Interpretation of Raman Spectra of Oxide Materials: The Relevance of

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Interpretation of Raman Spectra of Oxide Materials: The Relevance of Absorption Effects Anastasia Filtschew, and Christian Hess J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06105 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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The Journal of Physical Chemistry

Interpretation of Raman Spectra of Oxide Materials: The Relevance of Absorption Effects Anastasia Filtschew and Christian Hess* Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Darmstadt, Germany ABSTRACT: When using Raman spectra for structural characterization of solid powder materials such as catalysts, the interpretation needs to take into account the possible absorption of radiation. For example, the reduction of oxide materials may result in new UV-Vis absorption bands and therefore affect the Raman intensity. In resonance Raman spectroscopy the absorption correction of Raman intensity based on the Kubelka–Munk theory is widely established. In contrast, in Raman spectroscopy typically no absorption correction or normalization by a phonon mode is applied. Using a combined approach of Raman and UV-Vis spectroscopy, this study examines the effect of absorption on Raman spectra as well as the usefulness of different absorption corrections for Raman spectroscopy. The results show that a change in absorption of about 10% may result in a decrease in Raman intensity of up to onethird. Corrections based on the Kubelka–Munk theory predict large intensity changes for small changes in the low-absorption region, resulting in the largest deviations among different absorption corrections. On the other hand, the commonly used normalization by a phonon mode shows increasing deviations with increasing spectral distance of the investigated feature from the position of the phonon mode, since it is not wavelength dependent. To reduce these discrepancies, we have developed a new wavelengthdependent absorption correction. As illustrated for ceria materials, the corrected Raman intensity may vary significantly from the raw data intensity. Consequently, for a profound interpretation of Raman spectra of solid powder materials, the combined use of Raman and UV-Vis spectroscopy is recommended, allowing one to investigate and quantify the effect of absorption on Raman spectra, which may be of relevance even for Raman spectroscopy.

Raman spectroscopy is a powerful method for providing information on vibrational states of the bulk and the surface. It is therefore widely applied to structural characterization of solid powder materials such as catalysts. Raman spectroscopy has also been used to monitor surface and bulk reactions by both ex situ and in situ analysis. The most prominent techniques to measure Raman spectra are Raman spectroscopy and resonance Raman spectroscopy. In Raman spectroscopy the excitation wavelength is not or only weakly absorbed by the sample, while in resonance Raman spectroscopy it is chosen to be close to an electronic transition and therefore within the strong absorption region of the sample.1 It is well known that Raman spectra need to be corrected for absorption effects, since the strong absorption in resonance Raman spectroscopy may lead to a strong decrease of the scattered light.2-5 As part of the analysis of Raman spectra, typically either no absorption correction or a normalization by a phonon mode as an internal standard is applied6-13 without knowing, however, whether such a normalization by a phonon mode is justified. This normalization must be questioned and treated with caution, especially for materials that may change their absorption properties, such as reducible oxides. For example, the creation of defects may result in new absorption bands in the UV-Vis region, as is known for a variety of oxides, such as titanium oxide,14-15 cerium oxide,16-17 molybdenum oxide,13 and tin dioxide.18-19 Therefore, the question arises how absorption effects in Raman spectroscopy can be accounted for in a systematic manner, since they can be of major relevance for the interpretation of spectra. Kubelka and Munk provided a theory to describe the transmittance and diffuse reflectance of powders.20 By extension of the

INTRODUCTION

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Kubelka–Munk theory Schrader and Bergmann derived formulas for the Raman intensity of back-scattered or transmitted light of powder samples with finite thickness.4 Based on the Kubelka–Munk theory and the considerations of Schrader and Bergmann, Waters then deduced expressions for the Raman intensity of back-scattered light for infinitely thick powder samples.3 The work of Waters distinguishes two cases: (i) samples with a constant value of the scattering coefficient, but varying absorption coefficient, and (ii) samples with a constant value of the absorption coefficient, but differing scattering coefficient. As long as the particles do not change their size the first case is valid for most samples. In this case, the observed Raman Intensity Ψ is given by equation 1, where describes the exciting Raman laser intensity, the coefficient of Raman generation, s the scattering coefficient, and the diffuse reflectance of an infinitely thick powder.3 Therefore, to obtain the absorption-corrected Raman spectra (I0) the observed Raman intensity (Ψ ) has to be corrected by a function G( ) (eq. 2).2-3 can be determined by measuring the diffuse reflectance of the sample by UV-Vis spectroscopy. Hence for the correction of Raman spectra the measurement of UV-Vis spectra is necessary. However, the correction by the function becomes inexact for diffuse reflectance values higher than 0.9. Thus for oxide materials with a high diffuse reflectance at the excitation wavelength a new absorption correction needs to be developed. In case of the development of absorption bands in the UVVis, in principle, two scenarios may be distinguished: (i) the Raman and the UV-Vis bands originate from different species leading to an attenuation of the Raman bands measured using an excitation in the proximity of the UV-Vis absorption (ii) the Raman and the UV-Vis bands originate from the same species leading to a resonance enhancement of the Raman bands. An important oxide material with large diffuse reflectance values at a standard Raman excitation wavelength of 514 nm is ceria. Ceria is widely used for fuel cell and catalytic applications owing to its facile oxygen release.21-26 Such a defect creation is accompanied by a change in oxidation state and may result in changes of the UV-Vis absorption properties. Therefore, ceria was investigated as a representative of oxides with changing absorption properties. Owing to its relevance for the development of NOx absorbers24, 27-28 we chose NO2 storage in polycrystalline ceria to illustrate our approach. To account for absorption effects and to work out the timedependent reaction behavior we alternately recorded Raman and UV-Vis spectra during gas exposure. The evolution of the stored nitrate species was determined for the raw and corrected Raman spectra by analyzing different correction procedures, including normalization by the ceria phonon signal. The results will be critically discussed.

EXPERIMENTAL SECTION

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Material Synthesis The ceria sample was prepared by decomposition of cerium(III) nitrate hexahydrate (Sigma-Aldrich, 99.999% trace metal basis) using two cycles of calcination at 600°C for 12 h and a heating rate of 6°C/min. After calcination the sample was sieved to achieve particle sizes of 200–300 µm.

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Figure 1. Schematic drawing of the experimental setup.

Experimental setup The investigations were performed with a commercial CCR1000 catalyst cell (Linkam Scientific Instruments). The sample was loaded on a ceramic fiber fleece in a ceramic sample holder, which can be heated by a ceramic heating element (see Figure 1). The feed was flowed through the sample from top to bottom. Raman and UV-Vis spectra were recorded in a backscattering geometry. As indicated in Figure 1, the UV-Vis fiber was tilted by 30°C with respect to the Raman objective. Since the two spectroscopy methods interfere with each other, Raman and UV-Vis spectra were taken alternately. Raman spectra were obtained by using an argon ion laser (514 nm, Melles Griot) for excitation and a transmission spectrometer (Kaiser Optical) equipped with a charge-


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coupled device (CCD) detector for collection of the backscattered light. The spectrometer was calibrated by the emission lines of a standard argon lamp. The resolution of the spectrometer was 5 cm–1; the wavelength stability was better than 0.5 cm–1. The laser power was adjusted to 2.3 mW as measured at the location of the sample with a power meter (Ophir). UV-Vis spectra were recorded on an AvaSpec-ULS2048 spectrometer (Avantes) using a halogen and deuterium lamp (AvaLight-DHS, Avantes) for excitation. For calibration MgO powder was used as a white standard. Using the same in situ CCR1000 catalyst cell, Raman spectra were also recorded for NIR excitation delivered by an allsolid-state tunable titanium:sapphire laser, which is pumped by a diode-pumped frequency-doubled Nd:YLF laser (Evolution-15, Coherent). The titanium:sapphire laser is tunable within 770 to 900 nm. For our measurements we used a wavelength of 770.2 nm. The backscattered light was sent to a triple spectrometer (Princeton Instruments, TriVista 555), which operated in triple subtractive mode with the first two stages acting as a bandpass filter (300 g/mm) and the third stage as a spectrograph (3600 g/mm). The triple spectrometer is equipped with a back-illuminated CCD camera with UV antireflection coating (Princeton Instruments, Spec 10:2kBUV) and was cooled with liquid nitrogen. The spectral resolution of the spectrometer is 1 cm–1.

attributed to O2– vacancies and replacement of cerium(IV) atoms by cerium(III) atoms or impurity atoms in the bulk, respectively.29, 31-32 The two features at 833 and 1129 cm–1 are attributed to the O–O stretching vibration of peroxides and superoxide resulting from absorption of molecular oxygen on two- and one-electron defects, respectively.33-35 The feature at around 1170 cm–1 is a second-order Raman mode (2LO).30 As a result of the NO2 storage, during the in situ experiment, features at around 730, 1037, 1260, 1560, 1604, and 1620 cm–1 emerge. The features at 730 and 1037 cm–1 can be assigned to the bending and stretching vibrations of free nitrate, respectively,22, 36-37 whereas the shoulder at around 1260 cm–1 is characteristic of nitrite ions.38 The weak features at 1560, 1604, and 1620 cm–1 originate from bidentate and bridged nitrate species.36

RESULTS AND DISCUSSION

Figure 3. In situ UV-Vis spectra of ceria samples showing the temporal evolution during exposure to 500 ppm NO2 in 20% O2/N2 at 30°C. The wavelength positions of the excitation lasers used for Raman experiments are indicated. The inset depicts difference spectra. Spectra in the inset are offset for clarity.

Figure 2. In situ Raman spectra of ceria at 514 nm excitation showing the temporal evolution during exposure to 500 ppm NO2 in 20% O2/N2 at 30°C. The inset depicts the temporal behavior of the F2g mode.

Figure 2 depicts Raman spectra of the ceria sample at 514 nm excitation measured during exposure to a mixture of 500 ppm NO2 in 20% O2/N2. Figure S1 (in the Supporting Information) provides an enlarged view of the region 500-1800 cm–1. The Raman spectra are dominated by a feature at 465 cm–1 originating from a symmetrical stretching of the CeO8 cubes (F2g mode).29-30 Further characteristic ceria features could be observed at 250, 550, 595, 833, 1129, and 1170 cm–1.30 In earlier work the feature at around 250 cm–1 was assigned to a Ce–O surface vibration.17 The features at 550 and 595 cm–1 can be



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As can be seen in the inset of Figure 2, the intensity of the F2g mode decreases to about one-third during the first 2 min and subsequently increases to an intensity higher than at the beginning. Furthermore, a red-shift of 1.5 cm–1 was observed. A similar intensity behavior was also detected for the other ceria features (see Figure S1). Interestingly, the corresponding UVVis spectra show major changes in absorption within 410–670 nm on the timescale of the Raman changes (see Figure 3).

Figure 4. In situ Raman spectra of ceria at 770 nm excitation, showing the temporal evolution during exposure to 500 ppm NO2 in 20% O2/N2 at 30°C.

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Since the laser excitation of the ceria sample occurs at 514 nm, the changes in absorption result in a decrease in intensity of the ceria-related features as well as the signals of the adsorbed NOx species. It is remarkable that the temporal behavior of the diffuse reflectance at 527 nm strongly resembles the dynamics of the F2g intensity (see Figure S2). To exclude an overlay of absorption effects and intensity changes induced by reaction, a NO2 storage experiment was realized at 770 nm excitation (see Figure 4). At this excitation wavelength, changes in Raman intensity could only be induced by reaction, since neither significant initial absorption nor changes in absorption at 770 nm were observed (see Figure 3). As can be seen in Figure 4, the intensity did not change during reaction but a red-shift of the F2g mode (see inset of Figure 4) was observed. The intensity and shift of the F2g mode can be described with the Phonon Confinement Model (eq. 3), where represents the phonon dispersion for the selected mode, Γ = 9.2 cm–1 the natural Raman full width at half maximum (FWHM), q the wave vector, L = 5 nm the correlation length, and 2².30, 39-43 Equation 4 describes the shift of the F2g feature Δ , where a is the lattice constant and 1.24 the Grüneisen parameter.29, 40 As shown by the simulation based on equation 3 (see Figure 5), the shift of the F2g feature does not result in an intensity change, fully consistent with the experimental results at 770 nm excitation. Therefore, the observed changes in the Raman intensity at an excitation wavelength of 514 nm are induced only by absorption effects and thus have to be corrected for to obtain the correct temporal evolution of the adsorbed NOx species.

Figure 5. Simulation of the F2g feature based on the Phonon Confinement Model. An increase of the lattice parameter results in a red-shift of the F2g mode. The lattice parameter was varied between 0.000 and 0.005 Å in steps of 0.001 Å. For details see text.


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One approach to accounting for absorption effects is the correction by a function (eq. 2), as introduced by Waters.3 Since the function becomes very large for values higher than 0.9 (see Figure 6), this approximation is no longer valid for → 1.3 During NO2 storage in ceria the diffuse reflectance is 0.89 at the beginning, then decreases to 0.79 and increases again to 0.90 at the end. Hence the observed diffuse reflectance is close to the limit. In this range, a small variation in absorption results in a large change in Raman intensity, consistent with our observations. A change of about 10% in diffuse reflectance results in a decrease of about 63% in Raman intensity. Figure 7 depicts the Raman spectra of Figure 2 corrected for absorption effects by the function. The sharp decline in the F2g intensity in the first 2 min could be attenuated by the correction, but the spectra still show significant variations in intensity at the beginning. These variations are not real, since they could not be observed in the experiment at 770 nm excitation. To reduce this error we developed an alternative approach to correct the Raman data. We first plotted the Raman intensity of the F2g and 2LO features as a function of the absorption (see Figures S3 and S4). An approximately linear dependence of the Raman intensities on reflectance was observed. Since the detected intensity of each feature is affected by the focus and the scattering coefficient, the intensity was extrapolated to zero absorption to yield relative intensities ⁄ . As illustrated in Figure 8, the relative intensities of the F2g and 2LO modes show a linear dependence on the diffuse reflectance. The Raman intensities were then divided by the linear function of this straight line to yield corrected Raman intensities I0 (eq. 5):

Figure 7. In situ Raman spectra of ceria at 514 nm excitation corrected for absorption effects by the function. The inset depicts the temporal behavior of the F2g mode.

Figure 9 depicts corrected Raman spectra as obtained by equation 5. The sharp decline at the beginning could be largely attenuated by the correction. The residual variations in intensity are ascribed to the error of this correction. Comparison of this correction with the one based on the function reveals that the variations for the correction by the function (eq. 2) still amount to about 35% (see inset of Figure 7), while for our correction (eq. 5) the variations could be reduced to about 13% (see inset of Figure 9). Therefore, for low absorptions or high diffuse reflectance our approach seems to be more appropriate to correct Raman spectra for absorption effects.

Figure 6. Dependence of the function on .


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Figure 8. Dependence of the relative Raman intensity of the F2g and 2LO features on diffuse reflection. The red line represents a least squares fit to the data.

Figure 9. In situ Raman spectra of ceria at 514 nm excitation corrected for absorption effects according to eq. 5. The inset depicts the temporal behavior of the F2g mode.

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Figure 10. Enlarged view of the UV-Vis spectra shown in Figure 3 within the first 5 min.

Since the Raman spectra at 770 nm excitation showed no changes in intensity for the F2g mode, another possibility for the correction of absorption effects is normalization by the F2g feature. However, normalization by the F2g feature does not take into account that various peak positions can be subject to different absorptions. For example, for excitation at 514 nm, the F2g feature is located at 527 nm, while the ionic nitrate is located at 544 nm. This 17 nm difference already results in different absorptions during the first few minutes as illustrated in Figure 10. Hence normalization by the F2g feature implies also an error in the temporal evolution of the nitrate species during the first few minutes of the reaction. To compare the error induced by the normalization by the F2g feature with the other corrections, the temporal evolution of the integrated ionic nitrate and summed bidentate and bridged nitrate signals was determined for the raw data as well as for all corrections. Figure 11 depicts the results for the temporal evolution of the summed bidentate and bridged nitrate area (i.e. integrated intensity), while Figure 12 depicts the results for the temporal evolution of the ionic nitrate area. In both cases, the area of stored nitrate species is lower for the raw data during the first 15 min. At later times, it shows behavior similar to that of the corrected data. Consequently, without correction one would expect a slower formation of the nitrate species at the beginning. For the ionic nitrate area there are no noticeable differences between the three corrections within the error of the analysis (see Figure 12). In contrast, for the summed bidentate and bridged nitrate areas significant differences are observed (see Figure 11). In particular, the corrected area shows a large deviation during the first 10 min. This observation can be ascribed to the strong differences of the values for reflections close to the limit 0.9 as discussed above. The F2g normalized areas show variations from the linearly corrected areas mainly during the first 5 min. As can be seen in Figure 10, the absorption at the position of the F2g feature is higher than for the bridged nitrates. It also shows different absorption changes during the first 5 min. Since the F2g normalization considers only the absorption changes at the F2g peak position,


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whereas the linear correction is wavenumber dependent, the true bridged nitrate evolution is better described by the linear correction.

Figure 11. Temporal evolution of the summed bidentate and bridged nitrate area of raw (black), F2g normalized (blue),

corrected (green) and linearly corrected (red) Raman spectra. The analyzed bands are positioned within 1520-1660 cm-1.

function results again in an attenuation, but no compensation of the intensity variations caused by the absorption effects. The F2g normalized data shows a behavior different from that of the linearly and corrected data for the first 5 min. Since the standard deviation for the area of the Ce-O feature is sufficiently small, the absorption differences at the F2g and CeO peak positions lead to a deviation from the linear correction. However, the deviation of the F2g normalization from the linear correction during the first 10 min is significantly smaller for the Ce-O feature than for the summed bidentate and bridged nitrate species (compare Figure S5 and Figure 11), since the Ce-O feature is located closer to the F2g feature (see Figure 10). In the literature, either the F2g mode or the 2LO mode of ceria was used as an internal standard in the Raman spectra, without knowing if normalization by these modes masks any dynamics in intensity.6-12 However, at 514 nm excitation the Raman spectra range from 514 to 670 nm in the UV-Vis spectra and, consequently, different wavenumbers in the Raman spectra are subject to different absorptions (see Figure 10). Therefore, we examined the F2g normalization and its effect on the intensities of other features and compared it to other wavenumberdependent absorption corrections. While for some features normalization by the F2g feature can be sufficient (see Figure 12), for other features the error can be noticeable (see first 4 min of Figure 11 and Figure S5). The error of the F2g normalization increases with increasing spectral distance of the investigated features to the F2g feature. In resonance Raman spectroscopy, the function is commonly used for absorption correction. Hence we examined its usage for Raman spectroscopy. As predicted by the function for the high-reflection region, the recorded Raman spectra showed a large decrease in the Raman intensity, while only a small change in absorption could be observed (see Figure 6). However, correction by the function gave not only unsatisfactory results, but also the largest deviations, since Raman spectroscopy works at the limits of this correction.

CONCLUSIONS

Figure 12. Temporal evolution of the ionic nitrate area of raw (black), F2g normalized (blue), corrected (green) and linearly corrected (red) Raman spectra. The analyzed band is positioned at 1037 cm-1.

A similar behavior can be observed for the Ce–O surface stretching feature (see Figure S5). The absorption of NO2 is accompanied by a reaction of surface oxygen, resulting in a decrease of the Ce–O feature. The raw data of the Ce–O area shows a behavior similar to that of the F2g feature intensity owing to absorption variations. The correction by the

In the work reported in this contribution we studied the influence of absorption on the intensity of Raman spectra of solid oxide materials. Since the reduction of oxides may result in new bands in the UV-Vis region, the Raman intensity decreases as a consequence of these absorption bands. For illustration NO2 storage in ceria was chosen, since it is accompanied by a reduction of ceria. Upon exposure to NO2, the Raman spectra of ceria show variations of about 63% in the intensity and a red-shift of the F2g feature. To elucidate the origin of this behavior, we combined the Raman spectra with UV-Vis spectra to correct the Raman spectra for absorption effects. During reaction the absorption increased to about 10%. As a result of the combined spectroscopic approach the dynamics in Raman intensity could be ascribed to absorption effects. These studies were also assisted by Raman investigations at 770 nm excitation, where no absorption was observed. Consequently, not only in resonance Raman spectroscopy but also in Raman spectroscopy, adequate correction for absorption effects is crucial to reveal the correct intensities of the Raman features (see Figure 11 and Figure 12). It is shown in the context of the NO2 storage in ceria that the commonly used absorption corrections are only of limited use for a quantitative description of absorption effects. Therefore,



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a new wavelength-dependent absorption correction was developed and its application illustrated. In any case, simultaneous experiments by UV-Vis and Raman spectroscopy are recommended to elucidate to what extent absorption effects influence the Raman spectra and to allow for their quantification.

ASSOCIATED CONTENT Supporting Information Presentation of additional Raman and UV-Vis data; Figures S1 – S-5 (PDF)

AUTHOR INFORMATION Corresponding Author * E-Mail [email protected]; Tel 0049-61511621975

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Deutsche Forschungsgemeinschaft (DFG). The authors thank Philipp Waleska for help with the Raman experiments at 770 nm excitation.

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