Composites of Metal Nanoparticles and TiO2 Immobilized in Spherical

Feb 16, 2010 - Polymer brushes here, there, and everywhere: Recent advances in their practical applications and emerging opportunities in multiple res...
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Composites of Metal Nanoparticles and TiO2 Immobilized in Spherical Polyelectrolyte Brushes )

Yan Lu,*,† Thomas Lunkenbein,‡ Johannes Preussner,§ Sebastian Proch,^ Josef Breu,‡ Rhett Kempe, and Matthias Ballauff*,†

)

† F-I2 Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Glienicker Strasse 100, 14109 Berlin, Germany, ‡Inorganic Chemistry I, University of Bayreuth, 95440 Bayreuth, Germany, §Metallische Werkstoffe, University of Bayreuth, 95440 Bayreuth, Germany, ^Clusterphysik, Universit€ at Konstanz, Universit€ atsstrasse 10, 78464 Konstanz, Germany, and Anorganische Chemie II, Universit€ at Bayreuth, 95440 Bayreuth, Germany

Received August 11, 2009. Revised Manuscript Received February 7, 2010 The synthesis and the catalytic activity of nanocomposites consisting of metal nanoparticles (Au, Pt, Pd) and nanoparticles of TiO2 (anatase) is presented. These composite particles have been synthesized by reduction of the respective metal ions adsorbed on the surface of as-prepared TiO2 nanoparticles that are immobilized on spherical polyelectrolyte brush particles (SPB) as carrier system. The SPB particles consist of a polystyrene core from which long chains of poly(styrene sodium sulfonate) are grafted. We demonstrate that the metal nanoparticles (such as Au, Pt, and Pd) are only generated on the surface of the anatase particles having a size of ca. 10 nm. These metal NP/TiO2@SPB composite particles exhibit a high colloidal stability. They are excellent heterogeneous photocatalysts for the degradation of the dye Rhodamine B under UV irradiation. The photocatalytic activity of the composite particles is 2-5 times higher than that of the pure TiO2 particles. This finding is traced back to an enhanced adsorption of the dye on the metal@TiO2 composites.

1. Introduction Nanoparticles are of considerable interest due to their unique electronic, optical, sensing, and catalytic properties.1-3 Titanium dioxide (TiO2) nanomaterials have received much attention recently because of their photocatalytic activity, high chemical stability, low toxicity and possible applications in solar cells.4-6 In particular, TiO2 photocatalysis has been accepted as one of the most promising technologies for the complete elimination of organic compounds7,8 and the deactivation of micro-organisms.9,10 The degree of crystallinity of the TiO2 particles is of central importance for the photocatalytic activity.6 Moreover, it is wellknown that anatase TiO2 has a catalytic effect superior to other phases such as rutile.11 However, titania particles synthesized by *Corresponding author. E-mail: (Y.L.) [email protected]; (M.B.) [email protected]. (1) Nanoparticles and Catalysis; Astruc, D., Ed., Wiley-VCH: Weinheim, Germany, 2008. (2) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. Rev. 2008, 108, 845. (3) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (4) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891–2959. (5) Bavykin, D. V.; Friedrich, J. M.; Walsh, F. C. Adv. Mater. 2006, 18, 2807– 2824. (6) Testino, A.; Bellobono, I. R.; Buscaglia, V.; Carnevali, C.; D’Arienzo, M.; Polizzi, S.; Scotti, R.; Morazzoni, F. J. Am. Chem. Soc. 2007, 129, 3564–3575. (7) Wang, X. H.; Li, J. G.; Kamiyama, H.; Moriyoshi, Y.; Ishigaki, T. J. Phys. Chem. B 2006, 110, 6804–6809. (8) Padmanabhan, S. C.; Pillai, S. C.; Colreavy, J.; Balakrishnan, S.; McCormack, D. E.; Perova, T. S.; Gun’ko, Y.; Hinder, S. J.; Kelly, J. M. Chem. Mater. 2007, 19, 4474–4481. (9) Fu, G.; Vary, P. S.; Lin, C. T. J. Phys. Chem. B 2005, 109, 8889–8898. (10) Seo, J. W.; Chung, H.; Kim, M. Y.; Lee, J.; Chio, I. H.; Cheon, J. W. Small 2007, 3, 850–853. (11) Mao, Y.; Wong, S. S. J. Am. Chem. Soc. 2006, 128, 8217–8226. (12) Yoo, K.; Choi, H.; Dionysiou, D. D. Chem. Commun. 2004, 17, 2000–2001. (13) Zhang, H.; Finnegan, M.; Banfield, J. F. Nano Lett. 2001, 1, 81–85. (14) Niederberger, M.; Bartl, M. H.; Stucky, G. D. J. Am. Chem. Soc. 2002, 124, 13642–13643. (15) Han, S.; Choi, S.-H.; Kim, S.-S.; Cho, M.; Jang, B.; Kim, D.-Y.; Yoon, J.; Hyeon, T. Small 2005, 1, 812–816.

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a sol-gel method usually require the addition of acid or a heat treatment to obtain systems with photocatalytic activity.12-15 The obvious technical prospects of crystalline anatase nanomaterials have therefore lead to an intense research and to a concomitantly large number of publications in this field. Recently, nanoparticles of noble metals immobilized on TiO2 have received much attention since these composites not only retain the catalytic activity of the metal nanoparticles16,17 but also possess the intrinsic photocatalytic activity of TiO2.18,19 Thus, doping TiO2 with nobel metals could greatly enhance the photocatalytic activity of TiO2.20,21 Under UV irradiation, the photogenerated electrons may quickly transfer from TiO2 surface to the metal particles, leading to effective electron-hole separation and resulting in the improvement of photocatalytic efficiency.22,23 Tada et al.24 have reviewed recent developments in the design of highly efficient photocatalytic reactions by TiO2 that has been doped with metal nanoparticles. The synergy of the charge separation and electron pool effects of noble metal nanoparticles can lead to highly active and selective photocatalyzed reactions. Up to now, different methods have been reported for the preparation of noble metal/TiO2 composites, such as conventional impregnation25,26 and deposition-precipitation (DP) (16) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405–408. (17) Pietron, J. J.; Stroud, R. M.; Rolison, D. R. Nano Lett. 2002, 2, 545–549. (18) Dawson, A.; Kamat, P. J. Phys. Chem. B 2001, 105, 960–966. (19) Tada, H.; Teranishi, T. K.; Yo-ichi, I.; Ito, S. Langmuir 2000, 16, 3304. (20) Mohapatra, S. K.; Kondamudi, N.; Banerjee, S.; Misra, M. Langmuir 2008, 24, 11276–11281. (21) Nakato, Y.; Ueda, K.; Yano, H.; Tsubomura, H. J. Phys. Chem. 1988, 92, 2316. (22) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 11439–11446. (23) Subramanian, V.; Wolf, E.; Kamat, P. V. Langmuir 2003, 19, 469–474. (24) Tada, H.; Kiyonaga, T.; Naya, S. Chem. Soc. Rev. 2009, 38, 1849–1858. (25) Chen, Y. L.; Li, D. Z.; Wang, X. C.; Wang, X. X.; Fu, X. Z. Chem. Commun. 2004, 2304–2305. (26) Li, J.; Zeng, H. C. Chem. Mater. 2006, 18, 4270–4277.

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Figure 1. Schematic presentation for the synthesis of Metal NP/TiO2@SPB composite particles.

techniques,27,28 photodeposition,29 and sol-gel routes.30 Thus, TiO2 microspheres,31 nanotubes, nanowires,32 films33 or mesoporous structures34,35 have been chosen as the solid substrate for the deposition of metal nanoparticles. Here we report the synthesis and characterization of novel metal-TiO2 nanocomposites that exhibit excellent colloidal stability in aqueous systems. The starting point of the present study is our finding that highly crystalline anatase particles with dimensions of ca. 10 nm can be prepared at room temperature using spherical polyelectrolyte brush (SPB) particles as carrier system, no further heat treatment is necessary to induce crystallization.36 The SPB particles used as carriers consist of a solid polystyrene (PS) core onto which long anionic polyelectrolyte chains of 4-styrenesulfonic acid sodium salt (NaSS) are densely grafted. As shown in ref 36, the SPB act as “nanoreactors” inasmuch as the TiO2 nanoparticles are formed within the surface layer of the polyelectrolyte chains by a sol-gel route. The interaction between the precursors and the sulfonic acid groups then leads to the formation of crystalline anatase nanoparticles at room temperature. In the present investigation we demonstrate that metal nanoparticles such as Au, Pd, and Pt can be selectively deposited on these TiO2 particles by using different metal salts. Figure 1 shows the schematic presentation for the synthesis of such composite particles: First, TiO2@SPB nanocomposites are prepared as recently described.36 In the second step, metal ions are added to the aqueous suspension of these TiO2@SPB particles. Then metal nanoparticles are generated via chemical reduction employing sodium borohydride as reducing agent. The composites of metal nanoparticles@TiO2 are thus affixed to a stable colloidal carrier. We demonstrate that the composite particles exhibit a high photocatalytic activity that can be compared to other photocatalysts in a quantitative manner. This is due to its excellent colloidal stability that allows us to conduct the kinetic analysis in a homogeneous suspension. There is no sedimentation or coagulation in these suspensions that remain stable for months. Moreover, the average size and the total amount of the metal@TiO2 (27) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (28) Yan, W. F.; Mahurin, S. M.; Pan, Z. W.; Overbury, S. H.; Dai, S. J. Am. Chem. Soc. 2005, 127, 10480–10481. (29) Sano, T.; Negishi, N.; Mas, D.; Takeuchi, K. J. Catal. 2000, 194, 71. (30) Epifani, M.; Giannini, C.; Tapfer, L.; Vasanelli, L. J. Am. Ceram. Soc. 2000, 83, 2385. (31) Bian, Z. F.; Zhu, J.; Cao, F. L.; Lu, Y. F.; Li, H. Chem Commun. 2009, 3789–3791. (32) Elmoula, M. A.; Panaitescu, E.; Phan, M.; Yin, D.; Richter, C.; Lewis, L. H.; Menon, L. J. Mater. Chem. 2009, 19, 4483–4487. (33) Wang, X. C.; Yu, J. C.; Yip, H. Y.; Wu, L.; Wong, P. K.; Lai, S. Y. Chem.; Eur. J. 2005, 11, 2997–3004. (34) Li, H. X.; Bian, Z. F.; Zhu, J.; Huo, Z. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 4538–4539. (35) Wang, X. D.; Mitchell, D. R. G.; Prince, K.; Atanacio, A. J.; Caruso, R. A. Chem. Mater. 2008, 20, 3917–3926. (36) Lu, Y.; Hoffmann, M.; Yelamanchili, R. S.; Terrenoire, A.; Schrinner, M.; Drechsler, M.; M€oller, M. W.; Breu, J.; Ballauff, M. Macromol. Chem. Phys. 2009, 210, 377–386.

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Table 1. Characterization of the Anionic SPB Particles label

PS corea [g]

NaSS [g]

water [g]

Rcoreb [nm]

Lb [nm]

PS-NaSS 377.9 11.58 780 77.3 304.7 a Solid content for PS core dispersion is 5.16 wt %. b Rcore and L were measured by DLS at 25 C in water.

particles affixed to the SPB carriers are known precisely and the total surface of these particles can be calculated. Since the photooxidation takes place on the surface of the metal@TiO2 particles,37 the measured photocatalytic activity can be related to the total surface of the active particles and compared to other systems.

2. Experimental Section 2.1. Synthesis of Spherical Polyelectrolyte Brushes. First, polystyrene (PS) core covered with a thin layer of photo initiator was prepared by conventional emulsion polymerization, which has been described in detail previously.38 The PS-NaSS (polystyrene sodium sulfonate) polyelectrolyte brushes were prepared by photoemulsion polymerization. Diluted PS core solution (2.5 wt %) was mixed with a defined amount of functional monomer 4-styrenesulfonic acid sodium salt (NaSS, 30 mol % with regard to the amount of styrene) under stirring. Photoemulsion polymerization was done by use of UV irradiation at room temperature for 60 min. Vigorous stirring ensured homogeneous conditions. Thereafter the brush particles were cleaned by dialysis first against purified water (membrane: cellulose nitrate with 100 nm pore size supplied by Schleicher & Schuell) followed by dialysis against ethanol (membrane: regenerated cellulose with 200 nm pore size supplied by Schleicher & Schuell). All pertinent parameters, namely, the core radius R and the thickness L of the attached chains as determined by dynamic light scattering are shown in Table 1. 2.2. Synthesis of TiO2@SPB. A semibatch process has been made for the synthesis of TiO2@SPB nanocomposite particles.36 For a typical run, 1.09 g PS-NaSS brush particles solution (4.62 wt % solid content) was diluted in 32 g of ethanol. 0.15 mL of water was then added under stirring. The reaction was initiated by adding the solution of 0.15 mL titanium(þ4) ethoxide (TEOT) dissolved in 8 mL of ethanol. TEOT solution was added dropwise at a feeding rate of 0.08 mL/min. The reaction mixture was vigorously stirred for two more hours after the addition of TEOT solution. The TiO2 alcosols were then washed with ethanol and water by repeated centrifugation 3 times and redispersed into water after cleaning. TGA measurement indicates that 19.8 wt % TiO2 was embedded into the SPB system.

2.3. Immobilization of Metal Nanoparticles onto TiO2@SPB Composite Particles. The preparation of metal nanoparticles in the presence of TiO2 composite particles was carried out by reduction using sodium borohydride. In a typical experiment, (37) Na, Y. S.; Kim, D. H.; Lee, C. H.; Lee, S. W.; Park, Y. S.; Oh, Y. K.; Park, S. H.; Song, S. K. Korean J. Chem. Eng. 2004, 21, 430–435. (38) Ballauff, M. Prog. Polym. Sci. 2007, 32, 1135–1151.

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0.012 g of metal salts (HAuCl4, Na2PdCl4, or H2PtCl6) was added to TiO2@SPB aqueous solution (9.69 g of TiO2@SPB latex diluted with 110 g of water), and the mixture was stirred for 30 min under an N2 atmosphere. Thereafter, sodium borohydride (0.024 g dissolved in 5 g of ice-cold water) was quickly added to the solution, which was vigorously stirred for 1 h. Finally, the metal nanocomposite particles were cleaned by serum replacement against purified water (membrane: cellulose nitrate with 100 nm pore size supplied by Schleicher & Schuell). 2.4. Photocatalytic Activity of Composite Particles. The photocatalytic activity of metal NP/TiO2@SPB nanocomposites was measured by the decolourisation of Rhodamine B (RhB) under UV radiation. The UV source was a 150 W Hg lamp (44 mm long) with higher radiation intensity level in wavelength ranges 280-360 and 460-510 nm, which was surrounded by a circulating water jacket (Heraeus) to cool the lamp. All runs were conducted at ambient pressure and temperature. The distance between the Hg lamp and the reactor was 10 cm for each experiment. In a typical run, a certain amount of metal NP/TiO2@SPB composite particle dispersion and 20 mL of an aqueous solution of RhB (2  10-5 M) were mixed together in a quartz glass reactor with stirring. This dispersion was stored in the dark for ca. 30 min prior to irradiation to establish the adsorption/ desorption equilibrium of the dye on the catalyst surface. After a given irradiation time, UV-vis spectra were taken from the sample in the range of 400-650 nm. The rate constant of the reaction was determined by measuring the decrease in intensity of the peak at 552 nm with time.39 In the case of using P25 (Degussa) as photocatalyst, the photocatalytic reaction has been done under the same experimental conditions. For the reuse of the photocatalyst, the metal NP/TiO2@SPB particles were purified and separated by ultrafiltration with cellulose membrane (100 nm pore size supplied by Schleicher & Schuell) after the application in photocatalysis. 2.5. Characterization Methods. Photoemulsion polymerization was done in a UV-reactor (Heraeus TQ 150 Z3, range of wavelengths 200-600 nm). The UV-spectra were measured by Lambda 650 spectrometer supplied by Perkin-Elmer. STEM images and EDX measurements were done in a Zeiss Libra 200FE with in-column energy filter. TEM was operated at an acceleration voltage of 200 kV by Zeiss EM922 EFTEM. Thermogravimetric measurements (TGA) were performed using a Mettler Toledo STARe system. After drying in the vacuum at 30 C overnight, the composites were heated to 800 C with a heating rate of 10 C/min under air. X-ray diffraction (XRD) measurement was performed at 25 C on a Panalytical XPERT-PRO diffractometer in reflection mode using Cu KR (λ = 1.5418 A˚) radiation. The specific surface area S of TiO2 particles has been calculated from the total amount of TiO2 in the sample (19.8 wt % in the composite particles from TGA measurement), the average size of the particles (12 nm from analysis of TEM image), and the bulk density of anatase TiO2 (F = 3.90  103 kg/m3).36

3. Results and Discussions Previously, it has been demonstrated that the strong localization of the counterions within the surface layer of polyelectrolytes of the spherical polyelectrolyte brushes (SPB) can be used to generate and immobilize metal nanoparticles40,41 and nanoalloys.42,43 More recently, we have shown that anionic SPB particles can (39) Li, J.; Ma, W.; Chen, C.; Zhao, J.; Zhu, H.; Gao, X. J. Mol. Catal. A 2007, 261, 131–138. (40) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Chem. Mater. 2007, 19, 1062. (41) Mei, Y.; Sharma, G.; Lu, Y.; Drechsler, M.; Irrgang, T.; Kempe, R.; Ballauff, M. Langmuir 2005, 21, 12229. (42) Schrinner, M.; Proch, S.; Mei, Y.; Kempe, R.; Miyajima, N.; Ballauff, M. Adv. Mater. 2008, 20, 1928. (43) Schrinner, M.; Ballauff, M.; Talmon, Y.; Kauffmann, Y.; Thun, J.; M€oller, M.; Breu, J. Science 2009, 323, 617–620.

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serve as well-defined nanoreactors for the direct generation of crystalline anatase TiO2 nanoparticles at room temperature.36 The SPB particles used in this work consist of a solid polystyrene (PS) core from which long anionic polyelectrolyte chains of 4-styrenesulfonic acid sodium salt (NaSS) are densely grafted by covalent bonds.38 TiO2 nanoparticles are generated directly within the surface layer of polyelectrolyte chains, due to the association of sulfonate groups (SO3-) on the polyelectrolyte brushes with metallorganic compounds of titanium.44,45 Thus, the titanium precursor is hydrolyzed directly in the brush layer affixed to the surface of the core particles. Figure 2a displays images of the TiO2@SPB nanocomposites obtained by transmission electron microscopy (TEM). It demonstrates that TiO2 nanoparticles are homogeneously embedded into the SPB support and no TiO2 particles can be observed outside of the carrier particles. The observed TiO2 particle sizes range from 10 to 12 nm and thermogravimetric (TGA) measurements show that 19.8 wt % TiO2 have been deposited on the anionic SPB. The composite particles form stable colloidal suspensions, no precipitation or decomposition takes place after storing for months. After the immobilization of TiO2 nanoparticles on SPBs, TiO2@SPB composite particles can serve as effective nanoreactors for in situ chemical transformation of adsorbed metal ions into respective metal nanoparticles. This is due to the chemisorption of metal ions (such as AuCl4-, PdCl42-, and PtCl62-) onto the TiO2 nanoparticle surface.46,47 No metal nanoparticles are generated in bulk. The reduction to metallic nanoparticles in the presence of TiO2@SPB composite particles was done at room temperature via the addition of NaBH4 and could be followed optically by the color change of the suspensions. All systems remained stable during the reaction and the subsequent cleaning by ultrafiltration. Figure 3a shows the photo of Au/TiO2@SPB suspension before and after reduction. The appearance of purple red color indicates the formation of Au nanoparticles in the system. This is also confirmed by UV-vis spectra as shown in Figure 3b. Here a broad absorption band centered at 528 nm appears which is characteristic for metallic gold colloids.48,49 It is worth noting that the anionic SPB particles alone are not suitable for the deposition of metal nanoparticles (such as Au, Pd, and Pt) because of the electrostatic repulsions between the negatively charged polyelectrolyte brushes and metal ions.50 In this case, only coagulation of metal particles can be observed after reduction and the resulting dispersions are totally unstable as shown in Figure S1 in the Supporting Information. Parts b-f of Figure 2 show the TEM images of Au/TiO2@SPB, Pd/ TiO2@SPB, and Pt/TiO2@SPB particles, respectively. From Figure 2, it can be seen clearly that metal nanoparticles are homogeneously embedded onto the SPB-TiO2 particles. No secondary particles or aggregates of metal nanoparticles can be found outside the composite particles. Moreover, from a comparison of the TEM images of the TiO2 composite particles before and after deposition of metal nanoparticles it can be seen that metal nanoparticles are all located on the TiO2 particles embedded in the SPB. (44) Hosono, E.; Fujihara, S.; Imai, H.; Honma, I.; Masaki, I.; Zhou, H. ACS Nano 2007, 1, 273. (45) Yelamanchili, R. S.; Lu, Y.; Lunkenbein, T.; Miyajima, N.; Yan, L. T.; Ballauff, M.; Breu, J. Small 2009, 5, 1326–1333. (46) Ma, R.; Sasaki, T.; Bando, Y. J. Am. Chem. Soc. 2004, 126, 10382–10388. (47) Soejima, T.; Tada, H.; Kawahara, T.; Ito, S. Langmuir 2002, 18, 4191–4194. (48) Creighton, J. A.; Eadon, D. G. J. Chem. Soc. Faraday Trans. 1991, 87, 3881. (49) Hatab, N. A.; Eres, G.; Hatzinger, P. B.; Gu, B. J. Raman Spectrosc., in press (DOI:10.1002/jrs.2574). (50) Lu, Y.; Mei, Y.; Walker, R.; Ballauff, M.; Drechsler, M. Polymer 2006, 47, 4985–4995.

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Figure 2. TEM images for the TiO2@SPB (a) and Au/TiO2@SPB (b, c), Pd/TiO2@SPB (d, e) and Pt/TiO2@SPB (f) composite particles. (Arrows indicate the presence of metal nanoparticles.)

Figure 3. (a) Photo of suspensions of the TiO2 composite particles before and after deposition of Au nanoparticles. (b) UV-vis spectra of TiO2@SPB solution before and after Au deposition.

In order to investigate the particle size and distribution of metal nanoparticles immobilized in TiO2@SPB composites, dark field STEM images have been taken from the sample shown in Figure 4. The typical STEM images from the Metal NP/ TiO2@SPB composite particles reveal that metal nanoparticles which are seen as bright dots in the images are confined on the surface of TiO2 nanoparticles (Figure 4, parts a, c, and e). In addition, from the STEM images it can be obviously seen that Pd nanoparticles are homogeneously deposited onto the TiO2@SPB composite particles, while some aggregates can be found in the Langmuir 2010, 26(6), 4176–4183

case of Au and Pt composite particles. Analysis of STEM images demonstrates that the size of metal nanoparticles is different: For Au nanoparticles the diameter d is 2-5 nm, for Pd, d = 2-4.5 nm, and for Pt, we obtain d = 3-6.5 nm. The formation of metal nanoparticles on TiO2 is also supported by the energy dispersive X-ray spectrum (EDX) shown in Figure 4, parts b, d, and f. The signal of metal particles is always accompanied by that of Ti, which confirms that metal nanoparticles are selectively deposited on the titania particles. This phenomenon agrees with the results from other groups for the formation of DOI: 10.1021/la904227f

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Figure 4. STEM images and corresponding EDX to prove the elemental composition of mixed Au/TiO2 (a, b), Pd/TiO2 (c, d) and Pt/TiO2 composite particles (e, f), respectively. (Cu from the copper grid).

metal nanoparticles in the presence of TiO2.46,51 Similar spectra have been observed for Pd and Pt composite particles. TGA measurements show that 4.3 wt % Au nanoparticles, 3.1 wt % Pd nanoparticles and 3.8 wt % Pt nanoparticles have been embedded, respectively. (51) Nino-Martinez, N.; Martinez-Castanon, G. A.; Aragon-Pina, A.; MartinezGutierrez, F.; Martinez-Mendoza, J. R.; Ruiz, F. Nanotechnology 2008, 19, 065711– 065718.

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In order to confirm further the formation of metal nanoparticles in the presence of as-prepared TiO2@SPB composite particles, XRD measurements are made. The X-ray diffraction pattern of Pd/TiO2@SPB composite particles (Figure 5) shows that the TiO2 carrier particles consist of highly crystalline anatase as observed previously.36 Although only 3.1 wt % Pd has been embedded onto TiO2 composite particles, weak peaks around 2θ = 40.2 and 46.9 can be still observed, and are assigned to the Pd (111) and (200) diffraction, respectively. This demonstrates Langmuir 2010, 26(6), 4176–4183

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Figure 5. PXRD pattern of Pd/TiO2@SPB composite particles. Black and blue ticks indicate reference peaks for anatase-TiO2 (PDF No. 00-021-1272) and Pd, respectively. Figure 7. Influence of Pd/TiO2@SPB composite particles concentration on the photodegradation of RhB. The concentration of the reactants was as follows: [RhB] = 0.02 mmol/L, T = 20 C. Parameter of the different curves is the concentration of composite particles in the solution. Key: triangles, 0.66 mg/L; circles, 1.10 mg/L; diamonds, 1.54 mg/L; quadrangles, 2.20 mg/L.

Figure 6. Successive UV-vis spectra of RhB photocatalytic degradation in the presence of Au/TiO2@SPB composite particles under UV irradiation. The concentration of the reactants was as follows: [RhB] = 0.02 mmol/L, [Au/TiO2@SPB] = 1.60 mg/L, T=20 C.

that metal nanoparticles have been deposited onto the TiO2@ SPB composites. In addition, it is worth noting that the PXRD pattern shown in Figure 5 is background-corrected to exclude the influence of the reflection from pristine SPB-particles (see in Figure S2 in the Supporting Information), which leads to the increase of intensity toward 2θ = 20. The original PXRD patterns of Pd, Au and Pt/TiO2@SPB particles are shown in Figure S3 in the Supporting Information, from which peaks assigned to Au and Pt diffractions can be observed as well. The photocatalytic activity of the metal NP-TiO2@SPB nanocomposite particles was measured by the photodegradation of the organic dye Rhodamine B (RhB) in the presence of composite particles. The kinetics of this reaction can be monitored by UV-vis spectroscopy as seen from UV-vis spectra measured at different times shown in Figure 6. RhB shows a strong absorption band at 552 nm. Addition of metal NP-TiO2 nanocomposite particles leads to a decrease of the absorption at 552 nm with time. The blue shift of the spectra indicates the formation of N-diethylated intermediates during the photocatalytic degradation of RhB.36 The stepwise deethylation of the RhB molecule causes the wavelength position of its major absorption band to move to the blue region (RhB, 552 nm; N,N,N0 -tritetraethylated rhodamine, 539 nm; N,N-diethylated rhodamine, 522 nm; (52) Chen, F.; Zhao, J.; Hidaka, H. Int. J. Photoenergy 2003, 5, 209–217.

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N-ethylated rhodamine, 510 nm; rhodamine, 498 nm).39,52 The color of the dispersion disappeared after the reaction, indicating that the chromophoric structure of the dye was destroyed. Additional experiments demonstrated that degradation of RhB is negligible in the absence of photocatalyst. First order rate kinetics with respect to the RhB concentration could be used to evaluate the photocatalytic rate as done previously.36 The relative concentration of RhB can be determined from the relative extinctions A/A0 measured at 552 nm. Since the photocatalysis takes place on the surface of the TiO2 particles, the apparent rate constant kapp will be in first approximation proportional to the total surface S of the TiO2 nanoparticles present in the system36 -

dct ¼ kapp ct ¼ k1 Sct dt

ð1Þ

where ct is the concentration of RhB at time t, k1 is the rate constant normalized to S, the surface area of TiO2 nanoparticles normalized to the unit volume of the system. As shown in Figure 7, the plots of ln(C/C0) versus time gave straight lines in all cases indicating the decomposition of the dyes follows firstorder kinetics. Figure 8 shows the apparent rate constant kapp as a function of the concentration of TiO2 composite particles and theoretical specific surface area of TiO2 nanoparticles, respectively. For the calculation of the surface areas of TiO2 particles, the bulk density of anatase TiO2 (F = 3.90  103 kg/m3) has been used. From Figure 8a, it can be found that in all cases the increase of the TiO2 concentration in the system leads to a marked increase of the rate constant. Further increase in catalyst loading will cause opacity and light scattering and thus impede the photocatalysis.53 In order to compare the specific turnover frequencies (TOF) of metal@TiO2 composite particles with data taken from literature, the rate constant k1 normalized to the surface area of TiO2 nanoparticles in the unit volume of the system has been calculated from the linear part of the curves shown in Figure 8b. Table 2 gives the data of photocatalytic activity of metal NP/TiO2 nanocomposite particles. The commercial product P25 (Degussa) presents the lowest photocatalytic activity which is only 60% of (53) Chen, C. Y. Water Air Soil Pollut. 2009, 202, 335–342.

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Figure 8. Rate constant kapp as a function of the concentration (a) and the surface area S (b) of metal NP/TiO2@SPB composite particles normalized to the unit volume of the system, respectively. (c,d): Effect of metal NP/TiO2@SPB nanocomposite particles amount on the rate constant (kapp) fitted with Langmuir expression. Inset is the plot of reciprocal of kapp against reciprocal of K 3 S, in which all the measured data follow the same linear function. Key: circles, TiO2@SPB composite particles; diamonds, Pt/TiO2@SPB composite particles; triangles, Pd/TiO2@SPB composite particles; quadrangles, Au/TiO2@SPB composite particles. Table 2. Photocatalytic Activity of the TiO2 Composite Particles for the Photodegradation of RhB sample

metal

k1 (min-1 m-2 L)a

k0 (min-1)

K (L/m2)

P25 (Degussa) 0.0050 0.0084 0.019 0.39 TiO2@SPB Pt 0.0166 0.022 1.25 Pt/TiO2@SPB Pd 0.0293 0.024 1.64 Pd/TiO2@SPB Au 0.0425 0.020 4.20 Au/TiO2@SPB a k1: apparent rate constant normalized to the surface of the particles in the system.

the specific photocatalytic activity of the pure TiO2 composite particles.36 From Table 2, it can be seen that the rate constant k1 of Pd, Pt or Au doped TiO2 composite particles is 2-5 times of that of pure TiO2 particles. These data can directly be compared to the literature: Misra et al.20 have reported the functionalization of TiO2 nanotubes with Pd nanoparticles of 10 nm size. They found that the rate constant of Methyl Red degradation using Pd/ TiO2 is 2 times of that of pure TiO2 nanotubes. Li et al.26 have prepared highly active Au/TiO2 photocatalysts by in situ encapsulation of Au particles into core-shell TiO2 spheres. However, no kinetic data are given. Huang et al.54 have studied the photocatalytic activity of Pt modified TiO2 loaded on natural zeolites. Doping the system with 1.5 wt % of Pt with respect to TiO2, they demonstrated that the photocatalytic activity is (54) Huang, M. L.; Xu, C. F.; Wu, Z. B.; Huang, Y. F.; Lin, J. M.; Wu, J. H. Dyes Pigments 2008, 77, 327–334.

4182 DOI: 10.1021/la904227f

1.5 times of that of the undoped TiO2. Liu et al.55 have reported the formation of Pt (2-6 nm) nanoparticles on TiO2 nanoparticles (TP) and nanowires (TNW) by microwave irradiation. It is found that the rate constant of TP/Pt and TNW/Pt is 2.3 and 2 times of that of pure TP and TNW for the photodegradation of methyl orange, respectively. Caruso et al. have developed porous Au nanoparticles/TiO2 nanocomposites for the photodegradation of methylene blue.35 They observed a 40% enhancement of the photocatalytic activity for the optimal system in comparison with the undoped control system. Thus, photocatalytic activity of TiO2 nanoparticles doped with different metal nanoparticles found in the previous literature is twice at the most when compared to the respective undoped systems. The metal NP-TiO2 nanocomposites prepared in present work exhibit the highest enhancement of the catalytic activity of TiO2 particles so far. In addition to high activity, reuse of the photocatalyst is a very important prerequisite for its practical application. Thus, the metal NP/TiO2@SPB particles were purified and separated after the application in photocatalysis for reuse. It is found that no significant decrease in photoactivity of metal NP/TiO2@SPB particles was observed after reuse for four times (see in Figure S4 in Supporting Information). Hence, the present photocatalyst system could be easily recycled and reused. This demonstrates the high stability of the present composite particles. (55) Liu, Z. L.; Guo, B.; Hong, L.; Jiang, H. X. J. Photochem. Photobiol. A: Chem. 2005, 172, 81–88.

Langmuir 2010, 26(6), 4176–4183

Lu et al.

Article

Two reasons may be invoked for the higher activity of the metal NP/TiO2@SPB particles as compared to the pure TiO2 particles: First, there may be a synergistic effect of highly disperse metal nanoparticles deposited on TiO2 in the system and the transfer of the photogenerated electrons from TiO2 to the neighboring metal particles.24 This effect would then slow down the electron-hole recombination.18 A second reason may be located in a stronger adsorption of the dye on the surface of the particles. In the following we present the first step of an analysis of this problem. Figure 8b demonstrates that the dependence of kapp on the surface S is not strictly linear.56 This finding immediately points to role of adsorption of the dye on the surface of the TiO2 nanocomposite particles. As shown by Figure 8, parts c and d, the apparent rate constant kapp can be fitted with a Langmuir-type expression:57 k0 K½TiO2  1 þ K½TiO2 

ð2Þ

1 1 1 þ k0 K ½TiO2  k0

ð3Þ

kapp ¼ which can be linearized as 1 kapp

¼

where k0 is the rate constant and K is the adsorption equilibrium constant of TiO2 composite particles. As shown in Figure 8c and d, the linear function of 1/kapp vs 1/c (or 1/S) indicates that the photocatalytic degradation of RhB in the presence of TiO2 nanocomposite particles proceeds on the surface of the particles, that is, the dye must be adsorbed on the TiO2 surface in order to react. The values of k0 and K obtained from the fit of eq 3 for Figure 8d have been summarized in Table 2. The parameter k0 depends on the reaction conditions (such as temperature, UV intensity, etc.). In our study, similar k0 has been obtained for different TiO2 composite particles, because the small differences are within the experimental limits of error. This can be further confirmed by the plot of reciprocal rate constant against 1/KS (inset of Figure 8d) leading to a master curve for all data. However, the value for the adsorption constant K for the TiO2 composite particles doped with metal nanoparticles is much (56) Konstantinou, I. K.; Albanis, T. A. Appl. Catal. B: Environ. 2004, 49, 1–14. (57) Daneshvar, N.; Rabbani, M.; Modirshahla, N.; Behnajady, M. A. J. Photochem. Photobiol. A: Chem. 2004, 168, 39–45.

Langmuir 2010, 26(6), 4176–4183

higher than the one of pure the TiO2 particles. The increase of reagents adsorption maybe due to the reason that the metal deposition using NaBH4 reduction leads to negative surface charge of metal nanoparticles,50,58,59 which will be favorable for the adsorption of positively charged RhB molecules. This is in accord with the results reported by Zhao et al.60 that addition of the anionic surfactant sodium dodecylbenzenesulfonate (SDBS) to the aqueous TiO2 dispersions greatly enhanced adsorption and photodegradation of cationic RhB. Thus, doping the TiO2 particles with metal nanoparticles leads to a stronger adsorption of the dye and concomitantly higher reaction rates in photocatalysis.

4. Conclusions Colloidal stable novel metal-semiconductor nanocomposites are successfully synthesized by using spherical polyelectrolyte brush particles as carrier system. A uniform distribution of metal nanoparticles (such as Au, Pt and Pd) selectively deposited on as-prepared TiO2 nanoparticles immobilized in SPBs is observed. The metal NP/TiO2@SPB composite particles possess excellent photocatalytic activity for the degradation of RhB under UV irradiation in that the rate constant is 3-5 times higher than that of the pure TiO2 particles. A kinetic analysis suggests that the enhanced photocatalytic activity of the metal/TiO2 composites may be traced back to an enhanced adsorption of the dye on the surface of the particles. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft, SFB 840 “Mesotechnologie”, is gratefully acknowledged. Supporting Information Available: Figures showing a photo of the suspension and TEM image of the SPB particles after deposition of Au nanoparticles, PXRD patterns of pristine SPB particles, PXRD patterns of metal NP/ TiO2@SPB composite particles, and the photocatalytic activity of Au/TiO2@SPB particles. This material is available free of charge via the Internet at http://pubs.acs.org. (58) Behrens, S.; Wu, J.; Habicht, W.; Unger, E. Chem. Mater. 2004, 16, 3085– 3090. (59) Tan, S.; Erol, M.; Sukhishvili, S.; Du, H. Langmuir 2008, 24, 4765–4771. (60) Zhao, J.; Wu, T.; Wu, K.; Oikawa, K.; Hidaka, H.; Serpone, N. Environ. Sci. Technol. 1998, 32, 2394–2400.

DOI: 10.1021/la904227f

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