Research On Preparation And Properties Of γ-manganese Sulfides Powders And Thin Films By A Microwave Hydrothermal Process

Manganese sulfide is a wide gap （band gap energy, Eg≈3.7eV） VIIB-VIAdilute magnetic semiconductor （DMC） that usually exists in three forms: thestable rock-salt type structure α-MnS, metastable sphalerite type structuresβ-MnS and wurtzite type structure γ-MnS. Both the metastable phases, β-MnSand γ-MnS, only exist in a low temperature range, and will transform irreversiblyto the stable type at100⁴00°C or under high pressure. Metable forms of γ-MnSare expected to exhibit unique optical properties, as well as electrical andmagnetic properties, when compared to the corresponding stable phase. So it haspotential applications in shortwave photoelectric device, solar selective coatings,solar cell as a window/buffer material, photoconductors, catalytic materials,optical mass memories and dilute magnetic semiconductor.In this paper, well crystallized γ-MnS powders and thin films can beprepared by a high efficient, energy-saving and environment friendly microwavehydrothermal process in the low temperature and the short reaction time. Theinfluences of reaction temperature, reaction times, concentration and pH valuesof precursor solution on the phase composition, morphologies of the γ-MnS wereinvestigated without any additives assisted microwave hydrothermal. Then someadditives were used to control the microstructure of the products. On this basis,monophase γ-MnS thin films were successfully prepared on ITO conductiveglass substrate by this method. When using template assisted microvavehydrothermal method, the films microstructure and properties can be controled.The phase compositions, microstructure of the as-prepared powders and thinfilms were characterized by X-ray diffractometer （XRD）, Energy dispersiveX-ray spectroscopy （EDS）, X-ray photoelectron spectroscopy （XPS）, Scanningelectron microscope （SEM）, Transmission electron microscope （TEM）, Infrared spectroscopy （FT-IR）. The optical properties of the powders and thin films werecharacterized by Ultraviolet/visible/near-infrared spectroscopy （UV-VIS-NIR）and Photoluminescence spectroscopy （PL）. The results show that:（1） The γ-MnS powders were successfully prepared without any additiveassisted microwave hydrothermal process using MnCl₂·4H₂O and CH3CSNH2（TAA） as raw materials, the best synthesis process of monophase γ-MnSpowders is: reaction temperature is150°C, reaction time is90min, concentrationof reactants is0.1mol/L. As the pH increases from6to12, the morphology of theγ-MnS powders changes from hexagonal columns to hexagonal flakes, theparticle size of the γ-MnS became smaller and self-assembled quickly intospherical crystallites under the microwave induction. When pH is larger than8,crystallinity of the γ-MnS increases, but the optical band gap of the γ-MnSspherical microcrystals slightly decreases.（2） Special morphologies γ-MnS powders were prepared by PVP, citric acid（C₆H₅Na₃O₇） and HMTA assisted microwave hydrothermal process respectively.Using PVP as an additive, the morphology of γ-MnS is inflorescence shape.Citric acid was used to obtain tower cone-like morphology of γ-MnS. Whenusing HMTA as an additive, the morphology of the products is solid sphericalstructure. The influence of the HMTA amount on the morphorlogy of theproducts, the results show that the surface of the self-assembled spheres becomessmoother, the corresponding particle size decreases to2μm and the particle sizebecomes uniform with the amount of HMTA increases. The absorption peak ofthe products between300⁴00nm becomes more and more intensive and exhibitsa red shift when the amount of the HMTA increases.（3） Uniform monophase γ-MnS thin films were successfully synthesized onITO conductive glass without any template assisted microwave hydrothermalprocess using TAA as S source, and using Mn（CH₃COO）₂·4H₂O, MnCl₂·4H₂O asMn source respectively under the same reaction condition. Results indicate thatwell-crystallized γ-MnS thin films with much denser and uniform surfacemorphology can be achieved when using MnCl₂·4H₂O as Mn source. Theinfluences of different reaction temperature and S/Mn molar ration on thecrystallization, morphology and optical property of the as-prepared γ-MnS thinfilms by using MnCl₂·4H₂O as Mn source and TAA as S source were investigated. Results show that with the rise of reaction temperature and the increase of S/Mnmolar ratio from2.0to4.0, the crystallinity of γ-MnS thin films, the dense anduniform of the γ-MnS thin films are obviously improved, the as-prepared γ-MnSthin films have more absorbency to ultraviolet light between250nm³00nm andthe optical band gap of the thin film slightly increases.（4） The γ-MnS crystal thin films were prepared on ITO substrates usingMnCl₂·4H₂O and CH3CSNH2（TAA） as precursors, polyvinyllpyrrolidone （PVP）and trisodium citrate （C₆H₅Na₃O₇） as complexing agents by a microwavehydrothermal process. The results found that the use of the complexing agent isbeneficial to achieve well-crystallized and much denser and uniform γ-MnS thinfilms with different morphologies. The γ-MnS thin films with an interweavedwire-like morphology were obtained using PVP as the complexing agent, theobtained γ-MnS thin films show a pencil-like morphology with using C₆H₅Na₃O₇as the complexing agent. The optical properties of the γ-MnS thin films obtainedwith complexing agents are superior to those prepared without any complexingagent.