Ammonia Borane Nanospheres for Hydrogen Storage - ACS Applied

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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Ammonia Borane Nanospheres for Hydrogen Storage María-José Valero-Pedraza,† Didier Cot,† Eddy Petit,† Kondo-François Aguey-Zinsou,‡ Johan G. Alauzun,§ and Umit B. Demirci*,† †

Institut Européen des Membranes, IEM-UMR 5635, ENSCM, CNRS, Université de Montpellier, 34090 Montpellier, France Merlin Group, School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia § Institut Charles Gerhardt Montpellier, ICGM-UMR 5253, ENSCM, CNRS, Université de Montpellier, CNRS, ENSCM, 34095 Montpellier, France ‡

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ABSTRACT: Nanosizing ammonia borane NH3BH3 (AB) is an attractive approach to make it suitable for chemical hydrogen storage. Herein is reported the successful direct preparation of ammonia borane nanospheres. No scaffold is used. The preparation is performed via a simple process using alkaline water H2O as main solvent, a common and thermally stable surfactant like cetyltrimethylammonium bromide (CTAB), and dodecane C12H26 as counter-solvent. Interestingly the protic solvent H2O, when kept at pH > 7, has no hydrolytic effect on ammonia borane, allowing nanosizing with the safest solvent. Ammonia borane nanospheres with an average diameter of 110 nm can then be obtained via a protocol where the molar ratio AB/CTAB is 5060/1 and the volume ratio H2O/C12H26 is 1. Such an achievement should open new prospects for ammonia borane in the field of hydrogen storage but also as precursor of nanostructured B,N-based materials for energy and environmental applications. KEYWORDS: ammonia borane, CTAB, hydrogen storage, nanosizing, water

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catalyst;15−18 addition of a solid-state doping agent;19−22 insertion of AB into the porosity of a scaffold (i.e., nanoconfinement);23−26 and chemical modification toward synthesis of alkali amidoborane derivatives MNH2BH3 (M = Li, Na, K).27,28 Nanoconfinement is efficient for nanosizing AB and concomitantly modifying its thermal stability and reactivity.23−26 Typically, a porous scaffold like silica SBA-15 or a metal−organic framework is infiltrated with AB, leading to an AB@scaffold composite that shows a lower dehydrogenation temperature and a substantially decreased or annihilated evolution of gaseous byproducts. Depending on the nature of the scaffold, the improved dehydrogenation properties are explained by one or more of the following effects: nanosizinginduced defect sites initiating dehydrocoupling; interfacial reaction between surface Hδ+ (from X−O−H groups, with X = Si or C) and Hδ− of AB; formation of X−O−BH3 coordination between the lone pair of surface O and the vacant orbital of BH3; short diffusion path for released H2; catalytic effect of surface elements like N and/or B (in, e.g., carbon nitride and boron nitride), such elements acting then as Lewis base and acid respectively; and/or catalysis by metal centers for metal−

INTRODUCTION Ammonia borane NH3BH3 (AB) is a remarkable hydrogen storage material carrying 19.6 wt % of hydrogen.1 It is characterized by an equal number of protic Hδ+ (via −NH3) and hydridic Hδ− (via −BH3) hydrogens in intra- and intermolecular interactions. This results in heteropolar dihydrogen N−H···H−B bonds accounting for the solid state of the material at ambient conditions (in fact, up to ca. 100 °C).2−4 These interactions, together with the occurrence of homopolar dihydrogen B−H···H−B interactions under heating, are the driving forces of the intra- and intermolecular dehydrogenation of the NH3BH3 molecules (at >100 °C).5,6 Owing to such properties, AB (in thermolytic conditions) has been understandably much investigated in recent years. It is worth mentioning that, like sodium borohydride NaBH4,7 the precursor used to synthesize AB,8 it has also been much investigated for H2 generation in hydrolytic conditions (where water provides half of the generated H2).9−12 AB in the pristine state is however not suitable for solid-state hydrogen storage because of inappropriate dehydrogenation features.13,14 Destabilization strategies have thus been developed in order to decrease the onset dehydrogenation temperature (1 μm) of agglomerated AB nanoparticles (around 50 nm).34 More recently, Lai et al. reported the synthesis of AB nanoparticles (20−160 nm) by antiprecipitation (with tetrahydrofuran as solvent, cyclohexane as counter-solvent, and oleic acid as surfactant).35 These nanoparticles were decorated with nickel nanoparticles (1−7 nm; i.e., smaller than the AB particles), and destabilization took place; indeed, AB started its dehydrogenation from 50 °C. What this last work demonstrates is that nanosized AB particles could be stabilized to some extent without a scaffold, but a key remains the development of better approaches for the facile and controlled preparation of AB nanoparticles. To date, several methods have been reported for the synthesis of nanoparticles by wet chemical methods (e.g., (co)precipitation, sol−gel processing, microemulsion)36−41 for making, for example, free-standing particles and core−shell nanoparticles of metal oxides and transition metals. However, little is known on the translation of such methodologies to preparing hydride nanoparticles. The challenge is in the reactivity of common hydrides of interest for hydrogen storage with solvents and potential stabilizing ligands. For example, AB is a good reducing agent of, for example, ketones; is highly reactive with protic solvents; and is soluble in only a limited number of solvents, including ethers.42

In keeping with the aforementioned challenge, we report on a straightforward, easy, scalable strategy for the preparation of AB nanospheres. Attractively, no scaffold was used. Instead, we targeted the AB nanosizing via the formation of micelles through an emulsification approach, with cetyltrimethylammonium bromide (CTAB; [CH3(CH2)15N(CH3)3]+[Br]−)43,44 as the surfactant. Importantly and originally, an aqueous alkaline solution and CTAB as a common and commercial surfactant were used. This is the first report on AB nanosizing in aqueous solution, namely with the “greenest” solvent, water. Dodecane was used as counter-solvent. In doing so, we successfully prepared AB nanospheres with an average diameter of 110 nm which should open new prospects in the field.

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RESULTS AND DISCUSSION We explored the possibility of nanosizing AB (Figure 1) via the formation of micelles in alkaline aqueous solution of CTAB. Water (at pH 11) is a suitable solvent owing to the good stability of AB in alkaline solution.1 Water in a small amount may even act as promoter for synthesis of AB by metathesis in tetrahydrofuran.45 Dodecane C12H26 was selected as the CTAB-compatible counter-solvent, also because AB is not soluble in saturated hydrocarbons. Water and dodecane are favorably immiscible, and this was thought to be a good combination to allow the precipitation of nanosized AB in the reverse micelles. At room temperature, 1 mL of an aqueous alkaline AB solution was added dropwise (0.17 mL min−1) to 1 mL of an aqueous CTAB solution (Figure 1). The as-obtained solution was then added to 2 mL of dodecane in a similar way. An emulsion formed. Two preparations are reported hereafter. These are 1 and 2, for which the molar ratios AB/CTAB are 5060/1 and 506/1, respectively. For both, the volume ratio water/dodecane is 1. Here, it is worth mentioning two things. First, the aforementioned conditions in terms of molar ratio AB/CTAB and volume ratio water/dodecane were actually selected after a wider screening as detailed in Materials and Methods. Second, an additional preparation without CTAB was performed to show the importance of the surfactant in obtaining the nanospheres discussed below (Figure S1). For 1, the AB concentration in water was 4.9 M (i.e., 0.3 g of AB in 2 mL of H2O), which is slightly less than half of the B

DOI: 10.1021/acsanm.9b00176 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Figure 2. SEM micrographs of 1, dropped onto a holey carbon film on a copper grid (the grid was deposited onto a carbon tape).

Figure 3. Analysis by 11B NMR of 1 and 2 in solution state.

solubility of AB (∼11.4 M at room temperature).46 The CTAB concentration in 2 mL of water (9.6 × 10−4 M) was similar to the critical micelle concentration (9.45 × 10−4 M at 25 °C).47 A droplet of the emulsion (after solvent evaporation) was observed by SEM. Nanosizing of AB in the form of nanospheres of various sizes (from 65 to 155 nm) with an average diameter of 110 nm (Figures 2 and S2) was successful in our conditions. By taking into account this average nanosphere size and the density of AB (0.78 g cm−3), one may predict a specific surface area of about 70 m2 g−1. 1 was also analyzed by 11B NMR to verify the presence of AB and especially to ensure that no hydrolysis with formation of borates took place (Figure 3). No signal at δ ≥ 0 was observed, confirming the hydrolytic stability of AB in our experimental conditions. Analysis of the region between 0 and −20 ppm shows two multiplets centered at −4.8 and −11.6 ppm. They may be attributed to BHx (x = 1, 2) environments found in, for example, cyclic trimers of AB (B-(cyclo-

diborazanyl) amine-borane; B-(cyclotriborazanyl) amine-borane; cyclotriborazane).48−54 This indicates a slight evolution of AB. By integrating the peaks, a purity of 97.5% (if CTAB is not taken into account) was found. Demulsifying took place about 20 days after synthesis (Figure S3). The presence of AB in the dodecane phase was then discarded by 11B NMR (Figure S4). The localization of AB in the aqueous phase was also verified by catalyzed hydrolysis. The hydrolytic dehydrogenation allowed recovering ca. 97.5% of the hydrolyzable hydrogens, which is in good agreement with the purity determined above. It is worth mentioning that, after shaking the demulsified solution, an emulsion reappeared. For 2, the AB concentration in water was kept at 4.9 M but the CTAB concentration was increased (9.6 × 10−3 M) to be 10 times higher than the critical micelle concentration (9.45 × 10−4 M at 25 °C).47 In doing so, the molar ratio AB/CTAB was divided by 10 in comparison to the ratio for the previous sample. A droplet of the emulsion (after solvent evaporation) C


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Figure 4. SEM micrographs of 2: (a and b) spherical clouds of mostly isolated spherical nanoparticles; (c and d) spherical clouds of mostly aggregated spherical nanoparticles; (e and f) hollow spheres with irregular surface structures.

same two signals at δ ≤ 0 were observed, suggesting a purity of 98% (Figure 3). No demulsifying was observed (Figure S3), and no AB was detected in the organic phase (Figure S4). For the present work, and in fact in our overall project, we aim at getting homogeneous nanoparticles of AB for hydrogen storage. With 2, the size is of micrometric scale like for bulk AB, and thus, it is likely that it decomposes in a similar manner. As a consequence, for the present work, we focused on 1 only, but we keep in mind that 2 might open new prospects for nanosized AB, for example for preparing Pickering emulsions. As shown above, AB can be readily nanosized in water with the mediation of CTAB and dodecane. 1 was then further investigated in the solid state. The FTIR spectrum of 1 shows the fingerprint of AB (Figure 5), especially the bands in the stretching regions of the N−H (3400−3100 cm−1) and B−H (2500−2100 cm−1) bonds.56,57 Additional bands, but of small intensity, within the 3000−2600 cm−1 range are also observed. They favorably compare to the bands (C−H stretching)

was scrutinized by SEM. Different big spherical microstructures (0.3−7 μm) were observed (Figure 4), namely, spherical clouds of mostly isolated spherical nanoparticles, spherical clouds of mostly aggregated spherical nanoparticles, and hollow spheres with irregular surface structures. These big spherical microstructures would be formed by assembly of smaller (average size = 200 nm) spherical nanoparticles of AB similarly to Pickering emulsions.55 For 1, the concentration of CTAB was lower, and this led to nanospheres with an average diameter of 110 nm. When the CTAB concentration is increased significantly, as for 2, the particles tend to agglomerate in “capsules” of micrometric scale. These capsules are most probably formed through bilayer organization of CTAB molecules. This is a first explanation that may be proposed, but further investigations are required to control, modify, and characterize (by, for example, SAXS and DLS) such original agglomerates. With 2, no evolution (by hydrolysis) of AB was noticed by 11B NMR, and as for 1 the D


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resonance. It can be explained by a BH3 environment with a higher molecular degree of freedom.60 It is reasonable to attribute this signal to NH3BH3 entities at the surface of the nanospheres. Few other signals of very low intensity can be seen between δ = −15 ppm and δ = −10 ppm. They are typical of oligomeric species with BH 2 N 2 , BHN 3 , and BHN 2 environments.19−28 Further signals are observed at δ ≥ 0 ppm. We attribute them to cyclic species with BN4, BN3, and BHN2 environments like in borazine-based oligomeric compounds.61 Though such signals might also be due to the BO3 environment like in boric acid B(OH)3,62 the oxidation of AB is discarded because no borate was observed by 11B NMR analysis of 1 in solution (Figure 3). There is no BH4 environment in our conditions, which allows discarding the presence of the ionic dimer of AB, namely, the much less stable diammoniate of diborane [(NH3)2BH2]+[BH4]−. Our NMR results may be interpreted as follows: The high-speed rotation of the spectrometer rotor may have resulted in slight evolution of solid-state 1. However, dehydrocoupling of a little amount of AB (likely the surface molecules) cannot be discarded. This may have taken place because of the higher molecular degree of freedom of surface NH3BH3 molecules (being more sensitive to dehydrocoupling). Returning to the aforementioned main 11B NMR peaks (at about δ = −27 ppm; Figure 6), they are sharp, indicating high crystallinity. This is confirmed by powder XRD (Figure S6). The diffraction peaks of 1 belong to body-centered tetragonal AB (ref 00-013-0292), confirming the purity of 1 from a crystallographic point of view. By Scherrer analysis, a crystallite size of 88 ± 15 nm was found, which is in good agreement with the SEM observations (Figures 2 and S2). This is slightly smaller than the crystallites in bulk AB (124 ± 11 nm). As observed elsewhere,34,35 the diffraction peaks of 1 are sharper than those observed for bulk AB. In addition, they are split at for example 2θ = 34, 35.5, and 42.4°. Similar splitting was reported for AB nanoconfined in carbon cryogel, and it was explained by stress exerted on the AB crystal by the volume of the carbon matrix.63 A similar explanation can be proposed for 1; the splitting could be to the CTAB (strongly binding) on the AB nanoparticles surface, where the stress (strain-induced distortion in the AB structure) would be localized and exerted by CTAB. A facile approach to assess the nanosizing effect on the thermal stability of 1 is TGA. For comparison, the thermal stability of bulk AB and AB* (i.e., AB mixed with 4 wt % CTAB) was also checked up to 200 °C (Figures 7 and S7). Note that CTAB is stable over the targeted temperature range. The TGA profiles of 1, bulk AB ,and AB* are comparable, but there are also some important differences, as detailed hereafter and summarized in Table 1. From about 80 °C, 1 slightly decomposes, and up to 103.8 °C, it undergoes a weight loss of 1 wt %. Exactly the same behavior is observed for AB*. This may be interpreted as follows: the decomposition of 1 from 80 to 103.8 °C is due to the destabilization of NH3BH3 molecules in interactions with CTAB. Above 103.8 °C, the main decomposition of 1 takes place. Interestingly, there is an endothermic event occurring in the midst of the exothermic process. It starts from 110.4 °C and peaks at 112.2 °C. Melting of the borane in 1 takes place (Figure S8). It is concomitant to the decomposition of 1. With bulk AB, the melting event is known to precede the decomposition (the endothermic event peaking at 104

Figure 5. FTIR spectra of solid-state 1, bulk AB, and CTAB.

belonging to CTAB. Mapping by EDX confirmed the presence of uniformly distributed Br due to the surfactant CTAB (Figure S5). By ICP-AES and quantification of Br, the proportion of CTAB in 1 was found to be 4 wt %. Solid-state 1 was also scrutinized by solid-state MAS NMR of the nucleus 11B (Figure 6). The presence of the two typical

Figure 6. 11B MAS NMR spectrum of solid-state 1.

peaks (centered about δ = −27 ppm) of the BH3 group due to nuclear quadrupolar coupling (related to anisotropy around the boron atom) characterizes the NH3BH3 molecule58 and indicates that the nanospheres are mainly composed of AB. Note that the small hump at about −30 ppm is typical of AB and is generally explained as the result of some internal dynamics and/or disorder of AB.59 Like for the solution-state 11 B NMR spectrum (Figure 3), there are additional signals of smaller intensity. A first one at δ = −20.7 ppm has a symmetric E


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°C),1,7,8 and with nanoconfined/nanosized AB, melting is generally not observed.23−26,29−33 Even with our previous AB nanoparticles,35 the melting event preceded the decomposition and the TGA results of the AB nanoparticles were similar to those of bulk AB. With respect to AB*, the endothermic event peaks at 105 °C like for bulk AB. Our nanosizing approach is thus efficient for destabilizing AB to some extent. However, the nanosphere size would not be small enough to avoid, subsequently, melting. For the first main decomposition, the weight losses for 1 and bulk AB are close, with 11.7 and 12.5 wt %, respectively. The former value has to be corrected to 12.2 wt % by taking into account the weight of CTAB. This suggests comparable decomposition paths for both samples. It is worth notin that for AB* the decomposition is exacerbated as compared to bulk AB. Though the decomposition of 1 starts from about 80 °C (weight loss of 1 wt %), the first main event takes place at a temperature higher than the one observed for bulk AB and AB* (Figure S7). On the basis of previous observations,59 it may be assumed that, during the weight loss of 1 wt % (between 80 and 110 °C), the surface AB molecules (that would be more mobile as discussed above) of 1 would dehydrogenate toward the formation of a polymeric B,N-based “shell”; the as-formed shell would then act as a barrier, that is, either as a heat barrier (resulting in a gradient of reactivity in the grain) or as a gas barrier (making the generated H2 harder to expel). A second main decomposition takes place for 1. The onset temperature is 145 °C, and the weight loss for this event (over the range 145−200 °C) is 10.4 wt % (10.8 wt % if the weight of CTAB is subtracted). With bulk AB, the onset temperature is 127 °C and the weight loss is almost triple (29.3 wt %). With AB*, the onset temperature is 120 °C and the weight loss is almost triple also (31.2 wt %). The high weight losses of AB and AB* can be explained by the release of higher amounts of borazine, the main byproduct evolving at such temperature.13,14 As shown in the next paragraph, borazine is a minor byproduct of 1. According to the mechanistic paths proposed by Al-Kukhun et al.,64 borazine mainly forms from cyclic di/trimers as well as from acyclic polymeric species (based on the −NH2BH2− units). We may thus reasonably assume that for our sample 1, that is, with nanosizing, such intermediates form in much lesser extent as compared to what happens with bulk AB. In support of the TGA analysis, the evolving gases from 1 were analyzed by MS (Figure S9). Besides the mass-to-charge ratio m/z of 2 (H2), the following ones were also detected: m/ z = 17, 27, 28, 29, 53, and 80. Gaseous byproducts evolve.13−33 They rationalize a weight loss higher (22.1 wt %) than the theoretical amount of H in AB expected to evolve up to 200 °C

Figure 7. TGA and DSC profiles of solid-state 1, bulk AB and AB*.

Table 1. Summary of the TGA Results (Temperature T in °C and Weight Loss Δw in wt %) for 1, Bulk AB and AB*, with PD as a Possible Preliminary Decomposition (below 100°C), 1st MD as the First Main Decomposition, 2nd MD as the Second Main Decomposition, and OD as the Overall Decomposition over the Whole Temperature Range (80−200°C) sample 1a PD 1st MD 2nd MD OD

bulk AB

T (°C)

Δw (wt %)

80−104 104−145 145−200 80−200

1 12.2 10.8 24

T (°C) 102−127 127−200 102−200

AB* Δw (wt %)

T (°C)

Δw (wt %)

12.5 29.3 41.8

81−104 104−120 120−200 81−200

1 26.1 31.2 57.3

a

Corrected values having taken into account the weight of CTAB (4 wt % in 1). F


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ACS Applied Nano Materials (about 13 wt %). The main byproducts are ammonia NH3 (m/ z = 17) and diborane B2H6 (m/z = 27 and 28), followed by the species with m/z = 29 ascribed to aminoborane NH2BH2. There are two other byproducts in much lesser amounts and mainly evolving during the second decomposition step. The species with m/z = 53 would indicate a hydrogen-deficient dimer such as N2B2H4. The value m/z = 80 is typical of borazine B3N3H6. With bulk AB, the main decomposition product is borazine, leading generally to weight losses higher than 40 wt %.64 In our conditions, borazine is a minor byproduct, confirming the potential of these AB nanoparticles. Like for the nanoconfinement approach, this advantageous feature may be explained by one or more of the following effects: nanosizing-induced defect surface sites initiating dehydrocoupling, interfacial reaction between the surfactant head and the AB molecule, and catalytic effect of surface elements (acting as a Lewis base and/or acid). Accordingly, nanosizing is beneficial for mitigating the decomposition extent of AB as well as for improving its dehydrogenation properties. Especially, the release of borazine in the second decomposition is significantly reduced, which is an advantageous feature for AB in chemical H storage. The most developed strategy for nanosizing AB is nanoconfinement into a porous scaffold, but the use of a scaffold results in a mass penalty leading to decreased gravimetric hydrogen storage capacities (of at least by a factor of 2). Direct preparation of AB nanoparticles appears to be the best solution to avoid such an issue. In very recent years, Song et al. reported the formation of AB nanoparticles with an average size of 50 nm. The nanoparticles were agglomerated, but they showed slightly improved dehydrogenation properties in comparison to recrystallized AB.34 In comparison to 1, these nanoparticles are roughly comparable, especially for the second decomposition step (Table 2). In a previous work we synthesized AB nanoparticles by antiprecipitation using organic solvents.35 Nanosizing (20−160 nm) was effective, but the nanoparticles (Table 2) and bulk AB showed quite similar behaviors under heating. This is not the case in the present conditions. Elsewhere, the thermal stability of (bulk) AB was found to be dependent on the synthesis conditions (e.g., the nature of the precursors, the complexation ability of the solvent, the temperature, and the crystallization process), and the differences in thermal stability were tentatively discussed at the molecular scale.65 Similar reasons may explain the differences of thermal behavior of the aforementioned nanoparticles. Indeed, in our previous work, the nanoparticles were obtained by antiprecipitation, and tetrahydrofuran, cyclohexane, and oleic acid were used as solvent, countersolvent, and surfactant, respectively.35 Herein, the nanoparticles were obtained by micellization, involving alkaline water, dodecane, and CTAB. The difference in the surfactant head (−COOH vs. −[NH4+][Br−]) may, alone, affect the thermal stability of the corresponding AB nanoparticles. Further works, focusing on the molecular scale, are required to better highlight these differences. With the AB nanoparticles presented herein, three advantageous features (in comparison to bulk AB) can be seen. The onset temperature of dehydrogenation is lowered though caused by the presence of CTAB on the particles surface. The melting event is delayed, and the amount of byproducts is significantly reduced. Nanosizing has thus some positive effects on the thermal decomposition of AB, but further optimizations are still required. The average size of 110 nm seems to be still

Table 2. Summary of the DSC, TGA, and MS (m/z = 2 for H2) Results for 1 and for the Nanoparticles Reported in Refs 34 and 35a sample 1 melting preliminary decomposition

first main decomposition

second main decomposition

overall decomposition

peak temp (°C) temp. range (°C) peak temp. (°C) weight loss (wt %)b temp. range (°C) peak temp. (°C) weight loss (wt %)b temp. range (°C) peak temp. (°C) weight loss (wt %)b weight loss (wt %)b

sample in ref 34

112

sample in ref 35 110

80−104 85 1 104−145

86−124

91−135

120

101

118

12.2

6.7

28.5

145−200

124−200

135−200

151

149

160

10.8

18.2

44.8

24

24.9

73.3

a

The melting event is deduced from the DSC results where the peak temperature (°C) is given. The TGA results are discussed in terms of temperature range (°C) for each decomposition step. The peak temperature for each decomposition step is issued from the MS results. bFor the sample 1: Corrected values having taken into account the weight of CTAB (4 wt %).

high to avoid AB melting; smaller size should be targeted. The dehydrogenation properties of the AB nanospheres have to be further improved. A possible strategy would be to protect them with a porous layer (e.g., metallic, polymeric, ceramic, etc.) such as the core@shell nanostructure reported for sodium borohydride NaBH4.66 A controlled growth (in terms of both thickness and weight) of such a layer should stabilize the nanoparticles while not exacerbating any mass penalty. The nanostructures reported herein may open new prospects in preparation of nanostructures of boron nitridebased ceramics. Microporosity, high specific surface area, and the presence of other elements (e.g., C, O, etc.) may open application prospects in reversible H2 storage and water/air remediation.67−70

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CONCLUSION In summary, the concept of nanosizing AB in aqueous solution while using CTAB as surfactant and dodecane as countersolvent has been proven. This is all the more attractive that the proposed approach is novel and effective and uses the “greenest” solvent, namely, water. Depending on the experimental conditions, nanospheres with an average diameter of 110 nm or big spherical microstructures (0.3−7 μm) made of smaller spherical particles form. Such nanostructures show improved dehydrogenation properties in terms of onset temperature of decomposition and amounts of unwanted byproducts. These realizations open new prospects for nanosized AB in the field of hydrogen storage as the proposed approach avoids the important mass penalty that is specific to the use of a scaffold (via nanoconfinement of AB). This is extremely important in the context of developing novel and effective approaches for the design of better hydrogen storage G


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ACS Applied Nano Materials materials, and the present work may be seen as a first building block toward the development of capability and expertise to make outstanding hydrogen storage materials. Further works should allow going further with optimized AB nanoparticle sizes, by using, for example, other surfactants, and with surface protection with a thin, and inert or catalytic, layer. There may be prospects in other fields where AB would be employed as precursor of nanostructured polymeric and ceramic BN-based materials.

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X’Pert HighScore. This tool is based on the use of the Scherrer equation L = (K·λ)/(β·cos θ) where K is a dimensionless shape factor (i.e., constant related to crystallite shape, normally taken as 0.9), β the peak width of the diffraction profile at half the maximum intensity (in radians), and θ the Bragg angle. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis of 1 after solvent evaporation were performed with a Q500 apparatus (TA Instruments) and a Q20 device (TA Instruments), respectively. With these techniques, bulk AB and AB* (intimate mixture of bulk AB and 4 wt % CTAB prepared in the glovebox) were also analyzed. For both techniques, ∼3 mg of sample was put in a sealed aluminum crucible of 40 mL with a pinhole, and the applied heating rate was 5 °C min−1 from 30 to 200 °C under N2 (60 mL min−1). The preparation of the sample was systematically done in the argon-filled glovebox. For gas analysis, the TGA was coupled to a mass spectrometer (Agilent). Elemental analysis of 1 after solvent evaporation was performed by inductively coupled plasma atomic emission spectrometry (ICP-AES) after a mineralization step under acidic conditions. After preparation of 1, it was observed that demulsifying took place after about 20 days (Figure S4). The aqueous phase was then separated from the organic phase. The presence or absence of AB in the dodecane phase was analyzed by 11B NMR. The localization of AB in the aqueous phase was also verified by catalyzed hydrolysis: NH3BH3 + 3 H2O → NH3 + B(OH)3 + 3 H2. At room temperature, 15 mg of a platinum catalyst (previously prepared by reduction of K2PtCl6 by NaBH4) was added in the aqueous phase, and the H2 evolution was followed with the help of the inverted buret-based setup we developed for hydrolysis of boron hydrides. The experiment was repeated for 3 different batches. According to the volumes of H2 that were measured, the weight of AB that hydrolyzed was found to be 292 ± 9 mg. For the preparation of 1, 300 mg of AB was weighed.

MATERIALS AND METHODS

Ammonia borane (NH3BH3, denoted as AB; Sigma-Aldrich, 97%) and dodecane (C12H26; Sigma-Aldrich, ≥99%) were used as received. Cetyltrimethylammonium bromide ([CH3(CH2)15N(CH3)3]+[Br]− denoted as CTAB) was purchased from Sigma-Aldrich (≥98%) and dried under dynamic vacuum at 120 °C for 72 h before use. Both AB and CTAB were stored and handled under purified argon atmosphere (argon-filled glovebox Jacomex PBOX, H2O < 5 ppm, O2 < 5 ppm). Deuterium oxide (D2O; Eurisotop, ≥99.90%) was also used as received and kept in a fridge (4 °C). Deionized ultrapure water (MilliQ grade with a resistivity >18 MΩ cm) was used. Deionized water pH was adjusted with an aqueous solution of NaOH (1 M). The synthesis of 1 was performed as follows: Deionized water and dodecane were selected as immiscible solvents. CTAB was employed as surfactant. In a first step, an aqueous solution of AB was added dropwise (syringe pump: 10 mL h−1) to an aqueous solution of CTAB at room temperature. The concentration of AB was then 4.9 M (i.e., 0.30 g of AB in 2 mL of H2O). This is slightly less than half of the maximum solubility of AB in H2O (11.4 M). The CTAB concentration in the 2 mL of water (9.6 × 10−4 M) was similar to the critical micelle concentration (9.45 × 10−4 M at 25 °C). In a second step, the as-obtained aqueous solution was added dropwise (syringe pump: 10 mL h−1) to 2 mL of dodecane at room temperature. For 2, the same process was applied. However, the CTAB concentration (9.6 × 10−3 M) was increased to be 10 times higher than the aforementioned critical micelle concentration. In doing so, the molar ratio AB/CTAB was divided by 10 in comparison to the ratio for the previous sample. In fact, a wider screening was performed beforehand. The molar ratio AB/CTAB was varied from 16 to 10 120 (via 126, 253, 506, 759, 1012, 1265, 1518, 1771, 2530, 5060, 5186, and 5313). In addition, for sample 1, the volume ratio water/ dodecane was varied from 0.4 to 8 (via 0.5, 1, 5.5, and 8). Finally, the AB/CTAB ratios of 506 (for 2) and 5060 (for 1) resulted in reproducible results as reported above (in both cases with a volume ratio of 1). As an additional experiment, to show the importance of CTAB in getting well-dispersed and quite uniform nanospheres of AB, the aforementioned preparation was done in a similar way and without the surfactant. Micrographs were obtained by scanning electron microscopy (SEM) on a Hitachi S4800 microscope. A drop of the sample was left to dry onto a holey carbon film on a copper grid under dynamic vacuum overnight prior to acquisition. Elemental mapping was performed by energy-dispersive X-ray spectroscopy (EDX; Zeiss EVO HD15). Molecular characterizations of 1 and 1 after solvent evaporation were done by 11B nuclear magnetic resonance (NMR; Bruker AVANCE-300; probe head BBO10 operated at 96.29 MHz; D2O as reference); 11B MAS NMR (Varian VNMR 400 spectrometer; 128.4 MHz); Fourier transformed infrared spectroscopy (FTIR; Nicolet iS50 FT-IR Spectrometer Thermo Fisher Scientific; 4000− 700 cm−1; 64 scans). The structure of 1 after solvent evaporation was studied by X-ray diffraction by means of a PANalytical X’PERT Pro diffractometer using Cu Kα radiation (λ = 1.5406 Å) and working with a voltage and current of 40 kV and 30 mA. The data were collected with 2θ steps of 0.08° and accumulation times of 31 s. The sample was prepared onto a specific holder in the glovebox and protected from air and moisture using a Kapton film. The crystallite size (L, in nm) was determined using the tool available in the software

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00176. Additional experimental results (Figures S1−S9) as described in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kondo-François Aguey-Zinsou: 0000-0002-4239-6491 Johan G. Alauzun: 0000-0002-6531-0750 Umit B. Demirci: 0000-0003-3616-1810 Author Contributions

M.-J.V.-P. did most of the experimental work and contributed to the article writing. D.C. was in charge of the SEM imaging. E.P. supervised all of the NMR experiments and performed some. K.-F.A.-Z. provided his expertise in nanosizing and contributed to finalizing the article writing. J.G.A. supervised the nanosizing aspects, contributed to finalizing the article writing, and is the representative of the ICGM, the partner of the project. U.B.D. (from IEM) is the project leader and wrote the article. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was funded by the University of Montpellier (Université de Montpellier, AAP Post-Doc UM 2016; 50%) and the French State via the Agence Nationale pour la Recherche (ANR) and the program “Investissement d’Avenir” H


Article


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