Description of the technique
Infrared spectroscopy is certainly one of the most important analytical techniques available to today's materials scientists. One of the great advantages of infrared spectroscopy is that virtually any sample in virtually any state may be studied. Liquids, solutions, pastes, powders, films, fibres, gases and surfaces can all be examined with a judicious choice of sampling technique. As a consequence of the improved instrumentation, a variety of new sensitive techniques have now been developed in order to examine formerly intractable samples.
Typically when a molecule is exposed to infrared (IR) radiation, it absorbs specific frequencies of radiation. The frequencies which are absorbed are dependent upon the functional groups within the molecule and the symmetry of the molecule. IR radiation can only be absorbed by bonds within a molecule, if the radiation has exactly the right energy to induce a vibration of the bond. This is the reason only specific frequencies are absorbed.
Infrared spectroscopy focuses on electromagnetic radiation in the frequency range 400-4000 cm-1, where cm-1is known as wavenumber (1/wavelength), which is a unit of measure for the frequency. To generate the infrared spectrum, radiation containing all frequencies in the IR region is passed through the sample. Those frequencies which are absorbed appear as a decrease in the detected signal. This information is displayed as a spectrum of % transmitted radiation plotted against wavenumber.
In addition to identifying the molecule using IR spectroscopy, other information can be obtained. In particular the frequency of the stretching is related to the ratio of the strength of the bond and the reduced mass of the atoms involved. If the reduced mass is known then the strength of the bond within the molecule can be estimated. For a given value of the reduced mass, a vibration of long wavelength (small frequency) corresponds to a long bond (weak bond) and one of short wavelength (high frequency) corresponds to a short bond (strong bond).
The technique is also useful for quantitative analysis. For example the concentration of a solution can be estimated if the specific absorption of the solute is known in a spectral region where the solvent is transparent.
Examples of applications in the EMMI laboratories
1 Traditional Infrared Analysis
The traditional infrared analysis method for powdered samples is the collection of a KBr pellet spectrum of an aliquot of the powdered sample. However, preparation of KBr pellets requires some skill, especially for quantitative analyses. Attenuated Total Reflectance (ATR) accessories provide a simple and effective alternative, suitable for the infrared analysis of powders.
The powdered sample is simply placed onto the ATR crystal (in our case diamond) and the sample spectrum is collected. The sample is then cleaned from the crystal surface and the accessory is ready to collect additional spectra. ATR analysis is less complicated than using KBr pellets, is fast and a very small amount of the sample is needed. ATR spectra cannot be used with a quantitative method developed using transmission spectra due to differences in the relative peak intensity of the absorption bands - a result of the internal reflection mechanism of ATR accessories.
The traditional FTIR analysis is an important method for the characterization of the polymers and ceramics and their stability against hydrolysis and oxidation. The thermolysis products at different temperatures can be analyzed by FTIR in order to understand the mechanism of thermal transformation to ceramics.[2,3] Together with solid state MAS NMR and Raman, FTIR is one of the most versatile method for material characterization. [2,3] Moreover, FTIR and HRTEM facilitate the identification the crystalline phases in the ceramics.[4,5]
1. Ralf Riedel, Edwin Kroke, Axel Greiner, Andreas O. Gabriel, Lutz Ruwisch, and Jeffrey Nicolich, Inorganic Solid-State Chemistry with Main Group Element Carbodiimides, Chem. Mater. 1998, 10, 2964-2979
2. Ya-Li Li, Ralf Riedel, Jürgen Steiger, and Heinz von Seggern, Novel Transparent Polysilazane Glass: Synthesis and Properties, Advanced Engineering Materials 2000, 2 (5), 290-293.
3. Andreas O. Gabriel, Ralf Riedel, Wolfgang Dressler, and Silvia Reichert, Thermal Decomposition of Poly(methylsilsesquicarbodiimide) to Amorphous Si-C-N Ceramics, Chem. Mater. 1999, 11, 412-420.
4. Ya-Li Li, Edwin Kroke, Alexander Klonczynski, and Ralf Riedel, Synthesis of Monodisperse Spherica Silicon Dicarbodiimide Particles, Adv. Mater. 2000, 12 (13), 956-961.
5. Ralf Riedel, Axel Greiner, Gerhard Miehe, Wolfgang Dressler, Hartmut Fuess, Joachim Bill, and Fritz Aldinger, The First Crystalline Solids in the Ternary Si-C-N System, Angew. Chem. Int. Ed. Engl.1997, 36 (6), 603-606.
2 Adsorption of probe molecules and study by FTIR
The absorption of specific probe molecules (CO, pyridine, chloroform, NO, ammonia, etc.) allows evaluating the surface properties of materials such as acidity, basicity, redox properties, metal dispersion, etc. [6-7]. For example, the position and the quantification of the specific vibration bands of adsorbed pyridine at different temperatures give information about the nature (Lewis or Bronsted) and strength of acid sites .
6. Delsarte, S.; Mauge, F.; Lavalley, J.-C.; Grange, P., "Basic sites on mixed nitrided galloaluminophosphates"AlGaPON": infrared studies of SO2 and CDCl3 adsorption", Catal. Letters 68 (2000) 79-83.
7. Portela, L.; Grange, P.; Delmon, B., "The adsorption of nitric oxide on supported Co-Mo hydrodesulfurization catalysts: a review", Catal. Rev. - Sci. Eng. 37 (1995) 699.
8. Jung, S. M.; Grange, P. "TiO2-SiO2 mixed oxide modified with H2SO4. II. Acid properties and their SCR reactivity", Appl. Catal. A 228 (2002) 65.
3 In situ / operando FTIR
The « in situ » infrared spectroscopy study the behaviour of solid samples under static or dynamic flows such as inert, oxidant, reductant, nitriding, reaction media atmospheres. In the operando mode, the infrared analysis of the solid materials under flow is combined to the simultaneous measurement of the flow composition by mass spectrometry providing a characterization of the materials at work.
4 DRIFTS and environmental temperature-controlled chamber accessories
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is a technique that collects and analyzes scattered IR light. It is used for the measurement of fine particles and powders. DRIFTS offers the advantage of fast and easy sampling as no pelletizing is required and the powder is directly placed in a microcup. An environmental temperature controlled chamber allows performing in situ or operando analysis [9-10].
9. Centeno, M. A.; Delsarte, S.; Grange, P., "DRIFTS Study of the Surface Structure and Nitridation Process of Mixed Galloaluminophosphate Oxynitride (AlGaPON) Catalysts", J. Phys. Chem. B 103 (1999) 7214.
10. Demoulin, O.; Navez, M.; Ruiz, P., "Investigation of the behaviour of a Pd/g -Al2O3 catalyst during methane combustion reaction using in situ DRIFT spectroscopy", Appl. Catal. A 295 (2005) 59.
5 Grazing Incidence FTIR
This accessory is used to characterize planar surface via specular external reflection of the IR beam. Grazing incidence is used to minimize the analyzed depth, which is necessary for thin layers (i.e. self-assembled monolayers) characterization.
Example of application:
Grazing incidence spectrum of a monolayer of mercaptododecylphosphonic acid grafted on a silicon wafer coated by PVD with 20 nm titanium. d+ and d- correspond to the symmetric and assymetric CH2 stretching vibrations, respectively.
6 TGA-IR Analysis
TGA-IR (thermogravimetric analysis coupled with infrared spectroscopy) is used to analyze the evolution of gases from samples both by weight loss (quantitative) and IR spectroscopy (qualitative). In our laboratory, a Netzsch STA 409 TGA instrument is used with a Bruker Tensor 27 FTIR spectrometer outfitted with a flow-through gas cell and an LN-MCT external detector.
The TGA device of the instrument is programmed to control temperature, heating rates, and soak times. A flowing nitrogen atmosphere is maintained at all times in the instrument in order to provide a carrier gas in the system and to simulate the low vapor pressure of gases evolving during vacuum drying. While this is a valid simulation of the vacuum environment, some differences would likely be expected between the two cases and further study is warranted. IR spectra are continuously monitored and recorded at defined intervals.
Combined with mass spectrometry, TGA-IR studies provide additional insight into the crosslinking process and the thermal transformation of the polymers to ceramics. 
11. Edwin Kroke, Ya-Li Li, Christoph Konetschny, Emmanuel Lecomte, Claudia Fasel, Ralf Riedel, Silazane derived ceramics and related materials, Materials Science and Engineering 2000, 26, 97-199.
7 Combined Raman and FTIR Micro-Spectroscopy
HORIBA Jobin Yvon LabRAM/IR combines upon a single bench-top system, both confocal Raman microscopy and the complementary FTIR micro-spectroscopy. The instrument provides highly specific spectral fingerprints which enables precise chemical and molecular characterization and identification. Traditionally, only a single technique has been coupled with microscopy - either the Raman or Infrared. The combination of the two provides a full and complete vibrational spectroscopic characterization and maximizes the strengths of the two techniques.
The microscope offers the advantages of same spot analysis where the same position on the sample can be analyzed via both techniques and without the need for any sample positioning or instrument adjustment. The automated XYZ axis mapping gives the possibility to analyze microstructured systems such as MEMS (micro electro mechanical systems).
The FTIR module is hard coupled to the base unit and provides fast FTIR micro-analysis with both contact and non-contact IR optimized objectives.
Integration of the module enables quick and easy selection of the required spectroscopic mode.
The resultant Raman and FTIR analysis is optimal providing performance comparable to the very best single mode instruments available whilst the single unit design provides both space and cost savings over the conventional use of separate Raman and FTIR instrumentation.