Optically photoactive Cu–In–S@ZnS core-shell quantum dots/biopolymer sensitized TiO2 nanostructures for sunlight energy harvesting

Abstract Colloidal semiconductor quantum dots (QDs) have been intensively investigated because of their remarkable tunable optical and electrical properties for potential clean energy production through solar photoconversion devices. Herein, we present for the first time the design and development of a hybrid nanostructure for applications in sunlight energy photoconversion, relying on a “safe and sustainable by design (SSD)” strategy, as follows: a) “Cadmium-free” (non-Cd-containing) inorganic semiconductor quantum dots (QDs), as optically active nanomaterials composed of quaternary Cu–In–S/ZnS systems (ZCIS); b) carboxylic-functionalized cellulose derivative (CMC) as biopolymer ligand and macromolecular stabilizing agent; c) A green chemistry approach based on an aqueous synthesis at mild conditions (e.g., pH, temperature, “organic-free”, nontoxic precursors) for creating water-soluble supramolecular colloidal core-shell nanostructures (i.e., ZCIS@CMC, core: ZCIS; shell: CMC); d) As a proof-of-concept, the fabrication of a photoelectrochemical cell (PEC-cell) based on a solid-liquid heterojunction using the photoanode composed of TiO2 sensitized by ZCIS@CMC nanohybrids. Regarding optical properties, the results demonstrated that the ZCIS@CMC nanohybrids presented absorption/emission characteristics with maxima responses within the visible range of the light spectrum, which is prospectively appropriate for sunlight photoconversion applications. The solid-liquid junction photoelectrochemical cells (PEC-cell) fabricated using TiO2 films deposited onto ITO glass slides sensitized by ZCIS QDs evidenced photoactivity under visible light illumination using “dark” (“OFF”) and “light” (“ON”) cycles. Moreover, the PEC-cells confirmed the significant enhancement of photoinduced current density (over 150%) upon illumination when the relative sensitization density associated with the ZCIS QDs/TiO2 ratio was increased, and ethanol was used as the co-solvent. The results were credited to better contact between ZCIS QDs and TiO2 nanoparticles, tailoring a more favorable energy level alignment at the heterojunction interfaces, promoting the formation of a cascade of conduction band edges. Thus, these quantum dot-sensitized nanostructures developed through a sustainable and biocompatible approach produced strictly via green chemistry process can be anticipated as a sustainable alternative to the existing heavy metal-based sunlight photoactive optical nanomaterials after further optimization of conversion efficiency parameters.