Pt Janus Micromotors for Enhanced Nitroaromatic ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Fuel-Free Light-Powered TiO2/Pt Janus Micromotors for Enhanced Nitroaromatic Explosives Degradation Lei Kong,†,‡ Carmen C. Mayorga-Martinez,§ Jianguo Guan,*,‡ and Martin Pumera*,†,§

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Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, People’s Republic of China § Center for Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Nitroaromatic explosives such as 2,4,6-trinitrotoluene (2,4,6-TNT) and 2,4-dinitrotoluene (2,4-DNT) are two common nitroaromatic compounds in ammunition. Their leakage leads to serious environmental pollution and threatens human health. It is important to remove or decompose them rapidly and efficiently. In this work, we present that lightpowered TiO2/Pt Janus micromotors have high efficiency for the “on-the-fly” photocatalytic degradation of 2,4-DNT and 2,4,6-TNT in pure water under UV irradiation. The redox reactions, induced by photogenerated holes and electrons on the TiO2/Pt Janus micromotor surfaces, produce a local electric field that propels the micromotors as well as oxidative species that are able to photodegrade 2,4-DNT and 2,4,6-TNT. Furthermore, the moving TiO2/Pt Janus micromotors show an efficient degradation of nitroaromatic compounds as compared to the stationary ones thanks to the enhanced mixing and mass transfer in the solution by movement of these micromotors. Such fuelfree light-powered micromotors for explosive degradation are expected to find a way to environmental remediation and security applications. KEYWORDS: micromachines, microrobots, photocatalytic degradation, explosives, pollutants

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INTRODUCTION Nitroaromatic compounds 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) are common in military explosives, which may lead to serious soil and water pollution.1,2 In addition, they threaten human health indirectly through the food chain even at a very low concentration, which may cause many diseases like aplastic anemia, liver function disturbances, cataracts, etc.3 Hence, it is of high significance to develop methods that are able to degrade these nitroaromatic compounds rapidly and efficiently. In the past several years, significant efforts toward degradation of nitroaromatic compounds were reported. Thermal treatment of the hydrolysates of TNT or biodegradation through bacteria were studied;4,5 however, the efficiency is limited, and the degradation is not completed. Advanced oxidation processes (AOPs), such as the Fenton process,6 O3/H2O2 under UV irradiation,7,8 ultrasonic irradiation,9 etc., degrade the nitroaromatic compounds rapidly and completely by produced hydroxyl radicals (•OH) which are highly oxidative. However, the addition of H2O2 is not ecofriendly and cannot be used for remediation efforts of high© XXXX American Chemical Society

energy explosives when they are leaked to the environment. Electrochemical reduction of 2,4-DNT and 2,4,6-TNT in water or pH-buffered media is not practical beyond the laboratory setting.10,11 Photocatalytic degradation of organic pollutants in pure water or other solutions through photocatalytic materials under light irradiation is a conventional method.12−14 According to the early studies by Schmelling and Gray, the photocatalytic degradation of TNT with TiO2 contained both oxidative and reductive steps,15,16 which can degrade the pollutant by strong oxidative species such as superoxide (•O−2 ) or hydroxyl radicals (•OH). Taking into account the degradation efficiency, environmental compatibility, and convenience, photocatalytic degradation is an appropriate method for rapid and efficient removal of nitroaromatic compounds in pure water. Nano-/micromotors are micromachines that transfer the energy from the environment to autonomous movement,17−19 Received: April 12, 2018 Accepted: June 5, 2018

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DOI: 10.1021/acsami.8b05776 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces exhibiting potential applications in biomedicine,20−22 microengineering,23 and environmental detection and remediation.24−29 Some catalytic micromotors such as activated carbon Janus micromotors,30 zeolite micromotors, and magnesium micromotors have exhibited the “on-the-fly” removal or degradation of explosives and nerve agents.31,32 However, the toxic fuels used and limited lifetime impede their real applications. Light-powered photocatalytic micromotors, like TiO2,33,34 Fe2O3,35 Cu2O,36 ZnO,37 Si, and bismuthoxyiodide-based (BiOI-based) particles,38,39 have been attracting significant attention recently. These are based on tubular or wire-like-shaped bodies, propelled either by bubble ejection, concentration gradient differences (self-diffusiophoresis), or local electric field (self-electrophoresis) in pure water or in the solution containing a low concentration of hydrogen peroxide through the asymmetric photocatalytic redox reactions under light irradiation, which have been attracting significant attention. Furthermore, the velocity of the devices, their direction, and their behavior are easily controlled by adjusting the intensity, orientation, and mode of incident light. The wavelength of light can be varied from ultraviolet (UV) to visible light based on the absorption spectrum of photocatalytic materials,40 which make the use of sunlight to directly activate them possible. Meanwhile, the “on-the-fly” photocatalytic degradation of organic dyes in pure water with TiO2/ Pt, TiO2/Au, and Au-WO3@C Janus micromotors was reported and demonstrated high photocatalytic activity and enhanced mass transfer on the solution by movement.34,41,42 Thus, the light-driven photocatalytic micromotors have potential applications in the “on-the-fly” degradation of nitroaromatic compounds in pure water without any additions. In this work, considering the high stability and excellent photocatalytic activity of the TiO2/Pt heterostructure,43 we demonstrate “on-the-fly” photodegradation of explosives 2,4,6trinitrotoluene and 2,4-dinitrotoluene with TiO2/Pt Janus micromotors in pure water under UV irradiation (Scheme 1). The mechanism of movement is self-electrophoresis, and the local electric field is established by the asymmetric photo-

catalytic redox reactions on TiO2/Pt Janus particles. Meanwhile, 2,4-DNT and 2,4,6-TNT molecules can be photodegraded with TiO2/Pt Janus micromotors by UV irradiation. Even at a high concentration of 2,4-DNT and 2,4,6-TNT present in the solution, TiO2/Pt Janus micromotors moved rapidly, and their enhanced diffusion together with the photoinduced degradation effect of TiO2/Pt Janus micromotors led to fast photodegradation of the nitroaromatic explosives.

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RESULTS AND DISCUSSION The TiO2/Pt Janus micromotors were prepared using the partial surface modification method and characterized by scanning electron microscopy, dynamic light scattering, and Xray photoelectron spectroscopy. Figure 1a shows the schematic

Scheme 1. Schematic Representation of the Propulsion Mechanism of UV-Light-Powered TiO2/Pt Janus Micromotors and “On-the-Fly” Degradation of Nitroaromatic Explosives Figure 1. (a) Schematic of fabrication of TiO2/Pt Janus micromotors. SEM image (b) and EDX mapping (c−e) of TiO2/Pt Janus micromotors.

of the fabrication of TiO2/Pt Janus particles. First, the prepared TiO2 microspheres were placed on the glass slide.34,44 Then, Pt was sputtered on the surface of TiO2 particles. Finally, the TiO2/Pt Janus particles were released by sonicating from the glass slide in ethanol and washing with deionized water three times (see details in the Experimental Section). The high-magnification scanning electron microscopy (SEM) image shows a typical TiO2/Pt Janus particle (Figure 1b), showing a spherical shape with ∼1 μm diameter which is consistent with the result of dynamic light scattering (DLS) size distribution measurements (Figure S1, Supporting Information). The Pt coating on the side of the TiO2 particle clearly shows a well-defined asymmetric structure of this Janus microparticle. For further insight into the Janus structure of the as-obtained TiO2/Pt particle, energy-dispersive X-ray specB


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Figure 2. High-resolution XPS spectrum of the Ti 2p region (a), O 1s region (b), and Pt 4f region (c) for TiO2/Pt Janus microparticles.

Figure 3. (a−d) Trajectories of TiO2/Pt Janus micromotors in 1 s under UV irradiation with the intensity of 40 mW cm−2 in different concentrations of 2,4-DNT. (e) Average velocity of TiO2/Pt Janus micromotors moving in different concentrations of 2,4-DNT.

troscopy (EDX) mappings were carried out. Figure 1c,e indicates the presence of Ti and O well-distributed around the entire surface of the particle. Pt is shown to be concentrated on one side of particle (Figure 1d), which is consistent with the asymmetric morphology in the observed SEM image. Thus, we successfully obtained the typical Janus structure in the TiO2/Pt microspheres. High-resolution X-ray photoelectron spectroscopy (XPS) of TiO2 (Figure S2) and TiO2/Pt (Figure 2) microspheres was measured to gain insight into the exact chemical composition of the surface of the Janus particle. The Ti 2p regions in both materials are decomposed into two contributions that are attributed to TiO2.45 In the case of the O 1s region, the spectrum deconvoluted shows two components; the one around 528 eV corresponds to the Ti−O bond of TiO2, and the main peak is attributed to −OH,46 which is due to the high hydration of the particles. Therefore, the XPS analysis suggests that the ideal chemical compositions of TiO2 particles are close to that of TiOx(OH)4−2x, which is consistent with the reported study.44 The high content of −OH on the surface of the asprepared particles may provide a high hydrophilicity to react with water. The Figure 2c shows the Pt 4f regions that confirm first the well-deposited platinum and its pure metallic nature.47 The main idea of this article is based on the light-powered TiO2/Pt Janus micromotor photocatalytic activity which can move in pure water under UV irradiation through selfelectrophoresis and at the same time photodegrade nitroaromatic explosives.34 The local electric field is formed by the asymmetric photocatalytic redox reactions on TiO2/Pt Janus particles. As shown in Scheme 1, when the UV irradiation is on, photogenerated electrons in TiO2 will transfer to the Pt layer and react with hydrogen ions as in reaction R1. Moreover, the photogenerated holes promote the water oxidation onto the exposed TiO2 surface (reactions R2 and/ or R3).

2H+ + 2e− → H 2

(R1)

2H 2O + 4h+ → O2 + 4H+

(R2)

H 2O + h+ → •OH + H+

(R3)

According to the above equations, H+ is highly concentrated on the TiO2 side, and a local electric field pointing from the TiO2 end to the Pt end is formed. As a consequence, the negative TiO2/Pt Janus particles show autonomous movement in the local electric field toward the TiO2 side. At the same time, the redox reactions caused by photogenerated holes and electrons not only form the local electric field to propel the movement of micromotors, but also have the potential ability to degrade 2,4-DNT and 2,4,6-TNT.16 Furthermore, due to the micromixing and enhanced mass transfer by movement of the Janus motor, photodegradation efficiency will be promoted. Generally speaking, the TiO2/Pt Janus micromotors should provide a more efficient method for removal of 2,4DNT and 2,4,6-TNT in water without any fuel additives (such as hydrogen peroxide or glucose).48−51 We will demonstrate this in the following pages. To investigate the motion performance of as-prepared TiO2/Pt Janus micromotors, we measured the velocity in pure water with and without UV irradiation. Figure S3a demonstrates that the Janus particles exhibited only Brownian motions in pure water when UV irradiation was absent. In contrast, the autonomous movement was clearly observed under UV irradiation at an intensity of 40 mW cm−2 (see Figure S3b and Video S1 in the SI). The estimated velocity of the TiO2/Pt Janus micromotors is 9.7 ± 1.98 μm s−1, thus proving the photocatalytic activity of the as-obtained micromotors in water, showing that their movement could be conveniently controlled by switching the UV light on/off. Subsequently, for evaluating the application of TiO2/Pt Janus micromotors on pollutant degradation, first we tested the C


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Figure 4. (a) Cyclic voltammograms of 2,4-DNT [100 μM, concentration in borate buffer (BBS)] after UV irradiation (40 mW cm−2) for 1 h with TiO2/Pt Janus micromotors at different concentrations from 0 to 0.03, 0.06, and 0.12 mg mL−1. (b) Degradation efficiency of 2,4-DNT with different concentrations of TiO2/Pt Janus micromotors. (c) Cyclic voltammograms of solutions containing 2,4-DNT (100 μM, concentration in BBS), 2,4-DNT after UV irradiation (40 mW cm−2) for 1 h, TiO2/Pt Janus micromotors (0.12 mg mL−1) under UV irradiation (no 2,4-DNT contained), photodegradation of 2,4-DNT with TiO2/Pt Janus micromotors (0.12 mg mL−1), or stationary TiO2/Pt Janus particles (BBS contained in solution which hinders the movement of Janus particles), respectively. (d) Degradation efficiency of 2,4-DNT from part c. Conditions: scan rate of 50 mV s−1.

following previous publications.2,63,64 Figure S4a shows the cyclic voltammograms (CVs) of 2,4-DNT solutions at different concentrations prepared in borate buffer solution (BBS) pH = 9.2 (details are shown in the Experimental Section). Two reduction peaks appear at −787 and −629 mV that correspond to reduction of nitro groups of 2,4-DNT, and the intensity of both peaks were more evident when the concentration increased from 20 to 100 μM. The CV measured in absence of the analyte (black line) shows a reduction peak which corresponds to reduction of oxygen present in the solution.2 Therefore, the reduction peak at −629 mV was chosen for the quantitative analysis of 2,4-DNT, trying to avoid any interferences from the inherent oxygen reduction. The relationship between the concentration of 2,4-DNT and the current deducted background (blank buffer) is shown in Figure S4b. The revised current (In) was obtained through equation 1:

motion of micromotors in different concentrations of the 2,4DNT solution. Figure 3a−d shows the trajectories of Janus micromotors for 1 s under UV irradiation in a varied 2,4-DNT solution of 1 μM, 10 μM, 100 μM, and 1 mM (Video S2). Figure 3e summarized the average velocity calculated from 40 different particles for each 2,4-DNT concentration. Because of the fact that the TiO2/Pt Janus micromotors’ mechanism motion is based on self-electrophoresis, there is a strong dependence on the presence of ions in the solution, which influences ζ potential on the surface and therefore decreases the velocity.41,52,53 Nitroaromatic explosives are neutral compounds and therefore do not impact the micromotor motion under study. In this way the micromotor performance in terms of velocity was not compromised by the presence of DNT. There are many analytic methods for 2,4-DNT and 2,4,6TNT detection, including gas chromatography,54 highperformance liquid chromatography,55 Raman spectroscopy,56 infrared absorption spectroscopy,57 mass spectrometry,58 immunoassay techniques, and electrochemical techniques.59,60 Electrochemical methods have the advantage of sensitivity and selectivity.61 Particularly, the cyclic voltammetry results of 2,4DNT and 2,4,6-TNT show a set of reductive peaks that are ascribed to reduction of nitro groups present in the compound.62−64 Before studying the photodegradation of explosives, we implemented first the 2,4-DNT detection using cyclic voltammetry and a glassy carbon (GC) electrode,

In = |I | − |I0|

(1)

Here, |I| corresponds to the net current of each concentration measured at −629 mV, and |I0| corresponds to the net current measured at −629 mV of the blank buffer. The result indicates a good linear response (R2 = 0.9409) in a concentration range from 20 to 100 μM. As a previous work reported,16 the photocatalytic oxidative degradation of pollutants such as nitroaromatic compounds using semiconductor TiO2 particles involves their photoexcitation by UV light that produces a change in the energy D


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Figure 5. (a) Trajectories of TiO2/Pt Janus micromotors in 1 s under UV irradiation with an intensity of 40 mW cm−2 in 0.6 mM 2,4,6-TNT solution. (b) Cyclic voltammograms of 0.6 mM 2,4,6-TNT in borate buffer solution (pH of 9.2) and blank buffer. (c) Degradation efficiency of 2,4,6-TNT under different conditions. Conditions: scan rate of 50 mV s−1.

state of the electrons from the valence band to the conduction band, resulting in the electron and hole generation onto the surface of the particle. Accordingly, this active surface particle is able to produce hydroxyl, superhydroxyl, and perhydroxyl radicals from water splitting, oxidation of hydroxide ions, and/ or oxygen oxidation and adsorption. These radicals in consequence are responsible for the pollutant oxidation such as nitroaromatic compounds.65 For an evaluation of the photodegradation efficiency of 2,4DNT by TiO2/Pt Janus micromotors, solutions containing 1 mM of 2,4-DNT and TiO2/Pt Janus micromotors at different concentrations were UV irradiated by using a xenon lamp with an intensity of 40 mW cm−2 in a quartz cuvette during 1 h. After the solutions were centrifuged, the supernatants were collected and diluted 10 times with BBS buffer; then, cyclic voltammetry measurements were conducted. In the absence of the micromotors, the 2,4-DNT concentration remains the same, while, when the concentration of the micromotors increased, the 2,4-DNT concentration decreased (see Figure 4a). Figure 4b summarizes the degradation efficiency (DE) as a function of TiO2/Pt micromotor concentration; the error bar corresponds to three different sets of experiments of each concentration. The degradation efficiencies were obtained using equation 2: degradation efficiency =

In ,BD − In ,AD In ,BD

concentration were added into the 2,4-DNT solution under UV irradiation for 1 h, the CV response (magenta curve) in Figure 4c shows the lower intensity of the reductive peak at −629 mV, suggesting the certain 2,4-DNT degradation by TiO2/Pt Janus micromotors under UV irradiation. As mentioned previously, the mechanism of motion of TiO2/Pt Janus micromotors is sensitive to the presence of ions in the solution. In addition, as the ζ potential of microparticles changes with addition of ions, they will not be able to move, and they will precipitate quickly in solution when the ion concentration is higher than 10 mM.34 Thus, to investigate the effect of movement/absence of the movement of the Janus particle on 2,4-DNT photocatalytic degradation, we replaced the pure water with borate buffer (BBS) to create a high concentration of ions beyond 10 mM (for details, see the Experimental Section). The green curve in Figure 4c shows the CV response from the experiment performed in stationary conditions (high ion concentration) that suggests high reductive peak intensity compared with micromotors in movement. Figure 4d shows the degradation efficiency of 2,4-DNT of the control experiments. In absence of micromotors, the degradation efficiency is just 2.4%. However, the 2,4-DNT degradation efficiency is 27.4% in the presence of the moving TiO2/Pt Janus micromotors. Furthermore, stationary TiO2/Pt Janus micromotors do not possess such a high degradation efficiency as self-propelled micromotors because of the absence of enhanced diffusion and mixing of the solution; a degradation efficiency of ∼20.2% was observed in such a case. This clearly demonstrates that the movement of micromotors enhances the mass transfer in the solution and promotes the degradation efficiency. Additionally, we performed the recovery analysis of the cyclic degradation (4 runs) of 2,4-DNT by the same TiO2/Pt Janus micromotors (Figure S5). The recovery percentages of the second and third runs were around 98%, while for the fourth run it decreased a little, ∼88%. The obtained recovery data demonstrated the high stability and reused capabilities of TiO2/Pt Janus micromotors for 2,4-DNT photodegradation. The 2,4-DNT is the intermediate in commercial manufacturing of 2,4,6-TNT.61 The 2,4,6-TNT is more common, harmful, and difficult to remove. After having succeeded in 2,4-DNT photodegradation using TiO2/Pt Janus micromotors, we performed the photodegradation of 2,4,6-TNT under the same conditions. Figure 5a shows the trajectory of Janus micromotors for 1 s under UV irradiation with 40 mW cm−2

× 100% (2)

where In,BD and In,AD represent the current deducting background before and after degradation, respectively. The results in Figure 4b show an increase in the degradation efficiency from 2.37% to 15.4%, 17.7%, and 27.4% when the concentration of micromotors increases from 0 to 0.03, 0.06, and 0.12 mg mL−1, respectively. The control experiments under different conditions were performed, and the data are shown in Figure 4c. The black curve in Figure 4c represents the reductive peak of the CV of 100 μM 2,4-DNT without UV irradiation [DNT (no UV)]. The red curve corresponds to 2,4-DNT after UV irradiation for 1 h [DNT (UV)]; in both cases, no photodegradation of 2,4DNT was observed. For exclusion of the interference of TiO2/ Pt particles which were not removed completely after centrifugation, the solution only containing TiO2/Pt particles was also irradiated with UV light for 1 h; the resulting CV measured (blue curve) did not show any interference peak. When the TiO2/Pt Janus micromotors at a 0.12 mg mL−1 E


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dark for 24 h. Then, the TiO2 particles were collected after centrifugation at 10 000 rpm for 30 min and washed with ethanol and water three times. Finally, the TiO2 particles were sintered at 450 °C for 2 h. Subsequently, 0.3 mL of TiO2−ethanol suspension (1 mg mL−1) was dropped on a 22 × 22 mm glass slide freshly cleaned with nitrogen gas and dried at ambient temperature for 24 h. Then, TiO2 microspheres were partially covered with platinum by ion sputtering for 200 s with a current of 40 mA. Finally, the TiO2/Pt Janus microparticles were obtained by sonicating the glass slide in ethanol for 20 s and washing with deionized water (18.2 MΩ cm) several times. Preparation of 2,4-DNT, 2,4,6-TNT, and BBS Solution. The 2,4-DNT (97%, Sigma-Aldrich, Singapore) and 2,4,6-TNT (AccuStandard) were prepared as a stock solution in acetonitrile (99.9%, Merck, Singapore) with the concentration of 5 and 1 mM, respectively. Borate buffer (BBS, pH = 9.2−9.25) was prepared in three steps. First, 6.183 g of boric acid (H3BO3, Sigma-Aldrich, Singapore) was dissolved in 800 mL of deionized water. Second, the pH of the solution was adjusted to 9.2−9.5 with 5 mol L−1 of NaOH (97%, Sigma-Aldrich, Singapore) solution. Finally, the total volume of solution was increased to 1 L with deionized water. Operation of Micromotors. The motion study experiments of TiO2/Pt Janus micromotors in the presence of 2,4-DNT or 2,4,6TNT were performed in pure water using a glass slide freshly cleaned and dried with nitrogen gas. The UV light for activating the movement of micromotors is generated by a mercury lamp in the optical microscope. Photocatalytic Degradation and Electrochemical Detection of 2,4-DNT and 2,4,6-TNT. The photocatalytic degradation of 2,4DNT and 2,4,6-TNT was carried out in a 4 mL quartz cuvette. A mixture of 0.7 mL of 2,4-DNT (5 mM in acetonitrile), 1.75 mL of TiO2/Pt Janus particles (0.24 mg mL−1), and 1.05 mL of deionized water was placed into the cuvette. After standing in the dark field for 30 min to achieve an adsorption−desorption equilibrium and shaking for some time, UV irradiation with the intensity of 40 mW cm−2 from a xenon lamp was switched on. Subsequently, after 1 h of irradiation, the solution was centrifugation at 10 000 rpm for 10 min to remove the micromotors. Finally, the resulting solution above-obtained was diluted 10 times with BBS for cyclic voltammetry (CV) measurements. All CV measurements were performed with a scan rate of 50 mV s−1 in potential windows from 0 to −1.2 V. As previously reported,34 TiO2/Pt Janus micromotors were sensitive to high ion concentration, and cannot move when the ion concentration is beyond 10 mM. Therefore, we used BBS to substitute deionized water in the cuvette for creating stationary particles under UV irradiation. The blank controls of UV irradiation for 2,4-DNT solution or TiO2/ Pt suspension and the photocatalytic degradation with different concentrations of micromotors were tried in the same way as above. The 2,4,6-TNT degradation was processed in the same way as for 2,4DNT. In addition, the different concentrations of 2,4-DNT and 2,4,6TNT solutions were also measured. Because the measured solution with micromotors contains pure water, to ensure that all the conditions are the same, the measurements for different concentrations of 2,4-DNT and 2,4,6-TNT should supply the same percentage of pure water. Equipment. Sputtering was carried out with a JEOL JFC-1600 Auto Fine coater. Scanning electron microscopy (SEM) and energydispersive X-ray (EDX) spectroscopy were obtained by using a JEOL7600F semi-in-lens field-emission SEM instrument coupled to Oxford EDX. X-ray photoelectron spectroscopy (XPS) was performed with a Phoibos 100 spectrometer and a monochromatic Mg X-ray radiation source (SPECS). Dynamic light scattering (DLS) size distribution was obtained with a Zetasizer Nano (Malvern Instruments Ltd.) instrument. The UV light for activating movement of micromotors is generated by a mercury lamp in an optical microscope (Nikon Eclipse Ti-E microscope). Optical microscope videos and images were obtained with a Nikon Eclipse Ti-E microscope. The UV light for photodegradation of 2,4-DNT and 2,4,6-TNT in the cuvette is generated from a xenon lamp (7-Star Optical Instruments Ltd.,

intensity in 0.6 mM of 2,4,6-TNT solution (see Video S3 in the SI). The velocity was 8.5 ± 1.89 μm s−1 calculated from 40 different particles. Cyclic voltammograms in BBS and 0.6 mM of 2,4,6-TNT solutions were recorded (Figure 5b). In the presence of 2,4,6-TNT, three reduction peaks appear at −840, −676, and −529 mV that correspond to the reduction of nitro groups of 2,4,6-TNT.2,62 However, the CV in BBS shows the typical reduction peak of oxygen reduction (around −787 mV).2 The reduction peak at −529 mV was chosen for quantitative monitoring of the 2,4,6-TNT photodegradation using TiO2/Pt Janus micromotors (see details in the Experimental Section). Finally, the degradation efficiency of 2,4,6-TNT using TiO2/Pt micromotors was evaluated. The results of the degradation efficiency are shown in Figure 5c. The degradation efficiency for 2,4,6-TNT is 2.6% in the absence of the micromotors. In the presence of the TiO2/Pt micromotor, the degradation is much higher (16.8%). The experiment performed under stationary conditions (presence of TiO2/Pt Janus micromotors under UV irradiation but suppressed motion due to high ion concentration) with a degradation efficiency of ∼5.4% demonstrated the importance of enhanced mass transfer of the TiO2/Pt Janus micromotors during “on-the-fly” photodegradation of 2,4,6-TNT. The micromixing and enhanced mass transfer caused by movement are key for the high efficiency for decomposition of 2,4,6-TNT.

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CONCLUSIONS In conclusion, we have demonstrated highly efficient “on-thefly” photodegradation of 2,4-DNT and 2,4,6-TNT with UVlight-powered TiO2/Pt Janus micromotors in pure water. The mechanism of movement is self-electrophoresis by the asymmetric photocatalytic redox reactions over TiO2/Pt Janus particles. The TiO2/Pt Janus micromotors can still move quickly in a high concentration of 2,4-DNT (1000 μM) and 2,4,6-TNT (600 μM). The concentration of 2,4-DNT and 2,4,6-TNT can be accurately measured by cyclic voltammetry measurement with reduction peaks appearing at −629 and −529 mV, respectively. They can be degraded by photocatalytic reactions that occur on the TiO2/Pt Janus particle surface under UV irradiation, which manifested by the decrement of reduction peaks in CV measurements. Furthermore, the moving TiO2/Pt Janus micromotors show a higher degradation efficiency than stationary TiO2/Pt Janus particles because of the enhanced mixing and mass transfer in solution by movement. Such fuel-free light-powered micromotors for explosives decomposition shall find a way to environment remediation and security applications. To meet the requirements of practical applications, there still are some issues to be solved. The high concentration of ions or high viscosity in real polluted wastewater may slow and even stop the movement of self-electrophoretic micromotors. Thus, photocatalysts with higher photocatalytic activity should be developed. Additionally, cheaper cocatalysts can be used to replace Pt because of its high cost.

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EXPERIMENTAL SECTION

Synthesis of TiO2/Pt Janus Micromotors. TiO2 microspheres were prepared according to previous works.34,44 Briefly, 50 mL of ethanol (99.5%, Fischer, Singapore) was mixed with 0.2 mL of sodium chloride (99%, Sigma-Aldrich, Singapore) solution (0.1 M). After stirring for 5 min, 0.85 mL of tetrabutyl titanate (97%, Sigma-Aldrich, Singapore) was added into the above solvent. The stirring was stopped when the solution changed to emulsion and was stood in the F


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ACS Applied Materials & Interfaces Beijing, China). Voltammetry analyses were conducted at room temperature using an Autolab PGSTAT 101 electrochemical analyzer (Eco Chemie, Utrecht, The Netherlands) connected to a computer and controlled by NOVA version 1.0 software (Eco Chemie). The experiments were performed in an electrochemical cell using a threeelectrode system. Glassy carbon (GC, 3 mm in diameter) working electrode, platinum auxiliary electrode, and Ag/AgCl/1.0 M KCl reference electrode were obtained from CH Instruments (Austin, TX).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05776. Additional figures including DLS size distribution, highresolution XPS spectra, micromotor trajectories, cyclic voltammograms, current vs concentration, and stability test results (PDF) Video S1: motion of TiO2/Pt Janus micromotors in pure water under UV irradiation (AVI) Video S2: motion of TiO2/Pt Janus micromotors in 2,4DNT solution with concentrations of 1 μM, 10 μM, 100 μM, and 1 mM under UV irradiation (AVI) Video S3: motion of TiO2/Pt Janus micromotors in 2,4,6-TNT solution with a concentration of 0.6 mM under UV irradiation (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jianguo Guan: 0000-0002-2223-4524 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the project Advanced Functional Nanorobots (reg. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). The authors acknowledge A*Star Grant SERC A1783c0005 (Singapore). L.K. acknowledges the Scholarship Fund from China Scholarship Council (CSC 201606950043).

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REFERENCES

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