Bandgap Engineering in Novel Fluorite‐Type Rare Earth High‐Entropy Oxides (RE‐HEOs) with Computational and Experimental Validation for Photocatalytic Water Splitting Applications

Abstract

Five different rare‐earth‐based nanocrystalline high entropy oxides (HEOs) with fluorite structure and average crystallite sizes between 6 and 8 nm are prepared and their photocatalytic behavior toward azo dye degradation and photoelectrochemical water splitting for hydrogen generation is examined. The cationic site in the fluorite lattice consists of five equimolar elements selected from the group of rare‐earth elements including La, Ce, Pr, Eu, and Gd and second‐row transition metals, Y and Zr. The studied HEOs exhibit bandgaps in the range from 1.91 to 3.0 eV and appropriate valence and conduction bands for water splitting. They reveal high photocatalytic activity that is mostly attributed to the accessibility of more photocatalytic active sites, which provide radicals responsible for the azo dye degradation. The materials successfully produce hydrogen by photocatalytic water splitting, suggesting the potential of HEOs as new photocatalysts. The photocatalytic performances of all studied HEOs outperform the single fluorite oxides or equivalent mixed oxides. The Ce0.2Zr0.2La0.2Pr0.2Y0.2O2 (CZLPY) engender hydrogen in 9.2 µmol mg−1 per hour that is much higher content than for pristine CeO2 material which amounts to 0.8 µmol mg−1 per hour.

High-Entropy Layered Rare Earth Hydroxides

“…High-entropy oxides similar to other high-entropy compounds can act as catalysts in reactions of oxygen and hydrogen release from water . High-entropy RE oxides were also reported to be advanced catalysts for water splitting applications due to their high band gap flexibility …”

Evaluation of Photocatalytic Performance of Nano-Sized Sr0.9La0.1TiO3 and Sr0.25Ca0.25Na0.25Pr0.25TiO3 Ceramic Powders for Water Purification

“…The calculations show that the energy difference between the VB and CB in the case of the SCNPTO was 2.69 eV, while that in the case of the SLTO was 3.05 eV. Recently it has been reported that the presence of Pr ions tends to lower the band gap in high-entropy oxides [ 50 ]. The much higher band gap in the case of the La-doped material is beneficial in preventing the recombination of electrons and holes to a greater extent than the SCNPTO, leading to a more efficient photocatalytic degradation.…”

Structural and Optical Properties of High Entropy (La,Lu,Y,Gd,Ce)AlO3 Perovskite Thin Films

“…[ 34 ] There is an immense amount of high entropy oxide materials (200+) that may show interesting optical properties. [ 35 , 36 , 37 , 38 , 39 , 40 , 41 ] For example, the stabilization of a high entropy sesquioxide ceramic phosphor has been shown to exhibit multi‐wavelength emission with a high degree of optical transparency in the visible region. [ 42 ] On the other hand, Ce‐doped RE‐based oxides have been observed to have narrow band gaps with absorption over the entire visible spectra.…”

“…[ 36 , 39 ] Additionally, RE high entropy oxides with multiple optically active cations have shown improved photocatalytic performance when compared to similar single fluorite and equivalent mixed oxides. [ 41 ]…”

“…[36,39] Additionally, RE high entropy oxides with multiple optically active cations have shown improved photocatalytic performance when compared to similar single fluorite and equivalent mixed oxides. [41] Although high entropy materials design may allow for stabilization with more compositional tuning, achieving a singlephase with multiple stoichiometry materials can often be challenging. [37] Furthermore, the growth of complex perovskite oxides, particularly when a high growth temperature is used to achieve high crystallinity materials, can result in antisite defects, impurities, as well as oxygen vacancies that can degrade crystalline quality.…”