annular dark field (HAADF) images of a typical antimonene flake in Figure 2.4d, it can be seen that the crystal structure conforms to β-antimonene and the flake is highly crystalline with no major defects. Figure 2.4e shows the Raman spectra of exfoliated antimonene flakes with different thicknesses, it is observed that the Raman intensity has a certain dependence on the thickness, where the peak intensity increases with the increase of the thickness, only for the flakes with thicknesses greater than 70 nm.
Figure 2.4 (a) Optical image of a dispersion of exfoliated few-layer antimonene. (b) AFM image of few-layer antimonene flakes drop-casted onto a SiO2/Si substrate. (c) Height histogram of the image in panel (b). (d) Low-magnification (top left) and an atomic-resolution (down right) HAADF images of a typical antimonene flake taken along the [0–12] direction. (e) Single-point spectra of different thicknesses were measured by studying AFM images (inset). The TEM (f) and HRTEM (g) images of exfoliated antimonene nanosheets. Raman spectra (h) and high-resolution XPS (i) of Sbbulk and SbSE. (a–e) Reproduced with permission [19]. Copyright 2016, Wiley-VCH. (f–i) Reproduced with permission [21]. Copyright 2017, Wiley-VCH.
It usually takes a long time for the sonication process in the LPE. If the sonication power is increased during the LPE or the antimony crystals are pretreated before the LPE, the exfoliation time will be greatly shortened and the yield can be remarkably improved [20, 21]. High sonication power affords sufficient energy to break the van der Waals interactions between the Sb-Sb layers, while the pre-grinding of antimony crystals provides a shear force along the Sb-layer surface, and both processes are conducive to peel off thin antimonene flakes. By using ultrahigh sonication power (850 W) in the surfactant-free LPE, high-quality and high-stability antimonene was prepared in the ice-bath with ethanol as the best solvent, and the exfoliated flakes possessed narrow thickness distribution (0.5–1.5 nm) [20]. The high specific capacity (860 mAh g–1) of antimonene made it very suitable for the anode of a sodium ion battery (SIB), and the antimonene anode exhibited good cycling stability and high rate capability. Moreover, adding pre-grinding of antimony crystals in the 2-butanol solvent before sonication, uniform and smooth antimonene flakes were produced by the modified LPE method [21]. The micron-scale antimonene flakes had tunable thicknesses between 0.5 nm and 7 nm, specifically, their band gaps were also finely tuned from 0.8 eV to 1.44 eV. The antimonene was served as a hole transport layer (HTL) in a perovskite solar cell, due to its matched energy level with CH3NH3PbI3. Comparing with the HTL-free device, both of the highest short-circuit current density (Jsc) and external quantum efficiency (EQE) of antimonene-HTL device were increased by 30%.
In addition to generating shear force via pre-grinding antimony crystals before the LPE, shear force can also be obtained in the process of LPE by using rotating blades mixers. Gusmão et al. obtained arsenene, antimonene, and bismuthene exfoliated nanosheets by the surfactant-assisted LPE method under the action of shear force generated by rotating household kitchen blenders [22]. As the transmission electron microscopy (TEM) shown in Figure 2.4f, the morphology of exfoliated antimonene (SbSE) nanosheets consisted of shapes with defined angles and nanostripes because the preferential cleavage of antimony crystals was easy to occur in different crystallographic directions. The size distribution of antimonene nanosheets exhibited a broad range from 100 nm to 900 nm and the maxima value appeared at around 200 nm. Then, the high-resolution TEM (HRTEM) was employed to determine the structure of antimonene (Figure 2.4g). The Raman intensity of SbSE was lower than that of bulk antimony (Sbbulk), and the Raman peak shifted to higher frequency owing to partial oxidation (Figure 2.4h). The bonding states of Sbbulk and SbSE were also investigated by the high-resolution X-ray photoelectron spectra (XPS), it can be seen that both the element phase (Sb 3d5/2, 3d3/2) and partial oxidation phase (Sb2O3) existed in the SbSE (Figure 2.4i). As a pnictogen, SbSE can be used for the electrochemical detection of ascorbic acid due to its high electron transfer properties and low onset oxidation potential. Also, SbSE can be considered as a wide-pH-range catalyst for the hydrogen evolution reaction (HER).
2.3.3 Epitaxial Growth
Epitaxial growth is a method that a crystalline layer can be directly deposited onto a crystalline substrate [23]. This method is widely used in the preparation of high-quality crystalline 2D materials with large capacity. At present, van der Waals epitaxy (vdWE) and molecular beam epitaxy (MBE) are explored to grow few- or even single-layer antimonene on different crystalline substrates.
Traditional epitaxial growth requires very similar lattices to realize the matching between the substrate and the epitaxial layer. Koma et al. found that the vdWE proceeded with the van der Waals force can almost avoid the limitation of the substrate and can easily grow various layered materials on the substrates without dangling bonds [24]. Ji et al. first used the vdWE method to synthesize few-layer β-antimonene monocrystalline polygons on different substrates and highlighted their atomic structure and ambient stability [25]. The synthesis process was carried out in a two-zone tube furnace for 60 min, where antimony powders in the T1 zone was heated up to 660°C to generate antimony vapor and the substrates (fluorophlogopite mica, silicon, sapphire) were placed in the downstream T2 zone (380°C) (Figure 2.5a). Antimony vapor was carried from T1 zone to T2 zone by Ar/H2 (70%/30%) mixed gas and then deposited on the substrates to grow various antimonene polygons. The absence of dangling bonds and low migration energy barrier of antimony atoms on mica substrate are beneficial to a fast growth of antimonene on mica. From the optical microscope images shown in Figure 2.5b, it can be seen that few-layer antimonene polygons exhibit various shapes, including triangles, hexagons, rhombus, and trapezoids with lateral sizes about 5–10 μm. The thicknesses of these polygons are down to 4 nm, while the thinnest one is found to be 1 nm, corresponding to a monolayer antimonene. The HRTEM image of a typical antimonene polygon is shown in Figure 2.5c, it is extracted that the synthesized antimonene belongs to the rhombohedral structure, namely, β-antimonene. The Raman spectra of antimonene polygons also show a thickness dependence, in which the Raman peaks move to the higher frequency region with the decrease of thickness (Figure 2.5d). By comparing optical microscopy, AFM, Raman spectroscopy, and XPS results of antimonene before and after one-month aging, it is observed that few-layer antimonene show outstanding stability in ambient condition. Moreover, the synthesized antimonene polygons exhibited high electrical conductivity up to 1.6×104 S m−1 and good transparency in the visible range.
