Increase In Concentration Of Oxygen Vacancies Research Articles

Doping Pd2+ into NinCeOx nanofibers promotes low-temperature CO2 methanation

CO2 methanation at low temperatures is still a challenge. Herein, NinCeOx (n = 1–3) and Ni2.5Pd0.1CeOx nanofibers by electrospinning is reported. The main advantage of this method is to obtain the highly dispersed precursor of palladium, nickel, and cerium, overcoming the difficulty of uniform mixing in conventional preparation methods. The Ni2.5Pd0.1CeOx nanofiber catalyst exhibited outstanding catalytic performance at low temperatures (CO2 conversion rate = 90.4 %, CH4 selectivity = 99.6 % at 230 °C) along with exceptional stability over 300 h. EPR, Raman, and O 1s XPS confirmed that Pd2+ doping increased oxygen vacancy concentration. In-situ infrared spectroscopy indicated that CO2 methanation on Ni2.5CeOx and Ni2.5Pd0.1CeOx catalysts followed the formate pathways. Pd2+ doping increased the number of surface oxygen vacancies and hydroxyl groups, thus increasing the amount of bicarbonates and formates. DFT calculations suggested that Pd2+ doping increased CO2 adsorption energy, and confirmed surface hydroxyl groups and bicarbonate being beneficial for CO2 methanation, consequently enhancing the activity of Ni2.5Pd0.1CeOx catalyst especially at low temperatures.

Enhanced electrochemical performance of (PrBa)xFe1.9Nb0.1O5+δ symmetrical electrodes with A-site deficient for symmetrical solid oxide fuel cells

Symmetrical solid oxide fuel cells (SSOFCs) with symmetrical electrodes have many desirable characteristics compared with conventional SOFCs. However, the lack of electrode materials simultaneously satisfied the high electrocatalytic activity in oxidizing and reducing atmospheres. Herein, a novel perovskite oxide electrode (PrBa)xFe1.9Nb0.1O5+δ ((PB)xFN, x = 1, 0.97, 0.95 and 0.93) was reported as a promising symmetrical electrode material for SSOFCs. XRD results reveal that the (PB)0.95FN sample has a tetragonal structure and good chemical compatibility with Gd0.1Ce0.9O2-δ electrolyte. Among all the samples, (PB)0.95FN has the lowest polarization resistance (Rp). At 800 °C, compared with primary PBFN, the cathode Rp of (PB)0.95FN is reduced from 0.109 Ω cm2 to 0.063 Ω cm2 in air, whereas the anode Rp is decreased from 0.3808 Ω cm2 to 0.3109 Ω cm2 in H2. In addition, an electrolyte-supported single cell with (PB)0.95FN symmetrical electrodes achieves a maximum peak power density at 800 °C, which is enhanced by 14.67 % compared with that of PBFN. A-site deficiency on PBFN perovskite shows better electrochemical performance, which is attributed to an increase in oxygen vacancy concentration. The results indicate that (PB)0.95FN is a potential electrode material for SSOFCs.

Optimizing Low-Temperature CO2 methanation with Aluminum-Doped Ni/CeO2 Catalysts: Insights into reaction pathway adjustments and strong Metal-Support interactions

Revealing the logical relationship between the structure, reaction mechanism and performance of the catalyst is a key step in the design of low-temperature CO2 methanation catalyst. The elemental Al was introduced into Ni/CeO2 using a co-precipitation method to form a composite support CeO2-Al2O3 loaded nickel-based catalyst. It was found that the incorporation of Al significantly improved the catalytic performance of low-temperature CO2 methanation. Notably, at a molar ratio of Al/Ce equal to 0.1, the catalyst exhibited optimal CO2 conversion of 88.83% and CH4 selectivity of 99.96% at 240 ℃. The addition of appropriate amounts of Al not only increases the specific surface area of the catalyst but also maintains a moderate strong interaction between metal Ni-support, thereby promoting the formation of active metal Ni species. Furthermore, the presence of Al leads to an increase in oxygen vacancy concentration, which facilitates the generation of numerous hydroxyl groups. Consequently, this alters the distribution of basic sites on the catalyst surface, resulting in a shift from the CO* pathway to a dual pathway involving both CO* and formate pathways. Remarkably, these divergent reaction pathways correspond to distinct active sites, thereby maximizing the utilization of each region within the catalyst. The diversity of reaction pathways and active sites represents pivotal factors for enhancing the catalytic performance of low-temperature CO2 methanation catalysts.

Correlation of electronic structure and magnetic properties of [formula omitted] nanostructure using synchrotron soft XAS

The present study reports crystalline structure, electronic structure and magnetic properties for nanoparticles undoped CeO2 and Ce1-xSmxO2 (x = 0.02 to 0.10, @ 0.02). The nanoparticle samples are synthesized by the co-precipitation method. XRD data shows the successful incorporation of Sm3+ ions at Ce4+ sites in the face-centered cubic (fcc) lattice. The soft X-ray absorption spectroscopy technique is utilized to get an insightful overview of electronic structural properties due to structural defects, chemical states and the development of oxygen vacancies in Ce1-xSmxO2 nanoparticles. The soft XAS spectra show a significant Ce and Sm peak in the M4,5 domain, along with the well-defined O K-edge peak. Analysis of Ce M4,5-edges indicates that Ce-ions have both 3 + and 4 + oxidation states. With the incorporation of Sm+3 ions, there is a reduction of Ce4+ states and oxidation of Ce3+ states, as well as a substantial increase in oxygen vacancy concentration. The room-temperature magnetic properties of Ce1-xSmxO2 nanoparticles reveal ferromagnetic behavior. The origin of ferromagnetism is explained by F-center exchange (FCE) coupling mechanisms mediated via development of oxygen vacancy and defects.

Effect of oxygen vacancy of lignite-char-supported Co catalysts doped with In on efficient dry reforming of methane

In this study, a series of catalysts rich in oxygen vacancies were prepared through the introduction of indium (In) for application in the dry reforming of methane. Among these catalysts, one doped with 2 wt% In, named as Co-2In/C, showcases a higher surface area (417 m2/g) and smaller particle size (4.21 nm). The stability assessment for Co-2In/C catalysts conducted under conditions of 800 °C, 0.5 MPa and a flow rate of 30,000 mL·h−1·g-1cat over 100 h resulted in CH4 and CO2 conversions of 73 % and 83 %, respectively, and a ratio of H2/CO is 0.65 without evident deactivation. The generation of more oxygen vacancies balances the redox capability of the catalyst and prevents excessive metal agglomeration. Moreover, an increase in oxygen vacancy concentration accelerates carbon elimination and extends the catalyst life.

Facile Deficiency Engineering in a Cobalt‐Free Perovskite Air Electrode to Achieve Enhanced Performance for Protonic Ceramic Fuel Cells

AbstractAs a crucial component responsible for the oxygen reduction reaction (ORR), cobalt‐rich perovskite‐type cathode materials have been extensively investigated in protonic ceramic fuel cell (PCFC). However, their widespread application at a commercial scale is considerably hindered by the high cost and inadequate stability. In response to these weaknesses, the study presents a novel cobalt‐free perovskite oxide, Ba0.95La0.05(Fe0.8Zn0.2)0.95O3‐δ (BLFZ0.95), with the triple‐conducting (H+|O2−|e−) property as an active and robust air electrode for PCFC. The B‐site deficiency state contributes significantly to the optimization of crystal and electronic structure, as well as the increase in oxygen vacancy concentration, thus in turn favoring the catalytic capacity. As a result, the as‐obtained BLFZ0.95 electrode demonstrates exceptional electrochemical performance at 700 °C, representing extremely low area‐specific resistance of 0.04 Ω cm2 in humid air (3 vol.% H2O), extraordinarily high peak power density of 1114 mW cm−2, and improved resistance against CO2 poisoning. Furthermore, the outstanding long‐term durability is achieved without visible deterioration in both symmetrical and single cell modes. This study presents a simple but crucial case for rational design of cobalt‐free perovskite cathode materials with appreciable performance via B‐site deficiency regulation.

An oxygen vacancy-rich BiO2-x/COF heterojunction for photocatalytic degradation of diclofenac.

A BiO2-x/COF composite was successfully synthesized by simple mechanical ball milling. Compared to pure BiO2-x and COFs, the BiO2-x/COF composite (1 : 9) showed superior photocatalytic capability. Under visible light irradiation for 90 min, the photocatalytic degradation rate of DCF reached 97%. In addition, the characterization results showed that the formation of heterojunctions and the increase in oxygen vacancy concentration were the reasons for the enhancement of the photocatalytic activity. It is confirmed by free radical capture experiments that ˙O2- and h+ are the main reactive substances in the photocatalytic process. The photocatalytic degradation mechanism of the composite and the photocatalytic degradation pathway of diclofenac were deduced.

Designing Multidimensional and Interconnected Multi-Element Nanostructures by Exsolution-Self-Assembly in a Complex Concentrated Oxide

Complex concentrated oxides (CCOs) are an emerging material class that includes high-entropy oxide (HEOs) and entropy-stabilized oxides (ESOs), whose unprecedented properties stem from disorder-induced distributions in electronic structure and chemistry caused by stabilizing many-cation (typically > 5) solid solutions [1-2]. Integrating these materials into composites with nanoscale tunability will enable tailored (multi)functionality beyond what is possible in a single phase [3]. Here, we demonstrate a novel, highly extensible approach, exsolution self-assembly (ESA), to realize CCO-based nanocomposite thin films with intricate multi-element nanostructures [4]. Using pulsed-laser deposition (PLD), we selectively reduce cations in a 6-cation model perovskite CCO, inducing defect-interaction-driven exsolution and simultaneous self-assembly of metal nanorods and metal-oxide core-shell nanoparticles, depending on film growth conditions. ESA using a CCO precursor enables novel multielement composite morphologies with finely tuned sub-phase chemical composition that are not possible by conventional exsolution or self-assembly independently. By mapping the distribution of distinct sub-phase structure and chemistry with sub-nanoscale electron microscopy and atom probes, we show that Ellingham's model of cation reducibility predicts nanostructure chemical composition while mass transport via extended defects governs local morphology formation. Importantly for various applications, the electronic conductivity of our nanocomposites exceeds 0.1 S/cm at room temperature, with transport correlated to nanostructure formation primarily through B-site cation mixed valency and site occupancy in the CCO matrix.A correlated analysis using aberration-corrected scanning transmission electron microscopy (STEM) imaging, energy dispersive X-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS), geometric phase analysis (GPA) strain mapping, atom probe tomography (APT) with 3D mass spectrometry, and X-ray photoemission spectroscopy (XPS) was performed to elucidate the fundamental nanostructure formation mechanisms underlying the highly tailorable synthesis approach. We show that the method achieves uniform bulk exsolution with short process time and is manipulated by tuning readily accessible PLD conditions. Detailed characterization resolved the impact of increasing oxygen vacancy concentration and varying cation reducibility on nanostructure self-assembly. For example, metal nanorods grow from the bottom of the thin film to the top surface, with growth restricted by compressive stress exerted by the matrix in the in-plane direction. Metal-oxide core-shell nanoparticles embedded in the matrix form via seed growth effect triggered by metal nanoparticle formation followed by subsequent reduction of additional mobile cations. This work demonstrates a route towards vast morphological tunability and compositional complexity through nanocomposite design of CCO thin films. Given CCOs’ vast combinatorial space, ESA is expected to be highly extensible via application to novel compositions and crystal structures, with the understanding presented here enabling one to take advantage of chemical complexity in a rational way.

The effects of Co doping on the gas sensing performance of In2O3 porous nanospheres

Gas sensor constructed with metal oxide semiconductor (MOS) has attracted a growing concern owing to its feasibility for rapid detection of various harmful and pollutant gases. However, the sensors made of pure MOS usually suffer from the drawbacks of high working temperature, low sensitivity and poor selectivity. Herein, we propose a Co-doping strategy to upgrade the gas sensing performance of In2O3. To expound it, Co-doped In2O3 porous nanospheres (Co-In2O3 PNSs) with the diameter of ∼120 nm was prepared from home-made In(OH)3 NSs via a soaking-lyophilization-calcination method and their sensitive performance towards several volatile organic compounds (VOCs) was investigated in detail. Compared to pure In2O3 sensor, the best Co-In2O3 sensor (with optimized 0.5 mol% Co) exhibited outstanding improvements in triethylamine (TEA) sensing performance, especially including lower optimal working temperature (OWT: 220 °C vs 320 °C), higher response (Ra/Rg: 3500 vs 60 for 10 ppm TEA), and better selectivity (selectivity coefficient STEA/Sethanol: 400 vs 1850). Besides, an interesting selectivity transformation from ethanol to formaldehyde was also observed after Co doping. These improvements of Co-In2O3 PNSs were mainly attributed to their reduced crystallite size, increased oxygen vacancy concentration, narrowed bandgap, as well as upshifted Fermi level, whose mechanism was discussed.

Enhancing ORR activity and CO2 tolerance of Pr0.4Sr0.6Co0.2Fe0.8O3-δ-based SOFC cathode through synergistic doping and surface modification

The development of cathodes with high catalytic activity and stability remains a critical challenge for the commercialization of solid oxide fuel cells (SOFCs). In this study, a novel approach combining doping and surface modification was employed to enhance the catalytic activity and CO2 resistance of Pr0.4Sr0.6Co0.2Fe0.8O3-δ (PSCF) cathode. Initially, Ni was partially substituted for Fe in PSCF, resulting in the reduction of the average bond energy of metal–oxygen, thereby increasing oxygen vacancy concentration and electrochemical activity. The polarization resistance of PSCFN0.1 at 800 °C was found to be 0.025 Ω·cm2, representing a 32.4 % reduction compared to PSCF. Subsequently, Pr(NO3)3 solution was impregnated on the surface of the PSCFN0.1 using a spin-coating technique, resulting in the formation of a PrO2/PSCFN0.1 heterostructure that significantly improved the oxygen reduction reaction (ORR) kinetics. At 800 °C, the polarization resistance of PrO2-PSCFN0.1 was only 0.016 Ω·cm2, a 56.8 % reduction compared to PSCF. Moreover, the peak power density of PrO2-PSCFN0.1 reached 1190 mW·cm−2 at 800 °C, representing a substantial increase of 207.4 % compared to PSCF. Furthermore, PrO2-PSCFN0.1-based full cell exhibited excellent performance stability and CO2 resistance. This work provides an effective strategy to explore novel cathodes for SOFCs.

In-situ mullite whisker reinforced SiC porous ceramics with whiskers and bonding layers synchronously growing: Using CaF2 as a temperature-controlled whisker formation switch

Mullite-bonded SiC porous ceramics are of potential in hot gas infiltration while the insufficient mechanical performance limits its application. In-situ mullite whisker reinforcement is a potential solution but the formation temperature mismatch between mullite whisker and bonding layer is a challenge. In that case, mullite whiskers massively form ahead of the bonding layer excessively consuming alumina leading to poor bonding layers between SiC particles and thus, the mechanical performance. Herein, the mismatch was solved by using CaF2 as a temperature-controlled reaction switch. As a result, both the bonding layer and whisker synchronously formed. The material showed significant reinforcement effects, and exhibited a flexural strength of 124.8 MPa and a fracture toughness of 1.53 MPa·m1/2 with an open porosity of 36.6 vol%, realizing a breakthrough in the mechanical performance. Interestingly, an increase in oxygen vacancy concentration with the sintering temperature was first reported, which may benefit the whisker growth.

Regulation of oxygen vacancy concentration over ZnCr oxide in bifunctional catalyst for the direct conversion of syngas to light olefins

Metal oxides play a crucial role in catalytic activity of bifunctional catalyst for directly converting syngas into light olefins. A series of ZnCr oxides were synthesized with varying concentrations of precipitant and different atmospheres of precipitate treatment via co-precipitation method. The characterization results indicate that suitable concentration of precipitant and hydrogen calcination contribute to the generation of more oxygen vacancies that can promote the chemical adsorption of CO. Density function theory calculations confirmed that the adsorption energy of CO could be decreased with increasing concentration of oxygen vacancies, and that the formation energy of oxygen vacancies was lower under the hydrogen atmosphere as compared to the oxygen atmosphere. The catalytic activity demonstrated that more oxygen vacancies promoted the chemisorption and conversion of CO. The ZnCr oxide obtained through hydrogen calcination exhibited the highest concentration of oxygen vacancy and achieved a C2=-C4= selectivity of 86 % and a CO conversion of 43 % at 4 MPa and 5400 mL g−1h−1 combined with SAPO-34 zeolite. These findings suggest that increasing oxygen vacancy concentration on the surface of metal oxides is an effective approach for designing high-performance catalysts for directly converting syngas into light olefins.

Oxygen Vacancies Mediated Co‐Sputtered Ti‐Doped BiVO4 Thin Films‐Based Transparent Photovoltaic Device

AbstractBiVO4, a narrow bandgap material (2.5 eV), has been widely explored for photocatalytic applications, but its applications in the optoelectronic field are unexplored. This work explores BiVO4 for photovoltaic devices using the oxygen vacancies mediated co‐sputtered Ti‐doped BiVO4 (Ti:BiVO4) that exhibits on‐site power production by photovoltaics and see‐through features. The structural, chemical, and optical properties of Ti:BiVO4 are investigated for heterojunction formation with p‐type NiO film. The sputtering power of Ti plays a significant role in improving the light absorption capability by increasing oxygen vacancy concentration, enhancing the device performance. The devices show an open‐circuit voltage value of 676 mV and a short‐circuit current density value of 4.83 mA cm−2 with a maximum power production value of 122.2 µW under UV illumination of intensity 53.1 mW cm−2. The obtained device performances correlate with the Ti dopant's deposition power that tunes the structural and optical properties of BiVO4 films. Moreover, in self‐powered mode, the fabricated devices show a fast photoresponse speed of 0.8 ms with a high detectivity value of 2.22 × 1012 Jones. This work establishes the suitability of co‐sputtered Ti:BiVO4 for the next generation of transparent self‐powered optoelectronic devices.

Mn dopant-enabled BiFeO3 with enhanced magnetic and photoelectric properties

In this study, BiFe1−XMnXO3 (BFMXO, x = 0, 0.03, 0.05, 0.07, 0.10, and 0.15) nanoparticles were synthesized using the sol-gel technique. The XRD patterns confirm that the BFMXO nanoparticles exhibit a rhombic structure with the addition of Mn dopant, while structural distortion occurred as the doping concentration increased. In particular, Mn doping effectively induced lattice shrinkage and enhanced the lattice strain. The SEM and TEM images showed increased uniformity and grain size reduction as Mn doping concentration increased. The XPS spectra show that the conversion of Fe2+ to Fe3+ leads to the increase of oxygen vacancy concentration. Further, the M−H loops show that the saturation magnetization and residual magnetization of all Mn-doped samples were successfully improved with the gradual transformation from antiferromagnetism to ferromagnetism. Therefore, BiFe0.93Mn0.07O3 reached a maximum Ms and Mr of approximately 0.956 and 0.054 emu/g, respectively. Further, BiFe0.93Mn0.07O3 exhibited 4–5 times higher photoconductivity than the pristine sample. First-principles calculations further accounted for the improved magnetic and electrical properties of BFMXO induced by the Mn dopant. This study demonstrates an effective method for controlling the structure and characteristics of BFO-based nanomaterials for multifunctional device applications.

Preloaded oxygen vacancy conditioning Ni/TiO2 to enhance photocatalytic CO2 reduction

The regulation of oxygen vacancy (OV) defects plays a critical role in facilitating photocatalytic reactions. In this study, Nix/TiO2-OV photocatalytic materials with ultra-high oxygen vacancy concentrations (>33%) were prepared by preloading oxygen vacancies on Ni/TiO2 system. The experimental results show that the catalytic performance initially rises and subsequently declines with an increase in oxygen vacancy concentration. Among the samples, the best one (8Ni/TiO2-OV2) exhibits a noteworthy photocatalytic yield of CO (22.654 μmol·g−1·h−1) and CH4 (3.93 μmol·g−1·h−1). Density functional theory (DFT) shows that the excellent catalytic performance of 8Ni/TiO2-OV2 is due to that oxygen vacancy is in favour of the narrowing of the band gap, thereby promoting the excitation and efficient transfer of electrons. This study presents a promising novel method for obtaining ultra-high concentration of oxygen vacancies and explores the relationship between oxygen vacancy concentration and catalytic performance.

Impact of Bottom Electrode in HfO2-Based Rram Devices on Switching Characteristics

Resistive random-access memory (RRAM) devices with hydrogen plasma treated HfO2 have shown low power switching (1) and good conductance quantization with programing pulsed operation (2) that qualify them to be used for in-memory computing. Engineering the distribution of defects or oxygen vacancies near the top and bottom electrodes has a significant impact on reducing the switching power and improving the multi-level cell (MLC) characteristics of the device. A graded distribution with higher concentration of oxygen vacancies closer to the top electrode (TE) due to hydrogen plasma treatment and lower concentration near the bottom electrode (BE) reduces switching energy (power) (1). However, the impact of the bottom electrode is not well understood. In this work we investigated three RRAM stacks as shown in Figure 1 that have the same top electrode metal (50nm PVD TiN/5nm ALD TiN) and dielectric (HfO2 with hydrogen plasma treatment at the mid-point) but with different BE for each stack: TiN (device A (control)), TiN plus Mo with thermal anneal (device B), and TiN plus Mo with plasma nitridation (device C). Choosing a more inert metal as the BE provides a lower oxygen vacancy concentration at the interface between the BE and the dielectric (3) which in turn reduces the switching power as noted with device C (TiN plus Mo with plasma nitridation) which switches at a minimum compliance current (Icc) of 500pA. It is possible that the presence of nitrogen prevents the increase in oxygen vacancy concentration near the BE. On the other hand, a higher concentration of oxygen vacancies at the BE and the dielectric interface leads to a larger magnitude of band bending and increases the height of Schottky barrier. This possibly leads to an increase in the switching power in device A and device B as compared to device C (3-4). Note that device A (TiN) switched at Icc=30µA and device B (TiN plus Mo with thermal anneal) switched at Icc=8µA, which is superior compared to device A in switching power. This suggests that the alloy of TiN with Mo is reducing the oxygen vacancies near the BE. To study the MLC characteristics of the devices, we performed a train of 36 RESET pulses with fixed amplitude Vp= -1V and pulse width starting at 4µs and increasing by 20µs every 3 pulses, while we applied Vr=0.1V and tr=10µs after each pulse to measure the conductance. Figure 2 shows that device B (TiN plus Mo with thermal anneal) has good MLC with no overlap between the states while device A shows unstable MLC behavior due to time constant variation. Figure 3a shows the results of device C for the same RESET pulse train that was applied to devices A and B. Since device C switches at very low current and voltage, the conductance quantization was unstable after 15 pulses due to overstress of the conductive filament. Figure 3b shows the results after selecting the proper pulse set Vp= -0.5V and pulse width starting at tp=100ns and increasing by 100ns every 3 pulses, while we applied Vr=0.05V and tr=100ns after each pulse, resulting in a better conductance quantization compared to the previous setup. Appropriate engineering of the BE and managing of the oxygen vacancy distribution by hydrogen plasma treatment can significantly reduce the switching power.

Electrochemical activity of copper doped Sr2.7Pr0.3Fe2O7−δ cathode catalysts for solid oxide fuel cells

Due to their great efficiency, low cost and pollution, and flexible fuel choice, solid oxide fuel cells (SOFCs) have been applied to energy conversion filed. However, one challenge for developing SOFCs is sluggish oxygen reduction reaction (ORR) rate. Herein, a series of Ruddlesden-Popper-type Sr2.7Pr0.3Fe2–xCuxO7–δ (SPFCx) oxides are successfully synthesized and thoroughly assessed as potential SOFCs cathodes. Among all compositions, Sr2.7Pr0.3Fe1·9Cu0·1O7–δ (SPFC10) possesses the best electrocatalysis performance and improved oxygen reduction kinetics, with the lowest polarization resistance (Rp, 0.13 Ω cm2) at 700 °C. Cu-doping will contribute to improving oxygen transport kinetics, increasing oxygen vacancy concentration, and O 2p band centers close to the Fermi level, hence, enhancing the electrocatalysis performance. At 700 °C, the maximum output of the single cell with SPFC10 cathode is 704 mW cm−2. These results highlight a highly active SPFC10 cathode with Ruddlesden-Popper structure for SOFCs.

Chemiresistive room temperature NO2 sensor based on nitrogen doped zinc oxide nanowires

The prevalence of nitrogen dioxide (NO2) in the atmosphere is causing a steady increase in air pollution. As such, there is an urgent need to create a sensitive and energy-efficient NO2 gas sensor to ensure environmental and health safety. In this study, we explore the effectiveness of nitrogen-doped zinc oxide nanowires in detecting harmful NO2 gas. Through the hydrothermal method, we synthesized zinc oxide (NW-0) and nitrogen-doped zinc oxide nanowire sensors (NW-1, NW-2 & NW-4). The introduction of nitrogen induced changes in both the crystallite size and nanowire density in ZnO. Additionally, an increase in oxygen vacancy concentration was observed with nitrogen doping. At room temperature, the NW-2 sensor exhibited exceptional NO2 sensing performance (response of 110) compared to NW-0 sensor (response of 1.9) for 10 ppm of gas. The NW-2 sensor also demonstrated a rapid response/recovery time of 50 s/5 s for a 10 ppm NO2 concentration. Moreover, the sensor showed stability over an extended period, minimal response variation with changes in relative humidity, and selectivity towards NO2 over other gases. Complementing the experimental findings, density functional theory (DFT) calculations were employed to estimate the adsorption energies of NO2 on the sensor surface.

Analyzing the role of Ni dopant to change the structural, optical and photocatalytic properties of SnO2 nanoparticles

SnO2 and 5 wt% Ni doped SnO2 nanoparticles (SnO2:Ni NPs) were successfully synthesised by a template-free hydrothermal method. X-ray diffraction (XRD) patterns depicted polycrystalline nature of the NPs in rutile-type cassiterite phase with dominant (110) and (101) Bragg diffraction peaks. Rietveld refinement of XRD patterns supported single phase tetragonal crystal structure having space group P42/m n m. With Ni doping, crystallite size of NPs decreased from 39 nm to 35 nm whereas lattice strain increased from 3.56 × 10−3 to 3.99 × 10−3. This is attributed to the substitution of Sn4+ ion by Ni2+ ions. The morphology of the SnO2 NPs also changed from regular spherical shape to elongated irregular shape upon Ni doping. The dominant Raman peak obtained at 634 cm−1 matched with the signature peak for rutile SnO2 (Raman A1g mode). Further, we observed disappearance of E g mode due to Ni doping, which indicated the formation of oxygen vacancies. Also, XPS analysis indicated an increase of oxygen vacancy concentration in the doped NPs due to charge imbalance between Sn4+ and Ni2+. The direct optical band gap of SnO2 increased from 3.97 eV to 4.11 eV when doped with 5 wt% Ni and it is ascribed to Burstein–Moss effect. Irrespective of higher optical band gap of SnO2:Ni NPs, they showed enhanced photocatalytic activity to degrade Rhodamine B (RhB) dye molecules under UV-visible irradiation. The first order kinetic reaction rate constants for degradation of RhB were found to be 0.014 min−1 and 0.045 min−1 in case of SnO2 and SnO2:Ni NPs respectively. The enhanced photocatalytic activity in SnO2:Ni NPs is explained by relating to the formation of more oxygen vacancies and chemisorptions of O2 and H2O molecules followed by generation of radicals. This work demonstrates the superiority of SnO2:Ni NPs for use as photocatalytic material for industrial waste water treatment.

Enhanced oxygen reduction reaction activity and electrochemical performance of A-site deficient (La0.6Sr0.4)1-xFe0.8Ni0.2O3−δ cathode for solid oxide fuel cells

In this work, (La0.6Sr0.4)0.9Fe0.8Ni0.2O3-δ (LSFN90), a stable, highly ORR-active and cost-efficient perovskite oxide, is developed as cathode materials for solid oxide fuel cell (SOFC). The introduction of A-site deficiency results in the crystal expansion of the cubic perovskite phase and an increase in oxygen vacancy concentration at operating temperature. The LSFN90 cathode displays good oxygen reduction reaction activity and low polarization resistance values. The A-site deficiency facilitates the diffusion of oxygen ions in the electrode and accelerates the surface oxygen exchange reaction. LSFN90 is used as cathode materials for SOFC to prepare anode-supported single cells, achieving maximum power densities of 1.51, 1.27, 0.95 and 0.63 W cm−2 under wet hydrogen (3%H2O–97%H2) atmosphere at 850, 800, 750 and 700 °C, respectively. The introduction of A-site deficiency can greatly enhance the oxygen reduction reaction activity and electrochemical performance of the cathode, demonstrating that LSFN90 has significant potential as a cathode material for practical applications in solid oxide fuel cells.