Photoelectrochemical Water Splitting: Thermal Annealing Challenges on Hematite Nanowires

Abstract

Hematite is getting great attention as an environmentally friendly material for photoelectrochemical water splitting, due to its narrow band gap (1.9−2.2 eV), nontoxicity, low cost, high stability and wide availability. However, hematite shortcomings such as its low absorption coefficient, short hole diffusion length, or poor electrical conductivity lead to multiple electron−hole recombinations and efficiency losses. This work describes the preparation of nanostructured hematite photoelectrodes by a hydrothermal method followed by thermal annealing under different conditions. A large spectrum of materials science characterization techniques were used to unify the broad and underlying physical-chemical processes by which a material's structure and properties influence the performance of these photoelectrodes. In particular, Sn diffusion into hematite via a high-temperature annealing scheme is fairly analyzed by Rutherford backscattering spectrometry to assess the in-depth Sn distribution profiles and by extended X-ray absorption fine structure analysis for structural order analysis. The increase of photocurrent with annealing temperature and time, besides being related with percent Sn diffusion along the hematite photoelectrode, is also correlated with nanowires morphology, porosity features, and structural crystalline order enhancement. This study shows that an accurate combination of the semiconducting photoelectrode intrinsic properties, such as percent Sn profile content, one-dimensional nanowire diameter, porosity, and structural crystalline order, naturally leads to photoelectrodes with improved conductivity to photogenerated carriers and reduced band gap.

Nanoscale ZnO/α‐Fe2O3 Heterostructures: Toward Efficient and Low‐Cost Photoanodes for Water Splitting

“…In fact, the PEC performance is limited by a low electron mobility (≈10 −2 cm 2 V −1 s −1 ), short hole diffusion length of 2–4 nm compared to its light penetration depth (≈120 nm at λ = 550 nm), and very short excited‐state lifetime of ≈10 ps. [ 72,73 ] Additionally, the CB of α‐Fe 2 O 3 lies ≈0.4 V below the H + /H 2 redox potential, thus suggesting that a bias has to be applied for hydrogen reduction.…”

“…Furthermore, the hydrothermal method is one of the simplest, low cost, and scalable growth approaches. [ 73 ]…”

“…Increasing the annealing temperature, reduction of the film thickness was observed (from (330 ± 50) nm to (285 ± 37) nm for 550 and 800 °C, respectively), which is consistent with the fact that reduced thickness facilitates the electron pathway toward the charge collector. [ 73 ]…”

Effects of Film Thickness and Coating Techniques on the Photoelectrochemical Behaviour of Hematite Thin Films

“…Crystal growth of α-Fe 2 O 3 in the (104) plane has been associated with the presence of oxygen vacancies acting as a location for electron trapping and charge recombination. Also, increased intensity of (104) peak has been related to low charge mobility leading to reduced conductivity of α-Fe 2 O 3 films (Kment et al, 2015;Shinde et al, 2016;Quiterio et al, 2020). The high intensity of the (104) peak observed for spin coated films can limit their photocatalytic efficiency during water splitting due to poor charge mobility and increased electronhole recombination (Quiterio et al, 2020).…”

Revealing the synergy of Sn insertion in hematite for next‐generation solar water splitting nanoceramics

“…Especially to the 1000°C annealing, it is difficult to affirm whether the higher photocurrent density can be attributed to the diffusion process of Fe and Sn species in the vicinity of the hematite‐FTO interface or if the temperature has a greater impact on beneficial modifications, such as elimination of defects in a polycrystalline material, when compared to treatment at 700‐800°C. Considering that ion mobility tends to increase with increasing temperature at the same time that changes in crystallinity and adhesion are more likely to occur, postannealing properties of hematite nanoceramics can be the result of a synergy between these aspects, opening an opportunity for an important line to be investigated 68 …”