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. 2020). In 2006, Wang et al. (2016) 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 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 and so on (Fang 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. 2020), 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. 2020). Organic piezoelectric polymers such as polyvinylidene fluoride (PVDF) and their copolymer polyvinylidene fluoride-trifluoroethylene P(VDF-TrFE) (Li et al. 2020; Yang et al. 2020; Lovinger 1983), are flexible and light weight but of low piezoelectric coefficient. On the other hand, inorganic piezoelectric ceramics such as lead zirconate titanate piezoelectric ceramics (PZT) (Park et al. 2017), potassium sodium niobate (KNN) and barium titanate BaTiO3 (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 (Wu et al. 2019).
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. 2019; Wang et al. 2014; Song et al. 2021). It is a hydrophilic glucan biopolymer consists of a linear chain of two anhydroglucose rings joined via a β-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). They have been explored for applications in electronics and functional devices (Soba et al. 2016; Chen et al. 2020; 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).
Recently, as a novel high-performance piezoelectric material, monolayer or few-layer molybdenum disulfide (MoS2) 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 MoS2, cellulose and PVDF (Sahatiya et al. 2018). MoS2 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 MoS2 with odd-number layers could generate high piezoelectric voltage and current under stress, which increased with the number of layers decreased. MoS2 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, MoS2 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 MoS2 nanosheet, 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 few-layer MoS2 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 MoS2 nanosheets and cellulose molecules for energy harvesting. MoS2 nanosheets were one-step exfoliated by triethanolamine, and cellulose was dissolved in NaOH/urea/H2O solution. MoS2/cellulose nanocomposite films were obtained by blending exfoliated MoS2 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.