Article pubs.acs.org/IECR
Facile Method to Prepare Monodispersed Hollow PtAu Sphere with TiO2 Colloidal Sphere as a Template Rong Huang, Aimei Zhu,* Yi Gong, Qiugen Zhang, and Qinglin Liu* Department of Chemical & Biochemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, The College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *
ABSTRACT: Bimetallic hollow PtAu sphere (H-PtAu) was prepared with TiO2 (titanium glycolate spheres) as a template in the presence of citric acid, and the template was removed during the formation of H-PtAu. The morphology and surface structure of H-PtAu were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The results show that the structure of hollow sphere is strongly dependent on the concentration of citric acid. The H-PtAu with uniform size is well dispersed; the Pt shell thickness can be controlled by the amount of Pt precursor. Electrocatalytic activities of H-PtAu with different Pt/Au atomic ratios were investigated by cyclic voltammetry (CV) in 0.5 M NaOH + 0.5 M methanol aqueous solution. Results show that H-PtAu catalyst has higher methanol oxidation activity than the commercial Pt/C (JM) catalyst, and the optimal Pt/Au molar ratio is 1.0.
1. INTRODUCTION Hollow metallic or bimetallic spheres, in addition to advantages of high specific surface, low density, saving of material, and reduction of cost, have attracted considerable interest due to their fascinating catalytic activities different from their solid counterparts.1,2 Hollow bimetallic spheres have been reported to be used as an unsupported catalyst for fuel cells. It is reported that the cage-bell-structured Au−Pt nanomaterials display superior catalytic activity toward oxygen reduction in proton-exchange membrane fuel cells because of their relatively higher surface areas than their solid counterparts and the electronic coupling effect between the inner-placed Au core and the Pt shell.3 Moreover, catalytic and electronic properties are strongly dependent on the size and morphology of nanomaterials, and therefore, synthesis of hollow bimetallic spheres with well-controlled morphology and size could be critical for practical purposes in construction of nanostructured catalysts. Template synthesis remains a simple and general chemical method to prepare hollow spheres with size and morphology controlled. Commonly used templates mainly include silica colloids,4,5 polystyrene colloids,6,7 selenium colloids,8 silver nanoparticles,9 ceramic hollow spheres,10 and microemulsion droplets.11 Recently, galvanic displacement reactions involving sacrificial metal nanoparticles (Co or Ag) and suitable metal ions provided a novel process to synthesize hollow nanostructured materials. For example, hollow Pt,12 Au,13 and Au/Pt alloy14 nanostructures have been prepared by employing Co nanoparticles produced in situ as a sacrificial template. However, synthesis of Co nanoparticles usually needs an inert atmosphere and would lead to a complex process. Furthermore, Pt hollow nanospheres, Pt nanotubes, and PtAu nanotubes were prepared by galvanic displacement reactions using Ag sacrificial template,. while an AgCl precipitate formed in the procedure, which made the procedure complicated and influenced the yield of hollow nanospheres. Compared to the © 2013 American Chemical Society
other templates, silica colloids (SiO2) possess better monodispersity and wider size range (nanometer to micrometer) and are easy to be synthesized or commercially obtained.15 However, removal of SiO2 required operating in extreme alkaline (pH = 12) and high-temperature (≥140 °C) conditions5 or use of 10 M HF.16 Titania (TiO2), in addition to the similar advantages of SiO2, has attracted increasing attention and been used in many different fields due to the stability, nontoxicity, and low cost. To date, there are many works reported on their applications of catalyst support in the catalytic industry.17−19 However, there have been few reports on using TiO2 as a template to prepare hollow spheres. Guo and co-workers1 reported preparing AuPt NPs assembling hollow spheres with TiO2 as a template; however, they did not investigate the exact formation mechanism of hollow sphere further. In this paper, we presented the results of systematic investigations on different factors that are responsible for the formation mechanism of a hollow sphere with TiO2 as a sacrifice template. In this work, a facile strategy was adopted to synthesize HPtAu in mild reaction conditions in which TiO2 acted as a sacrifice template and citric acid is used as a protective agent and pH adjuster. Due to the pH dependence of TiO2 stability,20 the morphology of TiO2 in nanoporous or stable dense structure could be controlled by the pH of the solution. Citratestabilized Au NPs can be easily assembled on the functionalized TiO2 precursor with NH2 group. The template TiO2 is easily removed in the process of Pt NPs depositing on the surface of gold NPs. This method has obvious advantages, such as being quick, simple, and environmental friendly and having good Received: Revised: Accepted: Published: 7432
February 22, 2013 April 26, 2013 May 10, 2013 May 10, 2013 dx.doi.org/10.1021/ie400573c | Ind. Eng. Chem. Res. 2013, 52, 7432−7438
Industrial & Engineering Chemistry Research
Article
Figure 1. Typical SEM images of f-TiO2 (A), Au/TiO2 (B), and H-PtAu (C) spheres; EDX images of Au/TiO2 (B1) and H-PtAu (C1) spheres.
(SEM) operating at 20 kV was used. Transmission electron microscope (TEM) characterization was performed on a JEM1400 electron microscope operating at 120 kV. X-ray diffraction (XRD) analysis was performed by a Rigaku (miniflex) using Cu Kα radiation (Ni filter) at 35 kV and 15 mA. X-ray photoelectron spectroscopy (XPS) was measured on a PHI QUANTUM 2000 XPS system with a monochromatic Al Kα source and charge neutralizer. All binding energies were referenced to the C 1S peak at 284.8 eV of the surface adventitious carbon. Cyclic voltammetric experiments (CV) were performed using a CHI 832 electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). A three-electrode apparatus was used for all cyclic voltammetry procedures, including Ag/AgCl (1.0 M KCl, E0 = 0.22 V vs RHE) as the reference electrode, platinum wire as the counter electrode, and a glassy carbon electrode (geometric area 0.09 cm2) as the
reproducibility and no need for a complex post-treatment process. The obtained hybrid nanomaterial with hollow cavity will probably lead to high electrocatalytic activity.
2. EXPERIMENTAL SECTION 2.1. Chemicals. 3-Aminopropyltromethoxysilane (APTMS) was purchased from J&K. Nafion (perfluorinated ion-exchange resin, 0.5 wt % solution in a mixture of ethanol and water) was purchased from Aldrich. Water used in this work was purified using the Millipore system. All other chemicals, such as chloroauric acid (HAuCl 4 ·4H 2 O), chloroplatinic acid (H2PtCl6·6H2O), and tetrabutyl titanate (Ti(OC4H9)4), were purchased from the Shanghai Chemical Factory (Shanghai, China) and used without further purification. 2.2. Characterization. An energy-dispersive X-ray (EDX) analyzer attached to a LEO 1530 scanning electron microscope 7433
dx.doi.org/10.1021/ie400573c | Ind. Eng. Chem. Res. 2013, 52, 7432−7438
Industrial & Engineering Chemistry Research
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working electrode. In order to clean and activate the electrode surface, a series of CV experiments was conducted by cyclic voltammetry between −0.8 and 0.4 V vs Ag/AgCl in nitrogenpurged 0.5 M NaOH solution at a scan rate of 50 mV·s−1 until a steady cyclic voltammogram was obtained. Electrode was cycled between −0.8 and 0.4 V vs Ag/AgCl in 0.5 M NaOH + 0.5 M CH3OH solution at a scan rate of 50 mV·s−1. 2.3. Preparation of Microsphere and Modified Electrodes. 2.3.1. Preparation of Microsphere. (a) NH2functionalized TiO2 precursor spheres (f-TiO2) and Au NPs were synthesized according to the literature1,21,22 with some modifications. Details are presented in the Supporting Information. (b) The Au/TiO2 hybrid nanostructure was prepared via mixing 10.0 mg of NH2-functionalized TiO2 precursor sphere and 120 mL of gold nanoparticles solution and then sonicating for 60 min. Product was collected and dispersed in 4.0 mL of water. (c) For synthesizing a bimetallic H-PtAu, 1.0 mL of Au/TiO2 solution was added to 29 mL of water and heated to 100 °C. Afterward, 3.0 mL of 0.1 M citric acid and a specified volume of 0.034 M H2PtCl6 were added to the mixture, followed by dropwise addition of 1.5 mL of 0.1 M VC and heating for about 30 min. By centrifuging the mixture, product was collected and dissolved in 1.0 mL of Ultrapure water. 2.3.2. Preparation of Modified Electrodes. The GC electrode was polished with 0.1 and 0.3 μm alumina slurry sequentially and then ultrasonically washed in water and ethanol for a few minutes. The cleaned GC electrode was dried at room temperature for further modification. Catalyst ink was prepared by concentrating the catalysts (H-PtAu, Au, or Pt/C) to a calculated volume. A 10 μL amount of this slurry was pipetted out on the top of the GC electrode and dried in air to yield a metal loading of 116 μg·cm−2. Then, 20 μL of Nafion (0.5 wt %) was dropped on the surface of the modified electrode.
Figure 2. Typical TEM images of Au/TiO2 (A and A1) and H-PtAu (B and B1) spheres at different magnifications.
edge, confirms their hollow essence. Furthermore, the magnified image (Figure 2B1) shows that the hollow spheres are porous and have a flower-like structure. The effect of pH on removal of TiO2 was investigated during the synthesis process. A 0.1 M HCl solution was used to adjust the initial pH of the solution. Citric acid (0.1 M, 3 mL) or sodium citrate (0.1 M, 3 mL) was added to control the final pH of the solution. Detailed results are presented in Table 1, and Table 1. Effect of Citric Acid and Sodium Citrate on the pH of the Solution
3. RESULTS AND DISCUSSION The morphology of the products was characterized using SEM and TEM. As-prepared hybrid Au/TiO2 nanospheres are of uniform size with an average diameter of ca. 300 nm (Figure 1B). Compared with the f-TiO2 spheres (Figure 1A), hybrid Au/TiO2 spheres have a lot of light spots on their surface. These light spots should be Au NPs and are well dispersed on the surface of f-TiO2 spheres. The chemical composition of the Au/TiO2 sphere was determined by EDX, as shown in Figure 1B1. Obviously, the strong Ti and O peaks can be ascribed to TiO2. The Au peak is attributed to Au NPs decorated on TiO2 spheres, and the Si peak originates from the substrate. The typical SEM image of H-PtAu is shown in Figure 1C. It can be seen that the building blocks of H-PtAu are many interconnected Au/Pt NPs, and all of the hollow spheres are unbroken. The peaks of the EDX spectrum (Figure 1C1) are associated with Au and Pt elements without any indication of Ti and O elements, indicating TiO2 spheres have disappeared after the reactions. To reveal the detailed structure, TEM was employed to characterize the products. Figure 2A and 2A1 shows typical TEM images of the Au/TiO2 sphere at different magnifications. The TEM image at high magnification (Figure 2A1) indicates many Au nanoparticles with a diameter below 10 nm being evenly loaded on the surface of the TiO2 sphere. Figure 2B and 2B1 shows typical TEM images of H-PtAu at different magnifications. The strong contrast difference in all of the spheres, with a bright center surrounded by a much darker
sample
HCl
initial pH
reagents
final pH
A B C D
− − + +
4.81 4.98 2.26 2.26
citric acid sodium citrate citric acid sodium citrate
2.20 5.55 1.95 5.12
typical SEM and TEM images of the corresponding products are shown in Figure 3. It is clearly seen that when the final pH value is less than 3 (Figure 3A, 3A1, 3C, and 3C1), hollow structures can be obtained; and final pH value is greater than 5 (Figure 3B, 3B1, 3D, and 3D1) and TiO2 is still reserved. The results demonstrate the pH dependence of the structure of TiO2 precursor spheres. Su20 and Yin23 et al. determined the surface acidic/basic properties of TiO2 by chemisorption of H+ or OH−. The strong repulsive force among charged particles reduces the probability of coalesce, and thus, more stable sols can form in acidic or alkaline media. At the same time, the isoelectric point of TiO2 is about 5−7. Away from the isoelectric range, aggregation of the TiO2 sol should be slight. However, within the isoelectric range, the TiO2 particles would coagulate rapidly. Actually, Wan24 reported that titanium glycolate can react with water at refluxing temperature and form porous spheres with hierarchical structures, consisting of about 10 nm nanoparticles interconnected. With addition of citric acid, the pH of the solution (pH < 3) far away from the 7434
dx.doi.org/10.1021/ie400573c | Ind. Eng. Chem. Res. 2013, 52, 7432−7438
Industrial & Engineering Chemistry Research
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Figure 4. TEM images of products synthesized at different concentrations of citric acid: 0 (A), 0.4 (B), 1.0 (C), and 3.0 mL (D).
SEM and TEM images of H-PtAu with different thickness of Pt shell are shown in Figure 5. When the amount of H2PtCl6 was lower (e.g., 0.1 mL), most of the hollow spheres were incompact and fractured and some even completely collapsed. It may because the Pt nanoparticles were too few to sustain the structure of the hollow sphere when the TiO2 NPs escaped from the core, which resulted in formation of a fragment and half-baked hollow sphere. With the increase of the amount of H2PtCl6 precursor, the Pt shell thickness of the hollow spheres increased and the hollow spheres were not broken and collapsed at all. When the volume of H2PtCl6 precursor increased to 0.6 mL, the Pt shell was too thick to have obvious clearance in some hollow spheres. The weak Ti signals in the EDX images (Figure S2, Supporting Information) of samples may come from the trace amount of TiO2 in the inner shell and indicate the hollow feature of samples, which agrees with the results of TEM and SEM. In addition, the effect of trace Ti (
