ARTICLE pubs.acs.org/JPCC
Ultraviolet-Induced Grafting of Alkenes to TiO2 Surfaces: Controlling Multilayer Formation Ryan Franking and Robert J. Hamers* Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706, United States
bS Supporting Information ABSTRACT: Photochemical grafting of alkenes has emerged as a versatile way to form functionalized surfaces and enables multistep surface chemistry on a range of substrates. However, the self-terminating nature of the reaction remains poorly understood. Here, we use X-ray photoelectron spectroscopy (XPS) to explore multilayer formation during photochemical grafting of functional alkenes on single-crystal rutile (110) TiO2 surfaces. We demonstrate that simple alkenes such as 1-hexene stop at 1 monolayer, but alkenes with an additional alcohol group easily form multilayers. Atomic force microscopy (AFM) and XPS studies show that the multilayers are highly conformal and that the alcohol groups remain chemically accessible even within multilayer films. Small amounts of water enhance multilayer formation by facilitating formation of •OH or other radical species that initiate oligomerization processes, while oxygen has little effect. We show that the multilayer formation can be reduced or eliminated by using small amounts of a radical scavenger or by modifying the molecular structure of the alkene. Overall these studies show that multilayer formation is controlled by the ability to form alkyl radicals under ultraviolet (UV) illumination.
’ INTRODUCTION The ability to graft functional molecular layers to surfaces has emerged as an important tool to control interfacial properties and reactivity. The ultraviolet (UV)-induced photochemical grafting of terminal alkenes to semiconductors is of particular interest because it provides a facile, nondestructive method to covalently bond organic molecules to a wide range of semiconductors, including Si,1 5 Ge,6 diamond and other carbon-based materials,7 10 SiO2,11 SiC,12 GaN,13 and silicon nitrides.12 Recently, we extended the reaction to include metal oxides such as TiO2 and ZrO2.14 16 TiO2 is of interest for its widespread use in solar energy,17 selective molecular catalysts18 and biological materials.15,19 Photochemical grafting is an attractive way to link molecules to surfaces of TiO2 and other materials because it does not require high temperatures or ultrahigh vacuum, offers simple photopatterning procedures, and because the resulting layers can be very stable even in aqueous environments.14,20 Past studies of alkene grafting have focused primarily on straightchain, terminal n-alkenes with 10 carbons or more. On surfaces of hydrogen-terminated silicon1,2,5,21 and carbon-based materials,22,23 UV photons were initially proposed to create reactive surface sites that are attacked by the alkene nucleophile through either direct surface hydrogen-bond cleavage1 or near-surface excitons.2 More recent work suggests that at least in some cases, the alkene reacts with a persistent hole on the surface created by the photoinitiated ejection of electrons into an acceptor level of the adjacent medium.21,22 Although the grafting reaction is often self-terminating near one monolayer for the straight-chain alkenes on silicon,1,4,24 multilayer formation has been reported for alkenes on silica (SiO2)11 and for r 2011 American Chemical Society
alkenes bearing a ω-trifluoroacetamide group on amorphous carbon.25 Multilayer formation could potentially impact properties such as the stability of the layers (e.g., by forming cross-links between adjacent molecules), the reactivity of additional molecular functional groups that may be present, and other properties. However, little work has been done to understand the mechanisms underlying multilayer formation. Here, we report an investigation of multilayer formation and oligomerization during photochemical grafting of alkenes to single-crystal rutile (110) TiO2 surfaces. Our results show that while a simple alkene will self-terminate at one monolayer, reactant molecules bearing OH groups can undergo multilayer formation. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements show that the OH groups within the multilayer films remain chemically accessible and that the multilayer films are highly conformal to the surface. To gain a deeper understanding of the underlying mechanism of grafting, we report on the effects of trace water, the presence/ absence of dissolved oxygen, the addition of a radical scavenger, and the location of the alkene group within the molecule on the propensity to form multilayers. These results provide important insights into how multilayers form and how the structure of molecules can be designed to either minimize or enhance multilayer formation.
Received: May 29, 2011 Revised: July 22, 2011 Published: July 26, 2011 17102
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’ EXPERIMENTAL METHODS Rutile Single Crystal Preparation. Before use rutile (110) single crystals (CrysTec GmbH) were cleaned under UV illumination (λ = 254 nm, irradiance∼10 mW/cm2) for at least 30 min in deionized (DI) water to remove organic layers. The bare crystals were then etched in 48% HF for 15 min, rinsed with DI water and dried. Finally the crystals were annealed at 900 °C for 1 h inside a TiO2 box made from a 99.99% pure TiO2 sputter target (Kurt J. Lesker Company). Atomic force microscopy (AFM) images (vide infra) show flat terraces separated by straight, parallel single unit-cell steps (vide infra) with a root-mean-square (rms) roughness of 0.15 nm. Alkene Photochemical Grafting. The alkenes, 1-hexene, 10-undecen-1-ol, 3-buten-1-ol, 3-methyl-3-buten-1-ol, cis-3hexen-1-ol, and 4-methyl-3-penten-1-ol were obtained from Sigma-Aldrich, stored under argon, and used as received. Inside an argon glovebox, a few drops of the relevant alkene were placed on a freshly prepared sample. The sample was then placed against a UV-grade fused-silica window and sealed in an airtight stainless-steel reaction cell. The sample was then illuminated though the window by a low-pressure mercury quartz grid lamp (λ = 254 nm, 10 mW/cm2) for the reaction time specified. After UV reaction the sample was sonicated first in heptane for hexene or n-butanol for the alcohols and then acetone and electronic grade methanol for 5 min each. Finally the sample was blown dry under nitrogen. 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was obtained sublimed from Sigma-Aldrich; in TEMPO experiments the pure TEMPO was added to the neat alkene to produce a 0.1 M solution. Surface Alcohol Chlorination. To help identify chemically accessible OH groups after reaction, we used a chlorination procedure. Inside an argon glovebox, UV-grafted samples were sealed inside a 300 mL container with 0.05 mL of SOCl2 to allow for a vapor-phase reaction. The container was then heated to 80 °C for 2 h. Measurements performed for different chlorination times showed that very long exposures introduced sidereactions, but a shorter time of ∼2 h yields the best compromise of high chlorination efficiency with minimal side-reactions. Reacted samples were sonicated in acetone and methanol for 5 min each and blown dry under nitrogen flow. Characterization. X-ray photoelectron spectroscopy (XPS) experiments were performed in an ultrahigh vacuum XPS system using an Al KR source (nominally 1486.6 eV photon energy) with a quartz-crystal monochromator and a hemispherical analyzer, typically using an analyzer resolution of 0.1 0.2 eV. Unless otherwise specified the electron takeoff angle was 45° and peak positions and areas were calculated by fitting raw data to Voigt functions after a Shirley background correction.26 A low-energy electron gun was used to neutralize sample charging, and spectra were shifted to place the lowest C(1s) peak at 284.8 eV.27 The Cl(2p) peaks were fit using a double Voigt function, consisting of two Voigt functions summed with the area ratio constrained at 2:1 and shape and width parameters constrained to be equal between the peaks. Calculated areas were normalized by the corresponding atomic sensitivity factors for the X-rays incident at 90° from the analyzer (C(1s) = 0.296; O(1s) = 0.711; Cl(2p) = 0.77; Ti(2p) = 1.798).28,29 Molecular coverage was calculated from the peak areas taking into account electron attenuation through the organic film, with the latter measured directly as described previously.14 Atomic force microscopy (AFM) was performed using a Veeco Nanoscope IV. Images were taken in tapping mode with
Figure 1. XPS of rutile (110) grafted for 18 h with n-butanol, 1-hexene, 3-buten-1-ol, and 10-undecen-1-ol. (a) C(1s) peaks, (b) O(1s) peaks, (c) Ti(2p) peaks, (d) molecular coverage versus illumination time for 3-buten-1-ol (green squares) and 1-hexene (red triangles). In some cases, the spectra were multiplied by a scaling factor, indicated adjacent to each graph, to facilitate visual comparison.
diamond coated silicon probes (Budget Sensors). Layers were scratched in contact mode using a silicon probe with a force constant of 0.2 N/m (Budget Sensors).
’ RESULTS XPS Measurements of UV Photo-Grafted Molecules. We illuminated rutile (110) TiO2 surfaces covered with each of the alkenes (hexene, butenol and undecenol) and a butanol control (lacking an alkene group) for 18 h and characterized the resultant layers with XPS. Figure 1 shows the C(1s), O(1s), and Ti(2p) peaks for each of the molecules, and Table 1 summarizes peak areas and coverage calculations. We first grafted hexene (red traces) to the rutile (110) surfaces. The C(1s) spectra Figure 1a, show a main carbon peak (calibrated to 284.8 eV to correct for charging during measurements)27 a high-binding-energy shoulder at 286.2 ( 0.2 and a small satellite at ∼288.6 ( 0.1 eV. The peak at 284.8 eV is typical of that from CHx alkyl carbon. We expect grafting hexene to the surface via the alkene group to produce a Ti O C linkage similar to the Ti O C ethoxy groups formed by adsorbed ethanol. Since previous studies of ethanol adsorption on TiO2 showed that the C atom in a Ti O C linkage gives rise to a C(1s) feature 1.2 1.3 eV higher than the CHx alkyl carbon,30,31 we attribute the 286.2 eV peak to C O Ti linkages. Analysis of the C(1s) peak areas yields a COH/Ctotal area ratio of 0.13 ( 0.04. This is consistent with a single carbon atom from hexene oxidizing to a Ti O C linkage upon binding to the surface. The small satellite peak observed around 289 eV is also observed on unreacted control samples; we attribute this peak to carboxyl-containing contamination, mostly likely from adsorbed atmospheric CO2. We calculated the coverage of the grafted hexene molecules from the C(1s) peak area with correction for the effects of 17103
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Table 1. XPS Data for Three Molecules Grafted on Rutile (110) TiO2 Surface for 18 h Area Ratio molecule hexene undecenol butenol
coverage, molecules/nm2 4.2 ( 0.6 13.4 ( 2 60 ( 10
Coxidized/Ctotal
C(1s) area 284.8 eV
286 eV
O(1s) area 287.8 eV
530 eV
533 eV
Ti(2p) area
0.13 ( 0.04
769
112
1625
326
823
0.10 ( 0.01
3199
336
534
329
250
0.24 ( 0.02
3136
869
149
954
76
electron scattering in the organic layer achieved by direct measurement as described previously.14 After 18 h of illumination the hexene yields a coverage of 4.2 ( 0.6 molecules/nm2. Since studies of highly ordered crystalline n-alkanes show the minimum area needed for an alkyl chain is 0.18 nm2 (corresponding to a surface density of 5.4 molecules/nm2),32 we conclude that the hexene coverage is close to one complete monolayer. Figure 1d shows the extent of grafting as a function of illumination time. The data show growth to a monolayer after approximately 15 h of illumination followed by no further growth for up to 40 h of illumination. From these data we conclude that the UV grafting of hexene onto TiO2 is self-limiting and forms a nearly complete monolayer on the surface, consistent with previous work on grafting of alkenes onto TiO2.14,15 Next we grafted two alkenes with different chain lengths, both containing terminal alcohol groups opposite the alkene bonds, to the rutile(110) surface. Figure 1a shows the C(1s) spectra after grafting of butenol (green) and undecenol (blue). We again attribute the largest peak for both molecules (at 284.8 eV) to CHx alkyl carbon. From the area of these peaks, undecenol yields a coverage equivalent to 13.4 ( 2 undecenol molecules/nm2 and butenol yields a coverage equivalent to 60 ( 10 butenol molecules/nm2. Since crystalline alkyl chains have a density of 5.4 molecules/nm2,32 we conclude both undecenol and butenol must be forming multilayers. Both alcohols have a significant shoulder peak centered at approximately 286.3 ( 0.2 eV which we attribute to carbon in C OH groups. The undecenol multilayer shows a COH/Ctotal area ratio of 0.10 ( 0.01, equivalent to the 1:10 stoichiometry of neat undecenol. While the molecules directly bonded to the TiO2 substrate should also contribute some C(1s) intensity near ∼286 eV from the Ti O C surface linkage, the electrons emitted from the C atoms in these interfacial Ti O C linkages will be attenuated by the organic film and contribute only a negligible intensity to the overall 286 eV C(1s) peak of the multilayer film. Therefore the COH/Ctotal peak area ratio of 0.10 ( 0.01 shows that the alkyl chain of undecenol does not undergo significant additional oxidation during grafting. The spectrum from butenol exhibits a second small peak at 287.8 eV, which we attribute to formation of ketone or aldehyde groups.33 This Ccarbonyl peak represents 12% of the total oxidized carbon and if alcohol and carbonyl groups are combined (Coxidized = Ccarbonyl + COH) this yields Coxidized/Ctotal = 0.24 ( 0.02. This value is stoichiometrically equivalent to that expected from neat butenol. Again, in the multilayer film any C(1s) intensity from the interfacial Ti O C linkages is expected to be strongly attenuated by the thick carbon overlayer and is negligible compared with the other forms of oxidized carbon. We conclude that while ∼12% of the alcohol groups of butenol are oxidized further to ketones or aldehydes during grafting, the alkyl chain is not further oxidized. The XPS data show that the surface coverage after grafting of 1-hexene is very different from that obtained from butenol and
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undecenol, after identical grafting times of 18 h. This difference is quite apparent from the grafting kinetic curves shown in Figure 1d. While hexene self-terminates after approximately 15 h of illumination, butenol shows nearly linear growth through 18 h of illumination. Some slowing of the reaction may be occurring with longer reaction times but layer growth does not appear to terminate even after 40 h of illumination. The O(1s) peaks are consistent with the above interpretations of the C(1s) data. For each molecule a peak is observed at ∼530 eV that arises from the bulk TiO2. The exact energy of this peak shifts slightly between samples, which we attribute to slight variations in charging during the course of the XPS measurements that are not fully eliminated by referencing the main C(1s) peak to a fixed 284.8 eV. The absolute intensity of the 530 eV O(1s) peak is substantially lower for butenol and undecenol than it is for 1-hexene or n-butanol, suggesting that films grafted using butenol and undecenol both attenuate the electrons emitted from the bulk TiO2. Butenol and undecenol also both have a second peak at 533 eV that is consistent with O within alcohol functional groups.34 Quantification of the 533 eV peaks is complicated by exposure of the sample to atmospheric water during loading into the XPS. However, after correcting for differences in elemental sensitivity factors and escape depth, the XPS peak area ratio O530/Ti2p = 2.0 ( 0.2 for each of the samples (consistent with the stoichiometry of TiO2) and O533/Coxidized= 1.0 ( 0.1 for butenol and undecenol, indicating that the surface concentration of oxidized carbon species inferred from the C(1s) spectra and from the O(1s) spectra are in agreement. All of the titanium 2p spectra show the expected 2p1/2 and 2p3/2 peaks. The peaks are symmetric and there is no evidence for broadening or additional peaks on the low-binding-energy side of the main peak. This symmetric shape shows that the rutile surface is fully oxidized with few oxygen vacancies.35 The peaks for both butenol and undecenol are lower in absolute intensity than those for butanol and hexane, due to attenuation caused by the grafted layers. Figure 1 also shows XPS data for a sample that was exposed to the saturated alcohol, butanol, and illuminated for 18 h. The coverage of this molecule is low, limited to 5.0 ( 0.7 molecules/ nm2 or approximately one monolayer, and is close to the contamination level. We conclude that while some physisorption or attachment to the surface through elimination of water by the alcohol group may be occurring, oligomerization and multilayer formation do not occur with the alcohol group alone. Angle-Resolved XPS and Chlorination of Alcohol Groups. While the above data show that multilayer formation occurs with butenol and undecenol, it is difficult from XPS alone to identify whether the multilayer formation directly involves the C OH groups (for example, leading to multilayer formation with ether linkages between molecules) or whether the OH groups remain unchanged. We tested the chemical accessibility and location of the alcohol groups by reacting a butenol-grafted 17104
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Figure 2. Angle-resolved XPS of rutile (110) grafted with butenol for 4 h then reacted with SOCl2 for 2 h. (a) Spectrum acquired using electron escape angle of 15° from surface normal (less surface-sensitive) and (b) using grazing-angle electron escape angle of 80° from surface normal (more surface-sensitive).
sample (4 h) with SOCl2 vapor and then characterizing the composition of the multilayer film using angle-resolved XPS. SOCl2 reacts with C OH groups to form C Cl moieties but does not react with C O C linkages.36 38 Figure 2 shows the XPS C(1s) and Cl(2p) spectra for electron escape angles of 15° from the surface normal Figure 2a and grazing-angle emission at 80 °Figure 2b. Chemically accessible R OH groups are expected to react with SOCl2 to produce R-Cl groups, so that the Cl(2p) signal can be used to characterize the distribution of C OH groups within the film. As the electron collection angle approaches grazing emission, the signal becomes more surface sensitive. Therefore if the SOCl2 only reacted on the outer surface of the organic layer, the chlorine-to-carbon ratio would be expected to increase near grazing incidence. Based on the direct measurement of the electron scattering by the carbon layer described previously,14 we calculate that as the electron collection angle is increased from 15° (near normal-emission) to 80° (near grazing-emission), the ratio of chlorine to carbon peak areas should increase by a factor of 1.8 if the chlorine is confined to the surface but it should only increase by a factor of 1.03 if the chlorine is distributed evenly though the carbon film. Our XPS data shows the chlorine-to-carbon peak area ratio remains nearly constant, undergoing a small decrease from 0.15 to 0.l2 when we change the angle from 15° to 80°. Since this ratio does not increase with collection angle, the data show that the chlorine is not restricted to the outer surface of the film. We can compare the chlorine to oxidized carbon ratio to determine the percentage of C OH groups chlorinated. Since C Cl groups are expected to give rise to a carbon peak between 286 and 287 eV in XPS,39 their peaks are convoluted with the oxidized carbon peak. Cl/Coxidized = 0.36 indicating approximately 36% of the total oxidized carbon is reacted with the SOCl2 within the 2 h vapor phase reaction time. Combining these results with the fact the alkyl chain does not appear to oxidize during the reaction, we conclude that a significant portion of the C OH groups are chemically active and accessible
Figure 3. Tapping-mode atomic force micrographs of 3-buten-1-ol grafted to rutile(110) surfaces for 4 h (a) and 18 h (b). The organic layer was scratched away in the central region using AFM in contact mode. Line scans are shown averaged 200 nm wide along the direction of the dashed lines. The arrows in the line profiles represent the measured thicknesses, 0.9 nm for the 4 h reaction and 6.5 nm for the 18 h reaction. The scale bar in each AFM image corresponds to 1 μm.
throughout the thickness of the grafted films, even when present as multilayers. AFM Characterization of Grafted Surfaces. We performed AFM measurements in order to understand the morphology of the grafted layers. Figure 3 shows tapping-mode images of rutile (110) single crystals that were reacted with butenol for 4 h (Figure 3a) and 18 h (Figure 3b). The 1 μm squares in the images represent areas where the organic layer was layer scratched away using an AFM tip in contact mode. Clear unit-cell step edges (∼0.3 nm) are visible in both the scratched regions and the undisturbed layer. Line profiles across the scratched region show the layer thickness for the 4 and 8 h reactions to be 0.9 and 6.5 nm respectively. Surprisingly, the TiO2 step edges can still be seen in the organic layer even after over 6 nm of growth, demonstrating that the grafting reaction is highly conformal to the surface morphology. The rms roughness of the surfaces increases from 0.15 to 0.37 nm after grafting for 18 h. By dividing the AFM thickness measurements with carbon atom coverage calculated from XPS, we can estimate an average density for the grafted layers at 4 and 18 h. After 4 h of illumination XPS shows a carbon coverage of 43 ( 7 atoms/nm2 leading to a density of 48 ( 12 carbon atoms/nm3 or 12 ( 3 butenol molecules/nm3. After 18 h of grafting XPS shows 237 ( 36 carbon atoms/nm2, yielding an average density of 36 ( 6 carbon 17105
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atoms/nm3 or 9 ( 1.5 butenol monomers/nm3. For comparison, solid butanol has density of 11 molecules/nm3;40 and neat liquid butenol has ∼7 butenol molecules/nm3 based on its density at standard conditions.41 Effect of Trace Oxygen and Water. To understand the effects of water and oxygen impurities on multilayer formation, we compared the reactivity of three different samples: (1) the asreceived butenol sparged with Ar for 30 min, (2) butenol sparged with oxygen for 30 min to maximize the dissolved oxygen, and (3) butenol dried over molecular sieve for 24 h to remove trace water. Using gas chromatography and Karl Fischer measurements we determined the as-received butenol contained 10 000 ( 2000 parts-per-million (ppm) of water, while after drying only 10 ppm of water remain. Figure 4 shows XPS spectra after 18 h of grafting for each of the samples. The calculated peak areas and calculated coverage are summarized in Table 2. For the Ar-sparged sample the C(1s) spectrum shows the alkyl carbon peak at 248.8 eV with a shoulder peak at ∼286.4 eV, which we assign primarily to the alcohol functional groups. Integrating the area under these peaks yields an area ratio COH/Ctotal = 0.22 ( 0.03, similar to the value of 0.25 expected for unreacted butenol. Analysis of the C(1s) and Ti(2p) peak areas yields a coverage equivalent to 61 ( 10 butenol molecules/nm2. Grafting of the O2-sparged butenol yields a C(1s) spectrum showing the alkyl peak corrected to 284.8 eV, a shoulder at 286.2 and a new, broader shoulder at 287.6 eV. We attribute these shoulders to alcohol and carbonyl groups respectively and the total area of these peaks (Coxidized =COH + Ccarbonyl) represents the total oxidized carbon. The fraction of carbon in the oxidized form, Coxidized/Ctotal is 0.25 ( 0.02, stoichiometrically equivalent to that expected for butenol. The fraction of carbon in carbonyl form Ccarbonyl/Ctotal = 0.12, similar to that observed from grafting of the butenol shown in Figure 1. Grafting using the O2-sparged
butenol yields a coverage of 78 ( 12 molecules/nm2, only slightly higher than (and within experimental error of) the value of 61 ( 10 obtained from the Ar-sparged butenol sample. Overall the primary difference between the Ar-sparged and O2-sparged samples is that the O2-sparged sample latter has a larger fraction of the oxidized carbon in the form of carbonyl groups, while the total fraction of carbon in oxidized form (COH + Ccarbonyl) is nearly the same. Perhaps more importantly, the total coverage produced by grafting of Ar-sparged and O2-sparged butenol samples is nearly identical. Grafting of the sieve-dried butenol yields a C(1s) spectrum with peaks at 284.8 eV, a shoulder at 286.3 (from C OH groups) and a broader peak at 287.3 eV (from carbonyl groups). The total fraction of carbon in oxidized forms Coxidized/Ctotal is 0.25 ( 0.02. Once again this is equivalent to the value of 0.25 expected for unreacted butenol, indicating that the three alkanelike carbons within each butenol molecule are unaffected by grafting. The total coverage yielded is 25 ( 7 butenol molecules/ nm2. This coverage is only 40% of that obtained from the Arsparged sample, and the XPS spectra show significantly more intense Ti(2p) and O(1s) substrate peaks compared with the Arsparged sample. The above data show that oxygen has little effect on the total grafting reaction rate or total amount of carbon in oxidized forms, except for possibly shifting the distribution of oxidized carbon atoms to favor carbonyl groups at the expense of alcohol groups. In contrast, removal of water greatly reduces multilayer formation. However, the coverage produced by grafting of butenol containing 10 ppm H2O still exceeds that produced by grafting of hexene containing 15 ppm H2O under identical grafting conditions (see Figure 1). These data clearly show that water impurities are one important factor contributing to multilayer formation. Effect of TEMPO on Grafting. To test whether radicals play an important role in the initial grafting or subsequent multilayer formation, we investigated grafting of butenol solutions containing the molecule 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO), which is a widely used radical scavenger.42 Figure 5 shows C(1s) and N(1s) XPS spectra after a solution of 0.1 M TEMPO in butenol was grafted to the surface of rutile (110). With the addition of TEMPO, the butenol sample yields a total carbon coverage corresponding to 18 C atoms/nm2 that is comparable to the contamination limit and is much smaller than the value of 236 carbon atoms/nm2 obtained with neat butenol. Some oxidized carbon is observed, but at the low carbon levels here it is difficult to distinguish between grafted molecules and contamination. The N(1s) spectrum shows nitrogen coverage of only 0.3 ( 0.1 nitrogen atoms/nm2; since typical contaminants yield no detectable nitrogen signal, all the N(1s) signal comes from TEMPO. Since each TEMPO molecules has 9 C atoms, this indicates that there are ∼3 C atoms/nm2 that could be attributed to direct binding of TEMPO compared with the total of 18 C
Figure 4. XPS comparing 18 h grafting of 3-buten-1-ol that was argonsparged (green), oxygen-sparged (red), and dried over molecular sieve (blue).
Table 2. XPS Data Showing the Influence of Trace Oxygen and Water on Butenol Grafted to Rutile (110) TiO2 Surfaces for 18 h area ratio 2
butenol
coverage, molecules/nm
Ar-sparged O-sparged
61 ( 10 78 ( 12
sieve-dried
25 ( 7
Coxidized/Ctotal
C(1s) area
O(1s) area
284.8 eV
286 eV
287.8 eV
530 eV
533 eV
Ti(2p) area
0.22 ( 0.03 0.25 ( 0.02
2611 2857
716 847
123
148 35
782 830
37 18
0.25 ( 0.03
2097
617
78
867
701
366
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Figure 5. C(1s) and N(1s) XPS of rutile (110) grafted for 18 h with 0.1 M TEMPO solutions in 3-buten-1-ol. Dashed lines depict results of peak-fitting.
atoms/nm2 observed. We therefore conclude that any grafting of TEMPO to the surface is negligible. Our data show that the addition of TEMPO to the reactant alkene eliminates multilayer formation. The low nitrogen coverage in XPS indicates this cannot be explained by competitive binding of TEMPO to the surface. Furthermore, UV visible absorption spectra (shown in Supporting Information) of a thin film formed between quartz plates of the 0.1 M TEMPO in butenol show that the addition of TEMPO induces only a 4% decrease in transmitted light at 254 nm. This demonstrates that direct absorption of UV light by TEMPO is insufficient to account for the change in grafting. Therefore we conclude that the decrease in grafting induced by the addition of TEMPO is caused by TEMPO’s well-known abilities as a radical scavenger.42 Geminal Substitution and Internal Alkenes. We studied the effect of substitutions around the alkene bond by comparing grafting of 3-buten-1-ol with that of 3-methyl-3-buten-1-ol, cis3-hexen-1-ol, and 4-methyl-3-penten-1-ol. These molecules are depicted in Figure 6a. Figure 6b shows the grafting kinetics of these compounds as extracted from XPS measurement after different grafting times. Methyl-butenol grafts much slower than butenol, yielding 9 molecules/nm2 after 18 h of grafting. While this coverage is ∼15% of that achieved by butenol over the same illumination time, it is still greater than a single monolayer. The fraction of carbon in C OH groups (COH/Ctotal) for methylbutenol is 0.20 ( 0.02 and is equivalent to the molecule’s stoichiometry before grafting. From the coverage kinetics we conclude that while geminal substitutions reduce the rate of grafting over terminal alkenes, they do not prevent multilayer formation. In contrast, the internal alkenes 3-hexen-1-ol and 4-methyl-3penten-1-ol do not appear to undergo significant multilayer formation. Even after 40 h of illumination, neither molecule exhibits C(1s) intensity significantly above contamination level. While some grafting of the molecules (monolayer concentration or less) cannot be ruled out, we conclude that the internal alkenes are much less reactive than the geminal or terminal alkenes and do not propagate to form multilayers. Thus, the location of the alkene bond within the molecule plays an important role in controlling multilayer formation.
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Figure 6. (a) Structures of the substituted alkenes and (b) XPS coverage kinetics for rutile (110) grafted with 3-buten-1-ol (green squares), 3-methyl-3-buten-1-ol (blue triangles), cis-3-hexen-1-ol (purple crosses), and 4-methyl-3-penten-1-ol (red circles). The data for 3-hexen-1-ol and 4-methyl-3-penten-1-ol are overlapped near the bottom of the graph.
’ DISCUSSION Our data show several results important to understanding multilayer formation. First, simple alkenes such as 1-hexene graft to TiO2 and self-terminate at a coverage of 1 monolayer with little or no evidence for multilayer formation. Second, alkenes bearing OH groups can exhibit multilayer growth that is highly conformal with the underlying surface, but whose rate depends strongly on the detailed substitution pattern around the alkene group. Third, multilayer formation by butenol can be reduced but not completely eliminated by careful drying. Finally, TEMPO, a well-known radical scavenger, suppresses the reaction and multilayer growth. In order to understand the mechanisms of multilayer formation in our data, it may be important to note that, as with polymerization reactions, distinct processes can be involved in the initiation versus the propagation of the multilayers. Direct versus Indirect Involvement of the Alcohol Group. Since multilayers form upon the introduction of alcohol groups to the alkenes, we first address the question of whether multilayer formation involves direct reaction of the alcohol groups to form ether linkages between adjacent molecules. Ether bonds could form either through direct reaction of the alkene bonds with the alcohol groups or through condensation reactions between two alcohols. Several experimental results allow us to rule out both of these possibilities. First, our experiments exposing the grafted surfaces to SOCl2 show the alcohol groups can be readily replaced with Cl atoms. Previous studies have shown that alcohol groups will react readily with SOCl2, but ether linkages are inert under identical conditions.36 38 Thus our XPS results after SOCl2 treatment demonstrate that the alcohol groups within the multilayer film are intact and chemically accessible. Furthermore, the angle-resolved XPS measurements in Figure 2 show no significant change in Cl(2p)/C(1s) intensity ratio as a function of electron emission angle. Since the Cl atoms arise from displacement of the C OH groups, this implies that the C OH groups of the multilayers are not concentrated at the vacuum-exposed 17107
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Figure 7. Possible reaction mechanisms leading to multilayer growth. (a) Initiation by extraction of H atom by a radical, leaving an alkyl radical on the grafted layer. (b) Propagation by addition of alkenes to the alkyl radical; (c) termination via cross-linking; (d) termination via H extraction from alcohol group, forming carbonyl groups.
surface but are instead distributed within the bulk of the grafted film. Second, the C(1s) XPS spectra show that the ratio of oxidized to total carbon within the grafted multilayers matches the 0.25 fraction expected from the stoichiometry of the starting alkene molecules. If ether linkages were being formed through direct reaction with the alkene bond, this fraction would be expected to increase. Finally, in XPS spectra the peak area from C atoms in C O species (286.2 eV) and the peak area from O atoms linked to C atoms (533 eV) are equal. In contrast condensation reactions would form C O C linkages with two oxidized C atom for each O atom and would therefore increase the relative amount of C atoms in C O bonds. From these results, we conclude that the alcohol groups are not directly involved in forming new chemical bonds with carbon or undergoing condensation reactions and instead must indirectly catalyze the multilayer formation. Multilayer Formation: the Role of Alcohol Functional Groups and the Importance of Radicals. The data in Figure 5 show that by suppressing the formation of radicals using the radical scavenger TEMPO during the grafting, multilayer formation can be eliminated. This indicates that radicals play a key role in multilayer formation and presents the question of how radicals are generated during the reaction. Previous studies have shown radicals can be generated by the photoreaction of absorbed species with TiO2, including both alcohols and water.43,44 Photo-oxidation studies of alcohols on TiO2 in the absence of water show selective oxidation of the alcohols to carbonyl groups, with dissociatively adsorbed alkoxy and radical species proposed as intermediates. 45 47 When TiO2 is illuminated with supra-bandgap light the photogenerated holes act as potent oxidizing agents. The holes can be scavenged by adsorbed alcohols,48 facilitating dissociation of the alcohols into alkoxy radicals RCH2O• and protons H+.46 Alkoxy radicals are expected to quickly undergo a 1, 2 H-shift forming the R-alkyl radical RCH•OH.49 Thus the presence of alcohol groups during illumination is expected to lead to the formation of alkyl radicals.
We propose that once created the alkyl radicals can initiate and propagate the multilayer formation. Since the radicals are generated near the TiO2 surface, even if they are not initially directly on the grafted layers, there is substantial opportunity for the radicals to immediately react with the layers. Alkyl radicals are known to abstract H from alkyl chains to transfer the radical.50 52 In particular, H abstraction is known to be promoted on the R-carbon of alcohol functional groups.53 Once the radical is generated on the layer or transferred to the layer through H abstraction, it can attack the alkene double bond to promote layer growth. Alkyl radicals are well-known to react with alkenes to undergo radical propagated polymerization.54,55 In our experiments the higher reactivity of terminal alkenes compared with internal alkenes Figure 6 is consistent with the known trends in reactivity of radical addition processes, as internal alkenes are poor nucleophiles relative to terminal alkenes.56 Radicals can also lead to cross-linking and H abstraction from the alcohol groups to form carbonyl groups, as reported in photolysis experiments on polyvinyl alcohol.49 Our previous studies of alkene grafting to TiO2 suggest that the first layer of molecules likely grafts via nucleophilic attack by the alkene on surface Ti OH sites, which act as hole traps when photoexcited.14,57 The new experiments reported here suggest a likely mechanism for extending monolayer growth into the multilayer region, via the processes depicted in Figure 7. In this model, we assume that the alcohol groups adsorbed to the TiO2 lead to formation of alkyl radicals upon excitation with abovebandgap light. If the radicals are not directly formed on grafted layers, multilayer formation can be initiated (Figure 7a) by hydrogen abstraction by the alkyl radicals. Once initiated, propagation of the multilayers occurs though radical additions to the alkene molecules from solution as depicted in Figure 7b. In the anoxic environment of the reaction cells, the propagation likely terminates through crosslinking between chains (Figure 7c) or by hydrogen abstraction to a separate radical, leaving behind a carbonyl group (Figure 7d). A radical-induced propagation process has been reported previously for styrene on H-terminated silicon under vacuum conditions.24 17108
dx.doi.org/10.1021/jp205007z |J. Phys. Chem. C 2011, 115, 17102–17110
The Journal of Physical Chemistry C Multilayer Formation and the Importance of Trace Water and Oxygen. Our data in Figure 4 show a very strong reduction
in multilayer formation from 1-butenol as the trace water content is reduced from 10 000 to 10 ppm; this indicates that water substantially enhances the polymerization reactions. At first this is surprising since TiO2 is a well-known photocatalyst for degrading organics in the presence of water.43,58,59 When TiO2 is illuminated with supra-bandgap light, the photogenerated holes can readily oxidize water to form H+ and •OH (hydroxyl radicals). Prior studies have shown that in sufficient concentrations these •OH radicals are the primary active species responsible for photocatalytic degradation of organics at TiO2 surfaces.43,58,59 Even at trace concentrations, the •OH radicals can react with the local grafted layers. As with the alkyl radicals, hydroxyl radicals are well-known to extract hydrogen atoms from saturated alkyl chains and alcohols.49,60 62 Therefore the •OH radicals can act as an additional initiation pathway for generating alkyl radicals on the grafted layers increasing the layer growth rate. Even though the water content has a large impact on the growth rate, we must emphasize that water alone is not the sole initiator of multilayer formation and the alcohol groups must play a role; we still observe multilayers with 10 ppm of water in butenol while 15 ppm of water in hexene shows no multilayer growth. In addition to water, oxygen is also known to be photochemically active on TiO2.63 When the photoexcited hole oxidize surface adsorbed species, the electrons must induce a corresponding reduction reaction in order to maintain overall charge neutrality of the material.64 Under the highly oxygen-rich conditions typically used for TiO2 photocatalysis the predominant reduction reaction is the reduction of O2 to O2 (superoxide) ion.65 Our experiments were performed in Ar under oxygen-deficient conditions, and we find that intentional introduction of oxygen leads to only a small change in the coverage of grafted molecules. These experiments suggest that under the conditions of our experiments superoxide formation is not important. A second possible reduction reaction is the direct injection of electrons from the valence band or surface states into acceptor states of the reactant liquid, as previously observed from surface of diamond21 and silicon.21,66 Previous studies have shown that surface defect states just below the conduction band of TiO2 can lead to a work function of
