The Preparation And Application Of Graphene-based Adsorption And Photocatalytic Materials

Graphene is a two-dimensional carbon material with single layer of hexagonal carbonatoms, and it has the ideal lattice structure and unique optical, electrical, thermal andmechanical properties, which can be applied widely in the field of optoelectronic, energystorage devices, biomedical and environmental. The graphene based materials mainly includegraphene oxide (GO), reduced graphene oxide (rGO), heteroatom doped graphene andgraphene based composites, which are primarily used in the adsorption and catalysisprocesses of the environmental field. Graphene based materials as adsorbent can effectivelyremove organic compounds and metal ions from wastewater, but in addition to the difficultrecovery, they are easy to agglomerate, leading to the sharp decline of specific surface area,and thus resulting to the decrease of the adsorption performance. As a zero band gapsemiconductor material, graphene has excellent electron transmission characteristics, and itcan separate the charge effectively and has large specific surface area with good adsorptionability to organic pollutants. Therefore, graphene is considered to be a high efficiency carrierin photocatalytic reaction.Based on the structure and properties of graphene, and its application characteristics inthe adsorption and photocatalysis, the graphene was tried to be used as an adsorbent in thetreatment of industrial day wastewater, and these studies can provide theoretical researchfoundation for graphene materials in water treatment application. Considering theindustrialized application need huge amount of grapheme materials, while the productioncapacity cannot exceed gram level unless by using the chemical synthesis methods, so the GOand rGO prepared by chemical methods were used as adsorbents in this research. In order toavoid the agglomeration of grapheme materials in engineering application, the organicmodified GO (chemical modification), the layered compounds pillared rGO (physicalmodification) or the composite with photocatalyst (physical modification) were synthesizedand used as adsorbents, to obtain the industrial applied grapheme based materials toward thedye waste water treatment. Firstly, the adsorptive property of rGO towards three types of dyeswas conducted, and the relationship between rGO adsorption performance and molecularstructure of organic dyes and the adsorption mechanism of rGO were further studied.Meanwhile the agglomeration of rGO was confirmed. Next, starting from the idea ofimproving water soluble of rGO and avoiding the agglomeration phenomena, water-solublepolyethyleneimine was used to functionalize the GO, and water soluble GO-based adsorptionmaterial was build to use towards dye adsorption. This idea indeed solves the agglomeration problem of rGO, but it leads to the difficult reclaim of adsorbent. Then, starting from the ideaof preventing the reunion of rGO, layered zirconium phosphate was used to pillar rGO, and arGO-based adsorbent material with stable three-dimensional structure was obtained. Notably,this idea solves both the agglomeration problem and the difficult reclaim of rGO, and itpossesses high removal performance towards dye. Finally, starting from the idea of preventingthe reunion and avoiding the reclaim of rGO, bismuth oxychloride was loaded on porous rGO,that rGO and bismuth oxychloride can prevent reunion mutually. Results indicate thiscomposite, with stable three-dimensional structure, behaves excellent adsorption andphotocatalytic properties toward dye removal, and it exhibit remarking degradation efficiencyfor dye under the visible light, and most importantly, the composite does not need to reclaim.The detailed contents are described as follows:Firstly, rGO was prepared by chemical reduction method. The adsorption and desorptioncharacteristics of methyl orange (MO), methylene blue (MB) and rhodamine B (RhB) on therGO were investigated in the laboratory. The relationship between rGO adsorptionperformance and molecular structure of organic dyes and the adsorption mechanism of rGOwas further studied. The results show that the adsorption rate and the maximum adsorptionamount of MO, MB and RhB on rGO is increased with the increase of the conjugatedstructure of dye molecules, and the maximum adsorption amounts of MO, MB and RhB onrGO were~1.32g/g,~1.36g/g and~1.48g/g at30°C, respectively. The adsorption of RhBon rGO was a spontaneous process and higher temperature could facilitate the adsorptionprocess. The efficiency of rGO almost remained constant during the first six cycles ofadsorption-desorption process. In addition, the fluorescence spectra implied that theadsorption of RhB onto rGO was a π-π stacking adsorption process. Finally, theagglomeration of rGO was confirmed.Secondly, starting from the idea of improving water soluble of rGO, GO was chemicallymodified with poly(ethylene imine)(PEI) to improve its colloidal stability and wasinvestigated as a potential adsorbent for the removal of methyl orange (MO). The resultsshowed that the PEI could improve the colloidal stability of GO in aqueous solution, and theobtained PEI-GO showed a macroscopically homogeneous dispersion longer than threemonths. After standing for90days, the BET specific surface area of GO decreased from353m2·g-1to214m2·g-1, while that of PEI-GO almost remained unchanged (from432m2·g-1to413m2·g-1). The PEI-GO exhibited significantly faster kinetic and higher adsorption capacitytowards MO than that of GO. Moreover, PEI-GO had a good adsorption capacity even in theacidic range, and the highest adsorption of MO occurred at pH=6.0(~0.18g/g). The adsorption of MO on PEI-GO was an endothermic, spontaneous and physisorption process.But there are recycling problems for PEI-GO, and the adsorption quantity is lower than rGO,so it is not suitable for industrial application.Thirdly, starting from the idea of preventing the reunion of rGO, α-zirconium phosphate(ZrP) was loaded on the rGO. In order to prevent the reunion of rGO during storage,α-zirconium phosphate (ZrP) was used as the modifier, and a peculiar adsorbent ofZrP-pillared rGO (rGO-ZrP) was prepared. Then, rGO-ZrP was employed as the adsorbentand the adsorption characteristics of rGO-ZrP towards methylene blue (MB) were evaluatedunder laboratory conditions. The results showed that rGO-ZrP possessed a higher BETspecific surface area relative to rGO. The maximum adsorption quantity of MB onto the newprepared rGO-ZrP was~1.38g g-1at30°C, higher than rGO (~1.36g/g). With the increase ofstorage day, the BET specific surface area and the maximum adsorption capacity of rGO-ZrPapproximately remained unchanged. Under the maximum adsorption capacity, the adsorptionquantity of MB onto rGO-ZrP was dependent on the initial concentration of MB, and highertemperature could facilitate the adsorption process. The efficiency of rGO-ZrP almostremained constant during the first six cycles of adsorption-desorption process. In addition, thefluorescence spectra implied that the adsorption of MB onto rGO-ZrP was a π-π stackingadsorption process, and the pillared structure of rGO-ZrP greatly enhanced the noncovalentadhesion. In conclusion, rGO-ZrP could serve as a promising adsorbent for the removal ofMB in waste water, and it is suitable for industrial application.Fourthly, the composite material based on the bismuth oxychloride (BiOCl) and rGo wassynthesized by one-step reduction method, to further improve the photocatalytic acitivity ofBiOCl under visible light. The performance of photocatalytic degradation of RhB undervisible light of BiOCl-rGO composites was investigated. The results showed that theadsorption capacity of RhB of BiOCl-rGO composites was higher than BiOCl and itsadsorption capacity increase with the amount of rGO. The performance of photocatalyticdegradation of RhB by BiOCl-rGO composites reaches the maximum with the masspercentage of rGO at2%. The dynamics of photocatalytic degradation of RhB by usingBiOCl-rGO composites fit well with the classic first order kinetics model. There is a certaindegree of blue shift phenomenon for the absorption peak of the ultraviolet-visible absorptionspectrum of RhB solution with the degradation time. rGO does not directly involved in theprocess of photocatalytic degradation, while the photocatalytic activity enhancement ofBiOCl-rGO composites can be attributed to the combination of the chemical bonds betweenrGO and BiOCl, thus improving the separation efficiency of of electrons and holes. BiOCl on one hand, can prevent the reunion of rGO and improve the adsorption capacityof RhB, on the other, it can photocatalytic degradate of RhB, and the BiOCl-rGO does notneed to reclaim. This idea also solve the agglomeration problem and the difficult reclaim ofrGO, while the rGO does not need to be reclaimed, and the BiOCl-rGO is suitable forindustrial application.