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Gas sensing properties of carbon nanomaterials
Abstract
The sensing properties of carbon nanomaterials are studied and compared in this paper. Graphene oxide and multi-walled carbon nanotube films were used as active layers of chemoresistive gas sensors and characterized at room temperature towards ammonia detection in concentration levels from 100 ppm to 500 ppm in air. We conclude that graphene oxide based sensor has higher response (2.4% to 7.4% in range from 100 ppm to 500 ppm) than the multi-walled carbon nanotubes based one (1.0-2.4%). On the other hand, the carbon nanotubes based sensor possessed higher recovery rates in a wide range of used concentrations.
In this work, the parameters of catalyst synthesis by solution combustion method using oxalic acid as a reducing agent, were investigated. The catalysts activity in the process of obtaining hydrogen and carbon nanofibers by the catalytic decomposition of methane has been determined. The effectiveness of using this reagent for the preparation of a nickel catalyst (90% Ni/10% Al2O3) that does not require preliminary reduction with hydrogen was shown. Based on the regression analysis, it was found that among the catalyst synthesis parameters, the yields of carbon and hydrogen are most strongly influenced by temperature.
Metal-organic frameworks (MOFs) are porous materials that can be used for drug storage. Recent studies have shown MIL-101 MOF as potential fluorouracil (5-FU) storage medium. Apart from the large pore volume, MIL-101 possesses three types of windows; triangular, pentagonal and hexagonal pores. The absorption mechanism of 5-FU inside MIL-101 is therefore crucial to develop the drug carrier. Herein, the absorption of 5-FU inside a MIL-101(Mg) MOF was studied by means of molecular docking and semi-empirical energy calculations with the aim of determining the best conformational and absorption energy of the drug molecule within the pores. According to the molecular docking, 5-FU preferred to be located inside the MIL-101(Mg) triangular windows with the highest binding affinity of -3.8 kcal.mol⁻¹. This result was due to the appropriate combination in the size of the pore and the 5-FU molecule. However, based on the obtained complexes from molecular docking, 5-FU when positioned near the pentagonal pore ([email protected]) produced the most energetically stable complex because of better interactions. This was confirmed by the lowest difference between the energy level of HOMO and LUMO for [email protected] The highest reactivity between 5-FU and pentagonal pore was further indicated by global hardness (η) and chemical softness (S) analysis.
Ammonia gas is described as toxic gas because exposure to ammonia gas by humans at high concentrations results in certain health risk factors. Therefore, it becomes imperative to develop a highly responsive gas sensor for the detection of ammonia gas. In Nigeria, there is lot of agricultural waste in the form of rice husk as a result of rice cultivation in the Northern part of the country which results in excessive ammonia gas production. Ammonia gas is produced as a result of the nitrogen present in fertilizers used on farms. In this paper, it is proposed to utilize rice husk ash nanoparticles as the gas sensing material for the detection of ammonia gas. The rice husk ash will be milled into different nano sizes, characterization of the nano particle will be carried out using Scanning Electron microscopy (SEM), RAMAN spectroscopy, Energy Dispersive X-ray (EDX) and electrical conductivity test. The sensor will then be fabricated using thick film fabrication technology on a paper substrate. The sensor will be exposed to ammonia gas of different concentrations using computerized gas sensing set up with Aalborg mass controllers. Results obtained from measurements are expected to give good sensitivity, selectivity, fast response and recovery times.
Graphite oxide is synthesized using various reaction parameters. Stagewise evaluation of graphite oxidation dynamics in the course of synthesis is carried out using the method of sample collection and thermogravimetric analysis. Thermally expanded graphite with high texture characteristics was obtained from graphite oxide. Properties of graphite oxide and thermally expanded graphite were determined using scanning electron microscopy, X-ray diffraction, thermogravimetry, differential scanning calorimetry, energy dispersive spectroscopy, and low-temperature adsorption of nitrogen. The values of capacity of supercapacitors based on graphite oxide and expanded graphite are found.
We have demonstrated a NH 3 , NO 2 and water vapour sensor based on poly(m-aminobenzene sulfonic acid) functionalized single-walled carbon nanotube (SWNT–PABS) networks. The SWNT–PABS based sensors were fabricated by simple dispersion of SWNT–PABS on top of pre-fabricated gold electrodes. SWNT–PABS sensors showed excellent sensitivity with ppb v level detection limits (i.e., 100 ppb v for NH 3 and 20 ppb v for NO 2) at room temperature. The response time was short and the response was totally reversible. The sensitivity could be tuned by adjusting the sensor initial resistance. The sensors were also suitable for monitoring relative humidity in air.
The preparation of graphitic oxide by methods described in the literature is time consuming and hazardous. A rapid, relatively safe method has been developed for preparing graphitic oxide from graphite in what is essentially an anhydrous mixture of sulfuric acid, sodium nitrate and potassium permanganate.
This work presents the results obtained on the single-step route towards the synthesis of iron oxide nanoparticles in a microwave plasma torch. The torch is supplied by 660 sccm of Ar mixed with 1 sccm of Fe(CO)5 and a variable amount of O2. The influence of oxygen addition on the phase composition of the synthesized powder was studied. Magnetite and maghemite phases could not be distinguished using the standard X-ray diffraction (XRD) analysis. Therefore, a combined XRD and Raman spectra analysis had to be applied, which is based on fitting of selected diffraction peaks and spectral features. According to XRD and Raman spectroscopy, the powder synthesized from Ar/Fe(CO)5 consisted about 50 % of magnetite, Fe3O4, the rest being α-Fe and FeO. An increase in oxygen flow rate led to an increase in γ-Fe2O3 percentage, at the expense of α-Fe, FeO and Fe3O4. Almost pure γ-Fe2O3 was synthesized at oxygen flow rates 25–75× higher than the flow rate of Fe(CO)5. A further increase in the oxygen flow rate led to α-Fe2O3 and ε-Fe2O3 production. The distributions of nanoparticles’ (NPs) diameters were obtained using transmission electron microscopy (TEM) and dynamic light scattering (DLS). The mean diameter of the NPs measured by TEM was 13 nm while the DLS measurements led to the mean diameter of 12 nm. About 90 % of all particles had the diameter in the range of 5–21 nm but a few larger particles were observed in TEM micrographs.
In this study, an ammonia gas sensor structure for mass production is realized with an array of carbon nanotubes synthesized on an anodic aluminum oxide (AAO) template. The device is expected to yield a uniform performance due to the regular and uniform nano channels in the AAO. It is a vertical structure made with open-ended multi-wall carbon nanotubes through deposition on the inside wall of the AAO so that the gas would diffuse into the nano channels for adsorption. The carbon nanotubes are in the form of flaked carbons having many defect sites. The results show that the sensor has high sensitivity, fast response time of less than 1min and good reproducible recovery behaviors in atmospheric pressure at room temperature.
The chemical reduction of graphite oxide (GO) to graphite by either NaBH4 or hydroquinone and also its surface modification with neutral, primary aliphatic amines and amino acids are described. Treatment of GO with NaBH4 leads to turbostatic graphite that upon calcination under an inert atmosphere is transformed to highly ordered graphitic carbon, while the reduction with hydroquinone yields directly crystalline graphite under soft thermal conditions. On account of the surface-exposed epoxy groups present in the GO solid, its surface modification with neutral, primary aliphatic amines or amine-containing molecules (amino acids and aminosiloxanes) takes place easily through the corresponding nucleophilic substitution reactions. In this way, valuable GO derivatives can be obtained, like molecular pillared GO, organically modified GO affording in organic solvents stable organosols or hydrophilic GO affording in water stable hydrosols and possessing direct cation exchange sites. The potential combination of surface modification and chemical reduction of GO in producing novel graphite based materials is also presented.
Single-walled carbon nanotubes (SWNTs) produced by arc-discharge have been researched as resistive gas sensors for H2 detection. Raw as well as modified single-walled carbon nanotubes were utilised as sensor material. Two types of modification treatments were carried out on the as grown SWNT material; chemical functionalization with palladium and doping with palladium by sputtering.CNTs were deposited on alumina substrates by airbrush. Two different alumina substrates (with and without heater element) were used to obtain the sensor devices.Conductance measurements were carried out in a N2 constant flow for H2 detection. The results demonstrate that the CNTs are p-type semiconductor materials, increasing their resistance with reducing gases. SWNTs functionalized with Pd show the best response when exposed to H2 at room temperature.