Preparation of ZnO Nanorod/Graphene/ZnO Nanorod Epitaxial Double Heterostructure for Piezoelectrical Nanogenerator by Using Preheating Hydrothermal

The overall goal of this experiment is to synthesize a novel zinc oxide nanostructure on graphene, using hydrothermal technique and demonstrate that it exhibits enhanced piezoelectric performance. This method can help answer key questions in the nanomaterial field for electric and optoelectronic applications, such as photovoltics, biosensors, as well as wearable and implantable electronics. The main advantage of this noble nanostructure is that it could enhance not only the number of nanoload, but also the specifics of this area, in the key-boleez-an, for piezoelectric nanogenerators.

Generally, individuals new to this method for graphing transfer, we struggle because it is not easy to place the floating graphene on the giant position of the substrate. I first had an idea that I could apply this noble structure to a nanogenerator for improving the performance when I found that a zinc oxide nanoload can grow on both sides of the graphene. The graphene used in this study is grown on copper foil, using the thermal chemical vapor deposition, or CVD technique.

To begin, wash the copper foil with a mild flow of acetone, isopropyl alcohol, and distilled water, respectively. Place the cleaned copper foil in a two inch quartz tube in the furnace, and then purge the furnace chamber with vacuum for ten minutes, by using a rotary pump. Configure the temperature of a digitalized furnace to ramp up the furnace to 995 degrees Celsius, while maintaining the desired flow rates.

Introduce the 20 sccm of methane for 10 minutes to grow the single-layered graphene. Maintain the 80 sccm of argon and 20 sccm of hydrogen throughout the process. After allowing the furnace to cool down to room temperature, within five minutes, purge the chamber again with argon at 100 sccm.

Place the graphene grown copper foil on a glass slide and fix the edges by commercial tape. Spin coat a layer of polymethylmethacrylate, or PMMA, at 550 rpm for five seconds, and then 3, 000 rpm for 30 seconds. Next, bake the PMMA-coated copper foil substrate at 60 degrees Celsius for two minutes to remove the solvent residue.

Dice the PMMA-coated copper foil into a smaller, one centimeter by one point five centimeter piece using a razor blade. Immerse the PMMA-coated copper foil into a nickel etchant reservoir by placing the copper foil facedown for 30 minutes. This leaves the floating PMMA graphene layer on the etchant solution.

A piece of PMMA-coated copper foil is enough for the experiment forward. Scoop the PMMA graphene layer up on slide glass, and then immerse the PMMA graphene layer into a deionized water reservoir. After repeating this process twice, scoop the PMMA graphene layer up on the polyethylene terephthalate, or PET, substrate.

Then, bake the substrate at 105 degrees Celsius for two minutes, to remove water residue. Finally, move the PMMA layer by dipping the substrate in warm acetone for 10 minutes. Begin synthesis with preheating the precursor solution of 40 millimolar zinc nitrate hexahydrate, 40 millimolar hexamethylene tetramine, and nine millimolar polyethylenimine in deionized water for 60 minutes at 95 degrees Celsius in a convection oven.

Meanwhile, fully cover the graphene PET substrate with a five millimolar zinc acetate solution in ethanol. Spin coat a layer of zinc acetate on the substrate at 500 rpm for five seconds, and then 2, 000 rpm for 60 seconds. Then, bake the substrate at 200 degrees Celsius for 30 minutes.

After repeating this process twice, the thickness of the layer is approximately 30 nanometers. Next, immerse the seed coated graphene PET substrate into the preheated solution by placing the substrate facedown at 95 degrees Celsius. Determine the heating time for the desired nanostructure.

Carefully spray ethanol on the substrate and allow it to dry at room temperature for one hour. The piezoelectric nanogenerator fabricated in this study has three electrodes. The indium tin oxide, or ITO coated PET, is used as the bottom electrode.

Spin coat a layer of polydimethylsiloxane, or PDMS, onto the ITO PET substrate at 500 rpm for five seconds, and then 6, 000 rpm for 60 seconds to form the insulating layer between the ITO and the zinc oxide nanorod. The thickness of the layer is approximately three microns. Fully cure the substrate at 80 degrees Celsius for two hours in a convection oven.

Then, transfer the graphene to the PDMS-coated ITO PET substrate as done before. Next, synthesize the double heterostructure on the substrate by using the demonstrated method. Spin coat a layer of PDMS at 500 rpm for five seconds, and then 5, 000 rpm for 60 seconds, to improve the robustness and durability of the zinc oxide nanorod.

Then, fully cure the nanorod at 80 degrees Celsius for two hours. The thickness of the layer is approximately eight microns. As a final step, cover the substrate with ITO-coated PET as the top electrode.

Set up custom-made equipment for electrical performance characterization, using a linear motor, commercial scale, and oscilliscope. Build the frame for vertically supporting the linear motor and place the commercial scale under the linear motor. The scale should be sensitive to small weight.

Place the nanogenerator on a scale, and then connect the electrodes of the nanogenerator to the sensing probes of an oscilloscope. Set up the initial and final positions, as well as the speed of the linear motor, while carefully monitoring the weight on the scale. Configure the initial position where the nanogenerator is slightly in contact with the measurement setup.

Start the linear motor. The speed of the linear motor determines the strain rate. Here, a strain rate of 100 millimeters per second and an applied load of 50 newtons is used.

Monitor the voltage signal with time, saving the time-dependent voltage signal in flash memory. The SCM images shown here present the morphologies of hydrothermally-grown zinc oxide nanorods. The preheating hydrothermal technique can result in two different nanostructures, depending on the growth time.

Here is a typical image of the zinc oxide nanorod on a graphene PET substrate at a growth time of five hours. In contrast, the image shown here indicates that the growth of the zinc oxide nanorod at 12 hours is successful on the top and the bottom of the graphene. Both zinc oxide nanorods are vertically aligned to the graphene layer, with average lengths of two point four nine and zero point seven zero microns, respectively.

In addition, the length of the zinc oxide nanorod on the bottom of the graphene is increased approximately two times as the growth time increases up to 24 hours. The growth of the zinc oxide nanorod on the bottom of the graphene is carried out over a large scale, and results in the self-elevation of the zinc oxide nanorod graphene construct, thereby forming the freestanding double-hetero structure. The electrical output is originated from the piezoelectric nanogenerator, rather than the measurement system, which is confirmed by polarity switching measurement.

The output voltages from the arrays of the zinc oxide nanorod are observed up to zero point five volts on the top of the graphene, and up to zero point three volts on the bottom of the graphene, by applying the periodic compress release loads of 49 newtons. Once mastered, this technique can be done in one or two days if it is performed properly. After watching this video, you should have a good understanding of how to synthesize a noble nanostructure and fabricate the piezoelectric nanogenerator.