The possibility of controlling the photocatalytic activity of TiO2 nanoparticles by tailoring their crystalline structure and morphology is a current topic of great interest. In this study, a broad variety of well-faceted particles with different phase compositions, sizes, and shapes have been obtained from concentrated TiOCl2 solutions by systematically changing temperature, pH, and duration of the hydrothermal treatment. The guide to select the suitable experimental conditions was provided by thermodynamic modeling based on available thermochemical data. By combining the results of TEM, HRTEM, XRD, density, and specific surface area measurements, a complete structural and morphological characterization of the particles was performed. Correlation between the photocatalytic activity in the UV photodegradation of phenol solutions and the particle size was established. Prismatic rutile particles with length/width ratio around 5 and breadth of 60-100 nm showed the highest activity. The surface chemistry of the particles was also investigated. Treatments that decrease the surface acidity, such as washing the powders with ammonia solution and/or calcining at 400 degrees C, have detrimental effect on photocatalytic activity. The overall results suggest correlation between particle morphology and photocatalytic activity and indicate that both electron-hole recombination and adsorption at the surface can be rate-controlling processes. The systematic approach presented in this study demonstrates that a substantial improvement of the photocatalytic activity of TiO2 can be achieved by a careful design of the particle morphology and the control of the surface chemistry.
The sol-gel synthesis of TiO 2 from TiCl 4 assisted by the triblock copolymer EO 20 -PO 70 -EO 20 (EO ) -CH 2 CH 2 O-, PO ) -CH 2 (CH 3 )CHO-) as templating agent was carried out by systematically changing H 2 O:Ti (r w ) and HCl:Ti (r a ) molar ratios. Mesoporous and nanocrystalline TiO 2 samples with well-defined and controlled phase composition (anatase, rutile, and mixed phase) were obtained after calcination at 400 °C and characterized for the morphology, particle size, and shape using TEM, HRTEM, XRD, and surface area measurements. The role of r w and r a and influence of the copolymer in determining the phase composition was demonstrated. Rutile becomes the main phase by increasing r w . In fact, the decrease of Ti concentration slows down the condensation rate, favoring formation of rutile seeds in the gel. The photocatalytic activity of TiO 2 in the UV photomineralization of phenol depends on the phase composition and oxidizing agent, H 2 O 2 or O 2 . When the oxidation is performed by H 2 O 2 , rutile, formed by large crystalline rods with high aspect ratios (size 15-20 × 100-120 nm), shows higher catalytic activity with respect to the small, almost cubic, anatase particles (5-15 nm). If O 2 is used, the catalytic activity generally decreases and the behavior of polymorphous species is reverse. EPR investigation of the paramagnetic charge carriers, formed under UV irradiation at 10 K, showed the resonance lines of holes trapped at Olattice sites and electrons trapped at Ti 3+ and O 2 sites. The rutile crystalline rods present the largest quantity of Oand Ti 3+ centers. The overall results suggest correlation between TiO 2 particle size and shape and the photocatalytic activity and indicate that electron-hole recombination is the most probable rate-controlling process.
Nanostructured (3−6 nm) thin films (80 nm) of SnO2 and Pt-doped SnO2 were obtained
by a new sol−gel route using tetra(tert-butoxy)tin(IV) and bis(acetylacetonato)platinum(II)
as metal precursors. The results of glancing incidence X-ray diffraction (GIXRD) and X-ray
photoelectron spectroscopy (XPS) investigations demonstrated that platinum substituted
for tin(IV) in the cassiterite structure. Electron paramagnetic resonance (EPR) and XPS
analyses showed that singly ionized paramagnetic oxygen vacancies (VO
•) were formed on
pure SnO2 thin films by interaction with CO atmosphere; in Pt-doped SnO2 films, the same
defects VO
• fully transferred their electrons to the noble metal so that Pt(IV) became Pt(II)
and Pt(0). Such samples successively exposed to air, at room temperature, reduced O2 to
O2
-. The behavior was well-detected by EPR measurements, which showed on thin films
the presence of Sn(IV)−O2
- species. The surface reactivity agrees with the results of the
electrical measurements.
Ce-doped borosilicate (BSG), phosphosilicate (PSG), and borophosphosilicate (BPSG) glasses (B:P:Si molar ratios 8:0:92, 0:8:92, and 8:8:84; Ce:Si molar ratio 1 x 10(-)(4) to 1 x 10(-)(2)) were prepared by the sol-gel method. High-resolution transmission electron microscopy (HRTEM), (31)P, (29)Si, and (11)B magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), and UV-vis absorption investigations demonstrated that, in PSG and BPSG, Ce(3+) ions interact with phosphoryl, [O=PO(3/2)], metaphosphate, [O=PO(2/ 2)O](-), and pyrophosphate, [O=PO(1/2)O(2)](2)(-), groups, linked to a silica network. This inhibits both CeO(2) segregation and oxidation of isolated Ce(3+) ions to Ce(4+), up to Ce:Si = 5 x 10(-)(3). In BSG, neither trigonal [BO(3/2)] nor tetrahedral [BO(4/2)](-) boron units coordinate cerium; thus, Ce(3+) oxidation occurs even at Ce:Si = 1 x 10(-)(4), as in pure silica glass (SG). The homogeneous rare-earth dispersion in the host matrix and the stabilization of the Ce(3+) oxidation state enhanced the intensity of the photoluminescence emission in PSG and BPSG with respect to BSG and SG. The energy of the Ce(3+) emission band in PSG and BPSG matrixes agrees with the phosphate environment of the rare earth.
Abstraer. Paramagnetic singly ionized oxygen vacancies V(; and chemisorbed Sn4+-O 2 species were detected by electron paramagnetic resonance measurements on SnO 2 and transition metal (Pt, Ru)-doped SnO 2 thin film that had been reduced with CO at different temperatures and then brought into contact with oxygen. The amounts of the two paramagnetic species were evaluated and are discussed asa function of the film annealing temperature in air, the reduction temperature under CO, and the type and concentration of the doping transition element. A1so the structural properties of the film were identified through glancing incidence X-ray diffraction analysis. Measurements of the electrical sensitivity S (S = RJRco, where Ra~ , and Rco are the resistance under air and under CO(800 ppm)/air respectively) show that the trend of the sensitivity values vs. the reduction temperature with CO could be predicted by the parallel trend of the number of Sn4+-O~ centers.
The thermal evolution of sol−gel SnO2-based thin films was explored by investigating
their structural and morphological features. Nanostructured SnO2 and Pt-doped SnO2 layers
were obtained using tetra(tert-butoxy)tin(IV) and Pt(II) acetlylacetonate as precursors. Films
were prepared by spin coating from ethanol solutions with different viscosity. After drying
at room temperature, they were annealed in air at 673 and 973 K. The surface morphology
was analyzed by scanning electron microscopy, atomic force microscopy, and scanning near-field optical microscopy. The structural characterization was performed by means of glancing
incidence X-ray diffraction and microdiffraction. Both drying at room temperature and
thermal treatment at 673 K resulted in the formation of holes on the surface and inside the
films. Their distribution and average dimension were found to depend mainly on the viscosity
of the sol precursor, and on the presence of Pt in the films. After annealing at 973 K, surface
segregation of PtO
x
phases and partial filling of the surface holes occurred. The effects of
morphology on the electrical transport properties are discussed on the basis of sensitivity,
S, measurements (S = R
air/R
CO, where R
air and R
CO stand for the resistance in air and CO/air, respectively).
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