Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ZnO/ZnO2/Pt Janus Micromotors Propulsion Mode Changes with Size and Interface Structure: Enhanced Nitroaromatic Explosives Degradation under Visible Light Amir Masoud Pourrahimi, Katherine Villa, Yulong Ying, Zdeněk Sofer, and Martin Pumera* Center for Advanced Functional Nanorobots, Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague, Czech Republic
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ABSTRACT: Self-motile mesoporous ZnO/Pt-based Janus micromotors accelerated by bubble propulsion that provide efficient removal of explosives and dye pollutants via photodegradation under visible light are presented. Decomposition of H2O2 (the fuel) is triggered by a platinum catalytic layer asymmetrically deposited on the nanosheets of the hierarchical and mesoporous ZnO microparticles. The sizedependent motion behavior of the mesoporous micromotors is studied; the micromotors with average size ∼1.5 μm exhibit enhanced self-diffusiophoretic motion, whereas the fast bubble propulsion is detected for micromotors larger than 5 μm. The bubble-propelled mesoporous ZnO/Pt Janus micromotors show remarkable speeds of over 350 μm s−1 at H2O2 concentrations lower than 5 wt %, which is unusual for Janus micromotors based on dense materials such as ZnO. This high speed is related to efficient bubble nucleation, pinning, and growth due to the highly active and rough surface area of these micromotors, whereas the ZnO/Pt particles with a smooth surface and low surface area are motionless. We discovered new atomic interfaces of ZnO2 introduced into the ZnO/Pt micromotor system, as revealed by X-ray diffraction (XRD), which contribute to enhance their photocatalytic activity under visible light. Such coupling of the rapid movement with the high catalytic performance of ZnO/Pt Janus micromotors provides efficient removal of nitroaromatic explosives and dye pollutants from contaminated water under visible light without the need for UV irradiation. This paves the way for real-world environmental remediation efforts using microrobots. KEYWORDS: micromotors, mesoporous, water purification, diffusiophoresis, bubble propulsion
1. INTRODUCTION Self-propelled micro/nanomotors, which can move autonomously powered by a catalytic reaction, have shown great potential for different applications, e.g., drug delivery, sensing, and environmental remediation.1−10 Catalytically powered micromotors are designed into different morphologies, i.e., multimetallic nanorods, tubes, and Janus microspheres, that play a key role in their motion behavior.11−15 Although the tubular morphologies show high-speed forward thrust through bubble propulsion, their material development requires complex syntheses, which yield only a few milligrams of the micromotors.16 On the contrary, the synthetic methods for preparation of Janus microspheres are relatively versatile and inexpensive, provide high yields of the materials, and can be up-scaled for gram-scales production.1,17−20 However, the motion behavior of Janus microspheres, including the direction and speed, is difficult to control. One of the challenges in the development of Janus particles is to find high-speed micromotors with controlled directional motion. Catalytically powered Janus micromotors consist of two components showing two distinct faces, where one specific face, mainly functionalized with noble metal platinum (Pt), © XXXX American Chemical Society
catalyzes the fuel (H2O2) decomposition to provide the propulsion.1 The mechanisms responsible for the motion are reported to be high-speed bubble propulsion and low-speed diffusiophoresis depending on size and surface porosity of the micromotors.1,21,22 Other Janus micromotors based on Mg are able to reduce pure water and generate hydrogen bubbles without using any H2O2.23 The other face of Janus micromotors could act as cargo for drug delivery or cleaning agents for water-purification approaches. This side is mainly made of the high surface area and low-density material such as silica (2.65 g cm−3) or polystyrene (PS, 1.04 g cm−3), which are inactive toward the final application, i.e., drug delivery, sensing, and environmental remediation.1 A subsequent modification of the inactive side of the particle with active species in consecutive steps of the fabrication of Janus micromotors using active materials with high surface area, such as activated carbon,24 TiO2,3,23 WO3,25 and BiOI, is favorable.26 ZnO and TiO2 semiconducting particles with a large bandgap (∼3.3 eV) Received: September 17, 2018 Accepted: November 19, 2018
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DOI: 10.1021/acsami.8b16217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
show enhanced diffusion at low H2O2 concentrations (i.e., 3 times. It was reported that the high surface porosity and large cavities could reduce the slip velocity around the micromotors,21,22 and it is therefore required to design mesoporous and hierarchical micromotors with optimal surface porosity and enhanced speed. Here, we present mesoporous and hierarchical spherical Janus ZnO/Pt micromotors of various sizes and surface porosity. First, highly structured porous ZnO flower-shaped particles, with sizes between 1.5 and 5 μm, are designed based on nanosheet assemblies. The catalytic Pt layer is deposited into pores of the self-assembled ZnO nanosheets, and the catalytic decomposition of H2O2 fuel thus occurs inside the cavities in contrast to the conventional smooth Janus particles. Depending on the size and porosity of the particles, the motility behavior and the underlying mechanism are different, which is illustrated in Scheme 1. The particles that are 350 μm s−1 (corresponding to ca. 70 body-length s−1, assuming the micromotors are 5 μm in diameter) in the presence of a fairly low level of the fuel, i.e., 10 nm) D
DOI: 10.1021/acsami.8b16217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces may enhance the bubble nucleation similarly to micromotors with tubular morphology.21 The bubble-propelled ZnO/Pt Janus micromotors hereby showed remarkably high speed compared to other fast micromotors reported in the literature, as shown in Figure 3e. Our porous ZnO/Pt Janus micromotors, although they have higher bulk density, which might hinder their fast swimming motion, exhibited higher speed than the bubble-propelled micromotors based on either lower bulk density materials (i.e., carbon, silica, and PS)1,24,32 or tubular morphology (ZnO/Pt).16 It is, therefore, worthwhile to assess different parameters such as size, density, porosity, surface morphology, and catalytic activity in our ZnO/Pt micromotors from the molecular level to microscales, in order to shed light on underlying mechanisms of enhanced motion in these novel micromotors. To explore the effect of surface roughness on the motion of the particles, control experiments were carried out using clusters of PS/Pt Janus micromotors with similar sizes close to our bubble-propelled ZnO/Pt micromotors. The SEM images of these micromotors showed that 1 μm primary PS particles were fused in larger clusters (i.e., >3 μm), which is comparable to the size of our ZnO microparticles (see Figure S2). The deposited Pt on PS microparticle clusters exhibited a smooth surface on the contrary to our hierarchical ZnO microparticles, which was not able to form any bubbles for low H2O2 concentration (
