Carbon Nanostructures for Actuators: An Overview of Recent Developments

In recent decades, micro and nanoscale technologies have become cutting-edge frontiers in material science and device developments. This worldwide trend has induced further improvements in actuator production with enhanced performance. A main role has been played by nanostructured carbon-based materials, i.e., carbon nanotubes and graphene, due to their intrinsic properties and easy functionalization. Moreover, the nanoscale decoration of these materials has led to the design of doped and decorated carbon-based devices effectively used as actuators incorporating metals and metal-based structures. This review provides an overview and discussion of the overall process for producing AC actuators using nanostructured, doped, and decorated carbon materials. It highlights the differences and common aspects that make carbon materials one of the most promising resources in the field of actuators.


Introduction
In the past twenty years, the research of new high-performance materials has gained a great deal of attention for sensor and actuator applications [1], mainly due to the development of industrial automation processes [2]. Also, the field of electronic engineering has required better performance for a large number of applications [3,4]. Since their discovery, allotropic carbon forms [5,6] emerged as the best candidates for the production of input-output devices [7] and electric transducers [8].
The great interest in carbon nanotubes (CNTs), fullerenes, and graphene is due to their astonishing mechanical, electrical, and electronic properties [9]. The use of nanostructured carbon-based materials has proven fruitful for developing a wide range of micro and nanodevices [10][11][12], with a particular focus on those able to produce tunable outputs [13]. Moreover, the high conductivity of aromatic conjugate systems of nanostructured carbon makes them excellent for the production of electrodes and electrochemistry applications [14][15][16]. Despite these superior properties, the high cost and difficult dispersion in other media have reduced their use for large-scale applications. A reasonable and technically affordable solution is the incorporation of nanostructured carbon-based materials into a wide polymeric matrix, trying to join the bulk properties of the polymers with the superior theoretical properties of nanostructured carbon-based materials [17]. Also, controlled chemical functionalization improves the processability as in the case of CNTs [18] and fullerenes [19], or when converting graphene into graphene oxide [20]. This review discusses the main recent achievements in the use of nanostructured carbon-based materials with a particular focus on the materials and their applications, and is organized as follows: (i) CNTs, (ii) graphene and graphene oxides, and (iii) fullerenes. Since the development of the first nanoelectromechanical systems, CNTs have attracted great interest as they combine theoretical superior mechanical and electronic properties [31-33]. One of the most important properties of CNTs is that they are very good conductors of both electricity and heat. Their high aspect ratio also provides a valid advantage when compared to other carbon nanostructures such as cage compounds [34]. CNTs are also very strong and elastic molecules in certain directions, and all of these properties are difficult to find combined at the same time in one material. For current carbon nanotube actuators, multiwalled carbon nanotubes (MWCNTs) and bundles of MWCNTs have been widely used, mostly due to ease of handling and robustness if compared with SWCNTs, i.e., highly oriented CNTs on single molecule devices such as nanoscale rotation actuators [35-37].

Graphene and Graphene Oxide
Graphene is an extremely electrically conductive form of elemental carbon that is composed of a single flat sheet of carbon atoms arranged in a repeating hexagonal lattice. The term graphene is also used to identify carbon sheets formed by double or multilayers because high quality and defect free single layer graphene is extremely difficult to create and manipulate. Today, graphene and graphene oxide, as shown in Figure 2, are considered next-generation materials [

Fullerene-Based Actuators
Among all the exotic allotropic carbon forms, fullerene-shaped forms are the smallest in size and the highest in strain [44], allowing efficient cycloaddition reactions on their surface [45]. Fullerene was clearly identified using mass spectroscopy in 1985 by Kroto et al. [46] who aimed to explain adsorption observed in the interstellar medium. After its discovery, the first rational synthesis of C 60 was reported by Krätschmer et al. [47] in 1990.
Among different fullerenes illustrated in Figure 3, C 60 has gained much attention due to its chemical reactivity [48], hydrogen storage capability [49], and for the six one-by-one-electron reduction behavior [50]. These attractive properties are counterbalanced by a very high cost that inhibits C 60 use at a large scale. Nonetheless, fullerenes have been investigated for different applications as actuators because of their properties, such as high surface area, thermal stability, non-toxicity, biocompatibility, and hydrophilic functionalization. Despite these, fullerenes are characterized by a high resistivity, close to 1014 Ω/m [51], compared with CNT or high quality graphene that could reach a value of down to 10 −6 Ω/m [52,53].

Carbon Nanotube-Based Actuators
CNTs are used to produce actuators that exploit their different properties when pristine or in composites. The first actuation using CNTs was reported by Baughman et al. [54], where a sheet of aggregated SWCNTs named buckypaper achieved a displacement of 0.12% under a square wave potential of ±0.5 V. A few years later, Roth et al. [55] reported electrochemical actuation on a single CNT and a maximum isometric stress of up to 25 MPa [56]. Many studies have been devoted to investigating the relationship between electrical input and the actuations of CNTs [57,58], showing how the aspect ratio is an unneglectable factor together with the network properties of CNTs [49,59,60].
Further observations were reported by Senga et al. [61] using a transmission electron microscope to prove the transition between the flattened state and the tubular state in rather thick CNTs, enlightening the relationship with thermal energy.
Other examples of actuators based on nanotube structures use the electroactuation process that induces deformation on CNTs using electrical stimuli [62][63][64][65]. Some example of CNT-based actuators are shown in Figure 4. The combined use of ionic liquids together with CNTs and polymers is a well-established practice able to obtain well-dispersed gels [66,67], as reported by Sugino et al. [68]. They also described the benefits of the simultaneous use of SWCNTS/ionic liquids and a conductive polymeric additive, and reported a triplicate strain value [69] using a mix of ionic surfactants compared with a non-additive actuator. Terasawa et al. [70] used a non-ionic surfactant to produce a CNT-containing gel. In this case, actuation was performed using poly(vinylidene fluoride-co-hexafluoropropylene) or poly(ethylene oxide) materials, with the latter showing better performance. A model based on ionic transport number and the ionic volume was proposed to explain this behavior, proving that the self-diffusion coefficient, rate of ion transport in the electrodes, and electrolyte types are all critical factors for this type of actuation.
Nevertheless, among all CNT polymer composites, elastomers are among the most studied [71]. Fang et al. [72] used dielectric elastomers mixed with highly aligned SWCNTs sheets modified using a laser treatment to induce mechanical anisotropy. During the electrochemical actuation, this treatment showed an appreciable beneficial effect reaching a strain of up to 33%. Anisotropic actuation was also used to develop soft linear actuators [73] operating at a low electrical field (100 V/µm) comprising a SWCNT electrode supported onto an acrylic matrix, inducing a strain of up to 40% in dielectric electroactive polymers. Among these last polymers, sulfonated fluoropolymers (e.g., Nafion) have generated great interest due to their simple synthesis and filming properties [74]. Lian et al. [75] induced actuation using functionalized SWCNTs dispersed into a Nafion matrix with a concentration doubling the storage modulus and the strain compared with the neat matrix. Similarly, Lu et al. [76] used carboxylic derivatives of SWCNTs supported onto carbon fibers for the production of Nafion-silica-based shape memory polymers. In this case, both electrical and thermally induced actuation was performed and magnified by the presence of SWCNTs. Layering was a common technique in SWCNT-based actuation. Mukai et al. [77] reported the use of a millimetric layer of SWCNTs as support for the growth of poly(pyrrole) by electropolymerization, performing the actuation in a watery medium with a displacement of around 0.1%. A three-layered structure was studied by Sachyani et al. [78], which promoted an angular strain of 300 • attributed to unaligned SWCNTs together with a flexible polymer layer. As reported by Zhang et al. [79], non-conductive polymers could be used as components for actuators. In this recent study, a transparency actuator based on a single layer super-aligned SWCNT sheet and paraffin-poly(dimethylsiloxane) matrix was developed showing a displacement of up to 0.4% with a switchable transmittance. High-performing polymers, such as polyimides, could also be used to produce tough actuator devices. As suggested by Ning et al. [80], pre-dispersed SWCNTs could be used to produce highly aligned CNT matrices supported onto a polyimide film and used as a thermomechanical actuator. A multiresponsive actuator was described by Zhou et al. [81]. In this case, super-aligned SWCNTs were used to produce a U-shaped device able to show twisting deformation under both thermal and electrical stimuli. Moreover, SWCNTs could be combined with other carbon sources, such as carbides [82], or inorganic species [83] improving the responses of the actuators by up to 60% [84].
MWCNTs have been used more widely for actuation than SWCNTs. Capeluto et al. [85] described the use of MWCNTs combined with azopolymers to produce transparent film actuators. Recently, Ji et al. [86] used MWCNTs/poly(dimethylsiloxane) to produce a very thick film able to generate a linear strain actuation of up to 4% under high voltage (100 V). Poly(pyrrole) was also used with a high load of MWCNTs (of up to 25 wt %) with a displacement after actuation of up to 18% [87]. Shirasu et al. [88] reported an actuation by using a thermoset polymer. In their study, a U-shaped epoxy-based device was produced, dispersing aligned MWCNTs and inducing a bending displacement of up to 10% under an applied DC voltage of 6 V. Further, MWCNTs were obtained with ruthenium oxides and the resulting inorganic-containing carbon material was used as an actuator, doubling the performances of a unfunctionalized SWCNTs based device considering the strain and maximum generated stress [89]. Better performances of MWCNTs containing actuators were reported for 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid-based systems [90] compared with carbon black-based devices, showing a strain improvement of up to 0.8%.
The use of a randomly distributed CNT net is an affordable approach to combine actuation capability avoiding the alignment procedure reported above. Chen et al. reported two interesting examples using buckypaper/Nafion actuators [91], or without a polymer matrix under very high voltage (1100V) [92]. In both cases, a good actuation response was observed of up to 1%.

Graphene and Graphene Oxide Actuators
Graphene actuation was studied by Saane and Onck [93] using a Density Functional Theory (DFT) calculation based on an adaptive intermolecular reactive empirical bond order potential method. The authors demonstrated the better performances of graphene compared to the inorganic material used (nanoporous gold), achieving a smaller actuation stroke under an electrical field but generating an enhanced mechanical work. Graphene-based composites used as electroactive actuators are reported to be highly sensitive to electric stimuli showing a monotonic response to electrical solicitations [94]. As seen in Figure 5, graphene-Nafion membrane actuators represent a good benchmark for comparing properties, such as mechanical strength between devices, with a displacement of up to 60% [95]. As a result of this, graphene was also used to developed wearable poly(pyrrole)-based fibers employed for the production of net actuators [96]. Sen et al. [97] reported the actuation of cellulose using graphene nanoplatelets (0.1-0.5 wt %) under DC excitation voltages of 3-7 V, reaching a strain of up to 1%. Titanium-doped graphene was used to induce actuation into an elastomeric matrix with a strain of up to 72% using 39 V/µm [98]. Graphene-based actuators are able to induce actuation under different external stimuli. Liu et al. [99] performed a photochemical actuation on bilayered graphene nanoplatelets/poly(dimethylsiloxane) using a near infrared light, as shown in Figure 5. The main unsolved issue in the use of graphene is the poor dispersion in polymeric matrices. An affordable solution could be heteroatomic massive functionalization [100] as in the case of graphene oxide (GO). Oxidized graphite has been well known about since the first studies reported by Brodie in the middle of the 19th century [101], but oxidative protocols have only allowed the production of GO in recent years [102]. GO showed a better dispersibility with respect to common graphene, producing very stable solutions in watery and polar solvents media [103].
Actuators based on GO were easily produced using hydrogels receiving both thermal and UV stimuli [104]. Actuation using near infrared light was performed by Chen et al. [105] using GO and poly(N-isopropylacrylamide) with remarkably shape memory. Similar results were obtained using Nafion-based materials [106]. Effective interactions between residual groups of GO and polar functionalities was proved using actuators composed by sulfonated poly(styrene) [107] or poly(pyrrole) [108], reaching a bending angle greater than 360 • C using low voltage (around 1 V). Recently, Vural et al. [109] produced a tandem bimorph actuator based on proteins and GO, reaching a maximum curvature three times higher than conventional GO of up to 17%. Terasawa et al. [110], combined GO and vapor-grown carbon fibers in an ionic liquid medium obtaining an actuation improvement of 56% compared with the same system containing SWCNTs. The high capacity for oxygen-based functionalities of GO enabled production of a very interesting bioinspired high-performance dielectric elastomer actuator, based on titanate supported onto poly(dopamine) GO. This material mimics a mussel shell, showing a very high actuation response to electrical stimulation with a strain of up to 1%, applying a 2 V triangular voltage frequency. Wang et al. [111] described a multiresponsive PDMS/graphene composite able to perform the actuation under different chemical stimuli with an exceptional recovery property of up to 98% after three cycles.
Another interesting property of GO is related to the possibility of high-ordered material production through direct reduction of GO. Reduced GO (rGO) has properties quite close to pristine graphene together with a considerable price reduction and its versatility permits the production of various actuator devices. As an example, Selvakumar et al. [112] described a biomorphic electrical actuator based on an rGO analog of CNTs buckypaper. A similar study was reported by Wang et al. [113] using an epoxy matrix producing a shape memory material with very high recoverability after hundreds of cycles. Other research reported visible light actuation induced by rGO on chitosan polymer, producing high-performance actuation [114] of up to 4% displacement.

Fullerene-Based Actuators
As mentioned before, the interesting properties of fullerene (high surface area, porosity, thermal stability, non-toxicity, bio-compatibility, and hydrophilic functionalization) are used to produce actuators. Kaur et al. [115] reported the use of low fullerene species (C 20 ) studying actuation at a molecular level using a purely theoretical approach based on DFT non-equilibrium charge transport (DFT-NEGF) computation method.
A similar study was reported by Huang et al. [116] using scandium nitride encapsulated into C 80 , proposing its use for single-molecule memory and logic devices for parallel molecular computing architectures.
A proper device was described by Jung et al. [117] assembling a C 60 -Nafion membrane that showed a harmonic response to an oscillating electrical field as shown in Figure 6. A more complex system was produced by Panwar et al. [118] using oxygenated C 60 . The fullerene-based electrode was anchored to an elastomeric matrix composed of poly(vinylidene fluoride)/poly(vinylpyrrolidone)/sulfonated poly(styrene) enhancing the actuation response.

Challenges and Prospective of Actuators Based on Carbon Nanostructures
Actuators based on carbon materials represent a field that is evolving day by day. Challenges are found in the field of biology, such as in nanomanipulation, where actuators with high performance are requested. An important aspect to take in account is the economical aspect. CNTs, graphene, and fullerene are expensive materials with low reproducibility. These drawbacks become a problem for large-scale application, where the economical aspect is not secondary to the material reproducibility in the case of standardization processes. Researchers are studying new carbon materials with low costs and high reproducibility grades able to demonstrate comparable performances to expensive carbon materials such as biochar, that are actually used only for sensor production [119]. Moreover, the ecofriendly aspect-which is not currently being addressed for the production of CNTs, graphene, and fullerene-will play an increasing role in the future production of actuators [120].

Conclusions
Nanoscale and macroscopic carbon-based actuation is a vast and very promising field with potentially great impact on device applications [121][122][123][124]. Combined with the lack of suitable molecular actuators, the use of CNTs and graphene/rGO materials could exploit novel actuation technologies. The further development of allotropic-containing materials poses viable exciting challenges which lie ahead.
Nevertheless, the main issue to be taken into account in the use of CNTs, graphene, and fullerene in any containing material is the economical drawback. These considerations discourage the use of nanostructured carbon material, but the astonishing performances showed by all of the allotropic carbon forms leave the door open for future evolution.
In conclusion, nanostructured carbon-based actuators have been studied extensively under small-scale and lab conditions and shown promising performances, but have not yet achieved their full potential.