Reaction of Alkylamine Surfactants with Carbon Dioxide: Relevance to

Apr 17, 2009 - Sunghee Lee , Paul J. Sanstead , Joseph M. Wiener , Remon Bebawee and Aileen G. Hilario. Langmuir 2010 26 (12), 9556-9564...
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NANO LETTERS

Reaction of Alkylamine Surfactants with Carbon Dioxide: Relevance to Nanocrystal Synthesis

2009 Vol. 9, No. 5 2088-2093

Nataly Belman,† Jacob N. Israelachvili,‡ Youli Li,§ Cyrus R. Safinya,| Joel Bernstein,⊥ and Yuval Golan*,† Department of Chemistry, Department of Materials Engineering and Ilse Katz Institute of Nanotechnology, Ben-Gurion UniVersity of the NegeV, Beer-SheVa 84105, Israel, and Department of Chemical Engineering and Materials Department, Materials Research Laboratory, Materials, Physics and Molecular, Cellular, and DeVelopmental Biology Departments, UniVersity of California at Santa Barbara, Santa Barbara, California 93106 Received February 19, 2009; Revised Manuscript Received April 3, 2009

ABSTRACT Exposure of tetradecylamine, hexadecylamine, and octadecylamine to CO2 results in their transformation to alkylammonium alkylcarbamate (AAAC) pairs, which we find is a major source of irreproducibility in nanoparticle synthesis. Controlled exposure to CO2 allows for highly uniform, ultranarrow ZnS nanorods coated with tetradecylamine to be reproducibly obtained in a single step. The crystal structures of the alkylamines and their AAAC analogs were investigated by powder X-ray diffraction and their isostructural three-dimensional unit cells are reported.

Tetradecylamine (TDA, C14H29NH2), hexadecylamine (HDA, C16H33NH2), and octadecylamine (ODA, C18H37NH2) surfactants are widely used as capping agents for nanoparticle synthesis,1-23 where the spacing between the resulting alkylamine-coated nanoparticles can be varied by the surfactant chain length.17,24 Ordered arrays of alkylamine-coated ZnS nanorods and nanowires were prepared using singlestep benchtop methods by Pradhan et al.17 Nevertheless, the samples were reported to be heterogeneous and the same sample often contained mixtures of different morphologies, that is, rods and wires.17 In spite of the wide use of alkylamines in nanoparticles synthesis, no attention has been paid to date in the nanomaterials community to the effect of atmospheric conditions on the reactivity of alkylamines in air during nanoparticle synthesis,

space group P21ab. Axial lengths were a ) 5.60 Å, b ) 7.35 Å, and c ) 45.1 Å. There were four molecules per cell and each chain axis was inclined at 25.5° to the c-axis.25 Three notable points about Bradley’s paper were the following: (i) It is the only (conference) paper in the literature on the three-dimensional structure of ODA. (ii) No powder diffraction files exist for TDA, HDA, or ODA in the JCPDS database despite their wide use. (iii) No mention of reactivity with carbon dioxide (CO2) was reported in that paper. In another study focused on gel formation of alkylamines with CO2, alkylammonium-alkylcarbamate (AAAC) molecule pairs were shown to form by reaction of one CO2 molecule with two alkylamine molecules. The reaction is reversible by heating, according to26-29

Single crystal structure analysis performed by Bradley et al. on ODA showed a herringbone molecule arrangement,

2CH3(CH2)nNH2 + CO2 {\} NH3+(CH2)nCH3 +

* To whom correspondence should be addressed. Tel: +972-8-6461474. Fax: +972-8-6472944. E-mail: [email protected]. † Department of Materials Engineering and Ilse Katz Institute of Nanotechnology, Ben-Gurion University of the Negev. ‡ Department of Chemical Engineering, and Materials Department, University of California, Santa Barbara. § Materials Research Laboratory, University of California, Santa Barbara. | Materials, Physics, and Molecular, Cellular, and Developmental Biology Departments, University of California at Santa Barbara. ⊥ Department of Chemistry, Ben-Gurion University of the Negev. 10.1021/nl900534m CCC: $40.75 Published on Web 04/17/2009

 2009 American Chemical Society

∆

CH3(CH2)nNHCO2- (1)

Hence, TDA can react with CO2 under ambient conditions to give tetradecylammonium tetradecylcarbamate (TATC), HDA can react with CO2 under ambient conditions to give hexadecylammonium hexadecylcarbamate (HAHC), and ODA can react with CO2 under ambient conditions to give octadecylammonium octadecylcarbamate (OAOC). As mentioned above, the crystal structures of TDA, HDA, and the

Figure 1. (a) Schematic of TATC formation by reacting CO2 with two TDA molecules. The reaction is reversible by heating. The calculated molecular lengths are marked. (b) Optical micrographs showing CO2 bubbles (indicated by arrows) released from TATC powder immersed in silicone oil upon heating. (c) TGA curve of pure TDA (green) and TATC (purple) powders at a scan rate of 5 °C/min. TGA of TATC showed a transition shoulder near 80 °C due to loss of CO2 from the carbamate of 9.9%, indicated by the red arrow. FTIR absorption spectra of (d) TDA and (e) TATC powders.

three AAACs have not been published to date, and only their lamellar d-spacings were reported: dTDA ) 35.38 Å, dHDA ) 40.51 Å, dTATC ) 42.0 Å, dHAHC ) 47.6 Å, dOAOC ) 52.4 Å.26,27 Yet, it is evident that the nanomaterials community is not fully aware of these results and their far-reaching implications for nanoparticle synthesis. In the present work, the crystal structures of alkylamines and AAACs were studied by temperature-resolved powder X-ray diffraction (XRD), and their three-dimensional unit cells are reported. Since all samples showed an isostructural lamellar character with an orthorhombic unit cell, in this work we emphasize TDA and its TATC analog, while similar results were obtained for HDA, ODA, and their related AAACs. Thermal analyses were carried out to characterize the melting points of the surfactants, to identify phase transitions and to quantify CO2 content in the AAACs. ZnS nanocrystals were synthesized using alkylamine surfactant molecules as capping agents. Aging of the alkylamines in ambient air was found to have a strong influence on the nanocrystal morphology. Controlled exposure of alkylamines to the ambient and AAAC formation resulted in well-defined alkylamine/AAAC ratios and was found to have a remarkable Nano Lett., Vol. 9, No. 5, 2009

stabilizing effect on the resulting nanocrystal morphology while improving uniformity and reproducibility. This allowed, apparently for the first time, obtaining homogeneous samples containing uniquely ultranarrow TDA-coated nanorods in a single step synthesis, as well as developing systematic reproducibility in the synthesis of TDA-coated nanowires. Furthermore, the alkylamines mediate nanorod assembly into “supercrystalline” arrays, in which the interparticle separation can be controlled by varying the alkylamine chain length. TATC molecule pairs were formed by reacting CO2 with two TDA molecules, as illustrated in Figure 1a. The reaction is reversible by heating.26-29 The fully extended length of the TDA and tetradecylammonium molecules is approximately 2.0 + 1.256 × 13 + 3.1 ) 21.4 Å, while that of the tetradecylcarbamate molecule 2.0 + 1.256 × 13 + 5.2 ) 23.5 Å.24,30,31 Using an optical microscope equipped with a temperature controlled stage, we were able to monitor the release of CO2 bubbles as a function of temperature from TATC powder immersed in silicone oil (Figure 1b). The bubbles were initially observed at ∼60 °C and ceased at ∼100 °C upon sample melting (heating rate was 4 °C/min). 2089

Figure 2. (a) Temperature resolved powder XRD patterns of (1) pure TDA at room temperature, (2) TATC at room temperature, (3) metastable high temperature structure of TATC at 79 °C. Curves (4) and (5) depict diffractograms of TATC held for 4 min at 130 °C, then cooled to (4) 40 °C and (5) 27 °C. Note that curves (3), (4), and (5) were taken at shorter measurement times, i.e., the noisier curves do not necessarily indicate a less ordered nature of the samples. (b,c) DSC thermograms of heating and cooling cycle of (b) TDA and (c) TATC powders. Scanning rate: 5 °C/min. Inset in (c): second heating and cooling cycle. Arrows indicate the direction of temperature change. All three phases showed isostructural lamellar character with orthorhombic unit cells. On the basis of the XRD data, schematic representations of the unit cells of TDA, HDA, and ODA are shown for (d) pure alkylamines at room temperature, (e) AAACs at room temperature, and (f) AAAC high temperature structures.

Thermal gravimetric analysis (TGA) was carried out to quantify the CO2 content in TATC. Figure 1c shows the TGA curve of pure TDA (green) and TATC (purple) powders. The TGA behavior was analyzed using a Mettler TGA/SDTA 851E and TGA-50 instruments. Powder samples were examined in an N2 atmosphere at a heating rate of 5 °C/ min. TGA of TATC showed a transition near 80 °C due to loss of CO2 from the carbamate (measured weight loss ) 9.9%, theoretical for loss of one CO2 per 1 pair of TATC molecules ) 9.3%), indicated by an arrow. A sharp weight loss was observed above 150 °C due to evaporation of both types of molecules. Our TGA analysis for TATC is consistent with similar OAOC measurements of George et al.26,27 2090

The formation of carbamate was also evidenced by infrared (IR) spectroscopy performed with a Bruker IR Scope II. A Fourier transform infrared (FTIR) microscope was coupled with an Equinox 55 FTIR spectrometer using an aperture of 60 µm. FTIR spectra of pure TDA and TATC powders are shown in Figure 1d,e, respectively. The structural evolution of the TDA-TATC system was further studied using temperature-resolved powder XRD and differential scanning calorimetry (DSC). The powder XRD diffractogram of TDA at room temperature is shown in Figure 2a (curve 1). The presence of a large number of Bragg peaks in the diffractogram is indicative of the highly ordered surfactant assembly. The lamellar packing of the molecules Nano Lett., Vol. 9, No. 5, 2009

is evident from the set of intense high order peaks of the lamellar spacing,26,27 with over 7 orders of (00l) peaks observed in the diffractogram. All the peaks were indexed to an orthorhombic unit cell with lattice constants a ) 5.60 Å, b ) 7.35 Å, and c ) 36.03 Å (Figure 2d). Exposure of TDA to CO2 was carried out by placing the sample of pure TDA in a desiccator with dry ice until weight gain reached saturation. The fully converted TATC sample had different orthorhombic lattice constants: a ) 7.75 Å, b ) 9.66 Å, and c ) 42.06 Å (Figure 2e), as derived from the powder X-ray diffractogram shown in Figure 2a (curve 2). The error in the calculated lattice constants was (0.04 Å. DSC analyses were carried out in order to characterize the surfactant melting points and phase transitions. The thermograms of heating and cooling cycles of (fully converted) TATC powder are shown in Figure 2c (scan rate 5 °C/min). The powder sample was placed in an aluminum crucible constantly purged with N2 gas. Arrows indicate the direction of temperature change. On heating, an endothermic peak (A) was observed at 84 °C, indicating a structural transition. Correspondingly, a new, metastable high temperature structure was identified by XRD (Figure 2a, curve 3) at 79 °C and indexed as an orthorhombic unit cell with lattice constants a ) 6.46 Å, b ) 8.85 Å, and c ) 39.38 Å (Figure 2f). Peak (B) at 98 °C corresponds to melting of TATC. On cooling, two small exothermic peaks (labeled C and D in Figure 2c) were observed at 71.5 and 69 °C. The hightemperature phase was expected on cooling just before the 71.5 °C peak. However, it could not be detected by XRD since these two peaks are very close. It is of the interest that XRD measured after heating the sample to 130 °C (heating rate 10 °C/min), holding 4 min at this temperature, and then cooling to 40 °C at the natural cooling rate showed the same pattern obtained for TATC at room temperature (Figure 2a, curve 4) despite the fact that CO2 is not expected in the sample after melting. The small DSC peak at 69 °C indicated this crystallization. This is attributed to small clusters of molecules that retain their crystallographic arrangement as a result of insufficient or partial melting; upon subsequent cooling, these aggregates can act as pre-existing nucleation sites.34 Upon continued heating these clusters may be reduced or even eliminated. A strong exothermic peak (E) at 33 °C corresponded to a solid state phase transition to the pure TDA (CO2-free) phase, which was confirmed by XRD at 27 °C (Figure 2a, curve 5). In this diffractogram some TATC peaks can be still observed (this can be expected since the powder was exposed to air, and started to react again with the ambient CO2). In a separate control experiment, the sample was melted at 130 °C and held for 40 min to verify complete melting and to ensure the absence of TATC nuclei, then cooled at the natural cooling rate to 40 °C. The resulting XRD diffractogram was characteristic of an amorphous sample and the previous behavior totally disappeared (Supporting Information, Figure S1, curve 4). Upon further cooling to 27 °C, the powder sample crystallized and coexistence of the CO2-free TDA phase and TATC (as a result of reaction with ambient CO2) was observed (Supporting Information, Figure S1, curve 5). Nano Lett., Vol. 9, No. 5, 2009

The inset in Figure 2c shows the second heating and cooling cycle thermogram for TATC with melting (39 °C) and solidification (33 °C) points identical to pure TDA (Figure 2b), indicating a reversible phase transition due to CO2 evaporation. Note that, unlike in the XRD experiments, continuous purging with N2 in the DSC experiments ensured complete removal of traces of CO2 evaporated from the sample. DSC measurements of alkylamines and AAACs were published previously with wide deviations in melting and crystallization points, which were quoted without further comments (Supporting Information, Tables S1,2).26,28,33,35-39 Existence of the metastable high-temperature structure of AAACs and detailed description of all the DSC peaks are reported here for the first time. DSC thermograms of HDA, ODA and their related AAACs are shown in Supporting Information, Figure S2. Notably, the axial in-plane lengths of HDA and ODA were identical to those obtained for TDA, while the lamellar spacing scaled, as expected, with chain length: c ) 40.53 and 45.16 Å, respectively (Figure 2d). For ODA, the lattice constants are very close to the values previously reported by Bradley et al.25 Similarly, the HAHC and OAOC at room temperature had the same axial in-plane lengths as the TATC and the lamellar spacing scaled with chain length c ) 47.17 and 52.31 Å, respectively (Figure 2e). The lamellar dspacings are in agreement with the values previously reported by George et al.26,27 Lamellar spacing of the metastable hightemperature structure was c ) 44.23 and 47.75 Å, respectively (Figure 2f). The lattice constants were derived from XRD patterns provided in Supporting Information, Figure S3. Detailed structural characterization, including XRD peak indexing and their temperature dependent evolution for all three alkylamines and their AAACs will be published separately. The calculated lengths of two TDA molecules (42.8 Å) and of TATC (44.9 Å) in their extended conformations are longer (by 6.8 and 2.5 Å, respectively), than the lamellar spacings measured from XRD. Hence, it is likely that the molecules are tilted from the c-axis by ∼30 and 20°, respectively. The tilt angle calculated for TDA is close to that suggested by Bradley et al. for ODA (25.5°), and the tilt angle for TATC is close to that previously suggested by George et al. for OAOC (18°).25,27 The difference between the cell volume of AAAC and two cell volumes of alkylamine is ∼190 Å3 (calculated from the experimental results in Figure 2d-f and summarized in Supporting Information, Table S3), which is consistent with the calculated volume of 4 CO2 molecules ∼48 Å3 each (theoretical volume of each CO2 molecule is 46 Å3).30,31 Therefore, we can conclude that each alkylamine unit cell contains 4 alkylamine molecules (in agreement with Bradley et al.25) and each AAAC unit cell contains 8 molecules, 4 alkylammonium and 4 alkylcarbamate. The number of molecules in the unit cell of the alkylamines and AAACs was calculated using F ) Mz/(NAV) where F is the surfactant density in g/cm3, M is the molecular weight in g/mol, NA is Avogadro’s number, V is the unit cell volume in cm3, and z 2091

Figure 3. BF TEM micrographs of TDA-coated ZnS nanoparticles with various well-defined morphologies (a) curved nanowires and (b) straight nanowires, both synthesized from unexposed (CO2 free) TDA, at reaction temperatures lower and higher than 100 °C, respectively. (c) Nanorods synthesized at 105 °C in TDA that was exposed to air in a controllable manner (containing ∼4% CO2).

is the number of molecules in the unit cell. Using z ) 4 for alkylamines and for AAACs pairs, the calculated densities were 0.956, 0.961, and 0.963 g/cm3 for TDA, HDA, and ODA (in agreement with Bradley et al.25), respectively and 0.993, 0.991, and 0.989 g/cm3 for the related AAAC analogs. The handbook densities of alkylamines are 0.8079, 0.8129, and 0.8618 g/cm3 for TDA, HDA, and ODA, respectively,40 considerably smaller than the calculated values. In fact, the opposite trends observed for the dependence of the density on chain length for alkylamines and AAACs are expected. Increased mobility with chain length gives rise to an increase in density, just as observed for simple alkane chains.40 The same trend is observed for the handbook densities as a function of chain length (yet absolute values are substantially lower; unfortunately the CRC handbook provides no references on the origin of the data or details on how they were obtained).41 Interestingly, an opposite trend is observed for AAACs that can be explained as follows: Following CO2 exposure, AAAC pair formation results in an electrostatic interaction between the headgroups that rigidly holds the headgroups in place. With decreasing chain length, the contribution of the flexible and mobile component decreases, leading to increasing average density with decreasing chain length, as observed for the AAAC based on our XRD measurements described above. Alkylamine surfactants are widely used as capping agents for confining nanomaterials into ordered structures with desired sizes and shapes.1-23 Despite the wide use of alkylamines in the synthesis of nanoparticles, no attention has been paid to date on the reactivity of alkylamines with atmospheric CO2 and its effect on the synthesis products. Previously, the formation of ODA- and HDA-coated ZnS nanorods and of ODA-, HDA- and TDA-coated ZnS nanowires was reported by Pradhan et al.17 Nevertheless, the samples were heterogeneous and the same sample contained different morphologies of rods and wires, as explicitly mentioned in the caption of Figure 3 of Pradhans’ article.17 Furthermore, these authors could not produce TDA-coated nanorods by direct synthesis. This can now be understood in light of the faster kinetics of CO2 absorption with decreasing chain length of the alkylamine molecule.42 Controlled exposure of alkylamines to CO2 enables welldefined and reproducible formation of ZnS nanoparticles with uniform morphologies. AAAC formation was shown to have a remarkable stabilizing effect on nanorods obtained. This allowed us to obtain pure samples of TDA-coated ultra2092

narrow ZnS nanorods in a single step synthesis, as well as to develop a highly reproducible synthetic protocol for ultranarrow TDA-coated ZnS nanowires. The following example is shown to demonstrate the relevance of the reaction of TDA with atmospheric CO2 for nanoparticle synthesis. TDA-coated ZnS nanoparticles were prepared using a modified synthesis based on the method of Pradhan et al.17 For the synthesis, zinc-ethylxanthate was dissolved in molten TDA. The TDA-coated ZnS particles were harvested by flocculating the sample with methanol, centrifuging, and drying in ambient air. Bright field (BF) transmission electron microscope (TEM) micrographs of highly uniform TDA-coated ZnS nanoparticles arranged in large ordered domains are shown in Figure 3. TEM analyses were carried out using a Tecnai G2 TEM operating at 120 kV. The samples were prepared by placing a droplet of a solution of the nanoparticles in chloroform on a lacey carboncoated TEM grid. Using unexposed (CO2 free) TDA at a synthesis temperature lower than 100 °C, curved wires were formed (Figure 3a), while at synthesis temperatures higher than 100 °C straight wires were obtained (Figure 3b). The nanowire width was 1.0 ( 0.1 nm and length varied from 30 to 300 nm. The perpendicular distances between the nanowires were 3.0 ( 0.1 and 3.3 ( 0.1 nm, respectively. Finally, exposure of TDA to air in a controllable manner, so that TDA and TATC coexist in a well-defined ratio (TDA mass gain was 4% wt due to reaction with CO2 and TATC formation), increased the melting point of the surfactant (see Supporting Information, Table S1) to 105 °C and enabled formation of uniform TDA-capped ZnS nanorods. The morphology of nanorods formation can be explained by the presence of small clusters of TATC. Even after CO2 was released from the TATC powder, the molecular arrangement is different from that of the pure TDA. The nanorod width was 1.0 ( 0.2 nm and the length was 3.5 ( 0.5 nm. The rods assembled into highly ordered two-dimensional supercrystalline arrays organized in ribbonlike columns. Within each ribbon (width: 4.4 ( 0.2 nm) the ZnS nanorods were separated by a well-defined perpendicular distance of 3.0 ( 0.1 nm. More details on the synthesis are available in the Supporting Information. In summary, temperature-resolved powder XRD studies allowed us to determine the structures of pure alkylamines and two phases of their AAAC analogs at low and high temperature. Controlled exposure of alkylamines to CO2, as well as the reaction temperature, allowed reproducible Nano Lett., Vol. 9, No. 5, 2009

formation of specific morphologies of ZnS nanoparticles. Highly uniform TDA-coated ZnS nanoparticles with homogeneous morphologies of nanorods, straight and curved nanowires, arranged in large ordered domains were synthesized. Small, controlled amounts of AAAC in the synthesis can be desirable and allowed formation of nanorods, while using pure, unexposed alkylamines, curved (lower synthesis temperature) and straight (higher synthesis temperatures) nanowires were reproducibly formed. Acknowledgment. We acknowledge A. Rabkin and O. Sima for assistance in nanoparticle synthesis. The help of D. Mogilyanski (BGU) and I. Feldman (Weizmann Institute of Science) with XRD is highly appreciated. We thank A. Miliontshick for assistance with TGA and DSC measurements at BGU, K. Brzezinska (UCSB) for help with additional DSC analysis, and J. Doyle (UCSB) for help with TGA. This work was supported by the US-Israel Binational Science Foundation, Grant 2006032 (J.I. and Y.G.) and DOE Grant DE-FG02-06ER46314, NSF Grant DMR-0803103 (C.R.S.). This work made use of MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under Award No. DMR05-20415. Supporting Information Available: Detailed description of materials and methods. Powder XRD patterns and DSC thermograms of HDA, ODA, and their related AAACs. Melting and crystallization temperatures of the alkylamines and their AAACs derived from the DSC measurements, compared to the literature. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Acharya, S.; Patla, I.; Kost, J.; Efrima, S.; Golan, Y. J. Am. Chem. Soc. 2006, 128, 9294–9295. (2) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682–2692. (3) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262–1269. (4) Brink, M. V.; Peck, M. A.; More, K. L.; Hoefelmeyer, J. D. J. Phys. Chem. C 2008, 112 (32), 12122–12126. (5) Chen, X. Y.; Li, J. R.; Jiang, L. Nanotechnology 2000, 11, 108–111. (6) Chen, X. Y.; Li, J. R.; Jiang, L. Chin. Sci. Bull. 2000, 45 (20), 1850– 1853. (7) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M.-C.; Casanove, M.-J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41 (22), 4286–4289. (8) Dumestre, F.; Martinez, S.; Zitoun, D.; Fromen, M.-C.; Casanove, M.J.; Lecante, P.; Respaud, M.; Serres, A.; Benfield, R. E.; Amiens, C.; Chaudret, B. Faraday Discuss. 2004, 125, 265–278. (9) Efrima, S.; Pradhan, N. C. R. Chim. 2003, 6, 1035–1045. (10) Kahn, M. L.; Monge, M.; Snoeck, E.; Maisonnat, A.; Chaudret, B. Small 2005, 1, 221–224. (11) Li, Y.; Li, X.; Yang, C.; Li, Y. J. Mater. Chem. 2003, 13, 2641– 2648.

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