Vacancy Concentration Research Articles

CO2 hydrogenation to light olefins over highly active and selective Ga-Zr/SAPO-34 bifunctional catalyst

The production of light olefins from the hydrogenation of CO2 is an efficient way to utilize CO2, where the surface oxygen vacancy in metal oxide plays an important role in CO2 adsorption and activation. Here, the Ga-Zr metal oxides were prepared by hydrolysis of urea at different temperatures and combined with SAPO-34 to prepare the bifunctional catalyst for CO2 hydrogenation to light olefins. The surface oxygen vacancy content of Ga-Zr oxide increases with increasing urea hydrolysis temperature, and a high CO2 conversion of 26.4% and C2=–C4= hydrocarbon selectivity of 87.2% were obtained by a well-matched amount of desorbed CO2 and H2. Using the CO2 and H2/HCOOH/CH3OH as probe molecules, the in-situ DRIFT spectra reveal that the CO2 could be activated on surface oxygen vacancy and converted to CO3* and HCO3* species, which were further hydrogenated to HCOO* and CH3O* species. While the by-product CO mainly originates from the decomposition of HCOO* and the presence of SAPO-34 converts CH3O* to C2=–C4=. The current study illustrates that boosting the surface oxygen vacancy in defected surfaces of metal oxide and providing a matching H2 dissociation ability is the key to improve the performance of CO2 hydrogenation to light olefins.

Optimization of tribocatalytic performance by modifying the concentration of oxygen vacancies in KSr2Nb4TaO15 ceramics

Optimization of tribocatalytic performance by modifying the concentration of oxygen vacancies in KSr₂Nb₄TaO₁₅ ceramics

Improved-Performance Amorphous Ga2O3 Photodetectors Fabricated by Capacitive Coupled Plasma-Assistant Magnetron Sputtering

Ga2O3 has received increasing interest for its potential in various applications relating to solar-blind photodetectors. However, attaining a balanced performance with Ga2O3-based photodetectors presents a challenge due to the intrinsic conductive mechanism of Ga2O3 films. In this work, we fabricated amorphous Ga2O3 (a-Ga2O3) metal–semiconductor–metal photodetectors through capacitive coupled plasma assisted magnetron sputtering at room temperature. Substantial enhancement in the responsivity is attained by regulating the capacitance-coupled plasma power during the deposition of a-Ga2O3. The proposed plasma energy generated by capacitive coupled plasma (CCP) effectively improved the disorder of amorphous Ga2O3 films. The results of X-ray photoelectron spectroscopy (XPS) and current-voltage tests demonstrate that the additional plasma introduced during the sputtering effectively adjust the concentration of oxygen vacancy effectively, exhibiting a trade-off effect on the performance of a-Ga2O3 photodetectors. The best overall performance of a-Ga2O3 photodetectors exhibits a high responsivity of 30.59 A/W, a low dark current of 4.18 × 10−11, and a decay time of 0.12 s. Our results demonstrate that the introduction of capacitive coupled plasma during deposition could be a potential approach for modifying the performance of photodetectors.

Revealing the promotional effect of Zr and phosphate Co-decorated on δ-MnO2 catalysts for highly efficient catalytic elimination of chlorinated VOCs: An experimental and theoretical study

The development of advanced catalysts for catalytic oxidation of CVOCs still presents challenges in terms of low activity, chlorination poisoning and the formation of toxic byproducts. In this study, a series of nP-MnxZry catalysts with exceptional catalytic oxidation activity were developed for the removal of CVOCs in industry. Among them, 15P‐MZ exhibited the optimal catalytic activity and stability for CB degradation, achieving a T90 of 180 ℃. Notably, the CB oxidation rate of 15P‐MZ was found to be 5.76 times higher than that of MnO2 at 155 °C. Structure and properties analysis combining with DFT calculations revealed that the dual modification strategy involved Zr doping and phosphate loading induced the formation of high concentration of oxygen vacancy over 15P‐MZ, which enhanced oxygen mobility, facilitating the breakage of the aromatic ring. Meanwhile, this strategy introduced more Brønsted acid sites, promoting the dissociation of Cl in the form of HCl, thereby inhibiting the formation of polychlorinated byproducts. Moreover, the activation effect of phosphate on the dissociation of H2O further promoted the Cl dissociation, thereby regenerating more active sites and improving catalytic activity and stability in humid environment, making it more promising for industrial applications.

Simultaneous Improvement of Energy Storage Characteristics and Temperature Stability in K0.5Na0.5NbO3-Based Ceramics via LiF Modification.

The splendid energy storage performances with eminent stability of dielectric ceramics utilized in pulsed power devices have been paid more attention by researchers. This scheme can be basically realized through introducing Li+, Bi(Mg2/3Ta1/3)O3, NaNbO3, and LiF into KNN-based ceramics. Under the breakdown strength (BDS) of 460 kV/cm, an outstanding energy storage density (W) of 6.05 J/cm3 with a high energy efficiency (η) of 85.9% is implemented. Within the broad temperature range from 20 to 140 °C, the numerical fluctuations of energy storage characteristics can be maintained at a relatively stable level (ΔWrec ≈ 3.5%, Δη ≈ 2.8%). As for the charging-discharging performances, this component possesses a fast discharging speed (t0.90 ≈ 51 ns) and remarkable temperature stability (the variations are smaller than 3.5%). Additionally, the internal mechanisms of outstanding energy storage properties can be confirmed via crystal structures and domain structures, the content of oxygen vacancies, dielectric and impedance spectra, and phase simulation. Hence, the combination of outstanding energy storage with remarkable thermal stability can be fulfilled in one ceramic system according to this discovery, providing a research thought of developing the materials for dielectric capacitors.

Bimetal Fluorides with Adjustable Vacancy Concentration Reinforcing Ion Transport in Poly(ethylene oxide) Electrolyte.

The poor ambient ionic transport properties of poly(ethylene oxide) (PEO)-based SPEs can be greatly improved through filler introduction. Metal fluorides are effective in promoting the dissociation of lithium salts via the establishment of the Li-F bond. However, too strong Li-F interaction would impair the fast migration of lithium ions. Herein, magnesium aluminum fluoride (MAF) fillers are developed. Experimental and simulation results reveal that the Li-F bond strength could be readily altered by changing fluorine vacancy (VF) concentration in the MAF, and lithium salt anions can also be well immobilized, which realizes a balance between the dissociation degree of lithium salts and fast transport of lithium ions. Consequently, the Li symmetric cells cycle stably for more than 1400 h at 0.1 mA cm-2 with a LiF/Li3N-rich solid electrolyte interphase (SEI). The SPE exhibits a high ionic conductivity (0.5 mS cm-1) and large lithium-ion transference number (0.4), as well as high mechanical strength owing to the hydrogen bonding between MAF and PEO. The corresponding Li//LiFePO4 cells deliver a high discharge capacity of 160.1 mAh g-1 at 1 C and excellent cycling stability with 100.2 mAh g-1 retaining after 1000 cycles. The as-assembled pouch cells show excellent electrochemical stability even at rigorous conditions, demonstrating high safety and practicability.

Insight into insulation degradation mechanism of Al2O3 involved with positive and negative defects

Al2O3 with the desired electrical insulating properties and thermal shock resistivity has been extensively applied in the field of solid oxide fuel cells and oxygen sensors. However, degradation of the insulating Al2O3 layer is an intractable issue in practical applications. In this study, different point defect structures of Al2O3 were realized with the substitutional doping effect of ZrO2 and MgO. The MgO dopant provides positively charged oxygen vacancies, whereas the ZrO2 dopant tends to trigger negatively charged vacancy formation at Al3+ sites. The oxygen vacancy concentration of Al2O3 exhibits the following trend: MgO-doped Al2O3 > Al2O3 > ZrO2-doped Al2O3. Furthermore, the densification morphology, insulating properties, and oxygen vacancy migration of Al2O3 have been confirmed to be largely affected by the extrinsic factors. This study indicates that oxygen vacancy migration depends on the applied electric field at high temperatures. As the voltage and temperature increase, oxygen vacancy migration shows obvious electric-field-dependent characteristics, and its aggregation macroscopically shows hole defects. The defect position of Al2O3 is nonstoichiometric Al2O3-x with poor crystallinity. Therefore, it is believed that the oxygen vacancy migration triggered by the second phase directly determines the insulation performance and causes the degradation of Al2O3 materials.

Effects of electrical and mechanical properties of Ba0.92Ca0.08Zr0.05Ti0.95O3 ceramics with lanthanum dopants

Element doping is one of the important approaches to improve the performance of lead-free piezoelectric ceramics. In this study, Ba0·92Ca0·08Zr0·05Ti0·95O3-xLa (BCZT-xLa) piezoelectric ceramics were prepared by sol–gel method. The effects of La3+ contents on the phase structure, mechanical properties, and electrical properties of BCZT-xLa ceramics were investigated. Results showed that La3+ could affect the properties of ceramics by adjusting the concentration of oxygen vacancies and microstructure. When the La3+ contents were 0.008, the ceramics showed the best performance (the maximum permittivity was 8736.27, the remnant polarisation was 9.69 μC/cm2 and the coercive field was 3.44 kV/cm). The influences of Vickers hardness on ceramics were investigated using various La3+ contents. The lattice energy and oxygen vacancies were the main factors for the change in Vickers hardness. With an increment of x, the Vickers hardness values decreased from 6.271 GPa to 1.438 GPa. This study could provide a reference for BCZT piezoelectric ceramics doped with rare earth elements.

Enhanced oxygen evolution on A-site defect perovskite oxide through interfacial engineering

Perovskite oxides are emerging as promising alternative to precious metal-based electrocatalysts for oxygen evolution reactions. Despite their potential, their catalytic activity is often insufficient for practical applications. In this study, we demonstrate that introducing A-site defects in LaNiO3 perovskite oxides promotes B-site exsolution during the reduction process. Subsequent chemical vapor deposition introduces selenium, forming an electrocatalyst with a heterojunction structure. Comprehensive characterization and electrochemical testing reveal that the r-La0.9NiO3/NiSex heterojunction structure, resulting from B-site exsolution induced by A-site defects, significantly enhances the electrocatalytic performance of the La0.9NiO3 electrocatalyst. This novel structure not only increases oxygen vacancy concentration but also improves the wettability of the electrocatalyst, as indicated by a reduced bubble contact angle in water. These modifications lead to a notable improvement in the electrochemical performance of the r-La0.9NiO3/NiSex electrocatalyst. At a current density of 10 mA·cm−2, the electrocatalyst exhibits an overpotential of 297.6 mV, with substantially increased mass activity. This study presents a novel approach to catalyst design in electrocatalysis, leveraging A-site defect induced B-site exsolution in perovskite oxides. The strategy of reduction followed by doping offers a robust framework for developing more efficient electrocatalysts, paving the way for advancements in the field of heterogeneous catalysis.

Self‐Confined Construction of Facet Heterojunction with Tunable Band Alignment for Enhanced Photocatalytic CO2 Reduction

AbstractAlthough crystal facet engineering is extensively studied in energy and environmental technologies, the in‐depth understanding of intrinsic mechanisms governing crystal facet heterojunctions in tuning band alignments is still limited. Here, novel Bi5O7NO3 crystals exposing tailor {080} facets are synthesized via NH4+‐assisted self‐confined construction. It has been confirmed that NH4+ ions selectively adsorb on the {141} facets, inversely inducing the growth of desired {080} crystal facets, while the tailored {080} facets facilitate the generation of oxygen vacancies. The controllable concentration of oxygen vacancies, influenced by different exposed facets, can optimize the relative positions of Fermi levels and shift the photoelectron transfer route between the {141} and {080}‐OV facets from type‐II to S‐scheme, thus triggering rapid charge transport channels and effectively suppressing electron‐hole recombination. DFT calculation verifies that the energy barrier for the *COOH formation on Bi5O7NO3‐{080}‐OV is the lowest, thereby promoting the generation of CO. The well‐designed Bi5O7NO3 crystals with the optimal {080}/{141} facet ratio exhibit a 3.8‐fold enhancement in photocatalytic CO2 reduction, compared to traditional Bi5O7NO3 dominated by the {141} facets. This work offers new insights into the regulation of band alignments at heterointerfaces and the design of high‐efficiency photocatalysts.

Al-Doped CuS Colloidal Nanocrystals as a Highly Efficient Electrochemiluminescence Emitter for the Ultrasensitive Detection of Circulating Tumor DNA.

Herein, CuS colloidal nanocrystals (NCs) with adjustable band gap and good film forming ability have been synthesized as new ECL materials. Furthermore, the band gap and oxygen vacancy (OV) content of CuS NCs are regulated by Al3+ doping, which significantly improves the ECL response of CuS NCs. First, the band gap of CuS-Al NCs decreases after doping with Al3+, which makes it easier for electrons to transition across the band gap. At the same time, the oxygen vacancy of CuS-Al NCs increases, which is conducive to improving the conductivity and promoting charge transfer, thus improving the ECL performance of CuS-Al NCs. Circulating tumor DNA (ctDNA) is an important tumor marker, and its sensitive monitoring is of great significance for tumor diagnosis, treatment, and prognosis detection. Therefore, an ECL biosensor for ultrasensitive detection of circulating tumor DNA (ctDNA) was prepared by using CuS-Al NCs as luminescent material and combining multiple antidromic hybrid chain reaction (anti-HCR) strategy mediated by the target. Compared with the process of target-induced HCR generation, this strategy first forms multiple HCR products and then destroys the already formed HCR products by target-induced destruction, which enhances the sensitivity of target response and improves the reaction efficiency. The constructed biosensor has good detection performance, and the detection limit is as low as 2.74 aM. This work puts forward the luminescence phenomenon of colloidal nanocrystals as new ECL materials, which broadens the application of ECL technology in ultrasensitive biochemical detection.

Conduction in yttrium iron garnets with nickel dopant: Potential for sensor applications

Doped yttrium iron garnet materials exhibit semiconducting behavior and have potential applications in resistive-based sensors, including thermoresistive and gas sensors. In this study, we synthesized Ni-doped yttrium iron garnet samples using the sol-gel method combined with heat treatment. The crystal structure, morphology, and Ni incorporation into the crystal lattice were investigated using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy. The saturation magnetization of the samples in the ground state was measured in a Physical Property Measurement System (PPMS) at 5 K. The experimental magnetic moments deviate significantly from the values calculated based on the cation distribution models, suggesting the occupation of Ni2+ and Fe2+ at the octahedral and tetrahedral sites, respectively, with the latter due to oxygen vacancies. We investigated the resistivity, magnetoresistance effect, and hydrogen sensing properties to elucidate the conduction mechanism. At room temperature, the resistivity of the samples decreased sharply as the oxygen vacancy concentration increased, from ∼108 Ωcm in sample x = 0 to ∼106 Ωcm in sample x = 0.08. The resistance of the samples increased when exposed to hydrogen gas, supporting the prediction from the magnetization data that the samples exhibit n-type semiconductor characteristics. The samples showed good sensitivity to H2 gas. At 250°C, sample x = 0.08 exhibits a response of S =11.5, the response time (τres. ∼8 s), and recovery time (τrec. ∼22 s) at 100 ppm hydrogen concentration.

An efficient Au/ZnO catalyst for the photocatalytic conversion of methane to formaldehyde

The photocatalytic oxidation of methane to value-added chemical products under mild conditions is crucial for resource utilization and environmental protection. However, inefficient generation of reactive oxygen species partially limits methane activation. To tackle this issue, we have developed highly dispersed Au-modified ZnO nanosheets through photo-deposition method for the photocatalytic oxidation of methane to formaldehyde. The optimized Au0.1/ZnO catalyst (0.1 wt% Au on ZnO) achieves a HCHO yield of 19.14 μmol h−1 with a selectivity of 87.94 %. Electron paramagnetic resonance, Raman, X-ray photoelectron spectroscopy and density functional theory show that Au enhances the concentration of oxygen vacancies (OVs) in ZnO. This enhancement is attributed to a strong interaction between Au and ZnO, which facilitates the transfer of electrons from the ZnO surface to Au. Under irradiation, Au and OVs function as hole and electron acceptors, respectively. This interaction promotes carrier separation, reduces charge recombination, enhances oxygen adsorption, and effectively generates reactive oxygen species, such as ·OH. Subsequently, CH4 is oxidized to ·CH3 by holes from Au. Concurrently, O2 is reduced by electron from OVs, following the pathways O2 → OOH→H2O2 → ·OH. The rapid oxidation of CH3OH by ·CH3 and ·OH results in the formation of HCHO.

Interfacial thermal resistance in stanene/ hexagonal boron nitride van der Waals heterostructures: A molecular dynamics study

Recently, the stanene (Sn)/hexagonal boron nitride (h-BN) van der Waals heterostructure (vdW) has garnered significant attention among the scientific community due to its distinctive electrical, optical, and thermal characteristics. Despite the promising potential of this heterostructure, the interfacial thermal resistance (ITR) between the Sn and h-BN layers remains unexplored. Understanding and modulating this ITR are essential steps towards harnessing the maximum potential of these materials in practical nanodevices. This study aims to investigate the interfacial thermal resistance (ITR) between the Sn and h-BN layers through the use of conventional molecular dynamics (MD) simulation. The transient pump–probe heating technique, commonly referred to as the Fast Pump Probe (FPP) approach, is utilized to analyze the ITR of the Sn/h-BN heterostructure. The estimated ITR value of a 30 × 10 nm2 Sn/h-BN nanosheet is found to be around ∼ 7 × 10-8 K.m2/W at room temperature. This study comprehensively investigates the impact of various internal and external parameters including nanosheet size, system temperature, contact pressure, vacancy concentration, and mechanical tensile strain (uniaxial and biaxial) on ITR, providing an extensive understanding of how these factors collectively affect the thermal resistance between Sn and h-BN layers. The simulationresults demonstrate a consistent decline in ITR by approximately ∼ 93 %, ∼45 %, ∼65 %, and ∼ 33 % with the increasing system size, temperature, contact pressure, and defect concentration, respectively. In contrast, increasing mechanical strain leads to a substantial enhancement in ITR, with a maximum increase of approximately ∼ 47 % under uniaxial tensile strain and almost ∼ 99 % under biaxial tensile strain. Moreover, the pristine Sn/h-BN heterostructure exhibits no significant thermal rectification effect. The Phonon Density of States (PDOS) profile of the Sn and h-BN layer is calculated to elucidate this underlying behavior of ITR. The PDOS analysis reveals that heat is transported from h-BN to the Sn layer through efficient coupling of low-frequency flexural phonons between these two materials. This work will provide both theoretical support and logical guidelines for modulating thermal resistance across diverse dissimilar material interfaces, which will be necessary for the development of advanced nanodevices used in next-generation nanoelectronics, nanophotonic, and optoelectronics applications.

Crystal Structures and Piezoelectric Properties of Quenched and Slowly-Cooled BiFeO3-BaTiO3 Ceramics.

The BiFeO3-BaTiO3 (BF-BT) ceramics were here prepared through the solid-state reaction of Bi2O3, Fe2O3 and nano-sized BT powders. The crystal structures and piezoelectric properties were investigated in both quenched (AQ) and slowly cooled (SC) 0.7BF-0.3BT ceramics. Prior work has shown that rhombohedral and pseudo-cubic phases coexist in 0.7BF-0.3BT ceramics. In this work, the crystal structure of the pseudo-cubic phase was refined as a non-polar orthorhombic Pbnm phase in the SC sample and as a polar orthorhombic Pmc21 phase in the AQ sample. In addition to a sharp dielectric peak at about 620 °C, corresponding to the Curie temperature of the rhombohedral phase, a broad dielectric peak with strong frequency dispersion and a sharp frequency-independent dielectric peak were observed at around 500 °C in the SC and AQ samples, respectively. We determine that the dielectric anomalies around 500 °C were caused by a relaxor phase transition of the non-polar orthorhombic phase in the SC sample and a ferroelectric-paraelectric phase transition of the polar orthorhombic phase in the AQ sample. The AQ sample showed better ferroelectric and piezoelectric properties than the SC sample. The 0.7BF-0.3BT ceramic slowly cooled in a nitrogen atmosphere showed a well-saturated P-E curve and a similar temperature-dependent dielectric constant as the AQ sample. Our results indicate that large concentrations of oxygen vacancies produce a more distorted polar orthorhombic phase and better piezoelectric properties in the AQ sample than in the SC sample.

BiOBr/FeMoO4 composite achieves oxygen vacancy concentration adjustment to promote persulfate activation degradation of organic pollutants in saline water

The investigation about the mechanism of crystal plane regulation on the generation of oxygen vacancies remains a challenge. In this paper, BiOBr/FeMoO4 composites were synthesized by precise control of crystal plane growth, and it exhibited the enhanced concentration of oxygen vacancies due to lower formation energy of oxygen vacancies. The composite performs higher photo-Fenton-like ability for degrading oxytetracycline hydrochloride (OTC). Structural analyses and theoretical calculations reveal that crystal plane regulation induces significant changes in oxygen vacancy concentration. The BiOBr/FeMoO4/peroxydisulphate (PDS) /light system, which dominated by the non-radical pathway, degraded 96.8 % ± 1.0 % of OTC within 30 min. The activation mechanism of the system and the degradation pathway of OTC were elucidated. The intermediates in the degradation process of OTC were evaluated using liquid chromatograph-mass spectrometer (LC-MS), toxicity evaluation software tool (T.E.S.T) and soybean germination experiments. This work offers novel insights into the pivotal role of crystal plane directional regulation in the quantitative generation of oxygen vacancies.

High-performance Co-free dual-phase perovskite positrodes for protonic ceramic cells: Zn and Ni-doped BaCe0.2Fe0.8O3-δ system

Protonic ceramic cells (PCCs) can convert the energy between electricity and fuels efficiently, and are attracting increasing attention. However, the development of PCCs is restricted by the lack of proper positrode materials. In this work, we report a Co-free system with a nominal composition of BaCe0.2Fe0.8O3-δ, which is composed of a Fe-rich matrix phase decorated with Ce-rich nanoparticles. The performance of such a dual-phase positrode system is further improved by doping Zn and/or Ni to increase bulk and surficial concentration of oxygen vacancies. The hydration reaction and surficial adsorption of oxygen species are thereby promoted, benefitting the oxygen reduction reaction. The Ni and Zn co-doped composition Ba(Ce0.2Fe0.8)0.85(Ni0.5Zn0.5)0.15O3-δ exhibits lower polarization resistance than the pristine and singly doped systems, enabling the fuel cells to show high peak power densities of 400, 721, and 828 mW cm−2 at 550, 600, and 650 °C, respectively.

UV-assisted preparation of highly selective Ce-doped BiOBr-based triethylamine gas sensors

In this study, the Ce-doped BiOBr gas sensor was prepared for the detection of triethylamine by ultraviolet-assisted deposition. Characterization tests such as EDS and XPS verified the presence of Ce elements. The gas sensing test results show that adding Ce improves the selectivity of BiOBr to triethylamine gas. Moreover, the Ce/BiOBr sensor showed a 141 % improvement in response to 200 ppm of triethylamine at 300 °C compared to the BiOBr sensor. The enhanced sensing performance of Ce/BiOBr is mainly attributed to the increased concentration of oxygen vacancies.

A CuO/MoO3-based SO2 gas sensor with moisture resistance and ultra-fast response at 90°C

Sulfur dioxide (SO2) is a toxic and harmful decomposition product generated during arc discharge or partial discharge in sulfur hexafluoride (SF6) electrical equipment, which can corrode internal pipelines. Detecting SO2 is crucial for assessing the insulation condition of equipment and preventing faults. To overcome the challenges faced by traditional SO2 gas sensors, including high operating temperatures, slow response times, and poor moisture resistance, this paper proposes a novel SO2 gas sensor based on CuO/MoO3. The morphology, composition, and chemical states of the materials were analyzed using different characterization techniques. At 90°C, the S2 sensor (CuO: MoO3=1: 1) exhibited a fast response time (10 s) to 10 ppm SO2 and demonstrated good moisture resistance (a 1 % change in relative humidity (RH) has the same effect on the sensor as 25.9 ppb SO2). The response of the S2 sensor to 100 ppm SO2 was 26.5. Additionally, the voltage sensitivity of the S2 sensor at low (1–10 ppm) and high (10–100 ppm) concentrations are 658.74 mV/ppm and 33.43 mV/ppm, respectively. Therefore, the S2 sensor is more suitable for low concentration conditions. These good sensing characteristics of the S2 sensor are owing to the high content of oxygen vacancies (39.2 %) and the heterojunction effect in the composite material. Additionally, this sensor exhibits excellent anti-interference capabilities, including moisture resistance and high selectivity. Therefore, the CuO/MoO3-based SO2 gas sensor can be effectively applied to detect the decomposition of SF6.

Depth-Resolved X-Ray Photoelectron Spectroscopy Evidence of Intrinsic Polar States in HfO2-Based Ferroelectrics.

The discovery of ferroelectricity in nanoscale hafnia-based oxide films has spurred interest in understanding their emergent properties. Investigation focuses on the size-dependent polarization behavior, which is sensitive to content and movement of oxygen vacancies. Though polarization switching and electrochemical reactions is shown to co-occur, their relationship remains unclear. This study employs X-ray photoelectron spectroscopy with depth sensitivity to examine changes in electrochemical states occurring during polarization switching. Contrasting Hf0.5Zr0.5O2 (HZO) with Hf0.88La0.04Ta0.08O2 (HLTO), a composition with an equivalent structure and comparable average ionic radius, electrochemical states are directly observed for specific polarization directions. Lower-polarization films exhibit more significant electrochemical changes upon switching, suggesting an indirect relationship between polarization and electrochemical state. This research illuminates the complex interplay between polarization and electrochemical dynamics, providing evidence for intrinsic polar states in HfO2-basedferroelectrics.