Facile Photochemical Preparation of Amorphous Iridium Oxide Films

Feb 11, 2014 - The surfaces were dried under a stream of N2 and placed under a UV–ozone lamp (Novascan PSD-UV3) for 15 min prior to deposition of th...
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Facile Photochemical Preparation of Amorphous Iridium Oxide Films for Water Oxidation Catalysis Rodney D. L. Smith,†,‡ Barbora Sporinova,‡ Randal D. Fagan,‡ Simon Trudel,*,‡ and Curtis P. Berlinguette*,†,‡ †

Departments of Chemistry and Chemistry & Biological Engineering, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada ‡ Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive N.W., Calgary, Alberta T2N 1N4, Canada S Supporting Information *

ABSTRACT: Light-driven decomposition of Ir(acac)3 spin-cast on a conducting glass substrate produces a thin conformal film of amorphous iridium oxide, a-IrOx. The decomposition process, which was carried out under an ambient atmosphere at room temperature and tracked by Fourier transform infrared (FTIR) spectroscopy, appears to proceed by way of a ligand-to-metal charge transfer (LMCT) process. The amorphous nature of the films is based on the lack of any observable Bragg reflections by powder X-ray diffraction techniques; the elemental composition was corroborated by X-ray photoelectron spectroscopy (XPS) measurements. The films are found to be excellent electrocatalysts for mediating the oxygen evolution reaction (OER) in acidic media, as evidenced by the onset of catalysis at 130 mV and a Tafel slope of 34 mV dec−1. These parameters enable current densities of 1 and 10 mA cm−2 to be reached at 190 and 220 mV, respectively. Exposing the films to higher temperatures (500 °C) renders a film of crystalline iridium oxide, c-IrOx, which displays a Tafel slope of 60 mV dec−1, thus requiring an additional 50 mV to reach a current density of 1 mA cm−2. The film of a-IrOx reported here is among the best OER electrocatalysts reported to date.

1. INTRODUCTION A viable mechanism for storing renewable energy is the electrolysis of water into oxygen and hydrogen fuels.1−5 Because the efficiency of this process is inherently sensitive to the performance of the catalyst responsible for negotiating the oxygen evolution reaction (OER), there is a broad effort to develop new metal oxide materials that are compatible with commercial electrolyzer systems. Although a growing body of OER catalysts containing abundant, inexpensive transition metals have recently appeared in the literature,6−9 relatively expensive materials continue to be used by industry because the economics of electrolytic hydrogen production are governed primarily by catalytic performance rather than the cost of the catalyst and because earth-abundant metals are typically not as stable. Indeed, electrolysis in acidic media requires electrocatalysts based on noble metals to circumvent undesirable degradation. It is for these reasons that iridium oxide continues to find widespread use in proton exchange membrane electrolyzers as well as other electrocatalysis industries.10 Although the composition of the catalyst is obviously important with respect to reactivity, the phase of the electrocatalytic films can also have a profound effect on performance. In this regard, amorphous phases of metal oxides have become more prominent in the recent literature owing to the superior electrolytic properties they display relative to crystalline phases of the same compositions.11−13 There are, however, very limited methods available for synthesizing © 2014 American Chemical Society

amorphous metal oxide catalysts and thus documented examples of amorphous metal oxides are confined to those containing cobalt,6 iron,12 nickel,8 manganese,9,14 ruthenium,11 and lead.15,16 Given that crystalline iridium oxide is an efficient and robust electrocatalyst in acidic media, it would be hugely advantageous to be able to readily access amorphous iridium oxide (a-IrOx). Documented methods for making a-IrOx electrocatalysts include electrodeposition17,18 and sputtering.19 A thin film of a-IrOx prepared by electrodeposition of an organometallic precursor17,20,21 currently represents the best performing electrocatalyst for electrolysis, but this method is not readily conducive to scaling. Taking these collective points into consideration, we set out to develop a scalable method for generating films of a-IrOx that act as efficient and active electrocatalysts in acidic media. Building on our previous disclosure outlining the photochemical synthesis of Fe/Co/Ni oxide films,13 we show herein that the light-induced decomposition of a photoactive precursor, Ir(acac)3 (acac = acetylacetonate), deposited on a conducting substrate yields a thin conformal film of a-IrOx at room temperature in an ambient atmosphere. The resultant films exhibit excellent electrocatalytic properties for the oxygen evolution reaction in Received: November 27, 2013 Revised: February 3, 2014 Published: February 11, 2014 1654

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peaks observed during XPS were due to adventitious carbon species. A surface etching rate of 0.1 nm min−1 was used, as determined with a SiO2 standard. Both a-IrOx and c-IrOx revealed the loss of the C 1s signal within 2 nm of the surface. Analysis of XPS spectra was performed using CasaXPS. Spectra were corrected by calibrating all peaks to the major adventitious C 1s signal (284.8 eV). A Shirley-type background was used and spectra were fitted using peaks with a GL(30) profile. The O 1s region was curve-fitted using a recently published approach.22 The C 1s region was first fitted with peaks representing carbon−oxygen species: esters, ketones, ethers. The relative ratio of carbon−oxygen species acquired from the C 1s region was used to constrain the peak area of the O 1s component peak for each carbon−oxygen species. The binding energy of the ester component peak (C(O)OC) was constrained (533.5 ± 0.2 eV). The position of ether (COC) and carbonyl (C(O) and C(O)OC) peaks were then constrained to −0.9 and −1.5 eV, respectively, relative to the ester peak. Two additional O 1s peaks were then added, with no peak area or binding energy constraints. All O 1s peaks were constrained to a fwhm ≤ 1.6 eV.

acidic media. Importantly, this preparative method offers a costeffective and scalable approach for depositing a-IrOx on electrode surfaces.

2. EXPERIMENTAL SECTION 2.1. Materials. Ir(acac)3 (98%, Strem) and chloroform (Sigma-Aldrich, reagent grade) were used as received. Fluorinedoped indium tin oxide (FTO; TEC7) was obtained from the Hartford Glass Company and cleaned prior to use by ultrasonication in detergent solution (Extran MN01, distilled water), distilled water, and isopropanol. The surfaces were dried under a stream of N2 and placed under a UV−ozone lamp (Novascan PSD-UV3) for 15 min prior to deposition of the solutions. 2.2. Film Preparation. Solutions of Ir(acac)3 were prepared by dissolving the complex in chloroform to obtain an overall 5% w/w solution (0.15 mol L−1). Films were deposited on FTO by spin-casting at 3000 rpm for 60 s (Laurell WS-650MZ-23NPP-Lite) and then irradiated with UV light for 1 h (Atlantic Ultraviolet G18T5VH/U; 185/254 nm) before annealing at 100 (a-IrOx) or 500 °C (c-IrOx) for 1 h. Assuming a uniform 100 nm film thickness (estimated from SEM cross sections) and a bulk density equal to crystalline IrO2 (11.66 g cm−3), the catalyst loading can be approximated as 0.1 μg cm−2. When the disorder in the material relative to crystalline IrO2 is considered, this value should be considered to be an upper limit. 2.3. Physical Methods. Electrochemical measurements were performed with a CH Instruments 660D potentiostat. A three-compartment electrochemical cell, with a Luggin capillary and a glass frit separating the working electrode and counter electrode (Pt mesh) compartments, was used. The Ag/AgCl (sat. KCl) reference electrode was calibrated against an aqueous solution of Na4[Fe(CN)6]. Electrochemical potentials were corrected for uncompensated resistance (Ru, typically ca. 15 Ω) and are given in the text relative to the reversible hydrogen electrode. Steady-state j−E plots were acquired in triplicate using staircase voltammetry (10-mV steps, 50-s intervals). Three distinct electrodes were used. The values in the text represent the average between the three separate electrodes of a given composition and the uncertainty represents the standard deviation between the samples. Film stability was probed by applying a constant 1 mA cm−2 for 24 h while monitoring the necessary potential. All current densities were calculated using geometric surface area. UV−vis spectroscopy was performed with a Varian Cary 5000 spectrometer equipped with a diffuse reflectance accessory (DRA-2500). Fourier transform infrared reflectance (FTIR) spectroscopy was carried out with a Varian FTS7000 equipped with a Universal Reflectance Sampling Stage. X-ray diffraction patterns were obtained using a Rigaku Multiflex θ−2 θ diffractometer (scan speed = 0.016° min−1, Cu Kα tube, λ = 1.5406 Å). SEM images were collected using a Zeiss Σigma VP field-emission scanning electron microscope. A Physical Electronics PHI VersaProbe 5000-XPS was used to record XPS spectra. The spectra were acquired using a monochromatic Al Kα source (1486.6 eV, 49.3 W) and beam diameter of 200.0 μm. A double neutralization (low-energy electron beam and low-energy Ar+ beam) was used during spectrum acquisition. Survey scans (0−1350 eV) were performed to determine elemental composition. High resolution scans were collected with a pass energy of 23.50 eV. Arsputtering experiments were performed to confirm that C 1s

3. RESULTS AND DISCUSSION The photochemically induced decomposition of coordination complexes has been shown to proceed for transition metal compounds in a variety of ligand environments. Low molecular weight ligands (e.g., carbonyls,23 alkyl carboxylates,12,13,24,25 βdiketonates26) can be liberated from the metal by a lightinduced ligand-to-metal charge transfer (LMCT) transition to form a reduced metal film that subsequently reacts with oxygen to form amorphous metal oxide films. The rate of decomposition is sensitive to the intensity of the absorptivity of the metal precursor at the specific wavelength of irradiation.13 On this basis, the concomitant formation of ozone when photolysis is carried out in ambient conditions does not appear to govern the reaction. Although a number of metal oxides have been prepared using this protocol, the formation of a-IrOx has not yet been documented. The precursor Ir(acac)3 was chosen for this study because it is readily available by commercial sources, the complex is characterized by the requisite LMCT band in an accessible region of the electromagnetic spectrum, and the complex does not appear to form microscopic domains on the substrate upon deposition on substrates. The spin-cast films of Ir(acac)3 on FTO were photolyzed with a 5.8-W lamp characterized by intense peaks at 185 and 254 nm. The decomposition of the precursor upon exposure to light was tracked by monitoring the disappearance of the absorbance bands associated with the acac ligand using FTIR spectroscopy (Figure 1). Quantitative loss of the sharp absorbance bands at ca. 3000 and 1500 cm−1 indicate complete loss of the acac ligands within 60 min of irradiation. The concomitant growth of a broad absorbance band centered near 3200 cm−1 was observed during this procedure that appears to correspond to surface hydroxyl groups (Ir(O)OH).27 The resultant blue-gray films were then annealed in air for 1 h at 100 °C to render a-IrOx. Crystalline films of iridium oxide (c-IrOx) were prepared by heating the films at 500 °C for 1 h. The amorphous nature of a-IrOx was verified by the lack of any Bragg reflection peaks associated with crystalline phases of iridium oxide in the X-ray diffraction (XRD) patterns (Figure 2). The growth of diffraction peaks at 28.0°, 40.0°, and 53.4° in the diffraction patterns of the heated films is consistent with the formation c-IrOx on FTO (JCPDS 15-790). 1655

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Figure 3. Scanning electron micrographs at two different magnifications after one (a, b) and two (c, d) depositions of a-IrOx on FTO.

Figure 1. FTIR reflectance spectra, provided as a (a) normalized Kubelka−Munk function and (b) baseline-corrected Kubelka−Munk function, tracking the photoinduced decomposition of spin-cast Ir(acac)3 on FTO. Dashed green lines highlight the absorbance bands for Ir(acac)3 that disappear during photolysis.

Figure 4. XPS survey scans of a-IrOx (blue trace) and c-IrOx (red trace).

Figure 2. X-ray diffraction patterns for a-IrOx and c-IrOx on FTO (smoothed using a 25-point average). The Bragg reflections for IrO2 (JCPDS 15−790) and SnO2 (JCPDS 41−1445) are included for reference.

Scanning electron microscopy (SEM) of a-IrOx prepared from a single coating of Ir(acac)3 on FTO revealed incomplete coverage of the substrate (Figure 3). Full coverage could nonetheless be achieved by repeating the deposition process to produce a second layer of the metal oxide film (Figure 3). The first coating resulted in a smooth morphology with a-IrOx islands showing a low apparent porosity, similar to those previously reported for a-FeOx.12 Upon application of the second coating, the deposited material provided better coverage of the substrate and a less textured substrate. X-ray photoelectron spectroscopy (XPS) survey scans identified the film constituents to be Ir, O, and C for both aIrOx and c-IrOx, with a trace of Cl present in a-IrOx (Figure 4). High-resolution spectra revealed nearly superimposable features in the Ir 4f regions for both phases: c-IrOx was characterized by 4f7/2 peaks at binding energies of 62.21 and 65.22 eV, whereas signals for a-IrOx were found at 62.37 and 65.37 eV (Figure 5).28−31 The high-resolution spectra of the O 1s region showed

Figure 5. X-ray photoelectron spectra detailing the Ir 4f7/2, 5/2 and O 1s regions of a-IrOx (blue traces, top) and c-IrOx (red traces, bottom) films on FTO glass. Sum of the fitting components is provided in gray; oxygen components from adventitious carbon is given in black.

hydroxide and oxide signatures at 531.5 and 530.4 eV, respectively, for both phases.22 The spectrum for a-IrOx, however, was found to contain a substantially higher contribution of OH− groups (76%) relative to c-IrOx (62%). Such a trend of higher surface hydroxylation was also observed for OER catalysts containing first-row transition metals prepared by this photochemical decomposition method.13 1656

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The observation of C 1s peaks in Auger and XPS spectra for films prepared by this photolysis method have been documented to originate from surface contaminants enabling residual ligands from the synthesis to be excluded.25,26,32,33 To corroborate this scenario in the present work, XPS depth profiles were obtained for both a-IrOx and c-IrOx. A series of high-resolution XPS spectra were acquired following periods of Ar+-sputtering (Figure 6). A complete loss of the C 1s signal

Figure 7. Cyclic voltammograms recorded on films of a-IrOx (blue) and c-IrOx (red) at a scan rate of 10 mV s−1 in 1 M H2SO4. Inset: Steady-state current density plots recorded under the same conditions.

Figure 8. UV−vis reflectance spectra illustrating the electrochromic behavior for a-IrOx (blue) and lack thereof for c-IrOx (red) with applied potentials of +1.0 V vs RHE (solid) and 0 V vs RHE (dashed). Figure 6. XPS depth profiles reveal a complete loss of C 1s signal within 2 nm of the surface for both (a) a-IrOx and (b) c-IrOx. The Ir 4f signal is maintained throughout the experiment.

the reversible precatalytic peaks at ∼0.80 V vs NHE (at pH ∼3). Moreover, the cyclic voltammetric data of the title a-IrOx films track the behavior reported for anodized, metallic iridium electrodes immersed in 1 M H2SO4 that produce reversible redox processes at 0.90 and 1.35 V vs SHE.34 On the basis of the similarities in peak location and CV structure, we tentatively assign the reversible peak set observed for a-IrOx in this study to a reversible Ir(IV)/Ir(III) redox couple for hydrated iridium oxide,35 an assignment that is supported by XPS showing a high concentration of hydroxide groups. An Ir(V)/Ir(IV) redox couple was previously reported for anodized iridium electrodes,34 but only a reduction peak could be discerned for the films studied here; we therefore tentatively assign this response to an Ir(V)/Ir(IV) redox process based on similar peak locations with the caveat that Ir(VI) may also be active.34−36 The electrochemical behavior of c-IrOx deviated from that of the amorphous phase; it did not reveal any redox activity prior to water oxidation (Figure 7) nor did it exhibit electrochromic behavior (a dark blue-gray color was maintained under all conditions studied; Figure 8). This behavior appears to be universal for iridium oxides that are exposed to high temperatures20,34,37,38 and has been previously linked to a dehydration of the oxide surface (i.e., fewer Ir(OH) present at the oxide surface).34,37 Steady-state j−E experiments (Figure 7, inset) indicated that similar overpotential (η) requirements were needed to initiate catalytic water oxidation for a-IrOx (0.13 ± 0.01 V) and c-IrOx

was observed within 2 nm of the surface for both a-IrOx and cIrOx, whereas the Ir 4f region did not change. This finding, combined with the lack of ligand-based vibrational modes in the FTIR spectra (Figure 1), indicates that a complete loss of acac ligands occurs. We emphasize that this approach enables the fabrication of a-IrOx without exposure to high temperatures; the ligands are completely removed from the film through the photochemically induced reaction. The electrochemical properties of a-IrOx were recorded in 1 M H2SO4 by cyclic voltammetry and steady-state measurements of current density versus external bias (Figure 7). Cyclic voltammetry on a-IrOx revealed a set of peaks with E1/2 = 0.81 V vs RHE (Ep,a = 0.85 V, Ep,c = 0.77 V). A color change occurred during this redox event; the film was colorless at potentials below the E1/2 value and converted to blue-gray at higher potentials. Optical profiles at variable applied potentials document this behavior (Figure 8). A further oxidative sweep (Figure 7) leads to the onset of water oxidation catalysis at ca. 1.3 V, with the reverse cathodic sweep producing a reduction peak at Ep,c = 1.29 V. The electrochromic and voltammetric behavior of the amorphous films prepared by photochemical decomposition in this study corresponds to amorphous films produced by the electrodeposition from an organometallic precursor,20 including 1657

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(0.13 ± 0.02 V). The a-IrOx film, however, exhibited a Tafel slope of 34 ± 2 mV dec−1, which is superior to the 60 ± 1 mV dec−1 slope measured for c-IrOx. This improvement in Tafel slope enables a-IrOx to reach current densities of 0.5 and 1.0 mA cm−2 at η of 0.18 ± 0.01 and 0.19 ± 0.01 V, respectively. In contrast, c-IrOx required 0.24 ± 0.01 V and 0.26 ± 0.01 V to reach the same respective current densities. A 24-h galvanostatic electrolysis was carried out to probe the stability of the catalysts under working conditions. Following a ca. 10-mV increase in potential over the first 4 h, a-IrOx maintained a constant potential for the remainder of the 24-h test (Figure 9). Under identical testing conditions, c-IrOx required an additional 30 mV over the first 4 h to maintain a constant current, after which a stable potential was observed.

complicated by disparate reaction conditions, the a-IrOx films reported in this study nonetheless compare favorably to these state-of-the-art formulations. In view of the superior OER electrocatalysis demonstrated by the a-IrOx and the versatility of the synthetic method, investigations are underway to deposit conformal films of aIrOx on photoactive materials to exploit the favorable features of this film within composite photoanode/electrocatalyst architectures.

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ASSOCIATED CONTENT

S Supporting Information *

Steady-state j−E plots. This information is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*S. Trudel. E-mail: [email protected]. *C. P. Berlinguette. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank National Science & Engineering Research Council (Canada) and Mitacs for financial support. C.P.B. is also grateful to Canadian Research Chairs and the Alfred P. Sloan Foundation for support. This research used facilities funded by the University of Calgary and the Canadian Foundation for Innovation.

Figure 9. Electrochemical potential required to maintain a 1 mA cm−2 current density with a-IrOx (blue) and c-IrOx (red). Following an initial increase of approximately 10−30 mV over the first 4 h, both films exhibited stable behavior.

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REFERENCES

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4. SUMMARY We have demonstrated the light-driven decomposition of a readily accessible molecular precursor, Ir(acac)3, to be an effective technique for the fabrication of amorphous phases of iridium oxide films that act as effective OER catalysts. The a-IrOx films prepared herein compare favorably to those prepared by alternate methods. Anodically electrodeposited aIrOx reached 0.5 mA cm−2 at similar η values20 but produced an inferior Tafel slope of 60 mV dec−1 compared to the ∼30 mV dec−1 of the title films. Films produced by the electrochemical oxidation of metallic iridium produced a Tafel slope reasonably consistent with the films prepared in our study, but the onset of catalysis was higher and, therefore, an additional >50 mV is needed to reach 0.5 mA cm−2 compared to the films reported here.34 The catalytic behavior of the photochemically generated films in this report also compare favorably to nanoparticulate IrOx materials reported in the recent literature;36,39−42 for example, an electroflocculated film of nanoparticles is characterized by one of the lowest known η values yet still requires an additional ∼100 mV to reach 0.5 mA cm−2 compared to the title films.36 Other notable OER catalysts in the literature include Ba0.5Sr0.5Co0.8Fe0.2O3‑δa composition that sits at the peak of the so-called “volcano plot”which reaches 1 mA cm−2 at 1.55 V vs RHE and 10 mA cm−2 at 1.6 V,43 whereas electrically and thermally deposited first-row transition metal oxides (e.g., CoOx, NiOx, FeNiOx, NiLaOx, Ni0.9Fe0.1Ox) reach 10 mA cm−2 at potentials as small as 1.56 V vs RHE.22,44 With the caveat that a direct comparison of electrocatalytic properties is 1658

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