Carbon Nanotube-Stabilized Pickering

We engineered the system by using silver phosphate (Ag3PO4) as a photocatalytic active metal oxide semiconductor and multiwalled carbon nanotubes ...
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Silver Phosphate/Carbon Nanotube-Stabilized Pickering Emulsion for Highly Efficient Photocatalysis Wanying Zhai, Gaiping Li, Ping Yu, Lifen Yang, and Lanqun Mao* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100190, P. R. China ABSTRACT: This study demonstrates a first exploitation of the unique properties inherent in Pickering emulsions to develop a new kind of photocatalytic system. We engineered the system by using silver phosphate (Ag3PO4) as a photocatalytic active metal oxide semiconductor and multiwalled carbon nanotubes (MWNTs) as a hydrophobic conducting nanostructure to form the Pickering emulsions. The photocatalytic activity of the as-formed Ag3PO4/ MWNT-stabilized Pickering emulsion-based system is studied toward dye decomposition and oxygen evolution. Results imply that the Pickering emulsion-based photocatalytic system exhibits a much higher efficiency, as compared with traditional solution-dispersed photocatalytic system. This high efficiency is elucidated in terms of the unique properties inherent in Pickering emulsions including (i) the self-assembled Ag3PO4/MWNT nanohybrid at water/oil interface, well ensuring a large surface area of the photocatalyst, (ii) the use of MWNTs to facilitate the formation of amphiphilic nanostructures self-assembled at water/oil interface, promoting the charge separation of the semiconductor through the π−π network of MWNTs by shuttling and storing photogenerated electrons from the visible light irradiated Ag3PO4, and (iii) the separation of the product (e.g., O2 evolved from water oxidation) from the reactants during the photocatalytic process, well accelerating the photocatalytic reactions. In addition to the high efficiency, the fast and simple procedures employed for demulsifying (e.g., sonication or centrifugation) and re-emulsifying (e.g., shaking) essentially make our Pickering emulsion-based photocatalytic system technically simple and thus practically applicable. This study opens a new way to developing novel photocatalytic systems with high efficiency and good practical applicability based on Pickering emulsion science and technology.

1. INTRODUCTION Photocatalysis through photomediated redox reactions such as photocatalytic water splitting has recently been of great concern since it has been considered as a vanguard solution to meet the demand for clean energy technologies.1−4 Generally, development of photocatalytic system includes two parts. First is to design and synthesize nanometer-sized photocatalysts with high activity under visible light irradiation. The other part is to engineer a photocatalytic system creating large active area and facilitating effective mass transfer on photocatalysts, since the photocatalytic reactions mostly take place on the surface of the photocatalysts.5,6 To this end, some excellent approaches have been developed on both parts: the synthesis of advanced photocatalytic materials and the establishment of highly efficient photocatalytic systems.7−9 In most cases, photocatalytic systems have been established by either dispersing a nanometer photocatalyst into the solution to form a solutiondispersed photocatalytic system or immobilizing a nanometer photocatalyst onto the solid substrate to form a surfaceimmobilized photocatalytic system. The solution-dispersing approach well creates a large surface area for the photocatalyst.6 However, the separation of the nanometer photocatalyst from the solution remains to be solved. In the latter system, while separation of the photocatalyst from the bulk solution is readily © 2013 American Chemical Society

achieved, the limitation of mass transportation of substrate to the photocatalyst surface remains a problem yet.6 Herein, we demonstrate a Pickering emulsion-based photocatalytic system that is different from the existing solutiondispersed and surface-immobilized systems (Scheme 1). In Pickering emulsions, the solid particles are strongly adsorbed at the liquid−liquid interface due to the tendency to decrease total free energy.10−14 So far, Pickering emulsions have been employed as the approach for various applications such as preparation of supracolloidal structures, Janus materials, and metal−organic framework capsules.15−21 The unique properties of Pickering emulsions also make them potentially attractive as a novel kind of photocatalytic system with a higher efficiency compared with the traditional solution-dispersed system. First, the highly dispersed nanometer photoactive solid emulsifiers self-assembled at water/oil interface well overcomes the particle agglomeration problem, ensuring a large active area for photocatalysis. Second, the spatial separation of the reaction products from the reactants, due to their different solubility in the water/oil phases, accelerates the photocatalytic reactions.22 Received: May 6, 2013 Revised: June 27, 2013 Published: July 3, 2013 15183

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Ag3PO4 (0.4 mL, 7.85 g/L). The resulting mixture was then sonicated in a sonication bath for 20 min (KQ-100DA; Kunshan Ultrasonic Instruments Co., Ltd., China; 100 W, 40 kHz). After that, the water-to-oil volume ratio was adjusted to be 0.4:1 by adding an appropriate volume of isooctane into the mixture. A stable emulsion was finally formed by shaking the mixture for 2 min. The emulsions could be broken simply by centrifugation for 5 min at 8000 rpm or sonication at a sonication bath for 2 min (KQ-400 KDE; Kunshan Ultrasonic Instruments Co., Ltd., China; 400 W, 40 kHz) and reemulsified again by shaking the as-broken emulsion for 2 min. 2.3. Characterization. Field emission scanning electron microscopy (SEM S-4800, Hitachi) equipped with an energy dispersive spectrum (EDS) was employed for characterization of the morphology, size, and composition of the samples. Powder X-ray diffraction (XRD) data were collected on Rigaku D/Max 2500 X-ray diffractometer with Cu Kα radiation (18 kW). Transmission electron microscopy (TEM) images were performed on a JEM 200CX and a JEOL JEM-2011 transmission electron microscope. Energy dispersive X-ray spectroscopy (EDS) measurements were carried out on a JEOL JEM-2011 energy dispersive spectrometer. The images of the emulsions were recorded on an Olympus B×51 microscope and a Nicon CoolPix S60 digital camera. UV−vis spectra were recorded on a TU 1900 spectrometer. The contact angle θ of the nanohybrid at the water/oil interface was measured as follows. First, a film cast of Ag3PO4−MWNT nanohybrid was soaked in isooctane. Then, one water droplet was gently placed on the film with an isooctane environment around the droplet, and the contact angle was measured (JC 2000D1 goniometer). Raman spectra were recorded on a Renishaw inVia plus Raman microscope with a 514.5 nm argon ion laser. 2.4. Photooxidation of Methylene Blue. In a typical photocatalytic experiment, an aqueous solution of methylene blue (MB) (1.34 × 10−3 mol/L, 20 μL) was added to the emulsion system (total volume was 1.4 mL, volume ratio of water-to-isooctane was 0.4:1) at room temperature. Prior to illumination, the emulsion system was kept in dark for 30 min to achieve an equilibrium adsorption of MB on the surface of the Ag3PO4−MWNT nanohybrid to exclude the effect of the MB adsorption in the photodecomposition process. Then, the emulsion system was exposed to a 500 W xenon arc lamp equipped with an ultraviolet cutoff filter to provide visible light with λ ≥ 420 nm. After illumination, the emulsion system with water and oil phases were separated by centrifuging and the water phase containing MB was diluted by five times for UV− vis spectroscopic measurements. The concentration (C) of MB in water phase was determined by monitoring the absorbance at 664 nm in the UV−vis spectra. The photocatalytic efficiency of the Pickering emulsion-based system was then calculated by C/ C0 values (C was the instantaneous concentration of MB, and C0 was the concentration of MB after MB adsorption onto the hybrid in dark for 30 min). The C/C0 values were obtained from the absorbance A normalized to the initial absorbance A0 at 664 nm. For comparison, the decomposition of MB in the solution-dispersed system was conducted with raw Ag3PO4. In this case, the amount of Ag3PO4, the initial concentration of MB, and the volume of water phase were kept almost the same in both systems. The photocatalytic experiments were performed without stirring in both systems. 2.5. Oxygen Evolution. In a typical experiment for photocatalytic O2 evolution, the Pickering emulsion system (total volume was 105 mL, volume ratio of water-to-isooctane

Scheme 1. Schematic of a Highly Efficient Pickering Emulsion-Based Photocatalytic System Formed by SelfAssembling Ag3PO4−MWNT Nanohybrid at the Water/Oil Interface

Third, photoactive solid emulsifiers can be rationally designed through hybridization of nanometer photocatalyst and conducting nanostructure, in which the conducting nanostructure can be used to capture and shuttle photogenerated electrons, which will promote the charge separation of photocatalysis. Finally, Pickering emulsions can be simply demulsified by, for example, sonicating or centrifuging the mixture and re-emulsified by shaking the mixture. This feature essentially makes the Pickering emulsion-based photocatalytic system technically simple and thus highly applicable for practical uses. In spite of these advantages, the utilization of Pickering emulsion concept to develop a novel photocatalytic system with excellent properties has been little explored so far. Although niobate K4Nb6O17 has recently been demonstrated to have amphiphilic and photocatalytic properties and thus to form Pickering emulsions in the water/oil mixture, the enhanced performance and the underlying mechanism of Pickering emulsion-based photocatalytic system reported in this study have been scarcely investigated.23 This study essentially demonstrates a novel photocatalytic system with a high efficiency and less technical demanding based on Pickering emulsion science and technology.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. AgNO3 and Na2HPO4 were purchased from Beijing Chemical Factory (Beijing, China). Ag3PO4 was prepared with a simple precipitation method.24 In a typical synthesis, 400 μL of an aqueous solution of Na2HPO4 (0.15 M) was added to 8 mL of water, to which 1.2 mL of an aqueous solution of AgNO3 (0.15 M) was added drop-by-drop under sonication. The resulting yellow precipitate was washed with water for three times to give Ag3PO4 nanoparticles. Multiwalled carbon nanotubes (MWNTs) with a diameter of around 30 nm were purchased from Shenzhen Nanoport Co. Ltd. (Shenzhen, China). Prior to use, the nanotubes were purified by refluxing in 2.6 M nitric acid for 5 h, then centrifuging, redispersing, filtrating, and calcining in Ar atmosphere at 600 °C for 5 h. All aqueous solutions were prepared with Milli-Q water (18.2 MΩ cm−1). Other reagents were of analytical reagent grade. 2.2. Emulsion Formation. In a typical experiment, Ag3PO4 and MWNTs were mixed by adding a dispersion of MWNTs in isooctane (40 μL, 1.67 g/L) into an aqueous dispersion of 15184

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was 0.4:1) was kept in dark for 30 min at room temperature followed by light illumination. The amounts of O2 evolved in the Pickering emulsion system were continuously monitored every 2 min with a dissolved oxygen meter (Model 9173, Jenco Electronics, LTD). Control experiments on the O2 evolution in a solution-dispersed system were carried out under the same conditions as those employed for the Pickering emulsion-based system. 2.6. Transfer of Electrons from Photoexcited Ag3PO4 into MWNTs. To study the possible transfer of electrons from photoexcited Ag3PO4 into MWNTs, an Ag3PO4−MWNT nanohybrid-stabilized emulsion system was first formed under the procedures mentioned above, with the exception of using toluene as the oil phase for dissovling an oil-soluble electron acceptor, palladium acetate (Pd(OAc)2, 1.5 mM). MB (67 μM) was used as the electron donor in the aqueous phase. The resulting emulsion system was illuminated under visible light (λ ≥ 420 nm) for 20 min. After that, the formed nanocomposite was collected by centrifugation and washed thoroughly with water by repeating centrifugation. Then, the nanocomposite was analyzed by field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). A control experiment with the same amount of pristine MWNTs (i.e., without hybridization with Ag3PO4) in toluene solution containing Pd(OAc)2 was conducted under visible light irradiation (λ ≥ 420 nm) for 20 min. In addition, to exclude the possibility of the direct reaction between Pd(OAc)2 and MB, a control experiment was conducted as follows: 40 μL of an acetone solution of MB (0.4 mg/mL) was added into 3 mL of an acetone solution of Pd(OAc)2 (0.125 mg/mL). The resulting mixture was subjected to UV−vis spectrometric measurements to record the change in the absorbance of MB and Pd(OAc)2. Since Pd(OAc)2 has a poor solubility in water, acetone was used here as a solvent to dissolve Pd(OAc)2.

has a bandgap of 2.43 eV with a valence band position at +2.9 V (vs NHE) and has been used as an excellent photocatalyst for water oxidation under visible light irradiation.24,29 Unfortunately, as mentioned above, Ag3PO4 is inherently hydrophilic and thus unable to form a Pickering emulsion by itself. We solve this problem by using carbon nanotubes (CNTs) as a scaffold for semiconductor nanoparticles (e.g., Ag3PO4), and more importantly, CNTs provide an emulsifying ability.30−32 In addition, as one kind of carbon nanostructure, CNTs have been proved to possess a good ability to capture and shuttle electrons through the π−π network.33−42 As a consequence, the hybridization of metal oxide photocatalysts (e.g., Ag3PO4) with CNTs would provide a very effective approach to formation of a Pickering emulsion-based photocatalytic system, as schematically shown in Scheme 1. Figure 1 displays the scanning electron microscopy (SEM) and powder X-ray diffraction (XRD) of synthesized Ag3PO4

Figure 1. (A) SEM image of Ag3PO4 nanoparticles. (B) XRD patterns of Ag3PO4 nanoparticles (red), Ag3PO4−MWNT nanohybrid (black), and simulated Ag3PO4 (blue). (C) SEM image of Ag3PO4−MWNT nanohybrid (containing ∼2 wt % MWNTs). (D) Photograph of an emulsion stabilized by Ag3PO4−MWNT nanohybrid containing 6.7 × 10−4 mol/L MB (volume ratio of water-to-isooctane was 0.4:1, containing ∼18 wt % MWNTs). Inset: optical microscopy image of the emulsion.

3. RESULTS AND DISCUSSION 3.1. Formation of Pickering Emulsion-Based Photocatalytic System. In order to establish a Pickering emulsionbased photocatalytic system, a novel category of photocatalytic systems distinct from the traditional solution-dispersed and surface-immobilized systems, photoactive solid nanoparticles have to be designed to simultaneously possess both an emulsifying ability and a photocatalytic activity. However, almost all kinds of photocatalysts reported so far have a poor emulsifying ability although they have been demonstrated to have a high photocatalytic activity.7 To circumvent this problem, we have to consider another way for developing photoactive solid nanoparticles with an amphiphilic property. In fact, the emulsifying ability of solid nanoparticles is closely associated with their surface wettability.25,26 Based on this, water-in-oil (w/o) and oil-in-water (o/w) emulsions can thus be separately established by rationally combining hydrophobic/ hydrophilic solid nanoparticles with hydrophilic/hydrophobic components.10 For example, emulsions have often been established by controlling the hydrophobicity of solid particles by surface coating of, e.g., surfactants.27,28 By using this combination strategy, one may prepare photocatalyst-based photoactive solid nanoparticles with both an emulsifying ability and a photocatalytic activity. In this study, a Pickering emulsion-based photocatalytic system is demonstrated with Ag3PO4 as a model photoactive nanostructure. As one kind of excellent photocatalyst, Ag3PO4

and Ag3PO4−MWNT hybrid. The size of Ag3PO4 nanoparticles was 312.8 ± 94.6 nm (A), and a body-centered cubic lattice was observed in XRD patterns (B, red). The SEM image of Ag3PO4−MWNT nanohybrid (C) suggests the formation of Ag3PO4−MWNT nanohybrid. The addition of a blue watersoluble dye (i.e., MB) reveals that the emulsion is of water-inoil (w/o) type (D). Surface wettability is one of the main factors influencing the stability of Pickering emulsions formed by solid particles,25,26 which could be described by the contact angle (θ) between the solid particle and the water/oil interface. The strength, with which a solid particle is held at a water/oil interface, becomes maximal at a contact angle of 90°. Any deviation of contact angle from 90° decreases the stability of emulsion.25,26 In this study, we interestingly found that the surface wettability of Ag3PO4−MWNT nanohybrid was simply controlled by varying the content of MWNTs in the nanohybrid. The contact angle θ becomes closer to 90° with increasing the contents of MWNTs in the as-formed Ag3PO4−MWNT nanohybrids, as depicted in Figure 2B (black curve), indicating that the stability of 15185

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Figure 2. (A) Typical UV−vis spectra of the MB solution (6.7 × 10−5 mol/L) before (black curve) and after (curves with other colors) the solution was illuminated under visible light (λ ≥ 420 nm) for 10 min in the Pickering emulsion-based system with a water/isooctane volume ratio of 0.4:1. The Pickering emulsions were stabilized with Ag3PO4−MWNT nanohybrids with different contents of MWNTs as indicated in the figure. (B) Photodecomposition of MB (6.7 × 10−5 mol/L) and the contact angle (θ) with an error bar (n = 3) as a function of the contents of MWNTs in the Ag3PO4−MWNT nanohybrid. C/C0 values were taken and calculated from the results shown in (A). The contact angle was determined with Ag3PO4−MWNT nanohybrid as substrate and isooctane as a continuous phase.

Figure 3. (A) UV−vis spectra of the MB solution illuminated by visible light at different times (as indicated in the figure) in the Pickering emulsionbased system. (B) UV−vis spectra of the MB solution illuminated by visible light at different times (as indicated in the figure) with raw Ag3PO4 nanoparticles in the solution-dispersed system. The amounts of Ag3PO4, initial concentration of MB (6.7 × 10−5 mol/L), and the volumes of water phase employed in two systems were kept almost the same. (C) First-order linear transforms of MB decomposition in (A, black) and (B, red).

the light harvesting activity of MWNTs and the stability of the Pickering emulsions, we set the content of MWNTs in the Ag3PO4−MWNT nanohybrid as 2 wt % in our following studies. On the other hand, we found that the volume ratio of waterto-oil also was an important issue in preparing the emulsions, and such a ratio affects the photocatalytic activity of the asprepared system for MB degradation. Under the conditions employed in our study, almost all water was in a form of droplets, and as a result, the increase in the water-to-oil volume ratio could make the droplets larger in size. As a fact, we found that the droplets formed at water-to-oil volume ratio of 0.4:1 have an average diameter of 4.6 ± 1.4 μm. This value was obviously smaller than that of the droplets formed at water-tooil volume ratio of 1:1 (i.e., 14.4 ± 1.7 μm). Since the photocatalytic reaction takes place at the water/oil interface in the emulsions, a larger surface area of droplets could ensure more reactive sites for photocatalysis. After comparing the photocatalytic activities of the emulsion-based system toward MB degradation with water-to-oil ratios of 0.4:1 and 1:1, we found that photocatalytic activity at water-to-oil ratio of 0.4:1 was higher than that at water-to-oil ratio of 1:1 (data not shown). Thus, the volume ratio of water-to-oil was set as 0.4:1 in the following studies. 3.2. Photocatalytic Activity of Pickering EmulsionBased System. To demonstrate the enhanced photocatalysis

emulsions increases with increasing the content of MWNTs. Although stable emulsions were formed with considerably high content of MWNTs, the light harvesting of MWNTs was also needed to be taken into account since a high content of MWNTs could prevent the light absorption of Ag3PO4. Therefore, we chose the decomposition of MB as a model photocatalytic reaction to investigate the effect of MWNT content on the activity of the Pickering emulsion-based photocatalytic system, as typically shown in Figure 2A. In this case, the decomposition of MB was measured with the absorbance (A) at λ = 664 nm, with the value of A/A0 proportional to the instantaneous concentration (C) normalized to the initial concentration (C0) of MB. As displayed in Figure 2B (red curve), the decomposition rate of MB in the Pickering emulsion-based system reaches maximum at an intermediate concentration of MWNTs. As the content of MWNTs increases, the MB decomposition rate decreases (red curve goes up). This result suggests that the light harvesting by a larger proportion MWNTs leads to a smaller number of photons absorbed by Ag3PO4. Thus, to develop a highly efficient photocatalytic system, it remains very essential to decrease the content of MWNTs in the as-formed Ag3PO4− MWNT nanohybrid. However, we found that this process has a limit, beyond which the water droplets were no longer stabilized by the Ag3PO4−MWNT nanohybrid and the emulsions collapse. Therefore, by simultaneously considering 15186

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pure MWNTs into the solution, when Ag3PO4−MWNT nanohybrid was dispersed into MB solution and the mixture was exposed to visible light, almost ∼98% MB decomposed (pink curve), indicating that the decomposition of MB under visible light in our Pickering emulsion-based system was mainly driven by Ag3PO4, rather than by MWNTs. 3.3. Photocatalytic O2 Evolution. To demonstrate the unique feature of our Pickering emulsion-based system with respect to the spatial separation of the products from the reactants because of their different solubility in the water/oil phases, experiments on the photocatalytic O2 evolution were thus conducted as a typical example. This was essentially inspired by the facts that (1) Ag3PO4 is an excellent photocatalyst for O2 evolution and (2) O2 is a nonpolar molecule and is thus much more soluble in isooctane phase than in water. At the water/oil interface, H2O reacts with the hole of Ag3PO4 nanoparticles, resulting in the evolution of O2 under the illumination of visible light. The O2 molecules, once evolved, tend to migrate out of water phase and dissolve into the isooctane phase because of its higher solubility in isooctane phase compared with that in water phase. The removal of the products from the reaction system could essentially accelerate the photocatalytic reactions. To verify this hypothesis, the amount of O2 evolved in the photocatalysis was continuously monitored with a dissolved oxygen meter (Figure 5A). Prior to illumination, the emulsion system was kept in the dark for 30 min. During the period of time in the dark, O2 concentration in the oil phase was continuously monitored every 2 min (Figure 5B, gray part). After that, the system was illuminated under visible light, and the concentration of O2 was continuously monitored (Figure 5B, yellow part). For comparison, the O2 concentration in water with the presence of the same amount of the raw Ag3PO4 nanoparticles (i.e., traditional solutiondispersed photocatalytic system) was also monitored during both processes. As typically shown in Figure 5B, in the dark, the O2 concentration remains constant in both the Pickering emulsion-based (red curve) and the traditional solutiondispersed (black curve) photocatalytic systems (gray part) and increases clearly with light irradiation (yellow part). Strikingly, as demonstrated in Figure 5B (inset), the rate of O2 evolution in our emulsion-based system was faster (red curve) as compared with that in the solution-dispersed system (black curve). The observed remarkably enhanced rate for O2 evolution in the emulsion-based system was considered to result from the migration of O2 from the water phase into the oil phase, thereby making the reaction equilibrium shift toward the product side. Thus, the unique property of spatial separation of the reaction products from the reactants in the emulsion-based system could accelerate the photocatalytic reactions of which the products exhibit different solubility in the water phase and the oil phase. 3.4. Evidences for Electron Transfer from Visible Light Irradiated Ag3PO4 to MWNTs. When Ag3PO4 nanoparticles are subjected to visible light irradiation, they undergo a charge separation (eq 1), whereas the photogenerated electron gradually reduces Ag3PO4 to Ag (eq 2) because the conduction band potential of Ag3PO4 is more positive than the reduction potential of H+ and more negative than the reduction potential of Ag+.24 Thus, Ag3PO4 photocatalyst is easily corroded by the photogenerated electrons and decomposes into the weakly active Ag. As reported previously, the photocorrosion could be overcome by using an electron acceptor such as AgNO324,45 or modifying Ag3PO4 photocatalyst with AgX (X = Cl, Br, I), Ag

in our Pickering emulsion-based system, we compared the decomposition of MB in our Pickering emulsion-based system with the traditional solution-dispersed system under the same conditions. As depicted in Figure 3, the decomposition of MB in the Pickering emulsion-based system (A) was much quicker than that in the solution-dispersed system (B). To make a more convenient comparison, the reaction kinetics of both systems were fit into a pseudo-first-order rate law at a low dye concentration (Figure 3C), in accordance with a generally observed Langmuir−Hinshelwood mechanism,43,44 and the apparent rate constants (k) were calculated with the slopes of the apparent first-order linear transforms. The k values for our Pickering emulsion-based system and the solution-dispersed system were calculated to be 0.48 and 0.24 min−1, respectively. This comparison, although could not be totally normalized, still demonstrates that our Pickering emulsion-based system exhibits a higher photocatalytic activity than the traditional solution-dispersed system. We should note that the decreases in absorbance recorded in Figure 3A,B were due to the photocatalytic decomposition of MB, rather than to the simple adsorption of MB molecules onto MWNTs or Ag3PO4-MWNT nanohybrid, in spite of the facts of the adsorption of organic dyes on MWNTs and of the light harvesting property of MWNTs. We reasoned this from our control experiments of MB decomposition on pure MWNTs (i.e., without Ag3PO4) both in dark and under visible light irradiation. Figure 4 compares the UV−vis spectra of MB

Figure 4. UV−vis spectra of MB solution before (black curve) and after different treatments (curves with other colors): blue curve, with dispersion of MWNTs, in dark for 40 min; green curve, with dispersion of MWNTs, under visible light irradiation for 10 min (λ ≥ 420 nm); red curve, with dispersion of Ag3PO4−MWNT nanohybrid, in dark for 40 min; and pink curve, with dispersion of Ag3PO4− MWNT nanohybrid, under visible light (λ ≥ 420 nm) irradiation for 10 min. The experiments under visible light were performed after the solutions were kept in the dark for 30 min prior to light illumination.

solution before (black curve) and after different treatments (curves with other colors). In dark, the dispersion of MWNTs (blue curve) or Ag3PO4-MWNT nanohybrid (red curve) into MB solution clearly decreases the absorbance of MB (black curve), suggesting the adsorption of MB molecules onto both kinds of nanostructures. The amount of MB adsorbed onto each kind of nanostructure was roughly calculated to be ∼19% of the original value of MB. Under visible light illumination, the loss of MB from initial solution was still ∼19% with pure MWNTs (green curve), suggesting that the photocatalytic ability of MWNTs under our experimental conditions was actually negligible. Unlike the loss of MB with the addition of 15187

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Figure 5. (A) Schematic illustration of the determination of O2 evolution under the illumination of visible light (λ ≥ 420 nm). (B) O2 evolution under illumination in solution-dispersed system (30 mL) with the raw Ag3PO4 (25 mg) and Pickering emulsion-based system (volume ratio of water-to-isooctane was 0.4:1; water volume: 30 mL; Ag3PO4−MWNT nanohybrid: 25.5 mg). Inset: O2 yield in the solution-dispersed system (black curve) and Pickering emulsion-based system (red curve). The difference in the baseline in (A) and (B) was due to the different solubility of O2 in water and isooctane.

nanoparticles, graphene oxides, or carbon quantum dots to promote the charge separation on Ag3PO4 nanoparticles.46−50 Ag 3PO4 + hv → Ag 3PO4 (e− + h+)

(1)

Ag 3PO4 + 3e− → 3Ag + PO4 3 −

(2)

and MWNTs could act as an electron donor and an electron acceptor, respectively. Early attempts have demonstrated that the charge transfer process occurs from photocatalysts to carbon materials such as CNTs54 and graphene36 based on the increase in photocurrent or photoconductivity of semiconductor−CNT composites,54 transient absorption spectra,34 and electrochemical impedance spectra.36 To achieve a direct evidence for the charge transfer process occurring between Ag3PO4 semiconductor and MWNTs in our Pickering emulsion-based system, an oilsoluble electron acceptor, palladium acetate (Pd(OAc)2), was added into the oil phase, and MB was still used as an electron donor in water phase. The resulting emulsion system was irradiated under visible light (λ ≥ 420 nm) for 20 min. After that, the formed nanocomposite was collected by centrifugation, then washed thoroughly with water, and collected again by repeating centrifugation and analyzed by SEM, TEM, and EDS. As displayed in Figure 7A,B, Pd nanoparticles were deposited on the surface of MWNTs after the emulsions were illuminated for 20 min. Control experiments with the same amount of pristine MWNTs (i.e., without hybridization with Ag3PO4) in toluene solution containing Pd(OAc)2 were conducted under visible light irradiation (λ ≥ 420 nm) for 20 min. No Pd formation was observed in SEM image and EDS (Figure 7C,D). We shall note that Pd(OAc)2 could not be reduced by pristine MWNTs under the conditions employed here, although it was reported that Pd nanoparticles could be formed upon mixing Pd(OAc)2 with SWNTs in THF for 4 h.55 The reason could possibly be the short time (20 min) and/or different solvent employed here. In addition, we found that Pd(OAc)2 does not react with MB in the absence of Ag3PO4−MWNT nanohybrid under the conditions employed in this study. We reasoned this from the fact that no significant change was recorded in the UV−vis spectra of Pd(OAc)2 and MB as a function of time (data not shown). Therefore, the deposition of Pd nanoparticles onto MWNTs could be elucidated with the reduction of Pd(OAc)2 precursor in oil phase by the photogenerated electrons transferred from Ag3PO4 to MWNTs. All these results suggest the electron transfer from photoirradiated Ag3PO4 to MWNTs. Thus, the hybridization of Ag3PO4 with MWNTs would facilitate electron transfer from the excited semiconductor nanoparticles to carbon nanotubes, promoting the charge separation and thus alleviating the photocorrosion of

In addition to its great role in stabilizing Ag3PO4-based emulsions, MWNTs used in this study are expected to store and shuttle electrons from visible light-irradiated Ag3PO4 to alleviate the photocorrosion of Ag3PO4. To investigate such a possibility, Raman spectroscopy is employed in this case. As shown in Figure 6, a typical G-band of pristine MWNTs was

Figure 6. Typical Raman spectra of the G-band of pristine MWNTs (black curve) and Ag3PO4-MWNT nanohybrid (red curve).

clearly observed at ca. 1585 cm−1, which was consistent with the value reported previously.51 However, the G-band shifts by 7 cm−1 to a lower frequency at 1578 cm−1 for the Ag3PO4− MWNT nanohybrid, presumably suggesting the occurrence of charge transfer from Ag3PO4 to MWNTs (Raman spectra were recorded with a 514.5 nm argon ion laser). Generally, the Gband of CNTs shifts to a lower frequency when CNTs are hybridized with an electron donor component, while it shifts to a higher frequency when carbon nanotubes are hybridized with an electron acceptor component.52,53 Thus, we may draw a conclusion that charge separation and transfer might be achieved in our Ag3PO4−MWNT nanohybrid, where Ag3PO4 15188

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Figure 7. TEM (A) image and EDS (B) of the nanocomposite prepared from the emulsions with the addition of Pd(OAc)2 in oil phase and MB in water phase and irradiated under visible light for 20 min. SEM image (C) and EDS (D) of the nanocomposite prepared from Pd(OAc)2 toluene solution with the dispersion of the same amount of pristine MWNTs after illumination under visible light for 20 min.

2010CB33502 and 2013CB933704; 863 programs, 2010AA06Z302), and The Chinese Academy of Sciences (KJCX2-YW-W25).

Ag3PO4, which was responsible for the highly efficient photocatalysis toward MB degradation and O2 evolution as demonstrated above.

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4. CONCLUSIONS In summary, we have demonstrated a full exploitation of the unique properties inherent in Pickering emulsions which would facilitate forming a novel kind of photocatalytic system with a high efficiency. The highly enhanced photocatalytic efficiency of the as-formed photocatalytic system has been elucidated in terms of excellent features of Pickering emulsions including enlarged active surface area, facilitated continuous separation of products from reactants at the water/oil interface, and promoted charge separation by carbon nanotubes. This study not only opens a new avenue to development of novel photocatalytic systems with a high efficiency and practical applicability but also broadens the applications of Pickering emulsions.

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AUTHOR INFORMATION

Corresponding Author

*Fax (+86)-10-6255-9373; e-mail [email protected] (L.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by the NSF of China (Grants 20935005, 21127901, 21210007, 21190030, and 91213305 for L.M. and Grant 91132708 for P.Y.), the National Basic Research Program of China (973 programs, 15189

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