Stabilization of Polyoxometalate Water Oxidation Catalysts on

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Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition Sarah M. Lauinger, Brandon Deane Piercy, Wei Li, Qiushi Yin, Daniel Lynn CollinsWildman, Elliot N. Glass, Mark D Losego, Dunwei Wang, Yurii V Geletii, and Craig L Hill ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12168 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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ACS Applied Materials & Interfaces

Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition Sarah M. Lauinger,1 Brandon D. Piercy,2 Wei Li,3 Qiushi Yin,1 Daniel L. Collins-Wildman,1 Elliot N. Glass,1 Mark D. Losego,2 Dunwei Wang,3 Yurii V. Geletii,1 and Craig L. Hill1* 1

Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta Georgia 30322,

United States 2

School of Materials Science and Engineering, Love Manufacturing Building, Georgia Institute

of Technology, 771 Ferst Drive NW, Atlanta Georgia 30313, United States 3

Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street,

Chestnut Hill, Massachusetts 02467, United States Keywords:

Polyoxometalates,

Atomic

layer

deposition,

Hematite,

Water

oxidation,

Immobilization ABSTRACT: Fast and earth-abundant-element polyoxometalates (POMs) have been heavily studied recently as water oxidation catalysts (WOCs) in homogeneous solution. However, POM WOCs can be quite unstable when supported on electrode or photoelectrode surfaces under applied potential. This article reports for the first time that a nanoscale oxide coating (Al2O3) applied by atomic layer deposition (ALD) aids immobilization and greatly stabilizes this now large

family

of

molecular

WOCs

when

on

electrode

surfaces.

In

this

study,

ACS Paragon Plus Environment

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[{RuIV4(OH)2(H2O)4}(γ-SiW10O34)2]10- (Ru4Si2) is supported on hematite photoelectrodes, then protected by ALD Al2O3, and this ternary system was characterized before and after photoelectrocatalytic water oxidation by FTIR, XPS, EDX and voltammetry. All these studies indicate that Ru4Si2 remains intact with Al2O3 ALD protection, but not without. The thickness of the Al2O3 layer significantly affects the catalytic performance of the system: a 4 nm thick Al2O3 layer provides optimal performance with nearly 100% Faradaic efficiency for oxygen generation under visible light illumination. Al2O3 layers thicker than 6.5 nm appear to completely bury the Ru4Si2 catalyst, removing all catalytic activity, while thinner layers are insufficient to maintain long term attachment of the catalytic POM. Introduction A growing world population and global rises in the standard of living have intensified increases in CO2 emissions, negatively impacting the environment.1 Combustion of fuels for transportation is a major source of these CO2 emissions. Solar fuels—or renewable fuel sources generated from sunlight—are one solution to this society challenge. One such solar fuel solution is the photoelectrochemical splitting of water into H2 fuel and O2.2-5 The water oxidation step remains a key roadblock. Despite the current availability of many heterogeneous water oxidation catalysts (WOCs), it is highly difficult to elucidate the mechanism of the four-electron, four proton oxidation of H2O to O2 at the molecular level in these systems (e.g understanding the atoms and orbitals involved in the multiple proton-coupled electron-transfer (PCET) steps and oxygen-oxygen bond formation process).

In contrast, homogeneous WOCs provide the

opportunity to investigate water oxidation at the molecular level; however, the organic ligands are susceptible to oxidative and hydrolytic degradation.


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Polyoxometalates (POMs) have been extensively studied as both water oxidation and reduction catalysts.6 These molecular transition-metal-oxide clusters have been immobilized on many surfaces for water oxidation.6-13 The prototypical WOC POM, [{RuIV4(OH)2(H2O)4}(γSiW10O34)2]10- (Ru4Si2), has been incorporated into a triad system with a Ru-based chromophore covalently bound to a TiO2 support to demonstrate photoelectrochemical activity.14 In another study, Ru4Si2 was immobilized on TiO2 and the catalytic water oxidation studied under UV illumination without the use of a chromophore.15 Under oxidative conditions, the system proved to be efficient but lost ~15% of the initial potential over 24 hours of continuous photoelectrochemical operation. Immobilization of molecular catalysts has become a rapidly growing subfield of solar fuels research.14, 16-18 However, these heterogeneous catalysis systems are susceptible to degradation via detachment of molecular catalysts under operating conditions.19-20 One technique to improve the stability of surface-immobilized species is atomic layer deposition (ALD).21-24

This

technique, which has also been used for other energy conversion applications, including nanostructured photovoltaics25 and energy storage,26 is a self-limiting, sequential thin film deposition process that enables the growth of highly conformal coatings with atomic resolution. A typical ALD sequence includes four steps: dosing of the gas-phase precursor, purging with inert gas to remove excess precursor, dosing of gaseous co-reactants, and purging again to remove excess co-reactants and by-products.25,

27

This sequence is repeated to increase the

coating’s thickness. By selecting precursors with high vapor pressures and rapid reaction rates, ALD can be used at low temperatures where organic materials, including molecular catalysts, are chemically stable.20, 28-37 Prior studies of hematite photoanodes with Al2O3 deposited thin films have reported reduce photocurrent density, presumably due to prevention of hole injection from


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hematite to the electrolyte.38 To date, the use of ALD overlayers with the purpose of protecting catalysts on the surface has only been applied to coordination-compound catalyst assemblies.39 To our knowledge, the ALD stabilization of POM catalysts has never been explored. In this report, we evaluate the performance of an ALD protected POM WOC applied to a hematite photoelectrode. Hematite is a widely-studied, visibly active photoelectrode for water oxidation (band gap of 1.9 to 2.2 eV) because of its earth abundance and low cost.40 Here, we demonstrate considerable stabilization of POM WOCs on hematite photoanodes by applying 0 to 8 nm of ALD deposited amorphous Al2O3 “on top of” the POM WOC. The effect of Al2O3 thickness on the photocurrent density of the system as well as oxygen generation have been assessed, and ALD “encapsulation” layer shows promise for extending the durability of these POM catalysts. Experimental Section General Materials and solvents were purchased as ACS analytical or reagent grade and used as received. Rb8K2[{RuIV4(OH)2(H2O)4}(γ-SiW10O34)2] (Ru4Si2) was prepared as previously described.41 The isostructural but non-water-oxidation-active POM, K10[Zn4(H2O)2(PW9O34)2] (Zn4P2)

was

prepared

by

published

methods.42

The

silylating

agent,

3-

aminopropyltrimethoxysilane (APS), was purchased from Sigma Aldrich and used as received. Hematite nanoparticles 20-40 nm in diameter were purchased from SkySpring Nanomaterials, Inc. and modified with APS silylating agent. Synthesis of hematite films Fluorine-doped tin oxide (FTO) substrates (~7 Ω.sq-1, Sigma) were cleaned with acetone, methanol and isopropanol. Following cleaning, FTO substrates were soaked in a solution of 0.15


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M iron(III) chloride hexahydrate (FeCl3. 6H2O) and 1 M sodium nitrate (NaNO3), then dried in an oven at 100 °C for 1 h. The backside of the FTO substrates was protected by Kapton tape to prevent growth during the synthesis. Afterwards, the tape was removed, the substrate was rinsed with deionized (DI) water and dried with N2 gas. Films were then annealed at 800 °C for 5 min to convert the resulting FeOOH to hematite. All hematite photoelectrodes labeled as hematite in this manuscript refer to samples subjected to the growth-annealing cycles three times. Preparation of hematite-APS-Ru4Si2 films and nanoparticles Hematite films and iron oxide nanoparticles were soaked in a 10 mL toluene solution with 0.1 mmol 3-aminopropyltrimethoxysilane (APS) overnight at 70 °C. The films were removed from the APS solution and rinsed with toluene, acetone, and ethanol before being soaked in a solution of aqueous Ru4Si2 for several hours. Nanoparticles were collected from the toluene/APS solution by centrifuging. The nanoparticles were washed, sonicated and collected by centriguging in a series with toluene, acetone, and ethanol before being soaked in a solution of aqueous Ru4Si2 for several hours. The resulting films and nanoparticles were rinsed with water before all measurements and atomic layer deposition. Atomic layer deposition Al2O3 ALD was conducted using a custom-built, hot wall, flow-tube reactor with a 1.5-inch inner diameter. The reactor was operated at approximately 1.5 Torr using ultra-high purity nitrogen (99.999%, Airgas) as the process gas with a downstream O2 purifier (SAES MicroTorr). Trimethylaluminum (TMA, 99%, Strem) was used as the aluminum precursor, with deionized water as the co-reactant. The deposition zone was maintained at 120 °C for all depositions, while the process gas lines were fixed at 110 °C to prevent precursor condensation. The ALD operational parameters were as follows: 0.2 s TMA/30 s purge/0.2 s H2O/30 s purge, with


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process gas constantly flowing. Deposition was controlled by a LabVIEW sequencer.43 Film thicknesses were measured by spectroscopic ellipsometry on witness Si wafer pieces using a Cauchy model for the Al2O3 layer. For deposition on nanoparticles, a porous plastic bag was filled with dried nanoparticles and heat sealed prior to being placed in the ALD chamber. Characterization Fourier transform infrared spectroscopy (FTIR) spectra were collected on a Nicolet 6700 FTIR spectrometer with a diamond ATR smart accessory in the range of 4000 – 550 cm-1. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were conducted at Clemson University’s Advanced Materials Center using a Hitachi 3400 SEM. X-ray photon spectroscopy (XPS) measurements were performed using a Thermo K-Alpha XPS. A monochromated aluminum K-alpha source (1486.6 eV) was used for photoelectron excitation. The analysis chamber’s base pressure during collection was approximately 5x10-8 Pa. Survey scan spectra were measured over a pass energy of 200 eV at 1eV energy steps. High resolution scans were performed over a pass energy of 50 eV at 0.1 eV energy steps. All measurements were made on samples prepared on FTO/glass substrates. Samples were standardized against crystalline Ru4Si2. Samples were dried in a vacuum oven before introduction into the XPS chamber. The Thermo Kα flood gun was used for charge neutralization during all experiments. Analysis of all spectra were performed on CasaXPS software (version 2.3.18). A mixture of Gaussian and Lorentzian functions (GL(30)) were used to fit the high resolution peaks of W, Ru, and C. No quantitative fittings were done for the high resolution XPS of the other elements. Reasonable fit was achieved using symmetric line shapes for all elements. Fitting constraints were applied in such a manner as to be chemically sensible. Doublet peaks for tungsten were fit using a separation of 2.17 eV and a ratio of 4:3 between the 4f7/2 and the 4f5/2


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peaks; doublet peaks for ruthenium were fit using a separation of 4.2 eV and a ratio of 3:2 between the 3d5/2 and the 3d3/2 peaks. The C 1s peak from adventitious carbon contamination obfuscated the Ru 3d peaks of the POM. Thus, quantification of Ru was based on the deconvolution of the visible Ru 3d5/2 peak. The C 1s peaks are deconvoluted assuming the presence of C-C, C-O-C, and O-C=O states, as is common with adventitious carbon contamination. TEM images of iron oxide nanoparticles coated with Ru4Si2 and Al2O3 ALD were collected on an FEI Tecnai TEM. Particles were dispersed in ethanol and drop cast onto TEM grids (S10) Preparation of hematite/FTO electrodes coated with Ru4Si2 and varying layers of ALD The working electrodes were fabricated from the films prepared as discussed above. A sharp blade was used to scrape off the exterior ALD, Ru4Si2, and hematite to expose the conductive FTO. A copper wire or copper tape was fixed to the exposed FTO surface using conductive silver adhesive 503 (Electron Microscopy Sciences). The copper wire or copper tape was insulated using Epoxy adhesive (Henkel Loctite Hysol 1C Epoxi). Electrodes were then dried in an oven at 60 °C for 1 hour prior to use. Photoelectrochemical measurements Photoelectrochemical reactions were carried out using a standard three-electrode cell for measurements on a Pine Research Instrument WaveDriver 20 bipotentiostat. All potentials were measured against a 1 M KCl Ag/AgCl reference electrode (+0.237 V vs. RHE) purchased from Bioanalytical Systems, Inc. A platinum wire was used as the counter electrode and working electrodes were fabricated as discussed earlier. Bulk photoelectrolysis and linear scan voltammetry with a 10 mV/s scan rate were conducted in a custom rounded 50-mL PEC cell with a flat quartz window and four arms equipped with airtight adapters for electrodes and headspace


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access (Tudor Scientific Glass). The cell was purged with Ar gas prior to use and a blanket of Ar gas was provided during measurements unless otherwise mentioned. For white light and some visible light experiments, a 150-W Xe light source was focused on the flat quartz window with and without a 400-nm cutoff filter with 160 mW/cm2 power at the film. For oxygen measurements, unless otherwise stated, a 455-nm LED was used for illumination. It provided 9.5 mW/cm2 of power at the film’s surface. Measurements were conducted in 240 mM sodium borate buffer, pH 8.3 with 0.1 M KNO3 electrolyte. For bulk photoelectrolysis, the potential was held at 1.24 V vs. RHE for multiple hours under either visible or white light with gentle stirring and an argon gas blanket in a homemade “argon box” (Figure S9) to ensure an air-free atmosphere. Oxygen Measurements Oxygen measurements were performed in a 40 mm x 10 mm quartz cuvette (Science Outlet) with a custom-fitted Teflon lid containing holes for counter, reference, and working electrodes and a FOXY Forspor oxygen probe. Solutions of 240 mM borate buffer, pH 8.3 and 0.1 M KNO3 were purged with Ar gas for 30 minutes before use. The 1.0 M KCl Ag/AgCl reference electrode, Pt wire within a tube with a Nafion proton membrane tip, and working electrodes were inserted through the Teflon lid and secured with paraffin and vacuum grease. The Ar-purged solution was added into the cell through an open hole in the lid until the solution overflowed and the last open hole was sealed with grease. The closed cell was placed into the homemade “argon box” at constant argon flow with a quartz window for light passage and septa for the potentiostat electrode clips. The potential was held at either 1.24 V vs. NRHE or open circuit potential during oxygen measurements with visible and white light illumination by either a 455-nm LED light source or by a Xe arc lamp. The oxygen probe was placed directly in front of the working


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electrode with the initial oxygen level dissolved in solution serving as the baseline for the experiment. An airtight box with argon flow was utilized so that all oxygen sensed by the probe was due to the oxygen generation at the electrode. Faradaic yields were calculated from the current passed through the working electrode during bulk photoelectrolysis. Results and discussion In an earlier study15, the silylating technique to immobilize Ru4Si2 onto metal oxides using 3aminopropyltrimethoxysilane (APS) was characterized by ATR-FTIR, XPS, EDX and electrochemical techniques. Figure 1 shows FTIR spectra for hematite surfaces after various chemical treatments, including attachment of APS and APS-POM. The characteristic POM WOt, W-Oa,b,c-W stretches are present, wherein W-Ot represents the terminal oxygen stretching around 990 cm-1; the W-Oa-Si, W-Ob-W, and W-Oc-W of the POM are found between 876 and 945 cm-1. This surface functionalization is further verified by visual inspection of the material as demonstrated in Figure S1, which shows photographs of functionalized and unfunctionalized nanoparticles dispersed in Ru4Si2 in water, illustrating the direct binding of Ru4Si2 to hematite in the films and nanoparticles.


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Figure 1. FTIR-ATR of (a) Red line: hematite; (b) Green line: hematite with APS; and (c) Blue line: hematite-APS-Ru4Si2.

Figure 2 gives the photocurrent density for various hematite photoanodes with and without Al2O3 ALD protection. The red and black lines in these figures are reference scans of pure hematite photoanodes, with and without illumination, respectively. Comparing the solid blue (hematite-APS-Ru4Si2) and solid red (hematite only) curves in Fig. 2a, we conclude that adding the Ru4Si2 POM catalyst to the hematite photoanode only slightly enhances the photocurrent density. However, over prolonged electrolysis (1.24 volts for 3 hours), this photocurrent density degrades (dotted blue line). We do not observe such degradation for pure hematite at this pH (Figure S2). ALD “encapsulation” of the hematite-APS-Ru4Si2 structure (solid blue curve in Figure 2b) appears to modestly increase the photocurrent density and reduce the onset potential (0.8 V vs. NRHE versus 0.9 V vs. NRHE for bare hematite). However, more significant is that


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this photoanode can withstand prolonged cycling without a loss in performance (dotted blue line in Figure 2b). This result suggests that the Al2O3 ALD coating is protecting the attachment of the POMs on the surface under operating conditions. We also note that the addition of APS and/or an Al2O3 ALD layer by itself degrades the performance of the hematite photoanode (green curve in Figure 2a and orange curve in Figure 2b respectively). 3

3

(a)

2.5

(b)

2.5

2

J / mA cm-2

J / mA cm-2

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


1.5 1

2 1.5 1 0.5

0.5 0

0 0.6

0.9

1.2

1.5

V vs RHE / V

1.8

0.6

0.9

1.2

1.5

V vs RHE / V

1.8

Figure 2. Linear voltammograms for photoanodes (a) without and (b) with Al2O3 ALD protection. In both figures, the solid red line is the illuminated-hematite-only reference scan and the black curve is the hematite-only reference scan collected in the dark. (a) Green line: Hematite-APS; Solid blue line: initial hematite-APS-Ru4Si2; Dotted blue line: hematite-APSRu4Si2 after 3 hours of bulk electrolysis. (b) Orange line: hematite with a 1nm of Al2O3 coating; Solid blue line: hematite-APS-Ru4Si2-4nm Al2O3 initially; Dotted blue line: hematite-APSRu4Si2-4nm Al2O3 after 3 hours of bulk electrolysis. Conditions: 0.1 M KNO3, 240 mM sodium borate buffer, pH 8.3; white light illumination with an Xe light source (160 mW/cm2).


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To further understand the physical structure of these Al2O3 ALD “encapsulated” POM photoanodes, we conducted additional photoelectrochemical and structural measurements on a series of hematite-APS-Ru4Si2-Al2O3 structures of varying Al2O3 thickness ranging from 0 to 8 nm. (Note that all thicknesses are based upon ellipsometry measurements of monitor silicon wafers included in the reactor chamber with the hematite photoanodes.) Figure 3 presents the photoelectrolysis data for these photoanodes. Here, we observe POM-functionalized hematite photoanodes with Al2O3 coatings of nominally 1, 3, and 4 nm thickness to be photoelectrochemically active for water oxidation at 1.24 V vs. NRHE. However, at 6.5 nm, photocurrent density drops to near zero, suggesting that the Al2O3 layer has completely covered the POM catalyst blocking hole transfer to the electrolyte. Depictions of the hypothesized structures—at approximate scale—are shown along the right side of Figure 3.


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Figure 3. Current densities, in mA/cm2, and schematic illustrations of photoanodes with varying thickness of Al2O3 ALD under white light illumination (160 mW/cm2) of (a) Blue line: hematiteAPS-Ru4Si2-4nm Al2O3; (b) Orange line: hematite-APS-Ru4Si2-3nm Al2O3; (c) Red line: hematite-APS-Ru4Si2-1nm Al2O3; and (d) Green line: hematite-APS-Ru4Si2-6.5nm Al2O3. Conditions: 0.1 M KNO3, 240 mM sodium borate buffer, pH 8.3, potential held at 1.24 V vs. . We further confirm these hypothesized structures using chemical and structural analysis of the photoanodes before and after photoelectrolysis. Figure 4 provides bulk chemical analysis of these photoanodes using EDX before and after photoelectrolysis. The expected 5:1 W:Ru ratio


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is observed for POM functionalized hematite before and Al2O3 ALD. However, after 3 hours of photoelectrolysis, EDX signals for uncoated, 1 nm, 2 nm, and 3 nm Al2O3 ALD coated samples show no chemical evidence of any Ru4Si2 catalyst remaining. In contrast, photoanodes protected with 4 and 6 nm of Al2O3 ALD still show POM present (5:1 ratios of W:Ru). In fact, this POM is still detectable on the 4 nm coated photoanode after 12 hours of photoelectrolysis (The full EDX spectra for this photoanode is included in Figure S4). This analysis further suggests that Al2O3 ALD layers of a critical thickness (~4 nm) can maintain the attachment of POMs to a photoanode surface during photoelectrolysis.

Figure 4. EDX atomic percentages of W (green), Ru (red), and Al (light blue) for films Hematite-APS-Ru4Si2-x nm Al2O3 varying deposition thickness of Al2O3 ALD (0-6nm) before photoelectrochemistry (PEC), after 3 hours PEC and after 12 hours PEC. Each film varies in initial Ru4Si2 POM concentration.


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XPS analysis of the photoelectrode’s surface chemistry further confirms the presence of Ru and W after 4nm of Al2O3 ALD deposition (Figure S5). It is significant that these elemental constituents of the POM are detectable by XPS after Al2O3 ALD since XPS only probes a few nanometers below a material’s surface. The simple detection of these elements suggests that the Al2O3 coating may not entirely cover the POM at small layer thicknesses. These same elements are still detectable in XPS after 12 hours of photoelectrolysis, further confirming the ability to protect these catalysts (Figure S5). The binding energies of the Ru 3d5/2 (282 eV), and W 4f7/2 (35.2 eV) peaks are consistent with Ru(IV) and W(VI), respectively, as expected for the Ru4Si2 POM. Figure 5 compares XPS spectra for the W 4f signals before and after photoelectrochemical experiments. This data provides insight into the Al2O3 coverage and the accessibility of Ru4Si2 catalyst to the electrolyte. Increasing Al2O3 thickness from 4 nm to 6 nm, significantly decreases the W 4f peak intensities, suggesting that the Al2O3 layer is beginning to “overcoat” the POM. The increasing Al 2p signal intensity is shown in Figure S6. After 3 hours of bulk photoelectrolysis, W is no longer detectable on the uncoated photoanode, suggesting that all of the POM has detached from the surface. In contrast, a significant fraction of the W 4f intensity remains for the photoanode protected with 4nm of Al2O3 ALD.


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Figure 5. High resolution XPS spectra of the W 4f peak for hematite-APS-Ru4Si2-x nm Al2O3 photoanodes before and after 3 hours of photoelectrochemistry. Photoanodes have Al2O3 ALD coatings of 0 nm (red), 4nm (blue), and 6nm (green) nominal thickness.

Finally, oxygen measurements were made during photoelectrolysis to assess Faradaic efficiency for water oxidation. Figures 6 and S7 show the oxygen yield and the Faradaic efficiency under a biased potential and no applied potential, respectively. Under a biased potential and illumination, hematite electrodes generate oxygen, but the “theoretical” Faradaic yield is only 85% (Figure 6(a)).

The high photocurrent density, but less than 100% Faradaic

yields, is consistent with some competing electron-hole recombination within the film.

In

comparison, hematite-APS-Ru4Si2-4nm Al2O3 photoanodes exhibit 100% Faradaic yield under


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applied potential conditions (Figure 6b). Hematite photoanodes with just a 4 nm of Al2O3 ALD coating show nearly zero water oxidation (Figure 6(c)). As a second control, we examined oxygen generation when a sandwich-type POM, K10[Zn4(H2O)2(PW9O34)2] (Zn4P2), that is not a water oxidation catalyst was substituted for the Ru4Si2 on the hematite surface. Again, near zero oxygen generation was detected (Figure 6d). This result further confirms that the Ru4Si2 catalyst is the source for hole transfer at these photoanode/electrolyte interfaces. Table 1 further summarizes the oxygen yields and Faradaic efficiencies measured for these photoanodes under 455-nm LED light illumination. Figure S8 compares the oxygen generation under white light illumination and visible light illumination.


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Figure 6. Oxygen measurements (markers) and theoretical Faradaic yields (lines) for (a) Red squares, line: hematite; (b) Blue triangles, line: hematite-APS-Ru4Si2-4nm Al2O3; (c) Orange circles, line: hematite-4nm Al2O3; and (d) Green X’s, line: hematite-APS-Zn4P2-4nm Al2O3 under visible light illumination, 455 nm LED (9.5 mW/cm2). Conditions: 0.1 M KNO3, 240 mM sodium borate buffer, pH 8.3, potential held at 1.24 V vs. RHE. Table 1. Oxygen yield and Faradaic efficiency of electrodes after 2 hours of 455-nm LED illumination under OCP and 1.24 V applied potential. Applied Potential

Moles O2 (μmol/L)

Faradaic Efficiency

OCP

12.85

44%

1.24 V

107.37

85%

OCP

36.76

50%

1.24 V

70.02

98%

Hematite-APS-Ru4Si2-no Al2O3

1.24 V

0.54

8%

Hematite-4nm Al2O3

1.24 V

0.74

3%

Hematite-APS-Zn4P2-4nm Al2O3

1.24 V

0.18

2%

Electrode

Hematite

Hematite-APS-Ru4Si2-4nm Al2O3

Conclusions Atomic layer deposition was successfully used to protect the attachment of POM WOCs on the surface of visible-light absorbing hematite photoanodes. Without this protection layer, Ru4Si2 POMs rapidly detach from the hematite’s surface under photoelectrolysis conditions. For appropriately thin Al2O3 ALD coatings, the POM catalyst remains sufficiently exposed to the aqueous electrolyte to permit hole transfer and water oxidation with nearly 100% Faradaic


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efficiency. Beyond a critical thickness of 6.5 nm, the Al2O3 layer appears to “overcoat” the POM and photoelectrochemical activity degrades dramatically.

Thinner Al2O3 coatings ( 3 hrs). However, under optimized conditions (here observed at ~ 4 nm Al2O3 ALD), the Al2O3 layer can protect the POM catalysts for extended periods of photoelectrolysis (> 12 hours) without a loss in performance. The Al2O3 ALD protected Ru4Si2-functionalized hematite photoanodes show improved Faradaic efficiencies relative to unfunctionalized hematite photoanodes. Thus, the concept of protecting efficient molecular water oxidation catalysts on light absorbing supports using Al2O3 ALD has proven to be effective in this system and may be broadly relevant to the immobilization and protection of other molecular multi-electron-process catalysts for large-scale technological applications.

Associated Content SEM/EDX data on the derivatized semiconductor metal oxides (SMOs) with and without the POM water oxidation catalyst and before and after use; XPS of Ru 3d spectra before and after bulk photoelectrolysis; XPS of Al 2p spectra of varying Al2O3 ALD depths; bulk photoelectrolysis of hematite electrodes; long-term bulk photoelectrolysis of hematite-APSRu4Si2-4nm Al2O3; O2 evolution measurement under 455-nm LED illumination with no applied bias potential; O2 evolution measurements under white light illumination with an applied bias potential. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*Craig L. Hill, Email: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgment We thank the Department of Energy, Office of Basic Sciences, Solar Photochemistry program (grant number: DE-FG02-07ER15906) for support of this work. BDP acknowledges support by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. We thank the Clemson University Advanced Materials Center for support. This work was also performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). TEM images were collected by Jeremy Yoo and Dr. Joshua Kacher, School of Materials Science and Engineering, Georgia Tech. TEM analysis was conducted in the Institute for Electronics and Nanotechnology (IEN) at Georgia Tech.

Abbreviations POM, polyoxometalate; WOC, water oxidation catalyst; APS, 3-aminopropyltrimethoxysilane; ALD, atomic layer deposition


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Stabilization of Polyoxometalate Water Oxidation Catalysts on

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