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

Abstract

The often overlooked and annoying aspects of the propensity of no-oxygen semiconductor kesterite, Cu2ZnSnS4, to oxidation during manipulation and storage in ambient air prompted the study on the prolonged exposure of kesterite nanopowders to air. Three precursor systems were used to make a large pool of the cubic and tetragonal polytypes of kesterite via a convenient mechanochemical synthesis route. The systems included the starting mixtures of (i) constituent elements (2Cu + Zn + Sn + 4S), (ii) selected metal sulfides and sulfur (Cu2S + ZnS + SnS + S), and (iii) in situ made copper alloys (from the high-energy ball milling of the metals 2Cu + Zn + Sn) and sulfur. All raw products were shown to be cubic kesterite nanopowders with defunct semiconductor properties. These nanopowders were converted to the tetragonal kesterite semiconductor by annealing at 500 °C under argon. All materials were exposed to the ambient air for 1, 3, and 6 months and were suitably analyzed after each of the stages. The characterization methods included powder XRD, FT-IR/UV-Vis/Raman/NMR spectroscopies, SEM, the determination of BET/BJH specific surface area and helium density (dHe), and direct oxygen and hydrogen-content analyses. The results confirmed the progressive, relatively fast, and pronounced oxidation of all kesterite nanopowders towards, mainly, hydrated copper(II) and zinc(II) sulfates, and tin(IV) oxide. The time-related oxidation changes were reflected in the lowering of the energy band gap Eg of the remaining tetragonal kesterite component.

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

“…This is the case for the hydrated copper and zinc sulfates that were confirmed in the freshly made nanopowders in small quantities by FT-IR spectroscopy but were not detected by XRD. However, a few months-long exposure of the kesterite nanopowders to ambient air confirmed substantial oxidation with the hydrated metal sulfates as the dominant by-products that were confirmed by XRD [ 5 ].…”

“…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 ).…”

“…500–600 °C and decomposes at still higher temperatures to the metal sulfides and sulfur [ 14 ]. Temperature levels around 500 °C are commonly used in the synthetic work-up of kesterite, also, to stabilize the structure of the most stable tetragonal polytype in the materials’ forms of rather thick layers or nanopowders [ 1 , 5 , 6 , 7 , 8 , 15 , 16 ].…”

“…The organic remnants may then decompose to carbonaceous species upon thermal annealing under neutral gas conditions. In this regard, the thermally treated kesterite nanopowders were recently shown by micro-Raman spectroscopy to contain some residual carbon [ 5 ]. The carbon contaminant is expected to react with such oxygen-bearing gases as O 2 , H 2 O, and SO 2 /SO 3 , which will impact the nature of the gaseous species that evolve upon heating the nanopowders.…”

“…The raw products containing xylene were conventionally dried overnight in air, stored in a desiccator, and annealed at 500 °C to make the final tetragonal polytype nanopowders. These materials were used in the already-published investigation of their prolonged oxidation in ambient air [ 5 ]. Herein, we approached the kesterite oxidation phenomena from a specific direction offered by the application of thermogravimetry and thermal analysis coupled with mass spectroscopy (TGA/DTA-QMS), the latter for the detection of evolving gas/volatile compounds at rising temperatures.…”