Phthalocyanine‐Based 2D Conjugated Metal‐Organic Framework Nanosheets for High‐Performance Micro‐Supercapacitors

2D conjugated metal‐organic frameworks (2D c‐MOFs) are emerging as a novel class of conductive redox‐active materials for electrochemical energy storage. However, developing 2D c‐MOFs as flexible thin‐film electrodes have been largely limited, due to the lack of capability of solution‐processing and integration into nanodevices arising from the rigid powder samples by solvothermal synthesis. Here, the synthesis of phthalocyanine‐based 2D c‐MOF (Ni2[CuPc(NH)8]) nanosheets through ball milling mechanical exfoliation method are reported. The nanosheets feature with average lateral size of ≈160 nm and mean thickness of ≈7 nm (≈10 layers), and exhibit high crystallinity and chemical stability as well as a p‐type semiconducting behavior with mobility of ≈1.5 cm2 V−1 s−1 at room temperature. Benefiting from the ultrathin feature, the nanosheets allow high utilization of active sites and facile solution‐processability. Thus, micro‐supercapacitor (MSC) devices are fabricated mixing Ni2[CuPc(NH)8] nanosheets with exfoliated graphene, which display outstanding cycling stability and a high areal capacitance up to 18.9 mF cm−2; the performance surpasses most of the reported conducting polymers‐based and 2D materials‐based MSCs.


Introduction
2D conjugated metal-organic frameworks (2D c-MOFs), [1] which represent a new generation of layer-stacked MOFs with strong in-plane conjugation and weak out-of-plane van der Waals interactions, [2] have emerged as promising electrode materials for energy applications, [3] owing to the intrinsic conductivity, versatile structures, [3b] and well-defined active sites as well as tunable porosity. [4] Particularly, 2D c-MOFs have been reported to exhibit high performance in supercapacitors. [5] For instance, a Ni 3 (HITP) 2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) 2D c-MOF with diradical nickel-bis(o-diiminobenzosemiquinonate) active sites has shown a gravimetric capacitance of ≈110 F g −1 and an areal capacitance of ≈18 μF cm −2 in an electrochemical double layer (EDL) supercapacitor. [5b] Besides the EDL charge storage, another Ni 3 (HIB) 2 (HIB = hexaiminobenzene) 2D c-MOF has exhibited a pseudocapacitive charge storage with a volumetric capacitance up to 760 F cm −3 . [5c] Despite the great progress of 2D c-MOFs for supercapacitors, the utilization of 2D c-MOFs as thin-film electrodes [6] for flexible micro-supercapacitors (MSCs) [7] -a smaller and lighter micro-power source for next-generation portable electronic devices [8] -has been largely impeded by the lack of capability of solution-processing and integration into nanodevices arising from the rigid particle morphology of the bulk powders by solvothermal synthesis. To address this challenge, the delamination of bulk materials into thin nanosheets is an efficient approach that can maintain the 2D nature and intrinsic conductivity. [9] For instance, ultrasonic treatment has been demonstrated to the top-down synthesis of a 2D non-conjugated MOF (Zn 2 (TPA) 4 (H 2 O) 2 ⋅2DMF, TPA = terephthalate, and DMF = dimethylformamide) nanosheets, [9a] which are held together by the weak hydrogen-bonding interactions in the bulk crystals. Whereas the nanosheets allow the high utilization of active sites and facile solution-processing toward device fabrication, [10] top-down exfoliation of van der Waal stacked 2D c-MOFs into thin nanosheets and the related MSC devices have remained unexplored.
Herein, we report the synthesis and delamination of a novel phthalocyanine-based layer-stacked 2D c-MOF (Ni 2 [CuPc(NH) 8 ]) with Ni-bis(o-diiminobenzosemiquinonate) redox active centres into nanosheets. The highly crystalline bulk samples with the domain size of ≈200 nm were prepared by solvothermal synthesis, and presented high specific surface area up to 690 m 2 g −1 and high chemical stability in acid/alkaline aqueous solution and other polar solvents. Subsequently, NaCl-assisted ball milling was performed for the mechanical exfoliation of the bulk crystals into nanosheets. The achieved 2D c-MOF nanosheets feature with average lateral size of ≈160 nm and mean thickness of ≈7 nm (corresponding to ≈10 layers), which can be homogeneously dispersed in DMF for more than 6 months. The conductivity of the 2D c-MOF nanosheets was determined as 0.01 S m −1 based on a van der Pauw pattern. The Hall effect measurement revealed a p-type semiconducting behavior with mobility of ≈1.5 cm 2 V −1 s −1 at room temperature. Benefiting from the intrinsic conductivity and porosity, high crystallinity, and ultrathin feature that allows high exposure of active sites and fast ion diffusion as well as facile film processability of the nanosheets samples, MSC devices were fabricated based on Ni 2 [CuPc(NH) 8 ]/graphene hybrids, which exhibited outstanding cycling stability and a high areal capacitance up to 18.9 mF cm −2 . The achieved areal capacitance is superior to most of the reported conducting polymers and 2D materials for MSCs (Table S1, Supporting Information). Our work opens up an avenue on the top-down scalable synthesis of conductive 2D c-MOF nanosheets toward energy storage applications with high performance.

Synthesis and Characterization of Bulk Crystals of Ni 2 [CuPc(NH) 8 ]
The bulk Ni 2 [CuPc(NH) 8 ] was synthesized through the coordination reaction between 2,3,9,10,16,17,23,24-octaaminophthalocyaninato copper [II] (OAPcCu) ligand (Figure 1a Figure S1, Supporting Information). The broad peak at 27.51°-that is common in layered 2D framework materials-suggests less coherence of the stacked Ni 2 [CuPc(NH) 8 ] layers in the out-of-plane direction and corresponds to an interlayer distance of 3.24 Å. Additionally, we performed structural modeling with several possible interlayer arrangements (Figure 1c and Figure S2, Supporting Information) by density functional theory (DFT) method. As shown in Figure 1b, the experimental XRD pattern rules out the AA-eclipsed, AA-inclined as well as AB stacking, and can be reproduced well with AA-serrated geometry, which is energetically most favored (Table S2, Supporting Information). Therefore, the peak at 27.51° is assigned to the (002) lattice plane and Ni 2 [CuPc(NH) 8 ] can be fully defined by square unit cells with a = b = 18.0 Å. Field-emission scanning electron microscopy indicated that this MOF presented stacked sheet-like morphologies, where the size of sheets was estimated as 100-300 nm (Figure 1d and Figure S3, Supporting Information). Corresponding energy-dispersive X-ray (EDX) spectroscopy evidenced the uniform element distribution of C, N, Cu, and Ni ( Figure S4, Supporting Information). Transmission electron microscopy (TEM) imaging showed the layer-stacked crystallites ( Figure 1e and Figure S5, Supporting Information). Fast Fourier transform (FFT) analysis confirmed the (100) crystallographic plane, which agrees well with the XRD result.
Fourier-transform infrared (FT-IR) spectroscopy revealed the disappearance of the NH stretching vibration band from the monomer OAPcCu in the Ni 2 [CuPc(NH) 8 ] ( Figure S6, Supporting Information), which further demonstrated the successful coordination polymerization. The porosity of Ni 2 [CuPc(NH) 8 ] was evaluated by low-pressure CO 2 adsorption at 195 K ( Figure S7a, Supporting Information), which showed a type I isotherm and a BET surface area of 690 m 2 g −1 . According to the quench solid DFT, the average pore size was calculated as 0.9-1.7 nm. Analogous to CO 2 , nitrogen adsorption at 77.3 K revealed similar behavior with surface area of 659 m 2 g −1 and pore size of 0.6-1.5 nm ( Figure S7b, Supporting Information). Thermogravimetric analysis (TGA) exhibited no significant weight loss at temperature up to 310 °C in argon ( Figure S8, Supporting Information). To determine the chemical stability of Ni 2 [CuPc(NH) 8 ], various organic solvents and aqueous solutions, including DMF, ethanol, water, 1 m KOH, and 1 m HCl were examined; the related XRD patterns demonstrated the integrity of its crystalline structure after immersing for 24 h at room temperature ( Figure 1f). Notably, the (100) and (002) peaks were shifted from 4.91° and 27.51° to 5.58° and 27.06° after soaking in 1 m HCl, respectively, which is attributed to the protonation of imine units in MOFs.

Ni 2 [CuPc(NH) 8 ] 2D c-MOF Nanosheets
Ball milling has been demonstrated as an efficient top-down synthetic strategy [11] for preparing graphene, 2D carbon nitrides, and 2D transition metal dichalcogenides nanosheets. Using the low energy ball milling method, the well-defined bulk Ni 2 [CuPc(NH) 8 ] crystals were mechanically exfoliated into nanosheets with 40-50% yield. During the exfoliation, the addition of NaCl as controlling agent played a key role in inserting into the layers, reducing the shear forces and mildly separating the stacked MOF layers (Figure 2a), which resulted in larger-sized nanosheets compared with the contrast flakes obtained in the absence of NaCl. The resultant nanosheets can be homogeneously dispersed in DMF for more than 6 months after mild sonication (Figure 2b and Figure S9, Supporting Information). Figure 2c shows the XRD pattern of the MOF nanosheets, which present intensive (100) and (002) diffraction peaks. Notably, the shift of the (002) peak from 27.51° to 26.74° correlates with an increase in the c parameter from 3.24 Å to 3.33 Å, indicative of swollen layers after delamination by the milling process. SEM and TEM images (Figure 2d and Figure S10, Supporting Information) reveal thin nanosheets with size distribution ranging from 50 to ≈600 nm. Based on the statistical analysis of 220 individual nanosheets, the average size was estimated to be ≈160 nm ( Figure S11, Supporting Information). High resolution TEM image unambiguously confirmed the square unit cells at the molecular level with lattice parameter of a = b = 1.7 nm in the nanosheets (Figure 2e). Atomic force microscopy (AFM) images revealed the mean thickness of the nanosheets as ≈7 nm, corresponding to ≈10 layers of Ni 2 [CuPc(NH) 8 ] (Figure 2f).
After exfoliation, the composition in the nanosheet samples was remained, as suggested by FT-IR spectroscopy characterizations ( Figure S12, Supporting Information). The survey spectrum by X-ray photoelectron spectroscopy further revealed the presence of C, N, Cu, Ni, and O elements ( Figure S13, Supporting Information). Deconvolution of the N(1s) and Ni(2p) signals showed three types of N (N = CN, NNi, and NCu) and a single type of Ni (NiN) atoms. No other counter ions could be detected, which reveals that Ni 2 [CuPc(NH) 8 ] is a charge-neutral material. To investigate the charge transport property, macroscopic conductivity measurement was applied in a van der Pauw configuration on the assembled nanosheets ( Figure S14, Supporting Information). Linear current-voltage (I-V) curves confirm the ohmic contact during the whole measurement (Figure S15, Supporting Information); temperature (T)-dependent measurement indicated a semiconducting nonlinear drop of conductivity (σ) upon cooling from 310 to 100 K (Figure 2g). Based on the above data, we obtained a conductivity of ≈0.01 S m −1 at 300 K. As shown in the inset of Figure 2g, the plot of σ versus T −1/4 over the measured temperature region can be well fitted to the Mott variable range hoping (Mott-VRH) , which describes the temperature dependence of hopping conductivity in the polycrystalline nanosheets. Hall effect measurement was applied under a magnetic field (−4 to 4 T) perpendicular to the sample plane at 300 K. Ni 2 [CuPc(NH) 8 ] nanosheets exhibit a linear relationship of Hall resistance to the magnetic field ( Figure 2h); moreover, the polarity indicates a p-type semiconducting behavior with carrier (holes) density of ≈6 × 10 14 cm −3 and mobility of 1.6 ± 0.2 cm 2 V −1 s −1 .

Ni 2 [CuPc(NH) 8 ]-Based MSCs
Benefiting from the intrinsic conductivity and porosity, high crystallinity, and ultrathin feature that provide sufficient accessible active sites [12] and fast ion diffusion [13] as well as facile film processability, 2D c-MOF nanosheets hold great potential application in flexible energy storage devices. As a proof of concept, Ni 2 [CuPc(NH) 8 ] nanosheets were mixed with electrochemically exfoliated graphene (EG, conductive platform, exfoliation details seen in Supporting Information) nanosheets (in different mass ratios) and filtered subsequently with a home-made mask into interdigital electrodes for flexible MSCs (Figure 3a,b and Figures S16 and S17, Supporting Information). The electrochemical behavior of MSCs based on pure EG thin film and Ni 2 [CuPc(NH) 8 ]/EG-x (x represents the mass ratio of graphene/ Ni 2 [CuPc(NH) 8 ]) were investigated firstly by cyclic voltammetry (CV) measurement. It is clearly shown that the integral area of CV curve for Ni 2 [CuPc(NH) 8 ]/EG-2 based MSC (abbreviated as Ni 2 [CuPc(NH) 8 ]/EG-2) presents the maximum value compared with other fabricated devices (Figure 3c). Therefore, the weight ratio of graphene/Ni 2 [CuPc(NH) 8 ] = 2 was found to be the optimized one for the construction of Ni 2 [CuPc(NH) 8 ]/ EG-based MSC electrode. The CV profiles of Ni 2 [CuPc(NH) 8 ]/ EG-2 display the nearly rectangular shape at scan rates from 2 to 50 mV s −1 (Figure 3d), resulting from the high specific surface area and well-defined structure of Ni 2 [CuPc(NH) 8 ]. In addition, the capacitive behavior of Ni 2 [CuPc(NH) 8 ]/EG-2 was investigated through galvanostatic charge-discharge (GCD) measurement at various current densities of 0.04-0.4 mA cm −2 (Figure 3e).

Conclusion
In summary, we demonstrated the efficient synthesis and delamination of phthalocyanine-based 2D c-MOF Ni 2 [CuPc(NH) 8 ] into ultrathin nanosheets by top-down ball milling exfoliation. The ultrathin Ni 2 [CuPc(NH) 8 ] nanosheets possess intrinsic conductivity, porosity and high crystallinity as well as high chemical stability. For the utilization as energy storage material, the Ni 2 [CuPc(NH) 8 ] 2D c-MOF based MSC device exhibited outstanding mechanical flexibility, cycling stability, and a high areal capacitance of 18.9 mF cm −2 . Our work provides a guideline for the top-down exfoliation of 2D c-MOF nanosheets and sheds light on realizing their function in flexi ble MSCs. By further modifying the chemical structures with redox active sites and improving the crystallinity and porosity as well as conductivity, we highlight the great potential of 2D c-MOFs for developing flexible electronic devices.