Synthesis and properties of CdSe Quantum Dot sensitized ZnO nanocomposites

In this work, zinc oxide nanocrystals with an average particle size of 13–22 nm are readily synthesized in aqueous medium by the wet synthesis method. Different sized nanocrystals obtained with change in calcination temperature are characterized by PL photoluminescence (PL) and UV–vis absorption spectroscopies, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The average crystal size of the as prepared ZnO nanopowder is determined by XRD and was found to be in good agreement with the UV–vis absorption analysis. The quality of different ZnO nanopowders is confirmed by XRD spectra. On the basis of different characterizations, ZnO calcined for 1 h (due to its large size and less agglomeration) is chosen for synthesis of ZnO–CdSe nanocomposites with variable sized CdSe QD's (Quantum Dots). Nano-composites are synthesized using bifunctional linker molecule Mercaptopropionic Acid (MPA), and by directly adsorbing CdSe QD's over the surface of ZnO nanocrystals. The difference in charge transfer mechanism in ZnO–CdSe nanocomposites due to different crystallite size of CdSe QD's is studied. Higher crystallinity of ZnO–CdSe nanocomposites can be determined from XRD characterization. Size and mode of attachment in various ZnO–CdSe nanocomposites are determined by SEM studies.

Graphical AbstractIn this work, by means of spectroscopic (XRD, PL, UV–vis and FTIR) and scanning electron microscopic (SEM) techniques, we have demonstrated Quantum Dot (QD) size and linker-dependent charge transfer from different-sized CdSe QD's into ZnO nanoparticles. A model is proposed, which elucidates the charge separation mechanism across the ZnO–CdSe nanocomposites. The diagram shows the main energy levels involved in the charge transfer of electrons and holes from the QD to the semiconductor matrix. Ef denotes the Fermi level. The band gap of 2.32 eV corresponds to the smallest CdSe QD's used (size ∼5 nm).Figure optionsDownload as PowerPoint slideHighlights
► In this work, zinc oxide nanocrystals with an average particle size of 13–22 nm are readily synthesized in aqueous medium by wet synthesis method.
► ZnO calcined at an optimum time of 1 h is chosen for synthesis of ZnO–CdSe nanocomposites.
► Nano-composites are formed either by directly adsorbing CdSe QD's over the surface of ZnO nanocrystals or using MPA bifunctional linker molecule.
► The difference in charge transfer mechanism in ZnO–CdSe nanocomposites for different crystallite size of CdSe QD's are studied and explained in terms of a model.
► It is conjectured that MPA-linker aids in strong anchorage between ZnO and CdSe quantum dots leading to an effective charge transport.