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Materials and Interfaces

Fine Tuning the Heterostructured Interfaces by Topological Transformation of Layered Double Hydroxide Nanosheets Yanqi Xu, Zelin Wang, Ling Tan, Yufei Zhao, Haohong Duan, and Yufei Song Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02246 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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Industrial & Engineering Chemistry Research

Fine Tuning the Heterostructured Interfaces by Topological Transformation of Layered Double Hydroxide Nanosheets Yanqi Xu,a Zelin Wang,a Ling Tan,a Yufei Zhao,a* Haohong Duanb* and Yu-Fei Songa* a

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory

of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029 P. R. China. b

Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford, OX1 2TA, U.K.

Email: [email protected]; [email protected]; [email protected] or [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Rational design of highly active, stable and inexpensive catalysts with abundant interfaces can have great potential to increase catalytic performance. Topological transformation of layered double hydroxides (LDHs) to the corresponding mixed metal oxides (MMO) offers an efficient strategy to achieve such interfaces. However, the formation and conversion of these heterostructured interfaces is lack of study and remains to be elusive. Herein, we report a detailed investigation of the topological transformation of LDHs. The as-prepared MMO with abundant interfaces can be modulated by calcination of ZnCo-LDH at different temperatures. Among them, the ZnCo-LDH calcinated at 200°C reveals the most abundant interfaces and exhibits excellent performance in electrochemical water oxidation. This work will deepen the understanding of the LDHs topological transformation from the molecular level. KEYWORDS：：Layered double hydroxides, topological transformation, heterostructured interfaces

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INTRODUCTION The transition metal oxides (such as Co3O4, NiO, Fe3O4) have been investigated as a class of promising candidates for electrochemical water oxidation reaction owing to their special electrochemical and catalytic properties such as low energy density and long-term corrosion resistance in alkaline solution.1-6 Recent efforts in improving their catalytic activity have been developed by adopting various pathways including partially doping, morphology

control,

interface

engineering,

and

integrating

oxides

on

other

carbon-supports.7-9 Interestingly, an efficient strategy has been developed to synthesize lateral heterostructure of high-energy faceted NiO or Co3O4 nanosheets stabilized by TiO2 or ZnO support, and the heterostructured interfaces play a significant role for excellent electrochemical activity.10-11 Despite that, the formation and conversion of these heterostructured interfaces is lack of study and remains to be elusive. Layered double hydroxides (LDHs) are a class of lamellar materials with the general composition [M2+1-xM3+x(OH)2]x+(An-x/n)•yH2O, where M3+ and M2+ are trivalent and divalent metal cations respectively.12-15 LDHs with tunable chemical composition can be used as a robust platform to synthesize highly dispersed and morphology-controllable mixed metal oxides (MMO) through topological transformation process,16-21 as first reported by L. Markov and co-workers in 1990 for the advantage in synthesis (111) faceted CuCo2O4 spinel from CuCo-LDH.22 The resultant interfaces between MMO phases were demonstrated to be the active sites to facilitate charge separation and electrons transfer.10,

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Indeed, calcination of LDHs has been demonstrated to be an

alternative way to traditional methods for fabrication of MMO nanocomposite materials.24-28 For example, Duan et al reported the structural, composition and

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morphology evolution from ZnAl-LDH to ZnO-supported Al2O3, and then to ZnO and ZnAl2O4 MMO on different calcination temperatures.29-31 However, the interface change during such topological transformation process and its influence on the properties were hardly touched. Bearing the above in mind, a better understanding the formation and conversion of the MMO and interfaces during the topological transformation of LDHs at different temperatures may open a new pathway for fine tuning the interfaces of MMO and thereby help to elucidate the structure-function relationship. In this work, the heterostructured interfaces by topological transformation of ZnCo-LDH on calcination from 200 to 800°C have been investigated carefully in detail. A large variety of microscopic and spectroscopic techniques have been utilized to monitor the structure, composition, and morphology evolution from ZnCo-LDH to MMO heterostructure composed of zincite-structured ZnO and spinel-like Co3O4 at different calcination temperatures. The ZnCo-MMO calcined at 200°C exhibits higher oxygen evolution reaction (OER) activity than that calcinated at other temperatures and pure Co3O4 or ZnCo2O4, which can be attributed to the formation of numerous interfaces in the ZnCo-MMO, allowing efficient electronic transfer as evidenced by theoretical calculation. EXPERIMENTAL SECTION Synthetic procedures of ZnCo-LDH, CoCo-LDH, ZnCo-x and CoCo-x. The ZnCo-LDH and CoCo-LDH were prepared by co-precipitation method as reported in literature.11, 32-33 The corresponding calcined derivatives were denoted as ZnCo-x and

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CoCo-x (x refers to the different calcination temperatures at 200, 400, 600 and 800°C), respectively. The experimental details were presented in the supporting information. Characterization. The Powder X-ray diffraction (XRD) patterns of the materials were recorded on a Rigaku UltimaIII with Cu-Kα radiation. Thermo-gravimetric-differential thermal analysis (TG-DTA) were carried out using a TG/DSC 1/1100 SF from METTLER TOLEDO. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was performed on a Shimadzu ICPS-7500 instrument. High resolution transmission electron microscopy (HRTEM) and Scanning electron microscopy (SEM) images were collected on a JEOL JEM-2010 and a Zeiss Supra55. X-ray absorption fine structure (XAFS) measurements were obtained from the 1W1B beam line of Beijing Synchrotron Radiation Facility (BSRF). X-ray photoelectron spectroscopy (XPS) measurements were using PHI Quantera SXM. The surface area of catalysts were measured on Quantachrome Autosorb-1 system at liquid nitrogen temperature. Electrochemical measurements. All electrochemical measurements were performed in 1.0 M KOH aqueous solution using a CH Instruments (CHI 660E) work station with three-electrode system (counter electrode: Platinum (Pt) electrode, reference electrode: saturated calomel electrode (SCE)). All potentials were quoted with respect to the reversible hydrogen electrode (RHE) and converted to RHE according to the Nernst equation (Evs RHE = Evs SCE + EoSCE + 0.059pH). The working electrode was prepared by loading 0.6 mg of sample on 1×1 cm2 carbon-fiber paper from its ethanol dispersion with 50 µL Nafion. Cyclic voltammetry (CV) was conducted with a scan rate of 50 mV s-1. The scan rate was 5 mV s-1 with 95% iR drop compensation for linear sweep 5



voltammetry (LSV).34 A series of CV measurements to probe the estimation of effective electrode surface area were performed with different scan rates (1, 5, 10, 20, 30, 40 and 50 mV s-1) in 0.10 to 0.15 V, and the sweep segments of the measurements were 50. The electro-chemical impedance spectroscopy (EIS) was carried out at 10 mA cm-2 current density with scanning frequency ranging from 100 KHz to 0.01 Hz. Computational details. Plane-wave density functional theory (DFT) + U calculations were utilized to model ZnCo2O4, Co3O4 and Co3O4 partially doped by Zn in tetrahedral sites using CASTEP module in Material Studio. The details of calculation were the same as those reported in our previous paper.11, 35 RESULTS AND DISCUSSION

Figure 1. XRD patterns of ZnCo-LDH and ZnCo-MMO obtained at calcination temperature from 100 to 800°C, respectively. The Co-containing LDHs (ZnCo- and CoCo-LDH) are chosen as precursors for synthesis of spinel-like Co3O4 during the topological transformation process. The XRD patterns of

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the ZnCo-LDH and CoCo-LDH precursors (Figure 1 and S1) exhibit a series of (00l) Bragg peaks, confirming that the ZnCo-LDH and CoCo-LDH possess the characteristic features of the layered structure.32-33 Thermogravimetric-differential thermal analysis (TG-DTA) are carried out to depict the thermal behavior of the ZnCo-LDH and CoCo-LDH (Figure S2). The TG-DTA results reveal that the ZnCo-LDH exhibits higher thermal stability than the CoCo-LDH, indicating the modulation of second component (Zn) in the LDH layers leads to the increased thermal stability.11, 36 Moreover, the XRD patterns show the transformation process of ZnCo-LDH calcined at different temperatures from 100 to 800°C (Figure 1). At 100°C, the XRD patterns show a serial of (00l) peaks referred to the characteristic diffraction of LDH, indicating the remained layered structure of LDH.21,

27, 37-38

The decreased intensity and the broadening of

diffraction peaks is related to the loss of interlamellar water.27 When the temperature is increased to 200°C, XRD patterns exhibit that the LDH layers has decomposed in accompanied by formation of the spinel-like Co3O4 phase at the same time. Further increase temperature to 350°C, the increased intensity of those broad peaks confirms the better crystallinity of the spinel-like Co3O4 nuclei. Note that no obvious ZnO peaks can be detected due to the high dispersion/low crystallinity degree. Further calcination at 400°C yields in the formation of zincite-structured ZnO phase as evidenced by XRD pattern. The crystallinity of spinel-structured and zincite-structured phases enhances with the increasing temperature to 800°C. Furthermore, some reflections (such as (220), (311), (422), (400), (511) and (440)) of the spinel-like Co3O4 gradually shift to high 2θ angles, indicating the lattice parameters of spinel-like Co3O4 is slightly smaller with the increase of calcination temperature (Table S1). This shift is attributed to Zn2+ ions segregated

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from spinel-like Co3O4, as also reported by Li’s group of Zn-doped Co3O4 phase.39 In contrast, the diffraction peaks in the XRD patterns of CoCo-LDH derived samples remain unchanged at different temperatures (Figure S1). Furthermore, the Zn/Co ratio of the as-prepared ZnCo-LDH and ZnCo-200 are determined to be the same as 0.99/1 by ICP-AES analysis. From the above results, it can be concluded that the second element (Zn) in Co-containing LDH play great roles on the topological transformation of LDH to the corresponding mixed metal oxide (MMO) phases, which will be discussed below. High-resolution transmission electron microscope (HRTEM) images are utilized to acquire the morphologies and the structures of the calcined ZnCo-MMO. The ZnCo-LDH precursor reveals a size of ~300 nm with the thickness of about 10 nm (Figure S3). In the case of ZnCo-200, HRTEM images show a nano-platelet with several domains (Figure 2a), in which the set of (111) planes and the set of (220) planes can be identified to be spinel-like Co3O4 with the dominant exposed (112) facets (Figure 2b-2c).40 In addition, the zincite-structured ZnO nanosheets show lattice spaces of 0.281 and 0.281 nm assigned to the (100) and (010) planes with the angle of 120o, respectively, suggesting the dominant (001) faceted zincite-structured ZnO nanosheets (Figure 2b and 2d).29 And abundant interfaces between these two phases can be clearly observed in this MMO structure (Figure 2b). It's worth noting that numerous porosity is induced in the ZnCo-200 (Figure 2a and Figure S4a-b), mainly due to the collapsed layer with the evolution of intercalated H2O and small molecules during transformation of LDH to MMO.41 Further increasing the calcined temperature to 400°C, the morphology of the ZnCo-400 also contain (001) exposed zincite-structured ZnO structure and (112) faceted spinel-like Co3O4 structure with slightly increased particle size (Figure 2e-h and Figure

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S4c-d). For the ZnCo-600, the crystallinity of the MMO heterostructures increases and the particle sizes of the each component (spinel-like Co3O4, zincite-structured ZnO) grow with clear interface between two components (Figure 2i-l and Figure S4e-f). The tendency of increased lateral size of MMO heterostructures also become obvious when the calcination temperature reaches 800 °C, and the spinel-like Co3O4 shows a high-crystalline character, and the zincite-structured ZnO phase also grows up and is separated from spinel phase (Figure S5). Due to the low crystallinity, abundant interfaces between the as-formed spinel-like Co3O4 and zincite-structured ZnO phases appear firstly at 200°C. Further increasing the calcination temperature to 400°C, the interfaces gradually decrease with the increase of particles size and crystallinity of spinel-like Co3O4 and zincite-structured ZnO phase. When the calcined temperature reaches to 600 and 800°C, the phase separation with the slight interfaces can be well observed between these spinel-like Co3O4 and zincite-structured ZnO phases. From the above results, it also can be observed visually that the interfaces between these phases decrease with the increased calcination temperatures (Figure 2, Figure S4, S5). Without the existence of Zn, the calcination treatment of CoCo-LDH leads to the formation of a pure (112) faceted Co3O4 phase without obvious interfaces (Figure S6). Increase of the calcination temperature from 200 to 800°C results in an improved crystallinity degree with an increased size. Due to the interfaces are usually the active sites of reactions, the abundant formed interfaces at ZnCo-200 may improve the catalytic activity as discussed below. The scanning electron microscopy (SEM) images (Figure 3) show that both the ZnCo-LDH and CoCo-LDH precursors commonly exhibit the plate-like morphology, which is a typical feature of LDHs. After heat-treatment at 200°C, the plate-like shapes of

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the LDHs changes into aggregation of smaller nanosheets. As the calcined temperature increases, the average size of the MMO heterostructures become larger, which is in good agreement with the above results. N2 adsorption-desorption is utilized to characterize the surface areas of LDHs and their derived catalysts (Figure S7 and Table S2). Upon calcination, ZnCo-200 shows a highest surface area (117.9 m2 g-1), which can be attributed to the abundant mesopores and macropores and will have great effect on the further catalysis.42-44

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Figure 2. (a)-(d), (e)-(h) and (i)-(l) HRTEM images and corresponding FFT patterns of ZnCo-200, ZnCo-400 and ZnCo-600, respectively.

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Figure 3. (a-1) - (a-5) and (b-1) - (b-5) The SEM images of ZnCo-LDH and ZnCo-x, CoCo-LDH and CoCo-x, respectively.

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During the calcination process, the valence states and coordination environments of the Zn and Co have been clearly confirmed by X-ray absorption fine structure (XAFS) measurements to further track the topological transformation process of ZnCo-LDH. Spinel Co3O4 phase contains two types of cobalt ions: one Co2+ in the tetrahedral site (Td) and the other two Co3+ located in the octahedral site (Oh) with an average oxidation state of cobalt ~ +2.67.45 For ZnCo2O4 reference, the Zn2+atom fully substitute Td site of spinel-Co3O4, leading the oxidation state of cobalt as +3,45 and the oxidation states of cobalt in ZnCo-LDH and CoCo-LDH are estimated as +2.59 and +2.26 by the iodometric redox titration, respectively.33 As shown in Figure S8, in comparison to the reference Co3O4 sample, there is no change in the position of the absorption edge for CoCo-x, demonstrating the oxidation state of Co ions in CoCo-x is the same as reference Co3O4, and not any remarkable change in the chemical coordination of cobalt ions during the calcination of CoCo-LDH process. Similar spectral features in the main-edge region (Figure 4a) can also be observed for both ZnCo-x and spinel references (Co3O4 and ZnCo2O4). More detail, the valence state of cobalt ions as seen from the position of the absorption edge in XANES spectra follows the order: ZnCo2O4 > ZnCo-200 > ZnCo-400 > ZnCo-600 ≈ ZnCo-800 > Co3O4 > ZnCo-LDH (Figure 4a). The corresponding extended XANES oscillation functions of the ZnCo-x are similar, except ZnCo-200 and ZnCo-400 show fewer oscillations, indicating structural differences in the coordination environment surrounding the Co atoms (Figure 4b).

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Figure 4. (a) Co K-edge XANES, (b) Co K-edge extended XANES oscillation functions k3χ (k) and (c) magnitude of k3-weighted Fourier transforms and corresponding fitting of the Co K-edge XANES spectra for ZnCo-LDH, ZnCo2O4, ZnCo-x and Co3O4, respectively; (d) curve-fitting results of Co-O and Co-Co shell for ZnCo2O4, ZnCo-x and Co3O4, respectively; (e) Co K-edge XANES spectra and (f) magnitude of k3-weighted Fourier transform of ZnCo-LDH, ZnCo2O4, ZnCo-x and ZnO, respectively. 14


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The EXAFS of the ZnCo-LDH illustrates explicit octahedral coordination of Co-O and Co-Co/Zn at about 1.7 Å and 2.8 Å (Figure 4c).46 Three main peaks below 4 Å can be observed for the ZnCo-x samples, which can be indexed to the first Co-O shell at ~1.5 Å, the second coordination octahedral sites (Oh) shell (Co-Co) at ∼2.5 Å, and the third tetrahedral sites (Td) shell (Co-Co/Zn) around ~3.0 Å.47 Compared with the reference ZnCo2O4, ZnCo-200 and ZnCo-400 show decreased coordination number (denoted as N) of Co-O and Co-Co, which can be attributed to the existence of defects in the spinel-like Co3O4 at the low calcination temperatures (Figure 4d and Table S3).10 Increasing the heating temperature to 600°C, the N of Co-O and Co-Co in ZnCo-600 increases, mainly due to the increased crystal with decreased distortion (Figure 4d and Table S3). The N of Co-O and Co-Co further decreases along with the increasing heating temperature to 800°C (Figure 4d and Table S3). Besides, for the third Co-Co/Zn shell of ZnCo-x, a shift from Co-Zn (Td) of ZnCo2O4 to short distance of Co-Co (Td) in Co3O4 also indicates that the Zn ions gradually move out of spinel-like Co3O4 phase when increasing the temperature from 200 to 800°C (Table S3 and Figure S8d). The above results can be understood as follows: Zn ions have been doped into Co3O4 phase as located as Td sites at the low calcination temperature (200°C), resulting in the Co almost existed as octahedral sites (Co3+). Further with increasing the temperature to 400, 600 and 800°C, the Zn ions gradually move out of spinel-like Co3O4 phase, leading to the Co ions located in both Td and Oh sites the same as the Co3O4 phase.47 Besides, the effect of calcination on the coordination environments of zinc ions in ZnCo-LDH and its calcined derivatives is also investigated with Zn K-edge XANES and EXAFS analysis (Figure 4e and 4f).32 Compared with those of ZnO and ZnCo2O4 standards, the ZnCo-LDH shows similar edge

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position, indicating the maintenance of the zinc oxidation state. After calcination, the ZnCo-200 and ZnCo-400 exhibit the resolved features like those of ZnO and ZnCo2O4 references, confirming the crystallization of ZnO and ZnCo2O4 phases, which indicates the co-existence of zincite-structured ZnO phase and Zn doped into spinel-like Co3O4 phase. Further increasing the heating temperatures to 600 and 800°C, the decreased peak at ~ 9675 eV shows the gradually disappeared ZnCo2O4 phase, demonstrating that the zinc ions segregate from spinel-like Co3O4, in accordance with the XAFS result of Co and XRD results (Figure 4a and 1), indicating Zn species mainly exist as ZnO. Furthermore, in our previous work, only slight Co ions (< 5%) can be doped into zincite-structured ZnO phase, further increased Co ions lead to the formation of ZnCo2O4.11 Besides, the ratio of the surface Co2+ in Co (Co2+ + Co3+) ions enlarge with increasing calcination temperature, indicating the decreased Co valance state of ZnCo-x, as characterized by X-ray photoelectron spectroscopy (XPS) (Figure S9 and Table S4), in accordance with the EXAFS analyses. While the ratio of the surface Co2+ in CoCo-x remains unchanged, demonstrating the oxidative states of Co are nearly invariable (Figure S9). From the above results, the faceted zincite-structured ZnO and spinel-like Co3O4 with lower crystal degree and abundant interfaces, are formed at the low calcination temperature (200°C), which is mainly due to the homogeneous dispersion property of ZnCo-LDH. The interfaces between zincite-structured ZnO and spinel-like Co3O4 phase can be identified to the partly Zn ions doped into the spinel-like Co3O4 as the formed ZnCo2O4, with slight Co ion doped zincite-structured ZnO phase. And the Zn ions gradually segregate from interfaces with the decreased interface between zincite-structured ZnO and spinel-like Co3O4 when increasing calcination temperature.

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On the basis of above investigations on the topological transformation of the ZnCo-LDH, a proposed process from ZnCo-LDH to ZnCo-x is shown in Figure 5. During the calcination from room temperature to 200°C, the collapse of the LDH structure occurs with the co-formation of (112)-faceted spinel-like Co3O4 and (001) exposed zincite-structured ZnO nuclei with abundant interfaces, which is confirmed by XRD patterns and HRTEM analysis (Figure 1, 2). Specially, the interfaces can be identified to the Zn2+ ions doped in tetrahedral sites (Td) into the spine-like Co3O4 as ZnCo2O4, and Co-doped ZnO phase with a slight doped content (

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