CeO2 Thickness-Dependent SERS and Catalytic Properties of CeO2

Jul 31, 2014 - CeO2 Thickness-Dependent SERS and Catalytic Properties of CeO2-on-Ag Particles Synthesized by O2-Assisted Hydrothermal Method...
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CeO2 Thickness-Dependent SERS and Catalytic Properties of CeO2‑on-Ag Particles Synthesized by O2‑Assisted Hydrothermal Method Sujie Chang, Shigang Ruan, Erlong Wu, and Weixin Huang* Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: Oxide-on-metal particles with exposed interfaces exhibit wealthy structures and functions, but their facile synthesis remains as a challenge. Herein we report a facile O2assisted hydrothermal method to synthesize CeO2-on-Ag particles with different CeO2 thicknesses. In this novel approach Ag particles catalyze the O2 + H2O reaction to form surface hydroxyls that induce the preferential nucleation of Ce(OH)3 on the surfaces of Ag particles, eventually forming CeO2 adlayers on Ag particles. CeO2-on-Ag particles exhibit the CeO2 thickness-dependent SERS effect in which their best SERS effect is 2 orders stronger than that of Ag particles. They also exhibit CeO2 thickness-dependent catalytic performance in CO oxidation in which the best one is as active as traditional CeO2-supported Ag catalyst. These results open up new opportunities to synthesize oxide-on-metal particles and explore their functions by tuning the oxide adlayer thickness.

1. INTRODUCTION Oxide-on-metal nanostructures with either buried interfaces (metal (core)/oxide (shell) particles) or exposed interfaces (oxide-on-metal particles) are attracting great interest because of their wealthy structures and functions. For examples, Au nanoparticles coated with SiO2 and Al2O3 thin shells realized the shell-isolated nanoparticle-enhanced Raman spectroscopy;1 monolayer-thick FeO islands on Pt nanoparticles exposed unique coordination-unsaturated Fe(II) sites at the FeO−Pt interface that were highly active in catalyzing the preferential oxidation of CO in excess H2;2 Al2O3 adlayers selectively blocking the low-coordinated Pd atoms on Pd nanoparticles resulted in the sintering-resistant and coke-resistant, highly stable Pd catalysts for oxidative dehydrogenation of ethane.3 Nevertheless, the facile synthesis of oxide-on-metal nanostructures remains a challenge. The key to synthesis oxide-onmetal nanostructures is to control the selective nucleation and growth of oxide on metal particles. Thus, utilizing chemical reactions that selectively occur on metal particle surfaces and eventually form oxides is the main strategy, such as thermal decomposition of metallorganic compounds4 and atomic layer deposition.1,3 However, these methods involve volatile poisonous compounds or complex procedures. Hydrothermal synthesis has wide applications in the structure-controlled preparations of various nanomaterials and nanocomposites, but few examples have been reported for the controlled synthesis of oxide-on-metal nanostructures. Meanwhile, the thickness of oxide thin films have been observed to © 2014 American Chemical Society

affect their surface chemistry and catalytic property in oxide thin films grown on flat metal substrates5,6 but seldom in oxideon-metal particles. Herein we report a novel and facile O2assisted hydrothermal method to synthesize oxide-on-Ag particles with different CeO2 thicknesses and their CeO2 thickness-dependent surface-enhanced Raman scattering (SERS) and catalytic properties.

2. EXPERIMENTAL SECTION 2.1. Preparations. Chemicals such as AgNO3, Ce(NO3)3· 6H2O, NaOH, FeCl2, CO(NH2)2, and EG were purchased from Sinopharm Chemical Reagent Co. Ltd. TiOSO4 was purchased from Aladdin Chemistry Co. Ltd. Poly(vinylpyrrolidone) (PVP, Mw ∼ 55 000) was purchased from Sigma-Aldrich. All chemicals were used as received. Ag particles were synthesized using a polyol method.7 In a typical synthesis, 25 mL of EG was placed in a 100 mL threenecked flask that was then capped and heated with stirring in an oil bath at 160 °C for 1 h. AgNO3 solution (100 mM in EG, 15 mL) and PVP solution (600 mM in EG in terms of the repeating unit, 15 mL) were then simultaneously injected with a two-channel syringe pump at a rate of 45 mL/h to the stirring solution. The mixture was then heated at 160 °C for about 1 h. Received: June 21, 2014 Revised: July 31, 2014 Published: July 31, 2014 19238

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HRTEM/STEM images and EDS mapping profiles were taken on a JEOL JEM-2100F field-emission high-resolution transmission electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer using monochromatized Al Kα (hν = 1486.7 eV) as the excitation source. The likely charging of samples was corrected by setting the binding energy of the adventitious carbon (C 1s) to 284.8 eV. H2 temperature-programmed reduction (H2-TPR) experiments were carried out on a Micromeritics ChemiSorb 2750, in which 0.04 g of catalyst was heated at a heating rate of 10 °C/min in a 5% H2−Ar mixture with a flow rate of 20 mL/min. 2.3. Catalytic Activity Test. The catalytic activity of various samples in CO oxidation was evaluated with a fixed-bed flow reactor. The catalyst experienced no pretreatment prior to the catalytic reaction. The catalyst weight was 0.05 g, and the reaction gas consisting of 1% CO in dry air was fed at a flow rate of 30 mL/min. The composition of the effluent gas was detected by an online GC-14C gas chromatograph equipped with a TDX-01 column (T = 80 °C, H2 as the carrier gas at a flow rate of 30 mL/min). The steady-state conversion of CO was calculated from the change of CO concentrations in the inlet and outlet gases.

The formed suspending solids were isolated by centrifugation at 11 000 rpm and then adequately washed with ethanol and water to remove excess EG and PVP. The synthesis of CeO2−Ag particles followed the hydrothermal methods. Typically, 0.05 g of Ag particles and 0.07 g of NaOH were dispersed in 35 mL of ultrapure water (resistance >18 MΩ) under stirring at RT into which a calculated volume of Ce(NO3)3 aqueous solution (0.01g/mL) was then added dropwise. The mixture was adequately stirred for an additional 30 min and then transferred into a 50 mL Teflon bottle. For the traditional hydrothermal synthesis, the Teflon bottle was tightly sealed in a stainless-steel autoclave and hydrothermally treated at 180 °C for 24 h. For our novel O2-assisted hydrothermal synthesis, the Teflon bottle was bubbled with oxygen with a flow rate of 25 mL/min for 30 min and then was tightly sealed in a stainless-steel autoclave and hydrothermally treated at 180 °C for 24 h. After the hydrothermal reaction the mixture was cooled, and the formed precipitate was collected, washed with ultrapure water, and dried in vacuo. Ag particles (denoted as Ag(O2)) was also prepared by the O2-assisted hydrothermal method without the addition of Ce(NO3)3 and NaOH for the comparison. The preparations of TiO2−Ag and Fe2O3−Ag particles were performed with similar hydrothermal methods to that of CeO2−Ag particles. During the preparation of TiO2−Ag particles TiOSO4 and CO(NH2)2 were used; during the preparation of Fe2O3−Ag particles FeCl2 and NaOH were used. Traditional 0.54 wt % Ag/CeO2 catalyst was prepared by the deposition−precipitation (DP) method. First CeO2 support was prepared by a traditional precipitation method. A calculated amount of Ce(NO3)3·6H2O was dissolved in ultrapure water, and the pH value of the solution was adjusted to ∼12 with 10% NaOH aqueous solution. The precipitate was filtrated, washed with deionized water, and dried in vacuo at 80 °C for 16 h, and the acquired yellow powder was calcined in muffle oven at 500 °C for 4 h to produce white CeO2 powders. Then CeO2 powders (0.3 g) were dispersed in 100 mL of ultrapure water under stirring, and 100 mL of aqueous NaOH solution (1 M) was then added. Then a calculated amount of AgNO3 aqueous solution (12.75 g/L) was added dropwise to the above mixture. The mixture was stirred at RT for 4 h, and the acquired precipitate was filtered, washed with ultrapure water, dried in vacuo at 80 °C overnight, and finally calcined in muffle oven at 500 °C for 4 h. 2.2. Structural Characterizations. Compositions of CeO2−Ag particles and Ag/CeO2 catalyst were determined by an Optima 7300 DV inductively coupled plasma atomic emission spectrometer (ICP-AES). The N contents in Ag particles and CeO2−Ag particles were measured by an Elementar Vario EL cube elemental analyzer. Powder X-ray diffraction patterns were recorded on a MXPAHF X’Pert PRO diffractometer using nickle-filtered Cu Kα (wavelength: 0.154 18 nm) radiation source with the operation voltage and operation current being 30 kV and 160 mA, respectively. UV− vis absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer at room temperature using ethanol as the reference solution. Laser Raman spectra were obtained in backscattering configuration on a LABRAM-HR confocal laser Raman spectrometer. The Ar+ (514.5 nm) was employed as the excitation source to obtain the visible−Raman spectra. The integration time was 10 s for the visible−Raman spectra. Scanning electron microscopy (SEM) images were taken using a field emission scanning electron microscope (FEI, Sirion200).

3. RESULTS AND DISCUSSION 3.1. CeO2-on-Ag Particles Synthesized by a Novel O2Assisted Hydrothermal Method. As-prepared Ag particles exhibit polyhedral shapes (Figure 1a,b) and a size distribution

Figure 1. (a) SEM image, (b) TEM image, and (c) the size distribution of as-synthesized Ag particles.

of 271 ± 50 nm, agreeing with a previous report.7 Figure 2 compares typical TEM images of 0.75 and 3.56 wt % CeO2−Ag particles prepared by traditional and O2-assisted hydrothermal synthesis methods under the same condition. For the traditional hydrothermal synthesis without O2 bubbling (Figure 2a,b), CeO2 was observed mostly to nucleate and grow in the aqueous solution and the prepared CeO2−Ag particles were physical mixtures of CeO2 and Ag particles. However, for our novel O2-assisted hydrothermal method (Figure 2c,d), CeO2 was observed to selectively nucleate and grow on Ag particles, forming nice CeO2-on-Ag particles with CeO2 adparticle size of 4.0 ± 0.8 nm. These observations demonstrate that the presence of enough O2 during the hydrothermal synthesis 19239

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were obviously observed to form. In the HRTEM images lattice fringes of 0.24 and 0.20 nm observed on the acquired particles respectively arise from the Ag {111} and {200} planes, and those of 0.31, 0.27, and 0.19 nm could be assigned to that of the CeO2 {111}, {200}, and {220} planes, respectively. It could be thus identified that the adparticles are CeO2. Meanwhile, the Fourier-transformed pattern of adparticles (the inset in Figure 3b3) also exhibits a typical electron diffraction pattern of CeO2. These results clearly demonstrate the growth of CeO2 adparticles on the starting Ag particles by our O2-assisted hydrothermal method. As shown in the large-scale TEM images (Figure 3a1−d1), CeO2 particles selectively nucleate and grow on Ag particles for the samples with CeO2 weight percentage up to 3.56%, forming CeO2-on-Ag particles. When the CeO2 weight percentage was increased to 5.68%, the nucleation and growth of CeO2 particles were observed to occur both on Ag particles and in the aqueous solution, forming the mixture of CeO2-on-Ag and CeO2 particles. The formed CeO2 adparticles at large CeO2 percentages constitute uneven CeO2 adlayers on Ag particles. This was further corroborated by the HAADFSTEM-EDS mapping images of a single 0.75 wt % CeO2-on-Ag particle (Figure 4). It can be clearly seen that the Ag particle is

Figure 2. TEM images of 0.75 wt % CeO2−Ag particles prepared by traditional (a) and O2-assisted (c) hydrothermal synthesis methods under the same conditions and 3.56 wt % CeO2−Ag particles prepared by traditional (b) and O2-assisted (d) hydrothermal synthesis methods under the same condition.

processes can switch the nucleation sites of CeO2 particles from the aqueous solution completely to the surfaces of Ag particles. CeO2−Ag particles with various CeO2 weight percentages synthesized by O2-assisted hydrothermal method were further examined by TEM and HRTEM (Figure 3a−d). The morphologies of Ag particles change a lot on which adparticles

Figure 3. TEM and HRTEM images of 0.21 wt % (a1−a3), 0.75 wt % (b1−b3), 3.56 wt % (c1−c3), and 5.68 wt % (d1−d3) CeO2−Ag particles prepared by the O2-assisted hydrothermal synthesis method and their Ag particle size distributions (e) and Ce adlayer thickness distributions (f). 19240

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Figure 6. O 1s XPS spectra of various CeO2−Ag samples.

we synthesized Ag(O2) particles by the O2-assisted hydrothermal treatment of as-synthesized Ag particles without the addition of Ce(NO3)3 and NaOH for the comparison. In the O 1s spectrum of Ag (O2) particles, the O 1s component from PVP weakens, and a strong new O 1s feature emerges at 532.6 eV. The O 1s feature with its binding energy at 532.6 eV can be assigned to adsorbed water stabilized by O adatoms on Ag surfaces on the basis of previous results on single crystals.9 This observation thus indicates that O2 should dissociate on Ag particles to form O adatoms during the O2-assisted hydrothermal process. For the CeO2−Ag particles, the O 1s feature of lattice O from CeO2 at 529.5 eV10 appears and increases with CeO2 amount, the O 1s feature at ∼531.2 eV is contributed not only by PVP but also by lattice O related to Ce3+ and surface hydroxyls of CeO2,10 and the O 1s feature at ∼532.6 eV contains the contribution from chemisorbed water on CeO2.10 The existence of ∼20% Ce3+ in CeO2 adlayers of all CeO2−Ag samples was demonstrated by their Ce 3d XPS spectra (Figure 7A).10 Ag in all samples exhibits Ag 3d5/2 binding energies at 368.2 ± 0.1 eV (Figure 7B), a typical value for metallic Ag.11 The N 1s XPS spectra of the samples (Figure 7C) show a feature of PVP at 399.6 eV8 whose intensity is weaker in Ag(O2) and CeO2−Ag particles than in assynthesized Ag particles. This demonstrates that some PVP capping ligands on Ag particles were removed during the O2assisted hydrothermal process. The actual N weight ratio was measured to decrease from 0.18% in Ag particles to 0.04− 0.05% in CeO2−Ag particles. This could partly explain the morphological changes of Ag particles after the O2-assisted hydrothermal process observed in TEM images. The reversible surface reaction of O adatoms and water to form hydroxyls (O(a) + H2O ↔ 2OH(a)) is well-established on Ag surfaces.9 Thus, the observation of adsorbed water stabilized by O adatoms on Ag(O2) particles surfaces suggests that the surfaces of Ag particles should be partially hydroxylated during the O2-assisted hydrothermal process. Thus, we proposed a likely mechanism for the selective formation of CeO2-on-Ag particles during the O2-assisted hydrothermal process (Scheme 1). The formation of CeO2 initiates from the reaction between Ce3+ and OH− to form nuclei. In the traditional hydrothermal process, both reactants are in the aqueous solution, resulting in the nucleation and formation of CeO2 dominantly in the aqueous solution. In the O2-assisted hydrothermal process, Ag particles catalyze the O2 + H2O reaction to form hydroxyls on their surfaces, and in addition to

Figure 4. HAADF-STEM (a) and Ag (b), Ce (c), and Ag + Ce (d) EDS mapping images of a single 0.75 wt % CeO2−Ag particle.

surrounded by a CeO2 adlayer. We counted the size of Ag particles and the thickness of CeO2 adlayers in the acquired CeO2-on-Ag particles (Figure 3e,f). The Ag particle sizes are 269 ± 44, 255 ± 46, 283 ± 54, and 295 ± 51 for 0.21, 0.75, 3.56, and 5.68 wt % CeO2−Ag particles, respectively, and their corresponding CeO2 adlayer thicknesses are 3.2 ± 2.1, 6.7 ± 4.6, 13.6 ± 5.3, and 20.4 ± 4.2 nm. Therefore, the sizes of Ag core in all CeO2-on-Ag particles are similar to those of the starting bare Ag particles, and the CeO2 adlayers become more homogeneous with the increasing CeO2 weight percentage. CeO2−Ag samples were also characterized by spectroscopic techniques. In the XRD patterns (Figure 5) Ag diffraction peaks

Figure 5. XRD patterns of (a) as-prepared Ag particles, (b) 0.21 wt %, (c) 0.75 wt %, (d) 3.56 wt %, and (e) 5.68 wt % CeO2−Ag particles prepared by the O2-assisted hydrothermal synthesis method.

dominate, and a very weak CeO2 (111) diffraction peak could be observed only in the samples with 3.56 and 5.68 wt % CeO2. The surface compositions of various samples were examined by XPS. In its O 1s XPS spectrum (Figure 6), as-prepared Ag particles exhibit a major O 1s peak at 531.2 eV with a shoulder at 533.2 eV that could be respectively assigned to PVP capping ligand and adsorbed water.8 In order to examine the influence of O2 employed in the O2-assisted hydrothermal process on the surface composition/structure of as-synthesized Ag particles, 19241

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Figure 7. (A) Ce 3d, (B) Ag 3d, and (C) N 1s XPS spectra of XRD patterns of (a) as-prepared Ag particles, (b) Ag(O2) particles, (c) 0.21 wt %, (d) 0.75 wt %, (e) 3.56 wt %, and (f) 5.68 wt % CeO2−Ag particles prepared by the O2-assisted hydrothermal synthesis method.

Scheme 1. Likely Formation Mechanism of CeO2-on-Ag Particles during the O2-Assisted Hydrothermal Process

OH− in the aqueous solution, the surface hydroxyls on Ag particles can also react with Ce3+ to form Ce(OH)3 nuclei on Ag particles. Moreover, it is well-known that the nucleation process is kinetically much more facile to occur on a solid surface than in an aqueous solution; therefore, Ce3+ will preferentially react with surface hydroxyls on Ag particles to nucleate on their surfaces, resulting in the selective formation of CeO2-on-Ag particles. Such a preferential nucleation process on Ag particles depends on the oversaturated concentration of Ce(OH)3 in the aqueous solution. The nucleation will also occur in the aqueous solution at large Ce3+ concentrations in the O2-assisted hydrothermal process, as observed in the case of 5.68 wt % CeO2−Ag particles. Our O2-assisted hydrothermal method was also successfully used to selectively synthesize TiO2-on-Ag and Fe2O3-on-Ag particles (Figure 8). During the traditional hydrothermal synthesis without O2 bubbling (a and b), TiO2 and Fe2O3 mostly nucleated and grew in the aqueous solution, but during our novel O2-assisted hydrothermal

synthesis (c and d), TiO2 and Fe2O3 selectively nucleated and grew on Ag particles. Therefore, the O2-assisted hydrothermal method offers a facile and green strategy to synthesize oxide-on-Ag particles. 3.2. CeO2 Thickness-Dependent SPR and SERS Properties of CeO2-on-Ag Particles. Ag particles are well-known to exhibit strong surface plasmon resonance (SPR) and SERS properties, and we found that the CeO2 adparticles on Ag particles influence their SPR property and significantly enhance their SERS effect (Figure 9). As-prepared Ag particles exhibit a SPR peak maximum at 465 nm (Figure 9A). The SPR peak does not shift for Ag(O2) particles but is broadened likely due to the morphological changes of Ag particles. The presence of CeO2 adlayer on Ag particles results in the red-shift of their SPR peak maximum to ∼504 nm for 0.21 wt %, ∼510 nm for 0.75 wt % CeO2−Ag particles, and ∼536 nm for 3.56 and 5.68 wt % CeO2−Ag particles. Since the sizes of Ag core in all CeO2on-Ag particles are similar to those of as-synthesized Ag particles and no shift in the SPR peak occurs for Ag(O2) particles subjected to the O2-assisted hydrothermal treatment, the observed SPR peak shift in CeO2-on-Ag particles should be attributed dominantly to the high-refractive-index CeO2 adlayer. The SPR peak of Ag core of CeO2-on-Ag particles with a thicker CeO2 adlayer red-shifts more largely. Similar results were previously reported for the influences of TiO2 and ZrO2 thin films on the SPR properties of Au and Ag cores.12 The SERS property of various samples was demonstrated by the Raman spectra of PVP capping ligands on Ag particles (Figure 9B). Probed with laser at 514.5 nm PVP powders barely show any vibrational features in the Raman spectroscopy (Figure S1). As-prepared Ag particles exhibit distinct Ag−N stretch peak at 240 cm−1 and vibrational features arising from PVP capping ligands (Table 1). For Ag(O2) particles, the Ag− N stretch peak attenuates, but the CO stretch peak of PVP capping ligands at 1596 cm−1 slightly grows. For CeO2−Ag particles, the F2g and defects-related vibrational modes of CeO2 appear respectively at 458 and 557 cm−1, and the intensity of

Figure 8. Representative TEM images of TiO2−Ag particles (a, c) and Fe2O3−Ag particles (b, d) prepared by traditional and O2-assisted hydrothermal synthesis methods under the same conditions. 19242

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Figure 9. (A) UV−vis diffuse reflectance spectra, (B) Raman spectra, and (C) normalized CO stretch intensity of PVP capping ligands at 1596 cm−1 of (a) as-prepared Ag, (b) Ag(O2), (c) 0.21 wt %, (d) 0.75 wt %, (e) 3.56 wt %, and (f) 5.68 wt % CeO2−Ag particles.

strongest SERS signal. The intensity of ν(CO) peak of PVP in 0.75 wt % CeO2−Ag particles is as large as 34.2 times of that in as-prepared Ag particles (Figure 9C); moreover, demonstrated by the actual N weight ratio, the PVP amount in asprepared Ag particles is as large as 3.6−4.5 times that in CeO2− Ag particles. Therefore, the SERS effect of 0.75 wt % CeO2−Ag particles is 2 orders stronger than that of as-prepared Ag particles. Thus, 0.75 wt % CeO2−Ag particles exclusively with the CeO2-on-Ag structure can serve as excellent substrates for SERS application with high sensitivity. Physical and chemical effects were proposed to explain the SERS property of coinage metal particles.13,14 First, the chemical effect13 is not likely involved in the enhanced SERS effect of CeO2−Ag particles because few differences were observed among the Raman spectra of PVP capping ligands in as-prepared Ag particles and CeO2−Ag particles. Second, the SERS hotspot effect14 induced by the morphological changes of Ag particles subjected to the O2-assisted hydrothermal treatment should be involved. Although the size distributions of Ag particles in all our samples do not vary much, Ag particles subjected to the O2-assisted hydrothermal treatment exhibit obvious morphological changes that could create additional SERS hotspots. This can be indicated by the enhanced SERS property of Ag(O2) particles over as-synthesized Ag particles.

Table 1. Assignment of Vibrational Peaks Observed in the Raman Spectra of Ag and CeO2−Ag Particles Raman shift (cm−1)

assignment

references

1596 1369 1294 1163 1002 933 842 809 774 658 557 458 240

CO stretch aromatic C−C stretch C−C bridge-bands stretch CH2 twist CH, CH2 rock C−C, ring breathing C−C, ring

17 18 18 19 20 21 19

C−C chain N−CO bend CeO2 defect CeO2 F2g Ag−N stretch

22 19 15 15 22

F2g mode increases with the CeO2 amount; meanwhile, all vibrational features of PVP capping ligands are greatly strengthened, demonstrating the enhancement effect of CeO2 adlayers on the SERS property of Ag particles. Its enhancement effect exhibits a volcano-shaped dependence on the CeO2 amount with 0.75 wt % CeO2−Ag particles showing the

Figure 10. (A) H2-TPR profiles and (B) CO oxidation light-off curves of (a) 0.21 wt %, (b) 0.75 wt %, (c) 3.56 wt %, and (d) 5.68 wt % CeO2−Ag particles, (e) CeO2, (f) 0.54 wt % Ag/CeO2, and (g) as-prepared Ag particles. The first and second in (B) respectively means the curves measured during the 1st and 2nd cycles of activity evaluation. 19243

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particles is obviously thickness-dependent and that the reducibility of >20 nm thick CeO2 adlayers on Ag particles approaches that of CeO2 particles. The reduction of surface and “near surface” regions is more difficult in thin CeO2 adlayers than in thick ones, indicating that the migrations of surface and subsurface lattice oxygen become difficult as the thickness of CeO2 adlayers decreases. Similar results were previously observed for oxide thin films grown on flat metal substrates and attributed to the strong modulation effect of the underlying metal on the property of oxide thin film.5,6 Such an effect dissipates quickly with the increasing thickness of oxide adlayer, and thus a thick oxide film exhibits properties similar to the bulk oxide. Figure 10B compares the catalytic performances of all samples in CO oxidation. As-prepared Ag particles and CeO2 are very poor in catalyzing CO oxidation whereas 0.54 wt % Ag/CeO2 with Ag-CeO2 interfacial sites is active, agreeing with previous reports.15 The catalytic performances of CeO2−Ag particles were evaluated for two consecutive cycles. The catalytic activity measured in the second cycle is better than in those measured in the first cycle, particularly at low reaction temperatures. This could be attributed to the gradual burn off the PVP capping ligands on Ag surfaces in CeO2−Ag particles during the first cycle of activity evaluation up to 240 °C. XPS measurements (Figure S3) detected no N 1s signal on CeO2− Ag particles after the first cycle of activity evaluation (denoted as PVP-free CeO2−Ag particles). The catalytic activities of PVP-free CeO2−Ag particles are higher than those of asprepared Ag particles and CeO2 and increase with the CeO2 amount. 5.68 wt % PVP-free CeO2−Ag particles exhibit almost the same catalytic behavior as 0.54 wt % Ag/CeO2. The Arrhenius plots were made using the CO conversion data below 40% for PVP-free CeO2−Ag particles and 0.54 wt % Ag/ CeO2 (Figure 11), from which the apparent activation energy

Third, the CeO2 adlayers must contribute significantly to the enhanced SERS effect of Ag cores of CeO2-on-Ag particles because the SERS signals of Ag(O2) particles are much weaker than those of CeO2−Ag particles. In addition to the hotspots induced by the morphological changes of Ag cores, uneven CeO2 adlayers on Ag particles could also create SERS hotspots at the CeO2−Ag interfaces; meanwhile, depending on the CeO2 adlayer thickness, the SPR peak maximum of Ag cores in CeO2on-Ag particles shifts toward or away from the wavelength of the laser (514.5 nm) employed in the Raman spectrometer. This could strongly affect the SPR excitation of Ag cores during the Raman spectrum measurements and subsequently the local electromagnetic field that plays a key role in the SERS effect.13 Ag particles in 0.75 wt %CeO2−Ag exhibit their SPR peak maximum at ∼510 nm, in a nice coincidence to the wavelength of the laser of the Raman spectrometer. During the Raman spectrum measurement, Ag particles in 0.75 wt %CeO2−Ag particles could exhibit very intense local electromagnetic fields due to the resonant laser-excited SPR peak and thus very strong SERS signals. The SPR peak maxima of other CeO2−Ag particles are away from 514.5 nm more or less, resulting in their reduced local electromagnetic fields and SERS signals. 3.56 wt % and 5.68 wt % CeO2−Ag particles exhibit similar SPR peaks, but the SERS effect of 3.56 wt % CeO2−Ag particles is much stronger than that of 5.68 wt %CeO2−Ag particles. The CeO2 adlayers of 5.68 wt %CeO2−Ag particles are thicker and more uniform than those of 3.56 wt % CeO2−Ag particles, likely exhibiting a weakened hotspots enhancement effect on the SERS signals of PVP capping ligands on the Ag cores. 3.3. CeO2 Thickness-Dependent Reducibility and Catalytic Activity of CeO2-on-Ag Particles. The thickness of oxide thin films have been observed to affect their surface chemistry and catalytic property for oxide thin films grown on flat metal substrates,5,6 but such effects have been seldom explored for powder samples. We thus investigated the reducibility of CeO2 adlayers in CeO2−Ag particles and the catalytic activity of CeO2−Ag particles in CO oxidation (Figure 10). Corresponding results of CeO2 and traditional 0.54 wt % Ag/CeO2 supported catalyst were also included as the comparison. In the H2-TPR spectra (Figure 10A), CeO2 powders exhibit a surface reduction peak starting at ∼350 °C and reaching the maximum at ∼560 °C. The surface reduction of CeO2 support in 0.54 wt % Ag/CeO2 is significantly facilitated by supported Ag nanoparticles to start at ∼80 °C and evolve into a sharp and symmetric peak with the maximum at ∼194 °C. The surface reduction of CeO2 adlayers in all CeO2− Ag samples starts at the similar temperature (∼110 °C). Thus, the Ag particles in CeO2−Ag samples promote the reduction of supported CeO2 adlayers, but not so significantly as Ag nanoparticles in 0.54 wt % Ag/CeO2. This could be attributed to the stronger ability of fine Ag nanoparticles in 0.54 wt % Ag/ CeO2 (Figure S2) to activate H2 molecules than very large Ag particles in CeO2−Ag samples. Different from the sharp surface reduction peak of CeO2 in 0.54 wt % Ag/CeO2, the surface reduction peak of CeO2 adlayers in CeO2−Ag particles except 5.68 wt % CeO2−Ag comes across a maximum and then evolve into a broad peak covering the whole temperature range. It is noteworthy that PVP capping ligands on Ag particles decompose between 400 and 500 °C during H2-TPR experiments, producing H2. Thick CeO2 adlayers in 5.68 wt % CeO2−Ag particles exhibit a similar sharp surface reduction peak to CeO2 in 0.54 wt % Ag/CeO2. These observations demonstrate that the reduction kinetics of CeO2 adlayers on Ag

Figure 11. Arrhenius plots of CO oxidation catalyzed by PVP-free CeO2−Ag particles and 0.54 wt % Ag/CeO2.

(E(a)) was calculated. 0.21, 0.75, and 3.56 wt % PVP-free CeO2−Ag particles exhibit an E(a) of ∼67 ± 8, 64 ± 7, and 66 ± 1 kJ/mol while 5.68 wt % PVP-free CeO2−Ag particles exhibit a much smaller E(a) of 46 ± 2 kJ/mol, which is close to the E(a) of 0.54 wt % Ag/CeO2 (41 ± 1 kJ/mol). On one hand, these results demonstrate that the Ag-CeO2 interfacial sites on CeO2−Ag particles with thin CeO2 adlayers are intrinsically less active than those on CeO2−Ag particles with thick CeO2 adlayers. The thickness-dependent activity of Ag19244

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CeO2 interfacial sites on CeO2−Ag particles can be well correlated with above thickness-dependent reducibility of CeO2 adlayers. It has been well-established that the reducibility of CeO2 is positively related to the catalytic activity of CeO2supported catalysts in CO oxidation.16 On the other hand, these results demonstrate that the Ag−CeO2 interfacial sites formed by thick CeO2 adlayers on Ag particles can be as same catalytically active as those traditionally formed by Ag nanoparticles on CeO2 supports. Thus, oxide-on-metal particles are likely promising catalysts as active as traditional oxidesupported metal catalysts.

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4. CONCLUSIONS In summary, we have successfully developed a facile O2-assisted hydrothermal method to synthesize oxide-on-Ag particles with tunable oxide adlayer thicknesses. In this novel approach Ag particles catalyze the O2 + H2O reaction to form surface hydroxyls that induce the preferential nucleation of Ce(OH)3 on the surfaces of Ag particles, forming CeO2 adparticles on Ag particles. CeO2-on-Ag particles exhibit CeO2 thickness-dependent SERS and catalytic properties. The best SERS effect of CeO2-on-Ag particles is 2 orders stronger than that of Ag particles, and the best catalytic performance in CO oxidation of CeO2-on-Ag particles is as active as traditional CeO2-supported Ag catalyst. These results open up new opportunities to synthesize oxide-on-metal particles and explore their functions by tuning the oxide adlayer thickness.

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectrum of PVP, TEM image of 0.54 wt % Ag/CeO2 catalyst, and N 1s XPS spectra of 3.56 wt % CeO2−Ag particles before and after the first cycle of catalytic activity evaluation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (W.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Basic Research Program of China (2013CB933104) and National Natural Science Foundation of China (U1332113, 21173204). The assistance of Prof. Ming Gong with the TEM measurements was gratefully acknowledged.

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REFERENCES

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dx.doi.org/10.1021/jp506187d | J. Phys. Chem. C 2014, 118, 19238−19245