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Top: The schematic illustration of the design principle and fabrication procedure of a moisture responsive RGO/GO bilayer film. Bottom: A crawler made of the flash-treated graphene-oxide film moves 3.5 mm in 12 seconds when humidity is switched on and off several times. [Images: Y.-Q. Liu et al., Opt. Mater. Express, doi: 10.1364/OME.7.002617]

Researchers from Jilin University, China, have reported a new method for making moisture-responsive graphene oxide (GO) films for smart actuators (Opt. Mater. Express, doi: 10.1364/OME.7.002617). The team, led by Yong-Lai Zhang, used a flash of light to remove oxygen-containing groups (OCGs) from one side of a GO film, reducing that side’s ability to absorb water. Therefore, in the presence of moisture, the non-reduced GO side takes in more water and expands, causing the sheet to bend toward the reduced GO (RGO) side. The researchers used this force-mismatch to power actuators—a crawler and a claw—that crept and grasped with changes in humidity.

While using light to reduce GO structures and make them moisture-responsive is nothing new, previous methods have lacked efficiency in fabrication and in precise control over the photoreduction gradient. Now, according to the Jilin team members, this new method could be used to create large-area moisture-responsive RGO/GO bilayer films in less than one second and also precisely reduce the top layer with a camera flash.  

Flash-reduced GO film

Graphene materials are particularly well-suited for actuators because they have mechanical strength, flexibility, good stability, high electron-mobility and are generally biocompatible. To make their bilayer material, the researchers first prepared a basic GO film using vacuum filtration. Next, they subjected one side of the film to a single flash from a xenon lamp 20 to 30 mm away. This flash of light, typically 2 J/cm2, reduced one side of the GO film while the other side went untouched.

The structural transformation of the GO film into an RGO/GO bilayer film was confirmed via scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and naked-eye observations. All found that the flash-reduced side of the film changed from yellow-brown to black.

During demonstrations with varying relative humidity (RH) at room temperature, the team found that the maximum bending angle of the RGO/GO bilayer film is about 85 degrees in RH of 86 percent. The researchers also found that the film’s response and recovery times were about 16 seconds and 17 seconds, respectively, and that bending-unbending cycles were highly repeatable.

Crawlers and claws

To test their material’s usefulness in creating smart actuators, the researchers cut four-legged crawlers and asterisk-shaped claws out of the moisture-responsive film. The crawler actuator moved 3.5 mm in 12 seconds when the humidity was switched on and off several times; the claw actuator went from open to closed in 12 seconds when exposed to increasing levels of RH. The team captured the actuator motion in a short video:

Zhang and his colleagues say that actuators like these that are powered by external stimulations—for example, changes in light, moisture, temperature or solvents—have “great potential” in many applications, including adaptive optics, tissue engineering and biomimetic actuation.