Application of Fourier Transform Infrared Spectroscopy to

Chapter 22

Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch022

Application of Fourier Transform Infrared Spectroscopy to Nanostructured Materials Surface Characterization Study of an Aluminum Nitride Powder Prepared via Chemical Synthesis 1

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Marie-Isabelle Baraton , Xiaohe Chen , and Kenneth E. Gonsalves 1

Laboratory of Ceramic Materials and Surface Treatments, Unité de Recherche Associée 320, Centre National de la Recherche Scientifique, University of Limoges, 123 Avenue Albert Thomas, F-87060 Limoges, France Polymer Science Program at the Institute of Materials Science and Department of Chemistry, University of Connecticut, Storrs, CT 06269 2

Due to unsaturation or strain, equilibrium of the interatomic forces on a surface is reached by adsorption of surrounding molecules. Therefore, the chemical composition of thefirstatomic layer may be different from that of the bulk. The importance of the surface with respect to the bulk in nanostructured powders makes the exact knowledge of the surface composition critical. Fourier transform infrared (FT-IR) spectrometry is a powerful tool to determine the nature of the chemical surface species as well as the reactive sites. As an example of an FT-IR surface study, a nanostructured aluminum nitride powder was analyzed and its surface was compared with the γ-alumina surface.

The surfaces of oxide nanopowders have been extensively studied either for characterization purposes or for potential catalytic properties investigation. As for non-oxide ceramic powders, references on their surface studies are scarce. Due to hydrolysis in ambient atmosphere, the unavoidable presence of oxygen in the first atomic layer of these nanostructured powders may drastically modify their expected properties. After briefly introducing our characterization technique of nanosized powder surfaces, we will present as an example, the Fourier transform infrared (FTIR) surface analysis of a nanostructured aluminum nitride powder obtained via sol-gel type chemical synthesis.

0097-6156/96/0622-0312$15.50A) © 1996 American Chemical Society

In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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BARATON ET AL.

FTIR Spectroscopy and Nanostructured Materials


General Scope Importance of the surface characterization for nanostructured powders. In the bulk of materials, the interatomic forces are balanced whereas on the surface dangling bonds and defects lead to unsaturation or strain. As a consequence, equilibrium may be reached either by rearrangement of the surface atoms or by adsorption of surrounding molecules. Thus the chemical composition of the first atomic layer of any material is different from that of the bulk. The surface atoms can be present as free bonds, free bonding orbitals with affinity to electrons or occupied bonding orbitals with a low ionization potential (/). Thus a wide range of different types of adsorption centres may result on a given surface. Moreover these centres determine the nature of the adsorption and, in return, the adsorbed molecules may change the properties of the surface. On the other hand, the nature of these adsorption centres and their relative concentration depend on the synthesis and collection conditions of the materials. All these remarks bring emphasis on the complexity of a surface structure and the close interdependence of the influencing parameters. In the case when the specific surface area of the powder is increased, the surface plays a non negligible part in the overall powder properties, and the need for a specific surface characterization then becomes critical. Many technological processes depend on the chemical composition of these surfaces and the increasing production of nanostructured powders raises the demand for well controlled surfaces. FT-IR Surface Spectrometry. Fourier transform infrared spectrometry is one of the most convenient techniques for the surface characterization of ceramic nanopowders. Indeed, the vibrational spectra bring information on the nature of the bond formed between the surface and the adsorbed molecules and consequently on the nature of the adsorption centres. Moreover, chemical species irreversibly grafted on the surface have specific absorption bands in the powder spectrum, and are considered as intrinsic probes since they may be perturbed by molecular adsorption. To fully characterize a surface, the experimental process summarized in Flow Chart 1 must be followed. During the first step, the surface is activated. The activation consists in heating the sample under dynamic vacuum for one or two hours. This thermal treatment cleans the surface from physisorbed and weakly chemisorbed species according to the temperature. Since hydrolysis is the most probable reaction to produce saturated bonds and balanced forces on any surface, many surface bonds involve hydrogen atoms. Consequently, an isotopic exchange H/D by deuterium addition will discriminate between hydrogen-containing groups located on the surface and the ones in the bulk. Moreover the vibrational frequencies of the exchanged groups shift toward lower values due to the higher molecular weight of deuterium. In other words, deuterium acts as a marker of the hydrogen vibrations. Methanol is a useful probe molecule to check the lability of OH surface groups. The reaction of methanol on these O H groups leads to the formation of methoxy groups. But other adsorption processes can also occur when acidic and basic sites are present on the surface.


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Characterization Process of Surface Species on Nanosized Powders |

«70 K, 2 h dynamic vacuum (ΙΟ^-ΙΟ" mbar)

Sample Activation \ -

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Isotopic Exchange H/D =>Internal and External Species

Identification of Surface Groups OH, NH, N1I ,CH... 2

-CH3OII

Addition =>X-OH + CH3OH

I

X-OCH3 + H 0 2

Lewis Acid (electron acceptor): A l

3 +

Probe-Molecules: C H 3 C N , CO, NH3, II 0, C5H5N... Brcmsted Acidity (proton donor): H 2

Acidity of Surface Sites

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Probe-Molecules: NH , C H N, C H6, C H ... 3

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Lewis Base (electron donor): O " Probe-Molecules: C0 ,CH CN,C H5N... 2

Basicity of Surface Sites

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Brensted Base (proton acceptor): OH" Probe-Molecules: CH3CN...

Flow Chart 1. FT-ER surface characterization process.


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Lewis acidic and basic sites, respectively, correspond to electron acceptor and electron donor sites whereas Bronsted acidic and basic sites correspond to proton donor and proton acceptor sites. For a complete characterization, different molecules should be used to probe these sites, but it must be kept in mind that a molecule can probe different types of sites at the same time. A striking example is the C H C N probe-molecule which can form at least four different species by adsorbing on different sites (hydrogen bonding, coordination on metallic ion, coordination on O H ' ion, reaction with O " ion). Moreover, acidic and basic characteristics are interdependent, since their formation depends on the stoichiometry, the crystalline phase, the synthesis conditions, and the impurities or the œntarninants. Therefore, several experiments have to be successively run with different probe molecules to get a good knowledge of the surface reactivity. In our experimental procedure the transmission spectra are recorded in situ using a specially designed cell. This cell enables heating the sample from room temperature to 873 Κ under atmosphere, vacuum or controlled pressures of various gases (2). This cell is placed in the sample chamber of the spectrometer so that it is possible to exactly follow the spectrum modification at any step of the experiment. Another advantage of this in situ analysis is the possibility of performing difference spectra. The difference between two spectra recorded at different steps can make the evolution of the surface species clearer. However, to avoid temperature effects, the two spectra must be recorded at the same temperature. It must also be noted that, even for nanosized powders, the contribution of the surface species to the spectrum is minor. Therefore the large amount of powder needed to get a good surface spectrum obscures the wavenumber region of the bulk vibrations and thus prevents simultaneous surface and bulk analysis. 3


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Experimental Synthesis conditions. The synthesis method for the nanostructured A1N powders was similar to that reported previously (5). Aluminum chloride hexahydrate and urea in equimolar ratio were thoroughly dissolved in deoxygenated water. Under vigorous stirring, anhydrous ammonia was bubbled through the aqueous solution. The reaction temperature was then increased gradually to 363 Κ over a period of 24 hours. A white gel formed and the solution viscosity increased. The reaction was completed by heating the mixture to 393 Κ for another 24 hours. The precursor gel was obtained by removing the solvent under vacuum Pyrolysis of the precursor gel was accomplished by high temperature processing at up to 1373 Κ under a continuous flow of anhydrous N H gas for 10 hours. Prior to pyrolysis, the chamber was evacuated to about 10" mbar, then repeatedly flushed and back filled with ultra pure nitrogen. The assynthesized white powders were stored and handled under argon. 5

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Bulk characterization tools. The formation of nanostructured AIN powders have been confirmed by FT-IR spectroscopy (Nicolet 60SX), Raman spectrometry (Dilor microprobe), X-ray diffraction (XRD) (Norelco X-ray diffraction unit with wide range goniometer), and transmission electron microscopy (TEM), along with the corresponding electron diffraction in a JEOL 200CX microscope.


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Wavenumbers Figure 1. FT-IR spectrum of the as-synthesized nanostructured AIN.

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Figure 2. Raman spectrum of the as-synthesized nanostructured AIN (the * mark refers to a plasma line).


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For IR bulk characterization, the AIN powder was dispersed in K B r and pressed according to the conventional sampling technique. The powder was analyzed as received under the Raman microscope. The X R D test sample is made by adhering the AIN powder onto an adhesive tape and for the T E M analysis the grid is dipped in a suspension containing the AIN powder. XPS and surface FT-IR measurements. X-ray photoelectron spectroscopy (XPS, Perkin-Elmer Physical Electronics PHI 5300) was also used to extract information about the surface species. The XPS instrument is equipped with a monochromatic A l K Q X-ray source (1486.6 eV) and hemispherical analyzer. The sample was prepared for XPS analysis by sprinkling the powders on an adhesive tape so as to obtain uniform and complete coverage. The IR surface spectra were recorded using a FT-IR Nicolet 5 D X spectrometer from 4 0 0 0 to 4 0 0 cm with a 4 cm resolution. The AIN powder was slightly pressed into a grid supported pellet (2) and placed inside the cell described above for trarismission analysis. A l l the spectra were recorded at room temperature unless otherwise stated. Deuterium (99% pure) was from Air Liquide and underwent no further purification. Methanol and pyridine from Merck-Uvasol were dried over molecular sieves. -1

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Bulk Characterization. -1

FT-IR characterization. The strong absorption band (Figure 1) at 6 9 0 c m is characteristic of the aluminum nitride transverse optical (TO) mode (4). Another broad band was observed in the range of 3 2 0 0 to 3 4 0 0 cm" . This corresponds mainly to the stretching vibrations of hydroxy1 and amine groups present on the powder surface. The band was carefully investigated and will be discussed below. 1

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Raman analysis. Raman peaks (Figure 2) were observed at 898, 660, and 248 cm . According to the literature (5), the peak at 660 cm (strong) corresponds to amorphous aluminum nitride, while the peaks at 898 and 248 cm" (weak) correspond to those observed for porycrystalline aluminum nitride. This strongly suggests that the nanostructured AIN sample contains a mixture of amorphous and noncrystalline phases. -1

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XRD measurements. Only aluminum nitride peaks were observed in the X R D patterns of the as-synthesized powders (Figure 3). The spectra also indicated that hexagonal crystals were obtained. Average crystallite diameter was estimated as 6 0 nm through a line broadening calculation of the X R D data (

Application of Fourier Transform Infrared Spectroscopy to

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