James P. Lewis

Origin of Photocatalytic Activity of Nitrogen-Doped TiO2 Nanobelts

Experiments combined with the density functional theory (DFT) calculation have been performed to understand the underlying photocatalysis mechanism of the nitrogen-doped titania nanobelts. Nitrogen-doped anatase titania nanobelts are prepared via hydrothermal processing and subsequent heat treatment in NH(3). Both the nitrogen content and the oxygen vacancy concentration increase with increasing the NH(3) treatment temperature. Nitrogen doping leads to an add-on shoulder on the edge of the valence band, the localized N 2p levels above the valence band maximum, and the 3d states of Ti(3+) below the conduction band, which is confirmed by DFT calculation and X-ray photoelectron spectroscopy (XPS) measurement. Extension of the light absorption from the ultraviolet (UV) region to the visible-light region arises from the N 2p levels near the valence band and from the color centers induced by the oxygen vacancies and the Ti(3+) species. Nitrogen doping allows visible-light-responsive photocatalytic activity but lowers UV-light-responsive photocatalytic activity. The visible-light photocatalytic activity originates from the N 2p levels near the valence band. The oxygen vacancies and the associated Ti(3+) species act as the recombination centers for the photoinduced electrons and holes. They reduce the photocatalytic activity although they contribute to the visible light absorbance.

Shape-enhanced Photocatalytic Activity of Single-crystalline Anatase TiO2 (101) Nanobelts

Particle size is generally considered to be the primary factor in the design of nanocrystal photocatalysts, because the reduction of particle size increases the number of active sites. However, the benefit from the size reduction can be canceled by a higher electron-hole recombination rate due to the confined space in sphere-shaped nanoparticles. Here we report a mechanistic study on a novel nanobelt structure that overcomes the drawback of sphere-shaped nanoparticles. Single-crystalline anatase TiO(2) nanobelts with two dominant surfaces of (101) facet exhibit enhanced photocatalytic activity over the nanosphere counterparts with an identical crystal phase and similar specific surface area. The ab initio density functional theory (DFT) calculations show that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O(2), facilitating the generation of superoxide radical. Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres due to the following three reasons: (i) greater charge mobility in the nanobelts, which is enabled along the longitudinal dimension of the crystals; (ii) fewer localized states near the band edges and in the bandgap due to fewer unpassivated surface states in the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O(2) on the (101) facet. Our results suggest that the photocatalysis efficiency of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocrystals, which provides a new concept for rational design and development of high-performance photocatalysts.

Second-generation photocatalytic materials: anion-doped TiO2

Previous experimental studies describe an efficient photoresponse in the visiblelight region for anion-doped TiO2 .D oping with carbon, nitrogen, as well as sulfur, yields promising second-generation photocatalysis with TiO2 .W e present a theoretical investigatio no fs ubstitutional anion doping in TiO2 and discuss doping effects on the electronic structure, and subsequently the photoactivity. The resulting bandgap narrowing predicted in this work is consistent with experimental observations. Furthermore, we discuss the effects of doping concentration on the localization properties of the valence band edge. Ou rs ystematic study of anion-doped TiO2 implies that the carbon-doped TiO2 is the most promising due to a significant overlap between the O 2p state and the carbon states near the valence band edge. Additionally, carbon dopants produce th el argest valence band red shift of the three anion-doped TiO2 studied.

Multicenter approach to the exchange-correlation interactions in ab initio tight-binding methods

An approximate method to calculate exchange-correlation contributions in the framework of first-principles tight-binding molecular dynamics methods has been developed. In the proposed scheme on-site (off-site) exchange-correlation matrix elements are expressed as a one-center (two-center) term plus a correction due to other neighboring atoms. The one-center (two-center) term is evaluated directly, while the correction is calculated using a generalization of the [Sankey-Niklewski Phys. Rev. B 40, 3979 (1989)] approach valid for arbitrary atomiclike basis sets. The proposed scheme for exchange-correlation terms, called the multi-center weighted exchange-correlation density approximation (McWEDA), permits the accurate and computationally efficient calculation of corresponding tight-binding matrices and atomic forces for complex systems. We calculate bulk properties of selected transition (W,Pd), noble (Au), and simple (Al) metals, a semiconductor (Si), and the transition metal oxide

Effects of dopant states on photoactivity in carbon-doped TiO2

\mathrm{Ti}{\mathrm{O}}_{2}

Visible light photocatalytic activity in nitrogen-doped TiO2 nanobelts

with the method to demonstrate its flexibility and accuracy.

First-principles calculations for nitrogen-containing single-walled carbon nanotubes

Recently an effective photoresponse in the visible-light region was experimentally observed in carbon-doped TiO2. We contribute a theoretical understanding of this phenomenon. Our ab initio density functional theory investigations show that substitutional carbon dopants incorporated into TiO2 drastically affect the electronic structure of the material, thus improving its photoactivity. The resulting bandgap of 2.35 eV predicted in this work agrees with the available experimental observations for carbon concentration around 5%. We also address the effects of doping concentration on the photoresponse of this material.

Product Selectivity Controlled by Zeolite Crystals in Biomass Hydrogenation over a Palladium Catalyst

We present a comprehensive experimental and theoretical study of the electronic properties and photocatalytic activity of nitrogen-doped anatase TiO2 nanobelts. UV-visible spectra show enhanced absorption in the visible light range for nitrogen-doped nanobelts compared to the pristine sample. The nitrogen-doped nanobelts exhibit improved photocatalytic activity compared to the pristine sample upon visible light irradiation. Furthermore, the incorporation of nitrogen introduces localized states in the band gap.

Selective Catalytic Production of 5-Hydroxymethylfurfural from Glucose by Adjusting Catalyst Wettability

We present calculations for possible configurations of nitrogen-containing single-walled carbon nanotubes and their electronic properties obtained with the ab initio tight-binding FIREBALL method. It is found that nitrogen atoms can be energetically incorporated into the carbon network in three forms: Substitution, substitution with formation of a vacancy structure, and chemical adsorption. The different forms exhibit different local densities of states near the Fermi levels, which might suggest a potential method to control the electronic properties of nitrogen-doped carbon nanotubes.

Linear-scaling quantum mechanical calculations of biological molecules: The divide-and-conquer approach

This work delineates the first example for controlling product selectivity in metal-catalyzed hydrogenation of biomass by zeolite crystals. The key to this success is to combine the advantages of both Pd nanoparticles (highly active sites) and zeolite micropores (controllable diffusion of reactants and products), which was achieved from encapsulation of the Pd nanoparticles inside of silicalite-I zeolite crystals as a core-shell structure (Pd@S-1). In the hydrogenation of biomass-derived furfural, the furan selectivity over the Pd@S-1 is as high as 98.7%, outperforming the furan selectivity (5.6%) over conventional Pd nanoparticles impregnated with S-1 zeolite crystals (Pd/S-1). The extraordinary furan selectivity in the hydrogenation over the Pd@S-1 is reasonably attributed to the distinguishable mass transfer of the hydrogenated products in the zeolite micropores.