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Live spectroscopy to observe electrochromism in viologen based solid state device
Solid State Communications 261 (2017) 17–20
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Communication
Live spectroscopy to observe electrochromism in viologen based solid state device
MARK
Suryakant Mishra, Haardik Pandey, Priyanka Yogi, Shailendra K. Saxena, Swarup Roy, ⁎ P.R. Sagdeo, Rajesh Kumar Material Research Laboratory, Discipline of Physics & MEMS, Indian Institute of Technology Indore, Simrol 453552, India
A R T I C L E I N F O
A BS T RAC T
Keywords: Raman spectroscopy UV–vis spectroscopy Viologen Electrochromism
In-situ Raman and UV–vis spectroscopies have been carried out from viologen based electrochromic device fabricated in two simple geometries. Ethyl viologen diperchlorate in polyethylene oxide matrix was used as the active layer in the device without any counter material to understand the effect of bias on viologen. The device, which is transparent otherwise, changes its color to navy blue when a bias of 2 V is applied. Reduction of viologen to its free radical state as revealed from Raman spectroscopy recorded in-situ in both the geometries appears to be the mechanism for this color switching. It is also observed that absorbance of the device is perturbed when bias is applied with maximum change in absorbance corresponding to green wavelength which is giving the blue tint of the device in on state. We establish that UV–vis and Raman spectroscopies prove to be the best method for understanding the mechanism of color switching in viologen based electrochromic device because it gives the advantage to see a device ‘live’ while operating.
1. Introduction Materials which show reversible change in color by the application of bias are known as electrochromic materials [1]. Devices, fabricated using above mentioned material, are called electrochromic devices. Phenomena of Electrochromism recently are attracting much attention because of its diverse application in smart windows to display system [2,3]. Many literatures recently have published on this phenomenon of color change using different material in organic as well as inorganic [4]. Electrochromism, observed as a consequence of reversible redox reaction in the given material prove to be a better option for realizing in devices. Origin of this property of material lies in the molecular change by redox reaction and this change in molecular structure modulate optical property of the material. In bulk and nano forms, various inorganic compounds like WO3, V2O5, NiO etc. are used for the fabrication of electrochromic device with good performance parameter like coloration efficiency, cycling stability and color switching time [5–7]. These metal oxides lack sufficient transparency and ease of fabrication processes to be used in a wide range of applications. In contrast, viologen based materials, being an organic material, are solution processable and can yield flexible electrochromic devices [8,9]. In order to achieve an electrochromic response in the solid state, an ionically conducting matrix is required in addition to a redox counter-reaction [10]. Instead of ⁎
Corresponding author. E-mail address: [email protected] (R. Kumar).
http://dx.doi.org/10.1016/j.ssc.2017.05.020 Received 25 October 2016; Received in revised form 20 March 2017; Accepted 23 May 2017 Available online 27 May 2017 0038-1098/ © 2017 Elsevier Ltd. All rights reserved.
inorganic, organic compounds like conjugate polymers (PPY, PANI,etc), where free radical will form and freely move from one side to other, shows electrochromic property. One particularly intriguing class of materials in this context is bipyridinium species, which are formed upon N,N-diquaternization of 4,4′- bipyridine, also known as viologen. Various materials and device geometries have been already reported [11–16] for successful operation of an electrochromic device but more investigations are required to be sure about the exact mechanism during functioning of the device. Especially, characterization must be done under the same conditions and geometries as of the working device. Being non-destructive methods, spectroscopic techniques like UV–vis [17,18] and Raman [19,20] can be of importance for doing in-situ monitoring of the device while being operated, this makes the basis of the current study. In the present work we have carried out spectroscopic studies to understand electrochromism in viologen based organic material and try to find out physics behind this phenomenon by fabricating electrochromic device in two different basic geometries enabling us to carry out in-situ spectroscopies. Device fabrication (geometries and recipe) and characterizations has been detailed in the experimental section below. In-situ spectroscopic methods reveal not only the mechanism of color switching by identifying the active species but also the polarity condition under which the color change initiates. This will be helpful in designing a device which is robust and can work under almost any
Solid State Communications 261 (2017) 17–20
S. Mishra et al.
opaque state may be due to reduction of viologen molecule (by accepting electron from the negative electrode) and the reverse process (by losing electron after bias removal) is due to oxidation of it. Due to reversible redox reaction of viologen, device switches its color. The blue color visible as a result of bias has only qualitative meaning and needs more scientific approach to identify the presence of different wavelength components in the transmitted light when the device is seen in a visible light. When potential across the device is applied EV2+ and (ClO4−) ions start moving under applied field. EV+2 ions move toward the negative electrode and its counter ions move toward the positive electrode. At the negative electrode, viologen ion (EV+2) accepts electron and reduces to EV•+. As voltage increases more reduction take place near the electrode interface and more number of FRC are generated which gives rise to intense blue color on the surface of negative electrode as shown in Fig. 1. This mechanism will be further confirmed by detecting the FRC and other viologen species later using Raman spectroscopy. Color change in x-bar geometry, it is not clear, from which electrode the color change starts. Device has been fabricated in open face geometry (OFG) to investigate the special distribution, if any, of the color change under the applied bias. This geometry is also helpful in understanding involvement of ions layers near interfacial electrodes in the color switching mechanism. Fig. 2(a) shows the schematic of the device in OFG where E1 & E2 are gold electrodes. Electrode E1 and E2 connected to the positive and negative terminal of a power source. When battery polarity in the positive direction with amplitude 2 V, edge of the E2 terminal darken and when polarity reverse with amplitude −2 V edge of E1 darken. It is also important here to mention that with change in bias polarity the color switching also changes reversibly. At a given finite bias, only one electrode connected to the –ve terminal is colored. By above mention observation one can conclude that species responsible for the navy blue color are generate at the –ve electrode which is capable of reducing the material which is in its contact. It appears that the EV2+ ion present in the neutral device reduced to EV•+ at the –ve electrode and gives rise to the navy blue color as can be seen in the OFG. The same reductions were observed in the x-bar geometry but due to lack of knowledge of special ion distribution along the electrodes, it appears the overall color change as can be seen in the Fig. 1. To establish the fact that free radical EV•+ is responsible for the color change Raman spectroscopy has been carried out in the ON & OFF states as shown in the Fig. 3. Raman spectra shown in Fig. 3 have been recorded in-situ form the x-bar geometry of the device, so that species responsible for color change can be identified. Raman spectra are recorded in two ranges R1 (800–1150 cm−1) and R2 (1550– 1750 cm−1) in the fetch of responsible color switching species. Appearance and disappearance of Raman peaks is observed in R1 where blue shift in R2 was observed. When device is in its virgin state two peaks can be seen in the range R1 at 823 and 934 cm−1 which disappear and one peak appear at 1028 cm−1 after the application of bias (ON state). Furthermore, a blue shift is observed in range R2 from 1647 to 1654 cm−1 when device goes to ON state. These observations can be understood as follows. Raman peaks at 823,934 and 1647 cm−1 from an OFF device correspond to EV(ClO4)2 [10]. Raman spectra give in Fig. 3(b) have been recorded under bias of 2 V on E2 (negative). The observed peaks at 1028 cm−1 and 1654 cm−1 are the vibrational modes of the restructured molecule after the reduction of viologen which leads to the formation of EV•+. The exact vibration modes responsible for appearance of these peaks has been available in literature [21–24]. After establishing the bias induced redox process, taking place in the electrochromic device, it will be interesting to correlate the perceived color with the absorbance spectrum (UV–vis) of the device similar to Raman spectroscopy. From UV–vis spectra one can understand the actual wavelength of light which is more affected by the bias induced reduction of viologen molecule. Fig. 4 shows absorbance (%) spectra from the device under with and without bias conditions. It is clear from Fig. 4 that absorbance of the device in the visible region
atmosphere. Raman spectroscopy clearly establishes connection between electrochromic doping and Raman features. Both UV–vis and Raman spectroscopic data reconcile the correlation between charge carriers and color changes in viologen based devices. It has been observed by getting electrons from negative electrode viologen converts into its free radical form and this species shows more absorption in green window and no specific change in blue color window. 2. Experimental details Chemical used in device fabrication were purchased from Alfa Aesar and Sigma Aldrich. Polyethylene oxide (PEO, Alfa Aesar, MW = 100,000), Ethyl viologen diperchlorate (98%, Sigma Aldrich), Sodium borohydride (NaBH4, Alfa Aesar), and Acetonitrile (ACN, anhydrous, 99.8%, Sigma Aldrich) were used as received. Material for electrochromic device fabrication was prepared by mixing 4 wt% ethyl viologen diperchlorate [EV(ClO4)2] in acetonitrile and 5 wt% PEO in acetonitrile. The PEO solution was filtered through a 0.45 µm PTFE filter before adding the viologen solution. The solution prepared above has been deposited over the electrode by dropping 3 μl of solution in between both the electrode and spin the device upto 3000 rpm for betterment of thin film. Thin film of EV+PEO was prepared on transparent ITO electrode using spin coating. After spin coating, second ITO electrode was laminated face to face on the spin coated substrate. The extreme part of both the electrodes was painted by silver glue for making connection with external power supply. In the cross-bar device EV in matrix has been sandwiched between two transparent electrodes. For the understanding of ion diffusion and electrode-molecule interface need to fabricate one more device of open face geometry. Device with open geometry were fabricated by drop casting EV in between two gold electrodes separated by few microns where both the electrodes are on the same plane. UV–vis spectroscopy was performed using Cary 60 UV–vis spectrophotometer of Agilent whereas Raman spectroscopy was done using LABRAM HR spectrometer using a 633 nm excitation source. 3. Results and discussion Fig. 1 is the schematic representation of cross-bar device associated with its biasing arrangement and actual photographs of devices under bias and no bias conditions. Initially at zero voltage, the device is in OFF state (say) and is purely transparent. By the application of bias, transparency of the device starts reducing as potential increases. Typical optical properties of chemically reduced viologen show that blue color originates due to the presence of free radical cation (FRC) that is EV•+, which can be obtained by chemically reducing ethyl viologen dication (EV2+)which is transparent. Similar redox process can take place as a result of electric bias instead of chemical redox process. In its ON state (say) at 2 V, device becomes opaque (with a navy blue tint). Conversion of device from its transparent state to pure
Fig. 1. Schematic illustration of cross-bar geometry of the device connecting through the external biasing arrangement. The images below the schematics are the actual photographs captured when seen through an electrochromic device kept on a paper printed with IITI logo without and with bias.
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Fig. 2. (a) Schematic representation of open face geometry device with its external biasing arrangement in its top and cross-sectional views (b) Microscopic optical images of actual device while operating in forward and reverse bias conditions. Voltages shown correspond to the one applied at E1 with respect to E2.
Fig. 4. Bias dependent UV–vis spectroscopy showing change in absorbance. Top inset spectra shows magnify absorption edge shifting with bias and bottom inset shows fractional change in absorbance (%) in ON and OFF states.
these peaks are more sensitive during application of bias. From the graph one can appreciate maximum absorbance of orange color window and there is unaffected blue region (436 nm) in ON state of device. This means that in the ON state device will allow only blue color to pass through it and suppress all the other color giving the navy blue tint appearance of the device in the ON state [25].
Fig. 3. In-situ Raman-spectra recorded in the cross bar geometry of the device under (a) without bias and (b) with bias conditions.
varies on the application of potential across the device. Absorbance of the film depends upon the band-gap of the material so the change in absorbance can be understood by the band-gap modulation (top inset, Fig. 4). During in-situ measurement, potential across the device was increased gradually upto 2 V and absorbance of the device is measured in the range between 300 to 800 nm. Initially in OFF state a hump (large dip) is observed around 550–560 nm. When device is in ON state absorption increases corresponds to all wavelengths with some mountain in the green region. By calculating ratio of absorbance in ON (AON) and OFF (AOFF) state at a given wavelength one can understand which wavelength is more responsive to the applied bias. The inset of Fig. 4 shows the ratio of absorbance in its ON and OFF states, two peaks appear at 398 nm and 559 nm. The physical significance of these peaks is that the wavelength corresponding to
4. Conclusions In summary, in-situ UV–vis and Raman spectroscopies prove to be an excellent tool to characterize electrochromic device in an endeavor to understand the origin of electrochromism. It is revealed here that viologen based electrochromic material changes color due to redox switching between two species EV2+ and EV•+ as a result of electric bias. The viologen ions in plastic (PEO) matrix alone do not make the complete device for operation due to which, the color goes back to transparent state after removal of bias. This is possibly due to atmospheric water, playing role in the redox process. An appropriate addition of oxidizable material (e.g, polythiophene) between viologen 19
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[8] C. Yan, W. Kang, J. Wang, M. Cui, X. Wang, C.Y. Foo, K.J. Chee, P.S. Lee, Stretchable and wearable electrochromic devices, ACS Nano 8 (2014) 316–322. http://dx.doi.org/10.1021/nn404061g. [9] W. Kang, C. Yan, C.Y. Foo, P.S. Lee, Foldable electrochromics enabled by nanopaper transfer method, Adv. Funct. Mater. 25 (2015) 4203–4210. http:// dx.doi.org/10.1002/adfm.201500527. [10] B. Liu, A. Blaszczyk, M. Mayor, T. Wandlowski, Redox-switching in a viologen-type adlayer: an electrochemical shell-isolated nanoparticle enhanced raman spectroscopy study on Au(111)-(1×1) single crystal electrodes, ACS Nano 5 (2011) 5662–5672. http://dx.doi.org/10.1021/nn201307g. [11] L.M. Loew, S. Scully, L. Simpson, A.S. Waggoner, Evidence for a charge-shift electrochromic mechanism in a probe of membrane potential, Nature 281 (1979) 497–499. http://dx.doi.org/10.1038/281497a0. [12] A.I. Fishman, A.A. Stolov, A.B. Remizov, Vibrational spectroscopic approaches to conformational equilibria and kinetics (in condensed media), Spectrochim. Acta Mol. Spectrosc. 49 (1993) 1435–1479. http://dx.doi.org/10.1016/0584-8539(93) 80051-B. [13] L.M. Loew, Design and characterization of electrochromic membrane probes, J. Biochem. Biophys. Methods 6 (1982) 243–260. http://dx.doi.org/10.1016/0165022X(82)90047-1. [14] X.G. Wang, Y.S. Jang, N.H. Yang, L. Yuan, S.J. Pang, XPS and XRD study of the electrochromic mechanism of WOx films, Surf. Coat. Technol. 99 (1998) 82–86. http://dx.doi.org/10.1016/S0257-8972(97)00415-5. [15] A. Jin, W. Chen, Q. Zhu, Z. Jian, Multi-electrochromism behavior and electrochromic mechanism of electrodeposited molybdenum doped vanadium pentoxide films, Electrochim. Acta 55 (2010) 6408–6414. http://dx.doi.org/10.1016/j.electacta.2010.06.047. [16] R.J. Mortimer, A.L. Dyer, J.R. Reynolds, Electrochromic organic and polymeric materials for display applications, Displays 27 (2006) 2–18. http://dx.doi.org/ 10.1016/j.displa.2005.03.003. [17] A.S.N. Murthy, A.P. Bhardwaj, Electronic absorption spectroscopic studies on charge-transfer interactions in a biologically important molecule: n,n′-dimethyl4,4′-bipyridylium chloride (paraquat or methyl viologen) as an electron acceptor, Spectrochim. Acta Mol. Spectrosc. 38 (1982) 207–212. http://dx.doi.org/10.1016/ 0584-8539(82)80198-0. [18] M.L. Rodriguez-Mendez, R. Aroca, J.A. DeSaja, Spectroscopic and electrochemical properties of thin solid films of yttrium bisphthalocyanine, Spectrochim. Acta Mol. Spectrosc. 49 (1993) 965–973. http://dx.doi.org/10.1016/0584-8539(93)80215V. [19] Wiley: Raman Spectroscopy for Chemical Analysis - Richard L. McCreery, (n.d.). 〈http://as.wiley.com/WileyCDA/WileyTitle/productCd-0471252875.html〉 (Accessed 28 March 2016). [20] M. Momose, Y. Furukawa, Non-destructive Raman evaluation of a heavily doped surface layer fabricated by laser doping with B-doped Si nanoparticles, Mater. Sci. Semicond. Process. 39 (2015) 748–754. http://dx.doi.org/10.1016/ j.mssp.2015.06.022. [21] B. Han, Z. Li, T. Wandlowski, A. Błaszczyk, M. Mayor, Potential-induced redox switching in viologen self-assembled monolayers: an ATR-SEIRAS approach, J. Phys. Chem. C 111 (2007) 13855–13863. http://dx.doi.org/10.1021/jp073208g. [22] Z.Q. Tian, W.H. Li, B.W. Mao, J.S. Gao, Surface-enhanced Raman spectroscopic studies on structural dynamics of coadsorption of thiourea and ClO4− at Ag electrodes, J. Electroanal. Chem. 379 (1994) 271–279. http://dx.doi.org/10.1016/ 0022-0728(94)87148-5. [23] T. Lu, T.M. Cotton, In situ Raman spectra of the three redox forms of heptylviologen at platinum and silver electrodes: counterion effects, J. Phys. Chem. 91 (1987) 5978–5985. http://dx.doi.org/10.1021/j100307a033. [24] S. Ghoshal, T. Lu, Q. Feng, T.M. Cotton, A normal coordinate analysis of the vibrational modes of the three redox forms of methylviologen: comparison with experimental results, Spectrochim. Acta Mol. Spectrosc. 44 (1988) 651–660. http://dx.doi.org/10.1016/0584-8539(88)80124-7. [25] C.M. Amb, A.L. Dyer, J.R. Reynolds, Navigating the color palette of solutionprocessable electrochromic polymers, Chem. Mater. 23 (2011) 397–415. http:// dx.doi.org/10.1021/cm1021245.
and electrode will support the bias induced oxidation of viologen without relying on ambient moisture when it comes to make the real device. The complete device fabricated then will be able to work in any atmosphere. Appropriate choice of matrix (like PEO) material for the additional layers will allow one to fabricate the device so that the layers make good contacts, necessary for making the device more stable and robust. In-situ Raman spectroscopy of device clearly shows the generation of free radical cation which is responsible for the dark color of the device. UV–vis spectroscopy of device, switching between transparent and navy blue color, shows the change in absorbance (%) as a result of bias change due to conversion of viologen molecule into free radical. It is also revealed that maximum change in absorbance take place for 556 nm as a result of bias which effectively gives the blue appearance of the device. Acknowledgements Authors acknowledge financial support from Department of Science and Technology (DST), Govt. of India (Award number SB/FTP/PS024/2014). Authors are thankful to Prof. R.L. Mccreery (University of Alberta) for gold electrodes, Professor V.D. Vankar (IIT Delhi) for ITO electrodes and Dr. V. Sathe (UGC-DAE CSR, Indore, India) for Raman measurements. Authors thank Dr J. Jayabalan (RRCAT, Indore) for useful discussions. Authors are also thankful to MHRD, Govt. of India for providing fellowships. References [1] V.K. Thakur, G. Ding, J. Ma, P.S. Lee, X. Lu, Hybrid materials and polymer electrolytes for electrochromic device applications, Adv. Mater. 24 (2012) 4071–4096. http://dx.doi.org/10.1002/adma.201200213. [2] M.L. Moser, G. Li, M. Chen, E. Bekyarova, M.E. Itkis, R.C. Haddon, Fast electrochromic device based on single-walled carbon nanotube thin films, Nano Lett. 16 (2016) 5386–5393. http://dx.doi.org/10.1021/acs.nanolett.6b01564. [3] C. Faure, A. Guerfi, M. Dontigny, D. Clément, P. Hovington, U. Posset, K. Zaghib, High cycling stability of electrochromic devices using a metallic counter electrode, Electrochim. Acta 214 (2016) 313–318. http://dx.doi.org/10.1016/j.electacta.2016.08.055. [4] G. Cai, P. Darmawan, M. Cui, J. Chen, X. Wang, A.L.-S. Eh, S. Magdassi, P.S. Lee, Inkjet-printed all solid-state electrochromic devices based on NiO/WO3 nanoparticle complementary electrodes, Nanoscale 8 (2015) 348–357. http://dx.doi.org/ 10.1039/C5NR06995E. [5] G. Cai, X. Wang, M. Cui, P. Darmawan, J. Wang, A.L.-S. Eh, P.S. Lee, Electrochromo-supercapacitor based on direct growth of NiO nanoparticles, Nano Energy 12 (2015) 258–267. http://dx.doi.org/10.1016/j.nanoen.2014.12.031. [6] G. Cai, J. Tu, D. Zhou, L. Li, J. Zhang, X. Wang, C. Gu, Constructed TiO2/NiO core/ shell nanorod array for efficient electrochromic application, J. Phys. Chem. C 118 (2014) 6690–6696. http://dx.doi.org/10.1021/jp500699u. [7] S. Paul, A. Ghosh, A. Chakraborty, L. Petaccia, D. Topwal, D.D. Sarma, S. Oishi, S. Raj, Temperature dependent photoemission spectroscopy on lightly-doped sodium tungsten bronze, Solid State Commun. 152 (2012) 493–496. http:// dx.doi.org/10.1016/j.ssc.2011.12.045.
20
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Communication
Live spectroscopy to observe electrochromism in viologen based solid state device
MARK
Suryakant Mishra, Haardik Pandey, Priyanka Yogi, Shailendra K. Saxena, Swarup Roy, ⁎ P.R. Sagdeo, Rajesh Kumar Material Research Laboratory, Discipline of Physics & MEMS, Indian Institute of Technology Indore, Simrol 453552, India
A R T I C L E I N F O
A BS T RAC T
Keywords: Raman spectroscopy UV–vis spectroscopy Viologen Electrochromism
In-situ Raman and UV–vis spectroscopies have been carried out from viologen based electrochromic device fabricated in two simple geometries. Ethyl viologen diperchlorate in polyethylene oxide matrix was used as the active layer in the device without any counter material to understand the effect of bias on viologen. The device, which is transparent otherwise, changes its color to navy blue when a bias of 2 V is applied. Reduction of viologen to its free radical state as revealed from Raman spectroscopy recorded in-situ in both the geometries appears to be the mechanism for this color switching. It is also observed that absorbance of the device is perturbed when bias is applied with maximum change in absorbance corresponding to green wavelength which is giving the blue tint of the device in on state. We establish that UV–vis and Raman spectroscopies prove to be the best method for understanding the mechanism of color switching in viologen based electrochromic device because it gives the advantage to see a device ‘live’ while operating.
1. Introduction Materials which show reversible change in color by the application of bias are known as electrochromic materials [1]. Devices, fabricated using above mentioned material, are called electrochromic devices. Phenomena of Electrochromism recently are attracting much attention because of its diverse application in smart windows to display system [2,3]. Many literatures recently have published on this phenomenon of color change using different material in organic as well as inorganic [4]. Electrochromism, observed as a consequence of reversible redox reaction in the given material prove to be a better option for realizing in devices. Origin of this property of material lies in the molecular change by redox reaction and this change in molecular structure modulate optical property of the material. In bulk and nano forms, various inorganic compounds like WO3, V2O5, NiO etc. are used for the fabrication of electrochromic device with good performance parameter like coloration efficiency, cycling stability and color switching time [5–7]. These metal oxides lack sufficient transparency and ease of fabrication processes to be used in a wide range of applications. In contrast, viologen based materials, being an organic material, are solution processable and can yield flexible electrochromic devices [8,9]. In order to achieve an electrochromic response in the solid state, an ionically conducting matrix is required in addition to a redox counter-reaction [10]. Instead of ⁎
Corresponding author. E-mail address: [email protected] (R. Kumar).
http://dx.doi.org/10.1016/j.ssc.2017.05.020 Received 25 October 2016; Received in revised form 20 March 2017; Accepted 23 May 2017 Available online 27 May 2017 0038-1098/ © 2017 Elsevier Ltd. All rights reserved.
inorganic, organic compounds like conjugate polymers (PPY, PANI,etc), where free radical will form and freely move from one side to other, shows electrochromic property. One particularly intriguing class of materials in this context is bipyridinium species, which are formed upon N,N-diquaternization of 4,4′- bipyridine, also known as viologen. Various materials and device geometries have been already reported [11–16] for successful operation of an electrochromic device but more investigations are required to be sure about the exact mechanism during functioning of the device. Especially, characterization must be done under the same conditions and geometries as of the working device. Being non-destructive methods, spectroscopic techniques like UV–vis [17,18] and Raman [19,20] can be of importance for doing in-situ monitoring of the device while being operated, this makes the basis of the current study. In the present work we have carried out spectroscopic studies to understand electrochromism in viologen based organic material and try to find out physics behind this phenomenon by fabricating electrochromic device in two different basic geometries enabling us to carry out in-situ spectroscopies. Device fabrication (geometries and recipe) and characterizations has been detailed in the experimental section below. In-situ spectroscopic methods reveal not only the mechanism of color switching by identifying the active species but also the polarity condition under which the color change initiates. This will be helpful in designing a device which is robust and can work under almost any
Solid State Communications 261 (2017) 17–20
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opaque state may be due to reduction of viologen molecule (by accepting electron from the negative electrode) and the reverse process (by losing electron after bias removal) is due to oxidation of it. Due to reversible redox reaction of viologen, device switches its color. The blue color visible as a result of bias has only qualitative meaning and needs more scientific approach to identify the presence of different wavelength components in the transmitted light when the device is seen in a visible light. When potential across the device is applied EV2+ and (ClO4−) ions start moving under applied field. EV+2 ions move toward the negative electrode and its counter ions move toward the positive electrode. At the negative electrode, viologen ion (EV+2) accepts electron and reduces to EV•+. As voltage increases more reduction take place near the electrode interface and more number of FRC are generated which gives rise to intense blue color on the surface of negative electrode as shown in Fig. 1. This mechanism will be further confirmed by detecting the FRC and other viologen species later using Raman spectroscopy. Color change in x-bar geometry, it is not clear, from which electrode the color change starts. Device has been fabricated in open face geometry (OFG) to investigate the special distribution, if any, of the color change under the applied bias. This geometry is also helpful in understanding involvement of ions layers near interfacial electrodes in the color switching mechanism. Fig. 2(a) shows the schematic of the device in OFG where E1 & E2 are gold electrodes. Electrode E1 and E2 connected to the positive and negative terminal of a power source. When battery polarity in the positive direction with amplitude 2 V, edge of the E2 terminal darken and when polarity reverse with amplitude −2 V edge of E1 darken. It is also important here to mention that with change in bias polarity the color switching also changes reversibly. At a given finite bias, only one electrode connected to the –ve terminal is colored. By above mention observation one can conclude that species responsible for the navy blue color are generate at the –ve electrode which is capable of reducing the material which is in its contact. It appears that the EV2+ ion present in the neutral device reduced to EV•+ at the –ve electrode and gives rise to the navy blue color as can be seen in the OFG. The same reductions were observed in the x-bar geometry but due to lack of knowledge of special ion distribution along the electrodes, it appears the overall color change as can be seen in the Fig. 1. To establish the fact that free radical EV•+ is responsible for the color change Raman spectroscopy has been carried out in the ON & OFF states as shown in the Fig. 3. Raman spectra shown in Fig. 3 have been recorded in-situ form the x-bar geometry of the device, so that species responsible for color change can be identified. Raman spectra are recorded in two ranges R1 (800–1150 cm−1) and R2 (1550– 1750 cm−1) in the fetch of responsible color switching species. Appearance and disappearance of Raman peaks is observed in R1 where blue shift in R2 was observed. When device is in its virgin state two peaks can be seen in the range R1 at 823 and 934 cm−1 which disappear and one peak appear at 1028 cm−1 after the application of bias (ON state). Furthermore, a blue shift is observed in range R2 from 1647 to 1654 cm−1 when device goes to ON state. These observations can be understood as follows. Raman peaks at 823,934 and 1647 cm−1 from an OFF device correspond to EV(ClO4)2 [10]. Raman spectra give in Fig. 3(b) have been recorded under bias of 2 V on E2 (negative). The observed peaks at 1028 cm−1 and 1654 cm−1 are the vibrational modes of the restructured molecule after the reduction of viologen which leads to the formation of EV•+. The exact vibration modes responsible for appearance of these peaks has been available in literature [21–24]. After establishing the bias induced redox process, taking place in the electrochromic device, it will be interesting to correlate the perceived color with the absorbance spectrum (UV–vis) of the device similar to Raman spectroscopy. From UV–vis spectra one can understand the actual wavelength of light which is more affected by the bias induced reduction of viologen molecule. Fig. 4 shows absorbance (%) spectra from the device under with and without bias conditions. It is clear from Fig. 4 that absorbance of the device in the visible region
atmosphere. Raman spectroscopy clearly establishes connection between electrochromic doping and Raman features. Both UV–vis and Raman spectroscopic data reconcile the correlation between charge carriers and color changes in viologen based devices. It has been observed by getting electrons from negative electrode viologen converts into its free radical form and this species shows more absorption in green window and no specific change in blue color window. 2. Experimental details Chemical used in device fabrication were purchased from Alfa Aesar and Sigma Aldrich. Polyethylene oxide (PEO, Alfa Aesar, MW = 100,000), Ethyl viologen diperchlorate (98%, Sigma Aldrich), Sodium borohydride (NaBH4, Alfa Aesar), and Acetonitrile (ACN, anhydrous, 99.8%, Sigma Aldrich) were used as received. Material for electrochromic device fabrication was prepared by mixing 4 wt% ethyl viologen diperchlorate [EV(ClO4)2] in acetonitrile and 5 wt% PEO in acetonitrile. The PEO solution was filtered through a 0.45 µm PTFE filter before adding the viologen solution. The solution prepared above has been deposited over the electrode by dropping 3 μl of solution in between both the electrode and spin the device upto 3000 rpm for betterment of thin film. Thin film of EV+PEO was prepared on transparent ITO electrode using spin coating. After spin coating, second ITO electrode was laminated face to face on the spin coated substrate. The extreme part of both the electrodes was painted by silver glue for making connection with external power supply. In the cross-bar device EV in matrix has been sandwiched between two transparent electrodes. For the understanding of ion diffusion and electrode-molecule interface need to fabricate one more device of open face geometry. Device with open geometry were fabricated by drop casting EV in between two gold electrodes separated by few microns where both the electrodes are on the same plane. UV–vis spectroscopy was performed using Cary 60 UV–vis spectrophotometer of Agilent whereas Raman spectroscopy was done using LABRAM HR spectrometer using a 633 nm excitation source. 3. Results and discussion Fig. 1 is the schematic representation of cross-bar device associated with its biasing arrangement and actual photographs of devices under bias and no bias conditions. Initially at zero voltage, the device is in OFF state (say) and is purely transparent. By the application of bias, transparency of the device starts reducing as potential increases. Typical optical properties of chemically reduced viologen show that blue color originates due to the presence of free radical cation (FRC) that is EV•+, which can be obtained by chemically reducing ethyl viologen dication (EV2+)which is transparent. Similar redox process can take place as a result of electric bias instead of chemical redox process. In its ON state (say) at 2 V, device becomes opaque (with a navy blue tint). Conversion of device from its transparent state to pure
Fig. 1. Schematic illustration of cross-bar geometry of the device connecting through the external biasing arrangement. The images below the schematics are the actual photographs captured when seen through an electrochromic device kept on a paper printed with IITI logo without and with bias.
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Fig. 2. (a) Schematic representation of open face geometry device with its external biasing arrangement in its top and cross-sectional views (b) Microscopic optical images of actual device while operating in forward and reverse bias conditions. Voltages shown correspond to the one applied at E1 with respect to E2.
Fig. 4. Bias dependent UV–vis spectroscopy showing change in absorbance. Top inset spectra shows magnify absorption edge shifting with bias and bottom inset shows fractional change in absorbance (%) in ON and OFF states.
these peaks are more sensitive during application of bias. From the graph one can appreciate maximum absorbance of orange color window and there is unaffected blue region (436 nm) in ON state of device. This means that in the ON state device will allow only blue color to pass through it and suppress all the other color giving the navy blue tint appearance of the device in the ON state [25].
Fig. 3. In-situ Raman-spectra recorded in the cross bar geometry of the device under (a) without bias and (b) with bias conditions.
varies on the application of potential across the device. Absorbance of the film depends upon the band-gap of the material so the change in absorbance can be understood by the band-gap modulation (top inset, Fig. 4). During in-situ measurement, potential across the device was increased gradually upto 2 V and absorbance of the device is measured in the range between 300 to 800 nm. Initially in OFF state a hump (large dip) is observed around 550–560 nm. When device is in ON state absorption increases corresponds to all wavelengths with some mountain in the green region. By calculating ratio of absorbance in ON (AON) and OFF (AOFF) state at a given wavelength one can understand which wavelength is more responsive to the applied bias. The inset of Fig. 4 shows the ratio of absorbance in its ON and OFF states, two peaks appear at 398 nm and 559 nm. The physical significance of these peaks is that the wavelength corresponding to
4. Conclusions In summary, in-situ UV–vis and Raman spectroscopies prove to be an excellent tool to characterize electrochromic device in an endeavor to understand the origin of electrochromism. It is revealed here that viologen based electrochromic material changes color due to redox switching between two species EV2+ and EV•+ as a result of electric bias. The viologen ions in plastic (PEO) matrix alone do not make the complete device for operation due to which, the color goes back to transparent state after removal of bias. This is possibly due to atmospheric water, playing role in the redox process. An appropriate addition of oxidizable material (e.g, polythiophene) between viologen 19
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and electrode will support the bias induced oxidation of viologen without relying on ambient moisture when it comes to make the real device. The complete device fabricated then will be able to work in any atmosphere. Appropriate choice of matrix (like PEO) material for the additional layers will allow one to fabricate the device so that the layers make good contacts, necessary for making the device more stable and robust. In-situ Raman spectroscopy of device clearly shows the generation of free radical cation which is responsible for the dark color of the device. UV–vis spectroscopy of device, switching between transparent and navy blue color, shows the change in absorbance (%) as a result of bias change due to conversion of viologen molecule into free radical. It is also revealed that maximum change in absorbance take place for 556 nm as a result of bias which effectively gives the blue appearance of the device. Acknowledgements Authors acknowledge financial support from Department of Science and Technology (DST), Govt. of India (Award number SB/FTP/PS024/2014). Authors are thankful to Prof. R.L. Mccreery (University of Alberta) for gold electrodes, Professor V.D. Vankar (IIT Delhi) for ITO electrodes and Dr. V. Sathe (UGC-DAE CSR, Indore, India) for Raman measurements. Authors thank Dr J. Jayabalan (RRCAT, Indore) for useful discussions. Authors are also thankful to MHRD, Govt. of India for providing fellowships. References [1] V.K. Thakur, G. Ding, J. Ma, P.S. Lee, X. Lu, Hybrid materials and polymer electrolytes for electrochromic device applications, Adv. Mater. 24 (2012) 4071–4096. http://dx.doi.org/10.1002/adma.201200213. [2] M.L. Moser, G. Li, M. Chen, E. Bekyarova, M.E. Itkis, R.C. Haddon, Fast electrochromic device based on single-walled carbon nanotube thin films, Nano Lett. 16 (2016) 5386–5393. http://dx.doi.org/10.1021/acs.nanolett.6b01564. [3] C. Faure, A. Guerfi, M. Dontigny, D. Clément, P. Hovington, U. Posset, K. Zaghib, High cycling stability of electrochromic devices using a metallic counter electrode, Electrochim. Acta 214 (2016) 313–318. http://dx.doi.org/10.1016/j.electacta.2016.08.055. [4] G. Cai, P. Darmawan, M. Cui, J. Chen, X. Wang, A.L.-S. Eh, S. Magdassi, P.S. Lee, Inkjet-printed all solid-state electrochromic devices based on NiO/WO3 nanoparticle complementary electrodes, Nanoscale 8 (2015) 348–357. http://dx.doi.org/ 10.1039/C5NR06995E. [5] G. Cai, X. Wang, M. Cui, P. Darmawan, J. Wang, A.L.-S. Eh, P.S. Lee, Electrochromo-supercapacitor based on direct growth of NiO nanoparticles, Nano Energy 12 (2015) 258–267. http://dx.doi.org/10.1016/j.nanoen.2014.12.031. [6] G. Cai, J. Tu, D. Zhou, L. Li, J. Zhang, X. Wang, C. Gu, Constructed TiO2/NiO core/ shell nanorod array for efficient electrochromic application, J. Phys. Chem. C 118 (2014) 6690–6696. http://dx.doi.org/10.1021/jp500699u. [7] S. Paul, A. Ghosh, A. Chakraborty, L. Petaccia, D. Topwal, D.D. Sarma, S. Oishi, S. Raj, Temperature dependent photoemission spectroscopy on lightly-doped sodium tungsten bronze, Solid State Commun. 152 (2012) 493–496. http:// dx.doi.org/10.1016/j.ssc.2011.12.045.
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