Defect-rich spinel ferrites with improved charge collection properties for efficient solar water splitting

Abstract

Spinel zinc ferrite (ZnFe 2 O 4 , ZFO) is a potential photoanode material for photoelectrochemical (PEC) water splitting because of its ideal bandgap (1.9-2.1 eV) and superior chemical stability in aqueous solutions. However, the low charge collection efficiency significantly hinders the improvement in PEC activity. Herein, we report an ultrafast and effective flame activation route to enhance the charge collection properties of ZFO. First, high-temperature flame (> 1300 ℃) facilitated surface and grain boundary diffusions, increasing the grain size and connectivity of the ZFO nanoparticles. Second, the reducing atmosphere of the flame enabled the formation of surface defects (oxygen vacancy and Fe 2+ ), thereby increasing the charge carrier density and surface adsorption sites. Significantly, these two factors promoted charge transport and transfer kinetics, resulting in a 10-fold increase in the photocurrent density over the unactivated ZFO. Furthermore, we deposited a thin Al 2 O 3 overlayer to passivate the ZFO surface and then the NiFeO x oxygen evolution catalyst (OEC) to expedite hole injection into the electrolyte. This surface passivation and OEC deposition led to a remarkable photocurrent density of ~1 mA/cm 2 at 1.23 V versus the reversible hydrogen electrode, which is the highest value among all reported ZFO photoanodes. Notably, the NiFeO x /Al 2 O 3 /F-ZFO photoanode achieved excellent photocurrent stability over 55 h (96% retention) and superior faradaic efficiency (FE > 94%). Our flame activation method is also effective in improving the photocurrent densities of other spinel ferrites: CuFe 2 O 4 (93 times), MgFe 2 O 4 (16 times), and NiFe 2 O 4 (12 times).

Effective charge separation in photoelectrochemical water splitting: a review from advanced evaluation methods to materials design

“…Defects can be observed and veried through different experimental methods. [71][72][73][74] Fang et al used a chemical vapor deposition technique to engineer Mo-doped BiVO 4 (Mo:BVO). The crystal orientation reconstruction in the samples could be detected by cross-section high-resolution transmission electron microscopy (HRTEM), as shown in Fig.…”

Enhanced Sensitivity of ZnFe2O4 Based on Ordered Magnetic Moment Induced by Magnetic Field: A New Insight into Mechanism

“…Zinc ferrite (ZnFe 2 O 4 ), as a kind of iron-based spinel ferrite (AB 2 O 4 ), is of interest for its wide range of applications in magnetic materials, gas-sensitive materials, energy, biomedicine, catalysis, and other fields. [21,22] The magnetic properties of a material are usually determined by the oxygenanion-mediated superexchange interactions between the A and B sites. [23] When materials are sized at the nanometer scale, the shape, particle size, and structure of the material have a significant impact on its magnetic properties.…”

Using hollow dodecahedral NiCo-LDH with multi-active sites to modify BiVO 4 photoanode facilitates the photoelectrochemical water splitting performance

“…However, given challenges such as slow water oxidation kinetics and severe charge recombination, the actual photocurrent density of BiVO 4 remains considerably lower to its theoretical value (7.5 mA/cm 2 ) [20]. In the pursuit of PEC performance of BiVO 4 enhancement, researchers have developed diverse solution strategies, including morphology control [21][22][23], defect engineering [24][25][26][27], metal doping [28,29], heterostructure construction [15,30,31], and cocatalyst loading [32,33]. For instance, Chang et al create a Co 3 O 4 /BiVO 4 pn heterojunction to improve the PEC performance by enhancing charge transport and separation through the built-in electric field [15].…”