Nanostructured Antireflective Iridium Oxide Coating for Water

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A Nanostructured Anti-Reflective Iridium Oxide Coating for Water Oxidation David Baker, Milena Graziano, and Brendan Hanrahan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03874 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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The Journal of Physical Chemistry

A Nanostructured Anti-Reflective Iridium Oxide Coating for Water Oxidation

David R. Baker*, Milena B. Graziano, Brendan M. Hanrahan Sensors and Electron Devices Directorate, U.S. Army Research Laboratory Adelphi, MD 20783, USA

*Corresponding author; [email protected]

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Iridium-oxide (IrOx) is one of the best catalysts for the aqueous oxygen evolution reaction (OER), and its activity is greatly impacted by surface characteristics. By reactively sputtering in a high O2 flow-rate environment, vertically oriented IrOx nano-platelets grow several hundred nanometers high exhibiting large surface areas, and anti-reflective optical properties across the visible spectrum. The nanoplatelet IrOx surface is electrochemically compared to other morphologies of IrOx surfaces for OER activity. It was found that the nano-platelet IrOx surface outperforms all other tested morphologies and planar Ir metal by exhibiting higher currents and lower overpotentials. Longevity testing of catalytic activity shows that the nano-platelet surface is more stable in a wide pH range. Characterization with x-ray diffraction and x-ray photoelectron spectroscopy shows that the stoichiometry and oxidation states are similar between the different morphologies of IrOx, but the preferred crystallographic orientation of the rutile IrOx film changes at higher O2 flow-rates. The change from a (110) to a (101) growth direction corresponds with higher OER activity. Nano-platelet IrOx films are therefore found to expose active sites preferable for the OER, and when combined with their anti-reflective properties these surfaces are promising for solar watersplitting applications.


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Introduction High energy efficiency (>20%) splitting of water into hydrogen and oxygen gasses is one of the most sought after technological goals in the renewable energy field.1-2 It would allow clean energy storage from many renewable sources to provide a stabilizing stopgap during periods of low energy production. Stored hydrogen gas could be used by fuel cells to efficiently deliver electricity on-demand, or even in internal combustion engines for mechanical energy.3 The large potential benefits of developing a successful technology have driven many research groups to employ both exotic and earth abundant materials for increasingly efficient electrolyzers.4-7 Light absorption efficiency is one of the primary factors contributing to the overall solarto-hydrogen conversion efficiency (STH) for solar energy driven water splitting systems. One method used to improve the amount of light absorbed by a photoelectrode is the incorporation of an anti-reflective layer. Typically, anti-reflective surfaces are created by patterning or roughening semiconductor light absorbers in such a way that light is trapped by the features instead of being lost due to reflection off a planar surface.8-10 Catalyst nanoparticles can then be dispersed onto the patterned surface to reduce overpotentials for the water splitting reactions, and any photogenerated carriers are transferred from the underlying semiconductor to the catalysts which then perform the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Common patterning techniques like ion etching, chemical etching, and photolithography can dramatically impact structural properties of the surface of semiconductor films, such as chemical composition, strain, and crystallinity. These parameters are intimately linked to the optical characteristics of the semiconductor material, which must be finely tuned to ensure optimal STH performance.11-12 Creating nano-patterned arrays by marring the surface of



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semiconductors, therefore, may not be feasible for many of the highly specialized materials that have shown recent promise.11 Since catalysts are still required to perform the water-splitting reactions on the surface of an electrode the above concerns could be alleviated if the desired antireflective properties were designed into a catalyst structure instead of the semiconductor absorber. Between the HER and OER water-splitting half reactions OER is the more difficult due to its 4-electron requirement expressed in equation 1: 2H2O ⇄ 4e- + 4H+ + O2, E° = 1.23 V vs RHE

(1)

This high density requisite of positive surface charge carriers results in large overpotentials ranging from 0.3 V to over 1 V for common OER materials like TiO2, ZnO, and Pt at 1 mA/cm2.13-14 In contrast, iridium oxide (IrOx) has demonstrated the lowest overpotentials, routinely under 0.5 V, as a catalyst for the OER, and has also been shown to be extremely resilient to the oxidative environment of the OER in a wide range of aqueous media, especially acidic environments where most other metal-oxides corrode.15-17 As such, IrOx proves to be the most promising OER catalyst for high efficiency solar water-splitting devices. Typically, IrOx is incorporated in the photoelectrochemical system by depositing a layer of small (~2 nm) nanoparticles onto the surface of an electrode as depicted in the schematic of Figure 1A. These nanoparticulate films have been deposited either by electrochemically controlled oxide deposition from an iridium-salt directly onto a working electrode,18-20 oxidizing an iridium metal surface,21-22 or by casting a colloidal suspension onto the working electrode.23-24 However, another benefit of IrOx is its high conductivity,25-26 so a more substantial surface


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structure would be able to pass charges more efficiently with minimal loss and potentially even prevent some interfacial recombination. Reactive sputtering is another common practice for creating IrOx coated surfaces. The method provides flexibility of thickness, orientation, crystal phase and other attributes based on simple sputtering parameters.27-30 The sputtering atmosphere is often an Ar/O2 mixture whereby the flowrates of each gas impact many characteristics of the surface morphology.31-32 As the O2 flow rate increases beyond the compound sputtering rate (i.e. when the metal target is also oxidized) and nano-structures of vertically oriented platelets have been shown to form on the desired sample which can extend several hundred nanometers above the substrate.31, 33-35 These nano-platelets grow from the top of a thin condensed layer, composed also of IrOx. The shape and orientation of these structures make the surface visibly dark, almost black to the human eye, acting as anti-reflection coatings, as depicted in the schematic of Figure 1B. Most commonly, this nano-platelet “black-IrOx” (b-IrOx) has been used in bioengineering fields to coat medical devices for sensor applications and electrode stimulation because of its activity, conductivity, and stability in many relevant pH environments.33,

36

However, the literature is scant for

performance characteristics of b-IrOx as a catalyst for the OER. Because of its potential to simultaneously be an efficient OER catalyst and an anti-reflective coating, this study examines the OER catalytic activity and stability of sputtered b-IrOx nano-platelets in various aqueous media.



Figure 1: A schematic of light interacting with the surface of a nano-particle catalyst-coated semiconductor whereby a portion of the incident light is reflected (A), and a semiconductor coated with an anti-reflection catalyst design (B). The reflectance of the sputtered films is shown visually (C), and spectroscopically (D). Experimental Fabrication Ir and IrOx thin films were deposited by 70 s of reactive sputtering on commercially supplied single crystal (100) Si wafers with 500 nm of thermally grown SiO2. The sputtering target was a 300 mm disk of iridium (99.9%) metal. Depositions were performed using a DC cathode power of 1 kW, an Ar flow rate of 100 sccm, and a substrate temperature of 250°C. The oxygen flow rate was set to 0 sccm, 60 sccm, and 100 sccm to deposit three surface morphologies: metallic Ir, a surface of grey appearance (g-IrOx), and an anti-reflective black surface (b-IrOx), respectively. A sputtered 100 nm layer of Ir was first deposited before the


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deposition of the g-IrOx and b-IrOx surfaces for electric continuity. Target conditioning via presputtering in an Ar atmosphere was conducted prior to each film growth. Characterization Imaging of as-deposited films was performed with scanning electron microscopy (SEM) using a ZEISS Auriga FE-SEM at 2 kV accelerating voltage, and a FEI-Quanta 200F environmental FE-SEM in high-vacuum mode at 20 kV accelerating voltage. The structural properties of the sputtered films were investigated using grazing incidence x-ray diffraction θ-2θ scans recorded in a Bruker D8 Discover system with a Cu tube source (λ = 1.5405 Å) and a VÅNTEC-500 area detector. The chemical states of the sputtered IrOx surfaces were analyzed with x-ray photoelectron spectroscopy (XPS) using a ULVAC-PHI Versaprobe III calibrated with sputtered Ag and Au targets. A 25 W monochromatic Al-Kα x-ray beam was focused to a spot size of 100 µm. Surfaces were neutralized with an Ar-ion beam and BaO electron neutralizer, in an operating vacuum 200 mV/dec) for all surfaces in NaClO4 may indicate that in the more neutral conditions the first chemical adsorption steps are slowing the OER kinetics, and there may be some adsorbed species blocking the active sites on all the surfaces preventing facile water adsorption. This may then also be the case for g-IrOx in NaOH and g-IrOx in NaOH point to this step being the rate determining step. Interstitial cations formed during fabrication have been noted to impact the OER activity, but in this case no contaminants were observed during XPS survey scans of asprepared electrodes, Figures S1-3.18

Figure 6: Representative Tafel plots of the different IrOx morphologies in 0.1M HClO4 (A), 0.1M NaClO4 (B), and 0.1 M NaOH. Each trace was obtained while bubbling with N2 and represents the Tafel region of the OER prior to polarization.


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Exposed crystal faces and edge sites have a dramatic impact on the OER activity for IrO2 catalysts. A DFT study by Sen and co-workers calculated that the adsorption energy of molecular oxygen was -3.22 eV/O atom on the (110) face compared with -3.06 eV/O atom on the (101) face.45 Some corner sites were even calculated to have adsorption energies 1.4 eV lower than the (110) face. Since g-IrOx grew preferentially in the (110) direction as a coherent film with only some surface roughness most of its surface would hold onto any generated oxygen longer than the high density of edges and corners which composed the b-IrOx films. The crystal orientation of the flat faces parallel to the electrode surface seen in Figure 2D is indeterminate based on the XRD data, and may contribute to the overall increase in activity through increased adsorbate mobility, but more information is required to determine the activity and orientation of those faces. By having a higher density of edge sites b-IrOx electrodes required less energy to desorb oxygenated products or, other adsorbed species at catalytic sites thereby lowering the onset potentials and overpotentials.

The active gravimetric surface area of IrOx electrodes has been shown to be proportional to the voltammetric charge passed when scanning CVs of IrOx between the water splitting reactions.17,

46-47

This relation is used for the electrochemical active surface area (ECSA)

measurement which can be used to relatively compare the surface areas of various electrodes by simply comparing the integrated charge within a CV cycle, as long as the experimental parameters of scan speed and potential range remain equal. The charge includes capacitance over the surface as well as any charge passed with respect to Ir redox processes, which showed a redox wave at 0.8 V vs Ag/AgCl. It should be noted that due to the minimal amount of material on each sputtered film reported here, standard gravimetric surface area measurement techniques,



such as BET, were attempted, but were found to have significant error in weight measurements, and were therefore deemed unreliable, thereby requiring the electrochemical method instead. From CV’s run at 50 mV/s between 0.4 and 1.0 V vs Ag/AgCl in the HClO4 solution the voltammetric current density was integrated to obtain a total charge passed for the three different morphologies of IrOx, see Figure 7. As expected from its roughly planar surface, Ir-metal showed the lowest surface charge at 0.15 mC/cm 2 followed by g-IrOx at 0.44 mC/cm2. With 2.48 mC/cm2 b-IrOx showed the highest ECSA, an increase of almost 17 times that of the planar Irmetal, and 6 times that of g-IrOx. All voltammetric charge densities were with respect to the geometric area. As mentioned above, this method is proportional to the actual surface area, it is not the absolute value, but between the samples it is clear that b-IrOx, in addition to its antireflective properties, also has significantly more active surface area on a given geometric area consistent with the SEM images of Figure 2.


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Figure 7: Effective capacitance surface area analysis of the three sputtered IrOx surfaces. Values are the integrated voltammetric current from 50 mV/s scans, CVs of these scans are shown in the inset, and were performed in 0.1 M HClO4. The electrodes’ activities were compared by normalizing with respect to relative ECSA areas. By holding the electrodes at 1.49 V vs RHE for five minutes, and while under N2 bubbling, the resulting current densities were compared to account for variations caused by increased surface area and is tabulated in Table 1.17 Open circuit potentials before and after are presented in the supplemental (Figure S7). Across all electrolyte solutions g-IrOx was found to have the highest activity per ECSA whereas b-IrOx and Ir-metal were similar to each other, but with 2-3 times less activity than g-IrOx (normalized with respect to Ir-metal in Figure S6). One reason for the unintuitive higher activity of g-IrOx over b-IrOx may be due to the structure of bIrOx limiting the OER reaction deep into the pores. The ECSA measurement reacts to the surface redox properties and surface capacitance. These are influenced by the electrolyte only nanometers into solution whereas while performing the OER an entire pore may fill with a bubble and become inactive. This would imply that the b-IrOx electrodes at high current densities may not have been performing at their maximum potential. Efforts are currently being made to modify the fabrication of b-IrOx in order to grow a more open structure which may result in fewer trapped O2 bubbles and an activity approaching g-IrOx. It is also evident that the current density with respect to ECSA is much higher for HClO4 than NaClO4 or NaOH elelctrolytes. If indeed there is a blocking species preventing water adsorption onto active sites or limiting surface diffusion then Na+ ions may be a culprit. This is further evidenced by the presence of Na on the surface of used IrOx samples in XPS survey scans, Figures S1-3.



To test the stability of the catalyst surface over an extended operational period, electrodes were held at 1 mA/cm2 for 2 h. Figure 8 shows the potential required to maintain that current density for each surface morphology in the three media. Any sharp discontinuities are the result of bubbles breaking from the electrode surfaces. The lower overpotential of b-IrOx compared to the other two morphologies previously observed with LSV was evident in all three media. At steady state the b-IrOx was 0.3-0.4 V lower in overpotential than the metallic Ir electrodes, and 0.1-0.3 V lower than g-IrOx despite similar growth conditions and chemical composition. Additionally, b-IrOx was the only one of the three surface morphologies which maintained a constant operating potential over the course of the tests, demonstrating that in addition to having the highest activity it is also the most stable surface. A common trend observed across all three media was that metallic Ir surfaces displayed a large growth in overpotential until a relatively stable state was reached; after ~0.5 h for HClO4 and NaClO4, and after ~1 h for NaOH. In all three media g-IrOx drifted to higher overpotentials over time without finding a steady state within the 2 h experiments. The differences in overpotential can partly be attributed to the increased nano-scale edge-site surface area improving the activity per geometric area. This high activity in combination with its stability across a wide pH range shows that b-IrOx has great potential as an OER catalyst. The chemical nature of the surface, based on XPS analysis, after 2 h of OER was the same before testing and after in all cases, importantly with no change in oxidation observed for any surface including the metal and g-IrOx. If not chemical, the source of the unstable response is likely to be structural just as the difference in initial catalytic activity was based on the abundance of expose edge sites.


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Figure 8: Longevity studies holding electrodes at 1 mA/cm2 for 2 h in 0.1 M HClO4 (A), 0.1 M NaClO4 (B), and 0.1 M NaOH (C).

Conclusions IrOx films were sputtered onto Ir-metal coated SiO2 surfaces which created a catalytic layer to perform the OER. By changing the flow-rate of O2 in the sputter chamber from 60 to 100 sccm the surface morphology changed from a rough continuous film, g-IrOx, to a thick vertically-oriented array of nano-platelets, b-IrOx, which displayed anti-reflective properties. Despite more O2 in the reaction chamber, XPS analysis showed the two IrOx coatings had similar valence electronic structure and stoichiometry. GI-XRD analysis of b-IrOx and g-IrOx displayed rutile-IrO2 peaks, but the preferential orientation of the crystal growth direction changed from (110) to (101) corresponding with an increase in O2 flow-rate, and the growth of nano-platelets. The catalytic activity of the nano-platelets in b-IrOx performing the OER was far superior to that of the rough g-IrOx surface, a metallic-Ir surface, and a Pt disc electrode. Tafel slopes confirmed that b-IrOx films had more favorable OER kinetics than g-IrOx and Ir-metal likely caused by



exposing more catalytically favorable edge sites, and an increased surface mobility of intermediates. After being held at 1 mA/cm2 the surfaces of g-IrOx and metallic Ir displayed drift in the applied potential indicating that the Ir surfaces were changing in the oxidative environment of the OER. On the other hand b-IrOx was stable in all three pH media tested for the full 2 h tests. Its anti-reflective properties allow b-IrOx to absorb >70% more light across the visible spectrum than the metallic Ir surface which makes it attractive as an OER catalyst for solar driven water-splitting semiconductors. The b-IrOx surface was therefore found to be an extremely promising catalytic coating for the water splitting OER. Acknowledgements The authors would like to thank Dr. Blair Connelly for her assistance with the reflectivity measurements. Also, the authors would like to thank Dr. Thomas Parker for his assistance with the glancing angle x-ray diffraction measurements. Funding was provided internally by the U.S. Army Research Laboratory. Supporting Information XPS survey scans before and after use, XPS scans of the Ir 4f and O 1s transitions before and after Ar-ion milling, XPS peak fitting of O1s, the current density at 1.49 V vs RHE normalized to the Ir –metal, open circuit potential before and after potentiostatic testing. References 1.

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TOC Graphic


Nanostructured Antireflective Iridium Oxide Coating for Water

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