Journal of Catalysis 348 (2017) 256–264

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Journal of Catalysis

journal homepage: www.elsevier .com/locate / jcat

Optimization of plasmon-induced photocatalysis in electrospun Au/CeO2

hybrid nanofibers for selective oxidation of benzyl alcohol

http://dx.doi.org/10.1016/j.jcat.2016.12.0250021-9517/� 2017 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: libx@mail.ustc.edu.cn (B. Li).

Benxia Li a,⇑, Baoshan Zhang b, Shibin Nie c, Liangzhi Shao b, Luyang Hu b

aDepartment of Chemistry, College of Science, Zhejiang Sci-Tech University, Hangzhou 310018, Chinab School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, Chinac School of Energy Resources and Safety, Anhui University of Science and Technology, Huainan, Anhui 232001, China

a r t i c l e i n f o

Article history:Received 5 December 2016Revised 30 December 2016Accepted 30 December 2016Available online 20 March 2017

Keywords:Au nanoparticlesCeO2

Hybrid nanofibersSurface plasmon resonancePhotocatalysisAerobic oxidation

a b s t r a c t

Exploiting photocatalysts with improved properties for solar-driven chemical reactions is of great signif-icance in developing green chemistry. Here, a series of Au/CeO2 hybrid nanofibers with different Au load-ings have been fabricated by a simple method of electrospinning followed by calcination in air. Theparticle size and plasmonic absorption of Au nanoparticles (NPs) loaded in the nanofibers were analyzedand found to vary with the dosage of chloroauric acid in the precursor solution. The Au/CeO2 hybrid nano-fibers were used as photocatalysts for selective oxidation of benzyl alcohol to benzaldehyde with O2

under simulated sunlight and visible light (>420 nm), respectively. The results showed that introducingAu NPs into CeO2 nanofibers induced a great improvement in photocatalysis. The degree of improvementincreased first and then decreased with the increase in Au loading, reaching an optimal level over 0.5 wt.%Au-loaded CeO2 nanofibers. The photocatalytic reaction presents a very high selectivity of 100% for ben-zaldehyde, which is important for organic synthesis. The transient photocurrent responses of the Au/CeO2

catalysts were also tested for corroborative evidence. After detailed discussion of various factors includ-ing plasmonic absorption, charge transfer, and surface activity of the photocatalyst, a possible mechanismfor the photocatalytic oxidation of benzyl alcohol occurring at the Au–CeO2 interface was proposed.

� 2017 Elsevier Inc. All rights reserved.

1. Introduction

Driving chemical reactions with sunlight instead of traditionalheating methods is of great significance for energy conservationand environmental protection. It represents a kind of sustainablechemistry and can potentially be applied to various industrialchemical processes [1–3]. Thus, the photocatalytic technique isvery attractive as one of the most important strategies of greenchemistry for fuel production [4], chemical synthesis [5,6], andenvironmental remediation [7]. Exploiting photocatalysts withnew and improved properties in terms of efficient solar harvestingand catalytic activity is an overarching concern in this field [8,9].Among various oxide semiconductors, ceria (CeO2) is of specialinterest due to its many fascinating properties, such as abundantoxygen vacancy defects, a special Ce4+/Ce3+ redox cycle, high oxy-gen storage capability and oxygen mobility, good chemical stabil-ity, and biocompatibility [10]. In particular, nanoceria hasexhibited outstanding catalysis in many reactions, such aswater–gas shift reactions [11], CO oxidation [12], and selective oxi-

dation [13], which benefits from its capability of adsorbing andreleasing oxygen by shuttling between Ce(IV) and Ce(III) redoxcycles [14]. However, the photocatalytic activity of ceria is usuallyunsatisfying because the wide bandgap energy (�3.2 eV) and poorcarrier conductivity restrict the efficiency of solar energy utiliza-tion and sunlight-driven chemical reactions [15]. Therefore,improving the photocatalytic activity of CeO2 is important for highefficiency of solar-driven chemical reactions.

In recent years, plasmonic photocatalysts based on the localizedsurface plasmon resonance (LSPR) of noble metal nanoparticleshave gained increasing attention due to their great potential forimproving the conversion of solar energy to chemical energy[16–18]. The unique LSPR properties of noble metal (e.g., Ag andAu) nanoparticles endow them with distinct advantages in absorb-ing and scattering light at specific wavelengths across a wide rangeof the optical spectrum [19,20]. Moreover, noble metal nanoparti-cles have been used widely as catalysts in conventional heat-driven catalysis for organic transformations because of theirwell-known surface catalytic properties [21]. Specifically, goldnanoparticles (Au NPs) have been recognized as powerful catalystsfor various oxidative reactions, as in the oxidation of alcohols, ami-nes, and hydrocarbons and in the epoxidation of alkenes [22]. Sup-

B. Li et al. / Journal of Catalysis 348 (2017) 256–264 257

ported Au NPs have been applied as effective catalysts, makingthem a highly popular research topic in heterogeneous catalysis[23]. As a result, Au NPs supported on semiconductors have raisedgreat expectations for plasmon-enhanced photocatalysis, becauseof their strong light–matter interactions and well-known surfacecatalytic properties [24–26]. In Au nanocrystal–semiconductorcoupling photocatalysts, the charge generation can be enhancedin the semiconductor by energy transfer from Au nanocrystals tothe semiconductor, which is generally attributed to three mecha-nisms: LSPR-induced light focusing, hot electron/hole transfer,and plasmon-induced resonance energy transfer (PIRET) based onthe near field [27,28]. Therefore, improved photocatalysis can beexpected in Au/CeO2 hybrid nanostructures because of improve-ment in several key aspects, including increased carrier conductiv-ity, visible light response triggered by plasmonic Au nanocrystals,and synergetic catalytic effects between CeO2 and Au NPs[29,30]. Some efforts have been made in preparing some Au/CeO2

composite photocatalysts [31,32], understanding their actionmechanism [33], and exploring more applications [34].

Continuous one-dimensional (1D) nanofibers of semiconduc-tors are always attractive for use in solar energy conversion dueto their large surface area, high charge carrier mobility, and easyassembly in devices [35]. Moreover, the photocatalysts of 1D nano-fibers are easy to recycle in practical applications. Aiming at thepreparation of CeO2 nanofibers modified with controllable AuNPs, we used the electrospinning technique, which plays anincreasingly important role in fabricating various functional nano-fibers [36]. Electrospun CeO2 nanofibers have high surface area andporosity, through adding appropriate amounts of polymers thatcould subsequently be removed. Au NPs were uniformly incorpo-rated into the mesoporous CeO2 fibers by in situ conversion fromthe metallic precursors. The Au loading amount in the hybrid nano-fibers could be regulated by varying the dosage of chloroauric acidin the precursor solution. The obtained Au/CeO2 hybrid nanofiberswere used as photocatalysts for selective oxidation of benzyl alco-hol to benzaldehyde with O2 under simulated sunlight and visible(>420 nm) light, respectively. The photocatalytic activities of Au/CeO2 hybrid nanofibers with different loading amounts of Au NPs(i.e., 0.25, 0.5, 1.0, and 2.5 wt.%) were studied. The photoelectro-chemical responses of the Au/CeO2 catalysts were also tested forcorroborative evidence. Various influencing factors including plas-monic absorption, charge transfer, and surface catalysis have beenanalyzed and discussed to provide a reasonable explanation aboutthe photocatalytic performance of the Au/CeO2 nanofiber catalysts.

2. Experimental

2.1. Materials

Cerous nitrate hexahydrate [Ce(NO3)3�6H2O], polyvinylpyrroli-done (PVP,Mw = 1,300,000), chloroauric acid tetrahydrate (HAuCl4-�4H2O), and N,N-dimethylformamide (DMF) were commerciallyobtained from Aladdin Industrial Corporation. All of them are ana-lytical reagent. Acetonitrile, benzyl alcohol, and benzaldehydewere purchased from Shanghai Sinopharm Chemical Reagent Co.,Ltd., and they are of chromatographic grade.

2.2. Materials preparation

The preparation of Au/CeO2 hybrid nanofibers with different Auloadings were fabricated by a facile electrospinning process withfollowing calcination in air. A precursor solution was prepared bydissolving 2.6054 g of Ce(NO3)3�6H2O and 1.6 g of PVP in 12 mLof DMF and adding a certain amount of HAuCl4 solution (0.1 M).The precursor solution was stirred for 24 h at room temperature

and then loaded into a plastic syringe with a 21 gauge stainlesssteel needle. The syringe was placed in a syringe pump and the dis-tance between the needle tip and the product collector was 15 cm.The positive voltage applied to the tip was 20 kV, and the solutionfeed rate was set to 0.5 mL/h. The ambient temperature andhumidity are controlled at around 20 �C and below 30%, respec-tively. The electrospun nanofibers were then calcined in a mufflefurnace that was kept at 200 �C for 1 h and then heated at 500 �Cfor 3 h to remove the organic components, with a heating rate of2 �C/min. Finally, the Au/CeO2 hybrid nanofibers were obtained.The as-obtained Au/CeO2 samples containing different Au amountswere denominated as CeO2–xAu, where x represents the Au loadingmass percent calculated from the experimental dosage of HAuCl4,as shown in Table S1.

2.3. Materials characterization

The phase composition and crystallinity were characterized bypowder X-ray diffraction (XRD) patterns recorded on a Japan Shi-madzu XRD-6000 diffractometer with monochromatized CuKa(k = 0.15418 nm) radiation. The morphologies and microstructuresof the samples were observed by field emission scanning electronmicroscopy (FESEM, JEOL JSM-6700F, Japan) and transmission elec-tron microscopy (TEM, JEOL-2010, Japan). HRTEM and HAADF-STEM images associated with the energy-dispersive X-ray (EDX)mapping spectra were carried out on a Hitachi S4800. The actualAu contents were measured by inductively coupled plasma opticalemission spectra (ICP-OES) on a Perkin Elmer Optima 8300 opticalemission spectrometer (USA). X-ray photoelectron spectroscopy(XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an exciting source of MgKa.The nitrogen adsorption/desorption measurements were per-formed using a Micromeritics ASAP 2020 V4.01 analyzer (USA) at77 K. Diffuse reflectance spectra (DRS) were recorded on a UV–vis Perkin Elmer Lambda 950 spectrophotometer using BaSO4 asthe reference.

2.4. Photocatalytic aerobic oxidation of benzyl alcohol

The photocatalytic activity of the Au/CeO2 hybrid nanofiberswas evaluated by aerobic oxidation of benzyl alcohol under simu-lated sunlight (UV–visible) and visible light (k > 420 nm), respec-tively. The photocatalytic reactions were carried out in atransparent quartz test tube having an inner diameter of 1.5 cmand a length of 12 cm. The suspensions for photocatalytic reactionswere prepared by dispersing the catalyst (1 mg) and injecting ben-zyl alcohol (10 lL) in acetonitrile (2 mL), sequentially. A balloonfull of O2 at a pressure of �1 atm was used to seal the test tubeand provide an oxygen atmosphere. Then the reaction solutionwas irradiated using a Xe lamp (CEL-HXF300, Beijing China Educa-tion Au-light Co., Ltd.) without or with a cutoff filter for experi-ments carried out under simulated sunlight (UV–vis) or visiblelight, respectively. After 5 h of irradiation, the reaction solutionwas centrifuged to remove the catalyst. The amounts of benzylalcohol, benzaldehyde, and other byproducts in the solution weredetermined by GC analysis on a gas chromatograph (Jiedao TECH,GC1690A, China).

2.5. Photoelectrochemical tests

The photocurrent response of each sample was tested on a CHI660D electrochemical workstation (Shanghai Chenhua, China)using a standard three-electrode quartz cell with a platinum wireas a counter electrode and a saturated calomel electrode as a refer-ence electrode. The electrolyte was Na2SO4 solution (0.1 M) thatwas bubbled with nitrogen. A Xe lamp (CEL-HXF300) without

Table 1Actual Au content from ICP analysis and the measured average diameter and estimated surface area of loaded Au NPs from their corresponding size distributions.

Sample Theoretical Au loading (wt.%) Actual Au loading (wt.%) Average diameter (nm) Surface area (m2 g�1)

CeO2–0.25Au 0.25 0.23 37 8.39CeO2–0.5Au 0.5 0.47 51 6.09CeO2–1Au 1.0 0.91 66 4.71CeO2–2.5Au 2.5 2.4 107 2.90

Fig. 1. XRD patterns of various CeO2–xAu samples.

258 B. Li et al. / Journal of Catalysis 348 (2017) 256–264

and with a cutoff filter was used to provide the simulated sunlightand visible (k > 420 nm) light, respectively. To prepare a workingelectrode, 10 mg of the catalyst was ground and suspended in1 mL of ethanol. The mixture was ultrasonically scattered for30 min and then spin-coated on a pretreated 25 � 30 mm indiumtin oxide (ITO) glass. After natural drying in air for 12 h, the cata-lyst film on ITO glass was dried at 120 �C for 5 h and finally usedas the working electrode. The photocurrent–time curves weremeasured at the applied voltage of 0 V under intermittent irradia-tion with light and dark phases of 30 s.

Fig. 2. SEM images of CeO2–xAu samples: (a) CeO2–0Au, (b) CeO2–0.25Au, (c) CeO2–0.5Au, (d) CeO2–1Au, and (e) CeO2–2.5Au. (f) Average size distribution of Au NPsloaded in the nanofibers.

3. Results and discussion

3.1. Au NP loadings in CeO2 nanofibers

The Au/CeO2 hybrid nanofibers used in this study were pre-pared by a simple electrospinning method followed by calcinationin air. Various Au loading amounts (0.25–2.5 wt.%) in CeO2 nanofi-bers were adjusted expediently by changing the dosage ofchloroauric acid (Table S1) in the precursor solution during electro-spinning. The actual numbers of loaded Au NPs in the hybrid nano-fibers were measured by ICP analysis and are summarized inTable 1. The ICP-measured Au contents are very close to those cal-culated theoretically from the experimental dosages of chloroauricacid used in the preparation, indicating that almost all of theHAuCl4 in the precursor solution was completely transferred intoAu NPs loaded in CeO2 nanofibers. The loaded Au particle sizes alsovaried with their loading contents, which will be discussed below.

3.2. Compositions and microstructures of Au/CeO2 photocatalysts

The phase composition and crystallinity of all the catalysts werecharacterized by XRD patterns (Fig. 1). The CeO2–0.25Au andCeO2–0.5Au samples present the identical patterns to the pureCeO2 (CeO2–0Au), which can be indexed to a cubic fluorite struc-

ture (JCPDS 04-0593). No peaks associated with Au species weredetected in their XRD patterns, probably due to the low loadingcontent as well as the high dispersion of Au NPs in CeO2 fibers[37]. With increasing Au loading amount, the diffraction peaks ofthe face-centered cubic (fcc) Au phase (JCPDS No. 04-0784) aredetectable in XRD patterns of CeO2–1Au and CeO2–2.5Au samples,besides those of the cubic CeO2 phase. Moreover, the diffractionpeaks of metallic Au in the Au/CeO2 hybrid nanofibers get strongeras Au loading content increases. Fig. 2a–e shows SEM images of theAu/CeO2 hybrid nanofibers with various Au loading contents (0,0.25, 0.5, 1, and 2.5 wt.%). Pure CeO2 and all of the Au/CeO2 hybridnanofibers present continuous 1D structure after calcination, withdiameters in the range of 200–500 nm and lengths of severalmicrometers. Au NPs are well loaded on CeO2 nanofibers, and theybecome more observable with increased loading from 0.25 to2.5 wt.%. No obvious agglomeration of Au NPs is observed evenafter the 500 �C high-temperature calcination, suggesting that

Fig. 3. (a) TEM image, (b) HRTEM image, and (c) EDX spectrum recorded on a CeO2–0.5Au nanofiber. (d) HAADF-STEM image and (e–g) elemental maps of O, Ce, and Au,respectively, in the CeO2–0.5Au nanofiber shown in (d).

Fig. 4. XPS spectra of the CeO2–0.5Au and CeO2–1Au samples: (a) Ce3d and (b) Au4f.

B. Li et al. / Journal of Catalysis 348 (2017) 256–264 259

CeO2 supports can prevent the agglomeration and particle growthof Au NPs. In fact, some Au NPs were incorporated into CeO2 nano-fibers during the in situ formation process, but they are hardlyobserved by TEM (Fig. S1 in the Supporting Information) becauseof the large diameters of CeO2 fibers. The Au NP size distributionwas roughly estimated by inspection of a number of SEM images(Fig. 2f). Specifically, with an increase of Au loading from 0.25 to2.5 wt.%, the average size of Au NPs grows from 37 to 107 nm.Obviously, the average particle sizes of Au NPs estimated fromSEM images are much larger than the actual average sizes, becausethe Au NPs that are too small to observe have been ignored.

The crystalline phase and elemental composition of the Au/CeO2 hybrid nanofibers were further characterized by HRTEMimaging and EDX analysis on a CeO2–0.5Au nanofiber (Fig. 3). Closeobservation by TEM imaging (Figs. 3a and S2 in the SupportingInformation) reveals that the CeO2 support is constructed by ran-dom assembly of CeO2 crystallites with sizes of 5–10 nm and pre-sents a mesoporous structure. Some darker nanoparticles withsizes of 10–40 nm are possibly Au NPs loaded in CeO2 nanofibers.

The HRTEM image (Fig. 3b) taken of this nanofiber depicts two dis-tinctive interplanar spacings of 0.32 and 0.24 nm, which corre-spond to the (111) planes of face-centered cubic CeO2 and Au,respectively. The EDX spectrum (Fig. 3c) recorded on this nanofibershows strong signals of Ce along with weak signals of Au. The ele-mental mappings (Fig. 3e–g) confirm the distributions of O, Ce, andAu in the Au/CeO2 hybrid nanofibers. Au NPs are well dispersedthroughout the nanofiber.

XPS measurements were performed typically on CeO2–0.5Auand CeO2–1Au catalysts to obtain further information about thesurface atomic compositions and valence states of the Au/CeO2 cat-alysts. The survey spectra (Fig. S3 in the Supporting Information)exhibit obvious signals of Ce and O, and the signals of Au speciesare very weak due to the low loading contents. In Ce3d XPS spectra(Fig. 4a), the eight binding-energy positions denoted by ‘‘a” and ‘‘b”

can be assigned to Ce3d3/2 and Ce3d5/2 states, respectively. Thepositions denoted as a1, a2, a4, b1, b2, and b4 are indexed to Ce4+,and the other two, denoted as a3 and b3, correspond to Ce3+, indi-cating that the Ce oxidation state is mainly Ce4+ in the samples

Fig. 5. N2 adsorption/desorption isotherms (a, c, e) and pore size distributions (b, d, f) of the samples: (a, b) CeO2–0Au, (c, d) CeO2–0.5Au, and (e, f) CeO2–1Au.

300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2 CeO2-0AuCeO2-0.25AuCeO2-0.5AuCeO2-1AuCeO2-2.5Au

Abso

rban

ce

Wavelength (nm)

Fig. 6. UV–vis diffuse reflection spectra (DRS) of the various Au/CeO2 catalysts.

260 B. Li et al. / Journal of Catalysis 348 (2017) 256–264

[38]. The Au4f spectra (Fig. 4b) show two individual peaks at83.8 eV for Au4f7/2 and 87.4 eV for Au4f5/2, respectively, verifyingthe nature of Au0 within both the samples. The XPS spectra of bothCeO2–0.5Au and CeO2–1Au catalysts almost present identical Ce3dand Au4f signals. Therefore, the valence effect on catalytic activityof the present Au/CeO2 catalysts can be excluded.

3.3. Physicochemical properties of Au/CeO2 photocatalysts

The specific surface areas and pore size distributions of the Au/CeO2 hybrid nanofibers are characterized by the N2 adsorption–desorption method, taking CeO2–0Au, CeO2–0.5Au, and CeO2–1Auas representative samples. The N2 adsorption–desorption iso-therms and the pore size distributions of the three samples areshown in Fig. 5. All samples exhibit a type II isotherm [39] witha hysteresis loop typical of mesoporous materials (Fig. 5a, c,and e). The specific surface area was calculated through the BETequation. Pure CeO2 nanofibers (CeO2–0Au) present a surface areaof 64.5 m2 g�1. The surface areas of CeO2–0.5Au and CeO2–1Au are63.6 and 60.4 m2 g�1, respectively. The pore size distribution

curves (Fig. 5b, d, and f) were obtained using the BJH method tothe desorption branch of the isotherms. All the samples show thepore size distribution with peaks centered in the range of 2.0–3.5 nm. These results indicate that textural properties of the nano-fibers were not affected obviously by Au NP loading.

The optical absorption properties of the Au/CeO2 catalysts withdifferent Au contents are characterized by UV–vis diffuse reflec-tance spectra (DRS) (Fig. 6). Pure CeO2 nanofibers show only theabsorption band below 450 nm due to the intrinsic bandgapabsorption of ceria. After Au NPs are loaded, a broad SPR absorp-tion peak appears in 500–800 nm with its center at ca. 605 nm.The SPR absorption becomes more intensive as the Au loadingincreases from 0.25 to 2.5 wt.%. The increased visible light absorp-tion from Au SPR should help enhance the harvesting of solarenergy and promote related photocatalytic processes.

3.4. Photocatalytic selective oxidation of benzyl alcohol

Selective oxidation is an important reaction in organic synthesisand plays a significant role in the production of valuable chemicals.The application of heterogeneous photocatalysis and molecularoxygen to oxidation reactions promises to avoid the use of tradi-tional, toxic chemical oxidants, offering a green and energy-efficient technology for organic synthesis [40,41]. Benzaldehydeis an important precursor for the production of perfumes, dye-stuffs, and pharmaceuticals [13]. The selective aerobic oxidationof benzyl alcohol to benzaldehyde driven by visible light is of vitalimportance for being ecofriendly and economically efficient[42,43]. Therefore, we chose the selective oxidation of benzyl alco-hol with molecular oxygen in acetonitrile as a model reaction(Fig. 7a), to examine the photocatalytic activity of the Au/CeO2

hybrid nanofibers with various Au loadings. The reaction was car-ried out under either UV–visible light or visible light (>420 nm)from the Xe lamp, which has maximal emission intensity in therange from 500 to 600 nm. The conversion yields after 5 h of reac-tions over various photocatalysts under UV–visible and visiblelight are listed in Tables S2 and S3 of the Supporting Information.The photocatalytic reactions present a very high selectivity of

Fig. 7. (a) Photocatalytic reaction and (b) normalized conversion yields ofbenzaldehyde over the different photocatalysts under UV–visible and visible light,respectively.

Fig. 8. Recyclability of the CeO2–0.5Au photocatalyst for the photocatalyticoxidation of benzyl alcohol in three cycles under simulated sunlight and visiblelight (k > 420 nm), respectively.

B. Li et al. / Journal of Catalysis 348 (2017) 256–264 261

100% for benzaldehyde, which is important for organic synthesis.For a more intuitive comparison, the conversion yields have beennormalized against the total mass of each photocatalyst and thereaction time and are illustrated by a histogram in Fig. 7b. Notably,under both UV–visible and visible light, the benzyl alcohol oxida-tion has been obviously promoted by introducing Au NPs. UnderUV–visible light, the normalized conversion yield presents anincrease first as the loaded Au NPs increase. The highest value isachieved over 0.5 wt.% Au-loaded CeO2 nanofibers (CeO2–0.5Au),which is nearly fourfold the value over pure CeO2 nanofibers.When Au loading further increases to 1 wt.%, the normalized con-version yield decreases greatly, and decreases continuously withmore Au loading in CeO2–2.5Au catalyst. Under visible light(>420 nm) irradiation, the normalized conversion also presents a

humplike variation tendency with varied Au loading content. TheCeO2–0.5Au catalyst shows the highest visible light photocatalyticactivity, but pure CeO2 nanofibers present very weak photocataly-sis for benzyl alcohol oxidation under visible light. The normalizedconversion yield increases with loaded Au amount and attains amaximum value of 392 lmol h�1 g�1 over CeO2–0.5Au catalyst,which is nearly 10-fold the value over pure CeO2 nanofibers undervisible light. For each catalyst, the normalized conversion yieldobtained under UV–visible light is much higher than that undervisible light because of the difference in available photon energy.Moreover, the TiO2 and SiO2 nanofibers loaded with 0.5 wt.% AuNPs (TiO2–0.5Au and SiO2–0.5Au), which were prepared by thesame method of electrospinning followed with calcination in air,have been used as photocatalysts for the selective oxidation ofbenzyl alcohol. By comparison, the photocatalysis of TiO2–0.5Auis obviously lower than that of CeO2–0.5Au, and SiO2–0.5Au showsthe lowest photocatalysis among the Au-loaded nanofibers. Theresults indicate that the combination of Au NPs and CeO2 supportcan generate a synergetic enhancement of the photocatalysistoward selective oxidation of benzyl alcohol. In addition, the recy-clability of the CeO2–0.5Au photocatalyst was examined under thesimulated sunlight and the visible light (k > 420 nm), respectively,in three cycles. The results are shown in Fig. 8, indicating that thephotocatalyst has good stability and reusability in the photocat-alytic reaction.

3.5. Photocurrent response of Au/CeO2 photocatalysts

To help understand the photo-response of Au/CeO2 hybridnanofibers, the transient photocurrent–time (Fig. 9) curves of var-ious CeO2–xAu photoelectrodes were recorded under UV–visibleand visible light. Fig. 9a indicates that the CeO2–xAu electrodeswith lower Au loadings (x = 0.25–0.5) present current switchessimilar to that of pure CeO2 under intermittent irradiation withUV–visible light. When Au loading is increased to 1 and 2.5 wt.%,the CeO2–xAu electrodes exhibit a little spikelike shape of the tran-sient current when the light is switched on or off, which may bedue to charge storage and recombination in the system [44].Fig. 9b shows the transient current–time curves of the CeO2–xAu(x = 0–2.5) electrodes under visible light (k > 420 nm) irradiation.As expected, pure CeO2 (CeO2–0Au) presents negligible photocur-rent response because of its weak absorption in the visible lightregion. The CeO2–xAu (x = 0.25–2.5)-coated electrodes give obvi-ous photocurrent responses under visible light, which is in accor-dance with the plasmonic absorption of loaded Au NPs (Fig. 6).However, the periodic on/off photocurrents of the CeO2–xAu(x = 1–2.5) electrodes with further increased Au loadings presenta pronounced spikelike shape. After the initial rise of photocurrentimmediately after the light is switched on, an obvious decay ofphotocurrent is observed due to recombination of surface-trapped electrons with holes [44], suggesting that the chargerecombination is more severe as Au loading increases.

3.6. Photocatalytic mechanism

From the above results, a further increase in Au loading from 0.5to 2.5 wt.% does not induce continuous enhancement in the photo-catalytic activity for benzyl alcohol oxidation over the Au/CeO2 cat-alysts, though the plasmonic intensity increases with Au loading.The declining photocatalytic performance upon excess Au loading(1–2.5 wt.%) is partly due to photon scattering and charge recom-bination, as suggested by transient photocurrents (Fig. 9). On theother hand, the size-determined surface catalysis of metal NPsshould also be considered simultaneously [45]. Generally, smallermetal NPs have a higher fraction of coordinatively unsaturated sur-face atoms, which are highly catalytically active [46]. Therefore,

Fig. 9. Transient current–time (i–t) curves of various CeO2–xAu electrodes recorded under (a) UV–vis and (b) visible light.

Fig. 10. Integrated correlations among Au NP loading amounts, particle size,plasmonic absorption, specific surface area of Au NPs, and the conversion of benzylalcohol over different Au/CeO2 nanofibers.

262 B. Li et al. / Journal of Catalysis 348 (2017) 256–264

the smaller Au NPs would show higher surface activity toward cat-alytic oxidation of benzyl alcohol, which involves adsorption andactivation of molecular oxygen as well as benzyl alcohol. To obtain

Fig. 11. Schematic mechanism of the photocatalytic oxidation of benzyl alcohol over Aucharge transfer in the photocatalyst and (b) photocatalytic reaction at Au–CeO2 interfac

a comprehensive understanding of the effect of Au loading on thephotocatalytic performance for benzyl alcohol oxidation, the plas-monic absorption, average diameter, and specific surface area ofloaded Au NPs in CeO2 nanofibers are shown in Fig. 10. The specificsurface areas of Au NPs were calculated by their average diameters(Table 1), taking the Au NPs as spheres (Eqs. S1–4 in the SupportingInformation). As expected, the specific surface areas of Au NPsgreatly decrease with the continuous increase of Au loading from0.25 to 2.5 wt.%, because of the increased Au NP sizes, leading toa decrease in the exposed active sites. However, the CeO2–0.25Aucatalyst does not present the highest photocatalytic activitydespite its having the largest Au surface area, because its plas-monic absorption is too weak. Therefore, the humplike variationin the photocatalytic performance of our Au/CeO2 catalysts is aresult of the balance between plasmon resonance and surface cat-alytic activity of the loaded Au NPs.

As a consequence, a mechanism for the photocatalytic oxidationof benzyl alcohol over the Au/CeO2 nanofibers is illustrated inFig. 11 and explained as follows. First, the charge generation andtransfer in the photocatalyst are shown in Fig. 11a. Under UV–visirradiation, CeO2 can be excited to generate electron–hole pairsin its conduction band (CB) and valence band (VB). Meanwhile,the plasmonic Au NPs act as photon antennas that can effectivelyconcentrate the visible light energy and transfer it to the adjacentCeO2 through near-field interaction, namely, the plasmon-inducednear-field enhancement effect. The electron–hole pair formation inadjacent CeO2 will be strongly enhanced, generating improvedphotocatalysis. In this case, Au NPs also serve as electron trappersto improve the charge separation. When the Au/CeO2 photocatalystis irradiated only visible light alone (k > 420 nm), CeO2 is barelyexcited to produce electron–hole pairs because of its wide bandgap. The enhanced photocatalysis of Au/CeO2 under visible light

/CeO2 catalyst under UV–visible and visible light, respectively. (a) Plasmon effect one.

B. Li et al. / Journal of Catalysis 348 (2017) 256–264 263

is mainly attributed to a plasmonic hot electron injection process[47], which allows the transfer of high-energy plasmonic electronsfrom Au NPs to CeO2 across the junction. Subsequently, the photo-catalytic oxidation of benzyl alcohol occurred at the Au–CeO2

interface, shown in Fig. 11b. Both Au NPs and CeO2 specifically con-tributed to promote the photocatalytic reaction. Benzyl alcohol isready to be adsorbed on Au NPs, forming a metal–alkoxide inter-mediate via O–H bond cleavage [48]. Oxygen molecules may beprone to be adsorbed on the CeO2 support because of its abundantoxygen vacancies [49] and then be activated by electron transfer.Finally, the activated oxygen captured b-hydrogen of the adsorbedbenzyl alcohol, forming benzaldehyde and H2O as products. Thus,the photocatalytic oxidation of benzyl alcohol was completed viasynergistic interactions between loaded Au NPs and CeO2 support.When Au loading and particle size increase, the plasmon-inducedcharge generation should be enhanced, but the surface active sitesfor catalytic reaction would greatly decrease because of thereduced Au surface area. As a result, the Au/CeO2 catalysts dis-played a humplike variation tendency in their photocatalytic per-formance with increased Au loading amount.

4. Conclusions

Au/CeO2 hybrid nanofibers with different amounts of Au NPs(0.25–2.5 wt.%) were prepared through electrospinning followedby calcinations in air. The particle size and plasmonic absorptionof Au NPs loaded in the nanofibers were determined by the dosageof chloroauric acid added in the precursor solution. The photocat-alytic activity of Au/CeO2 hybrid nanofibers were evaluated byselective oxidation of benzyl alcohol to benzaldehyde with O2

under simulated sunlight and visible light (>420 nm), respectively.Incorporation of Au NPs in CeO2 nanofibers induced a greatenhancement of photocatalysis for benzyl alcohol oxidation. Theenhancement is related to the Au loading amount, getting an opti-mal level over 0.5 wt.% Au loaded CeO2 nanofibers. The photocat-alytic performance of Au/CeO2 hybrid nanofibers is determinedby multiple factors including plasmonic absorption, charge trans-fer, and surface catalysis. Finally, a mechanism for the photocat-alytic oxidation of benzyl alcohol occurring at the Au–CeO2

interface was proposed on the basis of synergistic interactionbetween Au NPs and CeO2 support. Therefore, this work shedssome light on optimizing the photocatalysis of plasmonic metal/semiconductor photocatalysts and provides a potentially impactfulphotocatalyst system for solar-driven chemical reactions as well.

Acknowledgments

This work is supported by the National Natural Science Founda-tion of China (21471004) and the Excellent Youth Talents SupportPlan in Colleges and Universities of Anhui Province.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2016.12.025.

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