Browsing by Author "Tsere, Melkizedeck"

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Betalain-Based Dyes As Photosensitizers For Energy Harvesting Application: A Theoretical Investigation
(NM-AIST, 2025-07) Tsere, Melkizedeck
Betalain dyes are natural pigments that have attracted growing research interest as promising materials for light-harvesting applications. In the present work, the modification of betalain dyes was performed through a computational approach, aimed at screening the wide spread of blended triphenylamine-betalain dyes for dye sensitized solar cell applications. In the first modification approach, betalamic acid was joined to the phenyl ring and further grafted with electron donor groups to enhance light absorption and charge transfer ability. The second approach involved the introduction of new anchoring groups featuring carboxylic acid CH=C(X)COOH for the A1-X dyes and nitro group -CH=C(X)NO2 for the A2-X dyes, where X = CN, CH3, CCl3, CF3. In addition to molecular grafting, the solvent effects on the optoelectronic parameters and dye adsorption to the semiconductor surface were evaluated. The geometrical structures, optoelectronic properties of designed dyes and binding to the semiconductor have been explored. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods coupled with B3LYP and CAM-B3LYP at 6- 31+G(d,p) basis set were used for ground and excited state calculations, respectively. In the UV-Vis spectra, the maximum absorption wavelengths range between 400 ‒ 442 nm based on grafting with an electron donor. However, further modifications by introducing new anchoring units resulted in huge redshifts in wavelengths: 400 ‒ 700 nm for CH=C(X)COOH and between 500 and 1000 nm for the -CH=C(X)NO2. The attachment of the dyes to the semiconductor was modelled via binding to (TiO2)6H3 cluster; A2-X dyes exhibited more stable Dye@TiO2 complexes with binding energies (BEs) ranging between ‒4.08 and ‒2.88 eV compared to A1- X dyes with BEs from ‒1.11 to ‒0.05 eV in water solution. The results evince that the blended triphenyl-betalain dyes with the CH=C(X)NO2 anchoring groups could be promising materials for light harvesting applications. Among the CH=C(X)NO2 dyes, the CH=C(CN)NO2 demonstrated superior optoelectronic properties and binding stability, thus, would be expected the most favourable candidate for light-harvesting applications in dye sensitized solar cells.
Effect of Electron Donor Groups on Optoelectronic Properties of Betalain Dyes: A DFT Study
(wiley, 2025-05) Tsere, Melkizedeck; Costa, Rene; Deogratias, Geradius; Pogrebnaya, Tatiana; Pogrebnoi, Alexander; Machunda, Revocatus; Al-Qurashi, Ohoud; Wazzan, Nuha; Babu, Nambury
A sensitizer is a vital component of dye-sensitized solar cells(DSSCs); it absorbs incident photons, excites electrons, and facil-itates charge transfer to the semiconductor. In the present work,modification of betalain dyes through grafting of electron donorgroups has been performed. The reported optoelectronic properties of the investigated dyes are determined using densityfunctional theory (DFT) and time-dependent DFT methods. The investigated sensitizers exhibit maximum absorption between400 and 442 nm with light-harvesting efficiencies exceeding 93%.Favorable interactions are observed between the dyes and the hydrogenated TiO2 cluster, (TiO2)6H3. The obtained binding energies range from 1.39 to 0.97 eV in the gas phase and 0.31 to 0.03 eV in water. The electronic spectra of the dye@TiO2 complexes show broader and intensive bands with bathochromicshifts when compared to the individual dyes. The charge density distribution in the complexes indicates appropriate ability of thedyes for charge injection to the semiconductor. Among the considered dyes, the most promising candidates for use in DSSCs have been selected.
Enhancing the optoelectronic properties of blended triphenylamine-betalain based dyes through tailoring the anchoring unit: a theoretical investigation
(Taylor & Francis, 2024-11-25) Tsere, Melkizedeck; Costa, Rene; Deogratias, Geradius; Al-Qurashi, Ohoud; Wazzan, Nuha; Pogrebnaya, Tatiana; Pogrebnoi, Alexander; Machunda, Revocatus
A series triphenyl-betalain organic dyes featuring carboxylic acid and nitro anchoring groups CH = C(X)COOH for the A1-X dyes and -CH = C(X)NO2 for the A2-X dyes, respectively, where X = CN, CH3, CCl3 and CF3 was evaluated for dye sensitised solar cells application. The geometrical structures, molecular orbitals and energies, light absorption patterns, free energies of electron injection and dye regeneration and binding to the semiconductor have been explored using DFT/TD-DFT methods. The nitro-based anchoring group resulted in pronounced red-shift in absorption spectra between 111 and 317 nm compared to carboxylic acid-based dyes. Attachment of the dyes to the semiconductor was modelled via binding to (TiO2)6H3 cluster; A2-X dyes exhibited more stable Dye@TiO2 complexes with binding energies (BEs) ranging between −4.08 and −2.88 eV compared to A1-X dyes with BEs range of −1.11 to −0.05 eV. The results evince that the dyes with CH = C(X)NO2 anchoring groups could be promising materials for light harvesting application.
Glycerol Carbonate Synthesis Using a Catalyst Derived from Oldoinyo Lengai Volcanic Ash: Porosity, Process Optimization, Kinetic and Thermodynamic Analysis
(Wiley-VCH GmbH, 2025-11-08) Kandola, Isack; Tsere, Melkizedeck; Kichonge, Baraka; Jacob, Fortunatus; Bakari, Ramadhani; Pogrebnoi, Alexander; Mpumi, Nelson; Ekeoma, MO; Okoye, Patrick; Hilonga, Askwar; Kivevele, Thomas
This study investigates the use of natural catalysts from volcanic lava ash (VLA) to promote the conversion of glycerol (GL) to glycerol carbonate (GC). The surface basicity was characterized by temperature-programmed desorption of carbon dioxide (TPD-CO2), and textural properties by Brunauer–Emmett–Teller (BET). Kinetic modeling and thermodynamic calculations were performed to understand the reaction system. The VLA catalyst exhibited a surface area of 32.16 m2 g−1 with a total pore volume of 0.076 cm3 g−1 and an average pore diameter of 18.60 nm. Textural properties of volcanic ash lava revealed abundant active sites for adsorption and a mesoporous structure, which propagated diffusions of reactants, reactions at the active sites, and desorption of the products. High yield of glycerol carbonate and conversion of glycerol was attributed to high surface basicity of 32.40 mmol g−1 based on temperature-programmed desorption of carbon dioxide analysis. Process optimization based on response surface methodology using the Box-Behnken design revealed that 97.0 ± 0.1% of glycerol and 91.00 ± 0.15% of glycerol carbonate can be achieved at optimal conditions of 80 °C, 4.5 wt% catalyst loading, dimethyl carbonate to glycerol molar ratio of 3:1, under 90 min reaction time. The regression models exhibited R2 = 0.98 for glycerol conversion and R2 = 0.99 for glycerol carbonate yield. Kinetic studies revealed pseudo-first-order behavior with a rate constant of 0.1 min−1 at 348.15 K, and an activation energy of ∼33 kJ mol−1. Thermodynamic parameters (ΔG* = 92 kJ mol−1, ΔH* = 31 kJ mol−1, ΔS* = −174 J mol−1 K−1) suggest an endergonic and nonspontaneous reaction. The characterization results revealed that the catalyst possesses a basic and mesoporous structure. These properties played a crucial role in enhancing its catalytic efficiency and sustainable route for optimized glycerol carbonate synthesis with defined kinetics.