Synthesis of Cobalt Oxide Nanostructures by Microwave Assisted Solvothermal Technique Using Binary S

Physical Chemistry Communications, Volume 2 Issue 1, May 2015 www.bacpl.org/j/pcc/

Synthesis of Cobalt Oxide Nanostructures by Microwave Assisted Solvothermal Technique Using Binary Solvent System Manish Shinde*1, Nilam Qureshi2, Sunit Rane2, Uttam Mulik2, Dinesh Amalnerkar3* Centre for Materials for Electronics Technology (C‐MET), Panchwati, Off Pashan Road, Pune – 411008, India *

amalnerkar@ment.gov.in

Abstract Synthesis of cobalt oxide nanostructures using a facile route still remains a challenge. Herein, we propose a simple, economical and ‘green’ route for the synthesis of cobalt oxide nanostructures by using a two step microwave assisted solvothermal route.

Binary solvent system of water and ethylene glycol (EG) was used as a medium for solvothermal heating in a household microwave oven. We studied the effect of change of microwave power on the morphological properties of the resultant nanostructures. The physico‐chemical investigations reveal formation of nanostructures having faceted growth with different shapes. These nanostructures might prove useful for catalytic and gas sensor applications.

Keywords Cobalt Oxide; Binary Solvents; Solvothermal Synthesis; Nanoparticles; FESEM

I. Introduction Nanostructures of transition metal oxides, especially magnetic metal oxide are very important class of materials owing to their various properties like catalytic, superpamagnetic, gas sensing etc [1‐3]. Cobalt oxide is one of the most versatile materials among the transition metal oxides. Cobalt oxide exists in five different forms such as CoO2, Co2O3, CoO(OH), Co3O4 and CoO [4]. However, cobalt oxide with a valency more than three is unstable in the natural environment which restricts its usage. Other cobalt oxides viz. Co3O4 and CoO are more stable and useful in industry. Cobalt oxyhydroxide, CoO(OH), has a hexagonal structure and can be used as the conductive network in rechargeable alkaline batteries [5]. CoO is an antiferromagnetic material whose magnetic characteristics [6] and application as gas‐sensors [7] have been extensively studied. Co3O4 adopts the normal spinel structure, with Co2+ ions in tetrahedral interstices and Co3+ ions in the octahedral interstices of the cubic close‐packed lattice of oxide anions and exhibits high catalytic activity in CO oxidation [8]. It is a p‐type semiconductor and antiferromagnetic material with an energy band‐gap of 1.4 – 1.8 eV [4]. It is stable up to 800 °C and decomposes to CoO above 900 °C. Spinel Co3O4 in the nanosized regime is expected to lead to even more attractive applications in the conjunction of their traditional arena and nanotechnology. It has a gas sensing behavior and solar energy reflecting properties [ 9].

Due to its function in the reduction of SO2 with CO [10], ammonia oxidation [11] and the reduction of NO with methane [12], cobalt oxide can also be used as an effective catalyst in environmental protection and chemical engineering processes. Furthermore, it is also a traditional precursor of an anode material in Li‐ion rechargeable battery [13].

Therefore, it would be highly desirable to prepare nanocrystals of spinel cobalt oxide with well defined morphologies and a narrow range of size distribution. So far, there have been many efforts to synthesize various nanostructures of Co3O4. An efficient microwave‐assisted reflux route has been used to synthesize layered nanostructures of cobalt hydrotalcite like compound spheres. These cobalt hydrotalcite like compounds can be transformed into the spinel structure of Co3O4, which keeps a uniform spherical shape [14]. Co3O4 nanotubes and nanorods have been prepared by using solvothermal and hydrothermal techniques [15‐17]. By properly controlling the thermal decomposition thermodynamics and kinetics of a single molecular precursor viz. cobalt acetylacetonate, cobalt oxide nanocrystals have been prepared [18]. A two step polyol process was developed to synthesize Co3O4 with controllable superstructures [19, 20]. In recent years, preparations of single‐phase Co3O4 at

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temperatures ranging from 150 to 260 °C have been made via spray pyrolysis, chemical vapor deposition and sol‐ gel routes [21‐23]. We, herein, propose a simple, fast and two step microwave‐assisted semi‐solvothermal process to synthesize nanostructures of Co3O4. The intention of the present study was to study the effect of variation of microwave power on the morphological properties of the resultant nanostructures.

II. Experimental Work All the reagents were of analytical grade and were used as received without any further purification. The synthesis was carried out in a simple household microwave oven (Godrej GEM‐30E, maximum output power of 900 W).

Cobalt acetate (0.05 M) was used as the precursor of cobalt ions for all the experiments. Ethylene glycol and DI water (DIW) in the ratio of 1:19 were used as the solvent mixture. In a typical procedure, cobalt acetate and sodium carbonate were mixed (1:2 molar ratio) in the solvent mixture under stirring for 5 minutes. The resultant admixture was then transferred to the Teflon container and subjected to the microwave irradiation for different durations and different microwave powers. For the first reaction, microwave power was initially maintained at 270 W for first 8 minutes. For second and third reaction, initially the microwave power was maintained at 270 W for 6 minutes and then increased to 450 W and 900 W, for 2 min duration, respectively, to obtain pinkish black coloured solution mass.

The resultant solution mass was subjected to repeated washing with water and ethanol to remove the un‐reacted salts by using centrifugation. Finally, a jelly‐like semi‐solid mass was obtained. The formation of the jelly‐like mass may be attributed to the formation of a complex between EG and the cobalt oxide seeds. This mass was calcinated at 400 °C for 2 hrs to obtain black coloured powder. The samples prepared using this technique are labeled as Co3O4_A, Co3O4_B and Co3O4_C, respectively. The details of the experimental conditions as a function of microwave power are summarized in TABLE 1.

TABLE 1 SUMMARY OF EXPERIMENTAL CONDITIONS AS A FUNCTION OF MICROWAVE POWER

Calcination Sample Code

Solvents Used

Co3O4_A

Co3O4_B

Co3O4_C

Precursor Salt

Micro‐wave Power [W]

Time [min]

EG + DIW (1:19)

CoAc + Na2CO3

270

8

EG + DIW (1:19)

CoAc + Na2CO3

270

6

450

2

EG + DIW (1:19)

CoAc + Na2CO3

270

6

900

2

Temp [0C]

Time [hr]

400

2

400

2

400

2

The phase identification of the resultant powder products was conducted using X‐ray diffractometer (Brucker D5005), with CuKα radiation. Raman spectra of the samples were recorded on a homemade setup equipped with an Ar+ ion laser (Spectra‐Physics Stabilite 2017) as an excitation beam source, a spectrometer (Horiba Jobin Yvon TRIAX 550), and a CCD detector (Horiba Jobin Yvon Symphony) cooled at ‐196 C. The wavelength of the excitation beam was 514.5 nm. Infra red (IR) spectroscopy was performed by using a Nicolate Avatar 330 FTIR instrument. UV‐vis spectra of the samples were recorded on a Varian Cary 5000 UV‐vis‐NIR spectrophotometer equipped with an integrating sphere. Barium sulfate was used as the reference. The diffuse reflectance spectra were converted into the Kubelka‐Munk (K‐M) formalism. Field emission scanning electron microscopic (FESEM) examination was conducted out using HITACHI S‐4800 at an accelerating voltage of 5 kV. A field emission JEOL JEM 2100F transmission electron microscope (TEM), operating at 200 kV was utilized to determine the size and morphology of the nanophosphors. The samples for TEM measurement were prepared by dispersing a small amount of each sample in ethanol by ultrasonication; spreading a drop of the colloid over carbon coated copper grids, and drying under UV lamp. Diffuse‐reflectance spectra of the samples in powder form has been obtained using Shimadzu UV‐3600 UV‐VIS‐NIR Spectrophotometer.

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III. Results and Discussion 1. X‐Ray Diffraction The X‐ray diffractograms for calcinated samples, Co3O4_A, Co3O4_B and Co3O4_C, respectively, are shown in FIG. 1.

It is evident from these diffractograms that after calcination the resultant samples show crystalline nature.

However, the peak broadening in cases of all the samples is a clear indication of formation of nanocrystalline products. The XRDs match with JCPDS card No. 76‐1802, each peak corresponding to cubic Co3O4 having a spinel structure. The peaks corresponding to planes (311), (440), (511), (200), (400), (422), (533), (111) can be clearly identified. The sharpness of the peaks is found to change in the order ‐ Co3O4_A < Co3O4_B < Co3O4_C. In addition, minor peaks like (111) and (422) begin to appear more prominently from sample Co3O4_A to Co3O4_C. Since the calcination temperature is kept constant, apparent increase in the sharpness of the XRD peaks for the samples Co3O4_A, Co3O4_B and Co3O4_C is rather surprising. In these samples, increasing amount of microwave power is used. Hence, it is speculated that the supersaturation and subsequent nucleation conditions could have been different in all the three cases which may lead to formation of clusters having different critical sizes. Resultant growth of these clusters upon calcination treatment might have resulted into the nanostructures having increased particle size and crystallinity, ultimately leading to increasingly sharper peaks. All the samples show preferred orientation in the direction of (311) which is also the highest intensity plane in case of bulk Co3O4.

800

(311)

Intensity (A. U.)

Co3O4_A 600

Co3O4_B

(440) (220)

(511)

(400)

(111)

Co3O4_C

(422)

(533)

400

200

0

10

20

30

40

50

60

70

80

90

2 theta (deg.)

FIG. 1 TYPICAL X‐RAY DIFFRACTOGRAMS OF Co3O4 POWDERS CORRESPONDING TO Co3O4_A (BLACK LINE), Co3O4_B (RED LINE) AND Co3O4_C (GREEN LINE)

1420 1400

A1g

Co3O4 _A

Intensity (a.u.)

1380 1360 1340 1320

Eg

1300

F 2g F2g

1280 1260 300

400

500

600

700

800

900

1000

-1

Ramam Shift (cm )

FIG. 2 REPRESENTATIVE RAMAN SPECTRUM OF Co3O4_B SAMPLE

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2. Raman Spectroscopy The representative Raman spectrum of the Co3O4_B sample is shown in FIG. 2. Different bands are observed at 482, 519, 621, 690 cm‐1. These prominent Raman peaks correspond to the Eg (482 cm‐1), F2g (519 and 621 cm‐1), A1g (690 cm‐1) signature modes of the Co3O4 crystalline phase [24, 25]. Since the peaks at 367, 599, 809 cm‐1 corresponding to CoO(OH) and at 455, 675 cm‐1 corresponding to CoO are not present in the given spectrum, it can once again be concluded that Co3O4 is the only phase present in the sample Co3O4_B. 3. FTIR Spectroscopy The FTIR spectra of the samples Co3O4_A, Co3O4_B, Co3O4_C are displayed in FIG. 3. These spectra exhibit two distinct bands (as indicated by the arrows) that originate from the stretching vibrations of the metal‐oxygen bonds [4, 26]. The first band (1) at 570cm‐1 is associated with the OB3 vibration in the spinel lattice, where B denotes Co3+ in an octahedral hole. The second band (2) at 661cm‐1 is attributed to the ABO3 vibration, where A denotes the Co2+ in a tetrahedral hole. Other peaks at 584 cm‐1 corresponding to COO(OH) and at 507 cm‐1 corresponding to CoO are not detected. The peaks at 1569 and 1425 cm‐1 typical for stretching vibrations of carboxylate group, γa(COO‐) and γs (COO‐), respectively are absent. The peaks at 2882 and 2853 cm‐1 ascribed to γa (CH2) and γs (CH2), and the strong peak around 1080 cm‐1 for C‐OH are also not detected. Absence of all these peaks in the FTIR spectra means that precursors as well as traces of solvent (i.e. EG) are absent in the samples. These results further confirm that the synthesized Co3O4 has only one phase possessing spinel structure. The peaks in the vicinity of 3500 cm‐1 are attributed to the O‐H bonding while the peaks in the region of 2300 cm‐1 are attributed to CO2.

Intensity (a. u.)

Co3O4-A Co3O4-B Co3O4-C

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm )

Absorbance (a.u.)

FIG. 3 FTIR SPECTRA OF COBALT OXIDE POWDERS CORRESPONDING TO Co3O4_A (BLACK LINE), Co3O4_B (RED LINE) AND Co3O4_C (GREEN LINE) 1 .3

799

537

1 .2

281

1 .1

1 .0

0 .9

0 .8 200

300

400

500

600

700

800

900

1000

W a ve le n g th (nm ) FIG. 4 REPRESENTATIVE UV‐VISIBLE SPECTRUM OF Co3O4_A SAMPLE

4

1100

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4. UV‐visible Spectroscopy The representative UV‐visible (DRS) spectrum of Co3O4_A sample is displayed in FIG. 4. Three absorption peaks at 281, 537, and 799 nm are observed which provide three Eg values of 2.27, 1.25, and 1.00 eV, respectively. As has been investigated in the literatures [27], the band gap of 2.23 eV can be associated with the OII− → CoII charge transfer process (valence to conduction band excitation) while the 1.25 eV bandgap relates to the OII− → CoIII charge transfer (with the CoIII level located below the conduction band). However, the origin of the 1.00 eV band gap is unclear. The multiple band gap energies for the Co3O4 particles may also suggest the possibility of degeneracy of the valence band.

FIG. 5 FESEM IMAGES OF Co3O4_A NANOPOWDER AT (a) LOW MAGNIFICATION & (b) HIGH MAGNIFICATION AND (c) HRTEM IMAGES OF Co3O4_A NANOSTRUCTURES (INSET: SAED PATTERN) AND (D) CORRESPONDING LATTICE IMAGE

5. FESEM and TEM FESEM images of Co3O4_A sample are displayed in FIG. 5 (a and b). It may be noted that all the particulate powders have been directly placed on the conducting carbon tape without putting them in an appropriate solvent and subsequent ultrasonication treatment. Hence, the particles appear to be agglomerated. At lower magnification, the image shows the small porus bunches of the particles. Size of each bunch is around 500 nm (FIG. 5a). At higher magnification, each bunch shows particles attached to each other to form such bunches. They appear to have particles having cubical, rectangular and other shapes with sharp corners which imply that the particles exhibit faceted growth. Size of these particles varies from 50 nm to 150 nm (FIG. 5b).

The question whether these bunches like structures are just agglomerates or are made up of complex hierarchical structure has been answered by carrying out the TEM imaging of this particular powder sample as the representative case. Small amount of powder sample was placed in ethanol and then subjected to ultrasonication (20 min) for obtaining well dispersed solution. Subsequently, a drop of the resultant solution mixture was put on the carbon coated copper grid for TEM analysis. The morphological features obtained from TEM imaging are shown in FIG. 5c. It shows the formation of nanostructures having different regular and irregular shapes like

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spheres, triangles, rods, rectangles, cubes etc. Sizes of these different shapes and structures vary from 20 nm to 200 nm. The noticeable feature of TEM examination is occurrence of rectangular and cubic particles with one truncated corner (indicated by the red coloured circles) which gives an appearance of ‘sim‐card’ of the mobile. The inset of the FIG. 5c shows the SAED pattern. SAED pattern displays clear ring like structure. These clear rings suggest the highly crystalline nature of the sample. One very prominent aspect of TEM image is that no hierarchical growth is observed. The lattice image indicates the single cryatalline nature of the sample (FIG. 5d). The lattice spacing value of 2.43 nm matches well with the reported JCPDS value corresponding to the (311) plane.

FIG. 6 FESEM IMAGES OF ‐ Co3O4_B NANOPOWDER AT (a) LOW MAGNIFICATION & (b) HIGH MAGNIFICATION AND Co3O4_C POWDER AT (A) LOW MAGNIFICATION & (B) HIGH MAGNIFICATION

FESEM images of Co3O4_B sample are displayed in Fig. 6 a,b. At low magnification (FIG. 6a), it is observed that the sizes of bunches is increased. (The sizes of bunches vary from 600 nm to 1.5 μm). The particle size is also found to be increased. At higher magnification, these bunches show particles having elongated shapes (FIG. 6b). Most of the particles exhibit faceted growth and rod like structure. However, few rectangular structures are also observed.

The irregular shaped bunches reveal formation of elongated rod like structures having thickness of 20 nm with length varying from 50 nm to 250 nm. Other shapes like cubes and rectangles having the nanoscale sizes are also observed. The origin and nature of these bunches is unclear. It is still perplexing to understand that whether they are formed as complex hierarchical nanoscale structures or simply are attached to each other by weak physical bonding. Our studies pertaining to understand these structures using TEM imaging are underway.

FESEM images of Co3O4_C sample are presented in FIG. 6c,d. At lower magnification, the FESEM image of the bunches shows that the porosity as well as the sizes of the bunches have been reduced and compactness has been increased (FIG. 6c). However, the size of these bunches is intermediate between that of the samples Co3O4_A and Co3O4_B. It varies from 500 nm to 1μm. The higher magnification FESEM image exhibits particles having predominently rectangular shapes. The particles appear to grow thicker but the length is reduced. The results are intriguing and interesting. The lengths of these structures vary from 100 nm to 200 nm and thickness varies from 50 nm to 100 nm (FIG. 6d). The morphological features give a feeling that the sintering of earlier structures might have taken place in such a way that thickness of these structures is increased at the expense of the length.

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It may be speculated that during the reaction, initially cobalt acetate forms cobalt hydroxyl acetate in presence of water and ethylene glycol. Addition of sodium carbonate in the above solution leads to the formation of cobalt hydroxycarbonate. Sodium acetate is a byproduct of the reaction. The resultant solution admixture, when subjected to the heat treatment, culminates into cobalt hydroxycarbonate ‐ ethylene glycol complex which precipitates in the form of jelly‐like mass. This complex breaks down to form cobalt oxide upon calcinatetion at 400 °C. The schematic showing the above processes is depicted in FIG. 7. From all the FESEM and TEM images, it is observed that variation in solvents, oxidizing agents, microwave power as well as duration of application of microwave power can drastically alter the morphological features of the resultant micro/nano scale structures. When ethylene glycol is used as solvent along with water at 1:19 ratio, the variation in microwave power plays a very vital role in determining the morphology of the final product after calcination process (at fixed temperature of 400 °C for 2hr).

For obtaining the smaller sized particles, during nucleation, the temperature should be low. However, as the microwave power is increased, the effective absorption of the microwave radiation of the solvent might increase thereby increasing the temperature of the system. This rapid increase in temperature might facilitate rapid nucleation as well as growth increasing the critical size of the clusters. Since these clusters have not grown significantly to achieve the crystalline nature, they do not exhibit crystallinity in the XRD before calcination (as observed for all the samples). The subsequent growth of these clusters during calcination may be rapid in case of bigger clusters thereby increasing their particle sizes and crystallinity even further. This might be plausible reason for the growth of particle size as well as the increase increasing in crystallinity in the case of samples where increasingly higher amount of microwave power is used during synthesis .

Precipitate

FIG. 7 SCHEMATIC SHOWING THE FORMATION OF Co3O4 NANOSTRUCTURES VIA DECOMPOSITION OF COBALT‐HYDROXY CARBONATE ‐ ETHYLENE GLYCOL COMPLEX

IV. Conclusions In nutshell, various nano structures of cobalt oxide have been obtained via microwave assisted semi‐solvothermal techniques and by using binary solvent system comprising of EG and water. It is observed that the variation in microwave power affects the supersaturation conditions (and hence nucleation and growth) which play a very important role in determining the morphological features of the resultant powder products. These cobalt oxide nanostructures may be useful in gas sensing and in catalytic converters of vehicles pertaining catalysis applications

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due to the reduction of carbon monoxide. Since the main goal of the present work was confined only to prove the concept of synthesis of nanostructures of cobalt oxide, study related to the magnetic characterization of the synthesized products was deliberately omitted.

ACKNOWLEDGMENT

Generous funding from DeitY, Govt. of India is gratefully acknowledged.

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