Monolithic, crystalline and highly oriented coordination network compound (CNC) Prussian blue (PB) thin films have been deposited though different routes on conductive substrates. Characterization of the monolithic thin films reveals a long-term stability, even after many redox cycles the crystallinity as well as the high orientation remain intact during the electrochromic switching process.
© 2015 Optical Society of America
With regard to the fabrication of highly functional coatings, the use of layer-by-layer or liquid phase epitaxy (LPE) methods have recently attracted considerable attention . In particular for metal organic frameworks (MOFs) [2,3], this approach has led to a number of materials, referred to as surface-anchored MOFs (SURMOFs), to be successfully fabricated . Whereas for the powder form of the porous MOFs, applications in gas storage and separation were – and still are – the most important ones, in case of MOF thin films more advanced applications can be foreseen based on their mechanical , dielectric , and optical  as well as their magnetic properties. Metal-organic complexes [8,9] and coordination network compounds (CNC)  have been extensively used in optoelectronic devices, like display/window application or organic light emitting diodes (OLEDs).
Up to today, a variety of new and novel electrically conducting MOFs have been reported , consisting of transition metals bonded to organocyanide ligands such as tetracyano-quinodimethane (TCNQ) , tetracyanoethylene (TCNE) as well as dicyanoquinonediimine (DCNQI) . SURMOF porous thin films like HKUST-1 have been already shown to be tunable in their electric conductivity e.g. by loading with ferrocene  or TCNQ . In fact, with regard to electrical applications, the larger class of CNC also possesses a number of interesting properties; their conductivity is expected to be larger than that of the highly porous MOF materials on account of the closer molecule-molecule contacts. As a result of this closer packing, CNC materials exhibit a number of advantages with regard to electrical applications , in particular in connection with electrochemistry .
Prussian blue (PB) is a particularly interesting CNC example which has been extensively investigated for its optical, magnetic and electrical and dielectric properties . PB crystallizes in a simple cubic structure with CN cyanide ligands coordinating Fe2+ (low spin) and Fe3+ (high spin) ions in an alternating highly ordered cubic unit cell . The blue coluor in mixed valence Fe4III[FeII(CN)6]3 arises from a broad absorption band at around 700 nm and is based on a metal-to-metal charge transfer transition from the C-coordinated Fe2+ ions across the CN organic ligand to N-coordinated Fe3+ ions. Such charge transfer processes in CNC metal-organic coordination networks are quite common and can be described through a “donor-bridge-acceptor” terminology. These electron transfer properties as well as the electric conductivity and redox properties of PB are of huge interest for the fabrication of new monolithic CNC functional materials. The epitaxial preparation of monolithic films of molecular magnetic materials such as PB enables determining orientation-dependent properties. However, monolithic and epitaxial PB thin films produced on TCO (Transparent Conductive Oxides, e.g. ITO) electrodes have not been reported and only the electrodeposition of epitaxial PB thin films on Au substrates (110)  have been published before.
Here we demonstrate that the crystalline, highly oriented and monolithic thin films of PB CNC were fabricated on conductive substrates by layer-by-layer/LPE spray method as well as through spin-coating process. The PB thin film coated TCO substrate can then be used without any further modification in an electrochemical cell device.
2. Experimental methods
2.1 Materials and functional substrates
All employed chemicals, reagents and solvents here were commercially available and used as supplied without further purification.
The gold substrates were obtained from Georg Albert PVD (Silz, Germany), which were produced by thermal evaporation of 100 nm of gold (99.99% purity) onto polished single crystal silicon (100) wafers (Silicon Sense) using a 5 nm titanium adhesion layer. The MHDA (16-mecaptohexa-decanoic acid) solution (20 μM) was prepared by dissolving MHDA in acetic acid/ethanol (1/9, v/v). The gold substrates were immersed into the MHDA solution for 3 days and then rinsed with absolute ethanol, and finally dried under nitrogen flux to obtain MHDA SAMs. ITO substrates were purchased from Sigma Aldrich. To clean and functionalize the surface, the ITO as well as silicon substrates were pretreated by either O2 plasma or irradiated with UV light for 30 min to remove any organic impurities from the surface.
2.2 Preparation of the Prussian blue thin films
1% (wt%) Prussian blue powder was dissolved in H2O/ethanol (1/9, v/v) mixture with ultrasonic for 20 min. For the spin-coating method, a few drops of the solution were dropped onto the functional substrates at room temperature with a speed at 4000 rpm for 30 s to prepare one layer of PB thin films. For the spray method, the functional substrates were placed on the sample holder and subsequently sprayed with above mentioned PB solution for 15 s, then waited for 20 s before next spray. The number of spraying cycles determines the final thickness of the deposited PB film. After 50 cycles, the blue monolithic films were obtained.
X-ray Diffraction (XRD): Each sample was characterized by using a Bruker D8 Advance equipped with a Si-strip detector (PSD Lynxeye©; position sensitive detector) with Cu Kα1,2 radiation (λ = 0.15418 nm) in θ–θ geometry, variable slit on primary circle. Scans were run over various ranges with step width of 0.024° 2θ and 84 seconds, for higher order peaks up to 336 seconds per step.
Electrochromic (EC) switching: Cyclic Voltammetry (CV) and Amperometric Detection (AD) techniques were used to perform the EC switching of the PB film on a potentiostat: PalmSens (Palm Sens, Netherland).
UV-Vis spectroscopy: A Cary 5000 UV-VIS-NIR Spectrometer (Agilent Technologies, Santa Clara CA, USA) equipped with an Agilent Universal Measurement Accessory (UMA) was used to record the spectra in transmission mode. The scan range was from 1000 nm to 350 nm (scan rate of 600 nm/min). An EC-cell with two clean ITO-substrates filled with electrolyte was measured as 100% background and the dark current of the detector was taken as 0% background.
Raman: The Raman-spectra were recorded with a Bruker Senterra Raman microscope (Bruker Optics, Ettlingen, Germany) using a green laser at 532 nm for excitation. The integration time was 60 seconds with 3 co-additions. The mapping was recorded on 17 x 17 mm2 spots (in the highlighted area shown in Fig. 4(c)). Before integration on the assigned areas the Raman spectra were normalized (min - max) for the band at 2155 cm−1. A baseline correction was conducted: concave rubber band correction; numbers of iterations 24 and 64 baseline points.
Infrared reflection absorption spectroscopy (IRRAS): IRRA-spectra were recorded using a Bruker Vertex 80 FTIR spectrometer under a fixed angle of incidence of 80° purged with dried air. The data were collected on a liquid nitrogen cooled narrow band MCT detector. Perdeuterated hexadecanethiol-SAMs on Au/Ti/Si were used for reference measurements, baseline correction was not necessary.
Scanning electron microscope (SEM): SEM cross-sectional measurements have been performed on a Zeiss HR-SEM (Gemini Class) at 3-5 kV to check the continuity, compactness, and homogeneity of different monolithic PB thin films.
3. Results and discussion
As a result of the electrochromic (EC) properties of Prussian blue, the colour of the corresponding EC cell device can be easily switched, and the reversible EC behaviour of the thin film shows the pronounced stability during the switching, see Fig. 1. In our study, all monolithic PB thin film samples were characterized by X-ray diffraction (XRD) after preparation. The results showed an epitaxial and oriented growth on the conductive substrates (transparent ITO and modified Au substrates), see Figs. 3(a) and 3(b).
For the EC switching experiments, a standard three-electrode setup was used in this system and allowed the electrochemical study. Combining electrochemistry with UV-Vis spectroscopy, in situ EC-UV-Vis measurements were conducted in a home-built cell, see Fig. 2(a). The body of the cell is made of teflon, which is sandwiched by transparent working electrode (WE) and counter electrodes (CE), so that the light beam can pass through the windows of the cell. A pin hole at the top of the cell enables electrolyte solution to be filled with a syringe, and also, reference electrode to be inserted. Optical measurements were performed in transmission mode during the EC switching.
The UV-Vis spectra of the 5 layers PB CNC thin film during EC switching is shown in Fig. 1(a), and significant changes in the optical absorption properties were observed between the oxidized and reduced forms, as shown in Fig. 1(b). In our experiment, ITO substrate and spin-coated PB thin film on ITO function as CE and WE. The area of the window is about 0.79 cm2, and the volume of the electrolyte solution is around 0.4 mL (aqueous solution of 40 mM KCl). A platinum wire is used as reference electrode. To define the trigger potential of the electrochromic CNC smart window material, the cyclic voltammetry (CV) experiment was carried out firstly in the above mentioned EC-UV-Vis cell, as shown in Fig. 1(c). A reversible redox reaction is observed from the spin-coated CNC epitaxial PB thin film on ITO. The anodic peak at –0.065 V contribute to the oxidation of Fe2+ to Fe3+, and vise versa for the cathodic peak at –0.290 V.
By employing the Pulsed Amperometric Detection (PAD) method, a potential was applied to trigger the reversible switching ON and OFF of the smart window in the same EC-system, and the current was detected during the switching in Fig. 1(d). The time corresponding to full colour change from blue to transparent of the smart window is around 5 seconds, and the switching time is roughly 1 second, which corresponds to a rapid decrease of the current, as presented in Figs. 1(e) and 1(f).
The cycling behaviour of PB CNC thin films during EC switching can be directly observed spectroscopically by the in situ EC-UV-Vis measurements. Applying a negative bias voltage (–0.4 V), the PB thin film gets reduced to Prussian white (PW) K4Fe4II[FeII(CN)6]3 and changes to the bleached/transparent state while when applying a positive bias voltage (0.1 V) the PB thin film gets oxidized to a mixed-valence component PB Fe4III[FeII(CN)6]3 which leads to a strong and blue coloured thin film. Applying the potential alternatively for cycling the EC switching, UV-Vis spectra were recorded at intervals for each potential switch. The change of the absorbance (at 694 nm) during cycling of the EC switching for the spin-coated 1 layer PB thin films is shown in Fig. 1(g). No changes can be seen after 10 switching cycles at the applied potential. This shows the stable and reversible behaviour of the PB CNC thin film during EC long-term switching.
The crystal structure of the oxidized and reduced state of PB thin film by EC switching was characterized by XRD. Prussian blue crystallizes in the cubic space group #221 Pm3m . Comparison of XRD patterns of PB powder with PB thin film revealed the presence of a crystalline film oriented along the [00l] direction because no other (hkl)-peak was detected, see Fig. 3(b). Lattice cell refinement by the Pawley method  yielded a lattice parameter ao of 1.0221 +/− 0.0003 nm. Through measuring the specific angular ranges of the (00l) peak-series (with l = 2,4,6,8) each with high enough intensities, line profile analyses according to the Williamson-Hall method  could be applied for both the oxidized and reduced states of PB, as shown in Figs. 3(c) and 3(d).
Oxidation and reduction led to a change in the peak position of the PB CNC thin films. Williamson-Hall analysis, see Fig. 3(d) revealed that there was no significant change of the coherent scattering domain size Lvol and the stress/strain parameter εo (~Δd/d, with d lattice space parameter) from 30 ox/red-cycles to 40 ox/red-cycles with Lvol = 82 nm an 81 nm respectively, εo remaining constant at 0.5%. This is based on a dynamic, flexible and fully reversible expansion and contraction between the PB FeІІ–C≡N∙∙∙Fe3+ (ao = 1.0205 ± 0.0003 nm) and PW FeІІ–C≡N∙∙∙Fe2+ (ao = 1.0151 ± 0.0001 nm) unit cells: with a difference of 0.0054 nm, it shows a 0.5% dynamic change of the unit cell parameters during the EC switching process.
The PB thin films have also been characterized through infrared (IR) and Raman measurements before and after EC switching. The IR spectra of spin-coated 3 layers PB CNC thin film is shown in Fig. 4(a) with the strongest band at around 2120 cm−1 corresponding to a CN group vibration. A shift of around 14 cm−1 was observed between the oxidized and the reduced state of the PB thin films, and resulted from the change in the valence state in the Fe-ion to which the CN-groups are bound.
By changing the FeІІ–C≡N∙∙∙Fe3+ bridge into FeІІ–C≡N∙∙∙Fe2+ bridge, we observed a red-shift in the IR spectra. This significant red-shift to a longer wavelength can be explained by the reduced influence of the trivalent Fe ions by their reduction . The contraction of the whole cubic-unit PB/PW cell of around 0.5% is due to incorporation of K+ ions in the lattice was determined by XRD analysis . Therefore, vibrational spectroscopy as well as data from XRD measurements can be described in a coherent manner in regard to the change of PB into PW.
Figure 4(b) presents the Raman spectra of the oxidized and reduced state of spin-coated 3 layers PB thin film. A typical band corresponding to the vibration of CN group (2156 cm−1) for the oxidized state of PB thin film is observed. A new band appears at 2123 cm−1 after the reduction of PB to PW. This change reveals a conversion of PB (FeІІ–C≡N∙∙∙Fe3+) to PW (FeІІ–C≡N∙∙∙Fe2+) [25,26].
For 2D Raman characterization, an interface between oxidized and reduced state of the PB thin film is produced by using electrochemical cell for full-oxidation and thereafter half-reduction, as presented in Fig. 2(b). By integrating the Raman spectra of the CN-band between 2132 and 2108 cm−1, recorded from the inner area in Fig. 4(c), a clear interface between reduced and oxidized state of the PB thin film is observed from the 2D Raman-MAP, where the colour green represents the reduced state and blue represents the oxidized state of PB thin film.
To check continuity, compactness and homogeneity as well as a physical thickness of ≈150 nm (for the 3 layers PB) and ≈250 nm (for the 5 layers PB) of the deposited monolithic, crystalline and epitaxial CNC PB thin films, we performed HR-SEM cross-sectional measurements, as presented in Figs. 5(a) and 5(b). The observed morphology and physical thickness did not show any change after the electrochemical redox-cycling process.
In conclusion, crystalline and highly oriented thin films of the coordinative network compound (CNC) Prussian blue (PB) have been grown on conductive substrates, and were used successfully as electrodes to fabricate an electrochromic (EC) device. By applying an electrical potential, the PB layer was switched between two states which were characterized by XRD, IR and Raman measurements. The highly oriented, crystalline and monolithic CNC thin films are shown to be robust, stable and can be switched multiple times, thus demonstrating a simple way to saleable optoelectronic and display devices as well as -smart windows materials. Presently, in our laboratory, we develop further monolithic CNC materials with improved properties to eventually yield materials with “tunable porosity” as well as with “tunable potential barriers”.
E.R. thanks the Alexander von Humboldt (AvH) Foundation as well as KIT and CMM for financial support and funding. J.L., W.Z. and Z.W. thank the China Scholarship Council (CSC) for financial aid. This work was funded within the priority program SPP 1362 of the German Research Foundation (DFG). J. Liu and W. Zhou have contributed equally to this work; both have also to be considered as first authors here.
References and links
1. H. K. Arslan, O. Shekhah, J. Wohlgemuth, M. Franzreb, R. A. Fischer, and C. Wöll, “High-throughput fabrication of uniform and homogenous MOF coatings,” Adv. Funct. Mater. 21(22), 4228–4231 (2011). [CrossRef]
3. O. M. Yaghi, H. Li, M. Eddaoudi, and M. O’Keeffe, “Design and synthesis of an exceptionally stable and highly porous metal-organic framework,” Nature 402(6759), 276–279 (1999). [CrossRef]
5. S. Bundschuh, O. Kraft, H. K. Arslan, H. Gliemann, P. G. Weidler, and C. Wöll, “Mechanical properties of metal-organic frameworks: An indentation study on epitaxial thin films,” Appl. Phys. Lett. 101(10), 101910 (2012). [CrossRef]
6. S. Eslava, L. P. Zhang, S. Esconjauregui, J. W. Yang, K. Vanstreels, M. R. Baklanov, and E. Saiz, “Metal-organic framework ZIF-8 films as low-κ dielectrics in microelectronics,” Chem. Mater. 25(1), 27–33 (2013). [CrossRef]
7. E. Redel, Z. B. Wang, S. Walheim, J. X. Liu, H. Gliemann, and C. Wöll, “On the dielectric and optical properties of surface-anchored metal-organic frameworks: A study on epitaxially grown thin films,” Appl. Phys. Lett. 103(9), 091903 (2013). [CrossRef]
8. H. Xu, R. Chen, Q. Sun, W. Lai, Q. Su, W. Huang, and X. Liu, “Recent progress in metal-organic complexes for optoelectronic applications,” Chem. Soc. Rev. 43(10), 3259–3302 (2014). [CrossRef] [PubMed]
9. F. Carpi and D. De Rossi, “Colours from electroactive polymers: Electrochromic, electroluminescent and laser devices based on organic materials,” Opt. Laser Technol. 38(4-6), 292–305 (2006). [CrossRef]
10. V. Vinni, L. K. Rao, and N. Munichandraiah, “High contrast optical switching in electrochromic Prussian blue films for display/window applications,” Proc. SPIE 1622, 278–282 (1992).
12. C. A. Fernandez, P. C. Martin, T. Schaef, M. E. Bowden, P. K. Thallapally, L. Dang, W. Xu, X. Chen, and B. P. McGrail, “An electrically switchable metal-organic framework,” Sci. Rep. 4, 6114 (2014). [CrossRef] [PubMed]
13. X. Zhang, Z. Zhang, H. Zhao, J. G. Mao, and K. R. Dunbar, “A cadmium TCNQ-based semiconductor with versatile binding modes and non-integer redox states,” Chem. Commun. (Camb.) 50(12), 1429–1431 (2014). [CrossRef] [PubMed]
14. A. Dragässer, O. Shekhah, O. Zybaylo, C. Shen, M. Buck, C. Wöll, and D. Schlettwein, “Redox mediation enabled by immobilised centres in the pores of a metal-organic framework grown by liquid phase epitaxy,” Chem. Commun. (Camb.) 48(5), 663–665 (2012). [CrossRef] [PubMed]
15. A. A. Talin, A. Centrone, A. C. Ford, M. E. Foster, V. Stavila, P. Haney, R. A. Kinney, V. Szalai, F. El Gabaly, H. P. Yoon, F. Léonard, and M. D. Allendorf, “Tunable electrical conductivity in metal-organic framework thin-film devices,” Science 343(6166), 66–69 (2014). [CrossRef] [PubMed]
17. C. W. Kung, T. C. Wang, J. E. Mondloch, D. Fairen-Jimenez, D. M. Gardner, W. Bury, J. M. Klingsporn, J. C. Barnes, R. Van Duyne, J. F. Stoddart, M. R. Wasielewski, O. K. Farha, and J. T. Hupp, “Metal-organic framework thin films composed of free-standing acicular nanorods exhibiting reversible electrochromism,” Chem. Mater. 25(24), 5012–5017 (2013). [CrossRef]
18. K. Itaya and I. Uchida, “Nature of intervalence charge-transfer bands in Prussian blues,” Inorg. Chem. 25(3), 389–392 (1986). [CrossRef]
19. B. S. Brunschwig, C. Creutz, and N. Sutin, “Optical transitions of symmetrical mixed-valence systems in the Class II-III transition regime,” Chem. Soc. Rev. 31(3), 168–184 (2002). [CrossRef] [PubMed]
20. S. Nakanishi, G. Lu, H. M. Kothari, E. W. Bohannan, and J. A. Switzer, “Epitaxial electrodeposition of Prussian blue thin films on single-crystal Au(110),” J. Am. Chem. Soc. 125(49), 14998–14999 (2003). [CrossRef] [PubMed]
21. H. J. Buser, D. Schwarzenbach, W. Petter, and A. Ludi, “The crystal structure of Prussion Blue: Fe4[Fe(CN)6]·×H2O,” Inorg. Chem. 16(11), 2704–2710 (1977). [CrossRef]
22. G. S. Pawley, “Unit-cell refinement from powder diffraction scans,” J. Appl. Cryst. 14(6), 357–361 (1981). [CrossRef]
23. G. K. Williamson and W. H. Hall, “X-ray line broadening from field aluminium and wolfram,” Acta Metall. 1(1), 22–31 (1953). [CrossRef]
24. A. Hamnett, P. A. Christensen, and S. J. Higgins, “Analysis of electrogenerated films by ellipsometry and infrared spectrometry,” Analyst (Lond.) 119(5), 735–747 (1994). [CrossRef]
25. A. Pitarch, A. Alvarez-Perez, K. Castro, J. M. Madariaga, and I. Queralt, “Raman analysis assessed by Fourier-Transformed infrared and X-ray flouresence spectroscopies: a multi-analytical approach of ancient chromolithographs from the 19th century,” J. Raman Spectrosc. 43(3), 411–418 (2012). [CrossRef]
26. H. A. Khorami, J. F. Botero-Cadavid, P. Wild, and N. Djilali, “Spectroscopic detection of hydrogen peroxide with an optical fiber probe using chemically deposited Prussion blue,” Electrochim. Acta 115(1), 416–424 (2014). [CrossRef]