Heterooctamolybdate-Based Clusters H - ACS Publications

Sep 22, 2017 - Department of Chemistry, The University of Reading, Whiteknights, Reading RG6 6 AD, United Kingdom. §. State Key Laboratory of Molecul...
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Heterooctamolybdate-Based Clusters H3[(Cp*Rh)4PMo8O32] and H5[Na2(Cp*Ir)4PMo8O34] and Derived Hybrid Nanomaterials with Efficient Electrocatalytic Hydrogen Evolution Reaction Activity Vikram Singh,† Pengtao Ma,† Michael G. B. Drew,‡ Jingyang Niu,*,† Jingping Wang,*,† and Guo-Xin Jin*,§

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Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P. R. China ‡ Department of Chemistry, The University of Reading, Whiteknights, Reading RG6 6 AD, United Kingdom § State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: Polyoxometalates (POMs), emerging as a new class of porous molecular materials, play a promising role in homo- and heterogeneous catalysis. Among them, noble-metal-decorated POMs have a profound impact as catalytic materials. Thus, it is imperative to design and structurally explore new catalysts including noble metals. Herein, two new clusters, H3[(Cp*Rh)4PMo8O32]·14H2O (1) and H5[Na2(Cp*Ir)4PMo8O34]·13H2O (2) (Cp* = pentamethylcyclopentadienyl), based on a heterooctamolybdate anionic core were successfully obtained via a one-pot reaction using [Cp*MCl2]2 [M = Rh (1) and Ir (2)] and Na2MoO4 in acidic conditions. Compounds 1 and 2 were well characterized in the solid state by single-crystal X-ray diffraction, IR, and thermogravimetric analysis and in solution by UV−vis, electrospray ionization mass spectrometry, and electrochemistry. Compounds 1 and 2 represent an important class of structurally isolated organometallic POMbased clusters that were successfully nanostructured onto Ni foam and electrochemically reduced after 48 h of electrolysis to M/ MoO2, where M = Rh (3) and Ir (4), nanocomposite hybrid materials on a Ni foam surface in a 0.1 M KOH solution. The modified electrocatalysts (3 and 4) show efficient hydrogen evolution reaction activities almost comparable to those of highgrade Pt/C at 0.1 M KOH. The nanostructured POMs [1- and 2@NF (Ni foam)] and their corresponding reduced products (3 and 4) were observed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and Xray photoelectron spectroscopy and further proven by transmission electron microscopy (TEM) and high-resolution TEM.

1. INTRODUCTION Polyoxometalate (POM) chemistry is now rising rapidly as an expanding area of research in inorganic and organometallic chemistry. In recent years, POMs have been proven to have a cutting-edge role in diverse areas such as single molecular magnets, conducting materials, sensing, optoelectronic devices, and water oxidation catalysis.1 Noble-metal-decorated POMs are far less reported, particularly the organometallic derivatives.2 POMs with high-nuclearity and multifaceted structures have been widely implemented as photo- and electrocatalysts in the splitting of water into dihydrogen and dioxygen. It is still a great challenge for the chemist to design and develop efficient, robust, sustainable, and ecofriendly catalytic systems in order to support increasing global energy consumption. As per the earlier reports on molecular water-reducing systems, POMs that include transition metals such as iron, cobalt, and nickel have been used as molecular electrocatalysts for photo- and electrochemically driven hydrogen production.3 It is worth © 2017 American Chemical Society

noting that there are only a few reports on organometallicfunctionalized POM-based electrocatalysts, mainly including the noble metals.4 POMs based on noble metals are particularly promising, being highly active because of their tendency to stabilize a wide range of oxidation states owing to their propensity to form metal oxide/hydroxide units. Consistent efforts have been made to improve the electrocatalytic efficiencies of these rare state-of-the-art molecules. In this context, immobilizations of POMs and metal oxide nanoparticles on various electroactive metal surfaces such as metal foams, organic surfaces, carbon nanomaterials, and bioinspired catalysts have frequently been carried out in order to investigate their properties as electrocatalytic activators with the aim of obtaining new materials with interesting applications.5 Surface immobilization of POMs on inorganic and organic matrixes is Received: August 6, 2017 Published: September 22, 2017 12520

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Inorganic Chemistry an important strategy that could be frequently accessed by various covalent and noncovalent interactions between the substrates and POM clusters.6 These covalent and nonionic forces may be useful in the transformation and tuning of the POM properties, but indeed they play a significant role in improving the accessibility of active sites on the POMs and substrate interface and providing surface stability.7 Various methods were developed to obtain nanoassemblies with efficient hydrogen evolution reaction (HER) and oxygen evolution reaction activities. Earlier, carbon-supported porous microstructures were frequently obtained, and their catalytic performance has been explored. To this end, pioneer works were performed by Nadjo et al., as they widely explored POMs to transform the properties of several electrodes.8 Yu et al. used a POM-based metal−organic framework to obtain highly dispersed MoO2-porous-doped nanoparticles on graphene oxide sheets.9 Qiao et al. successfully obtained metal−organic frameworks derived from hybrid Co3O4−C porous nanowire arrays, which acted as reversible oxygen evolution electrodes on a Cu foam surface.10 Recently, Huang et al. explored the formation of monodispersed IrM (M = Ni, Co, and Fe) bimetallic nanoclusters as bifunctional electrocatalysts for an acidic overall water splitting.11 Previously, metal foams preferably using Ni were consistently used to obtain nanoassemblies because of the well-established conducting and porous nature of Ni foam (NF).12 Recently, some outstanding contributions have been made to achieved excellent HER performance by the synergistic effect of bifunctional electrocatalysts in both acidic and alkaline media.13 Taking these facts altogether, we herein synthesized and structurally obtained two novel organometallic-functionalized heterooctamolybdate clusters 1 and 2, which were successfully nanostructured onto an NF via hydrothermal treatment using protocol involving catalytic triethylamine and nafion. It is worth noting that heteropolyoxomolybdates bearing organometallic functionality bonded to noble metals Rh3+ and Ir3+ indeed play a significant role in inducing nanoaggregation onto the NF via distortion of the organometallic moieties, which brings the appended C−H bonds of the methyl groups in close proximity to the Ir3+ and Rh3+ metal centers and favors a C−H activation process.14 This strategy was successfully used to obtain nanostructured POMs 1@NF/2@NF, which were effectively reduced to electroactive hybrid materials M/MoO2 [M = Rh (3) and Ir (4)] onto the NF. Detailed investigations of these results are presented in this work.

Scheme 1. Structural Isolation of 1 and 2 by Changing the Reaction pH Conditions

valence-sum (BVS) calculations (Table S1 and S2 in the Supporting Information). Furthermore, 1 and 2 are air-stable, maintaining their crystallinities for at least several days, and no efflorescence was observed. Clusters 1 and 2 are soluble in water and organic solvents such as dimethyl sulfoxide (DMSO), chloroform, and dichloromethane. Remarkably, these POMbased organometallic clusters are also stable in acidic and basic aqueous solutions in the pH range of 5−9 at room temperature, as confirmed by subsequent electrospray ionization mass spectrometry (ESI-MS) studies at variable pH (Figures S1 and S2 in the Supporting Information). 2.2. Crystal Structures. Structures 1 and 2 represent rare examples of structurally characterized heteropolyoxomolybdate organometallic clusters in which the tetrarhodium/iridium {Cp*M}n+ [M = Rh (1) and Ir (2)] fragments are grafted onto the anionic core of the molybdate anion. The structures of 1 and 2 are shown in Figures 1 and 2, respectively. The essential

2. RESULTS AND DISCUSSION 2.1. Synthesis. Needle-shaped orange crystals of H3[(Cp*Rh)4(PMo8O32)]·14H2O (1) and yellowish crystals of H5[Na2(Cp*Ir)4PMo8O34]·13H2O (2) were obtained in 10−12 days via a one-pot reaction of Na2MoO4 and [Cp*MCl2]2 [M = Rh (1) and Ir (2)] at slightly acidic conditions using 2 M H3PO4 to maintain pH ≈ 5−5.2 (Scheme 1). In order, to optimize the reaction conditions for the successful structural isolation of 1 and 2, a series of parallel experiments were performed, and it was found that minor changes in the stoichiometries of the reactants and slight fluctuations in the pH of the reaction resulted in the formation of ionic fragments instead of these novel heterooctamolybdatebased organometallic clusters (1 and 2). Interestingly, we found that, in 1, the RhIII atoms were reduced to RhII when coordinated to a {PMo8O32}n− anion, whereas, in 2, the iridium remains stable at its 3+ oxidation state according to bond-

Figure 1. (a) Ball-and-stick model of 1, which has crystallographic mirror symmetry. Color code: Rh, green; Mo, blue; O, red; C, black. The Rh atoms are located at the corner of the quasi-cubane core. Only one orientation of the disordered Cp* ring bonded to Rh (1) is shown. H atoms are not shown for clarity. Solvent water molecules are also omitted.

features of the two clusters are similar, although 1 has crystallographic mirror symmetry and 2 contains two Na ions on the outside of the cluster (vide supra). Selected bond lengths and angles for 1 and 2 are presented in Table S3 in the Supporting Information. In 1, the crystallographic mirror plane passes through Rh(1) and Rh(3), as illustrated in Figure 1. However, all three Rh atoms have similar geometries, being bonded to three O atoms and a Cp* ring. Each metal shows one Rh−O bond at 2.216(7), 2.173(7), and 2.216(10) Å, respectively, which are 12521

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Figure 2. (a) Ball-and-stick model of 2. Color code: Ir, green; Mo, bright blue; Na, dark blue; O, red; C, black. Ir atoms are located at the corner of the quasi-cubane core. (b) Polyhedral representation of 2. H atoms are not shown for clarity. Solvent water molecules are also omitted.

Figure 3. (a) Chair-shaped conformation formed at the four Rhbonded corners via Rh−Mo bonds in cluster 1. (b) 12-membered metallacycle ring via metal−metal interactions Rh−Mo−Mo−Rh− Mo−Mo−Rh−Mo−Mo−Rh in 1.

significantly longer than the other two, which are in the range 2.085(7)−2.101(7) Å. The Rh−C distances are in the range 2.073(14)−2.159(15) Å. The four unique Mo atoms within {PMo8O32}n− have similar distorted octahedral environments. The metals are bonded to two terminal O atoms [in the range 1.699(8)−1.724(8) Å] and four further O atoms with different bond lengths. Of these, the shortest Mo−O bonds [type 1: in the range 1.853(7)−1.883(7) Å] involve O atoms shared with two other Mo atoms, next to those that bridge the Rh atoms [type 2: in the range 1.982(4)−2.009(7) Å], followed by those shared with Rh and Mo atoms [type 3: in the range 2.167(7)− 2.189(7) Å] and the longest, those shared with P and Mo atoms [type 4: in the range 2.315(6)−2.337(6) Å]. The eight Mo atoms are in the 6+ oxidation state (BVS calculations; Table S1 in the Supporting Information). The main distortions from octahedral geometry around the Mo atoms are to be found in the O (type 1)−Mo−O(type 2) angles, which range between 147.3(3) and 148.0(3)°. For the Rh atoms, the O(type 3)−Rh−O(type 3) angles with the range 87.2(4)−87.3(4)° are larger than the O(type 2)−Rh−O(type 3) angles at 74.5(3)− 74.7(2)°. The central P atom is bonded to four O atoms in a tetrahedral arrangement. The asymmetric unit contains 14 independent water molecules mostly refined with reduced occupancies. The molecular structure of the polyoxoanion in 1 (Figure 1) has a number of unique features. First, the methylcyclopentadienyltetrarhodium cation grafted on heterooctamolybdate, i.e., [{(Cp*Rh)4PMo8O32}]n−, corresponds to a rare class of structurally characterized organometallic POM-based clusters. There are 16 bridging μ3-O atoms and 16 terminal O atoms. In 1, the Rh atoms strongly interact with the MoVI atom, and this facilitates the formation of a chair-shaped conformation at the four corners of the cluster via Rh−Mo interactions including Rh2Mo2O2 atoms in the six-membered ring (Figure 3). Also, the metal atoms in 1 sustain a 12-membered metallacycle ring via metal−metal interactions Rh−Mo−Mo−Rh−Mo−Mo− Rh−Mo−Mo−Rh (Figure 3). The Rh atoms interact with the Mo atoms within the chair-shaped conformation through Mo···Rh distances of 3.281(1)−3.283(1) Å, which lies close to the sum of the van der Waals radii [rw(C) = 1.70; rw(O) = 1.52 Å]. By contrast, the Mo−Mo distances are 3.221(1) and 3.227(1) Å. There are significant differences in the packing of the molecules in the two structures. In 1, there are three independent Cp* rings. For two of these, the closest contacts are to the O atoms of the anions, while the third ring is involved in π···π stacking via a symmetry element (1/2 + x, −1/2 + y, 1/2 − z) with a distance of 3.70(1) Å between centroids (Figure S3 in the Supporting Information).15

For 2, for all four independent Cp* rings, the closest contacts are with H atoms of methyl groups of other Cp−Me rings; thus, there are short contacts of 3.0−3.2 Å between H atoms and the centroids of the rings. There is no π···π stacking of the Cp rings. The clusters 1 and 2 form a supramolecular network through solvent-assisted hydrogen bonding and show ABA types of packing motifs (Figure S4 in the Supporting Information). Structure 2 crystallizes in the monoclinic space group P21/n and has a structure very similar to that found in 1 although there is no imposed crystallography symmetry so that there are four independent {Cp*Ir}3+ cations grafted at the four corners of the heterooctamolybdate {PMo8O32}n− anionic core. The geometry around the Ir and Mo atoms is very similar to that found in 1, as shown in Figure 2. For each Ir atom, the short Ir−O bonds are in the range 2.08(2)−2.11(2) Å and the long Ir−O bonds in the range 2.18(2)−2.19(2) Å. The Ir−C bonds are in the range 2.09(4)−2.14(4) Å. For the Mo atoms, the Mo−O terminal bonds are in the range 1.69(2)−1.74(2) Å, with bonds to O atoms in a pattern similar to that found in 1, with O(type 1) = 1.86(2)−1.89(2) Å, O(type 2) = 1.99(2)− 2.03(2) Å, O(type 3) = 2.16(2)−2.24(2) Å, and O(type 4) = 2.30(2)−2.37(2) Å. However, for this structure, there are also two Na atoms in similar positions, both being bonded to seven O atoms, including one water molecule, four terminal O atoms, and two type 4 O atoms. The water molecules form the shortest bonds at 2.31(3) and 2.33(3) Å with the other bonds in the range 2.48(3)−2.63(3) Å. In this structure, there were 13 water molecules, with two being bonded to the Na ions and the remainder being refined with reduced occupancy. 2.3. ESI-MS. In order to investigate the stability of 1 and 2 in solution, ESI-MS analysis was performed. Single crystals of both clusters were dissolved separately in a mixed solvent of water and acetonitrile [v(H2O):v(CH3CN) = 3:1]. As shown in Figure 4, the ESI-MS spectra of cluster 1 show four prominent envelopes that correspond to the intact [(HCp*Rh)4PMo8O32]− cluster (1), which differ only in their degree of hydration. Similarly, cluster 2 also exists in hydrated ionic form in a mixed-solvent mixture (Figure S5 in the Supporting Information), thus revealing the structural stability of POMs 1 and 2 in solution. Furthermore, the self-buffering pH of aqueous solutions of 1 and 2 is about 7 and stable across a range of pH 4.5−9, which is determined by pH-dependent ESI-MS (Figures S1 and S2 in the Supporting Information) and UV−vis (Figure S6 in the Supporting Information) for both clusters 1 and 2. 2.4. Preparation, Characterization, and Electrocatalytic HER of POMs Nanostructured onto NF. The POMs 1 12522

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Figure 4. ESI-MS spectra of cluster 1.

and 2 were adsorbed onto the NF surface using nafion polymer and catalytic triethylamine via hydrothermal treatment at 120 °C for 3 days to impede their successful chemical adsorptions over the surface of NFs (Scheme 2). A detailed inspection of

Figure 5. (a) IR and (b) Raman spectra and (c) PXRD patterns of compound 1 and after its chemical adsorption onto the NF surface.

Scheme 2. Preparation of POMs 1 and 2Nanostructured onto NF for Electrocatalysis Experiments

were uniformly dispersed and chemically adsorped over the NF via distortion of the organometallic moieties, which brings the appended C−H bonds of methyl groups in close proximity to the Ir3+ and Rh2+ metal centers and favors the formation of nanoaggregation of POMs 1 and 2 onto the NF surface (Figures 6 and S8 in the Supporting Information).

Figure 6. (a) SEM image of NF. (b and c) SEM images of hydrothermally obtained nanomolecular needlelike aggregations of nanostructured POM 1 on an NF surface. (d) EDX spectrum of the cross-sectional area of the nanomolecular needle of 1 on NF.

Raman and IR spectra of nanodispersed POMs 1 and 2 onto the NF surface after hydrothermal treatment for 3 days was performed. The Raman bands appear with the splitting of the aliphatic C−H stretching bands together with slight shifts in the peaks toward lower wavenumbers; observation supports the different bonding environments of adsorbed POMs 1 and 2 (Figures 5 and S7 in the Supporting Information). Furthermore, IR spectra for nanostructured POMs 1 and 2 onto the NF surface showed subsequent shifts in the characteristic peaks toward lower wavenumbers. The characteristic peaks for νas of P−O at 1080 and 1075 cm−1 (1 and 2) appeared with three clear split bands at 1068, 1075, and 1120 cm−1 and 1062, 1070, and 1105 cm−1 for the nanostructured POMs 1 and 2, respectively. Also, the peak νas of Mo−Ot and bridged Mo−Ob−Mo vibrations appeared broad and shifted to lower wavenumber, i.e., from 998 to 992 cm−1 and from 980 to 975 cm−1 and from 870 to 860 cm−1 and from 860 to 855 cm−1 for both nanostructured POMs 1 and 2, respectively, onto the NF surface. It is worth noting that hydrothermally treated POMs 1 and 2 over an NF surface using catalytic triethylamine

The electrocatalytic activities of the nanostructured POMs onto the NFs, i.e., 1@NF and 2@NF, were examined for HER in H2-saturated basic (0.1 M KOH) solutions. These metal foams are widely used for obtaining hybrid nanostructured electrode materials.12 Furthermore, nanostructured POMs 1@ NF and 2@NF were reduced to a M/MoO2 [M = Rh (3) and Ir (4)] nanocomposite hybrid material on an NF surface after 48 h of electrochemically catalyzed HER in a H2-saturated 0.1 M KOH solution using a typical three-electrode cell at room temperature (≈25 °C). The significantly enhanced HER catalytic activities of 1 and 2@NF after 48 h of electrolysis indicate the formation of reduced hybrid products 3 and 4, with almost comparable HER rates to that found for a high-grade 20% Pt/C catalyst at similar reaction conditions. The nanomolecular assemblies of composites 3 and 4 obtained on the NF surface were observed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS), and their nature was further established from 12523

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The PXRD pattern at 48 h depicts the formation of reduced products 3 and 4 from their parent POMs 1 and 2, respectively. The hybrid materials 3 and 4 gave appreciably higher electrocatalytic HER rates than their parent POMs 1 and 2, presumably because they contained electroactive catalytic sites. Furthermore, subsequent LSV curves obtained after 48 h of electrolysis show no change in their overpotentials, i.e., the values in their LSV curves remain the same even after 1000 cycles, thus proving that the electrochemically derived reduced hybrid composite materials 3/ 4@NF remain stable for longer durations and are active enough for consistent electrochemical HER. The polarization curves of 1@NF and 2@NF and their corresponding electrochemically obtained reduced products 3 and 4 (after 48 h of electrolysis of 1 and 2@NF), commercial Pt/C(20%)@NF, and bare NF were acquired without iR correction at a voltage sweeping rate of 5 mV s−1 in H2saturated 0.1 M KOH solutions (Figure 8). The HER kinetics

transmission electron microscopy (TEM) and high-resolution TEM. As shown in Figure 7, the nanostructured POM cluster 2@ NF shows better electrocatalytic activity for the HER, i.e.,

Figure 7. Changes in the LSV curves for 1 (a) and 2 (b) obtained after every 6 h of electrolysis using Pt as the counter electrode. Relative changes in the LSV curves after 1 and 1000 cycles in 1 (c) and 2 (d). Figure 8. (a) LSV curves and (b) Tafel plots for both nanostructured POMs 1 and 2@NF and hybrid composites 3 and 4 obtained after 48 h of electrolysis of 1 and 2@NF, respectively, Pt/C on NF, and bare NF in H2-saturated 0.1 M KOH solutions.

−2

achieved a current density of 10 mA cm at an overpotential of 179 mV, whereas 1@NF reached a current density of 10 mA cm−2 at an overpotential of 193 mV. However, significant changes in the linear-sweep voltammetry (LSV) curves of nanostructured POMs 1@Ni and 2@NF in their HER electrocatalytic activities were mentioned after every 6 h of electrolysis, and they improved appreciably up to 400 cycles; i.e., the overpotentials get reduced to 121 and 53 mV to achieve a current density of 10 mA cm−2 for both nanostructured POMs 1 and 2 on the NF surface. In order to see the role of any electrode codeposition for enhanced HER activities for both nanostructured POMs 1 and 2 on the NF, their values were also examined using graphite as a counter electrode in H2saturated 0.1 M KOH solutions for 1 and 2 under the same electrolysis experimental conditions. However, the LSV curves for both nanostructured POMs 1 and 2 on the NF feature changes of up to 48 h of electrolysis similar to those obtained when Pt was used as a counter electrode (Figure S9 in the Supporting Information), thus ruling out the possibility for any anodic dissolution of Pt from the counter electrode. The significantly improved LSV curves for both the nanostructured POMs 1@NF and 2@NF after 48 h of electrolysis were due to the formation of reduced M/MoO2 (3 and 4) nanocomposite hybrid materials, which were subsequently identified by SEM, TEM, PXRD, and XPS analysis. Furthermore, the slow electrocatalytic formation of reduced products 3 and 4 from their parent POMs in an alkaline medium was subsequently observed by PXRD of characterizing samples in different time intervals during electrocatalytic HER experiments within 48 h (Figure S10 in the Supporting Information). The changes in the PXRD patterns were critically observed in every 12 h of electrocatalytic HER experiments.

of the above-mentioned catalysts were further evaluated by the corresponding Tafel plots (log j ∼ h). As shown in Figure 8, the Tafel slopes of 31, 42, 63, 84, 110, and 149 mV dec−1 were measured for commercial Pt/C, Ir/MoO2 (4), Rh/MoO2 (3), 2, and 1 onto the NFs and bare NF catalysts, respectively. The exchange current densities (j0), an important parameter, have been calculated carefully from the Tafel plot (log I vs V; then extrapolate the current to V = 0). The magnitude of the exchange current density for these efficient POM adsorbed/ reduced (1 and 2/3 and 4) products on the NF was mentioned (Table 1). The HER catalytic behavior was also tested in a higher alkaline medium, i.e., 1 M KOH; the compounds show similar trends in their HER catalytic rates, and the changes in Table 1. Catalytic Details of Synthesized HER Electrocatalysts onto the NF at H2-Saturated 0.1 M KOH Solutionsa catalyst

Tafel slope

ηHER at 10 mA cm−2 [V]

Pt/C@NF NF 1@NF 2@NF 3@NF 4@NF

31 149 110 84 63 42

50 276 190 179 125 70

current density (j0) 3.4 1.3 1.5 1.9 2.1 2.7

× × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4

a

[V] = potential measured versus RHE. The j0 values were calculated from Tafel curves using an extrapolation method with equation η = a + b log j, where a is the intercept on the y axis and b is the Tafel slope. The exchange current density j0 is calculated at η = 0 V.

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the catalysis experiment. The surface electronic state composition of the M/MoO2 [M = Rh (3) and Ir (4)] nanosheath-like aggregations was further substantiated by XPS. The XPS wide-scan spectral peaks for 3 and 4 obtained after 48 h of electrolysis feature characteristic peaks for Rh 3d5/2 (307.02 eV), Rh 3d3/2 (312.12 eV), and Ir 4f7 (60.03 eV), Ir 4f5 (63.05 eV) that correspond to Rh/Ir-doped nanoassemblies 3 and 4, as shown in Figures 10 and S14 in the Supporting

the LSV curves were almost comparable to those obtained at a 0.1 M KOH electrolyte. The HER activity of Ir/MoO2@NF (4) in a higher alkaline medium, i.e., 1 M KOH, is almost comparable to that of the commercial (20%) Pt/C catalyst@ NF and higher than most of the reported HER catalysts (Table S4 in the Supporting Information). The TEM images (Figure 9b) obtained after 48 h of electrolysis experiments clearly reveal

Figure 9. (a) SEM image of 3. (b) TEM image showing MoO2 accumulation of the nanoparticles onto differently layered polymeric carbon sheaths over an NF surface. (c) HRTEM image showing lattice fringes that established the presence of MoO2 nanoparticles trapped in a polymeric carbon sheath over NF.

that a number of nanoparticles in the darker contrast with a size of ∼10−12 nm were trapped in a uniformly spread polymeric carbon sheath. The HRTEM images (Figures 9c and S11 in the Supporting Information) provide evidence that the polymeric carbon sheath facilitates the entrapment of well-crystallized MoO2 nanoparticles that are well connected to the polymeric carbon sheath. It can also be seen from the surface texture of the SEM and TEM images of 3 and 4 that the carbon sheath appears with plenty of differently layered nanosheaths, which may be due to the Rh/Ir-doped (3 and 4) and P-doped regions within the polymeric carbon framework over the NF surface. The zoomed-in TEM images for the corresponding darkened spherically shaped nanoparticles 3 and 4 show the apparent lattice fringes within the darkly shaded region with nanofringes of 0.34 nm, which corresponds to the (110) crystallographic planes of hexagonal MoO2 and shows that these darker nanoparticles are likely to be MoO2 nanocrystals, whereas the Rh/Ir-doped (3 and 4) and P-doped regions within the carbon sheaths were further substantiated from the PXRD patterns. The PXRD patterns of 3 and 4 show similar characteristic peaks, which corroborates the identical nature of the reduced products 3 and 4 (Figures S12 and S13 in the Supporting Information). The well-resolved intense peaks were correlated to the (100), (101), and (220) planes of MoO2 (JCPDS 500739) in 3 and 4, while the diffraction peaks associated with the (111) and (200) planes in both reduced products 3 and 4 were mainly associated with the NFs (JCPDS 65-2865). The PXRD patterns also identify the presence of Rh/Ir in 3 and 4 via peaks that were consistent with the the (200), (220), and (311) planes of the pure metals, thus showing that 3 and 4 feature similar catalytic behavior although they slightly differ in their HER rates presumably because of the different metallic properties of Rh (3)/Ir (4). The small peaks at 25.55 and 29.46° indexed for the unreduced NiMoO4 impurities on the surface might have been formed during the first step of the reaction via POMs 1 and 2 on the NF by hydrothermal treatment, which was further authenticated from the PXRD patterns of POMs 1 and 2 that were initially recorded before

Figure 10. XPS wide-scan spectra of reduced products 3 (a) and 4 (b) obtained after 48 h of electrocatalysis of 1 and 2, respectively, and high-resolution Mo 3d (c), O 1s (d), C 1s (e), and Rh 3d (f) spectra of the reduced product 3.

Information). It is noted that the peaks at binding energies of 307.02 and 60.03 eV for 3 and 4, respectively, predominate, which shows the presence of Rh0/Ir0-enriched phases in the hybrid materials mainly due to the existence of nanostructured POMs 1 and 2 on the NF surface that subsequently provides reaction sites due to the close proximity of the Rh/Irnanostructured POMs 1 and 2 and the NF surface to obtain stable hybrid nanomaterials 3 and 4 on the NF surface. It is anticipated that the activation energy of the reaction is likely to be decreased, thereby aiding completion of the nanoparticle stabilization stage via the hydrothermal treatment of POMs 1 and 2 over an NF surface and their further electrocatalysis at H2-saturated 0.1 M KOH solutions for HER. These results indicate that the POMs 1 and 2 were successfully nanostructured on the NF, which subsequently leads to the formation of 3 and 4 over a carbon sheath@NF surface after 48 h of electrolysis HER experiments. Moreover, the reduced products 3 and 4@NF proved to be efficient electrocatalysts for HER. Interestingly, the XPS peak at 853.24 eV and a small peak at 854.42 eV that appear in both the POM-derived hybrid materials 3 and 4 provide further evidence for NF surface reactivity of POMs 1 and 2, which further facilitates the formation of hybrid materials 3 and 4. The C 1s peak at ∼284.7 12525

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Inorganic Chemistry

H5[Na2(Cp*Ir)4PMo8O34]·13H2O (2) were obtained via onepot reactions. The clusters 1 and 2 exist intact in solution, as confirmed from negative-ion ESI-MS. The clusters 1 and 2 were chemically adsorbed and successfully nanostructured on the NF surface and proved to be active electrocatalytic materials for HER because they could be subsequently reduced to hybrid nanocomposite materials M/MoO2 [M = Rh (3) and Ir (4)] after 48 h of electrocatalytic HER reaction in a 0.1 M KOH solution. The reduced hybrid materials show excellent catalytic HER activities almost comparable to those of Pt/C working electrodes. This work provides insight into a new direction in which to obtain novel energy materials that could prove to be a fruitful contribution to meeting the requirements of ideal electrocatalysts required to ameliorate the global energy crisis. Moreover, novel metal-decorated organometallic POMs might be useful in obtaining new energy materials that could be further improved via their grafting onto a suitable conducting metal surface.

eV is observed for both 3 and 4, which indicates the presence of polymeric graphitelike sheaths onto the NF surface due to the surface reactivity of NF with the distorted organometallic moieties of 1 and 2. Besides these, very small broad band peaks at 290.3 and 290.5 eV were observed, which correspond to a π−π* satellite, thus providing evidence for the formation of a polymeric carbon sheath over the NF surface in the nanocrystalline 3 and 4. The C−P peak at 285.5 eV also appears in the C 1s spectra of 1 and 2, which gives further evidence for the presence of P-doped regions in the hybrid material. The high-resolution Mo 3d spectrum contains four peaks. The two main peaks at 229.9 and 233.2 eV are related to the Mo 3d5/2 and Mo 3d3/2 binding energies of the MoIV oxidation state, respectively. In addition, the peaks at 232.3 and 236.1 eV could be assigned to MoVI 3d5/2 and MoVI 3d3/2 of MoO3, respectively, derived from the unreduced POMs 1 and 2 on the NF surface. Furthermore, the peaks at 60.12 and 63.20 eV and at 307.03 and 312.02 eV authenticate the presence of Rh/Ir-doped nanosheaths in 3 and 4, respectively. The EDX spectrum (Figures 11 and S15 in the Supporting Information)

4. EXPERIMENTAL SECTION 4.1. Materials and Methods. All chemical reagents were purchased from commercial sources and used without further purification. The Fourier transform infrared spectra were performed in the range 2500−450 cm−1 using KBr pellets on a Bruker VERTEX 70 IR spectrometer. PXRD patterns were recorded at room temperature on a Bruker D8 ADVANCE diffractometer with Cu Kα (λ = 1.54056 Å). The C and H elemental analyses were obtained with a PerkinElmer 2400-II CHNS/O elemental analyzer. Metal element analysis was conducted on a PerkinElmer Optima 2100 DV inductively coupled plasma optical emission spectrometer. ESI-MS measurements were performed on an AB SCIEX TRIPLE TOF spectrometer. During their measurements, 10 μL samples were dissolved and introduced into the ESI source through a Finningan surveyor auto sampler. The mobile phase (H2O) flowed at a rate of 5 μL min−1. The MS scans were run up to 2.5 min, and the spectra obtained are averages of 8−10 scans. Electrochemical measurements were made on a CHI 620c electrochemical analyzer. The corresponding work voltage and current are 40 kV and 100 mA, respectively. The morphology of the reduced hybrid products 3 and 4 was confirmed by a high-resolution transmission electron microscope (JEM-2100) at an acceleration voltage of 200 kV. Raman scattering was collected on a Renishaw RW1000 confocal microscope with a 514 nm line of an argon-ion laser as the excitation source. Morphology analysis was conducted on a scanning electron microscope (JSM-7600F) at an acceleration voltage of 10 kV. Elemental mapping and EDX were performed with a Vantage JSM-5160LV energy spectrometer. XPS was performed on a scanning X-ray microprobe (PHI 5000 Verasa, ULAC-PHI, Inc.). Thermogravimetric analysis (TGA)−MS analysis of the samples was performed using a NETZSCH STA 449 F5 analyzer heated from 30 to 1200 °C under nitrogen at a heating rate of 10 °C min−1. UV−vis was measured from 200 to 800 nm in a 1 × 10−4 M aqueous solution on a Hitachi UV−vis spectrophotometer. 4.2. Single-Crystal X-ray Crystallography. Single crystals of 1 and 2 [CCDC 1555190 (1) and 1555191 (2)] were recorded on a Bruker Apex-II CCD diffractometer at 296(2) K with monochromated Mo Kα radiation (λ = 0.71073 Å). Structure solution and refinement were carried out by using the SHELXS-97 and SHELXS-97 program packages. Refinements were full-matrix least squares against |F|2 using all data. The metal atoms were refined anisotropically, as were the carbon and oxygen atoms in the clusters, but the solvent oxygen atoms were refined isotropically. Hydrogen atoms bonded to oxygen could not be located. In 1, the cluster has mirror symmetry and one of the MeCp rings was disordered over a mirror plane. Crystallographic details are given in Table S5 in the Supporting Information. 4.3. Synthesis of Compounds 1 and 2. Synthesis of H3[(Cp*Rh)4PMo8O32]·14H2O (1). An aqueous solution (5 mL) of [Cp*RhCl2]2 (0.309 g, 1.00 mmol) was slowly added to an aqueous solution (20 mL) of Na2MoO4·2H2O (0.241.95 g, 1 mmol), and the

Figure 11. (a−f) Selected-area elemental mapping of 3, which shows the successful reduction of nanostructured 1@NF to the reduced hybrid product 3@NF after 48 h of electrolysis. (h) EDX elemental analysis of a selected cross section proving formation of the reduced hybrid product 3@NF.

confirmed that the composite catalysts MoO2 [M = Rh (3) and Ir (4)] were composed of C, O, Mo, and Rh (3)/Ir (4) elements, but there are only small amounts of the P dopants. The EDX elemental mapping images of composites 3 and 4 show that the C, O, Mo, and Rh (3)/Ir (4) elements are uniformly distributed in the materials (Figures 11 and S15 in the Supporting Information), a result that is consistent with the XPS results. Furthermore, the percent composition of Mo/O in 3 and 4 was found to be in the ratio 1:2.25, a value that suggests the presence of some unreduced molybdates on the NF surface.

3. CONCLUSIONS Two novel heterooctamolybdate-based clusters H 3 [(Cp*Rh)4PMo8O32]·14H2O (1) and 12526

DOI: 10.1021/acs.inorgchem.7b01819 Inorg. Chem. 2017, 56, 12520−12528

Inorganic Chemistry

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pH of the reaction mixture was maintained at 5−5.5 by using 2 M H3PO4. The reaction mixture was refluxed at 80 °C for 3 h with constant stirring. The resulting reaction mixture for 1 turned yellowish-red, was then allowed to cool, and was subsequently decanted by centrifugation. The resulting clear solution was kept in an open vial for slow evaporation. The red crystals for 1 were collected after 3 weeks. Yield: 0.165 g (38.41% based on Mo). Anal. Calcd for H3[(Cp*Rh)4PMo8O32]·13H2O: C, 19.22; H, 3.59; P, 1.24; Mo, 30.70; Rh, 16.46. Found: C, 19.09; H, 3.16; P, 1.12; Mo, 30.56; Rh, 16.38. IR (KBr pellets, cm−1): 1628(s), 1110(s), 1080(s), 1031.48(m), 998(s), 870(m), 821(w), 765(w), 590(w), 552(w). UV−vis [λmax, nm (ε, M−1 cm−1)]: 380 (1 × 104). Synthesis of H5[Na2(Cp*Ir)4PMo8O34]·13H2O (2). An aqueous solution (5 mL) of (Cp*IrCl2)2 (0.280 g, 1.00 mmol) was slowly added to an aqueous solution (20 mL) of Na2MoO4·2H2O (1 mmol, 241.95), and the pH of the reaction mixture was maintained at 4.5−5 using a 2 M H3PO4 solution. The reaction mixture was refluxed at 80 °C under constant stirring for 3 h. The resulting solution turned greenish-yellow and was then cooled. To this reaction mixture were added 2−3 drops of a 0.5 M NaOH(aq) solution until the pH increased to 6, and then the mixture was further stirred for 3 h at room temperature. The obtained reaction mixture was subsequently decanted by centrifugation, and the clear greenish-yellow solution was kept for crystallization. The greenish-yellow crystals were collected after 3 weeks. Yield: 0.175 g (41% based on Mo). Anal. Calcd for H5[Na2(Cp*Ir)4PMo8O34]·11H2O: C, 16.56; H, 3.02; P, 1.07; Mo, 26.45; Ir, 26.50. Found: C, 16.44; H, 2.96; P, 1.02; Mo, 26.34; Ir, 26.42. IR (KBr pellets, cm−1): 1628, 1110(s), 1075(s), 1031.48(m), 980(s), 860(m), 821(w), 765(w), 590(w), 552(w). UV−vis [λmax, nm (ε, M−1 cm−1)]: 410 (1 × 104). 4.4. POM 1 and 2 Grafting over an NF. A piece of NF was washed several times with HCl, ethanol, and deionized water to ensure that the surface of the NF was well cleaned prior to its use for the successful nanostructured growth of POMs 1 and 2 on its surface. The obtained 2 mg POM 1 and 2 crystals were finely powdered and dissolved in 1 mL of ethanol, followed by the addition of 1 drop of nafion and catalytic triethylamine under a N2 flow. The solution was further transferred to a 50 mL Teflon-lined stainless steel autoclave with a piece of pretreated NF (1 × 1 cm2) maintained at 120 °C for 48 h in an electric oven. After the autoclave cooled slowly at room temperature, the sample was collected, washed with water and ethanol several times, and then dried at 60 °C for 12 h. The NF color changes to brown and light yellowish for 1 and 2, respectively. The mass of the nanostructured POM catalyst on an NF was calculated directly as the weight increment (x mg) of NF after nanostructured POMs 1 and 2 were grafted onto the NF surface. The POM nanostructured loading obtained on the NFs was 2 mg for 1 and 2.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.N.). *E-mail: [email protected] (J.W.) *E-mail: [email protected] (G.-J.X.). ORCID

Jingyang Niu: 0000-0001-6526-7767 Guo-Xin Jin: 0000-0002-7149-5413 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grants 91222102 and 21573056). V.S. thanks Henan University for a postdoctoral fellowship award.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01819. ESI-MS spectra at variable pH for compounds 1 and 2, IR, Raman, and PXRD of compounds 2 and 2@NF, BVS calculations and crystallographic details of 1 and 2, SEM, TEM, elemental mapping, XPS, and EDX of 4, UV−vis and TGA of 1 and 2, and additional figures and tables (PDF) Accession Codes

CCDC 1555190−1555191 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 12527

DOI: 10.1021/acs.inorgchem.7b01819 Inorg. Chem. 2017, 56, 12520−12528

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