Langmuir 2007, 23, 8053-8060
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Influence of Alkyl Chain Length on Phosphate Self-Assembled Monolayers Doris M. Spori, Nagaiyanallur V. Venkataraman, Samuele G. P. Tosatti, Firat Durmaz, Nicholas D. Spencer,* and Stefan Zu¨rcher Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland ReceiVed February 17, 2007. In Final Form: May 8, 2007 A series of alkyl phosphates with alkyl chain lengths ranging from C10 to C18 have been synthesized. Self-assembled monolayers (SAMs) of these molecules were prepared on titanium oxide surfaces by immersion of the substrates in alkyl phosphate solutions of 0.5 mM concentration in n-heptane/isopropanol. The SAMs were characterized by means of dynamic water contact angle (dCA) measurements, variable-angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), and polarization-modulated infrared reflection-absorption spectroscopy (PMIRRAS). A higher degree of order and packing density within the monolayers was found for alkyl phosphates with alkyl chain lengths exceeding 15 carbon atoms. This is reflected in a lower dCA hysteresis, as well as a film thickness measured by VASE and XPS close to the expected values for SAMs with an average alkyl chain tilt angle of 30° to the surface normal. Additionally a shift of the symmetric and antisymmetric C-H stretching modes in the PMIRRAS spectra to lower wave numbers was observed. These findings imply a higher two-dimensional crystallinity of the films derived from alkyl phosphates with a longer alkyl chain length.
1. Introduction Self-assembled monolayers (SAMs) represent an easy, accurate and precise approach to the modification of surface properties.1-3 Consequently, a significant amount of research has been dedicated to the investigation of the fundamentals associated with the spontaneous adsorption and assembly of monomolecular layers.1-6 Most work has been carried out with alkyl silanes adsorbing on silicon oxide7 or thiols or disulfides adsorbing on gold.5,8,9 A further class of self-assembling materialssphosphonic and phosphoric acidsshas gained interest due to their ability to bind to a wide range of metal oxide surfaces and form robust SAMs of a similar quality to that of thiol SAMs on gold. This has been demonstrated for the system of octadecyl phosphate deposited from a heptane/2-propanol solution on tantalum oxide,10-13 for example. It is known that long-chain phosphonic and phosphoric acids lead to dense, well-ordered SAMs.10,14-17 Some comparison studies between different lengths of alkyl * To whom correspondence should be addressed. E-mail: spencer@ mat.ethz.ch, fax: +41 44 633 10 27. (1) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (2) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1-68. (3) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (4) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Anal. Chim. Acta 2006, 568, 84-108. (5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1169. (6) Schwartz, D. K. Annu. ReV. Phys. Chem. 2001, 52, 107-137. (7) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (8) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (9) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481-4483. (10) Brovelli, D.; Ha¨hner, G.; Ruiz, L.; Hofer, R.; Kraus, G.; Waldner, A.; Schlosser, J.; Oroszlan, P.; Ehrat, M.; Spencer, N. D. Langmuir 1999, 15, 43244327. (11) Ha¨hner, G.; Hofer, R.; Klingenfuss, I. Langmuir 2001, 17, 7047-7052. (12) Hofer, R.; Textor, M.; Spencer, N. D. Langmuir 2001, 17, 4014-4020. (13) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Ha¨hner, G.; Spencer, N. D. Langmuir 2000, 16, 3257-3271. (14) Foster, T. T.; Alexander, M. R.; Leggett, G. J.; McAlpnie, E. Langmuir 2006, 22, 9254-9259. (15) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429-6435. (16) Pawsey, S.; Yach, K.; Reven, L. Langmuir 2002, 18, 5205-5212. (17) Helmy, R.; Fadeev, A. Y. Langmuir 2002, 18, 8924-8928.
phosph(on)ates have been carried out,18-21 but there are still many open questions with respect to order, packing density, simplicity, and reproducibility of adsorption and stability, as well as binding mechanism and resulting bond-architecture to the corresponding metal oxide surfaces. The aim of this paper was to investigate the SAM quality, in terms of binding mechanisms, density, and order, of alkyl phosphoric acids with alkyl chains ranging from C10 to C18 deposited from heptane/2-propanol solution on titanium oxide, in order to determine if there is a transition from liquid-like SAMs to more ordered and crystalline structures, as has been found for silanes and thiols on silicon or gold, respectively.8,22,23 Research on aspects such as film thickness, tilt angle, and water contact angle as a function of the chain length has been carried out on various systems,19,24-28 but, so far, no research has been carried out systematically for alkyl phosphates on titanium oxide. Changing from the commonly used octadecyl phosphate to shorter chain length improves the solubility of the molecule,19 but, at the same time, the van der Waals interactions between the chainss a major stabilizing factor within the monolayersare reduced. Titanium oxide was chosen as the substrate due to its biocompatibility and its potential for use in biosensor applications. For (18) Chen, Y. X.; Liu, W. M.; Ye, C. F.; Yu, L. G.; Qi, S. K. Mater. Res. Bull. 2001, 36, 2605-2612. (19) Pellerite, M. J.; Dunbar, T. D.; Boardman, L. D.; Wood, E. J. J. Phys. Chem. B 2003, 107, 11726-11736. (20) Tosatti, S.; Michel, R.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3537-3548. (21) Zwahlen, M.; Tosatti, S.; Textor, M.; Ha¨hner, G. Langmuir 2002, 18, 3957-3962. (22) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (23) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (24) Barrena, E.; Palacios-Lidon, E.; Munuera, C.; Torrelles, X.; Ferrer, S.; Jonas, U.; Salmeron, M.; Ocal, C. J. Am. Chem. Soc. 2004, 126, 385-395. (25) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys. 1997, 106, 1600-1608. (26) Frey, S.; Shaporenko, A.; Zharnikov, M.; Harder, P.; Allara, D. L. J. Phys. Chem. B 2003, 107, 7716-7725. (27) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350-4358. (28) Zharnikov, M.; Kuller, A.; Shaporenko, A.; Schmidt, E.; Eck, W. Langmuir 2003, 19, 4682-4687.
10.1021/la700474v CCC: $37.00 © 2007 American Chemical Society Published on Web 06/15/2007
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a complete understanding of the structure of the deposited films, a combination of surface-characterization techniques was applied: dynamic water contact angle (dCA), variable-angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), and polarization-modulated infrared reflectionabsorption spectroscopy (PM-IRRAS). It was found that, for chain lengths above 15 carbon atoms, alkyl phosphates form crystalline structures. 2. Materials and Methods General Considerations. All reactions were carried out under a N2 atmosphere using standard Schlenk techniques. The used alcohols (H(CH2)nOH, n ) 10-18) and POCl3 were obtained from Fluka (Switzerland) in a purity equal to or higher than 95%. Petrolether for recrystallizations (bp fraction 80-110 °C) was freshly distilled before use. Routine lH, 31P, and 13C NMR spectra were recorded with a Bruker 300 MHz spectrometer. Elemental analyses were performed with a Leco CHN-900 at the Laboratory for Organic Chemistry, ETH Zurich. 2.1. Phosphate Synthesis. The alkyl phosphates were all synthesized according to a slightly adjusted protocol developed in the late 1970s by Imokawa and Tsutsumi (Scheme 1).29 Briefly, an Scheme 1. Synthesis of Alkyl Phosphates from Alcohols and Phosphorus Oxychloride.
alcohol with the desired chain length was added directly to a 1.5fold excess of neat phosphorus oxychloride under an atmosphere of nitrogen. The resulting mixture was warmed to 50 °C for 5 h, and the evolving hydrogen chloride was removed from the flask by a stream of nitrogen. After cooling to room temperature, the final mixture was poured into a water/ice mixture and stirred for several hours. The products were isolated by extraction into diethyl ether and purified by recrystallization from freshly distilled petroleum ether (bp 80 °C). This procedure readily allows several grams of product to be synthesized in good quality in a single step. Using an excess of phosphorus oxychloride reduces the formation of dialkyl phosphates, which, together with the phosphoric acid anhydride, constitute the two main side products and can be easily detected by 31P NMR. Several recrystallizations from petroleum ether remove the side-products to a level below the NMR detection limit. The purity of the final compounds was determined by 1H, 13C, and 31P NMR and elemental analysis. 2.2. Substrates. We used 8 × 6 mm2 single-side-polished silicon wafers coated by physical vapor deposition (PVD) sputtering of 20 nm TiO2 for XPS, VASE, and CA studies. PM-IRRAS substrates were obtained by PVD sputtering of 8 nm TiO2 onto flat gold samples prepared as reported previously.30 2.3. SAMs. SAMs were prepared by immersion of the wafers into 0.5 mM solutions of the corresponding phosphate dissolved in a heptane/isopropanol mixture (99.2/0.8 v/v for C10 to C17 and 99.4/ 0.6 for C18). After 48 h of immersion, they were removed from the solution, rinsed thoroughly with isopropanol, and blown dry with a stream of nitrogen. 2.4. XPS. XPS analyses were performed using either a PHI 5700 spectrophotometer equipped with a concentric hemispherical analyzer (CHA) (Physical Electronics, Eden Prairie, MN) or a SAGE 100 system (Specs, Berlin, Germany) in the standard configurations. On the PHI 5700, spectra were acquired at a base pressure of 10-9 mbar or below, using a non-monochromatic Al-KR source operating at 350 W and positioned 10 mm away from the sample. The instrument was run in the minimum-area mode using an aperture of 0.4 mm diameter. The CHA was used in the fixed-analyzer-transmission (29) Imokawa, G.; Tsutsumi, H. J. Am. Oil Chem. Soc. 1978, 55, 839-843. (30) Venkataraman, N. V.; Zu¨rcher, S.; Spencer, N. D. Langmuir 2006, 22, 4184-4189.
Spori et al. mode. The pass energies used for survey scans and detailed scans were 187.85 and 46.95 eV, respectively, for titanium Ti2p, carbon C1s, oxygen O1s, and phosphorus P2p. Under these conditions, the energy resolution (full width at half-maximum height, fwhm) measured on silver Ag3d5/2 is 2.7 and 1.1 eV, respectively. Acquisition times were approximately 5 min for survey scans and 15 min (total) for high-energy-resolution elemental scans. These experimental conditions were chosen in order to have an adequate signal-to-noise ratio in a minimum time and to limit beam-induced damage. Under these conditions, sample damage was negligible, and reproducible analyzing conditions were obtained on all samples. In addition, only one sample was introduced into the analyzing chamber at a time. The measurements were performed with a takeoff angle (detection angle to the surface) of 45° with respect to the surface plane. In addition and for qualitative comparison, a series of samples were also measured on a SAGE 100 system (Specs, Berlin, Germany) using recently published conditions.20 The main difference between these measurements and those recorded on the PHI 5700 is the 90° takeoff angle (detection angle to the surface) with a concomitantly lower surface sensitivity and lower signalto-noise ratio. Data acquired with this instrument are included in the Supporting Information. All recorded spectra were referenced to the aliphatic hydrocarbon C1s signal at 285.0 eV. Data were analyzed using the program CasaXPS [version 2.3.5, www.casaxps.com]. The signals were fitted using Gaussian-Lorentzian functions and leastsquares-fit routines following Shirley iterative background subtraction. Sensitivity factors were calculated using published ionization cross sections31 corrected for the energy dependence of the transmission function of the instrument and the attenuation-length dependence on kinetic energy. 2.5. VASE. The monolayer thickness was measured, using a VAS ellipsometer (M-2000FTM, J.A. Wollam, Inc., Lincoln, NE), and the data were evaluated using the software WVASE32 (WexTech Systems, Inc., New York). The measurement was conducted in the spectral range of 370-1000 nm at three angles of incidence (65, 70, and 75°) under ambient conditions immediately before and after monolayer formation. The parameters for data evaluation are summarized in the supporting materials. 2.6. dCA Measurements. Surface wettability was investigated by measuring advancing and receding contact angles in a sessile drop (water) experiment (contact angle measuring system, G2/G40 2.05-D, Kru¨ss GmbH, Hamburg, Germany). The measurements were performed in an automated way by increasing and decreasing the drop size at a speed of 15 µL per minute. Averaged data and error bars refer to a total of 480 points for the advancing angle and 240 points for the receding angle, resulting from measurements taken at three different locations on each sample. Data were analyzed using the tangent method 2 fit routine of the Drop-Shape Analysis program (DSA version 1.80.0.2 for Windows 9x/NT4/2000, (c) 1997-2002 KRUESS) 2.7. PM-IRRAS. PM-IRRAS measurements were carried out on a Bruker IFS66v spectrometer equipped with a PMA50 photoelastic modulator accessory. The interferogram from the spectrometer was modulated with a ZnSe photoelastic modulator (Hinds Co.) at a frequency of 50 kHz and analyzed with a lock-in amplifier (SR380, Stanford Research, USA). Typically 1024 scans were acquired at a resolution of 8 cm-1. The sample chamber was purged continuously with dry air during the measurements. The resulting interferograms were processed using OPUS (Bruker Optics) software, baseline corrected with a polynomial background, and normalized to the highest intensity band. The spectra showed strong bands in the frequency region below 1000 cm-1 arising from the TiO2 layer. However, the C-H stretching region, which is of interest in this study, was free of any interfering features from the TiO2 layer underneath.
3. Results 3.1. dCAs. One of the most sensitive techniques to probe the outermost surface of a sample is the measurement of dynamic (31) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8.
Influence of Alkyl Chain Length on Phosphate SAMs
Figure 1. dCA measurements on the C10 to C18 series of phosphate SAMs. Only a small increase in the advancing CA (closed symbols) can be observed by increasing the chain length. In comparison, the receding CA (open symbols) is very low for C10 and increases rapidly with chain length up to C14. For the longer-chain phosphates, the receding CA remains more or less constant at around 90°.
contact angles. This technique gives information not only on surface energy but also on the degree of order of the molecules directly located at the solid-air interface.20,32 Advancing water contact angles for all measured SAMs were found to be above 110°sa clear sign that the hydrophobic alkyl chains are exposed at the SAM-air interface (Figure 1). A small but significant increase was observed in going from C10 to C18. The receding contact angles, in contrast, show a large increase from approximately 60° for C10 to above 85° for SAMs having alkyl chains longer than C13. The decreasing hysteresis between advancing and receding contact angles as the carbon chain length is increased is an indication of the increasing order of the SAMs.20,21,32 An influence of substrate roughness on hysteresis can be excluded, since all samples were manufactured under the same conditions and possessed a highly reproducible subnanometer morphology, similar to that of ultraflat gold substrates.20,21 3.2 XPS Analysis. QualitatiVe XPS Analysis of SelfAssembled Alkyl Phosphates on Titanium Oxide. The SAMs for all nine investigated phosphates C10 to C18 were analyzed at a 45° takeoff angle. In the survey spectra, the only elements that could be detected were titanium, oxygen, carbon, and phosphorus, as expected for these SAMs on TiO2 (Figure 2a). The highresolution detail spectra for O 1s, Ti 2p, C 1s, and P 2p were resolved into their components using a fitting procedure as described below. The spectra are displayed in Figure 2, and the parameters used are presented in Table 1. In respect to each element, it was possible to observe that P 2p: The phosphorus signal (Figure 2b) was fitted as a single doublet with P 2p3/2 at 134.5 ( 0.1 eV and P 2p1/2 with a bindingenergy difference of 0.81 eV and a fixed-area ratio of 2:1. On a Ta2O5 substrate, the binding energy of a octadecylphosphoric acid SAM was found to be 134.2 ( 0.1 eV.13 The P 2p binding (32) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surfaces; John Wiley & Sons: New York, 1994.
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energy for the free, nonbonded acid (octadecylphosphoric acid powder) was detected at a slightly higher binding energy of 134.7 ( 0.1 eV.13 Ti 2p: Only one chemical species could be detected for the Ti 2p signal (Figure 2c) with a binding energy corresponding to that of TiO2 at 458.6 eV for Ti 2p3/2.33 With increasing length of the phosphates, the intensity of Ti 2p decreases due to greater attenuation by the thicker organic overlayer. C 1s: The main contribution of the C 1s signal originates from aliphatic carbon with a binding energy of 285 eV. A shoulder at a higher binding energy of 286.6 eV can be attributed to the CH2 group connected to the phosphate. Whereas this higher binding energy contribution remains more or less constant in intensity (the signal being too small to observe attenuation effects), the aliphatic carbon contribution increases with increasing chain length, as expected (Figure 2d). O 1s: At a takeoff angle of 45°, the main contribution to the O 1s signal originates from the oxygen in the TiO2 substrate (binding energy 530.1 eV), (Figure 2e). The remaining oxygen contributions should all therefore originate at the substrate-SAM interface and can be assigned to alkyl-O-P, PdO, P-OH, P-O-Ti, and Ti-O-H. Since the amount of Ti can be measured from the Ti 2p peak, the amount of oxygen from the substrate can be calculated and subtracted from the O 1s signal. The remaining peak clearly shows two shoulders on each side (Figure 2f). It was therefore modeled using three contributions assigned to R-O-P at 532.8 eVsP-O-H, P-O-Ti, and Ti-OHs which are assumed to have very similar binding energies and are fitted with a peak centered at 531.4 eV and an fwhm of 2.0 eV, and PdO at 530.8 eV. Since the resolution of the O 1s peak does not allow us to distinguish between all these contributions, some additional constraints had to be implemented in the fitting routine to obtain consistent results. Knowing that R-O-P and PdO are located at a depth in the structure similar to that of phosphorus, and since the amount of phosphorus can be measured independently, the areas of these two peaks were also calculated and constrained. This then only allows the area of the component assigned to P-O-H, P-O-Ti, and Ti-OH to vary freely. For this contribution, a clear trend can be observed: It decreases more with increasing chain length than would be expected from attenuation effects alone. This observation can be explained with a decreasing amount of free Ti-OH and an increasing phosphate density. As expected, the relative amount of the substrate oxygen contribution decreases with increasing carbon chain length, which is a clear sign that the overlayer thickness is increasing. The first indications of a structural change with increasing chain length can be clearly observed in Table 2. Indeed, although the C/Ti ratio increases with increasing chain length, a change in the (interface O)/P ratio, as well as a change in the P/Ti ratio with increasing alkyl chain length, suggests a change in the surface coverage and packing density of the alkyl chains in addition to a simple increase in the adlayer thickness. Finally, the oxygen found at the SAM substrate interface also decreases with increasing carbon chain length. If one calculates the excess oxygen at the interface, it can be attributed to the number of Ti-OH species per phosphate group. There is clearly more Ti-OH present in the case of the shorter molecules than for the longer ones. This suggests that, for longer chain lengths, the phosphate density of the monolayers is increasing at the cost of either the surfacebound water or hydroxyl groups. QuantitatiVe XPS Analysis of Self-Assembled Alkyl Phosphates on Titanium Dioxide. Since the studied SAMs on titanium dioxide (33) NIST Data Gateway. http://srdata.nist.gov.
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Figure 2. (a) XPS survey spectra for the series of C10 to C18 phosphate SAMs on TiO2. (b) P 2p detail spectra with their deconvolution into P 2p3/2 and P 2p1/2. (c) Ti 2p detail spectra with deconvolution into Ti 2p3/2 and Ti 2p1/2. With increasing chain length of the phosphate SAMs, the Ti 2p intensity decreases due to higher attenuation. (d) C 1s detail spectra. An opposite trend compared to that of Ti 2p with increasing chain length is observed. (e) O 1s detail spectra. For an explanation of the different components, see text. (f) O 1s spectra after subtraction of the oxygen contribution originating from the TiO2 substrate. All detail spectra are displayed after Shirley background subtraction.
are not homogeneous in the z-direction, the layered structure of the system has to be taken into account, in order to carry out quantitative XPS analysis. The expected layered structure involves the polar phosphate groups located at the TiO2-SAM interface on top of the bulk TiO2, and the nonpolar alkyl chains constituting the top surface. The contact-angle data are the most direct indication of this structure (see above). The attenuation of electrons emitted from buried layers by inelastic scattering in layers above leads to a lower detected intensity and accounts for (a) the lower calculated atomic percentage for phosphorus and oxygen and (b) the higher atomic percentage for carbon, in comparison to the values calculated according to the stoichiometry of the adsorbates (Table 3). Three-Layer Model. Data from an XPS experiment are usually specified as atomic concentrations, calculated using an equation
of the form
XA )
IA/RA
∑j Ij/Rj
(1)
where XA is the atomic composition of element A in a sample containing j components, IA is the measured spectral intensity for element A, and RA is the relative sensitivity factor for element A.34 However, these coefficients are only strictly correct for homogeneous samples. For nonhomogeneous samples in the z-direction, one needs a method to calculate the effective intensity of the emitted electrons after being attenuated by all the overlayers. This depends on the structure of the sample as well as the (34) Smith, G. C.; Livesey, A. K. Surf. Interface Anal. 1992, 19, 175-180.
Influence of Alkyl Chain Length on Phosphate SAMs
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Table 1. XPS Binding Energies and Fitting Parametersa binding energy (BE) [eV]
fwhm [eV]
532.8 ( 0.1
1.53 ( 0.02
531.4 ( 0.1
constraintsb
line shape %Gauss-Lorentz and asymmetry [lit. Casa]
1.98 ( 0.06
fwhm ) fwhm(O 1s TiO2); area ) area(O 1s PdO) none
GL(78)
530.8 ( 0.1
1.53 ( 0.02
fwhm ) fwhm(O 1s TiO2);area ) area(P 2p) × RSF
GL(78)
O 1s TiO2
530.1 ( 0.1
1.53 ( 0.02
area ) area(Ti 2p) × RSF × 2
GL(78)
Ti 2p1/2 Ti 2p3/2
464.4 ( 0.1 458.6 ( 0.1
2.45 ( 0.04 1.33 ( 0.02
none none
GL(1) GL(82)T(2.1)
C 1s C-O-P C 1s aliphatic
286.65
1.51 ( 0.05
GL(75)
285.0
1.51 ( 0.05
fwhm ) fwhm(C 1s aliphatic); BE ) BE(C 1s aliphatic) +1.65 none
P 2p1/2
134.6 ( 0.1
1.90 ( 0.08
P 2p3/2
133.8 ( 0.1
1.90 ( 0.08
O 1s R-O-P O 1s P-O-Ti P-O-H Ti-O-H O 1s PdO
a
fwhm ) fwhm(P 2p3/2); area ) 0.5 × area(P 2p3/2);BE ) BE(P 2p3/2) + 0.81 none
GL(78)
GL(75) GL(30) GL(30)
Data measured on PHI 5700. RSF ) relative sensitivity factor. b
Table 2. XPS Intensity Ratios Measured at a 45° Takeoff Angle on a PHI 5700 XPS Machinea C10H21PO4 C11H23PO4 C12H25PO4 C13H27PO4 C14H29PO4 C15H31PO4 C16H33PO4 C17H35PO4 C18H37PO4
C/Ti
P/Ti
interface O/P
Ti-OH/P
1.8 2.1 2.4 2.6 3.6 4.0 4.9 5.5 6.4
0.16 0.17 0.19 0.19 0.22 0.24 0.26 0.28 0.30
6.0 6.0 5.3 5.1 5.0 4.8 4.5 4.4 4.5
2.0 2.0 1.3 1.1 1.0 0.8 0.5 0.4 0.5
a All numbers are averaged over at least 3 samples with the same chain length and have a relative accuracy of ( 10%.
Table 3. Atomic Concentrations of Elements in the Nine SAMs Compared to Calculated Values for the Stoichiometry of the Moleculesa C10H21PO4 C11H23PO4 C12H25PO4 C13H27PO4 C14H29PO4 C15H31PO4 C16H33PO4 C17H35PO4 C18H37PO4
At% C
At% P
At% O (PO4)
69.5 ( 1.4 (66.7) 68.1 ( 4.5 (68.8) 72.3 ( 2.1 (70.6) 70.2 ( 2.3 (72.2) 75.7 ( 2.3 (73.7) 76.5 ( 1.6 (75.0) 78.8 ( 0.5 (76.2) 80.0 ( 0.4 (77.3) 80.8 ( 1.7 (78.3)
6.1 ( 0.3 (6.7) 6.4 ( 0.9 (6.3) 5.5 ( 0.4 (5.9) 6.0 ( 0.5 (5.6) 4.9 ( 0.5 (5.3) 4.7 ( 0.3 (5.0) 4.2 ( 0.1 (4.8) 4.0 ( 0.1 (4.6) 3.8 ( 0.3 (4.4)
24.4 ( 1.1 (26.7) 25.5 ( 3.6 (25.0) 22.1 ( 1.7 (23.5) 23.9 ( 1.8 (22.2) 19.4 ( 1.9 (21.1) 18.8 ( 1.3 (20.0) 16.9 ( 0.4 (19.1) 16.0 ( 0.3 (18.2) 15.4 ( 1.4 (17.4)
a The numbers are average values of at least three samples. Calculated values are written in brackets.
attenuation length of the electrons in each layer. To do so, we use the method described by Smith and Livesey.34 A detailed description of our calculations with all the parameters listed can be found in the Supporting Information for this article. In brief, we used a three-layer model similar to that used by Textor et al. on tantalum oxide,13,35,36 with a layer of titanium oxide, a layer containing all the phosphate and surface oxygen, and a final layer of the aliphatic carbon (Figure 3). With this model, where (35) Elsener, B.; Rossi, A. Electrochim. Acta 1992, 37, 2269-2276. (36) Rossi, A.; Elsener, B. Surf. Interface Anal. 1992, 18, 499-504.
only the thickness of the aliphatic layer was allowed to vary, we calculated calibration curves for the takeoff angles of the electrons (45° and 90°) in the two spectrometers used (for data using the 90° angle, see Supporting Information). The thickness d of the alkyl part of the SAM was obtained by finding the best fit of the measured average atomic composition for each chain length (Figure 3). As one can see, the layer thickness observed for the series of alkyl phosphate SAMs is not simply proportional to their chain length. The biggest deviations from the model occur for the short alkyl phosphates and show up mainly as an increased value for the interface oxygen, suggesting again a higher amount of free Ti-OH per phosphate molecule at the TiO2-SAM interface for shorter alkyl chains. Plotting the measured thickness versus the number of carbon atoms, an S-shaped trend is obtained with the measured thicknesses (XPS) of the alkyl phosphates C10 to C13 following a calculated thickness for a mean tilt angle of 45°. This value has, of course, no physical meaning for poorly ordered monolayers but is similar to that previously published for poorly ordered SAMs,21 where the chains are in a liquid-like conformation. For the longer molecules, the measured thickness more closely follows the calculated value with a tilt angle of 30° or below, which is again an indication for more densely packed and well-ordered layers (Figure 4). A similar trend can be found for the measured thickness using VASE. 3.3. VASE. As in XPS, the data obtained from VASE depend on the model parameters used to fit the measured data. Since VASE is very sensitive to small deviations in the layer thickness and optical parameters of the substrates used, each sample was measured individually before and after adsorption of the SAM. The difference between these two measurements was fitted as a Cauchy layer (see Supporting Information) with the only parameter to be fitted being d, the layer thickness on top of a layered structure of TiO2/SiO2/Si. For the underlying substrate the parameters have been fitted individually for each sample measured before adsorption and then fixed. Since the refractive index for the phosphate SAM is not known, the obtained layer thicknesses are not absolute. Nevertheless, the value used is unlikely to deviate from the true refractive index by more than
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Figure 3. Calculated calibration curves of apparent normalized atomic concentrations as a function of alkyl chain overlayer thickness using a three-layer model and a 45° electron takeoff angle. Measured apparent atomic concentrations for each chain length were then fitted to these curves in order to measure the alkyl layer thickness d.
10%, and therefore the measured thicknesses should be close to true values. This assumption has been confirmed by the close match to the XPS data (Figure 4). For SAMs obtained with C18, C17, and somewhat less for C15, quite large deviations from sample to sample were observed. Especially in the case of C18, it turned out to be quite difficult to obtain a reproducible thickness. Small deviations in solvent composition seemed to have an especially large influence on measured layer thickness. A possible explanation for this behavior could be coadsorption of solvent, leading to a higher apparent thickness. This is not found in XPS since the unbound solvent will evaporate during pumping down to ultrahigh vacuum. 3.4. PM-IRRAS. Infrared spectroscopic measurements on phosphate SAMs were carried out to gain further insight into the structure of these SAMs, especially the chain-length dependence of alkyl chain conformational order within the monolayers. PMIRRA spectra were carried out on phosphate monolayers deposited onto TiO2-covered gold substrates, owing to the poor quality of the infrared spectra obtainable on bare TiO2 substrates. Such an optically thin layer of TiO2 is essentially transparent in the wavelength region of interest, and thereby allows for the measurement of the spectrum of the adsorbed phosphate
Figure 4. SAM thickness versus number of carbon atoms measured for the alkyl phosphate monolayers. The data are compared with a theoretical thickness calculated for 30° and 45° tilt angles of the alkyl chains. The data are obtained by XPS (filled symbols) (using a three-layer model) and VASE (open symbols) measurements.
Influence of Alkyl Chain Length on Phosphate SAMs
Figure 5. PM-IRRAS spectra in the C-H stretching region of the series of phosphate SAMs. The number of carbon atoms (n) in the alkyl chain of the phosphates is indicated alongside the spectra.
monolayers with practically the same sensitivity as for monolayers adsorbed onto a metallic substrate. Such methods using a thick metallic layer underneath thin layers of a dielectric, often referred to as buried metal layer37 substrates, have been used in the studies of adsorbates on silicon oxide surfaces with a metallic Ni or Cu layer below.38,39 The SAMs on TiO2/Au substrates used in IR measurements were thoroughly characterized by means of XPS, VASE, and dCA measurements as described above, and were found to be identical in film composition, thickness, and order to those formed on TiO2/SiO2/Si substrates. The PM-IRRAS spectra, in the wavelength region of 28003000 cm-1, of the series of phosphates are shown in Figure 5. The spectra show four prominent bands assignable to the C-H stretching modes of the methylene (CH2) chain and that of the terminal methyl (CH3) group. The asymmetric and symmetric stretching modes of the methyl groups appear at 2960 and 2870 cm-1, respectively. The symmetric and antisymmetric stretching modes of the methylene chains appear in the regions of 2850 and 2920 cm-1, respectively. The most interesting feature of the series of spectra shown in Figure 5 is the shift in the positions of these bands. These bands show a clear shift to lower frequencies with increasing chain length, whereas the positions of the methyl stretching bands remain almost invariant. The methylene stretching modes are known to be sensitive to the conformational order of alkyl chains shifting to higher frequencies with increasing conformational disorder.40-42 For the series of phosphates studied here, the positions of these two bands are plotted as a function of alkyl chain length in Figure 6. The positions of the methylene stretching modes show a sharp transition at chain lengths above 15. For the shorter chains (n < 14) these bands appear at 2927 and 2856 cm-1, indicating that the alkyl chains adopt a disordered, liquid-like structure, whereas, for higher chain lengths (n > 15) they appear at 2920 and 2950 cm-1, indicating that they adopt a more ordered crystalline structure. For intermediate chain lengths, these bands appear at a value between these two extremes (n ) 14,15). These results are in agreement with the XPS and ellipsometric measurements described above. As mentioned in the discussions above, the SAMs formed from n ) 18 alkyl (37) Gardner, P.; LeVent, S.; Pilling, M. Surf. Sci. 2004, 559, 186-200. (38) Finke, S. J. S., G. L. Spectrochem. Acta 1990, 46A, 91-96. (39) McGonigal, M. B. V. M.; Butler, J. E. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 1033-1044. (40) Snyder, R. G.; S., H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 51455150. (41) Macphail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334-341. (42) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.
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Figure 6. Positions of the methylene symmetric (open symbols) and antisymmetric (filled symbols) stretching modes in the PMIRRA spectrum of the series of phosphates. The horizontal dotted line corresponds to the typical values of these modes observed in crystalline ordered alkyl chain assemblies.
phosphates showed some inconsistencies in the position and the intensity of the stretching bands, presumably due to solvent incorporation. However, the spectra of alkyl phosphates of other chain lengths remained consistent in their peak positions.
4. Discussion and Conclusions All measurement methods show a clear difference between short (C10 to C13) and long (C16 to C18) alkyl phosphate SAMs. We believe that the shorter molecules assemble into a less dense, liquid-like structure. This is supported by the fact that there is an excess of oxygen found for those alkyl phosphates with XPS, which we interpreted as free surface Ti-OH groups. The phosphate heads therefore must be assumed to be more widely spaced, leading to a model of the surface where the average tilt angle is higher than in a densely packed layer, as measured by XPS and VASE. PM-IRRAS also supports this widely spaced structure, in that more “liquid-like” C-H stretching vibrations are observed for the alkyl phosphates with shorter chain lengths. Increasing the chain length increases the van der Waals interaction between the alkyl chains, which leads to a more crystalline, all-trans conformation of the chains. At the same time, the packing density increases and therefore more surface OH groups are replaced with phosphate groups. The higher order is also evident in the smaller hysteresis of the dCAs and the shift of the C-H stretching frequency to values that are similar to those of crystalline aliphatic chains. A sketch of the proposed structures for short versus long alkyl phosphates on TiO2 is depicted in Figure 7. Since we do not exactly know the atomic surface structure of our amorphous or nano crystalline TiO2 films, the exact binding mode of the phosphate groups to the TiO2 surface remains speculative. Nevertheless, we can propose a surface structure, which is in accordance to our findings. In the case of thiol SAMs on gold, even short decanethiols form densely packed ordered monolayers.5,8,30 Whereas thiols have only one possible binding interaction with gold surfaces, phosphate groups can, in principle, bind in a mono-, bi- or even tridentate manner to the TiO2 surface. In single-crystal structures of layered alkyl phosphates, the phosphate groups bind almost exclusively in a bridged binding mode with each oxygen atom binding to a different metal ion. Two- and 3-fold coordination can be found, while a chelating binding mode with two oxygen atoms binding to the same metal is very uncommon. Therefore, we can most probably exclude such a chelating binding mode, since this would
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Figure 7. Sketch of the proposed binding structure for short (C10) and long (C18) alkyl phosphates self-assembled on TiO2.
lead to coordination numbers for the titanium of larger than six, and a highly strained four-numbered Ti-O-P-O cycle, which should not be energetically favorable. We believe, in view of the coverages observed, that a realistic picture involves a mixture of single and double, but not triple coordination, especially for the long alkyl phosphates. This is in accordance with our XPS data evaluation, which implies the presence of a free PdO unit. The short alkyl phosphates (C10-C13) are mainly stabilized due to their interaction with the surface. This might be the reason that they do not pack as densely in order to bind at an optimal place in a strong bridging bidentate manner, which can be further stabilized by hydrogen bonds to free surface Ti-OH groups. For the longer alkyl chains, the enthalpy gain due to van der Waals interactions between the alkyl chains becomes more important and also makes monodentate binding energetically favorable. Therefore additional molecules can squeeze between the bidentate
bonded molecules and almost double the final surface density (Figure 7). This study has shown for the first time that there is a strong influence of the chain length on the final structure in alkyl phosphate SAMs. While we have chosen identical adsorption protocols for all chain lengths in order to be able to compare the spectroscopic data, an optimization of the adsorption parameters for each molecule could potentially further improve the layer quality, order, and reproducibility of SAM formation. Acknowledgment. We thank Michael Horisberger of the PSI (Villigen, Switzerland) for support of substrate coating. Supporting Information Available: A table with the parameters used for the fitting of the VASE data as well as a detailed description of the calculation of the calibration curves for the XPS evaluation. This information is available free of charge via the Internet at http://pubs.acs.org. LA700474V
