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Posted: Oct 01, 2015

Holey nanosheets with defined edges and holes

(Nanowerk Spotlight) Synthesis of holey two-dimensional (2D) nanosheets with defined hole morphology and hole edge structures remains a great challenge for graphene. It is also an issue for other 2D nanomaterials, such as hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2).
The in-plane heterostructure of these monolayer materials is of great interest because it greatly influences the nanosheets' tunable bandgap and other unique properties, allowing them to outperform their intact counterparts in many applications. For instance, boron nitride nanosheets with defined hole shapes and edges have significant potential applications in energy storage, catalysis, sensing, transistors, and molecular transport/separation.
In new work, researchers have reported a facile, controllable, and scalable method to carve, for the first time, geometrically defined pit/hole shapes and edges on h-BN basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts.
The results have been reported in the September 29, 2015 online edition of Scientific Reports ("Oxidative Etching of Hexagonal Boron Nitride Toward Nanosheets with Defined Edges and Holes").
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Acid-purified Ag-BN samples from etching at 900°C. (a-c) are SEM images from as-purified samples, while (d-f) are TEM images for DMF-exfoliated BNNSs after purification. (Reprinted with permission by Nature Publishing Group) (click on image to enlarge)
"In comparison to graphene which consists of a single type of atom, heteroatomic 2D nanomaterials, such as hexagonal boron nitride and molybdenum disulfide, have more diversity in terms of the edge structure and chemistry," Zhongfang Chen, a professor in the Department of Chemistry at the University of Puerto Rico, tells Nanowerk. "While various chemical methods were demonstrated to obtain holey graphene in large quantities, scalable approaches to synthesize etched heteroatomic nanosheets such as boron nitride (BNNSs) have been less explored."
The team, led by Yi Lin, a senior research scientist at National Institute of Aerospace and Chen, developed a facile silver-catalyzed etching method to carve the highly inert h-BN basal plane surface, resulting in geometrically defined edges and holes which have not been reported previously. The sizes and shapes of the holes were dependent on the condition used, with geometry and edge chirality becoming increasingly defined at higher temperatures.
They also observed that etching at a lower reaction temperature could occur selectively at the platelet perimeter with the basal plane remaining intact.
"Upon purification and exfoliation, the resultant etched BNNSs with much reduced thicknesses inherited the hole and edge structural motifs and exhibited significant change in electronic properties as indicated from optical absorption spectra", Chen describes the outcome of the fabrication process.
Lund
Fine atomic edge structures of hexagonal holes of exfoliated BNNS from purified Ag-BN sample etched at 1000°C/3 h: (a,c) HR-TEM images and (b,d) the corresponding atomic structural schematics. (a,b) an edge with A orientation exhibiting mostly triangularly-tipped fine structures, which are mostly Z terminations. ZB-, ZN- and A-terminated edges are marked with red, blue, and green lines, respectively; (c,d) an edge with Z orientation exhibiting hexagonally-tipped fine structures, which are all Z-terminated. Note the instrument resolution was insufficient to differentiate ZB vs. ZN, thus the assignments of ZB- and ZN-terminated edges are interchangeable with ZN and ZB-terminated edges, respectively. (Reprinted with permission by Nature Publishing Group)
A previous paper also recently published in Scientific Reports("Opening of triangular hole in triangular-shaped chemical vapor deposited hexagonal boron nitride crystal") by a different group of scientists reported on the production of very large triangle holes using a different approach.
In contrast with this paper, Chen's team reports the oxidative etching to get defined edges and holes.
"Our method opens up an exciting scalable approach to further explore the properties and applications of BNNSs with defined holes and edges for potential applications such as catalysis and molecular transport/separation," Chen notes.
He and his collaborators envision that this synthetic strategy can be further extended to the synthesis of other edge/vacancy-enriched 2D nanomaterials as a means to further explore their structure/property relationships and identify applications that can take advantage of their unique structures and performance characteristics.
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