Journal of Materials Science and Nanomaterials
Open Access

Grain growth; Interfaces; Nanocomposites; Shaping; Oriented attachment; Electrospinning

Introduction

Organizing metal oxide nanoparticles inside spatially confined continuous phases is a critical prerequisite for advanced architectures and microstructure engineering in ceramics and catalysis, as well as a timely and intriguing goal in materials chemistry and physics. In the science and technology of functional materials, formulation and electrospinning of metal oxide nanoparticle/polymer blends have received increasing attention. Sensing, the design of scaffolds and membranes, and the development of materials components for energy storage and conversion are all related application fields [2, 3]. The concentration and spatial distribution of the embedded metal oxide nanoparticles are important for many of the underlying functional properties. Particle-particle and particle-polymer interactions determine structure formation inside the confined volume of the fibres at different length scales. In this study, we used topotactic decomposition of layered Mg(OH)2 to create flat nanoribbons made up of ensembles of staggered MgO nanocubes. MgO nanocube powders are well-established model systems for surface chemistry, catalysis, microstructure evolution in ceramics, and the investigation of nanoparticle self-organization in transforming chemical environments [4]. Water adsorption and subsequent dissolution of MgO nanoparticles at such nanostructures were discovered to produce 1D and 2D structures of interpenetrated nanocubes as primary building blocks. We used the spatial confinement of electrospun fibres in this study to gain more control over nanoparticle organisation [5]. Polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP) were chosen as polymers due to their solubility in H2O and methanol, respectively. The digital images show homogeneously appearing mats made up of nanofiber networks.

Aside from the inherent differences between the two polymers, PVA and PVP, we used identical spinning parameters such as applied potential, pumping rate, and nanoparticle concentrations within the polymer solution. These result in cylindrical polymer fibres with average diameters of 270 5 nm for the MgO/PVP formulation and 210 5 nm for the MgO/PVA formulation.

Transmission electron microscopy (TEM) and X-ray diffraction were used to examine the dispersion of metal oxide-related components within polymer matrices (XRD). As a result, the morphology and structure of the dispersed phases within the fibres were discovered to be drastically different. While the particulate nature of the MgO nanocrystals is retained in the PVP-formulated nanofibers, the MgO particles transform into layered and elongated structures that correspond to flat and sometimes scrolled-up Mg(OH)2 nanosheets in the water-based PVA dispersion. X-ray diffraction confirms MgO nanoparticle transformation into crystalline hydroxides, indicating that Mg(OH)2 contributions dominate, whereas PVP exhibited an exclusively MgO-specific diffraction pattern [6].

Materials and Method

Fabrication of preforms and fibres

MCVD combined with solution doping was used to create silicabased preforms doped with Sr-based nanoparticles. SrCl2, which can be purchased commercially from Sigma-Aldrich, was used as a precursor for the growth of Sr-based nanoparticles at two different concentrations, 5 and 3.25 mM. The porous silica layer was soaked in water with SrCl2 solutions and vitrified at 1800 and 1700 degrees Celsius for the 5 mM and 3.25 mM solutions, respectively [7]. These preforms are referred to as preform #1 and preform #2. A magnetic stirrer was used to properly homogenise the solution. Because SrCl2 is soluble in water at these concentrations, the soot was impregnated uniformly. Because of the spontaneous phase separation that alkalineearth elements undergo in SiO2-based systems at the temperatures reached during the preform fabrication process, an in situ growth of Sr-rich nanoparticles occurred in the core of the preform during the process [8]. The soaking solution concentration and vitrification temperature, as we will see later, determine nanoparticle size and density. Both preforms had 15 mm diameters and 1.2 mm core diameters. GeO2 and P2O5 were also introduced in gas phase during MCVD preform fabrication to improve the refractive index of the core in relation to the cladding and create a refractive index difference (n),as shown for both preforms. In terms of the silica-based composition of the preform core, preform #1 and #2 contain approximately 7 and 5 mol% GeO2, respectively, while both preforms contain 2 mol% P2O5. According to previous research, the GeO2, P2O5, and nanoparticle concentration in the preform core all contribute to the measured n. According to the previously mentioned and listed preform fabrication conditions, the slightly larger n for preform #1 than preform #2 is explained by higher GeO2 and nanoparticle concentration. As a result, as the soaking concentration and vitrification temperature decrease, the transparency of the preform core improves noticeably, as seen in the photographs included as an inset for a section of each preform. This observed behaviour is strongly influenced by nanoparticle properties, and as a result, a thorough investigation is being conducted [9].

Characterization of microstructures

A FEI QUANTA 3D FEG Scanning Electron Microscope with a resolution of 1.5 nm at 30 kV in Secondary Electron (SE) and low vacuum mode was used to characterise the microstructures of preforms and fibres. The composition of the preform core was investigated using an energy dispersive X-ray (EDX) detector.

Optical characterization

A Photon Kinetics PK2600 Preform Analyzer was used to measure the refractive index profile of fabricated preforms. A commercial Luna OBR 4600 was used to perform OBR measurements on the Rayleigh scattering enhanced nanoparticle-doped optical fibres [10]. The various nanoparticle-doped fibres were spliced to an SMF-28 fibre pigtail terminating in an FC/APC connector and connected to the OBR 4600 for the measurements. The fibres were tested using a laser input centred at 1550 nm and a wavelength range of 43 nm, with 16384 sensing points collected per analysis.

Results and Discussion

As shown, the solid-phase ripening of HNS nanoparticles was captured using an in situ AFM at 60oC for 870 minutes. OR (blue rectangles) and SR (red circles) processes of HNS nanoparticles were observed concurrently. The size of the HNS nanoparticles in the blue rectangles increased gradually over time, but no other particle was attached to them. OR is thought to have occurred in the blue rectangles. This process may have occurred due to HNS molecule diffusion through the crystal surface, as the low vapour pressure of HNS limited HNS molecule diffusion in the gas phase. The red circles depict the distinct particle migration and subsequent coalescence processes that were considered SR. The aggregated particles in red circles 4 and 5 roughly retained their primary particle shapes, whereas the shape of the aggregated particles in red circles 6 changed over time, indicating that the OR process occurred. During the non-classical crystallisation of inorganic nanoparticles in solution, similar OR growth pathways after oriented attachment have been widely observed. 14 As a result, the in situ AFM provided experimental evidence that two mechanisms, OR and SR, exist for the solid-phase ripening of HNS nanoparticles. OR and SR could occur in different particles at the same time, or OR and SR could occur in the same particle after SR. Although solid-phase ripening was observed in several particles, many particles retained their sizes and shapes throughout the test. The solid-phase ripening, including OR and SR, appears to have occurred at random in HNS nanoparticles, with the dominant factor determining the ripening kinetics still unknown [11].

Temperature is one of the most important factors influencing ripening kinetics, and as temperature rises, so does the ripening rate. 6 This study used in situ SAXS to track the decreasing trend in the SSA of HNS at various temperatures ranging from 60oC to 150oC for up to 48 hours. At each testing temperature, an initial rapid linear drop followed by a slow decrease in the SSA was observed. The effects of HNS nanoparticle thermal expansion on SSA variation were negligible in the first 5 hours, and the decrease in SSA was irreversible. The variation of the SSA with time in the first 5 hours was fitted to a line, and the slope of the fitted lines denotes the ripening rate. The slope decreased as temperature increased, indicating an increase in ripening rate. Ripening rate did not increase linearly with increasing temperature but increased significantly at 90oC and 150oC. The larger slope error bars at 90oC and 150oC were caused by the linear fitting process, and they have no bearing on the discussion of temperature variation in ripening rate. Because the melting temperature (Tm) of HNS is 318oC, the significant increases occurred between 0.3 and 0.5 Tm [12]. Similar phenomena have been observed during the sintering of inorganic nanocatalysts, and they are attributed to nanoscale atomic mobility. 28 At 0.3Tm, also known as Hutting temperature, atoms on the surface of nanocatalysts became mobile, and at 0.5Tm, also known as Tamman temperature, all atoms in a nanocatalyst crystallite became mobile with respect to each other. As a result, the significant increases in the ripening rate of HNS nanoparticles with increasing temperature may be related to the nanoscale mobility of HNS molecules [13].

Conclusion

We demonstrated that the sweet Amapá-latex natural chelating agent can be used to synthesise ZnO nanoparticles with defects-driven photocatalytic performance in a sustainable and environmentally friendly manner. ZnO nanoparticles with a wurtzite-like structure and crystallite size ranging from 13.5 to 16.4 nm were formed by chelating Zn2+ ions by pentacyclic triterpenes present in the sweet Amapá-latex chelating agent structure, according to XRD and FTIR analyses. TEM analysis revealed that the ZnO nanoparticles are polycrystalline with particle sizes ranging from 14.1 to 16.5 nm, which is consistent with XRD results. The optical bandgap of 3.02 eV of the sample ZnO#10 was tuned by the coexistence of VZn, VO++, Oi+, and ZnO structural defects, according to our PL analysis. Finally, the photocatalytic experiment revealed that the ZnO#10 sample had the best degradation rate of the MB dye in an aqueous solution after 240 minutes of irradiation with visible light. The small particle size, high surface area, and unique defective structure of ZnO#10 dictated the significant production of hydroxyl radicals, which led to the destruction of MB dye organic molecules. Furthermore, the cyclic photodegradation experiment demonstrated that the ZnO#10 sample can be used for 5 degradation cycles without losing photocatalytic performance significantly. As a result, these findings show that the contents of the sweet Amapa-latex used in the synthesis can control the defective structure of ZnO nanoparticles, directly influencing the production of chemical species that can attack and destroy organic dye molecules.

Acknowledgement

None

Conflict of Interest

None

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