Eric R. Waclawik

An Efficient Photocatalyst Structure: TiO2(B) Nanofibers with a Shell of Anatase Nanocrystals

A new efficient photocatalyst structure, a shell of anatase nanocrystals on the fibril core of a single TiO(2)(B) crystal, was obtained via two consecutive partial phase transition processes. In the first stage of the process, titanate nanofibers reacted with dilute acid solution under moderate hydrothermal conditions, yielding the anatase nanocrystals on the fiber. In the subsequent heating process, the fibril core of titanate was converted into a TiO(2)(B) single crystal while the anatase crystals in the shell remained unchanged. The anatase nanocrystals do not attach to the TiO(2)(B) core randomly but coherently with a close crystallographic registry to the core to form a stable phase interface. For instance, (001) planes in anatase and (100) planes of TiO(2)(B) join together to form a stable interface. Such a unique structure has several features that enhance the photocatalytic activity of these fibers. First, the differences in the band edges of the two phases promote migration of the photogenerated holes from anatase shell to the TiO(2)(B) core. Second, the well-matched phase interfaces allow photogenerated electrons and holes to readily migrate across the interfaces because the holes migrate much faster than excited electrons, more holes than electrons migrate to TiO(2)(B) and this reduces the recombination of the photogenerated charges in anatase shell. Third, the surface of the anatase shell has both a strong ability to regenerate surface hydroxyl groups and adsorb O(2), the oxidant of the reaction, to yield reactive hydroxyl radicals (OH(.)) through reaction between photogenerated holes and surface hydroxyl groups. The adsorbed O(2) molecules can capture the excited electrons on the surface, forming reactive O(2)(-) species. The more reactive species generated on the external surface, the higher the photocatalytic activity will be, and generation of the reactive species also contributes to reducing recombination of the photogenerated charges. Indeed, the mixed-phase nanofibers exhibited superior photocatalytic activity for degradation of sulforhodamine B under UV light to the nanofibers of either pure phase alone or mechanical mixtures of the pure phase nanofibers with a similar phase composition. Finally, the nanofibril morphology has an additional advantage that they can be separated readily after reaction for reuse by sedimentation. This is very important because the high cost for separating the catalyst nanocrystals has seriously impeded the applications of TiO(2) photocatalysts on an industrial scale.

Single Atom (Pd/Pt) Supported on Graphitic Carbon Nitride as an Efficient Photocatalyst for Visible-Light Reduction of Carbon Dioxide

Reducing carbon dioxide to hydrocarbon fuel with solar energy is significant for high-density solar energy storage and carbon balance. In this work, single atoms of palladium and platinum supported on graphitic carbon nitride (g-C3N4), i.e., Pd/g-C3N4 and Pt/g-C3N4, respectively, acting as photocatalysts for CO2 reduction were investigated by density functional theory calculations for the first time. During CO2 reduction, the individual metal atoms function as the active sites, while g-C3N4 provides the source of hydrogen (H*) from the hydrogen evolution reaction. The complete, as-designed photocatalysts exhibit excellent activity in CO2 reduction. HCOOH is the preferred product of CO2 reduction on the Pd/g-C3N4 catalyst with a rate-determining barrier of 0.66 eV, while the Pt/g-C3N4 catalyst prefers to reduce CO2 to CH4 with a rate-determining barrier of 1.16 eV. In addition, deposition of atom catalysts on g-C3N4 significantly enhances the visible-light absorption, rendering them ideal for visible-light reduction of CO2. Our findings open a new avenue of CO2 reduction for renewable energy supply.

Correlation of the catalytic activity for oxidation taking place on various TiO2 surfaces with surface OH groups and surface oxygen vacancies

Three catalytic oxidation reactions have been studied: The ultraviolet (UV) light induced photocatalytic decomposition of the synthetic dye sulforhodamine B (SRB) in the presence of TiO(2) nanostructures in water, together with two reactions employing Au/TiO(2) nanostructure catalysts, namely, CO oxidation in air and the decomposition of formaldehyde under visible light irradiation. Four kinds of TiO(2) nanotubes and nanorods with different phases and compositions were prepared for this study, and gold nanoparticle (Au-NP) catalysts were supported on some of these TiO(2) nanostructures (to form Au/TiO(2) catalysts). FTIR emission spectroscopy (IES) measurements provided evidence that the order of the surface OH regeneration ability of the four types of TiO(2) nanostructures studied gave the same trend as the catalytic activities of the TiO(2) nanostructures or their respective Au/TiO(2) catalysts for the three oxidation reactions. Both IES and X-ray photoelectron spectroscopy (XPS) proved that anatase TiO(2) had the strongest OH regeneration ability among the four types of TiO(2) phases or compositions. Based on these results, a model for the surface OH group generation, absorption, and activation of molecular oxygen has been proposed: The oxygen vacancies at the bridging O(2-) sites on TiO(2) surfaces dissociatively absorb water molecules to form OH groups that facilitate adsorption and activation of O(2) molecules in nearby oxygen vacancies by lowering the absorption energy of molecular O(2). A new mechanism for the photocatalytic formaldehyde decomposition with the Au/TiO(2) catalysts is also proposed, based on the photocatalytic activity of the Au-NPs under visible light. The Au-NPs absorb the light owing to the surface plasmon resonance effect and mediate the electron transfers that the reaction needs.

Controlled synthesis of CuInS2, Cu2SnS3 and Cu2ZnSnS4 nano-structures: insight into the universal phase-selectivity mechanism

Well-shaped CuInS2 nanopyramids and nanodisks were synthesized by a wet-chemical method. The phase structure was controlled by the coordination strength between solvent and metal precursors. Zincblende CuInS2 structure was obtained when copper iodide, indium acetate and 1-dodecanethiol were heated at 220 °C in 1-octadecene (ODE) or oleic acid (OA). When the solvent was replaced by oleylamine (OLA) or trioctylphosphine oxide (TOPO), the thermodynamically metastable wurtzite phase was obtained. Interestingly, zincblende phase can also be synthesized in OLA solvent by injecting 1-dodecanethiol into the reaction solution at 315 °C. It was demonstrated that the CuInS2 phase structure selectivity was determined at the initial formation step of a CuIn(SR)x intermediate. An intermediate with high crystallinity will give metastable wurtzite CuInS2 structure, while a low crystallinity intermediate transforms into zincblende CuInS2 phase. This understanding of the crystal formation mechanism enabled us to extend the same synthetic method to another two attractive nanomaterials, Cu2SnS3 and Cu2ZnSnS4. In this work, Cu2SnS3 and Cu2ZnSnS4 nanocrystals with zincblende or wurtzite structures were readily synthesized using similar reaction conditions to CuInS2 nanocrystals.

Colloidal semiconductor nanocrystals: controlled synthesis and surface chemistry in organic media

Colloidal semiconductor nanocrystals (CS-NCs) possess compelling benefits of low-cost, large-scale solution processing, and tunable optoelectronic properties through controlled synthesis and surface chemistry engineering. These merits make them promising candidates for a variety of applications. This review focuses on the general strategies and recent developments of the controlled synthesis of CS-NCs in terms of crystalline structure, particle size, dominant exposed facet, and their surface passivation. Highlighted are the organic-media based synthesis of metal chalcogenide (including cadmium, lead, and copper chalcogenide) and metal oxide (including titanium oxide and zinc oxide) nanocrystals. Current challenges and thus future opportunities are also pointed out in this review.

Preparation of graphene oxide/epoxy nanocomposites with significantly improved mechanical properties

The effect of graphene oxide (GO) on the mechanical properties and the curing reaction of Diglycidyl Ether of Bisphenol A/F and Triethylenetetramine epoxy system was investigated. GO was prepared by oxidation of graphite flakes and characterized by spectroscopic and microscopic techniques. Epoxy nanocomposites were fabricated with different GO loading by solution mixing technique. It was found that incorporation of small amount of GO into the epoxy matrix significantly enhanced the mechanical properties of the epoxy. In particular, model I fracture toughness was increased by nearly 50% with the addition of 0.1 wt. % GO to epoxy. The toughening mechanism was understood by fractography analysis of the tested samples. The more irregular, coarse, and multi-plane fracture surfaces of the epoxy/GO nanocomposites were observed. This implies that the two-dimensional GO sheets effectively disturbed and deflected the crack propagation. At 0.5 wt. % GO, elastic modulus was ∼35% greater than neat epoxy. Differential ...

Iodine and chlorine nuclear quadrupole coupling in the rotational spectra of Ar⋯ICl and ICl: intramolecular charge transfer induced in ICl by Ar

Abstract Accurate values of the ground-state spectroscopic constants B 0 , D J , χ aa (I), χ aa (Cl) and M bb (I) were determined for the two isotopomers Ar⋯I 35 Cl and Ar⋯I 37 Cl of a linear complex of argon and iodine monochloride from analyses of their rotational spectra. Interpretation of the changes in the nuclear quadrupole coupling constants χ aa (I) and χ aa (Cl) of Ar⋯ICl from those χ 0 (I) and χ 0 (Cl), respectively, of the free molecule shows that the electric charge redistribution within ICl when Ar⋯ICl is formed is equivalent to the transfer of 5.4×10 −3 e from I to Cl. The intermolecular distance r (Ar⋯I) and stretching force constant k σ were determined to be 3.576 A and 3.20 N m −1 , respectively.

Self-Assembled 3D ZnO Porous Structures with Exposed Reactive {0001} Facets and Their Enhanced Gas Sensitivity

Complex three-dimensional structures comprised of porous ZnO plates were synthesized in a controlled fashion by hydrothermal methods. Through subtle changes to reaction conditions, the ZnO structures could be self-assembled from 20 nm thick nanosheets into grass-like and flower-like structures which led to the exposure of high proportions of ZnO {0001} crystal facets for both these materials. The measured surface area of the flower-like and the grass, or platelet-like ZnO samples were 72.8 and 52.4 m2·g−1, respectively. Gas sensing results demonstrated that the porous, flower-like ZnO structures exhibited enhanced sensing performance towards NO2 gas compared with either grass-like ZnO or commercially sourced ZnO nanoparticle samples. The porous, flower-like ZnO structures provided a high surface area which enhanced the ZnO gas sensor response. X-ray photoelectron spectroscopy characterization revealed that flower-like ZnO samples possessed a higher percentage of oxygen vacancies than the other ZnO sample-types, which also contributed to their excellent gas sensing performance.

A comparative study of single-walled carbon nanotube purification techniques using Raman spectroscopy.

Raman spectroscopy has been utilized to show the increase of single-walled carbon nanotubes (SWCNTs) content in commercial grade samples synthesized by the chemical vapour deposition (CVD) technique with a minimization of impurities using both hydrochloric acid treatment and surfactant purification. Surfactant purification methods proved to be the most effective, resulting in a three-fold increase in the percentage of SWCNTs present in the purified product as determined by Raman spectroscopy.

Regioregular poly(3-hexyl-thiophene) helical self-organization on carbon nanotubes

Mixtures of regioregular poly(3-hexyl-thiophene) (rrP3HT) and multiwall carbon nanotubes have been investigated by scanning tunneling microscopy in ultrahigh vacuum. Carbon nanotubes covered by rrP3HT have been imaged and analyzed, providing a clear evidence that this polymer self-assembles on the nanotube surface following geometrical constraints and adapting its equilibrium chain-to-chain distance. Largely spaced covered nanotubes have been analyzed to investigate the role played by nanotube chirality in the polymer wrapping, evidencing strong rrP3HT interactions along well defined directions.