Researchers at Indian Institute of Technology (IIT) Guwahati have been able to synthesise highly crystalline carbon dots by doping them with nitrogen, sulphur and phosphorus. The amount of phosphorus defined the extent of crystallinity. Unlike an amorphous material, less light was scattered or reflected from crystalline carbon dots on shining light. Instead, the crystalline material efficiently converted the absorbed light energy into heat energy.
Simulated sunlight
The team led by Prof. Arun Chattopadhyay from the Department of Chemistry successfully used the crystalline carbon dots for desalinating seawater by exposing the carbon dots to simulated solar conditions. “The doped carbon dots were not only able to convert light into heat energy but were also able to interact with water and transfer the heat energy to water thus raising its temperature,” says Prof. Chattopadhyay.
Compared with carbon dots that were doped with all the three elements, those doped with only nitrogen and sulphur were amorphous in nature. “When only nitrogen and sulphur are present the polycyclic carbon does not arrange in a particular manner, making it amorphous. But phosphate esters that form when phosphoric acid is added bond the polycyclic fragments. That is what makes it crystalline,” says Dr. Gayatri Natu from the Department of Chemistry, IIT Guwahati and a coauthor of a paper published in Journal of Materials Chemistry A.
The three- and two-element carbon dots added to water (with concentration up to 250 mg per millilitre) and exposed to simulated sunlight under reduced pressure showed wide variability in their ability to transfer heat energy to water.
There was 43.5% evaporation of water within 15 minutes in the case of carbon dots doped with three elements and only 38.3% with carbon dots doped with only nitrogen and sulphur. “When we calculated the solar thermal evaporation efficiencies under standard atmospheric pressure, carbon dots doped with three elements had nearly 84% efficiency. It was about 44% with carbon dots doped with only nitrogen and sulphur,” Prof. Chattopadhyay says.
Thorough testing
The team tested the ability of the doped carbon dots to desalinate seawater samples from Bay of Bengal, Persian Gulf and a sample with average sea water salinity. Maximum desalination was achieved with carbon dots doped with three elements in the Bay of Bengal water sample — 43% evaporation of the initial volume in 15 minutes. Only 35.5% seawater evaporated during the same time in the case of carbon dots with two elements.
They tested the doped carbon dots’ ability to desalinate seawater even when the salt concentration was in excess. To do this, more seawater was added to the residual seawater after each cycle and the desalination efficiency was tested for nine cycles.
“The doped carbon dots with nitrogen, sulphur and phosphorus elements retained up to 78% of the original desalination efficiency even at the end of the ninth cycle,” says Ayan Pal from the Department of Chemistry, IIT Guwahati and first author of the paper. “The doped carbon dots can be reused by removing excess salt through dialysis.”
Recalling how they stumbled upon doped carbon dots for desalination, Prof. Chattopadhyay says: “We have been working with doped carbon dots and investigating their properties. We found that one set of doped carbon dots was highly crystalline. Since we were interested in the production of hydrogen from water using sunlight we tested these crystalline carbon dots. Though it didn’t produce hydrogen gas, it was evaporating water quickly. So we started testing it for desalination.”
“We are now trying to make doped carbon dots that are super crystalline so that energy is even more efficiently converted into heat. We also trying to make a film or sponge-like device that floats on water and evaporates water by converting sunlight into heat,” Prof. Chattopadhyay says.
