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

Abstract

The doping of foreign cations and anions is one of the effective strategies for engineering defects and modulating the optical, electronic, and surface properties that directly govern the photocatalytic O 2 and H 2 evolution reactions. BaTaO 2 N (BTON) is a promising 600 nm-class photocatalyst because of its absorption of visible light up to 660 nm, small band gap (E g = 1.9 eV), appropriate valence band-edge position for oxygen evolution, good stability under light irradiation in concentrated alkaline solutions, and nontoxicity. Although the photocatalytic and photoelectrochemical water-splitting efficiencies of BaTaO 2 N have been progressively improved, it is still far from the requirements set for practical applications. Here, we employ a 5% B site-selective doping of aliovalent metal cations (Al 3+ , Ga 3+ , Mg 2+ , Sc 3+ , and Zr 4+ ) to enhance sacrificial visible light-induced photocatalytic H 2 and O 2 evolution over BaTaO 2 N. The results of physicochemical characterizations reveal that no significant change in crystal structure, crystal morphology, and optical absorption edge is observed upon cation doping. Therefore, the difference observed in O 2 and H 2 evolution during the photocatalytic reactions over pristine and doped BaTaO 2 N photocatalysts is explained by examining optical, electronic, and surface properties. Also, molecular dynamics (MD) is used to gain insights into the respective effect of cation doping on adsorption energy of water molecules and formed intermediates (H* for H 2 evolution and HO*, O*, and HOO* for O 2 evolution) on the BaTaO 2 N surfaces terminated with TaO 6 , TaN 6 , and TaO 4 N 2 octahedra. Finally, the experimental reaction rates for H 2 and O 2 evolution are correlated well using a linear energy−performance relationship, elucidating the doping and surface-termination trends observed in the BaTaO 2 N photocatalysts.

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

“…Next, the effect of Al 3+ -Mg 2+ (co)substitution at the Ta site in BaTaO 2 N on electronic band structures was studied by DFT-HSE12s. As shown in Figure 5, the estimated bandgap energy of pristine BaTaO 2 N is 1.49 eV (direct-type), which is slightly lower than the experimentally 32,55 and theoretically 56 by a significant decrease in the band gap with an increase in the Al content, which is beneficial to absorb a significant fraction of visible light. Among the three cosubstituted models, the BaTa 0.5 Al 0.375 Mg 0.125 O 2 N model has the narrowest band gap (1.36 eV).…”

“…Apparently, the magnified XRD patterns indicate a slight alteration in the 2 angle position of the 110 reflection toward higher or lower 2 angles when Ta 5+ is partially substituted by Al 3+ (BTON2) or Mg 2+ (BTON3) in the octahedral coordination, indicating the lattice volume contraction or expansion, respectively. 32 As the ionic radius of Al 3+ is smaller than that of Ta 5+ , while the ionic radius of Mg 2+ is larger than that of Ta 5+ , the 2 angle position of the 110 reflections of the Al 3+ -Mg 2+ dual-substituted samples (BTON4-6) is near to that of pristine BaTaO 2 N (BTON1). Also, the concurrent substitution of smaller O 2-(126 pm) for larger N 3-(157 pm) to compensate charge balance in the Al 3+ -Mg 2+ dualsubstitution for Ta 5+ may have lessened the lattice expansion.…”

“…In the XPS core-level spectra of Ta 4f in Figure 4, the overlapping peaks of the Ta 4f 5/2 and Ta 4f 7/2 states of the Ta 5+ species bonded to N parameters of pristine BaTaO 2 N model were found to be in good agreement with the experimental data reported previously. 32,40 It can be noted that Al substitution leads to a contraction of lattice constants due to a smaller ionic radius of Al 3+ (53.5 pm) than that of Ta 5+ (64 pm). An opposite trend is observed in Mg substitution because the ionic radius of Mg 2+ (72 pm) is larger than that of Ta 5+ .…”

“…Substituents with different valences act as either electron donors or acceptors and change the carrier concentration when introduced into the host lattice. 30 Although divalent (Mg 2+ , Ca 2+ , Sr 2+ , and Zn 2+ ), trivalent (Al 3+ , Ga 3+ , and Sc 3+ ), tetravalent (Ti 4+ and Zr 4+ ), and hexavalent (Mo 6+ and W 6+ ) cations were singly introduced into the BaTaO 2 N lattice to improve its photocatalytic and photoelectrochemical water splitting efficiency, 17,28,[31][32][33][34][35] a dualsubstitution effect on water splitting efficiency of BaTaO 2 N has not been explored yet.…”

First-principles calculations of the structural, energetic, electronic, optical, and photocatalytic properties of BaTaO2N low-index surfaces

“…They found that the experimental photocatalytic reaction rates were well described by using the adsorption energies of the reaction intermediates. 25 To the best of our knowledge, some basic questions about the properties of BaTaO 2 N surfaces are still unsolved, such as (i) detailed electronic properties have not been presented and the corresponding bonding mechanism has not been clearly understood; (ii) the optical properties have not been calculated, particularly in their anisotropic behavior; and (iii) the effect of exposed termination on photocatalysis-related properties has not been fully elucidated. Therefore, to solve these issues, we have carried out DFT calculations to determine the structures and surface energies of low-indexed surfaces of BaTaO 2 N by considering different terminations, to study their electronic and optical properties, to explore the adsorption of molecular and dissociated water on surfaces in detail, and to reveal the mechanisms of the HER and the OER.…”