MXene-based chemical gas sensors: Recent developments and challenges.

The long-running Covid-19 pandemic has forced researchers across the globe to develop novel sensors and sensor materials for detecting minute quantities of biogenic viruses with high accuracy in a short period. In this context, MXene galleries comprising carbon/nitride two-dimensional nanolayered materials have emerged as excellent host materials in chemical gas sensors owing to their multiple advantages, including high surface area, high electrical conductivity, good thermal/chemical conductivity and chemical stability, composition diversity, and layer-spacing tunability; furthermore, they are popular in clinical, medical, food production, and chemical industries. This review summarizes recent advances in the synthesis, structure, and gas-sensing properties of MXene materials. Current opportunities and future challenges for obtaining MXene-based chemical gas sensors with high sensitivity, selectivity, response/recovery time, and chemical durability are addressed. This review provides a rational and in-depth understanding of the relationship between the gas-sensing properties of MXenes and structure/components, which will promote the further development of two-dimensional MXene-based gas sensors for technical device fabrication and industrial processing applications.

Effect of Pd-Sensitization on Poisonous Chlorine Gas Detection Ability of TiO2: Green Synthesis and Low-Temperature Operation.

Ganoderma lucidum mushroom-mediated green synthesis of nanocrystalline titanium dioxide (TiO2) is explored via a low-temperature (≤70 °C) wet chemical method. The role of Ganoderma lucidum mushroom extract in the reaction is to release the ganoderic acid molecules that tend to bind to the Ti4+ metal ions to form a titanium-ganoderic acid intermediate complex for obtaining TiO2 nanocrystallites (NCs), which is quite novel, considering the recent advances in fabricated gas sensing materials. The X-ray powder diffraction, field emission scanning electron microscopy, Raman spectroscopy, and Brunauer-Emmett-Teller measurements etc., are used to characterize the crystal structure, surface morphology, and surface area of as-synthesized TiO2 and Pd-TiO2 sensors, respectively. The chlorine (Cl2) gas sensing properties are investigated from a lower range of 5 ppm to a higher range of 400 ppm. In addition to excellent response-recovery time, good selectivity, constant repeatability, as well as chemical stability, the gas sensor efficiency of the as-synthesized Pd-TiO2 NC sensor is better (136% response at 150 °C operating temperature) than the TiO2 NC sensor (57% at 250 °C operating temperature) measured at 100 ppm (Cl2) gas concentration, suggesting that the green synthesized Pd-TiO2 sensor demonstrates efficient Cl2 gas sensing properties at low operating temperatures over pristine ones.

High-Performance Platinum-Free Dye-Sensitized Solar Cells with Molybdenum Disulfide Films as Counter Electrodes.

By using a radio-frequency sputtering method, we synthesized large-area, uniform, and transparent molybdenum disulfide film electrodes (1, 3, 5, and 7 min) on transparent and conducting fluorine-doped tin oxide (FTO), as ecofriendly, cost-effective counter electrodes (CE) for dye-sensitized solar cells (DSSCs). These CEs were used in place of the routinely used expensive platinum CEs for the catalytic reduction of a triiodide electrolyte. The structure and morphology of the MoS2 was analyzed by using Raman spectroscopy, X-ray diffraction, and X-ray photoemission spectroscopy measurements and the DSSC characteristics were investigated. An unbroken film of MoS2 was identified on the FTO crystallites from field-emission scanning electron microscopy. Cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel curve measurements reveal the promise of MoS2 as a CE with a low charge-transfer resistance, high electrocatalytic activity, and fast reaction kinetics for the reduction of triiodide to iodide. Finally, an optimized transparent MoS2 CE, obtained after 5 min synthesis time, showed a high power-conversion efficiency of 6.0 %, which comparable to the performance obtained with a Pt CE (6.6 %) when used in TiO2 -based DSCCs, thus signifying the importance of sputtering time on DSSC performance.

Sulfur ion-exchange strategy to obtain Bi2S3 nanostructures from Bi2O3 for better water splitting performance.

A two-step simple and efficient ion-exchange chemical strategy is proposed to obtain nanostructured Bi2S3 electrodes of different surface morphologies from the Bi2O3. In the first step, nanoplates of the Bi2O3 are obtained on nickel-foam using successive ionic layer adsorption and reaction method at room-temperature (25 °C). In the second phase, as-obtained nanoplates of the Bi2O3 are transferred to the Bi2S3 using four autoclaves containing different sulfur precursor solutions at 120 °C for 8 h for phase change, structural conversion and surface morphological modification (i.e., walnuts, network-type, nanowires, and nanoflowers). Due to higher surface area and conductivity, lower charge transfer resistance, and reduced band gap caused by ionic and phase conversion, the Bi2S3 surpasses the Bi2O3 in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities. The overpotential of 112-370 mV for the Bi2S3 network is much lower than that of the nanoplates of the Bi2O3 (275-543 mV), and walnuts (134-464 mV), nanowires (125-500 mV), and nanoflowers (194-520 mV) of the Bi2S3. The Bi2S3 network-type Bi2S3 electrode shows considerable chemical stability through cycling measurement, suggesting the importance of the present study in obtaining metal sulfides from metal oxide with better water splitting activities.

Energy storage and water splitting applications of self-grown Na2O-NiCl2 upright standing nanoplates: a process of 3D nickel surface modification using seawater.

The recent trend in research fosters the use of abundant seawater for modifying metal surfaces as electrode materials for energy generation, storage, transport, and water-splitting technologies. Economic and ecofriendly seawater is used as a solvent for modifying the surface of 3D nickel-foam (NiF) to Na2O-NiCl2@NiF as an electrode material in electrochemical supercapacitors and water-splitting electrocatalysis applications. The phase of the as-obtained Na2O-NiCl2 is confirmed from the proposed reaction mechanism, followed various physical measurement tests such as X-ray photoelectron spectroscopy and Fourier transform infrared analysis. The formation of Na2O-NiCl2 is caused by a high operation temperature and pressure of seawater solvent, the presence of lone pair electrons on oxygen, and more reactivity of Na for combining with dissolved oxygen than the lone-pair free Cl (towards Ni). In addition to exceptional HER and OER electrocatalytic activities, i.e., 146.3 mV cm-2 and 217 mV cm-2 at a scan rate of 5 mV s-1 to attain the 10 mA cm-2 current density, the Na2O-NiCl2 has demonstrated moderate energy storage ability with considerable durability, i.e., 2533 F g-1 specific capacitance at 3 A g-1 current density even after 2000 redox cycles. The as-assembled Na2O-NiCl2//Na2O-NiCl2 symmetric electrochemical supercapacitor device has ignited a "CNED" panel consisting of nearly forty LEDs with full brightness, offering applied importance in home appliances. In nutshell, seawater-modified metal surfaces can be used for energy storage and water-splitting applications.

A NiS2/C composite as an innovative anode material for sodium-ion batteries: ex situ XANES and EXAFS studies to investigate the sodium storage mechanism.

The successful deployment of sodium-ion batteries (SIBs) requires high-performance sustainable and cost-effective anode materials having a high current density. In this regard, sodium disulphide (NiS2) has been prepared as a composite with activated carbon (C) using a facile hydrothermal synthesis route in the past. The X-ray diffraction pattern of the as-prepared NiS2/C composite material shows well-defined diffraction peaks of NiS2. Most carbonaceous materials are amorphous, and the Brunauer-Emmett-Teller (BET) study shows that the surface area is close to 148 m2 g-1. At a current density of 50 mA g-1, the NiS2/C composite exhibits a high capacity of 480 mA h g-1 during the initial cycle, which subsequently decreases to 333 mA h g-1 after the completion of the 100th cycle. The NiS2/C composite electrode provides an exceptional rate capability by delivering a capacity of 270 mA h g-1 at a high current density of 2000 mA g-1, suggesting the suitability of the NiS2/C composite for SIBs. Ex situ X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses at the Ni K-edge have been used to examine the type of chemical bonding present in the anode and also how it changes during electrochemical redox cycling. The understanding of the sodium storage mechanism is improved by the favorable results, which also offer insights for developing high-performance electrode materials for rechargeable SIBs.

Assessment of antibacterial and anti-biofilm effects of zinc ferrite nanoparticles against Klebsiella pneumoniae.

In response to the emergence of drug resistance and limited therapeutic options, researchers are in action to look for more effective and sustainable antimicrobial practices. Over few years, novel nanoparticles are proving to be potent and promising for effectively dealing with ever- evolving microbial pathogens and diseases. In the present investigation, antibacterial and anti-biofilm efficiencies of zinc ferrite nanoparticles (ZnFe2O4 NPs) are explored against opportunistic pathogens Klebsiella pneumoniae (K. pneumoniae). Results of the present study demonstrate that the ZnFe2O4 NPs endow an excellent antibacterial efficiency with a maximum zone of inhibition i.e.16 mm. The reactive oxygen species (ROS)-induced bacterial damage is caused by the ZnFe2O4 NPs. Subsequently, intracellular cytoplasmic leakage of sugar and protein confirms their ability to disturb the membrane integrity of bacteria. This study also demonstrates the prominent efficiency of ZnFe2O4 NPs in an anti-biofilm study by inhibiting biofilm formation up to 81.76% and reducing mature biofilm up to 56.22% at 75 µg/mL the minimum inhibitory concentration value. Therapeutic possibilities of the ZnFe2O4 NPs in antimicrobial applications are discussed which are helpful to overcome the challenges associated with biofilm infectivity.

PH dependent morphological evolution of beta-Bi2O3/PANI composite for supercapacitor applications.

Crystalline beta-Bi2O3 was synthesized through pH-dependent chemical bath deposition process, altering the morphology and evolution from nanoparticles (approximately 40 nm) at pH 9 to platelets (approximately 40 nm width and 0.8 microm length) at pH 12. In-situ aniline nucleation and growth at less basic condition on the beta-Bi2O3 increased the surface area and specific capacitance of the device. The morphological change of beta-Bi2O3/PANI composite from nanoparticles to platelets like nanostructure facilitated higher specific capacitance from 210 to 430 F/g at a scan rate of 10 mV/s with enhanced ionic diffusion and retention of specific capacitance up to 84% at higher scan rates.

Porous metal-graphene oxide nanocomposite sensors with high ammonia detectability.

Nickel oxide-graphene oxide (NiO-GO), zinc oxide-graphene oxide (ZnO-GO) and bismuth oxide-graphene oxide (Bi2O3-GO) metal oxide-graphene oxide nanocomposite (MO-GO NC) sensors, operable at room temperature, were synthesized via a simple and cost-effective microwave-assisted combustion method for chemiresistive gas sensor applications. From the measured structural, morphological, and elemental detection properties, the sensors are found capable of detecting various gases. The Bi2O3-GO NC sensor exhibited excellent response over NiO-GO (~20 at 50 ppm) and ZnO-GO NC (~60 at 50 ppm) sensors for detecting NH3. The response of the Bi2O3-GO NC sensor at 50 ppm NH3 in just 14 s operation duration was ~81.23, which is improved 25-fold and 13-fold compared to pristine GO sensors. Additionally, the as-developed Bi2O3-GO NC sensor demonstrates outstanding repeatability and recovery kinetics, attributed to porosity and the combined effects of MO and GO. The sensing mechanism of the Bi2O3-GO NC gas sensors is proposed herein. The superior sensing performance, including quick response and recovery of the Bi2O3-GO NC sensor is attributed to favorable charge transfer across the Bi2O3 and GO interface. The significance of relative humidity on sensing potential of the Bi2O3-GO NC sensor has also been studied and the sensor is confirmed to be unaffected by relative humidity.

Recasting Ni-foam into NiF2 nanorod arrays via a hydrothermal process for hydrogen evolution reaction application.

A promising electrode for hydrogen evolution reaction (HER) has been prepared via a reduction process to form NiF2 nanorod arrays directly grown on a 3D nickel foam. We reveal NiF2@Ni nanorod arrays for a stable hydrogen evolution reaction (HER) application. The computational analysis for H2O, OH and H and experimentally in aqueous KOH endow considerable shift in Fermi levels for Ni (111) unlike for NiF2 (110) on account of an effective coalition of p-orbitals of fluorine and d-orbitals of Ni in NiF2, NiF2 under pinning the reduced overpotential of 172 mV at 10 mA cm-2 compared to Ni (242 mV) in same electrolyte. The electrocatalytic mechanism has been proposed using density functional theory (DFT) and is found in well accordance with the experimental findings of the present study. The preparation of self-grown porous nanostructured electrodes on the 3D nickel foam via a displacement reaction is possibly valuable for other metal halides for energy storage and conversion applications as these materials have inherently smaller overpotentials.

Tungsten oxides: green and sustainable heterogeneous nanocatalysts for the synthesis of bioactive heterocyclic compounds.

Tungsten oxide (WO3) as an efficient heterogeneous catalyst was prepared via a simple hydrothermal route for the synthesis of a wide range of bioactive heterocyclic compounds. The present investigation deals with the rapid and low-cost synthesis of C-3-alkylated 4-hydroxycoumarin, chromene, and xanthene derivatives. WO3 nanorods (NRs) are successfully envisaged to catalyze desired transformations, demonstrating the wide range of their potential applications in catalysis. Synthetic transformation details, smallest catalytic amounts, excellent product yields, and plausible reaction mechanisms for the formation of these heterocyclic scaffolds are elicidated. As-prepared WO3 NRs are characterized to confirm their structural, chemical, and morphological parameters by X-ray diffraction, X-ray photoelectron spectroscopy, and field emission scanning electron microscopy measurements, respectively. We discuss the factors that govern the formation of products, and the active role of WO3 NRs, which are essential for the activation of substrates in the present study of thermal conditions. Herein, detailed synthesis and spectroscopic information of the prepared compounds are reported.

Coconut-Water-Mediated Carbonaceous Electrode: A Promising Eco-Friendly Material for Bifunctional Water Splitting Application.

The organic and eco-friendly materials are extended to prevail over the worldwide energy crisis where bio-inspired carbonaceous electrode materials are being prepared from biogenic items and wastes. Here, coconut water is sprayed over three-dimensional (3D) nickel foam for obtaining a carbonaceous electrode material, i.e., C@Ni-F. The as-prepared C@Ni-F electrode has been used for structural elucidation and morphology evolution studies. Field emission scanning electron microscopy analysis confirms the vertically grown nanosheets of the C@Ni-F electrode, which is further employed in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), where excellent OER and HER performances with small overpotentials of 219 and 122 mV and with stumpy Tafel slopes, i.e., 27 and 53 mV dec-1, are respectively obtained, suggesting a bifunctional potential of the sprayed electrode material. Moreover, sustainable bifunctional performance of C@Ni-F proves considerable chemical stability and moderate mechanical robustness against long-term operation, suggesting that, in addition to being a healthy drink to mankind, coconut water can also be used for water splitting applications.

High Stability and Long Cycle Life of Rechargeable Sodium-Ion Battery Using Manganese Oxide Cathode: A Combined Density Functional Theory (DFT) and Experimental Study.

Sodium-ion batteries (SIBs) can develop cost-effective and safe energy storage technology for substantial energy storage demands. In this work, we have developed manganese oxide (α-MnO2) nanorods for SIB applications. The crystal structure, which is crucial for high-performance energy storage, is examined systematically for the metal oxide cathode. The intercalation of sodium into the α-MnO2 matrix was studied using the theoretical density functional theory (DFT) studies. The DFT studies predict Na ions' facile diffusion kinetics through the MnO2 lattice with an attractively low diffusion barrier (0.21 eV). When employed as a cathode material for SIBs, MnO2 showed a moderate capacity (109 mAh·g-1 at C/20 current rate) and superior life cyclability (58.6% after 800 cycles) in NaPF6/EC+DMC (5% FEC) electrolyte. It shows a much higher capacity of 181 mAh·g-1 (C/20 current rate) in NaClO4/PC (5% FEC) electrolyte, though it suffers fast capacity fading (11.5% after 800 cycles). Our findings show that high crystallinity and hierarchical nanorod morphology of the MnO2 are responsible for better cycling performance in conjunction with fast and sustained charge-discharge behaviors.

Stereospecific growth of densely populated rutile mesoporous TiO2 nanoplate films: a facile low temperature chemical synthesis approach.

We report for the first time, using a simple and environmentally benign chemical method, the low temperature synthesis of densely populated upright-standing rutile TiO(2) nanoplate films onto a glass substrate from a mixture of titanium trichloride, hydrogen peroxide and thiourea in triply distilled water. The rutile TiO(2) nanoplate films (the phase is confirmed from x-ray diffraction analysis, selected area electron diffraction, energy-dispersive x-ray analysis, and Raman shift) are 20-35 nm wide and 100-120 nm long. The chemical reaction kinetics for the growth of these upright-standing TiO(2) nanoplate films is also interpreted. Films of TiO(2) nanoplates are optically transparent in the visible region with a sharp absorption edge close to 350 nm, confirming an indirect band gap energy of 3.12 eV. The Brunauer-Emmet-Teller surface area, Barret-Joyner-Halenda pore volume and pore diameter, obtained from N(2) physisorption studies, are 82 m(2) g(-1), 0.0964 cm(3) g(-1) and 3.5 nm, respectively, confirming the mesoporosity of scratched rutile TiO(2) nanoplate powder that would be ideal for the direct fabrication of nanoscaled devices including upcoming dye-sensitized solar cells and gas sensors.

NiF2 Nanorod Arrays for Supercapattery Applications.

A electrode for energy storage cells is possible directly on Ni foam, using a simple reduction process to form NiF2 nanorod arrays (NA). We demonstrate NiF2@Ni NA for a symmetric electrochemical supercapattery electrode. With an areal specific capacitance of 51 F cm-2 at 0.25 mA cm-2 current density and 94% cycling stability, a NiF2@Ni electrode can exhibit supercapattery behavior, a combination of supercapacitor and battery-like redox. The symmetric electrochemical supercapattery delivers 31 W h m-2 energy density and 797 W m-2 power density with 83% retention in a 1 M KOH electrolyte, constituting a step toward manufacturing a laboratory-scale energy storage device based on metal halides. Producing self-grown hierarchically porous nanostructured electrodes on three-dimensional metal foams by displacement reactions may be useful for other metal halides as electrodes for supercapacitors, supercapatteries, and lithium-ion batteries.

Room-temperature chemical synthesis of 3-D dandelion-type nickel chloride (NiCl2@NiF) supercapattery nanostructured materials.

A simple, room-temperature operable, glycerol-supported single beaker-inspired, and binder-free soft-chemical protocol has been developed to synthesize 3-D dandelion flower-type nickel chloride (NiCl2) supercapattery (supercapacitor + battery) nanostructured electrode material from solid 3-D nickel-foam (NiF). The dandelion flower-type NiCl2@NiF labeled as B electrode, demonstrates a battery-type electrochemical performance as obtained 1551 F·g-1 specific capacitance (SC) and 95% cyclability over 50,000 cycles is higher than that of a setaria viridis-type NiCl2@NiF electrode, prepared without glycerol labeled as A electrode. As a commercial market product, assembled NiCl2@NiF@ (cathode)// BiMoO3 (anode) pouch-type asymmetric supercapacitor energy storage device demonstrates moderate energy density and power density (28 Wh·kg-1 and 845 W·kg-1). By utilizing three devices in series, three different colored LEDs can be operated at full brightness. The as-proposed low temperature protocol impeccably effective and efficient on account of the low-cost, easy synthesis methodology for scalability, and high crytallinity as well as solvent-free and non-toxic as pyrolated gases were used while synthesis processing.

Phase controlled synthesis of bifunctional TiO2 nanocrystallites viad-mannitol for dye-sensitized solar cells and heterogeneous catalysis.

The crystal architecture of TiO2 was successfully tailored via a low-temperature (≤200 °C) hydrothermal process in the presence of d-mannitol for feasible applications in dye-sensitized solar cells (DSSCs) and heterogeneous catalysis. In the development of anatase-TiO2 (A-TiO2), d-mannitol does not merely acts as a complexing agent to manage the zigzag chains of octahedral TiO6 2- with dominant edge sharing but also performs as a capping agent by influencing the hydrolysis process during nucleation, as confirmed by Fourier-transform infrared spectroscopy and dynamic light scattering studies. After physical measurements, the as-synthesized nanocrystallites (NCs) of A-TiO2 were used in DSSCs, where a fascinating power conversion efficiency (PCE) of 6.0% was obtained, which showed excellent performance compared with commercial anatase-TiO2 (CA-TiO2: 5.7%) and rutile-TiO2 (R-TiO2) obtained without d-mannitol (3.7%). Moreover, a smart approach was developed via the A-TiO2 catalyst to synthesize pharmaceutically important C-3 alkylated 4-hydroxycoumarins through different activated secondary alcohols under solvent-free, and heat/visible light conditions. In addition, the catalytic activity of the so-produced A-TiO2 catalyst under solvent-free conditions exhibited remarkable recyclability with up to five consecutive runs with negligible reduction, which is superior to existing reports, and clearly reveals the novelty, and green, sustainable nature of the as-synthesized A-TiO2 catalyst. A plausible reaction mechanism of both coupling partners was activated through the interaction with the A-TiO2 catalyst to produce valuable C-3 alkylated 4-hydroxycoumarins with 95% yield and high selectivity.

Room-temperature synthesis and CO2-gas sensitivity of bismuth oxide nanosensors.

Room-temperature (27 °C) synthesis and carbon dioxide (CO2)-gas-sensor applications of bismuth oxide (Bi2O3) nanosensors obtained via a direct and superfast chemical-bath-deposition method (CBD) with different surface areas and structures, i.e., crystallinities and morphologies including a woollen globe, nanosheet, rose-type, and spongy square plate on a glass substrate, are reported. Moprhologies of the Bi2O3 nanosensors are tuned through polyethylene glycol, ethylene glycol, and ammonium fluoride surfactants. The crystal structure, type of crystallinity, and surface appearance are determined from the X-ray diffraction patterns, X-ray photoelectron spectroscopy spectra, and high-resolution transmission electron microscopy images. The room-temperature gas-sensor applications of these Bi2O3 nanosensors for H2, H2S, NO2, SO2, and CO2 gases are monitored from 10 to 100 ppm concentrations, wherein Bi2O3 nanosensors of different physical properties demonstrate better performance and response/recovery time measurement for CO2 gas than those for the other target gases employed. Among various sensor morphologies, the nanosheet-type Bi2O3 sensor has exhibited at 100 ppm concentration of CO2 gas, a 179% response, 132 s response time, and 82 s recovery time at room-temperature, which is credited to its unique surface morphology, high surface area, and least charge transfer resistance. This suggests that the importance of the surface morphology, surface area, and crystallinity of the Bi2O3 nanosensors used for designing room-temperature operable CO2 gas sensors for commercial benefits.

Protective antigen detection using horizontally stacked hexagonal ZnO platelets.

Anthrax toxin detection before bacteremia, when toxin concentration is low, improves the chances of efficient treatment and cure. We present a novel technique for ultrasensitive detection of a protective antigen (PA(83)) of anthrax using an array of zinc oxide nanorods in conjunction with a FITC-labeled PA affinity peptide. The nanorods are composed of horizontally stacked hexagonal platelets which are uniformly spaced and grown unidirectionally upon a glass substrate via a new and simple technique. Images taken under UV emission demonstrate fluorescence sensitivity to PA as a function of antigen concentration, and a negative control using bovine serum albumin produced no fluorescence signal. The fluorescence signal of the PA-peptide complex is also significantly reduced in the absence of the nanorods, suggesting that the presence of ZnO nanorods inhibits the self-quenching properties of the fluorophore. A lower limit of detection for the assay system for PA is estimated at 150 aM, which demonstrates the possibility of using ZnO nanorods in biological sensor systems.

Electrochemically intercalated indium-tin-oxide/poly(3-hexylthiophene): A solid-state heterojunction solar cell.

A heterojunction solar cell design composed of poly(3-hexylthiophene) (P3HT) and intercalated indium-tin-oxide (ITO) donor-acceptor system is explored for the first time. Substantial change in band edge of ITO is noticed after intercalation. Structural and surface morphological studies are reported. Due to tuned band gap of ITO, an increase in short circuit current from 0.0012 to 0.46 mA/cm(2), fill factor from 0.39 to 0.51, and power conversion efficiency from 0.000 367 to 0.3% is obtained for heterojunction solar cell when compared to P3HT alone. This novel, room temperature design approach would be of great scientific interest in current solid-state solar cell scenario.