ZnO@ZIF-8: Gas sensitive core-shell hetero-structures show reduced cross-sensitivity to humidity

A ‘lawn-like’ distribution of interconnected zinc oxide nanorods, coated with a metal-organic compound based on zeolitic imidazolate frameworks – ZIF-8 was prepared on microstructured thin-film interdigitated Pt-electrodes forming ZnO@ZIF-8 core-shell heterostructures and investigated as gas sensor material in relation to the identical, but uncovered pure ZnO-layer. This composite combines the gas sensing properties of the metal oxide ZnO with the specific properties of the metal-organic framework material which result in a distinct change of the conditions of gas sensing at the ZnO/ZIF-8-interface. Herein, for the first time it is reported that as prepared ZnO@ZIF-8 composite material is an attractive choice to reduce the cross-sensitivity to water vapour (humidity) in the gas sensing response towards propene and ethene. The observed change of sensitivity in relation to uncovered ZnO is discussed to be due to (i) the specific interaction of the ZIF-8 at the interface with the ZnO taking influence on the gas reaction processes, (ii) the diffusivity of ZIF-8 for the different gas components, and (iii) the sorption behaviour of the used gases at the ZnO interface and inside the ZIF-8 material

Mo-doped TiO2 photoanodes using [Ti4Mo2O8(OEt)10]2 bimetallic oxo cages as a single source precursor

Photoelectrochemical solar water splitting is a promising and sustainable technology for producing solar fuels such as clean hydrogen from water. A widely studied photoanode semiconductor for this application is TiO2, but it suffers from a large band gap (3.2 eV) and fast recombination of electrons and holes. Herein, we present a novel, facile and rapid strategy to develop Mo-doped TiO2 (Mo:TiO2) mixed anatase–rutile photoanodes using [Ti4Mo2O8(OEt)10]2 bimetallic oxo cages as a single source precursor. These cages dissolved in tetrahydrofuran deposit by spray pyrolysis at 150 C forming films with hierarchical porosity on the micrometer and nanometer scale. XPS, EDXS and UV-Vis spectroscopy reveal Mo atoms evaporate during annealing in air at temperatures 650–800 C, contributing to the formation of nanostructures and porosity. XPS depth profiling, XRD, EDXS, Raman, and electron paramagnetic resonance indicate that the remaining Mo atoms are well spread and incorporated in the TiO2 lattice, at interstitial or substitutional sites of the rutile or anatase phases depending on the annealing temperature. Photocurrent measurements show that Mo:TiO2 photoanodes optimized at 700 C outperform a TiO2 photoanode prepared in a similar manner by a factor of two at 1.23 VRHE. Finally, UV-Vis spectroscopy, conduction and valence band calculations, and incident-to-photon efficiency measurements show these Mo:TiO2 photoanodes possess a narrower band gap than TiO2 and higher efficiency in the visible light range (5% at 400 nm). These outcomes open a new avenue in the exploitation of titanium oxo cages and advance the development of photoelectrodes for water splitting and energy application

High-temperature CO / HC gas sensors to optimize firewood combustion in low-power fireplaces

In order to optimize firewood combustion in low-power firewood-fuelled fireplaces, a novel combustion airstream control concept based on the signals of in situ sensors for combustion temperature, residual oxygen concentration and residual un-combusted or partly combusted pyrolysis gas components (CO and HC) has been introduced. A comparison of firing experiments with hand-driven and automated airstream-controlled furnaces of the same type showed that the average CO emissions in the high-temperature phase of the batch combustion can be reduced by about 80 % with the new control concept. Further, the performance of different types of high-temperature CO / HC sensors (mixed-potential and metal oxide types), with reference to simultaneous exhaust gas analysis by a high-temperature FTIR analysis system, was investigated over 20 batch firing experiments (∼ 80 h). The distinctive sensing behaviour with respect to the characteristically varying flue gas composition over a batch firing process is discussed. The calculation of the Pearson correlation coefficients reveals that mixed-potential sensor signals correlate more with CO and CH4; however, different metal oxide sensitive layers correlate with different gas species: 1 % Pt / SnO2 designates the presence of CO and 2 % ZnO / SnO2 designates the presence of hydrocarbons. In the case of a TGS823 sensor element, there was no specific correlation with one of the flue gas components observed. The stability of the sensor signals was evaluated through repeated exposure to mixtures of CO, N2 and synthetic air after certain numbers of firing experiments and exhibited diverse long-term signal instabilities

In situ high-temperature gas sensors: continuous monitoring of the combustion quality of different wood combustion systems and optimization of combustion process

The sensing characteristics and long-term stability of different kinds of CO ∕ HC gas sensors (non-Nernstian mixed potential type) during in situ operation in flue gas from different types of low-power combustion systems (wood-log- and wood-chip-fuelled) were investigated. The sensors showed representative but individual sensing behaviour with respect to characteristically varying flue gas composition over the combustion process. The long-term sensor signal stability evaluated by repeated exposure to CO ∕ H2 ∕ N2 ∕ synthetic air mixtures showed no sensitivity loss after operation in the flue gas. Particularly for one of the sensors (Heraeus GmbH), this high signal stability was observed in a field test experiment even during continuous operation in the flue gas of the wood-chip firing system over 4 months. Furthermore, it was experimentally shown that the signals of these CO ∕ HC sensing elements yield important additional information about the wood combustion process. This was demonstrated by the adaptation of an advanced combustion airstream control algorithm on a wood-log-fed fireplace and by the development of a combustion quality monitoring system for wood-chip-fed central heaters

Miniaturized Single Chip Arrangement of a Wheatstone Bridge Based Calorimetric Gas Sensor

The design and fabrication of a miniaturized calorimetric-type gas sensor in a single chip arrangement is presented. Active and passive thin-film Pt meanders are integrated in a single platform (7 × 7 mm2) together with a temperature sensor and a thin-film microheater at the reverse side. Active meanders are covered by a porous Al2O3/2 wt % Pt thick-film layer. The selection of substrate, position of meanders, and active catalysts (especially their concentration) play a crucial role in directing sensor performance. The presented results show that the sensor signal (Wheatstone bridge voltage) is generated by diffusion-limited exothermic reactions which point towards catalytically enhanced combustion reactions mainly inside the active porous layer. By extrapolation of the linear sensitivity curves, the sensitivity limit was estimated to be 4 ppm for propene and to be 18 ppm for CO. In general, the one-chip-sensing concept has high potential to be used as a gas sensor for analysis of combustible gases; however, further optimization of the meander design and the catalyst material as well as investigations of the sensing behavior under varying ambient temperatures are necessary before such applications shall be considered

Hydrogen sensors based on Pt-loaded WO

In this letter we report the enhanced sensing of platinum (Pt)-loaded tungsten oxide (WO3) sensor compared to pure WO3 sensor towards hydrogen (H2) gas at low operating temperature of 200 °C. The hydrogen sensing of pure and Pt-loaded WO3 sensors is reported at operating temperatures 200, 300 and 400 °C. The presence of Pt promotes the spillover mechanism and the dissociated H2 atoms can react with adsorbed/lattice oxygen atoms to release electrons which in turn increases the conductivity of the WO3 film. The H2 sensing mechanism of Pt-loaded WO3 sensor is investigated using micro-Raman spectroscopy. The Raman spectrum of Pt-loaded sensing layer shows a shift of symmetric stretching band from 796 to 804 cm−1 with the introduction of H2 gas

ZnO@ZIF-8: Gas sensitive core-shell hetero-structures show reduced cross-sensitivity to humidity

A ‘lawn-like’ distribution of interconnected zinc oxide nanorods, coated with a metal-organic compound based on zeolitic imidazolate frameworks – ZIF-8 was prepared on microstructured thin-film interdigitated Pt-electrodes forming ZnO@ZIF-8 core-shell heterostructures and investigated as gas sensor material in relation to the identical, but uncovered pure ZnO-layer. This composite combines the gas sensing properties of the metal oxide ZnO with the specific properties of the metal-organic framework material which result in a distinct change of the conditions of gas sensing at the ZnO/ZIF-8-interface. Herein, for the first time it is reported that as prepared ZnO@ZIF-8 composite material is an attractive choice to reduce the cross-sensitivity to water vapour (humidity) in the gas sensing response towards propene and ethene. The observed change of sensitivity in relation to uncovered ZnO is discussed to be due to (i) the specific interaction of the ZIF-8 at the interface with the ZnO taking influence on the gas reaction processes, (ii) the diffusivity of ZIF-8 for the different gas components, and (iii) the sorption behaviour of the used gases at the ZnO interface and inside the ZIF-8 material