AVS2013 Session SS+EN-ThM: Photocatalysis and Photochemistry at Surfaces

Thursday, October 31, 2013 8:00 AM in Room 201 A

Thursday Morning

Time Period ThM Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS2013 Schedule

Start Invited? Item
8:00 AM SS+EN-ThM-1 Hindering Influence of Oxygen Vacancies for Photoactivity on TiO2(110)
Igor Lyubinetsky, Zhi-Tao Wang (Pacific Northwest National Laboratory); N.Aaron Deskins (Worcester Polytechnic Institute); Michael Henderson (Pacific Northwest National Laboratory)

In photoinduced processes, surface point-defects are expected to act as charge trapping and/or recombination centers. However, the direct impact of surface defects on photoreactivity is not well explored. We present the first observation of a suppressing effect of oxygen vacancy (VO) defects on photoreactivity of TiO2(110). Direct scanning tunneling microscopy imaging reveal a pronounced site-selectivity in the hole-mediated photooxidation of trimethyl acetate (TMA) on TiO2(110) upon ultra-violet light irradiation, wherein the reaction readily occurs at regular Ti sites but is completely inhibited at VO defects. Utilizing electron energy loss spectroscopy and density functional theory, we show that the lack of reactivity of TMA groups adsorbed at VO’s cannot be attributed to either a less active adsorption conformation or electron transfer from the VO defect. Instead, we propose that the excess unpaired electrons associated with the VO promptly recombine with photoexcited holes approaching the surface, effectively ‘screening’ TMA species at VO site. We also show that this screening effect is spatially short-ranged, being predominately localized at the VO, and only mildly affecting TMA’s at adjacent Ti sites. The direct impact of O vacancies on TMA photoreactivity over TiO2(110) is expected to have similar implications for other hole-mediated (e.g., photooxidation) reactions in which adsorption at or near electronic point-defects is possible. Furthermore, the localized influence of these defects on hole-mediated chemistry offers opportunities for additional study of site-selective photocatalysis on TiO2. The presented results also demonstrate that structure–reactivity relationships, a customary subject in heterogeneous catalysis, are clearly relevant to photocatalysis.

8:20 AM SS+EN-ThM-2 Anisotropic Photochemical Reactivity of Rutile (110)
Anqi Song, Dapeng Jing, Melissa A. Hines (Cornell University)
The surface chemistry of nanocrystalline titanium dioxide has garnered a tremendous amount of attention over the past decade due to a number of high-profile applications, including dye-sensitized solar cells and photoactivated self-cleaning or environmentally-remediating surfaces. Since most surface science investigations study pristine surfaces in ultrahigh vacuum conditions, little is known about the photoreactivity of surfaces in aqueous solutions. In particular, do certain sites on the surface dominate reactivity or are all sites equally reactive? To gain insight into the site specificity (or chemical anisotropy) of the reactions, we have used ex situ scanning tunneling microscopy to investigate the reaction of aqueous solutions of H2O2 with rutile (110). Contrary to naïve expectation, above band gap radiation appears to suppress certain chemical reactions while also increasing reaction anisotropy. This finding suggests that some sites on the surface are more photoreactive than others. A mechanism that explains this anisotropy will be proposed.
9:20 AM SS+EN-ThM-5 Energy Transfer and Photostimulated Desorption of Atoms and Molecules Co-adsorbed with Oxygen on TiO2(110) Surface
Nikolay Petrik, Greg Kimmel (Pacific Northwest National Laboratory)
Titanium dioxide is a widely used photocatalyst. However, fundamental aspects of the photochemistry, including the role of molecular oxygen in photooxidation reactions, are still actively debated. Here, we use weakly bound (i.e. physisorbed) atoms and molecules, such as Ar, Kr, Xe, CO2 and N2, to probe the photochemical interactions of O2 on rutile TiO2(110). UV irradiation of chemisorbed O2 along with the physisorbed probe species lead to photon-stimulated desorption (PSD) of the probe species. Without O2, the PSD yields of the probe species are very low or not observable. However, the Ar, Kr and N2 PSD yields increase with increasing coverage of chemisorbed O2. No PSD was observed for CO2, and the PSD yield for Xe is very low compared to the other probe atoms or molecules. The angular distribution of the photo-desorbing Kr, which is broad and cosine, is quite different from the O2 PSD angular distribution, which is sharply peaked along the surface normal. We propose a mechanism for the observed phenomena, where the chemisorbed O2 serve as photoactive centers, excited via electronic excitations (electrons and/or holes) created in the TiO2 substrate by UV photon irradiation. The photo-excited O2 may transfer its energy to neighboring co-adsorbed atoms or molecule resulting in their desorption. Simple momentum transfer considerations suggest that heavier adsorbates (like Xe) and adsorbates with higher binding energy (like CO2) would desorb less efficiently according to the proposed mechanism.
9:40 AM SS+EN-ThM-6 Adsorption and Photo-induced Decomposition of Formate on Rutile TiO2 (110)
Andreas Mattsson, Shuanglin Hu, Kersti Hermansson, Lars Österlund (Uppsala University, Sweden)

Adsorption of formic acid on rutile TiO2 (110) were studied with reflection absorption infrared spectroscopy (RAIRS) in UHV with both s- and p-polarized IR light, incident along either the <001> or <1-10> direction. Experiments were conducted on surfaces prepared with different pre-treatments to obtain stoichiometric (s-TiO2), oxidized (o-TiO2) and reduced (r-TiO2) surfaces. Experiments were compared with density functional theory (DFT) calculations as implemented in the Vienna ab initio simulation package (VASP).

With p-polarized light, transmission and absorption peaks are observed due to the symmetric and asymmetric O-C-O stretch and C-H wagging modes in formate bonded to the Ti-atoms between the bridging oxygen rows in the <001> direction, in agreement with the measurements made by Hayden and co-workers.[1] This orientation of the formate molecule is dominant on for all surface preparations studied here. Employing s-polarized light reveals that the C-H wagging occurs in the plane of the molecule, since it is only seen with s-polarized light incident in the <1-10> direction. In the earlier work by Hayden, weak absorption peaks were observed with p-polarized light incident in the <1-10> direction, and attributed to a minority specie, oriented perpendicular to the majority specie, and bonded to bridging oxygen vacancies. This interpretation is at variance with our results for s-polarized light, and furthermore an equally weak band is seen regardless of the surface preparation. We attribute this to rapid hydroxylation of the bridging oxygen vacancies in good agreement with recent STM studies, which show that bridging oxygen vacancies become hydroxylated within a few minutes at pressures of 3x10-10 mbar.[2] DFT calculations support the above assignments, and in particular show that the minority species inferred from RAIRS is due to formate bonded to OH groups and not to bridging oxygen vacancies. We discuss the implications of our results for the photo-induced decomposition of formic acid on TiO2(110).

[1] B.E. Hayden, A. King, M.A. Newton, Journal of Physical Chemistry B 103 (1999) 203-208.

[2] S. Wendt, et. Al. Surf. Science 598 (2005) 226-245.

10:00 AM BREAK - Complimentary Coffee in Exhibit Hall
10:40 AM SS+EN-ThM-9 TiO2 Nanoparticle Arrays Functionalized with Pt Photodeposition: Studies Using X-ray Spectroscopies under In Situ Heating and Hydrogen Annealing
Yu Liu, James Taing (University of California, Irvine); Cheng-Chien Chen (Argonne National Laboratory); Hendrik Bluhm, Zhi Liu (Lawrence Berkeley National Laboratory); Michel Veenendaal (Argonne National Laboratory); Thomas Devereaux (SLAC National Accelerator Laboratory); John Hemminger (University of California, Irvine)

Using ambient pressure x-ray spectroscopies, we report the electronic and surface structures of TiO2 nanoparticle arrays with and without Pt photodeposition under in-situ heating and hydrogen annealing. X-ray absorption and transmission electron microscopies indicate that the TiO2 nanoparticles are in the rutile phase, but the anatase phase also can exist after Pt photodeposition. Valence photoemission results demonstrate a band gap narrowing when Pt is loaded onto the surface of TiO2 nanoparticles. Upon heating the samples, surface defects and oxygen vacancies are formed, which could prevent the recombination of electron-hole pairs. Heating also enhances the occupation of metallic Pt on top of the TiO2. In contrast, introducing hydrogen at high temperature would enhance the Pt4+ species related to the strong metal support interaction. The reduced band gap and the increased contact surface in the Pt-photodeposited TiO2 nanostructures can potentially enhance the performance of these materials in solar absorption and photocatalysis applications.


  1. Taing, J.; Cheng, M. H.; Hemminger, J. C., Photodeposition of Ag or Pt onto TiO2 Nanoparticles Decorated on Step Edges of HOPG. ACS Nano 2011, 5, 6325-6333.
  2. Liu, Y.; Taing, J.; Chen, C.-C.; Sorini, A. P.; Cheng, M. H.; Margarella, A. M.; Bluhm, H.; Devereaux, T. P.; Hemminger, J. C., Narrowing of Band Gap in Thin Films and Linear Arrays of Ordered TiO2 Nanoparticles, to be submitted to ACS Nano.
11:00 AM SS+EN-ThM-10 Carrier Dynamics on Oxide Surfaces Studied by Time-resolved Soft X-ray Photoelectron Spectroscopy
Susumu Yamamoto, Ryu Yukawa (The University of Tokyo, Japan); Masato Emori (Sophia University, Japan); Kenichi Ozawa (Tokyo Institute of Technology, Japan); Manami Ogawa, Kazushi Fujikawa, Shingo Yamamoto, Rei Hobara, Iwao Matsuda (The University of Tokyo, Japan)

Photocatalytic reactions on semiconductor oxide surfaces can be divided into four processes: (i) photon absorption, (ii) electron-hole pair formation, (iii) transport of photo-excited carriers from bulk to surface, and (iv) surface redox reactions. It is important to understand the dynamics of photo-excited carriers in order to make more efficient photocatalysts. Despite of its importance, little is known about transient electronic structures of photo-excited semiconductor surfaces.

Photoelectron spectroscopy (PES) has been successful in providing direct access to electronic structures of materials with surface sensitivity. The extension of PES to time-domain, or time-resolved PES, is now realized by the use of brilliant short pulse (several tens ps) x-ray available at the state-of-the-art synchrotron radiation facilities [1]. This allows us to study transient electronic structures of materials.

In this talk we will introduce the newly developed time-resolved PES system at the high-brilliance soft x-ray beamline BL07LSU at SPring-8 [2]. In the time-resolved PES measurements, the transient electronic structures after optical excitation by fs-laser pump pulses are monitored by ps soft x-ray probe pulses. The time-resolved PES studies on the relaxation of surface photovoltage effect on oxide surfaces such as SrTiO3(001) and ZnO(0001) will be presented.


[1] S. Yamamoto, I. Matsuda, J. Phys. Soc. Jpn., 82, 021003 (2013).

[2] M. Ogawa, S. Yamamoto, Y. Kousa, F. Nakamura, R. Yukawa, A. Fukushima, A. Harasawa, H. Kondoh, Y. Tanaka, A. Kakizaki, I. Matsuda, Rev. Sci. Instrum., 83, 023109 (2012).

11:20 AM SS+EN-ThM-11 Atomic Scale Photochemistry with STM
Arthur Yu, Shaowei Li, Gregory Czap, YANNING Zhang, Haiyan He, RUQIAN Wu, Wilson Ho (University of California, Irvine)

The STM has proven to be an invaluable tool for studying surface chemistry at the atomic and molecular scale, such as bond formation, bond breaking, and photo-catalysis. Here we present STM studies on photon-induced desorption of H2 molecules from Au (110) surface. Upon laser irradiation, H2 is found to desorb from Au surface. We attribute the desorption mechanism to excitation of molecular vibrational modes by photon-induced hot electrons. We also study the photo-induced desorption and reaction of CO and O2 on Al2O3 / NiAl (110) surface. We hope to have a better understanding of various mechanisms for photon mediated reactions and effects of plasmon modes in nearby nanoparticles on chemical reactions.

11:40 AM SS+EN-ThM-12 Atomic Structure and Catalytic Activity of Size-Selected MoS2 Nanoclusters for Water Splitting
Richard Palmer, Martin Cuddy, Kenton Arkill, Zhiwei Wang, Neil Rees (University of Birmingham, UK)

The green production of hydrogen by photocatalytic splitting of water molecules requires the catalyst both to absorb solar photons and to supply excited carriers of the correct energy to split water. MoS2 is a new and abundantly available candidate catalyst material with a layered structure in the bulk; it is believed that quantum confinement in MoS2 nanoparticles will allow the band gap and energy levels to be tuned to maximize the efficiency of water splitting. Here we report the atomic structure of size-selected nanoparticles (clusters) of MoS2, generated by magnetron sputtering of a bulk target and condensation in helium gas, then size selection with a novel lateral time-of-flight mass filter [1] prior to deposition onto carbon supports. X-Ray Photoelectron Spectroscopy (XPS) of cluster ensembles confirms that approximately stoichiometric compound clusters, with average formula MoS1.95, are produced (although we find they are somewhat sensitive to oxygen).

Atomic-scale imaging of the deposited MoS2 clusters by aberration-corrected Scanning Transmission Electron Microscopy (STEM) [2] of the MoS2 clusters shows layered nanoparticle structures (as opposed to e.g. fullerene structures), presenting ordered hexagonal arrays of Mo atoms. The cluster growth is remarkably anisotropic, such that as cluster size increases from 150-1000 MoS2 units (always mass selected) the lateral diameter of the clusters increases but the mean vertical height (2.3±1.0 layers) remains constant. These clusters demonstrate efficient electrocatalytic activity in the hydrogen evolution reaction, probably at edge sites, confirming these new nanosystems as intriguing candidates for water splitting.

[1] S. Pratontep, S. J. Carroll, C. Xirouchaki, M. Streun, and R. E. Palmer, Review of Scientific Instruments 76, 045103 (2005).

[2] Z. W. Wang and R. E. Palmer, Nano Letters 12, 91 (2012).

Time Period ThM Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS2013 Schedule