Effect of post-treatments on the photocatalytic activity of Sm₂Ti₂S₂O₅ for the hydrogen evolution reaction

Abstract

The oxysulphide photocatalyst Sm₂Ti₂S₂O₅ was synthesized by sulphurizing an amorphous Sm₂Ti₂O₇ prepared using a polymerized complex method under a H₂S flow and was used as a H₂ evolution photocatalyst in the sacrificial H₂ evolution and Z-scheme water splitting reactions. The H₂ evolution activity of Rh-loaded Sm₂Ti₂S₂O₅ was improved by annealing with sulphur powder and etching with nitric acid. Characterization using XRD, SEM, DRS and XPS suggested that annealing with sulphur decreased the density of the reduced Ti species in Sm₂Ti₂S₂O₅ and etching with nitric acid removed the amorphous phases and excessive sulphur species on the surface. After the two post-treatments, platinized Sm₂Ti₂S₂O₅ combined with rutile-type TiO₂ and NaI showed activity for Z-scheme water splitting under UV irradiation. Although UV irradiation was necessary owing to the use of TiO₂, this work provided the first evidence that an oxysulphide photocatalyst was applicable to Z-scheme water splitting.

---

Introduction

Photocatalytic water splitting has attracted worldwide interest as a promising method for conversion of solar energy into chemical fuels.^1–4 In order to utilize solar energy effectively, a photocatalyst must harvest visible light of up to 600 nm or even longer.²Z-scheme water splitting systems based on two-step photoexcitation are typically composed of a H₂ evolution photocatalyst, an O₂ evolution photocatalyst, and electron mediators.^3,4 During photoexcitation, electrons reduce H⁺ to H₂ and holes oxidize the reduced form of redox mediators on the H₂ evolution photocatalyst, while on the O₂ evolution photocatalyst, electrons reduce the oxidized form of redox mediators and holes oxidize H₂O to O₂. It has also been found that Z-scheme water splitting can complete even in the absence of redox couples owing to the interparticle electron transfer, where holes in a H₂ evolution photocatalyst and electrons in an O₂ evolution photocatalyst recombine via physical contact.^4,5 Compared to the one-step excitation mechanism that uses a single visible-light-responsive photocatalyst, the two-step excitation mechanism has several advantages:⁶ it enables the use of a semiconductor that is active for either water reduction or oxidation alone as long as it is active for the oxidation or reduction of the redox mediators, respectively. Because the energy required to drive either photocatalyst can be lower, a wider range of visible-light-active photocatalysts are applicable. For example, WO₃, BiVO₄, and Rh-doped SrTiO₃ have been employed in Z-scheme water splitting although they are active only for H₂ or O₂ evolution in the presence of appropriate sacrificial reagents.^6–8 Photocatalysts that are capable of both the H₂ and O₂ evolution reactions with sacrificial reagents are also applicable to Z-scheme water splitting, as has been demonstrated with TaON^6,9 and Ta₃N₅.¹⁰ Moreover, H₂ and O₂ are generated on different photocatalysts, which could in principle offer a simple means of separating the gaseous products using an appropriate membrane filter.⁸ Much effort has been devoted to construction of efficient Z-scheme systems by developing new materials and efficient electron relays.

Sm₂Ti₂S₂O₅, an oxysulphide semiconductor photocatalyst, has a crystal structure similar to that of a Ruddlesden–Popper-type layered perovskite oxide.¹¹ Under visible light irradiation (λ < 650 nm), Sm₂Ti₂S₂O₅ can evolve H₂ or O₂ from aqueous solutions containing sacrificial electron donors (Na₂S–Na₂SO₃ or methanol) or acceptors (Ag⁺), respectively.¹¹ This indicates that Sm₂Ti₂S₂O₅ has band positions suitable for water splitting under visible light irradiation. However, Sm₂Ti₂S₂O₅ and other oxysulphides have not been applied to Z-scheme water splitting successfully. Earlier studies on the photocatalyst Sm₂Ti₂S₂O₅ involved the H₂S gas sulphurization method for the preparation of fine Sm₂Ti₂S₂O₅ particles,¹² and the loading and addition of various metal species to Sm₂Ti₂S₂O₅.^13–15 It was found that promoting both the photocatalytic reduction of water and the oxidation of sacrificial electron donors as well as reducing the defect density of Sm₂Ti₂S₂O₅ were important for improving the photocatalytic H₂ evolution rate on Sm₂Ti₂S₂O₅ from an aqueous solution of Na₂S and Na₂SO₃. In this study, methods of modifying Sm₂Ti₂S₂O₅ were investigated to improve the H₂ evolution activity. After appropriate post-treatments such as annealing with sulphur and etching with nitric acid, Sm₂Ti₂S₂O₅ loaded with Rh exhibited a higher activity for the sacrificial H₂ evolution reaction. The modified Sm₂Ti₂S₂O₅ was used as a H₂ evolution photocatalyst in Z-scheme water splitting. It was found that Pt-loaded Sm₂Ti₂S₂O₅ suspended with TiO₂ (rutile) in an aqueous NaI solution was capable of splitting water into H₂ and O₂ under UV irradiation (λ > 300 nm).

Experimental

Amorphous Sm₂Ti₂O₇ was synthesized as an oxide precursor by a polymerized complex method.¹² Titanium tetraisopropoxide (0.02 mol, Kanto Chemical Co., 97%) and anhydrous citric acid (0.3 mol, Wako Pure Chemicals, 98%) were dissolved in ethylene glycol (0.2 mol, Kanto Chemical Co., 99.5%) at room temperature, and the mixture was heated at 333 K until the reagents were completely dissolved. Subsequently, Sm(NO₃)₃·6H₂O (0.02 mol, Wako Pure Chemicals Co., 99.5%) and 20 mL methanol were added to the solution. The mixture was stirred at 403 K until a transparent gel was formed. It was polymerized and carbonized at 523, 573, and 623 K for 1 h each and finally calcined at 773 K for 12 h to remove the carbon completely. The resulting amorphous oxide was sulphurized at 1223 K for 1 h under a flow of H₂S (10 mL min⁻¹) and calcined for 2 h in air at 573 K.

As-synthesized Sm₂Ti₂S₂O₅ was sealed in evacuated quartz tubes with 5 wt% sulphur powder (High Purity Chemicals Co., 99.99%) with respect to Sm₂Ti₂S₂O₅ and annealed at 1223 K for 20 h.¹² Nitric acid treatment was conducted by dispersing Sm₂Ti₂S₂O₅ powder (ca. 0.5 g) in HNO₃ (Wako Pure Chemicals Co., 69.8 wt%) for 10 min at room temperature, followed by rinsing with distilled water, filtration, drying at 343 K, and heat treatment at 573 K for 1 h in air. The mass loss during the whole nitric acid etching procedure was approximately 40%, while the mass loss during the rinsing and filtration procedures was approximately 8%. When both treatments were carried out, the nitric acid etching was performed after the sulphur annealing. Sm₂Ti₂S₂O₅ samples subjected to the sulphur annealing alone, to the nitric acid etching alone, and to both procedures will hereafter be denoted as Sm₂Ti₂S₂O₅ (S), Sm₂Ti₂S₂O₅ (HNO₃), and Sm₂Ti₂S₂O₅ (S + HNO₃), respectively.

The samples were characterized using X-ray powder diffraction (XRD; RINT-Ultima III, Rigaku; Cu Kα), ultraviolet-visible diffuse-reflectance spectroscopy (DRS; V-670, JASCO), field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi) and X-ray photoelectron spectroscopy (XPS; JPS-9000, JEOL). The binding energy for each sample was corrected by the reference C 1s peak (285.0 eV). The peak areas of Sm 3d_5/2, Ti 2p, S 2p_3/2, and O 1s were acquired by integrating the signals over 1090.0–1077.0, 468.0–455.8, 171.8–157.4, and 535.7–526.5 eV, respectively.

The photocatalytic H₂ evolution reactions were carried out in an inner irradiation-type Pyrex reaction vessel connected to an air-tight closed gas circulation system made of Pyrex glass. An aqueous solution (400 mL) containing the photocatalyst sample (0.2 g), Na₂S–Na₂SO₃ (0.2 M each) was irradiated with a 450 W high-pressure Hg lamp through a NaNO₂ solution (2 M) as an optical filter, the cutoff wavelength of which was 400 nm. For the H₂ evolution reaction, Rh (1.5 wt%) was loaded as a H₂ evolution cocatalyst by photodeposition from the Na₂S–Na₂SO₃ aqueous solutions.^13,14 A calculated amount of RhCl₃·xH₂O (Aldrich, 38–40% as Rh) precursor added to the solution was reduced to metallic Rh by photoexcited electrons from the photocatalyst in the initial stage of the reaction. The reactant solution was maintained at room temperature by flowing cooling water during the reaction. The evolved gas was analyzed by gas chromatography (GC, TCD, Shimadzu).

For Z-scheme water splitting, Pt-loaded Sm₂Ti₂S₂O₅ (0.2 g), rutile TiO₂ (0.2 g), and NaI (50 mM) were used as a H₂ evolution photocatalyst, O₂ evolution photocatalyst, and redox mediator, respectively. Pt is employed as an effective H₂ evolution cocatalyst in Z-scheme water splitting involving the redox couple IO₃⁻/I⁻.^6,10,15–17 Pt (2 wt%) was photodeposited on Sm₂Ti₂S₂O₅ from a calculated amount of H₂PtCl₆·6H₂O (Aldrich, 38–40% Pt) in an aqueous NaI solution (pH = 6). The platinized sample was collected and used for Z-scheme water splitting. The pH of the solution containing the H₂ evolution photocatalyst, O₂ evolution photocatalyst, and redox mediator was adjusted using an aqueous NaOH solution. The solution was evacuated and irradiated with a 450 W high-pressure Hg lamp (λ > 300 nm). Note that conventional TiO₂ (rutile) was employed to assess the applicability of Sm₂Ti₂S₂O₅ as a H₂ evolution photocatalyst in Z-scheme water splitting, since rutile-type TiO₂ worked effectively as an O₂ evolution photocatalyst in Z-scheme systems involving IO₃⁻/I⁻ owing to preferential adsorption of IO₃⁻ ions onto rutile-type TiO₂ and its sufficient activity of for IO₃⁻ reduction even without elaborate modifications while necessitating UV illumination.¹⁷

Results and discussion

Effects of post-treatments

Fig. 1 shows XRD patterns of the Sm₂Ti₂S₂O₅ samples before and after the post-treatments. The diffraction profile of the Sm₂Ti₂S₂O₅ phase was unchanged and no impurity phase was generated by the post-treatments. The peak intensities of Sm₂Ti₂S₂O₅ and Sm₂Ti₂S₂O₅ (HNO₃) were identical, while those of Sm₂Ti₂S₂O₅ (S) and Sm₂Ti₂S₂O₅ (S + HNO₃) became somewhat stronger. This suggests that the sulphur annealing at 1223 K improved the crystallinity of Sm₂Ti₂S₂O₅ and that the nitric acid etching did not change the bulk crystallinity of Sm₂Ti₂S₂O₅.

Fig. 1 XRD patterns of (a) Sm₂Ti₂S₂O₅, (b) Sm₂Ti₂S₂O₅ (S), (c) Sm₂Ti₂S₂O₅ (HNO₃), and (d) Sm₂Ti₂S₂O₅ (S + HNO₃).

Fig. 2 shows SEM images of the corresponding Sm₂Ti₂S₂O₅ samples. Sm₂Ti₂S₂O₅ consisted of plate-like submicron particles, which formed massive secondary particles several micrometers in size. During the sulphur annealing, these aggregates were sintered into 0.5–1 μm particles with smoother surfaces. When the samples were etched with HNO₃, dramatic changes were observed in the surface morphologies of Sm₂Ti₂S₂O₅ and Sm₂Ti₂S₂O₅ (S). It appeared that the outer surface of the samples had dissolved, exposing the interior consisting of submicron Sm₂Ti₂S₂O₅ grains. This verified the speculation of our previous study that the large Sm₂Ti₂S₂O₅ (S) particles were polycrystalline.¹² The Sm₂Ti₂S₂O₅ (S + HNO₃) grains were, on average, larger than the Sm₂Ti₂S₂O₅ (HNO₃) grains.

Fig. 2 SEM images of (a) Sm₂Ti₂S₂O₅, (b) Sm₂Ti₂S₂O₅ (S), (c) Sm₂Ti₂S₂O₅ (HNO₃), and (d) Sm₂Ti₂S₂O₅ (S + HNO₃).

The surface compositions of the Sm₂Ti₂S₂O₅ samples before and after the post-treatments are tabulated in Table 1. Pristine Sm₂Ti₂S₂O₅ nearly exhibited the stoichiometric Sm/Ti ratio, while the S/Ti and O/Ti ratios were larger than the corresponding stoichiometric ratios. We obtained similar results in our previous study.¹² This is because the Sm₂Ti₂S₂O₅ surface was enriched with elemental and oxidized sulphur species when Sm₂Ti₂S₂O₅ was sulphurized under a H₂S flow and annealed in air. Sm₂Ti₂S₂O₅ (HNO₃) after the nitric acid etching exhibited a smaller Sm/Ti ratio than pristine Sm₂Ti₂S₂O₅. This indicates that the Sm₂Ti₂S₂O₅ surface was indeed dissolved by nitric acid, particularly the Sm component with weak basicity. The sulphur annealing did not change the surface Sm/Ti ratio significantly, while generating excessive sulphur on the surface of Sm₂Ti₂S₂O₅ (S). Sm₂Ti₂S₂O₅ (S + HNO₃) maintained the Sm/Ti ratio of Sm₂Ti₂S₂O₅ (S), unlike the case of Sm₂Ti₂S₂O₅ (HNO₃) after the nitric acid treatment. It is thought that a part of the pristine Sm₂Ti₂S₂O₅ sample was rather amorphous and thus the nitric acid etching resulted in preferential dissolution of Sm species. However, annealing in the presence of sulphur improved the crystallinity of Sm₂Ti₂S₂O₅, incorporating amorphous Sm species into the crystal structure, thereby suppressing the leaching of Sm species during the nitric acid treatment. Note that the surface of Sm₂Ti₂S₂O₅ (S) was still etched, as was evident from the SEM images. This resulted in the partial removal of the excess sulphur species on the surface.

Table 1 Surface atomic ratios of Sm₂Ti₂S₂O₅ before and after the post-treatments, as estimated by XPS

Sample	Sm/Ti	S/Ti	O/Ti
Sm2Ti2S2O5	1.1	1.9	6.6
Sm2Ti2S2O5 (HNO3)	0.6	2.2	5.8
Sm2Ti2S2O5 (S)	0.9	2.6	7.5
Sm2Ti2S2O5 (S + HNO3)	0.9	2.0	6.5

---

The DRS shown in Fig. 3 confirms that the pristine sample exhibited a light absorption onset at approximately 650 nm, which corresponded to the band gap transition of Sm₂Ti₂S₂O₅.¹¹ The background absorption was also observed, presumably because of reduced Ti species. The reduced Ti species may have been generated by the exposure to heated H₂S.¹² Sm₂Ti₂S₂O₅ (HNO₃) had a similar absorption onset and background absorption. The fact that etching of the surface did not weaken the background absorption indicates that the reduced Ti species were present in the bulk of the sample. Sm₂Ti₂S₂O₅ (S) and Sm₂Ti₂S₂O₅ (S + HNO₃), which were subjected to the sulphur annealing treatment, had similar absorption onsets but lower background absorption. Considering the dramatic changes in morphology that occurred during the sulphur annealing, it is reasonable to assume that the reduced Ti species in the bulk were oxidized by elemental sulphur upon annealing at 1223 K.

Fig. 3 UV-vis DRS of (a) Sm₂Ti₂S₂O₅, (b) Sm₂Ti₂S₂O₅ (S), (c) Sm₂Ti₂S₂O₅ (HNO₃), and (d) Sm₂Ti₂S₂O₅ (S + HNO₃).

Fig. 4 shows the reaction time courses of photocatalytic H₂ evolution using pristine and post-treated Sm₂Ti₂S₂O₅ samples. The photocatalytic activity of Sm₂Ti₂S₂O₅ was improved by the sulphur annealing and the nitric acid etching, and combining these post-treatments resulted in the highest activity. During the sulphurization of amorphous Sm₂Ti₂O₇ to Sm₂Ti₂S₂O₅, a certain amount of Ti³⁺ species was generated because of the reduction of Ti⁴⁺ under the heated H₂S atmosphere. The generation of reduced Ti species is accompanied by the formation of anion vacancies. These vacancies tend to act as recombination centres for photoexcited electrons and holes and suppress the photocatalytic activity.^12,13 The sulphur annealing can potentially produce a higher activity owing to the lower density of reduced Ti species in Sm₂Ti₂S₂O₅. In reality, however, hardly any activity enhancement was observed upon sulphur annealing. This is likely due to the presence of excess sulphur species on the surface, which blocked the surface active sites and thus prevented the deposition of Rh as H₂ evolution sites.¹² The sintering of Sm₂Ti₂S₂O₅ may also have led to the lower-than-expected photocatalytic activity.¹² The nitric acid etching improved the H₂ evolution activity by a factor of three although the Sm component was deficient on the sample surface. The SEM and XPS results suggest that the enhanced activity of Sm₂Ti₂S₂O₅ (HNO₃) is attributable to the elimination of the defective amorphous layers on the surface that would otherwise have prevented charge transfer. Sm₂Ti₂S₂O₅ (S + HNO₃) benefited from both post-treatments. Moreover, the excess sulphur species on Sm₂Ti₂S₂O₅ (S) were eliminated in part by the nitric acid etching, and the Sm deficiency on the surface observed in the case of Sm₂Ti₂S₂O₅ (HNO₃) was avoided owing to the improved crystallinity. As a result, Sm₂Ti₂S₂O₅ (S + HNO₃) showed a H₂ evolution rate that was 4.5 times higher than that of pristine Sm₂Ti₂S₂O₅.

Fig. 4 Time courses of H₂ evolution over Sm₂Ti₂S₂O₅ after various post-treatments: (a) Sm₂Ti₂S₂O₅, (b) Sm₂Ti₂S₂O₅ (S), (c) Sm₂Ti₂S₂O₅ (HNO₃), and (d) Sm₂Ti₂S₂O₅ (S + HNO₃). Reaction conditions: catalyst, 0.2 g; cocatalyst, 1.5 wt% Rh (in situ photodeposition); sacrificial electron donor, Na₂S + Na₂SO₃ (0.2 M each in 400 mL of water); light source, 450 W high-pressure Hg lamp (λ > 400 nm, 2 M NaNO₂ (aq.) as a filter solution).

Application of Sm₂Ti₂S₂O₅ to Z-scheme water splitting

Table 2 shows the amounts of H₂ and O₂ evolved in Z-scheme water splitting reactions over 5 h using Pt-loaded Sm₂Ti₂S₂O₅ samples as a H₂ evolution photocatalyst. Simultaneous evolution of H₂ and O₂ was observed only when Sm₂Ti₂S₂O₅ (S + HNO₃) was used as a H₂ evolution photocatalyst together with TiO₂ (rutile) as an O₂ evolution photocatalyst and NaI as a redox mediator. The ratio of H₂ to O₂ produced was higher than the stoichiometry of overall water splitting because of the presence of excess electron donors (NaI) in the beginning of the reaction. When one of the components, i.e., the H₂ evolution photocatalyst, O₂ evolution photocatalyst, or redox mediator, was missing, the gas evolution was negligible. This confirms that the water splitting reaction occurred via the Z-scheme mechanism.

Table 2 Photocatalytic activity for Z-scheme water splitting using Sm₂Ti₂S₂O₅^a

H2 evolution photocatalyst^b	O2 evolution photocatalyst	Redox mediator	Activity^c/μmol
			H2	O2
a Reaction conditions: photocatalysts, 0.2 g each; reaction solution, aqueous NaI solution (430 mL, 50 mM, pH = 12); light source, 450 W high-pressure Hg lamp. b Loaded with 2 wt% of Pt by photodeposition. c Estimated from the amount of gases evolved in 5 h.
Sm2Ti2S2O5	TiO2 (rutile)	NaI	10	0
Sm2Ti2S2O5 (S)	TiO2 (rutile)	NaI	13	0
Sm2Ti2S2O5 (HNO3)	TiO2 (rutile)	NaI	11	0
Sm2Ti2S2O5 (S + HNO3)	TiO2 (rutile)	NaI	45	16
None	TiO2 (rutile)	NaI	0	0
Sm2Ti2S2O5 (S + HNO3)	None	NaI	0.6	0
Sm2Ti2S2O5 (S + HNO3)	TiO2 (rutile)	None	0	0

---

The use of pristine Sm₂Ti₂S₂O₅, Sm₂Ti₂S₂O₅ (S), and Sm₂Ti₂S₂O₅ (HNO₃) did not lead to successful O₂ evolution, while producing a certain amount of H₂. In addition, the H₂ evolution ceased during the initial stage of the reaction. In Z-scheme water splitting in the presence of reversible redox couples, simultaneous evolution of H₂ and O₂ is often difficult because of backward reactions on the respective photocatalysts. IO₃⁻ produced by the oxidation of I⁻ on Sm₂Ti₂S₂O₅ could largely be reduced back to I⁻ unless the H₂ evolution activity was high enough to outweigh the thermodynamically favorable backward reaction. In addition, the oxygen evolution on rutile-type TiO₂ could be suppressed by the oxidation of I⁻ when the concentration of I⁻ was high.¹⁷ The absence of O₂ evolution thus suggests that the activities of the above three Sm₂Ti₂S₂O₅ samples for H₂ evolution and I⁻ oxidation were too low to provide a sufficient amount of IO₃⁻ in the reaction solution. Further characterization is necessary to determine why, among the Sm₂Ti₂S₂O₅ samples subjected to different post-treatments, only Sm₂Ti₂S₂O₅ (S + HNO₃) was active for Z-scheme water splitting. However, the present results suffice to show that the appropriate post-treatments enabled the oxysulphide photocatalyst to be used as a H₂ evolution photocatalyst for Z-scheme water splitting and that lowering the densities of both reduced Ti species and surface impurities was necessary.

Fig. 5 shows the dependence of the photocatalytic activity of the Z-scheme system for water splitting on the pH of the reaction solution. Z-scheme water splitting was enhanced significantly as the pH increased from 6 to 12 and was suppressed at pH > 12. O₂ evolution was almost negligible at pH < 9. This is because the main oxidative product on Pt-loaded Sm₂Ti₂S₂O₅ (S + HNO₃) was I₃⁻, which failed to serve as an efficient electron acceptor for O₂ evolution over rutile TiO₂. The photocatalytic activity decreased at pH > 12. This is probably because of the decreasing driving force of Sm₂Ti₂S₂O₅ for H₂ evolution with increasing pH. It has been found previously that the potential of the band gap of Sm₂Ti₂S₂O₅ was independent of the pH of the solution,¹¹ while the H₂ evolution potential shifted negatively with increasing pH. Thus, a basic solution at a pH of around 11–12 was the most favourable for Z-scheme water splitting using Sm₂Ti₂S₂O₅, TiO₂, and I⁻/IO₃⁻.

Fig. 5 pH dependence of photocatalytic activity of the Z-scheme system based on 2 wt% Pt-loaded Sm₂Ti₂S₂O₅ (S + HNO₃), TiO₂ (rutile), and NaI under UV light illumination. Reaction conditions: photocatalysts, 0.2 g each; solution, aqueous NaI solution (50 mM, 430 mL); light source: 450 W high-pressure Hg lamp (λ > 300 nm).

Fig. 6 shows the results of Z-scheme water splitting with intermittent evacuations. The ratio of H₂ and O₂ approached 2 : 1 after the first five-hour reaction and subsequent evacuation as the excess I⁻ was converted into IO₃⁻ during the reaction. The rate of gas evolution gradually decreased as the gas product accumulated in the closed system because the backward reaction from H₂ and O₂ to H₂O occurred on the Pt used as a cocatalyst. The rate of water splitting recovered to some extent upon evacuation. However, the photocatalytic activity of the system gradually decreased. XRD patterns presented in Fig. 7 show that the sample after the reaction was a mixture of Sm₂Ti₂S₂O₅ (S + HNO₃) and rutile-type TiO₂. This result suggests that the bulk of Sm₂Ti₂S₂O₅ (S + HNO₃) was mostly stable during the water splitting reaction. The deactivation may be due to the photo-oxidation of the surface of Sm₂Ti₂S₂O₅ by photoexcited holes under the intense UV illumination. Although UV illumination and stabilization of Sm₂Ti₂S₂O₅ were still necessary, this represents the first instance of Z-scheme water splitting achieved using an oxysulphide photocatalyst as a H₂ evolution photocatalyst. It is expected that dual cocatalyst loading on Sm₂Ti₂S₂O₅^18,19 and the use of visible-light-sensitive O₂ evolution photocatalysts will allow stable water splitting under visible light illumination.

Fig. 6 Time course of Z-scheme water splitting using 2 wt% Pt-loaded Sm₂Ti₂S₂O₅ (S + HNO₃), TiO₂ (rutile), and NaI under UV light illumination. Reaction conditions: photocatalysts, 0.2 g each; reaction solution, aqueous NaI solution (50 mM, pH = 11, 430 mL); light source: 450 W high-pressure Hg lamp (λ > 300 nm).

Fig. 7 XRD patterns of (a) rutile-type TiO₂, (b) Sm₂Ti₂S₂O₅ (S + HNO₃), and (c) after the Z-scheme water splitting reaction with rutile-type TiO₂ at pH 11.

Conclusions

The processes of annealing with sulphur and etching with nitric acid were studied to improve the photocatalytic activity of Sm₂Ti₂S₂O₅ for H₂ evolution in sacrificial water reduction and Z-scheme water splitting. The photocatalytic activity improved by a factor of 4.5 when the sulphur annealing and nitric acid etching were applied in combination. The effects of these post-treatments were attributed to the oxidation of reduced Ti³⁺ species and the removal of amorphous surface phases and excess sulphur species, respectively.

Following the sulphur annealing and the nitric acid etching, Sm₂Ti₂S₂O₅ could be used as a H₂ evolution photocatalyst in Z-scheme water splitting in combination with TiO₂ as an O₂ evolution photocatalyst and NaI as a redox mediator. All three of the components were necessary to drive Z-scheme water splitting. Employing both post-treatments was essential for successful Z-scheme water splitting, which indicated the importance of removing reduced Ti³⁺ species and surface impurities from Sm₂Ti₂S₂O₅. Although UV irradiation was necessary because of the use of TiO₂, the present study revealed that appropriate modifications allowed an oxysulphide photocatalyst to serve as a H₂ evolution photocatalyst in Z-scheme water splitting.

Acknowledgements

This work was financially supported by the Grant-in-Aid for Specially Promoted Research (no. 23000009) of the Japan Society for the Promotion of Science (JSPS) and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). Further financial support was provided by JSPS through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST), initiated by the Council for Science and Technology Policy (CSTP), and by an international exchange program of the A3 Foresight Program of JSPS. Z.W. gratefully acknowledges Dr Curulla-Ferre, Prof. Wimmer, and Dr Yagi of TOTAL S.A. for scientific discussions and financial support. K. M. is indebted to the Nippon Sheet Glass Foundation for Materials Science and Engineering for funding support.

Notes and references

1. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
2. K. Maeda and K. Domen, J. Phys. Chem. Lett., 2010, 2655.
3. R. Abe, J. Photochem. Photobiol., C, 2010, 11, 179.
4. K. Maeda, ACS Catal., 2013, 3, 1486.
5. Y. Sasaki, H. Nemoto, K. Saito and A. Kudo, J. Phys. Chem. C, 2009, 113, 17536.
6. K. Maeda, M. Higashi, D. Lu, R. Abe and K. Domen, J. Am. Chem. Soc., 2010, 132, 5858.
7. Y. Sasaki, A. Iwase, H. Kato and A. Kudo, J. Catal., 2008, 259, 133.
8. Y. Sasaki, H. Kato and A. Kudo, J. Am. Chem. Soc., 2013, 135, 5441.
9. M. Higashi, R. Abe, A. Ishikawa, T. Takata, B. Ohtani and K. Domen, Chem. Lett., 2008, 37, 138.
10. M. Tabata, K. Maeda, M. Higashi, D. Lu, T. Takata, R. Abe and K. Domen, Langmuir, 2010, 26, 9161.
11. A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and K. Domen, J. Am. Chem. Soc., 2002, 124, 13547.
12. A. Ishikawa, Y. Yamada, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and K. Domen, Chem. Mater., 2003, 15, 4442.
13. F. Zhang, K. Maeda, T. Takata and K. Domen, J. Catal., 2011, 280, 1.
14. F. Zhang, K. Maeda, T. Takata, T. Hisatomi and K. Domen, Catal. Today, 2012, 185, 253.
15. K. Sayama, K. Mukasa, R. Abe, Y. Abe and H. Arakawa, Chem. Commun., 2001, 2416.
16. K. Sayama, K. Mukasa, R. Abe, Y. Abe and H. Arakawa, J. Photochem. Photobiol., A, 2002, 148, 71.
17. R. Abe, K. Sayama and H. Sugihara, J. Phys. Chem. B, 2005, 109, 16052.
18. F. Zhang, K. Maeda, T. Takata and K. Domen, Chem. Commun., 2010, 46, 7313.
19. R. Li, Z. Chen, W. Zhao, F. Zhang, K. Maeda, B. Huang, S. Shen, K. Domen and C. Li, J. Phys. Chem. C, 2013, 117, 376.

---

Footnotes

† Current address: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.

‡ Current address: State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China.

---