AVS2011 Session SS-WeA: Adsorption & Reactions on Oxide Surfaces

The chemistry of oxygen on TiO₂ surfaces is an important component in many catalytic and photocatalytic processes, such as water splitting and waste remediation, and has been extensively studied. So far, the majority of fundamental research has been carried out on the model transition-metal oxide surface of the rutile TiO₂(110). The investigation of molecular adsorption of O₂ can be considered as a natural first step providing information about possible O₂ surface chemistry on TiO₂(110). Both experiment and theory have demonstrated that O₂ dissociatively adsorbs at bridging oxygen vacancies (V_O) sites and five-fold coordinated terminal titanium atoms (Ti_5c) at elevated temperatures. At sufficiently low temperatures, the majority of the ensemble-averaging technique studies suggested that O₂ molecularly chemisorbs at V_O sites on reduced surfaces (at T < 150 K). However, recent STM studies reported a contradict result that the O₂ dissociation at V_O sites has been observed at temperatures as low as ~ 110 K.

In this work, we investigated the initial stages of oxygen adsorption on reduced TiO₂(110) with high-resolution scanning tunneling microscopy (STM) at 50 K. Molecularly chemisorbed O₂ species, not directly observed until now on TiO₂(110), have been imaged at two distinctive adsorption sites (V_O and Ti_5c) using “extremely mild” tunneling conditions. While O₂ species at Ti_5c site appears as a single protrusion centered on the Ti_5c row, the O₂ at V_O manifests itself by a disappearance of the V_O feature. The dissociation of chemisorbed O₂ can be readily induced by tunneling conditions that are normally used for TiO₂(110) imaging, and the dissociation details strongly depend on the scanning parameters and the type of the O₂ adsorption site. The O₂ molecules chemisorbed at low temperatures at these two distinct sites are the most likely precursors for the two O₂ dissociation channels, observed at temperatures above 150 and 230 K at the V_O and Ti_5c sites, respectively. In general, our results provide a molecular level insight into the thermal chemistry of O₂ on reduced TiO₂, and assist in understanding of the surface reactivity of transition-metal oxides.

[1] M. C. Xu, Y. K. Gao, E. M. Moreno, M. Kunst, M. Muhler, Y. M. Wang, H. Idriss, C. Wöll, Phys. Rev. Lett. 2011, 106, 138302.

[2] Y. M. Wang, A. Glenz, M. Muhler, C. Wöll, Rev. Sci. Instrum. 2009, 80, 113108.

[3] P. Rahe, M. Nimmrich, A. Nefedov, M. Naboka, C. Wöll, A. Kühnle, Journal of Physical Chemistry C 2009, 113, 17471.

Using temperature programmed desorption (TPD), scanning tunneling microscopy (STM) and ultraviolet photoemission spectroscopy (UPS), we have observed very different adsorption dynamics for acetic acid on rutile TiO₂(110) and (011)-2×1 surfaces at room temperature. While the bidentate adsorption of carboxylic acids on the (110) surface is well-established, we find a monodentate adsorption on the (011)-2×1 surface as the most likely adsorption geometry. On the (011)-2×1 surface, the initial sticking of adsorbed acetic acid is low. It appears that initial adsorption occurs at defects. These adsorbed acetates then act as nucleation sites for further adsorption. This adsorption mechanism results in the formation of quasi-1D acetate clusters running along direction. The role of acetate adsorption in the formation or annihilation of excess charges in TiO₂ is also found to be different on these two surfaces. We find that bidentate adsorption of acetate on the (110) surface results in extraction of excess charges from the substrate, while mono-dentate adsorption on the (011)-2×1 surface causes net-charge donation to the substrate. More interestingly, a difference in the binding energy of excess charges, or Ti-3d band gap states, has been observed. On the TiO₂(011)-2×1 surface the binding energy is ~0.3 eV higher than on the (110) surface. This difference is explained by the different crystal fields on the reconstructed (011) surface compared to the bulk-truncated (110) surface. At the rutile TiO₂(011)-2×1 surface, Ti-ions are located in a distorted square pyramidal coordination environment, which we propose causes the shift in binding energy of excess electrons at the Ti-site. The differences in binding energy of electrons trapped at the surface for the two surfaces may contribute to the face dependent photocatalytic activity of rutile TiO₂.

References:

1. J. Tao, T. Luttrell, J. Bylsma, and M. Batzill, J. Phys. Chem. C 2011, 115, 3434

2. J. Tao and M. Batzill, J. Phys. Chem. Lett. 2010, 1, 3200.

ZnTPP derivatives are attractive candidates for photoinduced electron-transfer mediators in dye sensitized solar cells (DSSCs). Many fundamental properties of the dye/metal oxide interface are not known and need careful consideration. In particular, the influence on solar cells efficiency, of the energy alignment and of the molecular packing at the surface, remains unclear. In this work, using x-ray, UV and inverse photoemission spectroscopies in conjunction with density functional theory (DFT) calculations, we have determined the energy alignment of molecular levels with respect to the substrate band edges for several ZnTPP derivatives adsorbed on ZnO(11-20) and TiO₂(110) surfaces. The ZnTPP derivatives were functionalized with COOH anchoring groups, to allow a priori either upright or flat adsorption on the surfaces. While the energy alignment, a critical parameter to allow charge separation at the dye/semiconductor interface, is found similar for all of these systems, large differences in solar cells efficiencies are observed. We have thus explored the adsorption geometry of the same ZnTPPs at the surface of ZnO and TiO₂ using UV-visible absorption and NEXAFS spectroscopies and scanning tunnel microscopy. It is found that that dye/dye interactions is an important factor, for electron transfer to the substrate. For ZnTPPs, upright adsorption opens deleterious exciton delocalization pathways, due to dipole/dipole interactions competing with electron transfer to the substrate. Choosing the adsorption geometry is thus critical for the electronic pathway control.

The use of nanoparticles in energy, environmental and medical applications has been growing significantly in recent years. In most of these applications, the nanoparticles are being used in as-synthesized form and/or functionalized through ligand conjugation. When particle size decreases to nanometer scale, a large percentage of the atoms are at or near the surface which makes the surface highly dynamic and reactive in nature. Consequently, these particles exhibit unique properties that make their characterization more difficult by conventional spectroscopic methods. Furthermore, knowledge on how the ligand molecules bind to the surface of nanoparticles is very limited. To better understand the interactions between ligand molecules and the surface of nanoparticles, we used a model system approach to study the interaction between the carboxylate anchoring group from trimethylacetic acid (TMAA) and CeO₂(111) surfaces as a function of oxygen stoichiometry. The epitaxial CeO₂(111) thin films 50nm in thickness were grown on YSZ(111) by oxygen plasma-assisted molecular beam epitaxy at 650°C under 2.5x10^-5 Torr of oxygen plasma. The sample films from MBE system were transferred to X-ray photoelectron spectroscopy (XPS) system and sputter cleaned to remove any surface contamination during the transfer. Following sputtering, stoichiometric CeO₂(111) surface was obtained by annealing the thin film under 2.0x10^-5 Torr of oxygen at ~550°C for 30 min. In order to reduce the CeO₂(111) surface, the thin film was annealed in ~5.0x10^-10 Torr vacuum at 550°C, 650°C, 750°C and 850°C for 30 min to progressively increase the oxygen defect concentration on the surface. XPS was used to characterize these surfaces prior to and following dissociative adsorption of TMAA on these surfaces using a molecular doser. The saturated TMAA coverage and the oxygen defect concentration were determined from XPS elemental composition. The saturated TMAA coverage on CeO₂(111) surface is found to increase with increasing oxygen defect concentration. This is attributed to increase in under coordinated cerium sites on the surface with increase in the oxygen defect concentrations. In parallel, we studied the interactions of TMAA adsorbed at various sites on the stoichiometric CeO₂(111) surface using periodic density functional theory (DFT) calculations. Both energetics and electronic properties of the surface and TMAA will be presented and correlated with experimental observations.

Cu and Cu-oxide nanoclusters supported on ZnO are prototypical catalysts for the electrochemical reduction of CO and CO₂ to methanol. In this report we describe the interaction of CO and CO₂ with Cu oxide nanoclusters on ZnO(1010) with a combination of surface sensitive tools including STM for structural information, EELS for electronic and vibrational studies as well as synchrotron-based photoemission for electronic properties. Cu is deposited onto ZnO and oxidized with a combination of O-exposure and annealing procedures to result in two distinct Cu-oxide (CuI and CuII) clusters, which preferentially nucleate and grow at step edges. Photoemission shows a large charge transfer between the oxide cluster and the substrate surface as well as significant band bending. It is believed that the CO₂ adsorption, forming a carbonate species, and consequent reduction, is coupled to the induced defects and electronic perturbation of the CuO_x/ZnO nanoclusters, absent in the case of Cu/ZnO nanoclusters. In addition to vibrational EELS and TDS to characterize the adsorption of the CO and CO₂ adsorption species, similar results from Au on ZnO(1010), which shows a lack of cluster formation growth, will be compared and contrasted.

This material is based upon work supported as part of the Center for Atomic Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001058

Chemically selective sensors are required for detection of chemical warfare agents with ever increasing demands on the selectivity, sensitivity, lifetime, speed, and reduced power consumption of these devices. Strategies for reducing the scale of these sensors have been explored to produce microfabricated Nitrogen-Phosphorus Detectors (NPDs) to accommodate these many requirements. The device incorporates sol-gel derived alkali metal silicate thin films on low thermal mass silicon substrates for field portable gas chromatography applications. In spite of the long history of NPDs, the details of the chemically-mediated emission related to their selectivity are not well understood. The NPD signal current ultimately depends on the transfer of electrons across the surface potential barrier of the thermionic cathode emitter. Two classes of competing mechanisms have been described in the literature to account for the chemically-selective ionization observed in NPDs: (a) gas-phase ionization models and (b) surface mediated electron emission. The latter mechanism has been the focus of our measurements of the surface work function of candidate emitter materials as a function of composition, structure, temperature, and ambient atmosphere. Specifically, both the local work function variations by scanning probe measurements and effective average work function by measuring total emission will be discussed.