Local Geometry and Electronic Properties of Nickel Nanoparticles

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Article Cite This: Inorg. Chem. 2018, 57, 10072−10080

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Local Geometry and Electronic Properties of Nickel Nanoparticles Prepared via Thermal Decomposition of Ni-MOF-74 Akhil Tayal,*,† Yanna Chen,†,‡ Chulho Song,† Satoshi Hiroi,‡ L. S. R. Kumara,† Natalia Palina,†,∥ Okkyun Seo,†,‡ Megumi Mukoyoshi,⊥ Hirokazu Kobayashi,⊥,§ Hiroshi Kitagawa,⊥ and Osami Sakata*,†,‡

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†

Synchrotron X-ray Station at SPring-8, National Institute for Materials Science (NIMS), 1-1-1 Kouto, Sayo-gun Hyogo 679-5148, Japan ‡ Synchrotron X-ray Group, Research Center for Advanced Measurement and Characterization, NIMS, 1-1-1 Kouto, Sayo-cho, Sayo-gun Hyogo 679-5148, Japan ⊥ Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku Kyoto 606-8502, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi Saitama 332-0012, Japan ABSTRACT: Metal−organic frameworks (MOFs) provide highly selective catalytic activity because of their porous crystalline structure. There is particular interest in metal nanoparticle-MOF composites (MNP@MOF) that could take advantage of synergistic effects for enhanced catalytic properties. We present an investigation into the local geometry and electronic properties of thermally decomposed Ni-MOF-74 calcined at different temperatures and time durations. Pair distribution function analysis using high-energy X-ray diffraction reveals the formation of fcc-Ni nanoparticles with a mixture of MOF phase in samples heated at 623 K for 12 h. Elevating the calcination temperature and lengthening the time duration assisted complete precipitation of Ni nanoparticles in the MOF matrix. Local structures and valence states were investigated using X-ray absorption fine structure spectroscopy. Evidence of ligand-to-metal charge transfer and gradual reduction of Ni2+ is apparent for those samples heated above 623 K for 12 h. In addition, the Ni lattice was found to be slightly compressed as a result of surface stresses in the nanosized particles or surface ligand environment. Electronic structure investigation using hard X-ray photoelectron spectroscopy shows a significant narrowing of the valence band and a decrease in the d-band center (toward the Fermi level) when the heating temperature is increased, thus suggesting promising catalytic properties for NiNP@MOF composite.

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INTRODUCTION The term metal−organic framework (MOF) is commonly used to define hybrid porous solids that are formed from the reaction between inorganic and organic species to create a nanometer-scale, three-dimensional structural framework.1,2 These compounds have attracted considerable interest because of their wide application in petrochemistry, catalysis, and gas separations, among others. The fundamental characteristics of these compounds are large surface area and porous structure, which makes them potential candidates for the adsorption of important gases such as CO2, CH4, CO, and H2 in significant quantities.1,3,4 A primary focus on this class of material has been to improve their performance and explore routes for their facile synthesis. Recent developments in MOF research have suggested that their performance can be enhanced through improved understanding of the adsorption mechanism and knowledge of the active adsorption sites. One class of MOF, namely, M-MOF-74, has piqued particular interest because of the presence of one-dimensional © 2018 American Chemical Society

pores and a high density of open metal sites that confers advantage to selective gas separation properties. In earlier investigations using different metal analogues of M-MOF-74, such as M=Ni, Zn, Co, Fe, and Mg, it has been demonstrated that toxic gas removing capabilities can be remarkably improved.5−9 However, these compounds fall short in certain areas, including poor thermal/chemical stability and limited electrical conductivity, thus hindering the development of fully functional materials. Recently, it was shown that a composite material containing metal nanoparticles (NPs) and MOFs (MNP@MOF) can act as a multifunctional material,9−14 which mitigates the limitation of MOFs. Because their applicability covers a wide range of areas, such as gas storage, sensing, biomedicine, and catalysis, there is significant focus on their production. Various synthesis methods have been proposed, such as solid grinding, microwave irradiation, Received: May 5, 2018 Published: July 31, 2018 10072

DOI: 10.1021/acs.inorgchem.8b01230 Inorg. Chem. 2018, 57, 10072−10080

Inorganic Chemistry

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surface grafting, and solution infiltration.13,15−17 Among the available techniques, it has been demonstrated that partial thermal decomposition of M-MOF-74 produces metal NPs of tunable size within the MOF pore.18,19 An advantage of such a synthesis process is that the metal NPs are stabilized within the MOF pores. The formation of such composite structures could help to improve the properties of MOFs for various applications, as mentioned above. We have synthesized NiNP@MOF composite via a thermal decomposition method and observed that fcc-Ni precipitates out when the calcination temperature and time duration are increased.19 NiNP@MOF is of particular interest because the proven excellent catalytic properties of Ni NPs opens up multifold applications, accentuated by its magnetic nature.11,20−22 We observed that the synthesized composite shows superparamagnetic behavior associated with a single domain magnetic NP that remains even at high heating temperatures.19 However, there remains a knowledge gap regarding a clear understanding of the formation process and functionality of NP/MOF composites produced via thermal decomposition. Bridging that gap could open the door to improved optimization of this material. Concerning the formation process, it was proposed that the formation of metal NPs in quinone-based metal complexes is governed by the reduction of the metal via hydroquinone.23,24 However, any apparent experimental evidence for this is missing in the prepared NP/MOF composite. Moreover, the as-formed metal NPs accompany cleavage of the ligand−metal bonds, which could have a significant impact on the structure of the NPs, and which in turn could affect the catalytic properties of NP/ MOF composites. In this regard, investigation of the local structures and electronic properties is of paramount importance to gain insight into the function and formation process of these materials.1 In this work, we have investigated NiNP@MOF composite prepared via thermal decomposition of Ni-MOF-74. The structural (long- and short-range) and electronic properties were examined using various synchrotron X-ray-based techniques. Pair distribution function (PDF) analysis using high-energy X-ray diffraction (HEXRD) was performed to investigate the short- to medium-range structure and average coordination. To probe the valence state and local structure of the material, X-ray absorption fine structure spectroscopy (XAFS) was used. XAFS is known for its sensitivity to the local atomic structure without interference from the host matrix (which in the present case is MOF) and provides structural information that could show significant deviation from the average long-range structure in the NP systems.25 Moreover, its sensitivity and selectivity for dilute samples aids in gaining information on the mixed valence states present in the composite. Hard X-ray photoelectron spectroscopy (HAXPES) was utilized to investigate the electronic properties in the valence band region. The electronic properties of an active catalytic material are conventionally explained in terms of the d-band center theory developed by Hammer and Nørskov26,27 that correlates the observed phenomenon based on narrowing of the valence band, shifts in the d-band center, and electron transfer between the alloy components. Hence, the HAXPES technique can provide vital information on NP function because of the sizable probing depth, which in the present case exceeds the total NP size (see Experimental Section).

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RESULTS AND DISCUSSION High-Energy X-ray Diffraction. Figure 1 shows the total structure factor (S(Q)) obtained from HEXRD data of Ni-

Figure 1. Structure factor calculated from the high-energy X-ray diffraction data of the indicated samples. For ease of comparison, the individual spectra are translated vertically.

MOF-74, bulk Ni, and thermally decomposed Ni-MOFs samples (for samples labeling refer to Experimental Section). The Ni-MOF-74 structure consists of helical O5M chains connected by the 2,5-dioxidoterephthalate linkers to give a hexagonally packed three-dimensional (3D) structure.9,28 All of the diffraction peaks for the as-prepared Ni-MOF-74 samples were indexed according to reported values,28 suggesting the formation of a single-phase compound. Rietveld refinement of the XRD data of the present samples was carried out previously.19 For the Ni36 sample, intense XRD peaks observed above 3 Å−1 were indexed to fcc metallic Ni and matched with the HEXRD data of bulk Ni, indicating structural transformation in the sample. However, two peaks below 1 Å−1 are still visible, and this corresponds to a NiMOF-74 structure, suggesting the formation of mixed phases of Ni NPs and Ni-MOF-74 at this heat treatment condition. The HEXRD data of Ni39 and Ni40 show similarities in terms of peak positions, and any Ni-MOF-74 peaks are below the detection limit, suggesting complete precipitation of the fcc-Ni metallic cluster. The particle sizes of Ni NPs calculated from the high-resolution scanning transmission electron microscopy images (not shown here) are found to be 4.3 ± 1.4 nm, 4.5 ± 1.2 nm, and 5.0 ± 1.2 nm for the Ni36, Ni39, and Ni40 samples, respectively.19 To gain further insight into how the atomic coordination varies with heat treatment, Fourier transform (FT) of the structure factor was performed to obtain the pair distribution function, g(r), that can be expressed in terms of the total correlation function, (T(r)),29 which is defined as 4πρrg(r) and shown in Figure 2. Here, ρ is the density in units of Å−3. For Ni-MOF-74, the Ni−O bond distance in the O5Ni structural motif varies between 1.99 and 2.04 Å, and the Ni− Ni distance in the helically packed O5Ni chains is in the range 2.89−2.9 Å. However, the Ni−Ni distance in the first coordination shell of fcc-Ni is 2.49 Å. From T(r) it can be seen that the intensity of the peaks corresponding to the Ni−O and Ni−Ni coordination in Ni-MOF-74 is reduced in Ni36 with respect to the as-prepared sample. This is at the expense of an increase in intensity at 2.49 Å corresponding to the Ni− 10073

DOI: 10.1021/acs.inorgchem.8b01230 Inorg. Chem. 2018, 57, 10072−10080

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Inorganic Chemistry

seen from (Table 1) that the coordination number for the first shell increases toward the bulk value for the samples heated at 673 K. Within the error limit, the CN for Ni39 and Ni40 is close to similar. In a study by Falicov et al.,31 it was proposed that the first shell coordination number could act as an index to determine configurational fluctuations that arise when the occupation number of a single-electron atomic orbital no longer remains a good quantum number. It was observed that in the NP state the number of d-holes is higher for those metal atom sites having a coordination number close to the bulk value, and, hence, it could act as a highly catalytically active material.31 It is apparent that the samples Ni39 and Ni40 could show enhanced catalytic activity. For Ni-MOF-74, the observed value of CN of the Ni−O shell shows a departure from the bulk value that may be attributed to some missing O in the O5M structural motif that could mimic the O environment of other native nickel oxides that have lower Ni−O coordination. To gain further information on the local electronic and crystallographic structure, we subsequently carried out XAFS and HAXPES measurements that are presented in the following sections. X-ray Absorption Fine Structure Spectroscopy. Figure 3a shows normalized X-ray absorption near-edge structure

Figure 2. Fourier transform of the structure factor in terms of the total correlation function for the indicated samples.

Ni coordination in fcc-Ni. Samples Ni39 and Ni40 show identical reduction in intensity in the Ni−O peak; however, the peak intensity at 2.89 Å is slightly higher for the Ni39 sample relative to the Ni40 case. In contrast to this, the peak intensity at 2.49 Å corresponds to the fcc Ni−Ni shell, and this shows inverse behavior with a marginally higher intensity for the Ni40 sample. It is well-known that a peak intensity in the FT of a total structure factor is related to the coordination number of a particular shell. These findings suggest that orderly growth of fcc-Ni metal nanocluster within the MOF matrix is obtained by lengthening the time duration of the calcination at 673 K. For Ni36, the signature for the presence of both Ni-MOF-74 and fcc-Ni is clearly evident. For Ni39 and Ni40, the presence of a Ni-MOF-74 phase is not clearly evident, but a compound with analogous composition could be present in the nonstoichiometric form in a minute amount in noncrystallized form below the diffraction limit. Quantitative estimation of the coordination number (CN) was done by calculating the pair correlation function, g(r), from the S(Q) data using the following expression30 (tabulated in Table 1). CN =

∫r

rmax

g (r )4πr 2ρ dr

(1)

min

Table 1. Coordination Number Estimated from the g(r) Data shell distance in (Å) 2.49 3.52 4.33

2.01 (Ni−O) 2.89 (Ni−Ni)

Ni39

Ni40

bulk Ni

7.1 8.7 7.1 6.9 18.9 20.5 Ni-MOF-74 (experimental)

8.8 6.9 20.5

10.0 6.1 20.5

Ni36

bulk Ni (crystallographic) 12 6 24 Ni-MOF-74 (crystallographic)

0.9

5

2.2

2

Figure 3. XANES profiles of indicated samples (a), for ease of clarity, individual XANES spectra are vertically translated. Comparison of the edge position was performed on the first derivative of the XAFS signal (b). For sample Ni40 and bulk Ni, the first derivative signal is multiplied by a factor of 10 to make a comparison with the other samples. The inset of (a) shows an overlay of the XANES spectra (top) and O5Ni structural motif (bottom).

(XANES) profiles and the first derivative (b) of the indicated samples measured at the Ni-K edge. Normalized spectra with similar coordinate values are also compared in the inset of Figure 3a. Edge positions were defined using the steepest inflection point in the first derivative of the XANES signal, as shown in Figure 3b. For comparison of the edge positions, Ni foil was used as the standard with E0 = 8333.0 eV (indicated by an arrow in Figure 3b), and the obtained values are listed in

In eq 1, rmin and rmax, are the radii of a particular coordination shell. For fcc-Ni, the value of ρ is estimated to be 0.0917 Å−3. For the multicomponent system, the number density is modified by the concentration of the individual elements; its value is 0.0045 Å−3 for the Ni-MOF-74. Typical error bars in the estimation of CN are ∼0.5 atoms. It can be 10074

DOI: 10.1021/acs.inorgchem.8b01230 Inorg. Chem. 2018, 57, 10072−10080

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formation of the mixed valence Ni(2−δ)+ ion is clearly evident. For the Ni40 sample, the near-edge spectral features match well with those observed for the bulk Ni, thus indicating complete precipitation of the Ni metal cluster in the MOF matrix. A slightly higher intensity (white line) for the Ni40 compared to the bulk Ni (inset Figure 3a) shows that Ni is not entirely reduced to a zero-valence state even at this heat treatment condition, and there is the possibility of a somewhat large number of d-holes compared to the case of bulk Ni. In the Ni-MOF-74 structure, the first nearest shell is comprised of the Ni−O shell with bond lengths ranging between 1.99 and 2.04 Å. The second nearest shell is formed by Ni−Ni, which have bond distances of 2.89−2.90 Å. In the presence of random M−OH sites, the ranges of metal−ligand and metal−metal distances could be further extended. For bulk fcc-Ni, the first four shells have bond distances of 2.49, 3.52, 4.31, and 4.98 Å. The last shell has contributions both from single scattering and multiple scattering paths. However, multiple scattering paths provide a more significant contribution because of the collinear chains of the Ni atoms present in the fcc structure enhancing the overall forward-scattering amplitude of the photoelectrons (focusing effect).36 Extended X-ray absorption fine structure (EXAFS) analysis could shed light on the variations in these distinct Ni environments afforded by different heat treatment conditions. Figure 4 shows the phase-shift (ϕ) uncorrected Fourier transform moduli and real-components of EXAFS for all

Table 2. The value of the edge position shows a systematic shift toward the bulk Ni value from the pristine Ni-MOF-74 Table 2. Absorption Edges Calculated from the Inflection Point in the First Derivative of XAFSa sample

E0(eV)

bulk Ni Ni-MOF-74 Ni36 Ni39 Ni40

8333.0 8342.0 8342.2 8338.9 8333.1

Typical error bar value in the estimation of the edge position is ±0.2 eV.

a

sample for the successively calcined samples at higher temperature and longer time duration. This indicates a reduction in Ni2+. Moreover, the observed distinct spectral features in the near-edge region can be used to deduce valuable information on a possible local geometry around the Ni atoms. For Ni-MOF-74, the observed two features are labeled as A and C (see Figure 3a), where feature A corresponds to the 1s → 3d transition that in general is Laporte forbidden. However, it becomes allowed either due to direct quadrupole coupling or dipole mixing with the Ni 4p and/or 2p orbital of the ligand(O) with the Ni 3d orbital. In the latter case, it provides valuable information on the local geometry of the metal ion wherein the non-centrosymmetric environment gives rise to observation of an intense pre-edge feature. In the Ni-MOF-74 sample, Ni is linked to five oxygen atoms in a distorted square pyramidal geometry (Figure 3a). This results in the observed pre-edge feature (A). The sharp absorption peak at 8342.0 eV (C) corresponds to the dipole allowed 1s → 4p transition. The appearance of the pre-edge peak (A) and the energy position of feature C indicate that the Ni atoms are in a +2 valence state32 in the Ni-MOF-74 structure. Thermal decomposition of this precursor phase leads to distinct structural transformation, as seen in the XANES profiles of samples Ni36, Ni39, and Ni40. For samples Ni36 and Ni39, two main features, B and C, are observed. The energy positions of these features systematically shift to lower values going from Ni36 to Ni39, as seen in the inset of Figure 3a. Moreover, the XANES spectra of Ni36 and Ni39 could not be represented as linear combination of Ni-MOF-74 and Ni bulk. This indicates formation of a mixed valence state in the Ni(2−δ)+ ion. Feature B is broad for both samples; however, of the two the relative broadening is more pronounced for Ni39. The position of this feature covers the energy range of pre-edge feature A observed for Ni-MOF74, but it does not form a well-separated peak, as seen for the unheated sample. This feature B can be generally explained in terms of molecular orbital theory in which the shoulder is most likely observed as a result of the 1s → 4pz + shakedown transition.33 In this process, the 1s → 4pz transition is followed by ligand-to-metal charge transfer to create a final state with lower energy than the main absorption peak.34,35 For the Ni39 sample, feature C is shifted significantly toward a lower energy value at 8338.9 eV, indicating that it is getting closer to that of the Ni metal. The observed XANES spectrum for Ni39 illustrates that valence reduction for Ni does not occur abruptly to provide metallic Ni. The HEXRD data show a signature of only metallic fcc-Ni being present in this sample. Following this, XAFS measurements provided additional information on the local electronic/structural properties, and

Figure 4. Fourier transform moduli (left) and real-components (right) EXAFS of indicated samples. A region highlighted between 2.2 and 3.1 Å in the real-component spectra is dominated by the Ni−Ni shell in Ni-MOF-74.

samples studied. For Ni-MOF-74, because of the low Ni concentration, the FT signal is weak. This is despite the fact the two peaks around R−ϕ = 1.6 Å and 2.6 Å, corresponding to the first Ni−O shell and Ni−Ni shell, can clearly be distinguished. As a result of the lower signal-to-noise ratio beyond 3.5 Å, it is difficult to extract any meaningful 10075

DOI: 10.1021/acs.inorgchem.8b01230 Inorg. Chem. 2018, 57, 10072−10080

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Inorganic Chemistry information after the second shell which contains a distance of 3.52 Å. The FT moduli of the Ni36, Ni39, Ni40, and bulk Ni show nearly similar features up to 6 Å. The first intense peak corresponds to the Ni−Ni shell at D = 2.49 Å, and it can be seen that the peak position is marginally shifted to a lower value for the Ni36 and Ni39, suggesting deviation from the bulk lattice value in these cases. Spectral features beyond 2.9 Å in the Ni40 and bulk Ni are mostly identical and correspond to higher order Ni−Ni shells. For the Ni36 and Ni39, although the overall shape of the FT modulus envelopes between 3.2 and 5.5 Å shows some similarity to that for fcc-Ni, complete overlap is not apparent. This is expected because the HEXRD results show the formation of a mixed phase for the Ni36 sample with the percentage of the secondary phase, other than fcc-Ni and that may arise from the Ni-MOF-74, is below 10%. The fraction of the secondary phase is further reduced in the Ni39 sample and was found to be below the detection limit of the present diffraction experiment. However, similarity in the spectra of the real components around 2.4−3.2 Å, highlighted between Ni36 and Ni-MOF-74, provides additional information on the presence of a secondary Ni-MOF phase in the Ni39 sample as well. This may be in a nanocrystalline or amorphous state, providing limited signature in the diffraction pattern, as further discussed in the next paragraph. Because the fraction of the secondary Ni-MOF-74 phase is below 10% in both samples, any spectral features around 1.6 Å, corresponding to the Ni−O shell in Ni-MOF-74, is mostly obscured under the higher weighting of the EXAFS signal coming from the fcc-Ni shells. The contribution from the Ni−Ni shell at 2.7 Å, due to a large scattering amplitude, could contribute to the composite EXAFS signal. A slightly higher intensity in the FT modulus around this region can be seen for Ni36; however, its presence is not clearly apparent for the Ni39 sample. Minimal variation in the FT moduli limits any ability to show clear evidence for the presence of additional shells. Consequently, further comparison is made in the real component spectra to gain higher sensitivity for the presence of other shells. In the real component spectra (Figure 4 (right)), a region between 2.2 and 3.1 Å is highlighted, where this is dominated by the Ni−Ni shell at D = 2.89−2.90 Å. This corresponds to the first metal−metal distance in the Ni-MOF-74 structure. In the as-prepared sample, this region is dominated by two wellresolved peaks of unequal intensity (the first being lower and the second being higher) and a shoulder component to the peak at 3.2 Å. In the bulk Ni, in this region, one can see a highintensity peak followed by two marginally resolved peaks that are relatively less intense. A similar observation can be made for the Ni40 sample, indicating that the shell environment is identical to that in fcc Ni. For the Ni36 and Ni39 samples in this region, one can see two nearly equal intensity peaks followed by a shoulder component to the peak at 3.2 Å. For Ni39 the shoulder peak is slightly merged with the second peak. These features may indicate a partial inclination toward the observed real component features in Ni-MOF-74. This analysis of the real component EXAFS shows that, for Ni36 and Ni39, there could be a fraction of a secondary Ni-MOF-74 component in these samples. Quantitative analysis of the EXAFS data was made by performing nonlinear least-squares fits (shown in Figure 5) to obtain the shell distance (D), coordination number (N), and mean square variation in bond length (σ2). For Ni-MOF-74, Ni40 and bulk Ni, the data were fitted in the k-range 4−13 Å−1 and R-range 1.0−5.6 Å. An exception to this is for Ni-MOF-74

Figure 5. Fourier transforms of EXAFS spectra and curve-fits of indicated samples (left). Corresponding χ(k) × k2 and fits are plotted on the right.

for which the R-range is between 1.0−3.5 Å. Fits for Ni36 and Ni39 were performed between the k-range 4.8−13 Å−1 and Rrange 1.0−5.6 Å. The number of independent fitting parameters was chosen to comply with the Nyquist criterion.37 The value of the passive electron reduction factor (S20 = 0.84) was obtained from fitting of the EXAFS data for bulk Ni. The obtained parameters are listed in Table 3. The shell distances for the bulk Ni and Ni40 samples are found to be in good agreement with the bulk Ni crystallographic value. However, a significantly lower coordination number is obtained for Ni40, and this is attributed to the NP size of Ni.25,38 For the Ni36 and Ni39 samples, as was apparent in the FT modulus, the EXAFS fits also show slight compression in the Ni lattice shell distances. The reduction in the lattice parameters for the Ni36 and Ni39 samples indicates the presence of surface stresses, which is common for NPs because of their large surface area and the accompanying cleavage of Ni−O bonds.39,40 Such contraction in the lattice parameter of Ni is not apparent by the HRXRD data. However, a clear difference in the XANES signal of the Ni36 and Ni39 samples compared to bulk Ni and Ni40 indicates that the observed contraction in the lattice could be attributed to the atomic pairs on the NP surface having a non-Gaussian bond length distribution.41 Such differences in the surface bond length was previously investigated using coherent diffraction studies on NPs.42 For the Ni39 sample, any additional shell corresponds to the Ni− Ni in Ni-MOF-74 does not provide any drastic improvement to the fit. However, for the Ni36 sample, addition of an extra shell at 2.90 Å leads to an improved goodness of fit. These observations indicate that, for the Ni36 sample, the volume fraction of a secondary Ni-MOF-74 phase is relatively higher compared to that for Ni39. Furthermore, a signal from any extra shell corresponding to that of Ni-MOF-74 is suppressed under the highly weighted EXAFS signal of the fcc Ni. For the Ni-MOF-74, three shells are used to perform the fitting, including two Ni−O shells and one Ni−Ni shell. Exclusion of any extra shell produces a significant residual component around 2 Å. The shell obtained at 2.3 Å is generally found in Ni(OH) 2 compounds because of nearby OH ligands 10076

DOI: 10.1021/acs.inorgchem.8b01230 Inorg. Chem. 2018, 57, 10072−10080

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Inorganic Chemistry Table 3. Parameters Obtained from EXAFS Fitsa sample bulk Ni (Crystallographic) bulk Ni D N σ ΔE Ni40 D N σ ΔE Ni39 D N σ ΔE Ni36 D N σ ΔE Ni-MOF-74 (crystallographic)

Ni-MOF-74 (experimental) D N σ ΔE

I shell

II shell

III shell

IV shell

D = 2.49 Å N = 12

D = 3.52 Å N=6

D = 4.31 Å N = 24

D = 4.98 Å (forward scattering)

2.47 ± 0.02 12 ± 2.6 0.07 2.4 ± 2.0

3.49 ± 0.02 7.1 ± 2.8 0.08 ± 0.01

4.33 ± 0.02 29 ± 6.4 0.09 ± 0.01

5.04 ± 0.02 29 ± 6.4 0.09 ± 0.01

2.47 ± 0.02 6.5 ± 1.7 0.08 ± 0.01 3.2 ± 2.6

3.48 ± 0.02 3.2 ± 1.0 0.08 ± 0.01

4.33 ± 0.02 12.5 ± 3.6 0.09 ± 0.01

5.04 ± 0.02 10.9 ± 4.1 0.09 ± 0.01

2.35 ± 0.02 5.8 ± 1.7 0.08 5.5 ± 3.1

3.43 ± 0.02 3.0 ± 0.8 0.08 ± 0.01

4.26 ± 0.02 7.1 ± 2.0 0.08 ± 0.01

4.67 ± 0.02 7.2 ± 2.3 0.09 ± 0.01

2.35 ± 0.02 4.4 ± 1.1 0.08 5.6 ± 3.1 D = 1.99 Å N=5 Ni−O

3.43 ± 0.02 2.5 ± 1.0 0.08

4.23 ± 0.02 6.2 ± 2.1 0.08

4.67 ± 0.02 5.1 ± 1.1 0.09

1.97 ± 0.02 1.0 ± 0.2 0.03 ± 0.01 3.2 ± 2.6

extra shell

2.90 ± 0.02 1.5 ± 0.5 0.05 ± 0.01

D = 2.89 Å N=2 Ni−Ni 2.9 ± 0.02 1.0 ± 0.2 0.03

2.3 ± 0.02 0.6 ± 0.2 0.03

Value of ΔE is fixed for all shells with respect to the first shell and constrained to vary between ±1. The error bar is shown on those parameters varied during the fits.

a

surrounding the NiO4 structural motif. This suggests that the fraction of an unsaturated O5Ni structural motif could be present in the Ni-MOF-74 sample, with some missing O being linked locally to a neighboring Ni−O cluster in a fashion similar to that observed in Ni(OH)2 compounds. However, from the limited data quality, this proposition cannot be clearly established. The presence of this extra shell could correlate with the observed coordination number of Ni−O via the HEXRD data, which was found to be somewhat reduced from the ideal crystallographic value. The presence of the O5Ni structural motif with some missing O atoms would reduce the average coordination number of the Ni−O shell for a given volume. Valence Band Measurements. We performed valence band (VB) measurements, as shown in Figure 6, to investigate the catalytic properties of the NiNP@MOF composite obtained via thermal decomposition. The inset shows the VB of bulk Ni for comparison. For the samples heated at 673 K, four spectral features that are associated with the VB of metallic Ni can be seen. These primarily arise due to the metal 3d-band43 (see inset of Figure 6). Furthermore, by increasing the heat treatment conditions, the VB intensity is decreased in the region between 4 and 10 eV. The opposite effect is observed in the region between 0 and 1.5 eV, indicating narrowing of the VB. The variation in the intensity of the VB in the region covering features A and B suggests that the number of d-electrons is increased by raising the calcination

Figure 6. Valence band spectra of indicated samples. Inset shows VB spectrum of bulk Ni.

temperature and lengthening the time duration. This can be attributed to the dissociation of Ni atoms from the MOF structure concomitantly occurring with the transfer of electrons from dissociated ligands to the metal NPs. This finding is corroborated by the XAFS data that show that the reduction of Ni2+ to Ni0 occurs gradually from Ni-MOF-74 → Ni36 → Ni39 → Ni40. For the Ni36 sample, the VB is mostly diffuse over the 8 and 1 eV region with no visibly distinct spectral features. This may be because of the presence of multiple features associated with different nickel-oxide complexes providing their signatures at a different binding 10077

DOI: 10.1021/acs.inorgchem.8b01230 Inorg. Chem. 2018, 57, 10072−10080

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Inorganic Chemistry energy position, thus obscuring the overall VB. This finding illustrates that the Ni in Ni36 is farthest from the Ni metal when compared to Ni39 and Ni40. It further confirms that the Ni ions are present in the mixed MOF and metallic phase at this temperature. MOF-74 is known as one of the important gas adsorbent MOF materials for CO2, toxic gases, etc.14,44−46 Recently, NiNP@MOF composite is found to be suitable for applications such as in target drug delivery.22 As Ni is an important catalyst in many types of reactions such as oxidation, methanation, and hydrogenation reactions, the NiNP@MOF composite in this report is expected to exhibit the enhanced catalytic activity, especially in CO2 methanation reaction or decomposition of toxic gas due to the synergistic effect between the MOF adsorbent and Ni catalyst.47−49 Particularly, information on the modification of the electronic properties of Ni NPs formed via the thermal decomposition of MOF-74 may also play an important role in better optimizing the catalytic activities due to its strong dependence on the electronic states of MNPs. In this regard, the position of the d-band center of active MNPs is a crucial parameter for understanding the catalytic properties of NiNP@MOF composite. Hybridization of a metal d-band with a bonding (σ) orbital of an adsorbate creates bonding and antibonding d−σ hybridized bands. Surface interactions of the Ni metal with an adsorbing gas leads to a modified filling of the antibonding states, which causes a higher shift in the metal dband center. A higher d-band center shift with respect to the Fermi level is attributed to weaker catalytic activity, resulting from the decreased filling of the antibonding state with stabilization of the metal-adsorbate bonds. The obtained value of the d-band center is 4.06 eV for Ni36 and 3.06 eV for both the Ni39 and Ni40 samples. If these values are compared with the Ni metal (estimated as 3.77 eV), it is apparent that from Ni36 to Ni40, the d-band center shifts toward the Fermi level, which corresponds to the role of sample changing from electron acceptor (for Ni36) to electron donor (for Ni39 and Ni40). In other words, for Ni36, the unfilled bands near the Fermi level will be filled by the electron from other reactants. For Ni39 and Ni40, the high density of states near the Fermi level is beneficial for Ni to donate electrons during the related reaction, thus demonstrating that these samples could serve as effective catalytic materials. Besides, a relatively larger d-band shift in the Ni39 sample implies better catalytic performance as compared to the Ni36 sample. Furthermore, regulation in the valence state and electronic properties with the notable deviation in the local and long-range structure of Ni NPs signify the manifestation of nontrivial interaction between MNP and MOF. Such interaction seems to affect valence state and electronic properties of MNP@MOF composite suitable for the catalytic application and provide better control on NP size via a synergistic effect. The observed controlled variation in the structure and electronic properties has scope for further optimization by adopting different calcination protocols (temperature/time-duration) in the intermediate region between Ni36 and Ni40.

fraction of Ni NPs grew at the expense of host phase NiMOFs. At the calcination temperature of 673 K for 24 h, fcc-Ni completely precipitated out in the form of a metallic nanocluster. However, even at this temperature the Ni valence state is not completely reduced to zero. XAFS data show that the edge position shifts to a lower energy value going from Ni36 to Ni39, and the near edge features of the Ni36 and Ni39 samples show a possible ligand-to-metal charge transfer process being present. The Ni lattice is slightly compressed for the Ni36 and Ni39 samples, indicating the presence of surface stresses that are common to nanosized particles or surface ligand environments. The local coordination number is found to be reduced for the nanosized Ni; however, it does get closer to the bulk value when the heat is increased. In addition, narrowing of the VB and a decrease of the d-band center are observed by increasing the heat treatment condition, thus illustrating that the effective catalytic activity of the NiNP@ MOF composite with optimum synergistic effect could be observed in the Ni39 sample.

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EXPERIMENTAL SECTION

NiNP@MOF composites were prepared via partial thermal decomposition of Ni-MOF-74, as described in ref 19. For this study we have used three samples of Ni-MOF-74 separately heated at 623 K for 12 h, 673 K for 12 h, and 673 K for 24 h. These are labeled as Ni36, Ni39, and Ni40, respectively. HEXRD measurements were performed at beamline BL04B2 at SPring-8, Japan, using a two-axis diffractometer. X-rays with 61.37 keV energy were monochromatized using a Si(220) monochromator. Data were collected by varying 2θ from 0.3−48.2° to allow a Q range of 0.3−25 Å−1. XAFS measurements at the Ni-K edge were performed at beamline BL01Bl at SPring-8, Japan, both in transmission and fluorescence modes. For the transmission measurements, a gas-flow-type ionization chamber was used with a mixture of Ar+N2 gas. The relative partial pressure was set to gain absorption percentages of 20% and 80% before and after the sample, respectively. A 19 element Ge solid-state semiconductor detector was used for measurements in the fluorescence mode. An energy scan was performed using a Si(111) monochromator. The pre-edge and post-edge background subtractions were performed following the standard procedure36 in the software prepared by Conradson et al.50 For the extraction of an EXAFS signal, a polynomial spline is subtracted, and the position of nodes was varied to remove the low-frequency background below R = 1.0 Å. Special care was taken in choosing the number of nodes and their location for all samples to remain nearly identical. EXAFS amplitudes and phases were calculated using FEFF-7 code.51 Parameters were calculated by performing nonlinear least-squares fits, and error bars were estimated by removing a particular shell and varying each parameter until the total error exceeds 10% more than its initial value.50 Valence band spectra using the HAXPES technique were measured at the National Institute for Material Science beamline BL15XU at SPring-8, Japan. An X-ray incident photon energy of 5.95 keV was used to measure the HAXPES to gain bulk sensitivity. The maximum NP size used in this study is ∼5 nm, which is an order of magnitude lower than the probing depth (3λ = 17−20 nm) estimated using the Tanuma, Powell, and Penn equation (TPP-2M)52 for the inelastic mean free path (λ). The spectra were collected using a highresolution hemispherical analyzer (VG Scienta R4000). The sample holder surface was set to 88◦ with respect to the takeoff angle of the photoelectrons. A pure Au sample (99.99%) was used to calibrate the binding energy with respect to its Fermi edge and to estimate the energy resolution (240 meV). For subtraction of the background intensity, a Shirley-type background function was used, and the spectra was normalized by the integrated area. The local structure of Ni in the Ni-MOF-74, shown in Figure 3a, was produced using VESTA software.53

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CONCLUSIONS In summary, we have characterized NiNP@MOF composite using a variety of techniques to investigate the local geometry and electronic properties of Ni NPs prepared via thermal decomposition of Ni-MOF-74. Thermal decomposition led to gradual precipitation of the Ni nanocluster, and the volume 10078

DOI: 10.1021/acs.inorgchem.8b01230 Inorg. Chem. 2018, 57, 10072−10080

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AUTHOR INFORMATION

Corresponding Authors

*(A.T.) E-mail: [email protected]. Phone: +81 (0) 791-58-1970. *(O.S.) E-mail: [email protected]. ORCID

L. S. R. Kumara: 0000-0001-9160-6590 Hiroshi Kitagawa: 0000-0001-6955-3015 Osami Sakata: 0000-0003-2626-0161 Present Address ∥

(N.P.) National Research Nuclear University Moscow Engineering Physics Institute (MEPhI), Kashirskoe sh. 31, 115409 Moscow, Russia. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS HEXRD and XAFS measurements were performed at BL04B2 and BL01B1 at SPring-8 under Proposal Nos. 2014A1321 and 2016A0130, respectively. HAXPES measurements were performed at the NIMS Synchrotron X-ray Station at SPring8 under Proposal Nos. 2014B4906 and 2016A4904 as part of the NIMS Nanotechnology Platform (Project Nos. A-14-NM0116 and A-16NM-0005). The authors thank Dr. Shigenori Ueda for helpful discussions. The authors also thank Hiroshima Synchrotron Orbital Radiation, Hiroshima University, and JAEA/SPring-8 for the use of the HAXPES setup at the NIMS BL15XU beamline of SPring-8. This work was also supported by Core Research for Evolutional Science and Technology (JST CREST) and by JST ACCEL (JPMJAC1501) from the Japan Science and Technology Agency (JST). This work was partly supported by MEXT/ JSPS Kakenhi Hojyokin (OS: Grants-in-Aid for Basic Research (C), No. 15K04616 and 18K04868). We thank Iain Mackie, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

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