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Langmuir 1999, 15, 2322-2326
Nanoscale Materials Synthesis. 1. Solvent Effects on Hydridoborate Reduction of Copper Ions Anne-Marie L. Jackelen, Michelle Jungbauer, and George N. Glavee* Department of Chemistry, Lawrence University, Appleton, Wisconsin 54914 Received June 22, 1998. In Final Form: January 21, 1999 The reduction of CuBr2 by NaBH4 in a variety of solvents yields nanoscale metallic copper as the primary product. Although the product of this reaction is largely solvent independent, the studies show that reaction times, the total volume of gaseous products, and nanoscale particle sizes are dependent on solvent properties. The reaction occurred instantaneously in polar, protic, and acidic solvents such as methanol and ethanol. In contrast, in polar, aprotic, and coordinating solvents such as tetrahydrofuran and acetonitrile, the reaction was slow. Particle sizes increased as the rate of reaction decreased. For solvents in which similar reaction times were observed, the reaction carried out in polar, aprotic, and coordinating solvents yielded copper samples with larger particle sizes. The acid properties of the solvent controlled the amount of gas evolved in these experiments.
Introduction Hydridoborate reduction of transition metal ions has been used extensively as a synthetic method in the rapidly growing area of nanoscale materials.1 Nanoscale, ultrafine, nanoclusters, and nanophase are all terminologies used to describe particles/materials in the size regime of 1-100 nm (approximately 100 to 6.0 × 107 atoms). Because the size regime is the transition between molecular and bulk solid state limits, novel and hybrid properties are often observed in these materials. The unique and dramatically improved magnetic, optical, and mechanical properties shown by materials with nanoscale building blocks may be useful in the electronic, magnetic, catalytic, and environmental industries.2 The various methods of particle preparation are currently being examined as part of an overall need to develop systematic schemes for nanoparticle production. Recent studies have shown that the composition of nanoscale materials generated through hydridoborate reduction of transition metal ions is solvent dependent.3 However, for Cu(II) ions, the reduction reaction in water and diglyme yielded metallic copper.4 As part of our efforts to explore the chemical kinetics of the reduction process, we have studied the effects of a variety of solvents on the hydridoborate reduction of Cu(II) ions. This paper shows (1) (a) Andres, R. P.; Averback, R. S.; Brown, W. L.; Brus, L. E.; Goddard, W. A.; Kaldor,. A.; Louis, S. G.; Moskovits, M.; Peercy, P. S.; Riley, S. J.; Siegal, R. W.; Spaepan, F.; Wang, Y. J. Mater. Res. 1989, 704. (b) Gesser, H. D.; Goswami, P. C. Chem. Rev. 1989, 89, 765. (c) Stucky, G. D. Nanochemistry and Nanoclusters: The Beginning of Matter. Naval Res. Rev. 1991, 3, 28. (d) Matijevic, E. Fine Particles: Science and Technology. MRS Bull. 1989, 14, 19. (e) Matijevic E. Fine Particles Part II: Formation, Mechanism and Applications. MRS Bull. 1990, 15, 20. (f) Dagani, R. Nanostructured Materials Promise to Advance Range of Technologies Chem. Eng. News 1992, November 23, 18. (2) (a) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Huffman, D. C. Science 1992, 257, 219. (b) McCandish, L.; Kear, B. H.; Kim, B. K. Mater. Sci. Technol. 1990, 16, 953. (c) Hahn, H.; Averback, R. J. Appl. Phys. 1990, 67, 1113. (d) Averback, R. S.; Hahn, H.; Hofler, H. J.; Logas, J. C. Appl. Phys. Lett. 1990, 57, 1745. (3) (a) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Inorg. Chem. 1995, 34, 28. (b) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1993, 9, 162. (c) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1992, 8, 771. (d) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Inorg. Chem. 1993, 32, 474. (4) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1994, 10, 4726.
that solvent properties affect the reaction time, gas evolution, and the particle/crystallite size of the nanoscale materials generated. However, in all the solvents, nanoscale metallic copper was produced. Experimental Methods General Procedures. All experiments were carried out using standard inert atmosphere techniques at room temperature.5 Reagents were stored and transferred in an inert atmosphere box and all reactions were carried out on a vacuum line under nitrogen or argon atmosphere using standard Schlenkware. All nonaqueous solvents were dried using appropriate drying agents6 and distilled under nitrogen. Nitrogen gas was bubbled through aqueous solvents and acetone for several hours prior to their use in washing the final products. Anhydrous CuBr2 and NaBH4 were used as received from Strem and Aldrich chemical companies, respectively. In a typical reaction, two Schlenk flasks were charged with CuBr2 and NaBH4 respectively in an inert atmosphere box. These flasks were closed using rubber septa, brought out of the box, and connected to the vacuum line. The appropriate solvent was added to dissolve the solids and the borohydride solution transferred into the CuBr2 solution. A black solid was isolated from the resulting black suspension by transferring the suspension onto a fritted filter flask. The solid was then washed using prepurged water and acetone and then dried in a vacuum. The dried solid was then collected and weighed in an inert atmosphere box. All liquids and suspensions were transferred using cannula techniques.5 Reactions were carried out using the following solvents: water, methanol, ethanol, n-propanol, n-butanol, (5) (a) Shriver, D. F.; Drezdon, M. The Manipulation of Air-Sensitive Compounds, 2nd ed.; Wiley: New York, 1986. (b) Herzoq, S.; Dehnert, J.; Luhder, K. Technique of Inorganic Chemistry; Johnassen, H. B., Ed.; Interscience: New York, 1969; Vol. VII. (6) (a) Riddick, J. A., Bunger, W. B., Sakaw, T. K. Organic Solvents: Physica Properties and Methods of Purification; John Wiley and Sons: New York, 1986; Vol II. (b) Vogel, Arthur. Vogel’s Textbook of Practical Organic Chemistry. Longman Scientific and Technical: New York, 1989. (7) (a) Bowser, J. R. Inorganic Chemistry; Brooks/Cole Publishing Co.: Pacific Grove, CA, 1993. (b) Lagowski, J. J. The Chemistry of NonAqueous Solvents, Acdemic Press: New York, 1966. (c) Serjeant, E. P.; Dempsey, B. Ionisation Constants of Organic Acids in Aqueous Solution; Pergamon Press: Elmsford, NY, 1979. (8) (a) Lide, D. R. Handbook of Chemistry and Physics; CRC Press Inc.: Boston, MA. 1990. (b) Angelici, R. J. Synthetic Technique in Inorganic Chemistry; Saunders Co.: Philadephia, PA. 1977. (c) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH: New York, 1990. (9) Munakata, M.; Kitagawa, S.; Miyazima, M. Inorg. Chem. 1985, 24, 1638.
10.1021/la9807311 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/04/1999
Nanoscale Materials Synthesis
Langmuir, Vol. 15, No. 7, 1999 2323 Table 2. Size of Copper Particlesa
Table 1. Effect of Solvent on Reaction Times and % Yield of Metallic Coppers solventa
reaction time, h
yield, g (%)b
water methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol diethyl ether tetrahydrofuran acetonitrile dichloromethane
instantaneous instantaneous instantaneous 5.5 20 52c 70c 120c 21 120d 144c
0.120 (95) 0.110 (87) 0.103 (82) 0.110 (87) 0.120 (95) 0.115 (92) 0.126 (100) 0.089 (70) 0.113 (89) 0.102 (80)
a
All reactions were carried out at ambient temperature. b Based on amount of metallic copper isolated. c Reaction time influenced by the solubility of the reagents in the solvent. d Stable complex forms and decomposes over time.
n-pentanol, n-hexanol, diethyl ether, tetrahydrofuran, acetonitrile, and dichloromethane. Products were characterized using X-ray powder diffraction (XRD), energy-dispersive X-ray (EDS) analysis, and scanning and transmission electron microscopies. The XRD data were obtained on a Siemen 500 instrument on material coated and immobilized on a glass plate using cyanoacrylate glue in an inert atmosphere box. Scanning and transmission microscopy data were obtained on a Hitachi S-2460 and an RCA EMU-4 microscope on samples placed on carbon-coated TEM grids. Microscopy samples were prepared by sonication of a few milligrams of the powder in toluene or ethanol. A drop of the dispersed material was placed on the grid and allowed to dry. EDS analysis were carried out following the SEM data collection on a KEVEX fission instrument. Differential scanning calorimetric (DSC) data were obtained on Perkin-Elmer instrument. A summary of the reaction times in the different solvents and yields is provided in Table 1. Slight modifications were made in the typical reaction procedure described in the general procedures depending on the solvent. These changes are described in the following paragraphs. Reaction in Methanol. A 25 mL colorless methanol solution of NaBH4 (0.151 g, 3.99 mmol) was added to a 75 mL yellowbrown methanol solution of CuBr2 (0.447 g, 1.99 mmol). The mixture yielded a black suspension following color changes from green-brown to milky white and then to light brown. Isolation of the black suspension using standard inert atmosphere techniques yielded 0.126 g a brown-black powder. Powder diffraction analysis indicated the product was predominantly metallic copper. However, EDS analysis of the powder indicated the presence of a significant amount of CuBr2. If the reaction in methanol was carried out by addition of methanol to solid NaBH4 (0.152 g) and CuBr2 (0.444 g) in Schlenk flask, an initial precipitation of a black solid was followed by the precipitation of a blue-green solid. Isolation of the product from this reaction yielded a mixture of a black and blue-green solid. A similar product was generated when the reaction was carried out by adding a methanol solution of CuBr2 (0.445 g), to a methanol solution of NaBH4 (0.151 g). An XRD analysis of thermally processed samples from the latter two reactions showed the presence of Cu and CuO. Reactions in the CuBr2-Soluble Solvent System (Ethanol, n-Propanol, n-Butanol, Tetrahydrofuran, and Acetonitrile). In general a 2.00 mmol solution of CuBr2 was allowed to react with 4.00 mmol of NaBH4. An instantaneous color change was observed with gas evolution. The rate of gas evolution was different in the different solvents. The resulting brown-black precipitates from the reactions were isolated as described in the General Procedures. In all cases, we produced black solids that were identified as metallic copper. To determine the total amount of gas evolved in these reactions, experiments were carried out in reaction flasks attached to a vacuum line system of a known volume. The line was equipped with a mercury manometer. The amount (Table 4) of gas evolved from the reactions was determined from pressure and temperature measurements using the ideal gas law.
solvent
XRDc
TEMb
water methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol eiethyl ether tetrahydrofuran acetonitrile dichloromethane
14.8 13.4 13.5 15.4 17.4 14.1 17.7 9.63 23.9 13.9 17.5
45.0 21.0 13.3 24.6 26.7 18.5 39.0 32.9 26.7
a All size measurements are given in nanometers. b Results obtained from electron microscope photographs. c Values based on X-ray diffraction data, calculated using Scherrer’s formula.
Table 3. Selected SEM-EDS Analysis of Product from the CuBr2 and NABH4 Reaction elements presenta solvent
Cu
O
Na
Br
methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol
90.90 99.78 99.89 99.71 99.89 99.99
0 0.17 0 0.05 0 0
0 0 0 0 .01 0
9.10 0.04 0.11 0.24 0.10 0.01
a
All values are weight percent.
Table 4. Total Gas Evolved in Selected Solvents and Solvent pKa Values7 solvent water methanol ethanol 1-propanol butanol acetonitrile tetrahydrofuran
total gas evolved, mmola
pKa
14.6 15.4 15.1 4.0 8.1 3.9
15.7 15.5 15.9 16.1 16.1 29.1
2.1
-2.08
a
All reactions were carried out using approximately 2.0 mmol of CuBr2 and 4.0 mmol of NaBH4.
Reactions in the CuBr2 Partially Soluble Solvent Systems (n-Pentanol, n-Hexanol, Diethyl Ether, and Dichloromethane). In general one Schlenk flask was charged with 2.00 mmol of CuBr2 and 4.00 mmol of NaBH4 in an inert atmosphere box. The solvent (∼100 mL) was added to the stirred mixture on a vacuum line. We observed slow gas evolution with the precipitation of a black solid in all the reactions. Nanoscale metallic copper was isolated from the resulting black suspension.
Results and Discussion The reaction of copper bromide and sodium borohydride in a variety of solvents results in the formation of a brownblack precipitate. Isolation solid under inert conditions resulted in very high yields of a black powder (Table 1). The nanoscale powders were characterized using X-ray powder diffraction (XRD) and electron microscopy in conjunction with energy dispersive electron microanalysis (EDS). The XRD data show that, in all the solvents used, the collected solid was exclusively metallic copper, except in the case of methanol where some CuO was also observed depending on the order in which reagents were combined. Figure 1 gives the spectral data for the product isolated in the reaction carried out in a sample series of alcohols. Phase analysis of the data show that all the lines observed in the spectra correspond to metallic copper. In addition, the EDS of the samples generally indicated that the percentages of copper in the samples were over 99 wt %
2324 Langmuir, Vol. 15, No. 7, 1999
Jackelen et al.
Figure 1. XRD of nanoscale metallic copper prepared in propanol and butanol.
with trace amounts of Na and Br (Table 3). Copper crystallite sizes ranging from 10 to 25 nm were calculated from X-ray data using the Scherrer equation, in which the breadth, B, of a diffraction peak located at 2θ is related to particle thickness, t as follows: t ) 0.9λ/B cos (θ), and λ is the wavelength of the X-ray. Scanning and transmission electron microscopy (SEM and TEM) pictures of the samples show that the materials generated in these reactions did not have well-defined form or structure (Figure 2). Particle sizes determined from TEM photographs ranged from 13 to 45 mm (Table 2). The DSC of a typical sample showed a single exothermic event at 480 °C corresponding crystallization of the nanoscale particles (Figure 3). Medium Effects on the Nature of the Products. The reaction of CuBr2 and NaBH4 resulted in very high isolated yields (Table 1) of nanoscale metallic copper in all the solvents used with exception of methanol. This suggests that for most solvents there are no competing reactions or reaction pathways, and if other reactions were occurring in the process, they did not interfere with the reduction process that yields nanoscale metallic copper. The reaction carried out in methanol was unique as indicated by the fact that the yields obtained in several trials were low. In addition, depending on the order in which reagents were mixed, the product generated was different. Metallic copper was obtained exclusively only when a methanol solution of sodium borohydride was added quickly to a methanol solution of copper bromide. A mixture of black and blue-green solid [presumably nanoscale metallic Cu and Cu{BHx(OCH3)4-x}2 or Cu(OCH3)2] was formed if the reaction was carried out in any other fashion. The low yields in the reaction described are a result of the obvious dissipation of NaBH4 apparent in the vigorous reaction between the solvent and the reagent (eq 1). This is apparent in the EDS analysis (Table 3) of the sample from the methanol reaction, which shows a significant amount of Br, an indication of the presence of CuBr2 that did not react in the product.
NaBH4 + 4CH3OH f NaB(OCH3)4 + 4H2
(1)
Medium Effects on Reaction Times. CuBr2-Soluble Solvent Systems. The reaction times given by Table 1 indicate that for the alcohols, which are polar, protic, and generally weakly coordinating solvents (Table 5), the reaction rate decreases with increasing alkyl chain length. Thus for methanol and ethanol instantaneous reactions
Figure 2. SEM photographs of nanoscale metallic copper: (a) prepared in propanol; (b) prepared in butanol.
were observed, whereas the reaction in butanol took 20 h. The decreasing trend in the reaction rates may be due to the decreasing solvent polarity from methanol to butanol (Table 5). Alternatively, the observed trend in the reaction rates in alcohol solvents may be related to the decreasing solvent acidity (Table 4). This trend suggests a mechanistic pathway that is facilitated by OH protons present in the alcohols. However, similar reaction times were observed for the reactions carried out in the aprotic solvent, THF, and butanol. These data will seem to suggest that other factors may also be important in determining the reaction times. In addition, the increasing reaction times in reactions mediated by alcohols parallels the increasing viscosity of the solvents (Table 5). The increase in viscosity presumably inhibits the mobility of the reactants thereby giving rise to longer reaction times.
Nanoscale Materials Synthesis
Langmuir, Vol. 15, No. 7, 1999 2325
Figure 3. Sample DSC of nanoscale metallic copper prepared in non-aqueous solvent. Table 5. Selected Physical Constants of the Solvents Used in This Study6a,8 solvent
Ea
DMb
CAc
visd
water methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol eiethyl ether tetrahydrofuran acetonitrile dichloromethane
78.5 32.6 24.3 20.3 17.5 13.9 13.3 4.3 7.3 36.2 8.9
1.85 1.70 1.69 1.68 1.66 1.7 1.55 1.15 1.75 3.92 9.08
6.10 1.30 0.75 0.73 0.75 0.85 0.86
0.89 0.54 1.08 2.00 3.38 3.35 4.59 0.25 0.55 0.38 0.45
1.00
a Dielectric constants at 25 C. b Dipole moment (debyes). c Coordination ability.9 d Viscosity coefficient × 103 (kg/m s).
For the nonalcohol solvents, the reaction times were apparently controlled by factors other than the polarity of the solvents. Significant differences in reaction times were observed for methanol, ethanol, and acetonitrile that are solvents with high dipole moments and dielectric constants (Table 5). For the aprotic solvents THF and acetonitrile in which the CuBr2 is soluble, the reaction proceeds much faster in the less polar solvent, THF. Acetonitrile, although polar, apparently stabilizes some form of an intermediate (perhaps a borohydride complex) better than tetrahydrofuran, THF. This suggests that in these two solvents the coordination ability, which may lead to stable intermediate complex, control the reaction times. CuBr2 Partially Soluble Solvent System. For the longer chain alcohols, pentanol and hexanol, as well as CH2Cl2 and Et2O, limited solubility required that we ran the reactions by adding the solvent to the solid reagents in a single flask. We observed even longer reaction times with increasing alkyl chain length in the alcohols. It is apparent that the polarity and viscosity effects are further compounded by the limited solubility. The reaction times observed in CH2Cl2 and Et2O are greatly dependent on the solubility of the reactants in the solvents. Medium Effects on Particle Sizes. Generally, the XRD crystallite sizes and the electron microscopy particle sizes increased with increasing reaction time. In the case of the alcohols, particle size increased as the alkyl chain length increased. The reactions carried out in pentanol and hexanol appear to be anomalies; however, these apparent anomalies are probably a simple function of the solubility of the reactants in the higher alcohols. CuBr2-Soluble Solvent Systems. The observed sizes
of the nanoscale copper generated indicate large crystallite and particle sizes in the systems in which adequate time was available for nucleation and growth. Also in the fast reaction more nuclei are likely to be present; more nuclei limit the available number of copper atoms in the growth step of the process. Thus for the series of solvent, HOCH3, HOCH2CH3, HOCH2CH2CH3, and HOCH2CH2CH2CH3, crystallite size increases from 13.4 to 17.4 nm, and TEM measured sizes range from 13.3 to 26.7 nm. The solvents HOH, HOCH3, and HOCH2CH3, in which instantaneous reactions were observed, yielded average copper particle sizes of 45.0, 21.0, and 13.3 mm, respectively. The XRD crystallite sizes of 14.8, 13.4, and 13.5 show similar but less pronounced decreases. These data suggests that the alkyl chain length limits particle growth for equivalent reaction times. Thus, for butanol and THF, for which equivalent reaction times, were recorded smaller particle and crystallite sizes (Table 2) were recorded for the copper samples generated in butanol. It appears as though the dangling four-carbon chain in butanol impedes crystallite and particle growth to a greater degree in comparison to the cyclic four-carbon unit in THF. A comparison of the solvent effect on particle size in the following solventssHOH, HOCH2CH3, and CH3CH2OCH2CH3, in which there is stepwise substitution of protons with ethyl groupssyields results that are somewhat distorted. The distortion is a result of the fact that the CuBr2 is only partially soluble in diethyl ether. The crystallite size of 9.63 mm observed for the copper sample is a direct function of the rate of Cu formation, which is limited by solubility of the reactants in diethyl ether. However, if THF (which has the same functional group and number of carbon atoms and is similar to Et2O in terms of the dipole moments and dielectric constant) is used in the above series of solvents, an increase in particle and crystallite sizes is observed from ethanol to THF. The reactants are soluble in the three media; however, the reaction in THF takes 21 h and yields nanoscale materials with larger crystallite and particle sizes. THF is much less acidic/polar than HOH or EtOH. It is reasonable to suggest that it stabilizes an intermediate that decomposes slowly or, because of its polarity, it stabilizes a nonpolar intermediate to a greater extent, allowing time for nucleation and growth of particles. However, as reaction time becomes very long as result of solvent ability to stabilize the intermediate (as seen in the case with acetonitrile) or limited solubility of reactants, crystallite, and particle sizes decreases. Medium Effect on Gas Evolution. CuBr2-Soluble Solvent Systems. In polar, protic, and relatively acidic solvents such as CH3OH and CH3CH2OH, the total amount of gas evolved in the reaction is nearly equivalent to the total amount of gas predicted by eqs 2, 3 and 4.
CuBr2 + 2NaBH4 f Cu + H2 + B2H6 + 2NaBr (2) B2H6 + 6ROH f 2B(OR)3 + 6H2
(3)
yielding
CuBr2 + 2NaBH4 + 6ROH f Cu + 7H2 + 2B(OR)3 + 2NaBr (4) Thus for a reaction carried out with 2 and 4 mmol of CuBr2 and NaBH4, respectively, we will expect 14 mmol of H2 gas. Analyses of the gas generated by gas chromatography indicate that hydrogen is produced in these reactions. Since these measurements were done in a static vacuum, any processes that are likely to introduce additional gases into the system will result in higher than expected values of evolved gas. Total volumes of gases of 4.1 and 8.1 mmol
2326 Langmuir, Vol. 15, No. 7, 1999
were obtained for the less acidic solvents, propanol and butanol, respectively. This suggests that in the less acidic solvents the transformation of borane to hydrogen is relatively slow. GC studies reactions carried out in diglyme shows the presence of borane. Also previous studies on the gases generated using mass spectroscopy has demonstrated the formation of borane.3b The gas evolution data indicates that very little conversion of borane to hydrogen occurs in the reaction carried out in propanol. A similar result was obtained in the reaction in acetonitrile. The absence of an OH proton makes the reaction described by eq 3 impossible for CH3CN and THF. The formation of THF-borane adducts in the reaction carried out in THF presumably further limits the observed total amount of gas.
Jackelen et al.
Summary This study demonstrates that there is a significant medium effect on the reaction rates and the total amounts of gaseous products formed in the borohydride reaction of copper(II) ions. The experiments show that the product of the reaction is largely solvent independent for the solvent systems explored. Even though the primary solid generated in all the reactions is metallic copper, there are differences in crystallite and particle sizes because of the differences in solvent properties and reaction times. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this research. The authors will also like to thank Dr. K. J. Klabunde for the very helpful comments and discussions. LA9807311
