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Tài liệu Adsorption of toxic gases on graphene sio2 and graphene h bn

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VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY PHAM BA LICH ADSORPTION OF TOXIC GASES ON GRAPHENE/SiO2 AND GRAPHENE/h-BN MASTER'S THESIS VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY __________________ PHAM BA LICH ADSORPTION OF TOXIC GASES ON GRAPHENE/SiO2 AND GRAPHENE/h-BN MAJOR: NANOTECHNOLOGY CODE: 8440140.11QTD RESEARCH SUPERVISORS: Dr. DINH VAN AN Dr. PHUNG THI VIET BAC Hanoi, 2020 ACKNOWLEDGEMENTS I do thank to all people who helped me and supported me during the completion of this report. Foremost, I especially would love to express my hearty appreciation toward Dr. Dinh Van An – my first supervisor and Dr. Phung Thi Viet Bac – my second supervisor for their continuous support, conscientious guidance, wonderful inspiration, and providing me with an excellent atmosphere during my thesis. I would also like to acknowledge Prof. Morikawa Yoshitada, Assoc. Prof. Ikutaro Hamada at Osaka University for their kind supports during my internship in Japan. I am gratefully indebted to them for their very valuable comments and supervision on this research topic. My sincere thanks also go to Ms. Ta Thi Luong, Mr. Pham Trong Lam, and Mr. Ngoc Thanh for their helpful instruction when learning how to use DFT and VASP. I also want to thank other lab mates and lab secretaries for all their kind supports. Last but not least, I am grateful to staff of Vietnam Japan University, Osaka University, and the Japanese International Cooperation Agency for their support with kindness. TABLE OF CONTENTS Page INTRODUCTION .......................................................................................................1 CHAPTER 1: LITERATURE REVIEW ....................................................................3 1.1. Graphene material ................................................................................................3 1.2. Heterostructure of Graphene/Hexagonal boron nitride (G/h-BN) .......................6 1.3. Heterostructure of Graphene/Silicon dioxide (G/SiO2) .......................................7 1.4. Gas molecules ......................................................................................................8 1.5. Physisorption mechanism of gas sensor.............................................................11 CHAPTER 2: COMPUTATIONAL METHODS AND MODELS .........................13 2.1. Density Functional Theory (DFT) .....................................................................13 2.2. The Kohn-Sham (KS) Method ...........................................................................14 2.3. The Local-Density Approximation (LDA) ........................................................17 2.4. VASP – Vienna Ab initio Simulation Package ..................................................18 2.5. Implemented computational scheme ..................................................................18 2.6. Heterostructure configurations ...........................................................................21 2.6.1. Unit cell of the graphene/substrate heterostructure.........................................21 2.6.1.1. G/h-BN heterostructure ................................................................................21 2.6.1.2. G/α-SiO2 heterostructure ..............................................................................22 2.6.2. Supercell of the graphene/substrate heterostructure .......................................23 2.6.2.1. G/h-BN heterostructure ................................................................................23 2.6.2.2. G/α-SiO2 heterostructure ..............................................................................27 2.6.3. Positions for adsorption of toxic gases on G/h-BN and G/α-SiO2 ..................28 CHAPTER 3: RESULTS AND DISCUSSION ........................................................30 3.1. Study and fabrication the Graphene/substrate heterostructures .........................30 3.1.1. The mismatch property between graphene and substrates ..............................30 3.1.1.1. G/h-BN heterostructure ................................................................................30 3.1.1.2. G/α-SiO2 heterostructure ..............................................................................31 3.2. Adsorption of different gases on Graphene/h-BN .............................................32 3.2.1. CO2 on graphene/h-BN ...................................................................................32 3.2.2. CO on graphene/h-BN.....................................................................................35 3.2.3. NO on graphene/h-BN ....................................................................................38 3.2.4. NO2 on graphene/h-BN ...................................................................................41 3.2.5. NH3 on graphene/h-BN ...................................................................................43 3.2.6. H2O on graphene/h-BN ...................................................................................45 3.2.7. Selectivity and sensitivity of gas adsorption on G/h-BN ................................47 3.3. Adsorption of CO and NO gases on Graphene/α-SiO2 ......................................49 CONCLUSIONS .......................................................................................................52 REFERENCES ..........................................................................................................53 LIST OF TABLES Page Table 1.1. Physical properties among graphite, SiO2, and h-BN ................................7 Table 2.1. The optimization results of AB stacking pattern with C on top B ...........24 Table 2.2. The optimization results of AB stacking pattern with C on top N ..........25 Table 3.1. Comparison of lattice mismatch with other articles ................................30 Table 3.2. Comparison with various DFT simulation methods ................................31 Table 3.3. Adsorption energy and adsorptive distance for CO2 on G/h-BN.............33 Table 3.4. Adsorption properties from optimization of CO/G/h-BN in four sites ....36 Table 3.5. Adsorption properties from optimization of NO/G/h-BN in four sites ...39 Table 3.6. Adsorption properties from optimization of NO2/G/h-BN in four sites ..41 Table 3.7. Adsorption properties from optimization of NH3/G/h-BN in four sites ..43 Table 3.8. Adsorption properties from optimization of H2O/G/h-BN in four sites ..45 Table 3.9. A comparison of adsorption energies of gases on G/h-BN .....................48 Table 3.10. A comparison of adsorption energies of gases on G/α-SiO2 .................50 LIST OF FIGURES Page Figure 1.1. Structure of monolayer of graphene and its applications .........................3 Figure 1.2. Structure and properties of other graphene’s derivatives .........................4 Figure 1.3. Structure and physical properties of other 2D materials ..........................5 Figure 1.4. Topology and charge density map of G/h-BN and G/SiO2 ......................5 Figure 1.5. Structure of h-BN .....................................................................................6 Figure 1.6. Configurations of (a) α-quartz and (b) cristobalite of SiO2 ......................8 Figure 1.7. Schematic mechanism of gas sensor and its signal measurement ..........11 Figure 2.1. Flow chart of the solution procedure of DFT .........................................13 Figure 2.2. Two different AB-stacking patterns of G/h-BN .....................................21 Figure 2.3. G/α-SiO2 heterostructure with (a) side view and (b) top view of G/SiO2 ...................................................................................................................................23 Figure 2.4. Electron distribution of G/h-BN heterostructure with C on top B .........26 Figure 2.5. Band structure of G/h-BN and DOS of (a) h-BN, (b) graphene and (c) total system................................................................................................................26 Figure 2.6. Optimized configuration and electronic density of G/α-SiO2 using revPBE-vdW. (a) top view and (b) side view. Yellow in (c) presents the locations of the electron cloud. .....................................................................................................28 Figure 2.7. Three adsorption sites and gas molecule orientations on G/h-BN .........29 Figure 2.8. Different adsorption sites on G/α-SiO2...................................................29 Figure 3.1. Atomic structures of all configurations for CO2 molecule adsorbed on G/h-BN. (H: horizontal orientation, U: upright orientation; red ball: oxygen, brown ball: carbon, blue ball: nitrogen) ...............................................................................32 Figure 3.2. Molecular orbital (MO) diagram of CO2 (1) and calculated DOS from this study (2) ..............................................................................................................34 Figure 3.3. DOS of CO2 before and after adsorption on G/h-BN .............................34 Figure 3.4. Partial DOS of a carbon on graphene mainly interacted with CO2 before and after adsorption ...................................................................................................35 Figure 3.5. Adsorption of CO molecule on G/h-BN in four different sites ..............36 Figure 3.6. Molecular orbital of CO (1) and DOS of CO before adsorption (2) ......37 Figure 3.7. DOS of CO before and after adsorption on G/h-BN ..............................38 Figure 3.8. Partial DOS of a carbon on graphene mainly interacted with CO before and after adsorption ...................................................................................................38 Figure 3.9. Adsorption of NO molecule on G/h-BN in four different sites ..............39 Figure 3.10. MO of NO molecule (1) and DOS of NO before adsorption (2)..........40 Figure 3.11. DOS of NO before and after adsorption on G/h-BN ............................40 Figure 3.12. Adsorption of NO2 molecule on G/h-BN in four different sites ..........41 Figure 3.13. MO of NO2 molecule (1) and DOS of NO2 before adsorption (2) .......42 Figure 3.14. DOS of NO2 before and after adsorption on G/h-BN...........................42 Figure 3.15. Adsorption of NH3 molecule on G/h-BN .............................................43 Figure 3.16. MO of NH3 molecule (1) and DOS of NH3 before adsorption (2) .......44 Figure 3.17. DOS of NH3 before and after adsorption on G/h-BN...........................44 Figure 3.18. Adsorption of H2O molecule on G/h-BN in four different sites ..........45 Figure 3.19. MO of H2O molecule (1) and DOS of H2O before adsorption (2) .......46 Figure 3.20. DOS of H2O before and after adsorption on G/h-BN...........................46 Figure 3.21. Adsorption energy and distance of each gas on G/h-BN from z-axis scanning .....................................................................................................................48 Figure 3.22. Adsorption of CO and NO on G/α-SiO2 in top and hollow sites .........50 LIST OF ABBREVIATIONS 2D DFT DOS FET G/h-BN G/SiO2 HOMO KS LUMO MO VSEPR vdW Two-dimensional Density Functional Theory Density Of State Field Effect Transistor Graphene/hexagonal Boron Nitride Graphene/Silicon Dioxide Highest Occupied Molecular Orbitals Kohn-Sham Lowest Unoccupied Molecular Orbitals Molecular Orbital Valance Shell Electron Pair Repulsion theory Van der Waals INTRODUCTION Recently, graphene, a two-dimensional (2D) monolayer of graphite, has drawn a great interest in public due to its potential electrical properties. It can serve as a core material in nano-electronic appliances. One of the most fascinating application of carbon-based material such as graphene is in gas sensors detecting gases on the atmosphere with a high sensitivity. Accordingly, the high mobility of carrier’s behavior of graphene may provide essential clues for gas adsorption properties. Current research efforts are mostly directed at the detection and remedy of air pollution by anthropogenic activities. Nonetheless, an issue with the high sensitivity of graphene with gas adsorption is that the selectivity in the study of gas adsorption is quite questionable. Hence, for the purpose of enhancing the selectivity of gas adsorption’s study, several novel approaches have been adopted by doping method, structural defect method, or substrate introduction. In this study, the substrate introduction on graphene is take into consideration to ameliorate the selectivity of pristine graphene. Heterostructure of graphene and a vdW interactive substrate has been studied and reported such as a second graphene layer, MoS2, SiO2 or h-BN in order to open band gap of graphene and help improve its electrical properties. It is demonstrated that a considerable improvement in chemical stability of graphene supported on such substrates. Therefore, hybrid structures of graphene with a substrate are of pivotal importance for both theoretically fundamental studies as well as applications of graphene. Additionally, the booming of electrical waste disposal is a critical problem for scientists and environmentalists. The manufacture of a variety of chemically singleuse gas sensors is one of typical examples of this. In order to solve that issue, the physisorption of adsorbates is necessary for the gas adsorption mechanism. Furthermore, the introduction of a substrate below graphene is still based on mainly vdW interactions, so that graphene and substrate could be really promising for achieving the requirement for green studies. In this work, the adsorption of gases including five toxic gases CO2, CO, NO, NO2, NH3, and water vapor H2O adopted for hygrometer (humidity sensor) on two constructed heterostructures (G/h-BN and G/SiO2) were investigated on particular sites (Top, Hollow, and Bridge). To fully understand the adsorption mechanism of toxic gases on hybrid structures, DOS analysis was conducted. The aims here were to (1) analyze the optimal positions in gas adsorption, (2) rationalize the adsorption properties (adsorption energy and adsorptive distance) of gases on heterostructures, (3) decipher the adsorption mechanism by exploiting DOS diagram and (4) analyze the selectivity and sensitivity of constructed materials. CHAPTER 1: LITERATURE REVIEW 1.1. Graphene material Graphene, silicene, germanene, phosphorene, hexagonal boron nitride (h-BN), molybdenum disulphide (MoS2), graphitic structures of carbon nitride (g-C3N4), and zinc oxide (g-ZnO) [12] which are typical representatives of two-dimensional (2D) ultrathin materials have recently exploited on a wide range of applications such as electrical appliances. As far as graphene was concerned, a 2D sp2-bonded carbon monolayer, has drawn tremendous attention owing to its notable electronic and mechanical properties [14]. It is known to have remarkable electronic properties, such as a high carrier mobility [8][19], but the absence of a band gap restricts its applications of large-off current and high on-off ratio for graphene-based electronic devices [14]. Fig. 1.1 illustrates the structure of monolayer of graphene and its applications [23]. Figure 1.1. Structure of monolayer of graphene and its applications Initially, it was believed before the accomplishment of experimental fabrication of graphene that strict 2D crystals could be difficult to stabilize from 3 theoretical and experimental perspectives due to the effects of thermal expansion. Nevertheless, in 2004, a single layer of carbon in atoms-level thickness was fabricated by Geim and Novoselov using micromechanical exfoliation and studied the electronic field effect and carried out a series of studies [8]. On the other hand, graphene oxide and its reduced form are graphene’s derivatives and are all semiconductors but with lower carrier mobility (Fig. 1.2) [24]. Figure 1.2. Structure and properties of other graphene’s derivatives In comparison with other 2D materials (Fig. 1.3) [12], graphene is considered to be an excellent sensor material with high conductivity, electron mobility and the gapless and approximately linear electron dispersion around the Fermi level. Although graphene exhibits a very promising material with excellent electronic properties, there will be quite questionable to adopt graphene into sensors using nanoelectric devices [19]. Therefore, the introduction of the substrates such as h-BN and SiO2 can be an innovative way to help open up the band gap of graphene, thereby enhancing the sensor properties of graphene. 4 Figure 1.3. Structure and physical properties of other 2D materials Figure 1.4. Topology and charge density map of G/h-BN and G/SiO2 A comparison of topology of G/h-BN and G/SiO2 is depicted in Fig. 1.4 [17][29]. It is clear that while a similar lattice structure of graphene with both h-BN and SiO2 is seen, h-BN seems to have a smooth surface without any charge traps and 5 SiO2 surface is usually impure and uneven. The surface optical phonon energy of hBN is two-fold magnitude greater than that of SiO2 [4]. It indicates that G/h-BN will have a better performance compared with G/SiO2. 1.2. Heterostructure of Graphene/Hexagonal boron nitride (G/h-BN) Hexagonal boron nitride (h-BN) is a representative of 2D material with a wide band gap. Hexagonal boron nitride (h-BN) is called “white graphene”, consists of alternative boron and nitrogen atoms in a sp2-hybridized 2D honeycomb arrangement and 2D h-BN monolayer is isolated from bulk BN (Fig. 1.5) [6]. The bond length of B-N is around 1.45Å. The interaction between boron and nitrogen is basically a covalent bond. Figure 1.5. Structure of h-BN Due to the fact that hexagonal boron nitride (h-BN) has a wide band gap, it could lead to its excellent properties of electrical insulations, high thermal conductivity and superior lubricant properties [28]. Additionally, it can be useful as an important complementary 2D dielectric substrate for graphene electronics. A direct band gap of h-BN which is the gap between HOMO and LUMO levels is attained by π and π∗ located on the N atom and B nucleus, respectively [9]. 6 Dean et al. studied the quantum Hall effect of graphene with h-BN substrate for the first time in 2011 [7]. Wang et al. conducted the combination of graphene and h-BN crystals by physical force [26]. Later on, other researchers have investigated and proposed G/h-BN heterostructure functional devices. G/h-BN heterostructure can act as a high-capacity cathodes with high voltage for Aluminum batteries in the study of Pretti Bhauriyal et al. [3]. Furthermore, gas sensing properties has been studied for 2D G/h-BN lateral interface, specifically for NOx gas in Paquin et al. studies [21]. 1.3. Heterostructure of Graphene/Silicon dioxide (G/SiO2) Table 1.1. Physical properties among graphite, SiO2, and h-BN Lattice constant Thermal conductivity (Wm-1K-1) Dielectric constant SiO2 Graphite Graphite diamond 0.3567 h-BN Orthogonal a =1.383 Å, b = 1.741 Å, c = 0.504 Å Face-centered 1.936 1.4 a=b= 2.46Å c = 6.67Å 25-470 22 25.1 3.9 8.7 5.7 4 a=b= 2.502Å; c = 6.61 Å First of all, SiO2 is known as nano-silica and has been used for a great deal of biomedical research owing to its functionalized ability and stability. Furthermore, SiO2 substrate was widely applied on integrated circuits working as a dielectric medium. It is explained by the good physical properties of SiO2 (Table 1.1) [10]. For graphene supported by SiO2, Ishigami et al. [13] indicated that single layer graphene mainly follows the underlying morphology of SiO2 and estimated the adhesion energy between graphene and SiO2. Otherwise, DFT calculations of graphene on SiO2 surface have been published. Whilst SiO2 is generally experimentally found in amorphous, DFT calculation typically limited to crystalline SiO2 structures. There are two kinds of SiO2 represented in Fig. 1.6 [8]. They are α-quartz and cristobalite with multiple layers of SiO2. In reality, αquartz is the most stable configuration under ambient conditions. X F Fan et al. [8] has studied the interaction between graphene and the surface of SiO2 in both α-quartz 7 and cristobalite using first principles DFT. It is found that by applying oxygen defect, the SiO2 surface could shift the Fermi level of graphene. Nguyen et al. [20] has also studied oxygen-terminated SiO2 (0001) surface interacted with graphene and concluded that G/SiO2 is a semiconductor. Furthermore, Zhimin Ao et al. [2] has also worked with G/α-SiO2 (0001) for interaction study and concluded that the vdW forces mainly serves as the interacting force in this system and is stronger than interaction of graphene layers in graphite. (a) (b) Figure 1.6. Configurations of (a) α-quartz and (b) cristobalite of SiO2 On the other hand, the studies of gas adsorption on G/h-BN and G/α-SiO2 are still limited and the gas adsorption mechanism is still a questionable problem for these systems even there have been a lot of studied investigating the gas adsorption on pristine graphene. Hence, the significance of depicting the gas adsorption has motivated us to study more deeply in these heterostructures. 1.4. Gas molecules These days, the process of industrialization and urbanization are booming sharply, one of the most pressing problem worldwide is air pollution. As a result, the detrimental effect of this serious phenomenon is that it can be linked to the damage of human health in a direct or indirect way. As WHO data reported, air pollution could cause 1 in 9 deaths worldwide while ambient air pollution caused 7.6% deaths 8 over the world in 2016 [27]. It is estimated that 4.2 million premature deaths universally are associated to the pollution of atmosphere [1]. It mainly causes some serious health problem related to breathing such as lung cancer, and acute respiratory infections in children as well as heart disease, stroke, chronic obstructive pulmonary disease, etc. For the purpose of mitigating the impacts of air pollution, the detection of toxic gases on air with a good sensitivity and favorable selectivity plays an essential role in research and manufacturing process. Thus, this study aims to discover and propose promising materials that can not only be economical but also produce a better sensitivity and selectivity with toxic gases in the air. The mechanism of gas adsorption is also one of the aspects drawing a lot of interest for understanding the interaction of material and gas adsorbate. In reality, outdoor air pollution can root from natural and anthropogenic sources. Regarding natural sources, the contribution of local air pollution is more prone to forest fires and dust storms [1]. On the other hand, human activities are the key factor leading to polluted air problems and far exceeds natural sources. Adverse health consequences of air pollution can occur as a result of short- or long-term exposure [27]. Herein, we investigate the adsorbability of 5 pollutants (CO2, CO, NO, NO2, NH3) which have strong impacts on human health as well as the earth climate i.e. global warming and water vapor H2O acting as an essential element in air. There are some physical properties and toxicity of 5 toxic gases and water vapor below: a) Carbon dioxide (CO2): CO2 gas is colorless gas and is called “greenhouse gas” due to its main contribution into greenhouse effect. CO2 is naturally traced in the Earth’s atmosphere and could be released from the dissolution of carbonate rocks in water and acids. It is a by-product of burning fossil fuels and land-use changes and other industrial processes. In geometric aspect, CO2 is a linear molecule with sp hybridization, with the bond length of C=O is 1.163 Å. b) Carbon monoxide (CO): CO is a colorless and odorless gas, which at high levels can be harmful to humans by binding with hemoglobin, so it will be easily absorbed through the lungs. That leads to hypoxic injury, nervous system damage, and even 9 death. The dizziness and nausea phenomenon are recognized in 45-min exposure of CO. The origin of CO in the atmosphere is believed coming from the exhaust of transport activities and machinery that burn fossil fuels. According to VSEPR, CO has a linear molecular shape and the bond length of CO is originally 1.128 Å. The hybridization of CO is sp with the bond order of 3. CO is a diamagnetic molecule with fully paired electrons. c) Nitrogen monoxide (NO): There is a natural spontaneous reaction transforming NO in the atmosphere environment into NO2 during oxidation process. Generally, nitrogen oxides are mainly produced from natural phenomenon such as lightning in thunderstorms. NO gas is a colorless gas and is a main factor in acid rain deposition. Some common symptoms with breathing of low levels of NO are cough, tiredness, and nausea. Nonetheless, NO can damage seriously our lung over the next one to two 5 days after breathing. Basically, NO has 1.15 Å of bond length and bond order is . NO is a paramagnetic molecule with one unpaired electron. 2 d) Nitrogen dioxide (NO2): NO2 is a reddish-brown gas with origin from motorvehicle emissions. The shape of NO2 molecule is bent. Due to acute toxicity of NO2, NO2 has a serious harm to health like chlorine and CO gases in equal measure. It can be directly adsorbed through human lungs and its inhalation, then cause heart failure and even to death. NO2 is also a paramagnetic molecule with single unpaired electron. The bonding angle of NO2 is 134.3o with 1.197 Å of bond length. e) Ammonia (NH3): Ammonia is a colorless alkaline gas with a characteristic of pungent smell. Ammonia is considered as one of the abundant nitrogen-based substances in the air. Breathing over the threshold of concentration of NH3 can expose to sinusitis, upper airway irritation, and eye irritation. Some diseases of the lower 10 airways and interstitial lung are found as acute exposures occur to concentrated ammonia [27]. The bond length of N-H is 1.017 Å. Geometrically, NH3 has one lone pair of electrons and has a tetrahedral structure in sp3 hybridization. f) Water vapor (H2O): The bond length of O-H in water vapor is 0.9584 Å. According to VSEPR, H2O is in a bent shape with sp3 hybridization. Geometrically, H2O has two lone pairs of electrons. In this study, H2O is in gas phase and is not classified as a toxic gas. Nonetheless, H2O serves as an indispensable factor in air, so it can be as a reference for other toxic gases studies. Apart from investigating the toxic gas adsorption on constructed heterostructures, the study on adsorption of water vapor could be adopted for hygrometer. 1.5. Physisorption mechanism of gas sensor Figure 1.7. Schematic mechanism of gas sensor and its signal measurement Toward developing gas sensor materials, the mechanism for physisorption is taken into account. Therefore, the rationale of signal measurement plays an essential role in understanding it (Fig. 1.7). Basically, there are three phases that physisorption 11 has occurred during detection (Fig. 1.7a). The first stage is the stage before gas adsorption (1). The gas molecule is in the proximity of the surface of sensor and there has not been any interaction yet. Then, the second stage is the stage being completely adsorbed on surface of sensor material (2). This is the time that the gas molecule is close enough to interact with surface of material and the interaction of gas and sensor fully achieved the most preferable positions for adsorption. After being completely adsorbed on sensor, the gas turns to desorption stage (3). The adsorption process will last for several seconds. From practical aspect, the signal measurement of gas sensor can be estimated by impedance measurement. It is based on the change of resistance (R). Due to ntype nature of graphene, if the gas molecule serves as an acceptor which gains electron from sensor material, the signal of resistance will immediately increase and reach a peak during being completely adsorbed and vice versa (Fig. 1.7b). In desorption phase, the value of resistance will suddenly come back the initial value due to the fact that there is not any interaction between gas molecule and surface. For each adsorption, the response of sensor will get a peak of resistance during a contact time. If the sensor is well qualified, the response should get the same height and wide of the peak at each different adsorption. 12
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