Advances in Inorganic Nanomaterials for Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) that utilize triboelectrification and electrostatic induction to convert mechanical energy to electricity have attracted increasing interest in the last 10 years. As a universal physical phenomenon, triboelectrification can occur between any two surfaces that experience physical contact and separation regardless of the type of material. For this reason, many materials, including both organic and inorganic materials, have been studied in TENGs with different purposes. Although organic polymers are mainly used as triboelectric materials in TENGs, the application of inorganic nanomaterials has also been intensively studied because of their unique dielectric, electric, piezoelectric, and optical properties, which can improve the performance of TENGs. A review of how inorganic nanomaterials are used in TENGs would help researchers gain an overview of the progress in this area. Here, we present a review to summarize how inorganic nanomaterials are utilized in TENGs based on the roles, types, and characteristics of the nanomaterials.


INTRODUCTION
The invention of triboelectric nanogenerators (TENGs) in 2012 1 turned the historic physical phenomenon of triboelectrification (contact electrification) 2 into a working principle for energy conversion. With the development of TENGs, additional applications 3 to energy conversion have been discovered, including sensors, 4−8 control interfaces, 9 functional systems, 10 and biomedical applications. 11,12 The nature of triboelectrification implies that it can occur between any two materials that have physical contact and separation, as charges can transfer between the surfaces. For this reason, diverse materials have been studied for use in TENGs with different purposes, utilizing unique physical and chemical properties.
There are many review articles 13−24 that have described the working modes, mechanisms, and applications of TENGs. Zheng et al. have reviewed the application of TENGs in biomedical applications. 13 Chen and co-workers 14 and Wang et al. 20 have reviewed energy harvesting and self-powered sensing using TENGs. Wang 15,16 has reviewed the theoretical progress of triboelectrification. Zhang and Olin 17 and Bai et al. 19 have reviewed the materials that are utilized in TENGs. Pan and coworkers 25 have reviewed the applications of TENGs for future soft robots and machines. In this Review, we skip this part of the information and focus solely on how inorganic nanomaterials are used to improve the performance of TENGs.
The performance of TENGs is highly dependent not only on the triboelectric materials and their dielectric properties but also on how to pair two triboelectric materials. With the aim of boosting performance, nanomaterials have been introduced to TENGs. Nanomaterials can serve as either electrode or triboelectric materials, depending on the types of nanomaterials and how they are utilized. Nanomaterials in all dimensions have been applied to TENGs with significant performance improvements.
This Review summarizes the utilization of nanomaterials via several aspects: (1) the roles of inorganic nanomaterials, (2) the types of inorganic nanomaterials, and (3) the composition of the nanomaterials. Perspectives of future studies are also given after the summary. This Review provides an overview of how inorganic nanomaterials that are used in TENGs can lead to further development in this area.

THE ROLES OF INORGANIC NANOMATERIALS IN TENGs
The roles of nanomaterials can be divided into two categories: electrode materials and triboelectric materials. Metallic nanomaterials and some carbon-based nanomaterials have been used as electrode materials in TENGs because of their excellent electrical properties. The differences in physical and chemical properties of the triboelectric materials place great importance on electrode selection. The most common nanomaterials used as electrode materials are thermally evaporated gold 26 or copper 27 nanofilms at the backside of triboelectric materials. Such deposited nanofilms have a larger contact area (Figure 1) between the electrode and the triboelectric material, which can enhance electrostatic induction. These nanofilms are of critical importance when materials with high roughness such as textiles 28 are used as triboelectric materials. Glue-or tapebased electrode attachment methods ( Figure 1) limit the contact area between the electrode and the triboelectric surfaces. Moreover, the direct deposition of a nanofilm eliminates the glue layer, which reduces the distance of electrostatic induction, resulting in a higher current output for the TENGs.
In addition to metallic materials, 29 carbon materials such as graphene 30 have also been used as electrodes in TENGs due to their excellent electrical conductivity, flexibility, and optical properties. Different techniques have been used to produce graphene, including chemical vapor deposition (CVD), 27,30−33 laser induction, 34,35 layer-by-layer assembly, 36 and doping. 37 Carbon nanotubes 31,32,38−40 have also been applied as electrodes in different forms. Carbon nanotubes could serve as pure electrodes in the form of network layers or be embedded in polymers 38−40 to enhance the flexibility of the TENGs.
The utilization of inorganic nanomaterials to improve the performance of TENGs has gained the most interest from researchers. The reasons are as follows: (1) surfaces with inorganic nanomaterials have different charge distributions on flat surfaces; (2) the high surface energy of inorganic nanomaterials can enhance the charge transfer between triboelectric surfaces; (3) the dielectric properties of inorganic nanomaterials can change the properties of composites containing inorganic nanomaterials; and (4) nanomaterials can bring extra characteristics to TENGs that can promote their application in specific areas. Of course, there are other advantages that nanomaterials have brought to TENGs. We review the utilization of inorganic nanomaterials below.

INORGANIC NANOMATERIALS USED IN TENGs
Inorganic nanomaterials that have been studied in TENGs include metallic nanomaterials, metallic oxides, 2D materials, perovskites, ferroelectric nanomaterials, and carbon nanomaterials. The utilization of these nanomaterials is due to their electrical, dielectric, and surface topographical properties; e.g., metallic nanomaterials have good conductivity, metallic oxides such as TiO 2 have strong positive charge affinities, 2D semiconductors have flat surfaces and mechanical flexibilities, and ferroelectric nanomaterials can provide piezoelectricity.
3.1. Metallic Nanomaterials 3.1.1. Gold Nanomaterials. Gold nanofilms are commonly used metallic nanomaterials as electrodes in TENGs due to their excellent conductivity and low contact resistance. In addition to the direct thermal deposition methods reviewed above, a sputter coat of gold on a prestretched elastomer film could result in a crumpled gold electrode. 41 Such a crumpled electrode (Figure 2a) could increase the contact area between the gold and the countertriboelectric layer. Such an advance has led to a maximum open-circuit voltage of 124.6 V, a maximum current of 10.13 μA, and a power density of 0.22 mW/cm 2 . The same increment of contact area has also been found on nanoflowers (Figure 2b), similar to gold electrodes deposited by electrochemical methods. 42 The top performances of the TENG are 110 V, 5.5 μA, and 150 μW/cm 2 .
Different from the gold nanofilms reviewed in the above section, where the films are used only as electrodes, gold nanofilms have also been used as triboelectric layers. 26 The reasons metallic films are used as both triboelectric layers and electrodes are their excellent electric conductivity, high permittivity, and charge affinity. 17 The presence of gold nanoparticles could increase the contact area 43 with the counterlayer, such as polymers, and enhance the stability of the output due to the high oxidation resistance of gold. The output power of a TENG (Figure 2c) in the presence of a 56 nm gold nanoparticle layer was enhanced 25 times 44 due to the increase in contact area and nature of the positively charged gold nanoparticle surfaces. An area power density of 313 W/m 2 , a volume power density of 54 268 W/m 3 , and a maximum opencircuit voltage of 1200 V were obtained on the TENG.
In addition to the nanofilms, gold nanoparticles have also been impregnated in PDMS mesoporous pores 45 and served as triboelectric layers to generate charges with a PDMS ( Figure  2d). The impregnation of gold nanoparticles enhanced the power of TENGs over 5-fold compared to a flat film. Similar to this impregnation process, gold nanosheets have been embedded in a PDMS film 46 to enhance the stability and stretchability of TENGs (Figure 2e) and in a PTFE film to enhance the surface charge density and the output current. 47 The stretchability of TENGs brought by the gold nanosheet allows them to be applied at the joints of the human body for energy harvesting and sensing. Tactile sensors have been made with embedded gold nanostructures 48 with potential application in human−machine interactions (Figure 2f). Table 1 summarizes the performances of the gold nanomaterial constituted TENGs, showing the nanomaterials, countertribolayers, open-circuit voltages, short current or current  densities, and power densities. 3.1.2. Silver Nanomaterials. Silver is another widely used metallic material in TENGs. Compared to silver nanoparticles, silver nanowires are more commonly used in TENGs. Silver nanowires are popular because they can be simply transformed into transparent, flexible, and low-resistance electrodes for use in TENGs. Another reason is that the methods for the synthesis of silver nanowires are generally simple. Table 2 summarizes the performances of the silver nanomaterial constituted TENGs.
Yang's group reported a silver nanoparticle film-based TENG 49 for harvesting energy from wind, where a silver nanoparticle film pasted on photograph paper was used as both an electrode and a triboelectric material. The positive charge affinity nature of the silver nanoparticles offers a rapid charging and discharge process and thus high output performance for a TENG. Brushing nanosilver ink 50 on photopaper has also been used to create a silver film for use in TENGs.
Although the enhancement of silver nanoparticles has been proven, more studies have utilized silver nanowires since more features, as described above, could be created. Example  features are the flexibility and transparency of silver nanowirebased electrodes. 51 These features are of great importance for the application of TENGs as sensors to be attached to the human body. 52,53 Pasting, 54 spin coating, 52 Meyer-rod coating, 55 and splashing 56 are commonly used methods for creating silver nanowire electrodes, as these methods allow easy control of the thickness of silver nanowire films. To improve the performance of the electrodes, an embedding technique 57 has been used to fix the silver nanowire networks inside polymers such as PDMS, 46 PVDF (Figure 3), 58 and PEDOT:PSS 53 so that the contacts among the silver nanowires will not be lost during physical deformation. The embedding of the silver nanowires maximizes the contact between the polymer, which enhances the electrostatic induction of the TENGs. To further increase the stability of the performance of the TENGs, researchers welded silver nanowires 52,59 before embedding.

Metallic Oxides
Metallic oxides such as TiO 2 and ZnO have high positive charge affinities, making them easy to pair with materials with negative charge affinities. The tunable surface morphologies and the work functions of these materials can ultimately affect the electrical performance 61 of TENGs. In addition, the permittivity of the metallic oxides 62 could also have an impact on the charge transfer process of the TENGs. Table 3 summarizes the performances of some of the TENGs constructed with metallic oxides.
3.2.1. ZnO Nanomaterials. The tunable surface morphologies of ZnO make it possible to design specific structures for TENGs. Nanoparticles, 63 nanorods, 60,61,64−66 nanoflowers, 67 nanoripples, 68 and microballon arrays 69 have been created and used in different TENGs. The open-circuit voltage of TENGs using ZnO nanostructures is usually lower than 100 V, 61,63,64,70 while the number can be improved to above 100 V 71 to several hundred volts 72 if specifically composited with polymers.
One unique feature of using ZnO nanomaterials is that one can dope the materials to change their electrical behaviors. A TENG performance boosting effect can be brought by charge transfer at the surface due to doping. 60 Chen et al. reported that 4 M % Sb-doped ZnO nanorod arrays can boost the output voltage and current by 24 and 5.5 times compared to undoped arrays. Sb doping bends the band of the ZnO nanorod arrays downward (Figure 4), leading to more electrons being transferred to the countertriboelectric surface.
A similar phenomenon has also been found on Ga-doped ZnO nanorod arrays. 64 Another TENG performance boosting effect brought by doping is the high trap density close to the conduction band edge, which results in shallow trapped electrons. 73 3.2.2. TiO 2 Nanomaterials. Similar to ZnO, TiO 2 nanomaterials have tunable shapes and sizes. TiO 2 nanoparticles 74−77 have been more widely studied in nanogenerators. In most studies, TiO 2 nanomaterials are embedded 75 or composited 74 with polymers to improve the TENG performance. The chemical modification of TiO 2 nanostructures 78 is another way to enhance the output of TENGs.
The high dielectric constant of TiO 2 nanomaterials is one of the factors that improves the output performance of TENGs. Moreover, the oxygen vacancies 79,75 in TiO 2 composites have also been suggested as a factor of performance enhancement.

Other Metallic
Oxides. In addition to ZnO and TiO 2 , other metallic oxide nanomaterials, such ITO, 80 Al 2 O 3 , 81 and Fe 3 O 4 , 82 have also been studied for their applications in TENGs. Chun et al. 80 reported a TENG based on an interlocked array of surface-functionalized ITO nanohelixes   and demonstrated an over 340 times output enhancement compared to a flat ITO. Such enhancement is attributed to the nanoscale roughness, negligible degradation against external force and strain, and efficient charge generation and induction. Kim's group reported a stoichiometric Al 2 O 3 -based TENG 81 that reveals meaningful electric power generation under mechanical friction.

2D Nanomaterials
The application of 2D nanomaterials has gained increasing attention from researchers due to their unique geometry and electrical and dielectric properties. Based on the difference in electrical and dielectric properties, 2D nanomaterials have been used as electrodes or triboelectric layers in TENGs. 83 3.3.1. Graphene and Graphene Oxide. Graphene is a monolayer of carbon atoms that exhibits excellent electrical conductivity. Such a property makes graphene a good alternative to metallic electrodes. Moreover, graphene is an inert material that makes it chemically stable under extreme conditions. The transparency of graphene brings more features to TENGs. From a sustainable perspective, graphene is a green material that can benefit the development of environmentally friendly TENGs.
Shankaregowda et al. 84 reported a flexible and transparent TENG based on a graphene layer grown by a CVD method, in which graphene was used as both an electrode and a triboelectric layer. Such a TENG has maximum outputs of 650 V, 12 μA, and 0.21 mW/cm 2 . By crumpling the graphene layer, 85 the output power density can reach 0.25 mW/cm 2 . By aligning the graphene sheets, 86 the output power density reached 4.8 mW/cm 2 . The alignment of the graphene sheets forms numerous microcapacitors and has a low dielectric loss that contributes to a high output power density.
A graphene layer has also been applied to convert mechanical energy from water droplets. Kwak et al. 87 reported a triboelectrification-induced large electric power generator based on a graphene/PTFE structure ( Figure 5). The transfer of graphene on the PTFE surface polarized the graphene with negative charges accumulated on the top surface that interacted with the droplet to generate electricity. An output power of 1.9 μW from a single droplet was measured based on the unique graphene/PTFE structure.
Although CVD-grown graphene has excellent electrical conductivity, the production rate is still quite low due to the limitation of the technique. Alternatively, researchers have used reduced graphene oxide (rGO) in the development of TENGs. Usually, rGO is produced using thermal 88,89 and optical 34 methods. rGO can be used as either electrodes or triboelectric layers, depending on the design. Stanford et al. 34 and Zhao et al. 35 reported two separate studies on laser-induced graphene as electrodes. The first group used a 75 W pulse laser, while the second group used an 8.1 W continuous laser. Li et al. 90 reported a TENG using thermally reduced rGO at 700°C as a triboelectric layer and successfully converted the mechanical energy between the rGO and electrolyte into electricity. Zhao and co-workers 88 reported a foam structure made from thermally reduced rGO and polyimide (PI) and used it as a pressure-sensitive electrode in a wind-driven TENG. Wu et al. 91 reported the electron-trapping behavior of rGO embedded in a PI film, which can enhance the output approximately 30 times.
In some cases, GO has not been reduced for use in a TENG. Harnchana et al. reported a TENG made from a composite of GO, PDMS, and SDS 92 that has an output of 438 V and 11 μA/cm 2 . Such performances are approximately three times those of a flat PDMS layer. GO can also be used directly as a triboelectric layer 93 for energy harvesting and dynamic force sensing. Such a simple construction enabled the TENG to have a surprisingly high performance, with a short-circuit current density of 3.43 μA/cm 2 , an open-circuit voltage of 1100 V, and a power density of 3.13 W/m 2 . Moreover, the TENG can also act as a force sensor with a sensitivity of 388 μA/MPa.  Reports have indicated that MXenes can be more negative than PTFE. 96 Due to the strong electronegativity of MXenes that attract electrons during triboelectrification processes, the open-circuit voltage of TENGs based on MXenes is usually high. For example, a TENG with a structure of MXene/glass:PEI-ITO 97 can have an open-circuit voltage of 650 V. However, the output power density was found to be approximately 0.05 mW/cm 2 , which could be improved. To improve the performance of TENGs, researchers have composited MXenes with or embedded in polymers. The presence of MXenes in polymer films has two effects, dielectric permittivity and percolation, that need to be balanced by the percentage of MXenes ( Figure 6) to obtain the maximum performance. 94 PDMS is one of the most commonly used polymers to composite with MXenes. 98 Fluoropolymers such as PTFE, 96 PVDF-TrFE, 103 and PVDF 94,106 have also been used in composites with MXenes. Theoretical analysis was reported by Zhang et al., 105 who indicated that the percentage of MXenes in the PVDF film can be tuned to maximize the output. Experimentally, 10% MXene in the PVDF film provided an output of 0.22 mW/cm 2 . The same percentage of MXene has also been reported by Bhatta et al. 106 where PVDF fibers were functionalized by10% of MXene. An output of 1.1 mW/cm 2 was obtained on the TENG operated in contact-separation mode.
Other polymers, such PVA, 107,108 rubber, 104 and PDOT:PSS, 105 have also been used in composites with MXenes for use in TENGs. An output power density of 0.11 mW/cm 2 has been reported 109 based on a triboelectric layer of PVA/MXene fibers with 30 vol % MXene. In another report, 108 4 wt % MXene revealed the best performance.
In addition to the composition with polymers, MXenes can also be composited inorganic materials for use in TENGs. Feng and co-workers 110 reported an alternate-layered MXene composite produced with Nb 2 CT x and Ti 3 CT x . They found that 15 wt % Nb 2 CT x in the composite could lead to the best performance of the TENGs.
The electrical conductivity of MXenes makes them not only good triboelectric materials but also alternative electrodes in TENGs. Cao et al. 111 produced a type of MXene liquid electrode for improving the performance of a TENG working in a single electrode mode. Enhanced outputs up to 300 V and 0.37 μA/cm 2 were achieved.
A summary of the performances of the TENGs made of MXenes and composites is given in Table 4. 3.3.3. 2D Semiconductors. 2D semiconductors used in TENGs are mostly transition metal dichalcogenide (TMD) materials, including MoS 2 , WS 2 , MoSe 2 , and WSe 2 . The use of these 2D materials could benefit the theoretical understanding of the mechanism of triboelectrification. Seol and co-workers 83 made a triboelectric series of 2D layer materials and pointed out that the triboelectric effects of the 2D studied triboelectric  materials are obviously related to the effective work functions. According to the maximum output voltage and current, the 2D materials can be listed in the order of (−) MoS 2 , MoSe 2 , WSe 2 , and WS 2 (+). Among these 2D semiconductors, MoS 2 is the most investigated in TENGs. MoS 2 materials can act solely 112,113 as a triboelectric layer for energy conversion or be composited with other polymers. 114−116 The output power densities of the MoS 2 constituted TENGs range from 74 nW/cm 2 to 50 mW/ cm 2 , while the open-circuit voltage ranges from 2.3 to 145 V. The highest output, 50 mW/cm 2 , is from a TENG 102 with both triboelectric layers composited with MoS 2 flakes and polarized afterward (Figure 7).
The unique semiconductive property of MoS 2 leads to different electrical contacts in the TENGs. In some cases, 116 the depletion layers that formed between MoS 2 and other materials, such as Au and PPy, could enhance the output of the TENGs. This mechanism can have an impact on other 2D semiconductors but needs further study.

Perovskites/Ferroelectric Nanomaterials
Perovskites ( Figure 8) have recently gained much attention due to their successful applications 117 in energy harvesting technologies, including solar cells and TENGs. Recently, Ippili et al., 118 have reviewed the progress of the halide perovskitebased triboelectric self-powered sensors, showing the great potential of the materials. In most of the studies, single crystalline system of perovskites have been utilized, while the binary crystalline system of perovskites remains are less focused.
Many perovskites are also ferroelectric. In some papers, perovskites are emphasized, while others emphasize ferroelec-tricity. Therefore, we created a combined section for perovskites and ferroelectric nanomaterials.
Originally, perovskite refers to calcium titanate (CaTiO 3 ). Recently, it refers to the class of compounds that have the same type of crystal structure, XII A 2+VI B 4+ X 2− 3 . The "A" in the form can be either metals or organic groups. However, more perovskite structures have been synthesized recently that have different formulas, where "B" can combine two or more elements, such as Sr 3 Co 2 WO 9 . 119 3.4.1. Lead Halide Perovskites. The organolead halide perovskite MAPbI 3 was first used in a triboelectric photodetector in 2015. 120 Taking advantage of the excellent light absorption capability 121 of MAPbI 3 , enhanced performance has been achieved on a TENG, where the open-circuit voltage, short-circuit current, and amount of electric charge have been increased by 11%, 11%, and 9%, respectively. The output increase was further improved by using a hybrid perovskite structure, 122 MAPbI x Cl 3−x . The structure couples the triboelectric and photoelectric conversion mechanisms and realizes enhancements of 55.7%, 50.8%, and 58.2%. Although the advantages of organolead halide perovskites in TENGs have been demonstrated in these studies, the output, especially the power density, remains modest.
The performance of TENGs has recently been improved by using inorganic lead halide perovskites, such as CsPbBr 3 , 124 CsPbBr 2.6 I 0.4 , 125 Co(OH)(CO 3 ) 0.5 /Pt/CsPbIBr 2 , 126 and Ba 2+doped CsPbBr 3 . 127 It seems that CsPbBr 3 has the best performance in TENGs with or without doping compared to the other perovskites. In the case of doping, the electron binding energy, surface potential, and dielectric property of CsPbBr 3 can be optimized to obtain the best performance. For example, a TENG 127 made of CsPb 0.91 Ba 0.09 Br 3 resulting from A recent study has shown that the halogen elements in perovskites can be tuned to change their polarizabilities. The polarizability of CsPbBr 3 is calculated at 0.47 using density function theory calculations, while it is 0.52 for CsPbCl 3 . 128 Spontaneous polarization of the perovskites could further enhance their built-in electric field, which could lately improve the triboelectrostatic electric field by retaining more triboelectric surface charges.
3.4.2. Lead-Free Perovskite/Ferroelectric Materials. In many reports, lead-free perovskites are referred to as ferroelectric materials instead. Therefore, we use ferroelectric herewith.
Ferroelectric materials such as BaTiO 3 exhibit spontaneous electric polarization that can be reversed by an external electric field. Spontaneous polarization gives the material an excellent pyroelectric effect that can be used in energy harvesting for different applications. 129,130 The application of ferroelectric materials in TENGs has recently gained increasing interest from researchers. In most studies, ferroelectric materials are composited with a polymer to form a triboelectric layer. The first application of such a layer in TENGs was reported in 2015, 123 where BaTiO 3 (<100 nm) and SrTiO 3 (<100 nm) nanoparticles were filled into sponge PDMS films (Figure 9). The high permittivity and the nanopores created by the nanoparticles resulted in enhanced performance of the TENG (SrTiO 3 filled) with a charge density of ∼19 nC/cm 2 , a maximum open-circuit voltage of 338 V, and a maximum power density of 0.647 mW/cm 2 . The BaTiO 3 -filled PMDS film yielded a charge density of ∼14.7 nC/cm 2 at a volume ratio of 10%. In another report 131 in which 120 nm sized BaTiO 3 nanoparticles were used to fill a PDMS film, the optimized volume ratio was 30%. An output power density of 0.14 mW/cm 2 was obtained on the TENG. For composites with PVDF, 132 the optimized ratio of BaTiO 3 was 11.25%, which led to an open-circuit voltage of 131 V and a short-circuit current density of 89 μC/m 2 . The performance was further boosted to 161 V and 112 μC/m 2 by reducing the composite layer thickness down to 5 μm. A more complex composite created by core−shell structured BaTiO 3 −poly(tertbutyl acrylate) nanoparticles and PVDF 133 has also been studied as a triboelectric material. The composite has a higher dielectric constant of 26.5 at 150 MV/m, which contributes to the high output of the TENG, which is 2.5 times higher than that of the pure PVDF-based TENG.
In addition to engineering polymers, BaTiO 3 has also been composited with natural polymers such as cellulose for use in TENGs. Through simple vacuum filtration, BaTiO 3 particles can be tracked among the cellulose fibers forming a composite film. 134 The presence of the BaTiO 3 particles increased the dielectric constant of the cellulose film, which improved the performance of the TENG assembled with the composite film. The optimized ratio of BaTiO 3 particles was 13.5%, which yielded an open-circuit voltage of 181 V, short-circuit current of 21 μA, and power density of 4.8 W/m 2 . The open-circuit voltage and the short-circuit current were enhanced by 150% and 310%, respectively. The addition of silver nanowires to cellulose−BaTiO 3 composites 135 could increase the conductivity of the composite film and could be used as both a triboelectric layer and an electrode. Simple poling treatment of the composite can further promote the TENG performance. An output power density of 180 μW/cm 2 was achieved with the film paired with fluorinated ethylene propylene (FEP). Moreover, long-term stability was realized on the TENG with no output reduction after 10 000 cycles. A similar enhancement of poling has also been found in a BaTiO 3 /PDMScomposited triboelectric film. 131 Recently, a type of cellulose/ BaTiO 3 aerogel paper 136 was made that uses BaTiO 3 to increase the permittivity and charge trapping capability.
The composition of BaTiO 3 nanoparticles and inorganic nanomaterials has also been studied to enhance the output of TENGs. Taking advantage of the synergistic effect of multiwalled carbon nanotubes and 70 nm sized BaTiO 3 nanoparticles, 137 the charge and power density of a TENG was boosted to 160 μC/m 2 and 204 μW/cm 2 . Larger sized BaTiO 3 particles (500 nm) lead to lower output, which indicates that the larger surface area of the nanoparticles plays an important role in the output enhancement. The reason for the size effect is that the smaller nanoparticles create a larger interfacial volume fraction between BaTiO 3 and PDMS.
Researchers have also studied other ferroelectric materials, such as BiFeO 3 and ZnSnO 3 . A film made of a BiFeO 3 -modified glass fiber fabric on a PDMS 138 layer has been studied as a triboelectric material for use in wearable hybrid nanogenerators. The nanogenerator can generate an opencircuit voltage of 110 V and a short-current density of 3.67 μA/ cm 2 at a low frequency of 1 Hz. The addition of BiFeO 3 increased the dielectric constant from approximately 2.75 to 4, which led to an output power density of 151.42 μW/cm 2 . Such a power output is based not only on the triboelectricity of the layer but also on the piezoelectricity. A similar approach has also been applied for ZnSnO 3 constituted TENGs, 139 where a piezoelectric membrane of PVDF−ZnSnO 3 and PA6 has been used as a triboelectric layer. The composite shows a piezoelectric coefficient d 33 of −65 pm/V, which is significantly higher than that of the pure PVDF film. With the enhancement from the piezoelectric effect, the TENG made of the composite  ACS Nanoscience Au pubs.acs.org/nanoau Review film generated a maximum open-circuit voltage of 520 V and a short-circuit current density of 2.7 mA/m 2 . Another report ( Figure 10) that uses smaller ZnSnO 3 nanocubes 140 showed a higher short-circuit density of 7 μA/cm 2 and output power of 3 mW. Such an output can easily drive 106 blue LEDs. Mechanisms of the higher output have been suggested from the designed macrostructures and controlled microdielectric materials of the TENG. Table 5 shows the summary of the performances of the perovskite-or ferroelectric nanomaterial constituted TENGs.

Carbon Nanomaterials
Carbon nanomaterials exist in different crystal structures and different dimensions and have different electric and dielectric properties. Therefore, the application of carbon nanomaterials in TENGs can vary based on the properties that have been utilized. Carbon nanomaterials can be amorphous or crystalline depending on their chemical structures. Carbon black is one of the typical amorphous carbons. Crystalline carbon includes graphene, graphite, carbon nanotubes, fullerenes, and diamonds.
Graphene and graphene oxide as 2D nanomaterials have been reviewed above and are excluded in this section. However, graphene quantum dots as 0D nanomaterials are included in this section. Graphite, as an important member of the carbon material family, is also reviewed here for its application in TENGs, although it is generally not considered a nanomaterial.
3.5.1. Amorphous Carbon. Amorphous carbon is usually produced by using flame methods that result in powders containing nanosized carbon particles. High-temperature carbonization has also been used for producing amorphous carbon. 141 Amorphous carbon can be directly deposited on a p-type Si wafer for use as a triboelectric layer 142 that has resulted in an open-circuit voltage of 8.5 V, a short-circuit current density of 0.24 μA/cm 2 , a power density of 0.5 mW/ cm 2 , and an instantaneous energy conversion efficiency of 7.71%. The output was further enhanced by embedding graphene sheets in the amorphous carbon layer. The embedded graphene sheets contribute two effects: an edge effect that can capture electrons and a channel effect that enhances the electron mobility in the layer. Such contribution has enhanced the output to 13.5 V, 0.35 μA/cm 2 , 0.63 mW/ cm 2 , and 8.61%.
Carbon black is characterized under an electron microscope as nanosized particles. The materials can contain more than 97% amorphous carbon that has a high surface-area-to-volume ratio as well as good electric conductivity. Carbon black is usually composited with another material to form a triboelectric film. The presence of carbon black can change the permittivity 143 and the work function 61 of the composited film. When composited with PDMS, the charge storage characteristics of a TENG can be improved because the composite can store dynamically inserted negative charges. Such an improvement can further lead to a long-term increase in electrical charges. The content of carbon black in the composite has been experimentally found to be important since it plays a role in trapping charges. The carbon black content of 1 wt % resulted in the best performance of the TENG: 235 V, 35.6 μA/cm 2 , and 0.133 mW/cm 2 . Physically mixed carbon black, ZnO nanorods, and PTFE have been used to coat nickel foams for use in TENGs. 61 Carbon black, in this case, can affect the work function of ZnO nanorods because conductive electrons can transfer from carbon black to ZnO. The optimized content of carbon black here was 20 wt %, which led to an open-circuit voltage of 28 V, a short-circuit current density of 4.5 μA/cm 2 , and a power density of 80 μW/cm 2 .
3.5.2. Graphene Quantum Dots. Graphene quantum dots (GQDs) are nanosized layered carbon structures that have many activated parts due to dangling bonds at the edge. Such dangling bonds allow the GQDs to bind to organic and inorganic structures.
The addition of GQDs to a silver nanowire network can significantly enhance electron transfer. 144 The mechanism of the enhancement is that the electron flows to the GQDs and then to the PDMS. Such an intermediation by the GQDs reduces the barrier caused by the band mismatch of the silver and the PDMS. The enhancement of the GQDs increased the sensitivity of the triboelectric electronic skin 3 times.
GQDs can be doped and embedded in PVDF to make composite nanofibers using an electrospun technique. 145 Nitrogen-doped GQDs are negatively charged and can form ion−dipole interactions with the positive −CH 2 dipoles in the PVDF chains. A content of 5% GQDs in the composite fibers produces the highest output of 2.7 μW/cm 2 .
A single-walled CNT (SWCNT) network layer has been used as a p-type semiconductor to form a metal−semiconductor junction 150 with an aluminum electrode. Such a junction can reduce the loss of triboelectric charge because the SWCNT layer acts as a hole transporting layer that affects the triboelectric charge separation. The presence of the SWCNT layer can improve the charge-repelling force and the holeblocking barrier at the interface. A wearable TENG was made based on the structure that achieved an output voltage of ∼760 V, a current of ∼51 μA, and a power density of 0.77 mW/cm 2 . The SWCNT network layer has also been used directly as an electrode. 151 However, the loss of contact among the nanowires may cause the loss of conductivity during the operation. A microwave-assisted welding process can fix SWCNTs on a polycarbonate film (Figure 11a) and can significantly enhance the stability of the electrode. 146 The stability allows the electrode to be used on flexible substrates for harvesting energy. For using SWCNT-based electrodes in cases where stretchability is required, a simple compositing process has been used to create a stretchable electrode together with silver flakes and PDMS (Figure 11b). 147 A stretchable TENG working in the freestanding triboelectric layer mode was made with the electrode showing a peak power density of 84.4 mW/cm 2 .
Multiwalled CNTs (MWCNTs) have been more popularly used than SWCNTs because they have electrical properties similar to those of SWCNTs but at a lower cost. MWCNTs have been used either as a form of pure layer or as a type of composite. The deposition of a MWCNT layer can enhance the contact area of the triboelectric interface. 152,153 Such an effect, plus the positive surface of the MWCNTs, can enhance triboelectric charge generation. The synergistic effects enhanced the output by a factor of 7-fold. By screen printing a layer of MWCNTs on a thermoplastic polyurethane nanofiber membrane (TPUNM), a type of stretchable TENG 154 has been made where the MWCNTs act as both an electrode material and a triboelectric layer. A maximum voltage of 218 V, a current density of 0.75 μA/cm 2 , and a power density of 22.5 mW/cm 2 were obtained on the TENG by dip coating CNTs on a fiber with a silver nanowire network layer.
Using the surface chemistry of MWCNTs, cross-linked structures of MWCNTs and other materials, such as PEI (Figure 11c), have been fabricated. 148 The introduction of amide groups from PEI increases the tribopositivity, which further enhances charge transfer during the triboelectrification process. The results of the enhancements are represented by a 10-fold improvement in the output voltage and current.
Instead of chemical treatment of the MWCNTs for functionalization, direct mixing of the MWCNTs with polymers is less specific but more convenient experimentally. A MWCNT/PDMS composite can be simply produced by mixing the MWCNTs in PDMS kits (Figure 11d). 149,155,156 The addition of nanoflower-like ZnO (Figure 11e) 67 to the composite film can take advantage of the piezoelectric properties of ZnO to boost the output of a hybrid nanogenerator, namely, PAT-HNG. By mixing MWCNTs with PVA and PDAP, which resulted in dual responsive hydrogels, 39 self-healable and deformable TENGs have been fabricated that can survive under 200% strain. The mixing of ureidopyrimidinone-functionalized MWCNTs with IU-PAM could also produce self-healable TENGs. 38 By compositing with PANI, a PANI-MWCNT film-based TENG 157 could sense ammonia gas with a detection limit of 0.01 ppm.
3.5.4. Graphite. Graphite is generally not considered a nanomaterial, although nanosized graphite materials have been widely used. Here, we review the use of graphite in TENGs because the material has great importance in several aspects, ACS Nanoscience Au pubs.acs.org/nanoau Review such as a nonmetal electrode material, an environmentally friendly material, a low-cost material, and an oxidation resist material.
In most cases, graphite is used as the electrode material. As an electrode material, 159 a thick graphite film has excellent conductivity and mechanical strength that in practice could be directly used as an electrode that can allow the TENG to have the same or even better performance than metallic electrodes. A fully green TENG (FG-TENG) 158 using only graphite as the electrode material has been reported recently. The output power of the FG-TENG using the graphite electrodes was 35% higher than that using the copper electrode ( Figure 12). A thin graphite layer could also be coated on other substrates, such as sandpaper, 160 pristine paper, 161 and paper cards, 162 with coating methods of brushing, rod coating, and drawing, respectively.
In addition to its role as an electrode, graphite has also been composited with PDMS to form triboelectric layers for use in TENGs. 163,164 A graphite to PDMS ratio of 2:5 could result in the highest output of the TENG, 150 where the maximum opencircuit voltage is 410 V and the short-circuit current is 42 μA.
3.5.5. Other Carbon Nanomaterials. In addition to the above reviewed carbon materials, other carbon nanomaterials, such as fullerene 165 and diamond-like carbon, 166,167 have also been applied in TENG studies. The high electron affinity of the fullerene 165 could enhance the output of the TENG to a maximum open-circuit voltage of ≈1.6 kV, a short-circuit current of ≈100 μA, and a power density of 38 W/cm 2 . A TENG made of a diamond-like carbon film 153 had a maximum open-circuit voltage of 38 V, a short-circuit current of 3.5 μA, and a power density of 57 mW/cm 2 .
There are also studies on 3D-structured carbon nanomaterial-based TENGs that enrich the application of carbon nanomaterials. A rubber/carbon nanofiber-composited 3D structure 168 was produced and applied as a TENG operating in single electrode mode with an open-circuit voltage of 91 V and a short-circuit current of 2.87 μA. By using the templating method, patterned 3D carbon electrodes 169 have been made with the aim of increasing the contact area with the counter triboelectric layers. Compared to metallic electrodes, 3D carbon electrodes have shown more robustness to humidity and mechanical frictions.

CONCLUSIONS AND PERSPECTIVES
The applications of TENGs include energy harvesting, a variety of sensor types, biomedical applications, the Internet of Things (IoT), and human−computer interactions. Different applications have different requirements for the materials and production technologies. 170,171 For example, a wearable TENG requires flexible materials. Many of the requirements could not be fulfilled by macroscale materials but by nanosized materials. Therefore, different types of nanomaterials have been studied to determine their applications in TENGs. The utilization of inorganic nanomaterials in TENGs has been proven very successful. The benefits of inorganic nanomaterials include the following: (1) Increase the contact area of the triboelectric surfaces.
The high surface area to volume ratio significantly increases the contact area. (2) Tune the dielectric properties of the composited triboelectric materials. Inorganic nanomaterials include many types of materials with dielectric properties, such as dielectric constants, spread in a very large range. By selecting the type, size, and content of the inorganic nanomaterials, the dielectric properties of the triboelectric materials could be tuned as expected. (3) Enhance the charge transportation. The interface between a triboelectric material and an electrode may have a barrier due to the misalignment of the band structure. The presence of the nanomaterials could reduce the barrier so that the charges could be transported more conveniently. (4) Tune the optical properties of the TENGs. Networks of nanowires such as silver and CNT nanowires can be used to fabricate transparent electrodes for use in TENGs. (5) Mechanical properties: flexibility and stretchability. The network structures of the nanowires enabled the triboelectric films to be flexible and stretchable but retained their electrical properties. (6) Hybrid nanogenerators. Some inorganic nanomaterials, such as ferroelectric nanomaterials, have piezoelectric properties that allow the fabrication of hybrid nanogenerators.
Although many works reviewed here have explored the advances of inorganic nanomaterials in TENGs, there are still some issues that need to be addressed in the future: (1) Quantitative understanding of the relationship between the performances of the TENGs and the sizes of the inorganic nanomaterials. Theoretical models need to be developed and optimized in the future. (2) Theoretical models of how the size and percentage of inorganic nanomaterials change the dielectric properties of the composites. (3) Applications of new types of inorganic nanomaterials 172 in TENGs. Only a small portion of inorganic nanomaterials have been studied in the last several years, requiring more effort. In summary, we have reviewed the recent advances of inorganic nanomaterials in triboelectric nanogenerators based on the roles, types, and characteristics of nanomaterials. The advantages of inorganic nanomaterials and the performance of TENGs promoted by inorganic nanomaterials have been reviewed. Some prospective studies have been suggested that could inspire future research in the area. This Review provides an overview of how and why inorganic nanomaterials are utilized in TENGs, which offers guidance for future studies.