ARTICLE pubs.acs.org/JPCC
Infrared Attenuated Total Reflection Spectroscopy of Quartz and Silica Micro- and Nanoparticulate Films Christian Menno M€uller,† Alexandra Molinelli,‡ Manfred Karlowatz,‡ Alexandr Aleksandrov,‡ Thomas Orlando,‡ and Boris Mizaikoff *,† † ‡
Institute for Analytical and Bioanalytical Chemistry, University Ulm, 89081 Ulm, Germany School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ABSTRACT:
Studies on particulate quartz and silica films were performed using Fourier transform infrared spectroscopy (FT-IR) via attenuated total reflection (ATR). Based on the fact that measurements in the visible and near-infrared (vis/NIR) spectral range are already applied to spectrally distinguish disturbed from undisturbed soils, measurements in the mid-infrared (MIR) regime were performed to further investigate the utility of longer wavelengths toward this analytical problem. Natural particulate quartz samples were selected to provide a simplified surrogate for the dominating component of most soil matrices; water was added to simulate weathering processes of this matrix. Adding water to the pristine quartz particulate film resulted in a strong spectral shift of the asymmetric Si O Si stretch vibration at 1090 cm 1, thus substantiating the hypothesis of a size-related shift of the absorption wavelength due to changes in the particle size distribution within the evanescent field that is extending a few micrometers into the particle layer near the ATR surface. Monodisperse soda lime glass spheres and silica microsphere samples were then investigated at simulated weathering conditions to corroborate this assumption. The obtained results indeed demonstrate that spectra recorded at monodisperse particles do not reveal any spectral shifts during simulated weathering, which again confirms a particle size-related effect. In addition, studies using unpolarized and polarized IR radiation revealed a distinct correlation between the shifts of the major absorption features, i.e., the TO modes, and the particle size.
’ INTRODUCTION The problem of “disturbed soil” and its applicability for the reliable detection of buried objects has initiated substantial interest in the remote sensing community.1,2 Approximate frequency ranges of common internal vibrations of silicates, oxides, and other functional groups within minerals have most intense features located within the atmospheric window (700 1250 cm 1), thus rendering them most useful for remote sensing of silicates.3 However, for exploiting this phenomenon during field applications there are still too many spectral uncertainties that have not yet been investigated. Analyzing spectral features of minerals in the mid-infrared (MIR) range via Fourier transform infrared attenuated total reflection (IR-ATR) spectroscopy enables the investigation of different soil components during scalable laboratory experiments and provides new evidence on the spectral properties of silica-based particulate films. The most intense spectral features of quartz occurring between 830 and 1250 cm 1 are generally simplified as fundamental r 2011 American Chemical Society
asymmetric Si O Si stretching vibrations. The second most intense silicate bands are broadly characterized as O Si O deformation or bending modes, which occur in the spectral region of 400 to 560 cm 1. Weaker bands between 670 and 830 cm 1 have been attributed to symmetric Si O Si stretching vibrations.4 It is well-known that glass spectra of mineral compounds show differences in their spectral response, which are generally attributed to broadening of the bands. Furthermore, the importance of Coulomb forces was first recognized by Galeener and Lucovsky, who reported transverse optical (TO) and longitudinal optical (LO) mode splitting in silica samples probed with polarized light, and thereby contributed considerably to the clarification of their spectral features.5,6 At perpendicularly Received: June 1, 2011 Revised: October 3, 2011 Published: November 01, 2011 37
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polarized (s) radiation, essentially only the TO mode was detected.7 In recent years, especially the interest in semiconductors and accordingly the demand for an improved analysis of thin α-SiO2 layers promoted significant productivity in the field of optical glass analysis. Nevertheless, despite an enormous amount of work dedicated to analyzing the structure of α-SiO2 using IR spectroscopy, the interpretation of some spectral features observed, especially in the spectral region of 1000 1300 cm 1, remains a matter of substantial debate.8 Although these spectral features are generally accepted to originate mostly from asymmetric stretching vibrations of Si O Si bridging sequences,9 different interpretations are still prevalent in the relevant literature. The vibrational spectra of inorganic glasses are multiband spectra with virtually always present overlapping bands. Therefore, the interpretation of the obtained individual bands remains a challenging task due to a multitude of effects related to the vast variety of vibrational active structures such as (i) particular types of polyhedra, (ii) R X R bridges (R being Si, P, etc. and X being O, F, S, etc.) and (RXm)n‑ terminal groups, and (iii) superstructural units (e.g., various SiO4 rings), among others.8,10 12 Due to the lack of translational symmetry in a glass network, it is difficult to specify the degree of interaction of vibrations within a network. Nevertheless, IR-ATR spectroscopy is a highly suitable, yet comparatively simple method providing infrared spectra of quartz sand. This method allows the investigation of a wide variety of samples including other minerals, clays, or soil samples at highly reproducible measurement conditions. To simplify the interpretation of FTIR-ATR studies at silicabased particulate films, we have investigated quartz sand, soda lime glass spheres, and silica micro- and nanospheres at controlled conditions including wetting/drying cycles to simulate weathering processes relevant to remote sensing. Natural quartz samples with nonuniform size distribution were investigated in order to simulate field conditions. Furthermore, monodisperse soda lime glass spheres were used as samples to prove effects of the particle size on the resulting ATR spectra. By using spherical particles signal dependencies on particle shape were effectively eliminated. Finally, studying silica microspheres facilitated the spectral interpretation of the observed features by eliminating modes resulting from variations of the mineral composition. The presented studies at silica-based particulate films provide experimental evidence for particle size-related spectral shifts of TO modes, emphasized if probing the particulate films with polarized light. For quartz sand and α-SiO2, the band positions and intensities depend at a basic level on a set of dominant parameters, which are well-defined for the experimental conditions during the presented studies.
Figure 1. Schematic of the IR-ATR setup.
tissues were purchased from Whatman (Whatman International Ltd., Maidstone, England) and were used for cleaning the ATR waveguide surface. IR-ATR Setup. Measurements were recorded at a Bruker IFS 66/s spectrometer (Bruker Optics Inc., Billerica, MA) using an electrically heatable trough top plate ATR crystal mounting accessory attached to a Specac Gateway in-compartment ATR unit (Specac Inc., Woodstock, GA). A trapezoidal ZnSe ATR element (72 10 6 mm, 45°; Macrooptica Ltd., Moscow, Russia) was used as a multireflection ATR waveguide. Furthermore, a holographic thallium bromoiodide (KRS-5) polarizer (0.25 μm, Specac, Smyrna, GA) mounted in a motorized polarizer unit (Bruker Optics Inc., Billerica, MA) was used for measurements at s- and p-polarized radiation conditions. Data was recorded from 4000 cm 1 to 400 cm 1; 100 scans were averaged for each spectrum with a spectral resolution of 1 cm 1. A schematic of the experimental setup is shown in Figure 1. In the presented ATR setup, the sample was interrogated with a broad angular distribution of incident light (∼45° ( 15°). Thus, a “bulk” response of the sample was obtained. IR-ATR Studies at Particulate Films. Prior to each measurement, the crystal was thoroughly cleaned with methanol. The temperature of the heatable trough top plate was set to 50 °C to accelerate the evaporation of methanol and to more rapidly obtain stable conditions for recording background spectra. Approximately 600 mg of sample (quartz, soda lime glass or silica spheres) were applied to the crystal surface at a layer thickness markedly exceeding the penetration depth of the evanescent field (>7 μm film thickness); thus, reproducible measurement conditions for all samples were achieved. Following the ATR waveguide cleaning procedure, a spectrum of, for example, pristine quartz samples was recorded; such spectra will be referred to as pristine spectra in the remainder of this study. To simulate weathering conditions, a few droplets of deionized water were poured onto the sample (wetting cycle). Setting the temperature of the top plate to 70 °C accelerated the evaporation of the water during the drying cycle. Spectra recorded after such a weathering cycle is referred to as dried spectra. Finally, after each weathering cycle (wetting followed by drying) the sample was “disturbed” by careful stirring, as ZnSe is easily scratched, using a plastic spatula. Spectra recorded after such simulated disturbance events are referred to as disturbed spectra. This recurring procedure was investigated for different particulate materials at unpolarized, p-polarized, and s-polarized illumination conditions and related to the corresponding reference spectra.
’ EXPERIMENTAL SECTION Samples. Quartz (+230, 35 mesh) was purchased from Sigma-Aldrich (Milwaukee, WI). Soda lime glass spheres with a particle size ranging from 1 to 3 μm and 4 10 μm were obtained from MO-SCI Corporation (Rolla, MO). Larger soda lime sphere fractions (25 32 μm, 112 125 μm and 400 425 μm) were obtained from Whitehouse Scientific Ltd. (Waverton, Chester, U.K.). Silica micro- and nanospheres (200 nm) were purchased from Kisker Biotech (Steinfurt, Germany), and in part obtained from Dr. C. P. Wong, Material Science and Engineering, Georgia Institute of Technology (100 nm, 2, 3, 7, and 15 μm). Methanol (p.A.) was obtained from Merck (Darmstadt, Germany); lens-cleaning
’ RESULTS AND DISCUSSION IR-ATR Studies at Natural Quartz Particulate Films. In the IR spectrum of natural quartz sample (see Figure 2) the major absorption band is located at 1200 950 cm 1, with the most 38
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Figure 2. IR-ATR spectra of particulate layers of pristine (a), dried (b), and disturbed (c) spectra of quartz sand.
Figure 3. ATR spectra of pristine and dried quartz sand samples recorded at different polarization states of infrared radiation: (a) s-polarized [pristine], (b) s-polarized [dried], (c) p-polarized [pristine], and (d) p-polarized [dried].
prominent feature at approximately 1090 cm 1. This feature is attributed to asymmetric SiO4 stretching vibrations. The remaining absorption features are located at approximately 800 and 690 cm 1 and are assigned to the SiO4 symmetric stretching and to the Si O Si bending transition, respectively. In Figure 2, an entire weathering cycle is shown and enables the comparison of pristine, dried, and disturbed quartz spectra. As is clearly evident, each absorption feature evident in the dried spectrum is much more intense compared to the pristine and disturbed spectra. The explanation for this circumstance is that by adding a few droplets of water, the initially loosely packed pristine sample is condensed into a more compactly packed layer near the ATR waveguide surface; thus, the amount of absorbing material within the evanescent field is increased giving rise to higher absorbance. Yoshidome et al.13,14 have proposed that films comprising larger spheres exhibit a substantial interstitial volume compared to layers of smaller spheres. This implies that the amount of sample, which is interacting with the evanescent field, is varying with the particle size, and is increasing with decreasing particle diameter. Consequently, the latter may pack more densely at and closer to the waveguide surface compared to larger particles. Furthermore, if the sample contains a distribution of small and larger particles, it is anticipated that during the simulated weathering process by the addition of water to the particulate sample, smaller particles slide toward the waveguide surface in between the interstitial spaces provided by the larger ones. Thus, more absorbing material is present within the evanescent field, as observed by an increase in absorbance. After redisturbing the sample, the smaller particles are again redispersed in between the larger ones, and the dense packing within the waveguide surface layer is again dispersed. This is clearly evident in Figure 2, as the absorption intensities return almost to the initial signal height after redisturbing the sample with the spatula. These findings are in fact in agreement with field and laboratory remote sensing studies, where similar changes in spectral contrast have been reported as the predominant difference between spectra of pristine and disturbed soils.1,2,15 Even more pronounced is the observed shift of the asymmetric SiO4 stretching vibration. Located at 1090 cm 1 in the pristine spectrum, this absorption is shifting to 1060 cm 1 (dried spectrum), and then back to 1090 cm 1 (redisturbed spectrum) during a wetting/drying/disturbing cycle. This phenomenon was reproducibly observed, if the same sample was cycled several times in the order wetting/drying/disturbing (data not shown). Following this observation and since the composition of the sample remains unaltered during such a weathering cycle, it was assumed that the spectral position of the major absorption feature in particulate films is apparently dependent on the size of the
particles that are making up the particulate film within the evanescent field. If confirmed, this apparently significant and pronounced spectral shift may potentially be a characteristic spectral feature useful for the remote detection of disturbed soil locations without compositional change of the soil matrix. Studies using polarized light when probing quartz sand revealed further interesting spectral aspects of such samples. Consequently, IR-ATR spectra of pristine and dried quartz sand samples were recorded with s-polarized (a and b) and p-polarized (c and d) infrared radiation, as shown in Figure 3 for pristine and dried quartz sand. Splitting of the dominant absorption feature at 1090 cm 1 is apparent. When probed with s-polarized radiation the peak maximum is displaced by ∼30 to 1060 cm 1, whereas the displacement at p-polarized conditions induces a minute shift of only a few wavenumbers toward higher frequencies. Repeating the weathering simulations with polarized light results once more in an increase in absorption intensity, and shifts of the absorption maxima. However, the peak shifts resulting from the weathering cycle are not as pronounced as observed at unpolarized illumination. Harrick et al.16 have investigated quartz powders at s- and p-polarizations conditions, and the obtained spectra show similar shifts when probed with polarized radiation. While a definite theoretical explanation for these observations is absent, the spectral behavior is predominantly attributed to changes of the refractive index of quartz in vicinity of the absorption bands, and to the birefringence of quartz. Although the spectra recorded with polarized light may not be deconvoluted in detail, it is evident in summary that an increase of absorption intensity, as well as a spectral shift are observed at simulated weathering conditions, which may be attributed to changes in particle size distribution within the evanescent field. In order to further investigate the relation of the observed effects on the vibrational signatures to the particle size of minerals, and in particular the observed peak shift, monodisperse soda-lime glass spheres and silica spheres were studied. IR-ATR Studies at Soda Lime Glass Sphere Films. To corroborate the hypothesis of a particle size related shift of the asymmetric stretching vibration feature in the corresponding IR spectra, monodisperse soda lime glass spheres were studied at the same experimental conditions previously discussed for particulate quartz samples. Using possibly monodisperse fractions, particle size related changes in the spectra during the wetting and drying cycles should be effectively eliminated. According to the spherical shape, it is assumed that the particles will arrange in a densely packed layer at the waveguide surface, and thus, changes in the resulting spectra should be associated only with the different discrete particle sizes of the monodisperse fractions. 39
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Figure 4. Optical microscopy image of the 25 32 μm size fraction of soda lime glass spheres (scale bar: 100 μm).
Figure 6. ATR spectra of pristine soda lime glass spheres with different diameters: fractions 400 425 (a), 112 125 (b), 25 32 (c), 4 10 (d), and 1 3 μm (e) using unpolarized IR radiation.
Figure 5. Pristine (a) and dried (b) ATR spectra of the 112 125 μm size fraction of soda lime glass spheres with unpolarized IR radiation. Figure 7. ATR spectra of pristine soda lime glass spheres with different diameters: 400 425 (a), 112 125 (b), 25 32 (c), 4 10 (d), and 1 3 μm (e). Data has been normalized in intensity.
The investigated samples had to be obtained from two different sources in order to cover particle sizes in a 1 to >100 μm size regime, as the entire dimensional range is not available from one provider. Optical microscopy images of the glass sphere batches reveal sufficient quality of the samples with the exception of few defects, as evident in the exemplary microscope image of the 25 32 μm fraction (Figure 4). Evaluating various images of each batch, the number of shape defects and dimensional outliers appears insignificantly small, and should therefore not affect the obtained IR-ATR spectra. However, given the synthesis route of these beads each sample batch varies in chemical composition, and thus, the resulting IR spectra may vary from batch to batch. In Figure 5, the ATR spectrum of 112 125 μm spheres are shown upon illumination with unpolarized IR radiation. As is clearly evident, the spectrum shows the anticipated band broadening related to the structural disorder of the glass matrix, as predicted by theory.17 20 The broad absorption band ranging from 1200 to 870 cm 1 is resulting from the overlapping asymmetric SiO4 stretching vibration and the nonbridging oxygen (NBO) vibration. The absorption band at ∼780 cm 1 is attributed to the symmetric SiO4 stretching vibration. Efimov and Abo-Naf11,12,21,22 described a semiempirical model, which complements the soda lime glass spectra recoded in this study. It was shown that within the spectra of alkali disilicate crystals the range of the Si O Si asymmetric stretches may extend to 900 cm 1, whereas the range of nonbridging oxygen vibrations extends from 1120 cm 1 to frequencies lower than 1000 cm 1. Figure 5 also shows the spectrum obtained after the repetition of the simulated weathering process for 112 125 μm soda-lime glass spheres at unpolarized light conditions. The only observable difference in the pristine and dried spectra of the sample is expressed by a small change in overall spectral intensity, which is related to minor rearrangements of the spheres during the wetting step. This result was expected given the narrow size distribution of the spheres, which appear already densely packed
at the ATR crystal surface after the initial deposition. These findings again support the hypothesis that spectral shifts upon wetting and drying are solely related to particle size, if a fraction of significantly smaller particles is present. As previously indicated, the intensity of the spectra should correlate with the particle size, which in turn affects the packing density within the evanescent field. Largely, this trend is evident in the spectra shown in Figure 6, however, not for the smaller particles (1 10 μm). A detailed evaluation of the recorded spectra reveal that, depending on particle size, a decrease in intensity of the NBO vibration band at around 870 cm 1 relative to the asymmetric stretch vibration (at around 1050 cm 1) is evident. However, this is not instantly obvious, as the intensity of the spectra changes with varying particle size as well. To illustrate this decrease, the spectra were normalized in intensity at 1040 cm 1 for enabling a comparison (Figure 7). As all spectra reveal an almost constant slope in this wavelength range after normalization, a fair comparison between the spectra of differently sized particles is enabled. Now, a decrease in intensity of the NBO absorption feature is clearly evident; however, after normalization the intensity changes have to be considered on a relative scale. A shift of this absorption feature is also clearly visible; located at 870 cm 1 for the 400 425 μm batch, the peak shifts to 1050 cm 1 for the 1 3 μm fraction. Assuming that the band assignment is correct, one may derive that the contribution of the vibration of the nonbridging oxygen atoms is decreasing with decreasing particle size, therefore the abundance of NBO sites in the 400 425 μm batch is apparently considerably higher compared to the other batches. In summary, although a peak shift is clearly evident, the assignment is not unambiguous due to the overlap of the NBO vibration with the asymmetric stretch vibration. Beside the initially assumed shift of spectral features depending on 40
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Figure 8. s-Polarized (A) and p-polarized (B) ATR spectra of soda lime glass spheres with different diameters: 400 425 (a), 112 125 (b), 25 32 (c), 4 10 (d), and 1 3 μm (e). Data has been normalized in intensity.
the particle size, for the case of soda lime spheres it is also possible that this effect is convoluted by a systematic change in chemical composition of the analyzed samples. Using s-polarized illumination conditions, the TO mode appears possibly enhanced, as discussed in detail in the next section. Concurrently, the TO mode should be partly suppressed, if p-polarized illumination is applied (Figure 8). However, the interpretation of the spectral behavior of the TO mode located at lower wavenumbers is complex for these samples due to overlapping NBO vibrations. Similar to the spectra recorded using unpolarized radiation, the obtained data were normalized at 1040 cm 1 for facilitating a spectral comparison. The most prominent feature evident in Figure 8A is a red shift (>70 cm 1) of the NBO vibration band with decreasing particle size. Although apparently revealing a spectral shift, a dependence on particle size cannot be unambiguously corroborated due to the overlap between the TO mode and the NBO vibrations. Hence, besides the initially anticipated particle size related shifts of the TO mode, also differences in chemical composition of the soda lime glasses may contribute to differences in absorption intensity and position of the NBO vibrational mode. This circumstance is further emphasized using p-polarized illumination. As the TO mode should be partly suppressed, the absorptions in the region 950 880 cm 1 should mainly be caused by the NBO vibration, which is evident in Figure 8B. The spectra are again normalized at 1040 cm 1, and the decrease of the NBO vibration is observable. Serra et al.20 have investigated the influence of different cation/ SiO2 ratios, and have reported that the intensity of the NBO vibrations decreases with decreasing amount of SiO2, which confirms the observed results. In summary, the obtained results clearly indicate a particle size related change in the spectra, as the absorption feature in the spectral region of 950 880 cm 1 is evidently shifting. However, this change may be convoluted by the varying chemical composition throughout the analyzed batches of soda lime glass spheres, and the correspondingly overlapping NBO vibration bands. Consequently, further studies would require soda lime glass spheres with absolute homogeneous
Figure 9. ATR spectra of pristine particulate silica nano- and microsphere films with unpolarized (A), s-polarized (B), and p-polarized (C) IR radiation. Monodisperse silica microspheres with different diameters: 15 (a), 7 (b), 3 (c), and 2 μm (d).
chemical composition for unambiguously quantifying particle size effects. In the absence of such samples, in the present study monodisperse silica microspheres were selected to further elucidate these observations. IR-ATR Studies at Silica Micro- and Nanosphere Films. Monodisperse silica samples (100% SiO2) were studied at the same experimental condition as the quartz particles and the soda lime glass samples. Exemplary spectra of the microspheres recorded with unpolarized radiation are shown in Figure 9A. In each spectrum, the broad absorption band extending from ∼1250 to 900 cm 1 is clearly evident. The interpretation of the observed spectral features proves again challenging due to additional overlapping vibrational signatures. First, the symmetrical stretching (SS) vibration of the oxygen atoms gives rise to a band located at ∼800 cm 1. The most prominent features however are two pronounced peaks at 1120 and 1020 cm 1. Usually, IR spectra of silica reveal only one absorption band located at ∼1050 cm 1 with a shoulder toward the shorter wavelength end.13 While a qualitative interpretation of the bands at 1120 and 1020 cm 1 may attribute these features to the LO and TO mode of the AS1 vibration given the similarity to the IR spectra of silica nanoparticles reported, e.g., by Kravets et al.,23 the difference in the wavelength position of the absorptions renders an LO mode highly unlikely. A clarification of the observed spectral features was only achieved when the nanometer sized spheres were investigated. Reducing the particle size of monodisperse silica spheres from the micrometer to the nanometer regime resulted in a single peak with a maximum located at ∼1109 cm 1, which was evident at unpolarized, p-polarized, and s-polarized conditions (Figure 11). 41
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Figure 11. ATR spectra of pristine 100 nm silica nanospheres films with unpolarized (a), p-polarized (b), and s-polarized (c) IR radiation. Figure 10. Shift of the peak maximum corresponding to the As1 TO mode according to the particle size of the monodisperse silica spheres.
Yet, the obtained results are somewhat inconsistent, as the 100 nm spheres (Figure 11) show an absorption band at 1109 cm 1 whether illuminated with s-, p-, or unpolarized radiation. This is in conflict with the results obtained at microspheres, because the bands that were assigned to the nanometer sized features are reduced only a shoulder, if the spectra were recorded with s-polarized light. Although no fully satisfying explanation for this aspect has been found to date, it is hypothesized that due to the difference in excitation geometry (i.e., the adjusted electric field component) by using s- or p-polarized radiation, the interaction of the nanometer sized and micrometer sized particles with the incident beam is changed. Harbecke et al. have suggested that the TO mode is suppressed if p-polarization is used to probe the sample due to the missing electric field component necessary to excite the TO mode, and enhanced if probed with s-polarized radiation.26,27 Hence, future clarification of this aspect is anticipated using more defined optical sampling conditions combined with well-defined IR light sources such as e.g., quantum cascade lasers in lieu of the FT-IR spectrometer.
In a seminal study, Shaganov et al.24 have investigated the influence of size effects on the optical properties of absorbing media, and have made similar observations. Silica nanoparticles were examined at p-polarized radiation using a Ge ATR prism, and it was shown that significant spectral changes occur, if the particle size is smaller than the probing wavelength. As an important consequence, peak positions recorded at bulk materials may be subject to major shifts, if nanoparticulate samples of the same material are spectroscopically studied. These observations are related to dielectric confinement in three spatial directions (i.e., 3D confinement), which is the case in randomly ordered systems such as micro- or nanometer sized spherical films. Scanning electron microscope (SEM) studies (not shown) have revealed that the investigated silica microspheres do not have (nano)smooth surfaces, but are covered with nanometer-sized features. Therefore, the obtained IR spectra show overlapping bands of two modes: (i) the band resulting from the TO mode of the asymmetric stretching vibration, which is located at 1000 1050 cm 1 and caused by the micrometer scale of the silica spheres, and (ii) the band observed at 1120 cm 1, which is related to the nanoscale surface structure of the sample. In Figure 9, panels C and B, spectra recorded at s- and p-polarized conditions are shown. In Figure 9C, the peak at 1020 cm 1 (TO mode) decreases dramatically, and only a shoulder at approximately 1050 cm 1 remains, whereas in Figure 9B the band resulting from 3D confinement decreases, and only a shoulder remains at approximately 1100 cm 1. As evident in Figure 9C, the band positioned at 1100 cm 1 shifts only slightly (max. 5 cm 1). When the results obtained for soda lime glass spheres at p-polarized conditions are revisited, almost no dependence of the peak position at similar wavelength on the particle size variation was evident. Therefore, it may be concluded that the observed shifts are most likely not related to modes occurring in this wavelength region, therefore, rendering these samples unsuitable to investigate spectral shifts of films comprising varying particle sizes. However, as clearly evident in Figure 9B the As1 TO mode shifts depending on the diameter of the silica microspheres. With increasing particle diameter, a pronounced shift toward lower wavenumbers is observed, which corroborates the hypothesis of a dependence of the TO mode signature on the particle size. The correlation between the peak position of the As1 TO mode mode and the particle size is shown in Figure 10. Innocenzi et al.25 have observed similar spectral shifts when probing self-assembled mesostructured silica films. However, the observed shifts were related to TO3 and TO4 modes, which are only transitioned in silica materials providing disordered and ordered domains such as, e.g., mesoporous films.
’ CONCLUSIONS The presented studies show that IR-ATR spectroscopy provides a reliable methodology for fundamental studies on the spectroscopic properties of quartz-related particulate materials, which is of substantial interest for the interpretation of IR spectroscopic data provided by the remote sensing community on soil samples. Besides the already established differences in spectral contrast of disturbed and undisturbed soil, a strong spectral shift for quartz particulate materials of the maximum of the main absorption feature at 1090 cm 1 could be observed. A less distinct shift could be observed with s- or p-polarized IR radiation, however, offer interesting aspects for interpreting remote sensing data. Complementary IR-ATR studies at monodisperse soda lime glass sphere films with sphere diameters in the range from 1 to 400 μm substantiate the findings obtained for quartz particulate films providing a heterogeneous particle size distribution. Wetting and drying cycles of samples comprising glass spheres with a narrow size distribution size revealed no differences between the IR spectra of pristine and dried samples, which is in agreement with the hypothesis that the spectral properties of disturbed and pristine soils may be related to the particle size. In addition, the pronounced particle size dependent shift of the As1 TO resonance is of particular interest for interpreting the spectral behavior of quartz particles and soil in remote sensing applications and data interpretation. 42
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’ AUTHOR INFORMATION Corresponding Author
*Tel: +49 731 50 22750. Fax: +49 731 50 22763. E-mail: boris. mizaikoff@uni-ulm.de.
’ ACKNOWLEDGMENT This work was in part supported by the Army Research Office under a MURI (Multi-University Research Initiative) project (DAAD19-02-2-0012). ’ REFERENCES (1) Johnson, J. R.; Lucey, P. G.; Horton, K. A.; Winter, E. M. Remote Sens. Environ. 1998, 64, 34–46. (2) McFee, J. E.; Ripley, H. T. Detect. Rem. Technol. Mines Minelike Targets II 1997, 2079, 738–749. (3) Walter, L. S.; Salisbury, J. W. J. Geophys. Res., Solid Earth Planets 1989, 94, 9203–9213. (4) Farmer, V. C. The Infrared Spectra of Minerals; Mineralogical Society: London, 1974. (5) Galeener, F. L.; Lucovsky, G. Phys. Rev. Lett. 1976, 37, 1474–1478. (6) Galeener, F. L. Phys. Rev. B: Condens. Matter 1979, 19, 4292–4297. (7) Huebner, K.; Schumann, L.; Lehmann, A.; Vajen, H. H.; Zuther, G. Phys. Status Solidi B 1981, 104, K1–K5. (8) Courtens, E.; Foret, M.; Hehlen, B.; Vacher, R. Solid State Commun. 2001, 117, 187–200. (9) Bell, R. J.; Dean, P. Nature (London) 1966, 212, 1354–1356. (10) Almeida, R. M. Phys. Rev. B: Condens. Matter 1992, 45, 161–170. (11) Efimov, A. M. J. Non-Cryst. Solids 1996, 203, 1–11. (12) Efimov, A. M. J. Non-Cryst. Solids 1999, 253, 95–118. (13) Yoshidome, T.; Kusumoto, H.; Kuroki, O.; Kamata, S. Chem. Lett. 1998, 747–748. (14) Yoshidome, T.; Fukuyama, N.; Fukushima, Y.; Higo, M. Anal. Sci. 2008, 24, 939–943. (15) Salisbury, J. W.; Wald, A.; D’Aria, D. M. J. Geophys. Res. 1994, 99, 897–911. (16) Harrick, N. J., Bloxsom, J. T.; . 1966. Study Program to Obtain the Infrared Internal Reflection Spectra of Powdered Rocks. prepared under NASA Contract #NASw-964 Mod.1. (17) Salisbury, J. W., Walter, L. S.; Vergo, N.; D’Aria, D. M. Infrared (2.1 - 25 Micrometers) Spectra of Minerals; Johns Hopkins University Press: Baltimore, 1991. (18) Sanders, D. M.; Person, W. B.; Hench, L. L. Appl. Spectrosc. 1974, 28, 247–255. (19) Dell, W. J.; Bray, P. J.; Xiao, S. Z. J. Non-Cryst. Solids 1983, 58, 1–16. (20) Serra, J.; Gonzalez, P.; Liste, S.; Serra, C.; Chiussi, S.; Leon, B.; Perez-Amor, M.; Ylanen, H. O.; Hupa, M. J. Non-Cryst. Solids 2003, 332, 20–27. (21) Efimov, A. M. J. Non-Cryst. Solids 1997, 209, 209–226. (22) Abo-Naf, S. M.; El, B.; Fatma, H.; Azooz, M. A. Mater. Chem. Phys. 2003, 77, 846–852. (23) Kravets, V. G.; Meier, C.; Konjhodzic, D.; Lorke, A.; Wiggers, H. J. Appl. Phys. 2005, 97, 084306/1–084306/5. (24) Shaganov, I. I.; Perova, T. S.; Melnikov, V. A.; Dyakov, S. A.; Berwick, K. J. Phys. Chem. C 2010, 114, 16071–16081. (25) Innocenzi, P.; Falcaro, P.; Grosso, D.; Babonneau, F. J. Phys. Chem. B 2003, 107, 4711–4717. (26) Harbecke, B.; Heinz, B.; Grosse, P. Appl. Phys. A: Mater. Sci. Process. 1985, A38, 263–267. (27) Nagai, N.; Hashimoto, H. Appl. Surf. Sci. 2001, 172, 307–311.
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