Improved Photocatalytic Activity of Shell-Isolated Plasmonic

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Improved Photocatalytic Activity of Shell-Isolated Plasmonic Photocatalyst Au@SiO/TiOby Promoted LSPR 2

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Jiun-Jen Chen, Jeffrey Chi Sheng Wu, Pin Chieh Wu, and Din Ping Tsai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp309901y • Publication Date (Web): 19 Nov 2012 Downloaded from http://pubs.acs.org on November 24, 2012

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Improved photocatalytic activity of shell-isolated plasmonic photocatalyst Au@SiO2/TiO2 by promoted LSPR

Jiun-Jen Chen1, Jeffrey C.S. Wu1*, Pin Chieh Wu2, Din Ping Tsai2 1

Department of Chemical Engineering, 2Department of Physics National Taiwan University, Taipei, Taiwan 10617

*Corresponding author, J. C. S. Wu, ph: 886-223631994, e-mail: [email protected]

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Abstract The effect Localized Surface Plasmon Resonance (LSPR) of Au nanoparticles was studied on the photocatalytic activity of TiO2 film. We used thermal hydrolysis method to prepare 300-nm TiO2 film, and sodium citrate reduced method to synthesize gold nanoparticles. The photocatalytic activities of Au/TiO2, Au@SiO2/TiO2, and TiO2 films were evaluated by the degree of MB photodegradation under similar conditions with simultaneous UV (365 nm) and visible-light (400 nm < λ < 700 nm) irradiation for 5 h. The degree of MB photodegradation was in the following order: Au@SiO2/TiO2>Au/TiO2>TiO2. Although SiO2 shell prevented the electron trap effect, MB photodegradation efficiency of Au@SiO2/TiO2 was superior to that of Au/TiO2 because its LSPR was much higher. To validate the experimental results, the electric field intensity around the gold nanoparticles was simulated by finite element method (FEM). In the presence of gold nanoparticles, the LSPR effect increased the surrounding intensity of electric field that enhanced the photocatalytic activities. Furthermore, from the simulation results, Au@SiO2/TiO2 showed EM field improvement of nearly 9 times comparing with Au/TiO2. The SiO2 coating significantly increased the LSPR effect of gold nanoparticles. We named this new core-shell structure "shell-isolated plasmonic photocatalyst". Keywords: photocatalysis; gold nanoparticle; local surface plasmon resonance; shell-isolated plasmonic photocatalyst.

1. Introduction Semiconductor photocatalysis attracts a considerable amount of studies due to its applications in recent catalytic-related research. In particular, the environmental applications of TiO2 have attracted much attention because it is one of the most affordable, stable, and active photocatalysts. Many researchers tried to improve the photocatalytic performance of

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TiO2, especially by the modification with noble metals.1-5 In photocatalytic reactions, noble metal nanoparticles loaded on TiO2 often behave as electron traps.4,6 The noble metal nanoparticles are different from their bulk counterparts because of their small size, featuring unique electrical, mechanical, magnetic and optical properties. In particular, the optical properties of gold nanoparticles are dominated by their Localized Surface Plasmon Resonance (LSPR), defined as the collective motions of the conduction electrons induced by light irradiation.7,8 This is associated with a considerable enhancement of the electric near-field. The resonance wavelength strongly depends on the size and shape of the nanoparticles, the interparticle distance, and the dielectric property of the surrounding medium.9,10 The LSPR effect of noble metal nanoparticles can also enhance the activity of photocatalytic reaction.6,11 Recently, the coating of silica shell on nanoparticles has been widely discussed because of the following advantages, acts as a bridge to provide the cores with different reaction sites; tunes the optical properties of the cores; and increases the stability of nanoparticle dispersion.12-14 These recent studies gave us insight into the composite structure of silica-coated nanoparticles, the easily modified silica surface, and the unique properties of nanoparticles. Compared to previous research on the thickness optimization of silica shells and the study of their surface morphology,15 little research has been done to explore the effect of SiO2 on LSPR effect for nanoparticle core. To further understand the role of Au nanoparticles in photocatalytic reaction and the effect of SiO2 coating on such nanoparticles, several plasmonic photocatalysts were fabricated. Plasmonic photocatalysts, Au@SiO2/TiO2, Au/TiO2, and TiO2 film were prepared and MB photodegradation was conducted to determine the photocatalytic activity. Furthermore, simulation was carried out to give insight into the possible changes of the LSPR effect on Au nanoparticles with SiO2 shell.

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2. Experimental 2.1. Preparation of photocatalysts A gold dispersion was prepared according to the sodium citrate reduced method,16 in which distilled water was added to a small amount of HAuCl4 containing 50 mg of gold to a total volume of 500 mL. Upon boiling, 50 mL of 1% sodium citrate solution was added into the above solution under vigorous stirring. After 30 min of continuous boiling, the solution was left to cool. This method produced a stable, deep-red suspension of gold particles. An aqueous solution of 3-aminopropyltrimethoxysilaneaps (APS, 5 mL, 1 mM) was added to 500 mL of the gold solution under vigorous magnetic stirring.17 The mixture of APS and gold dispersion was left to stand for 15 min to allow complete complexation of the amine groups with the gold surface. By progressive addition of cation exchange resin (Dualite C467), a solution of active silica was produced by lowering the pH of a 0.54 wt% sodium silicate solution to 10.5. Afterwards, 10 mL of active silica was released into 500 mL of the surface modified gold solution under vigorous magnetic stirring. The silica shell with a near 3 nm thickness was obtained by allowing the solution to stand for over 2 days prior to the removal of free silicates via centrifugation, so that further growth of silica could be prevented. The TiO2 sol was obtained by the thermal hydrolysis method.18 Tetrabutoxide titanate (TBOT) and polyethylene glycol (PEG) were added to 0.1 M nitric acid (HNO3) solution. The volume of HNO3 was six times of that of TBOT, and the weight of PEG was half of that of TiO2. The mixed solution was heated to 80°C and kept at the same temperature for 8 h. In order to prevent cracking of the film in the process of drying and calcination, PEG was added. A thin TiO2 film was prepared by dip coating on the quartz plate. The quartz plates were dipped into the above TiO2 sol vertically and then pulled out at a speed of 38 mm/min using a step motor. For successive coatings, the coated quartz plate was dried in the air before 4



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conducting the next dip coating process. The TiO2 film coated on quartz plate was then calcined. During calcination, the oven temperature was increased at a rate of 1°C/min from ambient temperature to 150°C and maintained for 3 h, followed by an increase of temperature at the same rate to 500°C and maintained for 5 h. A pipet was used to take either 4 mL of Au dispersion or 4 mL of Au@SiO2 dispersion solution and spread the solution on the TiO2 quartz plate. To increase the adhesion of nanoparticles on the plate, the resulting sample was heated from room temperature to 80°C. To confirm the effect of SiO2 itself on MB photodegradation, we also prepared the sample of SiO2/TiO2. Adding 5 mL 0.54 wt% sodium silicate solution in the TiO2 sol as previously prepared. Then a quartz plate was dipped into the SiO2/TiO2 mixed sol. The parameters of dip coating and calcination process are the same as that of the TiO2 film preparation previously described. 2.2. Characterization Transmission electron microscopy (TEM) of the nanoparticles was carried out on a Hitachi model H-7100 instrument. Dynamic light scattering (DLS) measurements were performed on a Zetasizer Nano ZS analyzer (Malvern). Backscatter detection technology was applied with a 173-degree detecting position to reduce the effect from the dust within the dispersant and to allow the measurement of high concentrations. The light absorption of nanoparticles was characterized by diffusive reflective UV-vis spectroscopy (Varian, Cary 100). The field-emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS) were performed on the Hitachi model S-800. The powder of TiO2, which underwent similar preparation procedure, was measured by X-ray diffraction (Rigaku-Ultima IV MAC Science). The X-ray photoelectron spectroscopy (XPS, Thermo Theta Probe) was carried out to determine the chemical status of the as-prepared Au@SiO2 particles. The nanoparticles were pressed into a pellet and stuck to the sample holder using a Cu tape. Carbon (1s, 284.5 eV) was used as an internal standard for binding energy calibration. 5


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2.3. Photodegradation of Methylene Blue The methylene blue (MB) aqueous solution of 2.4 × 10-5 M was photodegraded in a glass reactor at 25°C. The photocatalysts-deposited quartz plate (5 cm × 5 cm × 1 mm) was immersed in the solution and irradiated simultaneously by UV (365 nm) and the Xe lamp with filter (400 nm < λ < 700 nm). The intensity of the visible light was 150 klux. UV-vis spectroscopy (Cary 100, Varian) was used to measure the concentration of the MB aqueous solution based on the intensity of the absorption peak at 664.3 nm.

2.4. Simulation of Surface Plasmon Resonance All of the simulated spectra and electromagnetic field distribution were obtained by solving three-dimensional Maxwell equations with the commercial COMSOL Multiphysics software, which is based on finite element method (FEM). The periodic boundary condition was used for the simulation of many nanoparticles in water. The refractive index of water, TiO2 and SiO2 used was 1.33, 3.30 and 1.46, respectively. The permittivity of gold in the UV-visible regime is described by the Drude-Lorentz model

ε (ω ) = 1 −

ω p2 ω 2 + iΓ p ω

+∑ j

f jω 2j

ω 2j − ω 2 − iΓ jω

(1)

Where, the plasmon frequency ωp = 8.997 eV and damping constant Γp = 0.07 eV. ω is the angular frequency of incident electromagnetic wave. ωj and Гj is the resonant frequency and the damping constants of the jth Lorentz oscillator, respectively.19 By calculating the scattering parameters (S-parameters), we can obtain the absorbance spectrum A = 1 – R – T, where R is reflectance and T is transmittance. The definition of S-parameters is:

S11 =

power reflected from port 1 power incident on port 1 6


(2)


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S21 =

power reflected from port 2 power incident on port 1

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(3)

The electromagnetic wave incident from port 1 and port 2 is a receiver plane.20 Reflectance 2

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and transmittance were calculated by S11 and S 21 , respectively.

3. Results and discussion 3.1. Photocatalyst characterization There have been considerable studies regarding core-shell Au@SiO2 nanoparticles prepared by: (1) using silane coupling reagent APS as primer to modify the Au nanoparticles surfaces to make them vitreophilic, and (2) slow silica deposition in water from a sodium silicate solution.17 Before coating, the diameter of the Au core was 18 nm based on the TEM results shown in Figure 1(a). Figure 1(b) shows a TEM micrograph of the nanoparticles with average Au core diameter of 50 nm and silica shell thickness of near 3 nm. The reason why the Au core increased from 18 nm to 50 nm is because of the constant stirring of Au particles, causing the growth of Au particle size during the procedure. Complete silica shell was formed and the thickness was measured to be 3 nm for most spherical particles of Au cores resulting in a total diameter of 56 nm for the nanoparticles. The size distribution of the bare Au and Au@SiO2 was measured via DLS. As can be seen in Figure 2, the average size of bare Au is measured to be 35 nm and that of Au@SiO2 is measured to be 65 nm. The particle size determined from DLS is larger than that determined from the TEM measurement. This is due to the fact that DLS takes the hydrodynamic diameter into account, i.e. the diameter of a hypothetical hard sphere diffusing at the same rate as the particle in the fluid. Another reason may be the slight aggregation of the primary particles. It was difficult to obtain a large amount of Au@SiO2 nanoparticles with uniform size distribution. Compared to the TEM results, the DLS results were more precise and wider ranged. Thus, the size measured by DLS 7


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was used in further simulations. The size of the gold nanoparticles mainly affects the shift of maximum wavelength (ranged 500-600 nm) in the LSPR. The increase of gold nanoparticle size (18 nm to 50 nm) has little effect on the electric field intensity.9 Thus, in our case, the size difference of gold nanoparticles has little effect on photocatalysis. When it comes to the analysis of core-shell type nanostructures, XPS is vital because of its detection depth (~10 nm) to the size of these particles and its ability to correctly identify the chemical state of target particles. Figure 3 displays the XPS spectrum of the region ranging from binding energy of 120 to 70 eV, which includes the Si 2p, and Au 4f peaks. Chemical identity of the elements (determined from the measured binding energies) is straightforward and corresponds to Au0 and SiIV.21 Au is well known for having attractive optical properties. The optical extinction of Au is due to a collective oscillation of the free electrons known as the plasmon resonance. Figure 4 shows the bare Au nanoparticles giving a characteristic surface plasmon at 523.8 nm. As silica shell encapsulates the Au surface, there is an increase in the intensity of the plasmon absorption and a red shift in the position of the maximum band. The peak of Au was shifted to a longer wavelength. This is due to an increase in the local refractive index around the nanoparticles by the silica shell, resulting in the maximum adsorption peak of Au@SiO2 at 526 nm as shown in Figure 4. As depicted, the absorbance and the position of the maximum absorption of Au@SiO2 nanoparticles can be adjusted by adding a silica shell, or by controlling the thickness of the silica shell. These phenomena have been investigated in detail by Liz-Marzán and co-workers.17 In a word, such phenomena are extremely important if one wishes to use the optical properties of Au. Figure 5(a), (b) and (c) show the cross-section SEM micrographs of TiO2 films and Figure 5(d), (e), and (f) show their top views. Three TiO2 films of different thicknesses were prepared by 1~3 times of dip-coating process and the thickness increased with the number of 8



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coatings on the quartz plate. The sample shown in Figure 5(a) and (d) was coated once with a TiO2 thickness of 80 nm, while that in Figure 5(b) and (e) was coated twice with a TiO2 thickness of 300 nm. The sample shown in Figure 5(c) and (f) was dip coated three times and obvious cracks were observed on the surface as shown from the top-view image. The films were uniform and smooth without showing cracks in the first two coatings. In this study, the 300 nm-thick film was selected for further processing. The XRD pattern shown in Figure 6 indicates the TiO2 powder exhibited anatase phase after thermal treatment at 500°C for 5 h. This also implies that the TiO2 sol, which underwent the same process, formed well-crystalized anatase TiO2 films. The bright spots that can be seen in Figure 7 are the slightly aggregated Au@SiO2. The aggregation was caused by the heat from the drying procedure. The EDS analysis result in Figure 8 shows the presence of Au. In Table 1, a summary of the elemental analysis confirms that the Au@SiO2 dispersion is loaded onto the TiO2 film. Also, the Si detected may include those from the quartz plate rather than entirely from the coating around the Au particles. For the SEM and EDS analysis, the sample was sputtered by Pt prior to analysis in order to increase its conductivity. Therefore, the EDS analysis shows Pt content.

3.2 MB photodegradation Figure 9 shows the result of the photodegradation of MB solution under simultaneous UV and visible-light irradiation. Factors affecting MB degradation can be categorized into three kinds.6 The physical adsorption of MB on the photocatalysts, the photodegradation of MB by light irradiation alone, and the photocatalytic degradation of MB in the solution.4,22-24 When TiO2 is exposed to UV irradiation, pairs of electron and hole will be generated. The photogenerated electrons will either recombine with the holes or react with the adsorbed oxygen on the TiO2 surface. In the latter, oxygen ions will be produced, further reacting with the adsorbed H2O on the TiO2 surface to form hydroxyl group (-OH) and hydroxyl radical 9


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(OH•). Finally, MB will be degraded as it reacts with the radicals. In the research conducted by Yogi and his coworkers,4 Au nanoparticles were used as electron traps to suppress the recombination of electrons and holes in order to enhance the MB degradation. The control experiment with only UV+visible light irradiation achieved MB degradation efficiency of near 15% without the help of catalysts for 5 h, whereas the MB degradation efficiency was increased in the presence of different photocatalysts. When TiO2 film was immersed into the MB solution, the photocatalytic degradation efficiency reached near 44% under 5 h of UV+visible light exposure because TiO2 could only activated by UV light. From Figure 9, we also found that after 5 h of UV+visible light irradiation, there is little difference between the MB degradation caused by SiO2/TiO2 and TiO2. Without the Au nanoparticles, SiO2 itself does not increase MB degradation. The MB degradation efficiency of Au/TiO2 increased to near 80% under 5 h of UV+visible light exposure because the Au nanometals can serve as electron traps.4 Under UV light irradiation, the photogenerated electrons were transferred from the TiO2 conduction band to the Au, and the holes were accumulated in the TiO2 valence band. Hence, photogenerated electrons and holes were efficiently separated. This prolonged the lifetime of electrons and holes, thus increasing the photocatalytic reaction activity. Besides that, MB photodegradation was also enhanced by the LSPR effect from the Au as it was irradiated by visible light.6,11 On the other hand, the introduction of Au@SiO2/TiO2 achieved MB degradation efficiency of 95% under UV+visible light irradiation in 5 h. The coating of SiO2 insulated the Au particles, thus blocking out the photogenerated electrons from TiO2. In this case, Au can no longer play the role of electron trap. However, the MB photodegradation efficiency of the Au@SiO2/TiO2 is the highest among all. Therefore, we can deduce that the SiO2 coating further promotes the LSPR of Au compared to that of bare Au. Au nanoparticle is well known for the production of an enhanced spatially confined electrical field close to the 10



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particle surface. This is mainly due to the collective oscillation of the electrons on the conduction band. According to Kumar and his co-workers,25 the coupling of the LSPR of nanometals (Ag) with the band gap of TiO2 conducts energy transfer, in which the plasmon relaxation occurs via excitation of electron-hole pairs in the TiO2. Such EM field relaxation channel will appear if the plasmon energy is larger than the band gap of TiO2. However, the EM field will remain “inert” to the field if the surrounding medium is an insulator (e.g., SiO2) with a larger band gap than the plasmon excitation energy. For the optimal application of this system, a maximum overlap must be made with the LSPR band and the bandgap of TiO2. Adding a SiO2 shell promotes the overlap and is beneficial to the system. However, increasing SiO2 thickness may cause the reduction of penetration depth. Therefore, authors concluded that a 2-5 nm SiO2 interlayer between the Ag nanoparticles and the TiO2 film was optimal for reaching maximum EM energy coupling. In recent studies,26-28 the phenomenon of LSPR enhancement also applied in Raman scattering signal measurement. Synthesized metal nanoparticles ( e.g., Au, Ag, Pt ) coated by SiO2 or Al2O3 are efficient electromagnetic resonators, and can significantly enhance the electric field of the incident electromagnetic radiations, which leads to a substantial increase of the Raman signal from species being in the close proximity to core-shell structures. In some studies, the addition of SiO2 in TiO2 was reported to increase the surface area of TiO2 particles,29,30 enhance the thermal stability for the phase of TiO2 particles,31 and raise the surface acidity,32 resulting in enhanced MB photodegradation. These effects are due to the inclusion of SiO2 in the process of TiO2 fabrication. However, in our study, we did not add SiO2 until the TiO2 film was completely formed. The presence of SiO2 on TiO2 film would not change the chemical properties of TiO2. Moreover, the differences in MB photodegradation efficiency between TiO2, Au/TiO2, SiO2/TiO2, and Au@SiO2/TiO2 were not due to the different adsorption capacity of these photocatalysts. As can be seen in Figure 9, four photocatalysts have a similar MB removal of within 3~4% when they are placed in the 11


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solution without light irradiation. The 3-nm SiO2 shell has insignificant effect on increasing the adsorption of MB. Therefore, in the shell-isolated plasmonic photocatalyst (Au@SiO2/TiO2), the 3-nm SiO2 shell plays an important role to promote the LSPR, thus significantly increasing the photocatalytic activity.

3.3 LSPR simulation The calculated module is constructed according to the TEM image (Figure 1(b)) in order to simulate the real system of chemical reaction. Figure 10(a) shows the absorbance spectra in the case of Au nanoparticles only (blue line) and Au nanoparticles coated with SiO2 (red line). All the thickness of SiO2 is set to a constant of 3 nm and all of the nanoparticles are set in water. The electromagnetic wave is incident from z to -z with x polarization. Due to the fabricated samples are deviated slightly from that of the simulated design, there is a little mismatch between experimental and simulated results. However, comparing Figure 10(a) with Figure 4, the overall calculated results are in agreement with experimental results. The plasmonic resonant frequency of Au nanoparticles with/without SiO2 shell is 526 nm/523.8 nm in experimental results and 545 nm/540 nm in simulation. A red shift in absorbance spectra between Au and Au@SiO2 core-shell nanoparticles is attributed to the plasmonic resonant frequency, which will decrease as the refractive index of surrounding medium increases.33 Subsequently, in order to elucidate LSPR effect, we plotted the electric field intensity on the outer surface of Au nanoparticles and Au@SiO2 core-shell nanoparticles, which is shown in Figure 10(b). The definition of electric field intensity is

uv uv uv uv E = ( E x + E y + E z )1/2 , and the subscripts denote the component of the total electric field. Figure 10(b) clearly shows that the electric field intensity is enhanced in the both cases of Au and Au@SiO2 core-shell nanoparticles by their LSPR. As shown in Figure 10(b), the electric field intensity enhancement of Au@SiO2 core-shell nanoparticles is much larger than that of Au nanoparticles only. 12



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To gain a deeper understanding on the effect of the energy transfer from nanoparticles to TiO2, we simulated two nanoparticles on a TiO2 thin film and investigated the spatial distribution of electric field intensity. Figure 11 shows the FEM simulation results of electric field intensity in arbitrary unit at x-z plane for Au nanoparticle dimer and Au@SiO2 core-shell nanoparticle dimer at wavelength λ = 540 nm (left) and wavelength λ = 545 nm (right), respectively. The electromagnetic wave is incident from z to -z with x polarization in both cases shown in Figure 11. The diameter of Au nanoparticles is 35 nm and that of Au@SiO2 core-shell nanoparticles is 65 nm (including the thickness of SiO2 shell), which are the average dimensions obtained from Figure 2. As expected, due to the LSPR effect and near field coupling between nanoparticles, the spatial electric field enhancement can be observed in the both cases of Au and Au@SiO2 nanoparticles. Similar with the case of Figure 10(b), the electric field intensity enhancement of Au@SiO2 core-shell nanoparticles is much higher than that of bare Au nanoparticles. The electric field intensity on the outer surface of Au@SiO2 core-shell nanoparticle is 8.98 times higher than that of bare Au nanoparticle. In our previous work,6 we pointed out that the Au nanoparticles play two kinds of roles in enhancing photocatalytic reaction process: (i) an electron trap to decrease the probability in the recombination of electron-hole pairs, and (ii) enhance near EM field intensity by LSPR to increase the photoreaction. Due to the SiO2 insulator coated on the Au nanoparticles, the transfer of induced electron from TiO2 thin film to metallic nanoparticles is hindered, which should make photocatalytic reaction rate lower. However, according to our measured data, we found that the photocatalytic reaction rate in the case of Au@SiO2/TiO2 is higher than that of Au/TiO2. The plasmon induced oscillating electron in the case of Au@SiO2/TiO2 creates a higher and local electric field than the case of Au/TiO2. Such promoted LSPR can assist in generating more electron-hole pairs inside the TiO2 thin film to compensate for the loss of the electron trap effect due to SiO2 insulation. This evidence proves that near field intensity enhancement of surface plasmon effectively increases the photocatalytic process. 13


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4. Conclusion Plasmonic photocatalyst consisting of TiO2 film loaded with SiO2-shell Au nanoparticles were successfully prepared in this study. We have investigated the photocatalytic activity of different plasmonic photocatalysts, Au@SiO2/TiO2, Au/TiO2, and TiO2 by carrying out MB photodegradation. Three photocatalysts, TiO2, Au/TiO2, and Au@SiO2/TiO2 gave 44%, 80%, and 95% in MB photodegradation efficiency after 5 h of UV+vis light irradiation, respectively. Shell-isolated plasmonic photocatalyst Au@SiO2/TiO2 provided the best performance in the MB photodegradation. The reason why Au/TiO2 performed better than TiO2 film alone is because the Au particles acted as electron traps to inhibit electron-hole recombination, together with the LSPR effect of Au. On the other hand, the MB photodegradation efficiency of Au@SiO2/TiO2 was superior to that of Au/TiO2 because the LSPR effect of the Au@SiO2/TiO2 is higher than that of the Au/TiO2 even though the 3-nm SiO2 shell insulated the Au and nullified its electron trap effect. In compliment with our findings, the simulated results showed approximately 9 times of increase in EM field comparing SiO2-coated Au with bare Au. The SiO2 shell played the role of enhancing the LSPR effect of Au under proper light irradiation.

Acknowledgements The authors would like to acknowledge the National Science Council of Taiwan for financial support of this research under project number NSC 101-3113-P-002-021-.

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Chemical Society 2011, 133, 15922. (28) Kudelski, A.; Wojtysiak, S. The Journal of Physical Chemistry C 2012, 116, 16167. (29) Fu, X. Z.; Clark, L. A.; Yang, Q.; Anderson, M. A. Environ. Sci. Technol. 1996, 30, 647. (30) Meia, F.; Liua, C.; Zhanga, L.; Rena, F.; Zhoub, L.; Zhaob, W. K.; Fang, Y. L. Journal of Crystal Growth 2006, 292, 87. (31) Viswanath, R. N.; Ramasamy, S. Colloids and Surfaces A: Physicochemieal and Engineering Aspect 1998, 133, 49. (32) Doolin, P. K.; Alerasool, S.; Zalewski, D. J.; Hoffman, J. F. Catalysis Letters 1994, 25, 209. (33) Maier, S. A. Plasmonics: fundamentals and applications; Springer, 2007.

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Table 1. Elemental analysis of Au@SiO2/TiO2 by EDS Element

Weight%

Atomic%

CK

1.72

3.32

OK

44.58

64.54

Si K

25.67

21.18

Ti K

20.93

10.12

Pt M

5.54

0.66

Au M

1.57

0.18

Totals

100.00

17


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(a)

(b)

Figure 1. TEM photos of (a) bare Au with an average diameter of 18 nm, and (b) Au@SiO2 with an average diameter of 56 nm

12 Au Au@SiO2

10

8

Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

4

2

0 0.1

1

10

100

1000

10000

size (d, nm)

Figure 2. Particle size distribution of bare Au particles and Au@SiO2 particles by dynamic light scattering

18



1000

Au 4f7/2 Au 4f5/2

Count/s

800

600

Si 2p

400

200

0 120

110

100

90

80

70

Binding Energy(eV)

Figure 3. XPS spectrum of Au@SiO2 showing Si2p and Au4f peaks

4 Au Au@SiO2 3

2

Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

0

100

200

300

400

500

600

700

800

900

Wavelength (nm)

Figure 4. UV-vis spectrum of (a) bare Au with max peak at 523.8 nm, and (b) Au@SiO2 with max peak at 526 nm

19


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(a)

(d)

(b)

(e)

(c)

(f)

Figure 5. Cross-section and top-view SEM photos of TiO2 films on quartz plate for (a) (d) one, (b) (e) two, and (c) (f) three coatings

20



10000

*

8000

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

* anatase

6000

4000

2000

*

*

*

*

0 20

30

40

50

60

2Theta (degree)

Figure 6. XRD patterns of TiO2 powder

21


70

80

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Figure 7. Top-view SEM image of Au@SiO2/TiO2

22



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Figure 8. EDS result of Au@SiO2/TiO2

23


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Figure 9. MB photodegradation under various conditions

24



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Figure 10. (a) Simulated spectra for x-polarized illumination of Au nanoparticles only (blue line) and Au nanoparticles covered with 3 nm SiO2 (red line) in water. The inset shows the TEM image of Au nanoparticles covered with SiO2. (b) Simulated electric field intensity on the outer surface of Au nanoparticles (left) and Au/SiO2 core-shell nanoparticles (right) at plasmonic wavelength in logarithmic scale. The wavelength of incident light is 540 nm (left) and 545 nm (right), respectively.

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Figure 11. Simulation of electric field intensity under x-polarized illumination of gold nanoparticles only (left) and gold nanoparticles covered with 3 nm SiO2 on TiO2 in x-z plane (in arbitrary unit).

The diameter of nanoparticles is 35 nm (without SiO2) and 65 nm (with 3

nm SiO2), respectively.

26



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Improved Photocatalytic Activity of Shell-Isolated Plasmonic

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