**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

**Figure 4.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup> based all solid Z scheme when photoactivation when UV light activation occurs.

**Figure 3.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup>

**Figure 2.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup>

The synthesis of semiconductor/semiconductor nanocomposites provides an efficient way 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

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

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

surface [17, 27].


, as an effect of the difference in the redox potential between both

redshift the photoactivation of TiO<sup>2</sup>

308 Titanium Dioxide - Material for a Sustainable Environment

conduction band of TiO<sup>2</sup>

reduction process on the TiO<sup>2</sup>

photoactivation with UV light occurs.

photoactivation with visible light occurs.



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 TiO<sup>2</sup> , WO3 , ZnO, Fe<sup>2</sup> O3 , CeO<sup>2</sup> , AgI, BiVO<sup>4</sup> , CdS, CdSe, Bi<sup>2</sup> WO<sup>6</sup> , and ZnSe, while for p-type semiconductors Bi<sup>2</sup> O3 , V<sup>2</sup> O5 , BiOI, BiOBr, BiOCl, CuO, Cu<sup>2</sup> O can be mentioned [18]. 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.

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 heterojunc-

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. [48] prepared a type III hetero-

> and In2 S3

A wide variety of physical and chemical methods have been developed for the synthesis of TiO<sup>2</sup>

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 sec-

**Sol-gel:** The sol-gel method consists of the acidic or basic hydrolysis of an organometallic precursor, followed by a slow polymerization. The obtained material is dried, allowing the

nanocomposites have been synthesized by the sol-gel method using Ti(OBu<sup>4</sup>

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%

and Ce(NO3

materials were tested for the degradation of methyl orange, noting the enhanced activity of

**Hydrothermal/solvothermal method:** The solvothermal method allows the synthesis of crystalline materials by heating the precursors in solution inside a sealed reactor (autoclave). Water (hydrothermal) and several organic compounds such as ethylene glycol and glycerol (solvothermal) may be used as solvent during the reaction. The solvothermal method is one of the most common preparation techniques for heterostructures, since the variation of pressure and temperature parameters allows the formation of a wide diversity of crystal morphologies. The

)3 ∙6H2

decomposition and elimination of all the organic components present in the gel.

upon 5 h. Yang et al. [47] reported the synthesis of different CeO<sup>2</sup>

composite compared with unmodified TiO<sup>2</sup>

the sol-gel synthetic route, using Ti(OBu)<sup>4</sup>

 to In2 S3 .

, by the union with CdS [38], CdSe [39, 40], Bi<sup>2</sup>

**-based heterostructures**

[47].

http://dx.doi.org/10.5772/intechopen.76501

. The authors propose that the H3


/TiO<sup>2</sup>

.

and CeO<sup>2</sup>

O [45, 46], and CeO<sup>2</sup>

TiO2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

S3 [41], 311

PW12O<sup>40</sup>


) and

heterostructures by

O as precursors. The synthesized

tions have been synthesized with TiO<sup>2</sup>

O3

assists in the transfer of electronics from TiO<sup>2</sup>

**3. Synthesis methods to obtain TiO2**

tion, some of the most used methods for the synthesis of TiO<sup>2</sup>

[44], CuO [45], Cu<sup>2</sup>

PW12O40, TiO<sup>2</sup>

[43], Bi<sup>2</sup>

WS<sup>2</sup>

WO3

the CeO<sup>2</sup>

/TiO<sup>2</sup>

/TiO<sup>2</sup>

[42], V<sup>2</sup>

O5

junction by incorporating H3

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 [18]; this facilitates in turn the recombination of the charge carriers and decreases the photocatalytic activity. Obregon et al. [30] reported the formation of a type I heterojunction using monoclinic BiVO<sup>4</sup> and TiO<sup>2</sup> . The 1 wt. % m-BiVO<sup>4</sup> /TiO<sup>2</sup> 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 [31–33]. Other semiconductors that have been coupled to TiO<sup>2</sup> to form a type I heterojunction [18] are WO3 [34, 35], Fe<sup>2</sup> O3 [36], MoS<sup>2</sup> [37] and BiOI [36].

**Figure 5.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup> -sensitized materials under visible light activation (a) and in TiO<sup>2</sup> -graphene heterostructures when UV light occurs (b).

**Figure 6.** Classification of semiconductor/semiconductor heterostructures based on the band alignment of the components.

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 TiO<sup>2</sup> , by the union with CdS [38], CdSe [39, 40], Bi<sup>2</sup> S3 [41], WS<sup>2</sup> [42], V<sup>2</sup> O5 [43], Bi<sup>2</sup> O3 [44], CuO [45], Cu<sup>2</sup> O [45, 46], and CeO<sup>2</sup> [47].

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. [48] prepared a type III heterojunction by incorporating H3 PW12O40, TiO<sup>2</sup> and In2 S3 . The authors propose that the H3 PW12O<sup>40</sup> assists in the transfer of electronics from TiO<sup>2</sup> to In2 S3 .

#### **3. Synthesis methods to obtain TiO2 -based heterostructures**

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

> O3 , CeO<sup>2</sup>

mentioned [18]. 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 [18]; this facilitates in turn the recombination of the charge carriers and decreases the photocatalytic activity. Obregon et al.

was tested in the degradation of phenol. This system has been tested by other authors using different approaches and modification [31–33]. Other semiconductors that have been coupled

**Figure 6.** Classification of semiconductor/semiconductor heterostructures based on the band alignment of the

O3 , V<sup>2</sup> O5 , AgI, BiVO<sup>4</sup>

nanocomposite was prepared by a simple impregnation method and

O3

[36], MoS<sup>2</sup>


[34, 35], Fe<sup>2</sup>

, BiOI, BiOBr, BiOCl, CuO, Cu<sup>2</sup>

, CdS, CdSe, Bi<sup>2</sup>

WO<sup>6</sup>

and TiO<sup>2</sup>

[37] and BiOI [36].

. The

, and

O can be

, ZnO, Fe<sup>2</sup>

[30] reported the formation of a type I heterojunction using monoclinic BiVO<sup>4</sup>

, WO3

semiconductors include TiO<sup>2</sup>

1 wt. % m-BiVO<sup>4</sup>

to TiO<sup>2</sup>

in TiO<sup>2</sup>

components.

ZnSe, while for p-type semiconductors Bi<sup>2</sup>

310 Titanium Dioxide - Material for a Sustainable Environment

/TiO<sup>2</sup>

to form a type I heterojunction [18] are WO3

**Figure 5.** Photoactivation and charge carriers transfer in a TiO<sup>2</sup>


A wide variety of physical and chemical methods have been developed for the synthesis of TiO<sup>2</sup> 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 TiO<sup>2</sup> -based heterostructures are explored.

**Sol-gel:** The sol-gel method consists of the acidic or basic hydrolysis of an organometallic precursor, followed by a slow polymerization. The obtained material is dried, allowing the decomposition and elimination of all the organic components present in the gel.

WO3 /TiO<sup>2</sup> nanocomposites have been synthesized by the sol-gel method using Ti(OBu<sup>4</sup> ) 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. [47] reported the synthesis of different CeO<sup>2</sup> /TiO<sup>2</sup> heterostructures by the sol-gel synthetic route, using Ti(OBu)<sup>4</sup> and Ce(NO3 )3 ∙6H2 O as precursors. The synthesized materials were tested for the degradation of methyl orange, noting the enhanced activity of the CeO<sup>2</sup> /TiO<sup>2</sup> composite compared with unmodified TiO<sup>2</sup> and CeO<sup>2</sup> .

**Hydrothermal/solvothermal method:** The solvothermal method allows the synthesis of crystalline materials by heating the precursors in solution inside a sealed reactor (autoclave). Water (hydrothermal) and several organic compounds such as ethylene glycol and glycerol (solvothermal) may be used as solvent during the reaction. The solvothermal method is one of the most common preparation techniques for heterostructures, since the variation of pressure and temperature parameters allows the formation of a wide diversity of crystal morphologies. The products obtained are usually well dispersed in form and size. Also, some additives and templates may be added into de reaction mixture to favor a desired morphology or crystallite size. Xu et al. [50] synthesized a rutile/anatase TiO<sup>2</sup> heterostructure using titanium tetrachloride (TiCl<sup>4</sup> ), urea and cetyl trimethyl ammoniumbromide (CTAB) as a template. Reaction was carried out at 160°C for 12 h, resulting in rutile/anatase nanoflowers with high surface area —up to 106.29 m2 /g—. CdS/TiO<sup>2</sup> composites were synthesized by Wu et al. [27] via the microemulsionmediated solvothermal method, allowing the formation of anatase nanoparticles with highly dispersed CdS nanocrystals on the surface. The modification of TiO<sup>2</sup> with CdS nanoparticles increased the absorption of visible light irradiation at 550 nm. Zhu et al. [51] prepared Bi2 O3 / TiO<sup>2</sup> flower-like spheres, which displayed high photocatalytic activity due to an enhanced visible light absorption. TiO<sup>2</sup> @MoO3 core@shell structures were synthesized by Li et al. [52] using a one-step hydrothermal method, while Liu et al. [53] used this method to achieve the formation of a series of Cu<sup>2</sup> O@TiO<sup>2</sup> core@shell structures by coating different Cu<sup>2</sup> O polyhedral nanoparticles on the TiO<sup>2</sup> surface. Cu<sup>2</sup> O/TiO<sup>2</sup> hollow spheres (HS) were synthesized by both solvothermal and sol–gel methods [54]. In a first step, TiO<sup>2</sup> HS were synthesized by a sol–gel method, using carbon nanospheres as a template. In the second step, the HS were mixed with a glucose solution, containing CuSO<sup>4</sup> ∙5H2 O; then, the mixture was poured into a teflon-lined stainlesssteel autoclave. In this reaction, glucose took the role of reducing agent, which helped to reduce copper from Cu2+ to Cu1+. These heterostructures were tested for the photocatalytic degradation of Rhodamine B under visible and sunlight irradiation, showing promising results.

**Electrosynthesis:** The electrosynthesis method consists of the use of electrochemical cells to produce the desired material. Yang et al. [39] achieved the electrodeposition of CdSe nanoparticles

namely a Pt wire (counter electrode), a saturated calomel electrode (reference electrode) and the TNTs (working electrode). The three electrodes were submerged in an electrolyte solution

**Mechanical mixing:** This is one of the simplest synthesis methods, which involves the direct mixing of the heterostructure precursors. Manual mixing usually results in long reaction times and low homogeneity of the products. In certain cases, binding agents may be added to

ciency for the degradation of monocrotophos. The coupled photocatalyst showed a redshift in

**Precipitation and co-precipitation:** Both precipitation and co-precipitation methods consist of the formation of an insoluble material, starting from one or several solutions containing the soluble precursors. Usually, an increase in the pH value of the solution helps in the formation of insoluble hydroxides, allowing the precipitation. Yu et al. [61] prepared an Ag/AgCl/TiO<sup>2</sup> heterostructure by the impregnation of TNTs with a 1 M HCl aqueous solution, followed by

precipitated on the TNTs. Lastly, UV irradiation was applied to achieve a partial reduction of

**Thin films:** One of the main burdens of using powder photocatalysts is the need of including a separation step for the effluent in order to reuse the photocatalyst in further cycles. This step can become difficult and very expensive, making the photocatalytic process less viable in a plant scale approach. A feasible solution is the immobilization of the material in a suitable support, such as glass, quartz or polymer. Some synthetic routes for obtaining photocatalysts

**Dip-coating:** This is one of the most used methods for the synthesis of thin films, which consists of submerging, at a constant rate, the substrate in a solution containing the precursor of the semiconductor. After a certain dwell time, the substrate is pulled out of the solution. Lastly, the solvent is dried, and a thermal treatment can be applied to eliminate organic resid-

**Spin Coating:** This process consists of putting a small amount of a solution containing the precursor of the thin film material on the surface of the substrate. Then, the substrate is rotated at high speed, eliminating the excess solution and leaving a uniform film once the solvent is dried.

material. This collision causes that some atoms are ejected from the surface of the electrode.

.

nanotubes (TNTs). For this, three electrodes were used,

heterostructure using the mechanical mixing method.

powders were mixed in an agate ball milling tank. The

solution. As a result, AgCl nanoparticles were

) are accelerated into the surface of a cathodic

were obtained.

http://dx.doi.org/10.5772/intechopen.76501

313

presented the best effi-

. CdSe was deposited at −0.7 V, −6 V vs. reference electrode at room

TiO2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

on the surface and the inner space of TiO<sup>2</sup>

Shifu et al. [60] prepared the WO3

its light absorption compared to pure TiO<sup>2</sup>

a second impregnation with a 0.1 M AgNO3

as thin films are described below.

the formed AgCl nanoparticles into metallic Ag.

**Sputtering:** In this route, ionized atoms (e.g., Ar<sup>+</sup>

uals and induce crystallization of the semiconductor in the film.

The precise amounts of TiO<sup>2</sup>

and SeO<sup>2</sup>

temperature. Well-dispersed CdSe nanoparticles deposited on TiO<sup>2</sup>

the mixture in order to increase the stability of the heterostructure.

and WO3

/TiO<sup>2</sup>

two oxides were mixed for 12 h at 300 rpm. A loading of 3% wt. of WO3

containing CdCl<sup>2</sup>

**Impregnation:** This method consists of the saturation of one specific support—in this case TiO<sup>2</sup> with a solution containing the desired precursor, usually a metal salt; this allows the metal ions to fill the support pores. Then, the material is dried and exposed to a thermal treatment.

Perales-Martínez et al. [55] reported the formation of the InVO<sup>4</sup> /TiO<sup>2</sup> catalyst. In a first step, both InVO<sup>4</sup> and TiO<sup>2</sup> 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. [56] reported the formation of cobalt oxide nanoparticles supported on the surface of rutile TiO<sup>2</sup> (Co3 O4 /TiO<sup>2</sup> heterostructure). In this case, TiO<sup>2</sup> was impregnated with the Co(NO3 ) 2 ∙6H2 O solution, followed by a thermal treatment in air atmosphere. Fe<sup>2</sup> O3 /TiO<sup>2</sup> photocatalyst was prepared using a Fe(NO3 ) 3 ∙9H2 O ethanol solution, where TiO<sup>2</sup> powders were stirred and sonicated [57]. Peng et al. [58] reported that after calcination at 300°C for 6 h, the Fe<sup>2</sup> O3 was deposited on the surface of TiO<sup>2</sup> nanorods. In other report, the preparation of RuO<sup>2</sup> /TiO<sup>2</sup> heterostructures was achieved by Uddin et al. [59] using ruthenium(III) pentan-2,4-dionate as RuO<sup>2</sup> precursor.

**UV light irradiation:** This method consists of the reduction and precipitation of one or more soluble precursors over the surface of TiO<sup>2</sup> , which acts as a support. The presence of UV light irradiation allows the photo-formation of electrons in the support, which are responsible of reducing the chemical species in the solution.

MoS<sup>2</sup> /TiO<sup>2</sup> and WS<sup>2</sup> /TiO<sup>2</sup> have been synthesized by the photo-reduction of either (NH<sup>4</sup> )2 MoS<sup>4</sup> or (NH<sup>4</sup> )2 WS<sup>4</sup> , directly on the surface TiO<sup>2</sup> particles [42]. These materials showed good efficiency in the degradation of methylene blue and 4-chlorophenol.

**Electrosynthesis:** The electrosynthesis method consists of the use of electrochemical cells to produce the desired material. Yang et al. [39] achieved the electrodeposition of CdSe nanoparticles on the surface and the inner space of TiO<sup>2</sup> nanotubes (TNTs). For this, three electrodes were used, namely a Pt wire (counter electrode), a saturated calomel electrode (reference electrode) and the TNTs (working electrode). The three electrodes were submerged in an electrolyte solution containing CdCl<sup>2</sup> and SeO<sup>2</sup> . CdSe was deposited at −0.7 V, −6 V vs. reference electrode at room temperature. Well-dispersed CdSe nanoparticles deposited on TiO<sup>2</sup> were obtained.

products obtained are usually well dispersed in form and size. Also, some additives and templates may be added into de reaction mixture to favor a desired morphology or crystallite size.

mediated solvothermal method, allowing the formation of anatase nanoparticles with highly

one-step hydrothermal method, while Liu et al. [53] used this method to achieve the formation

core@shell structures by coating different Cu<sup>2</sup>

using carbon nanospheres as a template. In the second step, the HS were mixed with a glucose

steel autoclave. In this reaction, glucose took the role of reducing agent, which helped to reduce copper from Cu2+ to Cu1+. These heterostructures were tested for the photocatalytic degradation of Rhodamine B under visible and sunlight irradiation, showing promising results.

**Impregnation:** This method consists of the saturation of one specific support—in this case TiO<sup>2</sup>

to fill the support pores. Then, the material is dried and exposed to a thermal treatment.

with a solution containing the desired precursor, usually a metal salt; this allows the metal ions

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. [56] reported the formation of cobalt oxide nanoparticles supported on the

O solution, followed by a thermal treatment in air atmosphere. Fe<sup>2</sup>

structures was achieved by Uddin et al. [59] using ruthenium(III) pentan-2,4-dionate as RuO<sup>2</sup>

**UV light irradiation:** This method consists of the reduction and precipitation of one or more

irradiation allows the photo-formation of electrons in the support, which are responsible of

and sonicated [57]. Peng et al. [58] reported that after calcination at 300°C for 6 h, the Fe<sup>2</sup>

heterostructure). In this case, TiO<sup>2</sup>

flower-like spheres, which displayed high photocatalytic activity due to an enhanced visi-

increased the absorption of visible light irradiation at 550 nm. Zhu et al. [51] prepared Bi2

dispersed CdS nanocrystals on the surface. The modification of TiO<sup>2</sup>

O/TiO<sup>2</sup>

Perales-Martínez et al. [55] reported the formation of the InVO<sup>4</sup>

) 3 ∙9H2

@MoO3

∙5H2

), urea and cetyl trimethyl ammoniumbromide (CTAB) as a template. Reaction was carried out at 160°C for 12 h, resulting in rutile/anatase nanoflowers with high surface area —up to

composites were synthesized by Wu et al. [27] via the microemulsion-

core@shell structures were synthesized by Li et al. [52] using a

hollow spheres (HS) were synthesized by both solvother-

/TiO<sup>2</sup>

, which acts as a support. The presence of UV light

particles [42]. These materials showed good effi-

were prepared using the solvothermal method. Then, the composite was

O ethanol solution, where TiO<sup>2</sup>

nanorods. In other report, the preparation of RuO<sup>2</sup>

have been synthesized by the photo-reduction of either (NH<sup>4</sup>

O; then, the mixture was poured into a teflon-lined stainless-

HS were synthesized by a sol–gel method,

heterostructure using titanium tetrachloride

with CdS nanoparticles

O polyhedral nanopar-

catalyst. In a first step,

was impregnated with the

O3 /TiO<sup>2</sup>

powders were stirred

/TiO<sup>2</sup>

O3 /

—

photo-

hetero-

O3 was

)2 MoS<sup>4</sup>

Xu et al. [50] synthesized a rutile/anatase TiO<sup>2</sup>

312 Titanium Dioxide - Material for a Sustainable Environment

/g—. CdS/TiO<sup>2</sup>

O@TiO<sup>2</sup>

surface. Cu<sup>2</sup>

mal and sol–gel methods [54]. In a first step, TiO<sup>2</sup>

ble light absorption. TiO<sup>2</sup>

solution, containing CuSO<sup>4</sup>

and TiO<sup>2</sup>

 (Co3 O4 /TiO<sup>2</sup>

soluble precursors over the surface of TiO<sup>2</sup>

reducing the chemical species in the solution.

/TiO<sup>2</sup>

, directly on the surface TiO<sup>2</sup>

ciency in the degradation of methylene blue and 4-chlorophenol.

catalyst was prepared using a Fe(NO3

deposited on the surface of TiO<sup>2</sup>

and WS<sup>2</sup>

of a series of Cu<sup>2</sup>

ticles on the TiO<sup>2</sup>

both InVO<sup>4</sup>

Co(NO3 ) 2 ∙6H2

precursor.

MoS<sup>2</sup>

or (NH<sup>4</sup> )2 WS<sup>4</sup>

/TiO<sup>2</sup>

surface of rutile TiO<sup>2</sup>

(TiCl<sup>4</sup>

TiO<sup>2</sup>

106.29 m2

**Mechanical mixing:** This is one of the simplest synthesis methods, which involves the direct mixing of the heterostructure precursors. Manual mixing usually results in long reaction times and low homogeneity of the products. In certain cases, binding agents may be added to the mixture in order to increase the stability of the heterostructure.

Shifu et al. [60] prepared the WO3 /TiO<sup>2</sup> heterostructure using the mechanical mixing method. The precise amounts of TiO<sup>2</sup> 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 TiO<sup>2</sup> .

**Precipitation and co-precipitation:** Both precipitation and co-precipitation methods consist of the formation of an insoluble material, starting from one or several solutions containing the soluble precursors. Usually, an increase in the pH value of the solution helps in the formation of insoluble hydroxides, allowing the precipitation. Yu et al. [61] prepared an Ag/AgCl/TiO<sup>2</sup> heterostructure by the impregnation of TNTs with a 1 M HCl aqueous solution, followed by a second impregnation with a 0.1 M AgNO3 solution. As a result, AgCl nanoparticles were precipitated on the TNTs. Lastly, UV irradiation was applied to achieve a partial reduction of the formed AgCl nanoparticles into metallic Ag.

**Thin films:** One of the main burdens of using powder photocatalysts is the need of including a separation step for the effluent in order to reuse the photocatalyst in further cycles. This step can become difficult and very expensive, making the photocatalytic process less viable in a plant scale approach. A feasible solution is the immobilization of the material in a suitable support, such as glass, quartz or polymer. Some synthetic routes for obtaining photocatalysts as thin films are described below.

**Dip-coating:** This is one of the most used methods for the synthesis of thin films, which consists of submerging, at a constant rate, the substrate in a solution containing the precursor of the semiconductor. After a certain dwell time, the substrate is pulled out of the solution. Lastly, the solvent is dried, and a thermal treatment can be applied to eliminate organic residuals and induce crystallization of the semiconductor in the film.

**Spin Coating:** This process consists of putting a small amount of a solution containing the precursor of the thin film material on the surface of the substrate. Then, the substrate is rotated at high speed, eliminating the excess solution and leaving a uniform film once the solvent is dried.

**Sputtering:** In this route, ionized atoms (e.g., Ar<sup>+</sup> ) are accelerated into the surface of a cathodic material. This collision causes that some atoms are ejected from the surface of the electrode. Subsequently, the ejected atoms are condensed on the surface of the substrate (anode), forming the thin film.

**Chemical vapor deposition:** This method uses volatile precursors at high temperature. The gaseous species react forming intermediates which are diffused and adsorbed on the surface of the substrate. Further reactions can take place on the surface on the substrate.

#### **4. Photocatalytic activity of TiO<sup>2</sup> -based heterostructures under visible and simulated sunlight**

The coupling of TiO<sup>2</sup> 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 [62]. 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 [63]. 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 byproducts 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.

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.

**Heterostructure** 

**Architecture**

**Pollutant removed**

**Reaction conditions**

**Performance**

**Reference**

**components**

FeWO4/TiO2

Composite

Salicylic acid (6.9

 ppm)

300

10

mg of composite were

suspended in 50

120

min of irradiation time.

k\* = 0.099 h−1

 mL.

W Xe lamp (λ ≥ 420 nm).

k\* = 0.053 h−1

[69]

Fe:Ti ratio = 95:5 FeWO4/TiO2/CdS

Fe:Ti:Cd ratio = 5:94:1

60 Bi

O2 3/TiO2 Bi:Ti molar

ratio = 1:100

TiO2/Fe

O3 Carbon-doped

Composite

Methylene blue (3.2

 ppm) light.

0.06 solution.

60

min irradiation.

g of powder in 100

 mL

anatase/brookite TiO2

(80:20)

Rutile/anatase TiO2

Nanoflowers

Methylene blue (15

 ppm)

350

10 120 Adding 0.5

solution.

mL of a 30% m/v H

O2 2

~88% of degradation rate.

315

min of irradiation time.

mg of catalyst.

W xenon lamp.

72% degradation of

[50]

http://dx.doi.org/10.5772/intechopen.76501

degradation rate.

4 (30:70)

Cr(VI)

(20

ppm of K Cr

)

450 365 nm).

30 solution.

Solar simulator with filter for blue

k = 0.008 min−1

[73]

TiO2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

mg of catalyst in 100

 mL of

nm (maxima at 254, 312 and

wt. % Cu2S/TiO2

Orange II (15 ppm)

150

W tungsten-halogen-lamp

k\* ~ 6.1 × 10−3 min−1

[70]

with UV cutoff filter at 475

pH ~ 6.4.

*p-*chlorophenol (12.88

 ppm)

150 0.001 g

6

h of irradiation time.

Mercury vapor lamp of 125

with a broadband from 250 to

 W in 30 min. k = 0.91 ppm g−1 min−1

Complete reduction to Cr(III)

[72]

mL−1 of catalyst.

W xenon lamp (λ

 ≥ 420 nm).

Degradation yield of 49%

[71]

 nm.


Subsequently, the ejected atoms are condensed on the surface of the substrate (anode), form-

**Chemical vapor deposition:** This method uses volatile precursors at high temperature. The gaseous species react forming intermediates which are diffused and adsorbed on the surface

catalyst 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 [62]. 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 [63]. 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 byproducts 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

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

**-based heterostructures under** 

with low band gap semiconductors leads to the activation of the photo-

of the substrate. Further reactions can take place on the surface on the substrate.

ing the thin film.

The coupling of TiO<sup>2</sup>

**4. Photocatalytic activity of TiO<sup>2</sup>**

314 Titanium Dioxide - Material for a Sustainable Environment

**visible and simulated sunlight**

degrades into a low toxicity dioxin.

kinetic constants.

315


**Architecture**

**Pollutant removed**

**Reaction conditions**

**Performance**

**Reference**

**components**

Bi MoO 2 Bi:Ti ratio = 1:2.6

0.5

wt.% InVO4/TiO2

Polydopamine@TiO2

Core@shell

Geosmin (1000

 ppb)

nanoparticles and

brush-like structures.

Composite

Phenol (30 ppm)

6/TiO2

Nanofibers

Rhodamine B (10

 ppm)

150

0.1 100

5 300

W lamp with an intensity

100% of degradation rate.

[55]

TiO

k# = 8.1×10−4 s−1

irradiation of 38.4

0.05

50 80 350 a 420

120

Fluorene (1000

Ofloxacin (25 ppm)

Solar irradiation with average

light intensity of 70.3

 K lux. Catalyst concentration of 0.5

120 500

W halogen lamp with two

89% of degradation rate

96% of degradation rate

[80]

2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

cutoff filters (below 420

above 850 nm).

40 in 80 mL.

5

Ag/AlO2/TiO2

Composite

Formaldehyde (10

 ppm)

90:10 Al:Ti mass ratio

h of irradiation time.

Sunlight irradiation with an

97.8% of degradation rate

[81]

http://dx.doi.org/10.5772/intechopen.76501

intensity of 90 ± 15

0.1 100

90

min of irradiation time.

317

mL of solution.

g of catalyst was dispersed in

mW/cm2.

mg of catalyst were dispersed

nm and

min of irradiation time

 g/L.

Bi O2 3/TiO2

Composite

> Bi:Ti ratio = 10%

60

wt. % Sb2S3/TiO2

Nanorods

p-hydroxyazobenzene

(10 ppm). Methyl orange

 ppb)

min of irradiation

98% degradation

92.4% degradation

[79]

nm cutoff filter.

W xenon lamp equipped with

90% degradation

[78]

min of irradiation time

mL of solution.

g of catalyst dispersed in

 W m−2.

h of irradiation.

g of catalyst was suspended in

mL of the compound solution.

W xenon lamp.

92% degradation

[77]


**Architecture**

**Pollutant removed**

**Reaction conditions**

**Performance**

**Reference**

**components**

BiVO @TiO 4

2

Core@shell

Methylene blue (5

 ppm)

Osram Dulux S67 blue light

~85% of Degradation rate.

[74]

Bulbs, with maximum emission

at 450 nm.

50

mg of the catalyst were

dispersed in 150

120

> 5

12 TiO2

Cd:Zn ratio = 3:1

Cu O/TiO 2 Cu O@TiO 2

2

Core@shell Polyhedra

Methylene Blue (3.2

 ppm)

300 filter (λ > 400

of 23 mW cm−2. Concentration of 0.2

photocatalyst mL−1.

4

4-Nitrophenol (10

 ppm)

0.02 dispersed in 50

mL of solution.

g of photocatalyst was

Up to 50% degradation for

octahedra core-shell structure.

h of irradiation time.

 mg

W xenon lamp with a glass

Up to 80% degradation for

[53]

octahedra core-shell structure.

nm). Light intensity

(cubes, cuboctahedra

and octahedra)

2

Cu

2

O nanospheres

Methyl Orange (30

 ppm)

decorated with TiO2

nanoislands.

wt. % Zn Cd x 1-xS/

Composite

Rhodamine B (4.8

 ppm)

500 (420 nm < λ < 800 nm).

0.04

80 120

min of irradiation time.

Light intensity of ~23

(λ > 400 nm).

0.03

150

40

min of irradiation time.

mL of solution.

g of catalyst was dispersed in

 mW cm−2

~97% degradation

[76]

r+~12.825 mg g−1 min−1

mL of solution.

g of the catalyst was added to

W halogen lamp

~95% degradation

[75]

wt. % WO3/TiO2

Composite

Methyl orange (20

 ppm)

2,4-dichlorophenol (20

 ppm)

1000 (λ > 420 nm). Catalyst loading of 1.1

5

h of time irradiation.

 g L−1.

W halogen lamp

85% degradation

73% degradation

[34]

316 Titanium Dioxide - Material for a Sustainable Environment

min irradiation.

 mL.


**Architecture**

**Pollutant removed**

**Reaction conditions**

**Performance**

**Reference**

**components**

Cu O/TiO 2

Bi2

Cu O/TiO 2

2

Hollow spheres

Rhodamine B (5

 ppm)

300

0.1 200 120 300 cut off filter.

300

> 1

wt. % Ag CO 2

3/TiO2

Composite

Methyl orange (20

 ppm)

300 0.05 in 80 solution.

5

\*Degradation rate constants approximated to the pseudo first-order kinetics.

#Kinetic constants of photodegradation estimated by the Langmuir-Hinshelwood model according to a first-order reaction.

Degradation performance using TiO2-based heterostructures under different experimental conditions.

+Reaction rate (degradation rate).

**Table 1.**

h of irradiation time.

mL of the compound

g of catalyst was suspended

k\* = 0.24 min−1

W iodine tungsten lamp.

~75% degradation

[88]

TiO2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

http://dx.doi.org/10.5772/intechopen.76501

319

min time irradiation.

W Xenon lamp with a 420

 nm

~ 92% of degradation rate.

k\* = 0.0081 min−1

min irradiation time

g of catalyst was dispersed in

k\* = 0.0165 min−1

mL of the compound solution.

W xenon lamp.

S3/TiO2

Fungus-like

2,4-dichlorophenoxyacetic acid

Xenon lamp with a radiation

intensity of 85 mW cm−2.

180

min of irradiation time.

360

min of irradiation)

81% degradation for 2,4-D

Complete Cr(VI) reduction.

~ 88% of degradation rate.

[54]

mesoporous Bi2

TiO2 nanotube

S3/

(2,4-D) (10 ppm) 2,4-D and Cr(VI) (10

 ppm)

2

Nanoparticles/

p-nitrophenol (20

 ppm)

Xe lamp (λ > 420

intensity of 100

210

min of irradiation.

 mW cm−2.

nm). with an

35–40% of degradation rate

[87]

under visible light irradiation.

>90% of degradation rate

under sunlight irradiation.

71% degradation

[41]

~75% mineralization (after

nanotubes


**Architecture**

**Pollutant removed**

**Reaction conditions**

**Performance**

**Reference**

**components**

Cu O/TiO 2

2 AgBr/Ag PO3 Ag:Ti molar ratio

 = 1:5

4/TiO2

Spheres

Methyl orange (8.2

 ppm).

Nanotubes

Rhodamine B (5

 ppm)

500

W tungsten halogen lamp

31% of degradation rate

[82]

with an optical filter (λ

120

min of irradiation time.

Simulated sunlight lamp with

~90% of degradation rate

[83]

318 Titanium Dioxide - Material for a Sustainable Environment

k\* = 0.1329 min−1

intensity of 4

0.001

in 6

50

Microcystin-LR (50

7 Cu O/TiO 2

2

Octadecahedron/

Methyl orange (30

 ppm)

500 filter (λ > 400

100 mW cm−2.

25 in 100

60 300

W Xenon lamp with a

99.9% of degradation after

[86]

cut-off glass filter (λ

 > 420 nm).

10 k = 0.49 min−1 88.5% mineralization.

95.4% of conversion after

120 k = 0.025 min−1

min irradiation.

min of irradiation.

Illumination intensity of

7 × 103 mW cm−2.

70 dispersed in 70

Cr (VI) (50 ppm K Cr

solution)

mL of solution.

mg of photocatalyst were

BiOBr/TiO2

Nanorods

Rhodamine B (15

 ppm)

molar ratio Ti:Bi

 = 2:1

min of irradiation.

mL of solution.

mg of catalyst were dispersed

nm) and intensity of

k\* = 0.055 min−1

W xenon lamp with glass

97% of degradation rate

[85]

Quantum Dot

wt. % Fe

O2

@TiO 3

Core@shell

Rhodamine B (10

 ppm)

350

5 50

5

h of irradiation time.

mL of aqueous solution.

mg of catalyst were dispersed in

k\* = 0.1605 h−1

W xenon lamp.

2

 ppm)

0.01

30

mL of solution.

g of catalyst was dispersed in

100% of degradation after

5 min. k\* = 0.6371 min−1 60% of degradation rate

[84]

min of irradiation time.

mL of solution.

g of catalyst was dispersed

W

m−2.

 > 420 nm).

k\* = 0.00312 min−1

> **Table 1.** Degradation performance using TiO2-based heterostructures under different experimental conditions.

Even when for the results shown in **Table 1**, direct comparisons are difficult to be established, it seems clear that tailored heterostructures formed by TiO<sup>2</sup> 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 Cu<sup>2</sup> O/TiO<sup>2</sup> 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 Cu<sup>2</sup> O-TiO<sup>2</sup> and Bi2 O3 /TiO<sup>2</sup> 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/Ag3 PO<sup>4</sup> / 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@ TiO<sup>2</sup> 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 chromium has been reported using Fe2 O3 /TiO<sup>2</sup> and Bi2 S3 /TiO<sup>2</sup> 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.

**5. Conclusions**

Even when TiO<sup>2</sup>

very few TiO<sup>2</sup>

ZnS, Ag<sup>2</sup>

TiO<sup>2</sup>

new schemes, beyond the TiO<sup>2</sup>

**Acknowledgements**

respectively.

O or graphene.

displays an outstanding performance as photocatalyst, its limitation to absorb

TiO2-Low Band Gap Semiconductor Heterostructures for Water Treatment Using Sunlight…

nanoparticles

321

http://dx.doi.org/10.5772/intechopen.76501

, and recom-

light and photoactivate under visible light irradiation makes necessary to develop a set of strategies

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

to overcome this handicap. Coupling of low band gap semiconductors with TiO<sup>2</sup>

it only works when tiny loadings of semiconductor particles are deposited on TiO<sup>2</sup>

and space consumption in photocatalytic water treatment systems.

mize the energy consumption and minimize the use of harmful reagents.

bination 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,



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 opti-

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,


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> - InVO<sup>4</sup> 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 methyl orange was achieved in 8 h of visible light irradiation by using BiOCl-TiO<sup>2</sup> 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. [68] tested the transfer of charge carriers in the BiVO<sup>4</sup> - TiO<sup>2</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 were prominent when TiO<sup>2</sup> was in contact with the aqueous matrix.
