Piezoelectric Effect of Cellulose Nanocrystals Thin Films

Ultrathin films of aligned cellulose nanocrystals (CNCs) were assembled on mica supports by using electric field-assisted shear. The relationship between polarization gradients and strain mechanics of the obtained films was examined by monitoring their deflection with an atomic force microscope operated in contact mode. The piezoelectric response of the films was ascribed to the collective contribution of the asymmetric crystalline structure of the cellulose crystals. The magnitude of the effective shear piezoelectric constant (d25) of highly ordered CNC films was determined to be 2.1 Å/V, which is comparable to that of a reference film of a piezoelectric metal oxide. A crystalline structures can display inhomogeneous deformation of strain gradients, associated with the piezoelectric response due to an applied electric field. Biopolymer structures with such property include cellulose, which can be used as soft electroactive material. Thus, cellulose nanocrystals (CNCs), nanoparticles of low density, high mechanical strength, thermal stability, chemical resistance, and biocompatibility, can be potentially used in components requiring a piezoelectric response, including sensors and actuators, biomedical devices, and so forth. Piezoelectricity is related to the change in polarization density and the occurrence of dipole moments within a material. It has been generally considered of significance only in highly crystalline materials. The piezoelectric effect in wood was first reported by Bazhenov in 1950. However, the magnitude of the piezoelectric constant in fibers and wood is small mainly due to the random, heterogeneous distribution and a relatively small amount of crystalline cellulose in the lignocellulose matrix. The experimental verification of both direct and inverse piezoelectric effects and quantification of the constants in the piezoelectric matrix were carried out by Fukada in 1955. Only the shear piezoelectric constants −d14 = d25 are finite while the other components are zero, according to uniaxially oriented system of cellulose crystallites. Different wood species show considerably different piezoelectric properties. Further, the piezoelectricity of a given species varies depending on factors such as density, percentage of latewood, and so forth. The piezoelectric modulus upon heat treatment of spruce increases initially and then decreases, following changes in crystallinity. Hydration also plays a role since it has been shown that the piezoelectric constant of bamboo in the dry state is larger than that in hydrated form. The piezoelectricity in chemical wood pulps, cotton, and cellulose derivatives such as cellophane, celluloid, and viscose rayon has been reported to depend on the fibril orientation. The piezoelectric constant of regenerated nanocrystalline cellulose (II) was measured to be 35−60 pC/N, which was considered suitable for energy harvesting and power generation. Ultrathin films of CNCs have been manufactured by several methods. Therefore, given the native crystalline nature of CNCs it is reasonable to ask the question if they can collectively yield a large piezoelectric effect. Could this specially be the case in films of highly aligned CNCs? Could such films induce high energy conversion and piezoelectricity? If this was the case, films or materials made with aligned CNCs could be useful to produce and detect sound, to generate voltage, or to manufacture nanosensors, actuators, microbalances, devices for ultrafine optical focusing, and so forth. The deconstruction of fibrillar cellulose by acid hydrolysis yields cellulose nanocrystal rod-like, highly crystalline nanoparticles. In studies related to the piezoelectric behavior of cellulose fibers, different preparation and modification routes as well as characterization techniques have been considered. Corona poled electro-active paper made from cellulose, cyanoethylated cellulose, and LiCl-DMAc modified cotton (0.32 index of crystallinity) were reported to have piezoelectric constants of 0.167, 0.1−0.2, and 0.16 Å/V, respectively. Such previous work involved the use of cellulose (in fibers or in composites) combined with chemical additives or electrolytes to allow the piezoelectric response; however, to our knowledge ultrathin films of CNCs has not been considered yet. Therefore, our present work explores the effective piezoelectric coefficient d25 Received: May 13, 2012 Accepted: June 21, 2012 Published: June 25, 2012 Letter pubs.acs.org/macroletters © 2012 American Chemical Society 867 dx.doi.org/10.1021/mz300234a | ACS Macro Lett. 2012, 1, 867−870 of CNCs assembled in ultrathin films which were previously manufactured by a combination of shear and electric fields. The degree of alignment of the CNCs within the films (as a function of voltage, frequency, and shear used during their manufacturing) is proposed to allow control of the piezoelectric behavior of the system and produce a large piezoelectric response. The dielectrophoretic properties of CNCs were investigated and reported in a recent contribution. The dipole density or polarization of CNCs was calculated by summing up the dipole moments per volume of the crystallographic unit cell. The Clausius−Mossotti factor allowed the description of the critical and characteristic frequencies as well as the peak dielectrophoresis of CNCs. We also determined the optimal field strength for isotropic alignment in thin films. Using the same methods of our previous work, we obtained ultrathin films of aligned CNCs. By using shear forces coupled with externally applied electric fields we investigated the effect of alignment on the piezoelectric response of the CNC film. The polarizability of CNCs under uniform electric fields and shear forces during withdrawal of a deposition plate induced alignment. Mica was used as solid support for the CNCs. Two reference films were obtained, without application of electric field, and used to elucidate the influence of the solid support. Film formation was observed to depend on the withdrawal rate as well as rate of solvent (water) evaporation. Homogeneous CNC deposition was observed when the solid support was modified with a positively charged polymer layer. Thus, preadsorption of low molecular weight polyethyleneimine (PEI) was used to facilitate a linear growth of ultrathin films of CNCs on mica. The buildup of single or multiple layers of CNCs depended on the concentration of the dispersion and other factors. The length of the deposited CNC films on mica with preadsorbed PEI was 5 cm. The typical film thickness and root-mean-square roughness (atomic force microscope, AFM) were of the order of 38 and 2.5−3 nm, respectively. The piezoelectric response from the CNC film was monitored by measuring the height deflection by using a conductive AFM diamond tip. The 10 Hz signals of low and high voltage resulted in deflection perpendicular to the zdirection of the film, as observed in Figure 1. Three different sections are shown in this figure to represent the cyclic (on− off) response of the film subject to three different alternating voltages (10 Hz): 10 V (upper section), 15 V (middle section), and 0 V (bottom section). According to the shift in height as a result of changes in AC electric fields, the strain response of the film was found to be linear and nonhysteretic. To our knowledge, no detailed work related to piezoelectricity of crystalline cellulose or CNC films is available to date. Thus, this contribution provides the first experimental results showing that CNCs display such piezoelectric effects. Piezoelectric experiments were performed on four different supported CNC films, with different degrees of particle alignment. For a given voltage 7−10 repetitions were performed and the average used to calculate the piezoelectric constant (Figure 2). A linear correlation between the measured effective displacement and the applied voltage was observed. The values reported in Figure 2 were corrected for the contribution from the solid support (mica sheet on gold-coated glass wafer). Films of partly aligned CNCs (obtained by electric field assisted-shear at 800 V/cm, 45 Hz) yielded a piezoelectric constant of 0.97 Å/V. A similar value, 1.10 Å/V, was obtained with films manufactured under slightly lower electric field strength and higher frequency (400 V/cm and 200 Hz). The respective degree of alignment for these films was 42 and 46%, respectively (Table 1). CNC films with a higher degree of alignment (88% alignment degree obtained under assembly at 800 V/cm and 2 kHz) yielded a higher piezoelectric response, Figure 1. Map showing the extent of CNC film displacement (z direction) as a result of their piezoelectric effect. The extent of displacement is indicated by lighter or darker fields as monitored by an AFM (conductive) diamond tip in contact with the film. A single point was monitored under given intermittent electric fields (10, 15, and 0 V). The deflection measured was used to calculate the piezoelectric constant of the films. The x and z scales in the image are dimensionless but indicate film deflection evolution with time as the voltage is turned on and off (see Figure 3 for the experimental setup). Figure 2. Vertical displacement of CNC films subject to externally applied electric fields. Included are results for films produced under four different conditions during electric field-assisted shear (films i−iv, Table 1). The films with the higher degree of alignment produced a higher piezoelectric response, as indicated by the slopes of the profiles. The displacements and voltages are both peak-to-peak values. Table 1. Field Strength and Frequency Used during the Manufacture of CNC Films (i−iv) by Using an Electric Field-Assisted Shear Assembly Setup sample field strength (V/cm) /frequency (Hz) degree of alignment of CNCs (%) piezoelectric coefficient (d25)