1098
J . Phys. Chem. 1991, 95. 1098-1 105
of the pressure-broadening cross section^.^' The ratios of the spin relaxation cross sections to the geometric cross sections are shown in Table VI. These ratios are small compared to those obtained for C 0 2 . It takes two or three “hard-sphere” collisions to cause a change in rotational angular momentum of the CHI molecule. Since the first nonvanishing electrical moment in CHI is the octopole moment, the long-range anisotropy of CH4 is weaker than that of C02, making CH, a less effective collision partner. The efficiencies of CH4-X collisions increase with increasing polarizability of X, except for the enhanced efficiency of CH4-CH4collisions, which may be attributed to the matching of rotational energy levels which increase the likelihood of rotational energy exchange. Empirical potentials for CH4-N2 and CH4-CO2 and CH4-He, Ne, Ar have been derived from IH relaxation times by Lalita et al.384 using the Bloom-Oppenheim theory of spin relaxation, with additional assumption^.^^ The correlation time of the spin rotation interaction for a given J state is assumed to be equal to the average lifetime of the molecule in that J state, the latter being calculated from transition rates obtained by using the “constant acceleration” approximation. Three parameters in the anisotropy were obtained while using effective t and ro from viscosity data in the isotropic potential. Our results lead us to question the reliability of their empirical potentials. It appears that their procedure is feasible only if the temperature dependence of T , differs markedly from T3I2. We find, however, that for CHI-X the deviations from T3i2 dependence are relatively small, smaller than those observed by us in C02-X or N2-X for almost all X.23*64Furthermore, the comparison of the temperature dependence reported for CH4-N2 and CH4-C02 by Lalita et al. with ours in Table IV indicates that there may have been some systematic errors in their mea(61) Trengove, R. D.; Robjohns, J. L.; Dunlop, P. J. Eer. Bunsen-Ges. Phys. Chem. 1984. 88.450-453. (62) Turfa, A. F.; Marcus, R. A. J. Chem. Phys. 1979, 70, 3035-3040. (63) Bloom. M.; Oppenheim, 1. Adu. Chem. Phys. 1967, 12, 549-599. (64) Jameson, C. J.; Jameson, A. K.; Smith, N. J. Chem. Phys. 1987,86, 6833-6838.
surements or data analysis. Their method of extrapolating to a composition that is 100%in the buffer gas could give rise to larger errors in the temperature dependence of the probe-buffer relaxation times. The ratio of the two cross sections obtained from NMR relaxation times, u0/uJ= 2.9 means that T* = ~ ~ / 2 in. 9the gas phase at 300 K. This can be compared with a limiting case of an extended diffusion model of fluids. Gordon’s model, which assumes that collisions change the angular momentum of the molecules and that the molecules rotate freely between collisions, has been applied to spherical top molecules.65 In this model rJ is the mean time between collisions that either randomize both the magnitude and the orientation of the angular momentum of the molecule ( J diffusion) or only its orientation ( Mdiffusion). For diffusion in the weak friction limit (low gas densities), McClung finds T~ = 7 J / 4 , 6 5in reasonably good agreement with our T~ = sJ/2.9 for CH, gas. ConcIu sions We have established that spin-rotation dominates both ‘H and I3C relaxation in CH4 in the gas phase. Therefore we can express our experimental results in terms of a single temperature-dependent cross section a,( T ) . This cross section is well-defined according to Gordon’s theory in terms of the scattering matrix elements for calculations on a trial potential energy surface. These aJ(7‘) can be used together with other experimental data such as virial coefficient^,^^ diffusion coefficients,I4 total differential scattering cross sections,”-13 collision-induced microwave absorption,44 collision-induced rotational Raman scattering,2 and the various cross sections given in Table V, to fit a CHI-CHI or CH4-X potential surface. Acknowledgment. This research was supported in part by the National Science Foundation (Grants CHE85-05725 and CHE89-01426). (65) McClung, R. E. D. J . Chem. Phys. 1969, 51, 3842-3852. (66) Jameson, C. J.; Jameson, A. K. J . Chem. Phys. 1990.93,3237-3244.
Preparation of Copper-Poly (2-vinylpyrldine) Nanocompositest A. M. Lyons,*’$S. Nakahara,t M. A. Marcus,$ E. M. Pearce,§ and J. V. Waszczakt AT& T Bell Laboratories, Murray Hill, New Jersey 07974, and Polytechnic Unioersity, Brooklyn, New York I I201 (Received: October 9, 1989; I n Final Form: August 21, 1990) Ultrafine copper particles were prepared in a poly(2-vinylpyridine) matrix by the thermal decomposition of a copper formate-poly(2-vinylpyridine) complex. The steric bulk of the polymeric ligand precludes complexation of more than one pyridine moiety around copper and ensures solubility of the complex. At temperatures above 125 OC, a redox reaction occurs where Cu2+ is reduced to copper metal and formate is oxidized to C 0 2 and H2. The decomposition reaction was studied by thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry. Copper concentrations up to 23 wt % have been incorporated into the polymer by this technique. The presence of the polymeric ligand induces the redox reaction to occur at a temperature 80 OC lower than in uncomplexed copper formate. Incorporation of the reducing agent (formate anion) into the polymer precursor enables the redox reaction to occur in the solid state. Films of the polymer precursor were prepared, and the formation of metallic copper particles was studied by visible and infrared spectroscopy, X-ray diffraction techniques, transmission electron microscopy, and extended X-ray absorption fine structure (EXAFS). Results from these measurements indicate that spherical copper particles with an average diameter of 35 8, are isolated within the polymer matrix. The particles are thermodynamically stable at temperatures up to the decomposition of the polymer matrix ( ~ 3 5 0 “ C ) but oxidize rapidly upon exposure air. Introduction Nanocrystalline composites are composed of ultrafine particles, with diameters ranging from 1 to 20 nm, dispersed in a matrix. ‘Taken from the dissertation of A. M. Lyons submitted to the Faculty of the Polytechnic University in partial fulfillment of the requirements for the degree Doctor of Philosophy (Chemistry), June I , 1987. AT&T Bell Laboratories. f Polytechnic University.
0022-3654/9l/2095-l098$02.50/0
These materials exhibit unique properties, different from normal crystalline or glassy materials,ls2 which can be attributed to the enormous surface area of the ultrafine particles. A metal particle several nanometers in diameter has roughly half of its constituent ( I ) Andres, R. P.; et al. J . Morer. Res. 1988, 4 , 704. (2) Birringer, R.; Herr, U.; Gleiter, H . Proc. JIMIS 4 (1986). Suppl. Trans. Jpn. Ins!. Met. 1986, 27, 43.
0 1991 American Chemical Society
Copper-Poly(2-vinylpyridine) Nanocomposites atoms on the surface. In addition, the small size of the particles may inhibit the formation of an electronic band structure. Properties resulting from quantum size effects have been observed by magnetization, electron spin resonance, nuclear magnetic resonance, and optical and infrared spectroscopy measurements. Several reviews of these subjects have been p u b l i ~ h e d . ~ - ~ A wide variety of synthetic approaches have been applied to the preparation of ultrafine metal particles systems. The earliest reports involve the formation of gold colloids in More recently, numerous techniques have been reported including the formation of nanocrystalline metals by the condensation of metal vapors onto a coldfinger,2*sevaporation of metal atoms into solvent mat rice^,^-'^ cosputtering metal and a matrix forming oxide,I4 sol-gel techniques,IsJ6 and the reduction of absorbed metal salts on Alternatively, nanocomposite materials can be prepared where ultrafine particles are dispersed in a polymer matrix. One of the first reports of metal dispersions in polymers was the preparation of cobalt colloids in solution by the thermal decomposition of cobalt carbonylsm Cobalt particles in the size range from 5 to 100 nm were formed. The particle size and size distribution were found to depend upon the polymer and solvent chemistry. Particle size decreased with increasing polymer polarity and molecular weight. When no polymer was present in solution, the largest cobalt particles were formed. Smith and co-workers21-22 studied a similar system, Le., the thermolysis of iron pentacarbonyl solutions, to form colloidal iron dispersions. The polymer was found to serve as a loci for the catalytic decomposition of iron carbonyl and subsequent particle nucleation. Active and passive polymers were identified on the basis of the ability of the polymer to complex with the iron precursor. In this system, half of the initial iron reacted to form ultrafine particles (5-1 5 nm). The remaining metal precipitated as relatively course particles. The formation of ultrafine transition metal particles in polymer solutions has also been investigated for metals such as copper,23 rhodium,24and platinum2s by Hirai and co-workers. In this work,
(3) Halperin, W. P. Reo. Mod. Phys. 1986, 58, 533. (4) Perenboom, J. A . A. J.; Wyder, P.; Meier, F. Phys. Rep. 1981, 78, 173. ( 5 ) Morokhov, I. D.; Petinov, V. 1.; Trusov, L. 1.; Petrunin, V. F. Sou. Phys. Usp. 1981, 24, 295. (6) Weyl, W. A . Colored Glasses; Society of Glass Technology: Scheffield, England, 1951. (7) Petrov, Yu. 1. Opt. Specfrosc. 1969, 27, 359. (8) Birringer, R.; Gleiter, H.; Klein, H . P.; Marquardt, P. Phys. Lett. 1984, 102A. 365.
(9) Marquardt, P.; Borngen, L.; Nimtz, G.; Sonnberger, R.; Gleiter, H.; Zhu, J. Phys. Leu. 1986, 114A, 39. (10) Kimura, K.; Bandow, S. Bull. Chem. SOC.Jpn. 1983, 56, 3578. ( I 1) Moskovits, M.; Hulse, J. E. J . Chem. Phys. 1977, 67, 4271. Moskovits, M. Acc. Chem. Res. 1979, 12, 229. (12) A h , H.; Charle, K . P.; Tesche, B.; Schulze, W. Chem. Phys. 1982, 68, 137. ( 1 3 ) Andrews, M. P.; Ozin, G. A . J . Phys. Chem. 1986, 90, 2929. (14) Liou, S. H.; Chien, C . L. Appl. Phys. Left. 1988, 52, 512. (15) Roy, R. A.; Roy, R. Mater. Res. Bull. 1984, 19, 169. (16) Hoffman, D.; Komameni, S.; Roy, R. J . Mater. Sci. Leu. 1984, 3, 439. ( I 7) Sinfell, J . H . Reo. Mod. Phys. 1979, 51, 569. (18) Roberts, 0.L.; Griffin, G. L. Appl. Surf. Sci. 1984, 19, 298. (19) Davis, S.C.; Klabunde, K. J. Chem. Reo. 1982, 153. ( 2 0 ) Hess, P. H.; Parker, P. H., Jr. J . Appl. Polym. Sci. 1966, 10, 1915. (21) Smith, T. W.; Wychick, D. J . Chem. Phys. 1980, 84, 1621. (22) Griffiths, C . H.; O’Horo, M. P.; Smith, T. W. J . Appl. Phys. 1979, 50. 7108. (23) Hirai, H.; Wakabayashi, H.; Komiyama, M . Bull. Chem. Soc., Jpn. 1986, 59, 367. (24) Hirai, H.; Nakao, Y . ;Toshima, N . J . Macromol. Sci. Chem. 1978, A12, I 117.
The Journal of Physical Chemistry, Vol. 95, No. 3, I991
1099
TABLE I:
Reactant Specifications for the Preparation of Cu( HC00)2:P2VPy Complexes
complex (Cu:Py ratio)
CU(HCO,),. 2H,O, mol
PZVPY, mol
1 :4 1:2
1 . 2 5 x 10-3 2.50 x 10-3
5.00 x 10-3 5.00 x 10-3
methanol, m L 50 1
ow
‘50 mL of methanol used for low-MW polymer.
a solution of a metal salt (e.g., CuS04) is reduced in the presence of poly(N-vinyl-2-pyrrolidone)or other polymers, by a reducing agent such as sodium borohydride (for copper) or methanol (for the platinum metals). A polymer-protected metal colloid results with particle diameters in the range from 5 to 15 nm. This paper describes a unique method for the preparation of copper-polymer nanocomposites in the solid state (initial results were previously published).26 A soluble precursor was synthesized by complexing poly(2-vinylpyridine) with copper formate in methanol. Thermal decomposition of the complex results in a redox reaction whereby Cu(I1) is reduced to copper metal and the formate anion is oxidized to CO, and H2. By incorporating a reducing agent into the complex, the thermal decomposition reaction is not diffusion limited, and nanocrystalline copper particles could be prepared in the solid state. Thermal analytical techniques, such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), TGA mass spectrometry (TGA-MS) as well as temperature-programmed visible and infrared spectroscopy, X-ray diffraction, transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) were used to characterize the formation of the copper-polymer nanocomposite. Experimental Section
Materials. Poly(2-vinylpyridine) (P2VPy) was purchased from Aldrich Chemical Co. (MW 36000) and from Polysciences Inc. with a nominal molecular weight of 300000-400,000. In both cases, the polymer was purified by two precipitations from ethanol into deionized water, ground to a fine powder, dried under vacuum, and characterized by nuclear magnetic resonance and IR spectroscopies. Poly(4-vinylpyridine) (P4VPy) was used as received from Aldrich. Cu(HCO2)92H20was used as received from Pfaltz and Bauer. The copper salt was characterized by X-ray diffraction, and the water content of the material was verified by TGA. Preparation of Polymer Complexes. Complexes of different ratios of Cu to pyridine moiety were formed in freshly distilled methanol. High molecular weight P2VPy was employed except where noted. The concentration of reagents was varied as shown in Table I . Complexes were prepared by addition of Cu(HC02)2.2H20crystals directly to stirred methanol solutions of the polymer. Solutions of C U ( H C O ~could ) ~ not be used in the concentration range of interest as they were found to be unstable, resulting in the precipitation of anhydrous cupric formate. The stability of the complexes over time was found to be a function of P2VPy molecular weight. Complexes prepared with low molecular weight P2VPy remained soluble for several days, whereas complexes prepared with the high molecular weight polymer formed a small amount of C U ( H C O ~precipitate )~ during the first hour after addition of the copper salt. These solutions were gravity filtered through medium-fine paper and 0.4-rm syringe filters and remained stable to further precipitation for 1-2 days after which additional precipitation occurred. After 24 h of stirring, the complexes were isolated by freezing the solutions in liquid nitrogen and removing the solvent under a vacuum of =IO0 HmHg as the solution thawed. Films were prepared on either quartz or CaF, substrates by pipeting a few drops of solution on the substrate and evaporating the solvent under vacuum. Physical mixtures were prepared by grinding Cu(HCO2),-2H2Oand P2VPy ( 1 :2 (25) Hirai, H . J . Macromol. Sci. Chem. 1979, A13, 633. Hirai. H.; Nakao, Y.; Toshima, N . J . Macromol. Sci. Chem. 1979, A13, 727. (26) Lyons, A . M.; Nakahara, S.;Pearce, E. M. Mater. Res. Soc. Symp. froc. 1989, 132, I I I .
Lyons et al.
1100 The Journal of Physical Chehistry, Vol. 95, No. 3, 1991
ratio) in a mortar and pestle until a fine powder of homogeneous blue color was obtained. Chemical analysis of the complexes was performed by Schwartzkopf Laboratories, Woodside, NY. The magnetic susceptibility of the copper complexes was measured from 4.2 to 300 K by the Faraday method as previously described.27 The average effective moment per Cu atom (pelf) was obtained from C, by the equation p L e=~(8C,MW)1/2,where MW is the molecular weight of the complex. Spectroscopic Studies. Electronic spectra were recorded on either a Perkin-Elmer Model 575 or a Hewlett-Packard Model 8452A diode array spectrophotometer. Infrared spectra were recorded on a Nicolet Model 5DX Fourier transform spectrophotometer as mulls in Nujol or fluorolube on KBr or as films prepared on CaF, substrates as described above. Temperature-programmed spectroscopy was performed in the visible and the infrared regions. Films prepared on the appropriate substrates were mounted into a heated cell (Janis Research) equipped with either quartz or calcium fluoride windows. Experiments were conducted under both static and dynamic vacuum at 1.5 X Torr. A heating rate of 10 OC/min was used for the UV-visible experiments. Samples were heated by using a stepwise function for the infrared studies; the films were heated at IO OC/min to 100 OC, held a t that temperature for 15 min, ramped to I50 OC, held for 15 min, and finally ramped to 200 OC and held for 15 min before cooling to room temperature. Thermogravimetric Analysis ( T G A ) . The TGA experiments were performed on a Perkin-Elmer TGS-2 thermobalance in N2 purified by passing the gas over molecular sieves, followed by an Oxyclear purifier, purchased from Diamond Tool and Die. A flow rate of ==30 cm3/min and a sample size of either 10 or 30 mg were employed. A heating rate of IO OC/min was used except where noted. In certain experiments, an lnficon Model 14200 quadrupole mass spectrometer was connected to the thermobalance by a 10-ft, differentially pumped, 0.020-in.-i.d. stainless steel capillary tube. The capillary was heated to ~ 2 0 0OC with a heating tape to prevent condensation. I n this manner, the volatile species generated during decomposition of the sample in an inert gas ambient were monitored. Helium (Matheson 99.999%) was used for the TGA-MS experiments. The helium flow rate through the balance was increased to 80 cm3/min to compensate for the consumption of gas by the sampling system. Data were collected and plotted by using a Teknivent Vector/One interface with an AT&T 631 2 PC . Differential Scanning Calorimetry (DSC). DSC was performed on a Perkin-Elmer DSC-4 calorimeter in 99.999% N2 (purchased from Matheson Co.) at a flow rate of ==30 cm3/min and heating rate of 10 OC/min. Samples were weighed into aluminum volatile sample pans that were crimped closed and then punctured with a pin. Samples of P2VPy were heated to 200 OC and cooled to 50 OC before glass transition temperatures were measured. X-ray Diffraction. The composition of the polymer complexes was investigated by X-ray diffraction using a Phillips automated powder diffractometer. Samples were prepared as films on oxidized silicon substrates that were heated in a Lindburg tube furnace at the indicated temperature for 1 h. Diffraction patterns were recorded in air, immediately after removing the samples from the furnace. Transmission Electron Microscopy (TEM). TEM micrographs were taken of the 1 :2 copper formate complex using a Phillips 420 electron microscope operated at the accelerating voltage of 120 kV. Samples were prepared by placing one drop of the 1 :2 solution (see Table I) onto a 3-mm copper grid coated with a 200-A carbon film. The sample was pyrolyzed directly in the Torr by using a heating stage TEM under a vacuum of = I X equipped with a type S (Pt, 10% Rh) thermocouple. The sample was heated through a series of steps at 46, 76, 1 1 1, 150, 193, 237, and 306 O C over several hours. Particle size distributions were determined by using a Joyce-Loeble imaging system, the accuracy (27) Lyons, A . M.; Vasile, M . J.; Pearce, E. M.; Waszczak, J . V. Mocromolecules 1988, 21. 3125.
TABLE If: Elemental Analysis@Results of the 1:2 Complex and PZVPy Compared to Calculated Values compd C H P2VPy 78.4 (80.0) 6.6 (6.7) 1:2 Cu(COOH),:P2VPy 51.7 (52.8) 4.5 (4.4)
N 12.9 (13.3)
cu
7.6 (7.7)
16.5 (17.5)
"Observed (calculated), weight percent.
TABLE 111: Magnetic Susceptibility Data for the PZVPYCopper Formate Complexes
complex
fit, K
xo X IO6
C. X lo3
6
1:4 1:2
20-300 25-300
-0.093 0.251
0.469 0.836
8.7 14
g.frgB
1.47 1.56
05-
--.O.O
460
258oDC 500
RATE' I O W M I N
600 700 WAVELENGTH (nml
BOO
AMBIENT- VACUUM
Figure 1. Visible spectra of the 1:2 complex recorded from 25 to 230 O in a static vacuum.
C
of which was verified by manual measurements. Extended X-ray Absorption Fine Structure (EXAFS). The EXAFS was done at NSLS, Beamline X-I5A, using Si 11 1 crystals. The sample was prepared by dropping a solution of the complexed polymer onto Be foil coverslips and decomposing the film under vacuum ( Torr) at approximately 200 OC. The slips were then sealed into sandwiches in a drybox and stored under vacuum until use. Fluorescence detection was used for the sample and a Cu-N model compound, Alcian blue. This compound is a copper phthalocyanine derivative with side groups to render it water soluble. The model was prepared by dispersing it in polystyrene, being careful to avoid clumps large enough to exhibit self-absorption. For Cu-Cu, the model was Cu foil; photoelectron detection was used. Results Preparation of Polymer Complexes. Upon addition of Cu(HCO2),-2H2Oto P2VPy, the colorless solution turns a deep bluegreen. Bright blue hygroscopic powders and transparent, continuous, blue films were isolated with nominal copper to pyridine ratios of 1:4 and 1:2. No cross-linking of the P2VPy systems occurred as the complexes could be redissolved after isolation. Addition of cupric formate to P4VPy solutions, however, formed cross-linked insoluble blue gels. Results from the chemical analysis of P2VPy and the l:2 complex are in good agreement with theoretical values as shown in Table 11. Note that the weight percent values observed for carbon are low for both samples analyzed. Magnetic susceptibility results are shown in Table I11 for the 1:2 and 1:4 complexes. Moments of ~ 1 . pB 5 were calculated, which are low compared to the spin-only value of 1.73 pB for Cu(1l). We observed similar values of the magnetic moment for copper in CuClz complexes with P2VPy.,' Electronic Spectra. All spectra recorded of the complexes in solution exhibit only one broad, weak peak in the visible region. M cupric formate The maximum in the visible spectra of 3 X solutions shifts from 735 to 730 nm upon complexation (1:2 ratio) with P2VPy; however, the exact position of this peak is dependent upon solution concentration. Spectra of films of the 1:2 material are qualitatively similar, with a broad, multicomponent, absorption band observed with a maximum at approximately 740 nm. Spectra of a 1:2 film heated from 25 to 230 OC in static vacuum are shown in Figure 1. No change in the spectra is observed below
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1101
Copper-Poly(2-vinylpyridine) Nanocomposites
-.-
3n
1.8 \
E f
1 m 4
1.6 1.4
-----
\
-
\
\
-
23OOC XUAX 554 nm
4.2 -
1.0
0 c0 '
I
I
I
I
200.C 45min
640
Figure 2. Shift in the absorption maximum of a 1:2 complex film upon cooling from 230 "C. 1.6 t
I
I
I
I
V
I
I
1
I
I
is00 1400 WAVENUMBERS (em-9
1700
I 1300
1600
Figure 4. Fourier transform infrared spectra of the 1:2 complex between copper formate and P2VPy recorded from 100 to 200 "C.
TIME (mlnr.)
Figure 3. Shift in the absorption maximum vs time (solid line) and
decrease in the absorbance at 564 nm vs time (dashed line) of a 1:2 complex film exposed to air after heating to 180 "C in a dynamic vacuum. 80 O C . At 125 OC, films turn a deep red color and the spectra exhibit an intense ultraviolet absorption, with a shoulder at 4 5 0 nm. The intensity of the shoulder increases with increasing temperature until a well-defined maxima at 554 nm appears in the temperature range 180-230 OC. When the film was cooled, the intensity of the maxima increases, and the position shifts to 568 nm, as shown in Figure 2. For a sample prepared under dynamic vacuum (maximum temperature 180 "C),the position of the maxima after cooling occurs at 564 nm. Spectra of uncomplexed P2VPy show no absorption in the visible for films heated to temperatures from 30 to 350 OC. Heating bhe films to higher temperatures resulted in further changes. At 350 OC, the color of bulk copper began to appear. Reflective, continuous copper-colored films were formed after pyrolysis at temperatures 2500 O C . After the films were cooled to room temperature, air was introduced into the cell. This results in the rapid oxidation of the copper particles and significant shifts in the position and intensity of the visible absorption spectra. These shifts are plotted in Figure 3 for a film prepared at 180 OC under dynamic vacuum. An exponential decrease in intensity of the absorption at 564 nm is observed. The position of the maximum increases from 564 to 605 nm in 15 min and then levels off at 61 5 nm at longer times. Infrared Spectra. Infrared spectra of the 1 :2 complex heated under static vacuum are shown as a function of time and temperature in Figure 4. As prepared, broad vibrations due to the formate anions are observed at -1600 (s) and 1360 (m) cm-'. Complexation of copper by the pyridine moities is indicated by shifts in the ring vibrations which are on the order of 2-3 cm-'. The magnitude of these shifts is much smaller than those found ~' may indicate weak for CuC12 complexes with P ~ P V Y ,which complexation. The spectra of the complex does not change significantly, even after heating at 100 O C for 15 min. When the temperature was increased to I50 OC, significant changes in the spectra occurred due to loss of formate. The evolution of formate is almost complete after I min, and no formate is observed after 15 min at this
Ob
e'0
1;O
?.io 3;O
460'480 560 6;O TEMPERATURE ( C )
720 A 0
Figure 5. Thermograms of P2VPy and complexes between copper formate and P2VPy (1:4 and 1:2 ratios).
loo,ool -7 I I-
'Y
aQ
87.50-
-
-
Figure 6. Thermogram of a 1 :2 complex ( IO-mg sample) showing the two weight loss regions that occur before polymer decomposition. Insert shows a scale expansion of 2.5.
temperature. No further changes are observed after heating to 200 OC for 15 min; however, water is observed at higher temperatures due to desorption from the heated cell surfaces. It is interesting to note that the vibrations of the polymercopper composite are unchanged from the vibrations of the polymer alone. The lack of observable shifts in the IR frequencies of the polymer indicates that bonding between the pyridine groups and the copper particles is either extremely weak or infrequent. ThermogravimetricAnalysis ( TGA). Thermograms for P2VPy and the 1:4and I :2 complexes are shown in Figure 5. Moisture absorbed on the samples ("2 wt %) was previously removed by standing in N2 for 2 h at 30 OC in the thermobalance. P2VPy is stable to elevated temperatures in N2;an extrapolated decomposition onset temperature of 396 OC is observed. A new weight loss event is observed for the complexes with an extrapolated onset
1102 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 TABLE IV: Weight Loss of the Copper Formte:PZVPy Complexes Recorded by TGA in the Temperature Regimes from 35 to 220 OC and from 35 to 300 "C' HCOO, wt % obsd complcx MW calcd 220 OC 300 "C I :4 ~3.X 5 IO5 15.7 14.3 15.1 3.6 x 104 13.9 14.9 I :2 ~ 3 . X5 IOs 24.8 22.9 24.1 3.6 x 104 23.9 25.4
Lyons et al. 4
4
501
SCAN RATE: I O D E G / M I N
-
w V
25-
"Values are compared to the calculated quantity of formate in the complcxes. I
f
i FFORMATE O R M A T i DECOMPOSITION
~
1
WT: 15 50 mq
60
80
120
100
140
TEMPERATURE
160
I80
200
(OC)
Figure 9. DSC of the 1:2 complex with a total sample size of IO mg.
----\/
0.80 WT: 9 m Q SCAN RATE: 10 DEG/MIN
g w
g
COMPLEX+\,
85
$ 0.40
9 ' 0
30
60
90 120 150 180 210 240 270 300 TEMPERATURE ('C)
Figure 7. Thermograms of a I :2 complex and physical mixture of Cu(HC02)2.2H20with P2VPy (1:2 ratio); IO mg of each material was used.
0.00
t ~
60
80
I00
120 140 160 TEMPERATURE ('CI
I80
200
Figure 10. DSC of a 1:2 complex (15.5-mg total initial weight, 9.0 mg of P2VPy) and P2VPy (9.2 mg). The 1:2 complex was previously heated to 200 OC to decompose the complex, then cooled, and immediately reheated to 250 'C. The P2VPy sample was previously heated to 200 "C and cooled to 50 OC before the curve was recorded. A TBat 1 IO "C was observed for the Cu-P2VPy nanocomposite, whereas a T, of 105 OC was observed for pure P2VPy.
20,000 8,000
t
> 45,000 CARBON MONOXIDE
Differential Scanning Calorimetry (DSC).Results for the 1:2 copper formate:P2VPy complex are shown in Figure 9. An exothermic transition begins at approximately 110 OC, which is followed by an intense endotherm. As for the complexes between CuCI2 and P ~ V P Ythe , ~exotherm ~ is attributed to the reduction of copper, but in this case, the reduction is complete from Cu" to Cuo. The subsequent endotherm results from the evolution of C 0 2 and H2. No Tg was observed for the complex, even if the sample was previously annealed at 120 OC in the calorimeter.. Either the transition resulting from the onset of the redox reaction obscures the more subtle glass transition, or the Tg for the complex occurs at higher temperatures where the redox transitions dominate (Le., above 120 "C). This is consistent with the glass transition temperature of copper chloride complexes with P2VPy, which occurs at 130 0C.27 When the sample used in Figure 9 was cooled to 50 OC and rerun, the Tg of the P2VPy-copper composite was observed at 110 OC; this is a 5 OC increase over the Tg of the uncomplexed polymer (105 "C) as shown in Figure 10. In addition, the breadth of the transition region is greater. X-ray Diffraction. X-ray diffraction patterns for films pyrolyzed at 200, 250, and 350 OC for 1 h are shown in Figure 1 1. No peaks, other than those attributed to the silicon substrate, are observed in the X-ray diffraction patterns for films pyrolyzed at 200 O C ; however, at 250 O C , a broad copper peak begins to be visible. Films pyrolyzed at 350 OC and higher result in X-ray patterns with sharp peaks attributed to copper metal. Transmission Electron Microscopy (TEM). A micrograph of the 1 :2 copper formate complex, pyrolyzed to 306 OC, is shown in Figure 12. The particles are uniformly distributed throughout the film and isolated from one another. A distribution of particle sizes was measured, with a mean particle diameter of 35 A, as shown in Figure 13. Selected area diffraction results (Figure 14) demonstrate that these particles are crystalline. Diffuse rings, characteristic of noncrystalline materials, were observed for the
n 20*om
65,000 CARBON DIOXIDE 40,000 '44 a ) 50
100
150
200
TEMPERATURE ("Cl RATE I O W m i n
AMBIENT HELIUM
Figure 8. Evolution of selected ions for a 1:2 complex, recorded by mass spcctromctry. vs decomposition temperature in the thermobalance.
temperature of 125 OC. A second, much smaller, event occurs at =250 O C as shown in Figure 6 . The weight lost in the these regions corresponds closely to the calculated formate content for each complex, as shown in Table IV. The weight lost at 250 O C increases the proximity of the calculated and observed values. The onset temperature for formate decomposition in the complexes is 80 OC lower than for either Cu(HC02),.2H20 or a 1:2 physical mixture of Cu(HC02),.2H20with P2VPy a s shown in Figure 7. The decomposition of formate to C 0 2 and H2 was confirmed by mass spectroscopy. Peaks at 2 and 44 amu, corresponding to H2 and C02, were observed at 130 OC in the thermobalancemass spectrometer system as shown in Figure 8. No evidence of CO was observed. At 250 "C, small quantities of H2 (2 amu) were detected. The second principle region of weight loss of the complexes occurs at approximately 400 OC and is attributed to P2VPy decomposition .28.29 (28) Lyons, A. M.; Pearce, E. M.; Mujsce, A . M. J . Polym. Sci. 1990, 28, 245.
(29) Lyons, A. M.; Mujsce, A. M. Manuscript in preparation.
Copper-Pol y (2-vi n y I pyrid i ne) N anocom posi tes
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1103 60
200oc
49.8
n
16.5
13.2 I-
€
I
Ill
250OC
a lL
t
20
6.6
40
3.3
OO
4 0 2 0 30 4 0 50
6 0 10 8 0 90 100
PARTICLE DIAMETER
(8)
Figure 13. Distribution o f particle sizes determined from Figure 12.
COPPER
I 40.0
43.0
46.0 49.0 DEGREES (28)
52.0
55.0
Figure 11. X-ray diffraction patterns for films o f the 1:2 complex pyrolyzed at 200,250. and 350 "C. T h e pattern for bulk copper metal is shown for comparison.
Figure 14. Selected area diffraction pattern for the area shown in Figure
12.
Figure 12. TEM micrograph o f the 1:2 formate complex pyrolyzed to 305 O C .
films before heating. Thus there is no evidence for the presence of copper formate crystallites, or copper metal, before pyrolysis. Extended X-ray Absorption Fine Structure (EXAFS). The analysis was done via the standard AT&T least-squares fitting methodsm We were able to analyze shells 1-3 for Cu-Cu. The results for the first shell were r - rbulk E [-o.o~s, -0.001A], coordination number indistinguishable from 12. For the first shell, we were marginally able to pick up Cu-ligand pairs. The second shell also came in with near-bulk distance and strength. The third shell showed a coordination number 70% that of the bulk value and a distance expansion of between 0 and 0.01 7 A. The major difference between the Cu-Cu shells of bulk and particle is that the mean-square relative displacement u2 was greater in the particles than in the bulk by about 0.002 A.2 With care in filtering and fitting, a Cu-ligand shell could be detected at a distance of 2.08 f 0.05 A. This shell had an average Cu-ligand coordination of 0.4, to within a factor of 3. The uncertainty is due to the small magnitude of the Cu-ligand contribution relative to that of Cu-Cu and to the coupling of coordination number to mean-square relative ~
~~
(30) Lee. P. A.; Citrin. P. H.; Eisenberger, P.; Kincaid, B. M. Reu. Mod. Phys. 1981, 53. 769.
displacement typical in least-squares fits of EXAFS data, especially with regard to minor shells. The Cu-ligand bond distance detected in the third shell may be attributable to either Cu-nitrogen or Cu-oxygen interactions. The bond distance, 2.08 f 0.05 A, is somewhat greater than that observed for either CU-N or Cu-O bond distances, which are both approximately 2.0 A. For example, the Cu-N bond length is I .934 A in Alcian Blue (which was taken to be the same as in copper phthalocyanine) and 1.98-2.02 A in copper bis(2-methylpyridine) di~hloride.~'Also, for most oxygen-containing species absorbed on copper surfaces, the Cu-O distance is approximately 2.0 From this experiment it is not possible to distinguish between absorption of pyridine moities onto the surface of the copper particles and partial oxidation of copper. That the spectra were so similar to those in the bulk is probably due to polydispersity, together with the mass weighting characteristic of EXAFS. Also, if the smaller clusters are more disordered than the large ones, their contributions to the EXAFS are proportionately small. Therefore, the average size as EXAFS sees it is much larger than the number-average particle size as measured by TEM.
Discussion Structure of the Complexes. A weak complex is formed between cupric formate and P2VPy as indicated by the small shifts in the visible and infrared spectra as well as the enhanced solubility (31) Jeter, D. Y.; Hodgson, D. J.; Hatfield, W. E. Inorg. Chim. Acru 1971, 5, 251. (32) Crapper, M. D.; Riley, C. E.; Woodruff, D. P.; Puchmann, A.; Haase, J. Surf. Sci. 1986, 171, I . Outka. D. A.; Madix. R. J.; Stohr. J. Suq. Sci. 1985. 164, 235.
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The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
Lyons et al.
copper metal and formate is oxidized to C 0 2and H2. This reaction was observed by changes in the UV-visible and infrared spectra and by transitions in the calorimeter, thermobalance, and mass spectrometer. The formation of C02and H2 during the thermal decomposition has been observed for all forms of copper formate i n ~ e s t i g a t e d . ~In~ ,one ~ ~ study, the C 0 2 / H 2ratio was found to decrease during the course of the reaction.43 This change was L JI attributed to the formation of HCOOH as an intermediate. In Figure 15. Proposed structure of the 1:2 Cu(HCOO),:PZVPy complex. our work, the formation of CuH as an intermediate could be inferred as H2was detected in the TGA-MS system at 250 "C. of copper formate in P2VPy-methanol solutions. The coordination It is interesting to note that even though a Cu-0 bond exists geometry about the copper atom is difficult to determine precisely, in the complex, no Cu,O was detected in the char. This did not however. In most cupric formate compounds, each copper atom occur in complexes prepared between poly(acry1ic acid) and is coordinated to four bridging formate anions in a square-planar copper(Il), where a mixture of Cu and C u 2 0was formed.44 This arrangement.33 In the presence of heterocyclic amines, an observation is reasonable, considering the carboxylate ligand is elongated octahedral geometry occurs about the copper, with the covalently bonded to the polymer and thus cannot be volatilized amines occupying positions above and below the square plane.34 (e.g., as C02) without breaking a C-C bond. In some cases, heterocyclic amines induce dimer formation where The particle size distribution of the copper particles formed in two copper atoms are bonded together by bridging formate groups the TEM reveals a mean particle diameter of 35 A. Although in the syn-syn configuration and the amines occupy the terminal the results for such a thin film (
