School of Sciences and Aerospace Studies, Department of Mathematics, Physics and Computing
Moi University
Single walled carbon nanotube (SWCNT) and alkaline metal oxide have been identified as potential materials for management of CO2 emission. Yet the underlying operating mechanism is still not well understood, while an in-depth understanding would possibly lead to development of superior CO2 monitoring, capture, and storage devices. Here we present ab initio density functional theory calculations to provide a comprehensive description of CO2 gas interaction with SWCNT and CaO surface. In particular, our results revealed that CO2 is chemisorbed on CaO surface with negligible effect on electronic properties of the absorbent, while CO2 interaction with SWCNT can be categorized as physisorption interaction a process that can be easily reversed using thermal treating of the tube at 150 °C. Thus CaO is found to be ideal for long term storage of CO2 while SWCNT reported superior performance in CO2 sensing and capture. This work may guide the development of better devices based on CaO and SWCNT for CO2 sensing, capture, and storage.
Understanding interface properties of heterojunction is an important step towards controllable and tunable interfaces for photocatalytic and photovoltaic based devices. To this aim, we propose a thorough study of a double heterostructure system consisting of two semiconductors with large band gap, namely, wurtzite ZnO and anatase TiO2. We demonstrate via first-principle calculations two stable configurations of ZnO/TiO2 interfaces. Our structural analysis provides a key information on the nature of the complex interface and lattice distortions occurring when combining these materials. The study of the electronic properties of the sandwich nanostructure TiO2/ZnO/TiO2 reveals that conduction band arises mainly from Ti3d orbitals, while valence band is maintained by O2p of ZnO, and that the trapped states within the gap region frequent in single heterostructure are substantially reduced in the double interface system. Moreover, our work explains the origin of certain optical transitions observed in the experimental studies. Unexpectedly, as a consequence of different bond distortions, the results on the band alignments expect different electron accumulation between the two interfaces. Such behavior provides more choice for the sensitization and functionalization of TiO2 surfaces.
Niobium carbides and nitrides have been proposed as potential candidates for hardness and related applications, however, comprehensive studies are still needed for better understanding that may pave way for their re-engineering for the ultra hard industry. Here we present ab initio Density Functional Theory calculations that provide a comprehensive description of various hardness characterization parameters. Our results show that NbC in rocksalt (RS) had a higher shear modulus, Young's modulus, and Voigt-Reuss-Hill shear modulus compared to other phases of NbC and NbN considered in this work. Further, it was noted that NbC in RS had a higher value of Vickers hardness amongst the various phases NbC and NbN studied, thus identified as a potential candidate for hardness and related application. Finally, we showed that compounds with Vickers hardness (Hv) > 20 GPa were found to be brittle while those with Hv < 20 GPa were ductile.
The effect of nitrogen doping on the structural, electronic and optical properties of hexagonal and cubic Ge2Sb2Te5 (GST) has been investigated from first principles. The nitrogen content was set to 10 at.% and 25 at.%, in which it was found that the hexagonal phase becomes more stable, whereas the cubic phase becomes more unstable with increasing nitrogen content. A difference in optical reflectivity of about 8% was calculated for pure hexagonal and cubic phases. Pure GST was found to possess a higher reflectivity contrast in the infrared spectral range, whereas doped GST had a higher reflectivity contrast, which increased with rising nitrogen content, in the visible and ultraviolet spectral range. The optical conductivity was found to fall with increasing nitrogen content, in agreement with experiment and other theoretical studies.
In this work, ZnO thin films were investigated to sense NO2, a gas exhausted by the most common combustion systems polluting the environment. To this end, ZnO thin films were grown by RF sputtering on properly designed and patterned substrates to allow the measurement of the electrical response of the material when exposed to different concentrations of the gas. X-ray diffraction was carried out to correlate the material’s electrical response to the morphological and microstructural features of the sensing materials. Electrical conductivity measurements showed that the transducer fabricated in this work exhibits the optimal performance when heated at 200 °C, and the detection of 0.1 ppm concentration of NO2 was possible. Ab initio modeling allowed the understanding of the sensing mechanism driven by the competitive adsorption of NO2 and atmospheric oxygen mediated by heat. The combined theoretical and experimental study here reported provides insights into the sensing mechanism which will aid the optimization of ZnO transducer design for the quantitative measurement of NO2 exhausted by combustion systems which will be used, ultimately, for the optimized adjustment of combustion resulting into a reduced pollutants and greenhouse gases emission.