The Theoretical Study Of Sensor And Adsorption Mechanism For Some Nano-materials

As the development of industry civilization, the humans have to face the increasingly serious problems of resource shortage and environmental pollution, while enjoying the convenient life. The toxic wastes emissions in the process of industrial production and living, not only destructed the ecological environment, but also seriously threatened the human health; many malignant diseases are associated with the toxic pollutants. As the demands of environmental and health protection, more and more attention has been attracted on the real-time monitoring and efficiently removal of the toxic pollutants. Due to the unique physical and chemical properties, the nano-materials have been widely studied as sensors and adsorbents. Based on the surface effect and quantum size effect, the nano-mateials are more effective in adsorption, thereby, improving their sensor sensitivities and response speed. The semi-conductor nano-materials are quite promising gas sensors for detecting toxic pollutants in environment, due to their simple working principle, portability, high sensitivity, and low cost.In the present doctoral dissertation, by using density functional theory calculations (DFT), we studied the adsorption behaviors of some common toxic pollutants (chlorophenols compounds, formaldehyde and Dioxins compounds) on the ZnO nano-materials, Carbon nano-tubes/Graphene and Self-assembled monolayers (SAMs). We also investigated the adsorption mechanisms and the changes of their electronic structures. Furthermore, we studied the influence of morphology, defect, doping, covering rate and different functional group to adsorptions, from which we expect to provide valuable guideline for searching potential resources in detecting/concentrating toxic pollutants.The detailed information of the dissertation is listed as follows.(1) The adsorption of chlorophenols/chlorophenols radicals on ZnO[100] surfaceWe first studied the adsorption of2-chlorophenols (2-CP)/2-chlorophenols radicals (2-CPR) on the ZnO[100] surface without and with an O vacancy (Vo-ZnO[100]). The results indicated that, after the adsorption of2-CP, the H atom in the hydroxyl of the2-CP is dissociated, and transferred to the O atom on the ZnO[100] surface. The activated2-CP combined to the Zn atom of the surface through a stable ionic bond. As a result, the adsorption energy of2-CP on ZnO[100]/Vo-ZnO[100] is very large. But a slight charge transfer is observed during the adsorption, and the band structure had no significant change. Which indicated that the ZnO[100]/Vo-ZnO[100] may not act as the sensor of2-CP.Because there is no H atom dissociated in the adsorption, the adsorption energy of2-CPR on ZnO[100] is less than the2-CP. And the electronic structure after the adsorption had no difference. The adsorption energy of2-CPR on Vo-ZnO[100] is nearly twice than its on ZnO[100], which is arising from the formation of Zn-O bond between the O of the2-CPR and the Zn on the Vo-ZnO[100] surface. Consequently, the electronic structure of system is dramatically changed.Summary, the ZnO[100]/Vo-ZnO[100] may be used to enrich the chlorophenols compounds and their radicals. And the Vo-ZnO[100] could be used as potential resource for detecting chlorophenols radicals.(2) The adsorption of chlorophenols/chlorophenols radicals on single-walled ZnO nano-tube (SWZONT)The physical and chemical properties of nano-materials are highly depended on their morphology. Compared with the ZnO[100] surface, the SWZONT is a one-dimensional nano-material. In this section, we studied the geometry and electronic structures of the SWZONT without and with an O vacancy (Vo-SWZNOT). After that, the adsorptions of2-CP/2-CPR on the SWZONT/Vo-SWZONT were also investigated. According to the results, the SWZONT is a direct semi-conductor, which has the wider band gap than the ZnO bulk. The average Zn-O bond length in SWZONT is also larger than it in ZnO bulk. When the SWZONT contains one O vacancy, the geometry structure of SWZONT is partly deformed; the three Zn atoms near the vacancy site may have formed stable metal-metal bonds. The band gap of Vo-SWZONT became larger than the SWZONT.Different from the adsorption on ZnO[100] surface, the H atom in hydroxyl was not dissociated during the adsorption of2-CP on SWZONT/Vo-SWZONT. The interaction between2-CP and SWZONT/Vo-SWZONT is arising from the hydrogen bond, which was formed between the hydroxyl and the O atom in the ZnO tube. The adsorption is physical process, and the electronic structures of SWZONT/Vo-SWZONT have no significant change. The adsorption energy of2-CP on Vo-SWZONT is slight larger than it on SWZONT, which is due to the charge rearrangement of Vo-SWZONT. The2-CPR and2-CP have nearly the same adsorption energy on the SWZONT, however, the density of states (DOS) of SWZNT is more sensitive to the2-CPR. The interaction between2-CPR and Vo-SWZONT is much stronger than that between2-CP and Vo-SWZONT, and the electronic structure of Vo-SWZONT changed dramatically after the adsorption of2-CPR.Summary, the results suggested that, both SWZONT and Vo-SWZONT, especially the last one, could be used as potential resources for detecting chlorophenols radicals and enriching chlorophenols compounds. Furthermore, without losing their electronic structure, SWZONT/Vo-SWZONT could be functionalized via their interaction with molecular containing hydroxyl groups.(3) The adsorption of formaldehyde on the pristine and aluminum doped SWZONTDoping is the mean approach to improving the properties of nano-materials. In this section, we studied the geometry and electronic structures of the aluminum doped SWZONT (Al-SWZONT). After that, the adsorption of formaldehyde (HCOH) on the pristine and aluminum doped SWZONT was investigated. The results indicated that, the doping of Al atom in SWZONT dramatically changed its electronic structure. The Al-SWZONT was turned to be a conductor. But both SWZONT and Al-SWZONT has no magnetic moment. The nano-tube was partly deformed after the doping with the Al atom slightly sank into the tube surface.The adsorption of HCOH on SWZONT showed little charge transfer and medium adsorption energy. The interaction between HCOH and SWZONT is the coulomb attraction, so the electronic structure of SWZONT had little change after the adsorption. The HCOH had no influence to the magnetic moment of SWZONT; therefore, the SWZONT is not suitable to detect HCOH. But the medium adsorption energy suggested that the SWZONT may be used as the adsorbents to HCOH. The interaction between HCOH and Al-SWZONT was much stranger, and made dramatic changes to the band structure of the system. The Fermi level was shifted down after the adsorption, but the system was still conductor. The significant change of electronic structure after the adsorption was the appearance of spin polarization, and the HCOH/Al-SWZONT system showed obvious magnetic moment. According to the total spin density distribution, the magnetic moment was mainly contributed from the C and0atom in the HCOH. Furthermore, the higher covering rate of HCOH could increase the magnetic moment of the system. Summary, the Al-SWZONT is a promising material for detecting HCOH. Additionally, the Al-SWZONT could monitor the concentration of HCOH in the environment.(4) The adsorption of dioxins compounds on the graphene/carbon nano-tubesLong and Yang reported that the carbon nano-tubes (CNTs) are significantly better adsorbents than activated carbon for dioxin removal. In this section, we systematically investigated the adsorption of2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD) on graphene and single-walled carbon nano-tubes (SWNT) with different radius. Additionally, in order to evaluate the contribution of each part in TCDD to its adsorption, we divided the TCDD to four molecular fragment models: dichloroethylene (DCE), dichlorobenzene (DCB), benzene and1,4-dioxin (Dioxin) to represent the different parts of TCDD. After that, we studied the interaction between the Pryene (the model of graphene) and these four molecular fragment models using the M062X hybrid density functional, which is more accurate in describing non-bonding interaction. The results are as follows:(a) When the TCDD adsorbed on the graphene with parallel configurations, the π-π stacking area reached its maximum, and TCDD showed the largest adsorption energy. When the TCDD adsorbent with perpendicular configurations, the system showed obvious σ-π interaction, and the adsorption energy was the half of its in parallel configurations.(b) The benzene, Dioxin and DCE showed obvious π-π interaction with the Pyrene, and the DCB showed obvious σ-π interaction with Pyrene, which indicated that all the parts of TCDD were contributed to its physical adsorption.(c) When the TCDD adsorbed on the SWNT along the tube axis, the adsorption energy reached its maximum. The π-π stacking area was decreased due to the curve of the SWNT surface, and the adsorption energy of TCDD on SWNT was lower than its on graphene.(d) When the radius of SWNT was large enough, the TCDD preferred to encapsulated in the cavity of the SWNT. The adsorption energy of TCDD in the cavity of SWNT was significantly increased, which was even large than it on graphene. That was because both π-π and σ-π interaction were contributed to the adsorption.(e) The pristine CNTs could be promising materials for enriching dioxins compounds. Using the open-ends CNTs and increasing the tube radius could improve the adsorption efficiency.(5) The adsorption of chlorophenols on SAMs in aqueous solutionUsing the COSMO salvation model, we studied the adsorption of2-CP on the SAMs with three different hydrophilic head groups (-COOH,-NH2,-CN). The results indicated that, two stable configurations of2-CP molecule were existed in aqueous solution. When the H atom in the hydroxyl group of2-CP interacted with the Cl atom (configuration a), the molecule was more energy favorable, as the existence of intramolecular hydrogen bond. But its binding energy with H2O was smaller than the other configuration (configuration b), in which the H atom in the hydroxyl group was kept away from the Cl atom.When the2-CP interacted with the hydrophilic head groups in configuration a, the banding energies of head groups and2-CP were smaller than that of head groups and H2O, except the-NH2. When the2-CP interacted with the three head groups in configuration b, the binding energy of all the groups with2-CP was larger than that of head groups and H2O. But compared to the binding energy of2-CP with H2O, we found that the binding energy of-CN and2-CP was lower than that of H2O with2-CP in configuration b. So the SAM with-CN head group could not separate the2-CP from the aqueous solution. Moreover, the difference of binding energy between-COOH with2-CP and-COOH with H2O was not very distinct.These results suggested that the SAM with-NH2head group was a promising material to enriching and separating the2-CP from the aqueous solution.