Surface Chemistry of CdTe Quantum Dots Synthesized in Mixtures of

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Surface Chemistry of CdTe Quantum Dots Synthesized in Mixtures of Phosphonic Acids and Amines: Formation of a Mixed Ligand Shell Antti Hassinen,†,‡ Raquel Gomes,†,‡,# Kim De Nolf,†,‡ Qiang Zhao,§ André Vantomme,§ José C. Martins,∥ and Zeger Hens*,⊥,‡ †

Physics and Chemistry of Nanostructures, Ghent University, Ghent, B-9000 Belgium Center for Nano and Biophotonics, Ghent University, Ghent, Ghent, B-9000 Belgium § Instituut voor Kern- en Stralingsfysica, KU Leuven, Ghent, B-9000 Belgium ∥ NMR and Structure Analysis, Ghent University, Ghent, B-9000 Belgium ⊥ Physics and Chemistry of Nanostructures, Ghent University, Krijgslaan 281-S3, B-9000 Belgium ‡

S Supporting Information *

ABSTRACT: Solution nuclear magnetic resonance (NMR) spectroscopy is used to analyze the surface chemistry of CdTe quantum dots (QDs) synthesized in the presence of tetradecylphosphonic acid and oleylamine. The resulting CdTe QDs have a mixed capping, composed of tightly bound phosphonate anhydride and amine ligands. The overall ligand density is relatively low, which concurs with a low Cd excess in the QDs. We find that upon addition of oleic acid, neither amine or phosphonate anhydride ligands are released from the QDs. On the other hand, the addition of phosphonic acids to CdTe QDs partially capped by oleic acid moieties through a high temperature ligand exchange process leads to the ready, oneto-one release of oleic acid. A full thermodynamic analysis of the phosphonate/carboxylate exchange is however not possible since the exchange appears to be modified by the forced ligand exchange used to replace phosphonate by carboxylate ligands.

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bonds to the CdSe surface.9,10,18 Moreover, the number of negatively charged oleate or phosphonate ligands balances the positive charge related to the cadmium excess in the QDs, a result that stresses the picture of colloidal nanocrystals as inorganic/organic hybrid materials.9,18 In addition, direct phosphonic acid/carboxylic acid exchange experiments indicated that the binding of phosphonates to the CdSe surface is considerably stronger than that of carboxylates.18 Synthesis procedures can also involve coordinating solvents or additives such as tri-octylphoshine oxide (TOPO) and amines, which can act as L-type ligands. Especially in the case of amines, it was shown that these are dynamic ligands, rapidly exchanging between a bound and a free state.19,20 As a result, these ligands are easily removed from the ligand shell during the purification of the reaction product.8 Next to CdSe, CdTe was one of the first materials synthesized using the hot injection approach.1,21 By now, various synthesis approaches have been described wherein the originally used combination of dimethyl cadmium and coordinating solvents has been replaced by cadmium carboxylate or phosphonate complexes and noncoordinating solvents.13,22−24 This has resulted in an excellent control over the size and shape25 and the incorporation of CdTe QDs as

INTRODUCTION Over the last 20 years, the hot injection method1 has become a widely used approach for the synthesis of colloidal nanocrystals. It offers high precision in terms of size and shape control of the synthesized nanocrystals and it is applicable to a wide range of materials, including semiconductors, metals, and metal oxides.2−4 Colloidal nanocrystals synthesized by this approach are inorganic/organic hybrid materials where the inorganic nanocrystal core is capped by a monolayer of organic ligands.5 Especially in the case of semiconductor nanocrystals or quantum dots (QDs), research has long been focused on the inorganic core and its size- and shape-dependent optoelectronic properties, where especially cadmium and lead chalcogenides have been widely studied. Only over the last 5−10 years have detailed and systematic studies addressing the organic/inorganic interface of colloidal nanocrystals appeared. These involve the analysis of the ligands of as-synthesized nanocrystals,6−8 the understanding of the nanocrystal/ligand bond,9−11 and the in depth analysis of ligand exchange procedures.12 Taking the example of CdSe QDs, typical synthesis procedures make use of a reaction mixture containing cadmium carboxylate or cadmium phosphonate complexes with an excess of the free acid forms of these ligands.13−17 With both carboxylic and phosphonic acids, it has been found that, after purification, as-synthesized CdSe QDs have a ligand shell composed of carboxylates or phosphonates that bind via X-type © XXXX American Chemical Society

Received: January 25, 2013 Revised: June 13, 2013

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Figure 1. (A) 1H NMR spectrum of CdTe/TDPA (4.4 nm, [CdTe] = 340.5 μM) in THF-d8. Resonances are attributed to (†) residual THF, (‡) methanol, and (*) butylated hydroxytoluene and the protons of TDPA and OLA are labeled as indicated in the figure. Inset: enlargement of the same spectrum, visualizing the low intensity resonances. (B) Detailed view on the alkene resonance of the 1H NMR spectrum of CdTe/TDPA (resonance 2 in (A)).

octadecene (ODE). This mixture was first degassed for 1 h at 110 °C under nitrogen flow and heated to ∼300 °C to dissolve the CdO. Next, the temperature was decreased to 270 °C where 1 mL of the TOP−Te mixture was injected, and the QDs were allowed to grow at 250 °C for 45 min. The reaction was stopped by thermal quenching using a water bath, and the obtained particles were precipitated using tetrahydrofuran (THF) as the solvent and an isopropanol/methanol mixture as the nonsolvent under a inert atmosphere. The thus obtained precipitate was purified by four successive purification steps using THF and methanol as the solvent and the nonsolvent, respectively. Fixed reaction times of 45 min produced stock solutions of ∼100 μM of 4.4 nm CdTe QDs in 8 mL of THF. The QD size and concentration were determined from the absorbance spectrum, where the QD size is obtained from the position of the first exciton peak through a sizing curve, and the QD concentration is calculated from the absorbance at 410 nm.32 Solution 1H NMR Spectroscopy. NMR tubes (5 mm) were used for all NMR experiments. Typically, the QD concentration was kept >250 μM in NMR samples to obtain satisfactory 31P NMR spectra. The NMR samples were prepared by drying a suspension of CdTe QDs under a strong nitrogen flow, followed by redispersing the nanocrystals in THF-d8. NMR data were collected using a Bruker AVANCE II 500 spectrometer equipped with a 5 mm TXI-Z (1H, 13C, and 31 P) 3-channel probe (maximum Z-gradient strength of 0.504 T m−1). The temperature was set to 298.15 K. All 1D and 2D spectra were recorded using standard pulse sequences from the Bruker pulse program library, and all processing was performed using TopSpin 2.1. Quantitative 1H spectra were recorded with 16 scans and a 20 s delay between scans to allow full relaxation of all NMR signals. Digital ERETIC (Bruker BioSpin software), which is a practical implementation of the PULCON33 (pulselength-based concentration determination) method, was used for quantification. 1D 31P NMR spectra were recorded using 30° flip angle, with 12 288 scans and a 2 s interscan delay between the scans. The FID was sampled using 65 536 points for both 1H and 31P NMR measurements. 2D NOESY spectra were recorded using 512 t1 increments, consisting of 16 scans of 4096 sampled data points each. NOE mixing time for

cores in heteronanocrystals.26,27 Although less widespread than CdSe, CdTe QDs are nowadays used in for example LEDs,28 bioimaging,29 and photovoltaics.30 Nevertheless, detailed studies on the surface chemistry of CdTe nanocrystals are scarce. In the case of CdTe QDs synthesized in a coordinating solvent mixture of tri-octylphosphine (TOP) and dodecylamine (DDA), dynamic stabilization by amines similar to CdSe,19 has been demonstrated.31 On the other hand, no study has addressed the surface chemistry of CdTe nanocrystals synthesized using Cd-carboxylate or Cd-phosphonate complexes in noncoordinating solvents. In this paper, we use solution NMR to study the inorganic/ organic interface of CdTe QDs synthesized in the presence of tetradecylphosphonic acid (TDPA) and oleylamine (OLA). Using solution 1H and 31P NMR in combination with ligand stripping,10 we find that purified, as-synthesized nanocrystals are stabilized by a mixture of TDPA anhydride (TDPAanh) and, to a lesser extent, OLA. While free OLA tends to be present in the dispersion, solution NMR gives no indication of exchange between both OLA pools. Assuming that TDPA anhydride acts as an X-type ligand, its charge balances that of the Cd excess within the error of the measurement. Upon addition of oleic acid, no desorption of either OLA or TDPAanh is observed. On the other hand, addition of TDPA to CdTe QDs with a mixed TDPA/OA ligand shell leads to the ready release of oleic acid. Apart from the presence of bound OLA, the surface chemistry of these CdTe QDs appears very similar to that of CdSe QDs synthesized in the presence of octadecylphosphonic acid (ODPA).18

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EXPERIMENTAL METHODS CdTe Synthesis. Monodisperse colloidal CdTe QDs were synthesized by using the hot injection approach where one precursor solution is injected in a hot solution of the second precursor. The tellurium precursor was prepared by dissolving 0.18 g of elemental tellurium (Te) in 3 mL of trioctylphosphine (TOP) under an inert atmosphere at 100 °C for 45 min. The cadmium precursor was prepared in situ in the reaction flask prior to the hot injection by mixing 0.6 mmol of cadmium oxide (CdO), 1.9 mmol of tetradecylphosphonic acid (TDPA), and 0.8 mmol of oleylamine (OLA) with 10 mL of B

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nanocrystal samples was set to 600 ms. A polynomial baseline correction was applied during processing. Diffusion measurements were performed using double stimulated echo for convection compensation and LED, using bipolar gradient pulses for diffusion. 2D DOSY spectra were recorded using 64 t1 increments, consisting of 32 scans of 32 768 sampled data points each. The diffusion parameters, consisting of the gradient pulse length δ = 1800 μs and the diffusion delay Δ = 0.15 s, were chosen such that a signal decay of roughly 90% was obtained at the highest gradient strength used. The gradient strength was varied from 2 to 95% of the maximum strength.

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RESULTS AND DISCUSSION Figure 1 shows the 1H NMR spectrum of 4.4 nm CdTe QDs in THF-d8 synthesized in octadecene using a cadmium TDPA complex, OLA and TOP-Te (abbreviated as CdTe/TDPA). The sharp resonances in the spectrum can be attributed to (†) residual THF, (‡) methanol, and (*) butylated hydroxytoluene (BHT, see the Supporting Information). BHT is added as an antioxidant to protonated THF, which was used to store the CdTe QDs. The two pronounced broad resonances at 1.37 and 0.96 ppm typically correspond to the main CH2 pool and the methyl resonance of the aliphatic chain of bound ligands. In addition, three broad resonances, albeit with lower intensity, appear at 5.4, 2.85, and 2.1 ppm. A resonance in the range 5−6 ppm is indicative of alkene protons, which suggests that these lower intensity broad resonances come from bound OLA species. This is supported by their integrated intensities, which have a 2:2.3:4.1 ratio [5.4 ppm (alkene), 2.85 ppm (α-CH2), and 2.1 ppm (protons next to the alkene protons)], close to the expected ratio of 2:2:4. An enlargement on the 5−6 ppm region (Figure 1B) shows that the alkene resonance is more complex. The sharp features at 5.37 ppm indicate that some free OLA is present in the sample while the broad feature at 5.46 ppm points toward a second bound species. On the basis of a combined 13C and 1H NMR study on a blank sample of OLA in THF-d8 (see the Supporting Information), we conclude that this additional species is the trans isomer of OLA. Apart from OLA, both TDPA and TOP can contribute to the broad CH2 and CH3 resonances at 1.37 and 0.96 ppm in the CdTe/TDPA NMR spectrum. Since both species contain phosphorus, a more detailed investigation of the organic/ inorganic interface is possible using 31P NMR. The 31P spectrum of a CdTe/TDPA suspension (Figure 2) shows a very broad asymmetric resonance in the range 15−35 ppm. After ligand stripping using bis(trimethylsilyl) selenide,10 two sharp resonances appear at 14.2 ppm and 13.6 ppm in the 31P NMR spectrum (inset Figure 2). These correspond to the racemic and meso forms of (O,O′)-bis(trimethylsilyl)tetradecylphosphonic acid anhydride (SiTDPAanh). In contrast to CdSe, there is no indication of silylated phosphonic acid (SiTDPA) which would give a single resonance at 14 ppm.10 In the synthesis of the CdTe QDs the temperature is raised to ∼300 °C which, based on the 31P NMR measurements, suffices to transform all TDPA molecules to TDPAanh.34 Using diffusion ordered spectroscopy (DOSY), we find that the various resonances in the CdTe/TDPA 1H NMR spectrum correspond to four different diffusion coefficients (see Figure 3). The largest diffusion coefficients ((2.4 ± 0.2) × 10−9 and (1.26 ± 0.10) × 10−9 m2 s−1) correspond to THF-d8 and BHT, respectively, while free OLA has a diffusion coefficient of (0.83 ± 0.08) × 10−9 m2 s−1, a typical value for such a molecule in a

Figure 2. 31P NMR spectrum of CdTe/TDPA (4.4 nm, [CdTe] = 340.5 μM) in 450 μL of THF-d8 before (blue trace) and after (red trace) of bis(trimethylsilyl) selenide. Inset: enlargement of the 31P NMR showing two sharp resonances corresponding to racemic and meso forms of SiTDPAanh.

Figure 3. DOSY of a suspension of CdTe/TDPA (4.4 nm, [CdTe] = 340.5 μM) where four different diffusion coefficients are indicated. In the DOSY spectrum overlapping resonances cause streaking of the peaks along the diffusion axis, especially since a low cutoff is required to simultaneously visualize all relevant species.

low viscosity solvent. The diffusion coefficient related to the broad OLA and TDPAanh resonances (Dobs) amounts to (1.07 ± 0.09) × 10−10 m2 s−1, which translates into a hydrodynamic diameter of 9.0 nm. This number is slightly larger than the 8.0− 8.5 nm expected for a 4.4 nm QD with a 1.8−2.1 nm thick ligand shell. Since the volume fraction Φ of CdTe QDs in the NMR samples amounts to ∼5.5% (assuming a 1.8 nm ligand shell), this difference could be due to nonideal diffusion, which results in a reduced diffusion coefficient according to:35 Dobs (1) 1 − 2Φ Using the previously mentioned volume fraction of 5.5%, we find that the observed diffusion coefficient corresponds to a diffusion coefficient D at infinite dilution of (1.20 ± 0.09) × 10−10 m2 s−1. This yields a hydrodynamic diameter of 8.0 nm, a value expected for a 4.4 nm QD with a 1.8 nm thick ligand shell. Overall, we conclude that both TDPA and OLA have a D=

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Information, some of the BHT resonances show weak negative NOE correlations close to the noise level to the pool of bound CH2 at 1.37 and 2.85 ppm (OLA α-CH2). Also the methanol resonance at 3.3 ppm shows a weak negative NOE correlation to the pool of bound CH2 at 1.37 ppm. The combined results of 1D 1H and DOSY on CdTe/TDPA dispersions and 31P on stripped ligands indicate that the ligand shell of CdTe/TDPA QDs is a mixture of tightly bound (i.e., slow or no exchange) TDPAanh and OLA. This contrasts with CdSe QDs, where a mixture of TDPAanh and TDPA ligands was found. Similar to CdSe QDs,10,18 bis(trimethylsilyl) selenium strips the ligands from the CdTe surface and results in the formation of the silylated SiTDPAanh. We therefore conclude that also in this case, TDPAanh ligands are bound as X-type ligands. The behavior of OLA is more surprising. For CdTe QDs synthesized from dimethyl cadmium in a TOP/ DDA mixture, it was found that DDA is a dynamic ligand, exhibiting fast exchange between a bound and a free state.31 Opposite from this, we find that OLA behaves as a tightly bound ligand with CdTe/TDPA. The origin of this remains unclear. In the case of CdSe QDs, amines have been described as L-type ligands that are easily lost upon sample purification.8 An indication that OLA behaves as an L-type ligand also with CdTe/TDPA is the presence of free OLA after a purification procedure that removes all free TDPA. This hints at a dynamic adsorption desorption behavior that was also found with InP/ TOPO36 but not with typical X-type ligands. Possibly, the TDPAanh offers adsorption sites for amines to bind via hydrogen bonding between the amine proton and the TDPAanh oxygens. By a quantitative analysis of the alkene resonance of OLA and the methyl resonance of OLA+TDPAanh, and counting each TDPAanh as two ligands, we find a ligand shell composition of 90% TDPAanh and 10% OLA. Using the same quantitative NMR analysis of the methyl resonance, we obtain a total ligand density of 1.67 ± 0.24 nm−2, where again, we have counted each TDPAanh ligand for two as every TDPAanh is composed of two methyl groups. This is a relatively low value compared to typical densities found for cadmium or lead chalcogenide QDs, which typically range between 3 and 4.6 nm−2.6,9,18,37 Based on Rutherford backscattering spectrometry (RBS, see the Supporting In-

diffusion coefficient expected for ligands tightly bound to the CdTe QDs. To finalize the analysis as to what molecules do or do not interact with the CdTe surface, we complement the 1D 1H and DOSY measurements by a nuclear Overhauser effect spectroscopy (NOESY) study. Opposite from free ligands, bound ligands show a rapid buildup of intense and negative NOE cross peaks between ligand resonances, which is related to their reduced tumbling rate in solution.31 Moreover, the rapid build up of these intense negative NOE cross peaks in the bound state easily outweighs the slow buildup of positive NOEs when the molecule is in the free state, such that ligands in fast exchange generally show negative NOEs as well.31 Figure 4

Figure 4. (A) NOESY spectrum of CdTe/TDPA (4.4 nm, [CdTe] = 340.5 μM) showing negative NOE correlations for bound OLA and TDPAanh. (B) Enlargement on the alkene resonance of OLA where bound resonances of cis and trans isomers show negative NOE correlations and free OLA does not.

represents the NOESY spectrum of a 4.4 nm CdTe/TDPA dispersion in THF-d8 (Figure 4). As expected, the broad resonances of bound TDPAanh and OLA show negative NOE correlations. On the other hand, no negative NOEs connecting the resonances of the free OLA species are observed, which implies that chemical exchange, if any, between a pool of free and bound OLA is slow. As shown in the Supporting

Figure 5. Sample: 4.4 nm CdTe/TDPA QD suspension ([CdTe] = 289.8 μM) in 450 μL of THF-d8. (A) 1H NMR enlargement of the alkene resonance of OLA. The broad resonance of bound OLA species persists after additions of OA. (B) DOSY spectrum after addition of OA in 1:1 ratio relative to OLA+TDPAanh in the sample. (C) NOESY under the same conditions as DOSY. Added OA has positive NOE cross-peaks (blue traces) typical to free ligands whereas the bound OLA has negative NOE cross-peaks (red traces) indicating the interaction with CdTe QDs. D

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Figure 6. (A) 1H NMR spectrum of CdTe/TDPA after forced OA exchange (4.35 nm, [CdTe] = 253 μM) in 450 μL of THF-d8. (B) 31P spectra comparing samples with original ligands and after forced ligand exchange. Addition of free TDPA ligands to the CdTe/TDPA after the forced OA exchange raises the intensity of 30 ppm resonance and leaves resonance at 20 ppm unchanged. On the other hand the addition of bis(trimethylsilyl) selenide to CdTe/TDPA after forced ligand exchange (4.35 nm, [CdTe] = 221.9 μM) shows a single resonance in the 31P spectrum corresponding to the SiTDPA (inset). (C) Enlargement of the broad alkene resonance of OA. The addition of free TDPA ligands lowers the intensity of the broad resonance and a sharp resonance appears indicating release of OA. (D) Bound OA mole fraction as a function of the equivalents of added TDPA. The experimental points are fitted to an exchange model (full line) which yields an equilibrium constant for the exchange reaction.

solution (D = (1.01 ± 0.08) × 10−9 m2s−1) and broad OLA and TDPAanh resonances diffuse slowly with the same diffusion coefficient value as originally observed. Similarly, the lack of negative NOE cross-peaks for OA in the alkene region of the spectrum (Figure 5C) demonstrates that the added OA molecules do not interact with the CdTe/TDPA QDs. Also no change was observed in the 31P NMR measurements (not shown here). We thus conclude that the surface of the as synthesized and purified CdTe QDs does not leave free adsorption sites for carboxylic acid molecules to bind to. To force a ligand exchange to oleic acid, a CdTe/TDPA sample was precipitated using methanol as the nonsolvent followed by the dissolution of the QDs in pure OA and the heating of the mixture at 100 °C for 17 h.18 The resulting product was purified with a procedure similar to the one used for the purification of the original CdTe/TDPA synthesis products. Figure 6A shows that the 1H NMR spectrum in THFd8 after the exchange procedure is similar to the original one, yet the intensities of the broad resonances at 5.4 and 2.1 ppm have increased considerably. Also the complex structure of the alkene resonance typical of bound OLA (i.e., cis and trans) has disappeared. As shown in the Supporting Information, DOSY and NOESY give no indication of free ligands. A quantitative comparison of the alkene resonance (5.4 ppm) to the methyl resonance (0.96 ppm) reveals that the fraction of bound ligands with an alkene group has increased to ∼40%. As QDs were exposed to OA only, we conclude that we can partially exchange bound TDPAanh for OA. Moreover, upon ligand stripping with bis(trimethylsilyl) selenium, only a single sharp resonance at 13.9 ppm (Figure 6B inset) is obtained, which corresponds to the silylated phosphonic acid (SiTDPA). Thus, we conclude that the forced exchange results in the addition of oleic acid moieties to the ligand shell, the hydrolization of all TDPAanh ligands to TDPA ligands and the removal of bound

formation), we find that the 4.4 nm CdTe QDs used here have a Cd:Te ratio of 1.08 ± 0.06. Combined with the quantitative information on the ligand density, this results in a Cd2+:ligand1‑ ratio of 0.57 ± 0.4, when we only take the fraction of TDPAanh into account. Hence, within the error of the measurement, the positive charge on the excess Cd2+ is balanced by the negative charge on the TDPAanh ligands, indicating that the lower than expected ligand density correlates with a relatively small Cd2+ excess. To obtain further information on the binding of TDPAanh and OLA to CdTe/TDPA, we progressively added oleic acid (OA) to a CdTe dispersion. Both with CdSe18 and PbS,6 carboxylic acids were found to bind as X-type carboxylates that form, in the case of CdSe, a weaker ligand-nanocrystal bond than phosphonates or phosphonate anhydrides.18 We first added OA in a 1:1 ratio to the bound OLA species (Figure 5A) to analyze its interference with the amine ligands. Focusing on the broad signals underlying the additional sharp resonances, we find that the intensity ratio between the resonances of cis and trans OLA does not change upon addition of OA. Binding of OA would increase the intensity of the 5.4 ppm resonance as the resonances of the cis isomer of bound OLA and bound OA overlap. On the other hand, if OLA would be released, the trans isomer of bound OLA would lose intensity. We thus conclude that the addition of OA in this case has no effect on the ligands shell. In a second stop, OA was added in a 1:1 ratio relative to OLA+TDPAanh (Figure 5A) and the additions were continued until a 5:1 OA:(OLA+TDPAanh) ratio. Even then, the 1H NMR measurements do not indicate a loss of the original ligands and their replacement by OA. This was confirmed by DOSY and NOESY measurements. The DOSY spectrum of OA added in a 1:1 ratio relative to OLA+TDPAanh (Figure 5B) shows that the added carboxylic acid remains free in the E

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OLA. On average, we find a ligand density after exchange of 1.26 nm−2, i.e., about 25% lower than with purified CdTe/ TDPA. Figure 6C shows that upon addition of free TDPA to a sample of OA exchanged CdTe/TDPA, a sharp multiplet appears next to the alkene resonance of bound OA, which becomes more intense the more TDPA is added. The chemical shifts and resonance multiplicity of the sharp resonances corresponds to free OA as determined for a blank sample of OA in THF-d8. The alkene resonance can be decomposed in a linear combination of the bound OA (Figure 6C, dark trace) and the free OA (Figure 6C, lightest trace) resonances, such that we can plot the fraction of bound OA as a function of the amount of TDPA added (Figure 6D). For the initial additions of TDPA, Figure 6D shows that each TDPA molecule added releases a single bound OA from the CdTe surface. This means that the exchange reaction can be written as TDPA f + OA b ⇌ TDPA b + OA f

(2)

Here, the indices f and b refer to the free and bound form of the TDPA and OA species, respectively. Under the assumption that OAb corresponds to an oleate ligand,6,9 the exchange reaction indicates that the TDPA added binds as a hydrogen phosphonate (TDPA−), similar to what was found in the case of CdSe.18 A notable feature in Figure 6B is the presence of two resonances at ∼20 and ∼30 ppm in the 31P NMR spectrum (Figure 6B) of the CdTe QDs after the forced OA exchange, while the 31P spectrum of purified CdTe/TDPA QDs only shows a resonance at 20 ppm, albeit with a tail extending to 30−35 ppm. A similar observation has been made in the case of CdSe QDs stabilized by TDPA.18 Importantly, the addition of free TDPA ligands raises the intensity of the 30 ppm resonance, until the appearance of a sharp resonance marks the presence of free TDPA ligands in solution. This indicates that the second resonance reflects an intrinsic property of the bound TDPA ligands as well and not a chemical shift due to the presence of neighboring carboxylate moieties. Similar to the case of CdSe, a tentative interpretation is that the two resonances reflect two different binding sites at the CdTe surface, where the resonance at ∼30 ppm reflects a binding site induced by the forced ligand exchange. In that case, the selective increase of this resonance during the TDPA titration would imply that only on these newly created sites, exchange between OA and TDPA is possible. Hence, the free energy difference for ligand exchange (eq 2) would be different for both sites. Taking a closer look at the titration curve, it follows that at the point where the amounts of TDPA added and OA originally present are equal, still about 20% of the OA remains bound. This is surprisingly high since the addition of free OA in a 5:1 excess relative to bound TDPA does not induce the release of TDPAb (see Figure 5A), suggesting that the equilibrium expressed by eq 2 is strongly shifted toward the side of bound TDPA. To analyze the OA/TDPA exchange in more detail, we performed rotating frame NOE spectroscopy (ROESY) on a CdTe sample with a mixed OA/TDPA ligand shell, after the addition of free TDPA with a TDPA:OA ratio of 0.4. An enlargement of the ROESY spectrum in the region of the alkene resonance as recorded in THF-d8 and toluene-d8 is represented in Figure 7A and B, respectively. A cross peak between the alkene protons of OAb and OAf with a sign equal to that of the diagonal peak is vaguely discernible in THF-d8 and clearly present in toluene-d8, as indicated by the arrows in

Figure 7. (A) Enlargement of the alkene region of the ROESY NMR spectrum of CdTe/TDPA after forced OA exchange and addition of free TDPA with a TDPA:OA ratio of 0.4 (4.16 nm, [CdTe] = 183 μM) in 450 μL of THF-d8. (B) Enlargement of the alkene region of the ROESY NMR spectrum of the same sample as in Figure A but now in toluene-d8. (C) Variation of the fraction of free OA when cycling the sample temperature from 25 to 55 °C and back for a sample with a TDPA:OA ratio of 1.0 (4.68 nm, [CdTe] = 331 μM in 450 μL of toluene-d8). (D) NOESY spectrum of CdTe sample (4.77 nm, [CdTe] = 227 μM in 450 μL of toluene-d8) gone through the forced ligand exchange, full TDPA titration sequence, purification and after re-exposing to free OA (OA:TDPA 0.2).

Figure 7A and B. A ROESY cross peak with the same sign as the diagonal points toward chemical exchange between the pool of free and bound OA.9 This shows that the ligand exchange is effectively dynamic, i.e., OAf competes with TDPAf for adsorption sites occupied by bound OA. On the other hand, Figure 7C shows that cycling the temperature of a similar CdTe sample with a mixed OA/TDPA ligand shell and exposed to an excess of TDPA in a TDPA:OA = 1.0 ratio from 25 to 55 °C and back leads to a small but progressive release of OAb. We thus conclude that, although OAb is involved in a dynamic exchange, a titration series as shown in Figure 6 may not reflect the fully equilibrated concentrations of bound and free species. Clearly, this could explain the difference between the forward titration on as-synthesized CdTe QDs and the backward titration on CdTe QDs after forced ligand exchange. On the other hand, when a CdTe sample put through the forced exchange and full TDPA titration sequence is purified and reexposed to free OA, the alkene protons of free OA show strong and negative NOE cross peaks (see Figure 7D) to other OA protons. This is a clear indication that they interact with the nanocrystals, opposite from OA added to as-synthesized CdTe QDs. We thus conclude that the forced ligand exchange indeed makes the replacement of TDPA by OA less unfavorable, a conclusion in line with the tentative interpretation of the 31P spectra.

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CONCLUSION We have used a combination of 1D and 2D NMR measurements to study the surface chemistry of CdTe QDs synthesized in the presence of phosphonic acids and amines. As-synthesized, purified CdTe QDs have a ligand shell that is a F

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mixture of tightly bound phosphonic acid anhydrides and amines, where amines account for ∼10% of the aliphatic chains stabilizing the nanocrystals. The observation of tightly bound amines is unique. Especially with CdSe and CdTe nanocrystals synthesized using coordinating solvents, amines were shown to be involved in a rapid adsorption/desorption equilibrium,19,31 which indicates that the presence of phosphonic acid anhydrides at the CdTe surface offers specific adsorption sites for amines. The combination of the ligand density with the Cd:Te ratio leads to a full picture of the CdTe surface chemistry where the relatively low density of phosphonate anhydride ligands matches the low excess of cationic species to yield overall neutral nanocrystals. Forcing a ligand exchange to oleic acid moieties, we demonstrate that it is possible to hydrolyze the phosphonate anhydride complex and partially exchange phosphonate for carboxylate ligands. However, we find that the forced exchange also affects to OA/TDPA exchange behavior, making the replacement of TDPA by OA less unfavorable, which makes the CdTe QDs with a mixed ligand shell thus formed unsuited for a thermodynamic analysis of the phosphonate/carboxylate exchange reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Additional details on the NMR (1H, DOSY, and NOESY) and PL spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel:+32(0)92644863. Fax: +32(0)92644983. Present Address #

Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Germany.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been funded by the FWO-Vlaanderen (G.0794.10 and G.0760.12), BelSPo (IAP 7.35, photonics@ be), EU-FP7 (ITN Herodot, Grant Agreement No. 214954), and the Hercules Foundation.

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