Research on the Aggregation and Adsorption of Graphene Oxide Based on Molecular Dynamic Simulation

Author:Tang Huan

Supervisor:cui fu yi zhao ying


Degree Year:2018





Graphene oxide(GO)has been receiving increasing attentions due to its exceptional properties.With many promising applications,there are growing concerns that GOs will enter natural water bodies.Thus,it is necessary to explore their behavior in aquous environment.Aggregation is one of the most important factors that ultimately control the behavior of GO,and therefore,it is necessary to explore the aggregation of GOs in water.GO has also served as a potential material for water treatments.Given that theπnetworks and oxygencontaining functional groups are valuable for the sorption of aromatic compounds(ACs),GO is an effective adsorbent for ACs decontamination.A deep understanding of the adsorption mechanism will allow the development of GO-related water treatments materials.Molecular dynamics(MD)simulations and quantum chemical calculations could contribute significantly to understanding microscopic processes and furnish many details,and therefore,were employed in our study to provid a deep understanding of the behabior of GO in water.Firstly,a comparative experimental and MD simulation study was carried out to investigate the macro and micro properties of GO,and then the aggregation and the adsorption torwads aromatic componds were explored.In the study of the aggregation of GO.Microscopic aggregation process and mechanisms behind the effects of solution chemistries(pH,metal ions,and natural organic matter(NOM))and GO topology(oxygen content,GO size,and GO thickness)were uncovered.For example,MD results showed that more hydrogen bonds formed between GO and water at higher pH,according well with the increased hydrophilicity of GO calculated based on contact angle measurements.Radial distribution functions analysis suggested Ca2+interacted more strongly with GO than Na+,which explained the experimental observations that Ca2+was more effective in accelerating the aggregation process than Na+.In addition,the adsorption-bridging and steric effects of NOM were simulated,and NOM was found to be unfolded upon wrapping on GOs,leading to an increased capacity for ion and solvent binding.The evaluations of contributions to GO hydrophilicity,electrostatic energy,and intensities of interactions with metal ions indicated carboxyl group is the essential functional group in mediating the stability of GO.The adsorption of ACs on GO was explored in two aspects:the factor that control the adsorption capacity and the most favorobale adsoportion sites.Experimental isotherm analysis indicated the order of adsorption capacity is nitrobenzene>BPA>phenol>salicylic acid>benzoic acid.To examine which mechanism(includingπ-π,hydrogen bonds,vdWs,and hydrophobic interaction)governs the adsorption capacity,π-πstacking ability,hydrogen bonds interaction energy,polarizability,and the interaction intensity of ACs with water were examined using MD simulations and density functional theory calculations.Atomic force microscopy and MD results showed that adsorption capacity was mainly guided by theπ-πstacking ability of ACs.Hydrophobicity,GO-ACs hydrogen bond,van der Waals,and electrostatic interaction may contribute to the adsorption of ACs on GO,but are not important in regulating the adsorption capacity.Local configurations of ACs adsorbed on GO were captured,and two patterns for multilayer adsorption were observed.Further analysis suggested that upon adsorbing on GO,the translational motion of ACs in water will be suppressed;however,the solvent accessible surface area will be increased,which may increase the bio-accessibility of ACs.Results show that ACs exhibit a strong preference in adsorbing near the wrinkles and edges.Further analyses reveal that the edge-adsorption is mainly guided by the strongerπ-πinteraction near edges,accompanied by a stronger hydrogen bond interaction between carboxyl groups and ACs.Additionally,the water-mediated steric hindrance and flexibility of carboxyl groups also contribute to the edge-adsorption.A higher density of atoms and electrons is the main mechanism for the wrinkle-adsorption,and structural investigations indicate that the roughness serving as a steric hindrance for the ACs migration also contributes to the wrinkle-adsorption.Overall,by combining computer simulations with experimentalmeasurements,we provided molecular-level understandings toward the behavior of GO in water,indicating MD,if used properly,can be applied as a useful tool to obtain insights into the behavior of nanomaterials in water-treatment fields.