Formation of iron oxide nanoparticles for the photooxidation of water: Alteration of finite size effects from ferrihydrite to hematite

Abstract

Iron oxide nanoparticles represent a promising low-cost environmentally-friendly material for multiple applications. Especially hematite (α-Fe2O3) nanoparticles demonstrate great possibilities in energy storage and photoelectrochemistry. A hydrothermal one-pot synthesis can be used to synthesise hematite nanoparticles. Here, the particle formation, nucleation and growth of iron oxide nanoparticles using a FeCl3 precursor over time is monitored. The formation of 6-line ferrihydrite seeds of 2–8 nm which grow with reaction time and form clusters followed by a phase transition to ~15 nm hematite particles can be observed with ex situ X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman and UV/Vis spectroscopy. These particles grow with reaction time leading to 40 nm particles after 6 hours. The changes in plasmon and electron transition patterns, observed upon particle transition and growth lead to the possibility of tuning the photoelectrochemical properties. Catalytic activity of the hematite nanoparticles can be proven with visible light irradiation and the use of silver nitrate as scavenger material. The generation of elementary silver is dependent on the particle size of iron oxide nanoparticles while only slight changes can be observed in the oxygen generation. Low-cost nanoscale hematite, offers a range of future applications for artificial photosynthesis.

Additive-Free Rice Starch-Assisted Synthesis of Spherical Nanostructured Hematite for Degradation of Dye Contaminant

“…As shown in Figure 7 b, the indirect bandgap values were extrapolated at 2.38 eV. The obtained value was comparable to those synthesized α-Fe 2 O 3 nanoparticles that absorb visible light [ 51 ], suggesting that the Sp-HNP has a medium bandgap (at 1–3 eV) and promising semiconductor applications (i.e., as a photocatalyst) [ 52 ].…”

Nano-Sized Fe(III) Oxide Particles Starting from an Innovative and Eco-Friendly Synthesis Method

“…Actually, during the aging in water, the transformation occurs by a gradual oriented attachment of the ferrihydrite NPs that helps or is even necessary to trigger crystallization. During such gradual oriented attachment, the α-Fe 2 O 3 crystallization can occur by a topotactic transformation at solid phase, through dehydration of ferrihydrite and reorientation of iron ions favored, underwater, also at room temperature [8,17,19]. On the contrary, in dry heating conditions, with increasing temperature, ferrihydrite NPs tend to dehydrate and aggregate, but the reorientation of iron ions, and consequently the formation of α-Fe 2 O 3 , is limited, due to the restricted mobility of ions and it can occur only at high temperatures (i.e., 500 • C).…”

“…If produced in the form of nanoparticles, due to the low toxicity, stability and unique properties, hematite can be considered ideal for applications in a wide range of emerging fields. In particular, nowadays it represents a low-cost photocatalyst for water oxidation, but also an interesting opportunity for artificial photosynthesis, pigments, gas sensors, catalysts, magnetism, lithium ion batteries, electrochemical capacitors and biomedical applications [6][7][8][9][10][11][12][13][14]. Hematite nanoparticles can be synthesized by means of several methods, from physical, chemical, or biological routes.…”

“…These properties lead to diffuse applicability in several fields, such as in magnetic applications, wastewater treatments, metallurgical industry, lithium ion batteries, or in agricultural germination and growth of maize [24][25][26][27][28]. The methods of synthesis are typically based on chemical precipitation routes [8,[23][24][25], but they require extra purification steps and long times to obtain the final product, so that the possibility to produce large amounts of ferrihydrite nanoparticles (NPs) with high yields is still a challenge.…”

“…Then, the ferrihydrite NPs with the best features in terms of surface area are used as a precursor to obtaining the hematite nanoparticles. In particular, although the transformation from ferrihydrite to hematite has been studied for many years, the mechanisms at the base of the formation of hematite from ferrihydrite are still debated in recent literature, and this issue is indeed still current [8,17,29]. For this reason, the transformation of the ferrihydrite, obtained by this new method, to hematite NPs is investigated as well.…”