Colloidal Spheres Decorated with Au Nanoparticles Displaying Well

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A Facile Approach to TiO2 Colloidal Spheres Decorated with Au Nanoparticles Displaying Well-Defined Sizes and Uniform Dispersion Tatiana C Damato, Caio de Oliveira, Rômulo Augusto Ando, and Pedro HC Camargo Langmuir, Just Accepted Manuscript • DOI: 10.1021/la3045219 • Publication Date (Web): 11 Jan 2013 Downloaded from http://pubs.acs.org on January 21, 2013

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Revised Article to Langmuir, la-2012-045219

A Facile Approach to TiO2 Colloidal Spheres Decorated with Au Nanoparticles Displaying Well-Defined Sizes and Uniform Dispersion

Tatiana C. Damato, Caio C. S. de Oliveira, Rômulo A. Ando, and Pedro H. C. Camargo*

Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo-SP, Brazil *Corresponding author. E-mail: [email protected]

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Abstract

This paper describes a straightforward approach for the synthesis of hybrid materials composed of titanium dioxide (TiO2) colloidal spheres decorated with gold nanoparticles (Au NPs). In the reported method, monodisperse TiO2 colloidal spheres (~220 nm in diameter) could be directly employed as templates for the nucleation and growth of Au NPs over their surface using AuCl4-(aq) as the Au precursor, ascorbic acid as the reducing agent, PVP as the stabilizer, and water as the solvent. The Au NPs presented a uniform distribution over the TiO2 surface. Interestingly, the size of the Au NPs could be controlled by performing sequential reduction steps with AuCl4-(aq). This method could also be adapted for the production of TiO2 colloidal spheres decorated with other metal NPs including silver (Ag), palladium (Pd), and platinum (Pt). The catalytic activities of the TiO2-Au materials as a function of composition and NPs size was investigated towards the reduction of 4nitrophenol to 4-aminophenol under ambient conditions. An increase of up to 10.3 folds was observed for TiO2-Au relative to TiO2. A surface-enhanced Raman scattering application for TiO2-Au was also demonstrated employing 4-mercaptopyridin as the probe molecule. The results presented herein indicate that our approach may serve as a platform for the synthesis of hybrid materials containing TiO2 and metal NPs displaying well-defined morphologies, compositions, and sizes. This can have important implications for the design of TiO2-based materials with improved performances for photocatalysis and photovoltaic applications.

Keywords: titanium dioxide, gold, colloidal spheres, metal nanoparticles, monodisperse, catalysis, SERS.

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Introduction

Titanium dioxide (TiO2) has been widely employed in commercial products such as paints and heath care goods. It displays unique optical and electronic properties, biocompatibility, chemical stability, and low cost.1,2 In addition, TiO2 has attracted considerable attention in the area of sustainable energy and environmental remediation due to its semiconducting properties.1,2 Examples include the use of TiO2 as a photocatalyst for the water splitting reaction, for the photodegradation of organic pollutants, and in the development of dye sensitized solar cells.3-9 However, TiO2 presents a relatively large band gap that requires ultraviolet light for activation. Also, the high recombination rates of the photo-generated electron-hole pair lead to detrimental effects over its photocatalytic and photovoltaic performances.1,10,11 It has been reported that a promising alternative to improve the photocatalytic and photovoltaic properties of TiO2 is to decorate its surface with noble metal nanoparticles (NPs) including gold (Au), silver (Ag), palladium (Pd), and platinum (Pt).10,12-19 In this context, the noble metal NPs can act as electron traps for the electron-hole separation, thus improving the quantum yield.3,8,13,20 Also, metals that support localized surface plasmon resonance (LSPR) in the visible range (such as Ag and Au)21,22 can augment the formation of electron-hole pairs in the semiconductor region comprised by the electric fields originated from the LSPR excitation, the so called plasmon-enhanced charge generation.19,23-30 It is important to note that noble metal nanostructures display distinctive chemical and physical properties that have been enabling a wealth of applications in areas such as catalysis, electronics, plasmonics, sensing, imaging, and medicine.31-34 Therefore, the formation of hybrid materials composed of TiO2 and noble metal NPs allows one to combine the attractive properties of these two classes of materials.


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Various methodologies have been described for the synthesis of TiO2 and metal NPs hybrids.

These

include

impregnation,13,23,35

UV

irradiation,12

electrodeposition,36

sonochemistry,37 and hydrothermal,24 sol-gel,38 and flame-spray synthesis.39 Although these methods have been effective, the precise control over particle size, monodispersity, particle distribution, and composition in the final materials still remains challenging. Moreover, many reported studies involve rigorous conditions or complicated experimental procedures. Hence, the development of simple strategies for the synthesis of TiO2 and metal NPs hybrids in which the aforementioned parameters can be tightly controlled are essential to optimize and design properties for both current and novel applications. We report herein a facile and environmentally friendly approach for the synthesis of TiO2 colloidal spheres decorated with Au NPs (TiO2-Au). More specifically, monodisperse TiO2 colloidal spheres ~220 nm in diameter composed of anatase nanocrystallites ~10 nm in size could be directly employed as templates for the nucleation and growth of Au NPs over their surface. The described method employed AuCl4-(aq) as the Au precursor, ascorbic acid as the reducing agent, PVP as the stabilizer, and water as the solvent. It is important to mention that no prior steps of chemical modification at the TiO2 surface were required to achieve Au NPs deposition. The deposited Au NPs presented a uniform distribution over the TiO2 surface as well as monodisperse sizes. Interestingly, the size of the deposited Au NPs could be controlled by performing sequential AuCl4-(aq) reduction steps. This approach could also be modified to enable the synthesis TiO2 materials containing Ag, Pd and Pt NPs. The catalytic activity of the TiO2-Au materials as a function of composition and particle size was investigated by employing the reduction of 4-nitrophenol as a model reaction. Finally, the application TiO2-Au as substrates for surface-enhanced Raman scattering (SERS) sensing was demonstrated.


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Experimental Section

Materials and instrumentation Analytical grade chemicals HAuCl4·3H2O, (hydrogen tetrachloroaurate trihidrate, 48% in gold, Synth), AgNO3 (silver nitrate, ≥ 99.0%, Sigma-Aldrich), K2PdCl4 (potassium tetrachloropalladate, 98%, Aldrich), H2PtCl6·6H2O (chloroplatinic acid hexahydrate, > 37.5% Pt basis, Sigma-Aldrich), PVP (polyvinylpyrrolidone, Sigma-Aldrich, M.W. 55.000 g/mol), EG (ethylene glycol, 99.5%, Vetec), C3H6O (acetone, 99.5%, Vetec), C2H4O2 (acetic acid, 99.7%, Synth), C6H8O6 (ascorbic acid, 99.0%, BioXtra), Ti(OBu)4 (titanium butoxide, 97%, Sigma-Aldrich), NaBH4 (sodium borohydride, 95%, Vetec), C6H5NO3 (4-nitrophenol, >98% Merck-Schuchardt), and C5H5NS (4-mercaptopyridine, 95%, Sigma-Aldrich) were used as received. All solutions were prepared using deionized water (18.2 MΩ). The scanning electron microscopy (FEG-SEM) images were obtained with a JEOL microscope FEG-SEM JSM 6330F operated at 5 kV. The samples for SEM were prepared by drop-casting an aqueous suspension of the nanostructures over a Si wafer, followed by drying under ambient conditions. Energy-dispersive X-ray spectroscopy (EDS, Thermo Electron Corporation) was performed at an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) images were obtained with a JEOL 1010 microscope at 80 kV. The sample for TEM was prepared by drop-casting an aqueous suspension of the sample over a carbon coated copper grid, followed by drying under ambient conditions. Inductively coupled plasma mass spectrometry was performed in a Spectro Ciros CCD ICP optical emission spectrometer at the Analytical Instrumentation Center of University of São Paulo. A 201.265 nm line was employed for acquisition and a 5-point calibration curve with a correlation coefficient R2 > 99.9 was prepared from analytical grade chemicals. The samples were prepared by suspending a known mass of material in 5 mL of water, followed by the addition


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of 5 mL of aqua regia to this suspension. This mixture was left under stirring for 1 h and a 1 mL aliquot was employed in the analysis. Three distinct measurements were performed in each case, and the average value was considered. X-ray diffraction (XRD) patterns of powdered samples were recorded on a Rigaku diffractometer model Miniflex using CuKα radiation (1.541 Å, 30 kV, 15 mA). UV-VIS spectra were obtained from aqueous suspensions containing the nanostructures with a Shimadzu UV-1700 spectrophotometer. The Raman spectrum for TiO2 was obtained directly from the powder in a FT-Raman Bruker RFS 100/S using the 1064 nm line of a Nd/YAG laser which was focused at the sample with 50 mW of laser power and spectral resolution at 2.0 cm-1.

Synthesis of TiO2 Colloidal Spheres In a typical procedure,40 1 mL of Ti(OBu)4 was added to 22.5 mL of EG. This mixture was kept under vigorous stirring at room temperature for 8h. This solution was then quickly poured into 100 mL of acetone containing 1.25 mL of deionized water and 0.4 mL of acetic acid. This mixture was kept under vigorous stirring at room temperature for another 3 hours, yielding a white precipitate that was harvested by centrifugation. The solid, comprised of titanium glycolate microspheres, was washed several times with ethanol by successive rounds of centrifugation and removal of the supernatant. In the next step, the titanium glycolate product was added to 50 mL of water and this mixture was stirred for 8h at 70 oC to produce TiO2 colloidal spheres, which were isolated by centrifugation, washed with water several times, and re-suspened in water.

Synthesis of TiO2 colloidal spheres decorated with Au NPs (TiO2-Au) Typically, 1 mL of an aqueous TiO2 suspension (~0.9 mg) was added to 6 mL of an aqueous solution containing 35 mg of PVP and 60 mg of ascorbic acid. This mixture was


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heated to 90oC and kept under stirring. After 10 min, 3 mL of a 1 mM AuCl4-(aq) solution was added and the reaction allowed to proceed for 3 h. This was the first reduction step. It is important to note that after the addition of 1 mM AuCl4-(aq), the color of the reaction mixture gradually changed from white to red, indicating the reduction of AuCl4-(aq) and the formation of Au NPs. A second reduction step was performed by adding another 3 mL of a 1 mM AuCl4-(aq) solution to the reaction mixture obtained at the end of the first reduction step, followed by stirring at 90oC for another 3h. Similarly, a third reduction step was carried out by adding 3 mL a 1 mM AuCl4-(aq) solution to the reaction mixture obtained at the end of the second reduction step, followed by stirring at 90oC for another 3h. The solids that were obtained after the first, second and third reduction steps were denoted TiO2-Au1, TiO2-Au2, and TiO2-Au3, respectively. They could be isolated by stopping the reaction at the end of each corresponding reduction step. In all cases, the products were harvested by centrifugation, washed several times with water, and re-suspended in water for further use.

Study of the catalytic activities: reduction of 4-nitrophenol to 4-aminophenol For the study of the catalytic activities of the produced materials, 150 µL of a 1.4×10-4 M 4-nitrophenol aqueous solution, 1 mL of 4.2×10-2 M sodium borohydride aqueous solution, and 80 µL of 1 mL aqueous suspensions containing TiO2, TiO2-Au1, TiO2-Au2 or TiO2-Au3 (containing 65.6, 27.2, 28.8, and 43.2 µg of solid, respectively) were added into a quartz cuvette. Then, the change in absorbance at 400 nm was monitored by UV-VIS spectroscopy as a function of time. In this case, the color of the solution gradually changed from yellow to transparent as the reduction of 4-nitrophenol to 4-aminophenol took place.

Surface-enhanced Raman scattering (SERS) application


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An aqueous suspension containing TiO2-Au3 was drop-casted over a Si wafer and dried under ambient conditions. Functionalization with 4-mercaptopyridine (4-MPy) was performed by immersing this substrate containing TiO2-Au3 in a 1 mM aqueos solution (5 mL) of 4-MPy for 1 h. The sample was then taken out, washed with copious amounts of ethanol, and finally dried under ambient conditions. All samples were used immediately for SERS measurements after preparation. SERS spectra were acquired on a Renishaw Raman InVia (Renishaw, New Mills, Wotton-under-Edge, UK) equipped with a charge-coupled device detector and coupled to a Leica microscope (BTH2, Leica Microsystems GmbH, Ernst- Leitz-Strabe, Germany) that allows the rapid accumulation of Raman spectra with a spatial resolution of about 1 µm (micro-Raman technique). The laser beam was focused on the sample by a ×50 lens. Laser power was always kept below 0.7 mW at the sample. The experiments were performed under ambient conditions using a backscattering geometry. The samples were irradiated with the 632.8 nm line of a Renishaw RL633 laser (Renishaw, New Mills, Wotton-under-Edge, UK).

Results and discussion

Synthesis of TiO2 colloidal spheres decorated with Au NPs Our studies began with the synthesis of monodisperse TiO2 colloidal spheres. This procedure followed a two-step polyol approach, which started with the preparation of titanium glycolate that was subsequently converted to TiO2 by its reaction with water at 70 oC for 8 h.40 It has been reported that the synthesis of monodisperse TiO2 colloidal spheres by conventional sol-gel methods is difficult to achieve as the hydrolysis rates of precursors are too fast, which in turn do not allow the separation between the nucleation and growth stages. In order to overcome this problem, glycols are attractive as they are reactive enough to form,


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when mixed with alkoxide precursors, glycolates or mixed alkoxide/glycolate derivatives displaying much lower hydrolysis rates.41,42 Here, these derivatives were obtained by the reaction between Ti(OBu)4 and EG at room temperature for 8 h. The ethylene glycolate derivative was then poured in water containing small amounts of acetone and acetic acid (1.25 and 0.4 %, respectively) to produce titanium glycolate spheres,40 which are shown in Figure 1A. It can be observed that they displayed spherical shape, smooth surfaces, and were monodisperse in size (228 ± 18 nm in diameter). Figure 1B shows a SEM image of the TiO2 colloidal spheres 231 ± 17 nm in diameter that were produced after the hydrolysis of titanium glycolate. Both the spherical shape and monodisperse size distribution were maintained. Moreover, the TEM image (inset in Fig. 1B) shows that the TiO2 colloidal spheres were comprised of nanocrystallites ca. 10 nm in size and displayed rough surfaces. The Raman spectrum and XRD pattern (Figure 1C and D, respectively) reveled that the as-obtained TiO2 was crystallized as anatase. Specifically, the Raman spectrum (Fig. 1C) displayed peaks at 151, 406, 515 and 635 cm-1 that can be assigned to the Raman active modes of anatase with symmetries Eg, B1g, A1g, and Eg, respectively.43-45 The detected shift of the Eg mode from 141 cm-1 in bulk anatase to 151 cm-1 in the TiO2 colloidal spheres can be ascribed to phonon confinement effects due to the decrease in the crystallite size.44,45 The crystallite size estimated by the Scherrer’s formula from the XRD pattern (Fig. 1D) was 10.3 nm. Both these observations are in agreement with the TEM results. After their synthesis, we turned our attention on the utilization of the TiO2 colloidal spheres as templates for the nucleation and growth of Au NPs over their surface. In particular, we aimed at producing TiO2 colloidal spheres decorated with Au NPs in which the size and coverage of Au NPs on the TiO2 surface could be tailored. Interestingly, we discovered that the size of the deposited Au NPs over the TiO2 surface could be controlled by performing successive reduction steps employing AuCl4-(aq) as the metal precursor. This


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approach is shown in Scheme 1. In the first reduction step, the TiO2 colloidal spheres served as templates for Au nucleation and growth. Here, 3 mL of a 1 mM AuCl4-(aq) solution was employed as the metal source, PVP as the stabilizer, ascorbic acid as the reducing agent, and water as the solvent. This procedure yielded TiO2-Au hybrid materials comprised of TiO2 colloidal spheres decorated with Au NPs (TiO2-Au1, Scheme 1). In the second step, the TiO2 colloidal spheres decorated with Au NPs (TiO2-Au1) were employed in situ as templates for further Au nucleation and growth. This was performed by adding, to the reaction mixture, another 3 mL of a 1 mM AuCl4-(aq) solution (yielding sample TiO2-Au2, Scheme 1). Similarly, a third reduction step was performed by adding another 3 mL of 1 mM AuCl4-(aq) to the reaction mixture, in which TiO2-Au2 served as templates for further Au deposition, producing sample TiO2-Au3 (Scheme 1). SEM images of the products TiO2-Au1, TiO2-Au2, and TiO2-Au3 are shown in Figure 2A-C, respectively. They could be separately isolated by stopping the reaction at the end of each reduction step, followed by successive rounds of centrifugation and removal of the supernatant. After the first reduction step (sample TiO2-Au1, Fig. 2A), the formation of Au NPs 12.2 ± 2.2 nm in diameter over the TiO2 surface was observed. The deposited Au NPs were relatively monodisperse, presented spherical shape, and their deposition over the TiO2 surface was uniform (no significant aggregation at the surface was detected). In this method, the nanoporous surface of the TiO2 colloidal spheres could serve as effective sites for the Au nucleation and subsequent growth. It is important to mention that the nanoporous surface was essential to enable the uniform distribution of Au NPs over the TiO2 colloidal spheres. For instance, when the titanium glycolate spheres (that had smooth surfaces) shown in Fig. 1A were employed as templates in this process, the deposition of Au NPs over the titanium glycolate was not uniform (Fig. S1). As more AuCl4-(aq) was added (second reduction step, Fig. 2B), the size of the Au NPs


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at the TiO2 surface increased to 19.3 ± 2.3 nm. In this case, the Au NPs deposited after the first reduction step (TiO2-Au1) served as seeds for additional Au deposition and growth. SEM images also indicate that almost no nucleation events took place directly at the surface of the TiO2, as the coverage of Au NPs over the TiO2 colloidal spheres in Fig. 2B was similar to Fig. 2A. In fact, it can be expected that the Au deposition over the TiO2 surface together with deposition at the pre-formed Au NPs would lead to an increase in the size distribution of the deposited Au NPs. After the third reduction step (Fig. 2C), the deposited Au NPs served as sites for further Au nucleation and growth, causing an increase in the diameter of the Au NPs to 24.5 ± 2.8 nm. Similarly to what was described for the second reduction step, no deposition of Au NPs directly over the TiO2 surface seemed to take place. ICP-MS analysis showed that the Au weight percentage for samples TiO2-Au1, TiO2-Au2, and TiO2-Au3 corresponded to 9.5, 16.4, and 33.7, respectively. This result is in agreement with the increase in the Au content in the TiO2-Au colloidal spheres after the successive reduction steps. Figure 2D shows UV-VIS extinction spectra recorded for TiO2, TiO2-Au1, TiO2-Au2, and TiO2-Au3 (black, red, blue and green traces, respectively). While TiO2 displayed no bands in the visible range, samples TiO2-Au1 and TiO2-Au2 presented a broad and weak signal centered at 530 nm. This band can be assigned the dipole mode of the surface plasmon resonance (SPR) excitation in Au NPs ~ 12 and 20 nm in size, respectively.46,47 For TiO2-Au3, the SPR peak red-shifted to 540 nm as a result of the increase in the NP size to ~25 nm. These results are in agreement with the size-dependent far-field properties of Au NPs.22,46-48 The inset in Fig. 2D depicts a digital photograph of TiO2, TiO2-Au1, TiO2-Au2, and TiO2-Au3 aqueous suspensions (from left to right, respectively). While the TiO2 suspension had a white color, samples TiO2-Au1, TiO2-Au2, and TiO2-Au3 were red and became gradually darker as the Au content was increased. In order to investigate the importance of the sequential reduction steps over the


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formation of TiO2-Au materials, we also carried out a control experiment in which a single reduction step was performed by increasing the concentration of the AuCl4-(aq) precursor solution from 1 to 5 mM and including the utilization of PVP as a stabilizing agent. Figure S2 shows SEM images of the TiO2-Au material obtained in this experiment. Conversely to what was observed by employing the 1 mM AuCl4-(aq) precursor solution, the Au NPs were not monodisperse and their deposition over the TiO2 colloidal spheres was not uniform. These observations indicate that a lower AuCl4-(aq) concentration (1 mM) was required in the first reduction step in order to achieve a uniform distribution of Au NPs over the TiO2 surface. It is possible that the increase in the AuCl4-(aq) concentration was enough to raise the Au concentration during the reduction stages so that nucleation and growth were not efficiently separated.

Applying this approach to other metal NPs: the case of Ag, Pd, and Pt We were also interested in investigating if we could utilize this method to produce TiO2 colloidal spheres decorated with other noble metal NPs, such as Ag, Pd, and Pt. We started by performing a series of experiments employing the TiO2 colloidal spheres as templates, PVP as the surfactant, ascorbic acid as the reducing agent, and 3 mL of 1 mM Ag+(aq), PdCl42-(aq), or PtCl62-(aq) solutions as the Ag, Pd, or Pt sources, respectively. These conditions were similar to those described in Scheme 1 for the first reduction step using AuCl4-(aq). The schematic representation and corresponding SEM images of the products obtained in these experiments are shown Figure S3. Our SEM results clearly showed that Ag and Pd did not deposit over the TiO2 surface and thus the formation of TiO2-Ag and TiO2-Pd colloidal spheres did not take place (Fig. S3A and B). While the Pd concentration in sample TiO2-Pd obtained by ICP-MS was minimal (the weight percentage was 0.7), the Ag weight percentage in sample TiO2-Ag was 10.5. However, we believe that the presence of Ag in this


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sample does not correspond to the formation of TiO2-Ag materials. The presence of Ag in this sample is likely due to the presence of isolated Ag NPs together with the TiO2 colloidal spheres in the product, in agreement with the SEM image (Fig. S3A). It is possible that, due to their relatively large sizes, these Ag NPs could not be efficiently separated from the TiO2 colloidal spheres during the washing steps (successive rounds of centrifugation and removal of the supernatant). Conversely to what was observed for Ag and Pd, Pt NPs could be uniformly deposited over the TiO2 surface to produce TiO2-Pt colloidal spheres (Fig. S3C). These results indicate that the formation of TiO2 colloidal spheres decorated with metal NPs was dependent on the nature of the metal. More specifically, the uniform nucleation and growth of metal NPs onto the TiO2 surface by our approach was favored only for Au and Pt. Although the fundamental reasons for this observation remains unclear, it is plausible that the utilization of the HAuCl4·3H2O and H2PtCl6·6H2O acids as the Au and Pt precursors, respectively, could lead to an increase in the H+ concentration in the reaction mixture promoting the formation of positive charges over the TiO2 surface. This could favor the adsorption of the negatively charged AuCl4- and PtCl62- species on the TiO2 surface and thus the nucleation and growth of Au and Pt NPs upon reduction. On the other hand, the AgNO3 and K2PdCl4 salts were employed as the Ag and Pd precursors, respectively. The utilization of these precursors is not expected to promote the formation of positive charges on the TiO2 surface (by the release of H+ in the reaction mixture), not enabling the adsorption of either Ag+ (which is also positively charged) or PdCl4- over the TiO2 under the described conditions. On the other hand, we could employ the TiO2-Au colloidal spheres obtained after the first reduction step (Fig. 2A, TiO2-Au1) as seeds in a second reduction step employing 3 mL of 1 mM Ag+(aq), PdCl42-(aq), or PtCl62-(aq) as the Ag, Pd, or Pt sources, respectively. Figure 3 shows a schematic representation as well as SEM images of the corresponding TiO2-AuAg,


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TiO2-AuPd, and TiO2-AuPt colloidal spheres obtained by this approach. Similar to what was discussed for Au, the NPs deposited in the first reduction step could serve as effective sites for the nucleation and growth of Ag, Pd or Pt NPs. The presence of the two metals was confirmed by EDS analysis. The Ag/Au, Pd/Au, and Pt/Au atomic percentages for the products shown in Fig. 3A-C were 0.5, 0.25, and 0.13, respectively. These observations indicates that this approach may serve as a general platform to the generation of TiO2 colloidal spheres decorated with a variety of metal NPs in which the size and composition of the final materials can be controlled.

TiO2-Au materials: applications in catalysis and SERS We also investigated the catalytic activities of the TiO2 colloidal spheres decorated with Au NPs. We were particularly interested in studying how catalytic activities of the produced TiO2-Au materials were dependent upon their composition and Au NPs size. To this end, we employed the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride as a model reaction.49 The reduction of 4-nitrophenol was chosen as it is wellestablished that it can be catalyzed by noble metal nanoparticles due to the particle-mediated electron transfer from borohydride ions to 4-nitrophenol.49-52 Also, the color changes associated with the conversion of 4-nitrophenolate to 4-aminophenol enables one to easily monitor the reaction kinetics by UV-VIS spectroscopy.49-52 Finally, the product from this transformation, 4-aminophenol, is an important intermediate in the synthesis of analgesic and antipyretic drugs.53 We employed an excess of sodium borohydride in our experiments. Under this condition, 4-nitrophenolate ions are the dominant species in the reaction solution and display an absorption peak at 400 nm. Thus, the decrease in the strong adsorption at 400 nm can be employed to monitor the extent of the reaction (consumption of 4-nitrophenolate ions) and to calculate the rate constants (k) from the slope of the linear correlation between ln


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(Ct/C0) and time.49 Figure 4A-D shows the plots of ln (Ct/C0) and the absorption spectra (insets) as a function of time registered at room temperature for the TiO2 (Fig. 4A) and TiO2Au colloidal spheres obtained after the first (TiO2-Au1, Fig. 4B), second (TiO2-Au2, Fig. 4C), and third (TiO2-Au3, Fig. 4D) reduction steps. After an induction period in which no reaction took place, the reaction started and could be described by a first-order rate law (segment in red). The calculated rate constants (k) in terms of total mass and in terms of the mass of Au present in the catalysts are depicted on Table 1. The rate constants in terms of total mass corresponded to 0.2, 1.0, 1.9, and 2.3 min-1mg-1 for TiO2, TiO2-Au1, TiO2-Au2, and TiO2-Au3, respectively. This corresponds to an increase of 4.5, 8.2, and 10.3 folds in the catalytic activity (in terms of total mass) for the TiO2-Au1, TiO2-Au2, and TiO2-Au3 colloidal spheres, respectively, relative to the pure TiO2. This indicates that the catalytic activity for the 4-nitrophenol reduction reaction in terms of total mass was significantly enhanced as the Au weight percentage on the TiO2-Au materials increased. As Au NPs act as a catalyst for the 4-nitrophenol reduction, it can be expected that an increase in the Au content in the TiO2-Au materials would lead to higher catalytic activities. If we consider the catalytic activity only in terms of the mass of Au present in the catalysts, the rate constants calculated for samples TiO2-Au1, TiO2-Au2, and TiO2-Au3 were 10.8, 11.3, and 6.9 min-1mgAu-1. Although the Au NPs size increased from 12.2 ± 2.2 to 19.3 ± 2.3 nm in samples TiO2-Au1 and TiO2-Au2, respectively, the catalytic activity in terms of Au mass were similar in these materials (insensitive to the change in Au NPs size). It is important to note that plasmonic effects may also influence the catalytic activities in TiO2-Au materials. This contribution should become more significant as the magnitude of the electric fields generated close the Au NPs surface as a result of the SPR excitation increases with Au NPs size. Therefore, it is plausible that the expected decrease in the catalytic activity due to increase in the Au NPs size was somewhat balanced by the enhancement due to plasmonic


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effects in sample TiO2-Au2 relative to TiO2-Au1. For sample TiO2-Au3, a drop of 38.9% in the catalytic activity (relative to TiO2-Au2) was observed as the Au NPs size increased to 24.5 ± 2.8 nm. This result suggests that the increase in Au NPs size was the dominant factor to determine the catalytic activity in this sample. It is important to note that this sample is comprised of Au NPs closely joined together at the surface of the TiO2 colloidal spheres, which can also contribute to the decrease in the catalytic activity by mass of Au. As the TiO2-Au3 colloidal spheres contained Au NPs 24.5 ± 2.8 nm in size that were closely joined together on the TiO2 surface (Fig. 2C), they are attractive for applications in SERS sensing.22 In this case, the junctions and gaps between the deposited Au NPs can enable the presence of a large number of electromagnetic hot spots on the surface of the TiO2-Au colloidal spheres.54, 55 In SERS, hot spots may refer to gaps or junctions between two or more particles in which colossal electromagnetic SERS enhancements are observed as opposed to their individual particles counterparts.56 As a demonstration of their SERS application, the TiO2-Au3 colloidal spheres were drop-casted onto a Si wafer, functionalized with 4-mercaptopyridine (4-MPy), and their SERS spectrum was recorded. Figure 5 shows the SERS spectrum obtained under these conditions (employing 632.8 nm as the excitation wavelength). The characteristic 4-MPy bands can be clearly observed in the spectrum as a result of a significant signal enhancement due to the utilization of TiO2-Au3 colloidal spheres as substrates.57 No peaks were observed under similar conditions without the utilization of TiO2-Au3. It is important to note that an accurate estimation of the enhancement factor is limited by the morphology of the TiO2-Au sample, which makes it difficult to precisely determine the available Au surface area and, consequently, the number probe molecules contributing to the detected SERS signals.


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Conclusions

We described a facile, versatile, and environmentally friendly strategy for the synthesis of monodisperse TiO2 colloidal spheres decorated with Au NPs. Our methodology was based on the utilization of successive reduction steps employing AuCl4-(aq) as the Au precursor, PVP as the stabilizer, ascorbic acid as the reducing agent, and water as the solvent. Interestingly, no prior surface modification steps at the TiO2 colloidal spheres were required for the deposition of Au NPs. In addition to achieving a uniform distribution of Au NPs at the TiO2 surface, the reported methodology enabled the control over the composition of the TiO2-Au materials and the size of the deposited Au NPs. This approach could also be expanded for the production of TiO2 colloidal spheres decorated with other metal NPs including Ag, Pd, Pt. In this case, the synthesis of TiO2-Pt, TiO2-AuAg, TiO2-AuPd, and TiO2-AuPt was reported. The catalytic activities of the TiO2-Au colloidal spheres as a function of their composition and Au NPs size was investigated towards the reduction of 4nitrophenol. Our results indicated that an increase of up to 10.3 folds in the catalytic activity (in terms of total mass of catalyst) could be obtained relative to pure TiO2. The application of the TiO2-Au colloidal spheres as substrates for SERS sensing was demonstrated employing 4-MPy as the probe molecule. In this case, a significant enhancement in the 4-MPy signals was observed in the presence of the TiO2 colloidal spheres. The results presented herein indicate that the reported approach may serve as a general platform for the synthesis of TiO2 colloidal spheres decorated with metal NPs displaying well-defined morphologies, compositions, and sizes. This can have important implications for the design of TiO2 based materials with improved performances for photocatalysis and photovoltaic applications.


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Acknowledgements

This work was supported by FAPESP (grant number 2011/06847-0), CNPq (grant numer 471245/2012-7), and start-up funds from Universidade de São Paulo (grant numbers 11.1.25042.1.0 and 2012-145). T.C.D. and C.C.S.O. thank the CNPq for the fellowships. We thank Prof. Denise de Oliveira Silva at USP for the XRD analysis.

Supporting Information Available

Additional SEM images. This information is available free of charge via the Internet at http://pubs.acs.org/

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Figure 1. SEM images of titanium glycolate (A) and TiO2 (B) colloidal spheres. Both products were uniform and displayed spherical shape. The titanium glycolate microspheres were smooth, the TEM image (inset in B) shows that the TiO2 colloidal spheres were comprised of nanocrystallites ca. 10 nm in diameter and displayed rough surfaces. The Raman spectrum (C) and XRD pattern (D) indicated that the TiO2 was crystallized as anatase.


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Scheme 1. Strategy for the synthesis of TiO2 colloidal spheres decorated with Au NPs having controlled sizes and uniform NPs distribution. This approach was based on successive reduction steps employing AuCl4-(aq) as the Au precursor, PVP as the stabilizer, ascorbic acid as the reducing agent, and water as the solvent.


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Figure 2. (A-C) SEM images of TiO2-Au colloidal spheres that were obtained after the first (A), second (B), and third (C) reduction steps employing 3 mL of 1 mM AuCl4-(aq) as the precursor solution (products TiO2-Au1, TiO2-Au2, and TiO2-Au3, respectively). The scale bars in the insets correspond to 100 nm. The Au NPs were uniformly deposited over the TiO2 surface and their sizes were 12.2 ± 2.2, 19.3 ± 2.3, and 24.5 ± 2.8 nm in (A-C), respectively. (D) UV-VIS extinction spectra recorded for the TiO2 (black trace) and TiO2-Au (red, blue, and green traces correspond to samples TiO2-Au1, TiO2-Au2, and TiO2-Au3, respectively). The inset depicts a photograph of TiO2, TiO2-Au1, TiO2-Au2, and TiO2-Au3 aqueous suspensions (from left to right, respectively).


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Figure 3. Schematic representation and SEM images of the TiO2 colloidal spheres decorated with (A) Au and Ag; (B) Au and Pd; and (C) Au and Pt NPs. These products were obtained by employing the TiO2-Au1 colloidal spheres shown in Fig. 2A (obtained after the first reduction cycle with 3 mL of 1 mM AuCl4-(aq)) as templates for a second reduction step employing 3 mL of 1 mM Ag+(aq) (A), PdCl42-(aq) (B) and PtCl62-(aq) (C) as the Ag, Pd and Pt precursors, respectively. The Ag/Au, Pd/Au, and Pt/Au atomic percentages for the products shown in (A-C) were 0.5, 0.25, and 0.13, respectively.


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Figure 4. Plots of ln (Ct/C0) as a function of time registered at room temperature for TiO2 (A) and TiO2-Au colloidal spheres obtained after the first (TiO2-Au1, B), second (TiO2-Au2, C), and third (TiO2-Au3, C) reduction steps, respectively. The rate constants (k) were calculated from the linear section of the curves (shown in red). The insets in (A-D) depicts the absorption spectra of the solution as a function of time. In this case, the band centered at 400 nm (from nitrophenolate ions) decreases as reaction takes place.


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Figure 5. SERS spectra recorded from TiO2-Au colloidal spheres (obtained after the third reduction step) that had been functionalized with 4-MPy. The excitation wavelength was 632.8 nm.


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Table 1. Summary of the Au weight percentages obtained by ICP-MS and rate constants (k) obtained from Fig. 6 for TiO2 and TiO2-Au colloidal spheres as a function of composition and NPs size Sample

Au weight percentage

k (min-1mg-1)

k (min-1mgAu-1)

TiO2

-

0.2

-

TiO2-Au1

9.5

1.0

10.8

TiO2-Au2

16.4

1.9

11.3

TiO2-Au3

33.7

2.3

6.9


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Colloidal Spheres Decorated with Au Nanoparticles Displaying Well

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