## 3 Results and Discussion
### Effects of Surfactant in Directing the Size and Morphology of Alanate Nanostructures
To investigate this, the effect of surfactants displaying hard and soft properties with the same carbon chain length, i.e., dodecylamine (RNH2), dodecanethiol (RSH), and tridecylic acid (RCOOH) were investigated. Generally, the relatively small non-polarizable donors such as N, O, and F hold a hard character, while the soft character corresponds to the polarizable donors such as P, S, Cl. Based on these considerations, dodecylamine (RNH2), tridecylic acid (RCOOH), and dodecanethiol (RSH) were considered to be hard base, hard acid, and soft base, respectively. The average particle size observed upon LiAlH4 and NaAlH4 evaporation with surfactants of hard and soft properties are summarized in Figure 3 and Figure S17. The significant differences observed in particle size were taken as indirect evidence that the functionality of the surfactant head group affected the size of LiAlH4 and NaAlH4 morphology. For example, LiAlH4 stabilized by DDA (RNH2), TDA (RCOOH), and DDT (RSH) have an average particle size of 64 \(\pm\) 29, 179 \(\pm\) 40, and 188 \(\pm\) 11 nm, respectively. Considering that LiAlH4 is a hard acid, it would tend to bind more strongly to -NH2 and -COOH (of hard character) compared to -SH which is a soft base. As such, LiAlH4 nanoparticles observed by TEM are smaller with the -NH2 and -COOH functionalities (Figure S17). Figure 2: Change in average particle sizes of (**a**) LiAlH4 stabilized by RNH2 and RSH and (**b**) NaAlH4 stabilized by RNH2, RSH, and R4 NBr with an increase in carbon chain length (surfactant concentration = 1 mM). Error bars represent standard deviation of the mean particle size. Typical corresponding TEM images are summarized on Figures S3–S16. A similar trend was observed with NaAlH\({}_{4}\), where the formation of small NaAlH\({}_{4}\) particles is favored with hard surfactants (i.e., dodecylamine (DDA), and tridecylic acid (TDA)). Again, strong coordination of NaAlH\({}_{4}\) with -NH\({}_{2}\) and -COOH was found to better hinder the growth and agglomeration of the NaAlH\({}_{4}\) nanoparticles as compared to the weaker NaAlH\({}_{4}\)/SH based surfactant interaction. It is noteworthy that the reducing ability of LiAlH\({}_{4}\) and NaAlH\({}_{4}\) may inevitably affect the size restriction efficiency [50; 51]. LiAlH\({}_{4}\) is capable of reducing carboxylic acids into primary alcohols while amines remain unreacted [52]. Potential reaction between LiAlH\({}_{4}\) and TDA are given below in Equation (1) [53; 54; 51]. \[\text{LiAlH}_{4}+\text{RCOOH}\rightarrow\text{RCH}_{2}\text{OH}+2\text{LiOH}+ \text{AlH}_{\text{x}} \tag{1}\]  
In contrast, although NaAlH\({}_{4}\) is capable of reducing COOH functions, NaAlH\({}_{4}\) is a milder reducing agent as compared to LiAlH\({}_{4}\)[51]. By XRD, no by-product from the potential reaction of TDA with the alanates was observed. This may be due to the small amount of the resulting products, or their amorphous nature (Figures S18 and S19). Upon FTIR analysis, -CO (1105 and 1124 cm\({}^{-1}\)) and -OH (3264 and 3322 cm\({}^{-1}\)) stretching vibrations corresponding to a primary alcohol were observed in LiAlH\({}_{4}\)-TDA and NaAlH\({}_{4}\)-TDA. This suggested that the COOH functionality in TDA was fully reduced by both LiAlH\({}_{4}\) and NaAlH\({}_{4}\), as the C=O stretching of carboxylic acid was not observed (Figure 4) [40; 41]. It is likely that the initial reduction of the carboxylic function of the surfactant, into an -OH will result in a different stabilization mechanism of the alanate, and thus different particle sizes. For example, it has been reported that the binding energy between Li\({}^{+}\) and the various functional groups follows the order of OH < NH\({}_{2}\) < COOH [55]. Therefore, TDA is expected to lead to the formation of smaller particles than DDA due to the stronger affinity of Li\({}^{+}\) to -COOH as compared to -NH\({}_{2}\). However, the reverse was observed (Figure 3), owing to the reduction of carboxylic groups to alcohols.