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J. Phys. Chem. C 2007, 111, 9794-9798
Adsorption of TiO2 Nanoparticles on Glass Fibers M. Khajeh Aminian,† N. Taghavinia,*,†,‡ A. Irajizad,†,‡ and S. Mohammad Mahdavi† Physics Department and Institute for Nanoscience and Nanotechnology, Sharif UniVersity of Technology, Tehran 14588, Iran ReceiVed: January 7, 2007; In Final Form: March 17, 2007
Titania was deposited on glass fibers with a partial epoxy layer coating from a solution containing TiO2 nanoparticles at T ) 90 °C, and the adsorption process was examined by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FT-IR) spectroscopy, and photocatalytic activity measurements. XPS data demonstrated that about 25% of the surface was covered with TiO2 nanoparticles, with 10% on the epoxy layer and 15% on the glass body. It was found that TiO2 nanoparticles can be readily adsorbed on epoxy groups, whereas they have a low tendency to adsorb on carbon polymer chains. This difference can be attributed to the hydrogen bonding between the hydroxyl groups on the surface of TiO2 nanoparticles and the hydroxyl groups resulting from the acid-catalyzed epoxy ring opening. This hypothesis was verified by removing the epoxy groups on top through preheating of the fibers at 500 °C and studying the surface coverage after adsorption. In this case, TiO2 was adsorbed on about 15% of the surface, and none of the polymer layer was covered with nanoparticles. FT-IR, SEM, and photocatalytic activity measurements confirmed the XPS results.
1. Introduction The treatment of wastewater and purification of polluted air have attracted extensive attention in recent decades. Photocatalytic degradation of harmful and toxic organic pollutants using semiconductors, such as TiO2, ZnO, Fe2O3, CdS, and ZnS, is a useful approach to air and water purification, as the energy for the reactions can be provided by sunlight.1,2 Among these semiconductors, TiO2 is the most widely used photocatalyst owing to its excellent photocatalytic activity, physical and chemical stability, and nontoxicity.3 Because TiO2 is mainly produced in powder form, which is technologically impractical in continuous engineering processes,4 many attempts have been made to prepare supported catalysts, using glass beads,5 glass fibers,6,7 silica,8 stainless steel,9 fiber textiles,10 honeycombs,11 activated carbon,12,13 and zeolites.14 One of the possible ways of impregnating a porous support with photocatalyst nanoparticles is simple wash coating. In wash coating, nanoparticles are adsorbed on the support surface and are typically kept bound by weak forces. The binding of nanoparticles to the surface is often enhanced by posttreatment processes. The adsorption tendency of nanoparticles on a surface depends on the chemical groups on the top layer of the surface and can be enhanced by surface treatments15-17 such as light irradiation18 and surface functionalization.19 Hydrophilicity/ hydrophobicity represents the chemical nature of the surface and is one of the factors determining the adsorption tendency on the surface.20 The electric charges on the nanoparticles and on the support surface are also key factors in the adsorption process. These electric charges depend on the pH of the solution.21 Nanoparticles can be readily adsorbed on oppositely charged surfaces, as well as on a surface with appropriate chemical groups. * To whom correspondence should be addressed. E-mail: taghavinia@ sharif.edu. † Physics Department. ‡ Institute for Nanoscience and Nanotechnology.
In a previous work, we reported the adsorption of TiO2 nanoparticles on cellulose fibers by the impregnation method and the subsequent formation of nanofibers by heat exclusion of cellulose.22 The TiO2 nanoparticles were estimated to cover most of the cellulose fiber surface. The surface of cellulose fibers is highly hydrophilic and well-coated with OH-terminated TiO2 nanoparticles. However, as cellulose fibers are attacked by the photocatalytic activity of the nanoparticles, they are not appropriate supports for photocatalyst nanoparticles. In contrast, glass fibers are resistant to photocatalytic degradation. In addition, they are inexpensive, and their fibrous structure allows for easy air and water flow, with the entire surface accessible to the reactant molecules. Glass fibers are usually coated with an epoxy layer, to make them applicable in the composite industry. The coating covers the surface of the fiber partially or completely and helps it bind to a polymer matrix.23 In this work, we describe the use of glass fibers that are partially coated with an epoxy layer as a support for TiO2 nanoparticles. We studied the role of three surface components in the adsorption of TiO2 nanoparticles: the epoxy groups of the epoxy layer, the carbon chains of the epoxy layer, and the bare glass surface. We mainly employed X-ray photoelectron spectroscopy (XPS) to study the adsorption mechanism and identify active sites for adsorption by the shadow effect of nanoparticles. XPS is capable of characterizing the chemical groups and bonds on the surface.24 The chemical groups on the surface can be detected to some extent by measuring the chemical shifts of surface element peaks.25,26 2. Experimental Methods A completely dispersed solution of TiO2 nanoparticles was prepared, as reported earlier.22 Briefly, tetraisopropyl titanate (TiPT) was added dropwise to acidified water (pH 1.8) with stirring, to make 100 mL of TiO2 sol with a concentration of 0.05 M. Glass fibers coated with an epoxy layer (Nantong Xiujun Decoration Material Co.) were used as the substrate.
10.1021/jp070116i CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007
Adsorption of TiO2 Nanoparticles on Glass Fibers
Figure 1. XPS analysis of glass fibers showing Si, O, C, and Na atoms on the surface. After treatment with HCl solution, the Na peak disappears, the C peak intensity decreases, and the other peaks are sharpened.
The glass fibers were immersed in the TiO2 sol at 90 °C for 2 h to be coated with TiO2 nanoparticles. Then, the glass fibers were washed with deionized water and dried. Some samples were also prepared by preheating the glass fibers at 500 °C for 3 h, before they were coated with TiO2 nanoparticles in the same way. The photocatalytic properties of the samples were measured under ultraviolet (UV) irradiation of a 4-W lowpressure Hg lamp by inserting NH3 in a 40-L photocatalytic reactor using an ammonia sensor (TGS826, Figarosensor Corporation). Scanning electron microscopy (SEM) and energydispersive spectroscopy (EDS) of the fibers were carried out using a Philips XL30 apparatus. Surface composition was measured by XPS using the Al anode of a V.G. Microtech XR3E2 X-ray source and a concentric hemispherical analyzer (Specs model EA10 plus). Photoelectrons were collected at a takeoff angle of 0° from the surface normal. Transmission electron microscopy (TEM) observations were made using a Zeiss 120 keV instrument, and FT-IR transmission spectra were measured on a ABB Bomem MB Series instrument. Particle size measurements based on dynamic light scattering were carried out on a Malvern Zetasizer Nano Series apparatus. 3. Results and Discussion Figure 1 displays the XPS spectra of glass fibers after immersion at 90 °C in a HCl solution of pH 1.8 for 2 h, compared to the as-received fibers. This pH and temperature are basic conditions for TiO2 nanoparticles coating experiments, as follows next. The XPS spectra show the presence of Si, O, C, and Na on the surface. According to Figure 1, Na is removed from the surface after treatment with HCl solution through the dissolution of surface Na atoms. The carbon peak intensity decreases after HCl treatment, whereas the Si and O peaks become sharper and more intense. It seems that the solution removes the adsorbed environmental contaminants from the surface. This exposes the Si and O atoms of the glass fibers, resulting in the higher intensities of these peaks. Using the intensities of the C and Si peaks and considering the XPS sensitivity factors of these elements, one estimates that about 40% of the surface of the fibers is coated with an epoxy layer. To measure the size of the nanoparticles grown in the solution, TEM observations were made by dropping a sample of the solution containing nanoparticles on a carbon-coated grid. Figure 2a shows a TEM image of TiO2 nanoparticles with an average size of about 15 nm. Size distribution of the nanoparticles measured by particle size measurements based on dynamic light scattering is displayed in Figure 2b. The average size of the nanoparticles calculated from this histogram is about 13.7 nm, which is in good agreement with the TEM observations. We have already demonstrated that the nanoparticles grown at pH
J. Phys. Chem. C, Vol. 111, No. 27, 2007 9795
Figure 2. (a) TEM image of TiO2 nanoparticles prepared in solution. (b) Histogram of these particles obtained by dynamic light scattering analysis. The average size of nanoparticles was measured to be about 15 nm from the TEM image and 13.7 nm by dynamic light scattering.
Figure 3. SEM images of glass fibers (a) before and (b) after the TiO2 coating process. The fibers originally have a discontinuous coating of epoxy layer. On the surface of the TiO2-coated fibers, the adsorbed TiO2 nanoparticles can be seen.
Figure 4. (a) Comparison of XPS spectra of glass fibers before and after being coated with TiO2. The inset shows that the intensity of the 287.5 eV peak decreases after the TiO2 coating process; however, no change in the 285 eV peak is noticeable. (b) Diagram of the surface concentrations of the elements extracted from XPS data, showing that TiO2 nanoparticles were adsorbed on the surface of both SiO2 and the epoxy layer.
1.8 are in the crystalline phase of anatase and are appropriate for photocatalytic reactions.22 Glass fibers were immersed in this solution at T ) 90 °C for 2 h, to be coated with nanoparticles and then washed with deionized water. Figure 3a and b shows the SEM images of uncoated and TiO2 coated glass fibers, respectively. The average diameter of the fibers is about 7 µm. EDS analysis demonstrates that SiO2 is the main component of the glass fibers, with Na2O as a major additive and minor amounts of MgO, CaO, and Al2O3. Line-scan EDS confirmed that some portions of the surface of glass fibers had been coated with a polymer layer (Figure 3a). As can be seen in Figure 3a, there is a discontinuity in the epoxy layer on the fiber surface. According to Figure 3b and the EDS analysis, after the TiO2 coating process, the surface of the glass fibers is partially coated with TiO2. The chemical compositions of the coated and uncoated glass fibers were probed by XPS, as shown in Figure 4a. The appearance of the Ti peak demonstrates the adsorption of TiO2 nanoparticles on the surface. It can be seen that both the Si and C peaks become weaker after the coating process. It is known
9796 J. Phys. Chem. C, Vol. 111, No. 27, 2007 that the carbon due to surface contamination was negligible and the carbon is mainly related to the epoxy layer. In a highresolution scan of the carbon peak, one can see that the C peak is composed of peaks at 285 and 287.5 eV (inset of Figure 4a). Also, the intensity of the 287.5 eV peak is reduced after the coating process, whereas no change in the 285 eV peak is noticeable. A diagram of the surface composition of the materials extracted from the XPS peak areas, calibrated for the elemental sensitivity factors, is shown in Figure 4b. Because XPS provides information on the top few monolayers on the surface (about 2 nm),27,28 which is a small region compared to the thickness of the epoxy or TiO2 coating, one can assume that, at any point on the surface, only one of the components SiO2, TiO2, and epoxy contribute to the XPS data. This means that the coatings have a complete shadow effect. To perform more precise calculations of surface concentrations, additional measurements at various takeoff angles are required. It is known that measurement of surface composition using XPS has an intrinsic uncertainty of about 10%. Repeating our measurements also showed that the data are reproducible within this 10%. In addition, for a more quantitative analysis, geometrical factors arising from the shadowing of the nanoparticles on the curved surface of the fibers should be taken into account. We made an estimation of this effect and found that, in our case, these geometrical effects cause about a 10% error in the surface composition estimations. This is on the order of the intrinsic XPS uncertainty. In summary, the uncertainty in the measurement of surface composition seems to be sufficiently low to maintain the validity of the analysis. In this diagram (Figure 4b), the sum of the portions of Si and O in the ratio of 1:2 was assumed to represent SiO2. The same routine was performed for TiO2. The carbon and the remaining portion of oxygen are related to the epoxy layer. According to Figure 4b, about 38% of the fiber surface is covered with an epoxy layer, and rest is the SiO2 body. After the adsorption of TiO2 nanoparticles, TiO2 takes some portions of both SiO2 and the epoxy layer. The coverage of TiO2 is about 25%, of which 10% is on the epoxy layer and 15% is on the SiO2 body. This implies that TiO2 nanoparticles were adsorbed partially on both the SiO2 and the epoxy layer of the fibers. To explain this behavior, we focus on the intermediate epoxy layer and present the following model for the adsorption of TiO2 nanoparticles on the surface. As mentioned above, the XPS peak of carbon is mainly composed of peaks at about 285 and 287.5 eV with different intensities. We attribute the C peak with the lower binding energy (285 eV) to the carbon chain of the layer and the C peak with the higher binding energy (287.5 eV) to the C atoms attached to O in the epoxy group.25,26 Reports in the literature demonstrate that, in the attachment of epoxy silane molecules to the surface of SiO2, the silane group is mainly attached to the surface whereas the epoxy groups lie on top.29,30 This is the reason that the C peak corresponding to the epoxy group (287.5 eV) is more intense than the carbon chain peak (285 eV) for the fibers. Carbon peaks due to surface contamination were negligible. In our case, at pH 1.8, it is known that the acid-catalyzed ring opening of the epoxy groups takes place, leading to the creation of hydroxyl groups on the surface of the epoxy layer.31 This makes the surface of the layer more hydrophilic, so that the surface shows better interactions with polar groups in aqueous solution. The surface of TiO2 nanoparticles in aqueous solutions is also covered with hydroxyl groups. In addition, at the present pH of 1.8, some H+ ions are adsorbed on the surface,
Aminian et al. SCHEME 1: Illustration of Interactions between a TiO2 Nanoparticle in Solution and the Surface of SiO2, the Opened Ring Epoxy Groups, and the Organic Chains of the Layer on the Surface
forming OH2+ groups on the surface. This is because pH 1.8 is well below the isoelectric point of TiO2, i.e., 5.5.21 Scheme 1illustrates the described conditions. One expects that the hydroxyl groups of the TiO2 nanoparticles and the hydrophilic surface of the hydrolyzed epoxy groups experience an attractive interaction through strong hydrogen bonding. This leads to the adsorption of the TiO2 nanoparticles on the surface of the opened ring of the epoxy groups. Because the size of the TiO2 particles in the solution is about 15 nm, which is more than the penetration depth of XPS,27,28 it is expected that the TiO2 nanoparticles adsorbed on the surface of the epoxy layer create a complete shadow, blocking the signal from the epoxy layer. As the nanoparticles are expected to adsorb on the hydrophilic parts of the layer, one expects that the intensity of the carbon peak at 287.5 eV, which corresponds to the carbon of the epoxy groups, should be reduced after TiO2 adsorption. This is, indeed, observed in the XPS spectra (inset of Figure 4a). It is expected that some portion of the polymer coating is terminated on top by non-epoxy groups, as depicted in Scheme 1. These carbon atoms constitute the carbon chains of the polymer. To calculate exactly the fraction of each carbon type on the surface, additional measurements at various takeoff angles should be performed, which is beyond our instrumentation capabilities. The fact that the peak for the carbon chains does not show a considerable reduction after the TiO2 coating process implies that these sites are not favored for TiO2 nanoparticle adsorption. The material of the glass fiber body is mainly SiO2. The surface of SiO2 is hydrolyzed in solution and is covered with -OH groups.21 Hence, the adsorption of TiO2 nanoparticles on the surface of SiO2 is feasible. This adsorption leads to a decrease in the intensity of the Si peak as shown in Figure 4. To assess the validity of the model, we tried removing the epoxy groups on the surface of the polymer layer and studied the changes in the adsorption behavior. For this purpose, the glass fibers were preheated at 500 °C for 3 h before being coated with TiO2. We found that this treatment results in the incineration of the top carbon layers, leading to the removal of the epoxy groups on top. After being preheated, the fibers were coated with TiO2 nanoparticles in the same way. Removal of the epoxy groups was evidenced by infrared spectroscopy, as depicted in Figure 5. FT-IR measurements were used to probe the chemical bonds present in the sample before and after heat treatment. The following band assignments were made: TiO2 and C-H stretching, 3000-2900 cm-1; SiO2, 1460 and 1380 cm-1; epoxy ring antisymmetric stretching, 840 and 910 cm-1; and C-O
Adsorption of TiO2 Nanoparticles on Glass Fibers
Figure 5. FT-IR spectra of the coated glass fibers with and without preheating at 500 °C. It can be seen that the peaks of the epoxy ring and C-O (due to opened rings) are reduced after heating.
Figure 6. SEM image of the surface of preheated fibers after being coated with TiO2 nanoparticles. The coating of the preheated sample has a discontinuity in comparison to that of the unheated sample.
stretching, 1150 and 1060 cm-1.32,33 According to Figure 5, FTIR spectra of preheated and unheated samples that have been coated with TiO2 nanoparticles are the same except for a considerable change in the epoxy ring and C-O groups. This reveals that preheating of the fibers results in epoxy ring removal and, consequently, reduction of the opened ring (C-O) groups. It has been reported that the contact angle of the surface of epoxy layer is increased after heat treatment;30 that is, the surface becomes more hydrophobic because of the organic chains on the surface. Figure 6 shows the SEM image of preheated glass fibers coated with TiO2. According to the figure and EDS analysis, in this case, the surface is partially coated with TiO2. One can see that the TiO2 coating on the preheated fibers is less continuous than the coating on the unheated fibers (Figure 3b). The adsorption tendency of TiO2 nanoparticles on the preheated fibers was investigated by XPS, as shown in Figure 7. Assuming the intensity of O peak as a reference, the Ti peak is weaker in this case than it is for the unheated sample (Figure 4a), indicating that less TiO2 is adsorbed on the surface. It can be seen that the Si peak was reduced after the TiO2 coating process. However, the intensity of the C peak showed no considerable change. In a high-resolution scan of the C peak (inset of Figure 7a), it can be observed that, for the uncoated sample, the 285 eV peak of the preheated fibers is more intense than that of the previous unheated fibers, whereas the 287.5 eV peak grows weaker after heating. This implies that the epoxy group carbons were removed after the heat treatment. By comparing the carbon peak intensities before and after the TiO2 coating process (inset of Figure 7a), one can see that the carbon on the surface is not much affected by the adsorption of TiO2 particles.
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Figure 7. (a) XPS spectra of preheated glass fibers before and after TiO2 coating. It can be seen that the Si peak was reduced after the TiO2 coating process whereas the intensity of the C peak showed no considerable change. (b) Diagram of the surface concentrations of the elements extracted from XPS data, showing that, after preheating of the fibers, TiO2 nanoparticles were adsorbed only on the surface of SiO2, whereas the adsorption of nanoparticles on the surface of epoxy layer was reduced.
Figure 8. Photocatalytic degradation of ammonia for preheated and unheated glass fibers coated with TiO2 nanoparticles.
The changes in the surface coverage after the TiO2 coating process are shown in the surface concentration diagram of Figure 7b. The coverage of the organic coating is still about 40%, unaffected by the heat treatment. This demonstrates that heat treatment burns away only the top parts of the epoxy layer. According to Figure 7b, TiO2 covers exactly the same portion of SiO2 (about 15%) as for the unheated sample, i.e., the adsorption of TiO2 on SiO2 surface is not influenced by the heat treatment. In contrast, TiO2 does not cover any portion of the organic part, indicating that the adsorption of TiO2 nanoparticles on the surface of the epoxy layer is considerably reduced after preheating of the fibers. This change is attributed to the removal of the epoxy groups of the layer, exposing the carbon chains on top. In this case, because of the removal of the epoxy groups, there are few OH groups on the epoxy layer surface. Reduction of the OH concentration on the surface leads to weak interactions between the TiO2 nanoparticles and the surface, as illustrated in Scheme 1. The fibers coated with TiO2 nanoparticles were examined for their photocatalytic properties. Figure 8 shows the photocatalytic degradation of ammonia on preheated and unheated glass fibers coated with TiO2 nanoparticles. The unheated fibers reduce the level of ammonia to 20% of the initial value in about 75 min, whereas this time is 145 min for the preheated sample. The degradation rate for the preheated fibers is therefore found to be approximately one-half that of the unheated fibers. It seems that applying a heat treatment before coating the TiO2 nanoparticles results in a reduction of the tendency of the surface to adsorb TiO2 nanoparticles. Assuming that the degradation rate is proportional to the coated surface, one expects that the TiO2coated area on the unheated fibers is about twice that of the preheated fibers. This result is in fair agreement with the XPS surface concentration data, which shows a ratio of about 1.7
9798 J. Phys. Chem. C, Vol. 111, No. 27, 2007 for the surface concentrations of TiO2 on the unheated and preheated fibers. 4. Conclusions In summary, TiO2 was deposited on commercial partially epoxy-coated glass fibers from a solution containing TiO2 nanoparticles. The interaction of the TiO2 nanoparticles and the surface of glass fibers was studied using XPS analysis. Two different types of sites were identified on the surface for the adsorption of nanoparticles, namely, SiO2 glass sites and epoxy layer sites. The surface of SiO2 glass has a rather good tendency to attract TiO2 nanoparticles. TiO2 nanoparticles were also found to readily adsorb on the hydrophilic surface of the epoxy layer because of the hydrolysis of the epoxy rings on top by acid catalysis. Preheating of the fibers was found to remove the epoxy groups and leave the polymer chains. This leads to a smaller concentration of OH groups on the surface and, hence, less TiO2 adsorption. This description of the adsorption process is evidenced by XPS, SEM, and photocatalytic measurements. On the whole, it is demonstrated that XPS analysis is an effective method for studying the tendency of a surface to adsorb nanoparticles. Contact angle measurements have also been utilized to study the adsorption of nanoparticles; however, XPS provides more insight into the different sites capable of adsorption and the influence of treatment conditions on the population of different sites. Acknowledgment. The financial support of Hitech Industries Center of Ministry of Industries and Mines is kindly appreciated. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Yang, G. J.; Li, C. J.; Han, F.; Ohmori, A. Thin Solid Films 2004, 466, 81. (4) Monneyron, P.; Manero, M. H.; Foussard, J. N.; Benoit-Marquie, F.; Maurette, M. T. Chem. Eng. Sci. 2003, 58, 971. (5) Xu, Y.; Chen, X. Chem. Ind. 1990, 6, 497. (6) Robert, D.; Piscoro, A.; Heinz, O.; Weber, J. V. Catal. Today 1999, 54, 291.
Aminian et al. (7) Mattews, R. W. Solar Energy 1987, 38, 405. (8) Sato, S. Langmuir 1988, 4, 1156. (9) Zhu, Y.; Zhang, L.; Wang, L.; Fu, Y.; Cao, L. J. Mater. Chem. 2001, 11, 1864. (10) Ku, Y.; Ma, C. M.; Shen, Y. S. Appl. Catal. B 2001, 34, 181. (11) Fernandes, A.; Lassaletta, G.; Jimenez, V. M.; Justo, A.; GonzalezElipe, A. R.; Herrmann, J. M.; Tahiri, H.; Ait-Ichou, Y. Appl. Catal. B 1995, 7, 49. (12) Nozawa, M.; Tanigawa, K.; Hosomi, M.; Chikusa, T.; Kawada, E. Water Sci. Technol. 2001, 44, 127. (13) Takeda, N.; Iwata, N.; Torimoto, T.; Yoneyama, H. J. Catal. 1998, 177, 240. (14) Green, K. J.; Rudham, R. J. Chem. Soc., Faraday Trans. 1993, 89, 1867. (15) Chan, E. W. L.; Yu, L. Langmuir 2002, 18, 311. (16) Yamanoi, Y.; Yonezawa, T.; Shirahata, N.; Nishihara, H. Langmuir 2004, 20, 1054. (17) Mattigod, S. V.; Fryxell, G. E.; Alford, K.; Gilmore, T.; Parker, K. EnViron. Sci. Technol. 2005, 39, 7306. (18) Ryan, D.; Nagle, L.; Fitzmaurice, D. Nano Lett. 2004, 4, 573. (19) Park, J. H.; Park, J. K.; Shin, H. Y. Mater. Lett. 2007, 61, 156. (20) Cai, K.; Frant, M.; Hildebrand, G.; Liefeith, K.; Jandt, K. D. Colloids Surf. B 2006, 50, 1. (21) Shaw, D. J. Introduction to Colloid and Surface Chemistry; Butterworth-Heinemann: Amsterdam, 1992; Vol. 7, p 176. (22) Aminian, M. Kh.; Taghavinia, N.; Iraji-zad, A.; Mahdavi, S. M.; Chavoshi, M.; Ahmadian, S. Nanotechnology 2006, 17, 520. (23) Provatas, A.; Matisons, J. G.; Smart, R. St. C. Langmuir 1998, 14, 1656. (24) Wagner, C. D. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: Chichester, U.K., 1994; Vol. 1, p 437. (25) Crist, B. V. Handbook of Monochromatic XPS Spectra, Layers and Layers Damaged by X-rays; VCH: Chichester, U.K., 2000. (26) Briggs, D., Ed. Handbook of X-ray and UltraViolet Photoelectron Spectroscopy; Heyden & Son Ltd.: London, 1977. (27) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (28) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M. Langmuir 1995, 11, 813. (29) Tsukruk, V. V.; Luzinov, I.; Julthongpiput, D. Langmuir 1999, 15, 3029. (30) Luzinov, I.; Julthongpiput, D.; Liebmann-Vinson, A.; Cregger, T.; Foster, M. D.; Tsukruk, V. V. Langmuir 2000, 16, 504. (31) Carey, F. A. Organic Chemistry, 5th ed.; McGraw-Hill: New York, 2004. (32) Tarducci, C.; Kinmond, E. J.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. Chem. Mater. 2000, 12, 1884. (33) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; John Wiley and Sons: New York, 1964.
