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TiO2 hierarchical nanostructures with secondary growth have been successfully synthesized on electrospun nanofibers via surfactant-free hydrothermal route. The effect of hydrothermal reaction time on the secondary nanostructures has been studied. The synthesized nanostructures comprise electrospun nanofibers which are polycrystalline with anatase phase and have single crystalline, rutile TiO2 nanorod-like structures growing on them. These secondary nanostructures have a preferential growth direction . UV–vis spectroscopy measurements point to better dye loading capability and incident photon to current conversion efficiency spectra show enhanced light harvesting of the synthesized hierarchical structures. Concomitantly, the dye molecules act as spacers between the conduction band electrons of TiO2 and holes in the hole transporting medium, i.e., spiro-OMeTAD and thus enhance open circuit voltage. The charge transport and recombination effects are characterized by electrochemical impedance spectroscopy measurements. As a result of improved light harvesting, dye loading, and reduced recombination losses, the hierarchical nanofibers yield 2.14% electrochemical conversion efficiency which is 50% higher than the efficiency obtained by plain nanofibers.
Culture of BC Pellicles and Preparation of BC Nanofibers: BC pellicles were cultured, purified, and stored as reported in literature . Small pieces of the BC pellicles were immerged into water and the obtained suspension was disrupted by Ultrasonic Cell Disruption System for about 20min at room temperature to separate the BC nanofibers from each other from the pellicles. Synthesis of Au–BC Nanocomposites and Au NPs: Au–BC nanocompo- sites were synthesized by the reduction of HAuCl4 with PEI in aqueous mixed BC nanofibers solution. In a typical synthesis, 0.5mL of 10gL 1
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A simple, effective, and economical approach to improve the light harvesting of electrospun nanofibers has been reported in this work. By employing hydrothermal route, nanorods are grown on electrospun nanofibers. The resulting TiO2 nanostructures consist of both anatase and rutile phases. The secondary growth of nanorods is in  orientation and are single crystalline in nature, a characteristic which plays a significant role in reducing the charge transport resistance throughout the film. Upon integration of the synthesized nanostructures as photoanodes for solid-state dye-sensitized solar cells, the hierarchical nanofibers exhibit 2.14% efficiency with Jsc and Voc values being 4.05 mA/cm2 and 0.92 V, respectively. The nanorods provide additional surface area for dye loading, which helps to improve the light harvesting of the fibers by 41%. In addition to dye adsorption, the presence of larger number and densely packed dye molecules offers greater extent of screening between the electrons injected into the TiO2 conduction band and holes in spiro-OMeTAD. Owing to their crystallinity and packing density, the hierarchical nanofibers exhibit superior properties as compared to the plain nanofibers for solar cell application. These nanostructures can also be employed in fuel cells or in water splitting applications, where high surface area is required with efficient transport in 1D nanostructures. Furthermore, the combination of hierarchical nanofibers with CH3NH3PbI3, as a sensitizer with high absorption coefficient, can lead to inexpensive yet high efficiency solid-state cells.
The calcined nanofibers and nanofibers with secondary nanostructures are employed as photoanodes in ssDSC. The thicknesses of the photoanodes are about 4 μm. The current densities vs. voltage curves for the fabricated ssDSC are shown in Figure a and the cell parameters are summarized in Table . IPCE spectra are also recorded to better understand the performance of ssDSC (inset of Figure a). The HNFs comprise anatase and rutile phases (Table ; the calculations are given in Additional file), and it is well established in literature[-] that DSCs fabricated using a mixture of anatase and rutile phases exhibit improved cell performance as compared to those of pure anatase phase. Hence, the synthesized HNF are believed to perform better. The HNF-based photovoltaic cells always outperformed the NF-based photovoltaic cells for various photoanode film thickness (Additional file: Table S1). This enhanced photovoltaic performance can be attributed to increased current density (Jsc), open circuit voltage (Voc), and fill-factor (FF). The rutile nanorods on anatase nanofibers provide additional dye anchoring sites, which is significant for generating high Jsc (inset of Figure a). The higher dye loading capability of the HNF is validated using UV–vis spectroscopy (Figure b). The amount of dye loaded on HNF is approximately 6.0×10-8 mol/cm2, which is 41.17% higher than the amount of dye adsorbed on NF (approximately 4.25×10-8 mol/cm2). Thus, the absorbance of dye on HNF photoanode is larger than the NF-based photoanode as seen in Figure b. The presence of more number of dye molecules in case of HNF clearly suggests that the nanorods impart higher surface area and thus are beneficial in improving light harvesting by generating more photoelectrons. This correlates well with the high IPCE observed in case of HNF cell. The dip in IPCE at 340 to 385 nm for the HNF cell had negligible contribution to the short-circuit current density as the solar photon flux in this wavelength is low. Thus, the short-circuit current density integrated from IPCE spectra is higher for the HNF-based cell with respect to that of the NF solar cell.
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With the objective of facilitating higher dye loading, the nanofiber scaffold is subjected to hydrothermal treatment to grow secondary structures on the surface of the nanofibers. We try to investigate the effect of reaction time on hydrothermal reaction and observe the morphology of the nanofibers. This study will also help in understanding the formation mechanism of such nanostructures. As shown in Figure , the nanofibers prepared using different reaction times exhibit varying surface morphologies. Figure a shows small nuclei centers on the nanofibers after 10 min of reaction time. These centers will act as the core from which the rod-like nanostructures will grow. Figure b shows the nanofibers which are subjected to hydrothermal treatment at 30 min. No growth of secondary structures is observed here. The diameter of the nanofibers is in the range of 150 to 200 nm. A close inspection of the FESEM image (inset of Figure b) reveals that the nanofibers have rough surface, which is instrumental in the growth of hierarchical nanostructures. The surface roughness leads to reduction in energy barrier for heterogeneous nucleation of nanostructures and thus aids further growth. In the present case, different size nanorods grow preferentially on the rough nanofibers. With prolonged reaction time to 45 min, the spherical morphology tends to form irregular aggregates (Figure c). Figure d shows that for the reaction time of 1 h, secondary growth looks like nanorods covering approximately 80% of the nanofiber surface area. These nanorod-nanofiber structures are designated as HNFs throughout this paper. The average diameter of HNF is in the range of 500 to 700 nm. These nanorods not only increase the diameter of the nanostructure but also make its surface coarse. With further increase in reaction time to 2 h, the density, length, and width of the secondary structures on the nanofiber scaffold increase to a greater extent as shown in Figure e, leading to the filling of pores between each fiber. These nanostructures appear nucleated from the nanofibers and spread outwards. From the inset image of Figure e, it can be observed that the small nanostructures are of tetragonal shape, with the tip having a morphology which is close to the square facets. The diagonal size of the tetragonal nanorod measures about 200 to 250 nm. For 3-h reaction time, the nanofiber morphology gives way to the flower-like nanostructures (Figure f). The growth of the flower-like nanostructures occurs at the expense of the seeding layer, which in this case is the nanofiber scaffold. This leads to complete dissolution of the nanofiber network. The diameter of flower-like nanostructures is approximately 240 to 280 nm. As the nanorods grow in size their tips become more tapered. It is clear that the length, diameter, and density of the secondary structures can be tuned by varying the reaction time during the hydrothermal growth. Since a porous network of nanofibers will aid easy and complete infiltration of HTM layer, HNF synthesized for a hydrothermal reaction time of 1 h are apt for solar cell application. These synthesized nanostructures are believed to not only retain the porous network but also display higher anchoring sites for the dye molecules, thereby leading to increased light harvesting.
DS and SA conceived the idea of the project and carried out the characterization measurements. DS synthesized the nanofibers and fabricated the devices. SA performed the hierarchical growth. SSP contributed to the TEM and SAED characterizations. SGM supervised the project. All authors read and approved the final manuscript.
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and and (2017)Electrospun Ceramic Nanofiber Mats Today: Synthesis, Properties, and Applications. Materials, 10 (11). pp. 1-43. ISSN 1996-1944
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