Techniques for Ultrathin Films on Powders and Fibers

A routine procedure for obtaining an IR spectrum of a film on a powder sample involves (i) measurement of a background spectrum of the uncovered powder, the mixture of the uncovered powder with KBr or pure KBr, and (ii) measurement of the spectrum of the analyte powder (or the mixture of the analyte powder with KBr in the same proportion as for the background spectrum). Normalizing the analyte spectrum with respect to the spectrum of a transparent matrix (KBr) decreases the spectral sensitivity to the adsorbate [118]. However, this approach can be useful in the following situations  

When absorption bands of the adsorbent and adsorbate are close to each other (or overlap) and the adsorbent is intrinsically heterogeneous (e.g., wood, fibers). In this case the spectrum measured against pure adsorbent is significantly distorted. 

The KBr spectrum is employed in the internal-reference technique. In this case, a weak absorption band of the adsorbent (e.g., the overtones of the silica or zeolite lattice vibrations, 2100-1600 cm [119]) acts as the internal reference, and the ratio of adsorbate absorption to the reference absorption is proportional to the quantity of molecules adsorbed. The advantage of this is that it does not require to control precisely the sample weights [120]. 

Several methods are used for preparing samples for the transmission spectrum measurement  

In pressed disks the particles are held together only by surface forces. Solids that can be easily pressed are silica derivatives (e.g., Cab-O-Sil and Aerosil), a number of oxides (e.g.. Alumina C and Titania P25) and supported metals. Disregarding the technical problems concerning the disk preparation and their mechanical stability, which are discussed by Parkyns and Bradshaw [123], the essential disadvantages of this method for in situ studies are the formation of zones within the pressed matter, which are inaccessible for reaction with the surroundings, and changes in the support structure when the disk is pressed. 

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