Temperature and humidity effects on the acetone gas sensing of pristine and Pd-doped WO3 clusters: A transition state theory study.

CONTEXT: The performance of pristine and Pd-doped WO3 acetone gas sensors is calculated theoretically and compared with available experimental results. Temperature, humidity, and acetone concentration variation are considered in the present work. Transition state theory calculates Gibbs free energy of transition, including its components enthalpy and entropy of transition or activation. The variation of Pd doping concentration is used to obtain the maximum response and lowest response time for the optimum performance of the gas sensor. The present theory considers the reduction of acetone gas concentration as acetone reaches its autoignition temperature. Acceptable agreement between theory and experiment is obtained. The acceptance includes the decrease of Gibbs free energy with doping percentage, variation of temperature exponent to the power twelve in the considered reactions, and reduction of response time with the increase of temperature. METHODS: Density functional theory at the B3LYP level is used. 6-311G** basis set (for O atoms) and SDD (for heavy Pd and W atoms) are used to optimize the structures examined in the present work. The Gaussian 09 program and accompanying software were used to perform the current tasks.

Investigation of thermoelectric properties of cadmium selenide CdnSen (n= 7, 11, 13) molecular junctions: a DFT study.

CONTEXT: The thermoelectric properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated before and after adding hydrogen atoms. The effects of hydrogen passivation on the transmission and thermopower curves were analyzed. CdSe-diamantane (Cd7Se7) and CdSe-tetramantane (Cd11Se11) junctions exhibited the best thermoelectric performance due to their low surface reconstruction energy, which is attributed to the number of dangling and unsaturated bonds. This study guides the design of new molecular junctions with desired thermoelectric properties. METHOD: The electrical and thermal properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated using a ballistic quantum transport method based on the non-equilibrium Green's function (NEGF) approach. Thermoelectric properties were calculated for the molecular junctions with different structures before and after hydrogen passivation. Density functional theory (DFT) calculations were performed at the B3LYP level with the 3-21G basis set for the Cd atoms and the 6-31G** basis set for the Se atoms. The SIESTA and GOLLUM codes were used to study the effect of changing the shape and size of each structure on its electrical and thermal characteristics.

Prospective utilization of boron nitride and beryllium oxide nanotubes for Na, Li, and K-ion batteries: a DFT-based analysis.

CONTEXT: In the present work, we investigated the adsorption mechanism of natural sodium (Na), potassium (K), and lithium (Li) atoms and their respective ion on two nanostructures: boron-nitride nanotubes (BNNTs) and beryllium-oxide nanotubes (BeONTs). The main goal of this research is to calculate the gain voltage for Na, K, and Li ionic batteries. Density function theory (DFT) calculations indicated that the adsorption energy between Na + is higher than that of the other cations, and this is particularly clear in the BeONT. Furthermore, gain voltage calculations showed that BNNTs generate a higher potential than BeONTs, with the most significant difference observed in BNNT/Na + . This research provides theoretical insights into the potential uses of these nanostructures as anodes in Na, K, and Li-ion batteries. METHOD: Density function theory used to compute the ground state properties for BeONT and BNNT with and without selected atoms and their ions (Li, K, and Na). B3LYP used for exchange correlation between electrons and ions, and 6-31G* basis set used for all atoms and ions. Gauss Sum 2.2 software used for estimate the density of state (DOS) for all structure under investigation.

The reaction of pristine and Rh-doped SnO2 clusters with acetone: Application of Evans-Polanyi principle to transition state theory.

CONTEXT: Using transition state theory, acetone sensing by pristine and rhodium-doped tin dioxide is discussed. The Evans-Polanyi principle is modified from its original formulation commensurate with the Arrhenius equation to be more suitable for transition state theory. The new formalism for the activation energy replaces enthalpy with Gibbs free energy in the original Evans-Polanyi principle. The new formalism considers reaction entropy, which is not considered previously in Evans-Polanyi principle. Response and response time of interaction of acetone with both pristine and Rh-doped SnO2 clusters is calculated. Variations of response in terms of acetone concentration and temperature are calculated and compared to the experiment. Acceptable agreement between theory and experiment that calls for more comparisons to demonstrate the modified approach. METHODS: The pristine and Rh-doped clusters and their interaction with acetone are simulated using density functional theory at the B3LYP level. 6-311G** and SDD (for heavy atoms) basis sets are used to optimize the structures examined in the present work. Gaussian 09 program and accompanying software performed the current tasks.

Effect of formaldehyde properties on SnO2 clusters gas sensitivity: A DFT study.

Formaldehyde (CH2O) properties such as flash point and autoignition temperature have a great effect on the temperature range of sensitivity of sensors applied to detect CH2O gas. Tin dioxide nanocrystal interaction with formaldehyde is investigated from room temperature to 500 °C using transition state and density functional theory. Gibbs free energy, enthalpy, and entropy of activation and reaction are evaluated as a function of temperature. The sensitivity and response time of SnO2 clusters towards formaldehyde are evaluated. Results show that the activation energy of SnO2 clusters with formaldehyde increases with the rise of temperature while the reaction energy decreases (in negative value) with the rise of temperature. Response time is inversely proportional to formaldehyde concentration. The highest CH2O gas-sensitive range of SnO2 is confined between the formaldehyde flash point at 64 °C and the autoignition temperature at 430 °C. The effect of partial oxidation and dissociation of formaldehyde is discussed.

Chlorine gas reaction with ZnO wurtzoid nanocrystals as a function of temperature: a DFT study.

In the present work, we applied density functional theory and temperature-dependent Gibbs free energy calculations to wurtzoid structures to explain the sensitivity of ZnO nanocrystals towards chlorine molecules. In agreement with experimental findings, our results revealed that chlorine sensing under ambient conditions is feasible. Higher temperatures increased the sensitivity of ZnO nanocrystals towards chlorine gas molecules. Peak calculated sensitivities were in the temperature ranges (167-220 °C), (447-578 °C) and (952-1159 °C), which is in good agreement with experimentally determined temperatures. According to the calculated Gibbs free energy, these three ranges correspond to the van der Waals attachment of Cl2 molecules on Zn-polar sites, van der Waals attachment of Cl2 molecules on O sites, and dissociation of Cl2 molecules on ZnO nanocrystal surfaces, respectively. The removal of chlorine atoms from the surface of ZnO nanocrystals is difficult at low temperatures because of the high electron affinity of chlorine gas atoms, which results in a long recovery time and accumulation of chlorine atoms and molecules on the ZnO surface. Atomic charges and charge transfer are depicted using natural bond orbital analysis to explain the present mechanisms.