Photoactive Heterostructures Based on α-Fe₂O₃ and CuO Thin Films for the Removal of Pollutants from Aqueous Solutions

Abstract

Heterostructured photoactive nanomaterials represent innovative construction to absorb UV and UV-vis light. This feature makes heterostructures exciting candidates for environmental photocatalytic applications such as organic pollutants degradation and removal of heavy metals, among others. Therefore, the efficient design of heterostructures based on thin films of oxide semiconductors will allow obtaining a novel material with outstanding properties. This work presents a review of the current heterostructures based on α-Fe2O3 and CuO thin films, which were deposited onto different substrates using physics and chemistry routes. Moreover, we will discuss the key factors to promote structural and morphology control and the drawbacks such as low absorption of the solar spectra, low active surface area, and charge carrier recombination. Finally, the relevance of the results and future directions of the heterostructures as materials for the purification of aqueous systems were discussed.

1. Introduction

The heterostructures represent innovative constructions that use a broad spectrum of sunlight, capable of efficiently absorbing UV and visible light. Besides, due to their response to the absorption of different light sources, heterostructures can be used for varied photocatalytic applications for environmental remediation such as water splitting, CO₂ conversion, photocatalytic degradation, and oxidation [1]. Heterostructures are composed of two or more semiconductor material structures with specific chemical compositions, which can be formed by an interface between two different materials with unequal bandgaps. Remarkably, the idea of combining various metal oxides to form heterostructures is relatively recent and was born as a response to the need to improve its morphological, structural, and functional properties [2, 3]. The development of semiconductor heterostructures has brought about a tremendous impact on our lives. The utilization of these devices has improved our quality of life due to their role in electronics, memory devices, photodetectors, and optoelectronic devices [4]. Transistors, photovoltaic cells, diodes, and sensors are some cases where heterostructures are present.

A heterostructure is essentially a physical, and therefore electronic, the bond between two different materials in the solid state [4, 5]. Whenever materials connect as a heterostructure, the Fermi levels (E_F) align: higher-energy electrons flow across the interface to lower-energy unoccupied states until the Fermi levels have equilibrated. This leads to creating a charge carrier depletion zone at the interface (depletion region). After that, an energy barrier potential is created at the interface due to band bending, caused by the difference in the initial Fermi levels of the materials that make up the heterostructure. Thus, charge carriers must overcome this potential energy barrier to cross the interface [6, 7]. The junction between two different materials is one of the most important aspects to consider in the behavior of the new heterostructured material [8].

Heterostructures can be classified according to the configuration and dimensions of the interface between the two components that conform: one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) heterostructures [9, 10, 11]. In a 1D heterostructure, the interface is similar to a line shape, being the contact area is only in one direction; usually, some 1D heterostructures are nanorods, nanotubes, nanofibers, and others [12]. 2D heterostructures have an interface plane-like shape and conform to layers for both components (“layer-by-layer” systems). These heterostructures (nanofilms) usually are formed on a substrate that can be conductive or not [13]. Finally, 3D heterostructures expand in all three directions and involve multiple and rare shapes, the most common considering nanoparticle components. Frequently, these heterostructures are conformed by agglomerates of crystallites of different components, the more popular is the core-shell structure [14]. There is a growing interest in the research for nanofilms (2D) materials with novel properties that permit stacking and combination of thin layer-layer reaching unexpected features such as technological goals [15]. These materials have an impressive performance concerning flexibility in their electronic role and convenient design. The thin film nanoheterostructures have a principal advantage: an efficient charge separation, which restricts the recombination of charge carriers and consequently increments their photoactivity performance [16]. However, they have a disadvantage: the loss of energy by the charge carriers, inhibiting the evolution of chemical reactions.

The nanosized thin films combined into heterostructures depend on purposes and the demanded functions. Nevertheless, the heterostructured photoactive materials are extensive and diversified; most attention is given to metal-semiconductor and semiconductor-semiconductor heterostructures [17]. In this line of thought, we focus on thin film semiconductor-semiconductor systems with interesting optical properties for various applications in advanced catalytic and healthcare fields.

A group of those nanoheterostructured photoactive materials can be presented in three different assemblies: hosted nanophase, when one phase grows onto another in many positions; segmented nanophase, which includes two materials within each particle; and a mixture of two nanophase materials [11]. Up to now, there have been many original articles with methodologies appropriate for heterostructures thin film fabrication. For example, heterostructure thin films could be assembled using electrodes and are easily routed for deposited films. On the other hand, inorganic thin films commonly use a wet-chemical bottom-up became to achieve worldwide attention, leading to a remarkable increase in the number of research papers and patents. Notwithstanding, there have been very few researches focusing on various solution-processes techniques used for optimum inorganic nanofilm fabrication [18].

Metal oxides have a wide range of crystalline structures and a variety of functional properties that convert into unattainable conventional semiconductors. Iron oxide is one of the most abundant compounds on the planet, and it is a low-cost and environmentally friendly resource [19]. As a nanomaterial has outstanding characteristics that include; (i) photocatalytic properties for conversion reactions; (ii) large energy storage capacity; (iii) reduced industrial process cost; and iv) long-term sustainability due to its availability and low toxicity, along with others [20]. In particular, hematite (α-Fe₂O₃) is an n-type semiconductor with a bandgap of 1.9–2.2 eV, which ensures the absorption of more than half of visible light (>600 nm), obtaining 40% of the incident energy of the solar spectrum [21, 22]. On the other hand, copper oxide (CuO) is a p-type semiconductor with a bandgap from 1.3 to 2.2 eV. It has a monoclinic structure with fascinating characteristics: super thermal conductivity, high solar absorbance, low thermal emittance, relatively good electrical properties, photovoltaic properties, high stability, and antimicrobial activity [23, 24]. This semiconductor is involved in many technological fields, for instance, catalysis, sensors, high-efficiency thermal conductivity material, magnetic recording media, selectivity, and solar cell applications [25].

Based on the abovementioned, in this chapter, we attempt to provide the readers with an essential introduction to the innovative construction of heterostructured photoactive nanomaterials. We focus on the designs of heterostructures based on α-Fe₂O₃ and CuO thin films, highlighting the recent advances in heterostructures fabricated using different physics and chemistry routes. Finally, the progress in constructing thin film heterostructures as environmental technologies for the remediation of harmful elements in aqueous systems was presented.

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, 27, 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, 31, 32]. The results revealed the successful construction of new materials with high photocatalytic efficiency in the degradation of various pollutants in wastewater [31, 32, 33, 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.

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].

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).

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.

Figure 2.

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

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

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].

$$\text{Degradation efficiency}\left(\%\right)=\left(\frac{C_0-C_t}{C_0}\right)\times 100$$E1

Where, C₀ 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 present in the atmosphere to produce oxygen radical anions (${\text{O}}_2^{-}$). 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.

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].

Furthermore, Figure 4b presents the degradation profile plotted as C/C₀ versus irradiation time, where C is the concentration of RhB at the irradiation time (t in h), and C₀ 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 (E_fb) 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.

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].

Heterostructures based on Fe₂O₃ can also be used to decompose toxic metal ions from wastewater as Cr, Pb, and Hg [45, 46, 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 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).

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].

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 ${\text{O}}_2^{-}$ 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.

Figure 7.

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

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 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, 57, 58, 59, 60, 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.

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].

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 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.

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].

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 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.

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].

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, 64, 65]. In particular, heterostructures based on CuO are suggested as promising materials for metal ion reduction under visible illumination [66, 67, 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 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.

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].

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.

Acknowledgments

The work described in this review was financially supported by the projects CONCYTEC under the contract numbers N° 032-2019-FONDECYT-BM-INC.INV, N° 120-2018-FONDECYT, N°08-FONDECYT-BM-IADT-MU-2018, N° 237-2015-FONDECYT and N° 059-2021-PROCIENCIA.

Conflict of interest

The authors declare no conflict of interest.