Platinum Janus Micromotors with Spatially Separated

Subscriber access provided by Gothenburg University Library

Article

Zero-valent iron/platinum Janus micromotors with spatially separated functionalities for efficient water decontamination Chung-Seop Lee, Jianyu Gong, Da-Som Oh, Jong-Rok Jeon, and Yoon-Seok Chang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00223 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Nano Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Zero-Valent Iron/Platinum Janus Micromotors with Spatially Separated Functionalities for Efficient Water Decontamination

Chung-Seop Leea, Jianyu Gongb, Da-Som Oha, Jong-Rok Jeonc, and Yoon-Seok Changa,*

a

Division of Environmental Science and Engineering,

Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea b

School of Environmental Science and Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, PR China

c

Department of Agricultural Chemistry & Life Sciences, IALS,

Gyeongsang National University, Jinju 660-701, Republic of Korea

*Corresponding author’s contact information: Phone: +82-54-279-2281 Fax: +82-54-279-8299 Email: [email protected]

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

ABSTRACT Multifunctional

ZVI/Pt

Janus

bubble-propelled

micromotors

with

high

decontamination efficiency and efficient self-propulsion properties were fabricated by asymmetric deposition of catalytic platinum (Pt) in one hemisphere of zero-valent iron (ZVI) microspheres. In the ZVI/Pt micromotors-H2O2 system, ZVI acts as a heterogeneous Fenton-like catalyst for the degradation of organic pollutants, while simultaneously, the hemispheric Pt layer catalytically decomposes hydrogen peroxide (H2O2) into water and oxygen, thereby resulting in an oxygen-bubble propulsion system. The ZVI/Pt Janus micromotors were bubble-propelled at the high speed of over 200 µm/s in the presence of 5% H2O2. In addition, complete oxidative degradation of methylene blue (MB) occurred in the presence of 5% H2O2 after 60 min of treatment, whereas ZVI microspheres removed only 12% of MB in 60 min. The magnetic controllable and reusable properties of the ZVI/Pt micromotors make the water purification process more attractive and feasible. Therefore, the application of ZVI with unique redox chemistry to a micromotor system could hold great promise for developing innovative water purification and remediation technologies in the future.

Keywords: Micromotor; Zero-valent iron; Platinum; Bubble propulsion; Fenton reaction; Water decontamination;


Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Water pollution due to a wide range of organic pollutants is a serious environmental problem for the global concerns. It is therefore essential to investigate the use of efficient catalytic materials to remove hazardous and toxic pollutants from potential water resources. Chemically powered micro/nanomotors are capable of effectively degrading a broad range of environmental contaminants while being selfpropelled in a fluid (i.e., fuel), and have recently attracted considerable interest.1-4 Various self-propelled micro/nanomotors have thus been developed based on the mechanisms of self-electrophoresis,5-7 self-diffusiophoresis,8-10 and bubble-ejection propulsion.11-13 Among these, particular attention has been directed to autonomous bubble-propelled micromotors for which the mechanism involves the ejection of bubbles by the catalytic decomposition of the fuel, which leads to an efficient autonomous motion. Platinum (Pt) is one of the most efficient catalytic materials that has been extensively used to fabricate differently-shaped bubble-propelling micro/nanomotors including bi- or trimetallic nanowires,14-16 tubular microjets,11,17,18 and spherical Janus particles12,19,20 because Pt can catalyze the decomposition of hydrogen peroxide (H2O2) fuel. The catalytic decomposition of H2O2 over Pt produces oxygen bubbles (H2O2 ! H2O + 1/2O2), generating a driving force and providing the thrust for the forward propulsion of Pt-loaded micromotors. Such bubble-induced propulsion of micromotors holds considerable promise for the development of innovative remediation technologies because these micromotors do not require external energy for their motion and can operate in complex channel



environments21 where other traditional environmental catalysts cannot easily reach. Thus, various remediation applications based on the bubble-propelled micromotors, involving the adsorption of heavy metals,12 removal of oil,22 photocatalytic degradation of warfare agents,23 and oxidative degradation of pollutants,11 have been demonstrated. To accomplish such specific tasks, it is desirable to construct a spatial structure divided into different regions (i.e., bilayer and Janus structure). The combination of a functional material with Pt that acts as a chemical engine is a general strategy because a variety of functionalities can be introduced by the material selection. Therefore, a wide range of materials, including activated carbon,12 graphene,2 and metal oxide (Zn, Ti, Fe, Si),11,24-26 were employed with Pt to fabricate multifunctional micromotors. Zero-valent iron (ZVI) is a promising catalytic material for environmental remediation because ZVI is capable of effectively degrading a broad range of environmental contaminants and can be easily synthesized in huge quantities. Wang et al. have reported the self-propelled ZVI nanoparticles, which generate H2 bubbles for propulsion.27 However, they required as high as 27.2 wt% of citric acid to generate H2 bubbles, and easily form micro-sized aggregates. Despite their unique potential, simple single-component ZVI nanoparticles also have inherent limitation in performing both bubble-propulsion and particular remedial task in one particle. To overcome these limitations, herein, we develop a half-Pt-coated ZVI micromotor (denoted as ZVI/Pt Janus micromotor), by taking advantage of the useful function of Pt that enables the self-propulsion combined with the initiation of a Fenton-type reaction for water purification on the surface of ZVI. ! The use of ZVI as a reagent for the Fenton reaction (the reaction between Fe2+ and


Page 4 of 29

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2O2) instead of iron (II, III) salts or iron oxides is a promising strategy for water treatment because the generation of in-situ ferrous ions in ZVI/H2O2 systems could initiate a homogenous Fenton reaction in acidic conditions (Eqs. 1-2). Fe0 + H2O2 + 2H+ ! Fe2+ + 2H2O

(1)

Fe2+ + H2O2 ! Fe3+ + OH" + •OH

(2)

The hydroxyl radical (•OH, E#("OH/•OH) = 2.7 VNHE) produced by the Fenton reaction is well-known as a strong oxidant and can rapidly oxidize many recalcitrant organic pollutants. The main advantages of using ZVI as a heterogeneous Fenton-like catalyst in micromotor system are as follows: (i) regeneration of ferrous iron on the ZVI surface, thus maintaining continuous homogeneous Fenton reaction (Eq. 3), (ii) H2O2 generation by the corrosion of Fe0 to Fe2+ accompanied by two-electron transfer to dissolved oxygen (Eq. 4), initiating the Fenton reaction even without high amount of H2O2 ,28 and (iii) the recyclability of catalyst for reuse. 2Fe3+ + Fe0 ! 3Fe2+

(3)

Fe0 + O2 + 2H+ ! Fe2+ + H2O2

(4)

Considering these reaction mechanisms and thereby combining ZVI and Pt for the structural design of spherical Janus particles, the new ZVI/Pt Janus micromotors are expected to utilize both the asymmetrical catalytic decomposition of H2O2 by the hemispheric Pt layer for oxygen-bubble propulsion and the Fenton-type reaction by ZVI for the efficient removal of pollutants in the presence of H2O2. In this study, we will discuss the fabrication and characterization of the ZVI/Pt Janus micromotors and



will demonstrate their highly efficient self-propulsion and remarkable water decontamination efficiency using methylene blue (MB) as a model pollutant.

MATERIALS AND METHODS

Materials. Triton X-100 (TX-100), Isopropanol (IPA), sodium fluoride (NaF), 1,4benzoquinone (BQ), trifluoroacetic acid (TFA), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 1,10-phenanthroline, and methylene blue (MB) were supplied by SigmaAldrich (USA). Hydrogen peroxide (H2O2) was purchased from Merck (Germany). ZV-Iron Microsphere 200 (composition: 97.5% Fe, 1.0% C, 1.0% N, and 0.5% O), a microscale zero-valent iron (ZVI), was purchased from BASF (Germany). Ultrapure water obtained from a water purification system (Millipore, France) with the specific resistivity of >18 M$ cm was used in all trials.

Synthesis of the ZVI/Pt Janus Micromotors. The ZVI/Pt Janus micromotors were prepared using ZVI (Fe0) microspheres as the base particles. The overall fabrication procedure for the preparation of the ZVI/Pt Janus micromotors is described in Figure S1. The surface of ZVI microspheres might be partially occupied by oxides, and therefore, acid washing of the ZVI microspheres was conducted with 1.0 M HCl to remove the surface oxides. Afterward, the acid-washed ZVI microspheres were rinsed several times by de-aerated distilled water. Acid-washed ZVI microspheres were dispersed into degassed isopropanol solution, then spread onto a square 1 mm-thick borosilicate glass substrate (75 mm % 25 mm) and coated with a 50 nm thick Pt layer using an electron beam evaporator (KVE-C300160, Korea Vacuum Tech). The deposition was performed at room temperature with a beam power of 1.37 kW and a


Page 6 of 29

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pressure of 7 % 10-6 Torr. A Pt layer of 50 nm was deposited at a rate of 1 Å s"1, monitored by a quartz crystal sensor equipped with a deposition controller software (Sigma Instrument, SQM-242). After a brief sonication of the substrates in degassed water, the ZVI/Pt micromotors were detached from the glass substrate and suspended in water. The ZVI/Pt micromotors were magnetically separated from the solution and vacuum dried at 60"C.

Characterization. The morphology and size of the ZVI/Pt Janus micromotors were observed by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX, JEOL JSM-7401F). The crystal structure of the ZVI/Pt Janus micromotors was examined before and after Pt deposition by X-ray diffraction (XRD, Rigaku Corporation, Japan) analysis with Cu K& radiation operating at 40 kV voltage and 100 mA current. X-ray photoelectron spectroscopy (XPS, Thermo Scientific KAlpha XPS spectrometer) was used to elucidate the elemental characteristics of the deposited platinum. Electron spin resonance spectrometer (ESR, A200 Bruker) was used to characterize the reactive oxygen species (ROS) generated during the reaction.

Batch Experiments and Analysis. Methylene blue (MB) degradation experiments were performed in a glass Petri dish (diameter 60 mm, height 15 mm) containing 5 mL of MB solution (C0 = 30 mg/L) with 0.01% TX-100. Compared to other studies, a low concentration of TX-100 (0.01%) was used to reduce toxic effect of surfactants.3,22,23 The initial pH was adjusted to 3.0, which is the optimal pH for the Fenton reaction catalyzed by ZVI.28 After optimization of the initial dosage of micromotors (Figure S2), a batch experiment was initiated by placing 10 mg of ZVI/Pt Janus micromotors on the solution surface. A series of experiments in a static



(un-agitated) condition were undertaken with varying H2O2 concentrations (0, 1, 3, and 5%). At given sampling time intervals, MB concentration was periodically measured by a UV-vis spectrophotometer (Varian, Palo Alto, CA, USA) at the maximum absorbance wavelength of 664 nm for MB. TX-100 was used as surfactant to facilitate bubble ejection from the micromotors.22 A control test including an H2O2 only solution was performed in parallel to evaluate the loss of MB by unintended reactions. Repetition experiments were also performed in order to examine the long-term reactivity of the ZVI/Pt micromotors by exposing them to four sequential spikings of MB (30 mg/L) at each 60 min intervals. In each cycle, the ZVI/Pt micromotors were propelled in the MB-contaminated water for the dye degradation. Then, the ZVI/Pt micromotors were collected using a magnet, washed with degassed water, and reused for the subsequent cycles without any rejuvenation treatments. The ferrous ion released from the ZVI/Pt micromotors in the solution was measured by the 1,10-phenanthroline method. The motions of the ZVI/Pt Janus micromotors were observed and captured by using an optical microscope (OM, Olympus MX51) with 20% and 10% objectives. Trajectories of the ZVI/Pt micromotors were tracked by a Java based image processing program, Image J (National Institutes of Health, USA).


Page 8 of 29

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION Characterization of the ZVI/Pt Janus Micromotors. Figures 1a and S3a show the spherical morphology of the ZVI/Pt Janus micromotor in the ~1 to 5 !m diameter range corresponding to the size of the used ZVI microspheres. The EDX mapping analysis illustrates the distribution of iron and platinum within the Janus spherical micromotor (Figure 1b). The EDX data also indicate that the microsphere has a distinct Fe-Pt binary heterostructure, with a hemispheric Pt layer coating half of the ZVI microsphere. We found that the ZVI/Pt micromotor is a nanocomposite in which the Pt nanoparticles are uniformly distributed on the ZVI surface (Figure S3b); this arrangement leads to dramatically increased catalytic surface area and improved movement behavior.12 Three prominent peaks were observed at 2! = 44.8°, 65.1° and 82.4°, respectively, in the diffraction pattern of the hemispheric Fe0 layer (Figure 1c), corresponding to the (110), (200) and (211) planes of body centered cubic &-Fe0. On the other hand, the XRD pattern for the hemispheric Pt layer (Figure 1c) shows the prominent Pt crystallite planes to be (111), (200), (220), (311), and (222) faces corresponding to the 2' values of 40.5°, 46.9°, 68.5°, 81.3°, and 86.6°, respectively, indicating a face-centered cubic (FCC) lattice structure characteristic of the Pt crystal. After Pt deposition, most of the diffraction peaks arising from ZVI microspheres were




Page 10 of 29

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Oxygen-Bubble Propulsion of the ZVI/Pt Micromotors. The autonomous oxygen bubble-driven movement of the ZVI/Pt micromotors was studied by an optical microscope. Figure 2a shows the consecutive optical microscope images taken from Video S1. Figure 2a displays the propulsion behavior of the ZVI/Pt micromotors and their tracking trajectories in the presence of 5% H2O2 over a 960 ms period. The circular motion of Janus micromotors was mainly observed (Videos S1-3), indicating that a recoil force (Fdrive) originating from the momentum change induced by the detachment of oxygen bubbles was not aligned with the drag force (Fdrag), which in turn introduced a torque.29 In addition, distinct tails of oxygen bubbles ejected continuously from the Pt side of the ZVI/Pt micromotors were clearly visualized, reflecting the spontaneous reaction of Pt with the surrounding H2O2. Such generation of oxygen bubbles was believed to induce a strong momentum that overcomes the Brownian motion, thus propelling the ZVI/Pt micromotors forward at a high speed.20 As expected, the speed of the ZVI/Pt Janus micromotors was strongly dependent on the hydrogen peroxide concentration (Figure 2b). The speed increased from 71.82 to 132.29 and 207.89 µm/s using 1, 3, and 5% peroxide fuel, respectively, indicating that the higher concentration of H2O2 accelerated the O2 bubble evolution. By contrast,



the ZVI microspheres barely moved with only Brownian motion in the presence of 5% H2O2 at 10% magnification (Figure S5a), demonstrating that the most dominant mechanism of micromotor propulsion was the Pt-catalyzed O2 bubble generation by the spontaneous catalytic reaction of Pt with the surrounding peroxide fuel. Furthermore, the mean square-displacement (MSD) was analyzed by tracking the ZVI/Pt micromotors to systematically investigate their self-propelled behavior. After obtaining the tracking trajectory of a micromotor, the MSD was calculated by the following equation MSD = , where x((t) and y((t) are the coordinates of the micromotor in the plane of motion after a fixed time interval (t. Figure 2c compares the MSD of the tracer microparticles over a 3.0 s period in different systems of ZVI micromotors-H2O2 and ZVI/Pt micromotors-H2O2. The MSD plot of the ZVI micromotors in this figure shows a straight line, indicating that the insignificant motion of the particles can be attributed solely to diffusion. In contrast, the upward curvature for the ZVI/Pt micromotors under the same experimental conditions reflects the effective bubble propulsion instead of Brownian motion.30 Therefore, it demonstrates the significantly enhanced diffusion of the ZVI/Pt micromotors due to the bubble-propelled motion, compared to the ZVI micromotors undergoing diffusion only. Finally, these data clearly demonstrate the advantages of the ZVI/Pt micromotors toward enhanced fluid transport and solution mixing.31 Such a movement of the ZVI/Pt micromotors across the solution, along with the long tail of microbubbles, was expected to provide micro-mixing and enhanced mass transfer, leading to enhanced water purification efficiency.12


Page 12 of 29

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




trajectory assisted by the ZVI/Pt micromotors. Video S1 shows the movement of the ZVI/Pt micromotors in 5% H2O2. (b) Dependence of the speed of the ZVI/Pt micromotors at various H2O2 concentrations. (c) Plots of MSD for the ZVI microspheres and the ZVI/Pt micromotors in 5% H2O2. The inset is tracking line (taken from Video S2) illustrating the motion and distance of the ZVI/Pt micromotors during a period of 3.0 s in 5% H2O2. Catalytic Degradation of MB. To prove the decontamination capability of the ZVI/Pt micromotors, we examined their catalytic activity for the degradation of MB used as a model pollutant. Figure 3a shows the dye degradation efficiency in different H2O2 concentrations. The removal of MB increased gradually by increasing the H2O2 concentration and the highest degradation efficiency was observed at 5% of H2O2. High concentrations of H2O2 allowed the ZVI/Pt micromotors to move faster across a contaminated solution and, consequently, the mass transfer of the reagents involved in the Fenton reaction was greatly improved due to continuous mixing,11 leading to the immediate degradation of MB. At H2O2 concentration )3%, the ZVI/Pt micromotors were active for at least 1 h during MB degradation. Interestingly, MB was slightly degraded by the ZVI/Pt micromotors in the absence of H2O2 (Figure 3a), even though there was no noticeable movement of the ZVI/Pt micromotors (Figure S5b). The MB degradation mechanism by the ZVI/Pt micromotors without H2O2 could be hypothesized by the in-situ generation of hydrogen peroxide (Eq. 4) and the subsequent Fenton reaction (Eq. 2). Under oxidizing conditions, the corrosion of Fe0 to Fe2+ accompanied by two-electron transfer to dissolved oxygen (E#(Fe2+/Fe0) = * 0.447 VNHE and E#(O2/H2O2) = +0.695 VNHE) produces H2O2, which is converted into •OH radicals by reaction with Fe2+ (Eq. 2), thereby enabling a slight MB degradation.28


Page 14 of 29

Page 15 of 29

(b)

1.0

1.0

0.8

0.8

0.6

0.6

C/C0

(a)

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZVI microspheres ZVI/Pt micromotors Control (only H2 O2 )

0.4

0.4 0% H2O2

0.2

0.2

1% H2O2 3% H2O2 5% H2O2

0.0 0

10

20

0.0 30

40

50

60

0

Time (min)

10

20

30

40

50

60

Time (min)

Figure 3. (a) Removal of MB in aqueous solutions with the ZVI/Pt micromotors at various concentrations of H2O2. (b) Removal of MB with the ZVI microspheres and the ZVI/Pt micromotors in 5% H2O2 solutions (ZVI/Pt micromotors = 10 mg; [TX100] = 0.01%; [MB]0 = 30 mg/L; pHi = 3.0).

As shown in Figures 3b and S6, the presence of the ZVI/Pt Janus micromotors greatly improves the degradation of MB compared with the result using the ZVI microspheres only. A complete removal of MB was achieved after 60 min of treatment using the ZVI/Pt Janus micromotors, but only 12% of MB was degraded in 60 min using the ZVI microspheres. The observed rate constants (kobs) of the ZVI microspheres and the ZVI/Pt micromotors were estimated to be 0.0031 and 0.0446 min-1, respectively, i.e., fourteen times faster removal of MB took place when the Fenton reaction occurred with the ZVI/Pt micromotors. The control experiment demonstrated that the MB concentration in the presence of 5% H2O2 remained almost constant and unchanged without the ZVI/Pt micromotors or ZVI microspheres,



indicating that this hydrogen peroxide fuel itself had no significant effect on the MB degradation. Reaction Mechanism. To obtain insight into the contribution of oxidative species to the oxidative MB degradation by the ZVI/Pt micromotors, a scavenging experiment was conducted by adding radical scavengers including BQ, IPA, NAF, and TFA to investigate the roles of superoxide radicals (O2•"),32 •OH generated in the bulk solution (•OHbulk)33, •OH on the surface of the ZVI/Pt micromotors (•OHads),33 and electrons (e"),34 respectively.

3.0

(b)

Control IPA NaF BQ TFA

2.5

2.0

1% H 2O2 3% H 2O2 5% H 2O2

Intensity (a.u.)

(a)

-ln(C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

1.5

1.0

0.5

0.0 10

20

30

40

50

60

Time (min)

330

332

334

336

338

340

Magnetic field (mT)

Figure 4. (a) Inhibition kinetics of MB degradation by the ZVI/Pt micromotors in the presence of various ROS scavengers. (b) ESR spectra of DMPO-•OH adducts in the ZVI/Pt micromotors-H2O2 system (ZVI/Pt micromotors = 10 mg; [H2O2] = 5%; [TX100] = 0.01%; [MB]0 = 30 mg/L; pHi = 3.0).

Figure 4a shows the MB degradation kinetics by the ZVI/Pt micromotors in the presence of each scavenger. When TFA was added to the solution, the degradation


Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


efficiency decreased slightly after 60 min, proving that electrons were not critical in the whole reaction process. However, the MB degradation efficiency decreased in the presence of BQ and NaF from 100% to 60% and 66% after 60 min, demonstrating that the O2•" and •OHads might be the active and potential ROS contributing to the reactions in this system. In the presence of IPA, the degradation efficiency of MB was significantly suppressed after 60 min of reaction, implying that •OHbulk were the most important oxidants responsible for the oxidative degradation of MB. Similar results were also observed in previous study showing that the O2•" and •OH were involved in the oxidative mechanism of the ZVI-assisted Fenton system.28 Therefore, the hydroxyl radicals expected on the basis of Fenton reaction (Eq. 2) and other reactive oxygen species accompanied by •OH in the ZVI/Pt micromotorsH2O2 system were further analyzed using ESR spectroscopy. DMPO was employed to identify the hydroxyl radicals, thus leading to the formation of spin-adducts DMPO•OH.35 The ESR spectra of DMPO-•OH adducts in the ZVI/Pt micromotors-H2O2 system as functions of H2O2 concentration are presented in Figure 4b. No characteristic signals of DMPO-•OH adducts were detected in the ZVI microspheresH2O2 system with H2O2 concentrations below 3%. Only the weak signals of DMPO•OH adducts were observed when the H2O2 concentration was raised to 5%, indicating that only a small amount of •OH was generated. Conversely, the characteristic signals of DMPO-•OH adducts with relative intensities of 1:2:2:1 were clearly observed in the ZVI/Pt micromotors-H2O2 system. Moreover, the peak intensity of DMPO-•OH in the ZVI/Pt micromotors-H2O2 system increased with the increase in the H2O2 concentration, suggesting that the •OH formation was enhanced by increasing the H2O2 concentration. Thus, the H2O2 concentration in the ZVI/Pt micromotors-H2O2 system was highlighted as the primary factor responsible for the




Page 18 of 29

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


•OH + H2O2 ! HO2• + H2O

k2 = 2.7 %107 M-1 s-1

(8)

HO2• ! H+ + O2•"

k2 = 1.6 %105 M-1 s-1

(9)

Fe2+ + O2 ! Fe3+ + O2•"

k2 = 5 %107 M-1 s-1

(10)

In the proposed reaction mechanism, the hydroxyl radicals are produced by the Fenton reaction (Eq. 2) and the peroxidase catalyzed reaction (Eq. 5). Fe2+ can be regenerated through the reduction of Fe3+ with H2O2 (Eq. 6, Fenton-like reaction) and with hydroperoxyl radicals (HO2•) (Eq. 7). The established iron redox cycle (Eqs. 1-4, 6-7) promotes the •OH radical generation. Although the generation pathways of the superoxide radicals cannot be clearly identified from our experimental results, the possibilities include the reaction between the hydroxyl radical and the excessive hydrogen peroxide (Eqs. 8-9) or the oxygenation of Fe2+ (Eq. 10).

Mixing Effect. Based on the aforementioned test results, the differences in the MB oxidation efficiency between the ZVI microspheres and the ZVI/Pt micromotors could be attributed to the micromotor-induced fluid mixing. Therefore, the mixing effect of the ZVI/Pt micromotors was investigated using an ink (Reactive black 5) to visualize the diffusion.11 The different distributions of ink in 5% H2O2 are shown over 40 min in Figure 5. In the control experiments, mixing of ink and H2O2 solution occurred entirely due to the diffusion of ink molecules (Brownian motion dominates the movement of ink molecules). A similar result was observed with the ZVI microspheres in the 5% H2O2 solution, indicating that the effect of only ZVI microspheres on the ink diffusion was negligible. However, in the solution containing the ZVI/Pt micromotors, the ink was spread out much faster and over larger area than




Page 20 of 29

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solution containing ZVI microspheres (center row), and solution containing the ZVI/Pt micromotors (bottom row) (ZVI/Pt micromotors = 5.0 mg; ink = 10 !L; pHi = 3.0).

Magnetic Guidance and Reusability of the ZVI/Pt Micromotors. From a practical point of view, the direction control and reusability of the catalytic ZVI/Pt micromotors should be very important. Control of the directionality of micromotors can be achieved by external stimuli including light40, magnetic fields22 and ultrasound41. In this study, the ZVI/Pt micromotors were manipulated by an external magnetic field due to the magnetic nature of ZVI in the ZVI/Pt micromotors. To demonstrate this, an external weak magnetic field was applied by placing a neodymium magnet 1.0 cm away near the micromotors as they were moving irregularly in the 5% H2O2 solution. The ZVI/Pt micromotors underwent random motion as depicted in Figure 6a (left column); however, under the external magnetic field, the ZVI/Pt micromotors followed a predetermined destination under the magnetic guidance as depicted in Figure 6a (right column). In addition, we have found that the magnet seems to have little influence on the micromotor speed, indicating that the propulsion of the ZVI/Pt micromotors is primarily due to the bubble ejection instead of the magnetic field, which is consistent with previous study.42 Therefore, such magnetic guidance could allow the ZVI/Pt micromotors acting as the smart magnetic controllable microcleaners that can navigate within the complex channel networks. Further, reusability and durability of the ZVI/Pt micromotors against potential inactivation were assessed because a reusable catalyst is important for the cost-




Page 22 of 29

Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


column) of an external magnetic field (i*iv). The scale bars are 10 µm. (b) The reusability of the ZVI/Pt micromotors over subsequent cycles ([H2O2] = 5%; ZVI/Pt micromotors = 10 mg; [TX-100] = 0.01%; [MB] = 30 mg/L; pHi = 3.0).

Conclusions. The ZVI/Pt micromotors-H2O2 system has proven to be an efficient for oxidation of MB, and is superior to the ZVI microspheres-H2O2 system. Due to the dual functionality of H2O2, which acts as both an important •OH precursor and as a main fuel for the autonomous self-propulsion of micromotors, a highly synergistic effect toward a more efficient oxidative degradation of MB was achieved in the ZVI/Pt micromotors-H2O2 system without external energy. The ZVI/Pt micromotors can be magnetically guided to their target pollutants by an external magnetic field, and can be reused several times. Most usefully, these ZVI/Pt micromotors can operate in small-scale environments (e.g., narrow pipes) that are difficult to reach using traditional methods, and can perform localized and effective decontamination. These results indicate that future efforts should try to eliminate the need for H2O2, and to replace expensive Pt to meet the requirements for practical applications. To date, the redox reaction of ZVI such as hydrogen gas production by the acid-metal reaction, and H2O2 production by the two-electron transfer reaction have been reported, so further improvements can be expected to develop ZVI-based micromotors that can propel without noble metals, or that can generate their own fuel.

ASSOCIATED CONTENT Supporting Information Supporting Information is available: Videos showing the movement of ZVI/Pt micromotors in 5% hydrogen peroxide (ZIP)



Fabrication procedure of the ZVI/Pt Janus micromotors, characterization of the ZVI/Pt micromotors, optical images of the ZVI microspheres in the presence of 5% H2O2 and the ZVI/Pt micromotors in anoxic water, and UV-vis absorption spectra of the degradation of MB using the ZVI microspheres and the ZVI/Pt micromotors (PDF)

ACKNOWLEDGMENTS# This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2B3012681) and ‘‘The GAIA Project” by Korea Ministry of Environment (RE201402059).

REFERENCES (1)

Soler, L.; Sánchez, S., Catalytic Nanomotors for Environmental Monitoring

and Water Remediation. Nanoscale 2014, 6, 7175-7182. (2)

Orozco, J.; Mercante, L. A.; Pol, R.; Merkoçi, A., Graphene-Based Janus

Micromotors for the Dynamic Removal of Pollutants. J. Mater. Chem. A 2016, 4, 3371-3378. (3)

Uygun, D. A.; Jurado-Sánchez, B.; Uygun, M.; Wang, J., Self-Propelled

Chelation Platforms for Efficient Removal of Toxic Metals. Environ. Sci.: Nano 2016, 3, 559-566. (4)

Zhang, Q.; Dong, R.; Wu, Y.; Gao, W.; He, Z.; Ren, B., Light-Driven Au-

WO3@C Janus Micromotors for Rapid Photodegradation of Dye Pollutants. ACS Appl. Mater. Interfaces 2017, 9, 4674-4683. (5)

Wang, Y.; Hernandez, R. M.; Bartlett Jr, D. J.; Bingham, J. M.; Kline, T. R.;

Sen, A.; Mallouk, T. E., Bipolar Electrochemical Mechanism for the Propulsion of Catalytic Nanomotors in Hydrogen Peroxide Solutions. Langmuir 2006, 22, 10451-


Page 24 of 29

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10456. (6)

Pumera, M., Electrochemically Powered Self-Propelled Electrophoretic

Nanosubmarines. Nanoscale 2010, 2, 1643-1649. (7)

Paxton, W. F.; Baker, P. T.; Kline, T. R.; Wang, Y.; Mallouk, T. E.; Sen, A.,

Catalytically Induced Electrokinetics for Motors and Micropumps. J. Am. Chem. Soc. 2006, 128, 14881-14888. (8)

Sabass, B.; Seifert, U., Dynamics and Efficiency of a Self-Propelled,

Diffusiophoretic Swimmer. J. Chem. Phys. 2012, 136, 064508-1"064508-16. (9)

Chaturvedi, N.; Hong, Y.; Sen, A.; Velegol, D., Magnetic Enhancement of

Phototaxing Catalytic Motors. Langmuir 2010, 26, 6308-6313. (10)

Yang, F.; Qian, S.; Zhao, Y.; Qiao, R., Self-Diffusiophoresis of Janus

Catalytic Micromotors in Confined Geometries. Langmuir 2016, 32, 5580-5592. (11)

Soler, L.; Magdanz, V.; Fomin, V. M.; Sanchez, S.; Schmidt, O. G., Self-

Propelled Micromotors for Cleaning Polluted Water. ACS Nano 2013, 7, 9611-9620. (12)

Jurado-Sánchez, B.; Sattayasamitsathit, S.; Gao, W.; Santos, L.; Fedorak, Y.;

Singh, V. V.; Orozco, J.; Galarnyk, M.; Wang, J., Self-Propelled Activated Carbon Janus Micromotors for Efficient Water Purification. Small 2015, 11, 499-506. (13)

Safdar, M.; Wani, O. M.; Jänis, J., Manganese Oxide-Based Chemically

Powered Micromotors. ACS Appl. Mater. Interfaces 2015, 7, 25580-25585. (14)

Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St. Angelo, S. K.; Cao,

Y.; Mallouk, T. E.; Lammert, P. E.; Crespi, V. H., Catalytic Nanomotors: Autonomous Movement of Striped Nanorods. J. Am. Chem. Soc. 2004, 126, 1342413431. (15)

Sattayasamitsathit, S.; Gao, W.; Calvo-Marzal, P.; Manesh, K. M.; Wang, J.,

Simplified Cost-Effective Preparation of High-Performance Ag-Pt Nanowire Motors.



Chemphyschem 2010, 11, 2802-2805. (16)

Zhao, G.; Pumera, M., Magnetotactic Artificial Self-Propelled Nanojets.

Langmuir 2013, 29, 7411-7415. (17)

Solovev, A. A.; Sanchez, S.; Pumera, M.; Mei, Y. F.; Schmidt, O. C.,

Magnetic Control of Tubular Catalytic Microbots for the Transport, Assembly, and Delivery of Micro-Objects. Adv. Funct. Mater. 2010, 20, 2430-2435. (18)

Gao, W.; Sattayasamitsathit, S.; Orozco, J.; Wang, J., Efficient Bubble

Propulsion of Polymer-Based Microengines in Real-Life Environments. Nanoscale 2013, 5, 8909-8914. (19)

Wheat, P. M.; Marine, N. A.; Moran, J. L.; Posner, J. D., Rapid Fabrication of

Bimetallic Spherical Motors. Langmuir 2010, 26, 13052-13055. (20)

Mou, F.; Chen, C.; Ma, H.; Yin, Y.; Wu, Q.; Guan, J., Self-Propelled

Micromotors Driven by the Magnesium-Water Reaction and Their Hemolytic Properties. Angew. Chem., Int. Ed. 2013, 52, 7208-7212. (21)

Zhao, G.; Pumera, M., Marangoni Self-Propelled Capsules in a Maze:

Pollutants 'Sense and Act' in Complex Channel Environments. Lab Chip 2014, 14, 2818-2823. (22)

Gao, W.; Feng, X.; Pei, A.; Gu, Y.; Li, J.; Wang, J., Seawater-Driven

Magnesium Based Janus Micromotors for Environmental Remediation. Nanoscale 2013, 5, 4696-4700. (23)

Li, J.; Singh, V. V.; Sattayasamitsathit, S.; Orozco, J.; Kaufmann, K.; Dong,

R.; Gao, W.; Jurado-Sanchez, B.; Fedorak, Y.; Wang, J., Water-Driven Micromotors for Rapid Photocatalytic Degradation of Biological and Chemical Warfare Agents. ACS Nano 2014, 8, 11118-11125. (24)

Dong, R.; Wang, C.; Wang, Q.; Pei, A.; She, X.; Zhang, Y.; Cai, Y., ZnO-


Page 26 of 29

Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Based Microrockets with Light-Enhanced Propulsion. Nanoscale 2017, 9, 1502715032. (25)

Mou, F.; Kong, L.; Chen, C.; Chen, Z.; Xu, L.; Guan, J., Light-Controlled

Propulsion,

Aggregation

and

Separation

of

Water-Fuelled

TiO2/Pt

Janus

Submicromotors and Their "on-the-Fly" Photocatalytic Activities. Nanoscale 2016, 8, 4976-4983. (26)

Zhang, Q.; Dong, R.; Chang, X.; Ren, B.; Tong, Z., Spiropyran-Decorated

SiO2-Pt Janus Micromotor: Preparation and Light-Induced Dynamic Self-Assembly and Disassembly. ACS Appl. Mater. Interfaces 2015, 7, 24585-24591. (27)

Teo, W. Z.; Zboril, R.; Medrik, I.; Pumera, M., Fe0 Nanomotors in Ton

Quantities (1020 Units) for Environmental Remediation. Chem. - Eur. J. 2016, 22, 4789-4793. (28)

Keenan, C. R.; Sedlak, D. L. Factors Affecting the Yield of Oxidants from

the Reaction of Nanoparticulate Zero-Valent Iron and Oxygen. Environ. Sci. Technol. 2008, 42, 1262-1267. (29)

Hu, L.; Miao, J.; Grüber, G., Temperature Effects on Disk-Like Gold-Nickel-

Platinum Nanoswimmer's Propulsion Fuelled by Hydrogen Peroxide. Sens. Actuator B-Chem. 2017, 239, 586-596. (30)

Dunderdale, G.; Ebbens, S.; Fairclough, P.; Howse, J., Importance of Particle

Tracking and Calculating the Mean-Squared Displacement in Distinguishing Nanopropulsion from Other Processes. Langmuir 2012, 28, 10997-11006. (31)

Orozco, J.; Jurado-Sánchez, B.; Wagner, G.; Gao, W.; Vazquez-Duhalt, R.;

Sattayasamitsathit, S.; Galarnyk, M.; Cortés, A.; Saintillan, D.; Wang, J., BubblePropelled Micromotors for Enhanced Transport of Passive Tracers. Langmuir 2014, 30 (18), 5082-5087.



(32)

Yin, M.; Li, Z.; Kou, J.; Zou, Z., Mechanism Investigation of Visible Light-

Induced Degradation in a Heterogeneous TiO2/Eosin Y/Rhodamine B System. Environ. Sci. Technol. 2009, 43, 8361-8366. (33)

Antonopoulou, M.; Karagianni, P.; Konstantinou, I., Kinetic and Mechanistic

Study of Photocatalytic Degradation of Flame Retardant Tris (1-chloro-2-propyl) phosphate (TCPP). Appl. Catal. B: Environ. 2016, 192, 152-160. (34)

Gong, J.; Lee, C.-S.; Chang, Y.-Y.; Chang, Y.-S., A Novel Self-Assembling

Nanoparticle of Ag–Bi with High Reactive Efficiency. Chem. Comm. 2014, 50, 85978600. (35)

Qi, C.; Liu, X.; Ma, J.; Lin, C.; Li, X.; Zhang, H., Activation of

Peroxymonosulfate by Base: Implications for the Degradation of Organic Pollutants. Chemosphere 2016, 151, 280-288. (36)

Liu, Y.; Wu, H.; Li, M.; Yin, J. J.; Nie, Z., pH Dependent Catalytic Activities

of Platinum Nanoparticles with Respect to the Decomposition of Hydrogen Peroxide and Scavenging of Superoxide and Singlet Oxygen. Nanoscale 2014, 6, 11904-11910. (37)

Samet, Y.; Mefteh, R.; Abdelhedi, R.; Savall, A., Improvement of the

Electrocatalytic Activity of Platinum in Oxidation of Aromatic Compounds. C. R. Chim. 2008, 11, 1254-1261. (38)

He, W.; Zhao, H.; Jia, H.; Yin, J. J.; Zheng, Z., Determination of Reactive

Oxygen Species from ZnO Micro-Nano Structures with Shape-Dependent Photocatalytic Activity. Mater. Res. Bull. 2014, 53, 246-250. (39)

Brillas, E.; Sirés, I.; Oturan, M. A., Electro-Fenton Process and Related

Electrochemical Technologies Based on Fenton's Reaction Chemistry. Chem. Rev. 2009, 109, 6570-6631. (40)

Safdar, M.; Simmchen, J.; Jänis, J., Light-Driven Micro- and Nanomotors for


Page 28 of 29

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Platinum Janus Micromotors with Spatially Separated

Recommend Documents