Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies

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Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies Ahmad Umar a,b,**, H.Y. Ammar c, Rajesh Kumar d, Tubia Almas a,b,e, Ahmed A. Ibrahim a,b, M.S. AlAssiri b,c, M. Abaker c, S. Baskoutas e,* a

Department of Chemistry, Faculty of Science and Arts, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia b Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia c Physics Department, Faculty of Arts and Sciences, Najran University, Najran, Saudi Arabia d Department of Chemistry, JCDAV College, Dasuya 144205, India e Department of Materials Science, University of Patras, Patras GR-26504, Greece

article info

abstract

Article history:

2D SnO2 disks with excellent purity and crystallinity were synthesized through a low cost,

Received 29 March 2019

facile hydrothermal process and were characterized in terms of their morphological,

Received in revised form

structural, optical and electrochemical properties. The 2D disk-like morphology of syn-

22 April 2019

thesized SnO2 presented the average thickness of ~1 mm and possessed the typical rutile

Accepted 28 April 2019

tetragonal phase for the SnO2 with preferred growth along (100) plane. As-synthesized

Available online xxx

SnO2 disks were used for the fabrication of gas sensors for reducing gases like H2, CO, and C3H8. With the optimized temperature at 400
C, the as-synthesized SnO2 electrode

Keywords:

expressed the gas responses of 14.7, 9.3 and 8.1 for H2, CO, and C3H8, respectively. Contrary,

SnO2

the reasonable response times of 4 s, 3 s, and 8 s and the recovery times of 331 s, 201 s, and

2D disks

252 s were recorded for H2, CO, and C3H8 gases, respectively. The DFT studies conducted

Gas sensor

herein suggest that the adsorbed oxygenated species act as a primary redox mediator for

Reducing gases

gas sensing reaction between reductive gases like H2, CO and C3H8, and SnO2 sensor. From

DFT calculations

DFT analysis, a very low heat of adsorption (0.2 eV) estimated which suggested the physisorption of the H2 molecules on the surface of the sensing material (i.e. SnO2). In contrast, the deposited oxygen atom forms strong chemical bonds with O2c and O3c sites. The oxygen atom bonded to O2c site control the conductivity of the sensor better than the O3c sites. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. ** Corresponding author. Department of Chemistry, Faculty of Science and Arts, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia. E-mail addresses: [email protected] (A. Umar), [email protected] (S. Baskoutas). https://doi.org/10.1016/j.ijhydene.2019.04.269 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Umar A et al., Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.269

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Introduction Due to the uncontrolled utilization of non-renewable petroleum sources for energy requirements, in almost every aspect of human life, the main focus of the researches all over the world is now to generate some alternative fuel sources. Solar energy, wind energy, and hydropower plants are considered to be the best renewable and nonpolluting future energy sources [1]. Dihydrogen (H2) is another potential cleaner, greener, nonpolluting and renewable energy source due to its low ignition temperature, large flame propagation energy, minimum ignition energy ~0.017 mJ, high heat of combustion (286 kJ/ mol) and wide air flammable range of 4e74% and thus finds its applications in fuel cells, spacecraft, automobiles, and in industries [2e5]. However, odorless, colorless, tasteless, highly combustible and explosive nature makes the handling, storage, and transportations of the H2 gas very difficult and unsafe. Hence, in order to detect even traces of H2 gas in air highly reliable, sensitive and selective gas sensors are required. Many sensor materials based on composite materials such as organic-inorganic hybrid hydrogels [6], SWNTs/ MWNTs synergistic composites [7], conductive thermoplastic polyurethane-graphene nanocomposites [8], conductive graphene/thermoplastic polyurethane foams [9], polylactic acid nanocomposites [10], cobalt ferrite-polyaniline nanocomposites [11], electrically conductive flexible polymer composites [12], polyurethane sponge coated with cracked cellulose nanofibril/silver nanowires [13,14] etc. has been reported in literature. As H2 is a reducing gas, many n-type semiconducting metal oxide nanomaterials such as ZnO [15e17], WO3 [18,19], SnO2 [20e22], TiO2 [23,24], CeO2 [25,26], and In2O3 [27] based sensor are reported in literature. All the metal oxide based sensors are generally classified into four major classes including optical, acoustic, resistance and work function based [4,28]. The sensor metal oxides based hydrogen sensors act as transducers which convert the altered physical and chemical characteristics, upon adsorption of the gas, into electrical signals [29,30]. Among the various n-type semiconducting metal oxides, SnO2 is considered to be the best resistance based real-time gas sensor material owing to its low band gap of 3.6 eV at room temperature, good thermal and chemical stability, etc [31e33]. The most common crystalline phase of the SnO2 is tetragonal rutile which is anisotropic and polar. Each Sn atom is surrounded by six O-atoms present at the corners of the octahedron, whereas each O-atom is coordinated to three Snatoms present at the corners of an equilateral triangle [34,35]. Taguchi recognized the gas sensing capabilities of the SnO2, long back in 1962 by measuring the changes in resistance on exposure to combustible and reducing gases [36]. Since then it has been explored as efficient gas sensor materials for sensing gases like H2S [37], NO2 [38], CO [39,40], C2H5OH [41], SO2 [42,43], LPG [44], CH4 [45,46], NH3 [47], formaldehyde [48,49] etc. The factors like large surface to volume ratio, porosity, gas transmission, grain size and morphology of the SnO2 nanomaterials greatly affect the gas sensing parameters [50,51]. SnO2 nanostructures have been synthesized through various methods such as calcinations [52], solvothermal [53],

hydrothermal [54e57], sol-gel [58,59] to give versatile morphologies with large surface to volume ratio, the key factor for the adsorption of larger extent of analyte gases and O2 from the surrounding air responsible for the generation of anionic species. Among the various methods, the hydrothermal method is the most versatile one for the synthesis of a variety of nanomaterials [60e62]. In order to control the morphology, hydrothermal synthesis is carried out in using a variety of surfactants as well as in the absence of surfactants [63,64]. A detailed study of the recent publications revealed that it is the morphology of the sensing nanomaterials which affects the gas sensing behavior of the SnO2 nanostructures. The morphology of the SnO2 nanostructures can be controlled and engineered through the choice of the synthetic method, doping and making composites with suitable materials. Recently, a variety of nano-SnO2-based H2 gas sensors have been reported in the literature [65e67]. These include undoped and Pd-doped SnO2 microspheres and nanoclusters [68,69], undoped SnO2 nanomaterials with different morphologies such as nanofilms, nanorods, and nanowires [70e72], Ptdoped SnO2 nanowires [73], Mn and Co-doped SnO2 nanoparticles [2], Au-loaded SnO2 composite nanoparticles [74], ZnOeSnO2 composites [75] and many more. Various dopants alter the morphology as well as the morphology of the SnO2 nanostructures. Recently, density functional theory (DFT) has been used as a promising tool to provide invaluable help for the analysis and interpretation of adsorption of the gases on the surfaces of the metal oxide based gas sensors. DFT is advantageous over the other techniques like post-HartreeeFock methods, VoigteReusseHill method, phase field method, as it provides a virtual and more accurate image without any additional increase in computational time for the interactions between the adsorbate gases and sensor surfaces which are otherwise difficult to study experimentally [76e84]. Herein, we report the fabrication of H2 gas sensor based on 2D SnO2 disks and theoretical aspects of sensing behavior using DFT analysis. The 2D SnO2 disks of excellent purity and crystallinity were synthesized through a low cost, facile hydrothermal process and were characterized using different

Fig. 1 e Typical XRD pattern of synthesized SnO2 disks.

Please cite this article as: Umar A et al., Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.269

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Fig. 2 e (aed) FESEM and (e and f) TEM images of synthesized SnO2 disks.

characterization techniques. Finally, the gas sensor fabricated using 2D SnO2 disks was explored for its gas sensing applications towards H2(target gas), CO(Interfering gas) and C3H8(Interfering gas). The sensor showed better gas response with lower response time for H2 gas as compared to other analyte gases.

Materials and method Synthesis of 2D SnO2 disks A hydrothermal process was adopted to synthesize 2D SnO2 disks using stannous chloride dehydrate (SnCl2.2H2O, SigmaAldrich Chemicals) as the precursor and ammonium

hydroxide (NH4OH). 0.05 M SnCl2.2H2O (1.13 g) and 0.05 M hexamethylenetetramine (HMTA, 0.705 g) were dissolved in 50 mL of deionized (DI) water to make a precursor solution. 5 mL of NH4OH was then added to maintain the reaction mixture's pH ¼ 8.7. The prepared mixture was stirred for 30 min and then transferred to Teflon lined stainless steel autoclave and properly sealed to avoid leakage. The temperature of the autoclave was maintained at 160
C for 12 h to complete the hydrothermal process. After the hydrothermal process, the obtained precipitates were washed and filtered thoroughly with DI water and methanol (at least 2e3 times). The filtered precipitates were dried at 60
C in a laboratory oven for 24 h and then annealed in a furnace at 450
C for 2 h to have SnO2 powder.

Please cite this article as: Umar A et al., Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.269

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Fig. 3 e (a) Typical FESEM image and its corresponding (b) EDS spectrum and elemental mapping of (c) tin and (d) oxygen of the synthesized SnO2 disks.

Characterization of 2D SnO2 disks Field emission scanning electron microscopy (FESEM; JEOL-JSM7600F) and High-resolution transmittance electronic microscopy (HRTEM) were used to elaborate the surface morphology and structural features. The crystal phases, purity and crystallite size of the 2D SnO2 disks were explored by X-ray diffraction technique (XRD; PAN analytical Xpert Pro.) analyzed at Cu-Ka radiation with wavelength 1.54
A in the diffraction angle range of 20e80
. Compositional, spectral and vibrational properties of 2D SnO2 disks were examined through FTIR spectroscopy (Perkin Elmer-FTIR Spectrum-100) between the scan range of 400e4000 cm1 at room temperature. To prepare the sample for FTIR, the small amount of nanoparticle was well-mixed with KBr followed by its palletization which was used for measurements. For the TEM measurement, small amount of the synthesized materials was ultrasonically dispersed in acetone and a drop of acetone solution, which contains the SnO2 material, was placed on a copper grid and examined. Raman spectrum (Perkin Elmer-Raman Station 400 series) in the was scanned in the range of 200e1000 cm1 for the analysis of vibrational and scattering properties.

Fabrication of H2 gas sensor To prepare the H2 gas sensor, a slurry of the synthesized 2D SnO2 disks was made by dispersion in DI water and then the coating of prepared slurry on the alumina substrate (area:

1.5  1.5 mm2, thickness: 0.25 mm) was done by simple casting method. To make the sensing electrode, two Au electrodes and micro-heater were built upon its top surface and its bottom surface, respectively. The sensor was examined at different sensing temperatures, i.e. in the range of 350e450
C. The temperatures for the sensor was measured using an IR temperature sensor. For the gas sensing test, the sensor was kept into a small quartz tube which was specially designed of volume ¼ 1.5 cm3. The gas responses (S ¼ Rg/Ra, Rg: resistance in gas, Ra: resistance in the air) to 100 ppm H2, CO, and C3H8 were measured in the temperature range of 350e450
C. The variation in the mixing ratio of parent gases and dry air was necessary to control the gas concentrations. An electrometer connected with a personal computer was used to evaluate the DC 2-probe resistances.

Results and discussion Characterization of 2D SnO2 disks The crystallographic structure, phase type and purity of the SnO2 powder were studied through X-ray diffraction analysis and the corresponding XRD patterns are shown in Fig. l. Well defined diffraction peaks (110), (101), (200), (111), (210), (211), (220), (002), (310), (112), (301), (202) and (321) are the characteristic peaks of the rutile tetragonal phase with point group 4/ mnm (D4h) and space group P42/mnm (D144h) of the SnO2 [85].

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Further, the diffraction planes are well matched with the JCPDS card No. 88e0287 and reported literature. The maximum intensity of the diffraction plane (110) suggests that the preferred growth of the SnO2 is along this plane. DebyeScherrer equation (Eq. (1)) was used to calculate the crystallite size of the 2D SnO2 disks: d¼

0:94l bð2qÞ cosq

(1)

where, b(2q) ¼ full width at half maximum (FWHM ¼ 0.55438) calculated for the most intense peak of 2D SnO2 disks corresponding to the diffraction planes (110), l ¼ wavelength of Xrays, q is the Bragg diffraction angle corresponding to the selected diffraction peak (26.48) and d ¼ crystallize size which is estimated to be ~15.39 nm. The morphological analysis of the synthesized SnO2 powder was carried out by using FESEM and TEM techniques. Fig. 2(aed) show the low and high magnification FESEM images, respectively. These images reveal that the SnO2 power has two-dimensional (2D) disks like morphology of different dimensions grown in high density. The average thickness of the 2d disks is about 1 mm. However, the surfaces and edges of 2D disks are not smooth which are decorated with small nanosized SnO2 particles. The low-resolution TEM images (Fig. 2(e and f)) are further supported the 2D disks like morphology observed in FESEM results. Fig. 3 represents the elemental analysis profile and mapping images for the synthesized 2D SnO2 disks, respectively. From Fig. 3(b), the presence of peaks only for Tin (Sn) and oxygen (O) in the EDS confirms the high purity of the 2D SnO2 disks. The absence of any other peak corresponding to any impurity again explains the purity of synthesized 2D SnO2 disks. Fig. 3(c) and (d) are representing the elemental mapping images for oxygen and tin elements, respectively. Uniform distribution of Sn and O contents throughout the SnO2 lattice is ascertained by the elemental mapping images. FTIR spectrum for 2D SnO2 disks scanned from 400 to 4000 cm1 at room temperature through KBr pelletization is shown in Fig. 4(a). Two well defined FTIR peaks can be seen at 3399.2 cm1 and 613.5 cm1, corresponding to stretching vibration modes of OeH bonds of the H2O molecules adsorbed on the surface of the nanostructures and metal-oxygen (M  O) bending modes confirming the formation of SneO bonds, respectively. The structural and crystal nature of assynthesized SnO2 is further studied through Raman scattering analysis. Fig. 4(b) represents the Raman spectra for the 2D SnO2 disk-like structures. The Raman peaks at 474, 633 and 773 cm1 can be assigned to Eg, A1g and B2g modes, respectively [86]. Eg mode is due to the vibrational modes of oxygen in the oxygen plane, whereas A1g and B2g modes are related to the induced by the elongation and compression vibration mode of SneO bonds. These are the characteristic peaks of the tetragonal rutile structure SnO2 [87].

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observed for H2 gas. The sharp decrease in the resistance on exposure to analyte gases is attributed to oxidation of the gas molecules and releases the electrons to the conduction band of the sensor. When the supply of the gases is vented from the test chamber, the 2D SnO2 disks based sensor retrieves the initial baseline resistance. A comparison of the gas sensing responses over 2D SnO2 disks based sensor at different operating temperatures is shown in Fig. 5 (d). 2D SnO2 disks based sensor recorded the maximum responses of 9.3 and 8.1 For CO and C3H8 gases, respectively when the operating temperature was 400
C. This observation explains that 400
C operating temperature is adequate to provide thermal energy in order to surpass the activation energy required for the chemisorption of the analyte gases such as CO and C3H8 on the surface of the sensor. At higher operating temperature, the rate of desorption increases, resulting in the decline in gas responses. However, for H2 gas, no significant change in the gas response has detected when the operating temperature has increased from 400
C (14.7) to 450
C (15.0). Another important and challenging issue related to the semiconducting metal oxide sensors is the selective detection of a particular gas. Therefore, the selective gas sensing behavior of the 2D SnO2 disks based gas sensor towards H2 gas

H2 gas sensing applications of 2D SnO2 disks The response transients for 100 ppm H2, CO and C3H8 gases at 350, 400 and 450
C are shown in Fig. 5(aec). At all the operating temperatures, the maximum resistance variation has

Fig. 4 e Typical (a) FTIR and (b) Raman spectra of synthesized SnO2 disks.

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Fig. 5 e Resistance variations for different gases as a function of time at (a) 350
C (b) 400
C, (c) 450
C temperatures and (d) variations of gas responses with temperature using SnO2 modified gas sensors.

was also studied and correlated to the relative response of CO and C3H8 gases. Selectivity (%) of a sensor is calculated using Eq. (2).

Selectivity ð%Þ ¼

ðGas responseÞðInterfering gasÞ ðGas responseÞðTarget gasÞ

 100

(2)

At an optimized temperature of 400
C, the gas responses for H2 gas are 1.58 and 1.81 times higher as compared to gas responses of CO and C3H8, respectively (as shown in Fig. 6). Fig. 7(a) and (b) represent the variation in the response time (the time required for attaining 90% of the equilibrium value of the resistance on gas exposure) and the recovery time (the time required for the resistance to regain 10% of the original resistance in air after the analyte gas is vented off), respectively measured at different temperatures for 100 ppm gases. At optimized 400
C temperature, the response times of 4 s, 3 s, and 8 s are recorded for H2, CO, and C3H8 gases, while the high recovery times of 331 s, 201 s, and 252 s are observed for H2, CO, and C3H8 gases, respectively.

The proposed gas sensor mechanism

Fig. 6 e Percentage selectivity of 2D SnO2 disks based gas sensor towards H2 as compared to other gases.

It has been reported that the sensing mechanism of the metal oxide semiconductor materials involves two key factors. The initial being the receptor function involving the recognition and adsorption of the analyte gas molecules on the solid sensor surface leading to altered electronic states and the second is the transducer function which involves electro-

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Fig. 9 e (a) The optimized structure and (b) DOS of Sn32O46 cluster. Fig. 7 e Effect of operating temperature on (a) Response time and (b) Recovery times for different gases. the 2D SnO2 disks thereby posing higher resistance values (Ra) (Eq. (3)). chemical changes altering the electrical resistance of the sensor material [88]. The ambient O2 molecules adsorbed on the surface of the sensor are converted to oxygenated anionic species by the electrons released from the conduction band of

Dissociation

eCB

2 O2ðgÞ / O2 ðchemiÞ ƒƒƒƒƒƒ!2OðchemiÞ ƒƒƒƒƒƒƒ!O ðchemiÞ þ OðchemiÞ

(3)

Fig. 8 e Proposed mechanism for gas sensing behaviour of SnO2 disks based sensor. Please cite this article as: Umar A et al., Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.269

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Fig. 10 e The optimized structures and DOS of H2/Sn32O46 complexes.

As a result of this reduction process, an electron depletion layer (Space charge layer) is formed near the exposed surface of the SnO2 sensor. Exposure of the reducing gases like H2, CO and C3H8 leads to an increased conductivity (Lower resistance) for n-type semiconductors. These gases are oxidized when come in contact with these oxygenated species releasing

electrons which are transferred back to the conduction band of the SnO2 [88] (Eqs. (4)e(8)). A proposed mechanism for gas sensing behavior of SnO2 disks based sensor is shown in Fig. 8. H2ðchemiÞ þ O

ðchemiÞ

/ H2 O þ e

(4)

Please cite this article as: Umar A et al., Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.269

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Table 1 e Adsorption of H2 molecule on Sn32O46.

H2/Sn5c H2/Sn6c H2/O2c H2/O3c

Eads

HOMO

LUMO

EHOMO-LUMO

0.027 0.029 0.057 0.028

6.252 6.263 6.263 6.252

2.803 2.820 2.821 2.801

3.449 3.443 3.442 3.451

H2ðchemiÞ þ O2

ðchemiÞ

/ H2 O þ 2e

(5)

2H2ðchemiÞ þ O 2

ðchemiÞ

/ 2H2 O þ e

(6)

/ CO2 þ e

(7)

COðchemiÞ þ O

ðchemiÞ

C3 H8ðchemiÞ þ 5O 2

ðchemiÞ

/ 3CO2 þ 4H2 O þ 5e

(8)

DFT calculations for understanding H2 interactions Furthermore, DFT calculations were done to simulate the sensing mechanism for justifying the obtained gas sensing results. As for as the DFT method is concern, it exhibits significantly greater accuracy compared to Hartree-Fock (HF) theory and far less in computation cost than Møller-Pleast (MP) and the full configuration interaction (CI) methods [89]. Moreover, DFT is a powerful tool for the prediction of geometrical structure, electronic states, bond lengths which exhibited excellent agreements with the experimental results [89,90]. The 2D SnO2 disk was represented by a quantum

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cluster of 78 atoms, Sn32O46 where an oxygen atom was deposited on deferent sites of SnO2 nanocluster to form O@Sn32O46 complexes. Whilst, hydrogen molecule was adsorbed on different sites of Sn32O46 and O@Sn32O46 to form H2/Sn32O46 and H2/O@Sn32O46 complexes, respectively. Full Geometrical optimizations were done for the adsorbent cluster Sn32O46, the adsorbed gas H2, the complexes H2/Sn32O46 and H2/O@Sn32O46. The density of states (DOS) analysis was performed for the adsorbent cluster and the complexes. Calculations were carried out using the B3LYP/LanL2dz level of theory [91]. All calculations were done using Gaussian09 suite of the program [92]. Densities of states (DOS) were graphed for alpha electrons using GaussSum2.2.5 program [93]. The optimized structure and the DOS for Sn32O46 are shown in Fig. 9(a) and (b), respectively. The calculated energy gap of SnO2 was 3.44 eV in good agreement with the experimental value mentioned above (3.6 eV). Further, the (110) surface is adopted to explore the surface sensing behaviors of SnO2 by firstprinciples calculations, since it is the preferred plane of growth. We constructed four hydrogens adsorb systems, where the H2 molecule adsorbed on the top of four different sites of SnO2 (Sn5c, Sn6c, O2c, and O3c), as illustrated in Fig. 10. H2 was fixed on top of selected atoms vertically before optimization. The adsorption energies were calculated with the Eq. (9). Eads ¼ EH2 =Sn32 O46  ESn32 O46 þ EH2




(9)

Where, EH2 =Sn32 O46 ; ESn32 O46 and EH2 are the energies of H2/ Sn32O46 complex, Sn32O46 adsorbent cluster and the free hydrogen molecule, respectively.

Fig. 11 e The optimized structures and DOS of O@Sn32O46 complexes. Please cite this article as: Umar A et al., Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.269

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Table 2 e Deposition of atomic oxygen on SnO2.

O@O2c O@O3c

Table 3 e Adsorption of H2 on O@Sn32O46

EB

HOMO

LUMO

EHOMO-LUMO

1.105 1.456

6.080 6.300

2.995 2.792

3.085 3.508

The optimized structures and the DOS for H2/Sn32O46 complexes are shown in Fig. 10 and the adsorption energies are tabulated in Table 1. From Fig. 10 it was seen that H2 molecule does not prefer to be adsorbed on Sn sites. As we see in Table 1, the hydrogen adsorption on SnO2 in all the suggested systems is physisorption in nature. A small heat of adsorption (0.2 eV) characterizes physisorption [94,95]. The H2 adsorption on SnO2 has a negligible effect on the band gap. As it was mentioned in section Conclusion, the pre-adsorbed oxygen atom has a considerable effect in gas sensing process. Hence, four deposited oxygen systems were suggested where the deposited oxygen atom was set on the previously selected adsorption sites, Sn5c, Sn6c, O2c, and O3c to form

O2c O3c

Eads

HOMO

LUMO

EHOMO-LUMO

0.123 0.292

6.085 6.197

2.993 2.799

3.092 3.393

O@Sn32O46 complexes. The binding energies EB for the deposited O atom were calculated by Eq. (10). EB ¼ EO@Sn32 O46  ESn32 O46 þ EO




(10)

Where, EO@Sn32 O46 ;ESn32 O46 and EO are the energies of O@Sn32O46 complex, Sn32O46 adsorbent cluster and oxygen atom, respectively. After the optimization process we found that the deposited oxygen atoms form a strong chemical bond with O2c, and O3c sites only. The optimized structures and the DOS of the two stable O@Sn32O46 complexes are shown in Fig. 11. The binding energies are listed in Table 2. One can see in Fig. 12 that, the deposited oxygen atom forms a bridge between the site (O2c or O3c) and the nearest Sn atom. As shown in Table 2, the oxygen atom binds at O3c site stronger than the O2c site and in case of O2c site the energy gap decreases and as a result, the conductivity of the sensor increases. In contrast in the case of O3c site the energy gap increases and as a result, the conductivity of the sensor decreases. The adsorption of H2 molecule on O@Sn32O46 was studied. The adsorption energies were calculated with the Eq. 11 Eads ¼ EH2 =O@Sn32 O46  EO@Sn32 O46 þ EH2




(11)

Where, EH2 =O@Sn32 O46 ; EO@Sn32 O46 and EH2 are the energies of H2/ O@Sn32O46 complex, O@Sn32O46 adsorbent cluster and the free hydrogen molecule, respectively. Fig. 12 shows the optimized structure and the DOS of the most stable H2/O@Sn32O46 complex. Comparing the adsorption values in Table 3, the Eads values for H2/O@Sn32O46 complexes are in the range of 0.123 to 0.292 eV rather than 0.027 to 0.057 eV for H2/Sn32O46. This means that, the presence of the deposited oxygen atom enhances the adsorption of hydrogen. One can notice that, the HOMO-LUMO energy gap values for H2/O@Sn32O46 complexes are in the range of 3.09e3.39 eV less than that for the pristine Sn32O46 cluster (3.44 eV). This means that, deposited oxygen atom increases the sensitivity of SnO2 sensor to hydrogen gas. These results are in compatibility with the proposed mechanism (Section Conclusion).

Conclusion

Fig. 12 e The optimized structures and DOS of H2/ O@Sn32O46 complexes.

In summary, the morphological, structural, crystalline and gas sensing characteristics of the 2D SnO2 disks synthesized through the hydrothermal process were analyzed. Different characterization techniques confirmed the tetragonal rutile phase for SnO2 with preferred growth along (110) plane. Gas sensing experiments revealed better sensing performances of the SnO2 based sensors for H2 as compared to interfering CO and C3H8 gases at an optimized temperature of 400
C. The DFT calculations suggested the physisorption of H2 gas molecules on the surface of the sensor. It was also suggested that

Please cite this article as: Umar A et al., Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.269

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the adsorption of the oxygen leading to the formation of oxygenated anionic species significantly affects the electronic structure and band gap energies of the SnO2.

Acknowledgment Authors would like to acknowledge the support of the Ministry of Education, Kingdom of Saudi Arabia for this research through a grant (PCSED-09-18) under the Promising Centre for Sensors and Electronic Devices (PCSED) at Najran University, Kingdom of Saudi Arabia.

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