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Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation
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Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation✩ Erica Pizzolato a,b, Stefano Scaramuzza a, Francesco Carraro a, Alessia Sartori a, Stefano Agnoli a, Vincenzo Amendola a,∗, Marcella Bonchio a,b, Andrea Sartorel a,b,∗∗
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Department of Chemical Sciences, University of Padova, via Francesco Marzolo 1, 35131 Padova, Italy Institute on Membrane Technology, unit of Padova, via Francesco Marzolo 1, 35131 Padova, Italy
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Article history: Received 22 October 2015 Revised 27 November 2015 Accepted 28 November 2015 Available online xxx Keywords: Water oxidation Electrocatalysis Iron oxide Laser ablation Nanoparticles
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Iron oxide nanoparticles (FeOx NPs, 5–30 nm size) prepared via laser ablation in liquid were supported onto Indium Tin Oxide conductive glass slides by magnetophoretic deposition (MD) technique. The resulting FeOx @ITO electrodes are characterized by a low amount of Iron coverage of 16–50 nmols/cm2 , and show electrocatalytic activity towards water oxidation in neutral phosphate buffer pH 7 with 0.58 V overpotential and quantitative Faradaic efficiency towards oxygen production. XPS analysis on the oxygen region of the FeOx films reveals a substantial hydration of the surface after catalysis, recognized as a crucial step to access reactivity. © 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
1. Introduction Artificial photosynthesis aims at the production of renewable solar fuels, via the light-induced water splitting into hydrogen and oxygen (Eq. 1) [1]. In this process, water oxidation to dioxygen is an essential step [2], since through this reaction (Eq. 2) water provides the electrons required to feed the reductive side of the overall process, where hydrogen is ideally generated from proton reduction (Eq. 3).
2H2 O + hv → 2H2 + O2
(1)
2H2 O → O2 + 4H+ + 4e–
(2)
4H+ + 4e– → 2H2
(3)
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This work was supported by the Italian Ministero dell’Università e della Ricerca (MIUR), (FIRB RBAP11C58Y, “NanoSolar” and PRIN 2010 “Hi-Phuture”); COST action CM1205 “CARISMA: CAtalytic RoutInes for Small Molecule Activation” is gratefully acknowledged. ∗ Corresponding author. Tel: +39 049 827 5673; fax: +39 049 827 5829. ∗∗ Corresponding author at: Department of Chemical Sciences, University of Padova, via Francesco Marzolo 1, 35131 Padova, Italy. Tel: +39 049 827 5252; fax: +39 049 827 5300. E-mail addresses: [email protected] (V. Amendola), [email protected] (A. Sartorel).
However, the complexity of water oxidation, associated to a 4e− /4H+ removal and to the formation of a new oxygen–oxygen bond, implies severe kinetic hurdles and high activation energy, which can be overcome by the employment of suitable catalysts [2,3]. In recent years, there has been an explosion of reports in this field, and extended relevance was given to water oxidation catalysts (WOCs) based on first row transition metals, due to their natural abundance and limited cost [4]. Among these, iron appears as an optimal choice, due to its biocompatibility and low environmental impact. Consequently, water oxidation catalysis by iron compounds was investigated with molecular complexes employing polydentate nitrogen based ligands [5], or with extended iron oxides (FeOx ) phases [6–14]. In this latter case, the FeOx layer is supported onto conductive (or semiconductive) electrodes, and water oxidation is electrochemically (or photoelectrochemically) accessed. Current state-of-the-art materials are amorphous FeOx films prepared by photochemical metal-organic deposition (PMOD) [6], by electro- or photoelectrochemical deposition [7–9,12,14], by Successive Ionic Layer Adsorption and Reaction (SILAR) [10,11], or by Pulsed-Laser deposition [13]. Typically, water oxidation was investigated in alkaline aqueous media (pH 9–13) to guarantee stability of the oxide layer, while only recently catalysis was investigated in ideal neutral phosphate buffer (pH 7), with a low loading FeOx film (10–12 nmols Fe/cm2 ) electrodeposited onto Indium Tin Oxide (ITO) slides by means of cyclic voltammetry in neutral acetate buffer [14]. This material catalyzes water oxidation at a low overpotential of 0.48 V, reaching a total turnover
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Please cite this article as: E. Pizzolato et al., Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.12.004
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number (TON: ratio between evolved O2 mols and deposited Fe mols) of 1.5–5.2×104 , with a turnover frequency (TOF is defined as the TON per unit of time) of 750–1500 h−1 . A limitation of the system is the dependence of its stability upon operating conditions, and in particular a major loss of activity at high applied bias. In this work, we investigate the electrocatalytic behavior of magnetic FeOx nanoparticles (NPs, d<30 nm) prepared by laser ablation in liquid and casted onto ITO electrodes by magnetophoretic deposition (MD) [15]. FeOx NPs provide a nanostructured morphology of the electroactive layer, with an enhanced surface area compared to a 2D compact film surface. Moreover, among different deposition techniques from liquid solution, MD does not require an additional application of an external bias. This method provides indeed a stable, low-amount loading of the FeOx NPs onto an ITO electrode; the resulting film catalyzes water oxidation in neutral media at a low overpotential of 0.58 V and with a quantitative Faradaic efficiency; a Tafel slope of 110 mV/decade suggests the first electron transfer to the electrode as the rate determining step; evolution of the catalyst with formation of surface Fe-OH moieties upon prolonged electrocatalysis is also discussed.
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2. Experimental
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2.1. General procedures
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Iron concentration of FeOx NPs was determined by the ophenanthroline method, previously calibrated against inductively coupled plasma assisted mass spectrometry (ICP-MS) measurements. Briefly, FeOx NPs were dissolved adding concentrated H2 SO4 and heating at 90 °C for 90 min. After setting the pH to 3.9 with NaOH, the o-phenanthroline solution (1 g/L) was added with a ratio of 250 μL per mL of FeOx NPs dispersion. A buffer of potassium hydrogenphthalate was used to bring the final volume to 5 mL. After 20 min, UV−vis spectra were recorded. The absorbance at 396 nm gives the total iron in solution. TEM analysis was performed with a JEOL JEM 3010 microscope operating at 300 keV and equipped with a Gatan Multiscan CCD Camera model 794 a. FeOx NPs were deposed by drop casting on a copper grid coated with an holey carbon film and left drying in air at room temperature, in the same ambient conditions exploited for deposition of FeOx @ITO films. Cyclic voltammetry and Controlled Potential Electrolysis experiments were performed using a BAS EC-epsilon potentiostat and an AMEL Potentiostat-Galvanostat, model 7050. A custom designed air-tight three-electrode cell was employed for all experiments. The custom designed cell used was assembled starting from a 100 mL commercial threated flask equipped with a Teflon cap, in which three copper wires (1.5 mm diameter) were welded, bearing the clips that allow connections with the electrodes. In the Teflon cap the FOXY-R oxygen probe was inserted, in order to allow oxygen measurement in solution. A silicon O-ring between the Teflon cap and the flask ensures the tightness of the cell. A Pt wire and an Ag/AgCl were used as counter and reference electrodes, respectively. Dissolved oxygen measurements were performed using an Ocean Optics FOXY-R fluorescence probe coupled with NeoFox-GT phase fluorimeter and NeoFox Viewer software. The oxygen probe was calibrated using the two points calibration procedure of NeoFox Viewer software, employing a 2.0 M sodium sulfite as 0 mg/L dissolved oxygen value, and an aerated aqueous solution for which the amount of dissolved oxygen is reported, depending on the temperature and pressure. Characterization of FeOx NPs, complementary electrochemical and XPS experiments are available in the Supplementary Material.
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Scheme 1. Schematic representation of deposition of FeOx NP, picture of a FeOx @ITO electrode, TEM image of FeOx NP and SEM image of FeOx @ITO electrode.
3. Results and discussion
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3.1. Preparation of the FeOx @ITO electrodes
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Synthesis of FeOx NPs via laser ablation in liquid solution (LASiS) was carried out according to a well-established protocol [16], exploiting the first harmonic (1064 nm) of a 9 ns Nd-YAG laser focused on bulk α −Fe target immersed in pure water. Remarkably, LASiS does not require the use of undesired chemical precursors or additives to obtain FeOx NPs [16,17]. Laser synthesized crystalline FeOx NPs are purified from the amorphous iron hydroxides byproducts with an heat treatment at 60 °C for 1 h in a disodium ethylenediaminetetraacetate (EDTA) aqueous solution, then collected by centrifugation and repeatedly washed with distilled water. By transmission electron microscopy (TEM, Scheme 1 and Figure S1), the particles size was observed in the 5–30 nm range, with a polycrystalline structure typical of LASiS products, and single crystal domains of the order of 5 nm [17a]. According to selected area electron diffraction (SAED, Figure S1b), Raman and FTIR spectroscopy (Figure S2a-b), final FeOx NPs are prevalently composed by the magnetic phase magnetite (Fe3 O4 ), although Fe(III) oxide and hydroxides are also present, especially at the particles surface, as expected after the heat treatment in water. The FeOx NPs were deposited onto glass coated with ITO (8–12 /sq surface resistivity) slides, by drop-casting 8–25 μL of a 1 mM (total amount of Iron) aqueous suspension, previously sonicated for 30 min, followed by slow evaporation of the solvent at room temperature. The magnetic particles in the electrodes (hereafter FeOx @ITO) were distributed in a round area (diameter = 0.8 cm, geometric surface area = 0.5 cm2 ) with the aid of a magnet kept below the glass slide, while water was let to evaporate overnight, at room temperature (Scheme 1). This procedure allowed to achieve an Iron coverage in the range
Please cite this article as: E. Pizzolato et al., Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.12.004
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Fig. 1. Cyclic voltammetry of FeOx@ITO (black (1), red (2), and blue (3) traces refer to 16, 32 and 50 nmol Fe/cm2, respectively) and of ITO (purple (4) trace) electrodes in 0.2 M phosphate buffer, pH 7.00. Counter electrode: Pt wire; reference electrode: Ag/AgCl; scan rate: 100 mV/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3.2. Cyclic voltammetry and controlled potential electrolysis
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In order to check their electrocatalytic activity towards water oxidation, the FeOx@ITO slides were employed as working electrodes in cyclic voltammetry experiments (Fig. 1). Upon application of an anodic bias, an intense current was observed to begin at an onset potential of 1.2 V vs. Ag/AgCl (Fig. 1) and corresponding to a low overpotential for water oxidation of 0.58 V, only 100 mV above the amorphous iron oxide WOC recently reported by Wu et al. [14]; testing different electrodes prepared with the same protocol resulted in only a minor variation (<10%) of the current response, see Supporting Information. Accordingly, the intensity of the current was proportional to the amount of FeOx deposited, in the range 16 and 50 nmols/cm2 . At the highest Iron coverage of 50 nmols/cm2 , that was selected for the following experiments, deposition of the same amount of commercial magnetite (SigmaAldrich, 5 μm particle size) gave lower anodic currents (Figure S5). Similar CV profiles but lower currents were observed by employing FeOx @ITO electrodes in 0.1 M NaHCO3 buffer, pH 7 as the aqueous reaction media (Figure S6), probably related to better proton acceptor ability of phosphate (for HPO4 2− pKb = 6.79) with respect to bicarbonate (for HCO3 − pKb = 10.4). In order to ascertain that the anodic current was associated to water oxidation, controlled potential electrolysis (CPE) experiments coupled to evolved oxygen measurement were conducted by employing an air-tight cell (Supporting Information). Application of a positive potential (1.20– 1.50 V vs. Ag/AgCl) to the FeOx @ITO electrodes was associated to intense anodic currents (Fig. 2); oxygen evolution was confirmed by an O2 -selective fluorescence probe (Neofox, see Supporting Information) dipped in solution, with a > 98% Faradaic efficiency at 1.40 V vs. Ag/AgCl. Accordingly, no H2 O2 was found to evolve in solution, by means of iodometric test. For the CPE at 1.5 V, a
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Fig. 2. Current profiles versus time in CPE experiments with a FeOx @ITO as working electrode, at applied potentials of 1200–1500 mV vs. Ag/AgCl (25 mV steps). Counter electrode: Pt wire; reference electrode: Ag/AgCl.
16 and 50 nmols/cm2 . The deposition protocol allowed a straightforward preparation of the working electrode with reproducible electrochemical response; the FeOx nanoparticles adhere to the ITO providing a coarse surface as shown by the Scanning Electron Microscopy (SEM) analysis in Scheme 1, and the layer is stable upon dipping the electrode in aqueous solution, as confirmed by UV-Vis characterization of the electrodes (Figure S3). With respect to literature systems, deposition of small nanoparticles rather than compact films could result in benefits related to the increase of the active area [18], and additionally, the magnetophoretic deposition technique avoids the need of application of electrochemical or photoelectrochemical power.
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Fig. 3. Tafel plot analysis: overpotential (V) vs log(i) plot, overpotential was corrected for the ohmic drop.
current of 1.9 mA corresponds to a notable turnover frequency per Iron center for oxygen production of 708 h−1 . The Tafel slope (overpotential η vs log(i), with η corrected for the ohmic drop) results in 110 mV/decade at low applied overpotential (Fig. 3); this value is close to the ideal value of 120 mV/decade for a first electron transfer to the electrode as the rate determining step [19], and results higher than values usually observed for iron oxide based WOCs [14]. At higher overpotentials the Tafel slope increases up to 330 mV/decade [19]; although changes in Tafel slopes are usually ascribed to change in reaction mechanism and in surface coverage of adsorbed intermediates, deviations from ideal values are attributable to mass transport limitation, potentially associated to the formation of oxygen bubbles at the electrode surface [19].
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3.3. FeOx @ITO electrodes after controlled potential electrolysis
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From CPE experiments, a progressive abatement of the anodic current is observed upon prolonged electrolysis (Figure S7), indicative of a loss of activity of the FeOx @ITO electrodes. This is also evidenced by CV characterization, where a decrease of intensity of the catalytic wave is observed for FeOx @ITO electrodes undergone to 15 h CPE experiments, and appears to be more enhanced at higher operating potentials (Figure S8), as was observed for an amorphous iron oxide film [14]; after 15 h of CPE at 1.35 V vs. Ag/AgCl a total charge of 21 C corresponds to ca 2170 turnover per iron center. UV-Vis analysis of the electrodes rule out a major leaching of the FeOx layer from the ITO electrode as the origin of abatement of activity (Figure S3); a diagnostic P 2p peak at 133 eV in the XPS analysis indicates phosphate coordination at the FeOx surface
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Please cite this article as: E. Pizzolato et al., Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.12.004
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Fig. 4. XPS analysis (O region, 527–537 eV) of pristine FeOx @ITO electrodes (a), and undergone 15 h CPE experiments at applied potential of 1.2 V (b), 1.25 V (c), 1.30 V (d) and 1.4 V (e) vs. Ag/AgCl.
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(Figure S9), as a potential source for the loss of activity of the FeOx layer [14]. It is worth to mention that the presence of phosphate is observed also for FeOx @ITO electrodes dipped in buffer without application of the electrochemical bias, thus indicating a different behavior with respect to a Cobalt/oxide/phosphate WOC, where phosphate ions have a fundamental role in the electrodeposition of the active layer [20]. XPS analysis on the FeOx @ITO electrodes undergone to 15 h CPE experiments, revealed no major changes in the features of the Fe region, where the maximum of the Fe 2p3/2 peak centered at 711.5 eV and the presence of a satellite shifted of about 8 eV confirm an average oxidation state of 3+ (Figure S9) [21]. Conversely, the analysis of the oxygen region (527–537 eV, Fig. 4) revealed a significant evolution of the spectra as a function of the working potential. After separation into chemically shifted single components, three different peaks can be identified: one at 530.3 eV associated with lattice oxygen in iron oxides, a second component at 531.5 eV related to surface hydroxyl species, and a third peak at 533.5 typical of adsorbed water molecules [22]. Promotion of surface hydration and hydroxylation, concomitantly to
a clear reduction of pure oxidic species was observed to follow a clear trend, by increasing the applied potential in CPE experiments (Fig. 4 and Figure S10). Surface hydration of iron oxide was indeed reported to be mandatory to access water oxidation catalysis [19c].
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4. Conclusions
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A novel strategy to access water oxidation catalysis with earth abundant metals is demonstrated herein: in particular, iron oxide nanoparticles (FeOx NPs) prepared via laser ablation in liquid were reported for the first time to be an effective WOC electrocatalyst in neutral aqueous media featuring an overpotential as low as 0.58 V. Fabrication of the working electrode is obtained by magnetophoretic deposition of the FeOx NPs, thus avoiding energyconsuming electrodeposition methods. Optimization of FeOx catalytic performance will require: (i) a suitable tuning of electron transfer rate to the electrode, which appears as the rate determining step according to the Tafel analysis, and (ii) stabilization of the catalytic activity, possibly by exploiting a co-deposition of the FeOx with other metal oxides [18]. These studies will be pivotal in the application of such materials onto semiconductors, in order to develop a light activated device.
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Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2015.12.004.
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References
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Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation✩ Erica Pizzolato a,b, Stefano Scaramuzza a, Francesco Carraro a, Alessia Sartori a, Stefano Agnoli a, Vincenzo Amendola a,∗, Marcella Bonchio a,b, Andrea Sartorel a,b,∗∗
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a
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b
Department of Chemical Sciences, University of Padova, via Francesco Marzolo 1, 35131 Padova, Italy Institute on Membrane Technology, unit of Padova, via Francesco Marzolo 1, 35131 Padova, Italy
a r t i c l e
i n f o
Article history: Received 22 October 2015 Revised 27 November 2015 Accepted 28 November 2015 Available online xxx Keywords: Water oxidation Electrocatalysis Iron oxide Laser ablation Nanoparticles
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Iron oxide nanoparticles (FeOx NPs, 5–30 nm size) prepared via laser ablation in liquid were supported onto Indium Tin Oxide conductive glass slides by magnetophoretic deposition (MD) technique. The resulting FeOx @ITO electrodes are characterized by a low amount of Iron coverage of 16–50 nmols/cm2 , and show electrocatalytic activity towards water oxidation in neutral phosphate buffer pH 7 with 0.58 V overpotential and quantitative Faradaic efficiency towards oxygen production. XPS analysis on the oxygen region of the FeOx films reveals a substantial hydration of the surface after catalysis, recognized as a crucial step to access reactivity. © 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
1. Introduction Artificial photosynthesis aims at the production of renewable solar fuels, via the light-induced water splitting into hydrogen and oxygen (Eq. 1) [1]. In this process, water oxidation to dioxygen is an essential step [2], since through this reaction (Eq. 2) water provides the electrons required to feed the reductive side of the overall process, where hydrogen is ideally generated from proton reduction (Eq. 3).
2H2 O + hv → 2H2 + O2
(1)
2H2 O → O2 + 4H+ + 4e–
(2)
4H+ + 4e– → 2H2
(3)
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a b s t r a c t
This work was supported by the Italian Ministero dell’Università e della Ricerca (MIUR), (FIRB RBAP11C58Y, “NanoSolar” and PRIN 2010 “Hi-Phuture”); COST action CM1205 “CARISMA: CAtalytic RoutInes for Small Molecule Activation” is gratefully acknowledged. ∗ Corresponding author. Tel: +39 049 827 5673; fax: +39 049 827 5829. ∗∗ Corresponding author at: Department of Chemical Sciences, University of Padova, via Francesco Marzolo 1, 35131 Padova, Italy. Tel: +39 049 827 5252; fax: +39 049 827 5300. E-mail addresses: [email protected] (V. Amendola), [email protected] (A. Sartorel).
However, the complexity of water oxidation, associated to a 4e− /4H+ removal and to the formation of a new oxygen–oxygen bond, implies severe kinetic hurdles and high activation energy, which can be overcome by the employment of suitable catalysts [2,3]. In recent years, there has been an explosion of reports in this field, and extended relevance was given to water oxidation catalysts (WOCs) based on first row transition metals, due to their natural abundance and limited cost [4]. Among these, iron appears as an optimal choice, due to its biocompatibility and low environmental impact. Consequently, water oxidation catalysis by iron compounds was investigated with molecular complexes employing polydentate nitrogen based ligands [5], or with extended iron oxides (FeOx ) phases [6–14]. In this latter case, the FeOx layer is supported onto conductive (or semiconductive) electrodes, and water oxidation is electrochemically (or photoelectrochemically) accessed. Current state-of-the-art materials are amorphous FeOx films prepared by photochemical metal-organic deposition (PMOD) [6], by electro- or photoelectrochemical deposition [7–9,12,14], by Successive Ionic Layer Adsorption and Reaction (SILAR) [10,11], or by Pulsed-Laser deposition [13]. Typically, water oxidation was investigated in alkaline aqueous media (pH 9–13) to guarantee stability of the oxide layer, while only recently catalysis was investigated in ideal neutral phosphate buffer (pH 7), with a low loading FeOx film (10–12 nmols Fe/cm2 ) electrodeposited onto Indium Tin Oxide (ITO) slides by means of cyclic voltammetry in neutral acetate buffer [14]. This material catalyzes water oxidation at a low overpotential of 0.48 V, reaching a total turnover
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Please cite this article as: E. Pizzolato et al., Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.12.004
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number (TON: ratio between evolved O2 mols and deposited Fe mols) of 1.5–5.2×104 , with a turnover frequency (TOF is defined as the TON per unit of time) of 750–1500 h−1 . A limitation of the system is the dependence of its stability upon operating conditions, and in particular a major loss of activity at high applied bias. In this work, we investigate the electrocatalytic behavior of magnetic FeOx nanoparticles (NPs, d<30 nm) prepared by laser ablation in liquid and casted onto ITO electrodes by magnetophoretic deposition (MD) [15]. FeOx NPs provide a nanostructured morphology of the electroactive layer, with an enhanced surface area compared to a 2D compact film surface. Moreover, among different deposition techniques from liquid solution, MD does not require an additional application of an external bias. This method provides indeed a stable, low-amount loading of the FeOx NPs onto an ITO electrode; the resulting film catalyzes water oxidation in neutral media at a low overpotential of 0.58 V and with a quantitative Faradaic efficiency; a Tafel slope of 110 mV/decade suggests the first electron transfer to the electrode as the rate determining step; evolution of the catalyst with formation of surface Fe-OH moieties upon prolonged electrocatalysis is also discussed.
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2. Experimental
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2.1. General procedures
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Iron concentration of FeOx NPs was determined by the ophenanthroline method, previously calibrated against inductively coupled plasma assisted mass spectrometry (ICP-MS) measurements. Briefly, FeOx NPs were dissolved adding concentrated H2 SO4 and heating at 90 °C for 90 min. After setting the pH to 3.9 with NaOH, the o-phenanthroline solution (1 g/L) was added with a ratio of 250 μL per mL of FeOx NPs dispersion. A buffer of potassium hydrogenphthalate was used to bring the final volume to 5 mL. After 20 min, UV−vis spectra were recorded. The absorbance at 396 nm gives the total iron in solution. TEM analysis was performed with a JEOL JEM 3010 microscope operating at 300 keV and equipped with a Gatan Multiscan CCD Camera model 794 a. FeOx NPs were deposed by drop casting on a copper grid coated with an holey carbon film and left drying in air at room temperature, in the same ambient conditions exploited for deposition of FeOx @ITO films. Cyclic voltammetry and Controlled Potential Electrolysis experiments were performed using a BAS EC-epsilon potentiostat and an AMEL Potentiostat-Galvanostat, model 7050. A custom designed air-tight three-electrode cell was employed for all experiments. The custom designed cell used was assembled starting from a 100 mL commercial threated flask equipped with a Teflon cap, in which three copper wires (1.5 mm diameter) were welded, bearing the clips that allow connections with the electrodes. In the Teflon cap the FOXY-R oxygen probe was inserted, in order to allow oxygen measurement in solution. A silicon O-ring between the Teflon cap and the flask ensures the tightness of the cell. A Pt wire and an Ag/AgCl were used as counter and reference electrodes, respectively. Dissolved oxygen measurements were performed using an Ocean Optics FOXY-R fluorescence probe coupled with NeoFox-GT phase fluorimeter and NeoFox Viewer software. The oxygen probe was calibrated using the two points calibration procedure of NeoFox Viewer software, employing a 2.0 M sodium sulfite as 0 mg/L dissolved oxygen value, and an aerated aqueous solution for which the amount of dissolved oxygen is reported, depending on the temperature and pressure. Characterization of FeOx NPs, complementary electrochemical and XPS experiments are available in the Supplementary Material.
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Scheme 1. Schematic representation of deposition of FeOx NP, picture of a FeOx @ITO electrode, TEM image of FeOx NP and SEM image of FeOx @ITO electrode.
3. Results and discussion
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3.1. Preparation of the FeOx @ITO electrodes
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Synthesis of FeOx NPs via laser ablation in liquid solution (LASiS) was carried out according to a well-established protocol [16], exploiting the first harmonic (1064 nm) of a 9 ns Nd-YAG laser focused on bulk α −Fe target immersed in pure water. Remarkably, LASiS does not require the use of undesired chemical precursors or additives to obtain FeOx NPs [16,17]. Laser synthesized crystalline FeOx NPs are purified from the amorphous iron hydroxides byproducts with an heat treatment at 60 °C for 1 h in a disodium ethylenediaminetetraacetate (EDTA) aqueous solution, then collected by centrifugation and repeatedly washed with distilled water. By transmission electron microscopy (TEM, Scheme 1 and Figure S1), the particles size was observed in the 5–30 nm range, with a polycrystalline structure typical of LASiS products, and single crystal domains of the order of 5 nm [17a]. According to selected area electron diffraction (SAED, Figure S1b), Raman and FTIR spectroscopy (Figure S2a-b), final FeOx NPs are prevalently composed by the magnetic phase magnetite (Fe3 O4 ), although Fe(III) oxide and hydroxides are also present, especially at the particles surface, as expected after the heat treatment in water. The FeOx NPs were deposited onto glass coated with ITO (8–12 /sq surface resistivity) slides, by drop-casting 8–25 μL of a 1 mM (total amount of Iron) aqueous suspension, previously sonicated for 30 min, followed by slow evaporation of the solvent at room temperature. The magnetic particles in the electrodes (hereafter FeOx @ITO) were distributed in a round area (diameter = 0.8 cm, geometric surface area = 0.5 cm2 ) with the aid of a magnet kept below the glass slide, while water was let to evaporate overnight, at room temperature (Scheme 1). This procedure allowed to achieve an Iron coverage in the range
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Fig. 1. Cyclic voltammetry of FeOx@ITO (black (1), red (2), and blue (3) traces refer to 16, 32 and 50 nmol Fe/cm2, respectively) and of ITO (purple (4) trace) electrodes in 0.2 M phosphate buffer, pH 7.00. Counter electrode: Pt wire; reference electrode: Ag/AgCl; scan rate: 100 mV/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3.2. Cyclic voltammetry and controlled potential electrolysis
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In order to check their electrocatalytic activity towards water oxidation, the FeOx@ITO slides were employed as working electrodes in cyclic voltammetry experiments (Fig. 1). Upon application of an anodic bias, an intense current was observed to begin at an onset potential of 1.2 V vs. Ag/AgCl (Fig. 1) and corresponding to a low overpotential for water oxidation of 0.58 V, only 100 mV above the amorphous iron oxide WOC recently reported by Wu et al. [14]; testing different electrodes prepared with the same protocol resulted in only a minor variation (<10%) of the current response, see Supporting Information. Accordingly, the intensity of the current was proportional to the amount of FeOx deposited, in the range 16 and 50 nmols/cm2 . At the highest Iron coverage of 50 nmols/cm2 , that was selected for the following experiments, deposition of the same amount of commercial magnetite (SigmaAldrich, 5 μm particle size) gave lower anodic currents (Figure S5). Similar CV profiles but lower currents were observed by employing FeOx @ITO electrodes in 0.1 M NaHCO3 buffer, pH 7 as the aqueous reaction media (Figure S6), probably related to better proton acceptor ability of phosphate (for HPO4 2− pKb = 6.79) with respect to bicarbonate (for HCO3 − pKb = 10.4). In order to ascertain that the anodic current was associated to water oxidation, controlled potential electrolysis (CPE) experiments coupled to evolved oxygen measurement were conducted by employing an air-tight cell (Supporting Information). Application of a positive potential (1.20– 1.50 V vs. Ag/AgCl) to the FeOx @ITO electrodes was associated to intense anodic currents (Fig. 2); oxygen evolution was confirmed by an O2 -selective fluorescence probe (Neofox, see Supporting Information) dipped in solution, with a > 98% Faradaic efficiency at 1.40 V vs. Ag/AgCl. Accordingly, no H2 O2 was found to evolve in solution, by means of iodometric test. For the CPE at 1.5 V, a
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Fig. 2. Current profiles versus time in CPE experiments with a FeOx @ITO as working electrode, at applied potentials of 1200–1500 mV vs. Ag/AgCl (25 mV steps). Counter electrode: Pt wire; reference electrode: Ag/AgCl.
16 and 50 nmols/cm2 . The deposition protocol allowed a straightforward preparation of the working electrode with reproducible electrochemical response; the FeOx nanoparticles adhere to the ITO providing a coarse surface as shown by the Scanning Electron Microscopy (SEM) analysis in Scheme 1, and the layer is stable upon dipping the electrode in aqueous solution, as confirmed by UV-Vis characterization of the electrodes (Figure S3). With respect to literature systems, deposition of small nanoparticles rather than compact films could result in benefits related to the increase of the active area [18], and additionally, the magnetophoretic deposition technique avoids the need of application of electrochemical or photoelectrochemical power.
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Fig. 3. Tafel plot analysis: overpotential (V) vs log(i) plot, overpotential was corrected for the ohmic drop.
current of 1.9 mA corresponds to a notable turnover frequency per Iron center for oxygen production of 708 h−1 . The Tafel slope (overpotential η vs log(i), with η corrected for the ohmic drop) results in 110 mV/decade at low applied overpotential (Fig. 3); this value is close to the ideal value of 120 mV/decade for a first electron transfer to the electrode as the rate determining step [19], and results higher than values usually observed for iron oxide based WOCs [14]. At higher overpotentials the Tafel slope increases up to 330 mV/decade [19]; although changes in Tafel slopes are usually ascribed to change in reaction mechanism and in surface coverage of adsorbed intermediates, deviations from ideal values are attributable to mass transport limitation, potentially associated to the formation of oxygen bubbles at the electrode surface [19].
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3.3. FeOx @ITO electrodes after controlled potential electrolysis
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From CPE experiments, a progressive abatement of the anodic current is observed upon prolonged electrolysis (Figure S7), indicative of a loss of activity of the FeOx @ITO electrodes. This is also evidenced by CV characterization, where a decrease of intensity of the catalytic wave is observed for FeOx @ITO electrodes undergone to 15 h CPE experiments, and appears to be more enhanced at higher operating potentials (Figure S8), as was observed for an amorphous iron oxide film [14]; after 15 h of CPE at 1.35 V vs. Ag/AgCl a total charge of 21 C corresponds to ca 2170 turnover per iron center. UV-Vis analysis of the electrodes rule out a major leaching of the FeOx layer from the ITO electrode as the origin of abatement of activity (Figure S3); a diagnostic P 2p peak at 133 eV in the XPS analysis indicates phosphate coordination at the FeOx surface
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Please cite this article as: E. Pizzolato et al., Water oxidation electrocatalysis with iron oxide nanoparticles prepared via laser ablation, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.12.004
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Fig. 4. XPS analysis (O region, 527–537 eV) of pristine FeOx @ITO electrodes (a), and undergone 15 h CPE experiments at applied potential of 1.2 V (b), 1.25 V (c), 1.30 V (d) and 1.4 V (e) vs. Ag/AgCl.
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(Figure S9), as a potential source for the loss of activity of the FeOx layer [14]. It is worth to mention that the presence of phosphate is observed also for FeOx @ITO electrodes dipped in buffer without application of the electrochemical bias, thus indicating a different behavior with respect to a Cobalt/oxide/phosphate WOC, where phosphate ions have a fundamental role in the electrodeposition of the active layer [20]. XPS analysis on the FeOx @ITO electrodes undergone to 15 h CPE experiments, revealed no major changes in the features of the Fe region, where the maximum of the Fe 2p3/2 peak centered at 711.5 eV and the presence of a satellite shifted of about 8 eV confirm an average oxidation state of 3+ (Figure S9) [21]. Conversely, the analysis of the oxygen region (527–537 eV, Fig. 4) revealed a significant evolution of the spectra as a function of the working potential. After separation into chemically shifted single components, three different peaks can be identified: one at 530.3 eV associated with lattice oxygen in iron oxides, a second component at 531.5 eV related to surface hydroxyl species, and a third peak at 533.5 typical of adsorbed water molecules [22]. Promotion of surface hydration and hydroxylation, concomitantly to
a clear reduction of pure oxidic species was observed to follow a clear trend, by increasing the applied potential in CPE experiments (Fig. 4 and Figure S10). Surface hydration of iron oxide was indeed reported to be mandatory to access water oxidation catalysis [19c].
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4. Conclusions
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A novel strategy to access water oxidation catalysis with earth abundant metals is demonstrated herein: in particular, iron oxide nanoparticles (FeOx NPs) prepared via laser ablation in liquid were reported for the first time to be an effective WOC electrocatalyst in neutral aqueous media featuring an overpotential as low as 0.58 V. Fabrication of the working electrode is obtained by magnetophoretic deposition of the FeOx NPs, thus avoiding energyconsuming electrodeposition methods. Optimization of FeOx catalytic performance will require: (i) a suitable tuning of electron transfer rate to the electrode, which appears as the rate determining step according to the Tafel analysis, and (ii) stabilization of the catalytic activity, possibly by exploiting a co-deposition of the FeOx with other metal oxides [18]. These studies will be pivotal in the application of such materials onto semiconductors, in order to develop a light activated device.
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Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2015.12.004.
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References
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