Citation

Wenjie He, Xingxing Wu, Yifei Li, Jing Xiong, Zhiling Tang, Yuechang Wei, Zhen Zhao, Xiao Zhang, Jian Liu. Z-scheme heterojunction of SnS₂-decorated 3DOM-SrTiO₃ for selectively photocatalytic CO₂ reduction into CH₄[J]. Chin. Chem. Lett., 2020, 31(10): 2774-2778  

Wenjie He^a,b, Xingxing Wu^a, Yifei Li^a,b, Jing Xiong^a,b, Zhiling Tang^a,b, Yuechang Wei^a,b,*, Zhen Zhao^a, Xiao Zhang^a, Jian Liu^a     

The shortage of fossil energy resources and the global climate warming is threatening the living environment of human beings. The photocatalysis technology for carbon dioxide (CO₂) reduction to sustainable hydrocarbon energy may perfectly addresses the above conundrums [1-8]. However, the artificial photosynthesis technique is faced with the low solar energy conversion efficiency as well as the rare products [9-12]. Thus, it is significant to develop high performance photocatalyst. The perovskite-type SrTiO₃ possesses the wide band gap whose conduction band is positive enough to reduce CO₂, hence it was been widely researched for photoreduction reaction [13-19]. In addition, CO₂ as the anhydride of carbonic acid is more likely to adsorb with alkalis strontium on the surface of SrTiO₃ [20-22], so that it is promising to further investigate. However, the wide band gap of SrTiO₃ plays a doubleedged sword, which can not only decrease the recombination of electron-hole pairs, but also restrains the visible light absorption and the photoexcitation. Thus, the pristine SrTiO₃ is faced with the low quantum utilization and less multi-electron products [23, 24]. The strategies should be proposed to optimize SrTiO₃ and construct a novel catalyst with outstanding photoreduction performance as well as multi-electrons product (CH₄) selectivity.

Three-dimensional ordered macroporous (3DOM) catalysts as photonic crystals have been extensively studied in the field of photocatalysis due to their slow light efficiency [25-31]. Photonic crystals have periodic refractive modulation in the optical wavelength range, which can cause stopband reflection through the Bragg diffraction of the material and prohibit light propagation at certain wavelengths. The contact time between the photon and the material was extended, and the incident light of a specific wavelength is stored in 3DOM materials, thus the light capturing efficiency could be improved. And 3DOM structure could be obtained by replicating a classic face-centered cubic opal structure template [32-34]. In addition, tin disulfide (SnS₂), whose band gap is about 1.9–2.4 eV, is one of the most important layered transition metal sulfides [35-37]. The unique atomic arrangement in SnS₂, two layers of S atoms in the middle of two Sn atomic layers formed a built-in electric field perpendicular to the (001) crystal plane, is help to promote the directional migration of electrons [38-42]. And SnS₂ as the n-type semiconductor was used to investigate the adjustment of electrons structure in photocatalytic system.

Herein, SnS₂ nanosheets were in-situ constructed on the surface of 3DOM-SrTiO₃, and novel SnS₂/3DOM-SrTiO₃ catalysts with Z-scheme heterojunction were obtained. SnS₂ can absorb the full spectrum light and plays a role of electrons contributor. The photogenerated electrons achieved Z-scheme transmission affected by built-in electric field between layers via the hybrid interface [43-45]. SnS₂/3DOM-SrTiO₃ catalysts exhibited high the activity for CO₂ photoreduction and altered the predominant products selectivity from CO to CH₄ under full spectrum light irradiation in comparison to 3DOM-SrTiO₃. Thus, the present work will propose a way for increasing multi-electrons reduction process and reach to the high yield of the multi-electron products of CO₂ photoreduction.

The 3DOM-SrTiO₃ was synthesized by colloidal crystal template (CCT) method and SnS₂ was induced by co-hydrothermal as shown in Scheme S1 (Supporting information). The (SnS₂)_n/3DOM-SrTiO₃ catalysts were obtained and the specific amount of reagent nd corresponding products are listed in Table S1 (Supporting information). For clarification of the prepared catalysts, Fig. 1 exhibits scanning electron microscopy (SEM) images of the catalysts. SnS₂ shows regular hexagonal nanosheet structure with diameter of ~200 nm in Fig. S1a (Supporting information). As shown in Figs. 1b and c, the catalysts exhibit the ordered 3D network structure connected by the porous windows, and SnS₂ nanosheets distribute to the ordered channels of 3DOM-SrTiO₃. The compact arrangement structure benefits to the photoinduced electrons transfer between SnS₂ and 3DOM-SrTiO₃, and the relatively high specific surface area provides the abundant sites for CO₂ adsorption and photoreduction in Table S2 (Supporting information). The X-ray diffraction (XRD) patterns were exhibited in Fig. S1 (Supporting information). The diffraction peaks of (SnS₂)_n/3DOM-SrTiO₃ catalysts perfectly matched with Hexagonal phase SnS₂ (JCPDS No. 23-0677) and Cubic phase SrTiO₃ (JCPDS No. 35-0734), respectively.

Figure 1

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Fig. 1. SEM images of SnS2 (a), 3DOM-SrTiO3 (b) and (SnS2)3/3DOM-SrTiO3 (c) catalysts.

Furthermore, the phase structures of SnS₂ and (SnS₂)₃/3DOM-SrTiO₃ catalysts were further analyzed by Raman spectroscopy using an excitation wavelength of 532 nm (Fig. S2 in Supporting information). The Raman spectrum of SnS₂ exhibits a strong band at 314 cm^-1, which corresponds to the A_1g vibration mode of SnS₂. Moreover, a weak vibration peak at 205 cm^-1 is shown in the inset of Fig. S2, it is attributed to the E_g vibration mode of SnS₂. Meanwhile, the strong Raman bands of (SnS₂)₃/3DOM-SrTiO₃ catalyst located at 145, 397, 517 and 638 cm^-1 are attributed to the Raman-active transverse optical phonons of TO₂, TO₃, TO₄ and longitudinal optical phonons of LO₄ of SrTiO₃, respectively. In addition, the A_1g vibration peak of SnS₂ detected at 314 cm^-1. It demonstrates that the original chemical bond structure of the catalyst SrTiO₃ support is not affected by the composite SnS₂ nanosheets, and the appearance of the A_1g vibration peak of SnS₂ also indicates that it is tight contact with SrTiO₃, resulting in heterojunction structure.

The light absorption property of the catalysts is precondition of well photocatalytic activity, the photoexcitation efficiency of charge separation depends on the light absorption efficiency. As shown in Fig. S3a (Supporting information), after SnS₂ nanosheets constructed on 3DOM-SrTiO₃, the optical absorption bands of (SnS₂)_n/3DOM-SrTiO₃ catalysts have red-shift compared with pristine SrTiO₃, indicating that the light absorption is enhanced in the visible region (from 400 nm to 800 nm). The photoluminescence (PL) spectra of the catalysts were used to provide evidences for further understand the photoinduced charge carrier migration and separation efficiency. Fig. 2 shows the PL spectrum of (SnS₂)_n/3DOM-SrTiO₃ catalysts excited at an wavelength of 390 nm. Obviously, there is one PL emission peaks of pristine SnS₂ located at 520 nm. It attributes to the recombination of electronhole pairs on the surface or inner of SnS₂ nanosheets. However, 3DOM-SrTiO₃ catalyst has not PL emission peak, indicating the low recombination. (SnS₂)_n/3DOM-SrTiO₃ catalysts possess one PL emission peaks near to 520 nm, furthermore, the intensity of emission peak decreases with increasing of the SnS₂ dosage, suggesting that the rapidly transfer of photoexcited electrons under irradiation could be suppressed. For the reason that the E_CB and E_VB of SnS₂ are between E_CB and E_VB of SrTiO₃, if carriers transfer through heterojunction Type Ⅰ path, they will rapidly recombine and fail to effective separation. So that electrons were located on SrTiO₃ while the holes on SnS₂ after separation and Z-scheme transmission. It is attributed to the Z-scheme heterojunction role of (SnS₂)_n/3DOM-SrTiO₃ catalysts. It is noticed that, when SnS₂ composite content is increased to 5%, the intensity of emission peak belongs to (SnS₂)₅/3DOM-SrTiO₃ is enhanced. It indicates that the suitable dosage of SnS₂ contributes to the charge transfer, and the excess SnS₂ leads to the formation of recombination center, hence the photogenerated electron-hole pairs recombination efficiency increased.

Figure 2

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Fig. 2. The photoluminescence spectra of SnS2, 3DOM-SrTiO3 and (SnS2)n/3DOM-SrTiO3 catalysts.

Photocatalytic CO₂ reduction tests were conducted to evaluate the activity of the catalysts. The carbon-containing organic compounds of CO and CH₄ as the predominant reduction products were detected, and their curves of output over time are shown in Figs. 3a and b, respectively. All the catalysts possess the activity for CO₂ photoreduction as well as clean-energy production under the accordant conditions. 3DOM-SrTiO₃ catalyst exhibits the largest formation rates of CO product, while (SnS₂)₃/3DOM-SrTiO₃ catalyst has the largest formation rates of CH₄ product. Actually, there are two reactions (CO₂ reduction and H₂O oxidation) in the whole photocatalysis system. The two reduction reactions require to consume photogenerated electrons. Hence, the photocatalytic reduction of H₂O to H₂ occurs as a competitive reaction to the CO₂ photocatalytic reduction. In detail, 3DOM-SrTiO₃ catalyst exhibits the higher selectivity (90.4%) for CO₂ photoreduction in comparison with P25 (67.2%) and SnS₂ (71.2%) shown in Fig. 3c. It contributes to the three-dimensional ordered macropores structure in favor of CO₂ adsorption. Both P25 and 3DOM-SrTiO₃ catalysts have the higher CO selectivity (Fig. 3d), which are 61.1% and 58.9%, respectively. It is attributed to the wide band gap limited the electron-hole pairs separation and hindered the multielectrons reduction process. In contrary, the pristine SnS₂ catalyst possesses the narrow band gap performance with slightly higher CH₄ selectivity (51.4%). After introduction of SnS₂ nanosheets with narrow band gap, it is noted that the yield of CH₄ product is enhanced while CO product is decreased (Fig. 3d). It suggests that the SnS₂ nanosheets are able to act as electrons supplier, providing enough electrons for CO₂ reduction as well as CH₄ production. With the increasing dosage of SnS₂, the activity of (SnS₂)_n/3DOM-SrTiO₃ photocatalysts present a volcano variation trend. As shown in Table S3 (Supporting information), the yield of CH₄ over (SnS₂)_n/3DOM-SrTiO₃ catalysts reached to the highest amount about 12.5 μmol g^-1 h^-1 when the SnS₂ dosage is 3.0%. In addition, (SnS₂)₃/3DOM-SrTiO₃ catalyst also exhibits the best CO₂ reduction property (91.4%) as well as CH₄ selectivity (74.9%). The significant improvement of CH₄ product selectivity can be affected by the binary compounds, indicating that SnS₂ nanosheets play a key role to provide electrons in this photocatalytic CO₂ reduction system. It is noteworthy that the binary catalysts consisted of the SnS₂ and 3DOM-SrTiO₃ inherited the preeminent CO₂ photoreduction selectivity from 3DOM-SrTiO₃. The formed Z-scheme heterojunction effectively increased the photogenerated electrons and greatly facilitates the enhancing selectivity and yield of CH₄ product. The photocatalytic activity for CO₂ reduction decreased with further increasing the amount of SnS₂, indicating that the proportion of SnS₂ in 3DOM-SrTiO₃ is also crucial, the excess SnS₂ possesses negative effect to the photocatalytic reaction, which is attributed to the direct recombination of photogenerated electron-hole pairs in the SnS₂ nanosheets for decreasing photocatalytic performance of CO₂ conversion. Although SnS₂ can absorb the ultraviolet and visible light, the rapid recombination of photogenerated electronhole pairs limited its performance for CO₂ photoreduction. It is consistent with the results of UV–vis and PL spectra. In addition, the half reaction of H₂O oxidation to O₂ caused by valence band of semiconductor is also significant. Therefore, the produced protons participate in the hydrogenation reactions for the formation of hydrocarbons.

Figure 3

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Fig. 3. CO (a) and CH4 (b) products over SnS2, 3DOM-SrTiO3 and (SnS2)n/3DOM-SrTiO3 catalysts. (c) CO2 photoreduction selectivity as well as the products formation rate of CO, CH4 and H2. (d) The CO and CH4 selectivity.

To understand the process of charge separation as well as CO₂ photoreduction in this binary system, the pathway of electrons transmission and reactant conversion were proposed. As describes above, the energy gap (E_g) values of SrTiO₃ and SnS₂ are 2.94 eV and 1.76 eV, respectively (Fig. S3b in Supporting information). As shown in Fig. 4, the conduction band potential (E_CB) of SrTiO₃ (-1.36 V vs. NHE at pH 7) [46] and SnS₂ (-0.56 V vs. NHE at pH 7) [47] is more negative than the reduction potentials of CO₂ to CH₄ (-0.24 V) and CO₂ to CO (-0.53 V), hence both SrTiO₃ and SnS₂ could reduce CO₂ to CH₄ and CO in theory, which is consistent with the results of CO₂ photoreduction. Moreover, the adsorption capacity of CO₂ was enhanced by constructing the threedimensional ordered macropores structure in the present work.The perovskite type SrTiO₃ catalyst containing alkalis was employed, for the interaction built between Sr and CO₂, and which plays a significant role in the adsorption and activation of CO₂ [48]. In order to improve the photon capture efficiency, the constructed 3DOM structure with unique pores which could appear the slow light effect [49], as well as the introduced SnS₂ cocatalyst with narrow band gap is contributed to the excellent light adsorption. In addition, the separation and transfer efficiency of photogenerated carriers were enhanced by establishing Z-scheme heterojunction between SrTiO₃ and SnS₂.

Figure 4

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Fig. 4. Schematic diagram of electron-hole pairs separation and the possible reaction mechanism of SnS2/3DOM-SrTiO3 under simulative solar irradiation.

Herein, the binary (SnS₂)_n/3DOM-SrTiO₃ catalysts exhibit higher performance for photocatalytic CO₂ reduction than the SnS₂ and 3DOM-SrTiO₃ catalysts. Furthermore, the exist of SnS₂ in the binary structure (SnS₂/3DOM-SrTiO₃) absolutely enhanced the CH₄ selectivity compared with 3DOM-SrTiO₃, especially altered the main selectivity from CO (S_CO = 58.9% over 3DOM-SrTiO₃) to CH₄ (S_CH4 = 74.9% over (SnS₂)₃/3DOM-SrTiO₃). The SnS₂ with the narrow band gap contributed to the large absorption in visible light region, resulting in the efficiency utilization of photons and more free electrons to be excited. The electrons separate with holes at the valence band of SnS₂ and transfer to the conduction band, then transmit through the interface (between SnS₂ and 3DOM-SrTiO₃) to the conduction band of SrTiO₃ which effected by the internal electric field of SnS₂. Meanwhile, SrTiO₃ excited by ultraviolet light and the photogenerated electrons transfer from valence band to conduction band. It demonstrates that the Z-scheme transmission of photoexcited electrons enabled the electron-hole pairs recombination decreased and enriched the high energy electrons at the conduction band of SrTiO₃. In addition, the alkalis strontium on the surface of SrTiO₃ contributes to the adsorption of CO₂, resulting in the adequate adsorption intensity for CO₂ accepting multielectrons. Thus, the multi-electrons process of CO₂ reduction occurred, and the enhanced CH₄ selectivity due to the localization of electrons and the suitable CO₂ adsorption ability on the surface of SrTiO₃. The binary compounds possess the higher performance for CO₂ reduction and more multi-electrons process to form CH₄.

In summary, the present work proposed a novel binary SnS₂/3DOM-SrTiO₃ with Z-scheme heterojunction improved the photocatalytic carbon dioxide reduction performance. The threedimensional ordered macropores structure of 3DOM-SrTiO₃ facilitated the utilization of light, and the in situ introduced SnS₂ with narrow band gap is benefits to the boosting absorption of visible light. The atom Sr as the alkalis in the perovskite type SrTiO₃ is favor the adsorption and excitation of CO₂. In addition, the Z-scheme transmission of photoexcited electrons enabled the electron-hole pairs recombination decreased and enriched the high energy electrons at the conduction band of SrTiO₃, resulting the higher carbon dioxide reduction performance and more multi-electrons process to form CH₄, hence the predominant products selectivity altered from CO to CH₄. The study in this binary system provides a novel in-situ construction method and photogenerated electrons transfer strategy, the work may shed new light on the photoreduction performance and multi-electrons process improvement.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was supported by the National Natural Science Foundation of China (Nos. 21673142, 21972166), Beijing Natural Science Foundation (No. 2202045), PetroChina Innovation Foundation (No. 2018D-5007-0505) and Science Foundation of China University of Petroleum, Beijing (Nos. 242017QNXZ02, 2462018BJC005).

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.07.019.