Defect-Mediated Growth of Noble-Metal (Ag, Pt, and Pd) Nanoparticles

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Defect Mediated Growth of Noble Metal (Ag, Pt and Pd) Nanoparticles on TiO2 with Oxygen Vacancies for Photocatalytic Redox Reactions under Visible Light Xiaoyang Pan, and Yi-Jun Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4064802 • Publication Date (Web): 09 Aug 2013 Downloaded from http://pubs.acs.org on August 14, 2013

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The Journal of Physical Chemistry

Defect Mediated Growth of Noble Metal (Ag, Pt and Pd) Nanoparticles on TiO2 with Oxygen Vacancies for Photocatalytic Redox Reactions under Visible Light Xiaoyang Pan†,‡, and Yi-Jun Xu†,‡,* †

State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou, 350002, P.R. China

‡

College of Chemistry and Chemical Engineering, New Campus, Fuzhou University, Fuzhou, 350108, P. R. China * Corresponding author: Prof. Yi-Jun Xu; E-mail Address: [email protected]; Tel. +86 591 83779326

Abstract A synergistic strategy involving oxygen vacancy generation and noble metal deposition is developed to improve the photocatalytic performance of TiO2 under visible light irradiation. Through a redox reaction between the reductive TiO2 with oxygen vacancies (TiO2-OV) and metal salt precursors, noble metal nanoparticles (Ag, Pt and Pd) are uniformly deposited on the defective TiO2-OV surface in the absence of any reducing agents or stabilizing ligands. The resulting M-TiO2-OV (M= Ag, Pt and Pd) nanocomposites are used as visible light driven photocatalysts for selective oxidation of benzyl alcohol and reduction of heavy metal ions Cr (VI). The results show that the oxygen vacancy creation obviously enhances the visible light absorption of semiconductor TiO2. Meanwhile, the noble metal deposition can effectively improve charge separation efficiency of TiO2-OV under visible light irradiation, thereby enhancing the photoactivity. In particular, Pd-TiO2-OV, having the average Pd particle size of 2 nm, shows the highest visible light photoactivity, which can be attributed to the more efficient charge carriers separation of Pd-TiO2-OV than Ag-TiO2-OV and Pt-TiO2-OV. The possible reaction mechanism for photocatalytic selective oxidation of benzyl alcohol and reduction of Cr (VI) over M-TiO2-OV (M= Ag, Pt and Pd) has also been studied. It is hoped that our work could offer a simple strategy on fabricating defect-based nanostructures and their applications in solar energy conversion.

Keywords: defect; oxygen vacancy; noble metal nanoparticles; visible light photocatalyst. 1

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1. Introduction Extensive efforts have been made to study semiconductor TiO2 based photocatalysts, owing to their wide applications in environmental remediation and solar energy conversion.1-4 However, for practical applications, pure TiO2 is not a good candidate, because it is only active under ultraviolet (UV) light irradiation due to its relative wide band gap (3.2 eV).1-2, 4-6 Therefore, various strategies have been developed to modify the band structure of TiO2 to make it sensitive to visible light.2-3, 7-11 In particular, recent studies have revealed that defect engineering can be utilized as an effective strategy for narrowing the band gap of TiO2.9, 12-18 By introducing oxygen vacancies in the lattice of TiO2, one can effectively extend the light absorption of TiO2 to the visible or even infrared region.15, 19

In addition to extending optical absorption of TiO2 to the visible light region, it is equally

important to optimize the photogenerated electron/hole separation characteristics over the TiO2 surface. Notably, noble metal/TiO2 (denoted as M-TiO2) nanocomposites could effectively reduce the photo-generated electron/hole recombination, because noble metals are able to serve as electron sinks to facilitate interfacial electron transfer.5, 20

Therefore, the strategy to couple oxygen vacancy creation and noble metal deposition on TiO2 would be highly promising to design efficient TiO2 based visible light photocatalyst. In this regard, it is worth noting that the photocatalytic activities of M-TiO2 nanocomposites are strongly dependent on the size, morphology and dispersion of metal nanoparticles.21-23 Especially, the size of noble metal nanoparticles critically influences the electronic properties of M-TiO2.24 For example, Kamat et al have revealed that the particle size of Au has a profound effect on the shift of the apparent Fermi level (EF*).24 The smaller Au particles induce greater shift in EF* than the larger particles.24 Such a shift in the Fermi level can improve the energetic of the composite system and enhance the efficiency of interfacial charge transfer process.24 In contrast, as the size of the noble metal nanoparticles increase, metal particle can even act as recombination centers and hamper the photoactivities of M/TiO2.23

To manipulate the size, morphology and dispersion of noble metal particles, various methods have been developed to fabricate M/TiO2 nanocomposites.25-26 The most used method to obtain 2


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noble metal particles is the chemical reduction of metal ions with the protection of organic stabilizers (e.g., surfactant or polymer) in a solution phase. The as-synthesized noble metal nanoparticles by such a well-known colloidal synthesis approach can be well controlled in a small particle size dimension. However, the organic ligands attached to the surface of noble metal nanoparticles often exhibit susceptibility to chemical oxidation, especially under the condition of photolysis.27 The other limitation is the insufficient interfacial contact between the organic ligands stabilized metal particles and supports due to the presence of organic layers at the interface between the metal particles and supports, which could not effectively transfer and separate the photogenerated charge carriers.28-29 Therefore, it is highly desirable to implement simple and direct process for depositing noble metal nanoparticles onto the semiconductor TiO2 surface without the involvement of reducing agents and organic ligands as stabilizers for noble metal nanoparticles.

On accounts of these perennial problems, we have developed a novel and simple method to in situ load gold nanoparticles on TiO2 with oxygen vacancies (TiO2-OV) without using any chemical reducing agents or stabilizing ligands.30 Through a redox reaction between the reductive TiO2-OV and gold ions, Au-TiO2-OV nanocomposite with intimate interfacial contact is obtained. Therefore, the resulting Au-TiO2-OV nanocomposite shows enhanced photoactivity and photostability toward Rhodamine B degradation. However, it remains unclear whether this method can be used to prepare other noble metal nanoparticles i.e., Ag, Pt and Pd. In addition, the enhancement of visible-light photoactivity of Au-TiO2-OV is still limited, as compared to bare TiO2-OV. This can be attributed to the relative large particle size of Au nanoparticles (15 nm), which can not effectively separate the photogenerated charge carriers.24 Moreover, a literature survey leads us to find that the utilization of defective TiO2-OV as photocatalyst for selective organic synthesis has not been reported so far. Besides the “non-selective” degradation of organic pollutants, synthesis of fine chemicals by photocatalytic selective redox process has shown the promising potential in the field of heterogeneous photocatalysis.31-34

Herein, oxygen vacancies formation and noble metal deposition are combined to achieve an efficient visible-light-driven photocatalyst. The well defined noble metal (Ag, Pt and Pd) loaded defective TiO2 with oxygen vacancies (TiO2-OV) is achieved by a facile two-step process in the 3



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absence of any reducing agents or stabilizing ligands. In the procedure, defective TiO2-OV is first prepared by a photocatalytic reaction. Then, the noble metal nanoparticles are uniformly deposited onto the framework of TiO2-OV through the direct redox reaction between the reductive TiO2-OV and metal salt precursors. In particular, Pd nanoparticles with an average size of 2 nm are well decorated on the TiO2-OV surface. The as-prepared M-TiO2-OV (M= Ag, Pt and Pd) nanocomposites are used for selective oxidation of benzyl alcohol to benzaledehyde and reduction of Cr (VI) under visible light irradiation. The results show that oxygen vacancies generation can enhance the visible light absorption and the incorporation of the noble metal can effectively promote the charge carriers separation, thereby enhancing the photoactivities of TiO2-OV. Especially, Pd-TiO2-OV shows remarkably enhanced activities for photocatalytic redox reactions, as compared to blank P25-OV, Ag-TiO2-OV and Pt-TiO2-OV, because of more efficient charge carrier separation over the Pd-TiO2-OV sample under visible light irradiation. The photocatalytic mechanism of selective oxidation of benzyl alcohol and reduction of Cr (VI) over M-TiO2-OV is also proposed. We hope that our work would provide useful information and promote further interest in preparation of defective semiconductor based nanomaterials for target applications in solar energy conversion.

2. Experiment 2.1. Materials Degussa P25 was purchased from Degussa, Hulls Corporation, Germany. Trifluorotoluene (BTF) with a purity of >99% was supplied by Alfa Aesar. Silver nitrate (AgNO3), hexachloroplatinic (IV) acid hexahydrate (H2PtCl6·6H2O), palladium (II) chloride (PdCl2), anhydrous ethanol (>99%), benzyl alcohol (>99%) (BA), hydrochloric acid (HCl), Ammonium oxalate ((NH4)2C2O4·H2O), tert-butyl alcohol ((CH3)3COH), potassium dichromate (K2Cr2O7), potassium preoxydisulfate (K2S2O8) and p-benzoquinone (C6H4O2) were purchased from Sinopharm chemical regent Co., Ltd. (Shanghai, China). Deionized water was supplied from local sources. All of the materials were used as received without further purification. 2.2. Synthesis P25 with oxygen vacancies (P25-OV) was prepared by a previously reported method.19 In a

4


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typical experimental procedure, commercial available Degussa P25 (0.05 g) was added into the solvent of trifluorotoluene (3 mL) which contained benzyl alcohol (0.4 mmol) in a Pyrex glass bottle for 2 h photo-oxidation under UV light irradiation by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) with band-pass filter (365±15 nm). After photocatalytic reaction, the yellow powder was separated by centrifugation and washed with ethanol and distilled water for several times. The precursor of Ag and Pt was dissolved in deionized water directly to produce the corresponding solution with the desirable concentration, 10 mM. Because PdCl2 is slightly soluble in water, hydrochloric acid was used to dissolve it. The resulting H2PdCl4 solution has the same concentration of 10 mM as that of precursor solution of Ag or Pt. The reduction of noble metal ions on the surface of P25-OV was carried out as follows: 0.05 g of the as-synthesized P25-OV was added into 30 ml distilled water under ultrasonication. A given amount (0.588 ml AgNO3, 0.513 ml H2PtCl6 and 0.564 ml H2PdCl4) of 10 mM metal ions aqueous solution was mixed with the P25-OV suspension under strong stirring. Then the suspension was transferred into a round-bottom flask and was heated to 100 oC in an oil bath with magnetic stirring for 2h. Subsequently, the products were cooled to room temperature and were collected, washed with ethanol and water, and dried at 60 oC in an oven. 2.3. Characterization The crystal phase properties of the samples were recorded on a Bruker D8 Advance X-ray diffractometer (XRD) with Cu Kα radiation. The optical properties of the samples were characterized by a Cary 500 UV-visible ultraviolet/visible diffuse reflectance spectrophotometer (DRS), during which BaSO4 was employed as the internal reflectance standard. The morphology of the as-prepared M-P25-OV was determined by a transmission electron microscope (TEM, FEI Tecnai

G2

F20

S-TWIN).

Nitrogen

adsorption-desorption

isotherms

and

the

Brunauer-Emmett-Teller (BET) surface areas were collected at 77 K on a quantachrome Autosorb-1-C-TCD system. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Scientific ESCA Lab250 spectrometer which consists of a monochromatic Al Kα as the X-ray source a hemispherical analyzer and sample stage with multi-axial adjustability to obtain the composition on the surface of samples. All the binding energies were calibrated by the C 1s peak of the surface adventitious carbon at 284.6 eV. Electron 5



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spin resonance (ESR) measurements of oxygen vacancy were operated at a Bruker EPR A300 spectrometer. Typically, a 0.1 g portion of sample powder was placed in a quartz-glass sample tube. Then the sample was analyzed by the ESR instrument under room temperature. The settings for the ESR spectrometer were: center field: 3580.26 G; modulation frequency: 100.00 KHz; microwave frequency: 9.50 GHz; power: 6.34 mW. ESR signal of the radicals spin-trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, supplied from sigma Co., Ltd.) was also recorded on a Bruker EPR A300 spectrometer. In detail, the sample (5 mg) was dispersed in the solvent benzotrifluoride (BTF, 5 mL). Then, 25 µL DMPO/benzyl alcohol solution (1:10, v/v) was added and oscillated to achieve the well-blending suspension. The visible light irradiation source was a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) system equipped with a UV cutoff filter (λ> 420 nm), which is the same light source for photoactivity test. The settings for the ESR spectrometer were: center field: 3507.36 G; modulation frequency: 100.00 KHz; microwave frequency: 9.50 GHz; power: 6.34 mW. The photoluminescence spectra for the samples were investigated on an Edinburgh FL/FS900 spectrophotometer, and the excitation wavelength was set at 365 nm. 2.4. Photoactivity Photocatalytic aerobic oxidation of benzyl alcohol was performed as the previous research works.32-33 Typically, the catalyst (8 mg) was added into the solvent of trifluorotoluene (BTF) (1.5 mL) which contained benzyl alcohol (0.1 mmol). Molecular oxygen from a gas cylinder was used to saturate the solvent before reaction. The above mixture was transferred into a 10 mL Pyrex glass bottle. The container was filled with pure oxygen at a pressure of 0.1 MPa, and a stirring bar was introduced to the Pyrex glass reactor. For photocatalytic reaction under visible light irradiation, the reactor was irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) with a UV-CUT filter (>420 nm) for 2 hours. The reaction temperature was controlled at room temperature by an air-cooling system. After the reaction, the suspensions were centrifuged; the remaining solution was analyzed with an Aglient Gas Chromatograph (GC-7820). Controlled photoactivity experiments using different radicals scavengers (ammonium oxalate as scavenger for photogenerated holes,34 AgNO3 as scavenger for electrons,34-35 tert-butyl alcohol as scavenger for hydroxyl radical species36 and p-benzoquinone as scavenger for superoxide radical species34, 37) were performed similar to the above photocatalytic oxidation of benzyl alcohol except 6


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that radical scavengers (0.1 mmol) was added to the reaction system. The catalysts after photocatalytic reaction were washed with anhydrous ethanol and deionized water carefully. Finally, the catalysts were dried in an oven at 60 oC overnight for the following cycling photoactivity test. The fresh benzyl alcohol and BTF solvent was mixed with this used catalyst subject to 2nd run photoactivity testing. In analogy, the recycled 3rd, 4th and 5th run photoactivity testing was performed. Conversion of benzyl alcohol and yield of benzaldehyde were defined as the follows: Conversion(%) = [(C 0 − C BA ) C 0 ] × 100

Yield (%) = C BAD C 0 × 100 Where C0 is the initial concentration of benzyl alcohol, CBA and CBAD are the concentration of benzyl alcohol and benzaldehyde, respectively, at a certain time after the photocatalytic reaction.

As for the photocatalytic reduction of heavy ions Cr (VI) in water, the photocatalyst (30 mg) and ammonium oxalate (10 mg) was dispersed into 60 mL Cr (VI) aqueous solution (10 ppm) in a quartz reactor at room temperature under visible light irradiation by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) with UV-CUT filter (>420 nm). Before light was turned on, the solution was continuously stirred for 30 min in dark to ensure the establishment of an adsorption-desorption equilibrium. The suspension was magnetically stirred before and during the illumination. A 4 mL sample solution was drawn from the system at a certain time interval during the experiment. After the irradiation and removal of the catalyst particles by centrifugation, the residual amount of Cr (VI) in the solution was analyzed on the basis of its characteristic optical absorption at 371 nm, using UV-vis spectrometer (Cary-50. Varian Co.) to measure the change of Cr (VI) concentration with irradiation time based on Lambert-Beer’s law. The percentage of reduction is indicated as C/C0. Here, C is the absorption of Cr (VI) solution at each irradiation time interval of the main peak of the adsorption spectrum, and C0 is the absorption of the initial concentration when the adsorption-desorption equilibrium was achieved. Controlled experiment using K2S2O8 as scavenger for photogenerated electrons38 was carried out similar to the above photocatalytic reduction of Cr (VI) except that K2S2O8 (10 mg) was added to the reaction system.

3. Results and discussion 7



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Noble metal (Ag, Pt, and Pd) nanoparticles deposited on P25 with oxygen vacancies (P25-OV), denoted as M-P25-OV, have been synthesized by a facile two-step process, as illustrated in Scheme 1. At the first step, the commercial available Degussa P25 is dispersed in trifluorotoluene contained benzyl alcohol and is subsequently irradiated by UV light. During the photocatalytic reaction, the oxidation of benzyl alcohol on TiO2 surface occurs at the expense of a surface lattice oxygen atom, which results in the formation of a surface vacancy.19 Using the standard Kröger-Vink notation, the formation of oxygen vacancies can be described by the following equilibrium: hv C6 H 5CH 2OH + OO× → VO•• + C6 H 5CHO + 2e − + H 2O

(1)

Where OO× denotes the lattice oxygen; VO•• denotes the oxygen vacancies; and e − denotes the electron located on the oxygen vacancies states.

The presence of oxygen vacancies in the as-formed P25-OV sample is further supported by ESR spectroscopy (Figure S1, supporting information). Before UV-light irradiation, P25 sample does not contain any paramagnetic sites as evidenced by a flat line shown in Figure S1. Whereas after UV-light irradiation in the presence of benzyl alcohol, an intense signal at g=2.004 shows up, which can be assigned to the single electron trapped on the oxygen vacancy states.15 It should be noted that the oxygen vacancies formation does not accompany with the formation of Ti3+ defects (Figure 1a). As can be seen from the X-ray photoelectron spectra (Figure 1a), the Ti 2p signals are highly symmetric and no shoulders appeared at the lower energy side of the Ti 2p signals, suggesting that almost no Ti3+ defects are formed during the oxygen vacancies formation.8 This result is in good accordance with our previous report.19 In addition, the O 1s spectrum of P25-OV is also shown in Figure 1b, two characteristic peaks located at binding energy of about 529.8 and 531.6 eV are assigned to the Ti-O and hydroxyl species, respectively.8

When the noble metal ions are added dropwise and ultrasonically dispersed, they could be easily absorbed on the defective surface of P25-OV. After heat treatment, these colloidal species will grow into nanocrystals though oriented aggregation and Ostwald ripening, which is confirmed by our earlier work.30 In this process, the electrons located on the oxygen vacancy states are released and the adsorbed metal ions are reduced (Figure S2).30 Finally, the noble metal nanoparticles would be 8


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loaded onto the surface of P25-OV and simultaneously P25-OV is partially oxidized by the metal ions as evidenced by the decreased electron spin signal of P25-OV at g= 2.004 after noble metal deposition (Figure 2). Based on the above discussion, we express the noble metal reduction mechanism by the following reaction: M n+ + VO•• + ne − heat →  VO•• + M 0

(2)

The X-ray diffraction (XRD) patterns of the as-synthesized P25-OV and M-P25-OV (M=Ag, Pt, and Pd) samples are shown in Figure 3. It is obvious that the M-P25-OV nanocomposites with different metal loading exhibit similar XRD patterns. The peaks at 2θ values of 25.3, 37.8, 48.0, 53.9, 55.1, 62.7, 68.8, 70.3, and 75.0 can be indexed to (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes of anatase TiO2 in P25, respectively.4 In addition, characteristic diffraction peak at 27.4 is also observed, which is attributed to the (110) face of rutile TiO2 in P25.4 Notably, no typical diffraction peaks belonging to the Ag, Pt and Pd are observed in the M-P25-OV nanocomposites. The reason can be ascribed to the even distribution and low weight loading of metal nanoparticles on the surface of defective P25-OV.29

Figure 4 shows the UV-vis diffuse reflectance spectra (DRS) of the M-P25-OV nanocomposites. It is clearly seen that P25-OV sample exhibits considerably large absorption tail in the visible region, owing to the presence of oxygen vacancies.39 The presence of different noble metal affects the optical properties of light absorption for the M-P25-OV nanocomposites significantly. The addition of noble metal induces the increased light absorption intensity in the visible region, as observed in all of the M-P25-OV nanocomposites. Meanwhile, a red shift to higher wavelength in the absorption edge of M-P25-OV nanocomposites has also been observed, therefore indicating a narrowing of the band gap of P25-OV.

Transmission electron microscopy (TEM) combined with an energy dispersive X-ray spectroscopy (EDX) probe can simultaneously analyze the morphology and microscopic elements of the sample.27 TEM observations (Figure 5a, d and g) show that, in all the samples, P25 is displayed as nanoparticles with a diameter of ca.25 nm, which is the mix phase of anatase and rutile as confirmed by the above XRD analysis. The Ag, Pt and Pd nanoparticles ingredients are

9



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distributed on the surface of TiO2 nanoparticles with an intimate interfacial contact (Figure 5b, e and h). The average diameters of noble metal (Ag, Pt and Pd) estimated from Figure S3 are 12 nm, 6 nm and 2 nm, respectively. High-resolution TEM (HRTEM) images in Figure 5b, e and h unambiguously show the characteristic lattice fringes of 0.352 nm for TiO2 and 0.242, 0.227, 0.224 nm for Ag, Pt and Pd nanoparticles, which can be indexed as the (111) plane of face-centered cubic (fcc) structures of noble metal (Ag, Pt and Pd) nanoparticles, respectively.40-41 The energy-dispersive X-ray spectroscopy (EDX) has also been performed. The results (Figure 5c, f and i) reveal that the M-P25-OV nanocomposites contain the elements of Ti, O, and Ag/Pt/Pd. The weight ratios of noble metal in M-P25-OV are roughly estimated by the semiquantitative analysis of EDX. They are 1.9, 1.8 and 1.9 wt% for Ag-P25-OV, Pt-P25-OV and Pd-P25-OV nanocomposites, respectively.

In order to distinguish the valence states of the noble metal nanoparticles, X-ray photoelectron spectroscopy (XPS) is employed. The survey spectra (Figure S4a) of the M-P25-OV nanocomposites show the pronounced featured signal of Ag 3d, Pt 4f and Pd 3d, indicating that the noble metal particles are successfully deposited on the surface of the defective P25-OV. The high-resolution XPS spectrum confined to the Ag window (Figure S4b) presents the binding energies of the Ag 3d5/2 and Ag 3d3/2 peaks corresponding to 368.0 and 374.0 eV, respectively. Therefore, the Ag nanoparticles exist predominantly in metallic form.42 Similarly, the doublet with binding energies of 70.5 (Pt 4f7/2) and 73.9 eV (Pt 4f5/2) (Figure S4c) can be assigned to metallic Pt0 and characteristic peaks at 334.6 and 339.9 eV in the Pd 3d spectrum (Figure S4d) should be assigned to the binding energies of Pd 3d5/2 and Pd 3d3/2 of metallic Pd0.43-44 Therefore, the results of the TEM, EDX and XPS analysis together confirm that the noble metal (Ag, Pt and Pd) nanoparticles on the P25-OV surface exist predominantly in metallic form. In addition, the weight amount of the noble metal determined by XPS is ca. 2.2, 2.0 and 2.0 wt% for Ag-P25-OV, Pt-P25-OV and Pd-P25-OV nanocomposites, respectively (Table 1). There is a relative discrepancy between EDX and XPS characterization results due to the instrument errors.

In addition, the nitrogen adsorption-desorption isotherms by the as-prepared samples have also been performed. As shown in Figure 6, all these samples possess type IV isotherms with a typical 10


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H3 hysteresis loop characteristic of mesoporous solids,30 which is confirmed by the corresponding pore size distribution as shown in the inset of Figure 6. In order to clearly see the variations of P25-OV after noble metal deposition, the Brunauer-Emmett-Teller (BET) surface area, pore volume and average pore size of the samples obtained from nitrogen adsorption-desorption isotherms are summarized in Table S1. The results show that the BET surface areas, average pore sizes and pore volumes in the M-P25-OV nanocomposites are decreased, as compared to the P25-OV. This is mainly because a part of pores of P25-OV may be blocked by the loaded noble metal nanoparticles.30

The photocatalytic activities of P25-OV and M-P25-OV nanocomposites are evaluated using selective oxidation of benzyl alcohol as a probe reaction at room temperature and ambient pressure. Blank experiments (without photocatalyst or visible light) reveal negligible photocatalytic activities, verifying that the selective oxidation reaction is truly driven by a photocatalytic process. As can be seen from Figure 7, all these samples are visible-light-active for the photocatalytic selective oxidation of benzyl alcohol into benzaldehyde. It is obvious that the decoration of the noble metal particles can effectively improve the photocatalytic performance of P25-OV, which can be attributed to efficient photogenerated charge separation by the noble metal nanoparticles.45 Notably, the influences of noble metal deposition on the charge carriers separation process are two fold. Firstly, the noble metal deposition can decrease the concentration of oxygen vacancies, the charge recombination sites,19 on the defective P25-OV, as evidenced by the ESR analysis (Figure 2). Secondly, the noble metal can serve as electron sinks to promote the separation of photogenerated electron-hole pairs. In addition, it can be clearly seen that different kind of noble metal deposition results in remarkably different photoactivities of M-P25-OV. Among all of the samples, the Pd-P25-OV nanocomposite shows the best visible light photoactivity toward the selective oxidation of benzyl alcohol, which is almost 14 times as high as that of blank P25-OV. In contrast, the photocatalytic activity of Pt-P25-OV is 6 times as high as that of blank P25-OV while the photocatalytic activity of Ag-P25-OV is only 2 times as high as that of blank P25-OV.

To understand the origin of the remarkably different photoactivities of M-P25-OV after decorated with different kind of noble metals (Ag, Pt and Pd). The efficiencies of charge separation in 11



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M-P25-OV nanocomposites are examined by photoluminescence (PL) analysis, as shown in Figure 8. It can be seen that the intensity of the PL emission follows the order: P25-OV > Ag-P25-OV > Pt-P25-OV > Pd-P25-OV. Since a lower PL intensity is generally indicative of a lower recombination rate of photogenerated charge carriers, i.e. electron-hole pairs.46 Thus, we may conclude that the charge separation efficiency follows the order: Pd-P25-OV > Pt-P25-OV > Ag-P25-OV > P25-OV. Therefore, it seems that the observed photoactivity order, i.e., Pd-P25-OV > Pt-P25-OV > Ag-P25-OV > P25-OV, can be well correlated with the charge separation efficiency. The highest photoactivity of Pd-P25-OV is consistent with its highest charge separation efficiency. In addition, from the DRS spectra in Figure 4, the M-P25-OV shows much enhanced visible light absorption intensity over that of P25-OV. Particularly, the light absorption intensity of Pd-P25-OV is the highest among the M-P25-OV (M=Ag, Pt and Pd). Therefore, it is reasonable to observe the highest photocatalytic activity over Pd-P25-OV among M-P25-OV toward selective oxidation of benzyl alcohol under visible light irradiation. Notably, the surface areas of P25-OV, Ag-P25-OV, Pt-P25-OV and Pd-P25-OV are determined to be 52 m2/g, 50 m2/g, 47 m2/g and 46 m2/g, respectively (Table S1). For all these samples, the surface areas are quite close to each other. Therefore, it seems that the observed photoactivity order, i.e., P25-OV< Ag-P25-OV< Pt-P25-OV < Pd-P25-OV, can not be attributed to the difference of surface area. We conclude that the improved photoactivities of the M-P25-OV compared to P25-OV are attributed to the enhanced visible light absorption as well as efficient charge carriers separation by noble metal deposition.

The more efficient enhancement of fate of photogenerated charge carriers over M-P25-OV than P25-OV can also be verified by the photocatalytic reduction of Cr (VI) under visible light irradiation, which is a well known heavy metal ions pollutant in wastewater produced from electrolyte and other industrial processes.47 As shown in Figure 9, the M-P25-OV nanocomposites, as well as blank P25-OV, are all visible light photoactive for the reduction of Cr (VI) in water. Controlled experiment using K2S2O8 as quencher for photogenerated electrons confirms that the photoreduction of Cr (VI) is driven by the photogenerated electrons (Figure S5).38 Similar to the case of the photocatalytic selective oxidation of benzyl alcohol as discussed above, the deposition of noble metal can effectively promote the photocatalytic reduction process, as compared to blank P25-OV. It can be seen that the kinetic rate constants for photocatalytic reduction of Cr (VI) follow 12


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the order: Pd-P25-OV (0.0998 min-1) > Pt-P25-OV (0.0216 min-1) > Ag-P25-OV (0.0123 min-1) > P25-OV (0.0052 min-1). This result is in good accordance with the charge carriers separation efficiency as verified by the photoluminescence (PL) analysis. Thus, it is clear that the Pd nanoparticles with smaller particles size is much more favorable for the enhancement of P25-OV photoactivity for both selective oxidation of benzyl alcohol and reduction of Cr (VI) than that of noble metal (Ag and Pt) with larger particle size. This is probably due to the fact that the smaller particle size of noble metal is generally more favorable for charge carriers separation of photogenerated electron/hole pairs.24

Controlled experiments using different radicals scavengers are carried out to probe the reaction mechanism of selective oxidation of benzyl alcohol over Pd-P25-OV under visible light irradiation. As can be seen from Figure 10 (entry a), the addition of AgNO3 as a scavenger for photogenerated electrons significantly suppresses the photocatalytic reaction.35 A similar and obvious inhibition phenomenon for photocatalytic reaction is also observed when the scavenger of p-benzoquinone for superoxide radical37 and ammonium oxalate scavenger for photogenerated holes34 are added into the reaction system (see entries b and c in Figure 10, respectively). These results indicate that the photogenerated electrons, holes and superoxide radical are all involved in the photocatalytic oxidation of benzyl alcohol over Pd-P25-OV under visible light irradiation. In addition, when tert-butyl alcohol (TBA) as radical scavenger for hydroxyl radicals (•OH) is added, the conversion of benzyl alcohol is almost not changed (entry d in Figure 10), indicating that hydroxyl radicals are not involved in the photocatalytic reaction, which is in agreement with the absence of hydroxyl radicals in the solvent of trifluorotoluene (BTF).36

Base on the above results, a reaction mechanism for photocatalytic selective oxidation of benzyl alcohol to benzaledehyde over M-P25-OV is proposed as the following, which is schematically displayed in Figure 11. Upon the irradiation of visible light, the oxygen vacancy states can promote the visible light absorption and the generation of the photoexcited electron-hole pairs over the surface of TiO2.48 Subsequently, the decorated noble metal particles can accept the photogenerated electrons and prolong the lifetime of charge carriers. Therefore, the efficiency toward photocatalytic redox process is improved. The adsorbed benzyl alcohol in solution interacts with photogenerated 13



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holes to form corresponding aldehyde. In addition, the photo-generated electrons trapped by noble metal can also be captured by oxygen, which produce superoxide radicals, as evidenced by the ESR analysis (Figure 12). These oxygen containing species (O2 or O2˙-) then are able to selectively oxidize the benzyl alcohol, giving rise to the formation of benzaldehyde.

The photocatalytic testing on used Pd-P25-OV shows that this photocatalyst is stable and reusable in this reaction system. As displayed in Figure 13, during five times recycling photoactivity test for selective oxidation of benzyl alcohol, the conversion of benzyl alcohol over used Pd-P25-OV is similar to that over fresh Pd-P25-OV. To learn if there is change in bulk composition and surface defect structure after the photocatalytic oxidation of benzyl alcohol, the crystal structure and oxygen vacancy concentration of the fresh and used photocatalyst are investigated by the XRD and ESR techniques. As can be seen from Figure S6, the used Pd-P25-OV exhibit almost the same XRD patterns as the fresh Pd-P25-OV, suggesting almost no change occurred in the bulk phase composition. In addition, the ESR results, as shown in Figure S7, further demonstrate that the oxygen vacancy concentration on the Pd-P25-OV sample does not change during the photocatalytic reaction. The reason for the stabilized oxygen vacancies by noble metal deposition is attributed to the decreased concentration of free electrons on the defective P25-OV,15 which is already evidenced by the ESR analysis (Figure 2). Therefore, the as-prepared Pd-P25-OV sample is a stable visible light driven photocatalyst for selective oxidation of benzyl alcohol under ambient conditions.

4. Conclusion In conclusion, noble metal nanoparticles (Ag, Pt and Pd) have been successively deposited on the defective TiO2 with oxygen vacancies (TiO2-OV) via a facile and easily accessible route. During this process, defective TiO2-OV is first synthesized by a photocatalytic reaction, and then the noble metal nanoparticles are deposited on the surface of TiO2-OV via a redox reaction between reductive TiO2-OV and noble metal precursors without any reducing agents or stabilizing ligands. The resulting M-TiO2-OV (M= Ag, Pt and Pd) nanocomposites show enhanced photoactivities toward selective oxidation of benzyl alcohol and reduction of heavy metal ions Cr (VI) under visible light

14


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irradiation, as compared to blank TiO2-OV. The enhanced photoactivity lies crucially on the contribution role of noble metal particles acting as electron reservoir in prolonging the lifetime of photogenerated charge carriers. In particular, the Pd-TiO2-OV with average Pd particle size of 2 nm shows remarkably enhanced photoactivities than TiO2-OV, which can be attributed to its efficient charge carriers separation and enhanced visible light absorption. The reaction mechanism of the photocatalytic reaction over M-TiO2-OV is also proposed by using different radical scavengers and ESR analysis. It is hoped that our work can provide useful information and inform ongoing effort for synthesizing defective semiconductor based nanocomposites photocatalysts for solar energy conversion in heterogeneous photocatalysis.

Acknowledgement The support by the National Natural Science Foundation of China (21173045, 20903023), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), Program for Returned High-level Overseas Chinese Scholars of Fujian province, and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged. Supporting Information Available: ESR spectra of the P25 and P25 with oxygen vacancies (P25-OV); proposed mechanism of defect-mediated reduction of noble metal ions (Ag, Pt, Pd) over the P25 with oxygen vacancies (P25); typical TEM images of Ag-P25-OV, Pt-P25-OV and Pd-P25-OV; size distribution plots of Ag, Pt and Pd nanoparticles; X-ray photoelectron spectroscopy (XPS) survey spectra of the M-P25-OV (M= Ag, Pt and Pd) nanocomposites and high-resolution XPS spectra of Ag 3d, Pt 4f and Pd 3d; the surface area and porosity of the P25 with oxygen vacancies (P25-OV) and M-P25-OV (M=Ag, Pt and Pd) nanocomposites; controlled experiment with and without K2S2O8 as quencher for photogenerated electrons in photocatalytic reduction of Cr (VI) aqueous solution over the Pd-P25-OV nanocomposite under visible light irradiation (λ> 420 nm); XRD patterns of the original and used Pd-P25-OV catalysts after photocatalytic selective oxidation of benzyl alcohol under visible light irradiation; ESR spectra of 15



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the original and used Pd-P25-OV catalysts after photocatalytic selective oxidation of benzyl alcohol under visible light irradiation. Appendix: UV-vis diffuse reflectance spectra (DRS) of the P25, benzyl alcohol adsorbed P25 and P25 with oxygen vacancies (P25-OV); photocatalytic selective oxidation of benzyl alcohol to benzaldehyde under the visible light irradiation for 2 hours, over the P25 and P25 with oxygen vacancies (P25-OV); photocatalytic reduction of Cr(VI) over the P25 and P25 with oxygen vacancies (P25-OV) under visible light irradiation (λ> 420 nm). This material is available free of charge via the Internet at http://pubs.acs.org.

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Scheme 1. Schematic illustration for the synthesis of P25 with oxygen vacancies (P25-OV) and M-P25-OV (M= Ag, Pt and Pd).

(a)

470

(b)

Ti 2p Ti 2p3/2

Ti 2p1/2

465 460 Binding Energy (eV)

O1s Ti-O

Intensity (a.u.)

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


455 536

OH

534 532 530 Binding Energy (eV)

528

Figure 1. The X-ray photoelectron spectra (XPS) of Ti 2p (a) and O 1s (b) and their peak deconvolution of the P25 with oxygen vacancies (P25-OV).

21



Intensity (a.u.)

Pd-P25-OV Pt-P25-OV Ag-P25-OV

P25-OV

2.04

2.02

2.00 g Value

1.98

1.96

Pd-P25-OV R(110) A(004) A(200)

A(204)

A(116) A(220)

A(101)

A(105) A(211)

Figure 2. ESR spectra of the P25 with oxygen vacancies (P25-OV) and M-P25-OV (M= Ag, Pt and Pd).

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

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A(215)

Pt-P25-OV

Ag-P25-OV

P25-OV

10

20

30 40 50 60 2Theta (degree)

70

80

Figure 3. XRD patterns of P25 with oxygen vacancies (P25-OV) and M-P25-OV (M= Ag, Pt and Pd).

22


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1.6 Absorbance (a.u.)

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P25-OV Ag-P25-OV Pt-P25-OV Pd-P25-OV

1.2 0.8 0.4 0.0

300

400 500 600 700 Wavelength (nm)

800

Figure 4. UV-vis diffuse reflectance spectra (DRS) of P25 with oxygen vacancies (P25-OV) and M-P25-OV (M= Ag, Pt and Pd).

23



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Figure 5. TEM and HRTEM images of Ag-P25-OV (a and b), Pt-P25-OV (d and e) and Pd-P25-OV (g and h); EDX images of Ag-P25-OV (c), Pt-P25-OV (f) and Pd-P25-OV (i).

Table 1. The weight ratios of noble metals in the M-P25-OV (M=Ag, Pt and Pd) nanocomposites determined by the X-ray photoelectron spectroscopy (XPS) analysis. Sample Ag-P25-OV Pt-P25-OV Pd-P25-OV

Metal contents (wt%) 2.23 1.98 1.96

24


0.014


0.012

3

300

Pore volume(cm /g)

3

350

250 200 150

0.010 0.008 0.006 0.004 0.002 0.000

100

0

5 10 15 20 Pore diameter (nm)

50 0 0.0

0.2

0.4

0.6

25

0.8

1.0

Relative Pressure (P/P0)

Figure 6. The N2 adsorption-desorption isotherms of P25 with oxygen vacancies (P25-OV) and M-P25-OV (M= Ag, Pt and Pd) nanocomposites; inset is the corresponding pore size distribution.

70 Conversion & Yied (%)

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


Volume adsorbed (cm /g.STP)

Page 25 of 29

60

Conversion Yield Selectivity> 99%

50 40 30 20 10 0 P25-OV

Ag-P25-OV

Pt-P25-OV Pd-P25-OV

Figure 7. Photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over P25 with oxygen vacancies (P25-OV) and M-P25-OV (M= Ag, Pt and Pd) under the visible light irradiation for 2 hours.

25




Intensity (a.u.)

400

500 600 700 Wavelength (nm)

800

Figure 8. Photoluminescence (PL) spectra of P25 with oxygen vacancies (P25-OV) and M-P25-OV (M= Ag, Pt and Pd).

1.0

6

(a)

(b)

0.4 P25-OV Ag-P25-OV Pt-P25-OV Pd-P25-OV

0.2 0.0

0

5

Pd -P 25 -O V

0.08 0.06 0.04 0.02


Pt -P 25 -O V

0.10

Ag -P 25 -O V

0.6

0.12

P2 5OV

ln(C0/C)

4

(min -1 )

0.8

Rate Constant

0.14

C/C0

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

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0.00

2

0

10 15 20 25 Irradiation Time (min)

30

0

5

10 15 20 25 Irradiation Time (min)

30

Figure 9. Photocatalytic reduction of Cr (VI) over the P25 with oxygen vacancies (P25-OV) and M-P25-OV (M=Ag, Pt and Pd) nanocomposites under visible light irradiation (λ> 420 nm) (a), and the corresponding kinetic rate constant curve (b).

26


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70 60 Conversion (%)

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


50 40 30 20 10 0 (a)

(b)

(c)

(d)

(e)

Figure 10. Controlled experiments using different radical scavengers for the photocatalytic selective oxidation of benzyl alcohol over Pd-P25-OV in the trifluorotoluene (BTF) solvent; (a) reaction with AgNO3 as a scavenger for photogenerated electrons, (b) reaction with p-benzoquinone (BQ) as a scavenger for superoxide radicals, (c) reaction with ammonium oxalate (AO) as a scavenger for photogenerated holes, (d) reaction with tert-butyl alcohol (TBA) as a scavenger for hydroxyl radicals, and (e) the reaction in the absence of radical scavengers under visible light irradiation for 2 hours.

Figure 11. Schematic diagram of the proposed mechanism for selective oxidation of benzyl alcohol to benzaldehyde over the M-P25-OV (M=Ag, Pt and Pd) nanocomposites under the visible light irradiation.

27


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in the light in the dark

3500

3520 3540 Magnetic Field(G)

3560

Figure 12. ESR spectra of radical adduct trapped by DMPO (DMPO-O2·-) over the Pd-P25-OV nanocomposite suspension in the BTF solution without or with the visible light irradiation.

75 Conversion (%)

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

Intensity (a.u.)


60 45 30 15 0

st

1

nd

run 2

rd

run 3

th

run 4

run 5th run

Figure 13. Recycled testing of photocatalytic activity of Pd-P25-OV toward the selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation.

28


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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


TOC Graphic

29


Defect-Mediated Growth of Noble-Metal (Ag, Pt, and Pd) Nanoparticles

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