Lanthanum Dioxide Carbonate La2O2CO3 Nanorods as a Sensing Material for Chemoresistive CO2 Gas Sensor

Sensitivity Enhancement of CO2 Sensors at Room Temperature Based on the CZO Nanorod Architecture

Cobalt-doped ZnO (CZO) thin films were deposited on glass substrates at room temperature by radio frequency (RF) magnetron sputtering of a single target prepared with ZnO and Co3O4 powders. Changes in the crystallinity, morphology, optical properties, and chemical composition of the CZO thin films were investigated at various sputtering powers of 45, 60, and 75 W. All samples presented a hexagonal wurtzite-type structure with a preferential c-axis at the (002) plane, along with a distinct change in the strain values through X-ray diffraction patterns. Scanning electron and atomic force microscopy revealed uniform and dense deposition of nanorod CZO samples with a high surface roughness (RMS). The Hall mobility and carrier concentration increased with the introduction of Co+ ions into the ZnO matrix, as seen from the Hall effect study. The gradual increase of the power applied on the target source significantly affected the morphology of the CZO thin film, which is reflected in the CO2-sensing performance. The best gas response to CO2 was recorded for CZO-60 W with a response of 1.45 for 500 ppm CO2, and the response/recovery times were 72 and 35 s, respectively. The distinguishing feature of the CZO sensor is its ability to effectively and rapidly detect the CO2 target gas at room temperature (∼27 °C, RT).

Highly selective CO2 sensing response of lanthanum oxide nanoparticle electrodes at ambient temperature

Lanthanum oxide nanoparticles (La₂O₃ NPs) are attractive rare earth metal oxides because of their applications in optical devices, catalysts, dielectric layers, and sensors. Herein, we report room temperature operative carbon dioxide gas sensing electrodes developed by a simple sonication assisted hydrothermal method. The physiochemical, morphological and gas-sensing properties of the prepared nanoparticles were studied systematically and their successful preparation was confirmed with the absence of impurities and high selectivity towards CO₂. The fabricated sensor showed a high sensitivity of 40% towards CO₂ at 50 ppm, and it can detect concentrations of up to 5 ppm with a quick response time of 6 s and recovery of 5 s. The electrode demonstrated long-term stability of 95% for 50 days when tested with an interval of 10 days. This simple and cost-effective method shows great potential for fabricating room temperature CO₂ gas sensors.

Component regulation in novel La-Co-O-C composite catalyst for boosted redox reactions and enhanced thermal stability in methane combustion

A novel La-Co-O-C (LC-C) composites were prepared via a facile co-hydrothermal route with oxides and glycerol and further optimized for methane catalytic activity and thermal stability via component regulation. It was demonstrated that Co₃O₄ phase was the main component in regulation. The combined results of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of oxygen (O₂-TPD), temperature-programmed reduction of hydrogen (H₂-TPR), temperature-programmed desorption of ammonia/carbon dioxide (NH₃/CO₂-TPD) revealed that component regulation led to more oxygen vacancies and exposure of surface Co²⁺, lower surface basicity and optimized acidity, which were beneficial for adsorption of active oxygen species and activation of methane molecules, resulting in the excellent catalytic oxidation performance. Especially, the (3.5)LC-C (3.5 is Co-to-La molar ratio) showed the optimum activity and the T₅₀ and T₉₀ (the temperature at which the CH₄ conversion rate was 50% and 90%, respectively) were 318 and 367°C, respectively. Using theoretical calculations and in situ diffuse reflection infrared Fourier transform spectroscopy characterization, it was also found that the catalytic mechanism changes from the "Rideal-Eley" mechanism to the "Two-term" mechanism depending on the temperature windows in which the reaction takes place. Besides, the use of the "Flynn-Wall-Ozawa" model in thermoanalytical kinetics revealed that component regulation simultaneously optimized the decomposition activation energy, further expanding the application scope of carbon-containing composites.

Nanocomposites Based on Cerium, Lanthanum, and Titanium Oxides Doped with Silver for Biomedical Application

Nanosized composites CeO2–Ag, La2O3–Ag, and TiO2–Ag are a class of nanomaterials suitable for photocatalysis, optical devices, and photoelectrochemical elements. Further, nanocomposites with several wt.% of silver can be used as creating materials for pathogenic virus inactivation with pandemic-neutralizing potential. Thus, CeO2–Ag, La2O3–Ag, and TiO2–Ag nanocomposites are prospective materials due to their optical and biological activity. In the present work, CeO2–Ag, La2O3–Ag, and TiO2–Ag nanocomposites were synthesized by the co-precipitation method. The morphological and optical properties and the structure of the prepared nanocomposites were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) with EDX, and nitrogen adsorption-desorption based on BET, Raman spectroscopy, and photoluminescence (PL). Both oxide matrixes corresponded to the cubic crystal lattice with the inclusion of argentum into the crystal lattice of oxides at relative low c(Ag) and reduction of silver on particle surface at 5 wt.% Ag and greater. The CeO2, TiO2, and La2O3 with a concentration of 4 wt.% Ag inhibited the growth processes of prokaryotic cells of E. coli, Bacillus sp., and S. aureus compared to pure oxides. Influenza A virus and herpes completely suppressed reproduction by nanocomposites of CeO2–Ag (2, 5 wt.%) and La2O3–Ag (2, 5 wt.%) action.

Carbon Dioxide Sensing-Biomedical Applications to Human Subjects

Carbon dioxide (CO2) monitoring in human subjects is of crucial importance in medical practice. Transcutaneous monitors based on the Stow-Severinghaus electrode make a good alternative to the painful and risky arterial "blood gases" sampling. Yet, such monitors are not only expensive, but also bulky and continuously drifting, requiring frequent recalibrations by trained medical staff. Aiming at finding alternatives, the full panel of CO2 measurement techniques is thoroughly reviewed. The physicochemical working principle of each sensing technique is given, as well as some typical merit criteria, advantages, and drawbacks. An overview of the main CO2 monitoring methods and sites routinely used in clinical practice is also provided, revealing their constraints and specificities. The reviewed CO2 sensing techniques are then evaluated in view of the latter clinical constraints and transcutaneous sensing coupled to a dye-based fluorescence CO2 sensing seems to offer the best potential for the development of a future non-invasive clinical CO2 monitor.

Flame-Made La2O3-Based Nanocomposite CO2 Sensors as Perspective Part of GHG Monitoring System

Continuous monitoring of greenhouse gases with high spatio-temporal resolution has lately become an urgent task because of tightening environmental restrictions. It may be addressed with an economically efficient solution, based on semiconductor metal oxide gas sensors. In the present work, CO₂ detection in the relevant concentration range and ambient conditions was successfully effectuated by fine-particulate La₂O₃-based materials. Flame spray pyrolysis technique was used for the synthesis of sensitive materials, which were studied with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and low temperature nitrogen adsorption coupled with Brunauer–Emmett–Teller (BET) effective surface area calculation methodology. The obtained materials represent a composite of lanthanum oxide, hydroxide and carbonate phases. The positive correlation has been established between the carbonate content in the as prepared materials and their sensor response towards CO₂. Small dimensional planar MEMS micro-hotplates with low energy consumption were used for gas sensor fabrication through inkjet printing. The sensors showed highly selective CO₂ detection in the range of 200–6667 ppm in humid air compared with pollutant gases (H₂ 50 ppm, CH₄ 100 ppm, NO₂ 1 ppm, NO 1 ppm, NH₃ 20 ppm, H₂S 1 ppm, SO₂ 1 ppm), typical for the atmospheric air of urbanized and industrial area.

An In Situ Temperature-Dependent Study of La2O3 Reactivation Process

Lanthanum-containing materials are widely used in oxidative catalytic and electrocatalytic reactions such as oxidative coupling of methane (OCM) and solid oxide fuel cells (SOFCs). However, many of these materials are highly susceptible to air contamination which means ex situ characterization results generally cannot be associated with their reactivity. In this study, the activation processes of an in situ–prepared bulk La₂O₂CO₃ sample and an ex situ as-prepared La(OH)₃ sample are in situ investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and online mass spectroscopy (MS). Results indicate that the La₂O₂CO₃ sample, during linear heating to 800°C, always contains some carbonates near the surface region, which supports a two-step model of bulk carbonate decomposition through surface sites. The La(OH)₃ sample structure evolution is more complex due to contaminations from air exposure. Together with TGA results, online mass analysis of water and CO₂ signal loss showed that three major catalyst structure phase change steps and a preheating up to 800°C are required for the as-prepared material to be transferred to La₂O₃. This process is carefully investigated combining the three in situ methodologies. XPS and XRD data further reveal transformations of variety of in situ surface structures and forms including hybrid phases with hydroxyl, carbonates, and oxide as the sample heated to different temperatures within the range from 200 to 800°C. The results provide useful insights on the activation and deactivation of La-contained materials.

Nanostructured metal oxide semiconductor-based sensors for greenhouse gas detection: progress and challenges

Climate change and global warming have been two massive concerns for the scientific community during the last few decades. Anthropogenic emissions of greenhouse gases (GHGs) have greatly amplified the level of greenhouse gases in the Earth's atmosphere which results in the gradual heating of the atmosphere. The precise measurement and reliable quantification of GHGs emission in the environment are of the utmost priority for the study of climate change. The detection of GHGs such as carbon dioxide, methane, nitrous oxide and ozone is the first and foremost step in finding the solution to manage and reduce the concentration of these gases in the Earth's atmosphere. The nanostructured metal oxide semiconductor (NMOS) based technologies for sensing GHGs emission have been found most reliable and accurate. Owing to their fascinating structural and morphological properties metal oxide semiconductors become an important class of materials for GHGs emission sensing technology. In this review article, the current concentration of GHGs in the Earth's environment, dominant sources of anthropogenic emissions of these gases and consequently their possible impacts on human life have been described briefly. Further, the different available technologies for GHG sensors along with their principle of operation have been largely discussed. The advantages and disadvantages of each sensor technology have also been highlighted. In particular, this article presents a comprehensive study on the development of various NMOS-based GHGs sensors and their performance analysis in order to establish a strong detection technology for the anthropogenic GHGs. In the last, the scope for improved sensitivity, selectivity and response time for these sensors, their future trends and outlook for researchers are suggested in the conclusion of this article.

Gold Nanoparticles on Nanosheets Derived from Layered Rare-Earth Hydroxides for Catalytic Glycerol-to-Lactic Acid Conversion

Layered rare-earth hydroxides (LREHs), as a series of special lamellar compounds having a similar structure to layered double hydroxides (LDHs), are becoming a new type of catalyst materials. In this study, we have prepared a series of uniform LREH (RE = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm) nanosheets through a reverse-microemulsion method. After deposition-precipitation of HAuCl₄ and calcination, supported Au catalysts (denoted as Au/LREO) were subsequently obtained. The catalytic properties of all the derived Au/LREO catalysts were evaluated by aerobic conversion of glycerol to lactic acid under mild conditions (90 °C, 1 atm). Among these catalysts, Au/LPrO displays the best performances, including the highest glycerol conversion, lactic acid, and C₃ product selectivity. Both the catalytic activities and the characterizations of the structure of Au/LREO indicate that the kind of rare-earth ions plays a key role in determining the Au particle size and its valence state and reducibility, which are the important factors correlated with the catalytic activities in glycerol conversion. In fact, the three features of gold particles, the extra-small size (∼3 nm), high content of Au⁰ species, and high reducibility, are the essential prerequisites for achieving the superior catalytic performance of Au/LPrO.

Low-Temperature Carbon Dioxide Gas Sensor Based on Yolk-Shell Ceria Nanospheres

Monitoring carbon dioxide (CO₂) levels is extremely important in a wide range of applications. Although metal oxide-based chemoresistive sensors have emerged as a promising approach for CO₂ detection, the development of efficient CO₂ sensors at low temperature remains a challenge. Herein, we report a low-temperature hollow nanostructured CeO₂-based sensor for CO₂ detection. We monitor the changes in the electrical resistance after CO₂ pulses in a relative humidity of 70% and show the high performance of the sensor at 100 °C. The yolk-shell nanospheres have not only 2 times higher sensitivity but also significantly increased stability and reversibility, faster response times, and greater CO₂ adsorption capacity than commercial ceria nanoparticles. The improvements in the CO₂ sensing performance are attributed to hollow and porous structure of the yolk-shell nanoparticles, allowing for enhanced gas diffusion and high specific surface area. We present an easy strategy to enhance the electrical and sensing properties of metal oxides at a low operating temperature that is desirable for practical applications of CO₂ sensors.

Realizing small-flake graphene oxide membranes for ultrafast size-dependent organic solvent nanofiltration

Membranes for organic solvent nanofiltration (OSN) or solvent-resistant nanofiltration (SRNF) offer unprecedented opportunities for highly efficient and cost-competitive solvent recovery in the pharmaceutical industry. Here, we describe small-flake graphene oxide (SFGO) membranes for high-performance OSN applications. Our strategy exploits lateral dimension control to engineer shorter and less tortuous transport pathways for solvent molecules. By using La³⁺ as a cross-linker and spacer for intercalation, the SFGO membrane selective layer was stabilized, and size-dependent ultrafast selective molecular transport was achieved. The methanol permeance was up to 2.9-fold higher than its large-flake GO (LFGO) counterpart, with high selectivity toward three organic dyes. More importantly, the SFGO-La³⁺ membrane demonstrated robust stability for at least 24 hours under hydrodynamic stresses that are representative of realistic OSN operating conditions. These desirable attributes stem from the La³⁺ cross-linking, which forms uniquely strong coordination bonds with oxygen-containing functional groups of SFGO. Other cations were found to be ineffective.

Cellulose Nanopaper Cross-Linked Amino Graphene/Polyaniline Sensors to Detect CO2 Gas at Room Temperature

A nanocomposite of cross-linked bacterial cellulose-amino graphene/polyaniline (CLBC-AmG/PANI) was synthesized by covalent interaction of amino-functionalized graphene (AmG) AmG and bacterial cellulose (BC) via one step esterification, and then the aniline monomer was grown on the surface of CLBC-AmG through in situ chemical polymerization. The morphological structure and properties of the samples were characterized by using scanning electron microscopy (SEM), and thermal gravimetric analyzer (TGA). The CLBC-AmG/PANI showed good electrical-resistance response toward carbon dioxide (CO₂) at room temperature, compared to the BC/PANI nanopaper composites. The CLBC-AmG/PANI sensor possesses high sensitivity and fast response characteristics over CO₂ concentrations ranging from 50 to 2000 ppm. This process presents an extremely suitable candidate for developing novel nanomaterials sensors owing to easy fabrication and efficient sensing performance.

Understanding of binding energy calibration in XPS of lanthanum oxide by in situ treatment

Rare earth oxides have seen increased usage over the years in batteries and catalysts. Due to their unique electronic properties, they are the subject of fundamental and practical interest. However, the complexity in their electronic structures makes unambiguous characterization, such as X-ray photoelectron spectroscopy (XPS), very challenging. Lanthanum oxide (La₂O₃) has attracted special attention as a promising catalyst for the oxidative coupling of methane (OCM) reaction. In this work, a new and reliable way of XPS calibration is developed by applying various in situ preparations for a nanorod La₂O₃ catalyst to intentionally form different lanthanum compounds, followed by XPS characterization and corroboration with first principles calculations. To form different compounds, five sample treatments were performed including heating in vacuum and treatment with O₂, CH₄, CO₂, and H₂O, which are all relevant to OCM reaction conditions. Adventitious carbon or lattice oxygen, as conventional calibration standard species for energy scale, is only suitable for one or few in situ prepared surfaces. Our results also clearly demonstrate the vital difference between performing the ex situ analysis after exposure of the sample to the atmosphere and the in situ analysis. By carefully comparing the spectra of various photoemission peaks of different compounds, we conclude that the binding energy of 102.2 eV for the La 4d_7/2 peak can be used as the internal calibration standard for all considered samples. Furthermore, different oxygen species were unambiguously identified by matching the oxygen 1s binding energies from the in situ measurements and first principles predictions.

Layered perovskite-like La2−xCaxNiO4±δ derived catalysts for hydrogen production via auto-thermal reforming of acetic acid

Layered perovskite La_2−xCa_xNiO_4±δ produced ordered porous structures with Ni dispersed over La–Ca–O oxides, which promoted transformation of CH₃COOH.

Active oxygen species on Mg–La mixed oxides: the effect of Mg and La oxide interactions

Mg–La mixed oxide catalysts with a strong interaction between Mg and La oxides had higher amounts of oxygen vacancies and surface adsorbed oxygen species, resulting in a better performance in H₂ oxidation and CH₄ oxidative reactions.

La2O2CO3 Encapsulated La2O3 Nanoparticles Supported on Carbon as Superior Electrocatalysts for Oxygen Reduction Reaction

Constructing nanoscale hybrid materials with unique interfacial structures by using various metal oxides and carbon supports as building blocks are of great importance to develop highly active, economical hybrid catalysts for oxygen reduction reaction (ORR). In this work, La2O2CO3 encapsulated La2O3 nanoparticles on a carbon black (La2O2CO3@La2O3/C) were fabricated via chemical precipitation in an aqueous solution containing different concentrations of cetyltrimethyl ammonium bromide (CTAB), followed by calcination at 750 °C. At a given CTAB concentration 24.8 mmol/L, the obtained lanthanum compound nanoparticles reach the smallest particle size (7.1 nm) and are well-dispersed on the carbon surface. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results demonstrate the formation of La2O2CO3 located on the surface of La2O3 nanoparticles in the hybrid. The synthesized La2O2CO3@La2O3/C hybrid exhibits a significantly enhanced electrocatalytic activity in electrocatalysis experiments relative to pure La2O3, La2O2CO3, and carbon in an alkaline environment, by using the R(R)DE technique. Moreover, its long-term stability also outperforms that obtained by commercial Pt/C catalysts (E-TEK). The exact origin of the fast ORR kinetics is mainly ascribed to the La2O2CO3 layer sandwiched at the interface of carbon and La2O3, which contributes favorable surface-adsorbed hydroxide (-OH(-)(ad)) substitution and promotes active oxygen adsorption at the interfaces. The unique covalent -C-O-C(═O)-O-La-O- bonds, formed at the interfaces between La2O2CO3 and carbon, can act as active sites for the improved ORR kinetics over this hybrid catalyst. Therefore, the fabrication of lanthanum compound-based hybrid material with an unique interfacial structure maybe open a new way to develop carbon-supported metal oxides as next-generation of ORR catalysts.

CO2 sensing properties of La1−xBaxFeO3 thick film and packed powder sensors

We compared the CO₂ sensing properties of La_1−xBa_xFeO₃ packed powder and thick film sensors.

High temperature CO2 sensing properties and mechanism of nanocrystalline LaCrO3 with rhombohedral structure: experiments and ab initio calculations

Both experiment and calculation results show that CO₂ captures electrons from the semiconductor surface.