Triboelectric Nanogenerators Based on 2D Materials: History
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Contributor:
Yukai Zhou
,
Jia-Han Zhang
,
Songlin Li
,
Hao Qiu
,
Yi Shi
,
Lijia Pan

The development and production of nanogenerators provide a promising solution to address the energy crisis. Triboelectric nanogenerators, in particular, have attracted significant attention due to their portability, stability, high energy conversion efficiency, and compatibility with a wide range of materials. Triboelectric nanogenerators (TENGs) have many potential applications in various fields, such as artificial intelligence (AI) and the Internet of Things (IoT). Additionally, by virtue of their remarkable physical and chemical properties, two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), MXenes, and layered double hydroxides (LDHs), have played a crucial role in the advancement of TENGs. 

  • triboelectric nanogenerators
  • 2D materials
  • sensing
  • mechanical energy harvesting

1. Introduction

The growing use of fossil fuels such as oil, natural gas, and other non-renewable energy sources in both industrial and daily activities has raised significant concerns regarding the energy crisis [1][2][3][4][5][6]. To mitigate the excessive dependence on these fossil fuels, humans have been exploring and utilizing various sustainable and renewable energy sources, such as solar, wind, nuclear, and tidal power, for several decades. However, there are inherent limitations to the use of these green energy resources. For example, solar energy is constrained by weather, time, and space [7].
In 2006, the first piezoelectric nanogenerator (PENG) was manufactured [8], and later, triboelectric nanogenerators (TENGs) were introduced [9]. These devices utilize the abundant and often overlooked mechanical energy present in daily life, including human activities, mechanical operations, water, and wind. They convert this energy into electrical power for use as an energy collector or to power sensors. In particular, in the era of the Internet of Things (IoT) and with the growing diversification and miniaturization of electronic devices, piezoelectric and triboelectric nanogenerators are becoming more widely used [10]. Compared to PENGs, TENGs utilize two materials with different charged properties to generate significantly enhanced electrical signals. Due to the advantages of a wide range of materials, low cost, industrial scalability, and high energy conversion efficiency, TENGs have considerable application potential in wearable systems, flexible electronics, and artificial intelligence.
TENGs are based on the triboelectric principle. However, due to the negative effects of triboelectricity in industrial production and daily life, such as the accumulation of friction charges on industrial equipment that can detonate surrounding flammable gases [11] and the breakdown of capacitors in high-friction voltage circuits [12], this ancient energy source, which dates back more than 2600 years to ancient Greek civilization, has not been systematically utilized. It was not until the successful design of TENGs that researchers began to pay much attention to these techniques. Although the triboelectric phenomenon is very common in daily life, the physical mechanism behind this universal phenomenon was not thoroughly understood until Wang et al. observed the generation of triboelectric potential by utilizing Kelvin probe force microscopy [13]; they found that electron transfer is the main mechanism of triboelectrification between solids, liquids, and gases. Electron transfer occurs only when the interatomic distance between two materials is forced to be shorter than the normal bond length by rubbing one material against the other through external force. The reduction in the interatomic barrier leads to the overlap of the strong electron cloud between the two atoms in the repulsion region, resulting in electron transition.
TENGs operate in a variety of modes, including contact–separation mode [14], lateral sliding mode [15], single-electrode mode [16], and freestanding triboelectric layer mode [17]. They have advantages in terms of energy collection range, energy conversion efficiency, preparation technology, and device service life. In pursuit of higher output performance, researchers designed different structures to expand the friction area and energy harvesting methods [18]. Proposed device structure designs include arch-shaped [19], disc [20], zigzag [21], and fiber-based 3D friction generators (FB-TENGs) [22] or the design of micropatterns onto the tribo-surfaces [23]. Increasing the charge density of the contact layer [24], introducing more charge–gain groups, or increasing the charge-trapping layer to block the charge combination [25] are also used to increase the output performance of TENGs.
As the key to output performance, the choice of triboelectric material is very important. A range of materials in the triboelectric series have been proposed, including organic polymers, metals, inorganic materials, etc. [26]. Because polymer-based TENGs have an electron with an inclination toward recombination with positive charges induced on the electrode and metal-based TENGs are subject to the issue of the corrosion, it is necessary to use new materials [27]. The fabrication and application of 2D materials provide a promising direction for improvement in material selection for TENGs. Since the successful stripping of two-dimensional graphene, 2D materials with an atomic layer thickness have attracted the attention of researchers around the world. Owing to their excellent electrical properties, transparency, flexibility, and high surface area, 2D materials have great application potential in TENGs [10][26][28], even in a stacked structure.

2. 2D Materials Used in TENGs

Since their discovery, 2D materials have been of great interest to researchers. Researchers have attempted to use mechanical stripping, chemical vapor deposition (CVD), the hydrothermal method, etc., to prepare large areas of materials [29]. The variety of 2D materials is also expanding and can be divided into the graphene series, 2D chalcogen compounds, transition metal dichalcogenides (TMDs), metal phosphorous trichalcogenides, metal monochalcogenides, transition metal trichalcogenides, etc. [30][31][32][33]. They are widely used in TENGs, given their respective advantages. The performance of the TENGs based on 2D materials is summarized in Table 1.
Table 1. Summary pf the performance of TENGs based on 2D materials. 

Negative

Friction Layer

Positive

Friction Layer

Electrode

Output Performance

Refs.

Graphene

PET

Graphene

5 V

0.5 µA cm−2

2.5 µW cm−2

[34]

AGS@PDMS

Copper foil

Copper foil

117 V

26 µA

480 µW cm−2

[35]

GN-PTFE

Al

Cu

96 V

3.66 μA

39 µW cm−2

[36]

Silicone

CG

Graphene

9.3 V

-

1.5 μW cm−2

[37]

PDMS

CG

CG

83 V

25.78 μA

250 µW cm−2

[38]

F-GO/PVDF

water droplet

Cu/ITO

16.5 V

18.1 μA

280 µW cm−2

[39]

PI(Kapton)/PI: rGO/PI

Al

Al

190V

-

630 µW cm−2

[40]

PVDF/rGO-AgNPs

Al

Al

200 V

-

430 µW cm−2

[41]

Al2O3/BN

Graphene

Graphene

1.2 V

150 nA cm−2

-

[42]

BNNSs/BoPET

Paper

Copper

200V

0.48 mA m−2

14 µW cm−2

[43]

PI/BNNS/PI

Al

Al

65.9 V

4.5 μA

21.4 μW cm−2

[44]

PI(Kapton)/MoS2:PI/PI

Al

Al

400 V

175 mA m−2

2570 µW cm−2

[45]

MoS2/SiO2

PDMS

Al/Si

25 V

1.2 μA

2.25 μW cm−2

[46]

LC-WS2

PET

ITO

12.2 V

-

13.8 µW cm−2

[47]

MXene

PET

ITO

650 V

-

52 µW cm−2

[48]

MXene

Skin

MXene

16.4 V

2.67 μA

2.89 µW cm−2

[49]

Kapton

Ecoflex

MXene/PVA

230 V

270 nA

-

[50]

MXene/Silicone

hand

Ag fabric

35 V

12.5 µA m−2

5.5 µW cm−2

[51]

PDMS/MXenes

PDMS/AgNPs

AgNPs

453 V

131 μA

-

[52]

Mg-Al LDH

water droplet

Al

13 V

1.6 μA cm−2

-

[53]

ZnAl-LDH-PVDF

PET

ITO

230.6 V

5.6 mA cm−2

430 µW cm−2

[54]

3. Applications of TENGs Based on 2D Materials

In the preceding Introduction and discussion, the researchers provided a summary of the use of 2D materials in the context of triboelectric nanogenerators. Specifically, 2D materials have been primarily employed as friction layers and electrodes in TENGs, particularly those exhibiting high electrical conductivity and those positioned at side positions within the triboelectric series, such as MXene and graphene. The exceptional mechanical stability, smooth surface, and facile surface functionalization of 2D materials lead to TENGs that possess enhanced electrical output an

3.1. Energy Harvesting

Energy harvesting was the original design intention of TENGs. With the increasing demand for energy and the proliferation of mobile intelligent devices, these miniature energy harvesting devices, which can maintain high output at low frequencies, have a wide range of potential applications in daily life. Due to the flexibility, robustness, and stability of the materials, TENGs based on 2D materials can be adhered to the surface of the human body or other complex structures to efficiently collect mechanical energy generated by human movements, wind, liquid flow, and other activities, which are used in many of the works described above. For instance, Chen et al. attached a TENG based on crumpled graphene to the finger joint [37]. When the bending angle changed from 30° to 90°, the device produced an output voltage of 32 to 56 mV, and the bending rate could also be measured. Dong et al. first utilized MXene to fabricate a TENG, which they attached to the thumb joint to capture energy surges in the joint, such as editing a text message or tapping a mouse [48]. The device showed a significant open-circuit voltage ranging from −80 to + 40 V. Luo et al. designed an MH-TENG that can convert mechanical energy generated by human joint activities into electric energy [50]. Different degrees of extrusion and stretching of the TENG can produce different output voltages when different joints are bent and restored, which can be used to monitor human posture.
Drawing inspiration from natural energy harvesting, Lan et al. developed a stretchable triboelectric nanogenerator capable of converting wind energy into electrical output while conforming to the surface of a plant leaf [55]. The authors utilized a combination of 1D AgNWs and 2D metallic MoS2 nanosheets to form highly conductive and flexible composite films as electrodes. The composite film was subsequently covered and packaged with PDMS, with the nanosheets and AgNWs tightly bonded by electrostatic and capillary forces. The inclusion of metallic MoS2 nanosheets served to cover the AgNWs network, reducing strain distribution and sliding during large tensile deformation, thereby promoting good electrical output and better attachment to the blade. Additionally, the output voltage signal of the stretchable TENG varies with blade vibration frequency at different wind speeds, making it a valuable tool for harvesting and converting wind energy.
Water energy harvesting represents a great innovation of TENGs in the field of energy harvesting. It generates electricity using water in direct contact with the friction material in the form of droplets or streams or by collecting the mechanical energy generated by water in motion, such as waves. The former uses the principle of liquid–solid contact electrification, and the latter, with a fully enclosed structure, makes use of mechanical impact and vibration under the action of water waves [56][57]. Ocean energy, also known as blue energy, is an important flied of TENGs in water energy collection. Among various forms of ocean energy, water wave energy stands out as a renewable source that is not affected by seasonal changes and has a significant reserve. It is estimated that the wave energy around coastlines can reach 2–3 TW (1 TW = 1012 W) [58]. However, the electromagnetic generators (EMGs) currently used for wave energy harvesting are expensive, bulky, and inefficient, hindering their effective utilization. In contrast, TENGs offer a new energy harvesting method that is lightweight, simple, low-cost, and easy to package and is particularly suitable for collecting low-frequency (<5 Hz) wave energy in complex ocean environments [59][60][61]. By forming a TENG network, water wave energy can be collected on a large scale, effectively alleviating energy problems [56]. In recent years, TENGs based on two-dimensional (2D) materials have been explored for harvesting of water energy. For instance, Vu et al. modified polyvinylidene fluoride (PVDF) by incorporating functionalized graphene oxide (GO) as a friction layer in contact with water droplets [39]. They employed an inclined structure to make water droplets slide on the friction layer and generate electricity through solid–liquid contact electrification. A single water droplet can produce an output of 0.14 mW, maintaining 90% of the output voltage after more than 4000 cycles. Based on the same principle, Cui et al. also managed to capture the triboelectric energy generated by water droplets sliding along an inclined surface. Layered double hydroxides (LDHs) grown in situ were used as the triboelectric layer, and modified LDHs were superhydrophobic. The peak output voltage was about 13 V, and the current density was 1.6 μA cm−2.

3.2. Self-Powered Monitoring Systems

TENGs are based on the fascinating principle of triboelectricity. This principle describes a unique electromechanical coupling effect that facilitates not only energy harvesting but also the creation of pressure sensors and arrays. The output signals of TENGs are closely related to the applied pressure and speed, enabling the design of sophisticated wearable sensing systems that can effectively monitor human activities, especially in electronic skin, human–machine interaction, and wisdom medical sectors [62][63]. The integration of TENG technology based on 2D materials in wearable sensing systems eliminates the need for an external power supply, offering improved portability and stability, which presents a significant advantage over other wearable monitoring systems. This feature reduces costs while simultaneously improving the user experience [64][65]. The development of miniaturized self-powered wearable pressure monitoring systems is an exciting area of research, with immense potential for the advancement of the IoT ecosystem. The growing interest in this field has attracted an increasing number of researchers to join in.
Tactile monitoring is an important task that electronic skin strives to achieve. Lee et al. developed an ultra-thin single-electrode triboelectric nanogenerator and integrated it into a wearable self-powered input system [66]. Using PDMS as a friction layer, two layers of graphene electrodes were used to generate output signals in x and y directions. The surface of PDMS was treated by plasma to improve the charge density. The overall structure of the device has a negative Poisson ratio auxetic mesh design, which can provide conformal contact on rough surfaces. The TENG tactile sensor can undergo a 13.7% and 8.8% elongation on the x- and y-axes, respectively, while maintaining both mechanical and electrical stability with less than 1% relative resistance and voltage variation, even when subjected to stretching. The touch sensor can clearly detect the touch of a single point and continuous sliding and displayed a character using a real-time trajectory mode. The authors envisaged its potential application in self-powered wearable communication systems. Recently, Cai et al. designed a self-healable (≤2 h), super-stretchable (>760%), and shape-adaptive TENG by doping and modifying PDMS as an e-skin to harvest energy and monitor human motion [67]. In the synthesis of PDMS, cross-linking agent IPDI and chain extender TPAL interact with the raw material to produce hydrogen bonds and imine bonds, which serve as cross-linking points and provide self-healing abilities. Different PDMS friction layers can be obtained by controlling the content of two kinds of bonds. By adding MXene into lightly cross-linked PDMS, a self-healable conductive composite layer was prepared by forming hydrogen bonds between them. This TENG is capable of conforming to the irregular surface of human skin due to its shape adaptability and can retain its original triboelectric performance even after being subjected to repeated damage. The sensor array can be used for real-time monitoring of finger pressure and position. In terms of medical wisdom, Wang et al. designed a self-powered strain sensor based on graphene oxide-polyacrylamide (GO-PAM) hydrogels [68]. They mixed graphene oxide into a self-healing hydrogel in order to enhance the conductivity and mechanical properties (toughness and stretchability) of the electrodes. In addition to improving the capacity of charge storage as charge-trapping sites in PAM, GO also causes PAM to produce an obvious fold structure and an enhanced friction area. They placed the strain sensor on an insole, integrated it with a data processing module and PC interface, and utilized a deep learning algorithm to identify and monitor human daily life and pathological gait. The highest recognition accuracy was achieved by the artificial neural network system, with accuracy rates of 99.5% and 98.2% for daily life and pathological gait, respectively. The researchers envision these smart insoles being utilized to diagnose Parkinson’s disease and hemiplegia, as well to adjust patients’ rehabilitation training based on monitoring data.
Zhang et al. developed a wearable self-powered toroidal triboelectric sensor (STTS) for self-powered human–machine interactions [69]. MXene/Ecoflex nanocomposites and flexible conductive fabric serve as the friction layer with the finger and electrode, respectively. In order to provide a comfortable contact–separation space between the finger skin and the negatively charged layer, a 3D printed model was used to fabricate pyramidal arrays on the MXene/Ecoflex nanocomposites. Electrical signals are generated when the muscles expand as the finger flexes, increasing the area of contact between the skin and the pyramidal structure. The triboelectric sensor exhibits exceptional properties, including a high peak-to-peak voltage of 19.91 V, an impressive sensitivity of 0.088 VkPa−1, and a wide pressure detection range spanning from 0 to 120 kPa. These remarkable attributes enable the generation of high-quality output signals, which facilitate the accurate detection of various finger movements. To achieve excellent flexibility and wearability of the sensor, a glove skeleton was created using 3D printing technology and flexible TPU material, while the fingers were made into a ring structure. This design allows the single-electrode TENG to be seamlessly attached to the inner surface of the ring, enhancing the user experience. When integrated with a signal transmission processing system, STTS can facilitate human–machine interaction, including control of racing games, switching of home appliances, and playing realistic balance table games.
In addition to monitoring pressure and physical activity, TENGs can also be applied for the detection of respiratory behavior and gases. Wang et al. developed a TENG-based sensor using Ti3C2Tx MXene/amino-functionalized multiwalled carbon nanotubes (MXene/NH2-MWCNTs) as the friction layer and electrode [70]. The homogeneous distribution of NH2-MWCNTs on organic MXene sheets is attributed to the effect of electrostatic adsorption. As the positive friction layer, one end of the nylon film is anchored in the center of the TENG. As air flows, the nylon film moves periodically, driving the TENG to generate electrical signals. As the wind speed increases, the nylon swings up and down more violently, causing more friction charges to be generated and the peak output voltage and current to increase, which can be used to monitor breathing behavior. Support vector machine algorithms were used to identify various breathing patterns, with an average prediction accuracy of 100%. Additionally, the triboelectric sensor can specifically detect formaldehyde gas concentration by analyzing the change in output voltage, as formaldehyde gas molecules react with oxygen ions and affect the conductivity of NH2-MWCNTs. The device exhibits an excellent gas-sensitive response (35% @ 5 ppm), a low detection limit (10 ppb), and a fast response/recovery time (51/57 s) as a self-powered formaldehyde sensor.

3.3. 2D Tribotronic Transistors

The potential generated by friction in TENGs can also provide an external power source for two-dimensional transistors. Since 2014, when electrostatic potential generated by TENG was used as gate voltage to regulate the electron transport characteristics in a transistor for the first time, the research and application of tribotronic transistor have been continuously deepened [71]. The integration of TENGs in friction transistors involves using one friction layer as a movable layer and the other as an induced voltage, typically placed at the gate of the transistor, to generate potential through friction and regulate the self-powered transistor. By integrating a two-dimensional transistor with a friction nanogenerator into a triboelectronic transistor, the induced friction potential can be used to easily regulate carrier transport characteristics in semiconductor channels. The friction potential generated by low-frequency and intermittent mechanical displacements can be ideal for regulating logic devices [72], photodetectors [73], artificial synapses [74], and other two-dimensional transistor devices [28].
Taking basic tactile switches as an example, Wang et al. designed a novel electronic transistor based on a single-motor mode of vertical coupling of a TENG and a MoS2 FET [75] for the first time. They attached a polytetrafluoroethylene (PTFE) film to the back of the FET to act as a friction layer. When PTFE came into contact with the Al electrode or finger skin, it generated a friction voltage, which was used as the gate voltage to regulate the electron concentration in MoS2. The contact–separation with the Al electrode is equivalent to generating a negative voltage on the FET gate, thereby reducing the concentration of charge carriers (electrons) in MoS2 and the corresponding source-drain current. Meanwhile, with the increase in the separation distance, the negative voltage increases, and the source-drain current continues to decline. When the separation distance increases from 0 to 30 mm, the source-drain current changes by 10 times. In addition, the prepared tribotronic transistor was is used as a smart tactile switch to control the trigger of the transistor through finger contact–separation. The switching ratio can reach 16, which can be used to control the switching state of an LED. Later, Li et al. used InSe as channel material and deposited an In layer on the top of InSe for doping to prepare a tactile switch with higher sensitivity [76]. As an n-type dopant, the In layer produces a surface charge transfer doping effect and excellent electrical transport characteristics in the conducting channel and protects the device as an encapsulation layer. This also makes the tactile switch more sensitive, and at a low Vds of 0.1 V, the switching ratio reaches 106, meeting the requirements of high signal resolution and low power consumption. This can be used to trigger LEDs or demonstrate Morse code.
This kind of self-powered structure can achieve low drive power and low energy loss of the transistor. In addition, using a TENG instead of a traditional external power supply as the grid voltage can better realize the interaction with the external environment and facilitate the preparation of multifunctional and highly integrated device structures [62][75]. Two-dimensional materials possess superior characteristics, such as diverse valence band structures and adjustable thickness, which demonstrate enormous potential in numerous functional tribotronic transistors. Although a TENG integrated in a transistor does not use 2D materials in most of current research, the emergence of high-performance TENGs based on 2D materials integrated in tribotronic transistors can be expected with further research and application.
d longer operational lifetimes in comparison to those utilizing other triboelectric materials. Moreover, due to the atomic layer thickness, the resulting device is lighter and thinner, even if stacking structures are used [7][10][32]. Additionally, extensive investigations have been conducted into the electron-capturing ability of 2D materials, with significant applications arising from the incorporation of these materials into other triboelectric materials. This integration enables regulation of the dielectric properties, work function, and other features of the resulting hybrid materials, ultimately increasing the work function difference and surface charge density and positively impacting the output performance of TENGs.

This entry is adapted from the peer-reviewed paper 10.3390/mi14051043

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