Sb substitution into ZnO nano-composite: Ferromagnetic behavior

Journal of Magnetism and Magnetic Materials 397 (2016) 79–85

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Sb substitution into ZnO nano-composite: Ferromagnetic behavior P. Nakarungsee a, G.S. Chen c, T.S. Herng d, J. Ding d, I.M. Tang b, S. Talabthong a, S. Thongmee a,n a

Department of Physics, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand Department of Materials Science, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand c Department of Physics, Faculty of Science, National University of Singapore, Singapore 117551, Singapore d Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore b

art ic l e i nf o

a b s t r a c t

Article history: Received 17 March 2015 Received in revised form 17 July 2015 Accepted 1 August 2015 Available online 19 August 2015

Properties of particles whose dimensions are of the nanoscale can be quite different from those of bigger dimension particles. We investigate here the effects of doping Sb5 þ ions into ZnO nanorods (NR's) on the formation of magnetic moments. The pure ZnO and Sb-doped ZnO NR's were grown by the hydrothermal process. The fabricated nanorods were then studied using X-ray diffraction (XRD), ultraviolet–visible light spectroscopy (UV–vis), Scanning Electron Microscopy (SEM), Photoluminescence (PL) Spectrometer and Vibrating Sample Magnetometer (VSM) spectrometry. Substitution of Sb into the ZnO leads to the formation of a defect (SbZn þ 2VZn) complex state which can capture the oxygen p-orbital electrons of the oxygen atoms and form a spin polarized state. The PL was used to monitor the defect formation in the ZnO NR's when the Sb is doped in. As more Sb was doped in, the visible blue portion of the PL spectrum decreased while the visible red portion increased. Magnetization measurements (VSM) showed that the pure ZnO NR's possessed a magnetic moment which initially increased as Sb is doped in but decreased as more Sb was doped in. We obtain a value of 1.015 memu/g for the saturation magnetization of the pure ZnO nanorod. & 2015 Elsevier B.V. All rights reserved.

Keywords: ZnO Nanorods Sb Doping Magnetic moments Photoluminescence Defect formations

1. Introduction The observation of ferromagnetism in thin films of HfO2 by Venkatesan et al. [1], was unexpected since neither the Hf4 þ in which the d and f shells of the Hf4 þ ions are either empty or full nor the O2  ions are magnetic ions. In general, magnetism arises when the d or f shells of transition metal impurity ions are partially filled. This indeed was seen when Sc, Ti, V, Fe, Co or Ni were doped into ZnO nanoparticles [2]. Sundaresan et al. [3], stated that ferromagnetism is a universal feature of nanoparticles of otherwise nonmagnetic oxides such as CeO2, Al2O3, ZnO, In2O3 and SnO2. Hong et al. [4], observed similar magnetic behaviors in undoped semiconducting and insulating oxide thin films of TiO2, HfO2 and In2O3. More recently, Gao et al. [5], reported room temperature ferromagnetism in pure ZnO nanoparticles (NP's). Choudhury and Choudhury [6] observed room temperature ferromagnetism in TiO2 NP's [6]. Mishra et al. [7], also observed room temperature magnetism in Al-doped MgO NP's. Several studies [8–10] suggested that the ferromagnetism was due to defect states formed on the surface of the ZnO NP's. Wang n

Corresponding author. E-mail address: [email protected] (S. Thongmee).

http://dx.doi.org/10.1016/j.jmmm.2015.08.002 0304-8853/& 2015 Elsevier B.V. All rights reserved.

et al. [8], found evidence that ferromagnetism in ZnO NP's might originate from surface defects due to Zn or O vacancies. Liu et al. [9], suggested that oxygen defects, especially singly ionized oxygen vacancies play a crucial role in mediating ferromagnetism in undoped ZnO NP's. Zhang et al. [10] found that the Zn interstitials play an important role in triggering magnetic ordering in undoped ZnO thin films. Rainey et al. [11], found that the magnetization in undoped ZnO NP's correlated with the number of interstitials and oxygen vacancies. Recent theoretical calculations [12] indicated that the zinc vacancies were the source of the magnetism in undoped ZnO NP's. The vacancies would lead to defect state energy levels which would become occupied by the p-electrons of the neighboring oxygen atoms. Peng et al., found that the total energy of the spin polarized state would be lowered than that of the nonspin polarized state. Performing similar calculations, Wang et al. [13], found the difference to be 43.77 meV. In order to achieve p-type conductivity in ZnO by either arsenic or antimony substitution into the Zn site, Limpijumnong et al. [14], pointed out that these substitutions would simultaneously induce two Zn vacancies. They found that defect complexes ((AsZn(SbZn)–2VZn)  formed by the Sb (As) and the two Zn vacancies would have low formation energies and would be stable. It was pointed out by Xiu et al. [15], that the Sb complex (SbZn–2VZn)  1 would have an ionization energy of about 160 eV

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Fig. 1. XRD pattering of Sb doped ZnO at different percentage of Sb.

Table 1 Lattice constant of ZnO:Sb nanorods. Sample

(100)

a

(002)

c

c/a

Pure ZnO Sb 5 mol% Sb 15 mol% Sb 25 mol% Sb 30 mol%

31.7285 31.7285 31.7062 31.7062 31.7285

3.2538 3.2538 3.256 3.256 3.2538

34.3501 34.3945 34.3723 34.3723 34.3945

5.2172 5.2106 5.2139 5.2139 5.2106

1.6034 1.6013 1.6013 1.6013 1.6013

Sb 45 mol% Sb 30mol% Sb 25mol% Sb 15mol% Sb 5mol% ZnO

50

Transmission (%)

40 30 20 10 0

400

500

600

700

800

Wavelength (nm) Fig. 2. UV–vis transmission of the various Sb doped ZnO nanorods.

and would therefore be an acceptor for creating a hole. Yi et al. [16], also using first order principle calculations found that doping Li1 þ in ZnO nanoparticles led to the formation of (Lii þLiZn þVZn ) defect complex which would lowered the formation energy of the Zn vacancies so that the vacancies would become more stable. Wang et al. [16], and Lu et al. [17], performed similar calculations for the situations where the dopants were Li, Mg and Al or were the group V elements, respectively. Both found the formation of the defect complexes stabilized the formation of the Zn vacancies. Lu et al., found that the ferromagnetic stability induced by the (XZn þ2VZn) complex (X being the group-V element) decreased as the electronegativity of the element X decreases, i.e., N oSb oAs oP. This trend is ascribed to the increased delocalization of the O-2p orbitals. Xue et al. [18], found that the formation of Zn vacancy and surface OH bonding complex (VZn þ OH) in hydrogenated ZnO nanoparticles would give rise to a magnetic moment of 0.57 mB. In two of their hydrogenated ZnO nanoparticles, Xue et al., observed very small saturated magnetic moments (of order

Fig. 3. Band gaps of the various Sb doped ZnO nanorods. The extrapolation of the linear portion of the absorption curve provides the band gap energy value. The values of the energy gaps are given in the parenthesis in the insert.

10  4 emu/g). Yi et al. [19], found that ferromagnetism in ZnO could be engineered by the Li doping. Awan et al. [20], have reconfirmed the findings of Yi et al., that the Li doped ZnO nanoparticles exhibited ferromagnetic behaviors. Ma et al. [21], have observed room temperature magnetism in in ZnO doped with Al. Room temperature magnetism was observed in a different nanoparticle, MgO when Al was doped into it. The authors are not aware of any studies on the effects of only Sb doping on the magnetic properties of ZnO nanoparticles. There have been studies on the effect of co-doping of Sb and Co and of Sb and Mn [22] into ZnO films. Samanta et al. [23], found that Sb doping increased the saturation magnetization and coercivity of the ZnCoO films while Ji et al. [24], found that the Sb doping converted the nonmagnetic ZnMnO films into films which exhibited ferromagnetic properties. To see how Sb substitution into ZnO nanoparticles affects the formation of magnetic moments in non magnetic ZnO, we have grown ZnO:Sb nanorods (NR's) containing higher amounts of Sb using the hydrothermal method. To check on the formation of the Sb-doped ZnO nano rods (NR's), we have performed X-ray diffraction (XRD) studies on the doped ZnO:Sb NR's. The changes in the optical properties due to Sb doping were monitored by looking at the enhancement of the visible light spectrum using photoluminescence spectroscopy (PL) and ultraviolet–visible spectroscopy (UV–vis). To monitor the changes in the growth of NR's which might occur with the additional doping, we have taken scanning electron microscopy (SEM) images of the ZnO NR's. Finally, we have measured the magnetization of the Sb doped ZnO NR's.

2. Methodology The Sb-doped ZnO powder were prepared by using aqueous solutions of zinc nitrate (Zn(NO3)2  6H2O), antimony trichloride (SbCl3), and NaOH as precursors. The reagents were dissolved in distilled water at room-temperature. The zinc solutions were mixed with antimony solution under various conditions. Then NaOH was added under continuous stirring conditions until the pH of mixed solution was adjusted to 10. The resulting solution was placed in a Teflon-lined autoclave. The hydrothermal process was conducted in an oven heated to 150 °C with a 20 h synthesis

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Fig. 4. SEM images of ZnO nanorods with different mol% of Sb: (a) pure ZnO, (b) 5 mol% Sb, (c) 15 mol% Sb, (d) 25 mol% Sb, (e) 30 mol% Sb and (f) 45 mol% Sb sample, now seen as flakes.

time. The autoclave was then removed from the oven and allowed to cool down in air (at room-temperature). The nanostructure deposits were washed several times with distilled water and dried in an oven at 110 °C. Finally, all the samples were analyzed using X-ray diffraction (XRD), Scanning electron microscopy (SEM) Photoluminescence spectroscopy (PL) and a Vibrating Sample Magnetometer (VSM).

3. Results and discussion The surface and structural properties of ZnO were analyzed with a XRD, an ultraviolet–visible spectroscopy (UV–vis) and SEM. In Fig. 1, the XRD patterns of an undoped ZnO and of various

Sb- doped ZnO NR's are shown. The XRD patterns were taken using the CuKα line (wavelength 0.154 nm) at a scanning rate of 0.3°/s in range of 2θ ¼ 20–80°. The patterns for the pure ZnO and 5 mol% could be indexed to the hexagonal wurtzite structure (JCPDS card no. 36–1451). The positions of the lines give the lattice parameters a ¼0.3249 nm and c ¼0.5206. The three ZnO diffraction peaks (100), (002) and (101) appear in all patterns except for the 45 mol% sample. These three peaks become less intense as the Sb doping level increases, indicating less crystallinity. New peaks begin to appear at 2θ ¼20.4°, 24.3° and 30.9° in the patterns of the 15 mol%, 25 mol% and 30 mol% These peaks belong to a Sb impurity phase Analysis of the XRD patterns of all the doped specimens show the changes in the lattice parameters a and c as given in Table 1. Since the radius of Sb5 þ (0.062 nm) is nearly identical

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Fig. 5. Photoluminescence spectrum at room temperature of (a) PL spectrum of the pure and 5 mol% doped nanorods and (b) PL spectrum of all the Sb doped nanorods.

to the octahedron interstice radius (0.061 nm) in the ZnO structure and of the Zn2 þ ions (0.072 nm), the Sb5 þ ion can either replace the Zn ion in the ZnO compound or go into the interstitial positions in the ZnO structure [25]. If the Sb substituted into the Zn sites, there would not be that much of a change in the lattice parameters, which is what, is seen in Table 1. This substitution needed to create the (SbZn–2VZn) complex will only occur at the low doping concentration since only at low concentrations will the Sb replace the Zn. Limpijumnong et al. [14], ruled out the possibility of the As(Sb) ions replacing the oxygen ions. At high Sb substitution, the Sb5 þ ions would substitute into the interstitial sites since the size of the Sb5 þ ions is nearly the same as that of the interstitial size. Peng et al. [12], reported that the Sb5 þ in their Sb-doped ZnO nanoparticles entered into the interstitial sites and this was responsible for the increase of the reflectivity of their particles. The Sb ions could also attach itself to the surface of ZnO crystallite. These substitutions would not cause any distortion of the wurtzite crystal structure. We note from Table 1 that the values of the lattice parameter c in the higher Sb doped ZnO are nearly the same. Fig. 2 shows the UV–vis spectra of the ZnO samples. As is observed, the transmission of light decreases when Sb is doped into the ZnO nanoparticles. As the doping is increased from 5 to 15 mol%, the decrease is continuous but when the Sb is increased to 25% an increase in the transmission is seen. This could be due to the Sb ions entering into the interstitial sites. Fig. 3 shows the change in the band gap as determined from the change in the UV–vis spectrum. The graph were plot between (αhν)2 with Energy gap (Eg ¼ hc/λ) where α is the absorption coefficient, h is the Planck constant, ν is the frequency, Eg is the energy gap, c is the speed of light and λ is the wavelength. The values of the band gap are the intercepts of the slopes of the UV– vis with the bottom axis. Looking at Fig. 3, we see the band gap value of pure ZnO is about 3.03 eV. Upon increasing the Sb doping level from 5 mol% to 15 mol %, the band gap decreases from 2.93 to 2.77 eV. Increasing the Sb doping levels to 25 mol%, 30 mol% and 45 mol%, the values of the band gaps increased to 2.88, 2.90 and 3.17 respectively. Rana, Singh and Kaur [26] reported that the band gap of the ZnO nanoparticles increased as the Sb (low levels of doping, i.e., up to 5 mol%) doping of the ZnO NP increased. The increase in the energy gap from 2.77 to 3.17 eV as the doping level is increased from 15 mol% might be due to the formation of the Sb

impurity phase noted in the XRD patterns and the formation of the (SbZn.þ 2VZn) complex. Fig. 4 shows the SEM images of ZnO and Sb-doped ZnO with different amount of Sb2O3. Fig. 4a is for undoped ZnO. It is seen to be block-shape with an edge. Fig. 4b is the image of 5 mol% Sbdoped ZnO sample. This doped ZnO particles appear as small nanorods, the same as those of the 15 mol % and 25 mol% specimens (See Fig. 4a–d). These NR's are about 30–70 nm in diameter and 100–300 nm in length. Fig. 4e is image of the 35 mol% Sb-doped ZnO NR. The sizes of these NR's have become quite big, about 100– 250 nm in diameter and longer than the previous samples. The surfaces of the nanostructures exhibit much roughness. The image of a 45 mol% Sb-doped ZnO sample is shown in Fig. 4f. The shapes have changed from those of NR's to those of nano-flakes. All of SEM images show that Sb doping has cause the size of zinc oxide NR's to increase. This has been accompanied by the loss in the crystallinity of the ZnO rods. The SEM images correlate with the XRD results. The PL spectrums of pure ZnO and of the different Sb-doped ZnO are shown on Fig. 5. The spectrums for the pure ZnO and 5% Sb–doped ZnO are shown on Fig. 5a. All the spectrums on both Fig. 5a and b exhibit a sharp peak near the band gap edge (NBE) 378–380 nm at room temperature. Fig. 5b (which are of the other Sb-doped ZnO) indicates that there is a blue-shift of the NBE peak as more Sb ions are doped in. The PL spectra of the pure ZnO also exhibits a broad peak in the visible light region from 450–650 nm Similar broad bands were seen by Tam et al. [26], and by Djurisic et al. [27], in some of the ZnO they had fabricated. In others, the visible light bands were suppressed. The defects responsible for the blue band are believed to be the Zn vacancies and Zn interstitials while the defects responsible for green–yellow bands are due to oxygen vacancies or O  interstitials. Tian et al., [28] took the red shift of the absorption band in the UV–vis spectrum to be direct evidence of the (SbZn þ2VZn) complex in the Sb-doped ZnO NP's. This connection of the (SbZn þ2VZn) complex to the PL emission around 600 nm had already been made by Xiu et al., [15], Chu et al.,[29], Benelmadjat et al. [30], and Mandalapu et al. [31]. Rana et al. [25], and T.Yang, et al. [32], found that Sb doping would passivity the Zn interstitials and the oxygen vacancies, thus leading to suppression of the 533 nm and 630 nm PL bands. The change in the PL emissions seen in Fig. 5a may be evidence of the passivity of the Zn interstitial defects. Enhanced emission in the Sb

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Fig. 6. Hysteresis Loops of Sb Doped ZnO Nanorods: (a) hysteresis loops of the pure, 5 mol%, 15 mol%, 25 mol%, 30 mol% and 45 mol% Sb doped NR and (b) expanded view of the nysteresis loop for the pure ZnO NR.

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vacancies through the creation of the (SbZn–2VZn) defect complex. Substitution of Sb into the interstitial sites which is what occurs at the higher levels of Ab doping, does not lead to more zinc vacancies being formed. To maintain the charge neutrality of the ZnO:Sb NP's, more oxygen ions would have to be absorbed. This could be done by oxidizing the Sb ions close to the surface, thus leading to the formation of the Sb2O3 seen in the XRD patterns of the higher doped ZnO:Sb NP's. This would also explain the increase in surface roughness of the NP's seen in the SEM images. The presence of the impurities could then interrupt the interaction between the NP's, thus leading to a decrease in the magnetization. The measured value of the saturation magnetization of the pure ZnO nanorods fabricated in this study is 1.015 memu/g. This is close to the magnetization of the ZnO nanocrystals fabricated by Wong et al. [10]. The average size of the nanocrystals obtained by them was around 25 nm. In Fig. 8, we have plotted the coercivity for the different Sb doped ZnO nanorods. The coercivity force is the field strength needed to reverse the direction of the spin. It is the intercept of hysteresis loop with the M ¼0 axis. The increase in the coercivity as the level of Sb doping increases as seen in Fig. 8 is due to the increased pinning by the increased amount of defects formed as evident by the behavior of the PL emission in the red range of the visible light PL emission spectra.

70

4. Conclusion

Ms(memu/g )

0.95 0.90 0.85 0.80 0.75 0

10

20

30

40

50

mol% of Sb

Coercivity (Oe)

Fig. 7. Saturation magnetization (Ms) versus mol% of Sb dopant. Fig. 3:.

65 60 55 0

10

20

30

40

50

mol % of Sb Fig. 8. Coercivity (HC) versus mol% of Sb dopant.

doped ZnO (Fig. 5b) above 600 nm (Note the PL emission of the pure ZnO begins to drop 600 nm below while the PL spectrums for the Sb-doped ZnO are still increasing) is due to the presence of Sb in the ZnO. The increase in the intensity of visible light PL spectrum and decrease in the intensity of the NBE peaks as the Sb doping increases reflect the loss of crystallinity with the Sb doping. This is also borne out by the XRD measurements (See Fig. 1). The results of our VSM measurements are seen in Figs. 6, 7 and 8. Fig. 6a shows the hysteresis loops of the pure, 5 mol%, 15 mol%, 25 mol%, 30 mol% and 45 mol% Sb doped ZnO nanorods. Fig. 6b shows a blow up of the hysteresis loop of the pure ZnO nanorods. The narrowness of the loop indicates that the amount of magnetic energy stored by the ZnO nanorods is very small. From the various hysteresis loops, we extract the saturation magnetization of the different Sb-doped nanorods and plot them in Fig. 7. As we see, the saturation magnetization (MS) initially increases as the Sb is doped in. Further Sb doping leads the saturation magnetization to decrease. The explanation of this behavior is that the original magnetization in the pure ZnO NP is due to the defects (VZn, VO, Zni or Oi) in the NP's [5,8,9,10] which theoretical calculations [12– 18] narrows the defect down to the zinc vacancies VZn. Substitution of low amounts of Sb into the zinc sites stabilizes the zinc

This work studied on the magnetic properties of zinc oxide nanorods doped with various amount of antimony. The nanorods were grown using the hydrothermal method with heating at 150 °C for 20 h. XRD results indicate that the obtained Sb-doped ZnO have hexagonal wurtzite structure with Sb associated peaks appearing in the XRD patterns for the higher Sb doped nanorods. SEM images showed that most the samples were nanorods in shape are nanorods (except for the 45 mol% system, where the shape was flake like) and that the sizes of nanorods increased with increasing amount of Sb. More importantly, the surfaces of the nanorods became rougher as more Sb was added. The PL results showed that pure zinc oxide has high crystallinity based on the intensity of the UV (exciton) peak in the PL spectrum. The intensity of this peak decreased as more Sb was doped NR's, indicating that the crystallinity had decreased. The change in the intensities of the visible light spectrum (see Fig. 3a) where the decrease in the intensity of the blue portion of the spectrum implies that the Zn defects are being passivity and the increased intensity of the PL spectrum in the red portion of the spectrum is evidence of the defect (SbZn þ 2VZn) complex being created (see Ref. [28]). According to the studies in Ref. [14–18], the 2p-orbitals of the neighboring oxygen ions would occupy the ‘virtual’ energy level and form into a spin polarized state. The magnetic moment would arise from the zinc vacancies. The role of the Sb substitution into the zinc sites is the formation of the defect (SbZn þ 2VZn) complex which not only creates more zinc vacancies but stabilizes the zinc vacancies which will attract the 2p-electons of the neighboring oxygen atoms. Insertion of Sb ions into the interstitial sites does not affect the zinc vacancies except to destroy the crystallinity of the structure. Note additional oxygen atoms would have to be absorb to maintain charge neutrality of the ZnO:Sb NP's. This would lead to the destruction of the crystal structure observed.

Acknowledgment We gratefully acknowledge a financial support from the Faculty of Science and Kasetsart university for the grant no. RFG 1-9,

P. Nakarungsee et al. / Journal of Magnetism and Magnetic Materials 397 (2016) 79–85

Department of Physics and Kasetsart University Research and Development Institute.

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