Enhanced neutral seawater splitting on less-defective, two-dimensional LaTiO₂N photoanodes prepared from layered perovskite BaLa₄Ti₄O₁₅

Abstract

Perovskite-type LaTiO₂N with a bandgap energy of 2.1 eV is a promising semiconductor for solar (sea)water splitting to produce renewable hydrogen. However, the synthesis of an oxynitride with low defect density to achieve strong charge separation for high solar-to-hydrogen energy conversion is still challenging. Herein, we report less-defective, two-dimensional (2D) LaTiO₂N crystals prepared from layered perovskite BaLa₄Ti₄O₁₅ and their highly improved photoelectrochemical (PEC) seawater-splitting activity at neutral pH. The (111)-type layered perovskite BaLa₄Ti₄O₁₅ has structural similarity to LaTiO₂N and excess Ba species with a strong basicity, thus guaranteeing complete and fast nitridation accompanied by suppressed Ti⁴⁺ reduction. The prepared oxynitride, capable of absorbing visible light up to 610 nm, was highly crystalline and less-defective in both bulk and surface states, which is favorable for efficient photoreactions. Oxynitrides were also prepared from other layered perovskite oxides, namely La₂Ti₂O₇ and La₄Ti₃O₁₂, for comparison. Photoanodes using LaTiO₂N were fabricated by spin-coating and then employed for driving PEC seawater splitting including chloride oxidation at neutral pH. The less-defective LaTiO₂N exhibited a photocurrent density of approximately 2.4 mA cm⁻² at 1.23 V_RHE (4.9 mA cm⁻² at 1.36 V_RHE) in 0.5 M NaCl electrolyte at pH 6.4 under AM 1.5G simulated sunlight, indicating hitherto unreported remarkable activity. This work presents the first demonstration of the complete conversion of layered perovskite BaLa₄Ti₄O₁₅ into perovskite LaTiO₂N as well as the types of starting oxides that reduce the defect density of the resulting oxynitride, thereby improving the hydrogen evolution rate at neutral pH.

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1. Introduction

Gaseous hydrogen has received great attention as an upcoming alternative energy source to prevent environmental pollution and overcome the global energy crisis. Photoelectrochemical (PEC) water splitting using semiconductor materials not only generates H₂ and O₂ without the emission of greenhouse gases such as CO₂ but is also simple and scalable for the mass production of hydrogen.^1,2 Since Fujishima and Honda demonstrated the PEC water splitting using a TiO₂ photoanode under UV irradiation, various oxide semiconductors, such as WO₃ (<2.6 eV), BiVO₄ (<2.4 eV), and Fe₂O₃ (<2.1 eV), have been reported for visible-light-driven water splitting.^3–8 However, these oxide semiconductors are impractical for harvesting intense-visible-light wavelengths longer than 600 nm. Perovskite oxynitrides with a general formula of AB(O,N)₃ (A = alkaline-earth cations of Ca, Sr, or Ba, rare-earth cation of La; B = transition metal cations of Ti, Nb, or Ta) are potential semiconductors because they are capable of harvesting a wide wavelength range of visible light up to 750 nm, resulting in a high theoretical solar-to-fuel energy-conversion efficiency.^6,9,10 Their band structures span various electrochemical redox potentials feasible for artificial photosynthesis, producing value-added energy sources such as H₂, formic acid, and ammonia.^11,12 Perovskite-type LaTiO₂N has a bandgap energy (E_g) of 2.1 eV (λ < 600 nm), harvesting approximately half of the visible-light spectrum. The narrow E_g of the oxynitride typically originates from the negative shift of its valence band maximum potential by the hybridization of N 2p and O 2p atomic orbitals during the nitridation of the starting oxide.¹³ It can also drive water and seawater splitting by including a chlorine evolution reaction (CER) according to its band structure.¹⁴ In addition, LaTiO₂N is composed of low-cost and earth-abundant elements, making it the most widely studied oxynitride with a perovskite crystal structure for photocatalytic and PEC water splitting.^15–17

The synthesis of LaTiO₂N, commonly using La₂Ti₂O₇ with a stoichiometric La/Ti ratio of unity, results in high defect density, including reduced Ti³⁺, excess O²⁻, and deficient N³⁻ species, because highly electronegative Ti⁴⁺ in the oxide is readily reduced during nitridation in a reducing NH₃ flow.^18,19 The presence of defects, acting as the recombination sites of photogenerated charge carriers, leads to a reduction in the crystallinity of LaTiO₂N and surface reconstruction of the amorphous oxide layer, thus hindering the water-splitting activity of the oxynitride under visible-light irradiation. Several strategies to reduce the defect concentration, e.g., doping foreign elements, treating surface defects, and reducing the grain boundaries of LaTiO₂N, have been reported for enhancing the water-splitting activity of the oxynitride. Lin et al. attempted to reduce the defect density of LaTiO₂N by Mg doping, which improved charge separation and photocatalytic activity with an apparent quantum efficiency as high as 13.02% at 420 ± 20 nm for the oxygen evolution reaction (OER).¹⁴ Akiyama et al. reported a photoactive LaTiO₂N powder prepared by the nitridation of La₂Ti₂O₇ and the uniform etching of its surface defects by acid treatment in an aqueous poly(4-styrene sulfonic acid) solution.²⁰ An electrode made of the modified LaTiO₂N powder served as a highly active photoanode for water splitting, producing a photocurrent of 8.9 mA cm⁻² at 1.23 V_RHE in 0.1 M NaOH electrolyte at pH 13 under AM 1.5G simulated irradiation. Feng et al. reported crystalline LaTiO₂N with high interparticle networking prepared from La₂Ti₂O₇via a solid-state reaction (SSR).²¹ The starting oxide was crystallized at a high temperature of 1523 K to form continuous interparticle connections. This preparation largely reduced grain boundaries and potential defects between the oxynitrides after nitridation. Co₃O₄-modified LaTiO₂N particles caused fast charge transport during water splitting and produced a high photocurrent of 4 mA cm⁻² at 1.23 V_RHE in 1 M NaOH electrolyte at pH 13.6 under AM 1.5G simulated sunlight. However, despite several remedies for the synthesis of less-defective LaTiO₂N, its water-splitting activity has rarely been reported and is still limited in neutral electrolytes, in which water-splitting systems such as solar panels and PEC cells usually operate (Table S1†).

During nitridation, the starting oxides play a vital role in reducing the defect density of the resulting oxynitrides. A-rich starting oxides with an A/B ratio of 1.25 or even higher have been found to be effective for preparing less-defective AB(O,N)₃, thus enhancing PEC water-splitting activity.^10,11,22 Layered perovskite A₅B₄O₁₅ (A = Sr, Ba; B = =Nb, Ta) was completely transformed into perovskite ABO₂N without any impurity traces during nitridation.^11,22 The excess Sr or Ba species remaining after the complete nitridation was readily removed by washing with distilled water.²² The Lewis-basic, A-rich concentrations in A₅B₄O₁₅ significantly suppressed the reductions of B-site cations during the nitridation.²³ Consequently, the less-defective ABO₂N photoanodes produced a high water-splitting photocurrent at the milliampere level, which has never been achieved previously in perovskite oxynitrides. La-rich oxide mixtures with La/Nb ratios of more than unity have also been found to be favorable for the synthesis of less-defective, small LaNbON₂ particles, although the excess La species is difficult to remove because of the slow reaction with water, leading to impurity traces of La₃NbO₇.¹⁰ The incomplete nitridation of the La-rich mixture causes limited improvement in the PEC water-splitting activity of the resulting LaNbON₂. Thus, the aforementioned results indicate that both the A-rich condition in the starting oxide and strong basicity of the A species are necessary to obtain less-defective, single-crystalline oxynitrides during nitridation.

On the basis of the abovementioned background, layered perovskite BaLa₄Ti₄O₁₅ can be proposed as a starting oxide for the synthesis of less-defective, crystalline LaTiO₂N. Lewis-base Ba cations as well as La jointly occupy the A sites in the oxide, leading to a (Ba + La)/Ti ratio greater than unity, which may lower the reduction of Ti⁴⁺ during nitridation. In addition, the basic Ba species remaining after the nitridation is de-intercalated to the exterior of LaTiO₂N, which may easily dissolve in water.²² Crystallographically, (111)-type layered BaLa₄Ti₄O₁₅ has structural distortion, i.e., tilting of the TiO₆ octahedron in the perovskite block, which is similar to that of the Ti(O,N)₆ octahedron in LaTiO₂N with an orthorhombic perovskite structure.²⁴ The structural similarity between the starting oxide and resulting oxynitride should guarantee complete and fast nitridation to obtain highly crystalline LaTiO₂N. In a study, layered BaLa₄Ti₄O₁₅ served as an oxide semiconductor itself to drive the photocatalytic reduction of nitrate ions to dinitrogen.²⁵ N-doped BaLa₄Ti₄O₁₅ prepared via mild, short nitridation was responsive to visible light; however, it showed limited photocatalytic OER.²⁶ Because there are no reports yet on the conversion of layered perovskite BaLa₄Ti₄O₁₅ into LaTiO₂N, a demonstration of the improvement in the hydrogen evolution rate by the resulting oxynitride at neutral pH is a scientifically meaningful endeavor for introducing a new synthesis route.

In this study, we first attempted the complete conversion of layered perovskite BaLa₄Ti₄O₁₅ as a starting oxide to yield less-defective, single-phase LaTiO₂N for sunlight-driven seawater splitting. BaLa₄Ti₄O₁₅ was crystallized by the flux-assisted calcination of BaCO₃, La₂O₃, and TiO₂, followed by nitridation at 1123 K for 20 h under an NH₃ flow and subsequent surface-annealing treatment, finally producing LaTiO₂N. The oxynitrides were also prepared from other layered perovskite oxides, namely La₂Ti₂O₇ and La₅Ti₄O_x, for comparison. The prepared LaTiO₂N photoanodes were fabricated by spin-coating and necking treatment and then employed for driving PEC seawater splitting including chloride oxidation as well as pure water splitting at neural pH. The oxynitride photoanode exhibited a photocurrent density of approximately 2.4 mA cm⁻² at 1.23 V_RHE (4.9 mA cm⁻² at 1.36 V_RHE) in 0.5 M NaCl (electrolyte) at pH 6.4 under AM 1.5G simulated sunlight. The oxynitride prepared from BaLa₄Ti₄O₁₅ exhibited much higher neutral photoactivity than those of oxynitrides prepared from La₂Ti₂O₇ and La₅Ti₄O₁₅. Its higher photocurrent also considerably surpassed our previous result for SrNbO₂N crystals grown on a Nb substrate in the same electrolyte.²⁷ Various physical and surface properties of the LaTiO₂N particles prepared from BaLa₄Ti₄O₁₅ were investigated to determine the reason behind the enhancement in the PEC seawater splitting by the oxynitride. In particular, the conversion of BaLa₄Ti₄O₁₅ was compared with those of other starting oxides, and the surface and bulk defect densities of the resulting oxynitrides were estimated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) analyses. The analysis results clearly demonstrate the types of starting oxide that reduce the defect densities of the corresponding oxynitride as well as enhance its PEC oxidation reaction activity.

2. Experimental section

2.1 Synthesis of LaTiO₂N

BaLa₄Ti₄O₁₅ as a starting oxide was prepared by flux-assisted calcination using KCl flux.²⁸ BaCO₃ (99%, Kanto Chemical), La₂O₃ (99.99%, Sigma-Aldrich), TiO₂ (99.9%, Kojundo Chemical), and KCl (99.5%, Kanto Chemical) were blended in a molar ratio of Ba : La : Ti : KCl = 1 : 2 : 4 : 30 and then ground in an agate mortar for 30 min. The mixture was calcined in air at 1273 K for 20 h with a heating rate of 10 K min⁻¹ and cooled to 1023 K with a cooling rate of 1 K min⁻¹ and then down to 298 K naturally. The resulting oxide, hereinafter denoted as BaLa₄Ti₄O₁₅, was dispersed in deionized water for 5 h and then filtered. This procedure was repeated several times to remove the remaining KCl. The filtered oxide was then dried on a hot plate at 373 K for 5 h. For comparison, La₂Ti₂O₇ and La₅Ti₄O_x were also synthesized in the same manner as BaLa₄Ti₄O₁₅. La₂O₃ (99.99%, Sigma-Aldrich), TiO₂ (99.9%, Kojundo), and KCl (99.5%, Kanto Chemical) were mixed in a molar ratio of La : Ti : KCl = 1 : 2 : 10 and 2.5 : 4 : 25 for La₂Ti₂O₇ and La₅Ti₄O_x, respectively, and then ground in an agate mortar for 30 min. The mixtures were calcined in air at 1273 and 1373 K for 20 h to obtain La₂Ti₂O₇ and La₅Ti₄O_x, respectively.

LaTiO₂N was synthesized by the nitridation of the prepared BaLa₄Ti₄O₁₅. BaLa₄Ti₄O₁₅ (0.3 g) was transferred to an alumina boat and heated in an NH₃ flow (6 N grade) of 250 mL min⁻¹ at 1123 K for 20 h at a ramping rate of 10 K min⁻¹ with intermediate grinding. Then, the resulting LaTiO₂N was washed with a large amount of deionized water for 1 h to remove excess Ba species, followed by acid treatment with a 10 mM hydrochloric acid (Samchun Chemicals) aqueous solution for 5 min to eliminate impurity traces. The surface-treated oxynitride was dried naturally and annealed in an Ar flow (5 N grade) of 100 mL min⁻¹ at 973 K for 1 h with a ramping rate of 10 K min⁻¹. The nitridation of the starting oxides La₂Ti₂O₇ and La₅Ti₄O_x was also performed under the same conditions as those mentioned above.

2.2 PEC seawater splitting

Particulate LaTiO₂N photoanodes were prepared by a spin-coating method reported in a previous study.²⁸ Briefly, the oxynitride powder was stirred and sonicated in 200 μL of isopropanol for 10 min each; this process was repeated twice to obtain a highly dispersed suspension. Thirty microliters of the suspension was dropped onto a fluorine-tin oxide (FTO) glass substrate (2 × 1.5 cm) mounted on a spin coater (Ossila), which was then rotated at 1000 rpm for 1 min. This procedure was repeated three times to uniformly deposit the oxynitride particles on the FTO glass substrate. The LaTiO₂N/FTO substrate was placed on a hot plate at 373 K for 30 min until it had completely dried. Then, the LaTiO₂N/FTO electrode was subjected to a necking treatment using 40 mM NbCl₅-methanol solution¹⁰ and subsequent annealing in an Ar flow (6 N grade) of 100 mL min⁻¹ at 673 K for 10 min. To boost seawater splitting, a Co(OH)_x electrocatalyst was deposited on the prepared electrode,⁹ which was then annealed in an H₂ flow (6 N grade) of 30 mL min⁻¹ at 573 K for 30 min with a ramping rate of 10 K min⁻¹.

The seawater-splitting activity of Co(OH)_x/LaTiO₂N/FTO photoanodes was measured in a typical three-electrode system in conjunction with a potentiostat (WPG100e, WonATech Co., Ltd) using a two-compartment PEC cell separated with a 115 Nafion membrane (NARA Cell-Tech Corporation) under AM 1.5G simulated sunlight (XES-50S2-TT, San-El Electric). The prepared photoanode, an Ag/AgCl electrode, and a Pt wire were utilized as the working, reference, and counter electrodes, respectively. The photoanode and Ag/AgCl electrode were mounted in one compartment, and the Pt wire was placed in another compartment of the PEC cell. An aqueous 0.2 M potassium phosphate (KPi) buffer electrolyte was prepared in both compartments, and 0.5 M NaCl was added into the photoanode side to produce artificial seawater. The electrode potentials were estimated as values versus a reversible hydrogen electrode (RHE) by using the Nernst equation, E_RHE = E_Ag/AgCl + 0.059 × pH + 0.197. Linear sweep voltammograms (LSVs) were recorded during seawater splitting by cathodically scanning the potential range from 1.4 to 0.4 V_RHE at a scan rate of 10 mV s⁻¹ at 298 K in an Ar-saturated 0.5 M NaCl electrolyte buffered with 0.2 M KPi at pH 6.4 and 13. The bulk charge separation efficiency, η_bulk, can be determined using the equation⁸

η_bulk (%) = J_sulfite/J_abs × 100 (1)

LHE(λ) = 1 − 10^−A(λ) (3)

where J_sulfite is the photocurrent density for sulfite oxidation, J_abs is the photon absorption rate expressed as photocurrent density, LHE(λ) is the light harvesting efficiency, λ is the wavelength (nm), λ_e is the absorption edge wavelength of the photoanodes, Φ_ph(λ) is the photon flux (mW cm⁻² nm⁻¹), and A(λ) is the absorbance of photoanodes. Also, the efficiency of surface charge separation, η_surface, can be estimated using the equation

η_surface (%) = J_water/J_sulfite × 100 (4)

where J_water and J_sulfite are photocurrent densities for water splitting and sulfite oxidation, respectively. It assumes that the sulfite oxidation in 0.5 M Na₂SO₃ electrolyte completely suppresses the recombination of photogenerated charges at LaTiO₂N surfaces, indicating that η_surface = 100%. The Mott–Schottky (MS) analysis of LaTiO₂N/FTO photoanodes was performed in Ar-saturated 0.5 M NaCl buffered with 0.2 M KPi at pH 6.4 under dark conditions at a frequency of 1 kHz. Electrochemical impedance spectroscopy (EIS; Versastat 3-200) of the LaTiO₂N photoanodes with an area of 0.05 cm² was carried out at an applied potential of 1.23 V_RHE in the frequency range of 10⁴–10⁻¹ Hz with an AC amplitude of 10 mV. The acquired data were best-fitted with ZView software (Scribner Associates, Inc) using an equivalent circuit.

2.3 Structural characterization

The crystal structures of the synthesized La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x and the corresponding LaTiO₂N were analyzed by X-ray diffraction (XRD; MiniFlex 600, Rigaku) using Cu Kα radiation at 40 kV and 15 mA. The Rietveld refinement of LaTiO₂N was calculated using the FullProf Suite program. The optical properties of the oxides and oxynitrides were measured using ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS; V-770, JASCO). Tauc plots of the oxynitrides were estimated from the corresponding UV-vis DRS spectra by using the equation⁷

αhν = A(hν − E_g)^n/2 (5)

where α, h, ν, A, and n are the absorption coefficient, Planck's constant, light frequency, and proportionality constant, and characteristic integer of optical transition in a semiconductor, respectively. Moreover, the surface morphologies of the starting oxides and oxynitrides were investigated using a field-emission scanning electron microscope (FE-SEM; Gemini 500, ZEISS) and field-emission transmission electron microscope (FE-TEM; JEM-2100F, JEOL) equipped with an energy-dispersive X-ray spectroscopy (EDS) module. Bulk La/Ti atomic ratios of LaTiO₂N particles prepared from different starting oxides were determined by inductively coupled plasma mass spectrometry (ICP-MS; X2, I-CAPQ, Neptune, Thermo Scientific) analysis. The surface areas and pore size distribution of the different LaTiO₂N particles were determined by the Brunauer–Emmett–Teller (BET; ASAP 2020; Micromeritics Instrument Corp.) method based on the adsorption of N₂ gas. The chemical states of Ti species in LaTiO₂N prepared by the nitridation of different starting oxides were determined by high-performance X-ray photoelectron spectroscopy (HP-XPS; K-Alpha+, Thermo Scientific) using a monochromatic X-ray source producing Al K emission with a current of 6 mA and an acceleration voltage of 12 kV. The concentrations of atoms with an unpaired electron in the different LaTiO₂N particles were estimated using an electron paramagnetic resonance (EPR) spectrometer (EMXplus-9.5/12/P/L system, Bruker) with a microwave frequency of 9.84 GHz at room temperature. PL spectra were measured using a spectrophotofluorometer with time-correlated single-photon counting (TCSPC; Fluorolog3 with TCSPC, Horiba Scientific) with an excitation photon wavelength of 467 nm in a liquid nitrogen cooler. Time-resolved PL decay spectra were acquired at the PL peak position of 640 nm.

3. Results and discussion

In this study, the nitridation of different layered perovskite oxides was attempted to synthesize less-defective, photoactive LaTiO₂N. Fig. 1 presents a schematic illustration of the conversion of three layered perovskite oxides, i.e., La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₄Ti₃O₁₂, into perovskite LaTiO₂N. UV-responsive oxides are expected to transform visible-light-responsive LaTiO₂N during nitridation in an NH₃ flow. All the layered perovskite oxides with different La/Ti ratios were prepared by flux-assisted calcination using KCl as a molten salt, and La₂Ti₂O₇ with a La/Ti ratio of unity, commonly available for nitridation to LaTiO₂N, was readily crystallized at 1273 K for 20 h. BaLa₄Ti₄O₁₅ with a La/Ti ratio of unity as well but a (Ba + La)/Ti ratio of 1.25 was calcined at different temperatures (1173, 1273, 1373, and 1473 K) and also became crystalline (PDF Card No. 1526777) at 1273 K for 20 h (Fig. S1†). However, a calcination temperature higher than 1473 K resulted in an impurity phase of La₂Ti₂O₇. Although BaLa₄Ti₄O₁₅ was previously synthesized by SSR at 1723 K²⁹ and the polymerized complex (PC) method at 1323 K,²⁶ its preparation via flux-assisted calcination has not been reported yet. In this study, the synthesis result of single-phase BaLa₄Ti₄O₁₅ at a relatively low temperature of 1273 K is remarkable. For comparison, La-rich La₅Ti₄O_x with a La/Ti ratio of 1.25 was also synthesized at 1273 K; however, its XRD pattern was assignable to La₂Ti₂O₇ (PDF card no. 2108044) (Fig. S2†). When the calcination temperature was increased to 1373 K, the La-rich oxide was crystallized primarily to La₄Ti₃O₁₂ (PDF card no. 1526720) with impurity traces of La₂Ti₂O₇. The crystalline La₄Ti₃O₁₂ particles prepared from La₅Ti₄O_x were subjected to nitridation to determine the effect of the La-rich starting oxide as well as BaLa₄Ti₄O₁₅ on the photoactivity of the resulting LaTiO₂N.

Fig. 1 Schematic illustration showing the conversion of (A) layered perovskite La₂Ti₂O₇, (B) BaLa₄Ti₄O₁₅, and (C) La₄Ti₃O₁₂ into (D) perovskite LaTiO₂N.

Fig. 2 shows the XRD patterns for LaTiO₂N and the intermediate derivatives generated during the nitridation of different layered perovskite oxides at 1123 K. The nitridation of La₂Ti₂O₇ into LaTiO₂N (PDF card no. 4002463) was completed in 5 h, indicating a rapid conversion of the oxide. Prolonged nitridation of the oxynitride for 20 h was performed to improve its surface and bulk crystallinities.³⁰ By contrast, BaLa₄Ti₄O₁₅ was gradually transformed into LaTiO₂N during nitridation for the first 10 h, with segregated crystals such as Ba₂TiO₄ (PDF card no. 2310344), La₂O₃, and TiO₂ appearing in 10 h. LaTiO₂N was further crystallized during subsequent nitridation for 10 h, accompanied by exclusive La₂O₃ impurity. Then, the acid treatment of the resulting oxynitride removed the La₂O₃ traces and finally left perovskite-type LaTiO₂N. Meanwhile, the nitridation of La₅Ti₄O_x for 20 h led to the incomplete conversion of primary-phase La₄Ti₃O₁₂, although the secondary-phase La₂Ti₂O₇ was converted into LaTiO₂N even in 1 h. The crystalline La₄Ti₃O₁₂ was fully transformed into LaTiO₂N along with La₂O₃ traces during nitridation for 50 h, indicating a relatively slow nitridation (Fig. S3†). The impurity traces were removed by the acid treatment as well as LaTiO₂N prepared from BaLa₄Ti₄O₁₅. These results demonstrate for the first time that perovskite LaTiO₂N was successfully synthesized from layered perovskite BaLa₄Ti₄O₁₅ including unnecessary Ba ions and La-rich La₄Ti₃O₁₂. It also reveals that the rate of nitridation to LaTiO₂N at 1123 K is dependent on the different types of layered perovskite oxides. Both La₄Ti₃O₁₂ and BaLa₄Ti₄O₁₅ adopt a (111)-type layered perovskite (A_n+1B_nO_3n+3) crystal structure; however, their perovskite blocks (n = 3 or 4) and intruding AO layers (A = La or Ba/La) are different as shown in Fig. 1.³¹ The subtle difference in the crystal structure might be a critical factor determining the nitridation rate of the oxides. According to the full-width-at-half-maximum (FWHM) values for the (121) peak of LaTiO₂N, the relatively fast nitridation using BaLa₄Ti₄O₁₅ caused the high crystallinity of the resulting oxynitride.

Fig. 2 XRD patterns of LaTiO₂N and intermediate derivatives during the nitridation of (A) La₂Ti₂O₇, (B) BaLa₄Ti₄O₁₅, and (C) La₅Ti₄O_x at 1123 K for (a) 0, (b) 1, (c) 5, (d) 10, (e) 15, and (f) 20 h and (g) after subsequent acid treatment. The full width at half-maximum (FWHM) values for the (121) peak are shown for each specimen.

The elemental compositions of LaTiO₂N particles prepared from the nitridation of different oxides at 1123 K for 20 h were estimated by ICP-MS analysis (Table S2†). The La/Ti atomic ratio of LaTiO₂N prepared from La₂Ti₂O₇ was close to its theoretical value of unity, while those of LaTiO₂N prepared from BaLa₄Ti₄O₁₅ and La₅Ti₄O_x deviated from unity. In the case of La₅Ti₄O_x, La₄Ti₃O₁₂ undoubtedly remained even after nitridation contributed to a La/Ti ratio of more than unity. As shown in Fig. 2B, the XRD pattern for the oxynitride transformed from BaLa₄Ti₄O₁₅ is assignable to the single phase of LaTiO₂N. The excess amorphous Ba species remaining after the nitridation of Ba₅B₄O₁₅ (B = =Ta, Nb) was highly soluble in water, leading to Ba/B ratios of unity in the corresponding BaBO₂N.^11,22 In this study, the washing of the oxynitride produced after nitridation of BaLa₄Ti₄O₁₅ with deionized water caused highly alkaline waste water, which was gradually changed to neutral water via the repetition of the washing procedure (Fig. S4†). This result proves that the excess Ba species, amorphous BaO, was removed by washing with deionized water due to its strong basicity. Nevertheless, a significant ratio of the Ba species, leading to the La/Ti ratio of less than unity, was detected in the oxynitride prepared from BaLa₄Ti₄O₁₅. The Ba₂TiO₄ temporarily appearing in 10 h nitridation (as discussed in Fig. 2B) could result in amorphous BaTiO₃ because it reacts with TiO₂ to produce BaTiO₃.³² The nitridation of perovskite BaTiO₃ was attempted for a comparison and clearly showed the generation of segregated phases such as Ba₂TiO₄ and TiO₂ (Fig. S5†). Therefore, although reaction (6) is assumable for the nitridation of BaLa₄Ti₄O₁₅, LaTiO₂N could be prepared in this study by chemical reaction (8)via reaction (7):

BaLa₄Ti₄O₁₅ + 4NH₃ (g) → 4LaTiO₂N + BaO + 6H₂O (6)

Ba₂TiO₄ + TiO₂ → 2BaTiO₃ (7)

3BaLa₄Ti₄O₁₅ + 6NH₃ (g) → 6LaTiO₂N + BaO + 9H₂O + 3La₂O₃ + 4TiO₂ + 2BaTiO₃ (8)

Because the amorphous BaO was washed with distilled water and the segregated La₂O₃ and TiO₂ were removed by acid treatment, the Ba concentration detected in the ICP analysis was attributable to amorphous BaTiO₃ traces. However, the nitridation of BaLa₄Ti₄O₁₅ produced exclusively crystalline LaTiO₂N, which is feasible for sunlight-driven water splitting. The unit-cell parameters of the oxynitride were estimated by Rietveld refinement using its XRD results (Fig. S6 and Table S3†). The experimentally acquired pattern for the prepared LaTiO₂N well matched with its calculated pattern, indicating an orthorhombic system, and it was also consistent with published results.³³ Thus, the abovementioned results prove that layered perovskite BaLa₄Ti₄O₁₅ was successfully converted into single-crystalline LaTiO₂N via the complete separation of Ba species during nitridation.

Fig. 3A presents the DRS spectra of different layered perovskite oxides and the corresponding LaTiO₂N prepared by nitridation and subsequent annealing in an Ar flow. All the starting oxides were responsive to UV light below the wavelength of 350 nm. Interestingly, the light-absorption edges of the oxides were easily red-shifted to visible-light wavelengths even in 1 h nitridation, regardless of the types of layered perovskites (Fig. S7†). After complete nitridation, all the oxides were transformed into LaTiO₂N capable of absorbing visible-light wavelengths up to approximately 610 nm. However, the background absorption at wavelengths longer than 610 nm were distinguishable for the respective LaTiO₂N. The apparent colors of LaTiO₂N prepared from La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x are brown, red-orange, and reddish brown, respectively. The difference in color may be attributed to the different levels of background absorption. The high background absorption typically indicates the presence of defect density in the oxynitrides, originating from the reduction of highly electronegative Ti⁴⁺ to Ti³⁺ during nitridation.⁶ According to the DRS spectra results, the bulk defect density in LaTiO₂N prepared from BaLa₄Ti₄O₁₅ was very low compared to those prepared from other oxides. Excess Lewis-base Ba species locally relieved a reducing atmosphere at BaBO₂N surfaces during nitridation, thus reducing the generation of defects.²³ In this study, the presence of Ba ions in BaLa₄Ti₄O₁₅ may have been helpful in reducing the defect concentration in LaTiO₂N during nitridation. It was also more effective than the presence of excess La ions in La₅Ti₄O_x owing to its stronger basicity. The E_g of the prepared LaTiO₂N estimated by Tauc plots converted from the DRS spectra is presented in Fig. 3B. The values of La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x were approximately 2.17, 2.13, and 2.12 eV for a direct allowed transition, respectively, which was in good agreement with reported results.⁶ In addition, the flat-band potentials, E_fb, of the LaTiO₂N particles were measured by MS analysis (Fig. S8†). The E_fb values of LaTiO₂N prepared from La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x were determined to be −0.22, −0.21, and −0.28 V_RHE, respectively. By assuming that the E_fb of an n-type semiconductor is commonly positioned immediately below its conduction band minimum potential, band structure diagrams for LaTiO₂N prepared from La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x were constructed as shown in Fig. 3C. The band structures of the oxynitrides were similar and spanned the water redox and CER potentials. Thus, these optical results demonstrate that LaTiO₂N prepared from the different layered perovskite oxides was visible-light-responsive up to 610 nm, which is theoretically feasible for realizing sunlight-driven (sea)water splitting. In particular, BaLa₄Ti₄O₁₅ as a starting oxide afforded a less-defective oxynitride at a bulk level.

Fig. 3 Optical characterization: (A) UV-vis DRS spectra, (B) Tauc plots, and (C) band structure diagram of LaTiO₂N prepared by the nitridation of (a) La₂Ti₂O₇, (b) BaLa₄Ti₄O₁₅, and (c) La₅Ti₄O_x at 1123 K for 20 h (50 h in the case of La₅Ti₄O_x).

Fig. 4 depicts the SEM images of different starting oxides, namely La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x, and the corresponding LaTiO₂N particles. La₂Ti₂O₇ particles in the 100–200 nm size range were uniform and rod-like. BaLa₄Ti₄O₁₅ particles were composed of 2D truncated octahedrons with widths of 1–6 μm in a thickness of approximately 500 nm and small particles of nanometer sizes. The 2D octahedral shape was a typical structure of layered perovskite BaLa₄Ti₄O₁₅, similar to that of Sr₅Nb₄O₁₅ in our previous work.²⁸ Although the small particles failed to grow the 2D structure, they were also assignable to BaLa₄Ti₄O₁₅ according to an XRD result. The La₅Ti₄O_x particles were rod-like or polyhedron-type, indicating that La₂Ti₂O₇ of nanometer size and La₄Ti₃O₁₂ of ∼10 μm size were crystallized simultaneously, consistent with an XRD pattern result. After nitridation, the surface morphologies, including the size and shape, of the resulting LaTiO₂N were unchanged from those of the corresponding oxides. The difference in the nitridation rate of La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x, discussed in the XRD results above, could be partly attributed to their different particle sizes. In particular, the small La₂Ti₂O₇ particles completed nitridation the earliest because the nitrogen diffused quickly inside the particle. Moreover, the smooth surfaces of the oxides became porous, which originated from the exchange of three O²⁻ with two N³⁻ ions during nitridation.¹² The porosity of LaTiO₂N prepared from BaLa₄Ti₄O₁₅ was much higher than that prepared from La₄Ti₃O₁₂, even though both the particles were relatively large (Fig. S9†). The BET surface area of the oxynitride was also larger than that prepared from La₄Ti₃O₁₂ and its pore size distribution was relatively shifted to larger pore diameter, which are consistent with the SEM results (Table S4 and Fig. S10†). The origin for the different porosities is still unclear; however, it may be due to the faster nitridation of BaLa₄Ti₄O₁₅ excluding sintering for 20 h. Consequently, the nitridation of BaLa₄Ti₄O₁₅ produced 2D truncated octahedral LaTiO₂N particles with high porosity, leading to numerous reaction sites, which is favorable for PEC (sea)water splitting.

Fig. 4 SEM images of layered perovskite oxides: (A) La₂Ti₂O₇, (B) BaLa₄Ti₄O₁₅, and (C) La₅Ti₄O_x, and (D–F) corresponding LaTiO₂N particles prepared by the nitridation of the oxides at 1123 K for 20 h (50 h in the case of La₅Ti₄O_x).

The surface morphology of LaTiO₂N particles prepared from BaLa₄Ti₄O₁₅ was further analyzed by FE-TEM and EDS mapping analyses as shown in Fig. 5. Both the large truncated octahedron and small particles were highly porous, consistent with SEM images. Fig. 5C displays clear lattice fringes with the d(121) spacing (PDF card no. 4002463) corresponding to a single LaTiO₂N phase, demonstrating the high crystallinity of LaTiO₂N. In Fig. 5D, the elemental mapping shows that a LaTiO₂N particle is composed of elemental La, Ti, O, and N, and Ba species is not detected in the single particle. It reveals that the excess Ba species detected in the ICP analysis resulted from amorphous BaTiO₃ impurity traces. Thus, this result proves that the nitridation of BaLa₄Ti₄O₁₅ produced single-crystalline LaTiO₂N particles.

Fig. 5 (A–C) FE-TEM images and (D) EDS elemental mapping result of a LaTiO₂N particle prepared by the nitridation of BaLa₄Ti₄O₁₅ at 1123 K for 20 h and subsequently annealed in an Ar flow at 973 K for 1 h.

The surface and bulk defect densities of LaTiO₂N prepared from layered perovskite oxides with different ratios of La/Ti were estimated by XPS, EPR, and PL analyses. Fig. 6A displays the surface cation defect densities of LaTiO₂N particles generated during the nitridation of BaLa₄Ti₄O₁₅ for 1, 5, 10, 15, and 20 h and after acid treatment. Those arising during the nitridation of La₂Ti₂O₇ and La₅Ti₄O_x were also measured for comparison. The significantly low-electronegativity La species was excluded from the surface cation defects. The chemical states of Ti species on LaTiO₂N were deconvoluted from narrow-scanned Ti 2p XPS spectra with two oxidation states of Ti⁴⁺ and Ti³⁺ positioned at binding energies of 458.0 and 456.7 eV for the Ti 2p_3/2 peak, respectively (Fig. S11†).³⁴ The fractions of reduced Ti species on the oxynitride series are summarized in Table S5† and also plotted in Fig. 6A. The surface defects of Ti³⁺ gradually increased during the nitridation of the different oxides, indicating that long-duration nitridation at high temperatures led to an increase in the surface defect density of LaTiO₂N. In addition, the A-site-rich starting oxides BaLa₄Ti₄O₁₅ and La₅Ti₄O_x lowered the surface defect density during nitridation compared to La₂Ti₂O₇ with the La/Ti ratio of unity. These phenomena were analogous to those during the nitridation of A-site-rich oxides to obtain Nb- and Ta-based perovskite AB(O,N)₃.^9–11 In particular, the defect densities on LaTiO₂N prepared from BaLa₄Ti₄O₁₅ were the lowest during the overall nitridation. The excess Ba species in the oxide was more effective at suppressing the generation of defects than La-rich La₅Ti₄O_x. The surface defect of Ti³⁺ temporarily decreased during the first 10 h of nitridation, which probably originated from the segregation of intermediate Ba₂TiO₄ and TiO₂ (having Ti⁴⁺) during the crystallization of LaTiO₂N as illustrated in Fig. 2. The acid treatment to remove the La₂O₃ and TiO₂ impurities also reduced the cation defect density on LaTiO₂N.

Fig. 6 (A) Cation defect densities acquired on LaTiO₂N surfaces prepared by the nitridation of different starting oxides, namely (a) La₂Ti₂O₇, (b) BaLa₄Ti₄O₁₅, and (c) La₅Ti₄O_x, at 1123 K for 1, 5, 10, 15, and 20 h (50 h for La₅Ti₄O_x) and after subsequent acid treatment. (B) EPR, (C) PL, and (D) time-resolved PL decay spectra obtained from LaTiO₂N prepared by the nitridation of different starting oxides at 1123 K for 20 h (50 h for La₅Ti₄O_x).

Fig. 6B shows the EPR spectra of LaTiO₂N particles prepared by nitridation of different layered perovskite oxides. The broad EPR signal at g = 2.005 was observed in all LaTiO₂N, which has previously been ascribed to unpaired electrons resulting from oxygen and/or nitrogen vacancies in the oxynitride bulk.^35,36 The normalized signal of LaTiO₂N, prepared from BaLa₄Ti₄O₁₅, was relatively large as compared with those of other oxynitrides. The large signal may be mainly due to oxygen vacancies in amorphous BaTiO₃ traces segregated during nitridation, judging by the lowest background absorbance of the oxynitride observed in DRS spectra. Meanwhile, another broad EPR signal was observed in LaTiO₂N prepared from La₂Ti₂O₇ (at g = 1.998) and La₅Ti₄O_x (at g = 1.986), respectively, which has previously been attributed to free electrons originating from the Ti³⁺ defect species.^35,36 However, it was absent in the oxynitride prepared from BaLa₄Ti₄O₁₅. Therefore, this result indicates that the synthesis of LaTiO₂N using BaLa₄Ti₄O₁₅ suppressed the generation of bulk cation defects as well as the surface cation defects shown in XPS data.

Fig. 6C depicts the PL spectra of LaTiO₂N particles prepared from different layered perovskite oxides. Emission peaks at 640 nm corresponding to the band gap of 1.94 eV were observed in all LaTiO₂N, indicating the presence of intermediate defect levels inside the band structure of the oxynitride. The defect levels were assignable to the shallow donor levels of the Ti³⁺ species, according to its positions close to the conduction band of LaTiO₂N.³⁷ Nevertheless, the bulk defect level of LaTiO₂N prepared from BaLa₄Ti₄O₁₅ was the lowest, consistent with DRS spectra results. Fig. 6D shows the time-resolved PL decay spectra of the different oxynitrides acquired at the PL peak position of 640 nm. The LaTiO₂N prepared from BaLa₄Ti₄O₁₅ had a longer average PL decay lifetime of band-edge emission compared to other oxides, definitely leading to a slower recombination rate of photo-carriers. These results demonstrate that BaLa₄Ti₄O₁₅ is favorable as a starting oxide to synthesize less-defective LaTiO₂N both in bulk and surface levels.

For PEC measurements, a Co(OH)_x electrocatalyst was loaded on particulate LaTiO₂N/FTO photoanodes prepared by the nitridation of different starting oxides (La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x) at 1123 K for 20 h. Fig. 7A displays the LSV curves obtained from the Co(OH)_x/LaTiO₂N/FTO photoanodes during seawater splitting in 0.5 M NaCl aqueous solution buffered with 0.2 M KPi adjusted to pH 6.4 under chopped AM 1.5G simulated sunlight. The as-prepared oxynitrides were annealed in an Ar flow prior to the fabrication of photoanodes to improve the surface crystallinity of the oxynitrides according to our previous results.^22,30 The annealing of LaTiO₂N was optimized at 973 K for 1 h (Fig. S12†). The LaTiO₂N photoanode prepared from BaLa₄Ti₄O₁₅ produced a much higher photocurrent density (2.4 mA cm⁻²) at 1.23 V_RHE than those prepared from La₂Ti₂O₇ (0.8 mA cm⁻²) and La₅Ti₄O_x (0.6 mA cm⁻²). Its anodic photocurrent at 1.36 V_RHE reached 4.9 mA cm⁻². The tendency in the activity was unchanged even during seawater splitting of bare LaTiO₂N photoanodes, although the photocurrent was decreased due to the absence of electrocatalyst Co(OH)_x boosting the photoreaction (Fig. S13†). The complete nitridation of La₅Ti₄O_x for 50 h increased the photocurrent to 0.8 mA cm⁻² at 1.23 V_RHE; however, it was still less than 2.4 mA cm⁻² (Fig. S14†). Because amorphous BaTiO₃ impurity traces were detected in the LaTiO₂N prepared from BaLa₄Ti₄O₁₅, the PEC seawater-splitting activity of nitrided BaTiO₃ was also measured in the same manner for comparison (Fig. S5†). Although the nitridation of the oxide for 20 h allowed it to absorb visible light, no photoresponse was observed during the seawater splitting of the oxide. This result clearly proves that visible-light-responsive LaTiO₂N should afford a high seawater-splitting photocurrent. The photocurrent onset of LaTiO₂N at 0.90 V_RHE was more negative by approximately 0.1 V_RHE than that of LaTiO₂N prepared from La₂Ti₂O₇. However, the initial photocurrent of the LaTiO₂N decreased during long-term seawater splitting for 1 h, probably because of the self-photo-oxidation of the oxynitride surface which was not fully decorated with the Co(OH)_x electrocatalyst (Fig. S15†). Nevertheless, the high photoactivity of LaTiO₂N at neutral pH, i.e., low onset potential and high photocurrent, was remarkable, considering that the oxynitride has been previously reported for water splitting at a strong alkaline pH 13.^20,21

Fig. 7 (A) LSV curves acquired on particulate Co(OH)_x/LaTiO₂N/FTO photoanodes prepared by the nitridation of different layered perovskites, namely (a) La₂Ti₂O₇, (b) BaLa₄Ti₄O₁₅, and (c) La₅Ti₄O_x, at 1123 K for 20 h and subsequent annealing in an Ar flow at 973 K for 1 h, during seawater splitting under chopped AM 1.5G simulated sunlight. All curves were acquired by sweeping the potential from 1.4 to 0.6 V_RHE at a scan rate of 10 mVs⁻¹ in an Ar-saturated 0.5 M NaCl aqueous solution buffered with 0.2 M KPi at pH 6.4. (B) Nyquist plots of the corresponding photoanodes. The EIS measurements were performed in the electrolyte identical to (a) at an applied potential of 1.23 V_RHE. The EIS fitting results are summarized in Table S4.† (C) LSV data for the corresponding Co(OH)_x/LaTiO₂N/FTO photoanodes during seawater oxidation in 0.5 M NaCl aqueous electrolyte (dashed curves) and sulfite oxidation in 0.5 M Na₂SO₃ aqueous electrolyte (solid curves) under chopped AM 1.5G sunlight. Both solutions were buffered with 0.2 M KPi and adjusted to pH 13. (D) Bulk and (E) surface charge separation efficiency, η_bulk (%) and η_surface (%), of the Co(OH)_x/LaTiO₂N/FTO photoanodes presented in (C).

The surface morphology, such as porosity and size, of LaTiO₂N particles prepared from different perovskite oxides was significantly distinguishable, as revealed by SEM images and BET analyses. The electrochemical surface areas of the different oxynitrides were estimated by double-layer capacitance C_dl computed with cyclic voltammograms acquired under darkness at non-faradaic potentials, because the electrochemical properties reflect the surface properties of material particles (Fig. S16†). The C_dl value of the LaTiO₂N photoanode prepared from BaLa₄Ti₄O₁₅ was similar to that from La₂Ti₂O₇, while it was more than four times larger than that from La₅Ti₄O_x. This implies that the former LaTiO₂N photoanodes possess relatively large electrochemical reaction sites compared with those from La₅Ti₄O_x. However, the trend in the photoactivity of different LaTiO₂N, discussed in Fig. 7A, was not in agreement with that in the electrochemical surface area of the oxynitrides. Fig. 7B shows the Nyquist plots of the LaTiO₂N photoanodes prepared from different oxides, which were measured by EIS in a neutral NaCl aqueous solution under simulated sunlight at an applied potential of 1.23 V_RHE. The Nyquist plots were best-fitted by an equivalent circuit model; the fitting results are summarized in Table S6.†R₁, R₂, and R₃ denote electrolyte resistance (R_s), charge transfer resistance (R_ct) between LaTiO₂N and the electrolyte, and electron transport resistance (R_et) from LaTiO₂N to the FTO substrate, respectively. R_et could originate from the loose adhesion between the oxynitride and substrate prepared via spin-coating despite the subsequent necking treatment. The results revealed that the R_ct of the LaTiO₂N prepared from BaLa₄Ti₄O₁₅ was much smaller than those of LaTiO₂N prepared from La₂Ti₂O₇ and La₅Ti₄O_x. In addition, its R_et was relatively reduced because of the high crystallinity of the oxynitride. Thus, the high seawater-splitting activity of LaTiO₂N at neutral pH is highly attributable to the semiconducting properties of the less-defective oxynitride prepared from BaLa₄Ti₄O₁₅, which increases the conductivity of photo-carriers and accelerates the photoreaction.

The thermodynamic potential of 1.36 V_NHE for the CER at pH 0 is more oxidative than the 1.23 V_NHE for the OER; however, the CER driven by a two-electron pathway is kinetically faster than the OER driven via four-electron transfer. The LaTiO₂N photoanode prepared from BaLa₄Ti₄O₁₅ produced a photocurrent density (1.3 mA cm⁻²) at 1.23 V_RHE during water splitting in 0.2 M KPi aqueous solution adjusted to pH 6.7 (Fig. S17†). The OER activity of the oxynitride was lower than the CER activity discussed in Fig. 7A, consistent with our previous result.²⁷ This obviously indicates that the CER and OER were competitive in the neutral NaCl aqueous electrolyte evaluated in this study. The photoactivity of LaTiO₂N prepared from BaLa₄Ti₄O₁₅ was accordingly evaluated during seawater splitting at pH 13 to determine the effect of the less-defective oxynitride on the kinetically boosted OER in a strongly alkaline electrolyte. Its anodic photocurrent was further increased to 3.3 mA cm⁻² at 1.23 V_RHE (5.2 mA cm⁻² at 1.36 V_RHE) during the alkaline seawater splitting (Fig. S18†). The photocurrent of oxynitrides prepared from other oxides was enhanced as well, although their photoactivity order remained unchanged. This result demonstrates that the high photoactivity at neutral pH is mainly attributable to the enhanced semiconducting properties of less-defective LaTiO₂N from BaLa₄Ti₄O₁₅, other than the faster CER kinetics. In addition, the sulfite oxidations of the LaTiO₂N photoanodes prepared from different starting oxides were evaluated in 0.5 M Na₂SO₃ aqueous solutions at pH 13 as shown in Fig. 7C. The PEC behaviors of the oxynitrides for the sulfite oxidation were much more activated, and the trend between the oxynitrides was the same as that during the seawater splitting at pH 13. This reveals that LaTiO₂N prepared from BaLa₄Ti₄O₁₅ was highly photoactive regardless of oxidation reactions.

The separation efficiency (η_bulk) of charges generated in the bulk of LaTiO₂N prepared from different oxides was evaluated, as presented in Fig. 7D. The photon absorption rate (J_abs) of the different oxynitrides prepared from La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x was determined to be 11.0, 10.0, and 10.0 mA cm⁻², respectively (Fig. S19†). The η_bulk of the oxynitrides was approximately 22%, 37%, and 11% at 1.23 V_RHE, respectively, calculated using the photocurrent density for sulfite oxidation. This result was consistent with the degree of crystallinity and bulk defect densities of the oxynitrides, discussed in XRD patterns and PL spectra. Undisputedly, the highest crystallinity and low bulk defect density of LaTiO₂N prepared from BaLa₄Ti₄O₁₅ led to the highest efficiency. Fig. 7E displays the separation efficiency (η_surface) of charges generated at the surfaces of LaTiO₂N prepared from different oxides, estimated by the difference in photoactivities between fast sulfite and slow seawater oxidations. The η_surface of oxynitrides prepared from La₂Ti₂O₇, BaLa₄Ti₄O₁₅, and La₅Ti₄O_x was approximately 41%, 90%, and 61% at 1.23 V_RHE, respectively. Remarkably, an efficiency of almost 100% was achieved at 1.36 V_RHE during the water splitting by LaTiO₂N prepared from BaLa₄Ti₄O₁₅. This definitely indicates that the water-splitting activity driven by the oxynitride was efficient and sufficiently high. The low defect density and high crystallinity of LaTiO₂N suppressed the recombination of photogenerated holes and electrons, thus causing the consumption of the holes entirely for the slow water oxidation. Although the PEC activity of LaTiO₂N prepared from La₅Ti₄O_x was lower than that of LaTiO₂N prepared from La₂Ti₂O₇, its separation efficiency became higher. The higher efficiency could be attributed to the 2D-type surface structure of LaTiO₂N possible for the short diffusion of charges inside a narrow thickness because a similar effect was observed in 2D octahedral SrNbO₂N doped with Zr.²⁸ As a result, the low defect density and 2D surface structure of LaTiO₂N prepared from BaLa₄Ti₄O₁₅ strongly promoted the separation of photo-charges, enhancing PEC seawater-splitting activity. Therefore, as illustrated in Fig. 8, this study clearly proves that the complete conversion of layered perovskite BaLa₄Ti₄O₁₅ including excess Ba species with high basicity was effective at preparing less-defective, 2D LaTiO₂N crystals in both bulk and surface states, realizing efficient neutral seawater splitting under sunlight.

Fig. 8 Mechanism diagram of less-defective, two-dimensional LaTiO₂N prepared via nitridation of BaLa₄Ti₄O₁₅ with Lewis base, A-site-rich concentration for efficient neutral seawater splitting under sunlight.

4. Conclusions

In this study, for the first time, a layered perovskite BaLa₄Ti₄O₁₅ prepared via flux-assisted calcination using KCl was successfully converted into perovskite LaTiO₂N with a 2D structure and low defect density. For comparison, LaTiO₂N was also prepared by the nitridation of different starting oxides, namely La₂Ti₂O₇ and La₅Ti₄O_x. The complete nitridation of BaLa₄Ti₄O₁₅, which was much faster than that of La₅Ti₄O_x, led to the synthesis of highly crystalline, single-phase LaTiO₂N. According to DRS spectra and MS analysis results, the oxynitride was capable of absorbing visible light up to 610 nm, and its band structure straddled the water redox and CER potentials, driving solar (sea)water splitting. In addition, XPS and PL results proved that the nitridation of BaLa₄Ti₄O₁₅ caused low surface and bulk defect densities of LaTiO₂N, thus increasing the number of long-lived photocarriers and activating the photoreaction. SEM and TEM images exhibited different surface morphologies of LaTiO₂N prepared from various starting oxides, revealing that the difference in the shape and size of oxides determines the nitridation rate. In particular, LaTiO₂N prepared from BaLa₄Ti₄O₁₅ was 2D truncated octahedral and highly porous, leading to a high surface area providing numerous photoreaction sites. The Co(OH)_x/LaTiO₂N/FTO photoanode fabricated via spin coating showed a significantly higher photocurrent density of 2.4 mA cm⁻² at 1.23 V_RHE (4.9 mA cm⁻² at 1.36 V_RHE) in 0.5 M NaCl aqueous solution buffered with 0.2 M KPi adjusted to pH 6.4 under AM 1.5G sunlight irradiation. This photoactivity was much higher than those of the photoanodes prepared from La₂Ti₂O₇ (0.8 mA cm⁻²) and La₅Ti₄O_x (0.6 mA cm⁻²). Furthermore, the separation efficiency (η_surface) of charges generated at the surfaces of LaTiO₂N was 90% at 1.23 V_RHE (almost 100% at 1.36 V_RHE) during the water splitting. This result definitely indicates that the water-splitting activity driven by the oxynitride was efficient and high enough. The greatly enhanced photoactivity mainly originated from the low surface and bulk defect densities as well as 2D surface structure of LaTiO₂N, distinguishable from those prepared from La₂Ti₂O₇ and La₅Ti₄O_x. Thus, this work demonstrates a new synthesis route wherein the nitridation of BaLa₄Ti₄O₁₅ as a starting oxide is appropriate for the synthesis of less-defective LaTiO₂N, thus largely enhancing neutral seawater-splitting activity under sunlight.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Thanh Tam Thi Tran: data curation, investigation, formal analysis, visualization, writing – original draft. Jeongsuk Seo: conceptualization, methodology, investigation, validation, visualization, writing – original draft, review & editing, funding resources, supervision.

Conflicts of interest

The authors have no conflicts of interest relevant to this study to disclose.

Acknowledgements

This research was financially supported by the Basic Science Research Program (No. RS-2023-00243439) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta01901f

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