Carbon nitride-based light-driven microswimmers with intrinsic photocharging ability

Abstract

Controlling autonomous propulsion of microswimmers is essential for targeted drug delivery and applications of micro/nanomachines in environmental remediation and beyond. Herein, we report two-dimensional (2D) carbon nitride-based Janus particles as highly efficient, light-driven microswimmers in aqueous media. Due to the superior photocatalytic properties of poly(heptazine imide) (PHI), the microswimmers are activated by both visible and ultraviolet (UV) light in conjunction with different capping materials (Au, Pt, and SiO2) and fuels (H2O2 and alcohols). Assisted by photoelectrochemical analysis of the PHI surface photoreactions, we elucidate the dominantly diffusiophoretic propulsion mechanism and establish the oxygen reduction reaction (ORR) as the major surface reaction in ambient conditions on metal-capped PHI and even with TiO2-based systems, rather than the hydrogen evolution reaction (HER), which is generally invoked as the source of propulsion under ambient conditions with alcohols as fuels. Making use of the intrinsic solar energy storage ability of PHI, we establish the concept of photocapacitive Janus microswimmers that can be charged by solar energy, thus enabling persistent light-induced propulsion even in the absence of illumination—a process we call “solar battery swimming”—lasting half an hour and possibly beyond. We anticipate that this propulsion scheme significantly extends the capabilities in targeted cargo/drug delivery, environmental remediation, and other potential applications of micro/nanomachines, where the use of versatile earth-abundant materials is a key prerequisite.

Morphology Control in 2D Carbon Nitrides: Impact of Particle Size on Optoelectronic Properties and Photocatalysis

“…The results presented here are not only instrumental in the context of PHI thin film and device fabrication, but they might also help to tune the properties of photocatalytic carbon nitride‐based microswimmers or solar battery applications. [ 53 ] Disentangling the interdependent structural, morphological, and optoelectronic variables, which together define light‐induced charge transport and transfer reactions is key to understand, address, and optimize general photocatalyst design.…”

Conductivity mechanism in ionic 2D carbon nitrides: from hydrated ion motion to enhanced photocatalysis

“…To reveal if those changes have a possible impact on ion mobility and photocatalytic behavior, we first studied the overall conductivity of PHI. [6,28] The mobility of ions in the PHI pores was probed by different techniques spanning various timescales and diffusion distances: Electrochemical impedance spectroscopy (EIS), galvanostatic polarization measurements, and 7 Li pulsed field gradient (PFG) NMR were used as long-range sensitive methods, [42][43][44] whereas NMR line shape analysis and relaxometry were employed for correlating the macroscopic ion diffusion. Its microscopic origin is probed by temperature and frequency dependence of the diffusion-induced NMR spin-lattice or spin-spin relaxation rates, which are influenced by local dipolar-magnetic or quadrupolar-electric fluctuations.…”

“…Photocatalytic propulsion of PHI Janus particles or a direct electric discharge of the photo-reduced, blue state is also possible, enabeling solar batteries to be realized from this abundant material. [6,7] However, the long-term charge storage process itself can limit the direct photocatalytic activity by enhancing parasitic recombination of the light-induced charge carriers as charge builds up on the system. [8] An in-depth study of charge storage mechanism is still missing while being of high interest for the targeted design of (photo)electrochemical energy conversion and storage.…”

Optoelectronics Meets Optoionics: Light Storing Carbon Nitrides and Beyond

“…Reproduced under the terms of the CC BY‐NC‐ND license. [ 25 ] Copyright 2020, The authors. d) Schematic illustration of a PHI particle acting as photomemristor or sensor to external stimuli which affect the presence of trapped electrons, including band gap illumination, alkali metal ions, SEDs or electron scavengers, such as oxygen.…”

“…After illumination has stopped, this accumulated energy can be used for sustained, ballistic propulsion for as long as 30 min in the dark (Figure 3c). [ 25 ] Put into perspective, the storage of photocatalytically accumulated energy has immediate relevance for smart matter systems such as molecular machines and robotics, where an on‐board energy storage unit allows for sustained function even in the absence of a fuel or light, just like mitochondria are able to sustain respiratory function even in the absence of oxygen, that is, under anaerobic conditions, for a certain period of time. In artificial microswimmers, the reduction of the system complexity by the intrinsic bifunctionality of light absorption and energy storage is vital for targeted applications in the “micro‐world,” where complex, wired multi‐material solutions fulfilling charging, power storage, and supply functions are technically challenging and of high‐cost while impeding upscaling.…”