Phase evolution of lead titanate from its amorphous precursor synthesized by the OPM wet-chemical route

Lead titanate was synthesized by the OPM wet-chemical route by the dissolution of Ti metal in H₂O₂ followed by the addition of Pb²⁺ at high pH, resulting in a reactive and amorphous precipitate with (Pb:Ti=1:1) mole ratio, which was heat treated between 400°C and 700°C. The amorphous precipitate was characterized by DSC, and all of the powders were characterized by X-ray diffraction, Raman and XAS (EXAFS and XANES) spectroscopy at the Ti K edge. A metastable, stoichiometric and cubic pyrochlore phase (Pb₂Ti₂O₆, Fd3m) was identified by XRD and Raman spectroscopy up to approx. 450°C. Only tetragonal PbTiO₃ was identified at higher temperatures. XAS spectra showed that the local structure around the absorbing Ti atom of the intermediate pyrochlore phase is similar to that observed in the amorphous precursor. This fact indicates that the metastable intermediate pyrochlore (Pb₂Ti₂O₆) is kinetically favored to be formed because of its similarity to the amorphous precipitate, instead of the slightly different and thermodynamically favored tetragonal (PbTiO₃, P4/mmm) perovskite structure that is only formed at higher temperatures, after the crystallization of the metastable intermediate pyrochlore.     

Heterometallic lead-titanium oxyalkoxide precursors to lead titanate thin films

Two lead titanium oxyalkoxides with composition Pb₂Ti₄O₂(O₂CCH₃)₂(OC₂H₅)₁₄ and Pb₂Ti₂O₂(O₂CCH₃)₂(OCH(CH₃)₂)₈ have been isolated and characterised by elemental analysis, IR, ²⁰⁷Pb NMR and molecular weight measurements. X-ray structural analysis of the 1:2 complex confirmed the presence of two Pb₂Ti₂O units which are linked by a common Pb···Pb edge and are held together by alkoxide and acetate bridges. The acetate groups have migrated from the Pb to the Ti centres during the reaction. Hydrolysis of the 1:1 Pb/Ti complex produced clear gels providing the H₂O/complex ratio was less than 4. Heat treatment of the gel results in loss of residual organics below 400°C. The XRD pattern indicates the presence of the pyrochlore and perovskite phases after heating at 500°C but the sample is poorly crystalline. Complete conversion to the perovskite phase of lead titanate occurs on heating to 600°C for 1 hour. Thin films of lead titanate were deposited by dip-coating a solution of this complex in isopropanol. Analyses of the films, carried out using electron microprobe, Scanning Auger Spectroscopy and Rutherford Backscattering, indicated that they were of excellent quality, crystalline after heating at 600°C and with relatively sharp substrate-coating interface.     

半化学法制备Pb(Fe2/3W1/3)O3陶瓷粉体的反应机理

铅系弛豫铁电陶瓷钨铁酸铅Pb(Fe2/3W1/3)O3(PFW)是一种重要的介电材料,具有较大的介电常数(8000)和较低的烧结温度(小于900℃),适用于制备低烧高介的多层陶瓷电容器犤1～4犦。在传统氧化物法合成PFW的过程中,易生成恶化介电性能的钨酸铅(PbWO4或Pb2WO5)或焦绿石相(Pb2FeWO6)等其他相犤3,4犦。尽管通过加入过量5%的Fe2O3可以消除这些其他相,但因含较多的变价铁离子(Fe3+和Fe2+)而产生介电老化的缺点犤5犦。虽然二次合成法被广泛用于制备铅系弛豫铁电陶瓷犤6犦,但对制备PFW陶瓷的效果并不明显,仍有少量的钨酸铅PbWO4存在,并且预烧…     

On the Entangled Growth of NaTaO3 Cubes and Na2Ti3O7 Wires in Sodium Hydroxide Solution

The entangled growth of sodium titanate Na₂Ti₃O₇ nanowires and sodium tantalate NaTaO₃ cubes was investigated with electron microscopy, X‐ray diffraction, and UV diffuse reflectance spectroscopy. Depending on the composition of the Ta₂O₅‐ and TiO₂‐particle‐based powder mixtures, which served as educts, we observed different types of hybridization effects. These include the titanium‐induced contraction of the NaTaO₃ perovskite‐type unit cell and the generation of electronic defect states in NaTaO₃ that give rise to optical subbandgap transitions and tantalum‐induced limitations of the Na₂Ti₃O₇ nanowire growth. The transformation from Ta₂O₅ to NaTaO₃ occurs through a dissolution–recrystallization process. A systematic analysis of the impact of different titanium sources on NaTaO₃ dispersion and, thus, on the properties of the entangled nanostructures revealed that a perfect intermixture of cubes and nanowires can only be achieved when titanate nanosheets emerge during transformation as reaction intermediates and shield nucleation and growth of isolated NaTaO₃ cubes. The here demonstrated approach can be highly instrumental for understanding the nucleation and growth of composite and entangled nanostructures in solution and—at the same time—provides an interesting new class of photoactive composite materials.     

Liquid‐Phase Epitaxial Growth of Two‐Dimensional Semiconductor Hetero‐nanostructures

Although many two‐dimensional (2D) hybrid nanostructures are being prepared, the engineering of epitaxial 2D semiconductor hetero‐nanostructures in the liquid phase still remains a challenge. The preparation of 2D semiconductor hetero‐nanostructures by epitaxial growth of metal sulfide nanocrystals, including CuS, ZnS and Ni₃S₂, is achieved on ultrathin TiS₂ nanosheets by a simple electrochemical approach by using the TiS₂ crystal and metal foils. Ultrathin CuS nanoplates that are 50–120 nm in size and have a triangular/hexagonal shape are epitaxially grown on TiS₂ nanosheets with perfect epitaxial alignment. ZnS and Ni₃S₂ nanoplates can be also epitaxially grown on TiS₂ nanosheets. As a proof‐of‐concept application, the obtained 2D CuS–TiS₂ composite is used as the anode in a lithium ion battery, which exhibits a high capacity and excellent cycling stability.     

A one‐dimensional mixed‐anion lead(II) coordination polymer: catena‐poly[[(1,10‐phenanthroline)lead(II)]‐μ‐benzoato‐μ‐nitrato‐[(1,10‐phenanthroline)lead(II)]‐di‐μ‐benzoato]

In the title polymeric compound, [Pb₂(C₇H₅O₂)₃(NO₃)(C₁₂H₈N₂)₂]_n, both independent Pb atoms adopt an eight‐coordinate geometry formed by one nitrate, three benzoate and one 1,10‐phenanthroline ligand. The one‐dimensional polymer consists of dimeric [Pb₂(C₇H₅O₂)₃(NO₃)(C₁₂H₈N₂)₂] units, in which all nitrate and benzoate ligands act in a bridging–chelating coordination mode.     

A Facile Synthetic Route to a New SHG Material with Two Types of Parallel π‐Conjugated Planar Triangular Units

A new SHG material, namely, Pb₂(BO₃)(NO₃), which contains parallel π‐conjugated nitrate and borate anions, was obtained through a facile hydrothermal reaction by using Pb(NO₃)₂ and Mg(BO₂)₂?H₂O as starting materials. Its structure contains honeycomb [Pb₂(BO₃)]_∞ layers with noncoordination [NO₃]^? anions located at the interlayer space. Pb₂(BO₃)(NO₃) shows a remarkable strong SHG response of approximately 9.0 times that of potassium dihydrogen phosphate (KDP) and the material is also phase‐matchable. The large SHG coefficient of Pb₂(BO₃)(NO₃) arises from the synergistic effect of the stereoactive lone pairs on Pb²⁺ cations and parallel alignment of π‐conjugated BO₃ and NO₃ units. Based on its unique properties, Pb₂(BO₃)(NO₃) may have great potential as a high performance NLO material in photonic applications.     

Three‐dimensional Hemidirected Mixed‐ligand Lead(II) Coordination Polymer, [Pb2(bpa)2(SCN)3(NO3)]n (bpa = 1,2‐di(4‐pyridyl)ethane)

A new 3D hemidirected mixed‐ligand lead(II) coordination polymer with the ligand 1,2‐di(4‐pyridyl)ethane bpa) and the two metal coordinated anions nitrate and thiocyanate, [Pb₂(bpa)₂(SCN)₃(NO₃)]_n ( 1 ), has been synthesized and characterized by CHN elemental analysis, IR‐, ¹H‐ and ¹³C NMR spectroscopy. The single crystal X‐ray data of compound 1 show that the complex is a three‐dimensional coordination polymer with two different Pb atoms with stereoactive electron lone pairs and six‐ and five‐coordinate hemidirected geometries, respectively.     

Hydrothermal Synthesis and Up‐conversion Luminescence of Ho3+/Yb3+ Co‐doped PbTiO3

Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals with different content of dopant were successfully prepared via a facile hydrothermal method. The purity, morphology, element distribution, chemical state and up‐conversion (UC) photoluminescence (PL) of PbTiO₃ nanocrystals affected by Ho³⁺ dopant are investigated systematically. X‐ray diffraction (XRD) results illustrate that PbTiO₃ samples with the doping Ho³⁺ concentration ranging from 0 to 5 mol‐% are perovskite structure. The doping Ho³⁺ ions have no change on the crystal structure of perovskite PbTiO₃. Owing to the non‐equivalent substitution of Ho³⁺ to Ti⁴⁺ in PbTiO₃, the particle size of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples is decreased as well as the particle agglomeration is detected. Moreover, Ho and Yb ions have uniform distributions in the PbTiO₃ nanoparticles as the presence of Ho³⁺ and Yb³⁺ cations. The up‐conversion spectra demonstrate that Ho/Yb co‐doped PbTiO₃ samples have up‐conversion emissions centered at 550 nm, 660 nm and 755 nm, corresponding to the transitions of ⁵F₄(⁵S₂)→⁵I₈, ⁵F₅→⁵I₈ and ⁵S₂(⁵F₄)→⁵I₇ of Ho³⁺ ions. Additionally, the effect of temperature on the UC PL property of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ system is further investigated. The sensitivity and the trend of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples in temperature from 298 k to 493K are calculated on the basis of fluorescence intensity ratio (FIR) method. Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals are verified the high potential in the optical temperature sensing.     

Hierarchical Titanate Nanostructures through Hydrothermal Treatment of Commercial Titania Powders

Hierarchical titanate nanostructures were hydrothermally synthesized in concentrated base solutions using commercial titania powders as starting materials. By varying the base concentration, nanowire arrays, flowers of nanosheets and nanotubes, and urchin‐like nanostructures of nanowires and nanotubes were sequentially fabricated. If the NaOH concentration was higher than 6 M , hydrated Na₂Ti₆O₁₃ nanowire arrays, with nanowire diameters of 20–90 nm and an aspect ratio of 1100–5000, were produced at suitable reaction temperatures over a large area. In 10 M KOH solutions, aligned nanowires with a diameter of 30 nm and a lenght of 80 μm formed. In 4 M NaOH solutions, micrometer‐sized flowers of nanotubes and nanosheets formed. Reactions in 2 M NaOH solutions produced urchin‐like materials with a size of ca. 10 μm that were composed of nanotubes and nanowires. The adsorption behavior of the urchin‐like materials resembled macroporous materials with micropores. Since both base concentration and reaction temperature affected the reaction rate, the formation of various titanate nanostructures was proposed as a growth speed controlled process.     

Dielectric properties of chemically co-precipitated tetragonal Pb(Mg1/3Nb2/3)0.65Ti0.35O3

Pyrochlore phase free Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ ceramics have been synthesized successfully by chemical co-precipitation method. It has been noted that formation of perovskite phase Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ without pyrochlore phase is tricky. The synthesized samples at optimized parameters were characterized using X-ray diffraction technique. Careful analysis of the XRD data predicts the tetragonal lattice structure. The morphological studies depict the presence of uniform grain size. Dielectric constant (ε′) and loss tangent (tan δ), at and well above room temperature, were studied. The variation of dielectric constant with temperature shows sharp peak at ferroelectric–paraelectric transition temperature. Further, the temperature dependent dielectric constant shows good fit with modified Curie–Weiss law, which suggests normal ferroelectric behavior of Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃.     

Identification of the Structural Boundary between [Ln(18‐crown‐6)(NO3)3] and [Ln(NO3)3(H2O)3] · 18‐crown‐6 Motifs in the Lanthanide Series

Recrystallization of Ln(NO₃)₃ (Ln = Sm, Eu, Yb) in the presence of 18‐crown‐6 under aqueous conditions yielded [Ln(NO₃)₃(H₂O)₃] · 18‐crown‐6. X‐ray crystallography revealed isomorphous structures for each of the lanthanide complexes where [Ln(NO₃)₃(H₂O)₃] is involved in hydrogen bonding interactions with 18‐crown‐6. The transition point where the structural motif changes from [Ln(18‐crown‐6)(NO₃)₃] (with the metal residing in the crown cavity) to [Ln(NO₃)₃(H₂O)₃] · 18‐crown‐6 has been identified as at the Nd/Sm interface. A similar investigation involving [Ln(tos)₃(H₂O)₆] (tos = p‐toluenesulfonate) and 18‐crown‐6 were resistant to crown incorporation. X‐ray studies show extensive intra‐ and intermolecular hydrogen bonding is present.     

An oxo‐bridged centrosymmetric tetranuclear titanium compound

The title compound, octa‐tert‐butoxybis­[μ₃‐2,2′‐(N‐methyl­imino)­diethanolato]­di‐μ‐oxo‐tetratitanium(IV), [Ti₂O{(OCH₂CH₂)₂(NCH₃)}{(CH₃)₃CO}₄]₂ or [Ti₄(C₅H₁₁NO₂)₂(C₄H₉O)₈O₂], lies about an inversion centre, and displays the less usual zigzag configuration. One O atom of the N‐methyl­diethoxo­amine ligand bridges the symmetry‐related Ti atoms, while the other bridges the two independent Ti atoms, with the N atom binding to give a facial configuration. Four ^tBuO⁻ ligands and a bridging oxide complete the respective five‐ and sixfold coordination of the two Ti atoms. The Ti—O bond lengths range in a self‐consistent fashion from 1.7624 (17) to 2.0878 (18) Å, while the Ti—N bond length is 2.374 (2) Å.     

One‐Dimensional Holodirected Lead(II) Coordination Polymer, [Pb(μ2‐TPPZ)(NO3)(ClO4)]n (TPPZ = 2, 3, 5, 6–tetra(2‐pyridyl)pyrazine)

A lead(II) complex with 2,3,5,6‐tetra(2‐pyridyl)pyrazine (TPPZ), nitrate, and perchlorate ligands has been synthesized and characterized by CHN elemental analysis and IR and ²⁰⁷Pb NMR spectroscopy. The single crystal X‐ray data of the [Pb₂(μ‐TPPZ)₂(NO₃)₂(ClO₄)₂] compound show that the complex is a one‐dimensional coordination polymer and that the Lead atom has a less‐common, ten‐coordinate holodirected geometry.     

Lead-Doped Titanium-Oxo Clusters as Molecular Models of Perovskite-Type PbTiO3 and Electron-Transport Material in Solar Cells

In this work we have successfully prepared two lead-doped titanium-oxo clusters with core structures that resemble isolated perovskite PbTiO₃ species. In the obtained highly symmetric Pb₈Ti₇-oxo cluster, the central TiO₆ octahedra are orthogonally extended to adjacent octahedra through corner-sharing and the eight dopant lead ions form a cubic arrangement, making it the first molecular model of perovskite PbTiO₃. Moreover, the clusters readily dissolved in chloroform and showed high solution stability, as confirmed by MALDI-TOF MS measurements. Based on such solution processability, they can be easily spin-coated to form homogeneous films, which were employed as electron-transport materials in perovskite solar cells to give an average power conversion efficiency of around 15 % and improved device stability. This newly developed bottom-up cluster assembly method provides an efficient approach to the construction of atomically precise models of perovskite metal oxides as well as potential molecular tools to extend their applications.     

Tuning the Surface Structure of Nitrogen‐Doped TiO2 Nanofibres—An Effective Method to Enhance Photocatalytic Activities of Visible‐Light‐Driven Green Synthesis and Degradation

Nitrogen‐doped TiO₂ nanofibres of anatase and TiO₂(B) phases were synthesised by a reaction between titanate nanofibres of a layered structure and gaseous NH₃ at 400–700 °C, following a different mechanism than that for the direct nitrogen doping from TiO₂. The surface of the N‐doped TiO₂ nanofibres can be tuned by facial calcination in air to remove the surface‐bonded N species, whereas the core remains N doped. N‐Doped TiO₂ nanofibres, only after calcination in air, became effective photocatalysts for the decomposition of sulforhodamine B under visible‐light irradiation. The surface‐oxidised surface layer was proven to be very effective for organic molecule adsorption, and the activation of oxygen molecules, whereas the remaining N‐doped interior of the fibres strongly absorbed visible light, resulting in the generation of electrons and holes. The N‐doped nanofibres were also used as supports of gold nanoparticle (Au NP) photocatalysts for visible‐light‐driven hydroamination of phenylacetylene with aniline. Phenylacetylene was activated on the N‐doped surface of the nanofibres and aniline on the Au NPs. The Au NPs adsorbed on N‐doped TiO₂(B) nanofibres exhibited much better conversion (80 % of phenylacetylene) than when adsorbed on undoped fibres (46 %) at 40 °C and 95 % of the product is the desired imine. The surface N species can prevent the adsorption of O₂ that is unfavourable for the hydroamination reaction, and thus, improve the photocatalytic activity. Removal of the surface N species resulted in a sharp decrease of the photocatalytic activity. These photocatalysts are feasible for practical applications, because they can be easily dispersed into solution and separated from a liquid by filtration, sedimentation or centrifugation due to their fibril morphology.     

Effect of biphase on dielectric properties of Bi-doped lead strontium titanate thin films

Pb_0.4Sr_0.6TiO₃ (PST) thin films doped with various concentration of Bi were prepared by a sol-gel method. The phase status, surface morphology and dielectric properties of these thin films were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance analyzer, respectively. Results showed that the thin films with the maximum dielectric constant and minimum dielectric loss were obtained for x=0.15. For x<0.15, only pure PST perovskite phase were in the thin films. For 0.2<x<0.4, the PST/Bi₂Ti₂O₇ biphase were obtained. The thin films with pure Bi₂Ti₂O₇ pyrochlore phase were obtained for x=0.67. The biphase thin films had high tunability and high figure of merit (FOM). The FOM of PST/Bi₂Ti₂O₇ biphase thin film was about 6 times higher than that thin films formed with pure perovskite phase or pure pyrochlore phase.     

Solution‐Based Synthesis of Layered Intergrowth Compounds of the Homologous PbmBi2nTe3n+m Series as Nanosheets

Layered intergrowth compounds in the homologous Pb_mBi_2nTe_3n+m family are interesting because they are examples of natural heterostructures. We present a simple solution‐based synthesis of two‐dimensional nanosheets of PbBi₂Te₄, Pb₂Bi₂Te₅, and PbBi₆Te₁₀ layered intergrowth compounds, which are members of the Pb_mBi_2nTe_3n+m [that is, (PbTe)_m(Bi₂Te₃)_n] homologous series. Few‐layer nanosheets exhibit narrow optical band gaps (0.25–0.7 eV) with semiconducting electronic‐transport properties.     

Crown‐Ether‐Like Structures Derived from a Ti8O8(Carboxylate)16 Metallacycle

Mixed‐metal clusters have been obtained from the reaction of titanium alkoxides with either strontium or lead acetate and methacrylic acid. The structures of the clusters are derived from the metallacycle Ti₈O₈(methacrylate)₁₆. The Sr and Pb atoms in Sr₂Ti₈O₈X₂(OOCMe)₂(methacrylate)₁₆ (X: acetate or OiPr) and Pb₂Ti₈O₈(OBu)₂X₂(methacrylate)₁₆(BuOH)₂ (X: acetate or methacrylate) occupy the central cavity of the Ti₈O₈ ring. In addition to the crown‐ether‐like coordination of the ring oxygen atoms to the Sr or Pb atoms, bridging carboxylate ligands support the coordination of the latter atoms. In the compound Pb₂Ti₆O₅(OiPr)₃X(methacrylate)₁₄ (X: OiPr or methacrylate), the lead atoms are coordinated by a fragment of the Ti₈O₈(methacrylate)₁₆ metallacycle.     

Two New Lead(II) Coordination Polymers with (Substituted) Bipyridine and Auxiliary Nitrate Ligands

The reaction of lead(II) nitrate with 4,4′‐bipyridine (4,4′‐bpy) and 4,4′‐dimethyl‐2,2′‐bipyridine (4,4′‐dm‐2,2′‐bpy) or 5,5′‐dimethyl‐2,2′‐bipyridine (5,5′‐dm‐2,2′‐bpy) resulted in the fomation of single crystals of [Pb₂(4,4′‐bpy)(5,5′‐dm‐2,2′‐bpy)₂(NO₃)₄] ( 1 ) and [Pb₃(4,4′‐bpy)₂(4,4′‐dm‐2,2′‐bpy)₂(NO₃)₆] ( 2 ). The new compounds have been characterized by single‐crystal X‐ray diffraction structure analysis as well as through elemental analysis, IR, ¹H‐NMR and ¹³C‐NMR spectroscopy and their stability has been studied by thermal analysis. In the crystal structure of ( 1 ) formula‐like dimers are further connected to a 2‐D network through the auxiliary nitrate ligands. The crystal structure of ( 2 ) exhibits two crystallographically independent Pb^II central atoms (in a ratio of 1:2). With the aid of the 4,4′‐bpy and the nitrate ions, a 3‐D polymeric structure is achieved.