Terpyridine-Based Monolayer Electrochromic Materials - ACS Applied

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Terpyridine-based Monolayer Electrochromic Materials Jesse T.S. Allan, Simone Quaranta, Iraklii I. Ebralidze, Jacquelyn G. Egan, Jade Poisson, Nadia O. Laschuk, Franco Gaspari, E. Bradley Easton, and Olena V. Zenkina ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11848 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Applied Materials & Interfaces

Terpyridine Terpyridine-based Monolayer Electrochromic Materials Jesse T.S. Allan, Simone Quaranta, Iraklii I. Ebralidze, Jacquelyn G. Egan, Jade Poisson, Nadia O. Laschuk, Franco Gaspari, E. Bradley Easton,† and Olena V. Zenkina* Faculty of Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON L1H 7K4, Canada. ABSTRACT: Novel electrochromic (EC) materials were developed and formed by a two-step chemical deposition process. First, a self-assembled monolayer (SAM) of 2,2':6',2''-terpyridin-4'-ylphosphonic acid, L, was deposited on the surface of a nanostructured conductive indium − tin oxide (ITO) screen-printed support by a simple submerging of the support into an aqueous solution of L. Further reaction of SAM with Fe or Ru ions results in the formation of a monolayer of the redoxactive metal complex covalently bound to the ITO support (Fe-L/ITO and Ru-L/ITO, respectively). These novel lightreflective EC materials demonstrate a high colour difference, significant durability, and fast switching speed. The Febased material shows an excellent change of optical density and coloration efficiency. The results of thermal gravimetric analysis suggest high thermal stability of the materials. Indeed, the EC characteristics do not change significantly after heating of Fe-L/ITO at 100°C for one week confirming the excellent stability and high EC reversibility. The proposed fabrication approach that utilizes interparticle porosity of the support and requires as low as a monolayer of EC active molecule benefits from the significant molecular economy when compared with traditional polymer-based EC devices and is significantly less time-consuming than layer-by-layer growth of coordination-based molecular assemblies. KEYWORDS: Electrochromic materials, nanoparticles, screen-printing, self-assembled monolayers, inorganic complexes.

INTRODUCTION “Smart” materials are able to reversibly change their properties in a controlled fashion by applying an external stimulus.1 Electrochromic (EC) materials are a class of “smart” materials that are designed to change their optical properties under an applied voltage. The ability of these materials to switch between two colours, usually to bleach and to revive the colour, has resulted in their widespread applications in smart windows/mirrors, reflective-type displays, battery charge sensors, and electrooptic modulators.2 EC materials are represented mostly by the products that incorporate EC redox molecules3 or transition metal oxides that have a mixed redox state at the metallic centre,4 in conductive polymers.5, 6 The properties of these composites drastically depend on the overall composition and fine distribution of the EC species in the polymer. In contrast to well-developed polymer chemistry,7-9 organic10, 11 and organometallic12-14 molecular assemblies with EC properties have only caught the attention of researchers in the past decade, with published reports being rare. Most of these assemblies are based on metal complexes of bipyridine and its derivatives, due to their excellent stability and direct correlation of the light absorption properties of these complexes to the metal oxidation state. While EC molecular assemblies are much more atom efficient than EC polymers, their fabrication

process on a flat surface requires precise layer-by-layer deposition sequences that can be time-consuming. When designing materials based on EC molecular assemblies, two limitations should be taken into account: the amount of EC molecular layers should be large enough so that colour change is visible by the naked eye; and the nature of the involved molecules, the positioning of layers, and overall thickness should permit bidirectional electrontransfer. We have recently shown that the Fe (II) complex of 2,2':6',2''-terpyridin-4'-ylphosphonic acid (L-Fe-L) that has an intense metal-to-ligand charge transfer (MLCT) absorption band at 561 nm15 when deposited as a selfassembled monolayer (SAM) on glass, yields an insignificant substrate coloration. In contrast, the formation of LFe-L SAM on a porous titanium oxide (TiO2) surface results in a drastic colour change.16 Phosphonate groups allow for efficient deposition of an L monolayer onto hydroxylated surface and might promote electronic communication between the donor orbitals of the L-based complex and the acceptor orbitals of a suitable semiconductor. The idea of utilization of molecular chromophores and molecular catalysts surface-bound to transparent nanoITO films was proposed in the seminal work of Meyer.17 McGehee reported that anchoring of paminotriphenylamine self-assembled monolayer on mes-

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oporous ITO results in a material able to switch from a transparent to a black state.18 A similar concept was recently implemented by Viñuales to create organic viologen-based monolayer on TiO2 porous films.19 Taking this into account, we have hypothesized that the implementation of a conductive nanostructured substrate for metalorganic EC molecules could result in controllable light-reflective devices where high coloration efficiency (CE), and enhanced contrast ratio (significant change of optical density) can be achieved on a single EC molecular layer. This could have the advantage of minimizing manufacturing time and improving atom efficiency of the metalorganic moiety required for the EC device. The size and crystallinity of particles that form nanomaterials deeply impact their optoelectronic properties such as band gap, conduction and valence band edges. On the other hand, reversibility of EC effect on semiconductors and degenerate semiconductors sensitized with organic redox groups or metal complexes depends on the position of the semiconductor conduction band with respect to the redox potential of the organic chromophore20 or the LUMO of the metalorganic complex. Therefore, nanoparticles of different size allow for tuning of the band edges position resulting in the possibility of building a large variety of EC devices by providing the proper combination of semiconductive oxide and EC metalorganic moiety. The use of a traditional spin-coating technology for NPs deposition has certain shape, size, and thickness limitations. While the thickness of the NP layer in the range of 3 μm to 16 μm can be roughly adjusted by varying NPs concentration,17 this often results in destabilization of the suspension and thus requires re-adjusting of the dispersion medium composition to improve the adhesion and avoid cracks. In contrast, screen-printing allows more precise thickness control by step-by-step deposition of NP layers using paste with optimized composition. The screen-printing procedure may use specially designed or commercially available metal oxide nanoparticles, neither requires complex equipment, nor involves any preparative steps conducted under vacuum or highly controlled atmosphere, thus leading to a considerable reduction of the costs of the product.21 Herein, we report on new EC materials prepared on indium− tin oxide (ITO) screen-printed supports by the dipcoating deposition of L followed by corresponding metal (Fe, Ru) complex formation. EXPERIMENTAL Materials. ITO-coated glass slides 15 × 25 × 1.1 mm, Rs = 5-15 Ω/sq were obtained from Delta Technologies. ITO nanoparticles (NPs) 30 nm and < 50 nm particle size (referred as ITO-30 and ITO-50, respectively), poly(methyl methacrylate) (PMMA), trifluromethylsulfonamide lithium salt, propylene carbonate (PC), and acetonitrile (AcN) were purchased from Sigma-Aldrich. The PC and AcN were dried before use. 2,2':6',2''-terpyridin-4'-ylphosphonic acid, L, was synthetized according to previously published

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procedure.22 Solvents were purchased from VWR. Type I deionized water (18 MΩ-cm) was obtained with Milli-Q Direct Millipore tap water purification system. Methods. ITO based screen-printing paste was prepared as follows. ITO NPs were dispersed in anhydrous ethanol by using a PQ-N04 planetary ball milling (by Across International, U.S.) system equipped with 75 mL agate jars and 6 mm agate beads. The milling time and speed were 24 h and 200 rpm, respectively. A mixture of 2-[2-(2methoxyethoxy)-ethoxy] acetic acid (EMD Millipore, U.S.) surface treating agent and Disperbyk-111 (a polyester co-polymer functionalized with acid groups kindly provided by “BYK Additives and Instruments”, Germany) in a ratio 4.7:1 was used as dispersing agent during the milling. The dispersing agent had been previously diluted with anhydrous ethanol. The mass ratio of dispersing agent to ITO NPs has been set to 1:3.5 (5.0 wt% Dispebyk 111 and 23.3 wt% based upon ITO solid). Disperbyk 111 amount was kept relatively low to avoid an excessive viscosity reduction during the printing step. Further steric stabilization of the dispersion was achieved by adding a mixture of high boiling point solvents (α-terpineol, 2-butoxyethanol, 2-(2-Butoxyethoxy)ethyl acetate, all from Sigma Aldrich) along with a proper amount of Butvar 98 Polyvynyl butyral (PVB, molecular weight 40000-70000 g/mol determined by Size Exclusion Chromatography, Sigma Aldrich). PVB had been pre-solubilized in anhydrous ethanol (48 h at 50°C) to obtain a 15 wt% binder solution. The stabilized ITO dispersion showed negligible settling after ball milling and was evaporated under reduced pressure for 2 h at 40°C to remove the excess of ethanol and obtain a printable paste. Screen-printing of the ITO based paste on ITO glass was performed with a 90 T polyester mesh screen (by Mismatic, Italy). The printing procedure was repeated 3 times to achieve a suitable thickness of the ITO support. A drying step of 5 min at 120°C was performed between each layer printing. To anneal the screen-printed film, it was placed into the Fisher Scientific Programmable Muffle Furnace and the temperature was increased (5°C/min) to 500°C and held at that temperature for 1 h, after which the temperature was increased (5°C/min) to 600°C, and held at 600°C for 1 h. Specific surface area (SSA) of ITO NPs and ITO screenprinted annealed films has been determined by N2 adsorption with Quantachrome Nova 1200e surface analyzer. Screen-printed and annealed ITO films were functionalized by immersing the plate in an aqueous solution of L (1 mM). The plates were left in the solution for 2 h for ITO-30 and 12 h for ITO-50; then removed, washed with isopropanol, and dried by an N2 stream. The Fe-L/ITO complexes were formed by adding known volumes of 10 ppm Fe(ClO4)2 solution onto the L/ITO plates as previously reported.16 The Ru-L complex was formed by immersing L/ITO plates in a solution of RuCl3 (1 mM) in ethanol and heating to 80°C for 2 h. The plates were then removed, washed with isopropanol, and dried.


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Thermogravimetric analysis (TGA) was performed using a TA Instruments Q600 SDT instrument. Samples were left on a vacuum line for 12 h before the TGA measurements. Measurements were performed under a flowing air stream using a heating ramp of 5°C/min. Roughness and profilometry measurements were performed using Profilm3D Surface Profiler by Filmetrics (Filmetrics Application Lab – Rochester, Fairport, NY, USA) equipped with 20x Nikon CF IC Epi Plan objective. XPS measurements were performed using a Thermo Scientific K-Alpha AngleResolved X-ray photoelectron spectrometer equipped with monochromated Al Kα (1486.7 eV) X-ray source and 180° double focusing hemispherical analyzer with 128 channel detector with effective charge compensation. A Shirley fit algorithm was used for background subtraction and a Powell peak-fitting algorithm for data analysis. The iron content in the Fe-L/ITO-30 film was determined using a Vista-MPX CCD inductively coupled plasma – optimal emission spectroscopy (ICP-OES) system. Cyclic Voltammetry (CV) was performed on the functionalized films using a Plate Material Evaluating Cell from BioLogic. The lower section of the cell was changed to an inhouse design glass section to directly observe colour changes during the CV measurements. A Solartron 1287 potentiostat was used to measure the CVs. The reference electrode employed was an Ag/AgNO3 (0.01 M) in 0.1 M tetrabutylammonium hexafluorophosphate (TBAHFP)/MeCN (anhydrous) solution. The (TBAHFP)/MeCN also served as the electrolyte. All CV’s are normalized to Fc/Fc+. The samples of interest were held in place by the Evaluating Cell, which also served as the working electrode (WE). A platinum wire served as the counter electrode (CE). The solution was purged with N2 gas for 600 seconds to remove any traces of oxygen. The CVs were swept from -0.355 V to 1.14 V vs. Fc/Fc+. For the first set of measurements, CVs were acquired from variable sweep rates ranging from 200 mV/s down to 1 mV/s. The peak anodic (ip,a) and cathodic currents (ip,c) were examined as a function of sweep rate to investigate the electrochemical process taking place. The second set of measurements were performed to study film stability,

which involved performing 400 CV cycles on the film using a sweep rate of 50 mV/s. The electrolyte used for the electrochromic devices was synthesized following the procedure by van der Boom.12 Polymethylmethacrylate (700 mg), trifluoromethylsulfonamide lithium salt (300 mg), anhydrous acetonitrile (8.9 mL), and anhydrous propylene carbonate (1.7 mL) were added to an oven-dried glass vessel under a nitrogen atmosphere. The weight percent of each component was maintained at 70:20:7:3 wt%, respectively. The electrolyte was stirred vigorously overnight, at which point it was a viscous liquid. Approximately 0.2 mL of this electrolyte was drop casted onto a functionalized ITO screen-printed film to ensure full coverage of the modified section of the ITO/glass slide. The plate was placed into an oven at 60°C for 10 minutes before a second unmodified ITO/glass slide was placed onto the electrolyte to create an electrochromic device. A schematic representation of the design can be found in Fig. 5A. Clear tape was wrapped around the device to ensure the device was held together tightly. A Perkin Elmer Lambda 750S Uv-vis spectrophotometer equipped with a 60 mm integrating sphere was used to collect Uv-vis spectra. Spectralon (Labsphere, U.S.) was used as a 100% reflectance standard. The sample was placed into the solid state clamp in such a way that the modified surface would interact with the UV-light first. A Pine Wavedriver 10 potentiostat was used to change the potential from -1 V to 3 V during the cycling experiment. First, a scan of a modified sample was performed between 320 nm to 800 nm holding the potential at -1 V. Afterwards, the potential was held at 3 V, with the scan repeated. Finally, the wavelength was held at the value at which the absorbance change was largest for a given sample (561 nm for Fe and 420 nm for Ru). The cycling began first by varying the cycle time from 60 s at a given potential down to 7.5 s. Afterwards, the cycling time was held at 60 s and 1500 cycles were performed (48 h of continuous switching).

Figure 1. Typical surface profiles and topographical cross-sections of the ITO-30 film prepared by ITO nanoparticles deposition and sintering on ITO glass substrates: A) a top profile, B) a stage profile showing the thickness of the film of 3.6 ± 0.2 μm.



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Figure 2. Durability of A) L/ITO-30 and EC materials using TBAHFP/MeCN electrolyte: B) Fe-L/ITO-30, C) Ru-L/ITO-30.

RESULTS AND DISCUSSION ITO NPs were screen-printed as 3 layers one over the other and stepwise annealed at 500°C to burn out the organic components and then at 600°C to sinter the ITO NPs. The resulting ITO-30 films are 3.6 ± 0.2 μm thick (Fig. 1), ITO50 films are 3.8 ± 0.2 μm thick. The specific surface area (SSA) of the initial ITO-30 NPs was determined to be 6.09 ± 0.2 m2/g and was found to increase slightly to 10.2 ± 0.5 m2/g when annealed to form the film (Fig. S1) due to the balance between particles comminution (ball milling) and necking (sintering). SSA of ITO-50 does not change significantly before (31.9 ± 0.9 m2/g) and after (30 ± 1 m2/g) annealing (Fig. S2). The sheet resistance of ITO-30 and ITO-50 films is 250 Ω/sq and 125 Ω/sq, respectively. For comparison, the sheet resistance of a TiO2 screen-printed film obtained from commercially available P90 (Evonic) TiO2 nanoparticles16 is 2.3 MΩ/sq, which is four order of magnitude larger. The amount of L molecules grafted onto the ITO NPs surface was determined indirectly from the Uv-vis spectroscopy analyses of the L solutions before and after deposition (Fig. S3A, S3C). While the monolayer grafting density of L on ITO-30 is 1.5 molecules per 1 nm2 (See SI) which is close to a single crystal packing density,15 a significant amount of ITO-50 surface area is L-inaccessible. ITO-30 adsorbs 24.7 μmol of L per gram, while one gram of ITO50 adsorbs 26.2 μmol of L. In addition, TGA of L-

functionalized ITO-30 NPs shows a weight loss of 0.78% (Fig. S3B), while the weight loss associated with L decomposition on ITO-50 is 0.87% (Fig. S3D); both in a good agreement with Uv-vis data and confirming the formation of a monolayer of L on ITO NPs. According to TGA results, the combustion of L takes place at the range of 200450°C and has 2 steps attributed to the combustion of the molecule.23,24 Although the samples of L/ITO-30 and L/ITO-50 were treated similarly before the TGA analysis, including drying on a vacuum line for 12 h, the TGA of L/ITO-50 demonstrates significant weight loss with the derivative centered at 95°C, which is associated with water loss (Fig. S3D). This is in a good agreement with an assumption that the surface of L/ITO-50 has Linaccessible but water-reachable areas. The L anchored to the ITO film (L/ITO) shows an increase in capacitance with increasing scan rate while no obvious redox process taking place. It shows very good stability and durability with no loss in capacitance for over 400 cycles in TBAHFP/MeCN (Fig. 2). Electrochemical studies of iron, and ruthenium complexes of L on ITO30 film (Fe-L/ITO-30 and Ru-L/ITO-30) show good reversibility at all scan rates from 1 mV/s to 200 mV/s. The E1/2 determined at 50 mV/s for the Fe-L/ITO and RuL/ITO are equal to 0.49 V and 0.54 V (vs. Fc/Fc+) respectively, which is consistent with previously reported values for similar complexes in solution25, 26 and on a solid support.27

Figure 3. Typical XPS spectra of the films: A) a survey spectrum and B) P2p region of L/ITO-30, C) Fe 2p region of Fe-L/ITO-30, and D) Ru 3d region of Ru-L/ITO-30.


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The durability of both Fe-L/ITO and Ru-L/ITO materials is very good, reduction in peak current just insignificantly decreases after 400 cycles compared to the 50 cycle durability test (Fig. 2). Both the peak current (ip) vs. sweep rate (v) plot and the limiting current vs the square root of the scan rate (v1/2) plot of the Fe-L/ITO-30 (Fig. S4) show two separate linear regions, one at the high sweep rate, and one at lower sweep rates. The limiting current of RuL/ITO-30 is proportional to v, but not to v1/2 (Fig. S5). This indicates that the redox processes in both systems are not simply diffusion controlled.28, 29 A typical XPS survey spectrum of L/ITO-30 (Fig. 3A) contains peaks of the ITO support (In, Sn, and O), and L (C, N). The low intensity P 2p peak of L that is not distinguished on the survey scan is centered at 133.5 eV (Fig. 3B). Fe 2p region of Fe-L/ITO-30 spectrum contains two characteristic Fe2+ peaks:30 Fe 2p3/2 at 709.4 eV and Fe 2p1/2 at 722.5 eV for the film left at the potential under 0.3 V. Two characteristic Fe3+ peaks:30 Fe 2p3/2 at 711.9 eV and Fe 2p1/2 at 725.0 eV are observed on the film left at the potential above 0.9 V (Fig. 3C). Ru 3d5/2 peak is observed at 281.4 eV when analyzing Ru-L/ITO-30 film left, after several CV cycles, at the potential under 0.3 V. This value is typical for Ru2+ 3d5/2 peaks in similar complexes.31 Ru 3d5/2 peak typical for Ru3+ is observed at 282.0 eV when analyzing the film left after several CV cycles at the potential above 0.9 V. While the Ru 3d3/2 peaks are under the strong C1s signal, the presence of Ru3+ species in the sample left at the potential above 0.9 V can be confirmed by the formation of a shoulder on the C1s spectrum (Fig. 3D). Peak area normalization for Fe-L/ITO-30 using relative XPS sensitivity factors as determined by Wagner32 gives an N:C:P:Fe ratio equal to 2.82:15.46:0.94:0.33. This is consistent with ICP measurements showing that ITO-30 adsorbs 6.68 μmol of iron per gram; and UV-vis measurements that give 24.7 μmol of L per gram thus resulting in L:Fe ratio as 1:0.27. The element composition analysis of Ru-L/ITO-30 results in N:C:P:Ru ratio of 3.0:15.70:0.92:0.31. The experimental N:C:P ratios are close to the expected 3:15:1 stoichiometric ratio for L. The ratios of Fe and Ru are in a good agreement with the fact that both metals form complexes containing two L molecules per one metal ion in a solution. Due to close packing and steric hindrance, not all L molecules anchored to the surface are involved in the complex formation. The oxidation-reduction processes on the Fe-L/ITO and Ru-L/ITO materials are accompanied by the reversible colour change. During oxidation of Fe2+ to Fe3+ the intensity of MLCT is reduced, leading to colour bleaching. Electrochemical reduction results in colour recuperation (Fig. 4, video 1, 2). The Fe-L/ITO is pink in the reduced state, changing to a yellowish in the oxidized state. On the contrary, Ru3+ has higher than Ru2+ absorption intensity at 420 nm (violet) resulting in visual bleaching of the Ru-L/ITO in oxidized state. The Ru-L/ITO is a peach colour in the reduced state, changing to a duller yellow in

the oxidized state. The ITO support layers can be screenprinted in close proximity, separately functionalized to form Fe-L/ITO and Ru-L/ITO, and then assembled into one device (Fig. 4 middle).

Figure 4. Optical images of Fe-L/ITO-30 (top), Ru-L/ITO-30 (bottom), and both Fe-L/ITO-30 and Ru-L/ITO-30 (middle) films assembled into Plate Material Evaluating Cell during electrochemical cycling with TBAHFP/MeCN electrolyte.

While in this particular case a simultaneous colour switching of both films occurs during the electrochemical cycling due to the close values of E1/2 (vide supra), this opens a door for the development of dual29 or even multicolored33, 34 EC devices. The feasibility of Fe-L/ITO and Ru-L/ITO EC devices was demonstrated by covering functionalized screen-printed ﬁlms with a gel-electrolyte and a counter ITO electrode (Fig. 5) to form a solid-state setup as previously reported.12 The change in optical density (∆OD) equal to the change of absorbance, is a function of the pulse width. For the Fe-L/ITO EC device, the largest ∆OD is observed with 60 s holds while remaining quite stable down to 15 s. There is a noticeable drop in the overall ∆OD with 7.5 s holds. The Ru-L/ITO demonstrates highest ∆OD with 60 s holds, which gradually decreases with shorter holds. The largest ∆OD for Ru2+-L/ITO and Ru3+-L/ITO was observed at 420 nm (Fig. S6). Cycling was possible, but the change in absorbance was much smaller in magnitude than that of the Fe-L/ITO materials. The long-term stability of these materials was tested by running 1500 chronoabsorptometric switching cycles (Fig. 6) within 48 h. Negligible optical deterioration was observed for screen-printed ITO-30 and ITO-50 devices during the test confirming long-term redox switching stability. The minimum absorbance does tend to increase from 0.33 A.u. to 0.38 A.u. for Fe-L/ITO-30, from 0.31 A.u. to 0.44 A.u. for Fe-L/ITO-50, and from 0.66 A.u. to 0.69 A.u. for Ru-L/ITO-30.



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counter ITO darkens, it would increase the overall absorbance background.

Figure 6. Stability of solid-state setup: A) Fe-L/ITO-30, B) Fe-L/ITO-50, and C) Ru-L/ITO-30. These experiments were carried at λ = 561 nm for Fe-L/ITO and λ = 420 nm for RuL/ITO.

Figure 5. Solid-state setup: A) schematic representation, and chronoabsorptometry of B) Fe-L/ITO-30, C) Fe-L/ITO-50, D) Ru-L/ITO-30, and E) Ru-L/ITO-50 at different switching times. These experiments were carried at λ = 561 nm for FeL/ITO and λ = 420 nm for Ru-L/ITO.

When the devices were disassembled after long-term chronoabsorptometry, it was found the counter plate (unmodified ITO plate) had darkened. The darkening of unmodified ITO plate might be due to partial degradation of the electrolyte during chrono-absorptometric cycling that leads to changes of lithium ions concentration at the counter-electrode/electrolyte interface, lithium intercalation into the counter electrode and partial indium reduction.35 This would explain why the minimum absorbance begins to increase during the long term testing; as the

Thermal stability was examined by placing the Fe-L/ITO30 film into an oven at 100°C for 170 hours. When assembled, the device shows just a minor decrease of ∆OD (Fig. S7A). For the fresh Fe-L/ITO device, the change between the metal oxidation states shows an almost vertical increase/decrease in absorbance (Figs. 6A, S7B). However, after 1000 cycles, the time required to reach the maximum and minimum absorption does appear to increase, with a gradual curve to maximum (Figs. 6A, S7C). The reduction in the switching speed, as well as the darkening of the counter ITO plate, would indicate that the electrolyte aging decreased the electron transfer and affected the kinetics of the device. To confirm this hypothesis, the electrolyte was carefully removed by immersing the device into AcN solution to dissolve the electrolyte. The modified plate was washed and the fresh electrolyte was reintroduced, as well as a new counter-ITO plate. Figure S7D shows that replacing both parts allows for the rapid return of the switching profile. To reduce the residual current and thus a stress placed on the system, preventing the decomposition of the electrolyte, the system can be switched to 3 V for 5 s, and then hold at 2 V for the rest of cycle (see the video of Fe-L/ITO-50 after 2000 cycles in SI). Storage of the devices at ambient conditions for 5 months does not affect their properties. Our Fe-L/ITO-30 and Fe-L/ITO-50 materials show the ∆OD of 44% and 30% for 60 second holds, respectively.


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These values are very impressive for monolayer- based materials prepared by one deposition cycle. It should be mentioned that despite the fact that ITO-50 formally has higher surface area, the area accessible by L is just slightly (6%) higher in that of ITO-30. Moreover, iron forms a LFe-L complex that require sterically accessible L molecules bound to ITO nanopowder surface (vide supra, Scheme S1). The decrease in ∆OD value of Fe-L/ITO-50 compared to Fe-L/ITO-30, suggests that less L molecules bound to ITO-50 can form the L-Fe-L complex. Furthermore, different surface chemistry (as verified by the different adsorption kinetics of L on the two different ITO screen-printed films) stemming from different point defects and distribution of OH groups on the surface may play a role in the formation of the iron complex on the ITO surface. Ru-L/ITO-30 and Ru-L/ITO-50 demonstrate the ∆OD of 1% and 4%, respectively. The larger ∆OD value of Ru-L/ITO-50 may be associated with the formation of L-Ru complex, along with L-Ru-L (Scheme S1). Having relatively low ∆OD values, Ru-based systems nevertheless confirm the generality of the approach, the possibility of producing various materials using our new strategy that utilizes the formation of monolayers of electrochromic molecules on conductive supports with an enhanced surface area. To precisely map the colors associated with EC materials, colorimetric analysis was performed according to L*a*b* standard10, 36 resulting in L*= 63.6, a*= 31.6, b*= 5.7 for Fe2+-L/ITO-30 and L*= 98.7, a*= -7.3, b*= 23.1 for Fe3+L/ITO-30, respectively. During oxidation process the lightness (L*) of the film increases, the red (positive a*) colour changes to greenish-gray (negative a*), and the yellow component (positive b*) increases. Similarly, L*= 36.0, a*= 24.6, b*= 18.1 for Fe2+-L/ITO-50 changes upon oxidation to L*= 66.4, a*= 4.3, b*= 34.2 for Fe3+-L/ITO-50. Colorimetric analysis of Ru-L/ITO-30 material results in L*= 48.6, a*= 11.2, b*= 36.9 for Ru2+-L/ITO-30 and L*= 75.5, a*= -1.76, b*= 36.7 for Ru3+-L/ITO-30, respectively. Similarly, L*= 48.2, a*= 5.6, b*= 12.5 for Ru2+-L/ITO-50 changes upon oxidation to L*= 54.2, a*= -3.3, b*= 22.3 for Ru3+L/ITO-50. Upon oxidation, the lightness of the material increases, the red component turns to almost gray, and the yellow component remains unchanged. The colour difference, ΔE was calculated as differences in all three colour coordinates of EC material at two states:37 Δ = Δ∗ + Δ∗ + Δ ∗

(1)

Diffuse reﬂectance Uv–vis spectra were recorded and the change of optical density (absorbance) upon a simultaneous change of applied external bias across the film was monitored as a function of time (Figs. 5, 6). The coloration efficiency, η was defined as the relationship between the change of optical density (∆OD) or absorbance change (ΔA) of the film between its colored (Ac) and bleached (Ab) states at a certain wavelength λ, vs. corresponding charge density (Qd), eqn. (2),38, 39

=

∆

=

/

=

∆

The colour difference of the Ru-L/ITO is moderate, however, the optical density and coloration efficiency of the Ru-L/ITO was found to be low. Nevertheless, this material has the potential to be used as a templating layer for alternating EC layers that opens the door for precise color tuning.14 In contrast, Fe-L/ITO shows high colour difference and high optical density at 561 nm that together with moderate charge density result in significant coloration efficiency. Both Ru-L/ITO and Fe-L/ITO materials demonstrate very high stability. The main EC properties of the materials are summarized in Table 1. Table 1. EC properties of the materials Material

∆E

∆OD

Qd, mC/cm2device

η, cm2 C−1

Fe-L/ITO-30

55

0.44

2.96

148.6

Fe-L/ITO-50

40

0.30

2.89

101.4

Ru-L/ITO-30

30

0.01

2.69

4.5

Ru-L/ITO-50

15

0.04

3.23

11.1

CONCLUSIONS In this work, we fabricated thick (3.6 μm and 3.8 μm) nanostructured supports with a moderate (10.2 m2/g and 30.0 m2/g) surface area by screen-printing and sintering of two types of ITO nanoparticles on ITO/glass substrate. The immersion of the support into an aqueous solution of 2,2':6',2''-terpyridin-4'-ylphosphonic acid, L, results in the rapid formation of self-assembled monolayer of L covalently attached to the scaffold material. Further treatment with Fe and Ru metal salts results in the formation of corresponding metal complexes anchored to the surface. Both systems were visually found to change colour during electrochemical cycling. The developed EC materials demonstrate significant colour difference at two states observable with the naked eye. The optical density can be changed as a function of the pulse width (5-60 s). A near maximum (>90%) can be achieved with a pulse width of 7.5 s for the Fe-L/ITO and with a pulse width of 30 s for Ru-L/ITO-30. The high coloration efficiency of the FeL/ITO is comparable with 3D Fe-based metallosupramolecular polymers,39 while the stability is on par to inorganic WO3 films.40 This study reports novel materials for electrochromic devices predicated on monolayers of metal-organic molecules chemisorbed on nanostructured oxide surfaces. Consequently, such investigation constitute a proof of concept for further research in the field. Our current and future work is focused on the development of other supports with enhanced properties (surface area, pore sise, conductivity, and transparency), EC molecules with other covalent linkers and their films with further optimized characteristics.

ASSOCIATED CONTENT

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The Supporting Information that includes BET, Uv-vis, electrochemical, and spectroelectrochemical data is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Web: http://faculty.uoit.ca/zenkina/ † [email protected]

ORCID Olena V. Zenkina: 00000000-00020002-23032303-4620 E. Bradley Easton: 00000000-00030003-14931493-0500 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through the Discovery Grants program (RGPIN-2016-05823, RGPIN003652-2015) and University of Ontario Institute of Technology (UOIT). The authors acknowledge equipment support from the Canada Foundation for Innovation. JA acknowledges support from the UOIT President’s Postdoctoral Fellowship for Indigenous Students. NL acknowledges NSERC Canada Graduate Scholarship - Master’s Program. The authors acknowledge Rebecca Andrew, Filmetrics Application Lab – Rochester, Fairport, NY, USA for surface profile measurements on Profilm3D Surface Profiler by Filmetrics.

ABBREVIATIONS AcN, acetonitrile; EC, electrochromic; L, 2,2':6',2''-terpyridin4'-ylphosphonic acid; ITO, indium − tin oxide; MLCT, metalto-ligand charge transfer; SAM, self-assembled monolayer; CE, coloration efficiency; SSA, specific surface area; TGA, thermal gravimetric analysis; L/ITO, The L anchored to the ITO film; Fe-L/ITO and Ru-L/ITO corresponding metal complexes of L anchored to the ITO film; ∆E, The colour difference; η, the coloration efficiency; Qd, the electrode area; ∆OD, the change of optical density; λmax, specific wavelength.

REFERENCES (1) Borodinov, N.; Soliani, A. P.; Galabura, Y.; Zdyrko, B.; Tysinger, C.; Novak, S.; Du, Q. Y.; Huang, Y. Z.; Singh, V.; Han, Z. H.; Hu, J. J.; Kimerling, L.; Agarwal, A. M.; Richardson, K.; Luzinov, I., Gradient Polymer Nanofoams for Encrypted Recording of Chemical Events. Acs Nano 2016, 10 (12), 1071610725. (2) Granqvist, C. G., Electrochromics for Smart Windows: Oxide-based Thin Films and Devices. Thin Solid Films 2014, 564, 1-38. (3) Moon, H. C.; Lodge, T. P.; Frisbie, C. D., Solution Processable, Electrochromic Ion Gels for Sub-1 V, Flexible Displays on Plastic. Chem. Mater. 2015, 27 (4), 1420-1425. (4) Barile, C. J.; Slotcavage, D. J.; McGehee, M. D., Polymer–Nanoparticle Electrochromic Materials that Selectively

Page 8 of 10

Modulate Visible and Near-Infrared Light. Chem. Mater. 2016, 28 (5), 1439-1445. (5) Gaupp, C. L.; Welsh, D. M.; Rauh, R. D.; Reynolds, J. R., Composite Coloration Efficiency Measurements of Electrochromic Polymers Based on 3,4-Alkylenedioxythiophenes. Chem. Mater. 2002, 14 (9), 3964-3970. (6) Teran, N. B.; Reynolds, J. R., Discrete Donor-Acceptor Conjugated Systems in Neutral and Oxidized States: Implications toward Molecular Design for High Contrast Electrochromics. Chem. Mater. 2017, 29 (3), 1290-1301. (7) Tahtali, G.; Has, Z.; Doyranli, C.; Varlikli, C.; Koyuncu, S., Solution Processable Neutral State Colourless Electrochromic Devices: Effect of the Layer Thickness on the Electrochromic Performance. J. Mater. Chem. C. 2016, 4 (42), 10090-10094. (8) Lombard, J.; Jose, D. A.; Castillo, C. E.; Pansu, R.; Chauvin, J.; Deronzier, A.; Collomb, M. N., Alternated Bimetallic [Ru-M] (M = Fe2+, Zn2+) Coordination Polymers Based on [Ru(bpy)3]2+ Units Connected to bis-Terpyridine Ligands: Synthesis, Electrochemistry and Photophysics in Solution or in Thin Film on Electrodes. J. Mater. Chem. C 2014, 2 (46), 98249835. (9) Walesa-Chorab, M.; Skene, W. G., Visible-to-NIR Electrochromic Device Prepared from a Thermally Polymerizable Electroactive Organic Monomer. ACS Appl. Mater. Interfaces 2017, 9 (25), 21524-21531. (10) Tremblay, M. H.; Gelle, A.; Skene, W. G., Ambipolar Azomethines as Potential Cathodic Color Switching Materials. New J. Chem. 2017, 41 (6), 2287-2295. (11) Kim, S. H.; Shim, N.; Lee, H.; Moon, B., Synthesis of a Perylenediimide-viologen Dyad (PDI-2V) and its Electrochromism in a Layer-by-layer Self-assembled Multilayer Film with PEDOT:PSS. J. Mater. Chem. 2012, 22 (27), 13558-13563. (12) Shankar, S.; Lahav, M.; van der Boom, M. E., Coordination-Based Molecular Assemblies as Electrochromic Materials: Ultra-High Switching Stability and Coloration Efficiencies. J Am Chem Soc 2015, 137 (12), 4050-4053. (13) Farran, R.; Jouvenot, D.; Gennaro, B.; Loiseau, F.; Chauvin, J.; Deronzier, A., Photoinduced Charge Separation within Metallo-supramolecular Wires Built around a [Ru(bpy)3]2+-Bisterpyridine Linear Entity. ACS Appl. Mater. Interfaces 2016, 8 (25), 16136-16146. (14) Elool Dov, N.; Shankar, S.; Cohen, D.; Bendikov, T.; Rechav, K.; Shimon, L.; Lahav, M.; van der Boom, M. E., Electrochromic Metallo-Organic Nanoscale Films: Fabrication, Color Range and Devices. J Am Chem Soc 2017, 139 (33), 1147111481. (15) Laschuk, N. O.; Ebralidze, I. I.; Spasyuk, D.; Zenkina, O. V., Multi-Readout Logic Gate for the Selective Detection of Metal Ions at the Parts Per Billion Level. Eur. J. Inorg. Chem. 2016, (22), 3530-3535. (16) Laschuk, N. O.; Ebralidze, I. I.; Quaranta, S.; Kerr, S. T. W.; Egan, J. G.; Gillis, S.; Gaspari, F.; Latini, A.; Zenkina, O. V., Rational design of a material for rapid colorimetric Fe2+ detection. Mater. Design 2016, 107, 18-25. (17) Hoertz, P. G.; Chen, Z. F.; Kent, C. A.; Meyer, T. J., Application of High Surface Area Tin-Doped Indium Oxide Nanoparticle Films as Transparent Conducting Electrodes. Inorg. Chem. 2010, 49 (18), 8179-8181. (18) Nguyen, W. H.; Barile, C. J.; McGehee, M. D., Small Molecule Anchored to Mesoporous ITO for High-Contrast Black Electrochromics. J. Phys. Chem. C 2016, 120 (46), 26336-26341. (19) Alesanco, Y.; Palenzuela, J.; Tena-Zaera, R.; Cabañero, G.; Grande, H.; Herbig, B.; Schmitt, A.; Schott, M.; Posset, U.; Guerfi, A.; Dontigny, M.; Zaghib, K.; Viñuales, A., Plastic Electrochromic Devices Based on Viologen-modified TiO2 Films


Page 9 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Prepared at Low Temperature. Sol. Energy Mater. Sol. Cells 2016, 157, 624-635. (20) Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A., Electrochromic Performance of Viologen-modified Periodic Mesoporous Nanocrystalline Anatase Electrodes. Nano Lett 2004, 4 (7), 1231-1235. (21) Bonomo, M.; Naponiello, G.; Venditti, I.; Zardetto, V.; Di Carlo, A.; Dini, D., Electrochemical and Photoelectrochemical Properties of Screen-Printed Nickel Oxide Thin Films Obtained from Precursor Pastes with Different Compositions. J. Electrochem. Soc. 2017, 164 (2), H137-H147. (22) Cargill Thompson, A. M. W., The Synthesis of 2,2′:6′,2″-terpyridine Ligands — Versatile Building Blocks for Supramolecular Chemistry. Coord. Chem. Rev. 1997, 160, 1-52. (23) Daou, T. J.; Pourroy, G.; Greneche, J. M.; Bertin, A.; Felder-Flesch, D.; Begin-Colin, S., Water Soluble Dendronized Iron Oxide Nanoparticles. Dalton Trans. 2009, (23), 4442-4449. (24) Basly, B.; Felder-Flesch, D.; Perriat, P.; Billotey, C.; Taleb, J.; Pourroy, G.; Begin-Colin, S., Dendronized Iron Oxide Nanoparticles as Contrast Agents for MRI. Chem. Commun. 2010, 46 (6). (25) Et Taouil, A.; Husson, J.; Guyard, L., Synthesis and Characterization of Electrochromic [Ru(terpy)2 selenophene]based Polymer Film. J. Electroanal. Chem. 2014, 728, 81-85. (26) Castor, K. J.; Metera, K. L.; Tefashe, U. M.; Serpell, C. J.; Mauzeroll, J.; Sleiman, H. F., Cyclometalated Iridium(III) Imidazole Phenanthroline Complexes as Luminescent and Electrochemiluminescent G-Quadruplex DNA Binders. Inorg. Chem. 2015, 54 (14), 6958-6967. (27) Hu, K.; Severin, H. A.; Koivisto, B. D.; Robson, K. C. D.; Schott, E.; Arratia-Perez, R.; Meyer, G. J.; Berlinguette, C. P., Direct Spectroscopic Evidence for Constituent Heteroatoms Enhancing Charge Recombination at a TiO2-Ruthenium Dye Interface. J. Phys. Chem. C 2014, 118 (30), 17079-17089. (28) Strycharz-Glaven, S. M.; Tender, L. M., Study of the Mechanism of Catalytic Activity of G. Sulfurreducens Biofilm Anodes during Biofilm Growth. ChemSusChem 2012, 5 (6), 11061118. (29) Takada, K.; Sakamoto, R.; Yi, S.-T.; Katagiri, S.; Kambe, T.; Nishihara, H., Electrochromic Bis(terpyridine)metal Complex Nanosheets. J Am Chem Soc 2015, 137 (14), 4681-4689. (30) Ahmadi, S.; Ebralidze, I. I.; She, Z.; Kraatz, H.-B., Electrochemical Studies of tau Protein-iron Interactions—

Potential Implications for Alzheimer’s Disease. Electrochim. Acta 2017, 236, 384-393. (31) Agnès, C.; Arnault, J. C.; Omnès, F.; Jousselme, B.; Billon, M.; Bidan, G.; Mailley, P., XPS Study of Ruthenium trisbipyridine Electrografted from Diazonium Salt Derivative on Microcrystalline Boron Doped Diamond. Phys. Chem. Chem. Phys. 2009, 11 (48), 11647-11654. (32) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H., Empirical Atomic Sensitivity Factors for Quantitative Analysis by Electron Spectroscopy for Chemical Analysis. Surf. Interface Anal. 1981, 3 (5), 211-225. (33) Bulloch, R. H.; Kerszulis, J. A.; Dyer, A. L.; Reynolds, J. R., An Electrochromic Painter's Palette: Color Mixing via Solution Co-Processing. ACS Appl. Mater. Interfaces 2015, 7 (3), 1406-1412. (34) Chen, B. H.; Kao, S. Y.; Hu, C. W.; Higuchi, M.; Ho, K. C.; Liao, Y. C., Printed Multicolor High-Contrast Electrochromic Devices. ACS Appl. Mater. Interfaces 2015, 7 (45), 25069-25076. (35) Bressers, P. M. M. C.; Meulenkamp, E. A., The Electrochromic Behavior of Indium Tin Oxide in Propylene Carbonate Solutions. J. Electrochem. Soc. 1998, 145 (7), 2225-2231. (36) Jordão, N.; Cruz, H.; Branco, A.; Pinheiro, C.; Pina, F.; Branco, L. C., Switchable Electrochromic Devices Based on Disubstituted Bipyridinium Derivatives. RSC Adv. 2015, 5 (35), 27867-27873. (37) Puodziukynaite, E.; Oberst, J. L.; Dyer, A. L.; Reynolds, J. R., Establishing Dual Electrogenerated Chemiluminescence and Multicolor Electrochromism in Functional Ionic TransitionMetal Complexes. J Am Chem Soc 2012, 134 (2), 968-978. (38) Li, G. L.; Zhang, S. H.; Guo, C. S.; Liu, S. Q., Absorption and Electrochromic Modulation of Near-infrared Light: Realized by Tungsten Suboxide. Nanoscale 2016, 8 (18), 9861-9868. (39) Hu, C.-W.; Sato, T.; Zhang, J.; Moriyama, S.; Higuchi, M., Three-Dimensional Fe(II)-based Metallo-Supramolecular Polymers with Electrochromic Properties of Quick Switching, Large Contrast, and High Coloration Efficiency. ACS Appl. Mater. Interfaces 2014, 6 (12), 9118-9125. (40) Xie, Z.; Gao, L. N.; Liang, B.; Wang, X. F.; Chen, G.; Liu, Z.; Chao, J. F.; Chen, D.; Shen, G. Z., Fast Fabrication of a WO3 . 2H2O Thin Film with Improved Electrochromic Properties. J Mater Chem 2012, 22 (37), 19904-19910.



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Terpyridine-Based Monolayer Electrochromic Materials - ACS Applied

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