Hierarchical Microspheres Based on Î±-Ni(OH)2 Nanosheets

ARTICLE pubs.acs.org/JPCC

Hierarchical Microspheres Based on α-Ni(OH)2 Nanosheets Intercalated with Different Anions: Synthesis, Anion Exchange, and Effect of Intercalated Anions on Electrochemical Capacitance Jeong Woo Lee,† Jang Myoun Ko,‡ and Jong-Duk Kim*,† †

Department of Chemical and Biomolecular Engineering, Center for Energy and Environment Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Applied Chemistry and Biotechnology, Hanbat National University, San 16-1, Dukmyoung-dong, Yuseong-gu, Daejeon 305-719, Republic of Korea

bS Supporting Information ABSTRACT: Hierarchical microspheres composed of wrinkled α-Ni(OH)2 nanosheets were synthesized from an aqueous solution containing nickel salts, hexamethylenetetramine, 1-butanol, and dodecyl sulfate. The exchange of intercalated dodecyl sulfate anions within the microspheres by smaller anions (Cl, NO3, OAc, and SO42) resulted in retention of morphology. A possible growth mechanism is proposed based on the observation of the effects the parameter variation (different nickel and hydroxide sources, several alcohols, with and without dodecyl sulfate as well as 1-butanol) had on microsphere formation. Electrochemical and capacitive properties of the anionexchanged microspheres were studied by cyclic voltammetry and galvanostatic chargingdischarging. Interestingly, the specific capacitance was determined by size of the intercalated anions and not by the basal spacing; for example, the largest anion, SO42, gave the lowest specific capacitance, whereas the smallest anion, Cl, gave the highest specific capacitance. Our data support the notion that larger anions impede the mobility of OH ions toward the surface of Ni(OH)2 sheets to a greater extent than smaller ions.

’ INTRODUCTION Metal oxides and metal hydroxides are interesting, low cost, and low toxicity energy storage materials for electrochemical capacitor applications. Many studies to date have been directed toward replacing RuO 2 1,2 with NiO, 3,4 Co3 O 4 , 5 MnO2 , 6 Co(OH)2,7 or Ni(OH)2.8 In recent years Ni(OH)2 has received much attention due to its superior theoretical electrochemical properties. In particular, β-Ni(OH)2 has been extensively studied. Despite being a metastable phase, α-Ni(OH)2 presents itself as a promising material for secondary battery9 and electrochemical capacitor applications.10 Two polymorphic forms of layered nickel hydroxide (Ni(OH)2) exist: the α- and β-phases. Control over morphology, basal spacing, and types of inorganic materials present is facilitated in the α-Ni(OH)2 phase; thus manipulation of magnetic, electrical, and ion transport properties can be realized. Both forms have hexagonal crystal structures in which Ni(OH)2 layers are stacked along the c-axis. A main feature differentiation between these phases is the presence of intercalated species (water and/or anions) within the interlayer galleries of α-Ni(OH)2. β-Ni(OH)2 has a brucite-like structure (a = 3.12 Å, c = 4.6 Å; JCPDS Card No. 14-0117) comprised of perfectly stacked Ni(OH)2 layers which do not contain water or any charge-balancing anions between its layers. α-Ni(OH)2, on the other hand, is a hydrotalcite-like structure r 2011 American Chemical Society

(JCPDS Card No. 38-0715) comprised of randomly stacked Ni(OH)2x layers along the c-axis intercalated with water or anions. The basal spacing of α-Ni(OH)2 can be adjusted from 31.7 to 7.5 Å by adjusting the species intercalated in the interlayer galleries.11 The size and morphology of α-Ni(OH)2 particles are affected by several factors: inorganic precursor, solvent, temperature, concentration, and surfactant template. Yang et al. reported the synthesis of α-Ni(OH)2 with nanoribbon and nanoboard-type structures by a hydrothermal process.12 Cao et al. reported various α-Ni(OH)2 morphologies for alkaline rechargeable batteries by reverse micelle/microemulsion and a hydrothermal method.13 Ida et al. reported the synthesis of layered nickel hydroxide intercalated with dodecyl sulfate, and then successfully eliminated dodecyl sulfate molecules between the Ni(OH)2 layers, resulting in the formation of hexagonal α-Ni(OH)2 nanosheets that are potentially useful in ultrathin film devices.14 In our previous paper we demonstrated a simple, effective coprecipitation synthetic method using alcohol and a dodecyl

Received: July 6, 2011 Revised: August 20, 2011 Published: August 22, 2011 19445

dx.doi.org/10.1021/jp206379h | J. Phys. Chem. C 2011, 115, 19445–19454

The Journal of Physical Chemistry C

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Figure 1. Schematic representation of the steps in the synthesis of α-Ni(OH)2 NBHMs.

sulfate template for growing layered hydroxide structures with high crystallinity.15 In this paper, we describe the use of this two-step process to prepare α-Ni(OH)2 nanosheet based hierarchical microspheres (NBHMs), which can contain a variety of intercalated anions. The homogeneous precipitation method of hexamethylenetetramine (HMT) hydrolysis in water/alcohol mixture was used to form a α-Ni(OH)2 intercalated with dodecyl sulfate (α-Ni(OH)2-DS). The effects of dodecyl sulfate molecules and alcohol on the morphology of α-Ni(OH)2-DS were investigated. Intercalated dodecyl sulfate molecules can be completely displaced by a desired anionic species (Cl, NO3, OAc, and SO42) by anion exchange while retaining the hierarchical morphology of the parent α-Ni(OH)2-DS. While most research has focused on the effect Ni(OH)2 morphologies have on the electrochemical capacitance properties, only a few studies have probed the effects various intercalated anions have on electrochemical capacitance. In this paper we present our electrochemical capacitance investigations, using cyclic voltammetry and the galvanostatic chargingdischarging method, into the effect different intercalated anions have on the electrochemical capacitance of α-Ni(OH)2 NBHMs.

’ EXPERIMENTAL SECTION Materials. Nickel chloride [NiCl2 3 6H 2 O], nickel nitrate [Ni(NO3 )2 3 6H2 O], nickel acetate [Ni(OAc)2 3 4H 2 O, Ni(CH3COO)2 3 4H2O], nickel sulfate [NiSO4 3 6H2O], hexamethylenetetramine [HMT, C6H12N4], urea [CH4N2O], ammonia solution [NH4OH], sodium hydroxide [NaOH], sodium dodecyl sulfate [SDS, C12H25SO4Na], sodium chloride [NaCl], sodium nitrate [NaNO3], sodium acetate [NaOAc, Na(CH3COO)], sodium sulfate [Na2SO4], 1-propanol [C3H5OH], 1-butanol [C4H7OH], 1-pentanol [C5H9OH], cetyltrimethylammonium bromide [CTAB, C19H42BrN], chloroform [CHCl3], and potassium hydroxide [KOH] were purchased from Sigma-Aldrich. All reagents were used as received without any further purification. Deionized water was used throughout the experiments. Preparation of α-Ni(OH)2 NBHMs. Figure 1 shows the procedure for synthesizing α-Ni(OH)2 nanosheet based hierarchical microspheres by a two-step reaction. First, α-Ni(OH)2-DS NBHMs were synthesized by reacting a soluble nickel source, SDS, and HMT in an aqueous solution including 1-butanol. The second step entails the exchange of dodecyl sulfate template anions in the Ni(OH)2

galleries, resulting in the product α-Ni(OH)2 NBHMs intercalated with the desired anions. Synthesis of α-Ni(OH)2-DS NBHMs. α-Ni(OH)2-DS NBHMs were synthesized by a homogeneous precipitation method using HMT as a hydrolysis agent. The representative procedure for the synthesis was as follows. Nickel nitrate (5 mmol), SDS (20 mmol), HMT (50 mmol), 1-butanol (10 mL), and deionized water (90 mL) were dissolved in a round-bottom flask. The temperature was raised to 90 °C and maintained with vigorous magnetic stirring for 2 h. After the reaction, the flask was allowed to cool to room temperature, and a green precipitate was isolated and washed with excess water to remove excess surfactant and other soluble byproducts. In order to investigate the effect of the nickel source, nickel chloride, nickel acetate, and nickel sulfate were used in equimolar amounts in place of nickel nitrate. Preparation of α-Ni(OH)2 NBHMs with Different Anions. An anion exchange process was conducted according to a previously reported method with minor modifications.16,17 CTAB (6 mmol), sodium nitrate (30 mmol), and chloroform (100 mL) were added to water (50 mL) containing the green precipitate (α-Ni(OH)2DS NBHMs). After extensive magnetic stirring for 1 h at room temperature, the suspension containing the green solid material in the aqueous (upper) and organic (lower) phases was isolated and washed (three times with chloroform, once with acetone, and three times with water, in that order). The green precipitate was separated from the solution by centrifugation and dried at room temperature under vacuum. To verify that different anions were successfully inserted into the interlayers, sodium chloride, sodium acetate, and sodium sulfate were substituted in equimolar amounts for sodium nitrate. The basal spacings of the prepared α-Ni(OH)2 NBHMs depending on the intercalated anions and nickel sources are shown in Table 1. Sample Characterization. The crystal structures were analyzed by X-ray diffraction (XRD; Rigaku D/MAX-IIIC) equipped with Cu Kα radiation (λ = 1.504 05 Å). To investigate the morphology and internal crystal structure, transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were taken on a JEM-2100F microscope at 200 kV. Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) data were obtained on a Nova 230 microscope (FEI Company). Fourier transform infrared spectroscopy (FT-IR) was carried out on an IFS66 V/S & HYPERION 3000 spectrometer (Bruker Optics, Germany) in the range 6004000 cm1. Thermogravimetric analyses (TGA) were 19446

dx.doi.org/10.1021/jp206379h |J. Phys. Chem. C 2011, 115, 19445–19454


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Table 1. Summary of d003 Values of α-Ni(OH)2 NBHMs Depending on Intercalated Anion and Nickel Source sample no.

intercalated anion

d003a (nm)

A1

NiCl2

Cl

0.94

B1 C1

Ni(NO3)2 Ni(OAc)2

Cl Cl

0.98 0.94

D1

NiSO4

Cl

0.94

A2

NiCl2

NO3

0.95

B2

Ni(NO3)2

NO3

0.95

C2

Ni(OAc)2

NO3

0.99

D2

NiSO4

NO3

0.94

A3

NiCl2

OAc

0.99

B3 C3

Ni(NO3)2 Ni(OAc)2

OAc OAc

0.95 0.99

D3

NiSO4

OAc

0.96

A4

NiCl2

SO42

1.09

B4

Ni(NO3)2

SO42

1.09

C4

Ni(OAc)2

SO42

1.10

NiSO4

SO42

1.11

D4 a

nickel source

Figure 2. XRD patterns of α-Ni(OH)2-DS NBHM samples as a function of the nickel source: (a) Cl, (b) NO3, (c) OAc, and (d) SO42.

In eq 2, C is the specific capacitance, I is the current, Δt is the discharging time, ΔV is the potential window, and m is the mass of electroactive material.

d003 (basal spacing) was calculated from the (003) plane.

’ RESULTS AND DISCUSSION

1

performed at a heating rate of 10 °C min in air on a thermal analyzer (NETSCH TG 209 F3, Germany). Elemental analyses (EA) were conducted with a Flash EA 1112 series instrument (Thermo Fisher Scientific). Preparation of Electrodes. To evaluate the electrochemical properties of α-Ni(OH)2 NBHMs, working electrodes were prepared as follows: α-Ni(OH)2 NBHMs, vapor-grown carbon fiber (VGCF; Showa Denko K.K, Japan, specific area 13 m2/g, aspect ratio 67) as a conducting material, and poly(vinylidene fluoride) (PVDF) as a binder were mixed in a weight ratio of 85:10:5, respectively, using N-methyl-2-pyrrolidone (NMP) as a solvent, yielding a paste. The obtained paste was incorporated into nickel foam (1 cm 1 cm). The mass of active materials in the working electrode was 23 mg. Working electrodes prepared in this way were dried at room temperature for 1 day and then in a vacuum oven for 1 day, in that order. Evaluation of Electrochemical Properties. Electrochemical studies were carried out by measurement of cyclic voltammetry (CV), galvanostatic chargingdischarging of half cells using an EZStat potentiostatgalvanostat (Nuvant Systems Inc.). A beakertype three-electrode cell was equipped with an α-Ni(OH)2 NBHMs on nickel foam working electrode, an Ag/AgCl electrode (BASi) as a reference electrode, and platinum wire as a counter electrode. For all electrochemical characterization experiments, 1 M KOH was used as the electrolyte solution at ambient temperature. The specific capacitance was calculated by intergrating the area under the CV curve to obtain the charge (Q) and then dividing by the mass of electroactive materials (m), the scan rate (v), and the potential window (ΔV = Va Vc) according to eq 1: Z Vc Q 1 ¼ C¼ IðV Þ dV ð1Þ ΔV mvðVa Vc Þ Va In addition, specific capacitance can be calculated from the galvanostatic chargingdischarging function according to eq 2: C¼

IΔt ΔVm

ð2Þ

Characterization of α-Ni(OH)2-DS NBHMs. Figure 2 shows XRD patterns of α-Ni(OH)2-DS NBHMs made from different nickel sources. Regardless of the nickel source, intense layer diffraction peaks (00l) were observed in the low angle range (2θ < 20°), providing clear evidence of a layered structure. However, this series of peaks was shifted to a lower angle (i.e., larger dspacing) relative to previous reports on other α-phases.18,19 This indicates that intercalated dodecyl sulfate molecules between Ni(OH)2 layers enlarged the interlayer spacing. For nickel sources NiCl2, Ni(NO3)2, Ni(OAc)2, and NiSO4, the basal spacings of α-Ni(OH)2-DS NBHMs calculated based on the (003) reflection were 2.70, 2.71, 2.68, and 2.72 nm, respectively. These values are similar to those of other dodecyl sulfate intercalated layered double hydroxides and α-Ni(OH)2.20,21 Furthermore, two broad and asymmetric peaks were observed at about 33.7 and 60.0°, corresponding to nonbasal spacing and which are present in turbostratic materials. Turbostratic disorder can be explained on the basis of a stacking order of brucitelike structures parallel to and equidistant along the c-axis of the hexagonal structure but randomly twisted relative to each other. This turbostratic behavior has also been observed in other α-Ni(OH)2 samples.22 In conclusion, all diffraction peaks in the XRD patterns of α-Ni(OH)2-DS NBHMs definitely indicated the formation of the α-phase. The choice of soluble nickel sources did not affect the basal spacing of α-Ni(OH)2DS NBHMs. Characterization of α-Ni(OH)2 NBHMs. To enhance the electrochemical capacity of the Ni(OH)2 sheets, the intercalated dodecyl sulfate molecules, which have poor ionic conductivity, should be removed. In order to displace these amphiphilic anions, a two-phase anion exchange process was carried out. The exchange process is driven by the formation of a salt between dodecyl sulfate (from the interlayer galleries) and the cationic surfactant CTAB, present in excess in the aqueous solution. The resulting salt can dissolve in the organic phase (chloroform). Charge-compensating anions, present in excess in the aqueous phase, are intercalated into the interlamellar space to neutralize 19447



Figure 3. XRD patterns of α-Ni(OH)2 NBHMs obtained after anion exchange (see Table 1).

the positively charged Ni(OH)2 layer while retaining the hierarchical morphology of the solid (vide infra). Figure 3 shows XRD patterns of α-Ni(OH)2 NBHMs containing different intercalated anions. All α-Ni(OH)2 NBHM samples also showed turbostratic disorder, similar to α-Ni(OH)2-DS NBHMs. Regardless of the intercalated anion species, intense diffraction peaks (00l) were observed in the low angle range (2θ < 20°) and clearly evidenced retention of the layered structure after the anion exchange process. However, the basal spacings of α-Ni(OH)2 NBHMs became smaller due to substitution of dodecyl sulfate molecules by smaller anions. The basal spacings of the samples were very similar with the small anions studied: 0.940.98 nm (Cl), 0.940.99 nm (NO3), 0.950.99 nm (OAc), and 1.091.11 nm (SO42) (see Table 1). However, the basal spacing of SO42 intercalated samples was larger than that of Cl, NO3, and OAc intercalated samples. The trend in basal spacing corresponds to the order of anionic radii (Cl, OAc, NO3, and SO42) in the interlayer space.23 It should be noted that the basal spacing is also dependent on the degree of hydration of the interlayer galleries, and varies somewhat from sample to sample. This phenomenon has been reported for other α-Ni(OH)2 samples.24 Figure 4a shows TEM images of α-Ni(OH)2 NBHMs, which are morphologically very similar to α-Ni(OH)2-DS NBHMs (Figure S1a in the Supporting Information). During the anion exchange process, the morphology of the particles was not changed, and all the α-Ni(OH)2 samples derived from α-Ni(OH)2-DS NBHMs were clearly composed of hierarchical microspheres. Figure 4b shows an SAED pattern taken at the thin edge of an α-Ni(OH)2 NBHM. The SAED displays hexagonally arranged spots as (hk0) reflections on the (00l) plane, which were well matched with the hexagonal α-Ni(OH)2 phase. In the case of α-Ni(OH)2-DS NBHMs, the SAED pattern also showed hexagonal spots (Figure S1b in the Supporting Information). Crystallization with amphiphilic anions between the Ni(OH)2 layers did not significantly affect the in-plane structure of the sheets. Figure 4c,d shows NiO nanoparticles formed by the electron beam around a very thin edge of αNi(OH)2 during TEM observation; the nanoparticles are 56 nm in size. The d-spacing was calculated to be 2.08 Å,

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Figure 4. TEM image and SAED pattern of sample A1: (a) TEM image and (b) SAED pattern along the [001] direction. (c) TEM image taken from a very thin edge after irradiation with an intense 200 kV electron beam and (d) NiO nanoparticles formed by electron beam during TEM observation.

corresponding to the (200) planes of a cubic NiO structure (JCPDS Card No. 47-1049). In the case of α-Ni(OH)2-DS NBHMs, nanoparticles were also formed (Figure S1c,d in the Supporting Information), but relatively more time was needed, and their size was 23 nm, smaller than in the case of αNi(OH)2 NBHMs. It is believed that dodecyl sulfate molecules located in the interlayer space offered a significant resistance to external conditions such as the electron beam. A similar result was found when layered zinc hydroxide was converted to ZnO nanocrystals of 15 nm size by an electron beam under TEM observation.25 Figure 5 shows SEM images of α-Ni(OH)2 NBHMs at different magnifications. Hierarchical morphology was clearly observed, and was well matched with TEM images. Numerous very thin nanosheet structures with irregular shapes gathered and formed hierarchical spheres with a diameter of 23 μm size. The thickness of the nanosheets was approximately 1530 nm (Figure 5d), which is smaller than that of α-Ni(OH)2-DS NBHMs (Figure S2d in the Supporting Information), indicating the removal of dodecyl sulfate molecules between Ni(OH)2 layers. In conclusion, based on the results of XRD patterns and SEM images, it is confirmed that dodecyl sulfate molecules were successfully eliminated from α-Ni(OH)2-DS NBHMs, resulting in the formation of typical α-Ni(OH)2. FT-IR spectra of samples A1, A2, A3, and A4 are shown in Figure 6. These samples were synthesized with NiCl2 as the nickel source and were chosen to confirm whether small anions were successfully inserted into the Ni(OH)2 layers during the anion exchange process. For all compounds studied, common vibration bands were observed at 3660, 3500, 2900, 2838, 1640, 1340, 1190, 1030, and 985 cm1, and are marked as lines in Figure 6. The peak at 3660 cm1 was assigned to a vOH stretching band, and confirmed the presence of a hydroxyl group of the brucite structure of α-Ni(OH)2 phase.26 The presence of a broad feature in the hydroxyl stretching region (32003600 cm1) corresponded to the hydrogen-bonded hydroxyl groups from the metal hydroxide and intercalated water located in the interlamellar space of the turbostratic structure of α-Ni(OH)2.27 The bands at 2900 and 19448



Figure 5. SEM images of sample C2 at different magnifications.

Figure 6. FT-IR spectra of samples A1, A2, A3, and A4, respectively.

2838 cm1 were attributed to a CH stretching vibration of HMT molecules.28 The broad band around 1640 cm1 was assigned to the δ(H2O) vibration of water molecules existing between Ni(OH)2 layers.15 In addition, the sharp bands at 1340, 1190, 1030, and 985 cm1 were characterized as CN vibration, indicating that HMT molecules were located in the interlayer space.22 Consequently, all samples showed the presence of HMT, a hydroxyl group, and water species. For sample A2, the bands detected at 1480 and 1390 cm1 were assigned to NO stretching, indicating the v3 vibration mode of NO3 in C2v symmetry and D3h symmetry, respectively. In C2v symmetry, nitrate ions could be grafted to the Ni(OH)2 layer, whereas for D3h symmetry nitrate ions could be free in the interlayer space.1 For sample A3, the bands at 1560 and 1400 cm1 were assigned to vas and vs of the CdO stretching mode of the acetate group,29 respectively. In the case of A4, the peaks at 1160 and 1050 cm1 could be assigned to free sulfate ions in the interlayer space.30 TGA curves of samples A1, B2, C3, and D4 are shown in Figure S3 in the Supporting Information. All samples underwent a two-step weight loss when the temperature was around 100 and 350 °C. During the thermal decomposition, it is known that αNi(OH)2 undergoes multistep and larger mass losses compared to β-Ni(OH)2 (generally 18%), which does not have any anion

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or water species.24 The observed first mass loss was removal of intercalated or absorbed water, and the second mass loss was the decomposition of HMT and intercalated anions and the dehydration of α-Ni(OH)2. In addition, in Figure S4 in the Supporting Information, the final products of α-Ni(OH)2 after calcination at 600 °C for 2 h clearly reveal a cubic NiO structure. From the results of FT-IR spectra, all prepared α-Ni(OH)2 NBHM samples showed the existence of HMT, water molecules, and different anion species used in the anion exchange process in the interlayer space. Therefore, the chemical formula of αNi(OH)2 NBHMs synthesized by our method can be written as follows: Ni(OH)2x(C6H12N4)y(Am)x/m 3 nH2O (A = Cl, NO3, OAc, and SO42). On the basis of the results of EDS, EA, and TGA data, calculated chemical formulas of α-Ni(OH)2 NBHMs were similar to each other and are provided in Table 2. Effect of Alkaline Sources, Aliphatic Alcohols, and Dodecyl Sulfate on the Formation of α-Ni(OH)2-DS NBHMs. From the foregoing, we determined that α-Ni(OH)2-DS NBHMs could be synthesized from a nickel source, SDS, and HMT in a 1-butanol/water mixture. The hierarchical morphology of αNi(OH)2 was retained during anion exchange (C12H25SO4 f Cl, NO3, OAc, and SO42) of the interlamellar galleries. Therefore, experiments to determine which factors played critical roles in the synthesis of α-Ni(OH)2-DS NBHMs as a precursor to α-Ni(OH)2 NBHMs were performed. Various experiments with different alkali sources and aliphatic alcohols were performed to shed light on the effects of dodecyl sulfate and alcohol. Reactions with Other Alkaline Sources. When urea was used in place of HMT as a source of base, a green precipitate was obtained after reaction at 90 °C for 2 h. The product was α-Ni(OH)2 with a basal spacing of 7.5 Å (Figure 7a), and was not hierarchical type but instead displayed a mostly irregular morphology (Figure 7b). It was determined that the template dodecyl sulfate did not insert into the interlayer spaces between Ni(OH)2 sheets, and thus α-Ni(OH)2 structures were not synthesized. When sodium hydroxide solution or ammonia was used as an alkali source, there was no precipitate after the reaction time of 2 h at 90 °C. In conclusion, except for HMT, no other bases afforded the α-Ni(OH)2-DS morphology with 2 h of processing time and at 90 °C. In earlier experiments, HMT was used as a hydrolysis agent in the preparation of highly crystalline α-Ni(OH)224 and α-Co(OH)2.31 The following reactions might occur in solution: C6 H12 N4 þ 6H2 O f 6CH2 O þ 4NH3

ð3Þ

NH3 þ H2 O f NH4 þ þ OH

ð4Þ

During the heating process above 60 °C, HMT hydrolyzes and releases OH ions into the solution, as shown in eq 4. Interestingly in our synthetic procedure, HMT alone produces the α-phase with hierarchical morphology, even though urea is also known to release OH ions through the hydrolysis to form ammonia and bicarbonate anions. Reactions without Alcohol or SDS. In addition to products prepared from 1-butanol, α-Ni(OH)2-DS synthesized with the same volume of 1-propanol or 1-pentanol also had hierarchical morphologies and basal spacings of about 2.70 nm (data not shown). In addition, when the volume of different alcohol species was 10 and 30 mL, there was no difference in the morphology or 19449



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Table 2. Summary of Chemical Formulas and Elemental Analysis Data of α-Ni(OH)2 NBHMs exptl (%) sample no.

chemical formulaa

Ni

A1

Ni(OH)1.85(C6H12N4)0.11(Cl)0.15 3 0.50H2O Ni(OH)1.88(C6H12N4)0.08(NO3)0.12 3 0.55H2O

48.52

7.72

3.91

4.95

0

48.97

6.60

3.48

5.14

0

48.11

5.83

2.31

5.21

0

48.48

4.76

2.40

5.09

0

50.31

9.71

3.01

4.13

0

50.69

8.91

3.30

4.35

0

50.43

4.54

3.99

3.25

3.21

49.52

2.43

3.09

2.83

3.24

B2 C3 D4 a

calcd (%)

Ni(OH)1.84(C6H12N4)0.09(OAc)0.16 3 0.21H2O Ni(OH)1.76(C6H12N4)0.04(SO4)0.12 3 0.71H2O

C

H

N

S

Ni

C

H

N

S

Chemical formulas were calculated from EDS, EA, and TGA data.

Figure 7. Sample obtained by reaction with urea as a source of base: (a) XRD pattern and (b) SEM image.

basal spacing of the final products (data not shown). The effect of alcohol to form α-Ni(OH)2-DS was investigated in a control experiment without alcohol. After reaction without alcohol at 90 °C for 2 h, no precipitate was found in the reactor flask. Therefore, the alcohol plays a critical role in the formation of α-Ni(OH)2-DS NBHMs. At the moment it is not clear how the alcohol affects the formation of this morphology. It is possible that, in the presence of alcohol, dodecyl sulfate micelles are dissolved in the aqueous phase, and the amphiphilic anions are thus available to form an interaction compound with Ni(OH)2 as it forms during the slow hydrolysis of HMT. To elucidate the effects of dodecyl sulfate on realizing α-Ni(OH)2-DS NBHMs, an experiment without SDS was performed. After reaction at 90 °C for 2 h, there was no precipitate. In the case of the experiment without 1-butanol and SDS, there was also no precipitate. In conclusion, when SDS was not employed, irrespective of whether 1-butanol was added, there was no precipitate. On the basis of these experimental results, it is believed that dodecyl sulfate plays an important role as a template to form α-Ni(OH)2-DS NBHMs. This result is similar to a previous paper in which rare earth oxide nanotubes were synthesized in the presence of a high SDS concentration in water.32 Although in that case it was not a hierarchical-type structure, the two syntheses are similar in the sense that a DS intercalated phase was first formed, and the amphiphilic anions were then easily displaced by exchange with smaller anions. Proposed Growth Mechanism of the Hierarchical Morphology. The mechanism underlying the formation of the hierarchical morphology was investigated with green powder collected after the temperature reached 90 °C. Figure 8 shows SEM images of the collected powder as the following times: (a) 1 min, (b) 5 min, (c) 15 min, and (d) 72 h, respectively. In the initial reaction, irregular morphologies with very thin sheets and

Figure 8. SEM images of α-Ni(OH)2-DS NBHMs collected at 90 °C for (a) 1 min, (b) 10 min, (c) 15 min, and (d) 72 h, respectively.

high curvature were observed (Figure 8a,b). As the reaction progressed (Figure 8c), hierarchical morphologies with a diameter of 23 μm were observed. At extended reaction times, the morphologies and size of α-Ni(OH)2-DS NBHMs were not significantly changed (Figure 8d) compared to those observed after 2 h, except for the observation of aggregated structures. Furthermore, the yield did not increase markedly. Through comparative experiments, a process time of 2 h was deemed appropriate to obtain α-Ni(OH)2-DS NBHMs. This series of images shows clearly that, as the reaction progressed, the thickness of the Ni(OH)2 layer increased and finally posed a limitation to forming nanosheet based hierarchical structures. From these results, a formation mechanism for hierarchical morphology can be proposed. At 90 °C processing temperature, OH ions released by hydrolysis of HMT drive the reaction of nickel ions, dodecyl sulfate, and OH ion in the water/1-butanol 19450



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Figure 9. Schematic illustration of the formation mechanism of α-Ni(OH)2 NBHMs.

mixture system. α-Ni(OH)2 is a hydrotalcite-like structure comprised of stacked Ni(OH)2x layers which are formed by Ni centers coordinated in an octahedral arrangement, with OH groups forming the vertices of the octahedra. These octahedra form two-dimensional sheets by edge sharing; the sheets stack by hydrogen bonding between the hydroxyl groups of adjacent sheets.33 However, Ni(OH)2 layers intercalated with dodecyl sulfate molecules should have much weaker hydrogen bonding due to their hydrophobic tail groups, yielding thinner and more flexible sheets. As the reaction progresses, sheet-type materials can be divided forming several layer branches, but the fractal branching would not be repeated in many steps because the edge-on growth process has the lowest activation energy. The proposed mechanism for hierarchical structure formation is illustrated in Figure 9. Although many hierarchical structures have been reported, the detailed mechanism for the formation of inorganic structures has not been explained definitively. Whatever the mechanism, these hierarchical structures have high surface areas consisting of basal planes and edge surfaces, so they can have a number of enhanced physical and electrochemical properties.34,35 Electrochemical Properties of α-Ni(OH)2 NBHMs. The electrochemical properties and specific capacitance of α-Ni(OH)2 NBHMs were determined by cyclic voltammetry (CV) and galvanostatic chargingdischarging measurements in a three-electrode system. The α-Ni(OH)2 NBHMs were coated onto nickel foam as the working electrode. The reference electrode and counter electrode were Ag/AgCl and platinum, respectively. All experiments were conducted in 1 M KOH as the electrolyte at ambient temperature. CV curves of α-Ni(OH)2 NBHMs intercalated with different anions are shown in Figure 10, and all CV curves have two intense peaks. One peak is anodic (positive current density) during the oxidation reaciton of Ni2+ to Ni3+, and the other is cathodic (negative current density) during the reverse process. These peaks come from fast and reversible redox processes that

Figure 10. CV curves of samples A1, B2, C3, and D4 at 5 mV/s scan rate.

occur at the interface of α-Ni(OH)2 and the electrolyte. These reactions can be represented by eq 5:10 α-NiðOHÞ2 þ OH T γ-NiOOH þ H2 O þ e

ð5Þ

Since the specific capacitance is proportional to the area under the CV curve, this aspect was analyzed through a comparison of CV curves. The area under the CV curves increased in the order D4 (SO42) < C3 (OAc) < B2 (NO3) < A1 (Cl), which indicates that Cl intercalated α-Ni(OH)2 NBHMs had the highest capacitance among the samples studied. The specific capacitances calculated at 5 mV/s scan rate were 221 (D4), 580 (C3), 720 (B2), and 805 (A1) F/g. CV curves, the galvanostatic discharging curve, and the corresponding specific capacitances of samples B2, C3, and D4 are shown in Figures S5S7 in the Supporting Information. The electron storage mechanism of Ni(OH)2 is dependent on redox reaction eq 5 via OH ions and protons at the interface between Ni(OH)2 and the KOH electrolyte.36 We assumed that 19451



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Figure 11. CV curves and galvanostatic discharging curves of sample A1: (a) cyclic voltammetry curves at scan rates of 5, 10, 20, and 40 mV/s; (b) specific capacitance at scan rates of 5, 10, 20, and 40 mV/s; (c) galvanostatic discharging curve at current densities of 1, 2, 4, and 8 A/g; and (d) specific capacitance at current densities of 1, 2, 4, and 8 A/g.

the specific capacitance would be slightly enhanced with an increase in the basal spacing of the samples, based on the idea that larger spacing would provide easy access of OH ions and protons to Ni(OH)2 interlayer galleries. Thus, we anticipated that the capacitance would be enhanced by the intercalation of larger anions. However, Cl intercalated α-Ni(OH)2 NBHMs have the highest specific capacitance, and SO42 intercalated α-Ni(OH)2 NBHMs have the lowest specific capacitance. This result can be tentatively explained in terms of the charge of the anion and to some extent by the degree of hydration of the interlayer galleries. Doubly charged SO42 ions bind strongly to the positively charged Ni(OH)2 sheets, and are not easily exchanged by singly charged OH ions. The interconversion of Ni(OH)2 and NiOOH formally involves the movement of protons into and out of galleries. However, at the high pH of the electrochemical experiments, the concentration of free protons is extremely low and thus it is likely that intercalated water and OH ions dominate the transport process according to eq 5. In contrast to SO42, Cl ions are singly charged and the interlayer galleries (from the basal plane spacing) are well hydrated, leading to more facile exchange and transport of OH ions. This result is similar to that of α-Co(OH)2 reported by Hu et al.23 They synthesized α-Co(OH)2 intercalated by different anions in a one-pot synthesis; in that case, the product morphologies were different depending on the intercalated anion, but they also found a dramatically lower capacitance with intercalated SO42 relative to Cl ions. The CV curve and galvanostatic chargingdischarging were used to calculate the specific capacitance of sample A1 at various scan rates and current densities. Parts a and b of Figure 11 show the CV curves at scan rates of 5, 10, 20, and 40 mV/s, and the corresponding specific capacitances are 805, 597, 441, and 269

F/g, respectively. The specific capacitance of sample A1 is 1494, 986, 600, and 453 F/g at 1, 2, 4, and 8 A/g current density, respectively (Figure 11c,d). The decrease in specific capacitance is attributed to the presence of Ni(OH)2 sheets that cannot sustain redox reactions at fast scan rate or high current density because of slow transport of charge-compensating OH ions in the interlayer galleries.36,37 Therefore, at slow scan rate or low current density, full utilization of the electroactive surface of Ni(OH)2 enhanced the specific capacitance. α-Ni(OH)2 NBHMs had higher specific capacitance than other reported forms of α-Ni(OH)2.37,38 We considered that there are two reasons for the high specific capacitance of α-Ni(OH)2 NBHMs. The first is utilization of α-Ni(OH)2, which has superior electrochemical properties relative to β-Ni(OH)2.39 The second is the unique morphology of α-Ni(OH)2 which provides good access of OH ions and water to the Ni(OH)2 edge planes and thus to the interlayer galleries. To date, electroactive materials with hierarchical morphologies based on nanosheets have generally shown higher specific capacitance relative to bulk-type materials.7,40 Therefore, dodecyl sulfate as a soft template played a crucial role in achieving higher specific capacitance. An endurance test of sample A1 was conducted using galvanostatic chargingdischarging between 0.0 and 0.45 V at a nominal 4 A/g charging rate for 500 cycles. The specific capacitance as a function of cycle number is plotted in Figure 12. The degradation of the specific capacitance was about 9.1% after 500 cycles. Cycle retention is higher than those in other tests for α-Ni(OH)241 and α-Co(OH)2;23 this result is attributed to the easy access of OH ions and water to the Ni(OH)2 interlayer galleries during chargingdischarging cycles within the potential window. Cl intercalated α-Ni(OH)2 NBHMs provided the best long cycle stability. 19452



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Figure 12. Cycle test of sample A1: (a) galvanostatic chargingdischarging curve at a norminal charging rate of 4 A/g and (b) variation in the specific capacitance as function of cycle number.

’ CONCLUSION We have demonstrated a route to produce α-Ni(OH)2 NBHMs via use of a dodecyl sulfate template, and the properties of α-Ni(OH)2 NBHMs have been analyzed and described. The dodecyl sulfate molecules were then completely removed, resulting in the formation of α-Ni(OH)2 NBHMs without any change in morphology. Subsequently, anion exchange was carried out, and desired anions (Cl, NO3, OAc, and SO42) were intercalated. For synthesis, HMT, SDS, and 1-butanol played crucial roles in making nanosheet based hierarchical morphology. Electrochemical experiments of α-Ni(OH)2 NBHMs were performed by CV and by the galvanostatic chargingdischarging method. Doubly charged SO42 anions prevent easy access of OH ions and water to the surface of Ni(OH)2 and, consequently, yield low specific capacitance. In contrast to SO42, Cl ions are singly charged and the interlayer galleries are well hydrated, leading to more facile exchange and transport of OH ions. Therefore, Cl intercalated α-Ni(OH)2 NBHMs showed higher capacitance compared to SO42 intercalated α-Ni(OH)2 NBHMs. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM and SEM images of α-Ni(OH)2-DS NBHMs, TGA data of α-Ni(OH)2 NBHMs, XRD patterns of calcined α-Ni(OH)2 NBHMs, and CV curves, the galvanostatic discharging curve, and corresponding specific capacitances of samples B2, C3, and D4. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +82-042-350-3910. Tel.: +82042-350-3921.

’ ACKNOWLEDGMENT This work was supported by the BK 21 program and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0076882). The authors thank Prof. Thomas E. Mallouk for English editorial help.

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