Crystal Growth of Bimetallic Oxides CuMnO2 with Tailored Valence

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Crystal Growth of Bimetallic Oxides CuMnO2 with Tailored Valence States for Optimum Electrochemical Energy Storage Sixian Fu, Liping Li, Yuancheng Jing, Yuelan Zhang, Xiyang Wang, Shaofan Fang, Jianghao Wang, and Guangshe Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00988 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Crystal Growth & Design

Crystal Growth of Bimetallic Oxides CuMnO2 with Tailored Valence States for Optimum Electrochemical Energy Storage Sixian Fu,† Liping Li,† Yuancheng Jing,† Yuelan Zhang,† Xiyang Wang,† Shaofan Fang,‡ Jianghao Wang,‡ and Guangshe Li*,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun 130012, P.R. China

‡ Fujian Institute of Research in Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P.R. China

ABSTRACT:

Bimetallic oxides ABOx (x = 2, 3, and 4) with multiple lattice sites have shown numerous properties, blossoming into diverse applications, while regulating the valence state at A or B site usually causes dramatic change in crystal structure, giving rise to uncertainties in comprehending the structure-property relationships. Herein, we synthesized bimetallic layered crednerite CuMnO2 with double-coordinate Cu cations at A site and hexa-coordinate Mn cations at B site via a CTAB modified hydrothermal method. By controlling the crystal growth temperature, valence states tailoring was implemented along with the stabilization of monoclinic layered structure. The regulation process was followed by morphology changes of CuMnO2 crystal in a 1

ACS Paragon Plus Environment

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sequence: triangular sheets (140 oC), nanowires (160 oC), hexagonal prisms (180 oC), and octahedrons (over 200 oC). Interestingly, the oxidation states of Cu2+ and Mn2+ are found for triangular sheets, which transformed to the mixed valency Cu+/Cu2+ and Mn3+/Mn2+ for nanowires, and then to the dominant Cu+ and Mn3+ oxidation states for hexagonal prisms and octahedrons. Among all morphologies, nanowires showed a higher aspect ratio, preferentially growing along (002) plane, which when first applied to supercapacitor, exhibited the highest specific capacitance of 921 F/g at the current density of 1 A/g. The synergistic effect between especial redox equilibrium and controlled one-dimensional crystal architecture is uncovered to optimize the electrochemical energy storage. The methodology reported in this work creates a new path of functional bimetallic oxides ABOx with tailored valence states, controllable crystal growth, and stable crystal structure for optimum energy storage applications.

1. INTRODUCTION

Bimetallic oxides ABOx (x = 2, 3, and 4) with multiple lattice sites possess numerous tuned properties such as crystal, defect, spin, and electronic structures, which could deliver diverse functional applications in the field of sensor, catalysis, optoelectronics, and energy storage.1-4 Multiple lattice sites of metal ions endow ABOx crystals with diversified adjustable possibilities including valence state, atomic occupation, and morphology.5-7 Many efforts have been focused on regulating the valence state at A or B site to realize an optimum performance, which usually cause dramatic change in crystal structure. For example, scheelite-type RVO4 (R = La, Ce,

2


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Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu, Y) precursors are reported to transform into perovskite-type RVO3 by treatments in reductive atmosphere, in order to achieve a conversion from V5+ to V3+ oxidation state, but causing a significant change in structure from scheelite to perovskite.8 In (La, Sr)MnO3 fuel cell cathode perovskites, the average Mn oxidation state modification induced by changing La/Sr cations ratio always as accompanied by oxygen octahedral rotation.9 Therefore, it is still a grand challenge to accurately tailor the valence state with the crystal structure of bimetallic oxides ABOx well maintained.

Motivated by this, we selected crednerite CuMnO2 as a prototype target ABO2 to study. This is because crednerite CuMnO2, just like many other bimetallic oxides ABO2, have multiple lattice sites (double-coordinate Cu cations at A site and hexa-coordinate Mn cations at B site).10 The unique monoclinic layered structure at room temperature contains edge-shared MnO6 octahedron layers with coordinated Cu cations in the interlayer.11-12 Based on the crystal field theory, Mn3+ cations with 3d4 (t2g3eg1) electronic configuration share octahedral field with Mn2+ cations which has 3d5 (t2g3eg2) electronic configuration, but with a slight distortion of MnO6 octahedron due to the Jahn–Teller effect, which still holds the overall octahedron layers.13-14 The low reductive/oxidative temperature of Cu2+/Cu+ also makes it possible to tailor the valence state at an accessible condition.13 The integral monoclinic layered structure increases tolerance of the ionic volume change from valence states variation. With these characteristic advantages, we suspected that crednerite CuMnO2 is most likely to simultaneously realize a valence state tailoring and crystal structure maintaining. 3



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In this work, we synthesized bimetallic layered crednerite CuMnO2 via a CTAB modified hydrothermal method. We designed a temperature controlling method that accomplished an effective valence states tailoring, accompanied by a library of morphology changes. More importantly, the monoclinic layered structure of crednerite CuMnO2 was well maintained. Combining the characterization analyses, we conclude that the synergistic effect between especial redox equilibrium and controllable crystal growth is the key that optimize the electrochemical energy storage. The present methodology can be extended to synthesize other bimetallic oxides ABOx (x = 2, 3, and 4) with controllable morphologies, tailored valence states, as well as stable crystal structure for energy storage applications.

2. EXPERIENTAL SECTION

2.1. Synthesis of CuMnO2 Crystals by Controlling Crystal Growth Temperature.

CuMnO2 crystals with oriented crystal growth were prepared by a typical hydrothermal method. 0.15 g of CTAB (99%, Sinopharm) was firstly added into 50 mL mixed solution of deionized water and ethyl alcohol (1: 1), then 5 mL of NaOH (2 mol/L) solution, 2.5 mL of Mn(CH3COO)2⋅4H2O (0.1 mol/L) solution, and 2.5 mL of Cu(NO3)2⋅3H2O (0.1 mol/L) solution were added into above solution in sequence. The obtained mixture was homogenized by magnetic stirring (600 rpm) for 2 h, and then poured it in a Teflon-lined autoclave (100 mL). Five parallel reactions were prepared. The five autoclaves were sealed and heated at 140°C, 160°C, 180°C, 200°C, and 220 °C, respectively, for 24 h. Then, the reaction systems were cooled down to room 4


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temperature. The obtained powders were collected by centrifugation, washed several times with deionized water and ethyl alcohol, eventually dried at 70 °C for 12 h.

2.2. Synthesis of Contrast Bulk CuMnO2 by Solid State Method.

Contrast bulk sample CuMnO2 was synthesized using stoichiometric amounts of CuO and MnO powders (purity >99.9%). The raw materials were grinded thoroughly to blend well, and then pressed into pellets by uniaxial press. Finally, the pellets were heated at 950 °C under pure N2 flow for 12 h.

2.3. Characterization of CuMnO2 Crystals.

Field-emission scanning electron microscopy (FE-SEM) (HITACHI SU8020) and high-resolution transmission electron microscopy (HRTEM) (Tecnai G2 S-Twin F20) were used to observe the morphologies of CuMnO2 crystals. Energy dispersive spectroscopy (EDS) (Oxford X-Max80) was employed to acquire the elemental analysis and mapping images for components in CuMnO2 crystals. Powder X-ray diffraction (XRD) (Rigaku miniflex600) with Cu Kα radiation (λ = 0.1518 nm) was used for recording structural information of CuMnO2 crystals. The step scanning is in a 2θ range of 10−80° with intervals of 0.02°. Phase formation of CuMnO2 crystals were investigated by confocal Raman system (LabRAM Aramis, Horiba Jobin Yvon). The laser is a He-Ne laser (λ = 632.8 nm). Fourier transform infrared spectroscopy (FT-IR) (IFS-66V/S) was used to detect the chemical bonds in CuMnO2 crystals. X-ray photoelectron spectroscopy (XPS) (ESCA-LAB MKII) with a monochromatic Al Kα (hν = 1486.6 eV) radiation source was performed to determine oxidation states 5



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for CuMnO2 crystals. The charging shifts were calibrated by a primary C 1s value at 284.8 eV. X-ray absorption near-edge structure (XANES) of O K-edge was collected at the BL12B-a beamline of the National Synchrotron Radiation Laboratory (NSRL).

2.4. Electrochemical Measurements.

Electrochemical workstation (CHI 760E, Chenhua Instruments, China) was used to measure all electrochemical performances. All working electrodes were made by following procedure: 80 wt% of the CuMnO2 crystal material (∼1.6 mg), 10 wt% of the acetylene black (∼0.2 mg) and 10 wt% of the poly-tetrauoroethylene (PTFE) were mixed and grinded thoroughly to a paste. Then, the paste was uniformly painted on nickel foam with a coating area of 1 cm2. Next, we dried the painted nickel foams at 60 °C for 4 h. Finally, they were pressed into thin films at 8 MPa. Electrochemical measurements were performed in a 6 M KOH electrolyte via a three-electrode cell. Pt foil was served as the counter electrode, and Hg/HgO electrode was used as the reference electrode. The gravimetric specific capacitances (Csc) were calculated based on galvanostatic charge–discharge (GCD) curves by following equation:

Csc = (I∆t)/(m∆V)

(1)

where I (A) represents the discharge current, ∆t (s) is the discharge time period, m (g) is the mass of the CuMnO2 crystal material, and ∆V (V) is the potential window. The electrochemical impedance spectra (EIS) were measured from 0.1 Hz to 1000 kHz with a potential amplitude of 5 mV. Cycling stabilities were tested by GCD with constant current density of 5 A/g for 1000 cycles. 6


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3. RESULTS AND DISCUSSION

The crystal growth route of crednerite CuMnO2 was illustrated in Figure S1. Cationic surfactant CTAB was selected as the critical solvent and directing agent15-17 to tune the morphology, size, and growth direction of CuMnO2 crystals. One crucial parameter for the formation reaction is hydrothermal temperature, which could induce an oriented growth that drive the morphology changes at different temperatures.

3.1. Various morphologies of CuMnO2 crystals grown by controlling temperature

Figure 1. FE-SEM images of the CuMnO2 crystals grown by controlling the synthetic temperatures: (a) 140 oC; (b) 160 oC; (c) 180 oC; (d) 200 oC; and (e) 220 oC. For comparison, FE-SEM image of bulk CuMnO2 is also given in (f).

The growth behaviors of CuMnO2 crystals by controlling reaction temperatures were monitored by FE-SEM (Figure 1), HRTEM (Figure 2), and EDS-mapping (Figure 3). CuMnO2 obtained at 140 oC had the morphology of triangular sheets with an edge length of 250-300 nm (Figure 1a, 2a1), which showed a clear lattice spacing 7



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of 0.27 nm in HRTEM image (Figure 2a2), corresponding to the (200) plane of CuMnO2. When the reaction temperature reached 160 oC, CuMnO2 grew into special one-dimensional nanowires with a thickness of 5-15 nm (Figure 1b, 2b1), and the lattice spacing of 0.29 nm corresponds to the (002) crystal plane (Figure 2b2), which illustrates that the CuMnO2 nanowires grow along the extended direction of MnO6 octahedron layers. CuMnO2 obtained at 180 oC exhibited the hexagonal prisms morphology (Figure 1c, 2c1) with exposed lattice spacing of 0.25 nm (Figure 2c2) corresponding to the (110) plane. CuMnO2 obtained at 200 oC and 220 oC shared the octahedral morphology (Figure 1d, 1e, 2d1, 2e1), while the CuMnO2 crystal obtained at 220 oC was relatively larger in size. They also shared clear lattice spacing of 0.57 nm which is identified to the (001) plane (Figure 2d2, 2e2). There is no specific morphology and uniform crystal size for the contrast bulk-CuMnO2 (Figure 1f) which was obtained by conventional solid state method. These results confirm that the CuMnO2 crystal seeds have oriented growth behaviors along specific crystal planes when we effectively controlled hydrothermal temperatures with CTAB as a directing agent.

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Figure 2. TEM images, HRTEM images, and corresponding FFT patterns of the CuMnO2 crystals grown by controlling the synthetic temperatures: (a1,a2,a3) 140 oC; (b1,b2,b3) 160 oC; (c1,c2,c3) 180 oC; (d1,d2,d3) 200 oC; and (e1,e2,e3) 220 oC.

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Table 1. Elemental analysis from EDS data for CuMnO2 crystals grown by controlling the synthetic temperatures. Wt% of elements 140 oC 160 oC 180 oC 200 oC 220 oC

Cu 41.59 41.19 44.62 44.16 42.56

Mn 33.29 33.45 33.65 36.39 39.48

At% of elements O 25.12 25.36 21.73 19.45 17.96

Cu 23.12 22.69 26.13 27.02 26.67

Mn 21.42 21.44 22.95 25.74 28.62

O 55.46 55.87 50.92 47.24 44.71

EDS spectra were measured to detect elemental information in CuMnO2 crystals grown by controlling the synthetic temperatures. The semi-quantitative data revealed that all CuMnO2 crystals had near composition with about 25 atom % of Cu, 25 atom % of Mn, and 50 atom % of O, which is close to the stoichiometric ratio for CuMnO2. We representatively selected the CuMnO2 crystal obtained at 180 oC with hexagonal prism morphology to conduct elemental mapping measurement (Figure 3). Obviously, Cu, Mn, and O elements homogenously distributed through the hexagonal prism (Figure 3b-d).

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Figure 3. (a) Representative SEM images and corresponding elemental mapping images for (b) Cu, (c) Mn, (d) O, and (e) EDS spectrum of CuMnO2 crystal obtained at 180 oC.

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3.2. Various valence states of CuMnO2 crystals grown by controlling temperature

Figure 4. XPS spectra of CuMnO2 crystals prepared at given temperatures: (a) Cu 2p; (b) Cu LMM; (c) Mn 2p; and (d) Mn 3p. For comparison, the relevant data for bulk CuMnO2 is also given.

The valence state changes of the CuMnO2 crystals during the morphology variation were examined by XPS (Figure 4) and XANES (Figure 5). The survey spectra of all the crystals were presented in Figure S2. The CuMnO2 obtained at 140 o

C exhibits Cu 2p spectrum (Figure 4a) of two spin-orbit signals at 933.9 eV and

953.8 eV which are assigned to Cu 2p3/2 and Cu 2p1/2, respectively.18 The strong 2p3/2 and 2p1/2 satellites demonstrate a Cu2+ oxidation state, as reported elsewhere,18 which can be also confirmed by Cu LMM spectrum (Figure 4b) with a kinetic energy of 917.5 eV, closer to that of 917.7 eV for standard CuO.19 The Mn 2p core level spectrum (Figure 4c) has an obvious satellite feature at around 648 eV, indicating a

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Mn2+ oxidation state,20 which can be also verified by Mn 3p spectrum (Figure 4d) with a featured peak at 49.8 eV, closer to the positions reported previously.21-22 As for the CuMnO2 obtained at 160 oC, the Cu 2p3/2 peak (Figure 4a) shifts towards lower binding energy with the satellites becoming weak, which is attributed to an decline in average valence of copper,23 in other words, a mixed oxidation state of Cu+/Cu2+. It is further demonstrated by the fact that the Cu Auger peak (Figure 4b) shifts to 917.2 eV. The characteristic satellite of Mn 2p core level spectrum (Figure 4c) obviously became weak, indicating a mixed oxidation state of Mn2+/Mn3+,21-22 which is also confirmed by the fact that Mn 3p peak (Figure 4d) shifts to 49.6 eV. CuMnO2 obtained at 180 oC had narrower Cu 2p3/2 peak (Figure 4a) with a further shift to lower binding energy, showing similar tendency reported in other systems,24-25 which corresponds to the presence of a dominant Cu+ in mixed Cu+/Cu2+ oxidation state. This conclusion is also confirmed by the fact that Cu Auger peak (Figure 4b) shifts to 916.7 eV. The Mn 2p satellite (Figure 4c) further became weak and the Mn 3p peak (Figure 4d) shifts to 48.9 eV, similar to the position reported elsewhere,18 illustrating the presence of a dominant Mn3+ in mixed Mn2+/Mn3+ oxidation state. When the growth temperatures reached 200 oC and 220 oC, the Cu 2p3/2 peaks (Figure 4a) kept on shifting to about 932.3 eV, which is assigned to a Cu+ oxidation state.18 The Cu Auger peaks (Figure 4b) shifted and stabilized at around 917 eV approximating that of 916.8 eV for standard Cu2O,19 which perfectly corresponded to the Cu+ oxidation state. This is also supported by the fact that Mn 2p satellites (Figure 4c) fade and Mn 3p peaks (Figure 4d) finally shifts to around 48.5 eV, relating to a Mn3+ oxidation 13



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state.21-22

Figure 5. O K-edge XANES spectra of CuMnO2 crystals prepared at given temperatures. For comparison, the relevant data for bulk CuMnO2 is also given.

O K-edge XANES normalized from 523 eV to 563 eV were detected to further corroborate the valence state changes (Figure 5). Peaks a and b can be indexed to the Mn-O hybridization according to previous reports.26 CuMnO2 obtained at 140 oC showed peak a at 528.6 eV, exhibiting an obvious shift towards higher energy at 529 eV when the crystal growth temperature exceeded 180 oC, which is originated from the rising valence state of Mn cations (Mn2+→Mn3+), as reported previously.26-27 Based on the crystal field theory, Mn2+ has the electronic configuration of 3d5 (t2g3eg2) 14


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and that of Mn3+ is 3d4 (t2g3eg1). So the electron will preferentially occupy the t2g orbital of Mn2+ with low energy (528.6 eV) while it will preferentially occupy the eg orbital of Mn3+ with high energy (529 eV). CuMnO2 nanowires obtained at 160 oC showed peak a at 528.8 eV between 528.6 eV and 529 eV, confirming the mixed oxidation state of Mn2+/Mn3+. The peak b at 530.1 eV can be assigned to the transition from O1s to the eg↓ orbital of Mn3+.26 CuMnO2 obtained by 140 oC showed no peak b, demonstrating almost no Mn3+ oxidation state. With the growth temperature increasing, peak b appeared at 160 oC and gradually became more and more intense, indicating an increase of Mn3+ content. Peaks c and d at 530.9 eV and 531.7 eV respectively corresponds to 3d orbital hybridization with O in Cu2+ and Cu+.28 The intensities of them represent the contents of Cu2+ and Cu+, respectively. CuMnO2 obtained at 140 oC exhibited the most intense peak c but no peak d, which demonstrated a pure Cu2+ oxidation state. When the growth temperature increased, peak c gradually faded whereas peak d appeared, which can be explained by the valence state change from Cu2+ to Cu+. Peaks e and f are attributed to 4s4p orbital hybridization with O in Cu and Mn, respectively.28 Peak e shifted to a higher energy while peak f shifted to a lower energy with the increasing crystal growth temperature. The opposite changing tendency of peak e and f coincides well with the discussion above. Relevant peak positions in XPS and XANES were summarized in Table 2. Based on the analysis about XPS and XANES, incredibly, the controlling crystal growth process is accompanied by the valence state changes of copper and manganese with especial redox equilibrium in this system. 15



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Table 2. Peak positions determined by XANES and XPS data for the CuMnO2 crystals grown by controlling the synthetic temperatures. For comparison, the relevant data for bulk CuMnO2 are also given. XANES o

140 C 160 oC 180 oC 200 oC 220 oC Bulk-CuMnO2

Peak a (eV) 528.6 528.8 529.0 529.0 529.0 528.9

Peak e (eV) 534.2 534.9 535.3 535.2 535.3 535.3

XPS Peak f (eV) 541.3 540.3 540.0 539.9 539.9 540.0

Cu LMM (eV) 917.5 917.2 916.7 917.1 917.0 916.9

Mn 3p (eV) 49.8 49.6 48.9 48.4 48.5 48.4

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3.3. Stable structures of CuMnO2 crystals grown by controlling temperature

Figure 6. (a) XRD patterns of CuMnO2 crystals prepared at given temperatures; (b) Enlarged XRD patterns from 30o to 38o; (c) Three-dimensional XRD patterns of CuMnO2 crystals to compare the relative intensity of (002) peak; (c) schematic diagram for the monoclinic layered structure of crednerite CuMnO2 crystal observed along a axis.

The changes in morphology and valence state usually lead to crystal structural

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change.29 We combined XRD (Figure 6a, 6b, and 6c), Raman (Figure 7a), and FT-IR (Figure 7b) technologies to monitor the structure in this material system. All the CuMnO2 crystals grown by controlling the synthetic temperatures showed pure monoclinic CuMnO2 (JCPDS No.50-0860) with C2/m space group symmetry and lattice parameters of a = 5.596 Å, b = 2.88 Å, c = 5.899 Å, and β = 104.02o in accord with previous reports,11, 30 implying that the crystal growth and valence state can be effectively controlled without significant changes in structure (Figure 6a). Distinctly, the relative intensity changes of (002), (200), and (11-1) diffraction peaks were found for the crystals grown at given temperatures in enlarged XRD patterns (Figure 6b). The three schematic insets from top to down corresponds to (002), (200), and (11-1) crystal planes, respectively. It is apparently that the (002) peak gradually shifts to a lower degree with the crystal growth temperature increasing, which is attributed to the increase of interlayer spacing according to Bragg equation. The valence state tailoring from Cu2+ to Cu+ certainly will leads to enlarged Cu ionic radius, which induced the lattice expansion along c axis. The XRD data were normalized by (002) peak to show the diffraction intensity ratios of 200 (I200) and 11-1 (I11-1) to 002 (I002), listed in Table 3. Differing from the bulk CuMnO2 prepared by solid state method, all CuMnO2 crystals grown by hydrothermal method show the most intense (002) diffraction peaks, suggesting preferred orientation along the (002) direction.31 When the reaction temperature exceed 160 oC, the ratios of I200 and I11-1 to I002 rise gradually, similar to other reports,29,

31

which means that the crystals obtained beyond 160 oC had

bidirectional oriented growth along (200) and (11-1) directions. Notably for CuMnO2 18


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obtained at 160 oC, the relative intensity of (002) is far stronger than other crystals, which can be intuitively observed in Figure 6c. It is attributed to the unidirectional oriented growth of CuMnO2 nanowires along the (002) direction, which is in agreement with SEM and HRTEM result. The monoclinic layered structure of crednerite CuMnO2 crystal was schematically shown observed along a axis (Figure 6d). The unique structure contains stable edge-shared MnO6 octahedron layers with coordinated copper cations in the interlayer, which has advantage on accommodating some ions.32 Table 3. Diffraction intensity ratios of 200 (I200) and 11-1 (I11-1) to 002 (I002) for CuMnO2 crystals grown by controlling the synthetic temperatures. diffraction intensities o

140 C 160 oC 180 oC 200 oC 220 oC Bulk-CuMnO2 JCPDS No.50-0860

I200 181 328 354 260 329 759 100

I11-1 325 473 531 422 500 802 80

ratios I002 415 2183 1233 798 548 542 76

I200/ I002 0.436 0.1503 0.2871 0.3258 0.600 1.40 1.32

I11-1/ I002 0.783 0.2167 0.4307 0.5288 0.912 1.48 1.1

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Figure 7. (a) Raman spectra and (b) FT-IR spectra of CuMnO2 crystals prepared at given temperatures. For comparison, the relevant data for the bulk CuMnO2 is also given.

Raman and FT-IR spectra of all these CuMnO2 crystals were recorded to further confirm the structural information (Figure 7). The Raman bands (Figure 7a) at around 680 cm-1, 386 cm-1, and 306 cm-1 are mainly ascribed to the crednerite-CuMnO2 phase.33 According to the literature,33 the crednerite-CuMnO2 phase is a delafossite-derivative structure with two Raman active modes of A1g and Eg. The A1g active mode is due to O-Cu-O vibration along c axis while the Eg active mode is attributed to MnO6 octahedron vibration along a axis.34 For crednerite-CuMnO2 phase, like other Cu-based ABO2 compounds, the A1g active mode is located at about 680 cm-1, while the Eg active mode is located at around 386 cm-1.35-36 Apparently, the A1g

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band positions of all CuMnO2 crystals synthesized by controlling hydrothermal temperatures have various deviations compared to bulk-CuMnO2, corresponding to changes in O-Cu-O bond length and relevant lattice energy, based on previous reports.5, 37 FT-IR spectra (Figure 7b) which possess bands in the range of 400-1200 cm-1 are usually assigned to active vibrations in bimetallic oxides.38 According to previous report, 39 manganese based oxides have stretching, bending and wagging vibrational modes in MnOn polyhedral, related to the ranges of 250-450 cm-1, 450-600 cm-1, and 600-750 cm-1. For our CuMnO2 crystals grown by controlling the synthetic temperatures, the bands at 424 cm-1 and 725 cm-1 are assigned to Mn-O stretching and wagging vibrations, respectively. The broad bands at the range of 450-600 cm-1 are obviously asymmetric, which is due to the overlying of bending vibration of Mn-O in MnO6 octahedron and vibration of Cu+ linearly coordinated by O2- ions, according to the literatures.10, 38 The Raman and FT-IR results further proved that the CuMnO2 fundamental structure remain stable while the morphology and the valence state get changed.

The results mentioned above strongly demonstrate that the crystal structure for CuMnO2 was retained in spite of a valence state change. To understand this abnormal phenomenon, we have to recall the structural feature of CuMnO2 and the sample characterization data. As we all know, valence state change certainly will leads to electronic structure and ionic radius change. The tailored process from Cu2+ to Cu+ corresponded to enlarged Cu ionic radius, whereas the process from Mn2+ to Mn3+ generated shrunken Mn ionic radius. It is the unique properties of CuMnO2 that 21



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tolerated the significant changes in electronic structure and ionic radius. When the crystal growth temperature increases, on the one hand, the layered structure with large interlayer space provides accommodation for Cu cations to expand, which only leads to an increase of interlayer spacing from XRD observation. On the other hand, Mn3+ cations with 3d4 (t2g3eg1) electronic configuration share octahedral field with Mn2+ cations with 3d5 (t2g3eg2) electronic configuration, which maintained the stable edge-shared octahedron layers from Raman and FT-IR observation. To hold the fundamental structure, such valence states tailoring of bimetallic oxides CuMnO2 with multiple lattice sites were bound to induce the lattice energy transformation which was behaved by specific oriented crystal growth.

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3.4. Electrochemical Energy Storage of CuMnO2 crystals grown by controlling temperature

Figure 8. (a) A comparison of CV curves at a scan rate of 50 mV/s for CuMnO2 crystals prepared at given temperatures; (b) Enclosed CV curve area and peak current from comparative CV curves; (c) A comparison of GCD curves at a current density of 2 A/g for CuMnO2 crystals prepared at given temperatures; (d) Specific capacitances comparison from GCD curves; (e) Nyquist plots for CuMnO2 crystals prepared at

23



Page 24 of 36

given temperatures; (f) Cycling stability at 5A/g for the CuMnO2 crystal obtained at 160 oC. For comparison, the relevant data for bulk CuMnO2 is also given.

The properties of the crystals closely connect with the structural change in general. However, we have controlled the crystal growth and valence state of CuMnO2 crystals with the structure unchanged to investigate the electrochemical energy storage abilities. A comparison of cyclic voltammetry (CV) curves (Figure 8a) of CuMnO2 crystals were tested in a voltage range of 0-0.5 V at a scan rate of 50 mV/s. CV curves at different scan rates i.e. 5, 10, 20, 50, and 100 mV/s of every electrode (Figure S3) showed typical redox peaks, implying ideal pseudocapacitive behaviors that involves reversible faradaic redox reactions of M-O/M-O-OH (M represents Cu and Mn) with the OH- ions in alkaline electrolyte.40-41 Especially for the CuMnO2 crystal obtained at 160 oC, the interval of redox peak positions significantly increased with the increasing scan rates, implying a dominant diffusion-controlled process during pseudocapacitive reaction, as reported elsewhere.27,

41

The

diffusion-controlled process is only limited by the ion diffusion rate, so that controllable oriented growth of one-dimensional CuMnO2 crystals can effectively liberate the surface limitation of the bulk material. A histogram (Figure 8b) that exhibited the graphing key indicators was given based on the comparative CV curves. The CuMnO2 crystal obtained at 160 oC showed the highest peak current and relatively larger enclosed CV curve area, therefore greater capacitive ability.42 A comparison of galvanostatic charge–discharge (GCD) curves (Figure 8c) were measured at a current density of 2 A/g. The CuMnO2 crystal obtained at 160 oC 24


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exhibited the longest charge-discharge platform among all the crystals, illustrating superior surface diffusion efficiency of the unique one-dimensional nanowires.43 The GCD curves at different current densities i.e. 1, 2, 5, and 10 A/g of every electrode (Figure S4) was asymmetric, corresponding to typical faradaic pseudocapacitive behavior.44-45 Specific capacitances at different current densities which were calculated by equ 1 were compared in Figure 8d. The CuMnO2 crystal obtained at 160 o

C exhibited remarkably optimum specific capacitance (921 F/g at 1 A/g), which was

distinctly superior to other crystals. EIS results were shown in Figure 8e. The CuMnO2 obtained at 160 oC showed a straight line with an obviously higher slope than other electrodes in low-frequency region, meaning the lowest ion diffusion resistance and the best capacitive behavior.46 This observation can be attributed to two reasons. One is that unidirectional oriented growth of one-dimensional CuMnO2 nanowires along the (002) direction maximized the diffusion efficiency of electrolyte ions. Another is that the especial redox equilibrium in Cu+/Cu2+ and Mn3+/Mn2+ increased the efficiency of reversible faradaic redox reactions, which promoted the charge transfer. Long term cycling stability is an important criterion to evaluate whether the material is suitable for practical application of supercapacitors. The long term cycling stability of CuMnO2 obtained at 160 oC was shown in Figure 8f. CuMnO2 crystal obtained at 160 oC retained 90.7% of the initial capacitance after 1000 cycles, far better than 59.2% capacitance retention for the bulk CuMnO2 after same cycles.

4. CONCLUSION 25



Page 26 of 36

In summary, a library of bimetallic layered crednerite CuMnO2 crystals with controllable morphologies and tailored valence states were synthesized via a CTAB modified hydrothermal method. By controlling crystal growth temperature, tailored valence states were implemented along with the monoclinic layered structure well stabilized. Various morphologies of CuMnO2 crystals were firstly obtained along the tuned process. The oxidation states of Cu2+ and Mn2+ are found for triangular sheets (140 oC), which transformed to the mixed valency Cu+/Cu2+ and Mn3+/Mn2+ for nanowires (160 oC), and then to the dominant Cu+ and Mn3+ oxidation states for hexagonal prisms (180 oC) and octahedrons (over 200 oC). The CuMnO2 nanowires were found to preferentially grow along (002) plane, which when first applied to supercapacitor, exhibited the highest specific capacitance of 921 F/g at the current density of 1 A/g. The synergistic effect between one-dimensional unidirectional oriented growth and especial redox equilibrium is the key to optimize the electrochemical energy storage. The methodology reported in this work not only inspires exploration of bimetallic oxides ABOx (x = 2, 3, and 4) with tuned valence states as well as stable crystal structure for high-performance energy applications, but also builds a bridge between the synergistic effect of morphology and valence state with the material performance.

ASSOCIATED CONTENT

Supporting Information Available:

Schematic diagram, XPS survey spectra, CV curves, and GCD curves of CuMnO2 26


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samples synthesized by different conditions (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work is financially supported by National Natural Science Foundation of China (NSFC) (Grants 21025104, 21671077, 21771171, 21571176, and 21611530688). We acknowledge the National Synchrotron Radiation Laboratory (NSRL) beamline BL12B-a for providing beam time.

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For Table of Contents Use Only

Crystal Growth of Bimetallic Oxides CuMnO2 with Tailored Valence States for Optimum Electrochemical Energy Storage Sixian Fu,† Liping Li,† Yuancheng Jing,† Yuelan Zhang,† Xiyang Wang,† Shaofan Fang,‡ Jianghao Wang,‡ and Guangshe Li*,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun 130012, P.R. China

‡ Fujian Institute of Research in Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P.R. China

Bimetallic layered crednerite CuMnO2 was prepared by a CTAB modified hydrothermal method. Fine-tuning on crystal growth temperature accomplished effective valence states tailoring and a library of morphology changes along with the stabilization of crystal structure for optimum electrochemical energy storage.

36


Crystal Growth of Bimetallic Oxides CuMnO2 with Tailored Valence

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