Langmuir 1994,10, 2444-2449
2444
A Novel Technique for Recording Infrared Spectra of Powders: Attenuated Total Reflection Immersion Medium Spectroscopy Ulrich Kunzelmann,? Helmut Neugebauer,* and Adolf Neckel Institute of Physical Chemistry, University of Vienna, Wahringer Strasse 42, A-1090 Vienna, Austria Received January 3, 1994. In Final Form: March 29, 1994@ A new recording technique for the measurement of infrared spectra of powders using ATR spectroscopy is presented in this paper. The method, called ATRIMS (attenuated total reflection immersion medium spectroscopy),circumvents some ofthe problems of conventionalmethods. With ATRIMS, an ATR reflection element is covered with a thin film of liquid paraffin (Nujol) as an immersion medium for the powder. Compared to ATR spectroscopy of dry powders on the surface of a reflection element, ATRIMS shows higher band intensities, lower detection limits, reduced necessary sample amount, easier handling, and shorter analysis time. The resulting spectra have the typical band intensity-wavelength relationship of ATR spectra. The method is well suited for the investigationof strong absorbing substances (e.g.magnetite) and strong scattering substances (e.g. hydrargillite).
Introduction FTIR spectroscopy is a powerful tool to provide information on the chemical composition of thin surface layers and is increasingly applied to the study of electrochemical pr0cesses.l Especially in situ ATR-FTIR spectroscopy combined with electrochemical investigations of the formation, modification, and characterization of thin layers are of increasing importance. The ATR reflection elements (ARES) are covered with a thin ( < l o nm) metal layer or a metal grid. These coatings, which are produced mostly by evaporation in vacuum, act as working electrodes. While spectroelectrochemical experiments are performed, the ARE covered with the working electrode is in contact with the bulk electrolyte in an appropriate in situ electrochemical cell2 Very often, the in situ experiments show only weak spectral effects; e.g. when the surface species have weak oscillator strengths and/or low surface concentrations. For example, only thin, weak absorbing layers are formed on iron surfaces which have been passivated in weak alkaline, weak acidic, or neutral electrolytes. In this case, typical characteristics ofthe in situ measurement techniques have strong influence on the resulting spectra. To elucidate the results of spectroelectrochemical experiments, it is necessary to measure reference spectra of the substances involved in the reactions. Usually, spectra of solid substances are recorded using KBr pellets or Nujol suspensions in transmission technique. However, KBr and Nujol transmission methods have several disadvantages: (1) Interactions between the analyte and the KBr matrix can cause drastic shifts ofthe band position and changes of the band shapes. (2)When Nujol suspensions are measured between two parallel, IR transparent windows, fringes due to thin film interference often occur; strong absorption bands of Nujol also disturb the spectra. These disadvantages cannot be neglected, especially for the interpretation of the results of spectroelectrochemical experiments with weak spectral effects. Present address: Institut fur Festkorper- und Werkstofforschung IFW Dresden e. V., Postfach, D-01171 Dresden, FRG. Abstract published in Advance A C S Abstracts, May 15,1994. (1)Neckel, A. Mikrochimica Acta [Wien] 1987,III, 263. (2) Neugebauer, H.; Moser, A.; Strecha, P.; Neckel, A. J.Electrochem. Soc. 1990,137, 1472.
To minimize misinterpretations, it is advantageous to use the same measuring technique for recording both the in situ and the reference spectra. When the ATR technique is used, some of its characteristic properties have to be considered: When light is totally reflected at the interface between the optical denser medium (ARE) and the optical rarer medium (environment), an evanescent wave penetrates the optical rarer medium. The penetration depth d, is defined as the distance from the ARE’Ssurface, where the amplitude of the electrical field vectorE ofthe evanescent wave is reduced to the value Ede (Eo, electrical field vector a t the interface; e, base of the natural logarithm). In the case of two dielectric, nonabsorbing media, d, depends on the angle of incidence q1, the wavelength in the optical denser medium (ARE)A1,the refractive index ofthe optical rarer medium (environment) no, and the refractive index ofthe optical denser medium (ARE)n1.3This dependence is expressed in eq 1 (1) Ifvery thin layers (layer thickness dl 2~uz
(4)
where a is the absorption coefficient and d the thickness of the sample. Expansion of the exponential function exp(-ad) leads for low values of (ad), (ad 0.11,to (1 -
ad). On the other hand, in the case of ATR, the reflectivity
R of a material on the surface of an ARE for a single reflection and low absorption is given by R = (1 - ad,), where de is an effective thickness. A comparison of the expressions for transmission and ATR, noting that the same absorption coefficienta appears in both cases, shows that the effectivethickness de corresponds to the thickness d of a film in a transmission experiment which would yield the same absorbance as that obtained in an ATR experiment using a semiinfinite bulk sample. Harrick3 derived an equation (eq 3) for the effective thickness de of bulk materials whose thickness is much greater than the penetration depth d , of the evanescent wave. The low absorption approximation for de and the expressions for the electric fields for zero absorption were used.
(3)
Eo is the electric field amplitude at the reflecting surface of the ARE for unit field amplitude of the incoming wave in the denser medium (ARE). It can be gathered from eq 3 that four factors determine the effective thickness de, whereby Eo and d , decrease with increasing angle of incidence q 1 . Besides the angle of incidence 91,the ratio of the refractive indices (n&) is an important factor,
In Table 1,the calculated values for the relative penetration depth, the relative effective thickness, and the ratio of the relative effective thickness to the relative effective thickness of air for some liquids, which could be used as an immersion medium, are listed. It can be seen from . these values that an increase of the refractive index no of the immersion medium from the value no = 1.000 (air) to no = 1.475 (Nujol) leads to an increase of the relative effective thickness by a factor of 3.29 (dew in Table 1). However, one has to consider that the effective refractive index of a medium consisting of the powder and the immersion medium is much higher in the range of an absorption band. An increase of the absorbanceby using immersion media of higher refractive indices was also observed by Lynch and Chisholm,Swho used this effect for the enhancement (4) Mirabella, F. M.; Hanick, N. J. InternaZReflectionSpectroscopy: Review and Supplement, Harrick Scientific Corp.: Ossining, 1985; Chapter 11. (5) Lynch, B. M.; Chisholm, S. L. Langmuir 1992,8,351.
Kunzelmann et al.
2446 Langmuir, Vol. 10, No. 7, 1994
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Figure 1. Schematic ATRIMS procedure.
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of the sensitivity of the ATR technique. These authors investigated the dependence of the absorption band areas of CaC03 powder on the refractive indices of several immersion media by using an overhead ATR cell with a ZnSe-ARE and a two-phase composite sample consisting of a constant amount of analyte (CaC03)in the respective immersion medium. The effect was explained by the dependence of the penetration depth on the refractive index of the immersion medium.
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Instrumentation and Sample Preparation A routine FTIR spectrometer, Model 320, from NICOLET, Inc. (Madison, WI),with a Model 620 workstation and a DTGS detector was used to record ATR spectra with 4 cm-l resolution (wavelength range 400-4000 cm-l). A 45"KRS-5 ARE (parallelepiped, dimension 50 x 10 x 2 mm) was mounted in a Model 300 ATR accessory from SPECTRA-TECH, Inc. (Stanford, CT). For each spectrum 128 scans were coadded before performing the Fourier transformation. As illustrated in Figure 1, the ATRIMS procedure consists of three steps: (i)measurement of the uncovered ARE, yields single beam spectrum 11;(ii) measurement of the ARE covered with a thin (3-5 pm)Nujol film at one face, yields single beam spectrum I2; (iii) measurement of an immersion of about 0.1- 1 mg of the analyte powder in the Nujol film of step (ii), yields single beam spectrum 13. When measuring a series of samples, step (i)has to be performed only once before the first sample measurement. The spectra of the samples are represented in absorbance units. The absorbance A is calculated according to eq 7 (7)
In eq 7, fis a factor applied to the pure Nujol spectrum (log11/12). Factor fcorrects for the change of the volume fraction of Nujol in the measurements (ii)and (iii)and is determined interactively during the subtraction of the Nujol bands from the sample spectrum (negative Nujol absorption bands always appear in the spectra calculated with only the fist term of eq 7, because of the lower volume fraction of Nujol in the suspension). In practice, a constant factor f in the whole spectral range cannot correct all spectral features caused by Nujol. The ATRIMS spectra shown in this work are corrected for the Nujol absorption around 720 cm-l, leaving some uncorrected negative bands around 1380, 1450,and 2900 cm-l. The ATRIMS measurements are compared with transmission spectra of Nujol suspensions as thin films between two KBr windows and of KBr pellets with 0.1 to 1 w t % analyte. Again, also in the case of the Nujol transmission spectra the Nujol bands
Figure 2. (top) ATFUMS spectrum of goethite (a-FeOOH), compared with (bottom) the Nujol transmission spectrum (spectral influences of C02 and H20 vapor are only partially compensated). could not be removed completely by a similar subtraction procedure as described in eq 7. Liquid paraflin (Nujol) in spectroscopicgrade purity from E. Merck (Darmstadt, FRG) was used. A number of substances [formula (mineral name, producer or reference of the synthesis)] were measured: Fe20mH20 (limonite, ref 6 )Fe304 (magnetite, ref 7), a-FeOOH (goethite, ref 8),andAl(OHh (hydrargillite, refs 9 and 10). All substances were used as minerals, as chemicals in reagent grade purity, or were synthesized by us. Just before each sample measurement they were freshly pulverized in an agate mortar (grain sizes 1-5 pm). The software package SPECTRA-CALC (Galactic Industries Corp., Salem, NH)was used for the calculation and drawing of the spectra.
Results One of the characteristic substances involved in corrosion processes of iron is a-Fe00H (goethite). The goethite spectrum shows OH-bending vibration bands a t 890 and 790 cm-l, a lattice vibration band at 630 cm-l, and a broad OH-stretching vibration band at 3150 cm-l. Figure 2a represents a typical goethite spectrum recorded with ATRIMS. Only small shifts in the band positionscompared to the absorbance spectrum (measured (6) Own mineral collection. (7) Kiinzelmann,U.;Jacobasch,HA.;Reinhard, G. Werkst.Korros. 1989,40, 723. (8)Nauer, G.;Strecha, P.; Brinda-Konopik,N.; Liptay, G.J. Therm. Anal. 1989,30,813. (9) Johnson Matthew AZppha-Products (Karlsruhe, FRG). (10) We are very gratefulto Dr. Hofmann (MineralogischeSammlung der TU Bergakademie Freiberg, FRG) and Dr. Niedermayr (Mineralogische Sammlung des NaturhistorischenMuseums Wien, Austria) for placing these minerals at our disposal.
Langmuir, Vol. 10,No. 7,1994 2447
Recording IR Spectra of Powders .35 .3 -
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Figure 4. ATRIMS spectra of synthetic aluminum hydroxide, M(0H)s: (a) original ATRIMS spectrum; (b)with ATR correction; (c) after baseline correction of spectrum b. I
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I'I WAVENUMBER / cm-' Figure 3. (top)ATRIMS spectrum of the mineral hydrargillite (y-M(OH)s) in comparison with (bottom) the transmission spectrum of the Nujol suspension.
in transmission technique) of a Nujol suspension (Figure 2b) are found. In the region of the bending and lattice vibrations (bands at 890, 790, and 630 cm-l) the absorbances of the bands are amplified at lower wavenumbers. The intensity of the OH-stretching vibration around 3150 cm-l is nearly completely suppressed. The different band intensities are due to the wavelength dependence of the penetration depth (eq 1). Similar effects can be seen in the ATRIMS spectrum of the mineral hydrargillite ( y Al(OH),; Figure 3a). The immersing effect of the Nujol layer makes the ATRIMS technique advantageous for substances which scatter light independent of the wavenumber. Smaller refractive index differences of both the analyte powder and the immersion medium depress the stray light and the baselines of the spectra are flat. In contrast, however, spectra collected by using other reflection techniques show high background absorptions and tilted baselines in the case of such substances. ATR spectra may be corrected with a wavenumber-dependent factor to get transmissionlike shapes of the spectra. The result of such an ATR correction for a spectrum of synthetic aluminum hydroxide is shown in Figure 4b. Compared with the uncorrected spectrum (Figure 4a), especially the OH-stretching vibrations are amplified. Since the baseline of the uncorrected spectrum deviates in most cases from zero, it becomes tilted by the ATR correction (Figure 4a,b). Therefore, a correction for the baseline should be performed. The result is shown in Figure 4c. However, the use of such corrections should be done with great care since it does not provide more spectral information. With strong nonspecific absorbing substances, traditional reflection and trans-
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Figure 5. (top) ATRIMS spectrum of synthetic magnetite (Fea04)compared to (bottom)transmissionspectrumoftheNujol
suspension.
mission techniques yield spectra with high background absorptions. Figure 5 shows a comparison of the spectra of magnetite (Fes04)measured with the Nujol transmission technique (b) and with the ATRIMS technique (a). When the transmission technique is used, the band which has its resonance wavenumber at 570 cm-' l1 is shifted for about 15 cm-l to higher wavenumbers because of the tilted spectral background (blue shift, Figure 5b). The use of the ATRIMS technique produces only a small red (11)Nyquist, R. A.; Kagel, R. 0. Infrared Spectra of Inorganic Compounds; Academic Press: New York,1971.
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Kiinzelmann et al.
2448 Langmuir, Vol. 10, No. 7, 1994
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Figure 6. ATR spectra of limonite (Fe2OsnH20, orange f o d China, 6 mg): (a) with a Nujol film as immersion layer; (b) measurement with dry powder.
shift to lower energies due to the wavenumber dependence of the penetration depth (Figure 5a). Negative Nyjol bands (around 2900,1450,and 1380cm-') occur in the ATRIMSabsorbance spectra shown. In principle, a compensation of these negative bands should be possible by weighted subtraction of a Nujol spectrum, as indicated in eq 7.In practice, only the weak band at 720 cm-l was removed completely. Similar problems are found in the Nujol suspension transmission technique. The wavenumber regions between 1350 and 1500 cm-l as well as between 2800 and 3050 cm-l are not available for analytical purposes. Measurements in this regions can be performed only when using other immersion media (i.e. perhalogenated hydrocarbons) instead of Nujol. Discussion Limonite spectra recorded by spreading 5 mg of the sample powder over one face of the ARE with (a) Nujol or (b) air as immersion medium are shown in Figure 6.For comparison, the same amount of sample was used with both methods. The immersion of limonite in Nujol yields a strong amplification of the vibrational bands. The followingconsiderations must be taken into account when discussingamplification effectsin the case of using a Nujol film at the ARE compared with ATR spectra of dry samples: According to eqs 1 and 3,respectively, the penetration depth of the evanescent wave and the effective thickness increase with an increasing refractive index no of the environment for a given angle of incidence. This angle must be greater than the critical angle of total reflection [qdt = arcsin(ndnl)l. Using Nujol (no = 1.475) as environment, KRSd (nl = 2.38)as ARE, an angle of incidence of 45",the penetration depth a t 1000cm-1 is 4.7 pm compared with 2.8pm when using air. "he average effective thickness increases from 3.0pm (air) to 10.0pm (Nujol). In the case of a maximum volume fraction of lo%, the sample grains in the immersion medium Nujol are isolated from each other. Having an average grain size of 1 to 5 pm, they are smaller than the wavelength of the light (1 = 10 pm at 1000 cm-'1. According to the general theory of light scattering, they are not effective as dispersion centers, but their absorption (imaginary part of the dielectric function DF of the composite analyte) is dominating. So, the increase of the imaginary part of the effective DF, resulting in a high polarizability, causes a stronger electromagnetic field at the totally reflecting
I
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0 ravenumber tcm"~ 500
Figure 7. Calculated ATR spectra of FeOOH in Nujol and in vacuum (according to Gro~se'~) for a volume fraction f = 0.1. interfa~e.~ The MAXWELL-GARNETTeffectivemedium approximationis advantageousfor calculatingthe effective DF of the composite under such conditions.i2 Figure 7 shows the calculated ATR-spectra of FeOOH in air and in Nujol according to Grosse.13 Even if the intensity ratio is less than that reached in practice (compare Figure 6), a significant amplification effect is confirmed. A drastic effect can be expected, when the effective refractive index n,R of the two phase composite analyte/ Nujol is close to the critical where the total reflection is just occurring (e.g. neR,dt= 1.675with a KRS-5 ARE, angle of incidence = 45"). Significant losses of the reflectivity occur due to absorptive processes which are producing values of n , ~ greater than neff,crit.At the wavenumber of bands, which satisfy this condition, no total reflection occurs and the ATR spectroscopy can be described as common reflection spectroscopy. Harrick and Carlson'* demonstrated that the ATR technique with angles of incidencejust above the critical angle is a very sensitive tool for measuring small changes of the effective DF. In addition to the amplification effects other phenomena can contribute to the signal intensity: When the ARE is covered by a thinfilm of the immersion medium (up to 3-5 pm for the systems described in this paper), we have to consider an effective medium model for the exact mathematical description of the optical wave motion. Referringto model calculationsdone by Grosse,13 the absorption signal intensities show a saturation effect with growing thickness dl of the immersion medium. For example, such a saturation occurs in the case of Nujol with 10% volume fraction of a-FeOOH on KRS-5 when dl exceeds 5 pm. If the particles are larger than the wavelength of the IR radiation, scattering losses occur. The differences of the refractive indices of the powder and the surrounding medium are smaller when usingNujol as immersion agent compared with air. In this case, the scattering losses are minimized in favor ofthe absorption parts of the dielectric interaction. This effect may cause a further increase of the ATRIMS signal using Nujol as immersion medium.
Conclusions The application of thin films of liquid paraffin (Nujol) as an immersion medium yields an enhancement of the sample absorbanceand consequently ofthe signal to noise ratio due to an increase of the effectivethickness compared (12) Evenschor; M.;Grosse, D.; Theiss, W. Vib.Spectrosc. 1990,I , 173. (13) Grosse, P.Publication in preparation. (14) Harrick, N. J.;Carlson, A. I. Appl. Opt. 1971, 10, 19.
Recording IR Spectra of Powders
t o air. Furthermore, the experimental expense for obtaining ATR powder spectra is significantly reduced. In practice, a thin (3-5pm) Nujol film is applied on one face of the KRS-5 ARE. After the reference spectrum is recorded, the analyte is immersed in this Nujol layer and the sample spectrum is measured. The resulting spectra show a characteristic intensity shift to lower wavenumbers compared to spectra which are recorded with a transmission technique using Nujol suspensions. This shift is due to the wavelength dependence of the penetration depth ofthe evanescent wave in the two-phase composite. Nujol bands in the negative direction are found in the spectra because of its lower volume fraction in the suspension. In the case of strong wavenumber nonspecific scattering (e.g. AI(OH)3) or absorbing (e.g. FeSOJ substances, the ATR immersion medium spectroscopy (ATRIMS) has a
Langmuir, Vol.10,No. 7,1994 2449 higher performance compared to other methods for measuring powder spectra.
Acknowledpent. The investigations have been supported by the Osterreichisches Bundesministerium fir Wissenschaft und Forschung in connection with the OstWest-F’rogramm der Osterreichischen Akademie der Wissenschaften. We are very grateful to Professor Dr. P. Grosse (I. Physikalisches Institut der Rheinisch-WestfalischenTechnischen Hochschule)for model calculations. Furthermore, we thank Dr. A. Roeseler (Inst. f. Spektrochemie u. Angewandte Spektroskopie, ISAS Dortmund e. V., Aussenstelle Berlin) and Dozent Dr. G. Liebmann (Inst. f. Angewandte Photophysik, IAP der TU Dresden) for helpful discussions.
