Transparent, Superparamagnetic K${{{\rm I}\hfill \atop x\hfill}}$Co${{{\rm II}\hfill \atop y\hfill}}$[Fe^III(CN)₆]–Silica Nanocomposites with Tunable Photomagnetism

Abstract

Moldable magnetism: Tunable photomagnetism is exhibited by homogeneous, optically transparent, moldable silica xerogel nanocomposites containing the ferromagnetic Prussian blue analogue KxCo[FeIII(CN)6]z. These materials also show a blue-shifted intervalence charge-transfer band and superparamagnetic behavior. The picture shows the temperature dependence of the field-cooled (FC) and zero-field-cooled (ZFC) dc magnetization μ of a xerogel sample (inset). There has been considerable interest in molecular magnets based on the Prussian blue class of transition-metal cyanide complexes.1 Pioneering work on these materials has realized a broad range of ferro- and ferrimagnetic solids with Curie points ranging from cryogenic to above room temperature.2, 3 A parallel interest also exists in the development of nanocomposite structures containing nanoscale magnetic particles exhibiting single-domain magnetic behavior.4–6 Investigations of magnetic nanocomposites is driven by their novel properties and their potential as new magnetic, optical, and electronic materials. With one exception,7 however, the Prussian blue class of magnetic materials has not been produced as nanoparticles, and, to the best of our knowledge, there have been no reports of their incorporation into composite structures. Herein we report the fabrication of a new nanocomposite material containing KxCo[FeIII(CN)6]z, a ferromagnetic analogue of Prussian blue, in a porous silica matrix. This material is made by a controlled multicomponent sol–gel synthesis in which the precipitation of the Prussian blue analogue is arrested at nanoscale dimensions by gelation of the silica network. The resulting materials are homogeneous, optically transparent, and exhibit superparamagnetic and tunable photomagnetic behavior. We believe that this study suggests a new approach for utilizing the Prussian blue class of magnetic materials in advanced optical and magnetic applications. Homogeneous silica xerogels containing cyanide-bridged CoII/FeIII centers were made by incorporation of both transition-metal components during the solution phase of the synthesis. To obtain homogeneous materials reproducibly, conditions were optimized for the amount of water and the concentration and molar ratio of CoII and ferricyanide by following previously developed procedures.8, 9 In the optimized preparation, CoII nitrate was dissolved in methanol and added to tetramethylorthosilicate. Aqueous potassium ferricyanide was added to this solution to give a 1:1 Co:Fe molar ratio. Upon mixing, the solution turns dark purple, which suggests the formation of the mixed-valence complex. At concentrations up to 0.03 mol % total-metal to silicon (Fe+Co/Si), the solution remained transparent through gelation, aging, and drying, and ultimately yielded a homogeneous, optically transparent xerogel (Figure 1, inset). The spectrum of this glass (Figure 1) is qualitatively similar to that of the bulk materials, with a broad intervalence charge-transfer band in the visible region between 450 and 650 nm and a sharp, higher energy peak around 400 nm. However, the maximum of the intervalence band lies at 452 nm (22 124 cm−1) which is blue-shifted by approximately 2900 cm−1 from that of the bulk materials. To determine whether the magnetic behavior was singular, the magnetic susceptibility of these materials was measured as a function of temperature and field strength.10 Bulk MCo[FeIII(CN)6]z (MI=alkali-metal cation) shows ferromagnetic behavior in its field-cooled (FC) magnetization, with a Curie temperature of 14.0 K.11, 12 However, this complex, along with other first-row Prussian blue analogues, also shows blocking behavior in the zero-field-cooled (ZFC) magnetization, which has been attributed to spin-glass properties arising from defects in the lattice.13, 14 While it might be expected that a dilute dispersion of KxCo[FeIII(CN)6]z in a xerogel would exhibit only simple paramagnetic behavior, the FC and ZFC magnetization as a function of temperature for a 0.03 mol % CoII/FeIII xerogel (Figure 2 a) indicates more complex behavior. The FC magnetization parallels that of the bulk complex above 5 K but has a lower Curie temperature (Tc≈13 K). The ZFC magnetization curve shows a well-defined peak at 10 K. Above this temperature, the FC and ZFC magnetization curves converge and become coincident. This magnetization behavior is characteristic of blocking in a superparamagnetic system of small, single-domain particles or, as it is in the bulk, the glass transition in a spin glass.15 Measurement of the magnetization as a function of field strength at 5 K, below the blocking temperature (Figure 2 b), reveals hysteretic behavior that reaches a magnitude of 7.08×103 emu mol−1 at 5 kOe with a coercive field of approximately 0.41 kOe. From room temperature down to the blocking temperature (10 K) no hysteresis is observed (Figure 2 c). These observations are also consistent with both spin-glass and superparamagnetism. However, the hysteresis loops measured under FC and ZFC conditions at 5 K are identical and symmetric about the center; this is one of the defining characteristics of superparamagnetism and suggests that this property, possibly superimposed by the inherent spin-glass properties of the bulk, is present in these materials.15, 16 This conclusion is also consistent with ac susceptibility data (Figure 2, inset), which show a significant frequency-dependent, thermally activated magnetization reversal dynamics with an activation energy of 29±1 K (160 J mol−1) and τ0=3×10−6 s. This frequency dependence is a unique property of these materials and is significantly different from that of the bulk, as frequency dependence has not been observed in this composition. The optical absorption spectrum of a 0.03 mol % CoII/FeIII xerogel (inset) shows an intervalence charge-transfer band in the range 450–650 nm with a maximum at 452 nm. a) Field-cooled (FC) and zero-field-cooled (ZFC) dc magnetization versus temperature curves at H=100 Oe [inset: frequency dependence of ac susceptibility: 100 (•), 1000 (○), 5000 (▪), and 10 000 Hz (□)]. b) FC (•) and ZFC (○) hysteresis loops at T=5 K. c) Hysteresis loop at T=13 K. Support for a superparamagnetic origin of the magnetic properties and an explanation of the blue shift in the absorption spectrum is provided by the observation of KCo[FeIII(CN)6]z nanoparticles in the silica matrix. These particles can be observed directly by transmission electron microscopy, which reveals small domains, approximately 8–10 nm in size, distributed in the amorphous matrix (Figure 3). The strong diffraction of these domains verifies that they are crystalline, and quantitative analysis of the electron diffraction pattern (Figure 3, inset) yields d spacings identical to those of bulk Co[FeIII(CN)6]2, and hence they can be unambiguously assigned to the mixed-valence compound. Since the synthesis of these materials is carried out by mixing solutions of the various components to form a homogeneous sol–gel solution, the formation of nanoparticles must occur spontaneously during some phase of the process. Given the large formation constants for Prussian blue analogues, the existence of nanoparticles in the matrix (as opposed to bulk phase-separated regions) strongly suggest that gelation of the silica effectively arrests the precipitation of KCo[FeIII(CN)6]z and sequesters the particles. We believe this to be the first time that nanoparticles of this type of materials have been generated in this way. TEM image of a powdered CoII/FeIII xerogel showing nanoparticle crystalline structures with a size of 8–10 nm (inset: indexed diffraction pattern of the nanoparticles shown). Among the most interesting properties of bulk CCo[FeIII(CN)6]z is its photomagnetism, which was first reported by Hashimoto.12 Specifically, incidence of red light (λ≈600 nm) at temperatures below 150 K results in a net increase in the magnetization of the bulk material. This increase in magnetization can be erased by heating or irradiation with higher energy light (λ≈450 nm). As shown in Figure 4 a, an analogous photomagnetic effect is observed in the nanocomposite system: irradiation of the glass (λ≥550 nm) results in an increase in magnetization, which, on returning to thermal equilibrium after the radiation source is extinguished, amounts to 7.6 %. As with the bulk material, this magnetization could be decreased by irradiation with blue light (λ=448 nm). Photomagnetic behavior in these materials arises from the presence of diamagnetic CoIII/FeII impurity sites in the CCo[FeIII(CN)6]z lattice. Absorption of light by these impurities at low temperature results in electron transfer to create a paramagnetic CoII/FeIII metastable state that increases the magnetization of the bulk.12 Detailed studies of the system indicated that, on increasing the ligand-field strength around the CoII centers, electron transfer from cobalt to iron occurs and generates more CoIII/FeII defect sites. These additional defect sites increase the magnitude of the photomagnetic effect.17–19 In the bulk materials modification of the ligand field around cobalt is accomplished by the addition of alkali-metal cations, which change the stoichiometry of the complex and increase the degree of nitrogen coordination around the metal centers.17, 20 We found that, because of the porous nature of the silica matrix, direct substitution of the cobalt centers by ammine and other higher field ligands can be accomplished cleanly and in a highly controlled fashion. As shown in Figure 4 b, passing ammonia over the composite results in continuous spectral changes that are highly isosbestic. The broad intervalence charge-transfer band is replaced by a single peak (470 nm) characteristic of hexammine CoIII centers, which suggests that diamagnetic defect sites have been created. Measurement of the magnetization of the composite as a function of ammine doping shows a steady decrease in magnetization, and the materials become diamagnetic at the highest levels of doping. Changes in the magnitude of the photomagnetic effect as a function of exposure to ammonia are shown in Figure 4 c. With small exposure increments the magnetization increases until a maximum is reached at about 9.4 % above that of the doped but nonirradiated composite. Above this threshold the photomagnetic effect decreases, and at high degrees of substitution the effect vanishes. This change is consistent with observations on the bulk material, which show that large numbers of the diamagnetic CoIII/FeII defect sites result in significantly diminished photomagnetism. In the bulk material this has been explained as arising from an increased rigidity in the lattice as the number of CoIII/FeII sites increases. This rigidity can no longer accommodate the bond-length increases that accompany the electron transfer of the CoII/FeIII sites to the metastable state.17, Although such a rigidity effect may also be acting in the nanocrystals, high degrees of ammine substitution may ultimately result in disruption of the CN bridge with concomitant deactivation of the electron-transfer pathway, which would also degrade the effect. a) Photomagnetic effect at λ≥550 nm, H=500 Oe, and T=5 K. b) A series of optical absorption spectra taken every 30 min while anhydrous ammonia flowed over a 0.03 mol % CoII/FeIII xerogel. c) Percentage increase in magnetization of NH3-substituted xerogels as a function of NH3 exposure time (10 μL dose). The 0.03 % CoII/FeIII xerogel was made in the following manner: a solution of H2O (4.6 mL), methanol (20.8 mL), and an aqueous solution of K3[Fe(CN)6] (2.2 mm; 13.9 mL) was added dropwise to a solution containing tetramethylorthosilicate (30.3 mL), methanol (16.4 mL), and Co(NO3)2 (2.2 mm; 13.9 mL) in methanol, which gives an overall concentration of 0.31 mM for each metal. Then 4 mL of solution was placed into polystyrene cuvettes for gelation, aging, and drying phases. After gelation the xerogel has 12 % of the original volume with a final concentration of each metal of approximately 3 mM.