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Au or Ag nanoparticle-decorated 3D urchin-like TiO2 nanostructures: Synthesis,characterization, and enhanced photocatalytic activity

Liqin Xiang, Xiaopeng Zhao ⇑, Chaohong Shang, Jianbo YinSmart Materials Laboratory, Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, China

a r t i c l e i n f o

Article history:Received 18 January 2013Accepted 1 April 2013Available online 23 April 2013

Keywords:Au or Ag nanoparticlesUrchin-like TiO2

Surface plasmonQuenched photoluminencePhotodegradation

a b s t r a c t

The semiconductors decorated with noble metals have attracted increasing attention due to their inter-esting physical and chemical properties. Here, 3D urchin-like hierarchical TiO2 nanostructures decoratedwith Au or Ag nanoparticles were prepared by wet-chemical process. The morphology and structure werecharacterized by different techniques. It shows that Au or Ag nanoparticles with narrow distribution areuniformly loaded on urchin-like TiO2 nanostructures, and the resulted composite nanostructures showdistinct surface plasmon absorption band and quenched photoluminence compared to pure TiO2 nano-structures. Photocatalytic tests show both Au-decorated TiO2 and Ag-decorated TiO2 exhibit enhancedphotocatalytic activity for photodegradation of methyl blue in water.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

The combination of semiconductors with noble metal has beendemonstrated to be effective in areas ranging from catalysis to op-tics to biotechnology [1–5]. Recently, the assembly of noble metalnanoparticles (NPs) on semiconductors with special morphology,such as nanospheres [6], tubes [7], and wires [1,8], has raised greatinterest. Such hybrid nanostructure can combine the unique prop-erties of the complex architecture with the surface plasmon reso-nance (SPR) properties of noble metal NPs [1,6]. Furthermore, theelectrical, optical, and chemical properties of the hybrid nanostruc-ture can be adjusted by controlling the chemical compositions andmorphologies of the noble metal NPs and semiconductors indepen-dently [9–11]. As a result, such hybrid nanostructure offers newopportunities to achieve multifunctionality and has shown poten-tial in a wide range of application areas.

Because of its nontoxicity, low-cost, and high stability, TiO2 hasbeen frequently investigated in diverse fields, such as complex flu-ids, solar cells, water splitting, and photodegradation of organicpollutant [12–16]. Specifically, the photocatalysis of TiO2 has at-tracted a significant attention. However, TiO2 is far from being aperfect photocatalyst for the lack of visible light response[17,18]. One promising strategy to enhance the photocatalyticactivity of TiO2 is to introduce noble metal NPs (e.g., Au, Ag, Pt,etc.) onto the surface because it has been demonstrated that theincorporation of noble metal NPs can either enhance light-harvest-ing efficiency by surface plasmon resonance or slow down the

recombination of surface radicals by capturing photogeneratedelectrons [19,20]. Up to now, various noble metal nanoparticle-decorated TiO2 nanostructures have been prepared by differenttechniques, such as 0D Au-decorated TiO2 nanospheres, 1D Pt-dec-orated TiO2 nanotubes, 2D TiO2 films decorated with Ag and AuNPs, and so on [6,7,21–24]. Compared with these low-dimensionalnanostructures, however, decorating with noble metal NPs onto 3DTiO2 nanostructures has attracted considerably less attentionthough the 3D TiO2 nanostructures have advantages includinglow aggregation, high-surface area, and easy separation of particlesfrom solution when compared with the 0D, 1D, or 2D ones.

Herein, we report a facile wet-chemical preparation of a kind ofAu or Ag nanoparticle-decorated 3D urchin-like TiO2 nanostruc-tures. It shows that Au or Ag NPs with narrow distribution are uni-formly decorated on urchin-like TiO2 nanostructures, and theresulted composite nanostructures show distinct surface plasmonabsorption band and quenched photoluminence compared to pureTiO2 nanostructures. Under UV–vis light illumination, the Au orAg-decorated TiO2 nanostructures show enhanced photocatalyticactivity compared to the pure TiO2 nanostructures and commercialP25. More importantly, the sample is easy to separate from solu-tion by the common filter or centrifugation.

2. Experimental section

2.1. Chemicals

All the chemicals were of analytical grade and used as obtainedwithout further purification. The chemicals titanium tetrachloride

0021-9797/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2013.04.015

⇑ Corresponding author.E-mail address: [email protected] (X. Zhao).

Journal of Colloid and Interface Science 403 (2013) 22–28

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(TiCl4), tetrabutyl titanate (TBT), and toluene were purchased fromKermel Chemical Reagent Co. Ltd. of China. The chemicals silver ni-trate (AgNO3), chloroauric acid (HAuCl4), and NaBH4 were pur-chased from the Shanghai Reagent Company (PR China). Methylblue (MB) was purchased from Tianjin Chemical Reagent Co. Ltd.of China. 3-Aminopropyl-triethoxysilane (APTES) and P25 werepurchased from Sigma–Aldrich (St. Louis, USA). Deionized waterwas used throughout the syntheses.

2.2. Synthesis

Firstly, urchin-like TiO2 nanostructures were obtained by asolvothermal method described in our previous articles [25,26].Typically, 4 ml of TBT was dissolved in 30 ml of toluene in anice-water bath, and subsequently, 4 ml of 38 wt% TiCl4 aqueoussolution was added dropwise into the TBT/toluene solution understirring. The mixture was transferred into a 50 ml Teflon-linedautoclave. After heating for 24 h at 150 �C, the precipitates were fil-tered and washed with ethanol several times and then dried to ob-tain urchin-like TiO2 nanostructures. Secondly, the TiO2

nanostructures were functionalized with APTES. Simply, 0.6 g ofTiO2 was dispersed in 100 ml of ethanol, and then, 1.0 ml of APTESwas added. The suspension was refluxed at 80 �C for 6 h and wascentrifuged and washed with ethanol to obtain the functionalizedTiO2 nanostructures. Finally, 0.3 g of functionalized TiO2 nano-structures was dispersed in 50 mL of HAuCl4 or AgNO3 aqueoussolution (0.75 mM) at 30 �C. Under N2 protection, 80 mL of freshlyprepared, ice-cold NaBH4 solution (5 mM) was added slowly (2 mL/min) into the above dispersion. After stirring for 40 min, the prod-uct was filtered and washed with water three times to obtainresultant Au or Ag nanoparticle-decorated TiO2 nanostructures.

2.3. Characterization

The morphology was observed by scanning electron microscopy(SEM, JSM-6700) and transmission electron microscopy (TEM,JEOL-3010). The composition was analyzed by energy-dispersiveX-ray spectroscopy (EDS, JEOL-3010). The crystal structure wascharacterized by the powder X-ray diffraction (XRD, Philips X’PertPro) with Cu Ka irradiation (40 kV/35 mA) and step size of 0.033�in the 2h range of 10–70�. The surface area and pore size were ana-lyzed by N2 adsorption isotherms (Quantachrome Nova2000e). Thespecial surface areas were calculated using the Brunauer–Emmett–Teller (BET) model from a linear part of the BET plot (p/p0 = 0.10–0.30). The chemical groups were determined by Fourier transforminfrared spectra (FT-IR, JASCO FT/IR-470 plus) within 4000–400 cm�1 at a resolution of 4 cm�1. Absorbance spectra were mea-sured using UV–vis spectrophotometer (HITACHI U-4100), and thephotoluminescence (PL) was measured using fluorescence spectro-photometer (HITACHI F-7000) with photoexcitation wavelengthset at 215 nm.

2.4. Photocatalytic test

Analytical-grade methyl blue (MB, molecular formula: C37H27N3-

Na2O9S3, supplier: Tianjin Chemical Reagent Co. Ltd. of China) wasserved as the target organic pollutant for photocatalytic experi-ments. The photocatalytic test was performed at room temperature.Typically, 10 mg of photocatalyst was added into 30 mL of MB aque-ous solution (40 mg/L). The suspension was stirred under completedarkness condition at least for 2 h to achieve the equilibriumabsorption of MB. Then, the suspension was exposed to UV–vis lightirradiation using a 20 W low pressure mercury lamp, which hadspectral energy distribution centered at 365 nm, 405 nm, 436 nm,547 nm, and 578 nm. After a regular interval, 2 ml of suspensionwas taken from the reaction vessels. The catalyst was separated

by centrifugation, and the concentration of MB was analyzed byUV–vis spectrophotometer (HITACHI U-4100). Total organic carboncontent (TOC) was analyzed by TOC analyzer (SHIMADZU TOC-LCPN) after 3 h of UV–vis light irradiation.

3. Results and discussion

3.1. Decoration Mechanism and structure characteristic

The synthesis process of the 3D urchin-like hierarchical TiO2

nanostructures decorated with noble metal NPs is shown inFig. 1. To grow the noble metal NPs on the TiO2, we began withurchin-like hierarchical TiO2 nanostructures with diameters 1–2 lm. First, the surfaces of the urchin-like TiO2 nanostructureswere functionalized with APTES which supplied amidocyanogenfor capturing the noble metal NPs. Following this process, the par-ticles were dispersed into an aqueous solution of HAuCl4 or AgNO3

by mechanical stirring. Then, fresh ice-cold NaBH4 solution wasadded. Previous studies on primary amine-functionalized Au nano-crystals had identified that the amidocyanogen could interact withgold surface by a weak covalent bond [27]. Owing to the presenceof amidocyanogen, gold or silver NPs were adhered on the urchinsof TiO2 nanostructures once they formed.

APTES plays important role for noble metal NPs depositing onTiO2. Hydroxyl groups on the external surface of the urchin-likeTiO2 allows it to be functionalized with a layer of APTES throughsiloxane linkages. The linkage of APTES on TiO2 is confirmed byFTIR, as shown in Fig. 2. Compared with the pure urchin-likeTiO2, the sample of TiO2 functionalized with APTES exhibits somenew vibration peaks, including the Si–O symmetric stretching at1032 cm�1 and 1129 cm�1, the C–H deformation vibration at1485 cm�1, and the N–H deformation vibration at 1556 cm�1.The vibration band at 3389 cm�1 corresponding to OH, which is ob-served obviously in the spectra of TiO2, disappeared in the spectraof TiO2 functionalized with APTES. The results suggest that thefunctionalization has been accompanied by the consumption ofOH on the surface of TiO2 [28].

3.2. Au nanoparticle-decorated 3D hierarchical TiO2 nanostructures

Fig. 3 shows the morphology and structure of the representativesample of TiO2 nanostructures decorated with Au NPs. After deco-ration with Au NPs, the urchin-like morphology is not influenced(Fig. 3a and b), but a lot of Au NPs have been formed on the surfaceof needles according to the TEM image (Fig. 3c–e). The Au NPs aremainly well separated by needles, and their size has a relativelynarrow distribution range of 2–10 nm. The high-resolution TEMimage in Fig. 3e shows that the TiO2 needles are rutile phase withgrowth along [001] direction, and most of Au NPs are rhombicdodecahedra structure [29]. The XRD pattern of the sample isshown in Fig. 4a. Besides the diffraction peaks of rutile phase ofTiO2, only two weak diffraction peaks corresponding to Au (111)and Au (220) can be found in the XRD pattern. This can be attrib-uted to the low content of Au compared with that of TiO2. The EDSin Fig. 4b shows the sample contains Ti, Au, Si, and O elements.Obviously, the signal of Si element comes from APTES. The mole ra-tio of Au/Ti in the Au-decorated TiO2 nanostructures is 1.98%,which is averaged from three consecutive measurements by theEDS. In addition, it is worthy to point out that the ratio of AuNPs on TiO2 seems to be much higher from TEM (Fig. 3d) andXRD visualizations (Fig. 4a) than the EDS result. This is mainlyattributed to the fact that most of Au NPs are mainly decoratedon the tip of nanoneedles of TiO2 nanostructures (Fig. 3c). Thenitrogen adsorption and desorption isotherms (Fig. 4c) show thatthe BET surface area of the Au-decorated urchin-like TiO2

L. Xiang et al. / Journal of Colloid and Interface Science 403 (2013) 22–28 23


nanostructures is �40.0 m2/g, which is slightly higher than that(�38.3 m2/g) of the urchin-like pure TiO2 ones. Although someinterspace between needles of the urchin-like structures is overlaidby coated with Au NPs, the largely exposed surface of Au NPs may

be given a positive contribution to higher BET surface area of Au-decorated urchin-like TiO2 nanostructures. However, the BET sur-face area of these urchin-like TiO2 nanostructures is slightly lowerto that (�53.0 m2/g) of commercial P25 TiO2 photocatalyst. TheUV–vis absorption spectra (Fig. 4d) show that there is not onlythe absorption band of TiO2 at the wavelength lower than400 nm but also the absorption band at about 530 nm correspond-ing to the surface plasmon resonance of Au NPs.

3.3. Ag nanoparticle-decorated 3D hierarchical TiO2 nanostructures

Ag NPs can also be loaded on TiO2 by the similar process. Fig. 5shows the morphology of typical samples. It is found that, besidessome Ag NPs with diameters about 20 nm decorated on the TiO2

nanoneedles, there are many Ag nanodots with diameters only�2 nm adhering to the surface of TiO2 nanoneedles homoge-neously (see Fig. 5b and c). Compared to that of decorated AuNPs, however, the size of decorated Ag NPs is significantly smaller.The EDS in Fig. 5d shows the sample contains Ti, Ag, Si, and O ele-ments. The mole ratio of Ag/Ti averaged from three consecutivemeasurements by the EDS is 2.15%. Furthermore, like Au-decoratedTiO2 nanostructures, most of Ag NPs are also mainly decorated onthe tip of nanoneedles of urchin-like TiO2 nanostructures (Fig. 5b

Fig. 1. Schematic illustration of the synthesis process of urchin-like TiO2 decorated with Au or Ag NPs.

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Fig. 3. SEM images (a and b) and TEM images (c–e) of Au-decorated 3D urchin-like TiO2 nanostructures.

24 L. Xiang et al. / Journal of Colloid and Interface Science 403 (2013) 22–28


and c). The XRD pattern shows that, in addition to the characteris-tic peaks corresponding to the rutile phase of TiO2, there are two

characteristic peaks at 38.4� and 64.5� corresponding to the crystalplanes (111) and (220) of Ag (Fig. 5e). The corresponding UV–vis

Fig. 4. XRD (a) and EDS (b) of Au-decorated 3D urchin-like TiO2 nanostructures; the nitrogen adsorption and desorption isotherms (c) and UV–vis absorption spectra (d) ofurchin-like TiO2 and Au-decorated 3D urchin-like TiO2 nanostructures.

Fig. 5. TEM images (a–c), EDS (d), XRD (e), and UV–vis absorption spectrum (f) of Ag-decorated 3D urchin-like TiO2 nanostructures.



absorption spectrum of Ag-decorated TiO2 nanostructures isshown in Fig. 5f. It is found that, besides the absorption peak ofTiO2 at �340 nm, the characteristic absorption corresponding tothe surface plasmon resonance of Ag NPs can be also observed ataround 500 nm. This surface plasmon band is distinctly broadenedand shows a red shift compared to that of pure Ag NPs, probablydue to a strong interfacial electronic coupling between TiO2 andAg NPs [30,31].

3.4. Photocatalytic degradation of MB

Our previous study has shown that urchin-like TiO2 nanostruc-tures can act as photocatalyst to cause the degradation of MB [25].Here, we compare the photocatalytic efficiency of Au or Ag-deco-rated TiO2 nanostructures with pure TiO2 ones and commercialP25 photocatalyst. The photocatalytic efficiency is evaluated byMB degradation under UV–vis light irradiation. The MB degrada-tion is monitored by the time-evolution of absorbance spectra ofMB aqueous solutions. Fig. 6 shows the typical absorbance spectraof MB solution with irradiation time when Ag-decorated urchinTiO2 nanostructures are used as photocatalyst. It is found thatthe absorbing intensity of MB decreases with irradiation time, indi-cating the rapid photodegradation of MB. Fig. 7 plots the change ofnormalized temporal concentration ratio (defined by C/C0, where Cis the MB concentration after UV–vis light illumination and C0 isthe original MB concentration of before UV–vis light illumination)of MB solution as a function of irradiation time for different phot-ocatalysts. After 3 h of UV–vis light irradiation, the percentages ofMB degraded by Ag-decorated TiO2, Au-decorated TiO2, pure TiO2

nanostructures, and commercial P25 are 78%, 68%, 36%, and 32%,respectively. It is noteworthy to point out that the photolysis ofsingle MB under UV–vis irradiation in the absence of photocata-lysts is relatively slow, and MB cannot be degraded under darkconditions in the presence of the above photocatalysts, confirmingthat the photocatalytic activity indeed originates from the photol-ysis. The photocatalytic degradation efficiency follows the order:Ag/TiO2 > Au/TiO2 > TiO2 > P25. Fig. 8 shows the TOC analysis forcorresponding photocatalysts after 3 h of UV–vis light irradiation.It is found that the TOC removal efficiency is in accordance withthe concentration change obtained by UV–vis absorbance spectraof MB, indicating that MB has been well transformed into inorganiccarbon, and no other intermediate organic products are formed inthe decomposition process. Specifically, compared to P25 and pureurchin-like TiO2, Ag or Au-decorated TiO2 nanostructures have sig-nificantly higher TOC removal efficiency, also indicating that

decoration with Au and Ag NPs does play an important role inenhancing the photocatalytic activity.

The possible mechanism for the enhanced photocatalytic per-formance of the urchin-like TiO2 nanostructures after decorationwith Au and Ag NPs is investigated by the photoluminescencespectra analysis as shown in Fig. 9. It is seen that under UV excita-tion, the pure urchin-like TiO2 nanostructures exhibit the broad-band emission spectra at around 450 nm, which originates from

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Fig. 6. Absorbance spectra of MB as a function of irradiation time in the presence ofAg-decorated TiO2 nanostructures (Ag/Ti = 2.15%) as photocatalyst. The inset is UV–vis light wave of used lamp for photodegradation.

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nanostructures, urchin-like TiO2 nanostructures decorated with Au (Au/Ti = 1.98%)or Ag (Ag/Ti = 2.15%) NPs under UV–vis light.

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Fig. 9. Photoluminence spectra (kex = 215 nm) of pure urchin-like TiO2 nanostruc-tures and TiO2 nanostructures decorated with Au (Au/Ti = 1.98%) or Ag (Ag/Ti = 2.15%) NPs.



the defect state in TiO2 crystal [17]. After decoration with Au or AgNPs, the emission intensity is significantly decreased, indicatingthat most of the photogenerated electrons have not been relaxedto the defect state. This can be attributed to the effective capturefor photogenerated electrons by decorated Au or Ag NPs on thesurface of TiO2 nanostructures [32]. The capture of photogeneratedelectrons is very favorable for photocatalyst because it can effec-tively decrease the electron/hole recombination, and as a result,the photocatalytic activity of hole-induced surface radicals (suchas �OH) is increased [32]. It is interesting to note that the emissionof Ag-decorated TiO2 nanostructures is significantly weaker thanthat of Au-decorated TiO2 nanostructures. The result can well ex-plain that the former is more efficient than the latter in photocat-alytic degradation of MB. On the other hand, it has been reportedthat, under light irradiation, the SPR of noble metal NPs cannot

only enhance light-harvesting but also induce photoexcited elec-trons in the noble metal NPs on TiO2 that can move through the no-ble metal-TiO2 interface into the conductive band (CB) of TiO2. Theelectrons in the CB of TiO2 can produce superoxide radicals (see theschematic interpretation in Fig. 10), which can be used for the deg-radation of organic dyes [33,34].

The decorated amount of Au or Ag NPs has an effect on the pho-tocatalytic activity. As shown in Fig. 11, the photocatalytic activityof both Au- and Ag-decorated TiO2 nanostructures increases withthe increase of Au/Ti or Ag/Ti mole ratio. But the photocatalyticactivity Au-decorated TiO2 nanostructures are easy to become sat-urate with the increase of Au/Ti mole ratio. This may be attributedto the fact that further increasing the Au/Ti mole ratio less in-creases the number of decorated Au NPs but increases the particlesize. Finally, it is worthy to point out that the Au and Ag-decorated3D TiO2 nanostructures are very easy to separate from solution bycommon filter or centrifugation.

4. Conclusions

We have used a facile wet-chemical process to prepare novel Auor Ag nanoparticle-decorated 3D urchin-like TiO2 nanostructures.The characterization shows the sample possesses high-surface areaand uniform Au or Ag nanoparticle-decoration. In particular, thephotoluminence of TiO2 nanostructures is significantly quencheddue to the decoration of Au or Ag NPs. Under UV–vis light irradia-tion, the Au or Ag nanoparticle-decorated TiO2 nanostructures ex-hibit enhanced photocatalytic activity compared with the pureurchin-like TiO2 nanostructures and commercial P25 photocatalystdue to Au or Ag nanoparticle-enhanced electron/hole separation.The 3D TiO2 nanostructures also possess the advantage of easy sep-aration from solution compared to the 0D or 1D nanostructures.The combination of noble metal with TiO2 with special 3D hierar-chical nanostructure can offer flexibility in the design of materialswith multiple functionalities.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (Nos. 50936002, 11174234 and 51272215) andNPU Foundation for Fundamental Research (No. JC201159).

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