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Research Table 1: The adsorption energies (in eV) of Li2S, Li2S2, Li2S4, Li2S6, and Li2S8 on 2-Pmmn, χ3, β12 borophene, graphene, and phosphorene [86, 100]. Reproduced from Ref. [99]. Copyright 2019 Springer Science. 15 charging and discharging cycle. On the other hand, the bor- ophene in phase 12 also showed good feature as a fixing material for lithium batteries. Moreover, the borophene of the β12 also exhibits metallic properties throughout the bat- tery cycle. Thus, χ3 and β12 are promising anchoring mate- rials for lithium batteries. 4.2. Optoelectronic Applications 4.2.1. Sensor Application (1) Gas Sensor. Conventional carbon nanotubes and gra- phene gas sensors are highly sensitive to several irritating gases, but highly toxic chemicals such as formaldehyde can- not be identified. In recent years, the wide application pros- pect of borophene in gas sensors has attracted extensive attention [101]. In recent years, based on the DFT, researchers have stud- ied the application prospect of borophene nanometer as formaldehyde sensor [101]. Through calculation and reason- ing analysis, when HCOH molecule exists, the conductivity of borophene increases significantly, thus generating elec- trical signals. With the adsorption of more formaldehyde molecules by borophene, the strength of electrical signals increases, indicating that the sensitivity of borophene to formaldehyde gas is relatively high. In addition, borophene has high surface volume ratios and can be used to detect low-concentration gas molecules [102]. Huang et al. which studied in recent years have been able to succeed with bending and linear defects of preparation phase of the 2D borophene command on the NO gas molec- ular adsorption ability, by bending test and linear borophene in the adsorption NO gas after the i-v characteristic curve, as shown in Figures 15(a) and 15(b); we can clearly see the sen- sitivity of the borophene to NO on electrical properties [102]. In addition, the borophene nanosheet also adsorbs etha- nol. The adsorption of nanostructure borophene on oxygen atoms and hydrogen atoms on ethanol molecules is shown in Figures 15(c) and 15(d), which is at positions 1 and 2, respectively. Due to the adsorption of borophene and oxygen atoms and hydrogen atoms on ethanol, the bandgap of nano- borophene decreases rapidly and the conductivity increases. Therefore, the changes of adsorption energy and bandgap can be used to detect ethanol vapor. 4.2.2. The Electrocatalytic Applications. In practical applica- tions, ideal gas adsorbents cannot meet the test requirements because of their weak adsorbability when adsorbing gas, nor can they affect the release after adsorption because of their strong adsorbability. Therefore, it is a great challenge to choose a material with appropriate adsorbability and good selectivity. By using density functional theory (DFT) calculation method, extra electrons to the adsorbent are added. Tan et al. show that the borophene with negative charge on CO2 adsorption is more than before; in the experimental condi- tion to achieve CO2 saturation coverage, adsorption amount has reached 6:73 × 10−14 cm to 2 × 10−14 cm [105, 106]. Species Li2S Li2S2 Li2S4 Li2S6 Li2S8 Ref. 6.18 [90] 2.87 [90] 1.36 [101] 0.73 [102] 1.12 [102] 2-Pmmn χ3 β12 3.34 Graphene Phosphorene 2.51 2.89 1.91 6.45 4.32 2.67 2.53 1.45 1.53 0.65 0.72 1.27 1.00 were far beyond traditional graphene, silicene, phosphorene, and Li4Ti5O12. The migration energy potential barrier of sodium borate ion in stage 12 is 330meV, which is lower than that of lithium ion (660meV). The migration energy barrier of sodium borate at phase 3 was much lower than that of lithium at 660meV. It can be seen from these ref- erence data that borophene containing borophene vacancy is used as the cathode material of sodium ion battery, which has higher performance than traditional lithium ion battery in all aspects. To sum up, theoretically, borophene has a high capac- ity, excellent conductivity, and efficient ion transport capac- ity, providing a broader prospect for the development of cathode materials for lithium, sodium, and magnesium ion batteries [94–98]. In recent years, the development of lithium-sulfur batte- ries has been seriously hindered; however, the right sulfur anchoring material can inhibit the shuttle effect [99]. The interaction between the lithium polysulfide and the anchor- ing material should be just right. As shown in Table 1, for graphene, the adsorption energies are 0.65eV and 0.72eV, respectively, for Li2S4, Li2S6, and Li2S8. Because of such inter- actions, graphene cannot be used as an anchoring material for polysulfide. Zhao et al. have explored the application of borophene benzene as a potential anchoring material for lithium-sulfur batteries, using a first-principles calculation method [86, 103, 104]. The adsorption energies on the 2-Pmmn phase of borophene benzene are 6.45, 4.32, and 6.18eV, respec- tively, for Li2S4, Li2S6, and Li2S8. Lithium polysulfide will be decomposed, and irreversible sulfur loss will occur due to interaction during absorbing and releasing electrical energy. Surprisingly, the mutual effect between lithium polysulfide and borophene benzene χ3 phase is much smaller than that between lithium polysulfide and borophene benzene 2- Pmmn phase. The energies of lithium Li2S4, lithium Li2S6, and lithium Li2S8 on the borophene benzene χ3 phase are 2.67, 2.53, and 2.87 eV, respectively, indicating that the boro- phene benzene χ3 phase is a perfect fixing material and can be used in lithium sulfur batteries. During the charging and discharging process of the battery, the appropriate adsorp- tion strength is beneficial to inhibit the adsorption of spindle carbon on the electrode and protect its cycling structure from decomposition. The three phases of the borophene are a use- ful grappling material for batteries because of the metal struc- ture characteristic of the borophene during the whole batteryPDF Image | Two-Dimensional Borophene
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