A High‐Performing, Visible‐Light‐Driven Actuating Material Responsive to Ultralow Light Intensities

Abstract

Light‐driven actuating materials are highly desirable for miniaturized, untethered devices and microrobots. However, current light actuators require high light intensity or the use of UV or IR light to operate, which greatly increase the cost and difficulty for real‐life applications. Here, a significant visible (vis)‐light‐driven actuation effect in cobalt oxides/hydroxides (C–O–H), inducible at low intensities from ≈3 mW cm−2 (≈0.03 sun), is reported. In the form of bilayered films, the actuators can curl into loops at 70–180 ms per loop, corresponding to a high intrinsic strain of more than 1% at strain rate 0.6% s−1, and actuating stress exceeding 60 MPa, at ≈50 mW cm−2 (≈0.5 sun). Linear actuators made from actuating hinges of C–O–H can weightlift objects ≈200 times heavier than the active material. A chemo‐mechanics model describes the actuation well, showing that the mechanism is light‐induced volume shrinkage assisted by diffusion, and C–O–H exhibits outperforming chemical driving force per unit light‐intensity stimulation.

Novel Stimuli‐Responsive Turbostratic Oxides/Hydroxides for Material‐Driven Robots

“…On the contrary, the actuation stress σ a and strain ε chem a are also governed by the elasticity of the bimorph cantilever. As shown in Figure 5a, the actuation strain ε chem a is effectively a self, plastic strain within the active layer that deforms the bimorph cantilever in an elastic manner, and mechanics considerations [65] lead to the following conditions for mechanical equilibrium…”

“…Typical actuation responses in turbostratic oxides/hydroxides. a) Light-triggered fast actuation response in COH (scale bar ¼ 10 mm).Reproduced with permission [65]. Copyright 2019, Wiley-VCH.…”

Epitaxial growth of light-responsive azobenzene molecular crystal actuators on oriented polyethylene films

“…While human muscles or liquid crystal actuators have strains around 20%, our actuators rather resemble conducting polymers, ionic polymer metal composites, 35 or metal/metal oxide actuators with strains around one percent. 36 In the case of the photo-thermal azobenzene 2 on oriented PE, a temperature up to 60 1C was recorded upon illumination with 405 nm light at 300 mW cm À2 accompanied by the disappearance of the azobenzene crystals. It is worthwhile to note that the epitaxial crystallites reappeared spontaneously in the dark.…”

Mechanism of Actuation in Nickel Hydroxide/Oxyhydroxide Photoactuators

“…The range of strain of 0.1–1% was chosen as previous studies have reported strain in the region of 0.1–1% for illumination between 10–50 mW cm −2 . [ 10 ]…”

“…To better understand and optimise the actuators, a chemo‐mechanical model, inspired by literature sources, [ 10 ] has been built to determine the effect of layer thickness and degree of hydration on the bending response. First hand data was collected to build and test the model, as well as data from literature sources.…”