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Formation of Molecular Monolayers on TiO2 Surfaces: A Surface Analogue of the Williamson Ether Synthesis Jixin Chen, Ryan Franking, Rose E. Ruther, Yizheng Tan, Xueying He, Stephanie R. Hogendoorn, and Robert J. Hamers* Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States
bS Supporting Information ABSTRACT: Strategies to modify metal oxide surfaces are important because of the increasing applications of metal oxides in catalysis, sensing, electronics, and renewable energy. Here, we report the formation of molecular monolayers on anatase nanocrystalline TiO2 surfaces at nearambient temperatures by a simple one-step immersion. This is achieved by an analogue of the Williamson ether synthesis, in which the hydroxyl groups of the TiO2 surface react with iodo-alkane molecules to release HI and form a TiOC surface linkage. The grafted molecules were characterized by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to confirm the formation of covalently bonded monolayers. Kinetic studies yielded an activation barrier of ∼59 kJ/mol for the grafting reaction. Measurements of hydrolytic stability of the grafted molecules in water show that approximately half the molecules are removed within minutes to hours at temperatures of 25100 °C with an activation energy of ∼82 kJ/mol, while the remaining molecules are stable for much longer periods of time. These different stabilities are discussed in terms of the different types of TiOC bonds that can form on TiO2 surfaces.
’ INTRODUCTION This paper describes a simple strategy to graft alkyl iodides to TiO2 surfaces via formation of TiOC surface linkages. This strategy is derived from the well-known Williamson ether synthesis,1 in which organic hydroxyl ROH and halide groups R0 X react to form an ether (ROR0 ) with liberation of HX. We show that the ubiquitous surface hydroxy groups25 on TiO2 surfaces will react with alkyl iodides to form covalently bonded molecular monolayers under ambient conditions, and we analyze the kinetics of grafting and hydrolysis of the TiOC bonds via collision theory and transition-state theory to glean fundamental insights into the nature of these reactions. The formation of well-defined organicinorganic interfaces is increasingly important in emerging areas such as hybrid field effect transistors,6,7 organic and dye sensitized solar cells,8 biosensing devices,9,10 and nanomaterials.11 A key component of many of these emerging applications is the need for stable molecular monolayers that can serve as controllable surface linkers and/or form passivation layers at interfaces.12 These layers are often referred to generically as self-assembled monolayers (SAMs). While much effort has been devoted to the formation of selfassembled monolayers on silicon1317 and gold,18,19 developments in renewal energy have placed increased emphasis on development of molecular interfaces to a wider range of materials, especially metal oxides.2022 Prior studies of metal oxides23 have focused on surface linkers such as carboxylic acids,21,22,24 phosphonates,25,26 hydroxamic r 2011 American Chemical Society
acid,27,28 thiols,29 silanes,26 and photochemical grafting of alkenes.30,31 Yet, there remains great interest in improving the stability of the layers and especially making monolayers that will be stable in protic solvents such as water. Previous studies of reactions on group IV semiconductors have shown that, in many cases, bond-forming reactions known from organic chemistry can be used as a basis for new bond-forming reactions at surfaces.13,16,17,3234 Metal oxides have been widely studied as catalysts for oxidation and reduction reactions of small molecules.35 In these studies, some species such as amines, pyridine, ketones, nitriles, and alcohols have been found to be surface poisons and sometimes used to react irreversibly with the active sites on catalysts.36 Generally speaking, bond-forming reactions that poison catalysts might also be used to make stable molecular layers. While catalytic reactions typically take place using gas-phase reactants at elevated temperatures and are often investigated under ultra-high-vacuum conditions, these reactions can give key insights into likely strategies for forming improved surface adducts. The reaction of alkyl halides with alcohol groups to form ether linkages and release HX is widely known as the Williamson ether synthesis.1 Previous studies conducted under ultra-high-vacuum conditions showed that exposure of a TiO2 surface to gas-phase methyl iodide and ethyl iodide produced surface alkoxy groups Received: March 6, 2011 Revised: April 2, 2011 Published: May 02, 2011 6879
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with TiOC ether-like linkages.37,38 These results suggest that the interaction of alkyl halides with hydroxylated TiO2 surfaces may be a general method for linking organic molecules to TiO2 surfaces. However, the extension of this approach to ambient (nonvacuum) conditions and longer molecular layers has not been reported previously. Here, we report an investigation of the interaction of alkyl iodides with TiO2 surfaces under ambient condition and the formation and stability of the resulting layers in air and in water, using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). We find that the molecules bind via formation of TiOC linkages and that a substantial fraction of the adsorbed molecules are highly resistant to hydrolysis in water. Our results suggest that the surprising stability of these molecules is likely connected with the presence of bridge-bonded oxygen atoms at the inorganicorganic interface. This represents a simple method for forming monolayers on TiO2 surfaces.
’ EXPERIMENTAL METHODS Surface Grafting. Nanocrystalline anatase TiO2 films were prepared as described previously31 by screen printing of TiO2 paste (TiNanoxide T20/SP, Solaronix, Switzerland) followed by heating at 500 °C (see Supporting Information). All TiO2 samples were cleaned by acid piranha solution for 20 min before reaction in order to provide a high concentration of surface hydroxyl groups. (Caution: piranha solution, H2SO4:H2O2(30%) = 3:1 by volume is extremely corrosive and potentially explosive upon meeting organics, and should be handled with extreme caution.) Previous studies have cleaned TiO2 samples via piranha solution and via UV-ozone cleaning.39 We observed that samples cleaned by acid piranha exhibited water contact angles approaching 0°, consistent with formation of surface hydroxyl groups and removal of trace organic molecules.40 XPS spectra of the cleaned sample confirmed that there is no sulfate contamination from the piranha cleaning. The hydroxyl-terminated TiO2 films were then soaked in neat n-iodooctane (Sigma-Aldrich, >98%) for surface grafting (caution: iodo-alkanes are irritants and should be handled in the fume hood with proper eye protection from the vapor). Acetone, acetonitrile, and methanol (Sigma-Aldrich) were used as received without further purification, and 18.0 MΩ.cm deionized water was obtained from a Barnstead Nanopure Infinity system. The samples immersed in reactant liquids were sealed in glass vials and heated in an oven. When grafted in diluted iodooctane solution, the samples were grafted in a reflux flask under a dry nitrogen environment. After reaction, the samples were rinsed with acetone for 30 s, then methanol for another 30 s, and then dried in a flow of N2 followed by several more cycles of methanol rinsing and drying. When a clean sample was immersed into iodooctane for several minutes at room temperature, then rinsed with the same method; negligible CH stretching signal was observed using infrared spectroscopy. This indicates that physisorbed iodooctane was effectively removed by the rinsing. Samples were sealed in small centrifuge tubes for storage. When clean TiO2 samples were stored under the same conditions with the tubes filled with air or with deionized water for several weeks, no CH band was detected under FTIR. This indicates that contamination of the sample during storage is negligible for IR measurements. Characterization and Data Analysis. Infrared spectroscopy experiments were performed using a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer with a Veemax II variable-angle singlebounce reflection accessory, using a liquid nitrogen-cooled HgCdTe detector. Measurements shown here were collected using p-polarized light at an incidence angle of 50° from the surface normal. As reference samples, we used both the clean TiO2 film itself and a clean Ti/Si
Figure 1. XPS spectra of the TiO2 films before and after grafting at 80 °C for 14 h (see Supporting Information for more spectra and fitting results). substrate baked under the same condition, forming a thin layer of TiO2 on top of the titanium metal. The flat TiO2 film was used as the reference instead of a clean nanocrystalline film when quantifying the peak area of the CH stretching mode, because the flat references are more reproducible reference samples for comparing spectra obtained on different days. Peak centers were identified by fitting to Voight peak shapes.41 A cubic polynomial baseline defined by the baseline data points adjacent to the absorption bands was used to eliminate residual sloping backgrounds.42 X-ray photoelectron spectroscopy (XPS) data were obtained using a Phi system with a monochromatized Al KR source (1486.6 eV) and a multichannel detector using a takeoff angle of 45°. The atomic sensitivity factors (ASF) for XPS of the elements are C(1s)ASF = 0.296, O(1s)ASF = 0.711, Ti(2p)ASF = 1.798, and I(3d5/2)ASF = 5.337.43 After subtracting a Shirley background to compensate for inelastic scattering,44 the peaks were fited with Voigt functions for quantitative analysis. All fit parameters were adjustable.
’ RESULTS XPS Measurements. Figure 1 shows high-resolution XPS spectra of the C(1s), O(2s), Ti(2p), and I(3d) regions before and after grafting at 80 °C for 12 h. Comparing the spectra before and after grafting, the primary change is an increase in intensity in the 285 eV region due to the grafted molecular layer. Some 6880
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Langmuir intensity is always observed near 285 eV unless the samples are cleaned in vacuum because the bare TiO2 samples readily physisorb CO2 and organic material from the atmosphere. The peak area of the oxygen, after correction for the different sensitivity factors and electron escape depths, is 2.1 times that of the titanium (see Supporting Information), as expected for stoichiometric TiO2. The peak shapes of Ti(2p3/2) are similar to those reported previously for high-quality stoichiometric surfaces.31,45,46 One particularly notable feature of the XPS spectrum is that there is no detectable signal from iodine near 620 eV where the I(3d) peaks occur, even though iodine has a high atomic sensitivity factor (ASF), (I(3d5/2)ASF = 5.337 compared with C(1s)ASF = 0.296).43 On the basis of the noise level of the I(3d5/2) region and our observed C(1s) signal from the grafted molecules, we estimate that the number of iodine atoms on the surface corresponds to less than 0.002 iodine atom/carbon atom. Since each iodooctane molecule has 8 C atoms, this corresponds to less than 0.016 iodine atom per grafted iodooctane molecule. The absence of detectable iodine signal is important because it shows that grafting of the molecules to TiO2 is accompanied by loss of iodine (presumably as HI) into the reactant mixture. This confirms the overall chemical reaction TiOH þ RI f TiOR þ HI. To evaluate the molecular coverage, we used a clean sample to measure the Ti(2p) intensity and the C(1s) intensity of the functionalized layer. The procedure for quantitatively analyzing the data is described in more detail in the Supporting Information. Using this approach, we determined that grafting typically yields a molecular density of ∼3.5 molecules/nm2. FTIR Characterization of Grafting. The rate of grafting at different temperatures was determined using a set of samples that were cleaned in piranha solution and then immersed in neat iodooctane in glass vials. The sealed vials were then heated in a water bath or oven. After the desired reaction time, a vial was removed from the hot water bath and cooled in a cold water bath. The sample was then rinsed several times with acetone, methanol, and blown dry by nitrogen before measurement with FTIR. The temperature uncertainty is (2 °C, and the time uncertainty is ∼1 min. Figure 2a shows an FTIR spectrum of a nanocrystalline anatase film grafted with iodooctane at 40 °C for 20, 60, 120, 270, and 450 min, and 25 h, respectively, (from bottom to top) using a clean nanocrystalline sample as the reference, while Figure 2bd shows enlarged views of specific regions. The baseline shows two extremely broad sinusoidal oscillations peaking near 3000 cm1 and 1800 cm1 that arise from interference effects within the TiO2 thin film.47,48 On the basis of prior studies,18,19,38,42,4953 we can identify four primary regions of importance to understanding the grafting reaction. Sharp features in the 36003720 cm1 arise from the OH stretching modes of TiOH species; the negative (downward) sign of these peaks demonstrates that grafting of iodooctane reduces the number of TiOH groups on the surface. Peaks in the 28003000 cm1 region arise from CH stretching vibrations, while peaks in the 13001500 cm1 region arise primarily from CH bending and twisting modes; these modes reflect alkyl chain of the adsorbed molecules. Finally, a very broad band around 10001200 cm1 is similar to that observed previously from alkoxy groups produced on TiO2 by direct adsorption of alcohols, and includes contributions from TiOC species (∼1170 cm-1) and a number of different CC skeletal modes and CH3 rocking modes (10301140 cm1).49,5355
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Figure 2. (a) FTIR spectra of TiO2 nanocrystalline film grafted at 40 °C for 20, 60, 120, 270, and 450 min, and 25 h, respectively, from bottom to top, using the clean nanocrystalline sample at 0 min as the reference. (bd) Enlarged regions after 25 h grafting. In the OH region of panel (b), the upper plot shows the absorbance of clean sample (before grafting) referenced to a flat oxidized Ti thin film to show the starting distribution of surface OH groups on the clean sample. The lower plot showing a nanocrystalline film after grafting, referenced to the identical sample before grafting, to show the change in OH intensity induced by grafting.
In addition to these chemically relevant modes, there are other modes associated with surface-adsorbed water (1630 cm1)54,56 and adsorbed CO2 (16001680 cm1).57 Our data suggest that grafting displaces some chemisorbed CO2 from the surface, evidenced by the broad negative peak near 1650 cm1. Broad bands at 32003600 cm1 arise from liquid and ice-like water near the TiO2 surface56 and also from water physisorbed on the optics and detector. Except for the 32003600 cm1 water peaks, all other features increased monotonically in strength during the grafting process and reached an obvious saturation behavior (see below). However, because the intensities of 32003600 cm1 peaks varied strongly with room humidity and did not appear to correlate with the surface reactivity, we did not consider them further. Figure 2bd shows expanded views of three chemically significant regions of the spectrum using the sample grafted for 25 h. To facilitate later analysis and eliminate the broad interference pattern, we fit the peaks to Voight line shape functions and used a baseline subtraction based on a cubic polynomial fit to an expanded spectral region outside of the region of interest. 6881
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Figure 3. Grafting of the n-iodooctane at 87 ( 2 °C. (a) FTIR spectra of a series of different samples with different soaking times in neat iodooctane using a clean planar TiO2 substrate as the reference. (b) Integrated absorbances of the bands in (a) as a function of time.
Figure 2b shows the TiOH data presented in two ways. The upper plot shows the absorbance of the TiOH region of the fresh, unreacted nanocrystalline TiO2 sample, referenced to a planar Ti sample; this spectrum reveals the distribution of Ti OH species on the starting surface. The lower panel of Figure 2b shows the change in absorbance induced by grafting, with the negative peaks reflecting the loss of TiOH intensity due to grafting. Both spectra show features near 3631, 3653, 3671, and 3695 cm1. These peaks each arise from slightly different forms of TiOH; however, the detailed atomic arrangements associated with each peak remain controversial35,5861 and will be discussed below. A comparison of the total absorbance and the graftinginduced change in absorbance yields two important observations: (1) that all the TiOH bands decrease by nearly the same fraction, thereby demonstrating that all the available forms of TiOH react with nearly equal probability, and (2) that the change in TiOH vibrational intensity represents a significant fraction of the total TiOH intensity of the starting sample. In repeated experiments with multiple samples at different temperatures, we found that the TiOH vibrational intensity always decreased, but in most cases, there was still some significant TiOH intensity remaining, typically representing a decrease of ∼50100% from the initial value. This indicates that the past history of the sample and/or environmental conditions such as ambient humidity level may influence the density of grafting if sufficient hydroxyl groups are not present. Figure 2c shows the CH region. The starting sample (Figure 3a) is free of virtually all hydrocarbons, while after grafting,
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there are multiple features visible. As shown by the dashed lines in Figure 2c, the spectrum can be fit to five peaks at 2963, 2930, 2902, 2880, and 2859 cm1. These frequencies are similar to those reported previously for molecular layers on other surfaces,18,19,42,50 and based on these studies, we can assign the features to the asymmetric CH3 (2963 cm1), asymmetric CH2 (2930 cm1), the symmetric CH3 (split by Fermi resonance19,62 into two modes at 2902 and 2880 cm1), and symmetric CH2 (2859 cm1) modes. Figure 2d shows the region below 1500 cm1; here, grafting leads new peaks at 1462, 1420, and 1381 cm1. These modes are similar to those reported previously from ethoxy and propoxy groups on TiO2 and attributed to CH bending and twisting modes.54 To evaluate the rate of formation of the moleculesurface bonds, we used FTIR to characterize the surfaces after reacting for different amounts of time and at different temperatures. When the nanocrystalline TiO2 film was soaked in neat niodooctane liquid at room temperature, the CH modes (28003000 cm1) increased in intensity, while the TiOH modes (36003720 cm1) decreased; however, under these conditions the CH band intensity did not reach a constant value until the sample was soaked for approximately 3 days. At higher temperatures, the reaction rate is greatly accelerated. Figure 3 shows FTIR spectra after a nanocrystalline TiO2 sample was heated in n-iodooctane at ∼87 °C for different times, and Figure 3b shows the time-dependent changes in total peak area of the TiOH and CH regions. At this temperature, the CH modes reach a saturation value after approximately 40 min. In this case, it is noteworthy that significant TiOH intensity remains even when the CH modes have stopped changing. Furthermore, the lowest-frequency mode (3631 cm1) decreases in intensity more slowly than the other modes. To confirm that the observed features arise from grafting of the molecules, we conducted control experiments in which clean samples were exposed to rinsing procedures identical to those used for the grafted samples. The FTIR spectra of the control samples showed no significant increase in CH vibrational intensity. Thus, we conclude that the features observed after iodooctane grafting are due to bonding of iodooctane to the surface and not from the small molecules used to rinse the surface. In addition, the ratio of the CH2 to CH3 intensity is consistent with 1-iodooctane and not consistent with what would be expected from binding of methanol or acetone to the surface. While we used the CH stretching modes to characterize the grafting, the other CH and TiOC vibrational features show identical trends (Figure 2). Measurements of Grafting Kinetics. In order to evaluate the activation barrier for grafting, experiments were conducted at different grafting temperatures. Figure 4a,b shows representative data obtained at 40 °C and at 76 °C. In experiments at these and other temperatures, we found that the total area of the CH modes increased according to ACH = B(1 exp(kt)) and the TiOH area decreased according to ATiOH = C exp(kt) where B and C are constants and k is a rate constant for adsorption onto the surfaces. The rate constant k obtained using the CH modes is the same (within experimental error) as that determined using the TiOH modes. These data are consistent with a simple reaction:63 k
TiOH þ RI f s TiOR þ HI
ð1Þ
The rate of grafting is then expected to be r = k[RI][TiOH] where k is the rate constant of grafting, [RI] is the iodooctane 6882
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Figure 5. Graft kinetics using diluted iodooctane on TiO2 surface. (a,b) Normalized CH peak area as a function of time using mixture of iodooctane/hexane = 1:2 and 1:4 by volume.
Figure 4. Reaction kinetics of TiO2 film with neat n-iodooctane. (a, b) FTIR band intensities of CH stretching as a function of reaction time at different temperatures (see Supporting Information for additional data at different temperatures). The peak areas shown are normalized to the areas observed at the longest grafting times measured. The curves are fit using eq 3, with the parameters T and ka listed. (c) Arrhenius plot for the grafting reaction.
concentration on the surface (assuming to be the bulk concentration), and [TiOH] is the coverage of surface hydroxyl groups. Since the bulk concentration is maintained during the grafting, the reaction is pseudo-first-order, and r = ka[TiOH] where ka is the effective rate constant. The coverage of surface hydroxyl groups at time t is expected to vary as ½TiOHt ¼ ½TiOH0 expð ka tÞ
ð2Þ
and the coverage of the surface grafted molecules [TiOR]t follows a simple Langmuir growth ½TiORt ¼ ½TiORsat ½1 expð ka tÞ
ð3Þ
where [TiOR]sat is the final concentration of adsorbed molecules when the surface has achieved complete saturation. According to the Arrhenius equation, it can be expected that the grafting rate constant ka follows the relationship ka = Aa eEa/RT, where Aa is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature. So lnðka Þ ¼ Ea =RT þ lnðAa Þ
ð4Þ
Figure 4c summarizes the values of ka measured at 5 different temperatures and plots them on such an Arrhenius plot. This analysis yields an activation energy ∼59 ( 3 kJ/mol and a preexponential factor Aa ∼1.3 106 M1 s1.
In order to identify the factors controlling the reaction rate, we also performed experiments in which the iodooctane was diluted with n-hexane with volume ratios of 1:2 and 1:4. A control sample of TiO2 heated in neat hexane showed only a very small CH signal, demonstrating that adsorption of the hexane under the experimental time scale is negligible (Supporting Information Figure S7c control). The data in Figure 4c show that grafting of neat iodooctane has a kinetic rate constant ∼3.2 104 M1 s1. Figure 5a,b shows reaction curves for iodooctane/hexane mixtures in a 1:2 volume ratio (Figure 5a) and a 1:4 volume ratio (Figure 5b). These yielded rate constants of 2.7 104 M1 s1 (Figure 5a) and 3.2 104 M1 s1 (Figure 5b), respectively, nearly identical to that observed in neat iodooctane. In addition, we performed similar studies using acetonitrile instead of hexane; those results showed that dilution in acetonitrile slowed down the grafting rate, primarily by direct coadsorption of acetonitrile to the surface (see Supporting Information). Thus, we conclude that diluting the 1-iodooctane does not significantly change the reaction rate in the concentration ranges investigated. Stability of the Organic Layer. The stabilities of the grafted layers were characterized from the intensities of the CH bands in FTIR. In each case, the same sample was used throughout each stability measurement. Air stability experiments were carried out in a sealed container at room temperature. Thermal stability experiments were carried out by putting a grafted sample in an air-filled kiln for 1 h at different temperatures. Water stability tests were carried out in a glass vial of water placed in the oven, with water degassed by boiling for several minutes before the test. We evaluated the stability of the grafted molecular layers using the integrated intensity of the CH stretching band as a probe of the concentration of organic molecules on the surface. Before each FTIR measurement, each sample was rinsed with methanol and dried with N2 for several cycles. Two control experiments were carried out first to characterize the stability at room temperature in air and the thermal stability in air. After storing for one month in air, the intensity of the CH band decreased to 6883
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Figure 6. Thermal stability of the grafted molecular layers in air. (a) FTIR spectra of the sample after heating in air for 1 h at different temperatures. (b) Intensities of the bands as a function of temperature. All measurements were performed after cooling to room temperature.
∼70% of its initial value (Figure S2 in Supporting Information). Figure 6 shows the changes in the CH and TiOH modes when samples were heated in air for 1 h at the indicated temperatures. Very little change in CH or OH modes was observed after heating to 100 °C. However, heating to higher temperatures causes significant changes in the CH and TiO H modes. A more detailed analysis of the individual peak intensities shows that the individual CH modes all decrease at the same rate, but the behavior of the TiOH bands is more complex. The TiOH band at 3631 cm1 decreased as the temperature increased while the TiOH peaks at 3695 cm1 and 3671 cm1 both increased. These results suggest that at elevated temperatures the molecules partially decompose, with hydrogen atoms from cleaved CH bonds forming additional TiOH species. Figure 7 summarizes temperature-dependent hydrolysis of the molecular layers in water at temperatures of 18, 40, 60, and 80 °C. Figure 7a,b shows detailed data at 40 and 80 °C, respectively (additional temperatures are shown in Supporting Information). At all temperatures, immersion in water induced a rapid decrease in the intensity of the CH stretching band to a nonzero value followed by a plateau region during which very little further change is observed. The TiOH features increase in an almost perfectly complementary manner. The decrease to a substantially nonzero value of CH intensity is surprising because it suggests that only some of the grafted molecules are easily removed from the surface by water. To verify that this residual CH intensity arises from the grafted molecules and not from other sources of contamination, we carried out several additional analyses and measurements with control samples. First, we analyzed the intensities of the individual CH vibrational modes and found that immersion in water does not change the relative intensities of the different CH vibrational modes; in particular, it does not change the ratio of the CH3 mode at 2963 cm1 compared to
Figure 7. Water stabilities of the grafted monolayers on TiO2 at different temperatures. (a, b) FTIR band intensities of CH stretching as a function of hydrolysis time at different temperatures (see Supporting Information for additional data at different temperatures). The intensities of CH stretching bands (red squares) were fit by an exponential decay function (eq 6), with the parameters T and kd listed. The intensities of the TiOH stretching bands (blue dots) are shown without fitting due to its relatively larger experimental errors. (c) Continued hydrolysis of the same sample from (a) in NaOH solution (pH = 10.2). (d) Arrhenius plot for the dissociation of the grafted molecules in deionized water.
the CH2 modes near 2930 and 2859 cm1 (see Supporting Information Figure S6 and Table S2). Similarly, we conducted experiments using a cyclic alcohol (thereby lacking methyl groups) and found that grafting this molecule to the surface yielded CH2 modes but no detectable CH3 vibrational features. The experiments show that the residual molecules have the same ratio of methyl (CH3) and methylene (CH2) groups as those that were removed more easily. We also conducted experiments with identical control samples that were freshly cleaned and then immersed in water in a manner identical to that for the grafted samples; experiments over time periods as long as one month showed no significant contamination on the control samples. 6884
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Finally, we note that the residual CH intensity is smaller when the sample is grafted for a shorter time (Figure S3 in Supporting Information). Consequently, we attribute the residual CH intensity observed on the grafted samples to the presence of a second type of grafted molecule that is not easily hydrolyzed from the surface by deionized water. To test whether more harsh conditions would remove the strongly bound molecules, the sample in Figure 7a was removed from water after 1600 min and then exposed to 40 °C NaOH solution (pH ∼10). Figure 7c shows the time evolution of the CH and OH intensities in NaOH. Under these conditions, a further decrease in molecular coverage was induced, with NaOH removing approximately 60% of the remaining molecules and further increasing the TiOH intensity. Under these conditions, a new peak at 3720 cm1 emerged and increased with the further decrease of CH band (see Supporting Information Figure S8). Figure 7d shows a summary of kinetic data for the faster, initial removal of molecules from the surface. For the reaction k
s TiOH þ HOR TiO-R þ H2 O f
ð5Þ
the dissociation rate r = k[H2O][TiOR] = kd[TiOR] where kd is the effective dissociation rate. The surface coverage of organic molecules is expected to be of the form ½TiO-RðtÞ ¼ ½TiO-R0 ½TiO-Req expð kd tÞ þ ½TiO-Req
Figure 8. Changes in individual TiOH modes during hydrolysis. (a) Areas of 3631, 3653, 3671, and 3695 cm1 peaks and total CH area, measured in deionized water at 60 °C (see SI Figure S5c). (b) Changes in peak areas upon immersion in NaOH solution (pH = 10) at 40 °C. A new peak at 3720 cm1 emerged during the NaOH treatment (see SI Figure S8 for the spectra).
ð6Þ where the [TiOR]eq term is used to account for the more strongly bound molecules that are not easily removed by deionized water. We fit our measurements of CH absorbance at short times to this equation to determine kd at four temperatures. Assuming Arrhenius-like behavior, the data yield an activation energy Ed ≈ 82 ( 6 kJ/mol, and a pre-exponential factor is ∼7.4 1010 M1 s1 for the initial removal of the organic layer. Correlation between Change in TiOH and CH Intensities during Hydrolysis. Figure 2a shows that grafting of the molecular layer is accompanied by loss of surface OH groups, while Figure 7 shows that exposure to water at various temperatures removes the organic layer and restores the surface OH groups. To determine whether differences could be observed in different TiOH modes, the TiOH band was deconvoluted into several peaks by fitting (Figure 8). Figure 8a shows the intensities of the individual OH features, along with the CH intensities, from the immersion of the grafted sample in deionized water at 60 °C (this is the same sample shown in Supporting Information Figure S5c). Within the error limits of the experiment, it appears that all OH modes increase with a similar time dependence, and the final distribution is similar to that of the starting sample. Under harsh basic conditions, however, the distribution of OH groups can be changed. Figure 8b shows the behavior when this sample was subsequently immersed in NaOH solution (pH ∼10). In this case, a new peak at 3720 cm1 emerged as the remaining CH intensity decreased (see Supporting Information Figure S8).
’ DISCUSSION The above experiments show that TiO2 surfaces can be modified with a monolayer of organic molecules via a reaction that is functionally equivalent to the Williamson ether synthesis.64
In the classical Williamson ether synthesis, an alkoxide (R1 ONaþ) and an alkyl iodide (R2I) will react via the reaction: R 1 O Naþ þ R 2 I f R 1 O R 2 þ NaI
ð7Þ
The reaction involves a nucleophilic attack of the alkoxide on the alkyl halide and occurs via an SN2 mechanism. While the alkoxide is the active species, the reaction can also proceed from the alcohol R 1 -OH þ R 2 -I f R 1 -O-R 2 þ HI
ð8Þ
In this case, the reaction is typically considered as two separate steps ðiÞ R 1 -OH f R 1 -O þ Hþ
ð9Þ
ðiiÞ R 1 -O þ R 2 -I f R 1 -O-R 2 þ HI
ð10Þ
Correlation between TiOH and TiOR Species. The above mechanism implies that the surface grafting is closely connected to the presence of OH groups on the TiO2 surface. TiO2 anatase surfaces are terminated with titanium and oxygen atoms whose coordination numbers are lower than those of the bulk.35,51 While in the bulk the Ti atoms have a coordination number of 6 and the oxygen atoms have a coordination number of 3, at the surface there are exposed fivefold and possibly fourfold coordinate Ti atoms and twofold O atoms (referred to as “bridging oxygens”). The relative amounts of these undercoordinated species vary between different crystal planes. Electron microscopy studies suggest that the (001) and (101) faces predominate on microcrystalline anatase,65 although recent studies suggest that nanocrystalline anatase has complex behavior.66,67 Figure 9a shows a model of a section of the anatase(101) surface, which consists of 5-coordinate Ti atoms (Ti54þ) and 6885
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Figure 9. Scheme of the two different binding sites for the hydroxyl groups (a) and the alkoxy groups (b) on the anatase TiO2(101) surface.
2-coordinate bridging oxygens (Ti54þ)2O. Water molecules can bind molecularly to the surface or can decompose to form OH groups. This produces at least two distinct types of OH groups (labeled here as Ti54þOH and (Ti54þ)2OH on the surface.65,68 Other crystal faces may expose additional lowsymmetry sites, giving rise to a number of chemically distinct forms of OH groups on the surface. In general, however, the two types depicted in Figure 9a are representative of the most common two types of hydroxyls present. These two types of hydroxyls have very different properties. For example, a recent combined experimental and computational study showed that Ti54þOH groups are well deprotonated in water at even low pH values, while the bridge-bonded (Ti54þ)2OH groups remain protonated even at pH ≈ 10.69 Our FTIR spectra show at least five sharp OH stretching peaks at 3631, 3653, 3671, 3695, and 3720 cm1. While a definitive assignment is precluded by the relatively large number of different types of surface sites present, it is generally agreed that the peak at ∼3631 cm1 arises from protonated bridging oxygen, and the peaks near ∼3695 cm1 and ∼3671 cm1 arise from Ti54þOH species.61,70,71 Another peak has sometimes been observed at ∼3717 cm1 (often on reduced samples) and attributed to Ti3þOH groups due to nonstoichiometry of the surface or due to surface defects.61,70 We do not observe a significant feature in this region on our clean or grafted samples, except on samples that have been immersed in NaOH solution (Supporting Information Figure S8). Overall, our FTIR data indicate that our samples are relatively stoichiometric and have relatively few defects, in agreement with our XPS observations. The decrease in intensity of the OH peaks at 3671, 3695, and 3653 cm1 during grafting and the reappearance of these peaks upon hydrolytic cleavage of the molecules from the surface
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(Figure 8a) indicate that the grafting reaction involves the Ti54þOH groups. This is consistent with the results of prior ultra-high-vacuum studies of alcohol adsorption onto TiO2.68 However, we note that attributing the reactions to specific types of surface OH groups is difficult because sample handling after grafting (such as rinsing and exposure to air) could change the distribution of TiOH groups and because previous studies have shown that surface OH groups can diffuse readily.68 However, exposure to NaOH solution produces significant increases of the peak at 3720 cm1 (Figure 8b), which has been attributed to surface defects.61,70 Rate-Limiting Step and Grafting Mechanism. Our results show that liquid-phase alkyl halides will react with hydroxyl groups on the surface of TiO2 to graft organic molecules to the surface. While this process appears equivalent to the Williamson ether synthesis, the formation of analogous products does not necessarily imply that the reaction on TiO2 surfaces follows the same mechanistic pathway. Of particular note is that the TiOH groups of TiO2 are much more acidic than those of typical alcohols. For example, in aqueous solutions the TiOH groups on TiO2 surfaces (both rutile and anatase) have pK values between 5.0 and 6.0,72 while organic alcohols typically have pK of ∼16.73 If we assume the reaction proceeds via an SN2 mechanism like the analogous liquid-phase reaction, then we anticipate that the rate-limiting step could be (1) the deprotonation of surface hydroxyl groups to form TiO sites, (2) the interaction of iodooctane molecules with the TiO sites, or (3) formation and leaving of HI from the intermediate. The key experimental evidence here is that our dilution experiments show that the rate of reaction does not depend on the concentration of iodooctane (Figure 5). This indicates that the rate-limiting step does not involve the free iodooctane molecules, but must involve another step. Some additional insights can be achieved through an analysis of the grafting kinetics using collision theory and transition-state theory.74 The small value of the pre-exponential factor suggests a very hindered transition state. A simple analysis using transitionstate theory yields an activation entropy of 128 J K1 mol1 at 300 K (see Supporting Information). In the context of collision theory, this can be described in terms of a steric factor, which has a value of ∼1 104 for the grafting reaction (see Supporting Information). The small steric factor and the fact that the grafting rate is not affected by dilution suggest that the grafting is limited by processes other than simple molecular collisions with the surface. While our data provide no direct information on the chemical structure of the transition state, the nonpolar nature of iodooctane is expected to favor nonpolar and/or less ionic pathways. In polar media, the Williamson ether synthesis typically involves nucleophilic attack of the RI bond by the deprotonated alcohol (RO). However, such ionic intermediates are expected to be much less favorable in nonpolar media such as iodooctane. Consequently, the low solvent polarity may favor a concerted mechanism in which the surface OH group interacts with the reactant CI group via a four-member ring before releasing HI. While our data are not sufficient to elucidate the detailed nature of the transition state, our data clearly demonstrate that it is highly hindered. Molecular Packing Density. The infrared data provide additional insights into the factors that control the packing density. The CH vibrational data in Figure 3 and Figure 4, along with additional data at different temperatures in Supporting Information, 6886
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Langmuir all show that at long times the CH vibrational intensity reaches a plateau; this behavior demonstrates that the reaction selfterminates. This self-terminating behavior could in principle arise either from a limiting reactant (such as the surface OH groups) or from steric constraints that limit the packing density. Examination of the infrared data in Figure 3 shows that, while grafting induces a decrease in the concentration of surface OH groups, this change is relatively modest compared with the total OH intensity. This strongly suggests that the self-terminating behavior does not arise from a lack of available OH sites. Quantitative analysis of the C(1s) and Ti(2p) intensities in XPS can be used to extract a molecular coverage, as described in Supporting Information. Applying this analysis to molecular layers grafted for 5 h at 80 °C yields a packing density of 3.5 1014 molecules/cm2. Previous studies of the formation of alkoxides by thermal decomposition of alcohols on TiO2 under ultra-high-vacuum conditions have yielded molecular densities of 2.8 1014 molecules/cm2 for methanol,75 3.2 1014 molecules/cm2 for ethanol and n-propanol,76 and∼4 1014 ethoxide groups/cm2 for ethanol.68 Our results suggest that molecular densities prepared under ambient conditions are comparable to those produced by thermal decomposition. Self-assembled monolayers of alkanethiols on gold also typically produce molecular densities of ∼4 1014 molecules/cm2.77,78 Possible Origin of Water-Stable Grafted Molecular Layers. One of the more surprising results of this work is that hydrolytic stability measurements suggest the presence of two distinct types of grafted molecules, or perhaps two distinct channels for waterinduced dissociation. Because the experiments were performed using nanocrystalline films, we conducted additional control studies to ensure that the unexpected stability of the molecular layers was not associated with incomplete wetting (i.e., small pores rendered inaccessible by the hydrophobic nature of the molecules) or by contamination. First, while iodooctane-functionalized samples could be soaked in hot water for weeks without seeing further removal of the molecular layers, when similar experiments were conducted using decanoic acid we observed that the decanoic acid molecules were totally removed by soaking in water for several hours. In addition, we do not observe a change in the shape of the CH stretching bands when different molecules adhered on the TiO2 film during the water treatment and rinsing (data not shown). We also performed a small number of experiments using 1,1,1,2,2-pentafluoro-4iodobutane (purity >99%) via XPS and found its behavior qualitatively similar to that of 1-iodooctane. Previous studies of alcohols adsorbed onto single crystal and nanocrystalline TiO2 surfaces in ultrahigh vacuum used temperature-programmed desorption (TPD) to characterize the stability of alkoxy groups on TiO2.3 In these studies, heating alcohol-exposed TiO2 typically yielded two sets of desorption peaks: one at 300400 K due to desorption of the alcohol via associative recombination (TiOH þ Ti-OR f TiO þ Ti þ ROH) and another peak near 600 K due to decomposition and production of various dissociation products, particularly dehydrogenation to form alkenes.68,75,76,79,80 One proposed explanation for this behavior was that, because OH groups can also recombine and desorb as water, there may be insufficient OH groups to react with the alkoxides to reform the alcohols.76 However, later studies showed that desorption of the more weakly bound molecules followed by dosing with extra water had little effect on the high-temperature desorption feature and
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consequently proposed that the difference in thermal stability was associated with two different binding conformations for the alkoxides. Figure 9b depicts these two conformations. corresponding to binding via atop (monodentate) Ti54þ OAlkyl and a stronger binding site via bridged (bidentate) oxygens, forming (Ti54þ)2OAlkyl, respectively.53,68,75 These monodentate and bidentate alkoxys have also been directly imaged under scanning tunneling microscopy (STM).81,82 Although the prior experiments were done in vacuum, the alkoxys on TiO2 should share common structures with those of our samples. Our results suggest that the rapidly desorbing species may be bonded to the surface as the Ti54þOAlkyl alkoxy groups, while the more stable form may be the bidentate (Ti54þ)2OAlkyl form. Our data for the more quickly desorbing molecules yields activation an an activation energy of ∼82 ( 6 kJ/mol and a preexponential factor of 7.4 1010 M1 s1 (Figure 7) for the hydrolysis reaction. For comparison, the activation energy for TiO2 nanoparticle growth from titanium(IV) isopropoxide in water was reported as ∼72 kJ/mol,83 likely within experimental error of the value we observe. This suggests that both cases may share the same hydrolysis step: when water attacks the alkoxy group on the surface, the free alcohol leaves and either oxygen or surface hydroxyl group remains exposed.
’ CONCLUSIONS We have demonstrated that a surface analogue of a wellknown organic reaction, the Williamson ether synthesis, can be applied to react organohalides with the titanol (TiOH) groups of TiO2 to graft organic molecules to the TiO2 surface. Our results show that molecules are bound in two different forms with very pronounced differences in hydrolytic stability. The less stable form undergoes rapid hydrolysis in water, desorbing within minutes to hours at modest temperatures. The more stable form survives even in hot water for much longer periods of time, but can be removed using NaOH solution. While the experiments reported here are not able to establish definitive reaction sites, there are intriguing similarities to prior studies in ultrahigh vacuum showing two different binding configurations of alkoxy groups having very different stabilities. Our results suggest that, despite a 1013-fold difference in the pressure of reactants used, there may be some strong analogies between the types of molecular structures observed in a vacuum and those that can be formed in ambient conditions. Ultimately, a deeper understanding of the stable molecular species observed in ultrahigh-vacuum studies may yield improved methods for forming stable molecular interfaces to TiO2 and other metal oxide semiconductors. ’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental procedures for film fabrication; XPS data and methods for calculation of the surface coverage; FTIR data for air stability, thermal stability, and water stability tests; calculations for the collision theory and the transition state theory. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (608) 262-6371. E-mail:
[email protected]. 6887
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’ ACKNOWLEDGMENT The work described here was supported in part by the National Science Foundation Grant CHE0911543 and the Dept. of Energy Office of Basic Energy Sciences DE-SC0002160. ’ REFERENCES (1) Williamson, A. Philos. Mag. 1850, 37, 350–356. (2) Arrouvel, C.; Digne, M.; Breysse, M.; Toulhoat, H.; Raybaud, P. J. Catal. 2004, 222, 152–166. (3) Farfan-Arribas, E.; Madix, R. J. J. Phys. Chem. B 2002, 106 10680–10692. (4) Henderson, M. A. Langmuir 1996, 12, 5093–5098. (5) Hugenschmidt, M. B.; Gamble, L.; Campbell, C. T. Surf. Sci. 1994, 302, 329–340. (6) Mathijssen, S. G. J.; Smits, E. C. P.; van Hal, P. A.; Wondergem, H. J.; Ponomareko, S. A.; Moser, A.; Resel, R.; Bobbert, P. A.; Kemerink, M.; Janssen, R. A. J.; de Leeuw, D. M. Nat. Nanotechnol. 2009, 4 674–680. (7) Ortiz, R. P.; Facchetti, A.; Marks, T. J. Chem. Rev. 2010, 110 205–239. (8) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595–6663. (9) Rickert, J.; G€opel, W.; Beck, W.; Jung, G.; Heiduschka, P. Biosens. Bioelectron. 1996, 11, 757–768. (10) Wong, L. S.; Khan, F.; Micklefield, J. Chem. Rev. 2009, 109, 4025–4053. (11) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435–446. (12) Marcinko, S.; Fadeev, A. Y. Langmuir 2004, 20, 2270–2273. (13) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Acc. Chem. Res. 2000, 33, 617–624. (14) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688–5695. (15) Wang, X.; Landis, E. C.; Franking, R.; Hamers, R. J. Acc. Chem. Res. 2010, 43, 1205–1215. (16) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37, 3257–3260. (17) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491–11502. (18) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52–66. (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (20) Galoppini, E. Coord. Chem. Rev. 2004, 248, 1283–1297. (21) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gratzel, M. J. Phys. Chem. B 2000, 104, 1300–1306. (22) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (23) Neouze, M.-A.; Schubert, U. Monatsh. Chem. 2008, 139 183–195. (24) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451–455. (25) Gawalt, E. S.; Avaltroni, M. J.; Koch, N.; Schwartz, J. Langmuir 2001, 17, 5736–5738. (26) Helmy, R.; Fadeev, A. Y. Langmuir 2002, 18, 8924–8928. (27) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813–824. (28) McNamara, W. R.; Milot, R. L.; Song, H. E.; Snoeberger, R. C.; Batista, V. S.; Schmuttenmaer, C. A.; Brudvig, G. W.; Crabtree, R. H. Energ. Environ. Sci. 2010, 3, 917–923. (29) Dibbell, R. S.; Soja, G. R.; Hoth, R. M.; Watson, D. F. Langmuir 2007, 23, 3432–3439. (30) Li, B.; Franking, R.; Landis, E. C.; Kim, H.; Hamers, R. J. ACS Appl. Mater. Interfaces 2009, 1, 1013–1022.
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(31) Franking, R. A.; Landis, E. C.; Hamers, R. J. Langmuir 2009, 25, 10676–10684. (32) Hovis, J. S.; Coulter, S. K.; Hamers, R. J.; D’Evelyn, M. P.; Russell, J. N.; Butler, J. E. J. Am. Chem. Soc. 2000, 122, 732–733. (33) Bent, S. F. Surf. Sci. 2002, 500, 879–903. (34) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Am. Chem. Soc. 1997, 119, 11100–11101. (35) Kung, H. H. In Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier Science Publishers B. V.: Amsterdam, 1989; pp 5557. (36) Kn€ ozinger, H. Adv. Catal. 1976, 25, 184–273. (37) Su, C.; Yeh, J.-C.; Chen, C.-C.; Lin, J.-C.; Lin, J.-L. J. Catal. 2000, 194, 45–54. (38) Wu, W.-C.; Liao, L.-F.; Shiu, J.-S.; Lin, J.-L. Phys. Chem. Chem. Phys. 2000, 2, 4441–4446. (39) Bullard, J. W.; Cima, M. J. Langmuir 2006, 22, 10264–10271. (40) Wang, C.-Y.; Groenzin, H.; Shultz, M. J. Langmuir 2003, 19, 7330–7334. (41) Economou, I. G.; Cui, Y.; Donohue, M. D. Macromolecules 1991, 24, 5058–5067. (42) Faucheux, A.; Gouget-Laemmel, A. C.; de Villeneuve, C. H.; Boukherroub, R.; Ozanam, F.; Allongue, P.; Chazalviel, J.-N. Langmuir 2006, 22, 153–162. (43) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1992. (44) Shirley, D. A. Phys. Rev. B 1972, 5, 4709–4714. (45) G€opel, W.; Anderson, J. A.; Frankel, D.; Jaehnig, M.; Phillips, K.; Sch€afer, J. A.; Rocker, G. Surf. Sci. 1984, 139, 333–346. (46) Pan, J.-M.; Maschhoff, B. L.; Diebold, U.; Madey, T. E. J. Vac. Sci. Technol., A 1992, 10, 2470–2476. (47) White, J. U.; Ward, W. M. Anal. Chem. 1965, 37, 268–270. (48) Freeman, E. C.; Paul, W. Phys. Rev. B 1978, 18, 4288–4300. (49) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1991, 87, 2655–2659. (50) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682–691. (51) Davydov, A. A. In Molecular Spectroscopy of Oxide Catalyst Surfaces; Sheppard, N. T., Ed.; John Wiley & Sons Ltd.: Chichester, U.K., 2003; pp 72, 103105. (52) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed.; John Wiley &Sons, Inc.: New York, 2001. (53) Wu, W. C.; Chuang, C. C.; Lin, J. L. J. Phys. Chem. B 2000, 104, 8719–8724. (54) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1991, 87, 2661–2668. (55) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans.1 1989, 85, 1723–1742. (56) Morterra, C. J. Chem. Soc., Faraday Trans. 1 1988, 84 1617–1637. (57) Rasko, J.; Solymosi, F. J. Phys. Chem. 1994, 98, 7147–7152. (58) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1216–1220. (59) Tanaka, K.; White, J. M. J. Phys. Chem. 1982, 86, 4708–4714. (60) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 25, 61–69. (61) Lobo-Lapidus, R. J.; Gates, B. C. Chem.—Eur. J. 2010, 16 11386–11398. (62) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70, 842–851. (63) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (64) Williamson, A. W. Q. J. Chem. Soc. 1852, 4, 229–239. (65) Martens, J. H. A.; Prins, R.; Zandbergen, H.; Koningsberger, D. C. J. Phys. Chem. 1988, 92, 1903–1916. (66) Zhang, H. Z.; Chen, B.; Banfield, J. F. Phys. Chem. Chem. Phys. 2009, 11, 2553–2558. (67) Zhang, H. Z.; Chen, B.; Banfield, J. F.; Waychunas, G. A. Phys. Rev. B 2008, 78. (68) Gamble, L.; Jung, L. S.; Campbell, C. T. Surf. Sci. 1996, 348 1–16. 6888
dx.doi.org/10.1021/la2008528 |Langmuir 2011, 27, 6879–6889
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ARTICLE
(69) Panagiotou, G. D.; Petsi, T.; Bourikas, K.; Garoufalis, C. S.; Tsevis, A.; Spanos, N.; Kordulis, C.; Lycourghiotis, A. Adv. Colloid Interface 2008, 142, 20–42. (70) Szczepankiewicz, S. H.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2000, 104, 9842–9850. (71) Vuk, A. S.; Jese, R.; Gaberscek, M.; Orel, B.; Drazic, G. Sol. Energ. Mater. Sol. C 2006, 90, 452–468. (72) Kosmulski, M. Adv. Colloid Interfac. 2009, 152, 14–25. (73) McEwen, W. K. J. Am. Chem. Soc. 1936, 58, 1124–1129. (74) Adamson, A. W. In A Textbook of Physical Chemistry, 3rd ed.; Academic Press, Inc.: Orlando, FL, 1986; pp 52, 574, 584. (75) Henderson, M. A.; Otero-Tapia, S.; Castro, M. E. Faraday Discuss. 1999, 114, 313–329. (76) Kim, K. S.; Barteau, M. A.; Farneth, W. E. Langmuir 1988, 4, 533–543. (77) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222–1227. (78) Bindu, V.; Pradeep, T. Vacuum 1998, 49, 63–66. (79) Kim, K. S.; Barteau, M. A. J. Mol. Catal. 1990, 63, 103–117. (80) Kim, Y. K.; Kay, B. D.; White, J. M.; Dohnalek, Z. J. Phys. Chem. C 2007, 111, 18236–18242. (81) Zhang, Z.; Bondarchuk, O.; Kay, B. D.; White, J. M.; Dohna, Z. J. Phys. Chem. C 2007, 111, 3021–3027. (82) Zhang, Z.; Rousseau, R.; Gong, J.; Kay, B. D.; Dohnalek, Z. J. Am. Chem. Soc. 2009, 131, 17926–17932. (83) Oskam, G.; Nellore, A.; Penn, R. L.; Searson, P. C. J. Phys. Chem. B 2003, 107, 1734–1738.
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