Effect Of Oxygen Vacancy Defects Research Articles

Effect of oxygen vacancy defects on electronic and magnetic properties of copper pyrophosphate material

The effect of oxygen vacancies (VO) on the electronic and magnetic properties of copper pyrophosphate dihydrate Cu2P2O7*2H2O (CuPPD) semiconductors is studied employing the DFT-based ab initio pseudopotential method. Site preference for vacancy formation on the selected oxygen sublattices is analyzed. Investigations revealed that the VO vacancies led to narrow bands of hybridized Cu-3d and O-2p states within the energy gap of perfect CuPPD semiconductors, significantly reducing the gap width from 2.17 eV to 0.19 eV and changing its character from indirect to direct one. Calculations have shown that the vacancies have no significant influence on the ferrimagnetic structure of CuPPD. The magnetic properties are driven by local magnetic moments of Cu atoms pointed oppositely. The oxygen vacancies slightly modify the spin polarization of d-orbitals of Cu atoms neighboring the vacancy, resulting in the non-zero total magnetic moment of 0.19 µB/FU (0.0 µB/FU in perfect CuPPD). The study's main finding, which can be exploited in nanoelectronics, is that the controlled implementation of oxygen vacancies can regulate the energy gap of CuPPD semiconductors.

Defect engineering of two-dimensional ultrathin Co3O4 nanosheets for wireless and high-performance monitoring of the sensing properties of volatile organic compounds

Volatile organic compounds (VOCs) as serious outdoor and indoor air pollutants have attracted great attention. Many detection technologies of VOCs have been investigated. However, it is challenging to develop an efficient and portable wireless sensor for monitoring VOCs. Here, we proposed a new strategy to obtain a high-performance VOC sensor based on two-dimensional Co3O4 nanosheets (NSs) with numerous oxygen vacancy defects. The Co3O4 NS-30 W sensor detected 200 ppm ethanol and exhibited a high response value (10.70) and fast response and recovery behavior (20 s and 17 s, respectively). The effect of oxygen vacancy defects on the sensitive properties was studied by theoretical calculations on the (110) plane of Co3O4. Furthermore, a unique VOC wireless sensor was fabricated in order to enable wireless early warning from multiple terminals and monitoring of alcohol in exhaled gas. The VOC sensor exhibited a promising potential for precisely monitoring the alcohol consumption of drivers.

The effect of oxygen vacancy defects in nano-CuO1−x on its catalytic properties in ammonium perchlorate thermal decomposition

The oxygen vacancy defect engineering has been identified as a valid strategy to modify the physicochemical and catalytic properties of semiconductor catalysts, but it’s rarely used in combustion catalyst. Herein, a novel oxygen deficient copper oxide (CuO1−x) nano-combustion catalyst was successfully prepared by a two-step process involving the controlled chemical precipitation and vacuum sintering. As expected, the CuO1−x exhibits enhanced catalytic activity towards the ammonium perchlorate (AP) thermal decomposition. Once CuO1−x nanocatalyst was added, the high-temperature decomposition peak temperature (THTD) and the apparent activation energy (Ea) of AP are respectively decreased by 109.2 °C and 79.2 kJ/mol, higher than those of commercial nano-CuO (80.6 °C and 56.4 kJ/mol), showing significant oxygen vacancy defect effects. In addition, the effect of oxygen vacancy defects in CuO1−x on the catalytic thermal decomposition behaviors and mechanism of AP were investigated by thermal analysis techniques and density functional theory for the first time. The results prove that the improved catalytic performance of CuO1−x towards the thermal decomposition of AP might relate to the enhanced adsorption and bonding capacity with NH3, which helps to prevent the accumulation of NH3 and consequently promote the thermal decomposition reaction of AP.

Confining manganese-based peroxymonosulfate activation in carbon nanotube membrane for phenol degradation: Combined effect of oxygen vacancy defects and nanoconfinement

In this study, an open ends carbon nanotube (OCNT) membrane, which confined fine-tuned oxygen vacancy defects (Odef) modified MnOx inside OCNT (MnOx-in-OCNT), was developed for enhancing peroxymonosulfate (PMS) activation towards phenol (PE) removal. The MnOx-in-OCNT membrane filtration coupled with PMS activation process (MFPA process) achieved outstanding removal (100%) of PE at a high permeate flux of 585.0 L m2 h−1 bar−1 (residence time of only 8.8 s). The unit degradation rate (2 min running time) and mass transfer rate of MnOx-in-OCNT were 2.0 and 8.4 times higher than that of CCNT (closed caps CNT) membrane with surface-loaded MnOx (MnOx-out-CCNT) MFPA process, and its catalytic performance was much superior to MnOx-in-OCNT powder reactions and MnOx-in-OCNT membrane batch reactions. The PMS activation mechanism experienced an obvious enhancement yield of reactive oxygen species (ROS) from MnOx-out-CCNT to MnOx-in-OCNT MFPA process. The enhanced PMS activation and boosted ROS yield of MnOx-in-OCNT was mainly attributed to the co-effect of the fine-tuned Odef and nanoconfinement. The contact frequency between PMS, pollutant and Odef was greatly enhanced via nanoconfinement effect, leading to the enhanced yield of SO4•− and •OH. Meanwhile, the O2 was enriched and reacts with enhanced Odef under nanoconfined-space to produce O2•− and 1O2. Then, the co-enhanced ROS (SO4•−, •OH, 1O2 and O2•−) displayed wide applicability for phenolic pollutants removal owing to their presence of –OH. Finally, the possible degradation pathway of PE was deduced by DFT calculations and HPLC analysis.

Synergistic effect of oxygen vacancy defects and TiO2/WO3 heterostructures in photocatalytic hydrogen production and dye degradation

The exploration of cost-effective and efficient photocatalysts for environmental remediation and hydrogen production is a topic of immense importance in contemporary research. Titanium dioxide (TiO2) has been extensively researched in the area of environmental and energy photocatalysis due to its outstanding photocatalytic activity. However, its high photogenerated carrier recombination rate and wide bandgap limit the application. In this work, a TiO2/WO3 heterostructure nanocomposite with oxygen vacancy defects was synthesized through a simple sol-gel method. The introduction of oxygen vacancies reduced the bandgap and expanded the light absorption range, while the TiO2/WO3 heterostructure increased the photogenerated carriers and facilitated their separation. The resulting TiO2/WO3 nanocomposites exhibited enhanced photocatalytic activity, producing 101.88 μmol of hydrogen in 2 h for 100 mg of the nanocomposites, while the reaction constant for degrading dyes reached 0.1414 min−1. Moreover, the nanocomposites showed excellent stability under sunlight. This exploration provides a novel approach for designing and preparing TiO2-based composite photocatalysts.

Effect of oxygen vacancy defects on electronic and optical properties of MgO monolayers: First principles study

The optoelectronic properties induced by oxygen vacancy defects in MgO (111) monolayers have been studied using hybrid level of DFT method. HSE calculations show significant reduction in electronic band gap of MgO monolayer as a result of introduction of oxygen vacancies. The pristine monolayer has a wide band gap (4.84 eV, indirect) semiconducting behaviour, which changes to 2.97 eV (indirect) and 2.28 eV (direct) with increment in oxygen vacancy defect concentrations of 6.25 % and 12.5 %, respectively. Consequently, presence of oxygen vacancies leads to energy red shift of the observed optical phenomena with reference to the pristine monolayer. Most importantly, the di-vacancy system with two consecutive vacancy sites displays the strongest optical absorption and also becomes optically responsive over the spectral range from visible to ultraviolet region in the electromagnetic spectrum. Thus creation of oxygen vacancies in MgO monolayers may be a fruitful technique for achieving a suitable solar energy material.

Efficient charge separation in sulfur doped AgFeO2 photocatalyst for enhanced photocatalytic U(VI) reduction: The role of doping and mechanism insights

Photocatalytic reduction of U(VI) in aqueous solutions has been considered as an efficient and promising technology to solve radioactive U pollution. In this work, density functional theory (DFT) calculations were firstly employed to optimize and compare the adsorption configurations combined uranium with four given photocatalysts, then their adsorption energies were − 0.97 eV for AgFeO2, − 1.15 eV for Zn doped AgFeO2, − 1.73 eV for Cu doped AgFeO2 and − 2.66 eV for S doped AgFeO2, respectively, indicating the sulfur doping plays a major role in U(VI) photoreduction. Herein, a visible light responsive efficient sulfur doped AgFeO2 photocatalyst (S doped AgFeO2) was synthesized and utilized to photocatalytic reduction of U(VI) in aqueous solutions. According to XRD, XPS and TEM analysis, the sulfur was successfully doped in AgFeO2 via the hydrothermal method. The batch experimental showed that S doping enhanced the U(VI) photoreduction activity of AgFeO2, and the S-AFO-3 photocatalyst exhibited the highest photocatalytic activity (92.57%), which was 1.5 times than that of pure AgFeO2. ESR, PL and DFT results demonstrated that the enhancement of adsorbed U(VI) photoreduction was attributed to the own unique effect of oxygen vacancy defects and efficient charge separation of S doped AgFeO2 photocatalyst. Due to its higher adsorption energies, fast-U(VI) photoreduction rate and superior chemical stability, the sulfur doped AgFeO2 photocatalyst is hoped for water remediation containing U(VI) wastewater.

Effect of (Zn, Mn) co-doping on the structure and ferroelectric properties of BiFeO3 thin films

BiFe1-2xZnxMnxO3 (BFZMO, with x = 0–0.05) thin films were synthesized via sol–gel method. Effects of (Zn, Mn) co-doping on the structure, ferroelectric, dielectric, and optical properties of BiFeO3 (BFO) films were investigated. BFZMO thin films exhibit rhombohedral structure. Scanning electron microscopy (SEM) images indicate that co-doping leads to a decrease in grain size and number of defects. Leakage current density (4.60 × 10−6 A/cm2) of BFZMO film with x = 0.02 was found to be two orders of magnitude lower than that of pristine BFO film. Owing to decreased leakage current density, saturated P–E curves were obtained. Maximum double remnant polarization of 413.2 μC/cm2 was observed for BFZMO thin film with x = 0.02, while that for the BFO film was found to be 199.68 μC/cm2. The reason for improved ferroelectric properties is partial substitution of Fe ions with Zn and Mn ions, which resulted in a reduction in the effect of oxygen vacancy defects. In addition, co-doping was found to decrease optical bandgap of BFO film, opening several possible routes for novel applications of these (Zn, Mn) co-doped BFO thin films.

Phase transformation and room temperature stabilization of various Bi2O3 nano-polymorphs: effect of oxygen-vacancy defects and reduced surface energy due to adsorbed carbon species.

We report the grain growth from the nanoscale to microscale and a transformation sequence from Bi →β-Bi2O3→γ-Bi2O3→α-Bi2O3 with the increase of annealing temperature. The room temperature (RT) stabilization of β-Bi2O3 nanoparticles (NPs) was attributed to the effect of reduced surface energy due to adsorbed carbon species, and oxygen vacancy defects may have played a significant role in the RT stabilization of γ-Bi2O3 NPs. An enhanced red emission band was evident from all the samples attributed to oxygen-vacancy defects formed during the growth process in contrast with the observed white emission band from the air annealed Bi ingots. Based on our experimental findings, the air annealing induced oxidation of Bi NPs and transformation mechanism within various Bi2O3 nano-polymorphs are presented. The outcome of this study suggests that oxygen vacancy defects at the nanoscale play a significant role in both structural stabilization and phase transformation within various Bi2O3 nano-polymorphs, which is significant from theoretical consideration.

Template assisted nanoporous TiO2 nanoparticles: The effect of oxygen vacancy defects on photovoltaic performance of DSSC and QDSSC

We present a detailed study on the photovoltaic performance of dye and quantum dot (QD) sensitized solar cells fabricated using nanoporous TiO2 photoanodes prepared by a template approach. Nanoporous TiO2 particles (TNPO) are prepared by sol-gel method using dextran (TNPO-D) and glucose (TNPO-G) as templates. The decomposition of templating agents during calcination introduces nanopores (∼10 nm) in TiO2 nanoparticles as evidenced from the HRTEM studies. Oxygen vacancies are also introduced in TiO2 nanoparticles due to prevailing reducing ambient during decomposition. Oxygen vacancy (VO) related defects are evidenced from EPR spectra from a strong absorption signal observed at g-value of 1.994 related to Ti3+-VO-Ti3+ defect sites. The defect concentration estimated from EPR spectra is found to be ∼5 to 15 times larger for nanoporous TiO2 particles compared to Degussa P25-TiO2. TNPO samples also show enhanced defect level photoluminescence emission. The photovoltaic efficiency (η) and the IPCE of DSSC and QDSSC are strongly influenced by the defect concentration and seen to decrease with increasing VO in TiO2 nanoparticles. Twofold lower η is found in both DSSC (∼6.38% to 3.66%) and QDSSC (∼1.69% to 0.46%) devices made using nanoporous TiO2 compared to P25-TiO2 photoanode solar cells. P25-TiO2 also shows significant decrease in η when oxygen vacancies are introduced by hydrogen annealing process, which reveal detrimental effect of VO on the photovoltaic properties. EIS studies suggest increased interfacial resistance in solar cells made from nanoporous TiO2 photoanodes.

Tremendous effect of oxygen vacancy defects on the oxidation of arsenite to arsenate on cryptomelane-type manganese oxide

Manganese oxide as a benign and low-cost oxidant has been widely investigated for the oxidation of As(III) to As(V), but its As(III) oxidation rate is easily hindered due to the thermodynamically favorable adsorption of the As(V) species produced and coexisting ions. Herein, we develop a novel strategy of tremendously increasing the As(III) oxidation activity via tuning the concentration of oxygen vacancy defects (OVD) in cryptomelane-type manganese oxides (OMS-2). The result of the batch experiment of the As(III) oxidation on OMS-2 shows that increasing the concentration of oxygen vacancy defects in OMS-2 remarkably increases the specific As(III) oxidation rate from 0.12 to 0.22μmolm−2min−1, and dramatically reduces the unfavorable effect of the As(V) adsorption on the As(III) oxidation activity. The origin of the tremendous effect of OVD in OMS-2 on the As(III) oxidation activity is experimentally and theoretically investigated by the adsorption of the As(III) and As(V) species on OMS-2, the point of zero charge (pHPZC), the evolution of the concentration of Mn2+ ions dissolved in the solution, and density functional theory (DFT) calculations. The results reveal that the presence of OVD in OMS-2 not only makes the adsorption of As(III) on OMS-2 more thermodynamically favorable, but also accelerates the charge transfer from As(III) to Mn atom, thus significantly improving the As(III) oxidation activity.

Photocatalytic properties of titania thin films prepared by sputtering versus evaporation and aging of induced oxygen vacancy defects

In order to understand the variations in photocatalytic efficiencies of titania thin films prepared by different physical vapor deposition techniques, we studied the microstructure and resulting properties for the two widely used PVD methods, electron beam evaporation and reactive pulsed DC magnetron sputtering. In addition, we investigated the effect of oxygen vacancy defects induced by tempering in reducing atmospheres and the aging behavior. After deposition, several tempering series in oxygen, air, argon, and forming gas were carried out to control the amount of oxygen vacancy defects. The films were characterized with respect to crystallinity, texture, grain growth, grain structure, surface roughness, light transmission as well as band gap energy; the photocatalytic efficiency was measured via methylene blue degradation. The results show different nucleation and growth mechanisms between evaporated and sputtered titania thin films, resulting in severe influence on photocatalytic efficiency. The evaporated thin films exhibited homogeneous nucleation and growth, and stayed in the anatase structure even after tempering at 800°C for 1h. In contrast, the sputtered thin films started to form grains at the interface to the substrate and showed heterogeneous nucleation and growth. Moreover, the sputtered films already formed rutile when tempered at 600°C for 1h. The gain in surface area due to tempering, which promotes adsorption of photocatalytic reactants, was more pronounced in sputtered thin films (+29%) compared to evaporated thin films (+6%). Films with oxygen vacancy defects, preserved by tempering in argon or induced by tempering in forming gas, showed a large improvement in photocatalytic efficiency. However, aging of the samples over a period of 19 months lead to a progressive decline in efficiency, finally reaching the level of thin films tempered in air, which had remained stable over the same 19 month period. This strongly questions widely applied concepts based on improving the photocatalytic efficiency via oxygen vacancy defects.

(Invited) Understanding the Switching Mechanism in RRAM Devices and the Dielectric Breakdown of Ultrathin High-k Gate Stacks from First Principles Calculations

RRAM devices received increased interest lately as advanced non-volatile memory technologies in terms of low operating power, high density, better non-volatility, fast switching speed, and compatibility with conventional CMOS process. However, up to date the fundamental physical principles controlling the switching are not well understood. We have employed first-principles simulations based on density functional theory (DFT) to elucidate the effect of oxygen vacancy defects on the electronic structure of rutile TiO2 and NiO using the local density approximation with correction of on-site Coulomb interactions (LDA+U). The vacancy filament induces several defect states within the band gap, which can lead to the defect-assisted electron transport and account for on-state low resistance conduction in bulk rutile TiO2 and NiO. For CMOS devices on the other hand the reliability of the gate stack is becoming a significant challenge with the continuous scaling of transistors, due to the ultrathin oxides and defects in the gate stack. The degradation of the gate oxides has been observed under electrical stress, due to traps generated by defects, e.g. oxygen vacancies present in these materials. First principles methods based on density functional theory are used to determine the location of the defect states in the band gap when these defects are at the various interfaces of the gate stack and how they contribute to the oxide breakdown in ultrathin gate stacks.