Clean Energy Activated Micro- and Nanocatalysts Towards Environmental Remediation

Abstract

Global water shortages due to rapidly shrinking clean water reserves, increasing industrial activity, and growing world population, is one of humanity’s biggest challenges that requires immediate attention. Over the years, our water resources in particular, have witnessed severe contamination with domestic wastes, insecticides and herbicides, food processing waste, pollutants from livestock operations, volatile organic compounds (VOCs), heavy metals, chemicals waste, and many other hazardous substances. Conventional water treatment in a wastewater treatment plant WWTP, has proven to be highly inefficient in removing many of these problematic contaminants. Due to this, these hazardous pollutants are released without sufficient treatment into our rivers and lakes. One of the most promising new technologies to purify water is the advanced oxidation process that relies on the in-situ generation of highly reactive radicals, such as the hydroxyl radicals, using UV-hydrogen peroxide (H2O2), ozone-H2O2, or heterogeneous photocatalysis. Unfortunately, these new technologies have many drawbacks that limit their practical use, such as high operational costs, use of energy intensive UV lamps, or use of highly oxidizing and corrosive fuels like ozone and H2O2. The goal of this thesis is to address some of the limitations of current water treatment approaches by developing a new class of smart materials that can deliver efficient water remediation by exploiting clean energy sources. Additionally, these materials should demonstrate good reusability and cost-effectiveness. In the first chapter, the need for enhanced water remediation is introduced by shedding light on the growing water contamination crisis. Additionally, limitations of conventional water treatment strategies in efficiently eliminating various organic contaminants are presented. This is followed by discussing the outline of the thesis. In the second chapter, the state-of-the-art in using micro- and nanotechnology for environmental remediation is discussed. Various fabrication techniques employed in fabrication of micro- and nanorobots for water remediation is presented. In this chapter, many examples of micro- and nanodevices for removal of oil, heavy metals, and degradation of organic pollutants is presented. In the third chapter, use of visible-light activated, micro- and nano photocatalysts for organic pollutant degradation is presented. First, the working mechanism behind photocatalysis is presented, followed by discussing the various progresses made so far in using novel photocatalytic materials for organic pollutant decomposition. Next, three types of novel UV-visible light photocatalysts that were developed in thesis are discussed. These include, bismuth oxide-bismuth oxychloride heterojunctioned microrobots, core-shell platinum-palladium@titanium dioxide tubular nanorobots, and finally, the bio-templated core-shell iron oxide@titanium dioxide microhelical robots. The photocatalysts presented in this chapter were activated in the presence of UV-visible light or direct sunlight, to create radical species for successful degradation of a variety of organic pollutants, without employing any harmful fuels. Moreover, these photocatalysts demonstrated excellent chemical stability, good reusability for multiple cleaning runs, cost-effectiveness, and easy separation after use. Additionally, the photocatalysts developed in this chapter could be remotely controlled and precisely steered by using wireless magnetic fields to enhance their photocatalytic cleaning performance. In the fourth chapter, use of mechanical stress activated, nano piezocatalysts for organic pollutant degradation is presented. First, the working mechanism behind piezocatalysis is explained, followed by discussing the various progresses made so far in using novel piezocatalytic materials for organic pollutant degradation. In this chapter, novel piezocatalysts composed of bismuth ferrite (BiFeO3) are discussed. Additionally, the piezocatalysts developed in this study also demonstrated excellent visible-light photocatalytic properties. These BiFeO3 based piezo-photocatalysts exhibited enhanced organic pollutant degradation when the stimuli of UV-visible light and mechanical deformations were applied to them, simultaneously. Moreover, these piezo-photocatalysts demonstrated excellent chemical stability, and good reusability for multiple cleaning runs. In the fifth chapter, developed of a novel class of catalysts that can be activated by using alternating magnetic fields are presented. First, the work ing mechanism behind using the using magnetic-field induced catalysis is explained, followed by discussing the state-of-the-art in using magnetic fields to influence chemical reactions. Next, the concept of magnetoelectricity is introduced and examples of its applications for biomedical applications are presented. Finally, development of novel core-shell cobalt ferrite@bismuth ferrite, CoFe2O4@BiFeO3(CFO@BFO) nanoparticles for degradation of organic pollutants under alternating magnetic fields is discussed. These nanoparticles demonstrated a unique ability to develop transient surface charges when placed under alternating magnetic fields. These surface charges then participated in a series of redox reactions for generation of highly reactive radical species. Under the magnetoelectriceffect induced catalysis, these nanoparticles were able to degrade a variety of organic pigments as well as a cocktail of hazardous pharmaceuticals with over 85% efficiency. Moreover, these core-shell nanoparticles were also successful in reducing the toxic heavy metal, hexavalent chromium (Cr (VI)) to the harmless trivalent chromium (Cr (III)) state. The magnetic catalysts developed in this study also demonstrated excellent chemical stability, and good reusability for multiple cleaning runs. Moreover, they demonstrated a good synergy towards degradation of organic pollutants and reduction of heavy metals, simultaneously. In Chapter 6, the novel catalysts activated under three different energy sources, including, UV-visible light, mechanical stress, and alternating magnetic fields, for organic pollutant degradation is presented. CFO@BFO nanoparticles were successfully activated by these three energy sources, and showed enhanced degradation performance when these energy sources were used in a combinatorial manner. Additionally, in this chapter, coreshell CFO@BFO nanoparticles having three distinct shapes were developed to study the influence of catalyst’s shape on the corresponding organic pollutant degradation performance. In chapter 7, the major conclusions drawn from the thesis are presented and discussed. This is followed by reporting on the outlook and future perspectives of the carried out during this thesis. The Appendix presents results obtained from two additional projects that were conducted during the PhD thesis, where novel materials were developed for biomedical applications. Appendix A presents the results obtained by developing smart piezoelectric nanostructures that mimic the functionalities of eels by generating surface charges during motion. These surface charges were generated under specific magnetic field parameters and allowed for targeted and on-demand drug delivery to cancer cells. Appendix B presents the results obtained by developing novel magnetoelectric inverse-opal scaffolds that are composed of a biodegradable polymer and CFO@BFO nanoparticles. These scaffolds demonstrated enhanced bone cell proliferation when placed under alternating magnetic fields, due to the wireless electrostimulation of cells.