Michael A. Henderson

The interaction of water with solid surfaces: fundamental aspects revisited

Abstract Water is perhaps the most important and most pervasive chemical on our planet. The influence of water permeates virtually all areas of biochemical, chemical and physical importance, and is especially evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled ‘The interaction of water with solid surfaces: fundamental aspects’ in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the molecular-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodology has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochemistry, photoconversion, mineralogy, adhesion, sensors, atmospheric chemistry and tribology. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chemical and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, experimental studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.

An HREELS and TPD study of water on TiO2(110): the extent of molecular versus dissociative adsorption

The interaction of water with the (110) crystal face of TiO2 (rutile) was examined with high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). Water desorption occurred in three sequential TPD features as a function of exposure at 270, 175 and 155 K, ascribed to monolayer, second layer and multilayer states, respectively. Estimation of the extent to which water molecularly versus dissociatively adsorbed on the TiO2(110) surface was attempted by comparing HREELS and TPD data. Very low water exposures (below 7 × 1013molecules/cm2) dissociatively adsorbed at 135 K to surface hydroxyl groups (v(OH) = 3690 cm− 1), but molecular adsorption (v(OH) = 3420 − 3505 cm− 1 and δ(HOH) = 1625 cm− 1) resulted from additional water exposure. The appearance of molecular water in HREELS coincided with a red-shift in the OH stretching frequency of the hydroxyl, presumably due to the formation of hydrogen-bonding interactions with the adsorbed water molecules. Little or no additional water dissociation occurred during heating of the monolayer as inferred from HREELS. Second layer water on TiO2(110) was evident in HREELS by OH stretching intensity below 3400 cm− 1 indicative of hydrogen-bonding. However, the OH stretching mode of monolayer water molecules was not red-shifted by the presence of second layer water suggesting that protons on monolayer water molecules were not involved in hydrogen-bonding second layer water molecules. The influence of second layer water on subsequent layers was evident in both HREELS and TPD.

Oxygen-induced restructuring of the TiO2(110) surface: a comprehensive study

We report a comprehensive experimental and theoretical study of the eVect of oxidizing a TiO 2 (110) surface at moderate temperatures. The surfaces are investigated with scanning tunneling microscopy (STM ), low-energy He+ ion scattering (LEIS ) and static secondary ion mass spectroscopy (SSIMS ). Flat (1◊1)-terminated TiO 2 (110) surfaces are obtained by sputtering and annealing in UHV at 880 K. These surfaces are exposed to oxygen gas at elevated temperatures in the range 470‐830 K. Formation of irregular networks of pseudo-hexagonal rosettes (6.5 A ˚ ◊ 6A ˚ ) and small (11:0] oriented (1◊1) islands along with {001}-oriented strands is induced at temperatures from 470 to 660 K. After annealing above 830 K, only regular (1◊1) terraces and white strands are observed. The composition of these oxygen-induced phases is quantified using 18O 2 gas in combination with LEIS and SSIMS measurements. The dependence of the restructuring process on annealing time, annealing temperature, and sample history is systematically investigated. Exposure to H 2 18O and air in the same temperature regime fails to induce the restructuring. UHV annealing of restructured, oxygen-enriched TiO 2 (110) surface smooths the surfaces and converts the rosette networks into strands and finally into the regular (1◊1) terraces. This is reported in an accompanying paper [M. Li, W. Hebenstreit, U. Diebold, Phys. Rev. B (1999), submitted ]. The rosette model is supported by first-principles density functional calculations which show a stable structure results, accompanied by significant relaxations from bulk-truncated positions. A mechanism for the dynamic processes of the formation of rosettes and (1◊1) islands is presented and the importance of these results for the surface chemistry of TiO 2 (110) surfaces is discussed.

Self-diffusion in ceria

Ceria (CeO2) is an oxygen storage material vital to the proper functioning of automobile three-way catalysts and is typically viewed as an anion conductor. Prior experimental work using temperature programmed static secondary ion mass spectrometry (TPSSIMS) has indicated that for rutile TiO2, a prototypical oxide, the mobile species are Ti cations rather than O anions. To further expand on the mobile species in CeO2 we have investigated the diffusion of both cerium and oxygen ions by TPSSIMS. The CeO2(111) film was heteroepitaxially grown by molecular beam epitaxy on a yttria stabilized zirconia substrate. Although high quality low-energy electron diffraction patterns and Auger electron spectroscopy spectra free of impurity signals were obtained after just a few sputtering and annealing cycles, further cleaning was necessary to remove intense alkali and alkaline earth signals observed in SSIMS. The CeO2(111) surface was slightly enriched in 18O by first annealing the film in UHV at 830 K and then exposing...

Oxygen-induced restructuring of rutile TiO2(110): formation mechanism, atomic models, and influence on surface chemistry

The rutile TiO2(110) (1×1) surface is considered the prototypical ‘well-defined’ system in the surface science of metal oxides. Its popularity results partly from two experimental advantages: (i) bulk-reduced single crystals do not exhibit charging, and (ii) stoichiometric surfaces, as judged by electron spectroscopies, can be prepared reproducibly by sputtering and annealing in oxygen. We present results that show that this commonly applied preparation procedure may result in a surface structure that is by far more complex than generally anticipated. Flat, (1×1)-terminated surfaces are obtained by sputtering and annealing in ultrahigh vacuum. When re-annealed in oxygen at moderate temperatures (470–660 K), irregular networks of partially connected, pseudohexagonal rosettes (6.5×6 Awide), one-unit cell wide strands, and small (≈tens of A) (1×1) islands appear. This new surface phase is formed through reaction of oxygen gas with interstitial Ti from the reduced bulk. Because it consists of an incomplete, kinetically limited (1×1) layer, this phenomenon has been termed ‘restructuring’. We report a combined experimental and theoretical study that systematically explores this restructuring process. The influence of several parameters (annealing time, temperature, pressure, sample history, gas) on the surface morphology is investigated using STM. The surface coverage of the added phase as well as the kinetics of the restructuring process are quantified by LEIS and SSIMS measurements in combination with annealing in 18O-enriched gas. Atomic models of the essential structural elements are presented and are shown to be stable with first-principles density functional calculations. The effect of oxygen-induced restructuring on surface chemistry and its importance for TiO2 and other bulk-reduced oxide materials is briefly discussed.

Electron-induced decomposition of methanol on the vacuum-annealed surface of TiO2(110)

The 100 eV electron-induced decomposition (EID) of methanol adsorbed on the vacuum-annealed surface of TiO2(110) at 135 K was examined with temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD). By annealing at 850 K, a TiO2(110) surface was reproducibly prepared with an oxygen vacancy coverage of about 0.08 ML (where 1 ML=5.2×1014 sites cm−2). In the absence of electron irradiation, CH3OH adsorbed on the vacuum-annealed surface in three main TPD states: a molecular state at 295 K and two dissociative states at 350 and 480 K. The 480 K state was assigned to methoxyls at oxygen vacancy sites, and the 350 K state was due to methoxyls at non-vacancy sites. The surface coverages in these states for the saturated monolayer were 0.40 ML (295 K), 0.15 ML (350 K) and 0.08 ML (480 K). Although CH3OH dissociated on the surface, no irreversible decomposition was observed, and CH3OH was the only desorption product in TPD. By heating a multilayer CH3OH exposure to 197, 310 and 410 K, followed by recooling to 135 K, methanol adlayers could be prepared containing only the saturated monolayer, only both types of methoxyl and only the methoxyls at vacancies, respectively. Given these preparation conditions, the 100 eV EID of each methanol-related species was examined. Using CD3OD, the major positive ESD ions detected from multilayer methanol were D+, O+/CD2+ and OD+/CD3+ (the latter were mostly O+ and OD+ based on results with CH3OH) with weaker signals from C+, CD+, CO+, DCO+ and CD2OD+. However, the monolayer gave D+ and O+, with weak signals from OD+ and CD+. The EID cross-section for molecularly adsorbed CH3OH (1.7×10−16 cm2) was only a factor of three less than the literature values for the total dissociative cross-section in the gas phase suggesting that the TiO2(110) surace had little or no influence on the dissociative ionization process. No carbon-containing surface products were detected in post-irradiation TPD associated with EID of molecular CH3OH, including no additional methoxyl formation. The initial EID cross-sections for the two types of methoxyls were approximately equivalent regardless of the surface condition, but were a factor of 5 greater in the presence of CH3OH (3.0–3.4×10−15 cm2) than in its absence (5.8–6.2×10−16 cm2). EID of both vacancy and non-vacancy methoxyl resulted in H2CO products bound at the same sites, but vacancy-bound H2CO was resistant to further EID, whereas non-vacancy H2CO was decomposed with further electron exposure. Total D+ ESD cross-sections were several orders of magnitude lower than those measured by post-irradiation TPD, suggesting that the major EID channels involved ejection of neutral species. These results demonstrated the ability of low-energy electrons to active organics adsorbed on oxide surfaces with high cross-sections, and suggest that the EID cross-sections and products for surface organics depend on the coverage, adsorption state and adsorption site as in the case of methanol in TiO2(110). Based on these conclusions, low-energy electrons produced from adsorption of ionizing radiation may play a significant role in the radiocatalytic destruction of organics over oxide catalysts.

Ion scattering study of the Zn and oxygen-terminated basal plane surfaces of ZnO

Abstract The zinc-terminated, ZnO(0001)–Zn, and the oxygen-terminated, ZnO( 000 1 )–O polar surfaces have been studied by low-energy alkali ion scattering. Single scattering analysis shows that both surfaces exhibit a surface structure that is bulk-terminated. No evidence is found for registry shifts at the surface or substantial quantities of point defects such as Zn or O vacancies as may be expected to relieve charge imbalance at the surface. Comparison with the results of computer simulation confirms the bulk-terminated structure, but fails to quantify the interlayer spacing. A less satisfactory agreement between experiment and computer simulation is obtained for the Zn-terminated surface than for the O-terminated surface. It is found that, due to interference of multiple scattering from second and third layers, layer spacings cannot be trivially extracted from single scattering analysis. However, specific scattering conditions are identified for the oxygen-terminated surface, where spacing in the first O–Zn double layer can be obtained. This spacing is found to be nearly identical to the bulk value.

Geometric and electronic structure of epitaxial NbxTi1−xO2 on TiO2(110)

Abstract We have investigated the detailed geometric and electronic structure of MBE-grown Nb x Ti 1− x O 2 on TiO 2 (110) by means of high-resolution transmission electron microscopy, X-ray photoelectron diffraction, ultraviolet and X-ray photoemission and electron energy loss spectroscopy. We find no measurable change in the NbO bond length relative to that for TiO bonds in TiO 2 in the dilute limit ( x = 0.05), and that the epitaxial layers remain strained and coherent with the substrate for x ⩽ ≈ 0.3. However, significant dislocation generation occurs for x > ≈ 0.3. Nb substitution for Ti in the lattice introduces an additional valence electron per atom. The resulting density of states falls in the valence band region, but no new state density occurs in the either the band gap or conduction band. This result is in contrast to what occurs in the very dilute limit (parts per thousand), where Nb electrons occupy a shallow donor level near the conduction band minimum. Based on the electron counting rule, the extra Nb electrons form a non-bonding band which is degenerate with the valence band. The significance of these results for enhanced thermal and photochemistry on Nb x Ti 1− x O 2 surfaces vis a vis TiO 2 is discussed.

Instability of Hydrogenated TiO2

Hydrogenated TiO2 (H-TiO2) is touted as a viable visible light photocatalyst. We report a systematic study on the thermal stability of H-implanted TiO2 using nuclear reaction analysis (NRA), Rutherford backscattering spectrometry, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy. Protons (40 keV) implanted at a ∼2 atom % level within a ∼120 nm wide profile of rutile TiO2(110) were situated ∼300 nm below the surface. NRA revealed that this H-profile broadened toward the surface after annealing at 373 K, dissipated out of the crystal into vacuum at 473 K, and was absent within the beam sampling depth (∼800 nm) at 523 K. Photoemission showed that the surface was reduced in concert with these changes. Similar anneals had no effect on pristine TiO2(110). The facile bulk diffusivity of H in rutile at low temperatures, as well as its interfacial activity toward reduction, significantly limits the utilization of H-TiO2 as a photocatalyst.

Thermal chemistry and photochemistry of hexafluoroacetone on rutile TiO2(110)

The ultraviolet (UV) photon-induced decomposition of hexafluoroacetone ((CF(3))(2)CO; HFA) adsorbed on the rutile TiO(2)(110) surface was investigated using photon stimulated desorption (PSD) and temperature programmed desorption (TPD). HFA adsorbs both molecularly and dissociatively on the reduced TiO(2)(110) surface. The initial approximately 0.2 ML (where 1 ML equates to the cation site density of the ideal surface) coverage of HFA thermally decomposes resulting in the formation of adsorbed trifluoroacetate groups, with further HFA exposure resulting in molecular adsorption. No evidence was found for HFA photochemistry on the reduced surface. HFA adsorbed and desorbed molecularly on a pre-oxidized TiO(2)(110) surface with only a minor amount (approximately 1%) of thermal decomposition in TPD. A new adsorption state at 350 K was assigned to the reversible formation of a photoactive HFA-diolate species [(CF(3))(2)COO]. UV irradiation depleted the 350 K state, resulting in evolution of CF(3), CO, and CO(2) in the gas phase and formation of surface bound trifluoroacetate groups. (18)O isotope scrambling experiments showed that the ejected CO(2) was from photodecomposition of the HFA-diolate species while the CO photoproduct was not. These results are in contrast to the photochemical behavior of acetone, butanone and acetaldehyde on TiO(2)(110), where UV irradiation resulted in the gas phase ejection of one of the carbonyl substituent groups as well as a stoichiometric amount of carboxylate left on the surface. We conclude that fluorination alters the electronic structure of adsorbed carbonyls on TiO(2)(110) in such a way as to promote complete fragmentation of the adsorbed carbonyl complex to form gas phase CO(2) as well as to open up additional photodissociation pathways leading to CO production.