In this study, one-dimensional hematite (?-Fe2O3) nanoneedles were synthesized using a facile hydrothermal method.
The morphology, structural and photocatalytic properties of prepared hematite nanoneedles were analyzed using a range of analysis techniques. In particular, the photocatalytic activity of prepared samples were evaluated by UV-light assisted degradation of Rhodamine B dye. It was shown that the prepared hematite nanoneedles can be potentially applied for wastewater treatment thanks to good photocatalytic properties.
The environment pollution is one of the major problems facing mankind. In particular, colored and toxic wastewater released by textile industry strongly pollutes the soil and water resources. At the moment, the most effective method of wastewater treatment is photocatalytic degradation of organic dyes with the help of semiconductor nanostructures. In particular, wide bandgap semiconductor nanostructures such as TiO2 1, 2, ZnO 3, and SnO2 4 are often used for these purposes. On the other hand, these wide bandgap semiconductors can absorb the UV spectrum, which is only about 5% of the total solar light radiation. Thus, researchers are developing new semiconductor nanostructures with narrow bandgaps for efficient solar light collection. Compared to the wide bandgap semiconductors, n-type hematite (?-Fe2O3) with a bandgap of 2-2.2 eV can absorb nearly 40% of the solar light radiation.
However, hematite has very poor electrical conductivity and high electron-hole recombination rate 5. Therefore, hematite is usually used in the form of nanostructures to attain a high specific surface area. To date, hematite nanoparticles 6 and flower-like microstructures 7 have been successfully applied for photocatalytic degradation of organic dyes. To the best of authors’ knowledge, the photocatalytic activity of one-dimensional hematite nanostructures has been not reported so far motivating us to perform this study. In addition, we also investigated the stability and reusability of prepared hematite nanostructures.Analytical grade FeCl3 (97.0 %) and NaNO3 (99.
0 %) were purchased from Sigma-Aldrich and used as received. During the synthesis procedure, FeCl3 (0.015 mol) and NaNO3 (0.1 mol) were dissolved in 50 ml of pure deionized water. The cleaned fluorine-doped tin oxide (FTO) glass (2×2 cm) was inserted into a glass bottle with conducting face up to serve as a seed layer for nanoneedles growth. The several drops of diluted HCl acid were added to the final solution to reach the pH=3. A uniform layer of FeOOH nanoneedles was formed on the FTO glass after heating the corresponding sealed glass bottle at 100 oC for 2 h. The as-prepared FeOOH nanoneedles were gently washed with pure deionized water and converted to ?-Fe2O3 nanoneedles by calcination in air at 600 oC for 1 h.
The structure of the prepared powders was examined by XRD (Bruker D8 Discover) using Cu-K? radiation (? = 0.15405 nm) at a 2? scan range of 20–60o. The morphology of the particles was characterized by transmission electron microscope TEM (JEOL JEM-2100F). During typical photocatalytic experiments, 50 mg of the sample was added to 100 mL of a 1.0 × 10-5 M Rhodamine B (RB) solution and then magnetically stirred in the dark for 30 min to reach the equilibrium of RB adsorption on the nanoneedles.
The solution was then exposed to UV irradiation with a distance of 10 cm from a 250 W high-pressure Hg lamp at room temperature. Every 15 min, 5 mL of solution was withdrawn from the beaker and then centrifuged. The concentrations of RB in the supernatants were analyzed using a UV-vis spectrophotometer (Evolution 220) at a wavelength of 553 nm. All measurements were performed at room temperature.The morphology of prepared hematite nanoneedles was examined using a TEM analysis. Figure 1 shows that prepared hematite nanostructures have needles morphology. According to the TEM image analysis, the average diameter of prepared nanoneedles varied in the range of 85-144 nm, while the lengths of the hematite nanoneedles were in the range of 740-1240 nm. The XRD analysis was further employed to analyze the structure and crystallinity of prepared nanoneedles.
Figure 2 shows the XRD pattern of the prepared sample. According to the XRD analysis, all detected peaks can be assigned to the rhombohedral phase of ?-Fe2O3 (JCPDS No. 33-0664) 5, 8. The obvious sharp peaks suggested that the obtained sample is highly crystalline. In addition, no other peaks related to impurity were detected. Thus, this method can be effectively used for a scalable production of pure and highly crystalline hematite nanoneedles.To test the photocatalytic activity of the prepared sample we investigated the degradation of an aqueous solution of RB dye with dispersed hematite nanoneedles under the constant UV-light irradiation. Figure 3 shows the normalized UV-Vis spectra of the RB dye that was measured at different time intervals.
One can see that the intensity of the main peak centered at 553 nm decreases with increasing the time of UV light irradiation. Thus, one can conclude that the RB dye concentration decreases in the aqueous solution due to dye molecules decomposition on the hematite nanoneedles.Figure 4 shows the changes in the concentration of RB dye vs. UV-light irradiation time.
For comparative purposes, the degradation of an aqueous solution of RB dye without the hematite nanoneedles was also investigated. The degradation rate can be calculated using the following formula: I=C/Co*100%Where, Co – initial concentration of RB dye, and C – real-time concentration under UV-light irradiation. Without the hematite nanoneedles, the concentration of RB decreased slightly and about 89 % of RB dye remained in the solution after 1 hour of constant UV-light irradiation. On the other hand, the concentration of RB dye decreased to 54 % in 1 hour when hematite nanoneedles were added.
Therefore, these hematite nanoneedles can be used as photocatalyst to enhance the dye degradation rate in polluted wastewaters.In addition, the photocatalytic performance of recycled hematite nanoneedles was tested three times under the same conditions. It was found that no meaningful changes (results deviation less than 4%) were observed in the photocatalytic activity of the sample, suggesting that this material has long-term stability. SummaryIn conclusion, a simple and straightforward hydrothermal method was used to prepare hematite nanoneedles. The prepared samples were thoroughly characterized by several analysis techniques. It was shown that prepared sample can be applied as a promising photocatalyst for potential dyes degradation in wastewaters.
Good photocatalytic properties of prepared hematite nanoneedles can be attributed to the synergetic effects of high specific surface area and stability.