Ag Nanoparticles Supported on Yttria-Stabilized Zirconia: A

Article pubs.acs.org/JPCC

Ag Nanoparticles Supported on Yttria-Stabilized Zirconia: A Synergistic System within Redox Environments Zhouying Zhao,† V. A. Vulcano Rossi,† John P. Baltrus,‡ Paul R. Ohodnicki,‡ and Michael A. Carpenter*,† †

SUNY Polytechnic Institute, Colleges of Nanoscale Science and Engineering, Albany, New York 12203, United States U.S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States

‡

ABSTRACT: In this work, we report a distinctive dynamic sintering-free redox behavior of Ag nanoparticles (AgNPs) on 30 nm thick yttria-stabilized zirconia (YSZ) films. The material system demonstrates reversible 200 nm shifts in the plasmonic spectra with an unprecedented particle phase/size/morphology oscillation during cyclic redox reactions in air and hydrogen/air at 300−400 °C. This is in significant contrast to the minor changes more commonly observed for AgNPs on quartz. It was found that the ionically active YSZ has a strong tendency to drive AgNPs to oxidize under oxidizing conditions, and upon surface oxidation a large differential surface energy is built up, forcing the core/shell particles to migrate and coalesce toward a lower system free energy. Most strikingly, once switched to a mixture of H2 and air, the previously formed large dewetted metal-core/thick oxide-shell particles collapse, and new small Ag nanoparticles quickly form and remain in a highly dispersed and sintering-free state on YSZ. This is found to likely be due to catalytic production of water over the material system, which plays a key role in the dynamic redox activities. It is hypothesized that the small metallic particle regeneration and sinter-free behavior take place through a fourstep process resulting from the synergistic behavior of AgNPs supported on YSZ within a redox environment: (I) production of a local humid environment via catalytic reactions of H2 and O2 mostly at the triple-phase boundary, (II) dissolution of Ag+ from both reduced Ag and the AgOx shell, (III) collapse and spillover of the AgOx shell/water layer with Ag ions onto the hydrous YSZ surface, and (IV) reduction, diffusion, nucleation, and growth to new small metallic AgNPs with a dynamic equilibrium quickly reached. The findings behind this novel system could set up an avenue for a new concept of catalysts operating in a selfcontrolled dynamic regime for governing chemical reactions via metal and ceramic synergized catalysis with high activities and stabilities.

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INTRODUCTION There are many examples of conventional supported catalysts utilizing noble metal particles of generally a few nanometers in size affixed to metal oxide or semiconductor supports of high surface area to achieve catalytic activity while maintaining their stability.1−5 In recent years, the strong resonant interaction of metal nanoparticles with UV−vis light through localized surface plasmon resonance (LSPR) excitation has been reported to enhance the rate of chemical reactions.6,7 This has led to extensive research interest in plasmonically active catalysts of supported metallic or bimetallic nanoparticles for improved energy efficiency.8−10 In catalyst design, creating multifaceted and active-site enriched surfaces through employment of small nanoparticles of various shapes is a central criterion in the design of active catalysts for defect-manipulated catalysis.11−13 However, the challenge is that both the small size and high surface defect density make the nanoparticles intrinsically unstable even if attached to a support material. With catalytic cycling, these materials tend to change in phase/structure/ morphology through thermodynamically favored particle aggregation or sintering, bonding with poisoning chemicals, © 2016 American Chemical Society

and possible interdiffusion with supports, resulting in a loss of performance and functionality.4,5,14,15 Developing advanced catalytic systems with high reactivity and selectivity and extended stability remains an active and challenging area due to the complexity of catalytic processes involving not only surface/interface heterogeneous reactions but also nanoparticle structural/morphological variations which modify the catalytic properties in a way that is highly dependent on the reactions. We have recently reported bimodal Ag/AgOx nanoparticles functionally engineered on a quartz (SiO2) substrate and demonstrated both strong localized surface plasmon resonance (LSPR) and redox activity from this system.16 In situ Raman studies reveal interstitially located bulk oxygen and surfacebound atomic oxygen for the Ag/AgOx particles, two important active species reported to be responsible for partial and complete catalytic chemical oxidations and conversion.17 Oxidation of CO and hydrocarbons on the particle surface Received: January 7, 2016 Revised: February 17, 2016 Published: February 18, 2016 5020

DOI: 10.1021/acs.jpcc.6b00189 J. Phys. Chem. C 2016, 120, 5020−5032

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The Journal of Physical Chemistry C

onto YSZ-quartz and quartz substrates within a vacuum chamber of base pressure ∼10−7 Torr from 99.99% pure Ag pellets loaded in a tungsten boat. After the Ag deposition, the samples were annealed in a stepwise manner so that metallic silver nanoparticles were formed on YSZ without extensive oxidation. The protocol developed to achieve these as-prepared materials includes individual anneals at 200 and 300 °C in Ar with 1% H2 at a total flow of 2000 sccm for 6 h in order to minimize oxidation and then anneals at 400 and 500 °C in Ar to obtain the desired particle size and uniformity on the sample substrate (99.999% purity for both gases). Prior to the anneal, the samples in the furnace were pumped down to ∼10−1 Torr for 10 min and pre-exposed to 2000 sccm Ar for 40 min. A 1″ diameter quartz tube furnace was used for the annealing processes and also for gas exposure tests using UV−vis spectrometry for plasmonic extinction studies as described below. For all tests, the tube furnace was initially pumped down to ∼10−1 Torr followed by a 30 min pre-exposure and 30 min warm up to desired temperatures in an air flow for oxidative treatment prior to subsequent processes. For ex situ analyses of samples, the oxidative and reductive treatments were performed at 400 °C in air for 300 min and in 1% H2/air or Ar carrier gases for 100 min, respectively. For in situ extinction studies, following a pre-exposure and 5 h air exposure at 300 °C, the sample was cyclically exposed to redox gas cycles of 1% H2 in air and pure air for 100 min for each cycle at temperatures from 300 to 400 °C with a 25 °C step. The gas flow in all cases is 2000 sccm. The extinction spectra were recorded every 10 s. The furnace was then allowed to cool for 200 min in air. This in situ characterization is designed to observe how samples behave as a function of the redox reactions at different temperatures. XRD was used to characterize the sample crystalline structure and phase after different treatments. A Scintag XGEN-4000 Xray diffractometer (XRD) equipped with a Cu Kα X-ray source (0.154 nm) and a horizontal wide-angle four-axis goniometer with stepping motors was used for these analyses. XRD patterns were collected with a normal scan at 0.02° step and a count time of 10 s. An FEI quanta scanning electron microscope (SEM) was used to provide environmental SEM imaging of the samples. H2O was used to produce a low Torr partial pressure in the specimen chamber to circumvent electrical charging issues on the insulating sample substrates. The analyses were carried out for oxidized and reduced samples and in the presence and absence of the YSZ base layer. In this characterization, fast imaging of a fresh spot on the sample with scanning dwell time of 45 μs was conducted under, e.g., 0.378 Torr H2O vapor pressure and 10 kV electron acceleration voltage at room temperature. Both ex situ and in situ plasmonic properties of the samples were collected to probe sample redox states and time-resolved interactions with gas exposures. A Varian Cary 50 UV−vis spectrometer was used for ex situ extinction spectral measurement in a wider wavelength range from 200 to 1100 nm afforded by its Si diode detectors. An Ocean Optics UV−vis spectrometer coupled with a fiber optics setup was used for in situ monitoring of extinction spectra as the fiber optics feature enables a convenient coupling to a reaction chamber or furnace. The sample was mounted on a Macor holder and placed into the quartz tube furnace. In this setup two detectors are preferentially used to monitor plasmonic spectra and light source stability for real-time correction. The wavelength detection range spanned from ∼400 to 1000 nm, obtained

was found to proceed readily even at room temperature. This motivated the coupling of the system onto a metal oxide for potentially enhanced novel functional characteristics of the material in the presence of reactive ions. Many metal oxide materials like alumina, silica, and ceria have been used to support Ag clusters or nanoparticles for catalytic conversions of industrially important chemicals,18,19 but so far reports on using YSZ as the support for AgNPs are rare.20 YSZ is a well-known ionic conductor due to doping of Y3+ in the Zr4+ lattice, generating oxygen vacancies that enable ionic activity and conductivity at elevated temperatures.21−24 Oxygen anions and other active species like O and OH− can easily be introduced onto the surface25 and into the bulk of YSZ through gas exposures or electrochemical pumping. For instance, in the harsh environment gas sensing of the AuNPs/YSZ system, there is a reversible formation and dispersion of oxygen anions in YSZ with the reversible transfer of Au conduction band electrons at the interfacial area to oxygen upon exposure to redox gases, while the plasmonic property changes with the modulation of the free electron density serve as gas sensing signals.26−28 In either H2 + YSZ + O or H2O + YSZ heterogeneous reaction schemes, appreciable amounts of H2O are determined to be “absorbed” into the bulk as interstitial hydroxyl and hydrogen species.29−31 In electrochemical membrane reactors or solid-state fuel cells, oxygen anions can be electrochemically generated and pumped through a YSZ electrolyte and participate in chemical reactions.32−36 Given the excellent catalytic activity of AgNPs with their strong plasmonic properties and the ionic activity of YSZ, we decided to perform a functional coupling of bimodal Ag nanoparticles with a nanothin layer of YSZ prepared on quartz substrates, considering that active oxygen species associated with YSZ may interplay with the adsorbates and the nanoparticles, thereby affecting the catalytic redox reactions. In the present work, we report on the distinctive sinteringfree property and redox activity obtained from the engineered AgNP/YSZ nanocomposite as mirrored by its exceptional plasmonic sensing response to redox gas exposures. We focused on exploration and elucidation of the corresponding mechanisms behind the redox-dependent and reversible particle phase/size/morphology alterations with concomitant changes in the LSPR spectra for this nanomaterial system. Timeresolved in situ plasmonic spectra were monitored and analyzed along with Mie theory based simulations for probing the complex and detailed redox reaction processes. Environmental scanning electron spectroscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) characterizations and analyses provide morphological, chemical compositional, and structural information as a function of redox treatment. The significance and implication of the phenomena and findings obtained within this study are also discussed.

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MATERIALS AND METHODS The bimodal AgNP film was fabricated using the same techniques as reported in our previous work.16 Quartz substrates were ultrasonically cleaned in acetone and ethanol for 10 min. A 30 nm thick film of YSZ was deposited onto the precleaned quartz substrates using an RF sputtering system after a base pressure of 2 × 10−6 Torr was reached. For increased crystalline structure and film density, after the YSZ deposition, the samples were annealed at 1000 °C in 2000 sccm flow of 99.999% Ar for 1 h followed by a natural cooling in the gas. Ag thin films of 10 nm thickness were thermally evaporated 5021


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Figure 1. (a) Scanning electron microscopy (SEM) image of bimodal Ag/AgOx nanoparticles coupled to YSZ nanothin film (30 nm) coated on a quartz substrate and (b) a histogram of the particle size distribution generated using ImageJ program (b).

Figure 2. (a) Progression of the LSPR spectrum of as-prepared Ag nanoparticles/YSZ sample after each annealing step. (b) X-ray diffraction patterns for the as-prepared (labeled as before sensing) state and reduced sample state from the oxidized state.

using a fiber coupled USB4000 CCD spectrometer and a tungsten halogen light source. Analysis of the material’s surface was carried out using X-ray photoelectron spectroscopy (XPS) with a PHI 5600ci instrument using monochromatic Al Kα X-rays. The pass energy of the analyzer was 23.5 eV, and the scan step size was 0.1 eV (Ag 3d−0.05 eV). The binding energies for the components of YSZ were referenced to the Zr 3d5/2 peak, which was assigned a binding energy of 181.5 eV based on its position relative to the C 1s peak for adventitious carbon (a common binding energy reference due to ambient air exposure that was not available after the first treatment with air) at 284.6 eV in the as-received samples. This resulted in measured Ag 3d5/2 peak binding energies of 368.0 ± 0.2 eV, and 367.7 ± 0.2 eV for Ag was in the reduced and oxidized state, respectively. The values were confirmed by measurements on Ag foil and bulk Ag2O. A PHI model 04-090 charge neutralizer was employed to minimize the effects of sample charging. Treatments with air and 1% H2/air gases were performed in a reaction chamber attached to the XPS instrument, which permitted sample transfer for analysis without exposure to room air. The detailed procedures were selected to be comparable with optical gas sensing tests performed at similar temperatures. The gas treatments were at atmospheric pressure using a flow rate of 30 cm3/min. The temperature was raised from room temperature to 400 °C at a rate of 10 °C/min,

where it was held for the specified length of time before cooling back to room temperature typically over a period of 1 h.

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RESULTS AND DISCUSSION An SEM image of the as-prepared sample is shown in Figure 1a, where small particles can be seen in the gap between the large particles on the inset. A histogram displays a bimodal particle size distribution (∼22 nm on average size for the large particles) along with small particles that have approximately 3 nm diameter (Figure 1b). UV−vis extinction spectra were taken at room temperature after each stepwise annealing step (Figure 2a) in order to correlate the thermal treatments with both the particle size distribution and their corresponding LSPR spectra. Starting at the first annealing step at 200 °C two peaks are clearly observable at 506 nm (500, 490, 513 nm for the subsequent annealing steps) and 356 nm (358, 355, 360 nm for the subsequent annealing steps). These peaks generally relate to two different average AgNP size distributions,37−39 which confirms the existence of the bimodal distribution of plasmonically active AgNPs that has been previously studied.16 The changes of the two LSPR bands, mainly the ∼500 nm peak, can be characterized by a blue shifting, narrowing, and typical intensity increase with a corresponding increase in annealing temperature from 200 to 400 °C. These trends are indicative of the formation and evolution of particles with narrowed size distribution and reduced surface dampening effects.40,41 A slight opposite spectral change in the LSPR bands 5022


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The Journal of Physical Chemistry C results from the anneal at 500 °C in Ar and is indicative of a weak Ostwald-ripening-induced growth of the particles at this higher annealing temperature.42,43 A pronounced difference is that the ∼500 nm LSPR band of AgNPs on YSZ for the asprepared sample is about 100 nm redder than the typical peak position for similar size metal nanoparticles in solution or on substrates like quartz.37,38,44 This is found to be due to the effects of not only the YSZ-induced effective dielectric constant change but also a formation of a surface oxide on the AgNP as determined below. The XRD spectral pattern confirms the formation of metallic AgNPs with (111) preferred orientation for the as-prepared sample (labeled as “Before sensing” in Figure 2b). The resulting sample is thus described as a YSZ-supported bimodal distribution of Ag nanoparticles (NPs) with a surface AgOx layer. The bimodal distribution of Ag/AgOx NPs serves two roles: (1) The smaller particles have a higher surface to volume ratio and can act as highly active catalyst centers. (2) The larger particles also participate in the catalytic reactions but with a more intense LSPR than that of the smaller particles, potentially enhancing their catalytic activity.6,16,45 Strikingly, when the sample was exposed to redox gas cycles, it demonstrated an unprecedentedly large and reversible plasmonic spectral shift. Figure 3a shows typical in situ reversible changes of the plasmonic extinction spectra of the sample with 100 min cyclic exposures to pure air and 0.5% H2 in air at 400 °C. The LSPR peak is initially at 720 nm for the oxidized sample, then shifting to ∼540 nm once the reduction

reaction occurred, and then back to its original position after the oxidation exposure is completed. The redox and catalytic properties of the system under reactive conditions are dramatically different from that of other material combinations like Ag/AgOx on SiO2 or AuNP on YSZ with an order of magnitude larger plasmonic spectral change than that of the latter in comparable high-temperature gas exposures.16,26,27,44 Figure 3b depicts how the extinction spectral changes progress, starting with the oxide spectrum, during admission of 1% H2 in air at 400 °C. After 20 s the metallic state peak has red-shifted from an ∼500 nm peak position and become clearer, while the oxide state peak at ∼800 nm peak position is barely noticeable. The red shift of the metallic band is indicative of regeneration and growth of AgNPs during reduction of the oxidized sample together with the dielectric constant effects of the surface species produced.16,37 The latter is likely also responsible for the red-shift of the weak oxide band noticeable in the figure. The (111) Ag peak in the XRD pattern confirms that the nanoparticles are reduced to a predominant metallic character for the hydrogen exposed sample (Figure 2b). The regenerated AgNPs have sizes (Figure 8a) smaller than or comparable to that of the as-prepared particles (Figure 1a). Figure 4a depicts the traces of the LSPR peak position and intensity over time to cyclic redox gas exposures of 1% H2 in air and air at temperatures from 300 to 400 °C. This was obtained by fitting the time-resolved LSPR spectra with a Lorentzian function to extract the peak position and intensity for the band that red shifts (e.g., from ∼540 to 720 nm) with sample oxidation and reappears in the bluer wavelength region with sample reduction. The fitting quality can be seen in Figure 4b with the LSPR spectrum representing the sample in a reduction reaction from the prior oxidized state. The double-band spectra exist/evolve only for a short time upon exposure to the reducing gas, and in this gas cycle the bluer peak is the one used in the fitting analysis as the redder peak disappears quickly. Thus, as shown in Figure 4a, once H2 is turned on, the LSPR band shows a sharp drop (blue shift) in the peak position and an increase in intensity as the sample transitions from an oxidized state to the reduced state. After a small amount of time, generally from a few seconds to a few minutes, the LSPR peak is fairly stable, although small differences in the peak position (520−540 nm) and intensity (0.68−0.64 au) are observed as a function of temperature. The consistent reappearance of the peak at ∼540 nm indicates the reversibility of the as-prepared sample within the reducing cycle of the redox gas exposures. When the gas flow is switched to pure air (hydrogen off), the LSPR band red-shifts and decreases while oxidation is occurring. The LSPR spectral changes during the oxidation process are sluggish except for some initial fast changes at the onset of the oxidative gas exposure. It can be seen that even after 100 min of air exposure the LSPR spectrum does not appear to reach equilibrium. Likewise, both the LSPR peak position and intensity in air show temperature dependencies with clearly a faster rate with temperature due to temperature-enhanced oxidation kinetics. Further inspection of these figures reveals the fine features in the LSPR sensing trace observable at the onset of the gas exposures (representatively marked in Figure 4a). These features change from spike to dip shapes with temperatures increasing from 300 to 400 °C for the air exposure cycles, while only dips are seen with increasing temperature for the 1% H2 exposure cycles. They were found not to be artifacts but real features as evidenced by the corresponding LSPR spectral

Figure 3. (a) Comparison of experimental LSPR spectra of the bimodal Ag/AgOx nanoparticles/YSZ system after air (300 min), 0.5% H2 mixed air (100 min), and air (300 min) exposure treatments for oxidation and reduction reactions at 400 °C, respectively. (b) Fast LSPR peak shift upon changing the exposure from air to 1% H2 mixed air at 400 °C for the same sample. 5023


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Figure 4. (a) Changes in the LSPR peak position and intensity of Ag/AgOx nanoparticles/YSZ samples as a function of exposure time of different gases at different temperatures. (b) Double Lorentzian fitting curve superimposed onto the experimental LSPR spectrum of the sample under reducing gas exposure.

progression monitored at the seconds-level time scale (Figure 3b). These temperature and gas concentration dependent fine features should mirror surface/interface chemical reactions and their associated surface species like H2O and OH− as detected in our previous study with Raman spectroscopy,16 as LSPR is sensitive to changes in the surface effective dielectric function.37,39,46,47 Study of these features along with the reactive kinetics is lengthy and will be detailed in a future report. The slow red shift, intensity drop, and broadening of the LSPR band in the oxidation cycles can be attributed to an increasingly thick AgOx shell surrounding a Ag core according to Mie theory.48 The study by Santillan et al. with an optical extinction-based parametric method for sizing a single core/ shell Ag/Ag2O nanoparticle shows that LSPR band maxima are strongly dependent on the oxide shell thickness, while the peak intensity and width depend on both the core and shell dimensions.49 Characterization of AgNP oxidation has also employed simulation assistance to resolve the experimentally observed profound red-shift of LSPR bands of wet-chemical synthesized AgNPs in the presence of oxidants.47,50−52 Figure 5 presents simulation results of plasmonic spectra based on Mie theory for core/shell particle structure and thick-shell embedded particle structures48,53 to help understand the experimental observations. The simulation uses the average particle sizes obtained from Figure 8a and b below as references, dielectric constants of Ag and Ag2O from Pakic54 and Pettersson,55 and effective media dielectric constants from YSZ and air. As shown in Figure 5a, the LSPR band progressively red shifts, dampens, and broadens with surface oxidation and core/shell structure formation for AgNPs with a decreased core size and an increased shell thickness. When the shell thickness to core size ratio is large enough (e.g., rs/rc > 3.5), the simulation shows a leveling LSPR peak position, similar to the leveling at rs/rc of ∼2 reported by Santillan et al.49 This means that the shell is sufficiently thick such that the core particles can be treated as being embedded in an oxide matrix. The simulation can then be reasonably switched from a core/ shell structural model to an embedded particle model (regular particles in a media) with a further increase in the shell to core ratio. As a result of these modeled parameters the simulation in Figure 5b indicates a further progressive LSPR band red shift and broadening with continuous surface oxidation, particle migration, and coalescence for increased core and thicker shell dimensions. Since the simulation in Figure 5b is based on a

Figure 5. Simulation based on Mie’s theory (a) for single size core/ shell structural particles and (b) for coalesced core/shell particles with thicker shells by treating the particles as embedded ones in the AgOx matrix, coupled to YSZ and at room temperature. The insets depict schematic evolutions of the particles.

noncore/shell structural model, the shell thickness is not a parameter required for this calculation and thus is not labeled in the figure, but the rs/rc is sufficiently large for the oxide to be treated as the dielectric surrounding media, such as the average particle size of the oxidized sample is up to ∼200 nm as shown later, while the largest particle diameter used in the simulation is 50 nm. Though the LSPR band intensifies with an increase in particle size, the overall plasmonic intensity of samples can decrease as the particle dispersion decreases and can be a dominant factor. The small band at ∼560 nm is a multipolar plasmon excitation possibly corresponding to the experimentally observed feature at ∼485 nm in Figure 3.53 The combination of Figure 5a and b explains the experimental data very well. Therefore, the substantial red shift and intensity drop in the LSPR band with oxidation time in Figure 4a is indicative of the growth of a thick silver oxide shell and the 5024


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The Journal of Physical Chemistry C core/shell particle coalescence. The simulation also suggests a presence of a thin oxide shell (∼2 nm) on the Ag cores for the reduced sample right before the oxidation reaction. In order to better understand and help confirm what was responsible for the observed LSPR peak shifts, XPS analyses was used to monitor the oxidation state of Ag during a series of gas exposures similar to those preceding the optical measurements; all gas treatments were conducted at 400 °C. Specifically, the Xray-induced Ag MVV Auger peak shape was monitored to indicate whether Ag was in an oxidized or reduced state. This peak was used as opposed to the Ag 3d peak because of the very small difference between the Ag 3d5/2 binding energies of Ag metal and oxides.56 Figure 6a shows the Ag MVV Auger spectra after exposure to alternating treatments in zero air and 1% H2/air. The spectra of the reference materials Ag metal and Ag2O are shown for comparison. It can be clearly seen that the spectra following treatment in zero air are nearly identical to that of the silver oxide. In contrast, the spectra following treatment in 1% H2/air are similar to that for metallic silver with the similarity increasing for subsequent treatments in the reducing gas. It does appear that the Ag spectra maintain an oxidative component even after the reducing treatment, likely indicative of the surface oxide on these small particles. Of interest is the observation of a thick (relative to the reduced sample) surface oxide following treatment in air at 400 °C because bulk silver oxides are not stable and typically decompose to the metal at such high temperatures,57,58 while nanoparticle forms of AgOx have both a size and temperature dependence on their associated stability.59 Therefore, another set of experiments was conducted to determine whether that decomposition was actually occurring and the surface oxide was simply forming upon cooling in air. The spectra in Figure 7 show that when the bulk reference compound Ag2O was heated in air and then cooled in air the surface was a mixture of Ag metal and silver oxide. If the sample was heated in air and then vacuum was applied during cooling, the spectra were nearly identical to metallic Ag. Thus, it would appear that oxide formation occurs during cooling in the XPS experiment using the bulk Ag2O sample, with the observation of some Ag metal for the reference material even after cooling in air being due to unfavorable oxidation thermodynamics combined with its larger overall bulk-like particle size and slower oxidation kinetics associated with the lower temperature in cooling. The same experiments were then performed with the AgNP supported on the YSZ sample. While cooling in air produced only the oxide, as already observed, cooling in vacuum yielded Ag with mixed oxidation states. The exposure to vacuum eliminates an oxidizing atmosphere, which can cause partial decomposition of already formed AgOx as the vacuum was applied at 400 °C. Clearly, the result points to the stability of thick AgOx for nanoparticle Ag-YSZ under oxidizing conditions, consistent with the optical measurements. The oxide stability likely indicates a significant role of YSZ as the average particle size observed under oxidizing conditions is ∼200 nm as noted in Figure 8b below, well above the critical size for intrinsic decomposition according to the thermodynamic study by Bi et al. for AgNPs supported in porous silica,59 although the size is much smaller than that of the bulk reference. Also striking is the dramatic reversibility in solid-state morphology with redox gas exposures for the samples. Shown in Figure 8 is a comparison of the SEM images after the sample was reduced in 1% H2 in air for 100 min and oxidized in air for 300 min at 400 °C. The reduced sample is seen to be

Figure 6. (a) XPS spectra of Ag/AgOx on the YSZ sample after cyclic and different redox treatments. Bulk Ag and Ag2O references are included for comparison. (b) Changes in XPS Ag 3d/Zr 3d intensity as a function of the treatment.

comprised of highly dispersed small AgNPs on YSZ with an average size of ∼20 nm (Figure 8a), which are almost as small as those of the as-prepared sample (Figure 1). The inset in the figure further reveals the presence of bimodal metallic particles for the reduced sample clearly visible as brighter large and small particles sitting at the dark grain boundaries of YSZ. Surprisingly, much larger particles with an average size of ∼200 nm were seen for the oxidized state, which have a Ag core/thicker Ag oxide shell structure as described above. The reversible changes in particle size/structure appear to be responsible for the observed plasmon spectral oscillation, which can be exploited as a sensing response. Of particular interest is 5025


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dispersion of Ag, which is observed during the H2/air gas exposure, while lower values are observed during the air gas exposure. The behavior of the intensity ratios is in agreement with the well-dispersed smaller particle sizes and the far separated large particle sizes observed in the SEM images (Figure 8a and b). A final treatment in 10% H2/Ar was included to measure the effect of a more severe reducing treatment in the absence of air/oxygen. The result shows that such treatments result in much poorer dispersion of Ag as the particles underwent a substantial coalescence under this condition and indicate that air/oxygen plays a role in helping to promote Ag dispersion under more mildly reducing conditions containing hydrogen. The most probable silver oxide is Ag2O, as it is a thermodynamically favorable phase due to the large free energy drop during Ag reaction with molecular oxygen as compared to AgO.60 Thus, the redox reactions of AgNPs in O2- and H2-containing environments can be expressed as 2Ag (s) +

1 O2(g ) ↔ Ag2O(s) 2

(1)

1 1 1 Ag O(s) + H2(g ) ↔ Ag (s) + H2O(g ) 2 2 2 2

(2)

where Ag and H2O are the reduction reaction products. As previously discussed, a thick layer of AgOx forms on the Ag core accompanied by particle coalescence during the oxidation cycle, leading to the broad red-shifted LSPR band (Figure 3a) for the particles supported on YSZ. However, this is very different from the commonly observed behavior of the particles on a quartz substrate under identical test conditions, indicating the importance of the YSZ support layer on the resulting reaction characteristics. As shown, both plasmonic properties (Figure 9a) and particle morphology (Figure 9b and c) exhibit only minor changes before and after oxidation treatments, and detailed SEM size analysis with ImageJ software, of 100’s of particles, reveals an average size of 43 and 44 nm before and after oxidation, indicating a lack of substantial oxidation and mass transfer for these samples without the YSZ underlayer. It is well-known that bulk silver oxide will undergo thermal decomposition at temperatures above 200 °C.57 Though AgOx NPs have higher decomposition temperatures, their stability reduces with an increase in particle size due to the inverse-sizedependent change in free energy for oxidation.59 Therefore, from a thermodynamics perspective oxidation of AgNPs is unfavorable at high temperatures except for very small particles with sizes below a critical radius for a given temperature, for example 2 nm for 500 °C.59 This comparison further suggests that the YSZ underlayer plays a critical role in driving the observed oxidation of the silver nanoparticles. Nanothin layers of YSZ can be assumed to be catalytically active with regard to its bulk form due to the high surface area to thickness ratio, leading to more surface oxygen vacancies and active sites. These active sites are rich at the step, kink, and corners of the constituent crystalline grains because of the reduced coordination number of Zr ions.22 The YSZ surface can adsorb oxygen ion species like O2− and O− to passivate the surface states.21,22,25 When the ionic transport characteristics of the YSZ thin film are activated or enhanced at elevated temperatures,23,24 it has a strong chemical potential or tendency to transport oxygen anions (O2−) into the bulk through surfaceadsorbed oxygen anion species at YSZ surface defect sites. These activated oxygen anion species are expected to increase

Figure 7. Comparison of XPS spectra of Ag/AgOx nanoparticles on YSZ-coated and -noncoated quartz substrates after oxidation treatments with cooling in air and vacuum, respectively. Bulk Ag and Ag2O references are included.

Figure 8. Morphology change of Ag/AgOx particles on YSZ nanofilm after (a) reduction and (b) oxidation reactions for 100 min at 400 °C, where the inset in (a) is a high-magnification image of the brighter particles sitting at the dark grain boundaries of YSZ.

how the small metallic particles are quickly formed from the large core/shell particles and how the latter slowly evolve with both silver oxidation and mass transfer or migration from the former, in a reversible or oscillating style between the two redox states as evidenced by the LSPR band evolution and also as confirmed by repeated SEM imaging. The oscillating behavior of the AgNP size between oxidizing and reducing environments was also observed in the XPS experiments described earlier. Figure 6b shows a plot of the Ag 3d/Zr 3d XPS peak intensity ratios for the series of redox treatments. A higher value for the ratio indicates better 5026


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and mass transfer effects are likely attributed to the activity of the YSZ film and the subsequent large differential surface energy formation from the reaction. Table 1 summarizes the Table 1. Surface Energies from the Literature with FirstPrinciples and Empirical Computational Methods, ab Initio Atomistic Thermodynamics Computation, and Calorimetric Measurement material

surface energy (J/m2)

Ag Ag2O Ag2O Ag2O

1.0 ∼1.84 ∼2−3.23 ∼5.85−4.65

YSZ (8% yttria) YSZ (8% yttria)

0.85 ± 0.07 1.16 ± 0.08

note/reference [61] stoichiometric [62] O poor/rich, Ag terminated [62] O poor/rich, Ag or O terminated [62] hydrous surface [63] anhydrous surface [63]

surface energies for the relevant materials discussed. After the particle oxidation on YSZ, upon switching to hydrogen mixed with air, fast reduction takes place, resulting in transformation of the large Ag core/AgOx shell particles into smaller metallic AgNPs (Figure 8a) with rapid LSPR band changes (Figure 4a). In this process the AgOx shell undergoes a quick reaction with H2 at high temperatures (300−400 °C),67,68 producing silver metal and water according to eq 2. Moreover, it is expected that at the triple-phase boundary (TPB) of the Ag core/AgOx shell particles, YSZ, and gases, Ag upon being reduced tends to reoxidize as a result of reaction with oxygen anions formed on YSZ, followed by rereduction, and so on, leading to dynamic redox reactions of silver and the corresponding catalytic production of water from H2 and O2 reaction over the system. The water produced can dissipate onto both the YSZ surface and the particle surfaces away from the TPB locations through spreading/evaporation/readsorption. TPB-enhanced redox reactions are well-known phenomena and have been widely observed in various material systems.69−72 At the same time, in the reducing gas mixed with air, the YSZ surface also takes part in reactions since the oxygen-enriched YSZ surface can readily adsorb H2, with OH−1 species produced.29−31 Continuous water production through the dynamic redox reactions may lead to formation of a water layer on the particle surfaces as known from YSZ-related reactions in fuel cells, where hydrous layers can form at much higher temperatures, for example, 900 °C.30,31 Such humid environments along with intermediate species like OH−1 on the reduced sample can be inferred from the redder and broader 540 nm LSPR band of the reduced sample (Figure 3a) taken in situ than that in Figure 2a at room temperature due to temperature effects (400 °C)73 and also the dielectric constant effects of the reactive surface species produced.16 The quick red shifts of the metallic and oxide LSPR bands with time during reducing gas exposures in Figure 3b are also partially indicative of the dielectric constant effects of catalytic water formation during the reduction reaction of the silver oxide shell. Further, the fine features, i.e., spikes to dips from 300 to 400 °C in air exposure cycles and dips only in H2/ air at the onset of the gas exposures in Figure 4a, should mirror surface/interface chemical reactions and the associated formation/removal of H2O and surface species, as LSPR is sensitive to changes in the surface effective dielectric function.37,44,46 The reaction-associated formation of water and intermediate species is a direct consequence of reaction (eq

Figure 9. LSPR extinction spectra (a) and SEM images (b) and (c) of Ag/AgOx particles on quartz before and after oxidation treatment. (1) and (2) in the legend of (a) represent two samples.

the potential for oxidation of the AgNPs, driving metallic particle oxidation. Even at room temperature, plasmonic spectra of AgNPs on YSZ are more red-shifted than those for Ag NPs on SiO2 (see Figures 2a and 9a), consistently pointing to the importance of the unique properties of YSZ. Upon formation of silver oxide shells on the Ag cores, the surface energy is increased from 1.0 J/m2 for Ag61 to 4.65−5.85 J/m2 for Ag2O with O termination.62 The surface energy of silver oxide is also higher than that for anhydrous YSZ (8%) (1.16 J/m2).63 Due to the differential surface energy, the Ag core/AgOx shell particles will tend to dewet on YSZ. With this reduced surface adhesion and enhanced mobility at elevated temperatures, the particles are inclined to migrate and coalesce to minimize system energy, one of the two sintering mechanisms widely reported,64−66 while oxidation is continuously taking place through oxygen diffusion into the metal core/oxide shell interface. Both migration and oxygen interfacial diffusion may explain why the LSPR band indexed oxidation process is sluggish. The enhanced rates and kinetics of the LSPR band change with temperature, as observed from Figure 4a, are consistent with this interpretation of temperature activation as the key to the diffusion and migration processes and the oxidation kinetics as well. Thus, the particle oxidation 5027


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The Journal of Physical Chemistry C 2) and has been confirmed by Raman spectrometry in previous studies.16,60 It is well-accepted that AgNPs and even bulk Ag can dissolve in water to yield Ag + , directly or through oxidative dissolution,74−76 with the small particles (e.g., ∼5 nm) readily and almost completely dissolving in water, while the larger ones (e.g., ∼50 nm) have a relatively lower dissolution rate.77 It is not unexpected that the surface Ag atoms formed upon reducing the AgOx shells may immediately dissolve in a surface H2O thin layer by thermodynamically favorable mixing. Moreover, the silver oxide shell can spontaneously fall off the Ag core and dissolve on YSZ in the humid environment, as the oxide is unstable in a water-enriched environment even at room temperature.78 We speculate that upon the collapse/dissolution of silver oxide in the reduction reaction Ag+ ions on YSZ undergo fast reduction, diffusion, nucleation, and growth into AgNPs with a high atomic mobility at the test temperatures,79,80 responsible for the fast appearance (Figure 3b) and then stable evolution of the ∼540 nm LSPR band observed (Figure 4a). The hypothsized particle regeneration and growth is mechanistically in agreement with the synthesis of silver particles in aqueous suspensions by reduction of Ag2O with H2.81,82This is also consistent with the work of Glover et al., who have shown that production of small nanoparticles in ambient humid environment (RH > 50%) is an intrinsic property of Ag that is less size dependent.83 During the equilibrium stage, in addition to the dynamic redox reactions at the TPB in the presence of O2 and H2, there should also be dynamic redox reactions on the regenerated small AgNPs due to the competition between the YSZenhanced oxidation reaction and the H2 exposure and hightemperature driven reduction reaction. These reaction paths should contribute to the yield of the catalytic reaction product for a water-enriched environment, which is key to distinctively maintaining the metallic particles at small sizes without sintering even at 400 °C temperatures. Correspondingly, the regenerated AgNPs are seen to be situated at the grain (∼50 nm) boundaries of the YSZ layer (the inset of Figure 8a), as these locations are likely more hydrous with more stabilizing species like OH−130 and serve as lower-energy sites for the particles. The fine dispersion of the reduced particles should also be driven by the overall tendency to reduce surface energy through formation of a Ag/YSZ interface for which interfacial energies are typically lower than the corresponding free surfaces. As discussed previously in other systems such as Au/TiO2, interfacial free energy arguments can also often explain preferential occuption of metallic nanoparticles at the oxide-phase grain boundaries.84 Table 1 notes surface energies from previous first-principles and empirical computational methods, ab initio atomistic thermodynamics computation, and calorimetric measurements found in the literature. From Table 1, the very close surface energy of AgNPs (1.0 J/m2) to that of the hydrous YSZ surface (0.85 J/m2) strongly supports the dispersing mechanisms discussed. To attest to the critical role of the water generation in the Ag/AgOx particle reduction and dissolution process, a similar sample was exposed to H2 in Ar following a few cycles of redox reactions using air as the carrier gas. As shown in Figure 10a, the extinction spectra reverse with the air and 1% H2 in air exposure cycles as that dipicted earlier in Figure 3a, but upon changing the carrier gas from air to Ar for 1% H2, there was no recovery of the blue-shifted LSPR band that is characteristic of metallic AgNPs with a thin oxide surface. SEM image analysis

Figure 10. LSPR extinction spectra (a) and SEM image (b), (c), and (d) of Ag/AgOx particles on YSZ after different redox treatments as labeled in the figures.

of these samples indicates the production of small metallic particles with the air−H2 gas exposure (Figure 10b), but large particles (Figure 10d) remained when the oxidized sample was exposed to H2 in Ar. Detailed inspection of Figure 10a reveals that the LSPR band following 1% H2 in Ar gas exposure becomes weaker and broader but blue-shifted to ∼660 nm relative to the ∼800 nm band from air exposure. This change in the LSPR spectra is indicative of smaller Ag/AgOx particles with a thinner oxide shell relative to the oxidized sample according to Mie theory simulation. This is consistent with their average particle sizes measured by SEM in Figure 10d and c and also agrees with the larger particle size distribution 5028


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between YSZ and the particles as a function of temperature and the respective gases. Compared to conventional supported catalysts, this system represents a new type of catalyst that operates in a selfcontrolled dynamic regime and has no constraint and no or little hindrance of surface active sites. The dynamic coupling of bimodal core/shell plasmonic particles and an ionic metal oxide underlayer offers both strong LSPR and catalytic activity from the former and ionic activity from the latter and, in particular, a strong synergy between the interfacial components to overall enhance chemical reactions. These properties suggest that the material system is promising for catalysis reactions within industrially important oxidative conversions of chemicals or pollutant mitigation and also for harsh environment optical gas sensing. The analyses indicate that oxygen has to be present in the reducing gas for the YSZ to drive production of water through its synergistic anion formation/reaction with the AgNPs. This helps to maintain a small size of the particles and thereby a corresponding dynamic sintering-free character for high catalytic activity and catalysis efficiency. In addition to the novel Ag/AgOx on the YSZ system that has been introduced, these findings suggest the presence and a mechanism of strong synergistic interactions of metal nanoparticles and supports, which is important for understanding the fundamentals in catalysis and other relevant fields. The work could open a new avenue for transferable design of novel heterogeneous catalysts and chemical gas sensors exploiting favorable energetics for advanced functionalities and performances. The mechanism behind the strong oxidation and substantial particle migration may be useful in fuel cell technology for design of YSZ-incorporated Ag electrodes with enhanced stability and reliability.

observable from Figure 10d for the broader plasmonic spectrum. Therefore, it appears that in the absence of O2 the dynamic redox reaction channels, found mostly at the TPB, for the continuous catalytic production of water and formation of a humid envrironment have been deactivated. Thus, the production of finely dispersed metallic particles during the reduction cycle has been inhibited. The lack of production of small metallic particles during the H2 in Ar suggests that simply a reduction of the existing oxide is insufficient to produce adequate water for creating a humid environment as there is only a limited O source with the silver oxide, and this in turn supports the importance of the redox reactions at the TPB for sufficient water production in the presence of both H2 and O2 sources. This appears to confirm the critical role of water in the process of the silver oxide collapse and transformation mechanism studied. On the basis of the above experimental data and analyses, we propose a mechanism for the morphology oscillation and the resultant plasmonic sensing response accompanying redox reactions of the YSZ nanolayer coupled Ag nanoparticle system as schematically shown in Figure 11. In the air-only exposure

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CONCLUSIONS This work demonstrates exceptionally large oscillations of the plasmonic properties and the unprecedented reversible transformation of large dewetted Ag core/AgOx shell particles to highly dispersed small metallic AgNPs on YSZ under redox gas exposures at temperatures of 300−400 °C. The driving force behind this behavior is found to be the strong synergy between the YSZ support layer and AgNPs as a function of ionic activity, particle size, temperature-dependent thermodynamics, and redox gases. This allows for a potentially unique gas sensing capability to be realized with an extreme sensitivity in harsh environments due to the large optical and morphological changes. In particular, this enables a distinctive dynamic sintering-free and highly active nanomaterial system to be introduced as a potentially novel functional heterogeneous catalyst. Catalytic water production, occurring at the TPB, in hydrogen containing air/oxygen is essential to dissoluting the large Ag core/AgOx shell particles into small metallic particles and maintaining sintering-free Ag particles on YSZ in equilibrium with Ag+ dissolved on the hydrous surface, which may lead to a promising catalyst stability and activity by the sintering-free and exceptional redox activity observed. We anticipate that the strong synergy with YSZ and the H2Oenabled sintering-free mechanism can be exploited for design of different ionic and plasmonic material combinations with advanced functionalities by tuning the nanoscale material properties and maximizing their redox potentials or activities. The designed system could induce specific interactions of the system with selected reactants, allowing controlled chemical reactions under sintering-free catalytic operating conditions.

Figure 11. Schematics of hypothesized mechanisms for YSZ synergy enhanced oxidation of AgNPs and formation of large core/shell particles (left panel) and catalytic water production enabled splitting of Ag core/AgOx shell particles and maintaining of sintering free small AgNPs regenerated (right panel).

cycle (the left panel), oxygen ions formed on YSZ enable surface oxidation of the AgNPs to form a metal core/oxide shell. In this process, anhydrous YSZ with an oxygen ionenriched surface and the Ag2O shell have large differences in surface energies, causing the particles to be dewetted, migrate, and coalesce into large core/thicker shell particles for an overall reduced system free energy. Upon exposure to H2 in O2 (the right panel), both a reduction reaction on the AgOx shell and dynamic redox reactions at the TPB take place, catalytically producing water. The resultant humid environment causes the collapse and dissolution/decomposition of the AgOx shell, while Ag ions produced with this dissolution/decomposition undergo fast reduction, diffusion, nucleation, and growth into small AgNPs on YSZ until a dynamic equilibrium is quickly (seconds time scale) reached at the high temperatures. The distinctive sintering-free property of the highly dispersed AgNPs on YSZ is maintained by the enhanced redox reactions of Ag and catalytic reaction of H2 and O2 to produce water at both the TPB and on the AgNPs, driven by the synergies 5029


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(16) Zhao, Z.; Carpenter, M. A. Support-free Bimodal Distribution of Plasmonically Active Ag /AgOx Nanoparticle Catalysts: Attributes and Plasmon Enhanced Surface Chemistry. J. Phys. Chem. C 2013, 117, 11124−11132. (17) Nagy, A.; Mestl, G. High Temperature Partial Oxidation Over Silver Catalysts. Appl. Catal., A 1999, 188, 337−353. (18) Zhang, X.; Qu, Z.; Li, X.; Wen, M.; Quan, X.; Ma, D.; Wu, J. Studies of Silver Species for Low-Temperature CO Oxidation on Ag/ SiO2 Catalysts. Sep. Purif. Technol. 2010, 72, 395−400. (19) She, X.; Flytzani-Stephanopoulos, M. The role of Ag−O−Al Species in Silver−alumina Catalysts for the Selective Catalytic Reduction of NOx with Methane. J. Catal. 2006, 237, 79−93. (20) Tsampas, M. N.; Zhao, C.; Serve, A.; Boréave, A.; Dole, H.; Baranova, E.; Vernoux, P. Synergy of Ag and Yttria-Stabilized Zirconia Catalysts during Diesel Soot Oxidation. 8th International Conference on Environmental Catalysis, Aug 2014, Asheville, United States < hal01058182>. (21) Goff, J. P.; Hayes, W.; Hull, S.; Hutchings, M. T.; Clausen, K. N. Defect Structure of Yttria-stabilized Zirconia and its Influence on the Ionic Conductivity at Elevated Temperatures. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 14202−14219. (22) Xia, X.; Oldman, R. J.; Catlow, C. R. A. Oxygen Adsorption and Dissociation on Yttria Stabilized Zirconia Surfaces. J. Mater. Chem. 2012, 22, 8594−8612. (23) Jiang, J.; Hu, X.; Shen, W.; Ni, C.; Hertz, J. L. Improved Ionic Conductivity in Strained yttria-stabilized Zirconia Thin Films. Appl. Phys. Lett. 2013, 102, 143901. (24) Kushima, A.; Yildiz, B. Oxygen Ion Diffusivity in Strained yttria Stabilized Zirconia: Where is the Fastest Strain? J. Mater. Chem. 2010, 20, 4809−4819. (25) Bak, T.; Rekas, M.; Nowotny, J.; Sorrell, C. C.; Adamski, A.; Dyrek, K.; Sojka, Z. Interactions of Oxygen with Yttria-stabilized Zirconia at Room Temperature. Ionics 2001, 7, 332−338. (26) Rogers, P. H.; Carpenter, M. A. Particle Size Dependence of Nanocomposites for Plasmonic-Based All Optical Sensing Applications. J. Phys. Chem. C 2010, 114, 11033−11039. (27) Rogers, P. H.; Sirinakis, G.; Carpenter, M. A. Direct Observations of Electrochemical Reactions within Au-YSZ Thin Films via Absorption Shifts in the Au Nanoparticle Surface Plasmon Resonance. J. Phys. Chem. C 2008, 112, 6749−6757. (28) Baltrus, J. P.; Ohodnicki, P. R.; Joy, N. A.; Carpenter, M. A. Examination of Charge Transfer in Au/YSZ for High-Temperature Optical Gas Sensing. Appl. Surf. Sci. 2014, 313, 19−25. (29) Kogler, M.; Kock, E. M.; Bielz, T.; Pfaller, K.; Klotzer, B.; Schmidmair, D.; Perfler, L.; Penner, S. Hydrogen Surface Reactions and Adsorption Studied on Y2O3, YSZ, and ZrO2. J. Phys. Chem. C 2014, 118, 8435−8444. (30) Gorski, A.; Yurkiv, V.; Starukhin, D.; Volpp, H.-R. H2O Chemisorption and H2 Oxidation on Yttriastabilized Zirconia: Density Functional Theory and Temperature Programmed Desorption Studies. J. Power Sources 2011, 196, 7188−7194. (31) Weber, A.; Utz, A.; Joos, J.; Ivers-Tiffée, E.; Störmer, H.; Gerthsen, D.; Yurkiv, V.; Volpp, H.-R.; Bessler, W.-G. Electrooxidation of Reformate Gases at Model Anodes. ECS Trans. 2011, 35, 1513− 1528. (32) Takehira, K.; Shimomura, J.; Hamakawa, S.; Shishido, T.; Kawabata, T.; Takaki, K. Partial Oxidation of CH to Synthesis Gas Using Ni-catalyst/Au|YSZ|Ag Electrochemical Membrane Reactor. Appl. Catal., B 2005, 55, 93−103. (33) Li, N.; Gaillard, F.; Boreave, A. Electrochemical Promotion of Ag Catalyst for the Low Temperature Combustion of Toluene. Catal. Commun. 2008, 9, 1439−1442. (34) Tsiplakides, D.; Vayenas, C. G. Temperature Programmed Desorption of Oxygen from Ag Films Interfaced with Y2O3-Doped ZrO2. J. Catal. 1999, 185, 237−251. (35) Choi, S. H.; Hwang, C. S.; Lee, M. H. Performance Enhancement of Freestanding Micro-SOFCs with Ceramic Electrodes by the Insertion of a YSZ-Ag Interlayer. ECS Electrochem. Lett. 2014, 3, F57−F59.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the United States Department of Energy National Energy Technology Laboratory under contract number DE-FE0007190. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the United States Department of Energy National Energy Technology Laboratory. This work was also supported by the National Science Foundation research project PN 1006399

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REFERENCES

(1) Claus, P.; Hofmeister, H. Electron Microscopy and Catalytic Study of Silver Catalysts: Structure Sensitivity of the Hydrogenation of Crotonaldehyde. J. Phys. Chem. B 1999, 103, 2766−2775. (2) Bogdanchikova, N.; Meunier, F. C.; Avalos-Borja, M.; Breen, J. P.; Pestryakov, A. Activity of Silver Catalysts in the Reaction of Nitric Oxide with Carbon Monoxide. Appl. Catal., B 2002, 36, 287−297. (3) Dellamorte, J. C.; Lauterbach, J.; Barteau, M. A. Palladium-Silver Bimetallic Catalysts with Improved Activity and Selectivity for Ethylene Epoxidation. Catal. Today 2007, 120, 182−185. (4) Zhang, X.; Qu, Z.; Jia, J.; Wang, Y. Ag Nanoparticles Supported on Wormhole HMS Material as Catalysts for CO Oxidation:Effects of Preparation Methods. Powder Technol. 2012, 230, 212−218. (5) Sun, J.; Ma, D.; Zhang, H.; Liu, X.; Han, X.; Bao, X.; Weinberg, G.; Pfänder, N.; Su, D. Toward Monodispersed Silver Nanoparticles with Unusual Thermal Stability. J. Am. Chem. Soc. 2006, 128, 15756− 15764. (6) Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H2 on Au. Nano Lett. 2013, 13, 240−247. (7) Christopher, P.; Xin, H.; Linic, S. Visible-light-enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructure. Nat. Chem. 2011, 3, 467−472. (8) Linic, S.; Christopher, P.; Xin, H.; Marimuthu, A. Catalytic and Photocatalytic Transformations on Metal Nanoparticles with Targeted Geometric and Plasmonic Properties. Acc. Chem. Res. 2013, 46, 1890− 1899. (9) Xu, P.; Kang, L.; Mack, N. H.; Schanze, K. S.; Wang, H.-S. Mechanistic Understanding of Surface Plasmon Assisted Catalysis on a Single Particle: Cyclic Redox of 4-aminothiophenol. Sci. Rep. 2013, 3, 2997/1−6. (10) Manjavacas, A.; Liu, J. G.; Kulkarni, V.; Nordlander, P. PlasmonInduced Hot Carriers in Metallic Nanoparticles. ACS Nano 2014, 8, 7630−7638. (11) Khalavka, Y.; Becker, J.; Sönnichsen, C. Synthesis of RodShaped Gold Nanorattles with Improved Plasmon Sensitivity and Catalytic Activity. J. Am. Chem. Soc. 2009, 131, 1871−1875. (12) Christopher, P.; Xin, H.; Marimuthu, A.; Linic, S. Singular Characteristics and Unique Chemical Bond Activation Mechanisms of Photocatalytic Reactions on Plasmonic Nanostructures. Nat. Mater. 2012, 11, 1044−1050. (13) Seo, D.; Park, G.; Song, H. Plasmonic Monitoring of Catalytic Hydrogen Generation by a Single Nanoparticle Probe. J. Am. Chem. Soc. 2012, 134, 1221−1227. (14) Bartholomew, C. H. Mechanisms of Catalyst Deactivation. Appl. Catal., A 2001, 212, 17−60. (15) Rawalekar, S.; Mokari, T. Rational Design of Hybrid Nanostructures for Advanced Photocatalysis. Adv. Energy Mater. 2013, 3, 12−27. 5030


Article


(56) Wagner, C. D. Handbook of X-ray photoelectron spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer, 1979. (57) Weast, R. C. CRC Handbook of Chemistry and Physics, 70th ed.; CRC Press: Boca Raton, FL, 1989, D-43. (58) Karakaya, I.; Thompson, W. T. The Ag-O (Silver-Oxygen) System. J. Phase Equilib. 1992, 13, 137−142. (59) Bi, H.; Cai, W.; Kan, C.; Zhang, L.; Martin, D.; Träger, F. Optical Study of Redox Process of Ag Nanoparticles at High Temperature. J. Appl. Phys. 2002, 92, 7491. (60) de Rooij, A. The Oxidation of Silver by Atomic Oxygen. ESA J. 1989, 13, 363−382. (61) Medasani, B.; Park, Y. H.; Vasiliev, I. Theoretical Study of the Surface Energy, Stress, and Lattice Contraction of Silver Nanoparticles. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 235436. (62) Li, W.-X.; Stampfl, C.; Scheffler, M. Insights into the Function of Silver as an Oxidation Catalyst by ab Initio, Atomistic Thermodynamics. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 165412. (63) Costa, G. C. C.; Ushakov, S. V.; Navrotsky, A.; Castro, R. H. R.; Muccillo, R. Calorimetric Measurement of Surface and Interface Enthalpies of Yttria-Stabilized Zirconia (YSZ). Chem. Mater. 2010, 22, 2937−2945. (64) Hansen, T. W.; Delariva, A. T.; Challa, S. R.; Datye, A. K. Sintering of Catalytic Nanoparticles: Particle Migration or Ostwald ripening? Acc. Chem. Res. 2013, 46, 1720−30. (65) Li u, Z.; Che, A.; El zatahry, A. A.; Zhao, D. Direct Imaging Au Nanoparticle Migration Inside Mesoporous Silica Channels. ACS Nano 2014, 8, 10455−10460. (66) Yang, F.; Chen, M. S.; Goodman, D. W. Sintering of Au Particles Supported on TiO2 (110) during CO Oxidation. J. Phys. Chem. C 2009, 113, 254−260. (67) Gao, X.; Zhao, M.; Zhang, Z.; Chen, C.; Ma, J.; Lu, J. Effects of Hydrogen Annealing on the Microstructure and Optical Properties of Single-phased AgO film Deposited Using Direct-current Reactive Magnetron Sputtering. Thin Solid Films 2011, 519, 6620−6623. (68) Jelic, D.; Skundri, J. P.; Majstorovi, D.; Mentus, S. TheThermogravimetric Study of Silver (I) Oxide Reduction by Hydrogen. Thermochim. Acta 2011, 526, 252−256. (69) Sato, K.; Nakamura, J.; Uchijima, T.; Hayakawa, T.; Hamakawa, S.; Tsunoda, T.; Takehira, K. Partial Oxidation of CH to Synthesis Gas Using an Rh |YSZ|Ag Electrochemical Membrane Reactor. J. Chem. Soc., Faraday Trans. 1995, 91, 1655−1661. (70) Hamakawa, S.; Koizumi, M.; Sato, K.; Nakamura, J.; Uchijima, T.; Murata, K.; Hayakawa, T.; Takehira, K. Synthesis Gas Production in Methane Conversion Using the Pd|yttriastabilized zirconia|Ag Electrochemical Membrane System. Catal. Lett. 1998, 52, 191−197. (71) Hamakawa, S.; Sato, K.; Hayakawa, T.; York, A. P. E.; Tsunoda, T.; Suzuki, K.; Shimizu, M.; Takehira, K. Selective Oxidation of Ethane Using the Au I YSZ I Ag Electrochemical Membrane System. J. Electrochem. Soc. 1997, 144, 1−5. (72) Nielsen, J.; Jacobsen, T. Three-phase-boundary Dynamics at Pt/ YSZ Microelectrodes. Solid State Ionics 2007, 178, 1001−1009. (73) Yeshchenko, O. A.; Bondarchuk, I. S.; Gurin, V. S.; Dmitruk, I. M.; Kotko, A. V. Temperature Dependence of the Surface Plasmon Resonance in Gold Nanoparticles. Surf. Sci. 2013, 608, 275−281. (74) Sotiriou, G. A.; Meyer, A.; Knijnenburg, J. T. N.; Panke, S.; Pratsinis, S. E. Quantifying the Origin of Released Ag+ Ions from Nanosilver. Langmuir 2012, 28, 15929−15936. (75) Yu, S.-J.; Yin, Y.-G.; Chao, J.-B.; Shen, M.-H.; Liu, J.-F. Highly Dynamic PVP-Coated Silver Nanoparticles in Aquatic Environments: Chemical and Morphology Change Induced by Oxidation of Ag0 and Reduction of Ag+. Environ. Sci. Technol. 2014, 48, 403−411. (76) Alissawi, N.; Zaporojtchenko, V.; Strunskus, T.; Kocabas, I.; Chakravadhanula, V. S. K.; Kienle, L.; Garbe-Schönberg, D.; Faupel, F. Effect of Gold Alloying on Stability of Silver Nanoparticles and Control of Silver Ion Release from Vapor-deposited Ag−Au/ polytetrafluoroethylene Nanocomposites. Gold Bull. 2013, 46, 3−11.

(36) Jeangros, Q.; Faes, A.; Wagner, J. B.; Hansen, T. W.; Aschauer, U.; Van herle, J.; Hessler-Wyser, A.; Dunin-Borkowski, R. E. In Situ Redox Cycle of a Nickel−YSZ Fuel Cell Anode in an Environmental Transmission Electron Microscope. Acta Mater. 2010, 58, 4578−4589. (37) Noguez, C. Surface Plasmons on Metal Nanoparticles: The Influence of Shape and Physical Environment. J. Phys. Chem. C 2007, 111, 3806−3819. (38) Agnihotri, S.; Mukherji, S.; Mukherji, S. Size-controlled Silver Nanoparticles SynthesizedOver the Range 5−100 nm Using the Same Protocol and Their Antibacterial efficacy. RSC Adv. 2014, 4, 3974− 3983. (39) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. Nanosphere Lithography: Tunable Localized Surface Plasmon Resonance Spectra of Silver Nanoparticles. J. Phys. Chem. B 2000, 104, 10549−10556. (40) Peng, S.; McMahon, J. M.; Schatz, G. C.; Gray, S. K.; Sun, Y. Reversing the Size-dependence of Surface Plasmon Resonances. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 14530−14534. (41) Lerme, J.; Baida, H.; Bonnet, C.; Broyer, M.; Cottancin, E.; Crut, A.; Maioli, P.; Del Fatti, N. D.; Vallee, F.; Pellarin, M. Size Dependence of the Surface Plasmon ResonanceDamping in Metal Nanospheres. J. Phys. Chem. Lett. 2010, 1, 2922−2928. (42) Zong, R.; Wang, X.; Shi, S.; Zhu, Y. Kinetically Controlled Seedmediated Growth of Narrow Dispersed Silver Nanoparticles up to 120 nm: Secondary Nucleation, Size Focusing, and Ostwald ripening. Phys. Chem. Chem. Phys. 2014, 16, 4236−4241. (43) Vetter, T.; Iggland, M.; Ochsenbein, D. R.; Hanseler, F. S.; Mazzotti, M. Modeling Nucleation, Growth, and Ostwald Ripening in Crystallization Processes: A Comparison between Population Balance and Kinetic Rate Equation. Cryst. Growth Des. 2013, 13, 4890−4905. (44) Hu, J.; Cai, W.; Zeng, H.; Li, C.; Sun, F. Substrate Dependent Surface Plasmon Resonance Evolution of Ag Nanoparticles Treated in Atmospheres. J. Phys.: Condens. Matter 2006, 18, 5415−5423. (45) Zhou, X.; Xu, W.; Liu, G.; Panda, D.; Chen, P. Size-Dependent Catalytic Activity and Dynamics of Gold Nanoparticles at the SingleMolecule Level. J. Am. Chem. Soc. 2010, 132, 138−146. (46) http://nanocomposix.com/pages/silver-nanoparticles-opticalproperties#local-refractive-index. (47) Henglein, A. Colloidal Silver Nanoparticles: Photochemical Preparation and Interaction with O2, CCl4, and Some Metal Ions. Chem. Mater. 1998, 10, 444−450. (48) Myroshnychenko, V.; Rodrguez-Fernandez, J.; Pastoriza-Santos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; Liz-Marzan, L. M.; Garcia de Abajo, F. J. Modelling the Optical Response of Gold Nanoparticles. Chem. Soc. Rev. 2008, 37, 1792−1805. (49) Santillan, J. M. J.; Scaffardi, L. B.; Schinca, D. C. Quantitative Optical Extinction-based Parametric Method for Sizing a Single Core− shell Ag−Ag2O Nanoparticle. J. Phys. D: Appl. Phys. 2011, 44, 105104. (50) Chen, M.; Wang, L.-Y.; Han, J.-T; Zhang, J.-Y.; Li, Z.-Y.; Qian, D.-J. Preparation and Study of Polyacryamide-Stabilized Silver Nanoparticles through a One-Pot process. J. Phys. Chem. B 2006, 110, 11224−11231. (51) Yin, Y.; Li, Z.-Y.; Zhong, Z.; Gates, B.; Xia, Y.; Venkateswaran, S. Synthesis and Characterization of Stable Aqueous Dispersions of Silver Nanoparticles Through the Tollens Process. J. Mater. Chem. 2002, 12, 522−527. (52) Kuzma, A.; Weis, M.; Flickyngerova, S.; Jakabovic, J.; Satka, A.; Dobrocka, E.; Chlpik, J.; Cirak, J.; Donoval, M.; Telek, P.; Uherek, F.; Donoval, D. Influence of Surface Oxidation on Plasmon Resonance in Monolayer of Gold and Silver Nanoparticles. J. Appl. Phys. 2012, 112, 103531. (53) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (54) Rakić, A. D.; Djurišic, A. D.; Elazar, J. M.; Majewski, M. L. Optical Properties of Metallic Films for Vertical-cavity Optoelectronic Devices. Appl. Opt. 1998, 37, 5271−5283. (55) Pettersson, L. A. A.; Snyder, P. G. Preparation and Characterization of Oxidized Silver Thin Films. Thin Solid Films 1995, 270, 69−72. 5031


Article

The Journal of Physical Chemistry C (77) Dobias, J.; Bernier-Latmani, R. Silver Release from Silver Nanoparticles in Natural Waters. Environ. Sci. Technol. 2013, 47, 4140−4146. (78) Oscar, A.; Gallardo, D.; Moiraghi, R.; Macchione, M. A.; Godoy, J. A.; Pérez, M. A.; Coronado, E. A.; Macagno, V. A. Silver Oxide Particles/silver Nanoparticles Interconversion: Susceptibility of Forward/backward Reactions to the Chemical Environment at Room Temperature. RSC Adv. 2012, 2, 2923−2929. (79) Jelic, D.; PenavinŠkundric, J.; Majstorovic, D.; Mentus, S. The Thermogravimetric Study of Silver (I) Oxide Reduction by Hydrogen. Thermochim. Acta 2011, 526, 252−256. (80) Jelic, D.; Mentus, S.; Penavin-Škundric, J.; Bodroža, D.; Antunovic, B. A Thermogravimetric Study of Reduction of Silver Oxide under Non-isothermal Conditions. Contemp. Mater. 2010, I-2, 144−150. (81) Evanoff, D. D.; Chumanov, G. Size-Controlled Synthesis of Nanoparticles. 1. “Silver-Only” Aqueous Suspensions via Hydrogen Reduction. J. Phys. Chem. B 2004, 108, 13948−13956. (82) Merga, G.; Wilson, R.; Lynn, G.; Milosavljevic, B. H.; Meisel, D. Redox Catalysis on “Naked” Silver Nanoparticles. J. Phys. Chem. C 2007, 111, 12220−12226. (83) Glover, R. D.; Miller, J. M.; Hutchison, J. E. Generation of Metal Nanoparticlesfrom Silver and Copper Objects:Nanoparticle Dynamics on Surfaces and Potential Sources of Nanoparticles in the Environment. ACS Nano 2011, 5, 8950−8957. (84) Ohodnicki, P. R., Jr; Wang, C.; Natesakhawat, S.; Baltrus, J. P.; Brown, T. D. In-situ and Ex-situ Characterization of TiO2 and Au Nanoparticle Incorporated TiO2 Thin Films for Optical Gas Sensing at Extreme Temperatures. J. Appl. Phys. 2012, 111, 064320.

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Ag Nanoparticles Supported on Yttria-Stabilized Zirconia: A

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