Core–Shell Double Doping of Zn and Ca on β-Ga₂O₃ Photocatalysts for Remarkable Water Splitting

Abstract

Photocatalytic water splitting reaction attracts considerable attention owing to their potential application to generate H 2 gas from H 2 O by using solar energy. However, further activity enhancement is indispensable for industrial use. Various approaches have been adopted to improve its activity; however, as these defects and impurities are the main obstacles that reduce quantum efficiency (QE), the fabrication of fine crystals with low defects and impurities has been essential for activity improvement. Here, we found that Zn and Ca core−shell double doping in polycrystalline β-Ga 2 O 3 photocatalysts is very effective to enhance the photocatalytic activity, and QE reaches 71% under 254 nm illumination. Time-resolved IR absorption spectroscopy and first-principles calculations revealed that Zn and Ca create shallow mid-gap states, and electron trapping at these states prevent the electron−hole recombination. STEM−EDS mapping analysis demonstrate that Ca is doped uniformly in the bulk, but Zn is doped on the surface. These findings herein indicate that the induced concentration gradient of the dopants effectively inhibit the recombination in the bulk and at the surface and assist the diffusion of trapped electrons from the bulk to the surface, thereby accelerating reactions at the surface. These cooperative effects provide an attractive strategy to enhance the photocatalytic activity, which can be applied to many other photocatalysts including rough polycrystalline powders. This method requires neither the fabrication of fine single crystals nor the precise control of the cocatalyst loading.

Zr-doped BaTaO2N photocatalyst modified with Na–Pt cocatalyst for efficient hydrogen evolution and Z-scheme water splitting

“…These data reflect the dynamics of the intraband transitions of free and/or shallowly trapped electrons and photoexcited holes, respectively. 23,24 The addition of 1 mol% Zr evidently increased the lifetime of shallowly trapped electrons, in agreement with earlier work by Hojamberdiev et al, 25 but had no appreciable effect on the dynamics of trapped holes. Doping with lower valence cations may suppress the formation of reduced Ta species acting as recombination centers.…”

Role of Oxygen Vacancy in the Photocarrier Dynamics of WO3 Photocatalysts: The Case of Recombination Centers

“…However, the decay of deeply trapped electrons was drastically accelerated by the formation of oxygen vacancies, even after a few picoseconds. It has been reported that defects such as oxygen vacancies introduced in TiO 2 , , SrTiO 3 , , and Ga 2 O 3 , could increase the lifetime of photocarriers since these defects capture photoelectrons and reduce the probability of their recombination with holes, as shown in Scheme a. These trapped electrons need to travel long distances between defects by repeated hopping and tunneling.…”

Simultaneously Tuning the Defects and Surface Properties of Ta3N5 Nanoparticles by Mg–Zr Codoping for Significantly Accelerated Photocatalytic H2 Evolution

“…The results of the study of defect species described above were further supported by the transient absorption (TA) kinetic profiles of charge carriers probed at 2000 cm –1 (5000 nm) on a microsecond time scale (Figure a), reflecting the intraband transition of free and/or shallowly trapped electrons in/near the conduction band of the various Ta 3 N 5 . − Undoped Ta 3 N 5 exhibited the lowest TA intensity along with fast decay due to the rapid trapping of electrons at deep states, most likely arising from Ta 3+ (Figure f), a well-known recombination center in Ta 3 N 5 . ,, Ta 3 N 5 :Zr, which had the fewest defect species, produced significantly higher TA intensity at 2000 cm –1 (5000 nm) with slower decay than Ta 3 N 5 :Mg, which contained more V N . However, defining the functionality of each individual type of V N (V N •• and V N ••• /V N • ) remains difficult due to the complex transformation from each other upon photoexcitation.…”