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Transmission FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to probe the details of sulfur dioxide adsorption and photooxidation on titanium dioxide nanoparticle surfaces. Adsorption sites, surface speciation and photooxidation chemistry have been determined from analysis of FTIR spectra in conjunction with isotope labeling experiments. These data show that surface hydroxyl groups are involved in the adsorption of sulfur dioxide, and in particularly, sulfur dioxide reacts with either one surface O-H group to yield adsorbed bisulfite or two surface O-H groups to yield adsorbed sulfite and water. Using (16)O-H, (16)O-D and (18)O-H labeled surface O-H groups, additional insights into the adsorption mechanism as well as shifts in the vibrational modes of adsorbed sulfite have been determined. Upon irradiation, adsorbed sulfite/bisulfite converts to adsorbed sulfate. The relative stability of adsorbed sulfite to adsorbed sulfate on TiO2 nanoparticle surfaces was also examined in the presence of increasing relative humidity (RH). It is shown here that adsorbed water can more easily displace sulfite compared to sulfate by forming a stable sulfur dioxide water complex in the presence of adsorbed water. These differences in the RH-dependent stability of adsorbed species that form as a result of surface heterogeneous reactions on oxide particles surfaces has important implications in the heterogeneous chemistry of mineral dust aerosol in the atmosphere.

The reactivity of O

The surface photochemistry of nitrate, formed from nitric acid adsorption, on hematite (α-Fe2O3) particle surfaces under different environmental conditions is investigated using X-ray photoelectron spectroscopy (XPS). Following exposure of α-Fe2O3 particle surfaces to gas-phase nitric acid, a peak in the N1s region is seen at 407.4 eV; this binding energy is indicative of adsorbed nitrate. Upon broadband irradiation with light (λ > 300 nm), the nitrate peak decreases in intensity as a result of a decrease in adsorbed nitrate on the surface. Concomitant with this decrease in the nitrate coverage, there is the appearance of two lower binding energy peaks in the N1s region at 401.7 and 400.3 eV, due to reduced nitrogen species. The formation as well as the stability of these reduced nitrogen species, identified as NO(-) and N(-), are further investigated as a function of water vapor pressure. Additionally, irradiation of adsorbed nitrate on α-Fe2O3 generates three nitrogen gas-phase products including NO2, NO, and N2O. As shown here, different environmental conditions of water vapor pressure and the presence of molecular oxygen greatly influence the relative photoproduct distribution from nitrate surface photochemistry. The atmospheric implications of these results are discussed.

Formic acid adsorption and photooxidation on TiO2 nanoparticle surfaces at 296 K have been investigated using transmission FTIR spectroscopy. In particular, the role of adsorbed water in surface coordination, adsorption kinetics, and photoproduct formation is examined. Gas-phase formic acid adsorbs on the surface at low exposures to yield adsorbed bridged bidentate formate and, at higher exposures, molecularly adsorbed formic acid as well. Upon exposure to water vapor, adsorbed formate becomes solvated by coadsorbed water molecules, and the coordinatin mode changes as indicated by shifts in the vibrational frequencies. Adsorbed water also impacts the adsorption kinetics for formic acid on TiO2 and increases the adsorption rate, potentially by providing a medium for facile ionic dissociation. Ultraviolet irradiation of adsorbed formate on TiO2 in the presence of molecular oxygen results in the formation of gas-phase carbon dioxide, which increases in yield in the presence of adsorbed water on the surface. Additionally, the dispersion of TiO2 nanoparticles in water suspensions is found to change if first exposed to gas-phase formic acid before dispersion. The environmental implications of these results are discussed.

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The need for the conformal deposition of TiO 2 thin films in device fabrication has motivated a search for thermally robust titania precursors with noncorrosive byproducts. Alkylamido-cyclopentadienyl precursors are attractive because they are readily oxidized, yet stable, and afford environmentally mild byproducts. We have explored the deposition of TiO 2 films on OH-terminated SiO 2 surfaces by in situ Fourier transform infrared spectroscopy using a novel titanium precursor [(EtCp)-Ti(NMe 2 ) 3 (1), Et = CH 2 CH 3 ] with either ozone or water. This precursor initially reacts with surface hydroxyl groups at ≥150 °C through the loss of its NMe 2 groups. However, once the precursor is chemisorbed, its subsequent reactivities toward ozone and water are very different. There is a clear reaction with ozone, characterized by the formation of monodentate formate and/or chelate bidentate carbonate surface species; in contrast, there is no detectable reaction with water. For the ozone-based ALD process, the surface formate/carbonate species react with the NMe 2 groups during the subsequent pulse of 1, forming TiOTi bonds. Ligand exchange is observed within the 250−300 °C ALD window. X-ray photoelectron spectroscopy confirms the deposition of stoichiometric TiO 2 films with no detectable impurities. For the water-based process, ligand exchange is not observed. Once 1 is adsorbed, there is no spectroscopic evidence for further reaction. However, there is still TiO 2 deposition under typical ALD conditions. Co-adsorption experiments with controlled vapor pressures of water and 1 indicate that deposition arises solely from 1/water gas-phase reactions. This striking lack of reactivity between chemisorbed 1 and water is attributed to the electronic and steric effects of the EtCp group and facilitates the observation of gas-phase reactions.

In situ Fourier transform infrared (FTIR) spectroscopy is used to investigate silicon dioxide deposition on OH-terminated oxidized Si(100) surfaces using two aminosilanes, di-sec-butylaminosilane (DSBAS) and bis­(tert-butylamino)­silane (BTBAS), with ozone as the coreactant. Both DSBAS and BTBAS readily react at 100 °C with surface −OH groups (loss at 3745 cm–1) with formation of Si–O–SiH3 and Si–O–SiH2–(NH t Bu), respectively, through elimination of secondary and primary amines. The (O−)­SiH3 structure is characterized by a strong Si–O–Si band at 1140 cm–1, and sharp (O−)­SiH3 stretch (2192 cm–1) and deformation (983 cm–1) bands. SiH3 remains stable up to 400 °C, at which point rearrangement into bidentate ((O−)2SiH2) and then tridentate ((O−)3SiH) bonding takes place through condensation reaction with neighboring OH or O groups. In contrast, the O–SiH2–(NH t Bu) structure obtained from BTBAS exposure at 100 °C loses its NH t Bu group at ∼350 °C, leading to a bidentate bonding ((O−)2SiH2) that remains stable up to 500 °C. In both cases, the transformation to bidentate and tridentate bonding depends on the initial OH concentration. The degree of ligand exchange during atomic layer deposition (ALD) with ozone also depends on the ozone flux. For a high enough flux (≥300 sccm, P ∼ 7.5 Torr), the ligand exchange is essentially complete, with the ozone pulse reacting with Si–H x [loss of vibrational bands at 2192 and 983 cm–1 for DSBAS, and at 2972, 2185, and 924 cm–1 for BTBAS] and forming surface Si–O–H (3745 cm–1). The initial Si–O–Si band at 1140 cm–1 broadens upon ozone exposure, consistent with the formation of a Si–O–Si network that extends the existing SiO2 substrate. In steady state, the ALD process is characterized by reaction of SiH x by ozone with the formation of OH, thus sustaining the ALD process, with densification of stoichiometric silicon oxide [transverse optical (TO) and longitudinal optical (LO) phonon modes at 1053 and 1226 cm–1].