In-situ micro-spectroscopy technique for chemical bonding during nucleation: A transition from soft bond to stiff bond

Abstract

In-situ technique has been widely used in recent years since the advent of high-resolution spectroscopy systems, which stimulates us to probe into the time-dependent, dynamic behaviors and relevant mechanisms during a series of physical processes [1]. Especially, in-situ analysis presents a unique real-time glimpse into the fascinating physical phenomena [2], which are very important to develop the materials for subsequent applications. Generally speaking, the nucleation process involves dynamic changes in the cluster structures, as well as the formation and transformation of chemical bonds, which requires in-situ analysis urgently [3].The idea of chemical bonding provides novel insights into the nucleation process [4,5] which can be expressed as a chemical equation as follows:where C is the cluster, N s is the small size nucleus and N l is the large size nucleus. This model contains two fundamental assumptions: entropy change approximation and chemical bond softening. In the first assumption, entropy change may be assumed negligible when individual small size clusters are combined into larger clusters. This means that reduction of the whole system energy comes from the released energy via forming chemical bonds when clusters become combined. This also makes it possible to use bond energy (bond enthalpy) [6] to describe free energy variation of the system. In the second assumption, ions with similar resonance status[7] often combine to form clusters, and the clustering structure is constantly changing due to chemical bonds softening. Different chemical bonding is the key to distinguish the state of the system. As shown in Figure 1, the asformed clusters via those soft chemical bonds belong to a kind of amorphous state. When the size is large enough, the soft chemical bonds transform into stiff ones, resulting in a further reduction in energy. This marks the formation of a nucleus with a crystalline structure which has the lowest energy and therefore no longer changes dynamically.We have proposed and experimentally demonstrated the in-situ micro Raman characterization technique of chemical bonding behaviors driven by electronegativity [8]. As shown in Figure 2, in a modified setup, the emission from a single droplet can be excited in-situ under a laser scanning. The sample information about chemical bonding in aqueous system can be characterized in-situ by a series of spectra. High spatial resolution and the design of the bottom droplets make it possible to observe nucleation in real time. Raman signal increased obviously with the increase of the laser intensity [9,10], and the use of droplet inversion and ultra-thin glass expand the application of high magnification short focus objectives in solution Raman testing, which further significantly improves the signal. Figure 3 is a typical Raman spectrum of hexahydrate nickel sulfate (NiSO 4 ·6H 2 O) aqueous solution which is divided into two regions to analyze variations of sulfate ions and water molecules [11,12]. Through rapid and continuous scanning of these two r...

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“…Its symmetry reflects the corresponding energy state of materials with a single size [31]. During nucleation, the chemical bonding modes vary rapidly and involve dynamic changes in the cluster structures and the formation and transformation of chemical bonds; materials finally become relatively stable when their length scale reaches the critical size [32,33]. Clusters formed via softened chemical bonds belong to a kind of amorphous state.…”

“…Clusters formed via softened chemical bonds belong to a kind of amorphous state. When their size is large enough, the softened chemical bonds transform into stiff ones [33].…”

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