Strong interface scattering induced low thermal conductivity in Bi-based GeTe/Bi2Te3 superlattice-like materials

The thermal conductivities of GeTe/Bi₂Te₃ superlattice-like materials are calculated based on density functional perturbation theory (DFPT) and measured using a 3ω method. The calculated results show that the lattice thermal conductivity or thermal diffusivity of GeTe/Bi₂Te₃ superlattice-like materials significantly decrease due to the effects of interfaces and Bi atoms in Bi₂Te₃. Our measured results are in line with the theoretical calculations, and reach an extremely low thermal conductivity at 0.162 W mK⁻¹ compared with published work on Ge–Sb(Bi)–Te, indicating the effectiveness of modulating the thermal properties of phase change materials by using Bi-based GeTe/Bi₂Te₃ superlattice-like materials. Our findings give a calculation method to modify the thermal characteristics of superlattice-like materials and confirm Bi-based GeTe/Bi₂Te₃ superlattice-like materials as promising candidates for phase change materials with lower thermal conductivity.

Strong interface scattering caused by Bi atoms in Bi-based GeTe/Bi₂Te₃ superlattice-like materials leads to the decrease of thermal conductivity.     

High-performance ferroelectric non-volatile memory based on La-doped BiFeO3 thin films

An ultrathin (6.2 nm) ferroelectric La_0.1Bi_0.9FeO₃ (LBFO) film was epitaxially grown on a 0.7 wt% Nb-doped SrTiO₃ (001) single-crystal substrate by carrying out pulsed laser deposition to form a Pt/La_0.1Bi_0.9FeO₃/Nb-doped SrTiO₃ heterostructure. The LBFO film exhibited strong ferroelectricity and a low coercive field. By optimizing the thickness of the LBFO film, a resistance OFF/ON ratio of the Pt/LBFO (∼6.2 nm)/NSTO heterostructure of as large as 2.8 × 10⁵ was achieved. The heterostructure displayed multi-level storage and excellent retention characteristics, and showed stable bipolar resistance switching behavior, which can be well applied to ferroelectric memristors. The resistance switching behavior was shown to be due to the modulating effect of the ferroelectric polarization reversal on the width of the depletion region and the height of the potential barrier of the LaBiFeO₃/Nb-doped SrTiO₃ interface.

An ultrathin (6.2 nm) ferroelectric La_0.1Bi_0.9FeO₃ (LBFO) film was epitaxially grown on a 0.7 wt% Nb-doped SrTiO₃ (001) single-crystal substrate by carrying out pulsed laser deposition to form a Pt/La_0.1Bi_0.9FeO₃/Nb-doped SrTiO₃ heterostructure.     

Interplay of piezoresponse and magnetic behavior in Bi0.9A0.1FeO2.95 (A = Ba,Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramics

Extensive piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM) measurements in conjunction with piezoresponse spectroscopy have been carried out on pellets of Bi_0.9A_0.1FeO_2.95 (A = Ba, Ca) and Bi₀.₉Ba₀.₀₅Ca₀.₀₅FeO_2.95 co-doped ceramic samples in order to characterize their ferroelectric and magnetic nature and correlate the findings with our recent far-infrared spectroscopic studies on these samples. We are able to clearly discern the switching behavior of the 71° and 109° ferroelectric domains as distinct from that of the 180° domains in both pristine and Ba-doped bismuth ferrite samples. While substitution of Ba at the Bi site in bismuth ferrite does not affect the ferroelectric and magnetic properties to a great extent, Ca-doped samples show a decrease in their d₃₃ values with a concomitant increase in their magnetic behavior. These results are in agreement with the findings from our far-infrared studies.

In the pristine as well as the doped BiFeO₃ samples, ferroelectric domains show switching behavior. The regions marked by yellow color loops show either 71° or 109°-domains, whereas those marked by white loops are 180°-domains.     

Ferroelectric properties and Raman spectroscopy of the [(C4H9)4N]3Bi2Cl9 compound

The exploration of ferroelectric hybrid materials is highly appealing due to their great technological significance. They have the potential to conserve power and amazing applications in information technology. In line with this, we herein report the development of a [(C₄H₉)₄N]₃Bi₂Cl₉ tetra-alkyl hybrid compound that exhibits ferroelectric properties. The phase purity was confirmed by Rietveld refinement of the X-ray powder diffraction pattern. It crystallizes, at room temperature, in the monoclinic system with the P2₁/n space group. The outcome of temperature dependence of the dielectric constant proved that this compound is ferroelectric below approximately 238 K. The dielectric constants have been fitted using the modified Curie–Weiss law and the estimated γ values are close to 1. This confirms classical ferroelectric behavior. Raman spectroscopy is efficiently utilized to manifest the origin of the ferroelectricity, which is ascribed to the dynamic motion of cations as well as distortion of the anions. Moreover, the analysis of the wavenumbers and the half-width for δ_s(Cl–Bi–Cl) and ω(CH₂) modes, based on the order–disorder model, allowed us to obtain the thermal coefficient and activation energy near the para-ferroelectric phase transition.

The exploration of ferroelectric hybrid materials is highly appealing due to their great technological significance.     

Investigation into defect chemistry and relaxation processes in niobium doped and undoped SrBi4Ti4O15 using impedance spectroscopy

The aurivillius family of compounds SrBi₄Ti₄O₁₅ (SBTi) and SrBi₄Ti_3.8Nb_0.2O₁₅ has been prepared using solid state reaction techniques. The niobium doping enhances the value of the dielectric constant, but decreases the phase transition temperature and grain size of SBTi. Grain conductivity evaluated from the impedance data reveals that Nb doping increases the resistance of grains which indicates the decrease in oxygen vacancies. The negative temperature coefficient of resistance shown by the grain boundary conductivity is explained using the Heywang–Jonker model. The variation of ac conductivity with frequency is found to obey Jonscher’s universal power law. The frequency exponent (n), pre-exponential factor (A), and bulk dc conductivity (σ_dc) are determined from the fitting curves of Jonscher’s universal power law. From the frequency exponent (n) versus temperature curve, we conclude that the conduction mechanism of SBTi changes from large-polaron tunneling (300–475 °C) to small-polaron tunneling (475–550 °C), and in that of the niobium doped it is small-polaron tunneling (300–375 °C) to correlated band hopping (375–550 °C). Activation energies have been calculated from different functions such as loss tangent, relaxation time, grain and grain boundary conductivities, and ac and dc conductivity. The activation energies reveal that conductivity in the sample has contributions from migrations of oxygen vacancies, bismuth ion vacancies, electrons ionized from strontium vacancies, strontium ion vacancies and valence fluctuations of Ti⁴⁺/Ti³⁺ ions.

From the analysis of impedance, ac conductivity we conclude that the donor ion Nb⁵⁺ doping in SrBi₄Ti₄O₁₅, reduces the oxygen vacancy and improves the dielectric and ferroelectric properties.     

Investigation of K-ion storage performances in a bismuth sulfide-carbon nanotube composite anode

Herein, we synthesize a nanostructured bismuth sulfide/carbon nanotube composite and demonstrate its potential use as a high-capacity anode for K-ion batteries, for the first time. The composite anode shows reversible K-ion storage capabilities that are supported by density functional theory calculations.

The Bi₂S₃-CNT composite anode shows reversible K-ion storage capability through the conversion and alloying reaction steps.

K-ion batteries (KIBs) have attracted attention as a promising alternative to commercial Li-ion batteries (LIBs) because of their low cost, earth abundance, and low redox potential (Li: −3.04 V and K: −2.93 V vs. standard hydrogen electrode), along with the similar chemical properties between K and Li.¹ KIBs also possess an electrical energy storage mechanism similar to LIBs, which has accelerated the development of KIBs.²To date, alloys of group 14 or 15 elements in the periodic table, as defined by their corresponding alloying reactions, have been considered as potential candidates for high-capacity anodes in KIBs.³ Owing to the large volume changes of the alloying anode in the K-system (along with the issue of the large ionic size of K⁺ ion (1.38 Å)),⁴ most alloying anodes have been designed and synthesized via structural modulation with carbon-conducting agents, which are very important for buffering volume strain as well as creating pathways for electrical conduction.^5–9 McCulloch et al. investigated the electrochemical behavior of Sb–C composite electrodes as KIB anode.¹⁰ Through the formation of cubic K₃Sb phase, the Sb–C composite electrode delivered a high reversible capacity of 650 mA h g⁻¹. Sultana et al. confirmed the alloying–dealloying reaction between Sn and K by forming the K₂Sn₅ and K₄Sn₂₃ compound.¹¹ Recently, Huang et al. determined the potassium-storage mechanism in Bi.¹² In the K₃Bi–K₃Bi₂–KBi₂–Bi dealloying–alloying mechanism, the Bi electrode delivers high potassium storage capacity of 400 mA h g⁻¹. On the basis of conversion and sequential alloying–dealloying reactions, SnS₂–rGO and Sb₂S–graphene composite electrodes delivered high reversible capacities as KIB anode.^13,14 On the other hand, in the previously reported LIB and sodium-ion battery (SIB) system, bismuth sulfide (Bi₂S₃) was also considered a potential electrode material for high capacity and low-cost electrode materials; however, their development is still infancy.^15–17 In addition, to the best of our knowledge, K-storage capability in Bi₂S₃ has not yet been reported. Herein, we synthesize nanostructured Bi₂S₃ carbon nanotube (CNT) composites and investigate their possible electrochemical K-ion storage behaviors in KIBs for the first time.The nanostructured Bi₂S₃ and Bi₂S₃-CNT composite was synthesized via one-step hydrothermal method. The synthesis details are illustrated in Fig. 1. In a typical process, Bi(NO₃)₃ was first dissolved in deionized (D.I.) water and mixed with CH₄N₂S. For the composite anode, to disperse the CNTs in the solution, isopropyl alcohol was added with ultrasonication for 15 min. Then, it was directly transferred to a Teflon-sealed stainless steel autoclave and held at 180 °C for 10 h. The resultant powder was washed with ethanol and D.I. water, and the final products were obtained after drying at 80 °C overnight. The structure of the as-prepared Bi₂S₃ and Bi₂S₃-CNT composite powder was confirmed using X-ray diffraction (PANalytical, Empyrean) with a Cu-Kα radiation source (λ = 1.5418 Å). To prepare composite electrodes, the Bi₂S₃-CNT powder, acetylene black, and poly(vinylidene fluoride) were mixed with N-methyl pyrrolidone at a ratio of 8 : 1 : 1 by weight. The obtained slurry was pasted on the copper foil as a current collector and dried without air exposure at 80 °C.Open in a separate windowFig. 1Schematic illustration of the synthesis process of the Bi₂S₃-CNT composite.To confirm the morphological properties and elemental distributions in the composite material, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy dispersive X-ray (EDX) mapping were conducted, respectively. The electrochemical behavior was examined in R2032-type coin cells with K metal as the counter electrode (CE) filled with 1 M mol dm⁻³ KFSI ethylene carbonate : diethyl carbonate (1 : 1 v/v) solution at room temperature. For comparison, bare Bi₂S₃ sample was also synthesized and characterized via same procedures. Fig. 2a shows the typical orthorhombic structure of Bi₂S₃ with Pbnm symmetry. Fig. 2b reveals the Rietveld refinement of the high-resolution XRD pattern of the Bi₂S₃ composite (JCPDS no. 00-017-0320). The Rietveld refinement of the XRD pattern of Bi₂S₃-CNTs yielded lattice parameters a, b, and c of 11.45592, 11.24392, and 4.01072 Å, respectively. The accuracy of the structural model obtained by Rietveld refinement was confirmed by the small reliability factors (R_wp: 16.6%). In addition to Bi₂S₃, the Bi₂S₃-CNT composite showed a pure orthorhombic structure belonging to the Pbnm space group without any impurities and secondary phase (Fig. S1^†). The weight content of CNTs in the composite was determined to be ∼10.2 wt% according to thermogravimetric analysis, as shown in Fig. S2.^† We characterized the Bi₂S₃-CNT composite by SEM and TEM. As seen in Fig. 2c (SEM image), spherical Bi₂S₃ nanoparticles in the range 70–100 nm are highly interconnected through a web of CNTs that ensure the flow of electrons. TEM and EDX mapping of the Bi₂S₃-CNT composite clearly reveal the existence of carbon, bismuth, and sulfur element in the composite.Open in a separate windowFig. 2(a) Illustration of typical orthorhombic structure of Bi₂S₃. (b) Rietveld refinement of the XRD pattern for Bi₂S₃-CNT powder. Morphology of Bi₂S₃-CNT: (c) SEM and (d) TEM images with EDX mapping. Fig. 3a shows that the predicted voltage curves using first-principles calculation overlap with the first charge–discharge voltage profile of the Bi₂S₃ electrode in the voltage range 0.01–3.0 V.Open in a separate windowFig. 3Comparison of experimental charge–discharge curves of (a) the Bi₂S₃ anode and (b) Bi₂S₃-CNT anode in the voltage range 0.01–3.0 V at 10 mA g⁻¹ and predicted voltage curves using first-principles calculation.In previous reports on LIB and SIBs,^15,16 Bi₂S₃ typically undergoes subsequent conversion and alloying reactions during the charge–discharge process. On the basis of those reaction mechanisms, we first predicted that the electrochemical reaction of Bi₂S₃ in KIB is likely proceeds as follows:Bi₂S₃ + 6K → 2Bi + 3K₂S1Bi + 3K → K₃Bi2Bi + K → KBi3In the first reaction step, Bi₂S₃ reacts with K to form metallic Bi and potassium sulfide. Then, metallic Bi and K react to form the K–Bi alloy compounds K₃Bi and KBi. Note that the discharge products of Bi, K₂S, K₃Bi, and KBi were selected based on stable species presence in the calculated phase diagrams of K–Bi–S ternary and K–Bi binary systems, which were obtained from Materials Projects.^18,19 Through the conversion and alloying reaction, the theoretical capacity of eqn (1), (2), and (3) were calculated as 312, 384, and 145 mA h g⁻¹, respectively. To validate our proposed mechanism, we performed voltage calculations using the two-phase coexistence method.²⁰ Our predicted theoretical voltages for the reactions Bi₂S₃ + 6K → 2Bi + 3K₂S, Bi + 3K → K₃Bi, and Bi + K → KBi were 1.28, 0.51, and 0.25 V, respectively. As seen in Fig. 3, the Bi₂S₃ electrode delivered a high discharge–charge (potassiation–depotassiation) capacity of 820 and 650 mA h g⁻¹, respectively. The calculated voltages are in good agreement with experimental voltage profiles of Bi₂S₃ anode. The calculated capacity values in each potassiation step were nearly identical to those obtained from density functional theory (DFT) calculations. The calculation methods are provided in the ESI.^†21–23 The Bi₂S₃-CNT composite electrode delivered much higher discharge–charge capacity of 900 mA h g⁻¹ and 750 mA h g⁻¹, respectively (see Fig. 3b). It should be noted that the irreversible capacity over the theoretical value during the potassiation step was mainly attributed to the formation of a solid-electrolyte interphase layer induced by CNTs.²⁴ As observed in SEM and TEM images, the interconnected CNT network in the composite provides effective conductive paths for rapid electron transport, which are likely to assist K⁺ ion storage in Bi₂S₃. In general, conversion-alloying-based electrodes are known to deliver high capacities but experience severe volume changes owing to the continuous self-pulverization of electrode materials during the electrochemical charge–discharge reaction. This phenomenon was observed in the Bi₂S₃ electrode upon cycling in LIBs,¹¹ which resulted in capacity degradation with prolonged cycles. In the case of KIB, our DFT calculations (Fig. 4) suggest that in the first stage, Bi₂S₃ formed the K₂S phase with unit cell volume of 403.74 ų. Further potassiation led to the formation of K₃Bi from metallic Bi with unit cell volume of 361.38 ų, followed by the formation of KBi phase in the final stage. This calculation revealed the high volume expansion of the Bi₂S₃ electrode in the KIB system. Such large volume changes often accelerate electrode damage, which leads to the loss of electrical contact, and subsequently rapid capacity fading.Open in a separate windowFig. 4Electrochemical K storage mechanism of Bi₂S₃ in the composite electrode and volume changes. The insets show the detailed crystalline parameters computed by DFT. Fig. 5a shows the cycle life test of Bi₂S₃ and Bi₂S₃-CNT electrodes in the voltage range 0.01–3.0 V at 200 mA g⁻¹. While Bi₂S₃ only delivered a charge capacity of 250 mA h g⁻¹ because of the sluggish kinetics of K-ions with Bi₂S₃, the capacity reached 450 mA h g⁻¹ for the Bi₂S₃-CNT composite electrode. After 10 cycles, the Bi₂S₃ electrode showed a drastic capacity loss; after 20 cycles, it had very limited capacity. Moreover, after the first few cycles, the CE of above 98% was stabilized for the Bi₂S₃-CNT composite electrode, while the CE for the Bi₂S₃ electrode was much worse; this corresponds to the typical trend of capacity decay in the cycling stability test for the two electrodes. The sudden capacity decay of the Bi₂S₃ electrode is mainly attributed to the huge volume changes during repeated charge–discharge process observed in the DFT calculation results in Fig. 4. In contrast, the CNT encapsulants provided a strong buffer effect that mitigated the large volume changes and pulverizations of Bi₂S₃ particles upon cycling, and enabled more efficient electronic conduction of the whole electrode during cycling. As a result, the Bi₂S₃-CNT electrode demonstrated much better cycling stability over 50 cycles. The electro-conducting CNT matrix enhances the electric conductivity of Bi₂S₃ and thus has been effective in overcoming the poor potassium intercalation ability in Bi₂S₃. To determine the correlation between the electrochemical performance and structural durability of these electrode materials, XRD, SEM, and TEM analyses were carried out with cycled (after 20 cycles) Bi₂S₃ and Bi₂S₃-CNT composite electrodes. As expected from the different cycling stability between the two electrodes, the Bi₂S₃ electrode experienced severe structural damage after cycling. A noticeable broadening of the XRD peaks for the Bi₂S₃ electrode relative to the Bi₂S₃-CNT composite electrode was observed (Fig. 5b). The Bi₂S₃-CNT composite electrode retained its original crystal structure after cycling, while an amorphous phase was found in the cycled Bi₂S₃ electrode. As seen in SEM images (Fig. 5c), the cycled Bi₂S₃ electrode had extensive cracking on the surface, whereas the cycled Bi₂S₃-CNT electrode exhibited no signs of cracking. TEM examination of the Bi₂S₃-CNT electrode between before and after cycling further revealed that CNTs play an important role in suppressing the periodic volume expansion of Bi₂S₃ during cycling (Fig. 5d).²⁵ Although the Bi₂S₃ particles in the composite experienced large volume changes between before and after 20 cycles, Bi₂S₃ nanoparticles were well encapsulated within the CNT matrix. The CNT encapsulants provided a strong buffer effect that mitigated the large volume changes and pulverizations of Bi₂S₃ particles upon cycling, and enabled more efficient electronic conduction of the whole electrode during cycling. Compare to the Bi₂S₃ electrode, the Bi₂S₃-CNT electrode showed the much lower resistances after cycling; this clearly explain that CNT plays an important role to improve electronic conductivity and fast K⁺ ion diffusion kinetics (Fig. S3^†). The XRD, SEM, TEM and EIS results clearly demonstrate the superior structural stability of the Bi₂S₃-CNT composite and its enhanced electrochemical K-storage performance.Open in a separate windowFig. 5(a) Cycle life test of Bi₂S₃ and Bi₂S₃-CNT electrodes in the voltage range 0.01–3.0 V at 200 mA g⁻¹. (b) XRD patterns (c) SEM images of cycled Bi₂S₃ and Bi₂S₃-CNT electrode after 20 cycles. (d) TEM images of cycled Bi₂S₃-CNT particles after 20 cycles.In summary, we demonstrated the potential use of Bi₂S₃-CNT as anode material for KIBs. A combination of experimental and first-principles calculations was used to investigate the overall K-storage mechanism of Bi₂S₃. The CNT matrix in this composite anode not only provides electronic conductivity but also decreases the absolute stress/strain, and accommodates a period of large volume changes during the potassiation–depotassiation process. As a result, the Bi₂S₃-CNT delivered high capacity and good cycling stability. We hope that this study provides insight on the design of new electrode materials and contributes toward the realization of KIBs as a sustainable next-generation battery.     

Investigation of temperature and frequency dependence of the dielectric properties of multiferroic (La0.8Ca0.2)0.4Bi0.6FeO3 nanoparticles for energy storage application

In this work we synthesized the multifunctional (La_0.8Ca_0.2)_0.4Bi_0.6FeO₃ material using a sol–gel process. Structural and morphologic investigations reveal a Pnma perovskite structure at room temperature with spherical and polygonal nanoparticles. A detailed study of the temperature dependence of the dielectric and electrical properties of the studied material proves a typical FE–PE transition with a colossal value of real permittivity at 350 K that allows the use of this material in energy storage devices. Thus, the investigation of the frequency dependence of the ac conductivity proves a correlated barrier hopping (CBH) conduction mechanism to be dominant in the temperature ranges of 150–170 K; the two observed Jonscher''s power law exponents, s₁ and s₂ between 180 K and 270 K correspond to the observed dispersions in the ac conductivity spectra in this temperature region, unlike in the temperature range of 250–320 K, the small polaron tunnel (NSPT) was considered the appropriate conduction model.

The plot of dε′/dT versus temperature for the (La_0.8Ca_0.2)_0.4Bi_0.6FeO₃ material proves four dielectric anomalies at 180 K, 240 K, 320 K and 350 K with a colossal dielectric constant at low frequency allowing its application in energy storage devices.     

Electro–opto–mechano driven reversible multi-state memory devices based on photocurrent in Bi0.9Eu0.1FeO3/La0.67Sr0.33MnO3/PMN-PT heterostructures

A single device with extensive new functionality is highly attractive for the increasing demands for complex and multifunctional optoelectronics. Multi-field coupling has been drawing considerable attention because it leads to materials that can be simultaneously operated under several external stimuli (e.g. magnetic field, electric field, electric current, light, strain, etc.), which allows each unit to store multiple bits of information and thus enhance the memory density. In this work, we report an electro–opto–mechano-driven reversible multi-state memory device based on photocurrent in Bi_0.9Eu_0.1FeO₃ (BEFO)/La_0.67Sr_0.33MnO₃ (LSMO)/0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) heterostructures. It is found that the short-circuit current density (J_sc) can be switched by the variation of the potential barrier height and depletion region width at the Pt/BEFO interface modulated by light illumination, external strain, and ferroelectric polarization reversal. This work opens up pathways toward the emergence of novel device design features with dynamic control for developing high-performance electric–optical–mechanism integrated devices based on the BiFeO₃-based heterostructures.

The mutual interaction between polarization switching, light and piezoelectric strain.     

Gamma-Bi4V2O11 – a layered oxide material for ion exchange in aqueous media

Layered perovskite oxides have attracted considerable attention due to their potential application in photoelectricity and catalysis. The unique character of layered perovskites as ion-exchange materials provides the possibility of creating structural diversity. A new ion-exchange reaction in aqueous solution was observed in layered oxide gamma-Bi₄V₂O₁₁. When employed in ion exchange, gamma-Bi₄V₂O₁₁ is converted into the scheelite-type phase (ABO₄) by selectively discarding Aurivillius-type sheets, and is also converted into the A2X3 phase by selectively dissolving perovskite-like layers. Metal-doped BiVO₄ and Bi₂O₃ were obtained using such an ion-exchange reaction.

Layered oxides gamma-Bi₄V₂O₁₁ is found to undergo chemical transformations under room temperature. Gamma-Bi₄V₂O₁₁ can be transformed to ABO₄ in acid solution and A2X3 compounds in basic solution.

Recently, perovskite oxides and layered perovskite oxides have been widely and thoroughly studied because of their structural simplicity and flexibility, good stability, and their promising applications in solar cells, photocatalysts, and fuel cells, for example.^1–3 Perovskite oxides are a type of oxide with the chemistry formula ABO₃, where A is a large cation and B is a smaller cation. The skeleton of perovskite oxides comprises corner-sharing BO₆ octahedra, where the B-site cation is located in the center of an octahedron of oxygen anions with a coordination number of 6, and the A-site cation is located at the center of eight corner-sharing BO₆ octahedra. Layered perovskite oxides are stacks of perovskite oxide layers interspersed with other metal oxide layers. The layered perovskite oxides are well known as The Dion–Jacobson phases, A′[A_n−1BnO_3n+1]; the Ruddlesden–Popper phases, A2′[A_n−1BnO_3n+1]; and the Aurivillius phases, Bi₂O₂[A_n−1BnO_3n+1].^4–6 Unlike the perovskite oxides, the layered perovskite oxides have the unique characteristic of undergoing soft chemical reactions, such as intercalating molecules into their interlayer space, ion-exchange reactions, and exfoliation to nanosheets.^6,7 Many soft chemical reactions of layered perovskites have been reported, with the unique characteristic of replacing or modifying the interlayer cations under mild conditions.^7–9BIMEVOX materials are a group of compounds obtained by doping Bi₄V₂O₁₁ with other metallic ions. These materials exhibit high oxide ion conduction and have potential application at the membrane for oxygen separation.^10,11 Among BIMEVOX materials, the gamma phase of Bi₄V₂O₁₁ consists of Aurivillius-type Bi₂O₂²⁺ sheets alternating with perovskite-like oxygen-deficient layers (VO_3.5–0.5)²⁻.¹² Encouraged by the soft chemical reactions, and using the layered structure of Bi₄V₂O₁₁ as templates, it is feasible to design perovskites that retain the structural features of the precursor layered phases. Interestingly, we found that it is possible for Bi₄V₂O₁₁ to be converted by soft chemical reactions. Moreover, the compound is converted in alkaline solution, and the high crystal symmetry is inherited from the parent compound. We believed that this approach would provide a rational route to the design of materials with high symmetry at high temperatures, using soft chemical reactions at room temperature.Gamma-Bi₄V₂O₁₁ is synthesized by a standard solid-state reaction. Bi₄V₂O₁₁ possess three polymorphs: alpha, beta, and gamma. The phase-transition temperature of gamma-Bi₄V₂O₁₁ is 840 K (ref. 13). Gamma-Bi₄V₂O₁₁ can be synthesized by quenching in liquid nitrogen after sintering at 1073 K for 48 h or doping with a transition metal to stabilize the high-temperature phase. Gamma-Bi₄V₂O₁₁ constituted with Aurivillius-type Bi₂O₂²⁺ sheets alternating with perovskite-like oxygen deficient layers (VO_3.5–0.5)²⁻. The X-ray diffraction (XRD) pattern of gamma-Bi₄V₂O₁₁ is shown in Fig. 1.Open in a separate windowFig. 1XRD pattern of (a) gamma-Bi₄V₂O₁₁, (b) BiVO₄, and (c) Bi₂O₃ converted from gamma-Bi₄V₂O₁₁, (d) the product of a solid-state reaction between BiVO₄ and Bi₂O₃ converted from gamma-Bi₄V₂O₁₁.Inspired by the ion-exchange mechanism in layered perovskites, in which the Ruddlesden–Popper phases, Dion–Jacobson phases, and Aurivillius phases can be inter-converted,⁶ we conjectured that the interlayer cations in the layered structure of gamma-Bi₄V₂O₁₁ have the potential to be replaced. After ion exchange with protons (0.6 M HNO₃), gamma-Bi₄V₂O₁₁ is converted into a red powder. The red powder is indexed as the space group I21/a, with cell parameters of a = 5.1950, b = 11.701, c = 5.0920, and beta = 90.3800. These cell parameters are highly consistent with BiVO₄.¹⁴ However, according to the structure of gamma-Bi₄V₂O₁₁, which consists of Bi₂O₂²⁺ sheets and (VO_3.5–0.5)²⁻ layers, the sheets of Bi₂O₂²⁺ are thought to be removed in the ion exchange. Thus, the overall reaction could be represented as follows:(Bi₂O₂)₂(VO_3.5)₂ + 2HNO₃ → V₂O₅ + 2H₂OWhen examining the structure of gamma-Bi₄V₂O₁₁, we find that there are two Wyckoff positions for the Bi-atom: the 4e and 16m positions, respectively. Among the Bi₂O₂²⁺ sheets, the Bi-atom at the 4e position is connected to the oxygen atom in the sheets with a bond length of 2.3149 Å.¹⁵ The distance between the Bi-atom (4e) and the oxygen atoms in the (VO_3.5–0.5)²⁻ layers is as far as 2.7699 Å, which deviates from the typical Bi–O bond length.¹⁶ Thus, the Bi-atom at the 4e position and the oxygen atom in the sheets can be regarded as an integral part of the Bi₂O₂²⁺ sheets. For the Bi-atom at the 16m position, the distance between the bismuth and the oxygen atom in the Bi₂O₂²⁺ sheets is 2.6005 Å. Meanwhile, the Bi-atom (16m) connects to the oxygen atoms in the (VO_3.5–0.5)²⁻ layers with a bond length of 2.4193 Å. Compared with Bi-atoms at the 4e position, the Bi-atom at the 16m position prefers connecting (VO_3.5–0.5)²⁻ layers to Bi₂O₂²⁺ sheets. We believe the different Wyckoff positions for the Bi-atom leads to different behaviors when extracting Bi₂O₂²⁺ sheets in acidic solution. Consequently, BiVO₄ nanosheets, shown in Fig. 2, are obtained after ion exchange in acid solution.Open in a separate windowFig. 2Transmission electron microscopy (TEM) image (a) and atomic force microscopy (AFM) image (b) of BiVO₄ nanosheets.Traditional ion exchange often occurs under acidic conditions or through solid-state reactions with alkali or BiO_x at elevated temperatures. Interestingly, we find that the layered structure of gamma-Bi₄V₂O₁₁ is not only convertible under acidic conditions, but also in alkali solution. After reaction with 20 M KOH solution at room temperature, gamma-Bi₄V₂O₁₁ was converted to a light green powder. The powder was identified with XRD, and was composed of the alpha phase of Bi₂O₃ and a small fraction of the gamma phase of Bi₂O₃. Consistent with BiVO₄ leached from the acid treatment of gamma-Bi₄V₂O₁₁, Bi₂O₃, which can be regarded as perovskite-like layers (VO_3.5–0.5)²⁻, along with partial bismuth was removed from the layer structure. We believe that the Bi₂O₂²⁻ layers which lacked MO_x octahedra would keep growing under a high concentration of alkaline solution, which is regarded as a minimizer, and boost the crystal growth of Bi₂O₃. In this respect, most of the Bi₂O₂²⁺ layers leached from gamma-Bi₄V₂O₁₁ kept growing in the alkaline solution and formed the thermodynamically stable phase – alpha Bi₂O₃. As a result of crystal growth, the scanning electron microscope (SEM) image of Bi₂O₃ in Fig. 3 shows the micro-morphology of a single crystal.Open in a separate windowFig. 3SEM image of a Bi₂O₃ single crystal prepared via the ion-exchange mechanism.The structural evolution is demonstrated in Fig. 4, and the overall reaction is:Open in a separate windowFig. 4Structural evolution of gamma-Bi₄V₂O₁₁.Considering the structural evolution and the overall reaction, the reaction could form a closed loop, as shown in Fig. 5. The Bi₂O₃ and BiVO₄ are obtained by ion exchange from gamma-Bi₄V₂O₁₁, introducing further dopant, and form gamma-Bi₄V₂O₁₁, again by solid-state reaction:Open in a separate windowFig. 5Toolbox of metal-doped Bi₂O₃ and BiVO₄ derived from gamma-Bi₄V₂O₁₁.Regarding the large solid solution regions of metal-doped Bi₄V₂O₁₁ and the availability of most cations in the periodic table,^17,18 such a toolbox could construct a diverse set of structural architectures, with certain specific morphologies.Moreover, the toolbox provides the possibility to design BiVO₄ and Bi₂O₃ with dopants exceeding the equilibrium solid solubility. We first doped Mn into BiVO₄ and tuned the doping level at a wide range from 0 to 18% (mole ratio of Mn) by the modified two-step reaction shown in Fig. 7. Moreover, titanium-doped BiVO₄ was also realized. Accordingly, many transition metal-doped BiVO₄ materials were available by the two-step reaction, which could be employed to insert other elements to trigger various capabilities in the BiVO₄ mother compound. Fig. 6 shows the UV-vis absorption spectra of metal-doped BiVO₄. The bandgap as well as the band structure could be modified.Open in a separate windowFig. 6UV-vis absorption spectra of metal-doped BiVO₄.Open in a separate windowFig. 7Introducing dopant into BiVO₄ nanosheets by the modified two-step reaction.     

Promoting the photocatalytic activity of Bi4Ti3O12 microspheres by incorporating iron

Small amounts of Fe(NO₃)₃ were added to the synthesis mixture prior to the hydrothermal synthesis of Bi₄Ti₃O₁₂ microspheres. The physicochemical properties of the resulting materials were changed accordingly. The photocatalytic activities of several samples were studied through the photocatalytic degradation of organic pollutants. The samples with a theoretical Fe atomic percentage of 5.9% showed the highest photocatalytic activity among these samples. The main active species in photocatalytic degradation was demonstrated by radical capturing experiments as h⁺. The introduction of a suitable amount of Fe to the photocatalyst can facilitate the separation of electron–hole pairs generated upon light irradiation, inhibit their recombination efficiently, and prominently expand the light absorption region, thus leading to higher photocatalytic activity.

Small amounts of Fe(NO₃)₃ were added to the synthesis mixture prior to the hydrothermal synthesis of Bi₄Ti₃O₁₂ microspheres.     

Dielectric ceramics/TiO2/single-crystalline silicon nanomembrane heterostructure for high performance flexible thin-film transistors on plastic substrates

A dielectric ceramics/TiO₂/single-crystalline silicon nanomembrane (SiNM) heterostructure is designed and fabricated for high performance flexible thin-film transistors (TFTs). Both the dielectric ceramics (Nb₂O₃–Bi₂O₃–MgO) and TiO₂ are deposited by radio frequency (RF) magnetron sputtering at room temperature, which is compatible with flexible plastic substrates. And the single-crystalline SiNM is transferred and attached to the dielectric ceramics/TiO₂ layers to form the heterostructure. The experimental results demonstrate that the room temperature processed heterostructure has high quality because: (1) the Nb₂O₃–Bi₂O₃–MgO/TiO₂ heterostructure has a high dielectric constant (∼76.6) and low leakage current. (2) The TiO₂/single-crystalline SiNM structure has a relatively low interface trap density. (3) The band gap of the Nb₂O₃–Bi₂O₃–MgO/TiO₂ heterostructure is wider than TiO₂, which increases the conduction band offset between Si and TiO₂, lowering the leakage current. Flexible TFTs have been fabricated with the Nb₂O₃–Bi₂O₃–MgO/TiO₂/SiNM heterostructure on plastic substrates and show a current on/off ratio over 10⁴, threshold voltage of ∼1.2 V, subthreshold swing (SS) as low as ∼0.2 V dec⁻¹, and interface trap density of ∼10¹² eV⁻¹ cm⁻². The results indicate that the dielectric ceramics/TiO₂/SiNM heterostructure has great potential for high performance TFTs.

Dielectric ceramics/TiO2/single-crystalline silicon nanomembrane heterostructure for high performance flexible thin-film transistors.     

Phyto-inspired Cu/Bi oxide-based nanocomposites: synthesis,characterization, and energy relevant investigation

A modified and sustainable approach is reported in this research for the synthesis of a spherical-shaped CuO–Bi₂O₃ electrode material for electrochemical studies. Aqueous extract derived from the plant Amaranthus viridis L. (Amaranthaceae) (AVL) was used as a reducing agent for morphological control of the synthesis of CuO–Bi₂O₃ nanocomposites. The modified nanomaterial revealed an average crystal size of 49 ± 2 nm, which matches very well with scanning electron microscopy (SEM) findings. Furthermore, the synthesized material was characterized using Fourier-transform infrared spectroscopy, field emission SEM and energy-dispersive spectroscopy. The optical band gap energy of 3.45 eV was calculated using a Tauc plot. Finally, the bioorganic framework-derived CuO–Bi₂O₃ electrode was tested for energy generating and storage applications and the results revealed a capacitance of 389 F g⁻¹ by cyclic voltammetry, with a maximum energy density of 12 W h kg⁻¹ and power density of 5 kW kg⁻¹. Hydrogen evolution reaction and oxygen evolution reaction studies showed good potential of CuO–Bi₂O₃ as an electrocatalyst for water splitting, with maximum efficiency of the electrode up to 16.5 hours.

Spherical-shaped CuO–Bi₂O₃ electrode material and its electrochemical studies.     

Reaction induced robust PdxBiy/SiC catalyst for the gas phase oxidation of monopolistic alcohols

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity (92% conversion of benzyl alcohol and 98% selectivity of benzyl aldehyde) and stability (time on stream of 200 h) in the gas phase oxidation of alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species. TEM indicates that the agglomeration of the 5.8 nm nanoparticles is inhibited under the reaction conditions. The transformation from inactive PdO–Bi₂O₃ to active Pd⁰–Bi₂O₃ under the reaction conditions is confirmed elaborately by XRD and XPS.

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity and stability in the gas phase oxidation of monopolistic alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species.     

A Bi2WO6/Ag2S/ZnS Z-scheme heterojunction photocatalyst with enhanced visible-light photoactivity towards the degradation of multiple dye pollutants

A novel visible-light-driven Z-scheme heterojunction, Bi₂WO₆/Ag₂S/ZnS, was synthesized and its photocatalytic activity was evaluated for the treatment of a binary mixture of dyes, and its physicochemical properties were characterized using FT-IR, XRD, DRS and FE-SEM techniques. The Bi₂WO₆/Ag₂S/ZnS Z-scheme heterojunctions not only facilitate the charge separation and transfer, but also maintain the redox ability of their components. The superior photocatalytic activity demonstrated by the Z-scheme Bi₂WO₆/Ag₂S/ZnS attributes its unique properties such as the rapid generation of electron–hole pairs, slow recombination rate, and narrow bandgap. The performance of the Bi₂WO₆/Ag₂S/ZnS was evaluated for the simultaneous degradation of methyl green (MG) and auramine-O (AO) dyes, while the influences of the initial MG concentration (4–12 mg L⁻¹), initial AO concentration (2–6 mg L⁻¹), pH (3–9), irradiation time (60–120 min) and photocatalyst dosage (0.008–0.016 g L⁻¹) were investigated through the response surface methodology. The desirability function approach was applied to optimize the process and results revealed that maximum photocatalytic degradation efficiency was obtained at optimum conditions including 6.08 mg L⁻¹ of initial MG concentration, 4.04 mg L⁻¹ of initial AO concentration, 7.25 of pH, 90.58 min of irradiation time and 0.013 g L⁻¹ of photocatalyst dosage. In addition, a possible photocatalytic mechanism of the Bi₂WO₆/Ag₂S/ZnS heterojunction was proposed based on the photoinduced charge carriers.

A Z-scheme Bi₂WO₆/Ag₂S/ZnS heterojunction was successfully synthesized as a novel visible-light-driven photocatalyst for the degradation of multiple dye pollutants.     

Structure of bismuth tellurite and bismuth niobium tellurite glasses and Bi2Te4O11 anti-glass by high energy X-ray diffraction

Glass and anti-glass samples of bismuth tellurite (xBi₂O₃–(100 − x)TeO₂) and bismuth niobium tellurite (xBi₂O₃–xNb₂O₅–(100 − 2x)TeO₂) systems were prepared by melt-quenching. The bismuth tellurite system forms glasses at low Bi₂O₃ concentration of 3 to 7 mol%. At 20 mol% Bi₂O₃, the glass forming ability of the Bi₂O₃–TeO₂ system decreases drastically and the anti-glass phase of monoclinic Bi₂Te₄O₁₁ is produced. Structures of glass and the anti-glass Bi₂Te₄O₁₁ samples were studied by high-energy X-ray diffraction, reverse Monte Carlo simulations and Rietveld Fullprof refinement. All glasses have short short-range disorder due to the existence of at least three types of Te–O bonds of lengths: 1.90, 2.25 and 2.59 Å, besides a variety of Bi–O and Nb–O bond-lengths. The medium-range order in glasses is also disturbed due to the distribution of Te–Te pair distances. The average Te–O co-ordination (N_Te–O) in the glass network decreases with an increase in Bi₂O₃ and Nb₂O₅ mol% and is in the range: 4.17 to 3.56. The anti-glass Bi₂Te₄O₁₁ has a long-range order of cations but it has vibrational disorder and it exhibits sharp X-ray reflections but broad vibrational bands similar to that in glasses. Anti-glass Bi₂Te₄O₁₁ has an N_Te–O of 2.96 and is significantly lower than in glass samples.

Te–O, Bi–O and Nb–O bond lengths, co-ordinations in bismuth tellurite, bismuth niobium tellurite glasses and Bi₂Te₄O₁₁ anti-glass by HEXRD, RMC and Riteveld analysis.     

Effective doping control in Sm-doped BiFeO3 thin films via deposition temperature

Sm-doped BiFeO₃ (Bi_0.85Sm_0.15FeO₃, or BSFO) thin films were fabricated on (001) SrTiO₃(STO) substrates by pulsed laser deposition (PLD) over a range of deposition temperatures (600 °C, 640 °C and 670 °C). Detailed analysis of their microstructure via X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows the deposition temperature dependence of ferroelectric (FE) and antiferroelectric (AFE) phase formation in BSFO. The Sm dopants are clearly detected by high-resolution scanning transmission electron microscopy (HR-STEM) and prove effective in controlling the ferroelectric properties of BSFO. The BSFO (T_dep = 670 °C) presents larger remnant polarization (Pr) than the other two BSFO (T_dep = 600 °C, 640 °C) and pure BiFeO₃ (BFO) thin films. This study paves a simple way for enhancing the ferroelectric properties of BSFO via deposition temperature and further promoting BFO practical applications.

Sm-doped BiFeO₃ (Bi₀.₈₅Sm_0.15FeO₃) thin films were fabricated on (001) SrTiO₃ substrates by PLD over a range of deposition temperatures. The Sm dopants are clearly detected by high-resolution scanning transmission electron microscopy.

As a promising alternative lead-free piezoelectric material, BiFeO₃ (BFO) thin films have attracted enormous research interest due to their remarkable multiferroic properties and piezoelectric response.^1–4 However, the high leakage current and large coercive field are factors limiting the extensive use of BFO.⁵ Since the polarization is mainly induced by the Bi³⁺ ions, various rare-earth elements (e.g., La, Nd, Sm, and Gd) have been doped into the Bi-site to improve the overall ferroelectric response.^6–8 According to the site-engineering concept, the doping of foreign elements causes chemical pressure and controls the volatility of Bi atoms in BFO systems.^9,10 The structural information of rare-earth-doped BFO (Re-BFO) systems is therefore upgraded and a preliminary phase diagram is proposed.^11–14Sm-doped BFO (Bi_1−xSm_xFeO₃, BSFO) thin film has attracted research attention, due to the narrow concentration range at room temperature.¹⁵ It shows ferroelectric (FE) rhombohedral to antiferroelectric (AFE) orthorhombic phase transitions as the Sm doping amount increases. Morphotropic phase boundary (MPB) appears at around x = 0.13–0.15, and high values of out-of-plane piezoelectric coefficient (d₃₃ ∼ 110 pm V⁻¹) and enhanced dielectric constant at x = 0.14 are reported in such systems.^16,17 Structure studies show that three main phase, FE R3c phase, AFE PbZrO₃-like phase, and paraelectric Pnma phase, coexist at the MPB composition.¹⁸ In this regard, many efforts have been made on the investigation of phase transitions under external stimuli.^19,20 However, there are not much work on the structure analysis of BSFO at the atomic-scale level. It will be interesting to investigate the microstructure with high-resolution transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (HR-STEM). The detailed microstructure information will reveal how the Sm dopants distribute in the overall BSFO lattice. In addition, BSFO films in previous studies are mostly prepared by solid phase synthesis and sol–gel method^15,20,21 with very few reports using pulsed laser deposition (PLD).^17,22 In those prior reports, Sm-doping amounts in films were controlled by changing the composition of the deposition targets. Deposition temperature has been proven as an effective parameter for PLD in controlling the doping amount, thin film microstructure and the related properties. In the Ag-doped ZnO (SZO) system, deposition temperature was directly used to control the density of stacking faults and consequently affect the electrical transport properties.²³ Table S1^† lists several reported deposition temperatures for Re-BFO thin films, which is in the range of 520 °C to 850 °C. Here, we used three different substrate temperatures, 600 °C, 640 °C and 670 °C, for the BSFO film fabrication via PLD. Pure BFO film was also grown as a reference sample for comparison. Besides the detailed microstructure, the corresponding ferroelectric property measurements were conducted on the BSFO thin films to investigate how the Sm dopants affect the ferroelectric behavior of BSFO.In the present work, Bi_0.85Sm_0.15FeO₃ (BSFO) was selected as the model system. The BSFO target was synthesized by a conventional solid-state sintering method using high-purity Bi₂O₃ (99.99%), Fe₂O₃ (99.95%) and Sm₂O₃ (99.90%) powders. Thin films were grown on (001) single-crystal SrTiO₃ (STO) substrates epitaxially via PLD. KrF excimer laser with a wavelength of 248 nm was used as the laser source. Three different substrate temperatures, 600 °C, 640 °C and 670 °C, were applied in the deposition. For all depositions, oxygen partial pressure was kept at 200 mTorr and the deposition rate was 5 Hz. The films were cooled down to room temperature at a cooling rate of 10 °C min⁻¹ in 200 torr oxygen atmospheres. The growth condition and parameters of BFO film is same with our previous report.²⁴ Au top contacts with 100 nm thickness and 0.1 mm² were deposited by a custom-built magnetron sputtering system. The Au sputter target (99.99% pure) is made by Williams Advanced Materials.X-ray diffraction (XRD) spectra were collected by a PANalytical Empyrean system using Cu K_α radiation. The Raman spectra were measured by Renishaw''s inVia Raman microscope. The microstructure analysis was performed on FEI TALOS F200X TEM/STEM operated at 200 kV. The energy-dispersive X-ray spectroscopy (EDS) chemical mapping was acquired by the SuperX EDS system with four silicon drift detectors. Ferroelectric characterization was conducted by Precision LC II Ferroelectric Tester (Radiant Technologies, Inc.). Fig. 1(a) shows the θ–2θ XRD spectra of the as-prepared thin film samples deposited at 600 °C, 640 °C and 670 °C. All the films display BSFO (00l) diffraction peaks, indicating highly textured BSFO along c-axis. It is noted that BSFO (003) peak shifts from 71.62°, to 70.82° and to 70.70° with the deposition temperature increasing from 600 °C to 640 °C, and 670 °C. The corresponding out-of-plane lattice parameters are calculated to be 3.951 Å, 3.987 Å and 3.993 Å, respectively and are summarized in Fig. 1(b). Compared with the out-of-plane lattice parameter of pure BFO film (∼4.000 Å) on STO substrate,²⁴ three BSFO samples show smaller lattice parameters than that of pure BFO. This is because partial Bi³⁺ (radius = 1.030 Å) ions have been substituted by Sm³⁺ ions with smaller radius (radius = 0.958 Å).²⁵ The BSFO peak at around 32.155° (denoted as “*”) in Fig. 1(a) comes from rhombohedral (110) peak and the peak disappears when the deposition temperature increases to 670 °C. The above results indicate that the deposition temperature influences the Sm-doping amount and the BSFO crystal structure. Raman analysis has been conducted on all BSFO and pure BFO films. The Raman spectra were fitted with Lorentzian curves, as shown in Fig. S1.^† The reported data of bulk polycrystalline Bi_1−xSm_xFeO₃ was taken as reference.²⁶ The overall shape of peaks is the same, which implies that the main structure of BSFO sample is the same as the rhombohedral R3c structure of BFO.Open in a separate windowFig. 1(a) θ–2θ XRD spectra of BSFO film deposited at 600 °C, 640 °C and 670 °C. (b) The summary of out-of-plane lattice parameter. (c–e) Reciprocal space map (RSM) results of BSFO (103) peaks.To further analyze the detailed phase information, asymmetric reciprocal space mapping (RSM) measurements were performed around the (103) diffraction peak for all the three BSFO samples and the results are shown in Fig. 1(c)–(e). The RSM pattern of sample (T_dep = 600 °C) exhibits four domains, which are shown by red triangles. It indicates the existence of rhombohedral-like phase which has antipolar nature.^27–29 For the other two samples (T_dep = 670 °C and T_dep = 640 °C), peak split along Q_x direction is not apparent and the narrower width along Q_x direction is observed, indicating less structure distortion in BSFO films deposited at higher temperature. These results provide direct evidence that the deposition temperature significantly affects the domain structure of the BSFO film.In order to analyze the microstructure structure, TEM analysis has been applied on two samples (T_dep = 670 °C and T_dep = 600 °C). Fig. 2(a) and (c) show the overall films stacks of BSFO on STO substrates. The corresponding selected area electron diffraction (SAED) patterns of BSFO thin films only demonstrate the rhombohedral-like phases. It is interesting to note that the TEM image of high deposition temperature sample (T_dep = 670 °C) exhibits few dark lines. And the dark line density in the high deposition temperature sample is obviously higher than that in the lower deposition temperature one. The image contrast is proportional to ∼Z² (Z, atomic number) in TEM bright-field mode. The dark line is therefore proposed be related with Sm, owing to Z_Sm is larger than Z_Bi and Z_Fe. It also suggests that the higher deposition temperature introduce more Sm dopants in BSFO films than the lower one. The Fig. 2(b) and (d) are high resolution TEM (HR-TEM) images from local areas, which were analyzed by Fast Fourier Transform (FFT). The spots (marked by red arrows) in the FFT image of the sample (T_dep = 670 °C) correspond to the incommensurate phase, which is caused by the competition between ferroelectric (FE) and antiferroelectric (AFE) phases. Different phase emerges in the sample (T_dep = 600 °C), and the corresponding spots are marked by red circle. It has been proved as the antipolar orthorhombic AFE phase, linked to the macroscopic AFE behavior. The phase information of Bi_0.85Sm_0.15FeO₃ thin film in this study is different from previous reported BSFO with only AFE phases.¹⁶ The above microstructure analysis shows the effect of deposition temperature on the phase formation in BSFO system even with 15 atomic percent Sm.Open in a separate windowFig. 2(a and c) Cross-sectional TEM images of BSFO thin films (670 °C and 600 °C) on STO substrates. (b and d) High resolution TEM images. The insets in (a and c) show the corresponding (SAED) pattern of BSFO thin films. The insets in (b and d) show the fast-Fourier transformed (FFT) images from the blue squared region.STEM analysis was then performed to resolve the composition information. Fig. 3(a) shows the high-resolution STEM (HR-STEM) image of the 670 °C sample. The high angle annular dark field mode (HADDF) image intensity is proportional to the atomic number Z. Thus, the white line areas in STEM image correspond to the dark lines in TEM images. We further examined the composition distribution across this Sm layer using the intensity line profile (Fig. 3(b)), which provides a direct interpretation of composition information in the HAADF imaging mode. It is obvious that the position near white line area has higher intensity than other areas. This result proves that Sm³⁺ ion has been doped into BFO system effectively. Energy dispersive spectroscopy (EDS) measurements have been further applied on the same area. The EDS mapping of Bi and Sm elements are shown in Fig. S2(b) and (c).^† The EDS line-scan analysis of Sm is overlaid on the HADDF image and displayed in Fig. S2(a).^† The line profile reflects high content of Sm corresponding to the white line area. Therefore, both the TEM and STEM results show that the higher deposition temperature BSFO sample (T_dep = 670 °C) has higher Sm-doping amount.Open in a separate windowFig. 3(a) HR-STEM image of BSFO film deposited at 670 °C. (b) Enlarged view of the yellow dashed square area from (a). The intensity line profile is inserted along the marked blue line.The ferroelectric behaviors were characterized by the polarization–electric field (P–E) hysteresis loops. Fig. 4(a)–(c) show the P–E loops of BSFO samples with three different deposition temperatures, while Fig. 4(d) shows the loop of pure BFO as a comparison. The polarization measurement as a function of electric field measurement was carried out at room temperature for several times to ensure the reproducibility of the measurements. The BSFO sample (T_dep = 670 °C) with higher Sm-doping amount exhibits obvious enhanced polarization. The remnant polarization (Pr) for the film is determined to be 17 μC cm⁻², much larger than other two BSFO samples and the BFO sample. It was proposed that the incorporation of Sm could break the short-range dipolar regions, surmount the local barrier and transform it to the long-range polar structure.³⁰ The BFO film with four-variant domains exhibits a lower electric filed and remnant polarization Pr.³¹ It was also found that the formation of bridging phase could enhance piezoelectric and dielectric properties of BSFO.¹⁶ In this work, the BSFO sample (T_dep = 600 °C) shows four structural domain structure and the lowest Pr. With the increase amount of Sm-doping amount, incommensurate phase appears in the 670 °C sample. We conclude that the higher deposition temperature introduces the higher Sm-doping amount, which further assists the incommensurate phase formation and suppresses the AFE phase. In addition, the unsaturated P–E loop of BFO film indicates it suffers from high leakage current, which is generally due to the appearance of Bi deficiencies and oxygen vacancies.³² The Sm–O bond enthalpy (565 ± 13 kJ mol⁻¹) is stronger than the Bi–O bond enthalpy (337 ± 12.6 kJ mol⁻¹).³³ Therefore, the higher polarization exhibited by BSFO samples (T_dep = 670 °C) than the pure BFO proves that the Sm dopant could compensate for the Bi loss and suppress the formation of oxygen vacancies. The P–E loop results were compared with prior reports on Re-BFO films. As shown in Table S1,^† the remnant polarization is quite different for Re-BFO systems. It is due to the different ionic radii of the rare earth elements, which result in different structural distortion of BFO and diverse critical doping ratio for phase transitions. Besides the phase variants, the polarization is closely related with the orientation of crystalline structures.Open in a separate windowFig. 4(a–c) Polarization hysteresis measurements for BSFO film deposited at 670 °C, 640 °C and 600 °C. (d) Polarization hysteresis measurements for BFO film.This study demonstrates that Sm-doping amount in BSFO thin films can be effectively tuned via deposition temperature. The Sm dopants influence phase formation of BSFO and further control the macroscopic ferroelectric properties. The local incommensurate phase presented by Bi_0.85Sm_0.15FeO₃ with higher Sm-doping amount than the reported ones (Bi_0.86Sm_0.14FeO₃), which is extremely helpful in constructing phase diagram of BSFO. More interestingly, the existence and location of Sm dopants in BSFO thin film have been directly demonstrated by the HR-STEM and corresponding EDS analysis. This work is also beneficial for the exploration of other Re-BFO films with deposition temperatures and detailed structure analysis, which is an important step toward the practical applications of Re-BFO in electronic devices.     

Effect of Bi-substitution into the A-site of multiferroic La0.8Ca0.2FeO3 on structural,electrical and dielectric properties

(La_0.8Ca_0.2)_1−xBi_xFeO₃ (x = 0.00, 0.05, 0.10, 0.15 and 0.20) (LCBFO) multiferroic compounds have been prepared by the sol–gel method and calcined at 800 °C. X-ray diffraction results have shown that all samples crystallise in the orthorhombic structure with the Pnma space group. Electrical and dielectric characterizations of the synthesized materials have been performed using complex impedance spectroscopy techniques in the frequency range from 100 Hz to 1 MHz and in a temperature range from 170 to 300 K. The ac-conductivity spectra have been analysed using Jonscher''s power law σ(ω) = σ_dc + Aω^s, where the power law exponent (s) increases with the temperature. The imaginary part of the complex impedance (Z′′) was found to be frequency dependent and shows relaxation peaks that move towards higher frequencies with the increase of the temperature. The relaxation activation energy deduced from the Z′′ vs. frequency plots was similar to the conduction activation energy obtained from the conductivity. Hence, the relaxation process and the conduction mechanism may be attributed to the same type of charge carriers. The Nyquist plots (Z′′ vs. Z′) at different temperatures revealed the appearance of two semi-circular arcs corresponding to grain and grain boundary contributions.

(La_0.8Ca_0.2)_1−xBi_xFeO₃ (x = 0.00, 0.05, 0.10, 0.15 and 0.20) (LCBFO) multiferroic compounds have been prepared by the sol–gel method and calcined at 800 °C.     

Ag0/Au0 nanocluster loaded Bi2O4 photocatalyst for methyl orange dye photodegradation

Visible-light-sensitive Ag and Au nanocluster loaded Bi₂O₄ (Ag–Bi₂O₄ and Au–Bi₂O₄) semiconductor photocatalysts have been synthesized. The composite materials exhibited increased photocatalytic degradation of the azo-dye pollutant, Methyl Orange (MO). In addition, Au–Bi₂O₄ (Au-7% wt) showed the highest MO degradation rate (0.05904 min⁻¹) i.e. 7.69 times higher than the pristine Bi₂O₄ and 1.4 times higher than 1% Ag–Bi₂O₄. The optical properties of the composites showed that the band gaps of the composite samples 1% Ag–Bi₂O₄ and 7% Au–Bi₂O₄ were 1.96 eV and 2.09 eV, respectively. The increase in the degradation rate is attributed to the decrease of the recombination rate of photoinduced e⁻/h⁺, caused by the enhanced charge transfer between the metal nanoparticles and Bi₂O₄ as confirmed in the photocurrent measurements. The photocurrent measurements showed increase in the transients output by 8.25 times and 2.75 times for 1% Ag–Bi₂O₄ & 3% Au–Bi₂O₄, respectively as compared to that of the pristine Bi₂O₄. These features further aided the increase in the photocatalytic efficiency while retaining the original physical properties, thus showing the robustness of Bi₂O₄ as a photocatalyst.

Illustration of charge transfer between Bi₂O₄ and nanoclusters, and photocatalytic MO degradation by M/Bi₂O₄ under visible light irradiation (M = Ag or Au).     

Ti3C2 MXene: a promising microwave absorbing material

In this work, we demonstrate the enhancement of microwave attenuation capability of Ti₃C₂ enabled microwave absorbing materials (MAMs) within a frequency range of 2–18 GHz. Ti₃C₂ nano-sheet/paraffin composites exhibit enhanced microwave absorbing performance with an effective absorbing bandwidth of 6.8 GHz (11.2–18 GHz) at 2 mm and an optimal reflection loss of −40 dB at 7.8 GHz. Moreover, mechanisms for the dielectric responses of the Ti₃C₂ MXene nanosheets are intensively discussed. Three typical electric polarizations of Ti₃C₂ are illustrated with the Cole–Cole diagram. The enhanced microwave absorbing properties can be ascribed to the high dielectric loss accompanied with the strong multi-reflections between MXene layers.

A novel microwave absorbing material with Ti₃C₂ MXene nanosheets.