Direct synthesis of Co2Al(OH)7−2x(CO3)x·nH2O layered double hydroxide nanolayers by successive ionic layer deposition and their capacitive performance

Applied Surface Science 320 (2014) 609–613

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Direct synthesis of Co2 Al(OH)7−2x (CO3 )x ·nH2 O layered double hydroxide nanolayers by successive ionic layer deposition and their capacitive performance A.A. Lobinsky ∗ , V.P. Tolstoy, L.B. Gulina Department of Solid State Chemistry, Saint-Petersburg State University, Peterhof, 198504 Saint-Petersburg, Russian Federation

a r t i c l e

i n f o

Article history: Received 18 June 2014 Received in revised form 31 August 2014 Accepted 19 September 2014 Available online 30 September 2014 Keywords: LDH Supercapacitor Nanolayers Layer-by-Layer SILD

a b s t r a c t New method of synthesis of Co2 Al(OH)7−2x (CO3 )x ·nH2 O layered double hydroxide (LDH) films by successive ionic layer deposition (SILD) is presented in this paper. The obtained nanolayers were characterized by SEM, EDX, XRD, XPS, FTIR spectroscopy and electrochemical techniques. The results showed that the as-synthesized product is formed by nanosheets with a thickness of 3–5 nm, having hydrotalcite crystal structure. Electrochemical characterization of the sample prepared by 50 cycles of SILD indicated a capacitive behavior with the specific capacitance value of 900 F/g at a current density of 1 A/g and 950 F/g at 0.5 A/g in 1 mol/L KOH aqueous solution. Repeated cycling for 1000 charge–discharge cycles demonstrate that capacitance increases by 6%, so such electrodes may be used as electrodes of hybrid supercapacitors. The presented convenient route of synthesis may be used for the preparation of LDH films with high surface area and a large capacitance. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Supercapacitors, also known as electrochemical capacitors, electrochemically store and deliver energy at high charge–discharge rates and are a key emerging technology for energy storage. In general, supercapacitors can be classified into two types according to their energy storage mechanism: electrical double layer capacitor and pseudocapacitors, depending on whether Faradaic redox reactions exits or not during the charge and discharge process [1]. Faradaic processes allow pseudocapacitors to achieve greater capacitances (10–100 times higher) and energy densities than electrical double layer capacitor. As electrode materials for pseudocapacitor have been widely used few kinds of materials include MnO2 [2,3], RuO2 [4], MoO3 [5], NiO and Ni(OH)2 [6,7], Co3 O4 and Co(OH)2 [8–10], etc. Recently, layered double hydroxides (LDHs) began to attract attention as a new electrode materials for supercapacitors. A general formula for LDHs may be written as [M2+ 1−x M3+ x (OH)2 ][An− ]x/n ·zH2 O, where M2+ may be cations of Mg2+ , Co2+ or Ni2+ and M3+ may be Al3+ , Fe3+ , or Cr3+ , etc. and An− can be an interlayer anion, e.g. CO3 2− , SO4 2− , NO3 2− . Currently, the

∗ Corresponding author. Tel.: +7 8124284104. E-mail addresses: [email protected] (A.A. Lobinsky), [email protected] (V.P. Tolstoy), [email protected] (L.B. Gulina). http://dx.doi.org/10.1016/j.apsusc.2014.09.136 0169-4332/© 2014 Elsevier B.V. All rights reserved.

chemistry of LDH is well studied and they have a wide application as catalysts, adsorbents, ion exchange, optical and magnetic materials, drug delivery systems and electrodes components for chemical power sources and sensors [11]. Among LDHs the most promising for the creation of new supercapacitors are layers containing cations of Co2+ , for example Co2 Al(OH)7 ·nH2 O. Previously, the electrochemical properties of electrodes of this composition were examined for a sample obtained by precipitation [12], hydrothermal [13,14] and refluxing [15] methods. It was shown that the capacitance of supercapacitors based on these electrodes is in the range from 263 to 641 F/g at a current density of 1 A/g. New possibilities of thin film LDH synthesis rose after development of Layer-by-Layer (LbL) synthesis techniques that make possible to deposit LDH layers on the surface of samples with irregular shape, providing exact thickness of the layer [16]. The LbL methods have advantages compared to the more conventional coating methods, in particular the simplicity of the LbL process and equipment, the flexible application to objects with irregular shapes and sizes and control over the required multilayer thickness [17]. The reagents to LbL synthesis commonly been used are colloidal nanoparticles, charged inorganic substances, heteropolyacids, and polyelectrolyte solutions. The LbL provides a simple method for preparing thin-film electrodes for electrochemical capacitors [18]. One of the methods of LbL synthesis is the SILD method [19–21] or in other terminology Successive Ionic Layer Adsorption and

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A.A. Lobinsky et al. / Applied Surface Science 320 (2014) 609–613

Fig. 1. SEM images of Co2 Al(OH)7−2x (CO3 )x ·nH2 O layer on silicon.

Reaction (SILAR) [22]. The synthesis of SILD method is performed without the use of polyelectrolyte solutions and this makes it possible to simplify the procedure significantly and get the coating that contain only inorganic compounds, which is important for many tasks. In this study, on example Co2 Al(OH)7−2x (CO3 )x ·nH2 O, we report about new simple route of synthesis of LDH by SILD method, using solutions of salt cobalt and aluminum, and its application to supercapacitors electrode materials.

2. Experimental As a substrate for nanolayers synthesis 5 mm × 25 mm polycrystalline nickel foam (NF) plates (110 PPI) were used, on which electrochemical experiments were preformed, and also 10 mm × 25 mm × 0.35 mm single-crystal silicon plates with 1 0 0 orientation, were used for physical characterization. Extra pure water (Milli-Q) was used in all experiments. Substrates of silicon were cleaned in an ultrasonic bath filled with acetone for 10 min. Then plates were sequentially treated for 10 min in concentrated HF, water, 70% HNO3 , water, 0.1 M KOH and than flushed out by water. NF plates were treated according to the technique described in [23] for 15 min in 6 M HCl solution, then several times rinsed by water and dried on air at 120 ◦ C for 30 min. The reagents used for synthesis were aqueous 0.01 M solutions of analytical grade Co(NO3 )2 ·6H2 O (pH 6.5) and Al(NO3 )3 ·9H2 O (pH 9.5). pH of these solutions was controlled by addition of NaOH solution. The time between the preparation of solutions and synthesis of nanolayers was 1 h. For synthesis of LDH substrate nickel foam plates were fixed in a holder of special home made automatic setup and sequentially immersed for 30 s into solution of cobalt salt, further in water, solution of aluminum salt and again in water. The sequence corresponds to one SILD cycle, which is repeated 50 times to obtain the desired film thickness. The obtained samples were characterized by SEM, EDX, FTIR, XRD, and XPS methods. The morphology and composition of synthesized films were investigated by SEM at accelerating voltage of 5 kV on Zeiss Merlin microscope equipped with an EDX spectrometer (Oxford INCAx-act). FTIR transmission spectra of synthesized films on silicon surface were registered by Shimadzu IR Prestige21 spectrophotometer using differential technique with respect to spectra of bare silicon plate. XRD patterns were obtained using a Bruker D8 DISCOVER X-ray diffractometer with CuK␣ radiation in grazing incidence diffraction geometry ( = 0.3◦ ). X-ray photoelectron spectroscopy of sample (XPS) was obtained using ESCALAB 250Xi electron spectrometer, with Al K␣ radiation (14,866 eV). All electrochemical measurements of electrodes with synthesized films were made by Elins P-30I potentiostat using a

three-electrode cell with 1 mol/L KOH solution as an electrolyte. Platinum plate and Ag/AgCl (aq. KCl sat.) were used as counter electrode and reference electrode, respectively. Electrochemical characterization of the films was made by cyclic voltammetry and galvanostatic charge–discharge techniques. Cyclic voltammograms were recorded at different scan rates of 5, 10 and 20 mV s−1 with a potential sweep range between 0 and 0.55 V. Specific capacitance C of a nickel foam electrode with LDH layer was determined according to [24] as: C=

I , (V/t)m

where I (mA) is a galvanostatic current, V (mV) is the potential window, t (s) is the discharge time of a cycle and m (g) is the mass of the active material in the film electrode. LDH formation on nickel surface was controlled by OHAUS PioneerTM PA54C precise balance. 3. Results and discussion The SEM images of the Co-Al LDH layers are shown in Fig. 1. As can be seen, the Co-Al LDH layers are formed of nanosheets with a thickness of 3–5 nm. Energy dispersive X-ray spectra analysis (EDX) (Fig. 2) show significant energy signal intensity of the Co, Al, O and C elements in the LDH nanolayers. The atomic concentration ratio of Co/Al is equal to 2:1. The X-ray diffraction (XRD) patterns of the LDH film is shown in Fig. 3. The series of diffraction peaks of (0 0 3), (0 0 6), (0 1 2), (0 1 5)

Fig. 2. EDX spectrum of Co2 Al(OH)7−2x (CO3 )x ·nH2 O layer on silicon.

A.A. Lobinsky et al. / Applied Surface Science 320 (2014) 609–613

Co2Al(OH)7.nH2O nanosheets

Co2+aq cations

1400 (003)

NaAl(OH)4

Co(NO3)2

1200

Ni foam

Ni foam

Ni foam

Intensity, CPS

611

1000

Co(NO3)2 N SILD

800 (006)

cycles

600

Ni foam

(012) (015)

400

(018)

"Vertically" oriented Co2Al(OH)7.nH2O nanosheets

200 10

20

30

40

50

60

70

Fig. 3. XRD pattern of Co2 Al(OH)7−2x (CO3 )x ·nH2 O layer on silicon.

110 100

1640

80

777

60 50

30

1358

40

403

611

70

3430

20 10 0 4000

3500

3000

2500

2000

Wavenumber, cm

1500

1000

500

-1

Fig. 4. FTIR transmission spectrum of Co2 Al(OH)7−2x (CO3 )x ·nH2 O layer on silicon surface.

and (0 1 8) correspond to hydrotalcite crystal structure of the layers [12]. On FTIR spectra (Fig. 4) of the films one can note that the valence band (3430 cm−1 ) and the deformation band (1640 cm−1 ) correspond to vibrations of O H bond, and the band with peaks at 1358 cm−1 and 777 cm−1 vibrations of C O in anion CO3 2− [14], which may be attributed to the adsorbed CO2 from the atmosphere. Other absorption bands in the region of 700–400 cm−1 may be assigned to the valence vibrations Co O in LDH [15].

The XPS spectra of Co 2p, O 1s and Al 2p are presented in Fig. 5. The peaks at 781.2 eV, 531.5 eV and 74.0 eV binding energy refer to the Co 2p, O 1s and Al 2p signal, respectively. Comparison with the data [25,26] showed the main peaks of Co 2p, O 1s and Al 2p are assigned to Co Al LDH. Investigation by XPS gives the atomic concentration ratio of Co/Al is equal to 2:1, that correspond to the date of EDX analysis. As a result of the investigation of synthesized nanolayers by SEM, EDX, XRD, FTIR and XPS methods it was shown that these nanolayers consist of Co Al LDH including water molecules. Based on these results, we propose a formula of synthesized layers as Co2 Al(OH)7−2x (CO3 )x ·nH2 O. In our opinion, reactions proceeding during the film synthesis can be explained using scheme (Fig. 6) of the chemical processes occurring on the surface at SILD synthesis. At the first step after dipping in the solution of Co(NO3 )2 on the surface adsorption occurs of cations of Co2+ aq . Then sample treatment in the solution of Al(NO3 )2 . At pH 9.5 aluminum exists in solution mainly in the form of Al(OH)4 − complex, this complex reacts with Co(OH)2 layer. As a result of reaction Co2 Al(OH)7 ·nH2 O nanolayer is formed on the surface. After treatment in the solution of Co(NO3 )2 on the second SILD cycle a positive charge is gained and for account of the mutual repulsion are mainly oriented vertically to the substrate surface. Important results were obtained by the electrochemical study of Co2 Al(OH)7−2x (CO3 )x ·nH2 O electrode layer on the surface of nickel foam prepared by SILD method. As demonstrated in Fig. 7, two redox processes takes place in the layer, including Co2+ → Co4+ transformation at 500 mV. Proportionality of currents to the square root of scan rate provides information that the film is thick enough, and charge transfer rate is limited by diffusion of charge carriers in the film. Charge–discharge curves of nickel foam/Co2 Al(OH)7−2x (CO3 )x · nH2 O electrode at different polarizing currents (Fig. 8) allow one 531.6

5000

2500

O 1s 781.2

Al 2p

Intensity, CPS

Intensity, CPS

Intensity, CPS

Co 2p

810

800

790

Binding energy, eV

780

770

74.0

200

Transmittance, %

90

Fig. 6. Schematic description of SILD synthesis of Co2 Al(OH)7−2x (CO3 )x ·nH2 O nanosheets.

545

540

535

530

Binding energy, eV

525

85

80

75

70

Binding energy, eV

Fig. 5. X-ray photoelectron spectra of Co 2p, O 1s and Al 2p from Co2 Al(OH)7−2x (CO3 )x ·nH2 O layer on silicon surface.

65

612

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Table 1 Comparison of the specific capacity, electrochemical stability and thickness of nanoparticles of the electrodes CoAl-LDH for supercapacitors obtained by different methods. Method of synthesis

Specific capacitance (at a current density 1 A/g) (F/g)

Specific capacitance retention

Thickness of nanoparticles CoAl(OH)x ·nH2 O (nm)

Refs.

Precipitation method Hydrothermal method Hydrothermal method Refluxing method

263 672 466.5 711.5

98% after 400 cycles 92.5% after 5000 cycles – 81.2% after 2000 cycles

40–60 40 40–60 100

[12] [13] [14] [15]

0.25

120

5 mV/s 10 mV/s 20 mV/s

0.20

100

Specific capacitance, %

0.15

I v-1/2 / A V 1/2 s-1/2

0.10 0.05 0.00 -0.05 -0.10 -0.15

0

100

200

300

400

500

600

E / mV vs. Ag/AgCl Fig. 7. Cyclic voltammograms of NF electrode, covered by Co2 Al(OH)7−2x (CO3 )x ·nH2 O film, recorded at different scan rates. Current is normalized by square root of scan rate.

to determine its capacitance. For instance, specific capacitance of a sample, formed by 50 SILD cycles, is 950, 900, 830 and 550 F/g obtained at current density 0.5, 1, 2 and 5 A/g respectively, in 1 mol/L KOH aqueous solution. The long cycle life of supercapacitors is also a crucial parameter for their practical application. The result shows that the capacitance (Fig. 9) increases by 6% after 1000 charge–discharge cycles at current density of 5 A/g. As compared with other papers (Table 1), electrodes based on LDH layers obtained by SILD method possess a higher specific capacity. These results can be explained by the formation of very fine nanocrystals (with a thickness of 3–5 nm) which increase the surface area of the electrode and thus the specific capacity.

500

Potential / mV vs. Ag/AgCl

60

40

20

-0.20 -0.25

80

0

200

400

600

800

1000

Cycle number Fig. 9. Dependence of specific capacitance of nickel foam electrode, covered by Co2 Al(OH)7−2x (CO3 )x ·nH2 O film, from the number of charge–discharge cycles at current density equal to 5 A/g.

4. Conclusions Repeated sequential treatment of substrate by SILD technique in Co(NO3 )2 and Al(NO3 )3 solutions leads to formation of Co2 Al(OH)7−2x (CO3 )x ·nH2 O layer on the surface, which has hydrotalcite crystal structure and is formed by nanosheets with a thickness of 3–5 nm. Electrochemical study of nickel foam electrodes modified by LDH film prepared by 50 SILD cycles demonstrates that specific capacitance of the film is 900 F/g at current density 1 A/g and 950 F/g at 0.5 A/g. The repeated cycling for 1000 charge–discharge cycles demonstrate that capacitance increases by 6%, so such electrodes may be used as electrodes of hybrid supercapacitors. Acknowledgments

5 A/g 2 A/g 1 A/g 0,5 A/g

400

0

The authors acknowledge the financial support of the SPbSU Grant (# 12.38.259.2014). We gratefully acknowledge the Centers for X-ray diffraction studies, Physical methods of surface investigation, Nanotechnology and Chemical Analysis of St. Petersburg State University, and also Ph.D. O.V. Levin for his assistance in the electrochemical measurements.

300

200

References 100

0 0

500

1000

1500

2000

Charge / C g

2500

3000

3500

-1

Fig. 8. Galvanostatic charge–discharge curves of NF electrode, covered by Co2 Al(OH)7−2x (CO3 )x ·nH2 O film, recorded at different current densities.

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