Preparation of Polyaniline-Based Nanocomposites for Photocatalytic Degradation of Pollutants and Hydrogen Production

Abstract

Chapter 1: Nanotechnology allows us to work with materials that are extremely small—about one-billionth of a meter. At this tiny scale, materials behave differently and can show special abilities. One important use of such materials is in photocatalysis, a process where sunlight helps break down harmful chemicals in water or produce clean fuel like hydrogen. This thesis focuses on creating new photocatalyst materials made by combining polyaniline (a conducting polymer) with different metal-based nanoparticles. Polyaniline can absorb visible light and help move electrical charges more efficiently, making the overall material work better under visible light. The goal of this research is twofold: i) To remove harmful dyes and pollutants from wastewater using sunlight, and ii) To generate hydrogen gas, a clean and renewable fuel, using the same sunlight-driven process. The materials developed in this work are tested using different scientific tools to understand their structure, how they absorb light, and how well they perform. The results show that these newly designed nanocomposites can clean polluted water more effectively and also produce hydrogen in a more efficient and eco-friendly way. In simple terms, this research aims to turn sunlight into a powerful tool—for cleaner water and cleaner energy. Chapter 2: Recently, polyaniline-based photocatalysts have gained great attention due to their narrow band gap (E = 1.77 eV), which enables them to absorb a significant amount of visible light (~ 43%) in the solar spectrum. In this study, we have synthesized a ternary nanocomposite (PANI/BiOCl/GO) through an oxidative polymerization approach, incorporating polyaniline (PANI), graphene oxide (GO), and bismuth oxychloride (BiOCl). Different weight percentages of composites (GO/BiOCl: 0.5%, 1%, 2.5%, and PANI: 99.5%, 99%, 97.5%) were prepared and named as 0.5PBG, 1PBG, and 2.5PBG, respectively. The ternary nanocomposite's successful development, crystallinity, purity, porosity, and optical properties were evaluated through several spectroscopic and surface analysis techniques, including UV-Vis-DRS, PL, XRD, XPS, BET, and EDS analysis. FESEM and HRTEM images unveiled the porous characteristics of PANI, the morphology of exfoliated GO layers, and the nanoplate-like structure of BiOCl. The ternary nanocomposite was finally tested for its ability to degrade an organic dye, rhodamine-b (Rhb), and in the process also generate solar-light-driven green hydrogen by water half-splitting. The composite achieved about 90% detoxification (assigned from GC-TCD by analyzing the evolved CO2 gas after degradation) and 96% color removal of Rhb dye within 120 minutes. The degradation of Rhb by 1PBG displayed a first-order reaction, featuring a rate constant 7.25 times higher than that observed for pure PANI, 3.8 times higher than for pure GO, and 3.9 times higher than for pure BiOCl. Thus, the ternary composites achieve a good amount of synergy. Significantly, this reaction rate constant is 4.7 times greater than the rate observed with commercially used TiO2-P25 photocatalyst. Various reaction parameters including solution pH, different illumination areas, catalyst dosage, and the study of scavengers were investigated to understand their effects on the degradation reaction. The photocatalyst's reusability effectiveness was evaluated over 6 cycles, and its stability was subsequently confirmed through XRD and ICP-OES analysis. The LC-MS study revealed the identification of various intermediates and end products following the degradation reaction. The nanocomposite also produced 1000 ppm of hydrogen gas with an apparent quantum efficiency (AQE) of 17.97% when CH3OH was used as a sacrificial agent, 500 ppm (AQE of 9.69%) in the acidic conditions, and 600 ppm (AQE of 11.63%) in the basic conditions. In a broader perspective, this endeavor paves the way for exploring fresh opportunities in the utilization of this ternary nanocomposite. Its potential extends beyond accelerating dye degradation to encompass diverse solar-driven applications as well. Chapter 3: Polyaniline-based photocatalysts have attracted attention due to their favourable bandgap (2.7 eV) and significant visible light absorption (~43%). In this study, a novel ternary nanocomposite, PANI/GO/MoO3, synthesized via oxidative in-situ polymerization, combining polyaniline (PANI), graphene oxide (GO), and molybdenum trioxide (MoO3) was presented with different wt./wt. %. Comprehensive characterization using XRD, BET, EDS, XPS, PL, and UV-Vis-DRS revealed crystallinity, porosity, and superior optical properties, respectively. FESEM image confirmed the porous morphology of PANI, exfoliated GO layers, and MoO3 nanorods (60-80 nm). Among the composites, 2.5PGMO (GO-MoO3: 2.5 wt.% and PANI: 97.5 wt.%) exhibited the highest electron lifetime (0.612 ms), significantly outperforming individual components like PANI (0.0495 ms), GO (0.023 ms), and MoO3 (0.022 ms). Photocatalytic activity was validated through both methyl orange (MO) degradation and solar-driven hydrogen production via water splitting. The 2.5PGMO composite achieved 98% MO removal and 70% detoxification within 120 minutes, with a reaction rate surpassing traditional photocatalysts. Optimal conditions, such as pH, catalyst dosage, and scavenger presence, enhanced performance. The composite shows 85% degradation of the pollutant over five cycles and stability of the nanocomposite was confirmed by XRD and ICP-OES. In solar hydrogen production, it delivered an apparent quantum efficiency (AQE) of 30.76% using CH3OH as a sacrificial agent, with nearly 28% AQE across varying pH conditions. This study underscores the PANI/GO/MoO3 nanocomposite as a promising multifunctional photocatalyst for simultaneous environmental remediation and sustainable hydrogen production, paving the way for advanced solar-driven technologies. Chapter 4: This study focused on creating a ternary nanocomposite (PANI/GO/MoS2) using an oxidative polymerization technique. The composite incorporated polyaniline (PANI), graphene oxide (GO), and molybdenum disulfide (MoS2) in different weight ratios. Comprehensive characterizations were performed using UV-Vis-DRS, PL, XRD, XPS, BET, and EDS to evaluate the material's crystallinity, purity, porosity, and optical properties. FESEM imaging revealed the porous nature of PANI, the exfoliated structure of GO, and the nanosphere morphology of MoS2 (35-55 nm in diameter). This composite was tested for its effectiveness in degrading methyl orange (MO) dye and generating green hydrogen via visible-light-driven water splitting. Within 120 minutes, it achieved around 81.23% detoxification and 99% removal of MO dye. The degradation process adhered to first-order kinetics with a rate constant 7.1 times higher than pure PANI, 22 times higher than GO, 6.35 times higher than MoS2, and 9.26 times greater than the commercial TiO2-P25 photocatalyst, indicating strong synergy among the components. The study also examined the impact of various reaction parameters like pH, illumination area, catalyst dosage, and scavengers on the degradation process. Reusability of the photocatalyst was assessed over six cycles, maintaining 80% stability, as confirmed by XRD analysis. GC-MS identified the intermediates and final degradation products. The nanocomposite achieved hydrogen production with an apparent quantum efficiency (AQE) of 26% using CH3OH as a sacrificial agent, and AQEs of 22%, 19%, and 15% under acidic, basic, and neutral conditions, respectively. This research highlights the potential of ternary nanocomposites for diverse applications beyond dye degradation, including various solar-driven technologies. Chapter 5: In this study, a binary nanocomposite comprising polyaniline (PANI) and nickel–aluminium layered double hydroxide (Ni-Al LDH) was synthesized via an oxidative polymerization method, with varying LDH loadings (2, 5, and 7 wt%). Comprehensive physicochemical characterization including UV-Vis DRS, photoluminescence (PL), XRD, XPS, BET, and EDS was employed to investigate the optical, structural, compositional, and textural attributes of the materials. From FESEM the porous morphology of PANI and the hierarchical, flower-like morphology of LDH were observed. The photocatalytic performance of the composite was evaluated for Congo red (CR) dye degradation and photocatalytic hydrogen evolution under visible light irradiation. After 120 minutes, the system achieved 98% dye removal and approximately 50% mineralization, as confirmed by total organic carbon analysis. Kinetic studies indicated pseudo-first-order behaviour, with the rate constant exceeding those of pristine PANI, LDH, and TiO2-P25 by factors of 6, 8, and 9, respectively, evidencing a pronounced synergistic interaction. Operational parameters such as pH, catalyst loading, illumination area, and the presence of scavengers significantly influenced activity. The composite maintained ~70% catalytic efficiency over six consecutive cycles. HRMS enabled identification of intermediate and final degradation products. Under methanol-assisted conditions, the composite exhibited a hydrogen evolution AQE of 20%, with AQEs of 18%, 21%, and 16% in acidic, basic, and neutral media, respectively. These results underscore the composite's bifunctionality for environmental remediation and solar-driven energy conversion.