Selective Deposition of Silver Nanoparticles onto WO3 Nanorods with

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Selective Deposition of Silver Nanoparticles onto WO Nanorods with Different Facets: the Correlation of Facet-induced Electron Transport Preference and Photocatalytic Activity Jing Ding, Yuanyuan Chai, Qianqian Liu, Xin Liu, Jia Ren, and Wei-Lin Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10580 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016

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The Journal of Physical Chemistry

Selective Deposition of Silver Nanoparticles onto WO3 Nanorods with Different Facets: the Correlation of Facet-induced Electron Transport Preference and Photocatalytic Activity Jing Ding†, Yuanyuan Chai†, Qianqian Liu†, Xin Liu†, Jia Ren†, Wei-Lin Dai*,†

†

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative

Materials, Fudan University, Shanghai 200433, P. R. China

KEYWORDS: Ag nanoparticles, WO3 nanorods, electron transfer, {001} facets, visible-light absorption

ABSTRACT：WO3 nanorods with regular hexagonal morphology and different exposed facets were fabricated by hydrothermal treatment, and then Ag nanoparticles (Ag NPs) selectively deposited onto hexagonal WO3 nanorods with different facets were also successfully synthesized through an in-situ photo-reduction method. The prepared samples were characterized by various analytical techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectra and so on. The results illustrated that the intrinsic nature of charge separation on the {001} facets of WO3-110 nanorods and the surface plasmon resonance (SPR) effect both contribute to the enhancement of visible-light

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absorption and the decrease of the recombination of the photogenerated electron-hole pairs. For comparison, Ag/WO3-110 catalysts with dominant exposed {001} facets exhibited much better photocatalytic activity than that of Ag/WO3-001 with high percentage of exposed {100} and {010} facets for the degradation of organic pollutants (including rhodamine B, methyl orange and so on) under the visible light irradiation. In addition, the underlying photocatalytic reaction mechanism was further investigated by the controlled experiments using radical scavengers.


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1. INTRODUCTION

Photocatalytic solar energy conversion is regarded as one of the most promising solutions to tackle the environmental problem and energy shortage crisis. Metal oxide semiconductors, as a kind of highly efficient photocatalysts, have been extensively investigated. However, many metal oxides possess a wide band gap, which limits their visible-light absorption. For instance, TiO2, as one of the most commonly studied photocatalyst, only absorbs 5% solar spectrum due to its wide band gap of 3.2 eV and displays low quantum yield1,2. To solve these problems, several visiblelight-driven photocatalysts have been successfully sought in recent years. Among these visiblelight photocatalysts, tungsten oxide (WO3) has been regarded as one of the most ideal candidates due to its stable physicochemical properties, such as relatively narrow band-gap energy (2.4-2.8 eV) and high oxidation power of valence band (VB) holes similar to that of TiO23,5. However, pure WO3 still has some disadvantages such as the lower conduction band level that does not provide a sufficient potential to react with strong electron acceptors and directly results in the fast recombination and the lower photocatalytic activity6,7. Therefore, a great vision has come toward the development of a novel visible-light-driven WO3-based photocatalysts which can work efficiently under a wide range of visible-light irradiation. Recently, special attentions have been paid to the crystal facet engineering of WO3 nanocrystals. Especially, the role of the crystal facet in the photocatalytic reaction has been intensively explored. Xie and co-workers have designed a couple of monoclinic WO3 crystals


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with different percentages of {002}, {200} and {020} facets. Through in-depth study of the electronic structural effects induced by crystal facet, they demonstrated that the {002} dominant rectangular sheet-like WO3 crystal with an elevated conduction band minimum exhibited a much higher photocatalytic reduction of CO2 to CH48. Zhang et al. reported that orthorhombic WO3 nanocrystals with the preferable exposure of the high-energy {001} facets exhibited enhanced photocatalytic activity, which could be attributed to the more effective production of active oxygen species and thus leaded to reducing the recombination of photo-induced electrons and holes9. Zhu et al. demonstrated that hexagonal WO3 nanorods grown along {110} axis with dominantly exposed {001} facets exhibited higher adsorption capacity for methyl orange (MO) and rhodamine B (RhB) dyes than those along {001} axis with {010} and {100} facets exposed10. However, to the best of our knowledge, the work on the preparation of visible-lightresponsive WO3 with different exposed facets using the surface plasmon resonance ( SPR) effect of noble metal has been rarely investigated up to now. Noble metals have attracted significant interest by virtue of their unique optical, electronic and physical properties. Particularly, noble metals (such as Au, Ag and Pt) exhibit excellent absorption of visible light and interfacial charge transfer because of their SPR effect. As we know, SPR is the resonant photon-induced coherent oscillation of charges at the metal-dielectric interface, which occurs when the frequency of exciting light matches the natural frequency of the surface metal electrons oscillating to resist the restoring force of positive nuclei11,12. Therefore, SPR greatly contributes to enhance the visible light absorbance and improve the solar-energy-


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conversion efficiency. Meanwhile, it offers a new opportunity to work out the problem of the limited efficiency of photocatalysts and develop a novel visible-light-driven photocatalyst as well. On the basis of this character, a number of noble metal modified semiconductor materials have so far been reported. For example, Ag@TiO213,14, Au@CeO215,17, Au@TiO218,21 and Pt@WO322 have been widely investigated and exhibited a remarkable photocatalytic activities. In consideration of the significantly lower cost and relatively higher stability than other noble metals, silver will remain as the primary choice in the future. In the present work, we investigated the relationships between the locations of silver nanoparticles on different facets of hexagonal WO3 nanorods and the photocatalytic performance of photocatalyst by tailoring the photoreduction deposition methods (as described in Figure 1). Two types of Ag/WO3 photocatalysts were prepared and their photocatalytic activities were evaluated by degrading organic dyes under visible light irradiation. Possible reasons for the correlation between the enhancement of photocatalytic performance and the structural characteristics of WO3 with different facets were discussed. Furthermore, the possible degradation mechanism of MO over Ag/WO3-110 and the active species in the photocatalytic process were further discussed and proposed in detail.


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Figure 1. Schematic representation of the synthesis process of Ag/WO3-110 photocatalyst.

2. EXPERIMENTAL SECTION

Materials. Ammonium tungstate (99.99%, AR), sodium tungstate (99.99%, AR) and silver nitrate (99.99%, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrochloric acid (37.5%, AR) and Methanol (99.0%, AR) were purchased from Aladdin Industrial Inc.. All the reagents were analytical grade and used without any further purification. All aqueous solutions were prepared with the deionized water.

Catalyst Preparation. Hexagonal WO3 nanorods growing along {110} direction with dominant exposed {001} facets (designated as WO3-110) and hexagonal WO3 ones growing along {001} direction with exposed {010} and {100} facets (designated as WO3-001) were synthesized by a previously reported method with some modifications10. WO3-110 nanorods were prepared via the hydrothermal method. Typically, 1.18 mmol (NH4)10W12O41·5H2O was dissolved in 120 mL distilled water. The pH value of solution was adjusted to pH of 1.5 with dropwise addition of 1.0 M HCl under constant magnetic stirring at 298 K for 20 min. Subsequently, the mixed solution was transferred into a 200 mL Teflon-lined stainless autoclave and heated at 453 K for 24 h. Upon leaving the solution cool to room temperature, the precipitates were separated by filtration, then washed with deionized water and ethanol several time, followed by drying at 353 K under vacuum for overnight. Finally, the solid products were


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heated at 573 K for 5 h in air to remove NH4+. To obtain WO3-001 nanorods, a similar way to synthesize WO3-110 nanorods was carried out by using 14.55 mmol Na2WO4·2H2O instead of 1.18 mmol (NH4)10W12O41·5H2O. The Ag supported WO3 nanorods were prepared by the photo-deposition method. AgNO3 was chosen as the Ag precursor and methanol was employed as the electron acceptors. Typically, 0.5 g of WO3 powder was suspended in 30 mL deionized water, 3.7 mL AgNO3 solution (10 g/L) was added. Subsequently, 3 mL methanol was added into the solution and the suspension was irradiated by a 300 W Xe lamp under continuous stirring. After 4 h photo-deposition, the suspension was filtered, washed with deionized water for three times and transferred to a vacuum oven to dry at 353 K for overnight. Catalyst Characterization. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 advance spectrometer with Cu Kα radiation (λ=0.154 nm), operated at 40 mA and 40 kV. UVVis spectroscopy measurement was carried out on a Shimadzu UV-3600 UV-vis-NIR spectrophotometer with BaSO4 as the reflectance standard. The FT-IR spectra were carried out on a Nicolet Avatar-360 FT-IR spectrometer. The Laser Raman experiments were performed with a Jobin Yvon Dilor Labram I Raman spectrometer equipped with a holographic notch filter and a CCD detector. Transmission electron microscope (TEM) images were obtained on a JEOL JEM 2010 transmission electron microscope. The samples were supported on carbon-coated copper grids for the experiment. The X-ray photoelectron spectra (XPS) were obtained on a RBD 147 upgraded PHI 5000C ESCA system equipped with a dual X-ray source, of which the Mg Kα


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(1253.6 eV) anode and a hemispherical energy analyzer were used. The background pressure during data acquisition was maintained at 420 nm). Visible irradiation was obtained from a 300 W Xe lamp with a 420 nm cutoff filter. In each experiment, 100 mg of photocatalyst was dispersed in 100 mL aqueous solution of MO (10 ppm) in an ultrasound generator for 5 min. Prior to irradiation, the suspension was magnetically stirred in dark for 30 min to obtain the absorption-desorption equilibrium. During the photodegradation reaction, 5 mL of MO solution with catalyst was sampled at the certain time intervals and centrifuged to remove the solid photocatalyst. The


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concentration of MO was determined by means of a UV-Vis spectrophotometer at a wavelength of 464 nm.

3. RESULTS AND DISCUSSION The XRD patterns of WO3 nanorods with different exposed facets and the corresponding 4.5wt.%Ag/WO3 nanorods are presented in Figure 2. Both WO3 nanorods prepared by a hydrothermal method exhibit hexagonal crystal structure with obvious diffraction peaks at 2θ values of 13.8°(100), 22.8°(002), 24.2°(110), 28.1°(200) and 36.5°(202), whose typical lattice constants are a=b=0.732 nm and c=0.766 nm (JCPDS 85-2459)

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. Compared with pure WO3-

001, the XRD pattern of 4.5wt.%Ag/WO3-001 shows the distinct peaks at 38.1°(111), confirming the presence of metallic silver in the catalyst. This phenomenon illustrates that Ag NPs appear aggregation on exposed {010} and {100} facets of WO3 nanorods to some extent. For comparison, however, we could not clearly observe any diffraction peaks assignable to metallic silver in 4.5wt.%Ag/WO3-110 samples, probably attributed to that very small Ag NPs are better dispersed on the exposed {001} facets of WO3 nanorods than that on the exposed {010} and {100} facets, which is in well accord with the results of TEM. The nitrogen adsorption and desorption isotherms in Table S1 clearly shows that the WO3-110 nanorods exhibit higher surface area than WO3-001, further implying that {001} facets of WO3 nanorods are more conducive for Ag NPs to be highly dispersed. In addition, based on the results of ICP-


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AES analysis, the weight percentages of Ag in 4.5wt.%Ag/WO3-110, and 4.5wt.%Ag/WO3-001 are measured to be 4.31 and 4.25, respectively.

★ ▲

WO3 Ag

WO3-110

Intensity / a.u.

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100

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002

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222 302 220 300211 301 221 311400

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2 Theta / degree

Figure 2. XRD patterns of WO3 nanorods with different exposed facets and the corresponding 4.5wt.%Ag/WO3 nanorods.

The morphology and nanostructure of the hexagonal WO3 nanorods with different exposed facets and the particle size distribution of the silver nanoparticles were visualized by TEM and the high resolution TEM (HR-TEM). As can be seen from Figure S1a-b, WO3-110 exhibits a rodlike structure with average diameter and length around 13 and 140 nm, respectively. Meanwhile, WO3-001 was also comprised of hexagonal single crystal nanorods, as shown in Figure S1d-e. These two morphologies of WO3 nanorods are similar to those previously reported10. HR-TEM and FFT images of hexagonal WO3-110 in Figure 3a and S1g display {010} and {100} facets with 120° angle. One lattice spacing of 0.635 nm can be indexed to the


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{100} direction of hexagonal WO3. The other with the almost same lattice space as that of {100} direction (d=0.635 nm) matches well with the crystal facet {010} of hexagonal WO3. Simultaneously, as shown in Figure 3b, the interfacial angle between {110} and { 110 } facets is 90°, whose lattice spaces are 0.365 and 0.316 nm, respectively. These data demonstrate that the hexagonal WO3-110 nanorods grow preferentially along the {110} axis with the dominant {001} facets exposed, as illustrated in Figure S1b (inset). WO3-001 can be obtained by using sodium tungstate instead of ammonium tungstate as a tungsten source and it also exhibits hexagonal rodlike morphology with 90° angle between {001} and {100} facets, whose lattice spaces are 0.315 and 0.384 nm, respectively (Figure 3e and S1d). However, by contrast, hexagonal WO3-001 nanorods (Figure S1d) are shown to have a lager average diameter and length around 60 and 400 nm and grow along {001} axis with {010} and {100} facets exposed. From Figure 3c and 3d, it can be clearly seen that the very small silver nanoparticle are highly dispersed on the {001} facets of WO3-110 support. Meanwhile, the corresponding TEM histogram of Ag NPs further displays a very narrow particle size distribution with sizes between 3-7 nm and the average diameter of silver nanoparticles deposited onto {001} facets of WO3 nanorods is approximately 4 nm (Figure 3c (inset)). In addition, the energy dispersive X-ray spectroscopy (EDX) shown in Figure S1c, resulting from selected area, revealed that the presence of Ag, W and O in the Ag/WO3-110 sample. Compared the TEM image of 4.5wt.% Ag/WO3-110 (see Figure 3c,f ), the TEM images of 4.5wt.% Ag/WO3-001

clearly reveal that vast Ag nanoparticles come to

aggregate on the {010} and {100} facets of WO3 nanorods and the sizes of Ag NPs are more


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than 100 nm, which is in accord with the results of XRD. EDX displays the presence of Ag, W and O in the Ag/WO3-001 sample in selected area of Figure S1f. This phenomenon further indicates that WO3-110 samples own the unique electronic structure and contributes significantly to a better dispersion on {001} facets of WO3 nanorods for silver nanoparticles, and thus Ag NPs on the {001} facets will be as effective antennas for incident light and the charge separation and transfer rate at the WO3 interface also will be dramatically enhanced.

Figure 3. HR-TEM (a,b) and FFT (inset,left) images of WO3-110; TEM image and size distribution of Ag nanoparticles (c) and HR-TEM image (d) of 4.5wt.%Ag/WO3-110 sample; HR-TEM (e) and FFT (inset) images of WO3-001; TEM (f) image of 4.5wt.%Ag/WO3-001 sample.


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To evaluate the optical absorption properties, the UV-Vis diffuse reflectance spectra (DRS) of pristine WO3 nanorods with different exposed facets and 4.5wt.% Ag/WO3 nanorods were shown in Figure 4. As demonstrated in Figure 4a, bare WO3-001 exhibits photoabsorption from the UV and visible-light regions. However, as for pure WO3-110, an obvious absorption edge appears approximately at 494 nm and the band absorption of WO3-110 has a red-shift for about 54 nm with respect to that of WO3-001. Notably, the high percentage of {001} facets in WO3-110 is responsible for this red-shift. At the same time, this phenomenon indicates that WO3-110 possesses the better visible-light-induced photocatalytic activity and more electron-hole pairs can be produced than that of WO3-001. In comparison with pristine WO3 nanorods before and after Ag loading, a clear red shift of the band gap absorption edge for 4.5wt.% Ag/WO3 is observed and the absorption is significantly enhanced in the visible region especially 4.5wt.% Ag/WO3110 catalyst, suggesting the SPR effect of metallic Ag NPs displays efficient plasmon resonance in the visible region and thus resulting in yielding more electron-hole pairs. Additionally, the bandgap energies of WO3-110 and WO3-001 in Figure 4b calculated based on the Oregan and Gratzel method are 2.51 and 2.82 eV, respectively24,25. Remarkably, the band gap energy of WO3-110 is lower than that of WO3-001, suggesting that WO3-110 with exposed {001} facets has different surface electronic band structure with WO3-001 with exposed {010} and {100} facets, thus resulting in the better optical property and more optimal visible light response ability of WO3-110. In addition, the valence band (VB) XPS was applied to determine the electronic structure. Figure S2 shows the edges of the VB of WO3-110 and WO3-001 are 2.78 and 2.86 eV,


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respectively. Meanwhile, the edges of the CB of WO3-110 and WO3-001 are thus estimated to be 0.27 and 0.04 eV. These results are in well accord with the literatures reported previously, further indicating that the conduction band level of pristine WO3 is too low to provide a sufficient potential to react with strong electron acceptors and thus resulting in fast recombination of photogenerated electron-hole pairs. Hence, Ag NPs will facilitate the separation of electrons and holes in light irradiation. (a)

(b) 4.5wt% Ag/WO3-110

Absorbance

2 ^ WO3-110

(α hν)

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2.51eV WO3-110

4.5wt% Ag/WO3-001

WO3-001

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Figure 4. (a) UV-vis absorption spectra of WO3 nanorods with different exposed facets and the corresponding 4.5wt.%Ag/WO3 nanorods samples and (b) Diffuse reflectance spectra of pure WO3 nanorods with different exposed facets.

To investigate in-depth the surface elemental composition and electronic structure of WO3 nanorods and 4.5wt.% g/WO3 nanorods, XPS experiments were carried out. As shown in Figure S3a, WO3-110 sample contains W and O elements with obvious photoelectron peaks at binding energies of 35 (W 4f), 533 eV (O 1s) in the survey spectrum, while the W, O and Ag signals


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appear in the spectra of 4.5wt.%Ag/WO3-110. In Figure S3b, W 4f spectrum for both samples can be deconvoluted into two peaks centered at around 34.6 and 36.8 eV. The two peaks are respectively ascribed to W 4f5/2 and W 4f7/2, suggesting that the tungsten in the tungsten oxide sample exists as W6+

26,27

. Simultaneously, the O1s spectra in Figure S3c clearly display the

presence of two distinct O species. The peaks located at approximately 531.0 and 533.3 eV are originated from the lattice oxygen O2- in the WO3-110 sample and the surface adsorbed water. Compared to those of the pristine WO3-110 sample, the W4f and O1s peaks of 4.5wt.%Ag/WO3110 samples show a small shift towards low binding energy, indicating that the presence of small Ag clusters can strongly interact with the support WO3-110 and electrons tend to transfer from Ag nanoparticles to the support WO3-110 nanorod28. Figure S3d clearly shows the presence of metallic silver in the 4.5wt.%Ag/WO3-110 sample from the corresponding Ag 3d5/2 and Ag 3d3/2 binding energy values of 367.4 and 373.4 eV, respectively. Based on the binding energy of W 4f and O 1s levels of WO3-001 sample in Figure S3e and S3f (O 1s, binding energy 530.3 eV; W 4f7/2, binding energy 34.2 eV; W 4f5/2, binding energy 36.3 eV), the oxidation states of the W and O elements can be identified as +6 and -2, respectively. Notably, in contrast with those of 4.5wt.%Ag/WO3-001, the W4f, O1s and Ag 3d peaks of 4.5wt.%Ag/WO3-110 sample in Figure 5b,d come to a certain shift to lower binding energy, further illustrating that the electrons can be more easily separated on {001} facets of WO3-110 than that of WO3-001 with exposed {010} and {100} facets. And then, some electrons transport from Ag NPs to {001} facets of WO3-110. This finding accords well with the rules of most of the metal-metal oxide composite and suggests


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the strong interaction between the Ag NPs and WO3 nanorods, and the fast electron transfer can prevent the photogenerated charge carriers recombination.

Figure 5. XPS spectra of 4.5wt.%Ag/WO3-110 (I) and 4.5wt.%Ag/WO3-001 (II): a) the survey spectrum; b) high-resolution W4f; c) high-resolution O1s; d) high-resolution Ag3d.

The photocatalytic activities of pristine WO3 with different exposed facets, as well as Ag/WO3 catalysts, were evaluated by the photodegradation of MO and RhB under visible light. As shown in Figure 6a, dark absorption of anionic dye (MO) was measured for 30 min to check the selfdegradation. Remarkably, there is no adsorption phenomenon in pure WO3 and Ag/WO3 systems. However, in Figure 6b, in terms of the degradation of cationic dye (RhB), it is obvious


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that the remaining dye concerntrations reduced with WO3-110 and Ag/WO3-110 as photocatalysts after reaching the adsorption-desorption equilibrium. This phenomenon illustrates that electrons tend to migrate to {001} facets of WO3-110, thus resulting in cationic dye (RhB) is easily adsorbed on the {001} facets of WO3-110 before adsorption-desorption equilibrium, coinciding with that of the related literature reported previously10. Notably, 4.5wt.%Ag/WO3-110 catalyst exhibits much higher photocatalytic activity than pure WO3-110 and 4.5wt.%Ag/WO3001, ulteriorly manifesting that Ag NPs loading on the {001} facets of WO3-110 nanorods, whose facets are gathered by electrons under visible light irradiation, is beneficial for improving the photocatalytic activity. Meanwhile, 4.5wt.%Ag/WO3-110 photocatalyst displays the highest rate constant, which is approximately 40 times and 8 times as high as that of 4.5wt.%Ag/WO3001 for the photodegradation of MO and RhB (see Figure S4a-b). 1.0

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0.4

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Figure 6. The photocatalytic performance of WO3 nanorods with different exposed facets and the corresponding 4.5wt.%Ag/WO3 nanorods catalysts for the degradation of MO (a) and RhB (b) under visible light.


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To check the photostability of 4.5wt.%Ag/WO3, the photocatalytic degradation of MO was repeated up to three cycles by adding the same amount of the recycled 4.5wt.%Ag/WO3-110 photocatalyst as fresh MO solutions (10 mg/L) under visible-light irradiation (see Figure S5a). The photocatalytic activity keeps essentially unchanged throughout the repeated cycles. Moreover, compared to the fresh catalyst (Figure S5b), the XRD pattern of the reused one does not show any obvious change and the amount of Ag and W in the reused catalyst after three recycles is almost the same as that of the fresh catalyst as estimated by ICP-AES. To further understand the involvement of active radical species in the photocatalytic process, control experiments have been carried out, and the corresponding results are shown in Figure 7. Ammonium oxalate (AO), isopropanol (IPA) and 1,4-benzoquinone (BQ) were used as the hole (h+) scavenger, hydroxyl radical (·OH) scavenger and superoxide radical (·O2-) scavenger, respectively29,31. The photocatalytic activity of 4.5wt.%Ag/WO3-110 decreases slightly by the addition of superoxide radical scavenger, while hole-scavenger and hydroxyl radical scavenger display a remarkable impact on the MO reaction rate. These results indicate that h+ and·OH are the main oxidative species for 4.5wt.% Ag/WO3-110 in these photocatalytic systems.


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1.0 0.8

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50 100 150 Irradiation time /min

200

Figure 7. The influence of various scavengers on the MO photocatalytic degradation in the presence of 4.5wt.%Ag/WO3 catalyst under visible light irradiation.

To understand the photogenerated electron-hole pair separation ability of WO3 nanorods with different exposed facets and Ag/WO3-110 nanorods samples, PL spectra were recorded. Figure 8a exhibits the PL spectra of pristine WO3-110, WO3-001, 4.5wt.%Ag/WO3-110 and 4.5wt.%Ag/WO3-001 samples under the excitation wavelength of 270 nm. All samples have similar emission profiles. The emission peaks centered at around 421 and 480 nm in the PL spectra for all of the samples originate from the localized states and defects in the band gaps, which is verified by the results from previous work32,33. Remarkably, the PL intensity of WO3110 sample is significantly lower than that of WO3-001 samples, indicating that the exposed facets could influence the recombination process of the photogenerated carriers in WO3. In other words, the photogenerated electron and hole pairs can be efficiently separated on the {001}


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facets of WO3 nanorods and electrons can be transferred to electron-rich {001} facets more easily34. In contrast, the PL intensity for the 4.5wt.%Ag/WO3-110 sample was lower than that of bare WO3-110. That is, in the Ag/WO3-110 system, electrons excited from light-activated WO3110 nanorods and Ag NPs can quickly migrate to the {001} facets of WO3 under visible-light irradiation. Here, holes prefer to gather on the surface of Ag NPs and electron will migrate to the electron-rich {001} facets of WO3-110 that could display lower recombination probability of photogenerated charge carriers and promote charge separation. This efficient charge separation greatly improved the photocatalytic activity of the Ag/WO3-110 systems. EIS analysis is a powerful technique to evaluate the interfacial properties between the electrode and the electrolyte, such as conductivity, structure and charge transport. Figure 8b shows the Nyquist plots of pure WO3-110, WO3-001 and the corresponding Ag/WO3 photocatalysts. Obviously, the Nyquist plots show an inconspicuous arc in the high frequency region and a straight line in the low-frequency region. The high frequency is related to the intrinsic electronic resistance of WO3 materials and the inconspicuous arc is probably due to the low Faradaic charge transfer resistances. Here, the magnitudes of the equivalent series resistance (ESR) of pure WO3-110, WO3-001, 4.5wt.%Ag/WO3-110 and 4.5wt.%Ag/WO3-001 samples are about 27, 29, 23 and 46 Ω, respectively, which can be obtained from the x-intercept of the Nyquist plots. Meanwhile, at low frequency, the more vertical the line is, the lower ion diffusion resistance will be35,36. It is obvious that WO3-110 has lower ESR and is more vertical than WO3001, indicating that {001} facets of WO3-110 are conducive for electrons to gather and transfer.


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Clearly, 4.5wt.%Ag/WO3-110 exhibits the lowest ESR and most vertical curve in all samples, further implying that Ag NPs deposited significantly enhanced the electron mobility and thus reducing the recombination of electron-hole pairs. WO3-001 4.5wt.% Ag/WO3-001 WO3-110 4.5wt.% Ag/WO3-110

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ex:270nm

300

350

400

450

500

Wavelength / nm

Figure 8. (a) Photoluminescence emission spectra of WO3 nanorods with different exposed facets and the corresponding 4.5wt.%Ag/WO3 nanorods samples; (b) Nyquist plots of the impedance spectra for WO3-110, WO3-001 and 4.5wt.%Ag/WO3 composites in a frequency range of 0.01 Hz to 100 kHz. The inset shows the enlarged impedance spectra at the highfrequency region.

Based on the experimental results above, a possible photocatalytic reaction mechanism for the MO degradation by 4.5wt.%Ag/WO3-110 photocatalyst was proposed and briefly described in Figure 9. Under visible light irradiation, photons can be absorbed by WO3-110, which subsequently produces photogenerated electrons and holes. The electrons tend to transfer to the {001} facets of WO3-110 in order to realize the effective separation of the photogenerated


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electrons and holes. However, the low conduction band level of WO3-110 also limits the photocatalyst’s ability to react with electron acceptors and increases the recombination of photogenerated electron-hole pairs, and thus leading to lower photocatalytic activity37. Conjugation of Ag nanoparticles with WO3-110 nanorods induces surface plasmon resonance which is produced by the collective oscillation of Ag surface electrons that enhances the local inner electromagnetic field38,39. Hence, the photogenerated electrons on the {001} facets of WO3110 photocatalyst and Ag NPs can be produced and electrons are quickly migrated from Ag NPs to the {001} facets of WO3-110. The remaining holes on Ag NPs can oxidize the hydroxide ion (OH-) to produce superoxide radicals (·OH) during the SPR process. These lead to the decrease of the recombination of the photogenerated electron-hole pairs40. In addition, the Agδ+ on the surface of Ag NPs can oxidize the dye molecular since Ag+ itself is a good oxidant. In the present work, the degradation of organic pollutants seems to be mainly initiated by ·OH and hole attack. Meanwhile, the holes accumulated on the valence band of WO3-110 can also oxidize organic pollutants (such as MO). Therefore, these observations show that the photocatalytic performance of 4.5wt.%Ag/WO3-110 photocatalyst is far superior to pure WO3-110 and 4.5wt.%Ag/WO3-001.


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Figure 9. Possible degradation mechanism of MO over 4.5wt.%Ag/WO3 photocatalyst under visible light irradiation.

4. CONCLUSIONS In summary, WO3 nanorods with regular hexagonal morphology and with the exposed {001} facets were prepared and Ag/WO3-110 catalysts with Ag NPs well dispersed on the WO3 {001} facets were successfully synthesized by an in-situ photoreduction approach. Such 4.5wt.%Ag/WO3-110 catalysts with high proportion of {001} facets shows excellent visiblelight-responsive photocatalytic performance, far exceeding that of 4.5wt.%Ag/WO3-001 catalysts with exposed{010} and {100} facets. Moreover, in-depth investigations show that the enhanced photocatalytic activity is attributed to not only the intrinsic nature of charge separation on the {001} facets of WO3-110 nanorods, but also the SPR effect that contributes to the


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decrease of the recombination of the photogenerated electron-hole pairs. The present study put forward the possibility of engineering of crystal-based photocatalysts by photodeposition of noble metals on different facets of visible-light asbsorbing semiconductors contributing to photocatalytic activity, which might open novel vistas for exploring crystallology in various optoelectronic applications.

ASSOCIATED CONTENT

Supporting Information. The physicochemical characterization, TEM images, XPS valence band spectra, XPS high-resolution spectra, the apparent rate constants, the reusability and the XRD patterns of 4.5wt.%Ag/WO3-110 photocatalyst are available. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program (Grant No. 2012CB224804), NSFC (Project 21373054, 21173052), State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC) and the Natural Science Foundation of Shanghai Science and Technology Committee (08DZ2270500).

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Selective Deposition of Silver Nanoparticles onto WO3 Nanorods with

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