1 Biomaterials for photocatalysis: an overview

Abstract

Light-responsive bioinspired materials are emerging materials for precise and controllable medical and biological applications. These light-responsive bioinspired materials have been studied to design light- induced modulators, nanovehicles and biopolymers to control cell behavior, as well as regulate environments. Photocatalysis uses sustainable, eco-friendly, natural light and power for effective chemical conversion. Biomaterial-assisted photocatalysis has become a hot spot for research due to its applications: degradation of pollutants in water and air, artificial photosynthesis, photochromism, cancer therapy, drug delivery, dissipation of heat and energy, sterilization and production of affordable energy sources. Photo-generated electrons, bandgap energy and generation of electron-hole pair are applied to degrade or create different compounds by absorbing or releasing energy or heat. This chapter will focus on the principle of photocatalysis along with biomaterial-assisted photocatalysts.

1.1 Introduction

A combination of light-responsive surfaces and nanoparticles has emerged as an innovation that provides positive sustainability to society. Biomaterials designed by mimicking natural materials have become a current topic for research. The idea to form biomaterial-assisted photocatalysts originated from photosynthesis and became an effective option to produce renewable-energy [1]. Such photocatalysts have gained much importance due to their applications in removal of pollutants from water and environment. Photocatalysts can be activated through both UV and visible light. When photocatalysts are biomaterial-based, they provide greater surface area, better functionality and porosity for not only better dispersion but also increased interaction with contaminants. Biomaterial-assisted photocatalysis uses natural materials to increase the compatibility of this approach with living systems [2].

For ideal photocatalysis, there should be strong interaction, larger surface area, greater adsorption, easy separation and higher resistance to denaturation of catalyst. The biomaterials can be biopolymers, biochars, biomolecules, immobilized enzymes, proteins or peptides for excellent photocatalytic activity [3]. Usually, inorganic photocatalysts have better activity than organic, due to optimum bandgaps and greater electron flow. Moreover, biomaterial-based photocatalysts are preferred over inorganic photocatalysts due to their environment-friendly nature and lowest particle size [4, 5].

Various factors such as improved light absorption and greater photon harvesting capability that can be related to size and structure of immobilized biomaterials and crystallinity [6, 7] influence the efficiency of photocatalytic activity. As regards physiochemical properties, the interactions and biomaterials are very important. During typical photocatalysis, photons are absorbed at the interface, and thereby, an electron-hole pair is generated to form hydroxide and superoxide anion radicals [8]. Electrons generated in light-responsive reactions undergo reduction reactions, and the charge carriers are involved in photocatalysis [9, 10]. Electrons play their role in reduction reactions, whereas in reduction of pollutants, holes are involved in oxidation reactions; they oxidize pollutants [11, 12].

The bandgap energy plays an important role in formation of free radicals to start redox reactions. Various bottlenecks can affect the efficiency, such as spectral mismatch, recombination of photogenerated electrons and low surface area [13, 14]. Therefore, different strategies have been developed to enhance the biomaterial-assisted photocatalysis under UV or visible light, so far [15, 16], by tuning the bandgap according to natural spectra to avoid spectral mismatch either by doping, modifying bandgaps or sensitization [17, 18].

Hence, biomaterials with engineered nanoparticle-based photocatalysts are used in various applications including detoxification, packaging and to combat environmental pollution [19, 20]. In subsequent sections of this chapter, the fundamental principle of photocatalysis will be discussed, followed by an overview of biomaterial-based photocatalysts as a presentation of recent discovery in the field of biomaterials. Different modes of synthesis and techniques to characterize biomaterials for photocatalysis will also be discussed. Challenges in modulation along with strategies to develop an efficient photocatalyst with future directions will be provided as a critical area for biomaterials.

1.2 Basic principle of photocatalysis

Photocatalysis involves light and a catalyst to proceed with a chemical reaction that involves valence band (filled) and conduction band (empty) with energy gaps (bandgap measured as electron volt, eV) [21]. Any photon having bandgap energy can activate an electron from ground state (valence band) to jump to excited state (conduction band) - an essential step during photocatalysis because electron of conduction band and the hole in the valence band tends to move in lattice [22]. These free electrons and holes are involved in ongoing and repeated redox reactions during photocatalysis. The redox reactions are useful either to remove toxic ions by dissolving them or mineralize dissolved toxic ions. Photocatalysis can be done through different mechanisms, based on the incident wavelength of light on the surface [23].

1.2.1 When the wavelength of light is >400 nm

Mostly, when dyes absorb visible light, photocatalytic degradation occurs through direct mechanism. In direct mechanism, a photon excites the electron from its ground state to excited state, thereby converting it to a semi-oxidized radial cation through electron transfer in conduction band. The trapped electrons tend to react with oxygen and form superoxide radical anion, converting it to a hydroxyl radical; this radical further undergoes oxidation reaction [24].

1.2.2 When the wavelength of light is <400 nm

1.2.2.1 Generation of electron-hole pair

When the wavelength of light is <400 nm, the electrons does not move into excited state; in that case, the photocatalyst helps the reaction to proceed. The bandgap is typically higher, and the available energy is greater than the bandgap. Hence, the photons from UV light tend to promote the movement of a photoelectron from valence band of a photocatalyst to conduction band, leaving a hole behind in valence band to generate an electron-hole pair [6].

Ionization of water

The holes generated at the valence band, in response to UV light, react with water to form hydroxide radical on the surface of a photocatalyst. It causes biomineralization of organic molecules by reacting at the surface of a photocatalyst. The extent of biomineralization depends upon the structure and stability of the respective organic molecule [20].

Sorption of oxygen ions

The electrons of the conduction band react with oxygen ions and generate anionic superoxide radicals, leading to an oxidation reaction, and prevent the generation of electron-hole pair, thus retaining the neutral nature within the molecule.

1.2.2.2 Protonation of superoxide

Apart from generation of an electron-hole pair, protonation of superoxides by forming hydroperoxyl radicals is another approach to photocatalysis. The hydroperoxyl radical produces hydrogen peroxide, further dissociates, and forms hydroxyl radicals, reacting with pollutants to form carbon dioxide and water [17].

1.3 Overview of biomaterials for photocatalysis

1.3.1 Biopolymers-based photocatalysts

Though synthetic polymers are commonly used photocatalysts, researchers were urged to develop sustainable and eco-friendly biomaterials using different biopolymers such as agar,...