Spontaneous Reduction of Copper(II) to Copper(I ... - ACS Publications

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Spontaneous Reduction of Cu(II) to Cu(I) at Solid-Liquid Interface Shammi Rana, Anupam Prasoon, Pampa Sadhukhan, Plawan Kumar Jha, Vasant Sathe, Sudipta Roy Barman, and Nirmalya Ballav J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02844 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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The Journal of Physical Chemistry Letters

Spontaneous Reduction of Cu(II) to Cu(I) at SolidLiquid Interface Shammi Rana,† Anupam Prasoon, † Pampa Sadhukhan,‡ Plawan Kumar Jha,† Vasant Sathe, ‡ Sudipta Roy Barman, ‡ and Nirmalya Ballav*,†,§ †

Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune

411 008, India ‡

UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452 001, India

§

Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Pune

411 008, India

Corresponding Author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Oxidation and reduction reactions are of central importance in chemistry as well as vital to the basic functions of life and such chemical processes are generally brought about by oxidizing and reducing agents, respectively. Herein, we report the discovery of an interfacial reduction reaction (IRR) – without the use of any external reducing agent. In course of metalligand coordination, spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface was observed – unlike in a liquid-phase reaction where no reduction of Cu(II) to Cu(I) was occurred. High-quality thin films of a new coordination network compound bearing Fe(II)-CN-Cu(I) link were fabricated by IRR and employed for efficient electro-catalysis in the form of oxygen reduction reaction. Also, thermally activated reversible structural phase transition modulated the electron transport property in thin film. This work unveils the importance of chemical reactions at solid-liquid interfaces which can lead to the development of new functional thin film materials.

K4[FeII(CN)6]

TOC Graphic

solid interface liquid

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CuII(OAc)2 CuII

SAM

CuI

IRR


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Chemical conversions involving oxidation and reduction reactions in liquid (also solid) phase are usually performed in presence of oxidizing and reducing agents, respectively. However, these seemingly simple chemical reactions become complicated in biological environments due to confinement effect. A notable example is the conversion of molecular oxygen to water (O2 → H2O) by cytochrome c oxidase (complex IV) in the power house of eukaryotes.1-2 Such an unprecedented chemical reduction of O2 is perhaps possible due to unique metal-ligand coordination as well as electron transfer processes in a confined geometry. Cyanide, a noncompetitive inhibitor of complex IV, electrostatically stabilizes both Fe and Cu centers at once upon forming the biredox Fe(II)-CN-Cu(I) coordination link with high affinity and hinders further enzymatic reduction – so called cyanide poisoning.2-3 Various synthetic modules mimicking complex IV were explored to understand the distinctive bonding motifs of the Fe-CN-Cu coordination link in different redox states: oxidized Fe(III)-CN-Cu(II), partially reduced Fe(II)CN-Cu(II) and fully reduced Fe(II)-CN-Cu(I).4-5 Apart from biology, such kind of biredox links generated a new class of porous solids by extending the coordination in three dimensions – Prussian blue and its analogues (PBAs).6 Perhaps, PBAs are the oldest synthetic porous coordination networks trailing down to recent days metal-organic frameworks (MOFs).7 Herein, we report the first observation on spontaneous reduction of Cu(II) to Cu(I) at a solidliquid interface – without the need of any extraneous reducing agent. As a consequence, thin film of coordination network bearing Fe(II)-CN-Cu(I) link, an analogue of PB, was deposited onto a functionalized Au substrate via layer-by-layer method. Specifically, an interfacial reduction reaction (IRR) of Cu(II) to Cu(I) was observed upon subsequent dipping of thiolate self-assembled monolayer (SAM)8 with –COOH tail group into the solutions of Cu(OAc)2 (Cu(II) ion) and K4[Fe(CN)6] (Fe(II) ion), respectively (Figure 1a and Supporting Information Figure S1).


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Interestingly, reaction of Cu(OAc)2 and K4[Fe(CN)6] in a liquid-phase did not produce the Fe(II)CN-Cu(I) coordination link, not even the Fe(III)-CN-Cu(I) coordination link which could in principle produce via electron transfer from Fe(II) to Cu(II). Instead, coordination network solid bearing the Fe(II)-CN-Cu(II) link, namely copper hexacyanoferrate (Cu-HCF),6 was precipitated out from the liquid phase reaction (Figure 1a and Figure S2). Furthermore, reaction of CuCl (Cu(I) ion) and K4[Fe(CN)6] in a liquid-phase as well as at a solid-liquid interface did not result into the formation of expected Fe(II)-CN-Cu(I) coordination link (Figure S3 and S4). Thus, IRR is anticipated to be very useful in generating new materials which are otherwise difficult to achieve via conventional liquid-phase reactions. SAM of mercaptoundecanoic acid (MUDA) on polycrystalline Au substrate was prepared by standard procedure.9 MUDA/Au SAM was dipped into ethanolic solution of Cu(OAc)2 for thirty minutes followed by washing with ethanol and drying it over stream of nitrogen gas. Subsequently, it was dipped into aqueous-ethanol solution of K4[Fe(CN)6] for thirty minutes to complete one cycle of the layer-by-layer growth (Figure S1).10-12 Cross sectional image revealed an average film thickness of ~400 nm resulting from the growth of twenty consecutive cycles (Figure S5a). After each 5 growth cycles, cross-sectional height was measured, and almost a linear trend was observed when the thickness was plotted vs number of cycles (Figure S5 b,c). Approximately 100 nm thin film was grown in each 5 cycles that means ~20 nm per cycle, is significantly greater than the thickness of a single Cu/K4[Fe(CN)6] layer, ca. 1 nm. Thus, here fabricated thin film showed growth of multi-layers in each cycle and our data is consistent with earlier reports on the interfacial growth of much-celebrated MOF – HKUST-1 whereby in each cycle of the layer-by-layer growth multi-layers of HKUST-1 was getting deposited.13


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a

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Figure 1. (a) Schematic of distinctive reactivity of surface supported (solid-liquid interface) and liquid-phase chemical reactions involving Cu(II) and K4[Fe(CN)6]. (b-c) High-resolution Cu 2p and Fe 2p XPS spectra of thin film sample (after 2 cycles of growth). Respective satellite spectral zones characteristics of Cu(II) and low-spin Fe(III) are marked by grey shades. (d) Au 4f XPS


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spectra of thin film sample (after 2 cycles of growth) showing the 4f7/2 and 4f5/2 doublet peak (due to spin-orbit coupling). (e) Valence band (VB) spectra recorded on pristine Au substrate and on the sample (after 2 cycles of growth) showing the 6s (green shade) and 5d (yellow and orange shades) states of Au. Cu 3d state is clearly visible (blue fill) in the difference spectrum. Dotted black line represents the Fermi level (EF).

At the first step, anchoring of Cu(II) ions onto the surface of MUDA/Au SAM took place – without the reduction of Cu(II) to Cu(I) (0.5 cycle) as was also evidenced in our XPS data showing Cu 2p3/2 signal at ~934.5 eV along with strong satellite features (Figure S6).14 XPS analysis of the sample after 2 cycles of growth was performed and appearance of a strong Cu 2p3/2 photoemission signal at binding energy (BE) of ~932.0 eV confirmed the presence of majorly Cu(I) in the sample resembling Cu(I)-CN bonding scenario (Figure 1b).14 Also, strong Fe 2p3/2 photoemission observed at BE of ~708.4 eV confirmed the presence of Fe(II) in the sample in majority (Figure 1c).14-15 Almost absence of satellite features in the XPS signals further supported the presence of Cu(I) as well as low-spin Fe(II) as expected in six coordinated complex with strong-field ligands like CN. Interestingly, Cu 2p and Fe 2p photoemission signals for the bulk (powder) Cu-HCF sample obtained from liquid-phase reaction did show predominant presence of Cu(II) and Fe(II) – neither oxidation Fe(II) to Fe(III) nor reduction of Cu(II) to Cu(I) occurred (Figure S2).14 To check the possibility of electron transfer from the Au substrate causing the interfacial reduction, we have measured core-level photoemission signal of Au. Indeed, appearance of a single Au 4f7/2 peak at BE of ~83.9 eV ruled out the possibility of oxidation of the Au substrate (Figure 1d).14, 16 Furthermore, valence-band (VB) photoemission signals from the sample (2 cycles of growth) appeared to be dominated by the 6s and 5d features of Au (Figure 1e).16 The observation


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of an unambiguous Fermi edge at 0 eV in Figure 1e, as expected for metallic Au, is another indicator of the presence of metallic Au substrate – not an oxidized species.16 Expected 3d features of Cu (and also Fe) within 2-4 eV were apparently merged with the VB signal of Au and upon subtraction of the VB signal of pristine Au, a prominent peak could be realized at ~1.7 eV which is characteristics of photoemission from Cu 3d states of Cu(I) species.17 Furthermore, S 2p XPS data on the sample (2 cycles of growth) clearly suggested that the thiolate (Au-S) interface was almost intact (Figure S7). Thus, Au substrate was not responsible for the interfacial reduction of Cu(II) to Cu(I). Earlier, in course of building molecular ruler stacks on Au surface, reduction of Cu(II) to Cu(I) was observed,18-19 for example, upon immersion of Cu(II) anchored SAMs of various mercaptoalkanoic acids/alkane dithiols into solutions of thiol/dithiol (-SH/-S-S-) of the respective ad-layers. The following interfacial reaction was proposed: (RCOO)2Cu(II) (s) + R’SH (liq)  (RCOO)Cu(I)SR’ (s) + RCOOH (liq). However, in all these interfacial reactions, the counterparts in liquid-phase did show reduction of Cu(II) to Cu(I) as thiols and dithiols are well-known reducing agents.20-21 The role of interface in the reduction process was redundant – unlike in the present study where the reaction at solid-liquid interface (reduction of Cu(II) to Cu(I)) markedly differed from the reaction in liquid-phase (no reduction of Cu(II) to Cu(I)). In order to understand the interfacial reduction reaction (IRR) of Cu(II) to Cu(I), we have employed Raman spectroscopy as a non-destructive analytical technique. CN stretching frequency is quite sensitive to counter ion and its coordination environment, for example, in the Raman spectra, the νCN bands at 2135 cm−1 and 2093 cm−1 distinctively revealed the presence of Fe(III)CN and Fe(II)-CN bonds in K3[Fe(CN)6] and K4[Fe(CN)6], respectively.22-23


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a

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*

Figure 2. (a) Raman spectra recorded step-wise in the layer-by-layer growth. Characteristic Cu(I)CN (2115 cm-1), Fe(II)-CN (2093 cm-1), K-CN (2073 cm-1) and Cu(I)---CN (2053 cm-1) vibrations are depicted with the help of Lorentzian fit (green line). (b) A schematic of the Cu(II) anchoring on MUDA/Au SAM template representing 0.5 cycle of the layer-by-layer growth. As a guide to eye, six approaching directions of ligands to a metal ion M is provided. (c) Addition of K4[Fe(CN)6] leading to reduction of Cu(II) to Cu(I) at the interface by the CN ligand (1 cycle of


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layer-by-layer growth). (d) Illustration of a synergistic Cu-CN bonding driving the interfacial reduction of Cu(II) to Cu(I): σ-bonding upon overlapping of filled 5σ orbital of CN ligand and partially filled 3dz2 orbital of Cu and π-back bonding from filled 3dzx orbital of Cu to empty π* orbital of CN.

It may also be used as fingerprints of the electronic structures in various biredox PBAs.23 Likewise the synergistic metal-carbonyl bonding,24 CN binds to the metal ion through a σ-bond upon donating electrons from 5σ orbital to the partially filled d-orbital of the metal ion; and a πbond is formed by accepting electrons from the filled metal d-orbital into empty π*-orbital of CN – so called σ-donor and π-acceptor.24 We have recorded Raman spectra at each step of the few initial cycles (Figure 2a and S8) and based on the Raman data we have proposed a mechanism for the layer-by-layer growth of the Fe(II)-CN-Cu(I) coordination network on SAM template (Figure 2b-c and S9). In Cu(II)-carboxylate systems, an open site (usually occupied by H2O) is available for further coordination with suitable external ligand. Upon dipping MUDA/Au SAM decorated with Cu(II) ions into K4[Fe(CN)6] solution, three distinctive Raman bands at 2115 cm-1, 2093 cm1

and 2053 cm-1 appeared (1 cycle) which are characteristics of Cu(I)-CN (strong interaction),

Fe(II)-CN and Cu(I)---CN (weak interaction) bonds, respectively (Figure 2a and S8).4-5,

25

Interestingly, no Raman band at 2153 cm-1 characteristic of Cu(II)-CN bond was observed (Figure S8)5 – a clear signature of the reduction of Cu(II) to Cu(I). Also, the absence of Raman band at 2135 cm-1, characteristic of Fe(III)-CN bond, excluded the possibility of electron transfer from Fe(II) causing the IRR to take place. The fourth weak Raman band at 2075 cm-1 ( after 2 cycles of growth) is assigned to K---CN bond.26


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In a liquid phase reaction, without any confinement effect, ideally six CN ligands of ferrocyanide can approach to Cu(II) ion from all six directions in an octahedral geometry (± x, ± y and ± z) and as a consequence, coordination network with Fe(II)-CN-Cu(II) link could easily precipitated– without the reduction of Cu(II) to Cu(I).27 However, in the present study of solid-liquid interface, CN ligand can approach to Cu(II) ion from only four directions – two directions (-x and + y) are pre-occupied upon surface-anchoring to SAM. Now, Cu(II) has one partially filled orbital in the z-direction (3dz2) and the CN ligands approaching in ±z direction will try to transfer electron density from its filled 5σ orbital to reduce Cu(II) to Cu(I). Once Cu(I) is formed, all 3d-orbitals are filled and can further stabilize the Cu-CN bond synergistically upon back filling electron density from its filled 3dzx orbital to empty π* orbital of CN (Figure 2d). Two more CN approaching from remaining directions (+x and -y) will interact with Cu(I) in a non-linear fashion leading to comparatively weaker Cu---CN bond.15 Thus, unlike one type of Cu(II)-CN bond in a liquid phase reaction, geometrical constraints at the interface will make two types of bonds – stronger Cu(I)-CN bond at 2115 cm-1 (linear) and weaker Cu(I)---CN bond at 2053 cm-1 (non-linear) – as clearly evident from the Raman data.15 Such distinctive Cu-CN bonding is further supported by the gradual modulation of the intensity of the Raman bands at 2053 cm-1 and 2115 cm-1 beyond 1 cycle (Figure 2a and S8). Upon adding Cu(II), the Raman band at 2053 cm-1 was significantly diminished while the Raman band at 2115 cm-1 was retained (1.5 cycle). Subsequent dipping into K4[Fe(CN)6] solution (2 cycles) again generated both aforementioned Raman bands of Cu(I)-CN and Cu(I)---CN bonds. Thus, our Raman data not only complimented the XPS data but also eliminated the possibility of reduction of Cu(II) to Cu(I) by secondary electrons that could generate in an event of very long exposure to X-rays which is not the case.20 Typically, distance between two Fe ions in Prussian blue analogues


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(PBAs) is ~10 Å which is almost matching to the next nearest neighbor distance of S/Cu atoms in thiolate SAMs on Au surface (~10 Å)6, 28 and thereby allowing metal-ligand coordination vis-àvis maximization of charge neutrality at the confined environment – resembling various enzymatic reactions. We have recorded the UV-vis spectra of K4[Fe(CN)6] solution before and after 20 cycles of LbL growth and realized an oxidation of K4[Fe(CN)6] to K3[Fe(CN)6] in solution (Figure S10) which could be the probable source of electron causing the reduction of Cu(II) to Cu(I) at solidliquid interface. PBAs have been the subject of general interest to the chemistry community since decades, perhaps because of their ease synthesis, and interesting optical, electronic, magnetic and catalytic properties which can be further tuned with physical perturbations giving rise to electro-chromic, electro-catalytic, and photo-magnetic effect.6, 29-30 The unique bonding motif of biredox link allows ligand-to-metal, metal-to-ligand and metal-to-metal charge transfer (LMCT, MLCT and MMCT) interactions across three dimensions and enables PBAs to exhibit spectacular physicochemical properties.6 Among various biredox states in PBAs, it is the partially reduced state, which is prone to exhibit prominent MMCT band in the visible region of the electromagnetic radiation.31 Indeed, solid-state UV-vis absorption spectrum of bulk Cu-HCF sample showed a broad MMCT transition at ~700 nm, possibly due to electron transfer from Fe(II) to Cu(II) (Figure S11).31 However, the thin film Cu-HCF sample did not show such MMCT transition thereby affirming the presence of fully reduced Fe(II)-CN-Cu(I) state (Figure S10). MMCT band was also absent in K4[Fe(CN)6] due to the presence of fully reduced Fe(II)-CN-Fe(II) state.31 Note here that coordination network having Fe(II)-CN-Cu(I) link could not be achieved from both solid-liquid interfacial as well as liquid phase reactions despite using Cu(I) salt as precursor (Figure S3 and S4).


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Cu

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(111)

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200 nm

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Figure 3. (a) FESEM image and EDXS mapping (elements are mentioned) on the thin film having Fe(II)-CN-Cu(I) coordination link. (b) XRD pattern of bulk (powder) Cu-HCF and thin film samples. (c) TEM image of materials extracted from thin film sample (inset: corresponding SAED pattern). (d) Schematic of cubic to rhombohedral transformation upon insertion of potassium ions in PBAs.


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The reaction of Cu(I) and K4[Fe(CN)6] in liquid-phase did not form the expected Fe(II)-CNCu(I) link which can be understood on the basis of soft-hard acid-base (SHAB) principles. On one hand, Cu(I) and Cu(II) ions usually behave as soft acid and borderline acid, respectively. On other hand, the softness of CN- base was influenced upon coordination with Fe(II) ion, so as to say, [Fe(CN)6]4- could behave as a borderline/hard base. Thus, in a liquid-phase reaction and in stoichiometric condition (unlike at solid-liquid interface), formation of an extended network of the Fe(II)-CN-Cu(I) link is thermodynamically less favorable than that of the Fe(II)-CN-Cu(II) link. Also, in a layer-by-layer growth, hard base nature of the –COO- ion did not allow successful anchoring of the soft acid Cu(I) ion onto the surface of MUDA/Au SAM in the first step and subsequently prevented the formation of thin film bearing Fe(II)-CN-Cu(I) coordination link. Field emission scanning electron microscopy (FESEM) image presented in Figure 3a shows uniform and high-quality thin film involving Fe(II)-CN-Cu(I) coordination link. Elemental mapping by using energy dispersive X-ray spectroscopy (EDXS) technique revealed homogeneous distributions of elements (K, Cu, Fe, C and N) across the thin film (Figure 3a). An elemental composition of KxCu4-x[Fe(CN)6]x(=1.5±0.2).yH2O in the thin film material was estimated from a thorough EDXS analysis (Figure S12). To check crystallinity and structure of our thin film materials, out-of-plane X-ray diffraction (XRD) measurements were performed and compared with that of the bulk Cu-HCF. Two prominent diffraction peaks at 2 = 15.3o and 17.9o in the XRD pattern of bulk Cu-HCF were characteristics of PBA in cubic phase (Figure 3b).32 Interestingly, thin film material was realized to be majorly in rhombohedral phase whereby characteristic diffraction peaks at 2 = 14.8o, 19.5o, 21.3o were observed (Figure 3b).33 A minor contribution of the cubic phase in our thin film material was also noted with concomitant appearance of a diffraction peak at 2 = 15.3o. Major presence of the rhombohedral phase in our thin film material


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was further confirmed by transmission electron microscopy (TEM) measurement – a portion of the thin films was mechanically extracted by scratching, dispersed in ethanol and drop casted on carbon-coated copper grid. Selected area electron diffraction (SAED) pattern over a large area suggested a high-degree of crystallinity and confirmed the major presence of rhombohedral phase (Figure 3c). Grazing incidence X-ray diffraction (GIXRD) pattern recorded on thin film complemented our analysis on the out-of-plane X-ray diffraction (XRD) data (Figure S13). Now the question arises, how did a rhombohedral phase getting spontaneously deposited from a solidliquid interfacial reaction while a cubic phase was precipitated out from a liquid phase reaction? PBAs have general formula AxMA[MB(CN)6]y.nH2O, where MA and MB are transition metal ions and A is an alkali metal ion. If the ratio of M A/MB is equal to 1 then structure is perfect cubic i.e. without any vacancy defect, but upon deviation of this ratio from 1, the [MB(CN)6] vacancies will distort the structure from perfect cubic.33 Some fraction of water molecules and alkali metal ions are sheltered in the nanopores of framework and the remaining fraction of water molecules reside in the vacancy of the [MB(CN)6] site and coordinate to the MA site. In our thin film sample MA/MB(Cu/Fe) ratio is approximately 3 which clearly suggests presence of a large number of vacancy defects in the material (each interstitial site is occupied by K+ ion).33 It has already been shown that increasing the concentration of alkali metal ion in PBAs induces phase transition from cubic to rhombohedral and also the system changes from fully oxidised state (Fe(III)-CN-Fe(III)) to fully reduced state (Fe(II)-CN-Fe(II)).34 Usually, higher alkali metal ion concentration and fully reduced biredox state in complementarily favour rhombohedral phase in PBAs (Figure 3d).34 Hence, higher K+ ion concentration (from EDXS) and fully reduced state (Fe(II)-CN-Cu(I)) (from XPS and Raman) in our thin film justify the formation rhombohedral phase structure from an interfacial growth. Also, formation of linear and non-linear Cu(I)-CN bonds at solid-liquid


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interface – as was evidenced from our Raman data – could lead to rhombohedral structure in the thin film material. Interestingly, in our bulk Cu-HCF sample, the concentration of K+ ion was observed to be very low (less than 0.3%) which supported the cubic phase structure (Figure S14).

Inclusion and exclusion of alkali metal ion as well as water molecule in PBAs are known to induce structural change in the material.34 However, temperature-induced reversible structural phase transition in PBAs is rare unlike flexible MOFs.35 To investigate structural dynamics in our thin film bearing the Fe(II)-CN-Cu(I) coordination link, we have performed variable temperature XRD measurements and a remarkably change in the XRD patterns was observed. Upon heating the thin film from 300 K to 400 K, characteristic diffraction peak of the rhombohedral phase at 2 = 14.8o disappeared and a new diffraction peak at 2 = 16.3o appeared which we would like assign to a modified-rhombohedral and/or orthorhombic phase of PBAs (Figure 4a).33 Upon cooling down the sample temperature from 400 K to 300 K, original diffraction peaks reappeared and thereby revealing a stimuli-responsive nature of our thin film material. Based on the Raman spectroscopy data, such a temperature-induced reversible structural phase transition was primarily assigned to loss of water molecules from the coordination network (Figure 4b). In Raman spectra, characteristics vibrations of Fe---O (476, 510, and 590 cm-1) and Cu---O (350 cm-1) bonds at 300 K were disappeared at 400 K and again reappeared at 300 K.36-37 Usually, structural phase transition in a material is accompanied by a change in the physical property and therefore we were motivated to study temperature-dependence of the electrical transport across the thin film. Variable temperature current-voltage (I-V) characteristics were recorded on the thin film by using eutectic gallium indium (EGaIn) as top electrode in two probe instrument (Figure 4c). Indeed, electrical conductance was enhanced by two-order of magnitude upon increasing the temperature from 300


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K to 400 K and such a transition in electrical conductance value was observed to be reversible thereby corroborating the reversible structural phase transition in the thin film.

a

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300 K 400 K 300 K

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Figure 4. (a) Out-of-plane XRD patterns of the thin film at 300 K (blue), 400 K (green), and 300 K (red). (b and d) Temperature dependent Raman spectra on the thin film at 300 K (blue), 400 K (green), and 300 K (red). (c) Current-voltage (I−V) characteristics of thin film at 300 K (blue), 400 K (green), and 300 K (red) (Inset: schematic of the I-V measurements on thin film (brown) by using EGaIn as the top-electrode (silver spheres)).


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PBAs with partially reduced biredox link exhibit interesting electronic and magnetic properties due to thermally activated charge-transfer, in particular MMCT, for example, Co(III)-CN-Fe(II) ⇌ Co(II)-CN-Fe(III).38-39 What about our fully reduced Fe(II)-CN-Cu(I) link? To probe thermally activated charge-transfer process in such a fully reduced biredox link we have again explored temperature-dependent Raman spectroscopy and monitored the metal-CN vibrations (Figure 4d). Interestingly, band at 2093 cm-1 (Fe(II)-CN) was shifted to 2103 cm-1 at high-temperature and a shoulder peak was appeared at ~2135 cm-1 which is characteristic of Fe(III)-CN bond.22 Also, intensity of bands at 2054 cm-1 (Cu(I)---CN) and 2115 cm-1 (Cu(I)-CN) were decreases and negligibly small band at ~2150 cm-1 was appeared at high-temperature which is characteristic of Cu(II)-CN bond (Figure S15).5 Therefore, at room-temperature (300 K) Fe(II) and Cu(I) were present in majority whereas at high-temperature (400 K) mixed-oxidation states Fe(II)/Fe(III) and Cu(I)/Cu(II) were present. In principle, thermally activated conversion of Fe(II) to Fe(III) could take place at the cost of Cu(I) to Cu(0) which is apparently not an energetically feasible conversion so as the conversion of Fe(II) to Fe(I) at the cost of Cu(I) to Cu(II). Thus, we would like to attribute this unusual observation to thermally activated charge-transfer to the antibonding * orbital CN causing the sharp metal-CN bands at 300 K to be smeared at 400 K and facilitating the electronic transport across the material. Design and development of durable, low-cost, and highly-active electro-catalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is very much useful to improve the performance of energy storage devices such as fuel cells and batteries.40 PBAs are emerging in the domain of large scale energy applications such as storage materials of alkali as well as multivalent metal ions, cathode materials in rechargeable ion batteries and electro-catalysts in metal-air batteries.28, 32, 41 So far, PBAs involving Co, Mn, Fe, and Cu ions are mainly explored for OER


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activity, perhaps due to stability of various biredox systems at their partially reduced states under ambient conditions.28 Since a fully reduced state is synthetically difficult to achieve, exploration of the ORR activity using PBAs was really restricted. The here fabricated thin film could exhibit ORR activity due to the presence of a fully reduced state Fe(II)-CN-Cu(I) and therefore we have tested the ORR activity by employing cyclic voltammetry (CV) and linear scan voltammetry (LSV) techniques.

a

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Figure 5. (a) CV curves of thin film (blue lines) which was lastly dipped into K4[Fe(CN)6] solution and bulk Cu-HCF (black lines) samples (inset: schematic showing the ORR). (b) LSV plots


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recorded on the thin film (blue line) which was lastly dipped into K4[Fe(CN)6] solution and bulk Cu-HCF (black line) samples. (c) CV curves of thin film (red lines) which was lastly dipped into Cu(II)(OAc)2 solution (inset: schematic showing the ORR). (d) LSV plot recorded on the Cu(II) exposed thin film (blue line) which was lastly dipped into Cu(II)(OAc)2 solution. All measurements were performed in aqueous 0.1 M KOH electrolyte solution.

In the CV plot, onset potential for ORR was observed at around 0.820 V (vs RHE) (Figure 5a), which was further supported by the LSV curve exhibiting the onset potential at ~0.826 V (vs RHE) (Figure 5b). The over potential of Fe(II)-CN-Cu(I) system when compared to commercial catalyst (Pt/C) could be realized around 174 mV which is remarkable. The E1/2 value was found to be 0.723 V that indicates fast ORR kinetics.42 Overall, the onset potential and E1/2 value of our thin film material is comparable or greater than other reported electro-catalysts. ORR was also performed in N2 saturated electrolyte where oxygen reduction peak was absent (Figure S16a). Further, we have performed the stability test, even after 100 continued cycles the oxygen reduction peak position as well the features of the initial CV plot were almost retained thereby strongly suggesting high-durability of the thin film of Fe(II)-CN-Cu(I) coordination link in electro-catalytic reactions such as ORR (Figure S16b). Notably, under similar conditions, bulk Cu-HCF (bearing the Fe(II)CN-Cu(II)) drop casted on Au wafer did not show any ORR activity; flat curves were observed in both CV and LSV scans (Figure 5a,b). Therefore, redox centres Fe(II) and Cu(I) in thin film could be synergistically participating in the ORR activity and increasing the overall performance which is a rare example in the scenario of biredox and composite metal ion systems.43 Catalysis in general is an interfacial phenomenon and in a recent study, an important role of the interface on the ORR activity of the supported Ni/Pt catalyst has been elegantly demonstrated.44


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We were motivated to investigate the interfacial effect, if any, on the ORR activity of the thin films of Fe(II)-CN-Cu(I) coordination link supported on Au substrate. The aforementioned electrochemical measurements (CV and LSV) on the ORR activity were performed on thin film sample that was lastly dipped into K4[Fe(CN)6] solution. Remarkably, the sample that was lastly dipped into Cu(OAc)2 solution did not show any ORR activity, flat curves in the CV and LSV scans (likewise bulk Cu-HCF sample) were observed (Figure 5c,d). Since our thin films were fabricated by employing layer-by-layer approach, chemical composition of the outermost layer in thin films was distinctive in each step. There could be an admixture of Cu(II) and Cu(I) ions when the sample was lastly dipped into Cu(OAc)2 solution whereas Cu(I) ions were predominantly present in synergistic bonding with Fe(II) ions when the sample was lastly dipped into K4[Fe(CN)6] solution. Thus, our results clearly revealed an interfacial effect on the ORR activity and justifying the need of engineering of thin films of various other materials before applying for various electro-catalytic reactions. In conclusion, we have provided conclusive evidence for an interfacial reduction of Cu(II) to Cu(I) resulting in the fabrication of high-quality thin films of coordination network having Fe(II)CN-Cu(I) link. Electron transfer from neither Fe(II) nor Au substrate was found to be responsible for the interfacial reduction reaction; rather a confinement effect of restricted coordination at a solid-liquid interface is proposed to facilitate the reduction of Cu(II) by CN. Thin films fabricated by IRR exhibited interesting dynamics in structure and electronic property. Electro-catalytic activity of the fully reduced Fe(II)-CN-Cu(I) coordination link in the form of ORR was noticeable due to synergetic effect of both Cu(I) and Fe(II) and an important interfacial effect in ORR was demonstrated. Our thin film comprised of Fe(II)-CN-Cu(I) coordination link is a hitherto new material in the domain of PBAs and the concept of IRR could provide new opportunities and


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challenges in developing various electro-active coordination linkages upon simply arresting them at the solid-liquid interfaces which are otherwise difficult to achieve employing liquid-phase reactions.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Data on experimental section and layer-by-layer growth, XPS, Raman, EDXS, FESEM, and electro-chemical measurements (CV and LSV) (PDF) Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Financial support from SERB (India, Project No. EMR/2016/ 001404), MHRD-FAST (India, Project − CORESUM), DST Nanomission (India, Project No. SR/NM/TP-13/2016) and IISER Pune is thankfully acknowledged. S.R. and P.K.J. thank IISER Pune for providing Senior Research Fellowships. A.P. thanks DST-KVPY for a fellowship. The authors sincerely acknowledge the UGC-DAE Consortium for Scientific Research, Indore (India) for XPS and Raman data.

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Spontaneous Reduction of Copper(II) to Copper(I ... - ACS Publications

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