Highly crystalline Na_0.5Bi_0.5TiO₃ of a photocatalyst valence-band-controlled with Bi(III) for solar water splitting

Abstract

Na_0.5Bi_0.5TiO₃ (BG 3.1 eV) with a valence band formed by Bi(III) was found as a new photocatalyst for solar water splitting. The water splitting activity of highly crystalline Na_0.5Bi_0.5TiO₃ synthesized by a flux method was much higher than that of the samples synthesized by a solid-state reaction. The optimized RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%) gave a 5.1% apparent quantum yield at 350 nm and split water even under simulated sunlight irradiation with a 0.05% solar to hydrogen energy conversion efficiency. We have successfully achieved solar water splitting using a one-step photoexcitation type photocatalyst valence-band-controlled with Bi(III).

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Hydrogen production by photocatalytic solar water splitting is attractive, and important science and technology from the viewpoint of resource, energy and environmental issues. Therefore, many researchers have developed various powder-based visible-light-driven photocatalysts working by a one-step photoexcitation and a Z-schematic two-step photoexcitation for water splitting.^1–6 The one-step photoexcitation type photocatalyst especially possesses an advantage in practical use from the viewpoint of its simplicity. Domen and co-workers have reported that GaN–ZnO,⁷ ZnGeN₂–ZnO,⁸ LaMg_1/3Ta_2/3O₂N,⁹ TaON,¹⁰ CaTaO₂N,¹¹ Ta₃N₅¹² and Y₂Ti₂O₅S₂¹³ of (oxy)nitride and oxysulfide photocatalysts show activities for one-step photoexcitation type water splitting under visible light irradiation. It is also reported that g-C₃N₄ of an organic semiconductor photocatalyst can split water under visible light irradiation.¹⁴ We have also reported that IrO₂-loaded SrTiO₃ co-doped with Rh and Sb of a metal oxide photocatalyst shows activity for water splitting under visible light irradiation.¹⁵ Although some of these visible-light-driven photocatalysts can split water under simulated sunlight irradiation, their solar to hydrogen energy conversion efficiencies (STH) are still low due to their low apparent quantum yields (AQYs). In recent years, Domen and co-workers have developed an Al-doped SrTiO₃ (band gap (BG) 3.2 eV) photocatalyst loading Rh/Cr₂O₃ and CoOOH cocatalysts with almost 100% AQY, which was prepared by a flux treatment.¹⁶ Although the photocatalyst responds to only UV light, it shows the highest STH (0.65%) among one-step photoexcitation type photocatalysts.¹⁶ We have also reported that Rh_0.5Cr_1.5O₃-loaded AgTaO₃ (BG 3.4 eV) shows high AQY (38% at 340 nm) and STH (0.13%) for water splitting.¹⁷ The valence band maximum (VBM) of the Al-doped SrTiO₃ is formed by O2p orbitals, while that of AgTaO₃ is formed by Ag4d orbitals.¹⁸ A photocatalyst whose valence band (VB) is formed by orbitals of elements except oxygen like AgTaO₃ is called a valence-band (VB)-controlled photocatalyst.^3,17,19 The VB-controlled photocatalyst is very important from the viewpoint of extension of responsive wavelength. Although the STH of the Rh_0.5Cr_1.5O₃-loaded AgTaO₃ is the highest among the VB-controlled photocatalysts, the BG of the AgTaO₃ is wider than that of the Al-doped SrTiO₃. Therefore, it is important to achieve solar water splitting with high STH using a VB-controlled photocatalyst with a narrower BG than the Al-doped SrTiO₃.

Na_0.5Bi_0.5TiO₃ (BG 3.1 eV) with a perovskite structure is one of the candidates satisfying the conditions mentioned above, because Bi(III) is a suitable cation for the VB-controlled photocatalyst, as seen in BiVO₄, Na_0.5Bi_0.5WO₄ and Bi₄Ti₃O₁₂.^20–22 The VB of Na_0.5Bi_0.5TiO₃ is formed by hybridized orbitals consisting of O2p and Bi6s, leading to the narrower BG than SrTiO₃.²³ Na_0.5Bi_0.5TiO₃ showed activities for photocatalytic H₂ and O₂ evolution from aqueous solutions containing sacrificial reagents under UV irradiation as half reactions of water splitting.^24,25 Na_0.5Bi_0.5TiO₃ photocatalysts have also been used for degradation of dyes.^26,27 However, water splitting into H₂ and O₂ at a stoichiometric ratio over the Na_0.5Bi_0.5TiO₃ has not been achieved yet. The distortion of the perovskite framework in the crystal structure of Na_0.5Bi_0.5TiO₃ is similar to that of AgTaO₃ (Fig. S1 and Table S1, ESI†). Therefore, Na_0.5Bi_0.5TiO₃ is expected to possess potential for photocatalytic water splitting. Moreover, the activity of Na_0.5Bi_0.5TiO₃ will be improved by changing the synthesis method from a conventional solid state reaction to a flux method as well as that of SrTiO₃²⁸ because Na_0.5Bi_0.5TiO₃ is similar to SrTiO₃ from the viewpoints of a perovskite structure consisting of TiO₆ octahedra. A flux method is useful to control bulk properties of a material such as morphology and crystallinity.^29–32 In the present study, we synthesized Na_0.5Bi_0.5TiO₃ of a photocatalyst VB-controlled with Bi6s orbitals by a solid state reaction and a flux method, and investigated their photocatalytic water splitting with loading suitable cocatalysts.

Na_0.5Bi_0.5TiO₃ was synthesized by a solid state reaction (SSR) and a flux method (FM) using Na₂CO₃ (Kanto Chemical; 99.8%), Bi₂O₃ (Kanto Chemical; 99.9%) and TiO₂ (Kojundo; 99.99%) as starting materials. After the starting materials were mixed in an alumina mortar in a ratio of Na₂CO₃ : Bi₂O₃ : TiO₂ = 0.25 : 0.25 : 1, the mixture was calcined at 1173–1373 K for 5 h in air for the SSR. The obtained material was washed with distilled water. For the FM, 10 molar equivalence of NaCl (Kanto Chemical; 99.5%) to the objective was used as a flux reagent. The mixture of the starting materials and the NaCl-flux was heated at 1173–1473 K for 5 h in air. The obtained material was washed with distilled water to remove the NaCl-flux. Na_0.5La_0.5TiO₃ and Na_0.5La_0.25Bi_0.25TiO₃ were also synthesized by FM at 1273 K for 5 h with the NaCl-flux in order to compare their band structures with Na_0.5Bi_0.5TiO₃. Details of the experiment are described in the footnotes of the Figures and Tables, and the ESI.†

X-ray diffraction (XRD) patterns revealed that Na_0.5Bi_0.5TiO₃ was successfully synthesized by SSR and FM (Fig. S2, ESI†). The FWHMs of the (012) peaks of the sample synthesized by FM at 1273–1473 K were similar to each other and smaller than those of the other samples. This result indicates that these samples synthesized by FM at 1273–1473 K possessed higher crystallinity than the other samples. The change in width of the peak split between (104) and (110) depending on the calcination temperature suggests that distortion of the perovskite framework slightly changed. The effect of synthesis method on the morphology of Na_0.5Bi_0.5TiO₃ particles was investigated by scanning electron microscopy (SEM) measurements, as shown in Fig. 1. The particle surfaces of the samples synthesized by FM were more faceted than those of the samples synthesized by SSR. Particles prepared by FM were small comparing with SSR at the same calcination temperature. The surface area (S.A.) of the samples synthesized by FM was larger than those of the samples synthesized by SSR. All absorption edges of diffuse reflectance spectra (DRS) of the Na_0.5Bi_0.5TiO₃ were located at around 400–420 nm, as shown in Fig. 2, indicating that the BG of Na_0.5Bi_0.5TiO₃ was 3.0–3.1 eV. The BG of Na_0.5La_0.5TiO₃ in which the VB is formed by O2p orbitals was estimated to be 3.4 eV from its DRS. Therefore, the BG of Na_0.5Bi_0.5TiO₃ was narrowed about 0.3 eV due to the contribution of Bi6s orbitals to its VB. The absorption edge of Na_0.5La_0.25Bi_0.25TiO₃ was located between those of the Na_0.5La_0.5TiO₃ and the Na_0.5Bi_0.5TiO₃ (Fig. S3, ESI†). XPS spectra at the VB regions of Na_0.5Bi_0.5TiO₃ and Na_0.5La_0.5TiO₃ synthesized by FM at 1273 K were measured (Fig. S4, ESI†). The peak of the Na_0.5Bi_0.5TiO₃ was wider than that of the Na_0.5La_0.5TiO₃. The onset binding energy of the peak of the Na_0.5Bi_0.5TiO₃ shifted to a lower energy of about 0.3 eV than that of the Na_0.5La_0.5TiO₃. These results also suggest that Bi6s orbitals contributed to the valence band maximum (VBM) of the Na_0.5Bi_0.5TiO₃. DFT calculations also revealed that Bi(III) contributed to the VB of the Na_0.5Bi_0.5TiO₃.²³ The VBM formed by O2p orbitals of materials consisting of MO₆ (M = metal cation) such as Na_0.5La_0.5TiO₃ are located around +3.0 eV vs. RHE.³³ Thus, the conduction band minimum (CBM) of Na_0.5La_0.5TiO₃ was calculated to be −0.4 eV vs. RHE from its BG. CBM depends on the distortion of M–O–M bond angles.³⁴ Therefore, the CBM of Na_0.5Bi_0.5TiO₃ was also −0.4 eV vs. RHE because the Ti–O–Ti bond angle of the Na_0.5Bi_0.5TiO₃ was similar to that of the Na_0.5La_0.5TiO₃ (Table S1, ESI†). The CBM of Na_0.5Bi_0.5TiO₃ has an advantage in a driving force for reduction of water to hydrogen compared with that of SrTiO₃ (−0.2 V vs. RHE). The BG of the Na_0.5Bi_0.5TiO₃ was narrowed 0.3 eV by the contribution of Bi6s to its VBM compared with Na_0.5La_0.5TiO₃, as mentioned above. The VBM of the Na_0.5Bi_0.5TiO₃ was calculated to be +2.7 eV from its BG and CBM. The small shift in the absorption edges seemed to be due to the difference in the distortion of the crystal structure and the particle size depending on the synthesis conditions. Additionally, the molar ratios of Bi/Ti for the samples synthesized by FM at 1173–1373 K were slightly smaller than those for the other samples (Table S2, ESI†). The smaller molar ratios of Bi/Ti for the samples synthesized by FM at 1173–1373 K also seemed to affect the difference in the absorption edges because Bi6s orbitals contributed to the VBM of Na_0.5Bi_0.5TiO₃.

Fig. 1 SEM images and surface areas of Na_0.5Bi_0.5TiO₃ synthesized by SSR and FM at various temperatures.

Fig. 2 Diffuse reflectance spectra of Na_0.5Bi_0.5TiO₃ synthesized by SSR and FM at various temperatures and Na_0.5La_0.5TiO₃ synthesized by FM at 1273 K.

Photocatalytic water splitting over Na_0.5Bi_0.5TiO₃ under UV irradiation (λ >300 nm) was examined, as shown in Table 1. RhCr₂O_x is an excellent cocatalyst for water splitting.^35,36 We applied the cocatalyst to the present Na_0.5Bi_0.5TiO₃. All samples produced H₂ and O₂ in a stoichiometric ratio. The samples synthesized by FM at 1273 and 1373 K especially showed high photocatalytic activities, because these samples possessed good crystallinity as seen in XRD (Fig. S2, ESI†). Photogenerated carriers easily migrated to the surface of such well-crystallized particles, resulting in high activities. Although the sample synthesized by FM at 1473 K possessed similar crystallinity to the samples synthesized by FM at 1273 and 1373 K, it showed a quite low activity because of its very small S.A. (<0.1 m² g⁻¹). Non-loaded Na_0.5Bi_0.5TiO₃ synthesized by FM at 1273 K hardly showed photocatalytic activity for water splitting under UV irradiation (Table S3, ESI†). 0.1 mol% was the optimum loading amount of the RhCr₂O_x cocatalyst. Coloading of the RhCr₂O_x cocatalyst with a CoOOH of an O₂-evolving cocatalyst is effective for water splitting over the Al-doped SrTiO₃ photocatalyst.^16,37 The water splitting activity of the RhCr₂O_x(0.1 mol%)-loaded Na_0.5Bi_0.5TiO₃ was further improved by loading of the CoOOH cocatalyst. RhCr₂O_x(0.1 mol%) and CoOOH(0.02 mol%)-coloaded Na_0.5Bi_0.5TiO₃ showed the highest activity for photocatalytic water splitting (Table S3 and Fig. S5, ESI†). The binding energy of the Rh3d_5/2 peak in the XPS spectrum of the RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%) (308.9 eV) was similar to not that of Rh₂O₃ (308.3 eV) but those of Rh_0.5Cr_1.5O₃(0.2 mol%)/Na_0.5Bi_0.5TiO₃ (308.9 eV) loaded by an impregnation method and previously reported Rh_2−xCr_yO₃/(Ga_1−xZn_x)(N_1−xO_x) (309.0 eV) loaded by a simultaneous PD method (Fig. S6, ESI†).^35,36 Therefore, the RhCr₂O_x loaded on the Na_0.5Bi_0.5TiO₃ was suggested to be mainly a Rh_2−yCr_yO₃ of a mixed-oxide. The optimum Na_0.5Bi_0.5TiO₃ also showed much higher photocatalytic activity for water splitting than Na_0.5La_0.5TiO₃ and Na_0.5La_0.25Bi_0.25TiO₃ (Table S3, ESI†). This might be because photogenerated holes in the Na_0.5Bi_0.5TiO₃ could easily migrate to the particle surfaces because of the contribution of Bi(III) to the VB.

Fig. 3(A) shows an action spectrum of photocatalytic water splitting over the RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%). The onset wavelength of the activity agreed with that of the absorption spectrum of Na_0.5Bi_0.5TiO₃. This agreement indicates that water splitting proceeded with photoexcitation from the VB formed by Bi6s orbitals to the CB formed by Ti3d, as shown in Fig. 3(B). The AQY was 5.1% at 350 nm. As far as we know, among metal oxide photocatalysts with VB formed by Bi6s orbitals, the AQY is the highest for one-step photoexcitation type water splitting. Additionally, the AQY was slightly higher than that of RhCrO_x-loaded GaN–ZnO which shows the highest AQY among a visible-light-driven photocatalyst for water splitting.⁷ Moreover, RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%) responded to light up to 420 nm.

Table 1 Photocatalytic water splitting over RhCr₂O_x(0.2 mol%)-loaded Na_0.5Bi_0.5TiO₃ synthesized by SSR and FM under UV irradiation

Synthesis condition		FWHM of (012) peak/°	S. A./m^2 g^−1	Activity/μmol h^−1
Method	Temp./K			H2	O2
Photocatalyst: 0.3 g, reactant solution: distilled water (120 mL), cell: top-irradiation cell with a Pyrex window, light source: 300 W Xe-arc lamp (λ >300 nm). A RhCr2Ox(0.2 mol%) cocatalyst was loaded in situ by a photodeposition (PD) method.
SSR	1173	0.11	1.3	30	14
SSR	1273	0.10	0.7	33	16
SSR	1373	0.10	0.6	10	4.9
FM	1173	0.12	2.6	9	4
FM	1273	0.09	0.9	102	50
FM	1373	0.09	0.9	58	27
FM	1473	0.09	<0.1	1	0.4

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Fig. 3 (A) Action spectrum of photocatalytic water splitting over RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%), and diffuse reflectance spectra of Na_0.5La_0.5TiO₃ (dashed line) and Na_0.5Bi_0.5TiO₃ (solid line) synthesized by FM at 1273 K. Photocatalyst: 0.3 g, cocatalyst: PD, reactant solution: distilled water (120 mL), cell: top-irradiation cell with a Pyrex window, light source: 300 W Xe-arc lamp with band-pass filters. (B) Reaction scheme of photocatalytic water splitting over RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%).

Water splitting proceeded over the RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%) even under simulated sunlight irradiation, as shown in Fig. 4. H₂ and O₂ steadily evolved for a long time at the rates of 203 mL h⁻¹ m⁻² and 91 mL h⁻¹ m⁻², respectively. The STH was estimated to be 0.05%. Thus, we successfully achieved one-step photoexcitation type solar water splitting using RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%) as a photocatalyst VB-controlled with Bi(III) possessing a narrower BG than SrTiO₃.

Fig. 4 Photocatalytic solar water splitting over RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%). Na_0.5Bi_0.5TiO₃ was synthesized by FM at 1273 K. Photocatalyst: 0.3 g, cocatalyst: PD, reactant solution: distilled water (120 mL), cell: top-irradiation cell with a Pyrex window, light source: solar simulator (AM-1.5 G, 100 mW cm⁻²), irradiation area: 25 cm². *Volumes were calculated with the conditions of room temperature, supposing 1 atm and 1 m² of irradiated area.

In conclusion, Na_0.5Bi_0.5TiO₃, which is a photocatalyst VB-controlled with Bi(III), synthesized by a flux method at 1273 K has arisen as a new photocatalyst for solar water splitting in a suspension system when RhCr₂O_x(0.1 mol%) and CoOOH(0.02 mol%) were coloaded. Na_0.5Bi_0.5TiO₃ synthesized by FM was more highly crystalized and faceted than the samples synthesized by SSR. XPS measurements revealed that the VBM of Na_0.5Bi_0.5TiO₃ was formed by Bi6s orbitals. The highly crystalline Na_0.5Bi_0.5TiO₃ with facets obtained by FM showed higher water splitting activity under UV irradiation than Na_0.5Bi_0.5TiO₃ obtained by SSR. This seemed to be because photogenerated carriers were able to easily migrate to the particle surfaces due to its high crystallinity. Optimized RhCr₂O_x(0.1 mol%)/Na_0.5Bi_0.5TiO₃/CoOOH(0.02 mol%) responded to light up to 420 nm. This photocatalyst gave 5.1% AQY at 350 nm and 0.05% STH. The AQY and STH were the highest among photocatalysts VB-controlled with Bi(III). Thus, we have demonstrated solar water splitting via one-step type photoexcitation using a VB-controlled photocatalyst consisting of Bi(III) with a narrower BG than SrTiO₃.

This work was supported by JSPS KAKENHI Grant Numbers 17H06433 and 17H06440 in Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion (I⁴LEC)”, and 17H01217.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available: Experimental detail, crystal structure, XRD, XRF, XPS, DRS and a time course of an activity. See DOI: 10.1039/d0cc07371g

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