**The Relevance of ATR-FTIR Spectroscopy in Semiconductor Photocatalysis**

Mohamed Faycal Atitar, Hamza Belhadj, Ralf Dillert and Detlef W. Bahnemann

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/60887

#### **Abstract**

Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy has a high potential for investigating a wide range of samples and systems. In photoca‐ talysis, various interfacial phenomena can be studied using this technique, including pH-dependent adsorption and photodegradation of probe molecules. The analysis of the processes occurring at the interface of thin particle films deposited on the surface of an ATR crystal, either in the liquid or the gas phase, is perhaps the best way to elucidate the mechanism of adsorption and heterogeneous photocatalytic reactions. This chapter summarizes the recent advances and applications of ATR-FTIR techni‐ ques in semiconductor photocatalysis. A brief outlook at some of the possible investigations in this area is provided and the different proposed adsorption and photocatalytic degradation mechanisms are discussed.

**Keywords:** Adsorption, attenuated total reflection Fourier-transform infrared (ATR-FTIR), photocatalysis, semiconductor, spectroscopy, thin films, TiO2

### **1. Introduction**

The expanding interest in environmental and energy issues led to the consideration of heterogeneous photocatalysis as one of the most promising advanced oxidation processes. The interest in this scientific field has increased in the last decade since photocatalysis is assumed to be a powerful tool for the destruction and remediation of highly toxic pollutants, the

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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.

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 widespread applicability of SERS [14, 15].

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

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 surface interactions. An in-depth analysis of the different proposed adsorption and photoca‐ talytic reaction mechanisms on TiO2, as well as on other metal oxides also employed in photocatalysis will be given. To complement this overview, results and interpretations of quantum chemical calculations will also be presented.
