Highly dense nickel hydroxide nanoparticles catalyst electrodeposited from a novel Ni(II) paddle–wheel complex

Journal of Catalysis 329 (2015) 22–31

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Highly dense nickel hydroxide nanoparticles catalyst electrodeposited from a novel Ni(II) paddle–wheel complex Emiliano Martínez-Periñán a, Marcello Gennari b,1, Mónica Revenga-Parra a, José M. Abad a,c, Eva Mateo-Martí d, Félix Pariente a, Oscar Castillo e, Rubén Mas-Ballesté b, Félix Zamora b,c, Encarnación Lorenzo a,c,⇑ a

Departamento de Química Analítica y Análisis Instrumental, Universidad Autónoma de Madrid, 28049 Madrid, Spain Departamento de Química Inorgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain IMDEA-Nanoscience, Faraday 9, Campus Cantoblanco-UAM, 28049 Madrid, Spain d Centro de Astrobiología (CSIC-INTA), Ctra. Ajalvir, Km. 4, 28850, Torrejón de Ardoz, Madrid, Spain e Departamento de Química Inorgánica, Universidad del País Vasco (UPV/EHU), Apartado 644, E-48080 Bilbao, Spain b c

a r t i c l e

i n f o

Article history: Received 28 January 2015 Revised 24 March 2015 Accepted 3 April 2015

Keywords: Dinickel complexes Paddle–wheel complexes On-surface electrosynthesis Nickel hydroxide nanoparticles Oxidative electrocatalysis

a b s t r a c t Nickel (II) hydroxide nanoparticles with very high oxidative electrocatalytic activity toward sugars have been prepared on different electrodes by a simple electrochemical procedure consisting of the following: (i) electro-oxidation of a dinickel precursor that produces microstructures of a novel paddle–wheel tetrakis-acetato dinickel (II) complex and (ii) subsequent alkaline treatment of these microstructures to produce nickel (II) hydroxide nanoparticles on the electrode. The novel paddle–wheel complex has been characterized by energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and mass spectrometry. The proposed electrochemical mechanism for the surface generation of this complex involves coupling of homogeneous chemical reactions to heterogeneous electron-transfer reactions. The high and homogeneous coverage and the 1–3 nm size of the nickel hydroxide nanoparticles generated play key roles in the electrocatalytic efficiency and the reproducibility toward the oxidation of several sugars. The high sensitivity values obtained prove the utility of the method to develop nickel hydroxide-modified electrodes for sensing and electrocatalysis applications. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction Electrocatalytic materials are a subject of current intense study in a wide variety of research areas, such as chemical synthesis, fuel cell catalysis, energy storage, and electrochemical sensors [1–4]. In this context, nickel-containing materials are attractive because of their earth-abundant nature and its catalytic activity [5]. In particular, nickel hydroxide micro-/nanostructures have gained increasing research attention in view of their potential applications as fuel cell catalysts [6,7], alkaline rechargeable batteries, and electrocatalysts for organic synthesis [8,9]. However, most of these properties are connected to their morphology and size. The different chemical routes used to fabricate on-surface nickel hydroxide structures, such as sol–gel, solvothermal methods, and ⇑ Corresponding author at: Departamento de Química Analítica y Análisis Instrumental, Universidad Autónoma de Madrid, 28049 Madrid, Spain. E-mail address: [email protected] (E. Lorenzo). 1 Present address: Universite Grenoble Alpes, DCM and CNRS, DCM, F-38000 Grenoble, France. http://dx.doi.org/10.1016/j.jcat.2015.04.010 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

sonochemistry, have resulted on thin film’s formation [10,11]. Additionally, a variety of different nickel hydroxide morphologies including nanoplates [12,13], hollow spheres [14,15], ribbon-like and board-like structures [16,17], flower-like structures [18,19], and tubes [20] have been also isolated. In principle, the use of nickel hydroxide for electrocatalytic applications could be more efficient using porous nanosized structures deposited on surfaces. However, there are few reports in the literature that show the production of isolated Ni(OH)2 nanoparticles on an electrode surface via the electrochemical generation of nickel hydroxide directly in presence of Ni2+ [21]; rather, thin film structures tend to be formed. In this context, an intuitive approach to obtain nickel hydroxide nanostructures directly deposited on the electrode surface could be based on the in situ direct formation of nickel-based structures on the electrode as precursors to form on-surface nickel hydroxide nanoparticles by alkaline treatment. It is known the ability of some metal–organic compounds is to be electrochemically deposited on the surface electrodes [22]. Therefore, selected metal–organic compounds could be excellent precursors for this goal. On the basis of recent results [23], and

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in part motivated by the high potential of electrochemistry for electrogenerating organized nanomaterials on the electrode surfaces, we have attended the on-surface formation of metal–organic structures using electrochemistry as an alternative synthetic procedure that precludes contamination. The electrocatalytic ability of the Ni(II)/(III) redox couple toward the electro-oxidation of small organic molecules has been reported [24]. In this investigation, and later experiments by a number of researchers [24–27], it was determined that the Ni(OH)2, present at the electrode, is initially oxidized to the catalytically active NiOOH species, which subsequently irreversibly oxidizes small organic analytes. Although the use of nickel electrodes in glucose detection has been reported over the past few decades [28–30], the fabrication methods and electrode modifications have been quite elaborate, which is the exception of a few simpler procedures [24,31–33]. Herein, we have focused our attention on a particular paddle– wheel dinickel tetrakis-thioacetato complex, [Ni2(CH3COS)4EtOH] (1) (Scheme 1), as precursor in an electrodeposition process. We demonstrate the formation of nickel-based microstructures on different electrode surfaces. The electrodeposited structures have been characterized by X-ray photoelectron spectroscopy (XPS) and mass spectrometry suggesting the formation of a novel dinuclear Ni(II) acetato paddle–wheel complex (2). No methods for the chemical synthesis of this complex in solution are described so far. These structures are suitable to form nickel (II) hydroxide nanoparticles deposited on the electrode surface upon strong basic conditions. To the best of our knowledge, there are no reports in the literature that show the production of isolated Ni(II)-based structures on electrode surfaces via electrochemical-driven deposition from a Ni(II)-based precursor in non-aqueous media; rather, structures in aqueous solution tend to be produced [33]. In alkaline aqueous solutions, the dinuclear Ni(II)-acetate-based structures originate nickel (II) hydroxide nanoparticles on the electrode surface with a potent and persistent electrocatalytic activity toward the oxidation of different sugars. 2. Experimental 2.1. Materials All reagents and solvents were purchased from Sigma-Aldrich and used as received. Aqueous experiments were carried out with deionized water from a Millipore system. 2.2. Synthesis of [Ni2(CH3COS)4EtOH] (1) Compound 1 was prepared according to a slight literature modification [34]. A warm ethanolic solution of thioacetic acid (2.866 g,

Scheme 1. Schematic representation of the structure of starting dimetallic complex [Ni2(CH3COS)4EtOH] (1) and its tetracarboxylate dimetallic counterpart [Ni2(CH3COO)4] (2).

23

0.037 mol) was treated with [NiCO32Ni(OH)24H2O] (3.979 g, 0.018 mol) in a portion-wise fashion. Color changed from light orange to deep red in about 30 min, and the mixture was stirred further for 4 h. The suspension formed was filtered and the precipitated discarded, and filtrate was allowed to stand overnight at 10 °C to yield deep red plate-like crystals, 1.854 g (43%). 1H NMR spectra were recorded on a Bruker AMX-300 spectrometer. C, H, S elemental analyses were performed on a Perkin–Elmer 240-B microanalyser. Electronic absorption spectra were recorded on an Agilent 8452 diode array spectrophotometer over a 190–1100 nm range in 0.1-, 0.2-, and 1-cm quartz cuvettes thermostated by a Unisoku cryostat. Aqueous experiments were carried out with deionized water from a Millipore system. 2.3. X-ray diffraction data collection and structure determination The single-crystal X-ray diffraction data collection was done at 293(2) K on an Oxford Diffraction Xcalibur diffractometer with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). The data reduction was done with the CrysAlisPro program [35]. All the structures were solved by direct methods using the SIR92 program [36,37] and refined by full-matrix least squares on F2 including all reflections (SHELXL97) [38]. All calculations for these structures were performed using the WINGX crystallographic software package [39]. The specimen employed for crystal structure determination was non-merohedrically twinned with the following twin law: (1.000 0.000 0.000–0.023–0.535–0.767 0.047–0.931 0.535). The final results showed a percentage for the minor domain of 38%. Crystal parameters and details of the final refinement are summarized in Table S1. 2.4. Electrochemical measurements All electrochemical measurements and the electrosynthesis of compound 2 were performed with a potentiostat Autolab PGSTAT128N (EcoChemie, NL) using the software package GPES 4.9 (General Purpose Elec. Experiments). For conventional three-electrode experiments, a homemade single-compartment electrochemical cell was employed. Glassy carbon (GC) and gold (Au) electrodes from CH Instruments were used as working electrodes and Pt wire as counter electrode. Specific calomel electrode (1 M LiCl for organic media from Radiometer Analytical) and calomel electrode were used as reference electrodes for experiments in organic solvents and aqueous environment, respectively. For planar highly oriented pyrolytic graphite (HOPG) and gold AFM plates (12 mm  12 mm, Arrandee™ Supplies, Germany), a homemade Teflon electrochemical cell with an active window of 0.6 cm2, in which the HOPG or Au substrate electrodes were sandwiched between an O-ring in the Teflon body and the base, was used. Rotating disk-ring electrode (RRDE) measurements were carried out using a bipotentiostat CHI900B (CH Instruments) and a glassy carbon disk/platinum ring RRDE electrode from PINE. Prior to the experiments, all solutions were centrifuged at 7000 RPM during 5 min to avoid suspension particles and deoxygenated by bubbling nitrogen for 5 min. For sugar electrocatalysis studies, freshly prepared solutions were employed. All experiments were carried out at room temperature. Electrochemical quartz crystal microbalance (EQCM) experiments were carried out using a SRS QCM200 Quartz Crystal Microbalance from SRS Instruments (Sunnyvale, CA, USA) coupled with an Autolab PGSTAT 128N potentiostat using NOVA 1.5 software package. 5 MHz AT-cut gold–chromium-coated crystals (1 inch diameter) from SRS were used as working electrodes. Calomel and platinum wire coil were used as reference and counter electrode, respectively. Cyclic voltammetry scans at 0.01 V/s from 0 V to 1.5 V were applied on the working electrode immersed in

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a solution containing 2.5  104 M of 1 in 0.1 M tetrabutylammonium perchlorate (TBAP)/chloroform (CHCl3). 2.5. Microstructures electrogeneration Microstructures of compound 2 were grown on the electrode surface from a solution containing 2.5  104 M of compound 1 in 0.1 M TBAP/CHCl3 by applying 1.15 V during 150 s. For mass spectra analysis, an Au coil (with much more electrochemical active surface than an Au disk electrode or an Au AFM plate) was used as working electrode immersed in 10 ml of a solution containing 2.5  104 M of compound 1 in 0.1 M TBAP/CHCl3, and a constant potential of 1.15 V was applied during 7000 s (until it has consumed all the starting material and lost the initial color). After that, the electrodeposited material was dissolved in pure acetonitrile (CH3CN), and this solution was analyzed by ESI-MS.

S

O O O

S

Ni

Ni O O

S

S

2.6. Characterization measurements Field emission scanning electron microscopy (FE-SEM) measurements were performed with a NOVA NANOSEM 230 equipment (FEI-SEM) operating with a VCD (low voltage, high contrast) and using low landing potentials (as low as 300 V) in order to probe the surface features. FE-SEM was additionally equipped with an energy-dispersive X-ray (EDX)-Ametek detector that allowed semi-quantitative analysis of elements. X-ray photoelectron spectroscopy (XPS) analysis of the samples was carried out in an ultra-high vacuum chamber equipped with a hemispherical electron analyzer, and using an Al Ka X-ray source (1486.6 eV) with an aperture of 7 mm  20 mm. The base pressure in the chamber was 5  1010 mbar, and the experiments were performed at room temperature. The following core-level peaks were recorded under the same experimental conditions: O(1s), C(1s), S(2p), Ni(2p), and Au(4f). The pass energy applied for taking the overview sample was 30 eV, while 20 eV pass energy was applied for the fine analysis of the core-level spectra. The core-level binding energies were calibrated against the binding energy of the Au(4f7/2) peak set to 84.0 eV for the gold surface sample. The peak deconvolution in different components was shaped, after background subtraction, as a convolution of Lorentzian and Gaussian curves. Lorentzian and Gaussian widths of 0.1 and 1 eV, respectively, common for all the components, were used. Mass spectrometry analysis was carried out in an AB Sciex mass spectrometer with QTOF hybrid analyzer model QSTAR pulsar. Electrospray was used as ionization technique, and CH3CN was used as ionization phase. Atomic force microscopy (AFM) images were acquired in dynamic mode using a Nanotec Electronica system operating at room temperature in ambient air conditions. For AFM measurements, Olympus cantilevers were used with a nominal force constant of 0.75 N/m. The images were processed using WSxM. The surfaces used for AFM were HOPG (NTI Europe Company) and gold AFM plates (12 mm  12 mm, Arrandee™ Supplies, Germany). In order to obtain reproducible results, very flat substrates were used with precisely controlled chemical functionalities, freshly prepared just before the chemical deposition. HOPG was cleaved with adhesive tape.

Fig. 1. X-ray structure of the paddle–wheel [Ni2(l-thioacetate)4(EtOH)] (1) entity showing the labeling scheme.

through the apical position of one of the nickel (II) metal centers of the dimeric unit to give rise to discrete almost linear tetrameric [Ni4(l3-thioacetate)2(l-thioacetate)6(EtOH)] entities (Fig. S1). The two nickel atoms in the [Ni2(l-thioacetate)4(EtOH)] units are bridged by four thioacetate ligands, so that one metal atom is surrounded by four sulfur atoms in a square-planar fashion and the other by four oxygen atoms in a similar manner. The two planes are twisted about the Ni–Ni axis by 21–24° from an eclipsed configuration. The Ni–S distances, 2.216–2.235(2) Å, are characteristics of square-planar, low-spin complexes while the Ni–O distances, 2.003–2.073(4) Å, are compatible for those found in high-spin complexes [40]. The fifth coordination site of the second nickel atom is occupied by the oxygen atom of the ethanol moiety at a distance of 2.03 Å, whereas in the first nickel atom is occupied by a sulfur atom from an adjacent [Ni2(l-thioacetato)4(EtOH)] unit at a considerably longer Ni  S distance (2.682 and 2.839 Å, respectively) to form centrosymmetric tetramers. These tetramers are further linked together by means of strong O–H  O hydrogen bonds involving the ethanol hydroxyl group as donor to an in-plane oxygen atom of an acetate ligand from a neighboring molecule. It gives rise to almost linear supramolecular chains of the [Ni4(l3-thioacetate)2(l-thioacetate)6(EtOH)] entities in which the metal  metal distances across this supramolecular hydrogen bonding are relatively close (3.789 and 3.918(2) Å, respectively). The supramolecular chains are packed parallel by means of weak van der Waals interactions (Fig. S1). It is worthy to note that asymmetric paddle wheels of [M2(l-thiocarboxylate)4] complexes are very scarce being in most cases related to heterometallic complexes [41]. Taking into account thiocarboxylate paddle–wheel-shaped homometallic complexes, the symmetric configuration with NiO2S2 square-planar environments is found in all cases [42–45] except for a previous example of nickel–thiobenzoate complex [34,46] that shows a crystal structure closely related to that described here.

3. Results and discussion 3.2. On-surface formation of [Ni2(CH3COO)4] (2) 3.1. Structural description of compound 1 Compound 1 consists of two crystallography distinguishable paddle–wheel-shaped [Ni2(l-thioacetate)4(EtOH)] units (Fig. 1) that are held together by means of additional Ni–S interactions

The formation of structures electrochemically driven on electrode surfaces is a novel synthetic procedure, which provides alternative routes for the deposition of controlled structures precluding contamination. Taking as precursor the dimetal subunit compound

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1, we have studied its electrochemical behavior at GC, HOPG, and Au electrodes. It was investigated by cyclic voltammetry in 0.1 M TBAP/CHCl3. The sweep toward the positive region (from 0.0 to 1.5 V) reveals a well-defined oxidation peak at 1.20 V (Fig. 2A). The reverse cathodic scan exhibits no waves unless high sweep rates (0.5 V/s) are used. At this scan rate, a poor-defined and small wave can be observed. A detailed study of the cyclic voltammetric behavior at different scan rates confirms that the cathodic wave strongly depends on the sweep rate being undetectable at 0.1 V/s or lower. In contrast, the oxidation peak appears at all the scan rates assayed increasing in current as the sweep rate increases. These results suggest that the oxidation mechanism is irreversible and complex with homogeneous chemical reactions coupled to heterogeneous electron-transfer reactions at electrode surface since the effect of the chemical reaction depends on the sweep rate. Similar behavior was observed when Au and HOPG electrodes were employed. In order to evaluate the potential solvent effect, similar electrochemical measurements of 1 were carried out in CH3CN containing also 0.1 M TBAP as supporting electrolyte. The electrochemical behavior of 1 was similar to that obtained in CHCl3 but around 0.1 V less positive. The number of electrons (n) involved in the oxidation process was determined by both chronoamperometric and chronocoulombimetric experiments. According to Eq. (1), to estimate n, the value of diffusion coefficient (D⁄) is needed. It was determined using a RRDE following the method reported by Chatenet et al. [47,48] and described in the Supporting Information. From Eq. (2), a plot of ts versus K/x results in a straight line crossing the origin, whose slope gives a diffusion coefficient for [Ni2(CH3COS)4EtOH] of 6.80  106 cm2/s. Once determined D, the value of n was estimated from Eq. (1). It was found to be 4.

Q ¼ 2n  F  C  D1=2  t 1=2  p1=2  A 1=3

t s ¼ K  ðm=DÞ

=x

25

shape of a surface-confined redox process and a formal potential E° = 0.63 V. It is worthy to note that cycling repeatedly the potential between 0 V and 1.5 V did not cause a noticeable decrease in the peak currents (less than 1% after 100 cycles), confirming the good chemical and electrochemical stability of the resulting modified electrode. The electrodeposition of the compound 1 oxidation product was also investigated by electrochemical quartz crystal microbalance (EQCM), which is a powerful tool to study interfacial processes at electrode surfaces. The gold quartz crystal resonator was used as working electrode, providing simultaneously real-time monitoring of mass changes onto the gold electrode surface associated to the electrochemical process. After temperature and frequency had stabilized, cyclic voltammetry was carried out. Fig. 2B shows the resulting frequency changes as a function of successive potential scans. As can be seen on the subsequent scans, as the scan goes to oxidation potentials, a decrease in the resonant frequency of gold resonator occurs at potentials above 1.0 V in each cycle. This is in agreement with the formation of some electrogenerated material on the electrode surface. As can be observed, the decrease in frequency (Df) was almost constant in each cycle (95 ± 10 Hz), which confirms that the deposition process is electrochemically driven. From Sauerbrey´s equation, Eq. (3), the corresponding mass binding quantity (Dm) from decreases observed in the crystal frequency magnitude (Df) was found to be 1761 ng/cm2.

Dm ¼ Cf  Df

ð3Þ

The electrodeposition of material can also be performed by holding the potential at 1.15 V for a period of time.

3.3. Field emission scanning electron microscopy of electrodeposited material

ð1Þ ð2Þ

When in a solution of compound 1 in 0.1 M TBAP/CHCl3 the potential was successively cycled between 0 and 1.5 V at a scan rate of 0.010 V/s in the stationary conditions of electrode and electrolyte, the shape of the voltammograms shows a decrease in peak current upon the successive cycles, indicating that a non-conductive material is electrodepositing on the electrode surface. If the resulting modified electrode is removed from the solution, washed thoroughly with CHCl3 to remove material weakly adsorbed and placed in an electrochemical cell containing only the supporting electrolyte (0.1 M TBAP/CHCl3), the voltammogram obtained (Fig. S2) shows a redox couple with the characteristic

FE-SEM experiments verified the presence and morphology of the material electrodeposited onto the electrode surface. The FE-SEM images (Fig. 3) of the modified electrode show that the material is electrodeposited as micrometric three-dimensional structures such as crystals raised from a platform of about 2 lm of diameter randomly distributed on the gold surface. By simply controlling the electrodeposition time is possible to change the shape and size of the structures formed. Fig. 3 shows typical FE-SEM images of the modified electrodes obtained by holding the potential at 1.15 V during 150 s and 600 s, respectively. The FE-SEM images confirm that the structures generated within 150 s (Fig. 3A) consist of rod-like microstructures with dimensions ca. 2–3 lm in length and a width ca. 300 nm. Longer

Fig. 2. (A) Cyclic voltammetry scans of 2.5  104 M compound 1 at a GC electrode in 0.1 M TBAP/CHCl3 at different scan rates: 0.01, 0.05, 0.1, 0.2, 0.5 V/s. (B) In situ frequency changes with successive voltammetric cycles on a gold quartz crystal electrode in a 2.5  104 M solution of compound 1 in 0.1 M TBAP/CHCl3 (scan rate 0.01 V/s).

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Fig. 3. FE-SEM images of electrodeposited nanostructures at 1.15 V for 150 s (A) and 600 s (B) electrodeposition time, respectively.

electrodeposition times, 600 s, produce a general increase in the length while the width remains constant (Fig. 3B). Finally, EDX was carried out to determine the elemental composition of the resulting electrogenerated structures. The data obtained as atomic percentages C (20.74%), O (32.30%), Ni (13.53%), and S (0.00%) confirmed the presence of Ni, C, and O and the absence of S. These data suggest the transformation from thioacetate ligand into acetate in the electrodeposition process, as well as the formation of a nickel (II) acetate complex.

Table 1 Binding energies in eV found for C (1s), O (1s), Ni (2p1/2), and S (2p) of starting material [Ni2(CH3COS)4EtOH] (1), its product of electro-oxidation [Ni2(CH3COO)4] (2), and the final material Ni(OH)2 from further evolution in basic media.

C (1s) O (1s) Ni (2p1/2) S (2p)

(1)

(2)

Ni(OH)2

286.5 531.6 856.0 164.2

286.4 531.2 856.3 –

– 530.4 856.8 –

3.4. Electronic absorption spectra The electronic spectrum of compound 1 before and after 5500 s of electrolysis at +1.15 V was recorded in a CHCl3 solution in the range of 200–800 nm (Fig. S3A). It shows an intense absorption band at 370 nm and a weak band at 460 nm. Upon electrolysis, both absorption bands disappear. Since the electrodeposited material is soluble in CH3CN, its electronic spectrum in this solvent was recorded and compared to the spectrum of compound 1 in CH3CN (Fig. S3B). The electrogenerated material, compound 2, does not show the characteristic absorption bands of the precursor (at 390 and 500 nm), but shows bands below 300 nm, indicating the presence of different species in solution. In order to compare, we tried to obtain the electronic spectrum of mononuclear nickel (II) acetate but it was not soluble in CH3CN. Therefore, these data indicate that the electrogenerated compound 2 consists of a material with different structures of both the precursor 1 and the known mononuclear nickel (II) acetate.

3.5. On-surface X-ray photoelectron spectroscopy of 2 The chemical identity of the modified electrode surface was investigated by XPS. Analysis of both the precursor compound 1 (deposited by drop-casting on the gold electrode) and the electrogenerated compound 2 on gold electrodes was performed in order to achieve chemical information related to the electrochemical process that leads to the electrodeposition of microstructures. In the wide scan, XPS overview spectra of the precursor the following atomic species can be identified (Table 1): O, C, S, Au, and Ni. The binding energies of peaks at 286.5.eV and 531.6 eV, which correspond to the carbon and oxygen region respectively, can be assigned to the monothiocarboxylate group. The Ni 2p1/2 peak appears at ca. 856.0 eV, characteristic of Ni acetate compound, and sulfur at 164.2 eV [49]. Therefore, from the characteristic

binding energies of C, O, Ni, and S, we can confirm its composition upon adsorption on gold. After the compound 1 was electrochemically oxidized at 1.15 V for 150 s, the sulfur peak in this region of the XPS spectra disappears confirming the conversion from monothiocarboxylate into carboxylate group; furthermore, the binding energies of carbon and oxygen components at 286.4 and 531.2 eV respectively confirm the presence of the carboxylate group. The Ni 2p1/2 peak appears at ca. 856.3 eV characteristic of Ni(II) acetate compound [50]. Therefore, these results agree with the absence of sulfur atoms in 2 and support that the electrodeposited microstructures may correspond to a Ni(II) acetate complex. 3.6. Mass spectrometry of 2 Compound 2 was dissolved in CH3CN and analyzed by ESI-MS both at negative and at positive modes. The major part of the peaks just confirms the presence of Ni(II) as it is observed for m/z(+) = 90.5 assigned to [Ni(CH3CN)3]2+, m/z(+) = 111 assigned to 2+ [Ni(CH3CN)4] , m/z(+) = 239 assigned to [Ni(ClO4)(CH3CN)2]+, and m/z() = 357 assigned to [Ni(ClO4)2]. However, the signal detected at m/z() = 369 (Fig. S4), according to its position and isotope pattern distribution, can be assigned to [Ni2(CH3COO)4(OH)]–. Such ion suggests that compound 2 consists of a novel dimetallic paddle–wheel Ni(II) acetate complex. A recent search in the crystallographic structure database of Cambridge shows 33 structures reported on dimetallic paddle–wheel Ni(II) carboxylate complexes; however, the dimetal complexes were isolated by coordination of amines or phosphines at the axial position, and just two examples are reported with oxo-ligands at these sites (tetrakis (l2-benzoato-O,O0 )-bis(benzoic acid-O)-di-nickel [51] and tetrakis(l2-propionato)-diaqua-di-nickel [52]). Therefore, the

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electrosynthesis provides an alternative chemically clean reaction environment to produce generated compounds on surfaces.

scan 1

150

50

3.7. Suggested electrochemical mechanism

Scan

3.8. Electrochemical behavior of microstructure-modified electrodes in alkaline aqueous media The electrochemical behavior of GC, Au, and HOPG microstructures-modified electrodes in 0.1 M NaOH aqueous solution was investigated. Similar results were obtained in all the cases; thus, the discussion has been focused on the GCE. Nevertheless, the cyclic voltammograms obtained for Au and HOPG electrodes are presented in Fig. S6. In the absence of the

I / µA

100

The overall data presented above clearly indicate that a four-electron oxidation process results on the transformation of compound 1 into compound 2 electrodeposited on the electrode surface as microstructures. Thus, it is reasonable to assume that one electron is removed from each sulfur atom in the starting material, resulting on the formation of disulfide species. Analogously, it is reported for monometallic nickel (II) compounds spontaneous formation of S–S bonds generating perthioacetate species from the thioacetate starting material [53]. In the present case, it seems that the bimetallic structure imposed by the starting material imposes formation of bimetallic Ni(II) acetate instead of monometallic perthiocarboxylates, by replacing all the sulfur atoms by oxygen. The most available source of oxygen to substitute the sulfur atoms in [Ni2(CH3CSO)4EtOH], to form [Ni2(CH3COO)4], appears to be water, which is ubiquitous in experiments at open atmosphere as the ones described herein. Thus, several processes should concur: (a) electro-oxidation of sulfur centers, (b) Ni–S bond cleavage, (c) S–S bond formation, (d) S–C bond cleavage, (e) C–O bond formation, (f) proton transfer from the oxygen to the formed disulfide, and (g) Ni–O bond formation. At what degree this bond forming/cleavage succession is concerted is unclear, but the overall pathway appears in the CV as a single four-electron signal, indicating cooperative multielectron transfer which is coupled to a chemical process that finally results in formation of [Ni2(CH3COO)4] as shown in Scheme 2. In order to evaluate the potential role of water in the electrogeneration of compound 2, according to the proposed mechanism, we have carried out the electro-oxidation of compound 1 under the same conditions described above but using water-saturated dry chloroform in the electrolyte. The optical microscope image of the resulting electrode surface, compared to that obtained when dry chloroform was used (Fig. S5A), shows the generation of the same micrometric three-dimensional structures but in the presence of high water content higher microstructures surface coverage is observed (Fig. S5B). This fact confirms the important role that water plays in the electrogeneration of these microstructures.

1 50

0

-50 0.0

0.2

0.4

0.6

E/V Fig. 4. Cyclic voltammograms of compound 2 microstructures GC-modified electrode in 0.1 M NaOH.

modifying layer, the voltammetric response is a typical background for GCE, that is, essentially a capacitive response with no faradaic process over the same potential range. However, upon modification, the response (Fig. 4) is dramatically different. The first oxidation scan from 0.0 to 0.7 V exhibits a broad anodic peak at 0.55 V with a small pre-peak (at 0.45 V) and a broad cathodic peak at 0.35 V. On subsequent scans, the voltammetric waves drastically change. The broad anodic peak disappears after the second scan. In contrast, a pair of very well defined peaks whose intensity grows with the number of scans, and then levels off, appears at E° = 0.43 V. We believe that after the first scan in alkaline aqueous media, Ni(OH)2 is generated on the electrode surface from electrodeposited compound 2. Thus, the well-defined redox couple at 0.43 V can be ascribed to the following process, Eq. (4):

NiðOHÞ2 þ OH $ NiOOH þ H2 O þ e

ð4Þ

The Ni(OH)2 exhibited the voltammetric shape typical of oxidized surfaces and the significant DEp is typical of such process. In fact, counterion transport can be limiting and thus the large DEp value [54]. In addition, the peak current scales with the square root of the sweep rate over the range of 0.02–0.75 V/s, as anticipated for a diffusion-controlled process; again likely counterion transport. These results indicate that at low scan rates the system is electrochemically quasi-reversible, suggesting facile charge-transfer kinetics. When the potential is cycled at high scan rates, the DEp increases. This electrochemical irreversibility is indicative of serious limitations in the kinetics of charge transfer at these conditions.

Scheme 2. Suggested electrochemical mechanism of the [Ni2(CH3COO)4] electrodeposition process and further evolution to Ni(OH)2 in basic media.

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of the electrode (0.07 cm2). After 50 cyclic scans, the C value was calculated to be 6.53  109 mol/cm2. Considering the mass loading of compound 2 deposited on the electrode, measured by EQCM, and the final mass loading of Ni(OH)2, calculated from the C, we estimated that about 90% of prepared compound 2 was converted to Ni(OH)2.

Although the Laviron formalism is generally applicable to simple redox reactions, and in this case counterion transport could be rate limiting, taking into account that the redox species are immobilized on the electrode surface, the anodic and cathodic peak potentials were plotted vs log scan rate (v) in a typical Laviron’s plot (Fig. S7). At scan rates higher than 5 V/s, the peak potential depicts a linear dependence with log (v). From the ratio of the slopes of these straight lines, the apparent surface electron-transfer rate constant (ks), as well as the electron-transfer coefficient (a), was determined through the Laviron’s equations [55]; a and ks were found to be 0.4 and 24.5 s1, respectively. Calculation of the charge associated with oxidation (QOx) of Ni(OH)2 and subsequent reduction (QRed) of the electrogenerated NiOOH gives a value for QOx/QRed of 1.1. From QOx, the effective surface concentration, C, on the electrode surface can be calculated by the following Eq. (5):

C ¼ Q Ox =nFA

3.9. Structure and morphology of the Ni(OH)2-modified electrodes FE-SEM and AFM were carried out to characterize the surface morphology of the Ni(II) hydroxide-modified electrodes. FE-SEM images (Fig. S8) for the gold-modified electrodes (3.16  109 mol/cm2) show that instead the electrodeposited compound 2 microstructures described above aggregates of nanoparticles over the surface can be appreciated. EDX measurements were carried out to determine the elemental composition of the resulting Ni(OH)2 nanoparticles. The data obtained confirmed the presence of O and Ni at atomic ratios of Ni(1) and O(2) which matched well with the theoretical values for Ni(OH)2. These results indicate that the resulting nanoparticles correspond to a Ni(OH)2 formed on the

ð5Þ

where F is the Faraday constant (96485 C/mol), n is the number of electrons transferred in the redox process (n = 1), and A is the area

A

B

4.5

3 2.5

Z [nm]

Z [nm]

1.2

1.2

4 3.5

2 1.5 1 0.5 0

1

1

0.8

0.8

Z [nm]

C

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0.4

0.2

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0 0

100

200

300

400

500

0 0

10

X [nm]

20

30

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50

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0

1.4

Z [nm]

Z [nm]

Z [nm]

0.6 0.4

X [nm]

80

100

0.2 0

0 100 200 300 400 500 600

0.6 0.4

0.2 0

100

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1

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40

X [nm]

1

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0.8

0.6

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20

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X [nm]

60

80

100

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X[nm]

Fig. 5. (A) Large AFM image of the Ni(OH)2 nanoparticles formed on HOPG showing a rather homogeneous coverage with few aggregation areas, (B) a zoomed image to show morphological features of the nanoparticles, and (C) several height profiles measured in (B) showing nanoparticles’ heights between 1 and 3 nm.

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electrode surface in alkaline medium from the microstructures of compound 2. AFM was used to provide quantitative information on nanoparticles size. Fig. 5A shows a typical AFM image of a HOPG surface with excellent homogeneous coverage of nanoparticles. About 50% of the nanoparticles show a typical height of 1–3 nm, and additionally some aggregates are also observed with apparent heights ranging from 6 to 12 nm (Fig. S9). An analysis of the lateral dimensions of the particles suggests that they have a typical dimension close to the AFM tip resolution, which is about 20– 25 nm (Fig. 5B and C). Based on the full analysis of the AFM images, we can suggest that the deposited Ni(OH)2 nanoparticles have around 1–3 nm of diameter. AFM on gold electrode was more complex because of the higher tendency showed by the particles to aggregate. Additionally, the chemical identity of the surface-confined material obtained after electrochemical treatment of the electrodeposited microstructures of 2 in 0.1 M NaOH was investigated using XPS. The appearance (Table 1) of the oxygen (530.4 eV) and nickel (856.8 eV) signals, shifted in both cases with respect to both compound 2 and the mononuclear Ni(CH3COO)2 [50], suggests the conversion from nickel (II) acetate to Ni(OH)2 [56–58]. Furthermore, the carbon region shows a peak at 289.3 that can be assigned to CH3-COO formed as a side product of the reaction (CH3COONa in Scheme 2). Hence, it seems clear that Ni(OH)2 is formed on the electrode surface from the electrodeposited microstructures of compound 2 in 0.1 M NaOH aqueous solution via the pathway described in Scheme 2. We can conclude that the method described here is a highly efficient and simple procedure for an electrode surface production of high coverages of Ni(OH)2 nanoparticles of 1 nm. It has been reported that Ni(OH)2 with a small crystalline size has higher reaction efficiency than with large crystalline size [59]. 3.10. Electrocatalytic oxidation of sugars Direct oxidations of sugars represent an interesting alternative to enzymatic methods, which can be accomplished by using modified electrodes with a suitable electrocatalyst. Ni2+/Ni3+ redox couple, in particular nickel and nickel hydroxide-/oxide-modified electrodes [60,61], have been successfully employed to electrocatalyze glucose oxidation [62,63] in strongly alkaline solutions. Recently, nanomaterials or nanosized structures have been applied on the electrochemical sensing of carbohydrates to improve the sensitivity and anti-fouling capability of the electrode [33,64,65]. These results have aimed us to use the as-prepared Ni(OH)2 nanoparticles-modified electrodes for the electrocatalysis of

80

NiðOHÞ2ðsÞ þ OH ! NiOOHðsÞ þ e NiOOHðsÞ þ sugars ! NiðOHÞ2ðsÞ þ products Both the increase of the anodic peak current and the total decrease of the cathodic peak current in the cyclic voltammograms illustrated in Figs. 6A and 7 are attributed to the reaction between NiOOH and sugars, which rapidly converts NiOOH to Ni(OH)2. CVs at different scan rates (from 0.002 to 0.5 V/s) were recorded to verify how the scan rate affects the electrocatalytic process. The linear response of peak current versus the square root of scan rate confirms the diffusional character of the process. The catalytic constant was estimated by the method of Galus [66] (Table 2). Quite high-catalytic constants are obtained for all the sugars assayed, in particular in the case of glucose, which kCAT was found to be 5.3  105 M1 s1. This value is higher to those reported in the literature (7.4  102 – 2.1  104 M1 s1) for other Ni(II)-based compound-modified electrodes [67–70]. At the optimal electrodeposition conditions (1.0  104 M compound 1 at 1.15 V for 150 s), the current–time response to increasing concentrations of glucose in the range 1–2500 lM was obtained (Fig. 6B). As can be observed, as the glucose concentration increased the chronoamperometric current increased. A plot of steady state current iss against glucose concentration gives a linear relationship (y = 0.0203x + 0.1700, R2 = 0.9961). Similar plots were obtained for all sugars studied. From the slope, normalized to the surface coverage of catalyst, of the corresponding calibration plot, the sensitivity was determined. The detection (S/N = 3) and quantification limit (S/N = 10) as well as the linear range were also determined (Table 2). The potent catalytic activity of the 80

A

40

0

20

B 2.3 mM D-Glucose

60

2 mM D-Glucose

I / µA

I / µA

60

several sugars in 0.1 M NaOH aqueous solution. In particular, glucose, fructose, maltose, lactose, and sucrose were assayed. Fig. 6A shows cyclic voltammograms (CVs) of Ni(OH)2 GC-modified electrodes for the oxidation of glucose. Under these conditions, at bare GC electrodes, the oxidation of glucose requires very high positive potentials, leading to a poorly defined anodic wave involving very slow electrode kinetics (data not shown). At Ni(OH)2-modified electrodes (Fig. 6A), in the absence of glucose, the previously described voltammetric response ascribed to the Ni(OH)2/NiOOH redox couple can be observed. Upon addition of glucose to the solution, there is a dramatic increase in the anodic current concomitant with a total decrease in the cathodic current, which is characteristic of a potent electrocatalytic effect. The same behavior (Fig. 7) is observed when the other sugars assayed (fructose, maltose, lactose, and sucrose) are added to the solution. The mechanism for the electrocatalytic oxidation of sugars at Ni(OH)2-modified electrodes can thus be expressed as follows:

40

0

20 0 -20

0 0.0

0.2

0.4

E/V

0.6

0

20

40

60

t/s

Fig. 6. (A) Voltammetric responses s of Ni(OH)2/GC-modified electrodes in the absence (dotted line) or in the presence (solid line) of increasing concentrations of D-glucose (0.5, 1.0 and 2.0 mM) in 0.1 M NaOH at 0.01 V/s. (B) Current vs time curves at 0.5 V of Ni(OH)2/GC-modified electrode for increasing glucose concentrations.

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80

40 20

2 mM D-Fructose

0

40

0

20

0 -20

2 mM D-Maltose

60

I / µA

I / µA

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80

0

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2 mM D-Lactose

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E/V

E/V

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-20

0

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0.6

E/V

Fig. 7. Voltammetric responses of Ni(OH)2/GC-modified electrodes in the presence of increasing concentrations of different sugars in 0.1 M NaOH at 0.01 V/s.

Table 2 Analytical parameters for the determination of different sugars with the developed Ni(OH)2-based sensor. Sugar

Sensitivity (lA/ (mM cm2))

Detection limit (lM)

Quantification limit (lM)

Linear range (lM)

kCAT (M1 s1)

Glucose Fructose Maltose Lactose Sucrose

287 397 393 614 529

0.36 0.29 0.27 0.30 0.13

1.19 0.95 0.88 0.99 0.43

1–2500 1–3100 1–1700 1–3100 1–1700

5.3  105 9.2  104 4.0  103 2.8  104 5.3  104

electrodeposited Ni(OH)2 nanoparticles is reflected in the high sensitivity obtained for all the sugars assayed at relatively low potentials. In the case of glucose, this value is higher to those reported in the literature (0.86, 72, 120, 230 lA/mM cm2) for the most sensitive glucose sensors based on Ni(OH)2-modified electrodes [33,71–74], even at the low catalyst surface coverages employed in this work. Detection limits below 0.40 lM (Table 2) compare extremely favorable with other sensitive carbohydrate sensors based on Ni(OH)2-modified electrodes previously described [61,65,75].

the oxidation of sugars. Thus, we have obtained very high sensitivity and low detection limit values for all the sugars tested and the higher values ever reported for the electrocatalysis of glucose with Ni(OH)2-based sensor. This work opens a novel perspective in the electrosynthesis of metal–organic materials, the formation of on-surface structures, and the production of catalytic metal hydroxide/oxide nanoparticles with electrocatalytic activity. The development of modified electrodes with stable electrocatalyst is an important addition to the understanding and control of molecular electrocatalysis for sensing and energy conversion systems. Acknowledgments The authors acknowledge Ministerio de Ciencia e Innovación (Project No. CTQ2011-28157 and MAT2013-46753-C2-1-P) and Comunidad de Madrid (NANOAVANSENS Program) for financial support. The authors thank Professor H.D. Abruña from Cornell University for helpful discussions. E.M.P. gratefully acknowledges the FPU-2010 Grant from the Ministerio de Educación (Spain). Appendix A. Supplementary material

4. Conclusions Electrochemistry was used to generate in a rapid and clean way on-surface microstructures of a novel dimetallic paddle–wheel complex, the tetrakis-thioacetato dinickel (II), using as precursor the dinickel complex 1 via direct electrosynthesis. We have demonstrated that the electrogenerated material is suitable to produce modified electrodes with amorphous Ni(OH)2 nanoparticles by a simple procedure consisting of an alkaline treatment of the substrate. The high density and uniform coverage of the electrodes as well as the very small size of the Ni(OH)2 nanoparticles seem to be play a key role in its electrocatalytic activity and reproducibility toward

Experimental details of coefficient diffusion calculation, electrocatalysis conditions optimization, supporting figures for the characterization of both compound 2 and nickel hydroxide nanoparticles, supporting figures to calculate the apparent surface electron-transfer rate constant (ks) as well as CIF of compound 1 (CCDC 1029368). Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jcat.2015.04.010. References [1] G.C. Bond, D.T. Thompson, Catal. Rev. Sci. Eng. 41 (1999) 319. [2] C. Burda, X. Chen, R. Narayan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025.

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