Zhuoxin Li

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.

Strong photoluminescence of nanostructured crystalline tungsten oxide thin films

Strong photoluminescence (PL) is observed in nanostructured crystalline tungsten oxide thin films that are prepared by thermal evaporation. Two kinds of films are investigated—one made of nanoparticles and another of nanowires. At room temperature, strong PL emissions at ultraviolet-visible and blue regions are found in both of the films. Compared with the complete absence of emission of bulk phase tungsten oxide powder under the same excitation conditions, our results clearly demonstrate the quantum-confinement-effect-induced photoluminescence in nanostructured tungsten oxides.Strong photoluminescence (PL) is observed in nanostructured crystalline tungsten oxide thin films that are prepared by thermal evaporation. Two kinds of films are investigated—one made of nanoparticles and another of nanowires. At room temperature, strong PL emissions at ultraviolet-visible and blue regions are found in both of the films. Compared with the complete absence of emission of bulk phase tungsten oxide powder under the same excitation conditions, our results clearly demonstrate the quantum-confinement-effect-induced photoluminescence in nanostructured tungsten oxides.

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.

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.

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.

Embedding dopamine nanoaggregates into a poly(dimethylsiloxane) membrane to confer controlled interactions and free volume for enhanced separation performance

In this study, a series of hybrid membranes with high separation performance and superior swelling-resistance were fabricated by incorporating metal ion-chelated dopamine nanoaggregates into a poly(dimethylsiloxane) (PDMS) bulk matrix membrane. The concomitant hydrogen bond, metal-organic coordination and π-complexation interactions render the synergy of a favorable free volume property, reinforced chain rigidity and facilitated transport function within the membranes. The membranes displayed simultaneously enhanced permeation flux and enrichment factors when utilized for model gasoline separation. Especially, when the weight fraction of dopamine/Cu reached 5.0 wt%, the membrane displayed an optimum separation performance with a permeation flux of 7.42 kg m−2 h−1 (2.7 times as much as that of the PDMS control membrane) and an enrichment factor of 4.81 (11% more than that of the PDMS control membrane). Thanks to the elevated cohesive energy and the chain extension effect, the swelling degree of the PDMS-dopamine/Cu membranes decreased remarkably with the dopamine/Cu content. This study may provide a novel route to the design and fabrication of robust, high-performance hybrid membranes to meet diverse energy and environment-related application requirements.

Creation of hierarchical structures within membranes by incorporating mesoporous microcapsules for enhanced separation performance and stability

In this study, we present a novel approach for fabricating hybrid membranes with superior separation performance and physicochemical stability by incorporating multifunctional dopamine mesoporous microcapsules upon CaCO3 template. The microcapsules are synthesized via the synergy of surface segregation, metal–organic coordination and biomimetic mineralization. The micro-scale hollow lumen and the mesoporous wall decrease diffusion resistance of the membranes by endowing smaller effective membrane thickness and introducing additional shorter permeation pathways for the penetrants, which lead to a faster mass transfer within the membranes. Meanwhile, the microcapsules bridge the polymer chains mainly owing to the numerous mesopores and unique bio-adhesion, which render the optimal polymer–microcapsule interface. Due to the hierarchical structures spanning from microscale, nanoscale to molecular-scale within the membranes, the membranes display remarkably enhanced permeation flux and desired enrichment factor when utilized for model gasoline separation. In addition, due to the elevated cohesive energy and reinforced chain rigidity, the membranes display higher thermal and mechanical stability. This study can identify a facile, generic, and efficient route to design and fabricate a variety of robust, high-performance hybrid membranes for a broad range of energy and environment-related applications.

A novel fluoro-terminated hyperbranched poly(phenylene oxide) (FHPPO): synthesis, characterization, and application in low-k epoxy materials

A new AB2 monomer 4-hydroxyl-4′,4′′-difluorotriphenylmethane was successfully synthesized via a Friedel–Crafts alkylation of phenol from 4,4′-difluorodiphenylmethanol. Based on the AB2 monomer, novel fluoro-terminated hyperbranched poly(phenylene oxide)s (FHPPOs) were synthesized via the SNAr reaction by self-condensation in one step. The FHPPOs were characterized by various techniques, including NMR, FT-IR, GPC, TGA and DSC. It was found that the molecular weight and polydispersity index of the FHPPOs increased with monomer concentration and reaction time. The degree of branching of the FHPPOs, determined by 13C NMR and 19F NMR with the aid of model compounds, decreased from 0.63 to 0.53 as the molecular weight increased. The glass transition temperature (Tg) of the FHPPOs increased with increasing molecular weight, up to 164 °C when the Mn was over 6, 800. The FHPPOs showed excellent thermal stability up to a Td5 temperature of 559 °C. Because of the low polarity of the poly(phenyl oxide) (PPO) backbones, abundant fluoro-terminated groups, which have large molar free volume, low polarizability of C–F bonds, and inherent free volume or molecule-scale cavities in hyperbranched structures, the addition of FHPPO into diglycidyl ether of bisphenol A (DGEBA) could effectively lower the relative dielectric constant, the dissipation factor, and moisture absorption of the cured DGEBA/FHPPO composites. The free volume of the composites, which was quantified by positron annihilation lifetime spectroscopy (PALS), increased with increased FHPPO loading. The excellent dielectric and thermal properties make FHPPO a promising low-k modifier for epoxy resins.