Understanding the “Tailoring Synthesis” of CdS Nanorods by O2 - Carbon4Energy

Parameters such as solution concentrations and composition of the ambient atmosphere are known to be important in phase and morphology control in the solvothermal synthesis of CdS semiconductor nanorods (NRs), but a clear understanding of the underlying mechanisms involved is lacking. In this work, a series of experiments were performed to demonstrate that the key factor affecting the phase and morphology of CdS NRs is the amount of O2 in the space above the reaction solution in the sealed vessel relative to the amount of precursors in solution: O2-depleted or reagent-rich conditions resulted in more cubic phase CdS and thick NRs with an aspect ratio usually less than 3, which have small blue-shifts in band-edge emission and little surface trap emission; while O2-rich or reagent-poor conditions resulted in more hexagonal phase CdS and slim single crystal NRs, which have significantly blue-shifted band-edge emission and relatively strong surface trap emission. Thus increasing the amount of solution in the vessel, changing the ambient atmosphere from air to N2 (Ar), or increasing the reagent concentration all lower the molar ratio of O2 to reagents and lead to more cubic phase and thicker NRs. The results indicate that the composition of the “empty” section of the reaction vessel plays as important a role as the composition of the liquid in determining the phase and morphology, something that has been overlooked in earlier work. A mechanism to explain the effect of oxygen on the nucleation and growth stages has been proposed based on those results and further supported by shaking experiment and ZnS NRs synthesis manipulation. The CdS NRs synthesized under different conditions showed obvious differences in photocatalytic activity, which indicated that controlling the synthetic process can lead to materials with tailored photocatalytic activity.