Photoinduced reactivity of titanium dioxide

This article is a review of the current knowledge of the chemical physics of carbon dioxide (CO2) conversion to fuels using light energy and water (CO2 photoreduction) on titania (TiO2)-based catalysts and Ti-species in porous materials. Fairly comprehensive literature reviews of CO2 photoreduction are available already. However, this article is focused on CO2 photoreduction on Ti-based catalysts, and incorporates fundamental aspects of CO2 photoreduction, knowledge from surface science studies of TiO2 and the surface chemistry of CO2. Firstly, the current state of development of this field is briefly reviewed, followed by a description of and insights from surface state and surface site approaches. Using examples such as metal-doping of TiO2, dye-sensitization, oxygen vacancies in TiO2 and isolated-Ti centers in microporous/mesoporous materials, the utility of these approaches to understand photoinduced reactions involved in CO2activation is examined. Finally, challenges and prospects for further development of this field are presented. Enhanced understanding of the CO2 : TiO2 system, with a combination of computational and experimental studies is required to develop catalysts exhibiting higher activity towards CO2 photoreduction.

Photoinduced activation of CO2 on Ti-based heterogeneous catalysts: Current state, chemical physics-based insights and outlook

Black TiO2 attracts enormous attention due to its large solar absorption and induced excellent photocatalytic activity. Herein, a new approach assisted by hydrogen plasma to synthesize unique H-doped black titania with a core/shell structure (TiO2@TiO2-xHx) is presented, superior to the high H-2-pressure process (under 20 bar for five days). The black titania possesses the largest solar absorption (approximate to 83%), far more than any other reported black titania (the record (high-pressure): approximate to 30%). H doping is favorable to eliminate the recombination centers of light-induced electrons and holes. High absorption and low recombination ensure the excellent photocatalytic activity for the black titania in the photo-oxidation of organic molecules in water and the production of hydrogen. The H-doped amorphous shell is proposed to play the same role as Ag or Pt loading on TiO2 nanocrystals, which induces the localized surface plasma resonance and black coloration. Photocatalytic water splitting and cleaning using TiO2-xHx is believed to have a bright future for sustainable energy sources and cleaning environment.

H‐Doped Black Titania with Very High Solar Absorption and Excellent Photocatalysis Enhanced by Localized Surface Plasmon Resonance

The recent interest in the properties of ceramics with grain sizes less than 100 nm has created a need for processing routes with which to manufacture such ceramics. This article briefly reviews the variety of techniques currently used to manufacture ultrafine starting powders, compact the powders, and sinter them into bulk nanocrystalline ceramics. The unique challenges associated with processing such fine structures are discussed together with each technique. Major obstacles have included the difficulty (now largely overcome) of producing sufficient quantities of ultrafine powders; the strong tendency of nanocrystalline powders to agglomerate; the difficulty of manufacturing homogeneous, stress free compacts from ultrafine powders; and the ever present obstacle of unwanted grain growth during sintering. Despite numerous technical difficulties, researchers have managed to develop techniques for processing ultrafine powders into nanocrystalline ceramics which are both fully dense (or near fully de...

Processing of nanocrystalline ceramics from ultrafine particles

The reaction between TiCl4 and benzyl alcohol is a simple and nonaqueous procedure for the synthesis of highly crystalline titania nanoparticles at temperatures as low as 40 °C. XRD measurements prove the exclusive presence of the anatase phase. The particle growth depends strongly on temperature so that with the appropriate thermal conditions the particle size can be selectively adjusted in the range of 4−8 nm. Fine-tuning of the particle size is possible by a proper choice of the relative amounts of benzyl alcohol and titanium tetrachloride. Lowering the titanium tetrachloride concentration leads to a considerable decrease of particle size. BET measurements show particularly high surface areas, up to 345 m2/g for the smallest particles and 115 m2/g for the calcined material. TEM investigations reveal that the nanoparticles are nearly uniform in size and shape. The as-synthesized particles display only minor agglomeration, whereas the calcined material consists of completely nonagglomerated particles, wi...

Benzyl alcohol and titanium tetrachloride - A versatile reaction system for the nonaqueous and low-temperature preparation of crystalline and luminescent titania nanoparticles

Nanocrystalline TiO2 studied by optical, FTIR and X-ray photoelectron spectroscopy: correlation to presence of surface states

Nanophase alumina (Al2O3) was synthesized in a d.c. arc plasma reactor under isochronal oxygen flow conditions. Transmission electron microscopy revealed spherical particles (50 nm) and the corresponding electron diffraction showed a δ-Al2O3 phase. Structural morphology by X-ray diffraction (XRD) evidenced the phase changes. These measurements confirmed the existence of the nanophase structure. X-ray photoelectron spectrsocopy (XPS) of the core levels of as-deposited, and calcined powders were carried out for comparative study. Zeta potential as determined from electrophoretic mobility measurements, at a pH value of 4, indicated a high value (+42.7 mV) for the nanophase alumina as compared to that for the commercial alumina (−14.1 mV). This is a measure of the state of agglomeration, which is higher for the nanophase alumina. Fourier transform infrared spectra of the alumina powder revealed a broad band from 500 to 1000 cm−1, indicative of the complex Al–O vibration due to interactions of the octahedral and tetrahedral coordination groups. The FTIR spectra also revealed the intermediate route leading to alumina formation, as seen from the presence of gas-phase type sub-oxide bands. In addition the presence of a strained surface vibrational mode retaining itself even after complete transformation to α-Al2O3 is confirmed. Optical absorption spectroscopy was studied yielding a band gap for the nanophase alumina of >5 eV. The Infrared and the absorption spectrum is characteristic of a gas-phase type of reaction. Thus a dimensional evolution starting from a gas-phase precursor, which is molecular in nature, leads to a collisionally quenched structure resulting in very fine particulates condensing from the plasma.

Nanophase alumina synthesis in thermal arc plasma and characterization: correlation to gas-phase studies

On the deposition of Langmuir-Blodgett films of Q-state CdS nanoparticles through electrostatic immobilization at the air-water interface

Synthesis and structural characterization of nanocrystalline aluminium oxide

Nanocrystalline alumina was synthesized by the gas phase condensation using the DC arc plasma technique. Spherical particles ranging in diameter between 20 and 80 nm were obtained. The stoichiometric composition of the powder was investigated by X-ray photoelectron spectroscopy (XPS) and the technique of neutron activation analysis after calcination at different temperatures. Enhanced surface activity in the form of an increase in oxygen content was observed to be maximum in the powder calcined around 800°C. This behaviour is attributed to differences in microchemical stoichiometry, i.e. presence of active centers which are most likely localised in crystallographically defective configurations thereby leading to chemisorptive hydroxyl incorporation at these local sites. This is viewed against the process of microstructural transformation operative at higher temperatures. The stretching modes in the infrared spectrum due to the non-hydrogen bonded hydroxyl groups confirmed the surface activity of this nanocrystalline alumina. Thermogravimetric analysis was used to study the behaviour of nanocrystalline alumina vis-a-vis coarse grain powder, during the process of calcination.

P. Madhu Kumar

Papers

Nanocrystalline TiO2 studied by optical, FTIR and X-ray photoelectron spectroscopy: correlation to presence of surface states

Nanophase alumina synthesis in thermal arc plasma and characterization: correlation to gas-phase studies

On the deposition of Langmuir-Blodgett films of Q-state CdS nanoparticles through electrostatic immobilization at the air-water interface

Synthesis and structural characterization of nanocrystalline aluminium oxide

Enhanced surface activity in nanocrystalline alumina as studied by neutron activation analysis, X-ray photoelectron and infrared spectroscopy