z
Materials Science inc. Nanomaterials
& Polymers
Carbon Based Nanomaterials for High Performance
Optoelectrochemical Systems
Rukiye Ayranci
+,
[a]Gaye Bas¸kaya
+,
[b]Merve Gu¨zel,
[a]Sait Bozkurt,
[b]Fatih S¸en,*
[b]and
Metin Ak*
[a]Carbon nanotubes (CNT) and graphene are two significant carbon based nanomaterials which have extraordinary phys-icochemical properties, used in diverse fields of research. Recently, some actual studies were made to associate these carbon nanomaterials to produce CNT-graphene hybrid with synergic effects of graphene and CNTs. Addressed herein, EDOT-modified reduced graphene oxide (rGO), functionalized multiwalled carbon nanotube (f-MWCNT) and hybrid material (rGO-f-MWCNT) have been prepared. Then, PEDOT-modified
composite films were synthesized via electrochemical polymer-ization to examine enhancing electrochemical properties of PEDOT and characterized by SEM, AFM analysis. As a conclusive finding, an improved stability, charge density, electrochromic switching kinetics and optical contrast of polymer composite films were observed on presence of nanocarbon materials in comparison to the control PEDOT film due to facile ion transport, high surface area.
Introduction
Graphene, a mono layer of sp2
bonded carbon atoms with versatile mechanical, electronic, and thermal properties has been emerged as a promising material in diverse fields of research. Because of its interesting physicochemical properties, it meets extensive range of technological applications including supercapacitors, electronics, fuel cells, biosensors, and bat-teries.[1]
Lately, two dimensional carbon based nanostructures such as graphene, graphite nanoplatelets (GNPs), and graphene oxide (GO) have arisen as promising fillers for polymer matrices.[2-4]
Owing to large surface area and presence of oxygen containing groups on sheet edges and basal planes of the GO materials, reactive ions can be adsorbed onto the electrode surface. On the other hand, CNT observed as a rolled graphene sheets also displays unique electro catalytic, electrical
and mechanical properties.[5]
Especially, CNTs were found to significantly increased heat transport in polymer composites as a result of their one dimensional structure, high aspect ratio
and high thermal conductivity.[6]
However, there is a problem with a stable dispersion of CNT. For this purpose, it proved that GO could be a better dispersant to form a stable dispersion of
CNT which resulted a novel hybrid dispersion named as GO-CNT.[7]
Investigations showed that graphene/CNT and GO-CNT carbon based hybrid nanomaterials show large specific area, higher electrical conductivities and catalytic properties com-pared with either pristine GO/graphene or CNTs. This hybrid nanomaterials were produced by several techniques including
simple sonication method.[7-9]
, electrostatic spray technique[10]
and CVD method. Graphene and CNT make a 3D network
structure because of the strong p-p stacking interaction
between them and provide exceptional stability.[11]
Detailed studies on the classification of carbon nanostructures based on dimensional organization of their edge structures presented by Stoner et al.[12]
. In addition, three dimensional (3D) hierarchical arrangements of the graphene/CNT hybrids have both the high charge density of graphene and 3D network of CNT with large surface area. Consequently, they have outstanding character-istics with highest edge density per unit nominal area than the other carbon based nanostructures such as graphite, MWCNT, SWCNT (Single Walled Carbon Nanotube), graphene sheets, activated carbon, aligned CNT, HOPG (Highly Oriented Pyrolytic Graphite) and bamboo CNT. This property makes them as the best candidate owning outstanding properties among all other carbon based counterparts. Most of the investigations asserted that the synergic effect between CNT and graphene is responsible for the better capacitance of the graphene/CNT hybrid. Besides, the incorporation of CNT with GO provide this hybrid nanomaterial as a versatile platform for the
electro-chemical and electro catalytic applications[13]
.
Nowadays, there has been considerable interest in carbon based nanomaterials, such as GO, rGO, CNT to fabricate a new
class of materials and applications with unique properties[14-17]
. Conjugated polymers (CPs) have received tremendous atten-tion in recent years due to their interesting electrical or
electrochemical properties and conceivable applications. [18–28]
To enhance electrochemical properties of conjugated polymer,
[a] R. Ayranci,+M. Gzel, Prof. M. Ak
Pamukkale University
Faculty of Art and Science, Chemistry Department Denizli, Turkey
E-mail: [email protected] [b] G. Bas¸kaya,+S. Bozkurt, Prof. F. S¸en
Sen Research Group Dumlupinar University
Faculty of Art and Science, Chemistry Department Ktahya, Turkey
E-mail: [email protected] [+
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201601632
carbon based nanomaterials have been employed to making the composite materials due to strong interactions between conjugated polymer and nanocarbon materials. Addressed herein, rGO and f-MWCNT were prepared and hybrid materials (rGO-f-MWCNT) have been prepared as a blend of enhanced mechanical properties due to the synergistic effects of the rGO and f-MWCNT. These materials have been modified by EDOT, then EDOT-modified carbon nanomaterials were electrodepos-ited on ITO for enhancing electrochemical properties of composite films. Hybrid material and EDOT-modified polymer films were characterized and electrochemical activity, stability, coloration efficiency, and switching kinetics of polymer films were investigated comparatively.
Results and Discussion
2.1. Characterization of Hybrid and PEDOT-modified carbon based nanomaterial films
The structural and chemical analysis of rGO/f-MWCNT was clearly defined by characterization techniques. For instance, HRTEM micrographs of rGO/f-MWCNT were shown in Figure 1a. As shown in Figure 1a, the HR-TEM image of the rGO/f-MWCNT was clearly revealed its tubular structure. The surface morphol-ogy of rGO/f-MWCNT was identified by SEM in Figure 1b which indicates the tubular and sheet structure of the prepared materials. Besides, the Raman spectrum of rGO/f-MWCNT was shown in Figure 1c. The spectrum of rGO/f-MWCNT shows two
evident peaks at 1344 and 1573 cm 1
depend on the local
defects and the sp2
graphitized structure, respectively. The G peak formation which is related to the double degenerated
zone center E2g mode. The reduction in particle size of sp2
plane domains and the functionality can arise from the extensive oxidation. Therefore, ID/IG peak intensity ratios are defined inferior defects/disorders. Raman spectra indicate an intensity ratio of ID/IG at 0.95 for rGO and after rGO/f-MWCNT, intensity ratio of ID/IG is found to be 1.14 which confirms the formation of rGO/f-MWCNT.
The surface analyses of corresponding polymer films were defined by SEM. All the figures were taken after 10 cycles of electropolymerization on ITO surface. Figure 2 showed the SEM images of the conducting composite polymer coated ITO electrode surfaces. For the pure polymer PEDOT, Figure 2a depicts the porous structure of the materials. In Figure 2b, it can be observed that the PEDOTR composite film showed like cauliflower and it had homogenous surface. The surface of the polymer composite films changed drastically presence of carbon based nanomaterial and showed a porosity and moss view in PEDOTF (Figure 2c). And especially nanotubular morphology of PEDOTF provided good electron transfer in the composite films. In Figure 2d the highly connected network image of PEDOTH confirmed the good dispersion quality of f-MWCNT and rGO. PEDOTF shows more nanotubular structure while PEDOTH shows more compact structure because of the strong interaction between rGO and f-MWCNT. Furthermore, more nanotubular structure of PEDOTF compared with PEDOTH results better optical and electrochemical properties. Thus,
contrary to what is expected, PEDOTF has better optical and electrochemical properties. Porous network structure of PEDOT-modified carbon based nanomaterial films were demonstrated enhancing electrochemical properties via electron transporta-tion process.
To gain further information for the possible changes in the morphology and correlation between morphology and con-ductivity, AFM images have been performed. Figure 3 showed AFM images of a) PEDOT, b) PEDOTR c) PEDOTF and d) PEDOTH on ITO. The PEDOT film surface with root mean square (rms) roughness of 126.51 nm displayed a poorly structure, while the
Figure 1. a) HR-TEM image, b) SEM image and c) Raman spectrum of rGO/f-MWCNT hybrid.
electrochemically polymerized with modified carbon based nanomaterial PEDOTR,PEDOTF, PEDOTH are very rough with rms roughness of 119.77, 260 and 68.43 nm, respectively. This indicates that hybrid decreases the roughness of the surface of PEDOT. The decreased roughness likely increases the contact area between the PEDOT-carbon based materials and the active
layer. It believed that physical interactions between f-MWCNT and rGO have resulted in a more compact structure. The dark region refers to PEDOT rich areas; the bright area indicates the carbon based materials.
2.2. Cyclic voltammogram of PEDOT-modified carbon based nanomaterial films
The redox chemistry of polymer composite films were inves-tigated by use of cyclic voltammetry between -1.5 to + 1.5 V in
a nonaqueous 0.05 M LiClO4-ACN electrolyte-solvent couple at
50 mV s-1
. The measurements were performed at room temper-ature under inert atmosphere being calibrated by adding ferrocene. In order to investigate effect of carbon based nanomaterials for current increase, we carried out CV studies. Figure 4 showed the CV of polymer composite films indicate
characteristic properties of conducting polymers during poten-tiodynamic electrodeposition. All the polymer composite films were displayed well-defined redox response at their respective potential window. As the CV scan between -1.5 to + 1.5 V the polymer composite films were deposited on the ITO-coated glass surface from dark blue to transparent. Cyclic
voltammo-gram of EDOT in 0.05 M LiClO4-ACN demonstrated oxidation
peaks at 0.45 V and a reduction peak at -0.72 V. Then, CV of P(EDOTR) showed oxidation peaks at 0.47 V and a reduction peak at -0.70 V. P(EDOTF) and P(EDOTH) conducting composite film on the ITO were increasing and a broad oxidation region was observed at 0.49 V, 0.39 V and a reduction peak was shifted to -0.47 V, -0.50 V respectively. When the region between -1.5 V and + 1.5 V was examined, electro activity increased between consecutive cycles. It can be seen clearly from Figure 4 that
Figure 3. AFM image of a) PEDOT b) PEDOTR c) PEDOTF d) PEDOTH on ITO .
Figure 4. Cyclic voltammogram of (a) PEDOT (b) PEDOTR (c) PEDOTF (d) PEDOTH films recorded at 50 mV s-1
higher than that of control PEDOT film when all conditions are kept constant.
The maximum cathodic and anodic peak current densities
were observed in the PEDOTH film 1.33 mAcm-2
and -1.77
mAcm-2
, while in the control PEDOT film 0.79 mAcm-2
and -0.77
mAcm-2
respectively. These results were indicated quick diffusion of electrons and counter ions in PEDOTH matrix due to incorporated f-MWCNT which bridges between rGO as an electron transporter. Furthermore, Figure 5 showed the
oxida-tion onset potential of polymer composite films and in internal graphs and it was compared with third CV cycle of polymer
polymer composites were obtained much lower than EDOT, when all conditions kept the same. It was initiated at 1.33 V for EDOT; however, it was obtained 1.1, 1.05 and 1.17 for EDOTR, EDOTF and EDOTH respectively. These results demonstrated presence of carbon based nanomaterials which results in a lowering of the onset of oxidation. The acceptable reason for the important improvement in the electrochemical reduction of polymer composite films was due to the included nanocarbon materials enhance ion transport depends on highly porous morphology.
2.3. Stability of PEDOT-modified carbon based nanomaterial films
Electrochemical stability is a significant for conducting poly-mers (CPs) with long lifetimes. CV technique was used for investigate stability of CPs. For this reason, PEDOT and polymer composite films were swept continuously between -1.5 V and 1.5 V in a monomer-free solution at a scan rate of 500 mV/s comparatively, as shown in Figure 6. After 200 cycles, polymer
composite films were maintained almost all electro activity such as 94.27% for PEDOTR 95.51% for PEDOTF and 92.41% PEDOTH compared to 88.15% for PEDOT. It was clearly showed that presence of carbon based nanomaterial modified EDOT, polymer composite films have much better electrochemical stability upon continuous switching compared with PEDOT. The highest stability was detected for PEDOTF. The well embedded f-MWCNT by aid of carboxyl groups in composite film was increased the degree of electron delocalization. Therefore, change of volume in oxidation/reduction process of polymer composite films was reduced. PEDOTF composite film can be good candidate material for electrochromic device.
Figure 5. Cyclic voltammogram of (a) PEDOT (b) PEDOTR (c) PEDOTF (d) PEDOTH film recorded at 50 mV s-1
in 0.05 M LiClO4-ACN.
2.4. Investigation of charge densities of PEDOT-modified carbon based nanomaterial films
Charge density (Qd) is the total charge used for the conducting
polymers deposition between doped and undoped states. The
value of Qdwas defined comparatively, by integrating the area
under the cathodic peak of polymer composite films during the
characterization of the ITO electrode in the 0.05 M LiClO4-ACN
solution. As illustrated in Figure 7, when a PEDOT film with
charge density of 3.5 mC/cm2
, PEDOT composite films showed remarkably different charge densities approximately 2 times, it
was found 6.25 mC/cm2
for PEDOTR, 8.11 mC/cm2
for PEDOTF,
7.31 mC/cm2
for PEDOTH . PEDOTF composite film shows the best charge density again affirming the effective dopant effect of f-MWCNT between the components.
2.5. Spectroelectrochemical properties of PEDOT-modified carbon based nanomaterial Films
For the purpose of achieving the optical and electrochromic properties of conjugated polymers, spectroelectrochemical studies have been performed successfully between neutral and oxidized sites. PEDOT, PEDOTR, PEDOTF, PEDOTH were synthe-sized by electrochemical polymerization on the ITO surface and spectral changes were performed using a UV-vis spectropho-tometer. The optical and electrochromic properties of the resultant polymers were examined by applying potentials
between 0.7 and 0.5 V for PEDOT, 0.7 and 1.2 V for PEDOTR,
0.7 and 1.0 V for PEDOTF and -0.7 and 0.8 V for PEDOTH
composites in a monomer-free 0.05M LiClO4-ACN medium. In
Figure 8, all polymers composite films were showed typical PEDOT absorption in the UV region of the spectra which was assigned to the intrinsic band gap absorption of composite structure following from the electron transitions from the
valence band to the conduction band. When compared to control PEDOT film, both a considerable value of absorbance increasing and red shift to higher wavelength were observed in
Figure 7. Charge Density Graphs of (a) PEDOT (b) PEDOTR (c) PEDOTF (d) PEDOTH.
Figure 8. Optoelectrochemical spectra of (a) PEDOT (b)P(EDOTR) (c) P(EDOTF) (d) P(EDOTH).
polymer composite films in the same conditions. Therefore, band gaps of polymer composite film were narrowed indicating interaction between nano carbon and EDOT. Upon analyzing the optoelectrochemical spectra of films, the optical band gaps
(Eg) calculated from onset of absorptions of the thin films were
estimated as 1.55 eV for PEDOT, 1.49 eV PEDOTR,1.49 eV PEDOTF and 1.48 eV for PEDOTH. In carbon based PEDOT
composite polymers have lower value of Eg depend on
excellent electron transport between nanocarbon materials and PEDOT.
2.6. Electrochromic switching
Electrochromic switching capabilities of PEDOT-modified car-bon based nanomaterial films were detected by the between doped and dedoped transmittance values in kinetic plots. To
properties of polymer compo-site films, polymer compocompo-site films were prepared by electro-deposition on ITO at constant
potential. Optical contrast
(DT%) at lmaxand the switching
time which are important
pa-rameters for electrochromic
materials were determined using a UV–vis spectrophotometer. The polymer composite films and control PEDOT film were switched between -1.0 V and + 1.3 V with a switching interval
of 5 s in the monomer-free LiCIO4-ACN solution. Figure 9a was
demonstrated second cycle of kinetic plots of all polymer films between the highest and lowest transmittance comparatively.
Switching time and DT% values for PEDOT measured as 2.5 s
and 29.33, for PEDOTR measured as 2s and 44.53, for PEDOTF measured as 2s and 38.94 and for PEDOTH measured as 3s and 52.32. PEDOTH composite film was demonstrated excellent optical contrast that is approximately 1.8 times higher than that of the PEDOT film. Highest optic contrast of PEDOTH composite film was affected by both good ionic and electronic
transport ability (rGO) and p-p interaction between the
carboxyl groups of the f-MWCNT and the quinoid rings of PEDOT which can alter the electron density. Presence of nanocarbon materials, increasing the active sites will lead to substantial charge addition and to develop the overall redox reaction kinetic, which is considered as the short switching time.
Therefore, porous and nanotubular morphology of PEDOTF was permited faster ion movement and faster switching time. Figure 9b demonstrated optical absorbance change of PEDOTH during 120 s. PEDOTH film was confirmed that long life time and high operational stability depend on high surface area. Table 1 summarized electrochemical properties of polymer composite films compared to PEDOT film.
4. Conclusion
PEDOT-modified carbon based nanomaterial films were pre-pared by using electropolymerization. These composite films were exhibited high specific capacitance, good cyclic stability, fast electrochemical reaction kinetics and great optical contrast in comparison to control PEDOT films. PEDOTF film was showed excellent redox stability depend on acting as dopant ion by nanotubular and porous structure. Since physical interaction between rGO and f-MWCNT was very strong, EDOT was thought to be better modified to f-MWCNT than hybrid. Therefore, the polymerization of PEDOTF was found to be more effective than PEDOTH contrary to expectations. PEDOTH film was demon-strated good optical contrast which depends on high surface area and superior electron transfer ability.
Supporting Information
Supporting Information contains detailed experimental proce-dures, preparation of nanocarbon materials and modified EDOT
Figure 9. (a) First step of optical absorbance change of polymer composite films (b) optical absorbance change of PEDOTH in 0.05 M LiClO4-ACN
electrolyte–solvent couple.
Table 1. Comparison of electrochemical properties of polymer composite films
Polymer HOMO (eV) LUMOa (eV) Stability % CD (mC/cm2) Eg (eV) Switching Time (s) T%
PEDOT -4.47 +5.96 88.15 3.5 1.55 2.5 29.3
PEDOTR -4.47 +5.96 94.27 6.25 1.49 2.1 44.53
PEDOTF -4.43 +5.92 95.51 8.11 1.49 2 38.94
conducting polymers and characterizations of nanocarbon materials.
Acknowledgements
This research project was financially supported Pamukkale University as BAP [Project No: 2014FBE045], Dumlupinar University as BAP (2016-75).
Conflict of Interest
The authors declare no conflict of interest.
Keywords: EDOT-modified · Electrochemistry · Graphene · Hybrid Material · Spectroelectrochemistry
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Submitted: October 28, 2016 Accepted: January 27, 2017
