Solution Synthesis of Cu3PdN Nanocrystals as Ternary Metal Nitride

Oct 8, 2014 - Here we report the solution-phase synthesis and character- ization of antiperovskite-type Cu3PdN nanocrystals that are multifaceted, uni...
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Solution Synthesis of Cu3PdN Nanocrystals as Ternary Metal Nitride Electrocatalysts for the Oxygen Reduction Reaction Dimitri D Vaughn, Jose Araujo, Praveen Meduri, Juan F. Callejas, Michael A. Hickner, and Raymond E. Schaak Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5029723 • Publication Date (Web): 08 Oct 2014 Downloaded from on October 19, 2014

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Solution Synthesis of Cu 3 PdN Nanocrystals as Ternary Metal Nit ride Elect rocatalysts for the Oxygen Reduction Reaction Dimitri D. Vaughn II,† Jose Araujo,† Praveen Meduri,‡ Juan F. Callejas,† Michael A. Hickner,‡ Raymond E. Schaak†,* †

Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 (USA) ‡ Department

of Materials Science and Engineering and Penn State Institutes of Energy and the Environment, The Pennsylvania State University, University Park, PA 16802 (USA) KEYWORDS nanoparticles, nanoparticle synthesis, metal nitrides, intermetallic compounds, antiperovskite structure, electrocatalysis, oxygen reduction reaction

ABSTRACT: The synthesis of transition metal nitride nanoparticles is challenging, in part because the unreactive nature of the most common nitrogen reagents necessitates high temperature and/or high pressure reaction conditions. Here we report the solution-phase synthesis and characterization of antiperovskite-type Cu3PdN nanocrystals that are multi-faceted, uniform, and highly dispersible as colloidal solutions. Colloidal Cu3PdN nanocrystals were synthesized by reacting copper(II) nitrate and palladium(II) acetylacetonate in 1-octadecene with oleylamine at 240 °C. The Cu3PdN nanocrystals were evaluated as electrocatalysts for the oxygen reduction reaction (ORR) under alkaline conditions, where both Cu3N and Pd nanocrystals are known to be active. The ORR activity of the Cu3PdN nanocrystals appears to be superior to that of Cu3N and comparable to that of Pd synthesized using similar methods, but with significantly improved mass activity than Pd control samples. The Cu3PdN nanocrystals also show greater stability than comparably-synthesized Pd nanocrystals during repeated cycling under alkaline conditions.

er-temperature methods often require the design and utilization of more reactive nitrogen sources.19-21 For example, metal nitride thin films can be synthesized using chemical vapor deposition and atomic layer deposition,19 nanocrystalline metal nitrides can be synthesized solvothermally,22-25 and bulk metal nitrides can be accessed by reacting metal oxides with alternative nitrogen sources such as urea and cyanamide at intermediate temperatures.21 Transition metal nitrides also can be accessed using other synthetic pathways, including solid-state metathesis,26 flux reactions,27 and colloidal chemistry.14,28,29 For the most part, the synthetic challenges outlined above make it difficult to access transition metal nitrides as nanocrystals. It can be even more difficult to render them colloidally stable for solution-phase utilization and processing.25 Modifications to some of the synthetic pathways that produce bulk transition metal nitrides can achieve a limited palette of nanostructural features, including porosity and agglomerated nanoparticulates, which succeed at producing higher surface area materials.21,30 A growing number of high quality metal nitride nanowires are also accessible using nanoparticlecatalyzed vapor-liquid-solid (VLS) and related pathways.31,32 However, it remains a significant challenge to access colloidal transition metal nitride nanocrystals directly in solution25 using the synthetic protocols that are most common for generating

INTRODUCTION Transition metal nitrides represent an important class of materials with interesting and useful catalytic, optoelectronic, electrochemical, and structural functions.1-15 These properties underpin their widespread use as semiconductors for optoelectronic devices,2-4 photoactive materials for solar cells,5-6 refractory and hard materials for structural applications and coatings,7-9 heterogeneous catalysts for industrial chemical production and organic synthesis,10-12 and materials for energy conversion and storage.13-15 Nanostructures of transition metal nitrides are often desired for improving their properties and further expanding their applications. For example, metal nitride nanoparticles offer higher surface areas than their bulk counterparts, and this can improve catalytic performance by maximizing the number of exposed active sites while offering better dispersibility and increased catalyst-support interactions.10 The synthesis of transition metal nitrides is often challenging because of the generally unreactive nature of the most common nitrogen sources, which include N2 and NH3, at low and moderate temperatures.1,16 As a result, high temperature and/or high-pressure methods are often used,16,17 as are plasma, laser, and physical vapor deposition techniques.16,18 Low-

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metal, alloy, oxide, and chalcogenide nanocrystals. Such methods primarily involve thermally- or chemically-triggered decomposition of soluble reagents in high boiling organic solvents in the presence of ligands that bind to the nanocrystal surfaces to mediate their nucleation and growth, influence their morphology, and render them dispersible in liquid solvents. Among the small number of colloidal transition metal nitride nanocrystals that has been reported is Cu3N, which can be synthesized as nanocubes by decomposing copper(II) nitrate in octadecylamine at approx. 250 °C.14,28,33 These Cu3N nanocubes were reported to be highly active electrocatalysts for the oxygen reduction reaction (ORR) under alkaline conditions.14 The ORR, which is the cathode reaction that occurs in proton exchange membrane fuel cells and also underpins metal-air batteries, is typically facilitated by noble metals.34-36 The discovery of effective ORR catalysis by Cu3N nanoparticles is significant, because it introduces a new catalytic material that is comprised entirely of inexpensive and Earthabundant elements.14 Minimizing the noble metal content of ORR catalysts, while still achieving acceptable performance, is a high-priority research goal,36 as exemplified by recent examples of effective ORR electrocatalysis by metal alloys,37 metal nitrides,14 perovskite oxides,38 and carbon-based materials.39 One of the drawbacks of the Cu3N ORR electrocatalyst, however, is its high overpotential relative to noble metals such as Pt.14 A viable alternative strategy for designing new ORR electrocatalysts is to incorporate a noble metal catalyst in a composite or alloy matrix that is comprised of less expensive materials, effectively diluting the expensive metal in a less expensive matrix and increasing the mass activity of the catalyst through synergistic enhancement effects.34,40-43 This has been achieved for nanostructured systems such as Fe-Pt-M, where M = Pd, Au, Cu, and Ni,44,45 as well as Au-CuPt.46 However, there are fewer examples of structurally ordered binary or higher-order noble metal intermetallic compounds that catalyze the ORR. Notable examples include Cu3Pt,47 Pt-M, where M = Fe, Co, and Ni,48-49 and AuCu.50 The Cu3N-Pd system provides an interesting opportunity to design a new intermetallic ORR electrocatalyst. First, both Cu3N and Pd are known ORR catalysts.14,51-53 Second, Cu3N adopts an anti-ReO3 crystal structure, which consists of corner-shared N-centered Cu-N octahedra arranged in a cubic lattice (Figure 1).14 This structure contains a central void that, if filled, becomes the antiperovskite structure that is common for related ternary metal (“M”) nitrides such as Ca3MN,54 InNM3,55 and MFe3N56 (Figure 1). This suggests that the ternary intermetallic compound Cu3PdN is a viable target as a new ORR electrocatalyst.

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Indeed, antiperovskite-type Cu3PdN (and isostructural substoichiometric members Cu3PdxN) is known and has been studied both experimentally and theoretically.57-62 Most notable is that insertion of Pd into Cu3N induces a change from semiconducting to metallic behavior.60-62 Studies of Cu3PdxN are, however, limited, with most reports focusing on films. Bulk Cu3PdxN was prepared by reacting [Cu(NH3)2]NO3 and [Pd(NH3)4](NO3)2 in supercritical ammonia at 500 °C and 6 kbar.57 Cu3PdxN is metastable, decomposing at 470 °C to produce Cu3Pdx and N2.57 The observation that Cu3PdxN is synthesized at relatively low temperatures (temperatures below which most common nitrogen sources are reactive) and that it decomposes at similarly low temperatures suggests that Cu3PdxN is an ideal target for colloidal nanoparticle synthesis, which is an inherently low-temperature technique that is known to generate compounds that are metastable in bulk form.63,64 Accordingly, here we report the solution-phase synthesis of colloidal Cu3PdN nanocrystals that are single crystalline, multi-faceted, and uniform. The combination of known pathways to Cu3N and Pd nanocrystals yields Cu3PdN, which is, to our knowledge, the first report of high quality colloidal nanocrystals of ternary transition metal nitrides. Importantly, the Cu3PdN nanocrystals are active ORR electrocatalysts. The ORR activity of the Cu3PdN nanocrystals, which is improved substantially relative to that of comparable Cu3N nanocrystals, is similar to that of Pd, but with significantly enhanced stability under alkaline conditions.

EXPERIMENTA L SECTION Chemicals and Materials. Copper(II) nitrate trihydrate [Cu(NO3)2·3H2O, 99+%], oleylamine (70%), and 1-octadecene (90%) were purchased from Sigma Aldrich. Palladium(II) acetylacetonate [Pd(acac)2, 99.99%] was purchased from Alfa Aesar. Oleylamine and octadecene were degassed prior to use, and all other chemicals were used as received without further purification. All syntheses were carried out under Ar using standard Schlenk techniques, and the workup procedures were performed in air. Synthesis of Cu3PdN. Cu(NO3)2·3H2O (60 mg) and Pd(acac)2 (25.2 mg) were dissolved in 7.5 mL of 1-octadecene and 2.5 mL of oleylamine. The solution was then transferred to a 100-mL three-necked round-bottom flask fitted with a condenser, thermometer adapter, thermometer, and rubber septum and was degassed under vacuum at 120 °C for 10 min. The flask was then filled with Argon, heated to 240 °C, and kept at this temperature for 15 min. The flask was then removed from the heating mantle and allowed to cool to room temperature. A solid was precipitated by adding 25 mL of ethanol and then centrifuging at 12,000 rpm for 5 min. The product was washed three more times using a 1:1 toluene/ethanol mixture and was finally suspended in hexanes for further characterization. Synthesis of Cu3N. The synthetic procedure was analogous to that of Cu3NPd, except that there was no Pd reagent. Materials Characterization. Powder X-ray diffraction (XRD) data were collected using a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained using a JEOL 1200 EX II

Figure 1. Crystal structures of anti-ReO3-type Cu3N and antiperovskite-type Cu3PdN.

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TEM operating at 80 kV. High-resolution TEM (HRTEM) images were collected using a JEOL 2010 LaB6 microscope and a JEOL 2010F field-emission microscope, both operating at an accelerating voltage of 200 kV. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS) was performed using the JEOL 2010F, which was outfitted with an EDAX solid-state X-ray detector. Samples were prepared by suspending the washed products in hexanes and drop-casting onto Formvar-coated copper TEM grids. X-ray photoelectron spectroscopy (XPS) analysis was performed on a monochromatic Al Kα source Kratos Axis Ultra operating at 14 kV and 20 mA for an X-ray power of 280 W. Spectra were collected with a photoelectron take-off angle of 90° from the sample surface plane and were referenced to the C1s peak with a binding energy of 285 eV. Electrocatalytic Testing. Electrochemical testing was performed in a 0.10 M KOH solution with dissolved Ar and O2 using a Solartron potentiostat at room temperature. The working electrodes with controlled and consistent mass loadings were prepared by dropcasting a known volume of the catalyst nanoparticles at fixed solution concentrations onto glassy carbon electrodes, producing a thin film of the nanoparticles on the glassy carbon electrode after solvent evaporation. Reported mass loadings, which were constant at approx. 20 µg/cm2 for all samples, refer to the masses of the Cu3N, Cu3PdN, and Pd nanocrystals deposited onto the electrodes. The reference electrode was Ag/AgCl(saturated KCl) and the counter electrode was a Pt wire in the same solution. The measured currents were not corrected for the ohmic resistance of the solution since the electrodes were located in close proximity to one another and held at similar positions throughout all of the testing. Cyclic voltammetry (CV) was performed between 0 and 1.1 V (vs Ag/AgCl) for Cu3N and Cu3PdN nanoparticles whereas the limits for Pd nanoparticles were 0.1 and -1.0 V (vs Ag/AgCl) at multiple scan rates of 5 mV/s, 10 mV/s, 50 mV/s and 100 mV/s. ORR polarization curves were obtained between the potentials of 0 and -1.1 V (vs Ag/AgCl) for Cu3N and Cu3PdN nanoparticles 0.1 and -1.0 V (vs Ag/AgCl) for Pd nanoparticles (due to the lower equilibrium potential of Pd relative to Cu3N and Cu3PdN), all at scan rates of 10 mV/s at rotation rates of 400 rpm, 1000 rpm, 1600 rpm, and 2500 rpm.

varying the ratio of the copper and palladium reagents during the synthesis of Cu3PdN, various substoichiometric Cu3PdxN products could be formed. Figure S1a shows powder XRD data for Cu3PdxN samples with nominal compositions of x = 0, 0.25, 0.5, 0.75, and 1. The lattice constants and experimentally measured compositions (based on EDS analysis of the Cu:Pd ratio) increase monotonically with x, as shown in Figure S1b. This result provides evidence that the amount of Pd inserted into Cu3N can be varied, which is consistent with the compositional variability that is known for Cu3PdxN and related antiperovskite compounds.57,58,61 However, the Cu3PdxN samples with x = 0.25, 0.5, and 0.75 have a small Pd impurity, so only nominally stoichiometric Cu3PdN is used for the indepth characterization and electrocatalytic studies that follow.

Figure 2. Powder XRD data for Cu3N and Cu3PdN nanocrystals, along with the corresponding simulated diffraction patterns for comparison.

Figures 3a and 3b shows representative TEM images of the Cu3PdN nanocrystals. The particles are multi-faceted with a quasi-cubic morphology and have an average diameter of 16 ± 2 nm. The corresponding SAED pattern in Figure 3c exhibits diffraction rings with positions and intensities that are consistent with those expected for antiperovskite-type Cu3PdN, including the (110) and (211) superlattice reflections. Figure 4 shows a high-resolution TEM image of a representative Cu3PdN nanocrystal, indicating that the particles are polycrystalline. The observed lattice spacings of 2.2 and 1.9 Å correspond to the (111) and (200) planes of Cu3PdN, respectively. Scherrer analysis of the powder XRD pattern for the Cu3PdN nanocrystals indicates an average grain size of approx. 8 nm, which is smaller than the average particle size of 16 nm observed by TEM and therefore is consistent with the polycrystalline nature of the nanocrystals observed by HRTEM. STEM-EDS line scans and element maps, shown in Figure 5, were used to confirm that the Cu and Pd were co-localized in each nanoparticle. Figure 5a shows a STEM-EDS line scan across a single particle of Cu3PdN and indicates that the Pd and Cu signals are co-located, with an approximate 3:1 Cu:Pd ratio. The STEM-EDS element maps in Figure 5b also show that the Cu and Pd signals overlap spatially throughout two representative particles. Taken together, the XRD, TEM, HRTEM, SAED, and EDS data confirm the formation of antiperovskite-type Cu3PdN nanocrystals. XPS measurements further confirmed that Cu, Pd, and N were present in the nano-

RESULTS AND DISCUSSION Nanocrystals of Cu3PdN were synthesized by heating a mixture of Cu(NO3)2·3H2O, Pd(acac)2, octadecene, and oleylamine to 240 °C and holding at that temperature for 5 min. Figure 2 shows a representative powder XRD pattern for the product, which matches well with the simulated pattern for antiperovskite-type Cu3PdN that is shown for comparison. The powder XRD pattern for Cu3N nanocrystals, also shown for comparison in Figure 2, is related to that of Cu3PdN, but with significantly different relative peak intensities and measurably different lattice constants. For Cu3N, a = 3.799(1) Å, while for Cu3PdN, a = 3.822(1) Å, which matches well with that expected for Cu3PdN [alit = 3.854 Å].57 Cu3PdN also has a distinct XRD pattern and measurably different lattice constant relative to known Cu-Pd intermetallic compounds, including the structurally-related Cu3Au-type cubic phase Cu3Pd (a = 3.676 Å), the long-period ordered superstructure of tetragonal Cu3Pd (a = 3.710 Å, c = 25.655 Å), and the tetragonal ordered Cu4Pd phase (a = 5.826 Å, c = 7.328 Å).65 Interestingly, by

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crystals. XPS spectra taken from the Cu 2p, Pd 3d, and N 1s regions of a representative Cu3PdN nanocrystal sample (Figure 5c) match well with those expected based on previous studies of Cu3PdxN films.66 In the N 1s region, the peak near 397.5 eV is consistent with the N environment in Cu3N, and the higher-energy shoulder is attributed to the amine ligands. The Pd 3d region shows binding energies near 335.5 and 341 eV, and these are most consistent with zero-valent Pd. The Cu 2p region shows two main peaks at 932.5 and 952.5 eV, along with two weak satellite peaks, and these values and features are consistent with those previously observed in Cu3PdxN.66

Figure 5. (a) STEM-EDS line scan across a single Cu3PdN nanoparticle, showing the linear distribution of the Cu Kα and Pd Lα signals. (b) STEM-EDS element map, showing the spatial distribution of the Cu Kα and Pd Lα signals across two particles. (c) XPS data for a representative sample of Cu3PdN nanoparticles, showing the Cu 2p, Pd 3d, and N 1s regions.

To evaluate their performance as an electrocatalyst for the ORR in O2-dissolved 0.10 M KOH, the Cu3PdN nanocrystals were deposited as thin films onto the surface of a glassy carbon electrode at a mass loading of approx. 20 μg/cm2. The mass loadings, which were kept constant at 20 μg/cm2 for all experiments, were controlled by using known concentrations of catalyst nanoparticles in solution and known volumes of solution deposited onto the electrodes. Pd and Cu3N nanocrystals synthesized using a protocol analogous to that of Cu3PdN were also tested for comparison. Figure 6 shows polarization curves (current vs voltage, relative to an Ag/AgCl reference electrode) for these systems, with the half-wave potential (E1/2) used as a measure of the catalytic activity. Potentials between 0 and -0.3 V correspond to a mixture of both the kinetic and diffusion regions for both Cu3PdN and Pd. Pd is an active ORR catalyst as expected, with E1/2 = -0.07 V. Cu3N nanocrystals also catalyze the ORR, with E1/2 = -0.45 V; this observation is comparable to a previous report.14 The Cu3PdN nanocrystals are also active ORR catalysts, with E1/2 = -0.13 V. This E1/2 value is only 60 mV more negative than Pd, making it comparable to Pd in terms of ORR activity. The polarization curve for Cu3PdN shows a small hump between –0.35 and –0.6 V that is consistent with additive behavior between Cu3N and Pd. However, the materials characterization data presented in the preceding paragraphs, as well as TEM and SAED data for the particles after electrochemical cycling (presented below), indicate that the majority of the sample is Cu3PdN, although some contribution from separate Cu3N and Pd cannot be unambiguously ruled out. The Koutecky-Levich equation was used to determine the electron transfer number,

Figure 3. (a,b) TEM images and (c) corresponding SAED pattern for the Cu3PdN nanocrystals. Two of the superlattice reflections characteristic of the antiperovskite structure of Cu3PdN, are highlighted in (c).

Figure 4. HRTEM image of a representative Cu3PdN nanocrystal.

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which was calculated to be 3.7 at a potential of -0.3 V (Figure S2). (See Supporting Information for calculation details.) This observation most closely corresponds to the preferred ORR pathway involving a single step four-electron transfer to form H2O, rather than the less desirable two-step pathway that involves the formation of an H2O2 intermediate that can degrade the catalyst over time.

The Cu3PdN nanocrystals, along with Pd for comparison, were tested for their durability by cycling them 1000 times between 0.1 and -1.1 V vs Ag/AgCl. Figure 8a shows polarization curves for a representative sample of Cu3PdN nanocrystals cycled for 1, 250, 500, and 1000 times. The half wave potential exhibits a positive shift of 20 mV from the 1st to the 250th cycles, followed by a negative shift of 15 mV in subsequent cycles. Overall, the change in activity throughout the 1000 cycles is minimal. During cycling, the Cu3PdN nanocrystals remain intact and non-agglomerated, without any observable changes in morphology, size, crystallinity, or crystal structure. This is confirmed by TEM images for the assynthesized Cu3PdN nanocrystal sample that was used for the cycling studies (Figure 8b) and the same sample after 7 (Figure 8c) and 1000 (Figure 8d) cycles, as well as the corresponding SAED patterns (Figure S3). These observations demonstrate that the Cu3PdN nanocrystals are highly stable under repeated ORR cycling.

Figure 6. Polarization curves in 0.10 M KOH (room temperature, 1600 RPM) for Cu3PdN, Cu3N, and Pd nanoparticle ORR electrocatalysts deposited as thin films on glassy carbon electrodes. All mass loadings are 20 µg/cm2 based on the masses of Cu3N, Cu3PdN, and Pd deposited onto each electrode.

Figure 7 shows the mass activities of the Cu3PdN and Pd catalysts. The kinetic currents at a potential of -0.1 V (vs Ag/AgCl) were used to determine the activities. Mass activities, calculated per unit weight of the total catalyst, normalize the catalytic activity to practical issues of material mass and cost, which are most impacted by the noble metal content. Cu3PdN contains only 34% Pd by weight, but has a mass activity that is 22% higher than that of Pd on a total mass basis. When expressed on a per-atom basis rather than a total mass basis, the Pd in Cu3PdN has more than 300% higher activity than the pure Pd control when normalized per Pd atom.

Figure 8. (a) Polarization curves in 0.10 M KOH (room temperature, 1600 RPM) for Cu3PdN nanoparticles on glassy carbon electrodes after 1, 250, 500, and 1000 cycles. TEM images of the Cu3PdN nanoparticles (b) as-synthesized and after (c) 7 and (d) 1000 cycles.

To further probe their stability, the mass activity of the Cu3PdN nanocrystals over 1000 cycles was compared to that of Pd nanocrystals synthesized using an analogous procedure. Figure 9 reveals that for the Cu3PdN nanocrystals, the mass activity (calculated using the measured current) increases slightly during the first 100 cycles. This is consistent with the behavior observed in Figure 8a, and is attributed to the removal of the surface ligands and concomitant surface roughening. After 250 cycles, the activity decreases, returning to near the initial value after 500 cycles and ultimately plateauing. In contrast, the mass activity of the Pd nanocrystals quickly drops from an initial value of 0.21 mA/μgcatalyst to 0.1 mA/μgcatalyst after 100 cycles, followed by a continual slight decrease with further cycling. This suggests that Cu3PdN can offer similar ORR activity, but potentially greater stability, relative to Pd under alkaline conditions.

Figure 7. Comparison of the mass activities for Cu3PdN and Pd at –0.1 V (vs Ag/AgCl) using the kinetic-limiting current.

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Figure 9. Plot of mass activity vs number of cycles for Cu3PdN and Pd at 100 mV above their respective onset potentials (vs Ag/AgCl). Mass activities were calculated using the total measured current from the ORR polarization curves at 1600 rpm.

CONCLUSIONS In conclusion, we have synthesized uniform, multi-faceted, and highly dispersible colloidal nanocrystals of antiperovskitetype Cu3PdN, a ternary transition metal nitride. This approach integrates two known ORR catalysts, Cu3N and Pd, into a single crystalline compound. Cu3PdN was indeed found to be an active ORR catalyst under alkaline conditions. Cu3PdN exhibited higher ORR activity than Cu3N and comparable performance to the benchmark Pd system on a total mass basis, although the mass activity per Pd atom was much higher for Cu3PdN than for Pd. During repeated cycling, the Cu3PdN nanocrystals are highly stable, offering higher ORR mass activity and stability under alkaline conditions than that of comparable Pd nanocrystals.

ASSOCIATED CONTENT Supporting Information. Additional XRD, SAED, and electrochemistry data. This material is available free of charge via the Internet at

AUTHOR INFORMATION Corresponding Author *(R.E.S.) E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported primarily by funds from the Pennsylvania State University, with additional support (J.F.C., R.E.S.) from the National Science Foundation (NSF) Center for Chemical Innovation on Solar Fuels (CHE-1305124). D.D.V. acknowledges support from an NSF Graduate Research Fellowship. J.A. was supported by an NSF REU program under grant CHE-1263053. P.M. was supported by the Penn State Institutes of Energy and the Environment and the Pennsylvania State University. TEM imaging was performed in the Penn State Microscopy and Cytometry Facility (University Park, PA) and HRTEM imaging and XPS analyses were performed at the Materials Characterization Lab of the Penn State Materials Research Institute.

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