Continuous-Flow Synthesis of Nitro-o-xylenes: Process Optimization, Impurity Study and Extension to Analogues.

An efficient continuous-flow nitration process of o-xylene at pilot scale was demonstrated. The effects of parameters such as temperature, ratio of H2SO4 to HNO3, H2SO4 concentration, flow rate, and residence time on the reaction were studied. Under the optimal conditions, the yield of products reached 94.1%, with a product throughput of 800 g/h. The main impurities of this continuous-flow nitration process were also studied in detail. Compared with batch process, phenolic impurity decreased from 2% to 0.1%, which enabled the omission of the alkaline solution washing step and thus reduced the wastewater emission. The method was also successfully applied to the nitrification of p-xylene, toluene, and chlorobenzene with good yields.

A review on two-dimensional materials for chemiresistive- and FET-type gas sensors.

Two-dimensional (2D) materials have shown great potential for gas sensing applications due to their large specific surface areas and strong surface activities. In addition to the commonly reported chemiresistive-type gas sensors, field-effect transistor (FET)-type gas sensors have attracted increased attention due to their miniaturized size, low power consumption, and good compatibility with CMOS technology. In this review, we aim to discuss the recent developments in chemiresistive- and FET-type gas sensors based on 2D materials, including graphene, transition metal dichalcogenides, MXenes, black phosphorene, and other layered materials. Firstly, the device structure and the corresponding fabrication process of the two types of sensors are given, and then the advantages and disadvantages are also discussed. Secondly, the effects of intrinsic and extrinsic factors on the sensing performance of 2D material-based chemiresistive and FET-type gas sensors are also detailed. Subsequently, the current gas-sensing applications of 2D material-based chemiresistive- and FET-type gas sensors are systematically presented. Finally, the future prospects of 2D materials in chemiresistive- and FET-type gas sensing applications as well as the current existing problems are pointed out, which could be helpful for the development of 2D material-based gas sensors with better sensing performance to meet the requirements for practical application.

Correction: Enhanced room temperature NO2 response of NiO-SnO2 nanocomposites induced by interface bonds at the p-n heterojunction.

Correction for 'Enhanced room temperature NO2 response of NiO-SnO2 nanocomposites induced by interface bonds at the p-n heterojunction' by Jian Zhang et al., Phys. Chem. Chem. Phys., 2016, 18, 5386-5396.

Enhanced room temperature NO2 response of NiO-SnO2 nanocomposites induced by interface bonds at the p-n heterojunction.

Recently, heterostructured nanomaterials have attracted great attention in gas sensing applications. However, the sensing mechanism of the enhanced sensitivity of heterostructured nanomaterials remains unclear, which is not conducive to further improvements in their sensing performances. In order to detail the fundamental studies on the gas sensing mechanism of heterostructured nanomaterials and improve the room temperature NO2 sensing properties of NiO-based nanomaterials, NiO-SnO2 heterojunction nanocomposites were fabricated. It was found that the sensitivity of the nanocomposites was largely enhanced compared to the bare NiO. On the basis of the intrinsic characteristics of the p-n heterojunction and the band structure of the NiO-SnO2 heterojunction, the largely enhanced room temperature NO2 response of the nanocomposites could be attributed to two factors. One was the significantly decreased initial conductance, and the increase in the equivalent hole concentration of the nanocomposites after exposure to NO2, associated with the effective electron transfer via the interface bonds at the heterojunction. Another was that the variation of contact potential in the nanocomposites, before and after exposure to NO2, exerted a drastic effect on the transducer function for gas sensing. According to the differentiation in the sensitivity of the nanocomposites with different molar ratios, the important role of interface bonds in gas sensing properties was further illustrated by the dependency of the sensitivity on the interface bond number and the interface resistance. Here, we hope that this work could give us a better understanding of the gas sensing mechanism of the p-n heterojunction, and provide a proper approach for heterojunction materials to further improve their sensing performances.

Mechanistic insights into formation of SnO2 nanotubes: asynchronous decomposition of poly(vinylpyrrolidone) in electrospun fibers during calcining process.

The formation mechanism of SnO2 nanotubes (NTs) fabricated by generic electrospinning and calcining was revealed by systematically investigating the structural evolution of calcined fibers, product composition, and released volatile byproducts. The structural evolution of the fibers proceeded sequentially from dense fiber to wire-in-tube to nanotube. This remarkable structural evolution indicated a disparate thermal decomposition of poly(vinylpyrrolidone) (PVP) in the interior and the surface of the fibers. PVP on the surface of the outer fibers decomposed completely at a lower temperature (<340 °C), due to exposure to oxygen, and SnO2 crystallized and formed a shell on the fiber. Interior PVP of the fiber was prone to loss of side substituents due to the oxygen-deficient decomposition, leaving only the carbon main chain. The rest of the Sn crystallized when the pores formed resulting from the aggregation of SnO2 nanocrystals in the shell. The residual carbon chain did not decompose completely at temperatures less than 550 °C. We proposed a PVP-assisted Ostwald ripening mechanism for the formation of SnO2 NTs. This work directs the fabrication of diverse nanostructure metal oxide by generic electrospinning method.

Graphene-Wrapped ZnO Nanocomposite with Enhanced Room-Temperature Photo-Activated Toluene Sensing Properties.

Graphene-wrapped ZnO nanocomposites were fabricated by a simple solvothermal technology with a one-pot route. The structure and morphology of these as-fabricated samples were systematically characterized. The adding of graphene enhanced the content of the oxygen vacancy defect of the sample. All gas-sensing performances of sensors based on as-prepared samples were thoroughly studied. Sensors displayed an ultrahigh response and exceptional selectivity at room temperature under blue light irradiation. This excellent and enhanced toluene gas-sensing property was principally attributed to the synergistic impacts of the oxygen vacancy defect and the wrapped graphene in the composite sensor. The photo-activated graphene-wrapped ZnO sensor illustrated potential application in the practical detection of low concentrations of toluene under explosive environments.

Chemically bonded graphene/BiOCl nanocomposites as high-performance photocatalysts.

After the successful solvothermal synthesis of graphene (GR) from ethanol and sodium, we obtained chemically bonded graphene/BiOCl (GR/BiOCl) nanocomposite photocatalysts via a facile chemical-bath method. A significant enhancement was observed in the photodegradation of methylbenzene, which was largely ascribed to the chemical coupling effects between Bi and C, as shown by X-ray photoelectron spectroscopy. Raman spectroscopy also indicated an increased size of the sp(2) ring clusters and decreased disorder in the graphitic structure, as substitutions of defects like vacancies as well as oxygen containing carbonaceous groups with C-Bi attachment take place. Overall, information about chemical coupling effects between GR and BiOCl might take us a step further in GR-based hybrid materials, providing a very good reference to the fabrication of chemically bonded GR/semiconductor compounds and facilitating their applications in environmental protection, photo-electrochemical conversion and photocatalytic decomposition of water.

Synthesis and characterization of tetrapod-like Cu-ZnO nanowhiskers prepared by photo-reduction technique.

Tetrapod-like Cu-ZnO nanowhiskers were synthesized by a photo-reduction technique in an aqueous solution containing Cu2+ ions. The XRD and TEM analyses revealed that the as-prepared ZnO nanowhiskers were covered by Cu coating or surrounded by Cu nanoparticles. Pure Cu nanoparticles were also produced in the solution by increasing the duration of xenon irradiation. These nanoparticles were amorphous and could be converted to crystalline structure by further electron beam irradiation. The photoluminescence measurement showed that the visible emission from the Cu-coated ZnO nanowhiskers had a blue shift when compared with that of the pure ZnO.

Room temperature formaldehyde sensors with enhanced performance, fast response and recovery based on zinc oxide quantum dots/graphene nanocomposites.

Novel zinc oxide quantum dots (ZnO QDs) decorated graphene nanocomposites were fabricated by a facile solution-processed method. ZnO QDs with a size ca. 5 nm are nucleated and grown on the surface of the graphene template, and its distribution density can be easily controlled by the reaction time and precursor concentration. The ZnO QDs/graphene nanocomposite materials enhance formaldehyde sensing properties by 4 times compared to pure graphene at room temperature. Moreover, the sensors based on the nanocomposites have fast response (ca. 30 seconds) and recovery (ca. 40 seconds) behavior, excellent room temperature selectivity and stability. The gas sensing enhancement is attributed to the synergistic effect of graphene and ZnO QDs. The electron transfer between the ZnO QDs and the graphene is due to oxidation process of the analyzed gas on the ZnO QDs' surface. This proposed gas sensing mechanism is experimentally proved by DRIFT spectra results. The ZnO QDs/graphene nanocomposites sensors have potential applications for monitoring air pollution, especially for harmful and toxic VOCs (volatile organic compounds).