Induced phase transition from ZnO to Co3O4 through Co substitution

Nano-Structures & Nano-Objects 11 (2017) 20–24

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Induced phase transition from ZnO to Co3 O4 through Co substitution Anuraj Sundararaj ∗ , Gopalakrishnan Chandrasekaran Nanotechnology Research Center, SRM University, Kattankulathur, Kancheepuram District - 603 203, Tamil Nadu, India

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Article history: Received 7 November 2016 Received in revised form 6 April 2017 Accepted 1 May 2017

Keywords: Dilute magnetic semiconductor Topography Crystallinity Magnetism Optical property

abstract Physical properties of Co substituted ZnO nanoparticle systems were investigated by varying the ratio of cobalt from 1 to 95 at.%. The 1 at.% Co substituted ZnO system acquired ZnO’s wurtzite structure, however the crystallinity gradually reduced with further increase in ratio of Co up to 55 at.%. Upon 65 at.% Co substitution, the system contained ZnO (100), Co3 O4 (311) and Co (111) phases. Further increase in Co ratio up to 95 at.% lead to the formation of Co3 O4 (311) phase. Shape of the particles were cylindrical, fractal and hexagonal/rectangular, for the systems with 1, 15 and 85 at.% Co, respectively. The UV–visible absorption peaks corresponding to ZnO in 1 at.% Co substituted sample blue shifted to 376 nm, and Co3 O4 in 95 at.% Co substituted sample blue shifted to 494 nm. The magnetization studies on highly crystalline samples revealed the paramagnetic nature of the systems. An increase in magnetization of the system was observed with increase in Co from 1 at.% to 15 at.%, however above 75 at.% the magnetization reduced with an increase in Co and eventually became super-paramagnetic at 85 at.% and anti-ferromagnetic at 95 at.%, due to the formation of Co3 O4 phase. © 2017 Elsevier B.V. All rights reserved.

1. Introduction ZnO is a large band gap semiconductor that is widely studied and used in UV absorption, photocatalysis, gas sensing, UV light emitting diode, photovoltaic, anti-microbial and transparent electrode applications [1,2]. Co substituted ZnO have been extensively researched in the past decade, for its unique dilute magnetic semiconducting property that enables its application in spintronics [3]. In Zn(Co)O nanoparticles, at low Co ratio, the Co2+ ions substituted in Zn2+ sites introduce a net magnetic moment via multiple exchange interactions and also change the particle morphology, this in turn influence other physical properties [4,5]. Furthermore, the Co3 O4 is a tri-cobalt tetra-oxide (transition metal oxide) semiconductive material that is extensively investigated for its application in lithium-ion batteries, gas sensors, electrocatalysis and magnetic / electrochromic devices. The surface properties of Co3 O4 are highly dependent on the shape/surface area of the nanoparticles, therefore the structural modifications of Co3 O4 semiconductive nanoparticles have been widely investigated [6,7]. The Co substituted ZnO and Co3 O4 semiconductive nanoparticles with uniform shape and size distribution have been achieved via microwave, hydrothermal, sol–gel, thermal solid phase decomposition and vapor deposition [8–12]. Also, there are reports on crys-

∗

Corresponding author. E-mail address: [email protected] (A. Sundararaj).

http://dx.doi.org/10.1016/j.nanoso.2017.05.003 2352-507X/© 2017 Elsevier B.V. All rights reserved.

tallographic, shape and size modulation by transition metal substitution in similar semiconductive systems [13]. In this work, the ratio of Co substitute on ZnO system was varied from 1 at.% to 95 at.%, to understand the physical property changes that occur during a complete crystal phase transition from ZnO’s hexagonal wurtzite to Co3 O4 ’s face-centered-cubic. The large range of Co substitute ratio in this work will provide an insight on transitions that occur, specifically at higher Co ratios that are seldom reported. In this article, we report on the induced phase transition from ZnO to Co3 O4 through 1 at.% to 95 at.% Co substitution and the observed changes in topographic, chemical, optical and magnetic properties. 2. Materials and methods Co-precipitation reaction method was used to synthesize the Co substituted ZnO (ZCO) nanoparticle systems. Reagent grade Zn(O2 CCH3 )2 (H2 O)2 and CoCl2 6H2 O were procured from SigmaAldrich and used as precursors, NaOH and H2 O2 were used as reducing agent and pH moderator throughout this work. Using 20 ml of de-ionized water as the solvent 0.05 mol stoichiometric precursor and 0.1 mol NaOH solutions were made. The precursor solutions were titrated into the reducing agent at a rate of 1 ml/min to form the precipitate, while the solutions were stirred at 2500 RPM and the reaction temperature was maintained at 65 °C using a Clarkson’s ceramic top digital magnetic stirrer with hot plate. Table 1 shows the precursor concentrations that were used during the synthesis of nanoparticles throughout this work.

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Table 1 Precursor concentrations and calculated atomic percentage ratio of Co with Zn. Sample ID

Concentration of CoCl2 6H2 O (mol)

Concentration of Zn(O2 CCH3 )2 (H2 O)2 (mol)

Atomic percentage ratio of Co with Zn (at.%)

a b c d e f g h i j k

0.0005 0.0025 0.0075 0.0125 0.0175 0.0225 0.0275 0.0325 0.0375 0.0425 0.0475

0.0495 0.0475 0.0425 0.0375 0.0325 0.0275 0.0225 0.0175 0.0125 0.0075 0.0025

1 5 15 25 35 45 55 65 75 85 95

The Once the titrations were completed, 30% hydrogen peroxide was added to the precipitate to moderate the pH (to 12) and ensure the completion of reaction; as otherwise the solution will sustain acidic and hinder the desired reaction. The following equation represents the aforementioned chemical reaction. Zn(CH3 COO)2 + CoCl2 + 4NaOH

↓ Zn(OH)2 + Co(OH)2 + 2CH3 COONa + 2NaCl ∆

Zn(OH)2 + Co(OH)2 → Zn(Co)O + 2H2 O. Once the reactions were completed, the precipitates with low Co concentration displayed a pale green color which indicated the formation of Zn/Co hydroxides. It was also observed that the precipitate’s color became progressively darker for higher Co concentrations. The obtained precipitates were washed and filtered multiple times using deionized water and absolute ethanol, with Whatman Grade 1 paper in a Naugra’s Buchner funnel assisted by a Bio Technics’s diaphragm pump. The acquired precipitates were dried in hot air oven at 80 °C for 24 h. The Zn/Co hydroxide samples were detached from the Whatman paper and calcinated at 400 °C for 24 h before analyses. All the experiments gave a yield of ∼0.008 g. 3. Results and discussion Crystallographic analyses using PANalytical’s X’pert X-ray diffractometer showed, that the crystallinity of ZCO particles highly depends on Co ratio (Fig. 1). The X-ray diffraction (XRD) patterns acquired from ZCO with up to 45 at.% Co indicated that the samples have ZnO’s hexagonal wurtzite structure [ICDD Card No: 36-1451] without additional phase, suggesting the possibility of Co2+ ions being substituted in Zn2+ sites [14]. The particle growth orientation was significant along (101) plane, which is consistent with other reports [14]. However, it was also observed that the diffraction peak intensity corresponding to the hexagonal wurtzite structure reduced as the Co was increased from 1 at.% to 55 at.%, indicating the reduction in long-range crystallinity. The ZCO with 55 at.% Co was almost amorphous, with traces of peaks corresponding to a hexagonal wurtzite structure. The diffraction peaks acquired from 65 at.% Co substituted sample indicated the presence of ZnO (100), Co (111) fcc and Co3 O4 (311) ccp phases. Further increase in Co from 75 at.% to 95 at.% supported the formation of Co3 O4 (311) ccp spinel phases [ICDD card No. 781969], where the crystallinity increased with increase in Co. The topographic properties of the ZCO samples were analyzed using FEI’s Quanta FEG 200 field emission scanning electron microscope (FE-SEM). It was observed that the Co ratio significantly affected the shape and size of ZCO particles. When Co in the system was in the range of 1 at.% to 25 at.%, the particles featured ∼500 nm long elongated cylindrical shape with narrowing edges, identical to the shape of rice grains (Fig. 2(a)). The length of these cylindrical particles remained almost same however, the breadth

Fig. 1. The XRD patterns corresponding to ZCO samples with 1 at.% to 95 at.% Co with respect to Zn.

increased with increase in Co ratio. The average breadth of the cylindrical particles were ∼65, 70, 100, 110 nm for 1 at.%, 5 at.%, 15 at.%, 25 at.% Co ratio. Although the increase in Co ratio from 1 at.% to 25 at.% reduces the crystallinity, the increase in particle dimension suggests that Co may act as a catalyst for the particles growth. The elongated structures could be due to the preferred growth orientation along (101) plane, inferred from XRD. For 35 at.% to 65 at.% Co ratio, the particles were spherical in shape with an average diameter of 45 nm, however other secondary feature formations were observed. The reduced particle size may be associated with the amorphous nature that was inferred from XRD. The sample with 35 at.% Co was featuring irregularly shaped agglomerates of the spherical nanoparticles, where the sample with 45 at.% Co featured nano/micro rod-shaped agglomerates. The sample with 55 at.% Co featured few crystals of ∼500 nm lengths, where the shape and contrast of these crystals were similar to those of Co3 O4 particles which were found predominantly in the later stages. At 65 at.% Co ratio, the Co3 O4 particle dimensions were up to few microns. At 75 at.% Co substitution, hexagonal and rectangular shaped nanostructures were predominantly observed. The average dimension of these structures varied up to ∼350 nm. Smaller hexagonal

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Fig. 3. UV–visible absorption spectrum corresponding to ZCO with 1 at.%, 5 at.%, 85 at.% and 95 at.% Co.

Fig. 2. FESEM images corresponding to ZCO with (a) 1 at.%, (b) 5 at.%, (c) 15 at.%, (d) 25 at.%, (e) 35 at.%, (f) 45 at.%, (g) 55 at.%, (h) 65 at.%, (i) 75 at.%, (j) 85 at.%, (k) 95 at.% Co.

and rectangular structures, with around ∼300 nm average dimension were observed in samples with 85 at.% Co ratio. Where, at 95 at.% Co ratio, along with the ∼150 nm hexagonal structures, spherical and 2D nanostructures were also formed. The observed hexagonal structures are similar to the reports on hexagonal structures synthesized used hydrothermal method [15]. From the EDAX’s energy dispersive spectroscopic (EDS) analyses data presented in Table 2, it was confirmed that the zinc

dominant samples have featured rice grain shaped nanoparticles with lower oxygen stoichiometry compared to Co-dominant samples. Where, the Co-dominant samples have featured hexagonal/rectangular shapes with relatively more oxygen, suggesting the presence of Co3 O4 phase. From UV–Visible absorption analyses of the ZCO samples performed using Shimadzu’s UV-1800, it was observed that the absorption wavelength for ZCO with 1 at.% and 5 at.% Co is higher (∼376 nm) when compared to the previously reported values (∼361 nm). Simultaneously, a blue shift in the absorption peak was also observed when the Co ratio was increased as shown in Fig. 3. The peaks corresponding to ZCO blue shifted from 376 to 374 nm when the Co ratio was increased from 1 at.% to 5 at.%, which is consistent with an earlier report on ZnO with similarly low Co ratio [14]. Also the peak corresponding to Co3 O4 in ZCO with high Co ratio, blue shifted from 566 to 494 nm when the Co ratio was increased from 85 at.% to 95 at.%. The observed blue shifts in the absorption peaks suggests an increase in excitation energy and band gap (Burstein–Moss effect), possibly via a quantum confinement effect influenced by the shape/size, defects in stoichiometry or the change in crystallinity [16–18]. From the magnetic property analyses using Lake Shore’s 7400 Series vibrating sample magnetometer (VSM), it was observed that ZCO with 1 at.%, 5 at.% and 15 at.% Co expressed a weak paramagnetic behavior with a magnetization of 0.3, 0.5 and 3.1 emu/g, respectively. The sample with 15 at.% Co ratio showed the highest coercivity of 285 Oe, with a remnant magnetization of 0.2 emu/g as shown in Fig. 4. Several mechanism for the observed magnetism in ZCO with 1 at.% to 15 at.% Co have been proposed, where super-exchange, double-exchange and oxygen vacancies are proposed to be the possible cause of magnetism [17]. The observed increase in magnetization and coercivity could be due to the increase in Co2+ ratio and particle size or reduction in longrange crystallinity. As shown in Fig. 5, the samples with 75 at.% and 85 at.% Co showed a paramagnetic and super-paramagnetic behavior, showing a magnetization of 0.65/0.41 emu/g, and coercivity of 362/24 Oe, respectively. The sample with 95 at.% Co showed an anti-ferromagnetic nature. The antiferromagnetic nature of Co3 O4 nanoparticles is due to its spinel structure, where there is multiple exchange interaction between the Co2+ and Co3+ site ions at the tetrahedral and octahedral sites [8]. There are reports on ferromagnetism in Co3 O4 however, it is proposed to be originating from the unpaired spin state corresponding to Co2+ at

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Table 2 Mean atomic percentage of Zn, Co and O in ZCO samples measured using energy dispersive spectroscopy. Cylindrical and fractal CoCl2 6H2 O precursor concentration

0.0005 M

0.0025 M

0.0075 M

Zn at.%

41 (±1) 1 (±0) 59 (±3)

38 (±1) 4 (±0) 58 (±3)

33 (±1) 7 (±0) 60 (±3)

CoCl2 6H2 O precursor concentration

0.0375 M

0.0425 M

0.0475 M

Zn at.%

8 (±0) 32 (±1) 60 (±4)

4 (±0) 25 (±1) 71 (±5)

2 (±0) 35 (±1) 63 (±4)

Co at.% O at.% Hexagonal and rectangular

Co at.% O at.%

Fig. 6. Representation of crystal phase transition.

coercivity with an increase in Co ratio could be associated with the formation of Co3 O4 phase, inferred from XRD. 4. Conclusion

Fig. 4. VSM loops of ZCO with 1 at.%, 5 at.% and 15 at.% Co.

We have reported on the synthesis of 1 at.%–95 at.% Co substituted ZnO nanoparticles via co-precipitation method and the observed changes in their physical properties at different Co ratio. As shown in Fig. 6, with an increase in Co ratio a smooth crystal phase transition occurred from ZnO’s hexagonal wurtzite structure to Co3 O4 ’s cubic closely packed spinel, via an intermediate amorphous and mixed phase. The shape and size of the ZCO particles varied with Co ratio; for low (1 at.% to 15 at.%) Co ratio the structures were cylindrical/fractal with features identical to rice grains, and for higher (75 at.%, 85 at.% and 95 at.%) Co ratio the structures were hexagonal, rectangular and 2D flake shaped. The change in morphology of the ZCO particles with respect to the Co ratio could be due to the crystal phase changes. The absorption peaks blue shifted with an increase in cobalt ratio. This could be due to an increase in excitation energy and band gap via a quantum confinement effect due to the change in shape/size or defects in the stoichiometry of the nanostructures. The ZCO with 1 at.% to 15 at.% Co showed an increase in magnetization with increased Co ratio. However the ZCO with 75 at.% Co showed a weaker magnetization that may have originated from its Co3 O4 phase. The magnetization dropped further on ZCO with 85 at.% Co and became anti-ferromagnetic at 95 at.% Co substitution. The mechanism behind the observed magnetism could have originated from super/double exchange or oxygen vacancies. The observed crystal phase transition, change in shape and size, shifts in optical band gap and change in magnetization, suggests the possibility of achieving control over these physical properties via manipulation of Co ratio for various applications. References

Fig. 5. VSM loops of ZCO with 75 at.%, 85 at.% and 95 at.% Co.

the particles grain boundary. The spin state of Co2+ is influenced by valence state and coordination number, which is related to oxygen vacancy [19]. The observed reduction in magnetization and

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