Metal Precursor Dependent Synthesis of NiFe2O4 Thin Films for High

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Metal Precursor Dependent Synthesis of NiFe2O4 Thin films for High Performance Flexible Symmetric Supercapacitor Shubhangi Bandgar, Madagonda M Vadiyar, Yong-Chien Ling, JiaYaw Chang, Sung-Hwan Han, Anil V. Ghule, and Sanjay S Kolekar ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00163 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Energy Materials

Metal Precursor Dependent Synthesis of NiFe2O4 Thin films for High Performance Flexible Symmetric Supercapacitor Shubhangi B. Bandgar†, Madagonda M. Vadiyar†, Yong-Chien Ling‡, Jia-Yaw Chang§*, SungHwan Han£, Anil V. Ghuleǁ*, and Sanjay S. Kolekar†* †

Analytical Chemistry and Material Science Research Laboratory, Department of Chemistry,

Shivaji University, Kolhapur 416004, Maharashtra, India. E-mail: [email protected] ‡

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.

§

Department of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei 10607, Taiwan. E-mail: [email protected] £

Inorganic Nano-Material Laboratory, Department of Chemistry, Hanyang University, Seoul,

133-791, Republic of Korea. ǁ

Green Nanotechnology Laboratory, Department of Chemistry, Shivaji University, Kolhapur

416004, Maharashtra, India. E-mail: [email protected] KEYWORDS: Symmetric Supercapacitor, NiFe2O4 Nanosheets, Metal Precursors, Coordination Chemistry, Energy density, Metal oxides, Thin films.

ACS Paragon Plus Environment

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ABSTRACT

Herein, we report the chemical synthesis of NiFe2O4 thin films forming nano-sheet, nano-flower and nano-feather like morphologies using NiCl2.6H2O, Ni(NO3)2.6H2O and NiSO4.6H2O nickel salt precursors, respectively, while using the same iron salt precursor. A nanostructure formation mechanism is proposed in detail using coordination chemistry theory. Interestingly, nanostructures of NiFe2O4 nano-sheets revealed a maximum surface area of 47 m2 g-1, which was higher than nano-flower and nano-feathers (25 and 11 m2 g-1). Similarly, the supercapacitive properties of the individual NiFe2O4 nano-sheet-based electrode demonstrated maximum specific capacitance of 1139 F g-1, which is found to be better than that of NiFe2O4 nano-flowers (677 F g-1) and nano-feathers (435 F g-1) in 6 M KOH electrolyte. Furthermore, the symmetric device fabricated using NiFe2O4 nanosheet electrodes and PVA-KOH solid gel electrolyte shows higher specific capacitance of 236 F g-1 with 98 % retention after 7000 cycles and higher specific energy density of 47 Wh kg-1 at a specific power of 333 W kg-1.


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INTRODUCTION Energy storage technologies and their development have attracted great attention of the scientific community and society at large owing to the growing demands for portable electronics and hybrid vehicles. 1-6 However, most researchers have recently focused on the development of a variety of high-performance energy storage devices such as batteries, capacitors, supercapacitors etc. 7-11 Unfortunately, low power, low cycle stability, slow charging-discharging and safety issues have limited a breakthrough development in batteries

12,13

and thus

supercapacitor (SC) devices are looked upon as promising and safer alternative devices as compared to batteries.14,15 Among the supercapacitors, pseudocapacitors are a favored choice considering their low cost and high performance with the possibility of commercialization. As a result, metals and mixed metal oxides such as Co3O4, Co(OH)2, NiO, Ni(OH)2, Fe2O3, CuO, ZnFe2O4, NiFe2O4, CuFe2O4, NiCo2O4, and CuCo2O4 etc.16-24 are explored with great interest. Among these metal oxides; metal ferrites such as ZnFe2O4, NiFe2O4, and CuFe2O4 have demonstrated their potential as outstanding electrode materials in energy storage applications.25 Particularly, NiFe2O4 stands out as an excellent candidate due to its high theoretical specific capacitance, high surface area, redox behavior and its abundance in nature.26 There are many physical and chemical methods such as sputtering, physical vapor deposition, hydrothermal, solvothermal, chemical bath deposition (CBD), successive ionic layer adsorption (SILAR) etc. being explored for the synthesis of NiFe2O4 thin films.27-30 Very recently, the formation of ZnFe2O4 nano-flakes using rotational chemical bath deposition (R-CBD) has been reported by our group.31,32 Therefore, we have used this R-CBD approach for the growth of NiFe2O4 thin films. In addition, understanding the chemistry behind the nanostructure formation in nanomaterials is one of the important and interesting tasks for the scientific community. It is


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understood that the precursors and chemicals, and their associated chemical properties play an important role in governing the structural and morphological features of the nanomaterials and is the topic of great interest to the scientific community. With this motivation, we were interested to explore the Pearson acid-base concept and to understand the evolution of nanostructure formation by varying the ligand basicity with respect to metal salt. Pearson acid-base concept i.e. hard-soft acid base (HSAB) concept is widely used in chemistry for explaining the stability of compounds, reaction mechanisms, and pathways. In this, the terms 'hard' or 'soft', and 'acid' or 'base' are assigned to the chemical species. This concept states that hard acids prefer to bind hard bases and vice versa, and based on this concept, metal ions and ligands are classified into hard and soft acid-bases. Hence, such a acidic and basic properties of the ions (both metal and ligand) show controlled properties during the reaction specifically dissociation or association.33 In this work, we have synthesized NiFe2O4 thin films via R-CBD. Three different nickel salt precursors chloride, sulfate, and nitrate were used to investigate the applicability of Pearson’s hard soft acid-base (HSAB) principle and to understand the chemistry of formation of different morphologies such as nano-sheets, nano-flowers, and nano-feathers. NiFe2O4 thin film electrodes (nano-sheets, nano-flowers, and nano-feathers) on SSM substrates were explored in supercapacitor application in the presence of 6 M KOH electrolyte. The NiFe2O4 nano-sheet, nano-flower and nano-feather thin film electrodes were used for electrochemical study to evaluate the specific capacity at the same current densities for better comparison. The investigations revealed that the NiFe2O4 nano-sheet thin film electrode shows maximum specific capacity of 1139 F g-1 at 5 mA cm-2 current density, excellent cycle stability of 98 % retention over 7000 cycles. Thus, the solid-state flexible symmetric supercapacitor device was fabricated


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using two NiFe2O4 nano-sheet electrodes using PVA-KOH gel electrolyte and used for further electrochemical study. EXPERIMENTAL Materials and Chemicals Unless specified, all the chemical and reagents used in the present work are of AR grade. Nickel chloride hexahydrate (NiCl2.6H2O), nickel nitrate hexahydrate (Ni(NO3)2.6H2O), nickel sulfate hexahydrate (NiSO4.6H2O), ferrous chloride tetrahydrate (FeCl2.4H2O), and ammonia were procured from Merck chemicals and were used as such for the thin film deposition. The flexible stainless steel mesh substrate was purchased from micro mesh India Pvt. Ltd. Synthesis of NiFe2O4 nano-sheet, nano-flower and nano-feather thin films In the typical experiment, three different sets of aqueous solutions of Ni2+ precursor such as 0.1M NiCl2.6H2O (25 mL), 0.1M Ni(NO3)2.6H2O (25 mL) and 0.1M NiSO4.6H2O (25 mL) were taken in three separate 100 mL beakers, to which aqueous solution of Fe2+ ion precursor of 0.2M FeCl2.4H2O (25 mL) was added in each beaker while the mixture was heated at 50 oC.33 Subsequently, 4 mL ammonia solution was added drop-wise to each beaker to adjust the pH~10 (±0.5), which transformed the colorless solutions into blue colored solutions while the solution temperature was maintained at 50 oC (±2) using a water bath. The ultrasonically pre-cleaned four flexible stainless steel mesh substrates (3 cm x 1cm dimension) were then immersed into each of the above precursor solutions and then subjected to rotation using gear motor at a speed of 60 rpm and the deposition was carried out for 3 h to ensure completion of the reaction and uniform deposition. After deposition, the substrates were carefully removed and washed with double


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distilled water for 5-6 times to remove loosely bound and physically adsorbed particles. Then the films were dried in hot air for 30 min and annealed in an air atmosphere at 500 oC to transform hydroxides into a pure phase of NiFe2O4 thin films. Similarly, three different sets of samples were prepared using 0.1M NiCl2.6H2O (25 mL), 0.1M Ni(NO3)2.6H2O (25 mL) and 0.1M NiSO4.6H2O (25 mL) solutions which were further characterized for structural and electrochemical properties. Materials characterization Crystal phase structures of the three samples were studied using Bruker D2 Phaser X-ray diffractometer with Cu Kα1 radiation (λ=1.5418 Å) operated at a 10 mA current and voltage of 30 kV and Fourier transform infrared spectrometer (FTIR) (Perkin Elmer FRONTIER MIR/FIR + SP10 STD). The surface morphologies of the synthesized three samples were examined by field emission scanning electron microscope (FESEM, Quanta250F FEI) operated at 200 kV. Thermogravimetric analysis (TGA) was carried out in an air atmosphere using a thermal analysis TGA-8000 system (Mettler Toledo). The surface area and pore size were investigated by N2 adsorption-desorption isotherms curves (NOVA1000e Quantachrome, USA) with BrunauerEmmett-Teller (BET) method. Physical electronics PHI-5802 instrument is used for X-ray photoelectron spectroscopy (XPS) analysis of the samples. Electrochemical characterization Electrochemical measurements were carried out using electrochemical analyzer (CHI 608E, Shanghai Chen Hua instrument co., LTD, China) at room temperature. To evaluate the electrochemical properties, the conventional three-electrode configuration was constructed using NiFe2O4 thin films as working electrode, platinum (Pt) wire as a counter electrode and silver-


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silver chloride electrode (Ag/AgCl) as a reference electrode in 6.0 M KOH aqueous electrolyte solution. Furthermore, to fabricate solid-state flexible symmetric supercapacitor device, the solid PVA-KOH gel electrolyte was prepared by heating of a mixture of PVA powder (5 g), KOH pellets (5 g), and distilled water (50 mL) at 90 oC with constant stirring until the clear solution was formed.14 After preparation of PVA-KOH gel electrolyte, the corners and edges of the NiFe2O4 thin film electrodes (2 cm x 1 cm) were sealed with insulating tape to avoid exposure of SSM. The symmetric supercapacitor device was fabricated using two NiFe2O4 thin film electrodes separated by cellulose paper (as separator) which were further immersed into the PVA-KOH gel electrolytes for 30 min. Subsequently, the electrodes were dried overnight at room temperature and further assembled together by sandwiching cellulose separator. This device assembly was eventually packed with an insulating tape by applying small pressure. The CV, GCD and EIS measurements were performed using the symmetric supercapacitor device and the specific capacitance (F g-1), specific energy (Wh kg-1) and specific power (W kg-1) were calculated from the following equation (1).34 ×∆

= ∆ ×

--------------------------- (1)

where Csp is the specific capacitance (F g-1), I is the current density (mA cm-2), ∆t is the discharging time (s), ∆V is the potential window (V) and m represents mass of the NiFe2O4 thin film electrode (0.001 g) or total mass of the two electrodes for full cell device. In addition, to balance the charge between the negative and positive electrode in the symmetric cell (q+ = q-), the mass load was balanced using equation (2),34

=

--------------------------- (2)


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Where, m+ and m- are the mass of positive and negative electrodes (g), Csp+ and Csp- are the specific capacitances of positive and negative electrodes, respectively. The total weight of two electrodes was used for full symmetric cell performance calculations. In addition, the specific energy and specific power is calculated using equation (3) and (4),34

=

×∆ .

= ∆

--------------------------- (3)

--------------------------- (4)

Where E and P are the specific energy (Wh kg-1) and specific power (W kg-1), respectively. I is the discharging current (mA cm-2), ∆V is the discharging voltage (V), and ∆t is the discharging time in seconds. RESULTS AND DISCUSSIONS Pearson acid-base concept, i.e. HSAB, concept underlining the preferential binding of the hard acids to hard bases and vice versa is applied here in this work to understand the chemistry behind the formation of nano-sheet, nano-flower, and nano-feather morphologies.33 We choose three different Ni2+ precursors such as NiCl2.6H2O, Ni(NO3)2.6H2O, and NiSO4.6H2O by keeping Fe2+ precursor constant (FeCl2.4H2O) as shown in Scheme 1. The order of basicity of the ligands is considered as Cl- > SO42- > NO3- 33. The Ni2+ ions are referred as hard acid and show preferential binding affinity in the order Cl- > SO42- > NO3- ions. This binding affinity of metal-ligand directly influences the stability of metal complexes (dissociation or association) and also affects the availability of local concentration of Ni2+ ions, which is responsible for the reaction and formation of various nanostructures. Surfactants and complexing agents also play an identical role in the stability or availability of metal ions or ligand concentration in the reaction.


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Scheme 1 Schematic of the formation mechanism of NiFe2O4 thin films nano-sheet (nickel chloride salt), nano-flower (nickel sulfate salt) and nano-feather (nickel nitrate salt) morphologies employing Pearson acid-base concept.

This indicates that the coordination chemistry is also responsible for the formation of nanostructures in the absence of any surfactant or complexing agents. Ammonia is used as the source of hydroxide ions, which releases hydroxide ions slowly in the aqueous medium at room temperature. The simple reaction involved is as follows. 2 2 ↔ 2 2 ! --------------------------- (5) These hydroxide ions easily react with Ni2+ ions released from Ni precursors as discussed in the Pearson acid-base concept. The possible reaction mechanism for NiCl2, Ni(NO3)2 and NiSO4 with (FeCl2.4H2O) and hydroxide ions are as given below, ! &

2"#$ 4 ! '(() 2"#*+ → 2"# --------------------------- (6)


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! &

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.// ℃

-$ 2 ! '(() -*+ 2"# '(() -"# 2 -------------------- (7) !456

!

.// ℃

-1 2 → -*1 +*OH+ '((() -*+ 2"# '(() -"# 2 - (8) 75 8

.// ℃

-* + 2 ! '(() -*+ 2"# '(() -"# 2 ------------ (9) The morphological properties were controlled on the basis of the basicity of ligands (Cl- > SO42- > NO3-) and are confirmed from the colors obtained after formation of nickel hydroxide and iron hydroxide as presented in Scheme 1. The chloride ion forms a faint green colored complex which confirms the strong binding between Ni2+ and Cl- ions. Hence, the Ni2+ ions are slowly released from NiCl2 and are made available for the reaction providing enough time for achieving equilibrium and to orient the growth forming nano-sheets like morphology. On the other hand, the SO42- ion shows slightly dark colored complex due to fast dissociation of NiSO4 precursor supplying fast and sufficient Ni2+ ions, thereby providing less time for achieving equilibrium and orientation forming nano-flower like morphology. Lastly, the highly basic NO3ligands which release Ni2+ ions at relatively faster rate undergo sudden aggregation providing relatively less time for achieving equilibrium and orientation forming nano-feather like morphology. To further support the formation mechanism of different nanostructures and to investigate the thermal stability of synthesized NiFe2O4 thin films with nano-sheets, nano-flowers, and nanofeathers, the materials scooped from the thin films were analyzed using thermogravimetric analysis (TGA) and differential thermal analysis (DTA) as shown in Figure 1 (a-c).


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Figure 1 TGA and DTA curves of NiFe2O4 (a) nano-sheet synthesized using nickel chloride salt, (b) nano-flower synthesized using nickel sulfate salt and (c) nano-feathers synthesized using nickel nitrate salt.

The NiFe2O4 nanosheets sample obtained from Ni-chloride salt showed weight loss in three well-defined steps (Figure 1a), while the NiFe2O4 nano-flowers sample (Figure 1b) and nanofeathers (Figure 1c) obtained from nickel sulfate and nitrate precursors showed sluggish weight loss in five and four steps, respectively. As shown in Figure 1a, the first step of weight loss up to 100 oC accounting for 4.8% could be attributed to the loss of moisture trapped in the sample.

32

Subsequent weight loss of 18% observed between 100-380 oC could be assigned to the decomposition of metal salts and ammonium complex favoring the formation of NiO and FeOOH phases, which are the intermediate phase.32 Further heating beyond 380 oC shows a


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weight loss of 7% and is indicative of the transformation of intermediate species into pure and stable NiFe2O4 phase above 500 oC (Figure 1a). Similarly, Figure 1b shows weight loss of 3% in the first step between 50-150 oC which can be attributed to the loss of moisture trapped in nano-flowers. Furthermore, the second (7%), third (11%), fourth (5%) and fifth (5%) step of weight loss is observed in temperature range from 100-200 oC, 200-400 oC, 600-820 oC and 820920 oC, respectively, indicative of forming intermediates and subsequent sluggish weight loss due to decomposition of metal salts and ammonium complex forming pure and stable NiFe2O4 phase above 920 oC as shown in eq. 8. On the other hand, Figure 1c shows weight loss in four steps demonstrating 3% weight loss in the first step as a result of the loss of moisture trapped in nano-feathers up to 100 oC. The subsequent steps of weight loss i.e. in second step (5% between 100-200 oC), third step (11% between 200-380 oC) and fourth step (7% between 380-700 oC) are indicative of forming intermediates and subsequent sluggish weight loss due to decomposition of metal salts and ammonium complex forming pure and stable NiFe2O4 phase above 500 oC as shown in eq. 9. The TGA thermograms of nano-flowers and nano-feathers show sluggish weight loss in multiple steps because of formation of NiO and Fe2O3 from some of the unreacted intermediates Ni(SO4)OH and FeOOH. Furthermore, the TGA thermogram revealed that pure and stable nano-sheet NiFe2O4 phase is formed at a relatively lower temperature at 600 oC.32 The DTA thermogram (blue colored curves) in the Figure 1(a-c) show both endothermic and exothermic peaks indicating the steps of weight loss involving exothermic and endothermic reactions.32 The XRD measurements were carried out to study the crystal structure and phase identification of the prepared NiFe2O4 thin film samples using nickel chloride, nickel sulfate and nickel nitrate, respectively, and are presented in Figure 2(a).


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Figure 2 (a) XRD pattern of NiFe2O4 and (b) FTIR spectra of nanosheet synthesized using nickel chloride salt, nano-flower synthesized using nickel sulfate salt and nano-feathers synthesized using nickel nitrate salt, respectively.

The XRD patterns showed characteristic (311) plane at 2θ of 35.5o with the highest intensity which is indicative of the formation of face-centered cubic spinel phase of the NiFe2O4 structure. Additional smaller peaks are also observed at 2θ of 30.5o, 44.2o, 46.3o, 56.2o, 63.0o, 68.1o, and 74.0o corresponding to the (220), (400), (420), (440), (531) and (622) planes of the cubic phases. Moreover, the extra non-identified peaks and the shift of peaks towards higher 2θ appear in the spectra of nickel sulfate salt and nickel nitrate salt based NiFe2O4 nano-flower and nano-feather at 2θ of 42.8o, 47.2o, 49.1o, and 64.2o which correspond to unreacted SO42- and FeOOH. Furthermore, additional peaks of stainless steel are also observed at 2θ of 43.5o and 50.2o and are attributed to (SS) mesh substrate. The XRD spectra obtained from synthesized NiFe2O4 thin films with nano-sheets, nano-flowers, and nano-feathers morphologies were in good agreement with JCPDS No.22-1084, 10-0325, and 54-0964, respectively. The average crystallite sizes were calculated from the intense peak (311) and the obtained crystallite sizes from the Scherrer’s formula25 are ~9.0 nm, ~13.0 nm and ~14 nm for nano-sheets, nano-flowers and nano-feathers,


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respectively. Furthermore, FTIR spectra were also recorded to support the formation of cubic phase with metal-oxygen bonding in all the three samples as shown in Figure 2(b).35 The peaks appearing at ~413 cm-1 are attributed to the M-O vibrations (Fe3+-O and Ni2+-O bonding) in octahedral site. On the other hand, strong vibrational peak at 598 cm-1 is assigned for Fe3+-O bonding in the tetrahedral site of the inverse NiFe2O4 structure.35 The peaks around (1009, 1140 and 1433 cm-1) and (1620, 1702, 2333, 2853, 2934 and 3154 cm-1) are attributed to the unreacted FeOOH in the prepared samples. Moreover, the broad peaks at 3472 and 3570 cm-1 are attributed to -OH stretching vibrations.35 Interestingly, the FTIR spectrum of sulfate precursor shows very strong absorption peak at 1140 cm-1 due to strong basic SO42- ions, which strongly binds with Ni2+ ions and exhibits slow oxidation process.36 This indicates that the Pearson acid-base concept is applicable in the formation of nano-flower like morphology. Furthermore, the FTIR study confirms the inverse spinel cubic structure of the NiFe2O4 thin films. Surface morphologies of synthesized NiFe2O4 thin films are studied using field emission scanning electron microscopy (FESEM). The low and high magnification images of NiFe2O4 thin films with nano-sheets, nanoflowers, and nano-feathers morphologies are presented Figure 3. The NiFe2O4 thin films obtained from nickel chlorides precursor gives nano-sheet like morphology as shown in low and high magnification FESEM images (Figure 3a-c). These nanosheets are formed due to the slow reaction and more reactive sites.


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Figure 3 Low and high magnification FESEM images of NiFe2O4 thin films with (a-c) nano-sheet synthesized using nickel chloride salt, (d-f) nano-flower synthesized using nickel sulfate salt and (g-i) nano-feathers synthesized using nickel nitrate salt.

Nanosheets with length in µm and with a thickness of ~ 300 nm were obtained. On the other hand, the low and high magnification FESEM image in Figure 3(d-f) presents the formation of NiFe2O4 thin films with nano-flower like morphology having a thickness of ~ 100 nm which is highly stable at high-temperature due to the hard base (SO42- ion). The Figure 3(g-i) shows nano-feather (~350 nm) like morphology which is formed due to the hard base (NO3- ion).


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Figure4 (a-c) N2 adsorption-desorption isotherms and (a’-c’) BJH pore size distribution of NiFe2O4 nanosheets synthesized using nickel chloride salt, nano-flowers synthesized using nickel sulfate salt and nanofeathers synthesized using nickel nitrate salt.

The porous structures of the synthesized NiFe2O4 thin films were evaluated using BET analysis. The N2 adsorption-desorption isotherms are presented in Figure 4 (a-c). The entire NiFe2O4 thin films show type-IV hysteresis loop from relative pressure 0.4 to 0.8 indicating mesoporous nature of the samples.37 However, the multipoint BET surface area of NiFe2O4 nano-sheet (47 m2 g-1) is higher than that of NiFe2O4 nano-flowers (25 m2 g-1) and NiFe2O4 nano-feathers (11 m2 g-1). In addition, BJH pore size analysis of NiFe2O4 nano-sheets shows narrow pore size of ~1.8 nm with a maximum number of mesopores as shown in Figure 4(a’). On the other hand, NiFe2O4 nano-flower and NiFe2O4 nano-feathers show pore size of 2.2 nm and 2.9 nm, respectively, as shown in Figure 4 (b’ and c’).37 BET surface area analysis of NiFe2O4 nano-sheets shows higher surface area (47 m2g-1) compared to nano-flowers and nano-


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feathers, and hence representative XPS analysis of NiFe2O4 nano-sheets was performed to understand the chemical environment and oxidation states.

Figure 5 (a-c) XPS core level spectra of Ni2p, Fe2p, and O1s obtained from NiFe2O4 nano-sheets thin film sample.

The survey spectrum of NiFe2O4 nano-sheets confirms the presence of Ni, Fe and O elements along with adventitious C as shown in Figure S1(SI).38 The core level spectrum of Ni2p presented in Figure 5 (a) reveals two distinct peaks of Ni2p1/2 and Ni2p3/2 with binding energies of 873 eV and 855 eV along with two satellite peaks indicating the presence of Ni with +2 oxidation state.38 On the other hand, the core level spectrum of Fe2p (Figure 5 (b)) also shows two peaks (Fe2p1/2 and Fe2p3/2) corresponding to the Fe3+ oxidation state with binding energies of 711 eV and 725 eV, respectively. The core level spectrum of O1s (Figure 5 (c)) shows


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binding energy peak at 530 eV which confirms the metal-oxygen bonding.38 In addition to this, the EDS spectrum of NiFe2O4 nano-sheet presented in Figure S2 (a) (SI) confirms the atomic ratio of Ni: Fe as 1:2 in agreement with the chemical formula and composition of NiFe2O4. Furthermore, an elemental mapping (Figure S2 (b-d)) study was performed to investigate the uniform distribution of the elements within the material. Accordingly, signals of nickel, iron, and oxygen elements were mapped and found to show the uniform distribution in the composition as expected. Interesting structural and morphological properties of NiFe2O4 thin films encourage us to evaluate the electrochemical properties of NiFe2O4 thin films. Individual electrode charge storage properties were evaluated by constructing conventional three-electrode cell configuration with NiFe2O4 thin films as a working electrode (anode), Pt wire as a current collector and Ag/AgCl electrode as a reference electrode in 6M KOH electrolyte.32 Initially, the comparative CVs were collected at 10 mV s-1 scan rate for NiFe2O4 nano-sheets, nano-flowers and nanofeathers as shown in Figure 6 (a). From the figure, two distinct peaks corresponding to the oxidation and reduction reaction were observed which are indicative of redox charge storage behavior.34,39 The area under the CV curves for NiFe2O4 nano-sheet is noted to be higher than for nano-flowers and nano-feathers at constant scan rate (10 mV s-1) indicating that nano-sheetbased electrode is a promising material for supercapacitor application. Interestingly, a close look at the morphology revealed that the improved performance could be attributed to the formation of well interconnected and organized nano-sheets.


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Figure 6 (a) CVs. (b) GCD. (c) Specific capacitance (F g-1) at various current densities (mA cm-2). (d) Cycle stability with capacitance retention. (e) Nyquist plots of NiFe2O4 nano-sheets, nano-flowers and nano-feathers thin films. (f) Bode plot of NiFe2O4 nano-sheets thin films.

Similarly, the GCD curves were also recorded and are presented in Figure 6 (b). The figure represents the similar trends of capacitive curves for all the three samples indicating pseudocapacitive properties. The electrochemical reactions (Ni2+/Ni3+ or Fe3+/Fe2+) involved during energy storage mechanism are as follows.40 -"# 2# ! ↔ - 2"# 2 ! ------------------ (10) The specific capacitance of the samples was calculated using equation (1). The prolonged discharge curve of NiFe2O4 nano-sheet illustrates maximum specific capacitance (1139 F g-1) as compared to that noted for NiFe2O4 nano-flowers (677 F g-1) and NiFe2O4 nano-feathers (435 F g-1) at 5 mA cm-2 current density because of small thin layer and strong interconnection between the nano-sheets provides high surface area and active sites for electrochemical reactions. On the


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other hand, larger size and less interconnectivity of nano-flowers and nano-feathers might be responsible for the observed reduction in the specific capacitance. The specific capacitance is noted to decrease with increase in current densities from 5 to 30 mA cm-2 as shown in Figure 6 (c). The effect of scan rate and current density on the CVs and GCDs was evaluated using different scan rates (10-100 mV s-1) and current densities (5-30 mA cm-2) as shown in Figure S3 (a-c) and Figure S3(a’-c’)(SI). From the figure it is noticed that all the samples show the identical shape of CVs and GCDs even at higher scan rate (100 mV s-1) and a current density of 30 mA cm-2. This indicates that all the NiFe2O4 thin films have higher rate capability. The cycling stability is one of the important parameter for the electrode materials and therefore the charging-discharging curves were collected over 7000 cycles as presented in Figure 6 (d). From the figure it is noticed that the NiFe2O4 nano-sheets demonstrates, 98 % capacitance retention over 7000 cycles, indicating better cycle stability than NiFe2O4 nano-flowers and nano-feathers (93 and 80 % capacitance retention). This improved cycle stability of nanosheets is attributed to the high surface area and strong interconnection of the individual nano-sheets. This strong interconnection and fast transportation of electrolytic ions due to thin layers is observed in nanosheet morphology. Furthermore, there is a strong binding between Ni2+ and Cl- ions. Hence, the Ni2+ ions are slowly released from NiCl2 and are made available for the reaction providing enough time for achieving equilibrium and to orient the growth forming nano-sheets like morphology. The electrochemical impedance spectroscopy (EIS) is one of the important methods to understand frequency response of the electroactive materials. Hence, Nyquist plots of the three NiFe2O4 thin films are presented in the Figure 6(e). In high-frequency region, the small quasisemicircle was observed due to the charge transfer resistance (Rct) and a straight line with the small curve is attributed to Warburg resistance (Rw) and ion diffusion for the NiFe2O4 thin


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films.31 Interestingly, NiFe2O4 nano-sheets show smaller Rct (1.15 ohm cm-2) and ESR (2.45 ohm cm-2) as compared to NiFe2O4 nano-flower and nano-feathers (1.30 and 7.0 ohm cm-2) and (3.30 and 4.20 ohm cm-2), respectively. The Bode plot of the individual NiFe2O4 nano-sheet electrode is presented in Figure 6(f) showing lower phase angle (-35o) and relaxation time of 0.2s which confirms the redox charge storage behavior of the electrode.31 Based on the remarkable electrochemical properties of the NiFe2O4 nano-sheet electrode in 6M KOH electrolyte, we fabricated flexible symmetric supercapacitor device using two NiFe2O4 nanosheet electrodes (SS/NiFe2O4 nano-sheet (+)//NiFe2O4 nano-sheet (-)/SS) using PVA-KOH solid state electrolyte for practical applications.31,41 The cyclic voltammogram is collected at a cell voltage of 0.0 to 1.2V and various scan rates of 10-500 mV s-1 as shown in Figure 7(a). The CVs show Faradaic redox charge storage behavior. Even at high scan rate (500 mV s-1), the CVs shows symmetrical curve without apparent polarization, indicating fast charging-discharging and excellent reversibility of NiFe2O4 nanosheet thin film based symmetric supercapacitor. To confirm the energy storage behavior we have performed galvanostatic charge-discharge measurements at various current densities of 2-10 mA cm-2 with cell voltage from 0.0 to 1.2 V as shown in Figure 7(b). The identical shape of GCDs once again confirms fast chargingdischarging nature as well as high rate capability. The specific capacitance of the device is calculated using equation (1) and the device shows the maximum specific capacity of 236 F g-1 at a current density of 2 mA cm-2 which is higher than similar symmetric devices based on NiFe2O4 and other metal oxide materials as compared in Table 1. The specific capacity decreases (236, 212, 190, 175 and 100 F g-1) with an increase in current densities (2, 2.5, 4, 6 and 10 mA cm-2) as shown in Figure 7 (c).


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Figure 7 (a) CVs. (b) GCD curves. (c) Specific capacitance (F g-1) at various current densities (mA cm-2). (d) Cyclic voltammogram at various bending angles of 30o, 60o, 90o, 120o, 150o and 180o. (e) Cycle stability with 98% capacitance retention over 7000 cycles. (f) Ragone plot of specific energy density (Wh kg-1) and specific power density (W kg-1). (g) Schematic of device and (h) actual photograph of symmetric device with glowing LED fabricated using NiFe2O4 nano-sheet thin films.

The flexibility of the substrate and electrode is also one of the essential properties to make flexible devices. Hence, the symmetric device is subjected to CV measurement at 100 mV s-1 scan rate at various bending angles of 30o, 60o, 90o, 120o, 150o and 180o as presented in Figure 7 (d). The CV curves recorded at various bending angles were found to overlap exhibiting identical shape and area under the CV curves proving excellent flexibility of the electrode and device. The


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cycling stability of the symmetric device is recorded over 7000 cycles as presented in Figure 6(e). The symmetric device shows 98% capacitance retention over 7000 cycles indicating excellent cycle stability. Furthermore, the coulombic efficiency can be also estimated, according to the equation given below;42

9 = : × 100% ---------------------------- (11) ;

where, η is the coulombic efficiency, td, and tc are the discharging and charging times respectively. Coulombic efficiency of the symmetric device can be maintained at 94–98% over the entire cycling process as shown in Figure S4 (SI). The specific energy and specific power densities are also evaluated for SS/NiFe2O4 nano-sheet (+)//NiFe2O4 nano-sheet (-)/SS device using equations (3) and (4) and are presented in Ragone plot (Figure 7 f). The specific energy density of 47 Wh kg-1 can be achieved at a power density of 333 W kg-1 and 20 Wh kg-1 is maintained even at high power density of 1666 W kg-1, which is superior to most of the previously reported symmetric supercapacitor devices (a detailed comparison with other metal oxide devices from literature is shown in Table 1).


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Table 1 Comparative performance of the NiFe2O4//NiFe2O4 device with various symmetric devices. Symmetric Devices

Electrolyte

MnO2//MnO2

Carboxymethyl cellulose- Na2SO4 gel 3M KOH electrolyte 1 M Na2SO4 electrolyte 1M KOH electrolyte 1M KOH electrolyte 0.5 M LiNO3

NiO-In2O3//NiOIn2O3 V2O5/graphene aerogels MnFe2O4/G/PANI// MnFe2O4/G/PANI ZnFe2O4/NRG// ZnFe2O4/NRG MnOOH//MnOOH CoMn2O4//CoMn2O4 NiFe2O4//NiFe2O4

2M KOH electrolyte PVA-KOH gel

Specific Capacitance 145 F g-1

Cycle stability (% retention) 2500 cycles (88% retention)

Specific energy 16 Wh kg-1

Ref.

28.2 mAh g-1

1000 cycles (94% retention) 10000 cycles (80% retention) 2000 cycles (74% retention) 1000 cycles (84.4% retention) 10000 cycles (84% retention) 5000 cycles (51.6% retention) 7000 cycles (98% retention)

26.24 Wh kg-1

41

24 Wh kg-1

44

13.5 Wh kg-1

45

6.7 Wh kg-1

46

32.5 Wh kg-1

47

23.29 Wh kg-1

48

47 Wh kg-1

Present Work

484 F g-1 307 F g-1 244 F g-1 81 F g-1 46.5 mAh g-1 236 F g-1

43

The schematic of the symmetric device is presented in Figure 7 (g) and also actual photo (Figure 7(h)) of red LED glowing to demonstrate the performance of the device. The two electrodes were separated by using cellulose paper separator and the yellow dots represent the solid state PVA-KOH electrolyte. Thus, we believe that the symmetric supercapacitor device composed of NiFe2O4 nano-sheet could be a promising energy storage system filling the gap between the traditional batteries and capacitors. CONCLUSIONS In summary, we demonstrated the synthesis of NiFe2O4 thin film electrodes via rotational chemical bath deposition with varying nickel salt precursors by applying coordination chemistry principles. Nano-sheet, nano-flower and nano-feather like morphologies were obtained on the basis of ligand basicity in the order of Cl- > SO42- > NO3-, respectively. When employing the NiFe2O4 thin films as supercapacitor electrodes, the NiFe2O4 nano-sheet thin film electrode


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exhibit higher specific capacitance of 1139 F g-1 at 5 mA cm-2 current density and provides 98 % retention after 7000 cycles, indicating good cycling performance. In addition, the flexible symmetric supercapacitor is demonstrated for practical application. The flexible symmetric device provide a maximum specific capacitance of 236 F g-1 and also delivered a maximum specific energy density of 47 Wh kg-1 indicating excellent charge storage properties. We believe that these low-cost and high-performance NiFe2O4 nano-sheet based thin films would pave way for new opportunities for energy storage applications, particularly for flexible, lightweight and portable electronic devices. ASSOCIATED CONTENT Supporting Information. Figures of S1-S4 AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected] * Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Authors are thankful to UGC-SAP and DST-FIST, PURSE (File SR/FST/CSI-231/2011(G)) for financial support and instrument facilities at the Department of Chemistry, Shivaji University, Kolhapur. Authors SBB and MMV are thankful for financial support from DAE-BRNS,


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Government of India (File No.36(4)/14/31/2015/36002) and UGC-BSR Meritorious Students Fellowship (F.7-183/2007 (BSR)) respectively. REFERENCES (1)

Yan, J.; Wang, Q.; Wei, T.; Fan, Z. Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities, Adv. Energy Mater., 2014, 4, 1300816.

(2)

Wang, X.; Chen, Y.; Schmidt, O. G.; Yan, C. Engineered nanomembranes for smart energy storage devices, Chem. Soc. Rev., 2016, 45, 1308-1330.

(3)

Huang, P.; Lethien, C.; Pinaud, S.; Brousse, K.; Laloo, R.; Turq, V.; Respaud, M.; Demortière, A.; Daffos, B.; Taberna, P. L.; Chaudret, B.; Gogotsi, Y.; Simon, P. On-chip and freestanding elastic carbon films for micro-supercapacitors, Science, 2016, 351, 691695.

(4)

Chaikittisilp, W.; Hu, M.; Wang, H.; Huang, H.-S.; Fujita, T.; Wu, K. C. W.; Chen, L.-C.; Yamauchi, Y.; Ariga, K. Nanoporous carbons through direct carbonization of a zeolitic imidazolate framework for supercapacitor electrodes, Chem. Commun., 2012, 48, 72597261.

(5)

Salunkhe, R. R.; Young, C.; Tang, J.; Takei, T.; Ide, Y.; Kobayashi, N.; Yamauchi, Y. A high-performance supercapacitor cell based on ZIF-8-derived nanoporous carbon using an organic electrolyte, Chem. Commun., 2016, 52, 4764-4767.

(6)

Salunkhe, R. R.; Kamachi, Y.; Torad, N. L.; Hwang, S. M.; Sun, Z.; Dou, S. X.; Kim, J. H.; Yamauchi, Y. Fabrication of symmetric supercapacitors based on MOF-derived nanoporous carbons, J. Mater. Chem. A, 2014, 2, 19848-19854.

(7)

Xu, J.; Lin, F.; Doeff, M. M.; Tong, W. A review of Ni-based layered oxides for rechargeable Li-ion batteries, J. Mater. Chem. A, 2017, 5, 874-901.

(8)

Salunkhe, R. R.; Kaneti, Y. V.; Kim, J.; Kim, J. H.; Yamauchi, Y. Nanoarchitectures for Metal–Organic Framework-Derived Nanoporous Carbons toward Supercapacitor Applications, Acc. Chem. Res., 2016, 49, 2796-2806.

(9)

Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage, Nat. Rev. Mater., 2017, 2, 16098.

(10) Naveen, N.; Sohn, K.-S.; Pyo, M. Graphite Oxide as a Robust and Efficient Template for Co3O4-Based High Power Battery-Type Electrodes, J. Electrochem. Soc., 2017, 164, A214A220. (11) Xia, L.; Yu, L.; Hu, D.; Chen, G. Z. Electrolytes for electrochemical energy storage, Mater. Chem. Front., 2017,1, 584-618. (12) Lu, X.; Yu, M.; Wang, G.; Tong, Y.; Li, Y. Flexible solid-state supercapacitors: design, fabrication and applications, Energy Environ. Sci., 2014, 7, 2160-2181.


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Jiao, Y.; Pei, J.; Chen, D.; Yan, C.; Hu, Y.; Zhang, Q.; Chen, G. Mixed-metallic MOF based electrode materials for high performance hybrid supercapacitors, J. Mater. Chem. A, 2017, 5, 1094-1102. (14) Vadiyar, M. M.; Kolekar, S. S.; Chang, J.-Y.; Kashale, A. A.; Ghule, A. V. Reflux Condensation Mediated Deposition of Co3O4 Nanosheets and ZnFe2O4 Nanoflakes Electrodes for Flexible Asymmetric Supercapacitor, Electrochim. Acta, 2016, 222, 16041615. (15) Yang, W.; He, L.; Tian, X.; Yan, M.; Yuan, H.; Liao, X.; Meng, J.; Hao, Z.; Mai, L. Carbon-MEMS-Based Alternating Stacked MoS2@rGO-CNT Micro-Supercapacitor with High Capacitance and Energy Density, Small, 2017, 13, 1700639. (16) Cheng, J. P.; Chen, X.; Wu, J.-S.; Liu, F.; Zhang, X. B.; Dravid, V. P. Porous cobalt oxides with tunable hierarchical morphologies for supercapacitor electrodes, CrystEngComm, 2012, 14, 6702-6709. (17) Sun, G.; Ma, L.; Ran, J.; Shen, X.; Tong, H. Incorporation of homogeneous Co3O4 into a nitrogen-doped carbon aerogel via a facile in situ synthesis method: implications for high performance asymmetric supercapacitors, J. Mater. Chem. A, 2016, 4, 9542-9554. (18) Yuksel, R.; Coskun, S.; Kalay, Y. E.; Unalan, H. E. Flexible, silver nanowire network nickel hydroxide core-shell electrodes for supercapacitors, J. Power Sources, 2016, 328, 167-173. (19) Quan, H.; Cheng, B.; Xiao, Y.; Lei, S. One-pot synthesis of α-Fe2O3 nanoplates-reduced graphene oxide composites for supercapacitor application, Chem. Eng. J., 2016, 286, 165173. (20) Bhise, S. C.; Awale, D. V.; Vadiyar, M. M.; Patil, S. K.; Kokare, B. N.; Kolekar, S. S. Facile synthesis of CuO nanosheets as electrode for supercapacitor with long cyclic stability in novel methyl imidazole-based ionic liquid electrolyte, J. Solid State Electrochem., 2017, 1-7. (21) Vadiyar, M. M.; Bhise, S. C.; Patil, S. K.; Kolekar, S. S.; Chang, J.-Y.; Ghule, A. V. Comparative Study of Individual and Mixed Aqueous Electrolytes with ZnFe2O4 Nano– flakes Thin Film as an Electrode for Supercapacitor Application, ChemistrySelect, 2016, 1, 959-966. (22) Pendashteh, A.; Moosavifard, S. E.; Rahmanifar, M. S.; Wang, Y.; El-Kady, M. F.; Kaner, R. B.; Mousavi, M. F. Highly Ordered Mesoporous CuCo2O4 Nanowires, a Promising Solution for High-Performance Supercapacitors, Chem. Mater., 2015, 27, 3919-3926. (23) Owusu, K. A.; Qu, L.; Li, J.; Wang, Z.; Zhao, K.; Yang, C.; Hercule, K. M.; Lin, C.; Shi, C.; Wei, Q.; Zhou, L.; Mai, L. Low-crystalline iron oxide hydroxide nanoparticle anode for high-performance supercapacitors, Nat. Commun., 2017, 8, 14264. (24) Sen, P.; De, A. Electrochemical performances of poly(3,4-ethylenedioxythiophene)– NiFe2O4 nanocomposite as electrode for supercapacitor, Electrochim. Acta, 2010, 55, 46774684.


27


Page 28 of 30

(25) Vadiyar, M. M.; Kolekar, S. S.; Deshpande, N. G.; Chang, J.-Y.; Kashale, A. A.; Ghule, A. V. Binder-free chemical synthesis of ZnFe2O4 thin films for asymmetric supercapacitor with improved performance, Ionics, 2017, 23, 741-749. (26) Yu, Z.-Y.; Chen, L.-F.; Yu, S.-H. Growth of NiFe2O4 nanoparticles on carbon cloth for high performance flexible supercapacitors, J. Mater. Chem. A, 2014, 2, 10889-10894. (27) Klewe, C.; Meinert, M.; Boehnke, A.; Kuepper, K.; Arenholz, E.; Gupta, A.; Schmalhorst, J.-M.; Kuschel, T.; Reiss, G. Physical characteristics and cation distribution of NiFe2O4 thin films with high resistivity prepared by reactive co-sputtering, J. Appl. Phys., 2014, 115, 123903. (28) Zhou, J.; Ma, J.; Sun, C.; Xie, L.; Zhao, Z.; Tian, H.; Wang, Y.; Tao, J.; Zhu, X. LowTemperature Synthesis of NiFe2O4 by a Hydrothermal Method, J. Am. Ceram. Soc., 2005, 88, 3535-3537. (29) Liu, F.; Zhang, W.; Chen, W.; Wang, J.; Yang, Q.; Zhu, W.; Wang, J. One-pot synthesis of NiFe2O4 integrated with EDTA-derived carbon dots for enhanced removal of tetracycline, Chem. Engin. J., 2017, 310, Part 1, 187-196. (30) Gunjakar, J. L.; More, A. M.; Shinde, V. R.; Lokhande, C. D. Synthesis of nanocrystalline nickel ferrite (NiFe2O4) thin films using low temperature modified chemical method, J. Alloys Comp., 2008, 465, 468-473. (31) Vadiyar, M. M.; Bhise, S. C.; Kolekar, S. S.; Chang, J.-Y.; Ghule, K. S.; Ghule, A. V. Low cost flexible 3-D aligned and cross-linked efficient ZnFe2O4 nano-flakes electrode on stainless steel mesh for asymmetric supercapacitors, J. Mater. Chem. A, 2016, 4, 35043512. (32) Vadiyar, M. M.; Bhise, S. C.; Patil, S. K.; Patil, S. A.; Pawar, D. K.; Ghule, A. V.; Patil, P. S.; Kolekar, S. S. Mechanochemical growth of a porous ZnFe2O4 nano-flake thin film as an electrode for supercapacitor application, RSC Adv., 2015, 5, 45935-45942. (33) Shinde, D. V.; Ahn, D. Y.; Jadhav, V. V.; Lee, D. Y.; Shrestha, N. K.; Lee, J. K.; Lee, H. Y.; Mane, R. S.; Han, S.-H. A coordination chemistry approach for shape controlled synthesis of indium oxide nanostructures and their photoelectrochemical properties, J. Mater. Chem. A, 2014, 2, 5490-5498. (34) Khani, H.; Wipf, D. O. Iron Oxide Nanosheets and Pulse-Electrodeposited Ni–Co–S Nanoflake Arrays for High-Performance Charge Storage, ACS Appl. Mater. Interfaces, 2017, 9, 6967-6978. (35) Liu, F.; Zhang, W.; Chen, W.; Wang, J.; Yang, Q.; Zhu, W.; Wang, J. One-pot synthesis of NiFe2O4 integrated with EDTA-derived carbon dots for enhanced removal of tetracycline, Chem. Eng. J., 2017, 310, Part 1, 187-196. (36) Bazhan, Z.; Ghodsi, F. E.; Mazloom, J. Surface morphology, optical, and electrochromic properties of nanostructured nickel ferrite (NiFe2O4) prepared by sol–gel method: effects of Ni/Fe molar ratios, Appl. Phys. A, 2016, 122, 551. (37) Wang, J.; Yang, G.; Wang, L.; Yan, W. Synthesis of one-dimensional NiFe2O4 nanostructures: tunable morphology and high-performance anode materials for Li ion batteries, J. Mater. Chem. A, 2016, 4, 8620-8629.


28

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Gao, X.; Wang, J.; Zhang, D.; Nie, K.; Ma, Y.; Zhong, J.; Sun, X. Hollow NiFe2O4 nanospheres on carbon nanorods as a highly efficient anode material for lithium ion batteries, J. Mater. Chem. A, 2017, 5, 5007-5012. (39) Brousse, T.; Bélanger, D.; Long, J. W. To Be or Not To Be Pseudocapacitive? , J. Electrochem. Soc., 2015, 162, A5185-A5189. (40) Shao, J.; Li, X.; Ding, Y.; Wan, Z.; Liu, H.; Yun, J.; Liu, Y.; Qu, Q.; Zheng, H. Construction of symmetric aqueous rechargeable battery with high voltage based on NiFe2O4 hollow microspheres, Electrochem. Commun., 2014, 40, 9-12. (41) Padmanathan, N.; Shao, H.; McNulty, D.; O'Dwyer, C.; Razeeb, K. M. Hierarchical NiOIn2O3 microflower (3D)/ nanorod (1D) hetero-architecture as a supercapattery electrode with excellent cyclic stability, J. Mater. Chem. A, 2016, 4, 4820-4830. (42) Marichi, R. B.; Sahu, V.; Lalwani, S.; Mishra, M.; Gupta, G.; Sharma, R. K.; Singh, G. Nickel-shell assisted growth of nickel-cobalt hydroxide nanofibres and their symmetric/asymmetric supercapacitive characteristics, J. Power Sources, 2016, 325, 762771. (43) Chodankar, N. R.; Dubal, D. P.; Gund, G. S.; Lokhande, C. D. Flexible all-solid-state MnO2 thin films based symmetric supercapacitors, Electrochim. Acta, 2015, 165, 338-347. (44) Yilmaz, G.; Lu, X.; Ho, G. W. Cross-linker mediated formation of sulfur-functionalized V2O5/graphene aerogels and their enhanced pseudocapacitive performance, Nanoscale, 2017, 9, 802-811. (45) Xiong, P.; Hu, C.; Fan, Y.; Zhang, W.; Zhu, J.; Wang, X. Ternary manganese ferrite/graphene/polyaniline nanostructure with enhanced electrochemical capacitance performance, J. Power Sources, 2014, 266, 384-392. (46) Li, L.; Bi, H.; Gai, S.; He, F.; Gao, P.; Dai, Y.; Zhang, X.; Yang, D.; Zhang, M.; Yang, P. Uniformly Dispersed ZnFe2O4 Nanoparticles on Nitrogen-Modified Graphene for HighPerformance Supercapacitor as Electrode, Sci. Rep., 2017, 7, 43116. (47) Zhang, Y.; Hu, Z.; An, Y.; Guo, B.; An, N.; Liang, Y.; Wu, H. High-performance symmetric supercapacitor based on manganese oxyhydroxide nanosheets on carbon cloth as binder-free electrodes, J. Power Sources, 2016, 311, 121-129. (48) Peng, T.; Hou, X.; Liu, C.; Yu, Q.; Luo, R.; Yan, H.; Lu, Y.; Liu, X.; Luo, Y. Controlled synthesis of hierarchical CoMn2O4 nanostructures for flexible all-solid-state battery-type electrodes, J.. Solid State Electrochem., 2017, 1-9.


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TOC


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Metal Precursor Dependent Synthesis of NiFe2O4 Thin Films for High

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