Template-Assisted Hydrothermal Growth of Aligned Zinc Oxide Nanowires for Piezoelectric Energy Harvesting Applications

A flexible and robust piezoelectric nanogenerator (NG) based on a polymer-ceramic nanocomposite structure has been successfully fabricated via a cost-effective and scalable template-assisted hydrothermal synthesis method. Vertically aligned arrays of dense and uniform zinc oxide (ZnO) nanowires (NWs) with high aspect ratio (diameter ∼250 nm, length ∼12 μm) were grown within nanoporous polycarbonate (PC) templates. The energy conversion efficiency was found to be ∼4.2%, which is comparable to previously reported values for ZnO NWs. The resulting NG is found to have excellent fatigue performance, being relatively immune to detrimental environmental factors and mechanical failure, as the constituent ZnO NWs remain embedded and protected inside the polymer matrix.

H arvesting energy from ambient sources in our environment has drawn considerable attention in light of the demand for alternative energy solutions for "small power" electronic applications that currently mostly rely on traditional power sources such as batteries. Ubiquitous and easily accessible ambient vibrations could provide a convenient means to power devices such as wireless sensors, wearable and portable electronics, and even biomedical implants. 1,2 Energy harvesting (EH) devices based on piezoelectric nanogenerators (NGs) are well-suited for scavenging power directly from small-scale ambient vibrations such as body movement, heartbeats, wind and fluid flow, by directly converting mechanical energy to electrical energy. Piezoelectric NWs, in particular, are typically more sensitive to lowamplitude vibrations than their bulk or thin-film counterparts, allowing for their integration into small-scale and efficient NG devices. 3−5 Piezoelectric ceramic materials typically possess relatively high piezoelectric coefficients, 6 but these materials are prone to mechanical failure because they simultaneously possess high stiffness constants. 3 From a vibrational EH point of view, the brittle nature of piezoelectric ceramics renders them a less than ideal materials choice for NGs, whereas piezoelectric polymers can offer greater flexibility at the expense of output voltage for the same amount of strain. 3 We therefore aim to fabricate polymer-ceramic nanocomposites as NG devices, based on well-aligned piezoelectric ZnO NWs that are embedded and supported in a flexible polymer matrix. The low-temperature template-assisted hydrothermal synthesis method developed in this work is easily scalable 7,8 and results in flexible NGs with as-grown ZnO NWs that require minimal postdeposition processing and exhibit stable EH performance over time. We demonstrate the EH performance of a NG fabricated in this method by subjecting it to low-level periodic vibrations and recording the output voltage and current characteristics, and optimizing the output power across an impedance-matched load resistance. Importantly, the structure of our NG renders the ZnO NWs well-protected from environmental factors, including light, that are known to be detrimental to EH performance. 9−11 ZnO NWs have been previously synthesized by various techniques, but among them, wet chemical methods 7,12−14 such as sol−gel synthesis, 15,16 electrochemical deposition, 15,17 and hydrothermal synthesis 7,13,14,18−21 are more attractive as the growth occurs at a relatively low temperature and thus can be easily scaled up. 7 There have been many investigations into the growth of ZnO NWs onto different substrates, such as indium tin oxide (ITO) coated glass, 22,23 poly(ethylene terephthalate) 24 and silicon wafer substrates, 18 but hydrothermal synthesis was not adopted before to grow solid ZnO NWs directly within flexible porous polymer templates. Stassi et al. 25 reported the synthesis of hollow ZnO nanotubes (NTs) within PC membranes by aqueous chemical method where the reaction was conducted at atmospheric pressure within a beaker. The resulting NTs with wall thickness of the order of 10 nm had low mechanical stability as evidenced by their disintegration upon dissolution of the PC template. In this work, we have developed a novel method to grow high-quality solid ZnO NWs within flexible PC templates, resulting in particularly high structural integrity of the NWs so that they remain vertically aligned and intact even upon removal of the PC template. To the best of our knowledge, this is the first report on the growth of solid ZnO NWs within nanoporous polymer templates using a simple, scalable and low-temperature (<100°C) hydrothermal synthesis method that can be used to directly fabricate flexible NGs based on the ZnO-PC nanocomposite structure. The NWs obtained by this method are well-aligned, highly ordered and mechanically supported, with the pores of the PC template offering good control over the length and diameter of NWs. Although previously reported polymer ZnO NWs embedded in a polymer matrix 12 were realized using multiple fabrication steps, our template-assisted approach results in ZnO NWs embedded in a PC matrix fabricated from a single growth process, with minimal postprocessing steps required for the fabrication of a ZnO-PC nanocomposite NG.
For the hydrothermal synthesis of ZnO NWs, zinc nitrate and hexamethylenetetramine (HMTA) are the most widely used precursor solution. 18 The chemical reaction is given by Here, zinc nitrate is the source of Zn 2+ ions while HMTA steadily provides OH − for ZnO crystal growth. Traditionally, the so-called high-growth-rate aqueous solution 26 is often used to grow well-aligned ZnO NWs onto rigid substrates, which contains not only the precursor solution, but also polyethylenimine (PEI) and ammonium hydroxide that are required to effectively suppress the homogeneous nucleation in the bulk solution, facilitating the heterogeneous nucleation on the substrate and thereby allowing the growth of aligned ZnO NWs rather than formation of bulk particles. 21,23 However, the diameter and length of ZnO NWs obtained largely decreased, and various defects were induced by PEI and ammonium additions, leading to a relatively lower piezoelectric performance. 21 In this work, we avoided the inclusion of PEI and ammonium in our hydrothermal growth process by instead using the PC template pores (with average diameter ∼227 ± 92 nm and length ∼12 μm) to confine the growth of ZnO NWs, as demonstrated in Figure 1. The figure shows the field-emission scanning electron microscopy (FE-SEM; FEI Nova NanoSEM) images of the PC template before ( Figure 1a) and after ( Figure  1b) the hydrothermal synthesis of ZnO NWs, with energy dispersive X-ray mapping (EDX; Bruker XFlash 6100) in Figure 1c. These images confirm a large fraction of the PC pores were filled with ZnO, and the resulting distribution of ZnO NW diameter (average ∼256 ± 51 nm) is shown in Figure 1d. The NW-filled template can be partially dissolved using chlorobenzene (see Supporting Information part S1) to partly expose the ZnO NWs thus revealing the ZnO-PC nanocomposite structure, as shown in Figure S1a. The growth method detailed in Supporting Information part S1, comprised of two steps: (I) seeding of the PC template pores in the precursor solution, and (II) subsequent NW growth within a Teflon-sealed stainless steel autoclave reactor. As the NW growth process can be controlled by adjusting reaction parameters e.g. precursor concentration, growth temperature and growth time, it was found that the optimum conditions for the growth of ZnO NWs within PC templates is 95°C for 5 h. To obtain higher density and longer NWs, the precursor solution was "refreshed" twice by repeating the growth steps on the same PC template, with care being taken to remove ZnO crystals deposited on the surface of the PC template by gently washing in hydrochloric acid (see Supporting Information part S1). To visualize the structure and morphology of the ZnO nanowires freed from the PC template, the template was burned away at 500°C for 0.5 h at a slow heating rate of 1°C per min. Figure 2 shows SEM images of free-standing ZnO NWs that resulted upon complete removal of the PC template, revealing a dense array of

ACS Applied Materials & Interfaces
Letter vertically aligned ZnO NWs of uniform length grown within the confinement of the PC template pores. It was also found that the solution-refreshing treatment largely increased the length and density of NWs as more pores were filled ( Figure  S2f). Although our template removal method allows for easy collection and imaging of the NWs, some NWs were found to partially disintegrate because of the resulting thermal shock and the mechanical stress produced because of the deformation of the PC templates during the burning treatment. The average length of the free-standing NWs was found to be 8.4 ± 2.1 μm which is close to the nominal thickness of the PC template. The ZnO NWs grown within PC templates exhibited much higher aspect ratio compared with the NWs grown from the same precursor solution directly onto ITO-coated substrates, where NW lengths were found to be typically less than 4 μm ( Figure  S3).
The orientation and crystalline structure of the templategrown ZnO NWs were confirmed using X-ray diffraction studies (XRD; Bruker D8 advanced). The XRD patterns (Figure 3a) from a bare PC template as well as three separate PC templates subjected to different growth treatments (seeded, nonrefreshed and refreshed) are shown. It can be seen that there is no signal detected from the seeded PC template, indicating the seeding process only resulted in noncrystalline ZnO nuclei rather than NWs. Only the PC templates (refreshed and nonrefreshed) filled with ZnO NWs show the formation of a dominant peak from (100). The presence of higher order peaks suggest that these NWs may be polycrystalline. The refreshed ZnO-PC sample was found to exhibit a more intense signal than the nonrefreshed ZnO-PC sample, indicating that a higher density of NWs was obtained via the solution-refreshing treatment. This is due to the fact that as the Zn 2+ source is gradually depleted from the precursor solution, it is difficult to form long NWs in a single growth step, and thus refreshing the precursor solution is an effective approach to obtain longer NWs 12,27 under the confinement effect of PC templates. Note that that higher-order peaks are not evident in the XRD pattern of the nonrefreshed sample because of the much weaker signal arising from incompletely formed NWs in the nonrefreshed sample, as compared to the higher density of fully grown NWs in the refreshed sample.
High-resolution transmission electron microscopy (HRTEM; JEOL 4000EXII) images are shown in Figure 3b, c. As indicated from the HRTEM image, the polycrystalline ZnO NWs constitute well-formed crystals with sizes ranging between 22 and 30 nm (shown by arrows in Figure 3b). The average diameter of the ZnO NWs as calculated from the TEM images is 240 nm ±40 nm. A lattice fringe spacing of 0.28 nm reveals the dominant (100) plane growth in the ZnO NWs along their long-axis. Furthermore, the selected area electron diffraction (SAED) pattern (Figure 3c inset) characteristically shows the highly crystalline ring pattern, indicative of polycrystalline ZnO.
Compared with the above results, XRD patterns from ZnO films and bulk crystals grown hydrothermally from the same precursor solution exhibit no preferred orientation ( Figure  S4a). Furthermore, the ZnO NWs grown within PC templates exhibit an oriented a-axis growth along (100) (illustrated in the inset of Figure 3a) as compared to the ZnO NWs grown directly onto ITO-coated substrates which preferentially grow along a (002) axis ( Figure S4b). This indicates the fabricated ZnO NWs grown within PC templates show preferential c-axis orientation perpendicular to the pore walls as a result of being confined by the template pores, 25 and not simply due to the hydrothermal growth process. The NWs are thus formed of single-crystalline nanoparticles with the c-axis parallel to the plane of the template stacked on top of one another within each pore, with smaller crystallites of random orientations present as well, possibly arising due to interface imperfections. The shorter NWs grown onto ITO-coated substrates are likely to be single crystalline with very strong alignment exhibited in XRD patterns (Figures S3 and S4b), while longer ZnO NWs grown within PC templates show polycrystallinity due to fusing of the nanoparticles or nanorods together, corresponding to higher order peaks. The observed alignment of ZnO NWs is crucial for the NG applications as further electrical poling treatment is not required to align the piezoelectric domains. 28

Letter
Finally, flexible, lightweight, and inexpensive NGs were fabricated by sputtering gold electrodes of thickness ∼30 nm onto the surfaces of the ZnO NW-filled PC template (refreshed sample), with Cu wires attached using Ag paint for electrical access. The piezoelectric response of these flexible NGs were measured by using a vibrational energy harvesting system previously described by Whiter et al. 8 to record the output voltage and power under different loading conditions, in response to periodic mechanical impacting, as shown in Figure  4. Our NG was subjected to uniaxial compression in order to take advantage of the vertical alignment of the ZnO NWs with a-axis orientation along their lengths. The fabricated NGs were tested at frequencies ranging from 5 to 75 Hz, and different amplitudes of the oscillating arm ranging from 0.5 to 3 mm, while keeping the other parameter fixed (see Supporting Information part S3 for details of the energy harvesting measurement setup). The observed open-circuit voltage output V OC exhibited positive peaks corresponding to compression of the NGs, whereas the negative peaks arise due to the releasing of strain. 22 The peak-to-peak value of V OC increased with both increasing frequency (Figure 4a) as well as amplitude ( Figure  4b). On the basis of these frequency-and amplitude-dependent tests, a significant increase of V OC could be obtained through impacting forces of higher frequency and/or amplitude. Electrical output characteristics for an impacting periodic force of frequency 5 Hz and amplitude 1 mm, akin to gentle finger tapping ( Figure S6), are presented in Figure S9 for comparison. We have also measured the output voltage in response to simple bending by hand, as shown in Figure S7. A blank PC template was tested under the same impacting conditions as the ZnO-PC NGs to confirm that the measured electrical output was solely due to the piezoelectric ZnO NWs ( Figure S8a). Also, it was found that compared to the nonrefreshed samples (Figure S8b), the solution-refreshed samples could generate up to an order of magnitude higher output voltage as well as current, which was consistent with the observation of longer and higher-density NWs being formed.
To highlight the mechanical stability and robustness of our NG, we conduct rigorous fatigue testing of our device by subjecting it to up to 540 000 continuous impacting cycles, i.e., 30 h of continuous testing at 5 Hz, as shown in Figure 4c. We find that after an initial ∼10% drop in the peak value of V OC occurring after 10 h of testing, the output remains stable for up to 30 h of testing. Furthermore, our NG was tested over a period of 4 months with negligible degradation in performance. Our results are particularly significant given that such extensive fatigue data is rarely presented in the literature for ZnO NWbased NGs (see Table S1).
To evaluate how much electrical power the ZnO-PC NGs can generate in a practical application, we connected the NG to different load resistors R L (1 kΩ to 100 MΩ) to determine the maximum power output that can be achieved through impedance matching. 29 The output power was found to peak at a resistance of ∼1 MΩ with a value of power density ∼1600 mW/m 3 (Figure 4d), for an effective NG area of ∼3 cm 2 . Note that only 12% of this area comprises the piezo-active ZnO NWs, as this fraction corresponds to the porosity of the PC template. The output power can be greatly enhanced by stacking several devices in parallel and connecting them electrically in series, 8 or by subjecting the NG to a higher impacting force ( Figure S6a). It is generally difficult to quantitatively compare electrical output of NGs due to the large differences in geometry and mechanical excitations reported 29,30 (see Table S1) . However, from the measured electrical output characteristics, the energy conversion efficiency 29−31 of our NG was evaluated to be ∼4.2% (see Supporting Information part S5), which is comparable to previously reported ZnO NWs. 30

Letter
In summary, we have successfully developed a templateassisted hydrothermal synthesis method to grow dense and well-aligned ZnO NWs with uniform diameter and length within PC templates that provide flexibility and robustness to the resulting NGs. Because of the confined growth process, all fabricated ZnO NWs demonstrate strong alignment along (100) preferred orientation, which is beneficial to their piezoelectric performance under uniaxial compression. While higher piezoelectric performance is expected of c-axis oriented NWs, this is outweighed by the benefit of prolonged and robust piezoelectric performance as seen in the superior fatigue performance of our NG. Furthermore, the compliant and flexible PC template allows for effective straining of the ZnO NWs upon application of stress, and can be easily mounted on curved surfaces without causing the NG to fracture. A single NG of area 3 cm 2 and thickness ∼12 μm generates a peak output power density of ∼1600 mW/m 3 across a load resistance of ∼1 MΩ, and excellent fatigue performance with an energy conversion efficiency that is typically higher than ZnO thin films or bulk samples. The as-grown ZnO NWs are well-protected from detrimental environmental factors and mechanical failure within the PC template, and can be easily incorporated into NGs without the need to be removed from the template. The solution-based fabrication method is low-cost and easily scalable and can thus be used to produce cheap and stable NGs for EH applications.