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

Wednesday, November 2, 2011 2:00 PM in 107

Wednesday Afternoon

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2:00 PM SS-WeA-1 Direct Observation of O2 Molecular Chemisorption at Two Distinctive Sites of TiO2(110)
Zhi-Tao Wang, Yingge Du, Zdenek Dohnálek, Igor Lyubinetsky (Pacific Northwest National Laboratory)

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 (VO) sites and five-fold coordinated terminal titanium atoms (Ti5c) at elevated temperatures. At sufficiently low temperatures, the majority of the ensemble-averaging technique studies suggested that O2 molecularly chemisorbs at VO sites on reduced surfaces (at T < 150 K). However, recent STM studies reported a contradict result that the O2 dissociation at VO 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 (VO and Ti5c) using “extremely mild” tunneling conditions. While O2 species at Ti5c site appears as a single protrusion centered on the Ti5c row, the O2 at VO manifests itself by a disappearance of the VO 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 VO and Ti5c 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.

2:20 PM SS-WeA-2 The Interaction of Carboxylic Acids with Rutile TiO2 (110) Single Crystal Surfaces: Results from IR-Spectroscopy
Maria Buchholz (Karlsruhe Institute of Technology (KIT), Germany); MingChun Xu, Yuemin Wang (Ruhr-University Bochum, Germany); Alexei Nefedov, Christof Wöll (Karlsruhe Institute of Technology (KIT), Germany)
The role of oxides is central in many technological areas such as gas sensing, catalysis and thin film growth. Zinc oxide and titanium oxide are also important for photocatalysis and photooxidation, e.g. of CO to CO2[1]. In the Graetzel-cell, organic molecules bound to TiO2-substrates via carboxylate bonds effectively convert photons into electric energy. In last decades numerous IR investigations of oxide powders, including the different modifications of TiO2, have been reported. An unambiguous assignment of the features in the complex IR spectra recorded for molecules bound to the oxide powder particle surfaces, however, is only possible on the basis of data recorded for well-defined reference systems, e.g. surfaces of single crystals. Unfortunately, studies on oxide single crystals are extremely scarce due to the fact that the sensitivity of reflection IR-spectroscopy for molecular adsorbates is two orders of magnitude lower for oxides than for metal single crystals. Only recently was is possible to overcome these technical problems by employing a novel, optimized spectrometer.[2] Here, we will demonstrate the performance of this highly sensitive IRRAS-setup by presenting high-quality IR-spectra obtained for two molecules, benzoic acid and terephthalic acid, adsorbed on rutile TiO2 (110). Owing to the fact that many Dye Sensitized Solar Cells (DSSCs) consist of dyes grafted to the oxide support via carboxylate groups determining and controlling the adsorption of carboxylic acids on oxidic substrates is fundamental to understanding the energy transfer from the molecule to the substrate. For the present experiments, monolayers of terephthalic acid (TPA) and benzoic acid (BA) were first deposited under UHV-conditions on a rutile TiO2 (110) surface at room temperature. Subsequently the sample was transferred in the main chamber and subjected to an analysis in a highly sensitive UHV IRRAS system. While for BA the expected bidentate carboxylate bonding is observed, for TPA the presence of two carboxylic acid groups leads to interesting complications. The IR-spectra allow, in particular, answering the question whether for the flat-lying TPA species observed in scanning probe techniques[3] the carboxylic acid group is still protonated, a question which could not be answered by the results from x-ray absorption spectroscopy[3].

[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.

2:40 PM SS-WeA-3 The Adsorption Dynamics and Interfacial Charge Trapping Behavior for Acetic Acid on Rutile TiO2 Surfaces
Junguang Tao, Tim Luttrell, Matthias Batzill (University of South Florida)

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.

3:00 PM SS-WeA-4 Effect of the Adsorption Geometry of Zinc-Tetraphenylporphyrin Derivatives on ZnO and TiO2, on the Exciton Delocalization Pathways
Sylvie Rangan, Senia Coh, Robert Bartynski, Keyur Chitre, Johnathan Rochford, Elena Galoppini (Rutgers University); Cherno Jaye, Daniel Fischer (National Synchrotron Light Source)

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.

3:20 PM BREAK
4:00 PM SS-WeA-7 Adsorption of Trimethylacetic Acid on Stoichiometric and Reduced CeO2(111) Surfaces
Shail Sanghavi, Ajay Karakoti, Manjula Nandasiri, Weina Wang, Ponnusamy Nachimuthu, Ping Yang, Satyanarayana Kuchibhatla, Suntharampillai Thevuthasan (Pacific Northwest National Laboratory)

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.

4:20 PM SS-WeA-8 Reactivity Differences between CeO2(100) and CeO2(111) Thin Films
David Mullins, Florencia Calaza, Steven Overbury, Michael Biegalski, Hans Christen (Oak Ridge National Laboratory)
Cerium oxide is a principal component in many heterogeneous catalytic processes. One of its key characteristics is the ability to provide or remove oxygen in chemical reactions. The different crystallographic faces of ceria present significantly different surface structures and compositions that may alter the catalytic reactivity. The structure and composition determine the availability of adsorption sites, the spacing between adsorption sites and the ability to remove O from the surface.
 
To investigate the role of surface orientation on reactivity, CeO2 films were grown with two different orientations. CeO2(100) films were grown ex situ by pulsed laser deposition on Nd-doped SrTiO3(100). The structure was characterized by RHEED, XRD and reflectometry. CeO2(111) films were grown in situ by thermal deposition of Ce metal onto Ru(0001) in an oxygen atmosphere. The structure of these films has been studied by LEED and STM. Attempts to grow CeO2(100) in situ by physical vapor deposition on Pt(100) and Pd(100) failed due to preferential growth of CeO2(111) on these supports.
 
The chemical reactivity was characterized by the adsorption and decomposition of various molecules such as methanol, water and acetaldehyde. Reaction products were monitored by TPD and surface intermediates were determined by soft x-ray photoelectron spectroscopy. In general the CeO2(100) surface was found to be more active, i.e. molecules adsorbed more readily and reacted to form new products, especially on a fully oxidized substrate. However the CeO2(100) surface was less selective with a greater propensity to produce CO, CO2 and water as products. The differences in chemical reactivity are discussed in light of possible structural terminations of the two surfaces.
 
Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. Portions of this work were conducted at the National Synchrotron Light Source, Brookhaven National Laboratory, and Oak Ridge National Laboratory's Center for Nanophase Materials Sciences, which are sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy.
4:40 PM SS-WeA-9 Adsorption and Photo-Reactivity of CO on TiO2(110)
Nikolay Petrik, Greg Kimmel (Pacific Northwest National Laboratory)
We have studied the low-temperature adsorption and reactions of CO on reduced, oxidized, hydroxylated, and electron-irradiated TiO2(110) using temperature programmed desorption, photon-stimulated desorption (PSD) and reflection-absorption infrared spectroscopy (RAIRS). Changing the condition of the crystal surface and the adsorbate coverage provides insight into the interactions of adsorbed CO with 5-fold coordinated Ti sites, (Ti5c), bridge-bonded oxygen (BBO) sites, and defect sites (oxygen vacancies, bridging hydroxyls and radiation-induced surface defects). Infrared spectra were obtained for light with the plane of incidence parallel and perpendicular to the [001] azimuths of TiO2(110). For adsorption on Ti5c sites, the RAIRS spectra are consistent with CO adsorbed nearly perpendicular to the surface. For adsorption on BBO sites, the molecules adsorb parallel to the surface and perpendicular to the rows of BBO atoms. The reactivity of various molecular adsorption forms of CO is probed using PSD. In CO photo-oxidation, the PSD yields of CO and CO2 change dramatically with initial CO coverage, indicating the importance of the relative position and orientation of O2 and CO molecules for the photochemical reaction.
5:00 PM SS-WeA-10 Adsorption of Carbon Dioxide on Rutile TiO2(110): A Scanning Tunneling Microscopy Study
Xiao Lin, Bruce D. Kay, Zhi-Tao Wang, Igor Lyubinetsky, Zdenek Dohnalek (Pacific Northwest National Laboratory)
Understanding the fundamental aspects of CO2 adsorption and reaction on well-characterized oxide surfaces is critical in providing fundamental understanding on how to control catalytic carbon sequestration and CO2 conversion to fuels. A model oxide surface, rutile TiO2(110) is used to investigate the adsorption properties of CO2 using scanning tunneling microscopy (STM). STM images obtained before and after in-situ doses of CO2 at 50 K reveal that the CO2 molecules preferentially bind in bridge-bonded oxygen vacancy (VO) defect sites. We show that electron injection from the STM tip can induce CO2 reduction to CO and VO annihilation. After the saturation of VO’s, CO2 molecules preferentially adsorb on five-fold coordinated Ti sites, where they remain mobile even at 50 K. The mobile CO2 molecules may be corralled by other immobile species such as CO. The contrast observed in the STM images suggests that the distribution of mobile CO2 molecules tracks the distribution of the subsurface charge as demonstrated by the CO2 induced standing wave patterns along the Ti rows. The adsorption behavior of CO2 on hydroxylated TiO2 surfaces will also be presented.
5:20 PM SS-WeA-11 Interaction of ZnO-supported Cu Oxides with CO and CO2
Ziyu Zhang, Fei Wang, Minh Le, Maoming Ren, John Flake, Phillip Sprunger, Richard Kurtz (Louisiana State University)

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 CuOx/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

5:40 PM SS-WeA-12 Microfabricated Nitrogen-Phosphorus Detectors: Surface Work Function and Thermionic Emission
Michael Brumbach, Ryan Hess, Robert Simonson, Matthew Moorman, Timothy Boyle (Sandia National Laboratories)

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.

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