Zinc Oxide Nanocrystals Stabilized by Alkylammonium

Aug 13, 2009 - J. J. Benítez , M. A. San-Miguel , S. Domínguez-Meister , J. A. Heredia-Guerrero , and M. Salmeron. The Journal of Physical Chemistry C...
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Zinc Oxide Nanocrystals Stabilized by Alkylammonium Alkylcarbamates Bing Luo, Julia E. Rossini, and Wayne L. Gladfelter* Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455 Received May 22, 2009. Revised Manuscript Received July 22, 2009 Nearly monodispersed, spherical ZnO nanocrystals were synthesized from the reaction of an amide precursor, [Zn(NiBu2)2]2, with hexylamine followed by reactions of the as-formed solution in a moist air flow. Extensive experiments were conducted to optimize the synthesis and to characterize the nanocrystals. The room temperature reactions led to 3.35.3 nm nanocrystals with the sizes increasing in direct proportion to the relative humidity. Purification afforded high yields of free-flowing nanocrystals that were dispersible in nonpolar solvents. The overall synthesis requires several days, but it results in multigram quantities of stable, redispersible nanocrystals. The nanocrystals were characterized using elemental analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), solution and solid-state NMR, IR, UV-vis absorption, and photoluminescence spectroscopies. In addition to providing H2O to serve as the source of oxygen in the ZnO, the air flow adds CO2 that converts the alkylamine into an alkylammonium alkylcarbamate, which serves as the surfactant. Elemental analysis, TGA, and XPS results established that the total number of N-hexyl fragments on a 3.7 nm nanocrystal was 200, where they exist as an equal number of anionic carbamates and cationic ammonium ions. The addition of pure hexylammonium hexylcarbamate to ZnO nanocrystals prepared by literature methods resulted in the formation of a product that was similar to the ZnO formed using [Zn(NiBu2)2]2. Larger nanocrystals up to 7.3 nm were also obtained by heating smaller nanocrystals in a mixture of hexylamine and toluene at 119 °C.

Introduction Zinc oxide is a wide-bandgap semiconductor material with extensive applications, for example, in pigments, catalysts, and micro- and optoelectronics.1-3 Studies on nanocrystalline ZnO *Corresponding author. E-mail: [email protected]. (1) Brown, H. E. Zinc Oxide: Properties and Applications; International Lead Zinc Research Organization: New York, 1976; p 218. (2) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301/1–041301/103. (3) Jagadish, C. Zinc Oxide Bulk, Thin Films and Nanostructures: Processing, Properties and Applications; Elsevier: Amsterdam, 2006; p 589. (4) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789–98. (5) Haase, M.; Weller, H.; Henglein, A. J. Phys. Chem. 1988, 92, 482–7. (6) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826–33. (7) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B 1998, 102, 7770–7775. (8) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566–5572. (9) Carnes, C. L.; Klabunde, K. J. Langmuir 2000, 16, 3764–3772. (10) Shim, M.; Guyot-Sionnest, P. J. Am. Chem. Soc. 2001, 123, 11651–11654. (11) Rataboul, F.; Nayral, C.; Casanove, M.-J.; Maisonnat, A.; Chaudret, B. J. Organomet. Chem. 2002, 643-644, 307–312. (12) Pesika, N. S.; Hu, Z.; Stebe, K. J.; Searson, P. C. J. Phys. Chem. B 2002, 106, 6985–6990. (13) Guo, L.; Ji, Y. L.; Xu, H.; Simon, P.; Wu, Z. J. Am. Chem. Soc. 2002, 124, 14864–14865. (14) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287–1291. (15) Monge, M.; Kahn, M. L.; Maisonnat, A.; Chaudret, B. Angew. Chem., Int. Ed. 2003, 42, 5321–5324. (16) Lyu, S. C.; Zhang, Y.; Lee, C. J.; Ruh, H.; Lee, H. J. Chem. Mater. 2003, 15, 3294–3299. (17) Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2003, 125, 13205–13218. (18) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625–1631. (19) Gao, P. X.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1315–1320. (20) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235–238. (21) Boyle, T. J.; Bunge, S. D.; Andrews, N. L.; Matzen, L. E.; Sieg, K.; Rodriguez, M. A.; Headley, T. J. Chem. Mater. 2004, 16, 3279–3288. (22) Yin, M.; Gu, Y.; Kuskovsky, I. L.; Andelman, T.; Zhu, Y.; Neumark, G. F.; O’Brien, S. J. Am. Chem. Soc. 2004, 126, 6206–6207. (23) Wang, X.; Ding, Y.; Summers, C. J.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 8773–8777. (24) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Phys. Chem. B 2005, 109, 2638– 2644.

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emerged in the 1980s,4-6 and a number of methods are now available.4-27 Most solution syntheses use divalent zinc sources such as zinc acetate.4,6-8,12,17,22 Other precursors including diethylzinc,9,10 dicyclohexylzinc,11,15,25 and [MeZn(OiPr)]427 were also used. Surface capping using ligands containing longchain alkyl groups is necessary for obtaining organic-dispersible ZnO nanocrystals or nanorods. Neutral surfactants include octanethiol,12 trioctylphosphine oxide (TOPO),10,17 long-chain amines,11,15,25 and a combination of trioctylamine and oleic acid.22 Without the protection of these surfactants, transparent ZnO colloidal solutions could be obtained, typically in alcohols, but precipitation gradually took place due to particle growth and/ or agglomeration.4,6-8 In this paper, we report the synthesis and characterization of ZnO nanocrystals based on the amido zinc precursor [Zn(NiBu2)2]2 and hexylamine. Zinc oxide nanocrystals capped with long-chain amines were previously prepared by Kahn et al. using dicyclohexylzinc as the precursor.11,15,25 The striking discovery of the current work is that the amine, hexylamine in our case, was transformed into hexylammonium hexylcarbamate (HAHC) by the reaction with atmospheric CO2 and that this transformation was vital for the formation of monodispersed nanocrystals of ZnO. Belman et al. have recently suggested that alkylammonium alkylcarbamates play a role as a stabilizing surfactant in ZnS nanoparticles, and they may play a wider role in other alkylamine/nanoparticle systems exposed to the atmosphere.28 Alkylammonium alkylcarbamates have been found to (25) Kahn, M. L.; Monge, M.; Colliere, V.; Senocq, F.; Maisonnat, A.; Chaudret, B. Adv. Funct. Mater. 2005, 15, 458–468. (26) Pinna, N.; Garnweitner, G.; Antonietti, M.; Niederberger, M. J. Am. Chem. Soc. 2005, 127, 5608–12. (27) Hikov, T.; Rittermeier, A.; Luedemann, M.-B.; Herrmann, C.; Muhler, M.; Fischer, R. A. J. Mater. Chem. 2008, 18, 3325–3331. (28) Belman, N.; Israelachvili, J. N.; Li, Y.; Safinya, C. R.; Bernstein, J.; Golan, Y. Nano Lett. 2009, 9, 2088–2093.

Published on Web 08/13/2009

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runs 1 2 3 4 5 6

[Zn(NiBu2)2]2 concn (M)

hexylamine concn (M)

Et2O (mL)

relative humidity (%)

T (°C)

reaction time (h)

ZnO diameter (nm)

0.047 0.047 0.047 0.047 0.093 0.047

0.093 0.093 0.093 0.093 0.186 0.093

50 50 50 50 50 100

19 ( 1 36 ( 1 50 ( 1 68 ( 3 54 ( 2 50 ( 2

20.7 ( 0.5 21.2 ( 0.9 21.2 ( 0.6 20.8 ( 0.4 20.1 ( 0.5 20.0 ( 1.0

98 71 72 54 110 144

3.3 3.9 4.4 5.3 4.6 4.8

form thermally reversible organogels with a wide range of organic solvents.29-31

Experimental Section Materials. Toluene and hexanes were dried with alumina in an MBraun solvent purification system. Diethyl ether, pentane, benzene-d6, deuterated chloroform, and hexylamine were distilled over calcium hydride under nitrogen. Hexadecylamine was recrystallized from hexanes prior to use. Acetone, ethyl acetate, and ethanol were dried over 4A molecular sieves and distilled under N2. Bis(diisobutylamido)zinc, [Zn(NiBu2)2]2,32 N-hexylammonium N-hexylcarbamate (HAHC), and N-hexadecylammonium N-hexadecylcarbamate (HDAHDC) were prepared as described in the literature.33 ZnO Nanocrystal Synthesis Using [Zn(NiBu2)2]2. The syntheses were conducted at room temperature in a 250 mL single-neck, round-bottom flask equipped with a 24/40 joint. In a typical experiment, hexylamine (0.62 mL, 4.66 mmol) was added via a syringe to a solution of [Zn(NiBu2)2]2 (1.50 g, 2.33 mmol) in 50 mL of Et2O under N2 at room temperature. A white precipitate formed immediately, but it dissolved after stirring for 3 h. Stirring was continued for an additional 15 h. The N2 was discontinued, and a flow of air having a defined relative humidity (36 ( 1% in this example) was passed over the solution (not through) and vented through a T-shaped joint. The flow rate was ca. 100 L/min. The humidity, measured with a Fisher Scientific traceable hygrometer (measurable relative humidity range, 10-95%; 0.1% resolution; and 3% accuracy), was controlled by varying the relative flow rates of an air flow and a water-saturated air flow obtained by passing an air flow through two water bubblers. The moist air reacted with the solution, and the continuous flow removed the volatiles. After the solution was exposed to the wet air for a few hours, a white, insoluble material started to form. As the reaction proceeded to 2 days, the solution became clear, at which point ca. 10 mL of the solution remained. The solution remained clear to ca. 1 mL, and then a solid started to separate. At the end, all liquids were evaporated and a yellow, waxy residue was obtained. The total time of this process was 71 h, and the temperature during this period was 21 ( 1 °C. The raw product was purified according to the following steps. Hexanes (20 mL) were added to the residue, and the solution was filtered through a 0.2 μm frit to remove a small amount of solid (ca. 0.02 g). Hexanes were removed from the filtrate under vacuum to regenerate a yellow, waxy solid. Acetone (10 mL) was added to the waxy solid and stirred for 10 min. The desired white solid was separated via centrifugation and dried under vacuum. The solid was redispersed in 20 mL of hexanes, to which ca. 0.1 mL of hexylamine was added. Filtration was conducted again to remove trace insoluble materials, and hexanes were removed from the filtrate to yield a white, waxy solid. Finally, ethyl acetate (10 mL) was added to the waxy solid, stirred for 20 min, and filtered. The isolated solid product was dried under vacuum for 1 h, collected, and stored under dry N2. The product (29) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393–10394. (30) George, M.; Weiss, R. G. Langmuir 2002, 18, 7124–7135. (31) George, M.; Weiss, R. G. Langmuir 2003, 19, 1017–1025. (32) Schumann, H.; Gottfriedsen, J.; Girgsdies, F. Z. Anorg. Allg. Chem. 1997, 623, 1881–1884. (33) Holas, T.; Zbytovska, J.; Vavrova, K.; Berka, P.; Madlova, M.; Klimentova, J.; Hrabalek, A. Thermochim. Acta 2006, 441, 116–123.

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was a white, free-flowing solid and was characterized to be surfactant-stabilized ZnO nanocrystals (0.41 g, 83% yield based on Zn). The syntheses were also carried out under different humidities (19-68%), which was found to affect the nanocrystal size. The effects of precursor concentration and solvent volume were also studied. The experimental conditions and results are summarized in Table 1. An attempt to synthesize ZnO nanocrystals was conducted using a wet N2 flow (instead of air) in the second stage of the synthesis. The resulting solid was not dispersible in any common organic solvents or water. XRD and TEM characterization showed it contained polydispersed ZnO nanocrystals and rods. An attempt to synthesize ZnO nanocrystals in the absence of water vapor was conducted by passing the air flow through two columns of 3A molecular sieves. The resulting solid was amorphous as determined by XRD and was not soluble in hexanes and ether. After exposing this solid in air for a week under ambient conditions, dispersible ZnO nanocrystals formed.

Reaction of HAHC with ZnO Nanocrystals Prepared from Zinc Acetate. The synthesis of ZnO nanocrystals using

zinc acetate was adopted from Gamelin and co-workers.34 A 0.55 M ethanolic solution of NMe4OH 3 5H2O (4.25 mmol) was dripped (ca. 8 min) into a 0.1 M DMSO solution of Zn(CH3COO)2 3 2H2O (2.51 mmol). The solution was stirred for 30 min at room temperature, at which point 15 mL was transferred to a flask containing 25 mL of ethyl acetate to induce precipitation of the nanocrystals. From the turbid mixture, 20 mL was removed and centrifuged. After decanting the liquid, the nanocrystals were dispersed in ethanol (15 mL) to yield an optically clear dispersion. To this, HAHC (0.060 g) was added, and the dispersion was stirred for 24 h, during which time it became turbid. The ZnO nanocrystals were precipitated with acetonitrile, centrifuged, separated from the liquid phase by decanting, and allowed to dry in air for 30 min. The nanocrystals were redispersed in hexanes to yield an optically clear mixture that was stable for several days if a small amount (ca. 0.1 mg) of HAHC was added to the hexanes. The ZnO/HAHC could also be redispersed in ethanol over time and remained stable if a small amount of the HAHC was added to the dispersion. Note that the ZnO nanocrystals prepared from zinc acetate and subsequently reacted with HAHC were only used to compare dispersibility and IR spectroscopic results to the ZnO / HAHC samples prepared from [Zn(NiBu2)2]2. ZnO Nanocrystal Growth. Under N2, a solution of toluene (150 mL) and hexylamine (60 mL) was loaded into a 500 mL, three-necked, round-bottom flask equipped with a stirbar and a thermometer. After the solution was heated to reflux (120 °C) in an oil bath, a dispersion of 3.00 g of ZnO nanocrystals (3.7 nm) in 30 mL of toluene was injected via a syringe. The temperature dropped to 115 °C immediately and increased to 119 °C in 2 min. At this temperature a gentle reflux was maintained, and the process was continued for 18 h. In the first 9 h, aliquots of the solution (30 mL) were isolated at an interval of 1.5 h and immediately cooled to 0 °C. Volatiles of the isolated solutions were removed under vacuum, and the residues were dispersed in hexanes (30 mL for each) and filtered to remove insoluble materials (ca. 0.01 g for each). Hexanes were removed from the (34) Schwartz Dana, A.; Norberg Nick, S.; Nguyen Quyen, P.; Parker Jason, M.; Gamelin Daniel, R. J. Am. Chem. Soc. 2003, 125, 13205–18.

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Luo et al. filtrates under vacuum to afford the ZnO samples (ca. 0.4 g for each). The final ZnO sample (ca. 0.8 g) was obtained after 18 h. During the growth, the solution was essentially clear for the first 9 h and became slightly cloudy at 18 h. More nondispersible materials (ca. 0.04 g) were removed by filtration from the 18 h sample. Solution NMR, IR, and Elemental Analysis. The samples were handled using standard air-free techniques. The spectra were obtained in C6D6 or CDCl3 solutions at room temperature on a Varian INOVA 300 spectrometer. The 1H NMR spectra were referenced to the residual proton at 7.15 ppm in C6D6 or 7.27 ppm in CDCl3. The 13C NMR spectra were referenced to the carbon resonance at 128.39 ppm for C6D6 or 77.23 ppm for CDCl3. The IR spectra of solids (KBr pellets) or liquids (NaCl windows) were recorded on a Nicolet MAGNA-IR 560 spectrometer. The elemental analyses were performed by Desert Analytics, Tucson, AZ. Solid-State 15N and 13C NMR Spectroscopy. The solidstate NMR spectra were obtained with the magic angle technique on a Varian VNMRS spectrometer operating at a 1H frequency of 600 MHz. The 15N NMR spectra were obtained using the crosspolarization method with 1H decoupling and were referenced to an NH4Cl standard at 41.5 ppm. A one-pluse experiment with 1H decoupling was used for 13C NMR, and the spectra were referenced to the R-C atom of glycine at 43.5 ppm. Powder X-ray Diffraction (XRD). The XRD measurements were conducted on a Bruker-AXS microdiffractometer equipped with an area detector and using a Cu X-ray source operated at 45 kV and 40 mA. All data collections used a frame time of 100 s. A standard LaB6 sample was used to calibrate the instrumental peak broadening. The XRD data fitting and crystal size calculations used MDI Jade 7 software. All of the XRD sizes reported here were calculated with the Scherer equation using the (102) peak. Transmission Electron Microscopy (TEM). Most samples were prepared by depositing a drop of clear nanocrystal hexanes dispersions on 300 mesh Cu grids coated with lacey carbon. For nondispersible samples, ethanol was used as the solvent and the dispersions were sonicated for a few minutes before added to the TEM grids. The TEM and HRTEM images were collected on FEI Tecnai T12A and FEI Tecnai G2 30 microscopes using 120 and 300 kV accelerating voltages, respectively.

UV-Vis Absorption Spectroscopy and Photoluminescence (PL). The nanocrystals were dispersed in spectrophotometric grade hexanes. Quartz cuvettes with a 3 mm pass length were used. UV-vis data were collected on an Online Instruments Cary-14 conversion spectrometer. The steady-state photoluminescent emission spectra were recorded on a Spex Fluorolog 1680 0.22 m double spectrometer using a xenon source. Thermogravimetric Analysis (TGA). TGA was performed on a NETZSCH thermal analyzer (model: STA 409 PC Luxx) under a dry N2 flow at a heating rate of 10 °C min-1. Capped alumina crucibles with the lid having a 0.2 mm hole were used. The samples weighed 12-14 mg. During the experiment, the samples were handled under N2 except for the short period (ca. 2 min) involved in transferring the crucibles from N2 atmosphere to the analyzer. X-ray Photoelectron Spectroscopy (XPS). The XPS measurements were performed on an SSX-100 system (Surface Science Instruments) equipped with a monochromated Al KR X-ray source, a hemispherical sector analyzer (HSA), and a resistive anode detector. The samples were mounted on the sample stages using double-sided carbon tape. Care was taken to ensure the powder fully covered the surface. Quantitative run-to-run agreement in the carbon intensities verified that the substrate did not contribute to the spectra. A metal grid was used above the samples for charge neutralization. The X-ray source was 200 W, and the spot size was 1  1 mm2. The survey spectra were obtained at 150 eV pass energy using 1 eV/step, and the high-resolution Langmuir 2009, 25(22), 13133–13141

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Figure 1. XRD patterns of ZnO/HAHC nanocrystals prepared under a relative humidity of (a) 19 ( 1%, (b) 36 ( 1%, (c) 50 ( 1%, and (d) 68 ( 3%. See Table 1 for details. Scheme 1

spectra were collected at 50 eV pass energy using 0.1 eV/step. A low-energy electron flux (10 eV) was used for charge neutralization. The base pressure of the XPS system was 5.0  10-10 Torr. The peak fittings were conducted using the ESCA 2005 software provided with the XPS system. Charge compensation was done by setting the C-H/C-C peak to 285.0 eV. A Gaussian-Lorentzian model with Gaussian percentages of 80-100% was applied to the peak fittings.

Results Synthesis of ZnO Nanocrystals Capped with HAHC. The general synthetic procedure is outlined in Scheme 1, and the typical reaction conditions and results are summarized in Table 1. The reaction of [Zn(NiBu2)2]2 with 2 equiv of hexylamine for 18 h at room temperature afforded a clear solution containing [Zn(NiBu2)2]2, [Zn(N(H)Hx)2]n(HNiBu2)m, and HNiBu2. Detailed characterization of the intermediates will be reported separately. Exposure of this solution to a wet air flow at a relative humidity of 19-68% for a period of 2.5-4 days yielded yellow, waxy residues. After the multistep purification as detailed in the Experimental Section, 3.3-5.3 nm white, freeflowing ZnO nanocrystals capped with HAHC (hereafter, referred as ZnO/HAHC) were obtained in 75-85% yields (on a Zn basis). Characterization of the ZnO Core. Figure 1 shows the diffraction patterns of nanocrystals obtained from [Zn(NiBu2)2]2 at relative humidities ranging from 19 to 68%. Each pattern is consistent with zinc oxide, and a decrease in line width as a function of increasing humidity is evident. The breadth of the 102 reflection was used to calculate the nanocrystal diameter using the Scherer equation. More detailed modeling of the complete diffraction patterns suggested minor differences in coherent length along different directions, suggesting that the nanocrystals DOI: 10.1021/la901830n

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Figure 2. Dependence of ZnO/HAHC nanocrystal size on the relative humidity. Square: 0.047 M of [Zn(NiBu2)2]2 (3.0 g in 100 mL of Et2O). Triangle: 0.093 M of [Zn(NiBu2)2]2 (3.0 g in 50 mL of Et2O).

are not perfectly spherical. This was confirmed by TEM as described below. Table 1 summarizes the effect of reaction conditions on nanocrystal diameter, and Figure 2 illustrates the linear relationship between humidity and diameter. TEM of all nanocrystals prepared at room temperature showed that they were spherical or nearly spherical with a narrow size distribution. Examples are illustrated in Figure 3 for a ZnO/ HAHC sample with an XRD size of 3.7 nm. The size distribution in Figure 3(iv) was obtained by measuring 222 nanocrystals, from which the volume length mean diameter was calculated to be 4.0 nm with a standard deviation of 0.4 nm. Close inspection of the TEM images revealed that ca. 50% of nanocrystals were oval in shape with the ratios of the long axis to the short axis being in the range of 1-1.3. The oval-shaped nanocrystals were more clearly seen in the HRTEM image in Figure 3(iii). A few crystals, such as the one shown in the inset of Figure 3(iii), exhibited a welldefined, hexagonal shape. The (100) interplanar distance was measured to be 2.74 A˚, which was slightly shorter than that for the bulk ZnO zincite crystal (2.81 A˚) (see, for example, the zincite powder diffraction file 036-1451). The UV-vis absorption spectra of ZnO/HAHC dispersions in hexane are illustrated in Figure 4. Samples a-c, exhibiting sizes of 3.3, 3.7, and 5.3 nm (calculated from XRD), respectively, were obtained from the room-temperature synthesis and samples d-f (sizes 5.5, 6.5, and 7.0 nm, respectively) were obtained by growth of a 3.7 nm sample at 119 °C (see below). As seen in Figure 4, the absorptions of the nanocrystals exhibited a systematic blue shift as the sizes decreased. This is consistent with the theory that semiconductor bandgap is increased as the size decreases and has been well documented. The bandgaps of these nanocrystals were calculated using the wavelengths at the inflection points of the absorption spectra. The nanocrystal sizes were calculated using the effective mass model and a recently developed model based on the tight binding theory.35 The results are listed in Table 2 with the comparison to the XRD sizes. The tight binding method afforded size results closer to the XRD results, while the effective mass method gave significantly larger sizes, especially for the smaller nanocrystals. In PL measurements, all of the ZnO/HAHC samples exhibited strong “green” emission in addition to the band edge emissions. The emission spectrum of a 3.7 nm sample with an excitation (35) Viswanatha, R.; Sapra, S.; Satpati, B.; Satyam, P. V.; Dev, B. N.; Sarma, D. D. J. Mater. Chem. 2004, 14, 661–668.

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at 310 nm is shown in Figure 4. The emission was found at 351 nm, which was within 0.01 eV of that obtained from the UV-vis absorption measurement. The green emission peak centered at 560 nm was observed for various ZnO samples obtained from different synthetic methods and has been attributed to transitions associated with oxygen vacancies on the surface.36,37 Characterization of the Surfactant. In the ZnO synthesis using [Zn(NiBu2)2]2 the first evidence that CO2 was incorporated into the product came from synthetic runs initially designed to probe the source of oxygen in the ZnO. Replacing the flow of moist air with a flow of air passed through two columns of activated molecular sieves resulted in the formation of a glassy amorphous product that could not be dispersed. This suggested that water was the oxide source. When wet N2 (relative humidity=42 ( 1%) was used, however, the product was a colorless solid insoluble in common solvents. In striking contrast to the experiments using moist air, XRD and TEM revealed that the product from the wet N2 synthesis was crystalline ZnO in the form of rods with lengths up to 15 nm and widths up to 7 nm. Polydispersed spherical particles up to 13 nm were also present in this sample. These results prompted additional study (see below) that ultimately confirmed that the CO2 present in air had converted the amines into carbamates. Solution 1H and 13C NMR Spectroscopy. The 1H and 13C NMR spectra (Figure 5(i) and (ii)) compare pure hexylamine (spectrum a), purified ZnO/HAHC nanocrystals (spectrum b) prepared by the method described above, and purified ZnO/ HAHC nanocrystals prepared by dissolving sample b (0.3 g) in hexanes and adding 0.2 mL of hexylamine followed by removal of the volatiles under vacuum (spectrum c). The NMR spectra show the presence of hexyls in the purified nanocrystal sample (b). In the 1H NMR spectrum (i, b), the hexyls were manifested as broad resonances at 0.7-1.2 (CH3), 1.2- 2.2 ((CH2)4), and 2.2-4.0 ppm (N-CH2). The centers of the resonances were shifted downfield from the corresponding resonances for hexylamine, indicating these hexyl-containing ligands were bonded to ZnO. While the NH2 in pure hexylamine was found as a singlet at 0.53 ppm, the NH resonances in sample b were broadened and possibly shifted too much for distinct observation. The sharp resonance at 0.3 ppm in the 1H NMR spectrum, which is due to silicone stopcock grease, is unaffected by the presence of the nanocrystals. Sample c was prepared to probe the impact of excess hexylamine on the spectra. Despite the attempt to remove the excess amine under vacuum, several distinct resonances were observed in both the 1H and 13C NMR spectra. The additional resonances are both shifted and broadened compared to pure hexylamine, although to a lesser extent than the resonances of the strongly bound N-hexyl groups. It should be noted that these additional resonances could be removed by rinsing the nanocrystals with ethyl acetate. The ligand signals were more easily observed in the 13C NMR spectra (ii). In sample b, the hexyls exhibited six broad, but wellseparated peaks at 42.6, 32.0, 30.8, 27.3, 22.9, and 13.9 ppm. These resonances were close to the corresponding hexylamine resonances at 42.4 (C1), 34.1 (C2), 31.8 (C4), 26.6 (C3), 22.7 (C5), and 13.8 (C6) ppm and presumably are assigned to the corresponding carbon atoms. A weak peak at 164.5 ppm (marked with an arrow in Figure 5(ii, b)) appears in the region characteristic of (36) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983–7990. (37) Van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 1715–1723.

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Figure 3. TEM (i and ii) and HRTEM (iii) images of a 3.7 nm ZnO/HAHC sample and the size distribution (iv) obtained from 222 nanocrystals. Table 2. Comparison of ZnO/HAHC Sizes Calculated from UV-Vis Absorption and from XRD

Figure 4. (i) UV-vis absorption spectra of ZnO/HAHC samples with different sizes. Samples a-c were obtained in the room temperature synthesis, and samples d-f were grown from a 3.7 nm sample at 119 °C. (ii) The PL emission spectrum of the 3.7 nm ZnO/HAHC sample with λex = 310 nm. Langmuir 2009, 25(22), 13133–13141

samples

a

b

c

d

e

f

XRD (nm) Eg (eV) tight binding (nm) effective mass (nm)

3.3 3.56 3.7 4.7

3.7 3.52 4.2 5.1

5.3 3.46 5.1 5.7

5.5 3.47 5.1 5.7

6.5 3.42 6.5 6.5

7.0 3.42 6.5 6.5

the carbamate carbonyl carbon.38 Consistent with the 1H NMR results, a new set of hexyl resonances (33.6 and 26.8 ppm) that did not exist in sample b were revealed in the 13C NMR spectrum of sample c. The rest of the resonances were not observed due to overlapping with other hexyl signals. The breadth of the NMR signals of the surface ligands in the nanocrystal samples (b and c) could be attributed to chemical shift inhomogeneity, exchange broadening, or slow molecular motion that decreases T2 relaxation times. Given the diameter of the nanocrystals, spectral broadening due to slow particle tumbling is expected and would likely obscure the impact of small chemical shift differences or exchange. Solid-State 13C and 15N NMR Spectroscopy. Referenced to NH4Cl at 41.5 ppm, two 15N NMR signals with similar intensities were found at 27.6 and 88.5 ppm for the nanocrystal sample (Figure 5(iii)) and were assigned to the hexylammonium and hexylcarbamate ions, respectively. The solid-state 15N NMR spectrum of pure HAHC (i.e., no ZnO) exhibits resonances at 35.9 and 88.5 ppm. The ammonium ion chemical shift was in (38) Belli Dell’Amico, D.; Calderazzo, F.; Labella, L.; Marchetti, F.; Pampaloni, G. Chem. Rev. 2003, 103, 3857–3897.

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Figure 6. High-resolution X-ray photoelectron spectra of C 1s, O 1s, N 1s, and Zn 2p3/2 for the 3.7 nm ZnO/HAHC sample.

Figure 5. Solution 1H NMR spectra (i) and 13C NMR spectra (ii) of (a) hexylamine, (b) 3.7 nm ZnO/HAHC nanocrystals, and (c) 3.7 nm ZnO/HAHC nanocrystals prepared by adding hexylamine to a hexanes solution of (b) followed by removal of the volatiles under vacuum. The vertical dashed lines in (i) (c) indicate the alkyl peak positions of hexylamine (a). The arrow in (ii) (b) indicates a weak carbamate peak. Solid-state 15N NMR spectrum (iii) of the ZnO/HAHC sample (b).

agreement with literature values, where, for example, the butylamine 15N NMR signal was 20.8 ppm and the butylammonium chloride 15N signal was 31.4 ppm.39 There have been a few reports on the 15N NMR spectra of carbamate esters,40-42 and their chemical shifts were very close to those found in our ZnO nanocrystal sample. It should be emphasized that the only compounds that have a 15N NMR chemical shift around (39) Duthaler, R. O.; Roberts, J. D. J. Am. Chem. Soc. 1978, 100, 3889–95. (40) Burns, J. M.; Ashley, M. E.; Crockett, G. C.; Koch, T. H. J. Am. Chem. Soc. 1977, 99, 6924–8. (41) Lycka, A.; Cizmarik, J. Pharmazie 1987, 42, 271–2. (42) Olah, G. A.; Heiner, T.; Rasul, G.; Prakash, G. K. S. J. Org. Chem. 1998, 63, 7993–7998. (43) Buchanan, G. W. Tetrahedron 1989, 45, 581–604. (44) Marek, R.; Lycka, A. Curr. Org. Chem. 2002, 6, 35–66.

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90 ppm and can be derived from hexylamine are amides and carbamates.43,44 The comparison of the solid-state and solution 13C NMR spectra of the nanocrystal sample is provided as Supporting Information. The two spectra matched, except that the 164.5 ppm signal in the solution spectrum was not observed in the solidstate spectrum. X-ray Photoelectron Spectroscopy. The XPS survey spectrum of pure HDAHDC (studied in place of HAHC due to the higher volatility of the latter) and of ZnO/HAHC samples prepared from [Zn(NiBu2)2]2 are given in the Supporting Information and showed the presence of O, C, and N and of Zn, O, C, and N signals, respectively. Other non-hydrogen elements, if they were present, were below the detection limit (ca. 0.1 at. %). The high-resolution spectra of HDAHDC exhibited three carbon peaks with binding energies (BE) of 285.0 (C-H/C-C, used as the reference45), 286.1 (CH2N), and 288.6 (CO2N) eV. The relative intensities were 90.9, 7.5, and 1.6%, respectively. The carbamate carbon BE measured here agrees with that reported for a tert-butyl carbamate species (289 eV).46 In addition, carbamate carbon signals at 288.7-290 eV were found in the reaction of NH3 and CO2 on a Cu(100) or Zn(0001) surface47 and the reaction of CO2 and MeNH2, Me2NH or EtNH2 on a Cu(221) surface.48 The broad N 1s signal of HDAHDC was deconvoluted into two peaks at 400.0 and 401.4 eV with the area ratio of 1:1.1. The 400.0 eV peak is consistent with the energies observed for carbamates (399.5-400.3 eV),46-48 and the 401.4 eV is characteristic of an alkylammonium N 1s (401.0-401.5 eV).49,50 The O 1s spectrum exhibited one symmetric BE at 531.6 eV, consistent with the oxygens of the carbamate. The high-resolution spectra of ZnO/HAHC are illustrated in Figure 6. Curving fitting of the C 1s signal afforded two peaks (285.0 and 288.8 eV) instead of the expected three. We found that fitting the signals of all carbons in the alkyl substituents as a single peak gave a better result than attempting to separate the C-N (45) Barr, T. L.; Seal, S. J. Vac. Sci. Technol. A 1995, 13, 1239–46. (46) Petoral, R. M., Jr.; Uvdal, K. J. Phys. Chem. B 2005, 109, 16040–16046. (47) Davies, P. R.; Roberts, M. W. J. Chem. Soc., Faraday Trans. 1992, 88, 361– 8. (48) Davies, P. R.; Keel, J. M. Surf. Sci. 2000, 469, 204–213. (49) Lindberg, B. J.; Hedman, J. Chem. Scr. 1975, 7, 155–66. (50) Chernyshova, I. V.; Rao, K. H.; Vidyadhar, A.; Shchukarev, A. V. Langmuir 2000, 16, 8071–8084.

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Figure 7. (i) IR spectra of (a) hexylamine, (b) the 3.7 nm ZnO/ HAHC sample, and (c) a ZnO sample obtained after sample (b) was heated at 250 °C for 2 h. (ii) Enlarged portions of the above spectra.

peak that is typically located at 286.0 eV. The second C 1s peak at 288.8 eV was assigned to the NCO2 in the hexylcarbamate. In the nanocrystal sample, the ratio of carbon signals of the hexyl carbons to the carbamate carbon was 6.7:1 instead of the expected 12:1. The presence of some surface bound carbonate or bicarbonate may explain this observation. The Zn 2p3/2 BE was found at 1021.2 eV, typical for ZnO.12,51-53 The N 1s signal was deconvoluted to two peaks at 399.1 and 400.1 eV with the area ratio of 1.1:1. The 400.1 eV peak, located in the range of other characterized carbamates (399.5-400.3 eV),46-48 was assigned to the carbamate carbon. The 399.1 eV peak, however, was closer to the alkylamine N 1s binding energies (e.g., 398.6 eV in hexadecylamine54 and 399.5 eV in dodecylamine50), rather than the range expected for the alkylammonium N 1s (401.0-401.5 eV50,54). This evidence suggests that on the surface the hexylammonium is hydrogen bonded to a surface oxide. The O 1s signal of ZnO/HAHC was deconvoluted to two peaks at 530.2 and 531.9 eV. The higher binding energy peak is assigned to the carbamate oxygens and hydroxide, while the peak at 530.2 eV is assigned to the oxide ions. These assignments were based on the reported XPS data for ZnO (530.0-530.7 eV12,51-53) and OH on ZnO surfaces (532 eV12 and 531.5 eV55) as well as on the results from the HDAHDC sample. The O2- to OH/carbamate ratio was (51) Deroubaix, G.; Marcus, P. Surf. Interface Anal. 1992, 18, 39–46. (52) Islam, M. N.; Ghosh, T. B.; Chopra, K. L.; Acharya, H. N. Thin Solid Films 1996, 280, 20–25. (53) Du, Y.; Zhang, M. S.; Hong, J.; Shen, Y.; Chen, Q.; Yin, Z. Appl. Phys. A: Mater. Sci. Process. 2003, 76, 171–176. (54) Lindberg, B.; Maripuu, R.; Siegbahn, K.; Larsson, R.; Goelander, C. G.; Eriksson, J. C. J. Colloid Interface Sci. 1983, 95, 308–21. (55) Chaparro, A. M.; Maffiotte, C.; Gutierrez, M. T.; Herrero, J. Thin Solid Films 2000, 358, 22–29.

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2.6:1. It should be pointed out that the XPS analysis was conducted for several ZnO/HAHC samples, and the binding energy variations for C, N, O, and Zn were within 0.3 eV. Infrared Spectroscopy. The IR spectrum of the 3.7 nm ZnO/ HAHC nanocrystals (b) and a sample (c) obtained after heating the 3.7 nm ZnO/HAHC nanocrystals to 250 °C for 2 h under vacuum are shown in Figure 7 along with that of hexylamine for comparison. The IR features in spectrum b support the presence of hexylcarbamate and hexylammonium in the nanocrystal sample. In the OH and NH stretching region a broad absorption centered at 3250 cm-1 was detected. The characteristic carbamate absorptions were found at 1586, 1518, and 1333 cm-1 (Figure 7(ii)), corresponding to the NCO2 stretching, N-H deformation and CNCO2 skeletal vibrations.56,57 The absorption at 1518 cm-1 was characteristic of a carbamate formed from a primary amine and is not found in R2NCO2- anions.57 Because carbonates and bicarbonates could form from the interaction of CO2 with ZnO, we attempted without success to assign these IR absorptions to surface-bound CO32- or HCO3-.58 In particular, the 1518 cm-1 was not assignable to either species. In the region of 1300-1700 cm-1, CO32- bound in a bidentate mode to ZnO surfaces afforded two bands at 1665-1580 and 13481303 cm-1.58 These absorptions result from the splitting of the D3h doubly degenerate asymmetric stretching vibrations of CO32due to the lower symmetry on surfaces compared to the 1420 cm-1 peak observed in bulk ZnCO3.59 For CO32- bound in a tridentate mode on ZnO surfaces, the asymmetric and symmetric stretching vibrations were found at 1617-1609 and 1340-1323 cm-1, respectively.60 Bicarbonate vibrations differ to an even greater extent from the absorptions observed on ZnO/HAHC samples. In early studies of CO2 adsorption on ZnO, the HCO3- IR absorption bands were found at 1600, 1431, and 1230 cm-1 or 1635, 1424, 1229, and 835 cm-1.58,61 Late studies on Ga2O3 showed that the bidentately bonded HCO3- afforded IR peaks at 1630, 1455, and 1225 cm-1.62 The ZnO sample (c) exhibited IR absorptions at 1553, 1426, 1261, and 881 cm-1, which were consistent with the HCO3vibrations described above. The broad νOH at 3450 cm-1 was indicative of the presence of HCO3- or surface OH. The absorptions attributed to alkyl groups were weak, and the characteristic hexylcarbamate absorptions essentially disappeared. These results indicate that the 250 °C thermal treatment dissociated at least a large percentage of carbamate leaving bicarbonate on the surface. Thermogravimetric (TGA) and Elemental Analysis (EA). The TGA results are shown in Figure 8. Three mass loss events at 70-285 °C, 285-440 °C, and 440-ca. 650 °C were observed, which accounted for 23.4% of the original sample and left a residue of ZnO (76.6%). The EA of a sample taken from the identical ZnO/HAHC preparation used for the TGA experiments yielded a value of 60.5% zinc. Assuming that all zinc ions are present as ZnO, one calculates that ZnO would constitute 75.3% of the ZnO/HAHC sample, in good agreement with the TGA result. The C, H, and N values were 14.01, 2.68, and 2.39%, respectively. Because all of the nitrogen is present as N-hexyl (56) Katritzky, A. R.; Jones, R. A. J. Chem. Soc. 1960, 676–9. (57) Aresta, M.; Quaranta, E. Tetrahedron 1992, 48, 1515–30. (58) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89–126. (59) Adler, H. H.; Kerr, P. F. Am. Mineral. 1963, 48, 124–37. (60) Wang, Y.; Kovacik, R.; Meyer, B.; Kotsis, K.; Stodt, D.; Staemmler, V.; Qiu, H.; Traeger, F.; Langenberg, D.; Muhler, M.; Woell, C. Angew. Chem., Int. Ed. 2007, 46, 5624–5627. (61) Taylor, J. H.; Amberg, C. H. Can. J. Chem. 1961, 39, 535–9. (62) Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L. J. Phys. Chem. B 2006, 110, 5498–5507.

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Figure 8. Thermogravimetric analysis (TGA) curve of the 3.7 nm ZnO/HAHC sample.

groups, one can predict that complete loss of these (as hexylamine) would remove 17.3% of the initial sample. The close agreement between the TGA (17.5%) and EA allows assignment of hexylamine loss to the region from 70 to 285 °C. The mass loss events at 285-440 °C and 440-ca. 650 °C account for 5.9% of the original sample. Assuming that the remaining carbon is desorbed as CO2 and that residual hydrogen is lost as water, the EA data predict that these two events should account for 7.4% of the original sample. ZnO Nanocrystal Growth at Elevated Temperatures. Crystal growth at higher temperature was conducted in refluxing toluene and hexylamine (119 °C). The use of hexylamine as a cosolvent was necessary to prevent ZnO precipitation. In a period of 18 h, the nanocrystals grew from 3.7 to 7.3 nm in diameter. Consistent with the observed nanocrystal dispersibility, the HAHC in ZnO/HAHC was detected by NMR and IR spectroscopy after the growth. Elevated temperatures cause alkylammonium alkylcarbamates to lose CO2. The increased stability of the carbamate in ZnO/HAHC, which is also evident in the TGA result, is apparently caused by complexation to the ZnO surface. The XRD patterns (provided as Supporting Information) exhibited progressive narrowing of the diffraction peaks consistent with the nanocrystal growth. Unlike the nanocrystals obtained in the room temperature synthesis, TEM revealed that more than 95% of the high-temperature grown nanocrystals were spherical. For a sample with an XRD size of 5.5 nm, its TEM size distribution was from 202 nanocrystals, and the volume length mean diameter was calculated to be 5.8 nm with a standard deviation of 0.7 nm.

Discussion The approach to ZnO nanocrystals described here is related to the method developed by Kahn et al., in which dicyclohexylzinc was reacted with a long chain primary amine and subsequently hydrolyzed in an air stream.25 Replacing ZnCy2 with [Zn(NiBu2)2]2 allows one to work with a nonpyrophoric, crystalline zinc source that is stable indefinitely under air-free conditions. Both [Zn(NiBu2)2]2 and ZnCy2 are expected to react with primary amines, and both have ligands that can be volatilized as byproducts. With dodecylamine, ZnCy2 forms a complex ZnCy2(NH2C12H25). The amido complex, however, undergoes a rapid σ-bond metathesis that we suggest leads to a species containing a primary amido ligand (eq 1), which further reacts to form Zn(NHR)2-containing materials. Details of these reactions will be published separately. The ultimate source of oxygen in ZnO was determined to be water rather than O2 13140 DOI: 10.1021/la901830n

Figure 9. Relationship of the nanocrystal radius vs time for the ZnO growth at 119 °C. The solid curve was a fitting using the Lifshitz-Slyozov-Wagner (LSW) equation.

regardless of whether the starting complex was ZnCy2 and [Zn(NiBu2)2]2. ZnðNi Bu2 Þ2 þ RNH2 h ZnðNHRÞðNi Bu2 Þ þ i Bu2 NH ð1Þ The incorporation of CO2 into the product largely as a carbamate is an important part of the current synthesis and was probably involved in the earlier work. Indeed, we agree with Belman et al.;28 it is possible that carbamates may play a role with nanocrystals that are stabilized by primary or secondary amines and that have been exposed to the atmosphere. The elemental analytical data coupled with the TGA results provided a quantitative ratio of HAHC formula units/ZnO nanocrystal for a set of 3.7 nm particles. Based on this particle diameter, the surface area of the ZnO core is 4300 A˚2. The results indicate that a total of 200 hexyl groups, half of which are bound as carbamates and the other half as ammonium ions, occupy this surface area. On average each hexyl group would occupy ∼22 A˚2, which is the same amount of surface area occupied by straight chain alkyl thiols on gold surfaces63 and tetradecylamine on ZnS nanorods.64 The nature of the bonding between the surface and the carbamate or the ammonium ion could not be discerned completely from the spectroscopy. Molecular zinc carbamate complexes are well-known,65-67 and it is possible that a mono- or bidentate carbamate could coordinate to a surface zinc ion. A carbamate could also bridge two zinc ions as they do in the tetranuclear complex [Zn4O(O2CNR2)6]. In the XPS, while the 400.1 eV N 1s peak can be readily attributed to the carbamate, the 399.1 eV signal could not be assigned to the cationic hexylammoniums, [HexNH3]þ. In fact, both peaks are located lower in energy than those found in alkylammonium ions (401.0-401.5 eV). An explanation is that hydrogen bonding occurs from the hexylammonium ion to a surface oxide. In the 15N NMR spectrum, the comparison between the hexylammonium N signal (35.9 ppm) found in pure HAHC and the higher field shift in the ZnO/HAHC sample (27.6 ppm) is consistent with the suggestion of hydrogen (63) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853–6. (64) Belman, N.; Acharya, S.; Konovalov, O.; Vorobiev, A.; Israelachvili, J.; Efrima, S.; Golan, Y. Nano Lett. 2008, 8, 3858–3864. (65) Klunker, J.; Biedermann, M.; Schaefer, W.; Hartung, H. Z. Anorg. Allg. Chem. 1998, 624, 1503–1508. (66) McCowan, C. S.; Groy, T., L.; Caudle, M. T. Inorg. Chem. 2002, 41, 1120–7. (67) Dell’Amico, D. B.; Calderazzo, F.; Labella, L.; Marchetti, F.; Mazzoncini, I. Inorg. Chim. Acta 2006, 359, 3371–3374.

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bonding of the ammonium ion to the surface. The signal for butylamine occurs at 20.8 ppm. Adjustments of the cation to anion ratio in the surfactant could allow ammonium carbamate surfactants to respond to residual charges on a nanocrystal effectively neutralizing such charge buildup. The 15N NMR spectrum shows equal intensities for the carbamate and ammonium/amine groups. This measurement, however, is made on 0.2 g of a bulk sample, which may mask uneven ratios on individual nanocrystals. The reaction of ZnO nanocrystals prepared using zinc acetate with HAHC results in stabilized nanocrystals (referred to as ZnO þ HAHC) that appear similar, but not identical, to samples synthesized by the current method. The most notable difference is that ZnO þ HAHC samples can be dispersed in ethanol, yielding stable, transparent dispersions whereas ZnO/HAHC samples cannot be dispersed in ethanol. Both samples can be dispersed in hexane. Infrared spectroscopy indicates that the carbamate group is present on the ZnO þ HAHC samples, but there exist small differences in relative peak intensities. While more work is needed to define the differences, we suggest that the surfactant is more fully ordered forming a more stable shell in the ZnO/HAHC samples where the surfactant was present during the entire synthesis. The ordered shell does not rearrange in response to a change in solvent; thus, the ZnO/HAHC particles do not disperse in ethanol despite lengthy stirring and sonication. The ability to disperse the ZnO þ HAHC particles in ethanol may be due to the ability of a portion of less tightly bound surfactants to reorient with their polar head groups facing the solvent. Nanocrystal Growth. While relative humidity variation provided a small size range of nanocrystals, refluxing a 3:1 mixture of toluene/hexylamine containing ZnO/HAHC samples induced growth of the nanocrystals. A graph of the nanocrystal diameter as a function of time is shown in Figure 9. Refluxing for longer periods resulted in cloudy dispersions. The coarsening of particles is driven by a favorable reduction in the surface energy as the radius of curvature of the particles increases. The kinetics of such Ostwald ripening processes of ZnO have been modeled using the Lifshitz-Slyozov-Wagner (LSW) equation (eq 2)68-70 r3 - r0 3 ¼ Kt

ð2Þ

where r is the nanocrystal radius after the growth, r0 is the initial nanocrystal radius, K is a coefficient, and t is the growth time. In (68) Liftshitz, I. M. S.; V, V. J. Phys. Chem. Solids 1961, 19, 35. (69) Wagner, C. Z. Elecktrochem. 1961, 65, 581. (70) Viswanatha, R.; Sarma, D. D. In Nanomaterials Chemistry; Rao, C. N. R., M€uller, A., Cheetham, A. K., Eds.; Wiley-VCH: Weinheim, 2007; pp 139-170.

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this model, monomer diffusion is the growth-limiting step. The line in Figure 9 shows the best fit of eq 2 to the data, but other considerations suggest that diffusion-limited ripening is not a reasonable growth mechanism in this situation. Unlike the current system, most of the past studies in which eq 2 or a related version has been applied were conducted in alcohols often in the presence of added water. In addition, no surfactant was present to stabilize the nanocrystals. In the current study the fact that the growth was conducted in a rigorously dry, nonpolar solvent mixture and that the nanocrystals are stabilized by a surfactant greatly complicates the interpretation of any kinetic study. More work is needed to elucidate the processes involved in nanocrystal growth.

Conclusions Bis(diisobutylamido)zinc can be reacted with atmospheric moisture in the presence of primary alkylamines to form multigram quantities of monodispersed ZnO nanocrystals. The sizes of the nanocrystals range from 3.3 to 5.3 A˚ and can be controlled by the relative humidity of the air used in the reaction. Extensive spectroscopic studies established that the alkylammonium alkylcarbamates comprise a dense surfactant layer that prevents agglomeration and growth at room temperature and renders the nanocrystals dispersible in nonpolar solvents such as hexane and toluene. The ammonium carbamate is formed in situ from the corresponding alkylamine and atmospheric carbon dioxide. Nanocrystal growth is induced at elevated temperatures. Acknowledgment. This work was funded by a grant from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, under Award DE-FG02-07ER15913. We also thank the help of Dr. Nathan Traaseth for solid-state NMR measurements, Dr. Andreas Stein and Ms. Melissa Fierke for TGA measurements, and Dr. Ozan Ugurlu for HRTEM measurements. The XRD, XPS, and TEM instruments were located in the University of Minnesota’s open-access Characterization Facility, which is supported by operations funding from the Institute of Technology. Supporting Information Available: Solid-state 13C NMR spectrum, XPS survey spectra of the 3.7 nm ZnO/HAHC nanocrystals and HDAHDC, and XRD patterns of the samples obtained from the growth of the 3.7 nm ZnO/ HAHC sample at 119 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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