1. INTRODUCTION Among the classes of highly porous materials, metalÀorganic frameworks (MOFs) are unparalleled in their degree of tunability and structural diversity as well as their range of chemical and physical properties. MOFs are extended crystalline structures wherein metal cations or clusters of cations (\" nodes \") are connected by multitopic organic \" strut \" or \" linker \" ions or molecules. The variety of metal ions, organic linkers, and structural motifs affords an essentially infinite number of possible combinations. 1 Furthermore, the possibility for postsynthetic modification adds an additional dimension to the synthetic variability. 2 Coupled with the growing library of experimentally determined structures, the potential to computationally predict, with good accuracy, affinities of guests for host frameworks points to the prospect of routinely predesigning frameworks to deliver desired properties. 3,4 MOFs are often compared to zeolites for their large internal surface areas, extensive porosity, and high degree of crystallinity. Correspondingly, MOFs and zeolites have been utilized for many of the same applications

Metal–Organic Framework Materials as Chemical Sensors

Metal–Organic Frameworks for Separations

Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis

Electrospinning is a versatile and viable technique for generating ultrathin fibers. Remarkable progress has been made with regard to the development of electrospinning methods and engineering of electrospun nanofibers to suit or enable various applications. We aim to provide a comprehensive overview of electrospinning, including the principle, methods, materials, and applications. We begin with a brief introduction to the early history of electrospinning, followed by discussion of its principle and typical apparatus. We then discuss its renaissance over the past two decades as a powerful technology for the production of nanofibers with diversified compositions, structures, and properties. Afterward, we discuss the applications of electrospun nanofibers, including their use as "smart" mats, filtration membranes, catalytic supports, energy harvesting/conversion/storage components, and photonic and electronic devices, as well as biomedical scaffolds. We highlight the most relevant and recent advances related to the applications of electrospun nanofibers by focusing on the most representative examples. We also offer perspectives on the challenges, opportunities, and new directions for future development. At the end, we discuss approaches to the scale-up production of electrospun nanofibers and briefly discuss various types of commercial products based on electrospun nanofibers that have found widespread use in our everyday life.

Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Metal oxide gas sensors: Sensitivity and influencing factors

The electrical conductivity, sigma, of donor-doped and undoped strontium titanate (SrTiO3) ceramics and, in some cases, single crystals, Sr1-xLaxTiO3 (0 ≤ to x ≤ 0.1), was investigated in the temperature range of 1000°-1400°C under oxygen partial pressures, PO2, of 10-20-1 bar. In conjunction with Hall data and thermopower data from related papers, a set of constants for a defect-chemical model was determined, precisely describing point-defect concentrations and transport properties of these materials. In contrast to former works, temperature-dependent transport parameters and their non-negligible influence on the determination of the constants was considered, as well as the equilibrium restoration phenomena of the cation sublattice, which can be studied only at such high temperatures. It was shown that defects in the cation sublattice completely govern the electrical behavior of donor-doped and undoped SrTiO3. In the latter case, frozen-in strontium vacancies act as intrinsic acceptors, determining the sigma(PO2) curves at lower temperatures. This intrinsic acceptor concentration also can be calculated with this model. The very good agreement between calculation and measurement is shown in many examples.

Defect Chemistry of Donor-Doped and Undoped Strontium Titanate Ceramics between 1000° and 1400°C

Several metal-organic framework (MOF) materials were under investigated to test their applicability as sensor materials for impedimetric gas sensors. The materials were tested in a temperature range of 120 °C - 240 °C with varying concentrations of O2, CO2, C3H8, NO, H2, ethanol and methanol in the gas atmosphere and under different test gas humidity conditions. Different sensor configurations were studied in a frequency range of 1 Hz -1 MHz and time-continuous measurements were performed at 1 Hz. The materials did not show any impedance response to O2, CO2, C3H8, NO, or H2 in the gas atmospheres, although for some materials a significant impedance decrease was induced by a change of the ethanol or methanol concentration in the gas phase. Moreover, pronounced promising and reversible changes in the electric properties of a special MOF material were monitored under varying humidity, with a linear response curve at 120 °C. Further investigations were carried out with differently doped MOF materials of this class, to evaluate the influence of special dopants on the sensor effect.

Metal-Organic Frameworks for Sensing Applications in the Gas Phase

A novel ammonia exhaust gas sensor for use in automotive urea SCR systems has been successfully developed. The sensor combines outstanding sensitivity and low cross sensitivities with fast and stable sensor responses. It is manufactured using the cost effective and reliable thick film technology that is already in use for automotive exhaust gas sensor applications.

Selective ammonia exhaust gas sensor for automotive applications

Nowadays, ceramic exhaust gas sensors are installed in quantities of millions in automotive exhaust gas systems. Almost any automobile being powered by a gasoline combustion engine is equipped with at least one zirconia exhaust gas oxygen sensor (λ probe) for detection of the air-to-fuel ratio λ. The first part of this short overview focuses on potentiometric as well as on amperometric zirconia exhaust gas oxygen sensors. It is remarkable that in the past years a leap in manufacturing technology has occurred from classical ceramic technology to tape and thick-film technology. The advent of novel combustion concepts like lean-burn operating gasoline direct injection required novel exhaust gas aftertreatment concepts. It pushed the development of the NOx sensor, which is manufactured in the same technology. It is also shown how development of exhaust gas sensors has always to be considered in interaction with exhaust gas aftertreatment systems. This elucidates why novel sensors have gained in importance just recently when stricter emission regulations were announced, meaning that the time is ripe for novel exhaust gas aftertreatment concepts. Appropriate sensors—ammonia sensors, hydrocarbon sensors, and particulate matter sensors—are still in the R&D stage. Several possible sensor principles are discussed. The materials that are used for sensors in the automotive exhaust are electroceramics. Besides ion-conducting zirconia and zirconia cermets, electrically insulating alumina is used for substrate purposes. Novel functional materials in the R&D state are strontium–iron titanate for temperature-independent resistive oxygen sensing and zeolites for selective detection of specific gases like hydrocarbons or ammonia.

A Brief Overview on Automotive Exhaust Gas Sensors Based on Electroceramics

The electrical conductivity of Sr1−xLaxTiO3 ceramics (x≤0.4) and single crystals (x≤0.1) was investigated in the temperature range between 19 and 1673 K. The mobility was calculated from the carrier concentration, n, which was determined by Hall measurements as well as by a chemical Ti3+ titration. The mobility of the single crystals agrees well with a model originally developed for undoped strontium titanate. At low T and high n scattering by ionized donor centers predominates. Above room‐temperature phonon scattering becomes predominant. Sr1−xLaxTiO3 ceramics follow the same model, but only if they had been previously reduced in water‐free and oxygen‐free hydrogen atmospheres. The behavior during the reducing and cooling process can be explained by a defect chemical model, using a set of constants developed in a former work. However, reduced in water saturated hydrogen the conductivity at low T decreases by decades compared to single‐crystal data and cannot be explained anymore by the above‐mentioned models. In conjunction with impedance spectroscopy experiments it was found, that in this case the electrical behavior of the ceramics is completely governed by grain‐boundary phenomena. These high‐ohmic grain boundaries may be depletion layers, which are formed or annihilated depending on the conditions of the preceding high‐temperature process.

Ralf Moos

Papers

Defect Chemistry of Donor-Doped and Undoped Strontium Titanate Ceramics between 1000° and 1400°C

Metal-Organic Frameworks for Sensing Applications in the Gas Phase

Selective ammonia exhaust gas sensor for automotive applications

A Brief Overview on Automotive Exhaust Gas Sensors Based on Electroceramics

Electronic transport properties of sr1-xlaxtio3 ceramics