Congting Sun

Crystallization design of MnO2 towards better supercapacitance

Nanostructured manganese oxides with different crystallization behaviors were fabricated by a simple redox reaction between KMnO4 and NaNO2 aqueous solution. MnO2 nanostructures with sphere-, rod-, wire-, plate- and flower-like morphologies were crystallized, and the relationship between crystallization characteristics and their electrochemical performances were studied. The electrochemical energy storage behaviors of these samples were investigated by cyclic voltammetry and galvanostatic charge–discharge measurements processed using noncorrosive Na2SO4 as the electrolyte. A maximum specific capacitance 200 F g−1 was obtained for poorly crystallized α-MnO2 at a current density of 1 A g−1. For different crystallographic MnO2 phases, their specific capacitance values increase in the order: β < γ < δ < α, meanwhile, for any particular MnO2 phase, their electrochemical energy storage performances decrease with increasing crystalline nature and particle size.

Tunnel-dependent supercapacitance of MnO2: effects of crystal structure

A proportional relationship between the specific capacitance of MnO2 and the percentage of effective Mn centers that act as active sites in the Faradaic charge storage has been established on the basis of a tunnel structure–crystallization behavior correlation. A quantitative relationship between the effective Mn centers at the surfaces and in the tunnels can distinguish the specific capacitance values that arise from the adsorption/desorption and insertion/extraction processes, respectively, of different MnO2 crystallographic forms in the Faradaic charge storage. The different specific capacitance values between the MnO2 crystallographic forms are mainly attributed to the different effective utilizations of Mn centers in the size-limited tunnels. The present model demonstrates that increasing the percentage of effective Mn centers via decreasing the crystal size can facilitate obtaining MnO2-based electrode materials with higher specific capacitance values.

CRYSTALLIZATION OF OXIDES AS FUNCTIONAL MATERIALS

Crystallization is essential to the manufacture of functional materials as varies as electronic devices, energy storage and conversion devices, and highly reactive catalysts. As an important part of functional materials, metal oxides possess wide applications and the crystallization of oxide materials has thus received considerable attention from both fundamental and technological perspectives. With particular emphasis on our recent laboratory results, this feature article gives a brief review in the field of crystallization of oxides. On the basis of chemical bonding theory of single crystal growth, we have simulated thermodynamic growth behaviors of various functional oxides such as ZnO, MgO, Cu2O, Nb2O5, V2O5, MnO2, SnO2, NiO, KDP/ADP, LiNbO3, and NaNbO3. Quantitatively analyzing bonding conditions of controllable crystallographic faces enables us to design proper synthesis strategies and optimize growth parameters, consequently obtaining functional oxides with desirable crystallization behaviors.

Chemical bonding theory of single crystal growth and its application to ϕ 3′′ YAG bulk crystal

The growth of YAG bulk crystals was studied using both theoretical calculations based on the anisotropic chemical bonding conditions and practical growth via the Czochralski (Cz) method. The chemical bonding theory of single crystal growth quantitatively describes the anisotropic bonding behaviors of constituent atoms during crystallizing, which can be applied to the thermodynamic growth of YAG single crystals. Both bonding conditions and crystal symmetry determine the projection configuration along the pulling direction and crystal ridges in the crystal shoulder of YAG grown along [111] direction. During Cz growth process of YAG single crystals, the relative low growth rate along directions results in the exposure of surfaces normal to directions. However, the chemical bonding energy density at the intersection of two adjacent growth directions is higher, leading to the exposure of surfaces normal to directions and the truncated-hexagon configuration of YAG along [111] direction. ϕ 3′′ YAG single crystal was successfully grown. Our present work provides a promising approach to achieve controllable growth for functional bulk crystal via both thermodynamic and kinetic controls.

Polymorphic crystallization of Cu2O compound

Cu2O can crystallize into various polymorphs, such as cubes, rhombic dodecahedra, branching structures, and hopper cubes, instead of thermodynamically stable octahedra by designed kinetic-control routes instead of traditional thermodynamic control. The present results confirmed that Cu2O polymorphs have distinct physical and chemical properties, and morphology changes can occur due to polymorphic transitions in a kinetics-controllable reaction system. Both thermodynamic and kinetic factors on these polymorphism systems are believed to be significant for developing a polymorphism–property relationship, ultimately guiding the appropriate material selection for specific applications. Furthermore, we took Cu2O as an example to illustrate the development of “polymorphism” in modern materials science: polymorphism is an intrinsic physicochemical characteristic, and involves varying growth shapes, phase transformation and variation of physical properties. These findings can provide new insight to polymorphism–performance correlation and kinetic–thermodynamic control synthesis.

Direct in situ ATR-IR spectroscopy of structural dynamics of NH4H2PO4 in aqueous solution

Crystallization of KDP-family crystals depends on the chemical bonding behavior of the crystal constituents in aqueous solution, which are sensitive to solution conditions. We applied in situ ATR-IR spectroscopy combined with a morphology-evolution calibration to declare the structural dynamics of NH4+ and H2PO4− during the NH4H2PO4 crystallization in aqueous solution with different concentrations and pH values. For unsaturated NH4H2PO4 solution, both the H2PO4− stretching vibration mode and NH4+ bending vibration mode are enhanced with increasing concentration. When the NH4H2PO4 solution becomes a saturated and then supersaturated and crystalline state, H2PO4− ions undergo hydrated dimerisation and polymerisation, which can be recorded by the appearance and red shift of the P–O⋯H–O–P in-plane bending vibration mode from 1250 to 1263 cm−1. During this process, hydrated NH4+ ions bind to the (H2PO4−)n frame, reflected by the splitting of the HN4+ bending vibration mode at 1450 and 1400 cm−1. For the supersaturated NH4H2PO4 solution, HPO42− and H2PO4− coexist in solution with increasing pH value up to 6.64, whereas H3PO4 and H2PO4− coexist with decreasing pH value down to 1.52. Such an in situ recording strategy is of particular value in studying system dynamics, and in general to monitor the solution concentrations and compositions before and during the crystallization process.

In situ ATR-IR observation of nucleation and crystal growth of KH2PO4 in aqueous solution

In situ Attenuated Total Reflectance-Infrared (ATR-IR) spectroscopy was used to record and characterize the nucleation and crystal growth of KH2PO4 (KDP) from aqueous solution with pH = 1.01–6.96. In the crystallization of KDP, K+ and H2PO4− ions transform from hydrated ions to clusters and crystalline state. With increasing measurement time, the shift of v3(PO4) at 938 and 1145 cm−1 towards lower wavenumbers illustrated the breaking of hydrogen bonding between H2PO4− and H2O. IR bands of vO–H at 2370 cm−1 and βO–H at 1274 cm−1 indicated the formation of H2PO4− clusters possessing KDP lattice structural characteristics via hydrogen bonding. The disappearance of v3(PO4) at 1145 cm−1 indicated the chemical bonding between K+ and H2PO4− in the nucleation stage of KDP. In KDP supersaturated solution with pH = 2.02–6.96, H2PO4− groups with larger hydrated ionic radius initially form a framework structure, and then K+ ions with a relatively smaller hydrated ionic radius insert into the H2PO4− framework to form KDP. Moreover, the time needed for the appearance of KDP solids in IR measurement can be modified by varying the pH value of KDP supersaturated solution. Such an in situ visualization strategy can characterize the structural variations by spectroscopy, which deepens the insight into the nucleation and crystal growth from aqueous solution during the whole crystallization process.

Chemical bonding theory of single crystal growth and its application to crystal growth and design

The effects of crystallization on the formation of geochemical, biological, and synthetic materials have been motivating decades of research into crystal nucleation and growth processes. The development of crystal growth theories and models can deepen the understanding of physicochemical interactions during the crystal growth process, which facilitates the designing of crystallization approaches in material production. The chemical bonding theory of single crystal growth emphasizes the dominant role of dynamic chemical bonding mechanisms at the growing interfaces. In this paper, we highlight the chemical bonding theory of single crystal growth from the chemical reaction viewpoint, by focusing on the atomic level of the growing interface between the liquid and crystal phases. Using ZnO, CeO2, MnO2 and Y3Al5O12 as examples, we review some typical applications of the chemical bonding theory of single crystal growth in calculating crystal habits, evaluating crystal properties, and guiding practical single crystal growth. Microscopically speaking, the essence of crystal growth and design is to create ideal chemical bonding architectures at both the crystal surface and the growing interface via both thermodynamic and kinetic strategies.

Growth, spectroscopic characteristics and laser potential of Yb3+:Ca3La2(BO3)4 crystal

Crystal of Yb3+-doped Ca3La2(BO3)4 has been grown by the Czochralski technique. The room temperature absorption and fluorescence spectra of the crystal have been investigated. The result showed that this crystal exhibits broad absorption and emission with the FWHM of 11 nm at 978 nm and 66 nm FWHM at 1025 nm, respectively. The stimulated emission cross-section of Yb3+ ions were calculated using the reciprocity method and Fuchtbauer-Ladenburg method, respectively. The room temperature fluorescence decay curves of 2F5/2 manifold of Yb3+ ions were recorded for both crystal and powder samples. The effect of radiation tapping on the spectroscopic properties is discussed. The result that the lifetime of the powder sample is shorter than that of the bulk sample demonstrates the existence of radiation trapping effect. The laser potentiality was also evaluated and the results show that this crystal is a good candidate for tunable and ultrashort pulse lasers.

Hybridized valence electrons of 4f0–145d0–16s2: the chemical bonding nature of rare earth elements

Abstract The chemical bonding nature of rare earth (RE) elements can be studied by a quantitative analysis of electron domain of an atom. The outer electrons of RE elements are within the valence shell 4f 0–14 5d 0–1 6s 2 , which are involved in all chemical bonding features. We in this work found that the chemical bonding characteristics of 4f electrons are a kind of hybridizations, and classified them into three types of chemical bonding of 4f 0–14 5d 0–1 6s 2 , furthermore, the coordination number ranging from 2 to 16 could thus be determined. We selected Y(NO 3 ) 3 , La(NO 3 ) 3 , Ce(NO 3 ) 3 , YCl 3 , LaCl 3 , and CeCl 3 as examples to in-situ observe their IR spectra of chemical bonding behaviors of Y 3+ , La 3+ and Ce 3+ cations, which could show different chemical bonding modes of 4f and 5d electrons. In the present study, we obtained the direct criterion to confirm whether 4f electrons can participate in chemical bonding, that is, only when the coordination number of RE cations is larger than 9.