Synthesis of micro- and nano-sized zinc sulfide crystallites by the method of self-propagating high-temperature synthesis
Authors: Kozytskyi S. V. 
Published: December 2023

The paper presents the results that demonstrate how ZnS crystallites of different sizes can be obtained by the method of self-propagating high-temperature synthesis (SHS). The SHS is possible due to an exothermic reactions occurring in mixtures of Zn and S powders, due to a large amount of heat released (enthalpy of formation being 202 kJ/mol).
Pressed powders of Zn and S are placed in a stoichiometric ratio into a reactor filled with Ar or N under a pressure of P>0.5 MPa. After initiated by ignition, the chemical reaction propagates through the mixture to produce a sample in the form of an ingot. ZnS crystallites with a characteristic size of ~ 30 μm are obtained in the traditional solid-state reaction of the SHS. 
Аfter the passage of the combustion wave, the reaction of the components continues in parallel with crystallization processes, which causes the formation of an active medium where self-organization processes occur. Sometimes, abnormally large ("giant") crystallites with length ~ 1 mm, thickness 0.1-0.2 mm, and regularly-shaped spatial structures are observed.
The size of synthesized zinc sulfide crystallites can be changed with a dispersant, NH4Cl. When 5 wt.% of the dispersant is introduced, the monolithic sample is polycrystalline ZnS with a characteristic grain size of ~ 40 μm, and when 7 wt.% is introduced,  the grain size decreases to 20 μm. As the concentration of the dispersant is increased to 10 wt. %, the synthesized material becomes loose, the characteristic grain size decreasing to 5-10 μm. At concentrations greater than 10 wt. %, powder-like ZnS is formed.
The SHS method makes it possible to obtain meso- and nano-sized ZnS, with grain sizes of order 50 – 100 nm and 2 – 50 nm, respectively, in two ways. The first one consists in the condensation of the substance’s vapor in a rarefied inert atmosphere. When, during the SPS, the pressure of the inert gas changes in the interval from 40 to 400 Pa, a part of the reactants in the form of steam escapes from the ampoule into the reactor bulk. When the steam particles collide with the inert gas atoms, they quickly lose their kinetic energy to form particles with sizes of 2 to 100 nm. To form particles of the required size, it is necessary to adjust the pressure of the inert gas in the reactor.
The second method for obtaining nanoparticles by the SBC is based on the use of an inert diluent, which prevents the growth of the forming particles. The main requirement for the diluent material is its inertness to both the reagents in the charge and the synthesized material. The effect of changing the pressure of the inert gas on the size of the obtained nanoparticles manifests itself both directly and indirectly—through the size of the condensation zone.  As the gas pressure increases, its density increases, heat removal increases, the rate of crystallization center formation in the gas phase decreases, but the rate of crystal growth increases. As a result, so does the size of the resulting particles.
The shape of nanoparticles obtained by this method depends on their size. Nanoparticles of size <20 nm have a shape close to spherical, which is a result changing the relative fraction of the surface energy in the total energy of a nanoparticle as its size decreases. Larger-size particles are faceted.