Novel Piezoelectric Paper‐Based Flexible Nanogenerators Composed of BaTiO3 Nanoparticles and Bacterial Cellulose

A piezoelectric paper based on BaTiO3 (BTO) nanoparticles and bacterial cellulose (BC) with excellent output properties for application of nanogenerators (NGs) is reported. A facile and scalable vacuum filtration method is used to fabricate the piezoelectric paper. The BTO/BC piezoelectric paper based NG shows outstanding output performance with open‐circuit voltage of 14 V and short‐circuit current density of 190 nA cm−2. The maximum power density generated by this unique BTO/BC structure is more than ten times higher than BTO/polydimethylsiloxane structure. In bending conditions, the NG device can generate output voltage of 1.5 V, which is capable of driving a liquid crystal display screen. The improved performance can be ascribed to homogeneous distribution of piezoelectric BTO nanoparticles in the BC matrix as well as the enhanced stress on piezoelectric nanoparticles implemented by the unique percolated networks of BC nanofibers. The flexible BTO/BC piezoelectric paper based NG is lightweight, eco‐friendly, and cost‐effective, which holds great promises for achieving wearable or implantable energy harvesters and self‐powered electronics.


Introduction
With the increasing demand for sustainable and reliable energy for personal electronics and wireless nanosystems, it is envisioned as a promising approach to harvest energy from the ambient sources, such as body movements, air fl ow, acoustic waves, and even thermal fl uctuations which are available in most of the circumstances. [1][2][3][4] Nanogenerators (NGs) are emerging as novel devices which can convert various kinds of ambient energy into electric power via piezoelectric, [ 5 ] triboelectric, [ 6,7 ] or pyroelectric effect [ 4 ] both in nanoscale and www.MaterialsViews.com www.advancedscience.com Adv. Sci. 2016, 3,1500257 Despite the enormous performance improvement that has been achieved, more facile, inexpensive, and scalable approaches are still needed.
As a kind of natural material, cellulose has become an attractive candidate for paper based devices due to its fl exibility, biocompatibility, and low cost. Recently, various techniques have been explored to implement novel paper based devices. Compared to other fl exible materials, cellulose possess a much lower coeffi cient of thermal expansion, which is an advantage for the thermal stability of devices. [ 25 ] In addition, suitability for printed electronic device is another promising merit for paper materials. [ 26 ] Bacterial cellulose (BC), a kind of biopolymer produced by Gluconacetobacter strains, has higher mechanical strength and better chemical stability than regular paper due to its high purity and crystallinity. Due to its intrinsic textured nanofi brillated structure, BC can present itself as either reinforcement or matrix for functional materials such as fl exible transparent fi lm, [ 27 ] conductive polymers, [ 28 ] and antibacterial textiles. [ 29 ] It is also necessary to verify BC as building materials for fl exible and cost-effective NGs.
In this work, we fabricated a piezoelectric paper with piezoelectric BTO nanoparticles and BC through a facile vacuum fi ltration method. The BTO/BC piezoelectric paper based NGs showed an enhanced output performance compared to traditional BTO/PDMS based devices. The unique entangled BC nanofi ber networks enable BTO nanoparticles to be dispersed in the piezoelectric paper uniformly, which makes the excellent piezoelectric property of the paper. The BTO/BC piezoelectric paper based NG can generate voltage of 1.5 V in bending conditions, which can drive a commercial liquid crystal display (LCD) screen. The fl exible BTO/BC piezoelectric paper based NG is lightweight, eco-friendly, and cost-effective, which holds great promises for achieving wearable or implantable energy harvesters and self-powered electronic devices.

Results and Discussion
Figure 1 a-d shows the fabrication process of the BTO/BC piezoelectric paper. As can be seen in Figure 1 a,e, raw BC are transparent gel-like pellicles which are composed of cellulose nanofi bers with the diameter of ≈10-30 nm and all the nanofi bers are found to aggregate densely due to the loss of water. The X-ray diffraction (XRD) pattern of the BC membrane is shown in the inset of Figure 1 e. The diffraction peaks of 14.2°, 16.5°, and 22.5° are assigned to diffraction planes of (101), (10-1), and (002) for native cellulose I, respectively. [ 30 ] An aqueous suspension of BC fi bers can be obtained by mechanically breaking the interconnected nanofi ber networks with a high speed homogenizer. BTO nanoparticles were synthesized by hydrothermal method. [ 18 ] As illustrated in Figure 1 f, the average diameter of BTO nanoparticles is about 100 nm. Raman spectrum (inset of Figure 1  , which represent a high piezoelectric tetragonal phase of BTO. [ 31 ] The as-synthesized BTO nanoparticles were ultrasonically dispersed in distilled water and then mixed with BC aqueous suspension under vigorous stirring to guarantee suffi cient blending. BTO/BC piezoelectric paper can be formed by vacuum fi ltrating the blended suspension with a microporous membrane followed by a pressing and drying process. Figure 1 g demonstrates the fi eld-emission scanning electron microscopy (FE-SEM) image of the BTO/ BC piezoelectric paper. It is found that the disintegrated BC nanofi bers get associated again due to the strong interaction of hydrogen bonds and BTO nanoparticles are uniformly bounded within the BC matrix. Thermogravimetry analyses (TGA) in the inset of Figure   350 °C (weight loss of 70%) and 450 °C (weight loss of 99%). Compared with the pure BC which has almost no mass residue after calcination, the BC mixed with 0.5 g BTO nanoparticles presents a mass residue of nearly 80% even above 500 °C, which indicates that the mass fraction of BTO nanoparticles in the paper is very high. This loading percentage is much higher than that achieved by a layer-by-layer approach (48 wt%). [ 32 ] This distinct feature makes the BTO/BC piezoelectric paper as light as possible because most of the weight is concentrated on the piezoelectric component, which is an important merit for integration applications.
BTO/BC piezoelectric paper was used as an active layer to fabricate NGs, as shown in Figure 2 a. First, a thin layer of PDMS was spin-coated on both side of the piezoelectric paper to provide a smooth surface for the electrodes and to protect the paper from moisture in the air. A layer of Ti/Au (10 nm/100 nm) was then deposited on both sides of the paper as electrodes. Two conductive tapes were then connected to the top and bottom electrodes, respectively. All the devices are characterized with the same active size of 3 × 2 cm 2 . Basically, poling process is of great importance for the BTO/BC piezoelectric paper because disordered dipoles in ferroelectric BTO domains need to be aligned by an external electric fi eld so that the piezoelectric potential can be enhanced in a specifi c direction. To reveal the necessity of this process, a range of electric fi eld were applied to pole the devices. As shown in Figure 2 b,c, the unpoled device showed output voltage of only ≈1 V. As the poling electric fi eld increased from 50 to 200 kV cm −1 , the output voltage can be enhanced to more than 12 V. This can be ascribed to the rearrangement of the ferroelectric dipoles under high electric fi eld. Furthermore, the output performance of the NG device can be affected by the amount of BTO nanoparticles contained in the BTO/BC piezoelectric paper. As shown in Figure 2 d,e, when no BTO was contained in the paper, negligible output signals were detected which may originate from the capacitance change of the device. As the amount of BTO nanoparticles increasing from 0.2 to 0.5 g, the output voltage raised from ≈4 to ≈13 V. However, when BTO was further added to 0.8 g, the output voltage decreased to ≈8 V. This can be understood by the trade-off between the density of piezoelectric  points and the total permittivity of the piezoelectric paper. The increasing amount of BTO nanoparticles will undoubtedly provide more piezoelectric points in the paper which is helpful to generate high piezoelectric output. However, excessive amount of BTO nanoparticles can result in very high dielectric constant of the composite, which can weaken the electromechanical coupling effect of the piezoelectric paper. [ 33 ] The output performance of the device is investigated by measuring the open-circuit voltage and short-circuit current when the NGs are subjected to cyclic compressive stress in the normal direction. As shown in Figure 3 a,b, the open-circuit voltage and short-circuit current density can reach as high as 14 V and 190 nA cm −2 , respectively. The value discrepancy of each peak can be attributed to the different strain rate of the device during compressing and releasing process. [ 34 ] To verify the signals are induced by the piezoelectric potential in response to the deformation of the piezoelectric paper, it is essential to conduct switching-polarity test. [ 9 ] In the reverse  connection, the Vt signal exhibits a negative pulse followed by a positive pulse in response to a pressing and a releasing action with the average output voltage and current density maintained at similar magnitude with the forward connection. Therefore, the possible artifacts from triboelectricity and the measurement system can be ruled out. Figure 3 c illustrates the dependence of the output characteristic on external load resistance. With the increment of the load resistance from 1 MΩ to 1 GΩ, the output voltage increases gradually from about 0.16 to 14 V, while the current density decreases from 160 to 14 nA cm −2 . The output power density can be calculated by V 2 / R , where V and R represent the output voltage and the corresponding external load resistance, respectively. As plotted in Figure 3 d, the output power reaches the maximum value of 0.64 µW cm −2 at a matched resistance value of 60 MΩ.
To understand the enhancement effect of this BTO/BC structure for the performance of NGs, we have also fabricated devices with commonly used BTO/PDMS structure. As exhibited in Figure 3 e, both of the output voltage and current density are much lower than the BTO/BC based devices. The maximum output power generated by BTO/PDMS based devices is only 0.04 µW cm −2 , which is more than ten times lower. To reveal the remarkable merit of using BC as matrix for BTO nanoparticles to disperse, SEM characterization is used to compare the top, bottom, and cross-sectional structure of both kinds of the fi lm. As shown in Figure 4 a-c, BTO nanoparticles are uniformly embedded in the whole BC matrix without any obvious aggregation. However, for BTO/PDMS fi lm, limited amount of BTO nanoparticles are existed near the top surface of the fi lm while extensive nanoparticles are accumulated at the bottom of the fi lm. From the side view of the PDMS based fi lm in Figure 4 e, it is clear that most of the nanoparticles have aggregated into large clusters and distribute at the bottom of the PDMS body. Uniform dispersion of piezoelectric nanoparticles within a polymeric matrix is one of the key issues for piezoelectric fi lm to yield high output. The piezopotential distributions inside the piezoelectric fi lms are simulated by COMSOL software. As calculated by simplifying the fi lm and BTO nanoparticles as a rectangular model and six piezoelectric circles in Figure 4 g,h, it is predicted that homogeneous dispersion of BTO nanoparticles can lead to higher piezoelectric potential than the case that all the particles are distributed at the bottom of the matrix with other conditions unchanged. However, it is intrinsically diffi cult to disperse BTO nanoparticles into sticky PDMS for inevitable aggregation and settlement will happen before the whole body has fully cured. The percolated network of the BC nanofi bers enables well dispersion of piezoelectric nanoparticles in the fi lm; thus, the piezoelectric output can be optimized. Furthermore, cellulose is reported to have a Young's modulus in the range 78 ± 17 GPa, [ 35 ] which is properly higher than PDMS (1-2 MPa). [ 18 ] The stiff nanofi bers can transfer stress to the localized piezoelectric BTO nanoparticles effectively. As a result, the BTO nanoparticles will be deformed more signifi cantly in the BC matrix and enhanced output signals can be yielded.
To demonstrate the potential applications requiring fl exible properties of the NG device, the output performance was measured in bending/releasing conditions. The NG device was mounted on a Kapton fi lm which was bent and released repeatedly by a bending stage. As shown in Figure 5 a, a peak output voltage of 1.5 V was achieved with the deformation frequency of  1 Hz. Furthermore, the durability of the device was also examined by applying bending/releasing repeatedly (Figure 5 b). It was found that at the beginning of the test, minor degradation of output voltage was observed, which was caused by small relative displacement between fi xture and the device. The output voltage was maintained at about 1.2 V afterward and no obvious performance decline can be seen for up to 3000 cycles, indicating excellent robustness of the device. The BTO/BC piezoelectric paper based NGs can be applied to harvest mechanical energy and operate small electronics. A commercial LCD can be directly triggered by cyclic bending/releasing the device. Since the LCD is a nonpolar device, it can be driven by AC source without a rectifi er unit. Both the bending and releasing action of the fi nger can generate the fl ash of number "8" on the screen, while no number is displayed during the holding process (shown in a video, Supporting Information).

Conclusion
In this work, we demonstrate a piezoelectric paper by incorporating BaTiO 3 nanoparticles into bacterial cellulose nanofi ber networks to realize lightweight, fl exible, and cost-effective NGs. The piezoelectric paper is fabricated by a vacuum fi ltrating method, which is facile and scalable. The BTO/BC piezoelectric paper based NG shows outstanding output performance with the open-circuit voltage of 14 V and short-circuit current density of 190 nA cm −2 . The maximum power density generated by this unique BTO/BC structure is 0.64 µW cm −2 , which is more than ten times higher than BTO/PDMS structure. This enhancement can be ascribed to homogeneous distribution of piezoelectric BTO nanoparticles in the BC matrix, which is implemented by the percolated networks of BC nanofi bers. The BTO/BC piezoelectric paper based NGs also represent potential applications as fl exible energy harvesters. In the cyclic bending condition, the device can generate a peak voltage of 1.5 V with high stability and durability. A commercial LCD screen can be driven by the cyclic generated power. The fl exible BTO/BC piezoelectric paper based NG is lightweight, eco-friendly, and costeffective, which holds great promises for achieving wearable or implantable energy harvesters and self-powered electronic devices.

Experimental Section
Preparation of the BTO/BC Piezoelectric Paper : A high speed homogenizer was used to mechanically break the interconnected BC nanofi ber networks with the speed of 10 000 rpm for 30 min. An aqueous suspension with BC fi bers uniformly dispersed can be obtained. Hydrothermal BaTiO 3 nanoparticles were ultrasonically dispersed in distilled water and then mixed with BC aqueous suspension. The wellmixed dispersions were vacuum fi ltrated by a microporous membrane (Jingteng, 0.22 µm, Tianjin, China). At last, the fi ltrated fi lm was pressed by a 2 kgf load and dried at 70 °C for 24 h to get the piezoelectric paper.
Characterization and Measurements : An FE-SEM (Quanta3DFEG), X-ray diffractometer (TTRIII), and Raman spectroscopy (JY-HR800) with an Ar + laser source were used for materials characterization. Cyclic bending deformation was provided by a home-made bending stage. The opencircuit voltage signal was recorded by a digital oscilloscope (DS4052, RIGOL) and a low-noise voltage preamplifi er (Standard Research System Model SR560). The short-circuit current signal was recorded by a lownoise current preamplifi er (Standard Research System Model SR570).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.