Department of Physics
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Browsing Department of Physics by Author "Maphanga, R. R."
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Item Open Access Computational study of low index surface of an anatase TiO2 doped with ruthenium (Ru) and strontium (sr) for application in Dye sensitized solar cells(2019-05-18) Nemudzivhadi, Hulisani; Maluta, N. E.; Maphanga, R. R.Titanium dioxide (TiO2) is considered to be an ideal semiconductor for photocatalysis because of its high stability, low cost and safety towards both humans and the environment. Doping TiO2 with different elements has attracted much attention as the most important way of enhancing the visible light absorption, in order to improve the efficiency of the dye sensitized solar cells (DSSCs). In this study, first principle density functional theory was used to investigate electronic and optical properties of bulk anatase TiO2, undoped, and ruthenium (Ru) and strontium (Sr) doped anatase TiO2 (1 0 0) surface. Two different doping approaches i.e., substitutional and adsorption mechanisms were considered in this study. The results showed that absorption band edges of Ru and Sr-doped anatase TiO2 (1 0 0) surface shift to the long wavelength region compared to the bulk anatase TiO2 and undoped anatase TiO2 (1 0 0) surface. Also, the results revealed that the band gap values and the carrier mobility in the valence band, conduction band and impurity energy levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped anatase TiO2 (1 0 0) surface. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Ru and Sr-doped anatase TiO2 (1 0 0) surface. Thus, we conclude that the visible light response of TiO2 can be modulated by doping with both Ru and Sr. However, Sr-doped system shows higher photocatalytic activity than the Ru-doped system. The study has successfully probed the interesting optical response mechanism of TiO2 (1 0 0) surface.Item Open Access Density functional theory study of adsorption of cronconate dyes on TiO2 Anatase (010) and (100) surfaces(2019-05-18) Ranwaha, Tshifhiwa Steven; Maluta, N. E.; Maphanga, R. R.Currently the dye sensitized solar cells have attracted more attention due to their low cost, transparency and flexibility. These types of solar cells use the dye molecule adsorbed on TiO2 semiconductor in Nano architecture with the role of absorbing photons, in recent research attempts are being made to shifts the absorption spectral of TiO2 to visible and near infrared–region of solar spectrum to achieve maximum photo absorption which yields to an increase in the efficiency of the dye sensitized solar cells. In the current study, density functional theory (DFT) was used to model two croconate dyes (CR1 and CR2), one with an electron donating methyl group (CR1) and the other with an electron –withdrawing caboxyl group (CR2). The geometric, electronic and optical properties of these dyes were compared. The adsorption behaviour of the two dyes on (010 and 100) anatase TiO2 surfaces were investigated in this study by employing first principle calculation based on DFT using a plane-wave pseudo potential method. The generalized gradient approximation (GGA) was used in the scheme of Perdew-Burke Ernzerhof to describe the exchange -correlation function as implemented in the CASTEP package in Material Studio of BIOVIA. The adsorption results shows a spontaneous electron injection followed by efficient regeneration of the oxidized dye molecules by the electrolyte and strong binding ability of CR2 to the TiO2 surface, but also shows a comparable binding strength of CR1. The results of this study will help in the design of high efficient dye for DSSCs.Item Open Access Density functional theory study of copper zinc tin (Cu2ZnSnS4) with Calcium and Barium(2020-08-24) Mlotshwa, Thokozane Mxolisi; Maluta, N. E.; Maphanga, R. R.; Kirui, J. KThe sun is the most important source of renewable energy today. Producing energy from sunlight using cheap, abundant and non-toxic materials is considered a major challenge in the field of solar-electrical energy conversion. Fossil fuel combustion, depletion of non-renewable sources, global warming and environmental degradation are some of the push factors towards clean, non-toxic and environmentally friendly methods of producing electrical energy. To harvest solar energy, a thin film solar cell composed of the Cu2ZnSnS4 (CZTS) semiconductor is a candidate, which can harvest useful amounts of energy. Some of its advantages are the optical direct band gap and high absorption coefficients. In this study, CZTS is investigated as a material for solar cells using first principle method. Thus, structural, electronic and optical properties of pure CZTS and doped CZTS (112) surface were investigated using the density functional theory as implemented in the Cambridge Serial Total Energy Package code. Alkali earth metals, Calcium (Ca) and Barium (Ba) were adsorbed on the CZTS (112) surface using the adsorption locator module. The results suggest that doping with barium rather than calcium could improve the photocatalytic activity on the CZTS based solar cells. Doping using different elements yielded improved optical and electronic properties of the CZTS based solar cells.Item Open Access Density functional theory study of TiO2 Brookite (100), (110) and (210) surfaces doped with ruthenium (RU) and platinum (Pt) for application in dye sensitized solar cell(2018-05-18) Dima, Ratshilumela Steve; Maluta, E.N.; Maphanga, R. R.Since the discovery of water photolysis on a TiO2 electrode by Fujishima and Honda in 1972, TiO2 has attracted extensive attention as an ideal photocatalytic material because of its excellent properties such as high activity, good stability, nontoxicity and low cost. Hence, it has been widely used in the fields of renewable energy and ecological environmental protection. However, as a wide band gap oxide semiconductor (Eg = 3.14 eV), brookite TiO2 can only show photocatalytic activity under UV light irradiation (λ < 387.5 nm) that accounts for only a small portion of solar energy (approximately 5 %), in contrast to visible light for a major part of solar energy (approximately 45 %). Therefore, effectively utilizing sunlight is the most challenging subject for the extensive application of TiO2 as a photocatalyst. Due to the unique d electronic configuration and spectral characteristics of transition metals, transition metal doping is one of the most effective approaches to extend the absorption edge of TiO2 to the visible light region. This method of doping either inserts a new band into the original band gap or modifies either the conduction band or valence band, improving the photocatalytic activity of TiO2 to some degree. In this work, the structural, electronic and optical properties of doped and undoped TiO2 (100), (110) and (210) surfaces were performed using first principle calculations based on DFT using a plane-wave pseudopotential method. The generalized gradient approximation was used in the scheme of Perdew-Burke-Ernzerhof to describe the exchangecorrelation functional as implemented in the Cambridge Sequential Total Energy Package code in the Materials Studio of BIOVIA. The metal dopants shift the absorption to longer wavelengths and improves optical absorbance in visible and near- IR region. The un-doped (210) surface showed some activity in the visible and near IR region.Item Open Access First-principles study of Hematite (α-Fe2O3) surface structures doped with Copper (Cu), Titanium (Ti), nickel (Ni) and manganese (Mn)(2023-05-19) Mabaso, Clarence Vusi; Maluta, N. E.; Maphanga, R. R.Hematite has attracted research interest for many years due to its application in water splitting. Despite its attractive characters such as a reasonable optical band gap, the semiconductor is still faced with great uncertainty for the accomplishment of hematite based photoelectrochemical cells for water splitting. Doping with transition metals has shown to be a practical solution to overcome some of the limitations faced with hematite by modifying the energy band to improve its photo-electrochemical (PEC) activity. This study explored two surface structures of pure and transition metals (Ti, Cu, Ni and Mn) doped- α-Fe2O3 oriented in the directions (001) and (101). Calculations via the first principle using the density functional theory (DFT) were adopted, the results show that the doping of transition metals in α-Fe2O3 has an effect in modifying both the valence and conduction band edges. Specifically, doping Ti introduces more electrons in the conduction band and fills the unoccupied 3d states, which could improve the rate of charge transportation and likely enhance the electrical conductivity of α-Fe2O3. Doping with Mn, Ni, and Cu has effectively improved the absorption coefficient for α-Fe2O3 (001) and (101) surfaces, in the visible light region. The overall analysis of the results shows an opportunity for a successful photo-electrochemical water splitting application.Item Open Access Studies of interaction of dye molecules with TiO2 Brookite clusters for application in dye sensitized solar cells(2019-09-20) Elegbeleye, Ife Fortunate; Maluta, E. N.; Maphanga, R. R.Dye sensitized solar cells (DSSCs) have attracted rapid interest over the recent years with prospect of emerging as a viable alternative to conventional silicon based solar cells. The photoanode of DSSCs comprises of dye molecules anchored to the surface of semiconductors such as TiO2. However, the major drawback of Titanium dioxide (TiO2) is its wide band gap (3.0 eV to 3.2 eV) which limits its photocatalytic activities to the ultraviolet region of the electromagnetic spectrum. Understanding the interaction of dye molecules with the surfaces of TiO2 is crucial for optimizing light-harvesting, photoconversion function and photocurrent densities in DSSCs. The three polymorphs of TiO2 are anatase, brookite and rutile. The optical properties of brookite semiconductor have not been much studied although brookite has been reported to have good photocatalytic properties. In this work, Density functional theory (DFT) computational approach was used through various computational softwares which are CASTEP, GAUSSIAN, GAUSSUM, GPAW, ASE, and AVOGADRO with B3LYP, LANL2DZ, PBE, and GGA functional to explore the photocatalytic properties of the typical ruthenium N3 complex, polyenediphenyl-aniline dye moiety, croconate dye molecules and three modelled surfaces of brookite which are (TiO2)5, (TiO2)8 and (TiO2) 68 for application in DSSCs. We also studied the absorption of the corresponding dye molecules on the three surfaces of brookite TiO2. Our findings showed strong binding ability, good electronic coupling, efficient charge separation, spontaneous electron injection and good spectral properties upon adsorption of the dye molecules to brookite TiO2 semiconductor clusters. Our findings on the optical absorption spectra of ruthenium N3 dye, croconate dye and polyenediphenyl-aniline dye molecule absorbed on (TiO2)5 and (TiO2)8 brookite cluster shows bathocromatic shift of the absorption maxima to higher wavelength and improve optical response of TiO2 brookite cluster. A red spectra shift and absorption over a wide range of the solar spectrum in the visible and near infra-red region of the solar spectrum was achieved upon absorption of the ruthenium N3 complex and polyenediphenyl-aniline dye molecules on (TiO2)5 and (TiO2)8 brookite cluster. The results generally suggest that the absorption of dye molecules on TiO2 brookite cluster improves its spectra responsivity in the UV region and makes it possible to absorb over the whole spectrum range, that is, the UV, visible and near infra – red region of the solar spectrum. Our findings also showed good electron injection kinetics from the dye to TiO2 brookite clusters, which suggests higher photocurrents density and open circuit voltage in DSSCs.Item Embargo Studies on structural, electronic and optical properties of SnO2 doped with nitrogen, chloeine, antimony and indium(2024-09-06) Nekhwevha, Nditsheni; Maluta, N. E.; Maphanga, R. R.SnO2 has recently attracted a great deal of interest due to its many technological applications, including in solar cells as it possesses advantageous optical and electrical characteristics, outstanding chemical stability, and thermal stability. However, the photocatalytic activity and charge carrier mobility are constrained by the large band gap. A cost-effective and efficient method for reducing the SnO2 band gap and increasing the potential for photocatalytic applications is doping with different elements. Examining how mono-doping and co-doping impact the electronic, structural, electrical, and optical characteristics of the SnO2 supercell structure, the current theoretical study used Density Functional Theory (DFT) calculations of different metal and nonmetals (N, Cl, In, and Sb) and (N-Cl and In-Sb) as dopants and co-dopants, respectively. The results show that due to the band gap narrowing and the existence of impurity levels in the band gap, all mono-doped and co-doped SnO2 exhibit some small redshift. The results of the trials and the calculated optical characteristics, such as the dielectric function, reflectivity, absorption coefficient, and energy-loss spectrum, are in good agreement. According to the predicted absorption coefficient, doped SnO2 has a noticeable band of absorption. Doped SnO2 exhibits superior absorption in the visible area of the electromagnetic spectrum than undoped, In-doped, Sb-doped, and In-Sb co-doped SnO2.Item Embargo The adsorption of bidens pilosa dye molecules onto TiO2 nanoparicle surfaces for optimization of light harvesting efficiency in dye sensitized solor cell: an experimental and theory study(2024-09-06) Randela, Ronel Ronella; Maluta, N. E.; Mathomu, L. M.; Maphanga, R. R.The availability and high demand for electrical energy is a key global concern, as a result, Dye sensitized solar cells (DSSCs) have attracted a lot of attention in recent years due to their ease of preparation, low toxicity, and environmental friendliness. The current study describes the green synthesis of TiO2 nanoparticles as well as their characterization using ultraviolet-visible, Fourier transformed infrared spectroscopy and X-ray diffraction. Furthermore, the study used Density Functional Theory to describe the optical characteristics of produced nanoparticles. The UV-Vis results showed that the dye extracted using solvents such as water, methanol, and ethanol had a common absorbance at 665 nm among the solvents used ethanol had the highest absorption. The molecules responsible for a broader range of absorbance are known to be pheophytin and porphyrin, which are found in chlorophyll extracted from the B. pilosa plant. FTIR analysis of the prepared TiO2 revealed the absorbed functional groups of the synthesized B. pilosa extracts and confirmed the formation of TiO2 NPs with a vibrational band at 497 cm 1. The TiO2 NPs were heterogeneous in shape under TEM and SEM but spherical under SEM, indicating the formation of paste during agglomeration. XRD analysis confirmed that the polymorph formed is anatase with the highest peak of (101) surface, which was used to computationally adsorb the dye molecule. Pheophytin and porphyrin characteristics were optimized using DFT. For both experimentally and computationally, the UV-vis absorbance was found to be between 420 nm and 665 nm with a higher light harvesting efficiency. pheophytin and porphyrin exhibited energy gaps of 2.1 eV and 2.8 eV respectively. This study demonstrates that the dye molecule synthesized from B. pilosa is an efficient sensitizer for DSSCs. The adsorption results substantiate the spontaneous electron injection and subsequent efficient regeneration of oxidized dye molecules and the strong binding ability of porphyrin dye molecules to the TiO2 surface. The results of this study will be useful for the development of highly efficient organic dyes for DSSCs.