Current projects: 2. Photochemically powered active colloids
Among the many types of active colloids powered by chemical reactions, those that convert light into mechanical motion is attractive because it offers the advantage of easy control and modulation. We are particularly interested in active colloids made of photo-semiconductor materials, such as TiO2, Fe2O3 or ZnO, that upon irradiation of light of appropriate wavelengths generate electrons and holes, which induce chemical reactions on the particle surface and propel the micromotor as a result. Despite intense research interest on this type of colloid in the literature, we believe the ground is still fertile, and that there remains many unsolved scientific questions to be answered and interesting observations to be made.
1. Zuyao Xiao, Shifang Duan, Pengzhao Xu, Jingqin Cui, Hepeng Zhang, and Wei Wang, Synergistic Speed Enhancement of an Electric-Photochemical Hybrid Micromotor by Tilt Rectification, ACS Nano, Accepted (published on June 12, 2020)
A hybrid micromotor is an active colloid powered by more than one power source, often exhibiting expanded functionality and controllability than those of singular energy source. However, these power sources are often applied orthogonally, leading to stacked propulsion that is just a sum of two independent mechanisms. Here, we report that TiO2-Pt Janus micromotors, when subject to both UV light and AC electric fields, moved up to 90% faster than simply adding up the speed powered by either source. This unexpected synergy between light and electric fields, we propose, arises from the fact that an electrokinetically powered TiO2-Pt micromotor moves near a substrate with a tilted Janus interface that, upon the application of an electric field, becomes rectified to be vertical to the substrate. Control experiments with magnetic fields and three types of micromotors unambiguously and quantitatively show that the tilting angle of a micromotor correlates positively with its instantaneous speed, reaching maximum at a vertical Janus interface. Such “tilting induced retardation” could affect a wide variety of chemically powered micromotors, and our findings are therefore helpful in understanding the dynamics of micromachines in confinement.
2. Pengzhao Xu, Shifang Duan, Zuyao Xiao, Zhou Yang and Wei Wang, Light-powered Active Colloids from
Monodisperse and Highly Tunable Microspheres with A Thin TiO2 shell, Soft Matter, ASAP (published on June 5, 2020)
The emerging field of active matter, and its subset active colloid, is in great need of good model systems consisting of moving entities that are uniform and highly tunable. In this article, we address this challenge by introducing core-shell SiO2-TiO2 microspheres, prepared by chemically coating a thin layer of TiO2 on an inert core, that are highly monodisperse in size (polydispersity 4.1%) and regular in shape (circularity 0.93). Compared with similar samples prepared by a classic sol-gel method, Janus TiO2-Pt active colloids prepared with core-shell TiO2 spheres move faster and boast a much clearer Janus interface. Moreover, a unique feature of these core-shell TiO2 microspheres is their great tunability in colloid size, shell thickness, and even the type of the core particle. These advantages are highlighted in two examples, one demonstrating a TiO2-Pt active colloid with a magnetic core that enables magnetic manipulation, and the other demonstrating the collective expansion and contraction of a uniform cluster of core-shell TiO2 colloids under UV light illumination. We believe TiO2 microspheres produced by this core-shell technique compare favorably with many other types of active colloids being employed as model systems, and thus open up many research possibilities.
3. Zuyao Xiao, Jingyuan Chen, Shifang Duan, Xianglong Lv, Jizhuang Wang, Xing Ma, Jinyao Tang, and Wei Wang*, Bimetallic coatings synergistically enhance the speeds of photocatalytic TiO2 micromotors, Chemical Communications, 2020, 56, 4728-4731 (published on Mar. 2, 2020)
The design of powerful, more biocompatible microrobots calls for faster catalytic reactions. Here we demonstrate a two-fold increase in the speed of photocatalytic TiO2–metal Janus micromotors via a Au/Ag bi-layered coating. Electrochemical measurements show that such a bimetallic coating is a better photocatalyst than either metal alone. Similarly, an additional sputtered Ag layer could also significantly increase the speed of Pt–PS or TiO2–Pt micromotors, suggesting that applying bimetallic coatings is a generalizable strategy in the design of faster catalytic micromotors.
4. Sinan Du, Huaguang Wang, Chao Zhou, Wei Wang* and Zexin Zhang*, Motor and Rotor in One: Light-Active ZnO/Au Twinned Rods of Tunable Motion Modes, J. Am. Chem. Soc. 2020, 142, 5, 2213-2217 (published on Jan. 19, 2020)
Precise control of the motion of micromachines is the key to achieving their functions for practical applications. The main challenge is that a given micromachine can typically exhibit only one motion mode, i.e., translation or rotation, while having multiple modes of motion resulting from a simple actuation is still rare. Here we designed and synthesized photochemically powered zinc oxide/gold (ZnO/Au) rods that exhibit multiple motion modes. Under homogeneous UV irradiation, these ZnO/Au rods undergo a transition from ballistic motion to persistent rotational motion upon increasing the fuel concentration or the light intensity. In addition, the rods can switch modes from a circular motion to a helical motion and then a straight-line motion by tuning the angle of incident light. We envision that such attractive colloidal micromachines with controllable motions hold considerable promise for diverse practical applications.
We are also investigating the single particle dynamics of a TiO2/Pt Janus micromotor under light, as well as their collective dynamics.
1. Zuyao Xiao, Shifang Duan, Pengzhao Xu, Jingqin Cui, Hepeng Zhang, and Wei Wang, Synergistic Speed Enhancement of an Electric-Photochemical Hybrid Micromotor by Tilt Rectification, ACS Nano, Accepted (published on June 12, 2020)
A hybrid micromotor is an active colloid powered by more than one power source, often exhibiting expanded functionality and controllability than those of singular energy source. However, these power sources are often applied orthogonally, leading to stacked propulsion that is just a sum of two independent mechanisms. Here, we report that TiO2-Pt Janus micromotors, when subject to both UV light and AC electric fields, moved up to 90% faster than simply adding up the speed powered by either source. This unexpected synergy between light and electric fields, we propose, arises from the fact that an electrokinetically powered TiO2-Pt micromotor moves near a substrate with a tilted Janus interface that, upon the application of an electric field, becomes rectified to be vertical to the substrate. Control experiments with magnetic fields and three types of micromotors unambiguously and quantitatively show that the tilting angle of a micromotor correlates positively with its instantaneous speed, reaching maximum at a vertical Janus interface. Such “tilting induced retardation” could affect a wide variety of chemically powered micromotors, and our findings are therefore helpful in understanding the dynamics of micromachines in confinement.
2. Pengzhao Xu, Shifang Duan, Zuyao Xiao, Zhou Yang and Wei Wang, Light-powered Active Colloids from
Monodisperse and Highly Tunable Microspheres with A Thin TiO2 shell, Soft Matter, ASAP (published on June 5, 2020)
The emerging field of active matter, and its subset active colloid, is in great need of good model systems consisting of moving entities that are uniform and highly tunable. In this article, we address this challenge by introducing core-shell SiO2-TiO2 microspheres, prepared by chemically coating a thin layer of TiO2 on an inert core, that are highly monodisperse in size (polydispersity 4.1%) and regular in shape (circularity 0.93). Compared with similar samples prepared by a classic sol-gel method, Janus TiO2-Pt active colloids prepared with core-shell TiO2 spheres move faster and boast a much clearer Janus interface. Moreover, a unique feature of these core-shell TiO2 microspheres is their great tunability in colloid size, shell thickness, and even the type of the core particle. These advantages are highlighted in two examples, one demonstrating a TiO2-Pt active colloid with a magnetic core that enables magnetic manipulation, and the other demonstrating the collective expansion and contraction of a uniform cluster of core-shell TiO2 colloids under UV light illumination. We believe TiO2 microspheres produced by this core-shell technique compare favorably with many other types of active colloids being employed as model systems, and thus open up many research possibilities.
3. Zuyao Xiao, Jingyuan Chen, Shifang Duan, Xianglong Lv, Jizhuang Wang, Xing Ma, Jinyao Tang, and Wei Wang*, Bimetallic coatings synergistically enhance the speeds of photocatalytic TiO2 micromotors, Chemical Communications, 2020, 56, 4728-4731 (published on Mar. 2, 2020)
The design of powerful, more biocompatible microrobots calls for faster catalytic reactions. Here we demonstrate a two-fold increase in the speed of photocatalytic TiO2–metal Janus micromotors via a Au/Ag bi-layered coating. Electrochemical measurements show that such a bimetallic coating is a better photocatalyst than either metal alone. Similarly, an additional sputtered Ag layer could also significantly increase the speed of Pt–PS or TiO2–Pt micromotors, suggesting that applying bimetallic coatings is a generalizable strategy in the design of faster catalytic micromotors.
4. Sinan Du, Huaguang Wang, Chao Zhou, Wei Wang* and Zexin Zhang*, Motor and Rotor in One: Light-Active ZnO/Au Twinned Rods of Tunable Motion Modes, J. Am. Chem. Soc. 2020, 142, 5, 2213-2217 (published on Jan. 19, 2020)
Precise control of the motion of micromachines is the key to achieving their functions for practical applications. The main challenge is that a given micromachine can typically exhibit only one motion mode, i.e., translation or rotation, while having multiple modes of motion resulting from a simple actuation is still rare. Here we designed and synthesized photochemically powered zinc oxide/gold (ZnO/Au) rods that exhibit multiple motion modes. Under homogeneous UV irradiation, these ZnO/Au rods undergo a transition from ballistic motion to persistent rotational motion upon increasing the fuel concentration or the light intensity. In addition, the rods can switch modes from a circular motion to a helical motion and then a straight-line motion by tuning the angle of incident light. We envision that such attractive colloidal micromachines with controllable motions hold considerable promise for diverse practical applications.
We are also investigating the single particle dynamics of a TiO2/Pt Janus micromotor under light, as well as their collective dynamics.
Older papers on this topic:
1. Dylan Nicholls, Andrew DeVerse, Ra’Shae Esplin, John Castañeda, Yoseph Loyd, Raaman Nair, Robert Voinescu, Chao Zhou, Wei Wang, and John G Gibbs*, Shape-Dependent Motion of Structured Photoactive Microswimmers, ACS Applied Materials & Interfaces, 2018, 10(21), 18050-18056 (published May 3, 2018)
2. Chao Zhou, Hepeng Zhang, Jinyao Tang, and Wei Wang*, Photochemically Powered AgCl Janus Micromotors as A Model System to Understand Ionic Self-diffusiophoresis, Langmuir, 2018, 34(10), 3289-3295
3. Wang, Y.; Zhou, C.; Wang, W.; Xu, D.; Zeng, F.; Zhan, C.; Gu, J.; Li, M.; Zhao, W.; Zhang, J.; et al. Photocatalytically Powered Matchlike Nanomotor for Light‐Guided Active SERS Sensing. Angew. Chem. Int. Ed. Engl. 2018, 57 (40), 13110–13113.
4. Dekai Zhou, Liqiang Ren, Yuguang C. Li, Pengtao Xu, YuanGaoa, Guangyu Zhang, Wei Wang*, Thomas E. Mallouk*, and Longqiu Li*, Visible Light-Driven, Magnetically Steerable Gold/Iron Oxide Nanomotors, Chem. Comm., 2017, 53, 11465-11468 (published on Oct. 2, 2017)
5. Dekai Zhou, Yuguang C. Li, Pengtao Xu, Nicholas S. McCool, Longqiu Li*, Wei Wang and Thomas E. Mallouk*, Visible-light controlled catalytic Cu2O–Au micromotors, Nanoscale, 2017, 9, 75-78 (published on Nov. 28, 2016)