Chirality Affecting Reaction Dynamics of HgS Nanostructures Simultaneously Visualized in Real and Reciprocal Space.

Chirality involved reactions enable to probe features in the fields of asymmetric synthesis and catalysis, which allow to gain insight into the fundamental mechanisms of topochemically controlled reactions. However, in situ observation of the chirality-associated reaction dynamics with simultaneous structural determination of microscopic features has been lacking. Here, we report the direct visualization of the electron-beam-stimulated reaction dynamics of HgS nanostructures with chiral and achiral morphologies simultaneously in both real and reciprocal space. Under the electron-beam excitation of HgS nanostructures, the formation and evaporation dynamics of Hg nanodroplets were vividly pictured, while the reciprocal space imaging revealed the structural transformation from monocrystalline to polycrystalline. Interestingly, such induced changes were size dependent, which were slowed when involving the chirality in the nanostructures. The finding offers a fundamental understanding of topochemically controlled reaction mechanisms and holds promise of tuning asymmetric synthesis for catalysis-related applications.

wavelengths from ultraviolet to near infrared, the phase transformation from -to -HgS was observed by the micro-Raman spectroscopy and X-ray photoelectron spectroscopy methods 37 . Using the pumpprobe microscopy, both the -HgS and metallic liquid Hg were created by the ultraviolet light exposure of α-HgS 38 . However, in situ observation of such irradiation-induced chemical reaction process as well as its subsequent structural evolution in space and time has been lacking. Involving the chiral effect on the e-beam irradiation stability have been reported in multiwalled carbon 39 and boron nitride 40 nanotubes. Revealing simultaneously the chemical reaction dynamics in both real and reciprocal space, especially when involving chiral effect, is essential to provide an unambiguous understanding of chirality mediated reactions and phase transitions, which paves the way for controlling the chiralityassociated plasmonics and catalytic reactions.
Here, we report the direct observation of chirality-related chemical reaction dynamics of inorganic HgS nanostructures by using in situ transmission electron microscopy (TEM) simultaneously in real and reciprocal space. The technique enables the visualization of the formation and evaporation dynamics of liquid Hg nanodroplets as a result of the e-beam-stimulated chemical reduction reactions of Hg 2+ to Hg 0 , and allows the simultaneous determination of the structural transition from monocrystalline to polycrystalline. Interestingly and surprisingly, the HgS nanostructures, once involving chirality, present distinct reaction behavior under the e-beam excitation. The chiral nanostructures show dense surface steps, which are more tolerant against the e-beam irradiation than that of the achiral ones. Such e-beam-stimulated reaction behavior is size-dependent, namely, the tolerance reaches the highest as the size of the nanostructure is around 200 nm and then becomes weakened as the size increases. The chiral effect on the above phenomena is discussed. 5

Results
Synthesis and characterization of HgS nanostructures. The inorganic HgS nanostructures were synthesized using the seed-mediated epitaxial method with the growth rate controlled by a syringe pump (Fig. 1, top; see Methods, the left corner is an example of the as-synthesized product). The HgS seeds had a size of ~8-15 nm ( Supplementary Fig. 2), which were used for the subsequent epitaxial growth of various nanostructures. The nanostructures can crystallize in the chiral space group P3121 or its enantiomorph P3221. Shown in Fig. 1

Experimental implementation of reaction dynamics studies.
We investigated the chemical reaction dynamics of the HgS nanostructures with chiral and achiral morphologies in both real and reciprocal space using the in situ TEM (Fig. 2, left). An e-beam is employed to excite the specimen so that chemical reactions are triggered at targeted locations. The subsequent reaction dynamics are recorded simultaneously in real (structural imaging) and reciprocal (phase determination) space by a digital 6 camera. Fig. 2 (bottom right) shows the morphologies of the chiral HgS before (0 s) and after 300 s ebeam irradiation, respectively. In comparison with the initial one, the nanostructure after the irradiation presents significant changes of contrast and the occurrence of many dispersed nanoclusters with dark contrast (marked by the red arrow).
High resolution TEM image reveals that the nanostructure after the e-beam irradiation was still α-HgS phase, without the transformation to β phase ( Supplementary Fig. 4), which is different from that observed in the samples after the laser exposure 37 . It indicates that the behavior in this work might be associated with the chemical reactions. Because chemical species exhibit quite different mass loss rate, the overall chemical composition of the sample may change during the e-beam irradiation. Upon comparison of the EDX mapping results between the two states (0 and 300 s), the losses of both Hg and S were clearly observed after the irradiation ( Supplementary Fig. 5). The EDX spectra show that the intensities from both Hg and S elements decreased, where the ratio of Hg:S quantitatively changed from ~1:1 of the initial stoichiometric sample to ~1:2 of the e-beam-irradiated one (Fig. 2, upper right).
To understand the above phenomena, the reaction dynamical processes were in situ revealed in the following sections.
Reaction dynamics of chiral HgS nanobipyramids. The structural evolution dynamics of a single chiral HgS nanobipyramid were investigated during the e-beam exposure, as shown in Fig. 3 (Supplementary Movie 1). The nanobipyramid is ~60, 120 nm in width and length, respectively. For convenience, we neglected the time of the TEM alignment, and denoted the initial image as time zero (0 s). The image and its live fast Fourier transform (FFT) pattern indicate that the as-synthesized HgS is monocrystalline, corresponding to the trigonal structure (a= 4.145 Å, b= 4.145 Å and c=9.496 Å) 42 .
The direction along the length of the nanobipyramid is parallel to the [001] orientation. After the e- 7 beam excitation for 50 s, several Hg nanodroplets (~8 nm in size) were seen (marked by the dotted circles). The occurrence of the Hg nanodroplets might be due to the e-beam-induced chemical reaction of HgS. The nanodroplets gradually decreased their size during the e-beam irradiation, and finally vanished in about 31 s (Supplementary Fig. 6). With the prolonged irradiation (120 and 180 s), the nanovoids gradually grew and coalesced into the big ones. In comparison with the initial state, the diffraction spots became less and the intensity was weaker, indicating a lower symmetry of the HgS nanostructure. As the e-beam irradiation continued (240 s), the HgS nanostructure was severely destroyed and only several diffraction spots were still visible. Eventually, a hollow nanostructure with the twisting shell layer remained on the supporting film after the 300 s e-beam irradiation.

Size-dependent effect on reaction dynamics of chiral HgS nanobipyramids.
To understand the formation of the Hg nanodroplets, we further studied the HgS nanobipyramids with different sizes under the e-beam excitation. Fig. 4 shows the tendency of the stability of HgS nanobipyramids with respect to their sizes (length ranged from 80 to 400 nm; an example is displayed in the inset) under the same e-beam irradiation condition. The period from the initial state to the one that the morphology and corresponding FFT pattern of the nanostructure without change obviously during the irradiation was referred to the total reaction time in the figure. With increasing the size of the nanobipyramid, the total reaction time increased to ~1400 s for the nanobipyramid with the size of ~200 nm, and then decreased to ~400 s for the ~400 nm one (Fig. 4 changes of the morphology and the corresponding FFT pattern were observed in the ~200 nm nanobipyramid. In contrast, significant changes were seen in the nanobipyramids with other sizes. For 8 example, the hollow nanostructure was already formed in the ~120 nm nanobipyramid. For the large ones, a number of nanovoids appeared in the nanobipyramids with the sizes of ~240 and 340 nm. It suggests that the ~200 nm HgS nanobipyramid is more persistent to the e-beam irradiation than that with other sizes, resulting in the distinct chemical reaction rates observed above. It is noteworthy from the FFT pattern that the ~340 nm nanobipyramid presents obvious transformation from monocrystalline to polycrystalline after the irradiation for the same time.  Fig. 17), which was shorter than that for the chiral one (~31 s). The atomic force microscopy (AFM) results show that the surface of the chiral nanobipyramid is rougher than that of the achiral one (Fig. 5, bottom). Moreover, the surface step heights of the chiral and achiral nanobipyramids are estimated to be ~10 and 20 nm, respectively, indicating the denser surface steps on the chiral samples ( Supplementary Fig. 18). It 9 suggests that the chirality-affected chemical reaction dynamics might be associated with the surface characteristics of the nanostructures.

Discussion
Generally, sample damage caused by the e-beam can be through either elastic scattering such as knockon atomic displacements or inelastic scattering including specimen heating and radiolysis. It has been reported that continuous ultraviolet light exposure can generate Hg droplets through local heating of HgS 38 . Besides, for pure α-HgS sample, the phase transformation to β-HgS occurs at ~673 K and is completed at ~698 K 43 . Under the e-beam irradiation shown here, however, the e-beam-induced temperature rise is no more than 1 K 44 (see detailed discussion in Supplementary Information including Supplementary Fig. 1 and Table 1). This suggests that heating is impossible to cause phase transformation and has negligible effect on the formation of Hg nanodroplets.
It is seen from Figure 2 (top right) that the loss of Hg (higher atomic mass) is faster than that of S after the e-beam irradiation, suggesting that the knock-on mechanism is not the major factor for the sample damage. With the exposure to high-energy e-beam, the ionizing radiation (radiolysis) would cause the breakage of chemical bonds in materials. Based on these considerations and the experimental results, a pictorial understanding of the e-beam-induced chemical reactions in HgS system is illustrated in Fig. 6 (top). Under the e-beam stimulus, the cleavage of Hg-S chemical bonds and the creation of vacancies readily occur, which may induce the reactions of the reduction of Hg 2+ to Hg 0 and the oxidization of S 2to S 0 . The Hg 0 atoms diffuse and aggregate so that the Hg nanodroplets nucleate and grow. As seen from the FFT patterns ( Supplementary Fig. 7-16 harder for the transition when 1 < < 2 since ∆ > ∆ . As observed in Fig. 4, the high irradiation tolerance occurred in the HgS nanobipyramids with the sizes of 180~220 nm provides the information of size-dependent stability in HgS nanostructured system during the irradiation. 11 Chirality has a significant role in the surface morphology of HgS nanobipyramids, resulting in the distinct dynamics observed during the e-beam stimulus (Fig. 5). nanodroplets on the chiral (~31 s, Fig. 3 and Supplementary Fig. 6) and achiral (~12 s, Supplementary   Fig. 17) HgS surfaces, the contact angles were estimated from eq. (8-10) to be ~126° and ~66°, respectively. At these contact angles, the total free energy of the Hg nanodroplet was 2.31 eV for the chiral morphology while it decreased to 0.55 eV for the achiral one. An estimate of the nucleation rate on a surface with contact angle of ~66° about 10 29 times higher than that of ~126° surface from eq. processes. This approach has the potential for unique applications in chemistry and materials science, providing the key parameters accounting for the transient behaviour of topochemically controlled 14 reactions at the nanoscale.

Synthesis of α-HgS seeds
In h under stirring. The product was collected by centrifugation at 6,000 rpm for 10 min, washed three times using isopropanol, and finally dispersed in 5.0 mL of deionized water.

Seed mediated epitaxial growth of α-HgS with chiral and achiral morphology
The epitaxial growth of α-HgS with chiral morphology was performed using a syringe pump at the specific growth temperature (Figure 1, top).