Carbon quantum dots as nano-scaffolds for Α-Fe₂O₃ growth: Preparation of Ti/CQD@Α-Fe₂O₃ photoanode for water splitting under visible light irradiation

To improve the photoresponsivity of hematite-based photoanode via better charge transfer rate and short paths for the electron transport, carbon quantum dots (CQDs) were used as conductive nano-scaffolds for the growth of photoactive material on Ti substrate. CQDs@α-Fe₂O₃ nanoparticulates with the average diameter of 3–5 nm were uniformly grown on the substrate under the finely optimized experimental conditions to prepare Ti/CQDs@α-Fe₂O₃ photoanode. The photocurrent response of the resulted photoanode with a photocurrent density of 2.1 mA cm⁻² at applied E_bias of +0.5 V vs. Ag/AgCl was increased by a factor of 10 compared to Ti/α-Fe₂O₃, mainly due to the improvement in charge-transfer rate and suppression of electron-hole recombination derived from the increased hole-diffusion length in conducting nano-scaffold structure. The surface morphology of samples was investigated with FE-SEM and HRTEM. Charge transfer resistance (R_ct) of Ti/α-Fe₂O₃ and Ti/CQDs@α-Fe₂O₃ photoanodes were estimated to be about 90.9 and 3.7 KΩ, respectively. After the continuous 4 h illumination of Ti/CQDs@α-Fe₂O₃ photoanode under the visible light irradiation, the efficiency of water splitting process (i.e. the photocurrent) did not changed significantly (±5%), indicating the high stability of photoanode and tightly deposited CQDs@α-Fe₂O₃ on Ti substrate, which was confirmed by FE-SEM image of the sample after the experiment. The formation of carbon-oxygen chemical bonds between CQDs and hematite molecules was confirmed by X-ray photoelectron spectroscopy (XPS). Finally, based on XRD pattern and photoresponses of various photoanodes annealed at different temperatures, the results showed that the structure design is as significant as crystallinity in hematite-based photoelectrodes.