Magnetic properties of sol gel synthesized c DOPED zno nanoparticles

M ANUS CR IP T AC CE PT ED MAGNETIC PROPERTIES OF SOL-GEL SYNTHESIZED C-DOPED ZnO NANOPARTICLES Nguyen Duc Dung1, *, Cao Thai Son1, Pham Vu Loc1, Nguyen Huu Cuong1, Pham The Kien1, Pha

Magnetic properties of sol-gel synthesized C-DOPED ZnO nanoparticles

Nguyen Duc Dung, Cao Thai Son, Pham Vu Loc, Nguyen Huu Cuong, Pham The

Kien, Pham Thanh Huy, Ngo Ngoc Ha

PII: S0925-8388(16)30209-2

DOI: 10.1016/j.jallcom.2016.01.208

Reference: JALCOM 36559

To appear in: Journal of Alloys and Compounds

Received Date: 26 November 2015

Revised Date: 22 January 2016

Accepted Date: 25 January 2016

Please cite this article as: N.D Dung, C.T Son, P.V Loc, N.H Cuong, P.T Kien, P.T Huy, N.N.

Ha, Magnetic properties of sol-gel synthesized C-DOPED ZnO nanoparticles, Journal of Alloys and

Compounds (2016), doi: 10.1016/j.jallcom.2016.01.208.

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MAGNETIC PROPERTIES OF SOL-GEL SYNTHESIZED C-DOPED ZnO

NANOPARTICLES

Nguyen Duc Dung1, *, Cao Thai Son1, Pham Vu Loc1, Nguyen Huu Cuong1, Pham The Kien1,

Pham Thanh Huy1

1

Advanced Institute of Science and Technology (AIST), Hanoi University of Science and

Technology, No.1 Dai Co Viet, Hanoi, Vietnam

Ngo Ngoc Ha2

2

International Training Institute for Materials Science (ITIMS), Hanoi University of Science

and Technology, No.1 Dai Co Viet, Hanoi, Vietnam

*

Corresponding author: dung.nguyenduc@hust.edu.vn

Abstract: ZnO doping with Carbon (C-doped ZnO) materials were prepared by sol-gel

technique following with a heat treatment process Single phase of Wurtzite crystal structure

of ZnO was concluded via x-ray diffraction (XRD) with a large amount of excess C tracking

by energy dispersive X-ray spectroscopy (EDX) analysis Two types of ZnO crystals (twinning particles) with different grain sizes and shapes were identified via scanning electron microscopy (FE-SEM) The first type has a smaller grain size of about 20 nm and hexagonal shape And the second type has a larger grain size of about 80-120 nm and round shape C substitutions of both Zn and O sites to form C-O and C-Zn bonds were conclusively confirmed via x-ray photoelectron spectroscope (XPS) Experimental evidences for the co-existence of different ferromagnetic phases in the materials are reported and discussed Two Curie points at high temperatures (>500oC) are presented A metamagnetic transition was observed at magnetic field H = 19.2 kOe which was related to the co-existence of ferromagnetic phases These involve in the formation of twinning C-doped ZnO nanoparticles

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INTRODUCTION

Studies of the diluted magnetic semiconductors (DMS), in which ferromagnetism and semiconductor properties are both exhibited, receive much attention for spin-depend transportations in computer technology, in particular, semiconductor spintronics devices1,2 With a Wurtzite crystalline structure and large direct bandgap energy of 3.4 eV, ZnO is among promising candidates for room-temperature DMS materials3 Important results in the ZnO-based materials include magnetotransport behaviour4, stability of the feromagnetism properties5, and magnetic anisotropy6

Doping of transition metals (TMs) such as Fe, Mn, Ni, Co was often reported for realization of ZnO-based DMS7–11 In such systems, the magnetism derives from TMs dopants and ZnO would play the role of the semiconductor hosting material When doping with non-magnetic elements, e.g carbon, ferronon-magnetic properties of the ZnO were also presented12–18, however the reported origins of the magnetism were rather diversified and sometime

inconsistent In thin films of C-doped ZnO prepared by pulsed-laser deposition, Pan et al.19 theoretically and practically reported the intrinsic n-typed ferromagnetic behaviour of the

materials with Curie points well above room temperature The authors observed that the substitution of C atoms at O sites in ZnO Wurtzite crystalline structure could introduce holes

in O2p states, which coupled with the C2p localized spins by a p-p interaction By this p-p

interaction, holes mediated the spin alignment of C atoms, leading to the indirect

ferromagnetic coupling of C atoms The magnetic moment of about 2 µ B per carbon was also

concluded Peng et al.20 theoretically generalized the hole-induced ferromagnetism This could be attributed to the localized nature for acceptor doping, especially at anion cites Consistent results were also attained from calculations using first-principles methods21–23 The enhancement of ferromagnetism was presented with quantum confinement effects on C dopants in ZnO at nanoscale20,24 In another report, Zhang et al.25 observed the

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ferromagnetism in Znx(ZnO)1-x granular films It was speculated that the dopants in ZnO play

no essential role in carrying magnetic moment The ferromagnetism derived from their native defects such as Zn interstitials or O vacancies with a Curie temperature attained at the temperature of above 500oC

Mechanism of ferromagnetic properties of ZnO-based materials, especially at nanoscale, is often complicated and not always well-controlled A significant uncertainty of defects and local environments prevails in discerning the fundamental physical properties of the materials In this research, we report the co-existence of ferromagnetic phases in C-doped ZnO nanoparticles prepared by sol-gel method Two Curie points at high temperatures (>500oC) and a metamagnetic transition are presented These involve in the formation of twinning C-doped ZnO nanoparticles

EXPERIMENTAL

The studied ZnO doping with C (C-doped ZnO) powders were fabricated by sol-gel method followed with a heat treatment process Zinc acetate (ZnAc) and Diethanolamine (DEA) as precusors were completely dissolved in isopropanol to form a 0.2 M solution DEA and ZnAc have the molar ratio of 1:1 C fine powder with an average size of about 1 µm was added to the solution and vibrated for dispersion in an ultrasonic vibrator in 2 hours Solution then was stirred by magnetic stirrer for 5 minutes and heated at the temperatures in the range from 65–70oC for 3 hours In the next step, the as-prepared solution with additive C atoms was dried in the air at 65−70o

C for 3 hours for gelation Finally, the received gel was annealed

at 450oC in Ar gas for 5 hour to complete solid state reactions

Crystalline structures, morphologies and compositions of the prepared samples were characterized by Bruker D8 Advance x-ray diffraction (XRD) using Cu Kα line at 1.5418 Å, JEOL-JSM7600F field emission scanning electron microscope (FESEM) system combined

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with Oxford instrument energy dispersive x-ray (EDX) Bindings of C and O and Zn were investigated by x-ray photoelectron spectroscope (XPS), PHI VersaProbe II system The magnetism was measured by MicroSence EZ9 vibrating sample magnetometer (VSM)

RESULTS AND DISCUSSION

Fig 1 shows a XRD pattern of a C-doped ZnO sample in 2θ scanning angle range from 10÷70o

Peaks characterized with Wurtzite crystalline structure of ZnO26 at 2θ angles of around 31.9, 34.2, 36.1, 46.7, 56.3, 62.5, 66.2, 69 and 70.5o are corresponding to (100), (002), (101), (102), (110), (103), (200), (112) and (201) diffraction planes, respectively No peak for any other phases and crystalline structures, including C graphite, is shown Thus, formation of

a single phase Wurtzite crystalline structure of ZnO can be concluded

Figure 1 XRD pattern of the C-doped ZnO sample characterized for Wurtzite crystalline

structure

Fig 2 shows a FESEM image of the sample Two types of ZnO particles mixing

together can be observed The first type is identified for its smaller grain size of about 20 nm with a hexagonal shape While the second type has a larger grain size of about 80-120 nm and round shape The relative parts of the particles of first and second types in the mixture are about 40% and 60% in volume, respectively In the inset, the EDX spectrum indicates coexistence of three elements: Zn, O and C in the sample We can see a relatively

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large amount of C, not being crystallized as previously shown in the XRD measurements The amorphous C may show halo-ring diffraction that its intensity is much lower than the one of XRD diffraction peaks of ZnO nanoparticles Obviously, C atoms must distribute homogeneously in the sample, either forming amorphous C (free carbon) or incorporating with ZnO in the Wurtzite crystal structure (C dopants) In the former case, the amorphous C atoms are independent from physical properties of ZnO nanocrystals Amorhous C show In the latter case, C atoms may allocate at the interstitial sites in Wurtzite ZnO that do not form atomic bindings with host materials or they can substitute Zn cites or O cites to form O-C and Zn-C bonds However, we can not estimate the realistic ratio of C dopants in the current case

Figure 2 FESEM image with two types of ZnO particles The first type is identified for its

smaller grain size of about 20 nm with a hexagonal shape and the second type has a larger grain size of about 80-120 nm and round shape In the inset, the EDX spectrum indicates coexistence of three elements: Zn, O and C in the sample

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Figure 3 XPS spectrum of C-doped ZnO nanoparticles The Gaussians at binding energies

(BE) of 248.8 eV and 286.8 eV are identified for C1s orbital in the C (free carbon) and C-O/C=O bonds, respectively The Gaussian at BE of 283.3 eV is identified for C1s orbital in

the binding of C and Zn

In this part, allocations of C atoms in the material are further investigated Fig 3 shows a XPS

spectrum of C1s orbital The Gaussians at binding energies (BE) of 248.8 eV and 286.8 eV

are identified for C1s orbital in the C-C (free carbon) and C-O/C=O bonds, respectively The Gaussian at BE of 283.3 eV is identified for C1s orbital in the binding of C and Zn, which

was also reported in references19,20,27 Obviously, substitution of C atoms at both Zn sites and

O sites to form O-C and Zn-C bonds can be conclusively confirmed

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Figure 4 (a) Magnetization curves of the C-doped ZnO nanoparticles at room temperature (T

= 25oC) The measured magnetization curve (open rhombi) is the combination of ferromagnetic component of sample (open circles) and diamagnetic component of sample holder (solid rhombi) In the inset, a metamagnetic transition is also observed at magnetic

field H=19.2 kOe; (b) thermomagnetic curve with magnetic field H = 5 kOe Two magnetic

phase transition temperatures (Curie points) can be observed at temperature ranges of

520−560oC and 580−620oC

Fig 4(a) shows the magnetization curves of the C-doped ZnO nanoparticles at room temperature (T = 25oC) The measured magnetization curve (open rhombi) is the combination

of ferromagnetic component of sample (open circles) and diamagnetic component of sample holder (solid rhombi) The ferromagnetic component is attained from the subtraction of the experimental magnetization by the magnetization contributed by the diamagnetic component

of the sample holder It can be seen that C-doped ZnO nanocrystals have ferromagnetic properties characterized with two magnetization processes: magnetic moments rotation and

magnetic saturation The inset shows a metamagnetic transition observed at magnetic field H

= 19.2 kOe As previously mentioned, magnetism in C-doped ZnO could obtain from

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involve with Zn atoms in interstitial positions of ZnO crystals and Zn vacancies that change electron configuration of two adjacent oxygen atoms13 It is noted that substitution of C atoms

at Zn sites and interstitial C do not result in magnetism Quantum confinement of carbon dopants in ZnO nano-sized crystals of about few nanometers was theoretically demonstrated

to give the rise of magnetism10,20 However, it is not the case where the sizes of ZnO nanoparticles are orders larger

The existence of the metamagnetic transition at magnetic field H = 19.2 kOe is very

interesting However, mechanism for metamagnetic transitions in general is often complicated and not always well explained, especially for diluted magnetism systems In some cases, it was reported that metamagnetic transitions appeared in materials from complex magnetic structures or containing various magnetic interactions29–31 The theories of metamagnetic transitions are usually based on the minimum total energy principle with the competition of different magnetic energies co-existing in a material such as static magnetic energy, magnetic exchange energy and magnetic anisotropy energy In the DMS materials, the magnetic anisotropy energy was rarely taken into account and/or discussed The mechanism of ferromagnetism in C-doped ZnO or ZnO systems was often explained by using the Stoner theory This based on band theory of electrons in solid and considering magnetic carriers as itinerant or Bloch electrons19,22 The spontaneous ferromagnetism in the materials would

occurs when the Stoner Criterion was satisfied D(E F)⋅J > 1, in which D(E F) is the density of

state at Fermi level and J is the (indirect) exchange interaction energy

Thermomagnetic curve of the C-doped ZnO with magnetic field H = 5 kOe is presented

in Fig 4(b) Two magnetic phase transition temperatures (Curie points, T C) can be observed at

temperature ranges of 520−560oC and 580−620o

C These close Curie points correspond to two ferromagnetic phases in the sample with small differential exchange energy This

observation may explain different values of Curie points (T C) in the similar materials19,22 It

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seems natural that such difference of T C for the grains may be connected with the difference

of their sizes in the mixture with the above-mentioned bimodal size distribution However, the metamagnetic phase transition as shown in Fig 4 (a) implies that two magnetic phases exist in one morphological grain type with different exchange energies It means that a single ZnO nanoparticle of any type should contain two magnetic phases However, the contribution of each particle type on magnetization is not able to be determined While the substitution of C atoms at O sites could involve with both types of ZnO nanocrystals, defects in the C-doped ZnO nanoparticles are possibly higher in the smaller ZnO nanoparticles (type I) for larger number of surface states This is quite consistent with a recent reported results32 in which the authors present an increase of magnetic volume fractions with the reduced grain size of the samples Here, the two ferromagnetic phases co-exist, irrespective of the origin coming from

C dopping into ZnO and defects in ZnO, both of them satisfied the Stoner Criterion This is

induced by a large D(E F ), thus the spin-up and spin-down electron bands will change

intensively and sensitively with applied magnetic field The co-existence of two ferromagnetic phases brings a complicated competition of different magnetic energies In such a case, metamagnetic transition can occur as a consequence

CONCLUSION

In conclusion, twinning C-doped ZnO nanoparticles prepared by sol-gel method is concluded Single crystal phase Wurtzite is established The diluted ferromagnetism of the materials is confirmed We demonstrate by experiments clear evidences for the co-existence

of different ferromagnetic phases in the materials Two magnetic-phase transitional Curie temperatures of the materials in accordance with twinning nanoparticles of ZnO help to clarify and unify currently-reported origins of magnetism in ZnO materials14,15,19