Raman spectroscopy, silica surface studies

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

The methoxylation can be carried out by reacting silica with methanol vapor at 300-400°C, or by refluxing silica in methanol (21,36). Because the infrared spectrum of the modified surface is well understood (36) we chose to use this system as a model to test the feasibility of using Raman spectroscopy (21 ) for studying such surface modification procedures. 

There are, at present, two overriding reasons an experimentalist would choose to employ laser Raman spectroscopy as a means of studying adsorbed molecules on oxide surfaces. Firstly, the weakness of the typical oxide spectrum permits the adsorbate spectrum to be obtained over the complete fundamental vibrational region (200 to 4000 cm-1). Secondly, the technique of laser Raman spectroscopy is an inherently sensitive method for studying the vibrations of symmetrical molecules. In the following sections, we will discuss spectra of pyridine on silica and other surfaces to illustrate an application of the first type and spectra of various symmetrical adsorbate molecules to illustrate the second. 

Ten years ago one would have predicted that Raman spectroscopy could never be used to study monolayer adsorption on metals because either (a) the sensitivity of the technique would be too low to permit detection of signals using a single reflection from a smooth metal surface, or (b) oxide supported metal surfaces are black if the metal loading is high and therefore the laser light would be absorbed. Both of the above objections have been shown to be faulty insofar as the technique has now been used to study absorption on silica-supported nickel (51, 52,53) and on single crystal nickel (54). Moreover in the special case of silver,... 

The idea of TERS is to have a localized probe that, ideally, stimulates SERS but without perturbing the chemical system under study. This concept has been taken forward by Tian s group in recent work on silica- or alumina-coated Au nanoparticles in so-called shell-isolated nanoparticle-enhanced Raman spectroscopy, or SHINERS [208], The idea is that the thin (a few nanometers) silica or alumina coating renders the particles inert so that they can be scattered over the surface of interest to generate SERS signals without perturbing the surface chemistry, rather like a large random array of TERS tips. 

In the past few years, in situ Raman spectroscopy studies of supported metal oxide catalysts have focused on the state of the surface metal oxide species during catalytic oxidation reactions (see Table 2). As mentioned earlier, there has been a growing application of supported metal oxide catalysts for oxidation reactions. The influence of different reaction environments upon the surface molybdena species on Si02 was nicely demonstrated in two comparative oxidation reaction studies (see Fig. 4). The dehydrated surface molybdena on silica is composed of isolated species (no Raman bands due to bridging Mo—O—Mo bonds at —250 cm ) with one terminal Mo=0 bond that vibrates at —980 cm" The additional Raman bands present at —800, —600, and 500-300 cm in the dehydrated sample are due to the silica support. During methane oxidation, the surface... 

Sekhar et al. identified another means of using metal-coated silica nanowires for the detection of cancer [41]. Here, the metal-decorated silica nanowires under study acted as a filter, such that their high selectivity made them effective in the diagnosis of cancers and other diseases. Specifically, the nanowires could detect interleukin-10 (IL-10) and osteopontin (OPN). For a closer examination of the selectivity of the metal nanoclusters, Raman spectroscopy can be used to study the affinity of certain substrate surfaces towards biomarkers. The substrate itself must be chosen carefully as this will affect the Raman scattering from the biomarkers. 

The use of IR and Raman spectroscopy as complementary analytical techniques (to other physical and theoretical methods) is unlimited, especially in the case of characterizations. Also, a major use of theoretical calculations is the prediction and validation of experimental (including IR and Raman) spectroscopic data, and some applications have been reported earlier. Thus, in the surface binding of DMMP, DIMP, DFP, and Sarin to silica, a DFT comparison with the experimental IR shifts showed that the theoretically-modelled adsorption sites are similar to those found by experiment, and in the case of Cyclophosphamide, the IR and Raman spectra were assigned from DFT calculations, also a homologous series of aminobisphosphonates were studied and characterized using IR and NMR spectroscopy. ... 

---