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Graphene vs. reduced graphene oxide: A comparative study of graphene-based nanoplatforms on electrochromic switching kinetics
Accepted Manuscript Graphene vs. reduced graphene oxide: a comparative study of graphene-based nanoplatforms on electrochromic switching kinetics Bhushan Gadgil, Pia Damlin, Carita Kvarnström PII:
S0008-6223(15)30283-9
DOI:
10.1016/j.carbon.2015.09.065
Reference:
CARBON 10335
To appear in:
Carbon
Received Date: 4 July 2015 Revised Date:
15 September 2015
Accepted Date: 17 September 2015
Please cite this article as: B. Gadgil, P. Damlin, C. Kvarnström, Graphene vs. reduced graphene oxide: a comparative study of graphene-based nanoplatforms on electrochromic switching kinetics, Carbon (2015), doi: 10.1016/j.carbon.2015.09.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Letter to the Editor
Graphene vs. reduced graphene oxide: a comparative study of graphenebased nanoplatforms on electrochromic switching kinetics
a
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Bhushan Gadgila,b,*, Pia Damlina and Carita Kvarnströma,*
Turku University Centre for Materials and Surfaces (MATSURF), Laboratory of Materials
University of Turku Graduate School (UTUGS), FI-20014, Turku, Finland
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b
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Chemistry and Chemical Analysis, FI-20014, University of Turku, Finland
Abstract:
We report a systematic study on the effect of graphene vs. reduced graphene oxide interfacial
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layers coated onto ITO electrode platforms on the electrochromic properties of most widely used electrochromes; i.e. methyl viologen, PEDOT, Prussian blue and WO3. As a conclusive finding, an improved electrochromic switching kinetics is observed on reduced graphene
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oxide in comparison to the pristine graphene sheets possibly due to the heterogeneous
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electron transfer in graphene materials. _________________________________________________________________________ Corresponding authors: Email: *[email protected], [email protected] (Bhushan Gadgil), *[email protected] (Carita Kvarnström) Tel: +358 2 333 6729, Fax: +358 2 333 6700.
ACCEPTED MANUSCRIPT Electrochromism, an electrically-controlled reversible color change, find its role in diverse applications including energy-efficient windows, antiglare automobile mirrors, displays and solar control windows 1. Various electroactive materials such as redox active polymers and molecules, conducting polymers, metal oxides and transition metal complexes have been
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used as an electrochrome 2. In addition to the color diversity and durability, the response
times of the electrochromic materials is an important factor in device applications. While display devices require fast response times, a relatively slow response time is acceptable in
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smart windows and sunroofs. Due to the fast growth of optoelectronics industry in last few decades, there has been high demand of an inexpensive, flexible, transparent conducting
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electrode platforms for high-tech applications.
Graphene (G), a 2D carbon allotrope, is a material of this decade due to its fascinating physico-chemical properties. While graphene materials have been used for various applications, many scientists believe that its true potential lies in photonics and
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optoelectronics, where the combination of its exceptional optical and electronic properties can be fully explored, even in the absence of a bandgap 3. An important aspect of graphene as
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an optical component is its low sheet resistance, high transparency and flexibility. Graphene is produced by several ways. A high quality graphene can be obtained via CVD method; on
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the other hand, most of the graphene is produced from exfoliated graphite oxide (GO) and its reduction thereof (rGO). Although such reduction methods still cannot efficiently remove the oxygenated groups from GO surface, they can partly restore the sp3 network of pristine graphite. Such rGO is very promising electrode material due to its heterogeneous electron transfer properties, unlike in high quality graphene 4. Recently, Koh et. al reported the potential of a few layer graphene as replacement to the transparent ITO electrode in organic solar cells 5. Meanwhile graphene based electrode materials have been prominently used for electrochromic applications. Son et. al reported slower response times for Prussian blue (PB)
ACCEPTED MANUSCRIPT nanoparticles grown on transparent graphene 6. On the other hand, our group recently reported better endurance and fast switching kinetics in polyviologen/rGO composite films 7. Similarly, improved switching rates and coloration efficiency was reported for PEDOT-ionic liquid functionalized graphene 8. When an electrochromic performance of WO3/rGO
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nanocomposite is compared with pristine WO3, enhancements in terms of switching times, cycling and coloration efficiency was reported for nanocomposite 9. However, none of these studies simultaneously compared the EC performance on G vs. rGO supports.
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In this work, we report for the first time a conclusive comparison between G and rGO as a
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transparent interfacial platform for EC switchings of four different EC materials (methyl viologen, PEDOT, PB and WO3). Especially, due to the varying electron transport behavior in G and rGO, the response times of graphene based EC systems are compared. Commercially available graphene dispersion was used. rGO was made from GO by modified 10,11
. The hydrazine reduction method was preferred due to its
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hydrazine reduction method
extensive use for deoxygenating GO functionalities. XPS spectra confirms the effective reduction of GO to rGO (Fig. S1). G and rGO layers of few nm were spin casted on ITO
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substrates (Fig. S2, ESI). Due to their lower thicknesses, the conducting substrate (ITO) was preferred over glass. The transmittance spectra of G and rGO (Fig. S3, ESI) shows >95% T
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suggesting less than three layers of graphene were formed on ITO substrates. This is well supported by Raman measurements (Fig. S4, ESI). The absence of D band and the relative intensity ratio between G and 2D bands confirm mainly single or few layers in these graphene materials. Four different classes of EC materials were used in this study; viz. methyl viologen (MV; redox active molecule), PEDOT (PT; conducting polymer), (PB; inorganic complex) and WO3 (WO; metal oxide). The EC films were deposited on G and rGO coated ITO substrates by controlled potentiostatic electrodeposition (details in ESI). This method not only provides control over the material properties like thickness,
ACCEPTED MANUSCRIPT morphology or electronic/chemical structure but ensures conformal coatings of such electroactive materials. The amount of EC deposits was regulated by controlling the deposition times. 10, 20, 30, 40, and 50 s times were used to make EC1, EC2, EC3, EC4 and EC5 layers respectively. The corresponding four EC materials are labelled as MV1→5,
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PT1→5, PB1→5, and WO1→5. SEM analysis revealed that the thickness and the surface coverage of materials increases with increasing electrodeposition time (Fig 1). Moreover, the thicknesses were in nanoscale which suggests ultra-thin films of EC materials were
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deposited. The smaller thicknesses of EC films were deliberately chosen so that they are comparable to those of G and rGO layers. In order to achieve similar EC thin films with
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respective deposits (1→5) on both G and rGO layers, the concentrations of precursors were tuned carefully to ensure constant thicknesses all along (Fig. S5, ESI). The electrochemical responses of these materials on G and rGO were tested using CV and
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appended in Fig. 2a. All the EC films display a well-defined redox response at their respective potential window. CV of MV shows well documented redox curve due to the formation of intensely purple colored radical mono cation formation at ca. -1.0 V. A
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capacitive redox response can be observed for PT film with cathodic coloring state (at -0.4 V) and anodic bleaching state (+0.3 V). For PB, the CV comprises well resolved redox response
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with color change from blue (+0.6 V) to colorless (-0.3 V). When WO films were scanned, the CV shows typical redox reaction of tungsten oxide, causing cathodically deep blue coloration (at -1.0 V) and bleaching state at +1.0 V. The absorption spectra of the EC films at fully colored states show that all the films under study exhibit distinct absorption profile in the visible range of the spectrum with broad characteristic absorption bands (Fig. 2b). The absorption wavelengths observed for MV, PT, PB and WO were 550, 590, 720 and 632 nm respectively and are chosen as monochromatic wavelength for studying EC properties of the
ACCEPTED MANUSCRIPT films. Table 1 shows color/bleach characteristics with corresponding photographs of the studied EC films. The films were further subjected to switching kinetics measurements and the response times
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were calculated (Fig. S6, ESI). The response time is defined as the time required to achieve 90% of the color transition. Refereeing from Fig. 3, one can clearly observe the difference between underlying G and rGO interlayer towards the EC performance. The response times of MV, PT, PB and WO are summarized in Table 2 (Fig. S7, ESI). As the thickness increases
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from EC1→5, the response times gradually increased. This increase can be explained by the
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large grain sized particle deposition on the surface of G and rGO that might have created the space among the materials which allows the electrolyte to penetrate through the films. As a primary observation, one can reveal that the response times are slower on graphene coated substrates in comparison to those on rGO.
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The EC response times of MV1→5 are in the range of 7-20 s. Also coloring of the films is slower than decoloring. This is very typical for viologen materials because of their steady color/bleach characteristics 12. For PT, PB and WO, the profile of the kinetic switching plots
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looks very much analogues. Though, a close look at the response time values reveals remarkable differences in these different EC materials. PT films are quicker in bleaching
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compared to the coloring 13, and the responses are much steadier when scanned over G and rGO substrates. A remarkable difference can however be seen for PB materials. With faster tinting, the response times are in the range of 2-16 s on G coated electrode. However when the substrate changed to rGO, the times are very fast (0.1-0.4 s) and gradually increasing with increase in the thicknesses of EC films and almost identical in coloring/decoloring course. When referred to the previous study comparing G vs ITO substrate 6, the switching profile of PB on G looks almost identical to our work, but faster kinetics is achieved on rGO compared
ACCEPTED MANUSCRIPT to ITO, which also indicates prominence of rGO interlayer in improving the color/bleach responses. WO films show linear increase in their response times on G. When switched to rGO substrate, the response times decreased substantially to 0.1-0.5 s. As the EC layer thickness grows, the response times are steady. The time values indicate that the response
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times of inorganic materials can be accelerated significantly on rGO based platforms
compared to G. This is particularly important considering the fact that the response times of inorganic polymeric materials are much lower because of the low conductivity and slow
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charge transport 14. On the other hand, the organic electroactive materials like MV or PT which possesses comparatively rapid switching kinetics also showed noticeable difference
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when switched between G and rGO adlayer.
The significant variances in electro-optical properties of electrochromes lie in the structural differences and electron/charge transfer processes at the edge and basal planes of G and rGO 15
. As discussed before, the defects in rGO are extensive in comparison to that of pure
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graphene analogues. Therefore electron transfer is lethargic in G, resulting in slow ion/electron diffusion in between EC film and interfacial G layer 16. rGO in contrast possesses some unreduced oxygenated groups/impurities, making the electrolyte penetration
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easier and switching kinetics quicker for EC films 17. Although the response times of the EC
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materials depend very much on the material itself, judging from our study, we believe that the electrode properties play a crucial role in achieving the desired application goals in such systems. The comparative switching tests of the studied EC systems thus suggest that graphene materials can be useful as a transparent active electrode or an interfacial layer and desired EC performance can be achieved via interfacial modifications in such graphene based electrode systems.
References:
ACCEPTED MANUSCRIPT 1. Monk PM, Mortimer RJ, Rosseinsky DR. Electrochromism: Fundamentals and applications. John Wiley & Sons; 2008.
2. Mortimer RJ. Electrochromic materials. Annu. Rev. Mater. Res. 2011;41:241-268.
2010;4(9):611-622.
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3. Bonaccorso F, Sun Z, Hasan T, Ferrari A. Graphene photonics and optoelectronics. Nat. Photon.
4. Loh KP, Bao Q, Eda G, Chhowalla M. Graphene oxide as a chemically tunable platform for optical
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applications. Nat. Chem. 2010;2(12):1015-1024.
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5. Koh WS, Gan CH, Phua WK, Akimov Y, Bai P. The potential of graphene as an ITO replacement in organic solar cells: An optical perspective. IEEE J. Sel. Top. Quantum Electron. 2014;20(1):36-42.
6. Ko JH, Yeo S, Park JH, Choi J, Noh C, Son SU. Graphene-based electrochromic systems: The case of prussian blue nanoparticles on transparent graphene film. Chem. Commun. 2012;48(32):3884-3886.
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7. Gadgil B, Damlin P, Heinonen M, Kvarnström C. A facile one step electrostatically driven electrocodeposition of polyviologen–reduced graphene oxide nanocomposite films for enhanced
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electrochromic performance. Carbon. 2015;89(0):53-62.
8. Saxena AP, Deepa M, Joshi AG, Bhandari S, Srivastava AK. Poly (3, 4-ethylenedioxythiophene)-
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ionic liquid functionalized graphene/reduced graphene oxide nanostructures: Improved conduction and electrochromism. ACS Appl Mater Interfaces. 2011;3(4):1115-1126.
9. Fu C, Foo C, Lee PS. One-step facile electrochemical preparation of WO3/graphene nanocomposites with improved electrochromic properties. Electrochim Acta. 2014;117:139-144.
10. Botas C, Álvarez P, Blanco C, et al. Tailored graphene materials by chemical reduction of graphene oxides of different atomic structure. RSC Adv. 2012;2(25):9643-9650.
ACCEPTED MANUSCRIPT 11. Li D, Mueller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008;3(2):101-105.
12. Gadgil B, Damlin P, Dmitrieva E, Ääritalo T, Kvarnström C. ESR/UV-vis-NIR
bearing a pendant viologen. RSC Adv. 2015;5(53):42242-42249.
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spectroelectrochemical study and electrochromic contrast enhancement of a polythiophene derivative
13. Brooke R, Fabretto M, Vucaj N, et al. Effect of oxidant on the performance of conductive polymer
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films prepared by vacuum vapor phase polymerization for smart window applications. Smart Mater Struct. 2015;24(3):035016.
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14. Niklasson GA, Granqvist CG. Electrochromics for smart windows: Thin films of tungsten oxide and nickel oxide, and devices based on these. J. Mater. Chem. 2007;17(2):127-156.
15. Brownson DA, Kampouris DK, Banks CE. Graphene electrochemistry: Fundamental concepts
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through to prominent applications. Chem Soc Rev. 2012;41(21):6944-6976.
16. Brownson DA, Munro LJ, Kampouris DK, Banks CE. Electrochemistry of graphene: Not such a beneficial electrode material? RSC Adv. 2011;1(6):978-988.
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17. Tang Y, Wu N, Luo S, Liu C, Wang K, Chen L. One Step electrodeposition to Layer by Layer
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Graphene–Conducting Polymer hybrid films. Macromol Rapid Commun. 2012;33(20):1780-1786.
ACCEPTED MANUSCRIPT Table 1. Electrochromic parameters and corresponding photos of the studied EC films
Material Electrolyte λ/nm Bleaching Coloring ________________________________________________________________________ 0.1 M KCl + water
550
+0.0 V
-1.0 V
PT
0.1 M LiClO4 + acetonitrile
590
+0.3 V
-0.5 V
PB
0.1 M KCl + water
720
-0.3 V
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MV
+0.6 V
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+1.0 V -1.0 V WO 0.1 M LiClO4+propylene carbonate 632 ________________________________________________________________________
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Table 2. Response times (sec.) of EC materials on G and rGO
Materials Bleaching Coloring Materials Bleaching Coloring ___________________________________________________________________ Methyl viologen
Prussian blue
7.0 (5.9)
7.1 (6.5)
MV2
10.7 (7.0)
12.3 (8.7)
MV3
12.2 (8.2)
MV4
14.7 (8.8)
MV5
PEDOT
3.2 (0.13)
2.5 (0.1)
PB2
4.1 (0.22)
7.2 (0.12)
15.9 (10.6)
PB3
8.1 (0.26)
7.6 (0.24)
17.1 (12.9)
PB4
11.0 (0.27)
8.5 (0.21)
19.8 (11.5) 21.3 (15.8)
PB5
15.3 (0.35)
8.7 (0.29)
WO3 0.68 (0.23)
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PB1
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MV1
1.3 (0.6)
3.9 (1.11)
WO1
0.6 (0.13)
PT2
1.4 (0.72)
4.1 (1.2)
WO2
0.79 (0.22) 0.87 (0.45)
1.2 (0.76)
4.2 (1.1)
WO3
2.0 (0.3)
1.6 (1.2)
4.8 (1.2)
WO4
4.1 (0.25) 13.0 (0.25)
1.8 (1.7)
5.0 (1.2)
WO5
PT3 PT4 PT5
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PT1
3.2 (0.28))
11.0 (0.26) 16.2 (0.2)
The values in the parentheses correspond to the response times of EC materials with similar intensity changes on rGO materials.
_____________________________________________________________________
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Figure 1. SEM images of different EC films on graphene
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graphene
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Figure 2. a) CVs and b) UV-Vis absorption spectra at fully colored state of EC films on
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wavelengths on G and rGO coated substrates
ACCEPTED MANUSCRIPT Electronic Supplementary Information (ESI) for
Graphene vs. reduced graphene oxide: a comparative study of graphene based nanoplatforms on electrochromic switching kinetics Bhushan Gadgila,b,*, Pia Damlina and Carita Kvarnströma,* Turku University Centre for Materials and Surfaces (MATSURF), Laboratory of Materials Chemistry and Chemical Analysis, FI-20014, University of Turku, Finland b
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a
University of Turku Graduate School (UTUGS), FI-20014, Turku, Finland
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Experimental
Graphene dispersion in ethanol was obtained from Graphene Supermarket. rGO monolayers
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were prepared from GO solution by modified hydrazine reduction method 1,2.In brief, previously centrifuged GO solution3-5 (1 mg/mL) is mixed with hydrazine monohydrate (5 mL), ammonia (200 µL) and toluene (10 mL) and resultant dispersion is heated in oil bath at 100 0C for 24 h. The final solution is transferred in dialysis membrane to remove impurity
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traces. The final product is dispersed in ethanol for further use. Indium tin oxide (ITO) glass substrates (100 Ω□-1, Delta-technology Inc. active diameter of 10 mm) were cleaned with ultrasonication in acetone, ethanol and water successively for 10 min before use. The thin
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films of few nanometer sizes were spin casted on ITO substrates from graphene dispersions to obtain their transparent coatings. A coiled platinum wire was used as the counter electrode
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and Ag wire coated with AgCl, was used as the quasi-reference electrode. The Ag/AgCl electrode was calibrated vs. ferrocene (Fe/Fe+) (E1/2(Fe/Fe+) = 0.45 V) before each experiment. The electrode separation between working and counter/reference electrode was 3 mm. Characterizations All electrodepositions and CVs were made with IviumStat potentiostat (Ivium Technologies, The Netherlands). Raman measurements were made using Nexus 870 spectrometer (Nicolet)
ACCEPTED MANUSCRIPT equipped with a Raman accessory. Wavelength of the Raman laser used was 1064 nm and all spectra were recorded by collecting 640 scans with a 4 cm-1 spectral resolution. The topographical and cross-sectional images of the films were obtained by a Leo (Zeiss) 1530 Gemini FEG scanning electron microscope (SEM). XPS measurements were recorded with a
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Perkin–Elmer PHI 5400 spectrometer using Mg Kα radiation (1253.6 eV). For
electrochromic measurements, optical changes were recorded using Agilent Cary 60 UV-Vis spectrophotometer connected to an IviumStat potentiostat (Ivium Technologies, The
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Preparations of electrochromic (EC) films
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Netherlands) performing chronoamperometry measurement.
All electrochromic films were electrodeposited potentiostatically.
MV films were cathodically electrodeposited from methyl viologen hydrate (98%) aqueous solution at -0.8 V 6. Poly(3,4-ethylenedioxythiophene) (PEDOT) films were electrodeposited
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at 1.1 V from EDOT monomer solution in acetonitrile containing 0.1 M LiClO4 as electrolyte salt 7. PB film was potentiostatically deposited at a potential value 0.40 V from aqueous solutions of 1 mM K3[Fe(CN)6] + 1 mM FeCl3 in the supporting electrolyte 0.1 M KCl + 0.01
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M HCl 8. For WO3 deposition, W electrolyte is prepared by dissolving W metal powder in
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H2O2 (30%) in ice bath. The clear solution obtained upon filtration was refluxed at 550 C for 6 h to decompose off excess peroxide. The solution was treated with Pt black to remove traces of any remnant H2O2. The final solution is mixed equally with anhydrous absolute ethanol yielding a deep yellow colored deposition sol, which was warmed at 500 C before use. A constant cathodic potential of -0.45 V is used to deposit WO3 electrochromic films 9. The respective electrochromic films of methyl viologen, PEDOT, Prussian blue and tungsten oxide labelled as MV, PT, PB and WO respectively; are prepared at different deposition times and labelled accordingly; viz. 10 s (MV1, PT1, PB1, WO1), 20 s (MV2, PT2, PB3,
ACCEPTED MANUSCRIPT WO2), 30 s (MV3, PT3, PB3, WO3), 40 s (MV4, PT4, PB4, WO4) and 50 s (MV5, PT5, PB5, WO5). The electrochromic performances of these films was studied on graphene and reduced
chronoamperometry and UV-Vis absorption spectroscopy.
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Fig. S1. XPS spectra of a) GO and b) rGO samples.
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Figures:
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graphene oxide layers in their respective electrolytes using cyclic voltammetry (CV),
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Table S1. Fitted results of the C1s core level XPS spectra of GO and rGO samples. -----------------------------------------------------------------------------------------------------------Sample
C-C / C=C
C-O (epoxy & hydroxyl)
C=O (carbonyl)
/ C-H C-N -----------------------------------------------------------------------------------------------------------GO
51.8
35.2
13
rGO
66.7
25.8
7.5
------------------------------------------------------------------------------------------------------------
ACCEPTED MANUSCRIPT Fig. S2. SEM image of graphene (G) and reduced graphene oxide (rGO) layers on ITO glass
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and photographs showing transparent dispersions of G and rGO.
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Fig. S3. Transmittance spectra of G and rGO showing >95% T suggesting less than three layers of graphene 10.
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Fig. S4. Raman spectra of G and rGO materials.
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Raman spectra of graphene showing one, two and three layers of graphene 11.
ACCEPTED MANUSCRIPT Fig. S5. SEM images of electrochromic films on G and rGO and their respective thicknesses in the inset (from 1→5)
Methyl Viologen (MV) On rGO
On G
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On G
PEDOT (PT) On rGO
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Prussian Blue (PB) On rGO
On G
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On G
WO3 (WO) On rGO
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Figure S6. Comparisons of the response times of MV, PT, PB and WO on G vs. rGO.
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Fig. S7. Response times calculations for different EC materials on G (black line) and rGO (blue line)
ACCEPTED MANUSCRIPT References 1. Li D, Mueller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008;3(2):101-105.
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2. Botas C, Álvarez P, Blanco C, et al. Tailored graphene materials by chemical reduction of graphene oxides of different atomic structure. RSC Adv. 2012;2(25):9643-9650.
3. Viinikanoja A, Wang Z, Kauppila J, Kvarnström C. Electrochemical reduction of graphene
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oxide and its in situ spectroelectrochemical characterization. Phys Chem Chem Phys.
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2012;14(40):14003-14009.
4. Kauppila J, Kunnas P, Damlin P, Viinikanoja A, Kvarnström C. Electrochemical reduction of graphene oxide films in aqueous and organic solutions. Electrochim Acta. 2013;89(0):8489.
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5. Hassinen J, Kauppila J, Leiro J, et al. Low-cost reduced graphene oxide-based conductometric nitrogen dioxide-sensitive sensor on paper. Analytical and bioanalytical
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chemistry. 2013;405(11):3611-3617.
6. Thorneley RNF. A convenient electrochemical preparation of reduced methyl viologen and
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a kinetic study of the reaction with oxygen using an anaerobic stopped-flow apparatus. Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1974;333(3):487-496.
7. Zainal MF, Mohd Y. Characterization of PEDOT films for electrochromic applications. Polym Plast Technol Eng. 2015;54(3):276-281.
8. Zou Y, Sun L, Xu F. Prussian blue electrodeposited on MWNTs–PANI hybrid composites for H2O2 detection. Talanta. 2007;72(2):437-442.
ACCEPTED MANUSCRIPT 9. Deepa M, Srivastava AK, Saxena TK, Agnihotry SA. Annealing induced microstructural evolution of electrodeposited electrochromic tungsten oxide films. Appl Surf Sci. 2005;252(5):1568-1580.
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10. Lian W, Huang Y, Liao Y, et al. Flexible electrochromic devices based on optoelectronically active polynorbornene layer and ultratransparent graphene electrodes. Macromolecules. 2011;44(24):9550-9555.
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on copper foils. Science. 2009;324(5932):1312-1314.
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11. Li X, Cai W, An J, et al. Large-area synthesis of high-quality and uniform graphene films
S0008-6223(15)30283-9
DOI:
10.1016/j.carbon.2015.09.065
Reference:
CARBON 10335
To appear in:
Carbon
Received Date: 4 July 2015 Revised Date:
15 September 2015
Accepted Date: 17 September 2015
Please cite this article as: B. Gadgil, P. Damlin, C. Kvarnström, Graphene vs. reduced graphene oxide: a comparative study of graphene-based nanoplatforms on electrochromic switching kinetics, Carbon (2015), doi: 10.1016/j.carbon.2015.09.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Letter to the Editor
Graphene vs. reduced graphene oxide: a comparative study of graphenebased nanoplatforms on electrochromic switching kinetics
a
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Bhushan Gadgila,b,*, Pia Damlina and Carita Kvarnströma,*
Turku University Centre for Materials and Surfaces (MATSURF), Laboratory of Materials
University of Turku Graduate School (UTUGS), FI-20014, Turku, Finland
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b
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Chemistry and Chemical Analysis, FI-20014, University of Turku, Finland
Abstract:
We report a systematic study on the effect of graphene vs. reduced graphene oxide interfacial
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layers coated onto ITO electrode platforms on the electrochromic properties of most widely used electrochromes; i.e. methyl viologen, PEDOT, Prussian blue and WO3. As a conclusive finding, an improved electrochromic switching kinetics is observed on reduced graphene
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oxide in comparison to the pristine graphene sheets possibly due to the heterogeneous
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electron transfer in graphene materials. _________________________________________________________________________ Corresponding authors: Email: *[email protected], [email protected] (Bhushan Gadgil), *[email protected] (Carita Kvarnström) Tel: +358 2 333 6729, Fax: +358 2 333 6700.
ACCEPTED MANUSCRIPT Electrochromism, an electrically-controlled reversible color change, find its role in diverse applications including energy-efficient windows, antiglare automobile mirrors, displays and solar control windows 1. Various electroactive materials such as redox active polymers and molecules, conducting polymers, metal oxides and transition metal complexes have been
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used as an electrochrome 2. In addition to the color diversity and durability, the response
times of the electrochromic materials is an important factor in device applications. While display devices require fast response times, a relatively slow response time is acceptable in
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smart windows and sunroofs. Due to the fast growth of optoelectronics industry in last few decades, there has been high demand of an inexpensive, flexible, transparent conducting
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electrode platforms for high-tech applications.
Graphene (G), a 2D carbon allotrope, is a material of this decade due to its fascinating physico-chemical properties. While graphene materials have been used for various applications, many scientists believe that its true potential lies in photonics and
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optoelectronics, where the combination of its exceptional optical and electronic properties can be fully explored, even in the absence of a bandgap 3. An important aspect of graphene as
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an optical component is its low sheet resistance, high transparency and flexibility. Graphene is produced by several ways. A high quality graphene can be obtained via CVD method; on
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the other hand, most of the graphene is produced from exfoliated graphite oxide (GO) and its reduction thereof (rGO). Although such reduction methods still cannot efficiently remove the oxygenated groups from GO surface, they can partly restore the sp3 network of pristine graphite. Such rGO is very promising electrode material due to its heterogeneous electron transfer properties, unlike in high quality graphene 4. Recently, Koh et. al reported the potential of a few layer graphene as replacement to the transparent ITO electrode in organic solar cells 5. Meanwhile graphene based electrode materials have been prominently used for electrochromic applications. Son et. al reported slower response times for Prussian blue (PB)
ACCEPTED MANUSCRIPT nanoparticles grown on transparent graphene 6. On the other hand, our group recently reported better endurance and fast switching kinetics in polyviologen/rGO composite films 7. Similarly, improved switching rates and coloration efficiency was reported for PEDOT-ionic liquid functionalized graphene 8. When an electrochromic performance of WO3/rGO
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nanocomposite is compared with pristine WO3, enhancements in terms of switching times, cycling and coloration efficiency was reported for nanocomposite 9. However, none of these studies simultaneously compared the EC performance on G vs. rGO supports.
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In this work, we report for the first time a conclusive comparison between G and rGO as a
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transparent interfacial platform for EC switchings of four different EC materials (methyl viologen, PEDOT, PB and WO3). Especially, due to the varying electron transport behavior in G and rGO, the response times of graphene based EC systems are compared. Commercially available graphene dispersion was used. rGO was made from GO by modified 10,11
. The hydrazine reduction method was preferred due to its
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hydrazine reduction method
extensive use for deoxygenating GO functionalities. XPS spectra confirms the effective reduction of GO to rGO (Fig. S1). G and rGO layers of few nm were spin casted on ITO
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substrates (Fig. S2, ESI). Due to their lower thicknesses, the conducting substrate (ITO) was preferred over glass. The transmittance spectra of G and rGO (Fig. S3, ESI) shows >95% T
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suggesting less than three layers of graphene were formed on ITO substrates. This is well supported by Raman measurements (Fig. S4, ESI). The absence of D band and the relative intensity ratio between G and 2D bands confirm mainly single or few layers in these graphene materials. Four different classes of EC materials were used in this study; viz. methyl viologen (MV; redox active molecule), PEDOT (PT; conducting polymer), (PB; inorganic complex) and WO3 (WO; metal oxide). The EC films were deposited on G and rGO coated ITO substrates by controlled potentiostatic electrodeposition (details in ESI). This method not only provides control over the material properties like thickness,
ACCEPTED MANUSCRIPT morphology or electronic/chemical structure but ensures conformal coatings of such electroactive materials. The amount of EC deposits was regulated by controlling the deposition times. 10, 20, 30, 40, and 50 s times were used to make EC1, EC2, EC3, EC4 and EC5 layers respectively. The corresponding four EC materials are labelled as MV1→5,
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PT1→5, PB1→5, and WO1→5. SEM analysis revealed that the thickness and the surface coverage of materials increases with increasing electrodeposition time (Fig 1). Moreover, the thicknesses were in nanoscale which suggests ultra-thin films of EC materials were
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deposited. The smaller thicknesses of EC films were deliberately chosen so that they are comparable to those of G and rGO layers. In order to achieve similar EC thin films with
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respective deposits (1→5) on both G and rGO layers, the concentrations of precursors were tuned carefully to ensure constant thicknesses all along (Fig. S5, ESI). The electrochemical responses of these materials on G and rGO were tested using CV and
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appended in Fig. 2a. All the EC films display a well-defined redox response at their respective potential window. CV of MV shows well documented redox curve due to the formation of intensely purple colored radical mono cation formation at ca. -1.0 V. A
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capacitive redox response can be observed for PT film with cathodic coloring state (at -0.4 V) and anodic bleaching state (+0.3 V). For PB, the CV comprises well resolved redox response
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with color change from blue (+0.6 V) to colorless (-0.3 V). When WO films were scanned, the CV shows typical redox reaction of tungsten oxide, causing cathodically deep blue coloration (at -1.0 V) and bleaching state at +1.0 V. The absorption spectra of the EC films at fully colored states show that all the films under study exhibit distinct absorption profile in the visible range of the spectrum with broad characteristic absorption bands (Fig. 2b). The absorption wavelengths observed for MV, PT, PB and WO were 550, 590, 720 and 632 nm respectively and are chosen as monochromatic wavelength for studying EC properties of the
ACCEPTED MANUSCRIPT films. Table 1 shows color/bleach characteristics with corresponding photographs of the studied EC films. The films were further subjected to switching kinetics measurements and the response times
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were calculated (Fig. S6, ESI). The response time is defined as the time required to achieve 90% of the color transition. Refereeing from Fig. 3, one can clearly observe the difference between underlying G and rGO interlayer towards the EC performance. The response times of MV, PT, PB and WO are summarized in Table 2 (Fig. S7, ESI). As the thickness increases
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from EC1→5, the response times gradually increased. This increase can be explained by the
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large grain sized particle deposition on the surface of G and rGO that might have created the space among the materials which allows the electrolyte to penetrate through the films. As a primary observation, one can reveal that the response times are slower on graphene coated substrates in comparison to those on rGO.
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The EC response times of MV1→5 are in the range of 7-20 s. Also coloring of the films is slower than decoloring. This is very typical for viologen materials because of their steady color/bleach characteristics 12. For PT, PB and WO, the profile of the kinetic switching plots
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looks very much analogues. Though, a close look at the response time values reveals remarkable differences in these different EC materials. PT films are quicker in bleaching
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compared to the coloring 13, and the responses are much steadier when scanned over G and rGO substrates. A remarkable difference can however be seen for PB materials. With faster tinting, the response times are in the range of 2-16 s on G coated electrode. However when the substrate changed to rGO, the times are very fast (0.1-0.4 s) and gradually increasing with increase in the thicknesses of EC films and almost identical in coloring/decoloring course. When referred to the previous study comparing G vs ITO substrate 6, the switching profile of PB on G looks almost identical to our work, but faster kinetics is achieved on rGO compared
ACCEPTED MANUSCRIPT to ITO, which also indicates prominence of rGO interlayer in improving the color/bleach responses. WO films show linear increase in their response times on G. When switched to rGO substrate, the response times decreased substantially to 0.1-0.5 s. As the EC layer thickness grows, the response times are steady. The time values indicate that the response
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times of inorganic materials can be accelerated significantly on rGO based platforms
compared to G. This is particularly important considering the fact that the response times of inorganic polymeric materials are much lower because of the low conductivity and slow
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charge transport 14. On the other hand, the organic electroactive materials like MV or PT which possesses comparatively rapid switching kinetics also showed noticeable difference
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when switched between G and rGO adlayer.
The significant variances in electro-optical properties of electrochromes lie in the structural differences and electron/charge transfer processes at the edge and basal planes of G and rGO 15
. As discussed before, the defects in rGO are extensive in comparison to that of pure
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graphene analogues. Therefore electron transfer is lethargic in G, resulting in slow ion/electron diffusion in between EC film and interfacial G layer 16. rGO in contrast possesses some unreduced oxygenated groups/impurities, making the electrolyte penetration
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easier and switching kinetics quicker for EC films 17. Although the response times of the EC
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materials depend very much on the material itself, judging from our study, we believe that the electrode properties play a crucial role in achieving the desired application goals in such systems. The comparative switching tests of the studied EC systems thus suggest that graphene materials can be useful as a transparent active electrode or an interfacial layer and desired EC performance can be achieved via interfacial modifications in such graphene based electrode systems.
References:
ACCEPTED MANUSCRIPT 1. Monk PM, Mortimer RJ, Rosseinsky DR. Electrochromism: Fundamentals and applications. John Wiley & Sons; 2008.
2. Mortimer RJ. Electrochromic materials. Annu. Rev. Mater. Res. 2011;41:241-268.
2010;4(9):611-622.
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3. Bonaccorso F, Sun Z, Hasan T, Ferrari A. Graphene photonics and optoelectronics. Nat. Photon.
4. Loh KP, Bao Q, Eda G, Chhowalla M. Graphene oxide as a chemically tunable platform for optical
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applications. Nat. Chem. 2010;2(12):1015-1024.
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5. Koh WS, Gan CH, Phua WK, Akimov Y, Bai P. The potential of graphene as an ITO replacement in organic solar cells: An optical perspective. IEEE J. Sel. Top. Quantum Electron. 2014;20(1):36-42.
6. Ko JH, Yeo S, Park JH, Choi J, Noh C, Son SU. Graphene-based electrochromic systems: The case of prussian blue nanoparticles on transparent graphene film. Chem. Commun. 2012;48(32):3884-3886.
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7. Gadgil B, Damlin P, Heinonen M, Kvarnström C. A facile one step electrostatically driven electrocodeposition of polyviologen–reduced graphene oxide nanocomposite films for enhanced
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electrochromic performance. Carbon. 2015;89(0):53-62.
8. Saxena AP, Deepa M, Joshi AG, Bhandari S, Srivastava AK. Poly (3, 4-ethylenedioxythiophene)-
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ionic liquid functionalized graphene/reduced graphene oxide nanostructures: Improved conduction and electrochromism. ACS Appl Mater Interfaces. 2011;3(4):1115-1126.
9. Fu C, Foo C, Lee PS. One-step facile electrochemical preparation of WO3/graphene nanocomposites with improved electrochromic properties. Electrochim Acta. 2014;117:139-144.
10. Botas C, Álvarez P, Blanco C, et al. Tailored graphene materials by chemical reduction of graphene oxides of different atomic structure. RSC Adv. 2012;2(25):9643-9650.
ACCEPTED MANUSCRIPT 11. Li D, Mueller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008;3(2):101-105.
12. Gadgil B, Damlin P, Dmitrieva E, Ääritalo T, Kvarnström C. ESR/UV-vis-NIR
bearing a pendant viologen. RSC Adv. 2015;5(53):42242-42249.
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spectroelectrochemical study and electrochromic contrast enhancement of a polythiophene derivative
13. Brooke R, Fabretto M, Vucaj N, et al. Effect of oxidant on the performance of conductive polymer
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films prepared by vacuum vapor phase polymerization for smart window applications. Smart Mater Struct. 2015;24(3):035016.
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14. Niklasson GA, Granqvist CG. Electrochromics for smart windows: Thin films of tungsten oxide and nickel oxide, and devices based on these. J. Mater. Chem. 2007;17(2):127-156.
15. Brownson DA, Kampouris DK, Banks CE. Graphene electrochemistry: Fundamental concepts
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through to prominent applications. Chem Soc Rev. 2012;41(21):6944-6976.
16. Brownson DA, Munro LJ, Kampouris DK, Banks CE. Electrochemistry of graphene: Not such a beneficial electrode material? RSC Adv. 2011;1(6):978-988.
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17. Tang Y, Wu N, Luo S, Liu C, Wang K, Chen L. One Step electrodeposition to Layer by Layer
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Graphene–Conducting Polymer hybrid films. Macromol Rapid Commun. 2012;33(20):1780-1786.
ACCEPTED MANUSCRIPT Table 1. Electrochromic parameters and corresponding photos of the studied EC films
Material Electrolyte λ/nm Bleaching Coloring ________________________________________________________________________ 0.1 M KCl + water
550
+0.0 V
-1.0 V
PT
0.1 M LiClO4 + acetonitrile
590
+0.3 V
-0.5 V
PB
0.1 M KCl + water
720
-0.3 V
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MV
+0.6 V
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+1.0 V -1.0 V WO 0.1 M LiClO4+propylene carbonate 632 ________________________________________________________________________
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Table 2. Response times (sec.) of EC materials on G and rGO
Materials Bleaching Coloring Materials Bleaching Coloring ___________________________________________________________________ Methyl viologen
Prussian blue
7.0 (5.9)
7.1 (6.5)
MV2
10.7 (7.0)
12.3 (8.7)
MV3
12.2 (8.2)
MV4
14.7 (8.8)
MV5
PEDOT
3.2 (0.13)
2.5 (0.1)
PB2
4.1 (0.22)
7.2 (0.12)
15.9 (10.6)
PB3
8.1 (0.26)
7.6 (0.24)
17.1 (12.9)
PB4
11.0 (0.27)
8.5 (0.21)
19.8 (11.5) 21.3 (15.8)
PB5
15.3 (0.35)
8.7 (0.29)
WO3 0.68 (0.23)
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PB1
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MV1
1.3 (0.6)
3.9 (1.11)
WO1
0.6 (0.13)
PT2
1.4 (0.72)
4.1 (1.2)
WO2
0.79 (0.22) 0.87 (0.45)
1.2 (0.76)
4.2 (1.1)
WO3
2.0 (0.3)
1.6 (1.2)
4.8 (1.2)
WO4
4.1 (0.25) 13.0 (0.25)
1.8 (1.7)
5.0 (1.2)
WO5
PT3 PT4 PT5
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PT1
3.2 (0.28))
11.0 (0.26) 16.2 (0.2)
The values in the parentheses correspond to the response times of EC materials with similar intensity changes on rGO materials.
_____________________________________________________________________
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Figure 1. SEM images of different EC films on graphene
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graphene
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Figure 2. a) CVs and b) UV-Vis absorption spectra at fully colored state of EC films on
ACCEPTED MANUSCRIPT Figure 3. Absorption changes of EC materials at their respective monochromatic
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wavelengths on G and rGO coated substrates
ACCEPTED MANUSCRIPT Electronic Supplementary Information (ESI) for
Graphene vs. reduced graphene oxide: a comparative study of graphene based nanoplatforms on electrochromic switching kinetics Bhushan Gadgila,b,*, Pia Damlina and Carita Kvarnströma,* Turku University Centre for Materials and Surfaces (MATSURF), Laboratory of Materials Chemistry and Chemical Analysis, FI-20014, University of Turku, Finland b
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a
University of Turku Graduate School (UTUGS), FI-20014, Turku, Finland
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Experimental
Graphene dispersion in ethanol was obtained from Graphene Supermarket. rGO monolayers
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were prepared from GO solution by modified hydrazine reduction method 1,2.In brief, previously centrifuged GO solution3-5 (1 mg/mL) is mixed with hydrazine monohydrate (5 mL), ammonia (200 µL) and toluene (10 mL) and resultant dispersion is heated in oil bath at 100 0C for 24 h. The final solution is transferred in dialysis membrane to remove impurity
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traces. The final product is dispersed in ethanol for further use. Indium tin oxide (ITO) glass substrates (100 Ω□-1, Delta-technology Inc. active diameter of 10 mm) were cleaned with ultrasonication in acetone, ethanol and water successively for 10 min before use. The thin
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films of few nanometer sizes were spin casted on ITO substrates from graphene dispersions to obtain their transparent coatings. A coiled platinum wire was used as the counter electrode
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and Ag wire coated with AgCl, was used as the quasi-reference electrode. The Ag/AgCl electrode was calibrated vs. ferrocene (Fe/Fe+) (E1/2(Fe/Fe+) = 0.45 V) before each experiment. The electrode separation between working and counter/reference electrode was 3 mm. Characterizations All electrodepositions and CVs were made with IviumStat potentiostat (Ivium Technologies, The Netherlands). Raman measurements were made using Nexus 870 spectrometer (Nicolet)
ACCEPTED MANUSCRIPT equipped with a Raman accessory. Wavelength of the Raman laser used was 1064 nm and all spectra were recorded by collecting 640 scans with a 4 cm-1 spectral resolution. The topographical and cross-sectional images of the films were obtained by a Leo (Zeiss) 1530 Gemini FEG scanning electron microscope (SEM). XPS measurements were recorded with a
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Perkin–Elmer PHI 5400 spectrometer using Mg Kα radiation (1253.6 eV). For
electrochromic measurements, optical changes were recorded using Agilent Cary 60 UV-Vis spectrophotometer connected to an IviumStat potentiostat (Ivium Technologies, The
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Preparations of electrochromic (EC) films
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Netherlands) performing chronoamperometry measurement.
All electrochromic films were electrodeposited potentiostatically.
MV films were cathodically electrodeposited from methyl viologen hydrate (98%) aqueous solution at -0.8 V 6. Poly(3,4-ethylenedioxythiophene) (PEDOT) films were electrodeposited
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at 1.1 V from EDOT monomer solution in acetonitrile containing 0.1 M LiClO4 as electrolyte salt 7. PB film was potentiostatically deposited at a potential value 0.40 V from aqueous solutions of 1 mM K3[Fe(CN)6] + 1 mM FeCl3 in the supporting electrolyte 0.1 M KCl + 0.01
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M HCl 8. For WO3 deposition, W electrolyte is prepared by dissolving W metal powder in
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H2O2 (30%) in ice bath. The clear solution obtained upon filtration was refluxed at 550 C for 6 h to decompose off excess peroxide. The solution was treated with Pt black to remove traces of any remnant H2O2. The final solution is mixed equally with anhydrous absolute ethanol yielding a deep yellow colored deposition sol, which was warmed at 500 C before use. A constant cathodic potential of -0.45 V is used to deposit WO3 electrochromic films 9. The respective electrochromic films of methyl viologen, PEDOT, Prussian blue and tungsten oxide labelled as MV, PT, PB and WO respectively; are prepared at different deposition times and labelled accordingly; viz. 10 s (MV1, PT1, PB1, WO1), 20 s (MV2, PT2, PB3,
ACCEPTED MANUSCRIPT WO2), 30 s (MV3, PT3, PB3, WO3), 40 s (MV4, PT4, PB4, WO4) and 50 s (MV5, PT5, PB5, WO5). The electrochromic performances of these films was studied on graphene and reduced
chronoamperometry and UV-Vis absorption spectroscopy.
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Fig. S1. XPS spectra of a) GO and b) rGO samples.
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Figures:
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graphene oxide layers in their respective electrolytes using cyclic voltammetry (CV),
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Table S1. Fitted results of the C1s core level XPS spectra of GO and rGO samples. -----------------------------------------------------------------------------------------------------------Sample
C-C / C=C
C-O (epoxy & hydroxyl)
C=O (carbonyl)
/ C-H C-N -----------------------------------------------------------------------------------------------------------GO
51.8
35.2
13
rGO
66.7
25.8
7.5
------------------------------------------------------------------------------------------------------------
ACCEPTED MANUSCRIPT Fig. S2. SEM image of graphene (G) and reduced graphene oxide (rGO) layers on ITO glass
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and photographs showing transparent dispersions of G and rGO.
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Fig. S3. Transmittance spectra of G and rGO showing >95% T suggesting less than three layers of graphene 10.
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Fig. S4. Raman spectra of G and rGO materials.
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Raman spectra of graphene showing one, two and three layers of graphene 11.
ACCEPTED MANUSCRIPT Fig. S5. SEM images of electrochromic films on G and rGO and their respective thicknesses in the inset (from 1→5)
Methyl Viologen (MV) On rGO
On G
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On G
PEDOT (PT) On rGO
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Prussian Blue (PB) On rGO
On G
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On G
WO3 (WO) On rGO
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Figure S6. Comparisons of the response times of MV, PT, PB and WO on G vs. rGO.
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Fig. S7. Response times calculations for different EC materials on G (black line) and rGO (blue line)
ACCEPTED MANUSCRIPT References 1. Li D, Mueller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008;3(2):101-105.
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2. Botas C, Álvarez P, Blanco C, et al. Tailored graphene materials by chemical reduction of graphene oxides of different atomic structure. RSC Adv. 2012;2(25):9643-9650.
3. Viinikanoja A, Wang Z, Kauppila J, Kvarnström C. Electrochemical reduction of graphene
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oxide and its in situ spectroelectrochemical characterization. Phys Chem Chem Phys.
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2012;14(40):14003-14009.
4. Kauppila J, Kunnas P, Damlin P, Viinikanoja A, Kvarnström C. Electrochemical reduction of graphene oxide films in aqueous and organic solutions. Electrochim Acta. 2013;89(0):8489.
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5. Hassinen J, Kauppila J, Leiro J, et al. Low-cost reduced graphene oxide-based conductometric nitrogen dioxide-sensitive sensor on paper. Analytical and bioanalytical
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chemistry. 2013;405(11):3611-3617.
6. Thorneley RNF. A convenient electrochemical preparation of reduced methyl viologen and
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a kinetic study of the reaction with oxygen using an anaerobic stopped-flow apparatus. Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1974;333(3):487-496.
7. Zainal MF, Mohd Y. Characterization of PEDOT films for electrochromic applications. Polym Plast Technol Eng. 2015;54(3):276-281.
8. Zou Y, Sun L, Xu F. Prussian blue electrodeposited on MWNTs–PANI hybrid composites for H2O2 detection. Talanta. 2007;72(2):437-442.
ACCEPTED MANUSCRIPT 9. Deepa M, Srivastava AK, Saxena TK, Agnihotry SA. Annealing induced microstructural evolution of electrodeposited electrochromic tungsten oxide films. Appl Surf Sci. 2005;252(5):1568-1580.
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10. Lian W, Huang Y, Liao Y, et al. Flexible electrochromic devices based on optoelectronically active polynorbornene layer and ultratransparent graphene electrodes. Macromolecules. 2011;44(24):9550-9555.
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on copper foils. Science. 2009;324(5932):1312-1314.
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11. Li X, Cai W, An J, et al. Large-area synthesis of high-quality and uniform graphene films