Mercury Goes Solid At Room Temperature At Nanoscale: Toward An Effective Hg Waste Storage


 While room temperature bulk mercury is liquid, it is solid in its nanoconfiguration (Ønano−Hg ≤ 2.4 nm). Conjugating the nanoscale size effect and the Laplace-driven surface excess pressure, Hg nanoparticles of Ønano−Hg ≤ 2.4 nm embedded in a 2-D turbostratic BN host matrix exhibited net crystallization at room temperature via the experimentally observed (101) & (003) diffraction Bragg peaks of the solid Hg rhombohedric α-phase. The observed crystallization is correlated to a surface atomic ordering of 7 to 8 reticular atomic planes of the rhombohedric α-phase. Such a novel size effect on the phase transition phenomena in Hg is conjugated to a potential Hg waste storage technology. Considering the vapor pressure of bulk Hg, RT solid nano-Hg confinement represents a viable green approach for Hg waste storage derived from modern halogen-efficient light technology.


Introduction
Mercury is a unique metal that does not form diatomic molecules in the gas phase. Its bulk room temperature liquid property is correlated to its rare gas-like con guration (Xe) 6 s24f14d10 . More accurately, the relativistic contraction caused by the Dirac dynamics of the valence electrons (1). As a result of the relativistic mass increase m = m 0 /√(1-(v/c)), "v/c ~ 0.58", the radial shrinkage of the effective Bohr radius r 0 = (ε 0 h/m e e 2 ) of the inner "1 s" electrons is ~ 23%. Since the high-order "s" electronic shells have to be orthogonal against the lower shells, they will suffer a similar radius relativistic contraction, inducing a weak Coulomb interaction between neighboring Hg atomic sites.
Hg, as a singular liquid metal in its bulk form, has the highest elemental surface tension at room temperature, γ ~ 486 mN/m (2). The theoretical calculations on the liquid-vapor interface of simple metals in general (3)(4) and methods based on the jellium model in particular (5), and the perturbation expansion up to the second order in the surface "e-ion" pseudopotential (6-7), showed that excessive surface tension could stimulate a signi cant surface atomic layering of 3-5 atomic planes, as depicted in Fig. 1.a & the corresponding periodic surface-to volume electron density pro le. This surface atomic ordering, in full agreement with capillary wave theory, has been observed by X-ray re ectivity measurements on bulk liquid mercury surfaces by Pershan et al. (8). Likewise, Ba le et al (9)(10) showed that such atomic ordering was able to be segregated in bulk liquid mercury by examining the height and width in addition to the position of the main peaks of the static structure factor S(Q) under ambient conditions. Both X-rays and neutron diffraction S(Q) pro les revealed a structure up to 4-5 discernable peaks: a feature of local surface atomic ordering (10).
Such an RT surface atomic ordering observed on the at surface of bulk Hg could be enhanced signi cantly if not drastically on nano-Hg particles if one could engineer them. Indeed, as a result of their substantial surface/volume ratio and the 3-D symmetry breakdown, the surface atom population would be greater in nano-Hg. Henceforth, at such a scale, the surface phenomena dominate gravity effects in view of the signi cantly elevated surface tension of Hg ( Fig. 1.b). The enhanced surface ratio of nano-Hg of radius "Ø nano− Hg/2" should induce an excess of Laplace surface pressure ΔP ≈ 4γ/Ø nano−Hg of tens of MPa. As an estimation, if Ø nano−Hg ≈ 2.50 nm, ΔP ≈ 0.76 GPa at RT. Considering the mercury phase diagram of Fig. 1.c, such an excess surface pressure at RT should induce net crystallization out of the liquidus space to the solid a-rhombohedric phase (11) of nano-Hg ( Fig. 1.c). Hence, this atomic ordering phenomenon at RT should manifest itself through a signi cant crystallization of the liquid phase to the solid rhombohedral "α-type" phase.

1 Synthesis
For the rst 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 solidi cation 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(C 2 H 3 O 2 ) 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.
The ideal precursors for obtaining the BN matrix were ortho-boric acid "H 3 BO 3 " and urea "H 2 NCONH 2 ", while mercury acetate "Hg (C 2 H 3 O 2 ) 2 " was the optimal Hg precursor. The chemical reaction taking place was: While the initial H 3 BO 3 and H 2 NCONH 2 compositions were kept stoichiometric, the Hg (C 2 H 3 O 2 ) 2 was varied to obtain nano-Hg particles within the nal BN host matrix. The relative molar initial concentration to the BN matrix of Hg (C 2 H 3 O 2 ) 2 was varied accordingly. The smaller this molar concentration is, the smaller the nano-Hg size. Different solutions of H 3 BO 3 , H 2 NCONH 2 and Hg(C 2 H 3 O 2 ) 2 with molar fractions of 2,1 and ξ where "ξ" was varied from 1, 1/4 and 1/20 for Hg ( The corresponding samples are labeled Hg 1/1 -BN, Hg 1/4 -BN, and Hg 1/20 -BN. 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 Hg 1/1 -BN sample and freshly exposed to the incident electron beam, the Hg1/4-BN and Hg 1/20 -BN nanocomposites consisted of nanosized Hg isolated particles embedded in the BN host matrix. Their average diameter Ø nano−Hg at the early stage of electron beam exposure was estimated to be 3.83 and 2.41 nm in the Hg 1/4 -BN and Hg 1/20 -BN samples, respectively. Hg 1/1 -BN consisted of relatively very large Hg particles within the submicron range. Subsequent to the heat generated by the TEM electron beam, the primarily welldispersed 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 nal morphological state of Hg 1/20 -BN following an exposure duration of ∼14 s. Figure 2.b displays a slightly higher magni cation ultrashort exposure time of the Hg 1/20 -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 Hg 1/20 -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 Hg 1/ξ -BN nanocomposites were investigated by XRD. A noteworthy consideration was assigned to the Hg 1/20 -BN nanocomposite, as the TEM average size of the corresponding nano-Hg was Ø Hg TEM ∼2.4 nm. These encaged nano-Hg particles are undersized su ciently to undergo excess surface pressure above the threshold value of 0.76 GPa and hence would experience a diffraction feature. the liquid nitrogen (∼78 K) diffraction pattern of this latter (d), i.e., Hg 1/20 -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., Hg 1/1 -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. 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 Hg 1/1 -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 Ø nano−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 Hg 1/20 -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 Ø nano−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 Hg 1/20 -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 speci c 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 con rm 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 signi cant angular shift, with 3 new less intense Hg Bragg peaks tting with the Hg(110), Hg(104) and Hg(113) diffraction of solid a-rhombohedric solid Hg in addition to the presence of numerous BN turbostatic 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 Hg 1/20 -BN nanocomposite con rms the room temperature crystallization of the nonpercolated nano-Hg " Ø Hg 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 signi cant advance in the safe storage of Hg and minimization of its hazardous aspect, especially Hg waste derived from modern halogen e cient light technology systems.

Conclusions
A size effect in nanoscale Hg dispersed in a 2-D BN host matrix was observed at room temperature. Hg nanoparticles with a diameter smaller than the threshold value of 2.5 nm, as de ned by the P-T phase diagram, exhibit net crystallization manifesting itself through surface atomic layering of approximately 7-8 atomic layers. Below such a threshold value of 2.5 nm, Hg is solid at room temperature with an arhombohedric crystallographic structure with an average lattice parameter a ∼ 3.005 Å. Considering the vapor pressure of liquid bulk Hg, embedding Hg in its nanoscale form in chemically inert BN matrices could be of a signi cant advance in the safe storage of Hg and minimization of its hazardous aspect, especially Hg waste derived from modern halogen-e cient light systems. Figure 1 (a) Theoretical surface atomic ordering in liquid Hg with the corresponding in depth variation of the electronic density, (b) Schematic representation of volume & surface driven shape anisotropy in Hg droplets, c) Hg phase diagram (according to [11]), (d) Schematic representation of the Hg nanoparticles con ned in turbostratic BN chemically inert host matrix.

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