Colloidal-chemical mechanism of Zn(OH)$_2$–ZnO layer formation at the glass – ammonia solution – Zn(II) interface

Thermodynamic and experimental studies of Zn(OH)$_2$/ZnO particle formation conditions in the model of closed system Zn$^{2+}$–NH$_{3,\,aq}$–NH$_{3,\,gas}$–H$^+$–OH$^-$–H$_2$O–N$_{2,\,gas}$ (1), which often occurs in the process of synthesis of zinc oxide nanoparticles and films by chemical bath deposition (CBD) methods, were carried out. It was shown that the driving force for the formation and growth of Zn(OH)$_2$/ZnO particles in the initially homogeneous system (1) at 25$^\circ$C is the difference in the chemical potential of particles at the initial temperature (unsaturated system) and the synthesis temperature (supersaturated system). Using vibrational spectroscopy, X-ray phase and chemical analysis, diffuse light scattering and electrophoresis methods, it was found that the phase transformation of Zn(OH)$_2$ into ZnO takes place in the region of 85–90$^\circ$C. The colloid-chemical transformation of Zn(NH$_3$)$^{2+}_4$ ionic particles into colloidal polycrystals of Zn(OH)$_2$/ZnO composition was established for the first time to be a staged process. The first stage of the process in the solution volume is localized at the gas nanobubble-solution interface as a result of rapid formation, growth and removal of gas nanobubbles from the solution. The interaction of positively charged Zn(OH)$_2$ nanoparticles with the surface of larger negatively charged gas nanobubbles creates colloidal aggregates “bubblejjsurface film of hydroxide nanoparticles”. Their adhesion forms an openwork foam-like structure of the colloid in the solution and in the film on the interfaces at the first stage of synthesis. After degassing of the electrolyte solution, the second stage develops, consisting of the nucleation and ionic-molecular growth of Zn(OH)$_2$/ZnO particles from the supersaturated solution, their distribution between the solution and the electrolyte – reactor wall – air interfaces. The film growth at this stage is regulated by the difference in surface charges of the double electric layer of the interface and polycrystalline colloidal particles. In the solution and on the interface, columnar Zn(OH)$_2$/ZnO structures grow as volumetric stars with conical hexagonal spikes.