**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.

**Keywords:** heterostructure, semiconductor, α-Fe2O3, CuO, photoelectrochemical, photocatalysis, bandgap

### **1. Introduction**

The heterostructures represent innovative constructions that use a broad s pectrum 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 (EF) 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 [9–11]. 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.

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

Notwithstanding, there have been very few researches focusing on various solutionprocesses 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.
