Hollow Multi-Shelled Structures of Co3O4 Dodecahedron with Unique

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Hollow multi-shelled structures of CoO dodecahedron with unique crystal orientation for enhanced photocatalytic CO reduction 2

Li Wang, Jiawei Wan, Yasong Zhao, Nailiang Yang, and Dan Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13528 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Hollow Multi-Shelled Structures of Co3O4 Dodecahedron with Unique Crystal Orientation for Enhanced Photocatalytic CO2 Reduction Li Wang†, ‡, #, Jiawei Wan‡, #, Yasong Zhao†, ‡, Nailiang Yang‡, & and Dan Wang†, ‡, &, * School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, 150001, P. R. China State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China & University of Chinese Academy of Sciences, Beijing 100049, P. R. China † ‡

Supporting Information Placeholder ABSTRACT: Structure and facet control are always considered as the effective routes to enhance the catalytic performance. We successfully synthesized the hollow multi-shelled structures (HoMSs) of Co3O4 dodecahedron by adopting the metal-organic frameworks (MOFs) as templates with sequential templating approach (STA). Importantly, owing to the topologic arrangement of metal atoms in MOFs, the Co3O4 nanocrystals in HoMSs are assembled oriented, forming a unique shell with dominant exposure of (111) facets. This process is defined as “genetic inheritance” in this work. In addition, these exposed facets possess high activity for photocatalytic CO2 reduction. Accompany with the properties inheriting from HoMSs, i.e., multiple interfaces and strong solar light harvesting, these Co3O4 HoMSs presented a high catalytic activity for CO2 photoreduction. The catalytic activity of quadruple-shelled (QS) Co3O4 HoMSs was about five and three times higher than Co3O4 nanoparticles and Co3O4 HoMSs without facet control, respectively.

Constructing current materials with specific structures has demonstrated great effect on achieving outstanding properties within materials. Since our first report on sequential templating approach (STA) in 20091, the hollow multi-shelled structures (HoMSs) have been well developed and demonstrated outstanding properties in lithium-ion batteries2-6, supercapacitor7-9, electrocatalysis10-12, photocatalysis13-18, photoelectrocatalysis19, 20, drug delivery system21 and so on. Besides the structure control, regulation of surface fine structure often demonstrates significant effect on materials’ performance properties22, 23. For instance, the facet effect on catalytic activity and selectivity of catalysts24-28. Up to now, the building blocks are irregularly arranged within most HoMSs, which may loss some specific properties in practical applications. Herein, fabricating HoMSs with high activity facets exposed can combine both their benefits, which is very charming but challenging, and there is rare report. In this case, we suppose that if we can develop the traditional carbon sphere template to a new one with a topological metal atoms arrangement. As a result, we may inherit this arrangement to the final HoMSs, inducing the controllable facets exposure, which may possess good catalytic activity or selectivity in catalysis

application. Metal organic frameworks (MOFs) are rich in periodically arranged metal atoms and organic ligands29, 30, which can be adopted as the ideal templates to make the above design realizable. Zeolitic imidazolate framework-67 (ZIF-67) is constructed with periodically arranged Co atoms31, which demonstrate similar topologic arrangement of Co atoms to Co3O4 (111) (Scheme 1), which possess good activity for photocatalytic CO2 reduction, owing to the strong interaction between (111) facets of Co3O4 and CO232. Herein, we introduce the STA process by using ZIF-67 as templates to design Co3O4 HoMSs. We discover that the periodically arranged Co atoms and organic ligands within ZIF-67 can support the metal atoms sites and space for the formation of HoMSs, respectively. The topologic arrangement of Co atoms within ZIF-67 prefer to induce the formation of {111} facets in Co3O4 HoMSs, which is defined as a “genetic inheritance” in this work. Scheme1. “Genetic inheritance” from MOFs to Co3O4 HoMS, all atoms stand for Co and are marked with different colors (green, yellow, and red) for periodic units.

Scheme 1 displays the spatial distribution of Co atoms in (001), (011) of ZIF-67 and Co3O4 (111) HoMSs. For instance, the exposed (001) and (011) facets within ZIF-67 demonstrate the similar periodic Co atoms as the ones in the (111) facet on Co3O4, which demand only simple shifts of Co atoms to make the transformation

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take place. The organic chains and pores in MOF provide enough space for the shift and rearrangement. In detail, during the STA, the organic ligands are removed, and Co atoms are oxidized to form Co3O4 nanocrystals. At the same time, the Co atoms are shifted in the same plane (Scheme 1, in the dash square), which results in the final spatial distribution of Co atoms in Co3O4 (111). To achieve the preferred facets exposure and inherit transformation, a mild experimental condition, such as a slow heating rate or a relatively small oxygen partial pressure, will be more favorable. In a typical experiment, the templates of dodecahedron ZIF-67 are prepared (Figure 1a, 1b, and Figure S1S3). By tuning the heating rate and the oxygen partial pressure, we can design HoMSs with core-shelled (CS) (Figure S4), doubleshelled (DS) (Figure 1c) and triple-shelled (TS) (Figure 1d) structures (detailed process can be found in Figure S4-S6). Under a slow heating rate (0.5 oC/min) and a small oxygen partial pressure (10%), quadruple-shelled (QS) cobalt oxide HoMSs were successfully obtained (Figure 1e, 1f and Figure S7). The related powder X-ray diffraction (XRD) (Figure 1g, JCPDS card No.421467) proves the oxide is Co3O4. And we characterize surface component and porous structure of the samples by XPS spectra (Figure S8 and Table S1) and N2 adsorption-desorption isotherms (Figure S9). Meanwhile, we decrease the oxygen volume to zero and obtained solid particles (Figure S10). From SEM images of different Co3O4 HoMSs, all of them demonstrate the same dodecahedron structure as ZIF-67, indicating the morphology inheritance from MOFs to HoMSs. Moreover, all of the shells present the same dodecahedron structure from the outside to the inner side of the HoMSs. All of the samples with different shell numbers were sliced, and investigated by STEM in Figure S11, which were consistent with the structure observed by TEM. The synthesis processes were illustrated in Figure S12.

decreased. The (111) and (222) facets occupy 65% of all the exposed facets in QS-Co3O4 (ZIF-67) HoMSs. Interestingly, the morphology of Co3O4 HoMSs evolved into nanoparticles (Figure S13) after grinding and the preferred facets of (111) disappeared. It can be inferred that nanoparticles in HoMSs are oriented assembled, exposing the (111) facet in the shell. Different from the oriented attachment, these nanocrystals are “soft” attached, which can be easily separated and become randomly arranged by grinding. When the Co3O4 HoMSs were ground, other facets hidden in the contact interfaces between Co3O4 nanocrystals exposed, resulting in the increase of the relative intensity of (311) and the decrease of the relative (111) peak intensity. Importantly, this preferred facet exposing phenomenon cannot be observed on the QS-Co3O4 HoMSs with the same STA process but using carbon-microsphere (CMS) as templates (Figure S14 and S15) or commercial Co3O4 nanoparticles. In this case, we can predict that the preferred facets of (111) facets can only be obtained by using MOF as templates, indicating the ZIF-67 induced an orientation growth of Co3O4 nanocrystals and brought an oriented alignment during the shell formation process. More investigations on the synthesis process of QS Co3O4 HoMSs (ZIF-67) have been also performed (Figure S16-S21).

Figure 2. (a) Schematic energy-level diagram for the CO2 photoreduction process, (b) CO and (c) O2 yields of Co3O4 nanoparticles, dodecahedron Co3O4 (ZIF-67), sphere Co3O4 (CMS), and grinded dodecahedron Co3O4 (ZIF-67). (d) CO2 photoreduction cycle performance of dodecahedron QS-Co3O4 HoMSs (ZIF-67).

Figure 1. (a) TEM and (b)SEM images of dodecahedron ZIF-67; TEM images of (c) DS- and (d) TS-Co3O4 (ZIF-67); (e) TEM and (f) SEM images of QS-Co3O4 (ZIF-67); (g) XRD patterns of different Co3O4 samples. Interestingly, the XRD patterns (Figure 1g) present that the intensity of dominant (111) facets is strongly related to the shell numbers of Co3O4 HoMSs. With the shell number increasing, the intensity of (111) facets increased, while the intensity of (311) peak

Since the (111) crystal facet of Co3O4 often exhibit good catalytic properties in CO2 reduction systems33-34. We investigate the CO2 photoreduction activity with various samples under the same measurement condition. The schematic energy-level diagram (Figure 2a) were measured by Tauc plot (Figure S22) and MottSchottky plot (Figure S23) of dodecahedron QS-Co3O4 (ZIF-67), which displayed the reaction process of photocatalytic CO2 reduction on catalyst. Under the same test condition, we investigated different samples of the CS-, DS-, TS- and QS-Co3O4 (ZIF-67), Co3O4 nanoparticles, QS-Co3O4 (CMS), and grinded QSCo3O4 (ZIF-67) particles. From the results of chromatographic analysis, the samples were able to reduce CO2 100% to CO and O2. And by the Figure 2b and 2c, the dodecahedron Co3O4 (ZIF-67) has obvious advantages compared with Co3O4 nanoparticles and spherical Co3O4 (CMS). With the increase of the shell number, the yield of CO increase and the dodecahedron QS-Co3O4 (ZIF-67) exhibited the highest photocatalytic activity for CO2 reduction with a CO yield of 46.3 μmol·g-1·h-1, which is about five and three times

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Journal of the American Chemical Society higher than Co3O4 nanoparticles and spherical QS-Co3O4 (CMS). In addition, we also prepared grinded sample through grinding treatment with the dodecahedron QS-Co3O4 (ZIF-67). The morphology of the dodecahedron QS-Co3O4 (ZIF-67) were converted into nanoparticles (Figure S13) and possessed a CO yield of 10.5 μmol·g-1·h-1, which is similar to 9.83 μmol·g-1·h-1 of commercial Co3O4. The ratio of O2 to CO in the product is close to 1:2 for all tested samples (Figure 2c). The above discussions demonstrate the advanced structure properties of dodecahedron Co3O4 (ZIF-67). In addition, we made a comparison with other reported cobalt oxide-based photocatalysts in Table S2. Besides the catalytic activity, the stability of catalyst is another important performance parameter. We performed the recycling tests with dodecahedron QS-Co3O4 (ZIF-67) under the same condition. As shown in Figure 2d, the QS-Co3O4 (ZIF-67) retained the activity as the original one after four cycles without decreasing the yields of CO or O2. And the morphology (Figure S24) and crystal structure (Figure S25) still remain the same after four cycles’ test, which demonstrate a good catalytic stability of QS-Co3O4 (ZIF-67). A series of experiments were also performed to reveal the key factors of catalytic activity in our system. We investigated the electrochemical impedance spectroscopy of different samples (Figure 3a and Table S3), in which dodecahedron QS-Co3O4 (ZIF67) demonstrates the smallest electrochemical impedance among all samples, indicating the most favorable electron transport in CO2 reduction process. By contrast, the spherical QS-Co3O4 (CMS) and Co3O4 NPs samples demonstrate larger electrochemical impedance than those of Co3O4 (ZIF-67) samples (Table S3), demonstrating larger charge transfer resistance. Therefore, it can be concluded that the oriented exposed (111) facets play a crucial role in promoting the charge transfer process for photocatalytic CO2 reduction. The transient photocurrent response (Figure 3b) demonstrate the consistent result with impedance spectroscopy test. The QS-Co3O4 (ZIF-67) performs the smallest the charge transfer resistance, which brought the largest photocurrent. We also performed the external quantum efficiency of the dodecahedron QS-Co3O4 (ZIF67) to investigate its quantum efficiencies at different wavelengths (Figure S26).

Figure 3 (a) EIS Nyquist plots at a bias of -0.3V vs. SCE (saturated calomel electrode), (b) transient photocurrent response curves at a bias 0.05 V vs. SCE of different Co3O4 samples. In summary, we successfully achieve the special facets exposing within HoMSs. By adopting the ZIF-67 as templates, the topologic arrangement of Co atoms in ZIF-67 are well inherited to Co3O4 nanocrystals, enhancing the exposure of (111) facets in Co3O4 HoMSs, which is defined as “genetic inheritance” in this work. In addition, these exposed facets possess high activity for photocatalytic CO2 reduction. As a result, the catalytic activity of QS-Co3O4 (ZIF-67) was about five and three times higher than Co3O4 nanoparticles and spherical QS-Co3O4 (CMS), respectively. This work paves a new strategy for crystal orientation control on the shell of hollow structures, achieving a preferred facet exposure

that will bring broad applications in photonic, electronic, and catalytic fields.

ASSOCIATED CONTENT Supporting Information Experimental methods, Electrochemical measurements, Figures S1-S26 and Tables S1-S3 illustrating additional physicochemical and electrochemical characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected].

Author Contributions #These authors contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21590795, 21820102002, 51802306), the Scientific Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201623), Queensland-Chinese Academy of Sciences Collaborative Science Fund (122111KYSB20170001), Chinese Academy of Sciences (CAS) Interdisciplinary Innovation Team.

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