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Aerosol-Sprayed Gold/Ceria Photocatalyst with Superior Plasmonic Hot Electron-Enabled Visible-Light Activity Henglei Jia, Xiao-Ming Zhu, Ruibin Jiang, and Jianfang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15184 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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ACS Applied Materials & Interfaces

Aerosol-Sprayed Gold/Ceria Photocatalyst with Superior Plasmonic Hot Electron-Enabled VisibleLight Activity Henglei Jia,† Xiao-Ming Zhu,‡ Ruibin Jiang,§ and Jianfang Wang*,† †

Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

‡

State Key Laboratory of Quality Research in Chinese Medicine, Macau University of Science

and Technology, Avenida Wai Long, Taipa, Macau SAR, China §

Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and

Engineering, Shaanxi Normal University, Xi’an 710119, China

ABSTRACT: Integration of nanoscale plasmonic metals with semiconductors is a promising strategy for utilizing visible and near-infrared light to enhance chemical reactions. Here we report on the preparation of Au/CeO2 microsphere photocatalysts through aerosol spray and the study of their photocatalytic activity towards the aerobic oxidation of 1-phenylethanol under visible light. The microsphere catalysts exhibit a remarkable photocatalytic performance with their turnover frequency values reaching 108 h‒1, which is more than 23 times that of (Au core)@(CeO2 shell) nanostructures and much larger than those obtained previously for the visible-light photocatalytic oxidation of 1-phenylethanol. In addition, the Au/CeO2 catalyst

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shows the best performance among eight types of oxide semiconductor supports. Moreover, the photocatalytic mechanism of the Au/CeO2 catalyst is systematically investigated. This study offers insights for plasmonic hot electron-enabled photocatalysis, which will be valuable for the design of various efficient (plasmonic metal)/semiconductor photocatalysts.

Keywords: ceria, gold nanoparticles, mesoporous materials, metal oxides, photocatalysis, plasmon resonance, plasmonic photocatalysts, selective alcohol oxidation

1. INTRODUCTION The efficient conversion of solar to chemical energy has been considered as an environmentally friendly and energy-sustainable strategy in response to challenging energy demands and environmental issues.1,2 Photocatalysis, including photocatalytic water splitting, photodecomposition of organic pollutants and photosynthesis of organic molecules,3‒5 is of great potential in realizing this strategy. Semiconductor photocatalysts have proven promising in these reactions due to their chemical stability, nontoxicity and high reactivity. However, the wide band gaps of common semiconductor photocatalysts make them only absorb ultraviolet (UV) light, which accounts for only ∼5% of the total solar energy. In addition, semiconductor photocatalysts also face the challenge of rapid recombination of photogenerated electron‒hole pairs.6 In order to alleviate these inherent problems, a vast number of studies aiming on the increase of the visiblelight photocatalytic activity of semiconductor photocatalysts have been carried out, including metal or nonmetal atom doping, morphology tuning, and heterojunction formation.7,8 Recently, a growing number of experiments have been devoted to the design of photocatalysts by integrating semiconductors with plasmonic metals.9‒12 Nanoscale plasmonic metals, such as Au, Ag and Cu,


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possess localized surface plasmon resonance (LSPR), which can not only endow metal nanoparticles with large absorption cross-sections but also squeeze light in nanoscale regions.2,13 LSPR can dramatically enhance the photocatalytic activity of semiconductor photocatalysts under visible light once they are integrated with plasmonic metals, therefore bringing a bright prospect for photocatalysis. The uppermost attraction of plasmonic metal/semiconductor nanostructures is effective electron‒hole separation, which is crucial for various photocatalytic processes.1,14 When a plasmonic metal is in contact with an n-type semiconductor, a downward bending of the conduction band of the semiconductor will occur, resulting in the formation of a Schottky barrier. This barrier functions as a filter allowing for hot electrons with sufficient energy to pass across the metal/semiconductor interface, therefore resulting in effective electron‒hole separation.15 The incident photon-to-electron conversion efficiency has been estimated to be in excess of 22% for Au/TiO2 nanostructures.13 Such a high efficiency can dramatically enhance electron–hole separation, making plasmonic metal/semiconductor nanostructures an interesting alternative for photocatalysis. Moreover, to facilitate hot electron injection, proper alignment of the conduction band edge of the semiconductor with respect to the Fermi level of the plasmonic metal is required. Therefore, the electronic properties of both the plasmonic metal and the semiconductor need to be taken into account during the design of such photocatalysts. Apart from their electronic properties, the geometric arrangement of the two components in (plasmonic metal)/semiconductor nanostructures can also greatly affect their photocatalytic performances.16‒18 For example, the structural geometry of the hybrid photocatalysts has a great effect on the photocatalytic hydrogen evolution. Asymmetric Au/CdS heterodimers show a dramatic enhancement on the photocatalytic activity in comparison with concentric (Au


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core)@(CdS shell) nanostructures. The former exposes more surface sites of the Au nanoparticles, which facilitates the charge separation at the Au‒CdS interface.19 Regarding the offering of sufficient active sites, mesoporous structures are of great interest for photocatalysis due to their high specific surface areas.20‒22 The porous structure possesses a vast number of continuous channels, which is beneficial for the transfer of reactant and product molecules. In addition, the fabrication of the semiconductor component into superstructures can also remarkably improve the photocatalytic activity. For example, the decoration of Au nanoparticles on TiO2 mesocrystals has been found to remarkably improve the visible-light photocatalytic activity owing to effective charge separation caused by the well-oriented TiO2 nanocrystal network.23 Alcohol oxidation is one of the most widely exploited organic reactions due to its importance in both industrial syntheses and laboratory research.24‒27 Conventional catalysts, including chromates and permanganates, are environmentally unfriendly from the viewpoint of green and sustainable chemistry. When a plasmonic metal/n-type semiconductor photocatalyst is illuminated under visible light, hot electrons are generated and injected into the conduction band of the semiconductor, leaving hot holes in the metal nanocrystal. The holes possess a mild oxidation capability, which can be utilized for chemoselective oxidation of organic molecules. Au nanoparticles loaded at the interface of anatase/rutile TiO2 particles has been reported to be able to enhance aerobic oxidation reactions under visible light. Au nanoparticles function as the active sites in this architecture. They absorb visible light and generate hot electrons. Subsequent injection of hot electrons into TiO2 leaves hot holes in the Au nanoparticles, causing successful aerobic oxidation.28 Au/CeO2 samples prepared by multiple photodeposition exhibit larger reaction rates than those made by single-step photodeposition for selective oxidation of benzyl


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alcohol under illumination with a green light-emitting diode (LED), due to the strong light absorption of the former samples.29 We have previously synthesized (Au core)@(CeO2 shell) nanostructures and studied their synergistic effect on the photocatalytic activity towards the oxidation of benzyl alcohol.30 Although the core@shell nanostructures exhibit a relatively higher conversion rate in comparison with the corresponding uncoated Au nanoparticles and pure CeO2 nanoparticles, the turnover frequency (TOF) is still unsatisfactory. A possible reason is that the Au nanoparticles are buried inside, which makes the generated hot holes hardly accessible by alcohol molecules. Therefore, the synthesis of highly efficient plasmonic metal/semiconductor hybrid photocatalysts with readily available active sites has still remained attractive yet challenging. Although a number of plasmonic metal/semiconductor hybrid nanostructures with different morphologies and sizes have been reported, their applications as photocatalysts still suffer from several obstacles. Many of such nanostructures aggregate or disintegrate in acidic, alkaline, or organic solutions or at high temperatures. Most nanostructures can only be prepared in solutions under well-controlled conditions at low throughputs. In addition, even if photogenerated carriers possess thermodynamically suitable potentials for a photocatalytic reaction, they will recombine if they cannot take part in reactions on the surface of the photocatalyst. As a result, the photocatalyst will exhibit a low activity. Moreover, the working mechanism of (plasmonic metal)/semiconductor photocatalysts on the oxidation of alcohols, including the roles of hot electrons and the oxidation ability of holes, has still remained unclear. In this work, we prepared Au/CeO2 microsphere photocatalysts with different amounts of loaded Au though a facile, robust and high-throughput aerosol spray method. The Au/CeO2 photocatalysts possess large specific surface areas, which allows reactant and product molecules to readily diffuse in and out. In


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addition, the spectral matching of the light absorption property of the Au/CeO2 photocatalysts with visible light endows them with a superior visible-light photocatalytic activity. The photocatalysts exhibit a remarkable photocatalytic activity, with the TOF reaching 108 h‒1, which is ∼23 times that of (Au core)@(CeO2 shell) nanostructures. To the best of our knowledge, this TOF value is the highest for the photocatalytic oxidation of 1-phenylethanol under visiblelight illumination. Furthermore, we carried out an elaborate and systematic study on the photocatalytic mechanism of the Au/CeO2 sample. The possible reaction steps involving hot electrons and holes were determined.

2. EXPERIMENTAL SECTION 2.1. Synthesis of CeO2, Au/CeO2, Pt/CeO2 and Pd/CeO2 Microspheres. The microsphere samples were prepared according to our previously reported method with slight modification.31,32 To make the CeO2 sample, Ce(NO3)3 (2 mmol) was added to a solution prepared in advance by dissolving triblock copolymer (ethylene oxide)20‒(propylene oxide)70‒ (ethylene oxide)20 (P123, 0.25 g) in absolute ethanol (30 mL). After being stirred for 10 min, the resultant transparent solution was transferred to a household ultrasonic humidifier (1.7 MHz, 30 W) for aerosol spray. The generated liquid droplets (∼3‒8 µm) were carried by N2 into a glass tube that was placed in a 90-cm-long tube furnace set at 400 °C. The produced CeO2 microspheres were collected on a filter connected to a water aspirator. The preparations of the Au/CeO2, Pt/CeO2 and Pd/CeO2 samples were similar except that an appropriate amount of the metal salt (HAuCl4, H2PtCl6 or H2PdCl4) aqueous solution was added in the precursor solution. The names of the microsphere samples are defined according to the elemental molar ratio of the noble metal to Ce or other transition metals in the precursor solution. For example, 5 mol%


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Au/CeO2 refers to the molar ratio between Au and Ce being 5:100. Three Au/CeO2 samples (1, 2 and 5 mol% Au), one Pt/CeO2 sample (5 mol% Pt) and one Pd/CeO2 sample (5 mol% Pd) were prepared. The as-prepared CeO2, Au/CeO2, Pt/CeO2, and Pd/CeO2 samples were thermally calcined at 400 °C for 4 h at a heating rate of 1 °C min‒1. The 5 mol% Pt/CeO2 and 5 mol% Pd/CeO2 samples were further treated at 350 °C for 2 h under H2/N2 atmosphere (5/95, v/v) at a heating rate of 5 °C min‒1 to further induce the formation of Pt and Pd nanoparticles.32 2.2. Preparation of Au/Fe2O3, Au/Cr2O3, Au/CuO, Au/NiO, Au/ZrO2, Au/TiO2 (Anatase) and Au/TiO2 (Rutile) Microspheres. The preparations of the Fe2O3, Cr2O3, CuO, NiO, and ZrO2 samples that were doped with 5 mol% Au were similar to that of the 5 mol% Au/CeO2 sample. The corresponding metal nitrate salts, Fe(NO3)3, Cr(NO3)3, Cu(NO3)2, Ni(NO3)2 and ZrO(NO3)2, were utilized. P123 was employed as the surfactant for Au/CuO, while another triblock copolymer (ethylene oxide)106‒(propylene oxide)70‒(ethylene oxide)106 (F127) was used for the other four samples. The temperature of the furnace was preset at 500 °C for the preparation of Au/NiO, and at 400 °C for the others. The precursor solution for 5 mol% Au/TiO2 was made as follows. A mixture of titanium butoxide (Ti(OCH2CH2CH2CH3)4, 1 mL) and HCl (37 wt%, 3.2 mL) was first stirred for 2 h. The mixture was then added under stirring into a solution made in advance by dissolving F127 (0.6 g) in absolute ethanol (55.4 g). After being stirred for one more hour, HAuCl4 (1.43 mL, 0.1 M, in water) was added into the solution, followed by stirring for 10 more minutes. The as-prepared 5 mol% Au/Cr2O3, 5 mol% Au/CuO and 5 mol% Au/NiO samples were thermally calcined at 400 °C for 4 h at a heating rate of 1 °C min‒1. The calcination temperatures for the 5 mol% Au/Fe2O3 and 5 mol% Au/ZrO2 samples were 500 °C. In addition, the calcination temperatures for the 5 mol% Au/TiO2 (anatase) and 5 mol% Au/TiO2 (rutile) samples were 400 °C and 900 °C, respectively.


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2.3. Preparation of (Au Nanosphere Core)@(CeO2 Shell) Samples. The Au nanosphere samples were obtained from NanoSeedz. The (Au nanosphere core)@(CeO2 shell) nanostructures were prepared following our reported procedure.30 Three Au nanosphere samples of different diameters (2.0 mL, numbers of particles per mL: 22-nm nanospheres, 1.08 × 1013; 57-nm nanospheres, 4.92 × 1011; 140-nm nanospheres, 8.4 × 1010) were employed as the core. They were precipitated by centrifugation and redispersed into a cetyltrimethylammonium bromide solution (20 mL, 0.025 M). An ethylenediamine tetraacetic acid (EDTA)‒NH3 mixture solution (3.0 mL), which was prepared by adding ammonia solution (0.38 mL, 30 wt%) and EDTA (0.4 mmol) in water (40 mL), and an aqueous Ce(NO3)3 solution (0.3 mL, 0.1 M) were then added into the Au nanosphere solution and subsequently mixed by repeated gentle inversion for 1 min. The resultant solution was kept in an oven at 90 °C for 5 h. The obtained nanostructure samples were concentrated and then calcined at 400 °C for 4 h in air. The calcined core@shell nanostructure samples were redispersed into water for further use. 2.4. Photocatalytic Aerobic Oxidation of Alcohols under Visible Light. The photocatalytic aerobic oxidation reaction was performed in a transparent Pyrex test tube with an inner diameter of 1.5 cm and a length of 12 cm. For typical reaction runs, the Au/CeO2 photocatalyst (5 mg), 1-phenylethanol (1 mmol) and K2CO3 (3 mmol) were dispersed or dissolved in water (2 mL) by ultrasonication for 5 min. The test tube was then sealed with a rubber stopper with an inlet and an outlet. The solution was bubbled with O2 at a pressure of ∼1 atm for the entire reaction process. A continuous Xe lamp (300 W) with a 420-nm long-pass filter was utilized as the light source for illumination. The optical power density was 0.9 W cm‒2. After 10-h reaction, the catalyst was recovered by centrifugation and the amounts of 1phenylethanol, acetophenone, and other byproducts in the solution were analyzed using a high-


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performance liquid chromatography (HPLC) system (Waters 625 LC) equipped with a Waters 600 controller, a Waters 486 absorbance detector, and a C18 (250 mm × 4.6 mm, 4 µm particle size) analytical column (ACE-119 ACE-121). A mixture of acetic acid solution (5 mM, pH = 3.76) and acetonitrile (65:35, v/v) was used as the mobile phase. The diluted sample (15 µL) was injected into the analytical column at a flow rate of 1.0 mL min‒1 and monitored with a UV detector at 258 nm. The yield was determined using 1-phenylethanol and acetophenone as the external standards. We note that the yield represents the amounts of the reactant and product in the solution, because the catalyst can adsorb the reactant and product molecules. 2.5. Photocurrent Measurements. The photocurrent measurements were conducted in a three-electrode, single-compartment quartz cell using an electrochemical workstation (CHI 760E). The working electrode was fabricated by depositing the CeO2 or 5 mol% Au/CeO2 sample on a transparent fluorine-doped tin oxide (FTO) glass slide. Specifically, the calcined CeO2 or 5 mol% Au/CeO2 sample (10 mg) was dispersed in ethanol (1 mL). The suspension (40 µL) was drop-cast on the conductive surface (deposition area: ∼1 cm × 1 cm, ∼0.4 mg cm‒2) of a 1 cm × 2 cm FTO glass slide. The working electrode was first heated at 80 °C to evaporate the solvent and then calcined at 400 °C for 4 h to increase the adhesion. A Pt plate and a saturated calomel electrode served as the counter and reference electrodes, respectively. All measurements were carried out at room temperature in KOH electrolyte (80 mL, 0.1 M) that had been deoxygenated by purging with high-purity N2 gas for 30 min before measurement. The same Xe lamp with UV light cut off was utilized for illumination at 0.9 W cm‒2. The photocurrent was recorded as a function of time by chopping the light on and off at an interval of 20 s at the open circuit potential.


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2.6. Detection of Hydroxyl Radicals. The generation of •OH under visible-light illumination was examined by monitoring the fluorescence intensity with terephthalic acid (TA) as the probe molecule, because TA can react with •OH to generate a fluorescent product emitting around 425 nm. For detecting •OH, the catalyst (5 mg) was dispersed into a mixture solution (2 mL) containing 10 mM NaOH and 3 mM TA. The experiments were carried out at room temperature in Pyrex tubes (inner diameter: 1.5 cm, length: 12 cm) under stirring. The same Xe lamp with UV light cut off was utilized, with the optical power density adjusted at 0.9 W cm‒2. After 2-h illumination, the catalyst was recovered by centrifugation and the fluorescence spectrum of the supernatant solution was subsequently measured under excitation at 310 nm. 2.7. Photocatalytic Hydrogen Generation Reaction. The photocatalytic H2 generation reaction was used to study the oxidation ability of holes from the photocatalyst. The experiments were carried out in a Pyrex reaction cell connected to a closed gas circulation with an evacuation system. The same Xe lamp with UV light cut off was utilized for illumination, with its optical power density adjusted at 100 mW cm‒2. In a typical experiment, the photocatalyst (10 mg) was dispersed into a solution (50 mL) containing 5 vol% methanol or 1-phenylethanol as a hole sacrificial agent. Pt cocatalyst (5 wt%) was deposited by direct photoreduction of H2PtCl6 in the solution above. The H2 generation reaction was conducted at 12 °C under stirring. The amount of H2 was monitored using a gas chromatography (Techcomp GC7900) equipped with a thermal conductivity detector. High purity Ar gas was used as a carrier gas. 2.8. Characterization. Scanning electron microscopy (SEM) imaging was performed on an FEI Quanta 400 FEG microscope. Low-magnification transmission electron microscopy (TEM) imaging was carried out on an FEI Tecnai Spirit microscope operated at 120 kV. Highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging


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and elemental mapping were carried out on an FEI Tecnai F20 microscope equipped with an Oxford energy-dispersive X-ray analysis system. The diffuse reflectance spectra of the powder samples were measured on an Ocean Optics USB4000 UV/visible/near-infrared spectrophotometer. The extinction spectra of the solution samples were taken on a Hitachi U3501 UV/visible/near-infrared spectrophotometer with 1.0-cm quartz cuvettes. X-ray diffraction (XRD) patterns were acquired on a Rigaku SmartLab diffractometer equipped with Cu Kα radiation. N2 adsorption-desorption isotherms were measured using a Micromeritics Tristar II 3020 system. Electron paramagnetic resonance (EPR) spectra were carried out on a Bruker EMX EPR Spectrometer (Billerica, MA). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed on a Perkin Elmer Optima 4300DV system. Fluorescence spectra were taken on a Hitachi F-4500 spectrofluorometer.

3. RESULTS AND DISCUSSION The CeO2 and Au/CeO2 microsphere samples were synthesized using an aerosol spray method followed by calcination at 400 °C in air.31‒33 Au nanoparticles in different amounts (1 mol%, 2 mol% and 5 mol% Au/CeO2) can be readily introduced into the CeO2 microspheres by altering the molar ratio of HAuCl4 to Ce(NO3)3 in the precursor solution. The structure of the Au/CeO2 sample is schematically illustrated in Figure 1a, where Au nanoparticles (yellow) are randomly loaded both on the surface of and inside the CeO2 microsphere (pink). The typical SEM images of the obtained samples are displayed in Figure 1b‒e. The CeO2 particles possess a spherical shape with diameters ranging from ∼200 nm to ∼2 µm in the four samples. The loaded Au nanoparticles in the CeO2 microspheres can be readily observed by HAADF-STEM imaging (Figure 1f) and elemental mapping (Figure 1g‒j). With the increase of the Au amount from 1


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mol% to 5 mol%, the sizes of the Au nanoparticles roughly increase from 15 nm to 21 nm. The increases in the Au amount and nanoparticle size are also reflected vividly from the colors of the as-prepared samples, which change from light yellow to pink and then purple (Figure 1k).

Figure 1. Mesoporous Au/CeO2 microsphere samples. (a) Schematic of a mesoporous Au/CeO2 microsphere. (b‒e) SEM images of the calcined CeO2, 1 mol% Au/CeO2, 2 mol% Au/CeO2 and 5 mol% Au/CeO2 samples, respectively. (f) HAADF-STEM image of the 5 mol% Au/CeO2 sample. (g) HAADF-STEM image of a single 5 mol% Au/CeO2 microsphere. (h–j) Elemental maps of Ce, O and Au on the microsphere shown in (g), respectively. (k) Photograph of the asprepared CeO2 (top-left), 1 mol% Au/CeO2 (top-right), 2 mol% Au/CeO2 (bottom-left), and 5 mol% Au/CeO2 (bottom-right) samples. The diameter of the circular area containing each sample is ∼4.5 cm. (l) HAADF-STEM image of a single 5 mol% Au/CeO2 microsphere at high magnification.

The possession of high specific surface areas by CeO2 nanostructures is a crucial factor for their catalytic performance. In addition, CeO2 has been widely exploited in organic oxidation reactions due to its high oxygen storage capacity.34,35 The Brunauer-Emmett-Teller (BET) surface areas and Barrett-Joyner-Halenda (BJH) pore size distributions of the CeO2 and Au/CeO2 microsphere samples were characterized by N2 adsorption‒desorption measurements (Figures


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S1‒S4, Supporting Information). All of the four samples are mesoporous. They exhibit similar adsorption‒desorption isotherms and pore size distributions. Their BET specific surface areas are 178, 175, 176 and 164 m2 g‒1 and their BJH pore sizes are 5.5, 5.6, 5.7, and 5.6 nm, respectively. These results suggest that the loading of Au nanoparticles does not affect the mesoporosity of the CeO2 microspheres. The mesopores can also be clearly seen on the high-magnification HAADFSTEM images taken on a single 5 mol% Au/CeO2 microsphere (dark spots on the image in Figure 1l). The high porosity of these samples allows for reactant and product molecules to diffuse into and out of the CeO2 microspheres. Moreover, the intimate contact of Au nanoparticles with CeO2 facilitates charge transfer across the interface. Therefore, our Au/CeO2 samples are a good candidate for studying plasmonic photocatalysis by use of organic oxidation reactions. XRD and diffuse reflectance measurements were performed to characterize the crystalline phase and light absorption property of the obtained CeO2 and Au/CeO2 samples. The diffraction peaks of the CeO2 sample (Figure 2a) can be indexed according to the cubic structure of CeO2 (space group Fm‒3m, lattice constant 0.5412 nm). The crystallinity of the CeO2 microspheres after the loading of Au nanoparticles is maintained. In addition, the diffraction peaks of the Au nanoparticles can be indexed according to the cubic structure of Au (space group Fm‒3m, lattice constant 0.4068 nm). With the increase of the Au amount, the diffraction peaks of Au become stronger. With Au loading, a broad absorption peak attributed to the LSPR of the Au nanoparticles appears around 555 nm (Figure 2b). In addition, the absorption peak gets stronger with the increase in the Au amount. Notably, the light absorption peak of the mesoporous Au/CeO2 samples coincides spectrally with the emission peak of the Xe lamp used in our


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experiments (Figure S5, Supporting Information). This coincidence is beneficial for the absorption of light from the Xe lamp by these samples and therefore for photocatalytic reactions.

Figure 2. Structure and light absorption of the Au/CeO2 samples. (a) XRD patterns of the CeO2, 1 mol% Au/CeO2, 2 mol% Au/CeO2 and 5 mol% Au/CeO2 samples, respectively. (b) Corresponding diffuse reflectance spectra. The green and red curves in (a) are the standard powder diffraction patterns of Au and CeO2.

The photocatalytic activities of the mesoporous microsphere samples were evaluated using the aerobic oxidation reaction of 1-phenylethanol (Scheme 1), which is one of the most


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important and fundamental reactions in both industrial syntheses and laboratory research. The reaction was performed by stirring an aqueous solution (2 mL) containing 1-phenylethanol (1 mmol), K2CO3 (3 mmol) and the catalyst (5 mg) under the visible-light illumination with the Xe lamp (Figure S5, Supporting Information). For the convenience of light harvesting, the illumination light was focused to a spot at a diameter of 2 cm using a lens, with an optical power density of 0.9 W cm‒2. The solution was bubbled with O2 at a pressure of 1 atm throughout the reaction process. To study the effect of the Au amount on the photocatalytic activity, the photocatalytic performances of the CeO2 and three Au/CeO2 samples were all evaluated using this reaction (Table 1). The CeO2 sample is inactive for this reaction under the visible light (Table 1, entry 1). In addition, when the amount of loaded Au is low at 1 mol%, no detectable product is obtained in the solution (Table 1, entry 2). This can be attributed to the small amount of Au nanoparticles or the weak light absorption. The photocatalytic performance is greatly increased when the Au amount is increased to 2 mol% and above (Table 1, entries 3 and 4). 1phenylethanol is almost completely transformed into acetophenone at 5 mol% of Au (Table 1, entry 4). We note that no by-products were detected in this reaction system. So the selectivity of the Au/CeO2 photocatalyst is higher than 99%.

Scheme 1. Photocatalytic Aerobic Oxidation of 1-Phenylethanol to Acetophenone

To ascertain the interaction between Au and CeO2, various control experiments were carried out (Table 2). No products were obtained under the illumination of the Xe lamp in the


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absence of the photocatalyst (Table 2, entry 1) or in dark with 5 mol% Au/CeO2 (Table 2, entry 2). For comparison, the photocatalytic activities of a 22-nm Au nanosphere sample (Table 2, entry 3 and Figure S6, Supporting Information) and the mixture of the 22-nm Au nanosphere sample with the CeO2 microsphere sample (Table 2, entry 4) were evaluated. The masses of the Au nanosphere and CeO2 samples were adjusted to be the same as those of the Au and CeO2 ingredients in the 5 mol% Au/CeO2 sample by use of ICP-OES. The average diameter, 22 nm, of the used Au nanosphere sample is close to that of the Au nanoparticles in the 5 mol% Au/CeO2 sample, which is 20.8 ± 3.7 nm. We note that the Au nanospheres are unstable, tending to aggregate in the 1-phenylethanol solution. For better dispersion, an aqueous cetyltrimethylammonium bromide solution (150 µL, 0.1 M) was added as the stabilizing agent. The yields are only 0.4% and 1%, much smaller than that of 5 mol% Au/CeO2. This result reveals that the high photocatalytic activity of the 5 mol% Au/CeO2 catalyst is derived not only from the stability of the Au nanoparticles supported on the CeO2 microspheres, but also from the synergistic effect between the Au nanoparticles and mesoporous CeO2 microspheres.

Table 1. Photocatalytic Activities of the CeO2 and Three Au/CeO2 Samples towards the Oxidation of 1-Phenylethanola

a

entry catalyst

Au amount (µmol) yield (%)b

1

CeO2

0

2

1 mol% Au/CeO2 27

∼0

3

2 mol% Au/CeO2 55

26

4

5 mol% Au/CeO2 137

94

0

Reaction conditions: 1-phenylethanol (1 mmol), K2CO3 (3 mmol), photocatalyst (5 mg), H2O (2

mL), O2 (1 atm), 10-h visible-light illumination (420 nm < λ < 780 nm) with the Xe lamp at 0.9


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W cm‒2. bThe yield was calculated by multiplying the conversion with the selectivity, which were determined by HPLC.

Table 2. Comparison of the Photocatalytic Activities of the Various Samples under Different Conditionsa

a

entry

catalyst

yield (%)

1

none

0

2b

5 mol% Au/CeO2

0

3

22-nm Au nanospheres

0.4

4

mixture of 22-nm Au nanospheres 1 with the CeO2 microspheres

5c

5 mol% Au/CeO2

0

6

uncalcined 5 mol% Au/CeO2

39.5

7d

5 mol% Au/CeO2

70

8e

5 mol% Au/CeO2

36.4

9

5 mol% Pt/CeO2

0.4

10

5 mol% Pd/CeO2

0.3

11f

5 mol% Au/CeO2

5

12

22-nm Au nanosphere@CeO2

6

13


4.3

14


6.3

15g

5 mol% Au/CeO2

74

Typical reaction conditions: 1-phenylethanol (1 mmol), K2CO3 (3 mmol), photocatalyst (for the

microsphere catalysts: 5 mg; for the Au nanosphere and core@shell catalysts: the amount of Au was adjusted to be the same as that of the 5 mol% Au/CeO2 catalyst), H2O (2 mL), O2 (1 atm), 10-h visible-light illumination (420 nm < λ < 780 nm) with the Xe lamp at 0.9 W cm‒2. bIn dark.


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c

f

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Under Ar atmosphere. dIn air without bubbling O2. eLED as the light source at 16 mW cm‒2.

Water bath at 40 °C in dark. g2 mmol 1-phenylethanol.

To determine whether the alcohol molecules are directly oxidized by the Au/CeO2 sample or by ambient O2 with Au/CeO2 as the catalyst, a control experiment with 5 mol% Au/CeO2 under Ar was performed (Table 2, entry 5). No reaction occurred in Ar, which underscores the essential role of O2 in this oxidation reaction. We also compared the photocatalytic activities of the 5 mol% Au/CeO2 sample before and after calcination (Table 2, entry 6, Figures S7 and S8, Supporting Information). The yield with the as-prepared sample as the photocatalyst is 39.5%, much less than 94% for the calcined sample. There are several probable reasons for this result. First, the specific surface area is largely improved by removing the surfactant upon calcination. The inter-connected pore channels make the surface sites on Au and CeO2 more readily accessible to the reactant molecules. Second, calcination improves the crystallinity of both Au and CeO2 (Figure S7, Supporting Information) and therefore reduces the concentration of defect sites on the solid catalyst. The reduced defect concentration can retard the recombination of photogenerated charge carriers and enhance electron‒hole separation and their respective migration to activity sites for interaction with the reactant molecules. Third, calcination removes the organic species that are probably existent at the interface between Au and CeO2. The formation of an intimate junction can facilitate hot electron injection. Furthermore, from the perspective of green chemistry, we carried out the reaction directly in air without O2 bubbling (Table 2, entry 7). A good photocatalytic performance was also obtained. To show the performance of our photocatalysts at low optical power densities, a green LED with an optical power density of 16 mW cm‒2 (Figure S5, Supporting Information) was also tested as the light


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source (Table 2, entry 8). 36.4% of the reactant was transformed into the product under the illumination of the green LED. We carried out additional control experiments to examine the photocatalytic activities of different noble metal nanoparticles. Pt and Pd were similarly loaded into the CeO2 microspheres (Figure S9, Supporting Information). The calcined 5 mol% Pt/CeO2 and 5 mol% Pd/CeO2 samples were further treated at 350 °C for 2 h under H2/N2 atmosphere (5/95, v/v) to induce the formation of Pt and Pd nanoparticles. However, the Pt and Pd nanoparticles are too small to be observed under SEM imaging, which is consistent with our previous experiments.32 The Pt/CeO2 and Pd/CeO2 samples showed nearly no activity for the reaction (Table 2, entries 9 and 10). This result can be ascribed to either that the LSPR of the Pt and Pd nanoparticles is too weak to cause strong light absorption36 or that the photocatalytic activity is dependent on the metal type. Plasmonic photothermal conversion can also accelerate chemical reactions. The temperature of the reaction solution under the illumination of the Xe lamp is ∼40 °C. We therefore carried out the reaction in a water bath set at 40 °C in the absence of the light illumination (Table 2, entry 11). The yield obtained under this condition is 5%, which shows that the reaction cannot be simply accelerated by the plasmonic heating of the catalyst. To further ascertain the contributions of plasmonic photothermal heating and hot electron injection, we prepared three (Au core)@(CeO2 shell) nanostructures with Au nanospheres of different diameters as the core (Table 2, entries 12‒14, Figure S10, Supporting Information). In these nanostructures, the plasmonic photothermal heating effect is maintained, but electron‒hole separation is suppressed, because the complete coating of the Au nanospheres by CeO2 hinders plasmon-generated holes from being accessed by the reactant molecules. The low yields of these three core@shell nanostructure samples further confirm that the contribution of plasmonic


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photothermal heating to the reaction is low. We will further discuss the results of the core@shell nanostructures below when the reaction mechanism involving the roles of hot electrons and holes is proposed. The oxide support plays an important role in (plasmonic metal)/semiconductor nanostructure photocatalysts. Not only can it stabilize plasmonic nanoparticles and shuttle electron transfer, but also its surface acid-base properties have important impact on the catalytic activity.1 For example, the exposed surfaces of metal oxides usually possess broken metal‒ oxygen bonds and unsaturated sites. In the catalytic process, these sites are regarded as acid and base sites, which can facilitate the absorption of reactant molecules. The acidic property depends on the charge and radius of the metal cation, while the basic property is related to the ionic character of the metal‒oxygen bond. To examine the influence of the chemical nature of the oxide support on the photocatalytic activity, we prepared seven more mesoporous Au/oxide microsphere samples through aerosol spray. They were all characterized by SEM and XRD (Figures S11‒S18, Supporting Information). The oxides are Fe2O3, Cr2O3, CuO, TiO2 (anatase), TiO2 (rutile), NiO and ZrO2. The amounts of Au in these samples are all 5 mol% relative to the molar numbers of the metals of the oxides. The photocatalytic activities of these seven samples were evaluated and compared with that of 5 mol% Au/CeO2. To ease comparison, we defined the TOF as the molar number of the product acetophenone per mole of Au atoms in the oxide microspheres per unit time. Of these eight types of the photocatalysts, the Au/CeO2 sample delivers the highest TOF of 108 h‒1 (Figure 3). We note that in order to improve the accuracy of the TOF of the 5 mol% Au/CeO2 sample, the conversion yield of the reaction with 2 mmol 1phenylethanol (Table 2, entry 15) was employed.


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Figure 3. Comparison of the photocatalytic activities of the samples with Au nanoparticles supported on the different oxides. The TOF values for the different samples listed along the horizontal axis from left to right are 108, 56, 24, 19, 13.7, 12, 1.7 and 1.7 h‒1, respectively.

Our highest TOF value is ∼4−800 times those obtained in the previous works for the photocatalytic oxidation of 1-phenylethanol performed with different visible-light sources (Table S1, Supporting Information).28,29,37‒42 We would point out that this is just a simple comparison, because the reaction conditions in these works are different. However, 1-phenylethanol has been employed as the substrate for visible-light photocatalytic reactions in all of these previous works. The large TOF value suggests a high catalytic activity of out photocatalysts. The activities of the catalysts with the different oxide supports are in the order of CeO2 > Fe2O3 > Cr2O3 > CuO > TiO2 (anatase) > NiO > TiO2 (rutile) > ZrO2. There are four probable reasons for the high activity of the Au/CeO2 sample. First, the band structure is an important factor for the activity of (plasmonic metal)/semiconductor photocatalysts. A low Schottky barrier between the Femi level of Au and the conduction band edge of the semiconductor can benefit the injection of hot electrons. CeO2, Fe2O3, TiO2 (anatase)


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and TiO2 (rutile) are n-type semiconductors. The Schottky barriers between Au and them are approximately 0.8, 0.52, 1.09, 0.89 eV, respectively.30,43,44 Fe2O3 seems to be the most suitable material as the support. However, its low electron mobility results in a lower photocatalytic activity than CeO2.45 In addition, rutile TiO2 is prepared by thermally treating the as-prepared sample at 900 °C. The high treatment temperature causes the enlargement of TiO2 nanocrystals and the destruction of the mesoporous structure.46 Cr2O3, CuO and NiO are p-type semiconductors. Hot electrons cannot inject into the conduction band of the oxide support. Although hot holes might be generated upon plasmon excitation, they are difficult to inject into the valence band of the oxide support due to the large potential barrier between Au and the oxide for holes. Even if hot holes can inject into the valence band of the oxide support, they are difficult to participate in the reaction due to the small hole mobilities in these oxides. ZrO2 is an insulator. Both hot electrons and hot holes are difficult to inject into this oxide. Therefore, CeO2 is the most preferable candidate for hot-electron-induced plasmonic photocatalysis. Second, nanocrystalline CeO2 possesses stoichiometric oxidation sites for alcohols.47 Cerium alkoxide can form at these sites, which facilitates the transfer of photogenerated charge carriers and accelerates the transformation of the reactant molecules. Third, the high oxygen storage capacity makes ceria-based nanomaterials attractive for oxidation reactions.35,48‒50 Fourth, the BET surface area of the CeO2 microspheres is the second largest among the thirteen types of oxide microspheres we prepared previously through aerosol spray.31 The large surface area is advantageous for molecular transport and access to the active sites on the Au nanoparticles and oxide support. As a result, the integration of Au nanoparticles with CeO2 support offers the best photocatalytic activity towards the oxidation reaction of 1-phenylethanol. In addition to the factors mentioned above, the difference in the light absorption bands of the oxides relative to the


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LSPR band of the Au nanoparticles as well as the difference in the surface reactivities of the oxides towards the aerobic oxidation of 1-phenylethanol can also affect the photocatalytic activities of the Au nanoparticle-loaded oxide samples. For example, Fe2O3 is active under visible light. Its photocatalytic activity can be increased through plasmon-enhanced light absorption. In addition, the Au/Cr2O3 sample exhibits a good photocatalytic activity because Cr2O3 itself is a good catalyst for the oxidation of alcohols. To examine the recyclability of the photocatalyst, three successive cycles of the photocatalytic reaction were conducted with the 5 mol% Au/CeO2 microspheres (Figure S19, Supporting Information). After each run of the reaction, the catalyst was precipitated by centrifugation and washed with 0.5 M NaOH solution twice to remove possibly adsorbed molecules.47,51 Although the yield decreases slightly in each cycle, there is still ∼70% yield after three cycles. One factor causing the decrease in the yield might be the loss of the catalyst during centrifugation. To expand the applicability of our catalyst, photocatalytic oxidation of different alcohols were conducted under the same conditions as that for 1-phenylethanol (Table 3). The 5 mol% Au/CeO2 sample also exhibits an excellent activity in photocatalytic oxidation of these alcohols to corresponding ketones, suggesting the capability of our photocatalyst in the oxidation of different alcohols.

Table 3. Photocatalytic Aerobic Oxidation of Different Alcohols with the 5 mol% Au/CeO2 Sample under Visible Lighta entry 1

alcohol substrate

yield (%) 99


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a

2

95

3

93

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Typical reaction conditions: alcohols (1 mmol), K2CO3 (3 mmol), the photocatalyst (5 mg), H2O

(2 mL), O2 (1 atm), 10-h visible-light illumination (420 nm < λ < 780 nm) with the Xe lamp at 0.9 W cm‒2.

Although the mechanism for hot-electron injection has been discussed in many studies for (plasmonic metal)/semiconductor photocatalysts, a number of important questions have still remained unanswered for the photocatalytic oxidation of alcohols. For example, which reaction steps do plasmon-generated hot electrons and holes take part in? What are the species formed in the intermediate steps? What is the oxidation ability of hot holes? How do 1-phenylethanol molecules interact with the intermediate active oxygen species? In this context, we carried out extensive studies in order to elucidate the fundamental photocatalytic mechanism of the Au/CeO2 catalyst in the photocatalytic oxidation of 1-phenylethanol. Our study focused mainly on what roles hot electrons and holes play in the photocatalytic reaction. To investigate the role of hot electrons, we first verified hot-electron injection by detecting photocurrent at the open-circuit potential.52 Figure 4a shows the anodic photocurrents of the working electrodes fabricated out of the CeO2 and 5 mol% Au/CeO2 samples, respectively. The working electrode was illuminated with visible light from the Xe lamp. The photocurrent was recorded as a function of time while the light was chopped on and off at the open-circuit potential. Photocurrent measurements can reveal the transfer direction of hot charge carriers. An anodic photocurrent indicates that the generated hot electrons flow from the sample to the FTO


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electrode. The 5 mol% Au/CeO2 electrode is sensitive to the visible light. A significant enhancement in the photocurrent is detected once the light is turned on. On the contrary, the CeO2 electrode is insensitive to the visible light. The photocurrent is barely detected above the noise as the light is turned on. This result suggests that the improved photocatalytic activity of the Au/CeO2 sample originates from plasmon-mediated hot-electron injection.


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Figure 4. Control experiments for understanding the photocatalytic reaction mechanism. (a) Time-dependent photocurrents of the CeO2 and 5 mol% Au/CeO2 samples measured at the opencircuit potential under visible light. (b) EPR spectra of •O2‒ generated by the CeO2 (left) and 5 mol% Au/CeO2 (right) samples upon the visible-light illumination in air for different periods of time. The spectra for both samples are drawn at the same intensity scale. (c) Fluorescence spectra of the different sample solutions containing TA recorded after the visible-light illumination. (d) Dependence of the yield of acetophenone on the added amount of the •OH inhibitor. (e) Comparison of the photocatalytic activities of the mesoporous Au/CeO2 microspheres with the three (Au core)@(CeO2 shell) nanostructure samples. (f) Time-dependent H2 evolution amount with the 5 mol% Au/CeO2 sample under the visible-light illumination, where methanol and 1phenylethanol were used as the hole sacrificial agent, respectively. Pt (5 wt%) was deposited as a co-catalyst in the H2 evolution experiments by direct photo-reduction of H2PtCl6 in the solution.

Once hot electrons are injected from the Au nanoparticles into the conduction band of CeO2, they can be trapped by O2, causing the generation of free radical intermediates, such as superoxide anion radicals (•O2‒) and hydroxyl radicals (•OH). To check if •O2‒ can be formed in our system, low-temperature EPR measurements were conducted to detect spin-active •O2‒ with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap in aqueous solutions under the visiblelight illumination in air.53 The characteristic four-line spectrum of the adduct between DMPO and •O2‒ with an intensity ratio of 1:1:1:1 becomes clearer with increaing illumination time for the 5 mol% Au/CeO2 sample (Figure 4b). In contrast, the signals are very weak for the CeO2 sample. This result suggests that hot electrons generated on the 5 mol% Au/CeO2 sample are readily trapped by O2 to produce •O2‒ under visible light. The generated •O2‒ can subsequently


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react to give other free radical intermediates, such as •OH, which has also been considered to play an important role in photocatalytic reactions. To clarify the role of •OH in the oxidation of 1-phenylethanol, we next detected its generation and studied its effect on the photocatalytic activity. First, we detected the generation of •OH with TA as the probe molecule, because TA can react with •OH to produce 2hydroxyterephthalic acid, which fluoresces at ~425 nm.54 The measurement was performed with the 5 mol% Au/CeO2 sample. The CeO2 microsphere sample, the 22-nm Au nanosphere sample, and the mixture of the 22-nm Au nanosphere and CeO2 microsphere samples were also tested for comparison. The masses of the Au nanosphere and CeO2 microsphere samples were adjusted by ICP-OES to be equal to those of Au and CeO2 in the 5 mol% Au/CeO2 sample, respectively. The sample solutions were illuminated with the visible light from the Xe lamp. As shown in Figure 4c, there is almost no •OH generation in dark or in the absence of the catalyst. In addition, weak fluorescence signals are detected for the Au nanosphere, CeO2 microsphere and their mixture samples. In contrast, a strong fluorescence peak is observed for the 5 mol% Au/CeO2 sample. The fluorescence intensity of the 5 mol% Au/CeO2 sample is ∼4.5 times that of the CeO2 sample and 28 times that of the Au nanosphere sample, indicating the effective generation of •OH by the Au/CeO2 sample under visible-light illumination. Second, we studied the effect of •OH on the photocatalytic activity. Dimethyl sulfoxide (DMSO) can be readily oxidized by •OH into a stable compound, methanesulfinic acid (CH4O2S).55 In our study, we used DMSO as the inhibitor to quench •OH generated in the photocatalytic reaction. If •OH is involved in the reaction, the photocatalytic activity of the Au/CeO2 sample will be suppressed in the presence of DMSO. Three different amounts of DMSO (0.5, 2, 5 mmol) were added into the reaction with 5 mol% Au/CeO2 as the photocatalyst.


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A yield of 34% is obtained in the absence of DMSO after 3-h visible-light illumination. In contrast, the yield drops to 21%, 7.3%, and 2.1% as the DMSO amount is increased, respectively (Figure 4d). This result clearly shows that •OH participates in the photocatalytic process and that its amount in the solution affects the photocatalytic activity. On the other hand, hot holes are left in the Au nanoparticles after the injection of hot electrons into CeO2. The sluggish kinetics of hot holes has a great effect on the photocatalytic performance of plasmonic catalysts.56 We next studied the role of holes in the photocatalytic activity and their oxidation ability towards 1-phenylethanol. To ascertain the role of hot holes in the reaction, we prepared three types of (Au core)@(CeO2 shell) nanostructures with differently sized Au nanospheres and compared their photocatalytic activities with that of the 5 mol% Au/CeO2 microsphere sample (Figure 4e and Figure S10, Supporting Information). In these core@shell nanostructures, the electron‒hole separation and hot-electron injection mechanisms are similar to those in the Au-loaded mesoporous microspheres under visible light illumination. The difference lies in that holes left in the Au core in the core@shell nanostructures can hardly interact with the reactant molecules. If hot holes are involved in the reaction, the photocatalytic activity of the core@shell nanostructures will be suppressed. As shown in Figure 4e, the photocatalytic activities of these core@shell nanostructures are very low in comparison with that of the mesoporous microsphere sample. The TOF of the 5 mol% Au/CeO2 sample is more than 23 times those of the core@shell nanostructures, suggesting that the accessibility of hot holes in the Au nanoparticles by the reactant molecules is crucial for the photocatalytic activity. The analysis above shows clearly the participation of holes in the Au nanoparticles in the photocatalytic reaction. Another possible reason for the low photocatalytic activity of the core@shell nanostructure samples is their red-shifted plasmon bands. The plasmon resonance


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peaks of the three core@shell nanostructure samples are located at 576, 602 and 721 nm, respectively. These plasmon peaks are away from the center of the emission spectra of the Xe lamp, which reduces the photocatalytic activities of the core@shell nanostructure samples. Our study above indicates that holes are somehow involved in the photocatalytic reaction of 1-phenylethanol. However, the oxidation ability of hot holes in the Au nanoparticles and the oxidation product remain still unclear. To tackle these questions, we performed photocatalytic H2 evolution measurements using the 5 mol% Au/CeO2 sample under the visible light illumination (Figure 4f). Either methanol or 1-phenylethanol was utilized as the hole sacrificial agent. If hot holes in the Au nanoparticles could oxidize 1-phenylethanol to acetophenone, H2 would be generated. As shown in Figure 4f, there is no H2 production with 1-phenylethanol as the hole sacrificial agent, while H2 is generated when methanol is used as the hole sacrificial agent. As a result, holes in the Au nanoparticles can only oxidize methanol but not 1-phenylethanol. This result is consistent with that observed in the control experiment conducted under Ar (Table 2, entry 5). In the absence of O2, 1-phenylethanol cannot be oxidized. Taking the results above together, we can infer that holes in the Au nanoparticles interact with 1-phenylethanol in the photocatalytic reaction, but the oxidation product is not acetophenone. Therefore, the most possible oxidation product is believed to be a free radical intermediate, which is produced by abstracting one electron from 1-phenylethanol with one hot hole. Hydroxyl radicals can be generated either through the capture of hot electrons by O2 or through the oxidation of OH– ions by hot holes. Plasmon-generated hot electrons and holes have been demonstrated to cause simultaneous H2 and O2 evolution on Au/TiO2 hybrid nanostructures.57 However, in our case, the consumption of hot holes is believed to be mainly caused by the partial oxidation of 1-phenylethanol, because the concentration of 1-phenylethanol


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is estimated to be ∼30 times that of OH– ions in the reaction solution. Therefore, •OH is probably originated from the capture of hot electrons by O2. It has been well known that plasmonic metal nanocrystals in metal/semiconductor nanostructures can enhance the visible-light photocatalytic activity of semiconductors according to two different mechanisms, which are plasmonic enhancement of light absorption and plasmonic sensitization, depending on the type of semiconductors.10,15,44 In our work, CeO2 is an n-type wide-bandgap semiconductor. The plasmonic sensitization mechanism is dominant in the enhancement of the visible-light photocatalytic activity. In addition, the plasmonic field enhancement effect might also play a role in the increase in the generation of hot charge carriers due to the presence of a small amount of oxygen defects in CeO2. However, we believe that the concentration of oxygen defects is minimized during the calcination process (400 °C for 4 h in air). This is consistent with the fact that no clear light absorption is observed on the diffuse reflectance spectrum of the pure CeO2 sample in the spectral region around the plasmon peak (Figure 2b). Therefore, the contribution of the plasmonic field enhancement effect is much less than the plasmonic sensitization mechanism. The signals for the generation of superoxide anion and hydroxyl radicals for the pure CeO2 sample under the visible-light illumination are possibly associated with some weak, defect-associated absorption of the material. But the CeO2 sample shows no photocatalytic activity towards the aerobic oxidation of 1-phenylethanol. These results can be explained according to the mechanism proposed below, where the nucleophilic alkoxide loses one electron due to the attack by holes on the Au nanoparticles. The electron-abstraction process allows for the catalyst to return back to its original state for another cycle of reaction. For the CeO2 sample with oxygen defects, the oxidation ability of holes generated from the


30

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excitation of the defects might not be high enough for the electron-abstraction process, even though the oxygen-containing radicals can be generated through the excitation of the defects. All of the results and observations described above about the roles of hot electrons and holes can be summarized as follows: (i) Hot-electron injection takes place under visible light; (ii) Hot electrons are captured by O2 to generate •OH; (iii) The generation of •OH is correlated with the photocatalytic activity; (iv) The consumption of hot holes in the Au nanoparticles is crucial for the photocatalytic reaction; (v) The oxidation product of 1-phenylethanol by holes in the Au nanoparticles is probably a free radical intermediate. In view of these results, a possible reaction mechanism for the photocatalytic aerobic oxidation of 1-phenylethanol with the Au/CeO2 microsphere catalyst under visible light is proposed (Figure 5). The reaction process can be divided in three steps. The photocatalytic activation of the reactant molecules by the Au/CeO2 catalyst is the first step (Figure 5a). The crucial function of the catalyst is to transform the reactant molecules into their free radical intermediates, which decreases the activation energy of the overall reaction and facilitates the product formation. Specifically, the weak base, K2CO3, abstracts a proton from the hydroxyl group of 1-phenylethanol to form the corresponding alkoxide. The activation of the alcohol molecule by the base is an essential step for the entire process. Under visible light illumination, the Au nanoparticles in the Au/CeO2 sample absorb incident photons through LSPR excitation. The plasmon-generated hot electrons with sufficient energy inject into the conduction band of CeO2, leaving holes in the Au nanoparticles. The hot electrons are captured by O2 to generate •O2‒. On the other hand, the nucleophilic alkoxide attacks holes in the Au nanoparticles. The alkoxide loses one electron to form its free radical intermediate, with the Au/CeO2 catalyst returning back to its original state for another cycle. After this step, the reactant molecules, 1-


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phenylethanol and O2, are both catalyzed into their free radical intermediates with high activities. Superoxide anion radicals can react further to transform into other intermediates, such as •OOH, H2O2, and •OH (Figure 5b). In addition, H2O2 can be readily reduced by hot electrons to give •OH and OH–. The oxygen-containing radical species abstract α-H from the free radical intermediate of 1-phenylethanol to yield acetophenone (Figure 5c).

Figure 5. Proposed mechanism for the photocatalytic aerobic oxidation of 1-phenylethanol with the Au/CeO2 microsphere catalyst under visible light. The lines with a half-solid, half-empty arrow indicate charge transfer. The dashed, arrowed lines indicate the attack by a radical species. (a) Photocatalytic activation of the reactant molecules by the catalyst. (b) Transformation of superoxide anion radicals into other reactive intermediate species. (c) Production of acetophenone through the abstraction of α-H from the radical intermediate of 1-phenylethanol by the oxygen-containing radical species.


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4. CONCLUSIONS We have prepared mesoporous Au/CeO2 microsphere photocatalysts with different amounts of loaded Au nanoparticles using an aerosol spray method. These mesoporous (plasmonic metal)/semiconductor hybrid photocatalysts possess large specific surface areas and exhibit strong visible-light adsorption due to the LSPR of the Au nanoparticles. The photocatalytic aerobic oxidation reaction of 1-phenylethanol under visible light illumination has been used to evaluate the photocatalytic activity of the Au/CeO2 catalysts. The 5 mol% Au/CeO2 sample shows the highest photocatalytic activity, with the TOF reaching 108 h‒1, which is more than 23 times those of (Au nanosphere core)@(CeO2 shell) nanostructures. The high photocatalytic activity of the Au/CeO2 sample is derived from the interaction between Au and CeO2 and plasmon-induced hot charge carrier generation and separation. In addition, we have systematically studied the oxide support effect on the photocatalytic activity with eight types of Au/(oxide semiconductor) microsphere photocatalysts. Our Au/CeO2 catalysts also exhibit remarkable activities in photocatalytic oxidation of different alcohols. Moreover, the photocatalytic mechanism has been carefully investigated by considering the roles of hot electrons and holes in the photocatalytic reaction. A rational mechanism for the photocatalytic oxidation of 1-phenylethanol with the Au/CeO2 microsphere catalyst has been proposed. Our results will be highly useful in the understanding and design of (plasmonic metal)/semiconductor hybrid photocatalysts for the green synthesis of different value-added molecules.

ASSOCIATED CONTENT Supporting Information


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Nitrogen adsorption‒desorption isotherms and pore size distributions of CeO2 and Au/CeO2; emission spectra of the light sources; extinction spectra and TEM images of the Au nanosphere and Au@CeO2 samples; XRD patterns and HAADF-STEM image of 5 mol% Au/CeO2; SEM images of Pd/CeO2 and Pt/CeO2; SEM images and XRD patterns of other oxides loaded with Au; recyclability test result; TOF values of the alcohol oxidation reported in previous works. 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 The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Hong Kong RGC (NSFC/RGC, Ref. No.: N_CUHK440/14; TRS, Ref. No.: T23-407-13N) and NNSFC (Ref. No.: 21229101).

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