Structural, Electronic, Elastic, Mechanical, and Opto-Electronic Properties for ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 Photocatalyst Effort on Wastewater Treatment through the First Principle Study

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Introduction
For the last two decades, the photocatalytic degradation process has been used as a promising wastewater treatment method for the mitigation of organic wastewater pollutants due to its numerous merits [1]. As water contamination has inauspicious efects on both human living life and ecosystems, it is a drastic biodegradable problem and impossible to avoid the water pollution system or even stop this industrialization, a major source of water pollution and the backbone of the modern civilization of every country. Tis issue has become a pressing problem in certain countries, including Bangladesh, India, Pakistan, China, Indonesia, and some African nation, where a number of industries, including those producing ready-to-wear garments, industries, pharmaceuticals, paint industries, and household waste, have been expanding while disregarding the apex of environmental sustainability [2,3]. Additionally, the human population, as well as the unexpected growth in industry both rise and create the greatest demand for clean water resources, while wastewater is generated by several processes and endangers the quality of these resources. Te availability and quality of pure water support sustainable development, human and biodegradation environmental health, and food and energy security [4]. On that account, it is therefore highly imperative that wastewater is adequately managed prior to release from the industry to reduce the ghastly impact on the human body and ecosystems unless it poses threatening and alarming hazard to both developed and developing nation. It is reported that roughly 30% of the naturalistic ambiance has been destroyed after the Second World War, despite the hazardous organic-inorganic contamination has been purifed by biological and conventional processes [5,6]. However, it has been reported that some organic adulterants were not eliminated in these procedures. In this case, the researcher chose photocatalytic and photocatalysis methods as an emerging technique for the mitigation of organic pollutants using a sustainable oxidation process due to its efciency of photoactivity [7], high stability [8], low cost [9,10], low toxic [11], and safety to human health and environment [12,13]. Moreover, the oxidation process has a great impact on removing the toxic organic pollutants in wastewater and turning them into carbon dioxide and water. Additionally, its process has several advantages, including quick processing, a simple reaction system, recyclable materials, self-regenerated, nonconsumption of oxygen, and producing a high level of UV or visible light absorption [14]. Photocatalytic semiconductors, as well as other optoelectronic devices [15,16], are employed in the degradation process for many industrial dyes and antibiotics from pharmaceutical and other industries [17][18][19][20][21] because photocatalysis manipulated a variety of fundamental features, including maximum photocatalytic efciency, a large surface area, light harvesting, reusable, and facilitates the charge carrier separation or enhances the surface reaction of material [22][23][24][25][26]. Te method of photocatalysis produces the electron-hole pair due to induced phot-generation, which leads to the production of superoxide free radicals, and by reacting with oxygen, water molecules, and organic pollutants, causes OH, a free radical. Researchers have discovered some photocatalysts for the photocatalytic destruction of pollutants, such as TiO 2 , ZnO, BaTiO 3 , KNbO 3 , SrBi 4 , Ti 4 O 15 , and WO 3 [27][28][29][30][31]; these materials can be used to exploit UV or visible light as an endless supply of energy [32]. In general, the following materials can be used in rare-earth (Sm/Nd)-lanthanum ferrite-based perovskite ferroelectric and magnetic nanopowders: AFe2O4 (A � Co, Mg, and Mn) complexes for showing magnetostructural properties; Mn-doped LaFeO3; and nanocrystalline zinc ferrite particles as perovskite materials, which have not been seen to be photocatalytic but have good opto-electric properties [34]. Tere has been considerable theoretical, experimental, and synthetic research on the probable environmental and energy applications of stannite-type quarterly crystals such as BAg2CX4 (B � Zn, Cd, Pb, Fe, Mn, Hg; C � Si, Ge, Sn; X � S, Se, Te). [37][38][39]. Metal nanophotocatalysts are easily reusable due to their magnetic function and are strongly associated to organic pollutant mitigation. It is reported that the energy gap value was 2.2 eV for ZnAg 2 GeS 4 materials did not utilize all the range of visible light in order to achieve a wider band gap [17,26]. Additionally, ZnCu 2 SnS 4 semiconductor has been used as an ambiance-friendly photocatalyst as well as a photovoltaic solar cell, and perovskites implications have a straight band gap of 1.5 eV [40][41][42][43][44].
In this work, the new stannite type quarterly crystals, ZnAg 2 SnS 4 , have been designed and investigated their electronic structure and optical properties, and mechanical or electric properties have been calculated which were compared with established photocatalytic ZnAg 2 GeS 4 materials. Secondly, Fe atom has doped into Sn on ZnAg 2 SnS 4 and made a comparative study on how the photocatalytic nature can be changed. Next, the optical properties such as conductivity and absorption show how much of light can be absorbed by the materials, that is, the vital factor acting as the photocatalytic behavior even loss function which says how energy or system be suitable for the study. Finally, calculating the elastic properties can give the evidence of molecular and physical stability of designed crystal.

Computational Methods
According to the basis principle, generalized gradient approximation (GGA) is more physically consistent than a local-density approximations (LDA), which depends on the gradient of the density, and it is a true exchangecorrelation functional of DFT. In addition, the Perdew-Burke-Ernzerhof (PBE) function is very popular because it is a nonempirical function with reasonable accuracy over a wide range of systems. As a result, the PBE techniques were used for optimization for Cu 2 ZnSnS 4 , and ZnAg 2 GeS 4 crystal's structure at frst [45]. Te tetragonal type and space group I4̅ were chosen for theoretical calculation since it described identical experimental data in Table 1. As a frst step of the process, the electronic structures of ZnAg 2 GeS 4 , ZnAg 2 SnS 4 , and ZnAg 2 Sn 0.93 Fe 0.07 S 4 crystals were calculated using the GGA with PBE method, which was implemented in the CASTEP code [46] in Material Studio 8.0 [47]. For ZnAg 2 GeS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 simulations, the cutof was maintained at 523, the k point was fxed at 4 × 4 × 2, and the total energy was set to 1 × 10 −5 eV/atom with the normconserving pseudopotentials functional. Both the density of states and the optical characteristics were determined under these conditions. After simulation, the elastic stifness constants of ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 at atmospheric pressure by employing the stress-strain technique were calculated. On the other hand, band gap analyzed using GGA and PBE showed that GGA works on all crystals under the similar conditions, which implies that additional study in this area is possible. Finally, for the purpose of forecasting the structural, electrical, elastic, mechanical, and optical properties of ZnAg 2 GeS 4 , ZnAg 2 SnS 4 , and ZnAg 2 Sn 0.93 Fe 0.07 S 4 , as depicted in Figures 1(a)-1(c), the 2 × 1 × 1 supercell types were to be accounted.

Results and Discussion
3.1. Structural Properties. Te lattice parameter values of ZnAg 2 GeS 4 , ZnAg 2 SnS 4 , and ZnAg 2 Sn 0.93 Fe 0.07 S 4 were calculated by the material studio after optimizing their crystal structures, which are listed in Table 1. Furthermore, it should be noted that the optimization structure shown in  Table 2. Te bonding and antibonding states are indicated by the negative value and positive value of P μ , respectively. From Table 2, it evidences that all bonds including S-Zn, Ag-S, S-Sn, S-Sn, and Fe-S indicate the boding nature due to the positive value of P μ for both the compounds, ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 .

Electronic Structure.
Te electronic structure has been used to calculate the electronic properties of ZnAg 2 GeS 4 , ZnAg 2 SnS 4 , and ZnAg 2 Sn 0.93 Fe 0.07 S 4 . Te Fermi level (EF) between the valence and conduction band was indicated at 0 eV. Te energy gap between the maximum valence band and the lowest energy state of the conduction band is closely related to the LUMO-HOMO gap [48][49][50][51]. Te crystal materials have two types of energy bands, either direct or indirect band gaps. In Figures 2(a) and 2(b), it was observed that the momentum of the lowest energy state of the conduction band and maximum valence band was found at the same symmetry point G and it reacts as a direct band gap. Terefore, an electron can shift from the maximum energy state of the valence band to the lowest energy state of the conduction band without altering momentum for both ZnAg 2 GeS 4 and ZnAg 2 SnS 4 compounds. Te calculated band gap has been observed at 0.93 eV for the ZnAg 2 GeS 4 crystal. Figure 2(a) shows the electronic band gap, which has been reported to be 1.15 eV for ZnAg 2 SnS 4 crystal. After doping 7% of Fe atoms, the band started to decrease signifcantly, which has been recorded at 0.32 eV, as shown in Figure 2(c). It can be observed that this material follows an indirect band gap by revealing minimum conduction band and maximum valence band, which are completely diferent symmetry points for ZnAg 2 Sn 0.93 Fe 0.07 S 4 . Te electron cannot readily transit from the greatest energy level of the conduction band to the lowest energy state of the valence band without experiencing a change in momentum because of the indirect band gap. Te values of the electronic band gaps for ZnAg 2 GeS 4 , ZnAg 2 SnS 4 , and ZnAg 2 Sn 0.93 Fe 0.07 S 4 semiconductors are listed in Table 3.

Density of States and Partial Density of States.
Te density of states plays a crucial role in demonstrating the nature of electronic band structures and the scattering orbitals. Te suitable method GGA with PBE has been used to interpret total density of states (TDOS) and partial density of states (PDOS) of Zn, Ag, Sn, Fe, and S atoms for ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 compounds. Figure 3(a) depicts the comparative study of TDOS between ZnAg2SnS4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 crystals. It can be observed that the ZnAg 2 Sn 0.93 Fe 0.07 S 4 crystal has higher electron densities in both the valence and conduction bands compared to ZnAg 2 SnS 4 . Figure 3(b) illustrates the PDOS of ZnAg 2 SnS 4 which reveals that the nature of s and d orbitals for Zn; s and d for Ag; s, d, and p for Sn; and s and p for S elements have been examined for exploring electron transitions owing to hybridization by transferring from the highest energy state of the valence band to the lowest energy state of the conduction band. From Figures 3(b) and 3(c), it is also observed that robust hybridization is signifcantly afected by the d orbital in the valence band (VB) and that strong hybridization of the conduction band (CB) is responsible for the s orbital for both ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 crystals. Figures 3(d)-3(j) depict the contribution of individual atoms to the production of total density of states (TDOS) and partial density of states (PDOS), with Fe atoms responsible for decreasing the band gap between VB and CB. As can be seen, the Fe atom contributes to higher below-Fermi and above-Fermi energy levels, raising the valence band level while decreasing the conduction band level. For this reason, the band gap was declined by 7% due to Fe atom doping on ZnAg 2 SnS 4 .

Elastic Constants and Mechanical Properties.
A solid's mechanical characteristics and elastic constants, which afect things such as debye temperature, dislocation motion, and stress-strain behavior, are important parameters. In a tetragonal crystal system, there are six independent elastic constants, namely, C 11 , C 12 , C 13 , C 33 , C 44 , and C 66 . Some mechanical and dynamical properties of the material can be determined by its elastic constants. Te traditional mechanical stability conditions under isotropic pressure for a tetragonal crystal are given by the following equation: Compounds     Advances in Condensed Matter Physics (1)  [53] for B and G in their literature and listed in Table 5. From Table 5, it can be seen that  Table 5.
3.5. Photocatalytic Activity. As a result of the metal oxide's or metal crystal's catalytic action, the photocatalytic reaction proceeds via oxidation and reduction processes. Negative electrons are responsible for reduction, but positive holes join with water molecules from moisture to generate hydroxyl radicals, which are the byproduct of an oxidative reaction. Te operation is described in detail below.
Specifcally, electron-hole pairs are activated by UV light. When the photogenerated electrons come into contact with O 2 , they form reactive anion radicals, such as O 2 − . Appropriate photon irradiation initiates the creation of photogenerated electron-hole pairs on the photocatalyst surface. Active OH free radicals react with water to form holes, which can subsequently be used to degrade organic pollutants. Te hydrogen ions can be converted to hydrogen molecules by photoexcited electrons once they are in the conduction band. Te band gap is directly related to the amount of ultraviolet light that can pass through it. Most of the photocatalysts has a band gap of 3.2-2.8 eV, which is approximately 387.45-442.80 nm in wavelength. However, it was found that the excellent photocatalyst corresponds to 688.80 nm wavelengths, indicating a band gap of 1.8 eV or lesser. ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 were explored for their 1.15 eV and 0.32 eV band gaps, respectively. Further research has revealed that ZnAg 2 Sn 0.93 Fe 0.07 S 4 can absorb a variety of UV radiation, which raises the possibility that it could function as a photocatalyst.

Optical Properties.
Te photocatalyst is dependent on a number of active sites, including light absorption, charge mobility, and the magnitude of band gap and electron-hole transportation in terms of conductivity, refectivity, refractive index, and loss function. Furthermore, a material with a wide surface area is more efective at absorbing pollutants because it creates a greater number of active surface sites, which in turn speeds up the degradation or oxidation of the pollutant.

Optical Refectivity.
Refectivity is a signifcant optical property that describes the amount of light that strikes the surface of the photocatalytic material. Tis can be examined from the refectivity data, which is related to the absorbance of that material. In previous research examined, the higher absorption spectrum of UV or visible light indicates a lower refectivity. Figure 4

Absorption.
Te optical spectrum is an impactful process where light energy is converted to other forms of energy depending on the nature of the energy band gap. For direct band gap semiconductors, optical absorption occurs at a higher photon energy than the energy gap (Eg) between VB and CB. Te absorption spectrum is modest for indirect band gap semiconductors without exceeding the direct gap where c represent the speed of light in vaccum and v represent the speed of light in another medium. Te refractive index has two portions, namely, the real part, which indicates the phase velocity, and the imaginary part, which indicates the mass attenuation coefcient. Te comparative study of refractive index as a function of photon energy for ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.0.07 S 4 is illustrated in Figure 5(a). Te magnitudes of the refractive index for the real part are signifcantly higher at the initial state, while the imaginary part is reported to be almost zero for both doped and undoped. After that, the real part of ZnAg 2 Sn 0.93 Fe 0.0.07 S 4 declined moderately and ZnAg 2 SnS 4 increased slowly, whereas the imaginary part of ZnAg 2 Sn 0.93 Fe 0.0.07 S 4 increased gradually at 2.5 eV. After 3 eV, however, both materials increased in a similar trend.

Dielectric Function.
Some of the optical properties, including refectivity, refractive index, and absorption spectrum, have been investigated by dielectric function or relative permittivity, which describes the response of a material to the application of an alternating electric feld to a solid material.
Here, ε 1 (ω) and ε 2 (ω) denote the dielectric constant (real part) and the dielectric loss factor (imaginary part), respectively. Te probability of photon absorption for the band structure of any material is closely related to the imaginary portion of the dielectric function. Te real part of the dielectric constant maintains the energy storage potential in the electric feld, while the imaginary segment indicates the opposite; even this is true for electric potential energy. From Figure 5(b), we can see that the real portion is always higher than the imaginary part within the energy range from 4.3 eV to 5 eV, where the imaginary part shows a higher value than the real portion for both doped and undoped. At the Fermi energy level, the value of the real part of ZnAg 2 SnS 4 is reported at around 2.6, while after 7% of Fe atom doping, its value increased by 4.3 for ZnAg 2 Sn 0.93 Fe 0.0.07 S 4 , but the values of the imaginary part of doped and undoped crystals are accounted for almost zero.
3.6.5. Conductivity. Te conduction process of photocatalytic semiconductors takes place on the basis of the energy band and free electrons, which are closely related to the discrete space of orbital electrons. It is also produced owing to the presence of free electrons and holes and the transition of free electrons from the valence band to the conduction band in the crystal materials. Optical conductivity has two segments, frst, the real part, and second, the imaginary part. Te real part of conductivity attains the same information as the imaginary part of the dielectric function, which describes the convective current, and the imaginary part of conductivity indicates the displacement current. 3.6.6. Loss Function. Te loss function is a fundamental aspect of optical characteristics and consists of two photon energy zones for crystal materials. Inside the dielectric theory validation range, the energy loss function is strongly aligned with the photocatalyst dielectric function. Te dielectric function refects the response of a semiconductor to an external electromagnetic perturbation. Tis response is accounted for in the energy loss function. Te calculated exploration of loss function values for ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 is illustrated in Figure 6. It can be observed that the loss function of ZnAg 2 Sn 0.93 Fe 0.07 S 4 increased rapidly from 0 eV to 2.5 eV and peaked at 0.125 due to the splitting of the orbital. After 2.5 eV, it fell down again.   On the other hand, the loss function value for 7% doped Fe atom material increased gradually and reached by 0.075 at 5 eV.

Conclusion
Overall, frst-principle calculations using a suitable DFT functional have been used to explore the elastic, electronic, structural, mechanical, and optical properties of the stannite type quarterly crystals of ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 . Te Mulliken bond population analysis, which reveals the bonding nature of Zn-S, Ag-S, S-Sn, S-Sn, and Fe-S was calculated and used to investigate the obtained lattice parameters value and chemical bonding of ZnAg 2 SnS 4 and ZnAg 2 Sn 0.93 Fe 0.07 S 4 and confrmed the optimized structures. Secondly, the calculated band gaps of ZnAg 2 GeS 4 , ZnAg 2 SnS 4 , and ZnAg 2 Sn 0.93 Fe 0.07 S 4 were found at 0.93 eV, 1.15 eV, and 0.32 eV, respectively, using GGA with PBE function. In addition, the experimental band gap of the ZnAg 2 GeS 4 is 0.94 eV that is almost same to the calculated band gap (0.93 eV) in this study which indicates the accuracy of this study for all crystals. Another objective of this study is the doping on ZnAg 2 SnS 4 by the most available metals replacing Sn, where 7% Fe atoms were doped and found a very lower band gap, 0.32 eV, indicating the material is well-suited for absorbing all UV or visible light in the higher wavelength region and displaying enhanced photocatalyst performance in comparison to the ZnAg 2 SnS 4 crystal. Te extension for the other properties of designed crystals were evaluated by mechanical and elastic properties, which have shown to be ductile, stifer, and anisotropic, indicating that both semiconductors-based photocatalysts are ductile. Te mechanical and elastic properties have been shown to be ductile, stifer, and anisotropic, indicating that both semiconductors are ductile. To foretell efcient semiconduction in photocatalysts, we have computed their optical properties such as refectivity, absorption, refractive index, dielectric function, conductivity, and loss function. For Fe doped of ZnAg 2 Sn 0.93 Fe 0.07 S 4 , with its increased absorption spectrum in the UV visible region, it is found to be superior to ZnAg 2 SnS 4 as a photocatalyst for use in waste water treatment. Terefore, it is evident to conclude that semiconductor-based ZnAg 2 Sn 0.93 Fe 0.07 S 4 , after being doped with 7% Fe atoms, has enhanced photocatalytic performance to that of undoped ZnAg 2 SnS 4 .

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.