A comparative study of the adsorption and photoinduced degradation (PID) of acetone and acetic acid on thin films of anatase, brookite, and rutile TiO 2 nanoparticles is presented. The materials were thoroughly characterized by a wide range of methods, including X-ray diffraction, transmission electron microscopy, and Raman and UV-vis spectroscopies. In situ FTIR transmission spectroscopy was used to follow adsorption and PID reactions. Molecular adsorption of acetone and acetic acid is observed on anatase and brookite, whereas significant dissociation occurs on rutile. It is inferred that adsorbate-surface interaction increases in the order anatase < brookite < rutile, favoring formation of bridge-bonded species on rutile (acetate and formate). Illumination with simulated solar light readily dissociates acetic acid and acetone on all TiO 2 samples and produces polymorph-specific intermediate surface species, including acetate, formate, carbonate, and water. PID of surface coordinated acetate is rate determining for complete mineralization of acetic acid and prevents further photooxidation on rutile. On anatase and brookite surfaces, acetate formation is suppressed upon photooxidation of acetone, whereas on rutile, acetate readily forms. On anatase, intermediate species form, which are not observed on either brookite or rutile, suggesting different reaction pathways for the different TiO 2 polymorphs. Accurate quantum yield measurements were performed. The quantum yield for PID of acetone is larger for brookite than for anatase and much larger than for rutile. In contrast, the quantum yield for PID of acetate is lower for brookite than for anatase, whereas PID of acetate does not occur on rutile under our experimental conditions. The results are discussed in terms of a balance of strong adsorbate-surface interactions, moderate bonding of intermediate PID surface species, and efficient surface-adsorbate charge transfer of photogenerated electrons and holes.

Adsorption and solar light decomposition of acetone was studied on nanostructured anatase TiO2 and Nb-doped TiO2 films made by sol-gel methods (10 and 20 mol % NbO2.5). A detailed characterization of the film materials show that films contain only nanoparticles with the anatase modification with pentavalent Nb oxide dissolved into the anatase structure, which is interpreted as formation of substituted Nb=O clusters in the anatase lattice. The Nb-doped films displayed a slight yellow color and an enhanced the visible light absorption with a red-shift of the optical absorption edge from 394 nm for the pure TiO2 film to 411 nm for 20 mol % NbO2.5. In-situ Fourier transform infrared (FTIR) transmission spectroscopy shows that acetone adsorbs associatively with eta1-coordination to the surface cations on all films. On Nb-doped TiO2 films, the carbonyl bonding to the surface is stabilized, which is evidenced by a lowering of the nu(C=O) frequency by about 20 cm(-1) to 1672 cm(-1). Upon solar light illumination acetone is readily decomposed on TiO2, and stable surface coordinated intermediates are formed. The decomposition rate is an order of magnitude smaller on the Nb-doped films despite an enhanced visible light absorption in these materials. The quantum yield is determined to be 0.053, 0.004 and 0.002 for the pure, 10% Nb:TiO2, and 20%Nb:TiO2, respectively. Using an interplay between FTIR and DFT calculations we show that the key surface intermediates are bidentate bridged formate and carbonate, and H-bonded bicarbonate, respectively, whose concentration on the surface can be correlated with their heats of formation and bond strength to coordinatively unsaturated surface Ti and Nb atoms at the surface. The oxidation rate of these intermediates is substantially slower than the initial acetone decomposition rate, and limits the total oxidation rate at t>7 min on TiO2, while no decrease of the rate is observed on the Nb-doped films. The rate of degradation of key surface intermediates is different on pure TiO2 and Nb-doped TiO2, but cannot explain the overall lower total oxidation rate for the Nb-doped films. Instead the inferior photocatalytic activity in Nb-doped TiO2 is attributed to an enhanced electron-hole pair recombination rate due to Nb=O cluster and cation vacancy formation.

Adsorption of formic acid on rutile TiO2 (110) revisited: An infrared reflection-absorption spectroscopy and density functional theory study. Journal of Chemical Physics AbstractFormic acid (HCOOH) adsorption on rutile TiO 2 (110) has been studied by s-and p-polarized infrared reflection-absorption spectroscopy (IRRAS) and spin-polarized density functional theory together with Hubbard U contributions (DFT+U) calculations. To compare with IRRAS spectra, the results from the DFT+U calculations were used to simulate IR spectra by employing a three-layer model, where the adsorbate layer was modelled using Lorentz oscillators with calculated dielectric constants. To account for the experimental observations, four possible formate adsorption geometries were calculated, describing both the perfect (110) surface, and surfaces with defects; either O vacancies or hydroxyls. The binding energy was found to be E bind = 1.84 eV for the bridging bidentate formate species, which bonds with its IRRAS spectra measured on surfaces prepared to be either reduced, stoichiometric, or to contain surplus O adatoms, were found to be very similar. By comparisons with computed spectra, it is concluded that formate binds to in-plane Ti 5c atoms rather than to O vacancy sites. The results emphasize the importance of protonation and reactive surface hydroxylseven under UHV conditions -as reactive sites in e.g. catalytic applications.

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The photoelectrochemical properties of tungsten oxide thin films with different stoichiometry (WO 3-x ) and thickness were investigated. The films were sputtered in O 2 /Ar gas (ratio 0.43) on glass substrates coated with fluorine doped tin dioxide at two sputter pressures, P tot = 10 and 30 mTorr, yielding O/W ratios of the films, averaged over three samples, of 2.995 and 2.999 (x~0.005 and x~0.001), respectively. The films were characterized by x-ray diffraction, scanning electron microscopy and spectrophotometry. The 10 mTorr samples showed large absorption in the near infrared (NIR) range, whereas the 30 mTorr samples had a small absorption in this region. The concentration of oxygen vacancy band gap states was estimated from cyclic voltammetry, and was found to correlate with the optical absorption in the NIR region. The incident photon to current efficiency for illumination from the electrolyte side 1

We present a demonstration of online monitoring of gas-phase photocatalytic reactions. A cotton cloth impregnated with commercial titanium dioxide nanoparticles is used as a photocatalytic filter to clean air contaminated with a model pollutant. A fan forces air through the filter while it is irradiated by UV diodes. The concentration of the air pollutant is measured online by an inexpensive, commercially available semiconductor air quality sensor. The structural parts of the reactor were 3D printed in polylactide bioplastic. We provide all schematics, 3D printed model parts, hardware, firmware, and computer code of the reactor and control units. The device can be used for interactive learning of both gas phase photocatalysis and gas sensing, as well as in student laboratory classes for measuring air pollutants and their photodegradation. The experimental setup can also form the basis for a project work for chemical engineering university students, and it can be employed as a building block for development of other gas phase chemical reaction demonstrations.

Modern buildings tend to be ''energy guzzlers'' and have indoor environments with unhealthy air. Glazing with TiO 2 /VO 2 bilayer coatings (1) exhibits enhanced photocatalytic air purification compared with single-layer TiO 2 due to heating from the underlying infrared-absorbing VO 2 film, (2) is thermochromic thanks to the VO 2 and admits less solar energy inflow when there is a cooling demand, and (3) has significantly improved luminous transmittance as a result of antireflection due to TiO 2. These coatings were deposited by reactive DC magnetron sputtering onto heated glass; they were nanocrystalline, and the anatase phase prevailed in TiO 2. The VO 2 coatings showed well-developed thermochromism. The photocatalytic degradation rate of stearic acid was almost doubled for the TiO 2 /VO 2 bilayer film compared with that for single-layer TiO 2. Our results demonstrate an important, and hitherto unexplored, synergy between photocatalysis, thermochromism, and high luminous transmittance which exploits spectral-selective material properties for solar illumination.

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