Mussel-inspired Coating of α-AlH 3 : A Compact Structure with Highly Enhanced Stability

we present a novel surfacing coating to resolve the stability of α-AlH3. Inspired by the strong chemical adhesion of mussels, the polymerization of dopamine was rst introduced to coat α-AlH3 through a simple situ polymerization. The α-AlH3 was used as a substrate. In-depth characterizations conrmed compact formation with PDA on α-AlH3 surface. The coated α-AlH3 sample was characterized by XRD XPS and SEM . The results show that a strong PDA lm is formed on the surface of α-AlH3, the PDA@α-AlH3 retained primary morphology. The crystal form of α-AlH3 does not change after coated by PDA. The results of XPS analysis show that N1s appears on the material after coated by PDA, indicating that polydopamine is formed on the surface of α-AlH3. The moisture absorption tests show that the moisture absorption rate of α-AlH3 is greatly reduced after being coated with PDA. The excellent intact ability of PDA prevent α-AlH3 reacting with watered in the air. The thermal stability of α-AlH3 before and after coating was analyzed by DSC. This work demonstrates the successful applications of dopamine chemistry to α-AlH3, thereby providing a potential method for the metastable materials. surface of α-AlH 3 . The moisture absorption tests show that the moisture absorption rate of α-AlH 3 is greatly reduced after being coated with PDA. The excellent intact ability of PDA prevents α-AlH 3 from reacting with water in air. The thermal stability of α-AlH 3 before and after coating was analyzed by DSC. This work demonstrates the successful applications of dopamine chemistry to α-AlH 3 , thereby providing a potential method for metastable materials.


Introduction
We present a novel surface coating to resolve the stability of α-AlH 3 . Inspired by the strong chemical adhesion of mussels, the polymerization of dopamine was rst introduced to coat α-AlH 3 through simple situ polymerization. The α-AlH 3 was used as a substrate. In-depth characterizations con rmed compact formation with PDA on the α-AlH 3 surface. The coated α-AlH 3 sample was characterized by XRD XPS and SEM. The results show that a strong PDA lm is formed on the surface of α-AlH 3 , and PDA@α-AlH 3 retains its primary morphology. The crystal form of α-AlH 3 does not change after coating with PDA. The XPS analysis results show that N1 s appears on the material after coating with PDA, indicating that polydopamine is formed on the surface of α-AlH 3 . The moisture absorption tests show that the moisture absorption rate of α-AlH 3 is greatly reduced after being coated with PDA. The excellent intact ability of PDA prevents α-AlH 3 from reacting with water in air. The thermal stability of α-AlH 3 before and after coating was analyzed by DSC. This work demonstrates the successful applications of dopamine chemistry to α-AlH 3 , thereby providing a potential method for metastable materials. AlH 3 is a solid-state hydrogen storage, hydrogen provider and reducing agent. Combined with oxidizer(ADN, AP CL-20), it becomes a high energy material often used for aerospace and missile industry [1][2][3][4] . Theoretical studies have shown that the speci c impulse value of AlH 3 is higher in solid, liquid and solid-liquid propellants than in Al [5][6][7] . AlH 3 is a special compound. It has at least seven different crystalline structures depending on the synthesis conditions: α α, β γ δ ε and ζ. Among them, α-AlH 3 is the best-studied crystalline [8][9][10][11][12] . However, α-AlH 3 has a small enthalpy of formation and is in a metastable state. Hence, the problem related to stability remains unresolved [13][14][15][16] .
To date, various researchers worldwide have been tackling this problem in improving the stability of AlH 3 , including surface passivation, surface coating and doping with other substrates [17][18][19][20][21][22][23][24] . In spite of this, the appropriate coating material and the novel coating technique for coating α-AlH 3 should be explored.
Dopamine is a biological neurotransmitter that widely exists in living organisms 25 . The use of dopamine solution through the oxidation-polymerization of monomers has provided a facile and versatile method for modifying the surfaces of solid materials, which has led to the development of bioinspired poly(dopamine) (PDA) for the successful modi cation of various substrates, including metals, metals with native oxide surfaces, ceramics, semiconductors, carbon materials, and synthetic polymers. PDAmediated chemistry could provide a general method for the fabrication of numerous multifunctional substrates with speci c properties [26][27][28][29][30] . Dopamine chemistry is a straightforward and versatile coating strategy that may open a door for the surface processing of α-AlH 3 . However, few works have reported the use of PDA coating on α-AlH 3 .
Herein, we report a general and facile approach to the coating of α-AlH 3, which is bioinspired through the in situ polymerization of dopamine. To the best of our knowledge, this is the rst report about the application of dopamine chemistry to α-AlH 3  complex is basically the same as the characteristic diffraction peak of α-AlH 3 . It shows that the position of the characteristic diffraction peak remains unchanged before and after the modi cation, indicating that α-AlH 3 has good crystallization performance, without the characteristic diffraction peak of PDA with the coating agent, and the coating amount of the coating agent PDA is small, indicating that α-AlH 3 is coated and modi ed by PDA.
The surface compositions of PDA@α-AlH 3 and α-AlH 3 were analyzed by photoelectron spectroscopy (XPS), and the results are shown in Figure 2 and Table 1. Figure 2 shows that the element types on the surface of α-AlH 3 before and after coating changed. The surface elements of α-AlH 3 before coating only contain three elements: C, Al, and O. After coating, the surface elements of PDA@α-AlH 3 contained not only three elements: C, Al, and O but also the characteristic N element peak of the polydopamine lm. These observations indicate that the surface of α-AlH 3 was successfully coated with a polydopamine lm. However, the intensity of the peaks has changed. The intensity of the C1 s peak is signi cantly increased, indicating that the intensity of the O1 s peak and Al2p corresponding to the carbon element in the PDA coated on the surface of α-AlH 3 is signi cantly reduced. At the same time, it can be seen more speci cally from Table 1 that the content of C1 s increased from 18.17-52.37%, and the corresponding contents of O and Al were reduced. From Table 1, the content of O1 s is reduced from 38.58-22.83%, the content of Al2p is reduced from 43.25-21.27%, and the content of N1 s is characteristic of a polydopamine lm at 3.53%, indicating that the surface of α-AlH 3 is coated with PDA. To further study the in uence of PDA on the micromorphology of α-AlH 3 , the morphology analysis results of PDA@α-AlH 3 and α-AlH 3 by SEM are shown in Figure 3. Figure 3 shows the SEM images and atomic distribution as determined by the EDS mapping images of two samples obtained from different locations. This shows that some Cl minerals included in α-AlH 3 have been unremoved by cleaning.
After coating, the elements of PDA@α-AlH 3 contained not only Al and O but also the characteristic N and C elements of polydopamine. The C distribution is shown in blue patches and dots. The N distribution is shown in purple patches and dots. The composition of samples determined by EDS is given in Table 1.
As seen from Table 2, the C atomic percentage was increased by 24.55%, and when the N atomic percentage was increased by 2.44%, α-AlH 3 was coated by PDA, showing a signi cant enrichment of the C and N contents. At the same time, the Al and The O atomic percentages were decreased by 7.88% and 8.87%, respectively. To study whether a small amount of coating agent PDA can slow down the moisture absorption of α-AlH 3 , the moisture absorption rates of PDA@α-AlH 3 and α-AlH 3 were tested. The results are shown in Figure 4. The results show that at room temperature and 93% relative humidity, the moisture absorption rate of α-AlH 3 coated with PDA is signi cantly lower than that of α-AlH 3 g. With increasing time, the moisture absorption rate of uncoated α-AlH 3 increases rapidly with storage time and reaches the equilibrium point of moisture absorption after 12 days, which is as high as 13.3%. The moisture absorption rate of α-AlH 3 after being coated with PDA is only 0.05%, which shows that the polydopamine lm on the surface plays an important role in isolating moisture in the air. It is speculated that this is mainly due to the close combination of PDA, which effectively inhibits the reaction of AlH 3 with water vapor in the air.
Thermal stability is a key performance factor for novel materials. To investigate the thermal stability of the samples before and after modi cation, the thermal performance was determined by DSC. The test results of the samples are displayed in Figure 5. The characteristic parameters are summarized in Table   3. As shown in Fig. 5, the DSC curve of α-AlH 3 exhibits a single endothermic desorption process starting at and peaking at 181.2 ℃, which is in accordance with the endothermic reaction being attributed to the dehydriding of α-AlH 3 . However, the thermal decomposition temperature of PDA@α-AlH 3 is 191.1℃, indicating that the thermal stability of α-AlH 3 is greatly improved after PDA modi cation, which is due to the formation of organic PDA to improve its heat resistance. These results clearly demonstrate that PDA@α-AlH 3 completely decomposed into Al is more di cult than pure α-AlH 3 . For this reason, the prepared samples are naturally placed in a desiccator, and after a period of time, the hydrogen content is detected, and the decomposition rate is indirectly calculated through the change in hydrogen content. The test results of samples α-AlH 3 and PDA@α-AlH 3 are shown in Table 4 below. Table   4 shows that the hydrogen content of the product is reduced due to the decomposition of. The initial hydrogen content of sample #1 is 9.745%. From the initial hydrogen content data of #2, #3 and #4, we can see that there are varying degrees of reduction in hydrogen about α-AlH 3 modi ed by PDA. However, from the perspective of decomposition rate, the decomposition rate of α-AlH 3 is the highest, and the decomposition rate is reduced after modi cation by PDA. The decomposition of PDA@α-AlH 3 may be slowed due to the formation of the organic polymer PDA. The more organic content there was, the lower the decomposition rate. To investigate the in uence of PDA on the sensitivity of α-AlH 3 , different batches of prepared PDA@α-AlH 3 samples were numbered, and the impact, friction and electrostatic sensitivity were tested to study the in uence of PDA on the sensitivity of α-AlH 3 . The test results are shown in the table 5.  Table 5, it can be seen that the α-AlH 3 surface modi cation material polydopamine has an effect on α-AlH 3 . The impact sensitivity, friction sensitivity and electrostatic sensitivity have little effect. After being modi ed by PDA, it can meet the requirements of later application indexes.

Conclusion
Polydopamine (PDA) was successfully generated on the surface of α-AlH 3 by in situ dopamine (DA) polymerization, and the structure and morphology of the package were characterized by various test methods, such as XRD, XPS and SEM. The results showed that the coating layer was more uniform, the coating effect was better, and the crystal type of the material was not changed. The stability of the PDA@α-AlH 3 composite at room temperature and 93% high humidity is signi cantly higher than that of uncoated α-AlH 3 . This research provides new ideas for the long-term storage performance of α-AlH 3 and its application in propellants.

Methods
Materials α-AlH 3 was typically synthesized following a wet chemical method. LiAlH 4 and AlCl 3 react in diethyl ether to produce an alane-ether complex. Then, α-AlH 3 was crystallized from the crystallization solution by removing ether by heating the crystallization solution to a temperature ranging from approximately 80 ℃ to 90 ℃; additionally, the crystallization additive was also added to the crystallization solution. α-AlH 3 was synthesized in our institute. Dopamine hydrochloride was purchased from Sigma-Aldrich and used as received. The other chemicals were commercial, analytical grade and used without further puri cation.
Coating of α-AlH 3 with PDA A phosphate-buffered saline (PBS pH=7~7.5) solution was rst prepared after the as-prepared solution and α-AlH 3 was dispersed to the PBS solution, with stirring at 300 rpm for 30 minutes. Then, dopamine was added to the above suspension, with stirring at 300 rpm for 4 h. During the polymerization process, the color of the solution changed from white to dark brown as a result of dopamine polymerization. The obtained dark brown solution was ltered. The samples were rinsed with distilled water and then dried in a vacuum oven at 60 ℃. The entire operation process is shown in Figure 6.

Hygroscopicity test
Static hygroscopicity refers to the GJB772A-97 method. Place the saturated solution of potassium nitrate in a desiccator. After equilibrium, a hygrometer was used to determine the relative humidity in the desiccator to be 93%. Combine α-AlH 3 and PDA@α-AlH 3 at the same time. Place it in a dry place, and use a Mettler analytical electronic analytical balance for weighing, with an accuracy of 0.0001. The weight change was recorded every 24 hours, the change in moisture absorption rate was observed, and the moisture absorption rate was calculated as follows: In the formula, Q is the moisture absorption rate, ΔG is the weight gain of the sample after moisture absorption, and m is the initial weight of the sample.
characterization Structural characterization of the α-AlH 3 and PDA@α-AlH 3 samples was performed by powder X-ray diffraction (XRD, DMAX2400 with Cu K a radiation at λ = 1.5418 Å). The morphology of the samples was examined by eld emission scanning electron microscopy (SEM, Quanta600FEG). The surface chemistry was analyzed using X-ray elemental analysis (XPS, Thermo Fisher spectrometer equipped with monochromatic Al Ka radiation (1486.6 eV)). The thermal analysis was studied by DSC (NETZSCH STA 449C). The samples were heated from room temperature to the set temperatures with a heating rate of 10 ℃/min under an Ar 2 gas ow rate of 70 ml/min to prevent oxidation. The moisture absorption curves of α-AlH 3 and PDA@α-AlH 3 Page 14/15 Figure 5 the DSC curve of α-AlH 3 and PDA@α-AlH 3