Laser-Induced Fabrication of Hollow Platinum Nanospheres for

Article pubs.acs.org/JPCC

Laser-Induced Fabrication of Hollow Platinum Nanospheres for Enhanced Catalytic Performances Hyeri Lee, Jin-Ah Kwak, and Du-Jeon Jang* Department of Chemistry, Seoul National University, NS60, Seoul 151-747, Korea S Supporting Information *

ABSTRACT: The simple irradiation of 355 nm nanosecond laser pulses to SiO2@Pt core−shell nanospheres at fluence of 2.7 mJ cm−2 during the preparation process of hollow platinum nanospheres has been found to enhance the catalytic performances of platinum nanocatalysts on a large scale. Laser irradiation has transformed platinum nanoclusters topped on silica nanospheres into well-defined platinum nanoshells having uniform and smooth surfaces; the thickness of platinum nanoshells has been tuned easily by adjusting the irradiation time only. Laser irradiation increases the catalytic performances of hollow platinum nanospheres in the degradation of rhodamine B in the presence of KBH4 by five times via lowering the energy barrier. The energetically more favorable formation of the activated complexes in the nanocavity surfaces is suggested to reduce the activation energy substantially. The restructuring of surface atoms induced by photothermal annealing during laser irradiation has rendered the metallic surfaces much easier to chemisorb reactants and to facilitate electron relays, enhancing the catalytic performances of platinum nanocatalysts extensively.

1. INTRODUCTION Nanostructured materials with functional properties have been widely explored as they can be employed potentially in a variety of technologies; nanometer-sized materials have fascinating properties that are complementary or superior to those of bulk materials.1−4 In particular, noble metal nanostructures have attracted considerable attention from researchers scientifically as well as industrially5 because of their novel optical, physical, and chemical properties, which are substantially different from the respective properties of bulk-scale noble metals.6 One of the characteristic optical properties of noble-metal nanoparticles is the localized surface-plasmon resonance (SPR), caused by collective oscillations of free electrons excited resonantly by external electromagnetic waves at certain frequencies. The frequencies of SPR depend strongly on the shape, size, and environment of nanoparticles, making them very attractive for diverse practical applications.7,8 Among various noble-metal nanostructures, platinum nanoparticles have attracted widespread attention because platinum plays an outstanding role in multifunctional nanocatalysts for various industrial reactions. However, because there are some limiting factors such as low platinum-utilization efficiency and high cost, some ways must be found to reduce the amount of platinum used in specific applications in order to lower the overall cost.9,10 Hollow noble-metal nanoparticles11 have shown a range of interesting properties compared to their solid counterparts.12,13 In particular, hollow-structured14 platinum nanoparticles have high surface-to-volume ratios and low densities to exhibit a large enhancement in their catalytic activity.15,16 Reactions occurring in the cavities of hollow nanoparticles could be facilitated by two main factors: the increased concentration of © 2014 American Chemical Society

reaction intermediates within the cavity of the hollow nanocatalyst and the possibility of catalytically more active sites within the inner surface of the hollow nanocatalyst.12,17 Therefore, the rates of the reactions in cavities increase due to the nanocage effect, otherwise known as the confinement effect.18,19 However, it is quite difficult to produce complete metal nanoshells. Laser-induced fabrication is the simplest and cleanest technique among diverse synthetic methods.20,21 So, the interactions of noble-metal nanoparticles with pulsed lasers have been the subject of intensive research with the aim of improving the basic understanding of interactions between light and nanomaterials.22,23 Controlled heating with laser pulses has been employed to reshape metallic nanostructures.24 It has been reported23 that the excitation energy of SPR becomes thermalized in a few picoseconds to heat the lattice and then cooled slowly by transferring heat to the surrounding medium.25 For all metal nanostructures regardless of their initial shapes, heating and melting by either thermal26 or nonthermal sources will allow them to transform into their thermodynamically most-stable shape, that is a solid nanosphere. It is well-known that the melting points of lowdimensional solids such as nanoparticles, thin films, and nanofibers are considerably reduced relative to the those of their respective bulk solids;27 the melting point of a platinum nanostructure is much lower than that of bulk platinum.28 It has been also demonstrated that metal nanoparticles and their aggregates can be transformed into smaller or larger Received: July 23, 2014 Revised: August 29, 2014 Published: September 9, 2014 22792

dx.doi.org/10.1021/jp5073704 | J. Phys. Chem. C 2014, 118, 22792−22798

The Journal of Physical Chemistry C

Article

nanostructures by laser excitation.29,30 The excitation of a SPR band with picosecond or nanosecond laser pulses results in the melting and size-reduction of noble-metal nanoparticles in a solution.30 Thus, the thermalized photon energy of SPR excitation can induce rough nanoshells consisting of platinum seeds to be transformed into laser-smoothed platinum nanoshells. We have synthesized hollow platinum nanostructures via a hard-template method31 and smoothed their surfaces using nanosecond laser pulses. The fabrication procedure of the platinum nanoshells by irradiation with 355 nm laser pulses of 4 ns at fluence of 2.7 mJ cm−2 is shown in Figure 1. The hard-

Aldrich. Ultrapure deionized water (>17 MΩ cm) was obtained using a Millipore Milli-Q system. Synthesis of Platinum Nanocatalysts. Hard templates of silica nanospheres were prepared via the sol−gel process36 of TEOS under base catalysis following the Stöber method.37 A total of 25.00 mL of ethanol, 4.50 mL of water, 0.40 mL of 25% NH3(aq), and 1.55 mL of TEOS(l) were mixed and kept under vigorous stirring for 1 h. Produced silica nanospheres were centrifuged at 10 000 rpm for 10 min and then redispersed three times in 31.45 mL of ethanol.38 Platinum seeds were prepared by mixing 10.0 mg of H2PtCl6(s) with 10.00 mL of 35% HCl(aq), 10.00 mL of 0.20 M NaBH4(aq), and 10.00 mL of 0.30 g L−1 PVP(aq) at once,39 and they were centrifuged at 10 000 rpm for 10 min and redispersed in 30.00 mL of ethanol three times. For the functionalization of the surfaces of silica hard templates with amino groups, 10.00 mL of silica hard templates-dispersed ethanol and 5.00 mL of 1.0 M APTES(l) were mixed and kept under stirring for 2 h. Then, 5.00 mL of platinum seeds was added and stirred for 1 h to be attached onto the aminofunctionalized silica hard templates. A total of 4.00 mL of 2 days-aged 10 mM H2PtCl6(aq) and 2.00 mL of the above mixture solution were mixed and stirred for 1 h and then added with 0.32 mL of 0.10 M L-ascorbic acid(aq) and stirred for 1 h for the transformation of platinum seeds into platinum nanoshells.40 Laser-smoothed platinum nanoshells were fabricated by irradiating nanosecond laser pulses with a spot diameter of 12 mm to 1.0 mL of the above solution at fluence of 2.7 mJ cm−2 for 1 h (if not specified otherwise). A platinum colloidal solution was contained in a quartz cell having a path length of 10 mm and stirred vigorously during laser irradiation. The sample was irradiated with 4 ns pulses of 355 nm having an average energy of 3.0 mJ from a Q-switched Quantel Brilliant Nd:YAG laser. Finally, hollow platinum nanospheres were made by etching the silica nanocores of SiO2@Pt core−shell nanospheres with HF; 10.00 mL of 0.5% HF(aq) and 1.00 mL of the above solution were mixed and stirred for 2 min.41 Then, the products were centrifuged at 10 000 rpm for 10 min and redispersed in 1.00 mL of ethanol three times. Note that the molar concentration of platinum in the final colloidal solution is 6.3 mM. Characterization. The catalytic properties of platinum nanocatalysts were tested for the reduction reaction of rhodamine B in the presence of 1.3 mM KBH4. A total of 0.10 mL of the ethanol colloidal solution containing platinum nanocatalysts was added into 1.70 mL of 20 μM RhB(aq) and 0.90 mL of water contained in a polyphenyl cell having a path length of 10 mm, and then 0.40 mL of 10 mM KBH4(aq) was added rapidly. The absorption spectral changes of RhB were measured at scheduled intervals using a temperature-controllable Scinco S-3000 spectrophotometer. While transmission electron microscopic (TEM) images were obtained with a Carl Zeiss LIBRA 120 microscope, energy-dispersive X-ray (EDX) elemental profiles and high-resolution TEM (HRTEM) images were measured using a JEOL JEM-2100F microscope.

Figure 1. Schematic description for the fabrication of Y-HNS and NHNS.

template method used in our experiments involves six steps: the preparation of silica hard templates, the surface functionalization of the templates with amino groups, the coating of the templates with platinum seeds, subsequent laser irradiation to form smooth nanoshells, and the final selective removal of silica hard templates with hydrofluoric acid to obtain hollow nanostructures.32 We have investigated the catalytic activity of platinum nanospheres in the reduction of rhodamine B (RhB) in the presence of KBH4 by monitoring their catalytic rate constants as well as their catalytic induction times. It is found that hollow platinum nanospheres fabricated with laser irradiation (Y-HNS) exhibit much higher catalytic performances than hollow platinum nanospheres fabricated without laser irradiation (N-HNS) do. Industrial processes discharge large amounts of dye-containing wastewater which is hazardous and nonbiodegradable in most cases.33,34 In particular, a variety of N-containing dyes such as RhB undergo natural reductive anaerobic degradation to yield potentially carcinogenic aromatic amines.34 Therefore, the decomposition of organic pollutants via catalytic oxidation or reduction using nanocatalysts has attracted much attention to be a green method for the management of organic wastes.35

2. EXPERIMENTAL SECTION Materials. H2PtCl6 (s, 99.9%), tetraethyl orthosilane (TEOS, l, ≥99%), 25% NH 3 (aq), (3-aminopropyl)triethoxysilane (APTES, l, >98%), L-ascorbic acid(s), 35% HCl(aq), 50% HF(aq), polyvinylpyrrolidone (PVP, s, 25 000 g mol−1), NaBH4 (s, 99.9%), rhodamine B (RhB, s), KBH4 (s, ≥95%), and ethanol(l) were used as purchased from Sigma-

3. RESULTS AND DISCUSSION Sharp contrast between the brighter inner cores and the welldefined dark circular outlines in the TEM images of hollow platinum nanospheres N-HNS and Y-HNS in Figure 2 indicates that the platinum nanocatalysts are structurally hollow indeed. The sizes of hollow platinum nanospheres have been 22793

dx.doi.org/10.1021/jp5073704 | J. Phys. Chem. C 2014, 118, 22792−22798

The Journal of Physical Chemistry C

Article

Figure 2. TEM images of (a) N-HNS and (b) Y-HNS.

determined by spherical silica hard templates, whose average diameter is 130 ± 30 nm. The average shell thicknesses of NHNS and Y-HNS nanocatalysts have been found to be 8.7 ± 2.0 and 13.4 ± 3.3 nm, respectively (Figure 2). This indicates that the average nanoshell thickness of hollow platinum nanospheres increases with laser irradiation although the same amount of the platinum precursor H2PtCl6 has been used for the two cases. The TEM images of Figure S1 show clearly that platinum nanocatalysts undergo morphology changes as a function of laser-irradiation time; the average nanoshell thickness of Y-HNS increases with the increment of the laser irradiation. The average shell thicknesses of hollow platinum nanospheres fabricated with laser irradiation for 0, 15, 30, and 60 min have been found to be be 8.7 ± 2.0, 9.2 ± 1.4, 10.1 ± 1.5, and 13.4 ± 3.3 nm, respectively. Because the melting point of a platinum nanoparticle (600 °C) has been reported to be much lower than that of bulk platinum (1769 °C at 1 atm),28 nanoclusters in platinum seeds can be fused by laser irradiation. In other words, the thermalized photon energy of surface-plasmon excitation induces platinum nanoclusters adsorbed on silica hard templates to melt and fuse together, producing thicker and denser platinum nanoshells by laser irradiation even at the same concentration of chloroplatic acid. This suggests that our laser-induced method should be much simpler and more efficient than typical fabrication processes for core−shell nanocomposites of dielectric solid nanospheres covered by metallic nanoshells. Figure 2 also shows that the surfaces of hollow platinum nanospheres become smoother and more uniform by laser irradiation. The surfaces of SiO2@Pt nanopspheres fused by the thermalized photon energy of surface-plasmon excitation have been fused to become smoother and more uniform. One can note from Figure 2 that, compared to other methods, our laser-induced method brings about serious aggregation of hollow platinum nanospheres. The thermalized energy of laser irradiation is considered to induce the contacting surfaces of SiO2@Pt nanospheres to aggregate together. The line-scanned EDX elemental profiles of Figure 3 have been examined to verify the exact structures and compositions of Y-HNS. Compared with the HRTEM image of Figure 3 of ref 5, the HRTEM image of Figure 3a also indicates that the surface of Y-HNS is smoother than that of N-HNS. The

Figure 3. (a) Area-normalized line-scanned EDX elemental intensity profiles of Y-HNS scanned along the yellow line in the HRTEM image of the inset; the black line represents a theoretical fitting of a shell17 having a cavity diameter of 155 nm with a shell thickness of 12 nm. (b) HRTEM image and EDX elemental maps of Y-HNS.

elemental profile of platinum, examined along the yellow line of the inserted HRTEM image, exhibits two characteristic sharp peaks, indicating that Y-HNS has a well-defined hollow nanostructure. The line-scanned elemental intensity profile has been theoretically fitted by a equation derived simply from a hollow nanosphere;17 the platinum nanosphere of Y-HNS has been found to have a cavity diameter of 155 nm and the shell thickness of 12 nm. Elemental distribution profiles also designate that the interior of the platinum nanocatalyst has been removed almost completely during the etching process via HF. However, the intensity profiles of Si and O reveal that silica exists in the inside and the outside of the platinum nanoshell. The elemental mapping profiles of Pt, Si, and O in Figure 3b also display the detailed structure of Y-HNS. The elemental map of Pt indicates that intensity at the core of a nanosphere is much lower than that at the edge of the nanosphere, representing that the interior and the shell of the Y-HNS are completely hollow and well-defined, respectively. The elemental maps of Si and O show the outlines of circles similar to the outline of Pt but with wider diameters and greater thicknesses. This also suggests that some silica dissolved during the HF etching process has been precipitated again along the inner and outer surfaces of the platinum shell. We have evaluated the catalytic performances of platinum nanocatalysts N-HNS and Y-HNS by monitoring the timedependent absorbance changes of RhB reduced catalytically via the hollow platinum nanospheres in the presence of KBH4 (Figure 4). The catalytic performance of Y-HNS has been compared with that of N-HNS under the same conditions, revealing that the catalytic activity of Y-HNS is much higher than that of N-HNS. The absorbance of RhB at 554 nm has been found to decrease much more rapidly in the presence of Y-HNS (Figure 4b) than in the presence of N-HNS (Figure 4a); less than 54% of RhB decomposes in the presence of the 22794

dx.doi.org/10.1021/jp5073704 | J. Phys. Chem. C 2014, 118, 22792−22798

The Journal of Physical Chemistry C

Article

Figure 4. Absorption spectra at 30 °C of aqueous solutions containing 11 μM RhB and 1.3 mM KBH4 after addition of (a) N-HNS and (b) Y-HNS, measured at elapsed times indicated in the units of min.

N-HNS on the time scale of 1 h while more than 87% of RhB does in the presence of the Y-HNS during the same period. This suggests that laser irradiation enhances the catalytic efficiency of a nanostructure enormously. A similar phenomenon has already been reported for the catalytic reduction of 4nitrophenol via silica-coated gold nanorods.21 The progression of the catalytic reduction of RhB via nanocatalysts can be followed by monitoring changes in optical density at the wavelength of the absorbance maximum of RhB (554 nm) in the presence of KBH4. Since the concentration of potassium borohydride largely exceeds the concentration of RhB, the reduction rate can be assumed to be independent of the concentration of borohydride. So, in this case, the pseudofirst-order degradation kinetic profiles of RhB via platinum nanocatalysts in the presence of KBH4 at indicated four different temperatures in Figure 5 have been employed to extract the observed degradation rate constants (kobs) of platinum nanocatalysts; the pseudolinear plots of ln(A/A0) = −kt, where A is the optical density at 554 nm of RhB and t is the time of the reaction, yield kobs.42 The catalytic rate constants (k) of platinum nanocatalysts shown in Table 1 can be obtained by subtracting the degradation rate constants (k0) of RhB in the absence of any nanocatalysts from kobs; k = kobs − k0. Note that the kinetic parameters of platinum nanocatalysts in Table 1 have been calculated by using k values. As seen in Table 1, the degradation rate constants of Y-HNS are much higher (by a factor of 5.07 at 30 °C) than the respective ones of N-HNS. The pseudo-first-order kinetics profiles of Figure 5 also reveal that the catalytic performances of platinum nanocatalysts become enhanced as the reaction temperature increases. Figure 5 indicates that a certain period of time was required before the catalytic reaction was initiated. As observed in a number of systems,43−45 this period is called the induction time (t0) and usually ascribed to the diffusion time required for reactants to be adsorbed onto the catalyst’s surface before the reaction

Figure 5. First-order kinetics, ln(A/A0) vs t, for the catalytic degradation of 11 μM RhB(aq) via nanocatalysts of (a) N-HNS and (b) Y-HNS in the presence of 1.3 mM KBH4 at temperatures indicated in the units of °C.

could start. The induction time has been attributed to many factors such as the diffusion-controlled adsorption of reactants onto the catalytic surfaces46 and a dynamic restructuring of the surface atoms of the nanocatalysts.43,44 Figure 5 shows that in our experimental conditions, the induction time with Y-HNS is longer than that with N-HNS. The induction time also depends on temperature substantially, in a way that it decreases with increasing the temperature (see below).19 As shown in Figure 6a and Table 1, the activation energy (Ea) of the catalytic degradation reaction of RhB (16.5 kcal mol−1) via the nanocatalyst of Y-HNS is substantially smaller than that (22.3 kcal mol−1) via the nanocatalyst of N-HNS. On the other hand, the frequency factor (A) of the catalytic degradation via Y-HNS (2.2 × 1010 min−1) is also much smaller than that via N-HNS (1.1 × 1014 min−1). These results indicate that laser irradiation to SiO2@Pt core−shell nanospheres has enhanced the catalytic performances of hollow platinum nanospheres by lowering the energy barrier of the catalytic degradation of RhB, rather than by increasing the frequency factor of the reaction. The activation energies (Ea0) and the frequency factors (A0) of the induction process for the catalytic degradation of RhB via platinum nanocatalysts have been obtained from the Arrhenius plots of the inverse of the induction time (1/t0), which has been treated as the rate constant of the induction process. Figure S2 shows that Ea0 for Y-HNS (10.3 kcal mol−1) is higher than Ea0 for N-HNS (7.3 kcal mol−1) while A0 via Y-HNS (1.9 × 106 min−1) is also larger than A0 via N-HNS (1.6 × 104 min−1). The results suggest that the induction process with Y-HNS takes a longer time than the process with N-HNS does because laser irradiation has 22795

dx.doi.org/10.1021/jp5073704 | J. Phys. Chem. C 2014, 118, 22792−22798

The Journal of Physical Chemistry C

Article

Table 1. Rate Constants (k) at 30 °C, Induction Times (t0) at 30 °C, Activation Energies (Ea), Frequency Factors (A), Activation Enthalpies (ΔH‡), and Activation Entropies (ΔS‡) for the Catalytic Degradation of RhB via Platinum Nanocatalysts in the Presence of KBH4 nanocatalyst N-HNS Y-HNS a

k/min−1 (t0/min) a

0.0078 (8.55) 0.0398 (14.35)

Ea/(kcal mol−1) (Ea0/(kcal mol−1))

A/min−1 (A0/min−1)

ΔH‡/(kcal mol−1)

ΔS‡/(cal mol−1 K−1)

22.3 (7.3) 16.5 (10.3)

1.09 × 10 (1.60 × 104) 2.20 × 1010 (1.88 × 106)

21.7

−4.5

15.9

−21.4

14

The values of k0 and t0 at 30 °C in the absence of any nanocatalysts are 0.0047 min−1 and 0.00 min, respectively.

Eyring plot, the plot of ln(k/T) vs 1/T to reveal ln(k/T) = −(ΔH‡/R)(1/T) + ΔS‡/R + ln(kB/h).49 Figure 7 and Table 1

Figure 7. Eyring plots for the degradation reaction of RhB catalyzed via indicated platinum nanocatalysts in the presence of KBH4.

show that ΔS‡ for the catalytic degradation of RhB via Y-HNS (−21.4 cal mol−1 K−1) is much more negative than ΔS‡ via NHNS (−4.5 cal mol−1 K−1) whereas ΔH‡ via Y-HNS (15.9 kcal mol−1) is lower than ΔH‡ via N-HNS (21.7 kcal mol−1). Thus, the results suggest that the formation of the activated complex during the catalytic degradation reaction of RhB is favorable in energy but unfavorable in degree of freedom. We have shown that the irradiation of nanosecond laser pulses to SiO2@Pt core−shell nanospheres during the preparation of hollow platinum nanospheres enhances the catalytic performances of the platinum nanocatalysts largely by substantially reducing the activation energy of the catalytic degradation reaction of organic pollutants such as RhB. The catalytic degradation mechanism of RhB via platinum nanocatalysts in the presence of KBH4 could be explained as follows. RhB is electrophilic and BH4− is nucleophilic in comparison with platinum nanocatalysts, meaning that the nucleophilic BH4− can donate electrons to platinum nanocatalysts and that the electrophilic RhB can capture electrons from platinum nanocatalysts. Thus, the platinum nancatalysts facilitate electron transfer from BH4− (donor) to the RhB (acceptor) through their catalytic surfaces;50,51 platinum nanocatalysts serve as electron relays for the degradation reaction of RhB in the presence of KBH4. It has already been reported5 that the catalytic electron relays take place mainly in the nanocavity surfaces of platinum nanocatalysts. The nanoreactor confinement effect of hollow platinum nanospheres has been suggested5 to expedite electron relays from BH4− to the RhB enormously by reducing Ea extensively. In this work, we have shown that laser irradiation enhances the catalytic performances of the platinum nanocatalysts further on a large scale by lowering Ea substantially. Figure 7 has shown that Ea decreases by laser irradiation because the formation of the activated

Figure 6. (a) Arrhenius plots for the catalytic degradation reaction of 11 μM RhB(aq) in the presence of 1.3 mM KBH4 via platinum nanocatalysts indicated inside. (b) The compensation law plot of the activation energies and the frequency factors obtained from the Arrhenius plots.

increased the energy barrier of the induction process in hollow platinum nanospheres. As shown in Figure 2 and Figure S1, the average shell thickness of platinum nanocatalysts has been increased with laser irradiation. Thus, we consider that it is more difficult for reactant molecules to diffuse through the thicker nanoshells of Y-HNS. Based on our results, it is suggested that the induction time is determined together by the diffusion rate of reactant molecules into the cavity of the platinum nanocatalyst and by the adsorption rate of the molecules on the interior surface of the nanocatalyst.45 Figure 6b shows the compensation law plot of ln A = α + Ea/(RTθ), where α is a constant and Tθ is called the fictitious isokinetic temperature, at which the catalytic rates of all the nanocatalysts become equal.47,48 In our system, Tθ has been found to be 341 K. It is considered that compared with N-HNS, the nanocatalyst of Y-HNS has a smaller concentration of active sites that have low Ea for the same catalytic degradation reaction of RhB in the presence of KBH4. The enthalpy (ΔH‡) and entropy (ΔS‡) of activation can be extracted from the slope and intercept, respectively, of the 22796

dx.doi.org/10.1021/jp5073704 | J. Phys. Chem. C 2014, 118, 22792−22798

The Journal of Physical Chemistry C

■

complex for the catalytic degradation reaction of RhB is energetically more favorable in the nanocavity surface of YHNS than in the surface of N-HNS. The heterogeneous catalysis is mainly related to the chemisorption of reactant molecules, which is considered to depend on the structure of a nanocatalyst.52 When a reactant molecule is absorbed on the cavity surface of Y-HNS, interaction energy is suggested be strong enough for activation. Thus, in order to understand the enhancement of the catalytic performances of hollow platinum nanospheres by laser irradiation, we should consider the surface restructuring process of SiO2@Pt core−shell nanospheres, resulting from photothermal annealing during laser irradiation. The thermal-induced melting of platinum nanoshells starts at their surfaces, enabling surface atoms to get mobility.53,54 It is known that the diffusion of atoms on the surfaces of nanocatalysts becomes faster at the Hüttig temperature (0.3Tmelting).53 Thus, a restructuring of surface atoms induced by laser irradiation is suggested to render the metallic surfaces of Y-HNS much easier, than the surfaces of N-HNS, to chemisorb reactant molecules and to facilitate electron relays from BH4− to RhB,50 enhancing the catalytic performances of platinum nanocatalysts on a large scale.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 2 880 4368. Fax: +82 2 875 6624. E-mail: djjang@ snu.ac.kr. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by a research grant through the National Research Foundation (NRF) of Korea funded by the Korea government (2012-006345). D.J.J. is also thankful to the SRC program of NRF (2007-0056331).

■

REFERENCES

(1) Park, S. Y.; Lytton-Jean, A. K. R.; Lee, B.; Weigand, S.; Schatz, G. C.; Mirkin, C. A. DNA-Programmable Nanoparticle Crystallization. Nature 2008, 451, 553−556. (2) Xia, Y.; Halas, N. J. Shape-Controlled Synthesis and Surface Plasmonic Properties of Metallic Nanostructures. MRS Bull. 2005, 30, 338−348. (3) Kim, Y.; Jang, D.-J. A Facile Growth Mechanism of Wurtzite ZnS Nanostructures Showing Intense Ultraviolet Luminescence. CrystEngComm 2014, 16, 6989−6995. (4) Lee, J.; Han, K.; Jang, D.-J. Silica-Coated Silver/Gold Composite Nanoboxes Having Enhanced Catalytic Performances and Reusability. Appl. Catal. A: Gen. 2014, 469, 380−386. (5) Kwak, J.-A.; Lee, D. K.; Jang, D.-J. Facile Fabrication of Platinum Nanobubbles Having Efficient Catalytic Degradation Performances. Appl. Catal. B: Environ. 2013, 142, 323−328. (6) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (7) Hartland, G. V. Optical Studies of Dynamics in Noble Metal Nanostructures. Chem. Rev. 2011, 111, 3858−3887. (8) Noguez, C. Surface Plasmons on Metal Nanoparticles: The Influence of Shape and Physical Environment. J. Phys. Chem. C 2007, 111, 3806−3819. (9) Ataee-Esfahani, H.; Nemoto, Y.; Wang, L.; Yamauchi, Y. Rational Synthesis of Pt Spheres with Hollow Interior and Nanosponge Shell Using Silica Particles as Template. Chem. Commun. 2011, 47, 3885− 3887. (10) Lu, H. M.; Meng, X. K. Theoretical Model to Calculate Catalytic Activation Energies of Platinum Nanoparticles of Different Sizes and Shapes. J. Phys. Chem. C 2010, 114, 1534−1538. (11) Hu, J.; Chen, M.; Fang, X.; Wu, L. Fabrication and Application of Inorganic Hollow Spheres. Chem. Soc. Rev. 2011, 40, 5472−5491. (12) Mahmoud, A.; Mahmoud, R. N.; El-Sayed, M. A. Enhancing Colloidal Metallic Nanocatalysis: Sharp Edges and Corners for Solid Nanoparticles and Cage Effect for Hollow Ones. Acc. Chem. Res. 2013, 46, 1795−1805. (13) Wang, Z. X.; Chen, M.; Wu, L. M. Synthesis of Monodisperse Hollow Silver Spheres Using Phase-Transformable Emulsions as Templates. Chem. Mater. 2008, 20, 3251−3253. (14) Wang, Z. X.; Wu, L. M.; Chen, M.; Zhou, S. X. Facile Synthesis of Superparamagnetic Fluorescent Fe3O4/ZnS Hollow Nanospheres. J. Am. Chem. Soc. 2009, 131, 11276−11277. (15) Yang, T.; Liu, J.; Zheng, Y.; Monteiro, M. J.; Qiao, S. Z. Facile Fabrication of Core−Shell-Structured Ag@Carbon and Mesoporous Yolk−Shell-Structured Ag@Carbon@Silica by an Extended Stçber Method. Chem.Eur. J. 2013, 19, 6942−6945. (16) Deng, Z.; Chen, M.; Gu, G.; Wu, L. A Facile Method to Fabricate ZnO Hollow Spheres and Their Photocatalytic Property. J. Phys. Chem. B 2008, 112, 16−22. (17) Kim, Y.; Kim, H.-B.; Jang, D.-J. Facile Microwave Fabrication of CdS Nanobubbles with Highly Efficient Photocatalytic Performances. J. Mater. Chem. A 2014, 2, 5791−5799.

4. CONCLUSIONS Platinum nanoclusters topped on silica nanospheres have been transformed into platinum nanoshells having uniform and smooth surfaces by being irradiated with 355 nm nanosecond laser pulses; the thermalized photon energy of surface-plasmon excitation has induced platinum nanoclusters adsorbed on silica hard templates to melt and fuse together. The thickness of platinum nanoshells has been tuned easily by adjusting the irradiation time only. Hollow platinum nanospheres fabricated with irradiation (Y-HNS) have completely hollow interiors and well-defined platinum nanoshells. The catalytic activity of YHNS in the degradation reaction of rhodamine B (RhB) in the presence of KBH4 has been found to be 5 times greater at 30 ◦ C than that of hollow platinum nanospheres fabricated without irradiation (N-HNS). Laser irradiation enhances the catalytic performances of hollow platinum nanospheres by lowering the energy barrier of the catalytic degradation of RhB. The energetically more favorable formation of the activated complex in the nanocavity surface of Y-HNS has been attributed to reduce the activation energy. The induction process of the catalytic degradation of RhB with Y-HNS takes a longer time than the process with N-HNS does because the increment of the shell thickness by irradiation has increased the energy barrier of the induction process though platinum nanoshells. A restructuring of surface atoms induced by photothermal annealing during laser irradiation has been suggested to render the metallic surfaces of Y-HNS much easier to chemisorb reactant molecules and to facilitate electron relays from BH4− to RhB, enhancing the catalytic performances of platinum nanocatalysts extensively.

■

Article

ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images and Arrhenius plots. This material is available free of charge via the Internet at http://pubs.acs.org. 22797

dx.doi.org/10.1021/jp5073704 | J. Phys. Chem. C 2014, 118, 22792−22798

The Journal of Physical Chemistry C

Article

(18) Weng, G.; Mahmoud, M. A.; El-Sayed, M. A. Nanocatalysts can Change the Number of Electrons Involved in Oxidation-Reduction Reaction with the Nanocages Being the Most Efficient. J. Phys. Chem. C 2012, 116, 24171−24176. (19) Zeng, J.; Zhang, Q.; Chen, J.; Xia, Y. A. Comparison Study of the Catalytic Properties of Au-Based Nanocages, Nanoboxes, and Nanoparticles. Nano Lett. 2010, 10, 30−35. (20) Kim, S. J.; Ah, C. S.; Jang, D. J. Optical Fabrication of Hollow Platinum Nanospheres by Excavating the Silver Core of Ag@Pt Nanoparticles. Adv. Mater. 2007, 19, 1064−1068. (21) Son, M.; Lee, J.; Jang, D.-J. Light-treated Silica-Coated Gold Nanorods Having Highly Enhanced Catalytic Performances and Reusability. J. Mol. Catal. A: Chem. 2014, 385, 38−45. (22) Hashimoto, S.; Werner, D.; Uwada, T. Studies on the Interaction of Pulsed Lasers with Plasmonic Gold Nanoparticles Toward Light Manipulation, Heat Management, and Nanofabrication. J. Photochem. Photobiol. C: Photochem. Rev. 2012, 13, 28−54. (23) Hodak, H.; Martini, I.; Hartland, G. V. Spectroscopy and Dynamics of Nanometer-Sized Noble Metal Particles. J. Phys. Chem. B 1998, 102, 6958−6967. (24) Werner, D.; Hashimoto, S.; Uwada, T. Remarkable Photothermal Effect of Interband Excitation on Nanosecond Laser-Induced Reshaping and Size Reduction of Pseudospherical Gold Nanoparticles in Aqueous Solution. Langmuir 2010, 26, 9956−9963. (25) Ah, C. S.; Han, H. S.; Kim, K.; Jang, D.-J. Photofragmentation Dynamics of n-Dodecanethiol-Derivatized Silver Nanoparticles in Cyclohexane. J. Phys. Chem. B 2000, 104, 8153−8159. (26) Radloff, C.; Halas, N. Enhanced Thermal Stability of SilicaEncapsulated Metal Nanoshells. Appl. Phys. Lett. 2001, 79, 674−676. (27) Nanda, K. K.; Sahu, S. N.; Behera, S. N. Liquid-Drop Model for the Size-Dependent Melting of Low-Dimensional Systems. Phys. Rev. A 2002, 66, 013208. (28) Wang, Z. L.; Petroski, J. M.; Green, T. C.; El-Sayed, M. A. Shape Transformation and Surface Melting of Cubic and Tetrahedral Platinum Nanocrystals. J. Phys. Chem. B 1998, 102, 6145−6151. (29) Jin, R.; Cao, Y. C.; Hao, E.; Métraux, G. S.; Schatz, G. C.; Mirkin, C. A. Controlling Anisotropic Nanoparticle Growth Through Plasmon Excitation. Nature 2003, 425, 487−490. (30) Warner, D.; Hashimoto, S. Controlling the Pulsed-LaserInduced Size Reduction of Au and Ag Nanoparticles via Changes in the External Pressure, Laser Intensity, and Excitation Wavelength. Langmuir 2013, 29, 1295−1302. (31) Wang, Z.; Chen, X.; Chen, M.; Wu, L. Facile Fabrication Method and Characterization of Hollow Ag/SiO2 Double-Shelled Spheres. Langmuir 2009, 25, 7646−7651. (32) Lou, X. W.; Archer, L. A.; Yang, Z. C. Hollow Micro-/ Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987−4019. (33) Das, J.; Velusamy, P. Catalytic Reduction of Methylene Blue using Biogenic Gold Nanoparticles from Sesbania Grandiflora L. J. Taiwan. Inst. Chem. E 2014, DOI: 10.1016/j.jtice.2014.04.005. (34) Natarajan, T. S.; Thomas, M.; Natarajan, K.; Bajaj, H. C.; Tayade, R. J. Study on UV-LED/TiO2 Process for Degradation of Rhodamine B Dye. Chem. Eng. J. 2011, 169, 126−134. (35) Fu, H.; Pan, C.; Yao, W.; Zhu, Y. Visible-Light-Induced Degradation of Rhodamine B by Nanosized Bi2WO6. J. Phys. Chem. B 2005, 109, 22432. (36) Ataee-Esfahani, H.; Liu, J.; Hu, M.; Miyamoto, N.; Tominaka, S.; Wu, K. C.; Yamauchi, Y. Mesoporous Metallic Cells: Design of Uniformly Sized Hollow Mesoporous Pt−Ru Particles with Tunable Shell Thicknesses. Small 2013, 9, 1047−1051. (37) Stö ber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Collid Interface Sci. 1968, 26, 62−69. (38) Kim, M. R.; Kim, J.−Y.; Kim, S. J.; Jang, D.−J. Laser-Induced Fabrication of Platinum Nanoshells Having Enhanced Catalytic and Raman Properties. Appl. Catal. A: Gen. 2011, 393, 317−322. (39) Calderone, V. R.; SchützWidoniak, J.; Bezemer, G. L.; Bakker, G.; Steurs, C.; Philipse, A. P. Design of Colloidal Pt Catalysts

Encapsulated by Silica Nano Membranes for Enhanced Stability in H2S Streams. Catal. Lett. 2010, 137, 132−140. (40) Li, J.; Yang, C.; Zhang, L.; Ma, T. A Novel Fumed SilicaSupported Nitrogenous Platinum Complex as a Highly Efficient Catalyst for the Hydrosilylation of Olefins with Triethoxysilane. J. Organomet. Chem. 2011, 696, 1845−1849. (41) Liu, B.; Zhang, W.; Feng, H.; Yang, X. Rattle-Type Microspheres as a Support of Tiny Gold Nanoparticles for Highly Efficient Catalysis. Chem. Commun. 2011, 47, 11727−11729. (42) Langeroodi, N. S. Silica-Supported Vanadium Oxide Nanoparticles as Efficient Heterogeneous Catalyst for Hydrolysis of Sucrose. Russ. J. Phys. Chem. A 2012, 86, 628−631. (43) Wunder, S.; Lu, Y.; Albrecht, M.; Ballauff, M. Catalytic Activity of Facetted Gold Nanoparticles Studied by a Model Reaction: Evidence for Substrate-Induced Surface Restructuring. ACS Catal. 2011, 1, 908−916. (44) Hervés, P.; Pérez-Lorenzo, M.; Liz-Marzán, M. L.; Dzubiella, J.; Lu, Y.; Ballauff, M. Catalysis by Metallic Nanoparticles in Aqueous Solution: Model Reactions. Chem. Soc. Rev. 2012, 41, 5577−5587. (45) Mahmoud, M. A.; El-Sayed, M. A. Time Dependence and Signs of the Shift of the Surface Plasmon Resonance Frequency in Nanocages Elucidate the Nanocatalysis Mechanism in Hollow Nanoparticles. Nano Lett. 2011, 11, 946−953. (46) Kuroda, K.; Ishida, T.; Haruta, M. Reduction of 4-Nitrophenol to 4-Aminophenol over Au Nanoparticles Deposited on PMMA. J. Mol. Catal. A: Chem. 2009, 298, 7−11. (47) Somorjai, G. A. Introduction to Surface Chemistry and Catlaysis; John Wiley & Sonc Inc: New York, 1994. (48) Bligaard, T.; Honkala, K.; Logadottir, A.; Nørskov, J. K.; Dahl, S.; Jacobsen, C. J. H. On the Compensation Effect in Heterogeneous Catalysis. J. Phys. Chem. B 2003, 107, 9325−9331. (49) Zimmer, K. D.; Shoemaker, R.; Rumimski, R. R. Synthesis and Characterization of a Fluxional Re(I) Carbonyl Complex fac[Re(CO)3(dpop′)Cl] with the Nominally tri-dentate Ligand dipyrido(2,3-a:3′,2′-j)phenazine (dpop′). Inorg. Chim. Acta 2006, 359, 1478− 1484. (50) Jiang, Z.-J.; Liu, C. Y.; Sun, L. W. Catalytic Properties of Silver Nanoparticles Supported on Silica Spheres. J. Phys. Chem. B 2005, 109, 1730−1735. (51) Deng, Z.; Chen, M.; Wu, L. Novel Method to Fabricate SiO2/ Ag Composite Spheres and Their Catalytic, Surface-Enhanced Raman Scattering Properties. J. Phys. Chem. C 2007, 111, 11692−11698. (52) Rasmussen, A. M. H.; Groves, M. N.; Hammer, B. Remote Activation of Chemical Bonds in Heterogeneous Catalysis. ACS Catal. 2014, 4, 1182−1188. (53) Moulijn, J. A.; van Diepen, A. E.; Kapteijn, F. Catalyst Deactivation: Is it Predictable?: What to Do? Appl. Catal. A: Gen. 2001, 212, 3−16. (54) Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes. J. Phys. Chem. C 2010, 114, 8814−8820.

22798

dx.doi.org/10.1021/jp5073704 | J. Phys. Chem. C 2014, 118, 22792−22798