Attenuated Total Reflection Infrared Studies of Oleate and

Ligand exchange reactions at the surface of oleate- and trioctylphosphine oxide (TOPO)-capped CdS quantum dots have been studied with attenuated total...
0 downloads 0 Views 592KB Size
Download PDF
Langmuir 2008, 24, 3841-3849

3841

Attenuated Total Reflection Infrared Studies of Oleate and Trioctylphosphine Oxide Ligand Adsorption and Exchange Reactions on CdS Quantum Dot Films Aidan G. Young,† Najeh Al-Salim,‡ David P. Green,§ and A. James McQuillan*,† Departments of Chemistry, and Anatomy and Structural Biology, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand, and Industrial Research Ltd., P.O. Box 31-310, Lower Hutt, New Zealand ReceiVed NoVember 22, 2007. In Final Form: January 16, 2008 Ligand exchange reactions at the surface of oleate- and trioctylphosphine oxide (TOPO)-capped CdS quantum dots have been studied with attenuated total reflection infrared (ATR-IR) spectroscopy, using thin films deposited from organic solvent suspensions. The oleate and trioctylphosphine capping ligands were found to form highly ordered and densely packed monolayers on the CdS surface. Adsorbed oleate is coordinated to CdS in a chelating bidentate manner through the carboxylate functional group, while adsorbed trioctylphosphine oxide is coordinated though the PdO functional group and appears to have numerous adsorption environments on the CdS surface. Exposure of such films to aqueous solution was found to cause partial delamination of the films from the ATR prism interface which was reversible upon redrying. Ligand exchange reactions on the oleate- and trioctylphosphine-capped CdS films were studied in situ at room temperature by allowing the films to be exposed to dilute aqueous solutions of thiol-containing ligands. Oleate and trioctylphosphine oxide are both strongly adsorbed to the CdS surface, and ligand exchange with monothiol-containing ligands has been found to be highly dependent upon experimental conditions, in particular pH, where exchange is only observed at solution pH where the exchanging ligand is uncharged. This is attributed to the inability of a charged ligand to penetrate the hydrophobic polymethylene layer on the CdS surface.

Introduction Colloidal semiconductor nanocrystals which have sizedependent optical properties, often called quantum dots (QDs), have received much recent interest due to their potential in a variety of applications including biological imaging,1,2 lasers,3,4 and light emitting devices.5,6 The QD surface chemistry has a crucial role in these applications, and the ability to tune the QD surface chemistry and reactivity by exchange of surface capping ligands is important in the development of QD based luminescence applications. Contact angle measurements and atomic force and scanning tunneling microscopy (AFM and STM) are common techniques for surface analysis which provide macroscopic information, and they have been valuable in indicating surface morphology and measuring interfacial force fields. However, these methods generally need to be supplemented by spectroscopic techniques which can probe the molecular nature of interfacial structure and composition. Addressing the molecular nature of chemical reactions at solid-solution interfaces remains a challenging area of research, as there are few applicable * To whom correspondence should be addressed. Fax: +64 3 479 7906. Telephone: +64 3 479 7928. E-mail: [email protected]. † Department of Chemistry, University of Otago. ‡ Industrial Research Ltd. § Department of Anatomy and Structural Biology, University of Otago. (1) Kim, S.; Lim, Y. T.; Soltesz, E. G.; De Grand, A. M.; Lee, J.; Nakayama, A.; Parker, J. A.; Mihaljevic, T.; Laurence, R. G.; Dor, D. M.; Cohn, L. H.; Bawendi, M. G.; Frangioni, J. V. Nat. Biotechnol. 2004, 22, 93-97. (2) Lidke, D. S.; Nagy, P.; Heintzmann, R.; Arndt-Jovin, D. J.; Post, J. N.; Grecco, H. E.; Jares-Erijman, E. A.; Jovin, T. M. Nat. Biotechnol. 2004, 22, 198-203. (3) Artemyev, M. V.; Woggon, U.; Wannemacher, R.; Jaschinski, H.; Langbein, W. Nano Lett. 2001, 1, 309-314. (4) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendiz, M. G. Science 2000, 290, 314317. (5) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354357. (6) Coe, S.; Woo, W.-K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800803.

spectroscopic techniques. Surface-enhanced Raman spectroscopy (SERS),7 surface-enhanced infrared spectroscopy (SEIRS),8 and vibrational sum-frequency spectroscopy (VSFS)9 have all been used to address the solid-solution interface. However, SERS and SEIRS are principally applicable to metal substrates, and analysis of VSFS data is complex. Infrared (IR) spectroscopy has long been applied to surface chemical reactions and has greatly enhanced our knowledge of heterogeneous catalysis at the gas-solid interface. However, IR spectroscopy has only more recently been applied to particulate solid-solution interfaces.10-18 The use of deposited thin films of QDs on internal reflection prism materials facilitates the collection of IR spectra of species adsorbed on QDs.19 The in situ IR spectra from such particle films reveal the molecular nature of surface chemical interactions and reactions. While there has been much attention paid to the luminescence properties of QDs in solution, there have been far fewer in situ IR spectroscopic studies addressing the surface chemistry of QDs. The IR spectra of CdTe QDs that incorporated oleic acid in the synthetic procedure have shown that the adsorbed species was oleate as opposed to oleic acid.20 Kinetic studies of trioctylphosphine oxide (TOPO) exchange with pyridine on CdSe QDs have been carried (7) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163-166. (8) Osawa, M. Top. Appl. Phys. 2001, 81, 163-187. (9) Shen, Y. R. Nature 1989, 337, 519-525. (10) Lefevre, G. AdV. Colloid Interface Sci. 2004, 107, 109-123. (11) Gisler, A.; Burgi, T.; Baiker, A. J. Catal. 2004, 222, 461-469. (12) Chiem, L. T.; Huynh, L.; Ralston, J.; Beattie, D. A. J. Colloid Interface Sci. 2006, 297, 54-61. (13) Ferri, D.; Buergi, T.; Baiker, A. J. Phys. Chem. B 2001, 105, 3187-3195. (14) Li, H.; Tripp, C. P. Langmuir 2004, 20, 10526-10533. (15) Bailey, J. R.; McGuire, M. M. Langmuir 2007, 23, 10995-10999. (16) Hug, S. J. J. Colloid Interface Sci. 1997, 188, 415-422. (17) Axe, K.; Vejgarden, M.; Persson, P. J. Colloid Interface Sci. 2006, 294, 31-37. (18) Peak, D.; Luther, G. W.; Sparks, D. L. Geochim. Cosmochim. Acta 2003, 67, 2551-2560. (19) McQuillan, A. J. AdV. Mater. 2001, 13, 1034-1038. (20) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300-4308.

10.1021/la703655v CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008

3842 Langmuir, Vol. 24, No. 8, 2008

out21 using IR spectroscopy, but only a small spectral window from 1350-1450 cm-1 was reported. In the present work, films of deposited CdS QDs immersed in solution were used to simulate conditions for QD surfaces in bulk solution. Attenuated total reflection infrared (ATR-IR) spectroscopic techniques19 were used to probe the surface chemistry of films of deposited oleate- and TOPO-capped CdS QDs, focusing on the nature of the ligand adsorption on the CdS surface. Such IR spectroscopic studies of adsorption are not readily carried out on QDs in the bulk solution phase. Ligand exchange reactions of the deposited oleate- and TOPO-capped CdS QD films with the monothiol-containing ligands mercaptoacetic acid (MAA), mercaptopropionic acid (MPA), and mercaptoethanol (ME) were also undertaken. These reactions were performed at room temperature, predominantly in dilute aqueous solution, and have provided information about experimental conditions which affect the occurrence and kinetics of ligand exchange. Materials and Methods Materials. Cadmium acetate dihydrate (Merck, 99%), oleic acid (Sigma, ∼99%), sodium oleate (Sigma, 99%), 1-octadecene (ODE, Acrojs Organics, 90%), sulfur (BDH, 99%), oleyl alcohol (SigmaAldrich, 85%), trioctylphosphine oxide (Sigma, 90%), mercaptoacetic acid (Riedel-De Hae¨n, 90%), mercaptopropionic acid (Acrojs Organics, 99+%), and mercaptoethanol (Riedel-De Hae¨n, 99%) were all used as received. All water used in experiments was deionized (Millipore, Milli-Q RG, resistivity 18 MΩ cm). Isopropanol (IPA), methanol (MeOH), absolute ethanol (EtOH), acetone, and toluene were all AR grade and used as received. Preparation of Oleate-Capped CdS QDs. The precursor compound cadmium oleate was prepared according to a reported method.22 Oleate-capped CdS QDs were prepared similarly to a reported procedure by Yu and Peng.23 Specifically, a solution of sulfur (51.2 mg, 1.6 mmol) in ODE (15 mL) was prepared and then heated at 100 °C under vacuum for 1 h, after which the vacuum was replaced by argon gas flow and the temperature was slowly increased to 260 °C. A solution of cadmium oleate (0.8 mmol) in ODE (2 mL) was quickly added. After 10 min, the mixture was quickly cooled to room temperature and IPA/MeOH (1:1) was added. The lower yellow organic layer was separated and washed with IPA/MeOH (1:1) until a precipitate was obtained, which was centrifuged, washed with acetone, and dried under vacuum. The solid was dissolved in toluene and centrifuged to remove any nonreacted materials. The clear liquid was decanted, and the solvent was evaporated under vacuum (yield, 0.2 g of oily solid). The UV-vis absorption spectrum of a toluene solution showed the excitonic absorption peak at 410 nm (3.0 eV), indicating an average particle size of ∼3.3 nm.24,25 The X-ray diffraction pattern showed that the CdS had a cubic morphology with broad diffractions at 2θ ) 31.20° (111), 35.24° (200) shoulder, 51.70° (220), 62.07° (311), and 64.57° (331) shoulder. Preparation of TOPO-Capped CdS QDs. TOPO-capped CdS QDs were prepared from oleate-capped CdS QDs by a ligand exchange reaction. Specifically, oleate-capped CdS (0.2 g) was dissolved in a mixture of oleyl alcohol (2 g) and trioctylphosphine oxide (90% TOPO, 3 g), and the mixture was heated under argon at 215 °C for 1 h. Upon cooling to room temperature, the TOPOcapped CdS solid was precipitated by addition of EtOH. The solid was separated by centrifugation and then washed three times with MeOH before being dried (yield, 0.1 g). ATR-IR Spectroscopy. A DuraSamplIR triple reflection 3 mm diameter diamond-faced ZnSe prism (ASI SensIR Technologies) (21) Kim, B. S.; Avila, L.; Brus, L. E.; Herman, I. P. Appl. Phys. Lett. 2000, 76, 3715-3717. (22) Al-Salim, N.; Young, A. G.; Tilley, R. D.; McQuillan, A. J.; Xia, J. Chem. Mater. 2007, 19, 5185-5193. (23) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368-2371. (24) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221-5230. (25) Brus, L. E. J. Chem. Phys. 1983, 79, 5566-5571.

Young et al. was used to collect spectral data. The diamond surface of the triple reflection prism isolates the CdS and any potentially corrosive solutions (e.g., pH extremes) from the underlying ZnSe surface. Prior to preparation of the QD films for ATR-IR analysis, the prism surfaces were cleaned by polishing with 0.015 µm γ-alumina (BDH, polishing grade) on a wet polishing micro cloth (Buehler) and then rinsed with water. The oleate-capped CdS QD films were formed by depositing 2 µL of a 22.5 mg mL-1 suspension in toluene onto the triple reflection prism. The removal of solvent using a water pump vacuum (∼50 mbar) for ∼15 min produced a ∼0.25 cm2 film. The TOPO-capped CdS QD films were formed in a similar manner by depositing 1 µL of a 45 mg mL-1 suspension in toluene. The prism was interfaced via a rubber O-ring to a glass flow cell which was manufactured in the Department of Chemistry.26 The solutions were delivered to the flow cell using a Masterflex C/L peristaltic pump and Masterflex Tygon tubing at a constant flow rate of 1 mL min-1. The IR analysis of the CdS films under solution flow was conducted in the absence of laboratory lighting. A Digilab FTS 4000 infrared spectrometer equipped with a KBr beamsplitter, Peltier cooled DTGS detector, and WinIR Pro version 3.4 software was used to collect and analyze spectra. The optical bench was purged with dried air. ATR-IR spectra were obtained from 64 scans at 4 cm-1 resolution and were not corrected for dependence of absorbance on wavenumber. Spectra are shown as recorded at room temperature, and they were not further modified by baseline correction or subtraction. General Methods. Solution pH measurements were made with a Mettler-Toledo MP220 meter to a precision of (0.1. UV-vis spectra were recorded on a Varian Cary 500 Scan UV-vis NIR spectrophotometer. Scanning electron microscopy (SEM) imaging was performed with a JEOL 6700F field emission scanning electron microscope (JEOL, Japan). Samples were coated in an Emitech 575X highresolution sputter coater (EM Technologies Ltd., England) with 5 nm of chromium and viewed at a working distance of 3-8 mm with a 3-7 kV range of accelerating voltages. SEM experiments were conducted on films prepared from toluene suspensions, similarly to those used in the ATR-IR experiments, on glass microscope slides. To provide an estimate of the film thickness from cross-sectional images, the slides were scored on the reverse side using a diamond cutter and then fractured by hand through the film. Figure 1 shows cross-sectional images of typical oleate- and TOPO-capped CdS films, from which an average film thickness of ∼1.5-2.0 µm is observed for each film.

Results and Discussion Oleate-Capped CdS QD Films. Oleic acid is a commonly used coordinating ligand in the synthesis of QDs.22,23 Its relatively high boiling point of 286 °C (at 100 mmHg)27 allows it to be used in the high-temperature reactions from which QDs are generally synthesized. Spectrum (a) of Figure 2 is the ATR-IR spectrum of liquid oleic acid at room temperature. Spectral assignments are presented in Table 1. In the C-H stretch spectral region, the strong sharp peaks at 2922 and 2853 cm-1 and the weaker shoulders at 2954 and 2870 cm-1 are due to the antisymmetric and symmetric CH2 stretching and antisymmetric and symmetric CH3 stretching vibrations, respectively.28 The weak peak at 3005 cm-1 is due to the C-H bond adjacent to the alkene functional group.29 The O-H stretch of the carboxylic acid in dimerized form was also observed as a broad weak underlying peak (partially shown) in the 3300-2500 cm-1 range. In the lower wavenumber spectral region, the peak due to the (26) Dickie, S. A.; McQuillan, A. J. Langmuir 2004, 20, 11630-11636. (27) 6788 Oleic Acid. In Merck Index, 11th ed.; Budavari, S. Ed.; Merck & Co., Inc.: Whitehouse Station, NJ, 1989. (28) Lee, D. H.; Condrate, R. A., Sr. J. Mater. Sci. 1999, 34, 139-146. (29) Shukla, N.; Liu, C.; Jones, P. M.; Weller, D. J. Magn. Magn. Mater. 2003, 266, 178-184.

Oleate and TOPO Ligand Reactions on CdS QD Films

Figure 1. SEM images of cross sections through typical oleate (top) and TOPO-capped (bottom) CdS films deposited on glass slides from toluene suspensions.

CdO stretch of the acid dimer is observed at 1707 cm-1, and the weak shoulder at 1743 cm-1 indicates some carboxylic acid in monomer form is also present.28 The peak at 936 cm-1 is due to absorption by the out-of-plane O-H bending vibration of the dimerized acid. The peaks at 1413 and 1285 cm-1 have been assigned to the C-O-H in-plane bending and C-OH stretching vibrations, respectively.30 The CdC stretching vibration of the alkene is very weak at ∼1658 cm-1 and appears as a shoulder on the peak of the CdO stretching vibration. The spectral region from 1460 to 1410 cm-1 contains contributions from the CH2 scissoring and CH3 asymmetric bending vibrations, while the 1300-1200 cm-1 region contains weak peaks due to CH2 twisting and wagging vibrations. A solution of oleic acid was obtained in n-hexane, which provides sufficient solubility for an adequate spectrum to be acquired. Spectrum (b) of Figure 2 is that of a 1 × 10-1 mol L-1 oleic acid solution in n-hexane. The spectrum is similar to that of liquid oleic acid but shows a 6 cm-1 increase in the wavenumber of the CdO stretch peak of the acid dimer. This increase is attributed to an increase in the “oily” aliphatic nature of the environment about the carboxylic acid functional group. The C-H spectral region of this spectrum is distorted by the negative n-hexane peaks from use of a pure n-hexane background spectrum. The negative n-hexane peaks also distort the oleic acid absorption spectrum in the CH2 scissoring vibrational region at ∼1460 cm-1. IR spectral measurements of oleic acid in aqueous media were able to be carried out only at sufficiently high pH where the carboxylic acid functional group was deprotonated and solubility was increased. Oleic acid has a reported pKa of 4.78.31 Spectrum (c) of Figure 2 is that of a 1 × 10-1 mol L-1 aqueous solution of sodium oleate at pH ) 6.0. The C-H stretch spectral region shows similar spectral features as observed for liquid oleic acid, with the exception of the O-H stretching vibration of the carboxylic acid dimer in the 3300-2500 cm-1 range (partially shown). At lower wavenumbers, the spectrum is dominated by the antisymmetric and symmetric carboxylate stretching vibrations (30) Thistlethwaite, P. J.; Hook, M. S. Langmuir 2000, 16, 4993-4998. (31) Ziemann, P. J. Faraday Discuss. 2005, 130, 469-490.

Langmuir, Vol. 24, No. 8, 2008 3843

at 1543 and 1407 cm-1, respectively. The peak at 1466 cm-1 is due to the CH2 scissoring vibration. The baseline of this difference spectrum when compared to the background spectrum is distorted by the loss of some water absorption, which gives rise to the absorbance losses at 1627, below 1000, and upward of 3000 cm-1. Spectrum (d) of Figure 2 is that of a dry film of oleate-capped CdS QDs deposited from a toluene suspension. The C-H stretch spectral region shows similar features as those appearing in the spectrum of liquid oleic acid, with the exception of the absence of the O-H stretching vibration of the carboxylic acid dimer from 3300 to 2500 cm-1 (partially shown). The CH2 stretching peaks remain sharp as was observed for liquid oleic acid in spectrum (a) of Figure 2. This indicates ordered polymethylene chains and suggests that a densely packed monolayer of oleate ligands exists on the CdS surface. The peak wavenumbers of the antisymmetric and symmetric CH2 stretching vibrations have previously been used to infer details about the spatial arrangement of layers of polymethylene chains.34-36 Specifically, polymethylene chains packed in a crystal-like manner exhibit IR peak maxima at 2918 and 2850 cm-1, while in a more liquid-like environment these maxima are generally at a ∼6-8 cm-1 higher wavenumber.36 In spectrum (d) of Figure 2, the Vas(CH2) and Vs(CH2) peak wavenumbers are 2917 and 2850 cm-1, respectively, and these wavenumbers are indicative of polymethylene chains stacked in a densely packed crystal-like phase, rather than in a liquid-like environment. At lower wavenumbers, the peak due to the carboxylate antisymmetric stretch is observed at 1534 cm-1. This confirms that the carboxylic acid functional group has deprotonated during the QD synthesis. The wavenumber of this peak is lowered by 9 cm-1 when compared to the spectrum of sodium oleate, and this decrease is attributed to the interaction with CdS that arises upon coordination through the carboxylate functional group. The absorbance of the 1534 cm-1 peak relative to the various CH2 peaks has decreased significantly when compared with that of the corresponding peaks in the spectrum of aqueous sodium oleate. This absorbance decrease is attributed to the presence of some residual 1-octadecene from the CdS synthesis that is also residing within the packed polymethylene chains. The characteristic and strongest absorptions of liquid 1-octadecene are at 2924, 2854, 1642, 1467, 992, 909, and 721 cm-1.37 Close examination of spectrum (d) of Figure 2 reveals that the spectral features are essentially a linear combination of those of sodium oleate in spectrum (c) of Figure 2 and the characteristic peaks of 1-octadecene. The presence of 1-octadecene also accounts for the stronger than expected CH2 scissoring vibration at 1463 cm-1 in spectrum (d) of Figure 2. The similarity in chain length between 1-octadecene and oleate may facilitate the ordered packing of polymethylene chains of these two molecular species on the CdS surface. The peak due to the carboxylate symmetric stretch is partially obscured by the CH2 scissoring vibration at 1463 cm-1, and it appears as a shoulder on the lower wavenumber edge at ∼1416 cm-1. The difference in wavenumber between the antisym(32) Thistlethwaite, P. J.; Gee, M. L.; Wilson, D. Langmuir 1996, 12, 64876491. (33) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; John Wiley and Sons, Inc.: Singapore, 1998. (34) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150. (35) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34A, 395-406. (36) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D.; Siperko, L. M. Langmuir 1992, 8, 2707-2714. (37) AIST: Integrated Spectral Database System of Organic Compounds (data were obtained from the National Institute of Advanced Industrial Science and Technology (Japan)).

3844 Langmuir, Vol. 24, No. 8, 2008

Young et al.

Figure 2. ATR-IR spectra of (a) liquid oleic acid, (b) 1 × 10-1 mol L-1 oleic acid in n-hexane, (c) 1 × 10-1 mol L-1 sodium oleate in aqueous solution (pH ) 6.0), (d) a dry film of deposited oleate-capped CdS QDs, and (e) time-evolution processes during a pH ) 6.0 aqueous flow (at 1, 46, 101, and 146 min flow time) of a film of oleate-capped CdS QDs. The background spectra were from (a) and (d) bare diamond, (b) n-hexane on diamond, (c) water on diamond, and (e) water on oleate-capped CdS QD film (pH ) 6.0). Final spectrum of time-evolution spectra is bolded.

metric and symmetric carboxylate stretching peaks (∆) has been observed to correlate with the mode of carboxylate coordination,38,39 and it has been utilized in numerous studies upon varying adsorption substrates (e.g., hematite,40 goethite,40 rutile titania,30 FePt nanoparticles,29 silica glass,28 and other metal oxides41). Specifically, chelating bidentate structures exhibit ∆ values significantly less than ionic values, while values for bridging bidentate structures are larger than those of chelating bidentate and comparable to those of ionic structures.39 In spectrum (d) of Figure 2, ∆ is 118 cm-1 and is consistent with chelating bidentate carboxylate adsorption to the CdS surface. The weak peak due to the CdC stretching vibration of the alkene appears at 1641 cm-1 and is likely due to contributions from both 1-octadecene and oleate. Its presence indicates that the alkene (38) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227-250. (39) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B: Applications in Coordination, Organnometallic, and Bioinorganic Chemistry, 5th ed.; John Wiley & Sons, Inc.: New York, 1997. (40) Buckland, A. D.; Rochester, C. H.; Topham, S. A. J. Chem. Soc., Faraday Trans. 1 1980, 76, 302-313. (41) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89-126.

functional group remains intact upon oleic acid adsorption. A small peak at 1737 cm-1 is attributed to a small amount of adsorbed oleic acid in the monomeric form.28,32 This monomeric oleic acid is likely to be more loosely associated with the CdS surface through van der Waals interactions with the aliphatic chains of neighboring oleate and 1-octadecene molecules, rather than directly interacting with the CdS through the carboxylic acid headgroup. The absence of any peak at ∼1707 cm-1 means that no dimerization of adsorbed oleic acid exists, and this provides further evidence that the adsorbed ligands form only a monolayer on the CdS surface. Spectrum (e) of Figure 2 shows the time-evolution of difference spectra when the deposited film of oleate-capped CdS QDs was exposed to a constant flow of water (with the absence of any adsorbing species). These time-evolution spectra show an increase in the IR absorptions at 1643 and 10) at elevated temperature overnight. Xie et al.53 employed concentrated 1 mol L-1 MPA in chloroform/water solvent mixture at pH ∼ 5-7 over 2 h at room temperature. Kirchner et al.54 employed a 1 mol L-1 MPA/1 (50) Dawson, R. M. C.; Elliott, D. C.; Elliott, W. H.; Jones, K. M. Data for Biochemical Research, 2nd ed.; Clarendon Press: Oxford, 1969. (51) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016-2018. (52) Aldana, J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 88448850. (53) Xie, R.; Kolb, U.; Li, J.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2005, 127, 7480-7488. (54) Kirchner, C.; Liedl, T.; Kudera, S.; Pellegrino, T.; Javier, A. M.; Gaub, H. E.; Stoelzle, S.; Fertig, N.; Parak, W. J. Nano Lett. 2005, 5, 331-338.

Oleate and TOPO Ligand Reactions on CdS QD Films

mol L-1 ME mixture in DMF also over an extended time. These studies all report ligand exchange in significantly more “forcing” conditions, either higher temperature or ligand concentrations, than those in the current work. Furthermore, these studies all employ solvent mixtures with an organic component, and such mixtures may interrupt the van der Waals interactions between polymethylene chains of adsorbed ligands and allow greater ease of solution-phase ligand access to the QD surface. In the current work at pH ) 6.6, the solution-phase ligand exists predominantly in the carboxylate form, and the absence of adsorption at this pH is thought to be due to the inability of the charged species to efficiently penetrate the hydrophobic polymethylene layer. Spectrum (b) of Figure 5 shows the time-evolution of difference spectra when a deposited film of TOPO-capped CdS QDs was exposed to a constant flow of 1 × 10-3 mol L-1 MPA solution at pH ) 3.2. Similar spectral features to those of adsorbed MAA in spectrum (a) above are observed, with the Vas(COO-) and Vs(COO-) peaks of adsorbed MPA present at 1549 and ∼1397 cm-1, respectively. The close chemical similarity between MAA and MPA accounts for their similar ligand exchange characteristics. The slow exchange of these two ligands points to TOPO providing a more stable capping environment on the CdS surface. Ligand Exchange with Oleic Acid on TOPO-Capped CdS QDs. Oleate and TOPO are two of the most commonly used capping ligands in the synthesis of cadmium chalcogenide QDs, and manipulating QD surface chemistry by ligand exchange is important for the application of QDs in biological systems. At room temperature, MAA in dilute aqueous solution was able to displace some adsorbed TOPO but unable to displace any adsorbed oleate from the CdS surface. Thus, the exchange reaction of adsorbed TOPO at the CdS surface with oleate was investigated. An organic solvent was selected for this experiment, because neutral ligands have been found to better penetrate the polymethylene layer and oleic acid is not soluble in water at the required concentration. Figure 6 shows the ATR-IR spectra of a dry film of deposited TOPO-capped CdS QDs prior to and following exposure to a solution of 1 × 10-3 mol L-1 oleic acid in EtOH. The time-evolution difference spectra from this experiment were complicated by strong contributions from solution-phase EtOH which obscure the spectral regions of interest, and consequently these difference spectra are not presented. Comparison between the initial and final dry film spectra in Figure 6 shows an approximate 25% loss of adsorbed TOPO based on the absorbance of the PdO stretching vibration peak at 1039 cm-1. The broadening of the PdO stretch peak suggests that a greater variety of PdO environments are present on the CdS surface after adsorption of some oleic acid. This is

Langmuir, Vol. 24, No. 8, 2008 3849

expected as a result of adsorbed oleic acid influencing the binding sites of neighboring PdO functional groups. An increase in the absorbance of peaks due to COO- and COOH vibrations at 1543 and 1712 cm-1, respectively, shows that oleic acid was adsorbed to the CdS in both the protonated and deprotonated form. The spectral changes observed in Figure 6 demonstrate that ligand exchange of adsorbed oleate for TOPO can be carried out and can provide molecular information about the adsorbed ligands on the CdS surface. These changes occurred over a 6 h period, and this long time scale is indicative of the slow kinetics of ligand exchange when these reactions are conducted with dilute ligand solutions at room temperature.

Conclusions Our results have shown that with ATR-IR spectroscopy it is possible to study the exchange of capping ligands on QDs using deposited thin films. Both oleate and trioctylphosphine oxide form densely packed polymethylene layers capping the QDs with binding via carboxylate and PdO functional groups, respectively. Oleate- and trioctylphosphine-capped QD deposited films tend to partially delaminate from the diamond ATR prism surface when immersed in aqueous solutions, which is reversible on removal of water. The exchange of oleate and trioctylphosphine ligands on CdS QDs by the smaller monothiol-containing ligands, mercaptoacetic and mercaptopropionic acid, shows slow exchange kinetics, which reflects the more stable capping environments of the longer aliphatic chain capping molecules. Reported exchange reactions of these ligands on cadmium chalcogenide surfaces are usually carried out at high temperature and in highly concentrated ligand solutions to accelerate the kinetics and to increase the energetic favorability of these reactions. The current work has shown that such ligand exchange reactions can be studied at room temperature in dilute aqueous solution when the exchanging ligand is uncharged. Experimental conditions, including pH, temperature, exchanging ligand concentration, and solvent, are important factors in ligand exchange kinetics at QD surfaces. Acknowledgment. We thank Ms. Liz Girvan of the Otago Centre for Electron Microscopy, University of Otago, for SEM image collection. This research was funded by the NZ Foundation for Research Science and Technology, NERF Grant UOOX0403. Supporting Information Available: Figure showing the full spectrum (d) of Figure 4. This material is available free of charge via the Internet at http://pubs.acs.org. LA703655V