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Cite This: J. Am. Chem. Soc. 2018, 140, 10332−10343

Evidence of the Core−Shell Structure of (Photo)magnetic CoFe Prussian Blue Analogue Nanoparticles and Peculiar Behavior of the Surface Species Amélie Bordage,† Robinson Moulin,† Emiliano Fonda,‡ Giulia Fornasieri,† Eric Rivière,† and Anne Bleuzen*,† †

Institut de Chimie Moléculaire et des Matériaux d’Orsay, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France Synchrotron SOLEIL, L’Orme des Merisiers, St Aubin, BP 48, 91192 Gif sur Yvette, France

J. Am. Chem. Soc. 2018.140:10332-10343. Downloaded from pubs.acs.org by UNIV OF CAMBRIDGE on 08/20/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: We report on a comparative study of 5.5 nm (embedded in an ordered mesoporous silica matrix) and 100 nm (free) (photo)magnetic CoFe Prussian blue analogue (PBA) particles. Co and Fe K-edge X-ray absorption spectroscopy, X-ray diffraction, infrared spectroscopy, and magnetic measurements point out a core−shell structure of the particles in their ground states. In the 5.5 nm particles, the 11.5 Å thick shell is made of Fe(CN)6 entities and CoII−NC−FeIII linkages departing from the geometry usually encountered in PBA, whatever the oxidation state (CoIIFeIII or CoIIIFeII) of the CoFe pairs in the core. In the photomagnetic particles, the photomagnetic effect in the core of the particles is due to the same photoinduced CoIII(LS)FeII → CoII(HS)FeIII electron transfer whatever the size of the particles. The shell of the nanoparticles exhibits a peculiar photoinduced structural rearrangement, and the nanoparticles in their photoexcited state exhibit a superparamagnetic behavior.

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state change of the Mo ion.23 Despite the nanometric size of the MoCu nanoparticles (2.9 nm), a ferromagnetic behavior of the photoexcited state with a wide hysteresis loop was evidenced in this family of compounds,24 and a superparamagnetic behavior was detected in the photoexcited state of 3 nm CuNiMo trimetallic nanoparticles with a possible blocking temperature of 1 K.25 Lastly, several photomagnetic CoFe Prussian blue analogue (PBA) nanoparticles were prepared,26−29 and a superparamagnetic behavior of the ground state was evidenced in 10 nm particles.29 The CoFe PBA of chemical formula Rb2Co4[Fe(CN)6]3.3· 11H2O is among the first compounds in which a photomagnetic effect due to a metal−metal charge transfer was evidenced and one of the most studied photomagnetic compounds.30−32 The powder (called RbCoFe) is composed of 100 nm single-crystalline particles (SI1). The welldocumented photomagnetic effect in RbCoFe is due to the CoIII(LS)FeII → CoII(HS)FeIII electron transfer (LS: low spin; HS: high spin).30−32 Our group already evidenced a photomagnetic effect, different from that of the powder, in singlecrystalline 5.5 nm particles of the same chemical composition (called NanoRbCoFe) embedded in a silica matrix exhibiting a 2D hexagonal structure of the mesoporosity33−35 (SI1). Briefly, this first magnetometry study shows that the main differences between the photomagnetic effect exhibited by NanoRbCoFe

INTRODUCTION Switchable molecular materials are attractive for the design of multifunctional innovative devices since their physical properties can be tuned by the application of various external stimuli. Different physical (temperature,1 pressure,2 magnetic field,3 light,4 etc.) but also chemical (cations,5 molecules,6 even a solid,7 etc.) strains are likely to trigger the property change. Among them light is of particular interest because the associated electronic transitions, and therefore the addressing time, are extremely short.8−18 Nevertheless, the integration of such compounds into devices necessitates their processing as nano-objects and the knowledge of the effect of this processing step on the physical properties of the compounds. Among inorganic systems, spin-crossover9−13 and charge transfer compounds14−18 are often considered as the most promising and are therefore, by far, the most studied. Nevertheless, still few works evidence photoswitchable properties in nanoparticles, and the effect of size reduction on these properties remains an open question. In spin-crossover compounds, the light-induced excited spin state trapping (LIESST) effect is retained in small particles without an effect of the particles size on the thermal relaxation kinetics.19−21 Nevertheless, it has to be noted that the particles studied so far are larger than 30 nm. A photomagnetic effect was evidenced in nanoparticles of cyanide-bridged bimetallic MoCu coordination polymers.22−25 The photomagnetic effect, first assigned to a metal−metal charge transfer,22 was reassigned in some MoCu Prussian blue derivatives to a spin © 2018 American Chemical Society

Received: June 11, 2018 Published: July 23, 2018 10332

DOI: 10.1021/jacs.8b06147 J. Am. Chem. Soc. 2018, 140, 10332−10343

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Journal of the American Chemical Society and RbCoFe (SI2) are (i) a much higher magnetization at 2 K before irradiation in the nanoparticles and (ii) different profiles of the magnetization curves after irradiation below the magnetic ordering temperature (21 K) and over the temperature range corresponding to the thermal relaxation of the photoexcited metastable CoII(HS)FeIII state.33 In order to better understand these differences between the 5.5 nm particles confined within the silica mesoporosity and the 100 nm free particles (called “powder” in the following), we have conducted a comparative study of these compounds by X-ray absorption spectroscopy, X-ray diffraction, infrared spectroscopy, and magnetometry. As the two compounds in their ground state differ by both the electronic structures and local environment of the metal ions, the same comparative study was also conducted on the alkali cation free CoFe PBA of chemical formula Co4[Fe(CN)6]2.7·17H2O, which does not exhibit switching properties at ambient pressure. The powder and the 5.5 nm nanoparticles embedded in an ordered mesoporous silica monolith are respectively called CoFe and NanoCoFe. CoFe is known to be ferrimagnetic below its Curie temperature (16 K),36,37 and the magnetic behavior of the CoFe PBA nanoparticles contained in NanoCoFe slightly deviates from pure superparamagnetism due to weak interparticle interactions.34 The crystallographic and electronic structures of RbCoFe and CoFe are well known.18,32 RbCoFe is mainly made of CoIII(LS)FeII(LS) pairs, which result from the CoII(HS)FeIII → CoIII(LS)FeII electron transfer during its synthesis,32,38 so that its chemical formula taking into account the oxidation state of the ions is Rb2CoIII3.3CoII0.7[FeII(CN)6]3.3·11H2O. Due to the low spin state of the CoIII ion, the Co-to-ligand bonds are short (1.92 Å)39−41 and the lattice parameter of the face-centered cubic structure is also particularly small (9.96 Å)42 for a compound of the PBA family. CoFe is mainly made of CoII(HS) and FeIII(LS) ions, so that its chemical formula taking into account the oxidation state of the metal ions is CoII4[FeIII(CN)6]2.7· 17H2O.18,32 With a high spin state of the CoII ion the Co-toligand bonds are significantly longer (2.08 Å)39−41 and the lattice parameter of the face-centered cubic structure is 10.35 ± 0.05 Å.18 The chemical formulas for NanoRbCoFe and NanoCoFe are already published, and a first X-ray absorption study at the Co K-edge showed that NanoCoFe contains PBA nanoparticles mainly made of CoII(HS) ions, whereas the PBA nanoparticles contained in NanoRbCoFe are made of a mixture of CoII(HS) and CoIII(LS) ions.34,43 The results presented here shed new light on (i) the magnetic and photomagnetic particles, which can be described in terms of a core−shell structure in their ground state, (ii) the disappearance of the core−shell structure in the photoexcited state of the photomagnetic particles, which exhibit a superparamagnetic behavior, and (iii) the peculiar structure and behavior of the surface species.

Figure 1. Normalized room-temperature XANES spectra of NanoRbCoFe (orange) and NanoCoFe (pale blue) compared with those of RbCoFe (red) and CoFe (dark blue) (a) at the Co K-edge and (b) at the Fe K-edge.

At the Co K-edge (Figure 1a), the spectrum of NanoRbCoFe exhibits all the features of Co ions in octahedral symmetry, but it is significantly different from that of RbCoFe. The absorption maximum (1s a1g → 4p t1u transitions) on the spectrum of NanoRbCoFe exhibits two well-defined contributions with a comparable intensity at 7725 and at 7728 eV; they are the signature of the electronic state of the Co ions and can be assigned to CoII(HS) and CoIII(LS) ions, respectively. Therefore, in contrast to RbCoFe, which is composed of 80% CoIII(LS) and 20% CoII(HS),18 PBA particles in NanoRbCoFe are composed of a comparable amount of both ions, as previously reported.34 Additionally, the well-defined feature observed at 7719 eV on the spectrum of RbCoFe and due to 1s a1g → π*(CN) transitions is absent in the case of NanoRbCoFe, which indicates differences in the M−NC linkages (M = transition metal) between the nanoparticles and the powder. The low-energy shift of the first EXAFS (extended Xray absorption fine structure) oscillation at 7785 eV on the spectrum of NanoRbCoFe in comparison with that of RbCoFe is due to the lengthening of the average Co-to-ligand distance,

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RESULTS AND DISCUSSION Comparative Study of (Rb)CoFe PBA 100 nm Free Particles and 5.5 nm Nanoparticles Embedded in an Ordered Mesoporous Silica Monolith. The room-temperature XANES (X-ray absorption near-edge structure) spectra of NanoRbCoFe and NanoCoFe are compared to those of RbCoFe and CoFe in Figure 1a and b for the Co K-edge and the Fe K-edge, respectively. The XANES spectra of both PBA powders have already been analyzed and will not be discussed any further here.18,32,39−41 10333


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comes from an incomplete CoIIFeIII → CoIIIFeII electron transfer during the PBA synthesis step. Furthermore, although all the features that are present on the spectra of the powders are also present on the spectra of the nanocomposites, the spectra of the nanocomposites and of the powders differ significantly from one another, especially in the profile of the beginning of the absorption ramp and in the multiple scattering structures. These differences reflect, as at the Co K-edge, significantly different average local structures around the Fe ions in the nanocomposites and in the powders. All the differences observed between NanoRbCoFe (respectively NanoCoFe) and RbCoFe (respectively CoFe) at both edges reflect significantly different average local structures around the metallic ions in the nanocomposites and in the powders. It still has to be noticed that the modifications of the XANES spectra may be also due to the effect of missing bonds at the coordination polymer surface, also called surface truncation in the following. The X-ray diffraction patterns of NanoCoFe and NanoRbCoFe are compared to those of CoFe and RbCoFe in Figure 2a. The diffractograms of the two nanocomposites exhibit a

associated with the larger amount of CoII(HS) ions with longer Co-to-ligand bonds in the nanocomposite. We already showed18 that the spectra of CoFe PBA powders containing a mixture of CoIII(LS) and CoII(HS) ions are well reproduced by the linear combination of the spectra of reference compounds allowing the quantification of ions in each oxidation state.18 The spectrum of NanoRbCoFe was therefore compared to such linear combinations of reference spectra for CoIII(LS) and CoII(HS) species (SI3); the best agreement is obtained for 60% CoII(HS) and 40% CoIII(LS) and gives an estimation of both species in NanoRbCoFe. Nevertheless, in contrast to powders,18,44 none of the linear combinations completely satisfactorily reproduces the spectrum of NanoRbCoFe. Especially, the beginning of the absorption ramp and the multiple scattering contribution just above the absorption edge are different from those of the powders (Figures 1 and S3-3). These differences reveal, in addition to a different CoII(HS)/CoIII(LS) ratio and associated different Co-to-ligand bond lengths, a different average local structure around the Co ions in NanoRbCoFe and in RbCoFe. The spectrum of NanoCoFe with an absorption maximum at 7725 eV is close to that of CoFe18 and exhibits all the features of CoII(HS) ions in sites having octahedral symmetry. The main differences between both spectra are (i) different profiles of the beginning of the absorption ramp, (ii) a lower intensity of the absorption maximum and the presence of a shoulder at 7730 eV for the nanocomposite, and (iii) different profiles of the multiple scattering contribution just after the absorption edge. The different profiles of the beginning of the absorption ramp and of the multiple scattering contributions clearly indicate different average local structure around the Co ions in NanoCoFe and in CoFe, as already observed for NanoRbCoFe and RbCoFe. The lower intensity of the absorption maximum and the presence of a shoulder at 7730 eV can also arise from different local structures around the absorber atom, or they can reveal the presence of minority CoIII(LS) species in the nanocomposite. Nevertheless, the location of the first EXAFS oscillation at the same energy for both NanoCoFe and CoFe indicates that the amount of CoIII(LS) species with short Coto-ligand bonds is not larger in NanoCoFe than in CoFe. The differences between the spectra of NanoCoFe and CoFe can therefore be assigned to different average local structures around the CoII(HS) ions in both samples, as it is the case for NanoRbCoFe and RbCoFe. At the Fe K-edge (Figure 1b), the absorption maximum located at the same energy (7131 eV) on the spectra of NanoCoFe and of CoFe indicates that both compounds mainly contain FeIII ions; like CoFe, NanoCoFe is mainly composed of CoII(HS)FeIII pairs. The absorption maximum on the spectrum of NanoRbCoFe is slightly shifted toward higher energy with respect to RbCoFe, whose absorption maximum at 7130 eV reflects the +II oxidation state of the Fe ions.18,32,39−41 This intermediate energy between 7131 and 7130 eV for the nanocomposite reflects the presence of a mixture of FeII and FeIII ions. These supplementary FeIII ions in the nanocomposite can be related to the larger amount of CoII(HS) ions detected at the Co K-edge in NanoRbCoFe than in RbCoFe (see above). While RbCoFe is essentially made of CoIIIFeII pairs resulting from the CoIIFeIII → CoIIIFeII redox process during the synthesis,32,38 the counterpart nanocomposite contains a significant amount of CoII and FeIII ions. As all the metal ions are part of the PBA network in the nanocomposites,43 the larger amount of CoII and FeIII ions

Figure 2. (a) X-ray diffraction patterns of NanoRbCoFe (orange) and NanoCoFe (pale blue) compared with those of CoFe (dark blue) and RbCoFe (red) (asterisks (*) mark the peaks of the aluminum sample holder). (b) FT-IR spectra of NanoRbCoFe (orange) and NanoCoFe (pale blue) compared with those of RbCoFe (red) and CoFe (dark blue) over the 2050−2250 cm−1 cyanide stretching vibration range.

broad diffuse peak centered at 24° (2θ) arising from amorphous silica. In addition to this broad feature, the most intense diffraction lines of the well-known face-centered-cubic structure of Prussian blue analogues45 can be detected. These peaks are broader and weaker than those of CoFe and RbCoFe, in agreement with the size reduction and dilution of the PBA particles in the silica matrix. In CoFe PBA powders, the unit cell parameter is related to the oxidation and spin states of the Co ions.18,42 Thus, the value of the lattice parameter of CoFe mainly composed of CoII(HS)FeIII pairs (10.35 ± 0.05 Å)18 is significantly larger than that of RbCoFe, mainly composed of CoIII(LS)FeII pairs (9.96 ± 0.02 Å);42 the value of the lattice parameter of CoFe PBA powders containing a mixture of CoII(HS) and CoIII(LS) ions is intermediate and depends on the CoII(HS)/CoIII(LS) ratio.18,42 The weak intensity and the broadness of the diffraction peaks of the nanocomposites prevent a precise determination of their unit cell parameters. Nevertheless, it clearly appears that the position of the diffraction peaks of the nanocomposites is very close to that of the corresponding PBA powder, indicating 10334


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CoIII(LS)FeII environment is significantly more intense than that corresponding to the CoII(HS)FeIII one. A structural model compatible with all the experimental data based on a core−shell structure of the particles and schematized in Figure 3 can be proposed. In the following,

the presence of coherently diffracting domains mainly composed of CoIII(LS)FeII pairs in NanoRbCoFe and mainly composed of CoII(HS)FeIII species in NanoCoFe. This result is in line with the majority CoII(HS)FeIII species in NanoCoFe, but it contrasts with the 60% CoII(HS) and 40% CoIII(LS) in NanoRbCoFe detected by XANES spectroscopy. This means that the CoFe PBA nanoparticles in NanoRbCoFe are neither composed of a homogeneous mixture of CoIII(LS) and CoII(HS) ions, which would give rise to an intermediate position of the diffraction peaks, nor composed of two kinds of particles, respectively composed of CoII(HS)FeIII or CoIII(LS)FeII pairs, which would result in the splitting of each diffraction line into two peaks. The FTIR spectra of NanoCoFe and NanoRbCoFe are shown in Figure 2b, where they are compared to those of CoFe and RbCoFe over the 2050−2250 cm−1 cyanide stretching vibration range. Most of the bands have already been assigned in the literature. The bands at 2160 ± 6 cm−1, 2125 ± 6 cm−1, and 2090 ± 6 cm−1 are assigned to cyanide in the CoII(HS)FeIII, CoIII(LS)FeII, and CoII(HS)FeII environments, respectively.5a,37,46−50 According to its chemical composition (Rb2CoIII3.3CoII0.7[FeII(CN)6]3.3·11H2O)18,21 the spectrum of RbCoFe exhibits a main band at 2125 cm−1 and a shoulder at 2090 cm−1, respectively ascribable to the CoIII(LS)−NC−FeII majority linkages and the CoII(HS)−NC−FeII minority species. The main band on the spectrum of CoFe corresponds to the CoII(HS)−NC−FeIII majority species, and the weak bands at 2120 and 2092 cm−1 have been assigned to the presence of small amounts of cyanide in CoIII(LS)FeII and CoII(HS)FeII pairs. It has to be noticed that the bands corresponding to cyanide ions linked to FeII ions are always significantly more intense than the bands corresponding to cyanide ions linked to FeIII ions. The spectrum of NanoRbCoFe exhibits a main band at 2130 cm−1 ascribable to cyanide ions in the CoIII(LS)FeII environment. The spectrum of NanoCoFe exhibits a main band centered at 2166 cm−1, assigned to CoII(HS)−NC−FeIII species. The weak bands at lower energy can be ascribed either to the presence of small amounts of CoIII(LS)FeII pairs, as for CoFe, and/or to terminal FeIII−CN species. In addition, the spectrum of NanoCoFe exhibits a shoulder of significant intensity at 2190 cm−1, which has never been clearly ascribed before. This shoulder at 2190 cm−1 can also be seen on the spectrum of NanoRbCoFe, and, by scrutinizing the spectra of CoFe and RbCoFe, one can also observe this band, although much weaker (SI4). This band lies in the energy range corresponding to cyanide linked by the C side to FeIII ions;5a,48−50 therefore, it can arise either from CoIII(LS)− NC−FeIII linkages49 or from a second type of CoII(HS)−NC− FeIII species, different from those that are mainly contained in CoFe. XANES spectroscopy (see above) clearly shows that NanoCoFe essentially contains CoII(HS) species, so that the first hypothesis can be ruled out and the band at 2190 cm−1 assigned to a second kind of CoII(HS)−NC−FeIII linkages. As this band is more intense in the nanoparticles, we propose to assign it to surface species with very close spectral signature for all the compounds whatever their size and the oxidation state of the transition metal ions; the intensity of this band is much weaker in CoFe and RbCoFe than in the nanocomposites due to the lower surface-area-to-volume ratio. Its intensity seems weaker in NanoRbCoFe than in NanoCoFe, but one has to keep in mind that the cyanide band corresponding to the

Figure 3. Scheme of the structure proposed for the PBA particles in RbCoFe, CoFe, NanoRbCoFe, and NanoCoFe. The scale between the core and the shell is arbitrary.

the part of the sample made of surface species will be called the shell and the rest of the sample will be called the core. What differs between the powders and the nanocomposites with the same chemical composition is essentially the surface-area-tovolume ratio. The relative intensity of the contributions of the surface and core species depends (i) on the size of the particles and (ii) on the measurement technique. X-ray absorption spectroscopy at the transition metal (TM) K-edges probes all the absorber atoms. In the 100 nm particles, the surface-areato-volume ratio is so low that the contribution of surface species is negligible and the spectra are mainly those of core species. In contrast, surface species represent a significant part of the coordination polymer in the 5.5 nm particles. The XAS spectra of CoFe and NanoCoFe are close because the compounds are both composed of CoII(HS)FeIII species. The difference between the spectra arises from different local structures of the TM ions in the core and in the shell. The difference between the spectra of RbCoFe and NanoRbCoFe at the Co K-edge is more pronounced because, in addition to different local structures around the TM ions in the core and in the shell, the oxidation state of the TM ions and the spin state of the Co ions are different in the core (CoIII(LS)) and in the shell (CoII(HS)). The fact that the surface of particles with a chemical formula given by Rb2Co4[Fe(CN)6]3.3·11H2O and with a core mainly made of CoIIIFeII pairs is composed of CoII and FeIII species is not surprising. Indeed, the CoIIFeIII → CoIIIFeII redox process that accompanies the formation of RbCoFe is conditioned by a sufficient amount of Fe(CN)6 entities in the coordination sphere of the Co ions;38 this amount is necessarily reduced on the surface of the particles, which can prevent the redox process from happening. As the surface species form a thin shell around the particles, it is most probable that these species do not give rise to detectable X-ray diffraction peaks. Detectable X-ray diffraction peaks arising only from the core of the particles explain the same positions of the diffraction peaks for the nanocomposite and the powder counterpart. Such a core−shell structure is also supported by the magnetic behavior of the ground state of the RbCoFe and 10335


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Journal of the American Chemical Society NanoRbCoFe photoswitchable compounds. The zero field cooled (ZFC) and field cooled (FC) magnetization curves of NanoRbCoFe and RbCoFe were recorded under a magnetic field of 50 Oe; they are shown in Figure SI5. The FC/ZFC curves for NanoRbCoFe are completely different from those for NanoCoFe,34 which confirms the absence of nanoparticles mainly composed of CoIIFeIII pairs in NanoRbCoFe. If this was the case, a superparamagnetic behavior comparable to that of NanoCoFe would have been observed.34 For both NanoRbCoFe and RbCoFe compounds, the curves exhibit a ZFC/ FC thermal irreversibility, which reveals a collective behavior caused by magnetic interactions. In both compounds, the chemical composition of the core of the particles is given by Rb2CoIII3.3CoII0.7[FeII(CN)6]3.3·11H2O; this core is mainly made of diamagnetic CoIII(LS)−NC−FeII linkages and of a few isolated CoII ions. The average distance between these CoII ions is too long to give rise to magnetic interactions strong enough to induce a collective behavior of the magnetic moments. This part of the compounds is therefore expected to exhibit a paramagnetic behavior without ZFC/FC thermal irreversibility. The experimentally observed ZFC/FC thermal irreversibility can be explained by the presence of a shell made of CoII(HS) and FeIII metal ions linked by cyanide bridges, through which the exchange interaction is strong enough to induce a spin glass or a superparamagnetic behavior. Furthermore, the reduced differences between the FC and ZFC magnetization curves ((MFC − MZFC)/(MFC − MZFC)5K) for NanoRbCoFe and RbCoFe (SI5) are very close, meaning that the species originating this magnetic behavior in both compounds are very similar. Such similar magnetic behavior in the ground state of the both compounds strongly supports the presence of similar surface species made of magnetic CoIIFeIII pairs. To summarize, we propose that (i) NanoCoFe and CoFe consist of a CoII(HS)FeIII core, (ii) NanoRbCoFe and RbCoFe consist of a CoIII(LS)FeII core, and (iii) all four compounds have a similar CoII(HS)FeIII shell. Structure and Electronic Structure of the Core and Shell at Room Temperature. In the 100 nm particles, surface species represent a negligible part of the compound so that the spectra of RbCoFe and CoFe will be considered as those of pure particle cores: a core mainly made of CoIII(LS)FeII linkages for RbCoFe and mainly made of CoII(HS)FeIII linkages for CoFe. In the 5.5 nm particles, the shell represents a significant part of the compound so that, if the nanoparticles are composed of a shell made of surface CoII(HS)FeIII pairs and a core similar to that of the 100 nm particles, the X-ray absorption spectrum of NanoRbCoFe (respectively NanoCoFe) should be the sum of the two contributions, which are called ShellNanoRbCoFe (respectively ShellNanoCoFe) and CoreNanoRbCoFe (respectively CoreNanoCoFe) in the following. The spectra of the core and of the shell of the nanoparticles were obtained from linear combinations of the spectra of NanoRbCoFe, NanoCoFe, RbCoFe, and CoFe according to the procedure described in the supplementary data (SI6) and giving similar spectra for ShellNanoRbCoFe and ShellNanoCoFe. At the Co K-edge, the linear combinations used to obtain the spectra for ShellNanoRbCoFe, ShellNanoCoFe, CoreNanoRbCoFe, and CoreNanoCoFe are given by eqs 1, 2, 3, and 4, respectively:

μShellnanoRbCoFe = μShellNanoCoFe =

1 (μ − 0.2μRbCoFe ) 0.8 NanoRbCoFe

1 (μ − 0.3μCoFe ) 0.7 NanoCoFe

μCoreNanoRbCoFe =

(1)

(2)

1 (μ − 0.8μShellNanoCoFe ) 0.2 NanoRbCoFe (3)

μCoreNanoCoFe =

1 (μ − 0.7μShellNanoRbCoFe ) 0.3 NanoCoFe

(4)

These spectra are shown in Figure 4a, where they are compared to those of RbCoFe and CoFe. The spectra of CoreNanoRbCoFe and of CoreNanoCoFe are very close to

Figure 4. (a) Normalized XANES spectra at the Co K-edge of RbCoFe (red), CoFe (dark blue), CoreNanoRbCoFe (gray), CoreNanoCoFe (black), ShellNanoRbCoFe (pale green), and ShellNanoCoFe (dark green). (b) Moduli of the Fourier transform of the EXAFS signals of RbCoFe (red), CoFe (dark blue), ShellNanoRbCoFe (pale green), and ShellNanoCoFe (dark green) (the linear combination was performed in k-space prior to the Fourier transform). 10336


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NanoCoFe at the Fe K-edge (SI6). The linear combinations used to obtain the spectra for ShellNanoRbCoFe, ShellNanoCoFe, CoreNanoRbCoFe, and CoreNanoCoFe are given by eqs 5, 6, 7, and 8, respectively:

those of RbCoFe and CoFe, respectively, which confirms, as suggested by X-ray diffraction above, the similarity of the CoIII(LS)FeII core in NanoRbCoFe and in RbCoFe and the similarity of the CoII(HS)FeIII core in NanoCoFe and in CoFe. This also confirms that the contribution of the surface species to the XAS spectra at the Co K-edge is negligible in the powders. This clearly demonstrates that (i) the nanoparticles are formed of well-defined cores and shells and (ii) the electronic structure and local structure of the Co species forming the shell of both kinds of nanoparticles are very close despite the very different cores (mainly made of CoIII(LS)FeII pairs with a lattice parameter close to 9.96 Å for NanoRbCoFe and mainly made of CoII(HS)FeIII pairs with a lattice parameter close to 10.35 Å for NanoCoFe). On the XANES spectra at the Co Kedge of the shell species (ShellNanoRbCoFe and ShellNanoCoFe), the position of the absorption maximum at 7725 eV is the signature of CoII(HS) ions and the position of the first EXAFS oscillation at the same energy as on the spectrum of CoFe is in line with Co-to-ligand bond lengths for 6-foldcoordinated CoII(HS) ions. Nevertheless, the shape of the XANES spectra of the Co shell species, significantly different from that of CoFe and of CoreNanoCoFe, clearly indicates that, despite the same CoII(HS) electronic configuration, the local structure around the CoII(HS) ions in the shell is different from those forming the core. The disappearance of the structure at 7719 eV in the beginning of the absorption ramp on the spectrum of the shell species reflects different CoII(HS)-to-NC bonds, and the broadness of the absorption maximum suggests a lower symmetry of the Co site in the shell than in the core. In order to go ahead with the description of the local structure around the Co ion in the shell, the Fourier transforms (FT) of the EXAFS signal of ShellNanoRbCoFe and ShellNanoCoFe were extracted. The FT moduli are shown in Figure 4b, where they are compared to those for RbCoFe and CoFe. The FT moduli for RbCoFe and CoFe are composed of well-defined peaks.51 In RbCoFe (respectively CoFe), the Co ions in octahedral sites are linked to the N-side of an average of five (respectively four) cyanide bridges and one (respectively two) water molecule(s). Thus, the first peak of the FT moduli can be assigned to the first N and O neighbors, and the other main peaks can be assigned to the C and Fe neighbors. All the peaks are shifted toward a lower distance value in RbCoFe. This reflects the Co-to-first neighbor distance, which is significantly shorter in RbCoFe (1.91 Å),39−41 mainly made of CoIII(LS) with no electron in antibonding eg* orbitals, than in CoFe (2.08 Å),39−41 containing CoII(HS) ions with two electrons in antibonding eg* orbitals. The FT moduli of the shell species are very different from those of CoFe and RbCoFe. They display one main peak at the same distance as the first peak of the FT modulus for CoFe, attributable to first neighbors of 6-foldcoordinated CoII(HS) ions, which suggests that the coordination number of the Co ion is the same in the core and in the shell and therefore at the coordination polymer surface also. The very weak intensity of the next contributions probably reflects some disorder in the shell and the limited number of neighbors beyond the first nearest neighbors in line with a higher OH2/N−C ligands ratio in the coordination sphere of Co surface truncated species. The contributions of the shell and of the core were also extracted from the normalized spectra of NanoRbCoFe and

μShellnanoRbCoFe = μShellNanoCoFe =

1 (μ − 0.3μRbCoFe ) 0.7 NanoRbCoFe

1 (μ − 0.1μCoFe ) 0.9 NanoCoFe

μCoreNanoRbCoFe =

(5) (6)

1 (μ − 0.9μShellNanoCoFe ) 0.1 NanoRbCoFe (7)

μCoreNanoCoFe =

1 (μ − 0.7μShellNanoRbCoFe ) 0.3 NanoCoFe

(8)

These spectra are shown in Figure 5a, where they are compared to those of RbCoFe and CoFe. The same conclusions as at the Co K-edge can be drawn concerning the well-defined core and shell contributions, the similarity of the core of NanoRbCoFe (respectively NanoCoFe) and of RbCoFe (respectively CoFe), and the similarity of the electronic structure and local structure of the Fe species forming the shell of both kinds of nanoparticles. The XANES spectra of the Fe ions of the shells differ from the spectra of the cores by a weaker intensity of the absorption maximum and of the multiple scattering contribution as well as a stronger intensity of the pre-edge features. All together, this suggests that the octahedral Fe(CN)6 entities with linear Fe−C−N linkages in the cores undergo an important structural reorganization in the shells. The moduli of the FT of the EXAFS signal at the Fe K-edge of ShellNanoRbCoFe and ShellNanoCoFe are shown in Figure 5b, where they are compared to those of RbCoFe and CoFe. The FT moduli for RbCoFe and CoFe are very close because the local structure around the Fe ions is very close in both compounds. The two main contributions can be assigned to the six C first neighbors and the six N second neighbors. The comparable intensity of the first and second contributions is characteristic of the linear Fe−C−N linkages. The FT moduli for the shell species are completely different, with one main contribution situated at a significantly longer distance from the absorber atom than in the octahedral Fe(CN)6 species with linear Fe−C−N linkages. This indicates that the Fe(CN)6 entities in the shells strongly depart from the usual octahedral symmetry with linear or quasi-linear Fe−C−N linkages, and the FeIII ions could be in their HS state. The study of this peculiar structure is in progress. An in-depth EXAFS analysis for instance should provide valuable structural information on coordination number, bond lengths, bond angles, and tilt of the coordination polyhedra. The X-ray absorption spectra of RbCoFe, CoFe, NanoRbCoFe, and NanoCoFe and the linear combinations extracted from those spectra demonstrate the well-defined core−shell structure of the particles, and the spectral signature of the cores and of the shells could be extracted from these data. The quantitative information from the linear combination allows a more precise description of the core−shell structure of NanoRbCoFe and NanoCoFe. Assuming a spherical shape of a particle with a diameter of 5.5 nm (constrained by the size of the pore of the mesoporous silica monolith) and a constant density of the metal ions over the whole nanoparticles, an estimation of the dimensions of the shell and of the core can be 10337


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Figure 6. (a) Fe K-edge normalized XANES spectrum of NanoRbCoFe* (orange, filled circles) compared to that of RbCoFe* (red, filled circles). (b) Comparison of the Fe K-edge normalized XANES spectrum of RbCoFe* (red, filled circles) and RbCoFe (red), NanoRbCoFe* (orange, filled circles) and CoreNanoRbCoFe (gray), and NanoRbCoFe* (orange, filled circles) and ShellNanoRbCoFe (pale green).

Figure 5. (a) Normalized XANES spectra at the Fe K-edge of RbCoFe (red), CoFe (dark blue), CoreNanoRbCoFe (gray), CoreNanoCoFe (black), ShellNanoRbCoFe (pale green), and ShellNanoCoFe (dark green). (b) Moduli of the Fourier transform of the EXAFS signals of RbCoFe (red), CoFe (dark blue), ShellNanoRbCoFe (pale green), and ShellNanoCoFe (dark green) (the linear combination was performed in k-space prior to the Fourier transform).

increase in the magnetization of NanoRbCoFe (SI2) after irradiation at low temperature was ascribed to the CoIII(LS)FeII → CoII(HS)FeIII photoinduced electron transfer.33,34 Nevertheless, the FC magnetization curves of NanoRbCoFe before and after irradiation are different from those of RbCoFe. In order to better understand these differences, NanoRbCoFe and RbCoFe were studied by XAS at the Fe and Co K-edges at 10 K before and after irradiation. The spectra of NanoRbCoFe and RbCoFe at 10 K before irradiation are very close to those at 300 K described above, and so we consider in the following that the local structure and electronic structure of these compounds at 10 K are the same as those described above at 300 K. Thus, the structure of the particles in NanoRbCoFe and RbCoFe at 10 K can be described as a core mainly composed of CoIII(LS)−NC−FeII linkages surrounded by a shell made of CoII(HS)−NC−FeIII linkages, strongly departing from the usual geometry. Before irradiation, the magnetization value, higher in NanoRbCoFe than in RbCoFe, is in line with the

deduced from the percentages of the spectra of the core and of the shell contained in the spectra of the nanoparticles. Considering an average of 20% core and 80% shell, the particles can be reasonably described by a spherical core with a diameter of 32 Å approximately made of 20−25 unit cells corresponding to 70−80 CoFe pairs and a shell of 11.5 Å thickness corresponding to about three metal ions layers, as shown in Figure 8. The error bar for the average contribution of the shell can be overestimated at ±10%. This corresponds to an error bar of ca. ±3 Å for the thickness of the shell, which is within the metallic layer-to-layer distance in PBA (around 5 Å). Structure and Electronic Structure of the Core and the Shell in the Stable and Metastable States Involved in the Photoswitching Properties. Our group already evidenced a photomagnetic effect in NanoRbCoFe, and the 10338


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higher surface-area-to-volume ratio in NanoRbCoFe, which therefore contains a higher amount of magnetic (CoIIFeIII)shell species. NanoRbCoFe and RbCoFe at 10 K after irradiation are called NanoRbCoFe* and RbCoFe* in the following. The XANES spectra at the Fe K-edge of NanoRbCoFe* and RbCoFe* are compared in Figure 6a; additional comparison of the spectrum of NanoRbCoFe* to those of RbCoFe*, CoFe, and NanoCoFe are given in SI7. The spectra of NanoRbCoFe* and RbCoFe* are strikingly similar, which shows that the electronic structure and local structure around the Fe ions are very close in the excited state of both compounds. This is confirmed by the similarity of the FT moduli of the EXAFS spectra (SI7). This means that NanoRbCoFe* cannot be described in terms of a core−shell structure with a peculiar structure of the shell, as was the case before irradiation (Figure 8). The nanoparticles in their photoexcited states are made of one single phase, the spectra of which at the Fe K-edge is characteristic of octahedral FeIII(CN)6 entities with linear Fe− CN linkages. The Fe(CN)6 sublattice spreading over the whole volume of the nanoparticle is formed of Fe(CN)6 species corresponding to a “core-like” structure. One can notice here that the XANES spectra at the Fe K-edge of the nanoparticle and of the powder are very close. This shows that all the Fe ions of the nanoparticle, including surface species, are surrounded by six linear C−N ligands in Fe(CN)6 octahedral entities, which suggests that these Fe ions were also 6-fold coordinated in the ground state. This also allows us to conclude on the minor role played by a pure surface truncation effect on the XANES spectrum at the Fe K-edge of the nanoparticles. An EXAFS study and full multiple scattering simulations of PBA XANES spectra indeed demonstrated that the dominating influence on XANES is that of the first and second nearest neighbors,52 which are the same (C−N) for all Fe ions in the photoexcited state. This means (i) that in CoFe PBA nanoparticles Fe K-edge XANES is mainly determined by the local geometry of the Fe(CN)6 entity and (ii) that the unusual shape of the Fe K-edge XANES spectra of the ground state of the nanoparticles exclusively arises from a departure from the octahedral geometry of the Fe(CN)6 entity. The XANES spectra at the Co K-edge of NanoRbCoFe* and RbCoFe* are compared in Figure 7a; an additional comparison of the spectrum of NanoRbCoFe* to those of RbCoFe*, CoFe, and NanoCoFe is given in SI7. The spectrum of NanoRbCoFe* is similar to none of the three other spectra, and any attempt to extract contributions of a core and of a shell from the spectrum of NanoRbCoFe* failed. This confirms that NanoRbCoFe* cannot be described in terms of a core−shell structure with a peculiar structure of the shell, as was the case before irradiation. The nanoparticles in their photoexcited states are made of one single phase. The energy of the absorption maximum on the spectrum of NanoRbCoFe* is characteristic of CoII(HS) ions, but the spectrum slightly differs from that of RbCoFe*. The modulus of the FT of the EXAFS signal at the Co K-edge of NanoRbCoFe* is compared to that of RbCoFe* and of ShellNanoRbCoFe in SI7. It is very close to that of RbCoFe*, indicative that the local structure around the CoII(HS) ions is very close in the photoexcited state of the two compounds and corresponds to a “core-like” structure. One can notice that the maximum of the first peak of NanoRbCoFe* is slightly shifted toward longer distance and that of the third peak toward shorter distance. This could reflect a slightly narrower angle between the Co−N bond and the linear Fe−CN linkage in

Figure 7. (a) Co K-edge normalized XANES spectrum of NanoRbCoFe* (orange, filled circles) compared to that of RbCoFe* (red, filled circles). (b) Comparison of the Co K-edge normalized XANES spectrum of RbCoFe* (red, filled circles, bottom) with RbCoFe (red, bottom), NanoRbCoFe* (orange, filled circles, middle) with CoreNanoRbCoFe (gray, middle), and NanoRbCoFe* (orange, filled circles, top) with ShellNanoRbCoFe (pale green, top).

Figure 8. Structure of NanoRbCoFe before and after irradiation. Typical irradiation conditions are as follows: 10 K, λ = 635 nm, PW = 50 mW·cm−2, 30 min.

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Journal of the American Chemical Society NanoRbCoFe* than in RbCoFe*. This slight structural difference between the structure of the nanoparticles and of the powder certainly plays a role in the different XANES spectra of NanoRbCoFe* and RbCoFe*. In contrast to the Fe K-edge, a pure surface truncation effect on the XANES spectra at the Co K-edge cannot be totally excluded due to the variation of the number of second nearest neighbors at the coordination polymer surface (replacement of isocyanide ions by water molecules) even if it can be anticipated that this will certainly not be a dominant effect. The XANES spectra of the core and of the shell of NanoRbCoFe before irradiation (CoreNanoRbCoFe and ShellNanoRbCoFe) and the spectrum of NanoRbCoFe* (for which the distinction between core and shell do not stand anymore) are compared at the Fe K-edge in Figure 6b and at the Co K-edge in Figure 7b; they are also compared to the spectra of RbCoFe (before irradiation and corresponding to the core) and of RbCoFe*. The spectra of CoreNanoRbCoFe and NanoRbCoFe* correspond to the spectra of the core of the nanoparticles before and after irradiation. They are very close to those of respectively RbCoFe and RbCoFe*, also corresponding to core species (due to the size of the particles). They correspond to Co and Fe ions having an octahedral symmetry and linked through linear or almost linear Co−NC−Fe linkages. Irradiation induces a comparable change of the spectra, which has already been assigned to the CoIII(LS)FeII → CoII(HS)FeIII electron transfer accompanied by the spin change of the CoII ion. Therefore, one can conclude that the core of the particles is made of linear or almost linear Co− NC−Fe linkages in which irradiation induces the same CoIII(LS)FeII → CoII(HS)FeIII electron transfer accompanied by the spin change of the CoII ion, whatever the size of the core of the particle. Given the core−shell structure of the particles (Figure 8), it can be foreseen that such a core and therefore such a photomagnetic effect do not exist anymore in nanoparticles with a diameter smaller than 2.5 nm. The spectra of ShellNanoRbCoFe and NanoRbCoFe* correspond to the spectra of the shell of the nanoparticles before and after irradiation. Surprisingly, they are very different before and after irradiation. Whereas the Fe(CN)6 entities and FeIII−CN−CoII linkages in ShellNanoRbCoFe (before irradiation) strongly depart from the usual geometry (see below), in NanoRbCoFe* (after irradiation) they have recovered the geometry usually encountered in PBAs. This new structure for the nanoparticles is schematized in Figure 8. Therefore, irradiation induces, in addition to the well-known photoinduced electron transfer, a structural change that transforms the distorted Fe(CN)6 and/or FeIII−CN−CoII surface species into octahedral Fe(CN)6 entities and linear FeIII−CN−CoII linkages with a “core-like” structure. Such a photoinduced structural change of surface species has never been described before. It can be either the consequence of the photoinduced transformation of the core or another photoinduced phenomenon. Work is in progress to understand this new behavior. The magnetic properties of NanoRbCoFe* (photoexcited state) were further investigated by SQUID magnetometry. Field cooled and zero field cooled magnetization curves of NanoRbCoFe* and RbCoFe* (after irradiation at 10 K) are shown in Figure 9. The maximum of the ZFC curve, corresponding to the defreezing of the magnetic moments, is situated at lower temperature in NanoRbCoFe* (9 K) than in

Figure 9. FC and ZFC magnetization curves measured with a magnetic field of 50 Oe for NanoRbCoFe* (orange line) and RbCoFe* (red line) after irradiation at 10 K.

RbCoFe* (20 K), suggesting a superparamagnetic behavior of the nanoparticles with a blocking temperature of 9 K. In order to confirm this magnetic behavior, ac susceptibility measurements (zero dc and 3 Oe oscillating fields) of NanoRbCoFe* were performed (SI8). The maximum of the out-of-phase component, frequency independent for RbCoFe*, is frequency dependent with a Mydosh parameter ((ΔTmax/Tmax)/Δ(log f))53 of 0.055 for NanoRbCoFe*. The temperature dependence of the relaxation time of the magnetic moment in the photoexcited state, extracted from the maximum of the out-ofphase component, can be fitted by a Vogel−Fulcher law (τ = τ0 exp(Ea/kB(T − T0)); τ0 = 10−11 s, Ea/kB = 105 K, and T0 = 3.2 K), commonly used to describe the slow relaxation of magnetization for magnetically interacting nanoparticles.54 A fit with an Arrhenius law (τ = τ0 exp(Ea/kBT); τ0 = 5 × 1017 s, Ea/kB = 282 K) indeed leads to a value of τ0 out of range for isolated superparamagnetic particles. NanoRbCoFe* thus exhibits a superparamagnetic behavior modified by weak interparticle interactions, as already observed for the analogous nanocomposite containing alkali cation free CoFe PBA nanoparticles made of CoII(HS)FeIII pairs in their ground state.34

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CONCLUSION This comparative study by different techniques and especially by Co and Fe K-edges XAS of magnetic and photomagnetic CoFe PBA particles having different sizes sheds new light on the structure of the particles, which is of prime importance to understand the effect of size reduction on the properties and to control the properties of nanoparticles. The main findings can be summarized as follows: (1) In their ground states, the Co4[Fe(CN)6]2.7·17H2O and Rb2Co4[Fe(CN)6]3.3·11H2O PBAs have a core−shell structure. The core and its properties are well-known because, except in very small size particles (diameter smaller than a few tens of nanometers), the impact of the shell on the measurements performed to characterize them or to study their properties is generally negligible. The shell becomes non-negligible in 5.5 nm particles, which has allowed its evidence and first characterizations. The 11.5 Å thick shell is made of Fe(CN)6 entities and CoII−NC−FeIII linkages departing from the 10340


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present results suggest that surface truncation effects in coordination polymer nanoparticles differ from that observed in the cases of oxides and metals and that PBA could be a suitable model to study them. To conclude, these results bring new fundamental knowledge on the structural and (photo)magnetic properties of PBA nanoparticles and raise new questions also, which are of prime importance toward their integration into real application.

geometry usually encountered in PBAs, whatever the oxidation state of the CoFe pairs in the core. This shell exhibits peculiar magnetic and photomagnetic behaviors, still under study, and plays a major role in the properties of nanoparticles. As these two compounds are representative of the variety of chemical compositions (with and without alkali cations) and of electronic structures (oxidation and spin states of the TM ions), a comparable core−shell structure can be anticipated for the major part of the CoFe Prussian blue analogues family and probably also for other Prussian blue analogues and derivatives. It can be noticed that surface effects in coordination polymers and metal organic frameworks have rarely been reported in the literature so far. And yet, such effects should play an important role for instance in nanoparticles19−29,34,55−57 or heterometallic core−shell structures.58−62 (2) For a particle size of less than a few tens of nanometers and of more than 2−3 nm, it can be anticipated that the photomagnetic effect should be impacted by the core−shell structure of the CoFe PBA particles. The photomagnetic effect in the core of the particles is due to the same photoinduced CoIII(LS)FeII → CoII(HS)FeIII electron transfer whatever the size of the particles, although the lifetime of the photoexcited state seems to depend to a certain extent on the size of the particles. The surface species of the photomagnetic particles show a peculiar behavior before irradiation and under irradiation. Before irradiation, the distorted Fe(CN)6 and/or FeIII−CN−CoII surface species exhibit collective magnetic behavior different from the nearly diamagnetic core. Under irradiation, these distorted surface species are transformed into Fe(CN)6 entities and FeIII−CN−CoII linkages with a “corelike” structure. Work is in progress to explore this peculiar behavior, which can be either the consequence of the photoinduced change of the core or another photomagnetic effect that has not been reported before. (3) The nanoparticles in their photoexcited state exhibit a superparamagnetic behavior with the highest blocking temperature encountered so far in a photoinduced state of photoswitchable nanoparticles. Once the core−shell structure of the particles has been evidenced, some important questions emerge regarding the influence of alkali cations on the core and on the shell of the nanoparticles, the influence of the matrix on the nanoparticles, or the effect of the nanoparticles’ size on the properties. The answer to each of these questions is a challenging task, which necessitates varying each parameter independently. The synthesis strategy based on the use of ordered mesoporous silica monoliths that we have developed constitutes an appropriate platform to tackle these issues. Indeed, while keeping all the other parameters exactly the same, the nature and amount of alkali cations present during the PBA synthesis are directly fixed by their concentration in the impregnation solution, the chemical nature of the matrix can be varied by partially replacing the Si(OMe)4 silica precursors by MeSi(OMe)3 ones to get hybrid organic−inorganic (SiO2)x(MeSiO1.5)(1−x) polymers, and addition of swelling agents to the amphiphilic block copolymers is likely to adjust the diameter of the cylindrical pores and therefore the nanoparticles’ size. Work is in progress to address these new challenges. Lastly, this study also newly questions the effect of surface truncation in coordination polymer nanoparticles on the X-ray absorption spectra, since studies investigating coordination polymer nanoparticles by XAS are scarce in the literature. Our

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Sample Preparation. The synthesis of the samples was already described elsewhere, in ref 34 in the case of NanoRbCoFe and NanoCoFe and in ref 31 in the case of RbCoFe and CoFe. Their exact chemical formulas were determined by elemental analyses, and the size and shape of the particles were studied by transmission electron microscopy (SI1).33 The nanocomposites were ground for all measurements. FTIR Spectroscopy. FTIR spectra were collected in the transmission mode with a PerkinElmer Spectrum 100 spectrometer. X-ray Diffraction (XRD). Powder XRD data (Cu Kα radiation) were collected with a Philips X’Pert diffractometer at room temperature. All diagrams were recorded over the 2θ angle range of 10−50° in steps of 0.03. Co and Fe K-Edge X-ray Absorption Spectroscopy. X-ray absorption spectroscopy (XAS) spectra were recorded on the SAMBA beamline63 at SOLEIL (Gif sur Yvette, France). We used a Si(220) monochromator in transmission mode, and samples were available as pellets. Spectra were recorded in a step-by-step mode from 7000 up to 8700 eV, with a variable step size depending on the region; both edges (Co and Fe) were measured in a row to minimize experimental artifacts. Measurements were performed at room temperature, at 10 K, and at 10 K after irradiation; low temperature was achieved using a He-cooled cryostat, and irradiation was performed with a fully focused X-ray beam and no Al foil (at room temperature and 10 K, the X-ray beam was defocused and an Al foil was placed before the sample to avoid radiation damage). The charge transfer was monitored by measuring Co K-edge quick XANES spectra (2 min/scan), and irradiation was stopped when no more change was observed on the spectrum (which corresponds to ∼30 min of irradiation). No radiation damage was observed (neither at room temperature nor at 10 K), and the samples recovered the initial electronic structure when heated back to room temperature. Following the measurements, the spectra were energy-calibrated and conventionally normalized using the ATHENA software.64 The Fourier transform of the EXAFS signal was obtained at the Co K-edge with a Hanning window and a 3−9 krange and at the Fe K-edge with a Hanning window and a 2−8 krange; these parameters were arbitrarily chosen but reflect the general trend observed between the four compounds of interest, a trend that was tested for different windows and k-ranges. At the Fe K-edge, our experimental EXAFS range is limited to 9 k. We thus choose to apply a similar k-range for both edges. For both the XANES and EXAFS parts, the linear combination to extract the shell signal and test the cross-reconstruction was performed using the Kaleidagraph software. In the case of EXAFS, it has to be noted that the linear combinations were performed for the data in k-space, and a Fourier transform was then eventually applied.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b06147.

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Anne Bleuzen: 0000-0001-9885-4378 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank R. Saint-Martin (ICMMO) for help in XRD data collection and G. Alizon (SOLEIL) for technical support during the experiments. The authors also acknowledge SOLEIL for the provision of the synchrotron radiation facility on the SAMBA beamline through proposal 20140962.

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