Solar & Alternative Energy

Probing UV photo-oxidation on oxide surfaces

Christof Wöll

UV light directly oxidizes carbon monoxide on rutile titanium dioxide surfaces.

10 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003336

Photochemistry at oxide surfaces is important for solar-energy conversion.^1,2 The most promising candidate for solar-powered water splitting (i.e., hydrogen production) and, thus, benign energy production is titania (TiO₂)-based photocatalysis. Since the initial work of Fujishima and Honda,²numerous improvements related to water splitting have been reported.³ However, even after nearly 40 years of research, major issues concerning the photoactivity of this important oxide are still debated. For example, a convincing explanation as to why the anatase polymorph of TiO₂ shows much more photocatalytic activity than its rutile form, by an order of magnitude,^4,5 is unavailable.³

Even simple reactions, such as carbon monoxide (CO) oxidation (see Figure1), are not understood because of the almost exclusive focus on oxide-powder samples. Such samples are structurally undefined, while all possible surface orientations—often with a high defect density—are probed simultaneously.

The surface-science approach to understanding heterogeneous catalysis has been quite successful.⁶ Consequently, progress toward unraveling fundamental principles governing oxide-surface chemistry urgently necessitates detailed experimental and theoretical studies on model systems.

Figure 1.Photo-oxidation of carbon monoxide (CO) adsorbed on a rutile titania (TiO₂) (110) surface.⁷ C: Carbon. O: Oxygen. O₂: Molecular oxygen. O_2c: Bridging oxygen. CO₂: Carbon dioxide. Ti_5c: Fivefold coordinated titanium.

Especially for TiO₂, surface-oxide research within the past two decades has proceeded along a different direction than that of metal surfaces. Primarily driven by the tremendous success of microscopic interrogation, particularly by scanning-tunneling microscopy (STM), a wealth of structural information has become available about processes on the atomic scale, e.g., reactions occurring at defect sites on rutile^8–15 and, to a lesser extent, anatase.^16–18By employing density functional theory (DFT), many STM observations have been explained.⁹

Figure 2.Reflection-absorption IR spectroscopy data recorded for CO adsorbed on a rutile TiO₂(110) substrate. (A) Prior to UV exposure. (B)–(F) Increasing exposure to UV photons.⁷

However, identification of chemical intermediates by STM/ DFT is subject to some pitfalls (e.g., reliable identification of hydroxyl species at TiO₂ surfaces remains a challenge). It has become clear that spectroscopic methods, and in particular IR vibrational spectroscopy, are indispensable for establishing a reliable foundation of chemistry on oxide surfaces, as has been demonstrated with respect to chemistry on metal surfaces through the surface-science approach.

Unfortunately, specific optical properties of oxides lead to severe problems for applications of reflection-absorption IR spectroscopy (IRRAS), the standard experimental method in this field. Sensitivity to molecular vibrations within adsorbates on oxides is reduced by two orders of magnitude with respect to metals. Consequently, despite the availability of a large set of IR data recorded in transmission mode for powders, data for well-defined oxide model systems is virtually unavailable. This is one of the primary reasons why our understanding of chemistry and photochemistry on oxide surfaces is still relatively limited.

We recently overcame these intensity problems by employing a novel, carefully optimized apparatus, where an IR spectrometer is directly attached to an ultrahigh-vacuum chamber.¹⁹ We detected (for the first time) the internal CO-stretch vibration of a TiO₂single crystal surface through IRRAS⁷(see Figure 2) and consequently demonstrated that application of IRRAS on oxide surfaces offers huge potential for other molecular adsorbate species.²⁰When adsorbed CO is exposed to UV photons in the presence of molecular oxygen (O₂), photo-oxidation proceeds without any intermediates. Activated O₂ reacts directly with CO, yielding carbon dioxide.⁷ Furthermore, we determined the photo cross section of the photo-oxidation reaction and found that it agrees with previous data.⁷ In very recent measurements on anatase single crystals, we found that the corresponding cross section is much larger. This parallels observations reported for powders and indicates that the surprisingly large photo cross sections seen for anatase powders do not result from either special types of surface-active sites or a particular form of defects characteristic for anatase, as proposed previously. In contrast, they originate from special features of the (bulk) electronic structure, specifically the presence of an indirect band gap.²¹

These results have important implications for our fundamental understanding of photochemical energy conversion in general, as well as for fabrication of materials with high photochemical cross sections. For nanoparticles with diameters in the sub-10nm region, the electronic structure of particles will be strongly disturbed with respect to the bulk. Consequently, the longer lifetimes of electronic excitations (electron-hole pairs) of anatase are predicted to be reduced to the values characteristic of rutile, since the indirect band gap will no longer reduce the electron-hole recombination rate. We next plan to look at other photochemical reactions on TiO₂and zinc oxide surfaces, with particular emphasis on doping effects.

Nanorods in an electric field create liquid crystals for transformation optics

Nanotechnology Nanorods in an electric field create liquid crystals for transformation optics Oleg D. Lavrentovich, Andrii B. Golovin, Jie Xiang and Yuriy A. Nastishin Condensing and aligning metal nanorods in a fluid is a useful technique in fabricating reconfigurable metamaterials for applications such as invisibility cloaking and solar-energy collectors. 14 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003315 Advances in optical engineering are often associated with the development of new materials. One example is liquid crystals, organic fluids formed by rodlike molecules that align parallel to each other. Tailoring the temperature range of their existence, refractive indices, surface alignment, and dielectric and elastic properties made possible a multibillion-dollar liquid-crystal display industry. A newly emerging field is that of optical metamaterials for ‘transformation’ optics.1In these materials, the index of refraction varies from point to point in a broad range, including negative and zero values. It would be ideal if we could fabricate these materials with 3D reconfigurable architecture, as that would allow us to control light propagation with a finesse previously unknown, making possible applications such as sub-wavelength imaging, invisibility cloaking, and light concentrators. The challenge is that to direct the waves of light into desired trajectories, the local dielectric and magnetic properties of a metamaterial should vary so widely in space that no single natural substance can fit the bill. The metamaterials need to be constructed artificially, by selecting proper metallic and dielectric elements and then arranging them in space into predesigned architectures. The building units must be smaller than the wavelength of light to avoid scattering. Some metastructures can be fabricated by lithography, but this technique is of limited applicability for complex 3D and reconfigurable architectures. We propose an alternative approach,2,3 using a dispersion of metal nanoparticles in a fluid and controlling their concentration and orientation by a nonuniform electric field. Metal nanorods (NRs) can be dispersed in a dielectric fluid such as water or toluene. The electric field polarizes the NRs and aligns them along the field lines. If the field is nonuniform, it also creates a force moving the NRs, usually toward the region of stronger field. The particle does not need to be charged: the two ends of the rod with the two polarization charges of the same absolute value but opposite signs find themselves in slightly different fields. The effect is called dielectrophoresis. Previously, it was used to manipulate supramicron particles.4 In optical metamaterials, however, one prefers to work with much smaller (nanometer-scale) particles, to avoid light scattering. Since the dielectrophoretic force is proportional to the particle' volume, it was not clear whether it would be strong enough to manipulate the nanoparticles. Our work shows that the dielectrophoretic effect is in fact a very potent way to control the spatial location and orientation of nanoparticles and thus the optical performance of the metamaterial. We use a dispersion of gold (Au) NRs in toluene. Their diameter is 10–20nm and length 40–80nm. The volume fraction is low, 0.01–0.1%, and thus the dispersion is spatially uniform and isotropic, i.e., the NRs are distributed homogeneously in space far apart from each other and adopt arbitrary orientations. A nonuniform field transforms this fluid into a liquid crystal, as illustrated in Figure 1 in an experiment with two mutually perpendicular metal wires (2) and (3) in a glass cell (1). The dielectrophoretic force attracts the NRs to the electrode (2), where they align parallel to the electric field lines and condense into a cloud. The concentration of NRs is high near the electrode and low at the periphery of the cell. Consequently, the refractive index of the medium changes from a high value at the periphery (equal to the refractive index of pure toluene) to a smaller value near the electrode (2). The effect is most pronounced for light polarized perpendicularly to the electrode (2) and less so for light polarized parallel to it. This optical anisotropy is caused by the aligned assembly of the NRs, which resembles a liquid crystal formed by orientationally ordered rodlike molecules. The density variation represents a major difference compared with the normal liquid crystal, since all material properties, such as the refractive index, also change from point to point with the NR concentration.   Figure 1.Field-induced birefringent cloud of gold nanorods (Au NRs) concentrated and aligned in a high-electric-field region. A, P: Axes of the analyzer and polarizer. U, f: Amplitude and frequency of the applied voltage. The most interesting effect is produced when the sample represents a circular glass capillary: see (1) in Figure 2. One electrode is a metal wire (2) running along the axis. The second electrode is a transparent indium tin oxide layer (3) at the outer surface of the capillary. The field-induced radial pattern of NRs with the concentration that decreases from the electrode to the periphery (4) resembles the theoretical optical cloak proposed by Vladimir Shalaev's group.5 By applying the electric field, we reduce the visibility of the central wire in light polarized normally to the cylinder: see Figure 2 and video.6   Figure 2.(a) Field-induced radial gradient structure of an Au NR dispersion. The visibility of the central wire (b) is reduced when the field is on (c). The refractive index of toluene with the condensed cloud of NRs decreases when one moves from the NR-depleted periphery of the cylinder to the NR-rich region near the central electrode (2). The radial variation of the refractive index in the cloud (4) causes the light rays to bend around the wire (2), so its shadow is mitigated, representing an imperfect version of the cloaking effect. Theoretically, the refractive index at the inner surface of a cloaking shell should be 0. Our experiments2,3 have not yet reached this limit, as the estimated field-induced change in the refractive index is ∼0.1. The task is to increase it further, by a factor of 10. To make a 3D reconfigurable metamaterial, we need to learn how to reversibly intertwine the metal and dielectric elements at the scale of nanometers. Our approach based on the dielectrophoretic effect is only the first step forward. It shows that the architecture of metal and dielectric nanoscale elements can be controlled by the nonuniform electric field. The next steps would be to increase the concentration of NRs, to explore whether and how the dielectrophoretic forces can be supplemented by other forces such as electrophoretic and induced-charge electrophoretic ones, for a better control of the 3D architecture. The issues of light scattering and losses in the metal-dielectric composites also need to be addressed. We plan to apply the dielectrophoretic technique to NRs that are preassembled into chainlike and raftlike clusters7 to increase the initial concentration of NRs and to assemble clouds with differently oriented NRs in them. These tasks are formidable, but the rewards hold a major promise for optical engineering of the future. This work was supported by Air Force Office of Scientific Research grant FA9550-10-1-0527, Department of Defense Multidisciplinary University Research Initiative grant FA9550-06-1-0337, and Department of Energy DE-FG02-06ER46331. We thank N. A. Kotov and P. Palffy-Muhoray for providing us with Au NR dispersions, and A. Agarwal, J. Fontana, P. Luchette, H.-S. Park, L. Tortora, B. Senyuk, H. Wonderly, and L. Qiu for help in sample preparation. We thank V. M. Shalaev, P. Palffy-Muhoray, C. Y. Lee, A. V. Kildishev, S. V. Shiyanovskii, and V. P. Drachev for fruitful discussions. Oleg D. Lavrentovich, Andrii B. Golovin, Jie Xiang, Yuriy A. Nastishin Liquid Crystal Institute, Kent State University Kent, OH Oleg D. Lavrentovich received his PhD (1984) and DSc (1990) in physics and mathematics from the National Academy of Sciences, Ukraine. He is the director of the Liquid Crystal Institute at Kent State University and a SPIE Fellow. Andrii B. Golovin received his PhD in physics and mathematics from the National Academy of Sciences, Ukraine (1994). He is a research associate. He has conducted research in the areas of materials science, optical engineering, and laser physics. Jie Xiang received his MS in instrument science and technology (2009) from Harbin Institute of Technology, China. He is a graduate student at the Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University. Yuriy A. Nastishin received his PhD (1991) and DSc (2006) from the Institute of Physical Optics, Ukraine, where he leads the group on Optics of Liquid Crystals. Currently, he is a visiting scientist at the Liquid Crystal Institute, Kent State University.