Al-Si@Al(OH)3 Nanosheets Composite for Enhanced Ecient Strategy to Synthesize Al-Si@Al2O3 Core-shell Structure

Owing to the combined advantages of Al-Si alloy and Al 2 O 3 , Al-Si@Al 2 O 3 is widely utilized as a heat storage material, catalyst carrier and what adsorption host. Hence, the preparation of Al-Si@Al 2 O 3 and corresponding precursors is of utmost signicance. Herein, Al-Si@Al(OH) 3 precursor is investigated and Al(OH) 3 nanosheets are in-situ formed on the surface of Al-xSi alloy (x = 10, 20 and 30) in the presence of water. The inuence of Si content, diameter of Al-Si particles and heating parameters on morphology and thickness of Al(OH) 3 nanosheets is systematically explored using X-ray diffraction, electron microscopy, Fourier transform infrared spectroscopy and N 2 adsorption/desorption isotherms. The growth mechanism of Al(OH) 3 nanosheets is revealed and a pathway to obtain Al-Si@Al 2 O 3 nanosheets with desired structure and thickness is demonstrated.


Introduction
The term core-shell structure rst appeared in nanotechnology and extended to various subject areas. The core-shell structure is a two-component system that combines both components' respective characteristics to meet the intensive requirements of the advanced applications [1]. Recently, core-shell structured materials have garnered signi cant research attention due to their comprehensive properties and designability as photocatalysts [2], thermal storage material [3], and adsorption material [4]. Different methods can be used to obtain the core-shell structures, including hydrothermal method [5][6][7], sol-gel process [8][9][10], arc discharge method [11][12][13] and one pot approach [14][15][16].
The Al-Si alloy is a cost-effective, highly conductive and thermally stable candidate, which can easily form a core-shell structure with Al-Si particle as a core and Al 2 O 3 as a shell [17]. The low speci c area and high-temperature corrosion of Al-Si particles, however, greatly limit the practical applications. Hence, alumina (Al 2 O 3 ) is usually coated on the surface of Al-Si particles due to its high speci c area, chemical inertness and thermal stability [18]. For instance, Han et al. [19] have reported a facile hydrothermal process to synthesize γ-Al 2 O 3 on the surface of Al-40Si powder, revealing the growth mechanism of AlOOH nano-akes on the surface. Dai et al. [20] have prepared Al-Si@Al 2 O 3 by hydrothermal method and further investigated the effect of size and aluminum content in Al-Si powder on the catalytic performance of Pb/Al-Si@Al 2 O 3 . The results indicated that Pb/Al-Si@Al 2 O 3 catalyst possessed optimal hydrogenation performance when the size and aluminum content were 5 µm and 88%, respectively.
Al-Si powder, with different amounts of Si, exhibits different morphologies [21][22][23]. The eutectic Si can be observed when Si content in Al-Si powder is less than 12.6 at. %. In contrast, eutectic Si and primary eutectic coexist in Al-Si powder when Si content is higher than 12.6 at. %. Meanwhile, primary Si became more irregular and larger with the increase of Si content. These studies demonstrate that AlOOH nucleates at Si substrate, however, the in uence of Si morphology on nucleation of hydrated alumina has been neglected in these studies.
Herein, Al-Si powder, with different amounts of Si, is selected to explore the impact of Si morphology to Al(OH) 3 nucleation. Research showed that the morphology can be maintained while the phase change was arose from boehmite to γ-Al 2 O 3 [4]. Hence, instead of calcination temperature, the structure of Al 2 O 3 is mainly in uenced by the Si content. Silicon morphology of Al-Si powders has different characteristic in diverse particle size which is resulted from solidi cation rate [24], the particle size is also a signi cant research factor. Furthermore, Al(OH) 3 was prepared on the surface of Al-Si particles using the hydrothermal and water bath methods. In addition, the in uence of reaction time, Si content and particle size on the structure of Al-Si@Al(OH) 3 precursor is discussed, and the mechanism of in-situ formation of Al(OH) 3 nano-akes is investigated.

Experimental
Al-Si powder was treated using deionized water at 80 ℃ for 3 h under the hydrothermal method, followed by centrifugation, washing with absolute alcohol thrice and drying at 50 ℃ for 12 h. This process resulted in Al-Si@Al(OH) 3 sample. Moreover, Al-Si@Al(OH) 3 sample was prepared using the water bath method under the same conditions. With different amounts of Si and particle sizes, Al-Si powder was utilized to obtain different samples, as detailed in Table 1. The Al-Si powder, with 12 at. % of Si and a diameter of 75 µm, was labeled as A17. The A17 sample was hydrothermally treated for 3 h to synthesize Al-12Si@Al(OH) 3 and the product was labeled as Al-12Si@Al(OH) 3 (H) abbreviated to A17H3, meanwhile, the Al-12Si@Al(OH) 3 synthesized by water bath method for 3h was labeled as Al-12Si@Al(OH) 3 (W) abbreviated to A17W3. The experimental parameters, i.e., reaction time, silicon content and Al-Si particle diameter, were changed to investigate the in uence of different parameters on precursor microstructure. Al-20Si(A23) 20 38 Al-20Si(A27) 20 75 Al-30Si(A37) 30 75 The morphology of as-prepared core-shell composite was observed using a scanning electron microscope (SEM, JEOL JSM-6700F, Japan). X-ray diffraction (XRD) analysis was carried out using the Rigaku D/MAX-2500 X-ray diffractometer (Japan) with a scan rate of 10 °/min. The Fourier transform infrared (FTIR) spectroscopy was conducted using a Nicolet-5700 instrument. The pore structure and BET surface area were tested at 77 K via N 2 adsorption/desorption isotherms.

Results And Discussion
Analysis of samples Figure. 1 presents the XRD patterns of various precursors prepared by the water bath and hydrothermal methods. The characteristic peaks of Al and Si can be clearly observed, however, a weak peak of the bayerite phase (Al(OH) 3 , PDF# 20 − 0011) was observed in the XRD pattern of A17W6, which is consistent with the previous reports that the bayerite is formed by the reaction of Al with water [25]. The characteristic peaks of Al(OH) 3 phase in Figure. 1(c) and 1(d) were too weak and could not be observed in the XRD pattern. To further illustrate the existence of bayerite phase in hydrothermally-prepared precursors, the FTIR spectrum of A17H6 is shown in Figure. 2. The strong peak at 3435 cm − 1 illustrates the existence of Al(OH) 3 . Moreover, Al-O stretching vibrations are con rmed by the presence of peaks at 734 and 611 cm − 1 , con rming the existence of Al(OH) 3 [25]. It has been reported that Si is essential to form boehmite [19], i.e., the Al(OH) 3 is successfully formed in all precursors. Figure. 3 presents SEM images of A27 prepared using different methods and times. It is objected that the thickness of nanosheets prepared by the water bath method is thinner than the hydrothermal method. It is perceived that the growth of nanosheets on the surface of Al-Si@Al(OH) 3

(W) is abnormal and abundant
impurities appear around the core-shell structure, decreasing the active sites and compromising the catalytic performance. Compared with the water bath method, the hydrothermal method is a better way to prepare pure Al-Si@Al(OH) 3 . It is noteworthy that the increase in time only affects the thickness of nanosheets and we can decrease hydrothermal time to obtain equally distributed core-shell structures, shortening the preparation cycle.
As shown in Figure. 4, Al-Si@Al(OH) 3 can be synthesized using Al-Si powder with different amounts of Si.
The thickness distribution of A27H6 is more concentrated than others, showing the in uence of Si morphology on thickness uniformity of Al(OH) 3 nano-akes.
To further investigate the in uence of Al-Si particle size, two different grain sizes were utilized to form Al-20Si@Al(OH) 3 (H). It is observed that there is a little distinction in morphology, as shown in Figure. 5(a-b).
As the particle size of Al-Si powder in uenced the Si morphology, the density of primary Si on the surface of Al-Si particles increased with the decrease of mean particle size. It can be concluded that the particle size renders a negligible in uence on the structure of Al(OH) 3 nano-akes. Figure. 6(a) presents the adsorption/desorption isotherms of A27H6, which belong to type-of IUPAC and indicate the presence of meso-pores. The speci c surface area is found to be 19.5573 m²/g, which indicates that the nano-akes increased the speci c surface area of Al-Si particles. The pore diameter of A27H6 is mainly distributed below 25 nm, as shown in Figure. 6(b).

Impact of silicon morphology
In order to reveal the growth mechanism during the initial stage, the ground and polished Al-20Si alloy was treated with deionized water at 80 ℃ for 30 minutes and morphological changes were observed, as shown in Figure. 7. Overall, the distribution of nano-akes was mainly focused on the interphase between Si phase and Al matrix, which indicates that the nucleation of Al(OH) 3 mainly occurs at the Al/Si interphase and the nanosheets completely cover the surface of Al-Si particle with the increase of reaction time. In addition, more nanosheets were observed on the surface of eutectic Si than the primary Si phase, which indicates the in uence of Si area on Al(OH) 3 deposition.

Conclusion
In summary, the in uence of Si content and Al/Si particle size on the morphology and thickness of Al(OH) 3 nanosheets has been systematically investigated. The main conclusions can be given as: 1. The comparison of microstructure between hydrothermal and water bath methods revealed that the hydrothermal method is a better way to prepare Al-Si@Al(OH) 3 nanosheets than the water bath method.
Overall, the reaction time rendered a little effect on the structure of Al-Si@Al(OH) 3 nanosheets.