Zinc Oxide Nanorods Shielded with an Ultrathin Nickel Layer: Tailoring of Physical Properties

We report on the development of Ni-shielded ZnO nanorod (NR) structures and the impact of the Ni layer on the ZnO NR properties. We developed nickel-capped zinc oxide nanorod (ZnO/Ni NR) structures by e-beam evaporation of Ni and the subsequent annealing of the ZnO/Ni core/shell nanostructures. The core/shell NRs annealed at 400 °C showed superior crystalline and emission properties. More interestingly, with the increase of annealing temperature, the crystallinity of the Ni shells over the ZnO NRs gradually changed from polycrystalline to single crystalline. The presence of the Ni layer as a polycrystalline shell completely hindered the light emission and transmission of the ZnO NR cores. Further, the band gap of ZnO NRs continuously decreased with the increase of annealing temperature.

In the current paper, we report on the deposition of an ultrathin (10-nm thick) Ni layer over ZnO NRs, as well as its impact on the physical properties (particularly light emission and transmission) of aligned ZnO NRs. We developed vertically-aligned ZnO NRs by chemical vapor deposition; then, we deposited an ultrathin Ni layer by e-beam evaporation. In order to understand the influence of the crystalline characteristics of the Ni layer on the optical properties of the ZnO NRs, we also annealed the ZnO/Ni core/shell structures at different temperatures. From these studies, we observed that the ZnO/Ni core/shell NR structures annealed at 400 °C possessed significant crystalline and light emission properties, being prerequisites for the development of efficient photoelectrochemical devices.

Results and Discussion
Figure 1(a) shows field emission scanning electron microscopic (FESEM) cross-section images of as-grown ZnO NRs, revealing their vertically-aligned growth and uniform surface morphology. These NRs consisted of an average length of about 30 μ m, while their diameters varied between 50 and 100 nm. Figure 1(b) shows x-ray diffraction (XRD) studies of all the as-grown ZnO NRs, preferentially-oriented along the < 001> direction and having a full width at half maximum (FWHM) value of 0.25°; the calculated d-spacing value for the major peak diffracted at 2θ = 34.32° is 0.2611 nm, nearly matching with the hexagonal ZnO data (JCPDS data of ZnO #36-1451). We evaluated the dislocation density (DD) of the nanostructures using the following equation, where b s was the Burgers vector sizes of the screw-type dislocations (~0.5185 nm) and θ was the angle between the reciprocal vector and (001) surface normal: We found the DD for the as-grown ZnO NRs to be 1.6 × 10 9 cm −2 . We evaluated the lattice constant 'c' of the ZnO NRs using the d-spacing value of the (002) peak, found to be 0.522 nm. A minor peak diffracted at 16.94° (d = 0.523 nm) belonged to the ZnO (001) plane, whereas the other peaks diffracted at 37.98 and 41.4° belonged to the Au catalyst and sapphire substrate, respectively. These results indicated that though the RTCVD-grown ZnO NRs had uniform surface morphology and excellent crystalline quality, the structures contained a large number of dislocations.
In order to find the variation in the chemical composition of the ZnO NRs, we analyzed the as-grown ZnO/Ni core/shell NRs by energy dispersive spectroscopy (EDS) with the help of STEM. Figure 2(a) shows the spatial distributions of the elemental composition across the ZnO/Ni core/shell NRs, obtained by line-scan elemental mappings of Zn, O, and Ni. The profiles of Zn (blue) and O (red) showed a broadband-like peak at the center, while the profile of Ni (black) showed sharp peaks at both sides of the Zn and O profiles with a broad valley at the center. From these analyses, we extracted the average diameter of the ZnO NR cores and the thickness of the Ni shells as ~90 and ~10 nm, respectively. We also confirmed these results by elemental mapping of the ZnO/Ni core/shell NRs, as shown in Fig. 2(b), clearly emphasizing that the as-grown ZnO/Ni core/shell NRs purely consisted of Zn, O, and Ni elements with appropriate ratios, while also possessing a well-defined composition profile with an excellent interface between the ZnO and Ni components.
We examined the changes in the crystallinity of the as-grown and annealed ZnO/Ni core/shell NRs by XRD, providing a detailed descriptive report about these issues elsewhere 39 . In brief, the as-grown core/shell nanostructures exhibited similar structural characteristics as bare-ZnO NRs. However, these structures showed an additional diffraction peak, belonging to the (002) orientation of hexagonal Ni (JCPDS data of ZnO #36-1451, Ni #45-1027, Ni2O3 #14-0481). Upon annealing, noticeably two more diffraction peaks emerged (see Supporting Information, Figure SI  existence of these phases was very low since the relative intensity ratio between ZnO (110) [Ni 2 O 3 (202)] and ZnO (002) of ZnO/Ni core/shell NRs annealed at 400 °C was 0.03% [0.024%]. Figure 3 shows the variation of the FWHM and DD values of the (002) ZnO peak as a function of annealing temperature, indicating that after the deposition of the Ni layer over the ZnO NRs, both the FWHM and DD values slightly decreased from 0.25 to 0.20° and 1.6 × 10 9 to 1.0 × 10 9 cm −2 , respectively 40,41 . While increasing the annealing temperature of the ZnO/Ni core/shell NRs, both these parameters gradually decreased, with the structures annealed at 400 °C showing the lower values of 0.17° and 7.3 × 10 8 cm −2 . These observations emphasized that the deposition of the Ni layer over the ZnO NRs and their annealing significantly passivated the surface defects present on the ZnO core structures; as a result, the crystalline quality of the ZnO NRs (i.e., ordering of crystallites) strongly enhanced 42 .
In order to understand the interfacing characteristics of the Ni layer with the ZnO NRs, we examined the untreated and treated ZnO/Ni core/shell NRs with high resolution transmission electron microscopy (HRTEM), shown in Fig. 4; these images show that the as-deposited Ni nano-layer over the ZnO NRs had uniform surface morphology, while we found its thickness to be ~10 nm. The fast Fourier transformation (FFT) analysis of the HRTEM image, Fig. 4(d), confirmed that the as-deposited Ni nano-layer had polycrystalline nature. However, the Ni layer consisted of preferentially-oriented crystallites, oriented along the < 001> direction. Thus, the as-deposited Ni structures on the hexagonal facets of the ZnO NRs were composed of fine hexagonal polycrystalline nanocrystals with a preferential growth direction of < 001> . Figures 4(b,c,   However, with the further increase of annealing temperature to 400 °C, the polycrystalline Ni-layer converted to single-crystalline structures, as shown in Fig. 4(f). On the other hand, we observed the presence of Ni 2 O 3 phase, clearly noticed by electron diffraction studies (see Figure SI-2), exclusively over ZnO/Ni core/shell NRs surfaces. Figure 5(a) shows the cathodoluminescence (CL) spectra of pure ZnO NRs, as well as untreated and treated ZnO/Ni core/shell nanostructures. The pure ZnO NRs exhibited a strong emission peak at 386 nm and two weak peaks at 512 and 767 nm. We attributed these peaks to the transition of excited electrons from the conduction band: i) to the valence band (near band edge or ultra-violet emission, NBE or UV); ii) through various interstitial states (broadband, BB) 43 ; or iii) to the second harmonic emission of NBE (RE, red emission), respectively 44 . After deposition of the Ni layer and increasing the annealing temperature up to 200 °C, the NBE peak position  remained the same, while the I UV /I BB peak ratio decreased. After further increasing the annealing temperature, both the NBE peak position and the I UV /I BB ratio significantly increased, as shown in Fig. 5(b), emphasizing that deposition of an ultrathin Ni layer over the ZnO NRs drastically hindered their light emission characteristics (continuing up to the annealing temperature of 200 °C). We attributed the significant improvement in the emission properties and widening NBE band of the ZnO NRs annealed at higher temperatures (> 200 °C) to the formation of single-crystalline Ni crystals and a decrease of surface density states 45 . As a result, ZnO/Ni core/shell nanostructures annealed at 400 °C possessed excellent CL emission properties 42 . Figure 6(a) shows optical transmittance plots of as-grown and annealed ZnO/Ni core/shell NRs. After the deposition of the Ni metallic layer, the transmittance of the ZnO structures drastically decreased, while further decreasing with annealing temperature up to 200 °C. Above this temperature, the transmittance of the structures gradually improved. Variation in the transmittance of the structures mainly depended on the morphology of the ZnO/Ni structures, since the as-deposited and annealed structures up to 200 °C had a uniform metallic shield that reduced the light penetration as well as emission, resulting in a strong impact on their light transmission.
As observed in the XRD studies, the formation of single crystalline Ni phase (above 200 °C) played a crucial role in the improvement of light transmission, since the formation of crystalline phase with smaller openings gradually allowed ( Figure SI-3) the light through the shells. At near the fundamental absorption edge, the absorption coefficient (α = ln(100/T)/t) satisfied the following equation, where T was the transmittance; t was the thickness (height of the NRs); A was the proportional constant; E was the photon energy; E p was the phonon energy; and E g was the optical band gap: For direct transition (E p = 0), x was equal to 1/2 for allowed transition and 3/2 for forbidden transition 46,47 . For indirect transition, x = 2 for allowed transition and 3 for forbidden transition. In the present case, the square of the absorption coefficient on the incident photon energy, at above the fundamental absorption edge (α c ), was a straight line, since above α c it satisfied α 2 ∝ E (= hν ) relation (inset of Fig. 6(b)). Thus, the transition between valence band and conduction band near the fundamental band edge was direct. Therefore, we evaluated the energy band gap (E g ) of the structures using the equation (α hν ) 2 ~ hν −E g, , where 'h' was Planck's constant and 'ν ' was the incident photon frequency; the variation of the optical band gap of the structures with annealing temperature is shown in Fig. 6(b).
The as-grown ZnO NRs showed an optical band gap of ~3.24 eV, slightly lower (60 meV) than the bulk ZnO 1 . After the deposition of the Ni metallic layer, it decreased to 3.22 eV, while further decreasing with the increase of annealing temperature. We strongly attributed these changes in the optical band gap of the ZnO nanostructures to the dislocation density of the core nanostructures. As observed in the XRD studies, gradual improvement in the crystalline quality of the ZnO NRs due to the eradication of dislocations by Ni atoms probably led to lower values in the optical band gap 48 . On the other hand, according to the orbital hybridization theory, the formation of antibonding 3d-triplet states (with higher energies lying close to the conduction band of the ZnO) led to lower values in the band gap 49 . Therefore, surface passivation of the ZnO NRs by Ni atoms probably led to lower values in the overall band gap.
In summary, we investigated the impact of Ni-capping on the physical properties of vertically-aligned ZnO NR structures. We synthesized vertically-aligned ZnO NRs using the vapor-liquid-solid method with the help of a gold catalyst, while we deposited the Ni nano-layer using an e-beam evaporator. Upon annealing, the quality of the ZnO/Ni structures clearly improved due to the eradication of structural defects and/or passivation of surface The band gap of ZnO NRs gradually decreased with the increase of annealing temperature of the ZnO/Ni core/shell nanostructures. Based on these results, we emphasize that Ni-capped ZnO nanostructures annealed at higher temperatures could be utilized for different device applications, particularly for photoelectrochemical water-splitting devices due to their better structural and light emission properties.

Experimental Procedure
Synthesis. We achieved ZnO/Ni core/shell nanorod structures in three-steps, schematically-represented in Fig. 7. Initially, we cleaned c-plane Al 2 O 3 substrates (both sides polished sapphire substrates) by using ethanol, acetone, and deionized water under ultrasonic agitation for 10 min; then, we deposited the Au catalyst with a thickness of ~3 nm using e-beam evaporation. We synthesized the ZnO NRs using rapid thermal chemical vapor deposition (RTCVD) on Au-coated substrates by the vapor-liquid-solid (VLS) method. We carried out the growth of the ZnO NRs for 10 min at a temperature and pressure of 950 °C and 20 Torr, respectively. Here, we used Ar as a carrier gas and O 2 as a partial source of oxygen with flow rates of 100 and 2 sccm, respectively. Then, we carried out the deposition of the Ni layer on the ZnO NRs using an e-beam evaporator at room temperature. Finally, we annealed the as-grown ZnO/Ni core-shell NRs at different temperatures, varied from 100 to 400 °C in the RTCVD system under a vacuum of 10 −3 Torr for a fixed time of 5 min.
Characterization. We studied the structural properties of as-grown and annealed ZnO/Ni structures by powder X-ray diffraction (XRD) using Cu Kα 1 radiation in the range of 10-70°. We used field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) for the examination of surface morphology. We used scanning TEM (STEM) and energy dispersive spectroscopy (EDS) embedded with TEM for chemical composition analyses. We analyzed the crystalline characteristics of the Ni layers with high resolution transmission electron microscopy (HRTEM). We studied optical properties (e.g., emission and transmission) of the ZnO/Ni core/shell nanostructures by measuring their light emission and transmission versus wavelengths using mono-cathodoluminescence (CL) attached with FESEM and UV-Vis-NIR spectrophotometer. We estimated the light transmission of the structures in the wavelength range of 300-1500 nm with an incident light perpendicular to the growth direction of the ZnO NRs.