Octahedral Vacancies Research Articles

Exploring the Crystal Structure and Electronic Properties of γ-Al2O3: Machine Learning Drives Future Material Innovations.

For decades, researchers have struggled to determine the precise crystal structure of γ-Al2O3 due to its atomic-level disorder and the challenges associated with obtaining high-purity, high-crystallinity γ-Al2O3 in laboratory settings. This study investigates the crystal structure and electronic properties of γ-Al2O3 coatings under the influence of an external electric field, integrating machine learning with density functional theory (DFT). A potential 160-atom supercell structure was identified from over 600,000 γ-Al2O3 configurations and confirmed through high-resolution transmission electron microscopy and selected area electron diffraction. The findings indicate that γ-Al2O3 deviates from the conventional spinel structure, suggesting that octahedral vacancies can reduce the system's energy. Under an external electric field, the material's band structure and density of states (DOS) undergo significant changes: the bandgap narrows from 3.996 to 0 eV, resulting in metallic behavior, while the projected density of states (PDOS) exhibits peak broadening and splitting of oxygen atom PDOS below the Fermi level. These alterations elucidate the variations in the electrical conductivity of alumina coatings under an electric field. These findings clarify the mechanisms of γ-Al2O3's electronic property modulation and offer insights into its covalent and ionic mixed bonding as a wide-bandgap semiconductor. This discovery is essential for understanding dielectric breakdown in insulating materials.

A novel and efficient method for synthesizing reduced charge montmorillonite

Layer charge (LC), a paramount physiochemical characteristic of montmorillonite (Mt), has been garnered considerable attention on its regulating and impact on modified Mt products. Incorporating small cations such as Li+ into the tetrahedral/octahedral sheets of Mt, or employing acid treatment, are feasible methods to reduce its LC. However, conventional techniques for producing reduced charge Mt (RC-Mt) often suffer from prolonged processing times, complex procedures, and high consumption of water and energy. Moreover, the formation mechanisms of RC-Mt remain inadequately understood. To address these challenges, this study proposes a novel and efficient method combining dry lithiation and microwave irradiation to prepare RC-Mt, and compares it with a conventional acid treatment. The research also explores the applications of RC-Mt, focusing on organic modification and the evaluation of adsorption performance. It was found that the cation exchange capacity (CEC) of RC-Mt diminished with increasing microwave power and time, LiCl dosage, HCl concentration, acidification duration and temperature. It has been demonstrated that the dry lithiation & microwave method significantly reduces the total preparation time of RC-Mt to 12 min while eliminating the generation of wastewater. The pivotal advantages of this approach include its efficiency, time-saving, and minimal environmental impact. Analytical techniques such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and solid-state nuclear magnetic resonance spectroscopy (SSNMR) revealed that microwave irradiation facilitates the migration of Li+ into the hexagonal cavities and octahedral vacancies of the Mt tetrahedral sheets. Additionally, acid treatment results in the dissolution of octahedral cations from Mt. The modified products of RC-Mt with lower CEC, exhibited enhanced adsorption of perchlorate ions (ClO4−). These findings offer significant insights into the modification of Mt and its potential for advanced technological applications, presenting a promising avenue for future research.

Compressibility and thermal expansion of magnesium phosphates

Abstract. The ambient-temperature compressibility and room-pressure thermal expansion of two Mg3(PO4)2 polymorphs (farringtonite = Mg3(PO4)2-I, with 5- and 6-fold coordinated Mg, and chopinite = “Mg-sarcopside” = [6]Mg3(PO4)2-II), three Mg2PO4OH polymorphs (althausite, hydroxylwagnerite and ε-Mg2PO4OH, all with [5]Mg and [6]Mg) and phosphoellenbergerite ([6]Mg) were measured on synthetic powders using a synchrotron-based multi-anvil apparatus to 5.5 GPa and a laboratory high-temperature diffractometer, with whole-pattern fitting procedures. Bulk moduli range from 64.5 GPa for althausite to 88.4 GPa for hydroxylwagnerite, the high-pressure Mg2PO4OH polymorph. Chopinite, based on an olivine structure with ordered octahedral vacancies (K0 = 81.6 GPa), and phosphoellenbergerite, composed of chains of face-sharing octahedra (K0 = 86.4 GPa), are distinctly more compressible than their homeotypical silicate (127 and 133 GPa, respectively). The compressibility anisotropy is the highest for chopinite and the lowest for phosphoellenbergerite. First-order parameters of quadratic thermal expansions range from v1 = 2.19×10-5 K−1 for ε-Mg2PO4OH to v1 = 3.58×10-5 K−1 for althausite. Phosphates have higher thermal-expansion coefficients than the homeotypical silicates. Thermal anisotropy is the highest for farringtonite and the lowest for hydroxylwagnerite and chopinite. These results set the stage for a thermodynamic handling of phase-equilibrium data obtained up to 3 GPa and 1000 °C in the MgO–P2O5–H2O and MgO–Al2O3–P2O5–H2O systems.

Phase and Composition Engineering of Self-Intercalated 2D Metallic Tantalum Sulfide for Second-Harmonic Generation.

Self-intercalation in two-dimensional (2D) materials is significant, as it offers a versatile approach to modify material properties, enabling the creation of interesting functional materials, which is essential in advancing applications across various fields. Here, we define ic-2D materials as covalently bonded compounds that result from the self-intercalation of a metal into layered 2D compounds. However, precisely growing ic-2D materials with controllable phases and self-intercalation concentrations to fully exploit the applications in the ic-2D family remains a great challenge. Herein, we demonstrated the controlled synthesis of self-intercalated H-phase and T-phase Ta1+xS2 via a temperature-driven chemical vapor deposition (CVD) approach with a viable intercalation concentration spanning from 10% to 58%. Atomic-resolution scanning transmission electron microscopy-annular dark field imaging demonstrated that the self-intercalated Ta atoms occupy the octahedral vacancies located at the van der Waals gap. The nonperiodic Ta atoms break the centrosymmetry structure and Fermi surface properties of intrinsic TaS2. Therefore, ic-2D T-phase Ta1+xS2 consistently exhibit a spontaneous nonlinear optical (NLO) effect regardless of the sample thickness and self-intercalation concentrations. Our results propose an approach to activate the NLO response of centrosymmetric 2D materials, achieving the modulation of a wide range of optoelectronic properties via nonperiodic self-intercalation in the ic-2D family.

Determination of uric acid in serum by SERS system based on VO-MnCo2O4/Ag nanozyme

The level of uric acid is crucial to human health. Octahedral oxygen vacancy MnCo2O4/Ag (VO-MnCo2O4/Ag) nanozyme was successfully prepared by simple hydrothermal, calcination and self-reduction methods. VO-MnCo2O4/Ag nanozyme is rich in Mn2+/Mn3+ and CO2+/CO3+ redox electron pairs, large specific surface area and oxygen vacancies. VO-MnCo2O4/Ag nanozyme showed high uricase-like activity and peroxidase-like activity. At the same time, the SERS signal of the detected molecule could be significantly enhanced after the catalytic reaction of the VO-MnCo2O4/Ag nanozyme. The Km values of VO-MnCo2O4/Ag nanozyme for H2O2 and TMB were 0.04 mM and 0.027 mM respectively. Based on the uric acid oxidase-like and peroxidase-like activities of VO-MnCo2O4/Ag, we developed a label-free, sensitive, and reliable SERS uric acid detection system. The detection linear range of uric acid is 0.01 μM–1000 μM and the detection of limit is 7.8 × 10−9 M. The results show that the sensing system has good accuracy, sensitivity, selectivity, and stability. It can be applied to the determination of samples under different conditions. This study provides profound insights into the design of enzyme-like activity regulation and SERS properties regulation of nanozymes, provides guidance for the study of reaction kinetics and catalytic mechanism of nanozymes, and has broad application prospects in the field of nanozymes and SERS sensing analysis.

Molecular dynamics study of the exchange processes of heavy metals into montmorillonite: Characterization of hydrated edge surfaces and dynamic exchange mechanism

Molecular-level understanding of the exchange processes of heavy metals into the interlayer region of clay minerals help to predict the geochemical cycle of these elements. In this study, we conducted molecular dynamics by using the newly developed CLAYFF-MOH parameters to elucidate the hydration characteristics of edge surfaces. On this basis, we explored the exchange processes of a series of heavy metal cations. Results showed that Mg for Al isomorphous substitution in the secondary outermost positions would reduce the coordination number (CN) of edge Al on both edge surfaces; water films were formed onto the interface owing to H-bonding interaction. Moreover, the cation exchange rates were Pb2+ > Cr3+ > Cd2+ > Ag+ > Ni2+ > Cu2+; Ag+ showed the largest exchange amount, followed by Ni2+, Cd2+, Pb2+, Cu2+, and Cr3+. A small part of Cl− was exchanged along with multivalent cations. The simulations of Ni2+-Cd2+-Pb2+ system indicated that coexistence of divalent cations had no significant effect on the exchange, while the exchange process in Ag+-Cd2+-Cr3+ mixing system was more complicated. Electrostatic interaction was the dominant driving force responsible for exchange. Additionally, Na+ and Ag+ were complexed on the octahedral vacancy, Si–OH, and Al–OH sites. The mobilities of cations decreased with increased ionic valence increased, and overall the diffusion in the interlayer was more constrained than in external solution. This study provides fundamental molecular-level knowledge of the fate of heavy metals in natural environment.

Water-assisted-mechanical activation of copper pyrometallurgical tailings for molybdenum leaching and selective removal of environmentally-hazardous elements

Fayalite-rich and elemental crystalline blockade of copper pyrometallurgical slag flotation tailings (CSFT) make it difficult to dispose of safely and limits high-value reutilization. Water-assisted milling, a waste-free method, is exploratively employed for activation of CSFT and to improve elemental leachability. In comparison to traditional milling, water-assisted milling reduces the time required for amorphous transition and amorphous nano-mesoporous materials were obtained in 4 h-activated CSFT (SBET increased to 12.25 m2·g−1 from 0.99 m2·g−1; pore size: 2–50 nm). Formation and re-removal of strongly-bound water promoted the hydration expansion of mineral particles and generation of tetrahedral or octahedral silicon defects and vacancies, which provided a large number of active sites and effectively promoted the rapid diffusion and transfer of ions. Water-assisted milling improves the leachability of high-value-added Mo (from 53.09 to > 95 %). During the water-assisted milling process, the dissociation and REDOX reaction of Mo were accelerated by the co-effect of reactive metals (Ca and Al) and the electron migration in the friction environment. This procedure exposed and morphologically changed Mo, facilitated the separation of the iron phase from Mo, and enhanced its solubility in diluted H2SO4 agents. The separation and leaching recovery of high-toxicity arsenic (As) and cadmium (Cd) was enhanced and a preliminary process was proposed for selective leaching of As (>96.99 %) and Cd (96.15 %) by NaOH solution.

Improving thermal stability and textural properties of mesoporous γ-alumina granules by Zr-La dopants

Improving the thermal stability and modifying the porous properties of γ-alumina by incorporating various transition and rare earth metals in alumina have been widely studied. However, less attention has been paid to investigating the co-doping process for increasing the alumina thermal stability and surface area in temperatures ranging from 1000° to 1200°C. In this research, pure, Zr-doped, and Zr-La doped granules with an average diameter of 1.5–2 mm were synthesized using the sol-gel granulation method. XRD results indicated that the introduction of either Zr or Zr-La dopants improves the thermal stability of transition aluminas up to 1200 °C. DTA/TG results revealed that the incorporation of 1 wt% zirconium along with 1 wt% lanthanum retarded α-Al2O3 phase transformation temperature to around 1335 °C. Compared to the pure sample, the BET surface area of alumina containing 1 wt% zirconium, 1 wt% zirconium along with 1 wt% lanthanum, and 1 wt% zirconium along with 3 wt% lanthanum increased to 234.4, 232.4, and 215.6 m2/g at 750 °C, respectively. Moreover, the sample containing 1 wt% of Zr along with 1 wt% of La dopants maintained a higher specific surface area after calcination at 1000 °C (133.6 m2/g) and 1200 °C (64.7 m2/g). According to NMR and HRTEM investigations, the capability of the co-doped Zr-La alumina to preserve the mesoporous structure and thermal stability as well as suppressing α-alumina formation up to 1200 °C is the result of zirconium and lanthanum dopants occupying tetrahedral and octahedral vacancies in the structure of transition alumina. The reported Zr-Al co-doped alumina granules in this study with high thermal and porous structural stability can be good candidates as a support for industrial catalysts.

Keeping dysprosium in line: Trinuclear heterometallic MII2DyIII complexes with M=Cd, Co and Cu

AbstractFour new heterometallic trinuclear essentially linear (“in‐line”) MII2DyIII complexes (M=CdII (1), CoII (2), CuII (3, 4) were synthesised using the ligand 3,5‐di‐tert‐butylbenzoic acid (dtbbaH) in a MeCN : iPrOH (1–3) or in a MeOH : iPrOH : DMF (4) mixture. Single crystal structure analyses revealed the formation of [Cd2Dy(dtbba)6(iPrOH)2(H2O)2NO3] ⋅ iPrOH (1), [Co2Dy(dtbba)6(iPrOH)2(H2O)2NO3] ⋅ iPrOH (2), [Cu2Dy(dtbba)6(MeCN)4NO3] ⋅ MeCN (3) and [Cu2Dy(dtbba)6(iPrOH)1.68(DMF)1.32(H2O)NO3] ⋅ 3iPrOH ⋅ H2O (4). The M−Dy−M angles become more acute from (1) to (3) leading to decreased M⋅⋅M distances. The DyIII ions are 8‐coordinate with a distorted triangular dodecahedron geometry and a D2d symmetry whereas the geometry of the 5‐coordinate terminal MII ions can best be described as either distorted vacant octahedra or as spherical square pyramids with a C4v symmetry. Magnetic susceptibility measurements were performed on (1), (2) and (3) which allows a comparison of the effect of replacing highly anisotropic CoII with the quantum spin of CuII. Both (2) and (3) show effects of ferromagnetic interactions at low temperatures.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Thermal Migration Behavior of Na+, Cu2+ and Li+ in Montmorillonite

The main aim of this paper is to study the cation fixation sites in montmorillonite after heating at different temperatures. Montmorillonite was used to adsorb cations (Na+, Cu2+ and Li+) in the solution, and the montmorillonite-adsorbed cations were heated at different temperatures (unheated, 100 °C, 200 °C and 300 °C) for 25 h. Subsequently, the basal spacing of montmorillonite treated at different temperatures was monitored by X-ray diffraction (XRD). The exchangeable cationic content (Na+, Cu2+ and Li+) in montmorillonite was determined based on an inductively coupled plasma emission spectrometer (ICP-OES). In addition, the stretching and bending vibration changes in the OH group and the Si-O bond in montmorillonite were detected by Fourier transform infrared spectroscopy (FTIR). The vibration changes were related to the cation fixation sites. The XRD data showed that when the heating temperature reached 200 °C, the structure of montmorillonite adsorbing Li+ and Cu2+ ions completely collapsed, but the layer spacing of montmorillonite adsorbing Na+ decreased slightly, which indicated that Li+ and Cu2+ were more easily able to enter the crystal structure. The ICP-OES results showed that the contents of exchangeable Na+, Cu2+ and Li+ in montmorillonite decreased with the increase in heating temperature, and Li+ was more easily fixed by montmorillonite than Na+ and Cu2+. The FTIR data showed that when montmorillonite adsorbed with Li+ was heated at more than 200 °C, a new OH stretching vibration band appeared at 3971 cm−1, which may be caused by the migration of Li+ into the octahedral vacancy to form a local trioctahedral structure. Na+ has a large radius; it can only be fixed near the OH group and may not enter the tetrahedron/octahedron of montmorillonite. The number of charges carried by Cu2+ is high and the dehydration enthalpy of hydrated Cu2+ is high. When the heating temperature was greater than 200 °C, Cu2+ mainly entered the hexagonal cavity of the tetrahedron and caused slight changes in the OH bending vibration. The vibration of the Si-O bond hardly changed after montmorillonite adsorbed Na+, but the stretching vibration peak of the Si-O bond moved to the high value region after adsorbing Cu2+ and Li+, which was speculated to be related to the migration of Cu2+ and Li+ into the crystal structure.

Li5 SnP3 - a Member of the Series Li10+4x Sn2-x P6 for x=0 Comprising the Fast Lithium-Ion Conductors Li8 SnP4 (x=0.5) and Li14 SnP6 (x=1).

The targeted search for suitable solid‐state ionic conductors requires a certain understanding of the conduction mechanism and the correlation of the structures and the resulting properties of the material. Thus, the investigation of various ionic conductors with respect to their structural composition is crucial for the design of next‐generation materials as demanded. We report here on Li5SnP3 which completes with x=0 the series Li10+4x Sn2−x P6 of the fast lithium‐ion conductors α‐ and β‐Li8SnP4 (x=0.5) and Li14SnP6 (x=1). Synthesis, crystal structure determination by single‐crystal and powder X‐ray diffraction methods, as well as 6Li, 31P and 119Sn MAS NMR and temperature‐dependent 7Li NMR spectroscopy together with electrochemical impedance studies are reported. The correlation between the ionic conductivity and the occupation of octahedral and tetrahedral sites in a close‐packed array of P atoms in the series of compounds is discussed. We conclude from this series that in order to receive fast ion conductors a partial occupation of the octahedral vacancies seems to be crucial.

Surface Reconstruction of a Mullite-Type Catalyst via Selective Dissolution for NO Oxidation

We present a facile selective dissolution method for the surface modification of SmMn2O5 mullite (SMO) to increase the exposed sites for NO oxidation on the premise of excellent thermal stability. Surface Sm cations are partially removed during the treatment (SMO-H), leading to the exposure of B-site cations with higher electronegativity and oxygen vacancies. Under laboratory-simulated diesel combustion conditions, the SMO-H exhibits higher NO oxidation activity than the SMO and is comparable to that of the Pt/Al catalyst under a gas hourly space velocity (GHSV) of 120 000 mL g–1 h–1. In addition, the SMO-H possesses good thermal and steam stability during a 50 h test at 300 °C. X-ray absorption spectroscopy (XAS) and electron paramagnetic resonance (EPR) spectroscopy results reveal that the SMO-H presents more Mn4+O6 octahedral groups and oxygen vacancies than the SMO. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), and density functional theory (DFT) calculations show that the valence of surface Mn decreased after NO capture up to 300 °C and fulfilled the adsorption procedure as nitrite and/or nitrate species when O2 is involved in the flue gas. The results indicate that Mn-terminated surface rather than Sm-terminated one is the major adsorption site for NO.

Li4.3AlS3.3Cl0.7: A Sulfide-Chloride Lithium Ion Conductor with Highly Disordered Structure and Increased Conductivity.

Mixed anion materials and anion doping are very promising strategies to improve solid-state electrolyte properties by enabling an optimized balance between good electrochemical stability and high ionic conductivity. In this work, we present the discovery of a novel lithium aluminum sulfide–chloride phase, obtained by substitution of chloride for sulfur in Li3AlS3 and Li5AlS4 materials. The structure is strongly affected by the presence of chloride anions on the sulfur site, as the substitution was shown to be directly responsible for the stabilization of a higher symmetry phase presenting a large degree of cationic site disorder, as well as disordered octahedral lithium vacancies. The effect of disorder on the lithium conductivity properties was assessed by a combined experimental–theoretical approach. In particular, the conductivity is increased by a factor 103 compared to the pure sulfide phase. Although it remains moderate (10–6 S·cm–1), ab initio molecular dynamics and maximum entropy (applied to neutron diffraction data) methods show that disorder leads to a 3D diffusion pathway, where Li atoms move thanks to a concerted mechanism. An understanding of the structure–property relationships is developed to determine the limiting factor governing lithium ion conductivity. This analysis, added to the strong step forward obtained in the determination of the dimensionality of diffusion, paves the way for accessing even higher conductivity in materials comprising an hcp anion arrangement.

Fabrication and investigation of a new all-inorganic lead free perovskite Cs3Bi2I6 Br3 for ammonia detection at room temperature

A new all-inorganic lead free perovskite of Cs3Bi2I6Br3 has been synthesized by a simple liquid spin-coating method and applied for ammonia detection at room temperature. The successful formation was confirmed by XRD, FE-SEM, EDS and XPS. The sensing performance tests demonstrate that the responses of the device to ammonia are higher than those to ethanol and acetone. The top response value is of 11.8–500 ppm ammonia with satisfied recovery process, repeatability and improved stability of 14 days. Recoverable exchange between Cs+ ions located in octahedral vacancies and ammonia molecules should be in charge of the ammonia sensing mechanism. In contrast, electron transfer is responsible for the ethanol and acetone sensing processes. Our investigation offers a new strategy in the design of sensitive and potential green gas sensor in room temperature detection.

Microstructure, formation mechanism and mechanical properties of ZrC–Fe coating on cast iron via in situ solid-phase diffusion

Abstract To improve the mechanical properties of the surface of iron-based alloys, a ZrC–Fe coating is produced on cast ion via in situ solid-phase diffusion (ISSD) at 1130 °C. The coating is completely dense, reaching a thickness of approximately 18 μm after 10 h of ISSD. The volume fraction of the ZrC phase in the coating reaches approximately 92.3%. The size of the long axis (short axis) of columnar ZrC particles gradually decreases from approximately 4 μm (1.5 μm) to 1 μm (0.3 μm) as the thickness of the coating increases, displaying a gradient microstructure. The formation mechanism of the coating is mainly attributed to the carbon atoms diffuse into the octahedral vacancies of the zirconium lattice to undergo a diffusion-type solid state phase transformation to form ZrC, and α-Fe phase are distributed among the ZrC particles through the Fe atoms diffusion. Due to the carbon concentration gradient, the nucleation rate is reduced, thereby forming a gradient microstructure on the grain scale. The growth of the coating obeys the growth kinetic model d 2 = 30.5 t . In addition, the nanohardness and elastic modulus of the surface of the ZrC–Fe coating reach 29.7 GPa and 425.1 GPa, respectively, showing remarkable improvement compared to those of the cast iron substrate (4.5 GPa and 229.5 GPa, respectively). The fracture toughness gradually increases from 2.9 ± 0.1 MPa m1/2 to 3.6 ± 0.1 MPa m1/2 as the thickness of the coating increases. Moreover, the coating exhibits the superior coating/substrate adhesion. The excellent comprehensive mechanical properties are tightly related to a small amount of high toughness α-Fe phase, high volume fraction of in situ ZrC particles and the gradient change of the ZrC grain scale.

A Mechanism of Anti-Oxidation Coating Design Based on Inhibition Effect of Interface Layer on Ions Diffusion within Oxide Scale

In this paper, a mechanism of anti-oxidation coating design based on the inhibition effect of the interface layer on the diffusion of ions within oxide scale was introduced. The Fe2+ ions diffusion behavior in Fe3O4, Cr2FeO4, and FeAl2O4 were studied by molecular dynamics method of Nudged elastic bond. As the result shown, Fe2+ ions tended to diffuse through the vacancy at tetrahedral site in Cr2FeO4 and FeAl2O4, but diffuse through the octahedral vacancy in Fe3O4. When temperature ranged from 1073 to 1325 K, the energy barrier of Fe2+ ions diffusion in Cr2FeO4 was higher than that of FeAl2O4, and both of that were still obvious higher than that in Fe3O4. A new anti-oxidation coating was prepared based on the inhibition of interface layer consisted of FeAl2O4 to protect the carbon steel S235JR at 1200 °C for 2 h. The FeAl2O4 region was formed and observed at the interface between coating and Fe element diffusion area, and the mullite phase was distributed outside of the FeAl2O4 region. Comparing to the bare sample, the prepared coating exhibited an excellent anti-oxidation effect.

Fe-Mn Oxides Based Multifunctional Adsorptive/Electrosensing Nanoplatforms: Dynamic Site Rearrangement for Metal Ion Selectivity.

Achieving structural requirements for the exclusive selectivity of adsorbent to a specific metal remains challenging, as certain metal ions show similar adsorptive behaviors and preference toward a given site. We reported the morphology and oxidation state-dependent selectivity manipulating of layered oxides by controlling the dynamic evolution of different adsorptive sites. The computational investigation predicted the site-specific partitioning trends of metal ions at two sites of manganese oxide (MnO2) layers: the lateral edge sites (LESs) and octahedral vacancy sites (OVSs). In contrast to the predominant occupation of the OVSs for other metal ions, the binding of lead (Pb) ions was energetically favored at both the sites. We assembled ultrathin MnO2 nanosheets on the magnetic iron oxides to first enhance the accessibility of the LESs. A sequential ligand-promoted partial reduction of the atomic MnO2 layers induced the edge-to-interlayer migration of Mn atoms to block the nonspecific OVSs and activate the LESs, enabling a superior selectivity to Pb. In addition, the iron oxides helped construct a multifunctional adsorptive/electrosensing platform for Pb regarding their facile magnetic separation and electrochemical activity. Simultaneous selective adsorption and on-site monitoring of Pb(II) were achieved on this nanoplatform, owing to its satisfactory stability and sensitivity without an obvious matrix effect.

Hydration of Na-saturated synthetic stevensite, a peculiar trioctahedral smectite

AbstractSmectite interlayer water plays a key role in the mobility of elements and molecules and affects a variety of geological processes. In trioctahedral smectites, in contrast to saponite and hectorite, the layer charge of which originates from isomorphic substitutions, the stevensite layer charge originates from the presence of octahedral vacancies. Despite its common occurrence in lacustrine environments, stevensite hydration has received little attention compared to saponite and hectorite. Early reports mention a specific hydration behaviour, however, with the systematic presence of a low-angle reflection attributed to the regular interstratification of various hydration states. The present study aims to revisit this specific hydration behaviour in more depth. Within this scope, the hydration behaviour of the three smectite varieties above are compared using synthetic trioctahedral smectites of similar layer charge and various compositions of their octahedral sheets. The chemical composition of the octahedral sheet does not appear to influence significantly smectite hydration for saponite and hectorite. Compared to its saponite and hectorite equivalents, H2O content in stevensite is lower by ~2.0 mmol H2O per g of dry clay. Consistent with this lower H2O content, Zn-stevensite lacks a stable monohydrated state, with dehydrated layers prevailing from 60% to 0% relative humidity. The presence of the regular interstratification of 0W and 1W layers is responsible for the low-angle reflection commonly observed for stevensite under air-dried conditions. Finally, the stevensite identification method based on X-ray diffraction of heated and ethylene glycol-solvated samples is challenged by the possible influence of the octahedral sheet chemical composition (Zn or Mg in the present study) on hectorite swelling behaviour in synthetic Zn-smectites. The origin of this effect remains undetermined and further work is needed to propose a more general identification method.

The adsorption mechanism of Zn2+ by triclinic birnessite and the mechanism

ABSTRACT In this paper, the reaction mechanism for the adsorption of Zn2+ by synthetic triclinic Na-birnessite was studied by reacting synthetic triclinic Na-birnessite with Zn2+ in solution, thereby providing a theoretical basis for the purification of heavy metal ions in acid soil and water by triclinic birnessite. The adsorption effect of Zn2+ on Na-birnessite enhances with an increase in either reaction time or Na-birnessite dosage, as well as decrease of pH. Na-birnessite can effectively adsorb Zn2+ in acidic solutions without any secondary pollution, and the stronger the acidity, the better the treatment effect. In acidic conditions, H+ in solution exchanges with Na+ in the interlayer of triclinic Na-birnessite, then a small part of Zn2+ in solution exchanges with the Mn2+ produced during the triclinic-to-hexagonal phase transformation, and most of the Zn2+ forms a complex with OH− on the octahedral layer, which loses protons due to the consumption of H+ ions. Finally, Zn2+ adsorbs above and below octahedral vacancies in hexagonal birnessite in either an octahedral or tetrahedral coordination.