**2. History and brief overview of ATR-FTIR spectroscopy**

purification of polluted water and air, the development of self-cleaning surfaces coated with semiconducting metal oxide materials, and the conversion of solar energy into chemical energy [1–3]. Many semiconductor materials have been tested as photocatalysts, nevertheless, due to its low cost, abundance, high activity, and stability under a variety of conditions. Titanium dioxide (TiO2) is the most reliable and widely used material [4, 5]. Accordingly, there has been a tremendous amount of research on diverse aspects of TiO2 (nano)materials, ranging from their synthesis, characterization, and applications to atomic scale, to experimental and theoretical investigations of their fundamental physical and chemical properties [1, 5–7]. Despite these investigations, there remains a need to better understand the reaction mecha‐ nisms of the transformation of organic molecules occurring during TiO2 photocatalysis.

204 Emerging Pollutants in the Environment - Current and Further Implications

Surface science plays a prominent role in mechanistic investigations concerning the photoca‐ talytic process, providing a unique approach to understand bulk, surface, and interfacial phenomena occurring at the TiO2 surface [6, 8, 9]. According to several proposed photocatalytic mechanisms [10, 11], an important point for the conversion of the molecules on TiO2 is the physical and electronic structure of the adsorbed state of the molecules. How a molecule binds onto the TiO2 surface influences its electronic structure, as well as its redox properties. *Inter alia*, coverage, thermal stability, and reactivity, adsorption structure and site, are all important factors [8]. The interest in surface techniques to investigate liquid-solid and gas-solid interfacial chemistry has grown due to the importance of the information they provide. Few surface spectroscopic techniques are adequate to perform *in situ* analyses of interfacial interactions. For example, sum-frequency vibrational spectroscopy is restricted to planar solid-solution interfaces [12], infrared ellipsometry is considered mainly as a technique for the analysis of thin solid films rather than of interfacial species [13], and surface enhanced Raman spectro‐ scopy (SERS) requires the presence of metals on the surface. Thus, this complicates the more

IR spectroscopy is the oldest and most commonly used method for identifying both organic and inorganic chemicals, as well as for providing specific information on molecular structure, chemical bonding, and molecular environment. Being a powerful tool for qualitative and quantitative studies, it can be applied to study solids, liquids, or gaseous samples [16]. Recently, IR spectroscopy has been applied *in situ* to study surface reactions on immersed solids such as oxides. This has been achieved with particle films via internal reflection or attenuated total reflection (ATR-FTIR) methods. Investigating several metal oxide solid particles in suspensions or deposited as thin films on ATR crystals, these developments have led to *in situ* ATR-FTIR studies of adsorption and chemical reactions on a variety of solid-liquid and/or solid-gas interfaces in the photocatalysis context [17–24]. The ATR-FTIR technique has proved to be a powerful tool for probing binding mechanisms and for characterizing the

This book chapter focuses on the application of this technique in the above-mentioned context. An overview of the investigations that have been performed to date will be given, analyzing the different experimental procedures, and summarizing the performed investigation of

adsorption of organic molecules onto metal oxide surfaces in liquid media.

widespread applicability of SERS [14, 15].

Infrared absorption spectroscopy (IR) has contributed for more than fifty years to the molecular view on a wide variety of systems. The selection rule for a vibrational mode of a molecule to be IR active is that there is a change of the electric dipole moment of the molecule upon absorption of light. The absorption of infrared light due to the excitation from the ground vibrational energy level to a higher energy level provides information concerning molecular structure and molecular interactions [15, 25, 26]. Due to the existence of the wide absorption spectra database in the mid-infrared region (4000–400 cm-1), infrared spectroscopy is consid‐ ered as a universal technique since many molecules have strong absorbances in this region [27].

Fourier-transform infrared (FTIR) spectroscopy is a well-established technique based on the idea of the interference of radiation between two beams to yield an interferogram. The latter is a signal produced as a function of the change of path length between the two beams. The two domains of distance and frequency are interconvertible by the mathematical Fourier transformation method.

ATR spectroscopy was introduced simultaneously by Harrik [28] and Fahrenfort [29] based upon the total internal reflection phenomena. In this approach, IR spectra are recorded for a sample material that is in contact with an internal reflection element (IRE). The IR beam is focused onto the edge of the IRE, reflected through the IRE, and then directed to the detector (cf. Figure 1) [26, 27]. In this case, all the light reflects off the internal surface of the IRE, hence explaining the term total internal reflection [27]. The internal reflection element (IRE) or ATR crystal has, in most cases, a higher refractive index (*n1*) as compared to the sample (*n2*). Another important parameter is the incidence angle *θ* that can be determined from the refractive indexes of the sample (*n2*) and the IRE (*n1*):

$$\theta = \sin^{-1}(\frac{\eta\_2}{\eta\_1}) \tag{1}$$

The major applications of the ATR method are in the mid-IR region. However, the range has been extended to the near-IR, the far-IR, as well as to the UV and visible spectral regions. Therefore, it is of great importance to choose a suitable ATR crystal for a given application. The most common ATR crystals with their respective refractive indexes and some other relevant properties are summarized in Table 1.

**Figure 1.** Schematic diagram of a horizontal ATR sampling accessory illustrating the important parameters [16].


**Table 1.** Relevant properties of some common ATR crystals.

Since the IR beam should penetrate the sample, the penetration depth (*dp*) is one of the important parameters in ATR-FTIR spectroscopy. The measure of the depth that the infrared beam enters into the sample is defined by equation (2):

$$d\_{\rho} = \frac{1}{\left[2\pi Wn\_1 \left(\sin^2\theta - n\_{21}^2\right)^{\frac{1}{2}}\right]}\tag{2}$$

where *dp* is the depth of penetration, *W* the wavenumber, *n1* the refractive index of the ATR crystal, *θ* the angle of incidence, and *n*21 the fraction *n*<sup>2</sup> *n*1 .

Each of the parameters mentioned above has important messages to teach us about the ATR technique and its application. Readers interested in details of the theory of ATR should consult the respective literature [16, 25, 27].
