Layer-by-layer grown scalable redox-active ruthenium-based molecular multilayer thin films for electrochemical applications and beyond†

A layer-by-layer assembly method to prepare redox-active ruthenium based molecular multilayer thin fi lms with well controlled thicknesses was developed. The fi ne control of the physicochemical properties at the nanometer scale, with excellent stability of the layers under ambient conditions, makes the multilayer coatings of this type an excellent material for energy storage, energy conversion and molecular electronics applications. Nanoscale


Introduction
Recent developments in portable consumer electronics, such as transparent and/or flexible electronic devices and integrated microsystems, demand next generation energy storage devices, which must meet the requirements of high performance (rapid charge/discharge) and high stability (low performance loss) during the charging-discharging cycles as well as low cost. [1][2][3][4][5] Conventional electrochemical energy storage devices, such as batteries, are known to suffer from slow charge-discharge rates and difficulties in maintaining their efficiency upon scaling-down in size. 1,6 This creates a strong demand for small, flexible, high performance and low cost portable energy storage solutions. 4,[7][8][9][10][11][12] It was suggested that electrochemical supercapacitors, which store the electric energy by charging the electric double layer, may suit this purpose. Electrochemical supercapacitors based on graphene 7,13-15 and/or carbon nanotubes 3,4,[16][17][18] exhibit higher power densities, faster charge/discharge and longer cycling stability than conventional batteries. However, the practical use of carbon based supercapacitors is limited by their low charge storage capacities. 4 The gap between conventional batteries and supercapacitors 12 is bridged by electrochemical pseudocapacitors, 4 which accumulate charge in the bulk of a redox-active material during a fast oxidation/reduction reaction. To date, electrochemical pseudocapacitors based on conducting polymers, 19 covalent organic frameworks, 20 as well as transition metal oxides 4,21 and their composites 5,22-28 have been proposed. Pseudocapacitors employing thin films of transition metal oxides and their composites show very high capacitance, but suffer from low electric conductance and inefficient mass transport as well as from high material costs. 27,29 Highly-conductive, flexible and transparent films of conducting polymers can be easily formed by spin coating or (electro)chemical polymerization, but it is challenging to ensure their efficient binding to the conductive support and to control their morphology (thickness, porosity). As a result, pseudocapacitors based on conducting polymers often display poor cycling stability. 19 A better alternative to traditional conducting polymers for many applications is represented by coordination networks formed by redox-active metal organic frameworks and/or covalent organic frameworks. 20,30,31 The structure of coordination networks can be designed according to the desired functionality by a careful selection of metal centers and functional ligands. Different research groups demonstrated various applications of metal coordination network films, for example as interlayers for efficient inverted bulk heterojunction solar cells 32 and photocurrent generation, 33 as active layers in electrochromic devices, 34,35 integrated molecular logic 36 and memory 37 applications.
Metal coordination network films are highly conductive and exhibit nanopores which allow a good contact between the electrolyte and the metal ions inside the film, thus facilitating their oxidation/reduction. 31 Although these properties render them particularly suitable for energy storage applications, systematic studies exploring the redox-active porous coordination polymers on electrode surfaces with this goal are still scarce. 30,31 Facile, low cost, and environmentally friendly materials with improved stability are desired to enable better integration of coordination networks with electrodes for their practical energy storage applications. To address some of the aforementioned issues, we report here the first study on the energy storage application of surface-immobilized ruthenium complex multilayer thin films with anion storage capability, whose properties can be tuned as a function of film thickness.
Assembly of multilayer film of Ru-N on ITO 3.07 mg of Ru-N was dissolved in 5 ml of Milli-Q water by adding 3 drops of the 25% ammonia solution upon sonication. The pH of the solution was adjusted to 6 by the addition of 0.1% HCl while monitoring the pH with a pH meter. Then the solution was transferred to a volumetric flask and the volume was adjusted to 20 ml by adding Milli-Q water to obtain a 50 μM solution of Ru-N. Each Ru-N monolayer was assembled either on a bare or modified ITO sample by immersing the sample into the 50 μM Ru-N solution for 3 h. The Ru-N monolayers were connected by Zr(IV) ions by immersing the sample into a 20 mM aqueous solution of ZrOCl 2 for 30 min. After each assembly step, the sample was washed by immersing three times into a beaker filled with Milli-Q water (exchanged after each sample immersion) and dried under an Ar flow. The Ru-N/Zr assembly cycles were repeated until the desired number of layers, respectively thickness of the multilayer film was achieved (Fig. 1).

Cyclic voltammetry
Cyclic voltammograms were measured in a lab-made single compartment glass cell at room temperature in solutions deoxygenated by Ar and under Ar atmosphere. All cyclic voltammetry and spectroelectrochemical measurements employed a lab-built potentiostat; platinum wires were used as counter and quasi-reference electrodes. The potential of the platinum quasi-reference electrode was calibrated as −100 ± 10 mV vs. the equilibrium potential of the ferrocenium/ferrocene redox pair used as an internal standard and it is also calibrated as +765 ± 10 mV vs. Ag/AgNO 3 reference electrode.

UV-Vis and Raman spectroscopy
A Cary 5E UV-Vis-NIR spectrometer was employed for UV-Vis spectroscopy. LabRAM HR800 confocal Raman microscope (Horiba Jobin Yvon) was employed for Raman spectroscopy. In situ UV-Vis and Raman spectroelectrochemical measurements were carried out in Ar-deaerated 0.1 M HClO 4 in CH 3 CN using a custom-made optical quartz cell under a continuous Ar flow above the solution. For Raman spectroscopy we employed a long-working-distance objective lens (50 times magnification, 8 mm focal length) with a numerical aperture of 0.1 to focus DPSS laser (excitation wavelength 532 nm, power 1 mW) on the sample. The Raman signal was collected in a backscattering geometry.
Ex situ X-ray photoelectron spectroscopy Ex situ X-ray photoelectron spectra of ITO|(Ru-N) n multilayer samples were measured by using a Shimadzu/Kratos Axis HSi spectrometer with monochromated Al Kα radiation as an excitation source.

Galvanostatic charge/discharge measurements
Charging/discharging curves and capacitances of ITO|(Ru-N) 65 were measured in Ar-deaerated 0.1 M HClO 4 in CH 3 CN with Arbin 2000 battery test instrument using a three electrode cell and under a continuous Ar flow above the solution.

Results and discussion
Layer-by-layer assembly of ruthenium complex multilayer ITO|(Ru-N) n The multilayer thin films were prepared with the novel redoxactive dinuclear ruthenium complex [Ru 2 (tppz)(LPOH) 2 ](PF 6 ) 4 (Ru-N) on an indium tin oxide (ITO) substrate employing an aqueous-based layer-by-layer (LBL) assembly process (Fig. 1). After immobilization of the primer layer of Ru-N on the ITO, this involves bidentate/tridentate binding through P-O-In and/or P-O-Sn bonds. [38][39][40][41][42] Then the substrate was immersed in the solution containing Zr(IV) ions, resulting in the formation of Zr-phosphonate linkage with Ru-N in the primer layer. During the assembly process two Ru-N layers were connected by successive reaction of Zr(IV) ions with phosphonate linker groups through R-P(O)O 2 -Zr-O 2 P(O)-R bond, a concept developed by Mallouk et al. and others. [43][44][45][46][47][48][49] A similar assembly strategy has been applied for the fabrication of multilayers used in the implementation of high-k gate dielectrics for lowvoltage organic field effect transistors, 50 thin film transistors 51,52 and molecular electronics applications. 53,54 We describe the structure of the multilayers as ITO|(Ru-N) n , where "n" represents the number of Ru-N layers.

Ex situ characterization of ruthenium complex multilayers
To gain more insight into the structure and composition of the multilayers, we monitored the layer-by-layer growth of the ITO|(Ru-N) n multilayer film with ex situ UV-Vis, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS). Two growth mechanisms, namely linear and exponential growth, have been reported for the multilayer films assembled by LBL procedures in the literature. 22,[56][57][58][59][60][61] Fig. 2a shows the ex situ UV-Vis absorption spectra of ITO| (Ru-N) n with n = 1-65. The UV-Vis absorption spectra feature two bands. The band observed in the visible region (576 nm) corresponds to the metal to ligand charge transfer (MLCT) and the one in the UV region (357 nm) corresponds to intraligand ππ* transitions. Similar UV-Vis transitions for the basic backbone structure of Ru-N were also reported by Nagashima et al. 62 With an increasing number of layers, the integrated absorbance of the MLCT band at 576 nm increased linearly without any shift in peak position up to 65 layers (Fig. 2b). Similar observations were also reported by Shinomiya et al. 63 and van der Boom et al., 64 where the plots of absorbance vs. the number of immersion cycles exhibited a linear relationship, indicating linear growth of a layered Pd-based network.
The ex situ Raman spectra of the ITO|(Ru-N) n films with different amount of assembled layers (Fig. 2c and d) also show a linear increase of the Raman signal intensities of Ru-N with n. The layer thickness extracted from AFM scratching profiles increases linearly with n with a slope from which a thickness of 1.9 nm per layer can be derived. This value is very close to the length of Ru-N calculated from the molecular modeling (Fig. S5 †). Additionally, ex situ AFM images of bare ITO and of the thickest multilayer (n = 65, estimated thickness 123 nm) show surface structures with comparable roughness (Fig. S3 †).
Additionally, to get a clear insight into the layer composition we also monitored ex situ XPS spectra for ITO|(Ru-N) n (n = 1-13) multilayer film growth ( Fig. 3 and S12 †). The signal intensities for the O 1s, Sn 3d, In 3d, In 4s peaks from the ITO substrate were gradually decreased with increasing the ITO| (Ru-N) n layer thickness. On the other hand, the signal intensi- ties for N 1s, C 1s, Ru 3d, P 2p peaks were increased with increasing the thickness of the ITO|(Ru-N) n layers. Presence of Ru 3d signals confirmed the "+2" oxidation state of the ruthenium center in the multilayer film. 65 No Zr 3d signals were observed for bare ITO and n = 1 sample. While clear Zr 3d signals were observed from bilayers (n = 2) and other successive multilayers, confirming the Zr ion incorporation into the layers ( Fig. 3 and S12 †). Combining and comparing the results of ex situ UV-Vis, ex situ Raman spectroscopy, ex situ XPS, ex situ AFM strongly indicate a uniform and linear growth of the multilayers of Ru-N on ITO.

Cyclic voltammetry
Cyclic voltammograms of ITO|(Ru-N) n in 0.1 M HClO 4 in CH 3 CN (Fig. 4) show two reversible redox peaks corresponding to the oxidation of Ru(II)-Ru(II) to Ru(III)-Ru(II) and of Ru(III)-Ru(II) to Ru(III)-Ru(III). To avoid the degradation of ITO|(Ru-N) n multilayer films, the potential was always cycled between −0.25 to 0.7 V vs. Pt quasi-reference electrode. The total integrated charge obtained from cyclic voltammetry experiments increases linearly with the number of layers giving the slope of 12 × 10 −6 C cm −2 per layer (Fig. 4b), which is equivalent to the charge of one full Ru-N monolayer (ESI section 2 †). This indicates the addition of one full monolayer at each step of the layer-by-layer assembly and further confirms the formation of uniform multilayer films. The total integrated charge shows no saturation with the increase of layer thickness up to 65 layers. Moreover, the cyclic voltammogram of ITO|(Ru-N) 65 display well-defined reversible redox peaks even at a scan rate of 100 mV s −1 . This indicates the high accessibility of the redoxactive ruthenium ions for the electrolyte. 19,47,54,66 Similar results were also obtained in 0.1 M aqueous HClO 4 (ESI section 2 †).

Spectroelectrochemistry
The electrochemical reversibility and structural changes in ITO|(Ru-N) n multilayers during the redox transitions were investigated by employing in situ UV-Vis and Raman spectroelectrochemical experiments as well as electrochemical quartz crystal microbalance (EQCM) experiments (ESI section 3 †).
The results of UV-Vis spectroelectrochemistry (ESI section 5 †) and data reported by Nagashima et al. 62 show clear spectral changes due to the two well-separated one-electron redox processes which occurred in the multilayer, resulting in the formation of three dinuclear ruthenium complex with different oxidation states: Ru(II)-Ru(II), Ru(II)-Ru(III), and Ru(III)-Ru(III).    respect to the electrode potential is an ideal characteristic of functional materials that can be employed for electrochemical sensors, smart energy storage devices and molecular electronics applications. 4 Characterisation of the ruthenium complex multilayer ITO| (Ru-N) 65 in an electrochemical energy storage application Further we present a proof of principle application of ITO|(Ru-N) 65 multilayer for electrochemical energy storage. The charging of the ITO|(Ru-N) 65 electrode was first characterized by cyclic voltammetry in 0.1 M HClO 4 in CH 3 CN (Fig. 6a). The separation between respective oxidation and reduction peaks (Fig. 6b) is close to 25 mV for the two lowest scan rates, 10 and 50 mV s −1 . Theoretically, the oxidation and reduction peaks for a surface-confined redox-active species should be completely symmetric for a very fast charging or for very low potential scan rates. However, the residual hysteresis is rather small and constant at the low scan rate limit, and we attribute it to the imperfect potential distribution in the electrochemical cell and/or to a rather high resistance of the CH 3 CN-based solution. The kinetic limitations due to the slow mass transfer manifest themselves as an increase of the peak to peak separation at scan rates above 100 mV s −1 (Fig. 6b). A specific capacitance of 95.3 F g −1 was obtained for the ITO|(Ru-N) 65 multilayer from the voltammograms measured at 10 mV s −1 and is close to those reported for pseudocapacitors based on vertically aligned carbon nanotubes (70-120 F g −1 ), [68][69][70] covalent organic frameworks (48 ± 10 F g −1 ) 20 and conducting polymers (75-530 F g −1 ). 19,66 It decreases to 81.8 F g −1 at 100 mV s −1 and further by approximately 20% when the scan rate increases by one order of magnitude (Fig. 6c). This decrease, however, is quite small compared to that found for conducting polymers, 19 metal oxide materials 21 and their composite electrodes. 4,19,25 We attribute the fast oxidation/ reduction of the multilayer to two factors: (I) fast diffusion of the anions into and out of the multilayer film during the redox reaction, preventing kinetic limitations due to the slow charge compensation; and (II) fast charge transport within the multilayer itself, which might be due to the charge transfer mechanisms specific for the electrochemical environment. The long range electron transfer 53,71 in Ru-N multilayers is currently a subject of further studies. The stability of ITO|(Ru-N) 65 was characterized by continuous potential cycling with the rate of 100 mV s −1 (Fig. 6d). The capacitance retention after 2000 cycles was 80%, which is much higher than reported for the most conducting polymer based devices. 19,72,73 We also assayed the stability of these layers stored under ambient conditions (Fig. S11A †) and also the sample heated on a hot plate at 150°C (in air) by employing the cyclic voltammetry (Fig.S11B, ESI section 6 †). These results clearly demonstrated the excellent stability of ruthenium complex multilayer films under ambient conditions and reasonably good stability with the sample heated at 150°C. However a small change in the charge was observed in case of sample heated on a hot plate at 150°C for 30 minutes, which is may be due to a small material loss or surface oxidation during heating.
To characterize the charge/discharge properties of the ruthenium complex multilayer electrodes under standardized conditions, we further carried out galvanostatic charge-discharge experiments upon application of a constant current density varied from 10 to 100 μA cm −2 (Fig. 7a). Based on these curves, the electrodes display pseudocapacitor behaviour of ITO|(Ru-N) 65 with two small plateaus at 0.12 V and 0.37 V vs. Pt. These plateaus correspond to the reversible Faradaic reactions associated with the oxidation/reduction of the two ruthenium ions in the complex (cf. Fig. 4a and 6a). Similar electrochemical pseudocapacitor behaviour of conducting polymers ( polyaniline, polypyrrole, PEDOT, polythiophene, etc.), 19 74,75 and their composites has been reported in the literature. 4,5,76 For our electrodes, an excellent capacitance retention of ≈100% was observed after 10 cycles at all applied current densities (Fig. 7b). A maximum capacitance of 92.2 and 63.3 F g −1 was found for the smallest (10 μA cm −2 ) and the highest (100 μA cm −2 ) applied current density (Fig. 7c). Although the decrease of the capacitance with increasing current density is due to the diffusion limitations, it is much lower than for the other systems with only 35% at one order of magnitude increase in current density. This again indicates fast charge transfer kinetics upon the electrochemical oxidation/reduction of the multilayer.
The insert in Fig. 7c displays the Ragone plot (energy density vs. power density) of ITO|(Ru-N) 65 (ESI section 8 †). Our ruthenium complex multilayer electrode delivered a maximum power density of ∼3736 W kg −1 with an energy density of ∼6 W h kg −1 , as obtained for the characterized potential range of 0.9 V. We note that in order to achieve a larger operating voltage range and increase the energy and power density of multilayer-based supercapacitors, it is highly desirable to couple such electrodes with suitable counter electrodes. 4,19,77 The cycling stability of the electrode was also tested over 1000 galvanostatic charge-discharge cycles (Fig. 7d). At 50 μA cm −2 , a capacitance retention of 72% was found. A similar trend was observed for higher current density of 70 μA cm −2 for 400 cycles (Fig. 7d).

Conclusions
We successfully demonstrated the assembly of coordination networks based on a novel redox-active dinuclear ruthenium complex into multilayers on an ITO substrate. The multilayers were produced by a simple yet effective layer-by-layer assembly from an aqueous solution and are characterized by the welldefined orientation of the molecules, leading to a uniform growth. The thickness of the coating and the charge stored scale both with the number of layers assembled. The latter are limited only by the available assembly time, respectively deposition cycles. Even the thickest coating with 65 molecular layers demonstrates fast electrochemical oxidation/reduction. Without further device optimization, this multilayer exhibited an excellent performance and stability upon thousands of charging/discharging cycles in a prototypical energy storage application. Our results show that redox-active coordination networks grown on conductive substrates have a potential to unfold into a new class of promising materials for electrochemical energy storage as well as for other applications. The assembly procedure adopted in this study allows multilayer formation on various other oxide materials (Indium tin oxide, fluorine doped tin oxide, quartz/glass, sapphire etc.). The characteristics of the fabricated multilayers indicate that coordination networks of this type are promising materials for energy storage as well as for many other applications: as an effective linker between electrode and bacteria in microbial fuel cells [78][79][80] or as interlayers in organic photovoltaic devices, [81][82][83] for nanoelectronic devices with electrochemicallycontrolled charge transport properties, 84,85 or for fundamental studies of the long range electron transfer, 86 to mention only a few.