Pawar / Lee Heterogeneous Nanocomposite-Photocatalysis for Water Purification

1. Auflage 2015
ISBN: 978-0-323-39313-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 110 Seiten

In Heterogeneous Nanocomposite-Photocatalysis for Water Purification, the authors introduce various heterogeneous photocatalysts based on novel nanostructures of metal oxide semiconductors and graphene used for water purification, including TiO2, Fe2O3, SnO2, WO3 and g-C3N4, and outlines their advantages and drawbacks. The nanocomposite-photocatalysts ZnO and CdS are compared with reduced graphene oxide (rGO), a rapidly growing materials system. The authors describe how the photocatalytic activity of known nanomaterials can be improved by modifying the structural and optical properties (i.e., phase composition). - Introductory portion of the book includes a brief survey of all different kinds of heterogeneous photocatalysts - Discusses the possible photocatalysis mechanism occurring during the degradation of different toxic pollutants - Provides the photoelectrochemical measurement for synthesized catalysts, supporting the effective transportation of photoelectrons resulting into better catalytic properties

Autoren/Hrsg.

Pawar, Rajendra

Lee, Caroline Sunyong

Leseproben

Chapter 2 Nanomaterial-Based Photocatalysis
In this chapter, an introduction about nanostructured materials (NsM) and their applications is presented. Semiconducting nanomaterials are attractive because their physical properties are different from those of the bulk due to the quantum-size effect. Also, they provide opportunities to study the effect of spatial confinement and problems related to surfaces or interfaces, which is important for chemistry. Recently, one-dimensional (1D) nanomaterials, such as nanowires, nanobelts, nanorods, and nanotubes, have become the focus of intensive research owing to their potential applications in electronic, optoelectronic, electrochemical, electromechanical, and other fields. Further, various outstanding properties of NsM, such as optical absorbance, improved magnetism, and specific surface area, for efficient device fabrication have been summarized. Finally, applications of photocatalysis in different fields reported in the literature have been presented. Keywords
Nanostructured materials; photocatalysis; quantum size effect; chemical methods 2.1 Introduction
Since the last few decades, nanostructured materials (NsM) have been explored and investigated curiously worldwide in various applications including energy and environmental. NsM could be defined as the solids composed of structural elements—mostly crystallites—with a characteristic size (in at least one direction) of a few nanometers (1–100 nm). These materials exhibit outstanding and often superior physical and chemical properties compared with their bulk counterpart because of different chemical composition, arrangement of the atoms, and size. The growth of NsM in one, two, and three dimensions generates new interesting properties, which remarkably enhanced functions for device fabrication in various fields, including energy, medical, biological, opto-electronics, optics, magnetic, electronic, and many others. Particularly, metal and metal oxide nanostructures have been studied potentially due to their high specific surface to volume ratio, showed quantum size effect, properties can be controlled just adjusting shape and size and high interfacial reactivity. NsM could be classified into four types—zero: clusters of any aspect ratio from 1 to 8; one: multilayers; two: ultrafine-grained overlayers or buried layers; and three: nanophase materials depend on their dimensional grain growth (Figure 2.1). The properties of NsM are examined by their size distribution, shape, chemical composition and interfacial reactivity, and type of grains present at the interfaces [1]. The change in the size makes these materials have different electronic changes in terms of energy and number of levels. This makes these materials behave electronically different. The origin of the size-induced properties in nanomaterials depends basically on the surface phenomena (extrinsic contribution) and quantum confinement effects (intrinsic contribution). This chapter gives a detailed summary about NsM and their outstanding properties [2].
Figure 2.1 Schematic of the four types of nanostructured materials. Reproduced from Ref. 1. 2.2 Properties of NSM
2.2.1 Increase in Surface Area to Volume Ratio
First, nanomaterials have a relatively larger surface area compared with the same volume (or mass) of the material produces in a larger form. Let us consider a sphere of radius “r,” surface area, and volume of sphere given in Eqs. (2.1) and (2.2). The ratio of these two equations gives Eq. (2.3), which indicates the increase in surface to volume ratio with decrease in size of materials. Thus, when the radius of the sphere decreases, its surface area to volume ratio increases [3,4]. Figure 2.2 shows the relation between surface area and size of silica nanoparticles. It is seen that obtained surface area increased exponentially below 100 nm indicated crucial role of size in NsM. If a bulk material is subdivided into an ensemble of individual nanomaterials, the total volume remains the same, but the collective surface area is greatly increased. surfaceareaA=4pr2 (2.1) (2.1) volumeV=(43)pr3 (2.2) (2.2) areatoitsvolumeratioAV=3r (2.3) (2.3)
Figure 2.2 Effect of size on surface area of silica nanoparticles. 2.2.2 Quantum Confinement Effect
Contemporary literature on ultradisperse semiconductors distinguishes between the effects arising as a result of increase in surface area and the degree of surface imperfection with decrease in the size of the crystals and, separately, quantum size effects due to radical change in the electronic state of the semiconductor (Figure 2.3a) crystals less than a certain “critical” size, determined in turn by the extent to which the electron–hole pair photogenerated in the semiconductor is delocalized [5]. These effects also differ in the range of sizes in which they appear. Size effects are observed in semiconductor crystals measuring 10–100 nm, whereas quantum size effects are usually characteristic of nanocrystallites measuring less than 10 nm. In the literature semiconductor nanoparticles in which quantum size effects of one type or another appear, are often called quantum size particles or quantum points to emphasize their special electronic structure. It is necessary to mention the tentative nature of the classification in so far as the exact “critical” size after which the appearance of quantum size effects can be expected is largely determined by the chemical nature of the semiconductor and can vary from 0.5 (CuCl) to 46 (PbSe) or more nanometers. From the positions of quantum mechanics the “critical” size (the threshold for the appearance of quantum size effects) corresponds to the De Broglie wavelength of the free electron. During analysis of the interband absorption of the semiconductor nanoparticle the Bohr radius of the exciton (aB), which can be calculated from the electrophysical constants of the bulk semiconductor, can be used as such a criterion [6]. The data that have accumulated on size effects in semiconductor nanoparticles make it possible to examine them according to the nature of the effect on the properties of the nanocrystals.
Figure 2.3 (a) Dependence of a quantum-sized semiconductor’s band gap on particle size and (b) gold color in the nano form. Restriction of the free motion of the exciton in the bulk of the nanocrystal leads to an increase of its energy (gnano) in relation to the volume (bulk) semiconductor (gbulk). The increase of the energy of the exciton as a result of the quantum size effect (E=Egnano-Egbulk) can be calculated in the approximation of effective masses, which is based on assumptions about the parabolic nature of the permitted energy bands close to their edges and the invariability of the effective masses of the electron of the conduction band (e*) and the hole of the valence band (h*) in the transition from the bulk to the ultradisperse semiconductors. This approximation gives the following expression for ?E [7], E=p2?22R2(1me*+1mh*)-1.786e2eR-0.248Ry* (2.4) (2.4) The first term in Eq. (2.4) depends on R2 and corresponds to the increase in the energy of the exciton as a result of its spatial restriction in the potential box—the semiconductor nanocrystal. The second term determines the energy of columbic interaction of the electron and the hole in the composition of the exciton and increases with decrease in the size of the nanoparticle. The value of y* in the third term is called the Rydberg energy of the exciton and takes account of the correlation between the motion of the electron and the hole. As seen, the last two terms in Eq. (2.4) lead to a decrease in the energy of the exciton, which is restricted in the volume of the particle. Conventionally, it is known that the color of gold is golden, but at the nanoscale it starts to change dramatically due to quantum size effect. It is found that the colloidal solution of gold nanoparticles is no longer golden but ruby-red in color (Figure 2.3b). A thin film of gold deposit absorbs across most of the visible part of the electromagnetic spectrum and very strongly in the IR and at all longer wavelengths. It dips slightly around 400–500 nm, and when held up to the light, such a thin film appears blue due to the weak transmission of light in this wavelength range [8]. However, the dilute gold colloid film displays total transparency at low photon energies (below 1.8 eV). Its absorption becomes intense in a sharp band around 2.3 eV (520 nm). This kind of effect is known as surface plasmon, which is aroused from small size of metal particles [9]. Besides quantum size effects, the NsM behavior is different due to surface effects, which dominate as nanocrystal size decreases. Reducing the size of a crystal from 30 to 3 nm, the number of atoms on its surface increases from 5% to 50% beginning to perturb the periodicity of the “infinite” lattice. In that sense, atoms at the surface have fewer direct neighbors than...