Nanogenerators : A new paradigm in blue energy-harvesting

Abstract The depletion of conventional/natural energy sources demands nonconventional efficient alternatives with sustainable utility. The outstanding performance of nanomaterials resolves the energy scarcity issue through the birth of small-scale, energy-harvesting device called nanogenerator for the operation of nanosystems. A nanogenerator is an ambient energy-harvester with exotic features of being lightweight, sustainable, and stand-alone device, which promises efficient utilization of energy. Conversion of mechanical/thermal energy into electricity is the basic working principle of a nanogenerator. The nanogenerators employ piezoelectric or triboelectric properties of material to produce mechanical energy, whereas for thermal energy generation, the pyroelectric or thermoelectric properties are utilized. The unique and variant applicabilities of nanogenerators make them increasingly popular among the scientific communities with interdisciplinary research interest. The present chapter will summarize the fundamentals of different types of nanogenerators along with their applicability in different energy sectors.


Introduction
The progress of civilization has undoubtedly reached its unprecedented peak with the commencement of the fourth industrial revolution in the 21st century. The present age of industrial advancement quite expectedly is facing an elevated demand for energy resources.
Limited storage of natural energy resources is making the situation more critical day by day.
Consequently, modern industries are confronting enormous energy challenges. Moreover, the modern time of industrialization threatens the near future of the mankind with the rising level of carbon pollution that paved the path of unpredictable changes of Earth's climate possibly resulting in even scary extinction of life. The only way out of the critical situation may be the search for alternative and sustainable energy resources0 F 1 . In the perspective of climate change, the added feature that a sustainable energy resource should have is that the energy utilization needs to be pollution-free. Thus the futuristic growth of science and technology is confronting the dual challenge of energy crisis and carbon pollution (Fig 1). This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9

Origin of Nanogenerators:
The search for alternative energy resources particularly in terms of the non-conventional and sustainable form is the essential prerequisite of modern society to maintain the futuristic scientific, technological and industrial drive towards a new era of automation. The novel advancement of mankind would not be possible without the revolutionary concept of the Internet of Things (IoT). All of this technological progress is the phenomenal consequence of the technological achievement of miniaturization science. Nanotechnology is the utmost discovery of miniaturization science which even outperformed the projection of technological growth that had been foreseen by Moore, decades before in 1975. Nanotechnology gives birth to nanodevices that need the power to operate. However, in general, the market-available battery size is much larger than the nanodevices, thus in turn governing the size of the entire system. Consequently, the practical realisation of a nano battery is highly desirable, preferably employing alternative energy resources. The nano-size battery usually has This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 a small lifetime posing a great hurdle to scientists. The desirable alternative is to develop a nanosized battery that can harvest energy from the environment to become a self-sufficient green power resource for driving nanodevices. Nanogenerators are just fit in this scope. Firstly, they are of nano-size order and secondly, they are capable of converting naturally available mechanical energy into electrical energy. Moreover, this energy suffices to drive the nanodevice in self-power mode. The term nanogenerator was first coined in 19932 F 3 while the seed to the invention of nanogenerators is sowed long before in 1861 with the postulation of Maxwell's equation (Fig 2).
According to Maxwell's equation, the displacement current has two components. The first term of Maxwell's equation, 0 , postulates the origin of magnetic induction from the electric field. The term thus forms the basis of electromagnetic wave generation, the application of which extends to modern wireless communication technology. The second term defines the polarisation of medium and from this key term, the fundamental features of nanogenerator were originated3 F 4 . In nanogenerators, Maxwell's displacement current serves as the driving force for converting mechanical energy into electrical energy. This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 Presently, four primary effects, namely, piezoelectric, triboelectric, pyroelectric, and thermoelectric, are employed to develop nanogenerators which are widely utilized in different potential application sectors (Fig 3). This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 Fig 3a. Schematic illustration of nanogenerators5 F 6 based on (i) the piezoelectric effect, (ii) the triboelectric effect, (iii) the thermoelectric effect, (iv) the pyroelectric effect This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9

Piezoelectric nanogenerator (PENG)
In some materials, if stress is applied then their atomic structure alters which prompts the formation of dipole moment and leads to the generation of a voltage difference across the material. This type of material is called piezoelectric material and the said effect is known as the direct piezoelectric effect. However, if electrical polarisation occurs within piezoelectric material, then material deformation takes place. This is known as a converse piezoelectric effect (Fig 4). This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 The basic working principle of a PENG relies on the piezo potential, which is produced within the piezoelectric material. In general, two electrodes are connected at the opposite sides of the piezoelectric material. Initially, the Fermi levels of the two electrodes are balanced electrostatically. When strain is employed to the material by external means, the piezo potential is developed within the material producing a difference in-between the internal and external Fermi levels at the contacts. To nullify the differences in-between the Fermi levels, the electrons start to flow through external circuits in-between two electrodes till two electrodes are electrostatically rebalanced. External mechanical forces can trigger strain within PENG in such a way that the strain undergoes periodical variations. This will give rise to an alternating current (AC) at the PENG output and establishes the PENG as a promising power source for the nanosystems.
The external mechanical force can be applied in two directions: one is perpendicular to the nanowire and the other is parallel to the nanowire (Fig 5a). However, in both cases, the top This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 contact behaves as Schottky contact and the bottom contact behaves like an ohmic contact.
To boost the output power of the PENG, several nanowires should be integrated in such a way that the deformation of each nanowire is perfectly synchronized. Till now, there are two types of structures of PENG, namely, lateral-nanowire integrated NG (LING)9 F 10 and verticalnanowire integrated NG (VING)1 0F 11 (Fig 5b). In LING, the nanowires are grown parallel to a flexible substrate and during bending deformation all the nanowires are deformed synchronously with the substrate, resulting in elevated output power. While in VING, the device is fabricated directly on vertically grown nanowire arrays and it can produce energy from synchronous compression deformation of the NG. Different piezoelectric materials such as lead zirconate titanate1 2F 13 , barium titanate1 3F 14 , zinc oxide1 4F 15 , polyvinylidene fluoride1 5F 16 , cadmium sulphide1 6F 17 , molybdenum disulfide1 7F 18 were used for the fabrication of PENG.

Triboelectric nanogenerator (TENG)
Some materials get electrically charged when they make frictional contact with another dissimilar material1 8F 19 . This kind of contact-induced electrification is known as the triboelectric effect (Fig 6). After physical contact, the materials become oppositely charged, while, the strength of the charges is different for different materials1 9F 20 . This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 and freestanding triboelectric-layer (FT) mode2 5F 26 . In all the four kinds of TENGs, there is a minimum of one pair of triboelectric surfaces with at least two electrodes (For SE mode, the ground is the other electrode). In the triboelectric effect, transport of electrostatic charges occurs from one surface to the other. Upon displacement of one of the triboelectric surfaces, the electrostatic status of the system changes resulting in the generation of the potential difference in-between the two electrodes. Subsequently, the generated potential difference causes a current to flow via the external circuit to balance the electrostatic status.
Displacement of the triboelectric layers in the opposite direction creates a reverse potential difference in-between the electrodes, and therefore the current flows in the opposite direction.
Thus, an AC output is generated from the TENG via periodical displacement of triboelectric layers.
The triggering process is different in all four modes of TENG (Fig 7). Contact and separation process of the two triboelectric layers causes triggering of CS mode while relative sliding between triboelectric layers is responsible for the triggering of LS mode. In both CS and LS mode TENG, the external load is connected between the back electrodes of moving triboelectric layers and thereby imposing a limitation in the movement of the triboelectric layers. However, this limitation is waived off in the SE and FT mode of operation. In SE mode a triboelectric layer moves freely with respect to one static electrode, while in FT mode a triboelectric layer moves freely between the two static electrodes to trigger the NG. To increase the output power of TENG via structural optimization, different structures like the multi-layer integrations2 6F 27 and grating structures2 7F 28 have been fabricated.

Pyroelectric nanogenerator (PyENG)
The time-dependent temperature fluctuation often causes spontaneous polarization in certain anisotropic solids, which is known as the pyroelectric effect. The key parameter of the pyroelectric effect is the pyroelectric coefficient which can be described as the differential This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 change in spontaneous polarization owing to an alteration in temperature. The pyroelectric coefficient is the attribute of two phenomena, namely primary pyroelectric effect and secondary pyroelectric effect3 1F 32 . If all the dimensions of material remain constant (strain-free case), then the primary pyroelectric coefficient is employed to express the amount of charges generated due to an alteration in temperature. On the other hand, if the dimensions of a material are altered due to the temperature variation, then such anisotropic deformation of the material will produce a strain causing an additional contribution of piezoelectrically induced charges. This process is called secondary pyroelectric effect. In lead zirconate titanate, potassium niobate, barium titanate and some other ferroelectric materials the primary pyroelectric effect dominates while in zinc oxide, cadmium sulfide and some other wurtzitetype materials the secondary pyroelectric effect will predominate.
Based on the pyroelectric effect, Wang's group3 2F 33 devised the first pyroelectric nanogenerator (PyENG) in 2012. The principle of generation of current via primary pyroelectric coefficient can be explained through the example of potassium niobate nanowire PyENG. In potassium niobate dipole moments readily exist along with the spontaneous polarization density (Fig   8(a)-top). With increasing temperature, the random oscillations of the ions will intensify with a greater magnitude of spread around their corresponding aligning axes. This causes a reduction in effective dipole moments and consequently, the polarization density is decreased3 3F 34 . To regain the previous electrostatic status, charge carriers flow through the external circuit (Fig 8(a)-middle). Even if the PyENG is cooled down from ambient temperature, the oscillations of the ions are reduced with a smaller amplitude of spread across their respective aligning axes and thereby causing an enhancement in effective dipole moments. The resulting increase in polarization density prompts the flow of charge in the reverse direction (Fig 8(a)-bottom). This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 The principle of generation of current via secondary pyroelectric coefficient can be explained via the example of zinc oxide nanowire 33 PyENG were vertically grown zinc oxide nanowires are sandwiched between two electrodes (Fig 8b). With the increase in temperature, a pyroelectric potential is created along zinc oxide nanowire, thus one electrode becomes electrically positive and the other electrode becomes electrically negative. As the Fermi level of the electrically negative electrode is enhanced, it will drive the electrons to flow from the  Different materials like triglycine sulfate3 5F 36 , gallium nitride3 6F 37 , zinc oxide 33 , perovskite-based pyroelectrics were used to fabricate PyENG.

Thermoelectric nanogenerator (ThENG)
If two interconnected dissimilar conductors are maintained at different temperatures, then a potential difference is created between the conductors that are directly proportional to the temperature difference and such potential difference results in the flow of current in the loop.
This thermoelectric phenomenon is called the Seebeck effect (Fig 9). The Seebeck effect is utilized to fabricate ThENG. If a thermal gradient exists in the environment, then a ThENG can convert the heat into electrical energy. In 2012 Wang's group first fabricated ThENG using single Sb-Doped ZnO Micro/Nanobelts3 8F 39 (Fig 10a).
They had placed the ZnO belt on a glass substrate and used silver paste to fix the two ends, which eventually acted as electrodes with a separation of 3 mm. They found that 30 K temperature difference between two electrodes of ThENG can produce output current and voltage of about 194 nA and 10 mV, respectively.
To explain this phenomenon, a π-type model can be employed. In the π-type model, the structure is made up of one p-type and one n-type semiconductor, which is connected via a conducting wire/strip to complete the electrical circuit (Fig 10b). When one side of p-type and n-type material is heated and another one is cooled, the majority charge carriers (holes) of p-type materials and that (electrons) of n-type materials will diffuse from the hot side to This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 the cold side. This diffusion process results in building-up charge carriers at the cold end.
Such build-up of charge carriers at one end produces a potential difference across the semiconductor, proportional to the temperature difference across the semiconductor. If the cold ends of both the semiconductors are electrically connected, then a current flows from the p-type material to the n-type material.

Sb-doped ZnO micro belt NG. (bottom left) Calculated temperature distribution along a glass substrate. (right) I-V characteristic of a fabricated NG. 39 .
This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9

Fig 10b
A π-type model to explain the working principle of ThENG3 9F 40 .

Blue Energy and its harvesting using NG
As the futuristic growth of science and technology is confronting the dual challenge of energy crisis and carbon-pollution so renewable energy sources must be extensively explored and the use of fossil fuels should be restricted to a minimum.
Among the different renewable energy resources ocean energy is one of the least explored renewable energy sources on the earth. The total blue energy content of the Ocean was estimated to be 7.66 × 10 13 W4 0F 41 . Both the thermal and mechanical energy can be harvested from the ocean. The temperature gradient that exists across the warm surface waters and the cool deeper waters can be utilized to generate thermal energy and mechanical energy can be harvested from the tides, waves and currents of the ocean. Blue energy has several advantages over the other available renewable energy resources. Firstly, blue energy is invariant to weather, time of the day, or temperature4 1F 42 . Secondly, tides, waves and currents are predictable globally and more reliable than wind and solar energy. Finally, harvesting blue energy requires minimal use of land and small environmental interaction, thereby offering one of the most lenient techniques for large-scale, eco-friendly and sustainable electricity generation4 2F 43 .
Among different NGs, TENG has a huge potential for harvesting blue energy4 3F 44 . TENG can effectively harvest blue energy even at frequencies < 5 Hz which is perfectly suitable to harvest energy from a low-frequency ocean wave4 4F 45 . Since the invention of TENG, much effort has been made to harvest blue energy via various designs of prototypes.

Water-involved TENG
This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2. [4][5][6][7][8][9] In this type of TENG water is used as one of the triboelectric materials and liquid-solid contact electrification harvests the output energy (Fig 11). The first water-involved TENG was reported by Lin et al.4 5F  This is a post print which is published in "Nano Tools and Devices for Enhanced Renewable Energy " with DOI: https://doi.org/10.1016/B978-0-12-821709-2.00004-9 However, one major problem of water involved TENG is that the seawater can corrode polymer films via direct contact. Thus to overcome this problem researchers are intending to use fully enclosed TENG to harvest blue energy as discussed in the next subsection.

Non-water-involved TENG
In general, a waterproof layer is employed in the non-water-involved TENG (Fig 12). In July 2013, the first non-water-involved TENG was fabricated in a fully enclosed spherical shell by

Summary and perspective
Mankind moves towards modernization of every aspect of technology and modernization needs energy. However, the conventional energies are limited and thus the continuous proliferation of technological progress necessitates an alternative one. The process to harvest alternative energy should be inexpensive, self-powered, eco-friendly, sustainable and preferably maintenance free for a seamless growth of technology. The evolution of nanogenerator in 2006 created a revolution in harvesting alternative energy from the ambient environment by various means. The four physical processes, namely, piezoelectric effect, triboelectric effect, pyroelectric effect, thermoelectric effects were employed in the nanogenerators to harvest energy. The rapid growth in the development of the different nanogenerators has marked the foundation for self-powered modern electronic systems.
However, to harvest ambient energy like blue energy on a large scale, a nanogenerator with