Photobase effect for just-in-time delivery in photocatalytic hydrogen generation

Carbon dots (CDs) are a promising nanomaterial for photocatalytic applications. However, the mechanism of the photocatalytic processes remains the subject of a debate due to the complex internal structure of the CDs, comprising crystalline and molecular units embedded in an amorphous matrix, rendering the analysis of the charge and energy transfer pathways between the constituent parts very challenging. Here we propose that the photobasic effect, that is the abstraction of a proton from water upon excitation by light, facilitates the photoexcited electron transfer to the proton. We show that the controlled inclusion in CDs of a model photobase, acridine, resembling the molecular moieties found in photocatalytically active CDs, strongly increases hydrogen generation. Ultrafast spectroscopy measurements reveal proton transfer within 30 ps of the excitation. This way, we use a model system to show that the photobasic effect may be contributing to the photocatalytic H2 generation of carbon nanomaterials and suggest that it may be tuned to achieve further improvements. The study demonstrates the critical role of the understanding the dynamics of the CDs in the design of next generation photocatalysts.

normalized PL spectra of acridine in water at pH 3, 7 and 13. The integrated PL intensity at pH 7 is higher than at pH 13 by a factor of 1.77. The intensity at pH 3 is higher than at pH 7 by a similar factor, 1.73. Comparing the intensities, it is clear that only 3% of protonated acridine (percentage of protonated acridine at pH 7) would not account for the whole increase between pH 13 and 7. This control experiment provides evidence that some non-protonated acridine molecules when excited at pH 7, become protonated and emit as the acridinium cation.

a b
Supplementary Figure 4. Spectroscopic analysis of protonation of acridine. Comparison of (a) absorption and (b) emission spectra of acridine in ethanol, water and aqueous solution of NH4Cl, taken at the same concentration of acridine; (c) PL and (d) PLE spectra of acridine in water with and without addition of boric acid. Ethanol leads to a decrease in PL intensity. On the other hand, the PL intensity increases in the NH4Cl and boric acid, in line with the expected increased protonation. The PLE spectra with and without the boric acid are identical suggest that the nonprotonated species are being excited from the ground state in both cases. Figure 5. Absorption spectra at pH 7. Comparison of the non-normalized absorption spectra of CDs, acridine and CD-acridine taken at pH 7.  The negative values of 3 for pH 7 and pH 13 represent a growth term, rather than a decay term. This corresponds to a triplet state formation. The negative value of 3 for pH 13 represents a growth term, rather than a decay term. This corresponds to a triplet state formation.

Supplementary Note 1: Acridine protonation equilibria
The reaction of water deprotonation by acridine is given by: The equilibrium constant is then given by the Supplementary Equation 2: The reaction of dissociation of acridine conjugated acid can be written as: With the equilibrium constant Ka,Ac: Hence, in the ground state of acridine, K equals 10 -(14-5.5) = 3.2·10-9. In the excited state, K equals 10 -(14-10.7) = 5.0·10 -4 . From this the ratios of the concentrations of the protonated to deprotonated acridine can be calculated for any given pH: For instance, at pH 7 the ratio in the ground state is 0.032, so that around 3% of acridine molecules are protonated. In the excited state the ratio is ~5000, so that the majority of the acridine molecules are protonated.
Additionally, it should be noted that the deprotonation of water in a closed volume leads to the formation of OHanions that would increase the pH and decrease the protonation ratio. The upper limit of this effect is given by the concentration of the saturated solution of acridine in water which is only 0.25mM. Assuming all acridine molecules are excited, their protonation would produce 0.25mM of OH -, that is pH 10.3. This is still below the pKa of excited acridine, so that more than 50% of excited acridine molecules should be in the protonated form. In practice, this assumption is unrealistic, fewer acridine molecules would be excited, the pH shift would be smaller and the ratio of excited protonated acridine to excited non-protonated acridine closer the one calculated above for pH 7.

Supplementary Methods
Fitting procedure for TCSPC data: All TCSPC decay curves were fitted with biexponenital decay models given by the Supplementary Equation 9: Where I is the normalized PL intensity; Ai (i=1, 2) and i (i=1, 2) are the fractions and lifetimes of the two decay components, respectively. I0 is a baseline constant. The average lifetime (

Cyclic voltammetry:
Autolab N series Potentiostat/Galvanostat was used for the CV measurements, together with a standard three-electrode system with a commercial glassy carbon (CHI instrument, USA, 5.61 mm diameter) working electrode, Pt wire counterelectrode, and Ag/AgCl reference electrode. The pH of the deionized water was adjusted to 9 with sodium hydroxide to enhance the conductivity. Typically, 5 mg of the sample and 100 mL 5 wt% Nafion solution were dispersed in 1 mL water/isopropanol (3/1, v/v) solution and ultrasonicated for 30 min to form homogeneous slurry. Next, 4.5 mL of the slurry was drop-cast on the polished working electrode and dried in the room temperature. The electrodes were then immersed in the electrolyte for 20 minutes to reach equilibrium state. The cyclic voltammograms were acquired between -1.5 V to 1.5 V with a scan rate of 100 mV/s.