Dong-Dong Han

Moisture‐Responsive Graphene Paper Prepared by Self‐Controlled Photoreduction

A facile and cost-effective preparation of moisture-responsive graphene bilayer paper by focused sunlight irradiation is reported. The smart graphene paper shows moisture-responsive properties due to selective adsorption of water molecules, leading to controllable actuation under humid conditions. In this way, graphene-based moisture-responsive actuators including a smart claw, an orientable transporter, and a crawler paper robot are successfully developed.

Light-Mediated Manufacture and Manipulation of Actuators

Recent years have seen a considerable growth of research interests in developing novel technologies that permit designable manufacture and controllable manipulation of actuators. Among various fabrication and driving strategies, light has emerged as an enabler to reach this end, contributing to the development of actuators. Several accessible light-mediated manufacturing technologies, such as ultraviolet (UV) lithography and direct laser writing (DLW), are summarized. A series of light-driven strategies including optical trapping, photochemical actuation, and photothermal actuation for controllable manipulation of actuators is introduced. Current challenges and future perspectives of this field are discussed. To generalize, light holds great promise for the development of actuators.

Solvent-tunable PDMS microlens fabricated by femtosecond laser direct writing

Reported here is the fabrication of solvent-tunable polydimethylsiloxane (PDMS) microlenses using the femtosecond laser direct writing (FsLDW) technique. PDMS microlenses with equation-defined profiles, including both spherical microlens and aspheric hyperboloid microlens, have been fabricated according to preprogrammed models. In addition to excellent optical performance derived from the high accuracy and smooth surface, the resultant PDMS microlenses also show unique solvent-tunable properties; the focal length could be dynamically tuned by organic solvents of different solubility parameters. To obtain better control over the tunable property, a PDMS microlens has been flexibly integrated with a microfluidic device. Under the stimulation of different solvents, its tunable imaging performance has been demonstrated in a controlled manner.

Surface and Interface Engineering of Graphene Oxide Films by Controllable Photoreduction.

We report herein the engineering of the surface/interface properties of graphene oxide (GO) films by controllable photoreduction treatment. In our recent works, typical photoreduction processes, including femtosecond laser direct writing (FsLDW), laser holographic lithography, and controllable UV irradiation, have been employed to make conductive reduced graphene oxide (RGO) microcircuits, hierarchical RGO micro-nanostructures with both superhydrophobicity and structural color, as well as moisture-responsive GO/RGO bilayer structures. Compared with other reduction protocols, for instance, chemical reduction and thermal annealing, the photoreduction strategy shows distinct advantages, such as mask-free patterning, chemical-free modification, controllable reduction degree, and environmentally friendly processing. These works indicate that the surface and interface engineering of GO through controllable photoreduction of GO holds great promise for the development of various graphene-based microdevices.

Femtosecond Laser Direct Writing of Flexible All-Reduced Graphene Oxide FET

Reported here is the facile fabrication of all-reduced graphene oxide (RGO) field-effect-transistor (FET) on flexible substrates using a solo femtosecond laser direct writing (FsLDW) technology. By simply tuning the intensity of a femtosecond laser pulse, GO could be reduced in a controlled manner. Metallic and semiconducting RGO micro-patterns could be achieved by FsLDW under high and moderate laser power, respectively, which enables direct writing of source/drain and gate electrodes, as well as semiconducting channel of a FET on flexible substrates in ambient condition. In this way, a metal-free all-RGO FET was successfully fabricated by FsLDW without the use of any masks or chemical reagents. FsLDW of all-RGO devices shows unique advantages in both facile fabrication and flexible integration of graphene-based micro-devices, revealing great potential for the development of future electronics.

Preparation of a Fe3O4–Au–GO nanocomposite for simultaneous treatment of oil/water separation and dye decomposition

A nanocomposite capable of simultaneously controlling multiple water pollutants (soluble organic dye and insoluble chemical solvent) has been obtained. The Au and Fe3O4 nanoparticles (NPs) were modified on a graphene oxide (GO) surface via light reduction and covalent attachment. The obtained Fe3O4-Au-GO nanocomposite has magnetic driving ability and catalytic applications. The nanocomposite can form emulsions after wrapping an insoluble and volatile organic solvent inside; moreover, the multi-layer graphene shell structure may delay volatilization of the solvent, ensuring that the oil droplets are collected efficiently and completely by the Fe3O4-Au-GO nanocomposite. At the same time, the Au NPs on the surface of the composite can effectively catalyze the decomposition of an organic dye in water and the recovery of the nanocomposite catalyst can also be realized using an external magnetic field. The simultaneous treatment of non-soluble oil (organic solvents) and organic dyes in water can be realized by the Fe3O4-Au-GO nanocomposite. Therefore, based on surface modification of GO, one material with two types of water pollution treatment functions was realized. This provides a new way for the simultaneous treatment of oil separation and dye decomposition, and the assembled structure may result in emulsions to give new applications in fuel cells and other fields.

Reed leaf-inspired graphene films with anisotropic superhydrophobicity

Controlling the wettability of graphene and its derivatives is critical for broader applications. However, the dynamic dewetting performance of graphene is usually overlooked. Currently, superhydrophobic graphene with an anisotropic wettability is rare. Inspired by natural reed leaves, we report an ingenious fabrication process combining photolithography and laser holography technologies to create biomimetic graphene surfaces that demonstrate anisotropic wettability along two directions of grooved hierarchical structures, which are similar to reed leaf veins. Microgrooved structures with a period of 200 μm were fabricated via photolithography to endow the substrate with an obvious anisotropic wettability. Two-beam laser interference treatments of the graphene oxide (GO) film on the grooved substrate removed most of the hydrophilic oxygen-containing groups on the GO sheets and increased the surface roughness by introducing additional hierarchical micro-nanostructures. The combined effects endowed the resultant graphene films with a unique anisotropic superhydrophobicity similar to that of reed leaves. Superhydrophobic graphene surfaces with anisotropic antiwetting behavior might allow further innovations based on graphene in the fields of bionics and electronics.

Facile fabrication of moisture responsive graphene actuators by moderate flash reduction of graphene oxides films

We reported here a facile, green, and simple method to fabricate moisture-responsive graphene actuators by moderate flash reduction of graphene oxides (GO) films. Due to the limited light transparency and thermal relaxation, the oxygen containing groups (OCGs) on the GO sheets could be selectively removed from the radiation side, forming a photoreduction gradient along the lateral direction of the GO film. In this manner, we obtained a reduced GO (RGO)/GO bilayer film. The RGO/GO film can bend to the RGO side when exposed to moisture because RGO and GO layers show different absorption capabilities of water molecules. Taking advantage of this controllable deformation, we fabricated moisture-responsive actuators including a crawler and claw robots.

Facile Fabrication of High-Performance Humidity Sensors by Flash Reduction of GO

Graphene is a promising sensing material that has been widely used for high-performance sensors. However, the development of graphene-based sensing device is limited by complex manufacturing procedures. Here, we report a facile fabrication of humidity sensors by flash reduction of graphene oxides (GO) with the help of a photographic camera flash. The flash treatment triggers a drastic deoxygenation of oxygen functional groups (OCGs) from the GO sheets, which not only restores its conductivity partly but also induces the formation of a highly porous structure. The reduced GO sensor shows high sensitivity at the relative humidity atmosphere of 11%–95%, small hysteresis, rapid response, and enhanced durability. The sensitive mechanism was discussed through the investigation of complex impedance plots. This facile flash reduction strategy holds great promise for chemical-free production of graphene-based sensing devices.

Facile fabrication of flexible graphene FETs by sunlight reduction of graphene oxide

We reported here a facile fabrication of flexible graphene-based field effect transistors (FETs) by sunlight reduction of graphene oxide (GO) as channel material. As a mask-free and chemical-free method, sunlight photoreduction of GO without the use of any complex equipments is simple and green. The resultant FET demonstrated excellent electrical properties (e.g., an optimized Ion/Ioff ratio of 111, hole mobility of 0.17  cm2  V-1 s-1), revealing great potential for development of flexible microelectrics. Additionally, since the reduced GO channel could be fabricated by sunlight treatment between two pre-patterned electrodes, the process features post-fabrication capability, which makes it possible to integrate graphene-based devices with given device structures.