Accepted Manuscript

Preparation of SiO2@Au@TiO2 core-shell nanostructures and their photocata

lytic activities under visible light irradiation

Miaomiao Ye, Huihui Zhou, Tuqiao Zhang, Yiping Zhang, Yu Shao

PII: S1385-8947(13)00542-1

DOI: http://dx.doi.org/10.1016/j.cej.2013.04.064

Reference: CEJ 10679

To appear in: Chemical Engineering Journal

Received Date: 19 January 2013

Revised Date: 13 April 2013

Accepted Date: 15 April 2013

Please cite this article as: M. Ye, H. Zhou, T. Zhang, Y. Zhang, Y. Shao, Preparation of SiO2@Au@TiO2 core-shell

nanostructures and their photocatalytic activities under visible light irradiation, Chemical Engineering Journal

(2013), doi: http://dx.doi.org/10.1016/j.cej.2013.04.064

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1

Preparation of SiO2@Au@TiO2 core-shell nanostructures and 1

their photocatalytic activities under visible light irradiation 2

Miaomiao Ye, Huihui Zhou, Tuqiao Zhang, Yiping Zhang, Yu Shao* 3

Institute of Municipal Engineering, Zhejiang University, Hangzhou, 310058, P R 4

China 5

*Corresponding author. Tel.: +86-571-88206759; Fax: +86-571-88208721. 6

Email address: shaoyu1979@zju.edu.cn (Y. Shao) 7

8

Abstract 9

SiO2@Au@TiO2 core-shell nanostructures with tunable decoration amount of Au 10

nanoparticles have been successfully synthesized by combining three individual 11

synthesis steps with calcination. The as-prepared samples were characterized by 12

X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning 13

transmission electron microscopy (STEM), N2 adsorptiondesorption, X-ray 14

photoelectron emission spectroscopy (XPS), and UV-vis diffuse reflectance 15

spectroscopy. The photocatalytic activities of the samples were evaluated by 16

photocatalytic degradation of naproxen in aqueous solution under visible light 17

irradiation. Results show that the as-obtained core-shell nanostructure is composed 18

of a SiO2 core with an average diameter of ~337 nm, tunable content of Au 19

nanoparticles adsorbed on the surface of SiO2 core, and an outer layer of TiO2 with an 20

average thickness of ~7.0 nm. Photocatalysis experiments indicate that the 21

SiO2@Au@TiO2 core-shell nanostructures with Au decoration amount of 0.1 wt% 22

2

(denotes as SiAuTi-2) exhibit the highest photocatalytic activity since it has the 23

suitable decoration amount of Au nanoparticles for harvesting the visible-light energy 24

and for prohibiting the recombination of free excitons. For fast separation of the 25

catalysts, superparamagnetic cores of Fe3O4 were embedded in the SiO2@Au@TiO2 26

core-shell nanostructures and hence they can be separated from aqueous solution by 27

an external magnetic eld within 10 min. 28

Keywords: Core-shell nanostructures; Plasmonic effects; Photocatalysis; Titania; 29

Naproxen 30

1. Introduction 31

Heterogeneous photocatalysis using nanosized TiO2 catalyst under UV light 32

irradiation is a potential technique for elimination of organic pollutants in aqueous 33

solution [1,2]. However, there are two major barriers limit the widespread use of 34

TiO2 in the practical photocatalytic process. One is its relatively large optical band 35

gap (Eg = 3.2 eV), which limits its photo-response to visible light [3,4]; the other is its 36

small particle size, which makes the separation and recovery of those photocatalysts 37

from the treated solution very difficult [5]. To overcome its first limitation, 38

strategies including mental-ion doping [6], noble metal doping [7], nonmetal doping 39

[8], and compositing with other semiconductors [9] have been developed to extend 40

the absorption of TiO2 to visible light spectrum. Among them, it has been found that 41

noble metal (such as Au and Ag nanoparticles) doping can enhance the photocatalytic 42

activity in visible light because it not only can promote the photocatalytic activity by 43

slowing down the recombination of photogenerated electrons and holes but also can 44

3

induce a visible-light-driven photocatalysis due to their plasmonic effects [10-13]. 45

To overcome its second limitation, nanosized TiO2 is usually being immobilized on 46

supporting materials such as sand, glass, zeolite, and etc. to improve its separation 47

efficiency. Unfortunately, the overall photocatalytic activity will be significantly 48

decreased due to lowering of the surface-to-volume ratio [5]. In some case, TiO2 49

nanoparticles may easily detach from the support since the immobilization is typically 50

realized through physical adhesion. Therefore, it is still scientific challenges to 51

prepare TiO2-based photocatalysts both with high photocatalytic activity under visible 52

light irradiation and with easy recovery. 53

In this paper, we report the preparation of SiO2@Au@TiO2 core-shell 54

nanostructures with tunable decoration amount of Au nanoparpticles. The synthesis 55

process can be divided into the following three steps: firstly, monodisperse SiO2 cores 56

were synthesized through a classic Stber method; secondly, tunable amount of Au 57

nanoparticles were absorbed on the surface of SiO2 cores under sonication; finally, the 58

as-prepared SiO2@Au collides were coated with a thin TiO2 layer through the sol-gel 59

method, followed by calcination at 500 oC for 2h which converts the amorphous TiO2 60

layer into anatase phase (as shown in Fig. 1). This concept was first proposed by 61

Yin [16], who has reported the fabrication of the sandwich-structured SiO2/Au/TiO2 62

photocatalysts with high photocatalytic activity both under UV and visible light 63

irradiation. The designed core-shell nanostructures will possess a number of 64

important features: 1) a thin outer layer of anatase titania and a well-defined 65

SiO2/TiO2 interface can enhance the photocatalytic activity [17,18]; 2) the decoration 66

4

of Au nanoparticles can harvest the visible-light energy by their plasmonic effects, 67

and also can enhance charge separation by serving as an electron reservoir [10]; 3) the 68

titania outer layer can prevent the loss of Au nanoparticles from the nanostructures 69

after use [16, 19]; 4) the large size of the core-shell nanostructures can enhance the 70

separation efficiency by filtration or sedimentation. In addition, for fast magnetic 71

separation of the catalyst, highly field-responsive superparamagnetic cores of Fe3O4 72

were embedded in the center of the SiO2@Au@TiO2 nanocomposites to get the 73

Fe3O4@SiO2@Au@TiO2 core-shell nanostructures. Naproxen was selected as a 74

model pollutant for evaluation of the photodegradation process since it is a 75

representative of pharmaceutical and personal care products (PPCPs) that has been 76

newly detected in the aquatic environment in the past decade [20,21]. 77

2. Experimental 78

2.1. Chemicals 79

All reagents were directly used as received without further purification. 80

Tetraethyl orthosilicate (TEOS, 98%), 2-propanol (99.9%), tetrabutyl orthotitanate 81

(TBOT, 99%), and 3-aminopropyl-triethoxysilane (APTES, 99%) were obtained from 82

Sigma-Aldrich. Anhydrous ethyl alcohol, sodium citrate tribasic dihydrate (99%) 83

and ammonia (28%) were obtained from Sinopharm Chemical Reagent Co., Ltd 84

(China). Naproxen (99%) and hydrogen tetrachloroaurate (III) trihydrate 85

(HAuCl43H2O, 99.99%) were purchased from Alfa Aesar. Hydroxypropyl cellulose 86

(HPC, Mw = 100,000) were obtained from Acros. 87

2.2. Synthesis of SiO2 cores 88

5

The spherical SiO2 cores were prepared through the classic Stber method [22]. 89

Typically, 2 mL of TEOS was rapidly injected into a mixture of 8 mL of deionized 90

H2O, 2 mL of ammonia and 40 mL of 2-propanol under magnetic stirring rate of 800 91

rpm. After reacting for 4 hours, the colloidal spheres were collected by 92

centrifugation, washed with ethanol three times and re-dispersed in 40 mL of 93

2-propanol. 94

2.3 Synthesis of SiO2@Au 95

Firstly, 40 mL of above SiO2 aqueous solution was mixed with 0.5 mL of APTES 96

and heated to 80 C for 2 hours to functionalize the silica surface with amino (-NH2) 97

groups[16]. The -NH2 treated SiO2 were washed with ethanol twice and dried in 98

vacuum at 60 C overnight, then re-dispersed in 80 mL of deionized water. In a 99

separate process, Au nanoparticles with average diameter of ~ 15 nm were 100

synthesized by using the Turkevich process [23]. Finally, the SiO2@Au were 101

prepared by adding different amount of Au nanoparticles to the -NH2 modified SiO2 102

aqueous solution (20 mL) under sonication [24]. The collected samples were denoted 103

as Si, SiAu-1, SiAu-2, SiAu-3, and SiAu-4 with the addition of 0, 1, 2, 5, and 10 mL 104

of Au sols, respectively. All of the SiO2@Au colloids were centrifuged and 105

re-dispersed in 25 mL of ethanol. 106

2.4 Synthesis of SiO2@Au@TiO2 core-shell nanostructures 107

Typically, the above SiO2@Au aqueous solution with different amount of Au 108

nanoparticles (25 mL) were mixed with deionized water (0.12 mL), and HPC (80 mg) 109

under vigorous magnetic stirring. 0.25 mL of TBOT dissolved in ethanol (5 mL) 110

6

was introduced to the system drop by drop, followed by reuxing the solution at 80 111

C for 90 min [25]. The final products were washed with ethanol three times, dried 112

at 60 C for 8 h, and finally calcined in air at 500 C for 2 h. Therefore, the five new 113

samples were denoted as SiTi, SiAuTi-1, SiAuTi-2, SiAuTi-3 and SiAuTi-4, which 114

contains 0, ~0.05, ~0.10, ~0.25, ~0.50 wt% of the Au nanoparticles, respectively. 115

The content of the Au in the nanocomposites was calculated on the basis of all of the 116

precursors were completely convert to the final product. 117

2.5 Synthesis of Fe3O4@SiO2@Au@TiO2 core-shell nanostructures 118

The synthesis procedure of Fe3O4@SiO2@Au@TiO2 core-shell nanostructures is 119

similar to that of the SiO2@Au@TiO2 core-shell nanostructures except using the 120

Fe3O4@SiO2 core-shell nanostructures instead of the SiO2 spheres. The detailed 121

synthesis procedure of Fe3O4@SiO2 have been reported in our previous work [26,27]. 122

2.6. Characterization 123

The morphologies of the as-prepared samples were analyzed using a Phillips 124

Tecnai 10 transmission electron microscope (TEM) with an accelerating voltage of 125

100 kV. Elemental mapping was performed using STEM and EDX on a Tecnai G2 126

F20 S-TWIN transmission electron microscope with an accelerating voltage of 200 127

kV. The crystalline structures of all samples were evaluated by Xray diffraction 128

(XRD) analyses, carried out on a Rigaku D/maxrA diffractometer with Cu Ka 129

radiation ( 1.5405 ). The BET surface area and pore size distribution of the 130

products were measured by N2 adsorptiondesorption test on a Quantachrome, 131

ASIC-2 measuring instrument. The UVvisible diffuse reflectance spectra were 132

7

measured on a TU1901 spectrophotometer equipped with a labsphere diffuse 133

reflectance accessory. The X-ray photoelectron spectroscopy (XPS) measurements 134

were carried out using a VG ESCA Lab Mark II system with Mg K excitation. 135

2.7. Measurement of photocatalytic activity 136

Photocatalytic reactions for degradation of naproxen were carried out in a batch 137

photoreactor. The schematic illustration and the digital photo of the photoreactor are 138

shown in Fig. 2 and Fig. S1, respectively. The aqueous slurry with initial volume of 139

50 mL, prepared with a given amount of catalyst 1.0 g/L (in which the active TiO2 140

dosage is ~0.30 g/L) and naproxen in concentration of 1.0105 mol/L, was stirred in 141

the dark for 1.0 h to ensure that the naproxen was adsorbed to saturation on the 142

catalysts. A 250 W xenon lamp (as shown in Fig. S2, Supporting Information) was 143

used as the light source, the wavelength spectrum and intensity of the xenon lamp 144

used in our experiment are shown in Fig. S3 Supporting Information, and a cutoff 145

filter was used to block the UV light (< 420 nm). The reaction flask was put in a 146

cooling water system to keep the reaction at room temperature. Agitation was 147

provided by magnetic stirrer at a stirring rate of 800 rpm. 6 mL of suspension was 148

collected at timed intervals with a 10 mL syringe, then the sample was immediately 149

filtrated through a 0.22 m filter for analysis of naproxen concentration. The 150

concentration of naproxen were determined by HPLC (Agilent 1200, USA) provided 151

with a UVVis detector. A 4.6 mm 250 mm (5 m) XDBC18 column was used. 152

The analysis was carried out with a 70/30 (v/v) acetonitrile/water mobile phase and 153

the flow rate was set at 1.0 mL/min. 154

8

3. Results and discussion 155

3.1. Characterization of SiO2@Au@TiO2 core-shell nanostructures 156

3.1.1 TEM and STEM analysis 157

Firstly, the monodisperse spherical SiO2 cores with an average diameter ~337 nm 158

were synthesized by using the classical Stber process (as shown in Fig. 3a), then the 159

Au nanoparticles were adsorbed onto the 3-aminopropyl-triethoxysilane (APTES) 160

modified SiO2 cores under sonication (see Fig. 3b). Comparing Fig. 3b with Fig. 3c, 161

the smooth surface of the SiO2 spheres become rough, suggesting the successful 162

coating with a thin TiO2 layer outside the SiO2@Au particles. After calcination at 163

500 C for 2 h, the surface roughness of the SiO2@Au@TiO2 further increases (as can 164

be seen in Fig. 4d), indicating the crystallization of the TiO2 layer. It should be 165

noted that no apparent aggregations of the calcinated core-shell nanostructures have 166

been observed. The thickness of the TiO2 layer was estimated by calculating the 167

mean diameters of the SiO2@Au, SiO2@Au@TiO2, and calcined SiO2@Au@TiO2 168

core-shell nanostructures. In typical samples (as shown in Fig. 3), the average 169

diameter is ~337 nm for SiO2@Au, ~352 nm for SiO2@Au@TiO2, and ~351 nm for 170

calcined SiO2@Au@TiO2, suggesting the average thickness of the TiO2 layer is 7.5 171

and 7.0 nm before and after calcination, respectively. The thin thickness of the TiO2 172

layer will promote the photocatalytic acivtivy since our previous work and a number 173

of other researchers works have pointed out that the optimal size of anatase 174

nanocrystals for photocatalysis is around 10 nm [27-29]. The morphologies of other 175

samples decoration with different amount of Au nanoparticles can be found in Fig. S4, 176

9

Supporting Information. The structures of the SiO2@Au@TiO2 core-shell 177

nanostructures were further studied by scanning transmission electron microscope 178

(STEM) and the energy dispersive x-ray (EDX) silicon drift detector (SDD) elemental 179

mapping. The results are shown in Fig. 4, from which one can clearly see the Si, O, 180

Ti, and Au elements distribution of the selected particle. The EDX elemental 181

mapping results also can confirm the core-shell nanostructure of the selected sample. 182

3.1.2 XRD and XPS analysis 183

The XRD patterns of the calcined SiO2@Au@TiO2 core-shell nanostructures with 184

different decoration amount of Au nanoparticles are shown in Fig. 5. Take sample 185

SiAuTi-4 for an example, all diffraction peaks at 2 value of 38.3, 44.6, 64.8, and 186

77.6 can be indexed to the metallic gold (JCPDS 1-1172), suggesting the successful 187

decoration of Au nanoparticles in the SiO2@Au@TiO2 core-shell nanostructures. As 188

it can be observed, the intensity of the diffraction peaks increase with the increasing 189

decoration amount of Au nanoparticles. The diffraction peak at 2 value of 25.2 190

can be indexed to the anatase phase of TiO2 (JCPDS 21-1272). The relatively weak 191

diffraction peak is partly because of the small portion of the titania in the 192

nanocomposites and partly because of the influence from the amorphous phase of the 193

silica. To further confirm the chemical compositions of the surface of the 194

SiO2@Au@TiO2 core-shell nanostructures, X-ray photoelectron spectroscopy (XPS) 195

measurements were carried out. The results are shown in Fig. S5. In the spectrum, 196

elements of Si, Ti, C, Ag and O can be observed. The C 1s at 284.6 eV is due to the 197

adventitious hydrocarbon originated from the instrument itself. The Ag 3d5/2 and 198

10

Ag 3d3/2 is due to the high purity silver paint that has been used during XPS sample 199

preparation. All samples present binging energies for Ti 2p3/2 peaks at 458.2 eV 200

which can be assigned to Ti4+

in the TiO2 lattice [30], indicating the successful titania 201

coating outside the SiO2@Au colloids. Fig. 6 gives the high resolution XPS spectra 202

of Au 4f regions from the surface of the sample SiAuTi-3 and SiAuTi-4. Au 4f 203

region is characterized by a doublet of two spin orbit components corresponding to 204

Au 4f7/2 and Au 4f5/2 with a separation of about ~3.6 eV. The position of Au 4f7/2 205

peak at value of lower than 84 eV confirms the fact that Au is found as metallic Au 206

[31,32]. Noticeably, No Au 4f7/2 and Au 4f5/2 peaks were found in sample SiAuTi-1 207

and SiAuTi-2 may not only due to the low loading of the Au nanoparticles but also 208

due to the titania layer coated on the outside of the SiO2@Au collides (as shown in 209

the TEM and STEM images). 210

3.1.3 N2 adsorptiondesorption analysis 211

The surface areas and pore structures of the as-prepared samples with and without 212

Au nanoparticles decoration were investigated by N2 adsorptiondesorption analysis. 213

Fig. S6 shows the N2 adsorptiondesorption isotherms and the corresponding pore 214

size distribution curves of the calcined SiO2@Au@TiO2 core-shell nanostructures 215

with and without Au nanoparticles decoration. Without Au nanoparticles decoration, 216

the isotherms are of the typical type IV pattern with distinct H2 and H3 hysteretic 217

loops in the range of 0.50.9 P/P0 and 0.91.0 P/P0, respectively, indicating the 218

existence of ink-bottle- and slit-shaped pores according to the IUPAC classification 219

[33]. It is believed that the bimodal pore structures are beneficial to the 220

11

enhancement of photocatalytic performance due to the faster diffusion of various 221

reactants and byproducts and the increased harvesting of exciting light by multiple 222

scattering within the porous framework [34,35]. However, with Au nanoparticles 223

decoration, the hysteresis loop 1 disappeared and the hysteresis loop 2 shift to a 224

higher relative pressure (P/P0) range, which indicate that the increase of average pore 225

size (disappearance of the mesopores) and the decrease of total pore volume [36]. 226

The corresponding pore size distributions of all samples were determined using the 227

BarrettJoynerHalenda (BJH) method from the desorption branch of the isotherm. 228

Data concerning the BET surface area, average pore size and pore volume of the 229

calcined SiO2@Au@TiO2 core-shell nanostructures with and without Au 230

nanoparticles decoration are gathered in Table 1. The BET surface area and total 231

pore volume decreased with the increasing of decoration amount of Au nanoparticles. 232

This is mainly because a part of pores of the SiO2@Au@TiO2 core-shell 233

nanostructures may be blocked by the loaded Au nanoparticles. It has been reported 234

that the larger surface area and pore volume are beneficial to offer more active 235

adsorption sites and photocatalytic reaction centers result in improving the 236

photocatalytic activity [37]. Moreover, it is also believed that the developed pore 237

structures are beneficial to fast diffusion of the target pollutant and various byproducts, 238

thus improving the photocatalytic activity [38]. Therefore, the excessive decoration 239

of Au nanoparticles will lead to the decrease of the photocatalytic activity. 240

3.1.4 UVvis absorbance spectra analysis 241

UVvis absorbance spectra in the range from 200 to 800 nm of the calcined 242

12

SiO2@Au@TiO2 core-shell nanostructures with and without Au nanoparticles 243

decoration are shown in Fig. 7. It can be clearly observed that the samples with Au 244

nanoparticles decoration have stronger absorptions in the visible light region 245

compared with the sample without Au nanoparticles decoration, which is due to the 246

surface plasma resonance of Au nanoparticles decoration in the SiO2@Au@TiO2 247

core-shell nanostructures. With increasing the doping amount of Au, the absorption 248

intensity of Au surface plasma resonance peak at wavelength of ~560 nm [39] would 249

be gradually enhanced. This ensures that the SiO2@Au@TiO2 core-shell 250

nanostructures can be worked under visible light irradiation. In addition, the UV 251

part of the spectra were very similar for all the samples, indicating that Au 252

nanoparticles only deposits on surface of titania layer [10,30]. 253

3.2. Photocatalytic activity 254

The photocatalytic activities in visible light of the as-prepared samples were 255

evaluated by employing the photodegradation of naproxen as a model reaction. 256

Herein, the naproxen was selected as a model pollutant since it is a representative of 257

the PPCPs that has been newly and widely detected in the aquatic environment over 258

the past decade. Fig. 8a shows the photocatalytic oxidation of naproxen in the 259

presence of different samples under visible light irradiation. It can be found in Fig. 260

8a that visible light irradiation has no effect on the naproxen removal, no more than 261

2.2% of naproxen could be decomposed even after 6 h irradiation. As compared 262

with direct photolysis, 14% and 29.7% of naproxen could be removed in the presence 263

of the TiO2 spheres (denote as Ti) and SiO2@TiO2 core-shell nanostructures (denote 264

13

as SiTi), respectively. The synthesis process of TiO2 spheres can be found in 265

Supporting Information. Moreover, a higher removal rate was obtained when the 266

SiTi was replaced by SiO2@Au@TiO2 core-shell nanostructures (SiAuTi-2), thus 267

indicating the Au decoration on the interface of SiO2/TiO2 can enhance the 268

photocatalytic activity. It has been reported that Au nanopartilces decoration can 269

enhance the photocatalytic activity in visible light due to the following two reasons. 270

One is to help harvest the visible-light energy by their plasmonic effects, and the other 271

is to enhance charge separation by serving as an electron reservoir [40,41]. 272

Therefore, the photocatalytic activity under visible light irradiation enhanced with the 273

increasing decoration amount of Au nanoparticles. As shown in Fig. 8b, the 274

photocatalytic activity of SiAuTi-2 is higher than that of the SiAuTi-1. However, 275

with further increase in the decoration amount of Au, the photocatalytic activity 276

decrease dramatically. This is because excessive amounts of Au nanoparticles can 277

deteriorate the photocatalytic activity by increasing the occurrence of exciton 278

recombination [42]. Moreover, excessive decoration of the Au nanoparticles may 279

also break the connection of the TiO2 and SiO2 thus leading to decrease of the 280

photocatalytic activity. Therefore, sample SiAuTi-2 (Au content 0.1 wt%) in our 281

case processes the best photocatalytic activity for naproxen removal. In addition, the 282

visible light photocatalytic activities the of the as-prepared SiAuTi core-shell 283

nanostructures with different Au decoration amounts are higher than that of the 284

well-known commercial photocatalyst Degussa P25. 285

3.3. Recycling of the photocatalysts 286

14

Importantly, in order to prevent additional contamination, the nanosized catalysts 287

should be completely removed from the treated solution. It has been reported that 288

three-dimensional (3D) hierarchical structures have taken an advantage over powder 289

catalysts for separating the catalyst from solution by filtration or sedimentation [43]. 290

In our experimental, the SiO2@Au@TiO2 core-shell nanostructures can be separated 291

from an aqueous suspension in less than 4 h by sedimentation. For faster recycle of 292

the catalysts, highly field-responsive superparamagnetic magnetite (Fe3O4) cores were 293

embedded in the SiO2@Au@TiO2 core-shell nanostructures (see Fig. 9ac). Our 294

previous work have already demonstrate that this superparamagnetic magnetite can be 295

easily separated from solution in a low magnetic field gradient since it has a high 296

saturation magnetization [27]. A typical Fe3O4@SiO2@Au@TiO2 core-shell 297

nanostructure (denote as FeSiAuTi, see Fig. 9c) is composed of a central Fe3O4 core 298

with an average diameter of ~115 nm, an interlayer of SiO2 with an average thickness 299

of ~31 nm, tunable content of Au nanoparticles adsorbed on the surface of SiO2 shell, 300

and an outer layer of TiO2 with an average thickness of ~21.0 nm. Herein, we have 301

demonstrated that the majority of the superparamagnetic photocatalysts can be rapidly 302

collected within 2 min by using an external magnetic field (a 0.8 cm 1.8 cm 2.8 303

cm quadrate NdFeB magnet, as shown in Fig. 9d and Fig. 9f), and all of the 304

Fe3O4@SiO2@Au@TiO2 core-shell nanostructures can be completely removed after 305

10 min collection. The photocatalytic activity of the as-prepared superparamagnetic 306

photocatalyst was also evaluated by photocatalytic degradation of naproxen in visible 307

light under the same reaction condition as mentioned above. The result is shown in 308

15

Fig. 8b, from which one can see that the FeSiAuTi also has photocatalytic activity in 309

visible light. It is because the superparamagnetic magnetite in the core-shell 310

nanostructure just serve as the magnetic core for separation of the catalyst. 311

Unfortunately, the photocatalytic activity of FeSiAuTi is lower than that of the 312

SiAuTi-2 because of the lights scattering and screening effect [44,45] origin from the 313

turbid suspension of FeSiAuTi (as shown in Fig. 9d). It has been proved that when 314

the catalyst dose exceeds the optimum amount, the photocatalytic activity decreased 315

with the increasing dosage of the catalyst due to light scattering and screening effect 316

[44]. Many literature studies also pointed out that the catalyst amount beyond 1.0 317

g/L will result in the deterioration of the degradation [46]. In our experiment, in 318

order to keep the same amount of TiO2 (0.3 g/L) in each photocatalytic oxidation, 319

about 1.3 g/L of FeSiAuTi nanocomposites were used in the experiment. This may 320

be one of the reason to explain why the FeSiAuTi nanocomposites show lower 321

efficiency than the SiAuTi nanocomposites. Moreover, compare to the SiAuTi 322

suspension, the darker of the FeSiAuTi suspension may also lead to screening effect. 323

4. Conclusions 324

In summary, SiO2@Au@TiO2 core-shell nanostructures, with high photocatalytic 325

activities under visible light irradiation, were synthesized by combining three 326

individual synthesis steps with calcination. Sample SiAuTi-2 (with Au content of 327

0.1 wt%) possesses the highest photocatalytic activity since it has the suitable 328

decoration amount of Au nanoparticles for harvesting the visible-light energy by their 329

plasmonic effects and for prohibiting the recombination of free excitons by serving as 330

16

an electron reservoir. In addition, the as-prepared magnetic photocatalysts 331

(Fe3O4@SiO2@Au@TiO2 core-shell nanostructures) can be collected from aqueous 332

solution within 10 min by an external magnetic eld. The high photocatalytic 333

activity under visible light irradiation and the fast separation by magnetic separation 334

will make the application of these core-shell nanostructures in practice possible. 335

Acknowledgments 336

This work was financially supported by the National Natural Science Foundation of 337

China (No. 51108406 and No.51208457) and the Fundamental Research Funds for the 338

Central Universities (No. 2013QNA4026), to whom we are grateful. We also thank 339

Ms. Mao and Ms. Wu, the technician of 985-Institute of Agrobiology and 340

Environmental Sciences of Zhejiang University, for help with N2 341

adsorptiondesorption measurements. 342

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482

23

Figure Captions: 483

Fig. 1. Schematic illustration and typical TEM images of the synthesis process of the 484

SiO2@Au@TiO2 core-shell nanostructures 485

Fig. 2. Schematic of experimental apparatus for photocatalysis. (1) water out, (2) 486

water in, (3) xenon lamp, (4) quartz cold trap, (5) lamp protection sleeve, (6) cutoff 487

filter, (7) water out, (8) sample tube, (9) water in, (10) magnetic stirrer, (11) water 488

bath, (12) mechanical stirrer, (13) power switch. 489

Fig. 3. TEM images of (a) SiO2 spheres, (b) SiO2@Au, (c) SiO2@Au@TiO2, and 490

calcined SiO2@Au@TiO2 core-shell nanostructures at the same magnification. 491

Fig. 4. STEM image of a typical SiO2@Au@TiO2 core-shell nanostructure and the 492

corresponding EDX elemental mappings. 493

Fig. 5. XRD patterns of the calcined SiO2@Au@TiO2 core-shell nanostructures with 494

different doping amount of Au nanoparticles. 495

Fig. 6. High resolution XPS spectra of the Au 4f region taken on the sample SiAuTi-3 496

and SiAuTi-4. 497

Fig. 7. UVvis diffuse reflectance spectra of the calcined SiO2@Au@TiO2 core-shell 498

nanostructures with different doping amount of Au nanoparticles. 499

Fig. 8. a) Photocatalytic degradation of naproxen in the presence of TiO2, SiO2@TiO2, 500

and SiO2@Au@TiO2 under visible light irradiation; b) Influence of Au loading (0.05, 501

0.01, 0.25, 0.50 wt%, and FeSiAuTi) on the naproxen removal. 502

Fig. 9. TEM images of (a) Fe3O4@SiO2, (b) Fe3O4@SiO2@Au, (c) Fe3O4@SiO2@Au 503

@TiO2; (d) and (e) separation process of by using a quadrate NdFeB magnet. 504

Table 1 BET surface areas and pore structures of the SiO2@Au@TiO2 core-shell

nanostructures with different doping amount of Au nanoparticles

Samples

Au content

(wt%)

BET surface area

(m2/g)

Average pore size

(nm)

Total pore

volume (cm3/g)

SiTi 0 14.6 12.8 0.029

SiAuTi-1 0.05 12.0 20.9 0.031

SiAuTi-2 0.1 10.2 28.5 0.027

SiAuTi-3 0.5 8.9 36.2 0.027

SiAuTi-4 1.0 9.4 35.8 0.026

Table 1

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 9

Figure 8

SiO2@Au@TiO2 nanostructures with tunable amount of Au nanoparticles were

prepared.

SiO2@Au@TiO2 with 0.1 wt% Au content exhibit the highest photocatalytic activity.

Magnetic core was embedded in the nanostructure for fast separation of the catalyst.