Pramana – J. Phys. (2022) 96:73 © Indian Academy of Sciences https://doi.org/10.1007/s12043-022-02310-5
Reduction and adsorption of hydrogen peroxide in the oxygen and beryllium vacancies of beryllium oxide nanotubes
ALI A RAJHI, SAGR ALAMRI∗and GHAFFAR EBADI
Department of Mechanical Engineering, College of Engineering, King Khalid University, P.O. Box 394, Abha 61421, Saudi Arabia
∗Corresponding author. E-mail: salamri@kku.edu.sa
MS received 29 July 2021; revised 14 November 2021; accepted 17 November 2021
Abstract. The adsorption of the hydrogen peroxide (H2O2)molecule onto pure and (O or Be) vacancies of BeO nanotube (BeONT) was studied using density functional theory computations. As H2O2approaches the pure BeONT and Be-vacancy BeONT, their adsorption releases−8.3 and−31.3 kcal/mol, respectively, indicating physisorption.
Also, the electronic properties of the nanotube do not change significantly. But when H2O2approaches the O-vacancy BeONT (VO-BeONT), its adsorption releases−471.2 kcal/mol of energy, and electronic analysis showed that the VO-BeONT HOMO/LUMO gap reduces approximately about−29.9% and the electrical conductivity increases significantly. The reactivity of Be atoms of the defect is more towards H2O2 reduction to H2O compared with perfect ones. Throughout the process of adsorption, the diffusion of the O atom of the H2O2molecule was into the vacancy site, thereby dissociating the O–O and O–H bonds of H2O2and forming H2O. Therefore, VO-BeONT can generate electrical signals when the H2O2molecule approaches, being a hopeful sensor.
Keywords. Density functional theory; adsorption; reduction; BeO nanotube; nanostructure; hydrogen peroxide.
PACS Nos 03.65.-w; 03.65.Ta; 0.5; 20.Dd; 05.40.Ca
1. Introduction
In chemical industry, hydrogen peroxide (H2O2)is one of the commonly used cleaning agents and selective oxidants. H2O2 is produced as a result of biological reactions catalysed by enzymes, and therefore, detecting H2O2 is of paramount importance in medical diagnos- tics, preservation of the environment and food industry [1]. Numerous methods exist for the detection of H2O2. Electrochemical techniques are advantageous as they are simple, cost effective, highly sensitive and selective.
Hence, investigating the adsorption of H2O2as well as its reduction reaction onto suitable surfaces is electro- chemically important to design electrodes [2,3].
Carbon nanotubes (CNTs) have been celebrated by researchers [4]. CNTs are flexible and light with a large elastic modulus. The electronic characteristics of CNTs are commonly dependent on their curvature as well as the kind of chirality they have [5–11]. CNTs are candidates for different uses in nano-engineering [12–
23], and numerous researchers have synthesised and introduced several other similar inorganic nanotubes.
Different models of pure monolayer beryllium oxide
(BeO) nanotubes (BeONTs) as well as their cohesive, structural and electronic characteristics have been anti- cipated [24,25]. BeO compounds have characteristics that are different from SiC, C and BN compounds because the iconicity of the O–Be bond is more. Also, as a semiconductor, BeO has large elastic constants, high thermal conductivity, high melting point and large bandgap [26]. Moreover, it was found that BeONTs pos- sess higher HOMO–LUMO gaps than CNTs because BeONTs depend less on the diameter of tubes and chirality [27]. Furthermore, BeONTs have outstand- ing mechanical characteristics, i.e., Young’s modulus of BeONTs is comparable to CNTs [28]. Owing to these characteristics, BeONTs are considered as promising candidates to be used in nanoelectronic devices.
The adsorption of gases onto CNTs and nanotube bundles has been considered important in nanotube applications. Owing to their adsorptive properties in the gas phase, nanotubes have been employed as gas sensors, in fuel storage and in reducing dangerous pol- lutants from gas streams [29–34]. Nonetheless, most gases are physically adsorbed onto the suspended intrin- sic nanotubes [35]. On the contrary, dissociation and
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Figure 1. The optimised structure of BeONT and its DOS.
adsorption characteristics of molecules on nanotubes can be improved by defects and dopants [36–40], which show the importance of defects and doped atoms in nanotube applications. In this work, we investigated the possibility of using a pristine BeONT, as a viable sur- face for the adsorption and reduction of H2O2 using density functional theory (DFT) computations. Finally, the interaction of the H2O2 molecule with BeONT on oxygen (VO-BeONT) and beryllium vacancies (VBe- BeONT) was investigated.
2. Computational methods
All computations, including energy computations, geometry optimisation and analysis of state density were carried out through dispersion-augmented B3LYP, i.e, B3LYP-D. Here, Grimme’s “D” term is used for evaluating dispersion forces. As a basis set, we used 6-311G** (d) and GAMESS software for calculations [41]. GaussSum [42] software was also employed to draw the density of states (DOS) plots. B3LYP is often employed in nanostructures due to its remarkable per- formance in the field [43–47].
The energy of adsorption (Ead)of the molecule fol- lowing its adsorption onto the BeONT is computed as
follows:
Ead=E(H2O2/(BeONT)−E(BeONT)
−E(H2O2)+EBSSE. (1) HereE(H2O2)is the energy of the H2O2 molecules, E(BeONT) is the pristine BeONT energy and E(H2O2/BeONT) is the energy of the BeONT on which H2O2is adsorbed. Negative Ead shows the exothermic nature of the adsorption. Here, the basis set super- position error (BSSE) was also corrected for all the interactions under study. The HOMO/LUMO energy gap (Eg)was computed according to the following equa- tion:
Eg= ELUMO−EHOMO. (2) Here,ELUMOis the energy of the lowest unoccupied molecular orbitals (LUMO) and EHOMO is the energy of the highest occupied molecular orbitals (HOMO).
3. Results and discussion
3.1 The optimised structure of the pure BeONT 130 beryllium atoms and 130 oxygen atoms contained BeONT (10,0) was selected as an adsorbent for O3gas.
Pramana – J. Phys. (2022) 96:73 Page 3 of 8 73
Table 1. Calculated adsorption energy (Ead, kcal/mol), HOMO energy (EHOMO), LUMO energy (ELUMO), Fermi level energy (EF), HOMO–LUMO energy gap (Eg), work function (), and charge on the H2O2molecule (Q(e)) for the adsorption of H2O2molecule on the pristine BeONT (figures1and2). B3LYP method is used.
Structure Ead EHOMO EF ELUMO Eg Eg Q(e)
BeONT − −7.46 −3.95 −0.43 7.03 − 3.95 − −
N −7.1 −7.31 −3.90 −0.49 6.82 −3.0 3.90 −1.1 0.10 M −8.3 −7.27 −3.89 −0.51 6.76 −3.8 3.89 −1.4 0.12
Figure 2. Models for two stable adsorption states for an H2O2molecule on the pristine BeONT (distances are in Å). (a) ComplexNand (b) complexM.
We used hydrogen atoms to saturate its ends. The aver- age Be–O bond length is about 1.56 Å. The diameter and the length of the optimised intrinsic BeONT are predicted to be∼8.67 and 28.04 Å, respectively. From the DOS plot in figure 1, the energy levels of HOMO and LUMO of pristine BeONT are −7.46 and −0.43 eV, respectively (table 1). Hence, Eg was estimated to be∼7.03 eV, indicating semiconducting property.
3.2 H2O2adsorption onto the pure BeONT
To find the most stable complex of H2O2/BeONT, we considered various primary adsorption geometries. Fig- ure2shows the two most stable states. In configurations N and M, the H2O2 molecule is adsorbed onto the BeONT surface. In configurationN, the H2O2molecule
approaches via its two O atoms to the two Be atoms and via one H atom to the O atom with interacting distances 3.91 and 3.14 Å for O…Be and 2.97 Å for H…O.
In configurationM, H2O2interacts via one O and one H atom with one Be and O atom of the tube, respectively.
Be…O distance is 3.22 Å and H…O distance is 3.08 Å.
ConfigurationsNandMindicate poor interaction when the Ead =−7.1 and−8.3 kcal/mol, respectively (table 1). It should be noted that the weak interaction in com- plex Ncompared to Mis due to the higher structural strain and steric effect when three atoms simultaneously react with the surface of the BeONT. By the adsorption of H2O2, there is a slight change in the conduction and valence levels, which in turn reduces Eg of the com- plexes from 7.03 eV in the BeONT to 6.82 and 6.76 eV for N andM complexes, respectively (table1). Some
Figure 3. The optimised structure of (a) BeONT with Be vacancy (VBe-BeONT) and (b) BeONT with O vacancy (VO-BeONT).
Figure 4. Models for the most stable adsorption states of an H2O2molecule for (distances for VBe- and VO-BeONT are in Å) (a) complexSand (b) complexT.
Pramana – J. Phys. (2022) 96:73 Page 5 of 8 73
Figure 5. (a) The HOMO profile of VOnanostructure complex and (b) the HOMO profile of VBenanostructure complex.
Table 2. Calculated adsorption energy (Ead, kcal/mol), HOMO energy (EHOMO), LUMO energy (ELUMO), Fermi level energy (EF), HOMO–LUMO energy gap (Eg), work function ()and charge on the H2O2molecule (Q(e)) for H2O2molecule adsorption on the defected BeONT (figures3and4). B3LYP method is used.
Structure Ead EHOMO EF ELUMO Eg Eg Q(e)
VBe–BeONT – −7.01 −3.76 −0.50 6.51 − 3.76 – –
S −31.3 −6.32 −3.47 −0.61 5.71 −12.3 3.47 −7.7 0.24
VO-BeONT – −6.98 −3.75 −0.52 6.46 – 3.75 – –
T −471.2 −5.27 −3.01 −0.74 4.53 −29.9 3.01 −19.9 1.13
natural bond orbital (NBO) charge transfers from H2O2
to BeONT is about 0.12|e|. Thus, the adsorption of H2O2
on BeONT is a physisorption process. The electrical conductivity of the nanotube can change because of the change in Egbased on the following equation:
σ = AT3/2e
−2kTEg
. (3)
Here, A is a constant (electrons/m3K3/2), σ is the electrical conductance andkis the Boltzmann constant [48]. At a constant temperature, as Eg gets lower, the electrical conductivity becomes higher. So, the elec- trical conductivity of BeONT increases significantly following a decrease in Eg, which is caused by the adsorption process. Therefore, this change of Eg is very small and negligible which cannot generate a proper electronic noise to detect the presence of H2O2
molecule. Thus, pure BeONT cannot be a proper sensor for H2O2detection.
3.3 H2O2adsorption on vacancy-defected BeONT Next, we investigate the interaction of a vacancy- defected BeONT with an H2O2 molecule. Here, we focus on VBe and VO. Figure 3 shows the optimised geometric structures of the vacancy-defected BeONT.
Figure 6. Partial DOS plot of H2O2 and VO-BeONT nanostructure complex.
The formation energy of the defect (Ef) is also com- puted as follows:
Ef = E(BeONT)−E(defected BeONT)
−E(Be or O), (4)
whereE(defected BeONT) is the total energy of the VO
or VBe tube and E(Be or O) is the energy of a single vacancy-defected Be or O. For VO, Ef is computed to be +119.4 kJ/mol, but it is −117.6 kJ/mol for VBe. PositiveEfindicates the exothermic nature of the defect formation process for VO. Also, it suggests that VOis a more favourable process than VBein terms of energy.
Figure 7. The HOMO profile of H2O2/VOnanostructure complex.
Here, the adsorption of H2O2 is studied on VBe- BeONT. For this purpose, the H2O2molecule is located above the vacancy site on the vacancy-defected BeONT surface such as the top of the bond bridge, over the centre of a six-membered ring and on the Be or O atom. H2O2is located parallel to the surface of BeONT or perpendicu- lar to it. One local minimum is predicted for each VBeor VOafter the optimisation process (figure4). In config- urationS(VBe-BeONT), two H atoms of H2O2interact with two O atoms of the defected site BeONT. The cor- responding interaction distance between the O atoms of the defected tube and the H atoms of the H2O2is about 2.31 Å andEadis about−31.3 kcal/mol (table1). Some natural bond orbital (NBO) charge transfers from H2O2
to BeONT is about 0.24|e|. Thus, the adsorption of H2O2 on the VBe-BeONT is a physisorption process.
In H2O2adsorption on VO-BeONT (configurationT), throughout the optimisation, the diffusion of the O atom of the H2O2molecule was into the vacancy site, thereby dissociating the O–O bond of H2O2. The distances of interaction between the diffused O atom and three adja- cent Be atoms in the nanotube were∼1.50, 1.54 and 1.95 Å andEadwas∼ −471.2 kcal/mol (table2). Also, dur- ing the dissociation of H2O2,an O–H bond was formed and its length was 1.52 Å. Therefore, unlike pure and VBe-BeONT, H2O2was reduced to H2O when oxygen vacancies were present. The adsorption of H2O2in the vacancy-defected BeONT is more favourable than in the pure BeONT, because the localisation of the HOMO is mainly on the defected site of the defected-BeONT (fig- ure5).
After H2O2adsorption on VO-BeONT (figure6), the partial DOS plot illustrates that a new occupied orbital appears on the nanotube electron forbidden area (Eg) at−5.27 eV. The HOMO profile shown in figure7also
confirms that the HOMO of the complex shifts on the H2O2 by changing the HOMO energy. The Eg of the defected BeONT decreases significantly, following the adsorption of H2O2and the tube becomes more conduc- tive. Numerically, itsEgin complexTis decreased from 6.46 to 4.53 eV (∼ −29.9%) by the H2O2 adsorption process. According to eq. (3), the VO-BeONT sensi- tivity towards the H2O2 molecule increases. Thus, it is found that VO-BeONT can detect and reduce the H2O2
molecule.
To further evaluate the sensitivity of the surfaces, the changes in the work function () were investigated before and following the adsorption process. of a semiconductor is the least amount of work needed for extracting an electron from the Fermi level. The re- examination of the gas-induced by the suspended amplitude effect modifiers has been applied for the real- isation of a sensor operating system for many years [49].
Theoretically, in vacuum, we define the released elec- tron current density as follows:
j= AT2e
−kT
, (5)
where Ais the Richardson constant in A/m2, T is the temperature in Kelvin andis the work function.can be computed as follows:
=Einf−EF, (6)
whereEFis the energy of the Fermi level andEinfis the electrostatic potential, which is supposed to be equal to 0 at infinity. We subtractedof the nanotube from that of the complexes and obtained change in().for the pristine BeONT is about 7.03 eV which changed very slightly after adsorbing the H2O2 molecules, and this
Pramana – J. Phys. (2022) 96:73 Page 7 of 8 73 can be ignored. But when H2O2 is adsorbed onto VO-
BeONT,is significantly reduced from 3.75 to 3.01 eV.
According to eq. (5), there is an exponential relationship between the emitted current density and. Therefore, it can be said that after the adsorption of H2O2, by decreasing , the current density of the emitted elec- tron increases dramatically. Accordingly, we think that VO-BeONT is a better option to increase the sensitivity of BeONT toward H2O2than the pristine BeONT. The results are consistent with the results obtained using gas adsorption on vacancy defects in nanostructures [50–
54].
4. Conclusion
One-dimensional nanostructures have been significantly used in the sensor industry because of their unique electronic properties and surface-to-volume ratio. We studied the adsorption of the H2O2 molecule onto the pure and vacancy-defected BeONT by performing DFT computations. The H2O2 molecule was weakly adsorbed onto the pure and VBe-BeONT with low Ead (from −8.3 to −31.3 kcal/mol) and much inter- action distance. After the adsorption of this molecule, there was a slight change in the pure and VBe-BeONT electronic properties. However, the interaction of the H2O2molecule with the VO-BeONT was strong. For the VO-BeONT complex, Ead was −471.2 kcal/mol.
By H2O2molecule adsorption, the HOMO of the VO- BeONT was meaningfully destabilised. Throughout the process of adsorption, the diffusion of the O atom of the H2O2 molecule was into the vacancy site, thereby dis- sociating the O–O and O–H bonds of H2O2 forming an H2O. Therefore, there was a considerable reduction in its Eg which significantly amplifies the electrical con- ductivity. Thus, it was found that VO-BeONT can reduce and detect H2O2molecules.
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