Diffusion Of Gas Molecules Research Articles

Solid-liquid interfacial nanobubble nucleation dynamics influenced by surface hydrophobicity and gas oversaturation

Over the past two decades, extensive research efforts have been devoted to exploring the existence, stability, and applications of interfacial nanobubbles (INBs). However, investigations into the microscopic nucleation process of INBs have been relatively limited. In this study, we utilized molecular dynamics simulations to elucidate the nucleation dynamics of INBs, with a particular focus on examining the influence of surface hydrophobicity and gas oversaturation. Our findings revealed a distinct preference for INBs to form on hydrophobic surfaces compared to hydrophilic ones, which agrees well with the experimental results. Temporal evolution analysis of the interfacial density of gas and liquid near the solid–liquid interface indicated a gradual enrichment of gas molecules at the hydrophobic surface, accompanied by a gradual disruption of the hydration layer, phenomena not observed on hydrophilic surfaces. The affinity of gas molecules towards the hydrophobic surface was further confirmed by potential of mean force (PMF) analysis, which demonstrated a decrease in the energy barrier and an increase in the potential well with increasing surface hydrophobicity. Moreover, increasing bulk gas supersaturation and surface hydrophobicity both contribute to shortening the INB nucleation time, primarily due to the enhanced gas enrichment rate. Further studies indicated that the gas enrichment rate had a directly proportional linear relationship with the bulk gas concentration. However, as the surface hydrophobicity increased, the gas enrichment rate initially rose rapidly and then entered a plateau phase. This may be because, when surface hydrophobicity is strong, the gas enrichment rate becomes limited by the diffusion of gas molecules in the liquid. These findings offer valuable insights into the nucleation mechanism of INBs on hydrophobic surfaces under gas oversaturation, contributing to a deeper understanding of their behavior and potential applications.

Ultrasensitive and highly selective NO2 gas sensing of porous MXene nanoribbon assemblies

Nitrogen dioxide (NO2), a potential source for acid rain and photochemical smog, is known to harm human health, plant growth and crop yield, and the environment. Therefore, designing a room temperature operable NO2 gas sensor with exceptional sensitivity and selectivity is extremely important. Herein, we report that Ti3C2Tx MXene (Tx = –OH, –O, –F, etc.) nanoribbons containing inbuilt anatase TiO2 on their surfaces achieve highly sensitive detection of NO2 gas at room temperature with negligible cross-responses to other gas analytes. The MXene nanoribbons' ability to form porous gas sensor film facilitates the efficient diffusion of gas molecules and the sufficient utilization of surface-active sites, leading to a high response of −87.5 % to 50 ppm NO2 gas exposure with a response time of 9.2 s at room temperature. Signifying the low concentration NO2 detection abilities, MXene nanoribbons displayed a response of −4.4 % towards 500 ppb NO2 exposure. MXene nanoribbons' NO2 gas sensing ability is carrier gas dependent and declines monotonically as operating temperature increases. In addition, the performance comparison experiments conducted at 50 ppm NO2 gas exposure suggest that the MXene nanoribbons' response is 14.3, 1.6, and 45.3 times higher than the responses of multilayer MXene, MXene-TiO2 nanoplate, and MXene-TiO2 nanoparticle structures, respectively. The explicit selectivity and high sensitivity of MXene nanoribbons toward NO2 gas are expected to provide exciting opportunities for personal safety and environmental monitoring.

Three-dimensional Ni foam supported Pt/NiFe LDH catalyst with enhanced oxygen activation for room-temperature formaldehyde oxidation

Room-temperature catalytic oxidation of formaldehyde (HCHO) has been extensively investigated due to its high efficiency, convenience, and environmental friendliness. Herein, nickel-iron layered double hydroxide (NiFe LDH) nanosheets were synthesized in-situ on a nickel foil (NF) using a facile one-step hydrothermal method, followed by the deposition of ultra-low content (0.069 wt%) of Pt nanoparticles through NaBH4 reduction. The resulting three-dimensional (3D) hierarchical Pt/NiFe-NF catalyst exhibited exceptional activity for the complete decomposition of formaldehyde to carbon dioxide (CO2) at room temperature (∼95 % conversion within 1 h), as well as remarkable cycling stability. The 3D porous structure of Pt/NiFe-NF provides fast transport channels for the diffusion of gas molecules, making the active catalyst surfaces more accessible. Moreover, abundant hydroxyl groups in NiFe LDH serve as adsorption centers for HCHO molecules to form dioxymethylene (DOM) and formate intermediates. Furthermore, electronic interactions between NiFe LDH and Pt enhance the adsorption and activation of O2 on Pt surfaces, leading to the complete decomposition of intermediates into non-toxic products. This work presents new insights into the design and preparation of Pt-based 3D hierarchical catalysts with surface-rich hydroxyl groups for the efficient removal of indoor HCHO.

Molecular design of free volume structure in thermally rearranged polybenzoxazole membranes for gas separation studied by positron annihilation

AbstractIn this work, 2,2‐bis(3‐amino‐4‐hydroxyphenyl) hexafluoropropane (APAF) was used as the fixed diamine monomer, and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4′‐oxydiphthalic anhydride (ODPA), 3,3′,4,4′‐biphenyltetracarboxylic diandhydride (BPDA) and pyromellitic dianhydride (PMDA) were selected as four different dianhydride monomers, respectively, to prepare the thermally rearranged polybenzoxazole (TR‐PBO) gas separation membranes with different structures via a thermal rearrangement reaction at 450°C. The microstructure of TR‐PBO membranes was investigated using X‐ray diffraction and positron annihilation spectroscopy. All the TR‐PBO membranes exhibit bimodal free volume structure, containing ultramicropores (<0.7 nm) and micropores (<2 nm) with size of 3.6–4.6 and 7.6–8.4 Å, respectively. As the steric hindrance of the bridging groups in the dianhydride monomers increases, the radius and number of pores in the membranes increases, leading to an increase in fractional free volume (FFV). The pure gas permeation test shows a positive correlation between gas permeability and FFV in the TR‐PBO membranes, indicating that an increase in FFV is favorable for the diffusion of gas molecules. Particularly, the ultramicropores play a decisive role on the gas selectivity. All the TR‐PBO membranes exhibit excellent gas permeaselectivity. Among them, the H2/CH4 separation performance of APAF‐6FDA and APAF‐ODPA membranes approaches the 2008 Robeson upper bound, which is due to the high number of both micropores and ultramicropores. Our results demonstrate that the free volume structure of polymer membranes can be designed by selecting monomers with different structures and utilizing thermal rearrangement reactions, allowing for the preparation of gas separation membranes with high permeability and selectivity.

Hierarchical porous MOF/CTF hybrid frameworks used as protection against acidic harmful gases

Benefiting from the structural diversity, high porosity, controllable pore and functional design, porous organic frameworks (POPs) are attracting great attention in adsorption areas. In particular, metal–organic frameworks (MOFs) and covalent-organic frameworks (COFs) have exhibited fascinating potential in gas adsorption. However, in practical application, because the dominant micropores are unfavorable to the transport and diffusion of gas molecules, coupled with the unstable structure stemmed from weak coordination bonds, MOFs are restricted, while COFs are limited by the lack of sufficient active sites. The complementary advantages between MOFs and COFs provide ideas for designing novel hybrid frameworks. Here, a covalent triazine-based framework (CTF) was chosen as skeleton to in situ synthesize Zr-MOF-NH2, constructing an interpenetrated MOF/CTF hybrid. Further, Cu2+ ions were introduced to endow the hybrid with more active centers, resulting in a hierarchical porous Zr-MOF-NH2/CTF-Cu2+. The synergistic effect between MOF and CTF, coupled with Cu2+ active centers, endows the Zr-MOF-NH2/CTF-Cu2+ hybrid with exceptional adsorption performance for harmful acidic gases. The adsorption capacities of Zr-MOF-NH2/CTF-Cu2+ for SO2 and NO2 are 39.3 mg g−1 and 27.3 mg g−1, which are 2.6 and 3.1 times as high as that of Zr-MOF-NH2, respectively. Meanwhile, the breakthrough times of Zr-MOF-NH2/CTF-Cu2+ for SO2 and NO2 are 252.8 and 187.6 min, which are approximately 1.9 and 2.3 times as long as that of pristine MOF, respectively.

Vacancy–assisted exposed Sn atoms enhancing NO2 room temperature sensing of SnSe2 nanoflowers

NO2 is a hazardous gas extremely harmful to the ecosystem and human health, so effective detection of NO2 is critical. SnSe2 is a promising candidate for gas sensors owing to its unique layered configuration that facilitates the diffusion of gas molecules. Here, ultrathin self–assembled nanoflowers F–SnSe2 rich in defects were synthesized by a simple solvothermal method. It exhibits excellent gas sensing performances for NO2 at room temperature (25 °C), with a high gas sensing response of 8.6 for 1 ppm NO2 and a lower detection limit as low as 200 ppb, capable of sensitively detecting ppb–level NO2. DFT calculations revealed that the presence of Se vacancies assists the central Sn atoms to break through the shielding effect of the surface Se atoms and become exposed active sites. The higher reactivity leads to more charge transfer and higher adsorption energy, which strongly promoted the adsorption of NO2. This work verifies the important role of vacancies for the exposed active sites and provides new guidance for defect engineering to modulate the gas sensing performances of SnSe2.

Hierarchical PVDF/ZnO/Ag/ZIF-8 nanofiber membrane used in trace-level Raman detection of H2S

surface enhanced Raman scattering (SERS) detection of gases has always been difficult due to the low affinity and poor Raman cross section of the moving molecules. To mitigate the impact of these problems on detection of gases, a structure of zinc oxide/silver nanowires coated with zeolitic imidazolate framework-8 (ZnO NWs/Ag/ZIF-8) was constructed on polyvinylidene fluoride (PVDF) nanofiber membrane (PVDF/ZnO NWs/Ag/ZIF-8) and in detail researched in this work. Benefitting from the quadruple synergistic effect of efficient Knudsen diffusion of gas molecules inside ZIF-8, enrichment of ZIF-8 microsponges for gaseous molecules, regulation of ZIF-8 dielectric layer for light and reverse light scattering of ZnO NW/Ag tip, the structure was proven to have precise co-confinement on both hot spots and gaseous molecules. As a result, this PVDF/ZnO NWs/Ag/ZIF-8 achieved excellent detection for hydrogen sulfide (H2S), with a limit of detection of 1 × 10−10 v/v and the minimum relative standard deviation value of ca. 7.13 %. Furthermore, as a proof of concept, in practical application, we designed and assembled our substrate (3.5 cm × 3.5 cm) into a SERS face mask and realized efficient monitoring of H2S in human’s exhaled breath.

Effects of leucine on hydrate formation: A combined experimental and molecular dynamics study

Compared to conventional natural gas storage and transportation methods, hydrates offer a safer, more compact alternative with broad future potential. To enhance hydrate formation, there has been considerable interest in environmentally friendly and cost-effective amino acid promoters, such as leucine. This study utilized a transparent vessel for CH4 hydrate formation experiments, identifying the optimal 0.5–0.6 wt% leucine concentration, which led to enhanced gas consumption, shorter induction time, and maximum gas storage density. In the leucine system, hydrates exhibited distinctive branching growth on container walls, both upward and downward. The formation of numerous microchannels amplified capillary effects, enhancing gas–liquid contact and fostering hydrate formation. Simultaneously, isobaric-isothermal molecular dynamics (MD) simulations integrated system growth configurations, structural parameters, radial distribution functions, energy barriers and mean square displacements, unveiling leucine’s microscopic impact on hydrate formation. Simulation results unequivocally show that leucine significantly accelerates CH4 hydrate growth by momentarily adsorbing at the solid–liquid interface, expediting water molecule ordering into cage-like structures. Leucine also enhances water molecule structural order around the mobile leucine molecule. Moreover, leucine lowers the energy barrier for methane transitioning from gas to liquid, facilitating gas molecule diffusion into the liquid phase and affording plenty interaction time for water and methane molecules, thereby providing favorable conditions for hydrate formation. These findings provide crucial perspectives for future research in the field of eco-friendly hydrate promoters, paving the way for innovative solutions in natural gas storage and transportation.

Efficient separation of CO2/CH4 by covering ultrathin ionic liquid film on COF membrane

Covalent organic framework (COF) membranes show great potential for gas separation, but their large pore size make it hard to effectively separate gases with similar kinetic diameters. A promising strategy is to cover the surface of COF membranes with an ultrathin film of ionic liquids (ILs) to improve the selectivity of gas permeation. Here, we investigated the CO2/CH4 separation performance of composite membranes composed of COF (CTF-1) and 20 kinds of ILs, respectively, using molecular simulations. The [C4mim][PF6]/COF membrane is found to exhibit the best separation performance, with CO2 permeability up to 7.93 × 104 GPU and CO2/CH4 selectivity of 12.33 when the IL thickness is 8 Å. By studying the microstructure of the composite membrane, it was found that there is a relatively dispersed distribution of cations and anions on the COF surface, which is beneficial to the diffusion of gas molecules. The gas permeation process results reveal that CO2 rapidly achieves equilibrium in both the adsorption and dissolution layers simultaneously, thereby enhancing the permeation rate of gas molecules. The analysis of interaction energy and PMF indicates that the improved selectivity is due to the stronger interaction between IL and CO2 than that for CH4.

Competitive adsorption-diffusion coupling process of helium-nitrogen mixture in shale kerogen nano-slit

Helium, as a probe gas with low adsorption capacity, is widely used for measuring porosity, permeability, and specific surface area in reservoir rocks of unconventional gas such as shale and coal. The pretreatment duration of these rock samples determines the amount of residual nitrogen in their pores, and the competitive adsorption and diffusion of residual nitrogen with the probe gas has a significant impact on the measurement results of their physical properties. This work aims to reveal the mechanisms involved in the competitive adsorption-diffusion coupling process of nitrogen-helium mixtures. Brief experiments were conducted to demonstrate the competitive adsorption between nitrogen and helium in shale. Subsequently, molecular dynamics simulation was employed to investigate this process and provide necessary details. Meanwhile, a theoretical model was developed to describe the diffusion of gas molecules in this process. Our results indicate that helium rapidly permeates shale samples and reaches an equilibrium distribution. However, the competitive adsorption between nitrogen and helium can slow down the helium permeation process, which can be attributed to the main adsorption sites occupied by nitrogen. The agreement between FEM computations and MD simulations validates our model for various mechanisms. Furthermore, our model can be applied to a wider range of porous materials and gas mixtures in related fields.

Controlled adsorption of gas molecules by tuning porosity of titanium film

Within microelectromechanical system sensors, the establishment of a vacuum environment is a prerequisite for the control of specific residual gas molecules. At the wafer-level package stage, the interior of the sensor can be easily converted into a vacuum environment. However, after packaging, degassing occurs due to the accumulation of fumes with additional processing, resulting in a significant reduction in sensor reliability. To counteract this, non-evaporable getter (NEG) film is commonly packaged together with the sensor to absorb the outgassing gas molecules and maintain a vacuum environment within the sensor. Most NEG films require an activation process to migrate the adsorbed gas molecules from the surface to the bulk by thermal annealing. Recently, NEG films have been considered to reduce the activation temperature and time to avoid heat damage. Depositing an anti-oxidant layer on NEG film or alloying the NEG film with metallic materials through co-sputtering to create a distinct valence state during activation was found to prevent further oxidation of NEG film. However, these methods require expensive materials and fabrication equipment. In this study, we demonstrate that a much lower activation temperature (T = 350 °C) and time (t = 10 min) for Ti NEG film can be achieved by controlling the surface morphology depending on the deposition method and condition, without requiring further treatment such as the deposition of a capping layer or co-sputtering. Increasing the grain size of the Ti NEG film results in a larger surface area, which enables more efficient adsorption of gas molecules. Additionally, higher porosity in the film increases the diffusion of gas molecules, thus enhancing the overall gas adsorption capacity. Our experiments show that the Ti NEG film, which was deposited at 7.8 Å s−1 using a sputtering method, exhibited a grain size of 411 nm and a surface roughness of 59.185 nm. Furthermore, after an activation process at 350 °C for 10 min, the atomic ratio of the adsorbed gas molecules was 23.14%.

A Nanoporous Polymer Modified with Hexafluoroisopropanol to Detect Dimethyl Methylphosphonate.

The increasing threat of nerve agents has prompted the need for gas sensors with fast response, high sensitivity, and good stability. In this work, the hexafluoroisopropanol functional group was modified on a porous aromatic framework material, which served as a sensitive material for detecting dimethyl methylphosphonate. A nerve agent sensor was made by coating sensitive materials on a surface acoustic wave device. Lots of pores in sensitive materials effectively increase the specific surface area and provide channels for diffusion of gas molecules. The introduction of hexafluoroisopropanols enables the sensor to specifically adsorb dimethyl methylphosphonate and improves the selectivity of the sensor. As a result, the developed gas sensor was able to detect dimethyl methylphosphonate at 0.8 ppm with response/recovery times of 29.8/43.8 s, and the detection limit of the gas sensor is about 0.11 ppm. The effects of temperature and humidity on the sensor were studied. The results show that the baseline of the sensor has a linear relationship with temperature and humidity, and the temperature and humidity have a significant effect on the response of the sensor. Furthermore, a device for real-time detection of nerve agent is reported. This work provides a new strategy for developing a gas sensor for detecting nerve agents.

Effect of metal ion intercalation on the gas diffusion confined in two-dimensional MXene nanosheets

Designing and regulating the metal ion adsorption sites is an effective strategy to improve the gas separation performance of porous materials. The diffusion behavior of gas molecules (H2, H2O, CO2, N2, CH4, SO2 and COS) confined in ion intercalated (Li+, Na+, K+, Rb+, Cs+, Mg2+ and Al3+) MXene nanoslits was studied by molecular dynamics method. The results showed that, due to the increase of steric hindrance after ion intercalation, the diffusion of gas molecules in confined nanoslits is subject to varying degrees of restriction. The diffusion coefficients of CO2, SO2 and COS, which interact strongly with metal ions, are significantly reduced, thus significantly improving the diffusion selectivity between these molecules and weakly interacting molecules (H2, N2 and CH4). In particular, Al3+, which has a high charge number and a small ionic radius, has a significant effect on improving the diffusion selectivity of mixed gases. The effect of metal ion intercalated MXene on the selectivity of the gas mixture is the result of the synergistic effect of adsorption and steric hindrance.

Effect of kinetic properties of zeolites on the gas-sensing performance of zeolite-covered SnO2 sensors

ABSTRACT Zeolite additional layers coating on metal oxide surfaces to improve the selectivity and response values of the sensors are effective and widely used. The specific adsorption and diffusion of gas molecules in the zeolite are recognised to play an important role in gas sensing improvement. In this work, the adsorption and diffusion behaviours of formaldehyde and acetone in H-Y and H-A zeolites were calculated by Monte Carlo and molecular dynamics simulations. The loading of acetone is greater than formaldehyde in these two kinds of zeolites under the same temperature and pressure, and the water load is lower. The simulation results show that the effects of the nature of the guest molecule and steric hindrance of the diffusing molecule on diffusivity are significant. The diffusivity in H-A zeolite decreases by several orders of magnitude than that in H-Y zeolite However, in H-A zeolite, the internal diffusion dominates, and only the smaller diameter formaldehyde can diffuse between the pores, which is beneficial to improving the sensing properties of formaldehyde. In addition, in the competition diffusion with water molecules, formaldehyde dominates than acetone. The H-Y/SnO2 and H-A/SnO2 sensors were prepared to verify the reliability of the molecular simulations. Research highlights The adsorption and diffusion behaviour of formaldehyde and acetone in H-Y and H-A zeolites were investigated using molecular simulation. Molecular diameter mainly influences the kinetic behaviour. The sensitisation mechanism of H-Y/SnO2 sensor to acetone and H-A/SnO2 sensor to formaldehyde was proved. Focuses on the relationship between gas-sensing the performance of zeolite-covered SnO2 sensors and the adsorption.

Utilizing Gelatin Waste for Efficient Bimodal Porous Silica Adsorbents for Carbon Dioxide Capture.

This study explores the modification of pore structures in porous silica materials synthesized using sodium silicate and waste gelatin, under varying silica-to-gelatin ratios. At ratios of 1.0-1.5, bimodal porous silica with mesopores and macropores emerged due to spaces between silica nanoparticles and clusters, following gelatin elimination. The study further evaluated the obtained bimodal porous silica as polyethyleneimine (PEI) supports for CO2 capture, alongside PEI-loaded unimodal porous silica and hollow silica sphere for comparison. Notably, the PEI-loaded bimodal silica showcased superior CO2 uptake, achieving 145.6 mg g-1 at 90 °C. Transmission electron microscopy (TEM) revealed PEI's uniform distribution within the pores of bimodal silica, unlike the excessive surface layering seen in unimodal silica. Conversely, PEI completely filled the hollow porous silica's interior, extending gas molecule diffusion distance. All sorbents displayed nearly constant CO2 adsorption across 20 cycles, demonstrating outstanding stability. Notably, the bimodal porous silica displayed a negligible capacity loss, underscoring its robust performance.

Gas diffusion in cement pastes: An analysis using a fluctuating diffusivity model

This study proposes applying the concept of fluctuating diffusivity (FD) to the diffusion of gas molecules in cementitious materials, mainly through a two-state fluctuating diffusivity (2SFD) model. The 2SFD model was utilized to investigate oxygen diffusion in cement pastes. This analysis provides a reasonable description of the oxygen diffusion coefficient in cement pastes and suggests the presence of non-Gaussian diffusion, which is attributed to the heterogeneous microstructure. The presence of non-Gaussianity in the probability density of the displacement of the molecule, characterized by heavier tails than those of the Gaussian distribution, may significantly impact the durability of concrete structures.

The effects of rhamnolipid on the formation of CH4 hydrate and separation of CH4/N2 via hydrate formation

Rhamnolipid (Rha) is an efficient green surfactant that is able to promote hydrate generation, it can be applied to the hydrate method of separating. However, the effect of rhamnolipid on the separation of methane and nitrogen by the hydrate method is not clear. Therefore, in this paper, we investigated the effect of rhamnolipid on methane hydrate formation and the separation between methane and nitrogen via hydrate method. Firstly, the effects of five concentrations (0 ppm, 200 ppm, 400 ppm, 600 ppm and 800 ppm), temperatures and pressures on methane hydrate formation were studied separately. The rate of methane hydrate formation was positively correlated with experimental pressure. The gas consumption increased rapidly with the rhamnolipid concentration, and then increased slowly to a steady state. The final gas consumption above 200 ppm rhamnolipid concentrations could be increased more than five times compared with pure water conditions. The promotion of methane hydrate production is most pronounced at 274.15 K, mainly depending on the diffusion of gas molecules in solution and the ease of hydrate nucleation. Then, the interfacial tension at different concentrations was measured by the suspension drop method, and the relationship between the interfacial tension and the rate of hydrate formation was studied. Hydrate formation rate shows a multiplicative power function with the interfacial tension. What’s more, the kinetics of hydrate formation and gas separation effect of 3:1 CH4/N2 at 274.15 K and 7 MPa were studied with five concentrations. And 200 ppm was found to be the optimum concentration for separation, the separation factor is 8.25. Finally, the kinetics and thermodynamics of hydrate formation and gas separation effect of 1:1 CH4/N2 at 274.15 K and 9 MPa were also investigated, and the separation factor reached a maximum of 12.7 at 800 ppm, and the methane gas recovery rate also reached the maximum at 40.46%. The results of this study can provide some theoretical guidance for the optimal selection of green surfactants in separation by hydrate method.

Micro gas chromatography column coated with ionic liquid/metal–organic framework composites as the stationary phase

In this paper, the μGC-IL/MOF and the μGC-IL were prepared using [P66614][NTf2]/ZIF-8 and [P66614][NTf2] as the stationary phase, respectively. [P66614][NTf2]/ZIF-8 composite stationary phase material has high specific surface area and porous structure, which increases the diffusion of gas molecules in the column. Compared to μGC-IL, μGC-IL/MOF can separate n-alkanes mixture (C5–C12) and lung cancer biomarkers (pentane, isoprene, acetone, benzene, 2-pentanone) with higher resolution, and the resolution (R) of pentane and isoprene was increased by 257.00% in particular. The μGC-IL/MOF can separate lung cancer biomarkers in about 5 min with optimal carrier gas velocity and column temperature. The retention times of pentane, isoprene, acetone, benzene, and 2-pentanone were 0.884 min, 1.246 min, 1.634 min, 2.204 min, and 3.049 min, respectively. The resolutions of adjacent peaks were 1.785, 1.525, 2.521, and 3.514, respectively. Which meets the requirements of quantitative analysis (R > 1.5). Therefore, the μGC-IL/MOF is expected to be integrated into portable devices for environmental monitoring and medical diagnosis in the future.

Au- or Ag-Decorated ZnO-Rod/rGO Nanocomposite with Enhanced Room-Temperature NO2-Sensing Performance.

To improve the gas sensitivity of reduced oxide graphene (rGO)-based NO2 room-temperature sensors, different contents (0-3 wt%) of rGO, ZnO rods, and noble metal nanoparticles (Au or Ag NPs) were synthesized to construct ternary hybrids that combine the advantages of each component. The prepared ZnO rods had a diameter of around 200 nm and a length of about 2 μm. Au or Ag NPs with diameters of 20-30 nm were loaded on the ZnO-rod/rGO hybrid. It was found that rGO simply connects the monodispersed ZnO rods and does not change the morphology of ZnO rods. In addition, the rod-like ZnO prevents rGO stacking and makes nanocomposite-based ZnO/rGO achieve a porous structure, which facilitates the diffusion of gas molecules. The sensors' gas-sensing properties for NO2 were evaluated. The results reveal that Ag@ZnO rods-2% rGO and Au@ZnO rods-2% rGO perform better in low concentrations of NO2 gas, with greater response and shorter recovery time at the ambient temperature. The response and recovery times with 15 ppm NO2 were 132 s, 139 s and 108 s, 120 s, and the sensitivity values were 2.26 and 2.87, respectively. The synergistic impact of ZnO and Au (Ag) doping was proposed to explain the improved gas sensing. The p-n junction formed on the ZnO and rGO interface and the catalytic effects of Au (Ag) NPs are the main reasons for the enhanced sensitivity of Au (Ag)@ZnO rods-2% rGO.

Surface Morphology Enriching the Energy Barrier Leads to the Adsorption Characteristic of Nanobubbles.

With the emergence of nanobubble research, nanobubble distribution morphology at the interface and its stability control become the bottlenecks of nanobubble resistance reduction applications. In this paper, the evolutionary behavior of nanobubbles on smooth and step HOPG surfaces was compared through molecular dynamics studies. The results show that the surface energy barrier provided by the step HOPG surface restricts diffusion of gas molecules. Then, a method of multisolvent evaporation for preparing hydrophobic nanoindent surfaces was proposed, which can achieve phase separation through different evaporation rates of multisolvents, thus realizing the preparation of surface structures with uniform distribution of nanoindents. In this paper, the nucleation processes of nanobubbles on PS nanoindent hydrophobic surface, HOPG flat hydrophobic surface, and HOPG nanostep hydrophobic surface were compared by using atomic force microscopy in liquid experiment. The evolution of the volume and distribution morphology of nanobubbles on the three nanostructures was observed by 24 h in situ tests, revealing that the energy barrier effect arising from the uneven surface structure can effectively prevent adjacent nanobubbles from merging in close proximity to each other. It is also pointed out that the hydrophobic nanoindents prepared by using the multivariate solvent evaporation method in this paper can cover most of nanobubbles for stable adsorption. It can be seen from the results that the volume drop of the nanobubbles on the HOPG flat hydrophobic surface is 27% and that on the HOPG nanostep and the PS nanoindent hydrophobic surface it is reduced to 19% and 3% under the effect of structural energy barriers, respectively. The density of the nanostructures determines whether the existence of nanobubbles is stable. The coverage of nanobubbles on the HOPG flat hydrophobic surface was 3.313% when the existence of nanobubbles was mostly stable. The HOPG nanostep and PS nanoindent sizes were positively correlated with the morphological size of the nanobubbles, which increased the coverage of the nanobubbles on the hydrophobic surface of the HOPG nanostep and PS nanoindent to 5.229% and 4.437%, respectively, when the existence of nanobubbles was mostly stable.