Time-Dependent Behavior of Active Oxygen Species Formed on Photoirradiated TiO2 Films in Air (original) (raw)
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Oxygen release and structural changes in TiO2 films during photocatalytic oxidation
Journal of Applied Physics, 2006
Changes in the crystal structure and grain modifications in titanium oxide ͑TiO 2 ͒ thin films were observed during the photocatalytic oxidation of hydrocarbons. When the hydrocarbon and collodion films were irradiated, single crystalline titanium oxide transformed into polycrystals. The titanium oxide films gradually became network aggregates. These changes were analyzed with a dedicated in situ transmission electron microscope and observed three dimensionally by electron tomography. A detailed analysis of electron energy loss spectra of the samples also revealed that the changes were associated with the loss of oxygen atoms in the TiO 2 crystal lattice. Correlations between the polycrystalline grain size of TiO 2 and its catalyst activity were discussed based on the measured data.
Journal of Physical Chemistry B, 2001
The effect of calcination on photocatalytic activities was studied for highly crystalline TiO 2 powder, which was synthesized by the method of hydrothermal crystallization in organic media (HyCOM). The productions of O 2 •and H 2 O 2 under ambient condition were monitored by means of luminol chemiluminescence probing. The amount of produced O 2 •was decreased with the calcination at lower temperatures in parallel to the decrease of inner Ti 3+ signal observed at 77 K by ESR spectroscopy. The formation route for H 2 O 2 under aerated condition is mainly two-step O 2 reduction in the presence of oxidizable organic molecules such as luminol. Oxidization activity was significantly high when HyCOM-TiO 2 was calcined at higher temperatures, 973-1173 K, as monitored by the amount of oxidized luminol. The observation in ESR measurements and other evidences indicated that the luminol is oxidized via hydroxyl radicals rather than the direct oxidation by photoinduced surface trapped holes.
Role of Lattice Oxygen in Photocatalytic Oxidation on TiO2
Journal of Catalysis, 2000
ABSTRACT In the presence of low-intensity UV lights, lattice oxygen is extracted from the TiO2 surface at room temperature by adsorbed formic acid. Transient photocatalytic oxidation (PCO) and decomposition (PCD) of formic acid to CO2 and H2O were combined with interrupted reaction and temperature-programmed desorption to directly show that lattice oxygen is the oxidizing species during PCD, whereas adsorbed oxygen oxidizes organics during PCO. The rates of lattice oxygen extraction and diffusion of lattice oxygen from the TiO2 bulk to the surface to replenish the extracted oxygen were measured. The initial rate of decomposition (oxidation) of formic acid in the absence of gas-phase O2 is one-seventh the rate in 3% O2. Lattice oxygen vacancies are readily replenished by O2 in the dark at room temperature, but H2O does not re-oxidize them, even during UV exposure. Surface diffusion of formic acid to active sites does not limit photocatalytic reaction. Lattice oxygen extraction causes slow deactivation of TiO2.
The Journal of Physical Chemistry B, 2001
The effect of calcination on photocatalytic activities was studied for highly crystalline TiO 2 powder, which was synthesized by the method of hydrothermal crystallization in organic media (HyCOM). The productions of O 2 •and H 2 O 2 under ambient condition were monitored by means of luminol chemiluminescence probing. The amount of produced O 2 •was decreased with the calcination at lower temperatures in parallel to the decrease of inner Ti 3+ signal observed at 77 K by ESR spectroscopy. The formation route for H 2 O 2 under aerated condition is mainly two-step O 2 reduction in the presence of oxidizable organic molecules such as luminol. Oxidization activity was significantly high when HyCOM-TiO 2 was calcined at higher temperatures, 973-1173 K, as monitored by the amount of oxidized luminol. The observation in ESR measurements and other evidences indicated that the luminol is oxidized via hydroxyl radicals rather than the direct oxidation by photoinduced surface trapped holes.
Photooxidation of Organic Mixtures on Biased TiO2 Films
Environmental Science & Technology, 2001
Processes that occur in the TiO 2-photocatalysis of binary aqueous solutions containing model photolytes with different affinity for the TiO 2 surface (methanol and oxalic and salicylic acids) are analyzed from the photoelectrochemical response of TiO 2 films under bias in a time window of 1-100 s. Long-lived oxidized intermediates produced upon illumination at 0.6 V SCE are detected by cathodic sweep run in the dark after irradiation. The main conclusion derived from this work is that a scheme of competitive kinetics describes only those cases in which one of the components is weakly or nonadsorbed on TiO 2 , whereas for two photolytes with high affinity for the surface cooperative effects may occur. The methanol-oxalate system is quantitatively modeled by considering that oxalate forms surface complexes with different reactivity and a parallel pathway for hole transfer to-OH and adsorbed oxalate. In this case as well as for electrolytes containing methanol and salicylate photooxidation of methanol (with low affinity for the surface) via intermediates formed by reaction with trapped holes (-• OH) is partially or fully suppressed. For electrolytes containing oxalic and salicylic acids in which both components chemisorb on TiO 2 the photoelectrochemical response depends on preadsorption, the photooxidation pathways deviates those of single component systems, and there is remotion of salicylate adsorbed byproducts assigned to cooperative effects.
Photocatalytic reduction of oxygen molecules at the (100) TiO2 anatase surface
Surface Science, 2005
ABSTRACT The adsorption, photoreduction and chemical activity of oxygen molecules on the (1 0 0) anatase surface have been investigated here together with the effects that surface oxygen vacancies (VO) can have on these O2-related processes. We use an original approach by treating molecules on the TiO2 surface like surface defects in the same framework successfully used for defects in semiconductors. The achieved results: (i) give the first theoretical evidence of an acceptor behaviour of an adsorbed O2 molecule, which is at the origin of its photoreduction; (ii) show that the VO donor character is strongly affected by the interaction with O2; and (iii) suggest that the release of O2-1 radicals as well as the formation of O2-related radicals may be favoured by photogenerated electrons in presence of surface VO's.
Surface-Mediated Visible-Light Photo-oxidation on Pure TiO 2 (001)
Journal of the American Chemical Society, 2009
Usually only ultraviolet light must be used for photochemical reactions on TiO 2 because of its bulk band gap (3.0-3.2 eV). However, we used scanning tunneling microscopy (STM) to observe visible light photo-oxidation reactions of formic acid on the new ordered lattice-work structure of a TiO 2 (001) surface for the first time. Two photon photoelectron and electron energy loss spectroscopies and density functional theory calculations revealed that the nanostructured surface makes the band gap significantly smaller than 3.0 eV only at the surface layer and that the surface state of the crystal enables a visible light response. We report the first example of pure TiO 2 that shows visible light photochemical activity at the surface.
The Journal of Physical Chemistry C, 2009
The photocatalytic degradation activity of visible-light-responsive TiO 2 , which was developed as oxygendeficient TiO 2 , was improved by pretreatment with UV-light irradiation in the presence of oxygen and adsorbed water. The C 2 H 2 degradation rate under visible light was increased up to twice by the UV pretreatment. During the UV pretreatment, peroxo species was produced on the surface, and the visible-light absorbance of the photocatalyst between 400 and 500 nm was increased. The modification of the surface accompanied by the formation of peroxo species seems to enhance the photocatalytic oxidation of C 2 H 2. The increased activity was gradually diminished within 24 h under dark condition, suggesting that the modified surface is metastable and is reverted to the original surface.