Magnetite nanorod thermotropic liquid crystal colloids: Synthesis, optics and theory

Journal of Colloid and Interface Science 386 (2012) 158–166

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Magnetite nanorod thermotropic liquid crystal colloids: Synthesis, optics and theory Nina Podoliak a, Oleksandr Buchnev a, Dmitry V. Bavykin b, Alexander N. Kulak b,d, Malgosia Kaczmarek a, Timothy J. Sluckin c,⇑ a

Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, United Kingdom Materials Engineering and Energy Technology Research Groups, Engineering Sciences, University of Southampton, Southampton SO17 1BJ, United Kingdom c Mathematics, University of Southampton, Southampton SO17 1BJ, United Kingdom d School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom b

a r t i c l e

i n f o

Article history: Received 21 May 2012 Accepted 26 July 2012 Available online 3 August 2012 Keywords: Liquid crystals Fe3O4 Nanorods Nanostructures Ferronematics

a b s t r a c t We have developed a facile method for preparing magnetic nanoparticles which couple strongly with a liquid crystal (LC) matrix, with the aim of preparing ferronematic liquid crystal colloids for use in magneto-optical devices. Magnetite nanoparticles were prepared by oxidising colloidal Fe(OH)2 with air in aqueous media, and were then subject to alkaline hydrothermal treatment with 10 mol dm3 NaOH at 100 °C, transforming them into a polydisperse set of domain magnetite nanorods with maximal length 500 nm and typical diameter 20 nm. The nanorods were coated with 4-n-octyloxybiphenyl-4-carboxylic acid (OBPh) and suspended in nematic liquid crystal E7. As compared to the conventional oleic acid coating, this coating stabilizes LC-magnetic nanorod suspensions. The suspension acts as a ferronematic system, using the colloidal particles as intermediaries to amplify magnetic field–LC director interactions. The effective Frederiks magnetic threshold field of the magnetite nanorod–liquid crystal composite is reduced by 20% as compared to the undoped liquid crystal. In contrast with some previous work in this field, the magneto-optical effects are reproducible on time scales of months. Prospects for magnetically switched liquid crystal devices using these materials are good, but a method is required to synthesize single magnetic domain nanorods. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Liquid crystals (LCs) are widely used in electronic devices because of their remarkable optical and electrical properties. Key properties from a device point of view include the capacity to reorient molecules rapidly, as well as large optical and dielectric anisotropy. This enables their optical properties to be easily switched by an electric field. Driving voltages as low as a few volts are sufficient to control the optical response in LC devices, and as a result, most LC devices are driven by electric fields rather than by magnetic fields. . However, LC diamagnetic properties are also anisotropic, and thus LC’s can in principle be reoriented by a magnetic field [1]. But in contrast to their high sensitivity to electric fields, conventional liquid crystals are relatively insensitive to magnetic fields. For example, in a cell with the dimensions of a typical LCD device, a rather high field, of the order of 1–10 kOe, is required to trigger director reorientation. As a consequence, increasing the sensitivity of liquid crystals to magnetic probes represents an important technological challenge. We can envisage several magnetic device prin-

⇑ Corresponding author. Fax: +44 238059 5147. E-mail address: [email protected] (T.J. Sluckin). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.07.082

ciples that could in principle exploit such an effect. Examples might include optical switching triggered by magnetic rather than electric fields, as well as optical sensors for low magnetic fields driven by liquid crystal reorientation. Brochard and de Gennes [2] were the first to suggest a specific technique to increase the magnetic sensitivity of LC’s. Their procedure involved doping the LCs with very low volume concentrations of nanometre-sized magnetic particles. These suspensions have come to be called ‘‘ferronematics’’. The Brochard–de Gennes theory, and subsequent work by Burylov and Raikher [3], predicted that the ferronematics should respond to low fields, of the order of tens of Oersted. This low magnetic field would have no significant effect on the undoped nematic, but would be sufficiently strong to align or rotate the magnetic moments of the particles inside the ferronematic suspensions. The realignment or rotation effect of nanoparticles could then be transferred into the host nematic through coupling between the particles and the liquid crystal molecules. The first experimental realisation of ferronematics, carried out by Chen and Amer [4], seemed to confirm the theoretical expectation. Their ferronematic suspension was doped by c-Fe2O3 needlelike particles of 0.5 lm in length. However, their results have not been replicated in the literature, which may indicate that the suspension was not stable. Furthermore, the suspensions behaved

N. Podoliak et al. / Journal of Colloid and Interface Science 386 (2012) 158–166

anomalously at high magnetic fields, and an intriguing (and, so far, apparently unreproduced) cellular texture was observed. A detailed understanding is still lacking, but one interpretation is that the particles aggregated. Further work has been reported recently by Kopcˇansky´ et al. [5,6], in which ferronematic suspensions have been produced and characterised using small spherical particles, as well as rod-like and chain-like nanoparticles, in different nematic liquid crystals. To stabilize the suspension the particles were coated by surfactant, mainly oleic acid. Those experiments demonstrate that the properties of a ferronematic suspension depend on the coupling between the liquid crystal molecules and the nanoparticles showing stronger interaction in case of anisotropic particles. The importance of particle shape anisotropy on the magneto-optical properties of composite mixtures had already been predicted in early theoretical work [2,7]. In addition, Cordoyannis et al. [8] have found that the particle surface chemistry can also strongly influence the degree of interaction. With the development of different techniques in the synthesis of nanomaterials, new types of elongated nanomaterials, such as carbon nanotubes filled with a-Fe [9], or functionalized with magnetic particles [10], have been tested as liquid crystal dopants. Suspensions with such strongly anisotropic particles indeed demonstrated increased sensitivity to magnetic fields. However, the problem of particle aggregation remains one of the most important, unresolved challenges. Several different synthetic routes have been developed for the preparation of a wide range of ferromagnetic spheroidal particles, and for controlling their shape [11]. In this paper, on the other hand, we concentrate on elongated 1-D nanostructures, such as nanotubes, nanofibres, and nanorods. Such structures possess a high aspect ratio and are expected to align readily with a magnetic field [12]. Recent advances in the synthesis of elongated magnetite (Fe3O4) nanostructures suggest that this material may be promising for magneto-optic application [11]. Several different preparation methods for elongated magnetite (Fe3O4) nanorods have been reported, including template-assisted hydrolysis (using PEG [13,14], PVP [15], or ethylenediamine [16]), templateless hydrolysis [17], hydrolysis of dissolved iron salts or b-FeOOH nanorods [18] under hydrothermal conditions, gas phase reduction of aFe2O3 nanowires [19], thermal decomposition of iron (II) oxalate nanorods [20], pyrolysis of ferrocene in supercritical carbon dioxide [21], and reversed precipitation of magnetite under magnetic field [22]. Magnetite nanotubes have also been prepared by template [23] or templateless hydrothermal synthesis [24]. The arrangement of this paper is as follows: The experimental methods are explained in Section 2. Our experimental work includes synthesis, imaging, and magneto-optical measurements. We report a new facile method which enables the transformation of magnetite nanoparticles to nanorods using alkaline hydrothermal treatment without addition of any polymer templates. The nanorods are then characterised, and finally used to prepare ferronematic suspensions with low particle volume concentrations of the order of 101%. We also investigate the specific impact of surfactants on the interaction between nanorods and cyanobiphenyl liquid crystals (E7), and the stability of the resulting liquid crystal nanorod colloidal suspensions. Finally, we have studied the magneto-optical properties of the magnetite nanorods/E7 composite mixtures are studied by measuring the magnetic-field-induced Frederiks transition. Results are reported in Section 3. The key result is a decrease in the effective magneto-optical response threshold, corresponding to the magnetic Frederiks transition. This demonstrates an increased optical sensitivity of the ferronematic suspension to magnetic fields. We conclude Section 3 with a detailed discussion of our experimental results in the context of a theory which we have developed in previous work [25].

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Finally in Section 4, we present some conclusions. The main conclusion is that there is strong coupling between nanorod magnetization and the LC nematic director. But the nanorods themselves have a relatively low magnetization, limiting the total increase in the sensitivity of the LC colloid to magnetic fields. However, our studies suggest strongly that if the magnetic nanorods themselves could now be fabricated as single magnetic domains, then there are good prospects for constructing magnetically switchable nematic cells. 2. Experimental procedure 2.1. Reagents Sodium hydroxide (NaOH), hydrogen chloride (HCl), iron (II) chloride tetrahydrate (FeCl24H2O), hexane, oleic acid (OA) pure grade were all obtained from Aldrich. 4-n-octyloxybiphenyl-4-carboxylic acid (OBPh) was obtained from Alpha Aesar. Liquid crystal mixture E7 was purchased from Merck. All chemicals were used without further purification. 2.2. Preparation of magnetite nanoparticles The synthesis of the magnetite nanoparticles is an adaptation of a recent method of oxidative ageing of Fe(OH)2 in aqueous suspensions introduced by Vereda et al. [26]. A 3 g FeCl24H2O were dissolved in 250 cm3 of distilled water in a tall (800 cm3) beaker at pH 2.4, followed by slow dropwise addition of 150 cm3 of 0.1 mol dm3 NaOH solution in air at room temperature, accompanied by vigorous stirring (600 rpm). The addition rate was adjusted to maintain the solution pH within the range 7.00 ± 0.15. During the procedure the colour of the reaction mixture changed continuously from deep green to dark brown. If the solution pH exceeded 7.5, a solution of 0.1 mol dm3 HCl was added in order to maintain the solution pH within the allowed range. We note that if the solution pH > 8, the colour of the mixture rapidly changes to red brown, which is attributed to the formation of hematite. Once the solution pH stabilized, the mixture was stirred for 3 h. The precipitate nanoparticles were then separated by filtration, washed with water, and dried overnight at 120 °C. 2.3. Preparation of magnetite nanorods In order to convert the magnetite nanoparticles into nanorods, 200 mg of magnetite nanoparticles were mixed with 40 cm3 of 10 mol dm3 NaOH, transferred to PTFE-lined autoclave and heated for 48 h at 100 °C. The resulting dark brown solid precipitate was separated, washed with water using filtration, and finally subject to vacuum drying. 2.4. Preparation of magnetite nanorod suspension in liquid crystal In order to disperse the magnetite nanorods in the liquid crystal, 10 mg of nanorods were mixed with 10 cm3 of hexane. In order to stabilize the solutions, either solid OBPh (9 mg) or liquid OA (0.008 cm3) were added to the nanorod suspensions. The resulting solution was sonicated in an ultrasound bath for 60 min, so as to obtain a uniform nanorod dispersion. After sonication, the mixture was allowed to settle for 1 min; 5 cm3 of the liquid was then collected from the top and mixed with 0.3 cm3 of nematic liquid crystal E7. The hexane was removed from the resulting mixture by evaporating it in a rotary vacuum evaporator at 40 °C and 200 mbar. More liquid crystal was added to the suspension, thereby reducing the nanorod concentration by a factor of 100. In the

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suspensions used in our magnetic characterisation studies, the nanostructure volume fraction f = 3  105.

Magnet

Polariser

N

Analyser Hb

Photodiode

S

2.5. Sample characterisation HRTEM images of the products were recorded using a 100– 300 kV JEOL 3010 high resolution transmission electron microscope, with samples loaded onto a copper grid coated with a perforated carbon film (Agar Scientific). SEM images were recorded using a JSM 6500F thermal emission scanning electron microscope with an accelerating voltage of 0.5–30 kV, with samples deposited onto thin silicon wafers. FTIR spectra were recorded using a Nicolet iS10 spectrometer with a ‘‘golden gate’’ attachment for solid samples. The nematic–isotropic phase transition temperature (TNI) in the nanorod suspension was measured using a Mettler FP82 hot stage controlled by a Mettler FP80 central processor.

LC cell

H

Fig. 1. Schema of the experimental set up for measuring the magneto – optic response of the cells containing undoped liquid crystal or magnetite nanorod suspension. Dash line shows the path of He–Ne laser beam (see text for details).

2.6. Fabrication of liquid crystal cells Undoped liquid crystal and liquid crystal ferronematic suspensions were studied in standard, glass cell format. A typical cell consists of two glass slides with a gap 50 lm filled with a liquid crystal. The glass slides were covered with polyimide layers, and were then rubbed in such a way as to create a uniform, in-plane liquid crystal alignment with a small pre-tilt angle. The pre-tilt angle was measured to be approximately 3° using the crystal rotation method [27]. The optical quality of suspension and liquid crystal alignment in the cells was observed with a polarising optical microscope (POM) Olympus BX51. 2.7. Magneto-optic characterisation The sensitivity of the suspension to magnetic fields was investigated by measuring the magneto-optic response using the setup schematically shown in Fig. 1 [28]. The cell was placed between two crossed polarizers in a way that a LC director alignment made 45° angle with respect the polarizers axes. This optical system was held between two poles of an electromagnet, which created a test magnetic field, H, in the direction perpendicular to the cell plane. There was also a small bias magnetic field (Hb  15 Oe) along the liquid crystal director to orient the ferroparticles magnetic moment at the start of each experimental run. The reorientation of liquid crystal inside the cell with increasing test magnetic field strength was monitored by an optical method. We measured the intensity of a He–Ne laser beam (k = 632 nm) passing through the cell. The phase lag Du between the ordinary and extraordinary components of the laser beam, induced by the liquid crystal layer, could be extracted from the measured intensity I, using the relation [29] 2

I ¼ Imax sin

  Du : 2

ð1Þ

The phase lag change indicates reorientation of the liquid crystal by the magnetic field. 3. Results and discussion 3.1. Preparation and characterisation of magnetite nanorods Preparation methods for magnetite (Fe3O4) nanostructures can be divided into three groups. These are: oxidation of Fe (II) [20], reduction of Fe (III) [18,19] precursors, and hydrolysis of stoichiometric Fe(II) and Fe(III) mixture [16,17]. One technique for the control of nanorod morphology and size is achieved by the correct selection of template agent. A second technique involves the

Fig. 2. TEM images of magnetite (a) nanoparticles and (b) nanorods. Inset (a) shows higher magnification image; (b) left hand side: higher magnification image; (b) right hand side: SAED image for nanorod bundle.

precise control of the experimental environment [16], which may involve performing the reaction in anaerobic deoxygenated conditions. In order to reduce the manufacturing cost of nanostructured magnetite, the templateless synthesis of single crystal magnetite nanoparticles from Fe(II) aqueous solution [26] was modified, so as to use air oxygen as an oxidising agent. Fig. 2a shows a TEM image of magnetite nanoparticles obtained via facile oxidation of colloidal Fe(OH)2 with oxygen in aqueous solution at pH 7. The nanoparticles are characterised by a single crystal structure, an angular shape, and a wide distribution in particle sizes (see Fig. 3a). The average nanoparticle size is approximately 20 nm.

N. Podoliak et al. / Journal of Colloid and Interface Science 386 (2012) 158–166

From the HRTEM image (see inset in Fig. 2a) the interlayer spacing can be estimated as 0.48 nm, corresponding to the (1 1 1) planar spacing of spinel-type magnetite [30]. We find that the alkaline hydrothermal treatment of magnetite nanoparticles with 10 mol dm3 NaOH at 100 °C causes them to transform to magnetite nanorods, probably as a result of recrystallization (see Fig. 2b). According to the SEM data (see Fig. 3b), the nanorods are characterised by a wide distribution in lengths and diameters, with average values being approximately equal to 500 nm and 20 nm respectively. An HRTEM image is shown in an inset of Fig. 2b, and demonstrates conclusively that the nanorods are single crystals. The lattice fringes, estimated as 0.48 nm, are in agreement with the known distance between the (1 1 1) lattice planes in magnetite [30]. We note, however that magnetite appears black in colour, but our samples appear dark brown. This suggests that the sample also includes some degree of the chemically and crystallographically rather similar maghemite phase impurity. The [1 1 1] direction in magnetite crystals usually corresponds to the radial direction in nanorods. This suggests that the axial direction of a nanorod coincides with the [1 1 0] crystallographic vector. Wang et al. [14] have reported a similar case, in which magnetite nanorods grow in the [1 1 0] direction. In addition, narrow diffraction dots can be seen in an SAED image taken from a

Fig. 3. SEM images of magnetite (a) nanoparticles and (b) nanorods.

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nanorod bundle (right inset in Fig. 2b). This confirms that the magnetite/maghemite nanorods are characterised by high crystallinity.

3.2. Nanorod suspension in nematic liquid crystal When we first tried to prepare a suspension of magnetite nanorods in E7 liquid crystal, our mixtures were not very stable. Large agglomerates of nanofiber-like structures formed. A proportion of the magnetic materials precipitated, and those that remained were unevenly distributed in space. However, in these studies the suspension did not contain surfactants designed to avoid colloidal aggregation. A common stabilizing agent in colloidal suspensions is oleic acid (OA). This possesses a good affinity to iron oxide surfaces, caused by bonding through the carboxylic (COOH) group. As a result other workers, in particular Kopcˇansky et al. [5,6], have used this surfactant specifically as a stabilizing agent for magnetite nanoparticles in liquid crystal suspensions. However, we have found recently [25] that, despite the good adhesion between OA and magnetite nanoparticles, in liquid crystal suspensions large magnetic nanoparticle aggregates nevertheless form. Furthermore, even if large aggregates had not been formed, we might expect that OA adsorption on the surface of the magnetite nanostructures could also prevent interaction between the nanoparticles and the liquid crystal molecules, thus suppressing possible ferronematic behaviour. In order to improve the magneto-optic response of the composite mixture it is important to establish a strong interaction between the magnetic material and the liquid crystal molecules [31]. One general strategy which might be expected to strengthen this interaction is to employ specific surfactants which are known to interact with both the magnetite surface and the E7 liquid crystal. In our experiments we use the surfactant octyloxybiphenyl-4carboxylic acid (OBPh). This molecule contains one carboxylic group and one octyloxybiphenyl group. The carboxylic group binds to the iron oxide surface, while the octyloxybiphenyl group interacts with the liquid crystal, through a stacking mechanism. This polyfunctional surfactant might be expected to improve the overall affinity of liquid crystals to magnetite nanorods, as shown in Fig. 4. We note that OBPh has been used in a related system by Khatua et al. [32], who use it to stabilize a suspension of gold nanoparticles in the liquid crystal 5CB. In Fig. 5 we present FTIR spectra of samples of magnetite nanorods prepared under a number of conditions. Apart from the untreated nanorods, we examine nanorods treated in acetone solution either with E7, or with OBPh, or with a mixture of the two, in each case followed by drying in air. Spectrum (a) in Fig. 5 refers to the case of untreated nanorods, while in spectrum (b) the nanorods have been treated with 0.04 mol dm3 solution of

Fig. 4. Schematic illustration of the improvement in adhesion between E7 liquid crystal molecules and magnetite nanorods resulting from interaction with OBPh (via stacking mechanism). The OBPh is attached to the magnetite nanorod surface through the –COOH groups.

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Fig. 5. FTIR spectra of (a) untreated magnetite nanorods, and (b–d) magnetite nanorods treated for 48 h in acetone solution at room temperature, followed by drying in air. Treatments: (b) 0.04 mol dm3 E7, (c) 0.005 mol dm3 OBPh, and (d) mixture of 0.04 mol dm3 E7 with 0.005 mol dm3 OBPh.

E7 for 48 h. It can be seen that these spectra are almost identical, which suggests that the cyanobiphenyl molecules in E7 are only poorly adsorbed at the iron oxide surface. By contrast, the sample of nanorods treated with OBPh in acetone (see spectrum c in Fig. 5) exhibits new peaks at 1605 cm1 and 1523 cm1. These peaks are most likely to be associated with oscillations of the carboxyl group [33], thus confirming that OBPh adsorbs on the surface of magnetite nanorods more easily than E7. Finally, a small signal appears at 2226 cm1 in the FTIR spectrum from the suspension of nanorods in the OBPh – E7 mixture (see spectrum d in Fig. 5). This can be attributed to a characteristic stretch vibration of cyano groups in E7 liquid crystal [34]. This might suggest that E7 has an increased affinity to magnetite nanorods in the presence of OBPh, as a result of the mechanism schematically illustrated in Fig. 4. The FTIR results give grounds for optimism that OBPh can act successfully as a stabilizing agent for suspensions of magnetite nanorods in E7. Furthermore, unlike in most previously reported cases, in which nanorods have been obtained in the presence of templates [13–16], our magnetite nanorod samples are intrinsically free from templates. The absence of templates suggests that the surfactant molecules might be more easily able to access the nanorod surfaces. And indeed, our polarising optical microscope (POM) experiments do demonstrate this effect. Coating the nanorods with OBPh – as compared to OA – improves the optical quality of the corresponding nanorod suspension in E7 liquid crystal. In Fig. 6 we show POM images of cells containing suspensions of nanorods, coated in one case by OA, and in the other by OBPh. In both cells, bright and dark POM images indicate planar LC alignment, depending on the polarizer axis alignment with respect to the liquid crystal director. However, in Fig. 6a, corresponding to OA coated nanorods, large aggregates (10–100 lm in size) can easily be seen. In Fig. 6b, by contrast, in which the cell contains a suspension of OBPh coated nanorods, we see a much more homogeneous distribution of particles, and the large aggregates are now no longer present. Our optical experiments are thus carried out in nanorod–OBPh solutions in E7. We note, however, that not all OBPh would adsorb at the nanorod surfaces. It seems likely that any change in optical properties in the composite solution is due principally to the effect of the nanoparticles rather than the OBPh. However, it is necessary to eliminate the possibility that major changes are due to the effect of the OBPh rather than the nanorods. To test for this effect, the properties of some extra suspensions were also investigated. These extra suspensions contained E7

Fig. 6. Polarising optical microscopy images of cells containing ferronematic suspensions of magnetite nanorods coated with (a) OA, (b) OBPh. Crossed lines indicate the axis of polarizers, line with arrows indicates the direction of liquid crystal alignment. Scalar bars correspond to 200 lm.

Table 1 Clearing temperature of undoped nematic (E7), suspensions with OBPh and OA coated magnetite nanorods (E7 + MNRs + OBPh and E7 + MNRs + OA) and suspensions with surfactant molecules (E7 + OBPh and E7 + OA). Clearing temperatures are measured with the accuracy of 0.1 °C. LC/Suspension

Clearing temperature, °C

E7 E7 + MNRs + OBPh E7 + MNRs + OA E7 + OBPh E7 + OA

60.1 59.1 58.5 59.6 59.6

doped with surfactant at the same concentrations as in the nanorod suspensions. Table 1 lists the clearing temperatures of the undoped nematic, of the suspensions with OBPh and OA coated nanorods, and also of the suspensions containing surfactant molecules alone. In all suspensions a drop in the clearing temperature is observed. In our samples, adding either OBPh or OA to pure E7 causes (the same) phase transition temperature decrease, of 0.5 °C. In the case of ‘‘equivalent’’ suspensions including coated nanorods, however, the suspension with OA-coated nanorods possesses a lower clearing temperature than the suspension with the OBPh-coated nanorods. The difference in clearing temperature is 0.6 °C, a rather significant quantity. This difference indicates that

N. Podoliak et al. / Journal of Colloid and Interface Science 386 (2012) 158–166

the strength of the nanorod–liquid crystal molecule interaction depends strongly on the type of coating. Mixtures between nematic and non-nematic components have been much studied [35,36]. In general non-interacting colloid particles favour solution in an isotropic, rather than a nematic fluid. The director reorganisation around a colloid particle which does not actively itself favour nematic ordering involves considerable elastic energy on a scale of kBT, and is roughly proportional to the dimension of the object. The consequence is that in a colloidal suspension, the expectation is that the nematic–isotropic phase boundary slopes sharply downward as a function of concentration. Furthermore along the phase boundary the volume concentration of the isotropic phase is higher, by a large Arrhenius factor, than that of the nematic phase. This limits the ability of the nematic phase to successfully accept colloidal particles. However, when there is orientational coupling between the dopant solute colloidal particles and the solvent nematogen, this effect may be mitigated, and the decrease of the nematic–isotropic phase transition can be reduced. The most dramatic manifestation of this phenomenon occurs when the solute colloidal particles are ferroelectric [37–39], in which case the phase transition can increase with temperature. Analogous but weaker effects are expected for ferronematic particles. The different slopes of the phase transition as a function of temperature are a signal that coupling between the orientations of the magnetic and liquid crystalline particles is a sensitive function of shape and magnetic moment. Thus we expect that the higher the transition temperature, the stronger the orientational coupling between the nanorods and the liquid crystal molecules. In our case the higher temperature phase transition of the OBPh-coated-nanorod solution indicates a higher nanorod–liquid crystal molecule coupling than in the OA-coatednanorod solution, and a larger phase-transition-associated miscibility gap in the OA-coated-nanorod solution. This result is thus consistent with the lack of large aggregates in the OBPh-coatednanorod solution. 3.3. Magnetic-field-induced Frederiks transition in magnetite nanorod suspension In the next stage of our investigation, the magnetic-field induced Frederiks transition was measured in cells containing an OBPh-coated-nanorod suspension and undoped nematic, as described in Section 2. In Fig. 7 we show the dependence of phase lag changes on the magnetic field strength extracted from the experimental data, and also show a fit to a numerical model, as discussed further below. The Frederiks transition in a homogeneously aligned cell with undoped E7 would occur at a magnetic field of around 1500 Oe. In the magnetite nanorod suspension cell, by contrast, this effective Frederiks transition occurs at about 1200 Oe, which is around 20% lower than the threshold in the undoped E7 cell. Moreover, a noticeable optical response in the experimental undoped E7 cell can be detected at magnetic fields around 1000 Oe. The ferronematic suspension, however, responds at lower fields, at approximately 700 Oe (see Fig. 7). In Fig. 8 we show the time dependence of the normalised transmitted laser beam intensity, for the undoped nematic and the nanorod suspension, as magnetic field H is increased from 690 to 710 Oe, close to the point at which the nanorod suspension cell response can first be detected. In the undoped E7 cell, there is essentially no change in transmitted laser beam intensity as the field is changed. The transmitted laser beam intensity in the nanorod suspension, by contrast, changes noticeably with increasing field, indicating higher magnetic response of the suspension. To verify that the OBPh surfactant itself makes no explicit contribution to the magneto-optic properties of the suspension, we

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Fig. 7. Magnetic field dependence of phase lag changes, for cells containing undoped nematic (E7) (squares) and magnetite nanorod suspension (E7 + MNRs + OBPh) (circles). Dots represent the experimental data and lines represent the optimal model predictions.

Fig. 8. Optical signal collected from the cells containing undoped nematic (E7) and magnetite nanorod suspension (E7 + MNRs + OBPh) for field H = 710 Oersted.

performed a further additional magneto-optic measurement using a cell containing E7 + OBPh. Here we find the same results as in the undoped E7 cell; the OBPh by itself has no effect. 3.4. Fitting the magnetic-field-induced Frederiks transition to continuum theory The results of this magneto-optic experiment can be further analysed using a numerical model which we have recently introduced [25]. As discussed above, a detailed comparison with experimental results is given in Fig. 7. The model describes the magnetic-field-induced Frederiks transition in ferronematic cells. The theory is based on the Burylov–Raikher [3] continuum theory of ferronematics, which assumes a uniform distortion of the nematic matrix in a magnetic field, and a collective response to the external field. The theory neglects aggregation of ferromagnetic inclusions, assuming them to be uniformly distributed; in some way this is a rough approximation which limits the application of the model. However, this approximation seems to be valid [25] for low concentration suspensions. We also note that Burylov and Raikher [3] have discussed conditions for the validity of the collective response approximation. It is far from clear that our experiments satisfy their conditions, but nevertheless for suitable parameters, the experiments are consistent with a collective response picture. Moreover, the opposite individual nanoparticle picture would, for example, suggest depolarization and light scattering which were not observed in our magneto-optical experiments. The model distinguishes two separate processes through which the magnetic field can influence the nematic host. There is a direct

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effect, associated with the diamagnetic anisotropy of nematic molecules, and also an indirect effect, associated with the presence of colloidal inclusions. The latter effect includes the ferromagnetic properties of the nanorods, which permit alignment by low magnetic fields, and the coupling between the ferroparticles and the nematic medium, which transfer the nanorod rotation to the nematic host. These two processes are described in the model by two non-dimensional parameters j and x, known respectively as the magnetic and the coupling parameters. They contain all parameters of magnetic dopants in the following form:

X f D2

M fD

j ¼ pSffiffiffiffiffiffiffiffiffi and x ¼ ; K va K

ð2Þ

where Ms is the saturation magnetization of magnetite, f is the effective volume fraction of ferromagnetic inclusions, va is the diamagnetic anisotropy of nematic, and D is the cell thickness. K is a liquid crystal elastic constant; in this geometry K = K1, the splay elastic constant. The magnetic parameter j scales the effect of particle ferromagnetism with respect to nematic diamagnetism, while the coupling parameter x measures the ratio of the effective particle–liquid crystal coupling compared to the elastic energy of nematic deformations. The key quantity X is the effective coupling energy density, per unit nanorod volume, associated with orientational alignment between the nanorods and local liquid crystal director. The numerical model now calculates equilibrium orientation of the nematic and the ferroparticles inside the cell for given amplitude of the external magnetic field. It also simulates the light beam propagation through the liquid crystal layer, resulting in the calculated phase lag change between ordinary and extraordinary components of the beam depending on the external magnetic field. This dependence is then can be compared to the experimental curve, as shown in Fig. 7. There are only two fitting parameters in the model: the magnetic and coupling parameters. All other parameters either known from independent measurements (cell thickness, liquid crystal pre-tilt angle, bias magnetic field), or taken from the datasheet (elastic constants and refractive indices of nematic E7). The value of nematic diamagnetic anisotropy va = 1.2  107, however, is obtained from fitting magneto-optic experimental data in the undoped nematic cell. The best-fit theoretical curve shown in Fig. 7 is obtained using the following values of the fitting parameters: magnetic parameter j = 0.06 and coupling parameter x = 0.11. The ratio x/j between the fitting parameters is 1.83 affects the coupling energy density X (see Eq. (2)):

Ms x X¼ D j

sffiffiffiffiffiffi K1

va

;

ð3Þ

It should however be noted that the saturation magnetization of the nanorods, Ms, is not directly accessible from our experiments. The saturation magnetization of bulk magnetite material can be obtained from the literature, Ms = 480 emu cm3 [40,41]. But the saturation magnetization in small particles is usually lower than in bulk material, depending both on the particle size and on the method of synthesis [40,41,42]. Following Eq. (3), X is directly proportional to Ms. Using the bulk saturation magnetization yields an upper bound for the coupling energy density of X = 5.4  105 erg cm3. This calculation is rather rough approximation, as it is based on a fit of only one set of experimental data. Nevertheless, it shows that the effective coupling energy X between nematic E7 and the magnetite nanorods is more than one order magnitude higher than the coupling energy associated with spherical iron oxide nanoparticles, which the present team found in the earlier experiments [25]. We recall from our previous study [25], that from a theoretical point of view, a distinction is necessary between the cases of small and large magnetic parameter, specifically j < 1 and j > 1. In the

former case, the direct liquid crystal–magnetic interactions dominate the magnetic response, whereas in the latter case, indirect effects, mediated by the magnetic inclusions, prevails. In these experiments, we find a higher value of X for the nanorods than for the spherical nanoparticles studied previously [25]. Nevertheless, despite the higher coupling energy, the magnetic parameter j obtained from fitting the magneto-optic curves is lower than unity. Thus, in this ferrosuspension, the direct nematic effect still dominates the indirect effect. The low j implies that the nanorods should be only weakly aligned by a low magnetic field. This result holds, strictly speaking, only inside the liquid crystal, but we would expect that it would be robust and hold also in any solvent. The alignment of nanorods in a magnetic field in liquid phase was tested in the following experiment. A drop of acetone solvent containing a suspension of OBPh coated nanorods was placed on a flat substrate, and dried under a constant weak magnetic field. The alignment of nanorods was tested using SEM microscopy (see Fig. S1 In Supporting Information). The SEM image showed a random orientation of nanorods on the substrate. This suggests that, rather than consisting of single domains, the nanorods may contain many magnetic domains, whose magnetizations almost cancel, a result qualitatively consistent with the low fitted values of j. We note, however, that at this stage this result is not conclusive; it is also possible that the fields in this experiment were simply too low to exert a significant aligning effect. Thus, in this work, using magnetite nanorods functionalised with OBPh, we have observed an increased interaction between the magnetic inclusions and the nematic host in these ferronematic suspensions, as compared to our previous suspensions [25]. However, we observe that the magnetite nanorods cannot be fully aligned in a low magnetic field. This is also implied by the low value of the magnetic parameter j, which in turn leads to the relatively small response at a low field. The numerical model suggests that a higher response to a low magnetic field would be expected in a system characterised by a higher value of j. According to Eq. (2), this parameter is directly proportional to both the concentration of magnetic inclusions and their magnetization. It seems likely that if either the magnetic dipole moments or the ferroparticle concentration is increased, the magnetic dipole–dipole interaction between them would also increase. Unfortunately, this would lead to the possibility of nanoparticle aggregation, and under these circumstances, our numerical model is no longer valid. In aggregated suspensions, when aggregate size reaches tens or hundreds of microns, the uniform nematic texture is disrupted by local nematic distortions around aggregates (see Fig. 6a). In such systems, the magneto-optic response is likely to be driven by the individual reorientational behaviour of the aggregates, rather than by their collective response. An example of such a system is reported by Buluy et al. [9], who observe aggregated suspensions in which strong local director distortions cause great sensitivity to low magnetic fields, but in which, by contrast, the magnetic Frederiks threshold is only weakly affected.

4. Conclusions In this paper we have established a facile route for templateless preparation of single crystal magnetite nanorods by hydrothermal treatment of magnetite nanoparticles with 10 mol dm3 NaOH at 100 °C. The resulting nanorods are characterised by a single crystal structure. Typical nanorod dimensions are ca. 500 nm in length and ca. 20 nm in diameter. Nanorods possess good affinity to octyloxybiphenyl-4-carboxylic acid surfactant, which can dramatically improve the stability for colloidal suspension of nanorods and increases their affinity to cyanobiphenyl molecules. Using this surfactant increases the coupling between the liquid crystal

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director and the magnetic dipole of the nanorods. The result is a significant decrease in the magnetic Frederiks threshold in the magnetite nanorods/E7 liquid crystal composites, as compared to the undoped nematic E7. Interestingly, the magnetite nanorods in magnetic fields do not appear to orient in low magnetic fields. By contrast, when they are used to dope a nematic liquid crystal, the Frederiks transition occurs at a magnetic field significantly lower than in pure liquid crystal. We have explained this apparent paradox in terms of a theory in which there are two key parameters, which we have labelled the magnetic parameter j and the coupling parameter x. The system discussed in this paper possesses a low value of j – causing the negligible low field response, but nevertheless a higher value of x permits a higher threshold shift than in previous measurements in suspensions of spheroidal nanoparticles [25]. The theory we have used to model our results was also used successfully in this previous paper [25]. In principle, the Burylov– Raikher theory requires the assumption of a collective response. However, the conditions for this collective response given in the literature [3,7] do not seem to be satisfied in either set of experiments. Thus we have a satisfactory semi-empirical model but more theoretical work is clearly required. Our theory supposes well-aligned magnetic nanoparticles, and the initial alignment is achieved by imposing a bias field in the plane of the cell. But we have also carried out an experiment in which the ferronematic cells lack the bias field. In this case the Frederiks threshold is also reduced as compared to the pure samples, but the effects are much less pronounced than in the samples with a bias field. This reduction in threshold field has also been observed by Kopcˇansky´ et al. [5]. In principle the reason for this effect is clear. The effective mean nanorod magnetization is much reduced when there is no bias field, and hence the coupling to the magnetic field is lower than it would be if the particles were lined up by a bias field. The lower coupling implies a lower effective field, and hence a lower reduction in the Frederiks threshold. Unfortunately, at this stage our theory [25] is unable to deal with this case explicitly, because now the magnetic direction is not a good hydrodynamic variable. Rather, some account must be taken of the distribution of nanorod orientation, because in the low magnetic field case the nanorod magnetic dipoles almost cancel; they are said to be ‘‘compensated’’. Some initial progress has very recently been made to take account of this by Petrov and Zakhlevnykh [43], but in their theory the coupling parameter does not play a role, and so we are not in a position to fit the data to our experiments. The long-term goal of this work is the development of magnetooptical devices. Traditionally, research on ferronematics has been discussed in this context, but the original idea is now more than forty years old [2], but liquid crystal devices remain stubbornly electrically driven. Notwithstanding the early unrepeated work of Chen and Amer [4], the coupling between the nematic liquid crystal and the magnetic inclusions has been simply too weak. Our theoretical work focuses attention on the fact that two parameters need to be considered, one of which specifies the linear magnetic field susceptibility, and the other of which specifies the coupling itself. A successful device requires that both quantities must be optimised, while avoiding further aggregation which is likely to follow if the nanorods are too magnetic. This paper and its predecessor [25] have used different magnetic colloidal materials. In our last paper, there is a strong low field effect, but a weak shift in the Frederiks threshold. Here, by contrast, we report the contrary: a weak low field effect but a significant threshold shift. Without the theoretical picture we have developed, these results might appear paradoxical, but now we understand that, in contrast to the experiments discussed in [25], in the present low j case, the low field response is small, and the Frederiks shift is driven by x.

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In the introduction, we discussed the possibility of optical switching triggered by magnetic rather than electric fields, as well as optical sensors for low magnetic fields driven by liquid crystal reorientation. While the magnetic materials used in this experiment are surely not yet suitable for technological exploitation, the 20% reduction in the Frederiks threshold represents considerable progress. Moreover, an increase in j would have the effect both of further reducing the Frederiks threshold and of increasing the low field response. The most obvious route to increasing j is through an increase in the magnetization MS of the individual nanorods. It is somewhat of a surprise that in going from spheroidal nanoparticles to nanorods, the biggest effect has been to increase the coupling rather than to increase the particle magnetization. The suggestion must be that these magnetite nanorods are made up out of many magnetic domains. The low magnetization is the result of a near cancellation of high moments for individual domains. If further improvement in the magneto-optical performance of these suspensions is required, clearly some changes must be made in the synthesis, in such a way as to focus on better control and characterisation of not only the geometry of the magnetite nanostructures, but also of the size and distribution of magnetite domains, which could potentially be achieved, for example, by field induced preparation [22,44]. From an applications point of view, suspensions with individual particle behaviour could serve as active materials for sensing. However, for magnetically switched optical devices a collective response is required. To achieve it, both a high magnetic response from the individual particles and a strong nematic–ferroparticle interaction is required. Future work will also involve the thermodynamic properties of ferronematic samples. We expect, however, that the increased value of the coupling parameter x will be correlated with an increase in the nematic–isotropic phase transition TNI, as is predicted theoretically in systems with high magnetic susceptibility. Such an effect has been observed by Buluy et al. [45], in a study similar to ours, but using different surfactants. Acknowledgments DB and AK gratefully acknowledge financial support from EPSRC, UK (grant EP/F044445/1: ‘‘A hydrothermal route to metal oxide nanotubes: synthesis and energy conversion applications’’). NP, OB and MK acknowledge support through a UKERI EPSRC grant under the KTS scheme and thank Prof. M. Damzen and Midaz Lasers Ltd. for their contribution and support. We are also grateful to G. D’Alessandro, O. Buluy, K.R. Daly, V. Yu. Reshetnyak, and Y. Reznikov for continuing discussions through the course of this work, to Drs Buluy, Reshetnyak and Reznikov for communicating the results of work before publication, and to an anonymous referee for some extremely helpful remarks. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.07.082. References [1] V.G. Chigrinov, Liquid Crystal Devices: Physics and Applications, Artech– House, Boston–London, 1999. [2] F. Brochard, P.G. de Gennes, J. Phys. 31 (1970) 691. [3] S.V. Burylov, Y.L. Raikher, Mol. Cryst. Liq. Cryst. 258 (1995) 107. [4] S.H. Chen, N.M. Amer, Phys. Rev. Lett. 51 (1983) 2298. [5] P. Kopcˇansky´, N. Tomašovicˇová, M. Koneracká, M. Timko, V. Závišová, N. Éber, K. Fodor-Csorba, T. Tóth-Katona, A. Vajda, J. Jadzyni, et al., J. Magn. Magn. Mater. 322 (2010) 3696.

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