carbon nitride 2D heterostructure

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Materials and Interfaces

A specifically-exposed cobalt oxide/carbon nitride 2D heterostructure for carbon dioxide photoreduction Xingwang Zhu, Haiyan Ji, Jianjian Yi, Jinman Yang, Xiaojie She, Penghui Ding, Li Li, Jiujun Deng, Junchao Qian, Hui Xu, and Hua-ming Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04123 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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A specifically-exposed cobalt oxide/carbon nitride 2D heterostructure for carbon dioxide photoreduction Xingwang Zhua,b, Haiyan Jic, Jianjian Yia,b, Jinman Yanga,b, Xiaojie Sheb, Penghui Dinga,b, Li Lia, Jiujun Dengb, Junchao Qianc, Hui Xub,*, Huaming Lib a School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China b Institute for Energy Research, School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China c School of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou, 215009, China Corresponding Authors *Tel.: +86-511-88799500. Fax: +86-511-88799500. E-mail: [email protected]. Electronic

Supplementary

Information

(ESI)

available:

Characterization,

Photoelectrochemical measurement, AFM images, SEM images, SEM-mapping images of COCN, XRD patterns of the 5 wt% COCN before and after reaction, TEM images of Co3O4 exposed [011] facets, Production rate of CO, CH4 and H2 using Co3O4, 5 wt% COCN, Co3O4-[011], or 5 wt% COCN-[011] as catalysts, GC-MS analysis of the CO generated from the 13CO2.


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Abstract: Photocatalytic reduction of CO2 provides an opportunity to reach carbon neutrality, by which CO2 emissions from fuel consumption can be converted back to fuels. The challenge is to explore materials with high charge separation efficiency and effective CO2 adsorption capacity to boost the photoreduction of CO2. Here we report that 2D heterostructure comprised of Co3O4/2D g-C3N4 (COCN) can provide enhanced photocatalytic performance of reducing CO2 to CO, yielding a CO production rate of 419 μmol g-1 h-1 with selectivity of 89.4%, which is 13.5 and 2.6 times higher than that of pure 2D g-C3N4 and Co3O4. The enhanced photocatalytic performance arises from: (i) enhanced light absorption ability and charge separation efficiency originated from the unique 2D heterostructure connected through specifically-exposed facet interface and (ii) favorable CO2 adsorption capacity. The study may provide insight for the establishment of heterostructure-based photocatalytic system toward CO2 reduction. Keywords: Composite photocatalysts, CO2 conversion, Co3O4, g-C3N4 1. Introduction The increasing consumption of fossil fuel along with massive emission of carbon dioxide (CO2) make human face intractable crisis of energy and environment.

1, 2

Photocatalytic conversion of CO2 into hydrocarbon fuels represents a promising strategy to meet this challenge since it could reduce the amount of CO2 as well as obtain energy-bearing product.

3, 4

Actually, despite tremendous efforts invested over

the past decades, realization of rendering this technology commercially viable is still a mirage due to negligible solar to energy conversion efficiency.


5-7

Among various


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strategies to enhance photocatalytic CO2 reduction activity, construction of semiconductor heterostructures by combining two semiconductors with matched crystalline lattice and energy band structure is considered as one effective method, since it can not only improve the limited light absorption ability of bare wide-band semiconductors, but separate the charge carriers on different side of heterostructures, thereby enhancing the final performance. 8-10 Graphitic carbon nitride (g-C3N4), as a nonmetallic n-type organic semiconductor with layered structure, has been successfully employed in photocatalytic CO2 reduction reaction since the high conduction band (CB) position favors the reduction half-reaction.

11-13

However, the pristine g-C3N4 obtained by thermal polymerization

is generally bulky materials with low surface area ( 200 m2 g-1), short charge migration distance and increased reduction ability.

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However, 2D g-C3N4

possesses narrow photoresponse (< 450 nm) on account of the quantum confinement effect,

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and thereby limits the total CO2 reduction efficiency. Inspired by the

advantages of heterostructure discussed before,

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coupling 2D g-C3N4 with other

narrow band gap semiconductors will extend the photoresponse, promote the charge separation efficiency, and enhance the charge density on the surface-active sites. Cobalt oxide (Co3O4), as a typical p-type semiconductor with the feature of low-cost, thermodynamical/chemical stability, narrow band gap and tunable microstructure,


25,

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26

is a considered as a promising candidate to couple with 2D g-C3N4 satisfying the

requirements discussed above. In the hybrid structures, the quality and structure of interface where charge carriers transferred largely determine the charge migration efficiency, thus holding the key to the photocatalytic performance. 27 Generally, in semiconductor heterostructures, the formation of high-quality interface between two components requires well-matched crystalline lattices, imposing a fundamental constraint for the construction.

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2D heterostructure, unlike the case of conventional semiconductor

heterostructure, could construct high-quality interface regardless lattice mismatch.

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Moreover, the 2D-2D contact can also increase the interfacial contact area and shorten the charge migration distance from bulk to surface, which favors the enhancement of surface charge density, thus improving the photocatalytic performance. 30 Besides the quality of the interface, the interfacial structure such as interfacial facet is another important parameter which would hold great influence to the performance after rational design.

31

Taking Co3O4 as an example, the [112] exposed facets 2D Co3O4

hexagonal platelets are demonstrated to boost photocatalytic CO2 conversion efficiently along with an ideal photosensitizer.

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When Co3O4 is excited by sunlight,

high-density electrons tend to congregate on the surface [112] facet.

33

Similarly,

when an interface is formed on the exposed [112] facet of Co3O4, high-density electrons will congregate on the interface, then inject into the other component to participate in surface redox reaction based on matched band structure.

32, 33

Based on

the discussion above, we surmise that the construction of 2D heterostructure



comprised of 2D g-C3N4 and Co3O4 hexagonal platelets with exposed [112] facets would achieve a breakthrough in photocatalytic reaction. In such designed 2D heterostructure, high-density electrons excited by Co3O4 can congregate on the Co3O4 [112]-2D g-C3N4 interface, and then migrate and merge with the photogenerated electrons of 2D g-C3N4 through high-quality interface. Subsequently, these high-density electrons gathered on 2D g-C3N4 participate in the redox reaction to convert CO2 into fuel effectively. However, the controllable synthesis of 2D heterostructure with specifically exposed facet as interface is challenging. In this work, we precisely prepared the 2D heterostructure comprised of 2D g-C3N4 nanosheets and [112] facet exposed Co3O4 hexagonal platelets by a liquid nitrogen-assisted thermal oxidation method. As a proof of concept for the photocatalytic application, the CO2 reduction performance of as-prepared photocatalysts was measured. Significantly, we found that the Co3O4/2D g-C3N4 exhibits enhanced activity for CO2 reduction compared to bare 2D g-C3N4, in which the optimal Co3O4/2D g-C3N4 exhibits CO production rate reaching 419 μmol g-1 h-1 with selectivity of 89.4%. The rationally tailored 2D heterostructure enables high-density electrons excited from Co3O4 with wide photoresponse congregating and migrating through the Co3O4 [112]-2D g-C3N4 interface to the surface of 2D g-C3N4, thus greatly enhancing the surface charge density for CO2 reduction. Furthermore, the Co3O4/2D g-C3N4 (COCN) also possesses favorable CO2 adsorption capacity. This work may provide insight toward the design of heterostructure-based photocatalytic system for CO2 reduction or other applications.


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2. Experiment 2.1. Chemicals All the chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (China), and used directly without any purification. De-ionized (DI)water was used in all the experiments. 2.2. Preparation of the photocatalysts Synthesis of 2D g-C3N4:

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Firstly, the bulk g-C3N4 powder was synthesized by

thermal condensation of melamine under air atmosphere at 823 K with a heating rate of 2 K min-1 for 4 h. The obtained yellow bulk g-C3N4 (400 mg) powder was put into a combustion boat and heated at 823 K for ~120 min, with heating rate of 5 K min-1 under air atmosphere in a self-open muffle furnace. After the reaction, the obtained white powder is denoted as 2D g-C3N4. Synthesis of β-Co(OH)2: 0.58 g of Co(NO3)2·6H2O and 0.4 g of poly(vinyl pyrrolidone) (PVP, M.W.≈ 55000) were added into 20 mL of an aqueous solution containing 10 mL H2O and 10 mL ethanol under vigorous stirring for 30 min. Afterwards, 20 mL NaOH solution (0.4 M) was slowly dropped into it, with the color change from red to blue. Subsequently, the mixture was transferred into a 25 mL Teflon-lined autoclave and heated at 473 K for 12 h, then cooled down to room temperature. The obtained pink product was collected and washed using water and ethanol for several times and finally vacuum-dried for 12 h. Synthesis of Co3O4/2D g-C3N4: Firstly, 0.1 g of 2D g-C3N4 and a certain amount of β-Co(OH)2 were added in a beaker with 10 mL H2O under constant stirring to form



homogeneous mixture. Then the mixture was quickly soaked in liquid nitrogen to freeze the sample. The Co3O4/2D g-C3N4 was prepared by heating the obtained product at 573 K for 2 h in a muffle furnace in air atmosphere. The different weight ratios of Co3O4 in the 2D heterostructures were synthesized respectively and denoted as 1 wt%, 3 wt%, 5 wt% and 7 wt% COCN. With the same condition, when the temperature is 773 K, the preferential [011] facets of Co3O4-[011] and COCN-[011] are formed. Synthesis of Co3O4: Bare Co3O4 was prepared by directly heating β-Co(OH)2 in a muffle furnace without adding 2D g-C3N4 as shown above. 2.3. Photocatalytic activity measurement The photocatalytic CO2 reduction reaction was carried out in a 300 mL pyrex reactor based on an online system (Labsolar-6A, PerfectLight, Beijing) (Figure S5). The reaction system containing 2,2-bipyridine (bpy) (15 mg), catalysts (10 mg), 2 μmol CoCl2, and solvent (6 mL acetonitrile, 4 mL H2O and 2 mL TEOA) was stirred with a magnetic stirrer and irradiated

25, 34, 35

at 283 K under a 300 W Xenon lamp

(PLS-SXE 300C (BF), Perfectlight, Beijing). The reactor was first purged with high purity CO2 and the pressure of CO2 was regulated to 0.75 MPa. Gaseous products analysis was performed with a Shanghai KeChuang Chromatograph Instruments Co., Itd. GC-2002 gas chromatography system was equipped with a capillary column (length 30 m, inner diameter 0.32 mm) and a thermal conductivity detector. The selectivity in CO production is evaluated based on the required electrons using the following equation: Selectivity (%) = [2ν(CO)]/[2ν(CO) + 8ν(CH4) + 2ν(H2)] × 100%,


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where ν(CO), ν(CH4) and ν(H2), represent the yielding rates for ν(CO), ν(CH4) and ν(H2), respectively. 3. Results and discussion The construction route of COCN 2D heterostructure is illustrated in Figure 1. Firstly, the 2D g-C3N4 was prepared by exfoliation of bulk g-C3N4 obtained from polymerization of melamine. On the other hand, the β-Co(OH)2 hexagonal platelets as precursor of Co3O4 were prepared through a solvothermal reaction using Co(NO3)2 as precursor with the help of PVP and pH controlling agent (NaOH). Finally, The COCN was prepared employing a liquid nitrogen-assisted thermal oxidation method. The liquid nitrogen pre-treatment aims to prevent the stacking of g-C3N4 nanosheets, thus retaining the 2D structure, while the in-situ formation of Co3O4/2D g-C3N4 from β-Co(OH)2/2D g-C3N4 by thermal oxidation ensures the construction of tight interface between two components. The transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscope (AFM) were employed to observe the microstructure of as-prepared catalysts. The 2D g-C3N4 exhibits typical 2D structure with wrinkled and rough surface (Figure 2a), and the thickness is measured of ~1 nm (Figure S1a). The flexibility and large surface area of 2D g-C3N4 are beneficial for the imitate contact with other component, leading to good interface transport and immobilization of catalysts. As another component, the Co3O4 also shows 2D hexagonal platelets structure with the size of ~200 nm and thickness of ~28 nm (Figure 2b, Figure S1b, Figure, S2a). It can be clearly observed the Co3O4 platelets



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are uniformly distributes over the surface of 2D g-C3N4 nanosheets (Figure S2b, c). The chemical composition of COCN was probed by energy dispersive spectrometer (EDS) and elemental mapping (Figure S2d, Figure S3), which indicate that four elements including C, N, Co and O are homogeneously distributed over the 2D heterostructure. The large interfacial contact between Co3O4 and 2D g-C3N4 can be observed with the increase of magnification (Figure 2c). Furthermore, the dominant exposed facet of Co3O4 in COCN as interface is demonstrated by HRTEM (Figure 2d) and corresponding fast Fourier transform (FFT) image (Figure 2e). Obviously, the successful exposure of [112] facets with well ability to congregate high-density electrons is determined, which are normal to both the set of (220) and (222) planes with a lattice space of 0.276 and 0.231 nm respectively.

33

As discussed above, the

successful establishment of COCN 2D heterostructure with [112] facet of Co3O4 as interface is identified. After confirming the microstructure of COCN, the phase and chemical structure were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). In the XRD pattern (Figure 3a), the broad peak at ca. 27.3° can be indexed to the (002) plane of g-C3N4,

19

while all the other peaks can be attributed to hexagonal

Co3O4 (JCPDS: 43-1003) without other impurity peaks.

36

The disappearance of the

(100) peak ascribed to g-C3N4 at ca. 13.1° is due to the typical 2D structure.

19

The

chemical state of COCN was further analyzed by XPS (Figure 3b-e). Similar to the EDS results, the XPS survey spectrum indicates four elements of C, N, Co and O in the as-synthesized sample. In the high-resolution spectrum of N 1s, the peaks with


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binding energy of 398.8, 399.3, 400.9 and 404.7 eV are attributed to sp2 hybridized aromatic N (C=N-C), tertiary N (N-(C)3 or HN-(C)2, quaternary N (bonding to three carbon atoms in the aromatic cycles) and π excitation,

37, 38

respectively. And the

peaks at 296.8, 286.5 and 288.3 eV of C 1s corresponds to the graphitic carbon, C-O functional groups and N-C=N,

37

respectively. The spectrum of Co 2p can be

deconvoluted into three peaks located at 780.1, 781.8 and 796.3 eV, which are in good agreement with the oxidization states of Co3+ and Co2+, and the satellite peak of the oxidation state of Co3O4,

37, 39

respectively. As for the spectrum of O 1s, the

bonding energy of 529.3 eV is attributed to the oxygen atoms in Co3O4 lattice, while the binding energies of 530 and 532.3 eV are in agreement with C-O and O-C=O bonds. 40 The formation of C-O and O-C=O bonds also demonstrates the high-quality of the interface between the two semiconductors. 37, 41 To understand the charge separation and transfer in COCN, the band structures of prepared catalysts were then analyzed by the ultraviolet-visible absorption spectrometry (DRS) and XPS-VB measurement. As shown in Figure 4a, the bare 2D g-C3N4 displays limited visible light response with light absorption edge of ca. 435 nm, while Co3O4 shows good light response in the whole visible light region. Obviously, the introduction of Co3O4 can significantly enhance the photoresponse of 2D g-C3N4. The band gaps of 2D g-C3N4 and Co3O4 are calculated to be 2.88 and 1.35 eV by the UV-Vis absorption spectra (Figure 4b). On the other hand, the XPS-VB measurements (Figure 4c, d) show that the VB maximum energies of 2D g-C3N4 and Co3O4 are 1.85 and -0.09 eV, respectively. Based on the VB positions and the band



gaps measured above, the CB edge positions of Co3O4 (-1.03 eV) and 2D g-C3N4 (-1.44 eV) are obtained. This result suggests that upon light irradiation, the excited electrons of Co3O4 can migrate to 2D g-C3N4 through high-quality interface, enhancing the charge density on the dense surface active sites of 2D g-C3N4. On the other hand, the high CB position of 2D g-C3N4 can ensure the smooth proceeding of the photoreduction of CO2 to yield hydrocarbon products. The photocatalytic CO2 reduction performance of all the catalysts was evaluated under visible light irradiation in anaerobic environment in acetonitrile/H2O solvent (acetonitrile possess better solubility of CO2) with TEOA as hole sacrificial agent and [Co(bpy)3]Cl2 as photosensitizer

25, 42.

Figure 5a shows the photocatalytic CO2

reduction performance of the COCN heterostructure and control samples. The reduction of CO2 into CO is the first step toward the synthesis of more complex hydrocarbon fuels, for which this reaction hold great significance for the development of photocatalytic CO2 conversion. The CO yield rate of pure 2D g-C3N4 reaches only 31 μmol g-1 h-1 mixed with CH4 and H2 gas competing products, while pure Co3O4 shows a relative high CO yield (163 μmol g-1 h-1) without generating other gas product. After introducing Co3O4 into 2D g-C3N4 to construct COCN 2D heterostructure, the photocatalytic performance shows giant enhancement. The optimal photocatalyst (5 wt% COCN) exhibits a high CO evolution rate of 419 μmol g-1 h-1 with selectivity of 89.4%, and the yield of gas products increase with the prolonging of the irradiation time (Figure 5b). To make deep understanding of the catalytic behavior, a series of control experiments were performed (Figure 5c). When


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the reaction was carried out in the dark condition or without photocatalyst, negligible gas products can be detected, demonstrating the photocatalytic nature of the reaction (Figure 5b, columns 2 and 4). Besides, the [Co(bpy)3]Cl2 plays a dominating role to realize efficient catalytic reaction (Figure 5b, column 3). In addition, when using Ar as the reaction gas to replace CO2, only trace amount of H2 can be detected (Figure 5b, column 5), suggesting that the generation of carbon-based fuels derives from the reduction of CO2. Regarding the stability, there is only a mild decrease after 3 cycles probably due to the consumption of sacrificial agent. After adding additional TEOA (2 mL) into the reaction, the gas generation rate goes steady. On the other hand, the photocatalytic process did not destroy the intrinsic chemical structure of COCN, supported by the XRD measurements before and after the reaction (Figure S4). Having confirmed the high CO2 reduction performance, we focus on investigating the charge kinetics and CO2 absorption capacity of COCN, both of which are of great significance for the photocatalyic activity. In principle, the recombination of electron-hole (e-h) pairs will release energy, thereby forming PL emission.

43

The COCN shows quenched PL emission in contrast to the pure 2D

g-C3N4 (Figure 6a), suggesting the introduction of Co3O4 can suppress the recombination of electron and hole. In the time-resolved fluorescence measurement, the faster PL decay (shorter lifetime) means faster consumption rate of charges, and can reflect the charge separation efficiency. 43 The induction of Co3O4 into 2D g-C3N4 leads to faster PL decay with decrease of PL life time from 5.11 ns to 3.94 ns, suggesting a low possibility of the recombination of e-h pairs (Figure 6b).



Photocurrent response can also reflect the ability to generate e-h pairs.

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44

Figure 6c

shows that the photocurrent response of the as-prepared COCN is obviously higher than that of bare 2D g-C3N4 and Co3O4, indicating more charge carriers can be generated by establishing such 2D heterostructure, probably due to the electron injection from Co3O4. Besides the enhanced charge separation efficiency, the CO2 absorption ability is another key point for photocatalyic CO2 reduction. The CO2 adsorption measurement reveals that the COCN possesses a high CO2 adsorption capacity of ca. 18 cm3 g-1 at 780 mmHg and 283 K (Figure 6d) benefiting from the large surface area of the 2D heterostructure, which is favorable for the catalytic reaction. Based on the analysis above, both of the enhanced charge separation efficiency originated from the construction of 2D heterostructure and favorable CO2 adsorption capacity contribute to the enhanced CO2 conversion performance. In the typical phtocatalyic reaction, since the CB position of Co3O4 is higher than that of 2D g-C3N4, the electrons should migrate from Co3O4 to 2D g-C3N4 after their combination. Then, the photo-generated electrons of 2D g-C3N4 reduce CO2 to CO on the surface of [Co(bpy)3]Cl2. On the other hand, the VB position of Co3O4 is also higher than that of 2D g-C3N4. Therefore, upon light irradiation, the photo-generated electrons in Co3O4 will migrate to 2D g-C3N4 to participate in photoreduction reaction, whilst the holes in will gather on Co3O4 to reaction with the hole sacrificial agent (TEOA), achieving improved solar energy utilization and charge separation efficiency. The 2D heterostructure and specifically-exposed interface ensure the effective charge migration between two components. Finally, large amounts of electrons will react


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with the surface adsorbed CO2 molecules to produce CO fuel (Figure 7). 4. Conclusion In summary, a typical 2D heterostructure for the photocatalytic reduction of CO2 was successfully developed by combining 2D g-C3N4 and Co3O4. The construction of such 2D heterostructure can induce greatly enhanced photocatalytic reduction of CO2 to CO, with an evolution rate of 419 μmol g-1 h-1 and selectivity of 89.4%. The introduction of Co3O4 with narrow band gap can first broaden the light absorption, making more solar energy participate in the catalytic reaction. Besides, the intimate contact between two components ensures effective charge migration, and the specifically-exposed facet of Co3O4 could ensure high-density electrons migrating to 2D g-C3N4 through Co3O4 [112]-2D g-C3N4 interface, thus improving the surface chargedensity for photocatalytic reaction. Moreover, the COCN also possesses favorable CO2 adsorption capacity. Taken together, a high-efficiency photocatalyic CO2 reduction system can be realized. This work highlights the rational design of heterostructure-based phtocatalysts by optimization of synthetic method and interface engineering for CO2 reduction into hydrocarbon fuels.

Acknowledgements This study was supported by National Nature Science Foundation of China (21676128, 21776118, 21507046), Six talent peaks project in Jiangsu Province (2014-JNHB-014), and the high performance computing platform of Jiangsu University. A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education



Institutions. References 1.

Zhang, Z.; Yates, J. T., Jr., Band bending in semiconductors: chemical and physical consequences

at surfaces and interfaces. Chem. Rev. 2012, 112 (10), 5520-51. 2.

Rao, H.; Schmidt, L. C.; Bonin, J.; Robert, M., Visible-light-driven methane formation from CO2

with a molecular iron catalyst. Nature 2017, 548 (7665), 74-77. 3.

Voiry, D.; Shin, H. S.; Loh, K. P.; Chhowalla, M., Low-dimensional catalysts for hydrogen

evolution and CO2 reduction. Nat. Rev. Mater. 2018, 2 (1), 0105. 4.

Kou, J.; Lu, C.; Wang, J.; Chen, Y.; Xu, Z.; Varma, R. S., Selectivity Enhancement in

Heterogeneous Photocatalytic Transformations. Chem. Rev. 2017, 117 (3), 1445-1514. 5.

Li, K.; Peng, B.; Peng, T., Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to

Solar Fuels. ACS Catal. 2016, 6 (11), 7485-7527. 6.

Chang, X.; Wang, T.; Gong, J., CO2 photo-reduction: insights into CO2 activation and reaction on

surfaces of photocatalysts. Energy Environ. Sci. 2016, 9 (7), 2177-2196. 7.

Liu, X.; Inagaki, S.; Gong, J., Heterogeneous Molecular Systems for Photocatalytic CO2

Reduction with Water Oxidation. Angew. Chem. Int. Ed. 2016, 55 (48), 14924-14950. 8.

Li, P.; Zhou, Y.; Zhao, Z.; Xu, Q.; Wang, X.; Xiao, M.; Zou, Z., Hexahedron Prism-Anchored

Octahedronal CeO2: Crystal Facet-Based Homojunction Promoting Efficient Solar Fuel Synthesis. J. Am. Chem. Soc. 2015, 137 (30), 9547-50. 9.

Lin, Z.; Cai, L.; Lu, W.; Chai, Y., Phase and Facet Control of Molybdenum Carbide Nanosheet

Observed by In Situ TEM. Small 2017, 13 (35), 1700051. 10. Wang, H.; Chen, C.; Zhang, Y.; Peng, L.; Ma, S.; Yang, T.; Guo, H.; Zhang, Z.; Su, D. S.; Zhang, J., In situ oxidation of carbon-encapsulated cobalt nanocapsules creates highly active cobalt oxide catalysts for hydrocarbon combustion. Nat. Commun. 2015, 6, 7181. 11. Kuriki, R.; Sekizawa, K.; Ishitani, O.; Maeda, K., Visible-light-driven CO2 reduction with carbon nitride: enhancing the activity of ruthenium catalysts. Angew. Chem. Int. Ed. 2015, 54 (8), 2406-9. 12. Shi, H.; Chen, G.; Zhang, C.; Zou, Z., Polymeric g-C3N4 Coupled with NaNbO3 Nanowires toward Enhanced Photocatalytic Reduction of CO2 into Renewable Fuel. ACS Catal. 2014, 4 (10), 3637-3643. 13. He, Y.; Zhang, L.; Teng, B.; Fan, M., New application of Z-scheme Ag3PO4/g-C3N4 composite in converting CO2 to fuel. Environ. Sci. Technol. 2015, 49 (1), 649-56. 14. She, X.; Wu, J.; Xu, H.; Zhong, J.; Wang, Y.; Song, Y.; Nie, K.; Liu, Y.; Yang, Y.; Rodrigues, M.-T. F.; Vajtai, R.; Lou, J.; Du, D.; Li, H.; Ajayan, P. M., High Efficiency Photocatalytic Water Splitting Using 2D α-Fe2O3/g-C3N4 Z-Scheme Catalysts. Adv. Energy Mater. 2017, 7 (17), 1700025. 15. Yang, S.; Feng, X.; Wang, X.; Mullen, K., Graphene-based carbon nitride nanosheets as efficient metal-free electrocatalysts for oxygen reduction reactions. Angew. Chem. Int. Ed. 2011, 50 (23), 5339-43. 16. Zhu, X.; Liu, J.; Zhao, Z.; Yan, J.; Xu, Y.; Song, Y.; Ji, H.; Xu, H.; Li, H., Hydrothermal synthesis of mpg-C3N4 and Bi2WO6 nest-like structure nanohybrids with enhanced visible light photocatalytic activities. RSC Adv. 2017, 7 (61), 38682-38690. 17. Mo, Z.; Xu, H.; Chen, Z.; She, X.; Song, Y.; Yan, P.; Xu, Y.; Lei, Y.; Yuan, S.; Li, H., Gold/monolayer graphitic carbon nitride plasmonic photocatalyst for ultrafast electron transfer in


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solar-to-hydrogen energy conversion. Chinese J. Catal. 2018, 39, 760-770. 18. Mo, Z.; Xu, H.; Chen, Z.; She, X.; Song, Y.; Wu, J.; Yan, P.; Xu, L.; Lei, Y.; Yuan, S.; Li, H., Self-assembled synthesis of defect-engineered graphitic carbon nitride nanotubes for efficient conversion of solar energy. Appl. Catal. B: Environ. 2018, 225, 154-161. 19. She, X.; Wu, J.; Zhong, J.; Xu, H.; Yang, Y.; Vajtai, R.; Lou, J.; Liu, Y.; Du, D.; Li, H.; Ajayan, P. M., Oxygenated monolayer carbon nitride for excellent photocatalytic hydrogen evolution and external quantum efficiency. Nano Energy 2016, 27, 138-146. 20. Yi, J.; She, X.; Song, Y.; Mao, M.; Xia, K.; Xu, Y.; Mo, Z.; Wu, J.; Xu, H.; Li, H., Solvothermal synthesis of metallic 1T-WS2: A supporting co-catalyst on carbon nitride nanosheets toward photocatalytic hydrogen evolution. Chem. Eng. J. 2018, 335, 282-289. 21. Xu, H.; Yi, J.; She, X.; Liu, Q.; Song, L.; Chen, S.; Yang, Y.; Song, Y.; Vajtai, R.; Lou, J.; Li, H.; Yuan, S.; Wu, J.; Ajayan, P. M., 2D heterostructure comprised of metallic 1T-MoS2 /Monolayer O-g-C3N4 towards efficient photocatalytic hydrogen evolution. Appl. Catal. B: Environ. 2018, 220, 379-385. 22. Xu, H.; Yan, J.; Xu, Y.; Song, Y.; Li, H.; Xia, J.; Huang, C.; Wan, H., Novel visible-light-driven AgX/graphite-like C3N4 (X=Br, I) hybrid materials with synergistic photocatalytic activity. Appl. Catal. B: Environ. 2013, 129, 182-193. 23. Hao, Q.; Mo, Z.; Chen, Z.; She, X.; Xu, Y.; Song, Y.; Ji, H.; Wu, X.; Yuan, S.; Xu, H.; Li, H., 0D/2D Fe2O3 quantum dots/g-C3N4 for enhanced visible-light-driven photocatalysis. Colloids Surf. A 2018, 541, 188-194. 24. Chen, Z.; Xia, K.; She, X.; Mo, Z.; Zhao, S.; Yi, J.; Xu, Y.; Chen, H.; Xu, H.; Li, H., 1D metallic MoO2-C as co-catalyst on 2D g-C3N4 semiconductor to promote photocatlaytic hydrogen production. Appl. Surf. Sci. 2018, 447, 732-739. 25. Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; Li, D.; Yang, J.; Xie, Y., Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel. Nature 2016, 529 (7584), 68-71. 26. Cui, F.; Huang, Y.; Xu, L.; Zhao, Y.; Lian, J.; Bao, J.; Li, H., Rational construction of a 3D hierarchical NiCo2O4/PANI/MF composite foam as a high-performance electrode for asymmetric supercapacitors. Chem. Commun. 2018, 54 (33), 4160-4163. 27. Yu, Y.; Hu, S.; Su, L.; Huang, L.; Liu, Y.; Jin, Z.; Purezky, A. A.; Geohegan, D. B.; Kim, K. W.; Zhang, Y.; Cao, L., Equally efficient interlayer exciton relaxation and improved absorption in epitaxial and nonepitaxial MoS2/WS2 heterostructures. Nano Lett. 2015, 15 (1), 486-91. 28. Wang, H.; Sun, X.; Li, D.; Zhang, X.; Chen, S.; Shao, W.; Tian, Y.; Xie, Y., Boosting Hot-Electron Generation: Exciton Dissociation at the Order-Disorder Interfaces in Polymeric Photocatalysts. J. Am. Chem. Soc. 2017, 139 (6), 2468-2473. 29. Zheng, D.; Cao, X. N.; Wang, X., Precise Formation of a Hollow Carbon Nitride Structure with a Janus Surface To Promote Water Splitting by Photoredox Catalysis. Angew. Chem. Int. Ed. 2016, 55 (38), 11512-6. 30. Wang, H.; Jiang, S.; Chen, S.; Zhang, X.; Shao, W.; Sun, X.; Zhao, Z.; Zhang, Q.; Luo, Y.; Xie, Y., Insights into the excitonic processes in polymeric photocatalysts. Chem. Sci. 2017, 8 (5), 4087-4092. 31. Su, T.; Shao, Q.; Qin, Z.; Guo, Z.; Wu, Z., Role of Interfaces in Two-Dimensional Photocatalyst for Water Splitting. ACS Catal. 2018, 8 (3), 2253-2276. 32. Gao, C.; Meng, Q.; Zhao, K.; Yin, H.; Wang, D.; Guo, J.; Zhao, S.; Chang, L.; He, M.; Li, Q.;



Page 18 of 24

Zhao, H.; Huang, X.; Gao, Y.; Tang, Z., Co3O4 Hexagonal Platelets with Controllable Facets Enabling Highly Efficient Visible-Light Photocatalytic Reduction of CO2. Adv. Mater. 2016, 28 (30), 6485-90. 33. Hu, L.; Peng, Q.; Li, Y., Selective Synthesis of Co3O4 Nanocrystal with Different Shape and Crystal Plane Effect on Catalytic Property for Methane Combustion. J. Am. Chem. Soc. 2008, 130, 16136-16137. 34. Wang, S.; Guan, B. Y.; Lu, Y.; Lou, X. W. D., Formation of Hierarchical In2S3-CdIn2S4 Heterostructured Nanotubes for Efficient and Stable Visible Light CO2 Reduction. J. Am. Chem. Soc. 2017, 139 (48), 17305-17308. 35. Ran, J.; Jaroniec, M.; Qiao, S. Z., Cocatalysts in Semiconductor-based Photocatalytic CO2 Reduction: Achievements, Challenges, and Opportunities. Adv. Mater. 2018, 1704649. 36. Wang, T.; Shi, L.; Tang, J.; Malgras, V.; Asahina, S.; Liu, G.; Zhang, H.; Meng, X.; Chang, K.; He, J.; Terasaki, O.; Yamauchi, Y.; Ye, J., A Co3O4-embedded porous ZnO rhombic dodecahedron prepared using zeolitic imidazolate frameworks as precursors for CO2 photoreduction. Nanoscale 2016, 8 (12), 6712-20. 37. Guo, S.; Zhao, S.; Wu, X.; Li, H.; Zhou, Y.; Zhu, C.; Yang, N.; Jiang, X.; Gao, J.; Bai, L.; Liu, Y.; Lifshitz, Y.; Lee, S. T.; Kang, Z., A Co3O4-CDots-C3N4 three component electrocatalyst design concept for efficient and tunable CO2 reduction to syngas. Nat. Commun. 2017, 8 (1), 1828. 38. Han, C.; Ge, L.; Chen, C.; Li, Y.; Xiao, X.; Zhang, Y.; Guo, L., Novel visible light induced Co3O4-g-C3N4 heterojunction photocatalysts for efficient degradation of methyl orange. Appl. Catal. B: Environ. 2014, 147, 546-553. 39. Tahir, M.; Mahmood, N.; Pan, L.; Huang, Z.-F.; Lv, Z.; Zhang, J.; Butt, F. K.; Shen, G.; Zhang, X.; Dou, S. X.; Zou, J.-J., Efficient water oxidation through strongly coupled graphitic C3N4 coated cobalt hydroxide nanowires. J. Mater. Chem. A 2016, 4 (33), 12940-12946. 40. Yang, H.; Lv, K.; Zhu, J.; Li, Q.; Tang, D.; Ho, W.; Li, M.; Carabineiro, S. A. C., Effect of mesoporous g-C3N4 substrate on catalytic oxidation of CO over Co3O4. Appl. Surf. Sci. 2017, 401, 333-340. 41. Si, J.; Xiao, S.; Wang, Y.; Zhu, L.; Xia, X.; Huang, Z.; Gao, Y., Sub-nanometer Co3O4 clusters anchored on TiO2(B) nano-sheets: Pt replaceable Co-catalysts for H2 evolution. Nanoscale 2018, 10 (5), 2596-2602. 42. Wang, S.; Guan, B. Y.; Lou, X. W. D., Construction of ZnIn2S4-In2O3 Hierarchical Tubular Heterostructures for Efficient CO2 Photoreduction. J. Am. Chem. Soc. 2018, 140 (15), 5037-5040. 43. Li, J. X.; Ye, C.; Li, X. B.; Li, Z. J.; Gao, X. W.; Chen, B.; Tung, C. H.; Wu, L. Z., A Redox Shuttle Accelerates O2 Evolution of Photocatalysts Formed In Situ under Visible Light. Adv. Mater. 2017, 29 (17), 1606009. 44. Huang, S.; Xu, Y.; Zhou, T.; Xie, M.; Ma, Y.; Liu, Q.; Jing, L.; Xu, H.; Li, H., Constructing magnetic

catalysts

with

in-situ

solid-liquid

interfacial

photo-Fenton-like

Ag3PO4@NiFe2O4 composites. Appl. Catal. B: Environ. 2018, 225, 40-50.


reaction

over

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Figure caption

Figure 1. Schematic illustration of the synthetic process of Co3O4/2D g-C3N4 (COCN).

Figure 2. Morphology analysis of the prepared samples. TEM images of (a) 2D g-C3N4, (b) Co3O4 hexagonal platelets and (c) 5 wt% COCN. (d) HRTEM and (e) Fast Fourier transform (FFT) images of 5 wt% COCN.



Figure 3. Chemical structure characterization of the prepared samples. (a) XRD patterns of different samples. XPS spectra of (b) survey spectrum, (c) N 1s, (d) C 1s, (e) Co 2p and (f) O 1s in 5 wt% COCN.


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Figure 4. The optical property and band structure measurement. (a) UV–vis absorption spectra of the 2D g-C3N4, Co3O4 and COCN samples. (b) Estimated band gaps of the 2D g-C3N4 and Co3O4. (c-d) XPS VB spectra of the 2D g-C3N4 and Co3O4.



Figure 5. Photocatalyic CO2 reduction performance. (a) Photocatlaytic CO2 reduction rate over 2D g-C3N4, Co3O4 and COCN composites. (b) Time-dependent photocatlaytic CO2 reduction performance of 5 wt% COCN. (c) Control experiments under different reaction conditions. (d) Photocatalytic stability of 5 wt% COCN.


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Figure 6. Investigation of charge kinetics and CO2 absorption capacity. (a) PL spectra and (b) Fluorescence decay curves of the 2D g-C3N4 and 5 wt% COCN. (c) Time-dependent photocurrent responses over 2D g-C3N4, Co3O4 and 5 wt% COCN. (d) CO2 sorption isotherms of 5 wt% COCN at 283 K.



Figure 7. Schematic illustrating of charge-transfer behavior and CO2 photoreduction sites.


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carbon nitride 2D heterostructure

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