Degradation performance using TiO2-based heterostructures under different experimental conditions.
Heterogeneous photocatalysis is a promising advanced oxidation process for water purification, given its potential to fully oxidize organic pollutants and to inactivate microorganisms. Due to its versatility and high performance in a broad range of conditions, titanium dioxide (TiO2)-based photocatalysis has been systematically used at laboratory scale to treat water of different quality. Even though TiO2 is an exceptional photocatalyst, its broad band gap value (3.2 eV) makes necessary the use of UV light to achieve the photoactivation. This results in the underutilization of the material in sunlight-driven photocatalysis schemes. In order to overcome this handicap, the synthesis of heterostructures using low band gap semiconductors coupled with TiO2 has brought exceptional materials for visible light-driven photocatalysis. In this chapter, the fundamentals of the synthesis and photoactivation of TiO2-low band gap semiconductor heterostructures are explored. The mechanisms leading to the increase of the photocatalytic activity of such heterostructures are described. A summary of the available data on the photocatalytic performance of TiO2-based heterostructures is presented, in terms of degradation of organic pollutants in water using visible light and sunlight. A comparison of the depuration performance of powdered and thin film heterostructures is given at the end of the chapter.
- metallic oxides
- organic pollutants
- water treatment
Water and wastewater treatment brings new challenges due to the occurrence of new and refractory contaminants produced by anthropogenic activities [1, 2]. In the past few years, water depuration aimed to remove particles, the bulk of organic matter and inactivate microorganisms. However, nowadays, the degradation not only of organic pollutants at trace levels but its precursors and by-products is especially pursued in drinking water treatment systems. For instance, the biological and chemical degradation of fluoroquinolone antibiotics results in the generation of some by-products displaying antibiotic residual activity. Biological and abiotic degradation of personal care products, such as triclosan and triclocarban, leads to the emergence of polychlorinated biphenyls and dioxins. Some bacterial residues in surface water bodies, such as microcystin and geosmin, may impact in the organoleptic properties of drinking water as well as express toxicity, while some iodinated pharmaceuticals may be precursors of trihalomethanes. Under this scenario, the most advanced water treatment systems should aim to completely mineralize the organic pollutants, in order to take the risks of water contamination to the minimum. The complete mineralization of organic pollutants can be warranted by few processes, such as photocatalysis [3, 4, 5, 6, 7]. This process is based on the generation of highly reactive •OH radicals, which are able to fully oxidize organic molecules. Heterogeneous photocatalysis process has shown to be efficient in the degradation of organic pollutants in water, avoiding the transport and use of potentially hazardous materials, such as acids or H2O2, while the catalyst can be recovered and reused in several cycles [8, 9].
TiO2 is a widely used semiconductor in heterogeneous photocatalysis due to its high activity and stability [10, 11, 12]. This harmless material is currently used not only for water treatment but in food preparation and disinfection of surgical equipment. Anatase is the most photoactive phase of TiO2, followed by rutile; however, the wide band gap value of both phases (3.0–3.2 eV) results in the activation of these materials under UV-A light irradiation . In order to take the photocatalysis process toward sustainability, it is necessary to develop materials with high photo-response under sunlight irradiation. Given that sunlight comprises only 4–5% of UV-A light, the need to find nanostructures capable to absorb visible light—which composes 50% of the sunlight spectrum —becomes imperative. A growing number of modifications of TiO2 at nanometric scale have been performed in order to achieve the complete photoactivation of this semiconductor under visible light irradiation. Doping with nonmetal atoms has given partially positive results, since the redshift of the absorption edge of TiO2 increases in turn the recombination rate of the hole-electron pair. The development of heterostructures based on TiO2 coupled to low band gap semiconductors could be an efficient approach to improve the photocatalytic conversion of contaminants in water [12, 15, 16]. This chapter explores the fundamentals of the synthesis and photoactivation of the TiO2-low band gap semiconductor heterostructures, and presents some of the reported data on the photocatalytic activity of these materials, in order to bring light on their potential use for water purification at a higher scale.
2. Fundamentals on the formation and photoexcitation of TiO2-based heterostructures
A heterostructure or junction is defined as the interfacial union of two or more components. In photocatalysis, heterounions are formed to improve the efficiency of a semiconductor either by redshifting the light absorption or through the decrease of the recombination of the hole-electron pairs [12, 17, 18]. Heterostructures are commonly built up by combining a specific semiconductor with one or more materials, such as metals, semiconductors or organic molecules for example, azo dyes or polymers.
Semiconductor/metal junctions are heterostructures based on the deposition of metallic nanoparticles on crystalline semiconductors. Such union is able to hinder the electron/hole recombination through the sequestration of the photoelectrons from the conduction band of the semiconductor to the surface of the metallic nanoparticles  (Figure 1). The differences in the Fermi level reached in the semiconductor/metal junction triggers the sequestration of photoelectrons; although other factors, such as the higher work function and electronegativity of the metal, favor the transference of the charge carriers [16, 19, 20]. The defects created in the semiconductor/conductor junction are proposed as the main route of the electron transfer from one material to the other. The Schottky barrier created in the junction impedes the return of the charge carriers to the conduction band of the semiconductor. Typically, low loadings of metallic nanoparticles tend to be the optimal for achieving the highest potential of the electron trap process. However, when this optimal loading is surpassed, the electron traps are converted into charge carriers recombination sites, leading to the dramatic drop of the photocatalytic performance [21, 22, 23]. Even when the electron trap effect is unable to significantly redshift the light absorption of the heterostructure, some photocatalytic activity can be achieved by the surface plasmon resonance (SPR) effect, which is expressed by the oscillation of the electrons in the conduction band of the metallic nanoparticles when electromagnetic excitation at a determined wavelength is provided [24, 25, 26]. Some extent of photocatalytic activity has been attributed to SPR effect, although this cannot be considered as determinant in the photodegradation of pollutants in water when compared with that achieved under UV light irradiation.
The synthesis of semiconductor/semiconductor nanocomposites provides an efficient way to redshift the photoactivation of TiO2-based materials. When these materials are exposed to visible light, the low band gap semiconductor is activated, producing the hole–electron pair. Then, the photoelectrons migrate from the surface of the low band gap semiconductor to the conduction band of TiO2, as an effect of the difference in the redox potential between both semiconductors (Figure 2). The process leads to the drop of the hole-electron pair recombination rate, resulting in the oxidation process in the low band gap semiconductor, and the reduction process on the TiO2 surface [17, 27].
When the composite is photoactivated under UV light irradiation, the hole-electron pairs are produced in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of each semiconductor. Charge carriers are then transported and accumulated in the HOMO and LUMO of one of the semiconductors, as a function of gradient in the potential of the bands (Figure 3). For this kind of schemes, p-n heterostructures have shown the best results in the separation of the photo-formed charge carriers. However, in some cases, decreasing of the redox potential of charge carriers can occur, depending of the HOMO and LUMO positions of the semiconductors in the heterostructure, leading to the decrement in the photocatalytic performance [15, 17, 18].
In some advanced approaches, metallic nanoparticles are settled in the heterounion of the two semiconductors. Noble metal nanoparticles can act as electron mediators, transporting the charge carriers from one semiconductor to another, increasing with this the electron trap effect. Heterostructures based on p-n semiconductors have shown a notable increase in the charge separation when noble metal nanoparticles are added in the semiconductor heterounion. These materials, known as all solid-state Z schemes, provide highly reductive photoelectrons and highly oxidative photo-holes by the process shown in Figure 4 .
Photoelectrons are transported from semiconductor II to semiconductor I via the metallic nanoparticles. At the same time, photoelectrons are formed in semiconductor I and photoholes in the HOMO of semiconductor I are recombined with photoelectrons coming from the LUMO of semiconductor II. This kind of schemes impedes the recombination of the charge carriers with the highest oxidative and reductive potential, increasing not only the photocatalytic performance of the components, but bringing chances to photodegrade more recalcitrant pollutants because of the increment of the oxidative potential of the charge carriers.
When TiO2 nanoparticles are deposited on graphene sheets, the semiconductor/polymer junction displays higher photocatalytic activity due to the transfer of the charge carriers from the semiconductor to the polymer, as shown in Figure 5b. Even when the lifetime of the charge carriers is increased, visible light-driven activity is not improved; however, graphene materials can be used as an excellent electron mediator in all solid Z schemes. On the other hand, organic molecules, which are able generate the triplet state under visible light irradiation, can act as sensitizers when deposited on the TiO2 surface (Figure 5a). In this case, electrons are injected from the sensitized molecule to the LUMO of the semiconductor, triggering the photocatalytic process .
2.1. Classification of the semiconductors/semiconductor heterostructures
Semiconductor/semiconductor junctions can be classified depending on either the type of semiconductors that are being coupled or by the band structure they present. Considering the type of semiconductors, the semiconductor/semiconductor heterostructures can be classified either as anisotype—a p-n junction—, or isotype heterojunctions, in which both coupled semiconductors are n-type (n-n junction) or p-type (p–p junction). Some examples of n-type semiconductors include TiO2, WO3, ZnO, Fe2O3, CeO2, AgI, BiVO4, CdS, CdSe, Bi2WO6, and ZnSe, while for p-type semiconductors Bi2O3, V2O5, BiOI, BiOBr, BiOCl, CuO, Cu2O can be mentioned . When semiconductor/semiconductor heterostructures are classified depending on the band position of the components, three main groups can be mentioned. Figure 6 shows the three possible band alignments in semiconductor/semiconductor heterostructures.
In a type I heterostructure, also known as straddling alignment, the band gap value of the semiconductor B is smaller than that of semiconductor A (Figure 6). In this case, the potential of valence band of the semiconductor B is located at a higher position than that of semiconductor A, while conduction band of semiconductor A displays a lower potential than that of the conduction band of semiconductor B. In this scheme, electrons and holes are transferred from A to B, resulting in the accumulation of charge carriers in B ; this facilitates in turn the recombination of the charge carriers and decreases the photocatalytic activity. Obregon et al.  reported the formation of a type I heterojunction using monoclinic BiVO4 and TiO2. The 1 wt. % m-BiVO4/TiO2 nanocomposite was prepared by a simple impregnation method and was tested in the degradation of phenol. This system has been tested by other authors using different approaches and modification [31, 32, 33]. Other semiconductors that have been coupled to TiO2 to form a type I heterojunction  are WO3 [34, 35], Fe2O3 , MoS2  and BiOI .
Type II heterostructure, or staggered lineup, represents the most efficient configuration for the charge carriers transfer. In this case, photo-generated electrons will be transferred from semiconductor B to semiconductor A, while the holes are moving from semiconductor A to semiconductor B. A more efficient charge separation is achieved, having both photoelectrons and photo-holes distributed in the two semiconductors. A large number of type II heterojunctions have been synthesized with TiO2, by the union with CdS , CdSe [39, 40], Bi2S3 , WS2 , V2O5 , Bi2O3 , CuO , Cu2O [45, 46], and CeO2 .
Type III heterojunctions are similar in structure to the type II composites, but a wider difference in the position of bands between semiconductor A and B is observed. This type of heterojunctions is also known as broke-gap junction, which is highly recommended to the construction of all solid Z schemes. For instance, Heng et al.  prepared a type III heterojunction by incorporating H3PW12O40, TiO2 and In2S3. The authors propose that the H3PW12O40 assists in the transfer of electronics from TiO2 to In2S3.
3. Synthesis methods to obtain TiO2-based heterostructures
A wide variety of physical and chemical methods have been developed for the synthesis of TiO2-based heterostructures, using a vast diversity of structures and morphologies. Some examples of such methods are sol-gel, solvothermal, impregnation, sputtering, dip-coating, co-precipitation, mechanical synthesis and chemical vapor deposition, among others. In many cases, a combination of two or more synthetic methods and reaction steps are needed in order to create a specific photocatalyst with the desired characteristics. The use of photocatalyst powders for water purification has been of great interest. This is because of the simplicity in the synthesis, the high exposition of the particle surface area to the target pollutants, as well as the high dispersion of the catalyst. In this section, some of the most used methods for the synthesis of TiO2-based heterostructures are explored.
WO3/TiO2 nanocomposites have been synthesized by the sol-gel method using Ti(OBu4) and ammonium tungstate as main precursors [35, 49]. The photocatalytic activity of the material was tested via the degradation of malathion using natural sunlight. The complete degradation of the pollutant was achieved after a 2 h irradiation and a mineralization rate of 63% upon 5 h. Yang et al.  reported the synthesis of different CeO2/TiO2 heterostructures by the sol-gel synthetic route, using Ti(OBu)4 and Ce(NO3)3∙6H2O as precursors. The synthesized materials were tested for the degradation of methyl orange, noting the enhanced activity of the CeO2/TiO2 composite compared with unmodified TiO2 and CeO2.
Perales-Martínez et al.  reported the formation of the InVO4/TiO2 catalyst. In a first step, both InVO4 and TiO2 were prepared using the solvothermal method. Then, the composite was formed by suspending both oxides in methanol in order to achieve the impregnation assisted by ultrasonication. Lastly, the solvent was evaporated using a rotary evaporator. In another work, Maeda et al.  reported the formation of cobalt oxide nanoparticles supported on the surface of rutile TiO2 (Co3O4/TiO2 heterostructure). In this case, TiO2 was impregnated with the Co(NO3)2·6H2O solution, followed by a thermal treatment in air atmosphere. Fe2O3/TiO2 photocatalyst was prepared using a Fe(NO3)3∙9H2O ethanol solution, where TiO2 powders were stirred and sonicated . Peng et al.  reported that after calcination at 300°C for 6 h, the Fe2O3 was deposited on the surface of TiO2 nanorods. In other report, the preparation of RuO2/TiO2 heterostructures was achieved by Uddin et al.  using ruthenium(III) pentan-2,4-dionate as RuO2 precursor.
MoS2/TiO2 and WS2/TiO2 have been synthesized by the photo-reduction of either (NH4)2MoS4 or (NH4)2WS4, directly on the surface TiO2 particles . These materials showed good efficiency in the degradation of methylene blue and 4-chlorophenol.
Shifu et al.  prepared the WO3/TiO2 heterostructure using the mechanical mixing method. The precise amounts of TiO2 and WO3 powders were mixed in an agate ball milling tank. The two oxides were mixed for 12 h at 300 rpm. A loading of 3% wt. of WO3 presented the best efficiency for the degradation of monocrotophos. The coupled photocatalyst showed a redshift in its light absorption compared to pure TiO2.
4. Photocatalytic activity of TiO2-based heterostructures under visible and simulated sunlight
The coupling of TiO2 with low band gap semiconductors leads to the activation of the photocatalyst material under visible light irradiation, as established earlier, resulting in turn in the generation of materials with high efficiency and stability. An important number of studies have reported the photocatalytic performance of these heterostructures, showing high conversion rates of organic and inorganic pollutants in water. Some of these results are shown in Table 1. As observed in the table, conversion of azo dyes molecules is the most used way to assess the photocatalytic activity of the synthesized materials. This is due to the easy analytical determination of such molecules in water—most of them for UV–vis spectroscopy—in comparison with uncolored molecules—such as phenols—organochlorinated compounds and pharmaceutical substances. However, as was recently pointed out, using azo dyes molecules in the evaluation of the photocatalytic performance of semiconductors may result in an artifact because of the sensitization of the semiconductors by the adsorbed organic molecules . It is worth noting how the degradation rate constant is mostly determined using the pseudo first-order approach, forgetting the multiple phase conditions. In very few studies, other models—such as the Langmuir-Hinshelwood approximation—have been used . Degradation yields is the most reported parameter in this kind of experiments. Very few studies follow the content of the total organic carbon throughout the process, ignoring with this the mineralization yield of the pollutants. This may lead to a miscalculation of the risk that treated water pose on the exposed organisms, since some of the photodegradation by-products may be more toxic or recalcitrant than the parent compound. Examples of this are benzoquinone, which degradation requires more energy than phenol and triclosan, which degrades into a low toxicity dioxin.
|Heterostructure components||Architecture||Pollutant removed||Reaction conditions||Performance||Reference|
Fe:Ti ratio = 95:5
|Composite||Salicylic acid (6.9 ppm)||300 W Xe lamp (λ ≥ 420 nm).|
10 mg of composite were suspended in 50 mL.
120 min of irradiation time.
|k* = 0.053 h−1|||
Fe:Ti:Cd ratio = 5:94:1
|k* = 0.099 h−1|
|60 wt. % Cu2S/TiO2||Orange II (15 ppm)||150 W tungsten-halogen-lamp with UV cutoff filter at 475 nm. pH ~ 6.4.||k* ~ 6.1 × 10−3 min−1|||
Bi:Ti molar ratio = 1:100
|150 W xenon lamp (λ ≥ 420 nm).|
0.001 g mL−1 of catalyst.
6 h of irradiation time.
|Degradation yield of 49%|||
(20 ppm of K2Cr2O7)
|Mercury vapor lamp of 125 W with a broadband from 250 to 450 nm (maxima at 254, 312 and 365 nm).|
30 mg of catalyst in 100 mL of solution.
|Complete reduction to Cr(III) in 30 min.|
k = 0.91 ppm g−1 min−1
|Carbon-doped anatase/brookite TiO2 (80:20)||Composite||Methylene blue (3.2 ppm)||Solar simulator with filter for blue light.|
0.06 g of powder in 100 mL solution.
60 min irradiation.
|k = 0.008 min−1|||
|Rutile/anatase TiO2||Nanoflowers||Methylene blue (15 ppm)||350 W xenon lamp.|
10 mg of catalyst.
120 min of irradiation time.
|72% degradation of degradation rate.|||
|Adding 0.5 mL of a 30% m/v H2O2 solution.||~88% of degradation rate.|
|BiVO4@TiO2||Core@shell||Methylene blue (5 ppm)||Osram Dulux S67 blue light Bulbs, with maximum emission at 450 nm.|
50 mg of the catalyst were dispersed in 150 mL.
120 min irradiation.
|~85% of Degradation rate.|||
|5 wt. % WO3/TiO2||Composite||Methyl orange (20 ppm)||1000 W halogen lamp (λ > 420 nm).|
Catalyst loading of 1.1 g L−1.
5 h of time irradiation.
|2,4-dichlorophenol (20 ppm)||73% degradation|
|12 wt. % ZnxCd1-xS/TiO2|
Cd:Zn ratio = 3:1
|Composite||Rhodamine B (4.8 ppm)||500 W halogen lamp (420 nm < λ < 800 nm).|
0.04 g of the catalyst was added to 80 mL of solution.
120 min of irradiation time.
|Cu2O/TiO2||Cu2O nanospheres decorated with TiO2 nanoislands.||Methyl Orange (30 ppm)||Light intensity of ~23 mW cm−2 (λ > 400 nm).|
0.03 g of catalyst was dispersed in 150 mL of solution.
40 min of irradiation time.
r+~12.825 mg g−1 min−1
|Cu2O@TiO2||Core@shell Polyhedra (cubes, cuboctahedra and octahedra)||Methylene Blue (3.2 ppm)||300 W xenon lamp with a glass filter (λ > 400 nm). Light intensity of 23 mW cm−2.|
Concentration of 0.2 mg photocatalyst mL−1.
4 h of irradiation time.
|Up to 80% degradation for octahedra core-shell structure.|||
|4-Nitrophenol (10 ppm)||0.02 g of photocatalyst was dispersed in 50 mL of solution.||Up to 50% degradation for octahedra core-shell structure.|
Bi:Ti ratio = 1:2.6
|Nanofibers||Rhodamine B (10 ppm)||150 W xenon lamp.|
0.1 g of catalyst was suspended in 100 mL of the compound solution.
5 h of irradiation.
|0.5 wt.% InVO4/TiO2||Composite||Phenol (30 ppm)||300 W lamp with an intensity irradiation of 38.4 W m−2.|
0.05 g of catalyst dispersed in 50 mL of solution.
80 min of irradiation time
|100% of degradation rate.|
k# = 8.1×10−4 s−1
|Polydopamine@TiO2||Core@shell nanoparticles and brush-like structures.||Geosmin (1000 ppb)||350 W xenon lamp equipped with a 420 nm cutoff filter.|
120 min of irradiation
|Fluorene (1000 ppb)||98% degradation|
Bi:Ti ratio = 10%
|Composite||Ofloxacin (25 ppm)||Solar irradiation with average light intensity of 70.3 K lux.|
Catalyst concentration of 0.5 g/L.
120 min of irradiation time
|60 wt. % Sb2S3/TiO2||Nanorods||p-hydroxyazobenzene (10 ppm).||500 W halogen lamp with two cutoff filters (below 420 nm and above 850 nm).|
40 mg of catalyst were dispersed in 80 mL.
5 h of irradiation time.
|89% of degradation rate|||
|Methyl orange||96% of degradation rate|
90:10 Al:Ti mass ratio
|Composite||Formaldehyde (10 ppm)||Sunlight irradiation with an intensity of 90 ± 15 mW/cm2.|
0.1 g of catalyst was dispersed in 100 mL of solution.
90 min of irradiation time.
|97.8% of degradation rate|||
|Cu2O/TiO2||Nanotubes||Rhodamine B (5 ppm)||500 W tungsten halogen lamp with an optical filter (λ > 420 nm).|
120 min of irradiation time.
|31% of degradation rate|
k* = 0.00312 min−1
Ag:Ti molar ratio = 1:5
|Spheres||Methyl orange (8.2 ppm).||Simulated sunlight lamp with intensity of 4 W m−2.|
0.001 g of catalyst was dispersed in 6 mL of solution.
50 min of irradiation time.
|~90% of degradation rate|
k* = 0.1329 min−1
|Microcystin-LR (50 ppm)||0.01 g of catalyst was dispersed in 30 mL of solution.||100% of degradation after 5 min.|
k* = 0.6371 min−1
|7 wt. % Fe2O3@TiO2||Core@shell||Rhodamine B (10 ppm)||350 W xenon lamp.|
5 mg of catalyst were dispersed in 50 mL of aqueous solution.
5 h of irradiation time.
|60% of degradation rate|
k* = 0.1605 h−1
|Cu2O/TiO2||Octadecahedron/Quantum Dot||Methyl orange (30 ppm)||500 W xenon lamp with glass filter (λ > 400 nm) and intensity of 100 mW cm−2.|
25 mg of catalyst were dispersed in 100 mL of solution.
60 min of irradiation.
|97% of degradation rate|
k* = 0.055 min−1
molar ratio Ti:Bi = 2:1
|Nanorods||Rhodamine B (15 ppm)||300 W Xenon lamp with a cut-off glass filter (λ > 420 nm). Illumination intensity of 7 × 103 mW cm−2.|
70 mg of photocatalyst were dispersed in 70 mL of solution.
|99.9% of degradation after 10 min of irradiation.|
k = 0.49 min−1
|Cr (VI) (50 ppm K2Cr2O7 solution)||95.4% of conversion after 120 min irradiation.|
k = 0.025 min−1
|Cu2O/TiO2||Nanoparticles/nanotubes||p-nitrophenol (20 ppm)||Xe lamp (λ > 420 nm). with an intensity of 100 mW cm−2.|
210 min of irradiation.
|35–40% of degradation rate under visible light irradiation.|
>90% of degradation rate under sunlight irradiation.
|Bi2S3/TiO2||Fungus-like mesoporous Bi2S3/ TiO2 nanotube||2,4-dichlorophenoxyacetic acid (2,4-D) (10 ppm)||Xenon lamp with a radiation intensity of 85 mW cm−2.|
180 min of irradiation time.
~75% mineralization (after 360 min of irradiation)
|2,4-D and Cr(VI) (10 ppm)||81% degradation for 2,4-D|
Complete Cr(VI) reduction.
|Cu2O/TiO2||Hollow spheres||Rhodamine B (5 ppm)||300 W xenon lamp.|
0.1 g of catalyst was dispersed in 200 mL of the compound solution.
120 min irradiation time
|~ 88% of degradation rate.|
k* = 0.0165 min−1
|300 W Xenon lamp with a 420 nm cut off filter.|
300 min time irradiation.
|~ 92% of degradation rate.|
k* = 0.0081 min−1
|1 wt. % Ag2CO3/TiO2||Composite||Methyl orange (20 ppm)||300 W iodine tungsten lamp.|
0.05 g of catalyst was suspended in 80 mL of the compound solution.
5 h of irradiation time.
k* = 0.24 min−1
In most cases reported in the literature, halogen lamps are the light sources in photocatalysis schemes, while the loading of the catalyst is maintained below 1 g/L, in order to avoid the screening of light due to the high turbidity in the suspension. Concentration of the organic pollutants is normally set at levels of mg/L, which are one to three magnitude orders higher than those observed in the environment, even in wastewater [1, 64, 65]. The use of such concentration levels enables the determination of the kinetic constants in the photocatalytic degradation of the pollutants in water. However, when experimental conditions require the use of complex liquid matrices, such as superficial water, groundwater or wastewater, the degradation rate is reduced, bringing the opportunity for lowering the initial concentration of the target compound toward environmentally relevant levels, such as μg/L or even ng/L, with no impact in the study of the kinetic constants.
Even when for the results shown in Table 1, direct comparisons are difficult to be established, it seems clear that tailored heterostructures formed by TiO2 and low band gap semiconductors are 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 yields above 80% were obtained; except for the Cu2O/TiO2 material, which performance was 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 degradation yields lower than 50% when Cu2O-TiO2 and Bi2O3/TiO2 heterostructures were used 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/Ag3PO4/TiO2 nanospheres are used as photocatalyst, while 90% of degradation of environmentally relevant concentrations of geosmin was achieved upon 120 min using core@shell polydopamine@TiO2 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 TiO2 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 chromium has been reported using Fe2O3/TiO2 and Bi2S3/TiO2 heterostructures in a very short 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 the sum of the performance of the single components.
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 TiO2-InVO4 thin films , although the efficiency can be improved by the deposition of noble metal nanoparticles on the film surface. Conversely, in other study , the complete degradation of methyl orange was achieved in 8 h of visible light irradiation by using BiOCl-TiO2 thin films. This 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.  tested the transfer of charge carriers in the BiVO4-TiO2 heterostructure, noting the occurrence of oxidation reactions by photo-holes when BiVO4 was located on the surface of the thin films, while reduction reactions given by photoelectron were prominent when TiO2 was in contact with the aqueous matrix.
Even when TiO2 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 TiO2 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 TiO2, 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 TiO2-based Z schemes have been explored; thus, investigation should aim to develop new schemes, beyond the TiO2-Au-CdS approach, using nontoxic semiconductors, such as BiOI, ZnS, Ag2O or graphene.
TiO2-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.
The authors want to than to Secretaria de Ciencia, Tecnología e Innovación de la Ciudad de México and Dirección General de Asuntos del Personal Académico de la UNAM for the financial supporting given to this work through projects SECITI/047/2016 and IA101916, respectively.