چكيده فارسي :
In recent years, world societies have focused on clean and renewable sources of energy. Hydrogen energy production from photoelectrochemical (PEC) water splitting under solar irradiation is a useful technique. In common, a main component of a PEC cell is a semiconductor material as a photoanode. Under illumination, photoelectron and photohole pairs are generated by semiconductor and caused hydrogen and oxygen production on cathode and anode. Titanium dioxide (TiO2) is a n-type semiconductor with low cost, superior photocatalytic performance, longterm stability, and environmental compatibility but its photoactivity is limited in UV region of solar spectrum due to its wide band gap (3.2 eV). Visible light absorption of the TiO2 can be improved by doping of metals and nonmetal elements, sensitizing with narrow band gap semiconductor, coupling with cocatalysts and so on [1]. Graphene quantum dots (GQDs) as a new graphene based materials with low dimensions ( 100 nm), few-layer graphene sheets, high aqueous solubility and low cytotoxicity are utilized in energy conversion [2,3]. In this work, the TiO2 nanowires and GQDs were synthesized by hydrothermal technique on FTO substrate and green electrochemical approach, respectively. The prepared TiO2 nanowires on FTO were modified by the GQDs, using spin coating method. Physical and chemical properties of the GQDs/TiO2 nanowires photoanode were investigated by SEM, XRD, UV-vis spectroscopy and FTIR analysis. The GQDs/TiO2 nanowires electrode was utilized as photoanode in PEC cell and its photoresponse and PEC performance were compared with results from pure TiO2 nanowires photoanode. 14th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 12- 13 Dec, 2018 189 Based on our results, FESEM images confirmed the TiO2 nanowires formation, with average length of 4 μm and diameter range of 100-250 nm (Fig. 1 (a)). After deposition of the GQDs, the GQDs covered the surface of the TiO2 nanowires, uniformly, proving direct evidence for the successful fabrication of the GQD/TiO2 nanowires photoelectrodes (Fig. 1(b)). Photoelectrochemical properties of the TiO2 nanowires and the GQDs/TiO2 nanowires photoelectrodes were investigated in PEC cell with 0.1 M Na2S solution as electrolyte under light illumination of 100 mW/cm2. By using chronoamperometry technique (Fig. 2), the photocurrent density for the GQD/TiO2 nanowires photoelectrode was measured at about 30 A/m2, without any applied bias, 6 times larger than those of pure TiO2 nanowires photoelectrode. It was, thus, found that GQDs sensitized PEC cells greatly enhanced visible-light absorption due to favourable energy levels between the GQDs and TiO2 that induce directional transport charge for efficient charge separation. Therefore, the GQD/TiO2 nanowires provide a promising green photoelectrode for photoelectrochemical water splitting (H2 and O2 production) and other energy applications.
