For the first time, the synthesis of spatially isolated nano-Hg was demonstrated to be crystalline if its size was within the range of 2–5 nm. Such solidification was attributed to a size effect conjugated to the corresponding Laplace pressure excess. Apart from the safety aspect, the synthesis of nano-Hg was by itself an utmost challenge. The considered precursor was mercury(II) acetate Hg(C2H3O2)2. However, the foremost additional complexity remains in keeping the nano-Hg separated from each other; otherwise, the van der Waals/Otswald ripening type-induced agglomeration of the nano-Hg particles will generate larger Hg droplets and hence less surface pressure excess than the required threshold crystallization value of ≈ 0.76 GPa at RT. As schematically displayed in Fig. 1.d, a 2D boron nitride (BN)-isolated host matrix was used to prevent the coalescence process of nano-Hg. The deliberate choice of such a host matrix is its chemical inertness with Hg and its superior mechanical strength in addition to its 2-D structure.
2.2 Morphology & electron transmission studies
Figure 2.a reports a TEM of the Hg1/20-BN nanocomposite. The voltage/exposure time was shortened drastically (〈〈 20 secs) to minimize the agglomeration of nano-Hg. The observed rapid coalescence phenomenon was inherent to the insulating state of the nonpercolated Hg-BN nanocomposites and hence due to the lack of electronic charge and heat dissipation. Excluding the Hg1/1-BN sample and freshly exposed to the incident electron beam, the Hg1/4–BN and Hg1/20–BN nanocomposites consisted of nanosized Hg isolated particles embedded in the BN host matrix. Their average diameter 〈 Ønano−Hg〉 at the early stage of electron beam exposure was estimated to be 3.83 and 2.41 nm in the Hg1/4-BN and Hg1/20-BN samples, respectively. Hg1/1-BN consisted of relatively very large Hg particles within the submicron range. Subsequent to the heat generated by the TEM electron beam, the primarily well-dispersed nano-Hg began to coalesce promptly upon exposure to the electron microscopy beam even if the latter was kept at the minimum voltage possible. The TEM pattern of Fig. 2.a corresponds to the final morphological state of Hg1/20-BN following an exposure duration of ∼14 s. Figure 2.b displays a slightly higher magnification ultrashort exposure time of the Hg1/20-BN sample. If the Hg nanoparticles are almost quasi-spherical in shape with substantially truncated interfaces. The size polydispersity rose promptly subsequent to the slightly higher electron beam intensity. The new apparent diameter of the Hg nanoparticles ranges from 1.5 to 28.9 nm for the Hg1/20-BN sample. Few larger distorted nano-Hgs of ∼63–70 nm in diameter are also observed. This could be congruent with sample zones that were exposed to noteworthy heat from the probing e-beam.
2.3-Crystallographic & phase transition investigations
Thereafter, the Hg1/ξ-BN nanocomposites were investigated by XRD. A noteworthy consideration was assigned to the Hg1/20-BN nanocomposite, as the TEM average size of the corresponding nano-Hg was 〈ØHg〉TEM∼2.4 nm. These encaged nano-Hg particles are undersized sufficiently to undergo excess surface pressure above the threshold value of 0.76 GPa and hence would experience a diffraction feature.
Figure 3 displays the room temperature XRD profiles of Hg1/1-BN (a), Hg1/4-BN (b) and Hg1/20-BN (c) and the liquid nitrogen (∼78 K) diffraction pattern of this latter (d), i.e., Hg1/20-BN at ∼78 K. As shown in Fig. 3.a, excluding the (121) Bragg peak of the BN-t host matrix, the sample with the highest Hg concentration, i.e., Hg1/1-BN does not exhibit any Bragg peak structure proper to mercury but rather a wide amorphous bump and a very broad peak extending over 10 Deg (40–50 Deg). These are signatures of an amorphous liquid without any long- or mid-range crystalline order.
Figure 3.b displays the diffraction pattern of the second lowest Hg concentration, i.e., Hg1/4-BN. It exhibits 3 narrow diffraction peaks assigned to the BN-t host matrix (410), (132) and (203) Bragg peaks “ASTM Card 18–0251” (34). In addition, there is an intense but broad Bragg peak centered at 2Θ ∼ 32.719 Deg. This peak with a width at half maximum of ΔΘ ∼ 6.30 10− 2 rad, can be assigned only to crystallized mercury, more precisely to the a-rhombohedric (101) reticular orientation “ASTM Card 09-0253” (35). Compared to the diffraction pattern of Hg1/1-BN, yet broad, such a Bragg peak could be considered a signature of preliminary atomic ordering exhibited mostly by surface mercury atoms within the nonpercolated encaged nano-Hg. Using the Debye-Scherrer approximation for this Hg (101) Bragg peak, the average size of the corresponding Hg nanoparticles is 〈 Ønano−Hg〉 DS ∼ 2.43 nm. It is likely that such atomic-like ordering would originate from the surface atoms of the nano-Hg population and those with a smaller diameter according to the phase diagram of Fig. 1.c.
To conclusively corroborate the existence of this Hg (101) Bragg peak with surface atomic layering, the Hg1/20-BN nanocomposite was examined extensively at both 293.5 (Fig. 3.c) and 78 K (Fig. 3.d). As it is the sample with the smallest mercury volume concentration, the corresponding nano-Hg with an average diameter of 〈 Ønano−Hg〉TEM ∼2.4 nm according to the TEM measurements would display the largest surface/volume ratio. The relative Hg (101) intensity should be superior for the same Hg volume concentration. As illustrated in Fig. 3.c, not only is the relative intensity of the Hg (101) Bragg peak is relatively larger for the Hg1/20-BN nanocomposite but there is an additional Bragg peak centered at 2Θ ∼ 39.695 Deg. Figure 4.a & its inset zooms explicitly (Fig. 4.b & Fig. 4.c) on this additional diffraction peak with a width at half maximum of ΔΘ ∼ 3.580 Deg. Taking into account both its angular position and the relative intensity to the Hg (101) peak and the specific turbostratic structure of the host BN matrix (12), this Bragg peak could only be assigned to the 2nd intense crystalline Hg Bragg peak, i.e.. The Hg (003) crystallographic orientation of rhombohedric Hg α-phase “ASTM Card 09-0253”. To confirm that the indexed Hg (101) and Hg (003) are proper mercury Bragg peaks originating from the atomically ordered nano-Hg embedded in the BN-t host matrix, the sample Hg 1/20-BN was cooled to ∼78.0 K. The labeled Hg (101) and Hg (003) develop sharper peaks with a significant angular shift, with 3 new less intense Hg Bragg peaks fitting with the Hg(110), Hg(104) and Hg(113) diffraction of solid a-rhombohedric solid Hg in addition to the presence of numerous BNturbostatic diffraction peaks (Fig. 4a & zoom inset). Therefore, the coexistence of the two Bragg peaks, namely, Hg(101) and Hg(003), in the room temperature diffraction pattern of the Hg1/20-BN nanocomposite confirms the room temperature crystallization of the nonpercolated nano-Hg “〈ØHg〉TEM ∼2.4 nm” within the BN-t host matrix. These experimental observations support a surface atomic layering consistent with even 7 to 8 atomic plane ordering (〈a〉∼3.005 Å), as summarized in Fig. 4.d (13). Because of the vapor pressure of bulk Hg, embedding Hg in its nanoscale form in chemically inert BN matrices could be a significant advance in the safe storage of Hg and minimization of its hazardous aspect, especially Hg waste derived from modern halogen efficient light technology systems.