Effect of Microwave on the Nanowire Morphology, Optical, Magnetic

ARTICLE pubs.acs.org/JPCC

Effect of Microwave on the Nanowire Morphology, Optical, Magnetic, and Pseudocapacitance Behavior of Co3O4 Sumanta Kumar Meher and G. Ranga Rao* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

bS Supporting Information ABSTRACT: In the context of immense control of synthesis methods on the structural and functional characteristics of the materials, nanowire morphologies of Co3O4 are synthesized in conventional reflux and microwave-assisted methods, under homogeneous precipitation conditions. The Co3O4 sample synthesized by the conventional reflux method consists of randomly distributed thin nanowires while the microwave reflux method generates higher-dimensional and arranged Co3O4 nanowires. The surface area and pore structural analysis of the Co3O4 samples show significant difference in their meso- and macroporosity as well as specific surface area, due to differently crystallized products. The UVVis-DRS study shows crystallite size dependent optical transitions and band gaps. The magnetic study illustrates finite size effect and low temperature ferromagnetism in both samples; the lower-dimensional nanowires being more ferromagnetic than the higher-dimensional Co3O4 nanowires. Due to smaller crystallite size and more accessible surface sites, the Co3O4 sample synthesized by the conventional reflux method shows better charge storage, high Coulombic efficiency, and enhanced rate response during the pseudocapacitance studies. However the Co3O4 sample synthesized using the microwave-assisted method shows better high rate cyclic stability due to its more rigid orientated nanowire structure. Further, the Ragone plot exhibits considerably higher energy and power densities of lower-dimensional Co3O4 nanowires. Broadly, this study reveals that, under nonhydrothermal homogeneous precipitation conditions, the conventional reflux synthesized lower-dimensional Co3O4 nanowires bear superior surface properties than the microwave synthesized higher-dimensional Co3O4 nanowires, for electrochemical supercapacitor applications.

1. INTRODUCTION The greater role of nanostructured materials in dealing with the challenges in new sustainable and renewable energy is receiving wide attention.1 Especially, oxide nanostructures with infinite variety of structural motifs and manifold morphological features exhibit indispensable surface properties for energy harvesting, conversion, and storage.2 The classical synthetic pathway with reliable and scalable preparative approaches to obtain functional oxide nanomaterials is still a challenge. In the recent past, fundamental aspects of synthesis, processing, and control of multiscale oxide structures have been achieved for more efficient energy conversion and storage applications.13 In this context, the role of high surface area, optimal particle dimension and architecture, controlled pore channels, and the suitable alignment of nanocrystalline phases are highly emphasized.13 These material properties are directly related to the surface activity that essentially influences the electron and ion transport efficiency for charge storage.4 Since materials of same composition with different morphologies and microstructures exhibit substantial differences in their properties and surface activities, the focus has been mainly on the control of morphologies and microstructures of electro-active materials at the mesoscopic level.5 In this context, the fabrication of well-arranged one-dimensional (1-D) r 2011 American Chemical Society

nanostructures has been reported extensively.6 As the oriented growth of such type of nanostructures is rather difficult to achieve, it is important to develop facile, mild, and easily controllable methods to synthesize novel materials with a one-dimensional pattern through self-assembly of nanoparticles. Recently, electrochemical energy storage systems such as rechargeable batteries and electrochemical capacitors (ECs) are receiving ever-increasing attention due to their feasibility in increasing the energy utilization efficiency.7 Among these devices, the potential application of redox supercapacitors having better power and energy density has been methodically explored due to their functional ability to link the performance gap between batteries and dielectric capacitors (EDLCs).8 In addition, the chargedischarge efficiency and power system life of the pseudocapacitors are higher than batteries.79 The redox supercapacitor is also termed as a pseudocapacitor, where charge is stored using redox-based Faradaic reactions and is fundamentally different from an EDLC, where charge is stored in electrical double layer due to electrostatic charge separation between Received: September 22, 2011 Revised: November 10, 2011 Published: November 13, 2011 25543

dx.doi.org/10.1021/jp209165v | J. Phys. Chem. C 2011, 115, 25543–25556

The Journal of Physical Chemistry C electrolyte ions and electrodes.10 Theoretically, EDLCs can bear a typical capacitance value of only 1040 μF cm2, whereas pseudocapacitors can have capacitance values of 10100 times that of EDLCs.710 In materials context, transition metal oxides due to their variety of oxidation states for efficient redox charge transfer are generally considered suitable candidates as electrode materials for redox supercapacitors.8,11 Until now, the most versatile transition metal oxide in pseudocapacitor application has been hydrous ruthenium oxide (RuOxHy or RuO2 3 xH2O) because of its ultrahigh theoretical capacitance of ∼2000 F g1 in a wider applied potential window of ∼1.4 V, higher room temperature electrical conductivity, and fairly good chemical stability.12 On the downside, RuO2 is toxic and, being a noble metal oxide, is far too expensive for widespread commercial applications. Therefore, the efforts are quite active at present, to develop cost-effective pseudocapacitive materials with comparable performance to that of RuO2. The nanoscopic architectures of tailor-made electroactive materials possess better surface properties and strike a balance between electronic and ionic transportation of electrons/ions.13,13 Among the most active transition metals, Co3O4, due to its high surface area, good redox and easily tunable surface, as well as structural properties, has been studied extensively for supercapacitor applications.14 Co3O4 is equally important in heterogeneous catalysis,15 solidstate sensors,16 and magnetic materials.17 Especially, one-dimensional Co3O4 nanowire is fascinating for its widespread applications due to the reduced particle size and higher surface area, thus generating larger active interfacial sites. Therefore, the synthesis of one-dimensional Co3O4 nanostructures by using porous alumina18 or virus as templates,19 thermal conversion of cobalt hydroxide nanostructures using hydrothermal route,20 electrochemical deposition,21 and many other methods22 are widely explored and studied for numerous applications. The use of porous alumina or virus as template are however quite intricate and costly. The use of the hydrothermal method is widely accepted due to easily controllable parameters; however, the control of the shape of the nanoscale particles emerging from a hydrothermal process is quite tricky, due to the possibility of formation of added secondary products.23 Therefore, the synthesis of Co3O4 nanowire using a facile and nonhydrothermal method is still a reasonable challenge. Of late, microwavemediated synthesis method, due to microwave-induced accelerated kinetics, enhanced nucleation rate, and reduction in the reaction time, has drawn large attention in the synthesis of oxide materials.24 The microwaves are nonionizing electromagnetic radiations with a higher penetration depth, which leads to crystallites/particles with uniform dimensions and higher purity due to the absence of major thermal gradient in the reaction medium.24,25 In literature, microwave-mediated synthesis has been adopted to prepare Co3O4 with regular microstructures.26 However, most of the reported synthesis methods are hydrothermal-mediated, where the effect of the hydrothermal condition confines the clear understanding of microwave effect on the morphology of Co3O4. Therefore, synthesis of Co3O4 in a nonhydrothermal microwave method is essential to understand the effect of microwave on the crystallinity, morphology, and other properties of the material. In this work, we have synthesized Co3O4 samples with nanowire morphology, employing a homogeneous precipitation method, with urea as mineralizer, in conventional reflux as well as a microwave reflux method using P123 (poly(ethylene glycol)poly(propylene glycol)-poly(ethylene glycol)) as a structure

ARTICLE

directing agent. Further, the influence of the microwave on the dimensional growth of the nanowire morphology of Co3O4 and its effect on the physicochemical, optical, magnetic, and pseudocapacitance behavior have been thoroughly established.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Co3O4 Nanowires. Analytical grade Co(NO3)2 3 6H2O, P123 (poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol)), and urea (Thomas-Baker) were used as received. Triply distilled water was used during all the experimental processes of the synthesis. In a typical experiment, 100 mL of aqueous solution of 20 mmol of Co(NO3)2 3 6H2O was added dropwise to 100 mL of 1.0 mmol of P123, with vigorous stirring, to form a homogeneous solution. Solid urea was added to the resulting solution in a salt to urea molar ratio of 1:2 and stirred for an additional 3 h for complete homogeneity. The resulting solution was then divided in to two equal halves: one-half was subjected to refluxing at a fixed temperature of 120 °C for 12 h, and the other half was treated with a microwave irradiation using 250 W of power in a CEM Discover Bench Mate microwave reactor, at a fixed temperature of 120 °C for 15 min. The heat treatments resulted in the formation of fluffy light pink color solid precipitates. After autocooling the reaction mixtures to room temperature, the products were separated by centrifugation at 3000 rpm, with repeated washing using triply distilled water, followed by a mixture of absolute ethanol and water, and finally with absolute ethanol. The materials were then dried in a vacuum oven at 60 °C overnight. Finally, the samples were subjected to calcination in flowing air at a heating ramp of 5 °C min1 from room temperature to 300 °C and kept at that temperature for 3 h to get the final product. The samples prepared under reflux and microwave treatments before calcinations were denoted as Co3O4-ref-uc and Co3O4-mw-uc, respectively, and after calcinations as Co3O4-ref and Co3O4-mw, respectively. 2.2. Fabrication of Electrode and Electrochemical Measurements. The working electrodes for evaluating the electrochemical properties were fabricated by mixing 80 wt % Co3O4 with 15 wt % acetylene black in an agate mortar. To this mixture, 5 wt % polyvinylidene difluoride (PVdF) binder dissolved in 1-methyl-2-pyrrolidinone (NMP) was added to form a slurry. The slurry was coated (area of coating: 1 cm2) on a pretreated batterygrade Ni foil (0.2 mm thick) for electrical conductivity and vacuum dried at 60 °C for 8 h. Cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) studies were performed using a CHI 7081C electrochemical workstation in a three-electrode configuration, with a Co3O4 coated Ni plate, Pt foil (1 2 cm2), and Hg/HgO (1.0 M KOH) as working, counter, and reference electrodes, respectively. The measurements were performed using deoxygenated aqueous 1.0 M KOH as the electrolyte. The stability of the materials was evaluated from 2000 galvanostatic chargedischarge test cycles. The electrochemical impedance spectra were measured by imposing a sinusoidal alternating voltage frequency of 102 to 105 Hz, alternating current (ac) amplitude of 5 mV, and a constant dc bias potential of 0.45 V.

3. RESULTS AND DISCUSSION The powder X-ray diffraction (PXRD) patterns of Co3O4-ref-uc and Co3O4-mw-uc samples shown in Figure 1A are indexed as 25544

dx.doi.org/10.1021/jp209165v |J. Phys. Chem. C 2011, 115, 25543–25556

The Journal of Physical Chemistry C

ARTICLE

Figure 1. (A) PXRD patterns and (B) TGA profiles of the Co3O4 mw-uc and Co3O4-ref-uc samples; (C) PXRD patterns of the Co3O4-mw and Co3O4ref samples.

the characteristics peaks of orthorhombic Co(CO3)0.5OH 3 nH2O phase (JCPDS: 480083). In both reflux as well as microwavemediated heating methods, decomposition and successive hydrolyzation of urea in aqueous condition at elevated temperature leads to in situ release of OH and CO32 ions. The generation of OH and CO32 ions initiates the precipitation of Co2+ ions in the solution to cobalt hydroxide carbonate species. The reactions in the medium are expressed as follows:27 9 hydrolysis > > NH2 CONH2 sf NHþ þ NCO > 4 > = þ NCO þ 3H2 O f HCO þ NH þ OH 3 4 2 þ > HCO > 3 f CO3 þ H > > ; 2Co2þ þ OH þ CO2 þ nH O f 2CoðCO Þ OH nH O 2 3 0:5 3 3 2

ð1Þ The homogeneous precipitation process induces controlled generation of OH ions and slower crystal growth of the materials due to the slower hydrolysis rate of urea, which is considerably advantageous than the conventional precipitation process. The use of surfactant P123 in the synthesis process adds the advantage of preventing agglomeration of nascent crystallites during the initial stage of product formation and offers a particular shape to the material. The difference in the broadness and intensity of the respective peaks of the Co3O4-mw-uc and Co3O4-ref-uc samples are clear signatures of their variable crystallinity. The more intense and clear PXRD peaks of the Co3O4-mw-uc sample prepared using the microwave heating method demonstrate better crystallinity, phase purity, and larger crystallite size as compared to the Co3O4ref-uc sample. The uniform heating and accelerated kinetics during the microwave-mediated reaction process leads to monophasic product;24,25 however, in the case of conventional reflux treatment, the thermal gradient in the reaction medium and longer reaction time results in the ripening of some Co(CO3)0.5OH 3 nH2O nuclei to more stable secondary phases. The thermal behaviors of the Co3O4-mw-uc and Co3O4-ref-uc samples were determined by thermogravimetric analysis (TGA) as shown in Figure 1B. The typical TGA curves of the two precursors clearly display two-step weight loss each, due to dehydration and decomposition of the Co(CO3)0.5OH 3 nH2O phase. The first weight loss at the lower temperature (40250 °C) is due to the elimination of adsorbed and intercalated water while the second weight loss from 250 to 300 °C corresponds to the loss of structural water and thermal oxidative decomposition of Co(CO3)0.5OH 3 nH2O as well as desorption of residual nitrate and carbonate anions.27 Further, no major weight loss at higher temperature in the TGA curves indicate the absence of

additional structural changes in those regions. The appreciably different weight loss in the two temperature regions therefore corresponds to the difference in the adsorbed, intercalated, and crystalline water in the samples. The total weight loss of ∼28.3% for the Co3O4mw-uc sample as compared to ∼24.2% for the Co3O4-ref-uc sample is representative of the structural and possible surface morphological difference between the two samples. This also indicates the difference in crystallinity and structural organization of the precursor samples that undergo thermal decomposition via two different mechanisms. The composition and phase of the products after thermal decomposition of the precursors at 300 °C are confirmed by the corresponding PXRD patterns, and the profiles are shown in Figure 1C. The PXRD patterns show prominent peaks at 2θ values of 19.1°, 31.4°, 36.8°, 38.6°, 44.9°, 56.3°, 59.3°, and 65.4°, which are indexed respectively as (111), (220), (311), (222), (400), (422), (511) and (440) planes of FCC type Co3O4 with space group of Fd3m(227).27 Near absence of any secondary peaks in the PXRD patterns clearly illustrates the complete decomposition of precursors to highly pure Co3O4 samples. In the PXRD patterns, the peaks of the Co3O4-mw sample are more intense than those for the Co3O4-ref sample, which shows higher crystallinity and larger crystallite size of the Co3O4-mw sample as compared to the Co3O4-ref sample. The average crystallite sizes of the Co3O4-mw and Co3O4-ref samples calculated using the Scherrer equation are found to be ∼15 and 12 nm, respectively. This is a clear signature of super heating during the microwave treatment,24,25 which leads to accelerated urea hydrolysis along with enhanced crystal growth and better crystallinity of the uncalcined cobalt-basic-carbonate precursor, and the crystallinity is retained in the calcined product. The field emission scanning electron microscopy (FESEM) images of the uncalcined cobalt-basic-carbonate precursors, Co3O4mw-uc, and Co3O4-ref-uc samples are shown in Figure S1 in the Supporting Information, and the corresponding Co3O4-mw and Co3O4-ref samples are shown in Figure 2. The Co3O4-ref-uc sample shows one-dimensional nanowire morphology without any specific arrangement, whereas the Co3O4-mw-uc sample shows interconnected/agglomerated higher-dimensional nanowire structures. The FESEM images of the uncalcined precursor samples further show that the length distribution of the nanowires spans from hundreds of nanometers to several micrometers. However, a closer microscopic analysis shows that the 1-D nanowires of the Co3O4-ref-uc sample are longer than the higher-dimensional nanowires of the Co3O4-mw-uc sample. This provides primary insights into the rapid three-dimensional crystal growth phenomena under microwave treatment. The cause of the aggregation in microwave-mediated synthesis is due to creation of “hot surface” 25545



ARTICLE

Figure 2. (A, B, C) low- and high-resolution FESEM images of the Co3O4-mw sample; (D, E, F) low- and high-resolution FESEM images of the Co3O4-ref sample.

Figure 3. HRTEM images of the Co3O4-mw sample (A, B, C) and Co3O4-ref sample (D, E, F). The nonstatistical diameters of the Co3O4-mw and Co3O4-ref nanowires are indicated in (B) and (E), respectively.

on the initially formed cobalt-basic-carbonate nanowires.24,25 These “hot surfaces” induce mass transport and end to end fusion of nanowires to form agglomerated superstructures. The nonoccurrence of such superheating in the conventional reflux method leads to freely dispersed cobalt-basic-carbonate nanowires. The surface morphologies of the precursors are clearly retained in the respective calcined products and are shown in the FESEM micrographs in Figure 2. The Co3O4 samples clearly mimic the nanowire morphology of the respective cobalt-basic-carbonate samples with the microwave synthesized sample showing three-dimensionally grown structures as compared to the 1-D structures of conventional reflux synthesized sample. The nanowires of the Co3O4-mw samples endconnect with each other and provide a highly porous three-dimensional superstructure to the material as shown in Figure 2 A and C.

For better insights into the dimensionality of the nanowire morphologies, high-resolution transmission electron microscopy (HRTEM) is used, and the respective micrographs are shown in Figure 3. HRTEM shows that the Co3O4 nanoparticles appear as dark spots that are arranged randomly to form the nanowire structures. The interconnected nanowires of the Co3O4-mw sample and freely dispersed nanowire structures of the Co3O4ref sample are clearly shown in Figure 3A and D, respectively. The white patches seen in the high-resolution TEM images in Figure 3C and F are the intercrystallite voids that provide further evidence of three-dimensional connectivity of these Co3O4 nanoparticles forming Co3O4 nanowires. The HRTEM images in Figure 3B and E clearly demonstrate the multidimensional expansion of nanowires in the microwave synthesized Co3O4 25546


The Journal of Physical Chemistry C sample (nonstatistical diameter of ∼56.6 nm) as compared to its conventional reflux synthesized counterpart (nonstatistical diameter of ∼31.1 nm). Further, peak intensity analysis from the XRD patterns indicates that the [220] and [440] planes of the Co3O4-mw sample are stronger than the Co3O4-ref sample, which gives a clear indication of the difference in crystal shape and anisotropy of the two samples, and the crystalline direction of the Co3O4 wire during calcination is mainly along the [110] and [220] direction. The formation of cobalt-hydroxide-carbonate nanowires of different dimensionality from nascent crystallites involves a number of contributing factors such as hydrophobic interactions, hydrogen bonding, van der Waals forces, crystal-face attraction, electrostatic and dipolar fields, intrinsic crystal contraction, and Ostwald ripening.28 Due to the continuously proceeding slow reactions in the homogeneous precipitation process, the initially formed nascent cobalt-hydroxide-carbonate crystallite nuclei begin to impinge on other neighboring crystals and assemble along the specific orientation.28 The extrinsic crystal growth is modulated to some extent by adsorption of surface energy modifiers such as H2O and anions on certain crystallographic planes. The P123 surfactant during synthesis plays a major role in preventing the agglomeration of primary crystallites and acts as a morphology directing agent for the formation of nanowire structures. The surfactant molecules effectively shield further OH activity on the initially formed nascent cobalt-hydroxidecarbonate crystallites and induce the growth of nanowire structures. Since the initially formed crystallites essentially determine the nanowire dimensionality, the method of heating employed in the synthesis can have immense effect on the dimensionality of the nascent cobalt-hydroxide-carbonate crystallites. In the reflux method using the conventional heat treatment approach, cobalt-hydroxide-carbonate crystallites form initially and coalesce as the reaction proceeds. However, the surfactant restricts the three-dimensional agglomeration and promotes one-dimensional arrangement of nanocrystallites. This results in the one-dimensional growth and finally the nanowire morphology. In the microwave-mediated synthesis, the bulk temperature effects are entirely different as compared to the conventional heating process.24,25 This is due to a unique microwave dielectric heating mechanism that is absent in conventional heating.24,25 Microwave radiation accelerates generation of smaller size nuclei and crystal growth process.24,25 This is accomplished due to higher heating rate and the creation of hot spots during the reaction.24,25,29 All these factors contribute to the formation of more crystalline cobalt-hydroxidecarbonate in microwave synthesis as observed in the XRD analysis (Figure 1A). Further, due to the localized heating effect, the nascent cobalt-hydroxide-carbonate crystallites undergo rapid increase in size/dimensionality simultaneously with coalescence process.23,24,28 This is followed by the one-dimensional growth of the microwave ripened nanocrystallites under the influence of P123 surfactant to form nanowires of higher-dimensionality. Further, due to the creation of localized “hot surface”29,30 on the unprotected tip of nanowires during the microwave treatment, the individual nanowires tend to undergo end-on attachment/aggregation to produce hierarchical porous superstructures as shown in the HRSEM micrographs (Figure 2AC). The pictorial representations of the formation mechanisms of the nanowire morphologies under conventional reflux and microwave-mediated synthesis are presented in Schemes 1 and 2, respectively.

ARTICLE

Scheme 1. Plausible Formation Mechanism of the Co3O4-ref Sample

The physicochemical characteristics of the samples are investigated from BET analysis, and the results are given in Figure 4. The BET isotherms of the Co3O4-mw and Co3O4-ref samples in Figure 4A show H3 type hysteresis loops that signify the presence of aggregates of platelike particles with slit shape pores.31 This is further verified from the BJH pore size distribution plots in Figure 4B which clearly show that the pores are in the micro and meso size region with multimodal and bimodal pore size distributions in the Co3O4-ref and Co3O4-mw samples, respectively. The pore size distribution maxima of the Co3O4mw sample are centered at ∼3 and 25 nm and that of the Co3O4ref sample at ∼3, 8, and 18 nm. The higher mesopore fraction in the Co3O4-mw sample can simultaneously be established from the appreciably higher adsorption of N2 at the (p/po) = 0.91.0 for Co3O4-mw as compared to the Co3O4-ref sample, in the BET isotherms.32,33 The multimodal porosity of the samples is a contribution from various factors such as interparticle porosity, internanowire voids, and vacancies left over by the relief of H2O and CO2 during the thermal decomposition of cobalt-hydroxidecarbonate nanowires.33 The presence of mesoporosity however does not affect the crystalline continuity of the Co3O4 nanowire samples harvested from the respective cobalt-hydroxide-carbonate nanowires. The BET surface area of the Co3O4-ref and Co3O4-mw samples, respectively, are 69 and 60 m2 g1 with corresponding pore volumes of 0.21 and 0.17 cm3 g1. It is to be noted from TG analysis (Figure 1B) that the Co3O4-mw-uc sample loses slightly higher amounts of H2O and CO2 during the decomposition process. This in principle should result in generating more surface area and pore volume in the Co3O4-mw sample as compared to the Co3O4-ref sample.30,33 However, the development of a higher fraction of multimodal mesopores up to 10 nm and a lower fraction of mesopores beyond 10 nm in the Co3O4-ref sample as compared to the Co3O4-mw sample (Figure 4B) is the reason for higher BET specific surface area and pore volume of Co3O4-ref sample.30,33 To investigate the effect of different dimensionality of the nanowires on their band energy states, optical absorption study has been carried out in the UVvis region. The optical absorption profiles of the Co3O4-ref and Co3O4-mw samples shown in the insets of Figure 5A and B, respectively, reveal 25547



ARTICLE

Scheme 2. Plausible Formation Mechanism of the Co3O4-mw Sample

Figure 4. (A) BET isotherms and (B) BJH pore size distribution profiles of the Co3O4-mw and Co3O4-ref samples.

characteristic absorption bands centered at ∼400 and ∼700 nm, which are in agreement with the Co3O4 band structure.34 The relationship between optical density and Eg is given as αhv = constant (hv Eg)n.34 Assuming a direct transition with n equals to 1/2, (αhv)2 is plotted against hv (Tauc plot)34 as shown in Figure 5A and B for the Co3O4-ref and Co3O4-mw samples, respectively. The absorption band gap energies of the samples are derived by extrapolating hν to α = 0, in the respective plots. The Tauc plot profile of the Co3O4-ref sample gives two optical transitions at 0.83 and 2.11 eV, whereas the Co3O4-mw sample shows transitions at 0.75 and 1.92 eV. The lower band gap transitions are attributed to O2 f Co3+ (Eg1) charge transfer and the higher band gap transitions are assigned to O2 f Co2+ (Eg2) charge transfer processes.33,34 The presence of Co(III) centers in the Co3O4 spinals create sub-bands located inside the energy gap. Hence Eg2 is the true energy gap corresponding to interband transitions, as shown in Figure S2 in the Supporting Information.35a The optical band gap (Eg = (Eg2 Eg1) of the Co3O4-ref and Co3O4-mw nanowire samples are 1.28 and

1.17 eV, respectively. There is an inverse relation between the optical band gap energy and the crystallite size; the band gap energy increases as the crystallite size is decreased and vice versa.3436 Therefore, the above results corroborate the smaller crystallite size of Co3O4-ref sample as compared to the Co3O4-mw sample, which is consistent with the XRD and TEM analysis. Further, the optical band gap values of the Co3O4-ref and Co3O4-mw nanowires are higher than the optical band gap values reported for bulk Co3O4 materials.35,36 Thus, modification in the synthesis methods adopted in this study can effectively tune the crystallites to the quantum confinement regime that stimulate the finetuning of the material properties for several applications. The multiple band gap energies observed in the Co3O4 nanowires in Figure 5 suggest that the transitions are not solely from direct band gaps but from the discrete band gap and midgap transitions from the valence band region.3436 Finite-size effects, related to deviation from bulk properties with decrease in sample dimensions, in nanoparticles and nanowires consisting of antiferromagnetic (AFM) property have 25548



ARTICLE

Figure 5. Optical band gap energy of (A) the Co3O4-ref and (B) the Co3O4-mw nanowires obtained by extrapolation to α = 0. Insets show the respective UVvis absorption spectrum of the samples.

Figure 6. MH hysteresis loops of (A) the Co3O4-ref and (B) the Co3O4-mw nanowire samples measured at 20, 160, and 300 K in zero field cooled condition. The insets show the magnified hysteresis loops of the respective samples at 20 K; HC and MR represent coercive field and remnant magnetization, respectively.

been receiving much attention recently due to their huge potential in technological applications.37 In this context, finitesize effects of bulk and nanostructured Co3O4 on its antiferromagnetic behavior has been comprehensively studied.37 Therefore, to investigate the effect of dimensionality of nanowires on their magnetic behavior, the Co3O4-ref and Co3O4-mw samples were subjected to magnetic moment (M) versus magnetic field (H) measurements (MH) at different temperatures, that is, 20, 160, and 300 K, under zero field cooled (ZFC) conditions, and the results are shown in Figure 6. The MH characteristics of the samples at 160 and 300 K show no apparent hysteresis loop, which corresponds to the typical antiferromagnetic behavior of Co3O4. However, the MH measurements of both the samples at 20 K show clear hysteresis, coercive field (HC) and remnant magnetization (MR), thus exhibiting weak ferromagnetic behavior of Co3O4 nanowires.37 It is known that bulk Co3O4 is antiferromagnetic (AF) in nature (Neel temperature, TN ≈ 40 K) with each spin contributing Co2+ ion of the spinel (AB2O4 type) in the tetrahedral A-site having four neighboring Co2+ ions of opposite spins. However, in nanoscale regime and below Neel temperature, Co3O4 exhibits ferromagnetic behavior, which is due to the development of inverse spinel structure in the Co3O4 by “charge transfer” at lower temperature and “ion exchange” at higher temperature.38 The parametric change in the crystal structure and magnetic ordering induces fractional frustration in the corresponding AFM ordering that consequences in the ferromagnetism of nano-Co3O4. At 20 K ( > > > > 3CoðOHÞ2 þ 2OH f Co3 O4 þ 4H2 O þ e > = Co3 O4 þ OH þ H2 O f 3CoOOH þ e ð2Þ > > > CoOOH þ OH f CoO2 þ H2 O þ e > > > ; 4OH f O2 þ 2H2 O þ 4e 9 Cathodic scan/reduction (towards negative potential): > > > = CoO2 þ H2 O þ e f CoOOH þ OH 3CoOOH þ e f Co3 O4 þ OH þ H2 O > > > Co3 Oþ4H2 O þ 2e f 3CoðOHÞ2 þ 2OH ; ð3Þ 25550



ARTICLE

Figure 8. Discharge curves of (A) the Co3O4-ref and (B) the Co3O4-mw samples at different current density (Id); (C) comparison of current density dependent specific capacitance values of the Co3O4-ref and Co3O4-mw samples with cycling; and (D) comparison of current density dependent specific capacitance retention of the Co3O4-ref and Co3O4-mw samples with cycling.

Equations 2 and 3 clearly show that Co3O4 nanowires undergo electrochemical redox reactions, Co(II) T Co(III) T Co(IV), in alkaline medium that makes these materials active for potential charge storage (pseudocapacitor) applications.17b,44,45 The characteristic nonrectangular shapes of the CV patterns reveal that the charge storage occurs in Co3O4 nanowires due to the reversible redox reactions (pseudocapacitance process) and not due to the charge polarization (double layer capacitance).10,29,33,46 Although three redox couples (Co(OH)2/Co3O4, Co3O4/CoOOH, and CoOOH/CoO2) are involved during CV cycling of Co3O4 in an alkaline solution, the CV patterns of different Co3O4 samples show surface morphology dependent CV characteristics.14,17b,18,21b,44,45 Here, the Co(OH)2 species in the first oxidation step during the anodic scan is originated from the reduction of Co3O4 species in the last step of the cathodic scan during the continuous CV cycling. It should be noted that the redox peaks corresponding to individual redox couples are not distinguishable, which is due to broadening of some redox peaks because of slight surface modification of the Co3O4 nanowires by conductive carbon.45c In contrast, the electrodeposited Co3O4 samples, widely found in literature, show well-defined peaks of each redox couples due to maximum utilization of active material in the absence of carbon additives.44,45d In general, the CV characteristics of the Co3O4 nanowires in this study demonstrate the overall redox reactions 2 and 3 involved during the charge storage process. With increase in scan rate, it is observed for both the Co3O4-ref and Co3O4-mw samples that the measured specific current increases and the anodic and cathodic peaks shift to higher and lower potentials, respectively. This is due to the scan rate dependent diffusion of OH ions into the sample matrix. At lower scan rate, both the outer- and inner-pore surfaces are effectively utilized, while at higher scan rates mainly the outer surface of the pores is accessed by the OH ions.33,46,47 Further, the anodic sweeps of CV curves are not entirely

symmetric with corresponding cathodic sweeps, which indicates appreciable kinetic irreversibility in the redox process due to polarization and ohmic resistance during the Faradaic process.29,33,46,47 The CV curves of both the Co3O4 samples become prominently asymmetrical with increase in scan rates that indicates better high-rate response of the Co3O4 nanowires.33,48 For both the Co3O4 nanowire samples, an almost linear/quasi-linear relationship has been observed between anodic peak current density and the applied scan rate, as shown in the insets of Figure 7A and B, which indicates the occurrence of surface redox reactions during the charge storage process.29,33,46 However, the efficiency of the charge storage process in the Co3O4 nanowires of different dimensions is not pretty similar as observed from the difference in their respective peak patterns and measured specific currents of both the Co3O4-ref and Co3O4-mw samples. The redox peaks of the Co3O4-ref nanowires are better defined with higher specific current as compared to the Co3O4-mw nanowires, as clearly seen in Figure 7C. This is solely due to smaller crystallite size and higher surface area as well as pore volume of the lowerdimensional Co3O4-ref nanowires, which preferably provides additional accessible surface for OH ions for more redox reactions as compared to the higher-dimensional Co3O4-mw nanowires. The specific capacitance (Cs, Fg1) values of the Co3O4-ref and Co3O4mw nanowire samples at different scan rates (ν, V s1) from the CV measurements are calculated using the following equation:45b,49 Cs ¼

Z 1 νwðVa Vc Þ

Vc Va

I V dV

ð4Þ

where w (g) is the weight of the Co3O4 sample. The specific capacitance values were derived from the integration of potential (V versus Hg/HgO) versus specific capacitance (Cs, Fg1) graphs, as shown in Figure S3 in the Supporting Information. The specific 25551



ARTICLE

capacitance values for the Co3O4-ref sample at scan rates of 5, 10, 20, and 50 mVs1 are found to be 355, 350, 333, and 288 F g1, respectively. Similarly, the specific capacitance values for the Co3O4mw sample at the same scan rates are 268, 263, 248, and 194 F g1, respectively. In comparison, only 18% loss in specific capacitance value for the Co3O4-ref sample from the scan rate of 550 mVs1 is observed whereas for the Co3O4-mw sample the specific capacitance loss is as high as 28%, under the same scan rate (Figure 7D). This demonstrates higher current response of the lower-dimensional Co3O4-ref nanowires. The higher performance of the Co3O4-ref sample electrode, even at higher scan rate, is due to the active participation of nanochannels that assist maximum contact of OH ions for redox reactions. On the other hand, the Co3O4-mw nanowires with lower surface area and pore volume bear minimum number of nanochannels and expose lesser electroactive surface for OH ions for the redox reactions. Further, at higher scan rate, the diffusion of OH ions is still lower due to the aggregated nanowire structure that prevents the easy accession of OH ions inside the nanowire matrix. Therefore, the high rate capacitance retention of conventionally synthesized lower-dimensional Co3O4-ref sample is unexpectedly better than the microwave-mediated synthesized higherdimensional Co3O4-mw sample. These results suggest the significant variation in the efficiency of charge storage with mere change in the dimensionality and orientation of the nanowire structures. Since the cyclic performance and stability of an electrode material are equally important for the potential use of the materials in supercapacitor devices, extensive chargedischarge cyclic measurements were carried out on both the Co3O4 samples at different current densities. Figure 8A and B shows the rate dependent discharge profiles (taken from first cycle of each charge/discharge profiles shown in Figure S4 in the Supporting Information) for the Co3O4-ref and Co3O4-mw samples at gravimetric current densities of 1.0, 2.0, 4.0, 8.0, and 16.0 A g1. The nonlinearity of the discharge curves, irrespective of the applied current density, is characteristic of pseudocapacitance behavior of the Co3O4 electrode material due to electrochemical quasi-reversible redox reactions at the electrodeelectrolyte interface.9,10,14,29,33,46 The linear potential variation in the nonlinear discharge curve from 0.0 to ∼0.35 V indicates the double layer capacitance behavior while the linear potential variation from ∼0.35 to 0.5 V represents pure pseudocapacitance behavior.33 Further, the longer discharge duration of the Co3O4-ref sample as compared to the Co3O4-mw sample at the same current density conditions clearly signifies the reduced internal resistance and enhanced pseudocapacitance performance of the Co3O4-ref sample. From the chargedischarge measurements at different operating current densities, the specific capacitance values of the Co3O4-ref and Co3O4-mw samples have been calculated from the applied charge/discharge current (i), mass of the Co3O4 sample (m), discharge time (Δt), and operating potential (ΔV) using the following equation:9,10,14,29,33,46 Cs ¼

i mðΔV =ΔtÞ

ð5Þ

The derived rate dependent specific capacitance values of the Co3O4-ref sample at current densities of 1.0, 2.0, 4.0, 8.0, and 16.0 Ag1 are found to be 336, 328, 300, 268, and 227 Fg1, respectively. Similarly, the specific capacitance values for the Co3O4-mw sample at the similar current densities are 232, 227, 200, 168, and 125 F g1, respectively. The Co3O4-ref sample is found to maintain 68% specific capacitance with increase in the current density from

1 to 16 Ag1; however, for the Co3O4-mw sample, the capacitance retention under the same applied current density condition is only 54%. The monotonous decrease in specific capacitance values of the Co3O4-ref sample as compared to the Co3O4-mw sample with increased operating current density from 1 to 16 Ag1 indicates significantly higher rate performance of the former sample. This distinctively superior pseudocapacitance performance of the Co3O4-ref sample is attributed to better microstructural properties of lower-dimensional nanowires, which facilitates the reduction in mass transfer resistance, electrolyte penetration, ion diffusion, and electroactive surface utilization during the redox process.14,29,33,46 In addition, the higher BET surface area and pore volume of the Co3O4-ref sample provides suitable ion channels for plentiful chemically active surface sites for charge storage. Further, the smaller crystallites of the lower-dimensional nanowires of the Co3O4-ref sample ease the electron hopping between the neighboring nanoparticles during redox reactions.50 In addition, numerous nanochannels between the nanoparticles act as better “OH ion-buffering reservoirs” for reduced mean free path of OH ions facilitating faster electronic kinetics and maximum reversible redox reactions for charge storage.51,33 On the other hand, the higher-dimensional oriented nanowires of the Co3O4-mw sample, due to lower surface area, pore volume, and large crystallite size, are devoid of more electroactive surface sites, which impede the better electrolyte contact and higher number of OH ions to actively access its surface. Generally, the pore defects significantly scatter the ions due to frequent crashing against the highly disoriented pore walls. Therefore, the electrode kinetics is better for materials with higher pore regularity.1b,52 From Figure 4B, it is evident that the pore size distribution is rather narrow and regular for the Co3O4-ref sample than for the Co3O4-mw sample. Therefore, the residual ion transport resistance is expected to be lower in the Co3O4-ref sample that results in superior electrode kinetics and charge storage in the lower-dimensional nanowires. Accordingly, there should be higher material resistance for higherdimensional Co3O4-mw nanowires as compared to the lowerdimensional Co3O4-ref nanowires. This is clear in the chronopotetiometry experiments where a significant voltage (iR) drop is observed in the discharge profiles of the Co3O4-mw nanowires as compared to the Co3O4-ref nanowires at 16 Ag1, shown in (J) and (E) of Figure S4 in the Supporting Information, respectively. The increase in voltage drop as a result of nonuniform current distributions due to electrode resistance (Relectrode), bulk electrolyte resistance (Rbulk), and electrolyte resistance in pores (Rpores) is a signature of higher equivalent series resistance (ESR) (E = 2 I ESR, where E is the voltage drop and I is the applied current) of the cell.53 The higher voltage drop in the higher-dimensional Co3O4-mw nanowire sample due to substantial ohmic and pore resistance results in increased overall capacitance drop in the sample as compared to the lower-dimensional Co3O4-ref sample. An electrode material with high life-cycle stability is practically preferred for long-term use in electrochemical supercapacitor applications. Therefore, the rate dependent cyclic pseudocapacitance efficiency of both the Co3O4 samples has been tested for 400 galvanostatic chargedischarge cycles, each at current densities of 1.0, 2.0, 4.0, 8.0, and 16.0 A g1. The cyclic specific capacitance performance at different current densities are presented in Figure 8C, and the respective capacitance retention data are presented in Figure 8D. From the profiles in Figure 8C, it can be observed that both the samples exhibit striking cyclic stability; however, the decrease in the specific capacitance values with increasing applied current densities is considerably higher for 25552


The Journal of Physical Chemistry C the Co3O4-ref sample as compared to the Co3O4-mw samples. Quantitatively, from Figure 8D, the capacitance retentions of the Co3O4-mw sample after 400 charge dischargedischarge cycles at each current density of 1.0, 2.0, 4.0, 8.0, and 16.0 A g1 are found to be ∼99.1%, 98.2%, 97%, 95.8%, and 92.8%, respectively, whereas for the Co3O4-ref sample, the capacitance retentions at similar applied current densities are quantified as 97.9%, 94.8%, 93.3%, 90.6%, and 86.7%, respectively. It is interesting to note that the Co3O4-mw sample shows better capacitance retention and cyclic stability attributable to the oriented nanowire superstructures than the Co3O4-ref sample. The rigid superstructure due to interconnected nanowires undergoes reduced structural modification or degradation or crystallographic changes of the electroactive surface during the repeated insertion/extraction of OH ions into/from the nanowire matrix in the high rate redox process. In addition, the microwave-synthesized nanowire superstructures, due to highly porous matrix, are flexible enough to experience least strain during extended charge/ discharge cycles even at higher current density condition. However, the freely dispersed single-stranded lower-dimensional nanowire structures under numerous chargedischarge cycles undergo major crystallographic modification and flaking off, particularly at higher applied current density conditions. Hence, although the Co3O4-mw sample shows lower specific capacitance values at various current density conditions, the sample is particularly stable for extended duration during the chargedischarge cycles even at high rate operating conditions. The observed small increase in specific capacitance values of both the Co3O4 samples during the first 100150 chargedischarge cycles is usually due to initial surface activation of the nanostructures.28,32 For both Co3O4 samples, the chargedischarge curves as well as the corresponding duration of charging and discharging at various current densities are not entirely symmetrical ((AE) and (FJ) of Figure S4 in the Supporting Information). This is due to the surface related kinetic irreversibility of the OH ions during the quasi-reversible redox reactions,46b,47 which can be further substantiated from the quantitative charging (tC) and discharging (tD) time. The smaller Δt (tC tD) value is a lucid representation of higher OH ion redox reversibility due to the unimpeded OH kinetics resulting from the better surface accessibility of OH ions.29,33,46b This is related to the Coulombic efficiency η of the charge storage process that is a measure of competence of charge/ion transfer during an electrochemical reaction:29,54 tD η ¼ 100 ð6Þ tC The derived η value for the Co3O4-ref and Co3O4-mw samples from the first chargedischarge cycles at current densities of 1, 2, 4, 8, and 16 A g1 are 91.7%, 92.8%, 96.8%, 97.8%, and 97.5%, and 89%, 94.2%, 95.1%, 94.5%, and 92.8%, respectively, as shown in Figure 9. The higher η values of the Co3O4-ref sample at current densities up to 16 A g1 signify the lower internal resistance, shorter ion transport distances, and better electroactive surface utilization of lower-dimensional nanowires.29,54 The Coulombic efficiency of the Co3O4-ref sample almost reaches ∼100% during chargedischarge cycles at different current density due to the gradual surface activation during cycling. However, due to the poor surface properties of the higher-dimensional oriented nanowires of the Co3O4-mw

ARTICLE

Figure 9. Coulombic efficiency (η) derived from chargedischarge cycles of the Co3O4-ref and Co3O4-mw samples at various current densities of 1 Ag1 (black), 2 Ag1 (red), 4 Ag1 (green), 8 Ag1 (blue), and 16 Ag1 (purple).

sample, the surface activation is minimum, which results in sluggish increase of Coulombic efficiency during continuous chargedischarge processes. Nevertheless, under high current density conditions, the Coulombic efficiency of the Co3O4-ref remains at 97.5% while for the Co3O4-mw sample, it is at 92.8%. This observation further validates higher feasibility and surface accessibility of the Co3O4-ref sample even at higher current density conditions. The conventionally synthesized lower-dimensional Co3O4 nanowires due to better high rate Coulombic efficiency are therefore electrochemically preferred over microwave-synthesized higher-dimensional oriented nanowires for high rate supercapacitor applications. Due to the dominating effect of the kinetic features of the ions and electron transport in electrodes as well as at the electrode/ electrolyte interface on the pseudocapacitance performance, lower internal resistance (higher conductivity) is of great significance: for less energy will be wasted during charge storage process.10 Both the Co3O4 samples were subjected to AC impedance measurements in the frequency range of 0.01 105 Hz, to quantitatively evaluate their intrinsic resistance and relative pseudocapacitive performance. Figure 10A shows the complex-plane impedance plots (Nyquist plots; imaginary part, Z00 versus real part, Z0 ) of the Co3O4-mw and Co3O4-ref sample electrodes at a bias potential of 0.45 V. The impedance data plots were fitted (CNLS fitting method) to a Randle equivalent circuit (inset in Figure 10A) before analysis.55 The Nyquist plots of both the samples show similar profiles with slopes close to 90° along the imaginary axis (Z00 ), which is characteristic of a nonideally polarizable electrode, and can be attributed to qualitative pseudocapacitance process due to reversible redox reactions.10,33,55 The impedance data profiles have been analyzed assuming three major possible electrochemical processes in the electrode electrolyte interface at high, medium, and low frequency regions. The possible assumption of various electrochemical phenomena is in accordance with dissimilarity in the charge propagation phenomena during the Faradaic redox reactions ascribed to the difference in surface structures of the Co3O4-mw and Co3O4-ref nanowire samples. The impedance profiles reveal two partial semicircles in the high and medium frequency regions. The enlarged view of the higher frequency semicircles is shown in the inset of Figure 10A. At this high frequency, the 25553



ARTICLE

Figure 10. (A) Complex plane impedance plots (Nyquist plots) of the Co3O4 samples; insets show the equivalent circuit and impedance at high frequency region; (B) frequency dependent specific capacitance values of the Co3O4-mw and Co3O4-ref samples, derived from impedance measurements.

mass transfer can be neglected, and the semicircle signifies charge-transfer process at the outermost surface of the electrode in contact with the electrolyte.10,33,55 This can be modeled as a double-layer capacitor, Cd, in parallel with a charge-transfer resistor, Rict. The Rict arises due to the discontinuity in the charge transfer process at the electrodeelectrolyte interface because of conductivity difference between the solid oxide (electronic conductivity) and liquid electrolyte phase (ionic conductivity).33,55 The medium-frequency region is diffusion controlled, and the distributed capacitance and resistance are characteristic of surface redox reaction induced charge-transfer process. This phenomenon can be represented as a combination of film capacitor, Cf, in parallel with an electron-transfer resistor, Rect. From the semicircles in the medium-frequency region, the redox reaction induced chargetransfer resistance of the Co3O4-ref sample can be ascertained as lower than the Co3O4-mw sample. This is due to virtual reduction in the transport length of the OH ions inside the ideally porous Co3O4-ref sample. The near linear EIS plots of the Co3O4 samples in the lower frequency region are characteristic of Warburg impedance, W, where the resistance behaviors can be attributed to diffusion of the OH ion within the pores of the Co3O4 electrode during redox reaction.33,55 The characteristic nearstraight vertical line of the Co3O4-ref sample as compared to the Co3O4-mw sample demonstrates lower diffusion resistance in the former sample, due to easy accessibility of the OH ions into its highly regular mesopores. By and large, the AC impedance measurements substantiate the lower combination of ionic resistance of electrolyte, intrinsic resistance of active materials, and contact resistance at the active material/current collector interface for the Co3O4-ref sample as compared to the Co3O4-mw sample. The specific capacitance (Cs) values for both Co3O4 electrodes have been derived from the impedance data, using the following equation:10,29,33,46 Cs ¼

1 2πfZ00

ð7Þ

The impedance derived frequency dependent specific capacitance plots are shown in Figure 10B. At the entire frequency range, the specific capacitance values of the Co3O4-ref sample is substantially more than the Co3O4-mw sample. Moreover, at elevated operating frequency, the appreciably higher specific capacitance values of the Co3O4-ref sample (114 Fg1 at 1 Hz and 46 Fg1 at 10 Hz) as compared to the Co3O4-mw sample (80 Fg1 at 1 Hz and 20 Fg1 at 10 Hz) demonstrate the high power performance and excellent rate response of the Co3O4-ref sample. The Co3O4-mw sample

Figure 11. Ragone plot (power density vs energy density) of the Co3O4-ref and Co3O4-mw samples. The values are derived from the CV measurements at scan rates 5, 10, 20, and 50 mV s1.

obviously shows poor power performance and rate response due to diffusion limitations of OH ions into the matrix of the oriented nanowire superstructures. The better rate response of the Co3O4ref nanowire sample has been further verified from the capacitor response frequency:56 the frequency where ϕ = 45° (fϕ = 45), in the fϕ plot in Figure S5 in the Supporting Information. The marginally higher capacitor response frequency of the Co3O4-ref sample indicates slightly slower response time as compared to the Co3O4-mw sample. The power performance of the Co3O4-mw and Co3O4-ref samples is presented in the Ragone plot (power density vs energy density) in Figure 11. The energy and power densities were derived from CV measurements at the scan rates of 5, 10, 20, and 50 mV s1. The energy density values (dE) were obtained using the following equation:57 dE ¼

1 CV 2 2

ð8Þ

where C is the specific capacitance (F g1) at a particular scan rate under the potential window (V) of 0.6 V. The power density values (dP) were obtained by dividing the dE with time t for one sweep segment at a fixed scan rate:57 dP ¼

dE t

ð9Þ

The lower-dimensional Co3O4-ref nanowire delivers an energy density of ∼14.4 Wh kg1 at a power density of ∼4.3 kW kg1, which is superior to energy density of ∼10 Wh kg1 at a power density 25554


The Journal of Physical Chemistry C of ∼2.9 kW kg1 delivered by the higher-dimensional Co3O4mw nanowire sample. The considerably superior energy and power density of the Co3O4-ref sample over the Co3O4-mw sample relate to better surface properties and activity toward electrochemical supercapacitor application. Broadly, for the first time, this study authenticates the effect of microwave heating on the dimensionality of Co3O4 nanowires synthesized in a nonhydrothermal homogeneous precipitation condition. The results clearly demonstrate that the microwaveinduced dimensionality in the Co3O4 nanocrystallites and nanowires, which in turn, significantly affect its optical and magnetic behavior as well as charge storage efficiency.

4. CONCLUSIONS In the present work, we have demonstrated the tunable dimensionality of Co3O4 nanowires in nonhydrothermal condition, by employing different heating procedures, using conventional reflux and microwave-assisted reflux methods. The different heating approaches significantly influence the rate of crystal growth and interparticle attachment during the evolution of nanowire morphologies with different dimensionality. The well dispersed one-dimensional nanowire structures synthesized from the conventional heating approach shows improved physicochemical, optical, and magnetic properties compared to the higher-dimensional attached nanowire superstructures synthesized using the microwave-mediated method. Electrochemical studies on the Co3O4 nanowire samples show explicit contribution of pseudocapacitance to their energy storage performance. The cyclic voltammetry, chronopotentiometry, and impedance spectroscopy studies show improved high rate pseudocapacitance behavior of the conventional reflux synthesized sample due to the facile electrolyte/OH ion penetration and better Faradaic utilization of the electroactive smaller crystallite surface. The Ragone plot of power versus energy density further ascertains higher performance of the lower-dimensional freely dispersed nanowires compared to the higherdimensional oriented nanowire superstructures. From this study, it is evident that there is a significant effect of microwave on the dimensionality of Co3O4 nanowires during the synthesis under nonhydrothermal homogeneous precipitation conditions. This finding can be extended to microwave-controlled fine-tuning of the nanowire dimensionality of other materials for specific applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed characterization techniques, FESEM images of the uncalcined precursors, band structure model of Co3O4, specific capacitance plots derived from CV measurements, initial chargedischarge profiles of Co3O4 samples at different current densities, and frequency versus phase angle (ϕ) plot. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: (91) 44 2257 4226; fax: (91) 44 2257 0545.

’ ACKNOWLEDGMENT The authors gracefully acknowledge the financial support by DRDO, New Delhi, through grant no. ERIP/ER/0600319/M/ 01/1052.

ARTICLE

’ REFERENCES (1) Liu, J.; Cao, G.; Yang, Z.; Wang, D.; Dubois, D.; Zhou, X.; Graff, G. L.; Pederson, L. R.; Zhang, J.-G. ChemSusChem 2008, 1, 676. (2) Baxter, J.; Bian, Z.; Chen, G.; Danielson, D.; Dresselhaus, M.; Fedorov, A. G.; Fisher, T. S.; Jones, C. W.; Maginn, E.; Kortshagen, U.; Manthiram, A.; Nozik, A.; Rolison, D. R.; Sands, T.; Shi, Li.; Shollh, D.; Wu, Y. Energy Environ. Sci. 2009, 2, 559. (3) Patzke, G. R.; Zhou, Y.; Kontic, R.; Conrad, F. Angew. Chem., Int. Ed. 2011, 50, 826. (4) (a) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Schalkwijk, W. Nat. Mater. 2005, 4, 366. (b) Guo, Y.-G.; Hu, J.-S.; Wan, L.-J. Adv. Mater. 2008, 20, 2878. (5) (a) Manthiram, A.; Murugan, A. V.; Sarkar, A.; Muraliganth, T. Energy Environ. Sci. 2008, 1, 621. (b) Whittingham, M. S. MRS Bull. 2008, 33, 411. (c) Rolison, D. R.; Long, J. W.; Lytle, J. C.; Fischer, A. E.; Rhodes, C. P.; McEvoy, T. M.; Bourga, M. E.; Lubers, A. M. Chem. Soc. Rev. 2009, 38, 226. (6) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (7) Winter, M.; Brodd, R. J. Chem. Rev. 2004, 104, 4245. (8) (a) Shukla, A. K.; Sampath, S.; Vijayamohanan, K. Curr. Sci. 2000, 79, 1656. (b) Hall, P. J.; Mirzaeian, M.; Fletcher, S. I.; Sillars, F. B.; Rennie, A. J. R.; Shitta-Bey, G. O.; Wilson, G.; Cruden, A.; Carter, R. Energy Environ. Sci. 2010, 3, 1238. (9) (a) Conway, B. E.; Birss, V.; Wojtowicz, J. J. Power Sources 1997, 66, 1. (b) K€otz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (10) Conway, B. E. Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Press: New York, 1999; p 11. (11) (a) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (b) Naoi, K.; Simon, P. Electrochem. Soc. Interface 2008, 34. (12) Hu, C.-C.; Chang, K.-H.; Lin, M.-C.; Wu, Y.-T. Nano Lett. 2006, 6, 2690. (b) Mondal, S. K.; Munichandraiah, N. J. Power Sources 2008, 175, 657. (13) Zhao, X.; Sanchez, B. M.; Dobson, P. J.; Grant, P. S. Nanoscale 2011, 3, 839. (14) (a) Xiong, S.; Yuan, C.; Zhang, X.; Xi, B.; Qian, Y. Chem.—Eur. J. 2009, 15, 5320. (b) Wei, T.-Y.; Chen, C. -H.; Chang, K.-H.; Lu, S.-Y.; Hu, C.-C. Chem. Mater. 2009, 21, 3228. (c) Wang, G.; Liu, H.; Horvat, J.; Wang, B.; Qiao, S.; Park, J.; Ahn, H. Chem.—Eur. J. 2010, 16, 11020. (d) Zhu, T.; Chen, J. S.; Lou, X. W. J. Mater. Chem. 2010, 20, 7015. (e) Xia, X.-H.; Tu, J.-P.; Wang, X.-L.; Gu, C.-D.; Zhao, X.-B. Chem. Commun. 2011, 47, 5786. (f) Meher, S. K.; Ranga Rao, G. J. Phys. Chem. C 2011, 115, 15646. (15) Xie, X.; Li, Y.; Liu, Z.-Q.; Haruta, M.; Shen, W. Nature 2009, 458, 746. (16) Li, W. -Y.; Xu, L.-N.; Chen, J. Adv. Funct. Mater. 2005, 15, 851. (17) (a) Wang, R. M.; Liu, C. M.; Zhang, H. Z.; Chen, C. P.; Guo, L.; Xu, H. B.; Yang, S. H. Appl. Phys. Lett. 2004, 85, 2080. (b) Keng, P. Y.; Kim, B. Y.; Shim, I.-B.; Sahoo, R.; Veneman, P. E.; Armstrong, N. R.; Yoo, H.; Pemberton, J. E.; Bull, M. M.; Griebel, J. J.; Ratcliff, E. L.; Nebesny, K. G.; Pyun, J. ACS Nano 2009, 3, 3143. (18) Xua, J.; Gao, L.; Cao, J.; Wang, W.; Chen, Z. Electrochim. Acta 2010, 56, 732. (19) Nam, K. T.; Kim, D.-W.; Yoo, P. J.; Chiang, C.-Y.; Meethong, N.; Hammond, P. T.; Chiang, Y.-M.; Belcher, A. M. Science 2006, 312, 885. (20) Hu, L.; Peng, Q.; Li, Y. J. Am. Chem. Soc. 2008, 130, 16136. (21) (a) Chou, S.-L.; Wang, J.-Z.; Liu, H.-K.; Dou, S.-X. J. Power Sources 2008, 182, 359. (b) Deng, M.-J.; Huang, F.-L.; Sun, I.-W.; Tsai, W.-T.; Chang, J.-K. Nanotechnology 2009, 20, 175602. (22) (a) Xie, X.; Shen, W. Nanoscale 2009, 1, 50. (b) Cheng, H.; Lu, Z. G.; Deng, J. Q.; Chung, C. Y.; Zhang, K.; Li, Y. Y. Nano Res. 2010, 3, 895. (c) Gao, Y.; Chen, S.; Cao, D.; Wang, G.; Yin, J. J. Power Sources 2010, 195, 1757. (d) Wang, H.; Zhang, Li.; Tan, X.; Holt, C. M. B.; Zahiri, B.; Olsen, B. C.; Mitlin, D. J. Phys. Chem. C 2011, 115, 17599. (e) Yuan, Y. F.; Xia, X. H.; Wu, J. B.; Huang, X. H.; Pei, Y. B.; Yang, J. L.; Guo, S. Y. Electrochem. Commun. 2011, 13, 1123. (f) Xia, X.-H.; Tu, J.-P.; 25555


The Journal of Physical Chemistry C Mai, Y.-J.; Wang, X.-L.; Gu, C.-D.; Zhao, X.-B. J. Mater. Chem. 2011, 21, 9319. (23) Hould, N. D.; Lobo, R. F. Chem. Mater. 2008, 20, 5807. (24) (a) Bilecka, I.; Niederberger, M. Nanoscale 2010, 2, 1358. (b) Zheng, J.; Yang, R.; Xie, L.; Qu, J.; Liu, Y.; Li, X. Adv. Mater. 2010, 22, 1451. (25) Tompsett, G. A.; Conner, W. C.; Yngvesson, K. S. ChemPhysChem 2006, 7, 296. (26) (a) Ding, Y.; Xu, L.; Chen, C.; Shen, X.; Suib, S. L. J. Phys. Chem. C 2008, 112, 8177. (b) Lai, T.-L.; Lai, Y.-L.; Lee, C.-C.; Shu, Y.-Y.; Wang, C.-B. Catal. Today 2008, 131, 105. (c) Wang, J.; Niu, B.; Du, G.; Zeng, R.; Chen, Z.; Guo, Z.; Dou, S. Mater. Chem. Phys. 2011, 126, 747. (27) Xu, R.; Zeng, H. C. J. Phys. Chem. B 2003, 107, 12643. (28) (a) C€olfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (b) Deng, D.; Lou, X. W.; Lee, J. Y.; Archer, L. A. J. Mater. Chem. 2008, 18, 4397. (c) Grzelczak, M.; Vermant, J.; Furst, E. M.; LizMarzan, L. M. ACS Nano 2010, 4, 3591. (d) Mallavajula, R. K.; Archer, L. A. Angew. Chem., Int. Ed. 2011, 50, 578. (29) Meher, S. K.; Justin, P.; Ranga Rao, G. ACS Appl. Mater. Interfaces 2011, 3, 2063. (30) Hu, X.; Gong, J.; Zhang, L.; Yu, J. C. Adv. Mater. 2008, 20, 4845. (31) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999. (32) (a) Rege, S. U.; Yang, R. T. AIChE J. 2000, 46, 734. (b) Ghanem, B. S.; Msayib, K. J.; McKeown, N. B.; Harris, K. D. M.; Pan, Z.; Budd, P. M.; Butler, A.; Selbie, J.; Book, D.; Walton, A. Chem. Commun. 2007, 67. (c) Rose, M.; B€ohlmann, W.; Sabo, M.; Kaskel, S. Chem. Commun. 2008, 2462. (33) Meher, S. K.; Justin, P.; Ranga Rao, G. Nanoscale 2011, 3, 683. (34) (a) Wang, X.; Chen, X.; Gao, L.; Zheng, H.; Zhang, Z.; Qian, Y. J. Phys. Chem. B 2004, 108, 16401. (b) Xu, R.; Zeng, H. C. Langmuir 2004, 20, 9780. (35) (a) Barreca, D.; Massignan, C.; Daolio, S.; Fabrizio, M.; Piccirillo, C.; Armelao, L.; Tondello, E. Chem. Mater. 2001, 13, 588. (b) Gulino, A.; Dapporto, P.; Rossi, P.; Fragala, I. Chem. Mater. 2003, 15, 3748. (36) Yamamoto, H.; Tanaka, S.; Hirao, K. J. Appl. Phys. 2003, 93, 4158. (37) (a) Ambrose, T.; Chien, C. L. Phys. Rev. Lett. 1996, 76, 1743. (b) Zheng, X. G.; Xu, C. N.; Nishikubo, K.; Nishiyama, K.; Higemoto, W.; Moon, W. J.; Tanaka, E.; Otabe, E. S. Phys. Rev. B 2005, 72, 014464. (c) He, L.; Chen, C.; Wang, N.; Zhou, W.; Guo, L. J. Appl. Phys. 2007, 102, 103911. (38) (a) Nethravathi, C.; Sen, S.; Ravishankar, N.; Rajamathi, M.; Pietzonka, C.; Harbrecht, B. J. Phys. Chem. B 2005, 109, 11468. (b) Dong, Z.; Fu, Y.; Han, Q.; Xu, Y.; Zhang, H. J. Phys. Chem. C 2007, 111, 18475. (c) Dutta, P.; Seehra, M. S.; Thota, S.; Kumar, J. J. Phys.: Condens. Matter 2008, 20, 015218. (d) Ozkaya, T.; Baykal, A.; Toprak, M. S.; Koseoglu, Y.; Durmus, Z. J. Magn. Magn. Mater. 2009, 321, 2145. (e) Chen, W.; Chen, C.; Guo, L. J. Appl. Phys. 2010, 108, 073907. (39) Yokoyama, M.; Sato, T.; Ohta, E.; Sato, T. J. Appl. Phys. 1996, 80, 1015. (40) (a) Wang, Y.; Yang, C.-M.; Schmidt, W.; Spliethoff, B.; Bill, E.; Sch€uth, F. Adv. Mater. 2005, 17, 53. (b) Tomou, A.; Gournis, D.; Panagiotopoulos, I.; Huang, Y.; Hadjipanayis, G. C.; Kooi, B. J. J. Appl. Phys. 2006, 99, 123915. (c) Br€uck, S.; Sort, J.; Baltz, V.; Suri~ nach, S.; Mu~noz, J. S.; Dieny, B.; Baro, M. D.; Nogues, J. Adv. Mater. 2005, 17, 2978. (41) (a) Yi, J. B.; Ding, J.; Feng, Y. P.; Peng, G. W.; Chow, G. M.; Kawazoe, Y.; Liu, B. H.; Yin, J. H.; Thongmee, S. Phys. Rev. B 2007, 76, 224402. (b) Zeng, R.; Wang, J. Q.; Chen, Z. X.; Li, W. X.; Dou, S. X. J. Appl. Phys. 2011, 109, 07B520. (42) (a) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Phys. Rev. Lett. 1997, 17, 1393. (b) Li, S.; Bi, H.; Cui, B.; Zhang, F.; Du, Y.; Jiang, X.; Yang, C.; Yu, Q.; Zhu, Y. J. Appl. Phys. 2004, 95, 7420. (c) Wang, G.; Shen, X.; Horvat, J.; Wang, B.; Liu, H.; Wexler, D.; Yao, J. J. Phys. Chem. C 2009, 113, 4357.

ARTICLE

(43) (a) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Sch€uth, F. Nano Lett. 2006, 6, 2977. (b) Hong, J.-I.; Leo, T.; Smith, D. J.; Berkowitz, A. E. Phys. Rev. Lett. 2006, 96, 117204. (44) (a) Behl, W. K.; Toni, J. E. J. Electroanal. Chem. Interfacial Electrochem. 1971, 31, 63. (b) Svegl, F.; Orel, B.; Hutchins, M. G.; Kaicher, K. J. Electrochem. Soc. 1996, 143, 1532. (c) Casella, I. G.; Gatta, M. J. Electroanal. Chem. 2002, 534, 31. (45) (a) Spinolo, G.; Ardizzone, S.; Trasatti, S. J. Electroanal. Chem. 1997, 423, 49. (b) Srinivasan, V.; Weidner, J. W. J. Power Sources 2002, 108, 15. (c) Palmas, S.; Ferrara, F.; Vacca, A.; Mascia, M.; Polcaro, A. M. Electrochim. Acta 2007, 53, 400. (d) Hu, C.-C.; Hsu, T.-Y. Electrochim. Acta 2008, 53, 2386. (46) (a) Justin, P.; Meher, S. K.; Ranga Rao, G. J. Phys. Chem. C 2010, 114, 5203. (b) Meher, S. K.; Justin, P.; Ranga Rao, G. Electrochim. Acta 2010, 55, 8388. (47) Pell, W. G.; Conway, B. E. J. Electroanal. Chem. 2001, 500, 121. (48) Liang, Y.; Schwab, M. G.; Zhi, L.; Mugnaioli, E.; Kolb, U.; Feng, X.; M€ullen, K. J. Am. Chem. Soc. 2010, 132, 15030. (49) Wu, M.-S.; Huang, Y.-A.; Yang, C.-H. J. Electrochem. Soc. 2008, 155, A798. (50) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092. (51) Wang, D.-W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H.-M. Angew. Chem., Int. Ed. 2008, 47, 373. (52) Klein, M. P.; Jacobs, B. W.; Ong, M. D.; Fares, S. J.; Robinson, D. B.; Stavila, V.; Wagner, G. J.; Arslan, I. J. Am. Chem. Soc. 2011, 133, 9144. (53) Grupioni, A. A. F.; Lassali, T. A. F. J. Electrochem. Soc. 2001, 148, A1015. (54) Ye, C.; Lin, Z. M.; Hui, S. Z. J. Electrochem. Soc. 2005, 152, A1272. (55) Hu, C.-C.; Chang, K.-H.; Hsu, T.-Y. J. Electrochem. Soc. 2008, 155, F196. (56) (a) Sugimoto, W.; Iwata, H.; Yokoshima, K.; Murakami, Y.; Takasu, Y. J. Phys. Chem. B 2005, 109, 7330. (b) Lin, K.-M.; Chang, K.-H.; Hu, C.-C.; Li, Y.-Y. Electrochim. Acta 2009, 54, 4574. (c) Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. Electrochim. Acta 2009, 54, 2271. (57) (a) Li, J.; Gao, F. J. Power Sources 2009, 194, 1184. (b) Liu, C.; Yu, Z.; Neff, D.; Zhamu, A.; Jang, B. Z. Nano Lett. 2010, 10, 4863. (c) Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H. J. Am. Chem. Soc. 2010, 132, 7472.

25556


Effect of Microwave on the Nanowire Morphology, Optical, Magnetic

Recommend Documents