Adsorption of toxic gases on graphenesio2 and grapheneh BN

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 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 CHAPTER 1: LITERATURE REVIEW 1.1 Graphene material 1.2 Heterostructure of Graphene/Hexagonal boron nitride (G/h-BN) .6 1.3 Heterostructure of Graphene/Silicon dioxide (G/SiO2) 1.4 Gas molecules 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 NH 3/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 Figure 1.2 Structure and properties of other graphene’s derivatives .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 Figure 1.5 Structure of h-BN Figure 1.6 Configurations of (a) α-quartz and (b) cristobalite of SiO2 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 CO 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, MoS 2, 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 single-use 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/SiO 2) 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 3.2.4 NO2 on graphene/h-BN In NO2 adsorption, four basic positions (Top1, Top2, Hollow, and Bridge) are still constructed and optimized for adsorption study (Fig 3.12) Figure 3.12 Adsorption of NO2 molecule on G/h-BN in four different sites The formula for adsorption= energy calculation− is−also deduced as 266/ /ℎ− EG/h−BN = − /ℎ− 662 eV and ENO2 = −11.894 eV Eads = ENO2/G/h−BN + 278.556 (eV) Replacing these values, we have got: Table 3.6 Adsorption properties from optimization of NO2/G/h-BN in four sites where Position Distance d (NO2/G/h-BN) (Å) Eads (eV) Likewise, the adsorption of NO2 on G/h-BN gets the optimal position on top sites, especially T1 In comparison with other gases, NO has a closer adsorptive distance (around 3Å) although it is still a physisorption mechanism The adsorption energy for T1 is -0.403 eV Simultaneously, the MO of NO2 [22] and the DOS analysis will ameliorate the understanding of adsorption mechanism NO molecule is a paramagnetic molecule, so it is required to consider the spin polarized calculation like NO analysis That is why DOS of NO2 has a spin3πup and a spin down The band gap of NO2 is estimated and * equal to 2.902 eV The orbital is mainly located on oxygen atoms 41 Figure 3.13 MO of NO2 molecule (1) and DOS of NO2 before adsorption (2) Figure 3.14 DOS of NO2 before and after3π adsorption on G/h-BN * Due to the fact that LUMO of NO is in orbital which is located at below Dirac point after adsorption, it leads to a charge transfer between oxygen atoms of NO2 molecule and carbons on graphene 42 3.2.5 NH3 on graphene/h-BN Four positions of the ammonia molecule were investigated It includes Top1 (T1), Top2 (T2), Hollow (H), and Bridge (B) sites The top views of these configurations are illustrated below Figure 3.15 Adsorption of NH3 molecule on G/h-BN For NH3 adsorption, the energy adsorption Eads is calculated by the following formula: where Thenis stated as = The results of the calculations are given in the Table 3.7 From different adsorption site’s results, the adsorptive distance d is still over 3Å showing that NH is also physisorbed on G/h-BN and the optimal site for adsorption is bridge site with Eads = -0.185 eV Table 3.7 Adsorption properties from optimization of NH3/G/h-BN in four sites Position Distance d (NH3/G/h-BN) (Å) Eads (eV) Based on MO of NH3 [22], all energy levels (3a1, 1e, 2a1) of CO are found out and labelled in the DOS of NH3 The HOMO of NH3 is on 3a1 orbital of N atom and LUMO is on 4a1 There is a lone pair in HOMO of NH and it is placed on nitrogen atom The calculated band gap of NH3 is 4.601 eV 43 Figure 3.16 MO of NH3 molecule (1) and DOS of NH3 before adsorption (2) Figure 3.17 DOS of NH3 before and after adsorption on G/h-BN With respect to DOS of NH3 after being completely adsorbed on G/h-BN, the HOMO (3a1) was found in about -2 eV below Fermi level The shift of HOMO of N atom can be ascribed to the fact that HOMO (3a 1) of N atom had a dramatic overlap with the orbital of carbon of graphene and hence can impact a charge transfer It can be rationalized by the fact that NH has a lone pair of electrons, so NH molecule can be easily interacted with graphene 44 3.2.6 H2O on graphene/h-BN Similarly, this study constructed and optimized four different adsorption structures for H2O on G/h-BN There are Top1 (T 1), Top2 (T2), Hollow (H), and Bridge (B) positions (Fig 3.18) Figure 3.18 Adsorption of H2O molecule on G/h-BN in four different sites The calculations of adsorption= energy−were proceeded− with the formula below / /ℎ− /ℎ− where EG/h−BN = −266.662 eV and EH2O = −12.182 eV The formula is thenE computed=E by + 278.844 (eV) ads H2O/G/h−BN After optimization process, the results of adsorption properties are summed up in Table 3.8 It can be seen that the adsorptive distance d from four different sites is around 3Å It suggests that the adsorption of NO is a physisorption on G/h-BN From the values of adsorption energy, the most stable position for adsorption of H2O on G/h-BN is top position T2, with Eads = -0.155eV Table 3.8 Adsorption properties from optimization of H2O/G/h-BN in four sites Adsorption site Distance d (H2O/G/h-BN) (Å) Eads (eV) In the purpose of deciphering the charge transfer of H 2O during adsorption, the MO of H2O [11] and calculated DOS were compared and labelled each specific energy level Whilst HOMO is defined in b1 level and completely located on the O atom, LUMO is mainly situated on the H atoms The estimated band gap of H 2O is 5.548 eV 45 Figure 3.19 MO of H2O molecule (1) and DOS of H2O before adsorption (2) After the adsorption of H2O on G/h-BN, the DOS diagram of H2O moved backwards below Dirac point Although HOMO of H 2O is still dominant in oxygen atom, it is located on around -3eV below Fermi level The orbital mixing is elucidated by a closer in energy in HOMO and orbitals of carbon Furthermore, oxygen has two lone pairs of electrons which is a condition for interacting easily with carbons on graphene Figure 3.20 DOS of H2O before and after adsorption on G/h-BN 46 3.2.7 Selectivity and sensitivity of gas adsorption on G/h-BN As stated, the formula for gas adsorption before, the adsorption energy was conventionally computed by this formula: where molecule, respectively, As st udie d pre viously, t he tota l e ne rg y of G/h-BN he te rostruct ure -266.662 eV The adsorption energies of six different gases (CO NO, and NO2) on G/h-BN at optimal positions are illustrated on Table 3.9 Simultaneously, the z-axis scanning developedE by Dr V A Dinh has been investigated for determining =adsorption energy − ads / / , and the results of this method were also given in Fig 3.21 and Table 3.9 47 Figure 3.21 Adsorption energy and distance of each gas on G/h-BN from z-axis scanning Furthermore, in order to recognize the selectivity of adsorptive system, a cross referencing was performed with other research in Table 3.9 Table 3.9 A comparison of adsorption energies of gases on G/h-BN Gases Properties CO d(gas-G) (Å) Eads (eV) CO2 d(gas-G) (Å) Eads (eV) NH3 d(gas-G) (Å) Eads (eV) H2 O d(gas-G) (Å) Eads (eV) NO d(gas-G) (Å) Eads (eV) NO2 d(gas-G) (Å) Eads (eV) 48 In conclusion, the adsorption of all gases (CO 2, CO, NH3, H2O, NO, and NO2) on G/h-BN has been studied in hollow, top, and bridge sites with upright and horizontal orientations It is evident that the adsorption of all gases in horizontal orientation tends to more significant than upright orientation It is ascribed to the fact that parallel adsorption help gas molecule overlap symmetrically with carbon of graphene More interestingly, whilst CO2, CO, NH3 achieved the optimal position on bridge sites, H2O, NO, and NO2 were stably on top sites Furthermore, all gases (CO2, CO, NH3, H2O, NO, and NO2) are physically adsorbed on G/h-BN with around or over 3Å of adsorptive distance As far as adsorption energy is concerned, there was an analogy between site optimization method and z-axis scanning method From the results of both methods, it can be concluded that NO2 and NO are two gas molecules adsorbed the most on G/h-BN, −0.242 eV) with adsorption energy of NO2 ( even though NO E Nonetheless, H2O is the molecule that adsorbed the least with the value of adsorption classified as ads NO2 energy is In comparison with other research, the introduction of h-BN below graphene helps to open band gap of graphene (0.05eV) and increase the its sensitivity According to Leenaerts et al [15], the gas adsorption on G/h-BN is improved than its on pristine graphene The introduction of h-BN enhanced the sensitivity for all toxic gases and even water vapor To be particular, the adsorption energies of both NO, and CO were 10-fold higher than theirs on pristine graphene Then, while NO and NH3 on G/h-BN have ameliorated sensitivity of graphene six times, the adsorption of water vapor also achieved four-time higher 3.3 Adsorption of CO and NO gases on Graphene/α-SiO2 In this study, there are two configurations for CO and NO adsorption study on G/α-SiO2 There are one top and one hollow site for each gas which is represented below in Fig 3.22 49 Figure 3.22 Adsorption of CO and NO on G/α-SiO2 in top and hollow sites The adsorption energy is estimated by separating system in isolated gas system, G/α G/α E -SiO2 heterostructure and gas/ -SiO2 system For the isolated gas system, E CO and ENO are – 11.356 eV and – 8.146 eV, respectively Additionally, G/5SiO2 = - 567.051 eV Thus, the results of adsorption energy calculation for each gas is given specifically below Table 3.10 A comparison of adsorption energies of gases on G/α-SiO2 50 From the table summarizing the results of adsorption properties, it is obvious that the adsorptive distance d between CO or NO toward G/α-SiO2 is always over 3Å It shows that CO and NO gases are also physisorbed on G/α-SiO2 In addition, both CO and NO gases tend to stay more stability on top site compared with hollow site With regard to the selectivity, the NO molecule gives a better adsorption on G/α-SiO2 than CO molecule It can be rationalized by the single unpaired electron of NO increase the overlap interaction with orbital of carbons on graphene while CO is classified as a diamagnetic molecule Although the sensitivity of graphene has improved by using α-SiO2 as a substrate, the selectivity was highly recognized The signal of NO will be superior to its of CO which suggested that G/ α-SiO2 can be a promising material sensing for NO in the air 51 CONCLUSIONS In this work, we have already proposed two different heterostructures G/hBN and G/α-SiO2 and investigated the gas adsorption of different gases in the air at some typical sites with particular orientations Whilst only CO and NO gas were constructed to adsorb on G/α-SiO2 system in top and hollow sites, all toxic gases in the atmosphere CO2, CO, NH3, NO, NO2 and water vapor were studied the adsorption properties in T1,T2, H, and B sites on G/h-BN system There are some essential conclusions derived from this study The optimal positions and orientation for gas adsorption It is evident that all gases which are horizontally parallel with graphene gave the better adsorption compared with upright orientation in both G/h-BN and G/αSiO2 heterostructures With regard to G/h-BN system, the adsorption of CO 2, CO, and NH3 on bridge site was more favorable while H 2O, NO, and NO2 gained the optimum on top sites Adsorption nature: In both hybrid structures in this study, all gases are physically adsorbed on graphene-based surface Enhancement of sensitivity With respect to sensitivity, the introduction of substrate such as h-BN and αSiO2 has helped to ameliorate the sensitivity for gas adsorption significantly Notably, NO and NO2 adsorbed more effectively than other gases on G/h-BN It is ascribed to the fact that NO and NO are 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