Journal of Materials Chemistry Cstatic.tongtianta.site/paper_pdf/43ec7f64-5f83-11e9-bffa-00163e08b… · either an MPMS-5S or an MPMS-XL Superconducting Quantum Interference Device

This journal is©The Royal Society of Chemistry 2015 J. Mater. Chem. C

Cite this:DOI: 10.1039/c5tc01174d

Critical assessment of the nature and properties ofFe(II) triazole-based spin-crossover nanoparticles†

C. Bartual-Murgui, E. Natividad and O. Roubeau*

The shape and size of nanoparticles of the spin crossover compound [Fe(Htrz)2trz]BF4 obtained by the

reverse-micelle method and using as co-surfactants dioctylsulfosuccinate and behenic acid are described

through systematic transmission electron microscopy observations. A rod shape is systematically derived,

and the rod sizes, in particular, their width, are controllable through the surfactant concentration, although

a poor reproducibility is observed and ascribed to uncontrolled parameters in the micelle elaboration and

microemulsion formation and ageing. The influence of synthetic parameters and nanoparticle processing

on the spin crossover properties of nanoparticles is also reported, as characterized by both calorimetry

and magnetic measurements. These unravel original size and environment effects. On the one hand, the

hysteresis width of the thermal spin crossover exhibited by raw nanoparticles increases linearly with the

rod width, until it reaches a value of 40 K, close to that of the bulk material. A similarly good correlation is

found with the nanoparticle volume. On the other hand, the removal of the surfactant from the raw

nanoparticles is found to systematically reduce the hysteresis width in a drastic manner, by up to 16 K.

Introduction

The spin-crossover (SCO) phenomenon is a reversible entropy-driven transition between the low-spin (LS) ground state anda high-spin (HS) state of 3d4 to 3d7 metal ions in certaincoordination environments that can be triggered through eithertemperature, pressure or light.1 It may occur in a cooperativemanner in solid materials, giving rise to bistable materialswith interesting magneto-optical properties, especially in Fe(II)complexes.1,2 As such it provides one of the most appealingswitchable types of molecular materials. The subject has gainedrenewed interest in the last decade with the demonstration thatSCO and, in certain cases, magneto-optical bistability can bemaintained in nanoscale objects3 as well as in liquid crystallinephases,4 physical gels5 or thin films.6 Nanoparticles (NPs) are ofparticular interest, since their dispersibility can be very usefulto manipulate SCO materials, especially to make thin films,ideally without losing or modifying the bulk material properties.This aspect is the origin of our interest in SCO NPs, given that wefound that the sample thickness is critical to our calorimetricstudies7 of the reported light-induced switching within thebistability range of cooperative SCO materials.8 NPs of thefamily of one-dimensional triazole-based coordination polymers9

represent an ideal system in this respect, since they possess someof the largest known bistable thermal ranges and NPs havebeen reported for various compounds of this type, withoutsignificant residual HS fractions at low temperature.10 Actuallythe first SCO NPs reported were those of the generic compound[Fe(Htrz)2(trz)](BF4),

11 which exhibits a 40–45 K wide hysteresis inthe bulk.12 Their synthesis involves the use of a microemulsionformed with reverse-micelles, and has also been applied to formNPs of the related compound [Fe(Htrz)3](ClO4)2 as well as dilutionwith Zn(II).13 Different procedures have also yielded NPs of thesame or similar materials,14–17 including their implementationwithin porous silica monoliths.18 We decided to focus on thegeneric compound [Fe(Htrz)2(trz)](BF4) and the original reverse-micelle method for a number of reasons: (i) supposedly, the NPdispersions are stable and NPs maintain the hysteresis widthof the bulk material; (ii) there are studies of continuous andpulsed laser irradiation on the same and similar materials,including NPs;19 and (iii) the properties of the NPs seem to berobust as they have been used for advanced transport studies.20

However, surprisingly, we found that the shape and size ofthe NPs obtained following the published procedure differedfrom those originally reported,11,13 i.e. rods of various sizes areisolated instead of spherical NPs of 6 to 35 nm diameter,suggesting that the NP morphology had not been properlycharacterized.21,22 We therefore opted to study anew this highlyinteresting synthetic system, before going any further with ourplanned light-induced studies.

This work describes the results of this study and criticallyassesses the true shape and sizes of the obtained NPs, as well as

Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de

Zaragoza, Plaza San Francisco s/n, 50009 Zaragoza, Spain.

E-mail: [email protected]

† Electronic supplementary information (ESI) available: Additional TEM images;size distribution; IR spectra; powder diffraction data; and additional magneticand DSC data. See DOI: 10.1039/c5tc01174d

Received 25th April 2015,Accepted 21st May 2015

DOI: 10.1039/c5tc01174d

www.rsc.org/MaterialsC

Journal ofMaterials Chemistry C

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their relationship with synthetic conditions on the basis ofsystematic electron microscopy observations. The influence ofsynthetic parameters and the NP environment on the SCOproperties is also reported as characterized by both calorimetryand magnetic measurements.

Experimental sectionSynthetic procedures

General procedure for the synthesis of nanoparticles of[Fe(Htrz)2(trz)](BF4). Our goal being ascertaining the trueshape, size/form dependence and SCO properties of the pre-viously reported nano-materials, we used the same procedurebased on the reverse-micelle technique developed by Coronado,Galán-Mascarós and co-workers.11,13 First, two separate solutionsare prepared at room temperature: (i) 0.209 g of Fe(BF4)2�6H2Oand 1 mg of ascorbic acid are dissolved by stirring in 1.2 mLof deionized H2O and the resulting aqueous solution is addedto a previously prepared 20 mL n-octane solution of sodiumdioctylsulfosuccinate (2.04 g) and behenic acid (0.406 g), main-taining stirring fro 30 min to produce a stable microemulsion;(ii) 0.125 g of Htrz is dissolved by stirring in 0.6 mL of absoluteethanol and this solution is added to a previously preparedn-octane solution of sodium dioctylsulfosuccinate (2.04 g in20 mL), maintaining stirring for 30 min to produce a stablemicroemulsion. Then the two microemulsions are mixed,adding the second (ii) to the first (i), and stirred at roomtemperature for 4 hours. The characteristic pink colour of thecompound appears after a few seconds. The exact same pro-cedure was repeated several times to assess reproducibility(samples A0, A1, A2, and A3). A number of variations were doneto test the synthetic parameters: varying the amount of behenicacid to 0.306 g (B1), 0.102 g (B2) and none (B3); increasing theamount of the sodium dioctylsulfosuccinate surfactant to 3.0 gin both micellar solutions, i.e. reducing the water/surfactantratio o0 (C1).

In all cases, the resulting pink suspension is clear and homo-geneous and remains so for at least 2 days and up to a week,depending on the synthetic conditions used. However, afterstanding for long periods, the suspensions gradually settle anda purple fine deposit appears (see Fig. 1). We found no signifi-cant difference in the size/shape or properties of the NPs whenthe mixing is performed under Ar.

Raw and washed NPs were isolated in several manners atdifferent stages of the process described above (Fig. 1). Rawaged NPs were systematically obtained by evaporation anddrying in air of the whole heterogeneous dispersion (after 2 minof bath ultrasonication and manual agitation) once a signi-ficant amount of NPs was deposited. In addition, the rawsupernatant content after NP deposition was also isolated byevaporation and drying in air. The same process was alsoapplied to the freshly prepared homogeneous suspension inoctane, to obtain raw fresh NPs and assess the possible changesoccurring upon ageing. In all three cases, purple/pink filmswere obtained and used without further treatment for IR

spectroscopy and magnetic and DSC measurements. Raw agedNPs were also isolated in the powder form by centrifugation ofthe heterogeneous dispersion once a significant amount of NPswas deposited, at 15 000 RPM and 4 1C and drying in vacuum.Washed aged NPs were obtained by centrifugation at 15 000 RPMat 20 1C of the whole heterogeneous dispersion (after 2 min of bathultrasonication and manual agitation) once NPs were deposited.The supernatant (in general, colourless) was removed, and thesolid residue was washed with ethanol and bath ultrasonication.The resulting heterogeneous dispersion was centrifuged at15 000 RPM for 2 min and the slightly pink supernatant wasremoved. The process was repeated for a total of 3 times andthe final residue was dried in air.

Characterization

Transmission electron microscopy. The size and shape of theNPs were studied by transmission electron microscopy (TEM)using a JEOL 2000 FXII instrument working at an accelerationvoltage of 200 kV. Drops of either the adequately diluted clearand homogeneous octane dispersion of raw fresh NPs, theremaining supernatant of the aged raw dispersion once NPswere deposited, an octane re-suspension of these deposited NPsor an octane suspension of the washed NPs were put onto coppergrids with a carbon membrane film. Size statistics were obtainedusing DigitalMicrograph,23 and analysed through fits to a Gaussiandistribution (ESI†).

Calorimetry. Differential Scanning Calorimetry (DSC) measure-ments were performed using a Q1000 calorimeter from TAInstruments equipped with the LNCS accessory. The tempera-ture and enthalpy scales were calibrated with a standard sampleof indium, using its melting transition (156.6 1C, 3296 J mol�1).The measurements were carried out using aluminium panswith an empty pan as a reference at a scanning rate of 10 K min�1.The pans were either mechanically crimped for dry powdersamples or the lid was simply pressed without crimping for thefilm samples. Various temperature cycles in the range 293–413 Kwere systematically performed. The SCO temperatures, definedby the onset of the associated anomalies, as well as the transitionenthalpies, were determined using Universal Analysis 2000 fromTA Instruments.

Fig. 1 Schematic representation of the isolation of different types of NPsstudied in this work (right) with representative pictures of the original homo-geneous suspension obtained after the synthesis and stable for several days(left) and the heterogeneous system after ageing at RT (middle).

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Magnetometry. Magnetic measurements were done usingeither an MPMS-5S or an MPMS-XL Superconducting QuantumInterference Device magnetometer from Quantum Design. Thesamples were fitted within gelatin capsules, the top section ofthe capsule being reversed to press the powder or film sample.Variable-temperature measurements were performed under a0.5 T applied dc field in the settle mode. The overall averagescan rate was ca. 0.1 K min�1. Two cycles were systematicallyrecorded.

Infra-red spectroscopy. The spectra were acquired on neatsamples (both powder for washed NPs and films for raw NPs)using a Perkin Elmer Spectrum 100 apparatus equipped with anATR device. Data for sample A3 are shown in Fig. S12 (ESI†).

Powder X-ray diffraction. The crystalline phase of the nano-particles was identified by powder X-ray diffraction (XRD) onseveral batches of the washed aged NPs. Patterns were recordedusing a D-Max Rigaku diffractometer equipped with a Cu rotatinganode and a graphite monochromator to select the Cu Ka1,2wavelength, with 2y ranging from 3 to 601 at a step size of 0.03.Data for sample A3 are shown in Fig. S13 (ESI†).

Results and discussionNanoparticle shape and size and influence of syntheticconditions

Coronado, Galán-Mascarós and co-workers reported sphericalNPs of [Fe(Htrz)2(trz)](BF4) of diameters in the range 6–35 nm,by varying the amount of one of the co-surfactants, i.e. sodiumdioctylsulfosuccinate.11,13 No true correlation of size vs. syntheticconditions was however derived. The NP sizes were determined bydynamic light scattering of the fresh (1 day-old) homogeneousmicroemulsion suspension obtained upon mixing the two reverse-micelle solutions containing the Fe(BF4)2 and triazole precursors.Only one TEM image was however reported to support the statedspherical shape.21 The same microemulsion approach was laterused by two groups to form [Fe(Htrz)2(trz)](BF4)–silica composites,in which the shape of the [Fe(Htrz)2(trz)](BF4) NPs was clearly thatof rods of varying size and form factor.24,25 Our first attempt toreproduce the original synthetic conditions also showed rod-shaped NPs (sample A1, Table 1 and Fig. 2), more in line withthe shape of larger crystallites observed for the bulk material.26

Actually, an investigation focused on the transport properties ofNPs by the same group that reported the original spherical NPshas appeared very recently during the writing of the presentwork, and this study now shows HRTEM images of NPs with adifferent shape, similar to ours.27 It, however, does not mentionthe shape discrepancy or its possible origin. Moreover, althoughthis more recent study presents two NP sizes, called large andsmall volume NPs, neither the size variations have been corre-lated to systematic synthetic conditions, and more importantly,nor the potential effect of NP size/shape on the SCO propertieshas been studied.

Given the discrepancy within the available literature, wetherefore first sought to ascertain the true shape of the NPsobtained by the microemulsion method used, repeating exactly

the same synthetic conditions (samples A0–A4, see Table 1).The amount of one of the surfactants, behenic acid, was alsovaried (samples B1–B3, see the Experimental section and Table 1).Because the presence (excess) of a surfactant renders the imagingand, therefore, the shape and size assessment difficult, we firstfocus on washed NPs (see Experimental Section). Systematically,TEM observations of the washed NPs show the sole presence ofrod-shaped NPs of various widths/lengths, of a reasonable disper-sion (Table 1, Fig. 2 and ESI†). This rod shape therefore representsthe reproducible and true nature of the NPs obtained by themicroemulsion method for the compound [Fe(Htrz)2(trz)](BF4).Powder diffraction confirms the crystallinity and the phase of the

Table 1 Average length, width and volume of the rod-shaped NPs obtainedin this work, determined from TEM images of the washed aged NPs througha Gaussian distribution. The volume was calculated considering a cylinderusing the rod width as diameter and the rod length as height

Sample[Behenic acid]/[Behenic acid]0

cNPs avglength (nm)

NPs avgwidth (nm)

NPs avgvolume (mm3)

A0a 1 62 18 63.1B1a 0.75 32 15 22.6B2a 0.25 49 10 15.4B3a 0 34 10 10.7C1b 1 70 20 87.9A1 1 105 40 528A2 1 45 15 31.8A3 1 41 8 8.2A4 1 404 84 8955

a Performed on the same day by the same experimenter. b Made withan increased amount of the sodium dioctylsulfosuccinate surfactant(see the Experimental section). c [Behenic acid]0 is the concentrationused in the original synthetic conditions, and here those used for A0(see experimental).

Fig. 2 Representative transmission electron microscope images of washedNPs for samples A0, B1, B2 and A4. Additional images as well as the deriveddistribution in the width and the length are provided in the ESI.† Scale barsare 0.2 mm, except for sample A4 for which it is 1 mm.

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obtained NPs (Fig. S12, ESI†). Although some isolated NPs orgroups of few NPs can be observed, these are found as mostlyforming larger aggregates by stacking along the longer direc-tions of the rods. While this stacking may occur upon the TEMspecimen preparation, aggregates are likely to be present in thewashed NP dispersions, possibly resulting from the remainingtraces of the surfactant. Hints of this can be found in the imagesof sample B2 in Fig. 2.

By controlling the size of micelles through the water/surfactantratio, one can expect to control the size of the NPs formed withinthe microemulsion droplets. Indeed, we have found that dimin-ishing the amount of the co-surfactant behenic acid by 1/4 and3/4 and fully removing it result in a correlated reduction of theNP size, in particular, their width (Table 1 and Fig. 3). This is inagreement with the relatively larger volume of the NPs obtainedwhen the amount of the sodium dioctylsulfosuccinate surfactant isincreased, keeping the same amount of behenic acid (sample C1,Table 1) or no behenic acid is used.27 A true control of the NPvolume and the aspect ratio however would require a far bettercontrol of the synthetic conditions than that achieved here andpreviously, e.g. temperature, humidity, mixing and ageing condi-tions, since these strongly affect the thermodynamic parametersdriving the stability and the droplet size of the microemulsion.28

In particular, the formation of a highly monodispersed reverse-micelle would benefit from controlled and steady mixing, asusually done in advanced physico-chemical studies of such soft-matter phases. Indeed, we have found that repeating the presentsynthetic conditions on different days and by different experi-menters has produced a range of sizes and volumes of the NPs(Table 1). While the rod-shaped NPs are systematically obtained,this clearly indicates the poor reproducibility and thus control ofthe specific width and length of the rods eventually isolated.

Besides the likely role played by uncontrolled physico-chemicalconditions such as ambient temperature or the mixing used to

form the reverse-micelles, the observed variability in size of theformed NPs may also well originate in the TEM observation beingmade on NPs with a different history, for example, a variablestanding time in contact with the original microemulsion. It isalso unclear why the original (very few) TEM observations pub-lished seemed to indicate the presence of, smaller, spherical NPs.While the nucleation of the material’s NPs is necessarily occur-ring fast, as indicated by the almost immediate pink coloration ofthe microemulsion, the mechanism and kinetics of the furtherNP growth is unknown. The fact that the raw NP suspensions endup depositing upon standing may indicate an ageing effect uponmaintaining contact with the reaction microemulsion. Indeed,Ostwald ripening may occur between formed NPs and eitherunreacted micelles or micelles still containing remaining eitherprecursor. To assess these aspects, TEM observations were madefor the stable original suspension, the raw aged NPs depositedupon standing or isolated by centrifugation and the supernatantremaining once NPs were deposited (Fig. 4). Systematically, whenexcess surfactant is present, especially for the supernatant after

Fig. 3 Average rod-shaped NP volume and width as a function of theconcentration of the behenic acid co-surfactant, normalized to the valueoriginally used. The volume has been calculated considering a cylinder usingthe rod width as diameter and the rod length as height. All experiments wereperformed on the same day under the exact same conditions.

Fig. 4 Representative TEM images of: (a) NPs in the diluted fresh originalhomogeneous suspension of sample A3; (b) and (e) NPs in the supernatantof the respective samples A3 and A2, after deposition of NPs; (c) and(d) diluted deposited raw NPs and washed raw NPs of sample A3, respec-tively; (f) NPs of sample A1 after centrifugation and redispersion in octane. Allscale bars are 0.2 mm.


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NP deposition, the imaging of the (seemingly smaller) NPsis rendered difficult by a film formed by the surfactant uponevaporation/drying (images a, b and e in Fig. 4). Such a filmlikely allows the rod-shaped NPs to be vertical, and the imagesobtained for the supernatant indeed exhibit both (horizontal)rods or aggregates of a few packed rods and large aggregateswith seemingly spherical nano-objects, that are in fact most likelyrods packed vertically (see, in particular, image e in Fig. 4). This ispossibly what was originally reported as spherical NPs, giventhe similarity of our observations and the published image (seeESI†).21 Interestingly, the images of the raw suspension beforeany deposition has occurred only show rounded objects, eitherisolated or organized as packed chains within the surfactantfilm (image a in Fig. 4). This would be in agreement with thepresence of rod-shaped NPs, albeit of much smaller length.Deposited or centrifuged raw aged NPs do not suffer from thepresence of the excess surfactant film and are better imaged. Therod-shaped NPs are observed either isolated, in small aggregatesof few NPs stacked along their longer axis or as larger aggregatesof similarly stacked groups packed together (images c and f inFig. 4). In these aggregates, the surfactant covering the NPsis clearly distinguished, as lighter areas separating the NPs,making the estimation of the NP size possible, although lessaccurate than for washed NPs. Within these limitations, nosignificant differences have been observed between raw agedand washed aged NPs. The washing process therefore does notmodify the NPs themselves, while efficiently removing most ofthe surfactant. With all these observations, it appears likely thatthe freshly prepared NPs may still evolve in contact with thehomogeneous suspension. Indeed, a reasonable explanationfor the deposition of NPs upon prolonged standing is the increaseof the NP size, in particular, their length, until the larger contactareas favour the observed stacking along the longer surfaces ofthe NPs and result in the formation of larger agglomerates thatthen deposit.

Spin-crossover properties of NPs: reproducibility,size-dependence and effect of their environment

The signature of the SCO in both raw and washed aged NPs of thecompound [Fe(Htrz)2(trz)](BF4) was determined systematically byDifferential Scanning Calorimetry for all samples. The obtainedinformation was complemented, for most samples, by variable-temperature magnetic measurements. Table 2 gathers the SCOtemperatures and hysteresis widths derived from DSC for all rawand washed aged NP samples. A typical behaviour of raw agedNPs is shown for sample B1 in Fig. 5.

The wT vs. T data for raw aged NPs (films, see Fig. 1) system-atically exhibit abrupt transitions over ca. 10 K range, fromvalues in the range 0.1–0.2 cm3 mol�1 K at lower temperaturesto ca. 3.25 cm3 mol�1 K at higher temperatures, characteristicof a complete SCO of the Fe(II) ions from their LS (S = 0) state totheir HS (S = 2) state. The spin transitions occur in the range375–390 K upon warming and at 355–335 K upon cooling, thusgiving rise to the large hysteresis typically reported for the bulkmaterial.12 This is in agreement with the original report thatNPs obtained by the microemulsion method display magnetic

properties very similar to those of the bulk material.11,13

It should be however highlighted that the hysteresis width, DT,is in fact always smaller than the 40–44 K of the bulk material.For example, DT is 37.8 K for aged NPs of sample B1 as determinedfrom the temperature derivative of wT. In the DSC traces, a rathersharp endo/exothermic anomaly is detected upon warming/cooling in similar temperature ranges as for the magneticproperties. Because DSC data are dynamic in nature, i.e. acquiredat a fast temperature scan rate, the SCO temperature is definedas the onset temperature of these anomalies, in the case of

Table 2 SCO temperatures and related hysteresis widths (in K) of raw(films, see Fig. 1) and washed (powders, see Fig. 1) aged NPs, as derivedfrom DSC data. The reported data correspond to the second or third thermalcycle performed on the samplesa

Sample T upraw Tdownraw T

upwashed T

downwashed DTraw DTwashed

A0 375.9 337.6 375.5 353.6 38.3 21.9B1 372.9 341.3 372.9 349.7 31.6 23.2B2 369.0 344.3 375.6 354.0 24.7 21.6B3 366.9 341.6 364.8 346.8 25.3 18.0A1 374.6 334.8 374.0 352.0 39.8 22.0A2 371.8 339.3 374.7 352.1 32.5 22.6A3 361.7 342.2 363.7 345.5 19.5 18.2A4 378.7 337.7 379.4 346.5 41.0 32.7

a Virtually, no differences were observed between cycles for raw aged NPs,while a slight shift towards lower temperatures was systematically observedfor washed aged NPs, but becoming negligible after the third cycle.

Fig. 5 SCO properties of sample B1 raw aged NPs: wT vs. T plot (top) andDSC traces, with endotherms up (bottom).


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sample B1 at 373 and 341 K, respectively, upon warming andcooling and thus resulting in a hysteresis width DT of 32 K.Given that the agreement between magnetic and DSC data isreasonable to good, we have used extensively DSC, since it allowsperforming multiple temperature cycles in a reasonable time. Inthis respect, it is worth mentioning that no significant variationswere observed upon cycling the raw aged NPs, which togetherwith powder X-ray diffraction (Fig. S13, ESI†) clearly demonstratethat the material is the A phase of [Fe(Htrz)2(trz)](BF4).

12

The properties described so far correspond to raw agedNPs, isolated once the original homogeneous suspension hasdestabilized and large amounts of NPs have deposited (see theExperimental section). Because the NPs present in both the rawaged supernatant (that remains homogeneous and purple) andthe raw fresh suspension, i.e. within a few hours after thesynthesis, may present different properties, we have character-ized the latter, fresh raw NPs, and compared their SCO signa-ture with the raw aged NPs that subsequently deposited fromthe same sample (Fig. 6). Although subtle, several differencesdeserve to be mentioned. First, the massic energies associatedwith the SCO anomalies in the DSC traces are smaller for theNPs from the original homogeneous suspension, indicating asignificantly smaller concentration of [Fe(Htrz)2(trz)](BF4),which could be due to relatively smaller NPs. Then, while thereis no significant difference in the width or temperature of theSCO anomaly upon cooling, the transition that occurs uponwarming covers a broader range of temperatures for the NPsfrom the original homogeneous suspension, and is shiftedtowards lower temperatures with respect to the deposited rawNPs. The observation of this shift is reproducible, although itvaries from sample to sample, from 3 to 12 K. Clearly, thisindicates that the NPs continue to evolve until their suspensionbecomes unstable. Most likely, and in agreement with the TEMimages, these observations are to be related to a further growthof the NPs and the effect of the NP size on the SCO properties(see below).

Theoretical considerations have predicted that the environ-ment and the size of the NPs can have diverse strong effects ontheir SCO behaviour, including shifts of transition temperatureand reduction or suppression of hysteretic behaviour, i.e. lossof cooperative character.29,30 Experimental evidence has beenreported for NPs of the SCO Hoffman-like clathrate [FepzPt(CN)4],for which the modification of the chemical nature of the surround-ing matrix leads to the modulation of the cooperativity of thesystem, as well as to a shift of the SCO temperatures.31 Theeffect of the NP size has also been corroborated for the triazole-based system [Fe(NH2trz)3]Br2.

32 Here, raw NPs remain coatedwith an excess of surfactant, and this allows their redispersionin a number of solvents, as previously reported.11,13,22 Howeverboth the centrifugation typically used to isolate NPs and theredispersion may have an effect on the SCO, by condensingthe NPs for the former, and diluting the excess surfactant for thelatter. To ascertain such possibilities, we have determined theSCO behaviour of raw NPs centrifuged on the one hand (rawpowder in Fig. 1) and centrifuged and redispersed in octane onthe other (Fig. S15, ESI†). In both cases, the anomalies due to theSCO are more energetic with respect to the raw aged NPsobtained as films by evaporation, which can logically be ascribedto an increased concentration of the material, since the relative

Fig. 6 DSC traces of raw fresh NPs of sample A3 as isolated from theoriginal homogeneous suspension within 1 day after the synthesis (blueline) and raw aged NPs isolated once the suspension has become unstableand NPs have deposited (green line). Endotherms are up.

Fig. 7 SCO properties of raw (green circles and line) and washed (darkgrey squares and line) aged NPs of sample A0: wT vs. T plot (top) and DSCtraces, with endotherms up (bottom). The DSC trace of partially washedaged NPs (only one washing with ethanol and centrifugation, instead of 3)is also shown as an orange line.


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amount of the excess surfactant has been reduced in both cases.In addition, the anomaly upon cooling is shifted by ca. 15 and4 K, respectively, for centrifuged and centrifuged and redispersedraw NPs. With such effects for standard manipulation of NPs,it could then be expected that washed raw NPs would have theirSCO strongly modified. Indeed, the washing involves successivecentrifugation and dispersion in ethanol, allowing eliminationof the excess surfactant and obtaining NPs with a minimalcoating if any, as confirmed by IR spectroscopy (Fig. S12, ESI†).DSC and magnetic measurements show that the hysteresiswidth exhibited by aged NPs is largely reduced by this washingprocess, for example, by more than 16 K for sample A0, reachingonly 21 K (see Fig. 7). Such a striking reduction of the hysteresiswidth is observed in a reproducible manner on all samples andthus on NPs with varying sizes/volumes (Table 2). The largestreductions are observed for the samples having the largesthysteresis in their raw form. As there is no reason why, orindication that, the shape and size of the aged NPs are modifiedthrough washing, these variations can only be ascribed to theremoval of the surfactant and/or to the potential effect of ethanolon the SCO properties of the material. A ‘‘partial’’ washing (i.e.only one ethanol washing and centrifugation instead of three)results in a smaller reduction of the hysteresis width, similar tothat observed by redispersing in octane (see Fig. 7 and Fig. S15,ESI†). Therefore, it seems that the dominant effect is the removalof the surfactant. A first hypothesis for it would be that thesurfactant in some way gives rise to a cooperative behaviour

among NPs in contact with it, an effect that is lost upon washing.A deeper analysis along the available modelization of such amatrix or environment effect29,30 would however be required andis beyond the scope of the present study.

The availability of aged NPs of disparate sizes allows evalu-ating the possible size effects on their SCO. As a reference, thebulk material is known to form tiny sub-micrometric crystalswith a lath or rod shape,26,33 and has a typical 40–45 K widehysteresis.12 From the SCO temperatures determined by DSC forthe aged NPs studied here (Table 2 and Fig. 8), the most strikingsize effect is observed for raw NPs. Particularly, the hysteresiswidth is found to increase linearly with the width of the NPsuntil reaching values close to 40 K, similar to the bulk material(Fig. 8). A similarly good correlation is actually obtained withthe raw NP volume (inset of Fig. 8). On the other hand, for thewashed NPs, the hysteresis width remains overly similar for allsmall widths and increases significantly only for the largestNPs. It can be noticed that the actual aspect ratio of the NPshas little effect if any. The dominant parameter seems to bethe 3D volume of NPs over which the SCO can propagate.Indeed, when one or two dimensions of the NPs becomesmaller than that of the material’s coherent domain, thecooperativity exhibited by the material may be affected, sinceit necessarily results from long-range interactions within thiscoherent domain.

Conclusions

Nanoparticles of the spin crossover material [Fe(Htrz)2trz]BF4obtained by the reverse-micelle approach using sodium dioctyl-sulfosuccinate and behenic acid as surfactants systematicallyexhibit a rod shape, as inferred by transmission electron micro-scopy observations, refuting the original reports of spherical NPsin the literature. Their size, in particular, the rod width can bemodified by adjusting the surfactant concentrations. A strongvariability has however been observed, and ascribed to uncon-trolled parameters in the micelle elaboration and microemulsionformation and ageing. The hysteresis width of the thermal spincrossover exhibited by the obtained raw nanoparticles increaseslinearly with the rod width, until it reaches a value of 40 K, closeto that of the bulk material. A similar good correlation is foundwith the nanoparticle volume. The removal of the surfactant fromthe raw nanoparticles is found to reduce the hysteresis width in adrastic manner, by up to 16 K. This latter effect is particularlyrelevant for studies aiming at using NPs maintaining the large440 K hysteresis of the bulk material. Both the demonstratedsize and the environmental effects on the properties of spincrossover nanoparticles are highly original and will be analysedon the basis of available theoretical models.

Acknowledgements

This work was funded by the Spanish MINECO and FEDERunder project MAT2011-24284 and the Gobierno de AragónDGA through support to research group E98-MOLCHIP. CBM

Fig. 8 Effect of NP size on their SCO properties. Hysteresis width plottedvs. the average rod width and the average rod volume (inset, semi-log plot)for raw aged NPs (green full circles) and washed aged NPs (dark grey emptycircles). The volume has been calculated considering a cylinder using therod width as diameter and the rod length as height. The hysteresis width isderived from DSC data (see Table 2).


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acknowledges the Campus de Excelencia Iberus for funding hisresearch stay at ICMA-Universidad de Zaragoza. The use ofServicio General de Apoyo a la Investigación-SAI, Universidadde Zaragoza is also acknowledged.

Notes and references

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2 P. Gütlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed.Engl., 1994, 33, 2024–2054.

3 (a) A. Bousseksou, G. Molnár, L. Salmon and W. Nicolazzi,Chem. Soc. Rev., 2011, 40, 3313–3335; (b) H. J. Shepherd,G. Molnár, W. Nicolazzi, L. Salmon and A. Bousseksou, Eur.J. Inorg. Chem., 2013, 653–661.

4 (a) For a review, see A. B. Gaspar, M. Seredyuk and P. Gütlich,Coord. Chem. Rev., 2009, 253, 2399–2413; (b) our work onsimilar triazole-based materials as the subject of the presentstudy: P. Grondin, D. Siretanu, O. Roubeau, M.-F. Achard andR. Clérac, Inorg. Chem., 2012, 51, 5417–5426.

5 (a) O. Roubeau, A. Colin, V. Schmitt and R. Clérac, Angew.Chem., Int. Ed., 2004, 43, 3283–3286; (b) T. Fujigawa, D.-L.Jiang and T. Aida, Chem. – Asian J., 2007, 2, 106–113;(c) P. Grondin, O. Roubeau, M. Castro, H. Saadaoui,A. Colin and R. Clérac, Langmuir, 2010, 26, 5184–5195.

6 (a) For Langmuir–Blodgett films of a related triazole-basedmaterial, see O. Roubeau, E. Natividad, B. Agricole andS. Ravaine, Langmuir, 2007, 23, 3110–3117; (b) for layer-by-layer of clathrate [FepzPt(CN)4], see C. Bartual-Murgui,L. Salmon, A. Akou, C. Thibault, G. Molnár, T. Mahfoud,Z. Sekkat, J. A. Real and A. Bousseksou, New J. Chem., 2011,35, 2089–2094, and references therein; (c) for a review see,M. Cavallini, Phys. Chem. Chem. Phys., 2012, 14, 11867–11876.

7 M. Castro, O. Roubeau, M. Piñeiro-López, J. A. Real andJ. A. Rodrı́guez-Velamazán, J. Phys. Chem. C, submitted.

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9 (a) O. Kahn and C. J. Martinez, Science, 1998, 279, 44–48;(b) O. Roubeau, Chem. – Eur. J., 2012, 18, 15230–15244.

10 As opposed to NPs of the Hoffman-type clathrate[FepzPt(CN)4], see for example (a) F. Volatron, L. Catala,E. Riviére, A. Gloter, O. Stephan and T. Mallah, Inorg.Chem., 2008, 47, 6584–6586; (b) A. B. Gaspar, I. Boldog,V. Martinez, P. Pardo-Ibañez, V. Ksenofontov, A. Bhattacharjee,P. Gütlich and J. A. Real, Angew. Chem., Int. Ed., 2008, 47,6433–6437.

11 E. Coronado, J. R. Galán-Mascarós, M. Monrabal-Capilla,J. Garcı́a-Martı́nez and P. Pardo-Ibañez, Adv. Mater., 2007,19, 1359–1361.

12 J. Kröber, J.-P. Audière, R. Claude, E. Codjovi, O. Kahn,J. G. Haasnoot, F. Grolière, C. Jay, A. Bousseksou, J. Linarés,F. Varret and A. Gonthier-Vassal, Chem. Mater., 1994, 6,1404–1412.

13 J. R. Galán-Mascarós, E. Coronado, A. Forment-Aliaga,M. Monrabal-Capilla, E. Pinilla-Cienfuegos and M. Ceolin,Inorg. Chem., 2010, 49, 5706–5714.

14 (a) J.-F. Létard, O. Nguyen and N. Daro, WO Patent,2007/065996, 2007; (b) T. Forestier, S. Mornet, N. Daro,T. Nishihara, S.-I. Mouri, K. Tanaka, O. Fouché, E. Freysz andJ.-F. Létard, Chem. Commun., 2008, 4327–4328; (c) T. Forestier,A. Kaiba, S. Pechev, D. Denux, P. Guionneau, C. Etrillard,N. Daro, E. Freysz and J.-F. Létard, Chem. – Eur. J., 2009, 15,6122–6130.

15 C. Etrillard, V. Faramarzi, J.-F. Dayen, J.-F. Létard andB. Doudin, Chem. Commun., 2011, 47, 9663–9665.

16 C. Faulmann, J. Chahine, I. Malfant, D. De Caro, B. Cormaryand L. Valade, Dalton Trans., 2011, 40, 2480–2485.

17 (a) A. Tokarev, L. Salmon, Y. Guari, W. Nicolazzi, G. Molnárand A. Bousseksou, Chem. Commun., 2010, 46, 8011–8013;(b) I. A. Gural’skiy, C. M. Quintero, G. Molnár, I. O. Fritsky,L. Salmon and A. Bousseksou, Chem. – Eur. J., 2012, 18,9946–9954.

18 P. Durand, S. Pillet, E.-E. Bendeif, C. Carteret, M. Bouazaoui,H. El Hamzaoui, B. Capoen, L. Salmon, S. Hébert,J. Ghanbaja, L. Aranda and D. Schaniel, J. Mater. Chem. C,2013, 1, 1933–1942.

19 (a) F. Guillaume, Y. A. Tobon, S. Bonhommeau, J.-F. Létard,L. Moulet and E. Freysz, Chem. Phys. Lett., 2014, 604,105–109; (b) G. Gallé, D. Deldicque, J. Degert, T. Forestier,J.-F. Létard and E. Freysz, Appl. Phys. Lett., 2010, 96,041907.

20 F. Prins, M. Monrabal-Capilla, E. A. Osorio, E. Coronadoand H. S. Van der Zant, Adv. Mater., 2011, 23, 1545–1549.

21 A unique identical TEM image is found for undiluted NPs of[Fe(Htrz)2trz]BF4 in ref. 10 and 12 and in the relateddoctoral work: M. Monrabal Capilla, PhD thesis, Universidadde Valencia, 2011.

22 A recent study of thin films made of NPs obtained followingthe same procedure reports only SEM and AFM images thatdo not allow proper assessment of the shape and size ofthe NPs, see D. Tanaka, N. Aketa, H. Tanaka, T. Tamaki,T. Inose, T. Akai, H. Toyama, O. Sakata, H. Tajiri andT. Ogawa, Chem. Commun., 2014, 50, 10074–10077.

23 DigitalMicrograph, Gatan Inc.24 S. Titos-Padilla, J. M. Herrera, X.-W. Chen, J. J. Delgado and

E. Colacio, Angew. Chem., Int. Ed., 2011, 50, 3290–3293.25 I. Suleimanov, J. Sánchez Costa, G. Molnár, L. Salmon and

A. Bousseksou, Chem. Commun., 2014, 50, 13015–13018.26 From our own observations, as well as in earlier PhD thesis

of C. Etrillard, University of Bordeaux, 2011 and J. J. A.Kolnaar, Leiden University, 1999. See also ref. 33.

27 J. Dugay, M. Giménez-Marqués, T. Kozlova, H. W. Zandbergen,E. Coronado and H. S. J. Van der Zant, Adv. Mater., 2015, 27,1288–1293.

28 W. K. Kegel, J. T. G. Overbeek and H. N. W. Lekkerkerker,Structural aspects and characterization of microemulsions:Thermodynamics of microemulsion, in Handbook of micro-emulsions, ed. P. Kumar and K. L. Mittal, Marcel Dekker Inc.,1999, pp. 13–44.


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29 A. Atitoaie, R. Tanasa, A. Stancu and C. Enachescu,J. Magn. Magn. Mater., 2014, 368, 12–18, and referencestherein.

30 G. Felix, M. Mikolasek, G. Molnár, W. Nicolazzi andA. Bousseksou, Chem. Phys. Lett., 2014, 607, 10–14, andreferences therein.

31 Y. Raza, F. Volatron, S. Moldovan, O. Ersen, V. Huc, C. Martini,F. Brisset, A. Gloter, O. Stephan, A. Bousseksou, L. Catala andT. Malah, Chem. Commun., 2011, 47, 11501–11503.

32 A. Rotaru, F. Varret, A. Gindulescu, J. Linarés, A. Stancu,J.-F. Létard, T. Forestier and C. Etrillard, Eur. Phys. J. B,2011, 84, 439–449.

33 From which the long-sought crystal structure of this materialcould be determined by powder X-ray diffraction, thanks tothe high crystallinity of the resulting powder, see A. Grosjean,P. Négrier, P. Bordet, C. Etrillard, D. Mondieig, S. Pechev,E. Lebraud, J.-F. Létard and P. Guionneau, Eur. J. Inorg. Chem.,2013, 796–802.


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