Multi-dimensional micro-/nano TiO2 reactor spheres for simultaneous organic degradation and heavy metal recovery

• Main Author: Ang, Koon Keong, Jeremy
• Other Authors: Sun Delai, Darren
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Civil engineering
• Online Access: http://hdl.handle.net/10356/73377
• Institution: Nanyang Technological University

The intricate link between water and energy has raised the need for a pragmatic approach towards absolving the cycle for sustainable clean water purification. Therefore, significant attention has been drawn towards advanced materials which harness solar energy for effective water purification, and are also able to manage pollutants including pathogens, organics and heavy metals. This thesis explores the feasibility of fabricating titanium dioxide (TiO2)-based materials and its derivatives through sustainable synthesis approaches for the simultaneous degradation of organics and heavy metal recovery. In a preliminary study, TiO2 hollow spheres were fabricated using carbonaceous spheres as templates to investigate the structural morphology and photocatalytic oxidation of organic pollutants as well as changes to flux performance with and without photoinduced superhydrophilicity. The study demonstrated the formation of 300 nm Hierarchical TiO2 Hollow Spheres (HTHS) composed of 13.79 nm nanoparticulates. The HTHS revealed increased UV-absorbance which suggest light scattering by the hollow morphology which resulted in enhanced organic degradation. The flux evaluation observed increased flux due to superhydrophilicity for both HTHS and P25 nanoparticulate. The P25 demonstrated higher magnitude of flux increment due to higher surface to fluid contact. However, the advantage of larger micro-structures is the pragmatic recovery of material and prevention of release out of the treatment process. Subsequently, improvements were made to design a novel synthesis process as well as to modify the TiO2 based spheres. The synthesis method considered a green synthesis pathway which applied minimal additives and involved a 2-step process. Firstly, the highly-concentrated metal nitrate solution penetrated the pores of the carbonaceous spheres followed by a solvothermal treatment in Ti-glycerolate during which Ion Exchange occurs. The morphology of the spheres includes micro spheres consisting of hierarchical, multi-shell hollow spheres assembled by nanoparticulates. The succeeding study reports the novel synthesis of multi-dimensional hierarchical hollow heterojunctioned carbonaceous copper oxide (CuO)/ TiO2 spheres (HHHS) synthesized by the penetration of copper (Cu) ions through diffusion. The HHHS of 1.5 µm revealed carbonaceous TiO2 hierarchical thorns assembled by compact 20 nm nanoparticles and micro/nanostructured mesoporous hollow shells, consisting of heterojunctioned CuO/ CuTi3O8/ TiO2 assembled by 20 to 30 nm nanoparticulates. Experimental results revealed enhanced rate of degradation by 67% compared to the TiO2 counterpart. The HHHS initiated pollutant degradation when pollutants passed through channels of hierarchical thorn-like structures 20 nm nanoparticulates. Thereafter, the heterojunction shell filtered and adsorbed the pollutants/intermediates using the mesoporous bimodal pore distribution of 4 nm to 30 nm with an enlarged effective surface area of 30.6 m2/g. The charge and pollutant transfer were both aided by the mesoporous shell which heterojunctions mitigated the recombination of electron-hole pair while the superhydrophilicity of the surface enhanced fluid transfer. The enhanced charge separation and transfer were attributed to the mesoporous shell consisting of triple heterojunction of CuO/ CuTi3O8/ TiO2 and a staggering band structure of 1.65/ 2.8/ 3.1 eV, respectively. The generation of reactive oxygen species (ROS) maintained the porosity of the shell structure and the inner hollow sphere minimized the internal volume of the sphere. This reduced the transverse distance for ROS or intermediates, thereby maximizing their interaction. To further understand the synthesis approach, the use of Fe ions was applied at varying penetration duration to synthesize Hierarchical, Multi-Shell, Micro-/Nano, Fe-doped TiO2 Hollow Sphere (HMS). The bactericidal capability of the material was also studied. The novel Ion Exchange approach established in this study demonstrated the penetration of Ti into the pores of the carbonaceous spheres which was otherwise usually hydrolysed due to the functional groups. The experimental results revealed that lower concentration of Fe prompted an outward diffusion of Fe ions which created voids within the pores. These voids in turn draw in Ti ions via diffusion, and the solvothermal pressure, aided the overall diffusion into the pores of the carbonaceous spheres. Furthermore, the balance of concentration at the sphere’s surface created multi-shell of similar shell morphology. The reduced compaction of metal ions within the pores of the spheres resulted in porous shells as larger amounts of glycerolate were oxidized prior to the crystallization of the metal ions, permitting larger pore size and specific surface area. Conversely, compacted pores reduced pore size and specific surface area. The morphology of the HMS revealed agglomeration of 20 nm nanoparticulates and uniform dispersion of Fe-doped TiO2 with an enhanced visible light absorbance bandgap reduction to 2.7 eV. The bactericidal capability of the HMS revealed 40% physical lysis of bacteria under dark conditions due to the hierarchical thorn like structure and 70% bactericidal capability under visible light in 1 h. The visible absorbance of the HMS was aided by the TiO2 -Fe3+/ Fe2+ alignment which allowed the migration of electrons from the TiO2 to Fe3+ of the conduction and converted it to Fe2+ which also mitigated the recombination of electron-hole pair. The initial physical lysis and the subsequent degradation of the bacteria wall or expose RNA/DNA through the generation of reactive oxygen species. The simultaneous recovery of heavy metal and enhanced pollutant degradation was investigated using the 1.5V photoelectrocatalysis configuration and the previously fabricated HMS for clean water production. The cathode consisted of reduced graphene oxide (rGO) onto a flexible indium tin oxide (ITO)/ polyethylene terephthalate (PET) thin film using electrodeposition. The fabrication of the anode applies a novel low-temperature binding of TiO2 onto flexible thin film using graphene oxide (GO). The GO-HMS was prepared by sonication of GO and HMS. Subsequently, the mixture was dried atop the cathode to fabricate the anode. The binding of the GO-HMS was due to the GO and the interface of the rGO Cathode film. The photoelectrocatalysis configuration demonstrated enhanced degradation of Cu-Azo complex dye by 20% and concurrently recovered 68% Cu metal by reducing Cu2+ over a period of 1 h. The experimental mechanism illustrated the extraction of GO-HMS photoexcited electrons by the anode which mitigated the recombination of electron-hole pair, and enabled the stable formation of holes. In addition, the band alignment of the three-tier TiO2-Fe3+/Fe2+-GO interface offered high carrier mobility and facilitated the overall transfer of electrons. The excited TiO2 electron first migrated into the lower band structure of the Fe3+ which converted to Fe2+ and was subsequently released into the GO. The formation of holes enabled the production of hydroxyl radical (OH•) for the degradation of the Cu-Azo complex which concurrently allowed the extraction of Cu2+ ions. The resupply of electrons towards the cathode recovered Cu through the reduction of Cu2+ ions into Cu(s). The formation of 1 micron cubic structure was observed for samples which contain as low as 30 Cu ppm. The build-up into larger micro-rods was attributed to the conductive natural that allowed electrons to concentrate onto the surface of conductive Cu(s) and led to the nucleation of Cu ions. This outward growth of metal reduced the required metal to surface ratio needed for recovery. The lack of reducing agents such as the Cu2+ resulted in the recombination of cathode electrons which disrupted the OH• to form hydroxide (OH-). Therefore, the photoelectrocatalysis configuration demonstrated the combination of an intrinsic modification of TiO2 and the extrinsic employment of electrons to enhance photocatalysis while recovering heavy metals. In summary, the thesis explored the novel synthesis method to design and enabled the fabrication of modifiable TiO2 micro-/nano-spheres which observed high potential for scalability, as well as practical engineering applications. The TiO2 micro-/nano-spheres demonstrated enhanced pollutant or bacteria degradation, adsorption, and charge separation and transfer to produce clean water. By synergizing the TiO2 micro-/nano-spheres into the photoelectrocatalysis configuration, the simultaneous degradation of organics pollutant and recovery of heavy metal attained enhanced performance which could be applied to water treatment facilities to increase the availability of water globally.