To generate energy for nanoscale systems ZnO

To generate energy for nanoscale systems, ZnO based piezoelectric nanogenerators are an ideal source, especially in the case of small size and low power consumption batteries with a long life time. For a sustainable independent operation, a continuous power source is required and for this type of operations the energy source must be self powered. ZnO based nanostructures can harvest energy from external sources. These may include the stretching of muscles, blinking of an eye, joint movement, and expansion and contraction of blood vessels [8–11]. In this work we have used Pt as a sputtering electrode material rather than gold. In our previous work [12] we have sputtered gold to act as the top electrode and we have studied output voltage graphs with 2.5nm, 5nm and 7nm thick sputtered layers. We have used Pt in this study due to its high work function (6.1eV). External applied force is nearly 50nN as in an earlier reported work [12].
Materials and methods Zinc acetate dehydrate [Zn (CH3COO)2·2H2O], hexamethylenetetramine (C6H12N4), zinc nitrate hexahydrate [Zn(NO3)2⋅6H2O] and solvents of analytical grades with no further purification were used for synthesis. All reagents were purchased from Sigma Aldrich. An ITO coated PET substrate was ultrasonically cleaned with deionized water and then with acetone for 15min then dried in air for 30min. A droplet of 10mM zinc acetate was coated on PET substrates to grow a seed layer. Coated PET substrates were dried in the air for 20 to 30min. A well adhered and wurtzite structured ZnO seed layer was obtained by repeating the coating step two to three times then annealed at 60°C for an hour. Growth of well aligned ZnO nanowires was carried out in an equimolar nutrient solution of hexamethylenetetramine and zinc nitrate hexahydrate in a reaction flask. A 300mL nutrient solution was prepared and seed layer coated PET substrates were immersed upside down for 2.30h at 95°C. Substrates were removed from solution and for cleaning purposes they are rinsed with deionized water and dried in the air for 30min. ITO coated PET substrates have a resistivity of 10MΩ/sq, and their device structure is similar to that used in our earlier published work i.e. an ITO coated PET substrate has acted as a ptio electrode; it works as a bottom electrode due to its conductive coating. The middle portion is comprised of vertically grown ZnO nanowires and the upper electrode is a Pt sputtered electrode using Emscope SC 500 having an internal pressure of Ar 0.1Torr. A nutrient solution of HMTA (C6H12N6) and zinc nitrate hexahydrate [Zn(NO3)2·6H2O] was prepared in a canonical flask with a volume of 300mL; it is an equimolar solution and the molarity of the solution was adjusted by using a chemical relationwhere C is the molar concentration in mol/L, m is the mass of solute dissolved in a solvent of V liters and MV is the molecular weight in g/mol of the solute dissolved in solvent. The size of the device was around 500nm, and pure copper wires are connected to both the top and bottom electrodes by silver paste. Measurement of voltage and output power density was acquired by Picoscope 6204 (PC based instrument) i.e. we have recorded our voltage values by connecting our nanostructure with a picoscope and the picoscope to our computer. For the growth of ZnO nanowires Zn was provided by zinc nitrate salt hexamethylenetetramine which acted as a weak baseand gradually produced OH−. This slow release of OH− was found crucial for the proper growth of nanowires; if the release was fast then nutrient solution would have been consumed quickly and the further growth of nanowires would be stopped. The growth of nanowires was dependent on reaction parameters like temperature, time and the concentration of the precursor. The density of nanowires on the substrate was controlled by controlling the concentration of reactants in the precursor. We have carried out a series of experiments by varying the concentration of zinc salt and hexamethlenetramine and found that they should be equimolar. In initial stages of growth, nucleation growth was the deciding factor for the density of nanowires. These nuclei grew with the passage of time and initially converted in nanorods and then gradually transformed in nanowires. After initial stages the density of nanowires were not affected by further nuclei and these late coming nuclei were dissolved in a solution. Initially rod like structures were grown as reported [12] but we have grown a long wire structure by giving more and more time to a weak base so that it can slow down the growth process in the nutrient solution and wires can grow along their length.