Magnetic Properties of Well-Aligned ZnO Nanorod Arrays Grown by a Simple Hydrothermal Reaction

Well-aligned ZnO nanorod arrays with room temperature ferromagnetism were prepared on glass substrate through hydrothermal method. The as-prepared nanorod arrays were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), photoluminescence (PL) spectrum, and magnetization measurements. The XRD and SEM results indicated that the ZnO nanorods are with the wurtzite structure and exhibit preferential (002) orientation with c-axis perpendicular to the substrate surface. The PL results suggested that the possible defect in the as-prepared ZnO nanorod arrays might be VZn, Oi, or OZn. The first-principles calculations reveal that the room temperature ferromagnetism may result from the VZn defects present in the ZnOnanorod and the hybridization of the Zn 3d states withO 2p states is responsible for the half-metallic ferromagnetism in ZnO nanorod.


Introduction
Recently, one-dimensional (1D) ZnO nanorods have attracted great interest due to their tunable magnetic and optoelectronic properties [1][2][3][4][5][6][7][8].Particularly, for the potential applications in the spin electronics, the investigation to grow well-aligned ZnO nanorod arrays with room temperature ferromagnetism has been an urgent task because it is a key process toward realizing nanoscale devices [9][10][11].Synthesis methods, such as physical vapor deposition [12], chemical vapor deposition [13], and pulsed laser deposition [14], have been extensively used to obtain 1D well-aligned ZnO nanorod arrays.However, these vapor-phase processes fabrication techniques need vacuum condition, high energy consumption, sophisticated equipment, and rigid experimental conditions.Compared with the vapor-phase processes, the hydrothermal process is a low cost and environmentally friendly method, which is highly welcomed by numerous researchers [15][16][17][18].
In this study, well-aligned, single crystalline ZnO nanorod arrays with room temperature ferromagnetism and high packing density were achieved via a simple hydrothermal process.A possible origin of the ferromagnetism in the prepared ZnO nanorod arrays related to the intrinsic defect is proposed.

Experimental Details
The procedure of the ZnO nanorod arrays prepared under hydrothermal condition consists of two steps: (a) preparation of ZnO seed-layer and (b) growth of ZnO nanorod arrays.In the first step, a layer of ZnO seed crystal was deposited on the glass substrate via sol-gel method.In detail, equal molar Zn (CH 3 COO) 2 -2H 2 O and glycolic amide (0.35 mol/L) were solved in ethanol with stirring at 60 ∘ C for 12 h to yield a homogeneous solution.Subsequently, the precursor solution was dropped on the glass substrate, spinning at 3000 r/min for 30 s. Then the substrate was preheated in air at 80 ∘ C for 10 min.After that, the substrates were annealed at 500 ∘ C for 90 min in air to obtain a dense and transparent ZnO seed layer on the glass substrate by using an electronic furnace.In the second step, the glass substrate with a layer of ZnO seed crystal was placed into one autoclave filled with the precursor solutions of Zn (CH 3 COO) 2 -2H 2 O (0.04 M) and NaOH (0.08 M).After the autoclave is sealed safely, it is put into a bake oven at 120 ∘ C for 90 min.Finally, the glass substrate with white deposit is purged in deionized water for several times.

Results and Discussion
Figure 1 shows XRD pattern of the as-prepared sample.It is observed that all of the diffraction peaks can be well indexed to a wurtzite structure of ZnO.The strong and narrow diffraction peaks indicate that the material has a good crystallinity.In particular, the as-prepared sample exhibits preferential (0 0 2) orientation with -axis perpendicular to the substrate surface.
Figures 2(a) and 2(b) depict the SEM images of the prepared ZnO nanorod arrays.The low-magnification SEM image in Figure 2(a) demonstrates that the nanorod array is uniform and densely packed.From the high-magnification image in Figure 2(b), it can be seen that the prepared ZnO nanorod array exhibits typical wurtzite structure and preferential (0 0 2) orientation with -axis perpendicular to the substrate surface, which is reflected by the XRD pattern shown in Figure 1.Furthermore, energy dispersive spectroscopy (EDS) result indicates that the as-prepared ZnO nanorod array is only composed of Zn and O, and the atomic ratio of Zn and O is about 48.24 : 51.76.This suggests that the as-prepared nanorod arrays are nonstoichiometric and some defects may exist in the as-prepared nanorod arrays.
Figure 3 shows the room temperature PL spectra of the as-prepared ZnO nanorod arrays.A strong UV emission peak at 373 nm and three relatively weak and broad visible emissions centered at 425, 511, and 590 nm, respectively, can be observed.The UV emission band is usually attributed to the near-band edge emission of the wide band gap of ZnO due to the annihilation of excitons [8,[19][20][21].The visible emission is the most commonly observed and is often attributed to the defect emission [22][23][24][25][26][27].It is known that different defects may cause different electronic structures, which will be reflected on the corresponding optical properties observed in experiments.On the other hand, the electronic structure  approximately conforms to the yellow emission (590 nm, 2.10 eV).By analysis of the experimental phenomena and the calculation of the defect levels in ZnO, we suggest that the blue emission centered at 425 nm is attributed to V Zn [26,27], the green emission centered at 511 nm originates from O Zn [26,27], and the yellow emission centered at 590 nm is related to O i [26].Therefore the PL results reveal that the possible defect in the as-prepared ZnO nanorod arrays might be V Zn , O i , or O Zn .
Figure 4 shows the magnetization as a function of the applied field at 300 K for the as-prepared ZnO nanorod arrays.The diamagnetic contribution from the glass substrate has been subtracted from the raw data.It is interesting that the ZnO nanorod arrays exhibit room temperature ferromagnetism.The saturation magnetization M s and remanent magnetization M r are about 1.7 × 10 −3 and 4.33 × 10 −4 emu/g, respectively.The coercivity H c is about 60 Oe.As we know, neither doping nor evidence of secondary phase exit in the as-prepared ZnO nanorod arrays; the observed room temperature ferromagnetism in the as-prepared ZnO nanorod arrays probably results from the intrinsic defect.
In order to provide more insight into this issue, we employed first-principles calculation [28] on the electronic structure and magnetic properties for the ZnO nanorod.
Since the PL study results reveal that the possible defect in the as-prepared ZnO nanorod arrays might be V Zn , O i , or O Zn , thus, in the present calculations, the ZnO nanorods with V Zn , O i , or O Zn defect are investigated.The wurtzite supercell containing 96 atoms is used for calculation.For the ZnO nanorod with V Zn or O Zn defect, there are three inequivalent defect positions, which are denoted as V Zn1 -V Zn3 and O Zn1 -O Zn3 , as shown in Figures 5(a) and 5(b).However, there are two inequivalent O i defect positions, which are denoted as O i1 and O i2 as shown in Figure 5(c).The formation energy of V Zn at V Zn2 site is about 0.12 and 0.38 eV smaller than that at the V Zn1 and V Zn3 sites, respectively, and, thus, V Zn2 is the most stable site for V Zn defect.Similar to V Zn , O Zn also prefers at the O Zn2 site.While for the O i defect, it prefers at the O i1 site.Therefore, the magnetic properties of ZnO nanorod with V Zn , O Zn , and O i are calculated based on the V Zn2 , O Zn2 , and O i1 geometries, respectively.
The calculated densities of states (DOSs) for the ZnO nanorod with V Zn , O i , or O Zn defect are shown in Figure 6.It is clear that the spin-up and spin-down DOSs of pure ZnO nanorod are completely symmetrical, indicating that the pure ZnO nanorod is nonmagnetic.Similarly, the ZnO nanorod with O i or O Zn defect also exhibits nonmagnetic properties.As for the ZnO nanorod with V Zn defect, however, a strong spin splitting phenomenon is observed.The Fermi level passes through the band gap in the spin-down DOS and an energy gap of about 1.47 eV exists in the spin-up DOS.This demonstrates that the ZnO nanorod with V Zn behaves as halfmetallic.
By analysis of the partial densities of states (PDOSs) in Figure 7, it can be found that the metallic spin-down DOS near the Fermi level is mainly composed of Zn 3d and O 2p states.In particular, O 2p states make significant contribution to the magnetic moment.This suggests that the appearance of the half-metallic ferromagnetism in ZnO nanorod with V Zn originates from the hybridization of the Zn 3d states with O 2p states.

Conclusions
In conclusion, ZnO nanorod arrays with room temperature ferromagnetism were prepared on glass substrate through hydrothermal method.The as-prepared sample shows preferential (0 0 2) orientation with -axis perpendicular to the substrate surface.The room temperature PL measurements exhibit a prominent UV peak at about 373 nm which is attributed to the annihilation of excitons.Three relatively weak and broad visible emissions resulting from the defects can also be observed in the PL spectrum.By analysis of the calculated electronic structure of the ZnO nanorod with defects, we can get the conclusion that the V Zn defects present in the ZnO nanorods are responsible for the room temperature ferromagnetism.

Figure 5 :Figure 6 :Figure 7 :
Figure 5: The structure of ZnO nanorods (a) with V Zn , (b) with O i , and (c) with O Zn defect.