Trimethyl acetate on TiO2(110): preparation and anaerobic photolysis

Carboxylate Adsorption on Rutile TiO2(100): Role of Coulomb Repulsion, Relaxation, and Steric Hindrance

Understanding the adsorption and photoactivity of acetic acid and trimethyl acetic acid on TiO₂ surfaces is important for improving the performance of photocatalysts and dye-sensitized solar cells. Here we present a structural study of adsorption on rutile TiO₂(100)-1 × 1 and -1 × 3 using Scanning Tunnelling Microscopy and Density Functional Theory calculations. Exposure of both terminations to acetic acid gives rise to a ×2 periodicity in the [001] direction (i.e., along Ti rows), with a majority ordered c(2 × 2) phase in the case of the 1 × 1 termination. The DFT calculations suggest that the preference of c(2 × 2) over the 2 × 1 periodicity found for TiO₂(110)-1 × 1 can be attributed to an increase in interadsorbate Coulomb repulsion. Exposure of TiO₂(100)-1 × 1 and -1 × 3 to trimethyl acetic acid gives rise to largely disordered structures due to steric effects, with quasi-order occurring in small areas and near step edges where these effects are reduced.

The Mechanism of Adsorption, Diffusion, and Photocatalytic Reaction of Organic Molecules on TiO2 Revealed by Means of On-Site Scanning Tunneling Microscopy Observations

The interaction of organic molecules and titanium dioxide (TiO2) plays a crucial role in many industry-oriented applications and an understanding of its mechanism can be helpful for the improvement of catalytic efficiency of TiO2. Scanning tunneling microscopy (STM) has been proved to be a powerful tool in characterizing reaction pathways due to its ability in providing on-site images during the catalytic process. Over the past two decades, many research interests have been focused on the elementary reaction steps, such as adsorption, diffusion, and photocatalytic reaction, occurring between organic molecules and model TiO2 surfaces. This review collects the recent studies where STM was utilized to study the interaction of TiO2 with three classes of representative organic molecules, i.e., alcohols, carboxylic acids, and aromatic compounds. STM can provide direct evidence for the adsorption configuration, diffusion route, and photocatalytic pathway. In addition, the combination of STM with other techniques, including photoemission spectroscopy (PES), temperature programmed desorption (TPD), and density functional theory (DFT), have been discussed for more insights related to organic molecules-TiO2 interaction.

Photochemistry of Carboxylate on TiO2(110) Studied with Synchrotron Radiation Photoelectron Spectroscopy

We present a dedicated synchrotron radiation photoelectron spectroscopy (SR-PES) study of a photochemical reaction on the surface of rutile TiO₂(110). The photoreaction kinetics of carboxylate species (trimethyl acetate, TMA) upon irradiation by UV and soft X-rays were monitored, and we show that it is possible to control the reaction rates from UV light and soft X-rays independently. We directly observe Ti⁴⁺ → Ti³⁺ conversion upon irradiation, attributed to electron trapping at Ti sites close to surface OH groups formed by deprotonation of the parent molecule, trimethylacetic acid (TMAA). TMA photolysis on two surface preparations with different oxygen vacancy densities shows that the vacancy-related charge quenches the amount of charge that can be trapped at hydroxyls upon irradiation. During the initial stages of reaction the correlation between the amount of photodepleted TMA and the amount of charge trapped in the Ti 3d band gap state is nearly 1:1. A first-order kinetics analysis reveals that the reaction rate decreases with decreasing TMA coverage. There is also a coverage-dependent difference in the electronic structure of TMA moieties, primarily involving the carboxyl anchor group. These changes are consistent with a decreased hole affinity of the adsorbed TMA and hence a decreased reaction rate. This discovery adds to the previously presented picture of a reactivity that is inversely proportional to the number of surface hydroxyls, suggesting that the balance between the amounts of TMA, OH, and trapped charge needs to be considered.

Elementary photocatalytic chemistry on TiO2surfaces

In this article, we review the recent advances in the photoreactions of small molecules with model TiO₂surfaces, and propose a photocatalytical model based on nonadiabatic dynamics and ground state surface reactions.

Inhibitive influence of oxygen vacancies for photoactivity on TiO2(110)

Scanning tunneling microscopy results reveal a pronounced site selectivity in the hole-mediated photooxidation of trimethyl acetate (TMA) on TiO2(110), wherein the reaction readily occurs at regular Ti sites but is completely inhibited at oxygen vacancy (VV(O)) defects. Utilizing electron energy loss spectroscopy and density functional theory, we show that the lack of reactivity of TMA groups adsorbed at V(O)'s cannot be attributed to either a less active adsorption conformation or electron transfer from the V(O) defect. Instead, we propose that the excess unpaired electrons associated with the V(O) promptly recombine with photoexcited holes approaching the surface, effectively "screening" TMA species at the V(O) site. We also show that this screening effect is predominately localized at the V(O), only mildly affecting TMA's at adjacent Ti sites.

A theoretical investigation on photocatalytic oxidation on the TiO2 surface

The TiO(2) photocatalytic oxidation mechanism was theoretically investigated by using long-range corrected time-dependent density functional theory (LC-TDDFT) with a cluster model of the anatase TiO(2)(001) surface. We found that LC-TDDFT with the cluster model quantitatively reproduces the photoexcitations of the TiO(2) surface by calculating the electronic spectra of a clean TiO(2) surface and one with oxygen defects. We calculated the electronic spectra of a molecularly adsorbed TiO(2) surface for the adsorptions of phenol, methanol, and methane molecules as typical organic molecules. We obtained the surprising result that the main peak of the phenol-adsorbed TiO(2) surface, which overlaps with the main peak of the clean TiO(2) surface, corresponds to charge transfers from the phenol molecule to the TiO(2) surface. This indicates that the TiO(2) photocatalytic oxidation proceeds through direct charge transfer excitation from the substrate molecules to the TiO(2) surface. In contrast, we found slight and no charge transfer for methanol and methane adsorption, respectively, in agreement with the experimental findings for their reactivities. In light of these results, we propose a new mechanism for heterogeneous TiO(2) photocatalytic oxidations.

Hydrogen reactivity on highly-hydroxylated TiO2(110) surfaces prepared via carboxylic acid adsorption and photolysis

Combined scanning tunneling microscopy, temperature programmed desorption, photo stimulated desorption, and density functional theory studies have probed the formation and reactivity of highly-hydroxylated rutile TiO(2)(110) surfaces, which were prepared via a novel, photochemical route using trimethyl acetic acid (TMAA) dissociative adsorption and subsequent photolysis at 300 K. Deprotonation of TMAA molecules upon adsorption produces both surface bridging hydroxyls (OH(b)) and bidentate trimethyl acetate (TMA) species with a saturation coverage of nearly 0.5 monolayers (ML). Ultra-violet light irradiation selectively removes TMA species, producing a highly-hydroxylated surface with up to ~0.5 ML OH(b) coverage. At high coverages, the OH(b) species typically occupy second-nearest neighbor sites along the bridging oxygen row locally forming linear (2 × 1) structures of different lengths, although the surface is less ordered on a long scale. The annealing of the highly-hydroxylated surface leads to hydroxyl recombination and H(2)O desorption with ~100% yield, thus ruling out the diffusion of H into the bulk that has been suggested in the literature. In agreement with experimental data, theoretical results show that the recombinative H(2)O desorption is preferred over both H bulk diffusion and H(2) desorption processes.

Ethanol photo-oxidation on a rutile TiO2(110) single crystal surface

The reaction of ethanol has been studied on the surface of rutile TiO(2)(110) by Temperature Programmed Desorption (TPD), online mass spectrometry under UV excitation and photoelectron spectroscopy while the adsorption energies of the molecular and dissociative modes of ethanol were computed using the DFT/GGA method. The most stable configuration is the dissociative adsorption in line with experimental results at room temperature. At 0.5 ML coverage the adsorption energy was found equal to 80 kJ mol(-1) for the dissociative mode (ethoxide, CH(3)CH(2)O(a) + H(a)) followed by the molecular mode (67 kJ mol(-1)). The orientation of the ethoxides along the [001] or [110] direction had minor effect on the adsorption energy although affected differently the Ti and O surface atomic positions. TPD after ethanol adsorption at 300 K indicated two main reactions: dehydration to ethylene and dehydrogenation to acetaldehyde. Pre-dosing the surface with ethanol at 300 K followed by exposure to UV resulted in the formation of acetaldehyde and hydrogen. The amount of acetaldehyde could be directly linked to the presence of gas phase O(2) in the vacuum chamber. The order of this photo-catalytic reaction with respect to O(2) was found to be 0.5. Part of acetaldehyde further reacted with O(2) under UV excitation to give surface acetate species. Because the rate of photo-oxidation of acetates (acetic acid) was slower than that of ethoxides (ethanol), the surface ended up by being covered with large amounts of acetates. A reaction mechanism for acetaldehyde, hydrogen and acetate formation under UV excitation is proposed.

Photoreaction of the rutile TiO2(011) single-crystal surface: reaction with acetic acid

The reaction of acetic acid with stoichiometric and reduced rutile TiO(2)(011) single-crystal surfaces has been studied under dark and UV illumination conditions. The surface coverage after the dissociative adsorption of acetic acid with respect to Ti was found to be 0.55. Monitoring XPS Ti, O, and C lines revealed that the surface population decreased incrementally with temperature up to 650 K. The decrease in the slope of both the -CH(3)- and -COO- XPS peaks was not monotonic and followed two slopes in agreement with TPD results. The first channel involves the removal of surface acetates to acetic acid by recombinative desorption, and the second mainly involves dehydration to ketene. UV-light illumination was conducted at 300 K in the absence and presence of molecular oxygen at different pressures: in the 10(-6)-10(-9) Torr range. Acetate species were found to decrease with illumination time, and their decrease is seen to be dependent on the oxygen pressure. Plausible decomposition pathways are presented. Deliberately reducing the surface by electron bombardment prior to the adsorption of acetic acid did not affect the photoreaction rate within the experimental limits.

Epitaxial growth and properties of doped transition metal and complex oxide films

The detailed science and technology of crystalline oxide film growth using vacuum methods is reviewed and discussed with an eye toward gaining fundamental insights into the relationships between growth process and parameters, film and interface structure and composition, and electronic, magnetic and photochemical properties. The topic is approached first from a comparative point of view based on the most widely used growth methods, and then on the basis of specific material systems that have generated very high levels of interest. Emphasis is placed on the wide diversity of structural, electronic, optical and magnetic properties exhibited by oxides, and the fascinating results that this diversity of properties can produce when combined with the degrees of freedom afforded by heteroepitaxy.

STM study of glycine on TiO2(110) single crystal surfaces

The adsorption of glycine (NH2CH2COOH) was examined by scanning tunneling microscopy (STM) on TiO2(110) surfaces at room temperature. A (2x1) ordered overlayer was observed on the TiO2(110)-(1x1) surface. The adsorption of acetic acid and propanoic acid was also investigated on this surface and their STM images were quite similar to that of glycine. Since acetate and propanoate are formed by dissociative adsorption of these acids on TiO2(110), it is proposed that glycine adsorbs in the same way to form a glycinate. The amino group in the glycinate adlayer structurally analogous to those formed from aliphatic carboxylic acids would be extended away from the surface and potentially free to participate in additional reactions. The underlying structure of the TiO2 surface is important in determining the structure of the glycinate adlayer; no ordering of these adsorbates was observed on the TiO2(110)-(1x2) surface.

Thermal and Photochemistry of tert-Butyl Iodide on Rutile TiO2(110)

The thermal and photochemistry of tert-butyl iodide (t-buI) dosed at 100 K on rutile TiO2(110) has been studied using isothermal and temperature programmed desorption mass spectrometry. Nondissociative adsorption and desorption dominate the thermal behavior with dose-dependent t-buI desorption peaks at nominally 220 and 150 K. Ultraviolet photochemistry occurs readily, but the behavior of submonolayers and multilayers differ qualitatively. Ejection of t-buI and i-C4H8 dominate during submonolayer photolysis at 100 K. Multilayer photolysis results are also dominated by ejection during irradiation, but the t-buI component is strongly suppressed, and the maximum rates of i-C4H8 and HI ejection did not occur at the outset. A mechanistic model capturing the observations involves both direct and substrate-mediated electronic excitation of t-buI. According to this model, ejection of t-buI occurs only from transient substrate-mediated formation of anionic t-buI. For either excitation path, the C-I dissociation probability is significant, and the excited tert-butyl product rearranges readily to form i-C4H8 that is ejected. For any local region where there is multilayer coverage, products formed at the adsorbate-vacuum interface are ejected promptly, but products formed within the multilayer are trapped. Thus, ejection of t-buI is suppressed, and trapped primary photodissociation products, tert-butyl and I, react to either reform t-buI or rearrange to i-C4H8 and HI. The latter two products remain trapped and are subsequently induced to desorb by acquisition of momentum from collisions with subsequently formed translationally excited photodissociation products.