Mudar Ahmed Abdulsattar

Several factors frequently affect gas sensors, including sensing material, doping, temperature, detected gas properties, humidity, manufacturing method, etc. The present work studies WO3 doped SnS2 gas sensor response to NO2, considering the above factors using transition state theory formalism. The reaction rate equation in transition state theory was used to estimate the change in the number of vacancies in a 30 % WO3-doped SnS2 sensor when NO2 gas passed over its surface. Temperature dependence of Gibbs energy of adsorption and transition was evaluated at the effective temperatures. Effective temperatures were the temperatures after which NO2 gas dissociates and can no longer be detected. The effect of temperature and humidity was evaluated using logistic functions. Response, response time, and NO2 concentration were calculated and compared with the experiment. Interestingly, the lowest Gibbs energy of transition as a function of 30 % WO3 doping percentage was very close to the highest experimental response. WO3 doped SnS2 gas sensor is stable for an extended period, as proved experimentally and theoretically. Transition state theory enabled the calculation of changes in the number of vacancies and various experimentally obtained quantities that cannot be evaluated using density functional theory alone.

Context Chlorine properties that affect its reaction with the SnO₂ sensor surface are discussed. This includes temperature variation and Cl₂ reaction with humidity. Transition state theory formalism evaluates related thermodynamic properties such as Gibbs free energy and its components, enthalpy, and entropy. Logistic functions determine the effective concentration of Cl₂ gas due to its reaction with humidity and sensor material. The Gibbs free energy of adsorption and transition or activation is evaluated as a function of temperature. Results include SnO₂ sensor response to Cl₂ gas as a function of temperature and Cl₂ concentration. Results also include response time and the effect of humidity. An optimum response temperature can be between room temperature and 200 °C. A comparison with available experimental results is performed, which shows a good agreement between theory and experiment. The present model is the only available model that can successfully compare the theory and experiment of response and response time, including temperature and humidity effects. Methods Gaussian 09 software package is used with B3LYP level of DFT since most previous successful gas sensor calculations are performed using this version of DFT. 6-311G** basis sets are used to represent oxygen and chlorine atoms, while SDD functionals are used to represent heavier Sn atoms.

H2S is a poisonous gas that needs to be censored to protect humans from its exposure. H2S gas sensitivity and interaction with pristine and Al-doped ZnO clusters were studied using the transition state theory method. The reaction of ZnO with H2S gas is usually weak and can be enhanced by doping. Elements or their oxides, such as platinum group elements, are traditionally used as doping substances. However, the use of other cheaper elements or oxides such as Al or Mn also increases the reaction rate. The mechanism of increasing the reaction rate and sensitivity by Al doping is the subject of present work. The calculation results show that Al-doped ZnO increases the sensitivity towards H2S by growing the resistance of Al-doped ZnO due to lattice distortion. The Gibbs activation energy remains almost the same as calculated by the present model. In conclusion, it should be noted that good agreement between theory and experiment was achieved in terms of temperature-dependent reaction rate, gas response, recovery and response time for different doping ratios. For the first time, the autoignition reaction and temperature of H2S were considered in sensors. Further improvement in transition state theory is needed to include further gas sensor features.

Context The reaction of NO₂ with pristine and Pt-doped SnS₂ surfaces is investigated theoretically and compared with the experiment. Transition state theory formalism for gas sensors is adopted to present NO₂ gas sensing. The dissociation temperature at approximately 150 °C is found to be of great importance in NO₂ reactions. The adsorption and transition states of NO₂ with pristine and Pt-doped SnS₂ are calculated. Pt doping includes 0.5, 1, and 1.5% in accordance with available experimental results. The variation of thermodynamic quantities such as Gibbs free energy with Pt concentration and temperature is calculated. Transition state theory parameters that are suitable for the present sensor are determined. The results include the variation in response time with temperature, Pt concentration, and NO₂ concentration. Response and response time as a function of temperature are rarely investigated in theoretical calculations, which is one of the advantages of the present study. Optimum response temperature and Pt concentration are found. The results agree with available experimental results. Methods Density functional theory at the B3LYP level optimize molecular structures. 6-311G** basis set is used for all elements except Sn and Pt treated using SDD basis set. Gaussian 09 program and its facilities are used to perform present optimizations.

The sensitivity of nickel oxide doped zinc oxide to industrial gas acetylene was calculated and compared to available experimental data. The adsorption of C2H2 on the NiO-doped ZnO surface and the activation transition states formed afterward were studied computationally. B3LYP version of density functional theory with 6-311G** basis set, including dispersion correction (GD3BJ) was performed for the calculations with the help of Gaussian 09 software. Thermodynamic quantities of the reaction of C2H2 with NiO-doped ZnO surfaces, such as Gibbs free energy, enthalpy, and entropy, were used to interpret the reaction at the temperature range 25–325 °C. Response and response time variation with different NiO doping percentages were calculated. The calculations took into account the combustion of acetylene as it approached its autoignition temperature, which was not considered in previous works. The results show that the optimum response operating temperature of the C2H2 gas sensor is below the autoignition temperature of acetylene at 300 °C. A good agreement of theoretical response and response time with variation of temperature and acetylene concentration was obtained with the experiment. This study was the first to take into account the autoignition of C2H2 in gas sensor calculations and noted that gases can be easily distinguished by their autoignition temperature.

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.

H2 gas reaction with pristine and Au-doped SnO2 clusters is calculated and compared with the experiment. A new generalized version of the Evans–Polanyi principle is employed. As a function of temperature, the transition state is used to calculate the activation of Gibbs free energy, including its components enthalpy and entropy. H2 autoignition at elevated temperatures is considered using logistic functions. Reaction rate, response, response time, and recovery time are calculated and compared with the experiment. Results show a strong temperature dependence of H2 reactions, while O2 recovery reactions depend on temperature through the change of activation energy only. Promising results that need more comparisons between experiment and theory are obtained to validate the new formalism.