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  • ItemEmbargo
    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.
  • ItemEmbargo
    Investigation of covariability between energy fluxes and CO2 exchange over a semi-arid savanna (Kruger National Park) by Eddie Covariance Technique
    (2024-09-06) Takalani, Lufuno; Mulaudzi, T. S.; Maluta, N. E.; Mateyisi, M.; Thenga, H.
    South Africa faces climate change, natural disasters, and rising temperatures due to increased levels of carbon dioxide in the atmosphere, primarily caused by deforestation, burning fossil fuels, and releasing carbon dioxide into the atmosphere without additional carbon sinks. The gap in understanding lies in studying the connection between energy flows and Net Ecosystem Exchange (NEE) at the semi-arid savanna of Kruger National Park to gain a more detailed and accurate understanding of these processes, especially in semi-arid savannas that are susceptible to changes in environmental factors. By studying energy fluxes and NEE at Kruger National Park using the eddy covariance technique, the dissertation seeks to deepen our understanding of the mechanisms driving carbon exchange in semi-arid savannas and provide insights into the impact of environmental factors on ecosystem processes. The eddy covariance technique is a powerful tool that directly measures energy and carbon dioxide exchange between the land surface and the atmosphere. The study shows that the correlation between NEE and latent heat flux (LE) and net radiation (Rn) is generally the strongest, while ground heat flux (G) and sensible heat flux (H) have little impact on NEE. The dataset provides insight into the biometeorological and flow dynamics of the Skukuza ecosystem and how it responds to climate change. The study emphasizes the importance of considering seasonality, climatic variability, and precipitation when studying the surface energy balance and its components. The findings have implications for understanding the complex interactions between ecosystem processes and environmental factors.
  • ItemOpen Access
    Comparison and evaluation of empirical and machine learning models in estimating global solar radiation in Limpopo province
    (2023-10-05) Murida, Thalukanyo Witney; Mulaudzi, T. S.; Maluta, N. E.; Mphephu, N.
    This study investigated the performance of machine learning techniques as compared to the empirical models to forecast the global solar radiation in Limpopo regions. The machine learning techniques used in this study are Support Vector Machines, Random Forest, and Artificial Neural Network, and the empirical models used are the Clemence and Hargreaves- Samani models. To assess the efficiences of the machine learning models against the empirical models, the researchers calculated and compared the models performance evaluation using statistical equations such as Coefficient of determination, Mean Square Error, Mean Absolute Error, and Root Mean Square Error. Calibaration was done to improve performance of the empirical models. The present study found that machine learning techniques perform better than the empirical models when estimating the global solar radiation in the selected Limpopo regions.
  • ItemOpen Access
    Forecasting Minute Averaged Solar Irradiance Using Machine Learning for Solar Collector Applications
    (2023-05-19) Nemalili, Ronewa Collen; Jhamba, I.; Kirui, J. K.; Sigauke, C.
    Challenges in utilising fossil fuels for generating energy call for the use of renewable energy. This study focuses on modelling and forecasting solar energy and optimum tilt angle of solar energy acceptance using historical time series data collected from one of the South African radiometric stations, USAid Venda station in Limpopo province. In the study we carried out a comparative analysis of Random Forest and Bayesian linear regression in short-term forecasting of global horizontal irradiance (GHI). To compare the predictive accuracy of the models, k-Nearest Neighbors (KNN) and Long short-term memory (LSTM) are used as benchmark models. The top two models with the best performances were then used in hourly forecasting of optimum tilt angles for harvesting solar energy. The performance measures such as MAE, MSE, and RMSE were used and the results showed RF to have better performance in forecasting GHI than other models, followed by the LSTM and the third best model was the KNN whereas the BLR was the least performing model. RF and LSTM were then used in modelling and forecasting the tilt angles of optimal solar energy acceptance and as thus, the LSTM outperformed the RF by a small margin.
  • ItemOpen 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.
  • ItemOpen Access
    Development of a mathematical model for predicting bio-slurry temperature and subsequent gas production rate for underground brick-built biogas digester using ambient air temperature forecast
    (2022-11-10) Nekhubvi, Vhutshilo 1st Mountaineer; Tinarwo, David; von Blottnitz, Harro
    Background: Heat energy is essential for the anaerobic digestion of organic materials such as household, human or agricultural waste. Many developing countries have witnessed efforts to implement anaerobic digestion technology for biogas production as a strategy to enhance energy supply and poverty eradication in rural communities. Underground, brick, and mortar built fixed dome type digesters are the most deployed small-scale biogas technology in sub-Saharan Africa (SSA) countries such as Rwanda, Ethiopia, Tanzania, Kenya, Uganda, Burkino Faso, Cameroon, Benin, Senegal, and South Africa despite their relatively high initial costs. They have a long lifespan and no moving or rusting parts involved. The basic design is compact, saves space, is well insulated, and does not need additional heating, hence suitable for developing countries. The technology is labour-intensive that involves digging the pit and constructing the structure from underground, thus creating local employment. Unlike prefabricated biogas digesters, underground, brick, and mortar-built fixed dome type digesters are more robust than the latter, with minimal gas pipes corrosion experienced. However, little literature on this type of digesters' actual field operation and performance within the SSA context is available. The end-user must know what needs to be done and what the system's outcome is supposed to be. Besides determining parameters like total solids, volatile solids, carbon-nitrogen ratio, hydrolysis rate, organic loading rate, and hydraulic retention time, the temperature inside the digester becomes one of the metrics to evaluate the anaerobic digestion process. The digestion temperature critically affects the biogas yield, considering all other conditions unchanged. Knowing the operational temperature, one can estimate the maximum specific growth rate of the microorganisms and the biogas production rate. Prediction models for the internal operating temperature of these digesters under local conditions typical of Limpopo province of South Africa, where most of these digesters have been installed, are still lacking. To ordinary users in rural areas, the prediction of the possible 'duration of use,' for example, the duration of continuous cooking, is essential. However, regardless of fulfilling all other operational requirements to predict daily gas production, internal digester temperature remains the missing link to having a complete set for a quick and easy gas yield estimation. Aim of the study: The overall objective was to develop a locally applicable model for predicting the bio-slurry operating temperature of underground brick-built domestic size biogas digesters. The work established a correlation of ambient air temperature with the slurry temperature inside the digester using a heat transfer mechanism through the media between the fermenting slurry and the ambient air. Methodology: A thermodynamic study of a small-scale fixed-dome Deenbandhu biogas digester model was performed by monitoring the digester's temperature and surroundings. The K-type chromium-nickel temperature sensors with a sensitivity of 41 μV/°C and a response time of 0.8 s in liquids were positioned at the centre of the digester to measure the slurry temperature. Another temperature sensor was placed 2.0 m above the ground to measure ambient air temperature. The sensors were connected to the data logger and programmed to record temperature readings every second, automatically averaged hourly and daily. The soil surface heat flux was computed using Fourier's law of heat conduction to strengthen the model. Results: The average daily bio-slurry temperature of the digesters ranged between psychrophilic and mesophilic ranges. The results show a strong correlation between bio-slurry and ambient air temperature. A strong correlation was obtained between the measured and predicted temperature of the fermenting slurry inside the digester with a ()Pr|t|>value less than 2e-16 ***, showing that the model is most significant. A Q-Q plot was also used to measure the importance of each observation to the regression. Conclusion: The developed models can accurately estimate the bio-slurry temperature inside the digester using local ambient air temperature data. The set equation adds value as input to the research of small-scale household biogas digesters. Furthermore, the biogas production rate was calculated using data on predicted slurry temperature. It was found that the biogas production rate is satisfactory, given the condition of the study area. The biogas production rate varies from as low as 0.18 m3m-3d-1 during the cold month to 0.48 m3m-3d-1 during the warmest month. Temperatures above 20 ℃ were more conducive to a high biogas production rate.
  • ItemOpen Access
    Determination of global solar radiation using temperature-based model for different climate conditions for Limpopo Province of South Africa
    (2022-07-15) Mathebe, Sampie Mphagala; Maluta, N. E.; Mulaudzi, T. S.
    The research mainly focused on the determination of global solar radiation using temperature-based model by Hargreaves and Samani for the Northern regions of Limpopo Province of South Africa. The daily maximum and minimum temperature data measured at the following six (6) stations were used: Ammondale, Mutale, Nwanedi, Roedtan, Sekgosese and Xikundu for the period 2008 – 2010. The values of empirical coefficient Kr for the Inland stations of South Africa were computed and used as an input to the model. The observed and calculated global solar radiation data were compared on the basis of the statistical error tests that is mean bias error (MBE), the mean percentage error (MPE) and the root mean square error (RMSE). Based on the statistical results the model was found suitable to estimate monthly average daily global solar radiation for the regions listed above and elsewhere with similar climatic conditions and areas where the radiation data is missing or unavailable. Hence, the study will also help to advance the state of knowledge of global solar radiation to the point where it has applications in the estimation of monthly average daily global solar radiation across.
  • ItemOpen Access
    Estimation of Global Solar Radiation from SAURAN stations using air temperature-based models Hargreaves and Samani and Clemence models
    (2020) Shabangu, Charlotte Beauty; Maluta, N. E.; Mulaudzi, T. S.
    Knowledge of the amount of solar radiation available in a location is important for solar energy systems, architectural designs, agronomy, and installation of pyranometers. Some developing countries do not have good quality meteorological stations that can directly measure global solar radiation. Thus, several empirical methods were developed to estimate global solar radiation. This study uses two temperature-based models which are Hargreaves - Samani and Clemence models. Four selected stations from the Southern African Universities Radiometric Network (SAURAN) for this study are University of KwaZulu–Natal, Howard college (KZH), University of Stellenbosch (SUN), Nelson Mandela University (NMU) and University of Venda (UNV). A three-year (2014-2016) temperature data for each station were sourced from SAURAN. The performance of the two models was validated using statistical analysis that is, Mean Percentage Error (MPE), Mean Bias Error (MBE), Root Mean Square (RMSE), Coefficient of Determination (R2) and t-statistical value (t). Both models obtained acceptable values of MBE, MPE, RMSE, R2 and t in KZH, NMU and UNV stations. Both models achieved the best values of MBE from 2014 to 2016, ranging from -0.0099 to 0.0147 in KZH station, followed by NMU with MBE values ranging from - 0.0293 to -0.0014, -0.0104 to 0.0330 for SUN station, 0.0241 to 0.0245 for UNV station. The models achieved MPE values between ± 10 % in all the stations. The R2 values for both models are close to 1, while the t-statistic values of one, which is less than critical value, was achieved by the models from all selected stations. This suggests that both models have got capacity to estimate global solar radiation in all the selected areas of study. However, the higher values of MBE and RMSE also revealed high level of overestimation by the models in SUN station. Therefore, this study has found evidence that both Hargreaves - Samani & Clemence models can be best recommended for estimating global solar radiation in KZH, NMU and UNV stations and areas with similar climatic and meteorological conditions.
  • ItemOpen 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. K
    The 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.
  • ItemOpen Access
    Metal oxide nanostructures and hybrid perovskite semiconductor for photovoltaic application
    (2020-02) Olaleru, Solomon Alan; Kirui, J. K.; Jhamba, L.; Wamwangi, D.; Roro, K.
    Perovskite-based solar cells (PSC) is the fastest growing solar technology to date since inception in 2009. This technology has invigorated the photovoltaic (PV) community. While it has taken 15-42 years for traditional PV technologies to achieve maturity, PSC technology has accomplished the same within 10 years. As of late, hybrid perovskite materials have shown incredible possibilities for solar energy conversion and optoelectronics technologies by virtue of its benefits of high conversion efficiency, low-cost preparation and the application of earthabundant materials, which are basic determinants for massive production. The optical properties of lead halide perovskites are of basic significance for almost all applications. Based on the screening from literature, the greater part of reports centred on the fabrication of photovoltaic devices, while the photophysical processes of these materials are missing. In this work, we reported the photophysics of halide perovskites from materials to device. The charge dynamics and charge transport mechanisms were also investigated. At first, we optimized the properties of the perovskite materials before fabrication so as to identify optimal conditions for chemical and material synthesis. These optimal conditions of base perovskite preparations necessitated the use of powder samples to form single phases and also to determine the kinetics and energetics of phase formation. We explored the impact of antisolvent and additive on the photophysical properties and a better understanding of optical response of the perovskite materials. We hold the view that the performance improvement focused on material quality alone without complete understanding of the physics of the carrier-light interaction will not provide adequate solutions to existing problems. We used DMSO as an additive in DMF to regulate the crystal growth by dissolving the residue of PbI2 which can impede the crystallization and ethyl acetate as an anti-solvent to control the morphology of the perovskite film resulting in improved homogeneity. Their impacts on optical properties were examined along with consequent improvement on the light absorption property. Herein, we reported the charge transport mechanism and recombination phenomena in dopantfree HTM perovskite solar cell using I-V and EIS measurement with the focus on physical processes within the perovskite material as an active layer and the parameters that determine the photovoltaic performance. Impedance spectroscopy technique, which reveals the various interfacial processes in terms of resistive and capacitive elements, is used to get an insight into the charge transport through the junction and bulk of the FTO-perovskite solar cell. Again these values were consistent with the results obtained from I-V analysis. These analyses of current– voltage (I–V) characteristics and impedance spectroscopy technique provide essential insights into the performance parameters which determine the transport mechanism and location of electron hole recombination and the efficiency of the device. Perovskite-based solar cells (PSC) is the fastest growing solar technology to date since inception in 2009. This technology has invigorated the photovoltaic (PV) community. While it has taken 15-42 years for traditional PV technologies to achieve maturity, PSC technology has accomplished the same within 10 years. As of late, hybrid perovskite materials have shown incredible possibilities for solar energy conversion and optoelectronics technologies by virtue of its benefits of high conversion efficiency, low-cost preparation and the application of earthabundant materials, which are basic determinants for massive production. The optical properties of lead halide perovskites are of basic significance for almost all applications. Based on the screening from literature, the greater part of reports centred on the fabrication of photovoltaic devices, while the photophysical processes of these materials are missing. In this work, we reported the photophysics of halide perovskites from materials to device. The charge dynamics and charge transport mechanisms were also investigated. At first, we optimized the properties of the perovskite materials before fabrication so as to identify optimal conditions for chemical and material synthesis. These optimal conditions of base perovskite preparations necessitated the use of powder samples to form single phases and also to determine the kinetics and energetics of phase formation. We explored the impact of antisolvent and additive on the photophysical properties and a better understanding of optical response of the perovskite materials. We hold the view that the performance improvement focused on material quality alone without complete understanding of the physics of the carrier-light interaction will not provide adequate solutions to existing problems. We used DMSO as an additive in DMF to regulate the crystal growth by dissolving the residue of PbI2 which can impede the crystallization and ethyl acetate as an anti-solvent to control the morphology of the perovskite film resulting in improved homogeneity. Their impacts on optical properties were examined along with consequent improvement on the light absorption property. Herein, we reported the charge transport mechanism and recombination phenomena in dopantfree HTM perovskite solar cell using I-V and EIS measurement with the focus on physical processes within the perovskite material as an active layer and the parameters that determine the photovoltaic performance. Impedance spectroscopy technique, which reveals the various interfacial processes in terms of resistive and capacitive elements, is used to get an insight into the charge transport through the junction and bulk of the FTO-perovskite solar cell. Again these values were consistent with the results obtained from I-V analysis. These analyses of current– voltage (I–V) characteristics and impedance spectroscopy technique provide essential insights into the performance parameters which determine the transport mechanism and location of electron hole recombination and the efficiency of the device. Perovskite-based solar cells (PSC) is the fastest growing solar technology to date since inception in 2009. This technology has invigorated the photovoltaic (PV) community. While it has taken 15-42 years for traditional PV technologies to achieve maturity, PSC technology has accomplished the same within 10 years. As of late, hybrid perovskite materials have shown incredible possibilities for solar energy conversion and optoelectronics technologies by virtue of its benefits of high conversion efficiency, low-cost preparation and the application of earthabundant materials, which are basic determinants for massive production. The optical properties of lead halide perovskites are of basic significance for almost all applications. Based on the screening from literature, the greater part of reports centred on the fabrication of photovoltaic devices, while the photophysical processes of these materials are missing. In this work, we reported the photophysics of halide perovskites from materials to device. The charge dynamics and charge transport mechanisms were also investigated. At first, we optimized the properties of the perovskite materials before fabrication so as to identify optimal conditions for chemical and material synthesis. These optimal conditions of base perovskite preparations necessitated the use of powder samples to form single phases and also to determine the kinetics and energetics of phase formation. We explored the impact of antisolvent and additive on the photophysical properties and a better understanding of optical response of the perovskite materials. We hold the view that the performance improvement focused on material quality alone without complete understanding of the physics of the carrier-light interaction will not provide adequate solutions to existing problems. We used DMSO as an additive in DMF to regulate the crystal growth by dissolving the residue of PbI2 which can impede the crystallization and ethyl acetate as an anti-solvent to control the morphology of the perovskite film resulting in improved homogeneity. Their impacts on optical properties were examined along with consequent improvement on the light absorption property. Herein, we reported the charge transport mechanism and recombination phenomena in dopantfree HTM perovskite solar cell using I-V and EIS measurement with the focus on physical processes within the perovskite material as an active layer and the parameters that determine the photovoltaic performance. Impedance spectroscopy technique, which reveals the various interfacial processes in terms of resistive and capacitive elements, is used to get an insight into the charge transport through the junction and bulk of the FTO-perovskite solar cell. Again these values were consistent with the results obtained from I-V analysis. These analyses of current– voltage (I–V) characteristics and impedance spectroscopy technique provide essential insights into the performance parameters which determine the transport mechanism and location of electron hole recombination and the efficiency of the device. Perovskite-based solar cells (PSC) is the fastest growing solar technology to date since inception in 2009. This technology has invigorated the photovoltaic (PV) community. While it has taken 15-42 years for traditional PV technologies to achieve maturity, PSC technology has accomplished the same within 10 years. As of late, hybrid perovskite materials have shown incredible possibilities for solar energy conversion and optoelectronics technologies by virtue of its benefits of high conversion efficiency, low-cost preparation and the application of earthabundant materials, which are basic determinants for massive production. The optical properties of lead halide perovskites are of basic significance for almost all applications. Based on the screening from literature, the greater part of reports centred on the fabrication of photovoltaic devices, while the photophysical processes of these materials are missing. In this work, we reported the photophysics of halide perovskites from materials to device. The charge dynamics and charge transport mechanisms were also investigated. At first, we optimized the properties of the perovskite materials before fabrication so as to identify optimal conditions for chemical and material synthesis. These optimal conditions of base perovskite preparations necessitated the use of powder samples to form single phases and also to determine the kinetics and energetics of phase formation. We explored the impact of antisolvent and additive on the photophysical properties and a better understanding of optical response of the perovskite materials. We hold the view that the performance improvement focused on material quality alone without complete understanding of the physics of the carrier-light interaction will not provide adequate solutions to existing problems. We used DMSO as an additive in DMF to regulate the crystal growth by dissolving the residue of PbI2 which can impede the crystallization and ethyl acetate as an anti-solvent to control the morphology of the perovskite film resulting in improved homogeneity. Their impacts on optical properties were examined along with consequent improvement on the light absorption property. Herein, we reported the charge transport mechanism and recombination phenomena in dopantfree HTM perovskite solar cell using I-V and EIS measurement with the focus on physical processes within the perovskite material as an active layer and the parameters that determine the photovoltaic performance. Impedance spectroscopy technique, which reveals the various interfacial processes in terms of resistive and capacitive elements, is used to get an insight into the charge transport through the junction and bulk of the FTO-perovskite solar cell. Again these values were consistent with the results obtained from I-V analysis. These analyses of current– voltage (I–V) characteristics and impedance spectroscopy technique provide essential insights into the performance parameters which determine the transport mechanism and location of electron hole recombination and the efficiency of the device.
  • ItemOpen 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.
  • ItemOpen Access
    Evaluation of the regression coefficients for South Africa from solar radiation data
    (2019-09-20) Mulaudzi, Tshimangadzo Sophie; Maluta, N. E.; Kirui, J. K.
    The knowledge of solar radiation in this dispensation is crucial. The lack of grid lines in the remote rural areas of South Africa necessitates the use of solar energy as an alternative energy resource. Solar radiation data is one of the primary factors considered for the installation of renewable energy devices and they are very useful for solar technology designers and engineers. In some developing countries, estimation of solar radiation becomes a challenge due to the lack of weather data. This scenario is also applicable to South Africa (SA) wherein there are limited weather stations and hence there is a dire need of estimating the global solar radiation data for all climatic regions. Using a five year global solar radiation (𝐻) and bright sunshine (𝑆) data from the Agricultural Research Council (ARC) and South African Weather Service (SAWS) in SA, linear Angstrom – Prescott solar empirical model was used to determine regression coefficients. MATLAB interface was used whereby the linear regression plots were drawn. Annual empirical coefficients of 22 stations were determined and later the provincial values. The range of the regression coefficients, a and b were 0.216 – 0.301 and 0.381 – 0.512 respectively. The 2006 estimated global solar radiation per station in a province calculated from the modified models were compared with the observed and statistically tested. The root mean square errors were less than 0.600 MJm−2day−1 while the correlation relation ranged from 0.782 – 0.986 MJm−2day−1. The results showed the regression coefficients performed well in terms of prediction accuracy.
  • ItemOpen 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.
  • ItemOpen 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.
  • ItemOpen Access
    Low Field Microwave Absorption in Nano-Magnetic Participle - incorporated YBa2Cu3O7-z Superconducting Materials
    (2018-09-21) Nemangwele, Fhulufhelo; Vallabhapurapu, Vijaya Srinvasu; Maluta, Nnditsheni Eric; Kirui, Joseph Kiprono
    Understanding how and why superconductivity (SC) occurs in a given material has been very challenging for physicists for more than a hundred years, notwithstanding the major milestones, such as the London theory, the Landau-Ginzburg theory, and the BCS theory. The extreme challenge to predict the occurrence of SC is symbolized by the long string of unanticipated but breathtaking advances, i.e., the unexpected discoveries of cuprates and Fe-pnictides being the dramatic modern examples. Because of their incompatibility, the nucleation of SC near a ferromagnet is di cult and has never been realized except for the case that another superconductor provides proximity-boosted Cooper pairs. This perceived necessity to start with another superconductor is engrained in the exten- sive study of the proximity e ect in superconductor/ferromagnet (S/F) powder sample, where all the structures involve a superconductor with either stable or metastable struc- ture. Compounding the di culty, it is also generally recognized that SC with substantial Tc is favourable in low dimensionality because of strong quantum uctuation. In this thesis, we report a serendipitous nding of SC that emerges under the most implausible circumstances in low eld microwave absorption measurement. This new revelation may lead to unconventional avenues to explore novel SC for applications in superconducting spintronics. By means of a varienty of techniques, including EPR, SEM, FTIR, PPMS/VSM and XRD, nanonickel incorporated YBCO in di erent weighting factors have been studied. With its complex chemical structure and magnetic properties, Ni-YBCO is far from well understood and the magentic behavior of the system under di erent conditions is investi- gated. From the dilute mixture of nanonickel particles, it is found that groups of normal Josephson junctions (JJs) and JJs due to YBCO-nickel-YBCO interparticle weaklinks form as nickel is ferromagnetic. We experimentally show, for the rst time multiple phase reversals in the non-resonant microwave absorption (NRMA) spectra from Ni-YBCO pos- sibly, due to the formation of JJs. We also show that these multiple phase reversals then vii depend on microwave power and temperature. We argue that microwave power induced coherence among some groups of JJs and breaking of some of the weaker JJs can then lead to the disappearance of multiple phase reversals at higher microwave power levels. Further, we also report a role of pair breaking e ects that shall give a linear eld de- pendence of the derivative microwave absorption signal, which is essentially the NRMA signal. This pair-breaking e ect dominates at temperatures closer to Tc as expected thermodynamically. The presence of two peaks in the system, results in high permeability ferromagnet which acts as a magnetic short circuit for magnetic ux density and creates low reluctance path. A transition from normal to anomalous does not occur in this work, because of the possibility of junction in the sample. As predictable at the region around the origin where the weaklinks are supposed to be very strong for a very low doping or low nanonickel addition ( 0.5 % wt), not much e ect was observed. However, when the nanonickel addition is increased to 2 % and 3% we see a signi cant change in the magnetization and the associated hysteresis, indicating ux pinning.
  • ItemOpen 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.
  • ItemOpen Access
  • ItemOpen Access
    Analysis of the solar radiation data and the determination of regression coeffients for Vhembe Region, Limpopo Province
    (2012-12-11) Mulaudzi, Tshimangadzo Sophie; Sankaran, V.; Lysko, M. D.