Simple-Structured Micromotors Based on Inherent Asymmetry in

Subscriber access provided by UNIV OF LOUISIANA

Energy, Environmental, and Catalysis Applications

Simple-Structured Micromotors Based on Inherent Asymmetry in Crystalline Phases: Design, Large-Scale Preparation and Environmental Application Jianhua Zhang, Fangzhi Mou, Zhen Wu, Shaowen Tang, Huarui Xie, Ming You, Xiong Liang, Leilei Xu, and Jianguo Guan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03579 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 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 45 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

Simple-Structured Micromotors Based on Inherent Asymmetry in Crystalline Phases: Design, LargeScale Preparation and Environmental Application Jianhua Zhang, Fangzhi Mou*, Zhen Wu, Shaowen Tang, Huarui Xie, Ming You, Xiong Liang, Leilei Xu and Jianguo Guan

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China.

KEYWORDS ：

micromotors,

asymmetry,

homojunctions,

mass

production,

photocatalytic degradation

ABSTRACT: The key principle of designing a micro/nanomotor is to introduce asymmetry to a micro/nanoparticle. However, micro/nanomotors designed based on external

ACS Paragon Plus Environment

1

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

Page 2 of 45

asymmetry and inherent chemical and geometrical asymmetry usually suffer from tedious small-scale preparation, high cost and/or complexity of external power and control devices, making them facing insurmountable hurdles in practical applications. Herein, considering the possible distinct properties of different polymorphs, we propose a novel design strategy of simple-structured micromotors by introducing inherent asymmetry in crystalline phases. The inherent phase asymmetry can be easily introduced in spherical TiO2 particles by adjusting calcination temperature to control the phase transition and growth of primary grains. The as-designed anatase/rutile TiO2 micromotors not only show efficient autonomous motions controlled by light in liquid media stemming from the asymmetric surface photocatalytic redox reactions, but also have a promising application in environmental remediation due to their high photocatalytic activity in “on-the-fly” degradation of organic pollutants, facile large-scale fabrication and low cost. The proposed design strategy may pave the way for the large-scale productions and applications of micro/nanomotors.


2

Page 3 of 45 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

Synthetic micro/nanomotors can autonomously move in liquid media by harvesting surrounding physical and chemical energies, and may provide revolutionary solutions in biomedicine, micro/nanoengineering and environmental remediation.1–9 Generally, the key principle of designing a micro/nanomotor is to introduce asymmetry to a micro/nanoparticle, with which it can break the force balance and move.10–12 The asymmetry can be introduced to a micro/nanoparticle externally from the asymmetric environment. For instances, external fields can cause asymmetric distribution of chemical products or energies across the micro/nanoparticle, and substrate surfaces may induce frictional asymmetry.4,13 However, micro/nanomotors based on the external asymmetry suffer from the complexity of external power and control devices. Thus, major effort has been devoted to endowing micro/nanoparticles with inherent asymmetry, including asymmetry

in

chemical

composition

and

geometry.14–16

However,

these

micro/nanomotors usually have complex structures (such as Janus, multilayered microtubular, polymetallic rod-like, single-component microtubular and pot-like structures,


3


Page 4 of 45

etc.) involving expensive noble metals (Pt, Au and Ag). They also need to be fabricated by tedious small-scale preparation methods,17,18 such as template-electrochemical deposition,17,19–21 physical vapor deposition,22,23 rolled-up technology,24–26 threedimensional direct laser writing27,28 and assembly of materials.16,29 These greatly hinder their practical applications, especially in environmental remediation, owing to their low production and high cost. Thus, it is of great significance to develop a new design strategy of simple-structured micro/nanomotors that can be manufactured in large scale with low cost. Polymorphs with different phase structures, despite the same chemical composition, often exhibit different physical and chemical properties due to different atom arrangement. For instances, diamond and graphite are two crystalline polymorphs of carbon, but they exhibit distinct hardness, electrical resistance and optical transmissivity. The γ-Fe2O3 and α-Fe2O3 exhibit different magnetic properties and also electrochemical performance.30 The α-MnO2 nanorods show a higher catalytic activity than the γ-MnO2 and β-MnO2 nanorods in CO oxidation.31 If the asymmetry in crystalline phases is introduced into a micro/nanoparticle, asymmetric distribution of chemical products or energies are


4

Page 5 of 45 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


expected to be easily generated across it when physical energy conversions or surface chemical reactions are initiated. Hence, here we propose a novel design strategy of simple-structured micromotors by introducing asymmetry in crystalline phases (or phase asymmetry). Considering the high photocatalytic activity of TiO2 and easy control of its crystalline phases through calcination,32–34 we choose spherical TiO2 micromotors to demonstrate this design strategy.The phase asymmetry can be easily introduced in spherical TiO2 particles by adjusting calcination temperature to control the phase transition and growth of primary grains. Thanks to such phase asymmetry, the as-developed anatase/rutile TiO2 micromotors can autonomously move in liquid media under light irradiation because of asymmetric photocatalytic redox reactions on their surfaces. The motion state, speed of the TiO2 micromotors can be reversibly, wirelessly and remotely controlled by adjusting “on-off” switch and intensity of UV light. The anatase/rutile TiO2 micromotors are promising in environmental application because of their high activity in “on-the-fly” photocatalytic degradation of organic pollutants, facile large-scale fabrication and low cost.


5


Page 6 of 45

RESULTS AND DISCUSSION

To demonstrate the design strategy, TiO2 is an ideal choice for the chemical composition of the micromotors because of its high photocatalytic activity and easy crystalline-phase control.32-34 Anatase and rutile, which are two important crystalline polymorphs of TiO2 both consisting of TiO6 octahedral units with different unit distortions and the varying connectivity, have different energy band structures. As a result, at the interface of these two phases, the photogenerated electron-hole pairs will separate. The electrons inject from the conduction band of the anatase phase into that of the rutile phase, while the holes accumulate in the anatase phase.35 Then, they participate in the redox reactions with fuel (e.g. H2O2) in water according to equations (1)–(4).36–38 (1)

TiO2 + ℎ𝑣→h + + e ― H2O2 + 2h + (Anatase)→O2 +2H +

(2)

H2O2 +2e ― (Rutile) +2H + →2H2O

(3)

H2O2 + e ― (Rutile)→•OH + OH ―

(4)


6

Page 7 of 45 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


If anatase and rutile phases are asymmetrically distributed in a micro/nanosized TiO2 particle, the redox photochemical reactions will occur asymmetrically on its surface due to the charge separation at anatase/rutile homojunctions. The asymmetrical reactions then create a concentration gradient of products across the particle, which enables it to move autonomously through self-phoresis,39 as shown in Figure 1. Thus, the TiO2 micro/nanomotor based on phase asymmetry is highly feasible, but with an important question remains: How to realize the asymmetric distribution of anatase and rutile phases in a micro/nanoparticle? Temperature is considered widely as a key parameter in the growth of anatase grains and their phase transition to rutile.33 For an amorphous TiO2 micro/nanoparticle calcined at the temperature higher than 600 °C in air, the consisting short-range ordered TiO6 octahedral units arrange into long-range ordered anatase structure at first, and then the formed anatase grains gradually grow larger in size. There are always some grains became “coarser” which can provide nucleation sites to form rutile.32 Once rutile is formed, they grow fast by consuming surrounding anatase matrix grains.40–42 Due to the volume limitation and the growing size of the constituent grains over time, the particle in micro/nanoscale inevitably appears asymmetry in the


7


Page 8 of 45

distribution of anatase and rutile phases after calcination. Hence, the asymmetric distribution of anatase and rutile phases in a micro/nanoparticle is expected to be simply achieved by controlling the calcination temperature to control the phase transition and growth of primary grains. According to the design, TiO2 micromotors based on phase asymmetry have been fabricated by simply sintering amorphous TiO2 particles at 700 °C for 2 h. Due to the simple structure and easy fabrication of the micromotors, they can be fabricated in large scale with low cost. Figure 2a depicts the mass production of amorphous TiO2 microparticles in a 2000 mL beaker using cheap raw materials, and 6.08 g powder of the micromotors (Figure 2b) can be obtained in two batches. Figure 2c shows that almost all TiO2 particles are in spherical morphology with an average diameter of 0.6 μm. Compared with the smooth surfaces of the amorphous TiO2 microparticles (Figure S1, Supporting Information), there are plenty of large salient grains on the surfaces after calcination (Figure 2d). The X-ray diffraction (XRD) pattern clearly reveals that the particles have mixed phases of anatase and rutile (Figure 2e), from which the average size of constituent grains was calculated to be 55.2 nm via the Scherrer equation, and the mass fraction of


8

Page 9 of 45 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


anatase was confirmed to be 15.6 wt.%.43-45 As expected, due to the large size of grains, the arrangement of anatase and rutile grains in the particle exhibits obvious asymmetry. This phase asymmetry is verified by the close observation (Figure 2f–h and Figure S2, Supporting Information) on an ultrathin sectioned slice of a typical particle by transmission electron microscopy (TEM), which shows that the lower left region is concentrated with rutile grains (grains 1, 3 and 4) and the upper right one is rich in anatase phase (grains 2 and 5). Due to the inherent phase asymmetry, the anatase/rutile TiO2 micromotors show an efficient light-controlled autonomous motion because of the asymmetric surface redox reactions with H2O2 in aqueous media. When the anatase/rutile TiO2 micromotors are dispersed in the aqueous medium (1.0 wt.% H2O2), they only exhibit random Brownian motions without UV light irradiation (Figure 3a), while autonomous motions with an average speed up to 11 μm s−1 is observed when irradiated by UV light vertically from the objective of the optical microscope (so called vertical UV light hereafter) at an intensity of 80 mW cm−2 (Figure 3b and Video S1, Supporting Information). Besides the H2O2 fuel, the micromotors can also be powered by methanol and ethanol, which were usually used


9


Page 10 of 45

as hole scavengers in photocatalytic reactions, and the micromotors show an average speed of 3.5 and 4.2 μm s−1, respectively (Figure S3 and Video S2, Supporting Information). The stop-and-go motion of the three micromotors (Figure 3a and b) were tracked during three UV on-off cycles. The corresponding average speed over time is demonstrated in Figure 3c, reflecting the rapid response and reversible stop-and-go motions of the micromotors in response to “on-off” state of light irradiation. The speed distribution of the anatase/rutile TiO2 microparticles (Figure 3d), which is calculated from 254 micromotors randomly selected in the medium under UV irradiation, shows that over 90% of the particles are active with a speed ranging from 6 to 26 μm s−1 (over 43 body lengths per second). The wide speed distribution is attributed to the different degree of phase asymmetry of the micromotors. Light has been recently recognized as an excellent stimulus for controlling micro/nanomotors due to its high spatiotemporal resolution and remote manipulation, and thus various light-controlled micro/nanomotors have lately been designed by introducing asymmetry in chemical composition or geometry,10 such as TiO2/Pt,46 B-TiO2/Au,47 Au– WO3@C Janus micromotors,48 TiO2/Si nanotree,49 n+/p-Si@Pt nanowire,50 TiO2 and ZnO-


10

Page 11 of 45 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


Pt tubular micromotors.51,52 These micro/nanomotors have relative complex structures or involve expensive noble metals compared with the as-developed micromotors based on phase asymmetry, which inevitably result in difficulties in their large-scale fabrication and practical applications. No bubbles can be observed during the propulsion of the micromotors because the nucleation and growth of bubbles are highly inhibited owing to their small size and convex surface.53 In addition, the micromotors can autonomously move both in bulk and near the substrate (Figure S4 and Video S3, Supporting Information), suggesting the negligible contribution of substrate-involved phoresis (i.e. electroosmosis) on their propulsion. Thus, the propulsion mechanisms of the anatase/rutile TiO2 micromotors include selfelectrophoresis and nonelectrolyte diffusiophoresis (self-osmophoresis) based on their negatively-charged surfaces (zeta potential, −8.7 mV) and the neutral (O2, Figure S5, Supporting Information) and charged (H+ and OH-) products from the asymmetric photocatalytic redox reactions (equations (1)–(4)). However, the dominant propulsion mechanism of the micromotor is difficult to be determined because its motion direction relative to the phase configuration can not be effectively confirmed due to the negligible


11


Page 12 of 45

optical contrast between the anatase and rutile phases. Furthermore, we have previously reported that, by introducing external asymmetry in light irradiation, even isotropic TiO2 microparticles can move in aqueous media but only at a specific range of incident angles (β) of UV light.54 In contrast, due to their inherent asymmetry, the anatase/rutile TiO2 micromotors can move with a similar speed under UV irradiation in arbitrary directions (Figure S6 and Video S4, Supporting Information). This general applicability would simplify the setup of external light actuators for the micromotors and provide it with high flexibility in design according to application scenarios. The phase asymmetry of the TiO2 particles depends not only on the phase compositions but also the grain sizes, which can be easily modified by controlling the calcination temperature (T). The phase compositions and average grain sizes of the TiO2 particles obtained at different T are summarized in Table 1. The particles are composed of anatase only when T is under 500 °C, and rutile phase emerges and occupies larger and larger proportion as T increases from 600 to 700 °C. The average grain size increases from 7.9 to 55.2 nm with the increasing T from 300 to 700 °C. The particles consisting of the anatase phase only are isotropic, and thus they fail to autonomously move in the medium


12

Page 13 of 45 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


under vertical UV light irradiation due to the symmetric surface photocatalytic reactions (Figure 4a–b and Video S5, Supporting Information). In a sharp contrast, the particles obtained at 600 and 650 °C show fast autonomous motions in the medium due to their obvious asymmetry (Figure 4c–d and Video S5, Supporting Information). Accordingly, the average speed of the particles shows a speed jump at T = 600 °C due to the appearence of the phase asymmetry, and increases slowly from 10.3 to 11.7 μm s−1 with the increasing T from 600 to 700 °C due to the increasing degree of the phase asymmetry as the crystalline grains grow (Figure 4e). Since the anatase/rutile TiO2 micromotors are powered by photocatalysis, their motion speed can be controlled by regulating light intensity (I). As demonstrated in Figure 4f, the anatase/rutile TiO2 micromotors are ultrasensitive, and can be activated at a super low I of 1 mW cm−2. With the increasing I from 1 to 80 mW cm−2, their speed increases from 6.8 to 11.1 μm s−1 (Video S6, Supporting Information). Meanwhile, the dependence of the speed on I follows the Michaelis–Menten law (the black curve in Fig. 4g(i)), indicating that the maximum speed of the micromotor is limited by the total active sites of the micromotor for photocatalytic decomposition of H2O2. The increasing speed with I is attributed to that


13


Page 14 of 45

the increasing I promotes the photon flux (the number of incident photons per unit area per second) on the anatase/rutile TiO2 micromotors, and thus enhances the photocatalytic-reaction rate and the resulted product gradient across them.51 Furthermore, the mean squared displacement (MSD) of the micromotors versus time interval (Δt) were analyzed at different I of 0 and 80 mW cm−2 by tracking 10 micromotors over 12 s, respectively, as depicted in Figure 4g(ii). It can be clearly seen that the MSD of the micromotors under UV irradiation is much larger than that without UV light. Accordingly, the moving micromotors have a diffusion coefficient (D), which is calculated by the equation D = MSD/4Δt, over 16 times higher than the stationary ones (Figure 4g(iii)). Stemming

from

the

high

diffusivity

compared

to

traditional

stationary

micro/nanomaterials, active micromotors can largely boost their mass exchange with the environment, and thus exhibit enhanced performance in environmental remediation.7,55 Here we show that the anatase/rutile TiO2 micromotors can act as “swimming” photocatalysts (Video S7, Supporting Information) for high-efficient degradation of Rhodamine 6G on the fly (Figure 5). It can be seen that Rhodamine 6G (25 μM) in the medium is degradated over 90% in 10 min, and almost completely eliminated in 15 min,


14

Page 15 of 45 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


reflecting the high efficiency of the “swimming” anatase/rutile TiO2 micromotors (Figure 5a). In a sharp contrast, Rhodamine 6G in the medium can not be eliminated even after 25 min by the “stationary” micromotors (Figure 5b), of which the motion was quenched by adding 10 mM NaCl to retard their self-phoresis (Video S7, Supporting Information). The degradation kinetics of Rhodamine 6G by the “swimming” and “stationary” micromotors are summarized in Figure 5c and d, suggesting they follow the first-order kinetic model (equation 5), 𝑙𝑛

( ) = 𝑘𝑡 𝐶0 𝐶𝑡

(5)

where 𝐶0 and 𝐶𝑡 represent the original concentration and that at the UV irradiation time of 𝑡, and 𝑘 is the first-order rate constant. Without the addition of the micromotors and H2O2, the degradation of Rhodamine 6G is negligible (k = 4.7 × 10−3 min−1), and it degradates slowly with a k of 0.082 min−1 when 2.5 wt.% H2O2 was added. When the micromotors are present in the medium, the degradation of Rhodamine 6G is promoted, and the “swimming” micromotors show a much higher degradation efficiency (k = 0.26 min−1) than the “stationary” micromotors (k = 0.10 min−1). This higher efficiency is


15


Page 16 of 45

attributed to that the autonomous motions of the “swimming” micromotors facilitate the efficient dispersion of the photogenerated reactive oxidative species (•OH etc.) to degrade the target Rhodamine 6G molecules.56,57 Moreover, there is no noticeable decrease in the photocatalytic activity of the micromotors after 5 cycles (Figure S7, Supporting Information), indicating their good stability and reusability in photodegradation of organic pollutants. Even though numbers of micro/nanomotors have been discovered to be efficient for degradating organic pollutants in water thanks to their autonomous motions,

such

as

TiO2/Au/Mg,57

TiO2/Pt,46,58,

TiO2–Au,59

ZnO/ZnO2/Pt

Janus

micromotors60 and Fe/Pt tubular micromotors,61 the anatase/rutile TiO2 micromotors developed in this work have a remarkable advantage in water treatment because of their facile large-scale production and low cost.

CONCLUSIONS

In conclusion, we have proposed a novel design strategy of simple-structured micromotors by introducing asymmetry in crytalline phases. By controlling calcination temperature, the phase asymmetry can be easily introduced in spherical TiO2 particles


16

Page 17 of 45 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


due to the controlled phase transition and growth of primary grains. Thanks to the phase asymmetry, the spherical TiO2 particles can autonomously move in liquid media under light irradiation due to asymmetric surface photocatalytic redox reactions. The motion state and speed of the TiO2 micromotors can be reversibly, wirelessly and remotely controlled by adjusting “on-off” switch and intensity of UV light. The as-developed anatase/rutile TiO2 micromotors also show a promising application in water treatment because they not only can be fabricated in large scale due to their simple structures and easy preparation, but also show a high activity in “on-the-fly” photocatalytic degradation of organic pollutants. The design strategy proposed in this work is also expected to be applicable for the development of simple-structured micro/nanomotors with various chemical compositions, and it may pave the way for the large-scale productions and applications of micro/nanomotors.

EXPERIMENTAL SECTION

Preparation. The amorphous TiO2 microparticles with a size of 1.2 μm were prepared by a sol-gel method with some modifications according to the previous report.62 Briefly,


17


Page 18 of 45

2000 mL ethanol was mixed with 8 mL of 0.1 M sodium chloride (NaCl) solution and 34 mL tetrabutyl titanate (TBT, Sinopharm Chemical Reagent) at ambient temperature. After 18 min of stirring, the solution was allowed to stand for another 24 hours. The precipitant was separated, washed three times with alcohol and deionized water, then dried at 60 °C for 12 h. Then, the amorphous TiO2 microparticles were calcined at different temperature for 2 h to obtain TiO2 microparticles with different phase compositions. The anatase/rutile TiO2 microparticles were obtained by calcining the amorphous TiO2 microparticles at T of 600, 650 and 700 °C. The anatase TiO2 microparticles were obtained via similar processes at T of 300 and 500 °C. Characterization. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 Field-emission SEM (Japan) and the transmission electron microscopy (TEM) images were captured by JEM-F200 (Japan). To obtain ultra-thin section slices for high-resolution TEM (HRTEM) analysis, the TiO2 microparticles were at first embedded in epoxy resin,63 and then sectioned using a Leica EM UC7 ultramicrotome (Germany). The ultra-thin section slices were finally placed on the 200-mesh copper grids for the HRTEM analysis. X-ray diffraction (XRD) patterns of the samples were recorded on a


18

Page 19 of 45 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


Bruker D8 Advance X-ray diffractometer (Germany). The zeta potential of microparticles was obtained by Malvern Mastersizer 2000 (UK). Two low-angle peaks corresponding to the (101) plane of anatase and the (110) plane of rutile in the XRD patterns were selected to calculate the average grain sizes and phase compositions of the micromotors in Table 1 using Jade 6.5 and Highscore plus 3.0 software according to previous reports.43-45 Light-controlled motions. 50 µL of the TiO2 micromotor suspension was dropped onto a glass substrate, followed by adding 50 µL of 2.0 wt.% H2O2 solution. A fluorescence light source build-in the optical microscope (Leica EL6000, Germany) has a wavelength from 280 to 410 nm with an adjustable intensity (I) from 0–80 mW cm−2. The motions of the TiO2 micromotors under the continuous or pulsed UV irradiation were observed and recorded at room temperature through an optical microscope (Leica DMI 3000B, Germany). The motion behaviors of the micromotors in Rhodamine 6G solutions (25 µM) without or with NaCl (10 mM) were also investigated. Furthermore, a UV-LED light source (SZ Lamplic Technology, China) with a wavelength of 365 nm and I of 500 mW cm−2 was set above the substrate. The incident angle (β) was set to different values to study the


19


Page 20 of 45

motions of the micromotors depending on β. All videos of the motion of the micromotors were analyzed by using ImageJ and Video Spot Tracker V08.01 software. Quantitative detection of dissolved oxygen and pH value. 50 mL aqueous suspension with the TiO2 micromotors (0.5 mg mL−1) and H2O2 (0.01 wt.%) was put into a 50 mL beaker mounted with the probes of dissolved oxygen meter (Jenco 9173R, USA) and pH meter (Sartorius PB-10, China). An UV-LED light source (SZ Lamplic Technology, China) with a wavelength of 365 nm was set below the beaker. The concentration of the dissolved oxygen and pH value were measured when the UV light (I = 1 W cm-2) were turning on and off repeatedly. The concentration of the dissolved oxygen and pH value in a solution only with H2O2 (1 wt.%) were also measured by taking the same procedures. Photocatalytic degradation of Rhodamine 6G solution. The photocatalytic degradation of Rhodamine 6G solution was carried out in a 5 mL centrifugal tube. A mixture of 0.5 mL of Rhodamine 6G (100 mM), 0.5 mL of anatase/rutile TiO2 particles suspension (0.2 mg mL−1), 0.5 mL of H2O2 solution (10 wt.%) and 0.5 mL deionized water were placed into the tube. A UV-LED light source was placed under the centrifugal tube with a maximum I of 1800 mW cm−2. Before UV exposure, Rhodamine 6G was adorbed by the micromotors


20

Page 21 of 45 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


for 30 min to ensure the adsortion equlibrium. After UV exposure for a certain time, the concentration of Rhodamine 6G in sample solutions was analyzed by a UV-visible spectrophotometer (UV-2550, Shimadzu, Japan) immediately after separating the micromotors by centrifugation at 6000 rpm for 5 min. As a comparison, the photocatalytic degradation of Rhodamine 6G by “stationary” micromotors was also studied by adding 10 mM NaCl in the Rhodamine 6G solution. For cycling use experiments, after the asprepared micromotors were separated from solution by centrifugation, the supernatant was removed. Then, 0.5 mL of fresh Rhodamine 6G (100 mM) solution, 0.5 mL of H2O2 solution (10 wt.%) and 1 mL deionized water were added into the tube for another cycling use after the micromotors were re-dispersed by ultrasonication. ASSOCIATED CONTENT

Supporting Information.

Supporting video 1 (AVI)



21


Page 22 of 45






Supporting figures and descriptions of the videos (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

ORCID iD Fangzhi Mou: 0000-0002-9644-8277 Jianguo Guan: 0000-0002-2223-4524


22

Page 23 of 45 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


Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT

The authors are grateful for financial support from the National Natural Science Foundation of China (21875175, 21474078, 21705123, and 51521001), the Fundamental Research Funds for the Central Universities (WUT: 2017III028 and 2018III012), the Top Talents Lead Cultivation Project, and the Natural Science Foundation of Hubei Province (2015CFA003).

REFERENCES (1) 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, 13424–13431.


23


Page 24 of 45

(2) Wang, W.; Duan, W.; Ahmed, S.; Mallouk, T. E.; Sen, A. Small Power: Autonomous Nano- and Micromotors Propelled by Self-Generated Gradients. Nano

Today 2013, 8, 531–554.

(3) Sanchez, S.; Soler, L.; Katuri, J. Chemically Powered Micro- and Nanomotors.

Angew. Chem. Int. Ed. 2015, 54, 1414–1444.

(4) Chen, X.-Z.; Jang, B.; Ahmed, D.; Hu, C.; De Marco, C.; Hoop, M.; Mushtaq, F.; Nelson, B. J.; Pané, S. Small-Scale Machines Driven by External Power Sources. Adv.

Mater. 2018, 30, 1705061.

(5) Luo, M.; Feng, Y.; Wang, T.; Guan, J. Micro-/Nanorobots at Work in Active Drug Delivery. Adv. Funct. Mater. 2018, 28, 1706100.

(6) Guix, M.; Mayorga-Martinez, C. C.; Merkoçi, A. Nano/Micromotors in (Bio)chemical Science Applications. Chem. Rev. 2014, 114, 6285–6322.


24

Page 25 of 45 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


(7) Parmar, J.; Vilela, D.; Villa, K.; Wang, J.; Sanchez, S. Micro- and Nanomotors as Active Environmental Microcleaners and Sensors. J. Am. Chem. Soc. 2018, 140, 9317– 9331.

(8) Peng, F.; Tu, Y.; Wilson, D. A. Micro/nanomotors towards in Vivo Application: Cell, Tissue and Biofluid. Chem. Soc. Rev. 2017, 46, 5289–5310.

(9) Safdar, M.; Khan, S. U.; Janis, J. Progress toward Catalytic Micro- and Nanomotors for Biomedical and Environmental Applications. Adv. Mater. 2018, 30, 1703660.

(10) Xu, L.; Mou, F.; Gong, H.; Luo, M.; Guan, J. Light-Driven Micro/nanomotors: from Fundamentals to Applications. Chem. Soc. Rev. 2017, 46, 6905–6926.

(11) Song, M.; Cheng, M.; Xiao, M.; Zhang, L.; Ju, G.; Shi, F. Biomimicking of a Swim Bladder and Its Application as a Mini-Generator. Adv. Mater. 2017, 29, 1603312.


25


Page 26 of 45

(12) Yang, X.; Cheng, M.; Zhang, L.; Zhang, S.; Liu, X.; Shi, F. Electricity Generation through Light-Responsive Diving-Surfacing Locomotion of a Functionally Cooperating Smart Device. Adv. Mater. 2018, 30, 1803125.

(13) Xu, T.; Gao, W.; Xu, L.-P.; Zhang, X.; Wang, S. Fuel-Free Synthetic Micro/Nanomachines. Adv. Mater. 2017, 29, 1603250.

(14) Xu, B.; Zhang, B.; Wang, L.; Huang, G.; Mei, Y. Tubular Micro/Nanomachines: From the Basics to Recent Advances. Adv. Funct. Mater. 2018, 28, 1705872.

(15) Jurado-Sánchez, B.; Pacheco, M.; Maria-Hormigos, R.; Escarpa, A. Perspectives on Janus Micromotors: Materials and Applications. Applied Materials Today 2017, 9, 407–418.

(16) Mou, F.; Pan, D.; Chen, C.; Gao, Y.; Xu, L.; Guan, J. Magnetically Modulated PotLike MnFe2O4 Micromotors: Nanoparticle Assembly Fabrication and their Capability for Direct Oil Removal. Adv. Funct. Mater. 2015, 25, 6173–6181.


26

Page 27 of 45 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


(17) Wang, H.; Pumera, M. Fabrication of Micro/Nanoscale Motors. Chem. Rev. 2015,

115, 8704–8735.

(18) Wong, F.; Dey, K. K.; Sen, A. Synthetic Micro/Nanomotors and Pumps: Fabrication and Applications. Ann. Rev. Mater. Res. 2016, 46, 407–432.

(19) Wang, Y.; Hernandez, R. M.; Bartlett, 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, 10451–10456.

(20) Zhao, G.; Ambrosi, A.; Pumera, M. Self-propelled Nanojets via Template Electrodeposition. Nanoscale 2013, 5, 1319–1324.

(21) Fournier-Bidoz, S.; Arsenault, A. C.; Manners, I.; Ozin, G. A. Synthetic SelfPropelled Nanorotors. Chem. Commun. 2005, 441–443.

(22) Qin, L. D.; Banholzer, M. J.; Xu, X. Y.; Huang, L.; Mirkin, C. A. Rational Design and Synthesis of Catalytically Driven Nanorotors. J. Am. Chem. Soc. 2007, 129, 14870– 14871.


27


Page 28 of 45

(23) Baraban, L.; Makarov, D.; Streubel, R.; Monch, I.; Grimm, D.; Sanchez, S.; Schmidt, O. G. Catalytic Janus Motors on Microfluidic Chip: Deterministic Motion for Targeted Cargo Delivery. ACS Nano 2012, 6, 3383–3389.

(24) Solovev, A. A.; Mei, Y.; Bermudez Urena, E.; Huang, G.; Schmidt, O. G. Catalytic Microtubular Jet Engines Self-Propelled by Accumulated Gas Bubbles. Small 2009, 5, 1688–1692.

(25) Mei, Y.; Solovev, A. A.; Sanchez, S.; Schmidt, O. G. Rolled-Up Nanotech on Polymers: From Basic Perception to Self-Propelled Catalytic Microengines. Chem. Soc.

Rev. 2011, 40, 2109–2119.

(26) Harazim, S. M.; Xi, W.; Schmidt, C. K.; Sanchez, S.; Schmidt, O. G. Fabrication and Applications of Large Arrays of Multifunctional Rolled-Up SiO/SiO2 Microtubes. J.

Mater. Chem. 2012, 22, 2878–2884.

(27) Tottori, S.; Zhang, L.; Qiu, F. M.; Krawczyk, K. K.; Franco-Obregon, A.; Nelson, B. J. Magnetic Helical Micromachines: Fabrication, Controlled Swimming, and Cargo Transport. Adv. Mater. 2012, 24, 811–816.


28

Page 29 of 45 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


(28) Zeeshan, M. A.; Grisch, R.; Pellicer, E.; Sivaraman, K. M.; Peyer, K. E.; Sort, J.; Ozkale, B.; Sakar, M. S.; Nelson, B. J.; Pane, S. Hybrid Helical Magnetic Microrobots Obtained by 3D Template-Assisted Electrodeposition. Small 2014, 10, 1284–1288.

(29) Wu, Y.; Wu, Z.; Lin, X.; He, Q.; Li, J. Autonomous Movement of Controllable Assembled Janus Capsule Motors. ACS Nano 2012, 6, 10910–10916.

(30) Li, S. S.; Li, W. J.; Jiang, T. J.; Liu, Z. G.; Chen, X.; Cong, H. P.; Liu, J. H.; Huang, Y. Y.; Li, L. N.; Huang, X. J. Iron Oxide with Different Crystal Phases (alphaand gamma-Fe2O3) in Electroanalysis and Uftrasensitive and Selective Detection of Lead(II): An Advancing Approach Using XPS and EXAFS. Anal. Chem. 2016, 88, 906– 914.

(31) Liang, S.; Teng, F.; Bulgan, G.; Zong, R.; Zhu, Y. Effect of Phase Structure of MnO2 Nanorod Catalyst on the Activity for CO Oxidation. J. Phys. Chem. C 2008, 112, 5307–5315.


29


Page 30 of 45

(32) Kumar, K.-N. P.; Keizer, K.; Burggraaf, A. J.; Okubo, T.; Nagamoto, H.; Morooka, S. Densification of Nanostructured Titania Assisted by a Phase Transformation. Nature 1992, 358, 48–51.

(33) Hanaor, D. A. H.; Sorrell, C. C. Review of the Anatase to Rutile Phase Transformation. J. Mater. Sci. 2011, 46, 855–874.

(34) Mavracic, J.; Mocanu, F. C.; Deringer, V. L.; Csanyi, G.; Elliott, S. R. Similarity Between Amorphous and Crystalline Phases: The Case of TiO2. J. Phys. Chem. Lett. 2018, 9, 2985–2990.

(35) Low, J.; Yu, J.; Jaroniec, M.; Wageh, S.; Al-Ghamdi, A. A. Heterojunction Photocatalysts. Adv. Mater. 2017, 29, 1601694.

(36) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations. Chem. Rev. 2014, 114, 9987– 10043.


30

Page 31 of 45 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


(37) Konstantinou, I. K.; Albanis, T. A. TiO2-Assisted Photocatalytic Degradation of Azo Dyes in Aqueous Solution: Kinetic and Mechanistic Investigations - A Review. Appl.

Catal. B: Environ. 2004, 49, 1–14.

(38) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69–96.

(39) Moran, J. L.; Posner, J. D. Phoretic Self-Propulsion. Annu. Rev. Fluid. Mech. 2017, 49, 511–540.

(40) Gouma, P. I.; Mills, M. J. Anatase-to-Rutile Transformation in Titania Powders. J.

Am. Ceram. Soc. 2001, 84, 619–622.

(41) Gouma, P. I.; Dutta, P. K.; Mills, M. J. Structural Stability of Titania Thin Films.

Nanostruct. Mater. 1999, 11, 1231–1237.

(42) Koparde, V. N.; Cummings, P. T. Phase Transformations during Sintering of Titania Nanoparticles. ACS Nano 2008, 2, 1620–1624.


31


Page 32 of 45

(43) Patterson, A. L. The Scherrer Formula for X-Ray Particle Size Determination.

Phys. Rev. 1939, 56, 978–982.

(44) Hajjaji, W.; Jeridi, K.; Seabra, P.; Rocha, F.; Labrincha, J. A.; Jamoussi, F. Composition and Properties of Glass Obtained from Early Cretaceous Sidi Aich Sands (central Tunisia). Ceram. Int. 2009, 35, 3229–3234.

(45) Mohiuddin, K.; Strezov, V.; Nelson, P. F.; Evans, T. Bonding Structure and Mineral Analysis of Size Resolved Atmospheric Particles nearby Steelmaking Industrial Sites in Australia. Aerosol Air Qual. Res. 2016, 16, 1638–1650.

(46) 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.

(47) Jang, B.; Hong, A.; Kang, H. E.; Alcantara, C.; Charreyron, S.; Mushtaq, F.; Pellicer, E.; Buchel, R.; Sort, J.; Lee, S. S.; Nelson, B. J.; Pane, S. Multiwavelength Light-Responsive Au/B-TiO2 Janus Micromotors. ACS Nano 2017, 11, 6146–6154.


32

Page 33 of 45 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


(48) 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.

(49) Zheng, J.; Dai, B.; Wang, J.; Xiong, Z.; Yang, Y.; Liu, J.; Zhan, X.; Wan, Z.; Tang, J. Orthogonal Navigation of Multiple Visible-Light-Driven Artificial Microswimmers. Nat.

Commun. 2017, 8, 1438.

(50) Wang, J.; Xiong, Z.; Zhan, X.; Dai, B.; Zheng, J.; Liu, J.; Tang, J. A Silicon Nanowire as a Spectrally Tunable Light-Driven Nanomotor. Adv. Mater. 2017, 29, 1701451.

(51) Mou, F.; Li, Y.; Chen, C.; Li, W.; Yin, Y.; Ma, H.; Guan, J. Single-Component TiO2 Tubular Microengines with Motion Controlled by Light-Induced Bubbles. Small 2015, 11, 2564–2570.

(52) Dong, R.; Wang, C.; Wang, Q.; Pei, A.; She, X.; Zhang, Y.; Cai, Y. ZnO-Based Microrockets with Light-Enhanced Propulsion. Nanoscale 2017, 9, 15027–15032.


33


Page 34 of 45

(53) Fletcher, N. H. Size Effect in Heterogeneous Nucleation. J. Chem. Phys. 1958,

29, 572–576.

(54) Chen, C. R.; Mou, F.; Xu, L.; Wang, S.; Guan, J.; Feng, Z.; Wang, Q.; Kong, L.; Li, W.; Wang, J.; Zhang, Q. Light-Steered Isotropic Semiconductor Micromotors. Adv.

Mater. 2017, 29, 1603374.

(55) Jurado-Sanchez, B.; Wang, J. Micromotors for Environmental Applications: A Review. Environ. Sci.-Nano 2018, 5, 1530–1544.

(56) Gao, W.; Wang, J. The Environmental Impact of Micro/Nanomachines: A Review.

ACS Nano 2014, 8, 3170–3180.

(57) 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.


34

Page 35 of 45 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


(58) Kong, L.; Mayorga-Martinez, C. C.; Guan, J.; Pumera, M. Fuel-Free LightPowered TiO2/Pt Janus Micromotors for Enhanced Nitroaromatic Explosives Degradation. ACS Appl. Mater. Interfaces 2018, 10, 22427–22434.

(59) Wu, Y.; Dong, R.; Zhang, Q.; Ren, B. Dye-Enhanced Self-Electrophoretic Propulsion of Light-Driven TiO2-Au Janus Micromotors. Nano-Micro Lett. 2017, 9, 30.

(60) Pourrahimi, A. M.; Villa, K.; Ying, Y.; Sofer, Z.; Pumera, M. ZnO/ZnO2/Pt Janus Micromotors Propulsion Mode Changes with Size and Interface Structure: Enhanced Nitroaromatic Explosives Degradation under Visible Light. ACS Appl. Mater. Interfaces 2018, 10, 42688–42697.

(61) Soler, L. s.; Magdanz, V.; Fomin, V. M.; Sanchez, S.; Schmidt, O. G. SelfPropelled Micromotors for Cleaning Polluted Water. ACS Nano 2013, 7, 9611–9620.

(62) Eiden-Assmann, S.; Widoniak, J.; Maret, G. Synthesis and Characterization of Porous and Nonporous Monodisperse Colloidal TiO2 Particles. Chem. Mater. 2004, 16, 6–11.


35


Page 36 of 45

(63) Richardson, K. C.; Jarett, L.; Fink, E. H. Embedding in Epoxy Resins for Ultrathin Sectioning in Electron Microscopy. Stain Technol. 1960, 35, 313–323.


36

Page 37 of 45 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


GRAPHICAL ABSTRACT


37


Page 38 of 45

Figure 1. Schematic diagram of the TiO2 micromotor based on phase asymmetry. When irradiated by UV light, the TiO2 micromotor consisting of asymmetrically distributed grains of anatase (A) and rutile (R) produces a concentration gradient of products (oxygen molecules, H+ ions, etc.), and thus can autonomously move in the medium through self-phoresis.


38

Page 39 of 45 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


Figure 2. The mass production and characterization of the anatase/rutile TiO2 microparticles. (a) Optical images of the aqueous suspension of amorphous TiO2 microparticles prepared in a 2000 mL beaker by a sol-gel method, and (b) the anatase/rutile TiO2 microparticles (6.08 g) in a Petri dish, which are obtained in two


39


Page 40 of 45

batches. A one RMB coin was used for comparison. (c–d) SEM images, (e) XRD pattern and (f–h) TEM images of the as-prepared anatase/rutile TiO2 microparticles. The TEM image in (f) shows an ultra-thin section of an anatase/rutile TiO2 microparticle at low magnification, and it has a diameter of 450 nm slightly smaller than that of the particle (600 nm) because it was not sectioned at the equator of the particle. There are several notable grains, and 5 of them were further characterized by high-resolution TEM (HRTEM), suggesting grain 1 (g), 3, 4 were rutile, and 2 (h), 5 were anatase. HRTEM characterization of grain 3, 4 and 5 is given in Figure S2 (Supporting Information).


40

Page 41 of 45 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


Figure 3. Light-controlled autonomous motions. (a–b) Trajectories of three anatase/rutile TiO2 micromotors over 2 s in the aqueous medium with 1.0 wt.% H2O2 when UV light (I = 80 mW cm−2) is off (a) and on (b), respectively. Images are taken from Video S1 (Supporting Information). Scale bar, 5 μm. (c) The variation of the motion speed of a typical micromotor under the pulsed UV light illumination. (d) The speed distribution histogram of the micromotors obtained by tracking 254 particles.


41


Page 42 of 45

Figure 4. Autonomous motions of the TiO2 microparticles depending on the calcination temperature (T) and light intensity (I). (a–d) SEM images (i), XRD patterns (ii) and motion trajectories over 2 s (iii) under UV irradiation of the TiO2 microparticles prepared at T of 300 (a), 500 (b), 600 (c) and 650 °C (d), respectively. Images in (iii) of a–d are taken from Video S5 (Supporting Information). The scale bars in (i) of a–d are 200 nm, and the others in (iii) are 5 μm. (e) The speed of the micromotors obtained at different T, indicating the


42

Page 43 of 45 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


speed increases with T under UV irradiation (I = 80 mW cm−2). (f–g) Autonomous motions of the micromotors obtained at T of 650 °C at different I. (f) Trajectories of the micromotors over 3 s at I of 0, 1, 10, and 80 mW cm−2, respectively. Images are taken from Video S6 (Supporting Information). Scale bar, 5 μm. (g) Average speed (i), MSD (ii) and diffusion coefficient (iii) of the micromotors at different I, which are determined by tracking 10 micromotors. The concentration of H2O2 in the medium is 1.0 wt.%.

Figure 5. Photocatalytic degradation of Rhodamine 6G. (a–b) Absorbance spectra of Rhodamine 6G in the medium (C0 = 25 μM) before and after UV exposure (1.8 W cm−2) for different time with the “swimming” (a) and “stationary” (b) micromotors (obtained at T


43


Page 44 of 45

of 700 °C). Before UV exposure, Rhodamine 6G was adsorbed by the micromotors for 30 min to ensure the adsorption equilibrium. The concentration of the micromotors and H2O2 in the medium is 0.05 mg/mL and 2.5 wt.%, respectively. The inset in a depicts the trajectory (2 s) of a typical “swimming” micromotor, and that in b shows the trajectory of a “stationary” micromotor, whose motion was quenched by adding 10 mM NaCl solution in the medium. Images are taken from VideoS7 (Supporting Information). Scale bars, 5 μm. (c) Time dependence of the photocatalytic degradation of Rhodamine 6G in the medium containing the “swimming” (red dots) and “stationary” (blue squares) micromotors. (d) The corresponding degradation rate constant (k) obtained after fitting the experimental data by the first-order kinetic model. The degradation of Rhodamine 6G in the medium without micromotors and H2O2 (black triangles) or with only H2O2 (green triangles) were used as controls.

Table 1. The proportion of crystalline phase composition and average grain size of TiO2 microparticles obtained at different T.


44

Page 45 of 45 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


Calcination temperature (°C)

Anatase (wt.%)

Rutile (wt.%)

Average grain size (nm)

300

100

0

7.9

500

100

0

13.2

600

78.0

22.0

35.1

650

69.0

31.0

49.1

700

15.6

84.4

55.2


45

Simple-Structured Micromotors Based on Inherent Asymmetry in

Recommend Documents