Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J36: Focus Session: Materials for Photovoltaics and Photocatalysis I |
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Sponsoring Units: DMP Chair: Matthias Batzill, University of South Florida Room: Morial Convention Center 228 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J36.00001: Polycrystalline TiO$_{2}$ films with silver nanoclusters for photocatalysis Frank Womack, Fei Wang, Asoka Sekharan, Phillip Sprunger, Richard Kurtz Metallic Ag nanoclusters nucleate on TiO$_{2}$(110) due to weak substrate interactions and the need to minimize their surface free energy. EELS results on single crystal TiO$_{2}$ have shown that those nanoclusters have a distinct plasmon resonance at 3.7 eV which can be red-shifted when incorporated within a dielectric of titania. Polycrystalline oxides consist of mostly low surface free-energy faces and nanoclusters are expected to nucleate when dosed with Ag. We will present a combined synchrotron-based photoemission, Auger, and optical spectroscopy study of polycrystalline TiO$_{2}$ films functionalized with Ag. We have made thin films of TiO$_{2}$ by thermal evaporation of titanium followed by oxidation at elevated temperatures. Ag was then deposited via thermal evaporation of silver, and covered with additional layers of Ti that were subsequently oxidized and characterized with photoelectron and optical spectroscopies. We will discuss these data in the context of nanocluster formation \textit{vs} bandgap doping. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J36.00002: Buried silver nanoclusters on TiO$_{2}$(110) for photocatalysis Fei Wang, Frank Womack, Asoka Sekharan, Brendan Waston, Phillip Sprunger, Richard Kurtz Ag nanoclusters grown on TiO$_{2}$(110) can promote photochemistry by enhancing photoabsorption via their plasmon resonances. Overcoating the Ag clusters with a thin layer of titania red-shifts the plasmon to better match the solar spectrum and protects the nanoclusters from the environment. Our STM studies show that Ag clusters $\sim $5nm across and 2nm high nucleate on the TiO$_{2}$(110) surface at room temperature. Photoemission performed at the LSU CAMD synchrotron shows that the clusters interact weakly with the substrate, although there is charge transfer from surface defects to the first nanoclusters that nucleate. EELS shows that the bare clusters exhibit a plasmon resonance located at 3.8 eV. Ti overgrowth and subsequent oxidation gives rise to new losses at about 1.5eV as observed in EELS. We will discuss our work at incorporating the nanoclusters within the titania matrix in light of their potential for producing hot electron-hole pairs for surface chemistry. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J36.00003: Epitaxial In$_{2}$O$_{3}$ and Sn-doped In$_{2}$O$_{3}$ thin films with (100) and (111) orientation Erie Morales, Matthias Batzill, Ulrike Diebold In$_{2}$O$_{3}$ and Sn-doped In$_{2}$O$_{3}$ (Indium-Tin Oxide, ITO) have optical transparency and low electrical resistivity. Relatively little is known about their atomic-scale surface properties because of challenges in preparing single crystal samples. We have grown epitaxial In$_{2}$O$_{3}$ and ITO films on Yttrium Stabilized Zirconia. The (100) surface has polar character and the (111) orientation is non-polar. Films were prepared using oxygen-plasma assisted e-beam epitaxy under UHV conditions and the growth was monitored by RHEED. \textit{In-situ} characterization with XPS, ARXPS, LEED and synchrotron-based UPS was used. In$_{2}$O$_{3}$ (100) facets while ITO(100) stays with a 1x1 termination and Sn segregates to surface. In$_{2}$O$_{3}$ and ITO (111) exhibit a 1x1 termination. On both orientations valence band maximum is 2.7 eV below the Fermi level. For the ITO films resonant photoemission measurements indicate a Sn-derived band gap state. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J36.00004: Epitaxial growth of In2O3(100) on Y-stabilised ZrO2(100) by O-plasma assisted molecular beam epitaxy: a study by HRTEM and XPS. Anne Bourlange, David Payne, Russell Egdell, John Foord, Peter Dobson, John Hutchison Thin films of In2O3 have been grown on Y-stabilised ZrO2(100) by radiofrequency oxygen plasma assisted molecular beam epitaxy with a substrate temperature of 650\textsc{\r{ }C}. Ordered epitaxial growth was confirmed by HRTEM and selected area electron diffraction taken across the interface between the substrate and the epilayer. Excellent crystalline order was preserved up to the surface of the films. The valence band onset in the X-ray photoemission spectra of the epitaxial films was found at 2.90 eV relative to the Fermi energy. The discrepancy between this value and the widely quoted value of 3.75 eV for the bandgap will be discussed in relation to recent theoretical work [1]. \newline [1] A. Walsh \textit{et al.}, Phys. Rev. Lett. submitted. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J36.00005: Radiation-induced defect formation and reactivity of model TiO2 capping layers with MMA: a comparison with Ru Theodore E. Madey, Boris V. Yakshinskiy, M. Nejib Hedhili, Manish Chandhok Our goal is to provide insights into surface processes that affect the reflectivity of TiO2- and Ru-capped multilayer mirrors used in EUV lithography by 13.5 nm (92 eV) photons. EUV-generated secondary electrons from the substrates cause surface reactions that lead to mirror contamination in background vacuum. In our experiments, low-energy electron beams mimic excitations initiated by EUV radiation. Oxygen vacancies are produced at energies above 25 eV. Carbon accumulation is measured on both Ru and TiO2 surfaces during 20 eV and 100 eV electron bombardment in methyl methacrylate vapor (MMA). The initial rates on the clean surfaces are very different: a C film grows more rapidly on TiO2 than on Ru. However, the limiting growth rates are the same for C thicknesses greater than $\sim$1 to 1.5 nm, when MMA interacts with a C film. Irradiation of the C films in O2 gas has a mitigating effect. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J36.00006: In-situ vacuum studies of photocatalytic oxidation of isopropanol on nanometer thick TiO$_{2}$ films grown on silicon D. Kazazis, S. Guha, N.A. Bojarczuk, H.-C. Kim, A. Zaslavsky We report on measurements of the photocatalytic activity of ultra-thin TiO$_{2}$ films grown on $n$ and $p$ type Si wafers.~Using the oxidation of isopropanol to acetone as a model system, photocatalytic studies were carried out in-situ, in a high vacuum chamber equipped with leak valves for injecting isopropanol, oxygen and water vapor onto the TiO$_{2}$ sample.~The sample was irradiated through a quartz widow with a UV strobe light source.~The reaction was monitored with a line-of-sight mass spectrometer coupled to a lock-in amplifier tuned to the strobe frequency.~We find that the photocatalytic efficiency is enhanced as the TiO$_{2}$ thickness is reduced from 50\textit{nm} to 2\textit{nm}.~We also find that the efficiency is enhanced by lowering the substrate Fermi level in going from $n$ type to $p$ type Si.~The results strongly point to the hypothesis that only near surface electron-hole pair generation is relevant to the photocatalytic process; and that the reaction rate can be controlled by varying the substrate Fermi level which in turn changes the electrostatic potential variation within the heterostructure. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J36.00007: Generation of Organic Radicals During Photocatalysis on TiO$_{2}$ Invited Speaker: It is well-known that water-related radicals (such as OH$\cdot $ species) are produced by charge transfer events at UV-irradiated TiO$_{2}$ surfaces. In contrast, organic radicals are generally viewed as being formed by reactions with OH$\cdot $ groups and not by direct charge transfer events. Using rutile TiO$_{2}$(110) as a model photocatalyst, we show that organic radicals are generated in single-step charge transfer events during photodecomposition of adsorbed carboxylates and ketones. Some organic radicals (e.g., methyl) are ejected from the surface and, in high surface area catalysts, experience reactions away from the surface of origin. Other radicals (e.g., ethyl and t-butyl) have limited ability to escape the surface of origin without capture and subsequent thermal reactions. Understanding the chemistry associated with organic radical formation on TiO$_{2}$ opens the door for more detailed examinations of charge transfer dynamics and energy redistribution during photon-initiated reactions important to heterogeneous photocatalysis. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J36.00008: Photocatalytic reaction of catechol on rutile titanium oxide Peter Jacobson, Chundao Wang, Ulrike Diebold In an attempt to understand the fundamental aspects of photocatalysis we have studied the substituted benzene catechol on TiO$_{2}$(110). Previous studies have given detailed information about the catechol bonding configuration letting our group focus on molecular level interactions with scanning tunneling microscopy and X ray photoelectron spectroscopy. Under UV exposure (248 nm) in an oxygen background, catechol is observed to degrade via oxidation. This oxidation process results in removal of roughly 10{\%} of the initial monolayer. The removal of carbon from the TiO$_{2}$ surface is shown to depend upon the background gas. Formation of a residual carbon layer is achieved by annealing the catechol monolayer to 600C. This carbon layer is more difficult to remove by photocatalytic oxidation than a pristine catechol monolayer. Work supported by Intel Corporation [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J36.00009: Water on anatase TiO$_{2}$(101): a scanning tunneling microscopy study Y.B. He, O. Dulub, L.H. Ying, U. Diebold, C. Di Valentin, A. Tilocca, A. Selloni The discovery of photochemical water splitting on TiO$_{2}$ has motivated numerous studies of water on the surfaces of this important photocatalytic material. Previous temperature-programmed desorption and X-ray photoelectron spectroscopy studies of water on anatase TiO$_{2}$(101), the most stable surface of the photocatalytically efficient anatase form, have revealed that water adsorbs molecularly on the surface in accordance with theoretical predictions.$^{ }$In the present study, we have employed low-temperature scanning tunneling microscopy to study water adsorption on anatase TiO$_{2}$(101). We dose various amounts of water (0.2-1.8 Langmuir) at sample temperature T $\sim $ 130 K. Besides confirming that water favors molecular adsorption, atomically resolved STM images further reveal that water molecules adsorb at Ti$_{5c}$ sites forming preferentially one-dimensional chains with local doubling of the periodicity along the [010] direction. Near room temperature, the water molecules become mobile and hop between the Ti$_{5c}$ sites. Density Functional Theory calculations are under way to clarify the origin of the observed doubling of periodicity. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J36.00010: Chemical Deposition and Photoactivity of Anatase and Rutile TiO$_{2}$ Films on Si(111). John F. Anderson, Erie Morales, Ulrikie Diebold Dilute Aqueous Chemical Bath Deposition (CBD) from highly acidic (pH $<$ 1) TiCl3 HCl solutions at room temperature and slightly higher (23$^{\circ}$C - 40$^{\circ}$C) produced thin titanium dioxide films on clean Si(111).~ We report initial results of X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) measurements.~ The films thicknesses varied from 300 nm to $\sim $ 1$\mu $m.~ It was found that the films required annealing to ensure adherence to the Si(111) substrate.~ XRD showed that the anatase and rutile structures were present in the TiO2 as a function of post-deposition annealing temperature.~ Additionally, photo decomposition results of methyl orange and methyl blue on TiO2/Si(111) system under UV light is observed and reported. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J36.00011: Photoinduced Desorption of O$_{2}$ and Photooxidation of Organics from TiO$_{2}$(110) Surfaces David Sporleder, Daniel Wilson, Michael White We present here, a study of photoinduced O$_{2}$ desorption from, and reaction with coadsorbates on, a single crystal rutile TiO$_{2}$(110) surface. Translational energy distributions of O$_{2}$ photodesorbed with a photon excitation energy between 3.5 and 4.2~eV were measured using a pump-probe, time-of-flight (TOF) method. This method utilized a one-photon VUV ionization scheme for product detection that was developed in our lab. The translational energy distribution was found to be trimodal, indicating that different O$_{2}$ species (i.e. O$_{2}^{-}$ or O$_{2}^{2-})$ or binding sites may play a role. It was found that the O$_{2}$ translational energy distributions did not depend on the excitation energy over the range studied, which is consistent with a substrate mediated excitation mechanism involving hole capture. More recent experiments are exploring the mechanism for photooxidation of organic molecules. Specifically, we are determining translational energy distributions of small radicals originating from photoinduced fragmentation of simple ketones on a TiO$_{2}$(110) surface. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J36.00012: Influence of the Sensitizer Protonation and Adsorption Mode on the Efficiency of Dye-sensitized solar cells Annabella Selloni, Filippo De Angelis, Simona Fantacci, Moammhed Nazeruddin, Michael Graetzel Dye sensitized solar cells (DSSCs) represent a promising approach to the direct conversion of light into electrical energy at low cost and with high efficiency. In these devices, a dye sensitizer absorbs the solar radiation and transfers the photoexcited electron to a nanostructured TiO2 electrode. We have studied the electronic structure of different Ru(II)-polypyridyl dyes adsorbed onto a model TiO2 nanoparticle by means of first principles Density Functional Theory calculations. Our results suggest that two different electron injection mechanisms (adiabatic and non-adiabatic) may be present in DSSCs employing dyes carrying a different number of protons. We also found that sensitizers with inequivalent bipyridine ligands exert strong dipolar fields at the TiO2 surface, causing a conduction band down-shift and a reduction of the cell open circuit potential, thus resulting in a reduced DSSC efficiency. [Preview Abstract] |
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