Responsive Nanomaterials for Sustainable Applications

The global issue of the utmost exhaustion of fossil fuels on earth has driven research towards the development of alternative energy resources to meet the increasing demand for energy required in modern society. Among the different types of renewable sources, solar energy is the largest energy source which is unlimited and clean. Currently solar cells or photovoltaic (PV) technologies that generate electricity by harnessing sunlight is one of the fastest growing power generation sources in the energy sector. In this chapter we review the application of nanomaterials in some types of solar cells including dye-sensitized solar cells, quantum dots solar cells and perovskite solar cells. Semiconductor materials such as TiO2, ZnOx, SnOx, NiOx etc have been widely used as electron or hole transport materials in these type of solar cells. The morphology, shape, size, crystal structure of particles of these materials can significantly influence the device performance. The outlook of the future research direction is provided at the end of the review.

Microwave heating is a powerful and non-conventional energy source for heterogeneous catalytic reactions, which has attracted considerable attention during the past decades. With the presence of microwave-responsive catalysts, microwave can selectively heat the designed catalyst surface, and expedite the reaction rate at catalyst/solvent interface. This chapter strives to extensively review the recent work on microwave-responsive catalysts and their roles in heterogeneous catalytic reactions. The fundamental mechanism of microwave heating is illustrated to explain its functions in the catalytic reactions. The working principle of microwave-responsive catalyst and related evaluation methods are discussed in this chapter. Additionally, the advantages of microwave-responsive catalysts for specific reactions have been categorized and reviewed. At last, a few different strategies to enhance microwave thermal effects have been summarized. It is concluded that developing microwave-responsive catalysts is a practical method to expedite reaction rate, enhance energy-efficiency, and improve product quality. Therefore, designing microwave-responsive catalysts could be an effective strategy for highly efficient reactions and future industrial-scale applications.

The worldwide demand for green and renewable energy resources as well as the development of electronic devices has greatly boosted the improvement of energy storage systems. As one of the major types of energy storage devices, supercapacitors, with ultrahigh power densities, long-term cycling lives, and rapid charge and discharge capabilities, have been extensively investigated at the current stage, especially for those flexible or wearable electronic devices, which could be integrated into a smart system. In this chapter, the basic structures, the energy storage mechanisms, the categorization, and the characteristics of supercapacitors are comprehensively discussed. This chapter mainly focuses on different major components of flexible supercapacitors, ranging from the flexible electrode structure, the flexible substrates, and the improved electrolyte, to the construction of self-responsive flexible devices. Meanwhile, the emerging flexible integrated systems with these devices have also been illustrated, such as the energy sensor integrated systems and the energy collection-storage-sensing systems. Furthermore, the future trend of flexible supercapacitors based on future demands will be lastly discussed, focusing on the feasible and efficient strategies for designing novel and high-performance supercapacitors in future research.

The magnetisms of nanosized MnO₂ in different phases depend greatly on the morphology and the dimension of the nanoparticles and the interaction between the induced magnetism and the intrinsic magnetism. Different sharing modes of the basic structure in MnO₂ (i.e., MnO₆ octahedron) contribute to the low-temperature ground-state magnetism deviation from general antiferromagnetism. MnO₂ bulks possess an antiferromagnetic ordering between the corner-sharing MnO₆ octahedra and a ferromagnetic ordering between the edge-sharing MnO₆ octahedra. The essential reason is the change of interatomic distances, coordinate environment, and symmetry, resulting in the different surface states. For this reason, the effects of shapes, ions introducing, electronic structures, and exposed planes on magnetism are as important as sharing modes.

The stimuli-responsive hydrogels are three-dimensional hydrophilic polymeric networks with a fascinating property that they will undergo an obvious and reversible volumetric variation in response to a small variation of external environmental stimuli. In particular, combining of the stimuli-responsive hydrogels with photonic crystals (PCs) or Au nanoparticles (NPs), the volumetric variation responded to external stimuli could be converted into a color change, thus creating a kind of colorimetric sensors. These colorimetric sensors attract more and more interest of researchers in different fields due to their simple operation and visualized readout. Herein, after presenting a brief review on the basis concept, synthesis methods and sensitive mechanisms of the stimuli-responsive hydrogels, this chapter mainly focuses on their applications as colorimetric chemical sensors by combining with PCs. And some typical applications are proposed in detail, such as detecting pH value, ionic species, solvents, humidity, and biomolecules. In order to meet the increasing requirements of practical applications, the selectivity, response rate, and resolution ratio of these colorimetric sensors need to be improved in the near further.

Functional interfaces can be considered ‘materials within materials’ that are active at the nanoscale and are responsive to an applied external stimulus. At these interfaces, the material properties are not only distinct from the bulk, but new functionalities or phases can emerge. Notable examples of such functional interfaces include structural phase boundaries and domain walls in ferroelectric oxides. In this chapter, nano-mechanical–electromechanical properties of the morphotropic phase boundaries in bismuth ferrite thin films are reviewed. Specifically, using scanning probe microscopy techniques, factors governing nano-mechanical and electromechanical properties of morphotropic bismuth ferrite are identified and potential applications in oxide nanoelectronics are discussed.

Energy-saving buildings have drawn increasing interest worldwide in the past 30 years, during which the growing population and expanding urbanization significantly increased the energy intensity of numerous cities. In the modern energy-saving buildings, smart windows are playing an important role in the efficient utilization of daylight and the intelligent control of heat exchange between indoor and outdoor, eventually reducing the energy waste associated with lighting and air-conditioning. The “intelligence” of smart windows originates from the responsive materials of which the optical properties are adaptive to temperature or applied voltage. Recently, the development of smart windows has been greatly motivated by the burgeoning nanomaterials. This chapter focuses on the development of heat and electro-responsive nanomaterials-based smart windows which outperform the conventional ones and, more importantly, likely to cost less for commercialization.

Proton-conducting oxides receive more and more attention in the current world because of its wide applications in renewable and sustainable devices. Among all these applications, the use of proton-conducting oxide for fuel cells is becoming quite a hot topic as it avoids the problems for traditional fuel cells using oxide electrolyte and also lowers the working temperature of fuel cells, making them possible for practical applications. With the framework of proton-conducting solid oxide fuel cells, the utilization of nanomaterials now playing an essential part in the whole community, and this chapter will briefly summarize the nanomaterials for protonic oxide fuel cells that are also known as proton-conducting solid oxide fuel cells (SOFCs).

In the past decades, as an emission-free technique capable of realizing direct energy conversion between heat and electricity, thermoelectric materials/applications have attracted extensive attention. The efficiency of thermoelectric modules/devices is dominated by the material dimensionless figure of merit, zT. zT of thermoelectric materials can be enhanced through both optimizing carrier transportation properties and refraining the lattice thermal conductivity. Module design can also influence the energy conversion efficiency. Proper module design, such as segmented or cascade design, can effectively utilize the potential of composing materials. Furthermore, through proper device design, the thermoelectric modules can be designed as both flexible and rigid types and applied in both niche and macro-fields.