Selective NO2 Gas Sensors Employing Nitrogen- and Boron-Doped and Codoped Reduced Graphene Oxide

In this paper, we develop high-performance gas sensors based on heteroatom-doped and -codoped graphene oxide as a sensing material for the detection of NO2 at trace levels. Graphene oxide (GO) was doped with nitrogen and boron by a chemical method using urea and boric acid as precursors. The prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The obtained results proved the successful reduction of graphene oxide by doping effects, leading to the removal of some oxygen functional groups and restoration of an sp2 carbon structure. New bonds in honeycombs, such as pyridinic, pyrrolic, graphitic, B-C3, B-C2-O, and B-O, were created. Compared to the nondoped GO, the N/B-rGO materials exhibited enhanced responses toward low concentrations of NO2 (<1 ppm) at 100 °C. Particularly, the N-rGO-based device showed the highest sensitivity and lowest limit of detection.

Pickering Emulsions of Fluorinated TiO2: A New Route for Intensification of Photocatalytic Degradation of Nitrobenzene

Fluorination of the TiO₂ surface has been often reported as a tool to increase the photocatalytic efficiency due to the beneficial effects in terms of production of oxidizing radicals. Moreover, it is shown that the unique amphiphilic properties of the fluorinated TiO₂ (TiO₂-F) surface allow one to use this material as a stabilizer for the formulation of Pickering emulsions of poorly soluble pollutants such as nitrobenzene (NB) in water. The emulsions have been characterized in terms of size of the droplets, type of emulsion, possibility of phase inversion, contact angle measurements, and optical microscopy. The emulsified system presents micrometer-sized droplets of pollutant surrounded by the TiO₂-F photocatalyst. Consequently, the system can be considered to be composed of microreactors for the degradation of the pollutant, which maximize the contact area between the photocatalyst and substrate. The enhanced photocatalytic activity of TiO₂-F was confirmed in the present paper as the apparent rate constants of NB photodegradation were 16 × 10^-3 and 12 × 10^-3 min^-1 for fluorinated and bare TiO₂, respectively. At NB concentrations largely exceeding its solubility, the rate constant was 0.04 × 10^-3 min^-1 in the presence of both TiO₂ and TiO₂-F. However, unlike TiO₂, TiO₂-F stabilized NB/water emulsions and, under these conditions, the efficiency of NB photocatalytic degradation in the emulsified system was ca. 18 times higher than in the nonemulsified one. This result is relevant also in terms of practical applications because it opens the route to one-pot treatments of biphasic polluted streams without the need of preliminary physical separation treatments.

Photocatalytic Degradation Enhancement in Pickering Emulsions Stabilized by Solid Particles of Bare TiO2

Pickering emulsions provide a new way to enhance the efficiency of photocatalytic degradation of water-insoluble pollutants. Indeed, the semiconductor solid particles dually act as the photocatalyst and stabilizer of the emulsion droplets whose size dramatically affects the photocatalytic reaction. The present work aims at the validation of this concept by using bare TiO₂ without any surface modification. Nanostructured TiO₂ has been prepared by a simple sol-gel process and characterized by X-ray diffraction, specific surface area analysis, scanning electron microscopy, and diffuse reflectance spectroscopy. The emulsions were prepared by using 1-methylnaphthalene (1-MN) as a model organic contaminant scarcely soluble in water and bare TiO₂ as the photocatalyst/stabilizer. The emulsions have been characterized by electrical conductivity, optical microscopy, and light-scattering analyses. The photocatalytic degradation of 1-MN was 50 times faster in stable Pickering emulsions with respect to the case of biphasic liquid systems containing TiO₂. This finding allows us to propose Pickering emulsions stabilized by TiO₂ nanoparticles as an effective and novel way to intensify the photocatalytic degradation of water-insoluble organic pollutants.

TiO2-PANI/Cork composite: A new floating photocatalyst for the treatment of organic pollutants under sunlight irradiation

A novel photocatalyst based on TiO₂-PANI composite supported on small pieces of cork has been reported. It was prepared by simple impregnation method of the polyaniline (PANI)-modified TiO₂ on cork. The TiO₂-PANI/Cork catalyst shows the unique feature of floating on the water surface. The as-synthesized catalyst was characterized by X-ray diffraction (XRD), scanning electron micrograph (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectra (UV-vis DRS) and the Brunauer-Emmett-Teller (BET) surface area analysis. Characterization suggested the formation of anatase highly dispersed on the cork surface. The prepared floating photocatalyst showed high efficiency for the degradation of methyl orange dye and other organic pollutants under solar irradiation and constrained conditions, i.e., no-stirring and no-oxygenation. The TiO₂-PANI/Cork floating photocatalyst can be reused for at least four consecutive times without significant decrease of the degradation efficiency.