Graphite and Bismuth Selenide under Electrical Explosion in Confined Environment: Exfoliation, Phase Transition, and Surface Decoration

Electrical explosion, characterized by ultrafast atomization and quenching rate (dT/dt ≈ 1010–1012 K s–1) of the sample, is a unique approach for “one‐step” synthesis of nanomaterials. Experiments are carried out with layered graphite and Bi2Se3 under the action of electrical explosion in a confined reaction tube. High‐speed photography and electrophysical diagnostics are applied to characterize dynamic processes. SEM and EDS are used to characterize surface micro‐morphology of reaction products. The layered materials are first exfoliated to thin nanosheets/nanocrystals by shock waves and turbulent flow of the explosion. As the ionized explosion products (>10 000 K) contacts the sample, intense heat transfer happens, simultaneously atomizing the sample and quenching the plasmas. As a result, nanoparticles grow on the surface of thin sheets, forming “dot‐sheet” structure. The size distribution of the nanoparticles typically ranges from 10 to 100 nm, following Log‐normal distribution. The dotted graphite nanosheets gather together and form a stacked/cabbage‐like structure. By contrast, Bi2Se3 case accompanies with chemical reactions, causing surface corrosion and showing more possibilities: nanocrystals and nanotubes growth on different areas of the sample.

Recently, more and more synthesis method assisted by plasma technology has been reported. The introduced plasma brings the extra sources of electron/ion, heat/radiation, and even shock waves (SWs), leaving more luxuriant reaction products. Huang et al. adopted plasma-liquid technique to achieve scalable production of few-layered high-quality phosphorene with a fast speed (plasma treatment of ≈5 min). [17] Besides synthesizing in liquid phase, Yi et al. reported the technology of plasma-enhanced chemical vapor deposition (PECVD) for 2D material preparation, where the tubular furnace is collaborated with an ICP discharge plasma source. [18] However, the plasma sources mentioned above are usually mild and rely on the scheme of chemical reactions. Then we may ask: shall the physical effects of discharge plasmas be utilized for promoting the synthesis process? In fact, Luong et al. made a groundbreaking work to transfer the low-value carbon to graphene with the help of pulsed intense heating of arc plasma, which is called "flash graphene." [19,20] Adding a fine conductor between the two electrodes and applying pulsed discharge could generate stronger plasmas (>10 000 K) than the normal arc plasma (≈3000 K), along with strong outward-propagating shock waves, which is called the "electrical explosion." Wang et al. proposed a novel method of exploding graphite powder in a semiclosed tube with an intense pulsed discharge; as a result, nanosheets, fewlayer graphene, and Ag/C composite coatings were obtained. [21] Qiao et al. reported a transition from red phosphorus to BP powder via the compression of SW. [22] Mei et al. used conjoint treatment of acid plasma for micromachining carbon fibers. [23] In our previous research, a Cu wire is exploded in a confined tube with pre-placed graphite powders. [24] Therein, powders are under the action of SWs, plasma radiation, and finally enrolled into the plasma flow turbulence. Finally, after the action of intense discharge for ≈50 µs, the graphite powders become Cunanoparticle decorated thin nanosheets.
Typically for layered 2D material graphene, based on the above-mentioned researches, traditional methods can be roughly divided into two categories, namely "top-down" and "bottom-up." Although with development and improvement of these methods, there still have some limitations, including the production efficiency, quality, and cost. Meanwhile, pulsed plasma sources like electrical explosion method have been confirmed as useful tool for producing few-layers even monolayer graphene. [25,26] The exfoliation mechanism can be considered as a kind of "top-down" mechanism, which benefit from its considerable mechanical effects (shock wave). Like other traditional mechanical methods, the obtained graphene possesses relatively high quality. Noticeably, the whole process of the electrical explosion exfoliation can be fulfilled within µs to ms timescale, and the production efficiency is significantly improved through the "one-step" method. However, there still exist some limitation, due to the intrinsic nature of electrical explosion, it is difficult to control the middle processes which largely influence the quality of graphene. Therefore, it is necessary to further investigate the exfoliation mechanism for 2D layered materials.
In this work, typical layered materials of graphite and Bi 2 Se 3 were selected as the research object. A sealed reaction cavity (tube) was designed to facilitate the interaction between the exploding wire and raw material. Then, a series of experiments were designed and conducted to explore the in-depth reaction mechanisms. Electrical parameters and spatial-temporal resolved images were investigated to analyze the dynamic process inside the tube. Also, the reaction products (RP) after the explosion were characterized by SEM, EDS, TEM, XRD, and XPS analyses. Finally, the vital processes for the layered materials, including exfoliation, phase transition, and surface decoration, under the action of electrical explosion are ascertained, and main conclusions are proposed.

Electrical Explosion Device and Diagnostics
A 6 µF pulse capacitor charged up to positive 9.0 kV (about 250 J stored energy) was connected to a coaxial triggered switch to provide a microsecond timescale 10-20 kA current pulse, which passed through the metal wire to drive the wire explosion. A more detailed description of the equipment may be found elsewhere. [27] The schematic of the experimental setup and configurations are illustrated in Figure 1.
The load structure, as illustrated in Figure 1b), included an exploding copper wire and samples (bulks or powders) waiting for treatment which were placed in a polycarbonate (PC) tube with size of 12 mm diameter and 4 cm length, the tube was intact after explosion, and the sample can be directly collected (for bulks) or scraped from the tube wall (for powders).

Electrophysical Parameters
In the experiments, the wire is exploded in the sealed tube. The voltage drop U along the tube and current I through the tube were measured with a PVM-5 probe (bandwidth of 80 MHz) and Pearson 4997 coil (bandwidth of 15 MHz). The correctness of voltage and current measurement was examined by conducting the pulsed current with a low-inductance (nH level) ceramic resistor. [28] Then, resistive voltage drop U R of the load could be estimated by where U is the measured voltage and I is the circuit current. L S refers to the inductance of the load structure (obtained by an LCR bridge), and L W is the inductance of the wire. From monitoring the dynamic voltage and current, the stage of the electrical explosion can be divided and deposited energy calculated.

Optical Parameters
A high-speed camera (Photron, 220,000fps) was used to record dynamic process of the explosion, with spatial resolution <100 µm and an exposure time of 1 µs (per frame). Both selfemission and backlit images (via a continuous 200 W LED light) were taken for different purposes. Lens and filters were used to adjust the sampled light to a moderate level. The discharge and its diagnostic system were synchronized by a digital delay generator. All waveforms were recorded by a Tektronix DPO 4104B (bandwidth of 1 GHz) digital oscilloscope. The timing accuracy of synchronization, in this case, was less than 50 ns.

Characterization of Reaction Products
Several methods were used to characterize the morphological and crystallographic characteristics, including SEM (Regulus 8230), EDS, TEM/HETEM (FEI Talos F200X G2), XRD (Rigaku, Ultima IV) and XPS (PHI QUANTERA-II SXM). Therein, XRD pattern of the sample was taken using Cu Kα radiation (λ = 0.15406 nm) at the working voltage and current of 40 kV and 40 mA, and the recorded range at 2θ was from 5° to 80° with a step size of 0.02°. Micromorphology of the sample was obtained by SEM and TEM at an accelerating voltage of 5 kV and 200 kV, respectively. Before sample characterization, it was stored in the vacuum drying chamber to avoid oxidation as much as possible.

Statistical Analysis
Experiments related with electrical explosion were repeated 3-5 shots to ensure the reproducibility. The uncertainty of the electrophysical measurements was typically <5%. Besides, the statistical analysis of results was shown in scientific method, mainly refers to the nanoparticle distribution in this paper. The software for the statistics and curve fitting was based on Orig-inPro Learning Edition. www.advmatinterfaces.de

Experimental Methods
The arrangement of the experiments is shown in Table 1.

Electrical Explosion with Graphite Powder: Dynamic Process and Product Characterization
A 100 µm diameter and 4 cm length copper wire were installed in a 12 mm diameter PC tube with preplacing graphite powders. The electrical parameter waveforms and temporal-spatial resolution images were shown in Figure 2. The feedback signals of the camera were illustrated with blue lines. Pulses represent the shooting moment of each frame.
From the voltage and current waveforms, the explosion belongs to a periodical mode discharge due to the relatively smaller wire mass and sufficient stored energy. At early phase change stages, the current increases quickly because of the extremely high energy injection and the moderate variation of resistance. Until volumetric vaporization occurs which occurs at 0.7 µs, the current begins to decrease, correspondingly, the voltage drastically rises. With the decreasing current, the constraint caused by the Lorentz force is weakening, and the "phase explosion" happens, which is characterized by the drastic expansion of discharge channel; as a result, the arc-like breakdown occurs due to the increasing pd value. [29] The whole process lasts approximately 0.3 µs, and voltage reaches the peak value 17.8 kV. Sooner, breakdown occurs in the expanded metal vapor and a high-conductivity plasma channel is formed. The residual stored energy consumes in it in the form of RLC oscillation. Combined with the high-speed camera, the first frame is taken after breakdown, and the plasma has fully filled with the tube at this time. The graphite powders initially located at the bottom of tube are rolled up after explosion, manifested as the black shelters in high-speed images. The rapidly changed pressure and temperature field caused by explosive shockwave and plasma drive the movement of powders and make strong interaction between powders themselves and tube wall. The discharge ends at approximately 28 µs with the termination of current. The subsequent light emission from the channel is due to the high-temperature radiation of the metal vapor and undergoes the quenching process later. The cooled vapor or nucleated particles will have the secondary interaction with graphite powders and play a role in exfoliation and surface decoration.
The morphology of the explosive products and the copper particle size distribution are shown in Figure 3. From the SEM results, multilayer nanosheets were obtained with spherical copper nanoparticles adhering on it. The products characteristics are identical to former research. [24] Strong interactions occur between graphite powders and copper vapor or nucleated particles. Extremely high-temperature and intense flow field caused by explosion-induced SWs partly exfoliated the graphite into nanosheets. Figure 3a shows the SEM results of the macroscopic morphology of the treated graphite samples by electrical explosion. The top left corner of the first photograph shows the original graphite samples who are at the size of several micrometers and thickness of submicron. Obviously, after treatment, the graphite breaks into small-scale nanosheets with only small parts of micron-sized samples left. The middle picture gives an amplifying view of the composition. The exfoliated graphite nanosheets are stacked into multi-layered sheets. The size of  www.advmatinterfaces.de the treated nanosheets is discrepant and the curly and cracked structures are observed which should be related to the intense impaction of exploded shock waves. Some spherical copper nanoparticles formed by quenching and nucleating from the vapor state of the exploding wire adhere on the surface of the nanosheets randomly. The last photograph gives a typical morphology of the Cu/C composition; moreover, the statistic of copper particle sizes is made with more than 200 numbers. The statistic result is shown in the top left corner and the red line of particle size distribution is a fitting curve by using the lognormal distribution function, which is expressed as follows: where f(d) represents the log-normal distribution, D 50 is the median diameter, d is the particle diameter, n i is the number of particles with diameter d i , and σ g is the geometrical standard deviation, respectively. Particle size distribution fits well with log-normal distribution whose D 50 and σ g are 40.3 and 1.94 nm, respectively. The result is in perfect accord with former researches about synthesis nanoparticles via electrical explosion of wires in atmospheric air or inert gas ambiences which implies that the combination of copper nanoparticles and graphite nanosheets tends to be a mechanical and physical modality to some extent. Copper vapor quenches and nucleates in the free air within the tube or partly on the surface of graphite nanosheet; whatever, we deduce that no chemical reactions happen during this process. TEM and HRTEM results in Figure 3b give more details about the exfoliated products. After treatment, the thickness of the typical graphite nanosheet is about 12.8 nm which is composed by less than 40 individual monoatomic graphene layers with the interlayer spacing is 0.335 nm. A typical morphology is observed that the spherical copper nanoparticle with diameter only 21 nm adheres on the graphite nanosheet. The lattice spacing is 0.207 nm corresponding to the plane (111) of face-centered cubic Cu phase. The wrinkles composed of only 4 individual monoatomic graphene layers occur on the base nanosheet. The intense mechanical effect caused by the metal plasma turbulent flow should be responsible for such defect; meanwhile, considerable exfoliation effect is achieved. In conclusion, from the morphology results of the composite, we consider that the exfoliation of graphite samples mainly results from the intense mechanical effect (shock wave); moreover, the products of the exploding wire form nanoparticles and decorate on the surface of the graphite nanosheets base. Figure 4a shows the XRD results of the original and aftertreated samples, the original graphite powders have an intense and narrow diffraction peak at 2θ = 26.49° corresponding to the (002) crystal plane of graphitic carbon. After treatment of electrical explosion, the peaks of graphitic carbon are still alive with almost no shift which means the crystallographic structure is well preserved. The intensity of the (002) peak after treatment is weaker than that of the original sample which indicates that there might be some amount of the multilayer sheets after exfoliation; furthermore, the (004) peak is almost disappeared which implies that its sublattices almost completely exclude the long-range order greater than four layers. [30][31][32] These results suggest that exfoliation occurs to the raw graphite samples. Several diffraction peaks located at 43.28°, 50.42°, and 74.12° can be assigned to the (111), (200), and (220) planes of face-centered www.advmatinterfaces.de cubic (FCC) pure Cu, respectively. It has to say that even wire explodes in air, almost no obvious peaks are detected for oxides of Cu. As for XPS results in Figure 4b, from the survey spectrum in the first photograph, only Cu, C, and O elements are detected. The C 1s spectrum is deconvoluted into four peaks at 284.0, 284.5, 286.3, and 288.1 eV, corresponding to CC, CC, CO, and CO peaks, respectively. [33] The peaks located at 932.78 and 952.48 eV in the Cu 2p spectrum are from Cu 2p 3/2 and Cu 2p 1/2 , and the existence of pure metallic Cu can be proven by the two peaks. Due to the Cu wire explosion happening in air, the Cu is slightly oxidized into CuO, the satellite peak lies at 954.48 eV, indicating the appearance of Cu 2+ ; [34] however, because of its small amount, it is not detected in XRD results. The O 1s spectrum consists of two parts: the two peaks at 532 and 533 eV are indexed to CO and CO, respectively. Moreover, the remaining peak at 530.1 eV originated from CuO. [35] Combined with the morphology and crystallographic characterizations, the raw graphite sample treated by electrical explosion mainly includes two parts of effects, namely mechanical impacts (exfoliation) and physical deposition (surface decoration). Due to the chemical stability of graphitic carbon, no reactions occur between copper element, and it can also be verified by the subsequent XRD and XPS results.

Treatment with Bi 2 Se 3 Bulk-Sample under a Strong or Weak Plasma Process
Experimental results indicated that the treated sample placed in the tube had no effect on the explosion and discharge process; therefore, the electrical parameter waveforms and high-speed camera images are not repeatedly given in this section. The morphology of decorated products and elements distribution are more concerned. Considering the possible chemical reaction of Cu plasma and Bi 2 Se 3 , we introduced a by-pass branch for the exploding wire load, as seen in Figure 1b. When using this branch made of W wire, the plasma process inside the reaction tube will be largely weakened. And the influence of the discrepancy of interaction intensity is only considered for bulk-sample treatment which is illustrated in SEM results in Figure 5. Figure 5a shows the typical morphology and elements analysis of the Bi 2 Se 3 sample bulks treated by exploding a Cu wire in the tube. The final products are similar to the results when treatment with graphite samples. The Bi 2 Se 3 sample bulks are cracked into µm scaled plates and Cu nanoparticles formed by the exploding wire adhere on the surface, covering nearly all of the sample, even in the edge area. Cu particles are slightly agglomerated under the strong plasma process which can be seen in first photograph in the second row, the statistic of particle size is also made in the left bottom picture. It also fits a lognormal distribution, and the median diameter D 50 and geometrical standard deviation σ g are 54.4 and 1.55 nm, respectively. In the black dashed box in Figure 5a, new structure shown as nanotubes is observed which should result from the intense interactions between sample and exploding wire. Figure 5b gives results after treatment when adding extra shunt branch of W wire which means a weaker plasma process and interaction. Typical morphology manifested as the Bi 2 Se 3 sample base and the deposited Cu nanoparticles is also observed. In the last two photographs of SEM results in Figure 5b, between the cracked gaps, some polygonal submicron-sized particles are observed. From the corresponding EDS results, the polygonal particles seem to be composed by both Cu and Se, and new compound may form based on the two elements during treatment.
In conclusion, the effects of cracking, exfoliation as well as the deposition of nanoparticles (surface decoration) are common, and they all just come from the mechanical and www.advmatinterfaces.de physical effects in such treatment method of electrical explosion. Furthermore, due to the discrepant characteristics of material itself, novel structure like nanotubes and chemical reaction forming new CuSe compound occur when treated with Bi 2 Se 3 sample bulks, which not happens in graphite samples; however, it has to say that the above phenomena only happen locally, and they are not dominant. Discharge process during explosion is always inhomogeneous because of plasma and fluid instabilities which result in discrepant interactions between treated samples with different intensities (stronger or weaker); moreover, the effects inflicted on the bulks sample should be uneven and locally. Therefore, the Bi 2 Se 3 sample powders pre-grinded by bulks that possess larger reacting specific surface area was used to further investigate in the next 3.2.2 sections.

Treatment with Bi 2 Se 3 Powder-Sample (Larger Reacting Specific Surface Area) under a Strong Plasma Process
Pregrinding the Bi 2 Se 3 bulks into powders will give larger reacting specific surface area which is easy to investigate the possible reactions occurred during treatment. From the SEM images in Figure 6a, the morphology is totally different from the above results. Plate-like morphology of the raw samples (top right corner) almost disappears after treatment; instead, three typical characteristics are observed, namely submicronsize particles, nanoparticles, and nanotubes which can be seen in the first photograph. Furthermore, the nanotube structure is prevalent, and it verifies that the ground powder sample indeed has more intense interactions than bulk sample. Combining with element distribution mapping results in Figure 6c, the yellow dashed box shows a typical selected submicron-size particle, it clearly indicates that the particle composes of both three elements. From TEM results in Figure 6b, more details can be found. One end of the nanotubes always links with nanoparticles, and some tiny particles conglutinate on the tubes. The typical size of the tubes is only several nanometers. From the HRTEM result in yellow dashed box, the nanotube shows the lattice spacing of 0.4777 nm corresponding to (006) plane of Bi 2 Se 3 . Red dashed box shows the high-resolution image of the spherical nanoparticles, it seems that several lattice orientations are included in the particle.
To sum up, when using the ground powder as raw sample that possesses larger reacting specific surface area, the reactions occur drastically, and both new structure and compounds

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form. Due to the crystalline characteristic of the nanotubes still meeting Bi 2 Se 3 , for example, the same lattice spacing of a certain plane; therefore, we deduce that the nanotubes result from the strong mechanical effect of exploded shock wave. On the other hand, the rest structures may be partly formed as compound composed of the Cu with Bi and Se elements.
To confirm the crystallographic structure of the treated products under above three conditions (original samples, electrical explosion treatment with bulk-and powder samples), XRD patterns were analyzed in Figure 7. The XRD pattern of the original sample exhibits well-defined diffraction peaks of hexagonal Bi 2 Se 3 , with R-3m space group (JCPDS #33-0214). After treatment with the bulk sample, the diffraction peaks are still alive and in accord with the original sample, it still maintains as bulk, and decoration/reaction only occurs on the surface; therefore, the main phase still is pure Bi 2 Se 3 . Though Cu incorporated compound is detected from SEM and EDS results in Figure 5, the corresponding diffraction peaks of these phases are not obvious even missing. It may attribute to the reactions occur locally and inconsiderably, which has been explained in Section 3.2.1. Weak peak at 43.86° is detected under the situations of both bulk-and powder-samples which corresponds to the (111) plane of metallic Cu. When using powder samples who possess larger reacting specific surface area, the characteristics of the diffraction peaks are totally different. First, the intensity of peaks decreases drastically which is one or two orders of magnitude smaller than that of other two situations; moreover, the peaks are diffused which may indicate that the treated sample has poor crystalline order and exists  www.advmatinterfaces.de like amorphous. We deduce that this may attribute to the prevalence of the nanotubes. [36] XPS was used to characterize materials by way of identification of the elements and valence state. The condition of treated powder sample under strong plasma process (without a W shunt branch) is selected due to it can represent a comprehensive effect of treatment by electrical explosion rather than locally. As illustrated in Figure 8, from the survey spectrum, clearly peaks for Cu, Bi, Se, C, and O are observed, the C and O elements should originate from reaction or deposition with the ingredients of air, like CO 2 , O 2 , or H 2 O. For the highresolution Cu 2p, the peaks located at 932.78 and 952.48 eV are from Cu 2p 3/2 and Cu 2p 1/2 ; furthermore, no other obvious peaks are detected in Cu 2p spectra which indicates that there is an absence or insufficiency of chemical bond between copper and other elements. In high-resolution spectrum of Bi 4f, two strongest peaks at 158.3 and 163.6 eV correspond to Bi 4f 7/2 and Bi 4f 5/2 , respectively. The minor fitting peaks centered at 160.0 and 165.0 eV may show slight oxidation of the Bi 2 Se 3 . [37,38] The Se 3d spectrum can be deconvoluted into two peaks whose binding energy at ≈54.14 and 54.90 eV is attributed to Se 3d 5/2 and Se 3d 7/2 . The Bi 4f and Se 3d levels display a splitting of 5.3 eV and 0.76 eV, respectively, which is in well agreement with the previous reports. [39][40][41] Compared to the peak positions in the case of elemental Bi of Se, small chemical shift occurs (1.3 eV blue shift for Bi, and 1.4 eV red shift for Se), which may be caused by the SeBi bond and the charge transfer from Bi to Se. [42] In conclusion, after treatment, the majority of materials still maintain original component, and it also accords with the XRD pattern that most of the phases are composed of Bi 2 Se 3 and metal Cu. A fraction of oxide of Bi forms in this process; however, the evidences of existence of compounds are not found from XRD and XPS, which may be due to their small amounts. In addition, we can deduce that the prevalent nanotubes should be formed by the re-construction of Bi 2 Se 3 which is caused by the intense mechanical effect of the exploding wire.

Discussion
From the results above, it could be seen that the composites generally follow the pattern: metal nanoparticles distributed on the exfoliated thin layers of 2D materials, called the "dot-sheet" structure. The results lead us to the image that the raw material is bathed in a metal plasma environment, so the metal vapor will be quenched and coagulated on the surface of the treated material. It seems like a very simple mechanism, like a PECVD device, but a deeper analysis may reveal more problems. For example, the roles of SWs and thermal plasmas during the confined explosion process are not clear. More importantly, the high-speed expanding plasma channel (km s -1 level) of electrical explosion will punch on the material, causing high level of pressure and temperature gradient near the surface. In this section, more attention will be paid to the characteristics of the micro-morphology of the surface under different experimental

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conditions. Then the possible physical and chemical processes near the surface will be discussed.

Shock Waves for the Exfoliation
According to the theory of confined explosion, shock waves will be reflected by the wall for several times. [43] Without a doubt, those SWs and their subsequent complex turbulence will exfoliate some of the layered materials. A simple comparison of graphite samples before and after a Cu wire explosion is presented. The raw material powder we adopted is by mechanical milling, the powders are mainly in bulk form with a characteristic scale >1 µm, as seen in Figure 9a. After the explosion, quite a large amount of raw powder is transferred into the thin graphite nanosheets, as shown in Figure 9b, even though the observation area is lightly influenced by the metal plasma (fewer Cu particles). It was also observed that as the increase of the stored energy (from 250 J to 750 J), the ratio of graphite nanosheets appears an uprising tendency.

Plasmas for Heat Transfer and Formation of Dot-Sheet Structure
For the electrical explosion in the tube, the plasma can burn for hundreds of microseconds, as seen in Figure 10. It is anticipated the maximum temperature could be several eV (1 eV = 11 600 K) and radiative heat flux GW m -2 . After the action of SWs on the tube wall, some of the samples will be enrolled into the expanding plasma channel. Since the interaction time of plasma and sample is too short to fully evaporate the sample. The encounter of the plasma and sample results in the quenching of metal plasma and fast heating of the sample.
For different interaction degrees of the two objects, the micro-morphology may appear with different features. Figure 10 also gives the results of three typical micromorphology of the graphite raw powders under Cu wire explosion. Clearly, the high-intensity heat flux and ionized plasma-gas turbulent flow generate a sizeable influence on the graphite sheets. In fact, facing the heat and mass transfer from the plasma, the thin layers tend to crimp and shrink to a roll, as shown in the 1 st image of Figure 10. During this stage, the metal plasma will be cooled down and become nano-particles doted on the sheet's surface. Then, as the increase of contact degree, the deformation of the sheet will become severer. They fold and roll themselves first and then gather, resulting in complex stacked structure of the other two images in Figure 10. Namely, the dot-sheet structure is very much like a "cabbage" with extra dot decoration. An interesting phenomenon could be found that the average diameter of the nanoparticle increases from the 1 st SEM image to the 3 rd one. This is because the 3 rd case may come into the high-density plasma region with higher Cu vapor density and heat flux, resulting in a deeply deformed structure and longer time for nanoparticle's growth.

Chemical Reactions between Plasma and Layered Materials
As stated above, interaction between the plasma and sample is companied by intense heat and mass transfer. For the graphite and Cu, there is no chemical reaction between them. The variation is mainly physical processes, such as vaporization, condensation, etc. However, for the bismuth selenide, there may exist chemical reactions. Figure 11a gives more results of Bi 2 Se 3 bulk surfaces under the actions of Cu wire explosion. Similar to the graphite case, the surface Bi 2 Se 3 can be damaged by SWs and form smaller fragments. However, the successive thermal Cu plasmas will vaporize and atomize some Bi 2 Se 3 thin layers. After the end of discharge, the plasmas made of mixed atoms are cooling down and chemical reactions become obvious. The reaction products (RP) are mainly in the form of nano-crystals (Bi 2 Se 3 , CuSe, and Bi oxides), according to the EDS results in Figure 6 and XPS results in Figure 8. It seems like the Bi 2 Se 3 tends to break into small crystals, rather than thin sheets. It should be pointed out the chemical reaction could not exhaust all the metal plasma, so we can still find some typical Cu nanoparticles in the RP. This may attribute to that the electrical explosion is a transient nonequilibrium process. Figure 11b illustrates the main physical and chemical process of the electrical explosion and layered materials. Also, the characteristic time of each process is annotated. Although the electrical explosion is a typical one-step synthesis method, it provides a combined effects including SWs, plasma impact, heat transfer, and possible chemical reactions. The responses of the layered materials could be summarized in three aspects: exfoliation, phase transition, and surface decoration. Adjusting those physical and chemical processes may lead to different results, which will be our next-step research.

Novel Structures beside the "Dot-Sheet" Structure
Different species of metal wire experience discrepant discharge processes resulting in different temperature and pressure parameters in the tube. Replacing Cu with W metal as the exploding wire, flocculent amorphous structures are observed in Figure 12a. When treating with Bi 2 Se 3 bulks, large number of nanotubes with size less than 100 nm are formed whatever under Cu or W wire explosion, which can be seen in Figure 12b,c. Synthesized crystalline nanotubes of CuSe, WSe 2 , and Bi 2 Se 3 have been reported by other methods. [44,45] The extreme environmental condition  www.advmatinterfaces.de provided by electrical explosion can produce nanotubes; however, the formation mechanism and compositions need further investigation. More details of produced nanotubes can be seen in Figure 12c. Two forms, namely, hollow and nested nanotubes are observed, and the thickness of the outer shell is less than 20 nm. During explosion process, the long-life plasma can maintain the reaction system at a high temperature state. Vaporization and re-nucleation occur possibly for part of the sample, and the nuclei may bend due to some defects forming during nucleation. As the crystal growth continues, the bending nuclei grow into a ring, and further grow to nanotubes under sufficient system temperature. [46] Combined the above results, main characteristics of the interactions between electrical explosion and typical layered materials (graphite and Bi 2 Se 3 ) could be preliminarily concluded in Table 2. Generally, electrical explosion is a rapid (ms level) and effective one-step synthesis approach for processing the layered materials into nanoparticle decorated thin layers. It is also a hybrid methodology containing both top-down (exfoliation) and Bottom-up (nucleation/coagulation) steps. More importantly, it exhibits a wide variation of nanostructures under different physical/chemical conditions, broadening the domain of the possibility of graphite and Bi 2 Se 3 and their composites on nanoscale. For the application level, it could be expected that graphite and Bi 2 Se 3 layers will play a role in structural and functional materials. [37,47] It is anticipated the metal-plasma treated Bi 2 Se 3 layers could exhibit more interesting characteristics in the field of photoelectric detection (around 1.5 eV of photon energy) and catalyst design, which would be explored in our following studies.
Traditionally, electrical explosion is utilized to prepare nanoparticles or spray coatings on different surfaces, especially for processing the inner wall of the irregular workpieces. The two basic features of the electrical explosion are extremely high heating/quenching rate (up to 10 10 -10 12 K s -1 ) of the material with drastic entire phase transitions, as well as the arbitrarily combination of almost all elements. As more details of the physical/chemical processes of electrical explosion are revealed recently, the electrical-explosion-based synthesis route comes into more and more application scenarios. Therein, the application can be divided into three categories: a) novel alloy or compound powders/coatings, like high-entropy alloys; [48] b) modification and decoration of the material surface, like the dot-sheet structure in this study; (c) functionalization of materials for different purposes, such as metallized aerogel, antistatic materials, etc. To sum up, this method would be applicable in a wide range of materials and show more interesting properties as the advanced materials. A simple road-map of the electrical-explosion-based synthesis method could be seen in Figure 13.

Conclusion
Behaviors of graphite and bismuth selenide under the action of confined electrical explosion are investigated, including the dynamic plasma-material interaction process and the characterization of reaction products. Cu-decorated graphite/Bi 2 Se 3 thin layers (dot-sheet structure) are successfully synthesized, along with the formation of graphite nanosheet (<10 nm) or Bi 2 Se 3 nanocrystals (<50 nm). Through the results of high-speed photography of the explosion process and morphology and element distribution results of the reaction products, formation mechanism of dot-sheet structure is preliminarily ascertained. Particularly, the main physical and chemical processes related to the interactions between electrical explosion and layered materials are explored. In performing the electrical explosion, the generated and reflected shock waves will exfoliate quite a large amount of sample. Then the intense heat transfer happened, simultaneously atomizing some sample and quenching the metal plasma. During the cooling stage after the end of discharge, condensation along with the possible chemical reactions on the layered sample dominates, resulting in stacked or cabbage-like (dot-sheet) structure. Novel structures are also observed, such as flocculent amorphous structure by W wire explosion with Bi 2 Se 3 , nanotube clusters and hollowed nanotube/sphere of Bi 2 Se 3 , etc.