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

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][5][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 containingferromagnetic 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 Co II /Fe III 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 Co II and ferricyanide by following previously developed procedures. [8,9] In the optimized preparation, Co II 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 À1 ) which is blue-shifted by approximately 2900 cm À1 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 M

Magnetism and Photomagnetism of Prussian Blue Analogue Nanoparticles Embedded in Porous Metal Oxide Ordered Nanostructures

“…[59,70] A comparative study of the thermal relaxation in nanocomposites with various organizations of the porosity and chemical nature of the walls also suggests that the stability of the photoinduced state is sensitive to the chemical environment of the nanoparticles. [59] A photomagnetic effect was also evidenced in nanocomposites made of CoFe PBA particles (8-10 nm) embedded in silica xerogels [72,77] and of CoFe PBA particles with various sizes ranging from 3 to 13 nm wrapped in polyvinylpyrrolidone (PVP). [35] In one of the SiO 2 xerogel/CoFe PBA nanocomposites [72] the thermal relaxation of 8-10 nm RbCoFe nanoparticle aggregates in the photoexcited state is comparable with that of the nanoparticles embedded in the ordered mesoporous silica matrices.…”

“…This scarcity may be due to (i) the difficulty in finding preparation conditions allowing size reduction without killing the photoswitching properties, and (ii) a possible effect of size on the photoswitch- [59,70] 2D hexagonal mesoporous SiO 2 /CH 3 SiO 1.5 RbCoFe 5 nm [59] Cubic mesoporous SiO 2 RbCoFe 5 nm [59] Lamellar mesoporous SiO 2 RbCoFe <2 nm [59] SiO 2 xerogel K x Co y Fe z 8-10 nm [77] SiO 2 xerogel RbCoFe 8-10 nm [72] PVP K x Co y Fe z 3, 7, 10, 13 nm [35] PVP CuNiMo 3 nm [83] Direct synthesis CsCuMo platelet 17 × 9 nm [89,84] ing properties. All of the works on the photomagnetic effect in PBA nanoparticles embedded in porous metal oxide ordered nanostructures have been carried out by our team.…”

“…Whereas magnetic particles are known to be very sensitive to size reduction and environment, little is known about photomagnetic properties at the nanoscale. [35,39,59,62,72,[77][78][79][80][81][82][83][84] As a first step, in order to assess our approach, we also studied the effect of processing on the magnetic properties of PBAs. Indeed, the growth of magnetic PBAs in ordered nanostructured oxide matrices has already been achieved by several groups, as described above, and some magnetic studies of these compounds are available in the literature.…”

Synthesis of Fe4[Fe(CN)6]3·14H2O Nanopowder by Co-Precipitation Technique and Effect of Heat Treatment

“…Control of these factors allows one to observe unique properties of the materials. Amongst various inorganic compounds, Prussian blue (PB) [3,4] is emerged as one of the most useful and promising compounds in the area of nanomagnetic devices [5], electrochemistry [6] and optics [7]. General formula of PB family is A n [B(CN) 6 ] m ·xH 2 O, where A and B are transition metals.…”

Synthesis of Cobalt-Iron Prussian Blue Analogues Nanotubes by CTAB Soft-Template Method

“…In this case, the inherent structures become loose and the crystallization will be difficult. 3 In recent years, there have been quite a few techniques developed for preparing such materials, for example, porous alumina, 4 stearylamine, 5 ionic liquids, 6 mesostructured silica, 7 sodium hexametaphosphate, 8 apoferritin, 9 polyvinylpyrrolidone, 10 sol-gel, 11,12,13 anodic aluminum oxide 14 and microemulsion. 15,16 Although considerable efforts have been put on the syntheses of PBAs materials, relatively few attempts 17 have been on the produce of the tubular Prussian blue analogues, let alone the cobalt-iron PBAs nanotubes.…”