Light-Driven Au-WO3@C Janus Micromotors for Rapid

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Light-Driven Au-WO3@C Janus Micromotors for Rapid Photodegradation of Dye Pollutants Qilu Zhang, Renfeng Dong, Yefei Wu, Wei Gao, Zihan He, and Biye Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12081 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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ACS Applied Materials & Interfaces

Light-Driven Au-WO3@C Janus Micromotors for Rapid Photodegradation of Dye Pollutants Qilu Zhang,† Renfeng Dong, *,† Yefei Wu, † Wei Gao, ‡ Zihan He, †and Biye Ren*,† †

School of Materials Science and Engineering, South China University of Technology,

Guangzhou 510640, China ‡

Electrical Engineering and Computer Sciences, University of California, Berkeley, California

94720, United States KEYWORDS: light-driven; WO3@C; micromotors; self-diffusiophoresis; photocatalytic

ABSTRACT: A novel light-driven Au-WO3@C Janus micromotor based on colloidal carbon WO3 nanoparticle composite spheres (WO3@C) prepared by one-step hydrothermal treatment is described. The Janus micromotors can move in aqueous media at a speed of 16 µm/s under 40 mW/cm2 UV light due to diffusiophoretic effects. The propulsion of such Au-WO3@C Janus micromotors (diameter ~1.0 µm) can be generated by UV light in pure water without any external chemical fuels and readily modulated by light intensity. After depositing a paramagnetic Ni layer between Au layer and WO3, the motion direction of the micromotor can be precisely controlled by an external magnetic field. Such magnetic micromotors not only facilitate recycling of motors but also promise more possibility of practical applications in future. Moreover, the Au-WO3@C Janus micromotors show high sensitivity toward extremely low

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concentrations of sodium-2,6-dichloroindophenol (DCIP) and Rhodamine B (RhB). The moving speed of motors can be significantly accelerated to 26 µm/s and 29 µm/s in 5×10-4 wt. % DCIP and 5×10-7 wt. % RhB aqueous solutions respectively due to the enhanced diffusiophoretic effect, which results from the rapid photocatalytic degradation of DCIP and RhB by WO3. This photocatalytic acceleration of the Au-WO3@C Janus micromotors confirms the selfdiffusiophoretic mechanism and opens an opportunity to tune the motility of the motors. This work also offers the light-driven micromotors a considerable potential for detection and rapid photodegradation of dye pollutants in water.


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INTRODUCTION The motion of artificial nano/micro scale devices has attracted great attentions over the past decades owing to their potential applications in biomedical and environmental areas.1-9 Major research efforts have been stimulated to the use of a variety of chemical fuels to power the synthetic nano/micromotors including hydrogen peroxide,10-12 acids/bases,13 hydrazine,14 bromine/iodide15. Unfortunately, the requirement of high concentration of these chemical fuels greatly restrain the most practical utility of synthetic nano/micromotors. Therefore, it is highly desired that nano/micromotors can be powered by extremely low concentration of chemical fuels or biocompatible fluids such as water. Recently, extremly low level hydrazine fueled Janus mciromotors based on Iridium and water driven micromotors based on Al-Ga alloy and metal Mg has been reported.

16-19

However, the reactions between such kinds of Janus micromotors

and fuels are not controlled, in addition, the driven force of those water-fueled motors mostly comes from the chemical reaction of motors and water, and the constituent part of motors is consumed gradually as time progresses, which seriously impact the lifetime of the motors. Therefore, it is particularly attractive to fabricate synthetic micromotors which can continously move in aqueous media over a prolonged peorid and be controlled over their motion. Recently, light-driven synthetic micro/nanomotors have attracted considerable attention due to their potential applications and unique performances such as remote motion control and adjustable velocity.20-30 Several light-driven micro/nanomotors have been reported, such as lightpowered AgCl particles by photo-decomposed process, 28 TiO2-based micromotors propelled by photocatalytic reactions,23,

27, 29-30

and polymer-based microrockets driven by photothermal

effects.21 Light-driven micro/nanomotors based on photocatalytic reaction can move in water without photocatalyst being excausted. Therefore, more efforts have been devoted to


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photocatalytic nano/micromotors more recently.20-30 To the best of our knowledge, most of the light driven nano/micro motors propelled by photocatalytic reactions are based on TiO2, including plain TiO2 pariticles, TiO2 rockets and TiO2-based Janus micromotors.20, 23, 27, 29, 30 Therefore, it is appealing to fabricate novel light-driven micromotors based on different

Figure 1. (A) Schematic of driven mechanism of light-driven Au-WO3@C sphere Janus micromotors. (B) Schematic of driven mechanism of light-driven Au-TiO2 Janus micromotors.

photocatalytic materials and investigate their potential environmental applications. As an important n-type semiconductor materials, Tungsten oxide (WO3) and its hydrates have been widely used in many fields for photocatalysts, smart windows, gas sensors, and lithium battery due to their outstanding properties such as smaller band gap (2.4-2.8 eV) than TiO2 (3.2 eV), excellent photocatalytic performance, stable physicochemical properties, and


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good photocorrosion resistant feature.31-33 It is well known that the specific performance of photocatalytic materials is influenced not only by their composition and crystal phase, but also by their micro/nanoscale structure and morphology. Recently, colloidal carbon-based composite spheres have been paid great attention due to their enhanced physical performances and facile preparation. Such colloidal carbon spheres have functional groups such as -OH and -COOH groups distributed on their surfaces, which facilitate functional nanocomposite microspheres to bind cationic metal ions through electrostatic interactions and absorb nanoparticles on the surface of microspheres. Those functional carbon-based composite spheres promise eminent prospects and potentials for a wide variety of applications such as lithium ion batteries, gas sensitization, and photocatalysis.34-35 In order to enhance photocatalytic performance, colloidal carbon-based WO3 nanoparticle composite spheres (WO3@C) is particularly attractive. In this work, WO3@C composite microspheres are prepared by one-step hydrothermal treatment. Such WO3@C microspheres have several advantages of hydrophiphilic surfaces, light weight, nanosized porous structure, and high specific surface area, which could not only aid absorption and penetration of reactive species, but also enhance photocatalytic performance of WO3 nanoparticles absorbed on the sueface of carbon spheres for degradation of organic contaminants. In light of these advantages, we fabricate highly efficient photocatalytic AuWO3@C Janus micromotors (diameter ~1.0 µm) for the first time. Such spherical Janus micromotors

display

efficient

propulsion

in

pure

water

activated

by

UV

light.

In comparison with reported light-driven TiO2-based Janus micromotors, which move towards to TiO2 photocatalytic side through an electrophoretic mechanism,27 such Au-WO3@C Janus micromotors move towards to Au coated side through a dominative diffusiophoretic mechanism (seen in Figure 1, video 1). Moreover, the diffusiophoretic propulsion of WO3@C Janus


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micromotor can be accelerated by photocatalytic degradation of extremely low concentrations of dyes such as sodium-2,6-dichloroindophenol (DCIP) and Rhodamine B (RhB) because of enhanced diffusiophoretic effects. This dye-induced acceleration behavior offers considerable promises for environmental applications of micromotors ranging from environmental monitoring to environmental remediation.36-43 The work is therefore of interest for fabrication of new lightdriven micromotors in a variety of applications. EXPERIMENTAL SECTION Preparation of Carbon Microspheres. Carbon microspheres were prepared according to the procedure of Sun and Li.44 D (+)-Glucose anhydride (5.4 g, purchased from Shanghai Rich Joint Chemical Reagents Co., Ltd.) was dissolved in water (40 mL) to form a clear solution, which was transferred to a 45 mL Teflon-lined stainless steel autoclave and maintained at 180 ◦C for 12 h. After cooling, the products were isolated by centrifugation, cleaned by three cycles of centrifugation/washing/redispersion in water and in alcohol, and dried at 60 ◦C in vacuum oven for 4 h. Preparation of WO3@C Microspheres. WO3@C microspheres were prepared according to a method reported by Titirici et al with a slight modification.45-46 D (+)-Glucose anhydride (1.8016 g) and Na2WO4•2H2O (0.1649 g, purchased from Meryer Chemical Technology, Co., Ltd.) were dissolved in 10 ml of distilled water respectively. The two solutions were mixed together immediately with mild stirring, and transferred to a 45 mL Teflon-lined stainless steel autoclave, then maintained at 180 ◦C for 12 h. After cooling, the products were isolated by centrifugation, cleaned by three cycles of centrifugation/washing/redispersion in water and in alcohol, and dried at 60 ◦C in vacuum oven for 4 h.


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Preparation of WO3@C-based Janus Micromotors. Two WO3@C-based Janus Micromotors, i.e, Au-WO3@C and Al2O3-WO3@C microspheres were prepared. First, the prepared WO3@C spheres were calcined in air at 200 ◦C for 30 min. After cooling to room temperature, 1 mg WO3@C spheres were dispersed in 10 mL ethanol immediately, the sample was then spread onto glass slides and dried at ambient temperature. Thereafter, for Au-WO3@C Janus micromotors, a thin Au layer was deposited on top of the particle and formed hemispherical Au caps using Quorum Q150T with for 4 cycles with 60 s per cycle. The sputter current were 15 mA at the first two cycles and 30 mA at the rest two cycles. The thickness of Au layer on glass slide where particles were spread onto was found to be about 30 nm, as measured by the Veeco DEKTAK 150 Profilometer, and the Au layer on particles were estimated to be the same thickness with that on glass slide. Al2O3-WO3@C microspheres were prepared by coating with Al layer (about 30 nm) using ultra-high Vacuum Magnetron Sputter Coater JPG 560, then Al layer was oxidized in O2 atmosphere to form Al2O3 layer. Figure 2 shows the schematic of

Figure 2. Schematic of preparation of Au-WO3@C Janus Micromotors.

preparation of Au-WO3@C Janus Micromotors. Photocatalytic Activity of WO3@C Microspheres. Photocatalytic activities of WO3@C spheres were evaluated by the degradation of 2,6-Dichloroindophenol sodium salt hydrate (purchased from J&K Scientific Ltd.) and Rhodamine B (purchased from J&K Scientific Ltd.)


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under UV light irradiation of a 400 W Xe lamp. In a typical process, 20 mg of as-prepared WO3@C microspheres were dispersed in 10 mL deionized water. 250 µL WO3@C microspheres suspension were added into 15 mL of DCIP solution (25 µM) and RhB solution (6.25 µM) respectively. The solution was stirred for 0.5 h in the dark to reach adsorption equilibrium, and then was exposed to UV light irradiation. The samples were collected every 1 min intervals, and centrifuged to obtain clear solution to measure the DCIP adsorption by UV-Vis spectroscopy. Equipment. Scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) images were obtained by Zeiss VEO 18. Transmission electron microscopy (TEM) images were obtained by a JEM-2100HR Microscope with an accelerating voltage of 200 kV. X-ray diffraction (XRD) was performed in transmission geometry with an X’pert PRO diffractometer (40 kV, 40 mA) using Cu Kα radiation (wavelength λ = 0.154 nm) at room temperature. The 2θ ranged from 10º to 70º and the scan step was 0.02º in 2θ with a counting time of 1 s/step. XPS analyses were performed using KRATOS Axis Ultra (Kratos Analytical, Manchester, United Kingdom). An incident monochromated X-ray beam from the Al target (15 kV, 10 mA) was focused on a 0.7 mm × 0.3 mm area of the surface of the sample 45° to the sample surface. The electron energy analyser was operated with a pass energy of 20 eV enabling high resolution of the spectra to be obtained. The step size of 0.02 eV was employed and each peak was scanned twice. The content of element W is determineed by Hitachi Z-2000 Atomic Absorption Spectroscopy (AAS). UV light was generated by Mercury lamp sockets, dichroic mirror DM 400 and barrier filter BA420, intensity controlled by ND filters (4×, 8×, 16×) (all from Nikon), light intensity measured by UV radiometer UV-A (Videos were captured by an inverted optical microscope (Nikon Instrument Inc. Ti-S), coupled with 60× oil objectives, and a


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Zyla scmos digital camera (ANDOR) using the NIS-Elements AR 4.3 software. The UV−vis absorption spectra were measured using a U-3010 Spectrophotometer from Hitachi. RESULTS AND DISCUSSION The SEM image of WO3@C microspheres is shown in Figure 3A. These microspheres have relatively uniform spherical shape and

approximately an average size of 1 µm. As

F Figure 3. (A) SEM image of WO3@C microspheres with an average diameter around 1µm. Scal bar, 5 µm. The inset shows the high magnification SEM image of a WO3@C microsphere and carbon sphere respectively. scare bar, 0.6µm. (B) The XRD pattern of WO3@C microspheres. (C) TEM image of a spherical Au-WO3@C Janus micromotor. (D-F) The corresponding EDX mapping images for Au, W, O, respectively. Scale bar, 0.3 µm.


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aforementioned, the as-prepared WO3@C microspheres should have the bybrid structure with WO3 nanoparticles adsorbed on the shell of carbon microspheres.45-46 The SEM images clearly recognize the nanoparticles on the surfuce (seen in Figure 3A). As a control, the surface of carbon microspheres prepared by only D (+)-Glucose anhydride is relatively smooth (see also Figure 3A). The SEM results

suggest that the nanoparticles on the surfuce of carbon

microspheres should be WO3 nanocrystals. Furthermore, the crystal structure of WO3 absorbed on the shell of carbon spheres was characterized by XRD, as shown in Figure 3B. It is noted that all diffraction peaks are characteristic of the orthogonal WO3·H2O. Besides, XPS was employed to determine the surface composition of WO3@C spheres. Figure 4A shows the broad scan survey spectrum of WO3@C spheres, the main peaks correspond to W4f, O1s, and C1s, and the detail scan spectra are shown in Figure 4B-D. The W4f signal displays two kinds of W species: 4f7/2 (35.2 eV), 4f5/2 (37.3 eV), and 4f7/2 (36.3 eV), 4f5/2 (38.3 eV). The former (4f7/2 (35.2 eV), 4f5/2 (37.3 eV)) agrees with the signal of WO3, and the latter is attributed to the residual H2WO4 which is not completely converted to WO3·H2O nanocrystals yet. The above evidences clearly indicate that WO3 particles have been absorbed on the surface of carbon microspheres. The WO3 nanoparticles play a crucial role on the propulsion of Janus micromotors, thus the content of element W is determineed by AAS. The determined W content is 4.06 wt. %, which is in agreement with the results of XPS (Figure S1). In view of the above results, it is therefore important to demonstrate a reasonable formation mechanism of WO3@C microspheres. Here we deduce a possibly simplified reaction mechanism. In the first step, carbon microspheres are formed through polymerization of glucose and carbonization of linear or branchlike oligosaccharides.42 In the second step, water-insoluble H2WO4 is formed on the surface of carbon spheres by the displacement reaction of the functional -COOH groups on the surface of


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carbon spheres and Na2WO4, and then H2WO4 is further decomposed to produce WO3⋅H2O nanocrystals. Subsequently, WO3⋅H2O nanocrystals are absorbed on the surface of carbon microspheres. Finally, a hybrid WO3@C microsphere is obtained. When the surface of WO3@C microsphere is irradiated with UV light, negative electron (e-) and positive hole (h+) pair created by WO3 can react with water or oxygen to induce complex radical chain reactions,47-48 which

Figure 4. (A) Broad scan survey spectra of WO3@C microspheres. (B-D) Detail scan spectra for W4f, C1s, and O1s respectively.

make it possible to fabricate WO3@C-based Janus micromotors through a diffusiophoretic mechanism.


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In order to fabricate Janus particles, first, Au metal is deposited on the hemispherical surface of WO3@C spheres, as confirmed by the TEM and EDX mapping (Figure 3C-F). When exposed to the UV light, the electrons in the valence band of the photocatalyst are excited to the conduction band, while the holes are left in the valence band, thereby creating the negative electron (e-) and positive hole (h+) pairs.33 Usually the negative electron (e-) and positive hole (h+) can react with oxygen and water to initiate complex radical chain reactions and produce various 20, products such as H2O2, H+, ·OH, · O .

48

According to previously reported Au-TiO2 Janus

micromotors with electrophoretic mechanism,27 photo-excited electrons (e-) can be transferred from the conduction band of photocatalyst to metal coating deposited on its surface, the electron flow would take place and then result in electrophoretic effects. In present work, regarding the special hybrid structure of WO3@C microspheres, WO3 nanoparticles absorbed on the surface of microspheres may form uncontinuous structure, which significantly hinder the electronic conduction. More importantly, Na+ ions residued on the surface of WO3@C microspheres also significantly affect the electronic conduction, 46 as observed in EDS (Figure S1). Therefore, there is only diffusiophoretic driven-mechanisms toward the Au-WO3@C Janus micromotors. Our observation by optical microscope reveals that the dark region (coinciding with the Au side on the motors) is ahead during the motion of Au-WO3@C Janus micromotors (SI Figure S2 and SI Video S1), namely, the diffusiophoretic effect is a dominative driving force for the propulsion of the Au-WO3@C Janus micromotor, which is different from reported Au-TiO2 Janus motors via a self-electrophoretic mechanism.27 Understandably, there

will be a higher concentration of

photodegradative products around the WO3 side in comparison with the Au side, and the nanosized porous structure of WO3@C microspheres facilitates the diffusion of reactive products, leading to an osmotic flow from low to high solute concentration regions on the surface of Janus


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particles.49 This osmotic flow thus propels the micromotors with the Au side forward. To further comfirm this driving mechanism, Al2O3 (insulation layer) coated WO3@C Janus microsmotors were chosen as a control. As illustrated from SI Video S2, Al2O3-WO3@C Janus microsmotors also display efficient directional motion (~16 µm s-1) under 40 mW cm-2 UV light intensity in pure water. To validate the fact that the micromotors are not driven by thermophoresis, control experiments were conducted using spherical Au-C Janus particles (diameter ~1.0 µm). Even under 40 mW cm-2 intensity light, only slight directional movement (slightly faster than Brownian motion) is observed for Au-C Janus particles (SI Video S3) owing to thermophoresis effects. It is therefore clear that the propulsion of the Au-WO3@C Janus micromotor is mostly attributed to the diffusiophoretic effect. In general, non-electrolyte diffusiophoretic movement of Janus micromotors is caused by a concentration gradient of solutes across the particle surface. The velocity of Janus micromotors is given by Equation (1): 16 =

(1)

where U is the velocity of the Janus micromotors, K is the Gibbs absorption length, L is the length of the particle–solute interaction, k is the Boltzmann constant, T is temperature, η is the fluid viscosity, and ∇ C is the solute concentration gradient. In the present system, the solute concentration gradient (∇ C) is related to the speed of products generation, thus it is predictable that the velocity of the Janus micromotors (U) is able to be controlled by adjusting the UV light intensity (I). It should be mentioned that Au-WO3@C Janus particles also display slight spatial vertical movement because of light-generated temperature gradient on bottom and top of Janus


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particle. Therefore, we take approximately the speed of horizontal movement as the velocity U of Au-WO3@C Janus micromotors in the following discussion. Figure 5 A and SI Video S4 reveal that Brownian motion dominates the micromotor movement when I is smaller than 2.5 mW cm-2. When I increases from 5 to 10 mW cm-2, the speed of motors increases from about 6 to 11 µm s-1 (Figure 5 B-C). The micromotors can move at a speed of about 16 µm s-1 under a light intensity of 40 mW cm-2. Figure 5 E reveals that the speed of Janus micromotors changes as the light intensity (I). In general, the relationship of incident light (I) and the photon flux (Φ, the number of incident photons per unit area per second) can be described as Equation (2): 23 =

ℎ (2)


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where h is Plank’s constant (6.626 × 10−34 J s), c is the speed of light (3 × 108 m s-1) and λ represents wavelength of the UV light (330~380 nm in this case). Clearly, the photon flux Φ on the Janus micromotors increases with increasing I, thereby the number of the photogenerated holes and electrons increases to lead to a faster speed of product generation. Consequently, the

Figure 5. (A, B, C, D) Tracklines of micromotors (SI Video 4) with UV intensity 2.5, 5, 10, and 40 mW cm-2, respectively over 1 s. Scale bar, 10 µm. (E) The influence of the UV light intensity on the speed of water-fueled Janus micromotors. (F) Average MSD versus time interval (∆t) analyzed from tracking trajectories. (G) Diffusion coefficient of Au-WO3@C Janus micromotors under 0, 2.5 mW cm-2 UV light intensity in pure water. The values determined from the MSD plots (20 micromotors were analyzed).

speed of the micromotors is modulated by incident light intensity. To clearly illustrate that the micromotors exhibit enhanced directional mobility even under 2.5 mW cm-2 UV illumination, the mean square displacement (MSD) and diffusion coefficient


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(D) were calculated. The motors (number of motors n =20) were tracked over 10 s, MSD was calculated by the equation MSD = (x(∆t) − x(0))2 + (y(∆t) −y(0))2 and D was calculated by the equation D=MSD/i∆t,50 where ∆t is the time interval, i is the dimensional index ( here i is equal to 4 for the case of two-dimensional analysis from the recorded video). The plots of MSD versus ∆t and diffusion coefficient results were presented in Figure 5 F-G. In pure water, even extremely low UV intensity obviously exerts a positive influence on propulsion of the micromotors. Compared to reported bubble-generated Janus micromotors, the light-activated water-fueled Au-WO3@C sphere Janus micromotors obtain energy through photocatalytic reaction without Au-WO3@C consumed, and have two impressive advantages: repeatable light-controlled motion and long life time. As Figure 6 A illustrates, the movement of micromotors is activated and inactivated immediately by repeated on/off cycling of UV light illumination, which reflects their fast response to UV light. When exposed to 40 mW cm-2 UV light, the micromotors are activated and reaches a speed of around 16 µm s-1, but stop to move and only display Brownian motion immediately after removing the UV light. This “stop/go” propulsion behavior indicates that the repeatable and controllable motion of micromotors can be achieved by switching the UV irradiation on or off. After 30 cycles of such on and off controls, the Au-WO3@C Janus micromotor still show high repeatability because of its good photocorrosion resistant. Moreover, after 5 min UV illumination, the Janus micromotors still exhibit efficient propulsion. The direction control and reusability of motors are very important for their practical applications. The motion direction of micromotors also could be controlled by coating the particles with a paramagnetic Ni layer after the deposition of the Au layer. By exerting an external magnetic field, Ni-Au-WO3@C Janus micromotors can be precisely navigated following predetermined


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trajectories (Figure 6 B and SI Video S6) and readily recycled. Such magnetic micromotors offer the light-driven micromotor more possibility of practical applications in future. As aforementioned, WO3 is a good photocatalyst for decomposition of some hazardous organic compounds, and its applications in enviromental areas have been extensively studied

Figure 6. (A) Speed/time dependence illustrating the UV light-activated repeatable controlled motion of the spherical Au-WO3@C Janus micromotors. The speeds were measured at 1 s intervals from SI Video 5. UV light intensity 40 mW cm-2. (B) Time-lapse images (taken from SI Video 6) showing the magnetically guided propulsion of a Ni-Au-WO3@C Janus micromotor under 40 mW cm-2 UV light. Scale bar, 10 µm.

over the past decades.51-53 Recently, autonomously moving micromotors with high sensitivity toward extremely low concentrations of toxic pollutants have been shown to offer an innovative concept for environmental monitoring.7 Theoretically, WO3@C microspheres have similar properties to pure WO3. According to the self-diffusiophoretic mechanism, an acceleration should happen to Au-WO3@C Janus micromotors when a compound in aqueous solution is photo-decomposed by WO3@C microspheres. To further illustrate this phenomenon, DCIP, a chlorophenol derivative that is toxic and hardly biodegradable in the environment,54 was first


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used for this purpose. As shown in Figure 7, when the concentration of DCIP is higher than 5×10-13 wt. %, the micromotors are obviously accelerated compared to those in pure water, which should be attributed to the enhanced diffusiophoretic effect induced by the additional oxidized products when DCIP is photocatalytically degradated. At the same time, the porous structure of carbon microspheres also facilitates the adsorption and penetration of DCIP and diffusion of the oxidized products, which may play a positive role on the acceleration behavior of Janus motors. The micromotors move faster as the DCIP concentration increases, and the speed reaches the maximun (around 26 µm s-1) in 5×10-4 wt. % DCIP solution. This indicates that the motion of Janus micromotors is highly sensitive to extremely low concentrations of DCIP. This DCIP-based acceleration behavior will offer a potential for monitoring of extremely low concentrations of toxic pollutants in water. However, as the DCIP concentration increases further until 5×10-3 wt. %, the speed of micromotors cuts down to about 8 µm s-1. Previous reports have demonstrated that increasing salt concentrations reduced the speed of micromotors.16,

55

In

present work, Na+ ions accompanied with the degradation of DCIP are introduced into the solution. Accordingly, a high DCIP concentration will induce a high Na+ concentration in solution. Besides, the product Cl- ions may cause a chemophoretic effects that are opposite to the diffusiophoretic effect, and inhibit the motion of micromotors.16, 56 When the DCIP concentration reaches 5×10-2 wt. %, the micromotors begin to aggregate from all directions to the focus point of UV light. To demonstrate such a aggregation behavior of the micromotors, a control experiment is performed on polystyrene (PS) particles (2 µm diameter). As a result, the same phenomena happened to PS particles in the 5×10-2 wt. % DCIP solution upon UV illumination (SI Figure S3). It is clear that such a aggregation behavior of the micromotors in the 5×10-2 wt. %


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DCIP solution should be attributed to the thermal convection of surrounding liquid, because the deep blue DCIP solution will make the liquid at the focus point of UV light a higher temperature. The acceleration of Au-WO3@C Janus micromotors is originated from additional oxidized products produced by photocatalytic degradation of DCIP, namely, the degradation of DCIP happens during motion of micromotors simultaneously. To evaluate the photocatalytic activity of WO3@C microspheres, the UV−Vis absorption spectra are recorded for DCIP solutions, as shown in Figure 8 A. Initially, the solution of DCIP exhibits a strong absorption at 600 nm. After UV irradiation, the peak decreases as time progresses, indicative of a gradual decomposition of DCIP. After irradiation by UV light for 5 min, the absorption peak at 600 nm disappears. The

Figure 7. (A) The speed dependence of Au-WO3@C Janus micromotors upon the DCIP concentration. The inset shows scheme of degradation reaction of DCIP by WO3@C particles. (B) Tracklines of micromotors (SI Video S7) in 5×10-13, 5×10-11, 5×10-7, 5×10-4, 3×10-3, 5×10-3 wt. % DCIP solution with 40 mW cm-2 UV intensity respectively over 1 s. Scale bar, 10 µm.

UV-Vis

spectra clearly reveal that DCIP could be completely decomposed by WO3@C


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microspheres upon UV irradiation. Among the photocatalytic products, ·OH and · O are highly oxidative enough to decompose DCIP.57-58 To exclude the influence of UV irradiation on the DCIP solution, a control experiment is conducted for DCIP solution without WO3@C particles. As shown in Figure 8 B, after 5 min UV light illumination, the absorption peak at 600 nm only slightly reduces. Therefore, it is the WO3@C particles that work during the degradation process. To further validate such a dye-based acceleration behavir of Au-WO3@C Janus micromotors, Rhodamine B (RhB) was used as a comparision. As seen in Figure 9 A, the Janus

Figure 8. UV-vis spectra of 25 µM DCIP solution exposed to a 400 W Xe lamp for different time in the presence of WO3@C particles (A), and in the absence of WO3@C particles (B). micromotors also show high sensitivity to extremely low concentrations of RhB, but the RhB concentration at which the Au-WO3@C Janus micromotors begain to accelerate is higher than the corresponding DCIP concentration. The speed of Au-WO3@C Janus micromotors is accelerated at the concentration of 5×10-9 wt. %, and reach the highest speed of 29 µm s-1 in 5×10-7 wt. % RhB solution (SI Video S8). Figure 9 B shows that both DCIP and RhB can be


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photocatalytically decomposed by WO3@C microspheres, but WO3@C microspheres reveal higher photocatalytic activity to degradation of DCIP. Such a Au-WO3@C remediation process doesn’t require the addition of external peroxide fuel or decontaminating reagents. It is therefore of great interest for the development of in situ environmental applications of micromotors. The development of nano/micromotors has made a profound impact in the environmental field, and synthetic micromotors are shown to be useful for environmental applications ranging from environmental monitoring,36, 59 environmental remediation,41 to the removal of oil spills.60

Figure 9. (A) The speed change (%) of Au-WO3@C Janus micromotors in different concentration of DCIP and RhB solutions relative to that in pure water. (B) The photo degradation efficiency (%) of WO3@C toward 25 µM DCIP and 6.25 µM RhB exposed to a 400 W Xe lamp for different time.

Among those synthetic micromotors, most of them commonly require external fuels or surfactants for propulsion. Although water-fueled motors propelled by bubbles have advantages in fuels, such motors obtain their energy from the chemical reaction of motors and water, leading to gradual exhaustion of the constituent part of motors, which will seriously impact the lifetime


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of the motors and aslo greatly hinder their practical applications in environmental aeras. The present Au-WO3@C Janus micromotors have the following advantages: water-fuled, without other chemical fuels or surfactants, long life time, and controllable motion. Compared to most reported sythetic micromotors used for envrionmental mediations, the present Au-WO3@C Janus micromotors can be used not only for detection of extremely low concentrations of toxic pollutants but also for photocatalytic degradation of dyes in water. There is no doubt that such multiple funcitonal Au-WO3@C motors will open a new vision for resolution of environment issues in the future. CONCLUSIONS In conclusion, we describe a novel light-driven Au-WO3@C Janus micromotor for monitoring and rapid degradation of dyes in water. The Janus micromotor can move at a speed of 16 µm s-1 under 40 mW cm-2 UV light due to diffusiophoretic effects in pure water. Unlike previous water-fueled Janus micromotors propelled by bubbles, such Au-WO3@C Janus micromotors have two impressive advantages of repeatable light-controlled motion and long lifetime. The motion of Janus micromotors controlled by UV light and magnetic field could offer motors more possibility of practical applications. Moreover, Au-WO3@C Janus micromotors show high sensitivity toward extremely low concentrations of dyes such as DCIP and RhB. The speed of the micromotors can be significantly accelerated in low concentration of DCIP and RhB solutions by photocatalytic degradation of dyes due to enhanced diffusiophoretic effects. This dye-induced acceleration behavior offers the Janus micromotor a great potential for environmental applications such as detection and photodegradation of dye pollutants in water. ASSOCIATED CONTENT


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Supporting Information. The EDX of WO3@C microspheres. Time-lapse images of the moving Au-WO3@C Janus micromotors in water. The aggregation of WO3@C and PS microspheres in 5×10-2 % DCIP solution after UV irradiation with 40 mW/cm2 UV light for different times. Video S1: Motion of Ni-Au-WO3@C and Au-WO3@C Janus micromotors with the darker region ahead. Video S2: Motion of Au-WO3@C and Al2O3-WO3@C Janus micromotors in pure water under 40 mW/cm2 UV light. Video S3: Motion of Au-C Janus particles under 40 mW/cm2 UV light. Video S4: Motion of Au-WO3@C Janus micromotors under UV light with different intensities. Video S5: Repeatable go/stop propulsion behaviors of Au-WO3@C Janus micromotors controlled by UV light. Video S6: Directional motion of Ni-Au-WO3@C Janus micromotors under an external magnetic field. Video S7: Motion of Au-WO3@C Janus micromotors in different concentration of DCIP aqueous solutions under 40 mW/cm2 UV light. Video S8: Motion of Au-WO3@C Janus micromotors in different concentration of RhB aqueous solutions under 40 mW/cm2 UV light. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] (R. Dong) and [email protected] (B. Ren) Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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