Compositional and Structural Evolutions of Zn-Based Metal-Organic

Jul 16, 2018 - Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal–Organic Framework Fe(1,2,3-triazolate)2(BF4)x. Jour...
1 downloads 0 Views 8MB Size
Download PDF
Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Compositional and Structural Evolutions of Zn-Based Metal− Organic Frameworks During Pyrolysis Ang Li, Yan Tong, Huaihe Song,* and Xiaohong Chen State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China

Downloaded via DURHAM UNIV on July 24, 2018 at 09:54:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Metal−organic framework (MOF)-derived nanostructures for electrochemical applications have attracted tremendous attention; therefore, understanding of the decomposition mechanism of MOFs during thermal treatment is crucial for the design and synthesis of MOF-derived nanomaterials. Here, a systematic investigation was carried out to study the pyrolysis process of a Zn-based metal− organic framework (Zn-MOF), which revealed the compositional and structural evolution by in situ diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis−differential scanning calorimetry, and X-ray diffraction methods. The continuous change of the nature of surface of pyrolytic Zn-MOF at different temperatures was also studied by the cyclic voltammetry method, described by the fractal concept of electrochemical surface. The results show that the pyrolysis of Zn-MOF occurs at ca. 450 °C and the decomposition products are amorphous carbon and ZnO. The pyrolysis temperature plays a decisive role in the formation of the porous structures of carbon matrices and the evolution of the surface geometry of the products. The presented approach would be instructive and informative for the preparation of MOF-derived nanostructures.

1. INTRODUCTION Metal−organic frameworks (MOFs) are a kind of crystalline porous materials with tunable pore structures and molecular compositions, which have got tremendous attention recently.1 MOFs are assembled by metal-containing units, i.e., secondary building units (SBUs), and organic linkers.1 To date, more than 20 000 different MOFs have been reported, which is still growing by varying the SBUs and the functionalities of the organic linkers.2 The selectable element compositions and the optional structures have made MOFs a class of excellent precursors to prepare carbon nanomaterials and their composites.3−5 During heat treatment, the carbonization of the organic molecules and the collapses of the inherent ordered porous structures can provide highly porous carbon (PC) materials.3 On the other hand, by varying the organic linkers, one can introduce different atoms other than carbon to prepare heteroatom-doped porous carbon materials.4 With large specific surface area (SSA), the MOF-derived porous carbon and its functional hybrid materials are usually used as electrochemical electrodes for energy storage and conversion, such as supercapacitors,6,7 batteries,8−11 and fuel cells.12 Znand Al-based MOFs with aromatic acids as ligands are the most commonly used precursors to prepare pure porous carbon, among which MOF-5,8,12−15 ZIF-8,10,16−18 and AlPCP19,20 were the most commonly used precursors and templates in the earlier studies. Scientists usually adjust the © XXXX American Chemical Society

pore properties of the resulting carbon products by adopting different kinds of precursors. Xu et al first used MOF-5 as the template and furfuryl alcohol as the secondary carbon source to prepare porous carbon at 1000 °C.14 The product showed a SSA as high as 2872 m2 g−1 and a specific capacity of 204 F g−1 at a scan rate of 5 mV s−1 when used as electrode for supercapacitors.14 This kind of precursor holds a record that the porous carbon product prepared by carbonization of AlPCP showed an ultrahigh specific surface area of 5500 m2 g−1 after a post-acid wash treatment.19 To prepare the MOF-based products with desired properties, it is necessary to investigate the pyrolysis processes of MOFs. Many literature studies have reported the effect of pyrolysis temperature on the product structures.5,17,21,22 Some of the cases showed that the SSA of the MOF-derived porous carbon materials exhibited a positive correlation with the pyrolysis temperature.5,21 However, it seems that there is no unified regularity between the variation of pyrolysis temperature and the properties of the products. This might be ascribed to the differences of the pyrolysis mechanism of the MOF precursors with various molecular and structural compositions. In addition, some teams found a linear Received: May 15, 2018 Revised: July 13, 2018 Published: July 16, 2018 A

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Orderly texture evolution characterization by X-ray methods. XRD patterns of (a) the pyrolytic Zn-MOF (P-T series) and (b) the resulting carbon matrices (PC-T series); (c) 1-D Lorentz-corrected SAXS profiles of the resulting carbon matrices (PC-T series products).

relationship between the carbon content of ligand molecules in MOFs and the SSA of the resulting porous carbon products, when they synthesized porous carbon materials via pyrolysis of a series Zn-containing MOFs.23,24 With the increasing Zn/C ratio in MOF precursors, the SSA of the resulting porous carbon products increases linearly. They attributed this behavior to the pore-forming effect of the ZnO and the evaporation of the reduced Zn. Some works reported the controllable synthesis of carbon materials with different pore size distributions (PSDs) from MOFs by varying the ligands.23−25 They claimed that the strut lengths and the conformation of SBUs play an important role in the formation of the pore structures of the products.24 To promote the understanding of the pyrolysis mechanism of MOFs, more systematic investigations on the evolution of MOFs at the molecular level and on functional structures are needed. Usually, the MOF-derived porous carbons possess large surface area, which can provide large active area for the electrochemical reactions.3−5 Therefore, the MOF-derived porous carbon and their composites are widely used in the field of electrochemical energy storage and conversion.3,4 All of the energy storage and conversion reactions depend strongly on the surface features of electrode materials, such as specific surface area, pore distribution in size and space, pore volume, and surface functional groups.4,26−29 Therefore, it would be helpful to study the evolution of the electrochemical active surfaces. Fractals, as an effective and convenient concept, are widely used to evaluate the irregularities of the active surface of the electrodes.28,30,31 During the last three decades, scientists have developed a series of electrochemical techniques to determine the fractal dimension (Df) of electrode surfaces.28,30,31 Strømme and co-workers have provided an easy and practical method for determining the Df of an electrode surface from cyclic voltammograms.31 Many reports on fractal studies of electrodes are published, such as electrochemical surface of real electrodes,30 Li insertion into electrodes,32 and capacitance dispersion on the electrode.28 Eftekhari has investigated the surface structure features of the LiMn2O4 electrode by determining the fractal dimensions with the cyclic voltammetry (CV) method and found that the surface morphology of the electrode changes significantly during the Li insertion/ extraction processes.32 Strømme’s work also provided the fractal study of two completely different Li storage electrodes, the conversion materials and the intercalation materials, by the diffusion-limited current of a voltammetric experiment.31

Considering the lack of the study on compositional and structural evolution of MOFs during pyrolysis, a systematic investigation is strongly needed to provide reference for the design and preparation of MOF-derived nanostructures. In this work, in situ diffuse reflectance Fourier transform infrared spectroscopy (in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS)) and thermogravimetricdifferential scanning calorimetry (TG-DSC) were used to investigate the compositional transformation process of MOFs at the molecular level during heat treatment and X-ray methods and N2-sorption methods were employed to reveal the structural evolution. In addition, the surface irregularity and porous electrode geometry of the resulting carbon matrix were studied with the cyclic voltammetry method by determining the fractal dimension.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Zn-MOF. The Zn-MOF was synthesized according to the procedure reported in the literature.13 In a typical synthesis process, 2.3 g of benzene-1,4-dicarboxylic acid and 11.9 g of zinc nitrate hexahydrate were dissolved into 160 mL of dimethyl formamide (DMF), then transferred into a Teflon-lined stainless steel autoclave, and maintained at 120 °C for 24 h. After filtration and washing by DMF, the Zn-MOF was collected and dried under vacuum at 150 °C overnight. 2.2. Pyrolysis of Zn-MOF. The precursor Zn-MOF was pyrolyzed at different temperatures in N2 flow to obtain the products. The pyrolytic Zn-MOF was washed with 1 M HCl aqueous solution to obtain the resulting porous carbon (PC). The sample pyrolyzed at temperature T °C is named P-T (T is the pyrolysis temperature), and the corresponding porous carbon sample is marked as PC-T. 2.3. Material Characterization. Field emission scanning electron microscopy (FE-SEM; ZEISS SUPRATM 55 field emission microscope), X-ray diffraction (XRD; Rigaku D/max2500B2+/PCX system with Cu Kα = 1.5406 Å, 2θ = 5−90°), and Raman spectroscopy (using a 532 nm laser, Aramis, Jobin Yvon) were carried out to investigate the morphology and structure of the samples. N2 sorption isotherms were measured with ASAP2020 (Micromeritics) with a degassed process at 300 °C for 6 h. In situ DRIFTS was also conducted on a Nicolet iS50 spectrometer to investigate the molecular changes of the precursor during pyrolysis. TG-DSC (Netzsch STA 449C) was performed in the temperature range of 20−1000 °C with a heating rate of 5 °C min−1. B

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 2. N2 sorption isotherms of (a) Zn-MOF and (b−f) PC-T series products; pore size distributions of PC-T in the (g) micropore range and (h) meso- and macropore range.

structures of Zn-MOF during the pyrolysis process were examined by XRD, as illustrated in Figure 1a. The patterns of P-300 and P-400 have kept the characteristic peaks of ZnMOF, indicating that the basic crystalline structures of ZnMOF are still maintained in this temperature range. By comparing with the reported data, it can be found that the patterns of P-300 and P-400 show typical characteristics of the XRD pattern of MOF-5.14 The characteristic peaks of only wurtzite-type ZnO (JCPDS No. 36-1451) can be observed when the temperature is higher than or equal to 500 °C, indicating the collapse of the inherent crystalline structure of Zn-MOF. It can be judged initially from the sudden change of the patterns between P-400 and P-500 that the pyrolysis of the MOF precursor begins above 400 °C. All of the XRD patterns of PC-T series products show two broad peaks at around 22 and 43°, suggesting the typical structure of amorphous carbon, as can be seen in Figure 1b. The intensity of these peaks becomes increasingly stronger with the increase of pyrolysis temperature, which can be ascribed to the increasing crystal degree of the resulting carbon matrices. The SAXS method is a powerful tool in the characterization of the order degree of material structures, especially the investigation of long-range ordered structures.32,33 The one-dimensional (1-D) Lorentzcorrected SAXS profiles of the resulting carbon matrices are

The small-angle X-ray spectroscopy (SAXS) experiment was performed using synchrotron radiation as an X-ray source at the 1W2A station at Beijing Synchrotron Radiation Facility (BSRF). The normalization of the data and the removal of background were possessed by I = Is − (Ks/Kb)Ib, where I is the processed scattering intensity after background subtraction; Is and Ib are the scattering intensity of the sample and background, respectively; and Ks and Kb are the transmitted Xray intensity of the sample and the background, respectively. 2.4. Electrochemical Characterization. Electrochemical measurements were made in a standard three-electrode system using an Ag/AgCl electrode as the reference and a Pt plate as the counter electrode, and the working electrode was an indium tin oxide plate modified with the samples. The voltammogram data were obtained using a 0.002 M K4[Fe(CN)6] and 0.2 M KCl aqueous solution as electrolyte at several scan rates between 10 and 200 mV s−1 on a CHI660B electrochemical working station.

3. RESULTS AND DISCUSSION 3.1. Ordered Texture Evolution of Zn-MOF. The crystalline structure of the pyrolytic Zn-MOF and the longrange structure of the resulting carbon matrices were characterized by XRD and SAXS, respectively. The crystalline C

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C displayed in Figure 1c. Usually, the peaks in the Iq2−q curves can be indexed to the ordered structures, whereas the shoulders can be ascribed to the long-range structures with low order degrees.33 Both PC-500 and PC-600 show a shoulder at ca. q = 0.5, indicating the ordered structure with a low orderly degree, on the mesoscopic scale. With the increase of pyrolysis temperature, the peak becomes increasingly weaker and even disappears above 600 °C. It can be concluded that the original ordered structures of Zn-MOF collapsed at ca. 500 °C and the ordered structures were destroyed completely after 600 °C. 3.2. Porous Structures of the Resulting Carbon Matrices. The N2 sorption method was used to investigate the porous structures of the resulting carbon matrices (i.e., PCT series products). All of the samples show well-developed porosity and high surface area, and the isotherms of the precursor and the products are illustrated in Figure 2a−f. The N2 sorption isotherms of Zn-MOF exhibit obvious characteristics of type I adsorption isotherm, indicating a typical micropore sorption behavior.14 However, all of the isotherms of PC-T series products illustrate obvious features of type IV with a rapid rising at the low P/P0 region and a hysteresis loop at the high P/P0 region.34 The rapid rising of the curves can be ascribed to the adsorption of N2 in micropores, and the hysteresis loop implies the existence of mesopores in PC-T series products.13 The analysis results of the porosity parameters are listed in Table 1. It can be seen that the SSA

about different pyrolysis and carbonization reactions, giving rise to various molecular configurations.35 Raman spectrum is used to investigate the molecular configurations and the defect structures of the resulting carbon structures. All of the samples showed four characteristic bands of carbon, D at 1340 cm−1, G at 1594 cm−1, 2D at 2750 cm−1, and (G + D) at 2990 cm−1.36 The D band and the G band represent the defect and disordered structures and the atomic vibration of the sp2 electronic configuration, respectively.36 The ID/IG intensity ratio is a significant and widely used factor to quantify the disorder degree of carbon materials. On the basis of the calculation data shown in Figure 3, it can be considered that

Table 1. Porosity Parameters of Zn-MOF and PC-T Series Products samples

SSA (m2 g−1)

Smicroa (m2 g−1)

V (m2 g−1)

Vmicroa (cm3 g−1)

Vmeso/V (%)

Zn-MOF PC-500 PC-600 PC-700 PC-800 PC-900

2304 1331 1513 1454 1364 1420

1866 77 126 149 141 203

1.44 1.74 1.82 1.78 1.74 1.85

0.97 0.04 0.06 0.07 0.07 0.10

32.64 97.70 96.70 96.07 95.98 94.59

Figure 3. Raman spectra of the resulting carbon matrices (PC-T series).

the disordered degree is positively associated with the pyrolysis temperature. According to the phenomenological three-stage model proposed by Ferrari, the formation of tiny crystalline domains at the surface of the carbon matrix occurs at the transitional stage from the amorphous phase to nanocrystalline graphite and the decreased sp2 content in the carbonized products leads to the increased ID/IG ratio.11 In addition, to obtain more information from the results, the spectra were fitted with four bands: A1, D, A2, and G (Figure S1). The additional band A1 located at 1180 cm−1 represents the sp3rich phase, and A2 at 1500 cm−1 is due to the semicircle ring stretch vibration of benzene or condensed benzene rings or in the case of a-C:H contributions of C−H vibrations.35−37 The fitting results show that the area of A2 band decreases as the temperature rises, demonstrating the increasing degree of carbonization.37 3.4. Characterization of Evolution of Molecular Composition by TG-DSC and in Situ DRIFTS. The in situ detection techniques are the most direct way to observe the evolution of Zn-MOF during the pyrolysis process. This section used TG-DSC and DRIFTS to track these changes of Zn-MOF in real time, as displayed in Figure 4. Two major weight-loss steps can be observed from the TG curve with reference to DSC thermic peaks, as can be seen in Figure 4a. The first step, corresponding to a 29.7% weight loss, is related to the removal of guest molecules, occurred in the range of 100−300 °C.38 Two stages can be distinguished in the first weight-loss step, one appears in the range of 100−250 °C and the other appears in the range of 250−300 °C. The DRIFT absorbance spectrum (Figure 4c,d) exhibits aliphatic ν(C−H) bands at 2880 and 2930 cm−1 and ν(N−CO) bands at 1688

a

Calculated using the t-plot method.

of PC-T is much less than that of Zn-MOF and the SSA of ZnMOF is mainly contributed by micropores. After pyrolysis, both the specific surface area and pore volume of micropores (denoted Smicro and Vmicro, respectively) of the resulting carbon matrices increase with the increasing pyrolysis temperature. The amount of micropores in porous carbon PC-T products increases with the increasing pyrolysis temperature, which can be ascribed to the intensification of the pyrolysis reactions at a higher temperature and the participation of ZnO nanoparticles.15 In addition, the evaporation of the reduced Zn can also provide sites for an extra amount of micropores.13 The contribution of the mesopores also shows a decreasing trend with the increasing pyrolysis temperature. The nonlocal density function theory model was also used to calculate the pore size distribution (PSD) with a condition set of slit pores. The PSD of PC-T series products is displayed in Figure 2g,h. All of the products show a hierarchical pore structure with a wide PSD from 0.52 to 54.4 nm, and as the pyrolysis temperature increases, the PSD becomes increasingly wider in the range of micropores. 3.3. Molecular Configurations of the Resulting Carbon Matrices. Different heating temperatures will bring D

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 4. In situ characterization of evolution of molecular composition of the pyrolytic Zn-MOF by (a) TG-DSC and (b−d) in situ DRIFTS.

Figure 5. SEM images of the pyrolytic Zn-MOF at different temperatures: (a) Zn-MOF, (b) P-300, (c) P-400, (d) P-500, (e) P-600, (f) P-700, (g) P-800, and (h) P-900. All of the scale bars in (b−h) represent 100 nm.

cm−1, which can be ascribed to the presence of DMF molecules.39 The relatively weak band between 2200 and 2300 cm−1 suggests the existence of CO2.40 In addition, the DSC curve shows a narrow and strong peak at the second stage of the first step. One can deduce that the weight loss during the first step is attributed to the escaping of DMF molecules and the two stages in the first weight-loss step might be due to the escaping of DMF molecules absorbed in the pores of different sizes and with adsorption modes. A sloping region of the TG curve observed at 300−430 °C shows a tiny weight loss of ca. 1.1%, indicating that there are

few gas molecules escaped from the system. At this stage, the DRIFTS results show typical vibration modes of MOF-5,41 which are in line with the results of XRD. The vibration bands at 750−840 cm−1, as illustrated in Figure 4b, can be ascribed to the in-plane and out-of-plane deformation modes of the aromatic ring and the C−H groups, where intense and broad bands at 810−830 cm−1 are due to the C−H groups on 1,4disubstituted benzene rings.41,42 The bands in the 1300−1650 cm−1 interval are due to the asymmetric and symmetric modes of carboxylate, the benzene ring stretching modes, and the C− H bending vibrations.42 The band at 1690 cm−1 indicates the E

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 6. CV curves of the pyrolytic Zn-MOF at different temperatures for various scan rates: (a) Zn-MOF, (b) P-300, (c) P-400, (d) P-500, (e) P-600, (f) P-700, (g) P-800, and (h) P-900.

be seen in Figure 5a, the Zn-MOF particle shows a regular cubic shape with smooth facets. When the temperature increased to 300 and 400 °C, the surface became rough with an obvious grainy structure. As the P-300 and P-400 generally maintain the crystal structure of Zn-MOF according to the XRD results, it can be speculated that the loss of the guest molecules and the structural DMF has changed the texture of the Zn-MOF crystalline grain at this temperature range. The further heating makes the facet rougher and more irregular, and some particles emerged sporadically on the surface. By comparing the XRD spectra and the morphologies of the products before and after acid washing (for XRD patterns and SEM images of PC-T series, see Figures 1b and S2, respectively), it can be considered that these particles are composed of ZnO. Also, the results of energy-dispersive spectrometry (EDS) mapping also confirmed that the samples are composed of three elements, C, O, and Zn, with a uniform distribution (Figure S3). With the increasing pyrolysis temperature, the surface looks wrinkled and folded and the particles emerged at 500 °C. Therefore, we can conclude that the pyrolysis reactions and the formation of ZnO played key roles in the development of the morphologies. 3.6. Determination of Fractal Dimensions of the Pyrolytic Zn-MOF Surface. Since Strømme et al. provided a simple relationship between the peak current and the scan rate, the CV method has become a powerful tool to investigate the surface fractal properties by determining the fractal dimension,

existence of protonated BDC molecules that are not completely coordinated with Zn ions. In the range of 2850− 3100 cm−1, the intense bands of aromatic ν(C−H) modes are observed.42,43 The second weight-loss step in the TG curve occurs in the range of 450−520 °C, corresponding to a weight loss of 24.7%, which can be due to the decomposition of Zn-MOF.38 A strong endothermic peak can be observed at ca. 470 °C in the DCS curve, indicating the emergence of an endothermic reaction. At a higher temperature, no bands of any carboxylate groups can be observed in DRIFTS results, and only two bands at 1610 and 1450 cm−1 exist continuously after 450 °C, ascribing to the aromatic compounds and the C−C stretching modes of the aromatic rings, respectively.41 This can be attributed to the polymerization of aromatic compounds, leading to the formation of pyrolytic carbon, which is also confirmed by the exothermic reaction characterized by the exothermic peak at ca. 450 °C in the DSC curve.44 It can be concluded that the carbonization reactions start at ca. 450 °C, surprisingly happen before the decomposition of the SBU that composed of Zn-containing clusters involving carboxylate groups at ca. 470 °C. 3.5. Surface Morphologies and Compositions of Pyrolytic Zn-MOF. With the onset of the pyrolysis reaction, the reconstruction of the molecular composition and the structural configuration will lead to a major change in the morphologies of the precursor, as illustrated in Figure 5. As can F

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 7. Peak current vs scan rate for CV curves recorded for the pyrolytic Zn-MOF at different temperatures: (a) Zn-MOF, (b) P-300, (c) P-400, (d) P-500, (e) P-600, (f) P-700, (g) P-800, and (h) P-900.

Df, of the electrode surface.31 When the CV current is limited by the diffusion of the electroactive species adsorbed on the fractal surface of the electrode, the relationship of peak current Ip and scan rate v can be expressed by31

Ip ∝ ν α

This is due to the pyrolysis of the framework and the formation of the amorphous carbon and the ZnO nanoparticles, which can lead to the emergence of a rough surface and a disordered structure. Above 500 °C, the Df first decreases until it reaches a minimum value of 1.81 at 700 °C and then increases with the increasing temperature. According to the results of TG-DSC in Figure 4a, the pyrolysis and carbonization reactions completed preliminarily at ca. 520 °C and then experienced a further carbonization and regularization process until ca. 660 °C. These processes might lead to the formation of a relatively regular surface, showing a small Df at 700 °C. However, the value of Df increases again with the increase in temperature above 700 °C, indicating that the roughness and irregularity of the surface increased with temperature. It can be concluded that the pyrolysis temperature can strongly affect the surface geometry evolution of the products by promoting the crystalline regularity of the MOF precursor and generating pyrolysis and carbonization reactions.

(1)

where α = (Df − 1)/2. By determining the Df of the P-T series products, one can quantify the evolution of the surface properties of the pyrolytic Zn-MOF at different temperatures. We used an electrochemical redox couple, ferri/ferrocyanide, as the probe to detect the fractal properties of the electrode surface. Only one pair of well-defined current peaks can be observed in each CV diagram in Figure 6, and this pair of current peaks is assigned to the reversible reaction of the redox couple as follows45 [Fe(CN)6 ]3 − + e− ↔ [Fe(CN)6 ]4 −

(2)

Figure 6 shows the CV curves of the P-T series products at several scan rates between 10 and 200 mV s−1, and the value of Ip increases with the increasing scan rate ν. By fitting the log ν − log Ip plots, as displayed in Figure 7a−g, one can obtain the value of α that is related to the Df. The calculated results of Df are plotted in Figure 7h. It can be observed that the Df of the pyrolytic Zn-MOF decreases with the increasing temperature before the structural destruction of Zn-MOF. However, the Df jumps from 1.83 to 1.88 when the temperature rises to 500 °C.

4. CONCLUSIONS The pyrolysis process of Zn-MOF was studied systematically by an in situ investigation of the structural and compositional evolution process of the pyrolytic products. DRIFT and TGDSC were used to understand the decomposition mechanism of the precursor during thermal treatment at the molecular level, and the X-ray method and the CV method were utilized G

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

performance anode material for lithium-ion batteries. ACS Appl. Mater. Interfaces 2014, 6, 17067−17074. (9) Zhang, W.; Jiang, X.; Zhao, Y.; Carné-Sánchez, A.; Malgras, V.; Kim, J.; Kim, J.; Wang, S.; Liu, J.; Jiang, J.; Yamauchi, Y.; Hu, M. Hollow carbon nanobubbles: Monocrystalline MOF nanobubbles and their pyrolysis. Chem. Sci. 2017, 8, 3538−3546. (10) Li, A.; Song, H.; Bian, Z.; Shi, L.; Chen, X.; Zhou, J. ZnO nanosheet/squeezebox-like porous carbon composite synthesized by in-situ pyrolysisof a mixed-ligand metal-organic framework. J. Mater. Chem. A 2017, 5, 5934−5942. (11) Liu, S.; Zhou, J.; Song, H. Tailoring highly N-doped carbon materials from hexamine-based MOFs: Superior performance and new insight into the roles of N configurations in Na-ion storage. Small 2018, 14, No. 1703548. (12) Pandiaraj, S.; Aiyappa, H. B.; Banerjee, R.; Kurungot, S. Post modification of MOF derived carbon via g-C3N4 entrapment for an efficient metal-free oxygen reduction reaction. Chem. Commun. 2014, 50, 3363−3366. (13) Li, A.; Yan, T.; Cao, B.; Song, H.; Li, Z.; Chen, X.; Zhou, J.; Chen, G.; Luo, H. MOF-derived multifractal porous carbon with ultrahigh lithium-ion storage performance. Sci. Rep. 2017, 7, No. 40574. (14) Liu, B.; Shioyama, H.; Akita, T.; Xu, Q. Metal-organic framework as a template for porous carbon synthesis. J. Am. Chem. Soc. 2008, 130, 5390−5391. (15) Srinivas, G.; Krungleviciute, V.; Guo, Z. X.; Yildirim, T. Exceptional CO2 capture in a hierarchically porous carbon with simultaneous high surface area and pore volume. Energy Environ. Sci. 2014, 7, 335−342. (16) Zheng, F.; Yang, Y.; Chen, Q. High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metalorganic framework. Nat. Commun. 2014, 5, No. 5261. (17) Zhang, L.; Su, Z.; Jiang, F.; Yang, L.; Qian, J.; Zhou, Y.; Li, W.; Hong, M. Highly graphitized nitrogen-doped porous carbon nanopolyhedra derived from ZIF-8 nanocrystals as efficient electrocatalysts for oxygen reduction reactions. Nanoscale 2014, 6, 6590−6602. (18) Zhao, Y.; Liu, Y.; Kang, H.; Cao, K.; Wang, Y.; Jiao, L. Nitrogen-doped hierarchically porous carbon derived from ZIF-8 and its improved effect on the dehydrogenation of LiBH4. Int. J. Hydrogen Energy 2016, 41, 17175−17182. (19) Hu, M.; Reboul, J.; Furukawa, S.; Torad, N. L.; Ji, Q.; Srinivasu, P.; Ariga, K.; Kitagawa, S.; Yamauchi, Y. Direct carbonization of Albased porous coordination polymer for synthesis of nanoporous carbon. J. Am. Chem. Soc. 2012, 134, 2864−2867. (20) Hu, M.; Reboul, J.; Furukawa, S.; Radhakrishnan, L.; Zhang, Y.; Srinivasu, P.; Iwai, H.; Wang, H.; Nemoto, Y.; Suzuki, N.; Kitagawa, S.; Yamauchi, Y. Direct synthesis of nanoporous carbon nitride fibers using Al-based porous coordination polymers (Al-PCPs). Chem. Commun. 2011, 47, 8124−8126. (21) Chaikittisilp, W.; Hu, M.; Wang, H.; Huang, H. S.; Fujita, T.; Wu, K. C.; Chen, L. C.; Yamauchi, Y.; Ariga, K. Nanoporous carbons through direct carbonization of a zeolitic imidazolate framework for supercapacitor electrodes. Chem. Commun. 2012, 48, 7259−7261. (22) Liu, B.; Shioyama, H.; Jiang, H.; Zhang, X.; Xu, Q. Metalorganic framework (MOF) as a template for syntheses of nanoporous carbons as electrode materials for supercapacitor. Carbon 2010, 48, 456−463. (23) Lim, S.; Suh, K.; Kim, Y.; Yoon, M.; Park, H.; Dybtsev, D. N.; Kim, K. Porous carbon materials with a controllable surface area synthesized from metal-organic frameworks. Chem. Commun. 2012, 48, 7447−7449. (24) Aiyappa, H. B.; Pachfule, P.; Banerjee, R.; Kurungot, S. Porous carbons from nonporous MOFs: Influence of ligand characteristics on intrinsic properties of end carbon. Cryst. Growth Des. 2013, 13, 4195− 4199. (25) Fujiwara, Y.; Horike, S.; Kongpatpanich, K.; Sugiyama, T.; Tobori, N.; Nishihara, H.; Kitagawa, S. Control of pore distribution of porous carbons derived from Mg2+ porous coordination polymers. Inorg. Chem. Front. 2015, 2, 473−476.

to explore the transformation of the structure and the surface properties, respectively. The results show that the pyrolysis of Zn-MOF will finally lead to the formation of the amorphous carbon−ZnO composite, and the long-range ordered structures of the precursor on both the molecular and the mesoscopic scale were destroyed completely. All of the resulting carbon matrices of the products show highly porous structures with a hierarchical pore distribution. With the increase of pyrolysis temperature, the PSD becomes wider in the range of micropores, and both the surface area and the pore volume of micropores become increasingly larger. In addition, the pyrolysis temperature plays a decisive role in surface geometry evolution of the products, which has been quantified by determining the surface fractal dimension of the pyrolytic Zn-MOF products at different temperatures using the CV method.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b04606. Details of peak fitting of Raman spectra, EDS mapping of P-T series products, and SEM images of PC-T series products (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: 0086-1064434916. ORCID

Huaihe Song: 0000-0003-1547-0382 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51672021). REFERENCES

(1) Morozan, A.; Jaouen, F. Metal organic frameworks for electrochemical applications. Energy Environ. Sci. 2012, 5, 9269−9290. (2) Wang, L.; Han, Y.; Feng, X.; Zhou, J.; Qi, P.; Wang, B. Metalorganic frameworks for energy storage: Batteries and supercapacitors. Coord. Chem. Rev. 2016, 307, 361−381. (3) Chaikittisilp, W.; Ariga, K.; Yamauchi, Y. A new family of carbon materials: Synthesis of MOF-derived nanoporous carbons and their promising applications. J. Mater. Chem. A 2013, 1, 14−19. (4) Xia, W.; Mahmood, A.; Zou, R.; Xu, Q. Metal-organic frameworks and their derived nanostructures for electrochemical energy storage and conversion. Energy Environ. Sci. 2015, 8, 1837− 1866. (5) Tang, J.; Yamauchi, Y. Carbon materials: MOF morphologies in control. Nat. Chem. 2016, 8, 638−639. (6) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. Asymmetric supercapacitors using 3D nanoporous carbon and cobalt oxide electrodes synthesized from a single metalorganic framework. ACS Nano 2015, 9, 6288−6296. (7) Salunkhe, R. R.; Young, C.; Tang, J.; Takei, T.; Ide, Y.; Kobayashi, N.; Yamauchi, Y. A high-performance supercapacitor cell based on ZIF-8-derived nanoporous carbon using an organic electrolyte. Chem. Commun. 2016, 52, 4764−4767. (8) Yue, H.; Shi, Z.; Wang, Q.; Cao, Z.; Dong, H.; Qiao, Y.; Yin, Y.; Yang, S. MOF-derived cobalt-doped ZnO@C composites as a highH

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (26) Elleuch, A.; Halouani, K.; Li, Y. Investigation of chemical and electrochemical reactions mechanisms in a direct carbon fuel cell using olive wood charcoal as sustainable fuel. J. Power Sources 2015, 281, 350−361. (27) Liu, H.; Song, H.; Chen, X.; Zhang, S.; Zhou, J.; Ma, Z. Effects of nitrogen- and oxygen-containing functional groups of activated carbon nanotubes on the electrochemical performance in supercapacitors. J. Power Sources 2015, 285, 303−309. (28) Kim, C. H.; Pyun, S. I.; Kim, J. H. An investigation of the capacitance dispersion on the fractal carbon electrode with edge and basal orientations. Electrochim. Acta 2003, 48, 3455−3463. (29) Pajkossy, T. Electrochemistry at fractal surfaces. J. Electroanal. Chem. Interfacial Electrochem. 1991, 300, 1−11. (30) de Levie, R. Fractals and rough electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1990, 281, 1−21. (31) Strømme, M.; Niklasson, G. A.; Granqvist, C. G. Voltammetry on fractals. Solid State Commun. 1995, 96, 151−154. (32) Li, Z.-H. A program for SAXS data processing and analysis. Chin. Phys. C 2013, 37, No. 108002. (33) Wu, S.; Huang, G.; Wu, J.; Tian, F.; Li, H. Structural evolution of OBC/carbon nanotube bundle nanocomposites under uniaxial deformation. RSC Adv. 2015, 5, 32909−32919. (34) Song, R.; Song, H.; Zhou, J.; Chen, X.; Wu, B.; Yang, H. Y. Hierarchical porous carbon nanosheets and their favorable high-rate performance in lithium ion batteries. J. Mater. Chem. 2012, 22, 12369−12374. (35) Gong, Y.; Li, B.; Pei, T.; Lin, C.; Lee, S. Raman investigation on carbonization process of metal-organic frameworks. J. Raman Spectrosc. 2016, 47, 1271−1275. (36) Coccato, A.; Jehlicka, J.; Moens, L.; Vandenabeele, P. Raman spectroscopy for the investigation of carbon-based black pigments. J. Raman Spectrosc. 2015, 46, 1003−1015. (37) Ferrari, A. C.; Robertson, J. Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond. Philos. Trans. R. Soc., A 2004, 362, 2477−2512. (38) Sabouni, R.; Kazemian, H.; Rohani, S. A novel combined manufacturing technique for rapid production of IRMOF-1 using ultrasound and microwave energies. Chem. Eng. J. 2010, 165, 966− 973. (39) Sharma, A.; Kaur, S.; Mahajan, C. G.; Tripathi, S. K.; Saini, G. S. S. Fourier transform infrared spectral study of N,N′-dimethylformamide-water-rhodamine 6G mixture. Mol. Phys. 2007, 105, 117− 123. (40) Cui, S. Thermal degradation of as-synthesized MOFs studied by TG-FTIR. Spectrosc. Spectral Anal. 2012, 32, 131−132. (41) Civalleri, B.; Napoli, F.; Noël, Y.; Roetti, C.; Dovesi, R. Abinitio prediction of materials properties with CRYSTAL: MOF-5 as a case study. CrystEngComm 2006, 8, 364−371. (42) Bordiga, S.; Vitillo, J. G.; Ricchiardi, G.; Regli, L.; Cocina, D.; Zecchina, A.; Arstad, B.; Bjørgen, M.; Hafizovic, J.; Lillerud, K. P. Interaction of hydrogen with MOF-5. J. Phys. Chem. B 2005, 109, 18237−18242. (43) Reineke, T. M.; Eddaoudi, M.; Fehr, M.; Kelley, D.; Yaghi, O. M. From condensed lanthanide coordination solids to microporous frameworks having accessible metal sites. J. Am. Chem. Soc. 1999, 121, 1651−1657. (44) Kawahara, Y.; Otoyama, S.; Yamamoto, K. Direct carbonization of high-performance aromatic polymers and the production of activated carbon fibers. J. Text. Sci. Eng. 2015, 5, No. 1000219. (45) Neves, S.; Fonseca, C. P. Determination of fractal dimension of polyaniline composites by SAXS and electrochemical techniques. Electrochem. Commun. 2001, 3, 36−43.

I

DOI: 10.1021/acs.jpcc.8b04606 J. Phys. Chem. C XXXX, XXX, XXX−XXX