Time-Retrenched Synthesis of BaTaO₂N by Localizing an NH₃ Delivery System for Visible-Light-Driven Photoelectrochemical Water Oxidation at Neutral pH: Solid-State Reaction or Flux Method?

Abstract

Among 600 nm class transition-metal oxynitrides, BaTaO2N with a cubic Pm3̅m perovskite-type structure is promising for solar water oxidation due to its absorption of visible light up to 660 nm, narrower band gap (E g = 1.9 eV), appropriate valence band edge position for oxygen evolution, good stability in concentrated alkaline solutions, and nontoxicity. However, high defect density stemmed from long high-temperature ammonolysis limits the separation and transfer efficiency of photogenerated charge carriers in BaTaO2N. Here, a NH3 delivery system is specifically localized just above the synthesis mixture to reduce the synthesis time and defect density of BaTaO2N by a fresh supply of more active nitriding species and minimizing the generation of N2 and H2. Particularly, the effects of synthesis temperature (700–950 °C), synthesis time (1–8 h), and gas composition are systematically investigated to gain insights into the formation of single-phase BaTaO2N by solid-state reaction and flux method. Time-dependent experiments conducted at 950 °C show that single-phase BaTaO2N can be synthesized ≥6 and ≥4 h by solid-state reaction and flux method, respectively, revealing the advantage of the flux method over solid-state reaction in a localized NH3 delivery system. Subsequently, the separation and transfer efficiency and kinetics of photogenerated charge carriers are studied in BaTaO2N samples. Photoelectrochemical studies made it possible to resolve trends during visible-light-induced water oxidation, evidencing the inverse relationship between recombination and charge transfer phenomena. Transient absorption spectroscopy reveals that the dynamics of the photogenerated charge carriers in both types of BaTaO2N samples are different: (i) BaTaO2N synthesized by flux method has a greater number of holes despite the similar number of deeply trapped charge carriers and (ii) solid-state reaction led to the formation of a higher number of free electrons in BaTaO2N. The findings demonstrate the advantage of reducing the transfer distance of active nitriding species to the surface of the synthesis mixture for enhancing the photoelectrochemical water oxidation of BaTaO2N at neutral pH.

Unfolding the Role of B Site-Selective Doping of Aliovalent Cations on Enhancing Sacrificial Visible Light-Induced Photocatalytic H2 and O2 Evolution over BaTaO2N

“…74 This phenomenon would modify the potential drop across the Helmholtz layer in the electrolyte, leading to "unpinning the edge of the band." 74,75 Then, under these conditions, the phenomena that occur between the photocatalyst surface and the adsorbed intermediates could play a determining role. The role of surface phenomena is evidenced through the linear adsorption energy−performance correlations presented in Figure 6.…”

“…Finally, it has been argued that in the water splitting reaction over BaTaO 2 N-based photocatalysts, it is possible that the kinetics of electron transfer reactions are slow, and the concentration of minority carriers around the surface is increased to very high values . This phenomenon would modify the potential drop across the Helmholtz layer in the electrolyte, leading to “unpinning the edge of the band.” , Then, under these conditions, the phenomena that occur between the photocatalyst surface and the adsorbed intermediates could play a determining role. The role of surface phenomena is evidenced through the linear adsorption energy–performance correlations presented in Figure .…”

Exploring the effect of partial B-site Al3+–Mg2+ dual substitution on optoelectronic, surface, and photocatalytic properties of BaTaO2N

“…In the previous study, 32 the partial substitution of Mg 2+ for Ta 5+ in BaTaO 2 N similarly reduced the number of plate-like particles and led to the formation of particles with idiomorphic shapes in comparison to other substituents. Furthermore, the solid-state reaction here induced the formation of more irregular particles despite Al 3+ –Mg 2+ dual substitution in comparison to the flux method applied previously to synthesize BaTaO 2 N particles, 23,32,52 which may lead to the different photocatalytic activity. The EDX spectra of pristine and (co)substituted samples shown in Fig.…”

“…Therefore, the Al 3+ -Mg 2+ dual substitution can modulate the photocatalytic activity of BaTaO 2 N. The Al 3+ -Mg 2+ dual substitution in BaTaO 2 N leads to the improvement in the kinetics of photocatalytic processes as a result of efficient electron transfer and the reduction of recombination processes. The interrelation of both phenomena (electron transfer and recombination) in BaTaO 2 N photocatalysts has been discussed in previous works, 13,20,23 and the optoelectronic properties were presented to be responsible for the changes in the photocatalytic behavior. For instance, the codoping of Ta 3 N 5 with Mg and Zr 13 and the modification of BaTaO 2 N with Ca and cobalt oxide 17 significantly affected the optoelectronic properties in such a way that the co-doped photocatalysts could exhibit the lower onset potentials and higher photocatalytic activity for photoelectrochemical water splitting.…”

“…14 As a promising candidate for visible-light-driven water splitting because of its excellent visible light absorption up to 660 nm, narrow band gap, sufficient valence band potential for water oxidation, good stability, and nontoxicity, 15 perovskite BaTaO 2 N has gained significant research interest. Apparent quantum yields (AQY) of 0.06%, 16 2.1%, 17 6.8%, 18 and 11.9% 19 at 420 nm were progressively achieved for photocatalytic H 2 and O 2 evolution over BaTaO 2 N, respectively, and an incident photon-to-current efficiency of E43% at 1.23 V RHE was obtained in photoelectrochemical water oxidation over BaTaO 2 N. 20 In addition to other important strategies applied, such as flux growth, 21 particle morphology-and size-controlled synthesis, 22 time-retrenched synthesis, 23 thin-film fabrication, 24 facet-controlled synthesis, 25,26 solid-solution, 16,27 surface modification, 19 tensile uniaxial strain, 28 and particle transfer method, 29 the A-site or B-site substitution or partial substitution of atoms with different radii or valences have been proven to be effective in tailoring the surface local structure and anion ordering and modulating the optical, electronic, surface, and photocatalytic properties of BaTaO 2 N without altering its perovskite structure. Substituents with different valences act as either electron donors or acceptors and change the carrier concentration when introduced into the host lattice.…”