Robust Carbon-Stabilization of Few-Layer Black Phosphorus for Superior Oxygen Evolution Reaction

Few-layer exfoliated black phosphorus (Ex-BP) has attracted tremendous attention owing to its promising applications, including in electrocatalysis. However, it remains a challenge to directly use few-layer Ex-BP as oxygen-involved electrocatalyst because it is quite difficult to restrain structural degradation caused by spontaneous oxidation and keep it stable. Here, a robust carbon-stabilization strategy has been implemented to prepare carbon-coated Ex-BP/N-doped graphene nanosheet (Ex-BP/NGS@C) nanostructures at room temperature, which exhibit superior oxygen evolution reaction (OER) activity under alkaline conditions. Specifically, the as-synthesized Ex-BP/NGS@C hybrid presents a low overpotential of 257 mV at a current density of 10 mA cm−2 with a small Tafel slope of 52 mV dec−1 and shows high durability after long-term testing.


Introduction
Energy generation systems in modern industry mainly rely on the burning of fossil fuels, which causes environmental deterioration issues. Therefore, the development of new sustainable electrochemical energy technologies has received wide attention, among which the use of water electrolysis for hydrogen production has attracted much attention due to its efficient and clean energy source nature [1][2][3]. However, as a half reaction of electrocatalytic water splitting, the sluggish kinetics of the oxygen evolution reaction (OER) has become the most challenging issue that limits the efficiency of these electrochemical systems [4][5][6]. Electrocatalysts play important roles as key components in OER. Currently, the Ir/Ru-based noble metal catalysts possess the best OER electrocatalytic activity, but the scarcity of these metals on Earth and consequent high-cost severely limit their practical application [7]. Therefore, the development of highly efficient but more cost-effective noble metal-free electrocatalytic OER catalysts is an important research direction in this field.
More recently, two-dimensional (2D) ultrathin materials have considered as a type of ideal catalysts due to their distinctive physical and chemical properties [8][9][10]. Especially, black phosphorus (BP) is believed as a very promising candidate for OER owing to its remarkable electrical conductivity, large specific surface area, and effective electrocatalytic activity [11][12][13][14][15]. However, the few-layer BP is usually in a thermodynamic metastable state with high surface energy and poor antioxidant capacity in air or dissolved oxygen solutions [16][17][18]. In many applications, the surface oxidation of BP results in performance degradation due to reduced electrical conductivity and passivation of reaction interfaces. Unfortunately, it is difficult to avoid this undesirable phenomenon during the preparation of few-layer BP or its composites. This limitation has become one of the key obstacles towards the development and application of phosphorene-based materials. First, the bulk BP powder and NGS powder were mixed together with some sodium/potassium (NaK) alloy by a high-shearing treatment for 0.5 h. Bulk BP crystals were exfoliated into ultrathin nanosheets (Ex-BP) via simple shearing treatment in 1,2-dimethoxyethane (DME). Herein, the NGS was produced by directly chemical exfoliation of graphite in the presence of sodium-ammonia solution (experimental section). The liquid NaK alloy shows some intercalation properties for layered materials as well as a very strong reducibility [27,28]. As shown in Figure 1, Ex-BP nanosheets were directly exfoliated by the combined contribution of shearing-assistance and the chemical intercalation process. Second, the pure CO2 was subsequently added into this solution. Then, the amorphous

Electrochemical Measurements
The catalyst inks for electrochemical tests were made by dispersing catalyst (5 mg), carbon black (1 mg), ethanol (1 mL) and Nafion solution (5 wt.%, 15 µL) under sonication for 30 min. The above solution (8 µL) was pipetted through a pipette and transferred onto a freshly polished glassy carbon electrode. After drying the glassy carbon electrode, a working electrode with a uniform distribution of catalyst with a loading of 0.57 mg cm −2 can be obtained. Electrochemical tests were performed using an electrochemical workstation (CHI660E, Shanghai, China) and a three-electrode system in 1 M KOH electrode solution. An Hg/HgO electrode and a carbon rod were used as reference electrode and counter electrode, respectively. In all electrochemical tests, the electrode potential vs. Hg/HgO were converted to that vs. reversible hydrogen electrode (RHE) according to the Nernst equation (E(RHE) = E(Hg/HgO) + 0.059×pH + 0.098). The electrochemical impedance spectroscopy (EIS) measurements were performed on a PGSTAT302N system (Autolab, Switzerland) at a charged state with a 5 mV amplitude.

Results and Discussion
The production of the Ex-BP/NGS@C hybrid is schematically illustrated in Figure 1. To enhance the stability of Ex-BP, we introduced both a carbon coating layer and N-doped graphene sheets (NGS) to passivate Ex-BP. The whole process is performed in one-pot at room temperature.
First, the bulk BP powder and NGS powder were mixed together with some sodium/potassium (NaK) alloy by a high-shearing treatment for 0.5 h. Bulk BP crystals were exfoliated into ultrathin nanosheets (Ex-BP) via simple shearing treatment in 1,2-dimethoxyethane (DME). Herein, the NGS was produced by directly chemical exfoliation of graphite in the presence of sodium-ammonia solution (experimental section). The liquid NaK alloy shows some intercalation properties for layered materials as well as a very strong reducibility [27,28]. As shown in Figure 1, Ex-BP nanosheets were directly exfoliated by the combined contribution of shearing-assistance and the chemical intercalation process. Second, the pure CO 2 was subsequently added into this solution. Then, the amorphous carbon derived from CO 2 reduction was used as the carbon coating. The conversion of CO 2 into amorphous carbon coating was conducted by the following reactions: As shown in Figure 2, the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images illustrated the morphologies of the 2D Ex-BP/NGS@C hybrids. The 2D structure of Ex-BP/NGS@C can be clearly observed in Figure 2a. The interlayer spacing of 0.21 nm corresponds toto the (002) lattice plane of Ex-BP nanosheets. As shown in Figure 2b, the HRTEM image with an interlayer spacing of 0.37 nm revealed that the NGS nanosheets contained about five layers. Furthermore, the amorphous carbon that was coated on the Ex-BP nanosheets can be clearly observed in the HRTEM image in Figure 2b. The corresponding fast Fourier transform image shows a dispersion ring that also demonstrates the existence of the amorphous carbon. TEM image of Ex-BP in Figure S2 presents a free-standing nanosheet, which demonstrated the successful exfoliation of the bulk BP. The thickness of Ex-BP is determined to be about~3.6 nm by atomic force microscopy (AFM) observation ( Figure S3) that corresponds to a few layers (5-7 layers) of sheets. The selective-area electron diffraction pattern (SAED) reveals the crystalline performance of Ex-BP nanosheets with [010] preferential orientation [29]. Figure 2e shows the elemental mapping for C, N and P of Ex-BP/NGS@C hybrid (Figure 2c,f). The uniform distribution of all elements clearly indicates that Ex-BP nanosheets have good contact with NGS and amorphous carbon.
Coatings 2020, 10, x FOR PEER REVIEW 4 of 10 carbon derived from CO2 reduction was used as the carbon coating. The conversion of CO2 into amorphous carbon coating was conducted by the following reactions: As shown in Figure 2, the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images illustrated the morphologies of the 2D Ex-BP/NGS@C hybrids. The 2D structure of Ex-BP/NGS@C can be clearly observed in Figure 2a. The interlayer spacing of 0.21 nm corresponds toto the (002) lattice plane of Ex-BP nanosheets. As shown in Figure 2b, the HRTEM image with an interlayer spacing of 0.37 nm revealed that the NGS nanosheets contained about five layers. Furthermore, the amorphous carbon that was coated on the Ex-BP nanosheets can be clearly observed in the HRTEM image in Figure 2b. The corresponding fast Fourier transform image shows a dispersion ring that also demonstrates the existence of the amorphous carbon. TEM image of Ex-BP in Figure S2 presents a free-standing nanosheet, which demonstrated the successful exfoliation of the bulk BP. The thickness of Ex-BP is determined to be about ~3.6 nm by atomic force microscopy (AFM) observation ( Figure S3) that corresponds to a few layers (5-7 layers) of sheets. The selective-area electron diffraction pattern (SAED) reveals the crystalline performance of Ex-BP nanosheets with [010] preferential orientation [29]. Figure 2e shows the elemental mapping for C, N and P of Ex-BP/NGS@C hybrid (Figure 2c,f). The uniform distribution of all elements clearly indicates that Ex-BP nanosheets have good contact with NGS and amorphous carbon. Powder X-ray diffraction (XRD) was used to test the crystalline phases of all samples. Figure 3a shows the XRD patterns of bulk BP, NGS, Ex-BP/NGS and Ex-BP/NGS@C, respectively. The main characteristic of high crystalline BP was displayed as three strong intensity of (020), (040) and (060) peaks at 17.0°, 34.2° and 52.4°, respectively [30]. The peak at 26.4° is a characteristic peak of NGS. Figure 3b shows the Raman spectra of the bulk BP, NGS, Ex-BP/NGS and Ex-BP/NGS@C. The Raman peaks of bulk BP at 362.2, 439.3, and 466.2 cm −1 can be assigned to the characteristic Ag1, B2g, and Ag2 vibrational modes, respectively [31]. The Raman spectrum of Ex-BP/NGS and Ex-BP/NGS@C also demonstrates three peaks, indicating that both Ex-BP/NGS and Ex-BP/NGS@C maintain the characteristic vibration structure of bulk BP. The two strong bands centered at 1330 and 1580 cm −1 are Powder X-ray diffraction (XRD) was used to test the crystalline phases of all samples. Figure 3a shows the XRD patterns of bulk BP, NGS, Ex-BP/NGS and Ex-BP/NGS@C, respectively. The main characteristic of high crystalline BP was displayed as three strong intensity of (020), (040) and (060) peaks at 17.0 • , 34.2 • and 52.4 • , respectively [30]. The peak at 26.4 • is a characteristic peak of NGS.
Coatings 2020, 10, 695 5 of 11 Figure 3b shows the Raman spectra of the bulk BP, NGS, Ex-BP/NGS and Ex-BP/NGS@C. The Raman peaks of bulk BP at 362.2, 439.3, and 466.2 cm −1 can be assigned to the characteristic Ag1, B2g, and Ag2 vibrational modes, respectively [31]. The Raman spectrum of Ex-BP/NGS and Ex-BP/NGS@C also demonstrates three peaks, indicating that both Ex-BP/NGS and Ex-BP/NGS@C maintain the characteristic vibration structure of bulk BP. The two strong bands centered at 1330 and 1580 cm −1 are assigned to the in-plane vibrations of disordered amorphous carbon (D band) and crystalline graphic carbon (G band), respectively. The chemical states of Ex-BP/NGS@C were analysed by XPS measurements. The survey XPS spectrum of Ex-BP/NGS@C verifies the presence of P, N, C, and O peaks (Figure 3c). Figure 2d shows the high-resolution P 2p spectra of Ex-BP/NGS@C. Three prominent peaks located at 130.5, 131.3 eV are characteristic for P 2p 3/2 and P 2p 1/2 [32]. The peaks at 134.8, 135.5 and 136.4 eV are characteristic for oxidized phosphorus (P-O-C, P-O-P and O-P=O) binding energies (BE), respectively [33]. Moreover, the peaks at 133.6and 284.2 eV corresponding to P−C bond are observed in P 2p and C 1s XPS spectra in Figure 3d,e, which demonstrated that a strong contact is formed between the Ex-BP nanosheets and carbon coating layer [33]. The N 1s XPS spectra in Figure 3f can be indexed to quaternary N, pyrrolic N and pyridinic N in NGS. We also have further conducted an FTIR test on Ex-BP/NGS@C and demonstrated the characteristic peak of P-O and P-C in Figure S4 in the supporting information.
Coatings 2020, 10, x FOR PEER REVIEW 5 of 10 assigned to the in-plane vibrations of disordered amorphous carbon (D band) and crystalline graphic carbon (G band), respectively. The chemical states of Ex-BP/NGS@C were analysed by XPS measurements. The survey XPS spectrum of Ex-BP/NGS@C verifies the presence of P, N, C, and O peaks (Figure 3c). Figure 2d shows the high-resolution P 2p spectra of Ex-BP/NGS@C. Three prominent peaks located at 130.5, 131.3 eV are characteristic for P 2p3/2 and P 2p1/2 [32]. The peaks at 134.8, 135.5 and 136.4 eV are characteristic for oxidized phosphorus (P-O-C, P-O-P and O-P=O) binding energies (BE), respectively [33]. Moreover, the peaks at 133.6and 284.2 eV corresponding to P−C bond are observed in P 2p and C 1s XPS spectra in Figure 3d,e, which demonstrated that a strong contact is formed between the Ex-BP nanosheets and carbon coating layer [33]. The N 1s XPS spectra in Figure 3f can be indexed to quaternary N, pyrrolic N and pyridinic N in NGS. We also have further conducted an FTIR test on Ex-BP/NGS@C and demonstrated the characteristic peak of P-O and P-C in Figure S4 in the supporting information. The obtained Ex-BP/NGS@C hybrid was tested as an electrocatalyst for the oxygen evolution reaction (OER). Figure 4a shows the schematic diagram of the carbon-stabilization strategy by carbon-coated Ex-BP/N-doped graphene nanosheet nanostructures. In order to evaluate the OER activity of Ex-BP/NGS@C, Ex-BP/NGS, bulk BP and NGS, we used a three-electrode system for testing. All tests were performed in 1 M KOH. As shown in Figure 4b, compared with Ex-BP/NGS and Bulk-BP, Ex-BP/NGS@C exhibits the best OER performance. The onset potential of Ex-BP/NGS@C is about 1.43 V vs. RHE, which is higher than the sample without carbon derived CO2, whose onset potential is 1.53 V vs. RHE. Furthermore, in all the comparative samples Ex-BP/NGS@C shows the largest current density at the same voltage. This may be because the carbon coated on the surface of the sample makes the combination of black phosphorus and NGS more compact, which is beneficial to the electron transfer from the NGS surface to the black phosphorus surface. This phenomenon can also be proved from the Figure 5a. Ex-BP/NGS@C has a smaller charge transfer resistance (Rct ≈ 63 Ω), which is conducive to electron transport. In addition, we tested the oxygen evolution performance of bulk-BP. As can be seen from the linear sweep voltammetry (LSV) curve, bulk-BP has almost no oxygen evolution properties. The layer-to-layer stacking between them results in a smaller surface area, less active site exposure and poorer electron conductivity [34]. The Tafel The obtained Ex-BP/NGS@C hybrid was tested as an electrocatalyst for the oxygen evolution reaction (OER). Figure 4a shows the schematic diagram of the carbon-stabilization strategy by carbon-coated Ex-BP/N-doped graphene nanosheet nanostructures. In order to evaluate the OER activity of Ex-BP/NGS@C, Ex-BP/NGS, bulk BP and NGS, we used a three-electrode system for testing. All tests were performed in 1 M KOH. As shown in Figure 4b, compared with Ex-BP/NGS and Bulk-BP, Ex-BP/NGS@C exhibits the best OER performance. The onset potential of Ex-BP/NGS@C is about 1.43 V vs. RHE, which is higher than the sample without carbon derived CO 2 , whose onset potential is 1.53 V vs. RHE. Furthermore, in all the comparative samples Ex-BP/NGS@C shows the largest current density at the same voltage. This may be because the carbon coated on the surface of the sample makes the combination of black phosphorus and NGS more compact, which is beneficial to the electron transfer from the NGS surface to the black phosphorus surface. This phenomenon can also be proved from the . Ex-BP/NGS@C has a smaller charge transfer resistance (R ct ≈ 63 Ω), which is conducive to electron transport. In addition, we tested the oxygen evolution performance of bulk-BP. As can be seen from the linear sweep voltammetry (LSV) curve, bulk-BP has almost no oxygen evolution properties. The layer-to-layer stacking between them results in a smaller surface area, less active site exposure and poorer electron conductivity [34]. The Tafel slope of the material can be fitted from Figure 4c, 52, 54, 67, 90 mV dec −1 respectively corresponds to Ex-BP/NGS@C, Ex-BP/NGS, Ex-BP and RuO 2 (99.9% metals basis, Aladdin). The Tafel slope of sample Ex-BP/NGS@C is smaller, corresponding to a higher catalytic activity [35]. More importantly, derived carbon obtained by reducing carbon dioxide not only accelerates the transfer speed of electrons, but also contributes significantly to maintaining the stability of OER performance. Figure 4d,e show the stability of Ex-BP/NGS@C and Ex-BP/NGS. The LSV curves after 2800 cycles of the sample without carbon show its catalytic activity has a significant decline. The Ex-BP/NGS@C maintained a high degree of coincidence with the initial polarization curve after 2800 cycles, which confirmed the excellent stability of all samples we synthesized.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 10 Ex-BP/NGS@C is smaller, corresponding to a higher catalytic activity [35]. More importantly, derived carbon obtained by reducing carbon dioxide not only accelerates the transfer speed of electrons, but also contributes significantly to maintaining the stability of OER performance. Figure 4d,e show the stability of Ex-BP/NGS@C and Ex-BP/NGS. The LSV curves after 2800 cycles of the sample without carbon show its catalytic activity has a significant decline, The Ex-BP/NGS@C maintained a high degree of coincidence with the initial polarization curve after 2800 cycles, which confirmed the excellent stability of all samples we synthesized. The charge transfer resistances (Rct) of all catalysts were assessed to understand the electrode kinetics of the studied catalysts for OER. We use electrochemical impedance spectrum (EIS) to evaluate the electrode kinetic parameters of materials in the electrocatalytic oxygen evolution reaction [14]. Figure 5a shows the EIS Nyquist plots for Ex-BP/NGS@C, Ex-BP/NGS, Ex-BP, Bulk BP and NGS catalysts at the onset potential. It was found that the Rct of the Ex-BP/NGS@C sample (63 Ω) is much smaller than those of other samples such as Ex-BP/NGS (520 Ω). The material with the best catalytic performance has the lowest charge transfer resistance. This indicates highly efficient electron transport and favorable OER kinetics at the Ex-BP/NGS@C electrolyte interface. Figure 5b show the long-term stability of Ex-BP/NGS@C. In the 5.5 h during which the oxygen evolution reaction shown in Figure 5d, ∆j is linear growth with sweep speeds; and the slop of the line is twice of the double-layer capacitance [36]. As a result, Ex-BP/NGS@C (3.16 mF cm −2 ) is larger than the slope of Ex-BP/NGS (0.67 mF cm −2 ). According to ECSA test comparison results, sample coated with carbon dioxide-derived carbon exhibit greater electrochemically active area, representing an increase in the effective oxygen evolution activity of electrolytic water.  Compared with the sample which only has protection without carbon dioxide derived carbon ( Figure  5f) the electrochemical activity is maintained well. Figure 5g shows the oxygen evolution performance of a few-layer black phosphorus sample stripped by the same method. Since black phosphorus is easily oxidized and loses catalytic activity, the performance of the sample after longterm storage is significantly degraded. In the absence of any carbon protection, the catalytic performance of the black phosphorus layer is greatly reduced and most of the black phosphorus is oxidized. Figure 5h shows the current-time curve for Ex-BP/NGS@C. After four months, the current density retention time is still about 6 h, which is similar to the previous test results. There is a partial decrease in its intensity, which is consistent with the small changes in current density observed in Figure 5e.
In order to better evaluate the capability of Ex-BP/NGS@C, we have compiled the recent reported work of black phosphorus materials in OER (Table 1). In the above materials, the sample synthesized in this work Ex-BP/NGS@C has the smallest onset potential of 1.43 V, which is slightly less than 1.44 V of Co/BP. At the same time, compared with the materials mentioned in the table, Ex-BP/NGS@C can have a smaller Tafel slope without adding metal elements that have strong catalytic activity for oxygen evolution reaction.
By the above results, the excellent electrocatalytic performance of Ex-BP/NGS@C for OER could be due to good exfoliation of Ex-BP and the protection of both NGS and the carbon-coating layer. The carbon coating layer offers strong combination between Ex-BP and NGS in the hybrids, which increased electrical connectivity and maintained the stability of electrode. Better stability of the Ex-BP/NGS@C indicates that the amorphous carbon coating layer plays a key role in the hybrid materials. The combination of NGS and carbon-coating layer can prevent the oxidation of Ex-BP nanosheets. And, the adding of the NGS also plays an important role not only in enhancing the The charge transfer resistances (R ct ) of all catalysts were assessed to understand the electrode kinetics of the studied catalysts for OER. We use electrochemical impedance spectrum (EIS) to evaluate the electrode kinetic parameters of materials in the electrocatalytic oxygen evolution reaction [14]. Figure 5a shows the EIS Nyquist plots for Ex-BP/NGS@C, Ex-BP/NGS, Ex-BP, Bulk BP and NGS catalysts at the onset potential. It was found that the R ct of the Ex-BP/NGS@C sample (63 Ω) is much smaller than those of other samples such as Ex-BP/NGS (520 Ω). The material with the best catalytic performance has the lowest charge transfer resistance. This indicates highly efficient electron transport and favorable OER kinetics at the Ex-BP/NGS@C electrolyte interface. Figure 5b show the long-term stability of Ex-BP/NGS@C. In the 5.5 h during which the oxygen evolution reaction continues, there is no significant decrease in current density, indicating that it can maintain catalytic activity for a longer period of time at a higher current density. To further unveil the electrochemically active area, the CV curves of Ex-BP/NGS@C at different sweep speeds were tested ( Figure 5c). As shown in Figure 5d, ∆j is linear growth with sweep speeds; and the slop of the line is twice of the double-layer capacitance [36]. As a result, Ex-BP/NGS@C (3.16 mF cm −2 ) is larger than the slope of Ex-BP/NGS (0.67 mF cm −2 ). According to ECSA test comparison results, sample coated with carbon dioxide-derived carbon exhibit greater electrochemically active area, representing an increase in the effective oxygen evolution activity of electrolytic water. Figure 5e manifests that the oxygen evolution performance Ex-BP/NGS@C undergoes a slight drop after being stored in ethanol for 4 months owing to the double protection of NGS and C. Compared with the sample which only has protection without carbon dioxide derived carbon ( Figure 5f) the electrochemical activity is maintained well. Figure 5g shows the oxygen evolution performance of a few-layer black phosphorus sample stripped by the same method. Since black phosphorus is easily oxidized and loses catalytic activity, the performance of the sample after long-term storage is significantly degraded. In the absence of any carbon protection, the catalytic performance of the black phosphorus layer is greatly reduced and most of the black phosphorus is oxidized. Figure 5h shows the current-time curve for Ex-BP/NGS@C. After four months, the current density retention time is still about 6 h, which is similar to the previous test results. There is a partial decrease in its intensity, which is consistent with the small changes in current density observed in Figure 5e. In order to better evaluate the capability of Ex-BP/NGS@C, we have compiled the recent reported work of black phosphorus materials in OER (Table 1). In the above materials, the sample synthesized in this work Ex-BP/NGS@C has the smallest onset potential of 1.43 V, which is slightly less than 1.44 V of Co/BP. At the same time, compared with the materials mentioned in the table, Ex-BP/NGS@C can have a smaller Tafel slope without adding metal elements that have strong catalytic activity for oxygen evolution reaction. By the above results, the excellent electrocatalytic performance of Ex-BP/NGS@C for OER could be due to good exfoliation of Ex-BP and the protection of both NGS and the carbon-coating layer. The carbon coating layer offers strong combination between Ex-BP and NGS in the hybrids, which increased electrical connectivity and maintained the stability of electrode. Better stability of the Ex-BP/NGS@C indicates that the amorphous carbon coating layer plays a key role in the hybrid materials. The combination of NGS and carbon-coating layer can prevent the oxidation of Ex-BP nanosheets. And, the adding of the NGS also plays an important role not only in enhancing the electrical conductivity, but also in facilitating the ion transport, which offers efficient electrochemical reaction kinetics.

Conclusions
We have developed a facile method to fabricate a stable BP-based OER catalyst by combining carbon-coating and N-doped graphene nanosheets in a room temperature process. The Ex-BP nanosheets were produced by shearing exfoliation of bulk BP under chemical insertion assistance. The carbon coating layers was obtained by liquid NaK alloy reduction using CO 2 as the carbon source. The coated carbon-coating on Ex-BP nanosheets can avoid structural degradation and keep them stable. The Ex-BP/NGS@C hybrid presents excellent oxygen evolution electrocatalytic performance with an overpotential of 257 mV at 10 mA cm −2 , a small Tafel slope of 41.3 mV dec −1 and high durability under alkaline conditions. Moreover, our work provides a new way to make carbon coatings at room temperature using CO 2 as the carbon source that is green and environmentally friendly and also affords an effective way for the fixation and utilization of CO 2 greenhouse gas. We believe that carbon-coating layers made by this method will open new possibilities for fundamental studies and practical applications.