A New Flash Graphene Method

Doping graphene is possible through bottom-up approaches like chemical vapor deposition or synthetic organic processes, but these usually yield products in trace amounts or produce defects in the graphene. The Rice process is a promising route to produce large quantities of “heteroatom-doped” graphene quickly and without solvents, catalysts or water. Rice University chemists led by Professor James Tour have created a catalyst- and solvent-free flash Joule heating process for manufacturing bulk quantities of doped graphene with tailored properties for optical and electronic nanodevices. The new doped turbostratic graphene would lead to many more options for useful products concrete, asphalt or plastic. Moreover, it has enhanced electronic properties, making them better-suited for specific electronic and optical devices. The new process reported in the American Chemical Society journal ACS Nano.

The research team has modified its flash Joule heating process to produce doped graphene that tailors the atom-thick material’s structures and electronic states to make them more suitable for optical and electronic nanodevices. The doping process adds other elements to graphene’s 2D carbon matrix. The new general synthetic route to heteroatom-doped graphene by ultrafast and all-solid-state catalyst-free flash Joule heating (FJH) method within 1 s. It requires no solvent, no catalysts, and no water.

The authors successfully designed seven different heteroatom-doped FG, including single element doped N-FG, B-FG, O-FG, P-FG, S-FG, dual elements codoped B,N-FG, and multiple elements codoped B,N,S-FG. They were directly synthesized by an ultrafast and all-solid-state catalyst, solvent, and water-free FJH method. Different low-cost dopants, including elements, oxides, and organic compounds can be used regardless of boiling point and conductivity. According to the authors, the as-synthesized doped FG has good graphene quality, turbostratic structure, and expanded interlayer spacing. Therefore, they are dispersible in water-Pluronic (F-127) (1 wt %) solution, and then form stable concentrated dispersions.

There are many important industrial applications for the new graphene, for instance, heteroatom doping modifies the electronic structures, which improves the performance of doped FG as electrocatalysts and electrochemical energy storage materials. The research team managed to scale up the process to gram-scale synthesis of doped FG and this was demonstrated to show the scalability of the FJH method to bulk quantities. The whole process was economically sound where the electrical energy cost for heteroatom-doped FG synthesis was only 1.2–10.7 kJ g–1, which makes the FJH method suitable for low-cost and mass production of heteroatom-doped graphene. The authors are now working on scaling up the intrinsic FG to 1-ton per day production using an analogous FJH technology. The protocols and methods proposed by the authors can be easily translated to similar scales.

The researchers showed how graphene can be doped with a single element or with pairs or trios of elements. The process was demonstrated with single elements boron, nitrogen, oxygen, phosphorus and sulfur, a two-element combination of boron and nitrogen, and a three-element mix of boron, nitrogen and sulfur. The process takes about one second, is both catalyst- and solvent-free, and is entirely dependent on “flashing” a powder that combines the dopant elements with carbon black.

Graphene is turbostratic when stacks of the 2D honeycomblike lattices don’t align with one another. This makes it easier to disperse the nanoscale sheets in a solution, producing soluble graphene that is much simpler to incorporate into other materials, Tour said. The lab tested various doped graphenes in two scenarios: electrochemical oxygen reduction reactions (ORR) that are key to catalytic devices like fuel cells, and as part of an electrode in lithium metal batteries that represent the next generation of rechargeable batteries with high energy densities. Sulfur-doped graphene proved best for ORR, while nitrogen-doped graphene proved able to reduce nucleation overpotential during the electrodeposition of metallic lithium. That should facilitate more uniform deposition and improved stability in next-generation rechargeable metal batteries, the lab reported.