Elsevier

Chemosphere

Volume 286, Part 1, January 2022, 131651
Chemosphere

A critical review on relationship of CeO2-based photocatalyst towards mechanistic degradation of organic pollutant

https://doi.org/10.1016/j.chemosphere.2021.131651Get rights and content

Highlights

  • Recent efforts concerning ceria (CeO2) as a single photocatalyst were reviewed.

  • Several strategies modifications of CeO2-based and heterojunction were reviewed.

  • The modified CeO2-based photocatalyst exhibits high-performance than pure CeO2.

  • The modification of CeO2 play a vital role in obtaining desired heterojunction.

  • Several challenges and strategies for development have been proposed.

Abstract

Nanostructured photocatalysts commonly offered opportunities to solve issues scrutinized with the environmental challenges caused by steep population growth and rapid urbanization. This photocatalyst is a controllable characteristic, which can provide humans with a clean and sustainable ecosystem. Over the last decades, one of the current thriving research focuses on visible-light-driven CeO2-based photocatalysts due to their superior characteristics, including unique fluorite-type structure, rigid framework, and facile reducing oxidizing properties of cerium's tetravalent (Ce4+) and trivalent (Ce3+) valence states. Notwithstanding, owing to its inherent wide energy gap, the solar energy utilization efficiency is low, which limits its application in wastewater treatment. Numerous modifications of CeO2 have been employed to enhance photodegradation performances, such as metals and non-metals doping, adding support materials, and coupling with another semiconductor. Besides, all these doping will form a different heterojunction and show a different way of electron-hole migration. Compared to conventional heterojunction, advanced heterojunction types such as p-n heterojunction, Z–scheme, Schottky junction, and surface plasmon resonance effect exhibit superior performance for degradation owing to their excellent charge carrier separation, and the reaction occurs at a relatively higher redox potential. This review attends to providing deep insights on heterojunction mechanisms and the latest progress on photodegradation of various contaminants in wastewater using CeO2-based photocatalysts. Hence, making the CeO2 photocatalyst more foresee and promising to further development and research.

Introduction

The water quality has been significantly worsening recently with the pace of urban development and population growth (Im et al., 2021; Yuan et al., 2021). Various industrial processes discharged a high amount of wastewater into the environment containing high toxicity, thereby causing severe environmental pollution. For example, different types of organic pollutants, including phenols, antibiotics, benzene, and dyes that are high toxicity and challenging to degrade, could contaminate marine life and the ecosystem (Ma et al., 2019b). Therefore, developing an effective strategy to eliminate pollutants from wastewater is necessary.

Advanced oxidation processes (AOPs) are promising chemical treatment processes for wastewater pollutants by oxidation through hydroxyl radical reactions (·OH) in the presence of homogeneous and heterogeneous photocatalysis (Pham et al., 2021). Among AOPs, heterogeneous photocatalysis has emerged as an excellent tool for degradation of emerging pollutant due to effectiveness, eco-friendly and low cost (Acar and Dincer, 2018; Mehdi Sabzehmeidani et al., 2020). Semiconductor catalysts such as titanium dioxide (TiO2) (Badvi and Javanbakht, 2021), zinc oxide (ZnO) (Truong et al., 2021), iron oxides (Fe2O3) (Hitam and Jalil, 2020), and Copper oxide (CuO) (Liu et al., 2021) are often used in photocatalytic treatment because of their excellent characteristics, including chemical stability and superficial preparation.

In recent years, a rare earth oxide cerium oxide (CeO2) is promising for its wide application in catalysts (Yang et al., 2021a, Yang et al., 2021a). it has been extensively used in various application, for instance, electrocatalysis, fuel cells (Escudero et al., 2021; Wang et al., 2020a, Wang et al., 2020b), solar cells (Zhao et al., 2016), dry reforming of methane (Marinho et al., 2021) and photocatalysis (Labhane and Sonawane, 2020). This is due to the superior properties of CeO2, such as low toxicity, cheap, strong oxygen storage capacity, high chemical stability, and stable Ce3+/Ce4+ redox couple, which could be beneficial to the formation of abundant oxygen vacancies in CeO2 (Bui et al., 2021; Lin et al., 2020; Yang et al., 2018). Meanwhile, the existed abundant oxygen vacancies can reduce electron-hole recombination and enhance photocatalytic activity. However, the performance of single CeO2 is still unsatisfactory with many disadvantages, such as low specific surface area, large bandgap (2.8–3.1 eV) with lower absorption in visible regions up to 400 nm, and high electron-hole recombination (Wen et al., 2017a). Hence, the modification of CeO2 is needed to overcome these above shortcomings.

In recent years, various strategies such as adding support material (Phanichphant et al., 2016), controlling morphologies (Choudhary et al., 2020; Fang et al., 2015), doping metals, and non-metal and fabricating heterojunctions have been employed. Besides, these modifications may provide a different type of heterojunction and movement of charge carriers, which plays a crucial role in the photodegradation process. Despite the enormous interest in this modification, the properties and reaction mechanisms are still debatable and unresolved. Previously, most researchers did not critically explain the current CeO2 modification without doping, give a clear statement of the significance of CeO2 modification by doping other materials and provide a detailed mechanism for each heterojunction formed after doping.

Thus, inspired by the above-mentioned issues, the present review study reports the recent modification of CeO2-based materials for the photodegradation of various contaminants. The main objective of this study is to represent an appraisal of the current efforts in engineering various heterojunction photocatalysts and their corresponding mechanism in photocatalysis. The above-mentioned five crucial types of heterojunctions in photocatalysts are reviewed and discussed. Finally, the current status, opportunities, and future directions of these heterojunction photocatalysts are presented. It is believed that this research would draw more attention to the application of CeO2 photocatalysts and provide high knowledge on photocatalytic degradation in wastewater.

Section snippets

CeO2 as photocatalyst

Ceria (CeO2) is well known rare-earth metal oxides in the earth's crust. It is noteworthy that CeO2 minerals are commonly present in carbonates, phosphates, hydroxides, and silicates, and have traditionally been extracted and processed for pharmaceutical and industrial applications. As mentioned above, CeO2 is currently considered as one of the promising semiconductors for various applications, especially in photocatalytic degradation. This is due to unique structural characteristics, including

Modification of CeO2-based photocatalyst

It has been noted that the modification of CeO2 photocatalyst is required to overcome those shortcomings and enhanced the performance for targeted pollutants. Several attempts have been made to change the electronic band alignment to suppress electron-hole recombination and encourage electron-hole separation, which could dramatically improve photocatalytic efficiency. The efficiency of CeO2 can be enhanced by alteration using several alternatives such as non-metal doping, metal doping, adding

Conservative heterojunction

Based on the last part, it obviously can be noticed that the modification of CeO2-based photocatalyst by combining two or more materials to form heterojunction shows superior performance compared to a single structure. The performance enhancement was due to the synergistic effects between heterostructures, the high absorptivity of visible light, good stability and effective electron-hole separation and migration (Deng et al., 2015). In general, it is well known that a heterojunction is a

Conclusion and future prospect

Although all of the modification of CeO2 morphology has been done, the photocatalytic performance of a single CeO2 still dissatisfaction. Numerous modified CeO2 has been studied to enhance photodegradation efficiency, including metals and non-metals doping, adding support material, and construction with other semiconductors. In this review, the photodegradation mechanism on a single CeO2 photocatalyst and modified CeO2 was clarified. Subsequently, the latest developments in the modification and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Fundamental Research Grant Scheme FRGS/1/2019/STG07/UTM/01/1-5F192 and Collaborative Research Grant (No. 07G59 and 08G92) from Ministry of Higher Education Malaysia.

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