Photoelectrochemical Oxidation of Glycerol over g-C3N4 Nanosheets Coupled with Metal-Assisted Hybrid Carbon Quantum Dots

Abstract

Carbon quantum dots (CDs) are the most promising candidates of the carbon family with a size of less than 10nm. They owned superior properties such as high solubility in aqueous medium, low toxicity, biocompatibility, stable photoluminescence, spectral convertor, and act as electron reservoirs. These properties make CDs suitable for photo-electrochemical catalytic applications. Low quantum yield and activity in the visible light spectrum of CDs may restrict their applicability. To address this issue, doping and surface passivation of CDs is the only approach to improve their physicochemical characteristics, visible light absorption, and quantum yield by altering their size and structure. Metals are electron-efficient entities and act as electron donors. As a result, when metal ions are doped into CDs, the charge density and charge transition between the core of the CDs and the metal ions increases. This results in a greater improvement in the physicochemical properties of CDs as compared to non-metal doping[1]. Metal-assisted CDs i.e. hybrid CDs tend to modulate energy band positions, change the electronic structure, increase quantum yield, and generate new emissive traps in CDs. The photoluminescence property of CDs can be enhanced due to the surface plasmonic resonance (SPR) effects of metal nanoparticles (NPs). Hybrid CDs also show better catalytic performance as compared to the pristine CDs. It helps to enhance the active sites on the CDs surface to facilitate catalytic performance.By keeping these properties, the hybrid CDs were effectively utilized in the field of dye decolorization, degradation of water pollutants, storage materials, and as a sensor[2][3]. The photo-assisted electrochemical glycerol valorization is one of the prominent methods for producing value-added chemicals such as glycolic acid. However, it is a challenge to achieve high yield and selectivity of products. This research investigated the fabrication of heterojunction of g-C3N4 (CN) with CDs and Bi-assisted CDs (BiCD) as a sustainable photoactive material. The heterojunction was obtained by incorporating CDs and BiCD (5wt.% each) onto CN nanosheets by in-situ thermal polymerization treatment and named CDCN and BiCDCN. The prepared composite was analyzed by structural (XRD), functional (XPS) microscopic (FESEM, FETEM) spectroscopic (UV-Vis, PL, TRPL, EDS), and photoelectrochemical characterization techniques.The band gap of CN was obtained as 2.78 eV which was reduced to 2.69 eV for CDCN and 2.62eV for BiCDCN, increasing the absorbance in the visible light zone. The existence of CDs and BiCDs was confirmed by, FETEM investigation (Fig.1a). The XPS analysis of the composite revealed the presence of Bi 4f chemical state of bismuth metal. The calculated TRPL lifetime was 0.28ns for BiCDCN and 2.51 ns for CDCN which was lower than that of CN (9.31 ns) suggesting the rapid electron transfer ability of BiCDCN. In the presence of light and glycerol, the composite BiCDCN had the maximum photocurrent density of 12 mA/cm2, followed by CDCN (1.6 mA/cm2) and CN (1.3 mA/cm2) at 1.2 V. The photoelectrochemical oxidation of glycerol was performed at 0.8V for 3h. The detected products were glycolic acid, glyceric, glyoxylic acid, and oxalic acid. The addition of CD to CN increased the glycerol conversion and yield of glycolic acid & glyoxylic acid significantly. The composite exhibited 43% selectivity towards glycolic acid and 34% for glyoxylic acid. The glycerol conversion and yield of these two products were further enhanced by Bi doping. BiCDCN had a total glycerol conversion of 21%, followed by CDCN at 5.7% and CN at 3.8%. The BiCDCN had the highest yield of glycolic acid at 8.9 % and that for CDCN was 2.4 % and 1.2 % for CN (Fig.1b). The glyoxylic acid yield was 7.2%, 2.2%, and 0.8% for BiCDCN, CDCN, and CN, respectively. Figure 1