The use of Raman and FTIR Spectroscopy for the Analysis of Silica-based Nanofillers

C. Yeung, G. Gherbaz and A. S. VaughanUniversity of Southampton, Southampton, UK

Introduction

The interest in nanocomposites has grown exponentially since the early 1990s. With an increase in understanding of nanoparticle-matrix interactions, the ability to engineer materials and optimise desired properties became well established. However, detailed studies on the fundamental concept of varying surface functionalisations and how this can aid mixing, seems to have been neglected. A possible reason for this is that any such systematic study requires the surface chemistry to be quantitatively characterized.

For fillers such as nanosilica, the surface chemistry can easily be changed using commercial silanes. By using such technologies, it is possible to modify the chemistry and extent of the interphase regions, as well as modifying dispersion. Here we present the first step towards the design and quantification of nanoparticle surface chemistry in nanodielectrics.

cy3g09@ecs.soton.ac.uk

University of Southampton, Highfield, Southampton, SO17 1BJ, UKContact details :

Experimental procedure

Functionalising silica

Both nanosilica and micro-silica were functionalised for this experiment. Functionalised specimens were prepared by:

Dissolving the required quantity of the silane coupling agent (100 mg, 200 mg or 400 mg) in 3.0 g of methanol.

Adding 200 mg of the appropriate silica.

Samples were stirred to provide basic dispersion and left for 24 h to allow surface reactions to occur.

Excess silane was removed by repeated washing of the functionalised silica using methanol and evaporated in an oven.

Raman Spectroscopy

The resulting product was pressed to form a compacted disk-shape.

Data from these samples were obtained using a Renishaw Raman RM1000 spectrometer with a 785 nm CW diode laser of maximum operating power 25 mW.

FTIR Spectroscopy

After functionalising, the required mass of silica (10 mg, 20 mg or 40 mg) was dispersed into 90 mg of Nujol oil.

FTIR studies were performed using a Perkin Elmer Spectrum GX spectrometer with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector.

Results and Discussion

Raman Spectroscopy

Extended scans from 1300 cm-1 to 850 cm-1 for 500 accumulations at 10 s were acquired using a x50 lens.

Figure 1. shows Raman spectra obtained from the silane coupling agent Z-6040, the nanosilica and micro-silica.

A selection of Raman spectra obtained from nanosilica samples treated with the indicated quantities of Z-6040 are shown in Figure 3.

The epoxide peak is shown at C 910 cm-1.

The intensity of the epoxide peak increases monotonically with the concentration of the Z-6040 in the initial processing solution.

Although it is possible that some signal emanates from adsorbed molecules, the repeated washing process suggests that the signal is dominantly from covalently bonded chemical groups.

The results demonstrate strongly that the chosen chemical processing method has been successful in introducing epoxide an other organic groups into the system.

In addition, the results show that Raman spectroscopy is a viable means of probing the chemistry of such materials.

FTIR Spectroscopy

Each spectrum was collected from 400 cm-1 to 7800 cm-1 over 32 scans at 4 cm-1 resolution.

Figure 2. represents data for 10mg and 20mg of functionalised nanosilica in the carrier oil.

The amount of scattering increases with the amount of nanosilica present in the specimen.

Figure 4. shows raw FTIR data for micro-silica.

The 10/400 line represents functionalised micro-silica.

The 10/0 line represents the unfunctionalized micro-silica.

The solid line represents the difference.

Figure 5. shows absorbance spectra for 10mg of functionalised nanosilica with different loading levels of Z-6040.

At this loading level the absorbance was dramatically reduced compared to the equivalent micro-silica case.

The epoxide peak can no longer be distinguished from the noise.

The major feature around 1100 cm-1 increases monotonically with the degree of functionalisation.

Conclusions

Vibrational spectroscopy can provide information concerning the chemical state of functionalised silica.

In the case of Raman spectroscopy, the magnitude of the characteristic peaks scales with the degree of functionalisation

However, the approach is not easily adapted to provide absolute concentration data.

In the case of FTIR spectroscopy, optical scattering appears to compromise the simplistic application of the classical Beer Lambert equation

At present the FTIR technique is only capable of providing semi-quantitative data.

Raman and FTIR Spectroscopy

Raman Spectroscopy

Confocal Raman spectroscopy is a technique that takes advantage of the Raman effect.

The variations in vibrational and rotational energy of a system causes inelastic energy exchanges between incident electromagnetic radiation and matter.

The changes in frequency between incident and detected radiation is determined by the chemical composition of the system.

The intensity of Raman photons are detected and plotted as a function of frequency.

The spectrum observed allows the qualitative identification of the material.

FTIR Spectroscopy

Fourier Transform Infrared (FTIR) spectroscopy is a method of absorption spectroscopy which analyses the vibrational modes of a system in the infrared region of the electromagnetic spectrum.

Molecules absorb resonant frequencies that are specific to certain bonds within the system.

The amount of absorbance is detected and plotted as a function of frequency.

The spectra produced allow the quantitative identification of a material.

The above types of spectroscopy complement each other, one analysing a system qualitatively with great spacial resolution, whilst the other provides, in principle, a more quantitative approach.

Figure 1 : Raman spectra of untreated nanosilica, untreated micro-silica and Z-6040 coupling agent.

Figure 3. Variation in absorbance as a function of functionalised nanosilica loading level.

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Inte

nsi

ty

200 mg silane

100 mg silane

0 mg silane

400 mg silane

Figure 4. Raw FTIR absorbance for functionalised and unfunctionalised micro-silica and the derived difference spectrum

Figure 5. Variation in specimen absorbance as a function of functionalised nanosilica loading level.

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Inte

nsity

Micro-silica

Nanosilica

Silane Z6040

Figure 2 : FTIR data showing variation in specimen absorbance as a function of functionalised nanosilica loading level.