Morphological evolution of solution-grown cobalt-doped ZnO nanostructures and their properties

Chemical Physics Letters 700 (2018) 1–6

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

Morphological evolution of solution-grown cobalt-doped ZnO nanostructures and their properties Qui Thanh Hoai Ta, Gitae Namgung, Jin-Seo Noh ⇑ Department of Nano-Physics, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 461-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 8 March 2018 In final form 2 April 2018 Available online 3 April 2018 Keywords: Co-doped ZnO nanostructures Low-temperature solution method Morphology Crystal structure Magnetic property

a b s t r a c t It is demonstrated that the morphology of Co-doped ZnO nanostructures can be easily altered by controlling Co-doping concentration. A facile low-temperature solution method was employed for the nanosynthesis. The morphology of the nanostructures changed from nanorods to nanoparticles, and to needle-like structures as the molar ratio of Co2+ ions increased. No noticeable changes in structural and optical properties were caused by the low concentration of Co-doping, while a magnetic transition was observed. At the very low Co concentrations below 0.3 at%, the nanostructures showed diamagnetism, whereas a paramagnetic behavior was observed at a concentration of 2.5 at%. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction Diluted magnetic semiconductors (DMSs) have attracted big interest as a potential matter to allow for the simultaneous control of electrical charges and magnetic spins [1–3]. Their application areas include data storage, spin electronics, and magnetooptoelectronics [4–6]. To synthesize DMSs, magnetic dopants like iron (Fe) [7], cobalt (Co) [8], nickel (Ni) [9], and manganese (Mn) [10] have been doped into non-magnetic semiconductor matrices. One of major goals with the DMS-related researches was to achieve the ferromagnetic behavior at room temperature. For that purpose, relatively large amount of magnetic dopants that have a Curie temperature (TC) much higher than room temperature were necessary [11]. To this aim, Co that has a high TC of 1388 K and large magnetic moment of 1.75 Bohr magneton/atom has been intensively investigated as a dopant [12]. In the matrix side, oxide-based semiconductors such as ZnO, TiO2, and SnO2 have been preferred because they make ferromagnetism sustain to room temperature [11,13–15]. A typical matrix material is ZnO. ZnO is a wide band-gap semiconductor (Eg = 3.37 eV) with many unique features, including large excitonic binding energy (60 meV), high optical transmittance, nontoxicity, and piezoelectricity [16–18]. Magnetic, electrical, and optical properties of Co-doped ZnO have been closely studied both in bulk and at nanoscale [19,20]. In particular, nanostructured Co-doped ZnO has been paid more attention since its large surface-to-volume ⇑ Corresponding author. E-mail address: [email protected] (J.-S. Noh). https://doi.org/10.1016/j.cplett.2018.04.002 0009-2614/Ó 2018 Elsevier B.V. All rights reserved.

ratio could provide the DMSs with additional functionalities. A variety of Co-doped ZnO nanostructures such as nanoparticles, nanorods, and nanowires have been synthesized and examined by the employment of differing methods [21–23]. From the previous reports, it may be stated that the properties of Co-doped ZnO nanostructures significantly depend on the morphology [23,24]. For the further study, however, it would be desirable to ensure an experimental method that can afford to produce various nanostructures with little change in synthesis conditions. In this work, it is demonstrated that the morphology and doping concentration of Co-doped ZnO nanostructures can be simultaneously controlled in situ, using a facile low-temperature solution method. The standard condition for the synthesis of ZnO nanorods is kept constant for all experiments, while only the concentration of Co precursor added to the mixture solution is varied. To the best of our knowledge, this is the first report on the morphological evolution by the use of a single synthesis method. Furthermore, the Co-doping concentration is relatively low in this study, which is another dissimilarity to the conventional DMSs.

2. Materials and methods 2.1. Materials Two precursors, zinc nitrate hexahydrate (Zn(NO3)2
6H2O) and cobalt (II) nitrate hexahydrate (Co(NO3)2
6H2O) were purchased from Sigma-Aldrich. Ethyl alcohol (C2H5OH), ethylenediamine (C2H4(NH2)2, EDA), and sodium hydroxide (NaOH) beads were

2

Q.T.H. Ta et al. / Chemical Physics Letters 700 (2018) 1–6

purchased from Daejung Chem. All chemicals were used with no further treatment and deionized (DI) water was used as solvent. 2.2. Synthesis of Co-doped ZnO nanostructures Co-doped ZnO nanostructures were synthesized by a lowtemperature solution method. At first, a 0.297 g of Zn(NO3)2
6H2O and varying amounts of Co(NO3)2
6H2O were dissolved in 1 ml of DI water and stirred at 50 °C to prepare a salt solution containing both Zn2+ and Co2+ ions. Here, the amount of Co precursor was adjusted in the range of 1.46–29.1 mg to modulate the molar ratio of Zn2+ to Co2+ from 200:1 to 10:1. In the meantime, a 1.2 g of NaOH was dissolved in 1 ml of DI water to create NaOH solution. The NaOH solution was slowly added to the salt solution for 5 min under stirring, and then 4 ml of EDA was introduced into the mixture solution. At the next step, 20 ml of ethyl alcohol was put into the solution, and the reaction proceeded at 50 °C for 5 days. Also, pure ZnO nanorods were independently synthesized for comparison at the same conditions except no use of Co precursor. Finally, nanostructure powders were collected from the reaction solution through repeated centrifugation and washing. Detailed synthesis conditions and sample names are summarized in Table 1. 2.3. Characterization The morphologies and dimensions of Co-doped ZnO nanostructures were analyzed using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7500F). Additional SEM analyses were performed to confirm the morphologies and estimate Codoping concentration, using another FE-SEM (Hitachi SU-70) quipped with an energy-dispersive X-ray spectroscope (EDX). For the closer examination of the nanostructures, a transmission electron microscope (TEM, Tecnai G2 F30) was utilized. The crystal structures and qualities of Co-doped ZnO nanostructures were characterized by X-ray diffraction (XRD, Rigaku D/MAX 2200) with Cu Ka radiation. The optical property of the samples was examined using UV–Vis spectrophotometry (UV–Vis Cary 50 Bio). The magnetic measurements were conducted in the temperature range of 4.2 – 300 K, using superconducting quantum interference devicevibrating sample magnetometer (SQUID-VSM, Quantum Design MPMS 3).

(Zn2+:Co2+ = 200:1–100:1), resulting nanostructures resemble the morphology of pure ZnO nanorods, but both ends of individual nanorods are somewhat tapered (Fig. 1(b) and (c)). The average diameter and length of CZ2 samples are estimated at 440 nm and 5.1 lm, which are slightly thinner and longer than those of pure ZnO nanorods (500 nm and 3.1 lm). Provided that the cobalt nitrate concentration is raised, Co-doped ZnO nanostructures show the mixed morphology of short nanorods and nanoparticles (Zn2+:Co2+ = 70:1, Fig. 1(d)) and become agglomerated nanoparticles at a higher concentration (Zn2+:Co2+ = 20:1, Fig. 1(e)). Increasing the concentration further (Zn2+:Co2+ = 10:1), the morphology turns to needle-like structures (Fig. 1(f)). To verify the differing morphologies, TEM analyses were also conducted on the selected samples. Fig. 2 presents TEM images of CZ2 and CZ3 samples. It is clearly seen from Fig. 2(a) that CZ2 has nanorod morphology with tapered ends. On the other hand, CZ3 consists of a lot of nanoparticles and nanorods that have small aspect ratios (Fig. 2(b)), though image clarity is not good enough. These observations are in good agreement with those from SEM analyses. The experimental conditions for syntheses of all the nanostructures were the same except the concentration of Co precursor employed, signifying the important role of cobalt nitrate. One possible way to alter the morphology is to hinder the onedimensional (1D) growth of ZnO by the formation of pure Co or Corich phases. To diagnose this possibility, Co element mapping and concentration analyses were made using both TEM-EDX and SEMEDX. For all the measured samples, neither Co agglomerates nor Co-rich phases were found. Fig. 3 exhibits SEM-EDX spectra of CZ2 and CZ4 samples. From the two spectra, Co concentrations in CZ2 and CZ4 are calculated to be 0.23 and 2.5 at%, respectively. Considering the accuracy of SEM-EDX is about 1 at%, these values represent that little Co atoms are actually incorporated into the ZnO matrix. Similar results were obtained from TEM-EDX analyses too. The very low Co-doping concentrations fall behind the Zn2+ to Co2+ molar ratios in the original mixture solutions. It is stated from these results that very low concentrations of Co atoms are distributed throughout ZnO nanostructures without forming Co-rich phases. At the standard condition adopted in this study, 1D growth of ZnO is preferred since ZnO nuclei are homogeneously formed through the following reactions and EDA is preferentially coated onto the high-energy prismatic planes of ZnO nanocrystals.

3. Results and discussion

Zn2þ + 4OH— $Zn(OH)4 2—

ð1Þ

3.1. Morphologies and compositions

Zn(OH)4 2— $ZnO+H2 O + 2OH—

ð2Þ

Fig. 1 shows SEM images of Co-doped ZnO nanostructures along with pure ZnO nanorods. As can be seen in Fig. 1(a), welldeveloped hexagonal ZnO nanorods are synthesized when no dopants are used. The morphology begins to change once cobalt nitrate is introduced into the solution as a doping chemical. Most interestingly, the morphology of the Co-doped ZnO nanostructures gradually evolves depending on the molar ratio of Zn2+ to Co2+ ions. When low concentrations of Co precursor are employed

When cobalt nitrate is added to the zinc nitrate solution, similar reactions to the above are supposed to occur, leading to the formation of CoO and the substitution of Co cations into Zn2+ sites. However, it is inferred from the low Co-doping concentrations that the formation process of Co(OH)2 4 is not as efficient as in Eq. (1). Thus, at the low Zn2+ to Co2+ molar ratios (CZ1 and CZ2), the effect of Co precursor on the morphology is minimal. If the molar ratio is increased, a certain concentration of Co atoms are doped into

Table 1 Synthesis conditions of pure ZnO nanorods and Co-doped ZnO nanostructures. Sample

Zn2+:Co2+ molar ratio

Co(NO3)2 (mg)

Zn(NO3)2 (g)

NaOH (g)

EtOH (mL)

EDA (mL)

Temperature (°C)

Reaction time (days)

ZnO CZ1 CZ2 CZ3 CZ4 CZ5

NA 200:1 100:1 70:1 20:1 10:1

0 1.455 2.910 4.150 14.551 29.103

0.297 0.297 0.297 0.297 0.297 0.297

1.2 1.2 1.2 1.2 1.2 1.2

20 20 20 20 20 20

4 4 4 4 4 4

50 50 50 50 50 50

5 5 5 5 5 5

3

Q.T.H. Ta et al. / Chemical Physics Letters 700 (2018) 1–6

(b)

(a)

500 nm

.

(c)

500 nm

(d)

500 nm

500 nm

(f)

(e)

500 nm

500 nm

Fig. 1. SEM images of pure ZnO nanorods and Co-doped ZnO nanostructures with varying Zn2+ to Co2+ molar ratio (CZ1–CZ5). (a) Pure ZnO nanorods, (b) CZ1 (200:1), (c) CZ2 (100:1), (d) CZ3 (70:1), (e) CZ4 (20:1), (f) CZ5 (10:1).

(a)

(b)

1 µm

200 nm

Fig. 2. TEM images of two Co-doped ZnO nanostructures: (a) CZ2, (b) CZ3.

ZnO nanocrystals. This may cause some crystal strains in all directions, which weaken the tendency of plane-selective EDA coating and 1D crystal growth. The consequence is gradual morphology

change from nanorods to nanoparticles (CZ3 and CZ4). The reason why needle-like morphology appears at even higher molar ratio (CZ5) is not understood yet.

4

Q.T.H. Ta et al. / Chemical Physics Letters 700 (2018) 1–6

Zn

(a) Intensity(a.u.)

where D is the crystallite size, k is the wavelength of Cu Ka radiation (0.154178 nm), h is the Bragg’s diffraction angle, and b is the full width at half maximum of the preferred peak. The calculated average crystallite sizes of pure ZnO and CZ4 samples are 23.93 and 23.92 nm, suggesting that almost no lattice distortion was caused by the Co-doping. UV–Vis absorption spectroscopy was utilized to check the optical properties of the Co-doped ZnO nanostructures. Fig. 5 shows the absorption spectra of pure ZnO nanorods and four Co-doped ZnO nanostructures synthesized with different Zn2+ to Co2+ molar ratios. For all samples, the absorption peaks appear at an almost identical wavelength, which is about 370 nm. Unlike many literatures reported previously [24,29,30], there are no significant red shift or blue shift observed from our Co-doped ZnO nanostructures. It may be because such physical mechanisms as strong sp-d exchange interaction [31–33] and merge of dopant band and conduction band [34–36] are not activated due to the insufficient concentrations of Co atoms doped in ZnO lattice. The band-gap energy calculated from the central peak position (370 nm) is 3.35 eV, which is very close to the ideal value of ZnO crystal. The stronger absorption intensities at higher Co2+ molar ratios may arise from the facilitation of UV absorption through surface defects or intergap dopant sites produced by Co doping.

Co: 0.23 at%

10 8 6 4

O 2

Co 0 1

0

2

5

4

3

Beam Energy (keV) Zn

Intensity(a.u.)

(b)

Co: 2.5 at%

15

10

O 5

Co Zn Co

0 0

0.4

0.8

1.2

1.6

2.0

2.4

Beam Energy (keV) Fig. 3. SEM-EDX spectra of (a) CZ2 and (b) CZ4. The insets show Co-doping concentrations calculated from the respective EDX spectra.

3.2. Crystal structures and optical properties The crystal structures and qualities of pure ZnO nanorods and Co-doped ZnO nanostructures were characterized by XRD. As shown in Fig. 4, both pure ZnO and Co-doped ZnO nanostructures are highly crystalline (full width at half maximum of (1 0 1) plane 0.35°) and have the same crystal structure, which is assigned to the hexagonal wurtzite structure of ZnO crystal (JCPDS card No. 36-1451). No peaks corresponding to metallic Co or isolated Co oxides are observed. The diffraction peak appearing around 2h = 44° came from a Fe holder used in the measurement. The strongest intensity plane is (1 0 1) plane for all the samples, just like the prior reports [25–27]. These indicate that Co doping into ZnO incurs no significant change in the crystal structure and quality. The crystallite sizes of the samples were calculated using the Debye-Scherrer formula [28]:

0:9k b cos h

CZ5 CZ4 CZ3 CZ2 ZnO

Absorbance (a.u.)

ZnO

Magnetic properties of Co-doped ZnO nanostructures were also investigated using SQUID-VSM. Fig. 6a–c shows magnetizationmagnetic field (MH) hysteresis loops of three samples. The data were obtained at both 4.2 and 300 K with sweeping field at a rate of 700 Oe/s in the range of ±2 kOe. All MH curves generally obey the linear relationship over the measured field range. The slopes of the MH curves are negative for CZ2 and CZ3 samples, whereas CZ4 sample shows MH curves with positive slopes. The negative slopes originate from the diamagnetic behavior of ZnO [37,38]. Comparing two samples, however, the slope of MH curve of CZ3 is more negative than CZ2. The slopes of MH curves are calculated to be 0.055 and 1.88 emu/g
Oe for the respective CZ2 and CZ3 samples at 4.2 K, where magnetic moments are better aligned. From the TEM-EDX measurements, the actual Co concentrations contained in CZ2 and CZ3 were analyzed to be 0.18 and 0.11 at%. Although the Co concentrations are too small to overcome the diamagnetic nature of ZnO matrix, the slightly higher Co concentration of CZ2 may contribute to relaxing the diamagnetic feature of the sample. Furthermore, CZ4, Co concentration of which

370 nm (202)

(110)

(102)

Intensity (a.u.)

(103) (200) (112) (201)

ð3Þ

(100) (002) (101)

D¼

3.3. Magnetic properties

CZ2

CZ4 20

30

40

50

60

70

80

2 (Degree) Fig. 4. XRD patterns of pure ZnO nanorods and two Co-doped ZnO nanostructures (CZ2 and CZ4). All the samples exhibit the identical wurtzite structure of ZnO.

300

350

400

450

500

550

Wavelength (nm) Fig. 5. UV–Vis absorption spectra for pure ZnO and a set of Co-doped ZnO nanostructures (CZ2–CZ5).

Q.T.H. Ta et al. / Chemical Physics Letters 700 (2018) 1–6

5

Fig. 6. Magnetization-magnetic field (MH) hysteresis loops and magnetization-temperature (MT) curves of (a, d) CZ2, (b, e) CZ3, and (c, f) CZ4.

was analyzed to be higher (2.5 at%), exhibits a paramagnetic behavior characterized by the positive slope of its MH curve. At this Co concentration, Co cations can weakly interact possibly through exchange interaction between itinerant sp electrons and localized d electrons or through the formation of bound magnetic polarons [39–41]. However, the degree of such magnetic interaction is not strong enough to induce globally well-aligned state of magnetic moments, which will result in ferromagnetic behavior. Fig. 6d–f displays magnetization-temperature (MT) curves for the three samples. Here, magnetization was measured under the field of 1 kOe with a temperature interval of 40 K. As expected, the magnetization becomes larger as temperature decreases. This is attributed to the tendency of Co magnetic moments to be aligned more at lower temperature. 4. Conclusions Co-doped ZnO nanostructures have been synthesized by a lowtemperature solution method with simultaneous introduction of

zinc nitrate and cobalt nitrate. The relative amount of the two precursors were elaborately controlled to adjust the molar ratio of Zn2+ to Co2+ from 200:1 to 10:1. It was disclosed that the morphology of the nanostructure changed from nanorods, to nanoparticles, and in turn to needle-like structures as Co2+ ion concentration increased, while all the nanostructures showed well-developed wurtzite structure of ZnO. EDX analyses revealed that only small amount of Co atoms (Co concentration 2.5 at%) were incorporated into the ZnO matrices under the given synthesis conditions. In this low Co concentration range, no significant change in optical properties was observed. In the aspect of magnetic property, the Co-doped ZnO nanostructures in general exhibited diamagnetism, which is attributed to the diamagnetic feature of ZnO. However, magnetic alignment was found to gradually increase with an increase in Co concentration and a paramagnetic behavior was observed at the Co concentration of 2.5 at%. The morphological evolution of Co-doped ZnO in the very low Co-doping range may be indicative of a method to alter the morphology of ZnO with little changes in crystallographic, optical, and magnetic properties.

6

Q.T.H. Ta et al. / Chemical Physics Letters 700 (2018) 1–6

Acknowledgements This research was supported by the Gachon University research fund of 2017 (GCU-2017-0178). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03932515).

References [1] J.S. Kulkarni, O. Kazakova, J.D. Holmes, Dilute magnetic semiconductor nanowires, Appl. Phys. A 85 (2006) 277–286. [2] G. Mihály, M. Csontos, S. Bordács, I. Kézsmárki, T. Wojtowicz, X. Liu, B. Jankó, J. K. Furdyna, Anomalous hall effect in the (In, Mn)Sb dilute magnetic semiconductor, Phys. Rev. Lett. 100 (2008) 1–4. [3] K.M. Tripathi, T.S. Tran, Y.J. Kim, T. Kim, Green fluorescent onion-like carbon nanoparticles from flaxseed oil for visible light induced photocatalytic applications and label-free detection of Al (III) ions, ACS Sustain. Chem. Eng. 5 (2017) 3982–3992. [4] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices, Nature 409 (2001) 66–69. [5] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species, Sci. Transl. Med. 293 (2001) 1289–1292. [6] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Onedimensional nanostructures: synthesis, characterization, and applications, Adv. Mater. 15 (2003) 353–389. [7] L. Ge, C. Han, J. Liu, Novel visible light-induced g-C3N4/Bi2WO6 composite photocatalysts for efficient degradation of methyl orange, Appl. Catal. B Environ. 108–109 (2011) 100–107. [8] N. Volbers, H. Zhou, C. Knies, D. Pfisterer, J. Sann, D.M. Hofmann, B.K. Meyer, Synthesis and characterization of ZnO:Co2+ nanoparticles, Appl. Phys. A Mater. Sci. Process. 88 (2007) 153–155. [9] D.a. Schwartz, K.R. Kittilstved, D.R. Gamelin, Above-room-temperature ferromagnetic Ni2+-doped ZnO thin films prepared from colloidal diluted magnetic semiconductor quantum dots, Appl. Phys. Lett 85 (2004) 1395. [10] K. Omri, J. El Ghoul, O.M. Lemine, M. Bououdina, B. Zhang, L. El Mir, Magnetic and optical properties of manganese doped ZnO nanoparticles synthesized by sol–gel technique, Superlattices Microstruct. 60 (2013) 139–147. [11] S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, H.D. Drew, R.L. Greene, T. Venkatesan, High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO2, Phys. Rev. Lett. 91 (2003) 77205. [12] Kinetics of ordering and growth at surfaces, in: Max G. Lagally (Ed.), Plenum Press, New York, 1990. [13] J.-Y. Kim, J.-H. Park, B.-G. Park, H.-J. Noh, S.-J. Oh, J.S. Yang, D.-H. Kim, S.D. Bu, T.-W. Noh, H.-J. Lin, H.-H. Hsieh, C.T. Chen, Ferromagnetism induced by clustered co in co-doped anatase TiO2 thin films, Phys. Rev. Lett. 90 (2003) 17401. [14] R. Seshadri, Zinc oxide-based diluted magnetic semiconductors, Curr. Opin. Solid State Mater. Sci. 9 (2005) 1–7. [15] K.M. Tripathi, A. Sachan, M. Castro, V. Choudhary, S.K. Sonkar, J.F. Feller, Green carbon nanostructured quantum resistive sensors to detect volatile biomarkers, Sustain. Mater. Technol. 16 (2018) 1–11. [16] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott, M.A. Rodriguez, H. Konishi, H. Xu, Complex and oriented ZnO nanostructures, Nat. Mater. 2 (2003) 821–826. [17] M.-K. Liang, O. Deschaume, S.V. Patwardhan, C.C. Perry, Direct evidence of ZnO morphology modification via the selective adsorption of ZnO-binding peptides, J. Mater. Chem. 21 (2011) 80. [18] Y. Gao, M. Nagai, Morphology evolution of ZnO thin films from aqueous solutions and their application to solar cells, Langmuir 22 (2006) 3936–3940.

[19] L. Liu, S. Li, J. Zhuang, L. Wang, J. Zhang, H. Li, Z. Liu, Y. Han, X. Jiang, P. Zhang, Improved selective acetone sensing properties of Co-doped ZnO nanofibers by electrospinning, Sens. Actuators B Chem. 155 (2011) 782–788. [20] L.H. Van, M.H. Hong, J. Ding, Structural and magnetic property of Co-dopedZnO thin films prepared by pulsed laser deposition, J. Alloys Compd. 449 (2008) 207–209. [21] S. Fabbiyola, L.J. Kennedy, U. Aruldoss, M. Bououdina, A.A. Dakhel, J. JudithVijaya, Synthesis of Co-doped ZnO nanoparticles via co-precipitation: structural, optical and magnetic properties, Powder Technol. 286 (2015) 757– 765. [22] V. Gandhi, R. Ganesan, H.H. Abdulrahman Syedahamed, M. Thaiyan, Effect of cobalt doping on structural, optical, and magnetic properties of ZnO nanoparticles synthesized by coprecipitation method, J. Phys. Chem. C 118 (2014) 9715–9725. [23] M. Shatnawi, A.M. Alsmadi, I. Bsoul, B. Salameh, G.A. Alna’Washi, F. Al-Dweri, F. El Akkad, Magnetic and optical properties of co-doped ZnO nanocrystalline particles, J. Alloys Compd. 655 (2016) 244–252. [24] T.M. Hammad, J.K. Salem, R.G. Harrison, Structure, optical properties and synthesis of Co-doped ZnO superstructures, Appl. Nanosci. 3 (2013) 133–139. [25] B. Liu, H.C. Zeng, Room temperature solution synthesis of monodispersed single-crystalline ZnO nanorods and derived hierarchical nanostructures, Langmuir 20 (2004) 4196–4204. [26] B. Liu, H.C. Zeng, Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm, J. Am. Chem. Soc. 125 (2003) 4430–4431. [27] S. Kundu, S. Sain, B. Satpati, S.R. Bhattacharyya, S.K. Pradhan, Structural interpretation, growth mechanism and optical properties of ZnO nanorods synthesized by a simple wet chemical route, RSC Adv. 5 (2015) 23101–23113. [28] U. Holzwarth, N. Gibson, The Scherrer equation versus the Debye-Scherrer equation, Nat. Nanotechnol. 6 (2011), 534-534. [29] R. He, B. Tang, C. Ton-That, M. Phillips, T. Tsuzuki, Physical structure and optical properties of Co-doped ZnO nanoparticles prepared by coprecipitation, J. Nanoparticle Res. 15 (2013). [30] M.J. Chithra, K. Pushpanathan, M. Loganathan, Structural and optical properties of co-doped ZnO nanoparticles synthesized by precipitation method, Mater. Manuf. Process. 29 (2014) 771–779. [31] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations, J. Phys. Chem. Solids 63 (2002) 1909–1920. [32] P.I. Archer, S.A. Santangelo, D.R. Gamelin, Direct observation of sp-d exchange interactions in colloidal Mn2+ – And Co2+-doped CdSe quantum dots, Nano Lett. 7 (2007) 1037–1043. [33] K. Ando, H. Saito, Z. Jin, T. Fukumura, M. Kawasaki, Y. Matsumoto, H. Koinuma, Large magneto-optical effect in an oxide diluted magnetic semiconductor Zn1xCoxO, Appl. Phys. Lett. 78 (2001) 2700–2702. [34] G. Campet, M. Jakani, J.P. Doumerc, J. Claverie, P. Hagenmuller, Photoconduction mechanisms in titanium and rare earth n-type semiconducting electrodes with pyrochlore and perovskite structures, Solid State Commun. 42 (1982) 93–96. [35] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantumsized TiO2: correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. [36] J. Li, H. Fan, X. Jia, W. Yang, P. Fang, Enhanced blue-green emission and ethanol sensing of Co-doped ZnO nanocrystals prepared by a solvothermal route, Appl. Phys. A Mater. Sci. Process. 98 (2010) 537–542. [37] H. Li, J.P. Sang, F. Mei, F. Ren, L. Zhang, C. Liu, Observation of ferromagnetism at room temperature for Cr+ ions implanted ZnO thin films, Appl. Surf. Sci. 253 (2007) 8524–8529. [38] M.A. Garcia, J.M. Merino, E.F. Pinel, A. Quesada, J. De La Venta, M.L.R. González, G.R. Castro, P. Crespo, J. Llopis, J.M. González-Calbet, A. Hernando, Magnetic properties of ZnO nanoparticles, Nano Lett. 7 (2007) 1489–1494. [39] P. Nyhus, S. Yoon, M. Kauffman, S. Cooper, Z. Fisk, J. Sarrao, Spectroscopic study of bound magnetic polaron formation and the metal-semiconductor transition in EuB(6), Phys. Rev. B 56 (1997) 2717–2721. [40] V.G. Bhide, D.S. Rajoria, C.N.R. Rao, G.R. Rao, V.G. Jadhao, Itinerant-electron ferromagnetism in La1-xSrxCoO3: a Mossbauer study, Phys. Rev. B 12 (1975) 2832–2843. [41] J.H. Kim, H. Kim, D. Kim, Y.E. Ihm, W.K. Choo, Magnetoresistance in laserdeposited Zn1-xCoxO thin films, Phys. B Condens. Matter 327 (2003) 304–306.