Modification of Hematite nanorods for photocatalytic applications

Abstract

Its relatively narrow band gap falls within the 2.0–2.2 eV range, making hematite an attractive photocatalyst. Additionally, it can efficiently absorb light up to 600 nm, harnessing approximately 40% of the solar energy spectrum. Aside from its stability in water-based solutions with a pH greater than 3, hematite is also one of the most cost-effective semiconductor materials. Using nanostructuring, heterojunction, and surface modification with TiO2 techniques has been increasingly used to increase the performance of hematite PEC due to the short electron-hole pair lifetime (10 ps) and hole diffusion length (2-4 nm). In this thesis, 1D hematite nanorods were synthesized using a hydrothermal method, and by using ALD, we successfully deposited TiO2 continuous ultrathin films with variable thickness onto 1D hematite nanorod arrays. This study focuses on understanding the impact of the number of ALD TiO2 cycles on the morphology, optical properties, the charge excitation and transfer mechanism at the semiconductor - electrolyte interface, photoelectrochemical (PEC) performance, and photodegradation efficiency of these nanostructures. The optimal photoanode response is achieved with a 1.7 nm coating (TiO2/Fe2O3) under both visible and UV light. Specifically, the photocurrent density of TiO2/Fe2O3 reaches 0.27 mA cm-2 at 0.5 V vs. Hg/HgO under visible light, marking a significant 27-fold increase compared to pristine α-Fe2O3. These results highlight that, under visible light, the TiO2 overlayer serves as a passivation layer, facilitating the transfer of photogenerated holes and reducing surface recombination of electron-hole pairs, with Fe2O3 remaining the primary contributor to electron-hole pair generation. Under UV light, at lower thickness (≤ 6.8 nm), the TiO2 overlayer primarily acts as a passivation layer. However, at higher thickness, TiO2 functions as a light absorption layer, diminishing the contribution of Fe2O3. Also, these decorated hematite and pristine hematite nanorods were studied for their ability to degrade ciprofloxacin under simulated solar light. Compared to the PEC results, the pristine hematite nanorods demonstrated superior photodegradation capabilities, achieving 50% degradation within 6 hours. On the other hand, the TiO2-coated hematite nanorods displayed slightly lower photodegradation efficiency, with 45% degradation in the same period. The results highlight the importance of optimizing TiO2 overlayer thickness for PEC applications, with 20 ALD cycles being the ideal configuration. It is also important to note that increased PEC performance may not necessarily translate into enhanced photodegradation capabilities, as evidenced by the study's results. This study not only provides insights into the impact of a TiO2 overlayer on the photoelectrochemical water splitting performance of hematite but also holds promise for the design of high-performance photoanodes and casting light on their complex behavior about PEC and photodegradation applications.