IR Spectral Study of Mesoscale Process during Urea Crystallization from Aqueous Solution

Unravelling the mesoscale process and the dynamic heterogeneous structures that appear on the mesoscale in a crystallization system is important in designing and fabricating functional crystalline materials. Recent experimental observations show the existence of stable clusters and amorphous intermediates before the formation of a crystalline solid, which seems to contradict classical nucleation theory. Here we show by in situ infrared spectroscopy and theoretical calculation that the liquid/solid phase transformation of urea proceeds through the agglomeration of primary clusters. The phase transformation pathway of urea in solution has been identified, in which urea molecules initially aggregate into one-dimensional (1D) molecular chains, and then these 1D molecular chains assemble to 2D plane-like and 3D net-like clusters. Crystalline urea with <i>P</i>–42₁<i>m</i> symmetry can be formed when these 3D net-like clusters overcome a critical size. Both experimental and calculated results demonstrate that the liquid/solid phase transformation of urea in aqueous solution obeys the classical nucleation theory. Finally, a morphology diagram of urea is provided on the basis of relative chemical bonding energy density. This morphology diagram can be used to understand the multiple anisotropic geometries for how urea crystals in an aqueous solution system can be laid out. Our results demonstrate the concept of identifying a particular mesoscale process in a urea crystallization system by both in situ vibration spectroscopy observations and chemical bonding calculations

Physical Chemistry of Crystalline (K,NH₄)H₂PO₄ in Aqueous Solution: An in Situ Molecule Vibration Spectral Observation of the Early Formation Stage

The competing occupancy of cation position by NH₄⁺ and K⁺ during crystallization produces local distortions in structure, resulting in the difficulties in growing high-quality (K,NH₄)­H₂PO₄ single crystals with particular mixing concentration. Crystal properties such as structure, defect density, and purity always depend on the early formation stage of crystalline (K,NH₄)­H₂PO₄ molecule. Identification of the effect of K⁺/NH₄⁺ mole ratios in KH₂PO₄–NH₄H₂PO₄ aqueous solution on (K,NH₄)­H₂PO₄ molecule motion at the early stage of crystallization can provide the strategy to growing high-quality (K,NH₄)­H₂PO₄ crystals with particular mixing concentration. In situ molecule vibration spectroscopy was used to identify the early formation stage of crystalline (K,NH₄)­H₂PO₄ in KH₂PO₄–NH₄H₂PO₄ aqueous solution with various K⁺/NH₄⁺ mole ratios. (K,NH₄)­H₂PO₄ molecule motion was imaged via integrating the structural information on IR/Raman-active NH₄⁺, H₂PO₄^–, and hydrogen bonding infrastructures. K⁺<i>/</i>NH₄⁺ mole ratio in KH₂PO₄–NH₄H₂PO₄ aqueous solution determines the supersaturation in crystallization system as well as the competing incorporation of K⁺ and NH₄⁺ into the lattice. Both lower supersaturation and stronger competition between cations hinder the crystallization of (K,NH₄)­H₂PO₄, resulting in the remarkable spectral difference before and after the formation of crystalline (K,NH₄)­H₂PO₄. Our results demonstrate the concept of competed incorporation between different cations into the anionic framework from the molecular viewpoint

Crystal Growth and Design of Sapphire: Experimental and Calculation Studies of Anisotropic Crystal Growth upon Pulling Directions

The anisotropic growth of large-size sapphire single crystals along different pulling directions was studied on the basis of the chemical bonding theory of single crystal growth and practical Czochralski growth. The projection of thermodynamic morphology of sapphire single crystal respectively along [210], [110], [001], and [001] rotated 57.62° directions can be used to confirm the growth directions of surfaces that are preferred to be exposed thermodynamically in Czochralski growth. Starting from these thermodynamically preferred directions, the possible radial directions that are normal to the four typical pulling directions by kinetic controls have been identified by anisotropic chemical bonding distributions of sapphire single crystal. Chemical bonding calculations demonstrate that the lower pulling rate should be designed when <i>R</i>_axial/<i>R</i>_radial > 1, whereas the higher pulling rate should be designed when <i>R</i>_axial/<i>R</i>_radial < 1. The anisotropic chemical bonding conditions demonstrate the lowest chemical bonding density along the radial directions of sapphire single crystal when it grows along the [001] pulling direction. Taking [001] as the pulling direction in practical growth, a ϕ 2″ sapphire single crystal was grown via the Czochralski method with a growth rate of 2–3 mm/h. Our present work shows the effect of anisotropy on the Czochralski growth of large-size single crystals, which can provide a theoretical guide in practical growth from both thermodynamic and kinetic viewpoints

Phase Transformation of Ce³⁺-Doped MnO₂ for Pseudocapacitive Electrode Materials

Doping is one of the important methods to modify the physical and chemical properties of functional materials, which can be used to synthesize mixed ionic and electronic conducting metal oxides. Herein, the phase transformation of MnO₂ from β- to α-phase has been proven by doping Ce³⁺ ions. With the increase of the amount of Ce³⁺ ions, the sizes of MnO₂ nanorods were first decreased to 10–20 nm, then increased to 70 nm. The capacitive performance indicated that the specific capacitance of Ce-doped MnO₂ electrode materials increased 10-fold compared with undoped MnO₂, while the charge transfer resistance of Ce-doped MnO₂ decreased. The present results show that rare earth ions can be used as a promising dopant to modify the crystallization behavior and electrochemical performance of MnO₂ electrode materials

Solution-Phase Electronegativity Scale: Insight into the Chemical Behaviors of Metal Ions in Solution

By incorporating the solvent effect into the Born effective radius, we have proposed an electronegativity scale of metal ions in aqueous solution with the most common oxidation states and hydration coordination numbers in terms of the effective ionic electrostatic potential. It is found that the metal ions in aqueous solution are poorer electron acceptors compared to those in the gas phase. This solution-phase electronegativity scale shows its efficiency in predicting some important properties of metal ions in aqueous solution such as the aqueous acidities of the metal ions, the stability constants of metal complexes, and the solubility product constants of the metal hydroxides. We have elaborated that the standard reduction potential and the solution-phase electronegativity are two different quantities for describing the processes of metal ions in aqueous solution to soak up electrons with different final states. This work provides a new insight into the chemical behaviors of the metal ions in aqueous solution, indicating a potential application of this electronegativity scale to the design of solution reactions

Hydrogen Bonding Dependent Mesoscale Framework in Crystalline Ln(H₂O)₉(CF₃SO₃)₃

Structurally, hydrogen bonding is identified as a key factor to domain the construction of a crystallographic frame during the crystallization of the Ln­(H₂O)₉­(CF₃SO₃)₃ (Ln = La–Lu) system. In situ Raman spectroscopy is used to capture the hydrogen bonding dependent mesoscale frameworks that are formed during Ln­(H₂O)₉­(CF₃SO₃)₃ crystallization in aqueous solution by continuously collecting the spectra of structural fragments. The spectral characteristics show that the isolated Ln­(H₂O)₉³⁺ tricapped trigonal prisms cannot exist in the aqueous solution. With the concentration of aqueous solution, the hydrated Ln³⁺ and CF₃SO₃^– tend to share common H₂O molecules, and new hydrogen bonding will be built surrounding Ln³⁺. Especially, for the Nd, Eu, Yb, and Lu system, Ln­(H₂O)_<i>n</i>­(CF₃SO₃)₃ (<i>n</i> = 8–9) clusters instead of hydrated Ln³⁺ and CF₃SO₃^– are formed in the solution. Under the guiding of intermolecular hydrogen bonds, both bond lengths and bond angles of Ln–O may be regulated, leading to the initial formation of Ln­(H₂O)₆³⁺ prisms and the following Ln­(H₂O)₉³⁺ tricapped trigonal prisms. Meanwhile, the symmetry of both CF₃ and SO₃ groups decreases from <i>C</i>_3<i>h</i> to <i>C</i>₂ accompanied by the formation of Ln­(H₂O)₆³⁺ triprism. The present study opens up the chemical bonding behaviors of rare earth ions in aqueous solution, which provides basic data for the study of the coordination of rare earth complexes and the design of novel rare earth materials

Hydrogen Bonding Paradigm in the Formation of Crystalline KH₂PO₄ from Aqueous Solution

Revealing the hydrogen bonding paradigm is critical to clarify the formation mechanism of hydrogen bonded materials. The nucleation process of a typical nonlinear optical crystal, KH₂PO₄, is identified by in situ molecular vibration spectroscopy, which effectively demonstrates the oriented role of hydrogen bonding in local structure engineering. On the basis of the vibrational evolution of hydrogen bonds, the partition of different periods in the formation of crystalline KH₂PO₄ from aqueous solution becomes clear. In KH₂PO₄ aqueous solution, there are hydrated status, (H₂PO₄^–)_<i>n</i> aggregations, and prenucleation clusters. The prenucleation clusters exist in the solution with a metastable status over a period of time, and then they will transform into crystalline ones within a short time. Two distinct roles of P–O···H–O–P hydrogen bonding in the formation of crystalline KH₂PO₄ have been distinguished. At the initial stage of aggregation formation, P–O bond in H₂PO₄^– group guides the P–O···H–O–P hydrogen bonding, leading to the H₂PO₄^– that retains <i>C</i>_2<i>v</i> symmetry, whereas P–O···H–O–P hydrogen bonding guides the twist and rotation of H₂PO₄^– groups in prenucleation clusters, promoting the local structural evolution of (H₂PO₄^–)_<i>n</i> from <i>C</i>_2<i>v</i> to <i>D</i>_2<i>d</i> and the formation of crystalline KH₂PO₄ nuclei. The present work deepens the hydrogen bonding effect that can warrant much space to adjust the chemical bonding environment in constructing crystallographic frames

Microwave-Irradiation-Assisted Combustion toward Modified Graphite as Lithium Ion Battery Anode

A rapid method to high-yield synthesis of modified graphite by microwave irradiation of partially oxidized graphite (oxidized by H₂SO₄ and KMnO₄) is reported. During the microwave irradiation, electrical arc induced flame combustion of Mn₂O₇ and vaporization and decomposition of H₂SO₄ to form O₂ and SO₂, which helped to decompose graphite within 30 s. The modified graphite boosts its ability to support the intercalation and diffusion of Li⁺ ions. As an anode material for lithium ion batteries, the modified graphite displays high reversible capacity of 373 mA·h/g, approaching the theoretical value of 372 mA·h/g. Long cycling performance of 410 charge–discharge cycles shows the capacity is retained at 370 mA·h/g, demonstrating superior stability. The improved cycling stability is attributed to the formation of a stable solid electrolyte interface film with the help of in situ formed S-based compounds on a graphite sheet. This work demonstrated a simple and effective method to alter carbon structures for improving energy storage ability

Crystallization-Dependent Luminescence Properties of Ce:LuPO₄

The luminescence properties of Ce:LuPO₄ depend on both the Ce³⁺ center and the host lattice. In this article, we studied the dependence of the luminescence properties of Ce:LuPO₄ on both the doping concentration of Ce³⁺ and the size and morphology of the LuPO₄ matrix at micro- and nanosize regimes. The crystalline behavior of Ce:LuPO₄, including its size and shape, was investigated via precursor transformation crystallization. On the basis of this crystallization approach, Ce:LuPO₄ hollow nanospheres, nanorods, and regular tetrahedrons were obtained. For micro- and nanostructured Ce:LuPO₄, the surface-induced chemical bonding architecture can be effectively varied by controlling the size of the crystalline material and its geometry. Our experimental observations demonstrate that one-dimensional Ce:LuPO₄ nanorods doped with 0.1 mol % Ce³⁺ possess the best performance among the as-prepared samples. The significant anisotropy of Ce:LuPO₄ nanorods can result in a larger specific surface area and enhanced luminescence properties. Moreover, the improved luminescence property of Ce:LuPO₄ nanostructures can also be optimized by increasing the preferential anisotropic chemical bonding architecture to regulate the 5<i>d</i> level of Ce³⁺. Our work also shows that the photoluminescence emission intensity of Ce:LuPO₄ nanorods is increased as the surface area normal to their axial direction increases. From the standpoint of crystallization, the luminescence properties of Ce³⁺ in nano- and microsize matrixes can be well-optimized by controlling the crystalline behavior of the host lattice under proper synthesis conditions

Microwave–Hydrothermal Crystallization of Polymorphic MnO₂ for Electrochemical Energy Storage

We report a coupled microwave–hydrothermal process to crystallize polymorphs of MnO₂ such as α-, β-, and γ-phase samples with plate-, rod-, and wirelike shapes, by a controllable redox reaction in MnCl₂–KMnO₄ aqueous solution system. MnCl₂–KMnO₄ redox reaction system was for the first time applied to MnO₂ samples under the coupled microwave–hydrothermal conditions, which shows clear advantages such as shorter reaction time, well-crystallized polymorphic MnO₂, and good electrochemical performances as electrode materials for lithium ion batteries. For comparison, we also did separate reactions with hydrothermal only and microwave only in our designed MnCl₂–KMnO₄ aqueous system. The present results indicate that MnCl₂–KMnO₄ reaction system can selectively lead to α-, β-, and γ-phase MnO₂, and the as-crystallized MnO₂ samples can show interesting electrochemical performances for both lithium-ion batteries and supercapacitors. Electrochemical measurements show that the as-crystallized MnO₂ supercapacitors have Faradaic reactivity sequence α- > γ- > β-MnO₂ upon their tunnel structures, the intercalation–deintercalation reactivity of these MnO₂ cathodes follows the order γ- > α- > β-phase, and the conversion reactivity of these MnO₂ anodes follows the order γ- > α- > β-phase. MnCl₂–KMnO₄ reaction system can also lead to the mixed-phase MnO₂ (β- and γ-MnO₂), which can provide better anode performances for lithium-ion batteries. The current work deepens the fundamental understanding of several aspects of physical chemistry, for example, the chemical reaction controllable synthesis, crystal structure selection, electrochemical property improvement, and electrochemical reactivity, as well as their correlations