**3. The application of the boron-based clusters**

#### **3.1 Gas sensor**

From life safety point view, the design of sensitive materials to detect toxic gases in the environment is highly demanded. Among these hazardous gases, CO, NH3, NO, H2S, SO2, SO3, and CO2 are mainly produced through industrial applications and automobile exhaust, which represent a harmful threat to human life and the natural environment.

Hossain et al. [38] in a theoretical study using DFT calculations investigated the quasi-planar 2D borophene B35 (see **Figure 2(a)**) as an efficient gas sensor toward NO, NO2, N2O, and NH3 gases. Gases prefer to adsorb on the hexagonal hallow site of B35 where N2O gas is chemically adsorbed, and the other gases are physically adsorbed on this nanocluster. Also, after gas adsorption, the hardness and stability of all systems increased as a result of the increased highest occupied molecular orbital (HOMO) lowest unoccupied molecular orbital (LUMO) energy gap.

It is also demonstrated that the B36 (see **Figure 2(b)**) can be applied as a good detector for ammonia gas (NH3). The minimum energy configuration of this interaction is the adsorption of NH3 from N-head on a B atom of the B36. During this interaction, the enthalpy changes 90.5 kJ/mol and 0.35 ∣ *e* ∣ charge is transferred from ammonia to the B36. Also, the electrical conductance of B36 is found to increase after NH3 adsorption in which the HOMO-LUMO energy gap decreased from 1.55 to 1.35 eV [39].

Ploysongsri and Ruangpornvisuti [40] studied the adsorption of gases containing sulfur on B36 cluster i.e*.* H2S, SO2, and SO3. SO2 and SO3 gases adsorb from the oxygen

**Figure 2.** *The structure of some small clusters of boron. In the panels, two different views are shown and green balls represent B atoms.*

side to the edge of the cluster that is thermodynamically favorable, while H2S adsorption is not spontaneous on this cluster. The H2S, SO2, and SO3 gases can be adsorbed on the edge of B36 with an adsorption energy of 5.29, 43.85, and 80.57 kcal/mol, respectively.

Also, it is shown that metal-decorated B36 and its nitrogen-doped counterparts (M-N*x*-B36*<sup>x</sup>* (M = Fe, Ni, and Cu; *x* = 0, 3)) are also sensitive to detect CO, NO, O2, and N2 molecules. The substitution of three nitrogen atoms in the central ring of B36 can increase the stability and sensitivity of the B36 cluster. The adsorption energy of CO, NO, O2, and N2 gases for the most stable configurations changes in the range of 0.32 to 3.31 eV. Among the studied gases, Fe-N3-B33 and Ni-N3-B33 are more sensitive to CO and NO gases, which leads to reducing the energy gap between the highest occupied molecular orbital (HOMO) and the energy of the lowest unoccupied molecular orbital (LUMO) [41].

One of the dangerous gases emitted by industrial application is nitrogen dioxide (NO2) that puts human health at risk. Hou et al. [42] studied borophene as a highly sensitive and selective material for the NO2 detection. The borophene-based sensor can detect NO2 at a low concentration of 200 ppb, which has a fast response time of 30 s. The recovery time of the introduced sensor at room temperature was 200 s. The properties of this sensor were significantly better than those of other 2D materials such as phosphorene, MoS2, and graphene. For instance, this sensor demonstrates excellent flexibility, long-time stability, and outstanding stability under different bending angles.

Wang et al. [43] using first principles density functional calculations investigated hexagonal Cr-doped borophene (CrB6) as a potential sensor material for CO, CH4, and CO2 gases. The adsorption process of these gases on the CrB6 surface is different, in which for CO2 and CH4 gases it is physisorption while for CO it is chemisorption. CO adsorption remarkably affects the conduction bands of the CrB6 monolayer, and CH4 and CO2 adsorption affects these bands less. Since reversibility is an important property of gas sensors, CrB6 monolayer is recommended as a good material for CO2 and CH4 detection.

## **3.2 Electrode**

One of the efficient anode materials for Li-ion batteries is 2D borophene that is not stable as free standing form. Accordingly, Khan et al. [44] using DFT calculations investigated borophene in conjugation with boron nitride (B/BN) as a good anode for Li-ion battery. Using AIMD simulation, they found that the thermal and mechanical stability of the B/BN structure was dramatically improved compared to that of pristine borophene. Also, the specific charge capacity of B/BN increased compared to the other 2D material, which was 1698 mA h g<sup>1</sup> . Moreover, Li can easily diffuses into the B/BN interlayer due to the low energy barrier (ranging from 0.06 to 0.75 eV).

Kolosov and Glukhova [45] using first-principle calculations studied how surface decoration of single-walled carbon nanotubes (CNTs) by B12 icosahedral clusters can affect electronic properties, capacitance, and stability. They found that the B12 clusters (see **Figure 2(c)**) form a chemical bond with the wall of the CNTs, and the entire system demonstrates metallic behavior. The quantum capacitance and conductivity of the CNTs increased after binding the B12 icosahedral clusters to the inner and outer walls of CNTs. The latter was verified by calculating the transmission function near the Fermi level. They found that increasing boron concentration decreases the heat of formation that strongyle affects the stability of the system. After increasing the boron concentration, the proposed system illustrates attributes such as an asymmetric electrode.

Xie et al. [46] studied the 3D topological porous B4 cluster (H-boron) as a high ionic and electronic conductivity anode for lithium- and sodium-ion batteries. The electron-deficient boron atoms led to expose different adsorption sites for Li and Na ions that impose a low mass density (0.91 g/cm<sup>3</sup> ) and a high specific capacity (30 mAh/g). Li (Na) can readily migrate through this anode material with a low barrier energy of 0.15 (0.22) eV and small volume changes of 0.6% (9.8%). Suggesting that H-boron based anodes can operate with fast dynamic charge-discharge process and good cyclic life.

#### **3.3 Hydrogen storage**

Hydrogen storage as one of the clean energy sources is gaining tremendous attention from computational and experimental scientists. Hydrogen has some specific characteristics compared to gasoline, such as high energy content by weight and low energy content by volume, which offer hydrogen as a suitable fuel to obviate global energy and environmental concerns. However, there is a concern about the storage and safety of hydrogen-based technologies due to its fast burning feature. To resolve this important barrier, hydrogen can be stored on the material through chemisorption and physisorption mechanisms for future demands.

Studies indicated that metal-decorated boron clusters are potential candidates for hydrogen storage. Kumar et al. [47] studied the application of small boron clusters doped with two magnesium atoms (Mg2B*n*; *n* = 4–14) in hydrogen storage. The DFT

results show that all the clusters are stable and H2 molecules were adsorbed in molecular form on these clusters with an absorbtion energy in the range of 0.13– 0.22 eV/H2. Although, the Mg2B6 cluster indicated the maximum storage capacity of H2, MDs analysis indicates that after 200 fs H2 molecules are desorbed from the surface of all clusters except one H2 molecule adsorbed on the Mg2B11 cluster. Also, Liu et al. [48] predicted that the titanium-decorated B8 cluster (Ti2B8) has a capacity of 6.17 wt% for hydrogen storage with the average hydrogen adsorption energy of 0.247–0.358 eV/H2.

Kumar et al. [49] using DFT calculations investigated H2 storage capacity of lithium-doped B14 clusters (Li*n*B14; *n* = 1–5, see **Figure 2(d)**). These clusters are stable at room temperature and capable of storing hydrogen in molecular form. The Li5B14 cluster has a maximum H2 storage capacity of 13.89 wt%. However, based on the AIMDs results, most of the hydrogen molecules desorb from the clusters within 400 fs.

Esrafili and Sadeghi [50] studied hydrogen storage and adsorption of yttriumdecorated B38 fullerene using DFT calculations (see **Figure 2(e)**). They found that the Y atoms are tightly bound to the hexagonal cavities of the cluster, which makes Y@B38 stable and prevents aggregation of Y atoms. This suggests that Y@B38 is an efficient cluster for hydrogen storage. There are six H2 molecules per Y atom adsorbed on Y@B38 cluster with the gravimetric density of 4.96 wt% in which both polarization effects and Kubas mechanism play crucial role in the hydrogen adsorption process. They investigated a suitable energy range for hydrogen adsorption on Y@B38 cluster which is 0.180 to 0.249 eV/H2.

Wang et al. [51] using first-principal calculations investigated the ultrahigh hydrogen storage capacity for sandwich-like beryllium-doped boron clusters B6Be2 and B8Be2. Each Be atom in these clusters can adsorb seven hydrogen molecules which convert to a hydrogen storage capacity of 25.3 and 21.1 wt% for B6Be2 and B8Be2 clusters, respectively, which far exceeds the target gravimetric density of hydrogen adsorption (5.5 wt%). Consequently, both clusters are promising for H2 release and adsorption with adsorption energy in the range of 0.10 (0.11)–0.45 (0.50) eV/H2 for B6Be2 (B8Be2) clusters.

### **3.4 Catalyst**

Wang et al. [52] for the first time reported a spherical isomer of boron and phosphorus atoms that have high capability for overall water splitting (see **Figure 3 (a)**). This theoretically introduced isomer has 20 atoms and eight of them are boron, which can bare its spherical structure throughout the water-splitting process. The water molecule can adsorb on each BdP bond and strongly dissociates to OH + H. This step is the rate-limiting step with an energy barrie of 2.92 eVr.

Hamadi et al. [53] investigated the adsorption of iron atom on B40 fullerene (Fe@B40, see **Figure 3(b)**) and its application as a catalyst for carbon monoxide oxidation by DFT calculations. The iron atom prefers to be adsorb on top of the heptagonal and hexagonal rings of B40 with an adsorption energy of 4.39 and 3.45 eV, respectively. They found that when both CO and O2 molecules are injected into the B40, the surface must be covered by CO due to its higher adsorption energy. Also, the preferable mechanism of CO oxidation is predicted to be termolecular Eley-Rideal (TER) with a small energy barrier of 0.26 eV.

The most stable form of boron is *α*-boron which is capable of adsorbing singlemetal atoms and storing hydrogen molecules. Dong et al. [54] by using DFT

*Boron-Based Cluster Modeling and Simulations: Application Point of View DOI: http://dx.doi.org/10.5772/intechopen.105828*

**Figure 3.**

*The structure of some small clusters of boron that are used as a catalyst. In the figure, light gray, gray, red, green, yellow, plum, mustard, and fuchsia balls represent H, C, O, B, S, P, Fe, and Pt atoms, respectively.*

calculation studied the oxidation of methane (CH4) to form methanol on boron nanosheet/PdO (see **Figure 3(c)**). Initially, methane prefers to adsorb on the boron layer with the adsorption energy of �0.15 eV. Then, CdH bond of methane is broken through the interaction with the PtdO moiety of the catalyst and leads to the oxidation of CH4. This catalyst had high stability and offers excellent methanol selectivity.

Metal-free catalysts can be used instead of the toxic metal oxide catalysts. Amorphous boron (A-Boron) exhibited great catalytic merits for peroxymonosulfate (PMS) activation (see **Figure 3(d)**). The later is carried out by Duan et al. [55] to remove organic contaminants such as benzene, antibiotics, phenolics, and dyes from the water. Their results show that the performance of A-Boron is better than that of nanocarbons, transition metal oxides, and non-carbonaceous materials. They discovered through *in situ* radical capture analysis that both hydroxyl and sulfate radicals are responsible in the oxidation process of organic contaminants. In addition, a boric acid/ hydroxide can form on the surface of A-Boron during heat treatment, which can further deteriorate its catalytic performance. Moreover, DFT calculations revealed that PMC decomposition and peroxide OdO bond cleavage can occur directly on boron atoms along (1 0 0), (1 0 1), and (1 1 0) faces of A-Boron crystal.

Zhao et al. [56] by using DFT calculations proposed a mechanism for the ethanol decomposition on the surface of nano-boron (0 0 1). They found that (I) the ratelimiting step is the dehydrogenation of CH to form a carbon atom (CH + CO ! C + CO); (II) the oxygen dissociation can easily take place on the surface (0 0 1) of the boron crystal; and (III) the existence of O site on the surface (0 0 1) lowers the dehydrogenation energy barrier of CH3CH2O, CHCHO, and CHCH2O species in the ethanol decomposition pathway. The most favorable reaction pathway for the decomposition of methanol and corresponding species on this pathway is presented in **Figure 3(e)**.

### **3.5 Drug delivery**

Boron neutron capture therapy (BNCT) is a new cancer therapy technique that allows the elimination of tumor cells without harmful side effects for other healthy tissues. Harder-Viddal et al. [57] by using MDs studied the storage of the *ortho*carborane cluster (C2B10H12, see **Figure 4(a)**) within the right-handed coiled-coil (RHCC) *tetrabrachion* as a nanotube carrier for BNCT. Their results of binding free energies demonstrated that C2B10H12 can potentially enter and leave the *RHCCtetrabrachion*, which refers to the feasibility of diffusion of C2B10H12 cluster between solvent and carrier in the drug delivery process. They also found that there are about eight storage cavities along the central channel of the conveyor that lead to some stable configurations for this cluster within the conveyor.

The small boron-based cluster has also appeared in cancer therapies. Among the boron clusters, B40 (see **Figure 4(b)**) as the first all-boron fullerene has been investigated as a drug carrier in cancer therapy. For example, Zhang et al. [58] studied the adsorption of 5-fluorouracil (5-Fu) on B40 fullerene and M@B40 (M = Mg, Al, Si, Mn, Cu, Zn). The 5-Fu was adsorbed on the B atom in the corner of the B40 cage, forming the BdO bond. The adsorption energy of 5-Fu was 11.15 kcal mol<sup>1</sup> , which refers to the ease of release of this drug from the surface of B40 cage in an acidic environment of tumor tissues.

Shakerzadeh [59] studied Li- and Na-encapsulated B40 (Li(Na)@B40) fullerenes as carrier for anticancer drug nedaplatin (NdaPt, see **Figure 4(b)**). The energy gap decreased after drug absorbtion, which refers to the formation of stable complexes. The later is a chemical signal to describe drug adsorption and its effects on the electronic properties of B40 cage. The results demonstrated that in both gas and water phases, the adsorption of NdaPt altered more the electronic properties of Li- and Naencapsulated B40 fullerenes compared to bare B40 fullerene. The dipole moments of the Li(Na)@B40 complexes in water were high, suggesting that the solubility of these complexes in the polar medium. Moreover, the adsorption energy for NdaPt/Li(Na) @B40 complexes was 28 kcal/mol.

Zhang et al. [60] by using DFT calculations investigated the potential of B40 fullerene as a carrier for drug nitrosourea (NU, see **Figure 4(b)**). This drug was adsorbed from its N and O atoms on the fullerene surface with an adsorption energy of 25.18 kcal/mol. They showed that newly formed NdB and OdB bonds are strong polar covalent bonds. Also, it is investigated that the recovery time of NU drug under body temperature is 52 s due to the easy release of NU in the medium of cancer tissues.

#### **Figure 4.**

*The structure of some small clusters of boron that are used in drug delivery. The right panel two different views of B40 cluster are shown. In the figure, light gray, gray, red, green, dark blue, orange, and fuchsia balls represent H, C, O, B, N, F, and Pt atoms, respectively.*

They also found that B40 fullerene has a high loading capacity in which it can simultaneously transport five NU drugs.
