Dipole‐Moment Induced Phototaxis and Fuel‐Free Propulsion of ZnO/Pt Janus Micromotors

Abstract

Light‐driven micromotors have stimulated considerate interests due to their potentials in biomedicine, environmental remediation, or serving as the model system for non‐equilibrium physics of active matter. Simultaneous control over the motion direction and speed of micro/nanomotors is crucial for their functionality but still difficult since Brownian motion always randomizes the orientations. Here, a highly efficient light‐driven ZnO/Pt Janus micromotor capable of aligning itself to illumination direction and exhibiting negative phototaxis at high speeds (up to 32 µm s−1) without the addition of any chemical fuels is developed. A light‐triggered self‐built electric field parallel to the light illumination exists due to asymmetrical surface chemical reactions induced by the limited penetration depth of light along the illumination. The phototactic motion of the motor is achieved through electrophoretic rotation induced by the asymmetrical distribution of zeta potential on the two hemispheres of the Janus micromotor, into alignment with the electric field. Notably, similar phototactic propulsion is also achieved on TiO2/Pt and CdS/Pt micromotors, which presents explicit proof of extending the mechanism of dipole‐moment induced phototactic propulsion in other light‐driven Janus micromotors. Finally, active transportation of yeast cells are achieved by the motor, proving its capability in performing complex tasks.

Light-driven Au–ZnO nanorod motors for enhanced photocatalytic degradation of tetracycline

“…The simultaneous existence of E 0 and E resulted in an unstable motion of Au-ZnO nanomotors. Then, a phoretic torque (T ) would be generated to rotate the motor until E 0 coincided with E. 28,50 Photocatalytic degradation of TC After confirming the efficient motion behaviour of our nanomotors, photodegradation of TC was investigated under UV light irradiation to evaluate the photocatalytic activities of ZnO, Au-ZnO nanorod arrays (Au-ZnO-A), and Au-ZnO nanorod motors (Au-ZnO-M). As shown in Fig.…”

“…18 Additionally, ZnO was oxidized to produce Zn 2+ by the photoexcited holes through photocorrosion. 28 Therefore, an electric field directed from the ZnO side to the Au side was generated around the motor as a result of the asymmetric distribution of H + and Zn 2+ . Meanwhile, H + and Zn 2+ flowed along the concentration gradient to the Au side, generating a slip velocity and resulting in self-propulsion of the negatively charged Au–ZnO nanomotors ( ζ Au–ZnO = −9.45 mV, Fig.…”

“…18,26 ZnO is a widely used photocatalytic semiconductor with the advantages of low cost, high abundance, and nontoxicity. 27 Although a previous study showed the phototaxis of spherical ZnO/Pt Janus micromotors, 28 it is still difficult to control the distribution of spherical particles on the substrate into high-density monolayer particles for metal coating. In addition, ZnO can vertically align via epitaxy on various substrates to form large-scale nanorod arrays, and these neat nanorod arrays are conducive to the uniform sputtering of metals on their top.…”

Light-Activated Fuel-Free Janus Metal Organic Framework Colloidal Motors for the Removal of Heavy Metal Ions

“…The holes are responsible for oxidizing H 2 O or decomposing H 2 O 2 to form hydroxyl radicals, protons, and oxygen, and oxidizing MOF particles to generate Co 2+ ions (confirmed by ICP−MS), while the activated electrons will react with oxygen in the media or with H 2 O 2 to form superoxide. 19 The asymmetric distribution and the diffusivity contrast of the charged species such as H + and Co 2+ will induce a local electric field and chemopressure around the negatively charged Janus particle (ζ = −44 mV), leading to a self-propelled motion known as the ionic selfdiffusiophoresis. 48,49 To further test the mechanism, we studied the motion of colloidal motors in the presence of a neutral salt (NaCl).…”

“…Among these sources, light is an ideal and versatile physical stimulus, which can facilitate and regulate the propulsion of motors due to its ease in remote action and tunable excitation energy. − Photoactive colloidal motors have received progressive attention recently, which can convert light energy into motion via a photocatalytic reaction. There are many photo-responsive systems described as light-driven colloidal motors, including TiO 2 and its derivatives, , ZnO and its modified analogue, − BiOCl–Pt, and Pt- g -C 3 N 4 /Fe 3 O 4 . Despite their excellent performance under light irradiation, the main drawback is the requirement of incorporation of expensive metals for enhancing the propulsion, which significantly increases the cost of these micromachines for a large-scale production.…”