Visible-Light Driven Overall Conversion of CO2 and H2O to CH4 and

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Visible-light driven overall conversion of CO2 and H2O to CH4 and O2 on 3D-SiC@2D-MoS2 heterostructure Ying Wang, Zizhong Zhang, Lina Zhang, Zhongbin Luo, Jinni Shen, Huaxiang Lin, Jinlin Long, Jeffrey C. S. Wu, Xianzhi Fu, Xuxu Wang, and Can Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09344 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Journal of the American Chemical Society

Visible-Light Driven Overall Conversion of CO2 and H2O to CH4 and O2 on 3D-SiC@2D-MoS2 Heterostructure Ying Wang1,2, Zizhong Zhang1*, Lina Zhang1, Zhongbin Luo1, Jinni Shen1, Huaxiang Lin1, Jinlin Long1, Jeffrey C. S. Wu3, Xianzhi Fu1, Xuxu Wang1*, Can Li4* 1State

Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350108, China. 2Key Lab of Inorganic Synthetic and Applied Chemistry, State Key Lab Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology, Qingdao 266042, China. 3Department of Chemical Engineering, National Taiwan University, Taipei 10617, China. 4State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.

Supporting Information Placeholder ABSTRACT: A marigold-like SiC@MoS2 nanoflower with a

unique Z-scheme structure efficiently achieves the overall conversion of gas phase CO2 with H2O (CO2 (g) + 2H2O (g) = CH4 + 2O2) without any sacrificial reagents under visible light (λ ≥ 420 nm) irradiation. The CH4 and O2 evolution are 323 and 621 μL·g1·h-1, and stable throughout 5 cycle reactions of total 40 h. This work demonstrates a breakthrough in artificial photosynthesis with the Z-scheme 1D heterojunction constructed by combining 2D semiconductor and 3D semiconductor based on the transfer balance of photogenerated electron and hole.

Visible-light driven overall conversion of carbon dioxide with pure water to methane and molecular oxygen is one of the most challenging reactions in the photocatalysis field for artificial photosynthesis.1 Enormous efforts have been made in developing photocatalysts to tackle the reaction in the last decades.2-5 However, so far very few photocatalysts could simultaneously and efficiently reduce CO2 to CH4 and oxidize H2O to O2 under visible light.6 Some photocatalysts show the CH4 yield from tens of micromoles to thousands of micromoles per gram photocatalyst per hour,7-9 but they could only work in the presence of a hole-scavenger (e.g. triethanolamine or triethylamine). Such photocatalytic reductions of CO2 taking place with sacrificial electron donors are inherently short-term, as they only work until the donor is used up.6 More importantly, such conversions may not be true solar-to-fuel conversion reactions.10 Many inorganic semiconductor materials have been reported to be active for the photoreduction of CO2 with pure H2O without sacrifice agent under visible light irradiation, such as the La2O3-modified LaTiO2N,11 the colloidal zinc oxidecopper (I) oxide 12 and the Pt sensitized blue TiO2,13 but their efficiencies are very low and the dioxygen as an indispensable product is not detected or much lower than stoichiometric ratio. The CO2 reduction without O2 evolution is incompatible and hard to be clearly understood. So the overall photocatalytic reduction systems, with a stronger emphasis on the oxidation half-reaction (e.g. generation of oxygen or hydrogen peroxide), still is an outstanding question, and represents the development direction and the ultimate goal of photocatalysis to utilize the solar energy. There are two intrinsic factors to affect the efficiency of the overall photocatalytic reaction of CO2 with H2O. Both half-

reactions, CO2 reduction and H2O oxidation, are multi-charge transfer processes, which requires simultaneous accumulation of multiple electrons and holes at two different sites on the surface of photocatalyst.14, 15 Moreover, the two half-reactions are coupled and interacted,16 which requires not only the fast and comparable migration rates of the photogenerated electrons and holes towards the surface, but also the fast and comparable surface reduction and oxidation reaction. This determines that high reaction efficiency is very hard to achieve with a simple photocatalyst. Integrating two narrow-bandgap semiconductors into the all-solid-state artificial Zscheme heterojunction photocatalysts has been considered as the most efficient solution to these obstacles.17 However, the key of research lies in how to select two suitable semiconductors and assemble them into an efficient Z-scheme structure based on the thermodynamic and kinetic requirements, which has been the subject of exploration since this design concept was introduced by Bard in 1979.18 Herein, we reported a novel and unique marigold-like SiC@MoS2 nanoflower photocatalyst for gas-solid photocatalytic reaction, CO2 (g) + 2H2O (g) = CH4 (g) + 2O2 (g). The CH4 evolution as high as 323 μL·g-1·h-1 along with nearly stoichiometric O2 evolution of 621 μL·g-1·h-1 could occur efficiently and stably under visible light without any sacrificial reagents. This is a record for simultaneously photocatalytic CO2 reduction to CH4 and H2O oxidation to O2 under visible light. Such excellent photocatalytic behaviour can be attributed to the following reasons. (i) The novel direct Z-scheme heterostructure can utilize more negative conduction band of SiC and more positive valence band of MoS2. (ii) The component MoS2 responsible for H2O oxidation has just higher hole mobility, while the component SiC for CO2 reduction has just higher electron mobility. (iii) The 1D heterojunction formed by perpendicular fixing of one side edges of 2D-MoS2 on the surface of 3D-SiC and the marigold flower-like morphology of SiC@MoS2 make the surface of SiC and MoS2 exposed and accessible to reactants to the most degree. (ⅳ) The reaction is operated in the gas-solid reaction mode beneficial for adsorption/desorption of reactants/products. The SiC@MoS2-x nanoflowers (x stands for the mass fraction of MoS2) were synthesized by a self-assembly of MoS2 monolayer nanosheets (MoS2 NSs) and SiC nanoparticles (SiC NPs) at room temperature (Figure 1a). The MoS2 NSs and SiC NPs were selfprepared and characterized by XRD, SEM, TEM, and AFM (Figure

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Figure 1 SiC@MoS2 nanoflower: (a) schematically synthetic process, (b) SEM images, (c) TEM images, (d) HRTEM images, (e) STEM images, (f) EDX elemental mapping images, and the overlapped mapping of (g) C-K + Si-K and (h) Mo-L+ S-K.

S1-S4). This self-assembling process takes place owing to the strong electrostatic attraction between the positively charged SiC NPs and the negatively charged MoS2 NSs, as indicated by zeta potential (Figure S5). The electrostatic repulsion between the negatively charged MoS2 NSs allows themselves to bond erectly rather than recumbently onto SiC surface to form the nanoflower morphology.19 Morphology of the SiC@MoS2-60% nanoflower as a typical sample was characterized by SEM and TEM. The average diameter of nanoflower is ca. 1 μm (Figure 1b and Figure S6). The nearly transparent MoS2 NSs can be seen (Figure 1c and 1d) at the outside layer of nanoflower. The lattice spacings of 0.61, 0.22, and 0.25 nm match well respectively with that of MoS2 (002), SiC (220) and SiC (111) (Figure 1d), suggesting that both MoS2 NSs and SiC NPs maintain their well-defined crystal structures in SiC@MoS2. The energy-dispersive X-ray spectroscopy (EDX) mapping images of the selected area (Figure 1e) are shown in Figure 1f-h. The more compact aggregation of Si+C than Mo+S confirms the nanoflower morphology composed of SiC NPs (core) and MoS2 NSs (shell). It can be deduced accordingly that a 1D interface is formed by the

perpendicular fixing of one side edge of 2D-MoS2 to the 3D-SiC surface for the SiC@MoS2 heterostructure. Such a unique 2D/3D combination makes the surface of SiC and MoS2 exposed and accessible to reactants to the most degree, compared to the common core/shell structure. The photocatalytic CO2 reduction with H2O vapour on the SiC@MoS2-x photocatalysts was investigated under visible light irradiation (λ ≥ 420 nm). CH4 was determined as the major carbonous product (Figure 2a), and a small amount of H2 was detected as a side product (Figure S7). All the SiC@MoS2-x samples show much higher CH4 evolution than their individual components MoS2 or SiC. These facts lead us to conclude that the heterostructure of SiC@MoS2 plays a crucial role in enhancing the photocatalytic activity. The optimized SiC@MoS2-60% photocatalyst shows the CH4 evolution up to 323 μL g-1 h-1, and the O2 evolution about 620 μL g-1 h-1 (Figure 2b). Furthermore, the SiC@MoS2-60% keeps constant CH4 evolution within at least 5 cycles during a total of 40 h reaction, implying the high stability of the photocatalyst. It is noteworthy that the molar ratio of O2/CH4 is close to 2, strongly suggesting that the stoichiometric reaction of CO2 with H2O, i.e. CO2 (g) + 2H2O (g) = CH4 (g) + 2O2 (g), was reliably achieved on SiC@MoS2 photocatalyst under visible light irradiation. To the best of our knowledge, this is the highest efficiency of CO2 reduction with pure H2O stoichiometric to CH4 and O2 under visible light irradiation reported to date (Table S1). For the brevity, hereafter, the photocatalyst SiC@MoS2-60% is simply noted as SiC@MoS2. The control experiment with 12CO2 or 13CO2 as the reactant verifies that CH4 is produced from the photocatalytic reduction of CO2 (Figure S8 and S9). No other possible carbon containing compounds such as CO, CH3OH, HCHO, and HCOOH were detected by gas chromatography, indicating a near 100% selectivity of product CH4. Nevertheless, some tiny peaks of HCOOH, CH3OH and HCHO could be detected by mass-spectrometry (Figure S10). These compounds are likely to be strongly adsorbed on photocatalyst surface as the possible reaction intermediates. This is well verified by another control experiment using small amount of these compounds (HCOOH, HCHO and CH3OH) as starting reactants under the same reaction conditions (Figure S11). CH4 and O2 are main products likewise and the molar ratios of CH4 to O2 are close to the theoretical stoichiometric ratios 2:3, 1:1 and 2:1, respectively. The isotope tracer control experiment using H218O as a reactant suggests that the product molecular oxygen is derived from both initially added H218O and the C16O2 reduction (Figure S10 and S12). 20-22 The wavelength-dependent quantum efficiency (QE) of CH4 evolution proves that the reaction is indeed a light-driven process (Figure 2c). The QE at 400 nm is estimated as 1.75 %,

Figure 2 (a) CH4 evolution on the photocatalyst SiC@MoS2 with diferent MoS2 content for 4h reaction, Each datum is an average of five separate trials performed under the same conditions, and the standard deviation of these measurements is indicated by the error bar, (b) Stability of SiC@MoS2-60% during five photocatalytic cycles, and (c) Wavelength-dependent QE of CO2 reduction to CH4 on SiC@MoS2-60%.

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Journal of the American Chemical Society being approximately ten times of the latest reported result (0.18%).7 The linearly dependent of the CH4 evolution rate on light intensity indiactes that the photocatalytic reaction has a rate-determining step that is one electron-driven chemical process (Figure S13).23 The bandgaps and CB potentials of SiC@MoS2 were determined by the UV-vis diffuse reflectance spectrum (Figure S14 and S15) and Mott–Schottky plots (Figure S16). The visible light absorption of SiC@MoS2 is extended to ~700 nm and enhanced with increasing MoS2 contents. The bandgap and CB potential of SiC are ca. 2.48 eV and -1.10 V (vs. SHE), respectively. The bandgap and CB potential of MoS2 are 1.77 eV and -0.32 V (vs. SHE), respectively. These are completely consistent with the reported results.24, 25 The CB and VB positions are staggered between SiC and MoS2. Since the work function of MoS2 (5.2 eV)26-28 is larger than that of SiC (4.0 eV)29, the two semiconductors are suitable to form a Z-scheme structure (Figure 3a), which can be supported by the Kelvin probe force microscope (Figure S17) and the X-ray photoelectron spectroscopy results (Figure S18). Upon irradiation, both SiC and MoS2 in SiC@MoS2 are excited to generate electrons and holes. The photogenerated electrons transfer in principle via the Z-scheme path with the red arrow (Figure 3a).30 As a result, the photogenerated electrons accumulate at the CB of SiC, and the holes at the VB of MoS2. Obviously, the Z-scheme model of SiC@MoS2 takes advantage of the high photoreduction potential of SiC and the powerful photooxidation ability of MoS2. Moreover, the hole mobility of MoS2 NSs (200 cm2 V-1 s-1) is much higher than its electron mobility (72.2 cm2 V-1 s-1),31-33 while the electron mobility of SiC NPs (700 cm2 V-1 s-1) is much higher than its hole mobility (90 cm2 V-1 s-1).34-36 This allows the photogenerated electrons and holes to transfer quickly towards the surface of SiC and MoS2, respectively. Higher hole mobility of MoS2 than SiC accelerates water oxidation on MoS2 surface. Likewise, higher electron mobility of SiC than MoS2 facilitates CO2 reduction on SiC surface. The photoinduced redox probe reactions, Au3+ + e−→ Au and Mn2+ + h+ + OH− → MnOx, verify the Z-scheme electron transfer on SiC@MoS2 under visible light.37 The Au particles and MnOx particles are clearly deposited on the surface of SiC NPs and MoS2 NSs (Figure 3b), respectively. This result convincingly verifies that SiC NPs mainly serves as active sites of photocatalytic reduction while MoS2 NSs as the photocatalytic oxidation sites. The solid Zscheme model is also favorable for the photogenerated charge separation, which was understood by elevated photocurrent (Figure S19) and decreased photoluminescence intensity (Figure S20) of SiC@MoS2. Regarding the photocatalytic reduction of CO2 with H2O, two main pathways were speculated,38, 39 i.e. the hydrogenation (CO2 → HCOOH → HCHO → CH3OH → CH4) and the deoxygenation (CO2 → CO → C• → CH3• → CH3OH/CH4). Based on the above

Figure 4 A reaction pathway of photocatalytic CO2 reduction with H2O on SiC@MoS2.

experimental results, the photocatalytic reduction on SiC@MoS2 follows the hydrogenation pathway, as described in Figure 4. One CO2 molecule transforms into one CH4 molecule by four sequential hydrogenation half-reactions undergoing HCOOH, HCHO, and CH3OH on SiC surface (seen the detail mechanism in Supplementary Results and Discussion). Each of the half-reaction consists of two elementary reaction steps involving one proton and one electron. The one-electron transfer to form the adsorbed carboxyl species (COOH•)ad is the rate-determining step of overall reaction due to the massive energy cost.40 The oxidation halfreaction matching to every hydrogenation half-reaction is H2O splitting to 1/2O2 and two protons (adsorbed-state) by two photogenerated holes on the MoS2. This mechanism is similar to that suggested for the photoelectrocatalytic reduction system of CO2 and supported by the theoretical study.38, 41 A high electron mobility leads to a fast sequential reduction of CO2 on SiC surface and thus a high selectivity of the final CH4 product. Meanwhile, a high hole mobility leads to a fast oxidation reaction of H2O into O2 on MoS2 surface. The ability of MoS2 for the photocatalytic H2O splitting to O2 was affirmed by the photocatalytic experiment in an aqueous solution containing AgNO3 sacrificial reagent with the bare MoS2 NSs as photocatalyst under visible light irradiation (Figure S21). The high activity and stability of SiC@MoS2 photocatalyst could be attributed to the much higher stability of MoS2 NSs than the bulk one.42

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details on the synthesis process, characterization and photocatalysis control experiments.

AUTHOR INFORMATION Corresponding Author Figure 3 (a) Z-scheme model of SiC@MoS2 photocatalyst and its charge transfer process under light irradiation, (b) TEM image of the SiC@MoS2 sample after co-deposition of Au and MnOx in the Au3+ and Mn2+ solution under visible light followed by an ultrasound dispersion (different components are distinguished with colors: red (Au), blue (MnOx), green (SiC) and violet (MoS2)).

[email protected]; [email protected]; [email protected]. ORCID Zizhong Zhang: 0000-0002-9541-8981 Xuxu Wang: 0000-0003-3228-109X

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Can Li: 0000-0002-9301-7850

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The work is financially supported by the National Natural Science Foundation of China (Grants No. 21673043, 21673042, 51702053, U1305242)

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