Article pubs.acs.org/cm
Evidence for the Formation of Nitrogen-Rich Platinum and Palladium Nitride Nanoparticles Gabriel M. Veith,*,† Andrew R. Lupini,† Loïc Baggetto,† James F. Browning,‡ Jong K. Keum,‡ Alberto Villa,§ Laura Prati,§ Alexander B. Papandrew,⊥ Gabriel A. Goenaga,⊥ David R. Mullins,# Steven E. Bullock,|| and Nancy J. Dudney† †
Materials Science and Technology Division, ‡Chemical and Engineering Materials Division, and #Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge Tennessee 37831, United States § Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, 20133 Milano, Italy ⊥ Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States || Lockheed Martin, Marietta, Georgia 30063, United States S Supporting Information *
ABSTRACT: We report evidence for the formation of nitrogen-rich precious metal nanoparticles (Pt, Pd) prepared by reactive sputtering of the pure metal in a N2 plasma. The composition of the nanoparticles varies as a function of particle size and growth conditions. For the smallest particles the nitrogen content appears to be as high as 6.7 N atoms for each Pd atom or 5.9 N atoms for each Pt atom whereas bulk films have nominal compositions of Pt7.3N and Pd2.5N. The unusually large N content in the nanoparticles is balanced with H. The nanoparticles are metastable in air and moisture, slowly decomposing over several years. The catalytic properties of these N-rich nanoparticles were accessed by rotating disk electrode electrochemical studies, the liquid phase oxidation of benzyl alcohol, and gas phase CO oxidation, and support the experimental evidence for the materials composition.
KEYWORDS: precious metal nitride, reactive sputtering, metal-nitride, nitrogen-rich, gas-phase formation, size-dependent composition
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conduction bands via the introduction of the strongly polarizing N3− anion.6 This modifies the electronic structure by increasing the covalency of the metal−anion bond, which in turn lowers the band gap and increases conductivity.6 There have been relatively few studies of precious metal nitrides because the precious metal−N bonds are exceedingly difficult to create using standard solid-state synthesis methodologies because of their low predicted bond strength.21 However, reports have demonstrated the ability to prepare precious metal nitride phases and measure the bulk modulus, at high temperatures and pressures.22−26 Furthermore, the addition of N to Pt−Mo electrocatalysts was reported to increase the electrochemical stability significantly over the native metals.27 These results demonstrate the potential of N to influence precious metal materials properties in favorable ways, provided suitable synthesis approaches can be developed. Physical vapor deposition (PVD) processes, such as sputtering, are unique relative to nearly all synthesis methods in that they can produce materials that are not at equilibrium relative to the known thermodynamic phase diagrams.28 During
etal nitrides exhibit unique physical properties but have been relatively unexplored compared with the oxides,1 in part because of the stronger NN bonding in N2 as compared to the OO bond in O2, which makes the low-temperature synthesis of metastable materials significantly more challenging.2−9 Despite these synthetic challenges, nitrides can be produced by several methods, such as high-temperature ammonolysis, metathesis, solvothermal, molten fluxes, and vapor deposition.10−14 Most studies have focused on the synthesis of 3d transition metal nitrides, early 4d and 5d transition metals (i.e., Zr, Nb, Mo, Hf, Ta, W), and early pblock metals (B, Al, Ga, Si). These metal nitrides present a fascinating range of structures and oxidation states not typically observed in complex oxides and thus could provide a foundation for understanding and predicting materials with unique properties of particular relevance to energy applications.10,11,15,16 For example, Fe16N2 thin films prepared by sputtering are strong permanent magnets that may replace rareearth-based magnets.8,9 Fe−C−N17,18 and TaON19 are materials with electrocatalytic properties approaching those of platinum; these materials could be sources of lower-cost materials for fuel cells and oxygen reduction catalysts.20 Finally, a large number of interstitial nitrides are under investigation as photoconversion materials because of the ability to alter © 2013 American Chemical Society
Received: September 30, 2013 Revised: December 2, 2013 Published: December 3, 2013 4936
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metal from the samples for analysis. After reacting with the aqua regia, the samples were centrifuged to separate the support powder from the acidic liquid phase which was collected by decanting. Deionized water (18.3 MΩ) was used to wash the support and the centrifuge/ decanting/washing process was repeated two more times. The collected wash was combined then diluted and used for the ICP measurements. A series of ICP standards were prepared by the serial dilution of standards purchased from Alfa Aesar. The support material was collected, dried and analyzed with the XPS to confirm that all the metals were dissolved. The absolute N content was determined by Kjeldahl analysis performed by Galbraith Laboratories, Knoxville, TN, USA. The ICP and Kjeldahl results were combined and compared to the results obtained from the XPS. The second assessment of metal and nitrogen loading was performed using a PHI 3056 XPS with an Al anode source operated at 15 KV and an applied power of 350 W. Samples were manually pressed between two pieces of indium foil; the piece of In foil with the sample on it was then mounted to the sample holder with a piece of carbon tape (Nisshin E.M. Co. LTD). Indium was used as a support since the most intense C1s binding energy, from the support material, was used to calibrate the binding energy shifts of the sample (C1s = 284.8 eV).42 There were no treatments (i.e., cleaning, heating, processing) to the samples before the XPS measurements. High resolution data was collected at a pass energy of 23.5 eV with 0.05 eV step sizes and a minimum of 150 scans to improve the signal-to-noise ratio; lower resolution survey scans were collected at a pass energy of 93.5 eV with 0.5 eV step sizes and a minimum of 25 scans. The metal− nitrogen ratio was determined from the XPS data using standard sensitivity factors.42 Given the reproducibility and proximity of equipment, ICP and XPS measurements were used for routine analysis of the nanoparticles. The XPS was used to estimate the metal−nitrogen content of the films. Samples were imaged using an aberration-corrected Scanning Transmission Electron Microscope (STEM) UltraSTEM 200 from Nion Co. operated at 200 kV. Images were obtained using a high-angle annular dark field detector (HAADF), which provides Z-contrast imaging,43,44 where the image intensity depends on the thickness of the material and approximately the square of the atomic number of the elements. In this mode the metals show up brightly on the fainter background of the support and even single heavy atoms can be detected. Particle sizes were determined by measuring the widest point on the nanoparticle to account for particles that may not be orthogonal to the electron beam. Errors are associated with tilting of the nanoparticles to present a smaller cross-section and a pixels per image. X-ray absorption spectroscopy (XAS) data for the Pt-based nanoparticles were collected at the Pt LIII-edge (11564 eV) on beamline X19A at the National Synchrotron Light Sources at Brookhaven National Laboratory. A Si(111) double crystal monochromator was used and detuned by 30% to reject higher harmonics. The XAS was measured simultaneously in fluorescence and transmission modes. The fluorescence was measured using a large area passivated implanted planar silicon (PIPS) detector oriented perpendicular to the upcoming beam. Ion chambers for measuring Io and It were filled with nitrogen and argon, respectively. The Pt absorption was measured out to k = 16. The samples were ground to a fine powder, mixed with BN and pressed into a 13 mm diameter pellet. The measurements were recorded while the sample was in pure He at ca. −100 °C. PtO2 was spread as a thin layer on Kapton tape and then folded many times to minimize particle size and pinhole effects. It was measured at room temperature (19 °C). A Pt foil was placed downstream of It and in front of an additional ion chamber (Ir) during the measurements as an energy reference. Data reduction and analysis was done using the Athena/Artemis suite of programs.45 Nitrogen BET surface area measurements of the supports were performed using a Quantachrome Instruments Autosorb 1C instrument. Approximately 0.3 g of support material was dried at 300 °C under vacuum for 12 h prior to BET measurements. Neutron reflectometry (NR) was collected on PPM films deposited on silicon substrates using the Liquids Reflectometer (BL-4B) at the
sputtering, atoms are ejected from a target by impinging gas atoms with sufficient momentum to be transported to a substrate. During deposition onto a substrate, the atoms can be kinetically trapped in higher energy states when they condense into a solid structure. This process is known to lead to the formation and stabilization of new compositions with heretofore unobserved structures and properties that enable improvements in performance and challenge our fundamental understanding of materials chemistry and physics. For example, bulk indium nitride is a 0.7 eV semiconductor that adopts a classic hexagonal Wurtzite structure with tetrahedrally coordinated In and N atoms. InN films produced by PVD adopt stoichiometries of up to InN1.7.29−31 This is almost 70% excess N within the film structure, leading to a significantly increased bandgap, up to 1.18 eV, while the lattice expands only by 2% for the c and a axes.29−31 While InN films are some of the most intensely studied excess-N materials, numerous other nitride phases, e.g., GaN, AlN, TiN, Zn3N2, are also formed as films that contain significantly elevated N content.32−36 This paper describes the application of magnetron sputtering to prepare platinum and palladium nitrides (PPM). We will show that arresting the nucleation and growth of the platinum and palladium nitrides stabilizes nanomaterials with nitrogen content significantly larger than the corresponding thin film.
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EXPERIMENTAL SECTION
Materials Synthesis. The platinum and palladium nitrides samples were prepared by magnetron sputtering in a customized deposition chamber. Films were prepared by direct current (dc) magnetron sputtering of a pure platinum (Refining Systems Inc., Las Vegas Nevada − 99.99%) or palladium (Alfa Aesar − 99.99%) metal target at an applied power of 26 W. Pure metal films were grown in an Ar atmosphere (Research grade Argon, 99.9995% − Air Liquide) while the nitride films were prepared in a N2 atmosphere (Ultra High Purity grade, 99.999% − Air Liquide) where the N2 was used as the source of nitrogen in the samples. To prepare the nanomaterials 2 g of support material along with two-1 in. Teflon coated stir bars were placed inside a set of stainless steel (SS) cups attached to a vacuum compatible motor. The rotating cup and thin film substrates (silicon, alumina, or Kapton) were placed next to each other during the deposition such that the nanoparticles were prepared simultaneously with the film growth (i.e., with the same deposition conditions, distances (12 cm), gases, pressures etc.). Support materials include XC72 carbon black (Cabot, 272 m2/g), TiO2 (Degussa P25, 60 m2/g) and fumed silica (Cab-o-Sil, 232 m2/g). The cup was rotated causing the powder to tumble during the deposition, exposing new surfaces of the support to the deposition flux. During the deposition, the powder samples were within 5 °C of ambient temperature since the cup system acted as a thermal sink and there was a relatively low flux of atomic species. Deposition times were varied from 60 to 240 min to control the weight loading. Metal loading varied as a function of the support materials tamp-density and deposition time.37−41 Typical metal loadings were between 0.18 and 1 wt %. To evaluate the reactivity of the N2 plasma with the substrate powders, we used a pure gold target (Refining Systems Inc., Las Vegas Nevada − 99.99%) as the source material to prepare gold nanoparticles on the same support materials. Materials Characterization. Metal content was assessed using Xray photoelectron spectroscopy (XPS) and Inductively Coupled Plasma (ICP) Optical Emission Spectroscopy (ICP-OES) while the nitrogen content was assessed using the XPS and Kjeldahl analyses. These methods differ significantly in the physical mechanism of analysis but yield similar compositions, i.e., N and metal concentrations within 5−8 at %. Furthermore, the data were reproducible. First, quantitative metal loading was determined using a Thermo Jarrell Ash IRIS ICP-OES. Five mls of freshly prepared aqua regia (3:1 mixture of hydrochloric acid and nitric acid) was used to dissolve the 4937
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photoelectron spectroscopy (XPS) data collected for the films reveal the formation of PPM’s with nominal compositions of Pd2.5N and Pt7.3N (Table 1). Detailed spectroscopic inves-
Spallation Neutron Source (SNS), located at the Oak Ridge National Laboratory. The silicon wafers were obtained from the Institute of Electronic Materials Technology, Warszawa, Poland. Powder X-ray Diffraction (XRD) measurements were performed using a Scintag Pad V X-ray Diffractometer with a CuKα source in Bragg−Brentano geometry. Nanoparticle Catalytic Activity. To evaluate the properties of the PPM nanoparticles, we subjected the materials to rotating disk electrode electrochemical studies, the liquid phase oxidation of benzyl alcohol and gas phase oxidation of CO. These reactions were selected because of the sensitivity of Pd to N heteroatoms46−48 as well as the suitability of the base metals for gas phase catalysis.49,50 Electrochemical measurements were performed in a three-electrode configuration with a 0.1 M HClO4 electrolyte. The working electrode was a 5 mm diameter glassy carbon disc, the counter electrode was a Pt wire, and the reference electrode was a Hg/HgSO4 electrode in 0.5 M H2SO4. Catalyst inks were prepared by addition of 5 wt % Nafion solution (Ion Power) in a 40:60 ionomer:catalyst weight ratio. The reference electrode potential was 0.704 V with respect to a reversible hydrogen electrode established at a Pt ring in H2-saturated electrolyte. All subsequent voltages are referenced to the RHE potential. For each sample, cyclic voltammograms were recorded in N2-saturated electrolyte by sweeping the voltage at 10 mV s−1 from 1.2 to 0 V. The liquid phase oxidation of benzyl alcohol were carried out in a thermostatted glass reactor (30 mL) provided with an electronically controlled magnetic stirrer connected to a large reservoir (5000 mL) containing oxygen at 2 atm. The oxygen uptake was followed by a mass flow controller connected to a computer through an A/D board. The oxidation experiments were carried out in the presence of a solvent (0.0125 mol substrate, substrate/metal = 5000 (mol/mol), benzyl alcohol/cyclohexane: 50/50 (vol %), 80 °C, pO2 = 2 atm). Periodic withdrawal of samples from the reactor was performed. Recoveries were always 98% ± 3 with this procedure. For the identification and analysis of the products a GC-MS and GC (HP 7820A gas chromatograph equipped with a capillary column HP-5 30m × 0.32 mm, 0.25 μm film, made by Agilent Technologies), were used. For the quantification of the reactant-products an external calibration method was used where pure samples were used to generate calibration curves. For comparison purposes Pd/AC catalysts prepared via sol immobilization using polyvinyl alcohol (PVA) as protecting agent were also evaluated.51 Briefly, a mixture of Na2PdCl4· 2H2O (Pd:0.094 mmol) and freshly prepared PVA solution 1 wt % (Pd/PVA 1/1 wt %) were added to 100 mL of H2O. After 3 min, NaBH4 0.1 M solution (Pd/NaBH4 1/8 mol/mol) was added to the yellow-brown solution under vigorous magnetic stirring. The brown Pd(0) sol was immediately formed. An UV−visible spectrum of the palladium sol was recorded for ensuring the complete reduction of Pd (II). Within few minutes from its generation, the suspension was acidified at pH 2 by sulfuric acid and the support was added under vigorous stirring. The catalyst was filtered and washed several times with distilled water and dried at 80 °C for 2 h. The amount of support was calculated to obtain a final metal loading of 1 wt %. The catalytic activity of the PPM nanoparticles was determined using a custom built flow reactor to monitor the conversion of carbon monoxide to carbon dioxide (20−100 °C).38 Approximately 0.1 g catalyst were loaded into a quartz U-tube (i.d. = 4 mm) and supported on both ends with glass wool. A premade mixture of 6% O2, 1% CO, and 93% He (Air Gas, Knoxville, TN, USA) was metered through a Sierra Instruments Mass flow controller (25 mL/min). The product evolution was monitored with a Ametek Dymaxion mass spectrometer (0−200 AMU range). Calibration of the mass spectrometer was accomplished with several mixtures of CO, CO2, and O2. The temperature of the catalyst bed was determined using an embedded Ktype thermocouple.
Table 1. Elemental Composition Data Collected for the Sputter-Deposited PPM Films and Nanoparticles Grown on Carbon Blacka sample
film (XPS)
PPM nanoparticle (ICP:Kjedahl)
PPM nanoparticle (XPS)
Pt−N Pd−N
Pt7.3N Pd2.5N
PtN5.9±0.3 PdN6.7±0.3
PtN3.9−5.4 PdN3.1−6.2
a
Note N content in the carbon black support material without metal is below detection limits. The range in N content reported for the XPS analysis is the experimental variation between repeated reactions.
tigation using the XPS revealed the presence of a single metal species shifted by about +0.9 eV relative to the pure metal films (Pd, 335.5 eV; Pd2.5N = 336.4 eV; Pt, 71.3 eV; Pt7.3N = 72.2 eV) and a single N species with a binding energy of ∼398 eV, see Figure S1 in the Supporting Information. This binding energy is consistent with the formation of a metal-nitride. Our results indicate the presence of metal−N bonds along with the concomitant oxidation of the metal, confirming the formation of a PPN film and are in good agreement with the previous studies of laser-ablated materials.45,46 Growth of Au films prepared in a N2 plasma using similar conditions revealed the formation of only metallic Au, consistent with the stability of gold against oxidation.38 X-ray diffraction (XRD) studies of the nitride films showed a strong preferential texturing of the samples, with peaks close to the primary diffraction line for the base metals but shifted by 0.10−0.07 Å to larger lattice spacings (see Figures S2 and S3 in the Supporting Information). To try to investigate the structure and growth of these films, we performed neutron reflectometry (NR) experiments. NR is a neutron scattering technique highly sensitive to morphological and compositional changes occurring across surfaces and interfaces, including buried interfaces, over length scales from 5 Å to hundreds of nanometers. The reflectivity profile is measured as a function of the momentum transfer of the scattered neutrons. The resulting profile is proportional to the thickness, density, and composition of layered structures making up the film. NR is analogous to ellipsometry except that it is applicable to metallic films such as the ones investigated in this study; moreover it is sensitive to different isotopes such as 14N/15N.52,53 Nitrogen has a significantly larger neutron scattering length (14N = 9.36 × 10−4 nm) than Pd (5.9 × 10−4 nm) while being comparable to Pt (9.6 × 10−4 nm). This large contrast with Pd and difference with N and the likely differences in density of N-rich vs metal-rich films should enable us to identify gradients in density of nitrogen atoms toward the growth surface relative to the bulk of the film. Figure 1 shows the NR profile collected for thin films of nominal compositions Pd2.5N and Pt7N. Reflectivity profiles from both Pd2.5N and Pt7N films on Si substrates show minima at ∼0.07 Å−1 indicative of the formation of a thin film. Neither film, however, display the expected Kiessig fringes (interference pattern) of the nitride layer for a film of overall thickness 100 nm. The absence of these fringes from the profile could indicate the overall thickness of the film exceeds the resolution of the instrument (∼400 nm). However, if this were the case the appearance of the minima in the data would not be so pronounced. The absence of the Kiessig fringes is believed to
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RESULTS Platinum and Palladium Nitride Films. The platinum and palladium nitrides (PPM) films grown on a variety of substrates (Si, Al2O3, Kapton) were all stable in air. X-ray 4938
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shown in Figure 3 and Figure S5 in the Supporting Information. The metal atoms/clusters show up as bright
Figure 1. Neutron reflectivity profiles for Pt7N and Pd2.5N films.
result from strong off-specular scattering, Figure S4 in the Supporting Information, because of surface roughness (>10 nm) or lateral structure within the film. Platinum and Palladium Nitride Nanoparticles. The results collected for the PPM nanoparticles grown on the powder supports are significantly different than the thin-film samples. Figure 2 shows representative XPS data collected for
Figure 3. STEM images of Pt particles, Pd particles, Pt−N particles, Pd−N particles grown on carbon black. The bright white spots correspond to the heavy metal atoms. The light gray corresponds with the low Z carbon black.
white spots against the low contrast (gray) support material.43,44 The micrographs clearly reveal the formation of small nanoparticles confined to the outer surface of carbon black. The Pt and Pd metal particles have an average diameter of around 1.6 nm. In contrast the Pt−N nanoparticles appear to be dominated by very small metal clusters (0.8 nm clusters). The Pd−N similarly has very small metal clusters (
