Vapor Diffusion Sol-gel Method Research Articles

Composition-dependent surface chemistry of colloidal BaxSr1-xTiO3 perovskite nanocrystals.

BaxSr1-xTiO3 perovskite nanocrystals, prepared by the vapor diffusion sol-gel method and characterized by state of the art surface techniques, display significantly different O-H stretching frequencies and adsorption properties towards CO2 as a function of the alkaline earth composition (Ba vs. Sr). The difference of properties can be associated with the more basic nature of BaO-rich than SrO-rich surfaces.

Lanthanide-activated scheelite nanocrystal phosphors prepared by the low-temperature vapor diffusion sol-gel method.

A series of Eu3+-, Tb3+-, and Tm3+-doped CaWO4 phosphor nanocrystals have been synthesized under benign conditions using the vapor diffusion sol-gel method. The high degree of synthetic flexibility inherent to this approach has enabled the synthesis of a CaWO4:(Eu,Tb) dual-sensitized white light emitting nanocrystal phosphor upon commercial UV excitation at 366 nm with a long lifetime exceeding 1 ms.

Low-temperature synthesis of homogeneous solid solutions of scheelite-structured Ca1-xSrxWO4 and Sr1-xBaxWO4 nanocrystals.

A series of compositionally complex scheelite-structured nanocrystals of the formula A1-xAWO4 (A = Ca, Sr, Ba) have been prepared under benign synthesis conditions using the vapor diffusion sol-gel method. Discrete nanocrystals with sub-20 nm mean diameters were obtained after kinetically controlled hydrolysis and polycondensation at room temperature, followed by composition-dependent thermal aging at or below 60 °C. Rietveld analysis of X-ray diffraction data and Raman spectroscopy verified the synthesis of continuous and phase-pure nanocrystal solid solutions across the entire composition space for A1-xAWO4, where 0 ≤ x ≤ 1. Elemental analysis by X-ray photoelectron and inductively coupled plasma-atomic emission spectroscopies demonstrated excellent agreement between the nominal and experimentally determined elemental stoichiometries, while energy dispersive X-ray spectroscopy illustrated good spatial elemental homogeneity within these nanocrystals synthesized under benign conditions.

Complex perovskite oxide nanocrystals: low-temperature synthesis and crystal structure.

This Perspective reviews our recent efforts towards the low-temperature synthesis of complex perovskite oxide ABO3 (A = Sr, Ba; B = Ti, Zr) nanocrystals using the vapor diffusion sol-gel method and the determination of their room-temperature crystal structure. From a synthetic standpoint, emphasis is placed on demonstrating the ability of the vapor diffusion sol-gel approach to yield compositionally complex nanocrystals at low temperatures and atmospheric pressure without the need for postsynthetic heat treatment to achieve a crystalline and phase-pure oxide product. The ability to successfully achieve this is illustrated using Ba1-xSrxTi1-yZryO3 (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) and Eu(3+)-doped Ba(Ti,Zr)O3 nanocrystals as examples. From the standpoint of the structural analysis, emphasis is placed on highlighting how multiple and complementary spectroscopic techniques that probe atomic correlations in short (≤1 nm), intermediate (∼1-3 nm), and long (≥3 nm) length scales can be employed to gain insight into the atomic structure of the resulting nanocrystals. Examples that clearly illustrate this strategy of structural characterization are the investigation of the size- and composition-dependence of the structure of polar nanoregions in sub-10 nm BaTiO3 and sub-20 nm Ba1-xSrxTiO3 and BaTi1-yZryO3 nanocrystals, and the investigation of the distribution of rare earth dopants in sub-15 nm Eu(3+):BaTiO3 nanocrystals.

ChemInform Abstract: Low Temperature Synthesis and Characterization of Lanthanide‐Doped BaTiO3 Nanocrystals.

AbstractNanocrystals of BaTiO3 doped with up to 2 mol% Dy or La are prepared by the vapor diffusion sol—gel method from solutions of Ln(acac)3 (Ln: Dy, La) and BaTi(O‐CH2‐CH(Me)‐OMe)6 in BuOH/2‐methoxypropanol under a flow of N2/HCl/H2O vapor (25 °C, 72 h; 68—76% yield).

Low-Temperature Synthesis of AMoO4(A = Ca, Sr, Ba) Scheelite Nanocrystals

An extension of the vapor diffusion sol–gel method to the synthesis of the AMoO4 (A = Ca, Sr, Ba) scheelite family of materials is reported. Sub-30 nm quasispherical nanocrystals were obtained after vapor diffusion at room temperature, followed by thermal aging at 80 °C. Rietveld analysis of X-ray diffraction data, Raman spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy demonstrated that the vapor diffusion sol–gel method affords crystalline and phase-pure AMoO4 nanocrystals with excellent compositional control. The potential lithium storage capacity of the CaMoO4 nanocrystals versus Li at a rate of C/4 was also investigated. The nanocrystals exhibited an extremely large first discharge capacity of 1300 mA h g–1, which stabilized at 250 mA h g–1 after 25 cycles.

Structural Evolution of BaTiO3Nanocrystals Synthesized at Room Temperature

Sub-10 nm BaTiO(3) nanocrystals were synthesized at room temperature via the vapor diffusion sol-gel method, and their structural evolution during nucleation and growth stages was followed using a series of techniques that probe the atomic structure on different length and time scales. Special emphasis was placed on assessing the evolution of the local symmetry and structural coherence of the resulting nanocrystals, as these are the structural bases for cooperative properties such as ferroelectricity. Although the room-temperature crystal structure of the fully grown nanocrystals appears cubic to Rietveld analysis of synchrotron X-ray diffraction data, Raman spectroscopy and pair distribution function analysis demonstrate the presence of non-centrosymmetric regions arising from the off-centering of the titanium atoms. This finding demonstrates that accounting for diffuse scattering is critical when attempting the structural characterization of nanocrystals with X-ray diffraction. The local symmetry of acentric regions present in BaTiO(3) nanocrystals, particularly structural correlations within an individual unit cell and between two adjacent unit cells, is best described by a tetragonal P4mm space group. The orthorhombic Amm2 space group also provides an adequate description, suggesting both types of local symmetry can coexist at room temperature. The average magnitude of the local off-center displacements of the titanium atoms along the polar axis is comparable to that observed in bulk BaTiO(3), and their coherence length is on the order of 16 Å. The presence of local dipoles suggests that a large amount of macroscopic polarization can be achieved in nanocrystalline BaTiO(3) if the coherence of their ferroelectric coupling is further increased.

Vapor Diffusion Sol‐Gel Synthesis of Fluorescent Perovskite Oxide Nanocrystals

A model system consisting of Eu(3+) as the activator ion and BaZrO(3) as the host lattice is employed to demonstrate the potential of the vapor diffusion sol-gel method as a hydrolytic approach to the synthesis of fluorescent alkaline-earth perovskite oxide nanocrystals under ultrabenign conditions. The resulting nanocrystals are suitable precursors for nanostructured red-emitting phosphors.

Local structural distortion of BaZrxTi1−xO3nanocrystals synthesized at room temperature

Single crystalline, sub-15 nm BaZr(x)Ti(1-x)O(3) (0 ≤x≤ 1) nanocrystals were synthesized at room temperature via the vapor diffusion sol-gel method. As-prepared nanocrystals exhibit noncentrosymmetric regions whose volume fraction increases significantly upon substitution of small amounts of Zr(4+) for Ti(4+) and reaches a maximum for substitution levels ranging from 10 to 20 mol%.

Very Low-Temperature, Gram-Scale Synthesis of Monodisperse BaTiO3 Nanocrystals via an Interfacial Hydrolysis Reaction

AbstractA vapor diffusion sol-gel method is reviewed for the preparation of high-quality BaTiO3 nanocrystals on the gram scale at very low temperatures. The synthesis is based on the kinetically controlled introduction of water into a solution of the bimetallic alkoxide, BaTi(O2C4H9)6, where slow hydrolysis then occurs at the vapor-solution interface followed by nucleation and nanocrystal growth at 16 °C. The resulting 6-nm, quasi-spherical nanocrystals are both monodisperse (without stabilizing agents or size selecting purification) and highly crystalline (without any post-synthesis heat treatment), and are isolated in yields near 100%. Based on new permittivity and calorimetry data, the crystal structure of the nanocrystals is most likely in the paraelectric cubic phase (space group Pm3m) at room temperature, which corroborates previous diffraction data. It was also demonstrated that the BaTiO3 nanocrystals can be doped with trivalent lanthanum cations using the same low-temperature vapor diffusion sol-gel method to yield donor-doped Ba1−xLaxTiO3, which exhibits a considerable PTCR effect.