No abstract

Abstract:Graphite oxide has been investigated as a possible room-temperature chemiresistive sensor of ammonia in a gas phase. Graphite oxide was synthesized from high purity graphite using the modified Hummers method. The graphite oxide sample was investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, thermogravimetry and differential scanning calorimetry. Sensing properties were tested in a wide range of ammonia concentrations in air (10-1000 ppm) and under different relative humidity levels (3%-65%). It was concluded that the graphite oxide-based sensor possessed a good response to NH 3 in dry synthetic air (∆R/R 0 ranged from 2.5% to 7.4% for concentrations of 100-500 ppm and 3% relative humidity) with negligible cross-sensitivity towards H 2 and CH 4 . It was determined that the sensor recovery rate was improved with ammonia concentration growth. Increasing the ambient relative humidity led to an increase of the sensor response. The highest response of 22.2% for 100 ppm of ammonia was achieved at a 65% relative humidity level.

The influence of the interplay between central (Q C ) and secondary (Q S ) channel gas flow, as well as delivered microwave power (P MW ), during graphene nanosheet synthesis in a dualchannel electrode configuration of a microwave plasma torch at atmospheric pressure by ethanol decomposition was investigated. In the dual-channel configuration, plasma discharge can be sustained, even at high flow rates of ethanol, due to the separation of argon working and carrier gas. The plasma discharge instability was mainly influenced by an increase in the central channel flow, and a minor influence of secondary channel flow was also observed. With respect to the dependence on experimental conditions, the synthesized nanopowder consisted of amorphous carbon and nanocrystalline diamond nanoparticles, defective carbon nanosheets or few-layer graphene nanosheets. The synthesized nanosheets are rectangular in shape with a lateral size of several hundreds of nanometres and a few graphene layers thick, as shown by electron microscopy. Raman and x-ray photoelectron spectroscopy analysis of the synthesized nanosheets showed a good degree of graphitization, low oxygen content and increasing quality of graphene nanosheets with increasing microwave power. The number of defects in the synthesized nanosheets could be decreased by elongation of the graphene nanosheet assembly zone. An increase in the C 2 /C emission line intensity ratio correlated with a decrease in the number of defects in the graphene nanosheet structure. The achieved conversion yield of ethanol into graphene nanosheets was 8.3%, without negatively affecting the nanosheet quality.

The microwave (mw) plasma torch at atmospheric pressure has been studied for carbon nanotube (CNT) synthesis. The depositions were carried out on silicon substrates with 5-15 nm thin iron catalytic overlayers from the mixture of argon, hydrogen and methane. The optical emission spectroscopy of the torch showed the presence of C 2 and CH radicals as well as carbon and hydrogen excited atoms. The vicinity of the substrate influenced the relative intensities and increased the emission of C 2 . For fixed mw power, the temperature of the substrate strongly depended on its position with respect to the nozzle electrode and on the gas mixture, particularly the amount of H 2 . The speed of the substrate heating during an early deposition phase had a significant effect on the CNT synthesis. An abrupt increase of the temperature at the beginning increased the efficiency of the CNT synthesis. Areas of dense straight standing CNTs, 30 nm in average diameter, with approximately the same sized iron nanoparticles on their tops were found in accordance with the model of growth by plasma enhanced chemical vapour deposition. However, the deposit was not uniform and a place with only several nanometres thick CNTs grown on much larger iron particles was also found. Here, taking into account the gas temperature in the torch, 3100-3900 K, we can see similarities with the 'dissolution-precipitation' model of the CNT growth by high temperature methods, arc or laser ablation.

No abstract