Browsing by Author "Muthui, Z.W."

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Ab initio Investigation of the Structural and Electronic Properties of Alkaline Earth Metal - TiO2 Natural Polymorphs
(Hindawi Advances in Materials Science and Engineering, 2022) Mbae, J.K.; Muthui, Z.W.
Titanium (IV) oxide (TiO2) has gained much attention due to its application in technologies such as optoelectronics, electronics, sensors, photocatalysts, and sustainable energy generation. However, its optical absorption falls in the ultraviolet part of the electromagnetic spectrum, resulting in a low absorption ratio of solar light. In addition, rapid electron-hole recombination limits its photocatalytic activity. To extend the application range of TiO2, the structural and chemical properties can be modified by adding various dopants to tune its electronic structure for applications within a wider range of the solar energy spectrum and ideally extend towards the visible region, which forms the dominant part of the solar energy spectrum. In this study, the structural and electronic properties of three polymorphs of TiO2 have been studied using density functional theory (DFT) as implemented in the Quantum ESPRESSO simulation package. )e exchange-correlation potential has been treated with the generalised gradient approximation (GGA). Cationic substitution with non-toxic alkaline earth metal dopants Mg and Ca has been carried out with the aim of modifying the electronic structure of the polymorphs of TiO2. On 1–4% Mg and Ca cationic substitution, there is a slight expansion of the optimal unit cell volume and modulation of the band gap energy by raising the valence band maximum to higher energies. In addition, dopant inter and intra-band states are observed.
Density Functional Study of Structural and Electronic Properties of Ca And Mg Doped Tio2
(Chuka University, 2022) Mbae, J.K.; Muthui, Z.W.
The scarcity of affordable and environment-friendly sources of energy has led to emergence of photocatalysis to mitigate this problem. This is especially so if the photo catalysts are active in as much a wide region of the solar energy spectrum as possible. Environment-friendly, non-toxic and economical photocatalysts would find application in water treatment, providing another solution to the problem of clean water for domestic use. Of many materials available for photocatalysis is Titanium (IV) oxide (TiO2). It possesses many merits such as low cost, high photocatalytic activity, non-toxicity and high availability. However, drawbacks that limit its application include its optical absorption that falls in the ultraviolet part of the electromagnetic spectrum and rapid electron-hole recombination, which limits its photoquantum efficiency. To extend the optical absorption to a wider region of the solar energy spectrum, various dopants have been added to the oxide. In this study, the effect of doping rutile TiO2 with alkaline earth metals Ca and Mg is investigated using the Density Functional Theory (DFT) method as implemented in the Quantum ESPRESSO simulation package, treating the exchange correlation potential with the Generalised Gradient Approximation. The optimized cell parameters for pure rutile crystal system are 𝑎 = 𝑏 = 4.603Å 𝑐 = 2.992Å, with a volume of 63.393 (Å)3. On doping there is a slight expansion of the crystal structure and its volume slightly increases by 8.753 (Å)3 and 28.816 (Å)3 with Mg and Ca doping respectively. The calculated band gap of undoped rutile is 1.8 eV. Mg and Ca doping raises the valence band edge by 0.2 eV in both Ca and Mg doped rutile. Dopant inter band and intra band states are observed that would be useful in mitigating against charge recombination hence enhancing the efficiency of the photocatalysts. Isolated O 2p states are observed in the Projected Density of States (PDOS) of the doped systems which are normally attributed to enhanced optical absorption of photocatalysts in the visible region. Alkaline earth metal doping of TiO2 rutile modifies the structural and electronic properties of rutile TiO2 in a manner that would make it a more efficient photocatalyst. Co-doping with the alkaline metal dopants could be attempted to investigate the combined effect.
Search for Half Metallicity in Heusler Alloy Fe2nial for Spintronic Application using Density Function Theory.
(Chuka University, 2016) Muthui, Z.W.
Fast, durable, non-volatile and inexpensive data storage in electronic devices is greatly desired both for work and entertainment. Components that can provide these qualities continue to be developed by material scientists as companies continue to compete to produce devices possessing these qualities. Giant Magnetoresistance (GMR) and Tunnel Magnetoresistance (TMR) are properties that would greatly contribute to a device having these qualities. Half metals have attracted interest for their potential use in Metallic Tunneling Junctions (MTJ’s) fabrication, as well as spintronics due to their ability to provide full polarization of conduction electrons hence high GMR and TMR values. Heusler alloys, some of which exhibit half metallic character are intermetallic compounds which would be suitable in fabrication of MTJ’s with high GMR and TMR values. The electronic structure of Heusler alloy Fe2NiAl was investigated using first principle calculations of the density functional theory (DFT); the energy of exchange and correlation was treated by the generalized gradient approximation (GGA). Preliminary results point to the electronic structure of the alloy having a gap in the majority band, hence exhibiting desirable half metallic ferromagnetic character. This will contribute in design and fabrication of MTJ’s used in magnetic random access memory (MRAM) currently occupying a great deal of effort of material designers in spintronics technology.