**4. Application in environmental photocatalysis**

Following the ongoing demand for sustainable technologies, faceted TiO2 nanocrystals are primarily studied as possible photocatalysts in various environmental remediation processes. Due to the relatively higher photocatalytic activity, the majority of these studies focus on the anatase polymorph; however, some interesting findings are also reported for rutile and brookite.

Concerning fundamental aspects of reactivity of different facets, it is often desired to compare pristine photocatalysts with the majority of one specific facet exposed. Comparison of their relative activities can give the so-called activity order of the investigated surfaces [61–63]. Initially, it was generally noticed that an increase of the {0 0 1} content on the anatase nanoparticles increases its photocatalytic activity both for water splitting and for degradation of organic pollutants [64–66]. This was straightforwardly connected with the high surface energy of the (0 0 1) surface, which was expected to provide a high density of potentially active sites for the photocatalytic reactions. However, further studies have presented opposite results, leading to a significant reexamination of the problem. For example, studies by Gordon et al. [67], Pan et al. [61], Mino et al. [68], and Mao et al. [63] have shown relatively low photocatalytic activity of the {0 0 1} facets in different reactions. An interesting study was also reported by Günnemann et al., who studied a variety of different TiO2 surfaces cut from single crystal samples [69]. Their conclusions support observations of relatively lower photocatalytic activity of the anatase {0 0 1} facets, while {1 0 0} were the most active for OH generation (using terephthalic acid as a probe) and {1 0 1} showed the highest activity for methanol oxidation. Ultimately, these results showed that, at present, the photocatalytic activity of different crystal facets is hardly connected *a priori* with its surface energy or high density of undercoordinated species, as initially assumed. Instead, possible adsorption, detailed electronic interactions as well as density of charge trapping and transfer are further considered as crucial for the activity of a specific facet. This makes an overall problem very case-specific and due to our best knowledge, general conclusions are not possible to draw at this moment. Nevertheless, some of the recent details, key factors, and suggested mechanisms can be discussed for specific applications.

### **4.1 Water treatment from organic pollutants**

Concerning photocatalytic degradation of organic pollutants, it is first noteworthy that these studies can be sub-categorized into 3 categories: degradation of dyes, degradation of non-color compounds, and generation of reactive oxygen species (ROS). Particularly, it should be minded that due to possible sensitization, dye degradation can be initiated by a different mechanism than other pollutants, therefore producing possibly different results. In this regard, it is not recommended to use dyes as a model pollutant, when assessing photocatalytic activity toward the degradation of organic compounds in general [70]. Here, we will focus on the reports and mechanisms discussing the degradation of photochemically inactive compounds and the generation of ROS, which is the main issue for current advanced oxidation technologies.

Concerning degradation of persistent pollutants and ROS generation, water, oxygen, and pollutant itself are the main substrates that can react at the photocatalyst surface. Usually, it is assumed that the process is initiated by the photogenerated holes (h<sup>+</sup> ) that can either oxidize the pollutant, inducing its further transformation, or produce OH radicals from H2O [71, 72]. Simultaneously, excited electrons are often expected to reduce oxygen to the O2 , which can also contribute to the final degradation rate; however, their reactivity is much lower than h<sup>+</sup> or OH [73]. Based on this description, it could be expected that the photocatalyst with the highest photooxidation ability should achieve the highest degradation rates. Focusing on the anatase, this is in accordance with some of the reported studies showing that the {1 0 0} facets are highly active, especially concerning OH generation [45, 69]. This is also in accordance with the simulations performed by Ma et al., who have shown that h+ localization is the most favored on this facet, compared to the {0 0 1} and {1 0 1} [74]. However, many studies have also reported {1 0 1} facets to be the most photocatalytic active in the degradation process, which cannot be connected to higher h<sup>+</sup> reactivity on this surface. Moreover, our recent studies have shown that {1 0 1} facets revealed higher mineralization efficiency measured as a total organic carbon (TOC) removal during the phenol degradation process, independently of the degradation rate [41]. Interestingly, both of these facts can be attributed to the increased reduction ability of the {1 0 1} facets. First of all, while the reactivity of the O2 is lower than h<sup>+</sup> or OH, they are good ring-opening agents, which might promote the efficient conversion of the aromatic compounds to CO2 [75]. Moreover, possible multi-electron oxygen reduction can also promote the formation of the OH, as well as proton transfer from organic compounds to the adsorbed -OH groups, which might initiate the degradation. This problem was specifically investigated in detail for the anatase {1 0 1} facets, which have shown that a combination of O2 and H2O on the (1 0 1) surface results in the formation of surface -OH groups [76]. The process was especially favorable in the presence of two excess electrons in the reaction model, therefore connecting it with a possible 2-electron reduction. Importantly, this shows that on the reduced (1 0 1), H2O can dissociate, forming the final -OH, which is not occurring spontaneously on the perfect surface. These findings have a fundamental meaning for the reactivity of the {1 0 1} facets, especially for water treatment processes, as the -OH groups are a preferable source of the OH formation compared to H2O itself [77]. This is in good agreement with a recent study by Hwang et al., who confirmed that a significant

### *Crystal Facet Engineering of TiO2 from Theory to Application DOI: http://dx.doi.org/10.5772/intechopen.111565*

amount of free OH is formed through the oxygen reduction process, based on the 18O2 incorporation into the product [78]. Although utilized samples were not strictly faceted during this study, the exposition of the {1 0 1} structures might be expected due to their energetic stability. Furthermore, a recent study by Dudziak et al. showed that a very good correlation could be observed between the activity of the {1 0 1} enclosed anatase samples for degradation of aromatic compounds and higher probability of both h<sup>+</sup> and e trapping on these facets, compared to the {0 0 1} and {1 0 0} ones [79]. The combination of these studies suggests a possible mechanism of the {1 0 1} reactivity as the result of e induced H2O dissociation and further generation of OH with photogenerated holes. However, more detailed studies might still be needed to clarify it. Finally, recent reports have also shown that the application of nanostructures exposing {1 0 1} facets might result in lower toxicity of the final solution, during the naproxen degradation process, than {0 0 1} ones [80]. Therefore, at this moment, a combination of high reactivity, high TOC removal, and low toxicity makes {1 0 1} a preferable choice for the degradation of organic compounds, especially micropollutants with aromatic structure and high photostability.

Compared to the anatase facets, {1 0 1} in particular, other TiO2 structures are not studied in detail and generally show markedly lower photoactivity. Nevertheless, a few important findings are worth noticing. First of all, the same reductive pathway of OH generation, reported for anatase by Hwang et al., was not observed for rutile, suggesting that especially for the {1 1 0} rutile facets, OH generation occurs strictly through H2O oxidation [78]. Furthermore, the study by Günnemann et al. showed different activities of rutile {0 0 1}, {0 1 1}, and {1 1 1} facets in the methanol oxidation and OH generation. Specifically, they reported that {0 1 1} facets exhibited lower methanol oxidation ability, while {1 1 1} generated lower amounts of hydroxyl radicals. Besides, rutile activity in both reactions was fairly similar and generally worse than anatase [69]. Nevertheless, their study did not consider rutile {1 1 0} facets, which on the other hand, were studied by Kobayashi et al. for the oxidation of oxalic acid [52]. In this study, {1 1 0} revealed higher activity than {0 0 1}, which is also in some agreement with the oxidative OH generation by this facet. Finally, concerning brookite facets, it is especially worth mentioning that structures co-exposing {2 1 0}, {2 0 1}, and {1 0 1} facets result in significant activity increase for OH generation and methyl orange degradation, otherwise not observed for the control brookite samples [16].
