1 Introduction

Energy scavenging continues to develop in every sector as a means to supply power for applications ranging from the home to industry. The demand for energy resources is increasing rapidly as scientists continue to develop alternatives to batteries for low-power electronics and sensors to larger-scale power in an attempt to ensure a stable energy supply in the future. A unique door has opened up for scientists with the discovery of graphene through exfoliating graphite [1], and the investigation of 2D nanomaterials has become a develo** field. Researchers have been working to explore the future demand for energy in an effective and environment-friendly way. 2D nanomaterials are referred to as the materials having an ultra-thin layered crystalline structure with covalent bonding in the intra-layer and Van der Waals bonding in the inter-layer [2]. The large surface area-to-volume ratio and its atomically thin nature lead to 2D nanomaterials exhibiting dramatically different behaviors to its bulk state with novel characteristics due to quantum confinement [3].

In ultra-thin nanomaterials, such as graphene, with a defect-free crystal structure, its electrons need to pass a shorter path; this leads to a very high charge carrier mobility and an ultra-high electrical conductivity [4]. These properties make them an attractive option in fabricating a range of exciting nanoelectronic devices. The ultra-high transverse area and ultra-thin structure of two-dimensional nanomaterials provide the maximum amount of surface atoms, which creates an improved environment for specific applications such as photocatalysis, photovoltaics, and supercapacitors, where an ultra-high surface area represents a significant performance parameter for device effectiveness. As a result of these novel and unusual properties, 2D nanomaterials are being investigated for energy conversion and storage [5] with excellent performance and potential. Scientists are now moving to exploit these intriguing nanomaterials for real-life applications. For energy scavenging applications, 2D nanomaterials are being used in (i) solar energy scavenging such as photovoltaic cells [6], perovskites [7], photocatalysis [8]; (ii) mechanical energy scavenging such as triboelectric [9] and piezoelectric devices [10]; (iii) thermal energy scavenging such as thermoelectric [11] and pyroelectric systems [12]; and (iv) chemical energy scavenging such as osmotic power generation [13]. 2D nanomaterial-based nanogenerators are potentially an attractive option for large-scale power generation from sustainable sources such as wind power, ocean waves, and rolling wheels [14]. In addition, the generated power from these nanogenerators can supply power for the operation of portable electronic devices [15] that can facilitate multi-functionally in real-life applications such as body motion sensing and code transmission by a single generator which scavenges energy from the human body [16].

A number of excellent reviews have been published on 2D nanomaterials based on device fabrication for power conversion, storage technologies, and sustainable energy applications [17,18,19]. However, these existing reviews are limited to specific materials, mechanisms, and application areas. In this review, the area of 2D nanomaterials for energy scavenging devices for all the available energy resources is summarized with their performance analysis to provide a broad insight into this rapidly develo** area. In addition, the devices that are being fabricated using 2D nanomaterials for self-powered devices are discussed. This includes a range of sensors that aim to exploit 2D nanomaterials. In the first section of this review, an overview of current nanomaterials is discussed with their unique and essential properties, which are important for various energy scavenging techniques and suitable fabrication processes. In the second section, the range of energy scavenging mechanisms and device performance parameters will be explained in detail with their structure and effectiveness in replacing conventional energy sources. In addition, self-charging supercapacitors will be summarized based on 2D nanomaterials as a storage substitute. The third section describes specific real-life applications where 2D nanomaterial-based devices are being used as an alternative source of power for their operation in sensors both for industrial, health, and environmental monitoring purposes. The conclusions will provide insights into future directions for these new materials in energy scavenging research.

2 2D Nanomaterials for Energy Scavenging Devices

Among the range of 2D nanomaterials, graphene is the most investigated material for energy scavenging and device fabrication due to its excellent charge carrier mobility and low-cost production [20]. It is atomically thin with sp2 hybridization of its carbon atoms; Fig. 1a shows an image of a single-layer graphene sheet. Due to this configuration, graphene is highly transparent that can be an alternative to the transparent conductive oxide (TCO) in organic solar cells between the glass surface and organic materials [21]. It can also enhance electron transport and the dissociation of excitons between the heterojunctions of solar cells. A smooth and planar surface provides a low contact resistance that can reduce the potential drop and prevent leakage currents at the interfaces of p-type and n-type material solar cells [22]. Due to the high scattering of phonons in graphene, the thermal conductivity remains very high at room temperature, which is an important requirement for effective thermal energy scavenging [23]. The heterostructure of graphene enables it to metalize the 1D edges of the 2D layer graphene. Its surface geometry allows the complete separation of contacts, which leads to high-performance electronic devices [24]. The outstanding conductance in terms of electrical and thermal properties, with an ultra-wide surface area, leads graphene to be a material of interest for dye-sensitized solar and fuel cells [25]. Graphene-related materials, such as graphene oxide (GO), have a very high Young’s modulus and an excellent dielectric constant, enabling it to be a good option for the piezoelectric energy conversion process to utilize mechanical energies [26]. For a greater electronic device output, graphene-enabled or directed nanomaterials have been investigated for large-scale device integration to understand nanoelectronic industry-scale applications [27]. Considering the low cost, high lifetime, and modification capability of the properties, the future of graphene is mesmerizing both in terms of energy scavenging and in terms of storage technologies [25]. To meet commercial demands from a wide variety of patents and applications, graphene is to be produced at a scale of thousands of tons every year [28].

Fig. 1
figure 1

Introduction of some nanomaterials and some modifications. a A single-layer graphene sheet. b Transition metal dichalcogenide. c Heterostructure of 2D perovskite. d Ti3C2Tx MXene. e Diagram of 2D nanomaterial structure and type of vacancy(s). f CBM and VBM’s charge density in lateral heterostructure of MoS2/WS2. g Characterization techniques to detect abnormalities using optical methods. Figures reprinted with permission from: a Ref. [23], © 2014 American Institute of Physics; b Ref. [30] © 2016 Elsevier; c Ref. [37], © 2020 Nature; d Ref. [38], © 2019 Wiley; e Ref. [54], © 2020 Wiley; f Ref. [56], © 2015 American Chemical Society; g Ref. [28], © 2019 Nature

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2D transition metal dichalcogenides (TMDs) nanomaterials are similar to graphene. These nanomaterials have been investigated broadly in energy scavenging and sensor applications. Metal dichalcogenides (MXs): where M denotes the transitional metals and Xs represents S, Se, or Te, and it can be in both mono-layer and multilayer form [29]. During the exfoliation of multilayers, due to the interaction of s-Pz orbitals, the bandgaps of single-layer become wider, which enables a transformation of indirect bandgap into direct bandgap, thereby providing excellent photoluminescence. The wide bandgap is an important property to avoid current leakage in piezoelectric materials and photoluminescence provides a better solar energy harvesting mechanism [29]. A diagram of the single-layer crystal structure of tetragonal MoS2 is shown in Fig. 1b. The ultrathin two-dimensional nanomaterials based on MoS2 show highly favorable properties, such as an excellent density of states of electrons and holes with a very high Seebeck coefficient [30]. According to its electrical properties, its members are insulators such as HfS2, semiconductors such as MoS2, semi-metals such as WTe2, and true metal such as NbS2, thereby covering a great diversity of the electrical spectrum in response to the oxidation level along with the adjustments of transition metal atom [31]. TMD-based devices are also capable of high-frequency operations due to their significant bandgap, which is important for communication and data processing applications [32]. Two-dimensional TMDs offer many tunable properties functionalization with various polymers, thereby making them excellent candidates for flexible and transparent electronics applications with improved mechanical efficiency [33].

2D perovskite nanomaterials have elevated light-harvesting properties. Due to these properties, these materials are being extensively and increasingly investigated in solar energy scavenging applications. For stabilizing perovskites, both formamidinium (FA)- and phenylformamidinium (PFA)-originated cations are working as a substitute of surface patches. While formamidinium cations are better due to their attenuated band gaps and stability, the mixed cation 2D perovskites show excellent performance both in terms of device improvement and in terms of thermal stability. This performance is achieved as a result of the enhancement of surface flaws and avoiding the rearrangement of the hole transfer layer and perovskites layer at adjacent surface edges [34]. The nonlinear scattering and photocarrier recombination are comparatively reduced in ultrathin 2D perovskites, which is favorable for the investigation of intrinsic optical properties [35]. The triangle and hexagonal shapes of the high-quality 2D perovskite crystals facilitate tunable photoluminescence enabling wider applications in optoelectronics [36]. Charge-transfer excitons (CTEs) can be generated from the heterostructure of 2D perovskites, as shown in Fig. 1c, using a combination of PEA2PbI4: PEA2SnI4 which operates as a host and guest, respectively. This process leads to tuning of the properties of the perovskite that can provide opportunities to develop advanced optoelectronic devices [37].

Recently, 2D MXene nanomaterials are receiving significant interest as a result of the strength of their fascinating electrical, plasmonic, optical, and thermoelectric characteristics. These properties, in addition to their extended effective surface, have made them attractive materials for electronic, electrochemical, and catalytic applications for energy scavenging. MXenes are a group of 2D nanomaterials that consist of transition metals carbides, nitrides, and carbonitrides. Figure 1d shows Ti3C2Tx MXene synthesized from a MAX phase, in which M refers to a transition metal, A symbolizes the main element cluster, and X refers to carbon (C) or nitrogen (N) [38]. 2D thin films synthesized from the deposition of MXene dispersion in a layer-by-layer way show high optical transmittance (more than 80%), which is an important requirement for photoactive materials for the fabrication of displays and PV (photovoltaic) cells [39]. Due to the conductivity of carbide core and accumulation of dipoles on the surface of MXene fillers and polymer matrix, MXenes exhibit good electrical conductivity and dielectric properties under the applied electric field [40]. The outstanding surface chemistry makes MXenes and its nanocomposites an active field of study for electrochemical energy scavenging, such as electrolytic water splitting [41]. In addition, the increased surface charge density is of interest for extending the nanofluidic energy conversion such as osmotic power generation by playing an important role in modulation of the ion diffusion procedure between two liquids having a different level of salinity [13]. Existing published research from the web of science databases shows that MXenes have already attracted significant attention from the scientific community for energy scavenging technologies.

2D metal–organic frameworks (MOFs) have a structure of crystal-like materials, where the metallic node is bound with a variety of organic ligands. Due to their porous structure and surface chemistry, MOFs have excellent performance in electrolytic techniques such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CRR), oxygen reduction reaction (ORR), and urea oxidation reaction (UOR) [42]. The functionalization of MOFs with quantum dots (QDs) can extend the light-harvesting behavior, offering enhanced coverage of the light spectrum with adsorption of the visible region by QD-MOF hybridized fabricated devices [43]. Due to their tunable morphological structure, porous surface, and favorable electrochemical properties, 2D MOFs can be a convenient choice of material for energy scavenging, catalysis, and sensor applications with the opportunity of efficient device fabrication [44]. Due to the mixing of MOFs with a polymer matrix, its deposition on the support surface and applied mechanical compression exhibits limitations such as blockage of pores, destruction of the framework, or compound aggregation hampering device performance [45].

In addition to the mentioned 2D nanomaterials [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45], many others are being investigated for energy scavenging, and the number continues to increase. 2D nanomaterials synthesized from metal oxide and layered hydroxide materials are favorable for energy scavenging applications. The materials include PV (photovoltaic) cells, photocatalysis, piezoelectric power generation, and fuel cells [46]. ZnO nanoleaves have been synthesized through simple chemical vapor deposition (CVD) that shows optimistic electromechanical properties for the application in NEMS (nanoelectromechanical system) devices [47]. Some metallic non-layered 2D nanomaterials, such as several pure metals and alloys and layered 2D nanomaterials, include germanene, silicene, stanene, antimonene, which are being investigated for solar cells, catalytic, surface plasmon resonance process for sensing and energy conversion applications [48]. By stacking 2D unilamellar nanosheets from bulk materials, some 2D superlattices are being investigated since they provide very attractive electrical properties for energy scavenging such as water-splitting and energy storage technologies [53]. In some instances, different types of vacancies, as shown in Fig. 1e, can be formed during the synthesis of the 2D nanomaterials under specific harsh conditions that can be turned into the localization of electrons, lattice distortion, electronic compensation, and disbanding and reconstructing chemical bonds. These types of adjustments provide unique physical and chemical properties, which opens new opportunities for innovative research in photocatalytic and electrolytic energy scavenging applications [54]. In the context of exploiting cation and anion vacancies, distortion, and disorder on 2D nanomaterials, the process of surface defect engineering can significantly increase the photocatalytic properties by enhancing both charge separation and light adsorption [64], perovskites solar cells [65, 66], polymer cells [67], photocatalysis, and water-splitting systems [72, 2e represents the process of analyzing the cathode luminescence by applying an electron beam to find the undoped region with the level of excitonic emission [64]. The overall efficiency of the solar cell doped with As, without any anti-reflection coating, is 20.8%, which is shown in Fig. 2f along with the current density–voltage characteristics, and Fig. 2g delineates the related external quantum efficiency (EQE) [64].

Fig. 2
figure 2

Mechanism of solar energy scavenging through solar cells. a Diagram of nanodevice based on heterojunction of Graphene/n-GaN. b Schematic of solar cell fabricated through one- and two-step method. c Two-dimensional perovskite at grain boundary of device d Tracking of highest powerpoint of the device with no encapsulation under normal condition. e Sample excitation with electron beam. f Current density–voltage curve for the device with As do** without any Cu addition. g Relative EQE values with wavelengths. Figures reprinted with permission from: a Ref. [62], © 2017 American Institute of Physics; b Ref. [63], © 2016 Elsevier; c, d Ref. [66], © 2018 Nature; e, f, g Ref. [64], © 2019 Nature

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2D perovskites materials are becoming the epicenter of solar energy scavenging research as they are more favorable for many aspects, for instance the primary light-absorbing agent, passivation layer, cover coating, an anti-reflection coating, and more [65]. 2D nanomaterials have less volatility and have more hydrophobic organic ions than 3D materials, which leads to greater stability in thermal and chemical environments. Figure 2c provides a schematic of a solar device, where 2D formamidinium lead iodide (FAPbI3) is formed on the grain boundary of 3D perovskite. This structure provides better stability, as shown in Fig. 2d, which describes the photon conversion efficiency (PCE) measured by an encapsulated control and target device under continuous light [66]. The examination of Fig. 2d indicates that the target device shows less degradation of the maximum power point, which is only 20% rather than 68% for the case of the control device for 450 min. Thus, the efficiency of the perovskites has been extended to 19.77%. Figure 3a represents a photovoltaic device in which an atomically thin WOx layer is applied between the WSe2-MoS2 heterojunction to increase the photo-responsivity of the device [67]. The device characteristics are illustrated in Fig. 3b, wherein the output voltage, Voc (open-circuit voltage), is elevated to 0.48 V from 0.23 V. More importantly, the overall power conversion efficiency is enhanced dramatically to 5% from only 0.7%, as shown in Fig. 3c [67].

Fig. 3
figure 3

Solar energy harvesting mechanism and performances of the 2D nanomaterial-based scavenging devices. a Schematic of device with WOx layer. b Photovoltaic properties without (black) and with (red) WOx being excited by laser ray. c Electrical power plot with extended photoconversion efficiency to 5%. d Schematic of the fabrication of perovskite solar cell having an electron donor of rGO/PANI-Ru, e PSC’s energy-generating curve. f Electrical characteristics of device. g Current density voltage characteristics under both M2P and OMeTAD condition. h Plot of current density and voltage with spiro-OMeTAD along with M2P condition. i Stable performance of device under inert surroundings. j NADH regeneration. k Formic acid generation from CO2. l Photocurrent characteristics by applying simulated 1 sun illumination. Figures reprinted with permission from: a, b, c Ref. [67], © 2020 American Chemical Society; d, e, f Ref. [68], © 2017 Nature; g, h, i Ref. [69], © 2020 Wiley; j, k, l Ref. [77], © 2018 Nature. (Color figure online)

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A polymer solar cell based on pyridyl benzimidazole incorporated with Ru complex imprinted polyaniline assembly (PANI-Ru) is investigated using inoculating reduced graphene oxide (rGO). The device performance increases significantly because of the excellent electrical properties of rGO [68]. The designed polymer solar cell is shown in Fig. 3d, where the rGO/PANI-Ru: PCBM works as a photosensitive coating. The energy levels in the device are indicated in Fig. 3e, and the resulting value of HOMO and LUMO is − 4.8 and − 3.67 eV, respectively. In addition, Fig. 3f displays the current density–voltage (J–V) properties, where it is observed that there is no current density (Jsc) in the device under dark conditions [68]. The application of multi-functional 2D perovskites (M2P) with the hole transporting material (HTM) along with 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) light absorber increases both the effectiveness and the stability since M2P triggers faster hole transport and provides a passivation effect due to its randomly oriented crystal structure [69]. It is clear from Fig. 3g, h that the fabricated device is dominating in terms of performance for all the criteria such as open-circuit voltage (Voc), current density (Jsc), fill factor (FF), and photoconversion efficiency (PCE). This happens when it undergoes M2P deposition at both stages of perovskite solar cells in accordance with M2P and spiro-OMeTAD conditions. In addition, in the stage of spiro-OMeTAD on the basis of M2P requirements under the reverse and forward scan of the device, the stability of the perovskite solar cell improves by maintaining a maximum power point tracking to 80%, compared to 64%, in the case of applying M2P which is shown in Fig. 3i [69]. The highest efficiency from a perovskite solar cell to date is 25.2% [70]. Researchers are applying new materials and technologies such as applying additive engineering during film formation, introducing modifiers to elevate electron–hole transport, and changing the surface chemistry for a more efficient and stable perovskite solar cell [71].

In addition to PVs and PSCs, water splitting is a major concern for harvesting solar energy using 2D nanomaterials, in particular materials such as 2D phosphorene [72], 2D polymers [4m. In addition, for illuminating an 8-bit LCD, as illustrated in Fig. 4n the M-TENG has shown results in charging storage capacitors, as described in Fig. 4o, which indicates that it has potential as an excellent mechanical energy scavenging technology [83].

Rather than using dielectric materials, the gliding of a platinum-covered silicon AFM (atomic force microscope) tip as the external voltage source across multilayers of molybdenum disulfide (MoS2) leads to the creation of a TENG. In this case, triboelectricity is generated and MoS2 plays an important role in charge transfer [84]. Figure 5a shows three different triboelectric current responses of MoS2 grains where Igrain2 > Igrain3 > Igrain1, and Fig. 5b illustrates the cross-sectional current of Fig. 5a. Figure 5c represents the voltage-current behavior of grain 2 and the magnified measurement of voltage-current spectra of grain 2 from points 10–13 are shown in the inset. The reason for the variation in values is due to the variation of surface charge in different grains of MoS2 since the bottom electrode tends to be oxidized. The external voltage supplied by the AFM tip helps the nanogenerator to increase the generated triboelectric current density, which can be used in many large-scale mechanical energy harvesting purposes by the integration of many such devices [84]. A core–shell-like structure consisting of metal Ni as core and metalene (antimonene nanodendrite) shell-based nanogenerator is fabricated that can produce triboelectricity and provide electrochemical energy storage [85]. This dual functionality offers an optimistic possibility for the polymers for energy harvesting and storage applications. Figure 5d shows a schematic of the fabricated nanogenerator, which is fabricated with metalene by electrochemical deposition technique (EDT). The fabricated device can act as a capacitor with a specific capacity of 1618.41 mAh. The generated rectified output voltage under the mechanical force in the range from 0.2 to 20 N is shown in Fig. 5e where a maximum voltage is found at 20 N [85].

Fig. 5
figure 5

Schematics of structure and output performances of triboelectric nanogenerators. a Image of the adjacent MoS2 crystal divided into three grains from atomic force microscope and their current distribution in the three regions. b Cross-sectional current lines in the grains. c Spectrum of current–voltage characteristics of the grains 2. d Schematic of the triboelectric nanogenerator. e Rectified output voltage from the device. f Schematic of triboelectric nanogenerator with a single electrode based on reduced graphene oxide nanoribbons/ polyvinylidene fluoride. g Generated charges of Al foil, which decreases with the increasing gap between the film and foil. h Stable operation of triboelectric nanogenerator for 500 cycles. i Comparison of triboelectric generator voltage while fabricated by porous and solid nanocomposites. j Output voltage, current, and power. k Performance in powering a lithium coin cell. Figures reprinted with permission from: a, b, c Ref. [84], © 2018 Nature; d, e Ref. [85], © 2020, Elsevier; g, h, i Ref. [71], © 2016 Nature; j, k, l Ref. [72], © 2018 Nature

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Furthermore, thin-film reduced graphene oxide nanoribbons (rGONRs) incorporated within a polyvinylidene fluoride (PVDF) polymer have been applied in fabricating an arch-shaped TENG with a single electrode, as illustrated in Fig. 5f. The rGONRs exhibit charge negativity due to the larger diameter ratio, extreme edges, and electronegative oxygen-containing functional groups. This type of architecture that combines the surface roughness incorporated with the generating functional groups can improve the charge storage ability of the rGONRs/PVDF film [86]. The charge production on aluminum (Al) foil decreases with an increase in the gap between the rGONRs/PVDF film and aluminum (Al) foil, which is presented by Fig. 5g. This fabricated TENG has led to highly stable behavior after a stability test for 500 cycles, as illustrated in Fig. 5h, and this reliable behavior is important for applications in portable electronic devices [86]. A rapid fabrication technique with porous BaTiO3 or a polydimethylsiloxane (PDMS) nanocomposite-based TENG has been introduced, where the fabrication process is made more simple and effective [87]. A porous BaTiO3/PDMS-based nanogenerator was fabricated by microwave (MW) irradiation that can increase the output voltage by 83% and 130% for a positive and negative voltage, respectively. The resulting voltage from the TENG fabricated with a solid and porous nanocomposite is presented in Fig. 5i, which confirms the superiority of the porous nanocomposite. The voltage, current, and maximum power density at 100 MΩ load resistance are depicted in Fig. 5j. The fabricated device is capable of charging capacitors, rechargeable coin cells, and several hundred light-emitting diodes (LEDs). Figure 5k shows the capacitor charging efficiency and the TENG can charge 3 V lithium cells up to 1.6 V in 2500 s [87].

With additional investigation, such as utilizing a graphene/copper heterojunctions for electron transfer [88] and large-area graphene synthesis at the exterior side of the copper with the help of chemical vapor deposition (CVD) process [89], more stable and effective TENG fabrication processes are being developed. The functional nature of 2D nanomaterials such as graphene can avoid the oxidation of the Cu (copper) electrode at room temperature, which leads to higher device efficiency [88]. A chemical etching-free, residual polymer-free, and environmentally friendly method is used to grow graphene on copper by the CVD (chemical vapor deposition) process on the surface of transparent EVA (ethylene vinyl acetate) or PET (polyethylene terephthalate) [89]. The upcoming research on the synthesis and fabrication methods will develop this sector for the large-scale production of 2D nanomaterials, such as graphene, in an effective way. To concentrate on ambient energy sources and achieve a more efficient operation by the appropriate utilization of mechanical energy from both sides of the electrode, a robust thin film-based TENG array for scavenging bidirectional wind energy has been developed for self-powered devices [90]. The newly fabricated nanogenerator opens up a new way of thinking to utilize all available surrounding energies more effectively by develo** a more suitable device architectural design. Achieving an overall improvement is a continual process for exploiting 2D nanomaterials for triboelectricity generation. The tunable properties and flexibilities of 2D nanomaterials are making TENG more efficient in mechanical energy conversion from small-scale vibrational motion to heavy sea waves [5]. There are some demerits in terms of material synthesis complexity, slow growth rates, and high costs. In addition, the storage and fabrication costs are a barrier in triboelectric nanogenerator research for the scientific community [85, 86]. The application areas of these fabricated devices are highly versatile and cut across many fields, including device structure, functional materials, fabrication process, device outputs, reliability. Applications of the reported work are summarized in Table 2.

Table 2 Comparative summary of reported work on triboelectric energy harvesting including device structure, functional materials, fabrication techniques, device outputs, reliability, and applications
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3.3 Piezoelectric Power Generation

In a similar way to triboelectric power generation, the piezoelectric effect is a method where mechanical energy can be converted to electric energy. 2D nanomaterials are now being extensively used for both direct current (DC) and alternating current (AC) piezoelectric power generation. 2D nanomaterials exhibit excellent piezoelectric properties in energy scavenging, while they do not exhibit such properties in their bulk state [91]. In general, non-centrosymmetricity is a required property for a material to be piezoelectric because the electrical polarization is associated with it during the coupling of mechanical and electrical behaviors [92]. The device structure with fabrication methods and functional materials, outputs, and applications of the reported piezoelectric power generating devices based on 2D nanomaterials is summarized in Table 3.

Table 3 Comparative summary of reported work on piezoelectric energy harvesting including device structure, functional materials, fabrication techniques, device outputs, and applications
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A piezoelectric nanogenerator (PENG) has been fabricated using a 2D ZnO nanosheet, with its piezoelectric DC power generation process shown in Fig. 6a [93]. In this case, a gold (Au)-coated polyethersulphone (PES) has been used for the upper electrode and aluminum as the lower electrode. A 2D ZnO nanosheet/anionic nano-clay-layered heterojunction is synthesized amongst the two electrodes. When an external force acts on these nanosheets, it creates a positive potential on the outside of the expanded nanosheets, generating the negative electrical potential on the inside of the shrunk nanosheets, as shown in Fig. 6a. During the holding time, the positive potential of the nanosheets starts to decrease by attracting electrons, which are generated from the upper Au electrode connected to the compressed nanosheets until the potential becomes neutral when the force is released. As a result, the device generates only a DC pulse in every pushing and releasing period. The resulting current density and voltage depend on the level of the applied pushing force, which is represented in Fig. 6d where it can be found that a greater degree of force leads to a higher output. However, it is also necessary to consider the mechanical stability during operation of the fabricated PENG, and therefore, it is subjected to mechanical testing for comparison of the load–displacement curve between nanorods and nanosheets, as depicted in Fig. 6e [93]. Using a similar mechanism, a ZnO nanosheet-based PENG is represented in Fig. 6g [94]. The density of the nanosheets decreases the aggregation of freestanding of ZnO residues, which leads to an improved output performance. In this case, the voltage is increased up to 1.15 V, which is shown in Fig. 6h. It also shows an excellent power density of 600 nW cm−2 at 10 kΩ resistance, as illustrated in Fig. 6i. The fabricated device also provides a persistent output without any degradation in the output for up to 4000 cycles, indicating the durability of the device [94].

Fig. 6
figure 6

Device structure, working mechanism, and performance of piezoelectric energy generation. a A two-dimensional ZnO-based nanogenerator fabricated nano-clay-layered heterojunction and its working principle for energy scavenging. b A typical flexible nanogenerator based on single-layered MoS2 nanoflake, including device image in inset. c Operating mechanism of the piezoelectric. d Resulting voltage and current according to the external vertical. e Plot of load–displacement of nanorods and nanosheets derived from ZnO. f Relation between the piezoelectric response and applied external strain. g Three-dimensional structure of ZnO nanosheet-based piezoelectric nanogenerator. h Effect of applied force on voltage and current. i Piezoelectric current along with power densities of the device with an external circuit resistance. Figures reprinted with permission from: a, d, e Ref. [93], © 2013 Nature; b, c, f Ref. [94], © 2020 Elsevier; g, h, i Ref. [95], © 2014 Nature

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A single atomically thin-layered MoS2 has been investigated for piezoelectric harvesting and fabricated as a flexible PENG. Figure 6b displays a diagram of the fabricated device, and the working mechanism is represented by Fig. 6c [95]. When the PENG is stretched by applying a mechanical strain, both a positive and negative induction of the piezoelectric polarization charges occurs at the edges of the MoS2 adjacent to the electrodes, as shown in Fig. 6c. This results in a current flow through the external circuits. Similarly, during the relaxation period, electrons flow along with the load circuit in an alternating direction, as indicated by an arrow in Fig. 6c. The resulting voltage and current from the device are directly related to the maximum value of the strain that is applied, which is presented in Fig. 6f, and the fabricated PENG can easily sense the level of strain applied to it [95].

A new organic–inorganic ferroelectric perovskite material, known as Me3NCH2ClMnCl3 (TMCM-MnCl3), has been discovered with elevated piezoelectric properties that can be further modified by replacing with a new element or new molecular structure according to the application, which is attractive for nanosystems and self-powered devices [96]. This opens the door to solve the issues of rigidity and toxicity of piezoelectric materials to make them soft, lightweight, and biofriendly for applications in health monitoring sensors. For practical applications, piezoelectric nanogenerators are of interest when fabricated with nanomaterials that make them highly sensitive and lightweight. A stretchable graphene thin film-based yarn sensor having adjustable piezoelectric properties has been fabricated by combining a graphene oxide (GO) layer covering the polyester (PE) wound spandex yarns, which can provide any electrical signal from the small movement of human motion [97]. Here, the electron transport behavior of the graphene film is changed by varying the stretch level, which leads to a better sensitivity of the sensor. Hybrid power generating systems, such as a PENG coupled with a TENG, has been introduced recently for achieving better efficiency and flexibility of multi-level scavenging capability [98]. The fabricated hybrid nanogenerator can generate piezoelectrical power from the self-oscillating response of the impact-induced friction and triboelectric power is due to the effect of impact-induced friction directly. Effective piezoelectric generation depends on the behavior of the materials, where non-symmetricity is one of the most important. In this case, 2D nanomaterials such as single-layered, atomically thinned materials are capable of exhibiting non-symmetric properties that are not possible in the bulk state. Other properties such as flexibility, mechanical stability, conductivity can also be further improved by different methods. In terms of demerits, the piezoelectric materials can be toxic and fragile, which can be a challenge for environmental and health monitoring applications [96]. In addition, a vulnerable structure is not ideal for mechanical operations during piezoelectric generation.

3.4 Thermoelectric Power Generation

A convenient way of utilizing ambient and low-grade waste heat from industrial processes is to generate electrical energy by employing thermoelectric (TE) devices through a process termed thermoelectric power generation. Due to their excellent thermal conductivity, 2D nanomaterials are widely used for the fabrication of TE devices. However, the thermoelectric device performance is closely dependent in accordance with the thermoelectric figure of merit ZT where ZT = S2σ/ktotal; S denotes the Seebeck coefficient, σ symbolizes the electric conductivity, and ktotal refers to the total thermal conductivity [99]. In thermal energy scavenging, there can be four types of TE devices, which include the uncoupled thermal, thermoacoustic coupled, thermoelectric coupled, and the thermal and optical coupled devices; these are based on different materials, techniques for their synthesis, and application requirements [100]. A schematic of the mechanism of a thermoelectric generator is shown in Fig. 7a, where p-type and n-type semiconducting materials create a junction between them. For thermoelectric power generation in thermocouples, charge carriers are transported through the junction due to the temperature difference, ΔT (the Seebeck effect), which is the basic operational mode of thermoelectric nanogenerators [101]. High electrical conductivity and Seebeck coefficient can extensively increase the ZT, which is a crucial issue for obtaining higher outputs during the thermoelectric process.

Fig. 7
figure 7

Thermoelectric device structure, the working principle, and output performances of thermoelectric devices. a Mechanism of thermoelectric power generation due to Seebeck effect. b Device architecture for thermoelectric measurement. c Curve of Seebeck coefficient versus temperature. d Peak value of Seebeck coefficient according to temperature. e Thermoelectric nanogenerator based on p–n junction. f WS2 planar film as n-leg, while depositing in the fabrication process. g Current density–voltage characteristics of MoS2. h Current density–voltage characteristics of WS2. i Power factor of the MoS2 device. j Roll-to-roll arrangement for deposition on substrate. k Output voltage vs. temperature plots. l Plot of resistance and output power with changing temperature. Figures reprinted with permission from: a Ref. [101], © 2017, Royal Society of Chemistry; b, c, d Ref. [102], © 2019 Nature; e, f, g, h, i Ref. [136]. Figure 11g is an image of the photo-rechargeable hybrid capacitor with an applied bending force. Figure 11h shows the practical use in sensing human motion attached to clothing. Figure 11i shows the charging and discharging time of this capacitor, which takes 20 min to charge to 3 V and the same time for discharging, thereby indicating a stable energy harvesting operation. At different current densities, the capacitor takes a different time to fully discharge, which is shown in Fig. 11j. The overall efficiency and energy storage efficiency of the designed capacitor are presented in Fig. 11k, which indicates that the resulted efficiencies are stable over the cycles. The photovoltaic response of the fabricated device provides a high electrical potential of 3.95 V, along with a photoconversion efficiency of 10.2% that leads to the dual functioning of light-harvesting and energy storage. A comparison of this device with different integrated systems based on an overall efficiency and voltage is represented in Fig. 11l, which indicates the superior efficiency and power delivering capability of the device. The fabricated wearable strain sensor can offer accurate and uninterrupted data delivery from physiological signals without any extra power sources, which leads interest in its use for develo** smart sensor systems. Another flexible strain sensor has been developed by integrating a graphene/eco flex film with a meandering zinc wire that can provide an electrical potential and a current of 0.83 V and 75 µA, respectively [137]. The responsivity of the sensor is rapid without any need for an external power source, which can sense motion of a knee joint.

A graphene-based stretchable/wearable touch sensor powered by an inbuilt triboelectric nanogenerator (TENG) was investigated for perceiving its correlation with human skin motion, as illustrated in Fig. 12a. The bidirectional upper and lower electrodes with a dielectric layer in the auxetic form are located between them [138]. Figure 12b displays the physical shape of the device and after applying a tensile strain along x- and y-direction of the device at 13.7% and 8.8%, respectively, under a stretching interval of 1%. The resulting related resistance, along with the voltage generated, from the device with changing stretchability, is shown in Figs. 12c, d, respectively. By analyzing both parameters, it is clear that the fabricated TENG exhibits promising stability, with only a 1% deviation. This touch sensor can realize a simple touch, along with the velocity of touch sliding, and can show any character by using an actual trajectory mode in real time that can lead to develo** smart input devices for communication systems.

Fig. 12
figure 12

Nanogenerator-based sensors based on 2D nanomaterials. a Microscopic diagram of the fabricated device along with graphene electrodes. b Stretching behavior and extension lengths in both directions of the triboelectric nanogenerator sensor. c Relation between stretch level and resistance. d Variation of output voltage with applied stretch. e Chip photograph. f Microscopic image of the MoS2 sensor. g Effect of concentration of Cd2+ ions on sensor response. h Transmission electron microscopic photo of the selected area electron diffraction patterns and the orientation of planes. i Selectivity of the device. j Reproducibility of the fabricated sensor in ethanol exposure up to 10 cycles. Figures reprinted with permission from: a, b, c, d Ref. [137], © 2019 Elsevier; e, f, g Ref. [138], © 2019 Elsevier; h, i, j Ref. [139], © 2019 American Chemical Society

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A flexible MoS2-incorporated chemical sensor array that uses the functionalization of an ionophore has been demonstrated, which enables 29% elevated efficiency for label-free ion sensing [139]. Figure 12e shows a photograph of the chip, and Fig. 12f is an optical micrograph of the designed sensor on the PET substrate. The sensing performance, according to the changing Cd+ ion concentration, is illustrated in Fig. 12g, which indicates that an increased current is generated due to the increased concentration as it changes the resistivity of the MoS2 channel. The fabricated sensor device can detect Hg2+ and Na+ in samples, such as human sweat, enabling the opportunity for health or environmental monitoring sensor systems that can operate without any external power supply.

A new technique is implemented for fabricating a flexible and wearable ethanol gas sensor fabricated with titanium oxide (TiO2) grafted on 2D titanium carbide (TiC) nanosheets [140]. With a small noise-to-signal ratio, this device enables extended selectivity and also has the opportunity of tuning the sensitivity and selectivity within an extremely short period of response time. Figure 12h shows a TEM image of titanium carbide nanosheets and associated SAED (selected area electron diffraction) patterns. The designed sensor is more sensitive and selective for ethanol gas compared with other gases, as illustrated in Fig. 12i. Figure 12j represents the reproducibility and response time over 10 cycles of the fabricated sensor. The fabricated device can be used as a low-cost strip sensor. A further improvement can lead to playing a major role in printed electronic and biomedical applications for develo** smart sensing technologies. A flexible pressure sensor has been developed using piezoresistive graphene/P(VDF-TrFE) heterostructure combining with an extreme responsivity of graphene and a superior deformity of the PDMS substrate, which exhibits excellent performance [141]. An additional pressure sensor has also been fabricated with an MXene–textile network structure that extends the sensitivity up to 12.095 kPa−1 at 29–40 kPa and 3.844 kPa−1, which is at a minimum 29 kPa, providing an excellent responsivity of 26 ms along with improved stability of 5600 cycles [142].

5 Conclusion and Future Aspects

From the initial discovery of 2D nanomaterials, researchers are continuing to deepen their understanding of these fascinating materials, improve their fabrication and integration into device architectures, and seeking new applications in our daily life from home to industry and from human health to electronic devices. The electrical, chemical, optical, and mechanical properties, of these intriguing materials, are capable of further modification using a variety of approaches such as vacancy engineering, functionalization, surface defect engineering, do** with anions or cations, and hybridization for tunable capabilities of 2D nanomaterials. As a result, 2D nanomaterials have become an optimistic option in fabricating wearable, flexible devices of small size for energy scavenging, and sensing applications where a small amount of power is sufficient for analyzing and processing signals. While there are specific limitations, such as low power density and output, synthesis challenges, and high fabrication costs, 2D nanomaterials can play a significant role in future research to meet the future energy demand, if their synthesis and properties are optimized effectively.

Energy scavenging through the formation of 2D nanomaterial-based nanogenerator devices is a new technology, and the development of this field is rapid compared to other research areas. The future of this field ensure that nanoenergy and nanosystems will become an important research field, aiming at the synthesis of new materials for effective device output and device stability. This field is of interest for applications related to electronics to medical technology from simple chips to an industry-level application for various self-powered sensors. The recent and major achievement that leads nanogenerator is shown in Fig. 13. While the concept of piezoelectricity is well known, the first piezoelectric nanogenerator was proposed in 2006 [143], but within a short time frame, it was successful to draw the attention of the scientific community globally. More research has been carried away to design flexible nanogenerators based on piezoelectric, triboelectric, thermoelectric, and pyroelectric effects for wearable and portable electronics for replacing conventional batteries. After the invention of the triboelectric nanogenerator (TENG) in 2012, there has been significant interest in using the approach for scavenging mechanical energies. The hybridization of mechanical, thermal, and solar energy [111] in 2013, bidirectional energy harvesting TENG, and in vivo cardiac monitoring device based on TENG [144] in 2016 opens up diverse application areas for the technology. The blue energy harvesting concept by TENG is another major milestone in 2017 for solving the energy crisis from the sea waves [146]. Most recently, a triboelectric nanogenerator based on an implantable symbiotic heart pacemaker device was fabricated in 2019, which demonstrated the capability of nanogenerators in medical sectors [145]. In 2020, a human–machine interfacing tactile sensor has been fabricated based on a triboelectric nanogenerator, which can be an excellent opportunity to develop artificial intelligence to upgrade it to the next level as nanogenerator makes the power supply network simpler to reduce the complication of the sensor devices implanted in robots [147]. Therefore, nanogenerator-based scavenging devices have attracted siginficant attention and for the fabrication of nanodevices, and the importance of 2D nanomaterials will continue to increase in the coming future.

Fig. 13
figure 13

A brief timeline profile of major achievements in the field of nanogenerator research

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In this review, several types of 2D nanomaterials with unique properties have been described with the basic structure and approaches to improving the required properties by different techniques for energy scavenging. A wide range of possible methods of energy scavenging are possible using these materials. This includes solar, mechanical, thermal, and chemical energy using 2D nanomaterial-based PV cells, perovskite solar cells, water-splitting, triboelectric nanogenerators (TENG), piezoelectric nanogenerators (PENG), thermoelectric generators, pyroelectric generators, and osmotic power generation. These approaches have been explained, along with their basic mechanism and device descriptions. In addition to performance analysis, relevant modifications and extension of the devices are also discussed for achieving extended efficiency in energy scavenging. Finally, practical 2D nanomaterial-based energy scavenging devices for sensor design, nanosystems, and medical sciences are discussed. The major advantages of these devices are the opportunity to make them self-powered, wireless signal providers, miniature in size, and compatible with the human body during their operation. In addition, hybridization of these devices can be used for multi-functional applications by reducing the complexity in nanosystems and providing networks with higher flexibility in operation.

In summary, the energy scavenging techniques based on 2D nanomaterials utilizing sustainable sources are continuing to develop. A number of new materials, after the discovery of graphene, are being synthesized and studied. An significant number of investigations are being carried out by scientists to introduce highly efficient harvesting mechanisms and fabrication processes of energy scavenging devices. The performance of devices continues to improve with new and more favorable materials with fascinating properties and structures. However, further studies and investigations are needed on (i) the improvement of the performance and stable operation of these devices; (ii) the synthesis and fabrication techniques need to be more developed for practical applications; (iii) lowering the synthesizing and fabrication cost; (iv) hybridizing more scavenging techniques with a single device; (v) further investigation on new 2D nanomaterials as they develop. There is potential for energy scavenging devices based on 2D to be a major portion of self-powered sensors, nanosystems, and human–machine interaction applications. The dream of producing larger-scale energy will be also possible with more dedicated research and technological advancement in the future, which will meet the world energy demand, while also powering low power electronics without harming the ecological balance and human health.