Nitrogen controls the optical properties of carbon dots

Physicists at Ludwig-Maximilians-University (LMU) in Munich, Germany, have demonstrated that the optical and photocatalytic properties of so-called carbon dots can be precisely tuned by controlling the positions of nitrogen atoms introduced into their structure.

Thanks to their unusual optical properties, carbon particles with diameters on the order of a few nanometers – so-called C-dots – show great promise for a wide range of technological applications, from energy conversion to bio-imaging. Moreover, C-dots have several practical advantages over comparable materials in that they are easy to fabricate, stable and contain no toxic heavy metals.

Their versatility is largely due to the fact that – depending on their chemical composition and aspects of their complex structure – they can either act as emitters of light, in the form of photoluminescence, or function as photocatalysts by absorbing light energy and triggering chemical reactions, such as water splitting. However, the factors that determine these disparate capabilities are not well understood.

Now, physicists at LMU, led by Jacek Stolarczyk, have taken a closer look at the mechanisms underlying these very different properties. Their study, which appears in a paper in Nature Communications, shows that the physicochemical characteristics of these nanomaterials can be simply and precisely tuned by introducing nitrogen atoms into their complex chemical structure in a controlled manner.

“Up until now, C-dots have typically been optimized on the basis of the principle of trial and error,” says Stolarczyk. “In order to get beyond this stage, it is essential to obtain a detailed understanding of the mechanisms that underlie their diverse optical characteristics.”

The study was carried out as part of an interdisciplinary project called ‘Solar Technologies Go Hybrid’ (SolTech), coordinated by LMU’s Jochen Feldmann. SolTech is funded by the State of Bavaria to explore innovative concepts for the transformation of solar radiation into electricity and the use of non-fossil – and preferably non-toxic and abundantly available – fuel sources for the storage of energy. C-dots are in many respects ideally suited for such applications.

C-dots are made up of networks of polycyclic aromatic carbon compounds, whose complex interactions determine how they react to light. In the new study, the researchers synthesized C-dots by combining citric acid as a carbon skeleton with a branched, nitrogen-containing polymer, and then irradiated the mixture with microwaves. In a series of experiments, they systematically varied the concentration of the polymer in the mixture, such that different amounts of nitrogen were incorporated into the carbon networks.

They found that the precise synthesis conditions had a critical impact on the mode of incorporation of nitrogen into the carbon lattices that make up the C-dots. This influenced whether a nitrogen atom replaced one of the carbon atoms that form the interlinked 6-membered carbon rings resembling tiny graphene flakes, or instead replaced one of the carbon atoms in the 5- and 6-membered rings found on the free edges of the aromatic structures.

“Our investigation showed that the chemical environment of the nitrogen atoms incorporated has a crucial influence on the properties of the resulting C-dots,” says Santanu Bhattacharyya, the first author of the paper and a fellow in Feldmann’s research group. If nitrogen atoms are incorporated inside the graphene-like structures, which happens at intermediate polymer concentrations, this leads to dots that predominantly emit blue photoluminescence when irradiated with light of a suitable wavelength. On the other hand, if they are incorporated at edge positions, which occurs for either very high or very low amounts of the polymer, this suppresses photoluminescence and results in C-dots that photocatalytically reduce water to hydrogen.

In other words, the optical properties of the C-dots can be modified at will by varying the details of the procedure used to synthesize them. The members of the LMU team believe that these latest insights will stimulate further work on the use of these intriguing nanomaterials, both as photoluminescent light sources and as photocatalysts in energy conversion processes.

This story is adapted from material from Ludwig-Maximilians-University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.