Role Of Point Defects Research Articles

The Role of Point Defects in Heterojunction Photocatalysts: Perspectives and Outlooks

AbstractThe development of efficient photocatalysts is a mainstay for the advancement of photocatalytic technology. Heterojunction photocatalysts can overcome the inherent limitations of single photocatalysts, integrating the advantages of each component, and achieving efficient separation of photogenerated charges, making them a research focus in the field of photocatalysis. The role of point defects in photocatalytic reactions is diverse and can significantly influence the performance of photocatalysts. This review comprehensively summarizes the latest advancements in point defect engineering for the modulation of heterojunction photocatalysts. First, the types of point defects and their impact on photocatalytic processes are introduced. Subsequently, the common types of heterojunction photocatalysts are presented. Then, in accordance with recent research progress, the regulatory mechanisms of point defects within heterojunction photocatalysts are outlined, with a particular focus on their functions on surface/interfacial properties and photogenerated charge transfer processes. Finally, the challenges faced by point defect engineering in optimizing heterojunction photocatalysts and promising solutions are proposed. This review provides an in‐depth understanding of the role of point defect engineering in heterojunction photocatalysts, with the expectation of offering insights into the development of highly active photocatalytic materials.

Resistive Switching Mechanism in Ferroelectric Memristors with Thin Polycrystalline Barium Titanate Film

In a ferroelectric memristor, the manifestation of resistive effects is most often associated with the influence of polarization and dynamics of the domain structure on the charge transport. The role of point defects is either not taken into account or is reduced to the modulation of potential barriers at the interfaces with electrodes. However, the similarity of charge transport mechanisms in memristors based on thin ferroelectric and metal-oxide films suggests that the contribution of point defects in the anionic sublattice, namely, oxygen vacancies, to the resistive switching in ferroelectric memristors may be dominant. In order to identify the key factors responsible for resistive tuning in a ferroelectric memristor, a combined experimental and theoretical study of resistive switching effects in 10-nm polycrystalline barium titanate films was performed. X-ray photoelectron spectroscopy, piezoresponse force microscopy and scanning tunneling microscopy were employed to investigate the local resistive and ferroelectric properties and to estimate the stoichiometry of the films. A drift-diffusion numerical model of non-stationary processes in thin ferroelectric films was developed, including the Poisson equation and the continuity equation for each component of mobile charge carriers. Using the developed model as well as experimental I—V characteristics taken with scanning tunneling spectroscopy, the contribution of various mechanisms to the resistive switching in ferroelectric memristors with polycrystalline barium titanate has been studied. Non-stationary processes involving oxygen vacancies were found to govern the resistive switching.

Interaction between water and point defects inside volume-constrained α-quartz: An ab initio molecular dynamics study at 300 K

Quartz-based minerals in earth’s crust are well-known to contain water-related defects within their volume-constrained lattice, and they are responsible for strength-loss. Experimental observations of natural α-quartz indicate that such defects appear as hydroxyl groups attached to Si atoms, called Griggs defect (Si-OH), and molecular water (H2O) located at the interstitial sites. However, factors contributing to the formation of Griggs and interstitial H2O defects remain unclear. For example, the role of point defects like vacancy sites (O2− and Si4+), and substitutional (Al3+) and interstitial (Li+, K+, Ca2+, Mg2+, etc.) ions has remained largely unexplored. Here, we performed ab initio molecular dynamics at 300 K to examine the energetics and structure of water-related defects in volume-constrained α-quartz. Several configurations were systematically interrogated by incorporating interstitial H2O, O2− and Si4+ vacancies, substitutional Al3+, and interstitial Li+, Ca2+ and Mg2+ ions within α-quartz. Interstitial H2O defect was found to be energetically favorable in the presence of Substitutional Al3+, and interstitial Ca2+, Mg2+, and Li1+. In the presence of O2− and Si4+ vacancies, H2O showed a strong tendency to dissociate into OH—to form Griggs defect—and a proton; even in the presence of substitutional and interstitial ions. These ions distorted the α-quartz lattice and, in the extreme case, disrupted long-range order to form local amorphous domains; consistent with experimental reports. Our study provides an initial framework for understanding the impact of water within the crystal lattice of an anhydrous silicate mineral such as quartz. We provide not only thermodynamic and process-related information on observed defects, but also provides guidelines for future studies of water’s impact on the behavior of silicate minerals.

Unveiling the Interfacial Behavior of Au Contacted MoS2 Atomristor and the Role of Point Defects

Unveiling the Interfacial Behavior of Au Contacted MoS₂ Atomristor and the Role of Point Defects

Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO2 Photoanodes for Oxygen Evolution

AbstractAmong the various photoanode materials investigated for photoelectrochemical water splitting cells, TiO2 stands out due to its abundance, stability, and favorable valence band edge for water oxidation. In this study, the importance of introducing and combining oxygen and titanium vacancy point defects in anatase TiO2 photoanodes to improve their performance is unveiled, achieving a photocurrent density of 0.73 (±0.015) mA cm−2 at +1.23 VRHE under 100 mW cm−2 of simulated sunlight or 26.4 mA cm−2 at +1.23 VRHE under 100 mW cm−2 of 365 nm light. The characterization by X‐ray photoelectron spectroscopy, surface photovoltage, and electron paramagnetic resonance demonstrates that these oxygen and titanium vacancies can have both collective and localized positive effects on the material, leading to a narrowing of the bandgap, an increase in donor density, and an increase in hydroxyl groups on the surface of TiO2. These result in enhanced light absorption, conductivity, and photovoltage, as well as a more negative flat‐band potential and increase in hole flux to the semiconductor–electrolyte interface. These findings provide valuable insights into the role of point defects in modulating the properties of TiO2 and have important implications for the development of high‐performance TiO2‐based devices.

The effects of point defects on thermal-mechanical properties of BiCuOTe: a first-principles study.

Recently, BiCuOTe as a promising thermoelectric material has attracted extensive interest due to its lower thermal conductivity and higher electrical conductivity. However, little is known about the role of point defects in the growth, processing, and device degradation of this material. Moreover, the elastic properties which provide valuable information about the bonding characteristics, heat conductivity, and their anisotropic characters are investigated for effective design and characterization of new devices. Motivated by these considerations, a first-principles study about the stability of point defects and their effects on the thermal-mechanical properties of BiCuOTe was performed. The vacancies are found to be more stable than the interstitials. XO (here X occupying the O lattice site, with X = Cu, Bi or Te) are generally unfavorable among the considered point defects. Point defects generally have negative effects on elastic constants (except C66), suggesting that the resistance of defective systems to uniaxial and shear deformation is usually weaker than the that of ideal BiCuOTe. Similarly, point defects could deteriorate the ability to resist external compression. However, the introduction of point defects may improve the elastic compliances and depress the Debye temperature, which may increase the thermal expansion efficient of BiCuOTe. As compared with the ideal system, the point defects such as CuBi, TeBi, BiTe, OCu, CuTe and TeCu may generally reduce the phonon thermal conductivity. This study would provide insights into the effect of point defects on the elastic and thermal properties of BiCuOTe and has important implications in the rational design of superior thermoelectric materials.

Direct TEM Observation of Vacancy‐Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts

AbstractIncorporating hetero‐metal‐atom, e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions. However, the rational design of zeolites containing titanium at specific sites is difficult because the precise atomic structure during synthesis process remained unclear. Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE‐type zeolite precursor, based on a pre‐designed synthetic route including modification of vacancies followed by titanium insertion, where electron microscopy (EM) plays a key role at each step resolving the atomic structure. Point defects including vacancies in the precursor and titanium incorporated into the vacancy‐related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites.

Direct TEM Observation of Vacancy-Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts.

Incorporating hetero-metal-atom, e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions. However, the rational design of zeolites containing titanium at specific sites is difficult because the precise atomic structure during synthesis process remained unclear. Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE-type zeolite precursor, based on a pre-designed synthetic route including modification of vacancies followed by titanium insertion, where electron microscopy (EM) plays a key role at each step resolving the atomic structure. Point defects including vacancies in the precursor and titanium incorporated into the vacancy-related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites.

Role of point defects in stress-induced martensite transformations in NiTi shape memory alloys: A molecular dynamics study

Shape-memory properties of equiatomic NiTi rely on the thermal- or stress-induced reversible martensitic phase transformation between the $B2$ and $B{19}^{\ensuremath{'}}$ phases. Irradiation defects suppress thermal-induced transformations, but their effects on stress-induced transformations are poorly understood. We use molecular dynamics to investigate the effect of vacancies, vacancy clusters, interstitials, and antisite defects on stress-induced transformations. All defects suppress the transformation, but vacancy clusters do so to the greatest extent, while antisite defects do so to the least extent. The responsible mechanisms are grain boundary pinning and chemical disordering.

Decoupling the impact of bulk and surface point defects on the photoelectrochemical properties of LaFeO3 thin films.

Point defects (PDs) play a key role in the properties of semiconductor photoelectrodes, from doping density to carrier mobility and lifetime. Although this issue has been extensively investigated in the context of photovoltaic absorbers, the role of PDs in photoelectrodes for solar fuels remains poorly understood. In perovskite oxides such as LaFeO3 (LFO), PDs can be tuned by changing the cation ratio, cation substitution and oxygen content. In this paper, we report the first study on the impact of bulk and surface PDs on the photoelectrochemical properties of LFO thin films. We independently varied the La : Fe ratio, within 10% of the stoichiometric value, in the bulk and at the surface by tuning the precursor composition as well as selective acid etching. The structure and composition of thin films deposited by sol-gel methods were investigated by SEM-EDX, ICP-OES, XPS and XRD. Our analysis shows a correlation between the binding energies of Fe 2p3/2 and O 1s, establishing a link between the oxidation state of Fe and the covalency of the Fe-O bond. Electrochemical studies reveal the emergence of electronic states close to the valence band edge with decreasing bulk Fe content. DFT calculations confirm that Fe vacancies generate states located near the valence band, which act as hole-traps and recombination sites under illumination. Dynamic photocurrent responses associated with oxygen reduction and hydrogen evolution show that the stoichiometric La : Fe ratio provides the most photoactive oxide; however, this can only be achieved by independently tuning the bulk and surface compositions of the oxide.

An Ab Initio Study of Pressure-Induced Changes of Magnetism in Austenitic Stoichiometric Ni2MnSn.

We have performed a quantum-mechanical study of a series of stoichiometric NiMnSn structures focusing on pressure-induced changes in their magnetic properties. Motivated by the facts that (i) our calculations give the total magnetic moment of the defect-free stoichiometric NiMnSn higher than our experimental value by 12.8% and (ii) the magnetic state is predicted to be more sensitive to hydrostatic pressures than seen in our measurements, our study focused on the role of point defects, in particular Mn-Ni, Mn-Sn and Ni-Sn swaps in the stoichiometric NiMnSn. For most defect types we also compared states with both ferromagnetic (FM) and anti-ferromagnetic (AFM) coupling between (i) the swapped Mn atoms and (ii) those on the Mn sublattice. Our calculations show that the swapped Mn atoms can lead to magnetic moments nearly twice smaller than those in the defect-free NiMnSn. Further, the defect-containing states exhibit pressure-induced changes up to three times larger but also smaller than those in the defect-free NiMnSn. Importantly, we find both qualitative and quantitative differences in the pressure-induced changes of magnetic moments of individual atoms even for the same global magnetic state. Lastly, despite of the fact that the FM-coupled and AFM-coupled states have often very similar formation energies (the differences only amount to a few meV per atom), their structural and magnetic properties can be very different.

Role of Point Defects in the Formation of Relaxor Ferroelectrics

Relaxor ferroelectrics are engineered by doping a large amount of point defects into normal ferroelectrics. The point defects create nanoscale compositional heterogeneity, which in turn lead to local Curie temperature ( T C ) variation and random local electric and strain fields. However, it is still unclear about the individual roles played by each of these effects in converting a normal ferroelectric into a relaxor. In this study we distinguish these effects by carryout computer simulations using the phase field method. We find that although both the local-field and local- T C effects could lead to the formation of nanodomains in relaxors, it is the former that leads to the appearance of Burns temperature T B , the latter that leads to the appearance of the intermediate temperature T *, and a combination of the two allows one to model all three characteristic temperatures of a relaxor reported in experiments: T B , T * and the freezing temperature T f . This work unravels the detailed physical origin of the relaxor behavior and offers deep microscopic insight into relaxors.

Metastable solid 4He and the possible role of point defects

The metastable phase of solid 4He and the possible role of point defects in its destabilization are investigated by the introduction of a trial function of the shadow class with an explicit symmetrical kernel. This is a trial function that ensures the possible exchange of atoms and the delocalization of atoms and defects in a very effective manner. We show that the formation energy for vacancies is equal to zero at a pressure P c = 20 ± 2 atm, which is in excellent agreement with the experimental observation. The pressure at which a self-interstitial also has a formation energy equal to zero, is in agreement with the density where vacancies have the same property. Formation energies of a 3He interstitial or a substitutional impurity were estimated. Other properties of interest for systems made from 4He atoms are estimated and compared with results from the literature whenever available.

Zn ion diffusion in spinel-type cathode materials for rechargeable batteries: the role of point defects

Oxides with spinel structure are attracting increasing attention as potential cathode materials for multivalent ion batteries, such as those based on Mg2+, Ca2+ and Zn2+. Their electrochemical performances generally depend on synthesis conditions, as the intercalation properties of such materials can be highly influenced by the amount and distribution of point defects. In this work we employ atomistic modelling techniques to investigate the trends in the energetics of point defects formation and Zn2+ diffusion in spinel-type ZnCo2O4, in order to shed light on the correlation between structure, defects and intercalation properties. The results of this study indicate that ZnCo2O4 has the potential to be used as cathode material in Zn-ion rechargeable batteries and that synthesis approaches aimed at reducing the degree of spinel inversion and introducing cation under-stoichiometry are expected to improve the intercalation properties of this class of compounds.

Structure, stress, and mechanical properties of Mo-Al-N thin films deposited by dc reactive magnetron cosputtering: Role of point defects

In this work, the structural and mechanical properties of ternary Mo-Al-N alloys are investigated by combining thin film growth experiments and density functional theory (DFT) calculations. Mo1−xAlxNy thin films (∼300 nm thick), with various Al fractions ranging from x = 0 to 0.5 and nitrogen-to-metal (Al + Mo) ratio ranging from y = 0.78 to 1.38, were deposited by direct-current reactive magnetron cosputtering technique from elemental Mo and Al targets under Ar + N2 plasma discharges. The Al content was varied by changing the respective Mo and Al target powers, at a fixed N2 (20 SCCM) and Ar (25 SCCM) flow rate, and using two different substrate temperatures Ts = 350 and 500 °C. The elemental composition, mass density, crystal structure, residual stress state, and intrinsic (growth) stress were examined by wavelength dispersive x-ray spectroscopy, x-ray reflectivity, x-ray diffraction, including pole figure and sin2ψ measurements, and real-time in situ wafer curvature. Nanoindentation tests were carried out to determine film hardness H and elastic modulus EIT, while the shear elastic constant C44 was measured selectively by surface Brillouin light spectroscopy. All deposited Mo1−xAlxNy films have a cubic rock-salt crystal structure and exhibit a fiber-texture with a [001] preferred orientation. The incorporation of Al is accompanied by a rise in nitrogen content from 44 to 58 at. %, resulting in a significant increase (2%) in the lattice parameter when x increases from 0 to 0.27. This trend is opposite to what DFT calculations predict for cubic defect-free stoichiometric Mo1−xAlxN compounds and is attributed to variation in point defect concentration (nitrogen and metal vacancies) when Al substitutes for Mo. Increasing Ts from 350 to 500 °C has a minimal effect on the structural properties and phase composition of the ternary alloys but concurs to an appreciable reduction of the compressive stress from −5 to −4 GPa. A continuous increase and decrease in transverse sound velocity and mass density, respectively, lead to a moderate stiffening of the shear elastic constant from 130 to 144 GPa with increasing Al fraction up to x = 0.50, and a complex and nonmonotonous variation of H and EIT is observed. The maximum hardness of ∼33 GPa is found for the Mo0.81Al0.19N1.13 film, with nitrogen content close to the stoichiometric composition. The experimental findings are explained based on structural and elastic constant values computed from DFT for defect-free and metal- or nitrogen-deficient rock-salt MoAlN compounds.

Spontaneous relaxor to ferroelectric transition in lead-free relaxor piezoceramics and the role of point defects

The occupationally disordered structures and associated local polar fluctuations in lead-free relaxors determine their electrical properties that are also sensitive to external stimuli. These stimuli can lead to phase transitions, and the associated enhancement in the electro-mechanical responses necessitate a better understanding of these transitions. Here we report a non-canonical spontaneous phase transition from a relaxor to a ferroelectric phase in (Na1/2Bi1/2)TiO3-BaTiO3 with temperature. With the help of experiments (dielectric permittivity, diffraction, differential scanning calorimetry, polarization and Raman spectroscopy), a complete picture of the temperature evolution of relaxor behavior leading to this spontaneous phase transition has been reported. Furthermore, it has been shown that internal chemical pressure from oxygen vacancies can be utilized to tailor these phase transitions. Finally, an electric field-temperature phase diagram has been proposed with an emphasis on the influence of the defect chemistry. This work provides new insights into the origin of these spontaneous phase transitions.

ANALYSES OF STRUCTURE PHASE STABILITY OF U-Mo TARGET OF THE NEUTRON SOURCE

The paper presents analyses of structure phase stability of fuel materials by means of phase diagrams of martensite transformation method, proposed earlier for the system of “iron-carbon-vacancy”. It was shown that role of molybdenum in stabilization of uranium gamma-structure under irradiation is to hinder of the phase to phase transformations of martensite type. The role of point defects and electron structure in the process of homogenization of alloy structure in the process of irradiation was studied.

Influence of Vacancy on the Mechanical, Electronic and Thermodynamic Properties of Mo5SiB2 from First-Principles Calculations

Although Mo5SiB2 is a promising high-temperature material, the role of point defects on the structural stability, mechanical and thermodynamic properties of Mo5SiB2 is unclear. In this study, the first-principles calculations to study the influence of vacancies on the mechanical, electronic and thermodynamic properties of Mo5SiB2 were applied. Three vacancies: B-va, Si-va and Mo-va are considered. The results show that those vacancies are thermodynamically stable in Mo5SiB2. In particular, Mo-va is more thermodynamically stable than that of the B-va and Si-va. The calculated bulk modulus, shear modulus and Young’s modulus of the Mo5SiB2 are 271 GPa, 152 GPa and 384 GPa, respectively. Although those vacancies weaken the elastic deformation resistance of Mo5SiB2, they improve its deformation resistance along the c-axis. Importantly, the Mo-va1 improves the thermodynamic properties of Mo5SiB2 in comparison to the other vacancies. Finally, those vacancies result in a band shift from the conduction band to the valence band.

Role of point defects in hybrid phase TiO2 for resistive random-access memory (RRAM)

TiO2 thin films based resistive random-access memory was examined for resistive switching behavior. TiO2 thin films were deposited on FTO glass substrate using single step hydro-thermal method with different synthesis parameters. The x-ray Diffraction results confirmed the existence of both anatase and rutile phases (hybrid) in one film and only rutile phase in other film. Morphological studies of prepared TiO2 thin film were carried out using scanning electron microscope. The photo-luminescence results revealed the presence of Ti interstitial defects (Ti+4 & Ti+3) and oxygen vacancies in hybrid phase film. Whereas, Ti interstitial defects were absent in rutile phase film. Current voltage characteristics showed the presence of resistive switching in the hybrid phase film only. The switching characteristics have been attributed to originate from the presence of Ti interstitial defects, facilitating Ag+ ion migration in TiO2 dielectric layer. The observed resistive switching has been explained on the electrochemical metallization (ECM) model.

The role of point defects and defect gradients in flash sintering of perovskite oxides

The present study investigates the impact of point defects and their redistribution on the flash sintering process. Strontium titanate was chosen as a model system for the group of perovskite ceramics. The characteristics of flash sintering of strontium titanate were analyzed with different acceptor dopant concentrations. The onset of flash sintering was found to be dependent on the acceptor dopant concentration, as expected by the increasing conductivity. A gradient in the microstructure was found after flash sintering with larger grain sizes at the negative electrode. TEM-EDS measurements indicated Ti enrichment at the positive electrode for undoped strontium titanate and strong acceptor segregation for doped strontium titanate. In contrast, grain boundaries at the negative electrode were found to be stoichiometric for the undoped case and the acceptor segregation was less obvious for the doped case.Based on these results and the space charge behavior of strontium titanate, we infer that a gradient of the oxygen vacancy concentration is induced by the electric field during flash sintering: at the positive electrode the oxygen vacancy concentration is higher than at the negative electrode. For strontium titanate it is well known that a high oxygen vacancy concentration reduces the space charge and, hence, acceptor segregation, which agrees well with the experimental findings. Overall, the present study highlights the importance of point defect gradients and space charge for flash sintering of strontium titanate, which may be applicable for many other functional ceramics.