Ni doping effect on the electronic and sensing properties of 2D SnO2 Anjali Patel, Basant Roondhe, and Prafulla K. Jha
Citation: AIP Conference Proceedings 1961, 030039 (2018); doi: 10.1063/1.5035241 View online: https://doi.org/10.1063/1.5035241 View Table of Contents: http://aip.scitation.org/toc/apc/1961/1 Published by the American Institute of Physics
Ni doping effect on the electronic and sensing properties of 2D SnO2 Anjali Patel1, a), Basant Roondhe1,b), Prafulla K. Jha1, c) 1
Department of Physics, Faculty of Science, The M. S. University of Baroda, Vadodara, Gujarat, India-390002 a)
Corresponding author:
[email protected] b)
[email protected] c)
[email protected]
Abstract. In the present work using state of art first principles calculations under the frame work of density functional theory the effect of Nickel (Ni) doping on electronic as well as sensing properties of most stable two dimensional (2D) TSnO2 phase towards ethanol (C2H5OH) has been observed. It has been found that Ni atom when dope on T-SnO2 causes prominent decrement in the band gap from 2.26 eV to 1.48 eV and improves the sensing phenomena of pristine T-SnO2 towards C2H5OH by increasing the binding energy from -0.18eV to -0.93eV. The comparative analysis of binding energy shows that Ni improves the binding of C2H5OH by 5.16 times the values for pristine T-SnO2. The doping of Ni into 2D T-SnO2 reduces the band gap through lowering of the conduction band minimum, thereby increasing the electron affinity which increases the sensing performance of T-SnO2. The variation in the electronic properties after and before the exposure of ethanol reinforced to use Ni:SnO2 nano structure for sensing applications. The results indicate that the Ni doped T-SnO2 can be utilized in improved optoelectronic as well as sensor devices in the future.
Keywords: Ni doping, sensor, two dimensional T-SnO2, ethanol.
INTRODUCTION The past few years have witnessed a great revolution in the field of nanotechnology after the evolution of two dimensional material graphene due to high charge carrier mobility, large surface-to-volume ratio and outstanding electronic, phonon and mechanical performance [1-6]. These excellent properties of graphene show a great promise in future nanodevices application. However, the success in graphene leads researchers to explore other possible 2D materials [7-12] from their bulk material mainly due to its zero band gap which limits it uses in the electronic applications[13]. This has motivated researchers to continue search for other 2D materials which could exhibit properties better than or similar to graphene and can be used as new material for improved performance. Among various metal oxides, tin oxide (SnO2) nanostructures have attained a special position as a promising candidate for optoelectronic[14] as well as in gas sensing application due to its versatile personality generated by its exceptional properties like optical transparency, wide band gap and high chemical stability[15]. To the best of our knowledge despite great possibilities of two dimensional (2D) T-SnO2 only few studies are reported[16-17]. The bandgap tailoring which is important for electronic and sensing properties, no attempt to tailor bandgap of 2D SnO2 is yet reported. Many efforts in the direction to improve the sensing mechanisms of SnO2 have been explored because as from the previous study SnO2 has shown low selectivity at room temperature. However, it is known that the high temperature is required for better detection of gases [18]. To overcome these many efforts are made and one of the most common is to introduce a dopant. Previously noble metals like Pd and Pt are doped and found to create extensive improvement in the SnO2 sensing properties [19-20] but due to their high cost this has not been encouraged. Thus an alternative is explored from the less expensive 3d transition metals (Ni, Fe, Cu, Co) which also
International Conference on Nanomaterials for Energy Conversion and Storage Applications AIP Conf. Proc. 1961, 030039-1–030039-6; https://doi.org/10.1063/1.5035241 Published by AIP Publishing. 978-0-7354-1666-6/$30.00
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have high catalytic properties and can be utilized to enhance the sensing performance [21-22]. In this work, we present density functional theory based first principles calculation to address following issues: (1) to find the most stable geometry and adsorption energy for interaction of ethanol with pristine and Ni doped T-SnO2; (2) to tune the electronic properties of pristine T-SnO2 after adsorption of ethanol; (3) difference in adsorption mechanism due to Ni doping in T-SnO2.
COMPUTATIONAL METHOD The structural and electronic properties of pristine and Ni doped T-SnO2 have been investigated using first principles calculation based on density functional theory as implemented in Quantum Espresso code [23]. The Perdew – Burke – Ernzerhof (PBE) exchange correlation functional in generalized gradient approximation (GGA) is used for structural optimization [24]. The kinetic energy cut off 80 Ry was chosen in both configurations for plane wave basis set. The Brillouin zone integration of 13×13×1 with Γ-centered Monkhorst-Pack was taken [25]. The adsorption energy Ead is calculated according to the equation as follows: (1) ୟୢ ൌ ୱ୷ୱ୲ୣ୫ െ ሺୣ୲୦ୟ୬୭୪ ିୗ୬మ Ȁ୧ǣିୗ୬మ ሻ Here ୱ୷ୱ୲ୣ୫ is the total energy of ǣ െ ଶ Ȁ െ ଶ adsorbed ethanol, ୣ୲୦ୟ୬୭୪ is the total energy of the adsorbate, ିୗ୬మ Ȁ୧ǣିୗ୬మ is the total energy of ǣ െ ଶ Ȁ െ ଶ obtained from their fully optimized geometries.
RESULTS AND DISCUSSION Before calculating the electronic band structures of pristine and Ni decorated T-SnO2, we have optimized individually both pristine as well as Ni doped T-SnO2. The equilibrium geometry is shown in Fig. 1(a-b). As seen from the optimized structure presented in Fig. 1(a), T-SnO2 has honeycomb lattice structure same as graphene but with non-planarity in the structure with D3d symmetry. Stannic oxide (T-SnO2) is a centred hexagonal structure consisting of O-Sn-O where Sn atom lies between two layers of oxygen atom. Sn is attached octahedrally with oxygen which is the most stable structure of 2D T-SnO2 [16]. The calculated lattice constants are listed in Table 1 along with the previously reported work.
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(a)
(b)
(b)
Side view
Top view
FIGURE 1. (a) Side and top view of pristine 2D T-SnO2, (b) Side and top view of Ni doped 2D T-SnO2. Green color ball represents the Sn atoms, Red color ball represents the O atoms and Blue color ball represent the Ni atom
The calculated lattice constants are in good agreement with the available previous work [16]. For the doping/decoration of Ni a supercell of 3x3 is built. Doping causes effect on the structure which is clear as the significantly changes in lattice constant is observed.
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TABLE 1: Lattice parameter and band gap for T-SnO2 and Ni:T-SnO2 with available reported data Systems
T-SnO2(unit cell) T-SnO2(supercell-3X3) Ni:T-SnO2 (supercell 3X3) T-SnO2+C2H5OH Ni:T-SnO2+ C2H5OH
Work
a (Å)
Eg (eV)
Ead(eV)
Present Other[16] Present
3.242 3.224 9.253
2.48 2.55 2.48
-
Present
9.777
1.62
-
Present Present
9.729 9.762
0.19 1.8
-0.18 -0.93
Further to understand the electronic properties and the orbital contribution for the construction of conduction band minima (CBM) and valence band maxima (VBM), we have calculated the band structure as it unveils the nature of material. The electronic band dispersion of centred hexagonal SnO2 (T-SnO2) is shown in Fig. 2(a). The dispersion clearly depicts the semiconducting behaviour of T-SnO2 with an indirect band gap of 2.46 eV between ΓK, which is in good agreement with the previously reported work [16]. As it is expected for the case of 2D T-SnO2 the bandgap is significantly larger than bulk T-SnO2 value Eg= 0.79 eV. The reason behind this augmentation of band gap is the quantum confinement which occurs going from bulk to lower dimension. The band gap changes significantly by doping (substitution) of one nickel atom which is displayed in Fig. 2 (b). The CBM shifts from the Γ point to the M point, and the VBM moves to the point M from K, causing an indirect-to-direct band-gap transition. The band gap of pristine T-SnO2 decreases from 2.46 eV to 1.62 eV. Doping of Ni also alters the Fermi energy of TSnO2 as a significant change of 1.218 eV can be seen in the Fermi energy of T-SnO2. The Fermi level is shifted from -5.191 eV to -3.973 eV. The Dirac band [26] is observed at K wave vector which shows high mobility of electron at that point like graphene. The Dirac band disappears creating little gap when Ni is doped.
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(a)
(b)
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FIGURE 2. The electronic band structure of (a) T-SnO2 and (b) Ni/T-SnO2. The Fermi level set to zero.
To understand the electronic properties more clearly the density of states (DOS) along with the projected DOS (PDOS) is also calculated as it can describe more clearly the reason behind the band gap modulation. Figure 3(a-d) shows the DOS and PDOS of pristine and Ni doped T-SnO2. It is clear from the figure that the doping of Ni creates a sharp peak near Fermi level just above the valence band maxima (VBM) between 0 to -2 eV and some density contribution can be seen near the conduction band minima (CBM) in the range 1 eV to 2 eV. This extra peak immerges from the contribution from 2p orbital electrons present in Ni at both CBM and VBM. Little contribution of 3d orbit electron of Ni influences the VBM between -2 eV to -4 eV.
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(a)
(b)
(b)
(a)
(c)
(d)
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FIGURE 3. The Density of States (DOS) plot of (a) Pristine T-SnO2 and (b) Ni doped T-SnO2. Projected DOS of (c) Pristine TSnO2 and (d) Ni doped T-SnO2
This tailoring in the electronic property of T-SnO2 increases its sensing ability which is checked for C2H5OH. The interaction of C2H5OH has been studies on both pristine as well as Ni doped T-SnO2 by calculating binding energy and DOS. The adsorption of ethanol molecules is physical (physisorption) in nature for the pristine T-SnO2 with the distance 2.96 Å as can be seen from the Fig. 4(a). However, in the case of Ni:T-SnO2 the molecule binds covalently with the distance 1.78 Å shown in Fig 4(b). The binding energy calculated for pristine SnO2 is -0.18 eV which is comparatively lower than the case of Ni:T-SnO2. Our calculation depicts a dominance of Ni doped T-SnO2 over pristine T-SnO2 in the adsorption of ethanol.
(a)
(b)
FIGURE 4. The optimized geometry (a) Ethanol adsorbed on pristine T-SnO2 and (b) Ethanol adsorbed on Ni doped T-SnO2.
To further understand the effect of ethanol adsorption over both pristine as well as Ni doped T-SnO2, we present total electronic density of states (DOS) for both SnO2 and Ni:T-SnO2 with ethanol in Fig. 5(a-b).
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(b)
(a)
FIGURE 5. Density of States of (a) Ethanol adsorbed over pristine T-SnO2, (b) Ethanol adsorbed over Ni:T-SnO2
Fig. 6 (a-b) presents the projected DOS (PDOS) for ethanol adsorbed T-SnO2 and Ni:T-SnO2. The analysis of DOS together with the PDOS indicates that the contributions of ethanol molecules are localized around the top of the valance band. It can be clearly seen from the Fig. 6 (a) that the C(2p) and O(2p) orbitals apart from Sn(2p) orbitals have major contribution in the valance bands. But the major contribution of Sn(2p) orbitals C(2p), O(2p) along with Ni(2s) orbital is seen in both valance as well as conduction band region for the ethanol adsorbed Ni:TSnO2 depicted in Fig 6 (b).
(a)
(b)
FIGURE 6. Projected Density of States of (a) Ethanol adsorbed over pristine T-SnO2, (b) Ethanol adsorbed over Ni:T-SnO2
The analysis of present results on binding energy conferms that the ethanol molecules get physically adsorbed on pristine T-SnO2 but make bond when adsorbed on Ni:T-SnO2. The ethanol molecule make covalent bond with Ni:TSnO2 indicating useful for sensing of ethanol.
CONCLUSION The effect of Ni doping on the electronic and sensing properties toward ethanol of pristine 2D T-SnO2 is investigated by first principles calculations under the frame work of density functional theory. The decrement of the band gap together with the transition of indirect to direct band gap is observed in T-SnO2 after Ni doping. The existence of a direct band gap (1.62eV) can cause relatively high exciton binding energy which opens up the
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possibility of designing Ni:T-SnO2 based novel and advanced optoelectronic devices. Further, the Ni decoration of T-SnO2 improves electronic properties which will open up the material for other applications, such as in organic photovoltaics and gas sensing. By the analysis of our result we found that the binding of ethanol is very low in pristine form of SnO2 making it inefficient for the ambient condition operation. However, the binding of ethanol improves drastically when SnO2 is doped with Ni. A little change is observed in the structure after the doping of Ni however the central hexagonal structure of SnO2 is preserved. Large improvement in the binding energy of about 5.16 times is observed by Ni doping. Our results present an important advance toward tuning the bandgap and sensing properties of 2D SnO2 which can have implications for important applications. The reliable conclusions drawn in this study will encourage experimentalists to explore and use these nanostructures as chemical sensors
ACKNOWLEDGEMENT Authors are thankful to DST-SERB for financial support. Part of the calculations is carried out on Sayaji HPC cluster at The Maharaja Sayajirao university of Baroda, Vadodara.
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