Enhanced photocatalytic water splitting by gold carbon dot core shell nanocatalyst under visible/sunlight

Abstract

Hydrogen production from water using photocatalysts under sunlight still remains a huge challenge. The search for a suitable photocatalyst combines an ability to dissociate water molecules with a band gap that absorbs light in the visible range and also water stability. In this regard, our present study represents a facile method for fabricating carbon quantum dots (CQDs) and Au@CQDs, which are useful for photocatalytic hydrogen generation from water. The optical properties revealed a band gap energy (eV) for CQDs (2.78) and Au@CQDs (2.68); additionally, photoluminescence analysis of CQDs showed maximum emission at 460 nm while also exhibiting a red shift when excited at longer wavelengths. Spherical shaped CQDs, with an average size of 7 nm (d spacing of 0.22 nm), and core shell Au@CQDs, with a shell thickness of 6 nm, were observed by HRTEM analysis. Comparatively, both CQDs (260 μmol) and Au@CQDs (280 μmol) displayed a higher rate of hydrogen production under sunlight irradiation than other carbon materials reported in earlier literature. In photoelectrochemical analysis, current densities associated with Au@CQDs and CQDs photoelectrodes (PEs) were found to be 16 mA cm⁻² and 6 mA cm⁻², respectively at a very low bias of 0.16 V. Moreover, frequency response analysis (FRA) associated with Randel's equivalent circuit showed that polarization/charge transfer resistance for Au@CQDs was very low (2.74 Ohm) over that of CQDs PEs (88.8 Ohm), which was 12.9 kOhm for bare TiO₂ PEs. All these observations indicate that both CQDs and Au@CQDs are ample for preventing an electron–hole recombination processes, which ultimately leads to superior photocatalytic water splitting.