Advanced Strategies to Improve Performances of Molybdenum-Based Gas Sensors

Molybdenum-based materials have been intensively investigated for high-performance gas sensor applications.Particularly,molybdenum oxides and dichalcogenides nanostructures have been widely examined due to their tunable structural and physicochemical properties that meet sensor requirements.These materials have good durability,are naturally abundant,low cost,and have facile preparation,allowing scalable fabrication to fulfill the growing demand of susceptible sensor devices.Significant advances have been made in recent decades to design and fabricate various molybdenum oxides-and dichalcogenides-based sensing materials,though it is still challenging to achieve high performances.Therefore,many experimental and theoretical investigations have been devoted to exploring suitable approaches which can significantly enhance their gas sensing properties.This review comprehensively examines recent advanced strategies to improve the nanostructured molybdenum-based material performance for detecting harmful pollutants,dangerous gases,or even exhaled breath monitoring.The summary and future challenges to advance their gas sensing performances will also be presented.

Morphology control of aluminum nitride(AlN)for a novel high-temperature hydrogen sensor

Hydrogen is a promising renewable energy source for fossil-free transportation and electrical energy generation.However,leaking hydrogen in high-temperature production processes can cause an explosion,which endangers production workers and surrounding areas.To detect leaks early,we used a sensor material based on a wide bandgap aluminum nitride(AlN)that can withstand a high-temperature environment.Three unique AlN morphologies(rod-like,nest-like,and hexagonal plate-like)were synthesized by a direct nitridation method at 1400℃usingγ-AlOOH as a precursor.The gas-sensing performance shows that a hexagonal plate-like morphology exhibited p-type sensing behavior and showed good repeatability as well as the highest response(S=58.7)toward a 750 ppm leak of H2 gas at high temperature(500°C)compared with the rod-like and nest-like morphologies.Furthermore,the hexagonal plate-like morphology showed fast response and recovery times of 40 and 82 s,respectively.The surface facet of the hexagonal morphology of AlN might be energetically favorable for gas adsorption–desorption for enhanced hydrogen detection.

Introducing oxygen vacancies in TiO_(2) lattice through trivalent iron to enhance the photocatalytic removal of indoor NO

The synthesis of oxygen vacancies(OVs)-modified TiO_(2)under mild conditions is attractive.In this work,OVs were easily introduced in TiO_(2)lattice during the hydrothermal doping process of trivalent iron ions.Theoretical calculations based on a novel charge-compensation structure model were employed with experimental methods to reveal the intrinsic photocatalytic mechanism of Fe-doped TiO_(2)(Fe-TiO_(2)).The OVs formation energy in Fe-TiO_(2)(1.12 eV)was only 23.6%of that in TiO_(2)(4.74 eV),explaining why Fe^(3+)doping could introduce OVs in the TiO_(2)lattice.The calculation results also indicated that impurity states introduced by Fe^(3+)and OVs enhanced the light absorption activity of TiO_(2).Additionally,charge carrier transport was investigated through the carrier lifetime and relative mass.The carrier lifetime of Fe-TiO_(2)(4.00,4.10,and 3.34 ns for 1at%,2at%,and 3at%doping contents,respectively)was longer than that of undoped TiO_(2)(3.22 ns),indicating that Fe^(3+) and OVs could promote charge carrier separation,which can be attributed to the larger relative effective mass of electrons and holes.Herein,Fe-TiO_(2)has higher photocatalytic indoor NO removal activity compared with other photocatalysts because it has strong light absorption activity and high carrier separation efficiency.

Recent collaborative researches between Japanese universities and steel industry on the iron ore agglomeration process

The properties of iron ores used in ironmaking process have been drastically changed in the past couple of decades.Especially,the change has become significant in the last few years because of the considerable increase in the world steel production.The property change of the iron ore is mainly caused by the depletion of the hard and high-grade lump hematite ores.It has led to the increasing use of ores containing a larger amount of goethite/limonite,i.e.,hydro-oxides of iron.Typically,the proportion of pisolitic ores,which are course limonitic ores,has remarkably increased by several times in Japan.Further,large deposits of the fine goethite ores called Marra Mamba have been developed in Australia and exported to Asian countries.Such trends will be continued in future.Since the change of the ore properties affects not only to the productivity and yield of the sinter but also its metallurgical properties in the blast furnace,further improvement in the sintering technology/ process is required including the preliminary treatment process of raw materials.In order to make wide researches concerning the above issues,the research project 'New Sintering Process through Designing of Composite Granulation & Bed Structure' was formed in the ISIJ,which was the collaborative project between Japanese steel companies and several universities.The project was started in 2005 and carried on the wide range of studies for three and half years.Its main objects are the characterization of pisolitic/goethitic ores and the understanding the behavior during the iron ore sintering process.Further,considering the ore characteristics,some basic researches on the optimum designs of raw material blending,granulation,bed structure,and the metallurgical properties of the produced sinter were performed.The project have invented the technical principle of a new sintering process, namely MEBIOS(Mosaic EmBedding Iron Ore Sintering Process),characterized by the composite granulation and bed-structure,aiming to cope with the drastic shift of the ore properties.Another big issue fallen on the steel industry is the global warming.CO,emission from steelmaking industry occupies about 15%of the total value of the artificial emissions in Japan and therefore its reduction is urgently required.In order examine the possibility to minimize or to reduce further the CO_2 emission from the iron ore sintering process,the research project 'Technological Principle for Low-Carbon Sintering' has been formed since 2009 in the ISIJ.In this project,the analyses of the combustion rates of carbonaceous materials and heat transfer in the sintering bed are first examined by referring the previous studies.Further,experimental works will be conducted on the combustion/oxidation characteristics of biomass charcoal,some organic wastes,steel can scraps,mill scale and partially reduced iron ores as alternative agglomeration reagents of coke and anthracite coal.The effect of their use on the sintering process will be evaluated systematically.It is expected that the structural changes of the sintering bed is considerably different between carbonaceous materials,which disappear during combustion leaving a little amount of ash components and metallic iron bearing materials,which increase the mass and volume during its oxidation. Previous studies showed that the use of metallic iron bearing materials such as steel can scrap and mill scale led to significant decreases in the production rate.This project examines the characteristics of such changes of the sintering bed structure and mineral phases and main process parameters,which govern such phenomena.Further, it searches for a new process principle to overcome the demerits and realize the significant reduction of CO_2 emissions from the iron ore sintering process.In the symposium,summary of activities and the major results and progresses of the above two research projects will be introduced.

A facile method for preparation of uniformly decorated-spherical SnO2 by CuO nanoparticles for highly responsive toluene detection at high temperature

We reported a facile preparation of a uniform decoration of spherical n-type SnO2 by p-type CuO nanopa rticles as well as their utilization for enhanced performance on toluene gas detection.CuO nanoparticles and spherical SnO2 were synthesized by a facile non-hydrolytic solvothermal reaction,which could easily control their morphology.A uniform CuO nanoparticles decoration onto spherical SnO2 was achieved by a simple sonication and vigorous stirring at room tempe rature.We revealed orga nic solvents used in the oxide synthesis had a considerable influence on its surface charge that was beneficial for a uniformly electrostatic self-decoration between positively charged p-type CuO nanoparticles and negatively charged n-type spherical SnO2.Interestingly,CuO was partially reduced to Cu metal during high concentration of toluene exposure destroying p-n contact and developing new metal-semiconductor contact so-called ohmic junction,resulting in extraordinarily responsive and selective to toluene gas at 400℃as compared to a single p-CuO and n-SnO2.It was also found that the amount of particle decoration had an influence on sensor response and resistance.The optimum amount of CuO nanoparticle decoration was0.1 mmol on 0.5 mmol SnO2.The re s ponse(S=Ra/Rg)and selectivity of CuO/S nO2 based material toward the exposure of 75 ppm toluene had reached to such high as 540 and 5,respectively.The effect of p-n heterojunction and metal-semiconductor contact on the gas sensing mechanism of p-type CuO/n-type SnO2 was discussed.Furthermore,by decorating with CuO nanoparticles,CuO/SnO2 morphology was well-maintained after gas sensing evaluation demonstrated its excellency for high temperature toluene gas sensor application.

A simple and novel effective strategy using mechanical treatment to improve the oxygen uptake/release rate of YBaCo_(4)O_(7+δ) for thermochemical cycles

In recent years,oxygen storage materials(OSMs)have been widely used in many fields.It would be particularly important for researchers to design high-oxygen-uptake/release-rate materials.In this study,various synthesis processes were used to successfully synthesize YBaCo_(4)O_(7+δ)and comprehensively investigate their potential applications.Compa red with traditional solid-state reaction method and co-precipitation method,the results demonstrated that the utilization of mechanical ball milling treatment on co-precipitated precursors could lead to samples with reversible oxygen uptake/release under an oxidative atmosphere at low temperatures.The resultant materials exhibited fast oxygen absorption/desorption rate that could uptake/release oxygen directly to the equilibrium state within 9 min and20 min,respectively.The mechanochemically ball-milled sample possessed outstanding oxygen sto rage performance,which could be attributed to their small particle size,the active outer surface of particles,large specific surface area,and relatively low activation energy.Moreover,the ball-milled sample also exhibited excellent cycling stability during relatively short time spacing.TG results also demonstrated that the ball-milled samples could reversibly uptake/release 2.90 wt.%of excess oxygen(while only 0.70 wt.%for solid-state samples)by adjusting the ambient temperature under pure O_(2) atmosphere,which would make them promising candidates in various applications.This research demonstrated that mechanical treatment could be an effective strategy to tune the properties and oxygen storage capacity(OSC)performances of YBaCo_(4)O_(7+δ).

Building 2D quasicrystals from 5-fold symmetric corannulene molecules

The formation of long-range ordered aperiodic molecular films on quasicrystalline substrates is a new challenge that provides an opportunity for further surface functionalization. This aim can be realized through the smart selection of molecular building blocks, based on symmetry-matching between the underlying quasicrystal and individual molecules. It was previously found that the geometric registry between the C60 molecules and the 5- and 10-fold surfaces was key to the growth of quasiperiodic organic layers. However, an attempt to form a quasiperiodic C60 network on i-Ag-In-Yb substrates was unsuccessful, resulting in disordered molecular films. Here we report the growth of 5-fold symmetric corannulene C20H10 molecules on the 5-fold surfaces of i-Ag-In-Yb quasicrystals. Low-energy electron diffraction （LEED） and scanning tunneling microscopy （STM） revealed long-range quasiperiodic order and 5-fold rotational symmetry in self-assembled corannulene films. Recurrent decagonal molecular rings were seen, resulting from the decoration of specific adsorption sites with local pentagonal symmetry by corannulenes, adsorbed with their bowl-openings pointing away from the surface. They were identified as （Ag, In）-containing rhombic triacontahedral （RTH） duster centers and pentagonal Yb motifs, which cannot be occupied simultaneously due to steric hindrance. It is proposed that symmetry-matching between the molecule and specific substrate sites drives this organization. Alteration of the molecular rim by the introduction of CH substituents appeared to increase molecule mobility on the potential energy surface and facilitate trapping at these specific sites. This finding suggests that rational selection of molecular moiety enables the templated self-assembly of molecules leading to an ordered aperiodic corannulene layer.