Chun-ng Jia

Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.

Enhanced visible-light photocatalytic activity of g-C3N4/Zn2GeO4 heterojunctions with effective interfaces based on band match

Fabricating heterojunction photocatalysts is an important strategy for speeding up the separation rate of photogenerated charge carriers, which is attracting greater interest. However, the choice of three factors, individual materials, band offsets, and effective interfaces, is still important for fabricating efficient heterojunction photocatalysts. Herein, efficient g-C3N4/Zn2GeO4 photocatalysts with effective interfaces were designed by controlling the surface charges of the two individual materials inside the same aqueous dispersion medium, making use of the electrostatic attraction between oppositely charged particles. The g-C3N4/Zn2GeO4 heterojunction with opposite surface charge (OSC) showed higher visible-light photocatalytic activity for degradation of methylene blue than those of pure g-C3N4, pure Zn2GeO4, and the g-C3N4/Zn2GeO4 with identical surface charge (ISC). The investigation of the light absorption spectrum, adsorption ability, and photocurrent responses revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the OSC g-C3N4/Zn2GeO4 samples photocatalytic activity. By combining with theoretical calculations, we investigated the microscopic mechanisms of interface interaction and charge transfer between g-C3N4 and Zn2GeO4. The photogenerated electrons in the g-C3N4 N 2p states directly excited into the Zn 4s and Ge 4s hybrid states of Zn2GeO4. The strategy of designing and preparing a g-C3N4/Zn2GeO4 composite catalyst in this work is very useful for fabricating other efficient heterojunction photocatalysts.

Design of N-Coordinated Dual-Metal Sites: A Stable and Active Pt-Free Catalyst for Acidic Oxygen Reduction Reaction

We develop a host-guest strategy to construct an electrocatalyst with Fe-Co dual sites embedded on N-doped porous carbon and demonstrate its activity for oxygen reduction reaction in acidic electrolyte. Our catalyst exhibits superior oxygen reduction reaction performance, with comparable onset potential (Eonset, 1.06 vs 1.03 V) and half-wave potential (E1/2, 0.863 vs 0.858 V) than commercial Pt/C. The fuel cell test reveals (Fe,Co)/N-C outperforms most reported Pt-free catalysts in H2/O2 and H2/air. In addition, this cathode catalyst with dual metal sites is stable in a long-term operation with 50 000 cycles for electrode measurement and 100 h for H2/air single cell operation. Density functional theory calculations reveal the dual sites is favored for activation of O-O, crucial for four-electron oxygen reduction.

Exploring the effects of nanocrystal facet orientations in g-C3N4/BiOCl heterostructures on photocatalytic performance

Effective separation and migration of photogenerated electron-hole pairs are two key factors to determine the performance of photocatalysts. It has been widely accepted that photocatalysts with heterojunctions usually exhibit excellent charge separation. However, the migration process of separated charges in the heterojunction structures has not been fully investigated. Herein, photocatalysts with heterojunctions are constructed by loading g-C3N4 nanoparticles onto BiOCl nanosheets with different exposed facets (BOC-001 and BOC-010). The g-C3N4 nanoparticles with decreasing size and increasing zeta potential could induce stronger coupling and scattering in the heterojunction. The relationship between the crystal facet orientation in the BiOCl nanosheets and charge separation/effective migration behaviours of the materials is investigated. The visible light photocatalytic activity of the composites is evaluated by methyl orange (MO) and phenol degradation experiments, and the results show that ng-CN/BOC-010 composites exhibit higher photocatalytic performance than that of ng-CN/BOC-001 composites. Both photoelectrochemical and fluorescence emission measurements indicate that the different exposed facets in ng-CN/BiOCl composites could induce the migration of the photogenerated electrons in different ways, but do not significantly alter the separation efficiencies. The separated electrons in ng-CN/BOC-010 undergo a shorter transport distance than that of ng-CN/BOC-001 to reach the surface reactive sites. The study may suggest that the crystal facet orientation in polar semiconductors is a critical factor for designing highly efficient heterojunction photocatalysts.

Contributions of distinct gold species to catalytic reactivity for carbon monoxide oxidation

Small-size (<5 nm) gold nanostructures supported on reducible metal oxides have been widely investigated because of the unique catalytic properties they exhibit in diverse redox reactions. However, arguments about the nature of the gold active site have continued for two decades, due to the lack of comparable catalyst systems with specific gold species, as well as the scarcity of direct experimental evidence for the reaction mechanism under realistic working conditions. Here we report the determination of the contribution of single atoms, clusters and particles to the oxidation of carbon monoxide at room temperature, by the aid of in situ X-ray absorption fine structure analysis and in situ diffuse reflectance infrared Fourier transform spectroscopy. We find that the metallic gold component in clusters or particles plays a much more critical role as the active site than the cationic single-atom gold species for the room-temperature carbon monoxide oxidation reaction.

Transition metal nanoparticles dispersed in an alumina matrix as active and stable catalysts for COx-free hydrogen production from ammonia

Transition metal (Fe, Co, and Ni) nanoparticles dispersed in an alumina matrix as catalysts for NH3 decomposition have been synthesized by a facile co-precipitation method. The fresh and used catalysts were characterized by various techniques including powder X-ray diffraction (XRD), N2 adsorption/desorption, and transmission electron microscopy (TEM). Also, temperature-programmed reduction by hydrogen (H2-TPR) combining in situ XRD was performed to investigate the reducibility of the studied catalysts. For the ammonia decomposition reaction, 88% conversion of ammonia can be realized at the reaction temperature as low as 600 °C using a space velocity of 72 000 cm3 gcat−1 h−1 NH3 during a long term (72 h) catalysis test without any observable deactivation. The small amount of alumina (low to 10 at%) can act as the matrix in which the catalytically active transition metal nanoparticles were stabilized. Thus, the agglomeration of active transition metals under reaction conditions was significantly suppressed and the high activity of catalysts was maintained.

Hydroxyl-rich ceria hydrate nanoparticles enhancing the alcohol electrooxidation performance of Pt catalysts

The exploration of anode catalysts with high activity and stability for direct alcohol fuel cells (DAFCs) has been a big challenge for decades. Metal oxides, such as CeO2 with oxygen vacancies, are widely investigated in the promotion of Pt-based electrocatalysts for alcohol oxidation. Hydroxyl-rich CeO2·xH2O, which is capable of affording OH groups directly for reacting with CO to re-activate the nearby metal catalyst, may surpass CeO2 in alcohol electrooxidation reactions. Herein, small CeO2·xH2O nanoparticles with plenty of surface OH groups have been prepared by a tert-butylamine-assisted solvothermal method, and then ultrasonically mixed with Pt/CNTs (CNTs = carbon nanotubes). As a proof-of-concept experiment, the CeO2·xH2O/Pt/CNT catalyst containing the tert-butylamine-modified CeO2·xH2O nanoparticles achieves a high peak current density of 2304 mA mg−1 for methanol oxidation in the alkaline medium, which is remarkably higher than those of the calcined counterpart (CeO2/Pt/CNTs, 814 mA mg−1) and Pt/CNTs (520 mA mg−1). After a chronoamperometry test for 1200 s, the retained current density of CeO2·xH2O/Pt/CNTs (570 mA mg−1) is also much higher than those of CeO2/Pt/CNTs (163 mA mg−1) and Pt/CNTs (10 mA mg−1). The advantage of CeO2·xH2O nanoparticles over CeO2 nanoparticles has been further confirmed by ethylene glycol and glycerol oxidation reactions, and they have also been found to be effective over other alcohol oxidation catalysts such as Pt/C, Pd/C and PtRu/C. Therefore, this strategy may provide an alternative for the design of anode catalysts in DAFCs with high performance and low cost.

Pt-embedded-CeO2 hollow spheres for enhancing CO oxidation performance

Exploring high-performance catalysts has always been a general concern for promoting extended heterocatalysis reactions. Here, Pt embedded highly-porous CeO2 hollow sphere (Pt/CeO2 HS) composites are developed by a one-pot template-free solvothermal method. Evolution mechanism studies unravel that the Pt/CeO2 HS composites are derived from a self-assembly–reduction–Ostwald ripening process, where Pt nanoparticles (Pt NPs) are embedded into ceria mesoporous hollow spheres. The as-developed embedment strategy is more facile, sustainable and cost-effective than conventional deposition approaches, ensuring the precise control of the location, distribution, and uniformity of Pt NPs throughout the outer shell of hollow spherical CeO2. Importantly, the Pt NP embedding process could be determined to play a key role in creating oxygen vacancies and activating surface chemisorbed oxygen. Besides that, the aggregation of Pt NPs can be efficiently inhibited in the Pt/CeO2 HS composites, and more active sites should be involved in catalytic reactions. All these advantages contribute to the strikingly improved performance as well as excellent stability of the Pt/CeO2 HS composites toward CO oxidation reaction compared with mesoporous CeO2 nanospheres (CeO2 NS) and Pt/CeO2 NS reference catalysts. These findings present a decent protocol for desgining noble metal/oxide hollow structural composites in heterogeneous catalysis.

Synthesis and metal–support interaction of subnanometer copper–palladium bimetallic oxide clusters for catalytic oxidation of carbon monoxide

Subnanometer oxide clusters with distinct metal–support interactions have attracted great interest due to their possible superiority in catalytic performance compared to that of conventional metal–oxide nanoparticles. In this paper, we report the solution-based chemical synthesis of a new type of copper–palladium bimetallic oxide clusters anchored to the surface of ceria nanorods. Revealed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) techniques, we have identified that both copper and palladium species are fully oxidized with dominant metal–support interaction contributions by a strongly bound M–Ox–Ce (M = Cu or Pd) structure. However, no direct bond between copper oxide and palladium oxide clusters, i.e. Cu–Ox–Pd, has been verified by experimental evidences, and thus no synergistic effect on the catalytic activity of bimetallic copper–palladium oxide clusters, compared to that of single metal (palladium) oxide clusters, has been demonstrated for the CO oxidation reaction.

Fe- and Co-doped lanthanum oxides catalysts for ammonia decomposition: Structure and catalytic performances

Abstract In this paper, a series of Fe- and Co-doped lanthanum (hydr)oxides catalysts were prepared by a simple coprecipitation-hydrothermal method. The as-prepared catalysts were characterized with various techniques including powder X-ray diffraction (XRD), N2 adsorption/desorption, inductively coupled plasma (ICP) and transmission electron microscopy (TEM). The Fe-based catalysts exhibited consecutive phase changes of amorphous FeOx→FeLaO3→Fe2N under different stages (as-prepared→calcination→ammonia decomposition reaction); as for Co-based catalysts, the phase transformation followed a sequence of Co(OH)2→ Co3O4→metallic Co. It was revealed that Fe2N and metallic Co were most probably the active crystalline phase respectively for Fe- and Co-based catalysts in the decomposition of ammonia.