Surface Atomic-level Halogenation and Fluorescent Color Regulation of Liquid Metals at Ambient Conditions

Surface Atomic-level Halogenation and Fluorescent Color Regulation of Liquid Metals at Ambient Conditions | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 17 April 2025 V1 Latest version Share on Surface Atomic-level Halogenation and Fluorescent Color Regulation of Liquid Metals at Ambient Conditions Authors : Kaijin xu , Nannan Cui Cui , ting wu , Qingju Liu 0000-0003-2288-3417 , Xufeng Zhang , Guanghua Wang , Qian Li , Jing Liu 0000-0002-0844-5296 , Peizhi Yang , and liang fei duan 0000-0001-9981-1724 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.174492571.13349831/v1 Published Colloids and Surfaces A: Physicochemical and Engineering Aspects Version of record Peer review timeline 188 views 97 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Liquid metals (LMs) are the functional materials with both metallic and liquid properties, remaining in a liquid state at room temperature. The fluidity, electrical conductivity, printability, and self-healing capabilities provide liquid metals with significant advantages for applications in electronics. However, liquid metals generally exhibit a silver-white coloration, the single physical appearance restricted their applicability. Fortunately, the metal halide perovskites serve as fluorescent materials characterized by their abundant fluorescence properties. The atomic-level halogenation transformation of liquid metals surface will endow them with unique fluorescence properties, and providing groundbreaking opportunities for applications. Herein, the aluminum (Al) was dissolved and then dispersed in eGaSn to produce the eGaSn-Al materials. Subsequently, the solutions of KX + HX (where X = Cl, Br, I) were dropped onto the surface of the eGaSn-Al. Then, the metal halide fluorescent shells with different colors were formed, and endowing them with abundant fluorescent properties. Particularly, through the selection, regulation and combination of halogen elements, resulting colorful liquid metals with cyan, cyan-green, and yellow-green. This finding presents innovative concepts for the interface modification and special functional applications of liquid metals, and expands their application potential in electronic information devices, such as lighting and displays, anti-counterfeiting measures, sensing, and chameleon robotics. 1. Introduction Gallium-based liquid metals (LMs) represent a class of emerging functional materials that exhibit significant potential for application across various technological domains due to their unique physicochemical properties. In comparison to traditional metallic materials, liquid metals not only possess the characteristic excellent electrical and thermal conductivity typical of metals but also demonstrate features such as low melting points (approaching room temperature), good compliance, remarkable self-repairing capabilities, and biocompatibility. [1,2] These distinctive attributes enable liquid metals to overcome the technical limitations associated with conventional materials, facilitating their widespread use in cutting-edge fields including bionic soft robotics, [3] flexible electronic devices, [4] biomedical engineering, [5] additive manufacturing, [6] new energy technologies, [7] flexible sensors, [8] electronic skin, [9] and microfluidic systems. [10] However, despite the promising potential of liquid metals materials, several technical challenges persist and require urgent resolution. Notably, liquid metals inherently exhibit a uniform silver-white appearance under natural conditions and possess a high reflectivity of approximately 80–90% within the 400–2000 nm wavelength range. [11] This limited surface optical property significantly restricts their applicability in fields that demand diverse color expression and enhanced visual aesthetics. Such limitations are particularly critical in applications with stringent visual requirements, including information displays and decorative surface engineering. Therefore, imparting fluorescence functionality to the surface of liquid metals is of considerable importance. Metal halide perovskites (MHPs) constitute a class of materials with a perovskite crystal structure and the general chemical formula ABX₃. In this framework, the A-site typically accommodates organic or inorganic cations (e.g., Cs⁺, Rb⁺, CH₃NH₃⁺, CH(NH₂)₂⁺), the B-site hosts divalent metal cations (e.g., Cu²⁺, Pb²⁺, Sn²⁺, Bi³⁺, Sn⁴⁺), and the X-site comprises halide anions (e.g., Cl⁻, Br⁻, I⁻). [12] When the B-site metal ions include combinations of monovalent and trivalent cations, double perovskites with the chemical formula A₂B I B III X₆ can be synthesized. [13] Metal halide perovskites exhibit exceptional optical properties, characterized by a high absorption coefficient, tunable bandgap, solution processability, outstanding optoelectronic performance, a high photoluminescence quantum yield (PLQY), a charge carrier lifetime of several microseconds, and a charge carrier diffusion length of several micrometers. [14] These remarkable attributes have enabled the extensive application of halide perovskites in domains such as display devices, [15,16] light-emitting devices, [17,18] solar cells, [19,20] detectors, [21] lasers, [22] and sensors. [23] However, there are still numerous challenges associated with metal halide perovskite materials and their synthesis. These materials are susceptible to the effects of moisture, oxygen, heat, and light present in the environment. The use of lead as the B-site metal ion in many perovskite materials has raised concerns in terms of environmental protection and health. The preparation process is relatively complex, and there exist challenges in achieving Large-scale production. [24,25] In this work, considering the distinctive properties of liquid metals (LMs), including their fluidity, high solubility for solid metals, and inherent tendency to undergo surface oxidation, leading to the formation of a core-shell structure, Al was dissolved in the liquid metals. Subsequently, HX and KX solutions were dropped onto the surface. Through a straightforward solution-based reaction, a metal halide fluorescent layer with a double perovskite structure spontaneously formed on the liquid metals surface. The core-shell structure was closely linked and not liable to fall off. The obtained core-shell structure is closely linked and not prone to detachment. The liquid metals used in this study comprises gallium (Ga), tin (Sn), and aluminum (Al), while the metal halide shell consists of K₃AlX₆. Under ambient conditions, by modifying the solution environment of KX + HX (X = Cl, Br, I), which reacts with the eGaSn-Al surface, the halogen element at the X position can be precisely controlled to modulate the luminescent color of the metal halide shell. As a result, metal halide fluorescent materials with luminescent colors of cyan (K₃AlCl₆), green (K₃AlBr₆), and yellowish-green (K₃AlI₆) are obtained. Their corresponding photoluminescence emission peaks are located at 482 nm, 500 nm, and 555 nm, respectively. The atomic halogenation transformation on the surface of room-temperature liquid metals and the corresponding regulation of fluorescence colors overcome the previous limitation that liquid metals could only exhibit a silver-white physical appearance. This strategy enables precise control and tunable regulation of fluorescence colors, leading to significant advancements in the fluorescence functionality of liquid metals. Furthermore, it introduces innovative concepts for interface modification and the development of specialized functional applications. Notably, this approach expands the application potential of liquid metals in electronic information devices, including lighting and display technologies, anti-counterfeiting measures, sensing, and chameleon robots. Additionally, utilizing liquid metals as catalysts to lower the reaction potential energy and facilitate the directional synthesis of non-toxic perovskite materials under ambient conditions presents a novel strategy for the preparation of lead-free perovskites. 2. Experimental 2.1 Syntheses and processing of materials Eutectic gallium-tin (eGaSn) liquid metals (LMs) was prepared using high-purity gallium (Ga, 99.9%) and tin (Sn, 99.9%). The metals were accurately weighed in a mass ratio of 85% Ga to 15% Sn using an electronic balance (FA2204N, precision: 1.0 × 10⁻⁵ g). The weighed Ga and Sn were then heated to 60°C for 5 minutes to ensure complete melting, followed by vigorous stirring to achieve a uniform eGaSn alloy. After cooling to room temperature (25°C), aluminum (Al) powder was added and evenly dispersed within the eGaSn matrix, leading to the formation of the eGaSn-Al (LMs-Al) soft material. 2.2 Atomic Halogenation Transformation and Fluorescent Functionalization on the Surface of Liquid Metals The eGaSn-Al (LMs-Al) soft material was immersed in deionized water to remove residual surface impurities. After reacting for 20 seconds, the LMs-Al droplets were transferred onto a dry substrate for further processing. A mixed solution of 3 mol L⁻¹ HCl and 2 mol L⁻¹ KCl was applied to the surface of LMs-Al using a pipette. Upon completion of the reaction, a white metal halide layer formed on the LMs-Al surface, which exhibited distinct cyan fluorescence under 254 nm ultraviolet (UV) excitation. Similarly, when a solution containing 3.4 mol L⁻¹ HBr and 2 mol L⁻¹ KBr was introduced, a comparable white halide layer developed, emitting cyan-green fluorescence under 254 nm UV light. Further increasing the HBr and KBr concentrations to 4 mol L⁻¹ and 3 mol L⁻¹, respectively, resulted in a white halide shell that displayed yellow-green fluorescence when exposed to 365 nm UV illumination. This halide shell serves as a protective barrier, preventing water and oxygen in the ambient environment from further reacting with the LMs-Al core. Following natural drying, a stable core-shell structure is formed, with metal halides acting as the outer shell and LMs-Al as the inner core. 2.3 Characterization methods The X-ray diffraction patterns were acquired using the MiniFlex600 X-ray diffractometer (Cu Kα1, 40 kV, 50 mA) within a 2θ scan ning range of 10 to 80° to characterize the crystal structure of the product. The metal halide shell was analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), employing the Nova-Nano450 scanning electron microscope at an acceleration voltage of 15 kV. The X-ray photoelectron spectroscopy (XPS) were acquired using a PHI Versaprobe 4 X-ray photoelectron spectrometer, and the C1s standard spectrum (284.80 eV) was used as calibration. The photoluminescence (PL) spectra were acquired by means of the FLS 1000 fluorescence spectrometer (Edinburgh Instruments, UK), with a scanning rate of 1 nm s -1 . The widths of both the excitation slit and the emission slit were 1 nm. The excitation wavelengths utilized are either 254 nm or 365 nm, and the power output of the portable ultraviolet analyzer is rated at 6 W. 3. Results and discussion Figure 1 presents a flowchart illustrating the optimization of fluorescence characteristics and color modulation of the metal halide shell by modifying the environment of the HX + KX solution applied to the eGaSn-Al surface. The experiment is divided into two distinct phases: (1) dissolving gallium and tin to generate eGaSn, followed by dissolving aluminum powder in the eGaSn and stirring the mixture to ensure thorough blending, thereby forming the eGaSn-Al material; (2) placing the eGaSn-Al composite on a dry substrate and subsequently applying the mixed KX and HX solution onto the upper surface of the liquid metals using a dropper. After an adequate reaction period, a metal halide fluorescent shell is formed, as illustrated by the physical and model diagrams of the liquid metals halide reaction process in Figures 1a,b. As shown in Figure S1, LMs lack a crystalline lattice and exhibit water-like fluidity. They are typically silver-white in appearance with a metallic luster. A notable and widely observed feature of liquid metals is their core-shell structure. [26,27] Upon exposure to even trace amounts of oxygen, a thin oxide layer rapidly forms on the surface, primarily consisting of Ga 2 O 3 and SnO 2 . As depicted in Figure S2, when liquid metals dissolves Al, the surface shell of the LMs-Al soft material exposed to air at room temperature is predominantly composed of Al 2 O 3 , Ga 2 O 3 , and SnO 2 . [28] Figure S3 Upon the addition of HX and KX solution to the LMs-Al soft material, a distinct fluorescent shell formed on the surface. In contrast, when distilled water was applied to the LMs-Al soft material, a non-fluorescent Al₂O₃ film was generated, as shown in Figure 1c and Figure S4. Based on these observations, it can be inferred that the metal halide shell forms through the reaction between LMs-Al and the KX + HX solution. As illustrated in Movie S1, the KX + HX mixed solution is dropped onto the upper surface of the eGaSn-Al droplet using a dropper. On the surface of the eGaSn-Al droplet, since Al has the highest metal activity in eGaSn-Al, the KX + HX mixed solution mainly reacts with Al and rapidly undergoes a reaction to generate a large amount of gas, as shown in Equation (1). [29,30] 2Al+6HX = 2AlX 3 +3H 2 ↑ (X=CL, Br, I ) (1) The generation of hydrogen induces foam formation on the surface of LMs-Al upon interaction with the KX + HX solution, thereby preventing the solution from slipping off and fully reacting with LMs-Al. Under natural conditions, the LMs-Al shell dries rapidly and forms a core-shell structure, which can be attributed to two primary factors: (i) the hydrogen gas produced by the reaction between the KX + HX solution generates foam and increases the evaporation surface area。 (ii) residual water within the shell continues to react with the LMs-Al core, releasing heat and consequently accelerating the evaporation rate, as described by the formula. 2Al+3H 2 O = Al 2 O 3 +3H 2 ↑ △H=-1728 kJ/mol (2) During the drying process, the solution gradually diminishes, the shell layer continuously enriches and thickens. Meanwhile, it changes from the liquid state to the solid state, the fluidity weakens, and The recovery rate of hydrogen-induced pores gradually slows down. The rapid reduction in the fluidity and the rapid increase in the thickness of the shell layer result in the random distribution of wrinkling defects on its surface. [31] The reaction between the mixed solutions of KX + HX (X = Cl, Br, I) and the eGaSn-Al ultimately results in the formation of a dense white shell layer. This shell serves a protective function by preventing the liquid metals from reacting with oxygen and water in the ambient air. Upon natural drying, the white shell layer exhibits an uneven texture, which can be attributed to anisotropic shrinkage. [31,32] The white shell that forms on the surface of eGaSn-Al Liquid metals exhibits pronounced fluorescence characteristics when excited by ultraviolet lamps at wavelengths of 254 nm or 365 nm. Furthermore, the fluorescence color varies in response to changes in the solution environment titrated onto the surface of the Liquid metals. When the mixed solution of KCl + HCl is applied to the surface of eGaSn-Al and excited by a 254-nm ultraviolet lamp, the resulting fluorescent shell emits cyan fluorescence. When the mixed solution of KBr + HBr is utilized, the fluorescent shell emits cyan-green fluorescence under the excitation of 254-nm ultraviolet light. When the mixture of KI + HI is employed, the fluorescent shell emits yellowish-green fluorescence under the excitation of 365-nm ultraviolet light. By introducing the KX + HX mixed solution onto the surface of eGaSn-Al and varying the halogen element X, the fluorescence modification, color control, and optimization of the liquid metals can be accomplished. Figure 1. Flowchart of atomic halogenation transformation and fluorescent functionalization on the surface of liquid metals. a) Physical diagrams of the preparation process of fluorescence on the liquid metals surface. b) Physical images of the preparation process of fluorescence on the surface of liquid metals. c) Schematic illustration of the reaction of LMs-Al with water leading to the formation of an Al 2 O 3 film and the atomic halogenation on the surface of LMs-Al Resulting in the generation of a fluorescent layer. It is important to note that the preparation of Liquid metals fluorescent core-shell structures does not influence the intrinsic properties of eGaSn. After stripping the shell, the Liquid metals is washed with deionized water. After the remaining Al reacts fully with water, the Liquid metals eGaSn can be recovered and recycled. The Liquid metals serves as a catalyst throughout the preparation process and is almost not consumed during the reaction. The unit cell structure diagrams of K 3 AlCl 6 , K 3 AlBr 6 , and K 3 AlI 6 are presented in Figures 2a, b, c. respectively. All three metal halides are of a double perovskite structure, in which K + ions occupy the A site, while both K + and Al 3+ ions are positioned at the B I and B III sites. Cl⁻, Br⁻, and I⁻ reside at the X site. [33,34] The main component substances of the fluorescent shells on the surfaces of three types of eGaSn-Al were analyzed and identified through X-ray diffraction (XRD) analysis. Upon the dropwise addition of the KCl + HCl solution to the eGaSn-Al surface and the completion of the reaction, the XRD pattern of the resulting fluorescent shell layer exhibits a limited number of diffraction peaks corresponding to KCl and Al 2 O 3 , As depicted in Figure 2d. Notably, the predominant diffraction peak corresponds to K₃AlCl₆. Similarly, upon the addition of a KBr + HBr solution to the eGaSn-Al surface, the XRD pattern of the resulting fluorescent shell exhibits diffraction peaks associated with KBr, Al₂O₃ and K₃AlBr₆, with K₃AlBr₆ as the dominant phase, as depicted in Figure 2e. Likewise, following the application of a KI + HI solution to the eGaSn-Al surface, the XRD pattern of the generated fluorescent shell reveals diffraction peaks corresponding to KI, Al₂O₃ and K₃AlI₆, with K₃AlI₆ as the principal phase. as presented in Figure 2f. Accordingly, it can be preliminarily inferred that the luminescent substances formed on the surface following the reaction of the KX + HX (X=Cl，Br，I) mixed solution with eGaSn-Al are K₃AlCl₆, K₃AlBr₆, and K₃AlI₆, respectively. The valence states of each element in the fluorescent shell of metal halides were further disclosed by using X-ray photoelectron spectroscopy (XPS). Taking K 3 AlCl 6 as an example, the K2p peaks at approximately 296.35 eV correspond to K + 2p1/2 and those at approximately 293.58 eV correspond to K + 2p3/2 (Figure 2g). [35] The XPS spectrum of O1s was fitted into a single peak, with the peak position approximately at 532.18 eV, corresponding to the by-product Al 2 O 3 (Figure 2h). [36, 37] The XPS spectrum of A12p was fitted as a single peak, namely A1 3+ 2p at a peak position of approximately 75.23 eV. The Al element in the related products such as Al(OH) 3 , AlCl 3 , KAlCl 4 , and K 3 AlCl 6 all exists in the form of Al 3+ (Figure 2i). [38] The XPS spectrum of C12p can be fitted into two sub-peaks, where the position at 199.02 eV is C12p3/2 and the position at 200.60 eV is C12p1/2, (Figure 2j). [39] The XPS spectrum of Br2p can be fitted into two sub-peaks. The peak at 68.41 eV is Brp1/2, and the one at 75.21 eV is Br2p1/2 (Figure 2k). [40] The XPS spectrum of I2p can be fitted into two subpeaks. The position at 620.11 eV is I2p1/2 and the position at 631.59 eV is I2p3/2 ( Figure 2l). [41] Figure 2. XRD and XPS data graphs of liquid metal fluorescent shells of various colors. a) The crystal cell structure diagram of K 3 AlCl 6 . b) The crystal cell structure diagram of K 3 AlBr 6 . c) The crystal cell structure diagram of K 3 AlI 6 . d) The XRD pattern of the fluorescent shell resulting from the reaction of eGaSn-Al with KCl + HCl solution. e) The XRD pattern of the fluorescent shell produced by the reaction of eGaSn-Al with KBr + HBr solution. f) The XRD pattern of the fluorescent shell produced by the reaction of eGaSn-Al with KI + HI solution. g-j) The fitting results of the XPS spectra of K2p, O1s, Al2p, and Cl2p of K 3 AlCl 6 , respectively. k) The fitting result of the XPS spectrum of Br2p of K 3 AlBr 6 . l) The fitting result of the XPS spectrum of I2p of K 3 AlI 6 . As depicted in Figures S5, S6, S7, the XPS spectra of O1s for the three metal halide shells can all be fitted into a single peak, the XPS spectra of A12p can all be fitted into a single peak, while the XPS spectra of K2p can all be fitted into two subpeaks. The XPS spectra of Sn3d can all be fitted into four sub-peaks, namely, at 484.65 eV for Sn3d5/2, at 493.12 eV for Sn3d3/2, at 487.43 eV for Sn 4+ 3d5/2, and at 495.90 eV for Sn 4+ 3d3/2, suggesting that the fluorescent shell contains a small amount of metallic Sn and Sn ions. [42] The metal Sn is primarily introduced from the eGaSn-Al Liquid metals core when the metal halide shell is mainly stripped, suggesting that the metal halide photo shell is closely connected to the eGaSn-Al Liquid metals core and is not prone to detachment. The Sn 4+ ions are mainly produced during the halogenation process through the reaction of a small amount of metallic Sn with HCl to form SnCl 4 . The XPS spectra of Ga2p can all be fitted into two sub-peaks, namely, the position near 1119.08 eV corresponding to Ga 3+ 2p3/2, and the position near 1145.94 eV corresponding to Ga 3+ 2p1/2. They are mainly formed after the Ga on the surface of the eGaSn-Al Liquid metals reacts with the mixed solution of KCl + HCl or oxygen to form GaCl₃ or Ga 2 O 3 . [43] The complete XPS spectra of all samples are presented in Figure S8. Analysis of the valence states of various elements further verifies that the surface of eGaSn-Al undergoes transformation into a metal halide fluorescent shell upon halogenation with a mixed KX + HX solution. This process effectively enables the fluorescent functionalization of the liquid metals surface. To further confirm that the fluorescence characteristics of the eGaSn-Al surface after halogenation originate from the metal halide K₃AlX₆ with a double perovskite structure, the by-products generated during the reaction between the KX + HX mixed solution and eGaSn-Al, which forms the fluorescent shell, were irradiated under an ultraviolet lamp, as shown in Figure S9. It was observed that none of these by-products exhibited fluorescence characteristics. The by-products generated during the synthesis of K₃AlCl₆ include Al₂O₃, KCl, SnCl₂, SnCl₄, GaCl₃, and AlCl₃. Similarly, the synthesis of K₃AlBr₆ produces Al₂O₃, KBr, AlBr₃, SnBr₂, SnBr₄, and GaBr₃. The by-products generated during the synthesis of K 3 AlI 6 include Al 2 O 3 , AlI 3 , KI, SnI 2 , SnI 4 , and GaI 3 . These findings further confirm that the luminescent components formed by the reaction of the KX + HX (X = Cl, Br, I) mixed solutions with eGaSn-Al are K₃AlCl₆, K₃AlBr₆, and K₃AlI₆, respectively. This suggests that the fluorescent shell on the liquid metals surface is primarily composed of K₃AlX₆. As depicted in Figure 3, the micro-morphology of the metal halide fluorescent shell was inspected via scanning electron microscopy (SEM). Moreover, the distribution and composition of each element within the metal halide shell were characterized and analyzed through energy-dispersive spectroscopy (EDS). As presented in Figures 3a-c, they respectively correspond to the actual photos of K₃AlCl₆, K₃AlBr₆, and K₃AlI₆. The KX + HX mixed solution uniformly halogenated the surface atoms of eGaSn-A1, thereby forming a dense and rigid white shell layer. Macroscopically, the texture of the white shell layer is non-uniform. Under a high-magnification microscope, the surface of the halide shell layer is uniform and smooth, with fine white particles dispersed on the shell layer surface. Through the morphological analysis of the substances generated during the halogenation process, it is known that the white grains in Figure 3d are KCl, the white grains in Figure 3e are KBr, and the white particles in Figure 3f are KI. As depicted in Figures 3g-i, through the observation of the cross-section of the metal halide light shell, after the halogenation transformation of the eGaSn-A1 surface, a dense metal halide shell was formed. The metal halide shell and eGaSn-A1 formed a stable core-shell structure, which was closely connected and not prone to detachment. The dense metal halide shell serves a protective function for the Liquid metals, ensuring that the Liquid metals core is not oxidized. As depicted in Figure 3j and Figure S10, the three metal halide shells are rich in elements such as K, Al, X (Cl, Br, I), and O, while the distribution of Ga and Sn elements is relatively scarce. Based on the element distribution map, it can be observed that the X halogen elements (Cl, Br, I), Al, and O elements are distributed rather extensively on the surface of the shell layer. There is a significant correlation among the enrichment areas of K, Al, and X (Cl, Br, I) elements, further confirming that the main component in the shell layer of the three metal halide compounds is K₃AlX₆. Moreover, the enrichment regions of element O and element A1 exhibit overlap, as do those of element K and element X. This observation suggests that during the halogenation transformation of the eGaSn-A1 surface, a small quantity of KX and Al₂O₃ is generated. The formation of Al₂O₃ occurs simultaneously with that of the fluorescent shell. Owing to its intrinsic stability and hardness, Al₂O₃ functions as a self-protective sealing matrix for the metal halide shell, effectively shielding K₃AlX₆ from exposure to environmental moisture and oxygen. This protective barrier significantly enhances the stability of the core fluorescent material. F igure 3. SEM images and EDS data of the Liquid metals fluorescent shell. a-c) are the physical photographs of GaSn-Al droplets covered with a metal halide shell. d-f) represent the surface SEM images of K 3 AlCl 6 , K 3 AlBr 6 , and K 3 AlI 6 , respectively. g-i) represent the cross-sectional SEM images of K 3 AlCl 6 , K 3 AlBr 6 , and K 3 AlI 6 , respectively. j) represent the EDS images of K 3 AlCl 6 , respectively. To investigate the optical properties of the metal halide shell on the surface of the eGaSn-Al, the photoluminescence (PL) spectrum of the fluorescent shell was characterized and analyzed using a PL spectrometer, with both the excitation and emission slit widths set to 1 nm. Figures 4a,b respectively represent the actual physical photos of the K 3 AlCl 6 fluorescent shell and the K 3 AlBr 6 fluorescent shell under a 254 nm wavelength ultraviolet lamp. Figure 4c is the actual physical photo of the K 3 AlI 6 fluorescent shell under a 365 nm wavelength ultraviolet lamp. It can be observed from the room-temperature photoluminescence and photoluminescence excitation spectra of K 3 AlX 6 that K 3 AlX 6 demonstrates a long Stokes shift and a broad half-peak width. The Stokes shifts are approximately around 250 nm, and the full width at half maximum (FWHM) values are approximately within the range of 105-110 nm, as depicted in Figure 4d and Figure S11. The PL spectra of the fluorescent shells of the three colors were fitted using the Gaussian model, and could be deconvoluted into Gaussian peaks of various colors. Among them, the K3AlBr6 fluorescent shell layer exhibited three luminescence peaks corresponding to blue, green, and orange wavelengths, respectively (Figure 4e and Figure S12). Figure 4f respectively portrays the decay curves of the emissions of K 3 AlCl 6 , K 3 AlBr 6 , and K 3 AlI 6 , which are fitted through the double-exponential function providing the (τ1) and (τ2) components in real time, as well as the average recombination lifetime Method S1. The lifetimes calculated based on the fitting are 0.569 μs, 1.737 μs, and 0.651 μs, respectively, and they belong to long-lived materials within the realm of metal halide materials. [44] Figures 4g-i respectively display the emission spectra of K 3 AlCl 6 , K 3 AlBr 6 , and K 3 AlI 6 at different excitation wavelengths. The emission spectra all manifest pronounced asymmetry, suggesting the existence of more than one emission center. This luminescence phenomenon with more than one emission center might stem from the defects within the samples. [45] Figure 4. Luminescent characteristics of liquid metals halide core-shell structures. a-c) Photographs of the K 3 AlCl 6 , K 3 AlBr 6 and K 3 AlI 6 shells under ultraviolet lamps. d) The room-temperature photoluminescence excitation and emission spectra of the K 3 AlBr 6 fluorescent shell. e) The multi-peak fitting of the fluorescent shell of K 3 AlBr 6 using the gaussian model. f) The time-resolved photoluminescence (TPL) decay curves of the K 3 AlCl 6 , K 3 AlBr 6 , and K 3 AlI 6 fluorescent shells fitted with the double-exponential model. g-i) The excitation and emission spectra of the K 3 AlCl 6 , K 3 AlBr 6 , and K 3 AlI 6 shells at various wavelengths. Figure 5 illustrates the schematic principle underlying fluorescence regulation on the surface of liquid metals. As shown in Figure 5a, when the photoluminescence spectra of K₃AlCl₆, K₃AlBr₆, and K₃AlI₆ are plotted within the same coordinate system, it is evident that tuning the halogen element (Cl, Br, I) at the X-site induces a redshift in the emission peak of the metal halide shell, shifting from 484 nm to 500 nm and subsequently to 550 nm. Figure 5b presents the corresponding CIE chromaticity coordinates of K₃AlCl₆, K₃AlBr₆, and K₃AlI₆, which are (0.176, 0.278), (0.188, 0.364), and (0.343, 0.540), respectively. These results indicate that variations in halogen ions within K₃AlX₆ lead to substantial shifts in the CIE chromaticity coordinates, demonstrating a significant change in the fluorescence color of the liquid metals surface shell. This observation is consistent with the corresponding alterations in the photoluminescence spectra. The conduction band energy level of metal halide perovskite semiconductors is predominantly constituted by the porbitals of the B element, and the contributions of other orbitals can be disregarded. The valence band energy level is mainly composed of the s orbitals of the B element and the porbitals of the X element. Orbital hybridization results in anti-bonding coupling within the valence band, and a band gap is formed between the two anti-bonding orbitals. [46] The defect energy levels in metal halide materials with this structure will exist within the valence band, the conduction band, or form shallow levels, resulting in the impurities exerting a negligible influence on the optical properties of the metal halides. [47] Based on the PL spectra of the K₃A1X₆ fluorescent shell, it can be known that K₃A1X₆ possesses a large Stokes shift and a relatively wide full width at half maximum (FWHM > 60 nm). It can be inferred that the fluorescence emission of K₃A1X₆ is mainly attributed to the emission of self-trapped excitons (STEe). [48,49] Self-trapped excitons denote the excitons formed when free electrons are excited and interact with holes via Coulomb force. Nevertheless, owing to the strong coupling between electrons and phonons, transient lattice deformation is elicited, causing the excitons to be confined in new traps within the band gap and thereby becoming self-trapping excitons (STE). These self-trapping excitons return to the ground state through radiative transitions to produce light. [50] The luminescence mechanism of the metal halide shell is depicted as shown in Figure 5c. The physical model diagram for fluorescence regulation of liquid metals is presented as Figure 5d. The dissolution mechanism of Al in liquid metals is depicted as shown in Figure 5e. Al is incorporated into eGaSn, leading to the formation of eGaSn-Al. Within the eGaSn-Al system, the liquid metals gradually infiltrates the lattice structure of the aluminum powder, inducing its fragmentation into smaller particles and enabling its existence as individual atoms within the liquid metals. Additionally, the liquid metals effectively removes the passivating aluminum oxide layer, resulting in a more disordered arrangement of aluminum atoms compared to their original state in solid aluminum powder. This process enhances the mobility of aluminum atoms, granting them greater degrees of freedom. The dissolved aluminum atoms are randomly distributed within the liquid metals and exist in a state of dynamic equilibrium. Compared to solid aluminum, which directly participates in reactions, the dissolution of aluminum within the liquid metals lowers the reaction potential energy, thereby facilitating atomic interactions and enhancing chemical reactivity. [51] As illustrated in Figure 5f, when a mixed solution of KX + HX is applied to the surface of eGaSn-Al, the aluminum atoms on the eGaSn-Al surface first undergo a reaction with the KX + HX solution, as described by the following equation. 2Al+3H 2 O=Al 2 O 3 +3H 2 ↑ △H=-1728kJ/mol (2) 2Al+6HX+6KX = 2K 3 AlX 6 +H 2 ↑ (3) The eGaSn-Al reacts with the KX + HX solution and emits a considerable amount of heat. The temperature inside and on the surface of the liquid metals increases rapidly, facilitating the random dispersion movement of Al atoms within the eGaSn-Al. (Figure S13). Meanwhile, after the surface aluminum atoms participate in the reaction and form metal halides that are enriched in the surface solution, a concentration gradient of Al atoms exists in eGaSn-Al. The internal Al atoms will constantly diffuse to the surface and ultimately accumulate on the surface of eGaSn-Al. After reacting with the KX + HX solution, the formed metal halides are enriched in the solution. Eventually, after rapid drying, a metal halide fluorescent shell closely connected to the liquid metals is formed. Figure 5. Schematic illustrations of the principles of fluorescence regulation on the surface of liquid metals. a) Fluorescence spectra of three metal halides in the same coordinate system. b) International commission on illumination (CIE) coordinates of the fluorescence shell layer. c) Luminescence mechanism of metal halides. d) Model diagram of fluorescence modification on the surface of liquid metals. e) Schematic diagram of the mechanism of aluminum dissolution by liquid metal. f) Schematic diagram of the mechanism for the synthesis of metal halides via halogenation of surface atoms of liquid metal. 4. Conclusions In this work, we propose and validate a promising strategy for the transformation of atomic halogenation on the surface of liquid metals into K₃AlX₆ perovskite fluorescent layers at room temperature. By selectively modifying the halogen element at the X site within the K₃AlX₆ perovskite crystal structure, the color of the liquid metals shell can be precisely tuned. This tunability enables control over the surface coloration of the liquid metals, ranging from cyan and bluish-green to yellowish-green, with the photoluminescence peak shifting from 482 nm to 500 nm and subsequently to 555 nm. Our findings demonstrate that blending eGaSn with Al powder results in the formation of LMs-Al soft materials, wherein Al exists as individual atoms in a state of dynamic equilibrium and random motion within the liquid metals matrix. This structural transformation significantly reduces the activation energy of Al, thereby facilitating its rapid reaction with KX + HX solutions at room temperature to yield K₃AlX₆. Concurrently, the reaction generates hydrogen gas and heat, promoting the rapid formation and drying of the fluorescent shell. 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Acknowledgement This work was funded by the National Natural Science Foundation of China Projects (62465019), Yunnan Province University Service Key Industry Science and Technology Projects (FWCY-ZNT2024008), Yunnan Revitalization Talent Support Program; Spring City Plan: the High-level Talent Promotion and Training Project of Kunming (2022SCP005). Yunnan Industrial Technology Innovation Reserve Talent Project (202105AD160056). Yunnan Fundamental Research Projects (202301AU070142, 202401AT070306, 202501AT070011, 202501AT070010), the PhD Starting Fund program of Yunnan Normal University (2021ZB005, 01100205020503225), Wisdom Yunnan Project (202403AM140035), Project for Building a Science and Technology Innovation Center Facing South Asia and Southeast Asia (202403AP140015). Authors thank the Electron Microscopy Center, the Advanced Analysis and Measurement Center of Yunnan University for the sample testing service. ToC When aluminium (Al) dissolves in liquid metals (LMs), the reaction potential energy is decreased. Aluminium atoms diffuse to the surface of the liquid metals and react with KX + HX + H 2 O to generate K 3 AlX 6. After enrichment and natural drying, K 3 AlX 6 makes the surface of the liquid metals present a fluorescent color. Supplementary Material File (image2.emf) Download 2.09 MB File (image3.emf) Download 32.27 MB File (image4.emf) Download 50.42 MB File (image6.emf) Download 4.12 MB Information & Authors Information Version history V1 Version 1 17 April 2025 Peer review timeline Published Colloids and Surfaces A: Physicochemical and Engineering Aspects Version of Record 1 Dec 2025 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords distribution fluorescent functionalization interface engineering liquid metals surface atomic Authors Affiliations Kaijin xu Yunnan Normal University View all articles by this author Nannan Cui Cui Yunnan Normal University View all articles by this author ting wu Yunnan Normal University View all articles by this author Qingju Liu 0000-0003-2288-3417 Yunnan University View all articles by this author Xufeng Zhang Yunnan Normal University View all articles by this author Guanghua Wang Yunnan Normal University View all articles by this author Qian Li Yunnan Normal University View all articles by this author Jing Liu 0000-0002-0844-5296 Technical Institute of Physics and Chemistry View all articles by this author Peizhi Yang Yunnan Normal University View all articles by this author liang fei duan 0000-0001-9981-1724 [email protected] Yunnan Normal University View all articles by this author Metrics & Citations Metrics Article Usage 188 views 97 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Kaijin xu, Nannan Cui Cui, ting wu, et al. 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