FTIR and Raman Spectroscopy of Carbon Nanoparticles in SiO2, ZnO and NiO Matrices
© to the authors 2008
Received: 25 July 2008
Accepted: 11 September 2008
Published: 1 October 2008
Coatings of carbon nanoparticles dispersed in SiO2, ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings with a view to apply these as selective solar absorber surfaces in solar thermal collectors. Cross-sectional high resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Raman spectroscopy was used to estimate the grain size and crystallite size of the carbon clusters of the composite coatings. X-HRTEM studies revealed a nanometric grain size for all types of samples. The C–SiO2, C–ZnO and C–NiO coatings contained amorphous carbon nanoparticles embedded in nanocrystalline SiO2, ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The thermal emittances of the samples were 10% for C–SiO2, 6% for the C–ZnO and 4% for the C–NiO samples. The solar absorptances were 95%, 71% and 84% for the C–SiO2, C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have the best solar selectivity behaviour followed by the C–ZnO, and last were the C–SiO2samples. Raman spectroscopy studies revealed that both the C–ZnO and C–NiO samples have grain sizes for the carbon clusters in the range 55–62 nm and a crystallite size of 6 nm.
KeywordsFTIR spectroscopy Raman spectroscopy Carbon nanoparticles Oxide matrices
The original infrared spectroscopy instruments were of the dispersive type in which prisms and gratings were used to separate the individual frequencies of energy from the infrared source. From the 1980s Fourier transform infrared (FTIR) spectroscopy has been preferred  to the old dispersive infrared technologies. The main advantages of FTIR spectroscopy are: non-destructive technique, increased speed of collection of spectra, increased sensitivity with high resolution capability, increased optical throughput and mechanical simplicity.
Raman spectroscopy has been in existence since the late 1920s. It is a technique that is used for material analysis as a complement to infrared spectroscopy. Raman spectroscopy has been applied for the identification of a wide variety of compounds of pigments, minerals, drugs, etc. [2–4]. Lately, portable Raman spectrometers have been demonstrated as useful forensic and security tools for the rapid detection of illicit drugs at airports .
In the work reported here, we have used FTIR and Raman spectroscopy techniques to analyse the behaviour of carbon nanoparticles embedded in three different oxide matrices, namely SiO2, ZnO and NiO, intended for selective solar absorber applications in solar thermal collectors.
The C–SiO2, C–ZnO and C–NiO samples were prepared by sol–gel techniques whose details are presented elsewhere [6, 7]. FTIR reflectance spectroscopy studies were performed on the samples using a Bomem DA8 spectrometer in the near infrared and the infrared wavelength ranges (2.5–20 μm) and a Lambda 900 spectrophotometer in the ultraviolet and visible wavelength ranges (0.3–2.5 μm). Raman spectroscopy was also used to study these samples using a Jobin-Y von T64000 Raman spectrometer. The observations from the FTIR and Raman spectroscopy studies have been corroborated, wherever possible, by structural analysis techniques such as transmission electron microscopy (TEM), cross-sectional high resolution transmission electron microscopy (X-HRTEM), and selected area electron diffraction (SAED).
Results and Discussion
In the near infrared (NIR) and infrared (IR) regions, the reflectance of the samples with low SUC content is higher than that of samples with higher SUC content. The reason for this is that samples of low SUC content were thinner than those of higher SUC content. This is thought to be purely a viscosity effect of the sol during the spin-coating process. The net result of this behaviour is a lower thermal emittance by samples with low SUC content. It was thus clear that an optimum of SUC content had to be sought. The optimum for samples with high absorptance and low emittance was observed to be 11 g SUC based on the graphs in Fig. 1a.
Characteristic chemical bonds in the C–SiO2 samples were identified from the FTIR reflectance spectrum presented in Fig. 1b. Three distinct absorption bands are observed. The major band at approximately 1,050 cm−1 is assigned to stretching vibrations of Si–O–Si or Si–O–X, where X represents ethoxy groups bonded to silicon [8, 9]. The shoulder at about 1,200 cm−1 is assigned to either the transverse optical mode of the out of phase mode of the asymmetric vibration or to the longitudinal optical mode of the high frequency vibration of SiO2. On the other side of the major absorption band, at 900 cm−1, is an absorption band that can be assigned to the stretching vibration of Si–OH or Si–O− groups. A broad absorption band, situated between 3,000 and 3,600 cm−1, and another one around 1,600 cm−1 are assigned to O–H stretching and O–H bending vibrations, respectively [8, 9]. The latter absorption bands appear to indicate the hydrophilic nature of the sol–gel synthesized silica.
The reflectance spectra of the C–NiO samples are presented in Fig. 3b. It is clear that both modes of the O–H vibrations of the C–ZnO samples are absent in the C–NiO samples; this yields even better emittance characteristics. The step transition for all the samples is between 2 and 3 μm and is steeper than that of the C–ZnO samples. This gives the C–NiO samples the closest to a step transition at 2.5 μm expected for an ideal selective solar absorber surface for domestic water heating.
Calculations of Raman shifts and grain size estimations for the C–SiO2, C–ZnO and C–NiO samples
I D/I G
Here ω0if the peak position for the strain-free bulk sample. The symbolsa andr are, respectively, attractive and repulsive exponents in the Mie-Grunsen potentials in solids that govern the bond energies as a function of interatomic distances. (The values fora andr are respectively 6 and 12 for van der Waal’s forces in the 6–12 Lennard-Jones potential, 1 and 9 for ionic bonding anda + r = 3 for covalent bonding.) It can be clearly seen that a positive strain (tensile strain) reduces the observed peak ωvibleading to a red shift whereas a negative strain (compressive strain) leads to the opposite effect in peak position shift—a blue shift. This means that in all samples, carbon clusters are compressively strained by their respective host materials as expected.
In this equation, A 0 is a pre-factor to be determined from the fitting session, d is the carbon cluster size, q is the wave vector of the exciting light source in the Raman spectroscopy set-up, α is the scaling factor, ω(q) is the phonon dispersion relation for the material under study and Γ0 is the full width at half maximum of the Raman line-shape for bulk form of the same material. For materials of different shapes, a modification of the d 3 q is required. For instance, d 3 q becomes 2πqdq for nanorods  and proportional to q 2 dq for quantum dots .
Values extracted from the Richter equation fitting of the C–ZnO and C–NiO samples
Parameters in phonon confinement model
FWHM (bulk + broadening)
Background noise in intensity
Lattice parametera 0in graphite
Conclusions and Comments
We have been able to study C–SiO2, C–ZnO and C–NiO selective solar absorber materials using FTIR and Raman spectroscopy techniques. The thermal emittances of the samples were 10% for C–SiO2, 6% for the C–ZnO and 4% for the C–NiO samples. The solar absorptances were 95%, 71% and 84% for the C–SiO2, C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have the best solar selectivity behaviour followed by the C–ZnO, and last were the C–SiO2samples.
The Raman analysis of the selective solar absorber samples has shown that carbon behaves differently when placed in matrices of SiO2, ZnO and NiO. In all matrices, the D-band is broad but has no significant asymmetry. The G-band is indeed asymmetrically broadened in all cases. The ratio of the intensities of these phonon peaks yields the following grain sizes for the carbon clusters in the matrices, respectively: 56.2 nm, 58.3 nm and 61.6 nm. The difference in the grain sizes may be said to be insignificant. Using the phonon confinement model of Richter  yields a crystallite size of 6 nm which is responsible for the asymmetrical broadening for both the C–NiO and C–ZnO samples. The C–NiO samples have the least scattering intensity of the three. The highest scattering was observed in C–SiO2 samples. It was not possible to analyse the C–SiO2 samples due to the rising background intensity as the Raman shift increased.
Rudolf Erasmus of Witwatersrand University, South Africa, kindly assisted with the Raman experiments. The CSIR-National Laser Centre provided financial assistance.
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