In Situ and Simultaneous UV−vis/SAXS and UV−vis/XAFS Time-Resolved Monitoring of ZnO Quantum Dots Formation and Growth

The processes of formation, aggregation, and growth of ZnO quantum dots (Qdot) induced by the addition of KOH solution ([OH]/[Zn] = 0.5) to the zinc oxy-acetate ethanolic solution at 40 °C were examined in depth by in situ and simultaneous time-resolved monitoring of UV−vis absorption spectra combined with small-angle X-ray scattering (SAXS) and with X-ray absorption fine structure (XAFS) measurements. XAFS results recorded at the Zn K-edge evidence the presence of both the zinc oxy-acetate precursor and the ZnO nanocrystals. Each XAFS spectrum could be described by a linear combination of each reference spectra, the composition evolving from ∼47% to ∼82% of ZnO from the beginning to the end of time monitoring. The time evolution of the UV−vis spectra shows a pronounced red shift of the excitonic peak during the first 100 min of reaction, characterizing the ZnO Qdot growth. The SAXS curves obtained simultaneously with the UV−vis spectra follow the Porod law at the large q-range, indicating the presence of well-defined smooth surface particles. At the intermediate q-range, the SAXS profiles evolve from a Guinier pattern, typical of diluted sets of aggregates, to a power law dependence characteristic of fractal structures. For reaction times higher than 400 min, the slope of the fractal region of SAXS curves reaches a value characteristic of diffusion controlled cluster−cluster aggregation process. The results of this in situ and combined techniques investigation show that the kinetic of formation of colloidal ZnO nanocrystal is a step process composed of four main stages: (i) ZnO Qdot nucleation and growth; (ii) growth of compact ZnO Qdot aggregates; (iii) growth of fractal aggregates; and (iv) secondary nucleation and fractal aggregates growth.