Role of Surface Defects and Optical Band-gap Energy on Photocatalytic Activities of Titanate-based Perovskite Nanomaterial

*Izunna Stanislaus Okeke, Priscilla Yahemba Aondona, Amoge Chidinma Ogu, Eugene Echeweozo and Fabian Ifeanyichukwu Ezema*

## **Abstract**

In recent years, water pollution has become one of the major challenges faced by humans because of consistent rise in population and industrial activities. Water pollution due to discharge from cosmetics and pharmaceutical wastes, organic dyes, and heavy metal seen as carcinogens has the potential to disrupt hormonal processes in the body. Different approaches such as chlorination, aerobic treatment, aeration, and filtration have been deployed to treat wastewaters before being discharged into the streams, lakes, and rivers. However, more attention has been accorded to treatment approaches that involve use of nanomaterial due to non-secondary pollution, energy efficiency, and ease of operation. Titanate-based perovskite (TBP) is one of the most frequently studied nanomaterials for photocatalytic applications because of its stability and flexibility in optical band-gap modification. This chapter provided an overview of basic principles and mechanisms of a semiconductor photocatalyst, and current synthesis techniques that have been used in formulating TBP nanomaterial. The effect of reaction conditions and approaches such as doping, codoping, composites, temperature, pH, precursor type, surface area, and morphology on surface defects and optical band-gap energy of TBP nanomaterial was highlighted. Importantly, the impact of surface defects and optical band-gap energy of TBP on its photocatalytic activities was discussed. Finally, how to enhance the degradation efficiency of TBP was proposed.

**Keywords:** titanate-based perovskites, surface defects, optical bandgap, photocatalysis, organic pollutants

### **1. Introduction**

Currently, water pollution is becoming one of the most serious challenges confronting human beings due to steady increase in population, advancements in industrial activities, and urbanization [1]. Contamination of water bodies through discharge of organic pollutants such as dyes, heavy metals, and petroleum are posing a significant danger to humans as well as the aquatic ecosystem [2]. The effects of these pollutants differ and depend on the source and type; for example, organic dyes and heavy metals have been recognized as carcinogens while cosmetics and pharmaceutical waste products have been identified as endocrine disruptive agents [3]. These agents impede hormonal processes, thereby disturbing regular homeostatic reproduction, advancement, or behavior [4]. Furthermore, presence of dyes in the water bodies blocks sun from penetrating into the water bodies and lessens dissolved oxygen, therefore, causing death to photosynthetic organisms that live in the aquatic system [5].

To mitigate the impact of water pollution, scientists are making efforts to develop approaches to treat wastewaters before being discharged into rivers, streams, and underground water. Approaches such as chlorination, aerobic treatment, aeration, and filtration have been used to treat waste. However, greater attention has been given to treatment processes that involve the use of nanomaterials. Inorganic metal oxides nanoparticles (NPs) have benefits such as no secondary pollution, cost, and energy efficiency and are easily operated [6, 7]. Metal-oxide NPs have been studied for potential degradation of contaminants [8–10], heavy metals [11, 12], and inactivation of bacteria [13–15]. Many other compounds such as metal halides [16], metal nitrides [17], and metal chalcogenides [18, 19] have also been evaluated for photocatalytic applications. Among which, perovskite nanomaterial has drawn so much attention because of its wide variety of properties. Perovskite nanomaterial has indicated a broad range of electro-optical effects, piezo, ferro, and pyro-electrical properties that enable them to exhibit outstanding performances as structural, electronic, and magnetic material [20]. In addition, because of their crystalline structure, perovskites have unique chemical properties that contain a spectrum of cations to generate surface defects, which can balance unstable oxidation states [21].

Titanate-based perovskite material is among the most commonly studied perovskites for photocatalytic applications under visible light. This is because their optical band-gaps energy is easily modified and quite stable for a long period throughout a photocatalytic reaction [22]. A good number of titanium-based perovskites such as SrTiO3, MnTiO3, BaTiO3, ZnTi3, MgTiO3, and CaTiO3 have optical band-gaps energy above 3 eV [23–25], thereby allowing photocatalytic activities only under UV source. On the other hand, TBP, such as CoTiO3 and NiTiO3, have band-gap energy lower than 3 eV but their CB is below the oxidation potential [26, 27], which also limits their chances for photocatalytic applications.

Excellent photocatalyst semiconductor (SC) is expected to have excellent charge carrier mobility and charge separation to prevent recombination of electron (e) and hole (h) generated in the system [28]. Efforts have been made by scientists to enhance photocatalytic activities of SC photocatalyst through various means by modification of physiochemical properties of the nanomaterial. The modification can be done via doping (with metals, nonmetals, and salts), defect engineering, heterostructure, and cocatalysts [29–31]. Research has shown that doping greatly affects the electronic structure of SC nanomaterial [32]. Like TBP, the majority of SC photocatalysts have wide band-gaps energy, which makes it difficult for them to have photocatalytic activation with a visible light source [33, 34]. To improve the efficiency and quantum yield of SC photocatalysts such as TBP, their optical band-gap energy should be altered to respond to visible light sources. Another parameter that affects the photocatalytic activities of this material is presence of defects. Defects seen in a

*Role of Surface Defects and Optical Band-gap Energy on Photocatalytic Activities… DOI: http://dx.doi.org/10.5772/intechopen.106253*

material can be artificial or natural specific structure. Surface defects alter geometric structure, as well as the chemical environment of the host material [35]. It can also serve as charge carrier traps and adsorption sites; the induced electrons can be transferred to the sites and, therefore, prevent recombination of photogenerated e—h+ pairs.

This chapter highlighted fundamental principles and mechanisms of SC photocatalysis and recent synthesis techniques that have been deployed in preparing TBP nanomaterial. The influence of reaction conditions and approaches such as doping, codoping, composites, temperature, pH, precursor type, surface area, and morphology on surface defects and optical band-gap energy of TBP nanomaterial was noted. Ultimately, impact of surface defects and optical properties of TBP on its photocatalytic activities against organic pollutants was discussed. Considering progress recorded so far in this area, some perspectives on how to advance and improve degradation efficiency of TBP against organic pollutants were proposed.

#### **1.1 Fundamental principles and mechanisms of photocatalysis**

Generally, photocatalysis is initiated when a SC photocatalyst absorbs photons with energy equal to or higher than its optical band-gap energy. Consequently, electrons in the valence band (VB) are excited into the conduction band (CB) leaving holes in the VB. This excitation produces a potential difference midway CB and VB bands, creating reductive and oxidative entities at the CB and VB, respectively. These photo-activated charge carriers can react with H2O or dissolved oxygen to generate free radicals such as OH and O<sup>2</sup> that can degrade pollutants into smaller molecules [36] as shown in Eqs. (1)–(6). It is fundamental that the minimum material CB is located at a higher negative potential compared to the reduction potential for H<sup>+</sup> to H2, at the same time, it is also essential that the highest VB is located at a higher positive potential compared to the oxidation potential for H2O to O2 [22]. **Figure 1** describes the indirect organic pollutant degradation process by SC photocatalyst.

**Figure 1.** *Pictorial description of indirect organic pollutant degradation process by SC photocatalyst. Adapted with permission [37].*

$$\text{SC} + h\nu \to e^-\_{\text{CB}} + h^+\_{\text{VB}} \tag{1}$$

$$h\_{\rm VB}^{+} + H\_{2}O \rightarrow \bullet OH + H^{+} \tag{2}$$

$$h^+\_{\rm VB} + \bullet OH^- \rightarrow \bullet OH\_{ad} \tag{3}$$

$$
\bullet \text{ $e$ }\_{\text{CB}}^{-} + \text{O}\_{2} \rightarrow \bullet \text{ $O$ }\_{2}^{-} \tag{4}
$$

$$\text{\textbullet OH} + \text{pollutants} \rightarrow \text{CO}\_2, \text{H}\_2\text{O}, \text{salt} \tag{5}$$

$$\bullet OH\_{ad} \rightarrow \bullet OH\_{f\text{re}} + pollutant \rightarrow simple \text{oxid\\_product} \tag{6}$$

#### **1.2 Crystal structure of a perovskites**

Perovskite material is generally referred to as material whose crystal structure is described by the formula ABO3, A and B are ions that often have different sizes and O is an ion that is bonded to A and B. It has a cubic structure that contains B cations in a 6-fold orientation encircled by an octahedral of anion while the A cation in a 12-fold cuboctahedral orientation [38]. **Figure 2a and b** describes the idealized cubic perovskite structure. From the crystal structure, B site cations are firmly glued to the oxygen (or other anion) at the same time A site cations interaction with the oxygen is relatively weaker. Based on the nature of the cations residing in the lattice sites, various perovskite crystal geometries can be obtained by modifying these interactions.
