TiO2 for water treatment: Parameters affecting the kinetics and mechanisms of photocatalysis (original) (raw)
Abstract
The photocatalytic activity of TiO 2 is the result of an interplay between a considerable number of parameters, e.g., phase composition, electronic structure, particle size, exposed surface area, degree of aggregation, mobility of charge carriers, presence of impurities, amount and kind of defects, adsorption of molecules from gas or aqueous phase, lateral interactions between adsorbed species, nature of solvent, etc. Furthermore, these parameters can be broadly subdivided into those that are intrinsic to the photocatalytic material, and those that are extrinsic being influenced by the surrounding environment and conditions. The specific function and influence of a given feature for the photocatalytic performance of a TiO 2 sample is difficult to characterize since many of the before-mentioned parameters are strongly coupled. For example, while the degree of aggregation could be inherent to a given material, it is also simultaneously influenced by pH. The degree of aggregation can then influence adsorption of molecules, light scattering and photon adsorption, charge carrier dynamics etc. The plurality of variables driving the nature of the photocatalytic activity, presents a challenge when trying to understand the kinetics and mechanisms underlying photocatalytic processes. It is of primary importance to develop a method to understand and control these properties (or at least some of them). In this paper, we also discuss the relevance of quantum-integrated systems in which the local environment where the molecule is adsorbed is different from the "lonely" photocatalyst or the molecule in solution, and could be treated as a whole.
Figures (2)
![However, ultrafast kinetic studies continue to provide new insights into heterogeneous photocatalysts such as the role of cat- alyst aggregation and the occurrence of the antenna effect [10,11]. The Antenna effect is one of the few suggested mechanisms tak- ing place upon illumination beyond the well-accepted elementary steps. According to the antenna mechanism the energetic coupling in a long chain of nanoparticles, assumed to occur by topotactic attachments, enables electron transfer from the particle where the initial photon absorption took place to another TiO, particle within To use absorption spectroscopy to study reaction kinetics, it is necessary to clearly distinguish and assign the absorption bands of the transient species. Recently, several groups have revealed the transient absorption bands of trapped conduction band electrons in](https://figures.academia-assets.com/44684195/figure_001.jpg)
](However, ultrafast kinetic studies continue to provide new insights into heterogeneous photocatalysts such as the role of cat- alyst aggregation and the occurrence of the antenna effect [10,11]. The Antenna effect is one of the few suggested mechanisms tak- ing place upon illumination beyond the well-accepted elementary steps. According to the antenna mechanism the energetic coupling in a long chain of nanoparticles, assumed to occur by topotactic attachments, enables electron transfer from the particle where the initial photon absorption took place to another TiO, particle within To use absorption spectroscopy to study reaction kinetics, it is necessary to clearly distinguish and assign the absorption bands of the transient species. Recently, several groups have revealed the transient absorption bands of trapped conduction band electrons in
![Figure 2. Results from theoretical calculations with MSINDO [25,44,45] rutile (100) and anatase (100) in contact with an aqueous solution of oxalic acid. (a) 3D cluster for the simulation of the systems, an anatase cyclic cluster exposing the (100) face in contact with 100 water molecules at a liquid density, (b) and (c) the most abundant electroneutrally adsorbed oxalic acid in equilibrium in dark conditions: (b) bidentate species at rutile (110), and (c) monodentate species at anatase (100), (d) and (e) reaction mechanisms of the UV excited systems with the bidentate species (d) producing two COz molecules, and the homolytic scission of the excited monodentate species (e) yielding CO2 by oxidation or aldehyde by reduction. An interesting approach related to the system’s structure is one of Ozin and co-workers [75] where the coupling of both, the titania light absorption and the optical properties of photonic struc- tures, are used for amplifying photocatalytic reactions rates. Slow photons arising from the slow propagation of certain frequencies through an inverted opal structure of TiO are responsible for the enhancement of light absorption and thus an increase of electron- hole pairs with the consequent increase in the degradation rates of the model pollutant (methylene blue). The materials are tuned to generate, upon UV irradiation, slow photons at those frequen- cies at the titania edge of absorption which are normally not well absorbed. Such a promising technology needs further development and much research must be carried out, opening thus the doors for new ideas and possibilities of deeper and alternative exploration of the parameters affecting the photocatalytic systems. Quite a different point of view for the study of the photocat- alytic systems arises from theoretical calculations carried out on such systems. In theoretical calculations, normally done by quan- tum chemical methods, semi-empirical [76] or totally pure ones, such as ab-initio [77] all the parameters, intrinsic and extrinsic, discussed previously, are considered at once. That is, the parame- ters are linked one to the other, and mainly interconnected by the unavoidable presence of the interface when using a model cluster that includes a piece of a titania nanoparticle and the pollutant in the gas phase or in solution directly in contact. If the division of extrinsic and intrinsic is established by the interface, theoretical calculations of the system as a whole consequently integrate both. For example, the local environment where a molecule or an ion is](https://figures.academia-assets.com/44684195/figure_002.jpg)
](Figure 2. Results from theoretical calculations with MSINDO [25,44,45] rutile (100) and anatase (100) in contact with an aqueous solution of oxalic acid. (a) 3D cluster for the simulation of the systems, an anatase cyclic cluster exposing the (100) face in contact with 100 water molecules at a liquid density, (b) and (c) the most abundant electroneutrally adsorbed oxalic acid in equilibrium in dark conditions: (b) bidentate species at rutile (110), and (c) monodentate species at anatase (100), (d) and (e) reaction mechanisms of the UV excited systems with the bidentate species (d) producing two COz molecules, and the homolytic scission of the excited monodentate species (e) yielding CO2 by oxidation or aldehyde by reduction. An interesting approach related to the system’s structure is one of Ozin and co-workers [75] where the coupling of both, the titania light absorption and the optical properties of photonic struc- tures, are used for amplifying photocatalytic reactions rates. Slow photons arising from the slow propagation of certain frequencies through an inverted opal structure of TiO are responsible for the enhancement of light absorption and thus an increase of electron- hole pairs with the consequent increase in the degradation rates of the model pollutant (methylene blue). The materials are tuned to generate, upon UV irradiation, slow photons at those frequen- cies at the titania edge of absorption which are normally not well absorbed. Such a promising technology needs further development and much research must be carried out, opening thus the doors for new ideas and possibilities of deeper and alternative exploration of the parameters affecting the photocatalytic systems. Quite a different point of view for the study of the photocat- alytic systems arises from theoretical calculations carried out on such systems. In theoretical calculations, normally done by quan- tum chemical methods, semi-empirical [76] or totally pure ones, such as ab-initio [77] all the parameters, intrinsic and extrinsic, discussed previously, are considered at once. That is, the parame- ters are linked one to the other, and mainly interconnected by the unavoidable presence of the interface when using a model cluster that includes a piece of a titania nanoparticle and the pollutant in the gas phase or in solution directly in contact. If the division of extrinsic and intrinsic is established by the interface, theoretical calculations of the system as a whole consequently integrate both. For example, the local environment where a molecule or an ion is
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