NO2 and CO gas adsorption on carbon nanotubes: Experiment and theory

CO and NO selective adsorption by a C16Mg8O8 nanocage: A DFT Study

Density functional theory (DFT) calculations were performed to stabilize a representative C16Mg8O8 nanocage derived from C32 and Mg16O16 counterparts for selective adsorption of carbon monoxide (CO) and nitrogen monoxide (NO) gaseous molecules. After obtaining optimized structures, molecular features were evaluated for describing the model systems. Diagrams of density of states (DOS) revealed that the energy differences between frontier molecular orbital levels of the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) of the stabilized C16Mg8O8 nanocage could provide a more proper semiconductor in comparison with each of the original C32 and Mg16O16 cages. To explore the advantage of such C16Mg8O8 nanocage for CO and NO gases adsorption, molecular descriptors such as energies, geometries, and electronic structures were characterized for all possible adsorption configurations of bimolecular formation of gas . . .  nanocage. Significant changes of HOMO and LUMO levels besides the values of corresponding energy gaps of C16Mg8O8 nanocage in singular and bimolecular systems could help to recognize adsorption of each of CO and NO gaseous molecules. Furthermore, more variations of energy gaps in the process of gas . . .  nanocage bimolecular formation could lead to more sensitivity of nanocage for detection of adsorbed gases. As a consequence, the investigated C16Mg8O8 nanocage was introduced for differential recognition of CO and NO gases regarding several environmental health issues.

Role of Defect Engineering and Surface Functionalization in the Design of Carbon Nanotube-Based Nitrogen Oxide Sensors

Nitrogen oxides (NOx) are among the main atmospheric pollutants; therefore, it is important to monitor and detect their presence in the atmosphere. To this end, low-dimensional carbon structures have been widely used as NOx sensors for their outstanding properties. In particular, carbon nanotubes (CNTs) have been widely used as toxic-gas sensors owing to their high specific surface area and excellent mechanical properties. Although pristine CNTs have shown promising performance for NOx detection, several strategies have been developed such as surface functionalization and defect engineering to improve the NOx sensing of pristine CNT-based sensors. Through these strategies, the sensing properties of modified CNTs toward NOx gases have been substantially improved. Therefore, in this review, we have analyzed the defect engineering and surface functionalization strategies used in the last decade to modify the sensitivity and the selectivity of CNTs to NOx. First, the different types of surface functionalization and defect engineering were reviewed. Thereafter, we analyzed experimental, theoretical, and coupled experimental-theoretical studies on CNTs modified through surface functionalization and defect engineering to improve the sensitivity and selectivity to NOx. Finally, we presented the conclusions and the future directions of modified CNTs as NOx sensors.

Mechanical Performance and Applications of CNTs Reinforced Polymer Composites-A Review

Developments in the synthesis and scalable manufacturing of carbon nanomaterials like carbon nanotubes (CNTs) have been widely used in the polymer material industry over the last few decades, resulting in a series of fascinating multifunctional composites used in fields ranging from portable electronic devices, entertainment and sports to the military, aerospace, and automotive sectors. CNTs offer good thermal and electrical properties, as well as a low density and a high Young's modulus, making them suitable nanofillers for polymer composites. As mechanical reinforcements for structural applications CNTs are unique due to their nano-dimensions and size, as well as their incredible strength. Although a large number of studies have been conducted on these novel materials, there have only been a few reviews published on their mechanical performance in polymer composites. As a result, in this review we have covered some of the key application factors as well as the mechanical properties of CNTs-reinforced polymer composites. Finally, the potential uses of CNTs hybridised with polymer composites reinforced with natural fibres such as kenaf fibre, oil palm empty fruit bunch (OPEFB) fibre, bamboo fibre, and sugar palm fibre have been highlighted.

Titanium-benzene complex as a molecular oxide adsorbent: a first principles approach

CO, SO, NO, CO₂, SO₂, and NO₂ gas sensing properties of Ti-benzene (C₆H₆Ti) complex are studied with first principles calculations by analyzing change in structural parameters, electronic properties, and charge transfer. Adsorption of all six oxide molecules on C₆H₆Ti complex is found to be thermodynamically favorable at ambient conditions. The Gibbs free energy-corrected adsorption energy range for oxide molecules is found be 0.6-5.9 eV. The SO₂ transfers maximum charge to Ti metal, i.e., 0.36 (e^-) as compare to other oxides. The binding energy of Ti atom to benzene ring remains higher even after adsorption of oxide gas molecules. The higher values of HOMO-LUMO gap show that oxide-adsorbed complexes are chemically stable. The ADMP-MD simulations show that all oxide molecules remain adsorbed on Ti-benzene complex during the simulations for the temperature range 300-500 K. The Ti-benzene complex shows considerable gas sensing properties at ambient conditions.

Dry sonication process for preparation of hybrid structures based on graphene and carbon nanotubes usable for chemical sensors

The combination of graphene (G) and multi-walled carbon nanotubes (MWCNTs) creates three-dimensional hybrid structures particularly suitable as next-generation electrical interface materials. Nevertheless, efficient mixing of the nanopowders is challenging, unless previous disaggregation and eventual surface modification of both is reached. To avoid use of solvents and multistep purification process for synthesis of stable G/MWCNTs hybrids, herein, a novel dry method based on an air sonication process was used. Taking advantage from the vigorous turbulent currents generated by powerful ultrasonication in air that induces strong thermal convection or radiation to and from the particles, it simultaneously ensures disentanglement of the large MWCNT bundles and G exfoliation and their only mild surface modifications. By changing the ratio between MWCNTs and G, a range of hybrids was obtained, different in surface morphology and chemistry. These hybrids have shown great potential as sensing material for designing mass-based sensors for toxic gases and chemiresistor for vapors detection.

Encapsulation of monocyclic carbon clusters into carbon nanotubes: A continuum modeling approach

Carbon clusters are challenging to produce and isolate due to their highly reactive nature. One of the strategies for their isolation is to encapsulate the clusters into carbon nanotubes (CNTs) of appropriate radii. Herein, we have investigated the energetics for the encapsulation of the monocyclic carbon rings, [Formula: see text] ([Formula: see text], and [Formula: see text]) into CNTs of various radii using the continuum approximation. The encapsulation is driven by the non-covalent interactions between the carbon rings and the CNTs. The analyzes of the axial forces and the interaction energies at various orientations and positions of centers of mass of the rings with respect to the CNT axes clearly suggested the role of the tube radius in governing the energetics of encapsulation. Estimation of the acceptance and the suction energies as a function of CNT radius led to the prediction that the CNTs with radii of 5.38 Å, 5.83 Å, 6.25 Å, 6.68 Å, 7.07 Å, 7.51 Å, and 7.90 Å can efficiently encapsulate C10, C12, C14, C16, C18, C20, and C22 rings, respectively. In the limit of large tube radii, the numerical results lead to those obtained for carbon ring adsorption on graphene. Furthermore, the continuum approach enabled us to explore the potential energy surfaces thereby arriving at the equilibrium configurations of the rings inside the CNTs. Such an analysis is invaluable because of the enormous computational cost associated with quantum chemical calculations.

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

Nitrogen dioxide (NO2), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO2 sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials. Molybdenum disulfide (MoS2) has emerged as a potential candidate for developing next-generation NO2 gas sensors. MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

Buckybowls as gas adsorbents: binding of gaseous pollutants and their electric-field induced release

The adsorption of nitric oxide and nitrogen dioxide (NOx) to the Buckybowls sumanene and corannulene was investigated. Binding energies were up to 1.8× larger than for coronene as the planar analogue, demonstrating the advantages of Buckybowls for gas adsorption. In agreement with previous reports on carbon dioxide and methane adsorption, the favorable binding energies for NOx were shown to be associated with the curvature of the Buckybowls. It is shown that applying an electric field along the bowl symmetry axis modifies the bowl curvatures and impacts adsorbate binding energies, including the potential to desorb adsorbates. As a proof of concept, it is shown that applying electric fields of different strengths and orientations selectively controls sumanene's preference to bind nitric oxide, nitrogen dioxide, and carbon dioxide, suggesting potential applications for dynamically tunable gas adsorption. Moreover, it is demonstrated that adsorbates can be desorbed by applying suitable electric field strengths, allowing cleaning of the Buckybowls for renewed usage.

Molecular simulation of efficient removal of H2S pollutant by cyclodextrine functionalized CNTs

DFT-D3 calculations were carried out to investigate interaction of H2S and CH4 between numerous functionalized CNTs (f-CNTs), including hydroxyl, carboxyl, and cyclodextrin groups as potential candidates for selective adsorption and elimination of toxic pollutants. It was found that pristine CNTs as well as nanotube surface of functionalized CNTs cannot stably adsorb the H2S molecule (adsorption energy of -0.17 eV). However, H2S adsorption was significantly enhanced with different magnitudes upon the functionalization of CNT. For f-CNTs, H2S adsorption was accompanied by releasing energies in the range between -0.34 to -0.54 eV where the upper limit of this range belongs to the cyclodextrin-functionalized CNT (CD-CNT) as the consequence of the existence of both dispersion and electrostatic interactions between the adsorbate and substrate. Findings also demonstrated a significantly weaker interaction between CH4 and CD-CNT in comparison to the H2S molecule with adsorption energy of -0.14 eV. Electronic properties of the selected substrates revealed no significant changes in the inherent electronic properties of the CNTs after functionalizing and adsorbing the gas molecules. Moreover, DFTB-MD simulation demonstrated high adsorption capacity as well as CD-CNT ability for H2S molecules against the CH4 one under ambient condition.

Carbon Nanotube Chemical Sensors

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

XH3 (X=P or N) Adsorption on Pristine, Pt-Doped and Vacancy-Defective (8,8) Boron Nitride Nanotubes: DFT Calculations

The adsorption energies (E ad), interaction distances, changes of geometric and electronic structures of XH3 (X=P or N) gas molecule adsorption on pristine, platinum (Pt) doped and vacancy-defected single-walled (8,8) boron nitride nanotubes (BNNTs) have been calculated using the density functional theory (DFT). The effect of the Pt doping on B and N sites (PtB,N-doped) and the B and N vacancy defects (VB,N-defected BNNT) on the sensing behavior of pristine (8,8) BNNTs toward PH3 and NH3 gases have been examined. According to the obtained results, PH3 and NH3 molecules were more likely to be absorbed on the PtB,N-doped and VN-defected BNNT with relatively higher E ad compared with the pristine and VB-defected BNNTs. Therefore the order of the obtained E ad were PtB-doped BNNT/NH3>PtB-doped BNNT/PH3>PtN-doped BNNT/NH3>PtN-doped BNNT/PH3 for the PtB,N-doped BNNTs, and VN-defected BNNT/NH3>VN-defected BNNT/PH3>VB-defected BNNT/NH3>VB-defected BNNT/PH3 for the VB , N-defected BNNTs systems. The partial density of states (PDOS) of the adsorption systems indicated the strong interaction between the adsorbed PH3 and NH3 molecules and the substrates, i.e. PtB,N-doped BNNT and VN-defected BNNT. Therefore, it can concluded that the PtB,N-doped and VN-defected BNNTs have potential applicability in the gas-sensing detection of PH3 and NH3 with good sensitivity.

Lignocellulose-Chitosan-Multiwalled Carbon Nanotube Composites with Improved Mechanical Strength, Dimensional Stability and Fire Retardancy

A novel composite composed of lignocellulose (LC), glutaraldehyde crosslinked chitosan (GC) and multiwalled carbon nanotube (MWCNT) was fabricated by the hot-pressing process. The effect of the additional GC and MWCNT on the mechanical strength, dimensional stability and fire retardancy of lignocellulose composites was investigated. The results showed that LC/GC/MWCNT composite exhibited the maximum modulus of rupture (MOR) of 35.3 MPa, modulus of elasticity (MOE) of 2789.1 MPa and internal bonding (IB) strength of 1.2 MPa. Meanwhile, the LC/GC/MWCNT composite displayed improved dimensional stability with a thickness swelling (TS) value of 22.4%. Besides, the LC/GC/MWCNT composite exhibited improved fire retardancy with a limiting oxygen index of 29.0%. The peak heat release rate, the total heat release, the total smoke production and the maximum smoke production ratio of LC/GC/MWCNT composite decreased by 15.9%, 10.7%, 45.5% and 20.7% compared with those of LC composite, respectively. Therefore, the LC/GC/MWCNT composite may be a promising candidate for green wood based composites.

Quasi physisorptive two dimensional tungsten oxide nanosheets with extraordinary sensitivity and selectivity to NO2

Attributing to their distinct thickness and surface dependent physicochemical properties, two dimensional (2D) nanostructures have become an area of increasing interest for interfacial interactions. Effectively, properties such as high surface-to-volume ratio, modulated surface activities and increased control of oxygen vacancies make these types of materials particularly suitable for gas-sensing applications. This work reports a facile wet-chemical synthesis of 2D tungsten oxide nanosheets by sonication of tungsten particles in an acidic environment and thermal annealing thereafter. The resultant product of large nanosheets with intrinsic substoichiometric properties is shown to be highly sensitive and selective to nitrogen dioxide (NO₂) gas, which is a major pollutant. The strong synergy between polar NO₂ molecules and tungsten oxide surface and also abundance of active surface sites on the nanosheets for molecule interactions contribute to the exceptionally sensitive and selective response. An extraordinary response factor of ∼30 is demonstrated to ultralow 40 parts per billion (ppb) NO₂ at a relatively low operating temperature of 150 °C, within the physisorption temperature band for tungsten oxide. Selectivity to NO₂ is demonstrated and the theory behind it is discussed. The structural, morphological and compositional characteristics of the synthesised and annealed materials are extensively characterised and electronic band structures are proposed. The demonstrated 2D tungsten oxide based sensing device holds the greatest promise for producing future commercial low-cost, sensitive and selective NO₂ gas sensors.

Bio-Inspired Carbon Monoxide Sensors with Voltage-Activated Sensitivity

Carbon monoxide (CO) outcompetes oxygen when binding to the iron center of hemeproteins, leading to a reduction in blood oxygen level and acute poisoning. Harvesting the strong specific interaction between CO and the iron porphyrin provides a highly selective and customizable sensor. We report the development of chemiresistive sensors with voltage-activated sensitivity for the detection of CO comprising iron porphyrin and functionalized single-walled carbon nanotubes (F-SWCNTs). Modulation of the gate voltage offers a predicted extra dimension for sensing. Specifically, the sensors show a significant increase in sensitivity toward CO when negative gate voltage is applied. The dosimetric sensors are selective to ppm levels of CO and functional in air. UV/Vis spectroscopy, differential pulse voltammetry, and density functional theory reveal that the in situ reduction of FeIII to FeII enhances the interaction between the F-SWCNTs and CO. Our results illustrate a new mode of sensors wherein redox active recognition units are voltage-activated to give enhanced and highly specific responses.

Effects of Operating Temperature on Droplet Casting of Flexible Polymer/Multi-Walled Carbon Nanotube Composite Gas Sensors

This study examined the performance of a flexible polymer/multi-walled carbon nanotube (MWCNT) composite sensor array as a function of operating temperature. The response magnitudes of a cost-effective flexible gas sensor array equipped with a heater were measured with respect to five different operating temperatures (room temperature, 40 °C, 50 °C, 60 °C, and 70 °C) via impedance spectrum measurement and sensing response experiments. The selected polymers that were droplet cast to coat a MWCNT conductive layer to form two-layer polymer/MWCNT composite sensing films included ethyl cellulose (EC), polyethylene oxide (PEO), and polyvinylpyrrolidone (PVP). Electrical characterization of impedance, sensing response magnitude, and scanning electron microscope (SEM) morphology of each type of polymer/MWCNT composite film was performed at different operating temperatures. With respect to ethanol, the response magnitude of the sensor decreased with increasing operating temperatures. The results indicated that the higher operating temperature could reduce the response and influence the sensitivity of the polymer/MWCNT gas sensor array. The morphology of polymer/MWCNT composite films revealed that there were changes in the porous film after volatile organic compound (VOC) testing.

DFT simulation towards evaluation the molecular structure and properties of the heterogeneous C16Mg8O8 nano-cage as selective nano-sensor for H2 and N2 gases

Adsorption of hydrogen (H₂) and nitrogen (N₂) molecules was analyzed on a new fullerene-like C₁₆Mg₈O₈ nano-cage, composed of magnesium, oxygen, and carbon, using density functional theory. A detailed analysis of the energy, geometry, and electronic structure of various H₂ and N₂ adsorptions on the cluster surface was performed. The adsorption energies of H₂ and N₂ were estimated to ranging from -0.16 to -0.52eV, respectively. The most stable adsorption configurations were those in which the H or N atoms of the adsorbates were located near the Mg atom of the cluster surface at different sides. It was found that the heterogeneous C₁₆Mg₈O₈ nano-cluster selectively act against the H₂ and N₂ gaseous molecules. The electrical conductivity of the cluster, arising from HOMO/LUMO energy gap, was more sensitive to N₂ gaseous molecule rather than H₂ one, indicating that the heterogeneous C₁₆Mg₈O₈ nano-cage may be potential nano-sensor for N₂ molecule. These findings were specified by analyzing the characteristics in the electron density of states (DOS).

Shielding the chemical reactivity using graphene layers for controlling the surface properties of carbon materials

Graphene can efficiently shield chemical interactions and gradually decrease the binding to reactive defect areas. In the present study, we have used the observed graphene shielding effect to control the reactivity patterns on the carbon surface. The experimental findings show that a surface coating with a tiny carbon layer of 1-2 nm thickness is sufficient to shield the defect-mediated reactivity and create a surface with uniform binding ability. The shielding effect was directly observed using a combination of microscopy techniques and evaluated with computational modeling. The theoretical calculations indicate that a few graphene layers can drastically reduce the binding energy of the metal centers to the surface defects by 40-50 kcal mol(-1). The construction of large carbon areas with controlled surface reactivity is extremely difficult, which is a key limitation in many practical applications. Indeed, the developed approach provides a flexible and simple tool to change the reactivity patterns on large surface areas within a few minutes.

An Organocobalt-Carbon Nanotube Chemiresistive Carbon Monoxide Detector

A chemiresistive detector for carbon monoxide was created from single-walled carbon nanotubes (SWCNTs) by noncovalent modification with diiodo(η5: η1-1-[2-(N,N-dimethylamino)ethyl]-2,3,4,5-tetramethylcyclopentadienyl)-cobalt(III) ([Cp^CoI2]), an organocobalt complex with an intramolecular amino ligand coordinated to the metal center that is displaced upon CO binding. The unbound amino group can subsequently be transduced chemiresistively by the SWCNT network. The resulting device was shown to have a ppm-level limit of detection and unprecedented selectivity for CO gas among CNT-based chemiresistors. This work, the first molecular-level mechanistic elucidation for a CNT-based chemiresistive detector for CO, demonstrates the efficacy of using an analyte's reactivity to produce another chemical moiety that is readily transduced as a strategy for the rational design of chemiresistive CNT-based detectors.

Accelerating Gas Adsorption on 3D Percolating Carbon Nanotubes

In the field of electronic gas sensing, low-dimensional semiconductors such as single-walled carbon nanotubes (SWCNTs) can offer high detection sensitivity owing to their unprecedentedly large surface-to-volume ratio. The sensitivity and responsivity can further improve by increasing their areal density. Here, an accelerated gas adsorption is demonstrated by exploiting volumetric effects via dispersion of SWCNTs into a percolating three-dimensional (3D) network in a semiconducting polymer. The resultant semiconducting composite film is evaluated as a sensing membrane in field effect transistor (FET) sensors. In order to attain reproducible characteristics of the FET sensors, a pulsed-gate-bias measurement technique is adopted to eliminate current hysteresis and drift of sensing baseline. The rate of gas adsorption follows the Langmuir-type isotherm as a function of gas concentration and scales with film thickness. This rate is up to 5 times higher in the composite than only with an SWCNT network in the transistor channel, which in turn results in a 7-fold shorter time constant of adsorption with the composite. The description of gas adsorption developed in the present work is generic for all semiconductors and the demonstrated composite with 3D percolating SWCNTs dispersed in functional polymer represents a promising new type of material for advanced gas sensors.

Catalytic decomposition of gaseous 1,2-dichlorobenzene over CuOx/TiO₂ and CuOx/TiO₂-CNTs catalysts: Mechanism and PCDD/Fs formation

Gaseous 1,2-dichlorobenzene (1,2-DCBz) was catalytically decomposed in a fixed-bed catalytic reactor using composite copper-based titanium oxide (CuOx/TiO2) catalysts with different copper ratios. Carbon nanotubes (CNTs) were introduced to produce novel CuOx/TiO2-CNTs catalysts by the sol-gel method. The catalytic performances of CuOx/TiO2 and CuOx/TiO2-CNTs on 1,2-DCBz oxidative destruction under different temperatures (150-350 °C) were experimentally examined and the correlation between catalyst structure and catalytic activity was characterized and the role of oxygen in catalytic reaction was discussed. Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) generation during 1,2-DCBz catalytic oxidation by CuOx/TiO2-CNTs composite catalyst was also examined. Results indicate that the 1,2-DCBz destruction/removal efficiencies of CuOx (4 wt%)/TiO2 catalyst at 150 °C and 350 °C with a GHSV of 3400 h(-1) are 59% and 94% respectively and low-temperature (150 °C) catalytic activity of CuOx/TiO2 on 1,2-DCBz oxidation can be improved from 59 to 77% when CNTs are introduced. Furthermore, oxygen either in catalyst or from reaction atmosphere is indispensible in reaction. The former is offered to activate and oxidize the 1,2-DCBz adsorbed on catalyst, thus can be generally consumed during reaction and the oxygen content in catalyst is observed lost from 39.9 to 35.0 wt% after reacting under inert atmosphere; the latter may replenish the vacancy in catalyst created by the consumed oxygen thus extends the catalyst life and raises the destruction/removal efficiency. The introduction of CNTs also increases the Cu(2+)/Cu(+) ratio, chemisorbed oxygen concentration and surface lattice oxygen binding energy which are closely related with catalytic activity. PCDD/Fs is confirmed to be formed when 1,2-DCBz catalytically oxidized by CuOx/TiO2-CNTs composite catalyst with sufficient oxygen (21%), proper temperature (350 °C) and high concentration of 1,2-DCBz feed (120 ppm).

Hematite decorated multi-walled carbon nanotubes (α-Fe2O3/MWCNTs) as sorbents for Cu(ii) and Cr(vi): comparison of hybrid sorbent performance to its nanomaterial building blocks

Hybrid nanostructured sorbents were fabricatedviathe deposition and growth of hematite nanoparticles on carbon nanotubes, and fundamental aspects of their performance toward common heavy metal pollutants were evaluated.

Study on adsorption and desorption of ammonia on graphene

The gas sensor based on pristine graphene with conductance type was studied theoretically and experimentally. The time response of conductance measurements showed a quickly and largely increased conductivity when the sensor was exposed to ammonia gas produced by a bubble system of ammonia water. However, the desorption process in vacuum took more than 1 h which indicated that there was a larger number of transferred carriers and a strong adsorption force between ammonia and graphene. The desorption time could be greatly shortened down to about 2 min by adding the flow of water-vapor-enriched air at the beginning of the recovery stage which had been confirmed as a rapid and high-efficiency desorption process. Moreover, the optimum geometries, adsorption energies, and the charge transfer number of the composite systems were studied with first-principle calculations. However, the theoretical results showed that the adsorption energy between NH3 and graphene was too small to fit for the experimental phenomenon, and there were few charges transferred between graphene and NH3 molecules, which was completely different from the experiment measurement. The adsorption energy between NH4 and graphene increased stage by stage which showed NH4 was a strong donor. The calculation suggested that H2O molecule could help a quick desorption of NH4 from graphene by converting NH4 to NH3 or (NH3)n(H2O)m groups, which was consistent with the experimental results. This study demonstrates that the ammonia gas produced by a bubble system of ammonia water is mainly ammonium groups of NH3 and NH4, and the NH4 moleculars are ideal candidates for the molecular doping of graphene while the interaction between graphene and the NH3 moleculars is weak.

Influence of hexagonal boron nitride on the selective catalytic reduction of NO with NH3over CuOX/TiO2

hBN promoted NO conversion over CuO_X/TiO₂, hBN promoted the oxidation of NO to NO₂, hBN suppressed the NH₃oxidation, a de-NO_Xactivity of 90.6% was achieved at 275 °C and SO₂promoted the activity at 350 °C.

Fe2O3 nanoparticles anchored in situ on carbon nanotubes via an ethanol-thermal strategy for the selective catalytic reduction of NO with NH3

Fe₂O₃ nanoparticles anchored in situ on carbon nanotubes via an ethanol-thermal strategy present an excellent DeNO_x performance.

Advances in NO2 sensing with individual single-walled carbon nanotube transistors

The charge carrier transport in carbon nanotubes is highly sensitive to certain molecules attached to their surface. This property has generated interest for their application in sensing gases, chemicals and biomolecules. With over a decade of research, a clearer picture of the interactions between the carbon nanotube and its surroundings has been achieved. In this review, we intend to summarize the current knowledge on this topic, focusing not only on the effect of adsorbates but also the effect of dielectric charge traps on the electrical transport in single-walled carbon nanotube transistors that are to be used in sensing applications. Recently, contact-passivated, open-channel individual single-walled carbon nanotube field-effect transistors have been shown to be operational at room temperature with ultra-low power consumption. Sensor recovery within minutes through UV illumination or self-heating has been shown. Improvements in fabrication processes aimed at reducing the impact of charge traps have reduced the hysteresis, drift and low-frequency noise in carbon nanotube transistors. While open challenges such as large-scale fabrication, selectivity tuning and noise reduction still remain, these results demonstrate considerable progress in transforming the promise of carbon nanotube properties into functional ultra-low power, highly sensitive gas sensors.

Revealing the adsorption mechanisms of nitroxides on ultrapure, metallicity-sorted carbon nanotubes

Carbon nanotubes are a natural choice as gas sensor components given their high surface to volume ratio, electronic properties, and capability to mediate chemical reactions. However, a realistic assessment of the interaction of the tube wall and the adsorption processes during gas phase reactions has always been elusive. Making use of ultraclean single-walled carbon nanotubes, we have followed the adsorption kinetics of NO2 and found a physisorption mechanism. Additionally, the adsorption reaction directly depends on the metallic character of the samples. Franck-Condon satellites, hitherto undetected in nanotube-NOx systems, were resolved in the N 1s X-ray absorption signal, revealing a weak chemisorption, which is intrinsically related to NO dimer molecules. This has allowed us to identify that an additional signal observed in the higher binding energy region of the core level C 1s photoemission signal is due to the C ═ O species of ketene groups formed as reaction byproducts . This has been supported by density functional theory calculations. These results pave the way toward the optimization of nanotube-based sensors with tailored sensitivity and selectivity to different species at room temperature.

Low-temperature selective catalytic reduction of NO with NH₃ over nanoflaky MnOx on carbon nanotubes in situ prepared via a chemical bath deposition route

Nanoflaky MnO(x) on carbon nanotubes (nf-MnO(x)@CNTs) was in situ synthesized by a facile chemical bath deposition route for low-temperature selective catalytic reduction (SCR) of NO with NH₃. This catalyst was mainly characterized by the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N₂ adsorption-desorption analysis, X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR) and NH₃ temperature-programmed desorption (NH₃-TPD). The SEM, TEM, XRD results and N₂ adsorption-desorption analysis indicated that the CNTs were surrounded by nanoflaky MnO(x) and the obtained catalyst exhibited a large surface area as well. Compared with the MnO(x)/CNT and MnO(x)/TiO₂ catalysts prepared by an impregnation method, the nf-MnO(x)@CNTs presented better NH₃-SCR activity at low temperature and a more extensive operating temperature window. The XPS results showed that a higher atomic concentration of Mn(4+) and more chemisorbed oxygen species existed on the surface of CNTs for nf-MnO(x)@CNTs. The H₂-TPR and NH₃-TPD results demonstrated that the nf-MnO(x)@CNTs possessed stronger reducing ability, more acid sites and stronger acid strength than the other two catalysts. Based on the above mentioned favourable properties, the nf-MnO(x)@CNT catalyst has an excellent performance in the low-temperature SCR of NO to N₂ with NH₃. In addition, the nf-MnO(x)@CNT catalyst also presented favourable stability and H₂O resistance.

Detection of a CO and NH3 gas mixture using carboxylic acid-functionalized single-walled carbon nanotubes

Carbon nanotubes (CNT) are extremely sensitive to environmental gases. However, detection of mixture gas is still a challenge. Here, we report that 10 ppm of carbon monoxide (CO) and ammonia (NH3) can be electrically detected using a carboxylic acid-functionalized single-walled carbon nanotubes (C-SWCNT). CO and NH3 gases were mixed carefully with the same concentrations of 10 ppm. Our sensor showed faster response to the CO gas than the NH3 gas. The sensing properties and effect of carboxylic acid group were demonstrated, and C-SWCNT sensors with good repeatability and fast responses over a range of concentrations may be used as a simple and effective detection method of CO and NH3 mixture gas.