Synthesis of Ni-Doped ZnO Nanorod Arrays by Chemical Bath Deposition and Their Application to Nanogenerators

Nanogenerators (NGs) based on Ni-doped ZnO (NZO) nanorod (NR) arrays were fabricated and explored in this study. The ZnO films were grown on indium tin oxide (ITO) glass substrates, and the NZO NRs were prepared by the chemical bath deposition (CBD) method. The samples were investigated via field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) spectral analysis. The results showed that the growth of NRs presented high-density single crystalline structures and were preferentially oriented in the c-axis direction. The optical characteristics of the NZO NRs were also measured by photoluminescence (PL) spectra. All samples exhibited two different emissions, including ultraviolet (UV) and green emissions. ITO etching paste was used to define patterns, and an electrode of Au film was evaporated onto the ITO glass substrates by the electron beam evaporation technique to assemble the NG device. In summary, ZnO NRs with Ni dopant (5 mM) showed significantly excellent performance in NGs. The optimal measured voltage, current, and power for the fabricated NGs were 0.07 V, 10.5 µA, and 735 nW, respectively.


Introduction
The human demand for energy has reached an unprecedented level in recent decades because of the expansion of the earth's population and the industrialization of developing countries. At the same time, primary energy (PE) is also being exhausted daily (PE is an energy form found in nature that has not been subjected to any human engineered conversion process). Thus, this constitutes a very important reason to find novel renewable and ecologically pure alternative energy sources. Researchers are concerned with the limited electric equipment and power supply system used for collecting waste energy in our living surroundings [1,2]. At present, nanogenerators (NGs) are designed for transforming mechanical energy into electric energy [3,4]. These NGs are combined with one-dimensional (1-D) nanostructures for application in portable or wearable devices to collect energy for the supply of electric power in the future. In 1-D ZnO NG devices, NGs can be divided into three types, namely, piezoelectric (PZ), triboelectric, and pyroelectric applications [5][6][7]. NGs can also be applied in surface acoustic wave (SAW) devices [8]. Dopant and flexible substrates can improve the output power efficiency of ZnO piezoelectricity and enhance the optical and electric characteristics of ZnO [9].
Metal oxide semiconductors (MOSs) have been used interestingly in many research fields because of their outstanding properties, such as a low cost, excellent controllability, and good integration

Experimental
The NGs based on NZO ZnO NR arrays were fabricated in two steps. (1) During deposition, a ZnO target with a 3-inch diameter and purity of 99.99% was used. A 100 nm layer of ZnO film was grown on 1 × 1.5 cm 2 indium tin oxide (ITO) glass substrates via a radio frequency (RF) magnetron sputtering technique (vacuum of 5 × 10 −6 Torr). The RF power and gas mixing ratio (Ar/O 2 ) were 100 W and 10/1, respectively. Subsequently, the samples were dipped in 1:2 proportion of 0.015 M zinc acetate dihydrate (Zn[CH 3 COO] 2 •2H 2 O, 99.99%) and 0.03 M hexamethylenetetramine (C 6 H 12 N 4 , HMTA) with different concentrations of Ni(II) acetate (Ni[CH 3 COO] 2 , 99.8%) solution (0 and 5 mM); CBD was used to grow NZO NRs in a 95 • C oven for 3 h. The Ni dopant was supplied from Ni(II) acetate, and the reaction formulas of ZnO are as follows [27]: After the completion of the reaction, the samples were rinsed with deionized (DI) water and dried in an oven for 5 min at 100 • C. Then, the surface morphology of the NZO NRs was characterized with field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-3010). The elemental, crystallographic, and optical material properties of NZO NRs were analyzed via energy dispersive X-ray (EDX) spectrometry, X-ray diffraction (XRD) spectral analysis (MO3XHF22 MAC-Science), and photoluminescence (PL) spectrometry (Labram HR), respectively.
(2) The patterns were defined on the ITO glass substrates, and ITO etching paste was used as etchant for etching in an oven at 170 • C for 5 min. After this, the samples were cooled down for 10 min under room temperature and immersed in acetone, isopropyl alcohol, and DI water for individual cleaning for 1 min. A 100 nm Au layer was subsequently evaporated onto the surface as electrode by using electron beam evaporation technique. Lastly, the NZO NR arrays and electrode were combined to form NG devices and then placed in an ultrasonic oscillator machine with a water chamber. An ultrasonic wave was applied to drive the NG devices. A precision digital meter (Keithly 2400) was connected by copper tape to measure the average current, voltage, and power, as shown in Figure 1a-b. This experimental process was similar to that previously reported by Chen et al. [28].
After the completion of the reaction, the samples were rinsed with deionized (DI) water and dried in an oven for 5 min at 100 °C. Then, the surface morphology of the NZO NRs was characterized with field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-3010). The elemental, crystallographic, and optical material properties of NZO NRs were analyzed via energy dispersive X-ray (EDX) spectrometry, X-ray diffraction (XRD) spectral analysis (MO3XHF22 MAC-Science), and photoluminescence (PL) spectrometry (Labram HR), respectively.
(2) The patterns were defined on the ITO glass substrates, and ITO etching paste was used as etchant for etching in an oven at 170 °C for 5 min. After this, the samples were cooled down for 10 min under room temperature and immersed in acetone, isopropyl alcohol, and DI water for individual cleaning for 1 min. A 100 nm Au layer was subsequently evaporated onto the surface as electrode by using electron beam evaporation technique. Lastly, the NZO NR arrays and electrode were combined to form NG devices and then placed in an ultrasonic oscillator machine with a water chamber. An ultrasonic wave was applied to drive the NG devices. A precision digital meter (Keithly 2400) was connected by copper tape to measure the average current, voltage, and power, as shown in Figure 1a-b. This experimental process was similar to that previously reported by Chen et al. [28].

Results and Discussion
To prepare pure ZnO and NZO NR structures, all the regents were analytically uncontaminated and used without further purification. The NR structures were grown on ZnO films for 3 h at 95 • C by CBD. Figure 2 shows the nanostructural analysis of the as-grown pure ZnO and NZO NRs obtained by FE-SEM. It can be observed that pure ZnO NRs were vertically grown on the substrate. The height of NRs was 1.76 µm. With a Ni dopant of 5 mM, the cross-sectional surface of ZnO NRs strongly increased, and the length of NRs slightly decreased. The height of NRs was 1.72 µm. The images show that the surface of ZnO NRs changed and became dense because of the Ni dopant. The growth mechanism can be attributed to the higher bonding energy of Ni-O than Zn-O. The surface energy between the polar and nonpolar surfaces differed, thereby enhancing the lateral growth and inhibiting the vertical one. This growth process was similar to that previously reported by Kim et al. and Dong et al. [29,30].
With regard to the phase and lattice constant identification, the structural characteristics were determined via XRD analysis, including Cu Kα-1 radiation and λ = 0.15405 nm, as shown in Figure 3. The main diffraction peak was located at 34.58 • , which corresponds to the (002) diffraction plane of pure wurtzite ZnO structure. The pure ZnO sample displayed a hexagonal wurtzite phase (JCPDS Card No.  and no secondary phase. The same diffraction peaks can be also illustrated in the XRD patterns of the NZO NR arrays. The location of the impurity peaks was not found, thereby implying that the investigated pure ZnO and NZO samples have good crystalline properties [31]. Meanwhile, it is preferentially oriented along the c-axis, which means that all the samples were oriented perpendicular to the substrate. The red-shift of the diffraction peak was also derived from the decrease in d-spacing; this condition is defined by the following equation (Bragg's Law) [32]: where n is the order of diffraction, λ is the X-ray wavelength, θ is the angle of diffraction, and the value of d is the distance between planes. The shifting of the (002) peaks was toward the right side with Ni doping. This movement shows that a small Ni 2+ (ionic radius = 69 pm) has been effectively incorporated or substituted into the ZnO host structure at the Zn 2+ (ionic radius = 74 pm) site. These similar results were also observed in a previous report [33].

Results and Discussion
To prepare pure ZnO and NZO NR structures, all the regents were analytically uncontaminated and used without further purification. The NR structures were grown on ZnO films for 3 h at 95 °C by CBD. Figure 2 shows the nanostructural analysis of the as-grown pure ZnO and NZO NRs obtained by FE-SEM. It can be observed that pure ZnO NRs were vertically grown on the substrate. The height of NRs was 1.76 µm. With a Ni dopant of 5 mM, the cross-sectional surface of ZnO NRs strongly increased, and the length of NRs slightly decreased. The height of NRs was 1.72 µm. The images show that the surface of ZnO NRs changed and became dense because of the Ni dopant. The growth mechanism can be attributed to the higher bonding energy of Ni-O than Zn-O. The surface energy between the polar and nonpolar surfaces differed, thereby enhancing the lateral growth and inhibiting the vertical one. This growth process was similar to that previously reported by Kim et al. and Dong et al. [29,30]. With regard to the phase and lattice constant identification, the structural characteristics were determined via XRD analysis, including Cu Kα-1 radiation and λ = 0.15405 nm, as shown in Figure 3. The main diffraction peak was located at 34.58°, which corresponds to the (002) diffraction plane of pure wurtzite ZnO structure. The pure ZnO sample displayed a hexagonal wurtzite phase (JCPDS  incorporated or substituted into the ZnO host structure at the Zn 2+ (ionic radius = 74 pm) site. These similar results were also observed in a previous report [33]. All samples were annealed at 600 °C in air environment and characterized via PL spectrometer. PL spectroscopy was employed to measure the optical emissions from 300 nm to 700 nm with a 325 nm He-Cd laser with 25 mW increment at room temperature. Figure 4 shows that each sample exhibited two types of emission peak at wavelength of ~380 (3.26 eV) and 560 nm (2.21 eV), corresponding to a strong ultraviolet (UV) radiation (near band emission, NBE) and a visible-light emission (deep-level emission, DLE), respectively. The energy of the peak due to UV emission was almost equal to the band gap energy of the ZnO structure. By contrast, the visible-light emission of ZnO was yellow-green and depended on the availability of oxygen vacancies. The yellow-green emission region was attributed to the recombination of electrons trapped in singly ionized oxygen vacancies with holes and the inherent defects in the ZnO structure, such as zinc vacancy, interstitial zinc, oxygen vacancy (VO), and interstitial oxygen [34,35]. After Ni doping, the UV peak showed a slight red-shift from 379 nm to 383 nm, and the visible emission intensity gradually decreased. The above-mentioned phenomena can be attributed to the following: (1) the localized "d" electrons of Ni 2+ replaced those of Zn 2+ , (2) the sp-d exchange interactions between the electrons of band, and (3) VO regions were eliminated by annealing treatment at high temperature in air conditions. As a result, it was observed that Ni-doping can improve the optical properties of ZnO NR arrays; this phenomenon is similar to that reported earlier for NZO structure [36,37].  All samples were annealed at 600 • C in air environment and characterized via PL spectrometer. PL spectroscopy was employed to measure the optical emissions from 300 nm to 700 nm with a 325 nm He-Cd laser with 25 mW increment at room temperature. Figure 4 shows that each sample exhibited two types of emission peak at wavelength of~380 (3.26 eV) and 560 nm (2.21 eV), corresponding to a strong ultraviolet (UV) radiation (near band emission, NBE) and a visible-light emission (deep-level emission, DLE), respectively. The energy of the peak due to UV emission was almost equal to the band gap energy of the ZnO structure. By contrast, the visible-light emission of ZnO was yellow-green and depended on the availability of oxygen vacancies. The yellow-green emission region was attributed to the recombination of electrons trapped in singly ionized oxygen vacancies with holes and the inherent defects in the ZnO structure, such as zinc vacancy, interstitial zinc, oxygen vacancy (V O ), and interstitial oxygen [34,35]. After Ni doping, the UV peak showed a slight red-shift from 379 nm to 383 nm, and the visible emission intensity gradually decreased. The above-mentioned phenomena can be attributed to the following: (1) the localized "d" electrons of Ni 2+ replaced those of Zn 2+ , (2) the sp-d exchange interactions between the electrons of band, and (3) V O regions were eliminated by annealing treatment at high temperature in air conditions. As a result, it was observed that Ni-doping can improve the optical properties of ZnO NR arrays; this phenomenon is similar to that reported earlier for NZO structure [36,37].
Energies 2020, 13, x FOR PEER REVIEW 5 of 11 incorporated or substituted into the ZnO host structure at the Zn 2+ (ionic radius = 74 pm) site. These similar results were also observed in a previous report [33]. All samples were annealed at 600 °C in air environment and characterized via PL spectrometer. PL spectroscopy was employed to measure the optical emissions from 300 nm to 700 nm with a 325 nm He-Cd laser with 25 mW increment at room temperature. Figure 4 shows that each sample exhibited two types of emission peak at wavelength of ~380 (3.26 eV) and 560 nm (2.21 eV), corresponding to a strong ultraviolet (UV) radiation (near band emission, NBE) and a visible-light emission (deep-level emission, DLE), respectively. The energy of the peak due to UV emission was almost equal to the band gap energy of the ZnO structure. By contrast, the visible-light emission of ZnO was yellow-green and depended on the availability of oxygen vacancies. The yellow-green emission region was attributed to the recombination of electrons trapped in singly ionized oxygen vacancies with holes and the inherent defects in the ZnO structure, such as zinc vacancy, interstitial zinc, oxygen vacancy (VO), and interstitial oxygen [34,35]. After Ni doping, the UV peak showed a slight red-shift from 379 nm to 383 nm, and the visible emission intensity gradually decreased. The above-mentioned phenomena can be attributed to the following: (1) the localized "d" electrons of Ni 2+ replaced those of Zn 2+ , (2) the sp-d exchange interactions between the electrons of band, and (3) VO regions were eliminated by annealing treatment at high temperature in air conditions. As a result, it was observed that Ni-doping can improve the optical properties of ZnO NR arrays; this phenomenon is similar to that reported earlier for NZO structure [36,37].  The nanostructures and lattice spacing relations of NZO sample were explored by transmission electron microscopy (TEM) to further illuminate the detailed structure of the NZO NRs, as shown in Figure 5. In previous work and related literature, NZO NR arrays were obtained in a water solution system through CBD [38]. Figure 5a exhibits the fragments of NZO NR scraped from the sample with ethanol solution and then dropped onto a copper mesh with a dropping pipet. Lastly, the copper mesh was dried at 90 • C for 2 h in the oven. Figure 5b,c demonstrate that the HR-TEM image displayed the fringes of NZO planes with an interplanar spacing of approximately 0.259 nm. This manifestation indicates that the NZO NR is a single crystalline structure and can grow along the (002) direction, which is in accordance with selected-area electron diffraction (SAED) pattern shown in the upper right Energies 2020, 13, 2731 6 of 10 of Figure 5b. The figure shows that the NZO structure exhibited a hexagonal wurtzite phase and good crystallinity. Figure 5d-g depict the TEM elemental mapping images for a selected area (inside the pink rectangular frame) of the NZO NR arrays. Ni was uniformly doped into the ZnO structure with the visualized elemental distribution of Zn (red), O (green), and Ni (cyan). The NZO NRs were annealed at 600 • C and then analyzed via an EDX spectrometer. The presence of Ni was confirmed. Figure 5h exhibits the EDX spectra of the NZO structure. The spectra imply that three elements, namely, Zn, O, and Ni, exist in the NZO NR arrays. No traces of other elements were displayed in the spectra, thereby confirming the purity of the samples and successful doping of Ni 2+ . The atomic percentages of Zn, O, and Ni in the NR arrays were 61.64%, 37.87%, and 0.49%, respectively. Based on the above analysis, we can definitely conclude that the NZO NRs were obtained by the simple CBD method. In addition, it illustrates that the synthesis method is reliable for the doping process.
Energies 2020, 13, x FOR PEER REVIEW 6 of 11 The nanostructures and lattice spacing relations of NZO sample were explored by transmission electron microscopy (TEM) to further illuminate the detailed structure of the NZO NRs, as shown in Figure 5. In previous work and related literature, NZO NR arrays were obtained in a water solution system through CBD [38]. Figure 5a exhibits the fragments of NZO NR scraped from the sample with ethanol solution and then dropped onto a copper mesh with a dropping pipet. Lastly, the copper mesh was dried at 90 °C for 2 h in the oven. Figure 5b,c demonstrate that the HR-TEM image displayed the fringes of NZO planes with an interplanar spacing of approximately 0.259 nm. This manifestation indicates that the NZO NR is a single crystalline structure and can grow along the (002) direction, which is in accordance with selected-area electron diffraction (SAED) pattern shown in the upper right of Figure 5b. The figure shows that the NZO structure exhibited a hexagonal wurtzite phase and good crystallinity. Figure 5d-g depict the TEM elemental mapping images for a selected area (inside the pink rectangular frame) of the NZO NR arrays. Ni was uniformly doped into the ZnO structure with the visualized elemental distribution of Zn (red), O (green), and Ni (cyan). The NZO NRs were annealed at 600 °C and then analyzed via an EDX spectrometer. The presence of Ni was confirmed. Figure 5h exhibits the EDX spectra of the NZO structure. The spectra imply that three elements, namely, Zn, O, and Ni, exist in the NZO NR arrays. No traces of other elements were displayed in the spectra, thereby confirming the purity of the samples and successful doping of Ni 2+ . The atomic percentages of Zn, O, and Ni in the NR arrays were 61.64%, 37.87%, and 0.49%, respectively. Based on the above analysis, we can definitely conclude that the NZO NRs were obtained by the simple CBD method. In addition, it illustrates that the synthesis method is reliable for the doping process.  The NG device power density (P) is expressed in the following equation [39]: Energies 2020, 13, 2731 7 of 10 where I is the average current, V is the average voltage, and v is the volume of NRs. From the viewpoint of the total power, the ZnO NR arrays with 5 mM Ni content showed excellent performance. Figure 6e shows the current-voltage (I-V) curve property. The results displayed that the corresponding currents of pure ZnO NRs and NZO NRs were −0.043~0.80 mA and −1.32~3.66 mA, respectively. The Schottky contact behavior can be observed between the Au metal and the NR oxide material [37]. The electrical property of NZO NR arrays was stable and demonstrated a nearly symmetrically nonlinear behavior in the Schottky curve compared with pure ZnO. Simultaneously, it can be seen that the measured duration time of NZO samples is more stable than pure ZnO samples. The above-mentioned results indicate that NZO NGs can produce more output power than pure ZnO NGs. As we know, the length of NRs is too long; the NR structure may break easily while the sample is measured. On the other hand, an appropriate diameter of the NRs can produce a significant bite force between the electrodes. Additionally, some intrinsic defects may also be introduced by increasing the Ni concentration in ZnO NR arrays, Hence, the response for output power values of the NZO NGs may be greatly improved by selecting the optimal Ni content in ZnO NRs appropriately. By contrast, Ni can contribute to the formation of hole carriers, and the conductivity of NZO will be enhanced, thereby resulting in the fast transmission speed of NGs [40][41][42][43][44][45].

Conclusions
As previously demonstrated, NGs based on NZO NRs were successfully fabricated and characterized in this investigation. The NZO NR arrays were synthesized by CBD on ITO glass substrates. XRD and TEM analyses confirmed that the NZO NR arrays had a hexagonal wurtzite phase and a single crystalline structure. CBD was used to grow NZO NRs in a 95 °C oven for 3 h and the pattern was defined on the ITO glass substrates, and ITO etching paste was used as etchant for etching in an oven at 170 °C for 5 min. A 100 nm Au layer was subsequently evaporated onto the

Conclusions
As previously demonstrated, NGs based on NZO NRs were successfully fabricated and characterized in this investigation. The NZO NR arrays were synthesized by CBD on ITO glass substrates. XRD and TEM analyses confirmed that the NZO NR arrays had a hexagonal wurtzite phase and a single crystalline structure. CBD was used to grow NZO NRs in a 95 • C oven for 3 h and the pattern was defined on the ITO glass substrates, and ITO etching paste was used as etchant for etching in an oven at 170 • C for 5 min. A 100 nm Au layer was subsequently evaporated onto the surface as an electrode. Finally, the NZO NR arrays and electrodes were combined to form NGs to explore their characteristics. The results show that the synthesis of ZnO NRs with Ni concentration was predicted to exhibit enhanced performance of NGs compared with pure ZnO. In other words, while the NZO sample is measured, it can produce a large number of hole carriers and easily bend, which can significantly generate high output power values. In the future, the produced NZO nanostructure can also be combined with the Internet of Things for its incorporation into self-powered applications such as implantable biomedical devices, wireless sensors, and portable electronics.