Chapter 1
Nanophotocatalysts: Fundamentals, Mechanisms, and Advances in Remediation Technologies

Ehiaghe Agbovhimen Elimian1,2, Claude Kiki3, Andrew Nosakhare Amenaghawon4, and Fidelis Odedishemi Ajibade5*

1 Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, Ningbo, China

2 Department of Plant Biology and Biotechnology, Faculty of Life Sciences, University of Benin, Benin City, Nigeria

3 CAS Key Laboratory of Urban Pollutant Conversion, Fujian Key Laboratory of Watershed Ecology, Institute of Urban Environment, Xiamen, China

4 Department of Chemical Engineering, Faculty of Engineering, University of Benin, Benin City, Nigeria

5 Department of Civil and Environmental Engineering, Federal University of Technology Akure, Akure, Nigeria

*Corresponding author: foajibade@futa.edu.ng

1.1 Introduction

Sustainable and nontoxic technologies are being developed globally in response to the growing environmental pollution and the increasing energy demand. Photocatalysis is a highly effective technology for breaking down organic pollutants under light exposure in ambient conditions (Bianchi et al., 2015; Wu et al., 2021; Xu et al., 2019). In semiconducting materials, light energy excites electrons, resulting in the formation of electron-hole pairs. These pairs then engage in secondary reactions that produce radicals, which help detoxify contaminants. In "photocatalysis," light and a semiconducting substance initiate the reaction, which combines photochemical and catalytic processes (Su et al., 2018a; Maeda, 2011; Sano et al., 2006). In a nanophotocatalyst, electrons in the valence band are excited by incident photons and move to the conduction band. These charge carriers recombine to release energy and reactive species, which facilitate organic pollutant degradation (Fujishima et al., 2000; Carp et al., 2004; Duan et al., 2021). The crucial aspect of the photocatalysis reaction is the separation of charge carriers (Luo et al., 2020; Yang et al., 2011; Zhao et al., 2018). Nanocatalysts are derived from nanomaterials capable of catalyzing in both homogeneous and heterogeneous reactions (Moon et al., 2016; Tong et al., 2012). Nanomaterials, falling within the size range of 1-100 nm, exhibit a notable surface-to-volume ratio and exceptional reactivity. These nanomaterials can be broadly categorized into three groups based on their composition: carbon materials (carbon nanotubes, fullerene C60 and C540, and graphene) (Bhattacharjee et al., 2022; Liang et al., 2019; Hendriks et al., 2014; Wang et al., 2019), polymeric materials and inorganic (Long et al., 2012; Zhou et al., 2019; Deng et al., 2023) and metal and metal oxide materials (Guerra et al., 2018; Fattakhova-Rohlfing et al., 2014; Xu et al., 2012; Jemec et al., 2015). Among these, carbon nanoparticles are deemed the most effective in eliminating organic pollutants.

Over the last decade, nanocatalysts have gained significant importance due to their diverse applications in wastewater treatment, drug delivery, energy production, and environmental remediation (Kumar et al., 2020; Moore & Wei, 2021; Zhang et al., 2021). By employing condition-based synthesis of nanoparticles (NPs) and their corresponding composites, the activity of the nanophotocatalysts can be precisely adjusted and controlled at the nanoscale, influencing their physical, chemical, optical, electronic, and magnetic characteristics. The increase in surface area and the quantum effects are the primary factors contributing to the distinct properties observed in comparison to bulk materials (Baig et al., 2021). The variations in the properties of bulk materials primarily stem from two factors: the increase in surface area and quantum effects. Also, reduced catalyst particle sizes yield a larger surface area, resulting in heightened reactivity and improved catalytic performance. Additionally, quantum effects at the nanoscale influence electrical, optical, and magnetic properties, causing distinct behavior compared to bulk materials (Wu et al., 2021; Baig et al., 2021). These unique properties at the nanoscale can be optimized through various characterizations, owing to their intermediate relationship between macroscopic and nanoscopic levels. Given their numerous benefits and promising prospects, nanophotocatalysts have become essential assets in the remediation of environmental pollutants (Yu et al., 2018; Zhang et al., 2017). These catalysts play a critical role in pollution control, particularly in the treatment of hazardous waste, the purification of air and water, and the overall sustenance of a healthy ecosystem. Their importance lies in their ability to efficiently break down various contaminants, including organic dyes, antibiotics, and phenolic compounds, through photochemical reactions (Singh & Dutta, 2018; Kumar et al., 2021; Dinh et al., 2018; Araya et al., 2017; Badvi & Javanbakht, 2021). Carbon scaffolds are a kind of carbon nanostructure that contains open, interconnected microporous structures. Their properties may be tuned to efficiently break down various pollutants. Additionally, their capability to harness solar energy for photocatalytic processes presents a viable and environmentally friendly approach to remediating pollutants. Furthermore, their nontoxic nature and ability to function under ambient conditions make them a promising solution for addressing pollution issues without further impacting the environment. As nanotechnology continues to advance, nanophotocatalysts hold significant promise for transforming pollution control and environmental remediation efforts, providing a sustainable and effective pathway toward a cleaner and healthier environment.

1.2 Fundamental Principles of Semiconductor Photocatalysis

In the presence of irradiated semiconductors, photocatalysis plays the function of starting or accelerating certain reduction and oxidation processes (redox). Light is absorbed, and electron-hole pairs are subsequently photoexcited when the energy of the input photons is = the bandgap. The electrons (eCB-) play a reduction role with a chemical potential of +0.5 to -1.5 V per normal hydrogen electrode (NHE), whereas holes (hvb+) play an oxidative role with a chemical potential of +1.0 to +3.5 V (Rosman et al., 2021; Liu et al., 2020). The semiconductor stores photoexcitation-induced energy from incoming photons, which is then transformed into chemical form via surface/interface interactions and electronic processes. In contrast to traditional catalysis's thermodynamics, photocatalysis may facilitate both spontaneous and non-spontaneous processes (Liu et al., 2020; Yang & Wang, 2018). The activation barrier is broken down in spontaneous reactions by the input energy, which enables photocatalysis to occur more quickly or in more hospitable environments. Chemical energy that builds up in the reaction products during non-spontaneous reactions is partially transformed from the input energy. The typical sequence of a photocatalytic semiconductor cycle involves three main stages. Generally, heterogeneous photocatalysis involves the absorption of photons, excitation, separation, migration, transport, and possibly recombination of charges. The visual representation of the photocatalytic process is depicted in Figure 1.1.

Figure 1.1 The schematic diagram for the photocatalytic process.

Certain charge carriers have the ability to travel to the surface and instigate a sequence of surface reactions encompassing reduction and oxidation processes, ultimately resulting in the breakdown of pollutants. Consequently, the collective impact of these four successive steps significantly influences the overall efficacy of photocatalysis, a relationship that can be mathematically represented by Eq. (1.1) (Li et al., 2016).

Where solar energy conversion efficiency, light absorption efficiency, electron-hole pair excitation, migration and transportation, and utilization efficiency are denoted as , , , , and in Eq. (1.1) respectively.

The interplay between the redox potential of the substrate and the band edges of the semiconductor photocatalyst is crucial for the feasibility of reductive, oxidative, or nonreactive pathways (Fu et al., 2019). Specifically, if the redox potential of the substrate falls below the conduction band potential (CB) edge of the semiconductor photocatalyst, it enables reductive reactions. Conversely, when the redox potential exceeds the valence band potential (VB) edge, the substrate is primed for oxidative reactions. However, if the redox potential straddles the CB and VB edges, the substrate is precluded from participating in either reductive or oxidative reactions. Notably, if the redox potential simultaneously falls below the CB edge and exceeds the VB of the semiconductor photocatalyst, the substrate can engage in either reductive or oxidative reactions, illustrating the delicate balance that governs the outcome of photocatalytic processes (Fu et al., 2019; Liu et al., 2014a).

According to thermodynamics, the photogenerated e- or h+ are only necessary for the reduction and oxidation processes on the surface if their potentials lie between the semiconductor's VB and CB. For the effective photocatalytic degradation of a contaminant, the photogenerated charge carriers should possess adequate redox capacity that facilitates the generation of free radicals, which can further degrade the adsorbed pollutant (Yang & Wang, 2018; Li et...