Vacancy-Defect Modulated Pathway of Photoreduction of CO2 on Quaternary Single Atomically Thin AgInP2S6 Sheets toward Boosting Efficient and Selective Production of Value-Added Olefiant Gas

Wa Gao, ‡ Shi Li, ‡ Huichao He, Xiaoning Li, Zhenxiang Cheng, Yong Yang, Jinlan Wang,* Qing Shen, Xiaoyong Wang, Yujie Xiong,* Yong Zhou,* Zhigang Zou 1 Key Laboratory of Modern Acoustics (MOE), Institute of Acoustics, School of Physics, Eco-materials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China 2 State Key Laboratory of Environmental Friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, China 3 School of Physics, Southeast University, Nanjing 211189, China 4 Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China 5 Institute of Superconducting & Electronic Materials, Innovation Campus, University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia 6 Key Laboratory of Soft Chemistry and Functional Materials (MOE), Nanjing University of Science and Technology, Nanjing 210094, China 7 University of Electrocommunication, Grad Sch Informatics and Engineering, 1-5-1 Chofugaoka, Chofu, Tokyo 1828585, Japan. 8 School of Science and Engineering, The Chinese University of Hongkong (Shenzhen), Shenzhen, Guangdong 518172, China


Introduction
Photocatalytic conversion of CO2 with H2O into solar fuels would be like killing two birds with one stone in terms of saving supplying energy and environment, which occurs mostly on the surfaces of semiconductors through complicated processes involving multi-electrons/protons transfer reactions. 1 Photo-driving CO2 hydrogenation into C1 species have been well achieved in recent decade, 2 and our group has exploited a series of promising photocatalysts to converse CO2 to selectively form specific hydrocarbons, such as Zn2GeO4 ultrathin nanoribbons for CH4, 3 atomically thin InVO4 nanosheets for CO, 4 and TiO2-graphene hybrid nanosheets for C2H6 5 and so on. However, the controlled C-C coupling to produce high-value C2 or C2+ products still remains a great challenge. Olefiant gas (Ethylene, C2H4) is a chemical source of particular importance due to its high demand in chemical industry. C2H4 is usually derived from steam cracking of naphtha under harsh production conditions (800−900 °C). It is definitely desirable for realization of C2H4 synthesis through mild and environmentally benign pathways. 6 Transition metal thio/selenophosphates (TPS) is a broad class of van der Waals  AgInP2S6 crystal possesses appropriate bandgap structure (Eg = ~2.4 eV), which is favored for visible light absorption. 9 The low value of the effective mass of electrons and the high value of the effective mass of holes facilitate to accelerate the mobility dynamics of photogenerated electrons onto the surface prior to holes, 10 which may enhance local electron density, benefiting for photo-driving reduction reaction. The centrosymmetry structure of AgInP2S6 also enables the photoexcited electrons to distribute on the surface of the layer crystal uniformly, 11 which may remarkably reduce the energy barrier for catalytic molecule activation, alter the catalytic reduction pathway, and enhance yield and enrich species of products.
An atomically thin 2D structure is an ideal platform to provide atomic level insights into the structure-activity relationship. 12 Firstly, ultrathin structure allows the photo-generated carriers to easily transfer from the interior to the surface with shortened charge transfer distance, decreasing the bulk recombination. Secondly, large surface exposure renders rich catalytic active sites. Thirdly, transparency resulting from ultrathin thickness helps for light absorption. Creation of vacancy defects in the ultrathin structure can also additionally enrich the reaction intermediates, resulting in low-coordinated atoms on the surface of catalyst, which are known to facilitate to generate multi-carbon species from CO2 photoreduction. 13,14 Herein, we report the synthesis of the AgInP2S6 single atomic layer (abbreviated as SAL) of ~ 0.70 nm in thickness through a facile probe sonication exfoliation of the corresponding bulk crystal (abbreviated as BC). The sulfur vacancy (abbreviated as VS) defects were introduced in the resulting SAL through an etching process with H2O2 solution (abbreviated as VS-SAL), which was prospectively utilized for photocatalytic reduction of CO2 in the presence of water vapor. While BC and SAL dominantly produce CO, the implemented defect engineering changes the reaction 5 pathway of the CO2 photoreduction on VS-SAL, which allows to steer CO2 conversion into C2H4 with the yield-based selectivity reaching ~73% and the electron-based selectivity as high as ~89%, and the quantum yield of 0.51% at wavelength of 415 nm.
Both DFT calculation and in-situ FTIR and demonstrate that the key step for the CO production on BC and SAL follows a conventional hydrogenation process of CO2 to form *COOH, which further couples a proton/electron pair to generate *CO. *CO easily liberates from the defect-free AgInP2S6 surface with low absorption energy to become free CO gas. In contrast, the introduction of VS in AgInP2S6 causes the charge accumulation on the Ag atoms near VS. Thus, the exposed Ag site in VS-SAL can effectively capture the forming *CO, making the catalyst surface enrich with key reaction intermediates to promote C-C coupling into C2 species with the low binding energy barrier. This work may provide fresh insights into the design of atomically thin photocatalyst framework for CO2 reduction and establish an ideal platform for reaffirming the versatility of defect engineering in tuning catalytic activity and selectivity.

Results
Structure characterization of the AgInP2S6 related samples. BC was synthesized through physical vapor transport in a two-zone furnace, which displays bright yellowis-brown color ( Figure S1a). The SAL was produced through mechanical exfoliation in ethyl alcohol solution through a probe sonication technique. The well-defined Tyndall effect of the resulting transparent solution of SAL indicates high monodispersity of the ultrathin sheets ( Figure S1b). Etching of SAL with H2O2 solutions allows to deliberately create VS on the surface of SAL. 15 The powder X-ray diffraction (XRD) pattern of BC and SAL agrees with the simulated one from the crystal structure of ICSD 202185 well with the P 3 ̅1c space 6 group ( Figure S2), 12 and no impurity peaks were detected. The stronger SAL peak intensity ratio of (002) to (112) relative to BC indicates that the exfoliation of solution treatment for optimized 10 seconds, the sulfur atoms, which locate outermost in SAL, can be partially etched away from the surface to form VS. The generation of VS was confirmed with the electron paramagnetic resonance (EPR) spectra ( Figure   S4). The TEM image shows that the resulting VS-SAL10 displays no any morphology change in ultrathin structure ( Figure S5). The corresponding EDS reveals that Ag, In, 7 and P contents were nearly stoichiometric 1:1:2 of AgInP2S6, expect S element less than the stoichiometric ratio ( Figure S6). It indicates that H2O2 treatment mainly leads to VS, and has no etching effect on other moieties, which was also verified with the following XPS and the X-ray absorption near edge structure (XANES) spectra. The atomic resolution, aberration-corrected high-angle annular dark-field scanning TEM transitions energies between 2460 and 2500 eV, which originates from the excitation of an electron from a 1S inner orbital to a higher-energy orbital as a result of interaction with an X-ray. In comparison with BC, SAL shows a shift for S K-edge 8 peaks to lower energy side. This can be explained by the fact that the core electrons of S become more loosely bound after mechanical exfoliation due to the increased screening of the nuclear charge. Through VS engineering, the S K-edge of VS-SAL10 can have a further small moving to lower energy side (Figure 2a). Moreover, the K-edge peak of P between 2100 to 2250 eV exhibits almost no differences among BC, SAL, and VS-SAL10 (Figure 2b), which is in good agreement with the above-mentioned XPS results.
The UV-vis diffuse reflectance spectra show that the band gap of SAL was determined 2.66 eV, a little larger than that of BC (2.31 eV) ( Figure S8), exhibiting strong quantum size effect in the lateral direction. VS-SAL10 displays slightly narrowed bandgap (2.57 eV) with respect to SAL. It derives from that introduction of VS may tailor the electronic structure of SAL through generating impurity states near the conduction band (CB) edge, which can be overlapped and delocalized with the CB minimum edge, leading to a reduced bandgap that may broaden the light absorption edge. 18,19 The Mott-Schottky plots reveals that the CB edge of VS-SAL10 upshifts by ~ 0.06 eV and ~0.26 eV, relative to that of SAL and BC, respectively, as schematically illustrated in Figure S9. All BC, SAL, and VS-SAL10 were thus confirmed to possess suitable bandgaps as well as the appropriate band edge positions for photocatalytic CO2 reduction under visible-light irradiation.
Photocatalytic performance of the AgInP2S6 related samples toward CO2 photoreduction. The photocatalytic CO2 conversion was carried out in the presence of water vapor under simulated solar irradiation ( Figure 3). CO was detected the major product for BC and SAL (Figures 3a and 3b). BC shows the CO yield of 2.44 μmol g -1 for the first hour and a trace amount of CH4 of 0.63 μmol g -1 (Figure 3a). 9 The photogenerated holes in the VB oxidize H2O to produce hydrogen ions by the reaction of H2O → 1/2O2 + 2H + + 2e -. CO is formed by reacting with two protons and two electrons (CO2 + 2e -+ 2H + → CO + H2O), and CH4 formation through accepting eight electrons and eight protons (CO2 + 8e -+ 8H + → CH4 + 2H2O). SAL exhibits 6.9 and 14.3-time enhancement of production of CO and CH4 relative to BC, reaching 17.1 μmol g -1 and 9.0 μmol g -1 for the first hour, respectively (Figure 3b).
The prerogative of atomic ultrathin geometry of SAL may be mainly responsible for the enhanced photocatalytic activity besides larger surface area, allowing charge carriers to move from interior to the surface quickly to conduct catalysis, avoiding the recombination in body. Small amount of C2H4 was also detected for SAL with the yield of 5.3 μmol g -1 . C2H4 is generated through accepting twelve electrons and twelve protons (2CO2 + 12e -+ 12H + → C2H4 + 4H2O). With the H2O2 etching process, excitingly, C2H4 excitingly becomes the main product for VS-SAL10 with the yield of 44.3 μmol g -1 (Figure 3c). The calculated yield-based selectivity reaches ~73%, and the electron-based selectivity is as high as ~89% 20 (Figure 3e). Meanwhile, CO and CH4 minority products were also traced with the yields of 10.9 μmol g -1 and 5.6 μmol g -1 , respectively, both less than the case of SAL. It indicates that the surface of VS-SAL10 preferentially promotes the C1 intermediates to C-C couple into C2 product rather than liberate into free CO and CH4 gases. The quantum yield of VS-SAL10 was measured 0.51 % at wavelength of 415 nm using monochromatic light (See the details in SI). The etching process time was found determinative for the dominant production of C2H4. The EPR measurement shows that the signal intensity gradually increases with prolonging etching time from 5 seconds to 15 seconds ( Figure S4), indicating being raised number of VS in VS-SAL. Elongation of the etching time from 5s to 10s was favorable for increasing yield of C2H4 ( Figure S10).
However, much long etching time of 15s decreases activity negatively, which may be due to that an excess of VS defects may accelerate the recombination of photogenerated carriers. 21 Reduction experiment of CO2 preformed in the dark or absence of the photocatalyst shows no appearance of CO and hydrocarbon products, proving that the reduction reaction of CO2 is driven by light under photocatalyst.
Blank experiment with identical condition and in the absence of CO2 shows no appearance of C2H4, CO, and CH4, proving that the carbon source was completely derived from input CO2. An isotope labeling experiment using 13 CO2 confirms that the produced C2H4 originates from the input CO2 ( Figure S11a). The O2 production was also detected using the similar isotope H2 18 O tracer control experiment ( Figure   S11b).

Mechanism of the excellent photocatalytic performance of the VS-SAL. DFT
simulations were performed to explore the VS-mediated catalytic selectivity mechanism toward CO and C2H4 on AgInP2S6. CO2 molecules are initially adsorbed on the catalyst surface where H2O molecules dissociate into hydroxyl and hydrogen ions at the same time. The free-energy profile for the photocatalytic CO2-to-hydrocarbon process with the lowest-energy pathway on the perfect AgInP2S6 surface was calculated, as shown in Figure 4. The key step for CO production is the hydrogenation of CO2 to form *COOH, and the free-energy change of the step is 0.48 eV. Subsequently, the reaction intermediate (*COOH) further couples a proton/electron pair to generate CO and H2O molecules. An adsorption energy of -0.07 eV of the produced *CO on the defect-free AgInP2S6 surface implies the physical adsorption on the catalyst ( Figure S12a). It means that *CO molecules can easily liberate from BC and SAL to become free CO gas, allowing high CO catalytic selectivity. Additional parts of *CO were continuously reduced by the incoming electrons and the successive protonation process to transform into CH4. 19,22 While the charge density of the valence band (VB) for pristine AgInP2S6 is evenly located on all the S and Ag atoms, contrastingly, the charge density of the VB is mainly located on the Ag atoms near the VS for VS-AgInP2S6, ( Figure S13). That is to say, the presence of VS in VS-AgInP2S6 causes the charge enrichment on the Ag atoms near the VS, which would benefit for stabilizing the reaction intermediates. For VS-SAL, VS can act as a trap for the *CO molecule, that is, the *CO molecule can chemically adsorb at exposed Ag sites with an adsorption energy of -0.25 eV (CO can only physically adsorb on the exposed P and In sites with distance of 2.56 and 3.20 Å, See Figure   S13b-13d). The higher CO onset desorption temperature on VS-SAL10 than SAL affirms the stronger absorption ( Figure S14). The absorbed *CO can be further protonated to successively form a series of key reaction intermediates with unsaturated coordination, which was confirmed with in-situ FTIR measurement ( Figure S15). The other *CO molecules produced on the surface diffuses toward VS and couple with those reaction intermediates to produce C2H4. The C2H4 free energy diagrams are summarized in Figure 4c, while the corresponding C-C coupling barriers are presented in Figure 4b. The different C-C coupling energy barriers were evaluated for three unsaturated reaction intermediates (*COH, *CHOH, and *CH2) (Figure 4b).
The coupling energy barrier with a value of 0.84 eV (*CO-CHOH) is lower than that of other coupling pathways (*CO-COH, 1.01 eV and *CO-CH2, 1.84 eV), hence the C2H4 will be produced via CO-CHOH coupling and hydrogenation. The whole free energy diagram shows that the process of *CO to *COH is regarded as the potential determining-step (0.86 eV). It should be especially emphasized that the detected small amount of C2H4 on SAL possibly originates from potential existence of the tiny number of VS in SAL, resulting from mechanically detaching sulfur atoms from SAL during the probe sonication exfoliation process. The reaction process for reduction of CO2 into C2H4, CO, and CH4 over VS-SAL under light illumination is thus proposed in Figure S16 50 ns), confirming that the surface VS can serve as surface separation centers for charge carriers and further promote the charge separation, therefore offering more opportunities for photocatalytic CO2 reduction. Transient photocurrent shows that the photocurrent intensity of SAL was enhanced with steadily repeating course due to promoted charge separation, compared with BC ( Figure S19a).
The highest photocurrent intensity of VS-SAL10 implies that the VS also makes effective contribution for saving carriers. Electrochemical impedance spectra (EIS) reveal that VS-SAL10 manifests the smallest semicircle in Nyquist plots ( Figure S19b), suggesting the lowest charge-transfer resistance, which permits fast transport of photoinduced charge.

Discussion
In summary, single atomically-thin AgInP2S6 layers were successfully synthesized through a facile probe sonication exfoliation of BC. The atomically thin structure of SAL, relative to BC, enables more charge carriers to mobile from the interior onto the surface and survivingly accumulate onto the active sites to improve the photocatalytic activity. While SAL exhibits obvious conversion efficiency with CO as the major product, the presence of VS in VS-SAL changes the CO2 photoreduction pathway to allow the dominant generation of C2H4. This work not only paves an effective approach for selectively producing multi-carbon product from CO2 photoreduction but also provides a new insight for catalyst design through vacancy defect engineering. The ampoule was kept in a two-zone furnace (680 → 600 ℃) for 1 week. 23 After the furnace was cooled down to room temperature, the AgInP2S6 crystalline powders could be found inside the ampoule ( Figure S1a). SAL were prepared by sonication-assisted liquid exfoliation processes from synthetic AgInP2S6 crystalline powders. SAL was immersed in H2O2 solutions with the of concentrations 0.1 mol/L inside which SAL were allowed to react with H2O2 for 5, 10, 15 s, referred to VS-SAL5, VS-SAL10 and VS-SAL15, respectively, at 25 °C. All the obtained samples were carefully washed and dried before use.
Characterizations. X-ray diffraction (XRD) (Rigaku Ultima III, Japan) was used to investigate the purity information and crystallographic phase of the as-prepared The volume of reaction system was about 460 ml. Before the irradiation, the system was vacuum-treated several times, and then the high purity of CO2 gas was followed into the reaction setup for reaching ambient pressure. 0.4 mL of deionized water was injected into the reaction system as reducer. The as-prepared photocatalysts were 16 allowed to equilibrate in the CO2/H2O atmosphere for several hours to ensure that the adsorption of gas molecules was complete. During the irradiation, about 1 mL of gas was continually taken from the reaction cell at given time intervals for subsequent CO, CH4 and C2H4 concentration analysis by using a gas chromatograph (GC-2014C, Shimadzu Corp., Japan).    Three kinds of possible C-C coupling pathways over AgInP2S6 containing Vs. (c) Gibbs free energy diagrams for CO reduction to C2H4 over AgInP2S6 with Vs. The insets show the corresponding optimized geometries for the reaction intermediates during the CO2 reduction process. Sulfur, phosphorus, indium, silver, carbon, oxygen, and hydrogen atoms are yellow, purple, lilac, gray, black, red, and white, respectively.