One-step fabrication method for obtaining freestanding epitaxial double heterostructure is presented. This approach could achieve ZnO coverage with a higher number density than that of the epitaxial single heterostructure, leading to a piezoelectric nanogenerator with an increased output electrical performance.
Well-aligned ZnO nanostructures have been intensively studied over the last decade for remarkable physical properties and enormous applications. Here, we describe a one-step fabrication technique to synthesis freestanding ZnO nanorod/graphene/ZnO nanorod double heterostructure. The preparation of the double heterostructure is performed by using thermal chemical vapor deposition (CVD) and preheating hydrothermal technique. In addition, the morphological properties were characterized by using the scanning electron microscopy (SEM). The utility of freestanding double heterostructure is demonstrated by fabricating the piezoelectric nanogenerator. The electrical output is improved up to 200% compared to that of a single heterostructure owing to the coupling effect of the piezoelectricity between the arrays of ZnO nanorods on the top and bottom of graphene. This unique double heterostructure have a tremendous potential for applications of electrical and optoelectrical devices where the high number density and specific surface area of nanorod are needed, such as pressure sensor, immuno-biosensor and dye-sensitized solar cells.
Recently, the portable and wearable electronics devices became an essential element for a comfortable life owing to the nanotechnology development, which results in the tremendous demands for a power source in the range of microwatt to milliwatt. Considerable approaches for the power source of portable and wearable devices have been achieved by the renewable energy, including solar energy 1,2, thermal 3,4 and mechanical source 5,6. Piezoelectric nanogenerator have been intensively studied as one of possible candidate for energy harvesting device from environments, such as rustling the leaf 7, sound wave 8 and movement of human being 9. The primary principle underlying the nanogenerator is the coupling between the piezoelectric potential and dielectric material as a barrier. The piezoelectric potential generated in strained material induces the transient current that flows through the external circuit, which balances the potential at the interface between piezoelectric and dielectric material. The performance of nanogenerator would be improved by using nanostructure of piezoelectric material due to robustness under robustness under high stress and responsiveness to tiny deformation 10.
One-dimensional zinc oxide nanostructure is a promising component for piezoelectric materials in nanogenerator due to its attractive properties, e.g., its high piezoelectricity (26.7 pm/V) 11, optical transparency 12, and facile synthesis by using chemical process 13. Hydrothermal approach for growing the well aligned ZnO nanorod receives a great attention due to low cost, environmental friendly synthesis and potential for easy scaling up. Moreover, the preheating hydrothermal technique is easily controllable in experimental condition, resulting in many kinds of novel nanostructures, such as nanoleaves 14, nanoflowers 15 and nanotubes 16. The novel nanostructures enable a beneficial effect on performance of the electric and optoelectric devices wherever the high specific surface area of material is demanded.
In this protocol, we describe the experimental procedures for synthesis of more novel nanostructure (i.e., freestanding double heterostructure). The growth of ZnO nanorod at interface between graphene and polyethylene terephthalate (PET) substrate leads to the self-elevating the ZnO nanorod/graphene single heterostructure, yielding the freestanding double heterostructure. Furthermore, the feasible application of this unique nanostructure for electronic and optoelectric devices is demonstrated by fabricating a piezoelectric nanogenerator. Freestanding double heterostructure provides not only a high specific surface area but also a high number density of nanorod in a given area. This unique nanostructure has a tremendous potential for applications of electrical and optoelectrical devices, such as pressure sensor, immuno-biosensor and dye-sensitized solar cells.
1. Chemical Vapor Deposition (CVD) Growth of Single Layered Graphene
Note: The graphene used in this study was grown on copper (Cu) foil using the thermal chemical vapor deposition (CVD) technique (Figure 1A). Growth is uniform over an area of 2 cm x 10 cm for this system.
2. Preparation of Graphene/Polyethylene Terephthalate (PET) Substrate
3. Synthesis of ZnO Nanorod/Graphene/ZnO Nanorod Epitaxial Double Heterostructure
4. Fabrication of Piezoelectric Nanogenerator
Note: Piezoelectric nanogenerator in this study has the three electrodes (top, middle, bottom). Use the indium tin oxide (ITO) coated PET as the bottom electrode (Figure 4A).
5. Electrical Performance Measurement Setup
Note: We set up custom-made equipment for electrical performance characterization using linear motor, commercial scale and oscilloscope. Build the frame for vertically supporting the linear motor and place the commercial scale under the linear motor as shown in Figure 5A. The scale should be sensitive to small weight (0.02 kg – 20 kg).
The scanning electron microscopy (SEM) images shown in Figure 6 present the morphologies of hydrothermally grown ZnO nanorods. The preheating hydrothermal technique can result in two different nanostructures depending on the growth time. Figure 6A shows a typical image of ZnO nanorod on graphene/PET substrate at the growth time of 5 hr. In contrast, the image shown in Figure 6B indicates that the growth of ZnO nanorod at the growth time of 12 hr is successfully proceeded not only on the top of graphene but also on the bottom of graphene. Both ZnO nanorods are vertically aligned to the graphene layer, with average lengths of 2.49 and 0.70 µm, respectively. In addition, the length of ZnO nanorod on the bottom of graphene is increased approximately 2 times as the growth time increases up to 24 hr, as shown in Figure 6C. The growth of ZnO nanorod on the bottom of graphene is carried out over a large scale (>30 µm) and results in the self-elevation of ZnO nanorod/graphene construct, thereby forming freestanding double heterostructure (Figure 6D).
Figure 7A shows a photograph of the piezoelectric nanogenerator, which composed of the freestanding double heterostructure and two ITO-coated PET substrates as the top and bottom electrode. The electrical output is originated from the nanogenerator rather than measurement system, which is confirmed by polarity-switching measurement (Figure 7B and 7C). The output voltages from the arrays of ZnO nanorod on the top and bottom of graphene are observed up to 0.5 V and 0.3 V, respectively, by applying the periodic compress-release loads of 49 N (Figure 7E and 7F). Furthermore, the both arrays of ZnO nanorod produce the enhanced output voltage and current with average values of 0.9 V and 70 nA, respectively. The simulation result performed by COMSOL package reveals that the polarity of ZnO nanorod on the top and bottom graphene are reverse and good agreement between the results obtained from experiments and simulation (Figure 7D).
Figure 1. Growth of single layer graphene. (A) An image of experimental setup for thermal CVD. (B) Cu foil placement in growth furnace and (C) Growth step for single layer graphene. Please click here for a larger version of this figure.
Figure 2. Process flow of graphene transfer by using fishing method. Please click here for a larger version of this figure.
Figure 3. Synthesis process for the freestanding double heterostructure. Please click here for a larger version of this figure.
Figure 4. Schematic of the fabrication process for piezoelectric nanogenerator. Please click here for a larger version of this figure.
Figure 5. Electrical measurement setup. (A) Home-made measurement system schematic and (B) An photograph of measurement system. Please click here for a larger version of this figure.
Figure 6. SEM images of hydrothermally grown ZnO nanorod. (A) Single heterostructure after 5 hr of growth. Double heterostructure after 12 hr (B) and 24 hr (C) of growth and (D) Freestanding double heterostructure. Please click here for a larger version of this figure.
Figure 7. Representative results for piezoelectric nanogenerator. (A) An photograph of fabricated nanogenerator. Output voltages in forward (B) and reverse (C) connections. (D) COMSOL simulation of piezoelectric potential along a ZnO nanorod under axial strain. The dimensions of the nanorod are L = 600 nm and a = 60 nm and external force is 80 nN. Output voltages (E) and currents (F) of nanogenerator. Please click here for a larger version of this figure.
Please note that the high quality (>99.8%, annealed) of Cu foil should be considered as a substrate for successful growth of single layer graphene. Otherwise, the single layer graphene is not uniformly grown over the Cu foil, leading to dramatically decrease in conductivity of graphene. A 1 hr annealing at high temperature would help the improvement of the Cu foil crystallinity as well as removal of any contaminants from the Cu foil.
The growth of ZnO nanorod depends on the conditions for preheating hydrothermal, hence, the optimal concentration, temperature and time for growth have to carefully be calibrated. The concentration of zinc nitrate hexahydrate and HMT determines the diameter of ZnO nanorod, the additive PEI would not only provide the enhancement in aspect ratio of ZnO nanorod but also help continuous ZnO growth for long time 17,18.
The SEM images of typical freestanding single and double heterostructure are shown in Figure 6. As can be seen from these figures, the array of ZnO nanorods on the bottom of graphene begins to grow after 12 hr of growth, and their length linearly increase from 0.71 to 1.56 µm. Furthermore, the length of ZnO nanorods on the top of graphene is linearly increase before 12 hr, and then saturated approximately 2 µm after the growth of downward-grown ZnO nanorod. This result indicates that the ZnO nanorod growth obeys the manner of routine hydrothermal before 12 hr, but the double heterostructure begin to grow after 12 hr due to decomposed seed layer and the vacancy defect of graphene 19. Since we just regulated the growth time of typical hydrothermal technique to fabricate double heterostructure, the methodology proposed here can be used for other hydrothermally grown metal oxides, such as TiO2, SnO2 and Fe3O4.
For electrical measurements, it is necessary to examine whether or not the electric signal come from the fabricated nanogenerator. Triboelectricity between nanogenerator and measurement setup leads to electrical noise, and interrupts the accurate observation of piezoelectric performance. This is avoided by carefully configuring the initial position where nanogenerator is slightly contacted with measurement setup.
In summary, this manuscript details the facile protocols for fabricating a ZnO nanorod/graphene/ZnO nanorod epitaxial double heterostructure. The versatile approach to construct double heterostructure could enhance not only the number density of ZnO nanorod but also the specific surface area of double heterostructure in a given region. This approach thus utilizes two consecutive growth techniques to precisely construct unique nanomaterials into spatially arranged structures that can serve as functional materials for the number of electronic and optoelectronic applications such as touchpad electronics, smart gloves, implantable devices, and biosensors.
The authors have nothing to disclose.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2014R1A2A1A11051146). This work was also supported by National Research Foundation of Korea Grant funded by the Korean Government (NRF-2014R1A1A2058350).
Cu foil | Alfa Aesar | 13382 | |
poly(methyl methacrylate) (PMMA) | Aldrich | 182230 | |
zinc nitrate hexahydrate | Sigma-Aldrich | 228732 | |
hexamethylenetetramine (HMT) | Sigma-Aldrich | 398160 | |
polyethylenimine (PEI) | Sigma-Aldrich | 408719 | |
indium tin oxide (ITO) coated PET | Aldrich | 639303 | |
Silicone Elastomer Kit | Dow Corning | Sylgard 184 a, b | |
Nickel Etchant Type1 | Transene Company | 41212 |