CHAPTER 1
Introduction: Hierarchical Nanostructures for Energy Devices
SEUNG HWAN KO
Applied Nano and Thermal Science (ANTS) Lab, Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea
Email: maxko@snu.ac.kr
1.1 Introduction
Energy has been the hottest social issue for a long time. Energy issues have been related to the problems associated with current major energy sources such as fossil and mineral energy sources: (1) their inevitable exhaustion in the near future, (2) environmental problems such as global warming due to a commensurate increase in CO2 (a prominent greenhouse gas) emissions,(3) an energy shortage due to a recent dramatic increase in global energy consumption (between 2004 and 2030, the annual global consumption of energy is estimated to rise by more than 50%) and thus a price increase. Renewable energy sources, such as hydroelectric, solar, wind, hydrothermal, biomass and nuclear power, are expected to solve the problems associated with fossil fuels. However, energy issues are becoming more serious global problems in the aftermath of the Fukushima catastrophe.
Despite the projected persistent increases in oil and gas prices, less than 10% of the global energy production in 2030 is predicted to come from renewable energy sources. In order to moderate global reliance on exhaustible natural resources and their environmentally hazardous combustion, more scientific efforts should be directed toward reducing the cost of energy production from renewable sources.
Developing sustainable renewable energy sources has been a major research topic in an effort to solve the environmental problems caused by fossil fuels. Significant progress has been made in increasing the efficiency of various renewable energy technologies including solar cells, fuel cells, nuclear energy, wind power and so on. Since the nuclear power plant disasters at Japan and Ukraine, the safety issue has become the most important factor.
1.2 Energy Cycle
Energy devices do not mean only energy generation devices but also include energy storage and energy consumption devices. To fully understand efficient energy usage and to increase the efficiency, the term Energy Cycle should be understood. Energy Cycle is the complete life of energy from birth to death: energy generation, energy storage and energy consumption (Figure 1.1). Efficiency is a major concern in energy devices and the total efficiency of energy devices is limited by the one with lowest efficiency (just like a chemical reaction rate is dominated by the slowest process). Even though one may develop an extremely efficient energy generation device, if the generated energy is stored in a poor efficiency energy storage device or used for a poor efficiency energy consumption device, the efficiency will be low from the total energy cycle viewpoint. Therefore, to approach the energy problem more practically and effectively, the concept of an Energy Cycle should be introduced and the total efficiency of all energy devices involved should be counted systematically.
The most important factor is not just a simple number, such as the efficiency of a single energy device; the balance between many energy devices is very important. This may sound as though researchers in the energy field should know about all different types of energy devices (generation, storage and consumption) to increase an energy device's efficiency in the energy cycle. However, a closer look at the various energy devices may reveal that most of them have similar structures and requirements to make more efficient devices. The structures usually have an active layer sandwiched between two electrodes. The electrodes may be a transparent or non-transparent conductor depending on the application (optoelectronic devices need at least one transparent electrode, such as a solar cell and LED display). Furthermore, most of the energy devices are surface devices (using an interface) and therefore, the efficiency can be increased using a larger surface area. That is where nanomaterials can be useful. However, a larger surface area does not always yield a highly increased efficiency. Additional smart structuring, which can lead to better carrier transport, can boost up the efficiency along with an increased surface area.
1.3 Hierarchical Nanostructures for Efficient Energy Devices
The study of energy device materials is a field full of opportunities for practical and socially significant applications. Many potential renewable energy technologies in the form of solid-state devices and condensed matter phenomena involving the conversion of energy from one form to another exist, and some proceed with efficiency near unity. Within the last couple of decades, there has been an increase in interest in materials with nanometre-scale dimensions. Semiconductor nanowires, a subset of these materials, have received exceptional attention for their unique properties and complex structures. Many nanowire-based materials are promising candidates for energy conversion devices.
However, efficiency increases in the energy devices have been sluggish recently and there has been a need for new groundbreaking approaches, such as the design and fabrication of three-dimensional multifunctional architectures from appropriate nanoscale building blocks, including the strategic use of void space and deliberate disorder as design components to permit a re-examination of devices that produce or store energy. Recently, the importance of nanostructured materials in energy harvesting, conversion and storage technologies has been highlighted in several review articles. In particular, 3D branched nanowire structures with high surface areas and direct transport pathways for charge carriers are especially attractive for energy applications. For example, 3D branched nanowires improve light absorption due to the increased optical path as well as additional light trapping through reduced reflection and multi-scattering in comparison to 1D nanowire arrays, which are beneficial for solar energy harvesting applications. The high surface area can also increase surface activity and electrolyte infiltration in supercapacitors and batteries, and the direct charge carrier transport pathway in both the trunks and branches boosts the charge collection efficiency. These fascinating properties of 3D branched nanowire structures have therefore stimulated widespread interest in fabricating them. The bottom-up approaches, including vapour phase and solution-based routes, allow fabrication of a wide variety of 3D branched nanowires with diverse functions.
The appropriate electronic, ionic, and electrochemical requirements for such devices may now be assembled into nanoarchitectures on the benchtop through the synthesis of low density, ultraporous nanoarchitectures that meld a high surface area for heterogeneous reactions with a continuous, porous network for rapid molecular flux, for example, the three-dimensional design for batteries in Figure 1.2. Such nanoarchitectures amplify the nature of electrified interfaces and challenge the standard ways in which electrochemically active materials are both understood and used for energy storage. An architectural viewpoint provides a powerful metaphor to guide chemists and materials scientists in the design of energy-storing nanoarchitectures that depart from the hegemony of periodicity and order with the promise and demonstration of an even higher performance.
This book will focus on recent developments in hierarchical nano-structuring, especially for highly efficient energy device applications. Surface is a primary concern in most energy devices. Maximizing efficiency in energy devices can be achieved by either new material development or functional structuring. Hierarchical functional nanostructuring has rapidly gained interest to achieve increases in surface areas and favourable electrical properties. The energy devices covered in this book are: (1) energy generation devices (solar cells [DSSC, OPV], fuel cells, piezoelectric, thermoelectric, water splitting and so on), (2) energy storage devices (secondary battery, super capacitor, hydrogen storage), and (3) energy efficient electronics (display, sensors, etc). The hierarchical nanostructuring includes highly porous metal-organic frameworks, nanoparticle assembly with defined pore size, and multiple generation highly branched nanowire trees. This book is composed of four major parts as follows:
Part 1 (Chapters 1-3): Fundamentals — a general introduction to hierarchical nanostructures, characteristics, synthesis methods, and brief applications.
Part 2 (Chapters 4-8): Hierarchical nanostructures for high efficiency energy harvesting devices. Among the energy devices, this chapter will focus on high efficiency energy generation devices such as PV, fuel cells, thermoelectric devices and piezoelectric devices.
Part 3 (Chapters 9): Hierarchical nanostructures for high efficiency energy storage devices. Among the energy devices, this chapter will focus on high efficiency energy storage devices such as supercapacitors and secondary batteries. Mostly, anode and cathode structures will be discussed.
Part 4 (Chapter 10-13): Hierarchical nanostructures for high efficiency energy consumption devices. Among the energy devices, this chapter will focus on high efficiency energy consumption devices such as field emission devices, sensors and other applications.
These topics are currently among the major issues in society. Energy related issues have increased since the recent energy crisis. However, the widespread use of next generation green energy devices is still limited by efficiency and cost. Most of the energy related books focus only on the development of new materials. This book will cover the fundamentals to state-of-the-art functional hierarchical nanostructuring aspects.
CHAPTER 2
Fundamentals of Hierarchical Nanostructures
JINHWAN LEE AND SEUNG HWAN KO
2.1 Introduction
Due to a large surface-to-volume ratio and quantum confinement effects, nanoscale materials show distinctive optical, mechanical, chemical, thermal and electronic properties compared with their bulk counterparts. A large fraction of their atoms are located at the surface. For example, a material that is 5 cm3 possesses almost 0% (~10~%) surface atoms, but when the cube is divided 24 times into 1 nm-sized cubes, the percentage of surface atoms increases to 80% so that the same mass of nanomaterial will have enough surface area to cover an entire football field. The surface atom percentage explains why the nanomaterials' properties are size dependent. The atoms at the surface have weak bonding (because atoms or molecules on a surface possess fewer nearest neighbors) compared with bulk atoms, which means that the atoms at the surface have a tendency to react easily to external perturbation or energy. Because bulk materials have a very small fraction of surface atoms (almost 0%), they show bulk-atom-dominated material properties, which we know well. However, when the materials become nanometre sized, the percentage of surface atoms cannot be ignored anymore and the nanomaterial system starts to show characteristics of both surface atoms and bulk atoms. Depending on the size, the ratio between the surface atoms and bulk atoms and the resultant characteristics of the nanomaterials also show size-dependent properties. As the nanomaterial becomes smaller, the system shows more surface-atom-dominated characteristics with huge surface energy.
In nanomaterial systems, we need to decide when the surface atom becomes dominant and size-dependent phenomena start to show. However, this is not a simple problem because the length reference size scale to decide whether a system is small enough to be called a nanosystem depends on which characteristics you are interested in (e.g. optical, electrical, mechanical, thermal, chemical etc.) and material type.
In this chapter, we will explore the unique characteristics of nanomaterials and the simple physics behind them. These characteristics include thermal, electrical, phonon transport, mechanical, optical and magnetic properties.
2.2 Unique Characteristics of Nanomaterials
In light of the down-sizing trend in microelectronics, nanomaterials have received tremendous interest from various fields due to their unique characteristics. One may find various examples of miniaturization including magnetic and optical storage components with critical dimensions as small as tens of nanometres, size-dependent excitation or emission, quantized (or ballistic) conductance, Coulomb blockade (or single-electron tunneling, SET), and metal-insulator transition. It is generally accepted that quantum confinement of electrons by the potential wells of nanometre-sized structures may provide one of the most powerful (and versatile) means to control the electrical, optical, magnetic, and thermoelectric properties of solid-state functional materials. Additionally, some remarkable specific properties are related to other origins: for example, (i) large fraction of surface atoms, (ii) large surface energy, (iii) spatial confinement, and (iv) reduced imperfections. The following are a few examples suggested by G. Cao:
(1) Nanomaterials may have a significantly lower melting point or phase transition temperature and appreciably reduced lattice constants, due to a huge fraction of surface atoms in the total amount of atoms.
(2) Mechanical properties of nanomaterials may reach the theoretical strength, which is one or two orders of magnitude higher than that of single crystals in the bulk form. The enhancement in mechanical strength is simply due to the reduced probability of defects.
(3) Optical properties of nanomaterials can be significantly different from bulk crystals. For example, the optical absorption peak of a semiconductor nanoparticle shifts to a short wavelength due to an increased band gap. The color of metallic nanoparticles may change with their size due to surface plasmon resonance.
(4) Electrical conductivity decreases with a reduced dimension due to increased surface scattering. However, electrical conductivity of nanomaterials could also be enhanced appreciably due to the better ordering in microstructure.
(5) The magnetic properties of nanostructured materials are distinctly different from those of bulk materials. The ferromagnetism of bulk materials disappears and transfers to superparamagnetism on the nanometre scale due to the huge surface energy.
(6) Self-purification is an intrinsic thermodynamic property of nano-structures and nanomaterials. Any heat treatment increases the diffusion of impurities, intrinsic structural defects and dislocations, and one can easily push them to the surface nearby. Increased perfection would have an appreciable impact on the chemical and physical properties.
Many such properties are size dependent. In other words, the properties of nanostructured materials can be tuned considerably simply by adjusting the size, shape or extent of agglomeration.
2.2.1 Thermodynamic Properties and Thermal Stability
Due to their large surface area, all nanomaterials possess a huge surface energy and thus are thermodynamically unstable or metastable. Nanomaterials are found to have lower melting temperatures compared with their bulk counterparts when the systems' sizes decrease below a certain critical size. This melting point drop was found a long time ago by various researchers. Buffat and Borel presented results that are related to an essentially thermodynamic size effect, i.e. the reduction of the melting point of a small gold aggregate as a function of decreasing particle size as shown in Figure 2.1. The melting point drop is generally explained by the increasing surface energy relationship with decreasing system size. The decrease in the phase transition temperature can be attributed to a change in the ratio of surface energy to volume energy as a function of particle size. It is not always clear to determine or define the melting temperature of nanomaterials. For example, the vapor pressure of a small particle is significantly higher than that of its bulk counterpart, and the surface properties of nanomaterials are very different from those of the bulk materials. Evaporation from the surface would result in an effective reduction of nanomaterial size and thus would affect the melting temperature. For some materials, increased surface reactivity due to a large surface to volume ratio may promote the oxidation of the surface layer and thus change the chemical composition on the nano-material's surface through the surface chemical reaction, leading to a change of melting temperature. Buffet and Borel proposed clever experimental criterion to determine the size-dependent melting of nanomaterials: (i) the disappearance of the state of order in the solid, (ii) the sharp variation of some physical properties, such as evaporation rate, and (iii) the sudden change in particle shape. Bulk gold has a melting point of 1337 K and it decreases rapidly for nanoparticles with sizes below 5 nm as shown in Figure 2.1. Such size dependence has also been found in other materials such as copper, tin, indium, lead and bismuth, barium titanate (BaTiO3), lead titanate (PbTiO3) in the forms of particles and films.
