Oxygen Aspects in the High-Pressure and High-Temperature Sintering of Semiconductor Kesterite Cu2ZnSnS4 Nanopowders Prepared by a Mechanochemically-Assisted Synthesis Method

Abstract

We explore the important aspects of adventitious oxygen presence in nanopowders, as well as in the high-pressure and high-temperature-sintered nanoceramics of semiconductor kesterite Cu2ZnSnS4. The initial nanopowders were prepared via the mechanochemical synthesis route from two precursor systems, i.e., (i) a mixture of the constituent elements (Cu, Zn, Sn, and S), (ii) a mixture of the respective metal sulfides (Cu2S, ZnS, and SnS), and sulfur (S). They were made in each system in the form of both the raw powder of non-semiconducting cubic zincblende-type prekesterite and, after thermal treatment at 500 °C, of semiconductor tetragonal kesterite. Upon characterization, the nanopowders were subjected to high-pressure (HP) (7.7 GPa) and high-temperature (HT) (500 °C) sintering that afforded mechanically stable black pellets. Both the nanopowders and pellets were extensively characterized, employing such determinations as powder XRD, UV-Vis/FT-IR/Raman spectroscopies, solid-state 65Cu/119Sn NMR, TGA/DTA/MS, directly analyzed oxygen (O) and hydrogen (H) contents, helium density, and Vicker’s hardness (when applicable). The major findings are the unexpectedly high oxygen contents in the starting nanopowders, which are further revealed in the sintered pellets as crystalline SnO2. Additionally, the pressure–temperature–time conditions of the HP/HT sintering of the nanopowders are shown (in the relevant cases) to result in the conversion of the tetragonal kesterite into cubic zincblende polytype upon decompression.

Long-Term Oxidation Susceptibility in Ambient Air of the Semiconductor Kesterite Cu2ZnSnS4 Nanopowders Made by Mechanochemical Synthesis Method

“…The UV-Vis spectra were run for all nanopowders and, first, confirmed our earlier observations that the prekesterite nanopowders are defunct of semiconductor properties and yield no specific UV-Vis spectra [11][12][13][14]. Second, the typical spectra were successfully obtained for all three 500 • C-annealed kesterite nanopowders and the specific energy band gaps were calculated from them via Tauc (αhν)2 vs. hν [energy] plots as illustrated in Figure 6 (see, inserts in the spectra) [48].…”

“…In our recent paper on the oxygen-related aspects of the high-pressure and hightemperature sintering of kesterite nanopowders, we confirmed noticeable oxidation phenomena occurring in the synthesis as evidenced by the presence of crystalline tin(IV) oxide SnO 2 in the kesterite nanoceramics [14]. Additionally, oxygen-bearing copper and zinc sulfates were seen by FT-IR in the substrate nanopowders, which was supported by the evolution of sulfur(IV) oxide SO 2 in the TGA-DTA/MS experiments and by direct oxygen content determinations.…”

“…One of the convenient preparation methods in the solid state is a high-energy ball milling of suitable precursors, which yields the nanocrystalline kesterite powders via mechanochemical interactions [9,10]. In this area, in recent years, we have mastered the preparation of Cu 2 ZnSnS 4 nanopowders from a few precursor systems [11][12][13] as well as showed the feasibility of the nanopowders under Materials 2023, 16, 6160 2 of 17 high-pressure (7.7 GPa) and high-temperature (500 • C) for sintering towards mechanically robust kesterite nanoceramics [14].…”

Thermogravimetric/Thermal–Mass Spectroscopy Insight into Oxidation Propensity of Various Mechanochemically Made Kesterite Cu2ZnSnS4 Nanopowders

“…The chemical nature and plausible structure disorder also suggest that the compound may be susceptible to oxidation in an ambient humid air starting from the synthesis stage, going through storage/manipulations, and ending with viable applications. Indeed, our recent studies focused on this aspect, support such predictions for a range of kesterite-type nanopowders, with the conclusion that all manipulations of kesterite have to be carried out so as to minimize its exposure to air [4,5]. As for potential applications, it has yet to be proven that suitable engineering of a layered structure of a kesterite-based PV cell would prevent the cell from chemical deterioration in humid air even at ambient temperature conditions.…”

“…The preparation work-up and, specifically, the drying stage in air in the “wet” mechanochemical synthesis method, constitutes yet another oxygen source, possibly even more acute, since one deals at this stage with very reactive, high-surface-area nanopowders exposed to air for relatively long and variable periods of time. As a result, such oxidation by-products are detected for kesterite samples as metal sulfates including copper sulfate pentahydrate CuSO 2 •5H 2 O, zinc sulfate monohydrate ZnSO 4 •H 2 O, and, possibly, hydrated tin oxide SnO 2 •xH 2 O not only in the mechanochemical synthesis method [ 4 , 5 , 9 ] but also in other preparation routes [ 10 , 11 , 12 ]. It is instructive to note that in some kesterite studies, the presence of oxygen-bearing species is evident from inspection of the data, although it is not specifically acknowledged or appropriately commented on by authors [ 13 ] (e.g., XPS survey scans with the apparent O 1s band in the range of 529–531 eV for oxides or the S2p 3/2 /S2p 1/2 bands in the range of 167–172 eV for the sulfate group [SO 4 ] −2 ).…”