T. Maiyalagan, SRM Institute of Science and Technology, has been interested in nitrogen-doped graphene oxides for some years now. The material has immense potential for photocatalyzing water. For purifying water contaminated by industrial effluents, there is nothing like it – it’s low cost and highly efficient.
But you can’t just throw nitrogen-doped graphene oxide into water. The tiny particles diffuse in the water and cannot be easily recovered for reuse in other polluted water. T. Maiyalagan, in collaboration with researchers from Pakistan and South Korea, has now cracked the problem: grow nitrogen-doped graphene quantum dots on carbon fabric. one can recover the fabric coated with nitrogen-doped graphene quantum dots and reuse it many times.
The light weight, strong, flexible carbon fabric was woven with some 1000 carbon fibre bundles such that there are five warps and five wefts per centimetre.
You dip this fabric in graphene oxide dispersed in water, take it out, dry it, dip it again. And again. In four or five cycles, you get a uniform coating of graphene oxide on the fabric. Since both the fabric and graphene oxide are hydrophilic, they stick together well.
The researchers then reduced the graphene oxide coated on the surface of the carbon fabric using vapors of hydrazine monohydrate. This made it highly conducting and highly electrocatalytically active.
The next step was to get nitrogen-doped quantum dots on to the material. Doping reduces the band gap and the material will be able to absorb visible light.
Nitrogen doping is now a standard protocol: the hydrothermal cutting of graphene oxide sheets in the presence of hydrogen peroxide and ammonium hydroxide. The researchers dissolved the nitrogen-doped graphene quantum dots in a mixture of ethyl alcohol, acetone and water. This solution was used to drop cast the quantum dots as a thin film on the reduced graphene layer on the carbon fabric – a thin film that is not soluble in water.
Field emission scanning electron microscopic images show that the doped graphene oxide grows like nanoscale fractal ferns on the graphene-coated carbon fabric.
The edges of the surface of nitrogen-doped graphene quantum dots increase considerably in this morphology. Since these are active sites for both electrocatalytic and photocatalytic reactions, photocatalytic efficiency increases by about 50% more than with a uniform coating, says T. Maiyalagan.
Thus, you get a highly conducting (only 2.5 ohms per centimetre) flexible, stable material. Capable of degrading more than 60% of methylene blue in 90 minutes. In fact, it is better than platinum coated on fluorine-doped tin oxide glass!
The material has other uses besides cleaning polluted water. We are now examining its use in fuel cells, solar cells, supercapacitors, water splitting, and battery devices, says T. Maiyalagan, SRM Institute of Science and Technology.
Applied Surface Science, 480: 1035-1046 (2019);
Nehru Memorial College, Puthanampatti, Thiruchippalli