Understanding the fundamental electrical and photoelectrochemical behavior of a hematite photoanode

Abstract

Hematite is considered to be the most promising material used as a photoanode for water splitting and here we utilized a sintered hematite photoanode to address the fundamental electrical, electrochemical and photoelectrochemical behavior of this semiconductor oxide. The results presented here allowed us to conclude that the addition of Sn(4+) decreases the grain boundary resistance of the hematite polycrystalline electrode. Heat treatment in a nitrogen (N2) atmosphere also contributes to a decrease of the grain boundary resistance, supporting the evidence that the presence of oxygen is fundamental for the formation of a voltage barrier at the hematite grain boundary. The N2 atmosphere affected both doped and undoped sintered electrodes. We also observed that the heat treatment atmosphere modifies the surface states of the solid-liquid interface, changing the charge-transfer resistance. A two-step treatment, with the second being performed at a low temperature in an oxygen (O2) atmosphere, resulted in a better solid-liquid interface.

Unraveling the Role of Sn Segregation in the Electronic Transport of Polycrystalline Hematite: Raising the Electronic Conductivity by Lowering the Grain‐Boundary Blocking Effect

“…It is well known that under certain thermal and contaminant loading conditions, the electronic blocking effect associated with the solid–solid interface (grain boundary, GB) can be lowered . For instance, the addition of Sn to polycrystalline hematite has been shown to decrease grain boundary resistance . Hematite has also been modified by adding various other dopants (transition metals and nontransition metals).…”

“…. Among the nontransition metal dopants, Sn has been widely used to modify the electronic conductivity of hematite . Recently, Sn‐doping has also been used to suppress the surface defect states of hematite and enhance the electronic properties .…”

“…on the thermal history of the material . The sintering cycle, impurity concentration, and heat treatment atmosphere strongly influence the concentration of defects, altering the nature of the grain boundaries . Hence, to study in detail, the nature of the grain boundaries and their impact on electronic transport, the number of critical variables during sample preparation must be minimized.…”

“…Thus, n could assume different values on varying the parameters L or G. Varying L, with G constant, leads to the preparation of samples with different thicknesses, without modifying their thermal history, whereas varying G, with L constant, means variation of the thermal history. Leite and co‐workers in previous work investigated the role of Schottky defect and how the addition of Sn 4+ followed by N 2 /O 2 atmosphere affects charge transport in polycrystalline hematite electrodes with large grain sizes. In that work, the changes in chemical defects due to the combination of Sn 4+ and different atmospheres were exploited to impact the photoelectrochemical behavior of hematite samples.…”

Sunlight-driven water splitting using hematite nanorod photoelectrodes

“…In the case of PEC cells using an n-type semiconductor as a photoanode, after sunlight absorption a pair of electron-holes is generated and separated by the space-charge layer. The ideal scenario is represented by the schematic diagram in Figure 1c, which shows four fundamental steps: (1) light absorption, (2) charge separation, (3) charge injection to the back contact (for reducing water at the cathode surface), and (4) hole diffusion to the electrode-electrolyte interface to oxidize water (Sivula 2013, Zandi and Hamann 2015, Soares et al 2016. In summary, the minority carriers (holes, h + ) are injected into the SCLJ interface to promote the water oxidation reaction (OER) and release the O 2 molecule.…”