Effect of linker on the photosensitization of ZnO layers with CdSe quantum dots

Abstract

In this work, zinc oxide (ZnO) nanoparticles (size < 10 nm) were formed via precipitation in ethanolic solution. The zinc acetate and lithium hydroxide solutions in ethanol were mixed at 273 K temperatures under vigorous stirring. To study the effect of quantum dot (QD) coverage, we have prepared a colloidal suspension of capped CdSe QDs (size similar to 5 nm) by chemical route and anchored them to a nanoporous ZnO layer either by direct adsorption or through linker. Here a bifunctional molecule (mercaptopropionic acid, MPA, and thioglycolic acid, TGA) was previously adsorbed on the ZnO surface, which acted as a molecular cable. From TEM/SEM studies, it was observed that direct adsorption of CdSe QDs onto ZnO surface was not efficient. However, the bifunctional linker molecules particularly MPA facilitates binding of CdSe QDs to ZnO; and consequently, interparticle electron transfer is thus facilitated. The use of MPA linker despite of its long carbon chain also aids in the quenching of photoluminescence of CdSe on addition of ZnO in a more systematic manner indicating efficient charge transfer from CdSe into ZnO as compared with the without linker and with linker TGA case, respectively. Due to higher PL quenching and reduction in lifetime values, higher values of Stern-Volmer quenching constants were thus obtained for CdSe-ZnO composites with MPA as compared with TGA linker and without linker case, respectively. Nonlinear Stern-Volmer plots as observed for samples without linker case indicated heterogeneous quenching due to insufficient binding between CdSe QDs and ZnO. By means of spectroscopic (PL, UV-VIS, FTIR) and microscopic (TEM, SEM) techniques, we have demonstrated linker-dependent photosensitization mechanism of ZnO layers with CdSe QDs. Our data thus illustrate that interfacial-electron transfer kinetics in QD-linker-ZnO assemblies are almost independent of the length of alkyl-containing molecular linkers