Nanomaterials in Energy and Environmental Applications

Junhui He

• Publisher: Jenny Stanford Publishing
• Year: 2016
• Print ISBN: 9789814463782
• eBook ISBN: 9781315341293
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Biology

Nanoscience and nanotechnology are interdisciplinary fields that bring together physicists, chemists, materials scientists, and engineers to meet the potential future challenges that humankind will face, including the search for renewable energies for sustainable development and new technologies for carbon capture and environmental protection. Among the current subjects in nanoscience and nanotechnology, nanomaterials are developing fast and explosively and attracting a huge amount of attention. They continue to show promising potential and have found application in solar cells, fuel cells, secondary batteries, supercapacitors, air and water purification, and removal of domestic and outdoor air pollutants. To summarize the past developments and encourage future efforts, this book presents contributions from world-renowned specialists in the fields of nanomaterials, energy, and environmental science. It discusses the design and fabrication of nanostructured materials and their energy and environmental applications.

Chapter 1

Multifunctional Coatings for Solar Energy Applications

Lin Yao^a,b and Junhui He^a

^aFunctional Nanomaterials Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China

^bUniversity of Chinese Academy of Sciences, Beijing 100049, China [email protected]

Nanomaterials in Energy and Environmental Applications

Edited by Junhui He

Copyright © 2016 Pan Stanford Publishing Pte. Ltd.

ISBN 978-981-4463-78-2 (Hardcover), 978-981- 4463-79-9 (eBook)

www.panstanford.com

1.1 Introduction

Transparent surface coatings with suitable optical path differences can suppress the reflection of substrates. Such coatings are usually called “antireflective coatings (ARCs).” Many antireflective surfaces exist in nature. Taking some diurnal butterflies as an example, they use arrays of nonclose-packed nipples (about 300 nm) as antireflective structure (ARS) to reduce reflection from their compound eyes [1, 2]. The ARS on the cornea can increase light transmission and suppress reflection losses at the interfaces. Besides these excellent optical properties, the ARS arrays also exhibit self-cleaning capability because of the high fraction of air trapped between arrays [3, 4].

The antireflection phenomenon provides enormous inspirations for scientists to mimic for many important applications. Antireflection technology has been widely used in some high-precision optical components, solar cells, flat panel displays, and light-emitting diode lighting to increase the transmittance of incident light [5, 6, 7 and 8]. For solar cells, due to reflection at the air–glass interface of the packaging glass and scattering by accumulated dust on outdoor panels, part of incident energy on solar modules is lost. On the one hand, ARC on the glass can help alleviate reflection in systems; on the other hand, the self-cleaning property can, to some extent, solve the dust accumulation problem [5]. Tseng et al. systematically studied the antireflection and light trapping effects. Their results showed a 76.9% enhancement of short-circuit current density compared with that of bare silicon due to suppression of surface reflection [9].

It is common knowledge that smooth surfaces shine more than rough ones. It contains the very basic idea of antireflection that roughness is necessary to reduce reflection of surfaces. The reflection or optical disturbance will be zero if the medium for light propagation does not change or if the two media have the same refractive index (RI) [10]. Therefore, many materials with micro/nanostructure are perspective to fabricate ARC, including silicon, silica, titania, zirconia, zinc oxide, cobalt oxide, tin oxide, carbon, and poly(ethylene terephthalate) (PET), polystyrene (PS), gallium nitride [11, 12, 13, 14, 15, 16 and 17]. To date, two kinds of approach are available for fabricating ARCs. One is coating porous or multilayered films on the surface of devices, and the other is fabricating sub-wavelength ARS directly on the substrate [18, 19, 20 and 21]. The corresponding fabrication routes can be classified into bottom-up and top-down modes. The bottom-up technique usually uses nanoparticles as building block to form ARCs. The top-down approach relies on etching or lithography and so on techniques performed with or without masks. Recently, the two-step method with a combination of top-down and bottom-up approaches also attracts much attention [11].

When contamination or fogging occurs on ARCs, however, their optical properties would dramatically deteriorate. Contaminants accumulate and water molecules condense on the surface, leading to scattering and reflection of light. This problem may be solved by creation of a surface that has special wettability (superhydrophilicity or superhydrophobicity) and photocatalytic property. ARCs with self-cleaning property have developed rapidly in recent years from window glass to various devices [22].

In this review, we aim to provide recent developments in antireflective and self-cleaning surfaces, with particular emphasis on silicon and fused silica materials, as they are both commonly used in many functional devices. The current review is composed of four sections. In the first section, the basic concept and principle of antireflection and self-cleaning are briefly described. In the following section, the fabrication pathways and their mechanisms are discussed. Then, we introduce the latest typical progress in self-cleaning ARCs in recent 5 years. In the last section, some applications of the self-cleaning ARCs surfaces are introduced. A large number of research articles have reported the fabrication of ARCs or superhydrophobic/superhydrophilic surfaces within the past few years. This paper intends to give readers both an integrated tour of antireflective and self-cleaning coatings fabrication technologies and a basic realization of their great application prospects.

1.2 Theoretical Aspects of Antireflection and Self-Cleaning

1.2.1 Principle of Antireflection

1.2.1.1 Basic concept of antireflection

ARCs can effectively enhance the transmission of light. The Fresnel equation offers the basic mathematical model of the reflection of coatings [23]. We learn that RI that quantifies the speed of light in the current medium with respect to that in vacuum is the major parameter for characterizing the transmission of light in the medium.

For single-layer coatings, the Fresnel equation, which offers the basic preliminary mathematical model of reflection and refraction, has the following two assumptions:

(1) The reflected waves have the same intensity and one wave is reflected per interface;

(2) Other optical interactions such as scattering, absorption, and so on are negligible.

As illustrated in Fig. 1.1a, there would be no reflection if there is a destructive interference between light reflected from the coating-substrate and the air-coating interfaces. Therefore, n_c for an ideal homogeneous ARC has to meet the following two conditions:

(1) n_c = (n_an_s)^1/2, where n_a and n_s are the refractive indices of the air and the substrate, respectively.

(2) d = λ/4n_c, d is the thickness of the coating, λ is the wavelength of the incident light.

On the basis of the Fresnel’s theory, the antireflection property is also dependent on the s and p-polarization of the light [23]. The s-polarization and p-polarization have the electric field perpendicular and parallel to the incidence plane, respectively. Sunlight shows appreciable degree of polarization when light reflects at shallow angles due to the fact that light reflecting at shallow angles has the p-polarized light reflecting to the maximum.

Figure 1.1 Propagation of light rays through (a) a single-layer coating and (b) multi-layer coating on substrate (n_s > n_n, where n_s and n_n are the refractive indices of substrate and coatings, respectively) [4]. Copyright 2014 Elsevier.

For multi-layer coatings, the mathematical model has some differences. As shown in Fig. 1.1b, the reflected light from the interface ij between the adjacent layers i and j is [24]:

R_ij = |R_mn| exp [−2(δ_i + δ_j)]

in which, |R_mn| = [(n_i − n_j)/(n_i + n_j)], δ_i = 2πn_id_icosθ_i/λ (θ_i is the angle of refraction, d_i is the physical thickness of the layer)

For the entire coating:

R_sum = R₀₁ + R₁₂ + R₂₃ +…+ R_ns

By adjusting the reflective index and the thickness of each layer, a minimum R_sum can be obtained, and an ARC can be achieved.

In recent years, gradient-RI ARCs have received great attention. Different profiles for gradient-RI layers, including linear, parabolic, cubic, Gaussian, quintic, exponential, exponential-sine, and Klopfenstein, have been investigated for broadband and omnidirectional ARCs [25, 26 and 27]. Typical expressions for the continuous gradient-RI with linear, cubic, and quintic profiles are presented below [28]:

Linear index profile: n = n_o + (n_s − n_o)t, 0 ≤ t ≤ 1

Cubic index profile: n = n_o + (n_s − n_o)(3t² −2t³)

Quintic index profile: n = n_o + (n_s − n_o)t(10t³ − 15t⁴ + 6t⁵)

where n_o and n_s are the RIs of the incident and substrate media, respectively, and t is the thickness of graded interface region. Assembling a multilayer heterostructure that has a suited gradient-RI profile is beneficial to obtained AR properties within a wide range of wavelength.

1.2.1.2 Strategies to achieve antireflection

From the above discussion, we know that the antireflection property is obtained when n of a single-layer coating is (n_airn_s)^1/2. As for a glass substrate (n_s = 1.5), n of ARCs materials should be 1.22. Unfortunately, natural materials...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface
• 1. Multifunctional Coatings for Solar Energy Applications
• 2. Functionalization of Polyelectrolyte Multilayers via Counterions
• 3. Glass-Based Proton Exchange Membranes for Fuel Cell Applications
• 4. Silicon Nanowire Arrays: Fabrication, Properties and Energy Applications
• 5. Positron-Positronium Annihilation Studies of Energy- and Environment-Related Nanomaterials
• 6. Graphene: A New Star Nanomaterial in Energy and Environment Applications
• 7. Recent Advances in Synthesis and Applications of Metal-Added Carbon Nanotubes and Graphenes
• 8. Roles of Reduced Graphene Oxide in Improving Photocatalytic Hydrogen Generation Performance over Metal Sulphide Nanocomposites
• 9. Micro/Nanostructured Materials and Their Structurally Enhanced Performances for Environment
• 10. Utilization of Biological Polysaccharides as Eco-Friendly Structural Materials
• 11. Catalysts for Indoor Formaldehyde Control
• 12. Nanostructured Copper Oxide for Sensing Hydrogen Cyanide Gas
• 13. Nanomaterials and Health
• Index