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Journal of Crystal Growth 311 (2009) 508–511

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

� Corr

Kyungw

South K

E-m

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

Sonochemical deposition of nanosized Au on titanium oxides with differentsurface coverage and their photocatalytic activity

Daniel Yang a, Sang-Eun Park a,b, Joong-Kee Lee b, Sang-Wha Lee b,�

a Department of Chemical & Bio Engineering, Kyungwon University, San 64 Bokjeong-dong, Soojung-gu, Seongnam-si, Gyeonggi-do, South Koreab Advanced Energy Materials Processing Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea

a r t i c l e i n f o

Available online 24 September 2008

Keywords:

A1. Nanostructures

A2. Growth from solutions

B1. Metals

B1. Titanium compounds

B3. Photocatalyst

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.09.058

esponding author at: Department of Che

on University, San 65 Bokjeong-dong Soojung-g

orea. Tel.: +82 317505360; fax: +82 317505363.

ail address: lswha@kyungwon.ac.kr (S.-W. Lee

a b s t r a c t

Different surface coverage of nanosized Au on the TiO2 matrix was simply fabricated in an ultrasound-

driven cell by adjusting the loading volume of gold precursor. Non-annealed Au–TiO2 composites with

low surface coverage of nanosized Au exhibited a significantly enhanced photocatalytic activity in

comparison to pristine TiO2. However, a further increase of nanosized Au loading consequently led to a

decrease of photocatalytic efficiency of Au-deposited TiO2, probably due to the reduction of available

surface area for organic pollutant adsorption and light absorption. The X-ray photoelectron spectro-

scopy (XPS) analysis of Au-deposited TiO2 indicated the coexistence of metallic Auo and non-metallic Au

species that might play a role as an electron scavenger. The reduction of photoluminescence (PL)

intensity also indicated the efficient separation of electrons and holes even in Au–TiO2 composites

without heat treatment.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Nanoscale titanium oxides have been actively studied inoptoelectronics, solar cell, and photocatalysis due to their uniqueoptical, photochemical, and catalytic properties that are distinctfrom those of the bulk phase. The deposition of noble metals ontitanium oxide is advantageous to improve the photocatalyticactivity of their composites because the deposited metal can actas a reservoir of photo-generated electrons that can enhancethe interfacial charge-transfer process [1]. The better separationof photo-generated charge carriers consequently reduces theelectron–hole recombination and enhance the photocatalyticactivity [2].

Metal doping on titanium oxide has been achieved by a varietyof techniques such as impregnation, photo-deposition, electrolessplating, sol–gel method, and so forth [3]. In recent years, powerultrasound irradiation has been used for anchoring noble metalson various substrates (such as silica, carbon, titanium oxide, andpolystyrene) [4–7]. The reactivity of ultrasound irradiation isoriginated from acoustic cavitation phenomena involving thecreation, growth, and implosive collapse of a bubble in thesolution [8,9]. Ultrasound-induced cavitation is widely used todisperse nanoparticles within the pore of silica and/or to produce

ll rights reserved.

mical and Bio Engineering,

u, Seongnam-si Gyeonggi-do,

).

nanostructured materials in various forms [10,11], but it is hard tofind the systematic investigation of photocatalytic activity ofmetal–TiO2 composites with controllable surface coverage ofnanosized particles.

In this report, we present a sonochemical reduction method fora facile deposition of nanosized Au on the TiO2 matrix withcontrollable surface coverage simply by adjusting the loadingvolume of gold precursor (1.0 wt% HAuCl4). The photocatalyticactivity of Au–TiO2 composites with different surface coveragewas determined by the degradation of organic pollutants (such asmethyl orange (MO) and phenol) and discussed toward thedirection for attaining higher energy conversion efficiencies. Theresulting Au–TiO2 composites were characterized by transmissionelectron microscopy (TEM), X-ray photoelectron spectroscopy(XPS), photoluminescence (PL) spectroscopy, and UV–vis absorp-tion spectroscopy.

2. Experimental section

2.1. Sonochemical deposition

Degussa P25 (TiO2) was dried in a vacuum oven, followed byheat treatment at 773 K for 2 h. The annealed TiO2 (10 mg) and0.4 ml of 1.0 wt% HAuCl4 were added to 50 ml of distilled water ina sonication cell that was attached to the sonicator horn under theflowing N2 gas. Nitrogen gas was bubbled through the slurry toexpel any dissolved oxygen in the solution. Sonication of the

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D. Yang et al. / Journal of Crystal Growth 311 (2009) 508–511 509

slurry was carried out by adding 50ml of ammonia (28 wt% inwater) and 13ml of isopropanol (99.8%), immediately followed byultrasound irradiation in a sonicator cell (Sonics & Materials,VCX500, 20 kHZ, 150 W) at 313 K for 2 h [12]. The productsrecovered by centrifugation at 4000 rpm were washed withdistilled water three times to remove any residual precursors,and then stored in distilled water at room temperature for afurther characteristic analysis.

2.2. Photocatalytic activity

The photocatalytic activity of Au-deposited TiO2 was measuredin terms of degradation of MO and phenol. The photodegradationreaction was carried out with a homemade photoreactor equippedwith a 400 W high-pressure Hg lamp (lmax ¼ 365 nm) and an airventilation system. The temperature of aqueous solution is main-tained at 313 K under UV radiation intensity of 1240mW/cm2. Foreach experiment, 5 mg of photocatalyst was added into a quartztube containing 50 ml solution with MO (or phenol) of 30 ppm.The absorbance of residual MO and phenol in the solution wasmeasured with a UV–vis spectrometer at 464 and 269 nm,respectively [7,13].

2.3. Characterization

The XPS measurements were carried out on a Bruker AXISX-ray photoelectron spectrometer under the base pressure of1�10�8 Torr. For XPS measurements, Mg/Al Ka radiation with anenergy of 450 W was used for composition and binding state of Auspecies. The surface morphology and adherence of nanosized Auto titanium oxide were investigated with TEM (Model: HITACHI,H7600 microscope), working at 80 kV accelerating voltage. Tostudy the recombination of electrons/holes in the photocatalyst,the PL emission spectra of the samples were measured.Absorption spectroscopy (Model: HP 8453) was carried out inthe wavelength region of 200–1000 nm.

Fig. 1. Au-deposited TiO2 by the sonochemical reduction method with different

loading volumes of the gold precursor: (a) 0.4 ml of 1.0 wt% HAuCl4 and (b) 0.8 ml

of 1.0 wt% HAuCl4.

3. Results and discussion

3.1. Electron microscopic studies

Sonochemical reduction method is used to deposit nanosizedmetal on the various substrates including polymeric materials.The anchored metal nanoparticles are considered to form strongchemical bonds or interactions with the substrate. A plausiblemechanism for ultrasound-driven deposition is that nanosizedparticles are driven toward the substrate surface at very highspeeds, which cause the sintering and/or formation of strongchemical bonds [3,12].

We have prepared Au–TiO2 composites with different surfacecoverage of nanosized Au and then TEM analysis was conducted toinvestigate the morphology and average diameter of nanosized Auanchored on the TiO2 matrix. Fig. 1 shows the TEM images of Au-deposited TiO2 prepared at different loading volumes of goldprecursor (e.g., 0.4 and 0.8 ml of 1.0 wt% HAuCl4). The averagediameter of nanosized Au was estimated as the almost samevalues irrespective of loading volume of gold precursor, whereasthe surface coverage of nanosized Au on the TiO2 matrix was fairlyincreased with the increase of loading volume of the goldprecursor.

According to the analysis of TEM images, the average diameterof nanosized Au was estimated as 5.2771.03, 4.3371.26, and5.5370.92 nm, corresponding to a respective loading volume of0.2, 0.4, and 0.8 ml gold precursor. Highly dispersed gold

nanoparticles were observed at lower loading volume of goldprecursor. For loading volume more than 0.8 ml, TEM imagesdistinctly showed more than five Au nanoparticles deposited onthe TiO2 matrix, whereas one or two Au nanoparticles were onlyobserved on the TiO2 matrix for loading volume o0.2 ml. Highersurface loadings of metallic particles may result in an efficiencydecrease, possibly due to an optimal metal size and surfacecoverage that is available for light absorption and pollutantadsorption [14,15].

3.2. X-ray photoelectron spectroscopy (XPS)

XPS analysis was performed to elucidate the chemical oxida-tion state of nanosized Au anchored on the TiO2 matrix. Fig. 2(b)shows the doublet peaks of Auo located at 83.4 and 87.0 eV, whichare slightly lower than those of bulk metallic Au centered at 84.0and 87.7 eV, respectively [16]. The downshift of binding energiescan be understood as negative Au nanoparticles originated fromthe charge transfer from the TiO2 matrix that has higher workfunction [7]. As shown in Fig. 2(a), Au-deposited TiO2 withoutheat treatment exhibited an asymmetrical broadening and higherbinding energies in reference for annealed Au–TiO2 composites.The deconvoluted Au 4f peaks suggested the coexistence ofmetallic Auo and non-metallic Au (Au+ and/or Au3+) that might

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Fig. 2. XPS Au 4f7/2-5/2 core-level spectra: (a) Au-deposited TiO2 without heat

treatment and (b) Au-deposited TiO2 with heat treatment at 573 K.

Fig. 3. UV–vis spectra of Au-deposited TiO2 as a function of loading volume of the

gold precursor (1.0 wt% HAuCl4).

Fig. 4. The photoluminescence (PL) spectra of pristine TiO2 and non-annealed Au-

deposited TiO2 with different loading volumes of the gold precursor (1.0 wt%

HAuCl4).

D. Yang et al. / Journal of Crystal Growth 311 (2009) 508–511510

play a role as an electron scavenger [17]. Further investigationfor the detailed interaction between metal nanoparticles andtitanium oxide is currently underway and will be reportedelsewhere.

3.3. Optical spectra studies

Fig. 3 shows the UV–vis spectra of Au-deposited TiO2 with theincremental loading volume of gold precursor. Compared with thepristine TiO2, the absorption intensity of Au-deposited TiO2 wassignificantly enhanced with the increase of loading volume of goldprecursor. However, the absorbance band ascribed to the plasmonresonance of metallic Au was not observed in the UV–vis spectraof non-annealed Au–TiO2 composites. The intensity of absorptionbands of Au–TiO2 composites exhibited a strong dependence onthe surface coverage of nanosized Au on the TiO2 matrix. For lowerloading volume of gold precursor o0.2 ml, however, there was nodistinct difference of UV–vis spectra between samples. Fig. 3 alsoshowed the UV–vis spectrum of Au-deposited TiO2 with heattreatment at 773 K. The annealed Au–TiO2 composites exhibited abroad absorption peak around 520–550 nm due to the surfaceplasmon resonance of gold nanoparticles [18].

The PL emission spectra have been used to investigate thecharge carrier separation (or recombination of electron/hole pairs)[19]. As shown in Fig. 4, TiO2 powders show a distinct PL emissionband in the wavelength range of 400–700 nm. The intensity of PLspectrum of the pristine TiO2 is higher than that of Au-depositedTiO2 without heat treatment. The reduction of PL intensityindicates the retardation of recombination process probably dueto the efficient charge transfer into highly dispersed nanosized Au.The experimental results also indicated that PL spectra of Au–TiO2

composites are sensitive to the loading amount of Au on the TiO2

matrix. That is, PL spectrum of Au-deposited TiO2 (0.2 ml ofloading volume) is lower than that of Au-deposited TiO2 (0.8 ml ofloading volume).

3.4. Photocatalytic activity

The photocatalytic activity of Au-deposited TiO2 without heattreatment was determined by photodegradation of organics(MO and phenol) under UV light irradiation for 30 min. As shownin Fig. 5, photocatalytic degradation of organics exhibited the

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Fig. 5. Photodegradation of organics (MO and phenol) by non-annealed Au-

deposited TiO2 under UV light irradiation for 30 min.

D. Yang et al. / Journal of Crystal Growth 311 (2009) 508–511 511

maximal values at optimal loading of gold precursor (0.1–0.2 ml of1.0 wt% HAuCl4).

The photocatalytic degradation of MO showed strongerdependence on the nominal Au loadings in comparison to phenoldegradation. Low Au loadings on the TiO2 matrix exhibited theenhanced photocatalytic activity than Au-deposited TiO2 withrelatively high Au loadings. Zhang et al. [7] proposed the optimalsize of gold nanoparticles for the acquisition of highly photo-catalytic activity. If the particle is too small enough for the higherFermi level than that of the conduction band of TiO2 or too largeenough for the lower Fermi level than that of adsorbed O2, thephotoelectrons cannot be transferred to adsorbed oxygen species.

As previously mentioned, nanosized Au anchored on the TiO2

matrix was ranged in 4.3–5.5 nm of average diameter irrespectiveof nominal Au loadings (0.2–0.8 ml) of gold precursor. However,the photocatalytic activity of Au-deposited TiO2 was significantlydifferent depending on the surface coverage of nanosized Au, i.e.,maximal photocatalytic activity was obtained at optimal Auloadings. As a photocatalyst of Au–TiO2 composite, it has beenreported that higher loading of Au species have a negative effecton the photocatalytic process [20]. Even though electron–holerecombination was retarded by the electron scavenging and/orinterfacial charge transfer by Au+/Auo particles on the TiO2 matrix,further increase of Au loadings induced the decrease of theavailable surface area for organic pollutant adsorption and lightabsorption, consequently leading to a decrease of the photo-catalytic efficiency.

4. Conclusions

Au–TiO2 composites with different surface coverage of nano-sized Au was simply fabricated in an ultrasound-driven cell byadjusting the loading volume of the gold precursor. Nanosized Auanchored on the TiO2 matrix was estimated to have an averagediameter of ca. 5.0 nm according to the analysis of TEM images.The deconvoluted Au 4f peaks of non-annealed Au–TiO2

composites indicated the coexistence of metallic Auo andnon-metallic Au (Au+ and/or Au3+) species, which might play asan electron scavenger. The reduction of PL intensity alsoimplicated the efficient separation of electrons and holes inAu-deposited TiO2 without heat treatment. The coexistence ofmetallic Auo and non-metallic Au (Au+ and/or Au3+) on the TiO2

matrix was found to be beneficial for enhancing photocatalyticdegradation of organic pollutants (such as MO and phenol).

Acknowledgements

This work was partly supported by the GRRC program ofGyeonggi province and KOSEF grant funded by the Koreagovernment (MOST) (2006-05382).

References

[1] P.V. Kamat, J. Phys. Chem. 111 (2007) 2834.[2] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891.[3] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss,

A. Gedanken, Langmuir 18 (2002) 3352.[4] Z.Y. Zhong, Y. Mastai, Y. Koltypin, Y.M. Zhao, A. Gedanken, Chem. Mater. 11

(1999) 2350.[5] V.G. Pol, A. Gedanken, J. Calderon-Moreno, Y. Mastai, Chem. Mater. 15 (2003)

1378.[6] V.G. Pol, H. Grisaru, A. Gedanken, Langmuir 21 (2005) 3635.[7] B. Tian, Jinlong Zhang, T. Tong, F. Chen, Appl. Catal. B: Environ. 79 (2008) 394.[8] K.S. Suslick, G.J. Price, Annu. Rev. Mater. Sci. 29 (1999) 295.[9] A. Gedanken, Ultrason. Sonochem. 11 (2004) 47.

[10] M.V. Landau, L. Vradman, M. Herskowitz, Y. Koltypin, A. Gedanken, J. Catal.201 (2001) 22.

[11] N. Perkas, Y. Wang, Yu. Koltypin, A. Gedanken, S. Chandra-sekaran, Chem.Commun. (2001) 988.

[12] V.G. Pol, A. Gedanken, J. Calderon-Moreno, Chem. Mater. 15 (2003) 1111.[13] P. Han, S. Park, S. Lee, J. Korean Ind. Eng. Chem. 17 (2006) 100.[14] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647.[15] I.M. Arabatzis, T. Stergiopoulos, D. Andreeva, S. Kitova, S.G. Neophytides,

P. Falaras, J. Catal. 220 (2003) 127.[16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of

X-ray photoelectron spectroscopy, Physical Electronics, Inc., Minnesota,1995, p. 182

[17] Y.C. Liu, L.C. Juang, Langmuir 20 (2004) 6951.[18] S. Park, M. Park, P. Han, S. Lee, Bull. Korean Chem. Soc. 27 (2006) 1341.[19] X.Z. Li, F.B. Li, C.L. Yang, W.K. Ge, J. Photochem. Photobiol. A: Chem. 141 (2001)

209.[20] V. Subramanian, E.E. Wolf, P.V. Kamat, Langmuir 19 (2003) 469.