Fish-Scale-Like Intercalated Metal Oxide-Based Micromotors as

Subscriber access provided by ALBRIGHT COLLEGE

Surfaces, Interfaces, and Applications

Fish-Scales-Like Intercalated Metal-Oxide-Based Micromotors as Efficient Water Remediation Agents Wenjuan Liu, Hongbin Ge, Xiao Chen, Xiaolong Lu, Zhongwei Gu, Jinxing Li, and Joseph Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01095 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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 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 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.

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 25 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 Materials & Interfaces

Fish-Scales-Like Intercalated Metal-Oxide-Based Micromotors as Efficient Water Remediation Agents Wenjuan Liua*, Hongbin Gea, Xiao Chena, Xiaolong Lub*, Zhongwei Gua, Jinxing Lic*, Joseph Wangc a College of Materials Science and Engineering, Nanjing Tech University, Nanjing 211816, China b State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China c Department of NanoEngineering, University of California San Diego, La Jolla/CA 92093, USA

ABSTRACT With compelling virtues of large specific surface area, abundant active sites and fast interfacial transport, nanomaterials have been demonstrated to be an indispensable tool for water remediation applications. Accordingly, micro/nanomotors made by nanomaterials would also benefit from these properties. Though tuning the surface architecture on demand becomes a hot topic in the field of nanomaterials, there are still limited reports on the design of active surface architectures in chemically-driven tubular micro/nanomachines. Here a unique architecture composed of fish-scales-like intercalated (FSI) surface structure and active layer with 5 nm nanoparticles is constructed, which composes of Fe2O3 and ramsdellite MnO2, Mn2O3, in the tubular micromotor by using a versatile electrodeposition protocol. Tailoring the electrodeposition parameters enables us to modulate the active MnO2 surface structure on demand, giving rise to a pronounced propulsion performance and catalytic activity. Upon exposure to the azo-dye waste solution, the degradation efficacy greatly raises by around 22.5% with FSI micromotors treatment when compared to the normal compact motors, owning to the synergistic effect between Fe related Fenton reaction and a large catalytic area offered by the hierarchically rough inner surface. Such unique micromachines with large active surface area have great potential for environmental and biomedical applications. KEYWORDS: micromotors, architectured nanomaterials, active sites, MnO2, electrodeposition, water treatment

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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 becomes a global concern with the raising development of modernization.1 A strategy of advanced micro/nanomotors developed to tackle this conundrum has attracted considerable attentions in past years.2-4 Compared to the common methods, the propulsion of micro/nanomotors results in an efficient fluid mixing, thereby enhancing mass transfer which is usually dominated by passive diffusion in the micro scale.5-7 Up to now, micro/nanomotors have shown enormous promising perspectives in vast and varied chemical and biological species treatment, ranging from heavy metals ions, organic dyes, pharmaceutical pollutants, warfare agent residues, to bacterial and pathogenic microorganisms.8-14 Although investigation involving the fabrication, locomotion and functionalization of micro/nanomotors towards water remediation have been broadly reported, many of the researches remain at the proof-of-concept stage. The difficulty of practical water waste remediation with the least amount and lowest cost of the treatment materials, in other words, the invaders to the water system, is massive and still represents an unmet challenge. Therefore, design of micro/nanomotor emerges as a crucial factor in the upcoming water remediation applications. In this sense, choice of alternative materials to displace noble metals and construction of optimized surface structure turn into the typical and efficacious strategies. Tuning the functional materials and surface structure on demand has been extensively investigated in the field of nanomaterials, which brings to remarkable and unique advantages, relevant to numerous chemical and physical reactions in applications of catalysis, energy storage and conversion, environmental remediation, chemical sensing, biomedicine, etc.15-18 In the water remediation applications, the virtues of large specific surface area, abundant active sites for reactions, fast interfacial transport are highly desired. With the analogical constituent and geometry to nanomaterials, micro/nanomotors would correspondently benefit from the similar responsiveness. By far, tremendous efforts have been devoted to address those issues.19-28 On one hand, cost effective and eco-friendly materials become alternative candidates in fabricating micro/nanomotors, including activated carbon,19 MnO2,20,21


Page 2 of 25

Page 3 of 25 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


Fe 22,23 and even sporopollenin exine capsules extracted from sunflower pollen grains.24 On the other hand, optimizing surface structure of micro/nanomotors, especially extending the specific surface area, is capable of producing dramatic positive effects. For instance, reactive microporous Pt catalytic structures were created in one kind of graphene microengines and have led to an ultrafast bubble propulsion (as high as 170 body lengths/sec) and operation at extremely low levels (0.1%) of the peroxide fuel.25 Meanwhile, the outside mesoprous SiO2 layers in the tubular micromotors can effectively adsorb and collect the pollutants, and the adsorption capacity of the TiO2@mSiO2 tube is about 3 times higher than that of the TiO2 tube due to the presence of mSiO2 shell.26 In addition, a high surface area with a needle-like zirconia microstructure in the zirconia-graphene/Pt hybrid tubular micromotors is quite effective and selective to bind the organophosphate compounds.27 Modified existing porous materials (Silver-exchanged-Zeolite, MOF) as micro/nanomotors has been inspired and contributes to effective detoxification of chemical and biological threats.28 Since the first MnO2 micromotor reported in 2014 by Pumera,29 tremendous efforts have been made to explore its fabrication and functions. Janus micromotors composed of commercial MnO2 microparticles exhibited fast propulsion speed in H2O2 solution and have been utilized to remove the organic pollutants. 20,29 Later, tubular MnO2 based micromotors were obtained with either electrochemical deposition or chemical synthesis, demonstrating high efficacy in organic waste and heavy metals removal.21,30,31 Moreover, tubular MnO2 micromotors after surface functionalization are able to deliver drugs in biological media.32 In this paper, we demonstrate an affordable and simplified strategy to tune the surface structure of micro/nanomotors. MnO2 and Fe were also adopted as basic constitutes of our proposed tubular mircoengines due to their abundancy, convenient synthesis, and marvelous waste decontamination efficacy as confirmed in extensive researches.20,21,26,30,31 Recently, we disclosed the catalytic performance of PEDOT/MnO2 micromotors could be tailored by electrodeposition modes.33 Fast motion speed and highest degradation efficiency of azo-dye were achieved by micromotors in potentiostatic electrodeposition. Herein, we modulate the surface structure also with electrodeposition strategy. A mass production



of fish-scales-like intercalated (FSI) metal-oxide based micromotors were obtained by cathodic electrodeposition of Fe sandwiched layer following the electropolymerization of PEDOT outlayer, and subsequently anodic deposition of inner active MnO2 layer (as shown in Figure 1). Tailoring the sandwiched Fe layer enables us to tune the active MnO2 surface structure on demand, resulting in a pronounced propulsion performance and catalytic activity, aligning with the stable crystal structures of ramsdellite MnO2 (R-MnO2) and Mn2O3 in combination with Fe2O3. Compared to bilayer PEDOT/MnO2 (PM) micromotors, PEDOT/Fe-MnO2 (PFM) micromotors could be activated with minimum active MnO2, yet contribute to more efficient water remediation efficacy. This unique FSI structure enabled relatively fast speed of up to 111 µm·s-1 and 97.5% degradation ratio of 50 mg/L azo-dye in 20 mins with around 13 wt% Mn. On the contrary, 75% degradation ratio is achieved by the corresponding PM micromotors treatment and the motion speed is drastically drop to 65 µm·s-1 in 5% H2O2 solution. Overall, our reported FSI metal-oxide based micromotors exhibit considerable propulsion power and decontamination ability over corresponding normal PEDOT/MnO2 micromotors, resulted from synergetic effects of Fe-related Fenton reaction and high specific surface area. Our work provides a new insight on the design of economic and ecofriendly micro/nanomotors, extending the practical applications of artificial micro/nanomotors. RESULTS AND DISUSSION The fish-scales-like intercalated PEDOT/Fe-MnO2 (PFM) micromotors were synthesized by facile and scalable procedures as shown in Figure 1. The tubular structure was obtained via template-assisted electrochemical deposition protocol. First, EDOT monomer initially polymerize on the inner wall of the polycarbonate membrane and rapidly formed a supportive PEDOT outer layer because of solvophobic and electrostatic effects.34 With the assistance of PEDOT film, Fe and MnO2 layer were electrodeposited sequentially onto the inner walls of the pores. Herein, anodic electrodeposition was applied for MnO2 synthesis, which is extensively reported and confirmed a good success in significant improvement on electrochemical performance as obtained unique and specific morphology of MnO2, especially doping with Fe.35-39


Page 4 of 25

Page 5 of 25 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


The formation mechanism of MnO2 could be explained by the following equation:

Mn 2  4 H 2O  MnO2  4 H   2e  .

(1)

Figure 1. Schematic illustration of the fabrication procedure of FSI PEDOT/Fe-MnO2 tubular micromotors.

Figure 2. Characterization of FSI metal-oxide based micromotors. SEM images of a PEDOT/Fe microtube, the inset image illustrating the cross section morphology (a), SEM images of a FSI metal-oxide based micromotor and its cross section morphology (b)&(c), XRD pattern of the FSI metal-oxide based micromotors (d), the highresolution Mn 2p XPS spectra (e) and Fe XPS spectra (f), Scale bars: 5 μm. During the anodic electrodeposition, dissolution of Fe sandwiched layer and coelectrodeposition of Fe doping MnO2 active layer occurred simultaneously, which results in a distinct morphology as shown in Figure 2. Finally, the micromotors were insolated and dispersed after dissolving the membrane template, with active MnO2 as a catalyst to decompose H2O2 and generate bubble for propulsion. The morphology and structure of the obtained FSI metal-oxide based micromotors were characterized and displayed in Figure 2. In order to clarify the influence of the following anodic electrodeposition, the PEDOT/Fe microtube is also retrieved as a



control. It is clearly illustrated in Figure 2(a) that an individual PEDOT/Fe microtube about 14 μm long with a defined biconical geometry is well-formed. The smooth outer and inner surface is obviously observed, confirming formation of a compact PEDOT and a fine crystalline Fe coating layer (shown in the inset of Figure 2(a)). Once anodic electrodeposition applied (E=+1.0V vs Ag/AgCl with 3M KCl), the intermediate Fe layer subjects to intense anodic polarization (for Fe/Fe2+, E0ɵ=−0.44 V), starts to dissolve and releases Fe2+. Simultaneously, the formation of MnO2 crystal and incorporation with Fe oxide are occurred as reported,35,36 propitious to form a fishscales-like intercalated structure of composite metal-oxide layer in the tubular micromotors (Figure 2(c)) which are significantly different with our previous alveolate ones

33

and PM compact micromotors (Figure S1) when skipping cathodic

electrodeposition of Fe sandwiched layer. The outer surface of FSI metal-oxide based micromotor is little bit rougher than PEDOT/Fe microtube yet with identical dimension and geometry (Figure 2(b)). The developed FSI structure of inner MnO2 layer would be responsible for the efficient propulsion behaviors of the micromotor due to its large surface area for H2O2 catalytic decomposition (Figure 2(c)). Also, the rough surface favors the generation of the microbubbles. Furthermore, energy dispersive X-ray spectrum (EDX) mapping analysis confirms the uniform distribution of C, O, S, Fe, Mn in the as-synthesized nanomachine (Figure S2(a&b)). To be noticed, Fe element is not as distinguished as other elements due to its small quantity of 3.61 wt% (Table S1). Monodispersed FSI metal-oxide based micromotors are confirmed in Figure S2(c&d). Moreover, the chemical states of the deposited FSI metal-oxide based micromotors were deeply inspected by XRD analysis. The XRD patterns of the FSI metal-oxide based micromotors exhibit three distinct diffraction peaks, which are easily identified for (101) and (111) planes of ramsdellite MnO2 (R-MnO2) crystalline structure (JCPDS no. 42-1316) as well as (012) crystal plane of Mn2O3 (JCPDS no. 33-0900) (Figure 2(d)). Quantitative element analysis of FSI metal-oxide based micromotors was also conducted by XPS. The full XPS spectrum reveals the presence of carbon (C 1s peak), oxygen (O 1s, O Auger peaks), sulfur (S 2p peak), iron (Fe 2p peak) and manganese (Mn 2p, Mn Auger peaks) in the FSI micromotors (Figure S3). The oxidation state of


Page 6 of 25

Page 7 of 25 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


Mn was examined by Mn 2p high resolution XPS spectra (Figure 2(e)). As reported, the peaks at the binding energies of 641.7 eV and 653.5 eV are assigned to Mn2O3 and MnO2, confirming the Mn oxides are composed of both trivalent and tetravalent.33,36,40,41 As an important constituent, the chemical state of Fe was also inspected (Figure 2(f)). The Fe spectrum is decomposed into two peaks, corresponding to the specie of Fe2O3 with Fe 2p1/2 at 725.2 eV and Fe 2p3/2 at 710.8 eV.16,36 This finding elucidates only trivalent Fe oxide exists in the micromotors, which is resulted from a series of complicated oxidation reactions occurred during imposed anodic polarization. Although Fe0 is formed at the beginning, it is believed that all of Fe0 is oxidized to Fe2+ during MnO2 anodic electrodeposition (Equation 2). Afterward, the Fe2+ could either lose an electron from Equation 3 or take anodic oxidation with H2O to form Fe2O3 (Equation 4). Meanwhile, the Fe3+ could be reduced by Mn2+ in the solution following the reaction in Equation 5 and then was transformed to Fe2+.36

Fe  2e  Fe 2

(2)

Fe 2+  e  Fe3

(3)

2Fe 2+  3H 2O  2e  Fe2O3  6 H 

(4)

2Fe3+  2 Mn 2 +3H 2O  2 Fe 2  Mn2O3  6 H 

(5)

In addition, the acidic environment would encourage the electrochemical reduction of tetravalent Mn Oxide according to the following Equation. MnO 2  H + +e  Mn2O3  H 2O

(6)

Furthermore, to fully explore the fish scale intercalated structure and study iron oxide/manganese oxide interface, a series of transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements were conducted. As can be seen from Figure 3, the as obtained tubular metal-oxide based micromotors clearly show a fish-scale intercalated structure (Figure 3a and Figure 3c) which is further confirmed to be mainly composed of Fe (59.45wt %) by EDX mapping in Figure 3b and Figure S4. As noticed, Mn with 2.11 wt% is distributed randomly as small nanoparticles (Figure 3b & Figure S4).



Figure 3. The detailed morphology and crystal structure of FSI micromotors. TEM image of FSI micromotors and the inset illustrating the fish-scale intercalated structure (a), the elemental distribution on the fish-scale intercalated structure (b), TEM image of an enlarged fish-scale intercalated structure (c), TEM and HRTEM images of Mn oxides (d&e), TEM image of Mn oxides and their corresponding SAED pattern (f), TEM image of the individual fish scales and their corresponding SAED pattern (g). In addition, a detailed TEM image (Figure 3c) presents the FSI structure is closely adhered to an intermediate layer with numerous nanoparticles of around 5 nm diameter (Figure 3d).The high resolution transmission electron microscope (HRTEM) pattern (Figure 3e) of those small nanoparticles depicts four interlayer spacing of 0.120 nm, 0.213 nm, 0.228 nm and 0.138 nm, which can be well indexed to the (101), (111), (211) crystal planes of R-MnO2 and (012) plane of Mn2O3. Moreover, the SAED images (Figure 2f) also shows those four crystal patterns indicative of multiple crystallographic orientations which corresponded to mixed structure of MnO2 and Mn2O3, in accordance with XRD analysis. Furthermore, the FSI structure is confirmed to consist of maghemite (Fe2O3) (JCPDS 25-1402) with crystal planes of (205), (119), (0012) and (2115), although without significant signals in XRD pattern due to their rare amount (Figure 2g inset). A clear illustration of the individual fish scales is in Figure 2g. As proved, both Fe2O3 and Mn oxides are crystal, yet with amorphous PEDOT layer (Figure S5). This unique architecture of metal oxide micromotors with FSI structure and MnO2 nanoparticles could afford abundant specific surface area and is capable of providing fast ion transport channels, as a result, enhanced their


Page 8 of 25

Page 9 of 25 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


propulsion and catalytic performance. To get deep understanding of the impact of Fe, we conducted electrochemical impedance spectroscopy test on two polycarbonate membranes containing FSI and PEDOT/MnO2 based micromotors respectively (Figure S6). The FSI sample exhibits a superior electrochemical properties which attributes to abundant specific surface area (Figure 3), which will effectively contribute to the increase of electrolyte paths during the redox reactions and further enhance the catalytic performance.42,43 Overall, compared with alveolate MnO2 in our previous work,33 we successfully synthesize a new MnO2-Fe2O3 composited micromotor with distinct crystal structure and a high specific surface area by introducing an extra simple step. Subsequently, we examined the role of Fe deposition charge on the surface structure of the PEDOT/Fe-MnO2 micromotors. A great structural transformation of active composite metal-oxide layer obviously arises once cathodic electrodeposited other charges of Fe, yet with stable dimension and shape (Figure S4). To be noticed, the surface is modulated to granular (Figure S7(c)) and cratered (Figure S7(d)) morphology when imposed 0.3 C and 0.5 C Fe. It suggests that the dissolution of Fe is not sufficient to fully participate in formation of rough structure. The crystallization of fine MnO2 dominates in the anodic process in the case of 0.3 C Fe deposition. In contrast, overwhelmed Fe dissolution and reblending formation of Fe doping MnO2 layer occur when performed 0.5 C Fe, therefore leading to fine and tiny needles dispersed in the cratered MnO2 surface. Nevertheless, the deposition efficiency of MnO2 is declined with the increase of Fe deposition charges (Table S1), attributed to Fe related oxidation reactions in Equation (2)-(4) occupied the anodic process. The corresponding amount of Mn is decreased from 38.68 wt% (PM-0.5C) to 20.60 wt% (PFM-H), 13.66 wt% (PFM-M) and 8.22 wt% Mn (PFM-L) as increasing deposition charge of Fe from 0 C, to 0.3 C, 0.4 C and 0.5 C, respectively. In particular, the amount of Fe retains slightly increasing in spite of its dissolution during the anodic electrodeposition, confirming the dissolve Fe ions are continuously integrated to iron oxides and incorporated in the forming MnO2 layer. The

propulsion

performance

of

PEDOT/Fe-MnO2


micromotors

was


experimentally tested and their time-lapse images are plotted in Figure 4. First, Figure 4a displays the codependence of velocity of PFM micromotors upon iron deposition charges and H2O2 concentrations. As expected, the imposed anodic polarization on different Fe layers results in a rapid decline of MnO2, which influences the motion behavior of micromotors in turn. The more the Fe deposition charges are, the less the MnO2 forms. Correspondingly, the more concentrated the H2O2 is required to activate the micromotors. For instance, the motions of PM-0.5C (with 0.5 C MnO2, i.e. 38.68 wt% Mn in Table S1) micromotors can be observed in the peroxide fuel down to the concentration of 0.5%. In contrast, PFM micomotors deposited with 0.5 C Fe (i.e. 8.22 wt% Mn) need at least 5% H2O2 fuel for propulsion. Otherwise, the speed of the micromotors strongly depends on the concentration of the peroxide fuel. It clearly illustrates all micromotors exhibit faster propulsion as increasing the concentration of H2O2, as well as the decline of iron deposition charges, accordingly increasing of MnO2. The speed of PM-0.5C micromotors increases from 78 µm·s−1 to 147 µm·s−1 upon raising the peroxide level from 0.5% to 5%. The average velocities of PFM micromotors is respectively 131, 111 and 105 µm·s−1 in 5% H2O2 solution corresponding to 20.60 wt%, 13.66 wt% and 8.22 wt% Mn. Meanwhile, we compared the velocity of PFM and PM micromotors to clarify the importance of surface structure (Figure 4(b)). In comparison with PM micromotors, all PFM ones exhibited large electrochemical active surface area, and enhanced the catalytic properties. Hierarchically rough Fe2O3 structure and nano size of MnO2 are expected to provide an enlarged catalytic surface area and improve fuel accessibility and hence lead to a remarkably efficient propulsion. All PFM micromotors display superior propulsion velocities. The PM micromotors with 9.9 wt% Mn stay motionless, the maximum speed is only around 60 µm·s−1 with 21.4 wt% Mn. Nevertheless, PFM micromotors could reach more than 100 µm·s−1 even composed of 8.2 wt% Mn. The tracking trajectories in Figure 4(c) (taken from Video S1) show that the FSI micromotor (PFMM, 13.66 wt% Mn) travels at least three folds longer distance than PM-M one (with 14.61 wt% Mn) over a timeframe of 1.0 s. Other PFM micromotors with cratered and granular surface also propel farther than the corresponded PM ones (Figure S8 and S9


Page 10 of 25

Page 11 of 25 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


taken from Video S2 and S3). Therefore, modulating the surface structure could improve the propulsion performance. Moreover, released bubbles from the decomposition of H2O2 produce a complicated recoil force on the PEDOT/Fe-MnO2 (PFM) micromotors, propelling themselves at 6 sophisticated motion modes, including linear, orbital, rotary, helical, “8”-like and flower-like styles (Figure 5(a) taken from Video S4 and S5). Those complicated motion trajectories result in more violent fluid mixing in local region and ought to accelerate the mass transfer, furtherly enhance the water waste decontamination efficacy. It is believed the microtube geometry best suits the chemically driven micromotors, allowing efficient nucleation and growth of gas bubbles. As illustrated in Table 1,44-53 the versatile microstructural design of the micromotors, as well as the catalytic machine power, would contribute to the complicated motion trajectories. Normally, the smooth outer layers of PANI, PEDOT or PPy help to build a smooth Pt, Ag or Zn catalytic layer, giving to one or two relatively simple propulsion patterns (straight, curved or self-rotating).47,48,53 Nevertheless, asymmetric microstructure will lead to some unique motion trajectories. The size and shapes are crucial factors determining the trajectories for roll-up tubular micromachines, where five motion patterns of straight, curved, circular, spiral, and self-rotating motions were presented.45 In the other hand, the rough inner surface resulted from a supporting porous graphene oxide (GOx) or improved electrochemical parameters leads to an unbalance generation of oxygen gas bubbles along the tubular structures, caused an anisotropic distribution of the drag forces along the axial and radial directions, giving rise to distinct motion patterns.25,50,51 Besides, the tubular micromotors show irregular motion trajectories in very high concentration of fuels, because the violent catalytic reactions and the discontinuous bubbles ejection process will result in the turbulence of the propelling force vector and then affect the motion directions.51 To analyze the motion modality of our PEDOT/Fe-MnO2 micromotors, we assume each complex motion are combined from two basic modes: translational



motion and circular motion. These two motions are closely depended by the bottom structures of micromotors. One flat-bottom type micromotor and another tilt-bottom type micromotor are numerically simulated using COMSOL Multiphysics software (COMSOL, Stockholm, Sweden). The tested locomotion velocity generated by bubble propulsion, which is set to the bottom side of micromotors, is selected as the boundary conditions for calculating the flow field distribution. As illustrated in Fig. 5(b Top), there are two symmetrical ambient flow vortices around the micromotor, which means the flow resistance at both sides are identical. As a result, the micromotor will perform a linear motion as shown in Fig. 5a. In comparison, the ambient flow around tilt-bottom type micromotor (see Fig. 5(b Bottom)) shows an asymmetrical distribution, indicating the micromotor encounters a larger resistance from the left side and thus will move at a circular motion. Additionally, there are also some other motion styles combining linear and rotary motion together, which may be caused by the structure defects including cracks or holes formed during the electrodeposition.

Figure 4. Propulsion performance of PEDOT/Fe-MnO2 micromotors. Codependence of velocity of PFM micromotors with Fe deposition charges and H2O2 concentrations (a), Comparison of the velocity of PFM and PM micromotors with similar quality of Mn in 5% H2O2 solution(b), The propulsion schema and tracking trajectories showing the propulsion of FSI (also called PFM-M) and PM-M micromotors over a timeframe of 1.0 s in 5% hydrogen peroxide and 0.167% Triton X-100 (c), Scale bar: 20 μm.


Page 12 of 25

Page 13 of 25 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


Table 1 The propulsion performance of different bubble-recoiling propelled micro/nanoswimmers Velocity(bod y length/s)

Motion modality

Fabrication

Ref.

Ti/Fe/Au/Ag

20 in H2O2

Straight

Roll-up

44

Ti/Au/Fe/Pt

50 in 1.5% H2O2

Straight, Curved, Circular, Spiral, Selfrotating

Roll-up

45

Cu/Ag

20 in 3% H2O2

Curved

Electrodeposition

46

Polypyrrole/Pt Polyaniline/Pt PEDOT/Pt

160/240/340 in 5% H2O2

Nearly straight

Electrodeposition

47

CA:PEDOT/Pt

18 in 2% H2O2

Spiral, Self-rotating

Electrodeposition

48

TiO2/Pt

87 in 5% H2O2

Irregular

erGO/ Cu-Pt alloy

100 in 1% H2O2

Straight, Circular, Spiral

erGO/ Pt

40 in 1% H2O2

Circular, Flower-like Snake-like, Straight Spiral, Self-rotating

Electrodeposition

25

erGO/ MnO2

27 in 5% H2O2

Straight, Curved Spiral, Self-rotating

Electrodeposition

51

PEDOT/MnO2

12 in 5% H2O2

Straight, Circular Spiral, Self-rotating

Electrodeposition

33

Fe2O3/ SiO2MnO2

21 in 5% H2O2

Straight, Spiral

Sol-gel chemistry

21

Urease/SiO2

1 in 5M urea

Curved

Sol-gel chemistry

52

Polyaniline/Zn

50 in 1M HCl

Self-rotating

Electrosynthesized

53

Cr/Fe/Ti/Pd

6 in 30M NaBH4

Irregular

Roll-up

9

PEDOT/FeMnO2

10 in 5% H2O2

8”-like, Flower-like, Helical, Linear, Orbital Rotary

Electrodeposition

this work

Type of motor

Structure


Atomic layer deposition Electrodeposition

49 50


Figure 5. Time-lapse images depicting efficient propulsion of the PEDOT/Fe-MnO2 micromotors in different propulsion modes ((linear, orbital, rotary, helical, “8”-like and flower-like) at time intervals of 0.4 s in 5% H2O2 and 0.167% Triton X-100 (a), Scale bar: 20 μm. Simulation of oxygen concentration distribution produced by the PEDOT/Fe-MnO2 nanorocket in the presence of 5% H2O2 fuel (b).

Figure 6. The degradation of methylene blue using PEDOT/Fe-MnO2 micromotors. Schematic representation of the self-propulsion and catalytic degradation of MB by FSI MnO2 based micromotors (a),the absorbance spectra of 50 mg L-1 MB after 13 mins treatment with different micromotors (approximately 0.20 mg for each batch). Control experiments are MB (blank solution) and H2O2 (solution without any micromotors) (b), The degradation time of MB when the ratio reaches 0.99 (c), Comparison of degradation ratios between PFM and PM micromotors (d).


Page 14 of 25

Page 15 of 25 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


We further explored water decontamination capability of PFM micromotors as schematically illustrated in Figure 6(a). The new PFM micromachines are composed of Fe2O3 and MnO2 composite with high specific surface areas. These motors could generate highly oxidative species (such as HO·, HOO·, and O2·- ) from the Fentonlike reactions on MnO2 surface during their autonomous propulsion.3 Simultaneously, the Fe2O3 as heterogeneous Fenton regent could release Fe3+ in the acid condition and reduces to Fe2+ with H2O2,54,55 promoting the generation of free radicals in H2O2 solution. Furthermore, the PFM microswimmers produce significant mixing during the catalytic decontamination process and lead to a remarkably effective catalytic cleaning microsystem. Of particular significance, PFM micromotors display much higher catalytic surface areas. Consequently, a model pollutant of methylene blue (MB) is successfully degraded to environmental friendly byproduct due to the synergetic effects of Fe-related Fenton reaction and high specific surface area. The decontamination capability of PFM micromotors has been investigated by catalytic degradation of MB conducted in 5% H2O2 and 0.167 % Triton X-100 mixed solution. Figure 6(b) elucidates the effect of different components involved in the enhanced catalytic activity of the self-propelled micromotors. A dramatic decrease of absorbance signal, corresponding to 100% degradation, is observed following treatment with the moving PM-0.5C (without Fe deposition) micromotors. In contrast, the degradation ratios by PFM motors drop to 94%, 90% and 81%, corresponding to the Fe-0.3C, Fe-0.4C and Fe-0.5C (charge of deposited iron is 0.3 C 0.4 C and 0.5 C respectively) due to the sharp decline of Mn. And control experiment without the tubular motors was also carried out in parallel. A trivial removal (27%) of the organic dye is founded with only 5% H2O2. The presence of MnO2 as a catalyst promotes the generation of radicals and enhances the MB degradation process (Figure 6(c)). It respectively takes 13, 20 and 24 mins for PFM micromotors to degrade MB organic dye with the increasing charges of the deposited Fe from 0.3 C to 0.5 C. By contrast, MB solution without motors spent approximately 72 mins to complete the decontamination. The comparison between the PFM and PM micromotors with similar amount of Mn clearly indicates the presence of Fe2O3 and high surface area is crucial for the degradation process (Figure 6(d)). Compared to the



PM-M (with 14.6% Mn) motors, the FSI (with 13.6% Mn) motors display a 22.5% improvement on degradation capability (97.5%). The other two PFM motors also exhibit similar effect but with a lower promotion extent. These data clearly indicate the new FSI metal-oxide based catalytic micromotor platform offers significantly faster and greatly improved destruction of MB. Evidently, the large-scale collective motion and high surface areas of the FSI metal-oxide based micromotors could promote efficient fluid transport and mixing and accelerate the decontamination process. The FSI micromotors also exhibit good recycling performance as confirmed by XRD patterns before and after degradation (Figure S10). Overall, the synergistic effect between the composite of Fe2O3 and MnO2 and a large catalytic area offered by the hierarchically rough inner surface give rise to an extraordinary high performance for FSI metal-oxide micromotors.

CONCLUSIONS We have demonstrated that Fe doping MnO2 tubular micromotors with an exceptional high catalytic performance can be produced by anodic polarization following cathodic electrodeposition of Fe sandwiched layer. A unique architecture with fish-scales-like intercalated surface structure and active layer with 5 nm nanoparticles is obtained on demand by simply modulating the deposition charges of iron, along with the stable crystal structures composed of ramsdellite MnO2, Mn2O3 and Fe2O3. Compared to compact structure in the normal PM micromotors, the unique FSI architecture in PFM micromachines could enhance the propulsion speed and create versatile motion trajectories. The numerical simulation on motion modality reveals the anisotropic distribution of ambient flow vortices along the axial direction arising from the asymmetric microtube lead to a distinct motion trajectory. Furthermore, the asymmetric inner FSI catalyst structure induces unbalanced oxygen production along the tube, also affecting the motion patterns. Such enhanced catalytic activity attributed to synergistic Fe-related Fenton reaction accessibility and larger surface area enable a dramatic raise of azo-dye degradation efficacy with 25 % improvement outperformed the normal PM treatment. These FSI micromachines open a new avenue in building


Page 16 of 25

Page 17 of 25 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


robust motors capable of constructing diverse surface architectures using a simple electrochemical protocol. In addition, the large specific surface area holds considerable promises in biosensing, drug delivery, and other related applications.

METHODS Materials and Reagents Potassium nitrate (KNO3), sodium dodecyl sulfate (SDS), manganese(II) acetate tetrahydrate (MnAC2·4H2O), ferrous sulfate (FeSO4·7H2O), dichloromethane (CH2Cl2), ethanol (C2H5OH), boric acid (H3BO3), concentrated sulfuric acid (98% H2SO4), methylene blue (MB), hydrogen peroxide (30% H2O2) (Cat. 7722-84-1) were all purchased from Sinopharm Chemical Regents Co., Ltd. 3,4-ethylenedioxythiophene (EDOT) and polyethylene glycol octyl phenyl ether (Triton X-100) were obtained from Alfa Aesar Chemical Regent Co., Ltd. Polycarbonate (PC) membranes (Catalog no. 7060-2513) with an average pore diameter of 5 m were purchased from Whatman Inc., NY, USA. All reagents were of analytical grade and used as received without further treatment. Fabrication of MnO2 based Micromotors The MnO2 based micromotors were fabricated using a common template directed electrochemical deposition protocol. A polycarbonate membrane was employed as the template. A thin gold film was first deposited on one side of the porous membrane to serve as the working electrode using a JFC-1600 sputter coaster. The evaporation deposition was performed under high vacuum below 5×10-2 mBar at a current of 20 mA after evacuated twice and the deposition time is 110 seconds. A large stainless steel plate with an area of 16 cm2 and a Ag/AgCl with 3M KCl were used as the counter electrode and reference electrode, respectively. The membrane was then assembled in a self-designed plating cell with a stainless steel foil serving as a contact for the working electrode. Electrochemical deposition was carried out using a CHI 660E electrochemical workstation (CH Instruments. Ins, Shanghai, China). First, the poly(3,4-ethylenedioxythiophene) (PEDOT) layer was electropolymerized at +0.8V from a plating solution containing 15 mM EDOT, 7.5 mM KNO3 and 100 mM SDS,



with a total charge of 0.3 C (a platinum wire utilized as a counter electrode). Subsequently, a solution containing 20 mg mL-1 boric acid in 260 mg mL-1 of ferrous sulfate with a pH of 2.5 adjusted by concentrated sulfuric acid was prepared as the following electrolyte for the intermediate Fe layer electrochemical growth by applying voltage -1.0V with a total charge of 0.4 C, as well as 0.3 C and 0.5 C for comparison. Finally, the MnO2 plating solution was prepared by 100 mM Manganese(II) acetate (MnAC2). The inner MnO2 layer was oxidized using a potentiostatic method at a voltage of +1.0 V for 0.5 C of charge transfer. Afterwards, the sputtered gold layer was removed by hand polishing gently with 3~4 μm alumina slurry. The templates were then dissolved in methylene chloride for 10 min to release the micromotors. The later was collected by centrifugation at 7000 rpm for 3 min and washed 3 times with methylene chloride, ethanol and deionized water each, with a 3 min centrifugation after each wash. Centrifuge was carried out using a H/T18MM centrifuge (HX Instruments. Ins, Hunan, China). All the micromotors were stored in ultrapure water at room temperature (25 °C) for further use ultimately. Characterization and Motion Observation of Micromotors The morphologies and element distributions of MnO2 based micromotors were obtained by a ZEISS EVO18 Scanning Electron Microscopy (SEM), using an acceleration voltage of 15 kV, equipped with Oxford X-MAX Energy Dispersive Xray Spectrometer (EDX) analysis. The detailed TEM and SAED characterization were performed by a Jeol Transmission Electron Microscope (TEM) (Jem-3200FS). X-ray diffractometer with a Cukα x-ray source (ARL X’TRA) was employed to characterize the crystal structure of the micromotors. Moreover, X-ray photoelectron spectroscopy (XPS) was carried out using a Kratos AXIS Ultra DLD system with monochromatic Al-Ka X-rays at a photon energy of 1486.7 eV under a base pressure less than 8×10-9 mBar. The spectra were acquired with the pass energy of 40 eV and fitted using XPSPEAK41 software. Electrochemical impedance spectroscopy (EIS) test was conducted in a three-electrode configuration was used, with Ag/AgCl with 3 M KCl (φ=0.210 V) as a reference electrode, Pt wire as a counter electrode, and the deposited polycarbonate membranes containing FSI and PM MnO2 as the working electrodes.


Page 18 of 25

Page 19 of 25 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


The frequency range of EIS is from 100 kHz to 10 mHz at the OCP with a potential perturbation of ±10 mV and performed with a CHI660E potentiostat. In addition, optical microscopy images and motion videos of the micromotors in different H2O2 solutions were recorded by an inverted optical microscope (CKX53, Olympus Instrument Inc., Tokyo, Japan) coupled with a 20X and 40X microscope objectives and a MshOt digital camera (MSX2, Guangzhou Micro-shot Technology Co., Ltd., China). Degradation of Methylene Blue To evaluate the catalytic performance of FSI MnO2-based micromotors, degradation tests was carried out with methylene blue as a pollutant of representative. Before each test, precise digital balance (up to 0.01 mg) is utilized to weight each polycarbonate template so as to make sure that each solution contains the same amount of micromotors (~0.20 mg). During the catalytic degradation experiments of methylene blue (Co= 50 mg L-1), micromotors (~0.20 mg) dispersed in 100 μL ultrapure water was added to the mixed solution containing 300 μL methylene blue solution (250 mg L-1), 250 μL H2O2 solution (30%) and 250 μL Triton-X100 solution (1%). Thus, the concentration of all compounds in the ultimate solution was 133 mg L-1, 50 mg L-1, 5% and 0.17%, corresponding to micromotors, MB, H2O2 and Triton-X100 respectively. A UV-vis spectrometer (A590, AOE Instruments, Shanghai, China) was utilized to measure the absorbance of the mixed solution. Before each analysis, centrifugation was used to isolate micromotors from the mixture and the rest solution was calculated by measuring the absorbance at ~ 615 nm. Afterwards, the solution was re-blended in order to keep the degradation going on. The bland MB solution, H2O2 solutions with PM micromotors and without micromotors were used as control.

ASSOCIATED COTENTS The Supporting Information is available free of change on the ACS Publication website.

The SEM images and the EDX mapping results of PEDOT/MnO2 micromotors and PEDOT/Fe-MnO2 micromotors in different Fe electrodeposition coulombs. The TEM images and the EDX mapping results of the fish scale. The TEM and SAED pattern of



PEDOT layer. The electrochemical impedance spectroscopy of FSI micromotors and PM micromotors. XPS spectra of wide scan for FSI MnO2 micromotor, the chemical composition of various types of micromotors, the tracked trajectories showing the propulsion of MnO2 based micromotors with and without Fe (PDF) Video S1 (AVI) Video S2 (AVI) Video S3 (AVI) Video S4 (AVI) Video S5 (AVI) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

ORCID Wenjuan Liu: 0000-0003-0572-5310 Xiaolong Lu: 0000-0002-2049-8200

ACKNOWLEDGEMENT This work was supported by National Natural Science Foundation of China (Nos. 51501089 and 51505222), Nanjing Tech University Supported Program, PAPD-A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and Astronautics) (Grant No. MCMS-I-0318Y01). We acknowledge JuShang Testing & Analysis Center for providing HRTEM and SAED measurement. All authors declare no conflicts of interest.

REFERENCES


Page 20 of 25

Page 21 of 25 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


(1) Novotny V.; Olem H. Water Quality: Prevention, Identification and Management of Diffuse Pollution; Van Nostrand-Reinhold: New York, 1994. (2) Wang, J. Nanomachines: Fundamentals and Applications; WileyVCH: Weinheim, 2013. (3) Parmar, J.; Vilela, D.; Villa, K.; Wang, J.; Sánchez, S. Micro- and Nanomotors as Active Environmental Microcleaners and Sensors. J. Am. Chem. Soc. 2018, 140, 9317—9331. (4) Gao, W.; Wang, J. The Environmental Impact of Micro/Nanomachines: A Review. ACS Nano 2014, 8, 3170—3180. (5) 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. (6) Orozco, J.; Jurado-Sánchez, B.; Wagner, G.; Gao, W.; Vazquez-Duhalt, R.; Sattayasamitsathit, S.; Galarnyk, M.; Cortés, A.; Saintillan, D.; Wang, J. Bubble-propelled Micromotors for Enhanced Transport of Passive Tracers. Langmuir 2014, 30, 5082—5087. (7) Baldyga, J.; Bourne, J. R. Interactions between Mixing on Various Scales in Stirred Tank Reactors. Chem. Eng. Sci. 1992, 47, 1839—1848. (8) Vilela, D.; Parmar, J.; Zeng, Y.; Zhao, Y.; Sánchez, S. Graphene-Based Microbots for Toxic Heavy Metal Removal and Recovery from Water. Nano Lett. 2016, 16, 2860—2866. (9) Srivastava, S. K.; Guix, M.; Schmidt, O. G. Wastewater Mediated Activation of Micromotors for Efficient Water Cleaning. Nano Lett. 2016, 16, 817—821. (10) Esteban-Fernández de Ávila, B.; Lopez-Ramirez, M. A.; Báez, D. F.; Jodra, A.; Singh, V. V.; Kaufmann, K.; Wang, J. Aptamer-modified Graphene-Based Catalytic Micromotors: Off-On Fluorescent Detection of Ricin. ACS Sens. 2016, 1, 217—221. (11) 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. (12) Hoop, M.; Shen, Y.; Chen, X.; Mushtaq, F.; Iuliano, L. M.; Sakar, M. S.; Petruska, A.; Loessner, M. J.; Nelson, B. J.; Pané, S. Magnetically Driven Silver‐coated Nanocoils for Efficient Bacterial Contact Killing. Adv. Funct. Mater. 2016, 26, 1063—1069. (13) Kiristi, M.; Singh, V. V.; Esteban-Fernández de Ávila, B.; Uygun, M.; Soto, F.; Aktaş Uygun, D.; Wang, J. Lysozyme-Based Antibacterial Nanomotors. ACS Nano 2015, 9, 9252—9259. (14) Vilela, D.; Stanton, M. M.; Parmar, J.; Sánchez, S. Microbots Decorated with Silver Nanoparticles Kill Bacteria in Aqueous Media. ACS Appl. Mater. Interfaces 2017, 9, 22093—22100. (15) Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G.E.; Feng, Z.; Su, D.; More, K.L.; Wang, G.; Wang, Z.; Wu, G. Atomically Dispersed Manganese Catalysts for Oxygen Reduction in ProtonExchange Membrane Fuel Cells. Nat. Catal. 2018, 1, 935—945. (16) Zhou, Z.; Zhang, Q.; Sun, J.; He, B.; Guo, J.; Li, Q.; Li, C.; Xie, L.; Yao, Y. Metal–Organic Framework Derived Spindle-Like Carbon Incorporated α-Fe2O3 Grown on Carbon Nanotube Fiber as Anodes for High-Performance Wearable Asymmetric Supercapacitors. ACS Nano 2018, 12, 9333—9341. (17) Crudden, C. M.; Horton, J. H.; Narouz, M. R.; Li, Z.; Smith, C. A.; Munro, K.; Baddeley, C. J.; Larrea, C. R.; Drevniok, B.; Thanabalasingam, B.; McLean, A.B.; Zenkina, O.V.; Ebralidze, I.I.; She, Z.; Kraatz, H.; Mosey, N.J.; Saunders, L.N.; Yagi, A. Simple Direct Formation of Selfassembled N-heterocyclic Carbene Monolayers on Gold and Their Application in Biosensing. Nat. Commun. 2016, 7, 12654.



(18) Yang, G.; Xu, L.; Chao, Y.; Xu, J.; Sun, X.; Wu, Y.; Peng, R.; Liu, Z. Hollow MnO2 as a Tumor-Microenvironment-Responsive Biodegradable Nano-Platform for Combination Therapy Favoring Antitumor Immune Responses. Nat. Commun. 2017, 8, 902. (19) 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. (20) Wani, O. M.; Safdar, M.; Kinnunen, N.; Jänis, J. Dual Effect of Manganese Oxide Micromotors: Catalytic Degradation and Adsorptive Bubble Separation of Organic Pollutants. Chem.-Eur. J. 2016, 22, 1244—1247. (21) Villa, K.; Parmar, J.; Vilela, D.; Sánchez, S. Metal-Oxide-Based Microjets for the Simultaneous Removal of Organic Pollutants and Heavy Metals. ACS Appl. Mater. Interfaces 2018, 10, 20478— 20486. (22) Li, Z.; Wang, W.; Li, M.; Zhang, M.; Tang, M.; Su, Y.; Liu, Z.; Ju, X.; Xie, R.; Chu, L. Facile Fabrication of Bubble-propelled Micromotors Carrying Nanocatalysts for Water Remediation. Ind. Eng. Chem. Res. 2018, 57, 4562—4570. (23) Teo, W.; Zboril, R.; Medrik, I.; Pumera, M. Fe0 Nanomotors in Ton Quantities (1020 Units) for Environmental Remediation. Chem.-Eur. J. 2016, 22, 4789—4793. (24) Wang, H.; Potroz, M.; Jackman, J.; Khezri, B.; Marić, T.; Cho, N.; Pumera, M. Bioinspired Spiky Micromotors Based on Sporopollenin Exine Capsules. Adv. Funct. Mater. 2017, 27, 1702338. (25) Martín, A.; Jurado‐Sánchez, B.; Escarpa, A.; Wang, J. Template Electrosynthesis of HighPerformance Graphene Microengines. Small 2015, 11, 3568—3574. (26) Liang, C.; Zhan, C.; Zeng, F.; Xu, D.; Wang, Y.; Zhao, W.; Zhang, J.; Guo, J.; Feng, H.; Ma, X. Bilayer Tubular Micromotors for Simultaneous Environmental Monitoring and Remediation. ACS Appl. Mater. Interfaces 2018, 10, 35099—35107. (27) Singh, V. V.; Martin, A.; Kaufmann, K.; D. S. de Oliveira, S.; Wang, J. Zirconia/Graphene Oxide Hybrid Micromotors for Selective Capture of Nerve Agents. Chem. Mater. 2015, 27, 8162— 8169. (28) Singh, V. V.; Jurado‐Sánchez, B.; Sattayasamitsathit, S.; Orozco, J.; Li, J.; Galarnyk, M.; Fedorak, Y.; Wang, J. Multifunctional Silver-exchanged Zeolite Micromotors for Catalytic Detoxification of Chemical and Biological Threats. Adv. Funct. Mater. 2015, 25, 2147—2155. (29) Wang, H.; Zhao, G.; Pumera, M. Beyond Platinum: Bubble-Propelled Micromotors Based on Ag and MnO2 Catalysts. J. Am. Chem. Soc. 2014, 136, 2719—2722. (30) Safdar, M.; Minh, T. D.; Kinnunen, N.; Jänis, J. Manganese Oxide Based Catalytic Micromotors: Effect of Polymorphism on Motion. ACS Appl. Mater. Interfaces, 2016, 8, 32624— 32629. (31) Feng, X.; Zhang, Y.; Li, Y.; Huang, Z.; Chen, S.; Ma, Y.; Zhang, L.; Wang, L.; Yan, X. Graphene-Based Highly Efficient Micromotors. Chem. Lett. 2015, 44, 399—401. (32) Wang, L.; Chen, J.; Feng, X.; Zeng, W.; Liu, R.; Lin, X.; Ma, Y.; Wang, L. Self-Propelled Manganese Oxide-Based Catalytic Micromotors for Drug Delivery. RSC Adv. 2016, 6, 65624— 65630. (33) Liu, W.; Ge, H.; Gu, Z. W.; Lu, X. L.; Li, J.; Wang, J. Electrochemical Deposition Tailors the Catalytic Performance of MnO2-Based Micromotors. Small 2018, 14, 1802771. (34) Gao, W.; Sattayasamitsathit, S.; Orozco, J.; Wang, J. Highly Efficient Catalytic Microengines: Template Electrosynthesis of Polyaniline/Platinum Microtubes. J. Am. Chem. Soc. 2011, 133,


Page 22 of 25

Page 23 of 25 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


11862—11864. (35) Dubal, D. P.; Kim, W. B.; Lokhande, C. D. Galvanostatically Deposited Fe: MnO2 Electrodes for Supercapacitor Application. J. Phys. Chem. Solids 2012, 73, 18-24. (36) Lee, M.; Chang, J.; Tsai, W. Effects of Iron Addition on Material Characteristics and PseudoCapacitive Behavior of Mn-Oxide Electrodes. J. Electrochem. Soc. 2007, 154, 875—881. (37) Prasad, K. R.; Miura, N. Electrochemically Synthesized MnO2-Based Mixed Oxides for High Performance Redox Supercapacitors. Electrochem. Commun. 2004, 6, 1004—1008. (38) Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. Phase-controlled Synthesis of MnO2 Nanocrystals by Anodic Electrodeposition: Implications for High-Rate Capability Electrochemical Supercapacitors. J. Phys. Chem. C 2008, 112, 15075—15083. (39) Ye, Z.; Li, T.; Ma, G.; Dong, Y.; Zhou, X. Metal‐Ion (Fe, V, Co, and Ni)‐Doped MnO2 Ultrathin Nanosheets Supported on Carbon Fiber Paper for the Oxygen Evolution Reaction. Adv. Funct. Mater. 2017, 27, 1704083. (40) Strohmeier, B. R.; Hercules, D. M. Surface Spectroscopic Characterization of Mn/Al2O3 Catalysts. J. Phys. Chem. 1984, 88, 4922—4929. (41) Ivanov-Emin, B. N.; Nevskaya, N. A.; Zaitsev, B. E.; Ivanova, T. M. Synthesis and Properties of Calcium and Strontium Hydroxomanganates (III). Zh. Neorg. Khim. 1982, 27, 3101—3104. (42) Jia, J.; Ran, R.; Guo, X.; Wu, X.; Chen, W.; Weng, D. Enhanced Low-Temperature NO Oxidation by Iron-Modified MnO2 Catalysts. Catal. Commun. 2019, 119, 139—143. (43) Poonguzhali, R.; Shanmugam, N.; Gobi, R.; Senthilkumar, A.; Viruthagiri, G.; Kannadasan, N. Effect of Fe Doping on the Electrochemical Capacitor Behavior of MnO2 Nanocrystals. J. Power Sources 2015, 293, 790—798. (44) Mei, Y.; Huang, G.; Solovev, A. A.; Ureña, E. B.; Mönch, I.; Ding, F.; Reindl, T.; Fu, R. K. Y.; Chu, P. K.; Schmidt, O. G. Versatile Approach for Integrative and Functionalized Tubes by Strain Engineering of Nanomembranes on Polymers. Adv. Mater. 2008, 20, 4085—4090. (45) Solovev, A. A.; Mei, Y.; Ureña, E. B.; Huang, G.; Schmidt, O. G. Catalytic Microtubular Jet Engines Self‐propelled by Accumulated Gas Bubbles. Small 2009, 5, 1688—1692. (46) Teo, W. Z.; Wang, H.; Pumera, M. Beyond Platinum: Silver-Catalyst Based Bubble-Propelled Tubular Micromotors. Chem. Commun. 2016, 52, 4333—4336. (47) Gao, W.; Sattayasamitsathit, S.; Uygun, A.; Pei, A.; Ponedala, A.; Wang, J. Polymer-Based Tubular Microbots: Role of Composition and Preparation. Nanoscale 2012, 4, 2447—2453. (48) Uygun, M.; Singh, V. V.; Kaufmann, K.; Uygun, D. A.; D. S. de Oliveira, S.; Wang, J. Micromotor-Based Biomimetic Carbon Dioxide Sequestration: Towards Mobile Microscrubbers. Angew. Chem. 2015, 127, 13092—13096. (49) Li, J.; Liu, W.; Wang, J.; Rozen, I.; He, S.; Chen, C.; Kim, H. G.; Lee, H. J.; Lee, H. B. R.; Kwon, S.; Li, T.; Li, L.; Wang, J.; Mei, Y. Nanoconfined Atomic Layer Deposition of TiO2/Pt Nanotubes: Toward Ultrasmall Highly Efficient Catalytic Nanorockets. Adv. Funct. Mater. 2017, 27, 1700598. (50) Sánchez, S.; Solovev, ,A. A.; Mei, Y. F.; Schmidt, O. G. Dynamics of Biocatalytic Microengines Mediated by Variable Friction Control. J. Am. Chem. Soc. 2010, 132, 13144-13145. (51) Ye H.; Kang J.; Ma G.; Sun H.; Wang S. High-Speed Graphene@Ag-MnO2 Micromotors at Low Peroxide Levels. J. Colloid Interface Sci. 2018, 528, 271—280. (52) Ma, X.; Hortelao, A. C.; Miguel-López, A.; Sánchez, S. Bubble-Free Propulsion of Ultrasmall Tubular Nanojets Powered by Biocatalytic Reactions. J. Am. Chem. Soc. 2016, 138, 13782—13785.



(53) Gao, W.; Uygun, A.; Wang, J. Hydrogen-Bubble-Propelled Zinc-Based Microrockets in Strongly Acidic Media. J. Am. Chem. Soc. 2012, 134, 897—900. (54) Liao, Q.; Sun, J.; Gao, L. Degradation of Phenol by Heterogeneous Fenton Reaction Using Multi-walled Carbon Nanotube Supported Fe2O3 Catalysts. Colloid. Surf. A-Physicochem. Eng. Asp. 2009, 345, 95—100. (55) Gao, P.; Song, Y.; Hao, M.; Zhu, A.; Yang, H.; Yang, S. An Effective and Magnetic Fe2O3ZrO2 Catalyst for Phenol Degradation under Neutral pH in the Heterogeneous Fenton-Like Reaction. Sep. Purif. Technol. 2018, 201, 238—243.


Page 24 of 25

Page 25 of 25 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


TOC


Fish-Scale-Like Intercalated Metal Oxide-Based Micromotors as

Recommend Documents