Flow Alters the Interfacial Reactions of Upconversion Nanocrystals Probed by In Situ Sum Frequency Generation

Abstract

The interfacial interaction of the colloidal NPs in solution is closely related to the surface charge, which heavily influences the NP synthesis, colloidal stabilization and various applications. [2] In general, the inorganic core of colloidal NPs in solutions possess a net electric charge at the interface, which can attract dissolved heterogeneously-charged crystal-constituting ions and solvent molecules by electrostatic interaction, thus constructing the electrical double layer and rendering the NPs well-dispersed (Figure 1a). The presence of surface charge can also physically or chemically adsorb the dissolved molecules such as organic dyes at the NP interface, which restructures the energy dissipation pathways between the colloidal NPs and outside world. However, although much efforts had been devoted to understanding the interfacial interactions of NPs, comparable study taking into account the motion states of the particles is still rare. For example, the colloidal NPs could experience Brownian motion in stationary solution, flowing in fluid, as well as keeping static when being adhered on macroscopic objects, depending on the specific environments in applications such as biodetection, bioimaging, and printable electronics. [3] Albeit of its fundamental importance, the specific role of particle moving on the interfacial reactions of colloidal NPs is ambiguous up to now and as always been ignored for most cases.The sum frequency generation (SFG) vibrational spectroscopy, a second-order nonlinear optical technique, has been proved to be powerful for probing the interfacial molecules in view of its inherent interface selectivity and high sensitivity. [4] In this work, combined in situ spectroscopies based on SFG and photoluminescence (PL) characterization were adopted to reveal the unprecedented effect of particle moving to the interfacial adsorption of colloidal fluoride nanocrystals in great detail. Comparative results of the proof-of-concept experiments for the ligand-free NaYF 4 :Yb,Er upconverting (UC) nanocrystals reveal that the particle moving in aqueous solution reversibly alter the surface charge. More importantly, it is found that the motion state of the NPs determines molecular arrangement of Rhodamine B (RhB) at the particle interface, which significantly affect the energy transfer from the colloidal NPs to the adsorbed dyes.

Liquid flow reversibly creates a macroscopic surface charge gradient

“…Similar to Lis et al 4 and Xi et al 26 , we conducted flow experiments at the CaF 2 -water interface under acidic conditions, first at a single position in the channel. Figure 1a illustrates our experiment.…”

“…For each flow rate, the increase of the intensity upon applying flow varies monotonically between inlet and outlet. To the best of our knowledge, other spectroscopic flow experiments conducted so far only varied the flow rate in ranges where no dependency has been observed 4 , 26 .

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“…Figure 4a illustrates this view of the surface chemistry. While Lis et al 4 and Xi et al 26 only considered the charging reaction from Eq. ( 1 ) to qualitatively explain the flow-induced changes in v-SFG spectra, we argue that a consistent and quantitative description of the experimental results also requires to incorporate the dissolution as described by Eq.…”

In Situ Self‐Assembly of Ordered Organic/Inorganic Dual‐Layered Interphase for Achieving Long‐Life Dendrite‐Free Li Metal Anodes in LiFSI‐Based Electrolyte

“…[ 22,23 ] To verify these accurately, vibrational sum frequency generation (SFG) spectroscopy has been applied to detect molecular adsorption orientation states on the surface. [ 18,24 ] As shown in Figure 2d, as the self‐assembling time increasing from 0 to 360 min, two peak signals at 2940 and 2904 cm −1 assigned to CH 2 and CH 3 fermi resonance signals appeared gradually and finally got saturated. Along with the immersion time increasing longer, the coverage of organic layer changed little and became quite stable, demonstrating the adsorbed organic layer became saturated and formed a monolayer with sub‐nanosized thickness.…”

“…Interface‐sensitive sum frequency generation spectroscopy (SFG), X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy have been exploited to verify the evolution presences and the mechanical property of the organic and inorganic layers, respectively. [ 16–19 ] We demonstrate that the in situ self‐assembled ordered organic/inorganic hybrid SEI on lithium surface shows the advantages in prohibiting the formation of lithium dendrites and the resistance to the adsorption of solvent molecules, which has close relationship with its anticorrosion functionality as evidenced by SFG. As a result, the pretreated lithium metal anode exhibits excellent reversible stability at as high as 10 mA cm −2 , and it also maintains high Coulombic efficiency at the stripping/plating capacity of 3 mA h cm −2 , which is better than most reported literatures.…”