Highly Sensitive and Selective Gas Sensors Based on Metal Iodates: Material Characterization and Sensor Performance Evaluation

This study proposes the possibility of employing metal iodates as novel gas-sensing materials synthesized using a facile chemical precipitation method. An extensive survey of a library of metal iodates reveals that cobalt, nickel, and copper iodates are useful for gas sensor applications. Material analysis conducted using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, thermal gravity differential temperature analysis, and Raman spectroscopy enables us to understand the thermal behavior and optimize post-annealing conditions. The evaluation of the gas-sensing performance of the specified metal iodates indicates that all of them display p-type sensing behavior and exhibit a high gas response toward different gases: a gas response of 18.6 by cobalt iodate to 1.8 ppm of acetone, a gas response of 4.3 by nickel iodate to 1 ppm of NO2, and a gas response of 6.6 by copper iodate to 1.8 ppm of H2S. Further investigation of the temperature-programmed reduction of H2 and polarization–electric field hysteresis analyses elucidates that the high gas response originates from the inherent characteristics of metal iodates, such as the high oxygen-reduction ability of iodine, highlighting the potential of the iodates as novel gas-sensing materials

Nitrogen-Doped Graphene for High-Performance Ultracapacitors and the Importance of Nitrogen-Doped Sites at Basal Planes

Although various carbon nanomaterials including activated carbon, carbon nanotubes, and graphene have been successfully demonstrated for high-performance ultracapacitors, their capacitances need to be improved further for wider and more challenging applications. Herein, using nitrogen-doped graphene produced by a simple plasma process, we developed ultracapacitors whose capacitances (∼280 F/gelectrode) are about 4 times larger than those of pristine graphene based counterparts without sacrificing other essential and useful properties for ultracapacitor operations including excellent cycle life (>200000), high power capability, and compatibility with flexible substrates. While we were trying to understand the improved capacitance using scanning photoemission microscopy with a capability of probing local nitrogen–carbon bonding configurations within a single sheet of graphene, we observed interesting microscopic features of N-configurations: N-doped sites even at basal planes, distinctive distributions of N-configurations between edges and basal planes, and their distinctive evolutions with plasma duration. The local N-configuration mappings during plasma treatment, alongside binding energy calculated by density functional theory, revealed that the origin of the improved capacitance is a certain N-configuration at basal planes

Supercapacitors of Nanocrystalline Metal–Organic Frameworks

The high porosity of metal–organic frameworks (MOFs) has been used to achieve exceptional gas adsorptive properties but as yet remains largely unexplored for electrochemical energy storage devices. This study shows that MOFs made as nanocrystals (nMOFs) can be doped with graphene and successfully incorporated into devices to function as supercapacitors. A series of 23 different nMOFs with multiple organic functionalities and metal ions, differing pore sizes and shapes, discrete and infinite metal oxide backbones, large and small nanocrystals, and a variety of structure types have been prepared and examined. Several members of this series give high capacitance; in particular, a zirconium MOF exhibits exceptionally high capacitance. It has the stack and areal capacitance of 0.64 and 5.09 mF cm^–2, about 6 times that of the supercapacitors made from the benchmark commercial activated carbon materials and a performance that is preserved over at least 10000 charge/discharge cycles

Regenerating MXene by a Facile Chemical Treatment Method

A popular substance in the MXene family, titanium carbide (Ti3C2Tx), has received substantial attention mainly due to its high metallic conductivity, easy solution processability, and environment friendliness. However, the poor oxygen resistance nature of MXene has prevented its practical applications from being realized. Despite significant attempts to improve the oxidative stability of MXenes, a comprehensive understanding of the oxidation mechanism is still elusive, thus leaving an optimal strategy for recycling oxidized MXene in question. Here, by developing a facile hydrofluoric acid (HF) post-treatment, we have unraveled the regeneration kinetics of the oxidized Ti3C2Tx. A systematic and extensive investigation using a combination of Raman spectroscopy, scanning electron microscopy, X-ray diffractometer, and X-ray photoelectron spectroscopy revealed that HF post-treatment is critical for restoring the structure/morphology and surface composition of MXene nanosheets. These are ascribed to the oxidizing agent removal kinetics, while the generation of amorphous carbon and Ti(III) in fluorinated derivatives provides efficient electrical conductivity. Our findings suggested that HF post-treatment is sufficient to evade and reduce the degradation process while maintaining the conductivity for a longer time, which will not only be economically advantageous but also a step forward for the rational design of Ti3C2Tx-based devices and functional coatings

Three-Dimensional Metal-Catecholate Frameworks and Their Ultrahigh Proton Conductivity

A series of three-dimensional (3D) extended metal catecholates (M-CATs) was synthesized by combining the appropriate metal salt and the hexatopic catecholate linker, H₆THO (THO^6– = triphenylene-2,3,6,7,10,11-hexakis­(olate)) to give Fe­(THO)·Fe­(SO₄) (DMA)₃, Fe-CAT-5, Ti­(THO)·(DMA)₂, Ti-CAT-5, and V­(THO)·(DMA)₂, V-CAT-5 (where DMA = dimethylammonium). Their structures are based on the <b>srs</b> topology and are either a 2-fold interpenetrated (Fe-CAT-5 and Ti-CAT-5) or noninterpenetrated (V-CAT-5) porous anionic framework. These examples are among the first catecholate-based 3D frameworks. The single crystal X-ray diffraction structure of the Fe-CAT-5 shows bound sulfate ligands with DMA guests residing in the pores as counterions, and thus ideally suited for proton conductivity. Accordingly, Fe-CAT-5 exhibits ultrahigh proton conductivity (5.0 × 10^–2 S cm^–1) at 98% relative humidity (RH) and 25 °C. The coexistence of sulfate and DMA ions within the pores play an important role in proton conductivity as also evidenced by the lower conductivity values found for Ti-CAT-5 (8.2 × 10^–4 S cm^–1 at 98% RH and 25 °C), whose structure only contained DMA guests

Three-Dimensional Metal-Catecholate Frameworks and Their Ultrahigh Proton Conductivity

A series of three-dimensional (3D) extended metal catecholates (M-CATs) was synthesized by combining the appropriate metal salt and the hexatopic catecholate linker, H₆THO (THO^6– = triphenylene-2,3,6,7,10,11-hexakis­(olate)) to give Fe­(THO)·Fe­(SO₄) (DMA)₃, Fe-CAT-5, Ti­(THO)·(DMA)₂, Ti-CAT-5, and V­(THO)·(DMA)₂, V-CAT-5 (where DMA = dimethylammonium). Their structures are based on the <b>srs</b> topology and are either a 2-fold interpenetrated (Fe-CAT-5 and Ti-CAT-5) or noninterpenetrated (V-CAT-5) porous anionic framework. These examples are among the first catecholate-based 3D frameworks. The single crystal X-ray diffraction structure of the Fe-CAT-5 shows bound sulfate ligands with DMA guests residing in the pores as counterions, and thus ideally suited for proton conductivity. Accordingly, Fe-CAT-5 exhibits ultrahigh proton conductivity (5.0 × 10^–2 S cm^–1) at 98% relative humidity (RH) and 25 °C. The coexistence of sulfate and DMA ions within the pores play an important role in proton conductivity as also evidenced by the lower conductivity values found for Ti-CAT-5 (8.2 × 10^–4 S cm^–1 at 98% RH and 25 °C), whose structure only contained DMA guests