The Sulphide Based Chalcogenides
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Most metal sulfides possess a layered structure. One of the examples is MoS2, it has a layered structure of a two-dimensional (2D) segment comprising three atomic layers; a metal layer between two closed-packed sulfur layers. A total of two interactions exist on each layer and between layers which are strong covalent and weak van der Waals’ respectively. Moreover, the ability of sulphur layers sliding over each other with only a little resistance, resulting them being stable which allows them to be used as a high-temperature lubricant. Another example will be ZnS, where it has isotropic physical properties that are also known as layered. They can be viewed as consisting of a close-packed single layer of sulfur atoms which are stacked in different ways, with metal atoms occupying either the interstices or holes. Another crystal structure of ZnS, known as wurtzite with one-half of the tetrahedral holes filled by cations, is one of the examples of hexagonal close packing. Unlike the layered structure of MoS2 and ZnS, wurtzite has anisotropic properties because it crystallizes in a hexagonal space group.
Sulfide based chalcogenides are widely studied due to their desired properties of having large bandgap, high electrons mobility, suitable morphology with high surface area and stability. Not only that, sulphur-based chalcogenides quaternary semiconductors appear to have an ideal photovoltaic application due to their opto-electrical properties, environmentally friendly and earth-abundant composition. In addition, with suitable band engineering, they could prohibit rapid recombination of electron-hole (e−-h+) pairs, as well as backward reactions. This results in an enhancement on the photocatalytic activity.
Two of the most commonly studied sulfide based chalcogenides are ZnS and CdS. They possess a high photocatalytic activity which gives rise to the energetic feasibility for overall water splitting, where ZnS has a high conduction band level. As a result, water is reduced to hydrogen, in reaction involving hydrogen evolution from aqueous solutions that contain sacrificial reagents such as SO₃²⁻ and S2−, without any present of cocatalyst. Unfortunately, with ZnS having a large band gap, it minimizes its photocatalytic applications within the ultraviolet light regions. Thus, visible light responsive photocatalyst with a high photocatalytic activity is required to completely utilize the solar light energy. Hence, co-doping of ZnS can be performed to increase the light absorption ability to the visible light region while maintaining its high conduction band level. By implementing this method, it prevents photo corrosion and reduces the recombination rate of e−-h+pairs.
Today, the interest in chalcogenide glasses has increased significantly due to the implementation of alloys, crystals and glasses in a wide range of photonic devices. The glass-forming abilities is indirectly proportional to molar weight of constituent elements, where decreasing the element’s molar weight will increase the ability of glass-forming. Hence, the glass-forming abilities of sulphur chalcogenides are the highest as compared to selenium (Se) and tellurium (Te) chalcogenides; S > Se > Te. Sulphide chalcogenide glass is synthesized from a mixture of one or more sulfur by heating them to melt in an oxide free atmosphere. They are covalently bonded and classified as covalent network solids.
A common, most strong and stable chalcogenide glass, arsenic trisulfide (As2S3), has fiber that shows high transmission in the region below wavelength of 6 μm. Other non-sulphide based chalcogenide glasses such as As-Ge-Se and Ge-Se-Te have optical fibers with longer wavelengths and some deliver CO2 laser light at wavelength of 10.6 μm. As2S3 is the most popular application as optic fiber due to the ability of changing the refractive index of As2S3 by changing the quantity of arsenic and sulfur. Even with the high stability of As2S3, however, the highly toxic nature of arsenic produces harmful glasses, which are required to undergo complicated processes to limit the harmfulness of the material. Also, if dissolved in the human body, the toxicity may cause fatal problems. Hence, arsenic free glasses with lower toxicity, such as Gallium (III) Sulphide are developed. With Ga₂S₃ being a non-glass former, adding sodium or lanthanum sulphides will form a glass known as gallium lanthanum sulphide (GLS), making this process easier to fabricate into components.