Visible-Light-Driven, Nickel-Doped Cobalt Oxides/Hydroxides Actuators with High Stability

Abstract

Material-based, light-driven actuators have been a recent research focus for the development of untethered, miniaturized devices and microrobots. Recently introduced nickel hydroxide/oxyhydroxide (Ni­(OH)2/NiOOH) and cobalt oxides/hydroxides (CoO x (OH) y ) are promising light-driven actuators, as they exhibit large and rapid actuation response and are inexpensive to fabricate by fast electrodeposition. However, as their actuation is due to the volume change accompanying the light-induced desorption of intercalated water in their turbostratic structures, their actuation reduces over time as crystallization takes place slowly, which lowers the amount of water held. Here, we introduce nickel-doped cobalt oxides/hydroxides (NiCoO x (OH) y ) actuator that exhibits similar turbostratic crystal structures and actuation magnitude as CoO x (OH) y , but with much slower crystallization and hence significantly more stable actuation than CoO x (OH) y or Ni­(OH)2/NiOOH. The new actuator exhibits much better applicability than the Co and Ni counterparts, and the present work shows that a stabilized turbostratic structure is a key for achieving high light-driven actuation in transition-metal oxide/hydroxide actuators.

Mechanism of Actuation in Nickel Hydroxide/Oxyhydroxide Photoactuators

“…As shown in previous studies, [ 7,11,14 ] the Ni/Au/Ni(OH) 2 tri‐layer actuators exhibit repeatable actuation behaviour under pulsed light. This response is dependent on the light intensity, with a decline in actuation speed and degree of motion when the actuator is moved out of the focal point of the light.…”

“…The synthetic procedure also allows for easy patterning of the active material, giving rise to different actuating modalities. [ 7,11 ]…”

“…It is thought that volume shrinkage accompanying the removal of intercalated water in their turbostratic structure is the mechanism through which photoactuation occurs. [ 11,14 ]…”

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

“…Apart from the NHO system, COH and MnO x are the only two other systems reported to exhibit electrochemical and/or light actuation. [65][66][67][68] COH is a mixture of cobalt oxides (CoO x ) and hydroxides (Co(OH) y ) [65,67] with similar turbostraticity in their crystal structures as NHO. The COH system exhibits both electrochemical and light actuation, [67] and its light-induced actuation is intrinsically better than that of NHO (see Figure 6b and discussion in Section 3).…”

“…[65][66][67][68] COH is a mixture of cobalt oxides (CoO x ) and hydroxides (Co(OH) y ) [65,67] with similar turbostraticity in their crystal structures as NHO. The COH system exhibits both electrochemical and light actuation, [67] and its light-induced actuation is intrinsically better than that of NHO (see Figure 6b and discussion in Section 3). MnO x is known to exhibit only electrochemical but not light actuation.…”