Plasma-Activated Solutions Regulate Surface-Terminating Groups Enhancing Pseudocapacitive Ti3 C2 Tx Electrode Performance.

2D transition metal carbides and nitrides (MXenes) are actively pursued as pseudocapacitive materials for supercapacitors owing to their advantages in electronic conductivity and surface reactivity. Increasing the fraction of ─O terminal groups in Ti3 C2 Tx is a promising approach to improve the pseudocapacitive charge storage in H2 SO4 electrolytes, but it suffers from a lack of effective functionalization methods and stability of the groups in practical operation. Here a low-temperature and environment-friendly approach via the interaction of nonequilibrium plasmas with Ti3 C2 Tx dispersion is demonstrated to generate abundant and stable surface-terminating O groups. The impact of the discharge environment (Ar, O2 , and H2 ) on the structural characteristics and electrochemical performance of Ti3 C2 Tx nanosheets is studied. The Ti3 C2 Tx modified in Ar and H2 maintains their original morphology but a significantly lower F content. Consequently, an extraordinarily high content (78.5%) of surface-terminating O groups is revealed by the high-resolution X-ray photoelectron spectroscopy spectra for the Ti3 C2 Tx samples modified in H2 plasma-treated solutions. Additionally, the Ti3 C2 Tx treated using H2 plasmas exhibits the best capacitive performance of 418.3 F g-1 at 2 mV s-1 , which can maintain 95.88% capacity after 10 000 cycles. These results contribute to the development of advanced nanostructured pseudocapacitive electrode materials for renewable energy storage applications.

Unexpected rise of atmospheric secondary aerosols from biomass burning during the COVID-19 lockdown period in Hangzhou, China.

After the global outbreak of COVID-19, the Chinese government took many measures to control the spread of the virus. The measures led to a reduction in anthropogenic emissions nationwide. Data from a single particle aerosol mass spectrometer in an eastern Chinese megacity (Hangzhou) before, during, and after the COVID-19 lockdown (5 January to February 29, 2020) was used to understand the effect lockdown had on atmospheric particles. The collected single particle mass spectra were clustered into eight categories. Before the lockdown, the proportions of particles ranked in order of: EC (57.9%) < K-SN (13.6%) < Fe-rich (10.2%) < ECOC (6.7%) < K-Na (6.6%) < OC (3.4%) < K-Pb (1.0%) < K-Al (0.7%). During the lockdown period, the EC and Fe-rich particles decreased by 42.8% and 93.2% compared to before lockdown due to reduced vehicle exhaust and industrial activity. By contrast, the K-SN and K-Na particles containing biomass burning tracers increased by 155.2% and 45.2% during the same time, respectively. During the lockdown, the proportions of particles ranked in order of: K-SN (39.7%) < EC (38.1%) < K-Na (11.0%) < ECOC (7.7%) < OC (1.2%) < K-Pb (0.9%) < Fe-rich (0.8%) < K-Al (0.6%). Back trajectory analysis indicated that both inland (Anhui and Shandong provinces) and marine transported air masses may have contributed to the increase in K-SN and K-Na particles during the lockdown, and that increased number of fugitive combustion points (i.e., household fuel, biomass combustion) was a contributing factor. Therefore, the results imply that regional synergistic control measures on fugitive combustion emissions are needed to ensure good air quality.

Astigmatic dual-beam interferometric particle imaging for metal droplet 3D position and size measurement.

We propose astigmatic dual-beam interferometric particle imaging (ADIPI) to simultaneously measure the three-dimensional (3D) position and size of spherical metal droplets. A theoretical model reveals that the orientation and spacing of the ADIPI fringes generated from the two reflections propagating through an astigmatic imaging system relate to the depth position and size, respectively. Proof-of-concept experiments on micron-sized gallium droplets are performed, and the tilted fringes in elliptical patterns are observed in the ADIPI interferogram, confirming theoretical predictions. Droplet 3D position and size are determined with ADIPI, and the relative discrepancies are within 5% and 2% compared to those with a dual-view digital inline holography system, demonstrating the feasibility and high accuracy of ADIPI.

Sensing mechanism of the nano-confined space constructed by graphene.

Multilayer graphene with dense interlayer space is the most explored two-dimensional material (2DMs) in high performance gas sensor. Herein, the insertion and the diffusion behaviors of NO, NO2, NH3and H2S in the nano-confined space of graphene are investigated using density functional theory calculations. The optimum interlayer distance is found to be 6-7 Å, in which the interaction strength is enhanced by 2 -3 times compared to monolayer graphene. Based on the optimum interlayer spacing, a barrierless diffusion process is observed due to the negligible influence of adsorption sites on the adsorption energy. Besides, an enhanced adsorption of NO2is found at the edge, which leads to a small barrier (<0.15 eV) during the its inserting into graphene layers, while the barrierless process is observed for NO, NH3and H2S. As for sensing performance, an increased sensitivity is observed for NO and NO2at the edge because of the significant energy level shift and charge transfer. Meanwhile, multilayer graphene shows good selectivity towards NO2gas. Therefore, modulating the interlayer spacing of graphene layers is a promising strategy for fabricating practical low-cost gas sensors, which may facilitate future exploration of high performance gas sensor using multilayer 2DMs.

Stable and Effective Online Monitoring and Feedback Control of PCDD/F during Municipal Waste Incineration.

For the long-term operation of municipal solid waste incineration (MSWI), online monitoring and feedback control of polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) can be used to control the emissions to national or regional standards. In this study, 500 PCDD/F samples were determined by thermal desorption gas chromatography coupled to tunable-laser ionization time-of-flight mass spectrometry (TD-GC-TLI-TOFMS) for 168 h. PCDD/F emissions range from 0.01 ng I-TEQ/Nm3 to 2.37 ng I-TEQ/Nm3, with 44% of values below 0.1 ng I-TEQ/Nm3 (the national standard). In addition, the temperature of the furnace outlet, bed pressure, and oxygen content are considered as key operating parameters among the 13 operating parameters comprising four temperature parameters, four pressure parameters, four flow parameters, and oxygen content. More specifically, maintaining the furnace outlet temperature to be higher than 800 °C, or bed pressure higher than 13 kPa, or the oxygen content stably and above 10% are effective methods for reducing PCDD/F emissions. According to the analysis of the Pearson coefficients and maximal information coefficients, there is no significant correlation between operating parameters and PCDD/F I-TEQ. Only when there is a significant change in one of these factors will the PCDD/F emissions also change accordingly. The feedback control of PCDD/F emissions is realized by adjusting the furnace outlet temperature, bed temperature, and bed pressure to control the PCDD/F to be less than 0.1 ng I-TEQ/Nm3.

Surface tension and viscosity measurement of oscillating droplet using rainbow refractometry.

We extend rainbow refractometry to quantify the oscillations of a droplet in its fundamental mode. The oscillation parameters (frequency and amplitude damping), extracted using the time-resolved rainbow angular shift, are utilized to measure surface tension and viscosity of the liquid. Proof-of-concept experiments on an oscillating droplet stream produced by a monodisperse droplet generator are conducted. Results show that the relative measurement errors of surface tension and viscosity are 1.5% and 8.4% for water and 5.3% and 2.5% for ethanol. This approach provides an alternative mean for characterizing liquid surface properties, e.g., dynamic surface tension and viscosity, especially for liquids with a low Ohnesorge number.

SO2 Electrocatalytic Oxidation Properties of Pt-Ru/C Bimetallic Catalysts with Different Nanostructures.

Electrocatalytic oxidation of SO2 has been applied in many fields, and electrocatalyst is the focus of the research. Platinum-based electrocatalysts are the hot spot in this reaction. Although the properties of these materials have been optimized to a certain extent, there is still room for improvement in activity and long-term durability. In light of this, two kinds of carbon-supported Pt-Ru bimetallic electrocatalysts (PtRu/C alloy catalyst and Ru@Pt/C core-shell catalyst) were prepared by the microwave reduction method. The experiments demonstrate that the enhancement in the activity of bimetallic catalysts originates from the electronic effect and bifunctional effect between Pt and Ru. Bimetallic catalyst contains a large number of RuOxHy, which promotes the reaction. Because of the high Pt utilization, Ru@Pt/C catalyst with the Pt shell has a higher performance than alloy catalyst. The unit Pt mass activity of PtRu/C and Ru@Pt/C is 1.73 and 2.43 times that of Pt/C, respectively. Ru@Pt/C exhibits excellent stability in the high acid environment and is a promising SO2 electrocatalyst.

High-Performance Pt Catalyst with Graphene/Carbon Black as a Hybrid Support for SO2 Electrocatalytic Oxidation.

In the electrochemical Bunsen reaction, which is the key reaction of the sulfur-iodide cycle, the main overpotential corresponds to the oxidation of SO2. The catalysts currently used for the liquid phase electrocatalytic oxidation of SO2 are mainly based on noble metals, which have excellent corrosion resistance and catalytic activity in an acidic environment. To improve the performance of the commercial Pt catalyst, a Pt catalyst with reduced graphene oxide (RGO) and carbon black as a hybrid support was synthesized by the microwave-assisted polyol reduction process. The large two-dimensional planar structure of RGO was a better anchor for Pt nanoparticles, and a catalyst with fine and uniform distribution of Pt particles was obtained. The carbon black interior prevented the agglomeration of RGO, forming a stereoscopic catalyst structure. The electrochemical test results showed that the RGO/carbon black hybrid support catalyst possessed a higher electrocatalytic activity than the single support catalysts and could be used as an efficient and stable catalyst for the SO2 electrolyzer.

Sensitive detection of NO using a compact portable CW DFB-QCL-based WMS sensor.

This paper introduces a compact and portable sensor based on mid-infrared absorption spectroscopy for NO detection employing a room-temperature continuous wave (CW) distributed feedback quantum cascade laser (DFB-QCL) emitting at 1900.08cm-1. A software-based digital signal generator and lock-in amplifier, in combination with the wavelength modulation spectroscopy (WMS) technique, were used for the concentration measurement of NO. In addition, a Gabor filter denoising method was developed to improve the performance of the measurement system. As a result, a minimum detection limit of 42 ppbv can be achieved at 3 s integration time, and a measurement precision of 450 ppbv can be reached with a time resolution of 0.1 s. The performance of the compact portable sensor was verified by a series of experiments, denoting great potential of field application for sensitive NO sensing.

Phase critical angle scattering for measurement of transient nanoscale growth rate of a micron-sized bubble.

We developed phase critical angle scattering (PCAS) to simultaneously measure the spherical and transparent bubble size at the micron scale and transient bubble growth at the nanoscale. The theoretical derivation of PCAS reveals that the phase of the fine structure of critical angle scattering caused by reflection and first-order refraction is highly sensitive to and linearly shifts with bubble diameter growth. Experiments on a single growing bubble are implemented with a Fourier imaging system. The results show that the PCAS technique can measure the tiny bubble growth down to tens of nanometers, providing a promising tool for accurate characterization of bubble dynamics.

United Conversion Process Coupling CO2 Mineralization with Thermochemical Hydrogen Production.

In this work, to achieve both clean energy production and carbon emission reduction, a united conversion to couple CO2 mineralization with thermochemical hydrogen production is proposed. Natural magnesium silicate minerals are used to fix CO2 in the form of carbonate minerals, whereas H2O is dissociated to produce H2 in the thermochemical cycle. The integration provides a new solution to the challenges of the high energy consumption and poor economic value of conventional CO2 mineralization processes, and the technical feasibility has been proven. Moreover, the energy economy and CO2 conversion capacity were investigated. Hydrolyzation and carbonation experiments were performed in a homemade reactor, and it was found that an optimal MgI2 hydrolyzation rate of 75% could be achieved without alkali consumption. A detailed simulation of the whole system was also developed. The optimal energy conversion efficiency of the cycle reached 47.6%, which is higher than most of the published theoretical energy efficiency values for sulfur-iodine thermochemical cycles. A modified calculation of the net energy requirement for CO2 mineralization was carried out. Finally, a comparison and an evaluation of the energy efficiencies were made based on the calculation. In the optimal case, the modified net energy requirement is 1.4 MJ/kg CO2, which means that this method is competitive compared to those of previous works.

Accurate detection of small particles in digital holography using fully convolutional networks.

Particle detection is a key procedure in particle field characterization with digital holography. Due to various background noises, spurious small particles might be generated and real small particles might be lost during particle detection. Therefore, accurate small particle detection remains a challenge in the research of energy and combustion. A deep learning method based on modified fully convolutional networks is proposed to detect small opaque particles (e.g., coal particles) on extended focus images. The model is tested by several experiments and proved to have good small particle detection accuracy.

Three-dimensional dynamic measurement of irregular stringy objects via digital holography.

Dynamic stringy objects such as liquid rims and ligaments are frequently observed in important applications such as the multiphase breakup of fuel droplets. We develop a new method based on digital in-line holography to automatically measure complicated stringy objects. A static spring mounted on a rotator is measured to validate the effectiveness and accuracy of the method. The sections are extracted along the skeleton of the spring in a depth-of-field extended image and then sized and located as individual particles using a hybrid method. The surface points of sections are stitched together to visualize the entire spring. Local thickness errors smaller than 5.3%, and z errors smaller than 230 µm are achieved. This method is applied to characterize the spatial-temporal features of the liquid rim formed in the bag-type regime of the aerodynamic breakup of a falling drop. The evolution of the rim/ligament structures is continuously captured in seven frames, lasting in 1.58 ms. This Letter extends the application of digital holography as an effective 3D diagnostic tool.

Ab initio characterization and experimental validation on the roles of oxygen-containing groups in graphene based formaldehyde sensors.

The development of formaldehyde (HCHO) sensors employing reduced graphene oxide (rGO) as sensing materials calls for a profound, atomic level understanding on the roles of oxygen-containing groups. In this work, the performances of rGO-based HCHO sensors were investigated using ab initio calculations and experimental validation. Density functional theory (DFT) simulations were performed to calculate the adsorption energy (Eads) and charge transfer (ΔQ) for the adsorption of HCHO on pristine graphene, rGO with epoxides, rGO with hydroxyl groups, and rGO with carboxyl groups. The results show that the incorporation of oxygen-containing groups leads to an obvious increase of Eads and ΔQ values, with an order of carboxyl group > hydroxyl group > epoxides > pristine graphene. The increase of Eads and ΔQ values could increase the variation in the concentration of charge carriers, the change of conductance of the sensing materials, and hence the sensor response. The experimental measurements indicate that with a decrease in the C/O atomic ratio from 16.2 to 6.6, the sensor response to 1 ppm HCHO increases from 0.10% to 0.73%, confirming the DFT calculation results. Moreover, even with a certain C/O atomic ratio of ∼6.6, rGO with 6.80% carboxyl groups exhibits a distinctly larger response to 0.2-3 ppm HCHO, compared with the counterpart with 3.09% carboxyl groups. The as-obtained insights into the effects of oxygen-containing groups on the response of rGO to HCHO could be instructive for preparing rGO-based HCHO sensors for advanced performances.

Temperature dependence of ion diffusion coefficients in NaCl electrolyte confined within graphene nanochannels.

The behavior of ion diffusion in nano-confined spaces and its temperature dependence provide important fundamental information about electric double-layer capacitors (EDLCs) employing nano-sized active materials. In this work, the ion diffusion coefficients of NaCl electrolyte confined within neutral and charged graphene nanochannels at different temperatures are investigated using molecular dynamics simulations. The results show that ions confined in neutral nanochannels diffuse faster (along the graphene surfaces) than those in bulk solution, which could be attributed to the relatively smaller concentration in confined spaces and the solvophobic nature of graphene surfaces. In charged nanochannels where the electrostatic interactions between counter-ions and charged channel surfaces govern the motion of ions, the diffusion coefficients are found to be lower than those in the neutral counterparts. The increase of temperature will lead to enhanced vibrant thermal motion of ions. Due to the significant role of ion-surface interactions, ion diffusion coefficients in nano-confined spaces are more stable, that is, insensitive to the temperature variation, than those in bulk solution. The electrical conductivity is further estimated using the Nernst-Einstein equation. The findings of the current work could provide basic data and information for research studies on the thermal effects of graphene-based EDLCs.

Characterizations of transparent particle holography in near-field using Debye series.

The effects of the individual scattering process on the formations of both the particle hologram and its corresponding reconstructed three-dimensional particle image are investigated using the Debye series. A particle hologram model using the Debye series decomposes the object wave into different scattering modes and thus permits evaluating the effects of the individual scattering process [diffraction, reflection, transmission, refractions with (p-1) internal reflections] on the particle holography quantitatively. In the Gabor inline holography of a transparent droplet, the transmission light causes small discrepancies between the hologram fringes of an opaque particle (diffraction) and a transparent particle near the zero point of the Bessel-like modulation function, eventually giving rise to the glory spot in the center of the reconstructed dark particle image. For off-axis holography, this paper reveals the effects of reflection, particularly total reflection by bubbles, transmission, and refractions with (p-1) internal reflections of the scattered light on the formation and the reconstructed glory spot images of typical forward and backward off-axis holography.

Emerging energy and environmental applications of vertically-oriented graphenes.

Graphene nanosheets arranged perpendicularly to the substrate surface, i.e., vertically-oriented graphenes (VGs), have many unique morphological and structural features that can lead to exciting properties. Plasma-enhanced chemical vapor deposition enables the growth of VGs on various substrates using gas, liquid, or solid precursors. Compared with conventional randomly-oriented graphenes, VGs' vertical orientation on the substrate, non-agglomerated morphology, controlled inter-sheet connectivity, as well as sharp and exposed edges make them very promising for a variety of applications. The focus of this tutorial review is on plasma-enabled simple yet efficient synthesis of VGs and their properties that lead to emerging energy and environmental applications, ranging from energy storage, energy conversion, sensing, to green corona discharges for pollution control.

The effect of microbubbles on gas-liquid mass transfer coefficient and degradation rate of COD in wastewater treatment.

A commonly used aeration device at present has the disadvantages of low mass transfer rate because the generated bubbles are several millimeters in diameter which are much bigger than microbubbles. Therefore, the effect of a microbubble on gas-liquid mass transfer and wastewater treatment process was investigated. To evaluate the effect of each bubble type, the volumetric mass transfer coefficients for microbubbles and conventional bubbles were determined. The volumetric mass transfer coefficient was 0.02905 s(-1) and 0.02191 s(-1) at a gas flow rate of 0.67 L min(-1) in tap water for microbubbles and conventional bubbles, respectively. The degradation rate of simulated municipal wastewater was also investigated, using aerobic activated sludge and ozone. Compared with the conventional bubble generator, the chemical oxygen demand (COD) removal rate was 2.04, 5.9, 3.26 times higher than those of the conventional bubble contactor at the same initial COD concentration of COD 200 mg L(-1), 400 mg L(-1), and 600 mg L(-1), while aerobic activated sludge was used. For the ozonation process, the rate of COD removal using microbubble generator was 2.38, 2.51, 2.89 times of those of the conventional bubble generator. Based on the results, the effect of initial COD concentration on the specific COD degradation rate were discussed in different systems. Thus, the results revealed that microbubbles could enhance mass transfer in wastewater treatment and be an effective method to improve the degradation of wastewater.

[Simulation of Concentration Measurement of SO(2), NO(2) and Particles Simultaneously by Differential Optical Absorption Spectroscopy].

Our daily life is disturbed seriously by the haze weather now. It is very important to measure the haze composition quantificationally. The main composition of haze is SO(2), NO(2) and particles. At present, the research of measuring gas and particle simultaneously is rare relatively. This paper use differential optical absorption spectroscopy (DOAS) to simulate the concentration measurement of gas and particle simultaneously and obtain some meaningful results. Absorption spectral of many groups of different concentration of SO(2), NO(2) and particle were simulated, and each concentration was inverted by DOAS. In the first group of single component, the concentration change from 100 to 1 000 ppm, the inverted error of SO(2) is not greater than 0.17%, and which is 0.64% for NO(2). When the diameter of particle change from 100 to 500 nm, the inverted error is not greater than 2.08%. In the second group of multiple gases, when the concentration ratio of SO(2) and NO(2) is at the range of 1 : 10 and 5 : 1, the error of SO(2) is not bigger than 8%, and 5% for NO(2), relatively. But when the concentration of SO(2) is 10 times than NO(2), the error is higher than 10% for NO(2). In the third group of gas and particle, the error of gas concentration is lower than 10%, but the concentration error of particle is depended on signal to noise ratio (SNR) greatly. When SNR is higher than 40 dB, error can lower than 10% and when SNR is lower than 30dB, the error is bigger than 20%. From these results, we can see that DOAS can measurement SO(2), NO(2) and particles simultaneously effectively, and can applied to measure and analyze haze composition. However, when the absorption strength of the gases is different greatly, the strong absorption gas influent the weak absorption gas largely. And the SNR is lower, the error of inverted particle concentration increased greatly. The solution of these problems need better filtering and noise reduction method.

[The Experimental Research on Reducing the Minimum Measureable Limit of Tunable Diode Laser Absorption Spectroscopy with Wavelet Analysis].

To obtain the weaker second harmonic signal of low concentration, reduce the minimum measurable limit and improve the sensitivity and accuracy of absorption measurement, a serious of data processing methods are proposed based on tunable diode laser wavelength modulation spectroscopy. The experiment on lower NH3 concentration at 2.25 µm was carried out in a 10.13 m absorption cell with different concentration. The peak height of the second harmonic signal is maximum at m=2.2, which optimizes the signal-to-noise ratio. In order to guarantee the optimal signal-to-noise ratio, the experiment was carried out by loading the optimal high frequency modulation signal. WMS-2f was performed at a repetitive scan rate of 200 Hz and a current-modulation rate of 15 kHz, wavelength modulation spectroscopy with the optimal signal-to-noise ratio was adopted for its better noise immunity to measure different lower NH3 concentration in the Herriott cell. This survey is focused on the ν2+ν3 bands of absorption spectra near 2.25 µm in near-infrared region at ambient temperature and pressure, the line strengths of 2.25 µm are much larger than the absorption lines in the telecommunication bands, using stronger NH3 absorption lines can offer the potential of lower detection limits. During the data processing, the background signal of the original harmonic should be deducted at first, the second harmonic signal of 0.6×10-6 was obtained in a 10 m long-path Herriott cell after data processing, these signal processing mainly consist of cross-correlation analysis, multiple averages and wavelet transform analysis, the cross-correlation analysis was used to control the shift of center wavelength, the multiple averages and wavelet transform analysis were used to reduce influences of the environment noise, after that we get the revised second harmonic signal and improve the accuracy of the measurement results. The experimental results show that these data processing methods can obviously improve the signal quality and reduce the minimum measurable limit about 100 times lower than before. The experiment doesn't need to add any laboratory equipment and can well restrain the influence of the environmental noise and other disturbance, so these signal process combined with wavelength modulation technique will be more useful for on-line gas detection technology.