From Engineering Application to Environmental Implication: Reactive Species-Induced Transformation of Graphitic Carbon Nitride
Open AccessGraphitic carbon nitride (g-C3N4) has emerged as a promising photoreactive engineered nanomaterial for broad chemical, mechanical, environmental, biomedical, and energy applications in the recent decade. Its mass production and extensive use are expected in the foreseeable future. The stability of a nanomaterial is of great importance for practical applications. In addition, incidental release or inappropriate disposal of engineered nanomaterials could pose adverse environmental and health impacts. Aiming to promote nanomaterial robustness, safety regulations, and commercial acceptance, a thorough understanding of g-C3N4 stability in engineering systems and transformation in environments is urgently needed. This dissertation focuses on understanding the stability and persistence of g-C3N4 nanosheets under the attack of hydroxyl radicals (•OH) and reactive chlorine species (RCS). Furthermore, this dissertation will not only deepen understanding of the possible impacts of g-C3N4 on humans and ecosystems but also provide guidelines for rational design and optimization of robust and efficient nanomaterials for sustainable applications. The first part of this dissertation aims to understand the •OH-driven decomposition of g-C3N4 nanosheet. Two g-C3N4 nanosheet samples D and M2 were synthesized, among which M2 has more pores, defects, and edges, thus more vulnerable to oxidative attacks. Under the attack of hydroxyl radicals, both D and M2 were oxidized and decomposed to release nitrate and soluble fragments. Particularly, photoinduced holes (h+) increased the nitrate release rate by 3.37-6.33 times even though the steady-state concentration of •OH was similar. Molecular simulations highlighted that •OH only attacked a limited number of edge-site heptazines on g-C3N4 nanosheets and resulted in peripheral etching and slow degradation, whereas h+ decreased the activation energy barrier of C-N bond breaking between heptazines, shifted the degradation pathway to bulk fragmentation, and thus led to much faster degradation. The second part of this dissertation aims to explore the stability, aging, and environmental impact of g-C3N4 nanosheets under the attack of free chlorine and reactive chlorine species, a widely used oxidant/disinfectant and a class of ubiquitous radical species, respectively. g-C3N4 nanosheets were slowly oxidized by free chlorine even at a high concentration of 200-1200 mg L-1, but they decomposed rapidly when ClO• and/or Cl2•- were the key oxidants. Though Cl2•- and ClO• are considered weaker oxidants in previous studies due to their lower reduction potentials and slower reaction kinetics than •OH and Cl•, this dissertation highlighted their electrophilic attack efficacy on g-C3N4 nanosheets was on par with •OH and much higher than Cl•. A trace level of covalently bonded Cl (0.17-0.56 at%) was introduced to g-C3N4 nanosheets after free chlorine and RCS oxidation. The third part of this dissertation aims to elucidate the impacts of environmental transformations on the toxicity of g-C3N4 to bacterial cells and human cells. Our study indicated that g-C3N4 nanosheets did have high biocompatibility and low toxicity, but the toxicity was highly influenced by the transformation processes. After •OH-induced oxidation and decomposition, oxygen-containing groups were introduced to the surface of g-C3N4 nanosheets, and the •OH-aged g-C3N4 nanosheets became more benign to bacterial and human cells. By contrast, RCS-induced transformation not only increased the surface oxygen content but also led to the formation of C-Cl bonds. The trace level of covalently bonded Cl atoms significantly increased the toxicity of g-C3N4 nanosheets to bacterial cells, impairing cell membrane integrity and inhibiting the formation of biofilms, while the alteration of the toxicity to human cells was not statistically significant. In the last part of the dissertation, a chlorine-doped g-C3N4 was synthesized through a solvothermal method to effectively produce H2O2 with a rate of 1.19 ± 0.06 µM min-1 under visible light irradiation, which was improved by 104 times compared to pristine g-C3N4. Continuous net production of H2O2 was realized at a rate of 2.78 ± 0.10 µM min-1 up to 54 h with isopropanol as the hole scavenger, whereas H2O2 was only sustained for ~ 6 h without scavengers. Both molecular simulations and advanced spectroscopic characterizations elucidated that the Cl dopant increased the charge transfer rate, decreased the bandgap, and reduced the activation energy of the rate-limiting step of O2 reduction, all of which favored H2O2 production. This dissertation, for the first time, reveals the synergistic effect of photoinduced holes and hydroxyl radicals for the aging and transformation of g-C3N4 nanosheets. This dissertation also serves as the first to elucidate the essential role of reactive chlorine species in degrading and transforming g-C3N4 nanosheets. On one hand, this dissertation underlines the relationship between structural integrity and the photocatalytic performance of nanomaterials. It also underscores the urgent need for optimizing both the reactivity and robustness of photoreactive nanomaterials for engineering applications. On the other hand, this dissertation sheds light on understanding the stability and environmental transformation of other emerging photoreactive nanomaterials beyond g-C3N4 nanosheets. Moreover, it highlights the environmental implication of photoreactive nanomaterials in natural aquatic environments, particularly in saline water. Finally, this dissertation implements a novel metal-free photocatalyst for sustainable H2O2 production and elucidates the mechanism for promoting H2O2 production that can guide further optimization of photoreactive nanomaterials.
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