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Computational designing ultra-sensitive nano-composite based on boron doped and CuO decorated graphene to adsorb H2S and CO gaseous molecules

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2017 Mater. Res. Express 4 075039 (http://iopscience.iop.org/2053-1591/4/7/075039) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 94.184.182.3 This content was downloaded on 29/07/2017 at 09:19 Please note that terms and conditions apply.

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Mater. Res. Express 4 (2017) 075039

https://doi.org/10.1088/2053-1591/aa7c33

PAPER

RECEIVED

20 March 2017 RE VISED

7 June 2017

Computational designing ultra-sensitive nano-composite based on boron doped and CuO decorated graphene to adsorb H2S and CO gaseous molecules

ACCEP TED FOR PUBLICATION

28 June 2017 PUBLISHED

25 July 2017

Hamed Asadi and Majid Vaezzadeh Department of Physics, K N Toosi University of Technology, Tehran, Iran E-mail: [email protected] Keywords: grapheme, nano sensor, density functional theory (DFT), boron, copper monoxide, hydrogen sulfide, carbon monoxide

Abstract In this paper, interactions of carbon monoxide (CO) and hydrogen sulfide (H2S) with doped and decorated graphene were investigated by using density functional theory and quantum-espresso packages. First of all, impurity effects and properties like adsorption mechanism, the more probable position, binding energy, bond length, Lowdin charge analyze and density of state (DOS) have been determined and then the properties of CO and H2S adsorption calculated, and a brief comparison with other studies has been done. All of these lead to tuning the electronic structure of graphene sheet with impurities that show higher affinities with H2S and CO molecules in comparison to pristine graphene. The obtained results from DOS and charge transfer show that electrical conductance of the B doped graphene sheet and CuO Nano particle decorated—Boron doped graphene sheets are significantly changed compared to the pristine graphene sheet by an increase in the electronic states of near the Fermi’s energy states.

Introduction After identifying the pure graphene and trying to determine physical and chemical properties at the same time, the basic steps toward composing functional components and properties of these structures were begun. In the first action, researchers investigated the adsorption of various atoms on graphene. These works were done for almost all periodic table’s elements except lanthanide atoms and noble gases [1–8]. Graphene derivatives due to the high surface to the volume ratio, have large amount of surface energy. In turn, this energy leads to strong interaction with external molecules and atoms in the gas phase. The aims of theoretical studies and simulations are determining of adsorption mechanism, binding energy, fusion energy, bond length, location and type of physical and chemical bond, changes in band structure, density of states (DOS), electrical and thermal conductivity. In various studies, molecular adsorption of I2, NH3, H2S, CO, CO2, NO, H2O and NO2 on graphene in the term of adsorption energy, bond length and charge transfer processes have been investigated [9–14]. Indeed, some of the researchers were interested in the investigation of the doped graphene sheet to increasing graphene’s electronic, magnetic and mechanical performance. In this paper, Boron (B) doped with copper monoxide (CuO) Nano particle decorated graphene sheet designed to adsorb carbon monoxide (CO) and hydrogen sulfide (H2S). Mai et al investigated electric transport and H2S detection with β-AgVO3 nanowires [15]. Ao et al designed a CO Nano sensor by doping Al atom and lead to Al–CO bond with a large electric conductivity change after adsorption via introducing large amount of shallow acceptor states [12]. In addition, carbon nanotubes (CNTs) based devices are promising sensor to detect gas molecules with high sensitivity and fast response time at room temperature [16]. Following the success in construction of graphene-based sensors, a considerable number of theoretical and empirical reports about graphene- based gas sensors revealed. These achievements were consequences of understanding the effect of absorbent on physical and chemical properties of graphene sheet (GS) such as electrical conductivity by changing the charge carrier’s density. This density variation and the charge transfer between the absorbent and GS, are two significant factors that lead to © 2017 IOP Publishing Ltd

Mater. Res. Express 4 (2017) 075039

H Asadi and M Vaezzadeh

variation in the conductance of GS. One of the best methods available to study the structure and behavior of materials, is computer simulation of atomic and molecular structure of them. Compared with laboratory methods, simulation methods have priority because of availability, low-cost and controllability. Graphene is a good candidate and solution of ultra-high sensitive sensor for extensive applications [17]. Graphene based Nano sensors are working based on the changes of their electrical conductivity induced by surface adsorption, which act as either donors or acceptors associated with their chemical natures and preferential adsorption sites [12]. Moreover, because of small band gap and high electric conductance of graphene, the effect of Johnson noise (thermal noise) is minimum and negligible [18]. This fact leads to large conductivity changes due to the so small changes in charge carriers’ results of gas adsorption. In this paper, attentions are employed to designing an appropriate device to detection of H2S and CO toxic gases. These toxic gases cause abnormal disease and human illness, in the case of H2S concentration higher than 250 ppm result in death [19]. In order to increase adsorption affinity of GS, B atom replaced by a carbon atom in the center of super cell. Dopant rate to carbon atoms is 2% and eight adsorption position for CuO decoration have been considered. In the B-GS structure, B dopant, gives one electron less than the carbon atom in the system that leads to a P-type semiconductor that was shown by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [20]. However, the concentration of impurities could effect on the conductance of materials [21, 22]. The purpose of this research is determining the optimal adsorption position, binding energy, bond length and electronic structural changes of graphene after importing impurities, CO and H2S adsorption and make a comparison to illustrate effects of dopant and decoration on the system. All calculations and simulations are done with Quantum Espresso package and density functional theory (DFT) [23]. The CuO-composites, exhibit an enhanced electro catalytic property, high sensitivity, excellent selectivity, good stability, fast amperometric sensing, low operating voltage and good response to detection of H2S and NH3. Indeed, as the electrode material for lithium–ion batteries, CuO—graphene composite sheets, keep high active surface and prevent agglomeration of graphene sheets [24–26].

Simulations and calculations Aims of this study are determining the binding energy, the more probable adsorption position, bond length, charge transfer and density of states (DOS), by employing the DFT method and PW codes of Quantum Espresso packages. These packages, are collection of open source codes for calculating the electronic structure and modeling materials in Nano scale based on DFT method, plan wave and pseudo potential. Plan wave basis sets and pseudo potential presented as the norm-conserving and Ultra soft, which in this work Ultra soft has been selected. To describe the exchange-correlation potential, GGA (generalize gradient approximation) approximation with PBE (Perdew–Burke–Ernzehf) functional has been used. And also to describe the electron–ion interaction of O, Cu, B and C atoms, the rrkjus (Rappe Rabe Kaxiras Joannopoulos Ultra Soft) pseudo potential was employed. This procedure was done for a monolayer 5  ×  5  ×  1 super cell contains 50 atoms as shown in figure 1(a). The mainspring of selecting this number of atoms, is creating sufficient distance between impurities to ignore interactions of them together. In addition, distance from layers are taken to be 15 Å, because of obstructing interaction of neighbors. At the first step, to derive properties of pure graphene, super cell should be optimized. The bond length of C–C is 1.420 Å and lattice parameter a is equal to 12.290 Å. To find the optimal cut off energy, the value of Ecotoff has been changed in the SCF input file, alternatively, then the total energy relative to Ecotoff, had plotted which 476.200 eV for Ecotoff was derived. The same process were done to estimate the optimized K-point, which resulted in 7  ×  7  ×  1 mesh. Convergence threshold of total energy (a.u), force (a.u) and self-consistency (conv-thr) were taken to be 10−8. At the second step, one of the central carbon atoms replaced by a Boron atom. Z parameter of the B changed by adding 2.500 Å and then structure relaxed to optimize position of Boron atom and whole new structure, which is shown in figure 1(b). To realize more probable decoration position, eight positions were considered as shown in figure 2. The binding energy of adsorption positions, could be calculate by the following equation: Ebind = EB-GS – CuO – (EB-GS – E CuO). (1)

That the B-GS index refers to boron doped graphene sheet. After determining the more probable adsorption (decoration) positions and energies, the next step is invest­ igation of CO and H2S adsorption on the CuO/B-GS Nano-composite (hereafter called Nano composite). In the case of CO gas molecule, C–O triple bond length is equal to 1.1280 Å. Two likely positions have considered in the term of CO adsorption, which are placed 2.500 Å above and perpendicular to the Nano composite, as are shown in figure 3. For the H2S molecule, S–H bond length is 1.340 Å and the bond angel H–S–H is 92.1°. Figure 4, shows three adsorption positions of this molecule, which in the initial situations were placed 2.500 Å above the Nano composite. 2

Mater. Res. Express 4 (2017) 075039

H Asadi and M Vaezzadeh

Figure 1.  Pure GS contains 50 carbon atoms, and 25 unit cells (a), B (gray atom in the center) doped GS (B-GS) (b).

Figure 2.  More probable CuO adsorption positions on B-GS (Cu, O, B and C are characterized by brown, red, gray and yellow colors, respectively).

Figure 3.  More probable CO (C in yellow and O in red) adsorption positions on nano composite, (a) from carbon side and (b) from oxygen side.

Figure 4.  More probable H2S (H in light blue and S in olive color) adsorption positions on nano composite , from the S atom (a), from the two H atoms which S atom is top of them (b), from the one of H atoms (c).

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Table 1.  Bond length and binding energy of more probable adsorption positions. Bond length (Å) Adsorption positions

Binding energy (eV)

C–Cu

B–Cu

B–O

Cu–O (CuO)

O(CO)–Cu

O–C (CO)

S–H

S–Cu

TB1

−1.330

2.180

2.110

—

1.710

—

—

—

—

PBC1

−1.427

2.070

2.210

—

1.700

—

—

—

—

TC1

−1.402

2.040

2.300

—

1.700

—

—

—

—

TH1

−1.367

—

2.180

—

1.710

—

—

—

—

TB2

−1.590

—

—

1.530

1.750

—

—

—

—

PBC2

−1.569

—

—

1.530

1.760

—

—

—

—

TC2

−1.497

—

—

1.510

1.760

—

—

—

—

TH2

−1.339

—

—

1.520

1.730

—

—

—

—

CO(a)

−0.664

—

—

1.500

1.730

1.830

1.160

—

—

CO(b)

−2.386

—

—

1.480

1.710

1.740

1.150

—

—

H2S(a)

−1.518

—

—

1.490

1.750

—

—

1.360

2.100

H2S(b)

−1.308

—

—

1.530

1.770

—

—

1.370

2.150

H2S(c)

−1.297

—

—

1.500

1.770

—

—

1.360

2.120

Results and discussion After optimizing the PGS, the flat structure remains and bond length decreased to 1.422 Å, bond angle remains 120 degrees, also lattice constants a and c/a have been changed to 12.297 Å and 0.645 Å, respectively. The energy of entire structure was obtained 8128.830 eV. The PGS is not a suitable case to detect H2S and CO due to the low physisorption energy and large bond length. Indeed, in these cases, the energy, bond length and adsorption site were obtained, −0.022 eV, 3.150 Å above the hallow site for H2S and  −0.0102 eV 3.860 Å above the carbon atom for CO [27, 28]. By doping the Boron atom to the PGS, the total energy was decreased to 8050.610 eV and the B–C bond length of 1.489 Å was obtained. The effect of dopant, is removing one electron from super cell that leads to the p-type semiconductor. Although BGS is more appropriate option than the PGS to detect H2S and CO due to the results, but in this case, the adsorption energy is relatively weak and so not recommended as an appropriate Nano composite to adsorb them. The energy, bond length and more stable positions were obtained, −0.438 eV and 2.700 Å above the B atom for H2S adsorption and  −0.140 eV and 2.970 Å above the B atom from the carbon side of CO [29, 30]. The decoration energy, bond length and more probable site of CuO Nano particle on PGS had obtained, −0.870 eV, 2.160 Å and perpendicular to the C–C bond [24], before the B atom had been doped. Indeed, adsorption just had been occurred from the Cu side, and any adsorptions were found from the Oxygen side due to the negligible binding energy and long bond length. According to the calculations, after doping B, remarkable increase in binding energy and decrease in bond length has been found. Adsorption has become strongest either the Oxygen or copper sides. The full results of CuO decoration on BGS are shown in table 1. According to these results, CuO Nano particles could be adsorbed to the all of sites on B-GS, but the more probable site is above the B atom from Oxygen with binding energy equal to  −1.590 eV and O–B bond length about 1.670 Å that indicate the stable and strong bond. This position is shown in figure 5(a). In order to detect CO and H2S gases, the mentioned adsorption site was employed as Nano composite (CuO/B-GS). To ensure that this Nano composite is appropriate device to detect CO and H2S toxic gases, adsorption procedure and calculations have been done, also, results demonstrate that for both cases, this Nano composite works efficiently. The entire results are shown in table 1. According to this table, in the case of CO adsorption on Nano composite, adsorption from Carbon side of CO is significantly probable than the Oxygen side. The amount of binding energy  −2.386 eV and bond length of 1.740 Å resulted for this adsorption position, which is shown in figure 5(b). In the case of H2S, the binding energy of  −1.518 eV and the bond length of 2.100 Å were resulted for the more probable adsorption position which located on the top of the Copper atom from the Sulfur atom side of H2S molecule, which is shown in figure 5(c). Furthermore, the other adsorption positions of H2S, have shown a considerable enhancement of binding energy, so that this molecule could be adsorbed from the all orientations. These results indicate that the adsorption affinity of Nano composite is improved significantly by doping Boron atom and decorating CuO Nano particle. Electronegativity of B is 2.04 and C is 2.55, so after doping B, the bonding electron tend to the carbon more than Boron. This mechanism leads to appear an anchoring point located at dopant position. It’s like polarization 4

Mater. Res. Express 4 (2017) 075039

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Figure 5.  Optimized adsorption position of CuO-BGS (a), H2S/CuO-BGS (b) and CO/CuO-BGS (c) on the nano composite.

Figure 6.  DOS of BGS (a), CuO-BGS (b), CO/CuO-BGS (c) and H2S/CuO-BGS (d).

that Boron become positive and neighbor carbons become negative. According to the Lowdin charge analyze by using projwfc codes, each carbon atom on the graphene has 3.9528 charge which contribution of s and p orbitals are 0.8976 and 3.0552, respectively. Single B atom has 2.9997 charge. After doping B to the center of graphene, due to the higher electronegativity of the carbon, Boron donate 0.1496 charge to its neighbor carbons. These 3 carbons show more than 0.1496 increase in their charge because doping B made a positive potential that affected to the other neighbor carbons and attracted part of their charge. According to the DOS diagram of figure 6(a), a pick was appeared around  −1 eV under the Fermi’s level which is fixed to zero. This pick indicates acceptor states due to the elimination of one electron. By decorating CuO Nano particle, this particle attached to the anchoring point from Oxygen side and it was what was expected because Oxygen is more electronegative than Cu (1.9 for Cu and 3.44 for O) so O side is more negative than the Cu side and it makes CuO a polarized particle. Now by composing CuO to the B-GS, strong bond appears in the system. In the CuO/GS, due to the electronegativity of C atoms, surface of GS is negative so CuO attached to the 5

Mater. Res. Express 4 (2017) 075039

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surface from the Cu side. As shown in figure 6(b), DOS diagram dramatically has changed and numerous of electronic levels appeared near fermi’s level with small gap, which shows that system is more metal like and as a result enhancement of electric conduction is expected. Moreover, the surface is more active in the presence of CuO Nano particles. During adsorption of CO gas, according to the Lowdin charge analyze, 0.1357 charge transferred from CO to the Nano composite which contribution of carbon is significant and equal to the 0.139 but Oxygen atom of CO takes 0.033 charge. In the case of H2S, 0.3493 charge transferred to the Nano composite which p orbital of Sulfur atom donate majority of this charge by the amount of 0.2587. As shown in figures 6(c) and (d), overlap between before and after H2S and CO adsorption demonstrate strong hybridization near fermi’s level which means these molecules strongly adsorbed to the Nano composite (CuO/B-GS). Finally, these calculations reveals that the two procedure of H2S and CO adsorption, the transferred electrons from H2S and CO to CuO, are easily extracted by B-GS. Therefore, using the binding energy, bond length, Lowdin charge analyze and DOS, clearly, in the same conditions, our Nano composite is more effective in adsorption of H2S and CO gaseous molecules than the pure and only decorated GS.

Conclusions DFT method and Quantum Espresso package were used to study the optimal position, geometric structure, electronic structure, charge transfer and adsorption procedure of H2S and CO on the CuO Nano particle decorated, and Boron doped graphene sheet (B-GS) Nano composite. The effects of dopant and decoration were investigated by comparison with pure graphene sheet (PGS), and the other kinds of Graphene based Nano composite. The planar structure of the GS remains unchanged by doping Boron, but the electronic structure changed from semimetal to the semiconductor. An increase in the band gap observed because of symmetry breaking of subtleties. After decorating copper monoxide, the carriers’ population moved from the valence band to the conduction band and occupied d orbitals that cause to increase in the conductance of the system. Furthermore, after CuO adsorption, system loses the planar structure in the dopant location. Considerable changes were observed in the electronic structure once CO and H2S were adsorbed. These changes directly affected on the conductance and the other electronic properties due to the transferred charge, which prepare conditions to detect external impurities and also by proper calibration one can determine percent and kind of adsorbed materials. Indeed, by changing concentration of dopant and decoration, it’s possible to tune the sensitivity of Nano composite.

ORCID Majid Vaezzadeh

https://orcid.org/0000-0002-4159-9895

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