Carbon dots with tunable concentrations of trapped anti-oxidant as an efficient metal-free catalyst for electrochemical water oxidation

The challenging water oxidation reaction to generate molecular oxygen requires low-cost efficient catalysts for its application in renewable energy technologies. Carbon dot (C-dot) catalysts synthesized by microwave irradiation can trap an anti-oxidant, 5-hydroxymethyl-2-furaldehyde (5-HMF) inside the carbon framework. The C-dot with the highest concentration of 5-HMF acts as a stable metal-free oxygen evolution reaction (OER) catalyst which operates at a decently low 0.21(±0.03) V overpotential and can generate current density up to 33.6(±2.3) mA cm−2. With increased microwave reaction time, the concentration of 5-HMF inside the C-dots decreases at the cost of different furan derivatives which decreases the OER activity. The 5-HMF molecules in close vicinity to the catalytically active sites containing CO groups can extract the ˙OH/˙OOH radicals and can increase the in situ H2O concentration to facilitate the forward reaction of O2 evolution. During continuous electrolysis beyond 10 min, 5-HMF gets converted to 2,5-diformylfuran entities, which increases the catalytically active sites and thereby maintains the OER activity of the C-dots for at least 4 h. The ability of microwave irradiated sucrose derived C-dots to electro-oxidize water is generalized with C-dots and graphene dots (G-dots) prepared from different precursors.


X-ray Diffraction (XRD) Patterns
Figure S1: XRD patterns of the representative samples.Sample C5 burnt under the X-ray beam and hence XRD pattern could not be generated.

Quantum Yield (QY) of the C-dots and G-dots
Quantum Yield Determination: The QY of the C-dots and G-dots were determined with reference to Rhodamine 6G using a standard protocol. 28For this reason to stabilize the surface functionalities certain passivating agents are used in order to obtain high QY. 30C-dot synthesized in lesser time such as C5 has more surface functional groups and organic molecules than in others.With increased time of synthesis the amount of organic molecules and surface functionalities like C-OH, C-O-C and C=O reduces due to higher degree of carbonization with more time.Since these functional groups give rise to a series of emissive energy traps between ππ* of the C=C present in the particle resulting in a drop in QY. 31 Higher QY for C25 and C30

Results and Discussion
(2.63 and 2.60% respectively) is attributed to the formation of more polyaromatic structures with increased microwave time.It was previously observed that the formation of polyaromatic structures in C-dots helps in stronger emission which leads to a higher QY. 32On the other hand,

Electrochemical Set-up Test with a Known CuO Microsphere Catalyst
The electrochemical setup was tested with a known CuO catalyst and the results were found to be closely reproducible.The paper by Tilley et.al. deals with catalysts with three different morphologies namely CuO microspheres, nanosheets and nanowires. 35CuO microspheres were synthesized following the reported protocol and its oxygen evolution reaction (OER) activity was examined using cyclic voltammetry (CV).The comparative results between the reported and reproduced values of current density (J) and overpotential () are given in the table below.
Table S3: The reported and reproduced OER parameters using CuO microspheres.

5-HMF as the Catalyst
To check the actual contribution of 5-HMF, it was used alone as a catalyst for water oxidation using the following method.0.001g of pure 5-HMF was mixed with 250 µL distilled water, 250 µL of ethanol and 25 µL of 5% aqueous Nafion solution.5 µL of the resulting suspension was carefully drop casted on to the surface of GC disk followed by vacuum drying for 24 h.

Figure S2 :
Figure S2: UV-visible spectra of the C-dots and G-dots.The absorption maxima of C-dots at 270-283 nm corresponds to the -* transition of the C=C bond.The tail of the broad absorption peaks contain the n-* transition of the C=O bond.In G5 and G10, the -* transition is blueshifted to ~244 nm and the n-* transition at ~342 nm is distinctly observed.
G5 and G10 show the highest value of QY among all the C-dots as 15.1 and 18.1% as the result of fewer number of surface defects present in them since the higher synthesis temperature leads to less surface defects.C-D has a lower QY (1.6%) than that of C-E (2.5%) which indicates that the PVP used in the synthesis of the C-dots does not act as a passivating agent to the surface functional groups but only plays the role of a capping agent and limits the growth rate of the particles.The microwave assisted generation of C-dots also results in very low QY due to the large presence of surface functional groups, whereas the C-dots prepared by laser ablation display higher QY as the surface energy traps are stabilized by laser treatment.The extremely low QY indicates that the C-dots and G-dots are undoped species as very high QY is generally associated with N/S doped C-dots.30

Figure S4 :Figure S5 :
Figure S4: QY of the C-dots and G-dots.

Figure S16 :
Figure S16: Plot of dissolved oxygen formed with time for the C-dots C5 to C30.

Figure S20 :
Figure S20: Variation of nitrogen wt% and  (V) with the samples.No matching trend is observed between the nitrogen content and the OER activity.

Table S1 :
Selected database of the synthetic methods, precursors used, diameter (dia.), quantum yield (QY) and application of carbon dots.

Table S2 :
ICP-MS analysis data of the C-dots and G-dots.

Table S4 :
Double layer capacitance of C-dots obtained from cyclic voltammograms recorded at different scan rates.

Table S5 :
Amount of dissolved oxygen measured with Clark electrode

Table S7 :
Selected list of anti-oxidants obtained from carbohydrates.