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. 2023 Oct 4;145(42):23088–23097. doi: 10.1021/jacs.3c06191

Oxidative Activation of Small Aluminum Nanoclusters with Boron Atom Substitution prior to Completing the Endohedral B@Al12– Superatom

Tomoya Inoue 1, Miho Hatanaka 1, Atsushi Nakajima 1,*
PMCID: PMC10603816  PMID: 37792327

Abstract

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Elemental substitution and doping validate the optimization of chemical and physical properties of functional materials, and the composition ratio of the substituting atoms generally determines their properties by changing their geometric and electronic structures. For atomically precise nanoclusters (NCs) consisting of countable atom aggregates, the composition can be controlled accurately to provide an ideal model to study the heteroatom substitution effects. Since aluminum (Al) and boron (B) both belong to group 13 in the periodic table, the effect of B atom substitution on Aln NCs can be investigated while maintaining the total number of valence electrons in AlnBm NCs. In this study, oxidative reactivities of small Al NCs with B atom substitution are studied for AlnBm NCs (m = 1, n = 6–14 and m = 2, n = 11) supported on organic surfaces by using X-ray photoelectron spectroscopy and oxygen molecule (O2) exposure measurements. Before completing the endohedral B@Al12– superatomic NC, one B atom substitution in Al NCs (AlnB) enhances oxidative reactivities 3–20 times compared to those of Aln+1, particularly for n ≤ 11. When one Al atom of Al12B is further substituted by a B atom to form Al11B2, the reactivity drastically increases (6.6 × 102 times), showing that the B atom substitution makes the NC chemically active or inactive geometrically depending on the exohedral or endohedral site for the B atom in the Al NC. In addition, density functional theory calculations show that the electronegative B atom contributes to forming a locally positive Al site to facilitate O2 adsorption except in Al12B, in which the B atom is geometrically shielded by the surface of the Al12 cage in B@Al12.

Introduction

Aluminum (Al) is an important element from a materials science perspective, possessing lightweight, corrosion resistance, high electrical conductivity, and good heat resistance properties.1 In particular, its corrosion resistance is enhanced by the formation of an oxide film on the surface, which protects the interior of aluminum and contributes to its durability.2 Alloying expands the versatility of these unique properties, including machinability, wear resistance, and toughness, making aluminum alloys suitable for a wide range of demanding engineering applications.1 In fact, aluminum alloys are the second most widely used structural metal, after steel. In Al alloying, boron (B) acts as a grain refiner during solidification3,4 and enhances the electrical conductivity by removing impurity transition metals in Al,5,6 although the major alloying elements in Al are copper, manganese, silicon, magnesium, and zinc.

In addition to these macroscopic materials science perspectives of alloying, microscopic alloying at the atomic level is important for designing new material properties using the bottom-up approach. Among various alloying processes, an epitaxial growth method has been widely used as a representative and excellent technique.7,8 Another more atomically precise technique is the generation of nanocluster (NC) assemblies from atomic aggregates preformed as quasi-stable chemical species.9−13 To make this approach more promising, it is important to chemically stabilize the NCs to preserve their functional units during subsequent assembly formation. Very recently, it is demonstrated that electronically stabilized clusters known as superatoms of metal atom (M)-encapsulating silicon NCs, M@Si16, are suitable (“@” stands for the encapsulation) and also that geometric stability is desirable for stabilization, where alloying is specifically described as elemental substitution and doping.12−15

It is well-known that elements that adopt a face-centered cubic lattice structure in bulk, such as aluminum, form multiple twinned particles (MTPs) in their small atomic assemblies.16−19 One of the MTP structures is an icosahedron with a minimum of 13 atoms and with 20 faces. Particularly for Al, when 13 atoms, which are hybridized with the 3s and 3p orbitals (sp hybridization), come together,20 the resulting negative ion possesses a total of 40 valence electrons, satisfying a closed-shell electron configuration of 40 electrons, a highly stabilized superatom both electronically and geometrically.21−36 However, in this icosahedral structure, the bond length between the surface atoms is stretched by 5% compared to the distance between the central atom and the surface atoms, indicating that it is not geometrically stable enough.37,38 One effective way to atomically stabilize the icosahedral structure is to reduce the radius of the central atom by atom substitution. However, in this substitution process, the difference in electronegativity among the different atoms can induce charge separation within the NC, which may lower the chemical stability of the superatom itself; this must be considered when substituting atoms.

In this study, the size evolutions of small, B atom-doped Aln NCs over Al12B, AlnB (6 ≤ n ≤ 14), are evaluated in terms of chemical stabilities at their supported states with X-ray photoelectron spectroscopy (XPS) and molecular O2 gas exposures as they evolve into geometrically stabilized superatoms by substitution with the B atom. At the deposition of the Al–B NCs, the support effect is also controlled by predecorating the substrate with organic molecules, such as p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66),39−41 where NCs can be monodispersively immobilized on HB-HBC substrates. When a B atom is mixed with a small Aln NC less than 11-mer, the mixed AlnB NC becomes more reactive toward O2 owing to a charge transfer between Al and B atoms. However, at n = 12, the oxidative resistivity is drastically enhanced, showing that the B atom is encapsulated in an icosahedral structure as B@Al12.42−50 In this paper together with Al13 substituted with up to two B atoms (Al11B2), oxidative activation of small Aln NCs with B atom substitution, before completing the endohedral B@Al12 superatom, is discussed with theoretical calculations.

Results

Nanocluster Synthesis

Figure 1a shows B-doped Al NC anions, AlnBm–, synthesized by magnetron sputtering of the 1.2 wt % B-doped Al disk target. Single B atom-doped Al NC anions (AlnB–) were mainly generated with ion currents of a few hundred picoamperes (pA), while small amounts of two B atom-doped Al NC anions (AlnB2–) were observed in the enlarged view of the inset. The intensities of Al13– and Al12B– were rather enhanced compared to those of other neighboring Aln– and AlnB–, showing relatively higher stability of Al13– and Al12B– as the local maxima, resulting from both the 40-electron shell closure and the icosahedral geometrical packing.24,42,50 Since the mass difference between Aln– and AlnB– is m/z = 11, a specific size of AlnB– (n = 6–14) was mass-selected using the quadrupole mass filter and deposited on a molecularly decorated substrate at a low mass resolution m/Δm of ∼70.

Figure 1.

Figure 1

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Mass spectra of the NC anion of AlnBm– synthesized by magnetron sputtering; (a) AlnBm– (n = 7–22, m = 0 and 1) and (b) AlnBm– (n + m = 11–14), which were generated with 1.2 and 3.2 wt % B-doped Al disk targets, respectively. The mass spectra show that size-selective synthesis of (a) AlnB– (n = 6–14) and (b) Al11B2– can be performed with specific mass selection by a quadrupole mass spectrometer. The inset in (a) is an enlarged view from 400 to 500 m/z showing little AlnB2–.

To effectively synthesize Al NC anions doped with two B atoms (AlnB2–), the 3.2 wt % B-doped Al disk target was used. As shown in Figure 1b, the AlnB2– NCs were generated due to the increase in the B ratio in the target along with much less AlnB3–, while the amounts of AlnB– increased to nearly those of Aln–. In contrast to the enhanced Al12B–, the 13-mer Al11B2– exhibits no intensity enhancement compared with neighboring NCs, suggesting a loss of stability.41 During the selective deposition of Al11B2–, a higher mass resolution m/Δm of ∼160 was required to achieve the selective deposition of Al11B2– because the mass difference between Aln+1– and AlnB2– is relatively small (m/z = 5); the ion current was typically around 240 pA because of reduced ion transmission efficiency caused by a higher mass resolution.

Chemical State of AlnBm on the HB-HBC Substrate

Figure 2a shows the nascent XPS spectra for the Al 2p core levels for AlnB for n = 11–13, which were measured after the deposition on the HB-HBC substrate. The monodispersive morphology of the deposited NCs on the organic HB-HBC substrate has been confirmed by taking scanning tunneling microscopy images.15,36,51 The peaks are observed at 73.0 eV, which is the binding energy of the bulk metal of Al (Al0).52 XPS signals in O 1s core levels were hardly observed as shown in Figure 2b, showing that AlnB (n = 11–13) are supported on the HB-HBC-decorated substrate with negligible oxidation by residual gases. Moreover, Figure 2c shows the XPS spectra around B 1s core levels around 187.0 eV assignable to bulk B (B0).53 Since the peak width of B 1s for Al12B is relatively narrower than those of the other AlnB (n = 11 and 13), the B atom in Al12B seems to be located in an isotropic environment. Note that the signal-to-noise ratios of B 1s spectra are worse than those of Al 2p or O 1s not only because of the low ratio of the B atom in AlnB but also because of the small photoionization cross section of B 1s.54

Figure 2.

Figure 2

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Nascent XPS spectra for AlnB (n = 11–13) after deposition on the HB-HBC substrate in the regions of (a) Al 2p, (b) O 1s, and (c) B 1s core levels.

Although the chemical states of both Al and B atoms are apparently zerovalent (Al0 and B0), the charge state of AlnB NCs on HB-HBC could not be determined from the XPS spectra for Al 2p and B 1s core levels, whether anionic, neutral, or cationic. However, it seems that AlnB NCs would become negatively charged because the XPS spectra of C 1s for the organic HB-HBC show that HB-HBC in the substrate is positively charged by the deposition of NCs, as shown in Figure S1 in the Supporting Information, which shows the formation of a charge transfer complex consisting of AlnBm– and HB-HBC+.

Figure 3a shows the XPS spectra of Al 2p core levels for AlnB (n = 6, 8, 10, and 12) and also for nascent Al11B2 after deposition on the HB-HBC substrate. For AlnB, a sharp Al 2p peak for Al12B is observed at 73.0 eV representing Al0, while the Al 2p peaks are broadened and shifted to higher binding energies (∼76.0 eV) as the size n decreases from n = 12 to 6. Specifically for Al6B, the bulk metal component (Al0) completely disappears, and only a broad peak is nascently observed around 76.0 eV, originating from the surface oxide of Al (Al3+).55 In contrast to Al12B, Al11B2 becomes very oxidative: Al11B2 corresponds to one more B substitution in Al12B. The nascent Al 2p spectrum shows considerably oxidized Al atoms, where the oxidized peak (Al3+) is much larger than the metal peak (Al0).

Figure 3.

Figure 3

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XPS spectra for AlnB (n = 6, 8, 10, and 12) and nascent Al11B2 after deposition on the HB-HBC substrate in the regions of (a) Al 2p, (b) O 1s, and (c) B 1s core levels.

As shown in Figure 3b, correspondingly, the O 1s XPS peaks appear with a decrease in the size n or an increase in the B atom substitution number m, showing that AlnBm NCs become more easily oxidized even on an HB-HBC substrate. Since pure Aln NCs of any size can be deposited on the HB-HBC substrate without oxidation, the B atom substitution in Aln+1 to obtain AlnB makes Aln NCs reactive.36 The oxidative reactivity is particularly prominent in small Aln NCs below n = 10; the increase in the B ratio in the Aln NC and the B atom being exohedral enhance the reactivity.

Within the AlnB NCs, interestingly, the oxidative behavior of the doped B atoms is quite suppressed compared to that of Al atoms, although the gas-phase reactivities of Al and B atom vapors are similar.56,57 For AlnB (n = 8–12), the XPS spectra around the B 1s core levels exhibit relatively sharp peaks around 187.0 eV (B0) as shown in Figure 3c, while there is no peak at a higher binding energy derived from the oxidized component of B3+.58 For Al6B, however, a broad peak, corresponding to B3+, is apparently observed around 192.0 eV, although there is also a small residual component for B0 around 187.0 eV. For the small-sized Al6B, the B atom and Al atoms are nascently oxidized, suggesting that both Al and B atoms are exposed as exterior atoms. On the other hand, the oxidation peaks of B 1s are hardly observed for AlnB (n = 8–12), suggesting that the B atoms are geometrically protected by Al cages in the larger AlnB.

Oxidative Reactivity of AlnBm on HB-HBC

To evaluate the reactivity of the AlnBm NCs deposited on the HB-HBC substrate, we exposed O2 molecules to deposited AlnBm NCs. Figure 4a–d shows the XPS spectra around the Al 2p core levels for nascent AlnB (n = 10–12) and Al11B2 after deposition on the HB-HBC substrate (top) and after O2 molecule exposures (below), where exposure amounts were regulated to 10, 30, 1.5 × 102, 1.0 × 103, and 1.0 × 104 Langmuir (L). These Al 2p spectra were fitted with the Voight function, taking spin–orbit splitting (0.40 eV) into account,59,60 where the full widths at half-maximum (fwhm) for the Gaussian and Lorentzian profiles in the Voight function are 0.75 and 0.56 eV for Al0 (pink), respectively, and variable fwhm is adopted for Al3+ (blue). Compared to Al12B, the other AlnBm NCs are more reactive; for example, the spectral change from 0 to 30 L in Al10B is similar to the change from 10 to 1.5 × 102 L in Al12B. When they are nascently evaluated by their oxidative degree after deposition, the reactivity reduces in the order Al11B2, Al10B, Al11B, and Al12B.

Figure 4.

Figure 4

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(a–d) XPS spectra of the Al 2p core levels for AlnB (n = 10–12) and Al11B2 on the HB-HBC substrate before and after O2 exposures, where the amounts of O2 exposure are defined in Langmuir (L). The O2 exposure amounts are labeled on the right side of the figure: 10, 30, 1.5 × 102, 1.0 × 103, and 1.0 × 104 L. (e–h) Al0 ratios in Al 2p plotted against logarithmic O2 exposure amounts. In AlnB (n = 10 and 11) and Al11B2, the nascent oxidations seem more rigorous, and linear extrapolation is shown to evaluate the nascent oxidation rate.

To evaluate the reactivity quantitatively, the Al0 area ratio (pink color) is plotted against the logarithmic O2 exposure amounts, as shown in Figure 4e–h. For Al12B, a single linear relationship is obtained in all exposure ranges with a slope of 0.220 ± 0.002 (see Figure 4g and Figure S3). For Al10B, Al11B, and Al11B2, some of which are already oxidized at 0 L, the nascent oxidation rates are evaluated by considering the gas exposure levels during deposition and linearly extrapolating them. The evaluation provides two straight lines, and the first slopes are obtained as 0.396 (Al10B), 0.365 ± 0.042 (Al11B, Figure S4), and 0.579 (Al11B2) by taking into account the gas exposure during deposition (2.21, 0.62, and 21.1 L, respectively), as shown in Figure 4e, f, and h. The lowest reactivity is given by the slope of 0.22 for Al12B, while the highest reactivity is given by the slope of 0.58 for Al11B2. The change in the slope in Figure 4e, f, and h might be related to the disappearance of the active site as seen in the experiments in the gas phase,61 although it is necessary to evaluate the reactivities for the details.

Figure 5 shows the relative reactivities for Aln (n = 7–15; blue circle), AlnB (n = 6–14; red square), and Al11B2 (green triangle) deposited on the HB-HBC substrate obtained from the XPS spectra for Al 2p, where those for Aln (n = 7–15) and Al12B have been reported elsewhere.36 To completely oxidize Al12B, 3.51 × 104 L (10 to the power of 1/0.220) of oxygen molecules is required. For Al11B2, on the other hand, the exposure amount required for complete oxidation is 53.3 L (10 to a power of 1/0.579). The oxidative reactivity of AlnBm relative to Al12B can be calculated by dividing 3.51 × 104 L (the oxidative reactivity of Al12B) by the exposure amount required for the oxidation of AlnBm. The reactivity of Al11B2 relative to that of Al12B, calculated in this manner, is 6.6 × 102. Only for Al6B (red open square), the relative reactivity is estimated by assuming a linear extrapolation of the exposure amounts (65 L) during deposition; this is calculated as the product of the deposition time (12 h) and the base pressure of the deposition chamber (2.0 × 10–7 Pa). The other relative reactivities for AlnBm NCs were evaluated from Figure S5 and are summarized in Table S1 in the Supporting Information.

Figure 5.

Figure 5

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Relative reactivities for Aln (n = 7–15; blue circle),36 AlnB (n = 6–14; red square), and Al11B2 (green triangle) deposited on the HB-HBC substrate, which are normalized by the reactivity of Al12B. Only for Al6B (red open square), the reactivity is estimated from the exposure amount at nascent deposition, i.e., as the product of the deposition time (12 h) and base pressure of the deposition chamber (2.0 × 10–7 Pa).

Comparing the reactivities of AlnB and Aln+1, the behavior of B atom substitution changes around n = 11 (n + m = 12). When n ≤ 11 and, therefore, the B atom ratio in AlnB NCs becomes comparatively high, the reactivities of AlnB are enhanced 3 to 20 times compared to those of Aln+1. On the other hand, when n ≥ 12 (n + m = 13), the reactivities of AlnB are almost the same as those of Aln+1. In addition, the highest reactivity was observed for Al11B2, which has the highest B atom ratio in this study, suggesting that the AlnBm NCs with high B ratios are oxidatively activated by substitution with B atoms, while the activation reduces with the decrease in the B atom ratio associated with a further increase of the number of Al atoms. Specifically, the reactivity of Al12B (1.00) is smaller than that of Al13 (1.02), where the accuracy of the values is within 2%, as shown in Figure S3d–f in the Supporting Information. The results show that the B atom substitution also contributes to the deactivation against oxidation resulting from the geometrically closer packing of Al12B as compared to Al13 as well as the drawing of electrons from the surrounding surface Al atoms, the geometric relaxation of an icosahedral Al12 cage by encapsulating a smaller-sized isovalent B atom.42 As the surrounding Al atoms attain a complete packing of 12 Al atoms, the reactivity exhibits a gradual decline due to the geometric factor of the shielding effect and is additionally suppressed by the electronic factor of the 40-electron closed shell. The geometric shielding effect becomes evident in the size-dependent reactivity of the B atom in AlnBm NCs on HB-HBC, as detailed further in the subsequent discussion.

Reactivity of B in AlnBm on HB-HBC

Figure 6a,b shows the XPS spectra around B 1s core levels for AlnB (n = 6–14) and Al11B2 deposited on the HB-HBC substrate before and after O2 exposures. At 0 L (Figure 6a), the peak derived from nonoxidized B atoms (B0) is observed around 187.0 eV for AlnB on HB-HBC for n ≥ 7, while for Al6B and Al11B2 on HB-HBC, the oxidized B components (B3+) are also observed around 192.0 eV. At 1.0 × 104 L (Figure 6b), the peaks are broadened, and the oxidized component (B3+) grows, especially for small AlnB (n ≤ 9) and also for Al11B2.

Figure 6.

Figure 6

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(a,b) Nascent XPS spectra for B 1s core levels for AlnB (n = 6–14) and Al11B2 on the HB-HBC substrate after deposition and after O2 exposures, at 0 and 1.0 × 104 L. (c,d) Ratio of nonoxidized component B0 (at the binding energy of 185–189 eV) for AlnB (red square) and AlnB2 (green triangle) in the whole B 1s spectra before and after O2 exposures, for 0 and 1.0 × 104 L.

The ratio of the nonoxidized component (B0) at 0 L is plotted in Figure 6c, where the two areas from 185.0 to 189.0 eV and from 189.0 to 195.0 eV are regarded as nonoxidized (B0) and oxidized (B3+), respectively. The plot at 0 L (Figure 6c) shows that the B atoms of AlnB on HB-HBC (n ≥ 7) are not oxidized at all (R ∼ 1) at deposition. For Al6B on HB-HBC, the B atom is significantly oxidized compared to the B atoms of other larger AlnB NCs, implying a geometric change from the interior B atom to the exterior B atom in small AlnB (n ≤ 6). In fact, the B atom-encapsulating Al cage can be formed at Al7B–, as reported previously,50 and the structure will be discussed later. Note that the B atoms of Al11B2 (R ∼ 0.6) are apparently less reactive than those of Al6B on HB-HBC (R ∼ 0.3) because one of the two B atoms is protected by the rest of the Al11B cage in Al11B2, while the Al atoms in both Al6B and Al11B2 are almost fully oxidized just after the deposition. At 1.0 × 104 L in Figure 6d, the B atoms of AlnB NCs on HB-HBC (7 ≤ n ≤ 9) are more reactive than those of larger AlnB NCs on HB-HBC (n ≥ 10), indicating that the chemical protection of B atoms in AlnB (n ≥ 10) geometrically evolves with the number of Al atoms to form a cage large enough to encapsulate a B atom.

Discussion

Figure 7 shows the optimized structures and natural population analysis (NPA) charge distributions of Al7–, AlnB– (n = 6, 7, 10, and 12), and Al11B2– calculated with DFT. In the NPA charge distributions, negative and positive charges are represented by blue and red colors, respectively. Large negative charges are distributed on the B atoms (smaller spheres) in all AlnBm– NCs. While the negative charges are uniformly distributed in pure Al7– (Figure 7a), the substitution of the Al atom by the B atom results in the positive charges getting distributed on the Al atoms in Al6B– (Figure 7b). These positive charges are widely dispersed with an increase in the size n of the NCs, as shown in Figure 7, and the red colors on the Al atoms are diluted with size.

Figure 7.

Figure 7

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Charge distribution obtained from natural population analysis for optimized structures of (a) Al7– (C3v), (b–e) AlnB–: (b) Al6B– (Cs), (c) Al7B– (Cs), (d) Al10B– (D4h), and (e) Al12B– (Ih), and (f) Al11B2– (C5v). The smaller sphere represents the B atom in the structure, and the B atom is encapsulated for n ≥ 7.

An analysis of the optimized AlnBm– NC anions50 shows that the B atom is exposed to the surface as an exterior atom for m = 1, n ≤ 6 and m = 2, n ≤ 15. In other words, the B atom is encapsulated for m = 1, n ≥ 7, except for n = 9. The experimental results mentioned above consistently demonstrate the B atom behavior calculated for AlnBm– NCs; the exterior B atom in Al6B and Al11B2 (Figure 7b,f) is easily oxidized, while the interior B atom itself exhibits oxidative resistance for m = 1 and n ≥ 7 (Figure 7c–e), as shown in Figure 6c. In addition, the experimental results, in which the B atoms of AlnB on HB-HBC are hardly oxidized even at 1.0 × 104 L for n ≥ 10 (Figure 6d), are consistent with the calculation results because AlnB– (n ≥ 10) NCs take an endohedral B atom structure with a symmetrical cage (Figure 7d,e). Thus, it is expected that the AlnBm NCs on HB-HBC take similar structures with AlnBm– anions in the gas phase.

As shown in Figure 5, when the oxidative reactivity is evaluated by the XPS spectra of the Al 2p core levels, the reactivities of Al6B and Al11B2 on HB-HBC are specifically higher than those of other Aln and AlnB NCs on HB-HBC. This is because both the exterior B atom and the neighboring Al atoms in Al6B and Al11B2 on HB-HBC seem to be active sites for the O2 molecules (Figure 7b,f), where the dipole interaction seemingly enhances the oxidative reactivity in the case of Al6B– and Al11B2– (calculated dipole moments: 1.494 D for Al6B– and 0.574 D for Al11B2–). Generally, Al is oxidized through O2 dissociative adsorption in which O2– is generated with an electron transfer from Al during the reaction activation process.62,63 Since the electron is preferentially transferred from the negatively charged site and the generated O2– is stabilized by the positively charged site due to Coulomb attraction, the activation energy of the oxidation reaction is considered to be lowered by the coexistence of the negatively and positively charged sites at the surface. Note that energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied MO (LUMO) for Al10B–, Al12B–, and Al11B2– are calculated to be 2.09, 3.15, and 2.66 eV, respectively (Figure S6), and it seems that the magnitude of the energy gap alone cannot fully explain the oxidative reactivity.

On the other hand, the reactivity of AlnB (7 ≤ n ≤ 11) is higher than that of Aln+1 with the same number of constituent atoms, although the B atom is encapsulated as an interior atom (Figure 7c,d). This result suggests that the O2– precursor is stabilized by positively charged Al atoms. Since the electronegativity of the B atom (2.0) is more than that of the Al atom (1.6),64 the charge separation with B atom substitution generates positively charged Al atoms. In fact, with an increase in size (n) of the AlnB NCs, the charge transfers from the B and Al atoms are widely dispersed within the NCs, resulting in the decreased stabilization of O2– by the surface positive atoms, which causes the decreased reactivity of the Al atoms. Moreover, as the NC size becomes larger than n = 10, the AlnB NCs can form a more closely packed structure, finally resulting in an icosahedral structure at n = 12; with an increase in size (n), charges are uniformly dispersed in the Aln cage, reducing the influence of B atom substitution on reactivity (Figure 7e).

The thermodynamics of the oxidative reactions involving Al12B–, Al11B2–, and Al6B– NCs with O2 reactants was theoretically analyzed, despite limiting the orientation of the approaching O2. Electronic energy diagrams were calculated to provide three oxidative states of adsorbed O2: single bond O2, dual bond O2, and dissociated O2, as shown in Figure 8. Energy diagrams also covering the reaction of Al13– + O2 are presented in Figure S7 in the Supporting Information. In all NCs, the initial formation of triplet intermediates occurs with the adsorption of one of the O atoms followed by intersystem crossing (ISC) from triplet to singlet states. In the singlet states, the O2 molecule further adsorbs with each O atom, leading to dissociation into the two O atoms, where the reactions at the singlet O adsorbates are irreversible.

Figure 8.

Figure 8

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Electronic energy diagrams (in eV) depicting oxidative reactions for sequential reactions of three nanoclusters of (a) Al12B–, (b) Al11B2–, and (c) Al6B– with O2 reactants: adsorbed O2 (single bond), O2 (dual bond), and two dissociated O, starting from O2 adsorption. The singlet and triplet states are colored blue and red, respectively. In all cases, the triplet intermediates are first formed with the adsorption of one of the O atoms followed by the intersystem crossing (ISC) from triplet to singlet (double triangles), subsequent adsorption of these two O atoms, and eventual dissociation of two O atoms. Once the singlet O adsorbates are formed, these reactions are irreversible. The rate-determining steps of these reactions are the ISCs, and the calculated barriers at the ISCs are 0.312 eV for Al12B–, 0.116 eV for Al11B2–, and 0.082 eV for Al6B–, although the exact barrier could be much larger than the values in the figure due to the spin–orbit interactions.

The calculations for oxidative reactions show that the rate-determining steps for these reactions are the ISCs. The calculated barriers at the ISCs for Al12B–, Al11B2–, and Al6B– are 0.312, 0.116, and 0.082 eV, respectively, which qualitatively explains the oxidative reactivities, the higher oxidative reactivities of Al11B2– and A6B–. As shown in Figure S8 in the Supporting Information, consistently, the Wiberg bond indexes65 show that the stronger B–O bond in Al6B– and Al11B2– to the B atom could result in the smaller energy differences between triplet O adsorbates and ISCs, i.e., smaller activation barriers of ISCs. Moreover, note that the relatively reduced oxidative reactivity of Al12B– (1.00) compared to that of Al13– (1.02) can be rationalized by considering the ISC barriers, as shown in Figure S7. To provide a more quantitative evaluation, the precise barriers should take into account the spin–orbit interactions and molecular orientations of the embedded NCS and approaching O2 as well as the support effect of organic HB-HBC substrates.

Conclusions

B atom-doped small Al NCs of AlnBm (n = 6–14, m = 1 and n = 11, m = 2) were size-selectively supported on an HB-HBC-decorated organic substrate, and their reactivity toward O2 molecules was quantitatively evaluated by measuring the XPS spectra for Al 2p. From the analysis of the B 1s spectra in AlnB NCs, the threshold size for the encapsulated B atom was determined as n = 7. The B atom encapsulation is consistent with the result of the DFT calculation, indicating that the structures of AlnBm deposited on HB-HBC are similar to those of AlnBm– in the gas phase. An analysis of the Al 2p XPS spectra shows that AlnBm (m = 1, n ≤ 11 and m = 2, n = 11) NCs show higher reactivity than Aln+m, for the same number of constituent Al/B atoms in both NCs. In particular, the reactivities of Al6B and Al11B2 are very high compared to those of other AlnBm NCs, suggesting that the exterior B atom activates the NCs by acting as an active reaction site. The calculations for the oxidative reactions show that the rate-determining steps are the ISCs from triplet to singlet of the O adsorbates. On the other hand, Al12B shows the lowest reactivity to oxidation because of the relaxation of the icosahedrally exohedral Al12 cage by encapsulating a smaller B atom, which geometrically deactivates the Al12B NC. Thus, the results for the B atom-doped Al NCs show that substitution by hetero elements can activate the NCs, but they can also deactivate them by reducing the ratio of the replaced atoms and developing specific endohedral structures. Evaluating their properties in surface-supported systems is considered a significant contribution beyond gas-phase isolated systems,50 toward realizing the long-anticipated theoretical predictions concerning cluster-assembled materials, as pointed out by Khanna and Jena.66 These results suggest that the heteroatom substitution can provide the means to optimally create NC-assembled nanomaterials and freely design their physical properties.67

Experimental Section

Sample Preparation

The sample substrate of deposited AlnBm NCs was fabricated on an organic HB-HBC surface under vacuum-keeping conditions. The NC generation system was described in detail elsewhere.68,69 The AlnBm– NCs were synthesized by magnetron sputtering with Ar+ ions on Al–B composite disk targets (2 in. diameter), 1.2 wt % for m = 1 or 3.2 wt % for m = 2. The synthesis conditions of sputtering power and gas flow rates were optimized to maximize the amount of AlnBm– NCs, and the produced NCs were guided to a quadrupole mass filter by a series of ion optics. By adjustment of the mass separation conditions to lower the mass resolution while allowing only NCs with a specific number of constituent atoms as large as possible, the mass-selected AlnBm– NCs were deposited on an HB-HBC decorated substrate. The collision energy of AlnBm– was controlled by applying a bias voltage to the substrate (typically +5 V) to satisfy the soft-landing conditions (<10 eV/NC). The number of deposited AlnBm– ions was counted to be 2.4–4.8 × 1013; the coverage of AlnBm on the substrate was estimated as 0.6 monolayer (ML), assuming a deposition area of 2.8 × 1013 nm2 (6 mm in diameter), and the size of AlnBm was estimated to be the same as that of Aln+1 calculated from a cubic-root interpolation in Aln NCs.36 Deposition times were highly dependent on the average ion currents of the targeted AlnBm– NCs; the deposition time and the average ion current were typically 2 h and 650 pA for Al12B– and 12 h and 180 pA for Al6B–. The deposited NC samples were transferred to another chamber for photoelectron spectroscopy while maintaining ultrahigh vacuum (UHV) conditions.

The HB-HBC substrate was prepared by the thermal evaporation of synthesized HB-HBC39−41 on a highly oriented pyrolytic graphite (HOPG) substrate cleaned by cleaving in an atmosphere and heated to 350 °C under UHV conditions (<2 × 10–8 Pa). The thickness of the HB-HBC film was controlled to 5 monolayers (MLs) by monitoring the growth speed with a quartz crystal microbalance.

Photoelectron Spectroscopy and O2 Exposure Experiments

XPS measurements were conducted by using Mg Kα (hν = 1253.6 eV) as the X-ray source. Photoelectrons emitted from the sample surface were collected with a hemispherical electron energy analyzer (VG SCIENTA, R3000) at a detection angle of 45° from the surface normal. In the XPS analyses, peak fitting was performed by considering the instrumental broadening of the peak as determined from the Au 4f peak profile (Voight function with a full width at half-maximum (fwhm) of 1.09 eV; the Gaussian and Lorentzian fwhm values were 0.75 and 0.56 eV, respectively).

To examine the oxidative reactivities of the deposited AlnBm NCs, the samples were exposed to oxygen molecules (O2). The amount of O2 exposure was defined in Langmuir units (L = 1.33 × 10–4 Pa·s). The O2 gas was introduced into the XPS system with a variable leak valve. All XPS measurements and exposures to the O2 were performed at room temperature.

DFT Calculation

Geometry optimization for Al7–, AlnB– (n = 6, 10, and 12), and Al11B2– NCs with singlet spin states and for Al7B– with doublet spin states was conducted by the DFT implemented in the Gaussian 16 program.70 As a hybrid exchange–correlation function and a basis set, PBE071,72 and 6-311+G(d) were employed for DFT calculations, and all equilibrium geometries were optimized until no imaginary frequencies were found. Population analyses were conducted with natural population analysis (NPA)73 and Wiberg bond index65 for the total electron density obtained at the same level of DFT calculations.73 Electronic energy diagrams depicting oxidative reactions for sequential reactions were also calculated for Al13–, Al12B–, Al11B2–, and Al6B– NCs between O2 reactants, where all the geometry optimizations and reaction path searches were performed using the GRRM program74,75 with the energies and energy derivatives with the Gaussian 16 program.

Acknowledgments

This work is partly supported by JSPS KAKENHI Grants-in-Aid for Scientific Research (A) No. 19H00890, for Challenging Research No. 21K18939, and for Transformative Research Areas (A) “Hyper-Ordered Structures Science” (21H05573).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c06191.

  • Charge state of AlnBm on HB-HBC (Note S1); C 1s XPS peak positions after AlnBm deposition (Figure S1); chemical state of AlnBm on HB-HBC (Note S2); XPS spectra for Al 2p of AlnB (n = 6–14) and Al11B2 on HB-HBC (Figure S2); oxidative reactivity for Al12B on HB-HBC (Note S3); XPS spectra for Al 2p of Al12B and reactivity (Figure S3); oxidative reactivity for Al11B on HB-HBC (Note S4); XPS spectra for Al 2p of Al11B and reactivity (Figure S4); oxidative reactivity for AlnB on HB-HBC (Note S5); XPS spectra for Al 2p of AlnB (n = 7–9, 13, and 14) on HB-HBC and reactivity (Figure S5); relative reactivity for AlnBm on HB-HBC (Note S6); values for the slope and relative reactivity of AlnBm on HB-HBC (Table S1); energy gaps between the HOMO and LUMO for Al10B–, Al12B–, and Al11B2– (Figure S6); electronic energy diagrams for O2 reactions of Al13–, Al12B–, Al11B2–, and Al6B– (Figure S7); Wiberg bond indexes of O–Al/B atom and O–O bonds in the triplet O2 adsorbates (Figure S8); calculated states and energies for AlnBm– NCs and their O2 adsorbates (Table S2); coordinates of Al13–, Al12B–, Al11B2–, and Al6B– NCs and O2 (Table S3); coordinates of O adsorbates of Al13–, Al12B–, Al11B2–, and Al6B– NCs (Table S4) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja3c06191_si_001.pdf (1.7MB, pdf)

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