Article

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Alternative Low-Pressure Surface Chemistry of Titanium Tetraisopropoxide on Oxidized Molybdenum Alexis M. Johnson† and Peter C. Stair*,†,‡

† Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

*S Supporting Information

ABSTRACT: Titanium tetraisopropoxide (TTIP) is a precursor utilized in atomic layer depositions (ALDs) for the growth of TiO2. The chemistry of TTIP deposition onto a slightly oxidized molybdenum substrate was explored under ultrahigh vacuum (UHV) conditions with X-ray photoelectron spectroscopy. Comparison of the Ti(2p) and C(1s) peak areas has been used to determine the surface chemistry for increasing substrate temperatures. TTIP at a gas-phase temperature of 373 K reacts with a MoOx substrate at 373 K but not when the substrate is at 295 K, consistent with a reaction that proceeds via a Langmuir−Hinshelwood mechanism. Chemical vapor deposition was observed for depositions at 473 K, below the thermal decomposition temperature of TTIP and within the ALD temperature window, suggesting an alternative reaction pathway competitive to ALD. We propose that under conditions of low pressure and moderate substrate temperatures dehydration of the reacted precursor by nascent TiO2 becomes the dominant reaction pathway and leads to the CVD growth of TiO2 rather than a self-limiting ALD reaction. These results highlight the complexity of the chemistry of ALD precursors and demonstrate that changing the pressure can drastically alter the surface chemistry.

■ INTRODUCTION kinetics.4,7,8 Many of these experiments are summarized in a 4 Atomic layer deposition (ALD) is a technique that relies on dedicated review by Puurunen. Studies of other ALD metal two sequential gas−surface reactions to grow materials in a precursors and corresponding growth processes of metal oxide fi lms, such as Nb2O5, HfO2, ZrO2,Al2O5, and TiO2, have also layer-by-layer fashion. This method is employed in the − semiconductor industry and is most commonly used to deposit been performed.9 12 Additional experiments have explored the 1 fl ff metal oxides. However, ALD has become an emerging in uence of di erent oxidizing precursors including H2O, 13−15 technique for the fabrication of novel and complex catalytic ozone, and O2 plasma. systems and in the process has been extended to the growth of While TiCl4 is the common precursor for the growth of fi 2,3 metal lms and nanoparticles on catalytic supports. TiO2, titanium tetraisopropoxide (TTIP) is an important ALD employs multiple cycles that are combined to build alternative. TTIP is preferable for materials intended for materials with precise thickness control. One ALD cycle is catalytic applications since residual surface ligands can be composed of two half cycles that result in the regeneration of removed by calcination.16 In contrast with TiCl , residual the initial surface groups, providing new reactive sites for the 4 surface chlorine induces Brønsted acidity that imparts following cycle. Given a finite number of surface reaction sites and steric constraints imposed by the ligands, only a finite potentially undesirable catalytic functionality. While the thermal number of precursor molecules can react, resulting in the self- decomposition of TTIP for chemical vapor deposition (CVD) limiting nature of the technique.4 Reactive sites on oxides applications is the subject of a number of publications, a smaller 5,17−19 include hydroxyl groups, bridging oxygen atoms, and defect number have looked at the ALD chemistry of TTIP. An sites. enlightening study by Rahtu and Ritala combines mass There has been a significant amount of work directed toward spectrometry, quartz crystal microbalance (QCM), and TPD elucidating ALD chemistry, utilizing techniques such as X-ray experiments to explore the reaction mechanism of TTIP photoelectron spectroscopy (XPS), mass spectrometry, quartz deposition onto both a predosed Ti(OCH(CH3)2)4−x covered crystal microbalance (QCM), temperature-programmed de- surface and SiO2 for TPD and QCM experiments, respec- sorption (TPD), DFT, and ab initio calculations.5,6 The most studied system is the deposition of Al O from trimethylalu- 2 3 Special Issue: John C. Hemminger Festschrift minum (TMA) and H2O because it is widely used and considered an “ideal” deposition process. Significant work has Received: June 6, 2014 been done to establish a mechanism for the reaction, identify Revised: October 13, 2014 the source of self-limiting behavior, and understand the reaction

© XXXX American Chemical Society A dx.doi.org/10.1021/jp505653u | J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article tively.17 The current understanding is that the TTIP reaction dosing, transferring into the HPC, and performing analytical with a metal oxide surface follows eqs 1a and 1b measurements. The sample was a 1 cm diameter Mo(100) single-crystal disk i ° 2 −+OH(s) Ti(O Pr) 4(g) that was cut and mechanically polished to within 1 of the 100 plane. It was initially cleaned in a separate UHV chamber by i −7 →−( O − )Ti(OPr)22(s)32(g) + 2(CH)CHOH (1a) heating to 1500 K in 5 × 10 Torr of oxygen for 48 h to remove bulk carbon and sulfur. Two 0.25 mm Mo wires were i used to suspend the crystal between two thermally isolated (−−O)Ti(OPr)2HO2 2(s) + 2 (g) copper leads at the bottom of the manipulator shaft. The sample was resistively heated, and the temperature was →−( O − ) Ti( − OH) + 2(CH ) CHOH 22(s)32(g)(1b) measured with a K-type thermocouple spot-welded to the back of the crystal. Between TiO2 deposition experiments the In this mechanism, half of the precursor ligands are removed crystal was cleaned with argon ion sputtering using 2 keV ions upon the initial reaction with surface hydroxyl groups (eq 1a), at normal incidence followed by brief annealing to 1073 K. This and the other half are released with a subsequent water dose procedure was repeated until the crystal was mostly free of (eq 1b). titanium. The crystal was subsequently annealed at 773 K under Equations 1a and 1b can be generalized for many ALD 600 L of O2 to remove carbide species that had formed during processes on metal oxide surfaces for which the majority of the the cleaning process. XPS was used to verify the chemical state reactive sites are surface hydroxyl groups. However, as ALD and composition of the crystal surface. Peak areas were applications are broadening more nontraditional substrates are calculated with the appropriate sensitivity factor for comparison being used, and the chemistry involved is less well understood. between elements, and key elemental ratios are tabulated for In particular, only a few studies have explored reaction each experiment in the Supporting Information.25 mechanisms on noble metal surfaces, such as Cu and Pd, and TTIP Deposition. Titanium tetraisopropoxide (TTIP, these studies predominantly involve the TMA/H O proc- 2 Sigma-Aldrich 97 and 99.999%) was purified using several ess.20,21 freeze−pump−thaw cycles before it was deposited onto the In this study we use a slightly oxidized Mo(100) sample as Mo(100) single crystal in the UHV chamber by one of two our substrate to develop a better understanding of how experimental methods: (1) dosing lines and (2) the Gemstar-6 deposition proceeds on alternative surfaces compared to ALD system. 97% pure TTIP was used in the dosing lines, deposition on typical metal oxides such as Al O , TiO ,or 2 3 2 while 99.999% purity was used in the Gemstar-6 system; SiO . In addition, it is advantageous to use MoO as the base 2 x however, both systems produced similar deposition chemistry substrate in order to minimize reactions with the and are detailed below.26 Once a dose was complete, the ligands on TTIP. While molybdenum oxides can catalyze chamber was evacuated for 2−3 h until the pressure reached alcohol dehydration to through an alkoxide inter- − 10 9 Torr for analysis with XPS. A Mo(3d) satellite peak mediate, recent studies have shown that an alkoxide on the overlaps with the C(1s) region and was removed through surface of MoO would be more likely to recombine with a 3 subtraction of spectra that were taken of the base substrate hydrogen atom to desorb as the alcohol than dehydrate to the prior to precursor dosing. .22,23 As the Mo oxidation state decreases below 6+ Gas Delivery via Dosing Lines. The dosing lines were dehydration of the alkoxide is even less probable. X-ray used for experiments with the substrate heated to 573 and 473 photoelectron spectroscopy (XPS) was used to explore the K. The dosing line is evacuated with a mechanical pump to ∼10 nature of the chemistry during TiO deposition by TTIP onto 2 mTorr and is comprised of a precursor vessel, a leak valve, and MoO surfaces at different temperatures. The ratio of Ti(2p) x a sapphire tube. The precursor vessel is isolated from the and C(1s) XPS peak areas was used to determine whether the dosing line by a needle valve. A leak valve (Granville-Philips) TiO deposition followed CVD or ALD mechanisms at each 2 separates the low vacuum side of the line from the UHV system temperature. Surprisingly, we found that CVD can be the dominant reaction mechanism even at temperatures used in and is used to introduce gaseous precursors into the chamber. standard ALD growths that are below where TTIP decom- The end of the dosing line is a sapphire tube with a 0.25 mm poses. tantalum wire coiled around it for resistive heating to control the temperature of the gas entering the system. Prior to deposition, the head space of the precursor vessels and the ■ EXPERIMENTAL METHODS dosing lines were evacuated and purged several times with N2 Instrumentation and Sample Mounting. All experi- gas before directed dosing with either TTIP or H2Otoa − ments were performed in a previously described three-level pressure of 1 × 10 6 Torr. All precursor vessels and dosing lines ultrahigh vacuum (UHV) apparatus with a base pressure that were kept at room temperature, ∼295 K, during the duration of ranged from 5 × 10−10 to 2 × 10−9 Torr.24 Briefly, the upper these experiments, while the sapphire tube was heated to 383 level includes an Extrel quadrupole mass spectrometer, low K. The vapor pressures of TTIP and water at room temperature energy electron diffraction (LEED), ion sputtering gun, heated are 0.4 and 21 Torr, respectively.27,28 After each dose, the lines × −5 dosing lines, and a dual-anode Thermo/VG Microtech X-ray and the chamber were purged with 1 10 Torr of N2 for a photoelectron spectrometer with a hemispherical electron minimum of 15 min. The QMS was used to verify the presence energy analyzer. The middle level houses a high-resolution of the intact precursor during the dose and the disappearance of electron energy loss spectrometer, and the bottom level the precursor during the N2 purge. contains a high-pressure cell (HPC). Recently, an Arradiance Gas Delivery via the Gemstar-6. A commercial ALD Gemstar-6 ALD system was interfaced to the top level of the system (Arradiance Gemstar-6) was interfaced to the UHV UHV apparatus. A 360° rotating XYZ manipulator with a 1 m chamber through an auxiliary port on the system. One end of a Z-translator was used to position the sample for sputtering, gas 1/4″ OD stainless steel tube is inserted into the oven of the

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ALD system, and a fraction of the gas stream is directed into the UHV system toward the surface of the Mo substrate. A VCR gasket at the end of the stainless steel tube with a 75 μm diameter laser-drilled hole reduces the pressure from 1 to 10 Torr to ∼10−7−10−6 Torr. The connecting line could be evacuated with a turbo pump (Pfeiffer-Balzer 070) and heated to 383 K. The temperatures of the TTIP and water containers were 353 and 295 K, respectively. The corresponding vapor pressure for TTIP is ∼2.2 Torr.28 The head space of the precursor bottle was evacuated before dosing. Doses were performed by first closing the isolation valve between the ALD system and the mechanical pump evacuating it. The precursor was then dosed into the ALD reaction chamber followed by fl minimizing the carrier N2 ow to prevent large changes to the partial pressures within the reaction chamber over the course of the dose. After 15 s of equilibration time the gas was introduced into the UHV chamber. After each dose was complete the connecting line was purged with N2 for a minimum of 20 min, and the concentration of precursor remaining in the chamber was monitored with the QMS. The partial pressure of TTIP × −7 × and H2O entering the chamber was less than 3 10 and 3 10−6 Torr, respectively, based on the changes in the base pressure read by an ion gauge (Granville-Philips). This experimental setup was used for the set of deposition experiments in which the surface of the Mo crystal was heated to 373 K. ■ RESULTS Base MoOx Surface. A clean Mo (100) surface is extremely Figure 1. Representative (a) Mo(3d), (b) O(1s), and (c) C(1s) reactive and will dissociatively adsorb gases at room temper- spectra from the base Mo substrate. ature until a passivation layer is formed.29 The annealing temperatures used in the study temporarily removed the carbon, but were not high enough to completely desorb all of correspond to oxygen surface coverages of 0.7, 1.7, and 1.5 the oxygen. The base Mo surface in these experiments was monolayers. For oxygen coverages above 1.5 monolayers, the passivated by both carbon and oxygen, as shown in the two higher coverages in our case, it has been reported that the representative XPS data in Figure 1 for the base substrate used surface has chemical and physical properties similar to MoO2. in the 573 K experiments. The integrated peaks from panels a, Oxygen on surfaces with less than 1.5 monolayers is b and c corresponding to Mo(3d), O(1s) and C(1s) spectra, chemisorbed.36,37 Because the surface structure and hence the respectively, give a Mo:O:C ratio of 11:7:1. The binding stoichiometry is not known we refer to the surface as MoOx. energies for the Mo(3d5/2) peak from the base substrates in this The presence of chemisorbed oxygen implies that the Mo(100) work ranged between 227.9 and 228.0 eV with a full width half- surface is passivated by a layer of oxygen atoms onto which the maximum (fwhm) of ∼1.1 eV, Figure 1a. The binding energies adventitious carbon becomes adsorbed. for the O(1s) peak range from 530.4 to 530.7 eV with a TTIP Deposition onto a 573 K MoOx Surface. The − corresponding range of fwhm of 1.9 3.3 eV, Figure 1b. All results of TTIP deposition onto a 573 K base MoOx surface in O(1s) spectra are well fit with two Gaussians, with peaks at 300 L (1 Langmuir = 1 × 10−6 Torr s) intervals are shown in 530.5 and 531.4 eV, consistent with O(1s) spectra from Figure 2. 573 K is above the reported TTIP thermal molybdenum oxides found in the literature.30 The C(1s) peak decomposition temperature and would be expected to produce has a binding energy of 285.2 eV and a fwhm of 2.4 eV, chemical vapor deposition, whereby the growth of TiO2 is consistent with adventitious carbon. dependent only on the integrated precursor exposure. Panels a After a brief anneal to 573 K the C(1s) signal disappeared and b show the Ti(2p) and C(1s) spectra from the base surface from the base MoOx surface but grew back to the concentration (black) and after three 300 L doses of TTIP (red). The ffi fi shown in Figure 1c during the experiment. It is di cult with the Ti(2p3/2) peak from the base Mo surface is insigni cant, but current XPS orientation, θ =0°, to determine the exact after TTIP exposure there is a large peak with a binding energy elemental ratio of the top few layers of the substrate. Previous of 459.4 eV and a peak splitting of 5.7 eV which is indicative of ff work from our laboratory has explored the e ects of TiO2 formation; however, there is charging in the sample which chemisorbed oxygen on the structure, oxidation state, and leads to slightly higher binding energies.25 There is a persistent − acidity of Mo(100).29,31 34 These early experiments used CO C(1s) peak from adventitious carbon present on the base dissociation to calibrate the O(1s) signal to establish a metric surface, but this set of experiments also resulted in a for the expected intensity ratio of the Mo(3d5/2) to the O(1s) contribution at lower binding energies from carbidic carbon. peaks for increasing oxygen coverage ranging from 0.3 to 2 In Figure 2b, the C(1s) spectrum from the base Mo surface and monolayers on a Mo(100) surface.35 For this work, the O/Mo the total C(1s) spectrum taken after 900 L of TTIP are shown ratios are 0.06, 0.15, and 0.14 for the base substrate in the 573, in black and red. The C(1s) spectrum from the base surface 473, and 373 K experiments, respectively. These ratios was subtracted from the total C(1s) spectrum after 900 L of

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Figure 2. Results for increasing 300 L doses of TTIP onto a 573 K Mo substrate. (a) Ti(2p) spectra taken before (black) and after (red) a total of 900 L of dosing time, (b) C(1s) spectra taken of the base surface (black) and the total C(1s) signal (red) after a total of 900 L of TTIP, (c) the C(1s) signal added from 900 L of TTIP dosing (red) and the C(1s) spectrum from the isopropoxide in condensed vanadyl triisopropoxide (VOTP), and (d) Ti (black squares) and C (open blue circles) surface densities as a function of TTIP exposure.

TTIP to isolate the C(1s) signal due to the depositions alone. and is commonly used when growing TiO2 inside an ALD The reported C(1s) spectra have been corrected in this reactor using TTIP as the metal precursor.13 Panels a and b manner. The resultant C(1s) peak shape (Figure 2c) is bimodal show the respective Ti(2p) and C(1s) spectra for the base with similar peak ratios to that of the spectrum from an MoOx (black) and after four sequential 300 L of TTIP (red). isopropoxide ligand (blue) obtained in a previous study from While the peak area is significantly lower at this temperature condensed vanadyl triisopropoxide.38 than at 573 K, there is still a significant Ti(2p) signal from All elemental ratios and surface densities are summarized in deposited TiO2 which has a slightly lower binding energy of Table S1 (Supporting Information), with an error associated 459.0 eV, likely from reduced sample charging on a thinner with the integration and background subtraction process of deposited metal oxide film. The C(1s) spectrum from the base ∼8%. The Ti and C surface densities for the base substrate surface (black line in Figure 3b) is similar to the base signal in were 0.2 and 1.8 atoms/nm2, respectively. Figure 2d displays Figure 2b but is lacking a contribution from carbidic carbon. the change in Ti (black squares) and C (open blue circles) The red line represents the increase in C(1s) signal after four surface density above the base substrate value as a function of 300 L doses. The spectrum is bimodal and resembles the C(1s) precursor exposure time. As demonstrated by the graph, the Ti spectrum of condensed isopropoxide ligands, similar to Figure surface density increases linearly with exposure time, while the 2c. While the water doses reduced the intensity of the C(1s) C surface density is approximately constant. The ratio of peak signal, the shape of the peak remained the same, indicating the areas, C(1s):Ti(2p), from the spectra after 900 L of TTIP is loss of exchangeable isopropoxide ligands. ∼1:4.9. The Ti(2p):O(1s) ratio indicates that at 573 K the The elemental ratios and surface densities calculated after films are near stoichiometric. The reported density of ALD each dose are shown in Table S2 of the Supporting fi 3 grown TiO2 lms at 573 K is 4.1 g/cm , resulting in a Ti surface Information. The base substrate had a Ti and C surface density of 9.9 atoms/nm2.39 This is a conservative guide to density of 0.4 and 2.0 atoms/nm2. Figure 3c shows the changes determine when a monolayer of TiO2 has been deposited. For a in Ti (black squares) and C (open blue circles) surface substrate temperature of 573 K, after the second 300 L TTIP densities above the base substrate value after each dose. The C dose a monolayer of TiO2 has been grown with 11.3 Ti atoms/ surface density grows in a manner similar to the case of the 573 2 nm . K MoOx surface, in which it remains fairly constant with TTIP and Water Deposition onto a 473 K MoOx increasing TTIP exposure. On the other hand, the Ti surface fi Surface. TTIP was deposited onto the base MoOx surface density is almost linear with exposure time. These lms are held at 473 K. The results shown in Figure 3 are from four slightly deficient in oxygen, which is consistent with previous sequential 300 L doses (1200 total L) of TTIP followed by two studies of TiO2 ALD with TTIP onto another clean surface, sequential water doses of 300 and 600 L. The temperature of Si(001), at 373 K.18 The Ti atom surface densities suggest that the substrate is within the ALD window for TTIP deposition 40% of a full monolayer of titanium oxide was grown by the

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Reactions on MoOx at 295 and 373 K. TTIP was dosed into the UHV chamber and onto a 373 K base Mo surface using the Gemstar-6 system in three 20 L intervals followed by two 70 L intervals. After a total of 200 L of TTIP exposure the surface was dosed with 360 L of water followed by another 20 L dose of TTIP. The results are shown in Figure 4. The substrate

Figure 3. Results for increasing 300 L doses of TTIP onto a 473 K Mo substrate. (a) Ti(2p) spectra taken before (black) and after (red) a total of 1200 L of TTIP, (b) C(1s) spectra taken before (black) and after (red) a total of 1200 L of TTIP, and (c) Ti and C surface densities as a function of TTIP (black squares and blue circles) and water (red squares and circles) exposure.

Figure 4. Results for increasing three 20 L and two 70 L doses of end of the final dose. The C surface density is greater than the TTIP onto a 473 K Mo substrate. (a) Ti(2p) spectra taken before Ti surface density for the first two doses with a C(1s):Ti(2p) (black) and after (red) a total of 200 L of TTIP, (b) C(1s) spectra ratio of 1.3:1. This ratio implies that an average of 1.3 C atoms taken before (black) and after (red) a total of 200 L of TTIP, and (c) are deposited for every Ti atom. After 1200 L of TTIP exposure Ti (black squares) and C (open blue circles) surface densities as a the Ti surface density is larger than that of C with a ratio of function of precursor exposure. Surface density changes from H2O 1:2.3. doses are shown in red. The red squares and red circles in Figure 3c represent the Ti temperature is well below reported decomposition temper- and C surface densities, respectively, after 300 L and then 600 L fi atures for TTIP. Figure 4a shows the Ti(2p) spectra from a of H2O. There is a 47% decrease in the C density upon the rst base Mo surface and after 200 L of TTIP in black and red, 300 L of H2O, but there was no impact on the C density from the subsequent 600 L H2O dose. As expected, the Ti surface respectively. The binding energy, 458.9 eV, and peak splitting, density increases only slightly due to partial removal of the 5.9 eV, are similar to the results displayed in Figure 3 and carbon overlayer. consistent with a Ti−O containing species on the surface.

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Figure 4b displays the C(1s) peak from the base surface which control the temperatures of the gaseous precursors and the has a similar shape to previous adventitious carbon spectra. The substrate and therefore study substrate temperature effects in red line in Figure 4b represents the increase in C(1s) signal on more detail. In the current study we have explored the changes the surface after 200 L of TTIP dosing. While the peak shape in the chemistry of TTIP with increasing substrate temper- and binding energies are very similar to the spectra in Figures 2 atures while keeping the precursor gas temperature at 373 K. and 3, the signal-to-noise ratio is too low for further peak shape On the time scale of our experiments, there is no evidence of analysis. reaction with either water or TTIP when the substrate is held at Elemental ratios and Ti atom surface densities are reported room temperature (295 K). However, deposition was observed in Table S3 in the Supporting Information. Panel c depicts the when the substrate temperature was increased to 373 K. Our changes in the Ti and C surface densities (black squares and results indicate that thermal activation by the substrate is open blue circles) above the base substrate values as a function critical as increasing the gas-phase temperature alone to 373 K of precursor exposure time. Unlike the previous experiment at is insufficient for achieving growth. This means that the 473 K, the C surface density was consistently higher than the precursor first accommodates to the surface temperature prior corresponding Ti surface density. The C(1s):Ti(2p) ratio after to reaction and implies a Langmuir−Hinshelwood rather than the first 20 L TTIP dose was approximately 5.8:1 and dropped an Eley−Rideal mechanism for the surface chemistry. to 2.8:1 after a total of 200 L of TTIP exposure. Following 200 Temperature Dependence of TTIP Deposition onto L of TTIP exposure, the surface was dosed with 360 L of water, MoOx. The growth mode and chemistry of TTIP deposition and the results are represented by the red markers in Figure 4c. onto a base MoOx surface was studied with XPS as a function The Ti surface density remained the same, while the C surface of substrate temperature. The shapes of the Ti(2p) and C(1s) density decreased by ∼33%. The remaining signal had the same spectra, ratio of their peak areas, and Ti and C surface densities shape as shown in Figure 4b, consistent with the loss of after TTIP or water dosing are used to determine the type of isopropoxide from the surface. After the water another 20 L growth. dose of TTIP was applied which increased both the Ti(2p) and Atomic layer deposition and chemical vapor deposition are the C(1s) signals. Films grown at 373 K are significantly oxygen related processes for the growth of materials. As mentioned deficient. The Ti atom surface densities indicate that 17% of a previously, ALD is a process that is composed of two sequential full monolayer of titanium oxide was grown by the end of the self-limiting gas−surface reactions. Alternatively, CVD is the final dose. growth of materials from chemical reactions of gas-phase Two experiments with the surface at 295 K were also precursors. CVD can arise from a variety of mechanisms, performed. In the first experiment, 40 L of 388 K TTIP was including gas-phase bimolecular reactions, thermal decom- dosed onto a base room temperature MoOx substrate. No position of the precursor or surface reactions that regenerate titanium or carbon was deposited. The second experiment reactive sites. Since CVD and ALD are similar, it can be hard to involved dosing a room temperature TTIP-terminated surface distinguish between the two. One critical difference is self- with 720 L of 388 K water. No detectable changes in the C(1s) limiting growth in ALD. spectra were observed. Chemical vapor deposition through thermal decomposition − of TTIP has been well studied.17,28,46 52 CVD products can ■ DISCUSSION include TiO ,HO, propene, H , isopropanol, acetone, and 2 2 2 diisopropyl ether, depending on the reaction condi- Several studies have demonstrated that ALD can be carried out 17,47,48,53,54 under UHV conditions (10−6 Torr) with pressures that are 6−7 tions. A common thermal decomposition mecha- orders of magnitude lower than in traditional ALD reactors nism includes propene as a product, as represented in eq 2. − 12,18,40,41 (0.1 10 Torr). However, it has been suggested that Ti(OCH(CH ) )→+ TiO 4C H + 2H O the pressure difference between UHV conditions and tradi- 3 2 4(g) 2(s) 3 6(g) 2 (g) tional ALD can result in different chemistry.42,43 XPS, mass (2) spectrometry, secondary ion mass spectrometry (SIMS), For ALD of TTIP on a metal oxide surface, the reaction in the temperature-programmed desorption (TPD), IR spectroscopy, first half-cycle is eq 1a. It is important to note that the reaction and electron energy loss spectroscopy (EELS) are some of the terminates with some number of carbon-containing ligands methods that have been used to study and verify self-limiting remaining on the surface, covering potential reactive sites. This growth on flat surfaces at lower pressures.5,8,12,40,44,45 The most is responsible for the self-limiting behavior of ALD. Comparing ff studied process is the growth of Al2O3 by deposition of eq 1a and 2, one critical di erence between CVD and ALD is trimethylaluminum and water.4 IR and EELS experiments the lack of carbon expected to remain on the surface after CVD. conducted by Soto et al. and mass spectrometry experiments In our studies, we use this difference to determine the growth performed by Juppo et al. have successfully observed ALD-like mode. Not only do we observe surface carbon species but also behavior from lower-pressure dosing.8,43 A few other ALD the shape of the C(1s) spectra is similar to the C(1s) spectrum processes have been explored under UHV conditions, including of condensed vanadyl triisopropoxide observed in previous XPS studies by Roy et al. on the deposition of ZrO2 and studies performed in the same apparatus. This indicates that the coupled XPS and SIMS experiments by Lemonds et al. to carbon signal is predominantly composed of isopropoxide investigate the chemistry of TaF5 and Si2H6 half-cycle ligands remaining on the surface after the reaction. Although reactions.40,41 In both cases self-limiting deposition was the Ti(2p) or Mo(3d) spectra cannot be used to distinguish observed with substrate temperatures up to 523 K. Finally, whether the isopropoxide ligands are bound to Ti or Mo atoms, our own experiments investigating VOx deposition onto metal the decreases in C(1s) intensity after water dosing indicate that oxide surfaces have observed ALD-like growth of vanadia in an exchangeable ligands are present. UHV system.38 The more informative XPS results are the C(1s) to Ti(2p) Room Temperature TTIP Chemistry. Unlike traditional peak area ratio and corresponding surface densities which ALD reactors, our UHV setup allows us to independently should indicate the ratio of ligands to titanium atoms remaining

F dx.doi.org/10.1021/jp505653u | J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article after each dose. For ALD-like reactions, we expect that there dose, the total C(1s):Ti(2p) ratio immediately decreases to should be, on average, 1−2 isopropoxide ligands per Ti atom 2.8:1, indicating that less than one ligand remains on the remaining on the surface if there is less than one monolayer surface per reacted TTIP molecule and there was an overall loss coverage. This would be reflected in the XPS spectra as a of ligands from the first dose to the second. The ratio remains C(1s):Ti(2p) ratio that is greater than or equal to 3:1. Instead, relatively constant through 200 L of TTIP exposure. These for all doses of TTIP onto a 573 K MoOx surface the Ti(2p) ratiosaremoresuggestiveofCVD-likegrowthofTiO2 signal is greater than the C(1s) signal (Figure 2d), and the Ti compared to the first dose. These results suggest that ALD fi surface density increases linearly with TTIP exposure, therefore chemistry takes place during the rst dose because TiO2 is not indicating that TiO2 is being formed by CVD. This observation yet present on the surface, but following doses approach CVD is consistent with previous thermal decomposition stud- behavior as the TiO2 concentration builds up and the ies.17,19,46,47,49,51 dehydration reaction pathway becomes possible. Chemical Vapor Deposition of TTIP at 473 K. At 473 K We have considered the possible role of the MoOx surface in all doses of TTIP also resulted in low C(1s):Ti(2p) atomic this alternative reaction pathway. MoO3 and to a lesser degree ratios which decreased from 1.3:1 to 1:2.3 as the TTIP MoO2 are also alcohol dehydration catalysts; however, recent exposure increased. This indicates that CVD also occurs at 473 studies have shown that the surface preferentially K even though this is within the observed ALD temperature desorb as the alcohol rather than dehydrate to the alkene.22,23 13,55 window. Moreover, while thermal decomposition is a The presence of MoO3 was undetectable by XPS, and its common mechanism of CVD, 473 K is below the reported presence is not expected on Mo surfaces treated as described. TTIP thermal decomposition temperatures. We have consid- Therefore, substrate-catalyzed dehydration is not expected to ered the possibility of gas-phase reactions from residual water in contribute to the overall dehydration reaction. To further the chamber regenerating reactive sites from ligand exchange address the possibility of molybdenum oxide catalyzed causing uncontrolled growth, especially during the long dehydration TTIP was deposited onto a 473 K MoOx substrate evacuation and data collection periods. However, this can be with an oxygen coverage of 0.5 monolayer, a significantly lower ruled out by our experiment showing the absence of reaction oxygenconcentrationthaninthepreviousexperiment. between dosed water and the 295 K TTIP-covered surface. Vibrational spectroscopy and surface polarizability studies One possible CVD mechanism which explains our have shown that oxygen overlayers with this concentration do observations is that, under UHV conditions at 473 K, we are not have the structure or electronic properties of molybdenum 34−36 observing TiO2-catalyzed dehydration of TTIP molecules. As oxide but rather behave as chemisorbed oxygen. This far as we know, this reaction mechanism has not been proposed surface will have even less activity than MoO2 for dehydration before; however, it is well known that TiO is a dehydration of the alkoxide ligand. The elemental ratios, Ti atom surface 2 − catalyst that is active for alcohol dehydration above 500 K.56 60 densities, and a description of the experiment are detailed in Recent TPD experiments of isopropanol on TiO2(110) Table S4 of the Supporting Information. Due to the low reported a low-temperature dehydration pathway around 300 reactivity of TTIP only 45% of a complete TiO2 monolayer is K.58,59 It has also been established that the intermediate for the deposited after a total of 2040 L of TTIP exposure. However, reaction is the alkoxide which then undergoes β-hydride we observe low C(1s):Ti(2p) ratios, indicating CVD-like − elimination to form the alkene.56 58,60 While the adsorbed growth which is consistent with the results from the more species from TTIP deposition onto TiO2 is not exactly oxidized substrate. This suggests that the base MoOx substrate equivalent to the reaction intermediate for isopropanol does not play a significant role in the alternative reaction dehydration on TiO2, the same mechanism and products, pathway. Implications for Atomic Layer Deposition Processes. H2O and C3H6, have been reported for TTIP decomposi- tion.17,46,47,53,54 It is reasonable that the alkoxide ligand This alternative reaction pathway to ALD chemistry for TTIP chemistry of a deposited TTIP is similar to the chemistry of deposition is not restricted to MoOx substrates because the the intermediate alkoxide formed during alcohol dehydration. active dehydration site is predominantly TiO2. However, it is Thus, during low-pressure deposition, initially formed TiO2 improbable that the dehydration reaction pathway plays a major species catalyze dehydration of subsequent TTIP molecules, to role during ALD processes in traditional reactors because the fl 6− 7 form more TiO2 and regenerate reactive sites rather than precursor ux on the surface is 10 10 higher. Because of the terminating the deposition reaction by saturating the surface higher flux, all active dehydration sites are covered by deposited with isopropoxide ligands. This alternative dehydration path- TTIP molecules, and dehydration is quenched. This flux way is always possible during TTIP deposition onto/or near dependence has possible implications for substrate architectures with mass transport limitations, such as long nanoscale pores. TiO2 but is likely suppressed under the higher-pressure ALD conditions because the increased flux of TTIP molecules hitting Our results suggest that CVD, instead of ALD, occurs at the fl the surface leads to a more rapid surface saturation covering the bottom of these pores because the ux of TTIP molecules may ffi TiO sites active for dehydration. We propose that under the be insu cient to inhibit the dehydration pathway. The CVD 2 fi experimental conditions of low flux and moderate temperatures reactions would then lead to a lling of the pores rather than a the ALD reaction kinetics are slow enough that dehydration thin conformal coating. becomes the dominant pathway. ■ CONCLUSION Lowering the MoOx substrate temperature to 373 K causes a fi signi cant change in the growth mode. The integrated C(1s) The chemistry of TTIP deposition on to a MoOx surface was signal is larger than for the Ti(2p) signal for all TTIP and water explored at a series of substrate temperatures in UHV doses. After the first TTIP dose the calculated C(1s):Ti(2p) conditions. Compared to successful depositions onto a 373 K − ratio is 5.3:1 which means that on average 1 2 ligands are MoOx surface, the lack of reactivity of 373 K TTIP with a 295 remaining on the surface per reacted TTIP molecule, consistent K surface indicates that the surface temperature is a critical with ALD chemistry for the first dose. After the second TTIP parameter. TTIP molecules must accommodate to the surface

G dx.doi.org/10.1021/jp505653u | J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article prior to reaction and follow a Langmuir−Hinshelwood Reaction between Trimethyl Aluminum and Water. J. Vac. Sci. mechanism. Technol., A 1991, 9, 2686−2695. (9) Aarik, J.; Aidla, A.; Sammelselg, V.; Uustare, T.; Ritala, M.; CVD of TiO2 from TTIP was observed for substrate temperatures of 573 and 473 K, which are, respectively, within Leskela, M. Characterization of Atomic Layer Growth from Titanium Ethoxide and Water. Thin Solid Films 2000, and below the reported thermal decomposition range for TTIP. − −6 370, 163 172. It is believed that, under the conditions of low pressure (10 (10) Kukli, K.; Forsgren, K.; Aarik, J.; Uustare, T.; Aidla, A.; Torr) and moderate temperatures (473 K), dehydration of Niskanen, A.; Ritala, M.; Leskela, M.; Harsta, A. Atomic Layer TTIP molecules by nascent TiO2 not only becomes Deposition of Zirconium Oxide from Zirconium Tetraiodide, Water competitive with the ALD reaction but actually becomes the and Hydrogen Peroxide. J. Cryst. Growth 2001, 231, 262−272. dominant reaction. At lower temperature, 373 K, ALD-like (11) Kukli, K.; Ritala, M.; Sundqvist, J.; Aarik, J.; Lu, J.; Sajavaara, T.; chemistry is observed during the first TTIP dose, but XPS Leskela, M.; Harsta, A. Properties of Hafnium Oxide Films Grown by results suggest that for subsequent doses the growth mode Atomic Layer Deposition from Hafnium Tetraiodide and Oxygen. J. fl − transitions into CVD. In all cases, the in uence of the MoOx Appl. Phys. 2002, 92, 5698 5703. substrate appears to be negligible. While these results may not (12) Rahtu, A.; Kukli, K.; Ritala, M. In Situ Mass Spectrometry Study on Atomic Layer Deposition from Metal (Ti, Ta, and Nb) Ethoxides directly impact traditional ALD processes, this type of − alternative chemistry may be relevant to high surface area and Water. Chem. Mater. 2001, 13, 817 823. (13) Ritala, M.; Leskela, M.; Niinisto, L.; Haussalo, P. Titanium substrates with long nanopores, where mass transport to the Isopropoxide as a Precursor in Atomic Layer Epitaxy of Titanium bottom of the pore is limited and CVD occurs in preference to Dioxide Thin Films. Chem. Mater. 1993, 5, 1174−1181. typical ALD reactions. (14) Rai, V. R.; Vandalon, V.; Agarwal, S. Influence of Surface Temperature on the Mechanism of Atomic Layer Deposition of ■ ASSOCIATED CONTENT Aluminum Oxide Using an Oxygen Plasma and Ozone. Langmuir − *S Supporting Information 2012, 28, 350 357. Details on elemental ratios and surface densities and effects of (15) Rai, V. R.; Agarwal, S. Surface Reaction Mechanisms during the oxygen coverage on the base substrate on TTIP deposition. Ozone-Based Atomic Layer Deposition of Titanium Dioxide. J. Phys. Chem. C 2008, 112, 9552−9554. This material is available free of charge via the Internet at (16) Lu, J. L.; Kosuda, K. M.; Van Duyne, R. P.; Stair, P. C. Surface http://pubs.acs.org. Acidity and Properties of TiO2/SiO2 Catalysts Prepared by Atomic Layer Deposition: UV-visible Diffuse Reflectance, DRIFTS, and ■ AUTHOR INFORMATION Visible Raman Spectroscopy Studies. J. Phys. Chem. C 2009, 113, Corresponding Author 12412−12418. *E-mail: [email protected]. (17) Rahtu, A.; Ritala, M. Reaction mechanism studies on -water atomic layer deposition process. Chem. Vap. Notes Deposition 2002, 8,21−28. The authors declare no competing financial interest. (18) Lee, S. Y.; Jeon, C.; Kim, S. H.; Kim, Y.; Jung, W.; An, K.-S.; Park, C.-Y. In-Situ X-ray Photoemission Spectroscopy Study of Atomic ■ ACKNOWLEDGMENTS Layer Deposition of TiO2 on Silicon Substrate. Jpn. J. Appl. Phys. − This material is based upon work supported by the National 2012, 51, 031102/1 031102/3. (19) Cho, S.-I.; Chung, C.-H.; Moon, S. H. Temperature- Science Foundation under Grant No. CHE-1058835. The Programmed Desorption Study on the Decomposition Mechanism authors would also like to thank Kunlun Ding, Miles Tan, − of Ti(OC3H7)4 on Si(100). J. Electrochem. Soc. 2001, 148, C599 Charlie Campbell, and Bruce Rosen for helpful discussions. C603. (20) O’Neill, B. J.; Jackson, D. H. K.; Crisci, A. J.; Farberow, C. A.; ■ REFERENCES Shi, F. Y.; Alba-Rubio, A. C.; Lu, J. L.; Dietrich, P. J.; Gu, X. K.; (1) George, S. M. Atomic Layer Deposition: An Overview. Chem. Marshall, C. L.; et al. Stabilization of Copper Catalysts for Liquid- Rev. 2010, 110, 111−131. Phase Reactions by Atomic Layer Deposition. Angew. Chem., Int. Ed. (2) Lu, J. L.; Elam, J. 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