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,¹  (2) environmental problems such as global warming due to a commensurate increase in CO₂ (a prominent greenhouse gas) emissions,²  (3) an energy shortage due to a recent dramatic increase in global energy consumption²  (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.² 

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.³  Since the nuclear power plant disasters at Japan and Ukraine, the safety issue has become the most important factor.

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't 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.

The study of energy device materials is a field full of opportunities for practical and socially significant applications.²  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.⁴  Recently, the importance of nanostructured materials in energy harvesting, conversion and storage technologies has been highlighted in several review articles.^5–10  In particular, 3D branched nanowire structures with high surface areas and direct transport pathways for charge carriers are especially attractive for energy applications.^11,12  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.⁵  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.⁵  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.⁵ 

The appropriate electronic, ionic, and electrochemical requirements for such devices may now be assembled into nanoarchitectures on the bench-top 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.³ 

This book will focus on recent developments in hierarchical nanostructuring, 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.