Accepted Manuscript Preparation of Visible-Light Nano-Photocatalysts through Decoration of TiO2 by Silver Nanoparticles in Inverse Miniemulsions Zhihai Cao, Shudi Zhu, Hui Qu, Dongming Qi, Ulrich Ziener, Liu Yang, Yingjie Yan, Haitang Yang PII: DOI: Reference:
S0021-9797(14)00578-5 http://dx.doi.org/10.1016/j.jcis.2014.08.021 YJCIS 19756
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
28 June 2014 9 August 2014
Please cite this article as: Z. Cao, S. Zhu, H. Qu, D. Qi, U. Ziener, L. Yang, Y. Yan, H. Yang, Preparation of VisibleLight Nano-Photocatalysts through Decoration of TiO2 by Silver Nanoparticles in Inverse Miniemulsions, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.08.021
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Preparation of Visible-Light Nano-Photocatalysts through Decoration of TiO2 by Silver Nanoparticles in Inverse Miniemulsions Zhihai Cao1,2*, Shudi Zhu1, Hui Qu1, Dongming Qi2, Ulrich Ziener3, Liu Yang1, Yingjie Yan1, Haitang Yang1, * 1
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Xuelin Street 16, Hangzhou 310036, China
2
Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, 310018, China 3
Institute of Organic Chemistry III – Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, Ulm, 89081, Germany
Corresponding authors: [email protected]; [email protected]
ABSTRACT: Ag/TiO2 nanocomposites were prepared through combination of a sol–gel process of a titanium precursor in inverse miniemulsions and in situ reduction of silver ions in the “nanoreactors”. The morphological investigation shows that Ag nanoparticles are mainly located on the surface of TiO2 nano-supports because of the fast reduction rate of Ag ions by hydrazine. Ag/TiO2 nanocomposites with amorphous or anatase TiO2 phase displayed high visible-light catalytic activity for degradation of Rhodamine B. The photoactivity of Ag/anatase TiO2 nanocomposites could be influenced by the Ag content that could be conveniently tuned by the loading of silver salts. The influence of the loading of silver salts on the particle properties of the Ag/TiO2 nanocomposites was investigated systematically.
1
Keywords: Inverse miniemulsion; Ag/TiO2 nanocomposites; visible-light nano-photocatalyst; sol–gel; reduction.
1. INTRODUCTION Titanium(IV) dioxide (TiO2) has drawn intensive attention since its photocatalytic activity for decomposing water to produce H2 and O2 was discovered by Fujishima and Honda.[1] UV light is required for activation of the photocatalysis of TiO2 nanoparticles (NPs) because of their large band gap (3.0–3.2 eV). Therefore, the use of solar energy by plain TiO2-based materials is limited because only about 5% of sunlight is UV light. To improve the exploitation of the solar spectrum, a series of techniques has been developed to prepare TiO2-based photocatalysts possibly activated by visible-light, for example doping with non-metallic or metallic elements, or decoration with noble metal NPs.[2, 3] Noble metal/TiO2 nanostructured materials are promising because of their high visible-light photocatalytic activity and stability against photocorrosion.[4] Silver is one of the most widely investigated noble metals to prepare noble metal/TiO2 visible-light photocatalysts.[5-7] Ag/TiO2 nanocatalysts have been prepared through photodeposition of Ag NPs to TiO2 nano-supports[8], combination of the sol–gel technique and an azeotropic distillation[9], and one-pot sol–gel technique. [10] Microemulsion, a thermodynamically stable dispersion of two immiscible liquid phases, has been widely used to prepare nanocrystalline materials.[11] Noble-metal/TiO2 nanocatalysts could also be prepared through the microemulsion-based technique.[12,
13]
For example,
Zielińska et al. prepared a series of Ag/TiO2 nanocatalysts in microemulsion systems consisting of water/sodium bis-(2-ethylhexyl) sulfosuccinate/cyclohexane.[13] Typically, a 2
relatively large amount of surfactant has to be used to obtain a colloidally stable microemulsion with low interfacial tension and sub-100 nm droplets.[11] In comparison, a relatively lower amount of surfactant is required for stabilizing a miniemulsion system. Furthermore, there is a fast exchange of components between droplets in microemulsions. In recent years, miniemulsion has attracted intensive attention in recent years because of its high capability to prepare nanostructured polymeric, inorganic, and hybrid particles through versatile reactions for example polymerization and sol–gel processes.[14] Each droplet in miniemulsions can be regarded as a separated nanoreactor.[15] Hybrid NPs can be prepared conveniently through pre-introduction of a second moiety into the droplets and consequent encapsulation via various reactions.[16] Inverse miniemulsions composed of a polar dispersed phase and a low polarity continuous phase can be applied for preparing hydrophilic nanostructured materials, for example nanogels and inorganic nanocapsules.[17] Hydrophilic metal salts, which work as lipophobes are inherently required to improve the droplet stability in inverse miniemulsions.[17] Therefore, preparation of hybrid NPs containing hydrophilic metal salts can be performed conveniently. A series of nanogels and inorganic NPs containing various metal salts have been prepared in inverse miniemulsions.[18-20] The incorporated metal salts can be converted to metal NPs or metal oxide NPs to confer additional functionalities to hybrid NPs through certain reactions. For example, we have prepared magnetic hollow silica NPs successfully by converting iron salts to magnetic iron oxide NPs through heat treatment.[20] Recently, Heutz et al. prepared Au/TiO2 nanocomposites (NCs) by using AuCl4(NH4)7[Ti2(O2)2(cit)(Hcit)]2·12H2O
as
a
single-source
precursor
in
inverse
miniemulsions.[21] Two small molecular weight surfactants, sodium dodecylsulfate (SDS) or Triton X-100, were separately used to stabilize their inverse systems. However, a large 3
amount of surfactant was required, probably due to the high hydrophilic–lipophilic balance (HLB) value of these two surfactants. In addition, the adjustment of Au content in the Au/TiO2 NCs may be inconvenient due to the fixed ratio of Au to Ti in the precursor. Although deposition of Ag NPs to a preformed TiO2 nano-support for example P25 TiO2 NPs (Degussa) has been frequently used to prepare Ag/TiO2 NPs, the good dispersion of P25 TiO2 NPs in the reaction medium is relatively difficult to achieve. In the present work, we propose a technique based on the inverse miniemulsion technique to prepare Ag/TiO2 NCs that display good dispersibility and high visible-light catalytic activity. Most of the Ag NPs were attached to the surface of TiO2 nano-supports, advantageous for catalytic applications. The content of Ag NPs could be adjusted conveniently by varying the AgBF4 loading. The influences of the AgBF4 loading on the particle properties and photocatalytic activity of Ag/TiO2 NCs were investigated systematically.
2. Experimental Materials. Titanium ethoxide (TEO, 33–35% TiO2), dimethyl sulfoxide (DMSO, AR), Rhodamine B (RhB, 95%), and hydrazinium hydrate (AR grade, 80% solution) were purchased from Aladdin Chemistry Co. Ltd. Hexadecane (HD, 99%, Acros Organics) and AgBF4 (99%, Adamas Reagent Co, Ltd.) were used as received. The surfactant, poly(ethylene-co-butylene)-b-poly(ethylene oxide) (P(E/B)−PEO) with number-average molecular weight of 7100 g·mol−1 and a hydrophilic−lipophilic balance of 8.7 was synthesized according to the literature.[22] Demineralized water was used in all runs. Preparation of AgBF4/TiO2 NCs. AgBF4 (0.073−0.292 g) and water (0.2 g) were dissolved in DMSO (1.0 g) to form a polar solution for use as dispersed phase. P(E/B)−PEO (40.4 mg) 4
was dissolved in HD (12.5 g) functioning as continuous phase. After the mixture of both solutions was pre-emulsified under strong magnetic stirring for 15 min, the resulting crude emulsion was sonicated in an ice bath to prepare the inverse miniemulsions. Sonication was performed by applying a pulsed sequence (work 12 s, break 6 s) for 9 min, using a Scientz JY92-II DN sonifier at 42% maximum power. Thereafter, TEO (0.633 g) was directly introduced to the prepared inverse miniemulsions at 82 °C. The sol−gel process of TEO was run for 3 h with magnetic stirring at 300 rpm at 82 °C to obtain AgBF4/TiO2 NCs. Preparation of Ag/TiO2 NCs. Aqueous hydrazine solution (40 mg) was added to 2 g of the dispersion of AgBF4/TiO2 NCs. The reduction reaction was run for 2 h with magnetic stirring at 300 rpm at 40 °C to obtain Ag/TiO2 NCs with amorphous TiO2. Calcination of Ag/TiO2 NCs. Ag/TiO2 NCs were purified by a three-cycle centrifugation-redispersion in cyclohexane and a three-cycle centrifugation-redispersion in ethanol. The purified dry solid samples were heated to target temperatures (400 or 900 °C) at a heating rate of 2 °C·min−1, and then kept at the target temperature for 2 h in nitrogen to obtain Ag/anatase TiO2 (400 °C) and Ag/rutile TiO2 composites (900 °C). Visible-light catalytic activity of Ag/TiO2 NCs. Ag/TiO2 solid photocatalysts (5 or 30 mg) were dispersed in a 30 mL of an aqueous solution of RhB (2 × 10−5 mol·L−1) in an ultrasound bath. The reaction mixture was stored in the dark for 40 min to reach an adsorption equilibrium. The photocatalytic reaction was carried out in a test tube with agitation (1000 rpm) at 20 °C. The reaction medium was irradiated by a 400 W metal halogen lamp in a photochemical reaction instrument (XPA series, Nanjing Xujiang Instrument). UV light was cut off by a filter (λ < 400 nm). The RhB concentration during the photocatalytic reaction was measured at various time intervals by using a UV–vis spectrometer. The characteristic 5
absorption band of RhB at 554 nm was used to monitor the photodegradation of RhB in solution.[23] Characterization. Transmission Electron Microscopy (TEM). TEM measurements were performed on a Hitachi HT-7700 microscope operated at 80 kV. One droplet of NC dispersion was diluted in 2 mL of cyclohexane. One droplet of the diluted sample was placed on a 400 mesh carbon-coated copper grid and then allowed to dry at room temperature. The number-average particle sizes of AgBF4/TiO2 NCs, Ag/TiO2 NCs, and Ag NPs were obtained by counting at least 200 NPs Field Emission Scanning Electron Microscopy (FESEM). FESEM measurements were performed on a field emission scanning electron microscope (Supra 55, Carl Zeiss SMT Pte. Ltd.) by using powdered samples placed on a conductive film. The accelerating voltage was 2 kV. Energy-dispersive X-ray (EDX) spectroscopy was done on an energy-dispersive spectroscopy analyzer (Oxford Instruments INCA PentaFET*3) operated at 10 kV. The atomic content of Ag is reported as the average of four measurements. Nitrogen Sorption Analysis. Nitrogen sorption measurements were carried out at 77 K on a Quantachrome Autosorb-1-C automated gas sorption apparatus. The specific surface area was calculated through the Brunauer–Emmett–Teller equation. The pore size distribution was derived from the adsorption branch through the Barret–Joyner–Helenda method. UV–vis Spectroscopy. A 20 μL dispersion of AgBF4/TiO2 or Ag/TiO2 NCs was diluted with 2 g of cyclohexane. The UV–vis spectra were recorded on a UV-2450 UV–vis spectrometer from Shimadzu Corporation Company at a spectral range of 350 to 700 nm. X-ray Diffraction (XRD). Phase analysis of the samples by XRD was carried out on a 6
Bruker D8 Advance X-ray diffractometer with Bragg–Brentano θ–2θ geometry. The generator was a high-powered diffraction tube with a copper anode, which was operated at a working power of 1.6 kW (40 kV, 40 mA), and its Cu Kα radiation (λ= 1.5418 Å) was used for diffraction scans. Scan patterns were obtained at a resolution of 0.0195° from 20° to 90°. Powder samples for XRD were supported on a polymeric sample holder. The crystallite size of Ag and anatase TiO2 was calculated through the Scherrer equation. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were carried out on a Thermo K-Alpha spectrometer (Thermo Fisher Scientific Inc., MA) with a bass pressure lower than 1.0×10-9 Torr. All XPS spectra were collected at normal emission with a monochromatic Al Kα X-ray source (1486.6 eV). The binding energy of all XPS spectra were calibrated and referenced to the Fermi level of a sputtered clean gold sample and the scanning spot size on the sample is 100 μm.
3. RESULTS AND DISCUSSION Preparation of AgBF4/TiO2 and Ag/TiO2 NCs. To fabricate AgBF4/TiO2 NCs, AgBF4 was pre-introduced to the dispersed phase of the inverse miniemulsion; subsequently, AgBF4/TiO2 NCs were prepared through a sol–gel process of TEO in the AgBF4-containing inverse miniemulsion. The prepared AgBF4/TiO2 NCs showed a well-defined spherical morphology and a rough surface (Figure 1A). The number-average particle size of AgBF4/TiO2 NCs determined by TEM was 97 ± 27 nm. Ag/TiO2 NCs were obtained by in situ reduction of the Ag ions in the AgBF4/TiO2 NCs. The reducing agent, hydrazine, was added directly to the dispersion of AgBF4/TiO2 NCs. The reduction of Ag ions by N2H4 was confirmed by UV–vis spectroscopy. No obvious absorption peak could be observed in the range of 400–500 nm 7
before addition of hydrazine (Figure 2A, curve a), while a clear peak with a maximum absorbance at 434 nm appeared in the spectrum of the reduced sample (Figure 2A, curve b), consistent with the absorption of Ag NPs induced by localized surface plasmon resonance (LSPR).[24] The reduction of Ag ions by N2H4 is very efficient with a high conversion of around 90% already after 10 min and a maximum absorbance at about 50 min (Figure S1). The XRD results also indicate the formation of Ag NPs (Figure 2B). Compared with the XRD pattern before reduction (curve a in Figure 2B), five well-resolved diffraction peaks appear in the XRD pattern of the sample after reduction (curve b in Figure 2B). They can be well indexed with silver phase reflections according to the JCPDS Card Number 04-0783. A large number of small NPs as dark spots appears in the TEM image of the reduced sample (Figure 1B inset), which are assigned to Ag NPs. The particle size of Ag NPs determined by TEM was 12.3 ± 3.5 nm. The SEM result (Figure 1B) indicates that most of the Ag NPs are located on the surface of the TiO2 nano-supports. This characteristic is advantageous for the application as catalysts. The atomic ratio of Ag and Ti in the AgBF4/TiO2 NCs determined by EDX amounts to about 1 : 5, consistent with the employed materials for the synthesis. It also means that AgBF4 distributes homogenously over the AgBF4/TiO2 NCs. The atomic ratio of Ag and Ti in the Ag/TiO2 NCs determined by EDX is 1 : 3, higher than that of the AgBF4/TiO2 NCs (1 : 5). The somewhat higher ratio in the Ag/TiO2 NCs supports the fact that the Ag NPs mainly distribute on the surface. In addition, it is reasonable to propose that the Ag ions mirgrate to the surface from the interior of the TiO2 nano-supports to participate in the reduction reaction.
8
Figure 1. TEM (inset) and SEM images of the AgBF4/TiO2 (A) and Ag/TiO2 NCs (B). The amount of AgBF4 used in this run was 0.146 g.
The surface composition of the Ag/TiO2 NCs was further measured by XPS. Ag, Ti, O, F, and C could be detected in the sample (Figure 2C inset). F and C could be reasonably ascribed to the residual BF4- and surfactant. The bonding energies (B. E.s) of Ag are determined as 367.9 eV and 374.0 eV for Ag3d5/2 and Ag3d3/2 (Figure 2D), respectively, indicating the presence of Ag(0). The B. E.s of Ti are 459.1 eV and 464.8 eV for Ti2p3/2 and Ti2p1/2, respectively, with a difference of 5.7 eV (Figure 2C) corresponding to Ti(IV).[25] A
0.8
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0 470 468 466 464 462 460 458 456 454
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Figure 2. (A) UV–vis spectra of the dispersions of AgBF4/TiO2 and Ag/TiO2 NCs. (B) XRD patterns of the AgBF4/TiO2 and Ag/TiO2 NCs. (C and D) XPS spectra of the Ag/TiO2 NCs. The amount of AgBF4 used in this run was 0.146 g.
Discussion on the formation of Ag/TiO2 NCs. Before reduction, AgBF4 is dissolved in DMSO and therefore distributed presumably homogenously in the AgBF4/TiO2 NCs. Once hydrazine is added to the dispersion of AgBF4/TiO2 NCs, N2H4 molecules can diffuse to the AgBF4/TiO2 NCs via the continuous phase of the dispersion. The N2H4 molecules meet with Ag ions at the surface of the AgBF4/TiO2 NCs. The reduction of Ag ions by N2H4 is very fast, and therefore the N2H4 molecules are consumed completely at the surface before they diffuse into the AgBF4/TiO2 NCs. The formed Ag nuclei precipitate on the surface of the NCs. It has been reported that the reduction of Ag ions by N2H4 is an autocatalytic process.[26, 27] During the reaction process, the slow, continuous nucleation and fast, autocatalytic surface growth may concurrently occur.[26, 27] Therefore, it is reasonable to expect that the Ag ions diffuse to the surface to participate in the reduction reaction in the whole process. The surface of NCs is the main reaction locus, and consequently, the Ag NPs mainly locate on the surface of the TiO2 nano-supports.
10
Calcination of Ag/TiO2 NCs. As shown in Figure 2B, TiO2 synthesized through the sol–gel process appears amorphous. It has been reported that the phase of TiO2 can be converted from amorphous to anatase or rutile by heat treatment at different temperatures.[28] After calcination at 400 °C well-resolved diffraction peaks belonging to anatase TiO2 according to the JCPDS Card Number 21-1272 appeared in the XRD pattern (Figure 3A (curve a)). Thus, amorphous TiO2 has been converted to anatase TiO2. Treatment of the Ag/amorphous TiO2 NCs at 900 °C led to the formation of rutile TiO2 as confirmed by XRD (Figure 3A, curve b). It should be pointed out that no diffraction peaks of Ag2O appeared in the calcined samples.[13] Compared with the Ag/amorphous TiO2 NCs, the B. E.s of Ag in the Ag/anatase TiO2 and Ag/rutile TiO2 NCs did not change (Figure S2). XPS results also indicate that Ag NPs have not been oxidized during calcination. Before calcination, the nitrogen adsorption–desorption isotherm of Ag/TiO2 NCs displayed a type IV isotherm (Figure S3). Upon heat treatment the specific surface area decreased from 154 m2·g-1 (Figure S3) to 86 m2·g-1 for the Ag/anatase TiO2 NCs exhibiting a final mean pore size of about 12.5 nm (Figure 3B inset). After calcination at 900 °C, the Ag/rutile TiO2 NCs became nonporous. Compared with the sample before calcination (Figure 1B), most of the particles became irregular after calcination at 400 °C (Figure 3C). The atomic ratio of Ag and Ti determined by EDX was 1 : 2.9, not affected significantly by the heat treatment. The NCs merged to form micro-sized particles or even bulk objects after calcination at 900 °C (Figure 3D and inset).
11
B
A
150
Intensity (a.u.)
o
a: calcined at 400 C o b: calcined at 900 C anatase rutile Ag
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2θ (degree)
Figure 3. (A) XRD patterns of Ag/anatase TiO2 and Ag/rutile TiO2 composites. (B) N2 adsorption-desorption isotherm and pore size distribution (inset) of the Ag/anatase TiO2 NCs. SEM images of the Ag/TiO2 NCs calcined at 400 oC (C) and 900 oC (D).
Influence of AgBF4 loading on the particle properties. The photocatalytic activity of Ag/TiO2 NCs may be tuned by the content of Ag NPs.[27] One advantage of inverse miniemulsions is that the loading of salts into the dispersed phase can be tuned conveniently in a wide range.[19] Therefore, the content of Ag NPs in the NCs could be adjusted in a wide range through variation of the AgBF4 loading. In the present paper, the AgBF4 loading was varied in the range of 0.073–0.292 g, and the corresponding ratio of Ag to Ti was varied in the range of 0.14–0.54. It should be pointed out that the colloidal stability of the systems with various AgBF4 loadings was well controlled. In the following part, the influence of AgBF4 loading on 12
the particle properties of Ag/TiO2 NCs will be discussed systematically. a) Particle morphology and size. Spherical AgBF4/TiO2 NCs were obtained in the systems with 0.073–0.292 g of AgBF4 (Figure S4A, 4C, 4E, and 4G). After reduction, Ag ions were converted to Ag NPs, which attached to the surface of TiO2 nano-supports (Figure S4B, 4D, 4F, 4H, and S5A–D). Roughly estimated by TEM and SEM, the number of Ag NPs increased with the increase of AgBF4 loading (Figure S4A–H and S5A–D). This conclusion was confirmed by EDX. The content of Ag NPs in the Ag/TiO2 NCs could be tuned in the range of 4.2–13.5 atom% (Figure 4A). The Ag content determined by EDX was consistent with the employed content for the Ag/TiO2 NCs with 0.07 and 0.11 g of AgBF4, while the Ag content determined by EDX was lower than the employed content for the Ag/ TiO2 NCs with 0.146, 0.219 and 0.292 g of AgBF4. Considering the limited detecting thickness of EDX, the deviation of the Ag content indicates that some Ag NPs were located inside the NCs when a high AgBF4 loading was employed. The particle size of Ag/TiO2 NCs varied in the range of 100–140 nm, and did not show obvious dependence on the AgBF4 loading (Figure 4B). It has been reported that the particle size of NPs first decreases and then remains almost constant with the increase of salt content in the dispersed phase of inverse miniemulsions.[19] Therefore, the AgBF4 content used in the present study may lie in the range in which the particle size of NCs only weakly depends on the variation of AgBF4 loading.
13
A
20
determined by EDX feeding content
16 12 8 4 0 0.05
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Particle size (nm)
Ag content (atom%)
24
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150
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Figure 4. The Ag content in the Ag/TiO2 NCs with various AgBF4 loadings determined by EDX (A). Particle size of Ag/TiO2 NCs with various AgBF4 loadings (B).
b) Properties of Ag NPs. The optical property of the Ag/TiO2 NCs induced by LSPR was characterized by UV–vis spectroscopy. The results are shown in Figure 5A. With the increase of AgBF4 loading, the absorbance of the Ag NPs increased. The maximum absorption wavelength (λmax) of the Ag NPs remained at about 435 nm in the AgBF4 range of 0.073–0.219 g. In addition, the bandwidths of the absorption peak of the Ag NPs in the same range were similar. It is well accepted that the optical absorption of noble metal NPs depends on their particle size and particle shape.[29, 30] It means that the particle properties of the Ag NPs in the AgBF4 range of 0.073–0.219 g did not change obviously. This result is consistent with the independence of the number-average sizes of Ag NPs on the AgBF4 loading (Figure 5B). Although the size and shape of the Ag NPs of the sample with 0.292 g of AgBF4 is similar to the other samples, λmax of the Ag NPs shifted to about 444 nm. This red shift might be ascribed to the enhancement of interparticle coupling effects induced by the increase of particle number on the surface of one NC. [31, 32] Five well-resolved characteristic diffraction peaks belonging to the Ag NPs could be 14
observed in the NCs with various AgBF4 loadings (Figure 5C). The AgBF4 loading had only a slight influence on the crystallite size of Ag, which is varied in the small range of 7.9–10.8 nm (Figure 5D). The calcination process showed an obvious influence on the crystallite size of Ag, which increased to about 14 nm in the NCs with 0.073–0.219 g of AgBF4 after calcination at 400 oC (Figure 5D). For the NCs with 0.292 g of AgBF4, the crystallite size of Ag significantly increased from 10.1 nm to 21.8 nm. It means that the coalescence of Ag crystal grains took place during calcination in this sample.
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Figure 5. UV–vis spectra (A) and particle size (B) of Ag NPs of Ag/TiO2 NCs with various AgBF4 loadings. XRD patterns of Ag/TiO2 NCs with various AgBF4 loadings (C). Influence of AgBF4 loading and calcination on the crystallite size of Ag NPs (D).
15
c) Properties of TiO2 nano-supports. As shown in Figure 6A, the characteristic diffraction peaks belonging to anatase phase were observed in the XRD patterns of Ag/TiO2 NCs with various AgBF4 loadings after calcination at 400 oC. It means that the TiO2 phase has been converted from amorphous to anatase by this heat treatment. The crystallite size of TiO2 varied in the range of 15–23 nm, it first decreased and then increased slightly with the increase of AgBF4 loading (Figure 6B). The specific surface area of Ag/TiO2 NCs could be influenced by the AgBF4 loading. The surface area of the Ag/TiO2 NCs with 0.073 g of AgBF4 was about 210 m2·g-1. With the increase of AgBF4 loading, the specific surface area decreased. This reduction may be ascribed to the increase of the amount of Ag NPs, which may cover a part of the pores of TiO2. This assumption is confirmed by the fact that the pore volumes of the Ag/TiO2 NCs decrease with the increase of the AgBF4 loading (Figure S6). The N2 adsorption–desorption isotherms of the Ag/TiO2 NCs after calcination at 400 oC display a type IV isotherm (Figure 6C), indicating the presence of mesopores. The heat treatment led to the reduction of the specific surface area of TiO2 nano-supports (Figure 6D). This reduction is caused by the coalescence of TiO2 crystal grains under calcination conditions.[28] It should be pointed out that samples with rutile TiO2 were not fully characterized and discussed because of their low photocatalytic activity under irradiation of visible- light (see the next section).
16
A Ag Anatase
Crystallite size (nm)
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Figure 6. XRD patterns (A) and crystallite sizes of TiO2 (B) of Ag/TiO2 NCs with various AgBF4 loadings after calcination at 400 oC. N2 adsorption–desorption isotherms of the Ag/TiO2 NCs after calcination at 400 oC (C). Specific surface area of Ag/TiO2 NCs before and after calcination (D).
Photocatalytic degradation of RhB under irradiation of visible-light. Visible-light catalytic activity of Ag/TiO2 NCs was evaluated through the degradation of RhB under irradiation of visible-light. As shown in Figure S7, the characteristic absorption band of RhB at 554 nm was significantly reduced in the reaction catalyzed by the Ag/anatase TiO2 nanocomposites under the irradiation of visible-light. After 180 min reaction, only a very small absorption peak at 554 nm could be observed in the spectral range of 400–650 nm, indicating that most of RhB 17
has been mineralized (Figure S7). As shown in Figure 7A, Ag/amorphous TiO2 and Ag/anatase TiO2 NCs showed a high visible-light catalytic activity for photodegradation of RhB. Plain TiO2 NPs have only limited photocatalytic activity under irradiation of visible-light because of their large band gap. Therefore, the obvious visible-light catalytic activity of the Ag/TiO2 NCs could be ascribed to the cooperative contribution of Ag NPs and TiO2 nano-supports. The mechanism of visible-light catalysis of plasmonic-metal/TiO2 NCs has been investigated extensively in recent years. Three possible mechanisms have been proposed, including localized surface-mediated charge injection from metal to TiO2, near-field electromagnetic mechanism, and scattering mechanism.[33] The first mechanism requires the direct contact of plasmonic metal and TiO2. Besides of the direct contact systems, the latter two mechanisms may also take place in the systems in which plasmonic metal and TiO2 are separated. In our case, the Ag NPs are mainly distributed on the surface of the NCs, and the excited electrons appear in the conduction band of TiO2 either possibly by excitation through the near-field effect between Ag and TiO2 NPs or by LSPR-induced charge transfer from Ag NPs to TiO2. The excited electrons react with adsorbed O2 to form O2–· firstly, and then become ·OH radicals through a series of reactions.[34] The ·OH radicals degrade the RhB molecules to mineralized compounds. The Ag/rutile TiO2 composites did not show obvious visible-light catalytic activity. The low photo-activity may be ascribed to the nonporous structure of the composites and the large size of Ag and TiO2 particles (Figure 3D). The role of Ag NPs in the visible-light catalysis of Ag/TiO2 NCs could be as follows: a) enhancing the adsorption of visible-light through LSPR; b) improving the separation of excited electrons and holes; c) working as recombination sites.[23] The former two factors 18
promote the visible-light photocatalytic activity of Ag/TiO2 NCs, but the third factor will deteriorate the photocatalytic activity. As shown in Figure 7B, the Ag/TiO2 NCs with various AgBF4 loadings showed obvious photocatalytic activity under irradiation of visible light. The photodegradation rate increased with the increase of Ag content firstly, and then decreased, Figure 7C. The reduction in photoactivity of Ag/TiO2 NCs with a high Ag content may be caused by the increased exciton recombination of Ag NPs, similar to the findings of Wu et al.[[35]] In addition, the reduction of the adsorption of RhB and water soluble O2 on the surface of NCs because of the attachment of too many Ag NPs on the surface of TiO2 nano-supports may also reduce the photocatalytic degradation of RhB, as reported by Xiong et al.[36]
B 1.0
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AgBF4 0.073 g AgBF4 0.110 g
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Figure 7. Photodegradation kinetics of RhB under irradiation of visible-light in the presence
19
of Ag/TiO2 NCs with various TiO2 phases (A) and with various Ag contents (B). Relationship between c/c0 and the AgBF4 loading after reacted for 30 min (C). The feeding amount of AgBF4 in the Ag/TiO2 NCs used in Figure 7A was 0.146 g. The amount of Ag/TiO2 NCs used in photocatalytic experiments for Figure 7A and 7B was 30 mg and 5 mg, respectively.
4. CONCLUSIONS In this paper, we present a convenient method to prepare Ag/TiO2 NCs with high visible-light catalytic activity. AgBF4/TiO2 NCs were first prepared through a sol–gel process of TEO in inverse miniemulsions. The Ag/TiO2 NCs were obtained through reduction of Ag ions by hydrazine on the surface of AgBF4/TiO2 NCs. Ag NPs were distributed mainly on the surface of the Ag/TiO2 NCs, advantageous for catalytic applications. The TiO2 nano-supports synthesized through the sol–gel process were amorphous, but they could be converted easily to anatase or rutile phase through heat treatment. The heat treatment led to the coalescence of Ag and TiO2 crystal grains and reduction of surface area of the Ag/TiO2 NCs. The content of Ag NPs in the Ag/ TiO2 NCs could be conveniently tuned by varying the AgBF4 loading. The AgBF4 loading had significant influence on the particle properties of Ag/TiO2 NCs. The Ag/amorphous or anatase TiO2 NCs showed high visible-light catalytic activity for photodegradation of RhB. The photo- and radiolysis-deposition of Ag to TiO2 supports are often required to be carried out at a low concentration of silver ions to avoid the formation of separated Ag nanoparticles.[8, 37] By our proposed technique, the reduction of silver ions is carried out directly on the surface of silver salt/TiO2 NCs, and therefore the silver salt content does not influence the particle morphology and colloidal stability of the reaction systems significantly. Therefore, Ag/TiO2 NCs with different levels of Ag content can be synthesized 20
conveniently in a one-pot reduction. As a common heterophase reaction system, the miniemulsion is easy to scale up, advantageous for industrialization.[38] In addition, the proposed technique holds high potential to prepare various noble metal or metal alloy/TiO2 nano-photocatalysts by introducing different single or mixed noble metal salts to the dispersed phase of inverse miniemulsion.
ACKNOWLEDGEMENTS. Financial supports from National Natural Scientific Foundation of China (NNSFC) project (51003023), Zhejiang Province’s Xinmiao Talent Plan (2013R421041), and National Undergraduate Training Programs for Innovation and Entrepreneurship of China (201310346013) are gratefully acknowledged.
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2228. [9] S.A. Amin, M. Pazouki, A. Hosseinnia, Powder Technol. 196 (2009) 241. [10] H.J. Zhang, G.H. Chen, Environ. Sci. Technol. 43 (2009) 2905. [11] A.K. Ganguli, A. Ganguly, S. Vaidya, Chem. Soc. Rev. 39 (2010) 474. [12] R.B. Khomane, J. Colloid Interface Sci. 356 (2011) 369. [13] A. Zielińska, E. Kowalska, J.W. Sobczak, I. Łącka, M. Gazda, B. Ohtani, J. Hupka, A. Zaleska, Sep. Purif. Technol. 72 (2010) 309. [14] K. Landfester, Angew. Chem. Int. Ed. 48 (2009) 4488. [15] J.M. Asua, Prog. Polym. Sci. 27 (2002) 1283. [16] D.M. Qi, Z.H. Cao, U. Ziener, Adv. Colloid Interface Sci. 211 (2014). [17] Z.H. Cao, U. Ziener, Nanoscale 5 (2013) 10093. [18] Z.H. Cao, U. Ziener, K. Landfester, Macromolecules 43 (2010) 6353. [19] Z.H. Cao, Z. Wang, C. Herrmann, U. Ziener, K. Landfester, Langmuir 26 (2010) 7054. [20] Z.H. Cao, L. Yang, Q.L. Ye, Q.M. Cui, D.M. Qi, U. Ziener, Langmuir 29 (2013) 6509. [21] N.A. Heutz, P. Dolcet, A. Birkner, M. Casarin, K. Merz, S. Gialanella, S. Gross, Nanoscale 5 (2013) 10534. [22] H. Schlaad, H. Kukula, J. Rudloff, I. Below, Macromolecules 34 (2001) 4302. [23] Q. Zhang, D.Q. Lima, I. Lee, F. Zaera, M.F. Chi, Y.D. Yin, Angew. Chem. Int. Ed. 50 (2011) 7088. [24] Z. Cao, C. Walter, K. Landfester, Z. Wu, U. Ziener, Langmuir 27 (2011) 9849. [25] J.M. Du, J.L. Zhang, Z.M. Liu, B.X. Han, T. Jiang, Y. Huang, Langmuir 22 (2006) 1307. [26] V.V. Tatarchuk, A.P. Sergievskaya, T.M. Korda, I.A. Druzhinina, V.I. Zaikovsky, Chem. Mater. 25 (2013) 3570. 22
[27] M.A. Watzky, R.G. Finke, J. Am. Chem. Soc 119 (1997) 10382. [28] J.B. Joo, Q. Zhang, I. Lee, M. Dahl, F. Zaera, Y.D. Yin, Adv. Funct. Mater. 22 (2012) 166. [29] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410. [30] B.J. Wiley, S.H. Im, Z.-Y. Li, J. McLellan, A. Siekkinen, Y.N. Xia, J. Phys. Chem. B 110 (2006) 15666. [31] K.-H. Su, Q.-H. Wei, X. Zhang, J.J. Mock, D.R. Smith, S. Schultz, Nano Lett. 3 (2003) 1087. [32] Y. Lu, G.L. Liu, L.P. Lee, Nano Lett. 5 (2005) 5. [33] S. Linic, P. Christopher, D.B. Ingram, Nat. Mater. 10 (2011) 911. [34] J.G. Yu, G.P. Dai, B.B. Huang, J. Phys. Chem. C 113 (2009) 16394. [35] Y.M. Wu, H.B. Liu, J.L. Zhang, F. Chen, J. Phys. Chem. C 113 (2009) 14689. [36] Z.G. Xiong, J.Z. Ma, W.J. Ng, T.D. Waite, X.S. Zhao, Water Res. 45 (2011) 2095. [37] E. Grabowska, A. Zaleska, S. Sorgues, M. Kunst, A. Etcheberry, C. Colbeau-Justin, H. Remita, J. Phys. Chem. C 117 (2013) 1955. [38] J.M. Asua, Progr. Polym. Sci. DOI: 10.1016/j.progpolymsci.2014.02.009 (2014).
23
Graphical abstract
Ag/TiO2 nanocomposites were prepared through combination of a sol–gel process of a titanium precursor in inverse miniemulsions and in situ reduction of silver ions in the “nanoreactors”. Ag nanoparticles are mainly located on the surface of TiO2 nano-supports. Ag/TiO2 nanocomposites with amorphous or anatase TiO2 phase displayed high visible-light catalytic activity for degradation of Rhodamine B. The influence of the loading of silver salts on the particle properties of the Ag/TiO2 nanocomposites was systematically investigated. The proposed
technique
holds
high
potential
nano-photocatalysts.
24
to
prepare
various
noble
metal/TiO2
Highlights 1. 2. 3. 4. 5.
Ag/TiO2 nanocomposites (NCs) were prepared in inverse miniemulsions. Ag nanoparticles (NPs) are located on the surface of NCs. The content of Ag NPs could be easily tuned by the loading of silver salt. The loading of silver salt has significant influence on the properties of NCs. Ag/TiO2 NCs with amorphous and anatase TiO2 displays a visible-light photocatalytic activity.
25
S0021-9797(14)00578-5 http://dx.doi.org/10.1016/j.jcis.2014.08.021 YJCIS 19756
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
28 June 2014 9 August 2014
Please cite this article as: Z. Cao, S. Zhu, H. Qu, D. Qi, U. Ziener, L. Yang, Y. Yan, H. Yang, Preparation of VisibleLight Nano-Photocatalysts through Decoration of TiO2 by Silver Nanoparticles in Inverse Miniemulsions, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.08.021
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of Visible-Light Nano-Photocatalysts through Decoration of TiO2 by Silver Nanoparticles in Inverse Miniemulsions Zhihai Cao1,2*, Shudi Zhu1, Hui Qu1, Dongming Qi2, Ulrich Ziener3, Liu Yang1, Yingjie Yan1, Haitang Yang1, * 1
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Xuelin Street 16, Hangzhou 310036, China
2
Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, 310018, China 3
Institute of Organic Chemistry III – Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, Ulm, 89081, Germany
Corresponding authors: [email protected]; [email protected]
ABSTRACT: Ag/TiO2 nanocomposites were prepared through combination of a sol–gel process of a titanium precursor in inverse miniemulsions and in situ reduction of silver ions in the “nanoreactors”. The morphological investigation shows that Ag nanoparticles are mainly located on the surface of TiO2 nano-supports because of the fast reduction rate of Ag ions by hydrazine. Ag/TiO2 nanocomposites with amorphous or anatase TiO2 phase displayed high visible-light catalytic activity for degradation of Rhodamine B. The photoactivity of Ag/anatase TiO2 nanocomposites could be influenced by the Ag content that could be conveniently tuned by the loading of silver salts. The influence of the loading of silver salts on the particle properties of the Ag/TiO2 nanocomposites was investigated systematically.
1
Keywords: Inverse miniemulsion; Ag/TiO2 nanocomposites; visible-light nano-photocatalyst; sol–gel; reduction.
1. INTRODUCTION Titanium(IV) dioxide (TiO2) has drawn intensive attention since its photocatalytic activity for decomposing water to produce H2 and O2 was discovered by Fujishima and Honda.[1] UV light is required for activation of the photocatalysis of TiO2 nanoparticles (NPs) because of their large band gap (3.0–3.2 eV). Therefore, the use of solar energy by plain TiO2-based materials is limited because only about 5% of sunlight is UV light. To improve the exploitation of the solar spectrum, a series of techniques has been developed to prepare TiO2-based photocatalysts possibly activated by visible-light, for example doping with non-metallic or metallic elements, or decoration with noble metal NPs.[2, 3] Noble metal/TiO2 nanostructured materials are promising because of their high visible-light photocatalytic activity and stability against photocorrosion.[4] Silver is one of the most widely investigated noble metals to prepare noble metal/TiO2 visible-light photocatalysts.[5-7] Ag/TiO2 nanocatalysts have been prepared through photodeposition of Ag NPs to TiO2 nano-supports[8], combination of the sol–gel technique and an azeotropic distillation[9], and one-pot sol–gel technique. [10] Microemulsion, a thermodynamically stable dispersion of two immiscible liquid phases, has been widely used to prepare nanocrystalline materials.[11] Noble-metal/TiO2 nanocatalysts could also be prepared through the microemulsion-based technique.[12,
13]
For example,
Zielińska et al. prepared a series of Ag/TiO2 nanocatalysts in microemulsion systems consisting of water/sodium bis-(2-ethylhexyl) sulfosuccinate/cyclohexane.[13] Typically, a 2
relatively large amount of surfactant has to be used to obtain a colloidally stable microemulsion with low interfacial tension and sub-100 nm droplets.[11] In comparison, a relatively lower amount of surfactant is required for stabilizing a miniemulsion system. Furthermore, there is a fast exchange of components between droplets in microemulsions. In recent years, miniemulsion has attracted intensive attention in recent years because of its high capability to prepare nanostructured polymeric, inorganic, and hybrid particles through versatile reactions for example polymerization and sol–gel processes.[14] Each droplet in miniemulsions can be regarded as a separated nanoreactor.[15] Hybrid NPs can be prepared conveniently through pre-introduction of a second moiety into the droplets and consequent encapsulation via various reactions.[16] Inverse miniemulsions composed of a polar dispersed phase and a low polarity continuous phase can be applied for preparing hydrophilic nanostructured materials, for example nanogels and inorganic nanocapsules.[17] Hydrophilic metal salts, which work as lipophobes are inherently required to improve the droplet stability in inverse miniemulsions.[17] Therefore, preparation of hybrid NPs containing hydrophilic metal salts can be performed conveniently. A series of nanogels and inorganic NPs containing various metal salts have been prepared in inverse miniemulsions.[18-20] The incorporated metal salts can be converted to metal NPs or metal oxide NPs to confer additional functionalities to hybrid NPs through certain reactions. For example, we have prepared magnetic hollow silica NPs successfully by converting iron salts to magnetic iron oxide NPs through heat treatment.[20] Recently, Heutz et al. prepared Au/TiO2 nanocomposites (NCs) by using AuCl4(NH4)7[Ti2(O2)2(cit)(Hcit)]2·12H2O
as
a
single-source
precursor
in
inverse
miniemulsions.[21] Two small molecular weight surfactants, sodium dodecylsulfate (SDS) or Triton X-100, were separately used to stabilize their inverse systems. However, a large 3
amount of surfactant was required, probably due to the high hydrophilic–lipophilic balance (HLB) value of these two surfactants. In addition, the adjustment of Au content in the Au/TiO2 NCs may be inconvenient due to the fixed ratio of Au to Ti in the precursor. Although deposition of Ag NPs to a preformed TiO2 nano-support for example P25 TiO2 NPs (Degussa) has been frequently used to prepare Ag/TiO2 NPs, the good dispersion of P25 TiO2 NPs in the reaction medium is relatively difficult to achieve. In the present work, we propose a technique based on the inverse miniemulsion technique to prepare Ag/TiO2 NCs that display good dispersibility and high visible-light catalytic activity. Most of the Ag NPs were attached to the surface of TiO2 nano-supports, advantageous for catalytic applications. The content of Ag NPs could be adjusted conveniently by varying the AgBF4 loading. The influences of the AgBF4 loading on the particle properties and photocatalytic activity of Ag/TiO2 NCs were investigated systematically.
2. Experimental Materials. Titanium ethoxide (TEO, 33–35% TiO2), dimethyl sulfoxide (DMSO, AR), Rhodamine B (RhB, 95%), and hydrazinium hydrate (AR grade, 80% solution) were purchased from Aladdin Chemistry Co. Ltd. Hexadecane (HD, 99%, Acros Organics) and AgBF4 (99%, Adamas Reagent Co, Ltd.) were used as received. The surfactant, poly(ethylene-co-butylene)-b-poly(ethylene oxide) (P(E/B)−PEO) with number-average molecular weight of 7100 g·mol−1 and a hydrophilic−lipophilic balance of 8.7 was synthesized according to the literature.[22] Demineralized water was used in all runs. Preparation of AgBF4/TiO2 NCs. AgBF4 (0.073−0.292 g) and water (0.2 g) were dissolved in DMSO (1.0 g) to form a polar solution for use as dispersed phase. P(E/B)−PEO (40.4 mg) 4
was dissolved in HD (12.5 g) functioning as continuous phase. After the mixture of both solutions was pre-emulsified under strong magnetic stirring for 15 min, the resulting crude emulsion was sonicated in an ice bath to prepare the inverse miniemulsions. Sonication was performed by applying a pulsed sequence (work 12 s, break 6 s) for 9 min, using a Scientz JY92-II DN sonifier at 42% maximum power. Thereafter, TEO (0.633 g) was directly introduced to the prepared inverse miniemulsions at 82 °C. The sol−gel process of TEO was run for 3 h with magnetic stirring at 300 rpm at 82 °C to obtain AgBF4/TiO2 NCs. Preparation of Ag/TiO2 NCs. Aqueous hydrazine solution (40 mg) was added to 2 g of the dispersion of AgBF4/TiO2 NCs. The reduction reaction was run for 2 h with magnetic stirring at 300 rpm at 40 °C to obtain Ag/TiO2 NCs with amorphous TiO2. Calcination of Ag/TiO2 NCs. Ag/TiO2 NCs were purified by a three-cycle centrifugation-redispersion in cyclohexane and a three-cycle centrifugation-redispersion in ethanol. The purified dry solid samples were heated to target temperatures (400 or 900 °C) at a heating rate of 2 °C·min−1, and then kept at the target temperature for 2 h in nitrogen to obtain Ag/anatase TiO2 (400 °C) and Ag/rutile TiO2 composites (900 °C). Visible-light catalytic activity of Ag/TiO2 NCs. Ag/TiO2 solid photocatalysts (5 or 30 mg) were dispersed in a 30 mL of an aqueous solution of RhB (2 × 10−5 mol·L−1) in an ultrasound bath. The reaction mixture was stored in the dark for 40 min to reach an adsorption equilibrium. The photocatalytic reaction was carried out in a test tube with agitation (1000 rpm) at 20 °C. The reaction medium was irradiated by a 400 W metal halogen lamp in a photochemical reaction instrument (XPA series, Nanjing Xujiang Instrument). UV light was cut off by a filter (λ < 400 nm). The RhB concentration during the photocatalytic reaction was measured at various time intervals by using a UV–vis spectrometer. The characteristic 5
absorption band of RhB at 554 nm was used to monitor the photodegradation of RhB in solution.[23] Characterization. Transmission Electron Microscopy (TEM). TEM measurements were performed on a Hitachi HT-7700 microscope operated at 80 kV. One droplet of NC dispersion was diluted in 2 mL of cyclohexane. One droplet of the diluted sample was placed on a 400 mesh carbon-coated copper grid and then allowed to dry at room temperature. The number-average particle sizes of AgBF4/TiO2 NCs, Ag/TiO2 NCs, and Ag NPs were obtained by counting at least 200 NPs Field Emission Scanning Electron Microscopy (FESEM). FESEM measurements were performed on a field emission scanning electron microscope (Supra 55, Carl Zeiss SMT Pte. Ltd.) by using powdered samples placed on a conductive film. The accelerating voltage was 2 kV. Energy-dispersive X-ray (EDX) spectroscopy was done on an energy-dispersive spectroscopy analyzer (Oxford Instruments INCA PentaFET*3) operated at 10 kV. The atomic content of Ag is reported as the average of four measurements. Nitrogen Sorption Analysis. Nitrogen sorption measurements were carried out at 77 K on a Quantachrome Autosorb-1-C automated gas sorption apparatus. The specific surface area was calculated through the Brunauer–Emmett–Teller equation. The pore size distribution was derived from the adsorption branch through the Barret–Joyner–Helenda method. UV–vis Spectroscopy. A 20 μL dispersion of AgBF4/TiO2 or Ag/TiO2 NCs was diluted with 2 g of cyclohexane. The UV–vis spectra were recorded on a UV-2450 UV–vis spectrometer from Shimadzu Corporation Company at a spectral range of 350 to 700 nm. X-ray Diffraction (XRD). Phase analysis of the samples by XRD was carried out on a 6
Bruker D8 Advance X-ray diffractometer with Bragg–Brentano θ–2θ geometry. The generator was a high-powered diffraction tube with a copper anode, which was operated at a working power of 1.6 kW (40 kV, 40 mA), and its Cu Kα radiation (λ= 1.5418 Å) was used for diffraction scans. Scan patterns were obtained at a resolution of 0.0195° from 20° to 90°. Powder samples for XRD were supported on a polymeric sample holder. The crystallite size of Ag and anatase TiO2 was calculated through the Scherrer equation. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were carried out on a Thermo K-Alpha spectrometer (Thermo Fisher Scientific Inc., MA) with a bass pressure lower than 1.0×10-9 Torr. All XPS spectra were collected at normal emission with a monochromatic Al Kα X-ray source (1486.6 eV). The binding energy of all XPS spectra were calibrated and referenced to the Fermi level of a sputtered clean gold sample and the scanning spot size on the sample is 100 μm.
3. RESULTS AND DISCUSSION Preparation of AgBF4/TiO2 and Ag/TiO2 NCs. To fabricate AgBF4/TiO2 NCs, AgBF4 was pre-introduced to the dispersed phase of the inverse miniemulsion; subsequently, AgBF4/TiO2 NCs were prepared through a sol–gel process of TEO in the AgBF4-containing inverse miniemulsion. The prepared AgBF4/TiO2 NCs showed a well-defined spherical morphology and a rough surface (Figure 1A). The number-average particle size of AgBF4/TiO2 NCs determined by TEM was 97 ± 27 nm. Ag/TiO2 NCs were obtained by in situ reduction of the Ag ions in the AgBF4/TiO2 NCs. The reducing agent, hydrazine, was added directly to the dispersion of AgBF4/TiO2 NCs. The reduction of Ag ions by N2H4 was confirmed by UV–vis spectroscopy. No obvious absorption peak could be observed in the range of 400–500 nm 7
before addition of hydrazine (Figure 2A, curve a), while a clear peak with a maximum absorbance at 434 nm appeared in the spectrum of the reduced sample (Figure 2A, curve b), consistent with the absorption of Ag NPs induced by localized surface plasmon resonance (LSPR).[24] The reduction of Ag ions by N2H4 is very efficient with a high conversion of around 90% already after 10 min and a maximum absorbance at about 50 min (Figure S1). The XRD results also indicate the formation of Ag NPs (Figure 2B). Compared with the XRD pattern before reduction (curve a in Figure 2B), five well-resolved diffraction peaks appear in the XRD pattern of the sample after reduction (curve b in Figure 2B). They can be well indexed with silver phase reflections according to the JCPDS Card Number 04-0783. A large number of small NPs as dark spots appears in the TEM image of the reduced sample (Figure 1B inset), which are assigned to Ag NPs. The particle size of Ag NPs determined by TEM was 12.3 ± 3.5 nm. The SEM result (Figure 1B) indicates that most of the Ag NPs are located on the surface of the TiO2 nano-supports. This characteristic is advantageous for the application as catalysts. The atomic ratio of Ag and Ti in the AgBF4/TiO2 NCs determined by EDX amounts to about 1 : 5, consistent with the employed materials for the synthesis. It also means that AgBF4 distributes homogenously over the AgBF4/TiO2 NCs. The atomic ratio of Ag and Ti in the Ag/TiO2 NCs determined by EDX is 1 : 3, higher than that of the AgBF4/TiO2 NCs (1 : 5). The somewhat higher ratio in the Ag/TiO2 NCs supports the fact that the Ag NPs mainly distribute on the surface. In addition, it is reasonable to propose that the Ag ions mirgrate to the surface from the interior of the TiO2 nano-supports to participate in the reduction reaction.
8
Figure 1. TEM (inset) and SEM images of the AgBF4/TiO2 (A) and Ag/TiO2 NCs (B). The amount of AgBF4 used in this run was 0.146 g.
The surface composition of the Ag/TiO2 NCs was further measured by XPS. Ag, Ti, O, F, and C could be detected in the sample (Figure 2C inset). F and C could be reasonably ascribed to the residual BF4- and surfactant. The bonding energies (B. E.s) of Ag are determined as 367.9 eV and 374.0 eV for Ag3d5/2 and Ag3d3/2 (Figure 2D), respectively, indicating the presence of Ag(0). The B. E.s of Ti are 459.1 eV and 464.8 eV for Ti2p3/2 and Ti2p1/2, respectively, with a difference of 5.7 eV (Figure 2C) corresponding to Ti(IV).[25] A
0.8
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Binding energy (eV)
Figure 2. (A) UV–vis spectra of the dispersions of AgBF4/TiO2 and Ag/TiO2 NCs. (B) XRD patterns of the AgBF4/TiO2 and Ag/TiO2 NCs. (C and D) XPS spectra of the Ag/TiO2 NCs. The amount of AgBF4 used in this run was 0.146 g.
Discussion on the formation of Ag/TiO2 NCs. Before reduction, AgBF4 is dissolved in DMSO and therefore distributed presumably homogenously in the AgBF4/TiO2 NCs. Once hydrazine is added to the dispersion of AgBF4/TiO2 NCs, N2H4 molecules can diffuse to the AgBF4/TiO2 NCs via the continuous phase of the dispersion. The N2H4 molecules meet with Ag ions at the surface of the AgBF4/TiO2 NCs. The reduction of Ag ions by N2H4 is very fast, and therefore the N2H4 molecules are consumed completely at the surface before they diffuse into the AgBF4/TiO2 NCs. The formed Ag nuclei precipitate on the surface of the NCs. It has been reported that the reduction of Ag ions by N2H4 is an autocatalytic process.[26, 27] During the reaction process, the slow, continuous nucleation and fast, autocatalytic surface growth may concurrently occur.[26, 27] Therefore, it is reasonable to expect that the Ag ions diffuse to the surface to participate in the reduction reaction in the whole process. The surface of NCs is the main reaction locus, and consequently, the Ag NPs mainly locate on the surface of the TiO2 nano-supports.
10
Calcination of Ag/TiO2 NCs. As shown in Figure 2B, TiO2 synthesized through the sol–gel process appears amorphous. It has been reported that the phase of TiO2 can be converted from amorphous to anatase or rutile by heat treatment at different temperatures.[28] After calcination at 400 °C well-resolved diffraction peaks belonging to anatase TiO2 according to the JCPDS Card Number 21-1272 appeared in the XRD pattern (Figure 3A (curve a)). Thus, amorphous TiO2 has been converted to anatase TiO2. Treatment of the Ag/amorphous TiO2 NCs at 900 °C led to the formation of rutile TiO2 as confirmed by XRD (Figure 3A, curve b). It should be pointed out that no diffraction peaks of Ag2O appeared in the calcined samples.[13] Compared with the Ag/amorphous TiO2 NCs, the B. E.s of Ag in the Ag/anatase TiO2 and Ag/rutile TiO2 NCs did not change (Figure S2). XPS results also indicate that Ag NPs have not been oxidized during calcination. Before calcination, the nitrogen adsorption–desorption isotherm of Ag/TiO2 NCs displayed a type IV isotherm (Figure S3). Upon heat treatment the specific surface area decreased from 154 m2·g-1 (Figure S3) to 86 m2·g-1 for the Ag/anatase TiO2 NCs exhibiting a final mean pore size of about 12.5 nm (Figure 3B inset). After calcination at 900 °C, the Ag/rutile TiO2 NCs became nonporous. Compared with the sample before calcination (Figure 1B), most of the particles became irregular after calcination at 400 °C (Figure 3C). The atomic ratio of Ag and Ti determined by EDX was 1 : 2.9, not affected significantly by the heat treatment. The NCs merged to form micro-sized particles or even bulk objects after calcination at 900 °C (Figure 3D and inset).
11
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Intensity (a.u.)
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a: calcined at 400 C o b: calcined at 900 C anatase rutile Ag
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Figure 3. (A) XRD patterns of Ag/anatase TiO2 and Ag/rutile TiO2 composites. (B) N2 adsorption-desorption isotherm and pore size distribution (inset) of the Ag/anatase TiO2 NCs. SEM images of the Ag/TiO2 NCs calcined at 400 oC (C) and 900 oC (D).
Influence of AgBF4 loading on the particle properties. The photocatalytic activity of Ag/TiO2 NCs may be tuned by the content of Ag NPs.[27] One advantage of inverse miniemulsions is that the loading of salts into the dispersed phase can be tuned conveniently in a wide range.[19] Therefore, the content of Ag NPs in the NCs could be adjusted in a wide range through variation of the AgBF4 loading. In the present paper, the AgBF4 loading was varied in the range of 0.073–0.292 g, and the corresponding ratio of Ag to Ti was varied in the range of 0.14–0.54. It should be pointed out that the colloidal stability of the systems with various AgBF4 loadings was well controlled. In the following part, the influence of AgBF4 loading on 12
the particle properties of Ag/TiO2 NCs will be discussed systematically. a) Particle morphology and size. Spherical AgBF4/TiO2 NCs were obtained in the systems with 0.073–0.292 g of AgBF4 (Figure S4A, 4C, 4E, and 4G). After reduction, Ag ions were converted to Ag NPs, which attached to the surface of TiO2 nano-supports (Figure S4B, 4D, 4F, 4H, and S5A–D). Roughly estimated by TEM and SEM, the number of Ag NPs increased with the increase of AgBF4 loading (Figure S4A–H and S5A–D). This conclusion was confirmed by EDX. The content of Ag NPs in the Ag/TiO2 NCs could be tuned in the range of 4.2–13.5 atom% (Figure 4A). The Ag content determined by EDX was consistent with the employed content for the Ag/TiO2 NCs with 0.07 and 0.11 g of AgBF4, while the Ag content determined by EDX was lower than the employed content for the Ag/ TiO2 NCs with 0.146, 0.219 and 0.292 g of AgBF4. Considering the limited detecting thickness of EDX, the deviation of the Ag content indicates that some Ag NPs were located inside the NCs when a high AgBF4 loading was employed. The particle size of Ag/TiO2 NCs varied in the range of 100–140 nm, and did not show obvious dependence on the AgBF4 loading (Figure 4B). It has been reported that the particle size of NPs first decreases and then remains almost constant with the increase of salt content in the dispersed phase of inverse miniemulsions.[19] Therefore, the AgBF4 content used in the present study may lie in the range in which the particle size of NCs only weakly depends on the variation of AgBF4 loading.
13
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20
determined by EDX feeding content
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Figure 4. The Ag content in the Ag/TiO2 NCs with various AgBF4 loadings determined by EDX (A). Particle size of Ag/TiO2 NCs with various AgBF4 loadings (B).
b) Properties of Ag NPs. The optical property of the Ag/TiO2 NCs induced by LSPR was characterized by UV–vis spectroscopy. The results are shown in Figure 5A. With the increase of AgBF4 loading, the absorbance of the Ag NPs increased. The maximum absorption wavelength (λmax) of the Ag NPs remained at about 435 nm in the AgBF4 range of 0.073–0.219 g. In addition, the bandwidths of the absorption peak of the Ag NPs in the same range were similar. It is well accepted that the optical absorption of noble metal NPs depends on their particle size and particle shape.[29, 30] It means that the particle properties of the Ag NPs in the AgBF4 range of 0.073–0.219 g did not change obviously. This result is consistent with the independence of the number-average sizes of Ag NPs on the AgBF4 loading (Figure 5B). Although the size and shape of the Ag NPs of the sample with 0.292 g of AgBF4 is similar to the other samples, λmax of the Ag NPs shifted to about 444 nm. This red shift might be ascribed to the enhancement of interparticle coupling effects induced by the increase of particle number on the surface of one NC. [31, 32] Five well-resolved characteristic diffraction peaks belonging to the Ag NPs could be 14
observed in the NCs with various AgBF4 loadings (Figure 5C). The AgBF4 loading had only a slight influence on the crystallite size of Ag, which is varied in the small range of 7.9–10.8 nm (Figure 5D). The calcination process showed an obvious influence on the crystallite size of Ag, which increased to about 14 nm in the NCs with 0.073–0.219 g of AgBF4 after calcination at 400 oC (Figure 5D). For the NCs with 0.292 g of AgBF4, the crystallite size of Ag significantly increased from 10.1 nm to 21.8 nm. It means that the coalescence of Ag crystal grains took place during calcination in this sample.
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Figure 5. UV–vis spectra (A) and particle size (B) of Ag NPs of Ag/TiO2 NCs with various AgBF4 loadings. XRD patterns of Ag/TiO2 NCs with various AgBF4 loadings (C). Influence of AgBF4 loading and calcination on the crystallite size of Ag NPs (D).
15
c) Properties of TiO2 nano-supports. As shown in Figure 6A, the characteristic diffraction peaks belonging to anatase phase were observed in the XRD patterns of Ag/TiO2 NCs with various AgBF4 loadings after calcination at 400 oC. It means that the TiO2 phase has been converted from amorphous to anatase by this heat treatment. The crystallite size of TiO2 varied in the range of 15–23 nm, it first decreased and then increased slightly with the increase of AgBF4 loading (Figure 6B). The specific surface area of Ag/TiO2 NCs could be influenced by the AgBF4 loading. The surface area of the Ag/TiO2 NCs with 0.073 g of AgBF4 was about 210 m2·g-1. With the increase of AgBF4 loading, the specific surface area decreased. This reduction may be ascribed to the increase of the amount of Ag NPs, which may cover a part of the pores of TiO2. This assumption is confirmed by the fact that the pore volumes of the Ag/TiO2 NCs decrease with the increase of the AgBF4 loading (Figure S6). The N2 adsorption–desorption isotherms of the Ag/TiO2 NCs after calcination at 400 oC display a type IV isotherm (Figure 6C), indicating the presence of mesopores. The heat treatment led to the reduction of the specific surface area of TiO2 nano-supports (Figure 6D). This reduction is caused by the coalescence of TiO2 crystal grains under calcination conditions.[28] It should be pointed out that samples with rutile TiO2 were not fully characterized and discussed because of their low photocatalytic activity under irradiation of visible- light (see the next section).
16
A Ag Anatase
Crystallite size (nm)
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Figure 6. XRD patterns (A) and crystallite sizes of TiO2 (B) of Ag/TiO2 NCs with various AgBF4 loadings after calcination at 400 oC. N2 adsorption–desorption isotherms of the Ag/TiO2 NCs after calcination at 400 oC (C). Specific surface area of Ag/TiO2 NCs before and after calcination (D).
Photocatalytic degradation of RhB under irradiation of visible-light. Visible-light catalytic activity of Ag/TiO2 NCs was evaluated through the degradation of RhB under irradiation of visible-light. As shown in Figure S7, the characteristic absorption band of RhB at 554 nm was significantly reduced in the reaction catalyzed by the Ag/anatase TiO2 nanocomposites under the irradiation of visible-light. After 180 min reaction, only a very small absorption peak at 554 nm could be observed in the spectral range of 400–650 nm, indicating that most of RhB 17
has been mineralized (Figure S7). As shown in Figure 7A, Ag/amorphous TiO2 and Ag/anatase TiO2 NCs showed a high visible-light catalytic activity for photodegradation of RhB. Plain TiO2 NPs have only limited photocatalytic activity under irradiation of visible-light because of their large band gap. Therefore, the obvious visible-light catalytic activity of the Ag/TiO2 NCs could be ascribed to the cooperative contribution of Ag NPs and TiO2 nano-supports. The mechanism of visible-light catalysis of plasmonic-metal/TiO2 NCs has been investigated extensively in recent years. Three possible mechanisms have been proposed, including localized surface-mediated charge injection from metal to TiO2, near-field electromagnetic mechanism, and scattering mechanism.[33] The first mechanism requires the direct contact of plasmonic metal and TiO2. Besides of the direct contact systems, the latter two mechanisms may also take place in the systems in which plasmonic metal and TiO2 are separated. In our case, the Ag NPs are mainly distributed on the surface of the NCs, and the excited electrons appear in the conduction band of TiO2 either possibly by excitation through the near-field effect between Ag and TiO2 NPs or by LSPR-induced charge transfer from Ag NPs to TiO2. The excited electrons react with adsorbed O2 to form O2–· firstly, and then become ·OH radicals through a series of reactions.[34] The ·OH radicals degrade the RhB molecules to mineralized compounds. The Ag/rutile TiO2 composites did not show obvious visible-light catalytic activity. The low photo-activity may be ascribed to the nonporous structure of the composites and the large size of Ag and TiO2 particles (Figure 3D). The role of Ag NPs in the visible-light catalysis of Ag/TiO2 NCs could be as follows: a) enhancing the adsorption of visible-light through LSPR; b) improving the separation of excited electrons and holes; c) working as recombination sites.[23] The former two factors 18
promote the visible-light photocatalytic activity of Ag/TiO2 NCs, but the third factor will deteriorate the photocatalytic activity. As shown in Figure 7B, the Ag/TiO2 NCs with various AgBF4 loadings showed obvious photocatalytic activity under irradiation of visible light. The photodegradation rate increased with the increase of Ag content firstly, and then decreased, Figure 7C. The reduction in photoactivity of Ag/TiO2 NCs with a high Ag content may be caused by the increased exciton recombination of Ag NPs, similar to the findings of Wu et al.[[35]] In addition, the reduction of the adsorption of RhB and water soluble O2 on the surface of NCs because of the attachment of too many Ag NPs on the surface of TiO2 nano-supports may also reduce the photocatalytic degradation of RhB, as reported by Xiong et al.[36]
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Figure 7. Photodegradation kinetics of RhB under irradiation of visible-light in the presence
19
of Ag/TiO2 NCs with various TiO2 phases (A) and with various Ag contents (B). Relationship between c/c0 and the AgBF4 loading after reacted for 30 min (C). The feeding amount of AgBF4 in the Ag/TiO2 NCs used in Figure 7A was 0.146 g. The amount of Ag/TiO2 NCs used in photocatalytic experiments for Figure 7A and 7B was 30 mg and 5 mg, respectively.
4. CONCLUSIONS In this paper, we present a convenient method to prepare Ag/TiO2 NCs with high visible-light catalytic activity. AgBF4/TiO2 NCs were first prepared through a sol–gel process of TEO in inverse miniemulsions. The Ag/TiO2 NCs were obtained through reduction of Ag ions by hydrazine on the surface of AgBF4/TiO2 NCs. Ag NPs were distributed mainly on the surface of the Ag/TiO2 NCs, advantageous for catalytic applications. The TiO2 nano-supports synthesized through the sol–gel process were amorphous, but they could be converted easily to anatase or rutile phase through heat treatment. The heat treatment led to the coalescence of Ag and TiO2 crystal grains and reduction of surface area of the Ag/TiO2 NCs. The content of Ag NPs in the Ag/ TiO2 NCs could be conveniently tuned by varying the AgBF4 loading. The AgBF4 loading had significant influence on the particle properties of Ag/TiO2 NCs. The Ag/amorphous or anatase TiO2 NCs showed high visible-light catalytic activity for photodegradation of RhB. The photo- and radiolysis-deposition of Ag to TiO2 supports are often required to be carried out at a low concentration of silver ions to avoid the formation of separated Ag nanoparticles.[8, 37] By our proposed technique, the reduction of silver ions is carried out directly on the surface of silver salt/TiO2 NCs, and therefore the silver salt content does not influence the particle morphology and colloidal stability of the reaction systems significantly. Therefore, Ag/TiO2 NCs with different levels of Ag content can be synthesized 20
conveniently in a one-pot reduction. As a common heterophase reaction system, the miniemulsion is easy to scale up, advantageous for industrialization.[38] In addition, the proposed technique holds high potential to prepare various noble metal or metal alloy/TiO2 nano-photocatalysts by introducing different single or mixed noble metal salts to the dispersed phase of inverse miniemulsion.
ACKNOWLEDGEMENTS. Financial supports from National Natural Scientific Foundation of China (NNSFC) project (51003023), Zhejiang Province’s Xinmiao Talent Plan (2013R421041), and National Undergraduate Training Programs for Innovation and Entrepreneurship of China (201310346013) are gratefully acknowledged.
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[27] M.A. Watzky, R.G. Finke, J. Am. Chem. Soc 119 (1997) 10382. [28] J.B. Joo, Q. Zhang, I. Lee, M. Dahl, F. Zaera, Y.D. Yin, Adv. Funct. Mater. 22 (2012) 166. [29] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410. [30] B.J. Wiley, S.H. Im, Z.-Y. Li, J. McLellan, A. Siekkinen, Y.N. Xia, J. Phys. Chem. B 110 (2006) 15666. [31] K.-H. Su, Q.-H. Wei, X. Zhang, J.J. Mock, D.R. Smith, S. Schultz, Nano Lett. 3 (2003) 1087. [32] Y. Lu, G.L. Liu, L.P. Lee, Nano Lett. 5 (2005) 5. [33] S. Linic, P. Christopher, D.B. Ingram, Nat. Mater. 10 (2011) 911. [34] J.G. Yu, G.P. Dai, B.B. Huang, J. Phys. Chem. C 113 (2009) 16394. [35] Y.M. Wu, H.B. Liu, J.L. Zhang, F. Chen, J. Phys. Chem. C 113 (2009) 14689. [36] Z.G. Xiong, J.Z. Ma, W.J. Ng, T.D. Waite, X.S. Zhao, Water Res. 45 (2011) 2095. [37] E. Grabowska, A. Zaleska, S. Sorgues, M. Kunst, A. Etcheberry, C. Colbeau-Justin, H. Remita, J. Phys. Chem. C 117 (2013) 1955. [38] J.M. Asua, Progr. Polym. Sci. DOI: 10.1016/j.progpolymsci.2014.02.009 (2014).
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Graphical abstract
Ag/TiO2 nanocomposites were prepared through combination of a sol–gel process of a titanium precursor in inverse miniemulsions and in situ reduction of silver ions in the “nanoreactors”. Ag nanoparticles are mainly located on the surface of TiO2 nano-supports. Ag/TiO2 nanocomposites with amorphous or anatase TiO2 phase displayed high visible-light catalytic activity for degradation of Rhodamine B. The influence of the loading of silver salts on the particle properties of the Ag/TiO2 nanocomposites was systematically investigated. The proposed
technique
holds
high
potential
nano-photocatalysts.
24
to
prepare
various
noble
metal/TiO2
Highlights 1. 2. 3. 4. 5.
Ag/TiO2 nanocomposites (NCs) were prepared in inverse miniemulsions. Ag nanoparticles (NPs) are located on the surface of NCs. The content of Ag NPs could be easily tuned by the loading of silver salt. The loading of silver salt has significant influence on the properties of NCs. Ag/TiO2 NCs with amorphous and anatase TiO2 displays a visible-light photocatalytic activity.
25
