Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

TiO2 photocatalysis and related surface phenomena

Jenny Schneider,*,† Masaya Matsuoka,‡ Masato Takeuchi,‡ Jinlong Zhang, Yu Horiuchi,‡ Masakazu Anpo,‡ and Detlef W. Bahnemann*,† †Institut fur Technische Chemie, Leibniz Universitaẗ Hannover, Callinstrasse 3, D-30167 Hannover, Germany ‡Faculty of Engineering, Osaka Prefecture University, 1 Gakuen-cho, Sakai Osaka 599-8531, Japan Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

Understanding TiO2 photocatalysis: mechanisms and materials.

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

A novel and simple method for preparing highly photoactive nanocrystalline F--doped TiO2 photocatalyst with anatase and brookite phase was developed by hydrolysis of titanium tetraisopropoxide in a mixed NH4F−H2O solution. The prepared F--doped TiO2 powders were characterized by differential thermal analysis-thermogravimetry (DTA-TG), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV−vis absorption spectroscopy, photoluminescence spectra (PL), transmission electron microscopy (TEM), and BET surface areas. The photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air. The results showed that the crystallinity of anatase was improved upon F- doping. Moreover, fluoride ions not only suppressed the formation of brookite phase but also prevented phase transition of anatase to rutile. The F--doped TiO2 samples exhibited stronger absorption in the UV−visible range with a red shift in the band gap transition. The photocatalytic activity of F--doped TiO2 powders prep...

Effects of F- Doping on the Photocatalytic Activity and Microstructures of Nanocrystalline TiO2 Powders

We detect transient and persistent diffuse reflectance infrared signals from surface intermediates generated by band gap irradiation of polycrystalline, hydroxylated TiO_2 under controlled atmospheres. Irradiation in vacuo or in the presence of CD_3OD(ads) leads to a new absorbance at 3716 cm^(-1), the decline of a 3647 cm^(-1) band, and a reflectivity loss. Overall reflectivity, which gauges mobile charge, partially recovers in the dark after a few minutes but fully recovers upon exposure to O_2. The 3716 cm^(-1) feature persists for days in a vacuum or dry O_2 but is bleached by the action of (O_2 + H_2O + hν). It is assigned to a Ti(III)O−H^- vibration resulting from electron trapping at acidic Ti(IV)OH centers. Irradiation under O_2 yields a new band at 3683 cm^(-1), ascribed to surface-bound OH radicals that last indefinitely in a vacuum. None of the above events occur on thermally generated TiO_(2-γ) specimens subsequently exposed to H_2O(g). We infer that (1) O_2 is an efficient scavenger of conduction band electrons at the gas−solid interface, (2) the competition for holes between lattice oxygen photodesorption and OH^- oxidation depends on gas pressure and composition, and (3) the trapped carrier buildup eventually results in extended surface reconstruction involving Ti−OH functionalities, giving rise to restructuring hysteresis.

Infrared Spectra of Photoinduced Species on Hydroxylated Titania Surfaces

Free and trapped charge carriers in polycrystalline TiO2 following band gap irradiation are characterized by diffuse reflectance IR spectroscopy (DRIFTS). A spectrum-wide absorption signal proportional to λ1.7 (λ = wavelength/μm) indicates the presence of free conduction band electrons coupled with acoustic phonons in the lattice. Free electrons appear to decay according to saturation kinetics. The fitted parameters indicate a limited number of trapping states. The concentration of these states appears to be diminished by sequential UV treatments. The free carrier decay lifetime is lengthened as the samples are dehydrated, which suggests an excited-state relaxation event during electron trapping. Photogenerated free electrons are comparable to conduction band electrons injected from surface-bound chromophores, and the lifetime of these electrons can be extended across several orders of magnitude. A broad IR absorption peak centered at 3380 cm-1 is attributed to an electronic transition from an occupied su...

Slow Surface Charge Trapping Kinetics on Irradiated TiO2

Electric fields generated by photoexcited charge carriers in TiO_2 (anatase) produce Stark effect intensity and wavelength shifts for surface TiO−H stretching vibrations Deep electron-trapping states affect a single type of TiO−H stretch (Ti(IV)O−H at 3647 cm^(-1) ⇒ Ti(III)O−H at 3716 cm^(-1)) Shallow electron-trapping states, observed as broad absorption bands above 3000 cm^(-1), produce an apparently homogeneous electric field Intensity changes and corresponding wavelength shifts for ν(TiO−H) are proportional to the magnitude and polarity of the electric field O_2 is shown to reversibly abstract electrons from shallow trapping states The results suggest that shallow electron traps are not associated with localized structures, but rather are delocalized across the TiO_2 surface This behavior may explain the photon-dependent wettability of TiO_2 surfaces

Electron Traps and the Stark Effect on Hydroxylated Titania Photocatalysts

The adsorption and degradation of the nerve agent simulant dimethyl methylphosphonate (DMMP) over UV-irradiated TiO2 powders and thin films has been investigated. Adsorption of vapor-phase DMMP on TiO_2 powder is characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Photochemically assisted oxidation of adsorbed DMMP is carried out in situ by irradiation of samples in the DRIFTS accessory, giving kinetic data and information on specific site binding of DMMP and catalyst poisoning. Gas-phase intermediates from a static vapor phase reaction are identified by gas chromatography−mass spectrometry analysis, and surface-bound intermediates and products are analyzed by high-performance liquid chromatography−mass spectrometry, and ion chromatography of both aqueous and organic extractions from the TiO_2. Adsorbed DMMP is photodegraded in a stepwise fashion to give methylphosphonic acid, PO_4^(3-), H_2O, and CO_2 as products. A proposed reaction pathway is consistent with a rapid degradation of DMMP but with extensive poisoning of the catalyst by surface-bound phosphonate products.

Steven H. Szczepankiewicz

Papers

Infrared Spectra of Photoinduced Species on Hydroxylated Titania Surfaces

Slow Surface Charge Trapping Kinetics on Irradiated TiO2

Electron Traps and the Stark Effect on Hydroxylated Titania Photocatalysts

Adsorption and Photodegradation of Dimethyl Methylphosphonate Vapor at TiO2 Surfaces