Oxygen Vacancy-Induced Structural, Optical, and Enhanced

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Oxygen Vacancy-Induced Structural, Optical, and Enhanced Supercapacitive Performance of Zinc Oxide Anchored Graphitic Carbon Nanofiber Hybrid Electrodes Gowra Raghupathy Dillip, Arghya Narayan Banerjee, Veettikunnu Chandran Anitha, B Deva Prasad Raju, Sang Woo Joo, and Bong Ki Min ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12322 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 6, 2016

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ACS Applied Materials & Interfaces

Oxygen

Vacancy-Induced

Structural,

Optical,

and

Enhanced

Supercapacitive

Performance of Zinc Oxide Anchored Graphitic Carbon Nanofiber Hybrid Electrodes

Gowra Raghupathy Dillip1, Arghya Narayan Banerjee1,*, Veettikunnu Chandran Anitha1, Borelli Deva Prasad Raju2, Sang Woo Joo1,* and Bong Ki Min3

1

School of Mechanical Engineering and Technology, Yeungnam University, Gyeongsan, 712

749, South Korea. 2

Department of Future Studies, Sri Venkateswara University, Tirupati, 517 502, India.

3

Center for Research Facilities, Yeungnam University, Gyeongsan, 712 749, South Korea.

ABSTRACT Zinc oxide (ZnO) nanoparticles (NPs) anchored to carbon nanofiber (CNF) hybrids were synthesized using a facile co-precipitation method. This report demonstrates an effective strategy to intrinsically improve the conductivity and supercapacitive performance of the hybrids by inducing oxygen vacancies. Oxygen deficiency-related defect analyses were performed qualitatively as well as quantitatively using Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All of the analyses clearly indicate an increase in oxygen deficiencies in the hybrids with an increase in the vacuum-annealing temperature. The non-stoichiometric oxygen vacancy is mainly induced via the migration of the lattice oxygen into interstitial sites at elevated temperature (300°C), followed by diffusion into the gaseous phase with further increase in

the annealing temperature (600°C) in an oxygen-deficient atmosphere. This induction of 1

ACS Paragon Plus Environment


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oxygen vacancy is corroborated by diffuse reflectance spectroscopy, which depicts the oxygen-vacancy-induced bandgap narrowing of the ZnO NPs within the hybrids. At a current density of 3 A g-1, the hybrid electrode exhibited higher energy density (119.85 Wh kg-1) and power density (19.225 kW kg-1) compared to a control ZnO electrode (48.01 Wh kg-1 and 17.687 kW kg-1). The enhanced supercapacitive performance is mainly ascribed to the good interfacial contact between CNF and ZnO, high oxygen deficiency, and fewer defects in the hybrid. Our results are expected to provide new insights into improving the electrochemical properties of various composites/hybrids. KEYWORDS: Zinc oxide/Carbon nanofiber hybrid, oxygen deficiency, bandgap narrowing, supercapacitor *

Corresponding authors: Tel: +82 53 810 2453, e-mail: [email protected], [email protected] (A.N.B). Tel: +82 53 810 2568, e-mail: [email protected] (S.W.J). 1. INTRODUCTION Supercapacitors (SCs) exhibit many outstanding properties compared to conventional dielectric capacitors and batteries, such as higher energy and power density, fast charging and discharging, and long cycle life for a wide range of applications, such as consumer electronics, medical electronics, memory backup systems, hybrid electric vehicles, transportation, and military defense systems.1-4 However, to satisfy demands in the rapidly growing field of energy applications, more efforts have to be expended to the development of new electrodes and electrolytes without sacrificing the power density and cycle life.5,6. Over the past few decades, transition metal oxides/hydroxides have been explored for use as high energy-density pseudocapacitor electrodes because of their theoretical capacitance and abundance.7 Among the various metal oxides, capacitors based on ruthenium oxide (RuO2) show remarkably high specific capacitance and power.8 However, due to high 2


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toxicity and cost, the use of RuO2-based capacitors in practical large-scale production is limited.9 Therefore, much effort has been devoted to identifying inexpensive and low-toxicity metal oxide electrode materials with reasonable electrochemical properties as alternatives to RuO2.10 In practice, the most important characteristics required for using a metal oxide as a capacitor electrode are pseudocapacitive behavior, large surface area, high conductivity, and high electrochemical stability.9 Zinc oxide nanostructures are one of the promising candidates for supercapacitors due to their high specific energy density, improved biocompatibility, nontoxicity, good electrochemical activity, low cost, chemical stability, abundant availability and environmental friendliness compared to other transition metal oxides.11-13 Additionally, nanostructured ZnO possesses some unique physicochemical properties due to the unique spatial architecture and large aspect ratio (so also higher active surface area) compared to their bulk counterpart to meet some specific device-related demands for supercapacitor applications. Studies on the suitability of ZnO as a promising candidate for supercapacitors are limited, and there is a need to better understand the behavior of this material in order to improve its electrochemical properties. In order to exploit the power density of available metal oxide supercapacitors, several groups have recently fabricated composite/hybrid electrodes via the modification of carbonaceous materials with metal oxides.14,15 Carbon-based materials in the form of powders, fibers, aerogels, composites, sheets, monoliths, and tubes have been widely used as electrodes because of their low cost, variety of morphology/structure, easy processing, high electrical conductivity, improved chemical stability, relatively inert electrochemistry, extremely high mechanical strength, controllable porosity, and electro-catalytic active sites for a wide range of redox reactions.10,11,16-18 Among these materials, carbon nanofibers (CNFs) are attractive electrode additive materials for improving the performance of metal oxide supercapacitors. 3



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CNFs have high specific surface area, well-defined hollow cores, and a high aspect ratio greater than 106.19 Thus, a hybrid nanostructure exploiting the electric double layer capacitance of CNF and faradic pseudocapacitance of zinc oxide could be a suitable candidate for electrochemical capacitor with high specific capacitance and energy density. Although ZnO has been combined with additives like carbon nanotubes, fibers, aerogel, and graphene to enhance the pseudocapacitance of composites,20,21 there are several issues involved in these types of hybrid/composite materials. For example, the energy and power density of the related devices are far from satisfactory and do not meet current market demands. A majority of these issues are related to the presence of defects in the carbon nanostructures, surface/interfacial states within the nanocomposites, and poor crystallinity of the metal oxides. These issues lead to deterioration of the electrical transport properties of the hybrid nanomaterials. Therefore, novel composite/hybrid nanomaterials are needed to overcome the present obstacles. Generally, post-synthesis heat treatment of hybrid electrodes in a vacuum or in a controlled inert atmosphere is expected to improve the structure of the carbon nanomaterials or metal oxides by the removal of defects/surface states, leading to improved electrical characteristics of the nanocomposites.22 Very few reports describe the role of vacuum-annealing treatment of metal oxide/CNT composites (such as MnO2/CNT, RuO2/MWNT, and SnO2/MWNT) to improve cycle ability and energy density.23,24 To the best of our knowledge, there is no report on the electrochemical properties of ZnO/CNF nanohybrid electrodes that have been heat treated in an oxygen-deficient atmosphere for use in supercapacitors. In the present investigation, ZnO NPs that are well attached to CNF walls were synthesized via a precipitation process, followed by heat-treatment in a vacuum furnace. The novelty of the present work includes (i) improved specific capacitance and cyclic performance of the microelectrodes and (ii) a 4


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systematic study of the sample properties in terms of oxygen deficiency. The deficiency was studied in correlation with the surface morphology, structural/microstructural properties, and optical properties of the nanohybrid with the enhanced electrochemical properties for use in SC applications. The results could lead to the cost-effective fabrication of novel composite electrode materials for superior electrochemical supercapacitors.

2. EXPERIMENTAL PROCEDURE 2.1. Materials Commercially available graphitized CNFs (< 100 ppm iron content), synthesized by a vapor-growth method with an outer diameter of around 100 nm and a length of ~20 to 200 µm, Ni foam, poly(vinylidene fluoride) (PVDF), dimethylformamide (DMF), and carbon black were purchased from Sigma-Aldrich (USA). Precursors of high-purity zinc acetate dihydrate [Zn(OCCH3)2], sodium hydroxide (NaOH) (Extra pure, Duksan, Korea), and acids H2SO4 (60-61% mm-1, Junsei, Japan) and HNO3 (97% mm-1, Matsunoen, Japan) were used as received. Deionized water (DIW) with a resistivity of 18.2 MΩ-cm was used as a solvent.

2.2 Hybrid preparation The ZNO/CNF hybrid was prepared by a simple co-precipitation method according to previous work.25 The detailed synthetic process is provided in the Supporting Information.

2.3 Annealing in an oxygen-deficient atmosphere To study the properties of the as-prepared samples in an oxygen-deficient atmosphere, the room temperature (RT)-synthesized hybrid (CNFZnO-RT; Supporting Information) was annealed in a quartz tube vacuum furnace (E.M.S. Tech vacuum furnace, USA) at ~4 millitorr 5



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and temperatures of 300 and 600°C for 3 h. After the vacuum of the tube furnace was adjusted to the desired level using a standard rotary vane pump, the annealing was performed while increasing the temperature at a ramping rate of 20°C/min until it reached a preset level

(300°C/600°C). Next, the annealing was continued at this level for a predetermined length of time (3 h). The heater was then turned off and the samples were cooled to RT. The vacuum was then turned off, and the tube was vented out to atmospheric pressure to obtain the annealed samples for further characterization. For comparison with pristine and annealed hybrid samples, a control sample of zinc oxide nanoparticles was also prepared separately. A similar co-precipitation process was used with an equimolar ratio of zinc acetate and NaOH, followed by heat treatment at 600°C for 3 h in a vacuum. The annealed samples were named CNFZnO-300, CNFZnO-600, and ZnO-600 for the hybrids and the control sample, respectively.

2.4. Characterization The crystallinity of the samples was analyzed on a PANalytical X’Pert PRO X-ray diffractometer using Cu Kα radiation (0.154056 nm) at 40 kV and 30 mA. The data were collected in the range of 10 to 90° with a step size of 0.02°. The surface morphology of the samples was inspected by field emission scanning electron microscope (FE-SEM) (S-4200, Hitachi, Japan). The elemental composition of the hybrids was confirmed using an energy dispersive X-ray analysis (EDS) instrument attached to the FE-SEM. To improve the conductivity of the powdered samples during SEM measurements, an ultra-thin layer of platinum was sputter-coated on the sample surface (E-1030 Ion Sputter, Hitachi, Japan). The 6


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nanostructure of the powder was examined using a high-resolution transmission electron microscope (HRTEM) (Tecnai G2 F20 S-TWIN, USA) with a 200-kV field emission electron gun in Schottky mode. For HRTEM imaging, a small amount of powder was first dispersed in ethanol, sonicated for 5 min, drop-cast onto a commercially available carbon-coated copper grid, and dried under a visible lamp for 5 min. The TEM images were analyzed using commercial digital micrograph instrument software (version 1.82.366, Gatan Inc., USA). Thermogravimetric analysis (TGA) was recorded on a thermal analyzer (SDT-Q 600, TA instruments USA). The data were collected between RT and 700°C in N2 atmosphere. To obtain the bonding information, Fourier transform infrared (FTIR) spectra were recorded on a spectrophotometer (Model 5300, Jasco, USA) using KBr pellets in the wavenumber range of 4000 to 400 cm-1. The surface properties and chemical composition of the prepared samples were investigated by X-ray photoelectron spectroscopy (XPS) analysis on an X-ray photoelectron spectrometer (K-alpha, Thermo Scientific, USA). The samples were excited using monochromatic Al Kα X-ray radiation (1486.6 eV), and the data were recorded and processed using the commercial software Avantage (version 5.932, Thermo Scientific, USA). All experiments were performed with pass energies of 200 and 30 eV and step sizes of 1 eV and 0.1 eV for the survey and high-resolution spectra, respectively. Binding energies were measured using extrinsic carbon as an internal standard (C 1s = 284.8 eV). Each core level spectrum was first fitted with a Shirley-type background and then deconvoluted into various components using GL30 (a mixture of Gaussian (70%) and Lorentzian (30%)) in Avantage. The optical properties of the samples were measured through diffuse reflectance spectra (DRS) on a UV-Vis-NIR spectrophotometer (Jobin Varian Cary 5000, USA). The data were recorded between 800 and 200 nm using polytetrafluoroethylene (PTFE) as a reference. The topography of the microelectrodes was inspected using an atomic force microscope (AFM) 7



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(XE-series, Model: XE-100, Park systems, Korea) with a scanning probe microscopy (SPM) controller used in non-contact mode.

2.5. Microelectrode fabrication To fabricate a working electrode for electrochemical analysis, the active materials (fCNF/ZnO-600/CNFZnO-300/CNFZnO-600) were well dispersed in DMF along with PVDF and carbon black at a weight ratio of 80:10:10, respectively. The mixture was ultrasonicated for 30 min to drop-cast on Ni foam. Prior to use in all experiments, the 2.5 x 1 cm2 Ni foams were first ultrasonically cleaned with concentrated HCl solution (37 wt.%) to remove the surface layer, and then with DIW and absolute ethanol for 10 min each. They were then dried in a vacuum oven at 60°C for 3 h. Finally, about 5 mg of the mixture was coated onto the Ni foam over a working area of 1.5 x 1 cm2 and dried at 60°C for 6 h. All coated Ni foam electrodes were pressed at a pressure of 13 MPa to form a thin foil.

2.6. Electrochemical measurements Electrochemical

measurements

were

recorded

on

a

computer-controlled

electrochemical workstation (CHI 760 E, CH instruments, USA) with a conventional threeelectrode cell. The vacuum-annealed hybrid Ni foam electrode, a platinum wire, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. To evaluate the electrochemical performance of the CNFZnO hybrids, cyclic voltammetry and galvanostatic charge/discharge measurements were carried out using an aqueous electrolyte of 6 M KOH at RT. Electrochemical impedance spectroscopy (EIS) measurements were performed for all samples at open-circuit potential 8


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(OCP) between 0.1 and 100 kHz with an AC perturbation of 5 mV.

3. RESULTS AND DISCUSSION 3.1. XRD analysis Figure 1 shows a comparison of the XRD patterns of the synthesized hybrids with the standard XRD data of carbon and ZnO. For all hybrids and the functionalized CNF (f-CNF), there is a reflection around 26.31° (002) that can be indexed as graphitic carbon with hexagonal structure (Joint Committee on Powder Diffraction Standards - The International Centre for Diffraction Data (JCPDS-ICDD): 00-001-0640). This reflection originated from the graphitic layers at the surfaces of the CNF matrix. Similarly, the ZnO-600 and three hybrid samples reveal dominant reflections arising from the hexagonal wurtzite structure of zinc oxide (JCPDS-ICDD: 01-089-7102). The HRTEM analysis of CNFZnO-300/600 hybrids also shows the (002) planes at 26.31° (d = 0.34 nm) and 34.43° (d = 0.26 nm) for CNF and ZnO, respectively (Figure 3). The XRD pattern of CNFZnO-RT contains additional peaks indicated by * and # that probably arose from the formation of carbonates of zinc and sodium during synthesis of the hybrids. The impurities in the pattern were identified using X’Pert Highscore plus software and are shown in Figure S1 (Supporting Information). The figure shows that the percentage of impurities in this sample is very small, and these impurities are completely removed during vacuum annealing, as evidenced by the XRD patterns of the CNFZnO-300/600 samples. And it also verified by TGA (Figure 5). The X-ray line broadening method was applied with Scherrer’s equation26 to estimate the average crystallite size of the powders:

9



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D=

kλ β hkl cos θ

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(1)

where D is the crystallite size in nanometers, λ is the wavelength of the X-rays (0.154056 nm for Cu Kα radiation), k is the shape factor (0.9), θ is the peak position (°), and β hkl is the peak width at half-maximum intensity (radians). In general, the breadth of the Bragg peak is a combination of both instrumental and sample-dependent effects. Therefore, we recorded the diffraction pattern of a standard material (silicon) to determine the instrumental broadening (Figure S2 in the Supporting Information). The instrument-corrected broadening ( β hkl ) of the diffraction peaks of the hybrids was estimated using the following relation:27

β hkl = [(β 2 )measured − (β 2 )instrumental ]

1

2

(2)

The crystallite sizes were estimated using Eqs. (1) and (2) and are presented in Table 1. For the graphitic peak of the CNFs ((002) reflection at 26.31°), the average crystallite sizes of graphitic layers on the nanofiber surfaces of the hybrid samples were found to be around 14.3, 15.6, and 15.8 nm for CNFZnO-RT, CNFZnO-300, and CNFZnO-600, respectively. This indicates that under vacuum annealing, the graphitic nanocrystalline domains at the tube surfaces grow with increasing annealing temperature. Since the d-value of this peak remained at 0.34 nm for all samples, this increment of the crystallite size of the graphitic domains is manifested by the removal of some defects in the graphite layers.28 Similarly, for ZnO peaks in the hybrid samples, the average crystallite size determined from the dominant (101) peak reveals a decrease in full width at half-maxima. Hence, we observe an increase in the crystallite size with increasing annealing temperature (Table 1). This clearly indicates an improvement in the crystallinity of the samples with increasing annealing temperature. The average crystallite size of ZnO nanoparticles of the control sample (ZnO-600)

10


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was compared with that of hybrid samples, which revealed a nearly four-fold increase in the crystallite size of the control sample. This increase is due to the absence of a CNF matrix in the control sample, which allows the nanoparticles to agglomerate. The CNF matrix prevents particle agglomeration and maintains the nanostructure of the zinc oxide. Therefore, this type of carbon-nanostructure metal-oxide hybrid material should increase the effective surface area considerably for superior electrochemical and other interfacial applications, and hence, warrant considerable attention.

3.2. SEM analysis FE-SEM analysis was performed to identify the morphology and distribution of ZnO nanoparticles in the CNF matrix. Figure 2 and Figure S3 (Supporting Information) show FESEM images of all the samples. The f-CNF (Figure 2(a)) and b-CNF (Figure S3(a)) had similar morphology with hollow tubular structures aligned in random directions. Some of these are clearly seen with the graphitic layers on the surface, which is in line with the XRD measurements. ZnO nanoparticle formation at RT is indicated in the SEM images of the CNFZnO-RT sample (Figure 2(b); magnified version in Figure S3(b)). This is consistent with the XRD results. The SEM images of CNFZnO-RT suggest that the ZnO NPs were randomly attached to the walls of the CNFs to form the ZnO/CNF hybrids. Figures 2(c) and (d) show the SEM micrographs of the vacuum-annealed hybrids CNFZnO-300 and CNFZnO-600, respectively (magnified versions are in Figures S3(c) and (d)). Although they have similar morphologies to the CNFZnO-RT hybrid, the agglomeration increased with higher annealing temperature. Thermodynamically, a higher annealing temperature provides higher thermal energy to the system and increases the particle collision rate. These results from increasing the 11



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kinetic energy and the particle aggregation.29 Also, because of the random growth of clusters on the CNF matrix, the nanocluster size distribution becomes very wide. The morphology of the control ZnO-600 sample is shown in Figure S4(a) (Supporting Information). The nanoclusters are larger compared to the hybrid samples, which supports the XRD results. This again indicates that ZnO nanoparticle growth in the CNF matrix suppresses the particle agglomeration considerably, maintains the nanostructure of the hybrids, and provides a higher specific surface area for diverse interfacial applications.

3.3. TEM analysis The detailed nanostructures of the hybrid samples were further characterized using TEM. Figures 3(a) and (i) show low-magnification TEM images of the CNFZnO-300 and 600 hybrids, respectively. The hollow carbon nanofibers and the ZnO nanoparticles attached to the CNF walls are clearly seen in the figures. The CNFZnO-RT hybrid sample has a similar morphology (not shown here) with less particle agglomeration than the vacuumannealed samples. The particle size distributions are quite large for both samples, with an average particle/cluster size of approximately 60 and 50 nm for the CNFZnO-600 and CNFZnO-300 hybrids, respectively (although the exact mean size is not well defined). The attachment of ZnO NPs to the CNF walls is clearly shown in the HRTEM images in Figures 3(b) and (j) for the CNFZnO-300 and 600 hybrids. The inverse FFT patterns (Figures 3 (d, l) and (g, o)) reveal atomic planes with [002] growth directions of graphitic carbon and crystalline ZnO for CNFZnO-300/600. This indicates proper lattice spacing, as in the XRD studies. Also, the corresponding FFT images ((c, k) and (f, n)) and line intensity profiles ((e, m) and (h, p)) indicate proper phase formation of crystalline carbon and ZnO nanocrystal.

12


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3.4 EDS analysis EDS studies were carried out to identify the elemental composition of all the samples and the oxygen deficiency in the vacuum-annealed hybrids, as shown in Figures 4 (a), (d), (h), and (l). In general, EDS is a qualitative analysis tool and might not be sufficient for the quantification of elements. Nevertheless, we recorded the spectra in four different regions on each sample, and the average values are shown in the insets of the figures. The spectra confirmed an atomic variation of these synthesized CNFZnO hybrids that is close to the initial composition of the precursors described in experimental. The EDS spectrum of f-CNF is shown in Figure 4(a), and corresponding elemental mappings are shown in Figures 4(b) and (c). As expected, the spectrum reveals the presence of carbon and a small amount of oxygen resulting from functionalization. The CNFZnO-600 hybrid sample contains Zn and O along with elemental carbon (Figures 4 (l)–(o)). The only source of oxygen is considered to be ZnO nanocrystals because under high temperature vacuum annealing, the oxygencontaining functional groups attached to the CNFs are expected to be removed.30 Therefore, ZnO nanoparticles become oxygen deficient under high-temperature vacuum annealing. To support this, EDS analysis of the control ZnO-600 sample was performed. The data revealed a non-stoichiometric oxygen deficiency, indicating the formation of oxygen vacancies within the nanocrystals under high-temperature vacuum annealing (Figures S4(b)–(d), Supporting Information). For the CNFZnO-300 hybrid, the EDS analysis depicts a slight excess of oxygen compared to the stoichiometric value, as shown in Figures 4(h)–(k). This is mainly because of the partial removal of oxygen-containing functional groups at lower annealing temperature (300°C). Others have reported similar partial removal of oxygen-containing functional groups

13



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from a CNF surface under annealing at 300°C in an oxygen-deficient atmosphere.30 Therefore, these oxygen-containing functional groups also contribute to the EDS data to produce excess elemental oxygen over Zn. For the same reason, the CNFZnO-RT sample (which is not at vacuum annealed) also has excess oxygen, as shown in Figures 4(d)–(g). Although the EDS data do not convincingly indicate oxygen vacancies (especially for untreated and low-temperature annealed hybrid samples), the FTIR and XPS analyses (Figures 6 (a) and 7) indicate oxygen deficiency in the hybrid samples under vacuum annealing. Several groups have reported the formation of oxygen deficiencies within ZnO nanocrystals under high-temperature annealing, even in ambient atmosphere.31 This is because the formation energy of an oxygen vacancy is very low in ZnO.32 Hence, normally grown ZnO often becomes slightly oxygen deficient.33 In the present case, annealing at an elevated temperature (~300 °C) in an oxygen-deficient atmosphere leads to the desorption of lattice oxygen to the interstitial position to create an oxygen vacancy according to the following defect equilibrium:31 ∆

ZnO → VO + Zn Zn + 12 O2 1 2

O2 → Oint

  

(3)

where VO is the oxygen vacancy at the lattice site, ZnZn is the lattice zinc, and Oint is the interstitial oxygen. With further increase of the annealing temperature (600°C), the interstitial oxygen becomes unstable and diffuses off from the nanocrystal in the gaseous phase according to the following process: ∆

2Oint → O2 ( g )

(4)

Therefore, considerable oxygen vacancies can be induced in ZnO nanocrystals under high 14


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temperature vacuum annealing. 3.5. TGA analysis TGA analysis of the RT-synthesized hybrid (CNFZnO-RT) was measured further to quantify the content of CNF and ZnO in hybrid. Figure 5 shows the TGA thermograph of CNFZnO-RT from RT to 750°C in air and N2 atmospheres. In both the conditions, the sample had shown three stages of weight loss. The first two stages of weight losses were followed by a similar trend, measured in the air and N2 atmospheres. In the first stage, a slight weight loss (~ 1.2 wt.%) below 170°C, is assigned to the weight loss of moisture absorbed on the hybrid during the measurement of TGA. Second weight loss (around 2.2 wt.%) in the range of 170 to 250°C corresponds to the decomposition of impurities in the form of zinc/sodium carbonates present in the sample. These results indicated that the hybrid was formed above 250°C in the pure phase. These impurities were also identified by the XRD of CNFZnO-RT (Figure S1). Finally, beyond this region and up to 750°C, the gradual weight loss around 5.1 wt.% of the sample (under N2 atmosphere) was due to the desorption of –COOH groups that were attached to the surface of CNF during the functionalization process. On the other hand, in the same temperature region as stated above, the same sample (CNFZnO-RT) showed a major weight loss of about 15.1 wt.% when measured in air atmosphere, which is apparently due to the decomposition of CNF as well as –COOH groups.34,35 As all the CNF contents were decomposed from the hybrid at this temperature, the resulting residue (of CNFZnO-RT hybrid, measured in air condition) is considered as only zinc oxide (negligible content of trace elements in the CNF are not considered for comparison), which shows the ZnO content around 81.5 wt.% within the hybrid. Similarly, a relative comparison between TGA curves of CNFZnO-RT under N2 and air atmosphere (Figure 5) depicts the content of CNF around 10 wt.% within the hybrid. 15



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3.6. FTIR analysis FTIR measurements were carried out to substantiate the coordination surrounding the carbon and the effect of vacuum annealing on the ZnO/CNF hybrid. Figure 6 (a) presents the FTIR spectra of the pristine b-CNF/f-CNF, hybrid, and control ZnO samples. The peak absorption wavenumber and corresponding assignment of vibrations for all samples are tabulated in Table 2. For all samples, a peak near 3755 cm-1 and a broad band with a shoulder (~3690 cm-1) at about 3455 cm-1, which correspond to O-H stretching vibration of the surface-absorbed ambient water molecule and result from the chemical treatment during the functionalization process.

Vibrations for b-CNF and f-CNF: Pristine b-CNF and f-CNF were compared, which revealed a peak near 2910 cm-1 in f-CNF samples arising from symmetric C-H stretching vibration of an alkane group, indicating the stabilization of CNFs during functionalization. The small peak around 1645 cm-1 for both the b-CNF and f-CNF samples arises from asymmetric -C=O stretching vibrations of a carboxylic group (-COOH) along the side wall of the CNFs, and the intensity increases under functionalization. The peak near 1570 cm-1 for both the b- and f-CNFs is the graphene (C=C) peak. This peak indicates the presence of graphitic layers at the surfaces of CNFs, which also supports the XRD data. Similarly, the peak around 1120 cm-1 for both the b- and f-CNFs corresponds to the C-O stretching vibration. The small peak at 670 cm-1 for the b-CNF (which is absent in the f-CNF) is related to -C-S stretching vibration. It indicates the presence of some surface impurity sulfur in the as-received CNF samples, which is subsequently removed completely via the efficient functionalization procedure. Similarly, a small hump near 460 cm-1 for the b-CNF is identified as the metal bonding (Fe-O) of catalyst particles, which are removed by the 16


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oxidation treatment of b-CNF.36-39 The presence of –OH and –COOH groups on f-CNFs due to functionalization was clearly identified.

Vibrations for hybrids and control sample: Regarding the bonding information of ZnO in the hybrid (CNFZnO-RT/300/600) and control (ZnO-600) samples, the characteristic peaks at 1640 and 1420 cm-1 represent the asymmetric and symmetric stretching vibrations of zinc carboxylate. Similarly, a small peak at around 1570 cm-1 is due to the symmetric stretching vibration of a carbonyl (-C=O) bond of the carboxylate group. The presence of a carboxylate (-COO-) group is due to the use of a carbon-containing precursor during ZnO preparation and functionalization of the CNFs. Therefore, these FTIR-identified impurities are basically surface states present at the zinc oxide and/or CNF walls. The characteristic band of wurtzite ZnO between 600 and 400 cm-1 is the strongest for the control ZnO (ZnO-600). This is mainly because of the much higher crystallite size of the control sample against the hybrids (Table 1), which leads to more Zn-O bonds. The peak at 435 cm-1 is the Zn-O stretching vibration of hexagonal ZnO, while the peak at 500 cm-1 is the oxygen deficiency-related defect complex. Both of these peaks are related to the oxygen non-stoichiometry of the samples. The peak at 435 cm-1 is related to the oxygen sublattice vibration and hence is sensitive to the sublattice disorder in terms of non-stoichiometric oxygen vacancy in lattice sites. Therefore, an imposed oxygen vacancy within ZnO under vacuum annealing should decrease this peak intensity. A closer look clearly shows a weakening of this peak between the CNFZnO-RT/300 and CNFZnO-600 hybrids, indicating an increase in oxygen vacancies in the ZnO nanoparticles with increased vacuum annealing. On the other hand, the oxygen deficiency-related defect complex peak at 500 cm-1 is supposed to intensify with increasing oxygen deficiency. The corresponding peak intensity increases between the CNFZnO-RT and 17



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CNFZnO-300 samples, indicating the creation of oxygen deficiency in the ZnO nanoparticles under vacuum annealing. For the CNFZnO-600 sample, this peak intensity did not increase much in the expected direction, although the annealing temperature is higher than for the other two hybrids. This is probably due to the migration of some surface coordinated oxygen atoms at the CNF surface in the ZnO interstitial positions at higher temperature to compensate some of the oxygen deficiencies.40-42 The FTIR analyses qualitatively indicate the oxygen deficiencies/vacancies in the vacuum-annealed metal oxide nanoparticles, which is also supported by EDS and XPS measurements.

3.7. XPS analysis The hybrids (CNFZnO-RT/300/600) and control ZnO-600 samples were subjected to XPS analysis to determine the oxidative state and quantify the oxygen deficiency under heat treatment in a vacuum. The wide scans of the hybrids (CNFZnO-RT/300/600) and control ZnO-600 were recorded in the range of 1350 to 0 eV and are shown in Figures 7(a)–(d). C, O, and Zn are observed in the figure for the hybrid and control samples. However, the survey scan of CNFZnO-RT indicates a small amount of sodium impurity due to the carbonates. The high-resolution spectrum of Na 1s (CNFZnO-RT) is shown for reference in Figure S5 of the Supporting Information. This peak disappeared for CNFZnO-300/600, supporting the XRD and TGA results. Since the impurity did not appear at higher temperatures, the sodium content is neglected in the present case for comparison of the oxygen deficiency in the hybrids (CNFZnO-RT/300/600). Figures 7(e)–(o) show the high-resolution spectra of C 1s, O 1s, and Zn 2p for the hybrids (CNFZnO-RT/300/600) and control ZnO (ZnO-600), which were used to quantify the oxygen deficiency of the vacuum-annealed samples.

18


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C 1s spectra of hybrids: The asymmetrical spectra of C 1s were deconvoluted into various components using Thermo Scientific Avantage software to quantify the sp2/sp3 ratio and approximate the graphite concentration on the CNF surface resulting from vacuum annealing. The fitted C 1s spectra of the hybrids (CNFZnO-RT/300/600) are shown in Figures 7(e)–(g), and the estimated elemental percentages are presented in Table 3(a). The deconvoluted C 1s peak was fit to four contributions (C1, C2, C3, and C4) for all hybrids (CNFZnORT/300/600). Peaks C1 (284.35/284.65/284.82 eV) and C2 (285.29/285.28/285.48 eV) were assigned to the sp2 and sp3 hybridized graphite-like carbon on the walls of the CNF, respectively.43 sp2-Hybridized carbon is a good indicator of better graphitization and fewer defects in the lattice structure of a carbonaceous material.44 As the annealing temperature increases from RT to 600°C, the relative amounts of sp2 hybridized carbon increases (the narrowing of the C1 peak can be seen in Figures 7(e)–(g)) and is the highest for the CNFZnO-600 hybrid. This indicates that there are fewer defect sites on the graphitized carbon at higher vacuum-annealing temperature. The other peak observed at C3 (286.28/285.98/286.42 eV for CNFZnO-RT/300/600, respectively) might arise for several reasons. It might correspond to the carbon in the phenolic/alcoholic groups or (more likely) to the formation of a -C-O-Zn bond between carbon in the CNF and Zn in the ZnO nanoparticles.25,45 The ZnO NPs attached to the walls of the CNF are also shown in the HRTEM images of CNFZnO-300/600. The absence of a peak at ~282 eV indicates that there is no direct bonding between C and Zn (C-Zn) in the hybrids.46 Lastly, the C4 peak (290.37/289.82/290.90 eV) is ascribed to either the carbonyl group or a π-π* shake up satellite structure that is characteristic of conjugated systems.29,43 This peak seems to decrease as a function of annealing temperature, which was supported by the FTIR data (Figure 6(a)). At a higher annealing temperature in an oxygen-deficient atmosphere, the carbonyl groups 19



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disappear due to the increased crystallinity of the carbon materials. The decreased C4 peak intensity is shown in Figures 7(e)–(g).

O 1s spectrum of control ZnO-600: The core level spectra of the O 1s peak were deconvoluted to better understand the variation of oxygen vacancies in the hybrids and control ZnO under annealing in an oxygen-deficient atmosphere. The deconvoluted O 1s peak of the hybrids and control ZnO are shown in Figures 7(h)–(k). The asymmetric O 1s peak of the O1, O2, and O3 sub-spectra components were fitted for all samples, and the peak positions are listed in Table 3(b). This demonstrates that there is more than one chemical state in the samples. A low-binding-energy O1 peak of the control ZnO-600 is centered at about 530.16 eV (Figure 7(k)). This peak corresponds to the characteristic Zn2+ ion of the metal oxide in the hexagonal wurtzite structure of ZnO. The O2 peak at 531.48 eV is attributed to the O2- in the oxygen-deficient regions within the matrix of ZnO.47 The O3 peak at higher binding energy of 532.48 eV is usually assigned to the surface hydroxyl groups (O-H) adsorbed on the sample surfaces due to atmospheric contamination during sample handling.48

O 1s spectra of hybrids: Similar bands are observed for the CNFZnO-RT/300/600 hybrids (Figures 7(h)–(j)). However, the binding energy shift of the O1 peak (530.86/531.07/531.25 eV) is higher than that of the control ZnO-600 (530.16 eV), which results from the formation of zinc oxide NPs in the presence of the CNF matrix. This is due to the bonding between carbon (C) and zinc (Zn) through oxygen (O) in the ZnO/CNF composites/hybrids.46,48,49 Nayeri49 et al. reported the bonding between C and Zn via O in zinc oxide nanowire/multiwalled carbon nanotube heterojunction arrays using XPS analysis at 531.1 eV (C-O). They also reported that O atoms play an intermediate role for bonding between ZnO20


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NWs and CNTs. A -C-O-Zn bond between ZnO NPs and the CNF was observed at 530.86, 531.07, and 531.25 eV for the CNFZnO-RT, 300, and 600 hybrids, respectively. Notably, these results suggest that the interaction between the ZnO NPs and the CNF is stronger in the present hybrids, which leads to good interfacial applications. An O2 component at medium binding energy is located at about 531.53 and 531.81 eV for CNFZnO-300 and 600, respectively. This component is associated with O2− in the oxygen-deficient regions within the ZnO matrix. The intensity of this peak is related to the variations in the concentration of oxygen vacancies. Therefore, changes in the intensity of this component can be connected in part to the variations in the concentration of oxygen vacancies.50 In the O1s spectra, the O2 peak is obviously stronger in the annealed hybrids (CNFZnO-300/600) and control ZnO-600 sample and it gradually decreases with decreasing annealing temperature. This indicates that more oxygen vacancies are created at a higher annealing temperature, which is consistent with the FTIR and EDS results. The ratios of the O2 to O1 peak areas are about 0.51 and 1.27 for the CNFZnO-600 and control ZnO-600 samples, respectively. This suggests that more oxygen vacancies are created in the control ZnO-600 samples than in the hybrid CNFZnO600 samples. This is mainly because some of the CNF surface-coordinated oxygen functional groups might have migrated to the ZnO lattice sites at the elevated temperature to compensate for oxygen vacancies (as shown in the FTIR and EDS analyses). The O3 peaks at around 532.16, 532.70, and 532.72 eV are attributed to the surface hydroxyl groups (O-H) and/or the oxygen making ring bonds carbon (-C-O) in the hybrids (CNFZnO-RT/300/600). Additionally, in the CNFZnO-RT, an O4 peak is observed at about 533.18 eV due to the presence of loosely bound oxygen on the surface of ZnO or –C-O from chemisorbed water molecules. This band is not observed in the CNFZnO-300/600 and control ZnO-60047, apparently because of the high-temperature annealing. 21



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Zn 2p spectra of hybrids and control sample: The core level spectra of Zn 2p (Figures 7 (l)–(o)) show two sharp peaks fitted at about 1022.24/1022.56/1022.67/1021.44 eV and 1045.34/1045.66/1045.77/1044.54 eV for CNFZnO-RT/300/600/ZnO-600, respectively. The former group corresponds to the Zn 2p3/2 state, whereas the latter group belongs to the Zn 2p1/2 state, indicating Zn2+ in the normal state of the ZnO/CNFs hybrid. The peak positions and their corresponding atomic percentages are presented in Table 3 (c). For all samples, the binding energy difference between the spin-orbit interaction of two transitions (Zn 2p3/2 and Zn 2p1/2) was estimated to be ~ 23.1 eV, which is consistent with previous reports.46,49 The atomic ratios of O to Zn (O/Zn) were estimated to be 1.31, 0.84, 0.63, and 0.92 for CNFZnORT, 300, 600, and ZnO-600, respectively. The O/Zn ratio decreases in the hybrids due to the increased annealing temperature in a vacuum, confirming that the non-stoichiometric oxygen deficiency increases as a function of the vacuum annealing temperature. Similarly, the control ZnO-600 sample shows considerable oxygen deficiency, which appears to result from the high-temperature vacuum annealing. These results further confirm that more oxygen vacancies are created in the CNFZnO-600 hybrid under the influence of heat treatment in an oxygen-reduced atmosphere. Therefore, we suggest that the favorable interaction between C and Zn via O (-C-O-Zn) and oxygen non-stoichiometry within ZnO contributes significantly to the enhanced electrochemical activity of the hybrid nanostructures.

3.8. Diffuse reflectance spectra (DRS) UV-Vis DRS was used to evaluate the effect of temperature treatment under a vacuum on the fundamental energy gap of zinc oxide nanoparticles in ZnO/CNF hybrids, as shown in Figure 6 (b). The figure shows that the f-CNF has very low reflectance in the entire 22


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range of 200 to 800 nm, which appears to result from the high absorbance in the carbonaceous material. Similarly, CNFZnO hybrids also show low reflectance (< 20%) because of the high percentage of CNF. As expected, the control ZnO sample (ZnO-600) reveals considerable reflectance (~70 to 80%) in the visible part of the incident radiation, which is why ZnO is widely used (mostly in thin film form) as a transparent conducting oxide (TCO) in the electronics industry.51 A dip at 380 nm and below for both the control and hybrid samples corresponds to the fundamental absorption edge of wide-bandgap ZnO nanoparticles, indicating TCO properties. It is noted from the DRS that the fundamental absorption edges of the hybrid samples are shifted toward higher wavelength (i.e., the lower energy side) with increasing annealing temperature. This indicates that vacuum annealing has a considerable influence on the optical bandgap of the ZnO nanoparticles. To support this, the bandgap of the nanoparticles were determined using Kubelka-Munk (K-M) theory and the following equation:52 K/S = (1-R∞)2/2R∞ = F(R∞) ∝ α αhν = (hν - Eg)1/2

(5)

where K and S are the absorption and scattering coefficients, respectively, R∞ denotes the diffuse reflectance of an infinitely thick sample, F(R∞) is the K-M function, α is the linear absorption coefficient of the material, hν is the photon energy, and Eg is the bandgap energy for direct transition. The bandgap energies of all samples were estimated from the variation of the K-M function [F(R∞)hν]2 with photon energy (hν), which is represented in inset (i) of Figure 6(b). Temperature-dependent energy gap values are shown in inset (ii) of Figure 6(b) and Table 1. The energy gap values decrease with increasing annealing temperature. Several factors can affect the bandgap of ZnO (and similar semiconductor nanocrystals in general), such as 23



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size,53 strain,54 doping,55 amorphous domain in the crystalline phase,56 and oxygen vacancy.47,51,57 In the current case, although the ZnO nanoparticles/clusters agglomerate more with higher annealing temperature, the size distribution is very wide, and the approximate average particle size (50 to 60 nm; see the SEM and TEM images) is much higher than the excitonic Bohr radius (~2 nm) of ZnO.58 Thus, according to the confinement models, the size regime of the as-synthesized ZnO nanoparticles is beyond the weak-to-strong quantum confinement region of ZnO nanocrystals for effective bandgap enhancement.59,60 Regarding the bandgap modification due to the presence of an amorphous domain in the crystalline phase,56 our annealed samples reveal very high crystallinity (see the XRD and TEM images) with a negligible amorphous domain within the ZnO nanoparticles. Hence, its effect on the observed red shift of the ZnO bandgap against the increasing vacuum annealing temperature is discarded. The effect of strain on the bandgap modification54 is also discarded in the current case because the XRD data reveals negligible strain broadening of the respective ZnO peaks. An increase in the average particle size with higher annealing temperature is observed in the TEM images, which generally releases strain in bigger nanoparticles.61 However, the corresponding peak broadening effect is beyond the detectable range and hence neglected. Therefore, we suggest that the observed bandgap decrement of the vacuumannealed ZnO nanocrystals is due to the oxygen vacancy induced within the samples under elevated-temperature annealing in an oxygen-deficient atmosphere, as evidenced by EDS, XPS, and FTIR analyses. Physically, there are two reasons for the gradual red shift of the ZnO bandgap with increasing oxygen vacancies in the vacuum-annealed hybrid samples. First, as the density of the oxygen vacancies increases with the vacuum-annealing temperature, the defect states near the valence band edge become delocalized. Thus, they 24


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overlap with the valence band edge to shift the top of the valence band to the forbidden gap,51,47 leading to the decrease in the bandgap. Second, it is well known that a nonstoichiometric oxygen deficiency (or metal excess) induces n-type conductivity, whereas oxygen excess (or metal deficiency) induces p-type conductivity in binary, ternary, and tertiary-type TCOs.62 Therefore, as the concentration of oxygen vacancies increases gradually with the vacuum annealing temperature, the electron concentration in the conduction band increases considerably. This pushes the conduction band edge into the bandgap, which becomes narrower as a result.55

3.9. Electrochemical analysis In general, it is considered that heat treatment in an oxygen-deficient atmosphere will reduce structural defects of CNF and ZnO and improve the electrical transport properties of ZnO, which can greatly affect the electrochemical properties. Therefore, we investigated the cyclic voltammetry and galvanostatic charge/discharge curves of vacuum-annealed hybrids (CNFZnO-300/600) and control samples (f-CNF and ZnO-600) using 6 M KOH as an electrolyte. The cyclic voltammetry of electrodes was recorded at various scan rates ranging from -0.45 to 0.05 V, as shown in Figures S6 (a)–(d) (Supporting Information). All electrodes showed good oxidation and reduction peaks. In all electrodes, the increase in the scan rate shifted the anodic peak potential toward +ve potential, while the cathodic peak potential shifted toward –ve potential due to the enhancement of their respective peak currents. In order to illustrate the enhancement of electrochemical properties of the ZnO/CNF hybrid electrodes, CV curves were recorded for the f-CNF, ZnO-600, CNFZnO-300 and 600 hybrids at 5 mV s-1, as shown in Figure 8 (a). The electrodes coated with ZnO/CNF hybrids exhibited better cyclic voltammetry with large cathodic/anodic peaks compared to the control samples 25



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(f-CNF and ZnO-600). However, the lack of symmetry in the curves of the hybrid electrodes is probably due to the combination of a double layer (CNF) and pseudocapacitances (ZnO) contributing to the total capacitance. Moreover, the CNFZnO-600 hybrid had the highest cathodic/anodic reduction, indicating improved electrochemical performance corresponding to the highest maximum oxygen vacancy/deficiency (as revealed by the XPS, FTIR, EDS, and DRS analyses). Due to the increase in annealing temperature in the hybrids, the capacitive envelope of the CV curves also increased from the CNFZnO-300 to the 600 electrode due to the increase in oxygen vacancies at high temperatures. This could improve the oxygen diffusion rates due to decreased transport lengths and larger potential gradients across the ZnO particles.1 In general, the faradic capacitance (CF) of pseudocapacitors can be determined from the relation between the charge (∆q) stored in the capacitor to the change in potential (∆V):

CF =

∂ (∆q ) ∂ (∆V )

(6)

Experimentally, the specific capacitances can be estimated from the cyclic voltammetry and galvanostatic charge/discharge curves. The specific capacitance (C, in F g-1) derived from CV data is calculated according to the following equation:63 C=

1 I (v )dv 2 sm(v h − vl ) ∫

(7)

where s is the scan rate (V s-1), vh and vl are the high and low potential limits of the CV curves (V), respectively, I is the instantaneous current (A), v is the applied voltage (V), and m is the mass of the active material (g).

Several research groups have been trying to improve the specific capacitance of ZnO using additives such as graphene,18,35 and CNTs/CNFs.13,19 For instance, Lu18 et al. reported the specific capacitance of graphene-ZnO as 146 F g-1 at 2 mV s-1, whereas Li35 et al. 26


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obtained a C value of 156 F g-1 for a graphene-ZnO electrode at a scan rate of 5 mV s-1. There is a scarcity of specific capacitance values reported for ZnO-CNT/CNF hybrid nanomaterials. Recently, Aravinda et al.64 reported the specific capacitance of a ZnO/CNT nanocomposite near 60 F g-1 at a scan rate of 5 mV s-1. In the present study, the specific capacitances of fCNF, ZnO-600, CNFZnO-300 and CNFZnO-600 were calculated at a scan rate of 5 mV s-1 as 126.12, 141.63, 221.70 and 288.40 F g-1, respectively. The improved specific capacitance of the hybrid was compared to previous reports, which revealed a two-fold increase. This improvement is due to the capacitance arising from the synergistic effect of the double layer behavior of highly conductive CNFs and the pseudocapacitance of ZnO in the vacuumannealed hybrid (CNFZnO-600). The variations of specific capacitance C (F g-1) versus the scan rate of electrodes are shown in Figure 8 (c). Notably, the CNFZnO-600 hybrid microelectrode yielded the highest capacitance. The capacitance decreased with an increase in scan rate from 5 to 250 mV s-1. This might be due to the insufficient time available for ion diffusion and adsorption of electrodes at the higher scan rates. The charge/discharge (CD) curves were recorded to evaluate the effect of oxygen vacancies on the electrochemical properties of hybrids. Figures S6 (e)–(h) show the CD curves of fabricated microelectrodes at various current densities in the voltage range of -0.5 to -0.05 V. The curves indicate the existence of an electrochemical double-layer capacitance (EDLC) and pseudocapacitance process, consistent with the CV curves. The comparison of CD curves of f-CNF, ZnO-600, CNFZnO-300, and CNFZnO-600 at 0.1 A g-1 is shown in Figure 8 (b). It is clear that the charge and discharge time of the CNFZnO-600 hybrid is greater than that of other electrodes. Considering these results, the enhanced performance of the CNFZnO-600 hybrid might be explained as follows. (i) Due to the vacuum annealing, the defects in the carbon nanofibers for redox reactions and efficiency of electrolyte diffusion 27



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will decrease, which leads to rate performance enhancements. (ii) At higher oxygen deficiencies, the electrical conductivity of ZnO increases greatly and can improve the carrier transport for a better charge/discharge process.65 We measured the electrical properties of the CNFZnO-300 and 600 hybrids and control ZnO-600 microelectrodes by a standard two-probe method under ambient conditions (not shown here). Our preliminary studies showed that the RT conductivity values of the CNFZnO-300, CNFZnO-600, and control ZnO-600 were 59.27, 84.61, and 51.37 S m-1, respectively. Therefore, the conductivity increases in the following order: σZnO-600 < σCNFZnO-300 < σCNFZnO-600, thus supporting the preceding argument. The specific capacitance was also derived using the galvanostatic CD curves with the following relation: C=

I d ∆t (Fg −1 ) m∆ V

(8)

where I d is the discharge current (A), ∆t is the discharge time after the initial voltage drop (IR drop, VIR) (s), and ∆V is the potential range of a discharge (V).66 The specific capacitance of electrodes are shown with current density values in Figure 8 (d). The discharge capacitance decreases monotonically with increased current density, which might be due to the low penetration of ions in the entrances of the nanopores due to fast potential changes. At a current density of 0.1 A g-1, the estimated specific capacitances of the f-CNF, ZnO-600, CNFZnO-300, and CNFZnO-600 are 61.22, 100.91, 110.94, and 126.54 F g-1, respectively. Our results suggest that the hybrid sample (CNFZnO-600) have higher specific capacitance than the control samples (f-CNF and ZnO-600). Practically, the metal oxides (such as ZnO) give higher specific capacitance compared to the carbon materials (CNFs/CNTs) due to the faradic process. In our case also similar trend is obtained for f-CNF and ZnO-600, i.e. the ZnO-600 has higher value of specific capacitance than the f-CNF. For the case of hybrid, the 28


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total specific capacitance is the synergistic effect of the electric double layer capacitance (in CNF) and faradic capacitance (in ZnO). Our aim is to improve non-faradic (EDLC) and faradic (PC) processes for the enhancement of total capacitance of hybrid nanostructure by post-synthesis heat-treatment in vacuum. Therefore, we have demonstrated that the effect of annealing at oxygen deficient atmosphere improved both processes for the total capacitance of hybrids in the present case. Also, as the specific capacitance of f-CNF is very low compared to the other electrodes, the effect of f-CNF on the hybrid electrode is negligible and hence, is ignored for further comparison with the ZnO contained electrodes (CNFZnO-300, 600 and ZnO-600). The CNFZnO-600 hybrid electrode shows the highest capacitances, indicating the highest electrochemical reaction activity. This can be attributed to the high conductivity and better interfacial contact of the hybrid, which facilitates fast ion and charge transport for redox reactions.67 The specific capacitance of the CNFZnO-300 hybrid does not differ too much from the control ZnO-600 sample, as expected. This is mainly due to two competing factors. First, the CNFZnO-300 showed higher conductivity than the control ZnO-600 for a better ion transfer process. Second, AFM analysis depicts a higher root mean square (RMS) surface roughness (Rq) for the control ZnO-600 than the CNFZnO-300 sample, which leads to more active surfaces in the control sample for better charge/discharge sites. For example, Rq values for CNFZnO-300 and ZnO-600 were obtained as ~0.2 and 0.3 µm, respectively (Table S1, Supporting Information). This is because the control ZnO-600 microelectrode showed a wide variation in grain size (Figure S4 (a)), whereas CNFZnO-300 has a smaller cluster formation around the CNFs (Figure 2(c)). Therefore, the higher conductivity in CNFZnO-300 is compensated by less surface roughness and fewer active surface sites, resulting in comparable specific capacitance to that of the control ZnO-600 microelectrodes. Lu et al. 68 29



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reported similar results for TiO2 nanotube arrays. They enhanced the pseudocapacitance of hydrogenated TiO2 nanotubes by inducing oxygen vacancies without adding carbon materials. We obtained significantly enhanced specific capacitance of the CNFZnO-600 hybrid compared with the CNFZnO-300 hybrid and the control ZnO-600 microelectrodes. This is because of the higher conductivity of the CNFZnO-600 hybrid microelectrode and better RMS surface roughness (Rq = 0.45 µm, Supporting Information, Table S1) compared to CNFZnO-300 (Rq = 0.2 µm) and the control ZnO-600 (Rq = 0.3 µm). This results in better charge transport and charge/discharge sites for enhanced electrochemical properties. The cycle performance of the microelectrode was also recorded for up to 2000 cycles. The inset of Figure 8(e) illustrates the capacity retention of the CNFZnO-600 hybrid with the number of cycles at a current density of 2 A g-1. After 2000 cycles, the hybrid electrode retained 84.77% of original the specific capacitance, which clearly indicates the electrode is stable. Usually, a small IR drop occurs at the beginning of the discharge process due to the Ohmic internal resistance in the electrode Rs (Ω):35 Rs =

∆V IR 2I d

(9)

where ∆V IR is the magnitude of the voltage drop (V) and I d is the applied current density (A g-1). The factor of 2 is associated with the instant switching of the current density during transition from charge to discharge. At a current density of 3 A g-1, the internal resistances are 6.3, 5.1, and 3.9 Ω for ZnO-600, CNFZnO-300, and CNFZnO-600, respectively. A similar trend is observed in the ESR values calculated from the AC impedance analysis (Figure 9). The electrochemical performance of an electrode can be analyzed using the Ragone plot of the energy and power density, which were estimated from the following equations 30


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using galvanostatic CD curves:19 E=

C (∆V ) (Wh kg-1) 2

P=

E (W kg-1) ∆t × Rs

2

and

(10)

where E and P are the energy density (Wh kg-1) and power density (W kg-1), respectively, C (F g-1) is the specific capacitance derived from CD curves, ∆V = Vmax − V IR (in V), and Rs =

∆V IR (Ω) is the series resistance. The Ragone plot is shown in Figure 8 (e). At a 2I d

current density of 3 A g-1, the energy densities are 48.01 and 119.85 Wh kg-1 for ZnO-600 and CNFZnO-600, respectively, while the power densities are 17.687 and 19.225 kW kg-1. Recently, Chang and Kim13 reported the supercapacitor performance of ZnO/activated CNF electrodes, fabricated by electrospinning method, followed by thermal annealing. According to this study, the maximum specific capacitance and energy density were 178.2 F g-1 and 22.7 Wh kg-1 at a power density of 400 W kg-1. And the specific capacitance retained by 75 % after 1000 cycles at 1 mA cm-2. The enhanced electrochemical properties was explained in terms of the combinatorial effect of high surface area of CNF and large specific capacity of ZnO to manifest a synergistic effect of double layer capacitance of CNF and faradic capacitance of ZnO. In the present case, the hybrid (ZnO/CNF) was synthesized by a coprecipitation method and thereafter heat-treated the RT-synthesized hybrid at 600°C in vacuum to induce oxygen vacancy, which improves their charge transport, ion diffusion, rate capability and cyclic stability for redox reactions. The hybrid electrode (CNFZnO-600) retained its initial capacitance value of 84.77% after 2000 cycles at current density of 2 A g-1. These results show that the addition of CNF and induced oxygen non-stoichiometry in the ZnO significantly improve the capacitive performance of the ZnO/CNF hybrid electrode. 31



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Potentiostatic electrochemical impedance spectroscopy (EIS) measurements were carried out to better understand the performance of the ZnO/CNF hybrid. The Nyquist plots of the supercapacitor electrodes of ZnO-600 and CNFZnO-300/600 are shown in Figure 9 (a). As expected, all the electrodes show supercapacitor behavior in the low frequency region, which can be seen in the imaginary part that increased sharply towards the vertical direction. This indicates pure capacitive behavior of the electrodes. At higher frequency, the electrodes show semi-circular nature due to the Warburg resistance14,69,70 (Figure 9(c)). As seen from the curves, the diameter of the semi-circles decreases for the electrodes in the following order: CNFZnO-600 CNFZnO-600. The CNFZnO-600 hybrid electrode had the lowest ESR because the CNF reduces the aggregation of ZnO particles during the formation of the hybrid, and increases the contact area, and reduces the electrical resistance. Second, the conductivity of ZnO increases due to the effect of high-temperature vacuum annealing, and the defects in the grain boundary of the ZnO NPs and CNFs are minimized compared to the other samples. These provide a highly conductive path to the electrodes that reduces the electrical resistance of the CNFZnO-600 hybrid sample. As shown in the table, the CPE values decrease in the following order: CNFZnO-600 > CNFZnO-300 > ZnO-600. Therefore, we have successfully demonstrated that adding carbon materials with non-stoichiometric oxygen deficiency in metal oxides could affect the electrochemical properties greatly.

32


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4. CONCLUSION ZnO/CNF hybrid was prepared by a chemical precipitation method. Oxygen vacancies were induced by annealing in a vacuum to enhance the electrochemical properties of the hybrid. The induced oxygen vacancies and good interfacial contact in the hybrids remarkably enhanced the charge transport during cycling and provided a large reaction surface area to improve the specific capacitance compared to the control samples. At a current density of 0.1 A g-1, the specific capacitance and energy density of ZnO600/CNFZnO-600 were 100.91/126.54 F g-1 and 120.61/171.11 Wh kg-1, respectively. Therefore, we suggest that the improved electrochemical properties of the CNFZnO hybrid microelectrodes are due to a combined effect of several physicochemical properties: (i) A synergistic effect between double-layer capacitance of CNF and pseudocapacitance of ZnO. (ii) The high conductivity of graphitic CNF improves the charge transport. (iii) Hightemperature annealing provides defect-free interfacial contacts between CNF and highly crystalline ZnO NPs. (iv) Nanostructured surfaces of CNF and ZnO NPs produce a high active surface area for more reaction sites. (v) Vacuum annealing induces oxygen vacancies in ZnO to improve the oxygen diffusion rate and carrier concentration to better charge transport properties of the hybrid.

Supporting Information Functionalization of CNFs, synthesis of hybrid, impurity phase identification of RTsynthesized hybrid by X’Pert Highscore plus, XRD of standard Si, high magnification FESEM images of all samples, EDS and elemental mapping of ZnO-600, high-resolution spectrum of Na 1s, cyclic voltammetry and charge/discharge curves of all electrodes, surface roughness values of ZnO-600, CNFZnO-300 and 600 from AFM. 33



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ACKNOWLEDGEMENTS This work was funded by the grant NRF-2015002423 of the National Research Foundation of Korea.

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Nanotube Composite Electrode for Asymmetric Supercapacitors of High Energy Density. J. Power. Sources 2013, 241, 423-428.

Table captions Table 1. Variation of crystallite size (D) and energy gap (Eg) as a function of annealing temperature of synthesized samples

Table 2. Assignments of FTIR spectra of the synthesized samples Table 3. Atomic percentages of fitted high-resolution spectra of the C 1s (a), O 1s (b), and Zn 2p (c) of CNFZnO-RT/300/600 and control ZnO-600 samples

Table 4. Fitted parameters of AC impedance spectra of the fabricated electrodes

Figure captions Figure 1. Comparison of X-ray diffraction patterns of CNFZnO hybrids to JCPDS of bare ZnO and carbon.

Figure 2. FESEM images of synthesized samples: (a) functionalized CNFs, (b)–(d) CNFZnO-RT/300/600 hybrids, respectively.

Figure 3. (a) and (i) Low magnification TEM images, (b) and (j) HRTEM images of ZnO NPs attached to the walls of the CNF, corresponding FFT patterns of the CNF ((c) and (k)), and ZnO ((f) and (n)), inverse FFT patterns representing the crystalline carbon C (002) ((d) and (l)) and ZnO (002) ((g) and (o)) planes and line profiles ((e, m) and (h, p)) of CNFZnO300/600, respectively.

Figure 4. EDS spectra of f-CNF (a) and hybrids (CNFZnO-RT (d), 300 (h), 600 (l)) and their corresponding elemental mapping of carbon (b, e, i, and m), oxygen (c, f, j, and n) and zinc (g, k, and o), respectively. Inset shows the qualitative elemental percentage of samples. 42


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Figure 5. TGA profiles of CNFZnO-RT in air (black) and N2 (red) conditions. Figure 6. FTIR spectra (a) and DRS (b) of synthesized samples. Insets of (b) show the plot of [F(Rα)hν]2 versus photon energy (i) and the variation of the energy gap with annealing temperature (ii).

Figure 7. Survey scans (a)–(d) and high-resolution spectra of C 1s (e)–(g), O 1s (h)–(k), and Zn 2p (l)–(o) for hybrids (CNFZnO-RT/300/600) and control ZnO-600, respectively.

Figure 8. Comparison of cyclic voltammetry (a) at 5 mV s-1 and charge/discharge curves (b) at 0.1 A g-1, specific capacitance as a function of scan rate (c) and current density (d), Ragone plot (e) of fabricated electrodes, and capacity retention of CNFZnO-600 with number of cycles (inset of (e)) at 2 A g-1 current density.

Figure 9. (a) Nyquist plots of fabricated electrodes. Inset (b) shows the electrical equivalent circuit used for fitting AC impedance spectra and (c) shows the magnified view of impedance spectra at high frequency.

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Table 1. Sample D (nm) Eg (eV) CNFZnO-RT 18 3.282 CNFZnO-300 19 3.231 CNFZnO-600 21 3.165 ZnO-600 74 3.241

Table 2. Absorption wavenumber (cm-1) Assignment b-CNF f-CNF CNFZnO-RT CNFZnO-300 CNFZnO-600 ZnO-600 3755 3756 3750 3748 3750 3752 -OH 3692 3687 3685 3687 3689 3688 -OH 3445 3455 3440 3474 3460 3447 -OH 2917 2930 2925 2921 2931 2932 -CH/-CH2/-CH3 2364 2376 2375 2361 2376 2375 -C=NH/C=C 1656 1644 1636 1636 1643 1633 -C=O/Skeletel C-C 1565 1572 1562 1575 1573 1568 -C=O/Skeletel C-C -CH3/-CH2 1471 1467 1486 1473 1474 1476 -CO2 1399 1388 1404 1406 1396 1421 -C-O/C-O-C 1120 1121 1121 1118 1124 1119 -CH 1018 1034 1032 1031 1028 -C-S 670 O2- deficiency 493 506 494 458 482 431 433 434 M-O (M=Fe/Zn)

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Table 3. (a) Peak binding energy (eV)/ Relative atomic concentrations (%) Carbon C 1s Sample C1 C2 C3 C4 -C-O sp2-C at.% sp3-C at.% at.% O-C=O at.% (or -C-O-Zn) CNFZnO-RT 284.35 8.52 285.29 2.39 286.28 7.75 290.37 5.32 CNFZnO-300 284.65 8.50 285.28 3.75 285.98 9.18 289.82 4.42 CNFZnO-600 284.82 17.35 285.48 8.94 286.42 7.61 290.90 3.78

Table 3. (b)

Sample

CNFZnORT CNFZnO300 CNFZnO600 ZnO-600

Peak binding energy (eV)/ Relative atomic concentrations (%) O 1s O1 O2 O3 O4 O2 in O2-C-O/Oat.% at. % at.% at.% ZnO deficiency H COOH 530.86

9.36

-

-

532.16

16.80

533.18

8.06

531.07

11.02

531.53

5.94

532.70

11.86

-

-

531.25

12.75

531.81

6.55

532.72

8.67

-

-

530.16

21.60

531.48

27.53

532.48

2.00

-

-

Table 3. (c) Peak binding energy (eV)/ Relative atomic concentrations (%) Zn 2p Sample Z1 Z2 Zn2p3/2 at.% Zn2p1/2 at.% CNFZnO-RT 1022.24 19.94 1045.34 21.86 CNFZnO-300 1022.56 21.61 1045.66 23.72 CNFZnO-600 1022.67 20.16 1045.77 14.19 ZnO-600 1021.44 23.32 1044.54 25.55

Table 4. Sample Rs (Ω) RCT (Ω) CPE (mF) ZnO-600 0.756 8.64 0.318 CNFZnO-300 0.813 4.373 0.498 CNFZnO-600 0.567 1.398 0.809

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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Figure 8.

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Figure 9.

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A low-cost facile synthesis of ZnO/CNF hybrids and a simple post-annealing treatment in oxygen deficient atmosphere could improve its carrier transport properties to achieve better supercapacitor performance. Correlating structural, optical and electrical properties of the hybrids for the enhancement of electrochemical performance. 249x79mm (96 x 96 DPI)


Oxygen Vacancy-Induced Structural, Optical, and Enhanced

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