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.

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Titanium dioxide-based nanomaterials for photocatalytic fuel generations.

Electrical and optical spectroscopic studies of TiO2 anatase thin filmsdeposited by sputtering show that the metastable phase anatase differs in electronic properties from the well‐known, stable phase rutile. Resistivity and Hall‐effect measurements reveal an insulator–metal transition in a donor band in anatase thin films with high donor concentrations. Such a transition is not observed in rutile thin films with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self‐trapped excitons is observed in anatase thin films, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin films have a wider optical absorption gap than rutile thin films.

Electrical and optical properties of TiO2 anatase thin films

The anatase modification of TiO2 shows properties quite different from those of the extensively studied and used rutile phase. We have investigated the chemi-resistive behavior of anatase TiO2 thin films exposed to oxidizing and reducing gases (O2, H2 and ethanol) in the temperature range between 300 and 400 °C. The undoped anatase films demonstrate significant and rapid response to ethanol as well as an appreciable response to hydrogen. The high electron mobility and the low density of trapping or compensating centers are considered to have contributed to this good response. Doping the anatase thin films with Sr, Ce, Y and Tb makes then insensitive to hydrogen, but still sensitive to ethanol. The modification of the sensitivity and its dynamics by dopants have been measured and analyzed.

TiO2 anatase thin films as gas sensors

Amorphous Nb2O5 thin films of three different thicknesses (10, 100, 400 nm) were deposited onto SiO2/Si substrates by reactive sputtering in an Ar–O2 plasma. Thermal treatments were performed at different temperatures between 500 and 1100 °C. The structural and morphological evolution with temperature is shown to be dependent on the film thickness. At 600 °C, the films essentially crystallize in the TT phase. On the thickest films, the T phase also appears. Annealing at higher temperature progressively increases the concentration of the T phase. The films show large flat grains extending over the whole film thickness. In addition, a large number of polyhedral bubbles is present in the 100 and 400 nm films due to Ar atoms trapped during sputtering. After annealing at 1100 °C the Ar bubbles are no longer present and partial diffusion of the films into the substrate is observed. The modification at high temperature, explained either by the M or the H phase, is favored on the thickest films and leads to plate...

Structural and morphological characterization of Nb2O5 thin films deposited by reactive sputtering

The electronic structure of the hexagonal β‐Nb2N, hexagonal δ′-NbN, and cubic δ-NbN thin films have been investigated by x-ray photoemission spectroscopy. Calculated density of states for these phases are also presented and used in the interpretation of the experimental valence band spectra. The experimental energy distribution curves are consistent with the calculated densities of states. Comparing hexagonal to cubic phases, both β‐Nb2N and δ′-NbN are more covalent than the cubic phase NbN. Prominent covalent bonding in hexagonal β‐Nb2N and δ′-NbN nitrides can be related with the higher hardness values as compared to that of the cubic phase. This study discloses relevant observations important for both the technological applications and fundamental properties.

Electronic structure of Nb2N and NbN thin films

Cr1−xMoxNy thin films were deposited on silicon by radio-frequency reactive magnetron sputtering. The phase, texture, and residual stress were determined by x-ray diffraction analysis. All the films crystallize in the face-centered-cubic phase. The chemical composition was measured by electron probe microanalysis. Scanning tunneling microscopy revealed a finely grained surface morphology. Scanning electron microscopy showed a columnar crystal arrangement in every film of this study. The binary compounds Mo2N and CrN exhibit grain sizes of about 5 nm, whereas the ternary compounds show grain sizes up to 23 nm. The electronic structure was analyzed using x-ray photoelectron spectroscopy (XPS). The valence band N 2s and metal d peak separations show a minimum for films with about equal content of chromium and molybdenum. Hardness values, obtained by nanoindentation, exhibit also a pronounced minimum for x=0.4. The similarity between the trends of the XPS peak separation and hardness suggests that the differe...

R. Sanjinés

Papers

Electrical and optical properties of TiO2 anatase thin films

TiO2 anatase thin films as gas sensors

Structural and morphological characterization of Nb2O5 thin films deposited by reactive sputtering

Electronic structure of Nb2N and NbN thin films

Electronic structure and mechanical properties of resistant coatings: The chromium molybdenum nitride system