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Sep 25, 2018 - 2, Green Lake North Road, Kunming 650091 , China. ACS Appl. Mater. Interfaces , 2018, 10 (40), pp 34123–34131. DOI: 10.1021/acsami...
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Z‑Scheme Au@Void@g‑C3N4/SnS Yolk−Shell Heterostructures for Superior Photocatalytic CO2 Reduction under Visible Light Mengfang Liang, Timur Borjigin, Yuhao Zhang, Hui Liu, Beihong Liu, and Hong Guo* Yunnan Key Laboratory for Micro/Nano Materials and Technology, School of Materials Science and Engineering, Yunnan University, No. 2, Green Lake North Road, Kunming 650091, China

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S Supporting Information *

ABSTRACT: Au@g-C3N4/SnS yolk−shell Z-scheme photocatalysts are fabricated by a simple template-assisted strategy. The L-cysteine can offer the amine groups and meanwhile anchor on the surface of g-C3N4 during solvothermal reaction and thus contributes greatly to the enhanced carbon dioxide adsorption capability. This Zscheme photocatalytic reduction mechanism of Au@g-C3N4/SnS performs valuable functions in the reaction, leading to CH4 generation much earlier and higher concentration than that of Au@g-C3N4. Meanwhile, the unique yolk−shell structure can make the light bounce back and forth in the cavity and thus enhances the availability ratio of light. The application of small amount of noble metal cocatalysts and the large Brunauer−Emmett−Teller surface areas are also benefited for the enhanced photocatalytic activities. Hence, this novel material exhibits a distinguished reduction performance for CO2 reduction under visible light. The highest yields of CH4 (3.8 μmol g−1), CH3OH (5.3 μmol g−1), and CO (17.1 μmol g−1) can be obtained for the sample of Au@g-C3N4/SnS (SnS 41.5%), which is higher than other latest reported g-C3N4based photocatalysts for CO2 photoreduction including coupled with semiconductors and noble metal cocatalysts. This strategy might represent a novel way for the effective transition of CO2 to clean fuels and can also be enormous feasible utilization in the photocatalytic field. KEYWORDS: CO2 reduction, yolk−shell heterostructures, Z-scheme, Au@void@g-C3N4/SnS, visible light photocatalyst

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INTRODUCTION

and advanced reduction capacity to reduce carbon dioxides, remain urgently desirable. To work out these issues, developing the heterojunctions according to a direct Z-scheme-type mechanism has been proven to be an effective approach.12,17−21 In this process, the photogenerated electrons in the conduction band (CB) of coupled photocatalysts are injected into the valence band (VB) and submerge the holes of g-C3N4, which is benefited to facilitate the splitting of electron−hole and suppresses charge recombination as well. Furthermore, Z-scheme heterostructures show better photocatalytic performance compared with traditional heterojunctions because the e− on g-C3N4 and h+ on the coupled photocatalysts present enhanced reducibility and oxidability. For example, the previous synthesized Z-scheme g-C3N4/ ZnO21 and g-C3N4/MoS2 coupled semiconductors11 exhibit good performance for CO2 reduction and boosting solar-tohydrogen generation, respectively. Particularly, most recently, Yu et al. prepared g-C3N4/SnS2 photocatalysts, showing an enhanced photocatalytic capability for CO2 reduction.12 Undoubtedly, application of the Z-scheme-coupled semiconductors can significantly improve the photocatalytic

The reduction of carbon dioxide to methyl alcohol, methane, and carbon monoxide using the photocatalysis technique that converts solar energy into chemical fuels has gotten significant interest, on account of the sustainable strategy for the generation of clean fuels.1−4 Developing stable, high-efficient, inexpensive visible-light-driven photocatalysts can be considered as the critical factor for wide practical applications. Many inorganic semiconductor photocatalysts are employed for photocatalytic CO2 reduction, in which methods are on the basis of electron−hole pairs produced with semiconductor materials by illumination than semiconductor band gap.5−8 However, most of them cannot meet the requirements of real applications because of their poor response to visible light, and unsatisfied catalytic activities root in the rapid recombination rates of photoinduced carriers, such as TiO2 and CdS.9,10 Recently, a stable layered graphitic carbon nitride has been reported to show outstanding photocatalytic activity under visible light.11−17 Meanwhile, g-C3N4 that acted as a no metal photocatalyst can be prepared under mild reaction conditions, resulting of low cost. However, pure g-C3N4 exhibits insufficient photocatalytic performance because of its relative small surface areas and rapid charge recombination. Consequently, designing and researching the effective g-C3N4based vis-light photocatalysts, which own the steady structure © XXXX American Chemical Society

Received: June 6, 2018 Accepted: September 12, 2018

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DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces properties. Moreover, this unique Z-scheme-coupled semiconductors with the appropriate molecular architecture can also enhance remarkably their activity. In the midst of different nanostructures, hollow materials owning large specific surface areas and more active sites engage much attention. For instance, a hollow structure is better than solid particles of metal oxide photocatalytic properties.21−24 Particularly, because of well-defined internal cavities, large surface areas, and many active sites, hollow core−shell nanostructures have attracted wide attention compared to their solid counterparts. For example, the photocatalytic activity has prominently enhanced by using different sizes and morphologies TiO2based hollow materials.24−27 Our previously resultant yolk−shell Ag@TiO2 and hollow CeO2@Bi2WO6 microcapsules exhibited enhanced photocatalytic property.28,29 Despite these procedures are effective for the CO2 reduction, the provided improvements with photocatalytic activity are limited. Therefore, it is still very desirable to develop a controlled and durable hollow hybrid Zscheme g-C3N4-based photocatalyst to improve photocatalytic reduction of CO2. The two-dimensional transition-metal sulfides of SnS with confined band gaps of 1.0−1.3 eV have attracted wide attention on photocatalysis, and they can generate coupled semiconductor with g-C3N4 because they match with potential. For instance, SnS2/g-C3N430 and our fabricated SnS/SnS231 heterostructures exhibited improved photocatalytic activity under visible light. Though two-dimensional transition-metal sulfides have important application prospect in the fields of environment and energy, the reconstruction of the 2D structure seems inevitable because of its large surface energy and van der Waals force of attraction between the layers. Therefore, the nanosheets are easy to form large particles, which may reduce their catalytic activities. These problems can be solved by assembling two-dimensional nanoparticles into three-dimensional space construction. However, there have been no reports on the synthesis of 3D yolk−shell g-C3N4/SnS heterostructures for CO2 reduction under visible light so far. Herein, we design and prepare Z-scheme Au@g-C3N4/SnS yolk−shell heterostructures, which can enhance the photocatalytic activity for the reduction of carbon dioxide, as illustrated in Figure 1. As far as we all known, the manufacture of this yolk−shell Au@g-C3N4/SnS heterostructure has never been previously reported. First, Au@SiO2 was prepared, and then it acted as a template to manufacture uniform yolk−shell Au@g-C3N4 nanospheres. Subsequently, coupling SnS gave the desired Au@g-C3N4/SnS after heat treatment. Compared with other g-C3N4-based photocatalysts, this yolk−shell Au@gC3N4/SnS has more stable hollow configuration and higher surface area. Because of the internal electric field, noble metal effect, and surface electronegative property, this unique structure can improve the efficiency of charge separation and interfacial charge transfer. This empty frame leads to highefficient utilization of light sources through multistep reflections in its inner hole. With this material, it is expected to have a higher utilization rate, which is conducive to increasing the reduction of carbon dioxide in visible light.

Figure 1. Representative illustration of the assembling of Au@void@ g-C3N4/SnS yolk−shell heterostructures.

57.2% (wt %). The X-ray diffraction (XRD) patterns of the synthesized Au@SnS, Au@g-C3N4 references, and yolk−shell Au@g-C3N4/SnS samples with different ratios are shown in Figure 2a, indicating that most peaks of the product are lightly indexed in orthorhombic SnS (JCPDS no. 39-0354) and gC3N4. g-C3N4 exhibits two obvious peaks at 27.5° and 13.1°, which are allocated to the (002) and (100) interlayered reference, respectively. For the hybrids of g-C3N4/SnS, the patterns show the combination of two sets of diffraction results containing SnS and g-C3N4. With the increase of SnS loading, the intensity of g-C3N4 peaks becomes weaken. The Fourier transform infrared (FTIR) spectra of Au@g-C3N4/SnS with different concentrations are shown in Figure 2b. The peaks ranged of 1240−1600 cm−1 should be assigned to the characteristic carbon−nitrogen heterocyclic vibration in gC3N4, whereas the peaks at about 570 cm−1 are put down to Sn−S bond. These results are in consistent with XRD analysis and other reports.32−35 Scanning electron microscopy (SEM) images of Au@gC3N4/SnS (SnS 41.2 wt %) are shown in Figure 3a,b. The prepared samples are uniform nanospheres with an average size of 100 nm. The hybrid photocatalysts show a rough surface with coated g-C3N4. The Au core can be detected obviously, and its amount in coupled photocatalyst is measured as 0.52 wt % by atomic absorption spectroscopy (Hitachi Z-2000), in which loading is lower than most content of other reports. The product presents an obvious yolk−shell frame with the obvious inner core and void structure. Mass transport through the shells might lead to crack in some nanospheres. Meanwhile, the unique yolk−shell shapes are researched by a transmission electron microscope (Figure 3c−e). Its visible yolk−shell inner framework is detected distinctly. The size of the core is about 5 nm. SnS presents a nanosheet structure, and g-C3N4 displays an amorphous nature as expected, which is uniformly attached with SnS. A high-resolution transmission electron microscopy (HRTEM) micrograph (Figure 3e) of a portion of the sample also reveals that the value (0.21 nm) is assigned to (141) plane spacing of SnS, which is consistent with XRD results. The

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RESULTS AND DISCUSSION Microstructure of Yolk−Shell Au@g-C3N4/SnS Photocatalysts. The thermogravimetric analysis (TGA) shown in Figure S1 determines the Au@g-C3N4/SnS-coupled photocatalysts with SnS concentrations of 5.2, 19.4, 28.5, 41.2, and B

DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. XRD pattern (a) and FTIR spectra (b) of the synthesized Au@SnS, Au@g-C3N4 references, and yolk−shell Au@g-C3N4/SnS samples with different ratios.

Figure 3. SEM (a), TEM (b−d), HRTEM (e), and energy-dispersive X-ray mapping images [(f,g) the elements of Sn, C, and Au] of the fabricated yolk−shell Au@g-C3N4/SnS (SnS 41.2 wt %).

individually synthesized SnS presents a nanosheet aggregation, as shown in Figure S2, which is not as dispersive as the prepared Au@g-C3N4/SnS product. The main reason should be put down to the amino groups of g-C3N4-anchored Sn precursors. Meanwhile, the electrostatic attraction between the negative charge in g-C3N4 and Sn cations was also benefited to stabilize the Sn source. Therefore, nucleation centers of Sn are dispersive, and SnS presents confined growth with g-C3N4. The elemental mapping images (Figure 3g) detect the cooccurrence of Sn, C, and Au in the prepared yolk−shell Au@g-C3N4/SnS products. X-ray photoelectron spectroscopy (XPS) analyses are used to evaluate the chemical state and composition of yolk−shell Au@g-C3N4/SnS (Figure 4a). C 1s and N 1s peaks for g-C3N4 and Sn 3p, 3d, and S 2p peaks for SnS and all Au@g-C3N4/SnS composites with different contents are detected obviously. With the increase of SnS, the intensity of C peak was weaken, whereas that of Sn peak heightened. The Au 4f can also be detected in the products, which is in agreement with atomic absorption spectroscopy. High-resolution Sn 3d XPS peaks for all of the samples are shown in Figure 4b, displaying that the Sn 3d position splits to two parts at 485.6 (3d5/2) and 494.4 (3d3/2) eV. Its energy separation is 8.5 eV. Therefore, there are trace amounts of Sn4+ in the samples.36,37 The band energies of all of the Au@g-C3N4/SnS samples have a slight increase with the increase of g-C3N4 according to Figure 4c. The C 1s peak at 283.7 eV is corresponding to sp2 C−C bonds.38 The peak at 286.8 eV is attributed to N−CN, which shows a slight shift

to higher values for g-C3N4/SnS hybrids. This trend is as same as the other reported results.39 Figure 5 shows the adsorption/desorption isotherms of nitrogen and the pore size distribution of Au@g-C3N4/SnS (SnS 41.2 wt %) yolk−shell structures. The characteristic isotherm is as type IV. The Brunauer−Emmett−Teller (BET) surface area (85.32 m2 g−1) is larger than the reported g-C3N4and SnSx-based photocatalysts, which results in the obviously improved CO2 adsorption performance. The adsorption/ desorption curves of the Au@g-C3N4/SnS (SnS 41.2 wt %) sample without adding L-cysteine are shown in Figure S3. The BET surface area of the sample is 62.35 m2 g−1, and this value is lower compared with the prepared sample with adding Lcysteine. This result also implied that adding L-cysteine played an important role for the improved CO2 adsorption performance. Therefore, this unique yolk−shell-structured heterojunction adding L-cysteine has potential applications for enhanced reduction of CO2 in the process of photocatalystic reaction. Formation of Au@g-C3N4/SnS with Controlled Morphologies. We analyze the sample of Au@g-C3N4/SnS, as illustrated in Figure 1. Au nanoparticles are first fabricated as the core, and then the outer g-C3N4/SnS shell coated on the Au core by a general approach through the sacrificial silica layers. The Au@SiO2 core−shell structure is shown in Figure S4, displaying that the size of the prepared Au@SiO2 particle is C

DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Nitrogen adsorption/desorption isotherms of as-prepared yolk−shell Au@g-C3N4/SnS (SnS 41.2 wt %).

tions of 5.2, 19.4, 28.5, 41.2, and 57.2% (wt %) are exhibited in Figure 6. The visible photoabsorption intensity enhanced

Figure 6. UV−vis diffuse reflectance spectrum of Au@SnS, Au@gC3N4 references, and yolk−shell Au@g-C3N4/SnS samples with different ratios.

significantly with increasing the concentration of SnS. The band energies (Eg values) of Au@g-C3N4, Au@g-C3N4/SnS coupled photocatalysts contained SnS concentrations of 5.2, 19.4, 28.5, 41.2, and 57.2% (wt %), and Au@SnS are 2.65, 2.25, 2.12, 2.05, 1.98, 1.95, and 1.85 eV by Tauc relation, respectively. Therefore, they can promisingly serve on visible light photocatalysts. The unique hollow hybrid structure of Au@g-C3N4/SnS is the main reason for the narrowed band gaps, and XPS studies conform the formation of hybrid and also be in good accordance with other reports.12,31,32 This narrowed band gap is beneficial to accelerate the application of solar spectrum and enhance more effective photocatalytic activity over Au@g-C3N4/SnS hybrid photocatalysts. Photocatalytic Performance and Mechanism. The maximum CO2 adsorption capabilities of Au@SnS, Au@gC3N4, and Au@g-C3N4/SnS (SnS 41.2 wt %) are 0.17, 0.15, and 0.32 mmol g−1, respectively, as illustrated in Table 1, revealing that Au@g-C3N4/SnS exhibits higher value than Au@SnS and Au@g-C3N4. Combining with the above test of N2 adsorption/desorption isotherms, it could be concluded that the L-cysteine offered the amine groups and meanwhile anchored on the surface of g-C3N4 and thus contributes greatly to the enhanced CO2 adsorption performance. In fact, this

Figure 4. XPS spectra of the of as-prepared Au@SnS, Au@g-C3N4 references, and yolk−shell Au@g-C3N4/SnS samples with different ratios: (a) survey spectrum, (b) Sn 3d spectrum, and (c) C 1s spectrum.

about 100 nm, which is almost as same size as the final product. Silica is etched away by NH4HF2 and thus provides sufficient space for the crystallization of SnS in the shell. Our strategy offers a common approach to manufacture yolk−shell structures, and this tactics is also fitted for making other advanced yolk−shell hybrid structures. For example, Au@ TiO2/rGO (Figure S5), Au@SnO2/rGO (Figure S6), and Au@MoSe2/g-C3N4 (Figure S7) (in the Supporting Information for details), which all show excellent performance in the field of environmental catalysis and energy storage. Optical Properties. UV−vis absorption band edges of products with different mass ratios as Au@g-C3N4, Au@gC3N4/SnS coupled photocatalysts contained SnS concentraD

DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Maximum CO2 Adsorption Capabilities of the Au@ SnS, Au@g-C3N4, and Au@g-C3N4/SnS (SnS 41.2 wt %) sample

SBET (m2 g−1)

CO2 adsorption (mmol g−1)

Au@g-C3N4 Au@g-C3N4/SnS Au@SnS

78.25 85.32 93.35

0.15 0.32 0.17

pathway has been widespread used to improve the CO2 adsorption capacity of materials because of their large content of alkalinity.40 The enhanced CO2 adsorption capacity acts as the important role on the conversion of CO2 to hydrocarbons.12,41 The photocatalytic CO2 reduction is conducted with a 420 nm cutoff filter. No hydrocarbons are detected without photocatalysts or irradiation, suggesting that the illumination and effective photocatalysis are two necessary conditions in the photocatalytic process. The photocatalytic activities of CO2 reduction of Au@g-C3N4/SnS corresponding to different ratios are shown in Figure 7a, demonstrating that the reduction of CO2 occurs effectively with photocatalysts. The main photocatalytic products are important storable fuels of CO, CH3OH, and CH4. The reactions relative to CH3OH and CH4 needed higher redox potentials according to Table S1, and thus photocatalytic reduction involved more electrons. The analysis of apparent quantum efficiency (AQE) is shown in the Supporting Information. It is noticed that CH4 is not detected in the photocatalytic reduction products for individual Au@SnS, and the yields of CO and CH3OH are 3.02 and 5.07 μmol g−1, respectively. Though Au@g-C3N4 exhibits lower BET surface areas, weaker CO2 adsorption capacity, and visible light adsorption, as illustrated in Table 1, compared with Au@SnS, it shows better CO2 reduction effect. The yields of CH4 and CH3OH are 1.9 and 3.3 μmol g−1, respectively, implying that Au@g-C3N4 possesses intrinsic preeminent charge separation and transfer effect for the photoexcitation during the CO2 photocatalytic reduction. The low CB position and rapid electron−hole recombination of Au@SnS alone should be responsible for its inferior reduction ability of CO2. Particularly, the Au@gC3N4/SnS hybrids exhibit obviously enhanced photocatalytic CO2 reduction. With the content increasing of SnS, the reduction performance of CO2 remarkably increases and subsequently decreases. Au@g-C3N4/SnS (SnS 41.5%) delivers the highest yields of CH4 (3.8 μmol g−1), CH3OH (5.3 μmol g−1), and CO (17.1 μmol g−1), which are higher than other latest reported g-C3N4-based photocatalysts for CO2 photoreduction including coupled with semiconductors and noble metal cocatalysts,11−17,32−35 as shown in Table S2. Because the photocatalytic reduction difficulty of products of CH4 and CH3OH is higher than that of CO, we analyze the course of products of CH4 and CH3OH with different reaction times to study the photocatalytic mechanism under visible light, as shown in Figure 7b−d. For Au@SnS, only CH3OH is found from the initial time; however, CH4 is not detected at all even prolonged to 3 h. While for Au@g-C3N4, besides CH3OH, CH4 produces after 1 h under visible light irradiation. As for Au@g-C3N4/SnS (SnS 41.5%), both CH3OH and CH4 generated initially. Especially, the content of CH4 continuously increases obviously with the time prolongation, and the yield even arrives as high as 3.2 μmol g−1, which value is comparable to that of CH3OH in 3 h. Additionally, to investigate that methane and carbon monoxide are reduced from carbon dioxide, isotope tests with 13CO2 are developed according to

Figure 7. (a) Hydrocarbon generation rate in comparison with samples Au@SnS, Au@g-C3N4, and Au@g-C3N4/SnS for 4 h E

DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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specific surface area, and so forth. According to the above analysis, there is no significant difference in these factors. Therefore, the photocatalytic mechanism plays the key role during this process. On the basis of the results above, it is reasonable that the photoinduced electrons transfer to and accumulate on the g-C3N4 surface, resulting of the generation of CH4, which process needs much more electrons compared with the formation of CH3OH. As for the hybrid photocatalyst of Au@g-C3N4/SnS, the direct Z-scheme process should be responsible for the effective gathering of the photoinduced electrons, leading to CH4 generation much earlier and higher concentration than that of the Au@g-C3N4. Meanwhile, the rate of CH4 formation is faster than the product of CH3OH with the illumination time prolongation. The interactions between the hybrids contribute greatly to the separation electron−hole pairs and thus have significant effect on their photocatalytic activity. Therefore, photoluminescence (PL) spectra are applied to prove the enhanced separation efficiency of electron−hole pairs, as shown in Figure 10. The pure Au@g-C3N4 shows a strong emission peak

Figure 7. continued illumination. (b−d) Time courses of photocatalytic CH4 and CH3OH production over the prepared samples under visible light irradiation for 3 h.

other reports.42 The formed products are analyzed and should be assigned to 13CO and 13CH4, revealing that methane and carbon monoxide are photocatalytic-reduced from CO2 on the photocatalyst. The durability of Au@g-C3N4/SnS yolk−shell photocatalysts (SnS 41.2 wt %) was evaluated by five run cycles for CO2 photocatalytic reduction. The Au@g-C3N4/SnS sample was cleaned and dried before the cycling tests. The CH4 generation is more difficulty compared with CO and CH3OH, and thus the yield of CH4 was selected as products to investigate the stability (Figure 8). The yield of CH4 is about

Figure 8. Photocatalytic activity (yield of CH4) for CO2 reduction corresponding to cycling runs of Au@g-C3N4/SnS yolk−shell photocatalysts (SnS 41.2 wt %).

3.7 μmol g−1 after the fifth cycle without obvious decrease compared with the initial reaction (3.8 μmol g−1). Meanwhile, the XRD analysis also used to investigate the sample after 5 cycles as shown in Figure 9, revealing no transform of the photocatalyst after the reaction compared with the original fresh sample. These results exhibit that the prepared Au@gC3N4/SnS yolk−shell photocatalysts have good stability. As far as the different effects of the samples for CO2 reduction, photocatalytic activity is attributed to these factors such as optical properties, morphology, size, noble metal cocatalysts,

Figure 10. PL spectra of Au@SnS, Au@g-C3N4 references, and yolk− shell Au@g-C3N4/SnS samples with different ratios.

centered at about 460 nm, which is ascribed to the recombination of self-trapped excitations. The emission band becomes lower with the decreasing content of g-C3N4. The improved separation effect of e−−h+ pairs brings about the decrease of peak intensity. However, the content of SnS is larger than 30 wt % in the hybrids; no obvious change of intensity of emission peaks can be detected. The enhancement of separation efficiency of the photoinduced e−−h+ pairs is further implemented by surface photovoltage spectroscopy (SPS, Figure 11). Briefly, the strong SPS signal reflects high separation effect of photoinduced carriers in the light of SPS test.43 According to Figure 11, Au@SnS, Au@g-C3N4, and Au@g-C3N4/SnS (SnS 41.2 wt %) show strong response in the visible light zone. Particularly, Au@g-C3N4/SnS (SnS 41.2 wt %) shows the most intensive SPS responses. The electrochemical impedance spectroscopy (EIS) is also used to confirm the high efficiency in blocking the recombination of photoinduced e−−h+ pairs of Au@g-C3N4/SnS (Figure 12). The impedance significantly decreases than that of Au@SnS and Au@g-C3N4, suggesting that Au@g-C3N4/SnS exhibits improved separation effect of electron−hole pairs and the faster interfacial charge transfer. Therefore, this heterostructure

Figure 9. XRD pattern of the Au@g-C3N4/SnS yolk−shell photocatalysts (SnS 41.2 wt %) after 5 cycles. F

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respectively. The VB edges of Au@SnS and Au@g-C3N4 are 1.45 and 1.31 eV, respectively. So that, the CB potentials of Au@SnS and Au@g-C3N4 are calculated as −0.40 and −1.34 eV, respectively, according to ECB = EVB − Eg, in which values are much more negative than the reduction potentials of the formation of CH4, CH3OH, and CO. However, CH4 has not been detected in the products for Au@SnS alone even after 3 h, implying that the amassing and extraction of photoinduced electrons are not offered because of quick charge recombination. If according to the conventional heterojunction pathway, the photoinduced electrons of g-C3N4 of the Au@g-C3N4/SnS composites would transfer to the CB of SnS and thus, CH4 will not generate. However, CH4 can be obviously detected in the products because of the beginning of photocatalytic reduction in the first hour according to Figure 7d. Therefore, conventional heterojunction will be unfit for the prepared Au@g-C3N4/SnS hybrids. The transfer and separation of photoinduced e−−h+ pairs keep a direct Z-scheme route, and the quick combination is accomplished between the e− in the CB of SnS and the h+ in the VB of Au@g-C3N4. Therefore, the remainder e− in g-C3N4 secures its strong reduction ability and promotes the photocatalytic CO2 reduction under visible light irradiation. At the same time, it should be noticed that Au in the photocatalyst also has a significant effect on photocatalytic CO2 reducing as electron-transfer mediators.44−46 The photoexcited electrons in the CB of SnS of yolk−shell Au@g-C3N4/ SnS transferred to Au and then to the VB of g-C3N4 and subsequently recombined with photogenerated holes in gC3N4. Synchronously, photogenerated electrons in the CB of g-C3N4 and holes in the VB of SnS exhibited strong reduction and oxidation capacity. This ternary structure of Au@g-C3N4/ SnS is benefited for the separation and transplantation of photoexcited e− and h+, resulting enhanced catalytic performance.45 The electrons in g-C3N4 could reduct CO2 to form CO, CH3OH, and CH4, and the process is described as following47−49

Figure 11. SPS of Au@SnS, Au@g-C3N4 references, and yolk−shell Au@g-C3N4/SnS (SnS 41.2 wt %).

Figure 12. EIS of Au@SnS, Au@g-C3N4 references, and yolk−shell Au@g-C3N4/SnS (SnS 41.2 wt %).

of Au@g-C3N4/SnS makes increase of the separation efficiency for photoinduced charge carriers. On the basis of the above analysis and together with the former reckoned Eg value of samples, the approach of transference of photoinduced carriers is presented in Figure 13, showing the schematic exhibition of band structure diagram of Au@g-C3N4/SnS (SnS 41.2 wt %). The Eg values of Au@SnS and Au@g-C 3 N4 are 1.85 and 2.65 eV,

hν

g‐C3N4@SnS → g‐C3N4(e− + h+)/SnS(e− + h+)

(1)

g‐C3N4(e− + h+)/SnS(e− + h+) → g‐C3N4(e−)/SnS(h+) (2)

2H 2O + 4h+ → O2 + 4H+

(3)

CO2 + ne− + nH+ → CO or hydrocarbon + mH 2O (4)

Futhermore, the synthesized pristine Au@g-C3N4 and Au@ SnS might also boost the photoinduced electron−hole recombination because of the limited energy band gap. Therefore, their catalytic capabilities are not comparable to that of Au@g-C3N4/SnS. Moreover, the unique yolk−shell structure can make the light bounce back and forth in the hole, which improved the availability ratio of light. The application of small amount of noble metal can improve the transfer and separation of photoinduced e−−h+ pairs.50 The large BET surface areas are also benefited for the enhanced photocatalytic CO2 reduction performance. These results manifest that the present yolk−shell Au@g-C3N4/SnS is hopeful as superior vislight photocatalysts.

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CONCLUSIONS Yolk−shell Au@g-C3N4/SnS photocatalysts are synthesized by a simple template-assisted method. The Z-scheme mechanism,

Figure 13. Schematic illustration of band structure diagram and photoinduced carrier transfer of yolk−shell Au@g-C3N4/SnS under visible light irradiation. G

DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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heterostructures, noble metal effect, and large surface areas promote the efficiency of interfacial charge separation and transfer. The unique yolk−shell structure can make the light bounce back and forth in the cavity, which enhanced the availability ratio of light. Therefore, the outstanding photocatalytic effect for CO2 was observed. This study may offer a novel pathway to address the formation and charge transfer mechanism of g-C3N4-based photocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b09455. Experimental section, characterization, evaluation of photocatalytic properties, analysis of AQE, TGA pattern of the products; SEM images of the obtained pure SnS nanosheets; nitrogen adsorption/desorption isotherms of as-prepared yolk−shell Au@g-C3N4/SnS without adding L-cysteine (SnS 41.2 wt %); Au@SiO2 core− shell structures; TEM images of Au@TiO2/rGO; Au@ SnO2/rGO; Au@MoSe2/g-C3N4; the reduction potentials and comparison of the photocatalytic activity compared with other reports (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-871-65032180. Fax: +86-871-65036626. ORCID

Timur Borjigin: 0000-0002-7642-2088 Hong Guo: 0000-0001-5693-2980 Author Contributions

M.L. and T.B. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS National Natural Science Foundation of China (no. 21467030), Key Natural Science Foundation of Yunnan Province China (no. 2018FA028), and the Program for Outstand Young Talents (2018) of Yunnan University.

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DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b09455 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX