Structure, Stability, and Electronic Interactions of Polyoxometalates on

7 Dec 2012 - McGarvey , G. B.; Moffat , J. B. A Study of Solution Species Generated during the Formation of 12-Heteropoly Oxometalate Catalysts J. Mol...
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Structure, Stability, and Electronic Interactions of Polyoxometalates on Functionalized Graphene Sheets Jean-Philippe Tessonnier, Stephanie Goubert-Renaudin, Shaun Alia, Yushan Yan, and Mark A. Barteau* Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ABSTRACT: Polyoxometalates (H3PMo12O40, H3PW12O40, H4PMo11VO40) supported on oxygen- and alkyl-functionalized graphene sheets were investigated. Discrete molecular species were directly observed by electron microscopy at loadings below 20 wt.%. The interaction between the polyoxometalates and the graphene surface was found to significantly impact their vibrational spectra and a linear correlation between the frequency of the M-Oc-M vibration and the dispersion was evidenced by FTIR. While bulk-like electronic properties were observed for small aggregates (2−5 nm), UV−vis spectroscopy and cyclic voltammetry revealed changes in the electronic structure of isolated molecular species as a result of their interaction with graphene. Because of the ability to disperse alkyl-functionalized graphene in a variety of polar and nonpolar solvents, the materials synthesized in this work provide an opportunity to disperse polyoxometalates in media in which they would not dissolve if unsupported.

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INTRODUCTION Polyoxometalate (POM) anions represent an important class of nanosized polynuclear clusters consisting of d-block transition metals and oxygen atoms. The tunability of their size, structure, and elemental composition, and hence of their chemical and electronic properties, render them particularly interesting both for fundamental and applied science, especially for catalysis (redox, acid−base properties), materials science (magnetic, optical, and electrical properties), and medicine (antitumor, antiviral, and antimicrobial activities).1−5 Keggin-type POMs such as H3PMo12O40, H3PW12O40, and H4SiW12O40 (also known as heteropolyacids, HPAs) are the most popular, as they possess interesting redox and strong Brønsted acid properties that make them useful for electrochemistry,6,7 photochemistry,8,9 catalysis,3,10−22 environmental protection,23,24 and energy storage.25−28 There has been a resurgence of interest in HPAs recently as they have shown exciting performance for the production of chemicals from biomass,12−14,17−19 as well as for the storage of energy, for example, in pseudocapacitors25,26 and molecular cluster batteries.27,28 In battery applications, HPAs were found to act as electron sponges that outperform traditional lithium ion batteries where the cathode material is LiCoO2.28 HPAs dissolve in water and polar organic solvents, where they find application as homogeneous catalysts.3 They typically form secondary (crystalline) and tertiary (micrometer-sized aggregate) structures in the solid state.3 This hierarchical structure greatly influences the accessibility of HPA clusters (diffusion of reactants or electrolyte) and the location of the protons,3 both of which can impact HPA performance in heterogeneous catalysis.29 Recently, several groups have explored, both experimentally and theoretically, how factors © 2012 American Chemical Society

such as solvent characteristics and dispersion of HPAs on a substrate influence HPA properties.29−33 There is concurring evidence showing that the acid−base properties of HPAs are influenced by their environment: the interactions between HPAs (in condensed or dilute phases),30 as well as their interactions with solvents (hydration, ion pairing with alkali counterions)30 and surfaces, influence the rates of reactions that they catalyze.29 Dispersion of POMs on a support can improve the accessibility of the active sites, and hence increase apparent activity in gas−solid and liquid−solid catalytic reactions. However, theoretical calculations also showed that the anchoring of POMs on a surface is a complex process; the covalent or ionic nature of the interaction between the substrate and the POM clusters can also impact their physicochemical properties.31−33 Experimental evidence is scarce and it remains unclear whether it is possible to tune the POM properties indirectly, for example, by acting on the POM−support interaction. The samples reported in the literature are typically characterized by high POM loadings, with surface coverages close to 1 monolayer. Supported POMs in a more dilute state, ideally as discrete molecules, might show different properties due to their molecular dispersion and possible synergetic interactions with the support. The technique employed to synthesize supported HPAs can play a significant role in their catalytic activity.29 Several parameters are particularly important: the nature of the interaction between the HPAs and the support (covalent or Received: August 23, 2012 Revised: December 5, 2012 Published: December 7, 2012 393

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any adsorbed moisture. Extra-dry toluene (20 mL; Acros Organics) was added under argon (Argon 5.0, Keen Compressed Gas) and the suspension was tip-sonicated for 15 min (Branson Sonifier 200) in order to disperse the sheets. The mixture was then transferred to a 250 mL flask under Ar. Dry toluene (100 mL) and 20 mL of n-butyllithium (n-BuLi, 2.6 M in toluene, Acros Organics) were added. The mixture was stirred at room temperature under Ar for 2 h in order for the nBuLi to react with the RGO sheets and activate the surface. 1Bromodecane (11 mL; Acros Organics) was added and the mixture was kept at room temperature for another 1 h before it was heated to 70 °C in an oil bath. After 4 h at 70 °C, the suspension was allowed to cool to room temperature (≈1 h). The mixture was subsequently transferred into a 500 mL round-bottom flask containing 50 mL of cold methanol to neutralize the excess n-BuLi. A series of liquid phase extractions was performed with distilled water (1 L in total) to complete the neutralization and remove the LiBr formed during the reaction. The organic phase was recovered, filtered, and washed with 200 mL of fresh toluene and 200 mL of methanol. Finally, the recovered powder was dried at 60 °C overnight. Polyoxometalate Deposition. Polyoxometalates (H3PMo12O40·xH2O and H3PW12O40·xH2O, from Fisher Chemicals, and H4PMo11VO40, synthesized following the method of Tsigdinos and Hallada47) were deposited on RGO and on alkyl-functionalized RGO by the same technique. Graphene samples (80 mg) were placed in a vial. Methanol (4 mL) was added in order to produce a paste. Two milliliters of an aqueous solution containing the dissolved polyoxometalate was then added. The mixture was stirred and sonicated in order to produce homogeneous mixing of the POM and RGO sheets. Finally, the walls of the vial were rinsed with 1 mL of methanol and the vial was placed on a hot plate at 50 °C overnight in order for the sample to dry. The dried sample was finally crushed to a fine powder in an agate mortar. Aqueous solutions of H3PMo12O40 with different concentrations were prepared depending on the target HPA loading. The pH varied slightly when the HPA concentration decreased but it always remained below 2, conditions at which H3PMo12O40 is stable. In addition, it must be emphasized that methanol was also used to wet the graphene surface and that the stability of HPAs is significantly improved in water−alcohol mixtures.37 Finally, the pH in the aqueous film dropped as water evaporated. Samples with loadings of 5, 10, 20, 40, 50, 60, and 80 wt % H3PMo12O40 on RGO were synthesized (samples designated as xPMo12−RGO, with x = 5, 10, 20, 40, 50, 60, or 80). Samples with 20 and 50 wt % H3PMo12O40 on C10RGO were prepared for comparison (samples designated as xPMo12−C10RGO, with x = 20 or 50). Characterization. Attenuated total reflectance (ATR) FTIR measurements were performed on a Nicolet 8700 spectrometer equipped with a MCT-B detector and a Specac Golden Gate heated ATR stage with single reflection diamond crystal. Typically, 32 scans were accumulated with a resolution of 4 cm−1. The samples were rehydrated by keeping them for at least 1 day in a desiccator over an aqueous NH4Cl solution before measurement. UV−vis measurements were performed on a Jasco V-550 spectrometer with 10 mm optical path, far UV quartz cells (Spectrocell, Oreland, PA). Spectra were recorded with a scanning speed of 400 nm/min and a data pitch of 0.5 nm. Scanning electron microscopy (SEM) images were acquired on a JSM-7400-F microscope equipped with a field emission gun. The samples were loosely dispersed on conductive carbon tape. Images were typically acquired at an accelerating voltage of 2 kV and a working distance of 3 mm in order to better resolve surface features. For elemental analysis, energy-dispersive spectrometry (EDS) was carried out at 15 kV with an Oxford Instruments Inca detector connected to the SEM. Typically, 7−10 spot analyses of different regions of the sample were acquired and compared. A transmission electron microscope (TEM) equipped with a field emission gun (JEM 2010-F FasTEM) operated at 200 kV was used to perform overview images. High-resolution (HR) investigations were carried out on a Cs-corrected transmission electron microscope operated at 300 kV (FEI Titan).

ionic), the pH in the solution during impregnation, and the local pH at the surface of the support. Basic supports such as alumina have been reported to be unsuitable because the pH at the surface causes the degradation of H3PMo12O40 following the reaction34,35 PMo12O40 3 − + 3H 2O → PMo11O39 7 − + MoO4 2 − + 6H+

(1)

The degradation of H3PMo12O40 can be followed by Fourier transform infrared spectroscopy (FTIR), as the formation of PMo11O297− leads to the splitting of the peak at 1060 cm−1, which corresponds to P−O vibrations.36 Similar decomposition has also been observed for H3PW12O40.37 We focus in this work on the interaction between a representative HPA, H3PMo12O40, and functionalized graphene sheets (oxygen-functionalized and alkyl-functionalized). Graphene is an ideal support for the present work because of its exceptionally high surface area, which offers the possibility of submonolayer coverages even at high HPA loading. In addition, graphene is an interesting model nanocarbon to better understand the interaction of HPAs with other carbon materials studied in the past such as activated carbon, carbon black, HOPG (highly oriented pyrolytic graphite), or glassy carbon electrodes. The present work takes a different approach from the more common method to anchor HPAs to graphene and graphene oxide, which consists in first wrapping the surface with a positively charged polymer to provide a good interaction with the HPA anions.9,23 In order to deprotonate and positively charge the polymer, a strong base such as NaOH is typically employed. As with alumina, the high-pH conditions are expected to cause some degradation of the HPAs. In addition, while the polymer provides an interface between the support and the HPAs that helps to improve their dispersion, it also acts as a barrier that prevents the HPA clusters from interacting directly with the graphene surface. Although HPAs are present in their anionic form in the aqueous solution used for the impregnation of the support, a positively charged polymer is actually not necessary to achieve a good dispersion. HPAs display a natural affinity for carbon supports, including activated carbon,38 carbon nanotubes,27 graphene,7−9,20−22,24 HOPG,2,39−44 or even glassy carbon electrodes.45 In the special case of graphene, Li et al.8 recently showed that, during the photoreduction of graphene oxide (GO) with H3PW12O40, at least a portion of the HPAs bind to the reduced graphene oxide (RGO) surface. However, the strength of the interaction has not been measured. In the present work, the deposition of polyoxometalates on functionalized graphene sheets was carried out via a simple incipient wetness impregnation, without any polymer interface. Two different supports were tested: thermally reduced graphene oxide (RGO) and alkyl-functionalized RGO (C10RGO). The graphene sheets of both materials have the same size and same structure (defect density) but different hydrophilic−hydrophobic character. While RGO can be dispersed (for a short time) in aqueous solutions, C10-RGO is intrinsically hydrophobic and disperses only in organic solvents. Thus the latter supports provide a way to disperse POMs in organic media in which they are not soluble.

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EXPERIMENTAL SECTION

Alkyl Grafting Procedure. The alkyl grafting procedure was described in detail previously.46 Briefly, 100 mg of thermally reduced graphene oxide (Vor-X BK86X from Vorbeck Materials) was outgassed under dynamic vacuum at 120 °C overnight to remove 394

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containing groups are likely to be present to terminate sp3 carbon atoms. Although further investigation is necessary to determine whether this observation is affected by the higher apparent density of graphene within the field of view corresponding to folded or rumpled regions of the sample, this hypothesis is consistent with recent work from our group showing that metal oxide clusters preferentially nucleate at carbon nanotube defect sites.43 However, several groups, including this one, previously observed that HPAs also interact with different carbon surfaces in the absence of any oxygencontaining groups, such as on graphite and HOPG.39−44 These observations are further supported by theoretical calculations, which showed that the adsorption of H3PW12O40 on pristine graphene is energetically feasible with a charge transfer from the HPA cluster to graphene.48 Schwegler et al.38 reported that HPAs also interact strongly with oxygen-containing functional groups present on the surface of activated carbon, such as hydroxyls, carbonyls, and to some extent ether groups. While carboxyl groups are acidic and lose their proton to become negatively charged in contact with water, all the other groups are weak bases that can be protonated under strongly acidic conditions. Therefore, it is relatively easy to develop strong electrostatic interactions between the positively charged carbon surface (positive ζ potential) and the negatively charged HPA anions. ATR-FTIR. Keggin-type HPAs are formed by assembling three MO6 octahedra by edge-sharing oxygen atoms (M−Oc− M) to form M3O13 sets, which further condense by sharing corner oxygen atoms (M−Ob−M) around a central atom (typically P or Si) to form a cage. Each M3O13 also presents three terminal oxygen atoms (MOt). The ATR-FTIR spectrum of H3PMo12O40 is dominated by four characteristic peaks at 1051 cm−1 (νas P−O), 951 cm−1 (νas MoOt), 877 cm−1 (νas Mo−Ob−Mo), and 741 cm−1 (νas Mo−Oc−Mo), in good agreement with previously published FTIR spectra (Figure 3).34,49 Significant differences were observed in the spectra of the H3PMo12O40−RGO samples when compared to the corresponding unsupported HPA. In particular, the band at 741 cm−1 characteristic of Mo−Oc−Mo vibration shifted linearly to higher wavenumbers (hypsochromic shift) when the HPA loading decreased (Figure 4). Other bands representative of PMo12O40 were not significantly altered (±5 cm−1). The shift observed for the Mo−Oc−Mo band cannot correspond to the depolymerization of the Keggin structure or the reduction of the HPA with methanol (to form the corresponding heteropoly blue), as any major structural change would lead to a different spectrum in the 1100−700 cm−1 fingerprint region. A review of the literature on this topic shows an important discrepancy. While several groups observed a shift of the M− Oc−M band in the case of H3PW12O40 supported on activated carbon38 and carbon nanotubes,50 others did not report any shift for H3PW12O40 and H3PMo12O40 supported on graphite oxide and silica.23,51 It seems that any change in the M−Oc−M band also depends on the synthesis method and the surface area of the support. The frequencies of vibrations characteristic of HPAs are influenced by several parameters, such as structural isomerism, nature of the counterion, or anion−anion interactions. Thouvenot et al.52 found that band positions of the α and β isomers of H3PMo12O40 differ by only a few wavenumbers. The counterion (cation) has a more pronounced effect. Counterions with different electronegativity induce polarization, which

Cyclic voltammetric (CV) measurements were recorded on a multichannel potentiostat (Bio-Logic VMP2) with a three-electrode cell with a Pt wire counterelectrode, a commercial Ag/AgCl reference electrode in Ar-saturated 0.5 M H2SO4 (EMD OmniTrace Ultra). Inks of the functionalized graphene samples were prepared by dispersing 2.45 mg of the material in 1 mL of ethanol. An aliquot (8 μL) of the suspension was deposited immediately after its preparation onto a polished tip of a 5 mm diameter glassy carbon electrode (Pine Instruments) to reach a loading of 100 μg/cm2 and left to dry at room temperature. Afterward, 10 μL of 0.05 wt % Nafion (Liquion Solution from Ion Power, Inc.) was deposited onto the sample layer to ensure adhesion of the material onto the electrode. Fresh inks were prepared for each measurement to avoid any artifact that could arise from POM leaching or degradation during extended contact with the ethanol. All measurements were carried out at room temperature and under an argon atmosphere in 0.5 M H2SO4. Potentials are reported here versus Ag/AgCl. Voltammograms were recorded by scanning the applied potential between −0.23 and 0.52 V at different scan rates.

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RESULTS Electron Microscopy. Electron microscopy investigations of the PMo12−RGO samples as well as 20PMo12−C10RGO and 50PMo12−C10RGO did not show evidence for any large (>100 nm) HPA aggregates. Small particles of about 2−5 nm were observed by SEM only for the sample with the higher HPA loading (Figure 1). It is worth mentioning that, because of the

Figure 1. High-resolution SEM image showing 2−5 nm HPA aggregates on the surface of the 50PMo12−C10RGO sample.

difference in atomic number between carbon (Z = 6) and molybdenum (Z = 42), HPA aggregates would appear as easily detectable bright spots in SEM images. HRTEM images acquired at 200 kV showed that the sheets are homogeneously covered with HPA clusters. At high loading (sample 50PMo12−C10RGO), particles of 2−5 nm that correspond to the small HPA aggregates detected by SEM were observed, along with ≈1 nm particles; the latter can be unambiguously attributed to individual H3PMo12O40 molecules (Figure 2a). At a lower loading (20 wt %; sample 20PMo12− C10RGO), HRTEM (Figure 2b) as well as aberration-corrected scanning transmission electron microscopy−high-angle annular dark field (STEM-HAADF) (Figure 2c−e) images showed predominantly individual molecules along with a few dimers of 1 × 2 nm in size. Bright features corresponding to particles smaller than 1 nm in Figure 2d are the result of beam damage during the acquisition of the picture at high magnification. HAADF images appear to also show a higher cluster concentration at graphene edges and regions where the sheets are folded (Figure 2e), suggesting that HPAs preferably interact with the graphene surface at defect sites, where C atoms are not in the perfect honeycomb environment and where O395

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Figure 2. HRTEM images of (a) 50PMo12−C10RGO and (b) 20PMo12−C10RGO. (c−e) Aberration-corrected STEM-HAADF images of 20PMo12− C10RGO.

of different sizes but similar low polarizability. They found that anion−anion interactions significantly impact the position of several bands in the IR spectrum, in particular the M−Oc−M band. They demonstrated that Mo−Oc−Mo is actually not a pure stretching vibration and that it also presents some bending character. Shifts of the band are necessarily complex to understand. Anion−anion interactions, for example in HPA aggregates, lead to an increase in the stretching frequency. However, they would also induce a decrease of the bending vibrations frequencies. In addition, perturbations from water molecules and/or anion−cation interactions strengthen the bending contributions. In general, the decreasing effect is dominant and anion−anion interactions (when forming aggregates) induce a shift of the M−Oc−M band to lower frequencies. This explanation is consistent with the decrease in

changes the electron density of the POM and thus its redox properties.52 A change in the charge delocalization would also have a measurable effect on the IR spectrum. ATR-FTIR spectra of various Cs, Li, K, Bi and Cu salts of PMo12O403− were acquired to evaluate the effect of the counterion. Differences of almost 20 cm−1 in the position of the Mo− Oc−Mo band were observed, depending on the nature of the salt. In the case of H3PMo12O40 supported on RGO, H+ may be partially delocalized between the cluster and the graphene surface, thus taking part in the electrostatic bond. Therefore, it seems reasonable that at least part of the observed shift can be explained by the electrostatic interaction between the HPA anion and the protonated surface, acting as a counterion. In addition to polarization, the cations also act as spacers which affects anion−anion interactions. Rocchiccioli-Deltcheff et al.49 studied this effect in detail using alkylamonium cations 396

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Figure 3. ATR-FTIR spectrum of H3PMo12O40.

Figure 5. Band shifts measured from ATR-FTIR spectra of H4PMo11VO40 supported on RGO, for samples with different HPA loadings. Data for 100 wt % H4PMo11VO40 correspond to the unsupported HPA.

Figure 4. Band shifts measured from the ATR-FTIR spectra of H3PMo12O40 supported on RGO for samples with different HPA loadings. Data for 100 wt % H3PMo12O40 correspond to the unsupported HPA. Shifts measured for the Mo−Oc−Mo band were fitted by linear regression (R2 = 0.99).

Figure 6. Band shifts measured from the ATR-FTIR spectra of H3PMo12O40 supported on fumed silica, for samples with different HPA loadings. Data for 100 wt % H3PMo12O40 correspond to the unsupported HPA. Shifts measured for the Mo−Oc−Mo band were fitted by linear regression (R2 = 0.97).

Mo−Oc−Mo frequency observed when the loading increases in our sample series. To the best of our knowledge, this is the first time that a significant shift of the Mo−Oc−Mo band of up to 70 cm−1 has been observed when the HPA loading decreases on any support. However, this effect should not be limited to H3PMo12O40 or to the RGO support. In order to further test this hypothesis, samples with different HPAs (H3PW12O40 and H4PMo11VO40) supported on RGO, as well as H3PMo12O40 on different supports (alkyl-functionalized RGO and fumed silica) were synthesized. H3PW12O40/RGO led to a hypsochromic shift when the loading decreased from 60 to 40 wt %. Unfortunately, the bands were too weak at lower loadings to measure any shift precisely, even after rehydration of the sample. Better results were obtained with H4PMo11VO40−RGO (Figure 5) and H3PMo12O40−silica (Figure 6). The fitting of the data showed a similar trend as for H3PMo12O40−RGO. Small differences in the amplitude of the shift for a given loading were found between the samples, depending on the nature of the HPAs and of the supports, which is consistent with our hypothesis.

Similar shifts were also observed for H3PMo12O40 on alkylfunctionalized RGO (Figure 7), although about 30% of the Ocontaining groups present on the unfunctionalized RGO were involved in the grafting of the alkyl chains and despite the hydrophobic character of the surface.29 This observation is consistent with previous work from this group for POMs on highly oriented pyrolytic graphite (HOPG), which exhibits an unfunctionalized, graphite-like structure.40−44 POMs were shown to interact with the graphite surface, and to selfassemble into monolayers with a periodic structure, despite the hydrophobic character of the surface and the absence of any oxygen-containing groups that could act as anchoring points. Therefore, it appears the observed shift cannot be explained solely by the delocalization of H+ between the HPA and the surface O-groups (as on silica and RGO) but that the separation of the molecularly dispersed HPAs on these surfaces is probably the dominant factor. The present ATR-FTIR study demonstrated that the structure of HPA anions is influenced by their interaction 397

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Figure 7. ATR-FTIR spectra of RGO, alkyl grafted RGO, and H3PMo12O40 (20 and 50 wt %) supported on alkyl-grafted RGO. Bands characteristics of (*) alkyl chains and (+) HPAs are marked. The ATR-FTIR spectrum of RGO does not show any strong band corresponding to O-containing groups, consistent with its reduced state.

with a support. In addition to the loss of their tetrahedral symmetry, which induces a broadening of the peaks and loss in intensity,29,31 changes in the nature of the counterion (protonated surface) and of the anion−anion interaction (due to molecular dispersion at lower loadings, as observed by TEM) lead to a significant change in the M−Oc−M band position relative to that for bulk, unsupported HPAs. Differences in the electronic structure of the HPAs due to their dispersion are not clear from the FTIR results. UV−vis and electrochemical measurements were performed in order to investigate this aspect. UV−Vis Spectroscopy. UV−vis measurements were performed in order to investigate the electronic structure of the supported polyoxometalates. Attempts to measure the diffuse reflectance UV−vis spectra of the dry H3PMo12O40− RGO samples failed because of the strong absorption of the reduced graphene oxide in this spectral region. UV−vis measurements were therefore performed on H3PMo12O40 solutions and on RGO-supported H3PMo12O40 suspensions. Ethanol was chosen as a solvent because of its low UV cutoff value (205 nm) and its compatibility with both the HPA and the graphene support. We have previously demonstrated that RGO sheets functionalized with decyl (C10) chains give stable suspensions with a variety of nonpolar and polar organic solvents, including ethanol.46 Preliminary solubility tests showed that suspensions of 20PMo 12 −C 10 RGO and 50PMo12−C10RGO in ethanol were stable for several hours to several days depending on the concentration (Figure 8). Suspensions of C10RGO (without POMs) and PMo12− C10RGO were found to exhibit similar stability despite the intrinsic polarity of the POM clusters. In addition, it is worth noting that it was also possible to disperse 50PMo12−C10RGO in toluene (Figure 8), a common nonpolar organic solvent, despite the polarity of the HPAs. This result also showed that alkyl-functionalized RGO is a suitable carrier to disperse discrete POMs in nonpolar organic solvents in which they do not dissolve. HPAs typically show a strong ligand-to-metal charge transfer (LMCT) band in the 300−400 nm region. Ethanolic solutions of H3PMo12O40 with different concentrations (6 × 10−2 to 1.2 × 10−1 mg·mL−1; that is, 3 × 10−6 to 6.5 × 10−5 mol·L−1)

Figure 8. Photograph of 50PMo12−C10RGO suspended in ethanol (left, concentration 0.05 mg·mL−1) and toluene (right, concentration 0.1 mg·mL−1). The grafted alkyl chains allow the dispersion of HPAs in nonpolar organic solvents.

showed a sharp LMCT band at 309.5 nm with a molar extinction coefficient of 26 800 mol−1·L·cm−1, in good agreement with previously published values.53 The inflection point of the first derivative of the absorption spectrum was used to determine the edge position and obtain measurable shifts in the band gaps between the various samples. This technique is also commonly used to determine the edge position of suspended quantum dots and metal oxide nanocrystals.54,55 The edge energy in the case of the H3PMo12O40 solutions was calculated to be 2.59 eV (edge wavelength 480 nm), in reasonable agreement with values previously obtained for aqueous solutions (2.65 eV).56 The spectra acquired for H3PMo12O40−C10RGO showed peaks at the same positions as for unsupported H3PMo12O40, independent of the concentration (Figure 9a). Interpretation of the absorption spectra of H3PMo12O40−C10RGO samples required more processing because of the additional contributions of the support to the UV−vis spectrum. As the support is in a highly reduced state, the absorption spectrum of C10RGO is almost featureless (Figure 9). It was therefore necessary to subtract it to calculate the HPA contribution (Figure 9). H3PMo12O40 does not absorb at λ > 500 nm. Therefore, the C10RGO and H3PMo12O40−C10-RGO spectra were normalized at λ = 800 nm prior to the subtraction. The position of the edge was then determined from the first-derivative curve of the difference spectrum. Because of the additional processing, the derivative was noisy and the edge position was less well-defined. Therefore, an average edge energy was calculated from the edges obtained for spectra measured for suspensions with different concentrations. For the sample 50PMo12−C10RGO, the average edge was found to be at 2.56 eV (485 nm); that is, at a similar energy as for the unsupported HPA. At lower loading (sample 20PMo12−C10RGO), a significant shift of the edge energy to 2.9 eV (429 nm) was observed. 398

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We have previously shown that a linear correlation exists between the absorption edge energy measured for POM solutions and the potential of the first reduction in electrochemical measurements.56 In the present work, the strong absorption of graphene, as well as the broadening of the band observed when the loading of HPA decreased, precluded obtaining useful data for loadings below 20 wt %. A complete comparison between ATR-FTIR and UV−vis results was therefore not possible. The data did show that the edge energies for 50PMo12−C10RGO and for the H3PMo12O40 solution are similar (2.56−2.59 eV) and are actually higher than for H3PMo12O40 in the condensed phase (2.4 eV). The further increase of the edge energy for 20PMo12−C10RGO (2.9 eV) is consistent with the greater average dispersion of the HPAs at this loading; however, it is difficult to determine whether their interaction with the C10RGO support further modifies their electronic structure and thus their redox properties. Cyclic Voltammetry. Cyclic voltammograms of unsupported H3PMo12O40 immobilized on the glassy carbon electrode and recorded at 100 mV/s exhibited the typical three reversible redox peaks, each corresponding to a twoelectron process (Figure 10a).45 The first reduction peak (marked with an asterisk in Figure 10) was found to be at 0.305 V vs Ag/AgCl, in good agreement with previously published values.45,57−59 The H3PMo12O40 film was not stable during the measurement and the HPA progressively leached into the electrolyte, as demonstrated by the decreasing amplitude of the peaks with successive scans at the same scan rate. In contrast, no leaching was observed for the HPA/RGO samples when the potential was cycled between −0.23 and 0.52 V. Successive cycles overlapped almost perfectly, thus indicating that the HPA/RGO samples did not change significantly when the potential was cycled (Figures 10b,c). It seems that the interaction between the HPA and the RGO surface stabilized the anions and prevented them from leaching, even in the case of the 80PMo12−RGO sample (Figure 10b), as demonstrated by the stable amplitude of the CV spectra upon repeated scanning. For the highest loading sample, 80PMo12−RGO, there were indications of a lack of long-term stability in ethanol as the ink used to prepare the electrode turned green after a few hours. However, this was not observed for 50PMo12−RGO and samples at lower loadings. Schwegler et al.38 studied the leaching of H3PW12O40 supported on activated carbon. They found that the HPAs adsorb strongly and leaching is minimal in most solvents, except alcohols. The first cathodic peak (first reduction) was observed at 0.305 V vs Ag/AgCl for PMo12−RGO samples with a high HPA loading (>40 wt %); that is, at a similar value as for unsupported H3PMo12O40. Differences in the reduction potentials for the samples with 40, 50, 60, and 80 wt % HPA were not obvious. They all showed bulklike redox properties, in good agreement with the UV−vis data. A quantifiable shift of the first reduction peak was, however, observed for samples with greater HPA dispersions (20, 10, and 5 wt % HPA). The first reduction peak shifted to 0.295 V in the case of 20PMo12− RGO and further to 0.255 V for 5PMo12−RGO (Figure 10c). A shift to lower reduction potentials indicates that the individual HPA molecules are less oxidizing (their oxidation power is lower) than HPA aggregates and bulk solids.35,56,60,61 This observation is consistent with the UV−vis data that showed a shift of the edge to lower energies when reaching molecular dispersion.56

Figure 9. UV−vis absorption spectra of (a) 50PMo12−C10RGO, showing that the peak positions do not shift with concentration; (b) 50PMo12−C10RGO (0.05 mg/mL); and (c) 20PMo12−C10RGO (0.05 mg/mL), all dispersed in ethanol. The difference spectra in panels b and c were calculated by subtracting the normalized spectrum of the alkyl-functionalized graphene support (C10RGO) in ethanol from the spectrum of the HPA-containing sample. The edge energy was determined from the first derivative of the difference spectrum. 399

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hydrophilic and lipophilic. Notably, C10RGO enabled the dispersion of HPAs in both polar and nonpolar organic solvents, which allowed us to acquire UV−vis spectra of the suspended nanohybrid. We foresee that C10RGO will also be an interesting support for molecular species such as POMs to carry out catalytic reactions in organic solvents or for further processing into composite films for electronics.9 ATR-FTIR and electrochemical investigations showed that HPAs interact strongly with the RGO surface, which improved their stability against leaching during the CV tests. A measurable shift of up to 70 cm−1 was observed for the M− Oc−M band in the IR spectra when the HPA loading decreased, and the dispersion improved until the molecular level was reached. This shift was observed independent of the composition of the Keggin-structure HPAs (H3PMo12O40, H3PW12O40, and H4PMo11VO40) and of the support (RGO, C10RGO, silica). This shift most likely involves a combination of several factors: interaction between the molecules and the surface, which alters their symmetry (leading to a peak broadening); hydration (which impacts the bending frequencies); nature of the counterion (H+ is most probably delocalized between the HPA and the protonated surface, which induces a change in the polarization and cation−anion charge transfer); and anion−anion interactions (which influence both stretching and bending vibrations). UV−vis and CV measurements were carried out in order to further probe any change in the electronic structure of the HPA clusters that could result from their dispersion and electronic interaction with the surface. Both techniques showed that supported H3PMo12O40 exhibits “bulklike” redox properties for loadings above 40 wt %. However, for lower loadings (20 wt % and below), the edge energy increased from 2.56 to 2.9 eV and the first reduction peak shifted to a less positive potential, from 0.295 to 0.255 V. Both techniques are in good agreement with previous results and indicate that H3PMo12O40 species dispersed on graphene are less oxidizing than their bulk counterpart. While it was already well-known that the electronic properties of POMs can be tuned by acting on their structure and elemental composition (nature of the framework atoms and counterion), the present work opens a new route to further adjust their properties by acting on the support. The redox and catalytic properties of supported polyoxometalates depend both on their interaction with the support and on their molecular dispersion. The exact role of the support and the mechanisms involved remain unclear and will require further investigation. Recent theoretical calculations performed for PW12O403− supported on graphene showed that a charge transfer takes place from the HPA anion to the graphene.48 Such charge transfer would decrease the electron density on the POM anion and thus decrease its reducibility (the HPA would be less oxidizing), as observed in the present work. This hypothesis is particularly exciting as it raises the possibility of further tuning the electronic properties of the POMs by doping graphene with heteroatoms. Conversely, the electronic properties of graphene may be modified by carefully selecting the POM adsorbed and controlling its dispersion. Giovannetti et al.62 studied the interaction of graphene and metals. They showed that electron transfer occurs between both and that graphene can be either ndoped or p-doped depending on the work function of the metal. Similarly, Huang et al.63 managed to vary the work function of graphene between 5.1 and 3.4 eV by doping it with Cs. Controlling the interaction between POMs and (function-

Figure 10. Cyclic voltammograms of (a) unsupported H3PMo12O40, (b) 80PMo12−RGO, and (c) 5PMo12−RGO immobilized on a glassy carbon electrode in Ar-saturated 0.5 M H2SO4. The arrow in panel a emphasizes the decrease in intensity with increasing number of cycles. (*) Position of the first reduction peak.

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DISCUSSION HPAs (H3PMo12O40, H3PW12O40, and H4PMo11VO40) readily disperse on oxygen-functionalized (RGO) and decyl-functionalized graphene sheets (C10RGO). Electron microscopic investigations demonstrated that the HPA clusters are homogeneously dispersed on the carbon supports; while at low loading the HPAs are dispersed as single (discrete) molecular species, they tend to progressively form dimers and larger oligomers at loadings higher than 20 wt %. The HPA coverage on RGO did not intrinsically change the surface properties: HPA−RGO and HPA−C10RGO were respectively 400

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alized) graphene potentially opens new routes to design heterogeneous catalysts with optimal activity for redox reactions and to synthesize POM−graphene nanohybrids with controllable properties for organic optoelectronics and other applications.

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CONCLUSIONS RGO and alkyl-functionalized RGO sheets were synthesized and employed to support various HPAs. The high surface area of the graphene sheets allowed a high dispersion of the HPAs, down to the molecular level, as well as their suspension in organic solvents in which unsupported HPAs cannot be dissolved. Spectroscopic and electrochemical investigations demonstrated that the structure and the electronic properties of the HPA clusters are altered as a result of their high dispersion and strong interaction with the graphene sheets. Specifically, FTIR spectroscopy revealed a linear shift of the M−Oc−M vibration frequency as a function of the loading. UV−vis and electrochemical measurements respectively showed that the band edge position and the potential of the first reduction peak shifted, indicating that the HPAs became less oxidizing as a result of their dispersion. Our initial observations were generalized to various HPAs as well as different supports. These results demonstrate that metal oxide clusters are sensitive to their local environment, including the interaction with a support. The implications are important for catalysis but also for a variety of applications for which the redox and/or electronic properties of POMs are of importance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the ARRA/AFOSR for financial support (Grant FA9550-09-1-0523). We thank Professor Ilhan Aksay and Dr. Daniel Dabbs of Princeton University for supplying the RGO material. We also thank Dr. Libor Kovarik of the Environmental Molecular Sciences Laboratory for acquiring the Cs-corrected STEM-HAADF images.

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