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Microporous and Mesoporous Materials 110 (2008) 242–249

Highly dispersed gold nanoparticles assembled in mesoporoustitania films of cubic configuration

Yu Zhang a, Akhmad Herman Yuwono a, Jun Li b,c, John Wang a,*

a Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576, Singaporeb Division of Bioengineering, Faculty of Engineering, National University of Singapore, Singapore 119260, Singapore

c Institute of Materials Research and Engineering, Singapore 117602, Singapore

Received 21 August 2006; received in revised form 3 January 2007; accepted 10 June 2007Available online 14 June 2007

Abstract

Highly dispersed Au nanoparticles (NPs) of average size of 9–12 nm were successfully assembled in mesopores of titania thin films,through H2-reduction of AuCl�4 . The well organized mesoporous nanostructure was characterized by using small-angle X-ray scattering,transmission electron microscopy, scanning electron microscopy, X-ray diffraction, secondary ion mass spectroscopy and UV–vis spec-trophotometry. Au NPs embedded in the mesoporous nanostructure exhibited enhanced thermal stability, up to 450 �C, which is attrib-uted to the spatial confinement effect of the mesopores with cubic configuration. An obvious surface plasmon resonance was observed at570 nm in the UV–vis absorption spectra of Au NPs embedded in the mesoporous thin films. The red shift and broadening of plasmonband observed for Au NPs are accounted for by the interaction between Au NPs and mesoporous titania framework. The mesoporousTiO2/Au nanocomposite films show optically allowed direct transition of titania, where the incorporation of Au NPs evidently reducesthe bandgap of titania.� 2007 Elsevier Inc. All rights reserved.

Keywords: Mesoporous titania structure; Gold nanoparticles; Spatial confinement; Surface plasmon resonance

1. Introduction

Owing to the very high specific surface area, uniformpore size distribution in nanometer range and chemical sta-bility, mesoporous materials have recently been used ashost matrices for loading metal and metal oxide nanopar-ticles (NPs), which promise applications in several techno-logically important areas [1–7]. For example, Garcia et al.[1] incorporated well-defined magnetic Fe2O3 particleswithin mesoporous silica through a block-copolymer-basedself-assembly approach. Platinum and palladium NPs wereformed by H2- or photo-reduction and closely packed inthe mesopores of silica films and powders [2]. Yang et al.[3] synthesized highly dispersed Au NPs in SBA-15 show-ing catalytic activity in CO oxidation, via ion-exchange

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.06.009

* Corresponding author. Tel.: +65 6516 1268; fax: +65 6776 3604.E-mail address: msewangj@nus.edu.sg (J. Wang).

and subsequent reduction by NaBH4. Fan et al. [4] realizedhighly ordered, three-dimensional Au nanocrystal/silicaarrays through self-assembly of water-soluble micellesand silica. Shi et al. [5,6] employed template displacementstrategy to produce nanosized TiO2, Fe2O3 and palladiumparticles dispersed in SBA-15 matrices. Apparently, use ofa mesoporous oxide structure with high specific surfacearea for hosting NPs of metals or metal oxides can dramat-ically improve their functionalities, such as magnetic, cata-lytic and optoelectronic activities.

Due to the relatively weak interaction between silica andmetal or metal oxide NPs, mesoporous silica is unable toeffectively reduce the mobility and thus the tendencytowards coarsening for NPs when treated at high tempera-tures [3]. Moreover, as a rather inactive mesoporous sup-port, silica often does not participate in any reaction,which greatly frustrates many applications designed forthe nanostructures. In contrast, mesoporous titania as a

Y. Zhang et al. / Microporous and Mesoporous Materials 110 (2008) 242–249 243

supporting matrix for NPs can overcome some of theselimitations. Titania itself, even in nonporous structure, isa very useful functional material for several technologicallydemanding applications, including solar energy conversion[8–10], batteries and photocatalysis [11]. For example, tita-nia can be partially reduced and allows absorption of oxy-gen on the oxide surface for catalysis, which indeed makesit an excellent catalyst [3,12]. Moreover, due to the stronginteractions with metal NPs such as Au, Ag and Pt, nano-structured titania has long been considered as a supportingmatrix to prevent aggregation and thus improve the disper-sibility of NPs [13–15]. These titania-based nanocompositeshave been proven to exhibit unique optical and electricalbehavior, including photoluminescence, surface plasmonresonance (SPR), non-linear optical behavior, and catalyticactivities [16–19]. The unusual optical and chemical behav-ior arises from the quantum size effects of NPs embeddedin the host matrices and from the interactions at the inter-faces over varying lengthy scales.

Due to both scientific interest and technologic potential,some preliminary research work has been done to incorpo-rate metal NPs, especially Au NPs into TiO2-based meso-porous structures. For example, electrodeposition anddeposition–precipitation have been employed to synthesizeAu NPs, where a strong contact of Au NPs with titaniasupport was realized [20–22]. Overbury et al. [22] studiedthe catalysis behavior of Au NPs assembled on bothmesoporous titania and mesoporous silica, and concludedthat Au NPs assembled on titania without any functionalligand exhibited a much higher activity for CO oxidation,when essentially identical Au NPs were deposited on bothmesoporous supports. Both Sono- and photochemicalapproaches have been taken to encapsulate Au, Ag, andPt NPs in the pore channels of mesoporous TiO2 films,where the air entrapped in the mesoporous film matrixwas effectively removed by sonication and gold specieswere driven into the pores [23–25]. Au-, Cu-, Pd-loadedmesoporous TiO2 photocatalysts were also prepared viasol–gel route with surfactant templating. Indeed, mesopor-ous TiO2 loaded with 2 wt% Au exhibited enhanced photo-catalytic activity for hydrogen evolution from water [26].On the one hand, these previous studies have clearly dem-onstrated that mesoporous titania with high specific sur-face area is a very desirable candidate for metal-basedcatalysts. On the other hand, it is a technological challengeto synthesize the wanted Au NPs having narrow particlesize distribution and high thermal stability. Unlike mostprevious studies that employed mesoporous TiO2 thin filmsof wormlike or lamellar configurations [20–22], where thefilms exhibited a poor ability to prevent the aggregationof Au NPs especially at elevated temperatures, we investi-gate the feasibility of assembling Au NPs in highly orga-nized mesoporous TiO2 films of cubic configuration inthe present work. The resulting Au NPs with narrow par-ticle size distribution assembled in the mesopores werestudied for thermal stability up to 450 �C. Secondary ionmass spectroscopy (SIMS) was employed for the first time

to prove the uniform distribution of Au NPs throughoutthe film thickness. The interactions between Au NPs andthe TiO2 matrix were investigated by using transmissionelectron microscopy (TEM), X-ray diffraction (XRD),and UV–vis spectrometry.

2. Experimental

2.1. Synthesis

Mesoporous TiO2 thin films were prepared at room tem-perature by following the procedures detailed below: a pre-cursor solution was prepared by mixing appropriate ratiosof ethanol, hydrochloric acid (HCl), titanium tetraisoprop-oxide (TTIP, Aldrich, 97%), acetyl acetone (AcAc) anddeionized water (H2O), and then stirred for 2 h. Triblockcopolymer Pluronic F127 (designated as EO106PO70EO106,BASF) was then dissolved in ethanol and then mixed withthe precursor solution. Molar ratios of the ingredients werecontrolled as follows: TTIP/F127/AcAc/HCl/H2O/etha-nol = 1:0.004:0.5:0.5:15:40. After stirring for three morehours, the sol solution was deposited on glass and quartzsubstrates by spin coating (3000 rpm for 1 min). A gentleheating was then applied to enhance the inorganic poly-merization and stabilize the mesophases, typically at40 �C (48 h) and then 110 �C (24 h) in air. Finally, thecopolymer template was removed by calcination in air at350 �C (4 h, 1 �C/min ramp) (sample meso-TiO2).

To assemble Au NPs in the mesoporous TiO2 network,the meso-TiO2 sample was immersed in an aqueous solu-tion of KAuCl4 (0.005 M) at room temperature for 3 days.It was then rinsed with water to remove the surface-attached excess gold solution, before being dried at 60 �Cfor 24 h. To form Au NPs, thermal treatment was carriedout in Ar/H2 flow (Ar: 50 mL/min; H2: 5 mL/min) at350 �C (sample TiO2/Au-350 �C) or 450 �C (sample TiO2/Au-450 �C) for 2 h. Without Au incorporation, the meso-TiO2 sample was also annealed in Ar/H2 flow (Ar:50 mL/min; H2: 5 mL/min) at 450 �C for 2 h (sampleTiO2/H2-450 �C), to be taken as a reference.

2.2. Characterization

Small-angle X-ray scattering (SAXS) pattern for themeso-TiO2 sample was obtained by using a Bruker AXSNanostar SAXS System under transmittance mode, withCu Ka radiation (1.54 A), operated at 40 kV and 35 mA.Both TEM (JEOL JEM 2010F, 200 kV) and high resolu-tion TEM (HRTEM, JEOL JEM 3010, 300 kV) wereemployed to study the nanostructures of the TiO2/Aunanocomposite films. The Au NPs were also studied byusing energy dispersion X-ray spectroscopy (EDX, OxfordINCA) performed in HRTEM. Field emission scanningelectron microscopy (FE–SEM, JEOL JSM 6700F) wasperformed to characterize the surface morphology and toconfirm the locations of Au NPs in the mesoporous matrix,where the sample surfaces were tilted to various degrees.

244 Y. Zhang et al. / Microporous and Mesoporous Materials 110 (2008) 242–249

XRD (Bruker AXS D8 Advance, Germany) measurementswere carried out for identification of phases and crystal-linity, using Cu Ka radiation (1.5406 A) operated at40 kV and 40 mA with a step size of 0.05� and a time perstep of 20 s. The incidence angle between the beam and filmplane was fixed at 1.5�. The depth profile of the TiO2/Aufilms was investigated by using a SIMS (ION-TOF GmbH,TOF-SIMS-IV). Their optical properties were character-ized by using a UV–visible spectrophotometer (UV-1601,Shimadzu).

3. Results and discussion

3.1. Morphology and phase evolution

SAXS pattern for the meso-TiO2 sample in Fig. 1 showsa typical cubic mesostructure, as characterized by the(110), (200) reflections, with a d110 spacing of 13.6 nm.TEM studies confirmed the cubic pore configuration ofthe mesoporous TiO2 thin film with an average pore sizeof 9.9 ± 1.0 nm, as shown in Fig. 2. The meso-TiO2 thinfilm was subsequently immersed in an aqueous solutionof KAuCl4 with pH value of 3.4 ± 0.1, to allow AuCl�4 spe-

Fig. 2. TEM image of the meso-TiO2 without Au NPs. The inset showsthe size distribution of the mesopores.

Fig. 1. SAXS pattern of the meso-TiO2 without Au NPs.

cies to diffuse into the mesopores. In such acidic KAuCl4solution, the mesoporous TiO2 surface was positivelycharged, by considering the fact that the isoelectric pointof titania is about 6. The positively charged surface defi-nitely promoted the interaction with AuCl�4 ions, facilitat-ing the assembling process. Any remaining –OH polargroups at the mesopore surface could also act as graftingsites for the AuCl�4 species, and then as nucleation sitesduring the subsequent H2-reduction treatment [17]. Theapparently high surface area of the mesoporous structurefurther facilitated both the electric attraction and –OHpolar group grafting. A set of H2-reduction conditionswere then experimented, in order to investigate the growthbehavior of Au NPs, which were subjected to a confine-ment effect imposed by the mesoporous network.

Fig. 3a and b show the TEM micrographs for TiO2/Au-350 �C and TiO2/Au-450 �C, which were thermally treatedfor 2 h in Ar/H2 flow at 350 �C and 450 �C, respectively.There was no apparent change in the distribution of AuNPs in the mesoporous network with increasing tempera-ture from 350 to 450 �C. Evaluation of 200 Au NPsselected in the TEM studies gave a mean size of9.4 ± 1.5 nm and 12.7 ± 1.8 nm for TiO2/Au-350 �C andTiO2/Au-450 �C, respectively (Fig. 3a and b, insets). Obvi-ously, these Au NPs are comparable with the mesoporesizes of meso-TiO2, where only a slight increase in size dis-tribution occurs with increasing temperature over the tem-perature range studied, confirming the spatial confinementeffect of mesopores on Au NPs (Fig. 4).

Further analysis of the SAD patterns (Fig. 3a and b,insets) reveals that the diffraction rings correspond to(11 1), (200), (220), and (311) planes of fcc crystallineAu. The crystallinity of the Au NPs increased with increas-ing H2-reduction temperature. The XRD patterns in Fig. 5further confirm the formation of a well established nano-crystalline Au phase with the appearance of two diffractionpeaks at 2h = 38.2� and 44.4�, which are ascribed to (111)and (200) planes of fcc crystalline Au, respectively. Calcu-lation from the peak width of Au (111) reflection using theScherrer formula gave an average particle size of 15–20 nmfor the Au NPs assembled in the TiO2 mesoporous net-work, which is slightly larger than what was obtained fromthe TEM study. Such discrepancy is considered to relate tothe inhomogeneous strain imposed by the mesoporousTiO2 network on nanocrystalline Au particles [27].

On the basis of EDX studies as shown in Fig. 3d, thecontent of Au NPs in TiO2/Au-450 �C was calculated tobe �7 mol% (Au/Ti). Although the EDX analysis maynot give a quantitative result, the calculated Au contentis supported by the TEM image shown in Fig. 3b, whichexhibited a rather popular distribution of Au NPs in themesoporous TiO2 film. SIMS studies performed on thesame sample TiO2/Au-450 �C, which provides informationon the Ti–O, Au, Si and O concentration clearly indicatethat Au NPs were distributed rather uniformly throughoutthe film thickness, where the film thickness was about140 nm (Fig. 6). In order to further confirm the locations

Fig. 3. TEM micrographs for: (a) TiO2/Au-350 �C; (b) TiO2/Au-450 �C.The insets show the size distributions of Au NPs and the SAD patterns of(a) and (b), respectively; (c) HRTEM of TiO2/Au-450 �C and (d) EDX ofTiO2/Au-450 �C.

Fig. 4. Spatial confinement effect of the mesopore structure on the growthof Au NPs.

Fig. 5. XRD patterns of the mesoporous TiO2/Au nanocomposite films:(A) TiO2/Au-350 �C; (B) TiO2/Au-450 �C.

Y. Zhang et al. / Microporous and Mesoporous Materials 110 (2008) 242–249 245

of Au NPs in the mesoporous film matrix, they were care-fully examined by using both SEM and TEM. Fig. 7 shows

SEM and TEM images obtained for a selected area inTiO2/Au-450 �C, where the distribution of Au NPs wasclearly demonstrated by TEM. At the same time, theseAu NPs were observed to occur within the mesopores,instead of being attached on the film surface, as shownby the SEM image (30� tilted). Indeed, no Au particleextrusions were observed on the mesoporous film surface.

Fig. 8 shows the Raman spectra for the mesoporousTiO2 and nanocomposite films thermally treated at350 �C and 450 �C, respectively. Although the low fre-quency Eg mode of anatase phase appeared weak due tothe low crystallinity in association with low thermal treat-ment temperature, it was indeed strengthened by thermaltreatment at 450 �C. The nanocrystallinity of the anatase

Fig. 6. SIMS depth profiles for TiO2/Au-450 �C, showing the uniformdistribution of Au NPs in the entire film thickness: (A) Ti–O; (B) Au; (C)O and (D) Si.

Fig. 7. SEM and TEM micrographs for a selected area in TiO2/Au-450 �C, showing the occurrence of Au NPs inside the mesopores.

Fig. 8. Raman spectra for the mesoporous TiO2 and TiO2/Au nanocom-posite films: (A) meso-TiO2; (B) TiO2/Au-350 �C and (C) TiO2/Au-450 �C.

Fig. 9. UV–vis absorption spectra of the mesoporous TiO2/Au nanocom-posite thin films: (A) meso-TiO2; (B) TiO2/Au-350 �C and (C) TiO2/Au-450 �C.

246 Y. Zhang et al. / Microporous and Mesoporous Materials 110 (2008) 242–249

phase in TiO2/Au-450 �C was further shown by HRTEMstudies. Fig. 3c is a HRTEM micrograph showing a Aunanoparticle confined in a mesopore, where the four cor-ners of nanocrystalline domain of TiO2 are clearly visible,which provides information on the formation process ofAu NPs. AuCl�4 species, when incorporated into the mes-opores, were attached to the pore surface, such that thesesalt-containing interfaces acted as centers for nucleationand then growth of the Au particles due to their high sur-face area and sufficient growth space. With increasing ther-mal temperature, both the mesoporous TiO2 network andAu NPs underwent configuration changes in associationwith crystallization and particle coarsening. The twophases were confined with each other, leading to formationof well dispersed Au NPs in the mesoporous titania net-work, which were spherical in particle morphology and

narrow in size distribution, when they were thermally trea-ted at 450 �C. At the same time, the well defined mesopor-ous structure of TiO2 was preserved.

3.2. Optical properties

Au NPs can show typical SPR in the visible range due tothe collective modes of oscillation of the free conductionband electrons at the surface, when induced by an interact-ing electromagnetic field. Thus, UV–visible spectroscopywas performed to study the SPR behavior of Au NPsembedded in mesoporous TiO2 thin films in the presentwork. Fig. 9 shows the absorption spectra of both TiO2/

Y. Zhang et al. / Microporous and Mesoporous Materials 110 (2008) 242–249 247

Au-350 �C and TiO2/Au-450 �C in the visible range, with aband being centered at 570 nm. The Au SPR band was redshifted and largely broadened when compared with that ofunsupported Au with the peak position at �520 nm [28].Considering the fact that the Au NPs were confined inthe TiO2 matrix, an interaction between them wasexpected, which explains the red shift of the plasmonabsorption peak. Indeed, the Mie theory suggests that theAu plasmon peak shifts towards a longer wavelength, whenthe medium refractive index is increased. Moreover, theobserved broadening of the SPR peak can be accountedfor by considering the spatial spreading and scattering ofthe conduction electrons of Au NPs across the particle–matrix interface.

The absorption spectra in Fig. 9 also reveal that theplasmon band increased in intensity and decreased inbroadness as the Au NPs size increased from 9.4 ±1.5 nm (TiO2/Au-350 �C) to 12.7 ± 1.8 nm (TiO2/Au-450 �C), indicating that both peak intensity and peak widthare influenced by the NPs size, essentially due to the weak-ening of surface scattering of the conducting electrons inthe Au NPs when the particle size increases. In Fig. 9, itwas also observed that the absorption intensity of back-ground increased slightly when the film was calcined at rel-atively higher temperature. While the absorption generallyoriginates from d electrons transition of Au NPs or lightscattering from film substrate, film surface characteristicscan play an important role in the overall absorption, whichchange with the thermal treatment temperature. Indeed,the surface roughness is expected to increase with increas-ing thermal treatment temperature, in association withcrystallization of the anatase phase with increasing temper-ature. Both Raman spectroscopy and TEM studies haveconfirmed that the crystallinity of anatase phase increased,when the thermal treatment temperature was raised from350 to 450 �C. The slightly higher background absorptionobserved for TiO2/Au-450 �C could therefore be relatedto the change in surface roughness.

In addition, the absorption in ultraviolet range has beenmeasured by UV–vis absorption spectroscopy to reveal theeffect of Au doping on the optical properties and electronicstructure of the mesoporous TiO2 matrix. In general, theabsorption coefficient (a) can be expressed as a functionof the incident photon energy (hm), according to

a ¼ ðA=hmÞðhm� EgÞm ð1Þ

where A is a constant depending on the nature of transi-tions indicated by the m values, where m = 1/2 and 2 arefor allowed direct and allowed indirect transitions, respec-tively, and Eg is the corresponding bandgap [29]. From Eq.(1), one can write

lnðahmÞ ¼ ln Aþ m lnðhm� EgÞ ð2Þ

and

d½lnðahmÞ�=d½hm� ¼ m=ðhm� EgÞ ð3Þ

Eq. (3) suggests that a plot of d[ln(ahm)]/d[hm] versus hmwill indicate a divergence at hm = Eg from which the valueof Eg can be estimated. Once Eg is known, the value of m

can easily be obtained from the slope of the plot of ln(ahm)versus ln(hm � Eg) (Eq. (2)). Shown in Fig. 10a is the plot ofd[ln(ahm)]/d[hm] versus hm for the meso-TiO2 thin film with-out Au NPs where one could identify a bandgap of�3.35 eV. The value of m, obtained from the slope of theplot of ln(ahm) versus ln(hm � Eg), as shown in the insetof Fig. 10a, was 2.24 indicating the occurrence of an indi-rect optical transition. This is consistent with the indirectbandgap nature expected for anatase under normal condi-tions. However, the mesoporous TiO2/Au nanocompositefilms were found to exhibit rather different optical transi-tion behavior. As shown in Fig. 10b and c, there was adiscontinuity positioning at 3.73 eV and 3.84 eV forTiO2/Au-350 �C and TiO2/Au-450 �C, respectively. Theslopes of the ln(ahm) versus ln(hm � Eg) plots (Fig. 10band c, insets) gave 0.73 (TiO2/Au-350 �C) and 0.57(TiO2/Au-450 �C), respectively, for the m value. This indi-cates that the direct, and not indirect transition, is morefavorable in mesoporous TiO2/Au nanocomposite films.Direct transition is considered to be of advantage for theenhancement of optical absorption. For comparison pur-pose, meso-TiO2 without Au NPs was H2-reduced at450 �C. Fig. 10d and inset show that the bandgap and m

value for TiO2/H2-450 �C were �3.78 eV and 0.65, respec-tively. It suggests that a variation in transition nature fromindirect to direct mode arises from the annealing process inhydrogen atmosphere, due to the hydrogen surface termi-nation and creation of oxygen vacancies [30,31]. It is alsonoted that the m values for the above transitions as evalu-ated from the ln(ahm) versus ln(hm � Eg) plots were notexactly 1/2 or 2, suggesting a combined contribution fromindirect and direct transitions.

It is therefore of interest to determine both the directand indirect bandgaps. As shown by the UV–vis absorp-tion spectra in Fig. 9, the background absorption (�0.06)due to light scattering is rather low and negligible, as com-pared to the strong absorption of titania in the UV region(�1.7). In order to ensure a high accuracy, we repeated themeasurements by more than five times for each sample overdifferent film locations. For allowed indirect transition,bandgap was determined by extrapolating the linear por-tion of the plot of (ahm)1/2 versus hm to a = 0. Fig. 11a gavebandgap energies of 3.44, 3.28, 3.16 and 2.98 eV for meso-TiO2, TiO2/H2-450 �C, TiO2/Au-350 �C and TiO2/Au-450 �C, respectively. Although these extrapolations maynot give an exact value for each of the bandgaps, they dohowever show the effects brought about by the Au NPsin the mesoporous TiO2 thin films. Firstly, a blue shift ofapproximately 0.2 eV with respect to that of the bulk ana-tase TiO2, which has a bandgap of 3.2 eV, was evident forsample meso-TiO2. This is attributed to both the quantumsize effect in association with the very fine TiO2 nanocrys-tallites of the mesoporous film and the low crystallinityof TiO2. On the other hand, the bandgaps obtained from

Fig. 11. (a) Plots of (ahm)1/2 versus hm and (b) plots of (ahm)2 versus hm forthe mesoporous TiO2/Au nanocomposite films: (A) meso-TiO2; (B) TiO2/H2-450 �C; (C) TiO2/Au-350 �C and (D) TiO2/Au-450 �C. Insets show thevariation of bandgaps for the TiO2/Au systems.

Fig. 10. Plots of d[ln(ahm)]/d[hm] versus hm, where the insets showevaluation of transition mode from the variation of ln(ahm) versusln(hm � Eg). (a) meso-TiO2; (b) TiO2/Au-350 �C; (c) TiO2/Au-450 �C and(d) TiO2/H2-450 �C.

248 Y. Zhang et al. / Microporous and Mesoporous Materials 110 (2008) 242–249

the spectra in Fig. 11a gave an evident reduction of band-gap energy for the TiO2/Au nanocomposite films, as a

result of the incorporation of Au NPs which introducedintragap energy levels between the bandgap of titania.For allowed direct transition, the bandgap energies werecalculated to be 4.06, 4.03, 4.02 and 3.98 eV for samplesmeso-TiO2, TiO2/H2-450 �C, TiO2/Au-350 �C and TiO2/Au-450 �C, respectively, by extrapolating the straight linesof the plots of (ahm)2 versus hm to intercept the X-axis(Fig. 11b). Incorporation of the Au NPs into meso-TiO2

therefore not only enhances the UV light absorption, butalso extends the absorption into the visible region, as aresult of the SPR of Au NPs and the red shift of TiO2

bandgap. It suggests that the mesoporous TiO2/Au nano-composite films can be a promising candidate for visiblephotocatalytic applications.

Y. Zhang et al. / Microporous and Mesoporous Materials 110 (2008) 242–249 249

4. Conclusions

Au NPs were successfully assembled in the mesoporousTiO2 network, which was synthesized via a copolymer tem-plating route, by H2-reduction. The confinement effect ofmesoporous TiO2 network of cubic pore configurationtowards Au NPs was demonstrated by the enhanced ther-mal stability of Au NPs, up to at least 450 �C. The embed-ded Au NPs also helped to stabilize the mesoporous TiO2

structure at elevated temperatures. On the one hand, AuNPs embedded in mesoporous TiO2 thin films showedSPR, where the plasmon absorption peak was red shiftedand broadened due to the effluence of TiO2. On the otherhand, incorporation of the Au NPs changed the bandgapenergy of mesoporous TiO2 by introducing intragap energylevels between the conduction band and the valence bandof titania. In addition, thermal annealing in a hydrogenatmosphere triggered a variation in optical transition nat-ure from indirect to direct mode, arising from hydrogensurface termination. The optical absorption of TiO2/Aunanocomposite thin films in both UV and visible rangeswere enhanced, prompting applications for UV and visiblephotocatalysis.

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