The study of structural, morphological and optical properties of (Al, Ga)-doped ZnO: DFT and experimental approaches

ZnO is a widely studied material for several applications, such as a photocatalyst, a working electrode for dye-sensitized solar cells, and for thermoelectric devices. This work studies the effects of an increase in the number of carriers by doping ZnO with Al and Ga. The 6.25 mol% Al-doped ZnO, 6.25 mol% Ga-doped ZnO, and 12.5 mol% (Al, Ga)-co-doped ZnO nanoparticles were prepared using the combustion method. The prepared samples were then characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and UV–visible spectroscopy techniques. Moreover, the density functional theory (DFT) was also employed for computational study of Al and Ga doped ZnO. Optimized crystal structures, density of states (DOS) and band structure of these systems were calculated using Vienna Ab initio Simulation Package code. From this study, Al and Ga are found to play an important role in both the morphology and optical properties of the ZnO: Al and Ga doping can change the band gap and the Fermi level position in the ZnO. The prepared samples were characterized for their thermoelectric properties, and these were also modelled, using BolzTraP code, for ZnO, Al-doped ZnO, Ga-doped ZnO and (Al, Ga)-co-doped ZnO. The Seebeck coefficient, electrical conductivity, relaxation time, electronic thermal conductivity and power factor were all analysed. The experimental and computational results all point in the same direction, indicating that the thermoelectric properties of ZnO change because the semiconductor ZnO transforms into metallic ZnO when doped with Al and Ga. This leads to ZnO showing different thermoelectric properties, particularly Ga-doped ZnO and (Al, Ga)-co doped ZnO: they provide a high electrical conductivity and power factor. Therefore, it is expected that these favorable properties might promote the ZnO to be a potential candidate for improved efficiency thermoelectric devices

Hybrid density functional study of electronic and optical properties of phase change memory material: Ge2Sb2Te5\mathrm{Ge_{2}Sb_{2}Te_{5}}

In this article, we use hybrid density functional (HSE06) to study the crystal and electronic structures and optical properties of well known phase change memory material $\mathrm{Ge_{2}Sb_{2}Te_{5}}$. We calculate the structural parameters, band gaps and dielectric functions of three stable structures of this material. We also analyze the electron charge distribution using the Bader's theory of charge analysis. We find that hybrid density functional slightly overestimate the value of 'C' parameter. However, overall, our results calculated with the use of hybrid density functional (HSE06) are very close to available experimental values than calculated with the use of PBE functional. Specifically, the electronic band gap values of this material calculated with HSE06 are in good agreement with the available experimental data in the literature. Furthermore, we perform the charge analysis and find that naive ionic model fails to explain the charge distribution between the constituent atoms, showing the complex nature of this compound.Comment: 10 pages, 3 tables, 3 figure

Improved Thermoelectric Properties of SrTiO3 via (La, Dy and N) Co-Doping: DFT Approach

This work considers the enhancement of the thermoelectric figure of merit, ZT, of SrTiO3 (STO) semiconductors by (La, Dy and N) co-doping. We have focused on SrTiO3 because it is a semiconductor with a high Seebeck coefficient compared to that of metals. It is expected that SrTiO3 can provide a high power factor, because the capability of converting heat into electricity is proportional to the Seebeck coefficient squared. This research aims to improve the thermoelectric performance of SrTiO3 by replacing host atoms by La, Dy and N atoms based on a theoretical approach performed with the Vienna Ab Initio Simulation Package (VASP) code. Here, undoped SrTiO3 , Sr0.875La0.125TiO3 , Sr0.875Dy0.125TiO3 , SrTiO2.958N0.042, Sr0.750La0.125Dy0.125TiO3 and Sr0.875La0.125TiO2.958N0.042 are studied to investigate the influence of La, Dy and N doping on the thermoelectric properties of the SrTiO3 semiconductor. The undoped and La-, Dy- and N-doped STO structures are optimized. Next, the density of states (DOS), band structures, Seebeck coefficient, electrical conductivity per relaxation time, thermal conductivity per relaxation time and figure of merit (ZT) of all the doped systems are studied. From first-principles calculations, STO exhibits a high Seebeck coefficient and high figure of merit. However, metal and nonmetal doping, i.e., (La, N) co-doping, can generate a figure of merit higher than that of undoped STO. Interestingly, La, Dy and N doping can significantly shift the Fermi level and change the DOS of SrTiO3 around the Fermi level, leading to very different thermoelectric properties than those of undoped SrTiO3 . All doped systems considered here show greater electrical conductivity per relaxation time than undoped STO. In particular, (La, N) co-doped STO exhibits the highest ZT of 0.79 at 300 K, and still a high value of 0.77 at 1000 K, as well as high electrical conductivity per relaxation time. This renders it a viable candidate for high-temperature applications

Functionalized carbon nitride (g-CN) monolayer as a promising energy storage material: a density functional theory study

Two-dimensional graphitic carbon nitride (g-CN) sheet, functionalized with polylithiated molecules (CLi, OLi), has been investigated to study their structural, electronic and hydrogen (H) storage properties by van der Waals corrected first principles calculation. A strong binding of both CLi/OLi with two-sided coverage and large enough molecular distance ensures their uniform dispersion over the g-CN monolayer without forming clusters. Each Li in g-CN@2CLi (g-CN@2OLi) adsorbs 3H, due to its cationic nature through transferring a portion of its charge, resulting into a high H storage capacity of 10.34% (9.76%). The calculated H adsorption energies are well suited for practical applications

Functionalized carbon nitride (g-CN) monolayer as a promising energy storage material: A density functional theory study

Two-dimensional graphitic carbon nitride (g-CN) sheet, functionalized with polylithiated molecules (CLi2, OLi2), has been investigated to study their structural, electronic and hydrogen (H2) storage properties by van der Waals corrected first principles calculation. A strong binding of both CLi2/OLi2 with two-sided coverage and large enough molecular distance ensures their uniform dispersion over the g-CN monolayer without forming clusters. Each Li in g-CN@2CLi2 (g-CN@2OLi2) adsorbs 3H2, due to its cationic nature through transferring a portion of its charge, resulting into a high H2 storage capacity of 10.34% (9.76%). The calculated H2 adsorption energies are well suited for practical applications

Drastic Improvement in Gas-Sensing Characteristics of Phosphorene Nanosheets under Vacancy Defects and Elemental Functionalization

Efficient chemical gas detection is of great importance for various functionalities (such as leakage detection of hazardous and explosive gases in industrial safety systems). The recent discovery of 2D black phosphorene (BlackP) has created intensive interests toward nanosensors because of its maximized surface-to-volume ratio and exceptional carrier mobility that potentially deliver the superior performance than the conventional transition-metal oxides sensors. In this work, we have performed first-principles DFT calculations coupled with the statistical analysis to unravel the structural, electronic, and gas-sensing characteristics of pristine, defected, and metal-substituted BlackP toward toxic HS and SO gas molecules. Our findings have revealed that pristine BlackP weakly interacts with both HS and SO by van der Waals (vdW) forces characterized by the small binding energies. The analysis of electronic properties via the density of states (DOS) indicates that there is a negligible change in DOS after gas exposure, which confirms insensitive sensing. To intensify the binding energies, we have considered defects (mono-, di-, tri-, and quad-vacancy) and substitutional impurities (Ti, Si, Mn, and Fe) as the incentives. The presence of mono- and divacancies remains less energetically sensitive to both gas species because of the low adsorption energies. Meanwhile, tri- and quad-vacancies induce the dissociative adsorption, not suitable for the reversible adsorption-desorption cycles. Substitutional doping by Fe atoms is found to be a feasible approach to enhance the sensing resolution of SO detection because of the remarkable adsorption energy incorporated with the substantial variation in DOS after gas exposure. This modification in electronic properties is facilitated by the charge transfer mechanism from Fe 3d to P 3p which can generate the measurable electrical signal detected by the external circuit of the sensor

Graphitic carbon nitride nano sheets functionalized with selected transition metal dopants: an efficient way to store CO2

Proficient capture of carbon dioxide (CO2) is considered to be a backbone for environment protection through countering the climate change caused by mounting carbon content. Here we present a comprehensive mechanism to design novel functional nanostructures capable of capturing a large amount of CO2 efficiently. By means of van der Waals corrected density functional theory calculations, we have studied the structural, electronic and CO2 storage properties of carbon nitride (g-C6N8) nano sheets functionalized with a range of transition metal (TM) dopants ranging from Sc to Zn. The considered TMs bind strongly to the nano sheets with binding energies exceeding their respective cohesive energies, thus abolishing the possibility of metal cluster formation. Uniformly dispersed TMs change the electronic properties of semiconducting g-C6N8 through the transfer of valence charges from the former to the latter. This leaves all the TM dopants with significant positive charges, which are beneficial for CO2 adsorption. We have found that each TM's dopants anchor a maximum of four CO2 molecules with suitable adsorption energies (-0.15 to -1.0 eV) for ambient condition applications. Thus g-C6N8 nano sheets functionalized with selected TMs could serve as an ideal sorbent for CO2 capture

Drastic Improvement in Gas-Sensing Characteristics of Phosphorene Nanosheets under Vacancy Defects and Elemental Functionalization

Efficient chemical gas detection is of great importance for various functionalities (such as leakage detection of hazardous and explosive gases in industrial safety systems). The recent discovery of 2D black phosphorene (BlackP) has created intensive interests toward nanosensors because of its maximized surface-to-volume ratio and exceptional carrier mobility that potentially deliver the superior performance than the conventional transition-metal oxides sensors. In this work, we have performed first-principles DFT calculations coupled with the statistical analysis to unravel the structural, electronic, and gas-sensing characteristics of pristine, defected, and metal-substituted BlackP toward toxic H2S and SO2 gas molecules. Our findings have revealed that pristine BlackP weakly interacts with both H2S and SO2 by van der Waals (vdW) forces characterized by the small binding energies. The analysis of electronic properties via the density of states (DOS) indicates that there is a negligible change in DOS after gas exposure, which confirms insensitive sensing. To intensify the binding energies, we have considered defects (mono-, di-, tri-, and quad-vacancy) and substitutional impurities (Ti, Si, Mn, and Fe) as the incentives. The presence of mono- and divacancies remains less energetically sensitive to both gas species because of the low adsorption energies. Meanwhile, tri- and quad-vacancies induce the dissociative adsorption, not suitable for the reversible adsorption-desorption cycles. Substitutional doping by Fe atoms is found to be a feasible approach to enhance the sensing resolution of SO2 detection because of the remarkable adsorption energy incorporated with the substantial variation in DOS after gas exposure. This modification in electronic properties is facilitated by the charge transfer mechanism from Fe 3d to P 3p which can generate the measurable electrical signal detected by the external circuit of the sensor

Graphitic carbon nitride nano sheets functionalized with selected transition metal dopants: an efficient way to store CO2

Proficient capture of carbon dioxide (CO2) is considered to be a backbone for environment protection through countering the climate change caused by mounting carbon content. Here we present a comprehensive mechanism to design novel functional nanostructures capable of capturing a large amount of CO2 efficiently. By means of van der Waals corrected density functional theory calculations, we have studied the structural, electronic and CO2 storage properties of carbon nitride (g-C6N8) nano sheets functionalized with a range of transition metal (TM) dopants ranging from Sc to Zn. The considered TMs bind strongly to the nano sheets with binding energies exceeding their respective cohesive energies, thus abolishing the possibility of metal cluster formation. Uniformly dispersed TMs change the electronic properties of semiconducting g-C6N8 through the transfer of valence charges from the former to the latter. This leaves all the TM dopants with significant positive charges, which are beneficial for CO2 adsorption. We have found that each TM’s dopants anchor a maximum of four CO2 molecules with suitable adsorption energies (-0.15 to -1.0 eV) for ambient condition applications. Thus g-C6N8 nano sheets functionalized with selected TMs could serve as an ideal sorbent for CO2 capture

Efficient suppression of the shuttle effect in Na-S batteries with an As2S3 anchoring monolayer

Sodium-sulfur batteries (NaSBs) have emerged as a promising energy storage technology for large-scale stationary applications such as smart electrical grids due to their exceptionally high energy density and cost-effectiveness. However, one of the challenging problems impeding their practical applications is the sulfur shuttle effect by which the active redox intermediates are gradually dissolved in electrolytes. In this work, we have employed first-principles density functional theory (DFT) calculations to unravel the suppression of the shuttle effect in NaSBs with a two-dimensional (2D) As2S3 monolayer as the anchoring material. We show that semiconducting As2S3 is a suitable anchoring layer to inhibit the dissolution of the polysulfide intermediates in common electrolytes because of its stronger chemical binding with sodium polysulfides than with the electrolytes. The immense adsorption is attributed to the electron donation from the unfilled S-3p states of the polysulfides to As2S3. These mechanisms increase the carrier population and consequently improve the electrical conductivity of As2S3. Hence, the use of As2S3 can both reduce the shuttle effect and enhance the cathode electron conductivity to enable improved cycling stability and coulombic efficiency of the battery