**5. Conclusions**

Even when for the results shown in **Table 1**, direct comparisons are difficult to be established, it

efficient to achieve high photocatalytic degradation yields under visible light irradiation when azo dyes are used as target pollutants. For most of the reported heterostructures, degradation

as low as 31%. On the other hand, very low photocatalytic efficiency is observed for refractory industrial pollutants, such as nitrophenols and chlorophenols. These compounds displayed

O-TiO<sup>2</sup>

 nanospheres are used as photocatalyst, while 90% of degradation of environmentally relevant concentrations of geosmin was achieved upon 120 min using core@shell polydopamine@

 composites. Also, high loads of pharmaceutically active substances, bisfenol A and the widely used herbicide 2,4-D are efficiently removed from water under visible light by the TiO<sup>2</sup> based heterostructures. From these results, the use of these materials in advanced oxidation processes for ternary drinking water treatment sounds like a plausible option, keeping in mind that the high efficiency showed in these lab-scale studies can be affected by the complexity of the liquid matrix. Regarding heavy metals, the complete photocatalytic reduction of hexavalent

and Bi2

time lapse, while BiOBr-based composites showed a slightly lower activity. For the studies reported in **Table 1**, the occurrence of synergistic effects was observed when the photocatalytic performance of the heterostructures and their single components was compared. For some cases, the increment in the degradation rate and degradation yields was found in the order of 1.5–5 fold, demonstrating that the efficiency of the heterostructure was significantly higher than

When the photocatalytic performance of thin films is assessed, a clear decrease in the degradation rate of organic pollutants is observed. This is because of the decrease in the number of active sites exposed to the aqueous matrix due to the immobilization of the photocatalyst on a substrate. Degradation rates as low as 30% in 15 h for of azo dyes have been reported using TiO<sup>2</sup>

study reveals the importance of the charge carrier transference in the immobilized photocatalyst material. When this factor is taken into account, the reactivity of the thin film surface increases, leading to a higher photocatalytic activity and overcoming the mass transference hindrance.

In this sense, the arrangement of the heterostructure components is of high relevance since some approaches may favor the transfer of photo-holes or photoelectrons to the surface of the thin films. In this sense, Monfort et al. [68] tested the transfer of charge carriers in the BiVO<sup>4</sup>

 heterostructure, noting the occurrence of oxidation reactions by photo-holes when BiVO<sup>4</sup> was located on the surface of the thin films, while reduction reactions given by photoelectron

was in contact with the aqueous matrix.

 thin films [66], although the efficiency can be improved by the deposition of noble metal nanoparticles on the film surface. Conversely, in other study [67], the complete degradation of

S3 /TiO<sup>2</sup>

O3 /TiO<sup>2</sup>

methyl orange was achieved in 8 h of visible light irradiation by using BiOCl-TiO<sup>2</sup>

in photocatalytic assays under visible light irradiation. Some environmentally relevant pollutants, which are commonly found in surface water sources, are efficiently removed by the visible light-driven photocatalysis process. A high concentration of microcystin, a toxin produced by cyanobacteria, is fully degraded in 5 min under visible light irradiation, when AgBr/Ag3

O/TiO<sup>2</sup>

and Bi2

O3 /TiO<sup>2</sup>

and low band gap semiconductors are

material, which performance was

heterostructures were used

heterostructures in a very short

PO<sup>4</sup> /



thin films. This

seems clear that tailored heterostructures formed by TiO<sup>2</sup>

yields above 80% were obtained; except for the Cu<sup>2</sup>

degradation yields lower than 50% when Cu<sup>2</sup>

320 Titanium Dioxide - Material for a Sustainable Environment

chromium has been reported using Fe2

the sum of the performance of the single components.

TiO<sup>2</sup>

TiO<sup>2</sup>

InVO<sup>4</sup>

TiO<sup>2</sup>

were prominent when TiO<sup>2</sup>

Even when TiO<sup>2</sup> displays an outstanding performance as photocatalyst, its limitation to absorb light and photoactivate under visible light irradiation makes necessary to develop a set of strategies to overcome this handicap. Coupling of low band gap semiconductors with TiO<sup>2</sup> nanoparticles is an auspicious approach not only to redshift the light absorption of the composite, but to reduce the recombination rate of the hole-electron pair by the transference of the charge carriers from one semiconductor to another, increasing the photocatalytic performance. This leads to the generation of materials with high photoactivity and stability under visible light and sunlight irradiation.

In order to obtain functional heterostructures, care must be taken in the selection of the composite components, in order to get the best alignment of the semiconductors bands and thus the optimal transference of the charge carriers from one component to the other. Type II and III heterostructures have shown the highest efficiency in the separation of the hole-electron pairs. The defects formed in the heterounion act as transference sites for the charge carriers; although it only works when tiny loadings of semiconductor particles are deposited on TiO<sup>2</sup> , and recombination centers appear in the heterounion when the optimal loading is surpassed. The p-n heterostructures, specially the all solid Z schemes, have shown not only the efficient separation of the charge carriers in the composite, but the generation of highly oxidant photo-holes, which opens the opportunity to photodegrade highly recalcitrant organic pollutants in water. To date, very few TiO<sup>2</sup> -based Z schemes have been explored; thus, investigation should aim to develop new schemes, beyond the TiO<sup>2</sup> -Au-CdS approach, using nontoxic semiconductors, such as BiOI, ZnS, Ag<sup>2</sup> O or graphene.

TiO<sup>2</sup> -based heterostructures have shown a notable high performance in the photocatalytic removal of organic and inorganic pollutants in water under visible light irradiation. In some cases, removal yields surpass those observed for the individual components of the heterostructure, indicating the occurrence of a synergistic effect. New challenges are in the development of functional heterostructures in the form of thin films, which will optimize the energy and space consumption in photocatalytic water treatment systems.

The photocatalysis processes have the potential to move toward sustainability, through the development of sunlight active heterostructures, which simultaneously perform the oxidation of organic pollutants and the reduction of water molecule for hydrogen generation. This will lead to energy autonomous treatment systems based on sunlight-driven photocatalysis. Lastly, investigations should aim to the development of green synthesis methods, which optimize the energy consumption and minimize the use of harmful reagents.
