Flexible and environment-friendly regenerated cellulose/MoS2 nanosheet nanogenerators with high piezoelectricity and output performance

Flexible piezoelectric nanogenerators for energy harvesting are getting more and more attention nowadays by converting the mechanical energy to electric energy. Here, an environment-friendly piezoelectric nanogenerator based on the regenerated cellulose (RC)/MoS2 nanosheet nanocomposite successfully exhibited a relative high output voltage of 2 V and current of 150 nA under slight press which were 5 and 7.5 times higher than those of the neat RC film, i.e. 0.4 V and 20 nA, respectively. In particular, the MoS2 nanosheets were obtained through a simple, facile and low-cost pathway by mechanical exfoliation in triethanolamine. The nanocomposite film with MoS2 nanosheets content of 4% exhibited a high piezoelectric constant (d33) of 19 pC/N, which was 6.3 times higher than that of the neat RC film (i.e. 3 pC/N). Thus, the RC/MoS2 piezoelectric nanogenerator has great potential applications in the fields of energy harvester, sensors and is of great significance to environment protection.


Introduction
With the consuming of fossil energy and deteriorating of environment, the quest for clean and renewable energy has become an urgent challenge (Goldemberg 2007;Wang et al. 2019;. Energy harvesting technology is one of the potential methods in energy utilization. There are many sustainable energy sources that can be used in energy harvesting such as solar energy, wind energy (Fan et al. 2016). Piezoelectric materials have attracted more and more attention as a new pathway that can convert mechanical energy in ambient environment to electric energy and is promising to realize energy generation (Sappati and Bhadra 2018;Park et al. 2014;Zhao et al. 2020a, b). In 2006 developed a piezoelectric nanogenerator made of ZnO nanowire arrays for the first time, which could harvest mechanical energy and convert to electric energy. There is a potential difference when stress is applied to Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/ s10570-021-03962-z. piezoelectric materials, which is called positive piezoelectric effect. Besides, the converse piezoelectric effect is that the material deforms under electric stimulation (Wei et al. 2018). Energy harvesting technology inspired by piezoelectric effect can be used in nanogenerator (Ye et al. 2019), self-powered sensor (Raj et al. 2018) and so on (Fang et al. 2019;Purusothaman et al. 2019). The devices could convert mechanical energy from human daily life such as walking, bending at anytime and anywhere to achieve the purpose of energy utilization (Yang et al. 2016;Xu et al. 2010). Furthermore, when this kind of self-driven sensor used in vivo environment as medical device (Zhao et al. 2020a, b), the problem of battery replacement can be avoided because it can extract biochemical energy like heartbeat (Wang and Wu 2012;Vivekananthan et al. 2018).
So far, some types of piezoelectric materials have been studied (Chen et al. 2020a, b). Organic piezoelectric polymers, such as polyvinylidene fluoride (PVDF) and their copolymer polyvinylidene fluoridetrifluoroethylene P(VDF-TrFE) Yang et al. 2020;Lovinger 1983), are flexible and light weight. On the other hand, inorganic piezoelectric ceramics such as lead zirconate titanate piezoelectric ceramics (PZT) (Park et al. 2020), potassium sodium niobate (KNN) and barium titanate BaTiO 3 (Lv et al. 2020; Koda and Sodano 2014) exhibit superior piezoelectric properties but high brittleness, poor toughness and some high toxicity, thus their application is limited (Shi et al. 2019). For practical application, the polymer matrix usually composites with inorganic piezoelectric ceramic to prepare the flexible nanogenerator with optimized piezoelectricity Purusothaman et al. 2018).
Furthermore, most synthetic polymers, e.g. PVDF and P(VDF-TrFE), are not biodegradable and they would cause severe environmental problems after usage. Moreover, they are not renewable and biocompatible. Therefore, seeking for renewable, biodegradable, and biocompatible natural polymer for piezoelectric nanogenerator is urgent. Cellulose is the most abundant and widespread natural polymer in the nature (Kim et al. 2014;Wang et al. 2020;Song et al. 2021). It is a hydrophilic glucan biopolymer consists of a linear chain of two anhydroglucose rings joined via a b-1,4 glycosidic linkage (Pandey et al. 2010;Klemm et al. 1998). Compared with many synthetic polymers, cellulose has many excellent features: renewability, biocompatibility, biodegradability, high strength, and low thermal expansion (Tayeb 2019;Mishra et al. 2019). Especially, the biodegradability of cellulose is the key to solve the environmental pollution problems (Sannion et al. 2009). Cellulose has the same flexibility as other polymers and can be biodegradable under certain natural conditions which can be used without pollution. They have been explored for applications in electronics and functional devices (Soba et al. 2016;Chen et al. 2020a, b;Toroń et al. 2018;Zhai et al. 2015;Yin et al. 2020). However, cellulose cannot be dissolved in common solvent or melt because of its strong intra and inter hydrogen bonding (Zhang et al. 2007). Zhang et al. found that cellulose could be dissolved in NaOH/urea aqueous solution under low temperature (Cai and Zhang 2005). Cellulose films could be obtained through this non-toxic and low-cost solvent system after regeneration (Qi et al. 2009;Yang et al. 2018).
Recently, as a kind of novel high-performance piezoelectric material, monolayer or few-layer molybdenum disulfide (MoS 2 ) has attracted great attention (Sohn et al. 2019;Wu et al. 2021). Sahatiya et al. facricated a nanogenerator which employ both piezoelectricity and triboelectricity based on MoS 2 , cellulose and PVDF (Sahatiya et al. 2018). MoS 2 has layered structure in which a molybdenum plane is sandwiched between two sulfide planes connected by Van der Waals force, while the molybdenum and sulfide atom are covalently bonded (Acharya et al. 2018). Zhou et al. found that MoS 2 with odd-number layers could generate high piezoelectric voltage and current under stress, which increased with the number of layers decreased. MoS 2 with odd number has strong piezoelectric effect because of the asymmetry of positive and negative charge resulting from the deformation under external forces. On the contrary, MoS 2 with even number did not exhibit piezoelectricity owing to the presence of a projected inversion symmetry (Zhou et al. 2016), the positive and negative charge cancel each other out. There are several methods to obtain MoS 2 nanosheets, such as chemical vapor deposition (CVD) and mechanical delamination (Krishnamoorthy et al. 2016), but these methods are of low yield and efficiency. However, liquid exfoliation is a simple way to prepare 2D materials (Niu et al. 2016). Recently, we found that monolayer or fewlayer MoS 2 nanosheets could be obtained by mechanically stirring in triethanolamine, which is effective and low cost (Chen et al. 2017).
Here, we constructed a flexible, lead-free and biocompatible piezoelectric nanogenerator by compositing of MoS 2 nanosheets and cellulose molecules for energy harvesting. MoS 2 nanosheets were one-step exfoliated by triethanolamine, and cellulose was dissolved in NaOH/urea/H 2 O solution. MoS 2 /cellulose nanocomposite films were obtained by blending exfoliated MoS 2 nanosheets and cellulose, followed by regeneration, washing, and natural drying. Then, they were used to fabricate the piezoelectric nanogenerator with Al foil as the electrode, Cu wire as the connecting wire, and polyimide (PI) as the encapsulation layer. The piezoelectricity, output voltage and current of the nanogenerator under press were studied in details.
Exfoliation of MoS 2 nanosheets 0.5 g MoS 2 was added to 50 ml triethanolamine and stirred for 8 h at 1000 r/min, followed by centrifugation for 30 min at 4500 r/min. After washing the supernatant with deionized water and filtering several times to remove triethanolamine, the obtained product was dispersed in deionized water followed by freezedrying to prepare the MoS 2 nanosheets.

Preparation of RC/MoS 2 films
The NaOH, urea and deionized water were weighed at mass ratio of 7: 12: 81 and well mixed to make a NaOH/urea/H 2 O solution. The obtained MoS 2 nanosheets were homogeneously dispersed in the NaOH/urea/H 2 O solution by sonication and cooled to -13°C. Then a desired amount of cellulose was dissolved in the above dispersion to prepare the composite solution. After removing bubbles by centrifugation, the composite hydrogels were prepared by casting the composite solution on the glass plate and then immersing in dilute sulphuric acid (5%) for 5 min. The regenerated cellulose (RC)/MoS 2 nanosheet composite films with thickness of 20-30 lm were finally obtained after washing and immersing the composite hydrogels in water to remove residual reagent, followed by air-drying. These obtained films were denoted as RC, RC/ MoS 2 1, RC/MoS 2 2, RC/MoS 2 4, and RC/MoS 2 8, respectively, with incorporation of 0 wt%, 1, 2, 4 and 8 wt% MoS 2 nanosheets in the composites.

Preparation of RC/MoS 2 nanogenerators
The above obtained films were polarized under an electric field of 15 MV/m in silicone oil bath for 40 min at room temperature. Then for the part of construction of nanogenerator, the RC/MoS 2 films were sandwiched between Al foils which were used as electrodes, copper conductors were attached to the Al foils, finally the polyimide (PI) was used as encapsulation at outer layer.

Characterization
The microstructure of the sample was characterized by scanning electron microscope (SEM) (JSM-IT300, JEOL, Japan) after the sample was fractured in liquid nitrogen. The microstructure of the film was also observed by transmission electron microscope (TEM) (Tecnai G2 F30, FEI, USA). Fourier-transform infrared (FTIR) spectra of the samples were examined by FTIR spectrometer (Nicolet6700, USA). The X-ray diffraction (XRD) (D8 Advance, AXS, German) analysis was recorded with Cu Ka source radiation. The d-spacing was calculated by the Bragg's law: 2dsinh = nk, where d is the interplanar crystal spacing, h is the diffraction angle, n is the diffraction series, and k is the wavelength of the X-ray. The mechanical properties were carried out by universal tester (RGM-4100, Geger, China), the samples were selected from the uniform part of the films and cut into 30 9 5 mm, and the stretching speed was set at 2 mm/min. Thermal stability test (TGA) was measured by thermal analyzer (STA499C, Netzsch, Germany) in the nitrogen atmosphere and the temperature with the range of 0 to 1000°C at an increase rate of 10°C/min. Dielectric constant and loss was tested through Agilent equipment (HIOKI3532-50, HIOKI, Japan) in the frequency from 100 to 1 M Hz. The piezoelectric coefficient d 33 was obtained via quasi-static d 33 tester (ZJ-3A, Chinese academy of sciences) after polarization. The piezoelectric performance including output voltage and current of the nanogenerator was measured by electrochemical workstation under a slight press about 0.88 kPa (CHI660E, Shanghai, China).

Result and discussion
Structure of RC/MoS 2 nanocomposites Figure 1 shows the fabrication process of RC/MoS 2 film and construction of piezoelectric nanogenerator. The film was fabricated through blending, casting, regeneration, air-drying and the process is relatively simple and environmentally friendly. Besides, the obtained device has superior flexibility so that it can be used in many fields, e.g. detection of human physical signals. Figure 2 shows SEM images of the cross-sections of the RC and RC-MoS 2 4 films. The layered structure of the RC film which comprised of uniform and compact fibrils was observed in Fig. 2(a, b). Figure 2(c,d) reveal the morphology of RC-MoS 2 4 film, where with the incorporation of MoS 2 nanosheets, more obvious and compact layered structure was clearly observed. Moreover, there is no obvious aggregation and defect in the RC-MoS 2 composite film, and it shows good compatibility between MoS 2 nanosheets and cellulose molecules, probably owing to that the intercalation of cellulose molecules in MoS 2 nanosheets and formation of hydrogen bonding between them provide hindrance to prevent MoS 2 nanosheets from stacking. Fig. S1 shows the TEM image of RC-MoS 2 4. MoS 2 nanosheets were dispersed well in the cellulose matrix and exhibited nanolayered structure.
To further clarify the hydrogen bonding between cellulose molecules and MoS 2 nanosheets, and also their crystal structure (French 2014(French , 2020, FTIR and XRD measurements were used to characterize the RC-MoS 2 films. Figure 3 is FTIR spectra of RC-MoS 2 films with different filler content which was recorded from 4000 to 400 cm -1 . The broad absorption band around 3448 cm -1 is corresponding to stretching vibration of hydroxyl groups of cellulose (Marchessault and Liang 1960; Hishikawa et al. 2016). Compared with RC film, this broad band of RC-MoS 2 film shifted to lower wavenumbers which indicated that with the addition of MoS 2 , original hydrogen bondings in cellulose molecules were broken and new stronger hydrogen bondings between cellulose molecules and MoS 2 nanosheets were formed (Raghunathan et al. 2017). Besides, the peaks at 1418 cm -1 and 1017 cm -1 represent the bending vibration of C-H and stretching vibration of C-O-C group, which is characteristic of cellulose II and amorphous cellulose with the near total absence of crystalline cellulose I (Carrillo et al. 2004). This indicates that cellulose was successfully transformed into regenerated cellulose II. Fig. S2 shows the XRD patterns of the RC, MoS 2 , and RC-MoS 2 composite films. The XRD pattern of the bulk MoS 2 displays a (002) (French 2014). The relative higher intensity of the 2h diffraction peaks at 12°indicates the preferred orientation of the (1-10) plane parallel to the film surface, probably because of the regeneration of cellulose in the aqueous system and the film drying process (Isobe et al. 2011;Yang et al. 2015;Yin et al. 2020). For the RC-MoS 2 composite films, the feature peaks of XRD patterns are almost corresponding to the primary feature peaks of MoS 2 and RC. The d-spacing of the (002) plane of MoS 2 in the composites slightly increased to 0.62 nm from 0.61 nm of the neat MoS 2 . While the increase is relative low compared with 0.4 nm of the cellulose molecule thickness, it's probably owing to that some cellulose molecules were intercalated between MoS 2 nanosheets to form the ''brick-mortar'' structure.
Thermal stability Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses were conducted from 0 to 1000°C to study the thermal stability of RC-MoS 2 nanocomposite films. Fig. S3(a) reveals the thermal decomposition process: from 0 to 100°C, there is the evaporation of moisture in the films; between 300°C and 650°C, the mass loss occurs because of the cleavage of cellulose molecular chain and carbonization. Moreover, the residual weights of the composites were higher than that of RC, owing to the high thermal stability of MoS 2 . Fig. S3(b) is differential curve of TG curve and indicates the rate of mass loss. The mass loss and decomposition rate of RC-MoS 2 film decreased after addition of MoS 2 nanosheets for the reason that MoS 2 has good thermal stability and formed strong hydrogen bond with cellulose. Besides, thermal decomposition temperature of the RC-MoS 2 composite is a bit higher compared with RC. Homogenous dispersion of MoS 2 in RC enhances the thermal stability of RC which may inspire the high-temperature applications (Cao et al. 2019).
Mechanical property Figure 4 shows the mechanical properties of RC-MoS 2 films with different MoS 2 content. For RC-MoS 2 1, the tensile strength (Fig. 4b), Young's Modulus (Fig. 4c) and elongation at break (Fig. 4d) were evidently improved and gradually decreased as the MoS 2 content continuously increased. The tensile strength of composites depends much on the interaction between filler and matrix. It was increased from 57 MPa to 98.8 MPa with the increase of MoS 2 content from 0 to 4 wt%, because the strong hydrogen bond formed and stress transfer occurred between cellulose molecules and MoS 2 nanosheets. However, when the content of MoS 2 continuously increased, there would be more defects, so that the tensile strength decreased. Similarly, moderate addition of MoS 2 nanosheets were beneficial for the MoS 2 improvement of Young's Modulus and elongation at break as a kind of reinforcing material, but when the content was over 2 wt%, MoS 2 nanosheets may aggregate and the films became non-uniform and stress concentration happened, leading to poor mechanical properties. The tensile strength of the film decreased from 98.8 MPa to about 65.2 MPa, and the Young's modulus increased from 1.13 GPa to 2.86 GPa, with the increase of temperature from 20 to 100°C (Fig. S4), probably owing to the removal of moisture in the film at high temperature. The result indicated that the composite film still exhibits good mechanical properties at high temperature. Therefore, they are promising for application at high temperatures.

Piezoelectric property
Piezoelectric constant (d 33 ) is an important index to characterize the piezoelectric properties; it is the conversion coefficient of transform between mechanical and electric energy. As shown in Fig. 5, the d 33 of the neat RC film was 3 pC/N, which reveals that cellulose could be used as a kind of flexible piezoelectric matrix with good advantages of renewability, biodegradability, and biocompatibility. The d 33 of RC-MoS 2 films first increased and then decreased with the increased MoS 2 content. When the content of MoS 2 was 4%, the d 33 reached the maximum value of 19 pC/ N. When the MoS 2 content was higher than 4%, d 33 decreased probably owing to that the aggregation and stack of MoS 2 nanosheets occurred and led to actual lower proportion of MoS 2 nanosheets. To further study the practical application of the RC-MoS 2 piezoelectric films, the nanogenerators were constructed using these piezoelectric films through sandwiched structure. The films were first polarized and then sandwiched between Al foil, which acted as electrode. PI was used as encapsulation layer because it can prevent charges in surrounding environment from interfering the test owing to its insulation property. Figure 6(a, b) shows energy harvesting performance of the RC-MoS 2 nanogenerators with different filler content under a slight press about 0.88 kPa. The variation trend of output open-circuit voltage and short-circuit current was consistent with d 33 . The maximum open-circuit voltage was 2 V and short-circuit current was 150 nA, which were 5 and 7.5 times higher than those of the neat RC film, i.e. 0.4 V and 20 nA, respectively. Therefore, the incorporation of MoS 2 nanosheets greatly improved the piezoelectric and output properties of the RC film owing to the outstanding piezoelectricity of monolayer or oddlayer MoS 2 nanosheets. Fig. S5 shows the ''switch polarity'' test. When switch the device in the forward or reversed mode, the amplitude of output performance is reversed. It indicates that the performance of RC-MoS 2 is generated by piezoelectricity. These flexible RC-MoS 2 nanogenerators are easily fabricated, successfully convert mechanical energy to electric energy, and can be used in complex environment after encapsulation. They have potential applications in the fields of sensors, electronics, and electric skins owing to the good ability of energy harvesting. Besides, these devices have advantages of biocompatibility and biodegradability, which may inspire invivo applications.

Conclusion
In this work, cellulose was dissolved and blended with MoS 2 nanosheets in NaOH/urea aqueous solution to prepare the RC-MoS 2 piezoelectric films and nanogenerators. The structures of the RC-MoS 2 composite films were studied by SEM, FTIR and XRD, and the results indicated the formation of nanolayered structure and strong hydrogen bonding between cellulose molecules and MoS 2 nanosheets. Furthermore, the piezoelectric property of the films and their potential applications in energy harvesting were studied in details. The RC-MoS 2 4 film exhibited a high d 33 of 19 pC/N, output voltage of 2 V, and current of 150 nA under a slight press, which were 6.3, 5, and 7.5 times higher than those of the RC film, respectively. The incorporation of MoS 2 nanosheets thus greatly improved the piezoelectricity and energy harvesting of the RC film, which could inspire further research. Moreover, the incorporation of MoS 2 nanosheets also improved the mechanical properties and thermal stability of the RC film. As a whole, the flexible, renewable, biodegradable, and biocompatible RC- MoS 2 piezoelectric films with high piezoelectricity and output performance are of great significance to environment protection and have potential applications in high-performance energy harvester, wearable sensor, electric skin, etc.