Xingzhong Cao

Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile

The sole effect of surface/bulk defects of TiO2 samples on their photocatalytic activity was investigated. Nano-sized anatase and rutile TiO2 were prepared by hydrothermal method and their surface/bulk defects were adjusted simply by calcination at different temperatures, i.e. 400-700 °C. High temperature calcinations induced the growth of crystalline sizes and a decrease in the surface areas, while the crystalline phase and the exposed facets were kept unchanged during calcination, as indicated by the characterization results from XRD, Raman, nitrogen adsorption-desorption, TEM and UV-Vis spectra. The existence of surface/bulk defects in calcined TiO2 samples was confirmed by photoluminescence and XPS spectra, and the surface/bulk defect ratio was quantitatively analyzed according to positron annihilation results. The photocatalytic activity of calcined TiO2 samples was evaluated in the photocatalytic reforming of methanol and the photocatalytic oxidation of α-phenethyl alcohol. Based on the characterization and catalytic results, a direct correlation between the surface specific photocatalytic activity and the surface/bulk defect density ratio could be drawn for both anatase TiO2 and rutile TiO2. The surface defects of TiO2, i.e. oxygen vacancy clusters, could promote the separation of electron-hole pairs under irradiation, and therefore, enhance the activity during photocatalytic reaction.

A superior catalyst with dual redox cycles for the selective reduction of NOx by ammonia

An environmentally benign Cu-Ce-Ti oxide catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a broad operation temperature window. The dual redox cycles (Cu(2+) + Ce(3+) ↔ Cu(+) + Ce(4+), Cu(2+) + Ti(3+) ↔ Cu(+) + Ti(4+)) play key roles for the superior catalytic deNOx performance.

Oxygen vacancy–induced ferromagnetism in un-doped ZnO thin films

ZnO films became ferromagnetic when defects were introduced by thermal-annealing in flowing argon. This ferromagnetism, as shown by the photoluminescence measurement and positron annihilation analysis, was induced by the singly occupied oxygen vacancy with a saturated magnetization dependent positively on the amount of this vacancy. This study clarified the origin of the ferromagnetism of un-doped ZnO thin films and provides possibly an alternative way to prepare ferromagnetic ZnO films.

Highly Dispersed TiO6 Units in a Layered Double Hydroxide for Water Splitting

A family of photocatalysts for water splitting into hydrogen was prepared by distributing TiO(6) units in an MTi-layered double hydroxide matrix (M = Ni, Zn, Mg) that displays largely enhanced photocatalytic activity with an H(2)-production rate of 31.4 μmol  h(-1) as well as excellent recyclable performance. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping and XPS measurement reveal that a high dispersion of TiO(6) octahedra in the layered doubled hydroxide (LDH) matrix was obtained by the formation of an M(2+)-O-Ti network, rather different from the aggregation state of TiO(6) in the inorganic layered material K(2)Ti(4)O(9). Both transient absorption and photoluminescence spectra demonstrate that the electron-hole recombination process was significantly depressed in the Ti-containing LDH materials relative to bulk Ti oxide, which is attributed to the abundant surface defects that serve as trapping sites for photogenerated electrons verified by positron annihilation and extended X-ray absorption fine structure (EXAFS) techniques. In addition, a theoretical study on the basis of DFT calculations demonstrates that the electronic structure of the TiO(6) units was modified by the adjacent MO(6) octahedron by means of covalent interactions, with a much decreased bandgap of 2.1 eV, which accounts for its superior water-splitting behavior. Therefore, the dispersion strategy for TiO(6) units within a 2D inorganic matrix can be extended to fabricate other oxide or hydroxide catalysts with greatly enhanced performance in photocatalysis and energy conversion.

A hierarchical heterostructure based on Pd nanoparticles/layered double hydroxide nanowalls for enhanced ethanol electrooxidation

Finely dispersed Pd nanoparticles (PdNPs) anchored to CoAl layered double hydroxide nanowalls (LDH-NWs) have been fabricated via a facile in situ redox reaction between the LDH-NWs and the PdCl42− precursor. The integrated LDH-NWs play the roles of both a hierarchical support and a reductant without any external agent, ensuring the cleanness of the metal–support interface. Based on the effective exposure of the Pd active sites and the elaborate network architecture, the Pd/LDH-NW heterogenous material yields a largely improved catalytic activity as well as robust durability towards ethanol electrooxidation in comparison with the commercial Pd/C catalyst. Moreover, a density functional theory (DFT) calculation indicates that the enhancement in the electrocatalytic properties originates from the synergistic effect between the metal and support, in which the LDH support stabilizes the PdNPs via the formation of a Pd–HO bond which is accompanied by an electron transfer from the LDH to the PdNPs. This work provides a promising approach for the design and fabrication of highly efficient metal-supported nanocatalysts which can be used in fuel cells and other related catalytic reactions.

Lanthanum-doped ZnO quantum dots with greatly enhanced fluorescent quantum yield

The lanthanum-doped ZnO quantum dots (QDs) are synthesized by a modified sol–gel method under atmospheric conditions. The as-prepared quantum dots are characterized by X-ray powder diffraction, energy dispersion spectrum analysis and high resolution transmission electron microscopy. The optical properties of the products are studied by ultra-violet spectroscopy and fluorescent spectroscopy. The results show that the doped quantum dots exhibit greatly enhanced luminescent properties and their quantum yield centered around 495 nm is increased from 30.5% for un-doped ZnO QDs already improved by silane surface modification to 77.9% for La-doped ZnO QDs at a proper La-doping content, which is the highest reported so far for the green-emitting ZnO QDs. Positron annihilation spectroscopy is employed to probe the vacancy-type defects of ZnO QDs. Finally, anti-counterfeiting inks are prepared by incorporating La-doped ZnO QDs into the transparent oil and their possible potential applications are explored.

Enhanced Interfacial Interaction and CO2 Separation Performance of Mixed Matrix Membrane by Incorporating Polyethylenimine-Decorated Metal–Organic Frameworks

Polyethylenimine (PEI) was immobilized by MIL-101(Cr) (∼550 nm) via a facile vacuum-assisted method, and the obtained PEI@MIL-101(Cr) was then incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate mixed matrix membranes (MMMs). High loading and uniform dispersion of PEI in MIL-101(Cr) were achieved as demonstrated by ICP, FT-IR, XPS, and EDS-mapping. The PEI both in the pore channels and on the surface of MIL-101(Cr) improved the filler-polymer interface compatibility due to the electrostatic interaction and hydrogen bond between sulfonic acid group and PEI, and simultaneously rendered abundant amine carriers to facilitate the transport of CO2 through reversible reaction. MMMs were evaluated in terms of gas separation performance, thermal stability, and mechanical property. The as-prepared SPEEK/PEI@MIL-101(Cr) MMMs showed increased gas permeability and selectivity, and the highest ideal selectivities for CO2/CH4 and CO2/N2 were 71.8 and 80.0 (at a CO2 permeability of 2490 Barrer), respectively. Compared with the membranes doped with unfilled MIL-101(Cr), the ideal selectivities of CO2/CH4 and CO2/N2 for PEI@MIL-101(Cr)-doped membranes were increased by 128.1 and 102.4 %, respectively, at 40 wt % filler loading, surpassing the 2008 Robeson upper bound line. Moreover, the mechanical property and thermal stability of SPEEK/PEI@MIL-101(Cr) were enhanced.

Origin of the defects-induced ferromagnetism in un-doped ZnO single crystals

We clarified, in this Letter, that in un-doped ZnO single crystals after thermal annealing in flowing argon, the defects-induced room-temperature ferromagnetism was originated from the surface defects and specifically, from singly occupied oxygen vacancies denoted as F+, by the optical and electrical properties measurements as well as positron annihilation analysis. In addition, a positive linear relationship was observed between the ferromagnetism and the F+ concentration, which is in support with the above clarification.

Fabrication of Ultrathin Membrane via Layer-by-Layer Self-assembly Driven by Hydrophobic Interaction Towards High Separation Performance

A novel and facile layer-by-layer (LbL) self-assembly process driven by hydrophobic interaction and then reinforced by hydrogen bond was developed to prepare ultrathin membranes. Gelatin (GE) and tannic acid (TA) were alternately deposited on polyacrylonitrile (PAN) ultrafiltration membranes to obtain GE/TA membranes. The required number of deposition cycles for acceptable permselectivity of membrane was greatly reduced compared with that of the traditional LbL self-assembly process and could be ascribed to the rapid growth of membrane thickness and the integrity of the innermost gelatin layer. Higher surface hydrophilicity and more appropriate free volume characteristics were obtained for GE/TA multilayer membranes compared with pristine gelatin membrane. Moreover, the GE/TA multilayer membrane exhibited improved stability even at high water content of 30 wt %. The membrane separation experiments with pervaporation dehydration of ethanol aqueous solution as a model system demonstrated the GE/TA multilayer membrane achieved higher water permselectivity than the pristine gelatin membrane. High operation stability was acquired in the long-term membrane separation test.

Synergistic Phase and Disorder Engineering in 1T‐MoSe2 Nanosheets for Enhanced Hydrogen‐Evolution Reaction

MoSe2 is a promising earth-abundant electrocatalyst for the hydrogen-evolution reaction (HER), even though it has received much less attention among the layered dichalcogenide (MX2 ) materials than MoS2 so far. Here, a novel hydrothermal-synthesis strategy is presented to achieve simultaneous and synergistic modulation of crystal phase and disorder in partially crystallized 1T-MoSe2 nanosheets to dramatically enhance their HER catalytic activity. Careful structural characterization and defect characterization using positron annihilation lifetime spectroscopy correlated with electrochemical measurements show that the formation of the 1T phase under a large excess of the NaBH4 reductant during synthesis can effectively improve the intrinsic activity and conductivity, and the disordered structure from a lower reaction temperature can provide abundant unsaturated defects as active sites. Such synergistic effects lead to superior HER catalytic activity with an overpotential of 152 mV versus reversible hydrogen electrode (RHE) for the electrocatalytic current density of j = -10 mA cm-2 , and a Tafel slope of 52 mV dec-1 . This work paves a new pathway for improving the catalytic activity of MoSe2 and generally MX2 -based electrocatalysts via a synergistic modulation strategy.