-Co-LDH) can serve as an efficient electrocatalyst and catalyst for water and alcohol oxidation, respectively. In the electrochemical water oxidation, the material exhibits a lower overpotential, by ~100 mV, than monometallic Co-based solid-state materials (e.g., Co(OH)2 and Co3O4)-catalytic systems that were recently reported to be effective for this reaction. Moreover, the materials turnover frequency (TOF) per Co atoms is >10 times as high as those of the latter at the same applied potentials. The Zn-Co-LDH also catalyzes oxidation of alcohols to the corresponding aldehydes or ketones at relatively low temperature, with moderate to high conversion and excellent selectivity.

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.

Carbon-Armored Co9S8 Nanoparticles as All-pH Efficient and Durable H2-Evolving Electrocatalysts

Splitting water to produce hydrogen requires the development of non-noble-metal catalysts that are able to make this reaction feasible and energy efficient. Herein, we show that cobalt pentlandite (Co9S8) nanoparticles can serve as an electrochemically active, noble-metal-free material toward hydrogen evolution reaction, and they work stably in neutral solution (pH 7) but not in acidic (pH 0) and basic (pH 14) media. We, therefore, further present a carbon-armoring strategy to increase the durability and activity of Co9S8 over a wider pH range. In particular, carbon-armored Co9S8 nanoparticles (Co9S8@C) are prepared by direct thermal treatment of a mixture of cobalt nitrate and trithiocyanuric acid at 700 °C in N2 atmosphere. Trithiocyanuric acid functions as both sulfur and carbon sources in the reaction system. The resulting Co9S8@C material operates well with high activity over a broad pH range, from pH 0 to 14, and gives nearly 100% Faradaic yield during hydrogen evolution reaction under acidic (pH 0), neutral (pH 7), and basic (pH 14) media. To the best of our knowledge, this is the first time that a transition-metal chalcogenide material is shown to have all-pH efficient and durable electrocatalytic activity. Identifying Co9S8 as the catalytically active phase and developing carbon-armoring as the improvement strategy are anticipated to give a fresh impetus to rational design of high-performance noble-metal-free water splitting catalysts.

Metal-free B-doped graphene with efficient electrocatalytic activity for hydrogen evolution reaction

The chemical and physical properties of carbon nanomaterials such as graphene can be tailored by doping their structures with heteroatoms. However, substitutional doping of heteroatoms within carbon nanomaterials requires not only rational synthetic methods but also heteroatom-doping reagents that are easier to employ. Herein we report a facile, wet chemical synthetic method to metal-free, yet catalytically active, B-substituted graphene (B-SuG) by using BH3-THF—a simple and commercially available borylation reagent. Moreover, we show that the B-doped material (i.e., B-SuG) can serve as an efficient metal-free electrocatalyst for hydrogen evolution reaction (HER). Additionally, we demonstrate that BH3-THF is a better borylating agent in terms of producing the most effective electrocatalyst than other borylating agents such as NaBH4, B(OH)3, carborane, B2O3 and NH3-BH3. Compared with the conventional heteroatom-doping methods used for graphene, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), BH3-THF certainly also has the added advantage of being more amenable and easier to use. We expect that this work will stimulate future research on synthesis of other innovative and sustainable metal-free materials and catalysts and investigation of the fundamental structure–property relationships in metal-free catalysts/catalysis for renewable energy and other applications.

Metallic Co9S8 nanosheets grown on carbon cloth as efficient binder-free electrocatalysts for the hydrogen evolution reaction in neutral media

The development of efficient non-noble metal hydrogen-evolving electrocatalysts is of paramount importance for sustainable hydrogen production from water. Herein, we report the direct growth of metallic Co9S8 nanosheets on carbon cloth (CC) through a facile one-pot solvothermal method. We also show that the introduction of a tiny amount of Zn2+ ions (Zn : Co mol ratio of 0.5–1 : 100) in the synthesis system can reduce the thickness, improve the crystallinity, and optimize the surface structure of Co9S8 nanosheets, without Zn-doping. Furthermore, we show that the resulting Co9S8/CC materials can serve as efficient, binder-free, non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) under neutral conditions (pH 7). In particular, the Co9S8/CC material (synthesized in the presence of Zn2+ ions) affords a current density of 10 mA cm−2 at a low overpotential of 175 mV, has great catalytic stability as long as 100 h, and gives about 100% faradaic yield towards the HER in neutral media. The materials excellent catalytic performance toward the HER is attributed primarily to the synergistic effects of Co9S8s intrinsic catalytic ability, the ultrathin nanosheet array architecture and the self-supporting feature.

Heterometal Alkoxides as Precursors for the Preparation of Porous Fe– and Mn–TiO2 Photocatalysts with High Efficiencies

Transition-metal-doped titanium glycolates (M-TG, with M=Fe, Mn), which are the first non-stoichiometric heterometal alkoxides, have been synthesised through a solvothermal doping approach. X-ray diffraction, UV/Vis diffuse reflectance and ESR spectroscopy revealed that the dopant ion (Fe(3+) or Mn(2+)) is substituted for Ti(4+) in the TG lattice. Fe(3+) prolongs the crystallisation time of Fe-TG, whereas Mn(2+) has a smaller effect on the crystallisation time in comparison with Fe(3+). The as-synthesised M-TG materials were used directly as single-source precursors for the preparation of metal-doped titania (M-TiO(2)) through a simple thermal treatment process. The as-prepared M-TiO(2) materials maintain the rod-like morphology of the precursors and possess a mesoporous structure with high crystallinity. It has been proved that the dopant ions are incorporated into the TiO(2) lattice at the Ti(4+) positions. The photocatalytic activities of the M-TiO(2) materials obtained were evaluated by testing the degradation of phenol under UV irradiation. From the photocatalytic results, it was concluded that high crystallinity, a large surface area and appropriate transition-metal-doping are all beneficial to the enhancement of the photocatalytic performance of the doped TiO(2) material. In addition, it was noted that in comparison with Mn-TiO(2), Fe-TiO(2) shows higher photocatalytic activity due to the better inhibition effect of Fe(3+) on recombination of photogenerated electron-hole pairs. In contrast to the conventional nanosized TiO(2) photocatalyst, the micrometre-sized M-TiO(2) particles we obtained can be easily separated and recovered after the photocatalytic reactions.

Growth of molybdenum carbide micro-islands on carbon cloth toward binder-free cathodes for efficient hydrogen evolution reaction

Design and synthesis of efficient noble metal-free hydrogen evolution catalysts is of paramount importance for the practical application of water-splitting devices. Herein, we report a novel synthetic method to grow dispersed molybdenum carbide (Mo2C) micro-islands on flexible carbon cloth (CC). This method involves the controlled synthesis of a supramolecular hybrid between cetyltrimethyl ammonium cations and molybdate anions on CC, followed by simple thermal treatment of this supramolecular hybrid in Ar to form Mo2C on CC in situ. In this synthesis, the presence of cetyltrimethyl ammonium bromide is proven to be important because it effectively immobilizes molybdate ions on CC on the one hand and functions as a carbon source for the formation of Mo2C on the other. Moreover, the as-prepared Mo2C/CC composite material can serve as efficient binder-free cathodes toward the hydrogen evolution reaction (HER). The Mo2C/CC affords a current density of 10 mA cm−2 at a low overpotential of 140 mV and works stably in acidic media with a Faraday yield of ∼100%. The isolated island architecture of Mo2C ensures rich active sites to be exposed and allows the easy interaction of reactants (e.g., protons) with the active sites. Also, the strong adhesion between Mo2C and carbon cloth facilitates electron transport/transfer in the composite material and is helpful for the achievement of excellent catalytic stability.

Synthesis and photocatalytic activity of porous anatase TiO2 microspheres composed of {010}-faceted nanobelts

Porous anatase TiO(2) microspheres composed of {010}-faceted nanobelts were synthesized through simple thermal treatment of a titanium glycerolate precursor. The as-prepared TiO(2) nanomaterial was shown to serve as an efficient photocatalyst for H(2) evolution, and its activity was more than twice that of the benchmark P25 TiO(2).