## 3 Results and Discussion  
### Effects of Surfactant in Directing the Size and Morphology of Alanate Nanostructures  
The length of surfactants' carbon chain can play a significant role in controlling particle size during solvent evaporation. Our work on NaBH\({}_{4}\) and LiBH\({}_{4}\) revealed that longer linear surfactant chains lead to smaller nanoparticles due to the larger steric hindrance, whereas a reverse trend was found with particles stabilized with quaternary ammonium bromides [40; 41]. The effects of carbon chain length were thus investigated to determine whether the trends observed with borohydrides also apply to alanates. This was carried out by using linear surfactants with increasing chain length such as hexylamine (CH\({}_{3}\)(CH\({}_{2}\))\({}_{5}\)NH\({}_{2}\)), dodecylamine (CH\({}_{3}\)(CH\({}_{2}\))\({}_{11}\)NH\({}_{2}\)), and octadecylamine (CH\({}_{3}\)(CH\({}_{2}\))\({}_{17}\)NH\({}_{2}\)). For comparison, a quaternary ammonium bromide (R\({}_{4}\)NBr) such as TBAB ([CH\({}_{3}\)(CH\({}_{2}\))\({}_{3}\)]\({}_{4}\)NBr), TOAB ([CH\({}_{3}\)(CH\({}_{2}\))\({}_{7}\)]\({}_{4}\)NBr) and TDAB ([CH\({}_{3}\)(CH\({}_{3}\))\({}_{9}\)]\({}_{4}\)NBr) were also investigated. It should be noted that TBAB, TOAB, and TDAB are not soluble in diethyl ether. Therefore, the effect of these three surfactants was only investigated with NaAlH\({}_{4}\). Upon solvent evaporation, spherical-like nanoparticles were observed (Figure 1), and the average particle size of both LiAlH\({}_{4}\) and NaAlH\({}_{4}\) was found to decrease with increasing alkyl chain lengths. For example, with increasing chain length from 6 (HXA) to 18 (ODA) carbons, the particle size of LiAlH\({}_{4}\) decreased from 394 to 53 nm. Switching the surfactant from a linear amine to a thiol with different alkyl chain lengths revealed a similar trend, in agreement with the general observations that larger steric hindrance occurs with longer linear carbon chains (Figure 2 and Figure S15, Supporting information) [42; 43]. In contrast, stabilization of NaAlH\({}_{4}\) with a quaternary ammonium bromide showed a reverse trend (Figure 2b and Figure S16). NaAlH\({}_{4}\) particles of 9 \(\pm\) 3 nm were obtained upon stabilization with TBAB, whereas with TOAB (alkyl chain with 8 carbon atoms instead of 4 for TBAB), NaAlH\({}_{4}\) particles with an average size of 106 \(\pm\) 17 nm were observed (Figure 2b and Figure S16). With TDAB (i.e., 10 carbons) NaAlH\({}_{4}\) particles growth restriction failed (Figure S16) owing to the strong intrinsic steric hindrance of the surfactant which result in a low packing density of the surfactant at particle's surface and thus poor NaAlH\({}_{4}\) particle stabilization [44; 45]. Besides, the interaction of NaAlH\({}_{4}\) surface with the surfactants may  
Figure 1: Typical TEM images of (**a–c**) LiAlH\({}_{4}\) and (**d–f**) NaAlH\({}_{4}\) stabilised by (**a,d**) HXA, (**b,e**) DDA, and (**c,f**) ODA at 1 mM. be further weaken by the strong surfactant-surfactant interaction [46]. As such, poorer stabilizing ability was observed with longer R4 NBr chains. Interestingly, it can be observed that the ability of amine-based surfactants in leading to small particles and thus better stabilization effects is superior to that of thiol-based surfactants (Figure 2). Several studies have shown that the strength of the interaction between the surface of stabilized particles and surfactant's head group may affects the growth of nanoparticles [47; 48]. According to the Pearson's Hard and Soft Base (HSAB) principle, chemical substances can be categorized into hard, soft, and borderline character. The affinity between Lewis acids and Lewis bases would be stronger if they have similar hard or soft properties [24; 49]. Based on this, Li+, Na+, and AlH4+ can be considered as hard acids [24]. Therefore, LiAlH4 and NaAlH4 are expected to coordinate stronger with surfactants carrying a hard base head group.