Ultrathin Conductor Enabling Efficient IR Light CO2 Reduction

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Ultrathin Conductor Enabling Efficient IR Light CO2 Reduction Xiaodong Li, Liang Liang, Yongfu Sun, Jiaqi Xu, Xingchen Jiao, XiaoLiang Xu, Huanxin Ju, Yang Pan, Junfa Zhu, and Yi Xie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10692 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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Journal of the American Chemical Society

Ultrathin Conductor Enabling Efficient IR Light CO2 Reduction Xiaodong Li†, Liang Liang†, Yongfu Sun*, Jiaqi Xu, Xingchen Jiao, Xiaoliang Xu, Huanxin Ju, Yang Pan, Junfa Zhu, Yi Xie*

Hefei National Laboratory for Physical Sciences at Microscale, National Synchrotron Radiation Laboratory, Key Laboratory of Strongly-Coupled Quantum Matter Physics, University of Science and Technology of China, Hefei 230026, China. Fax: 86 551 63606266; Tel: 86 551 63603987; E-mail: [email protected], [email protected]. †These authors contributed equally to this work. KEYWORDS : conductor, ultrathin, intraband-interband transition, IR light, CO2 reduction

ABSTRACT: Concurrent transformation of carbon dioxide and water into hydrocarbons and oxygen by low-photonic-energy IR light still represents a huge challenge. Here, we design an ultrathin conductor system, in which the special partially occupied band serves as the mediator to simultaneously guarantee IR light harvesting and satisfied band-edge positions, while the ultrathin configuration improves charge separation rates and surface redox kinetics. Taking the low cost and earth abundant CuS as an example, we first fabricate ultrathin CuS layers, where temperature-dependent resistivities, valence-band spectrum and theoretical calculation affirm their metallic nature. Synchrotron-radiation photoelectron and ultraviolet-visible-near infrared spectra unveil metallic CuS atomic layers could realize a new cooperative intraband-interband transition under IR light irradiation, where the generated electrons and holes could simultaneously involve the carbon dioxide reduction and water oxidation reactions. As a result, CuS atomic layers exhibit nearly 100% selective CO production with an evolution rate of 14.5 μmol g-1 h-1 under IR light irradiation, while the catalytic performance shows no obvious decay after 96 h test. Briefly, benefiting from ultrahigh conductivity and unique partially occupied band, the abundant conductor materials such as conducting metal sulfides and metal nitrides hold great promise for applications as effective IR light responsive photocatalysts.

INTRODUCTION Through mimicking plants, artificial photocatalysts can directly transform carbon dioxide and water into useful fuels and oxygen at ambient conditions.1-5 To achieve this important goal, the photocatalysts need to satisfy the prerequisite requirement of band edges matching with the potentials of carbon dioxide reduction and water oxidation.6-8 In addition, considering additional overpotentials associated with the two half-reactions of carbon dioxide reduction and water oxidation, the suitable band gaps of photocatalysts are estimated to be 1.8– 2.0 eV, which theoretically precludes the utilization of low photonic energy IR light occupying ca. 50% of the solar radiation. In other words, it is quite hard for the traditional single-component photocatalysts to trigger concurrent transformation of carbon dioxide and water into hydrocarbons and oxygen under IR light irradiation, since the photonic energy of IR light is less than 1.55 eV.9 As such, designing band-new IR responsive photocatalyst systems based on novel basic principles is highly imperative, with efforts to achieve boosted IR light driven CO2 reduction performances. Interestingly, the great variety of conductors appear to be the most promising candidates for IR light responsive photocatalysts relative to the conventional semiconductors, thanks to their extremely high carrier density, almost no band gap and special partially occupied band. Generally speaking, one of the key parameters to promote photocatalytic activities

is how to increase the carrier concentration, in which the conductors often possess several orders of magnitude higher carrier density than the semiconductors.10 Moreover, in addition to possessing the fully occupied band (B-1) and the lowest unoccupied band (B1) similar to the semiconductors, the conductors also have an additional partially occupied band, which could be similarly termed as the conduction band (CB) (Scheme 1).11 Furthermore, the very small or even zero band gaps endow the conductors with very efficient IR light harvesting ability,12 which can afford one of the prerequisites for triggering IR-light-driven CO2 reduction; meanwhile, the electron-hole pairs could also be created by the partially occupied band mediated transition process, i.e. the individual transition process (I or II in Scheme 1) or the consecutive transition process (III or IV in Scheme 1). This implies that the conductors could potentially realize IR light induced concurrent splitting of carbon dioxide and water into hydrocarbons and oxygen as long as the potentials of electrons and holes, excited by any of the above transition processes, satisfy the redox potentials. More particularly, the consecutive transition process mediated by the partially occupied band could not only gain wider band edge positions in the condition of guaranteeing IR light absorption, but also mutually benefit each other through positive feedback effects. Taking the consecutive intrabandinterband transition process as an example (IV in Scheme 1),

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the IR light induced intraband transition process causes the electrons to accumulate near the Fermi level in CB band, where these metastable electrons could facilitate the following interband transition process from the Fermi level in CB band to the B1 band; at the same time, the intraband transition process could be further triggered once the electrons near the Fermi level are consumed by the interband transition process. In spite of these several advantages, to date, the single-component conductor has never achieved IR-light-driven photocatalytic reactions,10,11,13 especially for CO2 photoreduction reactions. This is probably ascribed to the fact that the electron-hole pairs are usually created by the individual transition process, which can not simultaneously achieve IR light absorption while keeping redox potential matching, and meanwhile the high carrier density inevitably causes the conductors to suffer from serious charge recombination, thus leading them unable to provide abundant electrons and holes for participating the photocatalytic redox reactions. Inspired by the above analysis, we design an ultrathin conductor system, in which the special partially occupied band serves as the mediator to simultaneously guarantee IR light harvesting and appropriate band edge positions for matching redox potentials, while the ultrathin configuration helps to improve the charge separation rates and surface redox kinetics. Taking the low cost and earth abundant metallic CuS as an example,14-16 we fabricate two-unit-cell thick CuS layers for the first time, where the temperature dependence of resistivities, valence-band XPS spectrum and DFT calculations demonstrate their metallic nature. Synchrotron-radiation photoemission and UV-vis-NIR spectra disclose the metallic CuS atomic layers could realize a new cooperative intraband-interband transition under IR light irradiation, where the generated electrons and holes could simultaneously involve the two half-reactions of CO2 reduction and H2O oxidation. In-situ Fourier transform infrared (FT-IR) spectroscopy unveils the COOH* is the key reduction intermediate during IR light irradiation, while Gibbs free energy calculations reveal the rate-limiting COOH* formation step possesses a modest barrier of 1.49 eV. Accordingly, CuS atomic layers possess nearly 100% selective CO production with an evolution rate up to 14.5 μmol g-1 h-1, which is the first report showing the sole conductor could trigger IR light CO2 reduction, while their photocatalytic activity has no obvious decay after testing for 4 days. Accordingly, this study affords a promising strategy for designing high photocatalytic CO2 reduction performance. RESULTS AND DISCUSSION In this work, hexagonal sheet-like metallic CuS atomic layers (Figure 1 and Figure S1) were first successfully synthesized, in which oleylamine (OM) and octylamine (OTA) played crucial roles in reducing metal ions as well as enforcing a hexagonal sheet-like morphology (Scheme S1). XRD pattern for the obtained samples in Figure S2A matched well with hexagonal phase CuS (JCPDS Card No. 78-0877). X-ray photoelectron spectra in Figure S3 illustrated the presence of Cu2+ and S2-,17 while the element mapping in Figure 1D-F verified the

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homogeneous distribution of Cu and S elements. Meanwhile, Raman spectrum in Figure S2B displayed a distinct peak at 474 cm-1, which could be ascribed to the S-S stretching of CuS crystals,18 while the absence of any other additional peaks suggested there was no carbon impurity, proved by the corresponding FT-IR spectrum in Figure S4. Moreover, the obtained products exhibited hexagonal sheet-like structures, as revealed by TEM image in Figure 1A. HRTEM image in Figure 1C depicted their lattice plane spacings were 0.326 nm with a dihedral angle of 60°, which demonstrated their orientation along the [001] projection. The side view of TEM image in Figure 1B confirmed the average thickness of the CuS atomic layers was ca. 3.25 nm, in accordance with the theoretical twounit-cell thickness of hexagonal CuS along [001] orientation (Figure S1). Thus, the above results demonstrated the fabrication of two-unit-cell thick CuS layers with hexagonal sheet-like morphology. As a contrast, CuS bulk was also fabricated by calcining the mixture of Cu and S powder in the vacuum quartz tube (Figure S2-3 and Figure S5). To disclose the metallic nature of the as-obtained CuS atomic layers with two-unit-cell thicknesses, theoretical and experimental characterizations were performed and shown in Figure 2. Figure 2A depicted that the electrical resistance for the CuS atomic layers increased with increasing temperatures, which implied their strong metallic character. Meanwhile, the resistance in the whole temperature range was very small, suggesting the high electron mobility in the CuS atomic layers. In addition, valence-band XPS spectrum in Figure 2B further validated the metallic nature of the CuS atomic layers, from which one could clearly see that the valence band maximum originated from the Cu 3d states and S 3p states passed through the Fermi level (0 eV).10,13 To further demonstrate the metallic feature of the as-prepared CuS atomic layers, DFT calculation was employed. As displayed in Figure 2C, the Fermi level in the calculated band structure of the two-unit-cell thick CuS slab crossed the conduction band, which further corroborated its metallic character. For such a metallic material of CuS atomic layers, the partially occupied band was considered as the conduction band (CB), while the highest fully occupied band and the lowest unoccupied band could be regarded as the B-1 and B1 bands, respectively.11 In addition, as displayed in Figure 2D, the corresponding calculated density of states (DOS) could be also divided into B1, CB and B-1 bands, respectively. In other words, the region above 1.4 eV could be attributed to the B1 band, while the region below -3.6 eV could be ascribed to the B-1 band. Meanwhile, the remaining CB band could be further divided into two regions: (a) the region of -3.6 ~ -1.1 eV was the non-bonding state arising from Cu 3d state with an obvious peak at -2.5 eV, which was in accordance with I XPS peak in Figure 2B; (b) the region between -1.1 eV and the Fermi level was stemmed from the strongly hybridized Cu 3d and S 3p states, showing another high peak at -0.4 eV. Most importantly, the DOS of 11.75 states eV-1 atom-1 at the Fermi level once again confirmed the metallic character of CuS atomic layers, which was well coincided with the valence-band XPS spectrum in Figure 2B. Thus, all the above results confirmed the metallic feature of the CuS atomic layers. For comparison, Figure S5

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also provided the temperature dependence of resistivities and calculated band structures for the bulk CuS, which also took on a metallic character. Ultraviolet-visible absorbance spectra and synchrotronradiation photoelectron spectroscopy (SRPES) were further conducted to unveil the electronic band structures of CuS atomic layers and CuS bulk (Figure 3A-B), with efforts to investigate the effect of metallic feature on their CO2 photoreduction performances (Figure 3C-E). Ultraviolet-visible absorbance spectra in Figure 3A described that both the CuS atomic layers and the CuS bulk possessed intense absorption ranging from ultraviolet region to infrared region, while their absorption energy gaps were about 1.01 and 1.53 eV (Figure S6), respectively. Meanwhile, according to the secondary electron cutoff (Ecutoff) measured by SRPES specrta in Figure 3B, we could gain the Fermi level potential of CuS atomic layers and CuS bulk was -4.97 eV and -4.55 eV (vs. Vacuum), respectively. Of note, as previously reported for metallic materials with small band gaps or without energy gaps, the electrons and holes could also be generated by virtue of the interband transitions.11 As for the CuS atomic layers (Figure 3F), it appeared unlikely that the photon transition for 1.01 eV was from the B-1 band to the Fermi level in the CB band (~4 eV, Figure 2C), while it was quite possible from the Fermi level to the B1 band (~1.4 eV). Contrastingly, the large bandgap energy of 1.53 eV for bulk CuS theoretically precluded its interband transition from the Fermi level to the B1 band under IR light irradiation (Figure S7). Interestingly, as indicated by the obvious absorbance in IR light region (Figure 3A), both the CuS atomic layers and bulk CuS could realize the intraband transition in the CB band under IR light irradiation.19-22 Based on the above analysis, the electronic band structures vs. normal hydrogen electrode at pH=7 for the CuS atomic layers and the bulk CuS could be achieved and displayed in Figure 3F and Figure S7, from which one can conclude that only the former could realize carbon dioxide reduction and water oxidation simultaneously (Figure 3C) under IR light irradiation.23 That is to say, as for the metallic CuS atomic layers, both the intraband transition and the interband transition processes could be realized under IR light irradiation (Figure 3F), where the generated electrons in the B1 band could reduce CO2 to CO, while the produced holes in the CB band could oxidize H2O to form O2. By contrast, the large band gap for CuS bulk failed to induce the interband transitions and hence it could not trigger IR light driven CO2 reduction (Figure S7). To evaluate the IR light catalytic activity for the as-obtained metallic CuS atomic layers, CO2 reduction experiments were conducted using Xenon lamp fitted with a 800 nm cut-off filter to simulate IR light (the detailed description of photocatalytic CO2 reduction measurement and the illumination spectrum of our light source was displayed in Methods and Figure S8). As shown in Figure 3C, the CuS bulk showed almost no IR light catalytic activities, which was fairly consistent with the prediction of its calculated band structure (Figure S7), where the interband transition (1.53 eV) from Fermi level in CB band to the B1 band was unable to be realized by absorbing the low-

energy IR light. By contrast, the metallic CuS atomic layers exhibited favorable IR light catalytic activities (Figure 3C), in which the CO yield monotonously improved with the increasing illumination time, and the rate of CO production (14.5 μmol g-1 h-1) was ca. 5 times larger than that of previously reported oxygen-deficient WO3 atomic layers.9 Notably, no any other gas or liquid products were detected in the reaction process, thus giving a near 100% CO selectivity. As far as we known, this is the first report showing the single conductor could trigger CO2 reduction under IR light irradiation. In addition, as revealed by apparent quantum efficiencies in Figure S9, the metallic CuS atomic layers showed an efficiency up to 0.05% at 800 nm, representing a new record for IR light driven CO2 reduction, while the bulk counterpart owned negligible efficiencies, which further affirmed the former's excellent IR light driven catalytic performance. Also, metallic CuS atomic layers did not show any gas or liquid products in dark or in N2 or without photocatalyst, corroborating the IR irradiation and the photocatalyst were imperative for carbon monoxide formation. To further validate the origin of the products, we employed synchrotron radiation vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). As displayed in Figure 3D and Figure S10, the 13CO2 labeling experiment ascertained the produced CO was stemmed from CO2 photoreduction. Moreover, in-situ thermographic photographs in Figure S11 clearly depicted that the 12 h IR light irradiation on the metallic CuS atomic layers only caused a slightly increased temperature from 16.6 to 17.9 ºC during a catalytic cycle, implying the IR light driven CO2 reduction could be realized at ambient temperatures. Besides, the CuS atomic layers could achieve water oxidation to form O2 (Figure 3C), further verified by the H218O labeling experiment (Figure S12), and the average evolution rate of O2 and CO was about 1:2, in consistent with the following reaction equations: CO2 + 2H+ + 2e- → CO + H2O (Eq. 1) H2O + 2h+ → 1/2O2 + 2H+ (Eq. 2) Furthermore, the IR light catalytic activities for the CuS atomic layers had no obvious decay after 8 consecutive cycling test up to 96 h (Figure 3E), which indicated their exceptional photostability, further confirmed by the corresponding postreaction characterizations of TEM images, XRD patterns, XPS spectra and Raman spectra in Figure S13-14. It is worth noting that the cycling curves "bend" down in each run, which may be caused by adsorbed CO molecules on the catalyst surface (Figure 4A-B), since the CO desorption step is an endothermic process (Figure 4C); when a new catalytic run begins, the gas in the system will be pumped and the adsorbed CO molecules will be desorbed from the catalyst surface in this process, so the catalytic performance can be maintained. Of note, the boosted IR light CO2 reduction performance may come from a new cooperative intraband-interband transition process in the metallic CuS atomic layers. As displayed in Figure 3F and Figure 4D, under IR light irradiation, electrons in the partially occupied CB band could be initially excited to the Fermi level via an intraband transition process, while these metastable electrons could be further excited to the lowest

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unoccupied B1 band by an interband transition process for participating the following CO2 reduction reaction; meanwhile, the remained holes in the CB band could involve the other halfreaction of H2O oxidation to produce O2 (Figure 3C). Intriguingly, the consumed electrons by the interband transition process could induce the formation of holes near the Fermi level, which in turn facilitates the intraband transition process, and vice versa. In other words, both of these intraband and interband transition processes could not only take place alone during IR light irradiation, but also mutually promote each other by positive feedback effects, which remarkably improve the IR light absorption as well as the carrier utilization. Besides, the IR light CO2 reduction is also benefited by the favorable CO2 reaction kinetics in the metallic CuS atomic layers. Moreover, in situ FT-IR spectra were conducted to probe the reaction intermediates during CO2 reduction under IR irradiation. As depicted by Figure 4A-B for the metallic CuS atomic layers, two obvious IR peaks gradually appeared and strengthened with extending the irradiation time, where the peak at 1545 cm-1 was attributed to the typical intermediate of COOH* for CO2 reduction to CO.24-26 Also, the peak at 1675 cm-1 could be attributed to asymmetric stretching of HCO3-*.25,27,28 According to the in situ FT-IR analysis, the CO2 photoreduction pathways for the metallic CuS atomic layers may be proposed as follows: * + CO2 → CO2* (Eq. 3) CO2* + H++e- → COOH*

(Eq. 4)

COOH* + H+ + e- → CO* + H2O

(Eq. 5)

CO* → * + CO↑ (Eq. 6) To achieve deeper insight into the reaction process, DFT calculations were further implemented on these possible reaction steps. The CO2 photoreduction process initiated with the proton-coupled electron transfer to the adsorbed CO2 molecules for producing COOH* intermediate, and the subsequent protonation of COOH* led to the formation of CO*, which would finally desorb from the surface of CuS atomic layers to generate free CO molecules. As shown in Figure 4C and Tables S1-2, the COOH* formation process could be regarded as the rate-determining step, in which the barrier energy of 1.49 eV was relatively mild for the CO2 photoreduction. Meanwhile, as for CO* formation, it was an exothermic process (-1.14 eV), which could occur spontaneously. And the finally desorption energy of CO molecules was fairly low (0.29 eV), which indicated the relatively easy desorption of CO* molecules from the surface of CuS atomic layers, further confirmed by their corresponding CO TPD spectrum in Figure S15. CONCLUSION In conclusion, we designed an ultrathin conductor system, in which the special partially occupied band acts as the mediator to simultaneously ensure IR light harvesting and satisfied bandedge positions, while the ultrathin configuration contributes to promote charge separation rates and surface redox kinetics. As an example, the low cost and earth abundant CuS atomic layers were first fabricated, in which temperature dependence of

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resistivities, valence-band XPS spectrum and DFT calculations affirmed their metallic nature. Synchrotron-radiation SRPES and UV-vis-NIR spectra disclosed the CuS atomic layers could realize a new cooperative intraband-interband transitions under IR light irradiation, where the generated electrons and holes could involve the CO2 reduction and O2 evolution reactions. In situ FT-IR spectra disclosed the COOH* is the key reduction intermediate under IR light irradiation, while Gibbs free energy calculations unveiled the rate-limiting COOH* formation step had a modest barrier of 1.49 eV. As an outcome, the CuS atomic layers performed nearly 100% selective CO production with an evolution rate up to 14.5 μmol g-1 h-1, which was the first report showing the metallic conductor could trigger IR light CO2 reduction, while their photocatalytic activity did not occur any obvious deactivation even after 4 days. The material design methodology as proposed in this work should be applicable to a variety of highly conducive materials that have tunable partially occupied band, for instance metal nitrides and sulfides.

EXPERIMENTAL SECTION Preparation of CuS Bulk 1 mmol Cu power and 1 mmol S powder were added into the mortar and grinded uniformly. Then the above mixture was transferred into quartz tube, vacuumized and sealed up. The system was kept at 450 °C for 48 h, and then cooled to room temperature. The collected blue-black powder was the CuS bulk. Preparation of CuS Atomic Layers 150 mg CuCl was mixed with 5 mL octylamine and 5 mL oleylamine in the flask (50 mL). The system was heated at 100 °C for 30 min with N2 flowing. Then the solution was heated up to 130 °C for 3 h. And then 144 mg S powder dispersed in 2.5 mL octylamine and 2.5 mL oleylamine was rapidly injected into the above system and kept at 95 °C for 5 h. The final product was washed with ethanol and cyclohexane for many times until the organic residuals were completely removed, and then dried in vacuum. Computational Details The calculations were conducted by the Vienna ab initio simulation package.29-30 The ion-electron interaction was shown by projector augmented wave (PAW) potentials. The generalized gradient approximation (GGA) in the Perdew– Burke–Ernzerhof (PBE) form was performed to treat the exchange–correlation between electrons.31 For accurate density of the electronic states calculations, the screened hybrid functional proposed by Heyd, Scuseria, and Ernzerhof (HSE)32 was adopted, and a 10 × 10 ×1 for sheet k-point mesh were used with 520 eV plane wave cutoff energy. Γ-point was used for gas phase molecules. The accuracy of the conventional energy (104 eV) and force (0.01 eV/Å) convergence criteria was required for ionic relaxations. The two-unit-cell slab along the [001] projection with 1.5 nm vacuum layer was built to simulate the

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synthesized 3.25 nm thick layers. All the calculations are nonspin-polarized calculations. To calculate the Gibbs free energies of each gaseous and adsorbed species, we adopted the expression as G = EDFT + EZPE – TS (Eq. 7) Where EDFT refers to the DFT calculated electronic energy, EZPE and TS stand for the zero-point energy and the entropy contribution, respectively. The EZPE and TS for each system were calculated at standard ideal gas conditions. To simulate catalytic process, the energy of the protonelectron pair for each reaction step is treated as a function of the applied potential relative to the computational hydrogen electrode (CHE).33 Thus, the corresponding free energy changes can be obtained by ΔG[COOH*] = G[COOH*] + G[H+ + e-] – (G[*] + G[CO2] + 2 × G[H+ + e-]) (Eq. 8) ΔG[CO*] = G[CO*] + G[H2O] – (G[*] + G[CO2] + 2 × G[H+ + e-]) (Eq. 9) G[H+ + e-] = 1/2G[H2] – eU (Eq. 10) Where the U refers to the applied overpotential and e stands for the elementary charge. In this study, U = 0 V vs. RHE. IR light CO2 reduction test Before performing the CO2 photoreduction performance, we fabricate the sample into a thin film: the sample was dispersed in deionized water to gain a concentration of about 1 mg mL−1; then, through spin-dropping 5 mL of the above dispersion on a quartz glass, followed by heat treatment at 65 °C for 30 min, the CuS-based thin film could be achieved. During the CO2 photoreduction process, the CuS-based thin film floated on the 50 mL water in a quartz glass vessel with the homothermal condensate water, which could enable the CuSbased thin film to remain a constant temperature of 290 ± 0.2 K. A CEL-HXF300 Xe lamp (Beijing China Education Au-light Co., Ltd.) with AM 1.5 G filter and 800 nm cutoff filter was used to simulate infrared light, in which the corresponding illumination spectrum compared with sunlight was displayed in Fig. S8. And the photon flux of IR light used in our photocatalysis measurements was obtained based on the following formulas: 𝑁

 = 𝑆𝑇 N=

𝑃𝑇𝜆

ASSOCIATED CONTENT Supporting Information Details of additional characterizations, DFT results, XRD patterns, TEM images and CO-TPD results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]

NOTES The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work was financially supported by National Key R&D Program of China (2017YFA0207301, 2017YFA0303500), National Natural Science Foundation of China (U1632147, 11621063, U1532265), Youth Innovation Promotion Association of CAS (CX2340000100), the Fundamental Research Funds for the Central Universities (WK2340000063, WK2340000073), the Fok Ying-Tong Education Foundation (161012), Key Research Program of Frontier Sciences of CAS (QYZDY-SSW-SLH011), and Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2017FXCX006). Supercomputing USTC and National Supercomputing Center in Shenzhen are acknowledged for computational support.

ℎ𝑐

REFERENCES

P =ES λ=

light, λ refers to wavelength,λ refers to an average wavelength, E is the average optical power density, E is total radiation intensity, E(λ) is the spectrum radiation intensity. According to theE measured by the optical power meter (Beijing China Education Au-light Co., Ltd), we could obtain the photon flux  of 1.78  1017 s-1 cm-2. Prior to irradiation, the reactor was washed with high-purity CO2 for several times to ensure it was only saturated with CO2. The gas products were quantified by Techcomp GC7900 gas chromatograph (FID detector, TDX-01 column). The liquid products were quantified by nuclear magnetic resonance (NMR) (Bruker AVANCE AV III 400) spectroscopy, in which dimethyl sulfoxide (DMSO, Sigma, 99.99%) was used as the internal standard. Recirculating cooling water system was carried out to keep the photocatalytic system at 290 ±0.2 K under light irradiation.

∫𝛥𝜆𝜆𝐸(𝜆)𝑑𝜆 𝐸

where  is the photo flux, N is the incident photon number, S stands for the illumination area, T refers to the illumination time, h corresponds to the Planck constant, c stands for the speed of

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Scheme 1. Possible phototransition processes (I to IV) in the conductors, holding promise for simultaneously ensuring IR light

harvesting and appropriate band edge positions matched with CO2 reduction and H2O oxidation potentials. B-1: the highest fully occupied band; B1: the lowest unoccupied band; CB: conduction band. h+ stands for the produced hole, erefers to the generated electron, and the black line in the CB band represents the Fermi level.

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Figure 1. Characterizations for the Hexagonal Sheet-like CuS Atomic Layers. (A) Top view and (B) side view of TEM images; (C) HRTEM image; (D)-(F) annular dark-field TEM images and the corresponding elemental mapping images.

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Figure 2. Theoretical and Experimental Results Concerning Band Structure and Electrical Property. (A) Temperature dependence of resistivities and (B) valence-band X-ray photoelectron spectroscopy (XPS) for the CuS atomic layers. (C) Calculated band structure and (D) density of states (DOS) of two-unit-cell thick CuS slab.

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Figure 3. Experimental Band Structure and IR-driven CO2 Properties for Metallic CuS Atomic Layers. (A) UV-vis-NIR diffuse reﬂectance spectra; (B) secondary electron cutoff (Ecutoff) measured by SRPES spectra, where the work function can be calculated by Φ = hν - Ecutoff, hν is the incident photon energy of 40 eV; (C) products of photocatalytic CO2 reduction; (D) the SVUV-PIMS spectrum of the products after 13CO2 photoreduction for the CuS atomic layers at hυ = 14.5 eV. (E) Cycling measurements for CO2 photoreduction to CO (each cycling for 12 h) of the CuS atomic layers. (F) Schematics illustrating the electronic band structures of the CuS atomic layers.

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Figure 4. CO2 Photoreduction Mechanism for Metallic CuS Atomic Layers. (A-B) In situ FTIR spectra for co-adsorption of a mixture of CO2 and H2O vapor, (C) free energy diagrams of CO2 photoreduction to CO, and (D) the corresponding scheme for the photoreduction CO2 into CO on the CuS atomic layers. Positive feedback: mutual promotion of interband and intraband transition.

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TOC

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Ultrathin Conductor Enabling Efficient IR Light CO2 Reduction

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