In-MIL-68 derived In2O3/Fe2O3 shuttle-like structures with n-n heterojunctions to improve ethanol sensing performance.

Metal-organic frameworks (MOFs) have a variety of structures and unique properties that make them suitable for use in gas sensors. Herein, In2O3/Fe2O3 was successfully synthesized using simple solvothermal and impregnation methods. The response to 100 ppm of ethanol gas reached 67.5 at an optimum working temperature of 200 °C, and the response/recovery time was 9 s/236 s. The composite also exhibited excellent selectivity, repeatability, and long-term stability. SEM, TEM, XRD, and XPS were used for the characterization of materials. The excellent sensing performance of the sensors is attributed to the construction of n-n heterojunctions, an increase in oxygen vacancies, and the unique structural characteristics of MOFs. The above experimental results indicate that In-MIL-68-derived In2O3/Fe2O3 is a promising ethanol sensing material.

Decade Milestone Advancement of Defect-Engineered g-C3N4 for Solar Catalytic Applications.

Over the past decade, graphitic carbon nitride (g-C3N4) has emerged as a universal photocatalyst toward various sustainable carbo-neutral technologies. Despite solar applications discrepancy, g-C3N4 is still confronted with a general fatal issue of insufficient supply of thermodynamically active photocarriers due to its inferior solar harvesting ability and sluggish charge transfer dynamics. Fortunately, this could be significantly alleviated by the "all-in-one" defect engineering strategy, which enables a simultaneous amelioration of both textural uniqueness and intrinsic electronic band structures. To this end, we have summarized an unprecedently comprehensive discussion on defect controls including the vacancy/non-metallic dopant creation with optimized electronic band structure and electronic density, metallic doping with ultra-active coordinated environment (M-Nx, M-C2N2, M-O bonding), functional group grafting with optimized band structure, and promoted crystallinity with extended conjugation π system with weakened interlayered van der Waals interaction. Among them, the defect states induced by various defect types such as N vacancy, P/S/halogen dopants, and cyano group in boosting solar harvesting and accelerating photocarrier transfer have also been emphasized. More importantly, the shallow defect traps identified by femtosecond transient absorption spectra (fs-TAS) have also been highlighted. It is believed that this review would pave the way for future readers with a unique insight into a more precise defective g-C3N4 "customization", motivating more profound thinking and flourishing research outputs on g-C3N4-based photocatalysis.

Hydrothermal synthesis of a bimetallic metal-organic framework (MOF)-derived Co3O4/SnO2 composite as an effective material for ethanol detection.

This study utilized a hydrothermal method and air calcination to prepare a bimetallic metal-organic framework (MOF) derived Co3O4/SnO2 nanocomposite material, which was employed as a sensing material for ethanol detection. The structure, elemental composition, and surface morphology of Co3O4/SnO2 nanocomposite materials were defined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Compared to SnO2 nanoparticles derived from metal-organic frameworks, the bimetallic metal-organic framework-derived Co3O4/SnO2 nanocomposite material exhibits significantly superior ethanol sensing performance. At 225 °C, the response value (R = Ra/Rg) to 100 ppm ethanol is 135, demonstrating excellent repeatability, selectivity and stability. Gas sensitivity assessment findings indicate that the 3 at% (Co/Sn) Co3O4/SnO2 nanocomposite is an excellent gas sensing material, providing strong technical support for ethanol detection and environmental monitoring.

Insights into Selective Mechanism of NiO-TiO2 Heterojunction to H2 and CO.

The construction of p-n heterojunctions has become a widely adopted strategy for achieving the selective detection of reducing gases, including H2 and CO. Nevertheless, the elucidation of the gas selectivity mechanism at the nanoscale remains elusive. First-principle calculations provide an attractive avenue for comprehending the influence of coordination structures on gas-sensitive selectivity, thereby unveiling the structure-activity relationship of p-n heterojunction sites. In this study, we investigate the selective adsorption behavior of H2 and CO on a NiO-TiO2 heterojunction using density functional theory. The results of d-band center analysis confirm that the NiO-TiO2 heterojunction with adsorbed oxygen significantly enhances the adsorption stability of reducing gases. Intriguingly, our calculations reveal that H2 has a higher affinity for adsorbed oxygen on the heterojunction surface compared to that of CO, corresponding to a lower H2 adsorption energy. Density of states (DOS) results indicate that the NiO-TiO2 heterojunction, with preadsorbed oxygen, exhibits ultrahigh selectivity with an n-type gas-sensitive response to H2, effectively eliminating the cross-sensitivity observed with CO, as confirmed by gas-sensitive characterization research. The sensing mechanism of the NiO-TiO2 heterojunction's selective detection of H2 without interference from CO can be visually explained by electron transfer and potential barrier changes, paving the way for future developments in novel, selective gas-sensitive materials.

A Review of Mn-Based Catalysts for Abating NOx and CO in Low-Temperature Flue Gas: Performance and Mechanisms.

Mn-based catalysts have attracted significant attention in the field of catalytic research, particularly in NOx catalytic reductions and CO catalytic oxidation, owing to their good catalytic activity at low temperatures. In this review, we summarize the recent progress of Mn-based catalysts for the removal of NOx and CO. The effects of crystallinity, valence states, morphology, and active component dispersion on the catalytic performance of Mn-based catalysts are thoroughly reviewed. This review delves into the reaction mechanisms of Mn-based catalysts for NOx reduction, CO oxidation, and the simultaneous removal of NOx and CO. Finally, according to the catalytic performance of Mn-based catalysts and the challenges faced, a possible perspective and direction for Mn-based catalysts for abating NOx and CO is proposed. And we expect that this review can serve as a reference for the catalytic treatment of NOx and CO in future studies and applications.

Conductivity and aging behavior of Sr(Ti0.6Fe0.4)1-x O3-δ mixed conductor materials.

Perovskite materials play a significant role in oxygen sensors due to their fascinating electrical and ionic conductivities. The sol-gel technique was employed to prepare various compositions of B-site-deficient Fe-doped SrTiO3 (iron-doped strontium titanate) or Sr(Ti0.6Fe0.4)1-x O3-δ , where x = 0.01, 0.02, and 0.03. The XRD results revealed that the principle crystalline phase of the samples was the cubic perovskite structure. The B-site deficiency improved the ionic and total conductivities of Sr(Ti0.6Fe0.4)1-x O3-δ . A small polaron conduction behavior occurred in the total electrical conductivity. The XPS results showed that the oxygen vacancy value decreased with the rise in the amount of B-site deficiencies. A lower B-site deficiency amount could produce more oxygen vacancies in the lattice but resulted in the ordering of vacancies and then lower ionic conductivity. The aging behavior was caused by the ordering of oxygen vacancies and resulted in a degeneration of electrical features under a long service time. Conversely, augmentation of the B-site deficiency amount inhibited the tendency for the ordering of oxygen vacancies and then promoted the electrical performance under a long usage time. The conduction mechanism of oxygen ions through oxygen vacancies was thoroughly investigated and discussed. The current study presents a feasible approach to ameliorate the physical features of conductors through doping the B-site of the perovskite layer with Fe, which would be a fruitful approach for numerous applications, including oxygen sensors and fuel cells anodes.

Tailoring activation of CoNiO nanoparticles/porous carbon nanofibers by atomic doping for high performance supercapacitors.

Metal-organic framework (MOF) materials are rich in active sites and have a high specific surface area, which make them potential electrode materials. In this work, a simple immersion method combined with a carbonization treatment process is applied to prepare MOF derived composite materials (CoNiO/PCNFs). Among them, cobalt-based MOFs (Co-MOFs) are selected as the precursor and doped with Ni atoms, and the ratio of Co and Ni is tailored to acquire a high-performance electrode. The electrochemical results show that when the ratio of Co to Ni is 2 : 2, the prepared CoNiO/PCNFs-2 electrode has high capacitance (912.4 F g-1 at 1 A g-1) and superior rate capability (retention is above 50% at 100 A g-1). Additionally, it is highly stable at 20 A g-1 (nearly no degradation after 6000 cycles). Density Functional Theory (DFT) calculations indicate that the Ni doping models present lower formation energy and better -OH group adsorption properties. Moreover, the density of electronic state (DOS) and differential charge density distribution demonstrate that Ni doping effectively enhances the charge transport during the charging and discharging processes, which is beneficial to enhance the energy storage of the electrode materials. In conclusion, this work presents a strategy to design MOF-derived composite electrodes. The experimental tests and theoretical calculations explore the energy storage process and prove that the CoNiO/PCNF electrode materials have great potential for applications.

Role of Mo doping and the interfacial interaction mechanism of Ni-Mo-S electrodes: experimental and computational study.

The metal element doping strategy is often used to optimize the electrode materials of supercapacitors as they can provide rich redox active sites and high conductivity; however, the synergistic effect between different metallic ions and the interfacial interaction mechanism during the energy storage process are still unclear. In this work, Mo-doped Ni-Mo-S (NMS) nanoflowers are prepared by one-step electrodeposition, and the ratios of Ni : Mo are tailored. Dynamics analysis shows that the Mo element occupies a prominent position in the capacitive behavior contribution. Meanwhile, density functional theory (DFT) reveals that Mo doping influences the electronic structure of the NMS materials and their affinity towards OH- in the electrolyte. All the electrodes (NMS-0, NMS-0.5, NMS-1, NMS-2, NMS-3, and NMS-4) exhibit excellent specific capacitance (1640.8, 1665.8, 1456.2, 1414.6, 1515.4 and 1214.6 F g-1, respectively) and good cycling stability (80.8%, 84.5%, 75.4%, 78.2%, 93.1% and 99.6%, respectively) for 5000 cycles. This work proposes an efficient method to synthesize NMS materials and theoretically studies the effect of the Mo element during the energy storage process.

New Analysis Method for Adsorption in Gas (H2, CO)-Solid (SnO2) Systems Based on Gas Sensing.

The chemisorption phenomenon is widely used in the explanation of catalysis, gas-solid reactions, and gas sensing mechanisms. Generally, some properties of adsorbents, such as adsorption sites and dispersion, can be predicted by traditional methods through the variation of the chemisorption capacity with the temperature, pressure, and gas-solid interaction potential. However, these methods could not capture the information of the interaction between adsorbents, the adsorption rate, and the competitive adsorption relationship between adsorbents. In this paper, metal oxide semiconductors (MOSs) are employed to study the adsorption behavior. The gas sensing responses (GSRs) of MOSs caused by the gas adsorption process are measured as a new method to capture some adsorption behaviors, which are impossible for the traditional methods to obtain. The following adsorption behaviors characterized by this new method are presented for the first time: (1) distinguishing the adsorption type using an example of two reducing gases: the adsorption type of the two gases is single-molecular layer adsorption in this work; (2) detecting the interaction between different gases: this will be a promising method to provide original characterization data in the fields of gas-solid reaction mechanisms and heterogeneous catalysis; and (3) measuring the adsorption rate based on the GSR.

First-principles calculations on the resistance and electronic properties of H2 adsorption on a CoO-SnO2 heterojunction surface.

Compared with pure metal oxides, heterojunctions greatly change the response to gas by the synergistic effect of the interface. In this work, density functional theory was used to reveal the adsorption performance of H2 on the heterojunction under oxygen conditions. First, we determined the most reasonable heterojunction structure based on the adhesion work. According to the adsorption energy, the presence of SnO2(100)(I)/CoO(110)(II) made the adsorption of H2 more stable. The DOS results showed that the resistance of the heterojunction increased with H2 adsorption, following the same trend as that of CoO(110) with H2 adsorption, although that of the heterojunction increased more. The electron density and electron density difference indicated that the heterojunction improved the reaction between H2 and oxygen ions on CoO(110). However, the resistance of CoO(110)(II)/SnO2(100)(II) increased after H2 adsorption, contrary to the resistance change of SnO2(100). Besides, the bonding energy between H2 and the adsorption site became worse. The above results demonstrated that the presence of the heterojunction could indeed change the response trend and the adsorption behavior of H2. Interestingly, the adsorption sites and effects of H2 were different when two metal oxides were used as the substrate of the heterojunction, respectively.

Opposite Sensing Response of Heterojunction Gas Sensors Based on SnO2-Cr2O3 Nanocomposites to H2 against CO and Its Selectivity Mechanism.

Metal oxide semiconductor (MOS) gas sensors show poor selectivity when exposed to mixed gases. This is a challenge in gas sensors and limits their wide applications. There is no efficient way to detect a specific gas when two homogeneous gases are concurrently exposed to sensing materials. The p-n nanojunction of xSnO2-yCr2O3 nanocomposites (NCs) are prepared and used as sensing materials (x/y shows the Sn/Cr molar ratio in the SnO2-Cr2O3 composite and is marked as SnxCry for simplicity). The gas sensing properties, crystal structure, morphology, and chemical states are characterized by employing an electrochemical workstation, an X-ray diffractometer, a transmission electron microscope, and an X-ray photoelectron spectrometer, respectively. The gas sensing results indicate that SnxCry NCs with x/y greater than 0.07 demonstrate a p-type behavior to both CO and H2, whereas the SnxCry NCs with x/y < 0.07 illustrate an n-type behavior to the aforementioned reduced gases. Interestingly, the SnxCry NCs with x/y = 0.07 show an n-type behavior to H2 but a p-type to CO. The effect of the operating temperature on the opposite sensing response of the fabricated sensors has been investigated. Most importantly, the mechanism of selectivity opposite sensing response is proposed using the aforementioned characterization techniques. This paper proposes a promising strategy to overcome the drawback of low selectivity of this type of sensor.

Y-doped CaZrO3/Co3O4 as novel dense diffusion barrier materials for a limiting current oxygen sensor.

Herein, we illustrate a feasible strategy to strengthen the gas sensing of Y-doped CaZrO3 (YxCa1-xZr0.7O3-δ (x = 0.05, 0.06, and 0.07))/0.1Co3O4 used as sensing materials. This compound was prepared via a solid-state reaction technique. The structural, morphological, electrical, and sensing features such as phase identification, microstructure, ionic conductivity, total conductivity and sensitivity of the fabricated sensors were evaluated via X-ray diffraction, scanning electron microscopy, electron-blocking method, electrochemical impedance spectroscopy and cyclic voltammetry. In addition, the influence of the Y-dopant on the properties of YxCa1-xZr0.7O3-δ/Co3O4 was thoroughly studied. XRD results revealed the formation of the orthorhombic perovskite phase of YxCa1-xZr0.7O3-δ. Moreover, the obtained results from the electrical properties elucidated high electronic and low ionic conductivities, and small polaron conduction of YxCa1-xZr0.7O3-δ/Co3O4. Furthermore, the results confirmed an excellent limiting current plateau for the fabricated oxygen sensor based on YxCa1-xZr0.7O3-δ/Co3O4. In particular, experimental observation indicates that Y-doping at the Ca site and/or Zr site might be difficult.

Conductivity and mixed conductivity of a novel dense diffusion barrier and the sensing properties of limiting current oxygen sensors.

First-principles calculations were used to explore the effect of various Y-doping levels on the electrical conductivity of SrTiO3. Herein, we prepared ((Y0.07Sr0.93Ti0.6Fe0.4-xO3-δ)/x/3Co3O4 (x = 0.1, 0.2, 0.3)) composites using a solid state reaction method. The properties of these sensing materials and the fabricated sensors including crystal phase composition, microstructures, oxygen ionic conductivity, total conductivity and sensor performance were investigated in detail. XRD demonstrates the formation of a highly cubic perovskite structure. The introduction of Co3O4 promotes remarkably the electronic conductivity of the Y0.07Sr0.93Ti0.6Fe0.4-xO3-δ/x/3Co3O4 composites due to the formation of n-type CoO and p-type Co2O3. A limiting current oxygen sensor based on (Y0.07Sr0.93Ti0.6Fe0.4-xO3-δ)/x/3Co3O4 as a dense diffusion barrier shows excellent sensing performance. The recovery time is less than the response time, indicating that Co2O3 promotes the gas desorption reaction which results in a shorter recovery time. The obtained results demonstrate a direct relationship between limiting current (IL) and oxygen content.

Discriminable Sensing Response Behavior to Homogeneous Gases Based on n-ZnO/p-NiO Composites.

Metal oxide semiconductor (MOS) gas sensors have the advantages of high sensitivity, short response-recovery time and long-term stability. However, the shortcoming of poor discriminability of homogeneous gases limits their applications in gas sensors. It is well-known that the MOS materials have similar gas sensing responses to homogeneous gases such as CO and H2, so it is difficult for these gas sensors to distinguish the two gases. In this paper, simple sol-gel method was employed to obtain the ZnO-xNiO composites. Gas sensing performance results illustrated that the gas sensing properties of composites with x > 0.425 showed a p-type response to both CO and H2, while the gas sensing properties of composites with x < 0.425 showed an n-type response to both CO and H2. However, it was interesting that ZnO-0.425NiO showed a p-type response to CO but an discriminable response (n-type) to H2, which indicated that modulating the p-type or n-type semiconductor concentration in p-n composites could be an effective method with which to improve the discriminability of this type of gas sensor regarding CO and H2. The phenomenon of the special gas sensing behavior of ZnO-0.425NiO was explained based on the experimental observations and a range of characterization techniques, including XRD, HRTEM and XPS, in detail.

Facile Synthesis of SnO2/LaFeO3-XNX Composite: Photocatalytic Activity and Gas Sensing Performance.

Here SnO2/LaFeO3-XNX composite was fabricated using a wet chemical method and was applied to pollutants degradation and gas sensing for the first time. The composite exhibits high performance for photocatalytic degradation of Rhodamine B (RhB) dye and selectivity sensing of various gases. On the basis of the completed experiments, the improved RhB degradation and selective gas sensing performance resulted from the extended optical absorption via N2 incorporated surface states and enhanced charge separation via coupling SnO2. Using the scavengers trapping experiments, the superoxide radical (O2•-) was investigated as the major scavenger involved in the degradation of RhB over SnO2/LaFeO3-XNX composite. In this paper, the probable reaction steps involved in the RhB dye degradation over SnO2/LaFeO3-XNX composite are proposed. This work will provide reasonable strategies to fabricate LaFeO3-based proficient and stable catalysts for environmental purification. In addition, the result of the selectivity of gas performance is also presented.