Nikolina A. Travlou

Cu–BTC MOF–graphene-based hybrid materials as low concentration ammonia sensors

Ammonia sensing capability of previously developed hybrid materials consisting of Cu-based MOF and either graphite oxide or aminated graphite oxide was investigated for the first time. The chips were exposed to continuous cycles of three different ammonia concentrations, followed by purging with dry air. The change in a normalized resistance was measured. All chips showed an irreversible increase in the resistance when initially exposed to ammonia indicating the chemical reaction of the target with their components. This resulted in the collapse of the MOF component. After signal stabilization/equilibration the chips were further tested for ammonia sensing and a reversible increase in the resistance was observed for all samples. Even though the crystalline porous structure of the sensing materials was no longer present, the ability of the resulting amorphous phase to weakly adsorb ammonia enabled the recording of electrical signal changes. The specific structure of the hybrid materials combined with the proximity of the graphene phase, resulted in carrier mobility. A hybrid material with the smallest content of graphene phase exhibited the largest signal change upon exposure to ammonia. This was linked to the more developed MOF units in the case of this material, and therefore to the larger involvement of the amorphous phase released by the collapse of MOF in the sensing mechanism. A linear relationship between the response of the sensors and the ammonia concentration was found. Combining the adsorption capacity of the hybrid materials with an electrical signal allows their application as components of safety devices.

Activated carbon-based gas sensors: effects of surface features on the sensing mechanism

Ammonia sensing capability of a commercial wood-based activated carbon (BAX) and its oxidized counterpart (BAX-O) was studied. The materials were exposed to continuous cycles of various ammonia concentrations, followed by purging with air, and changes in a normalized resistance were analyzed. When initially exposed to the reducing gas, chemical interactions between ammonia and the carbons generated an irreversible signal change, which was stronger in the case of the initial carbon. After equilibration and reactive adsorption the sensors were further tested with ammonia. During the reversible sensing, the two carbons exhibited opposite trends in their signal. This indicates that oxidation changed the electrical properties of the carbon sample, leading to a different sensing mechanism. For the initial material holes play the predominant role in a current transport, while their depletion upon the exposure to the reducing gas leads to a decrease in the conductivity. In the case of the oxidized sample, the presence of electron donating nitro groups causes a hole annihilation in the carbon itself converting it to a predominantly n-type material. Here the exposure to ammonia increases the conductivity of the chip owing to the electron donating properties of the gas. Physical interactions, such as hydrogen bonding, polar interactions with surface functional groups, and dispersive interactions of ammonia with the carbon matrix, lead to the pore filling, which governs the extent of the response signal. The sensor signal changes linearly with the ammonia concentration.

Oxidized g‐C3N4 Nanospheres as Catalytically Photoactive Linkers in MOF/g‐C3N4 Composite of Hierarchical Pore Structure

A unique composite of the copper-based metal-organic framework (Cu-benzene tricarboxylic acid (BTC)) with oxidized graphitic carbon nitride nanospheres is synthesized. For comparison, a hybrid material consisting of g-C3 N4 and Cu-BTC is also obtained. Their surface features are analyzed using Fourier transform infrared spectroscopy, X-ray diffraction, sorption of nitrogen, thermal analysis, scanning electron microscopy, photoluminescence, and diffuse reflectance UV-Vis spectroscopy. The results suggest that the formed nanospheres of oxidized g-C3 N4 act as linkers between the copper sites, playing a crucial role in the composite building process. Their incorporation to the Cu-BTC framework causes the development of new mesoporosity. Remarkable alterations in the optical properties, as a result of the coordination of oxygen containing functional groups of the oxidized graphitic carbon nitride to the copper atoms of the framework, suggest an increase in photoreactivity. On the other hand, for the hybrid material consisting of Cu-BTC and g-C3 N4 , the unaltered pore volume and optical properties support the formation of a physical mixture rather than of a composite. The tests on reactive adsorption and detoxification of G-series organophosphate nerve agent surrogate show the enhanced performance of the composite as catalysts and photocatalyst in visible light.

Self-organized nanostructured materials of alkylated phthalocyanines and underivatized C60 on ITO

Clicking thiols onto a core fluorous phthalocyanine (Pc) platform yields alkyl and fluoroalkyl derivatives. The Zn(II) Pc with 16 thioalkanes self-organizes with fullerene C60 into nearly monodispersed nanoparticles when cast on ITO surfaces. Particle size and surface density is controlled by varying the Pc/fullerene ratio and concentration. Fluorescence quenching indicates electronic interaction between the component molecules in the films of nanoparticles. The strong supramolecular interactions between the Pc with 8 or 16 fluorous alkanes inhibit incorporation of the fullerene.