Nanogenerator is a technology that converts mechanical/thermal energy as produced by small-scale physical change into electricity. Nanogenerator has three typical approaches: piezoelectric, triboelectric, and pyroelectric nanogenerators. Both the piezoelectric and triboelectric nanogenerators can convert the mechanical energy into electricity. However, the pyroelectric nanogenerators can be used to harvest thermal energy from a time-dependent temperature fluctuation.
![V_{\text{max}} = \pm \frac{3}{4(\kappa_0+\kappa)}[e_{\text{33}} - 2(1 + \nu) e_{\text{15}} - 2\nu e_{\text{31}}] \frac{a^3}{l^3} \nu_{\text{max}}](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_vjd3lw71ocLxM4AzlEWqN4aLRq6UtuabTLFA73FeIAd0YwKIJgdt_vdkOStv7C_SCp_QIRKBdFngRJrxTttREodumqK5KORZhtZny1AixFUsZnzx0hpuF5sJLg0QU9dQccZzYuYHYjxNRAx3Dc=s0-d)
Piezoelectric Nanogenerator[edit]
Piezoelectric Nanogenerator is an energy harvesting device converting the external kinetic energy into an electrical energy based on the energy conversion by nano-structured piezoelectric material. Although its definition may include any types of energy harvesting devices with nano-structure converting the various types of the ambient energy (e.g. solar power and thermal energy), it is used in most of times to specifically indicate the kinetic energy harvesting devices utilizing nano-scaled piezoelectric material after its first introduction in 2006.[1]
Although still in the early stage of the development, it has been regarded as a potential breakthrough toward the further miniaturization of the conventional energy harvester, possibly leading the facile integration with the other types of energy harvester converting the different types of energy and the independent operation of mobile electronic devices with the reduced concerns for the energy source, consequently.
Mechanism[edit]
The working principle of nanogenerator will be explained for 2 different cases: the force exerted perpendicular and parallel to the axis of the nanowire.
The working principle for the first case is explained by a vertically grown nanowire subjected to the laterally moving tip. When a piezoelectric structure is subjected to the external force by the moving tip, the deformation occurs throughout the structure. Thepiezoelectric effect will create the electrical field inside the nanostructure; the stretched part with the positive strain will exhibit the positive electrical potential, whereas the compressed part with the negative strain will show the negative electrical potential. This is due to the relative displacement of cations with respect toanions in its crystalline structure. As a result, the tip of the nanowire will have an electrical potential distribution on its surface, while the bottom of the nanowire is neutralized since it is grounded. The maximum voltage generated in the nanowire can be calculated by the following equation:[2]
, where κ0 is the permittivity in vacuum, κ is the dielectric constant, e33, e15 and e31are the piezoelectric coefficients, ν is the Poisson ratio, a is the radius of the nanowire, l is the length of the nanowire and νmax is the maximum deflection of the nanowire's tip.
The electrical contact plays an important role to pump out charges in the surface of the tip. The schottky contact must be formed between the counter electrode and the tip of the nanowire since the ohmic contact will neutralize the electrical field generated at the tip. In order to form an effective schottky contact, the electron affinity(Ea) must be smaller than the work function(φ) of the metal composing the counter electrode. For the case of ZnO nanowire with the electron affinity of 4.5 eV, Pt(φ=6.1eV) is a suitable metal to construct the schottky contact. By constructing theschottky contact, the electrons will pass to the counter electrode from the surface of the tip when the counter electrode is in contact with the regions of the negative potential, whereas no current will be generated when it is in contact with the regions of the positive potential, in the case of n-type semiconductive nanostructure (p-type semiconductive structure will exhibit the reversed phenomenon since the hole is mobile in this case). The formation of the schottky contact also contributes to the generation of direct current output signal consequently.
For the second case, a model with a vertically grown nanowire stacked between the ohmic contact at its bottom and the schottky contact at its top is considered. When the force is applied toward the tip of the nanowire, the uniaxial compressive is generated in the nanowire. Due to the piezoelectric effect, the tip of the nanowire will have a negative piezoelectric potential, increasing the Fermi level at the tip. Since the electrons will then flow from the tip to the bottom through the external circuit as a result, the positive electrical potential will be generated at the tip. The schottky contact will barricade the electrons being transported through the interface, therefore maintaining the potential at the tip. As the force is removed, the piezoelectric effect diminishes, and the electrons will be flowing back to the top in order to neutralize the positive potential at the tip. The second case will generate alternating current output signal.
Geometrical Configuration[edit]
Depending on the configuration of piezoelectric nanostructure, the most of the nanogenerator can be categorized into 3 types: VING,LING and "NEG". Still, there is a configuration that do not fall into the aforementioned categories, as stated in other type.
Vertical nanowire Integrated Nanogenerator (VING).
VING is a 3-dimensional configuration consisting of a stack of 3 layers in general, which are the base electrode, the vertically grown piezoelectricnanostructure and the counter electrode. The piezoelectric nanostructureis usually grown from the base electrode by various synthesizing techniques, which are then integrated with the counter electrode in full or partial mechanical contact with its tip.
After Professor Zhong Lin Wang of the Georgia Institute of Technologyhas introduced a basic configuration of VING in 2006 where he used a tip of atomic force microscope (AFM) to induce the deformation of a single vertical ZnO nanowire, the first development of VING is followed in 2007.[3]The first VING utilizes the counter electrode with the periodic surface grating resembling the arrays of AFM tip as a moving electrode. Since the counter electrode is not in full contact with the tips of the piezoelectricnanowire, its motion in-plane or out-of-plane occurred by the external vibration induces the deformation of the piezoelectric nanostructure, leading to the generation of the electrical potential distribution inside each individual nanowire. It should be noted that the counter electrode is coated with the metal forming the schottky contact with the tip of thenanowire, where only the compressed portion of piezoelectric nanowirewould allow the accumulated electrons pass through the barrier between its tip and the counter electrode, in case of n-type nanowire. The switch-on and –off characteristic of this configuration shows its capability of generating direct current generation without any requirement for the external rectifier.
In VING with partial contact, the geometry of the counter electrode plays an important role. The flat counter electrode would not induce the sufficient deformation of the piezoelectric nanostructures, especially when the counter electrode moves by in-plane mode. After the basic geometry resembling the array of AFM tips, a few other approaches have been followed for facile development of the counter electrode. Professor Zhong Lin Wang’s group have generated counter electrode composed of ZnO nanorods utilizing the similar technique used for synthesizing ZnO nanowire array. Professor Sang-Woo Kim's group of Sungkyunkwan University (SKKU) and Dr. Jae-Young Choi's group of Samsung Advanced Institute of Technology (SAIT) in South Korea introduced bowl-shaped transparent counter electrode by combining anodized aluminum and the electroplating technology.[4] They also have developed the other type of the counter electrode by using networked single-walled carbon-nanotube (SWNT) on the flexible substrate, which is not only effective for energy conversion but also transparent.[5]
The other type of VING has been also suggested. While it shares the identical geometric configuration with the aforementioned, such a VING has full mechanical contact between the tips of the nanowires and the counter electrode.[6] This configuration is effective for application where the force is exerted in the vertical direction (toward the c axis of the piezoelectric nanowire), and it generates alternating current (AC) unlike VINGs with partial contact.
Lateral nanowire Integrated Nanogenerator (LING).
LING is a 2-dimensional configuration consisting of three parts: the base electrode, the laterally grown piezoelectric nanostructure and the metal electrode for schottky contact. In most of cases, the thickness of the substrate film is much thicker than the diameter of the piezoelectricnanostructure, so the individual nanostructure is subjected to the pure tensile strain.
LING is an expansion of single wire generator (SWG), where a laterally aligned nanowire is integrated on the flexible substrate. SWG is rather a scientific configuration used for verifying the capability of electrical energy generation of a piezoelectric material and is widely adopted in the early stage of the development.
As of VINGs with full mechanical contact, LING generates AC electrical signal. The output voltage can be amplified by constructing an array of LING connected in series on the single substrate, leading the constructive addition of the output voltage. Such a configuration may lead to the practical application of LING for scavenging large-scale power, for example, wind or ocean waves.
Nanocomposite Electrical Generators (NEG).
"NEG" is a 3-dimensional configuration consisting three main parts: the metal plate electrodes, the vertically grown piezoelectric nanostructureand the polymer matrix which fills in between in the piezoelectricnanostructure.
NEG was introduced by Momeni et al.[7] It was shown that NEG has a higher efficiency compared to original nanogenerator configuration which a ZnO nanowire will be bended by an AFM tip. It is also shown that it provides an energy source with higher sustainability.
Other type. The fabric-like geometrical configuration has been suggested by Professor Zhong Lin Wang in 2008. The piezoelectric nanowire is grown vertically on the two microfibers in its radial direction, and they are twined to form a nanogenerator.[8] One of the microfibers is coated with the metal to form a schottky contact, serving as the counter electrode of VINGs. As the movable microfiber is stretched, the deformation of thenanostructure occurs on the stationary microfiber, resulting in the voltage generation. Its working principle is identical to VINGs with partial mechanical contact, thus generating DC electrical signal.
Materials[edit]
Among various piezoelectric materials studied for the nanogenerator, many of the researches have been focused on the materials withwurtzite structure such as ZnO, CdS[9] and GaN.[10] The greatest advantage of theses material arises from the facile and cost-effective fabrication technique, hydrothermal synthesis. Since the hydrothermal synthesis can be conducted in a low temperature environment under 100°C in addition to vertical and crystalline growth, these materials can be integrated in various substrates with reduced concern for its physical characteristics such as a melting temperature.
Endeavors for enhancing the piezoelectricity of the individual nanowire also led to the development of other piezoelectric materials based on Wurtzite structure. Professor Zhong Lin Wang of Georgia Institute of Technology introduced p-type ZnO nanowire.[11] Unlike the n-type semiconductive nanostructure, the mobile particle in p-type is a hole, thus the schottky behavior is reversed from that of n-type case; the electrical signal is generated from the portion of the nanostructure where the holes are accumulated. It is experimentally proved that p-type ZnO nanowire can generate the output signal near 10 times that of n-type ZnO nanowire.
From the idea that the material with perovskite structure is known to have more effective piezoelectric characteristic compared to that with wurtzite structure, Barium titanate (BaTiO3) nanowire has been also studied by Professor Min-Feng Yu of University of Illinois at Urbana Champaign.[12] The output signal is found to be more than 16 time that from a similar ZnO nanowire.
Professor Liwei Lin of University of California at Berkeley has suggested that PVDF can be also applied to form a nanogenerator.[13]Being a polymer, PVDF utilizes a near-field electrospinning for its fabrication, which is rather a different technique compared to other materials. The nanofiber can be directly written on the substrate controlling the process, and this technique is expected to be applied for forming self-powered textile based on nanofiber.
Comparison of the reported materials by 2010 is given in the following table.
| Material | Type | Geometry | Output voltage | Output power | Synthesis | Researched at |
|---|---|---|---|---|---|---|
| ZnO (n-type) | Wurtzite | D: ~100 nm, L: 200~500 nm | VP=~9 mV @ R=500MΩ | ~0.5 pW per cycle (estimated) | CVD, hydrothermal process | Georgia Tech. |
| ZnO (p-type) | Wurtzite | D: ~50 nm, L: ~600 nm | VP=50~90 mV @ R=500MΩ | 5~16.2 pW per cycle (calculated) | CVD | Georgia Tech. |
| ZnO-ZnS | Wurtzite (Heterostructure) | Not stated | VP=~6 mV @ R=500MΩ | ~0.1 pW per cycle (calculated) | Thermal evaporation and etching | Georgia Tech. |
| GaN | Wurtzite | D: 25~70 nm, L: 10~20 μm | Vavg=~20 mV,Vmax=~0.35 V@ R=500MΩ | ~0.8 pW per cycle (average, calculated) | CVD | Georgia Tech.[10] |
| CdS | Wurtzite | D: ~100 nm, L: 1 μm | VP=~3 mV | Not stated | PVD, Hydrothermal Process | Georgia Tech.[9] |
| BaTiO3 | Pervoskite | D: ~280 nm, L: ~15 μm | VP=~25 mV @ R=100MΩ | ~0.3 aJ per cycle (stated) | High temperature chemical reaction | UIUC[12] |
| PVDF | Polymer | D: 0.5~6.5 μm, L: 0.1~0.6 mm | VP=5~30 mV | 2.5 pW~90 pW per cycle (calculated) | Electro spinning | UC Berkeley[13] |
Applications[edit]
Nanogenerator is expected to be applied for various applications where the periodic kinetic energy exists, such as wind and ocean waves in a large scale to the muscle movement by the beat of a heart or inhalation of lung in a small scale. The further feasible applications are as follows.
Self-powered nano/micro devices. One of the feasible applications of nanogenerator is an independent or a supplementary energy source to nano/micro devices consuming relatively low amount of energy in a condition where the kinetic energy is supplied continuously. One of example has been introduced by Professor Zhong Lin Wang’s group in 2010 by the self-powered pH or UV sensor integrated VING with an output voltage of 20~40 mV onto the sensor.
Still, the converted electrical energy is relatively small for operating nano/micro devices; therefore the range of its application is still bounded as a supplementary energy source to the battery. The breakthrough is being sought by combining the nanogenerator with the other types of energy harvesting devices, such as solar cell or biochemical energy harvester.[14][15] This approach is expected to contribute to the development of the energy source suitable for the application where the independent operation is crucial, such asSmartdust.
Smart Wearable Systems. The outfit integrated or made of the textiles with the piezoelectric fiber is one of the feasible applications of the nanogenerator. The kinetic energy from the human body is converted to the electrical energy through the piezoelectric fibers, and it can be possibly applied to supply the portable electronic devices such as health-monitoring system attached with the Smart Wearable Systems. The nanogenerator such as VING can be also easily integrated in the shoe employing the walking motion of human body.
Another similar application is a power-generating artificial skin. Professor Zhong Lin Wang’s group has shown the possibility by generating AC voltage of up to 100 mV from the flexible SWG attached to the running hamster.[16]
Transparent and Flexible Devices. Some of the piezoelectric nanostructure can be formed in various kinds of substrates, such as flexible and transparent organic substrate. The research groups in SKKU (Professor Sang-Woo Kim’s group) and SAIT (Dr. Jae-Young Choi’s group) have developed the transparent and flexible nanogenerator which can be possibly used for self-powered tactile sensor and anticipated that the development may be extended to the energy-efficient touch screen devices. Their research focus is being extended to enhance the transparency of the device and the cost-effectiveness by substituting Indium-Tin-Oxide (ITO) electrode with a graphenelayer.[17]
Implantable Telemetric Energy Receiver. The nanogenerator based on ZnO nanowire can be applied for implantable devices sinceZnO not only is bio-compatible but also can be synthesized upon the organic substrate, rendering the nanogenerator bio-compatible in overall. The implantable device integrated with the nanogenerator can be operated by receiving the external ultrasonic vibration outside the human body, which is converted to the electrical energy by the piezoelectric nanostructure.
Triboelectric Nanogenerator[edit]
Triboelectric nanogenerator is an energy harvesting device that converts the external the ambient mechanical energy into electricity based on the nano-scale triboelectric effect. It is another mechanical energy harvesting technology besides the piezoelectric nanogenerator. This new type of nanogenerator was firstly demonstrated in Prof. Zhong Lin Wang’s group at Georgia Institute of Technology.[18] Although it is still in the early stage of the development for triboelectric nanogenerators, the output voltage has been enhanced to hundreds of volts,[19] and it has been proven to be a simple, cost-effective, robust and efficient approach to scavenge mechanical energy.[19][20]
The typical structure of a triboelectric nanogenerator is the stacking of two sheets with electrode layers on the back. The inner surfaces of the two sheets are covered with two different materials respectively, which should have large difference in their abilities to attract electrons. Also, it is always needed to have an intrinsic gap existing between the two sheets at strain free condition.
Mechanism[edit]
The working mechanism of the triboelectric nanogenerator can be described as the periodic change of the potential difference induced by the cycled separation and re-contact of the opposite triboelectric charges on the inner surfaces of the two sheets. When a mechanical agitation is applied onto the device to bend or press it, the inners surfaces of the two sheets will get into close contact and the charge transfer will begin, leaving one side of the surface with positive charges and the other with negative charges. This is just the triboelectric effect. When the deformation is released, the two surfaces with opposite charges will separate automatically, so that these opposite triboelectrc charges will generate an electric field in between and thus induce a potential difference across the top and bottom electrodes. In order to screen this potential difference, the electrons will be driven to flow from one electrode to the other through the external load. The electricity generated in this process will continue until the potentials of the two electrodes get back to even again. Subsequently, when the two sheets are pressed towards each other again, the triboelectric-charge-induced potential difference will begin to decrease to zero, so that the transferred charges will flow back through the external load, to generate another current pulse in the opposite direction. When this periodic mechanical deformation lasts, the alternating current (AC) signals will be continuously generated.
As for the pair of materials getting in contact and generating triboelectric charges, at least one of them need to be an insulator, so that the triboelectric charges cannot be conducted away but will remain on the inner surface of the sheet. Then, these immobile triboelectric charges can induce AC electricity flow in the external load under the periodic distance change.
Applications[edit]
The power generated by triboelectric nanogenrators under mechanical agitation can be used in a number of applications where there exists the mechanical motion. These applications will include, but not limited to, the following aspects:
Self-powered systems
One of the most important applications is to use the triboelectric nanogenerator as the power source for the independent and maintenance-free operation of electronic device and systems. The triboelectric nanogenerator can directly power the portable devices all by itself in some case; or the AC current pulses produced could be regulated by a rectifier and a storage unit in the first place, and then the stored energy will be able to power the regular electronics such as LED.[18]
The powering of personal electronics
Since there is abundant mechanical energy generated on human bodies in people’s everyday life, we can make use of the triboelectric nanogenerator to convert this amount of mechanical energy into electricity, to charge-up the personal mobile electronics such as cell phones and laptops. This will help to greatly improve the convenience of people’s life and expand the application of the personal electronics.
The power source for electrochemical processes
The triboelectric nanogenerator can be used as a direct power source to facilitate electrochemical process, such as pollution degradation, electrodeposition, water splitting and so on.[19]
The active (self-powered) pressure sensor
Since the triboelectric nanogenerators can efficiently produce electricity in response to external motion and vibration induced by pressure change, they can be used as an active sensor to detect the subtle pressure change with very high sensitivity. They will not rely on external power source to work.[20]
Pyroelectric Nanogenerator[edit]
Pyroelectric Nanogenerator is an energy harvesting device converting the external thermal energy into an electrical energy by using nano-structured pyroelectric materials. Usually, harvesting thermoelectric energy mainly relies on the Seebeck effect that utilizes a temperature difference between two ends of the device for driving the diffusion of charge carriers.[21] However, in an environment that the temperature is spatially uniform without a gradient, such as in outdoor in our daily life, the Seebeck effect can not be used to to harvest thermal energy from a time-dependent temperature fluctuation. In this case, the pyroelectric effect has to be the choice, which is about the spontaneous polarization in certain anisotropic solids as a result of temperature fluctuation.[22] The first pyroelectric nanogenerator was introduced by Prof. Zhong Lin Wang at Georgia Institute of Technology in 2012.[23] By harvesting the waste heat energy, this new type of nanogenerator has the potential applications such as wireless sensors, temperature imaging, medical diagnostics, and personal electronics.
Mechanism[edit]
The working principle of pyroelectric nanogenerator will be explained for 2 different cases: the primary pyroelectric effect and the secondary pyroelectric effect.
The working principle for the first case is explained by the primary pyroelectric effect, which describes the charge produced in a strain-free case. The primary pyroelectric effect dominates the pyroelectric response in PZT, BTO, and some other ferroelectric materials.[24] The mechanism is based on the thermally induced random wobbling of the electric dipole around its equilibrium axis, the magnitude of which increases with increasing temperature.[25] Due to thermal fluctuations under room temperature, the electric dipoles will randomly oscillate within a degree from their respective aligning axes. Under a fixed temperature, the total average strength of the spontaneous polarization form the electric dipoles is constant, resulting in no output of the pyroelectric nanogenerator. If we apply a change in temperature in the nanogenerator form room temperature to a higher temperature, the increase in temperature will result in that the electric dipoles oscillate within a larger degree of spread around their respective aligning axes. The total average spontaneous polarization is decreased due to the spread of the oscillation angles. The quantity of induced charges in the electrodes are thus reduced, resulting in a flow of electrons. If the nanogenerator is cooled instead of heated, the spontaneous polarization will be enhanced since the electric dipoles oscillate within a smaller degree of spread angles due to the lower thermal activity. The total magnitude of the polarization is increased and the amount of induced charges in the electrodes are increased. The electrons will then flow in an opposite direction.
For the second case, the obtained pyroelectric response is explained by the secondary pyroelectric effect, which describes the charge produced by the strain induced by thermal expansion. The secondary pyroelectric effect dominates the pyroelectric response in ZnO, CdS, and some other wurzite-type materials. The thermal deformation can induce a piezoelectric potential difference across the material, which can drive the electrons to flow in the external circuit. The output of the nanogenerator is associated with the piezoelectric coefficient and the thermal deformation of the materials.The output current I of the pyroelectric nanogenerators can be determined by the equation of I=p(dT/dt), where p is the pyroelectric coefficient, A is the effective area of the NG, dT/dt is the rate of change in temperature.
Applications[edit]
Pyroelectric nanogenerator is expected to be applied for various applications where the time-dependent temperature fluctuation exists. One of the feasible applications of the pyroelectric nanogenerator is used as a active sensor, which can work without a battery. One example has been introduced by Professor Zhong Lin Wang's group in 2012 by using a pyroelectric nanogenerator as the self-powered temperature sensor for detecting a change in temperature, where the response time and reset time of the sensor are about 0.9 and 3 s, respectively.[26] In general, the pyroelectric nanogenerator gives a high output voltage, but the output current is small. It not only can be used as a potential power source, but also as an active sensor for measuring temperature variation.
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