Underlayer engineering into the Sn-doped hematite photoanode for facilitating carrier extraction

Interfacial Se-O Bonds Modulating Spatial Charge Distribution in FeSe2/Nb:Fe2O3 with Rapid Hole Extraction for Efficient Photoelectrochemical Water Oxidation

Surface engineering is an effective strategy to improve the photoelectrochemical (PEC) catalytic activity of hematite, and the defect states with abundant coordinative unsaturation atoms can serve as anchoring sites for constructing intimate connections between semiconductors. On this basis, we anchored an ultrathin FeSe2 layer on Nb5+-doped Fe2O3 (FeSe2/Nb:Fe2O3) via interfacial Se-O chemical bonds to tune the surface potential. Density functional theory (DFT) calculations indicate that amorphous FeSe2 decoration could generate electron delocalization over the composite photoanodes so that the electron mobility was improved to a large extent. Furthermore, electrons could be transferred via the newly formed Se-O bonds at the interface and holes were collected at the surface of electrode for PEC water oxidation. The desired charge redistribution is in favor of suppressing charge recombination and extracting effective holes. Later, work function calculations and Mott-Schottky (M-S) plots demonstrate that a type-II heterojunction was formed in FeSe2/Nb:Fe2O3, which further expedited carrier separation. Except for spatial carrier modulation, the amorphous FeSe2 layer also provided abundant active sites for intermediates adsorption according to the d band center results. In consequence, the target photoanodes attained an improved photocurrent density of 2.42 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE), 2.5 times as that of the bare Fe2O3. This study proposed a defect-anchoring method to grow a close-connected layer via interfacial chemical bonds and revealed the spatial charge distribution effects of FeSe2 on Nb:Fe2O3, giving insights into rational designation in composite photoanodes.

Self-Diffusion versus Intentional Doping: Beneficial and Damaging Impact on Hematite Photoanode Interfaces

The comprehension of side effects caused by high-temperature thermal treatments in the design of (photo)electrodes is essential to achieve efficient and cost-effective devices for solar water splitting. This investigation explores the beneficial and damaging impacts of thermal treatments in the (photo)electrode design, unraveling the impact of self-diffusion and its consequences. The industrial-friendly polymeric precursor synthesis (PPS) method, which is known for its easy technological application, was chosen as the fabrication technique for hematite photoabsorbers. For substrate evaluation, two types of conductive glass substrates, aluminum borosilicate and quartz, both coated with fluorine-doped tin oxide (ABS/FTO and QTZ/FTO, respectively), were subjected to thermal treatments following the PPS protocol. Optical and structural analyses showed no significant alterations in substrate properties, whereas X-ray photoelectron spectroscopy (XPS) revealed the migration of silicon and calcium ions from the glass component to the FTO surface. This diffusion can be further mitigated by an oxide buffer layer. To track the potential ion diffusion on the photoabsorber surface and assess its effect on the photoelectrode performance, hematite was selected as the model material and deposited onto the glass substrates. From all the ions that could possibly migrate, only Si4+ and Ca2+ originating from the glass component, as well as Sn4+ from the fluorine-doped tin oxide (FTO), were detected on the surface of the hematite photoabsorber. Interestingly, the so-called "self-diffusion" of these ions did not result in any beneficial effect on the hematite photoelectrochemical response. Instead, intentional modifications showed more substantial impacts on the photoelectrochemical efficiency compared to unintentional self-diffusion. Therefore, "self-diffusion", which can unintentionally dope the hematite, is not sufficient to significantly impact the final photocurrent. These findings emphasize the importance of understanding the true effect of thermal treatments on the photoelectrode properties to unlock their full potential in photoelectrochemical applications.

Facile Zn and Ni Co-Doped Hematite Nanorods for Efficient Photocatalytic Water Oxidation

In this work, we report the effect of zinc (Zn) and nickel (Ni) co-doping of hydrothermally synthesized hematite nanorods prepared on fluorine-doped tin oxide (FTO) substrates for enhanced photoelectrochemical (PEC) water splitting. Seeded hematite nanorods (NRs) were facilely doped with a fixed concentration of 3 mM Zn and varied concentrations of 0, 3, 5, 7, and 9 mM Ni. The samples were observed to have a largely uniform morphology of vertically aligned NRs with slight inclinations. The samples showed high photon absorption within the visible spectrum due to their bandgaps, which ranged between 1.9-2.2 eV. The highest photocurrent density of 0.072 mA/cm² at 1.5 V vs. a reversible hydrogen electrode (RHE) was realized for the 3 mM Zn/7 mM Ni NRs sample. This photocurrent was 279% higher compared to the value observed for pristine hematite NRs. The Mott-Schottky results reveal an increase in donor density values with increasing Ni dopant concentration. The 3 mM Zn/7 mM Ni NRs sample produced the highest donor concentration of 2.89 × 10¹⁹ (cm^-3), which was 2.1 times higher than that of pristine hematite. This work demonstrates the role of Zn and Ni co-dopants in enhancing the photocatalytic water oxidation of hematite nanorods for the generation of hydrogen.

Interpretation of Mott-Schottky plots of photoanodes for water splitting

A large body of literature reports that both bismuth vanadate and haematite photoanodes are semiconductors with an extremely high doping density between 1018 and 1021 cm-3. Such values are obtained from Mott-Schottky plots by assuming that the measured capacitance is dominated by the capacitance of the depletion layer formed by the doping density within the photoanode. In this work, we show that such an assumption is erroneous in many cases because the injection of electrons from the collecting contact creates a ubiquitous capacitance step that is very difficult to distinguish from that of the depletion layer. Based on this reasoning, we derive an analytical resolution limit that is independent of the assumed active area and surface roughness of the photoanode, below which doping densities cannot be measured in a capacitance measurement. We find that the reported doping densities in the literature lie very close to this value and therefore conclude that there is no credible evidence from capacitance measurements that confirms that bismuth vanadate and haematite photoanodes contain high doping densities.

Dual-doping in the bulk and the surface to ameliorate the hematite anode for photoelectrochemical water oxidation

Aiming at the drawbacks of hematite like poor conductivity and tardy oxidation kinetics, herein, we utilized dual dopants in the bulk and surface to ameliorate the situation. Specifically, doping optimal amount of Zr⁴⁺ in the hematite (Zr:Fe₂O₃) enhances the conductivity of hematite due to the higher charge carrier density. Further, F:FeOOH could form p-n heterojunction in bulk where a potential barrier is built up that repels electrons but prompts holes transferring to F:FeOOH for water oxidation. What's more, the high electronegative of F^- would withdraw electron from the Fe site in FeOOH, and the enhanced positive electricity of Fe³⁺ is beneficial for adsorption of OH^- as well as enhance the conductivity of FeOOH to expedite holes transfer. As a result, the composite photoanode (F:FeOOH/Zr:Fe₂O₃) shows a 3.25-times enhanced photocurrent density comparing with α-Fe₂O₃. The special designation employs ultrathin F:FeOOH to act as both p-type semiconductor and efficient co-catalyst, avoiding redundant layer that would extend the migration distance of holes. On the top of that, the dual modification approach provides an extensive prospect for the further application of hematite.

Understanding the varying mechanisms between the conformal interlayer and overlayer in the silicon/hematite dual-absorber photoanode for solar water splitting

Dual-absorber photoelectrodes have been proved to have great potential in the photoelectrochemical (PEC) water splitting application due to their broadband absorption and suitable energy-band position, while the surface/interface issues are still not clearly resolved and understood. Here, during the preparation of a silicon/hematite dual-absorber photoanode achieved via synthesizing a Sn-doped hematite film on the silicon nanowire (SiNW) substrate, we separately introduced the conformal overlayer and interlayer of an Al₂O₃ thin film by atomic layer deposition. With the thickness-optimized interlayer (overlayer) of the Al₂O₃ thin film, the photocurrent density at 1.23V_RHE can be enhanced from 0.85 mA cm^-2 to 1.51 mA cm^-2 (1.25 mA cm^-2), and the on-set potential has a cathodic shift of ∼0.32 V. Although both the overlayer and interlayer modification can substantially improve the PEC performance, the underlying mechanisms are obviously different. The overlayer can only reduce the carrier recombination on the top surface and in the bulk of the hematite film; in contrast, the interlayer not only passivates the SiNW surface and bottom surface of the hematite film, but also the top surface of the photoanode due to Al³⁺ thermal diffusion from the bottom to the top surface of the hematite film and the resultant Al₂O₃ formation. This work deepens our understanding for the roles of the surface and interface engineering in the achievement of high-performance PEC systems based on dual or more absorbers.