PhD defense: Nitin Puthuval Prasad: Engineering Bi Based Multiferroic Ternary Oxides for Enhanced Photoelectrochemical Water Splitting

On Wednesday September 16 Nitin Puthuval Prasad will defend his thesis called " Engineering Bi Based Multiferroic Ternary Oxides for Enhanced Photoelectrochemical Water Splitting"

• 1e promotor: prof. dr. ir. E.J.M. Hensen
• 2e promotor: prof. dr. rer. nat. J.P. Hofmann
• Co-promotor: dr. Anja Bieberle-Hütter

Summary 
With rising greenhouse gas emissions and increasing energy demand, there is an urgent need to transition to a low-carbon or carbon-neutral energy landscape. This requires the adoption of technologies that can effectively convert renewable energy into storable fuels. Photoelectrochemical (PEC) water splitting offers such a pathway, directly coupling light absorption with fuel generation. Metal oxide semiconductors form the core of PEC systems, yet widely studied systems such as TiO₂, Fe₂O₃, and BiVO₄ suffer from intrinsic limitations that hinder further performance improvements. TiO₂ is chemically stable but possesses a wide bandgap (~3.2 eV), limiting light absorption largely to the UV region. Fe₂O₃ has a narrower bandgap (~2.1 eV) and is earth-abundant, but suffers from a short hole diffusion length, poor charge transport, and slow oxygen evolution kinetics. BiVO₄ exhibits visible-light absorption with a bandgap of approximately 2.4 eV, yet its performance is limited by poor electron transport, significant charge recombination, and photocorrosion during prolonged operation. These limitations motivate the search for alternative oxide photoelectrode materials. BiFeO₃ (BFO) and BiMn₂O₅ (BMO) were selected in this work as promising bismuth-based ternary oxides for PEC water splitting. BFO combines visible-light absorption with ferroelectric properties that may promote charge separation through the presence of an internal polarization field. BMO exhibits a narrower bandgap of approximately 1.8 eV, enabling broader solar-light absorption, and has been reported to be stable over a wide pH range. In addition, the presence of Mn redox centers may facilitate oxygen evolution reactions. These properties make BFO and BMO attractive candidates for PEC water splitting. However, the PEC performance of both materials remains relatively low, indicating scope for further improvement. In this thesis, we investigated several approaches to enhancing the PEC performance of BFO photocathodes and BMO photoanodes. The thesis addressed four research questions, each investigated in a dedicated chapter. In Chapter 2, the influence of Bi/Fe stoichiometry on the performance of BFO photocathodes was studied (Research Question 1). Two types of thin films, stoichiometric (BFO-s) and 10% Bi-excess (BFO-e), were deposited on fluorine-doped tin oxide (FTO) substrates using a spin-coating based sol-gel method. Bulk characterization confirmed phase-pure BFO with similar grain morphology (~100 nm) and a direct bandgap of ~2.7 eV for both compositions. Surface analyses using XPS, low-energy ion scattering, and ToF-SIMS revealed Bi enrichment at the surface, more pronounced in BFO-e. Cross-sectional TEM showed a thin Bi-oxide layer at the surface with a Fe-oxide-rich sublayer beneath. O 1s XPS spectra indicated that BFO-e contains fewer Bi and O vacancies than BFO-s. Under AM1.5G illumination at 0.6 V vs RHE, the photocurrent density of BFO-e was nearly 50% higher than that of BFO-s. Electrochemical impedance spectroscopy showed lower trapping resistance and lower surface charge-transfer resistance for BFO-e, indicating reduced recombination and more efficient interfacial charge transfer. The improved performance was attributed to the beneficial Bi-oxide surface layer and the lower vacancy concentration. These results show that a 10% excess of Bi in the precursor solution is an effective strategy to improve the PEC performance of BFO photocathodes. In Chapter 3, surface modification was explored as a strategy to improve the PEC performance of BMO photoanodes (Research Question 2). The effect of a bismuth oxide (BO) overlayer was examined by comparing pristine BMO (BMO-p) with BMO coated with a BO overlayer (BMO-o). Energy-dispersive X-ray spectroscopy confirmed that the BO overlayer forms surface nanoislands, while XRD identified α-Bi₂O₃ only in BMO-o. XPS showed pronounced Bi surface enrichment and a higher Mn³⁺/Mn⁴⁺ fraction in BMO-o. Under AM1.5G illumination (100 mW cm⁻²) in 0.1 M NaOH with Na₂SO₃ as a hole scavenger, chopped-light voltammetry yielded a tenfold increase in photocurrent density for BMO-o relative to BMO-p at 1.2 V vs RHE. Ultraviolet photoelectron spectroscopy combined with optical bandgap analysis confirmed the formation of a heterojunction between BO and BMO with band alignment that promotes hole transfer from BMO into BO. This favorable band alignment enhances charge separation and suppresses recombination, resulting in significantly improved PEC performance. These findings demonstrate that the formation of a BO/BMO heterojunction through nanoisland deposition is an effective interface-engineering strategy for improving the PEC performance of BMO photoanodes. Chapter 4 examined the role of annealing sequence in improving the PEC performance of stoichiometric and Bi-excess BFO thin film photocathodes (Research Question 3). Layer-by-layer (LL) and single-step (SS) annealing were compared for BFO-s and BFO-e films. All films were phase-pure BFO with identical bandgaps of ~2.7 eV. SEM revealed that LL-annealed films had fewer pores and improved microstructural uniformity. XPS and ToF-SIMS showed that SS-annealed films had a pronounced Bi/Fe surface-to-bulk gradient, while LL-annealed films displayed a more uniform distribution. Under illumination, LL-annealed BFO-e achieved a threefold higher photocurrent than SS-annealed BFO-e, while BFO-s showed comparable performance for both sequences. This was attributed to the simultaneous reduction of physical defects (pores) and chemical defects (vacancies) in BFO-e-LL, while compensating effects between surface and bulk defects cancelled out in BFO-s. These results establish that LL-annealing in combination with Bi-excess stoichiometry reduces both physical and chemical defect densities and thereby enhances the PEC performance of BFO photocathodes. In Chapter 5, doping was investigated as a strategy to improve the PEC performance of BFO photocathodes and BMO photoanodes (Research Question 4). For BFO, Bi-excess thin films containing 0%, 1%, and 5% Cr were prepared. PEC measurements revealed a systematic decrease in photocurrent with increasing Cr content. For BMO, Nd doping at 10% and 20% levels was investigated. Nd-doped BMO showed no detectable photoactivity under illumination, in contrast to the clear photocurrent response of undoped BMO. Overall, the results demonstrate that Cr doping of BFO and Nd doping of BMO, under the conditions studied, do not improve PEC performance and instead lead to a deterioration in photoactivity. While doping is frequently employed as an approach to enhance the performance of metal oxide photoelectrodes, its effectiveness is strongly dependent on the specific material system and dopant selected. In summary, this thesis shows that the PEC performance of bismuth-based ternary oxide photoelectrodes is closely linked to defect formation and interfacial properties. Stoichiometry control, Bi-oxide nanoisland modification, and optimization of the annealing sequence improved PEC performance by reducing defect-related recombination and/or promoting charge separation. In contrast, elemental doping with the dopants and concentrations investigated here did not improve the performance of either BFO or BMO. Across the successful strategies, the results indicate an important role of Bi surface composition and vacancy formation in determining the electronic and photoelectrochemical properties of these materials. Overall, the findings demonstrate that control of composition, surface modification, and thermal processing are important for improving the performance of bismuth-based photoelectrodes and provide directions for further optimization of these materials for solar-driven water splitting.