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Influence of Synthesis Temperature on the Growth and Surface Morphology of Co3O4 Nanocubes for Supercapacitor Applications Rashmirekha Samal 1,2 , Barsha Dash 1,2, *, Chinmaya Kumar Sarangi 2 , Kali Sanjay 1,2 Tondepu Subbaiah 3 , Gamini Senanayake 4 and Manickam Minakshi 4, * 1

2 3 4

*

ID

,

Academy of Scientific and Innovative Research (AcSIR), CSIR-Institute of Minerals and Materials Technology (CSIR-IMMT) Campus, Bhubaneswar 751013, India; [email protected] (R.S.); [email protected] (K.S.) CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India; [email protected] Office of the R&D, K. L. University, Vaddeswaram 522502, Guntur, India; [email protected] School of Engineering and Information Technology, Murdoch University, Murdoch, WA 6150, Australia; [email protected] Correspondence: [email protected] (B.D.); [email protected] (M.M.); Tel.: +61-8-93602017 (M.M.)

Received: 15 September 2017; Accepted: 24 October 2017; Published: 31 October 2017

Abstract: A facile hydrothermal route to control the crystal growth on the synthesis of Co3 O4 nanostructures with cube-like morphologies has been reported and tested its suitability for supercapacitor applications. The chemical composition and morphologies of the as-prepared Co3 O4 nanoparticles were extensively characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Varying the temperature caused considerable changes in the morphology, the electrochemical performance increased with rising temperature, and the redox reactions become more reversible. The results showed that the Co3 O4 synthesized at a higher temperature (180 ◦ C) demonstrated a high specific capacitance of 833 F/g. This is attributed to the optimal temperature and the controlled growth of nanocubes. Keywords: Co3 O4 nanostructures; morphology; temperature; synthesis; capacitors

1. Introduction Studies on the effects of morphology and crystal growth of the material synthesized on the microand nanoscale have been researched extensively since the discovery of carbon nanotube (CNT) [1]. It is well known that the shape and growth structure of the nanostructured materials strongly influence the properties, which are key factors to their performance in electrical applications [2–9]. Therefore, the application of one-dimensional structures, such as nanowires, nano-belts, and nano-tubes of different nano-materials are increasing due to the demand in technology owing to their excellent characteristics [3–9]. In the recent years, among the transition metal oxides, cubic Co3 O4 , is in much demand because of its broad practical applications in several important technological fields, such as solid-state sensors [10], electro chromic devices [11], heterogeneous catalysts [12], solar energy absorber [13], energy storage devices [14], and pigments [15]. The synthesis of spinel Co3 O4 has been the primary objective for material chemists [16,17]. In these reported articles, a range of synthesis routes to produce spinel Co3 O4 have been proposed, such as namely, the thermal decomposition of a solid cobalt nitrate (380 ◦ C) [18], chemical spray pyrolysis (350–400 ◦ C) [19,20], chemical vapour deposition (CVD, 550 ◦ C) [21], and the traditional sol-gel method (above 260 ◦ C) [22]. It is also reported that porous nanotubes of Co3 O4 can be synthesized by microemulsion method [23], and Co3 O4

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nanorods can be achieved by tweaking traditional molten salt synthesis [24] and solvothermal method [25]. Moreover, monodispersed Co3 O4 cubic nanocrystals are reported to be obtained through the salt-solvent process [26]. On the other hand, nanostructured Co3 O4 spherical particles that are formed by the aggregation of much finer particles in homogeneous aqueous solutions had also been reported [27]. Cubic Co3 O4 nanoparticles were reported to be successfully synthesized by a hydrothermal [28] as well as conventional method [29]. The mechanism of the cobalt nanocubes formation is proposed to be “surface wrapping” mechanism. Notably, all of the above said literature are limited to the synthetic technique but the research into the influence of temperature on the growth and surface morphology of Co3 O4 crystallites and its redox reactions are not widely researched. Interestingly, the temperature is one of the primary factor controlling the size and shape of the particles that influence the material characteristics for electrochemical applications. The electrochemical performance of Co3 O4 shown in the literature are varied and influenced by several factors, including particle size, surface morphology, and the ability of particles to adhere to conductive substrates. Recently, Co3 O4 self-supported nano-crystalline arrays that grown directly on conductive substrates exhibited better electrochemical performance [30–32]. These studies intrigued us to study the influence of synthesis temperature in a hydrothermal route on the surface morphology of Co3 O4 crystallites and to examine the corresponding redox reactions. Herein, interestingly, we have successfully synthesised Co3 O4 crystallites at different temperatures between 100 ◦ C and 180 ◦ C through hydrothermal route. The structural determination and the morphologies of the Co3 O4 crystallites were analysed through X-ray diffraction and electron microscopies. The significance and novelty of the current work lie in the effect of synthetic temperature on the crystal growth of the cobalt oxide nanostructure. The structural evolution of the crystal at different temperatures gives the purity of the compound with improved energy storage characteristics. The new insights into the synthesis temperature and its influence in redox reactions were observed through cyclic voltammetry experiments. The preliminary studies would bring the importance of temperature and its redox behaviour, which could be useful for supercapacitor applications. 2. Materials and Methods 2.1. Materials Cobalt(II) nitrate monohydrate (Co(NO3 )2 ·H2 O), urea ((H2 N)2 CO), absolute ethanol, acetone, and nafion, were of analytical reagent grade and obtained from SD Fine Chemicals Ltd, Mumbai, India. They were used without further purification. Double-distilled water was used for solution preparation. 2.2. Synthesis Procedure In a typical synthesis procedure, Co(NO3 )2 ·H2 O (0.291 g) and urea (0.3 g) was dissolved in 50 mL of deionised water under stirring for 30 min at room temperature. The resultant solution was then transferred to a 60 mL Teflon lined stainless steel autoclave. The synthesis temperature was varied from 100–180 ◦ C for 72 h. The resulted precipitate obtained at different temperature were centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final powders, and finally dried under vacuum at 60 ◦ C overnight. 2.3. Characterizations The physical characterization of the synthesized samples was carried out in different analytical instruments such as X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Infra-red spectroscopy (FT-IR), and Raman spectroscopy. The XRD patterns were generated in X’Pert PRO PAN analytical diffractometer (Model: PAN ANALYTICAL PW 1830 (Philips, Almelo, The Netherlands) using Cu-Kα radiation in the 2θ range of 10◦ to 80◦ . Transmission Electron Microscope (Model: FEI, TECNAI G2 20, TWIN, (FEI (Philips), Alemlo, The Netherlands) coupled with elemental anlysis chamber operated at 200 kV was used to study the internal features of the

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coated Cu grids after dispersing in acetone medium. The Field Emission Scanning Electron Microscope werea made in Zeiss, SupraTM scanning electron synthesised (FESEM) samples. measurements For TEM analysis, very dilute suspension of 55 themodel synthesised sample was microscope (Carl ZeissCu B. V. Sliedrecht, The Netherlands). prepare the samples,Scanning purified put on carbon coated grids after dispersing in acetoneTo medium. TheFESEM Field Emission nano powders were first dispersed in ethanolwere andmade then diluted, a dropletelectron of the Electron Microscope (FESEM) measurements in Zeiss, followed SupraTMby 55 placing model scanning solution onto aluminium The grid was then dried To in prepare desiccators for onesamples, day before imaging. microscope (Carl Zeiss B. V.foil. Sliedrecht, The Netherlands). the FESEM purified nano Infrared ofdispersed the samples formed KBrdiluted, platelets were recorded JASCO FT-IR 420 powders spectra were first in ethanol andinthen followed by placingwith a droplet of the solution spectrometer (JASCO, Tokyo, Japan). The Raman spectra were recorded using RENISHAW Raman onto aluminium foil. The grid was then dried in desiccators for one day before imaging. Infrared Spectrometer London, wherewere an argon ion laser wasFT-IR used420 as the excitation spectra of the(RENISHAW, samples formed in KBrUK) platelets recorded withbeam JASCO spectrometer source at 540 nm. (JASCO, Tokyo, Japan). The Raman spectra were recorded using RENISHAW Raman Spectrometer Electrochemical cyclic (CV) was performed using a CHI (RENISHAW, London,measurements UK) where an such argonasion laser voltammetry beam was used as the excitation source at 540 nm. 680C Electrochemical workstation (CHmeasurements Instrument, Austin, USA) with a typical three-electrode cell equipped such asTX, cyclic voltammetry (CV) was performed using a CHI with 680C an N2 gas flow system. The Co 3O4 samples synthesized at different temperatures were deposited workstation (CH Instrument, Austin, TX, USA) with a typical three-electrode cell equipped with an on N2 agas glassy as a working electrode, an Ag/AgCl electrode served as a reference flow carbon system.surface The Coserved O samples synthesized at different temperatures were deposited on a glassy 3 4 electrode and a platinum as a counter electrode. The electrolyte was 1 M KOH aqueous solution. carbon surface served as wire a working electrode, an Ag/AgCl electrode served as a reference electrode The active mass of the working cobalt oxide electrode was approximately 5 mg. and a platinum wire as a counter electrode. The electrolyte was 1 M KOH aqueous solution. The active mass of the working cobalt oxide electrode was approximately 5 mg. 3. Results and Discussion 3. Results and Discussion 3.1. Physical Characterization of Co3O4 Crystallites 3.1. Physical Characterization of Co3 O4 Crystallites 3.1.1. XRD Analysis 3.1.1. XRD Analysis The XRD patterns of the Co3O4 samples synthesized at various temperatures are shown in Figure XRD patterns of the Co3 O4 1a,d samples synthesized various temperatures areβ-Co(OH) shown in 1. AllThe of the X-ray reflections in Figure can be indexed to at mixed phases of hexagonal 2 Figure 1. All of the X-ray reflections in Figure 1 can be indexed to mixed phases of hexagonal (Wyckoff positions of space group: P-3m1 (164); 1a and cubic β-Co3O4 (Wyckoff positions of space β-Co(OH) positions space group: P-3m1 (164); and cubic β-Co 2 (Wyckoff 3 O4 (Wyckoff group: Fd-3m (227); 16c, whichofare in good agreement with1athe available database (JCPDSpositions No. 01of space group: Fd-3mNo. (227); 16c, which are in good agreement with the available database (JCPDS 074-1057) and (JCPDS 03-065-3103). No. 01-074-1057) and (JCPDS No. 03-065-3103). 180oC

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Figure 1.X-ray diffraction (XRD) patterns of the Co3O4 synthesized at a different temperature. Figure 1. X-ray diffraction (XRD) patterns of the Co3 O4 synthesized at a different temperature.

No diffraction peaks corresponding to α-phase compounds are detected in the XRD pattern. This No diffraction peaks corresponding α-phase are detected in the pattern. This indicates that the obtained product is of to high pure compounds β phase material that could beXRD achieved at low indicates that the obtained product is of high pure β phase material that could be achieved at low synthesis temperatures. The absence of X-ray diffraction peaks corresponding to other second phases, synthesis temperatures. Theconfirms absence of X-ray peaks to 3other second phases, such as CoO and CoOOH, that the diffraction final product is acorresponding mixture of β-Co O4 and β-Co(OH) 2. such as CoO and CoOOH, confirms that the final product is a mixture of β-Co O and β-Co(OH) 3 4 The sample synthesized at a relatively higher temperature exhibits peaks corresponding to β-Co3O42 . The sample intensity synthesized a relatively higher exhibits peaks corresponding to β-Co 3 O4 at at a higher (in at Figure 1), while the temperature cobalt hydroxide phase peaks are converted to cobalt


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a higher intensity (in Figure 1), while the cobalt hydroxide phase peaks are converted to cobalt oxide oxide phase. This shows that the optimal temperature for the synthesis of β-Co3O4 is around 180 °C. phase. This shows that the optimal temperature for the synthesis of β-Co3 O4 is around 180 ◦ C. To gain To gain more insight into the crystal structure, a range of spectroscopy and microscopic analyses more insight into the crystal structure, a range of spectroscopy and microscopic analyses have been have been performed and discussed in the subsequent sub-sections. performed and discussed in the subsequent sub-sections.

3.1.2. Infra-Red and Raman Analysis 3.1.2. Infra-Red and Raman Analysis The IR spectrum of the as-prepared Co3O4nanocrystals at different temperatures is shown in The IR spectrum of the as-prepared Co3 O4 nanocrystals at different temperatures is shown in Figure 2. To get more insights in the mid IR regions, magnified spectra is also presented in Figure 2b. Figure 2. To get more insights in the mid IR regions, magnified spectra is also presented in Figure 2b. The absorption peaks at 674 and 591 cm−1 are assigned to the ν (Co–O) modes [33], which indicates The absorption peaks at 674 and 591 cm−1 are assigned to the ν (Co–O) modes [33], which indicates the formation of cobalt oxide nanocrystals. It is observed that these two peaks started to evolve with the formation of cobalt oxide nanocrystals. It is observed that these two peaks started to evolve with an increase in synthesis temperature, indicating the formation of cobalt oxide is becoming significant an increase in synthesis temperature, indicating the formation of cobalt oxide is becoming significant at higher temperatures. The broad band centred at 3566 cm−−11 , 3380 cm−−11 and the peak at 1632 cm−−11 at higher temperatures. The broad band centred at 3566 cm , 3380 cm and the peak at 1632 cm correspond to the stretching and bending modes of the hydroxyls, respectively [34]. correspond to the stretching and bending modes of the hydroxyls, respectively [34].

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(b) 4 synthesized at different temperatures showing (a) near(a) (1000– Figure spectra of the Co3O Figure2.Infra-red 2. Infra-red spectra of the Co at different temperatures showing near 3 O4 synthesized −1 −1 −1 − 1 − 1 − 1 IR (400–1000 cm ) and ((400–1000 cm ). cm ). 4000 cm ) and (1000–4000 cm mid ) and mid IR (400–1000 cm(b) )mid andIR (b)region mid IR region ((400–1000

The peaks at about 1560 and 1051 cm−1 are attributed to the carbonate groups originating from the reaction of oxide with air forming CO2 during the analysis procedure. The peaks at 1351 and 865


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The peaks at about 1560 and 1051 cm−1 are attributed to the carbonate groups originating from the−1reaction of oxide air ν2 forming CO2 during analysis procedure. The peaks at 1351 and 3− intercalated cm are related to thewith ν3 and vibrational modes the of NO in the interlayers [35]. The − 1 3 − 865 cm are related to the ν3totheγ-Co–O–H and ν2 vibrational modes of NO intercalated in the interlayers [35]. −1 corresponds band at 514 cm bond vibration in the cobalt hydroxide. −1 corresponds totheγ-Co–O–H bond vibration in the cobalt hydroxide. The band at 514 cm To get further insights on the bonding structure Raman microscopy was carried out on these To get insightsRaman on thebands bonding structure Raman microscopy was carried out on these samples. Thefurther characteristic for Co 3O4 positioned at 475 cm−1, 518 cm−1, and 682 cm−1 are 1 , 518 cm−1 , and 682 cm−1 samples. characteristic Raman bandsto forbeCosignificant at sample 475 cm−synthesized 3 O4 positioned shown in The Figure 3. These bands appear for the at 180°C. The are shownofinthe Figure 3. These bands to be significant for the 180 ◦ C. intensity characteristic bandsappear progressively increased fromsample lowersynthesized to higher at synthetic The intensitySimultaneously, of the characteristic bandsatprogressively lower to higher synthetic temperature. the bands 229 cm−1 andincreased 1068 cm−1from are the characteristic bands of −1 and 1068 cm−1 are the characteristic bands of temperature. Simultaneously, the bands at 229 cm cobalt hydroxide Co(OH)2. These bands are prominent in the sample synthesized at 100°C where the cobaltphase hydroxide are prominent in the sample synthesized 100 ◦ C2 decreases where the 2 . These bands major is theCo(OH) cobalt hydroxide. The intensity of these bands corresponding toat Co(OH) major phase thesynthesis cobalt hydroxide. Theincreased intensity of these to Co(OH) decreases slowly whenisthe temperature from 100bands °C to corresponding 180 °C. From the Raman 2analysis, it ◦ C to 180 ◦ C. From the Raman analysis, slowly when the synthesis temperature increased from 100 is clear that the cobalt oxide phase gradually becomes prominent when the synthesis temperature it is clear on thatthe the cobalt oxide phase gradually becomes prominent when the temperature increases other hand cobalt hydroxide phase diminish. This confirms thesynthesis optimal temperature increases on the the Co other hand cobalt hydroxide phase diminish. This confirms the optimal temperature is 180 °C for 3O4 product to be produced. is 180 ◦ C for the Co3 O4 product to be produced. 682

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Figure 3. 3. Raman Ramanspectra spectra of of cobalt cobalt oxide oxide obtained obtained at at aa different different temperature. temperature. Figure

3.1.3. Field Field Emission Emission Scanning Scanning Electron Electron Microscopy Microscopy Analysis Analysis (FESEM) (FESEM) 3.1.3. The morphology morphology of of the the as-synthesized as-synthesizedproducts productsat atdifferent differenttemperatures temperaturesisisshown shownin inFigure Figure4.4. The The FE-SEM FE-SEM images images (Figure (Figure 4a,b) 4a,b) show show that that the the prepared prepared oxide oxide has has layered layered morphology morphology and and consists consists The of a large quantity of flake-like nanostructures having the initial growth of cubes on its surface. of a large quantity of flake-like nanostructures having the initial growth of cubes on its surface. The The particles are shown to have a uniform size, lower specific areathat thatisisassociated associatedwith with aa suitable suitable particles are shown to have a uniform size, lower specific area diameter and and thickness. thickness.The Thenano nanoflakes flakeshave havea athickness thickness less than about 10–20 and a size diameter ofof less than about 10–20 nmnm and a size of ◦C of 2–3 µm in the other two dimensions. While increasing the synthesis temperature from 100 2–3 µm in the other two dimensions. While increasing the synthesis temperature from 100 °C to 125 ◦ C, interestingly, the surface morphology changes gradually from flake to nodular growth to 125 °C, interestingly, the surface morphology changes gradually from flake to nodular growth (Figure (Figure 4c,d). The particles are agglomerated thatbe may not be for anyofdiffusion of ions, 4c,d). The particles are agglomerated that may not suitable forsuitable any diffusion ions, which is a which is a pre-requisite for electrochemical applications. Whereas, when thetemperature synthesis temperature is pre-requisite for electrochemical applications. Whereas, when the synthesis is increased increased 150 ◦ C4e,f) (Figure 4e,f) the nanostructured in a pyramidal-like to 150 °C to (Figure the nanostructured material material appears appears to be into a be pyramidal-like shape shape but it ◦ C to 180 ◦ C but it appears in uniformity. less uniformity. Upon, further increasing temperaturefrom from150 150°C appears to be to in be less Upon, further increasing thethe temperature to 180 °C nanostructures are found to be deposited on the surface surface of of (Figure 4g,h) a large quantity of cube-like nanostructures the material. The formation of cubes could be due to the change in the pH of the solution in presence the material. The formation of cubes could be due to the change in the pH of the solution in presence of urea urea along with the change in reaction temperature [36]. of

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Figure 4. 4. FE-SEM FE-SEM images images of of cobalt cobalt oxides oxidesat at(a,b) (a,b)100 100◦°C, (c,d) 125 125◦°C, (e,f) 150 150 ◦°C, and (g,h) (g,h) 180 180 ◦°C at Figure C, (c,d) C, (e,f) C, and C at low (left) and high (right) magnifications. low (left) and high (right) magnifications.

The possible second phase reactions involved in the synthesis of Co3O4 nanostructures at lower temperatures between 100 °C and 150 °C can be summarized as: Co(NO3)2 → Co2+ + NO32−

(1)


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The possible second phase reactions involved in the synthesis of Co3 O4 nanostructures at lower temperatures between 100 ◦ C and 150 ◦ C can be summarized as: Co(NO3 )2 → Co2+ + NO3 2− Nanomaterials2017, 7,

(1)

(H2 N)2 CO + H2 O → 2NH3 + CO2

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−2 (H2N)+2CO 2O → 2NH 3 + CO NH H2+OH→ NH4+ +OH 3

(2)

(3)

4+ − NH 2+ 3 + H2O−→ NH +OH

(3)

Co2++ 2OH− → Co(OH)2

(4)

Co + 2OH → Co(OH)2

(4)

It is clear from Figure 4 that the morphology of the obtained product has been progressively It is clear from Figure 4 that the morphology of the obtained product has been progressively changed during the thermal treatment in presence of urea. The obtained Co3 O4 crystalline size and its changed during the thermal treatment in presence of urea. The obtained Co3O4 crystalline size and growth inferred from the microscopic images are found to be in good conformity with the XRD results. its growth inferred from the microscopic images are found to be in good conformity with the XRD results.

3.1.4. TEM Analysis

3.1.4. TEM Analysis The morphology of the as-synthesized product and its crystal growth upon synthesis temperature was further characterized electron microscopy (TEM)growth technique. 5 shows The morphology by of the thetransmission as-synthesized product and its crystal uponFigure synthesis temperature was further characterized by4 powders the transmission electron microscopy (TEM) technique. the representative TEM images of the Co3 O that were synthesized at different temperatures. Figure 5prepared shows theatrepresentative TEM nano imagesflake of the 3O4 powders that in were synthesized at The samples 100 ◦ C exhibited likeCostructure as seen Figure 5a. A gradual different temperatures. The samples prepared at 100 °C exhibited nano flake like structure as seen in structural evolution from hydroxide to the cobalt oxide nanoparticles has been found to form at 180 ◦ C Figure 5a. A gradual structural evolution from hydroxide to the cobalt oxide nanoparticles has been (Figure 5g–j). The TEM images shows the evolution of fine Co3 O4 nanocubes, initially on the surface found to form at 180 °C (Figure 5g–j). The TEM images shows the evolution of fine Co3O4 nanocubes, of cobalt hydroxide (Figure 5a,b), while taking several other shapes ,including pyramidal (Figure 5c,d), initially on the surface of cobalt hydroxide (Figure 5a,b), while taking several other shapes ,including and transitional (Figure 5e,f), before forming cubical structure To summarise pyramidal (Figure 5c,d), and transitional (Figure 5e,f),like before forming(Figure cubical5g–j). like structure (Figure this, the structural evolution of the cobalt oxide on the surface of cobalt hydroxide β-Co(OH) [37] can be 5g–j). To summarise this, the structural evolution of the cobalt oxide on the surface of2 cobalt explained pictorially in a2 schematic diagram pictorially presentedinina Figure 6. diagram presented in Figure 6. hydroxide β-Co(OH) [37] can be explained schematic

Figure 5. Cont.

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Figure5.5.TEM TEM images of cobalt oxide nano particles synthesized at different temperatures (a,b) ◦ C; Figure oxide nano particles synthesized at different temperatures (a,b) 100 Figure 5. TEM images imagesofofcobalt cobalt oxide nano particles synthesized at different temperatures (a,b) 100 °C; (c,d) 125 °C showing pyramidal shape; (e,f) 150 °C showing transitional shape; and (g–j) ◦ ◦ ◦ (c,d)°C; 125(c,d) C showing pyramidal shape; (e,f) 150(e,f) C 150 showing transitional shape; shape; and (g–j) 100 125 °C showing pyramidal shape; °C showing transitional and180 (g–j)C 180 °C showing cubical geometry. showing cubical geometry. 180 °C showing cubical geometry.

Figure 6. Schematic representation of the process and morphology of cobalt oxide at different Figure 6. Schematic representation of the process and morphology of cobalt oxide at different temperature. Figure 6. Schematic representation of the process and morphology of cobalt oxide at different temperature. temperature.

3.2. Electrochemical ElectrochemicalCharacterization Characterization 3.2. 3.2. Electrochemical Characterization 3.2.1. Cyclic Voltammetry of Cobalt Oxide as Working Electrodefor forSupercapacitor Supercapacitor Applications Cyclic Voltammetry (CV) (CV) of Cobalt Oxide as Working Electrode Applications 3.2.1. Cyclic Voltammetry (CV) of Cobalt Oxide as Working Electrode for Supercapacitor Applications The electrolytes electrochemical analysis suchsuch as cyclic voltammetry must possess The electrolytes used usedininthethe electrochemical analysis as cyclic voltammetry must The electrolytes used in the electrochemical analysis such as cyclic voltammetry must possess an electrochemical stabilitystability with a with safe avoltage window beforebefore any decomposition occurs. The possess an electrochemical safe voltage window any decomposition occurs. an electrochemical stability with a safe voltage window before any decomposition occurs. The resistance of the oxide-working electrode depends on the of the usedused and The resistance of cobalt the cobalt oxide-working electrode depends on resistivity the resistivity of electrolyte the electrolyte resistance of the cobalt oxide-working electrode depends on the resistivity of the electrolyte used and the size of theofions the electrolyte that diffuse into and outand of the ofpores the electrode particles. and the size the from ions from the electrolyte that diffuse into outpores of the of the electrode the size of the ions from the electrolyte that diffuse into and out of the pores of the electrode particles. For instance, in the case have a higher and hence there willthere be a particles. For instance, in of, theorganic case of,electrolytes organic electrolytes have aresistance, higher resistance, and hence For instance, in the case of, organic electrolytes have a higher resistance, and hence there will be a power but drop that can usually by the gain higher voltage. The organicThe electrolytes will be adrop power butbe that can beoffset usually offset byinthe gain cell in higher cell voltage. organic power drop but that can be usually offset by the gain in higher cell voltage. The organic electrolytes also have safety concerns, they are as flammable. Whereas, this is not the for the an case aqueous electrolytes also have safetyasconcerns, they are flammable. Whereas, thiscase is not for also have safety concerns, as they are flammable. Whereas, this is not the case for an aqueous electrolyte, such as sodium hydroxide, potassium hydroxide, or sulphuric acid [38,39]. Moreover, electrolyte, such as sodium hydroxide, potassium hydroxide, or sulphuric acid [38,39]. Moreover, aqueous electrolytes are cheaper, easier to purify and have a lower resistance, but they limit the cell aqueous electrolytes are cheaper, easier to purify and have a lower resistance, but they limit the cell


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an aqueous electrolyte, such as sodium hydroxide, potassium hydroxide, or sulphuric acid [38,39]. of 12 Moreover, aqueous electrolytes are cheaper, easier to purify and have a lower resistance, but9 they limit the cell voltage to typically 1V, thereby limiting the maximum achievable power [40]. Therefore, voltage to typically 1V, thereby limiting the maximum achievable power [40]. Therefore, with these with these issues in mind, in the current work, the concentration of 1.0 M aqueous KOH electrolyte issues in mind, in the current work, the concentration of 1.0 M aqueous KOH electrolyte was prepared was prepared in freshly prepared double distilled water and used as an electrolyte for examining in freshly prepared double distilled water and used as an electrolyte for examining cobalt oxide as a cobalt oxide as a potential electrode for supercapacitor applications. potential electrode for supercapacitor applications. The influence of temperature in the synthesis of Co3 O4 crystallites and testing its suitability for The influence of temperature in the synthesis of Co3O4 crystallites and testing its suitability for energy storage applications was examined by keeping the scan rate and potential window constant. energy storage applications was examined by keeping the scan rate and potential window constant. Figure 7 exhibits the typical CV curves of cobalt oxide (Co3 O4 ) electrode at scan rate 50 mV s−1 Figure 7 exhibits the typical CV curves of cobalt oxide (Co3O4) electrode at scan rate 50 mV s−1 within within the potential range of +800 to −200 mV vs. Ag/AgCl in KOH electrolyte for the samples that the potential range of +800 to −200 mV vs. Ag/AgCl in KOH electrolyte for the samples that were were synthesized at a different temperature. All of the tested samples exhibit two oxidation curves synthesized at a different temperature. All of the tested samples exhibit two oxidation curves (A1 and (A1 and A2 ) in the anodic scan but one reduction curve (C1 ), while reversing the scan to cathodic A2) in the anodic scan but one reduction curve (C1), while reversing the scan to cathodic direction. direction. During the cathodic scan, Co3 O4 is reduced to form CoOOH. In the anodic scan, CoOOH During the cathodic scan, Co3O4 is reduced to form CoOOH. In the anodic scan, CoOOH undergoes undergoes a change in the oxidation state of the Co atom forming CoO2 (A2 ) and Co3 O4 (A1 ) governed a change in the oxidation state of the Co atom forming CoO2 (A2) and Co3O4 (A1) governed by faradaic by faradaic reactions. The voltammogram obtained for the samples synthesized at lower temperature reactions. The voltammogram obtained for the samples synthesized at lower temperature appeared appeared to have redox peaks with a lower intensity and did not show well-resolved redox peaks. to have redox peaks with a lower intensity and did not show well-resolved redox peaks. Hence, the Hence, the magnified curves for the samples synthesized at lower temperatures are also shown in magnified curves for the samples synthesized at lower temperatures are also shown in Figure 8. Figure 8. When the temperature was increased from 100 ◦ C to 180 ◦ C, the redox peaks were seen When the temperature was increased from 100 °C to 180 °C, the redox peaks were seen noticeably, in noticeably, in Figure 7, and have a well defined shape, illustrating that the material is more reversible Figure 7, and have a well defined shape, illustrating that the material is more reversible and the and the reactions are significant when the material is dominated by Co3 O4 nanoparticles. The specific reactions are significant when the material is dominated by Co3O4 nanoparticles. The specific capacitance ‘C’ was calculated [41] from the relation: capacitance ‘C’ was calculated [41] from the relation: R V2 V1 iV dv Css= (5) (5) = 2(V2 − V1) ν m 2 2− 1 Nanomaterials2017, 7,

where Cs is the the specific specific capacitance capacitance in in F/g. F/g. V1 V1and and V2 V2are are the the cut cut off off potential, potential, and and iV iV is is the R V2 instantaneous current. iV dv is the integrated area of the CV curve. (V2 − V1) is the potential instantaneous current. V1 is the integrated area of the CV (V2 − V1) is the V. m m is the mass mass of the active material material in the single electrode electrode in g, which which is the mass window in V. difference of the working electrode. ν is the potential scan rate (v/s) and the factor 2 corrects the difference of the working electrode. ν is the potential scan rate (v/s) and the factor 2 corrects the fact fact that the above integration includes both positivescan scanand andnegative negativescan. scan. The The maximum maximum that the above integration areaarea includes both thethe positive ◦ C. specific capacitance was obtained for the sample synthesized at 180 °C.

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-0.2

0.0

0.2

0.4

0.6

0.8

Potential / V vs. Ag / AgCl Figure 7. Cyclic Figure 7. Cyclic voltammetric voltammetric (CV) (CV) curves curves of of cobalt cobalt oxide oxide electrode electrode synthesized synthesized at at different different −1 ◦ ◦ ◦ ◦ − 1 temperatures (a) 100 °C, (b) 125 °C, (c) 150 °C, and (d) 180 °C at a scan rate of 50 mV s temperatures (a) 100 C, (b) 125 C, (c) 150 C, and (d) 180 C at a scan rate of 50 mV s .. CV CV peaks peaks showed prominent at at aa higher higher temperature. temperature. showed typical typical redox redox behaviour behaviour and and becoming becoming more more prominent

Nanomaterials 2017, Nanomaterials2017, 7, 7, 356

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60

Current / A

o

180 C

40

o

100 C

A1

20 A2

0 -20

-0.2

0.0

0.2

b

a

0.4

0.6

0.8

Potential / V vs. Ag / AgCl ◦ C, (b) 125 ◦ C, Figure 8. Cyclic voltammetric (CV) (CV) curves of of cobalt oxide electrode synthesized at (a) 100 at Figure 8. Cyclic voltammetric curves cobalt oxide electrode synthesized (a) 100 °C, (b) at a scan rate of 50 mV s−1 (magnified curves of Figure 7a,b). −1 125 °C, at a scan rate of 50 mV s (magnified curves of Figure 7a,b).

predicted values standard reduction reduction potentials of of thethe proposed redox couples [41] given The The predicted values ofofstandard potentials proposed redox couples [41]ingiven ◦ C. These values shifted to 0.442 and Equations (6) and (7) are 0.437 and 0.428 V, respectively, at 100 in Equations (6) and (7) are 0.437 and 0.428 V, respectively, at 100 °C. These values shifted to 0.442 0.346 V at 180 ◦ C respectively, and the following redox reactions are considered to be governing the and 0.346 V at 180 °C respectively, and the following redox reactions are considered to be governing pseudocapacitive properties of Co3 O4 .

the pseudocapacitive properties of Co3O4.

− − Co 4 4++H Co3 O 3O H2 2OO++OH OH− ↔ ↔ 3CoOOH 3CoOOH+ +e e−

CoOOH ++ OH CoOOH OH−− ↔ CoO CoO22 ++ H H22O O++ ee−−

(peaks A (peaks A11/C /C11))

(6) (6)

(peak (peakAA2 )2)

(7) (7)

electrochemical behaviourofofCo Co33OO44in inFigures Figures 77 and the quasi-reversible faradaic The The electrochemical behaviour and88exhibits exhibits the quasi-reversible faradaic 2+ 3+ − to process involves electrontransfer transfer process process ininCo involving the the ability of OH − to process thatthat involves electron Co2+//3+ redox redoxcouple couple involving ability of OH be reversibly intercalated into the reduced form of Co O for the improved charge storage (forming 3 4 be reversibly intercalated into the reduced form of Co3O4 for the improved charge storage (forming COOH; peak A /C1 ), which is attributed to pseudocapacitance. However, the peak A2 (Equation (7)) COOH; peak A1/C11), which is attributed to pseudocapacitance. However, the peak A2 (Equation (7)) is raised from the formation of cobalt oxide (CoO2 ) and adsorption of ions on the near-surface is raised from the formation of cobalt oxide (CoO2) and adsorption of ions on the near-surface corresponding to the non-faradaic process thatis not reversible during the reduction process but corresponding non-faradaicTherefore, process thatis not studies reversible during reduction process but contributes to to the the capacitance. the above indicate thatthe reduction of Co 3 O4 is contributes towhile the capacitance. Therefore, the above indicate Co− 3O4 is reversible the further reduction of CoOOH formsstudies a mixture of CoO2that and reduction adsorption of of OH reversible whileisthe further reduction of CoOOH forms a mixture of CoO 2 and adsorption of OH− ions, which irreversible. Based on this mechanism, the value of specific capacitances obtained for ◦ C is of ions,the which is irreversible. the samples synthesizedBased at 100 ◦on C, this 125 ◦mechanism, C, 150 ◦ C, and 180value 45 specific F/g, 155capacitances F/g, 506 F/g, obtained 833 F/g, for − 1 respectively, at a san rate mV°C, s 125 . Overall, the °C, enhanced performance the155 Co3 O influenced by F/g, the samples synthesized at50100 °C, 150 and 180 °C is 45 of F/g, F/g, F/g, 833 4 is 506 −1 the synthesis temperature, pH of the solution and the role of urea as a fuel in the typical hydrothermal respectively, at a san rate 50 mV s . Overall, the enhanced performance of the Co3O4 is influenced by process [42]. the synthesis temperature, pH of the solution and the role of urea as a fuel in the typical hydrothermal process [42]. 4. Conclusions A hydrothermal route for the synthesis of Co3 O4 nanocubes has been reported for supercapacitor 4. Conclusions applications. The effect of synthesis temperature on the crystal growth of Co3 O4 was studied A hydrothermal for the synthesis ofits Coelectrochemical 3O4 nanocubes has been reported for an supercapacitor systematically and route the preliminary studies of activity showed they are excellent applications. The effect of synthesis temperature on the crystal growth of Co 3 O 4 was studied material for energy storage. The Co3 O4 nanostructures exhibit cube-like morphology and have comprised and of nanocubes with the increase in electrochemical synthesis temperature and in the they presence of excellent urea systematically the preliminary studies of its activity showed are an as fuel. The studies showed that the higher the synthesis temperature, the faster the growth rate,have material for energy storage. The Co3O4 nanostructures exhibit cube-like morphology and and hence obtained with nanocubes. TEM analysis confirmed the morphology the individual O4 as comprised of the nanocubes the increase in synthesis temperature andthat in the presence Co of 3urea nanocubes consist of aggregated nanocrystals. Co O nanocubes demonstrated a high capacitance 3 4 temperature, the faster the growth rate, and fuel. The studies showed that the higher the synthesis

hence the obtained nanocubes. TEM analysis confirmed the morphology that the individual Co3O4 nanocubes consist of aggregated nanocrystals. Co3O4 nanocubes demonstrated a high capacitance value of 833 Fg−1 and the materials synthesized at an optimum temperature of 180 °C showed a high reversibility with adsorption and desorption of ions from the aqueous electrolyte.


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value of 833 Fg−1 and the materials synthesized at an optimum temperature of 180 ◦ C showed a high reversibility with adsorption and desorption of ions from the aqueous electrolyte. Acknowledgments: The authors are thankful to S.K. Mishra for providing all the facility to carry out the work. The authors are also thankful to MoES for financial support. Author Contributions: B.D. and K.S. designed the research and R.S. performed the experiments. B.D., C.K.S. and T.S. analyzed the data and participated in the discussion of results. B.D., G.S. and M.M. contributed the electrochemical part while R.S. and B.D. extended the equipment facilities and wrote the manuscript. M.M. contributed in revising the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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