NANO LETTERS
Intermetallic Phase Transformations during Low-Temperature Heat Treatment of Al/Ni Nanoparticles Synthesized within Thermally Evaporated Fatty Acid Films
2002 Vol. 2, No. 4 365-368
Chinmay Damle, Anamika Gopal, and Murali Sastry* Materials Chemistry DiVision, National Chemical Laboratory, Pune - 411 008, India Received November 26, 2001
ABSTRACT The phase evolution during low-temperature heat treatment of Al and Ni nanoparticles synthesized within thermally evaporated fatty acid films is described. Nanoparticles of aluminum and nickel were grown in thermally evaporated stearic acid (StA) films by immersion of the film sequentially in solutions containing Al3+ and Ni2+ ions followed by in-situ reduction of the metal ions to yield nanoparticles of Al and Ni of ca. 230 Å diameter within the fatty acid matrix. Thermal treatment of the StA − (Al + Ni) nanocomposite film at 100 °C resulted in the formation of a metastable η phase, Al9Ni2, which on further heating at 125 °C decomposed to form Al3Ni2.
The fact that nanoscale materials exhibit chemical and physical properties different from bulk materials, and that these properties could form the basis of new technologies has kindled a great deal of interest in the nano-realm.1 A variety of templates such as polymers,2 porous glasses,3 zeolites,4 self-assembled monolayers,5 and bio-templates such as the tobacco mosaic virus (TMV)6 and DNA7 have been used for the synthesis of nano-hybrid materials in the past decade. Organic thin films such as Langmuir-Blodgett (LB) films have also been investigated in fair detail for the insitu growth of metal/semiconductor/oxide nanoparticles either by chemical reaction or heat treatment of entrapped metal ions.8-11 Some years ago, we had shown in this laboratory that thermally evaporated fatty acid films such as arachidic acid when immersed in CdCl2 and PbCl2 salt solutions, exchanged the protons in the carboxylic acid groups with Cd2+ and Pb2+ ions in solution and, furthermore, that this process led to ordering of the originally disordered film into a lamellar, c-axis oriented structure similar to that obtained by the LB deposition method.12 Developing on this theme, we demonstrate herein the entrapment of Al3+ and Ni2+ ions in thermally evaporated stearic acid (StA, CH3(CH2)16COOH) films followed by the in-situ reduction of the metal ions to yield nanoparticles of Al and Ni within the fatty acid matrix. An interesting aspect of this study is the observation of lowtemperature alloying of the nanoparticles (at ca. 100 °C) * To whom correspondence should be addressed. E-mail: sastry@ ems.ncl.res.in. Ph: +91 20 5893044. Fax: +91 20 5893952/5893044. 10.1021/nl015676m CCC: $22.00 Published on Web 02/28/2002
© 2002 American Chemical Society
resulting in the formation of a metastable Al9Ni2 phase. Prolonged heat treatment at 125 °C resulted in the decomposition of this phase to form the intermetallic Al3Ni2. Considerable effort has been devoted in the last few decades to the study of the binary Al/Ni alloy system due to the technological importance of the intermetallic phases.13 Various techniques such as ion-beam mixing,14 reaction synthesis,15 electron beam evaporation,16 and mechanical alloying17 have been used in the synthesis and characterization of intermetallic compounds and alloys in the Al/Ni system. Because the first phase to form strongly influences the subsequent phase evolution, it is essential to identify the metastable phases in the early stages of annealing. Pohla et al. observed two metastable phasessmonoclinic Al9Ni2 and a decagonal phasesin the Al/Ni alloys synthesized by the arc melting method followed by high undercooling of the pure metals under argon.18 The metastable η phase, Al9Ni2, has previously been observed in multilayer samples prepared by ion beam deposition19 and in rapidly solidified microstructures.20 In this communication, we emphasize on the low-temperature phase evolution in the Al/Ni binary alloy system in thin lipid matrixes. That the initial metastable phase in this study was observed to form at the much lower temperature of 100 °C compared to the 350-400 °C range observed in multilayer films19 and bilayer studies21 is a salient feature of this investigation. Thin films of stearic acid [StA; CH3(CH2)16COOH, Aldrich, used as-received] of 500 Å thickness were thermally
Figure 1. (A) QCM mass uptake recorded ex-situ during Al3+ and Ni2+ ion incorporation in a 500 Å thick thermally evaporated StA film. The different cycles of ion exchange are indicated next to the respective curves. (B) FTIR spectra in the range 1500-1800 cm-1 recorded from an as-deposited 500 Å thick StA film on a Si (111) substrate (curve 1); the StA film after 1 cycle of immersion in Al2(SO4)3 solution (curve 2); the StA film shown as curve 2 after reduction of Al3+ ions and two cycles of Ni2+ ions incorporation (curve 3) followed by reduction (curve 4).
vacuum deposited in an Edwards E306A vacuum coating unit at a pressure of better that 1 × 10-7 Torr onto goldcoated AT-cut quartz crystals for quartz crystal microgravimetry (QCM) measurements and Si (111) wafers (for Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) measurements). One 500 Å thick StA film was also deposited on a carbon-coated transmission electron microscope (TEM) grid for TEM measurements. The film thickness and deposition rate were monitored in-situ using an Edwards FTM5 frequency counter. FTIR measurements of the StA films at different stages of ion incorporation and reduction were recorded on a Shimadzu FTIR-8201 PC instrument operated in the diffuse reflectance mode at a resolution of 4 cm-1. XRD studies were carried out in the transmission mode on a Philips PW 1830 instrument operating at 40 kV voltage and a current of 30 mA with Cu KR radiation. TEM measurements were carried out on a JEOL model 1200EX instrument operated at an accelerating voltage of 120 kV. Figure 1A shows the QCM mass uptake recorded as a function of time of immersion of a 500 Å thick StA film in 10-4 M Al2(SO4)3 solution. The pH of the electrolyte was adjusted to 6 so that maximum ionization of the carboxylic acid groups in the thermally evaporated StA film would occur thereby leading to maximum loading of the fatty lipid films with aluminum ions. The frequency measurements were made ex-situ after thorough washing and drying of the crystal. The change in the resonance frequency of the quartz crystal was measured using an Edwards FTM5 counter (resolution and stability of 1 Hz) and converted to a mass loading using the Sauerbrey formula.22 It can be seen that there is a rapid diffusion of Al3+ ions into the StA film and an equilibrium mass uptake due to the ions of ca. 5.8 µg/ cm2 was recorded. There is considerable overcompensation of the negative charge in the fatty acid matrix by the positively charged aluminum ions. This aspect is not well 366
understood at the moment but is a feature common to electrostatic layer-by-layer assembly protocols.23 On stabilization of the Al3+ concentration in the StA matrix, the metal ions were reduced in-situ by treatment with hydrazine vapor for a period of 1 h. Thereafter, the StA-(Al nano) film on the QCM crystal was immersed in 10-4 M NiSO4 solution at pH ) 6 and the mass uptake with time recorded in a similar fashion. It can be see from Figure 1A that the mass uptake due to Ni2+ ions (3.3 µg/cm2) is considerably less that that of the aluminum ions. Consequently, one more cycle of reduction of Ni ions (by hydrazine) followed by immersion in NiSO4 solution was done to enhance the concentration of Ni2+ ions in the film (Figure 1A), the mass uptake due to Ni2+ ions during this cycle being 1 µg/cm2. It is observed that the Ni2+ mass loading during successive cycles is reduced due to blockage of diffusion pathways by the already present nanoparticles in the film. The incorporation of Al3+ and Ni2+ ions in the StA film and their in-situ reduction is readily studied by FTIR spectroscopy. Figure 1B shows the FTIR spectra in the range 1500-1800 cm-1 recorded from the as-deposited 500 Å thick StA film on Si (111) wafer after various cycles of ion exchange and reduction. The carbonyl stretch vibration occurs at 1700 cm-1 in the as-deposited StA film (curve 1) and is characteristic of vibrations from the carboxylic acid group.24,25 The FTIR spectrum recorded from the StA film after one cycle of Al3+ incorporation is shown as curve 2 in Figure 1B. It is observed that on formation of the aluminum salt of stearic acid, the carbonyl stretch frequency shifts from 1700 cm-1 to ca. 1580 cm-1 clearly indicating the binding of the Al3+ ions with the carboxylate ions of StA. The shift in the carbonyl stretch frequency to lower wavenumbers followed by the disappearance of the 1700 cm-1 resonance is known to be a clear indicator of salt formation in such fatty acid films12,24,25 and indicates in this case complete aluminum stearate salt formation. Reduction of the Al3+ ions Nano Lett., Vol. 2, No. 4, 2002
Figure 2. (A) TEM picture recorded from a 500 Å thick StA film after various cycles of Al3+ and Ni2+ ion incorporation and reduction. Prior to measurement, the lipid matrix was removed by soaking the StA-(Al+Ni) nano composite film in chloroform for 15 min. (B) Particle size distribution histogram of the Al and Ni nanoparticles shown in Figure 2A.
in the aluminum stearate film followed by incorporation of Ni2+ ions resulted in the FTIR spectrum shown as curve 3 in Figure 1B. It is observed that the carbonyl stretch band now appears as a broad resonance centered at ca. 1600 cm-1 indicating the binding of the Ni2+ ions with the StA carboxylate ions and formation of the nickel stearate salt. Curve 4 in Figure 1B represents the FTIR spectrum recorded after reduction of the Ni2+ ions in the film shown in curve 3. The curve is essentially featureless, indicative of complete formation of Al and Ni nanoparticles within the acid film. The transmission electron micrograph recorded from a 500 Å thick StA film after one cycle of Al3+ and Ni2+ ion exchange and subsequent reduction with hydrazine is shown in Figure 2A. The StA matrix was removed from this composite film by soaking the film in chloroform for 15 min and carefully removing the TEM grid from the organic phase. A number of well-dispersed particles can clearly be seen in the TEM picture with a fairly even size distribution. The particle size histogram for this micrograph is plotted in Figure 2B. Although the individual aluminum and nickel nanoparticles cannot be distinguished, an average size of 230 Å was estimated for the nanoparticles from the TEM picture. XRD patterns recorded from a 500 Å thick StA film under different stages of ion exchange reduction followed by heat treatment are shown in Figure 3. The various curves correspond to a 500 Å thick StA film after immersion in Al3+ ion solution and reduction (curve 1); film shown as curve 1 after Ni2+ incorporation and reduction (curve 2); film shown as curve 2 after heating at 100 °C for 6 h (curve 3) and film shown as curve 3 after heating at 125 °C for 6 h (curve 4). Reduction of the aluminum ions by hydrazine treatment leads to the growth of the (111) Bragg reflection at a 2θ value of ca. 38.4° from the aluminum nanoparticles generated in-situ (Figure 3, curve 1, feature a). The size of the aluminum nanoparticles in this film was calculated from the broadening of the (111) reflection using the DebyeScherrer formula to be ca. 250 Å. The XRD pattern recorded from the reduced aluminum stearate film after two cycles of Ni2+ ion incorporation and reduction is represented as curve 2 An additional peak at a 2θ value of 44.2° is clearly Nano Lett., Vol. 2, No. 4, 2002
Figure 3. XRD patterns recorded from a 500 Å thick StA film after immersion in Al3+ ion solution and reduction (curve 1); film shown as curve 1 after Ni2+ incorporation and reduction (curve 2); film shown as curve 2 after heating at 100 °C for 6 h (curve 3) and film shown as curve 3 after heating at 125 °C for 6 h (curve 4). The features identified in the figure are discussed in the text.
seen in this diffraction pattern (feature b) and arises due to the (111) Bragg reflection from the Ni particles grown in the StA matrix. The size of the nickel particles was calculated from the broadening of the (111) Bragg reflection (feature b) to be ca. 220 Å and like the Al particles, the Ni nanoparticles are fairly large. These values are in good agreement with those estimated from the TEM studies (Figure 2). Even though the Al and Ni particles are well within nanoscale dimensions, we term the particles “large” because their dimensions are much in excess of the thickness of the StA bilayers (ca. 50 Å) in which they are embedded. There was no significant change in the size of the aluminum particles after formation of Ni nanoparticles indicating that the nickel particles nucleate and grow separately. The StA/(Al + Ni)-nano film was heated at 100 °C for 6 h and the XRD pattern obtained after this thermal treatment is represented by curve 3, Figure 3. A number of features corresponding to the η phase, Al9Ni2, can be seen (marked with *) in the XRD profile. These peaks agree well with those reported for multilayer films prepared by ion beam deposition19 and splat-cooled Al-Ni alloys.18 Prior to the thermal treatment at 100° C, we had measured the XRD spectrum from the StA/(Al + Ni)-nano film heated at 50 °C and 75 °C for 6h and did not notice any changes associated with the formation of an alloy phase. This may be a consequence of the fact that StA melts at close to 80 °C and heating to 100 °C provides sufficient mobility to the Al and Ni particles to diffuse within the StA matrix and form the intermetallic phase. After formation of the Al9Ni2 phase, the film shown as curve 3 in Figure 3 was heated for a further 6 h at 125 °C to check the stability of the intermetallic phase. The XRD pattern obtained from this film is shown in Figure 3 as curve 4. It is interesting to note the complete disappearance of the Al9Ni2 phase and the evolution of another intermetallic phase, Al3Ni2 (peaks shown with #). These Bragg reflections match well with those reported in the literature for this intermetallic phase.18,19,26 Apparently, 367
further heat treatment has resulted in the decomposition of the η phase, possibly due to an increase in the concentration of one of the components in the metastable phase and a consequent perturbation to the equilibrium of this phase. To the best of our knowledge, formation of the metastable η phase, Al9Ni2, followed by Al3Ni2 at nanoscale dimensions with significant lowering of the annealing temperature has hitherto not been observed. This result suggests that the high surface free energy of the nanoparticles is, to a large extent, responsible for this considerable lowering of the annealing temperature. It may also be possible that the StA matrix plays a role in the lowering of the alloy formation temperature and is another aspect that requires further investigation. We believe that it should be possible, in principle, to limit the degree of salt formation and consequently, the relative amounts and the size of the nanoparticles grown in thermally evaporated lipid films. This may significantly change the phase sequencing in the Al/Ni alloy system and is an aspect we are currently exploring. Acknowledgment. This work was partially funded by a grant from the Indo-French Center for the Promotion of Advanced Scientific Research (IFCPAR), New Delhi, and is gratefully acknowledged. The authors thank Mr. Rajesh Gonnade, Materials Chemistry Division, N.C.L, Pune for TEM assistance. References (1) See the Feb. 28, 2000 issue of Chem. Eng. News. and articles by R. Dagani therein for coverage of new applications envisaged for nanomaterials.
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(2) Beecoft, L. L.; Ober, C. K. Chem. Mater 1997, 9, 1302. (3) Justus, B. L.; Tonnucci, R. J.; Berry, A. D. Appl. Phys. Lett. 1992, 61, 3151. (4) Wang, Y.; Herron, N. J. Phys. Chem. 1988, 92, 4988. (5) Patil, V.; Mayya, K. S.; Sastry, M. J. Mater. Sci. Lett. 1997, 16, 899. (6) Shenton, W.; Douglas, T.; Young, M.; Stubbs, G.; Mann, S. AdV. Mater. 1999, 11, 253. (7) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 75. (8) Elliot, D. J.; Furlong, D. N.; Gegenbach, T. R.; Grieser, F.; Urquhart, R. S.; Hoffman, C. L.; Rabolt, J. F. Coll. Surf. A. 1995. 103, 207. (9) Taylor, D. M.; Lambi, J. N. Thin Solid Films 1994, 243, 384. (10) Paranjape, D. V.; Sastry, M.; Ganguly, P. Appl. Phys. Lett. 1993, 63, 18. (11) Amm, D. T.; Johnson, D. J.; Laursen Y.; Gupta, S. K. Appl. Phys. Lett. 1992, 61, 522. (12) Ganguly, P.; Pal, S.; Sastry, M.; Shashikala, M. N. Langmuir 1995, 11, 1078. (13) Alexander, D.; Was, G.; Rehn, L. J. Appl. Phys. 1991, 69, 2021, and references therein. (14) Eridon, J.; Was, G.; Rehn, L. J. Appl. Phys. 1987, 62 (5), 2145. (15) Morsi, K. Mater. Sci. Eng. A 2001, 299, 1. (16) Ma, E.; Thompson, C. V.; Clevenger, L. J. Appl. Phys. 1991, 69 (4), 2211. (17) Maric, R.; Ishihara, K. N.; Shingu, P. H. J. Mater. Sci. Lett. 1996, 15, 1180. (18) Pohla, C.; Ryder, P. L. Acta Mater 1997, 45 (5), 2155. (19) Edelstein, A.; Everett, R.; Richardson, G.; Qadri, S.; Atman, E.; Foley, J.; Perepezko, J. J. Appl. Phys 1994, 76 (12), 7850. (20) Tonejc, A.; Rocak, D.; Bonefacic, A. Acta Metall. 1993, 19, 311. (21) Colgan, E. Mater. Sci. Rep. 1990, 5, 1. (22) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206. (23) Decher, G. Science 1997, 277, 1232, and references therein. (24) Pal, S. Ph.D. Thesis, University of Poona, 1996. (25) Rabolt, J.; Burns, F.; Schlotter, N.; Swalen, J. J. Chem. Phys. 1983, 78, 946. (26) Powder Diffraction File (Inorganic Phases), International Center for Diffraction Data, 1989.
NL015676M
Nano Lett., Vol. 2, No. 4, 2002
