Structural evolution of alkaline earth metal stannates MSnO₃ (M = Ca, Sr, and Ba) photocatalysts for hydrogen production

Abstract

The alkaline earth metal stannates MSnO₃ (M = Ca, Sr, and Ba) photocatalysts with different morphologies are successfully prepared by hydrothermal method and their photocatalytic activities are evaluated by photocatalytic reforming of ethanol/water solution to hydrogen. All of the as-prepared samples are characterized in detail by X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance (UV-vis DRS), N₂ physical adsorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) before and after the photocatalytic hydrogen production to illustrate the effect of the photoreaction on the surface structure, and thus to make the photocatalysis clear. The results reveal that the greatest photocorrosion occurs on the surface of CaSnO₃, SrSnO₃, and BaSnO₃ samples. And the formed surface species have great influence on H₂ production from ethanol/water solution. The photocatalytic reaction can transform CaSnO₃ into CaSn(OH)₆, producing CaSn(OH)₆/CaSnO₃ composite where the photogenerated charges can be more efficiently separated and transferred, consequently enhancing the hydrogen evolution. As for SrSnO₃, the photocorrosion can cause the formation of Sn²⁺ self-doped SrSnO₃ nanoparticles on the surface to increase the hydrogen production efficiency. Unlike CaSnO₃ and SrSnO₃, the photocatalytic activity of BaSnO₃ is gradually decreased due to the conversion of BaSnO₃ to BaCO₃. As expected, the H₂ evolution rate decreased in the order of CaSnO₃ > SrSnO₃ > BaSnO₃ under UV light irradiation. It is well demonstrated in the present work that CaSnO₃ is a potential photocatalyst for the photocatalytic reforming of ethanol/water solution to hydrogen.

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1. Introduction

Photocatalytic hydrogen production is an ideal technology to solve the current energetic and environmental problems. At present, a number of studies have focused on the development of photocatalysts that can work for the reaction. Among the various photocatalysts, semiconductor materials used for photocatalytic hydrogen production have been most intensively studied because of their unique electrical behavior. However, the reaction efficiency doesn't meet the requirements of practical application, owing to two key factors: (1) the light absorption efficiency of the developed inorganic and organic semiconductor is low and cannot satisfy the actual requirements. (2) The interaction between light and solid catalyst is unclear. Semiconductor materials used to the chemical process must satisfy two requirements in theory. Firstly, the bandgap energy is greater than 1.23 eV. Secondly, the band structure must be compatible with the reduction and oxidation potentials of water. In addition to the thermodynamic factors, the kinetic factors concluding the over-potential of hydrogen release and the separation and migration of charge carriers, etc. should be considered for design and development of highly efficient photocatalysts.

A large number of semiconductor materials were already found to be active for the hydrogen production under UV irradiation, mainly including metal-oxide semiconductors based on d⁰ (such as Ti, Zr, W, and Ta)^1–4 and d¹⁰ (such as In, Ga, Ge, and Sn),^5–8 cerium oxide based semiconductor,⁹ and non-oxide semiconductors such as ZnS, AgBr, and InP.^10–12 A majority of tin-based oxides,^7,13–16 such as ZnSnO₃, CaSnO₃, SrSnO₃, and BaSnO₃, were proved to be the active photocatalysts due to the strong oxidation and reduction ability at the same time, on the basis of three points as follows: (1) they have a large band-gap energy. (2) The conduction band position is more negative than the reduction potential of hydrogen. (3) The valence band position is more positive than the oxidation potential of the majority of the organic pollutants. Thus, the band structure of the alkaline earths metal stannates MSnO₃ (M = Ca, Sr and Ba) completely satisfies the conditions of the hydrogen evolution and thus have received extensive attention. Chen et al.¹⁵ synthesized the dumbbell and rod-shaped SrSnO₃, and investigated the photocatalytic activity of the water reduction. Omeiri et al.¹⁷ successfully synthesized BaSnO_3−δ photocatalyst, and studied the activity of hydrogen production under visible light. Zhang et al. reported¹⁶ that alkaline earth metal stannates MSnO₃ had a good activity for hydrogen production with UV light, and the photocatalytic activity followed the order of CaSnO₃ > SrSnO₃ > BaSnO₃. The results showed that the activity sequence was consistent with the band gap size, and the difference of two potential values between the location of valence band (VB_CaSnO3 > VB_SrSnO3 >VB_BaSnO3) and the reduction potential of H⁺/H₂ of alkaline earth metal stannates MSnO₃. However, the alkaline earth metal stannates MSnO₃ photocatalysts often suffer from the great surface photocorrosion under UV irradiation, leading to the change in surface structure of catalyst. It may be one of important parameters making the photocatalytic activity different. So, it cannot be fully clarifies the nature of photocatalytic hydrogen production if we consider only from the band structure of alkaline earth metal stannates. A complete research of the evolution of the surface structure is necessary for the in-depth and comprehensive understanding of the photocatalysis of alkaline earth metal stannates MSnO₃.

In this work, the alkaline earth metal stannates MSnO₃ were synthesized by using the hydrothermal method. The physical and chemical properties of the materials before and after the reaction were characterized by X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance (UV-vis DRS), N₂ physical adsorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Photocatalytic hydrogen production from ethanol/water solution as a model photoreaction was used to evaluate the photocatalytic properties of the as-synthesized photocatalysts and the corresponding reaction mechanism was also proposed to clearly explain the different photocatalytic activity of alkaline earth metal stannates under the same conditions.

2. Experimental

2.1 Preparation of catalysts

The MSnO₃ (M = Ca, Sr, and Ba) photocatalysts were prepared by hydrothermal method. Typically, 3.9 g CaCl₂ (or SrCl₂·6H₂O = 9.4 g, BaCl₂·2H₂O = 8.6 g) and 12.3 g SnCl₄·5H₂O were first dissolved into 350 mL deionized water with the stirring speed of 800 rpm at room temperature for 30 min and marked A solution. Thereafter, 16 g NaOH was dissolved into 100 mL deionized water with the stirring speed of 500 rpm at room temperature for 30 min and marked B solution. Thirdly, the B solution was poured into the A solution at the stirring speed of 1000 rpm for 1 h. The obtained white turbid liquid was denoted as C. Finally, 80 mL C was transferred to 100 mL autoclave and heated at 180 °C for 24 h, and then cooled to room temperature. The resulting precipitates were filtered, washed with deionized water, dried, and then calcined at 700 °C for 6 h.

2.2 Catalyst characterizations

The X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA, using Cu Kα₁ radiation (λ = 1.5406 Å). The specific surface areas were determined at 77 K with a micromeritics ASAP 2010 instrument. UV-vis DRS spectra were obtained on a Varian Cary 500 Scan UV-VIS-NIR spectrophotometer using BaSO₄ as a reference. XPS spectra were carried out on a VG ESCALAB 250XPS system with a monochromatized Al Kα X-ray source (15 kV 200 W 500 μm, pass energy = 20 eV). All binding energies were referenced to the C 1s peak at 284.6 eV of surface adventitious carbon. Transmission electron microscopy (TEM) images were obtained by a JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. The surface morphologies of the as-prepared catalysts were observed using a field emission scanning electron microscope (FESEM: America, FEI Nova NanoSEM230).

2.3 Photocatalytic activity measurements

The photocatalytic reaction of hydrogen production was performed in a closed gas-recirculation system equipped with an inner irradiation quartz action vessel. In this typical experiment, 0.1 g photocatalyst and 5 mL ethanol used as the sacrificial reagent was suspended in 155 mL of distilled water by magnetic stirring and the temperature of the glass reaction cell was sustained at about 20 °C by circulating water. Prior to the reaction, the system was evacuated by a mechanical pump and then filled with 101 kPa of high-purity Ar (>99.99%). This process was repeated three times in order to completely remove O₂ and CO₂ from the system. Photocatalyst was dispersed in the ethanol/water by stirring with a magnetic stirrer and irradiated under a 125 W high-pressure Hg lamp with a main wavelength of 365 nm (the light intensity is ca. 17.3 mW cm⁻² and the irradiation area is ca. 100 cm²). The temperature of the solution was controlled at 293 ± 0.1 K by circulating condensate water through an interlayer around the reactor. The gas compositions were monitored by a GC 112A equipped with a thermal conduction detector (TCD) and TDX-01 column. The apparent quantum efficiency (AQE) was calculated by the H₂ production using the following equation:

3. Results and discussion

3.1. Characterizations of MSnO₃ samples

Fig. 1A represented the XRD patterns of MSnO₃ (M = Ca, Sr, and Ba) samples. It was found that the samples consisted of a single phase of CaSnO₃, SrSnO₃ and BaSnO₃ (JCPDS no. 31-0312, 22-1442, and 15-0780) with a perovskite structure, respectively. No other peaks derived from impurities were observed, suggesting the good crystalline for the as-prepared samples.

The UV-visible absorption spectra of the as-prepared MSnO₃ samples were showed in Fig. 1B. Three samples displayed a typical absorption feature of semiconductor in the UV region. The band-edge absorption of MSnO₃ (M = Ca, Sr, and Ba) samples is positioned at 307, 324, and 407 nm, respectively, with increasing the cationic radius. According to the computing method in literatures,¹⁶ the light absorption can be described as follows:

αhν = A(hν − E_g)^n/2

Fig. 1 (A) XRD patterns and (B) UV-vis DRS over MSnO₃ (M = Ca, Sr, and Ba) samples.

Therein, α, hν, A, and E_g defined the coefficient of absorption, the energy of incident light, the constant, and the width of forbidden band, respectively. CaSnO₃ is a direct band gap semiconductor with n = 1. SrSnO₃ and BaSnO₃ are indirect band gap semiconductors with n = 4. So the corresponding forbidden band widths of MSnO₃ (M = Ca, Sr, and Ba) samples are 4.30, 4.10, and 3.12 eV, respectively, as shown in Table 1. The data are different slightly from those reported in literatures.^15–17 This may be caused by the preparation method and the crystalline grain. These UV-vis DRS results indicated that the as-prepared MSnO₃ (M = Ca, Sr, and Ba) samples prepared under the same hydrothermal conditions have differences in optical absorption properties, which will be closely related to the different photocatalytic activities for H₂ production below.

Table 1 The physicochemical characteristics of MSnO₃ (M = Ca, Sr, and Ba) samples

	CaSnO3	SrSnO3	BaSnO3
BET (m^2 g^−1)	13.5	16.9	5.9
Eg (eV)	4.30	4.10	3.12
H2 production rate (μmol h^−1)	27.4	4.2	2.4

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Therefore, the as-prepared MSnO₃ samples were further characterized by SEM, as demonstrated in Fig. 2. The surface morphology of CaSnO₃ sample presented inhomogenous cubic structure with 1–4 μm (Fig. 2a and b). The surface morphology of SrSnO₃ sample has a nanorod-structure (Fig. 2c and d), while the BaSnO₃ sample was ruleless nano-particles (Fig. 2e and f). The results suggested that the processes of the crystal growth for three samples prepared under the same hydrothermal conditions were different, resulting in different morphologies. Thus, we inferred that the surface areas for three samples followed the order of SrSnO₃ > CaSnO₃ > BaSnO₃, as verified by the N₂ absorption isotherms. The surface areas of CaSnO₃, SrSnO₃, and BaSnO₃ samples were 13.5, 16.9, and 5.9 m² g⁻¹, respectively, as shown in Table 1.

Fig. 2 SEM images of CaSnO₃ (a and b), SrSnO₃ (c and d), and BaSnO₃ (e and f) samples.

XPS spectra of O 1s, Sn 3d, Ca 2p, Sr 3d, and Ba 3d obtained over the as-prepared MSnO₃ samples were shown in Fig. 3A. The O 1s XPS spectra of CaSnO₃ samples can be fitted three peaks at 529.8, 531.3, and 532.6 eV. The binding energy at 529.8 eV is ascribed to lattice oxygen, 531.3 eV for the surface hydroxyl, and 532.5 eV for adsorbed water.^18,19 Similarly, the O 1s core level peak of SrSnO₃ and BaSnO₃ samples are also composed by lattice oxygen, surface hydroxyl, and adsorbed oxygen. However, the binding energy of different lattice oxygen decreases in the order of CaSnO₃ > SrSnO₃ > BaSnO₃. This maybe originates from the different cation electronegativity that affects the binding energy of lattice oxygen. Fig. 3B presented the Sn 3d XPS spectra of MSnO₃ samples at ca. 486.4 and 494.8 eV that corresponded to Sn 3d_5/2 and Sn 3d_3/2, respectively. The different value of two binding energies is 8.4 eV. As reported by Kwoka et al.,²⁰ it can be obtained that the Sn element exist three oxidizing steps corresponding to the following binding energy values: Sn⁰ (485.0 eV), Sn²⁺ (485.9 eV) and Sn⁺⁴ (486.6 eV). Recently, our group detailedly studied the valence of Sn element by designing and preparing the Sn-grafted TiO₂, Sn-grafted Ru/TiO₂ and Sn, Ni-grafted TiO₂ photocatalysts.^21–23 Based on the previous reports and the above results, they clearly demonstrate that the valence of Sn in MSnO₃ (M = Ca, Sr, and Ba) is +4. As shown in Fig. 3C, the Ca 2p_3/2 and Ca 2p_1/2 binding energies are 346.3 and 349.9 eV, respectively. And the difference in these two values is 3.6 eV. The Sr 3d_5/2 and Sr 3d_3/2 binding energies of SrSnO₃ sample are 132.9 and 134.4 eV, respectively. The Ba 3d_5/2 and Ba 3d_3/2 binding energies of BaSnO₃ sample are located at 779.4 and 794.9 eV, respectively. This is in good consistency with that reported in literature.¹⁸

Fig. 3 XPS of spectra of MSnO₃ (M = Ca, Sr, and Ba) samples. (A) O 1s, (B) Sn 3d, (C) Ca 2p, Sr 3d, Ba 3d.

3.2 Photocatalytic activity for H₂ production

The photocatalytic activities for H₂ production over the as-prepared MSnO₃ photocatalysts were compared under UV light irradiation. As seen from Fig. 4A, it appeared that the MSnO₃ samples displayed very low photocatalytic activity for H₂ evolution before the photocatalytic reaction 1 h, whereas the photocatalytic activity was significantly enhanced as the reaction progressed, suggesting that there existed an induction process at the initial phase of photocatalytic reaction. This result is in good consistency with the phenomenon of H₂ production from photocatalytic splitting water over ZnSn(OH)₆ and Zn₂SnO₄ catalysts in our previous research work.^24,25 Compared to the SrSnO₃ and BaSnO₃ samples, the CaSnO₃ samples showed significantly enhanced photocatalytic activity, depending on the irradiation time. It can see from Fig. 4B that the H₂ evolution rate over CaSnO₃ sample reached 27.4 μmol h⁻¹, followed by a sharp decrease of 4.2 and 2.4 μmol h⁻¹ over SrSnO₃ and BaSnO₃ ones, respectively. It can be estimated by the eqn (1) that their AQE values are equal to 0.48%, 0.073%, and 0.042%, respectively. As expected, the H₂ evolution rate decreases in the order CaSnO₃ > SrSnO₃ > BaSnO₃. Obviously, the CaSnO₃ sample is more active for photocatalytic reforming of ethanol/water solution to hydrogen than those of SrSnO₃ and BaSnO₃ samples.

Fig. 4 (A) Time course of evolved H₂ over the as-prepared MSnO₃ (M = Ca, Sr, and Ba) photocatalysts under UV light irradiation. (B) The hydrogen evolution rate from the ethanol solutions over the as-prepared MSnO₃ (M = Ca, Sr, and Ba) photocatalysts.

The crystal structure, electronic structure and optical excitation of MSnO₃ (M = Ca, Sr, and Ba) catalysts have been studied by Zhang et al.¹⁶ Meanwhile, they clarified the relationships between the structure, the optical excitation property, and the photocatalytic H₂ production. They thought that the photocatalytic activities of hydrogen-releasing rate for MSnO₃ (M = Ca, Sr, and Ba) samples associated with not only the conduction band position, but the migration excitation energy. Through the analysis of the activity, two issues were presented: (1) why is the hydrogen-releasing rate small at the initial phase? And as the reaction proceeds, the activity increased suddenly. (2) The photocatalytic activities of MSnO₃ (M = Ca, Sr, and Ba) samples matched with the conduction band position. However, the relationship of the activity and the conduction band position is not linear. In fact, the conduction band position and the migration excitation energy of photocatalyst are the reflection of the capabilities of the photo-electronic reduction and migration. This is one of proofs of the catalysts with high activity, but it is not equivalent to the photocatalytic hydrogen-releasing activity. The effect of the structure on the activity cannot fully explain the difference in photocatalytic activity. The photocatalytic hydrogen-releasing activities are very closely related to the reaction kinetics and the catalyst surface microstructure during the reaction. Therefore, the surface microscopic process of CaSnO₃, SrSnO₃, and BaSnO₃ photocatalysts during photocatalytic reforming of ethanol/water solution to H₂ would be detailedly characterized below.

3.3 Structural evolution

To elucidate the difference of the photocatalytic activities for H₂ production over the as-prepared MSnO₃ photocatalysts, XRD was first used to investigate their structural evolution. The XRD patterns of CaSnO₃ sample before and after photocatalytic reaction were compared in Fig. 5A. As mentioned above, the XRD patterns consisted of a single phase of CaSnO₃ with a perovskite structure before the photocatalytic H₂ production. After UV illumination 10 h, some new diffraction peaks, indexed as CaSn(OH)₆ (JCPDS-09-0030), appeared in the XRD, suggesting the generation of CaSn(OH)₆ after the photocatalytic H₂ production. The microstructural change during photocatalytic reaction was further characterized by TEM, as shown in Fig. 6. The TEM images of the cube CaSnO₃ was gradually destroyed with the increase of illumination time. The results indicated that the phenomenon of light corrosion appeared in the photocatalytic reforming of ethanol/water solution to H₂, resulting in the formation of CaSn(OH)₆ crystals.

Fig. 5 XRD pattern changes of MSnO₃ samples before and after photocatalytic H₂ production.

Fig. 6 TEM images of CaSnO₃ over the photocatalytic process. (A) 5 h, (B) 15 h, (C) 20 h.

As such, the crystal structure of SrSnO₃ before and after the photocatalytic H₂ production has also been changed (Fig. 5B). Before the photocatalytic reaction, the SrSnO₃ sample is good crystalline with the perovskite structure. After illumination 10 h, the diffraction peak of Sn (JCPDS-04-0673) appeared in the XRD, indicating the generation of metal Sn after reaction. Unlike CaSnO₃ sample, the precursor of SrSn(OH)₆ was not generated.

Fig. 5C presented the XRD pattern of BaSnO₃ sample before and after the photocatalytic H₂ production. After reaction, in addition to the characteristic diffraction peaks of BaSnO₃, the diffraction peaks of Sn and BaCO₃ were also detected, suggesting the formation of Sn and BaCO₃ during the photocatalytic H₂ production. C₂H₅OH can provide the only C source in the reaction. Therefore, it was inferred that the mineralized product CO₂ was generated in the photocatalytic reaction, which was evidenced by adsorption of 0.01 M Ba(OH)₂ solution. Alkaline substances in water reacted with CO₂ to generate CO₃²⁻, and then combined with Ba²⁺ to generate BaCO₃.

3.4 XPS analysis

To further illustrate the structural evolution, XPS was used to investigate their chemical states. XPS spectra of O 1s, Sn 3d, Ca 2p, Sr 3d, and Ba 3d obtained over the as-prepared MSnO₃ samples before and after photocatalytic reaction were characterized. As for CaSnO₃ sample (Fig. 7A), the intensity of O 1s located at 529.9 eV was significantly weakened after reaction. However, the intensity of O 1s located at 531.3 eV obviously increased. The results showed that the surface lattice oxygen of CaSnO₃ was reduced, and hydroxyl oxygen species were increased. Meanwhile, the chemical environment of Ca 2p and Sn 3d changed due to the crystal phase transition during the reaction. The positions of Ca 2p and Sn 3d were slightly shifted to some extent (Fig. 7B and C). In the absence of alcohol as sacrificial agent, the surface structure of CaSnO₃ was similarly changed. This phenomenon verified the transition from CaSnO₃ to CaSn(OH)₆ under UV irradiation.

Fig. 7 XPS spectra change of CaSnO₃ over irradiation process. (A) Before reaction; (B) ethanol as sacrificial agent; (C) pure water.

Fig. 8 compared the XPS spectra of O 1s, Sn 3d, and Sr 3d obtained over SrSnO₃ sample before and after reaction. Similarly, the O 1s XPS spectra of the lattice oxygen (529.9 eV) drastically reduced after photocatalytic reaction (Fig. 8A). As seen from Fig. 8B, there appeared a new Sn 3d_5/2 peak located 484.8 eV on the spectrum of Sn 3d after photocatalytic reaction, which can be attributed to metal Sn.^26–28 The results proved that Sn was formed during photocatalytic reaction due to the photocorrosion on the surface of SrSnO₃ sample, which is consistent with the XRD results. However, at the absence of ethanol as the case of sacrificial agent, the peaks of Sn on Sn 3d spectrum was very small, indicating that the presence of ethanol can promoting the generation of metal Sn. The position of Sr 3d peak moved toward the high binding energy after the photocatalytic reaction. It indicated that there were electron-withdrawing species on the catalyst surface, increasing the binding energy of Sr 3d peak. The reason is possibly that a large number of surface defects resulted in the increase of Sn²⁺ under UV irradiation, changing the chemical environment of Sr, thus increasing the binding energy.

Fig. 8 XPS spectra change of SrSnO₃ over irradiation process. (A) Before reaction; (B) ethanol as sacrificial agent; (C) pure water.

Fig. 9 also displayed the XPS spectra of BaSnO₃ samples before and after the photocatalytic reaction. As seen from the O 1s spectrum of BaSnO₃ samples (Fig. 9A), the peak of the lattice oxygen (529.3 eV) decreased after reaction due to photocorrosion on the sample surface. Notably, there was no presence of the surface lattice oxygen in the sample after the photocatalytic H₂ production. However, the adsorbed water of the surface chemistry disappeared after the water photolysis. Similarly, the surface of BaSnO₃ sample formed the self-doped Sn²⁺. The peak of Sn 3d shifted slightly on the XPS spectra. The XPS peaks of metal Sn were not clearly seen, but observed from the XRD pattern after photocatalytic reforming of ethanol, implying that the Sn generated was distributed in the form of inhomogeneous particulates on the catalyst surface. As seen from Fig. 9C, there appeared the peak of Ba 3d_5/2 located at 779.9 eV after the photocatalytic reaction, attributed to BaCO₃ in Ba 3d_5/2.^29–31 This result was consistent with the XRD results.

Fig. 9 XPS spectra change of BaSnO₃ over irradiation process. (A) Before reaction; (B) ethanol as sacrificial agent; (C) pure water.

3.5 The pH change of the solution during photocatalytic H₂ production

To better illustrate the corresponding reaction mechanism, the pH values of the solution were monitored during the process of photocatalytic reforming of ethanol/water solution to H₂. Fig. 10 presented the pH change of the solution during photocatalytic H₂ production over MSnO₃. As seen from Fig. 10, the pH value of the solution for three catalysts was about 6.0 at the initial stages, indicating the weak acidity. The pH value did not substantially increase under the light irradiation within one hour, thus corresponding photocatalytic hydrogen-releasing rate was relatively small. With increasing the irradiation time, the pH value of the solution increased. For the CaSnO₃ sample, the pH value of the solution was stable to be about 8.6 when the exposure time was more than 6 h. As for the SrSnO₃ sample, the pH value of the reaction solution continued to increase during the light irradiation, and the increasing rate of the pH value became small after light irradiation of 5 h.

Fig. 10 The pH change of photocatalytic reaction over CaSnO₃, SrSnO₃, and CaSnO₃ samples.

The pH value was 10.3 after light irradiation of 10 h. When the photocatalyst was BaSnO₃, the pH value of the solution changed with “S” type vs. the illumination time. And the pH value of the solution was 10.1 after reaction. It is obviously implied that the solution for three samples was alkaline after the photocatalytic reaction.

3.6 Reaction mechanism

After clarifying the structural evolution and the process of photocatalytic reaction in detail, the corresponding reaction mechanism was proposed to clearly explain the different photoactivity of MSnO₃ samples under the same conditions. According to the TEM results mentioned above, the images of the cube CaSnO₃ were gradually destroyed with the increase of illumination time. The CaSnO₃ surface reacted with water to generate CaSn(OH)₆, as verified by XRD and XPS above. CaSn(OH)₆ is alkaline that also conforms to the pH value above. CaSn(OH)₆ can be reformed by ethanol to generate H₂ under UV light.²⁴ The photocatalytic activity for H₂ production over CaSn(OH)₆ was far less than that over CaSnO₃ one. However, the sharp increase of the activity was related to the species of CaSn(OH)₆ generated during photocatalytic reforming of ethanol/water solution. This is mainly because the nanocomposite structure CaSnO₃/CaSn(OH)₆ was formed through the in situ coupling CaSn(OH)₆ with CaSnO₃, promoting the efficient separation of photogenerated charge, consequently enhancing the photocatalytic efficiency. The process of photocatalytic reforming of ethanol/water solution to H₂ is as follows:

Hydrogen was produced by photocatalytic reforming of ethanol/water solution over SrSnO₃ catalyst under ultraviolet excitation. And methane and carbon monoxide were released. According to XRD and XPS, the light corrosion phenomenon occurred on the SrSnO₃ surface after the photocatalytic H₂ production. Sn²⁺ was formed on the surface of SrSnO₃ sample. At the same time, Sr(OH)₂ was produced in the solution. This phenomenon can be explained by the following three facts: (1) the results of XRD and XPS (Fig. 5B and 8B) showed that Sn in catalyst was transformed into metal Sn after the photocatalytic reaction. The reason is that the dismutation reaction was generated due to the intermediate species Sn²⁺. (2) The positions of Sn 3d peak of Sn²⁺ and Sn⁴⁺were very close. And it is difficult to distinguish them from XPS. However, Sr 3d peak shifted toward higher binding energy, verifying the existence of Sn²⁺. (3) As seen from Fig. 10, the pH value of the solution increased continually, and the pH value was 10.1 after light irradiation 10 h, suggesting that the solution was alkaline after the photocatalytic H₂ production. The presence of the surface Sn²⁺ enhanced the separation and migration of the photogenerated charge carriers, improving the photocatalytic activity. Therefore, the activity for H₂ production increased obviously after the catalytic reaction 2 h, which is consistent with the photocatalytic activity in Fig. 4. According to the results above, we think that the pathway of photocatalytic H₂ evolution over SrSnO₃ sample is as follows:

Like SrSnO₃, the Sn on the surface of BaSnO₃ sample was transformed into Sn²⁺ under UV irradiation. At the same time, Ba(OH)₂ was produced, which is alkaline (Fig. 10). The activity for BaSnO₃ catalyst increased after light irradiation of 1 h due to the presence of Sn²⁺. However, as shown in Fig. 4, the H₂ release rate over BaSnO₃ sample slowly increased and was much smaller than those of CaSnO₃ and SrSnO₃. This phenomenon may be caused by the generated BaCO₃ due to the photocatalytic reaction. On one hand, the BaCO₃ enrichment on the catalyst surface reduced the number of the surface active sites and the surface migration rate of the photogenerated carriers. On the other hand, it hindered the molecular adsorption on the catalyst surface. The process of the photocatalytic reforming of ethanol/water solution to H₂ over BaSnO₃ sample is as follows:

Therefore, it is concluded in the present work that only CaSnO₃ is a potential photocatalyst for the photocatalytic reforming of ethanol/water solution to hydrogen.

4. Conclusions

The alkaline earth metal stannates MSnO₃ (M = Ca, Sr, and Ba) photocatalysts prepared under the same conditions showed different structures, optical absorption properties, morphology, and activities. The great photocorrosion occurs on the surface of CaSnO₃, SrSnO₃, and BaSnO₃ samples. And the formed surface species have great influence on their photocatalytic activities for hydrogen production from ethanol/water solution. The in situ coupling of CaSnO₃ and CaSn(OH)₆ formed during photocatalytic reaction can greatly promote the efficient separation and transfer of photogenerated charges, consequently enhancing the photocatalytic efficiency. As for SrSnO₃, the photocorrosion can cause the formation of Sn²⁺ self-doped SrSnO₃ nanoparticles on the surface to improve the photocatalytic activity. Unlike CaSnO₃ and SrSnO₃, the photocatalytic activity of BaSnO₃ is gradually decreased due to the conversion of BaSnO₃ to BaCO₃, decreasing the number of the surface active sites and the surface migration rate of the photogenerated carriers. As expected, the H₂ evolution rate over CaSnO₃ sample reached the value of 27.4 μmol h⁻¹, followed by a sharp decrease of 4.2 and 2.4 μmol h⁻¹ over SrSnO₃ and BaSnO₃, respectively. It is well demonstrated in the present work that CaSnO₃ is more active for photocatalytic reforming of ethanol/water solution to hydrogen than those of SrSnO₃ and BaSnO₃.

Acknowledgements

This work was financially supported by the NSFC (Grants No. 21403035, 21373051).

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