Materials Letters 65 (2011) 3051–3054

Contents lists available at ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r.com/ locate /mat le t

Preparation and visible-light photocatalytic activity of Au-supported porous CeO2

spherical particles using templating

Akira Nakajima ⁎, Tomoki Kobayashi, Toshihiro Isobe, Sachiko MatsushitaDepartment of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo 152–8552, Japan

⁎ Corresponding author. Tel.: +81 3 5734 2524; fax:E-mail address: anakajim@ceram.titech.ac.jp (A. Nak

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.06.051

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 May 2011Accepted 13 June 2011Available online 25 June 2011

Keywords:CeO2

GoldPorousPhotocatalystLSPR

Porous spherical CeO2 particles were prepared by impregnation of a cerium precursor solution into organicmonolith sphere particles, with subsequent firing at 500 °C in air. The single-phase CeO2 powder had specificsurface area of greater than 140 m2/g. Photodeposition with UV illumination loaded Au onto the CeO2 particlesurface, which changed from yellowish to purple because of localized surface plasmon resonance (LSPR). TheAu-loading increased photocatalytic decomposition activity of the CeO2 powder for gaseous 2-propanol (IPA)under visible light. Thermal desorption of IPA, which was adsorbed to all porous spheres, provided flux to thephotocatalytic reaction field of the sphere outer surface.

+81 3 5734 3355.ajima).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ceriumdioxide (CeO2) has been the subject ofmany studies related toits use as a three-way catalyst for automobile exhaust gas treatment [1–4],as a chemical mechanical polishing agent [5–7], as a solid oxide fuel cell[8,9], and for ultraviolet (UV) filtration [10–12]. This wide band-gapsemiconductor material absorbs light in the near-UV and visible regions.One cause of its slightly yellowish color is the excitation of electrons in theO2p (VB of CeO2) to Ce4f: the band level difference between these two isca. 3 eV [13]. The potential level of Ce4f band is slightlymore positive thanCB of TiO2 [14]. Therefore, charge transfer to oxygen is feasible undercoexistencewithwater.Moreover, studies ofphotocatalytic applicationbyUV or visible light are increasing recently because CeO2 is abundant,inexpensive, and harmless [15–22]. Pure CeO2 has been investigatedunder UV illumination related to water splitting for generation ofhydrogen gas [17] and photodegradation of toluene in the gas phase[15]. However, many studies have been conducted of the decompositionof organic dye or compounds in aqueous solution, and studies of thedecomposition of gaseous organic pollutant remain limited, especiallythose conducted under visible light.

Very recently, Kominami et al. demonstrated excellent photocatalyticmineralization of formic acid in water by Au-supported CeO2 powderunder visible light illumination [22]. The overall activity of this materialexceeds that of Au-supported TiO2. They attributed this result to theelectron injection from Au to CeO2 by the absorption of light around550 nm, caused by localized surface plasmon resonance (LSPR) of Au,

and to the resultant active species formation on the CeO2 surface. For thisstudy, we prepared porous CeO2 sphere particles to increase the specificsurface area using organic monolith particles as a template. Then,subsequent to the Au-loading procedure, photocatalytic decompositionactivity was evaluated for the obtained powders for gaseous 2-propanol(IPA) under visible light.

2. Experimental

Cerium nitrate hexahydrate (Ce(NO3)3•6H2O, 13.0 g; Wako PureChemical Industries Ltd., Tokyo Japan) was dissolved into ethanol(26.7 g Wako Pure Chemical Industries Ltd.). The solution was mixedand impregnated to organic monolith particles (MBP-8, 3.0 g; SekisuiPlastics Co., Ltd., Osaka, Japan) under vacuum at room temperature.The suspension was stirred for one day. Then wet monolith powderwas obtained through centrifugation at 2000 rpm for 20 min. Thepowder was dried at 90 °C with stirring and was subsequentlycalcinated at 500 °C for 1 h. For comparison, powders were preparedusing direct calcination of Ce(NO3)3•6H2O under identical conditions.These are denoted in this report respectively as P-500 (templated)and C-500 (not templated).

Loading of Au on CeO2 was conducted according to proceduresdescribed in a report by Kominami et al. [22]. The obtained CeO2 powders(200 mg)were dispersed inwater (10 cm3). Citric acid (C6H8O7, 10 μmol;Wako Pure Chemical Industries Ltd.) and tetrachloroauric acid (HAuCl4,3.4 mg; Wako Pure Chemical Industries Ltd.) were dissolved into thesuspension; then N2 bubbling (100 ml/min) was conducted for 1 h. Thenphotoillumination was conducted using a Hg–Xe lamp (UV intensity: ca.10 mW/cm2 at 365 nm)with stirring for 1 h at room temperature. TheAusource was reduced by photogenerated electrons. Then Au metal was

0.5 µm

0.5 µm0.5 µm

(c)

(b)

(a)

2 µm

Fig. 1. SEM micrographs of starting organic monolith particles and the obtained

3052 A. Nakajima et al. / Materials Letters 65 (2011) 3051–3054

deposited on the light-reachable surface of CeO2 particles, engenderingthe formation of Au/CeO2. The powder was washed repeatedly withdistilledwater and then dried at 60 °C for 2 h in air. The powders after Au-loading are described, respectively, as P-500(Au) and C-500(Au).

The crystalline phase of the powders was evaluated using X-raydiffraction (XRD, XRD-6100; Shimadzu Corp., Japan). Using a UV–Vis-NIRscanning spectrophotometer (UV-2450; Shimadzu Corp., Tokyo Japan),the UV–Vis absorption spectra were evaluated. The powder morphologywas observed using a transmission electron microscope (JEOL JEM-2010;JEOL, Tokyo, Japan) and a field-emission scanning electron microscope(S-800; Hitachi Ltd., Tokyo, Japan). The concentration of Au on thepowder surface was measured using X-ray photoelectron spectroscopy(XPS, PHI Quantera SXM; PHI Co., U.S.A.) with an Al Kα X-ray line(1486.6 eV). Specific surface areas of all materials were evaluated usingBrunauer, Emmett and Teller (BET) method with N2 (Autosorb-1;Quantachrome Instruments, Boynton Beach, FL, USA).

The photocatalytic activity was evaluated according to thedecomposition of gaseous IPA (Wako Pure Chemical Industries Ltd.).The sample powder (40 mg) was set at the center of a quartz vessel(500 ml in volume) in the area of 5.7 cm2. Subsequently, the vesselwas sealed and IPA gas was injected into it. The injected gas amountwas equivalent to that for 1000 ppm concentration. The vessel wasthen stored in the dark for 18 h. After the adsorption equilibrium wasconfirmed, visible light illumination was conducted using a Xe lampwith an L-42 filter (absorbed UVb400 nm; Asahi Glass Co. Ltd. Japan)to cut a large amount of lightwithin UVwavelength range. In this case,the illumination intensity at the powder surface was 20,000 lx (LX-105; Custom Co., Japan). As described in an earlier report [23–25],molecules of gaseous IPA were decomposed initially into acetone andwere then finally decomposed to H2O and CO2. Therefore, in thisstudy, IPA, acetone, and CO2 concentrations were evaluated duringthis light illumination using gas chromatography (GC-14A; ShimadzuCorp., Tokyo, Japan) equipped with a flame ionization detector, amethanizer, and a Sunpak-A column (Shimadzu Corp.). The carrier gaswas nitrogen; the respective temperatures for the detector andinjection port were 230 °C and 200 °C. The initial column temperaturewas 50 °C, which was maintained for 2.5 min. During this period, CO2

peak was recorded. Then the temperature was increased 20 °C/min to130 °C to gain peak separation, and peaks of IPA and acetone wererecorded.

powders: (a) starting organic monolith particles, (b) C-500, (c) P-500.

3. Results and discussion

Fig. 1 presents SEM micrographs of starting organic monolithparticles and the obtained powders. The organic monolith particlesare porous spheres. The C-500 was fine particles forming irregularaggregates, whereas P-500 became porous spheres. X-ray diffractionrevealed that these powders were crystalline CeO2 and the secondaryphase was not identified (Supporting Information). The crystallinityof C-500 was greater than that of P-500. The specific surface areas ofthese powders were 144 m2/g (P-500) and 72 m2/g (C-500).

Fig. 2 portrays TEM micrographs and color change of P-500 beforeand after Au-loading. The color of P-500 was changed from thin yellowto purple by Au loading. Gold nanoparticles of less than 10 nm wereobserved on the P-500 surface. The Au nanoparticles were observedmainly in the topmost 100 nm surface of the CeO2 sphere particles.Similar color change andmicrostructurewere observed also fromC-500.The specific surface area of the powder decreased by Au loading;however the decrease was not remarkable (127 m2/g for P-500(Au),and 61 m2/g for C-500(Au), respectively). A peak attributable to LSPR ofAu was confirmed around 530 nm [26] in UV–Vis spectra of obtainedpowders (Supporting Information). The band gap obtained from thisresult for P-500 and C-500 by assuming indirect transition was around2.9 eV (Supporting Information), which was almost identical to thatdescribed in a previous report [13].

Fig. 3 shows photocatalytic activity evaluated for P-500 and C-500before and after Au-loading under visible light. Wavelength spectraof the light source are presented in Supporting Information. Results ofXPS analyses revealed that the Au concentration on the CeO2 surfacefor P-500 and C-500 were, respectively, 3.9 and 1.7 atom%. Initialconcentrations (C0) of IPA before light illumination were, respec-tively, 259 ppm, 818 ppm, 130 ppm, and 696 ppm, for P-500, C-500,P-500(Au), and C-500(Au). Irrespective of the decreased specificsurface area, the Au loading increases the amount of initial IPAadsorption, which is probably attributable to the high affinity of Auwith gaseous IPA [27]. Moreover, the difference of C0 values impliesthat the adsorption capability of P-500 is greater than that for C-500(P-500/C-500=741 ppm/182 ppm=~4). This difference is greaterthan the surface area ratio (P-500/C-500=144/72=~2), suggestingthat the surface states of CeO2 in P-500 and C-500 differ. Although theCe3+/Ce4+ ratio obtained on the surface of P-500 by XPS was almostidentical to that for C-500, the ratio inside of the porous spheresmight be different by reduction during firing under low O2 pressure.Further investigations are necessary in relation to this result.

For P-500, evenwhenAuwas not loaded, photocatalytic activitywasincreasedmarkedly. The increase of IPA concentration in the early stageof light illumination is probably attributable to the thermaldesorptionofIPA molecules from the powder surface by the IR-included visible light

100 nm

100 nm

Au

(a)

(b)

Fig. 2. TEM micrographs and color change of P-500 powder before and after Au loading: (a) before loading and (b) after loading.

3053A. Nakajima et al. / Materials Letters 65 (2011) 3051–3054

illumination and the resultant increase of temperature in the measure-ment vessel (around 6–7 °C). For C-500, Au loading increased thephotocatalytic activity slightly. The average IPA decrease rate in all timeranges (0–100 h) for C-500 and C-500(Au) were, respectively, 1.5 and1.7 ppm/h. Although the effect of thermal desorption is ignored, Au-loading increases theoverall photocatalytic activity despite thedecreaseof the surface area. Calculation of this rate for P-500 is difficult to dousing decomposition data because of the remarkable increase of IPAconcentration in the early stage of light illumination.

Regarding P-500(Au), the average IPA decrease rate in early stageof light illumination (up to 20 h) was calculated as 3.1 ppm/h. Theratio of this rate between P-500(Au) and C-500(Au) was 1.8 in thistime range. However, the acetone generation rate ratio in the sametime range for these two is 7.1 (Fig. 3(b); 10 ppm/h for P-500(Au) and1.4 ppm/h for C-500(Au)), which is much larger than the practicalsurface area ratio (ca. 2). Based on these results, it is presumable thatvisible light illumination increases the sample temperature anddecreases the equilibrium adsorption amount of IPA, and that theresultant thermal desorption of IPA provides its flux from the inside ofthe porous sphere to the powder outer surface, which is the practicalreaction field of photocatalytic decomposition (Fig. 4). Then, theentire acetone amount was increased remarkably in P-500(Au). Thechange of CO2 concentration during the measurement period wasquite small. Even for P500(Au), ΔCO2 up to 20 h was merely 42 ppm.This value corresponds to 14 ppm (~5%) of acetone. Therefore, we

ignored acetone decomposition in themeasurement period in the ratediscussion of this study.

Compared to that of C-500, Au loading remarkably increased thephotocatalytic activity of P-500. One plausible explanation is thedifference of Au concentration. It is noteworthy that acetone wasgenerated in this sample (P-500(Au)), even in the dark [28], althoughthe amount was smaller (around 3 ppm/h) than in the case underlight illumination (around 10 ppm/h). One more reason on this resultis a large adsorption field inside of the powder for P-500 and feedingto the photocatalytic reaction field by thermal desorption. The overallphotocatalytic performance of the porous spherical Au-loaded CeO2

powder is expected to be governed by adsorption amount, adsorptionstrength, Au amount, temperature, and light intensity. Optimalconditions depend on the target chemicals and their concentrations.Investigation of the practical effects of these factors on the entirephotocatalytic efficiency will be conducted in future works.

4. Conclusions

For this study, porous spherical CeO2 powder was prepared usingorganic monolith particles as a template. The powder was single-phaseCeO2, with specific surface area exceeding 140 m2/g. The powder colorwas changed from thin yellow to purple by Au-loading, probably becauseof the LSPR of Au confirmed by UV–Vis spectra. The photocatalyticdecomposition activity of obtained CeO2 powder for gaseous IPA under

Reaction field

Adsorption fieldCeO2 porous sphere

Au

IPA fluxby thermal desorption

Fig. 4. Schematic illustration of the photocatalytic decomposition of IPA under visiblelight on this sample.

(a)

(b)

Ace

tone

con

cent

ratio

n [p

pm]

C-500

Time [h]

P-500-Au

P-500

C-500-Au

0

100

200

300

400

500

Time [h]

C/C

o

P-500-Au

C-500

P-500

C-500-Au

0 20 40 60 80 100

0 20 40 60 80 1000

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Fig. 3. Photocatalytic activity for (a) decomposition of gaseous IPA, and (b) acetonegeneration under visible light.

3054 A. Nakajima et al. / Materials Letters 65 (2011) 3051–3054

visible light was increased remarkably by Au-loading. Photocatalyticactivity suggests that thermal desorption of IPA produces its flux frominside of the powder to its outer surface, providing high photocatalyticactivity.

Acknowledgments

The authors are grateful to Mr. Katsuaki Hori and Mr. Jun Kouki atthe Center of Advanced Materials Analysis at Tokyo Institute ofTechnology for TEM and SEM observations.

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.matlet.2011.06.051.

References

[1] Lambrou PS, Savva PG, Fierro JLG, Efstathiou AM. Appl Catal, B 2007;76:375–85.[2] Roberta DM, Paolo F, Jan KPR, Giuseppe G, Mauro G. Appl Catal, B 2000;24:157–67.[3] Panayiota SL, Angelos ME. J Catal 2006;240:182–93.[4] Kaspar J, Fornasiero P, Graziani M. Catal Today 1990;50:285–98.[5] Tsai MS. Mater Sci Eng B 2004;110:132–4.[6] Lim DS, Ahn JW, Park HS, Shin JH. Surf Coat Technol 2005;200:1751–4.[7] Kim YH, Kim SK, Kim N, Park JG, Paik U. Ultramicroscopy 2008;108:1292–6.[8] Wang JH, Liu ML, Lin MC. Solid State Ionics 2006;177:939–47.[9] Laosiripojan N, Sangtongkitcharoen W, Assabumrungrat S. Fuel 2006;85:323–32.[10] Karunakaran C, Dhanalakshmi R. Sol Energy Mater Sol Cells 2008;92:1315–21.[11] Song S, Xu LJ, He ZQ, Ying HP, Chen JM, Xiao XZ, et al. J Hazard Mater 2008;152:

1301–8.[12] Zhao XJ, Zhao QN, Yu JG, Liu BS. J Non-Cryst Solids 2008;354:1424--30.[13] Pfau A, Schierbaum KD. Surf Sci 1994;321:71–80.[14] Li FB, Li XZ, Huo MF, Cheah KW, Choy WCH. Appl Catal, A 2005;285:181–9.[15] Hernández-Alonso MD, Hungr´ıa AB, Mart´ınez-Arias A, Fernández-Garc´ıa M,

Coronado JM, Conesa JC, et al. Appl Catal, B 2004;50:167–75.[16] Zhai Y, Zhang S, Pang H. Mater Lett 2007;61:1863–6.[17] Chug K-H, Park D-C. Catal Today 1996;30:157–62.[18] Bamwend GR, Arakawa H. J Mol Catal A: Chem 2000;161:105–13.[19] Chaudhary YS, Panigrahi S, Nayak S, Satpati B, Bhattacharjee S, Kulkarnic N. J Mater

Chem 2010;20:2381–5.[20] Ji P, Zhang J, Chen F, Anpo M. Appl Catal, B 2009;85:148–54.[21] Mao C, Zhao Y, Qiu X, Zhu J, Burda C. Phys Chem Chem Phys 2008;10:5633–8.[22] Kominami H, Tanaka A, Hashimoto K. Chem Commun 2010;46:1287–9.[23] Ohko Y, Hashimoto K, Fujishima A. J Phys Chem A 1997;101:8057–62.[24] Larson SA, Widegren JA, Falconer JL. J Catal 1995;157:611–25.[25] Anbar M, Neta P. Int J Appl Radiat Isot 1967;18:493–523.[26] Kowalska E, Mahaney OOP, Abe R, Ohtani B. Phys Chem Chem Phys 2010;12:

2344–55.[27] Tsionsky V, Gileadi E. Langmuir 1994;10:2830–5.[28] Su F-Z, ChenM,Wang L-C, Huang X-S, Liu Y-M, Cao Y, et al. Catal Commun 2008;9:

1027–32.