Two-dimensional (2D) nanomaterials can serve as versatile templates that nucleate, orient and confine the assembly of ultrathin material architectures. In particular, graphene oxide (GO) has often been used to direct growth due to its rich surface chemistry, ease of aqueous processing, and large interlayer gallery spaces. By restacking GO sheets into a multilayer film and introducing precursors through intercalation, the 2D interlayer gallery spaces can confine and guide the assembly and conversion of precursors into lamellar material structures. This intercalation approach is particularly suited to metal oxides, which display excellent mechanical strength as well as thermal stability, and are stable under conditions where graphene templates can be removed by air oxidation.

… The present paper explores a templating approach for creating complex textured surface topographies in metal oxide films.

—Chen et al.

In their earlier work, to introduce wrinkle and crumple structures in graphene, the team compressed the sheets multiple times in multiple orientations. That process won’t work with metal oxides due to the materials’ stiffness; compression results in cracking, said Po-Yen Chen, a Hibbitt Postdoctoral Researcher in Brown’s School of Engineering who led the work.

The team developed a method of using the crumpled graphene sheets as templates for making crumpled metal-oxide films, transferring those surface features from the graphene onto the metal oxides.

Creation of hierarchical metal oxide topographies using highly textured GO as versatile ion-intercalation templates. Multi-generational GO films are fabricated by one-time uniaxial contraction (G1-1D-GO), one-time isotropic contraction (G1-2D-GO), and repeated isotropic-isotropic contractions (G2-2D2D-GO). The as-fabricated textured GO are then utilized as intercalation templates for hydrated metal cations (e.g., Ag+, Zn2+, Al3+, Mn2+, Cu2+). After immersion in aqueous precursor salt solutions, the metal ion-intercalated GO films are dehydrated, annealed, and calcined at high temperature to oxidatively remove the carbon-based template and create textured metal oxide films (ZnO, Al2O3, Mn2O3, CuO) with high-fidelity replication of the original graphene wrinkled, crumpled, and hierarchical topographies. Credit: ACS, Chen et al. Click to enlarge.

The team started by making stacks of crumpled graphene sheets using the method they had developed previously. They deposited the graphene on a polymer substrate that shrinks when heated. As the substrate shrinks, it compresses the graphene sitting on top, creating wrinkle or crumple structures. The substrate is then removed, leaving free-standing sheets of crumpled graphene behind. The compression process can be done multiple times, creating ever more complex structures.

The process also allows control of what types of textures are formed. Clamping shrink film on opposite sides and shrinking it in only one direction creates periodic wrinkles. Shrinking in all directions creates crumples. These shrinks can be performed multiple times in multiple configurations to create a wide variety of textures.

To transfer those patterns onto metal oxides, Chen placed the stacks of wrinkled graphene sheets in a water-based solution containing positively charged metal ions. The negatively charged graphene pulled those ions into the spaces between the sheets. The particles bonded together within the interlayer space, creating thin sheets of metal that followed the wrinkle patterns of the graphene. The graphene was then oxidized away, leaving the wrinkled metal-oxide sheets. Chen showed that the process works with a variety of metal oxides—zinc, aluminum, manganese and copper oxides.

Generalized synthetic routes for fabricating crumple/wrinkle-textured metal oxides. Wrinkled, crumpled, and hierarchical GO structures are fabricated via uniaxial (1D), biaxial (2D), and sequential deformation, respectively. The textured GO films can be then utilized as versatile intercalation templates for different metal ions (e.g., Zn2+, Al3+, Mn2+). After removing graphene-based templates, multiple textured metal oxides (ZnO, Al2O3, Mn2O3) are achieved with replicated wrinkled, crumpled, hierarchical structures. Credit: ACS, Chen et al. Click to enlarge.

Once they had made the materials, the researchers then tested them to see if, as was the case with graphene, the textured surfaces enhanced the metal-oxide properties.

They showed that wrinkled manganese oxide, when used as a battery electrode, had charge-carrying capacity that was four times higher than a planar sheet. The researchers suggested that result was probably because the wrinkle ridges give electrons a defined path to follow, enabling the material to carry more of them at a time.

The team also tested the ability of crumpled zinc oxide to perform a photocatalytic reaction: reducing a dye dissolved in water under ultraviolet light. The experiment showed the crumpled zinc oxide film to be four times more reactive than a planar film.

In addition to improving the properties of the metals, the process also represents a way of making thin films out of materials that don’t normally lend themselves to ultrathin configurations.

This intercalation templating approach has broad applicability for the creation of complex, textured films, and provides a bridging technology that can transcribe the wide variety of textures already realized in graphene into insulating and semiconducting materials. These textured metal oxide films exhibit enhanced electrochemical and photocatalytic performance over planar films and show potential as high-activity electrodes for energy storage, catalysis and biosensing.

— Chen et al.

In addition to Chen, Hurt and Wong, other authors on the paper were Muchun Liu, Thomas Valentin, Zhongying Wang, Ruben Spitz Steinberg and Jaskiranjeet Sodhi. The work was supported by the Hibbitt Engineering Postdoctoral Fellowship and seed funding from Brown University.

Resources

• Po-Yen Chen, Muchun Liu, Thomas M. Valentin, Zhongying Wang, Ruben Spitz Steinberg, Jaskiranjeet Sodhi, Ian Y Wong, and Robert H Hurt (2016) “Hierarchical Metal Oxide Topographies Replicated from Highly Textured Graphene Oxide by Intercalation Templating” ACS Nano doi: 10.1021/acsnano.6b05179