**2. Heterostructures based on α-Fe2O3**

Hematite (α-Fe2O3) is a typical n-type semiconductor that has been extensively researched in photoelectrochemical and photocatalytic fields due to its being earth-abundant, cheap, and high chemical stability [26–28]. In addition, it has a bandgap energy of 1.9–2.2 eV that allows increased absorption of visible light [21]. Nevertheless, the photocatalytic performance of α-Fe2O3 is adversely affected by certain intrinsic factors such as the high recombination rate of charge carriers, poor conductivity of majority carriers, the short diffusion length of minority carriers, and a short lifetime of the electron-hole pairs [29]. Therefore, to overcome these drawbacks, many efforts have been developed, especially the formation of α-Fe2O3 heterostructures with other semiconducting materials [30–32]. The results revealed the successful construction of new materials with high photocatalytic efficiency in the degradation of various pollutants in wastewater [31–34].

For instance, Fe2O3/ZnO heterostructures thin films are one of the best materials used as photocatalysts because of their band position and bandgap energy of Fe2O3 and ZnO, which one is low bandgap energy lying in the visible region (Fe2O3), while the other in the UV region due to large bandgap (ZnO). Yu et al. [35] synthesized heterostructured Fe2O3-ZnO films, onto alumina plates, by a deposition method named "Solution Precursor Plasma Spray." The heterostructures films with different architectures were prepared by different injection modes. The first architecture, Fe2O3-ZnO-M, was obtained from the deposit of 12 layers of a mixed solution of

the precursors of iron and zinc. Meanwhile, the samples labeled Fe2O3-ZnO-S3, Fe2O3-ZnO-S6, and Fe2O3-ZnO-S12 were prepared alternately, injecting zinc and iron precursor solutions with stipulated cycles. The layers deposited per cycle for the Fe2O3-ZnO-S3, Fe2O3-ZnO-S6, and Fe2O3-ZnO-S12 thin films were 3, 6, and 12, respectively.

The optical bandgap for the different Fe2O3-ZnO film architectures was obtained from the Kubelka-Munk function and ranged from 2.65 eV to 2.93 eV (**Figure 1a**). Although it is true that from the point of view of photon energy absorption, a higher photocatalytic activity under visible light irradiation will be obtained for the samples fabricated from the mixture of the precursor solutions, there are other parameters such as the morphology of the surface, which indicates that the samples prepared via the separated-injection mode can also enhance the photoactivity. Thus, the influence

of these parameters on photocatalytic activity will be compared and discussed later. The photocatalytic activities of Fe2O3-ZnO thin films were evaluated to degrade the Acid Orange 7 (Orange II) dye under UV and visible light irradiation. **Figure 1b** shows the photocatalytic degradation curves of the dye under UV light irradiation for Fe2O3-ZnO samples. The photodegradation efficiencies obtained follow the order: Fe2O3-ZnO-M (100%) > Fe2O3-ZnO-S12 (83%) > Fe2O3-ZnO-S3 (65%) > Fe2O3- ZnO-S6 (21%). Meanwhile, the corresponding photodegradation curves for the Fe2O3-ZnO samples under visible light irradiation are shown in **Figure 1c** and confirm that up to 95% degradation was achieved using the Fe2O3-ZnO-M film, followed by Fe2O3-ZnO-S12 (28.4%), Fe2O3-ZnO-S3 (22%) and Fe2O3-ZnO-S6 (15%). This order is the same as that obtained under UV light irradiation (**Figure 1b**).

#### **Figure 1.**

*(a) Kubelka-Munk plots for Fe2O3-ZnO films; (b) Photodegradation performances of the Orange II degradation under UV light irradiation for Fe2O3-ZnO films; (c) Photocatalytic test of Fe2O3-ZnO films photocatalysts for Orange II dye degradation under visible-light illumination; (d) FESEM images of the top view of Fe2O3-ZnO-S12 samples. Reproduced with permission from [35].*

### *Photoactive Heterostructures Based on α-Fe2O3 and CuO Thin Films for the Removal… DOI: http://dx.doi.org/10.5772/intechopen.105818*

The surface morphologies influence the different photodegradation performances for the obtained samples under UV and visible light irradiation [35]. For example, the good photocatalytic performance obtained for the Fe2O3-ZnO-S12 sample is related to its well-shaped rod-like structure and hierarchical microstructure, as shown in the marked red rectangle in **Figure 1d**. Indeed, it is well known that the hierarchical structures present a highly organized structure, which allows an increase in the rate of mass transfer for reactant adsorption, increases the specific surface, and strongly favors efficient harvesting of light [36]. Thus, enhancing the photocatalytic properties. Furthermore, the high photodegradation efficiency of the Fe2O3-ZnO-M sample is attributed to the synergistic effect of ZnO and Fe2O3 phases to form a heterostructured catalyst, which allows a better separation of photogenerated electron/hole pairs, and thus a better photodegradation performance.

In addition, Suryavanshi et al. [37, 38] successfully synthesized stratified Fe2O3/ZnO thin films onto FTO coated glass substrate by chemical spray pyrolysis technique for subsequent study as photoelectrode in photoelectrocatalytic degradation of benzoic acid (BA), salicylic acid (SA) and methyl orange (MO) under solar illumination. The FE-SEM image of Fe2O3/ZnO heterostructure thin film is shown in **Figure 2a**. A morphology composed of grains randomly distributed on the film's surface was observed, which was helpful in the case of photocatalysis for the degradation of organic pollutants. Besides, the heterostructure morphology was slightly different from bare Fe2O3 and ZnO thin films. The variation of the morphologies for the thin films could be related to the lattice structure and defects produced during the deposition of films and the subsequent nucleation and growth [39]. Besides, the cross-section image of Fe2O3/ZnO film shown in **Figure 2b** reveals that the thickness obtained was about 2.04 μm.

The photoelectrocatalytic degradation experiment was carried out using a photoelectrochemical reactor model under the solar light illumination to investigate the photocatalytic activity of a large area deposited Fe2O3/ZnO photoelectrode. A constant bias (1.6 eV) was applied during the experiment to decrease the electronhole recombination and increase the electrochemical reaction rate. During the experiments, it is observed that the concentration of pollutants decreases due to their photoelectrochemical oxidation. **Figure 3a-c** show the extinction spectra of benzoic acid, salicylic acid, and methyl orange for heterostructured Fe2O3/ZnO photoelectrode, respectively. It is observed that extinction peak intensity decreases over time due to the decomposition of pollutants. The inset plot in **Figure 3a-c** show the degradation efficiency versus reaction time of BA, SA, and MO dye for Fe2O3/

#### **Figure 2.**

*FESEM image of (a) top and (b) cross-section view of stratified Fe2O3/ZnO thin films. Reproduced with permission from [38].*

**Figure 3.**

*Extinction spectra of (a) BA; (b) SA, and (c) MO as a function of wavelength at various time intervals. The percentage degradation using the photoelectrocatalytic degradation process is shown as insets. Reproduced with permission from [37, 38].*

ZnO photoelectrode. Degradation efficiency (%) of pollutants was calculated using the following Eq. (1) [40]:

$$\text{Depradiation efficiency} \left(\% \right) = \left(\frac{C\_0 - C\_t}{C\_o}\right) \times 100 \tag{1}$$

Where, *C*0 represents absorbance at *t* = 0 min, *C*t is the absorbance at time *t* (reaction time). As shown in **Figure 3**, the prominent absorption peaks of BA, SA, and MO dye observed at 230 nm, 302 nm, and 462 nm, respectively, decrease continuously concerning the reaction time using stratified Fe2O3/ZnO photoelectrode. The degradation graph shows that there is 74% degradation of BA, 80% degradation of SA, and 98% of MO dye in 320 min for the first two and in 80 min for the dye, respectively.

The higher degradation efficiency for the heterostructure Fe2O3/ZnO occurs due to the photogenerated electron and holes can easily transfer from the conduction band (CB) of Fe2O3 to the CB of ZnO and from the valence band (VB) of ZnO to the VB of Fe2O3, respectively. This charge transfer is possible due to the valence and conduction band of Fe2O3 being located at higher positive potentials than that of ZnO. As a result, more separation of photogenerated electron-hole pairs takes place improving the photocatalytic performance. Then, on the surface of the photocatalyst, photogenerated holes directly interact with adsorbed H2O onto the surface of the photoelectrode to produce a large amount of hydroxyl ( •*OH* ) radicals; meanwhile, electrons transfer towards the counter electrode where they can interact with the oxygen

*Photoactive Heterostructures Based on α-Fe2O3 and CuO Thin Films for the Removal… DOI: http://dx.doi.org/10.5772/intechopen.105818*

present in the atmosphere to produce oxygen radical anions ( <sup>−</sup> O2 ). Finally, the resulting ( •*OH* ) radicals can react with organic pollutants present in the wastewater to degrade them into less harmful products such as H2O and CO2 [41].

TiO2 is another semiconductor used as a coupling for Fe2O3-based photocatalysts to improve their photocatalytic activity. Wannapop et al. [42] fabricated Fe2O3/TiO2 thin films for degradation of Rhodamine B (RhB) dissolved in water under UV light irradiation. In this research, photoluminescence (PL) measurements were carried out to evaluate the separation and recombination of photogenerated carriers, wherein a stronger emitted signal is related to a faster electron-hole recombination rate [42, 43]. The PL spectra obtained from the samples are shown in **Figure 4a**. The results indicated that most Fe2O3/TiO2 films showed a lower emission intensity. Hence, the Fe2O3/ TiO2 heterostructures would have a low recombination rate of electrons and holes, improving the photocatalytic activity of the obtained films.

Furthermore, **Figure 4b** presents the degradation profile plotted as *C*/*C*0 versus irradiation time, where *C* is the concentration of RhB at the irradiation time (*t* in h), and *C*0 represents the initial concentration of RhB. After 5 h of irradiation, rhodamine was degraded to a maximum of 63% using Fe2O3/TiO2 heterostructured photocatalysts. In addition, the stability of films was also studied, wherein after three cycles of use, the efficiency of RhB degradation was decreased by 28% compared to the first usage.

Costa et al. [44] investigated the photocurrent response of heterostructured photoanodes composed of Fe2O3/WO3 and WO3/Fe2O3, deposited onto FTO-glass. The heterostructure films were investigated as photocatalyst material for Rhodamine B (RhB) dye degradation in an aqueous solution. Significantly, the order of the semiconductor layer influences the flat-band potential (Efb) positions and consequently changes the charge mobility in the electrode (photoanode). Under this study's conditions, the heterostructured WO3/Fe2O3 film showed reduced charge recombination and increased photocurrent. Therefore, WO3/Fe2O3 heterostructure film showed superior photoelectrocatalytic efficiency for RhB dye degradation (32%) in comparison to Fe2O3 or Fe2O3/WO3 heterostructure film, as shown in **Figure 5**.

Heterostructures based on Fe2O3 can also be used to decompose toxic metal ions from wastewater as Cr, Pb, and Hg [45–47]. Particularly, the presence of Cr(VI) above 0.05 mg/L (WHO standard) in drinking water is lethal mutagenic and carcinogenic to human beings [48]. In comparison to Cr (VI), the trivalent chromium (Cr(III)) is less harmful. Therefore, reducing Cr(VI) to Cr(III) has become an emerging research

#### **Figure 4.**

*(a) Photoluminescence (PL) spectra of Fe2O3/TiO2 heterostructures and (b) Photodegradation curves of RhB over Fe2O3/TiO2 heterostructures photocatalysts. Reproduced with permission from [42].*

#### **Figure 5.**

*Efficiency in the degradation of RhB dye in aqueous solution during polychromatic irradiation by photolysis, heterogeneous photocatalysis (HP), and electro-assisted heterogeneous photocatalysis (EHP) using FTO/Fe2O3, FTO/Fe2O3/WO3, and FTO/WO3/Fe2O3 electrodes. Reproduced with permission from [44].*

#### **Figure 6.**

*(a) Photocatalytic reduction of 5 × 10−4 M Cr(VI) in the presence of NiO/Fe2O3 heterostructure thin films. (b) Reaction profile of Cr(VI) reduction against specific time intervals for the different samples obtained. Reproduced with permission from [49].*

topic in the environmental field. Consequently, Jana et al. [49] constructed a novel heterostructure based on NiO/Fe2O3 thin film, which was employed in the photoreduction of Cr(VI). Thin films were used in this research to overcome the drawbacks related to material recovery and agglomeration during the photocatalysis process observed when powders are used as photocatalysts [50].

The photocatalytic reduction spectra of aqueous Cr(VI) (5 × 10−4 mol/L) at pH 2 under visible light irradiation in the presence of NiO/Fe2O3 heterostructures is shown in **Figure 6**. The results reveal that the absorption spectra of Cr(VI) centered at 350 nm decrease with the exposure time (**Figure 6a**), as well as the NiO/Fe2O3 thin films show the remarkable photo-reduction ability of approximately 100% within 90 min of visible light irradiation as shown in **Figure 6b**. A blank experiment (i.e., only light irradiation without catalyst) showed that no change in absorption of Cr(VI) was observed under the control experimental conditions, corroborating that the removal of aqueous Cr(VI) was truly driven by a photocatalytic process, rather than simple physical adsorption of Cr(VI).

### *Photoactive Heterostructures Based on α-Fe2O3 and CuO Thin Films for the Removal… DOI: http://dx.doi.org/10.5772/intechopen.105818*

Photocatalytic reduction of Cr(VI) to Cr (III) is principally promoted by photoexcited electrons generated after light illumination. The proposed schematic diagram can explain the possible mechanism well, as illustrated in **Figure 7**. When the NiO/ Fe2O3 thin films are irradiated from the light source, photogenerated electrons on the conduction band (CB) of Fe2O3 are transferred to the CB of NiO to decrease potential energy, and holes on the valence band (VB) of NiO migrate to the VB of Fe2O3 simultaneously. After that, CB electrons of NiO readily react with O2 forming <sup>−</sup> O2 and other active species. Furthermore, CB electrons also directly react with Cr(V) and Cr(IV), in agreement with the observations proposed in previous works [51, 52]. This easy cyclic transport of electron-hole within NiO/Fe2O3 matrix enables the separation of photogenerated electrons and holes, which reduces electron-hole recombination, thus improving the photoreduction efficiency of the heterostructure.

Furthermore, the stability of the as-synthesized thin films after photocatalytic reduction of Cr(VI) was studied. The results reveal that the photocatalytic activity of NiO/Fe2O3 thin films decreases after six cycles of photoreduction of Cr(VI), attributed mainly due to the deposition of small amounts of Cr(III) onto the surface of NiO/Fe2O3 after each cycle.

### **3. Heterostructures based on CuO**

Copper oxide (CuO) is an important p-type semiconductor with a narrow bandgap of 1.3–2.2 eV, large absorption of visible light, low cost, uncomplicated fabrication, and low toxicity. It has been widely investigated as photocatalysts in the photocatalytic degradation of organic pollutants in wastewater [53, 54]. Besides, CuO generates a high photocurrent compared to α-Fe2O3 [54]. However, the photocatalytic activity of the bare CuO is still low due to its high photoexcited electron-hole recombination [55]. Therefore, prevention/slowdown of the rate of recombination of photogenerated electron-hole pairs is an important parameter in aiming for superior photocatalytic activity. Recently, creating a heterostructure by coupling an n-type semiconductor

#### **Figure 7.**

*Schematic representation of band diagram and photodecomposition pathway of NiO/Fe2O3 heterostructure photocatalyst; where ϕ, VD, EF, and Eg represent work function, contact potential, Fermi level and band gap. Figure adapted from [49].*

with p-type CuO may represent an effective strategy for enhancing the stability of the photogenerated electron-hole pairs. Indeed, many studies have been focused on developing heterostructure CuO-based composite photocatalysts for photocatalytic applications, such as CuO/ZnO, CuO/TiO2, CuO/Cu2O, CuO/WO3, CuO/CeO2 and CuO/SnO2 [56–61].

For example, Selleswari et al. [61] fabricated CuO/SnO2 heterostructure thin films by spray pyrolysis technique and found that the efficiency of photodegradation of Congo-red (CR) and Malachite green (MG) under UV light irradiation was improved compared to pristine CuO and SnO2. The degradation efficiency of the CuO/SnO2 (1:1 ratio) heterostructure was 90 and 97% for the MG and CR dyes, respectively (**Figure 8a** and **b**, respectively). Besides, after seven photocatalytic degradation cycles, no significant decrease in the photocatalytic activity of the heterostructure was observed (only a loss of 3–5%), as shown in **Figure 8c** and **d**.

The enhanced photocatalytic activity of the heterostructure is attributed to its synergistic action on the specific adsorption property and low recombination probability of photo-generated carriers due to the efficient charge transfer between CuO and SnO2.

Additionally, ZnO is another semiconductor used as a coupling for CuO-based photocatalysts. In particular, Nguyen et al. [62] investigated the photocatalytic activity of ZnO/CuO thin films fabricated onto a glass substrate by sputtering, thermal

#### **Figure 8.**

*Degradation profile of (a) Congo-red and (b) Malachite green dyes; seven cycles segment for degradation of (c) Congo-red and (d) Malachite green under UV light irradiation for CuO, SnO2, and CuO/SnO2. Reproduced with permission from [61].*

#### *Photoactive Heterostructures Based on α-Fe2O3 and CuO Thin Films for the Removal… DOI: http://dx.doi.org/10.5772/intechopen.105818*

annealing, spin coating, and simple hydrothermal methods. The optical absorption analysis shown in the spectra in **Figure 9a** reveals that the ZnO/CuO heterostructure film has a broad absorption range and the highest optical absorption compared to ZnO film and CuO film. This phenomenon can be explained by the attributions of the high surface roughness of the ZnO film in the heterostructured ZnO/CuO film, which can reduce the optical reflection on the surface of the composite film [62]. Furthermore, the photocatalytic activities of the fabricated samples were examined by the rate of degradation of RhB dye under simulated solar irradiation. **Figure 9b** shows the degradation efficiencies calculated for CuO, ZnO, and ZnO/CuO thin films, where the values obtained after 120 min of illumination were 55, 78, and 93%, respectively. The improvement in the degradation efficiency of this heterostructured film was mainly ascribed to the effective suppression of the electron-hole pairs recombination and its higher photon absorption. Besides, **Figure 9c** shows the cycling photodegradation of the ZnO/CuO thin film, obtaining that after three cycling experiments, the film maintains good degradation efficiency for dye contamination, demonstrating that this new material is a highly photostable and reusable photocatalyst.

Another heterostructures based on CuO and CuO2 thin films used in the degradation of organic pollutants was proposed by Khiavi et al. [58]. In this research, a photocatalyst was developed by integrating cupric oxide (CuO) and cuprous oxide (Cu2O) thin films, which showed superior performance for the photocatalytic degradation of methylene blue (MB) compared to CuO and Cu2O pristine photocatalysts. As shown in **Figure 10a**, Cu2O was deposited on top of the CuO thin films due to the lower optical absorption of Cu2O than CuO, a greater bandgap of Cu2O (~2.2 eV) than CuO (~1.6 eV), and a longer carrier diffusion length of Cu2O (~500 nm) than CuO (~200 nm). The MB concentration change as a function of the photocatalytic

#### **Figure 9.**

*(a) Absorption spectra; (b) photodegradation of RhB; (c) recycling photodegradation of CuO, ZnO film, and ZnO/CuO heterostructure thin films. Reproduced with permission from [62].*

degradation time under visible light was plotted and is shown in **Figure 10b**. The results showed that the MB degradation rate by the CuO/Cu2O heterostructure was almost twice that of the bare semiconductors. The enhanced photocatalytic activity for CuO/Cu2O heterostructures can be attributed to the improved separation of photogenerated electrons and holes, where the interface between Cu2O and CuO acted as a key parameter for this separation. Meanwhile, higher absorption of visible light was achieved based on the difference in the energy levels of their conduction bands and valence bands.

Photocatalytic reduction for removing toxic metals from wastewater is considered one of the most attractive methods because of its low cost and ease of operation [63–65]. In particular, heterostructures based on CuO are suggested as promising materials for metal ion reduction under visible illumination [66–68]. For instance, Ghosh and Mondal [69] developed a unique binary heterostructure-based photocatalyst consisting of Cu7S4/CuO and explored its application in the photocatalytic reduction of Ni (II) under visible light. The results observed in **Figure 11a** reveal that Cu7S4/CuO films exhibit better photocatalytic performance than pure Cu7S4 and CuO films. The removal rate of Ni(II) for pure Cu7S4 film is 84% at 90 min and 89% for pure CuO films at 75 min, while for the Cu7S4/CuO heterostructure, the removal rate increased to 95% at 60 min. Likewise, these authors fabricated a thin-film TiO2/CuO

#### **Figure 10.**

*(a) Cross-sectional TEM image of the fabricated heterostructure CuO/Cu2O thin film photocatalyst; (b) MB degradation profiles for different irradiation times for CuO, Cu2O, and CuO/Cu2O samples. Reproduced with permission from [58].*

#### **Figure 11.**

*Reaction profile for the photocatalytic reduction of Ni (II) in the presence of (a) Cu7S4/CuO and (b) TiO2/CuO heterostructure films under visible light irradiation. Reproduced with permission from [69, 70].*

#### *Photoactive Heterostructures Based on α-Fe2O3 and CuO Thin Films for the Removal… DOI: http://dx.doi.org/10.5772/intechopen.105818*

heterostructure on an FTO glass substrate [70] and found a faster reduction of Ni(II) than in single films. The total decrease of Ni2+ by pure TiO2 was only 23% after 45 min of light irradiation, and for pure CuO, it extended up to 78% for the same period (**Figure 11b**). Meanwhile, for TiO2/CuO heterostructure, the degradation further increased up to 95% simultaneously.

The increase in photocatalytic performance for heterostructures in both cases happened due to the effective separation of electron-hole pairs by the junction to lower their recombination rate, thus, paving the way for better reduction. Specifically, a possible mechanism of Ni2+ reduction by the Cu7S4/CuO heterostructure catalyst was described. When both semiconductors are simultaneously excited by the absorption of photons under visible light irradiation, electron-hole pairs are created. Subsequently, the photogenerated electrons from the conduction band of Cu7S4 get easily transferred to the conduction band of CuO. While, the respective holes in the valence band of CuO would move upward in energy to the VB of Cu7S4, which retards the recombination of photogenerated electron-hole pairs. Then, the reduction of Ni(II) ions to Ni(0) can occur by the photogenerated electrons on the heterostructure surface. At the same time, the holes oxidize water to oxygen or organic compounds to CO2.

Moreover, Pastrana et al. [71] fabricated heterostructures based on CuO/α-Fe2O3 by dip-coating technique. Due to their rough structure, these heterostructures present efficient and fast arsenic removal performances compared to pure oxides. The removal of arsenic was attributed to the direct absorption of As(III) on thin films and the photocatalytic oxidation of As(III) to As (V). The results shown in **Figure 12a** indicate that the highest absorption efficiency obtained for the CuO/Fe2O3 heterostructures was approximately 85% within the first 20 min of irradiation; after that, the arsenite removal efficiency remains relatively constant. Besides, removal tests in the darkness were performed, obtaining removal efficiency below 10% (see **Figure 12b**), corroborating that the heterostructures photoactivity effectively enhanced the adsorption. The higher removal efficiency of heterostructures compared to pristine oxides is mainly due to roughness and the slower recombination of electron-hole pairs for the heterostructures. Many researchers claim that the increase of the surface roughness may favor the adsorption of molecules on the surface films [71] and that the inhibition of recombination of charge carriers will produce a more significant amount of reactive oxygen species, which are intermediates to oxidize As(III) to As(V) [72].

#### **Figure 12.**

*Effect of contact time variation on As(III) removal efficiency on α-Fe2O3 and α-Fe2O3/CuO heterostructure thin films. Reproduced with permission from [71].*

### **4. Summary and outlook for future directions**

In this review, we presented an overview of research developments in the fabrication of heterostructures based on CuO and α-Fe2O3 thin films for photocatalytic applications in wastewater treatment, such as photodegradation of organic pollutants and photoreduction of metal ions. According to the present study, a great number of researches related to efficient heterostructured photocatalysts based on CuO and Fe2O3 thin films have been reported. The improvement in photocatalytic activity is mainly attributed to the better light absorption and the efficient separation/transport of electron-hole pairs, and some examples are highlighted in this review. However, even though significant progress has been achieved in studying CuO-based and Fe2O3-based heterostructured photocatalysts based, improving these thin film-type heterostructured photocatalysts still presents challenges. Therefore, to resolve these drawbacks, it is necessary to consider the following aspects carefully: (1) the quantum efficiency of these heterostructures is still low compared to utilizing solar energy with high efficiency. Thus, a better understanding of the dynamic behavior of photogenerated carriers in the interface and surface of these types of heterostructures is required; (2) the catalyst's reusability, longevity, and stability should be emphasized in terms of practical applications and (3) more research should be developed to improve the scale production of these heterostructures, as well as its economic feasibility and long-term durability for the treatment of real industrial wastewater in the coming years.

In addition, we also propose some areas for future investigation, for instance, (1) to overcome disadvantages and improve their catalytic performance, the heterostructures based on CuO and Fe2O3 thin films can be modified via doping and the formation of ternary heterostructures by combining them with other semiconductors, graphene or other carbon-based materials; (2) the production of these heterostructures onto a variety of substrates, could be another interesting approach for photoelectrocatalysis or photocatalysis applications, as well as convenient to reuse the catalyst; (3) guide future research in the use natural sunlight, due to its abundance, as a source of illumination instead of commercially simulated sunlight and (4) carrying out research related to computational calculation and design forms of heterostructures based on CuO and Fe2O3 thin films will us get an extensive perception of the system and charge transportation kinetics in these materials, as well as clarify the correct vision of photocatalytic processes in CuO-based and Fe2O3-based heterostructured photocatalysts thin films.

In conclusion, this article has summarized the current efficient developments in the fabrication and application of photocatalysts of CuO-based and Fe2O3-based heterostructures thin films. Both CuO and Fe2O3 are non-toxic, cost-effective, and photoactive materials under UV/visible/solar irradiation, making them recommended semiconductors for heterostructures formation. However, it is still necessary to improve certain drawbacks in search of the more remarkable development of heterostructured systems based on CuO and Fe2O3 thin films with high efficiency and stability. If these objectives are achieved, these heterostructures will be used in industrial water treatment systems in some years. Finally, the implicit final aim of this review was not only to summarize the state of the art but also to stimulate the readers and arouse their curiosity, leading them to investigate this particular class of nanomaterials in greater detail.

*Photoactive Heterostructures Based on α-Fe2O3 and CuO Thin Films for the Removal… DOI: http://dx.doi.org/10.5772/intechopen.105818*
