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Facile synthesis and high temperature ferromagnetism in Ni-ZnO microflowers
Physica B: Condensed Matter 569 (2019) 31–35
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Facile synthesis and high temperature ferromagnetism in Ni-ZnO microflowers
T
Prakhar Shuklaa, Jitendra Kumar Shuklab,∗ a b
Department of Chemistry, Indian Institute of Technology, Roorkee, 247667, India Sarla Dwivedi Mahavidyalaya, Chhatrapati Shahu Ji Maharaj University, Kanpur, 209101, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Sol gel preparation XPS Magnetic materials
In this work, we report the high temperature ferromagnetism of nickel doped zinc oxide (Ni-ZnO) microflowers, which were prepared by simple sol-gel method. X-ray diffraction pattern of Ni-ZnO microflowers shows the hexagonal wurtzite crystal structure similar to ZnO structure. X-ray photo electron spectroscopy of Ni-ZnO microflowers indicates the presence of all elements. Field emission electron microscopy (FESEM) reveals the flower like structure of Ni-ZnO composite quite different from ZnO structure which possess flakes and rods. Both ZnO and Ni-ZnO structure exhibit the soft ferromagnetic behavior. The optical absorption measurement shows the red shifting with respect to ZnO after the Ni doping in ZnO structure. From the magnetization measurements, the coexistence of two ferromagnetic phases in the Ni-ZnO composite were observed.
1. Introduction Diluted magnetic semiconductor (DMS), as a one type of the most important materials, has gained the wider recognition due to broad application in different areas of science. Moreover, low cost, faster functioning in electronic components and good storage capacity in conventional microelectronic devices favor them for extensive use in light-emitting diode, memory device and field effect transistor applications etc. [1,2]. DMS comprises small amounts of magnetic ion impurity such as Ni, Co, Fe, Mn, etc. These impurities substitutes at cation sites within the host semiconductor material and linked with free charge carriers giving rise the ferromagnetism [3–5]. However, to attain the room temperature ferromagnetism (FM) in DMS is challenging task in practical applications. In this context, the transition metal doped semiconductors (II-VI group semiconductor) have attracted attention due to the possibility of high temperature FM. Particularly, the magnetic doping within a large band gap semiconductor is more desirable for potential applications. Among all II-VI semiconductors, zinc oxide (ZnO) is widely used semiconductor due to direct band gap (3.3 eV), large excitation binding energy (60 meV) and ease of synthesis. Additionally, ZnO is also rich in morphologies and consisting good carrier mobility [1–5]. This is the reason that ZnO has been considered as one of the promising candidates for fabricating DMS [6]. Among the various ferromagnetic semiconductors, Ni-ZnO is the most understood and the promising DMS for spintronic application because Ni doping in ZnO
∗
matrix do not only alter the magnetic behavior but also affect the optical properties [5]. Therefore, tunable magnetic and optical properties of Ni-ZnO plays a crucial role in optical integrated circuit application with improved threshold [5]. Up to date, Ni-doped ZnO nanoparticles, nanorods and films have been reported with room temperature ferromagnetism [5,7]. It is important that the method of preparation and morphology of DMS is also having a great impact on their magnetic properties. Because surface anisotropy and lattice strain can arise due to their surface structure which may alter the magnetic properties of DMS. Herein, we report the simple way to prepare the Ni doped ZnO flowers using diethanolamine as a surfactant. The obtained product was then characterized by X-ray diffractometer (XRD), X-ray photo electron spectroscopy (XPS) and field emission electron microscopy (FESEM). Electron paramagnetic resonance (EPR) spectroscopy was performed for both samples. Absorbance spectra were obtained by UV–visible spectrometer for the wavelength range 300–800 nm. Magnetic measurements (M-H and M-T) were carried out by vibrating sample magnetometer (VSM). Instrumentation and materials details are available in the supplementary information [S1, S2]. 2. Synthesis procedure of Ni-ZnO microflowers Initially, 1.765 g of ZnO material (given in S3) with 30 ml di-ionized (DI) water was sonicated for 1 h. Subsequently 0.162 g NiCl2 precursor dissolved in 10 ml DI water and added with ZnO solution under the
Corresponding author. E-mail addresses: [email protected] (P. Shukla), [email protected] (J.K. Shukla).
https://doi.org/10.1016/j.physb.2019.05.028 Received 14 March 2019; Received in revised form 15 May 2019; Accepted 16 May 2019 Available online 24 May 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 569 (2019) 31–35
P. Shukla and J.K. Shukla
Fig. 1. Schematic representation of the fabrication of Ni-ZnO microflowers.
morphology with some exfoliated flakes. As we know that ZnO morphologies are highly dependent on the concentration of KOH/ NaOH. Different morphology to ZnO appears due to some changes in the basicity in the solution which might be the region of such morphology of ZnO. On the other hand, flowers like structure were observed in Ni-ZnO composites. Fig. 2 (b) shows the FESEM image of NiZnO flowers which comprises exfoliated plates or nanosheet. These flowers have a diameter of about 2–4 μm framed from curved microflakes/plates. Additionally, many intercrossed micro plates and flakes were found in Ni-ZnO microflowers. XPS analysis was performed to see the chemical state of elements. Fig. 3 (a) displays the full XPS survey of the Ni-ZnO microflowers. The wide scan XPS spectrum of Ni-ZnO display the zinc, oxygen and carbon peaks, but Ni peaks were not seen in this spectrum that may be due to the low content doping of Ni with ZnO materials. Carbon peak was found due to surrounding carbon. For more information, the high scan XPS spectrum of Zn in 2p region is depicted in Fig. 3 (b), clearly indicates that the presence of two peaks Zn 2p3∕2 and Zn 2p1∕2 at 1021.26 eV, 1044.34 eV. The energy difference of order 23.0 eV was obtained between above mentioned peaks. From here, it could be attributed that Zn found in Zn2+ valence state which matches with reported literature [6]. Fig. 3 (c) shows the O (1s) peak that comprises two deconvoluted components at 530.1 and 531.56 eV, attributed the lattice oxygen and chemisorbed oxygen occurs due to surface hydroxyl groups in the Ni-ZnO structures. The core spectra of Ni consist of two peaks 2p1∕2 and 2p3∕2 at 872.71 and 855.35 eV associated with divalent Ni ion (Ni(II) oxide) as depicted in Fig. 3 (d). Additionally, the presence of a satellite peak at 861.73 eV also evidence for Ni2+ state. The low valence state of Ni ion have been compensated by oxygen vacancy at substitutional site of ZnO. EPR measures the unpaired electrons. The source of these electrons may be the presence of intrinsic defects and impurities within in materials. Fig. 4 (a, b) shows the EPR spectra of ZnO and Ni-ZnO composites at room temperature. A broad and relatively higher intensity peak appeared in the EPR spectrum of Ni-ZnO, which corresponds to
continuous stirring condition. After 30 min, 4 ml diethanolamine (DEA) along with 2 ml hydrazine hydrate (HH) was added dropwise in the above solution. This mixture was heated from 80 °C to 90 °C without stirring, shown in Fig. 1. After this, above mixture was washed with DI water, then dried at 50 °C for overnight. The obtained samples were kept in vacuum for further measurements. 3. Result and discussion Fig. 2 (a) represents XRD pattern of the pure ZnO and Ni doped ZnO nanostructure. All diffraction peaks could be well indexed in hexagonal wurtzite crystal structure (JCPDS card no. 00-005-0664). Peaks obtained at 2θ values can be attributed to the planes (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202), respectively. In comparison to ZnO, high intensity peaks of Ni-ZnO in XRD confirm its micro-crystalline nature. According to JCPDS card no. 00-004-0850 of Ni, no Ni peaks were observed in Ni-ZnO. However, a weak peak at 43.53° corresponding to (200) plane of NiO was obtained in the XRD pattern of Ni-ZnO composites. As previously reported by researchers that 3% Ni doping gives a secondary phase of NiO which optimize the Ni substitutional limit into ZnO. The Ni doping with ZnO increases the 2θ value than pure ZnO. The lattice constant a, b and c of ZnO and NiZnO were calculated by the equation [8]:
Sin2θ =
λ2 ⎡ h2 k2 l2 + 2 + 2⎤ 2 ⎥ ⎢a 4 ⎣ b c ⎦
(1)
The lattice parameters of ZnO (a = b = 3.29 Å and c = 5.27 Å) were shrunk after Ni doping with ZnO (a = b = 3.18 Å and c = 5.24 Å). As similar changes in 2θ values after Ni doping with ZnO have been reported by others. The reason could be the distortions that occur in the host lattice sites (ZnO) [9]. That takes place in lattice relaxation/ compression processes in the ZnO matrix due to the presence of interstitial defects or vacancies.The morphology of the ZnO and Ni-ZnO were studied by FESEM. Pure ZnO structure, as shown in Fig. S4, does not appear in clear single morphology. It comprises some flaky rod
Fig. 2. (a) XRD pattern for ZnO and Ni-ZnO composite and (b) Morphology of Ni-ZnO microflowers. 32
Physica B: Condensed Matter 569 (2019) 31–35
P. Shukla and J.K. Shukla
Fig. 3. (a) Wide XPS spectrum, (b) core level spectrum of the Zn, (c) core level spectrum of the O (1s) and (d) core level spectrum of Ni in Ni-ZnO structure.
the Ni signal. The “g” values are calculated by using the following equation:
hν = gμB B
for the creating several defects centers from Zn to O vacancies/interstitials. For Ni-ZnO, high intensity peak indicates that number of spins are taking part in resonance while broadening of the EPR signal of NiZnO may be related to Ni centered radical sites [10–13]. The room temperature UV–Vis absorption spectra of the ZnO and Ni-ZnO are shown in Fig. 5 (a) which indicates the absorption peak of ZnO and Ni-ZnO at 357 and 373 nm, respectively. The red shift absorption in Ni-ZnO could be attributed to the formation of shallow levels into the bandgap owing to Ni doping [5]. The optical band-gap energy (Eg) of the Ni-ZnO flowers were calculated by following relation:
(2) −34
J s), ν is the frequency where h is the Planck's constant (6.63*10 (9.596*109 Hz), μB is the Bohr magneton (9.27*1024 JT−1) and B is the magnetic field in T. In pure ZnO, three distinct g values, i.e. g = 4.5, 2.0 and 1.27 were obtained while g value of Ni-ZnO was found to be 2.196, respectively. The EPR line with g = 2.0 belongs to the surface oxygen defects as previously reported by researchers. It is anticipated that the different g values of ZnO results from the ZnO morphology, which is responsible
1
(αhν ) n = C (hν − Eg )
Fig. 4. EPR spectra of (a) ZnO and (b) Ni-ZnO composites. 33
(3)
Physica B: Condensed Matter 569 (2019) 31–35
P. Shukla and J.K. Shukla
Fig. 5. (a) Absorbance plot of ZnO and Ni-ZnO composites, (b) Tauc's plot of ZnO and (c) Ni-ZnO composites, respectively.
where α is the absorption coefficient, h is Planck's constant, C is a constant, ν is the frequency of light, Eg is the band gap energy, and n = 1/2 and 2 for direct and indirect type of materials, respectively. The band gap of Ni-ZnO was observed 2.67 eV while ZnO possesses band gap around 3.23 eV as displayed in Fig. 5 (b, c). This shrinkage of band gap might be due to strain in the crystal lattice. Above band gap variation indicates that Ni2+ substitute Zn2+ within ZnO lattice [6]. Thermograms of ZnO and Ni-ZnO composites are displayed in Fig. 6. It is visible that weight loss of ZnO governed by two stages. First weight loss, in 35 oC-270 oC range, occurs due to the removal of moisture and expulsion of adsorbed water in ZnO. Second major weight loss begins from 399 °C which may due to the removal and decomposition of organic groups (e.g. hydroxyl group (OH)) derived during the chemical synthesis in the ZnO structure. Attaining the 550 °C, no further decomposition was observed. In compare with ZnO, the Ni-ZnO shows only 3% weight loss even at 700 °C. The result suggests that Ni doping with ZnO effectively decreases the weight loss of Ni-ZnO with respect to ZnO because of strong physical interaction at the interface of Ni-ZnO composites [14,15]. The hysteresis loop (M-H) curve for ZnO and Ni-ZnO flowers are shown in Fig. 7 (a). Both ZnO and Ni-ZnO possess a soft ferromagnetic nature rather ZnO shows weak magnetism. The ferromagnetism in NiZnO can be expected due to magnetic Ni doping with ZnO semiconductor, as reported in several DMSs. It is noteworthy that Ni-ZnO exhibits more remanent magnetization compared with ZnO, but less coercive field than ZnO. Table S5 contains values of saturation magnetization Ms, coercive field Hc and remanent magnetization Mr for ZnO and Ni-ZnO structures. The origin of room temperature ferromagnetism in ZnO and Ni-ZnO can be the result of a number of possibilities including the presence of intrinsic/extrinsic defects in the ZnO, Zn and O vacancies, interstitials, doping effect or realization of some nanoscale
Ni-related secondary phase like NiO etc. However, NiO phase can be easily ruled out, since it is antiferromagnetic. On other hand, metallic Ni may also be the source of this ferromagnetism. However, XRD and XPS show the absence of metallic Ni clusters in Ni-ZnO composites. It could be anticipated that synthesis method, presence of zinc/oxygen vacancies and defects highly influence to the magnetism of ZnO and NiZnO structures. In addition, magnetic anisotropy, crystal size, crystallinity and surface structure must be strongly affecting the form of hysteresis loop and leads the lower coercivity and higher remanence in Ni-ZnO composites with respect to ZnO structure. Apart from this, higher Ni content (≥3%) doping is also accountable for formation of larger magnetic moment which increases the saturation magnetization of Ni-ZnO composites. It is well established that there are two possible sources of magnetism: extrinsic and intrinsic magnetism. The clustering of d elements comes under the extrinsic source while exchange interactions are an intrinsic source of magnetism. For more details, Fig. 7 (b) shows magnetization-temperature (M-T) curve of Ni-ZnO composites. It is visible from M-T plot that initially magnetization gradually drops up to Curie temperature TC = 516 K after this a second sudden drop was found until 900 K (maximum measurement temperature). Thus, there are two magnetic phases: One TC occurs at 516 K and second TC could not be achieved even at 900 K. As previously reported by scientist that the bulk Ni possesses TC around 630 K while Ni in nano dimension consist of TC in the 350–560 K range [16,17]. Therefore, it is anticipated that the high TC in Ni-ZnO may arise from the hybridization and charge hooping from the donor impurity band to empty 3d states of Ni2+ ions near the Fermi level. However, all Ni2+ ions do not contribute overall in magnetism because isolated Ni2+ ion shows paramagnetism. While those Ni2+ ions which interact with neighbors lead to antiferromagnetism that could not be expected to contribute to total magnetization. Therefore, the second possibility of magnetism is the indirect exchange interactions in which Ni2+ located at appropriate distances produces the ferromagnetic ordering. This indirect coupling occurs between localized d electrons of Ni ions (Ni2+) and free charge carriers (generated due to Ni doping with ZnO) via defects and Zn/O vacancies [18,19]. This is convenient since Ni doping does not only provide local magnetic moments but also the carriers required for coupling with these moments. From here, it can be concluded that NiZnO microflowers can be useful in device fabrication. 4. Conclusion A flower like Zinc oxide (ZnO) structure doped with a little amount of magnetic Nickel (Ni) impurity was prepared by simple sol-gel method. XRD pattern of Ni-ZnO shows the hexagonal wurtzite crystal structure as similar to ZnO but good crystallinity was observed. XPS of Ni-ZnO indicates the presence of all elements. The optical absorption measurement shows the red shift with respect to ZnO after the Ni doping in Ni-ZnO composites. Magnetic measurements reveal the soft ferromagnetic nature of both ZnO and Ni-ZnO structures. The high temperature M (T) plot indicates the presence of two ferromagnetic
Fig. 6. Thermogravimetric analysis of ZnO and Ni-ZnO composites. 34
Physica B: Condensed Matter 569 (2019) 31–35
P. Shukla and J.K. Shukla
Fig. 7. (a) M-H loops of ZnO and Ni-ZnO structures and (b) normalized M-T curve of Ni-ZnO in the temperature range of 300–900 K.
phases. [7]
Author contributions
[8]
The proposed work was performed by P. Shukla. Both authors have equal contribution in writing and reviewing the manuscript.
[9]
Acknowledgements
[10]
P. Shukla thanks Ministry of Human Resource Development (MHRD), Government of India for financial support.
[11]
Appendix A. Supplementary data [12]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.physb.2019.05.028.
[13]
References
[14]
[1] J. Wang, G. Huang, X. Zhong, L. Sun, Y. Zhou, E. Liu, Raman scattering and high temperature ferromagnetism of mn-doped zno nanoparticles, Appl. Phys. Lett. 88 (25) (2006) 252502. [2] B. Pal, P. Giri, High temperature ferromagnetism and optical properties of co doped zno nanoparticles, J. Appl. Phys. 108 (8) (2010) 084322. [3] S. Zhou, K. Potzger, Q. Xu, G. Talut, M. Lorenz, W. Skorupa, M. Helm, J. Fassbender, M. Grundmann, H. Schmidt, Ferromagnetic transition metal implanted zno: a diluted magnetic semiconductor? Vacuum 83 (2009) S13–S19. [4] X. Wu, Z. Wei, L. Zhang, X. Wang, H. Yang, J. Jiang, Optical and magnetic properties of fe doped zno nanoparticles obtained by hydrothermal synthesis, J. Nanomater. 2014 (2014) 4. [5] S. Fabbiyola, V. Sailaja, L.J. Kennedy, M. Bououdina, J.J. Vijaya, Optical and magnetic properties of ni-doped zno nanoparticles, J. Alloy. Comp. 694 (2017) 522–531. [6] P. Shukla, J.K. Shukla, Fabrication of graphene-supported palladium nanoparticles-
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decorated zinc oxide nanorods for potential application, J. Supercond. Nov. Magnetism 31 (4) (2018) 1–8. P.V. Radovanovic, D.R. Gamelin, High-temperature ferromagnetism in n i 2+doped zno aggregates prepared from colloidal diluted magnetic semiconductor quantum dots, Phys. Rev. Lett. 91 (15) (2003) 157202. P. Shukla, J. K. Shukla, Facile sol-gel synthesis and enhanced photocatalytic activity of the v2o5-zno nanoflakes, J. Sci.: Adv. Mater. Dev.. A.K. Rana, P. Bankar, Y. Kumar, M.A. More, D.J. Late, P.M. Shirage, Synthesis of nidoped zno nanostructures by low-temperature wet chemical method and their enhanced field emission properties, RSC Adv. 6 (106) (2016) 104318–104324. S.K. Misra, S. Andronenko, A. Thurber, A. Punnoose, A. Nalepa, An x-and q-band fe3+ epr study of nanoparticles of magnetic semiconductor zn1- xfexo, J. Magn. Magn. Mater. 363 (2014) 82–87. R.C. Hoffmann, S. Sanctis, E. Erdem, S. Weber, J.J. Schneider, Zinc diketonates as single source precursors for zno nanoparticles: microwave-assisted synthesis, electrophoretic deposition and field-effect transistor device properties, J. Mater. Chem. C 4 (30) (2016) 7345–7352. S. Moribe, T. Ikoma, K. Akiyama, Q. Zhang, F. Saito, S. Tero-Kubota, Epr study on paramagnetic species in nitrogen-doped zno powders prepared by a mechanochemical method, Chem. Phys. Lett. 436 (4–6) (2007) 373–377. N. Kondal, S.K. Tiwari, Origin of polychromatic emission and defect distribution within annealed zno nanoparticles, Mater. Res. Bull. 88 (2017) 156–165. R. Sedghi, M.R. Nabid, M. Shariati, M. Behbahani, H.R. Moazzami, Preparation of pan-based electrospun nanofiber webs containing ni-zno as high performance visible light photocatalyst, Fibers Polym. 17 (12) (2016) 1969–1976. N. Tarwal, P. Shinde, Y. Oh, R.C. Korošec, P. Patil, Nickel-induced microwheel-like surface morphological evolution of zno thin films by spray pyrolysis, Appl. Phys. A 109 (3) (2012) 591–599. R.A. Serway, J.W. Jewett, Physics for Scientists and Engineers with Modern Physics, Cengage learning, 2018. L. Sun, P. Searson, C. Chien, Finite-size effects in nickel nanowire arrays, Phys. Rev. B 61 (10) (2000) R6463. J. Wang, F. Sun, S. Yang, Y. Li, C. Zhao, M. Xu, Y. Zhang, H. Zeng, Robust ferromagnetism in mn-doped mos2 nanostructures, Appl. Phys. Lett. 109 (9) (2016) 092401. J. Philip, A. Punnoose, B. Kim, K. Reddy, S. Layne, J. Holmes, B. Satpati, P. Leclair, T. Santos, J. Moodera, Carrier-controlled ferromagnetism in transparent oxide semiconductors, Nat. Mater. 5 (4) (2006) 298.
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Facile synthesis and high temperature ferromagnetism in Ni-ZnO microflowers
T
Prakhar Shuklaa, Jitendra Kumar Shuklab,∗ a b
Department of Chemistry, Indian Institute of Technology, Roorkee, 247667, India Sarla Dwivedi Mahavidyalaya, Chhatrapati Shahu Ji Maharaj University, Kanpur, 209101, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Sol gel preparation XPS Magnetic materials
In this work, we report the high temperature ferromagnetism of nickel doped zinc oxide (Ni-ZnO) microflowers, which were prepared by simple sol-gel method. X-ray diffraction pattern of Ni-ZnO microflowers shows the hexagonal wurtzite crystal structure similar to ZnO structure. X-ray photo electron spectroscopy of Ni-ZnO microflowers indicates the presence of all elements. Field emission electron microscopy (FESEM) reveals the flower like structure of Ni-ZnO composite quite different from ZnO structure which possess flakes and rods. Both ZnO and Ni-ZnO structure exhibit the soft ferromagnetic behavior. The optical absorption measurement shows the red shifting with respect to ZnO after the Ni doping in ZnO structure. From the magnetization measurements, the coexistence of two ferromagnetic phases in the Ni-ZnO composite were observed.
1. Introduction Diluted magnetic semiconductor (DMS), as a one type of the most important materials, has gained the wider recognition due to broad application in different areas of science. Moreover, low cost, faster functioning in electronic components and good storage capacity in conventional microelectronic devices favor them for extensive use in light-emitting diode, memory device and field effect transistor applications etc. [1,2]. DMS comprises small amounts of magnetic ion impurity such as Ni, Co, Fe, Mn, etc. These impurities substitutes at cation sites within the host semiconductor material and linked with free charge carriers giving rise the ferromagnetism [3–5]. However, to attain the room temperature ferromagnetism (FM) in DMS is challenging task in practical applications. In this context, the transition metal doped semiconductors (II-VI group semiconductor) have attracted attention due to the possibility of high temperature FM. Particularly, the magnetic doping within a large band gap semiconductor is more desirable for potential applications. Among all II-VI semiconductors, zinc oxide (ZnO) is widely used semiconductor due to direct band gap (3.3 eV), large excitation binding energy (60 meV) and ease of synthesis. Additionally, ZnO is also rich in morphologies and consisting good carrier mobility [1–5]. This is the reason that ZnO has been considered as one of the promising candidates for fabricating DMS [6]. Among the various ferromagnetic semiconductors, Ni-ZnO is the most understood and the promising DMS for spintronic application because Ni doping in ZnO
∗
matrix do not only alter the magnetic behavior but also affect the optical properties [5]. Therefore, tunable magnetic and optical properties of Ni-ZnO plays a crucial role in optical integrated circuit application with improved threshold [5]. Up to date, Ni-doped ZnO nanoparticles, nanorods and films have been reported with room temperature ferromagnetism [5,7]. It is important that the method of preparation and morphology of DMS is also having a great impact on their magnetic properties. Because surface anisotropy and lattice strain can arise due to their surface structure which may alter the magnetic properties of DMS. Herein, we report the simple way to prepare the Ni doped ZnO flowers using diethanolamine as a surfactant. The obtained product was then characterized by X-ray diffractometer (XRD), X-ray photo electron spectroscopy (XPS) and field emission electron microscopy (FESEM). Electron paramagnetic resonance (EPR) spectroscopy was performed for both samples. Absorbance spectra were obtained by UV–visible spectrometer for the wavelength range 300–800 nm. Magnetic measurements (M-H and M-T) were carried out by vibrating sample magnetometer (VSM). Instrumentation and materials details are available in the supplementary information [S1, S2]. 2. Synthesis procedure of Ni-ZnO microflowers Initially, 1.765 g of ZnO material (given in S3) with 30 ml di-ionized (DI) water was sonicated for 1 h. Subsequently 0.162 g NiCl2 precursor dissolved in 10 ml DI water and added with ZnO solution under the
Corresponding author. E-mail addresses: [email protected] (P. Shukla), [email protected] (J.K. Shukla).
https://doi.org/10.1016/j.physb.2019.05.028 Received 14 March 2019; Received in revised form 15 May 2019; Accepted 16 May 2019 Available online 24 May 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 569 (2019) 31–35
P. Shukla and J.K. Shukla
Fig. 1. Schematic representation of the fabrication of Ni-ZnO microflowers.
morphology with some exfoliated flakes. As we know that ZnO morphologies are highly dependent on the concentration of KOH/ NaOH. Different morphology to ZnO appears due to some changes in the basicity in the solution which might be the region of such morphology of ZnO. On the other hand, flowers like structure were observed in Ni-ZnO composites. Fig. 2 (b) shows the FESEM image of NiZnO flowers which comprises exfoliated plates or nanosheet. These flowers have a diameter of about 2–4 μm framed from curved microflakes/plates. Additionally, many intercrossed micro plates and flakes were found in Ni-ZnO microflowers. XPS analysis was performed to see the chemical state of elements. Fig. 3 (a) displays the full XPS survey of the Ni-ZnO microflowers. The wide scan XPS spectrum of Ni-ZnO display the zinc, oxygen and carbon peaks, but Ni peaks were not seen in this spectrum that may be due to the low content doping of Ni with ZnO materials. Carbon peak was found due to surrounding carbon. For more information, the high scan XPS spectrum of Zn in 2p region is depicted in Fig. 3 (b), clearly indicates that the presence of two peaks Zn 2p3∕2 and Zn 2p1∕2 at 1021.26 eV, 1044.34 eV. The energy difference of order 23.0 eV was obtained between above mentioned peaks. From here, it could be attributed that Zn found in Zn2+ valence state which matches with reported literature [6]. Fig. 3 (c) shows the O (1s) peak that comprises two deconvoluted components at 530.1 and 531.56 eV, attributed the lattice oxygen and chemisorbed oxygen occurs due to surface hydroxyl groups in the Ni-ZnO structures. The core spectra of Ni consist of two peaks 2p1∕2 and 2p3∕2 at 872.71 and 855.35 eV associated with divalent Ni ion (Ni(II) oxide) as depicted in Fig. 3 (d). Additionally, the presence of a satellite peak at 861.73 eV also evidence for Ni2+ state. The low valence state of Ni ion have been compensated by oxygen vacancy at substitutional site of ZnO. EPR measures the unpaired electrons. The source of these electrons may be the presence of intrinsic defects and impurities within in materials. Fig. 4 (a, b) shows the EPR spectra of ZnO and Ni-ZnO composites at room temperature. A broad and relatively higher intensity peak appeared in the EPR spectrum of Ni-ZnO, which corresponds to
continuous stirring condition. After 30 min, 4 ml diethanolamine (DEA) along with 2 ml hydrazine hydrate (HH) was added dropwise in the above solution. This mixture was heated from 80 °C to 90 °C without stirring, shown in Fig. 1. After this, above mixture was washed with DI water, then dried at 50 °C for overnight. The obtained samples were kept in vacuum for further measurements. 3. Result and discussion Fig. 2 (a) represents XRD pattern of the pure ZnO and Ni doped ZnO nanostructure. All diffraction peaks could be well indexed in hexagonal wurtzite crystal structure (JCPDS card no. 00-005-0664). Peaks obtained at 2θ values can be attributed to the planes (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202), respectively. In comparison to ZnO, high intensity peaks of Ni-ZnO in XRD confirm its micro-crystalline nature. According to JCPDS card no. 00-004-0850 of Ni, no Ni peaks were observed in Ni-ZnO. However, a weak peak at 43.53° corresponding to (200) plane of NiO was obtained in the XRD pattern of Ni-ZnO composites. As previously reported by researchers that 3% Ni doping gives a secondary phase of NiO which optimize the Ni substitutional limit into ZnO. The Ni doping with ZnO increases the 2θ value than pure ZnO. The lattice constant a, b and c of ZnO and NiZnO were calculated by the equation [8]:
Sin2θ =
λ2 ⎡ h2 k2 l2 + 2 + 2⎤ 2 ⎥ ⎢a 4 ⎣ b c ⎦
(1)
The lattice parameters of ZnO (a = b = 3.29 Å and c = 5.27 Å) were shrunk after Ni doping with ZnO (a = b = 3.18 Å and c = 5.24 Å). As similar changes in 2θ values after Ni doping with ZnO have been reported by others. The reason could be the distortions that occur in the host lattice sites (ZnO) [9]. That takes place in lattice relaxation/ compression processes in the ZnO matrix due to the presence of interstitial defects or vacancies.The morphology of the ZnO and Ni-ZnO were studied by FESEM. Pure ZnO structure, as shown in Fig. S4, does not appear in clear single morphology. It comprises some flaky rod
Fig. 2. (a) XRD pattern for ZnO and Ni-ZnO composite and (b) Morphology of Ni-ZnO microflowers. 32
Physica B: Condensed Matter 569 (2019) 31–35
P. Shukla and J.K. Shukla
Fig. 3. (a) Wide XPS spectrum, (b) core level spectrum of the Zn, (c) core level spectrum of the O (1s) and (d) core level spectrum of Ni in Ni-ZnO structure.
the Ni signal. The “g” values are calculated by using the following equation:
hν = gμB B
for the creating several defects centers from Zn to O vacancies/interstitials. For Ni-ZnO, high intensity peak indicates that number of spins are taking part in resonance while broadening of the EPR signal of NiZnO may be related to Ni centered radical sites [10–13]. The room temperature UV–Vis absorption spectra of the ZnO and Ni-ZnO are shown in Fig. 5 (a) which indicates the absorption peak of ZnO and Ni-ZnO at 357 and 373 nm, respectively. The red shift absorption in Ni-ZnO could be attributed to the formation of shallow levels into the bandgap owing to Ni doping [5]. The optical band-gap energy (Eg) of the Ni-ZnO flowers were calculated by following relation:
(2) −34
J s), ν is the frequency where h is the Planck's constant (6.63*10 (9.596*109 Hz), μB is the Bohr magneton (9.27*1024 JT−1) and B is the magnetic field in T. In pure ZnO, three distinct g values, i.e. g = 4.5, 2.0 and 1.27 were obtained while g value of Ni-ZnO was found to be 2.196, respectively. The EPR line with g = 2.0 belongs to the surface oxygen defects as previously reported by researchers. It is anticipated that the different g values of ZnO results from the ZnO morphology, which is responsible
1
(αhν ) n = C (hν − Eg )
Fig. 4. EPR spectra of (a) ZnO and (b) Ni-ZnO composites. 33
(3)
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Fig. 5. (a) Absorbance plot of ZnO and Ni-ZnO composites, (b) Tauc's plot of ZnO and (c) Ni-ZnO composites, respectively.
where α is the absorption coefficient, h is Planck's constant, C is a constant, ν is the frequency of light, Eg is the band gap energy, and n = 1/2 and 2 for direct and indirect type of materials, respectively. The band gap of Ni-ZnO was observed 2.67 eV while ZnO possesses band gap around 3.23 eV as displayed in Fig. 5 (b, c). This shrinkage of band gap might be due to strain in the crystal lattice. Above band gap variation indicates that Ni2+ substitute Zn2+ within ZnO lattice [6]. Thermograms of ZnO and Ni-ZnO composites are displayed in Fig. 6. It is visible that weight loss of ZnO governed by two stages. First weight loss, in 35 oC-270 oC range, occurs due to the removal of moisture and expulsion of adsorbed water in ZnO. Second major weight loss begins from 399 °C which may due to the removal and decomposition of organic groups (e.g. hydroxyl group (OH)) derived during the chemical synthesis in the ZnO structure. Attaining the 550 °C, no further decomposition was observed. In compare with ZnO, the Ni-ZnO shows only 3% weight loss even at 700 °C. The result suggests that Ni doping with ZnO effectively decreases the weight loss of Ni-ZnO with respect to ZnO because of strong physical interaction at the interface of Ni-ZnO composites [14,15]. The hysteresis loop (M-H) curve for ZnO and Ni-ZnO flowers are shown in Fig. 7 (a). Both ZnO and Ni-ZnO possess a soft ferromagnetic nature rather ZnO shows weak magnetism. The ferromagnetism in NiZnO can be expected due to magnetic Ni doping with ZnO semiconductor, as reported in several DMSs. It is noteworthy that Ni-ZnO exhibits more remanent magnetization compared with ZnO, but less coercive field than ZnO. Table S5 contains values of saturation magnetization Ms, coercive field Hc and remanent magnetization Mr for ZnO and Ni-ZnO structures. The origin of room temperature ferromagnetism in ZnO and Ni-ZnO can be the result of a number of possibilities including the presence of intrinsic/extrinsic defects in the ZnO, Zn and O vacancies, interstitials, doping effect or realization of some nanoscale
Ni-related secondary phase like NiO etc. However, NiO phase can be easily ruled out, since it is antiferromagnetic. On other hand, metallic Ni may also be the source of this ferromagnetism. However, XRD and XPS show the absence of metallic Ni clusters in Ni-ZnO composites. It could be anticipated that synthesis method, presence of zinc/oxygen vacancies and defects highly influence to the magnetism of ZnO and NiZnO structures. In addition, magnetic anisotropy, crystal size, crystallinity and surface structure must be strongly affecting the form of hysteresis loop and leads the lower coercivity and higher remanence in Ni-ZnO composites with respect to ZnO structure. Apart from this, higher Ni content (≥3%) doping is also accountable for formation of larger magnetic moment which increases the saturation magnetization of Ni-ZnO composites. It is well established that there are two possible sources of magnetism: extrinsic and intrinsic magnetism. The clustering of d elements comes under the extrinsic source while exchange interactions are an intrinsic source of magnetism. For more details, Fig. 7 (b) shows magnetization-temperature (M-T) curve of Ni-ZnO composites. It is visible from M-T plot that initially magnetization gradually drops up to Curie temperature TC = 516 K after this a second sudden drop was found until 900 K (maximum measurement temperature). Thus, there are two magnetic phases: One TC occurs at 516 K and second TC could not be achieved even at 900 K. As previously reported by scientist that the bulk Ni possesses TC around 630 K while Ni in nano dimension consist of TC in the 350–560 K range [16,17]. Therefore, it is anticipated that the high TC in Ni-ZnO may arise from the hybridization and charge hooping from the donor impurity band to empty 3d states of Ni2+ ions near the Fermi level. However, all Ni2+ ions do not contribute overall in magnetism because isolated Ni2+ ion shows paramagnetism. While those Ni2+ ions which interact with neighbors lead to antiferromagnetism that could not be expected to contribute to total magnetization. Therefore, the second possibility of magnetism is the indirect exchange interactions in which Ni2+ located at appropriate distances produces the ferromagnetic ordering. This indirect coupling occurs between localized d electrons of Ni ions (Ni2+) and free charge carriers (generated due to Ni doping with ZnO) via defects and Zn/O vacancies [18,19]. This is convenient since Ni doping does not only provide local magnetic moments but also the carriers required for coupling with these moments. From here, it can be concluded that NiZnO microflowers can be useful in device fabrication. 4. Conclusion A flower like Zinc oxide (ZnO) structure doped with a little amount of magnetic Nickel (Ni) impurity was prepared by simple sol-gel method. XRD pattern of Ni-ZnO shows the hexagonal wurtzite crystal structure as similar to ZnO but good crystallinity was observed. XPS of Ni-ZnO indicates the presence of all elements. The optical absorption measurement shows the red shift with respect to ZnO after the Ni doping in Ni-ZnO composites. Magnetic measurements reveal the soft ferromagnetic nature of both ZnO and Ni-ZnO structures. The high temperature M (T) plot indicates the presence of two ferromagnetic
Fig. 6. Thermogravimetric analysis of ZnO and Ni-ZnO composites. 34
Physica B: Condensed Matter 569 (2019) 31–35
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Fig. 7. (a) M-H loops of ZnO and Ni-ZnO structures and (b) normalized M-T curve of Ni-ZnO in the temperature range of 300–900 K.
phases. [7]
Author contributions
[8]
The proposed work was performed by P. Shukla. Both authors have equal contribution in writing and reviewing the manuscript.
[9]
Acknowledgements
[10]
P. Shukla thanks Ministry of Human Resource Development (MHRD), Government of India for financial support.
[11]
Appendix A. Supplementary data [12]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.physb.2019.05.028.
[13]
References
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