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Langmuir 2007, 23, 10924-10928
Adsorption of CdSe Nanoparticles to Thiolated TiO2 Surfaces: Influence of Intralayer Disulfide Formation on CdSe Surface Coverage Jonathan R. Mann and David F. Watson* Department of Chemistry, UniVersity at Buffalo, The State UniVersity of New York, Buffalo, New York 14260-3000 ReceiVed July 16, 2007. In Final Form: September 4, 2007 Mixed monolayers of hexadecanoic acid (HDA) and 16-mercaptohexadecanoic acid (MHDA) were adsorbed to nanocrystalline TiO2 films, and CdSe nanoparticles were attached to the mixed monolayer functionalized surfaces. IR absorption spectroscopy was used to characterize the equilibrium binding of HDA and MHDA to TiO2. Surface adduct formation constants (Kad) of (4 ( 2) × 103 M-1 and (6 ( 4) × 103 M-1 were measured for HDA and MHDA, respectively. CdSe nanoparticles were adsorbed to the terminal thiol groups of MHDA. The surface coverage of CdSe was greater on mixed monolayers, consisting of approximately 12% MHDA and 88% HDA, than on pure MHDA monolayers. A mechanism is proposed wherein intralayer disulfide formation between MHDA thiol groups causes decreased reactivity toward CdSe nanoparticles. Disulfide formation is less significant at low fractional surface coverages of MHDA. The mechanism is supported by an increase of CdSe adsorption upon chemical reduction of surface disulfides to thiols. Our findings highlight the effect of intermolecular interactions on the affinity of nanoparticles for monolayer-functionalized surfaces.
Introduction The deposition of uniform monolayers of nanoparticles onto substrate surfaces represents an important challenge in nanofabrication. Nanoparticle-functionalized substrates have high surface areas and exhibit tunable properties and reactivity. They have been targeted for applications in sensing and detection,1,2 catalysis,3,4 patterning and information storage,5-10 and energy conversion.11-14 A useful strategy for preparing nanoparticlefunctionalized surfaces is to tether nanoparticles to substrates through bifunctional molecular linkers.10,15-21 Coordinate co(1) Gerion, D.; Parak, W. J.; Williams, S. C.; Zanchet, D.; Micheel, C. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7070-7074. (2) Bailey, R. C.; Nam, J.-M.; Mirkin, C. A.; Hupp, J. T. J. Am. Chem. Soc. 2003, 125, 13541-13547. (3) Jin, Y.; Shen, Y.; Dong, S. J. Phys. Chem. B 2004, 108, 8142-8147. (4) Radhakrishnan, C.; Lo, M. K. F.; Warrier, M. V.; Garcia-Garibay, M. A.; Monbouquette, H. G. Langmuir 2006, 22, 5018-5024. (5) Liu, J.-F.; Zhang, L.-G.; Gu, N.; Ren, J.-Y.; Wu, Y.-P.; Lu, Z.-H.; Mao, P.-S.; Chen, D.-Y. Thin Solid Films 1998, 327-329, 176-179. (6) Demers, L. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 30693071. (7) Ryan, D.; Nagle, L.; Fitzmaurice, D. Nano Lett. 2004, 4, 573-575. (8) del Campo, A.; Boos, D.; Spiess, H. W.; Jonas, U. Angew. Chem., Int. Ed. 2005, 44, 4707-4712. (9) Sun, S.; Chong, K. S.; Leggett, G. J. Nanotechnology 2005, 16, 17981808. (10) Dibbell, R. S.; Soja, G. R.; Hoth, R. M.; Watson, D. F. Langmuir 2007, 23, 3432-3439. (11) Plass, R.; Pelet, S.; Krueger, J.; Gra¨tzel, M. J. Phys. Chem. B 2002, 106, 7578-7580. (12) Blackburn, J. L.; Selmarten, D. C.; Nozik, A. J. J. Phys. Chem. B 2003, 107, 14154-14157. (13) Blackburn, J. L.; Selmarten, D. C.; Ellingson, R. J.; Jones, M.; Micic, O.; Nozik, A. J. J. Phys. Chem. B 2005, 109, 2625-2631. (14) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385-2393. (15) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221-5230. (16) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996-10000. (17) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137-143. (18) Nakanishi, T.; Ohtani, B.; Shimazu, K.; Uosaki, K. Chem. Phys. Lett. 1997, 278, 233-237. (19) Nakanishi, T.; Ohtani, B.; Uosaki, K. J. Phys. Chem. B 1998, 102, 15711577. (20) Wanunu, M.; Popovitz-Biro, R.; Cohen, H.; Vakevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2005, 127, 9207-9215. (21) Bae, S.-S.; Lim, D. K.; Park, J.-I.; Cheon, J.; Jeon, I. C.; Kim, S. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1305-1310.
valent bonding interactions dictate the affinity of functional groups for surfaces. For example, thiols and thiolates bind strongly to gold, silver, and metal chalcogenides;15,22-25 isocyanides bind to platinum;26,27 carboxylic acids and carboxylates bind to metal oxides;28-31 and silanes and siloxanes bind to silicon and silica.32,33 By exploiting the specificity of these surface attachment interactions, substrates can be programmed to bind nanoparticles of varying composition, simply by choosing the appropriate functional groups within the molecular linker. Thus, tethering nanoparticles to surfaces through bifunctional molecular linkers is a powerful synthetic technique. This approach should enable the synthesis of a wide range of nanostructured materials with tunable composition, physical properties, and chemical reactivity. We and others have shown that CdSe nanoparticles adsorb to mercaptoalkanoic acid-functionalized TiO2 surfaces.10,14 Mercaptoalkanoic acids bind to TiO2 through the carboxylic acid group, yielding a thiolated surface for CdSe adsorption. In this manuscript, we report that the formation of disulfide bonds between surface-adsorbed 16-mercaptohexadecanoic acid (MHDA) molecules causes decreased reactivity toward CdSe nanoparticles. Higher CdSe surface coverages were measured on mixed monolayers of MHDA and hexadecanoic acid (HDA) than on (22) Porter, M. D.; Bright, T. D.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (23) 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. (24) Natan, M. J.; Thackeray, J. W.; Wrighton, M. S. J. Phys. Chem. 1986, 90, 4089-4098. (25) Swayambunathan, V.; Hayes, D.; Schmidt, K. H.; Liao, Y. X.; Meisel, D. J. Am. Chem. Soc. 1990, 112, 3831-3837. (26) Hickman, J. J.; Zou, C.; Ofer, D.; Harvey, P. D.; Wrighton, M. S. J. Am. Chem. Soc. 1989, 111, 7271-7272. (27) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357-359. (28) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (29) Meyer, T. J.; Meyer, G. J.; Pfennig, B. W.; Schoonover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Chen, X.; Peek, B. M.; Wall, C. C.; Ou, W.; Erickson, B. W.; Bignozzi, C. Inorg. Chem. 1994, 33, 3952-3964. (30) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451-455. (31) Ekstro¨m, N. G.; McQuillan, A. J. J. Phys. Chem. B 1999, 103, 1056210565. (32) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034-1044. (33) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087.
10.1021/la702127t CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
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Langmuir, Vol. 23, No. 22, 2007 10925
pure MHDA monolayers. The enhanced adsorption to mixed monolayers is attributed to decreased disulfide formation, caused by the dilution of thiol groups within the monolayer. These results have important implications for the preparation of quantum dot functionalized semiconductors for energy conversion and photocatalysis applications. More generally, our findings illustrate that intermolecular interactions within surfactant monolayers must be considered when developing strategies for tethering nanoparticles to surfaces through bifunctional molecular linkers. Experimental Section Materials and Instrumentation. Dimethyl cadmium (Me2Cd) and tri-n-octylphosphine (TOP) were obtained from Strem Chemicals. Tri-n-octylphosphine oxide (TOPO), selenium shot, and titanium (IV) tetraisopropoxide were obtained from Alfa Aesar. MHDA, HDA, and poly(ethylene glycol) were obtained from Aldrich. Methanol, tetrahydrofuran (THF), and nitric acid were obtained from a variety of sources. Reagents were used without further purification. FTIR spectra were obtained using a Nicolet Magna-IR 550 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector, with a resolution of 2 cm-1 with Happ-Genzel apodization. UV/vis absorption spectra were obtained using an Agilent 8453 spectrophotometer with a Labsphere reflectance spectroscopy accessory. CdSe Synthesis. CdSe nanoparticles were synthesized by adaptation of the method developed by Murray et al.34,35 The synthesis was only modestly changed from our previously reported method.10 In short, dimethyl cadmium (1.0 mL, 15 mmol) was added to a solution of selenium (0.79 g, 10 mmol) in TOP (50 mL) in an inert atmosphere. This solution was then quickly added to a lowtemperature melt of TOPO (50 g) under argon. The TOPO had been heated under vacuum. The resulting mixture was slowly heated (approximately 160 °C) until the solution was a deep red color. Heat was removed and, before it solidified, the solution was added to methanol and stirred. Centrifugation was used to separate the methanol from the oily CdSe nanoparticles. The nanoparticles were washed a second time with methanol before being suspended in THF and stored. XRD and TEM measurements have shown that CdSe nanoparticles prepared by this route have wurtzite structure, with particle diameters of 5.2 ( 0.3 nm.10 TiO2 Synthesis. The preparation of nanocrystalline TiO2 films was described previously.10 In short, titanium (IV) tetraisopropoxide (50 mL) was slowly added to rapidly stirring dilute nitric acid (300 mL, 0.5% v/v). The mixture was then boiled until the total volume was reduced to 90 mL. This solution was then heated at 200 °C in a sealed vessel for 15 h, after which poly(ethylene glycol) (5.4 g) was added, and the solution was stirred for an additional 8 h. The films were prepared by spreading a thin layer of this mixture over glass slides and then annealing (430 °C, 30 min). Cross-sectional SEM images revealed that the films are 4.1 ( 0.9 µm thick. XRD and SEM measurements have shown that the films consist of anatase TiO2 with average particle diameters of 20-30 nm.10 Equilibrium Binding Experiments. TiO2 films were immersed in freshly prepared THF solutions of HDA or MHDA for a period of 24 h, after which they were rinsed with THF and allowed to dry before IR spectra were collected. Adsorption of CdSe to Mixed Monolayer Functionalized Surfaces. THF solutions of MHDA (1.7 mM) and HDA (1.7 mM) were mixed to yield solutions with various MHDA mole fractions. TiO2 films were immersed in freshly prepared HDA-MHDA solutions for 24 h, after which they were rinsed with THF and allowed to dry. The films were then immersed in THF suspensions of CdSe nanoparticles for 24 h. Upon removal, the films were rinsed again with THF and allowed to dry before UV/vis spectra were collected. TOP (3% v/v) and water (3% v/v) were added to the CdSe suspension for the disulfide reduction study. (34) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (35) Bowen, Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109-4117.
Figure 1. IR spectra of TiO2 films functionalized with HDA (top, conc. of undiluted stock solution ) 1.7 mM) and MHDA (bottom, conc. of undiluted stock solution ) 1.5 mM) as a function of solution concentration. The absorbances of HDA films were increased by 0.09 absorbance units. Table 1. IR Frequencies and Assignments for HDA and MHDA on TiO2 Films freq (cm-1)
assignment
refs
HDA
2959 2927 2878 2855
CH3 asym. stretch (νa(CH3)) CH2 asym. stretch (νa(CH2)) CH3 sym. stretch (νs(CH3)) CH2 sym. stretch (νs(CH2))
28, 36, 37 28, 36, 37 28, 36, 37 28, 36, 37
MHDA
2927 2857
CH2 asym. stretch (νa(CH2)) CH2 sym. stretch (νs(CH2))
28, 36, 37 28, 36, 37
adsorbate
Results and Discussion Adsorption of Hexadecanoic Acid and 16-Mercaptohexadecanoic Acid to TiO2 Surfaces. IR spectroscopy was used to characterize the adsorption of HDA and MHDA to nanocrystalline TiO2 films. Our goal was to determine the relative binding affinities of HDA and MHDA for the TiO2 surface, thus enabling the preparation of mixed monolayers with variable molar ratios. IR spectra in the C-H stretching region (2750-3050 cm-1) were obtained for TiO2 films which had been immersed for 24 h in THF solutions of HDA or MHDA (Figure 1). Each spectrum represents the average of two such films. The spectra were measured in transmission mode with the TiO2-coated glass slides held perpendicular to the IR beam. The TiO2-coated glass slides absorbed strongly below 2300 cm-1, but were transparent throughout the C-H stretching region. Peak frequencies and assignments are presented in Table 1. The presence of symmetric and asymmetric C-H stretching bands in the spectra of HDAand MHDA-exposed TiO2 films indicates that both acids adsorbed to the TiO2 surface. We have previously shown that MHDA adsorbs to TiO2 from THF solutions as the deprotonated carboxylate.10 We presume that HDA also adsorbs as a carboxylate. To quantify the extent of surface adsorption, the extinction coefficients and integrated absorption coefficients of C-H stretching bands were measured for HDA and MHDA. IR spectra were obtained for HDA and MHDA solutions in carbon tetrachloride. Concentration-dependent absorption data were fit to the Beer-Lambert equation. The extinction coefficients at the νa(CH2) band maxima of HDA and MHDA were 856 M-1 cm-1 and 895 M-1 cm-1, respectively, in close agreement with
10926 Langmuir, Vol. 23, No. 22, 2007
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previously reported values for linear alkanes.38 The integrated absorption coefficients from 2800 to 3000 cm-1 for HDA and MHDA were 4.47 × 104 M-1 cm-2 and 4.04 × 104 M-1 cm-2. Equilibrium binding experiments were performed to determine the influence of solution concentration on the adsorption of HDA and MHDA to TiO2 surfaces. Surface coverages were calculated from the C-H stretching absorbances of HDA- and MHDAfunctionalized TiO2 films, using a modified form of the BeerLambert equation39
Γ)
∫A(ν) A × (1000 cm3/L)
(1)
where Γ is the amount of adsorbate per projected surface area of the TiO2 film (in mol/cm2), ∫A(ν) is the integrated absorbance from 2800 to 3000 cm-1, and A is the integrated absorption coefficient (in M-1 cm-2). We assumed that the integrated absorption coefficients for surface-adsorbed acids were equal to the solution values. For both acids, the surface coverage increased with solution concentration (Figure 2a). Saturation surface coverages were obtained for HDA and MHDA concentrations greater than 1.5 mM. At low surface coverages of HDA and MHDA, THF also adsorbed to the TiO2 surface. IR spectra of TiO2 films which had been immersed in neat THF contained peaks at 2856, 2888, 2928, and 2961 cm-1, corresponding to the C-H stretching bands of surface-adsorbed THF.40-42 In addition, IR spectra of TiO2 films derivatized with low coverages of MHDA contained a weak THF band at 2961 cm-1. (The THF bands at 2856 cm-1, 2888 cm-1, and 2928 cm-1 were obscured by the MHDA C-H stretching bands.) The 2961 cm-1 absorbance decreased with increasing MHDA solution concentration, and the band disappeared altogether at MHDA concentrations greater than 0.05 mM. In a control experiment, TiO2 films were immersed in THF solutions of oxalic acid. Because oxalic acid has no C-H bonds, it was possible to directly monitor the disappearance of the THF bands. THF was fully displaced from the TiO2 surface at oxalic acid concentrations greater than 0.18 mM. Taken together, the MHDA and oxalic acid data reveal that carboxylic acids displace THF from the TiO2 surface at concentrations well below those required for saturation surface coverage of HDA and MHDA. Equilibrium binding data for HDA and MHDA were accurately modeled by the Langmuir adsorption isotherm (Figure 2b).43 The data deviate from linearity at the lowest solution concentrations, because the measured C-H stretching absorbances are artificially increased by the contribution from THF bands. Therefore, only the data for solution concentrations from 0.1 to 7 mM were fit to the Langmuir adsorption isotherm. The resulting saturation surface coverages (Γo) were (1.6 ( 0.1) × 10-7 mol/ cm2 for HDA and (1.2 ( 0.1) × 10-7 mol/cm2 for MHDA. These values are 1.2-3 times greater than previously reported Γo values for carboxylic acid functionalized porphyrins and transition metal polypyridyl complexes on nanocrystalline TiO2 films.39,44,45 The (36) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 39803992. (37) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334-341. (38) Hastings, S. H.; Watson, A. T.; Williams, R. B.; Anderson, J. A. J. Anal. Chem. 1952, 4, 612-618. (39) Trammell, S. A.; Meyer, T. J. J. Phys. Chem. B 1999, 103, 104-107. (40) Tschamler, H.; Voetter, H. Monatsh. Chem. 1952, 83, 302-321. (41) Cadioli, B.; Gallinella, E.; Coulombeau, C.; Jobic, H.; Berthier, G. J. Phys. Chem. 1993, 97, 7844-7856. (42) Shurvell, H. F.; Southby, M. C. Vibr. Spectr. 1997, 15, 137-146. (43) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361-1402. (44) Watson, D. F.; Marton, A.; Stux, A. M.; Meyer, G. J. J. Phys. Chem. B 2004, 108, 11680-11688.
Figure 2. (a) Coverage of HDA (9) and MHDA (0) on TiO2 films as a function of solution concentration. The vertical line at 1.7 mM corresponds to the total carboxylic acid concentration used to prepare mixed monolayers. Inset: low-concentration data with linear x-axis. (b) Fits to the Langmuir isotherm for HDA (9) and MHDA (0).
increased surface coverages for HDA and MHDA are consistent with their smaller footprints on the TiO2 surface. The measured surface adduct formation constants (Kad) for HDA and MHDA were (4 ( 2) × 103 M-1 and (6 ( 4) × 103 M-1, respectively. These values are 10-100 times lower than for carboxylic acid functionalized porphyrins and transition metal polypyridyl complexes adsorbed to nanocrystalline TiO2 films from ethanol, acetonitrile, and toluene.39,44,46 We attribute the lower Kad values for HDA and MHDA to the competitive adsorption of THF to the TiO2 surface. Preparation of Mixed Monolayers on TiO2. Mixed monolayers of HDA and MHDA were prepared by immersing nanocrystalline TiO2 films in THF solutions containing both HDA and MHDA. The sum of HDA and MHDA concentrations was held constant at 1.7 mM, which was sufficiently concentrated to yield saturation coverages of carboxylic acids on the TiO2 surface. The fractional surface coverage of thiols was controlled by varying the mole fraction of MHDA (χMHDA) in the mixed solutions. Importantly, the Kad values of HDA and MHDA were, within the precision of our measurements, equal. Therefore, χMHDA within mixed monolayers on the TiO2 surface was approximately equal to χMHDA in solution. (45) Hoertz, P. G.; Staniszewski, A.; Marton, A.; Higgins, G. T.; Incarvito, C. D.; Rheingold, A. L.; Meyer, G. J. J. Am. Chem. Soc. 2006, 128, 8234-8245. (46) Wang, D.; Mendelsohn, R.; Galoppini, E.; Hoertz, P. G.; Carlisle, R. A.; Meyer, G. J. J. Phys. Chem. B 2004, 108, 16642-16653.
Letters
Figure 3. Surface coverage of CdSe nanoparticles (proportional to 420 nm absorbance) on mixed HDA-MHDA monolayers, as a function of MHDA mole fraction (χMHDA). Inset: smoothed reflectance spectra for selected χMHDA values.
Adsorption of CdSe to Mixed Monolayer Functionalized TiO2 Films. CdSe nanoparticles were adsorbed from THF suspensions onto TiO2 films derivatized with mixed HDAMHDA monolayers. CdSe adsorption was initially quantified by transmission-mode UV/vis spectroscopy. The absorbance at 420 nm, within the CdSe band gap absorption, ranged from 0.029 to 0.056 and depended on χMHDA. These absorbances correspond to surface coverages on the order of 10-8 mol/cm2.10 We have found that the surface coverage of CdSe nanoparticles on nanocrystalline TiO2 films typically ranges from 10-8 to 10-7 mol/cm2. The coverage varies with the concentration of tri-noctylphosphine in the CdSe suspension, as discussed below. We anticipate that the CdSe surface coverage may also depend on particle size, which has been shown to influence the electrontransfer reactivity of coupled semiconductor nanoparticles.47 The calculated surface coverages are an upper limit, because light scattering contributed to the total measured absorbance. Diffuse reflectance spectra provided a more accurate method for determining relative CdSe surface coverages as a function of χMHDA. Scattering-induced variations of the baseline absorbance were not observed in diffuse reflectance spectra. However, the measured absorbances were lower in diffuse reflectance spectra than in transmission spectra, presumably due to lower effective path lengths in reflectance measurements. Therefore, diffuse reflectance spectra were used only to determine relative CdSe surface coverages. We investigated the dependence of CdSe surface coverage (ΓCdSe) on the fractional surface coverage of MHDA within the mixed monolayers (Figure 3). Each data point in Figure 3, and each spectrum in the inset, represents the average of 16 CdSefunctionalized TiO2 films. Somewhat surprisingly, CdSe nanoparticles adsorbed to pure HDA monolayers (χMHDA ) 0). We previously reported that CdSe nanoparticles do not adhere to unmodified TiO2 films or to TiO2 films derivatized with propanoic acid.10 We speculate that CdSe nanoparticles adsorb to HDAfunctionalized surfaces through van der Waals interactions between HDA and CdSe-adsorbed tri-n-octylphosphine oxide (TOPO). TOPO-modified CdSe presumably binds more strongly to HDA-functionalized surfaces than to propanoic acid functionalized surfaces due to the longer chain length of HDA. At (47) Robel, I.; Kuno, M.; Kamat, P. J. Am. Chem. Soc. 2007, 129, 4136-4137.
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low fractional surface coverages of MHDA, ΓCdSe increased with χMHDA, reaching a maximum at χMHDA values of 0.1-0.14. The increase of ΓCdSe with χMHDA corresponds to the increasing fractional surface coverage of thiol groups, or CdSe binding sites, within the mixed HDA-MHDA monolayer. For χMHDA values greater than 0.14, ΓCdSe decreased with increasing χMHDA. The surface coverage of CdSe nanoparticles on pure MHDA monolayers (χMHDA ) 1) was just 64% of the maximum value. We attribute the decreased adsorption of CdSe at high χMHDA values to the formation of disulfide bonds between surface-adsorbed MHDA molecules. Solvated thiols are readily oxidized to disulfides in the presence of weak oxidizing agents, including molecular oxygen.48-52 In addition, two disulfide formation mechanisms have been reported on thiolated surfaces: intralayer reactions between surface-adsorbed thiols,53-56 and interlayer reactions between surface-adsorbed and solvated thiols to yield bilayers.55,57-60 For our mixed monolayer functionalized TiO2 films, either mechanism would lead to decreased reactivity toward CdSe. The intralayer mechanism would convert surfaceadsorbed thiols to disulfides. Solvated disulfides have been shown to bind one-tenth to one-hundredth as strongly as thiols to gold surfaces.23 Similarly, we have shown that CdSe nanoparticles do not adhere to TiO2 surfaces on which MHDA monolayers have been photocatalytically oxidized to disulfides.10 Therefore, the observed decrease of CdSe adsorption with increasing χMHDA is consistent with intralayer disulfide formation. Alternatively, the interlayer mechanism would yield MHDA dimers with terminal carboxylic acid groups, which should bind less strongly to CdSe than thiols. Two factors suggest that intralayer disulfide formation is favored. First, our equilibrium binding experiments revealed that the saturation surface coverages of MHDA and HDA were similar. Interlayer disulfide formation would cause an increased saturation coverage for MHDA compared with HDA. Second, we have previously shown that MHDA adsorbs to the TiO2 surface exclusively as a carboxylate.10 The absence of protonated MHDA implies that bilayer formation is negligible and, therefore, that disulfide formation proceeds through the intralayer mechanism. Intralayer disulfide formation should become favored with increasing fractional surface coverage of MHDA, or decreasing average distance between thiols, in agreement with the observed decrease of ΓCdSe with increasing χMHDA. We were unable to directly characterize the formation of disulfides by IR spectroscopy. Neither S-H stretching bands nor S-S stretching bands, both of which are typically weak,61-64 were present in the IR spectra of MHDA-derivatized TiO2 films (48) Connor, R., Organic Sulfur Compounds. In Organic Chemistry: An AdVanced Treatise, 2nd ed.; Gilman, H., Ed.; John Wiley and Sons: New York, 1943; Vol. 1, pp 835-943. (49) Field, L.; Lawson, J. E. J. Am. Chem. Soc. 1958, 80, 838-841. (50) Wallace, T. J.; Schriesheim, A. J. Org. Chem. 1962, 27, 1514-1516. (51) Yiannios, C. N.; Karabinos, J. V. J. Org. Chem. 1963, 28, 3246-3248. (52) Aida, T.; Akasaka, T.; Furukawa, N.; Oae, S. Bull. Chem. Soc. Jpn. 1976, 49, 1441-1442. (53) Brust, M.; Blass, P. M.; Bard, A. J. Langmuir 1997, 13, 5602-5607. (54) Esplandiu´, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828838. (55) Brower, T. L.; Cook, M.; Ulman, A. J. Phys. Chem. B 2003, 107, 1172111725. (56) Carot, M. L.; Esplandiu, M. J.; Cometto, F. P.; Patrito, E. M.; Macagno, V. A. J. Electroanal. Chem. 2005, 579, 13-23. (57) Joo, S. W.; Han, S. W.; Kim, K. Langmuir 2000, 16, 5391-5396. (58) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 2000, 104, 6218-6224. (59) Joo, S. W.; Han, S. W.; Kim, K. J. Colloid Interface Sci. 2001, 240, 391-399. (60) Brower, T. L.; Garno, J. C.; Ulman, A.; Liu, G.-y.; Yan, C.; Go¨lzha¨user, A.; Grunze, M. Langmuir 2002, 18, 6207-6216. (61) Trotter, I. F.; Thompson, H. W. J. Chem. Soc. 1946, 481-488. (62) Sheppard, N. Trans. Faraday Soc. 1950, 46, 429-439. (63) Bastian, E. J. J.; Martin, R. B. J. Phys. Chem. 1973, 77, 1129-1133. (64) Sugeta, H.; Go, A.; Miyazawa, T. Bull. Chem. Soc. Jpn. 1973, 46, 34073411.
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observed increase of ΓCdSe upon reaction with TOP to the reduction of disulfides to thiols and the corresponding increase of the number of CdSe surface-attachment sites. Thus, the decreased adsorption of CdSe at high χMHDA values and the increase with exposure to TOP provide strong evidence that disulfide formation blocks CdSe nanoparticle attachment at high fractional coverages of MHDA.
Conclusions
Figure 4. Adsorption of CdSe to mixed HDA-MHDA monolayers on TiO2 films from THF suspensions of CdSe with (0) and without (9) 3% TOP.
(measured as described above or as KBr pellets prepared by removing the TiO2 films from the glass substrate). To confirm the disulfide formation mechanism, we measured the influence of tri-n-octylphosphine (TOP) on CdSe adsorption. Trialkylphosphines and triarylphosphines are known to reduce disulfides to thiols in solution.65-67 We reacted mixed monolayer functionalized TiO2 surfaces with CdSe suspensions containing 3% TOP and 3% water (Figure 4). Each data point in Figure 4 represents the average of 12 films. For monolayers with χMHDA ) 0 and 0.15, the additional TOP had little or no effect on CdSe adsorption. However, for monolayers with χMHDA greater than 0.15, the TOP caused a 10-20% increase of ΓCdSe. Both results support the proposed mechanism. For the pure HDA monolayers (χMHDA ) 0), the negligible influence of TOP is consistent with the absence of thiols or disulfides on the TiO2 surface. For χMHDA ) 0.15, the lack of a TOP-induced effect implies that disulfide formation is minimal within the mixed monolayers, due to the low fractional surface coverage of MHDA and consistent with the high coverage of CdSe. For high χMHDA values, however, the average distance between bifunctional linkers is shorter, and disulfide formation may become significant. We attribute the (65) Overman, L. E.; O’Connor, E. M. J. Am. Chem. Soc. 1976, 98, 771-775. (66) Ru¨egg, U. T.; Rudinger, J. Methods Enzymol. 1977, 47, 111-116. (67) Burns, J. A.; Butler, J. C.; Moran, J.; Whitesides, G. M. J. Org. Chem. 1991, 56, 2648-2650.
We measured greater adsorption of CdSe nanoparticles to mixed HDA-MHDA monolayers than to pure MHDA monolayers on TiO2 films. The highest CdSe surface coverages were observed on monolayers consisting of 10-14% MHDA (9086% HDA). We proposed a mechanism wherein intralayer disulfide formation between MHDA thiol groups causes decreased affinity for CdSe nanoparticles. Disulfide formation is increasingly favored (and CdSe adsorption disfavored) at high fractional surface coverages of MHDA. The proposed mechanism is supported by the increase of CdSe adsorption upon reduction of surface disulfides by TOP. Thus, the adsorption of CdSe nanoparticles to thiolated surfaces can be maximized by either of two methods: (1) diluting thiol groups within mixed surfactant monolayers or (2) reducing surface-adsorbed disulfides to thiols prior to nanoparticle attachment. The latter method can be highly dependent on the concentration of residual TOP in CdSe suspensions prepared by TOP/TOPO syntheses with organometallic cadmium precursors. Our findings have significant implications for the preparation of quantum dot functionalized semiconductor surfaces, which may have applications in energy conversion and photocatalysis. Optimizing the quantum dot surface coverage is necessary to maximize the light harvesting and energy conversion efficiencies. In addition, our findings are relevant to the deposition of a variety of nanoparticles onto monolayer-functionalized surfaces. Deleterious intermolecular interactions within monolayers must be minimized to efficiently adsorb nanoparticles to substrate surfaces. Acknowledgment. This work was funded, in part, by the National Science Foundation (CHE-0645678) and the State University of New York. In addition, acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. We thank Prof. Joseph Gardella and co-workers for the use of their FTIR instrumentation, and Gregory R. Soja and Jeremy S. Nevins for their assistance in obtaining SEM images. LA702127T
