Accepted Manuscript Title: Facile Synthesis of Ag@CeO2 Core-Shell Plasmonic Photocatalysts with Enhanced Visible-light Photocatalytic Performance Author: Linen Wu Siman Fang Lei Ge Changcun Han Ping Qiu Yongji Xin PII: DOI: Reference:

S0304-3894(15)00519-1 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.06.062 HAZMAT 16917

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

28-4-2015 10-6-2015 25-6-2015

Please cite this article as: Linen Wu, Siman Fang, Lei Ge, Changcun Han, Ping Qiu, Yongji Xin, Facile Synthesis of Ag@CeO2 Core-Shell Plasmonic Photocatalysts with Enhanced Visible-light Photocatalytic Performance, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.06.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile Synthesis of Ag@CeO2 Core-Shell Plasmonic Photocatalysts with Enhanced Visible-light Photocatalytic Performance Linen Wu a, b, Siman Fang a,b, Lei Ge* a, b , Changcun Han b, Ping Qiu b, Yongji Xin b a

State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum Beijing, No. 18 Fuxue Rd., Beijing 102249, People’s Republic of China.

b Department

of Materials Science and Engineering, College of Science, China University of Petroleum Beijing, No. 18 Fuxue Rd., Beijing 102249, People’s Republic of China.

Abstract Novel Ag@CeO2 core-shell nanostructures with well-controlled shape and shell thickness were successfully synthesized via a green and facile template-free approach in aqueous solution. As-prepared samples were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflection spectroscopy (DRS), electron spin resonance (ESR) and photoluminescence spectroscopy (PL). The structures with different core shapes and controllable shell thickness exhibited unique optical properties. It is found that the nanoscale Ag@CeO2 core-shell photocatalysts exhibit significantly enhanced photocatalytic activities in the O2 evolution and MB dye degradation compared to pure CeO2 nanoparticals. The enhancement in photocatalytic activities can be ascribed to the localized surface plasmon resonance (SPR) of Ag cores.

Moreover,

larger

active

interfacial

areas

and

contact

between

metal/semiconductor in the core-shell structure facilitate transfer of charge carriers and prolong lifetime of photogenerated electron-hole pairs. It is expected that the Ag@CeO2 core-shell structure may have great potential in a wider range of lightharvesting applications. Keywords: Ag@CeO2; core-shell structure; water splitting; photocatalysis.

1

*

Corresponding author at: State Key Laboratory of Heavy Oil Processing, China University of

Petroleum Beijing, No. 18 Fuxue Road, Beijing102249, PR China. Tel/Fax.: +86 1089739096. Email address: [email protected] (L.Ge)

1. Introduction The solar-energy-driven water splitting and pollutants degradation through semiconductor photocatalysis have attracted increasing interest as it is a promising technique for energy production and environment purification [1-6]. However, charge recombination within the semiconductor nanoparticles always limits the efficiency of light energy conversion. Herein, the design of new functional materials is a driving force for the development of advanced semiconductor materials [7-8]. Among various novel structures, the core@shell nanostructures have received increasing attention because of their favorable physical and chemical properties that endow them with potentials for diverse applications, such as drug delivery, surface functionalization, photonic devices, size-selective reactions and space-confined catalysis [9-16]. Among various nanostructures, noble metal core incorporated in semiconductor shell is one of the most attractive because it combines the optoelectronic properties of semiconductor with the plasmonic effect and superior conductivity of noble metal NPs. As a unique core-shell structure, it possesses remarkable advantages as a heterogeneous photocatalyst. Firstly, due to the surface plasmon resonance (SPR) of plasmonic metal, they can enhance the light absorption of semiconductor through scattering, absorption enhancement, and hot-electron injection; Secondly, the noble metal nanoparticles encapsulated by the semiconductor shell can not only greatly enhance their stability against coalescence, but also avoid corrosion or dissolution in practical applications [24, 29-32]; Thirdly, the well-controlled 3D structures could

2

provide “three-dimensional” contact between the noble metal core and the semiconductor shell, which offers larger active interfacial areas that facilitate the transfer of charge carriers and prolong the lifetime of photogenerated electron-hole pairs [29, 36, 38]. On account of these advantages of the noble metal core@semiconductor shell architectures, great advances have been made in recent years to fabricate and utilize these nanocomposites [17-24]. For example, Xu et al. reported several core-shell structures, such as Pt@CeO2 [25], Pd@CdS [26] and M@TiO2 (M=Au, Pd, Pt) [27], and applied them in photocatalytic processes. Wang et.al investigated the charge-transfer steps in metal@ TiO2 core-shells with plasmonic hot spots, which allows for tunable light absorption over the visible-light region by changing the morphology and the shell thickness [28]. CeO2, as one of the most important semiconductor materials, possesses exceptional catalytic activity towards oxidation due to its abundant oxygen vacancy, high oxygen storage capacity and low energy barriers between III and IV oxidation states [39-40]. Most reports on metal core@ceria shell structures are focused on the catalysis in organics oxidation and CO oxidation [33-35, 40]. However, systematic investigations on photocatalytic activities in water splitting of metal core@ceria shell nanostructures are still limited. Herein, we report a novel and simple template-free approach to fabricate the Ag@CeO2 core-shell nanostructures in aqueous phase. Since the SPR is strongly dependent on the morphology of plasmonic metals, we designed Ag@CeO2 core-shell structures with novel core shape and controllable shell thickness. Their photocatalytic activities are evaluated by O2 evolution via water splitting and the MB dye degradation under visible-light irradiation. To the best of our knowledge, this is the first report on the oxygen evolution via water splitting through visible light photocatalytic activity of 3

Ag@CeO2 core-shell structures with well-defined morphology. The obtained coreshell composites showed much higher visible light photocatalytic activity than pure CeO2. The impressive improvement of photocatalytic oxygen evolution and MB degradation are investigated and discussed in detail. The proposed possible photocatalytic mechanisms are likely to pave the way to develop improved plasmonic photocatalysts for light energy conversion.

2. Experimental  Materials hexamethylenetetramine(HMTA, Alfa Aesar), cerium nitrate hexahydrate (Ce (NO3)3·6 H2O, 99.9% purity), silver nitrate (AgNO3, Sigma-Aldrich, 99%), trisodium citrate dehydrate (chemical, 99% purity) 1,2-Propylene glycol(1,2 PG), sodium chloride

(NaCl,

99.5%,A.R.)

,

4-mercaptobenzoic

acid

(ligand,

Aladdin),

polyvinylpyrrolidone (PVP, Sigma-Aldrich, K 29-32, average Mw=58000) and ethanol were used as received. All of the chemicals were analytical grade and used directly without further purification.  Synthesis of the photocatalysts (a) Synthesis of Ag seeds The Ag NPs with an average size of 30 nm were prepared by a modified method [41]. Typically, 150 ml of 1 mM AgNO3 was heated to the boiling point at 120 oC and then 10 ml of 1 wt% sodium citrate aqueous solution was quickly added. The mixed solution was kept boiling for 30 min, and cooled at room temperature. The resulting gray yellow Ag NPs solution was then obtained, ready for further use. (b) Synthesis of Ag NPs@CeO2 core-shell nanocomposites Ag@CeO2 nanoparticles were prepared through the reported method with slight 4

modification [42]. Typically, 50 μl of ligand solution (3 mM in ethanol) was added to 10 ml of as-prepared Ag seed solution under stirring, incubated at 60 oC for 2 h. Then the mixture was centrifuged at 8000 rpm for 10 min to remove the supernatant. Subsequently, the isolated NPs were dispersed in an aqueous PVP solution (Mw= 58000 g.mol-1, 110 mM). Hexamethylenetetramine (HMTA, 1 mM) was then added, followed by Ce(NO3)3 (50 mM) to induce CeO2 formation. After that, the reaction mixture was incubated at 95 oC for 3 h and the resulting Ag NPs@CeO2 core-shell sample was subsequently cleaned by UV irradiation (Perfectlight, PLS-SXE 300 UV Xe lamp) for 2 hours to remove the ligands prior to the photocatalytic experiment. The product was separated by centrifugation, washed with deionized water and anhydrous ethanol followed by a dry process at 80 oC. (c) Synthesis of Ag nanowires (Ag NWs) The Ag nanowires (Ag NWs) were fabricated based on a modified polyol process [43]. In a typical synthesis process, 10 ml of 1,2-Propylene glycol (1,2-PG) containing PVP ( Mw=58000, 150 mM as calculated in terms of the repeating unit) was kept in a 50-mL three necked flask, sealed and heated with magnetic stirring in an oil bath at 160 oC for 1 h. Then 1 mL NaCl solution (1 mM in 1,2-PG) was injected quickly. After 5 min, 4 ml of 0.15 M AgNO3 in 1,2-PG were added drop by drop to the stirring solution. The mixture was stirred at 160 oC for 40 min, resulting in the gray Ag NWs. The products were cooled at room temperature, washed with ethanol and acetone for three cycles by centrifugation ready for subsequent experiments. (d) Synthesis of Ag NWs@CeO2 core-shell nanostructures: The synthesis of Ag NWs@CeO2 was similar to the procedure of Ag NPs@CeO2 core-shell nanoparticles, only different volumes of precursor solutions were used. The other conditions were the same as the above procedure. The CeO2 shell thickness can 5

be tuned by varying the addition amount of Ce(NO3)3, the resulting samples were labeled as Ag NWs@CeO2-x, where x= 1, 2, 3. The synthesis of pure CeO2 was the same as the above process, except the addition of Ag seed solution. 2.3 Characterization The crystal phase of as-prepared samples was analyzed by X-ray diffraction (XRD; Bruker D8 Advance, X-ray diffractometer) with CuKa radiation at a scan rate of 5° min-1, in the 2 θ range of 20-70°. The acceleration voltage and the applied current were 40 KV and 40 mA, respectively. The morphology of the samples was examined by high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20, accelerating voltage 200 kV). The X-ray photoelectron spectroscopy (XPS) was measured in a PHI 5300 ESCA system. The beam voltage was 3.0 eV, and the energy of Ar ion beam was 1.0 keV. The binding energies were normalized to the signal for adventitious carbon at 284.8 eV. UV-Vis diffuse reflection spectroscopy (DRS) was performed on a Shimadzu UV-4100 spectrophotometer using BaSO4 as the reference material. The electron spin resonance (ESR) signals of spin-trapped oxidative radicals were obtained on a Bruker model ESR JES-FA200 spectrometer equipped with a quanta-Ray Nd: YAG laser system as the light source with a UV-cutoff filter (λ> 400nm). The photoluminescence (PL) spectra of the photocatalyst were obtained by a Hitachi spectrometer with excitation wavelength of 325 nm. 2.4. Photocatalytic activity The photocatalytic performance of Ag NPs@CeO2 and Ag NWs@CeO2 coreshell samples was evaluated based on direct water oxidation and the degradation reaction of MB aqueous solution. The photocatalytic O2 evolution experiments were performed in a 300 ml quartz reactor at 4 oC. The reactor is connected to a closed circulating system. As light 6

source, a PLS-SXE 300 UV Xe lamp with a UV-cutoff filter (

>400nm) was used. In

a typical photocatalytic experiment, 50 mg of photocatalyst powder was dispersed in 100ml of 50 mM silver nitrate (AgNO3) solution. Before experiments, the suspensions were stirred and evacuated for at least 30 min to remove the dissolved air and to ensure the anaerobic conditions. Then it was irradiated for 4 h using the 300 W Xe amp with a UV-cutoff filter (

>400 nm), and the O2 evolution were detected by gas

chromatography (Beifen 3420 A, high purity Argon as a carrier gas, 99.999%) equipped with a thermal conductivity detector at each time intervals of 30 min. The degradation reaction was taken under irradiation of a 300 W Xe lamp with a UV-cutoff filter (

>400 nm). Typically, 50 mg of catalyst powder was added into 100

mL of the above MB solution (10 mg/L). Prior to irradiation, the photocatalysts were suspended in the solutions with constant stirring in dark conditions for 30 min to ensure the establishment of an adsorption/desorption equilibrium. At given time intervals (20 min) during the irradiation, about 3 mL of aliquots were sampled and centrifuged to separate the photocatalyst powder from the solution, and used for the absorbance

measurement.

The

filtrates

were

analyzed

by

a

UV-1700

spectrophotometer and the characteristic absorption peak of MB at 664 nm was used to determine the extent of its degradation. The photocatalytic activity of Ag NPs@CeO2 and Ag NWs@CeO2 core-shell samples were compared with the pure CeO2 powder under the same experimental conditions.

3. Results and discussion 3.1. Characterization of Ag@CeO2 core-shell samples Scheme 1 presents the synthesis of Ag NPs@CeO2 and Ag NWs@CeO2 coreshell nanostructures in aqueous media. The Ag NPs and NWs seeds were synthesized 7

via a facile route based on reduction of AgNO3. The obtained Ag colloids were used as the seeds for the surface nucleation of CeO2 via the hydrolysis to form core-shell structures. More importantly, the Ag NP and NW seeds are capped by 4mercaptobenzoic acid ligand / PVP to tune the interfacial energy between Ag / CeO2, and to induce the formation of homogeneous shell. After hydrothermal treatment, the Ag NPs and NWs are uniformly wrapped by CeO2. By varying the amount of Ce(NO3) 3

precursor, the core-shell structure can be prepared with controllable shell thickness. Fig.1. shows the X-ray diffraction patterns of as-prepared pure CeO2, Ag

NPs@CeO2 and Ag NWs@CeO2 core-shell samples with different shell thicknesses. As shown in Fig.1 (A), the diffraction peaks at 2 θ values of 28.54, 33.08, 47.48, 56.33, 59.08, 69.4, 76.69 and 79.06°were assigned to the (111), (200), (220), (311), (222), (400), (331), (420) facets of CeO2, respectively, which are in good agreement with the CeO2 patterns reported in the literature [49]. For the core-shell samples, the XRD patterns are compared with standard patterns of CeO2 (JCPDS 34-0394) and Ag (JCPDS 04-0783), in which parts of diffraction peaks were indexed to the well crystalline face of CeO2, whereas the other marked peaks are assigned to face centered cubic (fcc) silver (111), (200), (220), (311) planes, respectively. Fig.1 (D-F) illustrates the XRD patterns of Ag NWs@CeO2 core-shell samples with different CeO2 shell thicknesses. The absence of other impurity peaks indicates that the Ag NWs @CeO2 core-shell nanostructures with the high purity are successfully prepared by the facile solution process. In addition, the relatively weak diffraction peaks of CeO2 phase indicate the low crystallinity of CeO2 shell compared to Ag cores in the core-shell nanostructures. TEM and HRTEM were employed to investigate the microstructure of Ag@CeO2 core-shell samples. Fig. 2 (A-B) clearly shows the Ag NPs@CeO2 core-shell 8

nanostructure, in which the Ag core is compactly encapsulated within a CeO2 shell. The average diameter of well-dispersed Ag core was about 30 nm. After being coated with the CeO2 shell, the particle increases to 100~200 nm. As indicated in Fig. 2 (CD), prior to the growth of CeO2, the Ag NWs possess very smooth surfaces with high interfacial energy. The ligand/surfactant molecules are introduced in the synthesis to tune the interfacial energy and induce oxide formation on Ag NW cores. The presence of ligand/surfactant is crucial, otherwise the cerium precursor can not form cerium hydroxide on Ag surface. After addition of Ce4+ precursor, the growth of CeO2 on Ag NWs occurred immediately with the assistance of ligand/surfactant molecules, accompanying by the rapid colour change of the reaction solution from gray yellow to black. Fianlly, the surfaces of Ag NWs are totally wrapped by the CeO2 nanoparticles. In addition, by increasing the volume ratio of the ligand and Ce4+ precursor, Ag NWs@CeO2 samples with different shell thicknesses were obtained, as shown in Fig.2 (H-J) and Table 1. The CeO2 shell thickness of Ag NWs@CeO2-1,2,3 was 26.15 nm, 66.12 nm and 28.92 nm, respectively. The interfaces between the Ag NWs and CeO2 NPs are also clearly observed, indicating the formation of homogeneous core-shell structures. The HRTEM image in Fig.2 (G) shows the lattice fringes of 0.312 and 0.24 nm, which correspond to CeO2 (111) and Ag (111), respectively. To further confirm the core-shell structure, EDX mapping technique was conducted to characterize the interfaces between Ag and CeO2. Fig.3 (A, B) illustrates the elementary composition of core and shell respectively by chemical compositions using EDX. As expected, element Ag is predominant in the inner-core. Oppositely, the outer-shell mainly contains cerium element. Moreover, the elemental mapping was performed for an individual Ag@CeO2 nanoparticle. As indicated in Fig.3 (C) and (D), the orange, green, and red colors represent the distribution of the element Ag, Ce and 9

O, respectively. The Ce L-line indicated by a bimodal Ce distribution is observed at the edge of the particle or wire, while the Ag L-line shows a maximum Ag concentration at the center of the particle and wire. These results demonstrate the formation of Ag@CeO2 core-shell structure, in which Ag acts as the core and the CeO2 is the shell. The X-ray photoelectron spectrum (XPS) analysis was carried out to gain insight into the chemical composition and binding states on the surface of the Ag NWs@CeO2 core-shell nanocomposite. The binding energies obtained in the XPS analysis were corrected for specimen changing by referencing C1s to 284.8 eV. The survey spectrum in Fig.4 (D) shows that the main elements on the surface of the nanostructure are Ce, O, and Ag. Fig.4 (A) presents the high-resolution XPS spectra of O1s, the peak located at around 529 eV is ascribed to the lattice oxygen in CeO2 [44], whereas the broad peak appeared around 531.3 eV can be assigned to the chemisorbed oxygen and hydroxyl groups caused by the absorption of H2O molecules to the surface [45]. Regarding the typical signals of Ce 3d XPS spectra in Fig.4 (C), the Ce 3d5/2 and Ce 3d3/2 peaks located at 881.8eV and 900.4 eV is in accordance with literature values, which can be attributed to peaks of Ce 3d orbital. The result approves that the cerium element is in the IV oxidation state [44, 47, 48]. Fig. 4 (B) presents the XPS spectrum of Ag. The binding energy of 367.3eV and 373.4eV is indicative of metallic Ag, corresponding to the 3d5/2 and 3d3/2 of Ag0 species, respectively [46]. Therefore, the joint results of XRD, HRTEM, EDX and XPS confirm the formation of desirable Ag NPs@CeO2 and Ag NWs@CeO2 core-shell nanostructures via a facile and templatefree approach. UV-Vis diffuse reflectance spectroscopy (DRS) measurements were performed to investigate the light-harvesting ability of the synthesized pure CeO2 and Ag@CeO2 core-shell samples. As shown in Fig.5, the pure CeO2 sample show absorption from 10

the UV through the visible range with the steep shape spectrum, indicating that the visible light absorption is ascribed to the band gap transition [49]. While the Ag@CeO2 core-shell nanocomposites exhibit significantly enhanced light absorption intensity in both the ultraviolet and visible light regions, and the Ag NWs@CeO2 sample has the highest light harvest efficiency, which can be attributed to the presence of plasmonic Ag core and the incorporation into the CeO2 shell. The DRS results also illustrate that the Ag@CeO2 core-shell nanostructures could improve the visible light absorption efficiency, and hence is expected to have superior photocatalytic performance. 3.2 Photocatalytic performance The photocatalytic activities of the Ag NPs@CeO2 and Ag NWs@CeO2 coreshell nanostructures were evaluated by O2 evolution via water oxidation in aqueous solutions with sacrificial reagents, as well as the degradation reaction of MB aqueous solution (10 mg/L) under visible light irradiation (

>400 nm). The resultant

Ag@CeO2 core-shell nanocomposites are expected to show higher photocatalytic activity than pure CeO2. Fig.6 illustrates the O2 evolution curves of Ag NPs@CeO2 and Ag NWs@CeO2 core-shell samples with different shell thicknesses, together with that of pure CeO2 for comparison using silver nitrate (50 mM) as sacrificial agent. The results demonstrate that the photocatalytic O2 evolution of the Ag NPs@CeO2 and Ag NWs@CeO2 coreshell nanostructures are significantly improved. The average O2 evolution rates from water oxidation are 35.8 μmol·g-1·h-1 for pure CeO2, 44.3 μmol·g-1·h-1 for Ag NPs@CeO2, 42.5 μmol·g-1·h-1 for Ag NWs@CeO2-1, 61.8 μmol·g-1·h-1 for Ag NWs@CeO2-2, and 53.5 μmol·g-1·h-1 for NWs@CeO2-3. Therefore, the Ag NWs@CeO2-2 sample presents the highest O2 evolving rate, which is about 1.7 times

11

higher than pure CeO2. The catalytic activity of Ag@CeO2 core-shell samples was further investigated by degradation of MB in aqueous solution. Fig.7 shows the photodegradation of MB by the Ag NPs@CeO2 and Ag NWs@CeO2 core-shell structures, together with that of pure CeO2 for comparison. In the blank experiment, a slight MB concentration decreased under visible-light irradiation, indicating that MB undergoes photosensitized degradation. It is well-known that MB dye can absorb visible light at 664 nm, which is attributed to the excited state of the dye. In the MB degradation system, parts of the photoelectrons of the excited state were immediately injected into the CB of CeO2, which accelerated the MB degradation. Thus, both the photocatalytic process and the photosensitized process would work concurrently in this case [50]. This process might be similar to the surface sensitization of TiO2 via adsorbed dyes [51]. As shown in Fig.7, it can be clearly seen that the core-shell nanocomposites show higher photocatalytic activity than the pure CeO2 nanoparticles, while the Ag NWs@CeO2-2 core-shell samples exhibit the highest catalytic efficiency. The first-order kinetic photodegrading constant of Ag NWs@CeO2-2 sample is 0.640 h-1, whereas the kinetic constants of Ag NPs@CeO2 and pure CeO2 samples are 0.460 h-1 and 0.366 h-1, respectively. The improved photocatalytic performance of Ag@CeO2 core-shell samples can be attributed to the ability of trapping electrons and SPR effect of Ag cores. The Ag nanowires, compared with Ag nanoparticles, exhibit higher local electric field enhancements. Their longitudinal plasmon light absorption can be enhanced in the visible light region, leading to the superior photocatalytic activity of Ag NWs@CeO2 nanostructures. In addition, the thickness of the shell can have a significant effect on the absorption band and the power of plasmon resonance, the highest photocatalytic 12

performance of Ag NWs@CeO2-2 sample can possibly absorb light in a larger proportion and has the largest surface area compared to others. All the results above clearly demonstrate that the coverage of CeO2 shells on Ag NWs and NPs cores has been proved to be a useful and hopeful strategy for improving the photocatalytic properties of pure CeO2 catalysts. 3.3 Photocatalytic mechanism discussion To understand the involvement property of active radical species in the photocatalytic reaction, the electron spin resonance (ESR) technique was performed to analyze the photocatalytic mechanism. The ESR technique can be used to detect free radicals in reaction systems. To explore the main reactive species responsible for the photocatalytic reaction over the Ag@CeO2 core-shell photocatalysts, a series of quenchers were employed to scavenge the relevant reactive species. Typically, DMPO (5, 5-dimethyl-1-dimethy N-oxide) is generally used as a radical scavenger due to the generation of stable free radical, DMPO-O2- or DMPO-OH-. Fig.8 shows ESR spectra obtained as the effect of light irradiation on the Ag NWs@CeO2-2 photocatalyst at room temperature in air. As shown in Fig.8, there is no ESR signal in the dark, a gradual evolution of ESR peaks for DMPO-·OH adducts was observed under visible light irradiation. In contrast, no signals of DMPO-·O2- adducts were detected under the same conditions. Bard et al. [52] also reported similar ESR results that the strong and non-selective hydroxyl radical is the decisive specie in photocatalytic processes. Therefore, it is well recognized that the ·OH radicals play a predominant role among the oxygen-containing radical species over the Ag@CeO2 photocatalysts toward the oxygen evolution and degradation of MB dye under visible light illumination. To further clarify the photocatalytic process, the photoluminescence emission 13

spectra (PL) was performed. The photoluminescence (PL) study is widely used to investigate the migration, transfer, and recombination processes of the photogenerated electron-hole pairs in semiconductor materials, since PL emission arises from the recombination of charge carriers [53]. Fig. 9 presents the PL spectra for pure CeO2 and Ag@CeO2 core-shell nanostructures at an excitation wavelength of 325 nm. At room temperature, the emission band for pure CeO2 was centered at 520 nm, which was attributed to the recombination process of self-trapped excitations. The positions of the Ag@CeO2 core-shell structures were similar to pure CeO2. However, the mission intensity of the core-shell structures is significantly decreased, the Ag NWs@CeO2-2 sample showed the weakest intensity. This result clearly indicated that the recombination of photo-generated charge carriers is agreed well with the discussion in photocatalytic experiments. The PL spectra showed that the presence of the Ag core could effectively inhibit electron-hole recombination during the photocatalytic reaction under visible light irradiation. Based on ESR and PL results, possible mechanisms for the oxygen evolution and the MB photodegradation over the Ag@CeO2 core-shell structure photocatalysts are proposed. The purpose is to guide the further improvement of the photocatalytic performance of the novel composites. Fig 10 (e) illustrates the Plasmon-induced charge transfer in the charge injection mechanism. On one hand, the semiconductor CeO2 absorbs photons and excites electron-hole pairs, on the other hand, due to the CeO2 shell is porous and loose for reactant and product molecules/ions to pass through, the Ag core thus can absorb resonance photons and generates hot electrons which have energy between 1.0 and 4.0 eV with respect to the metal Fermi level [54]. Subsequently the excited electrons in the SPR state which have sufficient energy can be directly injected into the conduction band of CeO2. Meanwhile, the holes in the VB 14

of CeO2 can also transfer to the Ag core to reach energy equilibrium, which prevents the direct recombination of electrons and holes to increase the photocatalytic efficiency. For the oxygen evolution reaction, as shown in Fig .10 (a, b), upon visible light irradiation, hot electrons are generated in Ag core due to its strong absorption by a SPR band, then they are injected into the conduction band of CeO2, leading a generation of holes in the Ag cores (step I). The semiconductor CeO2 can also absorb photons and excites electron-hole pairs. The injected electrons and the electrons generated in the conduction band reacted with the sacrificial reagent Ag+ to form Ag0 (step II). On the other hand, the holes with abundant amount are left in the Ag cores for the oxidation of H2O into O2 (step III) in the Ag/CeO2 interfaces, and the O2 can diffuse out through the mesoporous loose CeO2 shell [55]. The subsequently transfer of the generated hot electrons to the CeO2 shell effectively prevent the direct combination of electron-hole pairs, which improve the photocatalytic efficiency [3638]. In addition, the improvement of photocatalytic activity also ascribed to a synergistic effect of the Ag core and the surface CeO2 shell [56]. As evidenced in HRTEM images, there are clear “three-dimensional” interface between Ag NWs and CeO2 nanoparticles, which can offer a larger active surface area and maximize the electron transmission efficiency. Thus the core-shell Ag NWs@CeO2 photocatalysts exhibit higher O2 evolution upon visible light irradiation. In MB degradation process as shown in Fig. 10 (c, d), the electrons accumulated in the conduction of the CeO2 scavenge oxygen molecules to generate reactive ·OH radicals after series of reactions [57] (step I and II). Meanwhile, an abundant amount of photo-generated holes scavenge the absorbed water through the mesoporous shell, leading to the production of the hydroxyl radical (·OH), which is responsible for the 15

oxidation decomposition of MB molecules [58]. The result is consistent with ESR result in Fig. 8. In conclusion, compared with pure CeO2, the Ag@CeO2 core-shell structures exhibit a significant enhancement of the photocatalytic activity.

 Conclusions In summary, the Ag@CeO2 core-shell nanostructures with well-controlled shape and shell thickness have been successfully prepared via a facile and green template-free approach in aqueous phase. The Ag@CeO2 core-shell structures exhibit greater enhancement of photocatalytic activity in the O2 evolution via water splitting and MB dye degradation under visible light irradiation, and the highest efficiency was observed with Ag NWs@CeO2-2 sample, which is about 1.7 times enhancement than pure CeO2. The superior visiable-light photovatalytic performance could be ascribed to synergistic effects between the large surface area and well-defined morphology with controllable optical properties from plasmonic effects. It is clear that the Ag NWs@CeO2-2 exhibit the largest local electric field enhancements compared with others. The promising results demonstrate that the noble metal@semiconductor-oxide core-shell structure could bring about great potential in solar energy conversion, especially as a promising technique to improve photocatalytic performance.

Acknowledgements This work was financially supported by the National Science Foundation of China (Grant No. 21003157 and 21273285), Beijing Nova Program (Grant No. 2008B76), and Science Foundation of China University of Petroleum, Beijing (Grant No. KYJJ2012-06-20).

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Figure Captions: Table .1 Data summary for Ag@CeO2 core-shell samples with different CeO2 shell thicknesses. Scheme 1 Schematic Illustration of the plausible formation process of the Ag NPs@CeO2 and Ag NWs@CeO2 core-shell nanostructures. Fig.1 XRD patterns of as-prepared samples: (A) pure CeO2; (B) Ag NWs@CeO2-2; (C) Ag NPs@CeO2 core-shell sample; (D-F) XRD patterns of Ag NWs@CeO2 with different shell thicknesses. Fig.2 (A,B) The typical transmission electron microscopy (TEM) images of Ag NPs@CeO2 core-shell nanostructures; (C,D) TEM images of Ag NWs (E,F) TEM images of Ag NWs@CeO2 core-shell nanostructures; (G) HRTEM of an individual Ag NWs@CeO2 core-shell sample (H-J) Ag NWs@CeO2 with different shell thicknesses (H-26.15nm; I-66.12nm; J-28.92nm). Fig.3 (A) Energy Dispersive Spectrometer (EDS) of the CeO2 shell (B) Energy Dispersive Spectrometer (EDS) of the Ag core (C) Energy dispersive X-ray (EDX) elemental mapping analysis of Ag NPs@CeO2 core-shell sample, showing the TEM image of the analyzed sample. (D) EDX elemental mapping analysis of Ag NWs@CeO2 core-shell sample. Fig.4 XPS spectra of the Ag NWs@CeO2 core-shell nanocomposite (A) O 1s, (B) Ag 3d, (C) Ce 3d (D) the survey spectra. Fig.5 UV-vis diffuse reflectance spectra of the samples of Ag NPs@CeO2, Ag NWs@CeO2-2 core-shell nanostructure and pure CeO2 sample. Fig.6 Photocatalytic O2 evolution curves of pure CeO2, Ag NPs@CeO2 and Ag NWs@CeO2 core-shell nanostructures catalysts from 50 mM silver nitrate aqueous solution under visible light illumination (>400 nm). Fig.7 The degradation curves of MB over pure CeO2, Ag NPs@CeO2 sample and Ag 24

NWs@CeO2-2 sample; Fig .8 ESR spectra of radical adducts trapped by DMPO (A) DMPO-·OH; (B) DMPO-·O2- in the Ag NWs@CeO2-2 core-shell sample dispersion solution. Fig.9 Photoluminescence (PL) spectra of pure CeO2 and Ag@CeO2 core-shell composites. Fig.10 Schematic diagram illustrating of the proposed photocatalytic mechanisms of core-shell Ag NPs@CeO2 and Ag NWs@CeO2 nanocomposites under visible light irradiation: (a, b) oxygen evolution; (c, d) MB oxidation. (e) Processes involved in the hot-electron effect. CB and VB represent the conduction and valence band of the semiconductor, respectively. EF refers to the Fermi energy level.

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Table .1 Data summary for Ag@CeO2 core-shell samples with different CeO2 shell thicknesses. Sample

Volume of Ag

Ligand

Ce(NO3)3

Shell thickness

O2 evolution

seeds (ml)

(μl)

(ml)

d (nm)

(μmol·g-1·h-1)

Ag NPs@CeO2

10

50

0.5

50 ~150

44.3

Ag NWs@CeO2-1

30

150

1.5

26.15

42.5

Ag NWs@CeO2-2

30

300

3

66.12

61.75

Ag NWs@CeO2-3

30

400

4

28.92

53.5

26

Scheme 1 Schematic Illustration of the plausible formation process of the Ag NPs@CeO2 and Ag NWs@CeO2 core-shell nanostructures.

Fig.1 XRD patterns of as-prepared samples: (A) pure CeO2; (B) Ag NWs@CeO2-2; (C) Ag NPs@CeO2 core-shell sample; (D-F) XRD patterns of Ag NWs@CeO2 with 27

different shell thicknesses.

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Fig.2. (A,B) The typical transmission electron microscopy (TEM) images of Ag NPs@CeO2 core-shell nanostructures; (C,D) TEM images of Ag NWs (E,F) TEM images of Ag NWs@CeO2 core-shell nanostructures; (G) HRTEM of an individual Ag NWs@CeO2 core-shell sample (H-J) Ag NWs@CeO2 with different shell thicknesses (H-26.15nm; I-66.12nm; J-28.92nm).

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Fig.3 (A) Energy Dispersive Spectrometer (EDS) of the CeO2 shell (B) Energy Dispersive Spectrometer (EDS) of the Ag core (C) Energy dispersive X-ray (EDX) elemental mapping analysis of Ag NPs@CeO2 core-shell sample, showing the TEM image of the analyzed sample. (D) EDX elemental mapping analysis of Ag NWs@CeO2 core-shell sample.

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Fig.4 XPS spectra of the Ag NWs@CeO2 core-shell nanocomposite (A) O 1s, (B) Ag 3d, (C) Ce 3d (D) the survey spectra.

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Fig.5 UV-vis diffuse reflectance spectra of the samples of Ag NPs@CeO2, Ag NWs@CeO2-2 core-shell nanostructure and pure CeO2 sample.

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Fig.6 Photocatalytic O2 evolution curves of pure CeO2, Ag NPs@CeO2 and Ag NWs@CeO2 core-shell nanostructures catalysts from 50 mM silver nitrate aqueous solution under visible light illumination (>400 nm).

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Fig.7 The degradation curves of MB over pure CeO2, Ag NPs@CeO2 sample and Ag NWs@CeO2-2 sample;

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Fig .8 ESR spectra of radical adducts trapped by DMPO (A) DMPO-·OH; (B) DMPO-·O2- in the Ag NWs@CeO2-2 core-shell sample dispersion solution.

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Fig.9 Photoluminescence (PL) spectra of pure-CeO2 and Ag@CeO2 core-shell composites

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Fig.10 Schematic diagram illustrating of the proposed photocatalytic mechanisms of core-shell Ag NPs@CeO2 and Ag NWs@CeO2 nanocomposites under visible light irradiation: (a, b) oxygen evolution; (c, d) MB oxidation. (e) Processes involved in the hot-electron effect. CB and VB represent the conduction and valence band of the semiconductor, respectively. EF refers to the Fermi energy level. Graphical abstract

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Facile synthesis of Ag@CeO2 core-shell plasmonic photocatalysts with enhanced visible-light photocatalytic performance.

Novel Ag@CeO2 core-shell nanostructures with well-controlled shape and shell thickness were successfully synthesized via a green and facile template-f...
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