Review
Review on the criteria anticipated for the fabrication of highly efficient ZnO-based visible-light-driven photocatalysts

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Abstract

Among the popular photocatalysts, ZnO is one of the most potent photocatalysts considering its green properties, cheap price, and durability. However, the practical application of ZnO is limited because of its large band gap energy and rapid recombination of the photoinduced electron–hole pairs. This paper reviews the main advancements in overcoming the barriers accompanied by pure ZnO and the criteria for fabrication of effective visible-light-responsive ZnO-based photocatalysts. Herein, the binary ZnO-based nanocomposites with p–n heterojunctions, n–n heterojunctions, and ternary ZnO-based nanocomposites based on different heterostructures, and their mechanism for enhanced light harvesting and charge separation/transfer were thoroughly discussed.

Introduction

Water is one of the most important substances on the earth, which is critical for life-sustaining of all living creatures. Although about 71% of the earth surface is covered by water, only about 2.5% is fresh water [1], [2]. On the other hand, water consumption in various sectors has been dramatically overdone in the past decades, which caused diminution in fresh water required for wildlife and human life. In addition, billions of people access to limited clean water resources, that makes them vulnerable to water-borne infections [3]. It is estimated that prior to 2015, approximately 70% of the globally generated wastewater was not properly treated so that their discharge caused the pollution of receiving natural water-bodies. Correspondingly, by 2025, it is predicted that 50% of the people will face clean water crises [4].

In light of this, there is an urgent need to develop effective, dependable and economically viable methods to deal with the emerging contaminants and address the safety problems caused by them [5]. In the last decades, several technologies have been established for wastewater treatments [6], [7], [8]. The well-known methods are coagulation [9], sedimentation–flocculation [10], ion exchange [11], molecular sieves [12], [13], reverse osmosis [14], membrane filtration [15], adsorption processes [16], ozonation [17], chlorination [18], chemical precipitation [19], and chemical and electrochemical techniques [20], [21]. The conventional treatment technologies are reportedly not cost- and energy-efficient and inadequate for complete degradation of recalcitrant contaminants in wastewater [22]. Accordingly, novel advanced treatment methods are needed to destroy organic portion of wastewaters. Over the past two decades, we have been witnessing an unprecedentedly large number of research on the wastewater treatment through advanced oxidation processes (AOPs) [23]. The in-situ generation of energetic oxidizing agents, mainly hydroxyl radicals (radical dotOH), superoxide anion radical (radical dotO2), and photogenerated electron/hole (e/h+) pairs through AOPs lead to the complete degradation and mineralization of contaminants in polluted water [24]. Heterogeneous photocatalysis has been recognized as one of the most appealing and potentially efficient AOPs owing to its relative low-cost and high stability, nontoxicity, no resistance to mass transfer and no secondary pollution, operation under ambient conditions, and more importantly the potential for decomposing the recalcitrant organic pollutants at short reaction time into less harmful compounds [25], [26], [27], [28]. In this regard, the application of semiconductor-based photocatalysis has received much attention across various disciplines as it offers an efficient and green solution for environmental problems (Fig. 1).

Section snippets

Fundamentals and applications of heterogeneous photocatalysis

The term photocatalysis refers to a process in which the rate of the chemical transformation is enhanced by a substance, i.e. photocatalyst, under the illumination of light without its ultimate alteration [29]. In this process, the incidence of light generates e/h+ pairs in the conduction (CB) and valance bands (VB) of the semiconductor that take part in the redox reactions to produce the final products [30]. The photocatalytic reactions can be processed in a homogeneous and/or heterogeneous

Selection of photocatalyst

In general, semiconductors are the most potent photocatalysts known as a result of their appropriate band gap energies and well-defined electronic configuration with occupied VB and unoccupied CB [52]. Semiconductors with distinctive electronic, light absorption, and charge transfer properties, and porous structure have provided desirable activities for photocatalytic degradation of numerous organic pollutants [53], [54]. The recent investigations have revealed that semiconductors of the groups

ZnO as a traditional photocatalyst

Wurtzite ZnO was introduced as a unique n-type semiconductor possessing a direct and wide band gap of 3.37 eV and a big exciton binding energy (60 meV) at ambient temperature with high electrochemical stability, good isoelectric point (pH 9–9.5) [91], [92], [93], and desirable electro-optical characteristics [94]. There is a growing body of literature that recognizes ZnO as an outstanding semiconductor in photocatalytic processes owing to its low-price, appropriate redox potential, nontoxicity,

The limitations of ZnO as photocatalyst

The design and fabrication of a photocatalyst with appropriate band structure, able to absorb a wide-range of visible region, with high redox potential, and good stability are still a challenging practice. In conventional processes, the inadequacy in light absorption in the visible region and/or unsatisfactory separation of charge carriers is the major problems [98], [99]. Therefore, many efforts have been focused to get these limitations under control. Regardless of the versatility, ZnO as

ZnO-based photocatalysts with p–n heterojunction

The unification of n-type and p-type semiconductors for production of nanocomposites offers a new approach for combining various physico-chemical features of specific compounds all into a unit system [260]. The heterojunction composed of an n-type semiconductor as a donor and a p-type semiconductor as an acceptor is so-called p–n heterojunction. The Fermi level is located nearby the VB in p-type semiconductors where holes constitute major carriers. On the other hand, in the n-type

Ternary ZnO-based nanocomposites

So far, there are plenty of studies focusing on the ZnO-based binary nanocomposites for improved visible-light photocatalysis, as described in the previous sections. Apart from the binary nanocomposites, ternary or multi-component composites have also drawn the attention of the researchers to develop novel composites with great photocatalytic activity under visible-light irradiation. Coupling of ZnO with various materials to develop ternary heterostructures such as Bi2O3/CeO2/ZnO [343],

Summary remarks and perspective

In summary, this review aimed to compile the existing strategies and modifications developed to date to overcome the drawbacks encountered with application of pure ZnO in photocatalytic processes, such as deficient chemical and photo-stability, poor sunlight harvesting, and confined active surface area. Moreover, the employed preparation methods, potential application domains, mechanisms involved in charge transfer via various junctions and their effects on the photocatalytic processes were

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