Solution Processable Iridescent Self-Assembled Nanoplatelets with

Photonic structures that are processable in organic solvents are critical to large-scale fabrication of device components. To provide a viable alterna...
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Solution Processable Iridescent Self-Assembled Nanoplatelets with Finely Tunable Interlayer Distances Using Charge- and Sterically Stabilizing Oligomeric Polyoxyalkyleneamine Surfactants Minhao Wong,‡,† Ryohei Ishige,‡,§ Taiki Hoshino,∥ Spencer Hawkins,† Peng Li,† Atsushi Takahara,§ and Hung-Jue Sue*,⊥ †

Materials Science and Engineering Program and ⊥Department of Mechanical Engineering, Polymer Technology Center, Texas A&M University, College Station, Texas 77843-3123, United States § Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Erato Takahara Soft Interfaces Project, Japan Science and Technology Agency (JST), 744 Motooka, Nishi-ku, Fukuoka 819-0385, Japan S Supporting Information *

ABSTRACT: Photonic structures that are processable in organic solvents are critical to large-scale fabrication of device components. To provide a viable alternative to traditional lithographic methods, solution processable photonic structures are required to demonstrate fine control over critical device dimensions in the fabrication process. Photonic structures typically require long-range electrostatic forces that are effective only in aqueous solutions. Here we report a novel strategy of using oligomeric polyoxyalkyleneamine surfactants to prepare charge- and sterically stabilized nanoplatelets that can self-assemble into lamellar phases in nonaqueous solutions with finely tunable large interlamellar distances that can exceed 100 nm. Brilliant iridescence in the visible spectrum with tunable colors is demonstrated. The nanoplatelets are shown to circumvent the typical phase transition behavior from isotropic to nematic to columnar phase and transitioned into smectic phase at concentrations of ϕ < 0.01.

C

simplification, in the absence of strong screening effects, colloidal assembly in nonaqueous solutions is much more difficult as the effective range of electrostatic repulsion is greatly reduced due to the lower density of surface charges. Several techniques, such as lowering van der Waals forces through refractive-index-matching solvents6 or adding surfactants resulting in nanosized reverse micelles that interact via electrostatic repulsions7,8 can potentially overcome this limitation. These modifications are known to yield Debye screening lengths κ−1 up to micrometer scale; however, it remains a challenge to create stable and highly ordered photonic structures in organic solvents.9,10 To achieve iridescence, the periodic lengths of colloidal assemblies must satisfy the Bragg condition at visible wavelengths. Photonic structures formed from close-packed assemblies such as the famous Schiller layers11 often display beautiful colors. A limitation of these materials is that very high concentrations of particles, often exceeding a volume fraction ϕ = 0.5, is necessary to create such assemblies.12,13 Moreover, the assembly typically involves sedimentation or evaporation, which

urrent interest in 2D nanomaterials such as graphene, MoS2, etc. stems from the fact that these materials can serve as building blocks of useful photonic and optoelectronic devices.1 In contrast to lithographic methods which require complex instruments and processes, the wet chemistry approach can provide a cost-effective alternative if device dimensions can be controlled precisely. An example of the importance of dimension control is in photonic structures that are critical components in applications such as adaptive color coatings, sensors, color displays, and active optical devices. A nanometer scale change in the critical dimension can result in a dramatic change in iridescence wavelength. Through wet chemistry methods, further modification and processing is also possible in the solution state, allowing the incorporation of photonic structures into a polymer matrix while preserving its iridescence. This is an essential step in the scalable fabrication of 2D nanomaterials/polymer based devices.2,3 It is of particular value if such photonic structures can be prepared in organic solutions as many polymers important to device components or the manufacturing4 of such components are soluble only in organic solvents. The construction of photonic structures by the colloidal assembly of particles is commonly achieved via the control of long-range electrostatic repulsion in aqueous solutions.5 As a © 2014 American Chemical Society

Received: September 6, 2013 Revised: January 18, 2014 Published: January 21, 2014 1528

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Figure 1. α-ZrP nanoplatelets. a, Transmission electron micrograph of irregular hexagons of α-ZrP nanoplatelets. Average size is 625 ± 280 nm measured over 266 particles. b, Field emission scanning electron micrograph of α-ZrP nanoplatelets deposited on a silicon wafer. Scale bars indicate 1 μm.

is generally a slow process.10 Centrifugation may accelerate the formation of these assemblies in some instances12 but restricts the production of the photonic structures in centrifuge tubes. Finally, once assembled, the photonic structures are permanently formed and not processable into other shapes and forms, thus severely limiting their applicability. A more practical approach is to use lyotropic lamellar phases suspended in solutions to create photonic structures. Iridescence has been observed in aqueous solutions of surfactant molecules and inorganic nanoplatelets with highly ordered mesophases.14−16 Inorganic nanoplatelets are typically large monolayer sheets, with high D/L ratios (D = diameter, L = thickness) which can also form mesomorphic lamellar phases in aqueous dispersions and were first observed in H3Sb3P2O14 nanosheets by Gabriel et al.17 Concentrations on the order of ϕ = 0.01 are sufficient to produce brilliant iridescence, representing a significant reduction in the material required. In contrast to the aforementioned methods,6−8 the formation of these photonic structures is spontaneous and fast. The wide range of functional properties available to 2D layered materials such as graphene,18 graphene oxide,19 transition metal dichalcogenides,20 transition metal oxides,21 etc. expands the design space for applications. Not surprisingly, the preparation of iridescent lyotropic lamellar phases in nonaqueous solutions faces the same difficulties of colloidal assembly mentioned above. Achieving iridescence is nontrivial even in aqueous solutions, as a very low ionic concentration is necessary to extend the Debye screening length beyond the typical length on the order of ∼10 nm, requiring a careful and thorough deionization of water. The challenge becomes even more demanding in nonaqueous solutions as large interlayer distances on the order of ∼200 nm are typically needed. Here we report an alternative, simpler method to induce the assembly of photonic structures in nonaqueous solutions. Iridescence arises via the novel use of charged oligomeric polyoxyalkyleneamine surfactants tethered to nanoplatelets. The visible spectrum of colors is produced due to the formation of lamellar phases with d-spacings of 142 to 238 nm under long-range electrostatic repulsion in a low-ionic strength system aided by the presence of a brush layer tethered onto the nanoplatelets. To the best of our knowledge, this is the first demonstration of fine control of interlamellar distance over such a large magnitude in nonaqueous dispersions.14−17,22,23

We chose highly polar, aprotic solvents to enable charge stabilization at the surface of the nanoplatelets. Nonpolar solvents are unsuitable due to their inability to induce charge separation.24 Additionally, as pointed out in the experiments performed by Kleshchanok et al.,25 using such solvents alone may not provide sufficient repulsion between nanoplatelets. To overcome this limitation, we selected charged oligomeric molecules to attach onto the nanoplatelets. Sterically stabilized nanoplatelets have previously been shown to form lamellar phases only at very high concentrations,26 and they typically transition from isotropic to nematic to columnar phase, sequentially.13,26−28 There are two possible effects which may result in a larger d-spacing. First, lowering the local dielectric constant can increase the Debye screening length by reducing the concentration of free ions surrounding each nanoplatelet. This is shown explicitly according to eq 1, κ −1 =

1 4πBn

(1) 2

B is the Bjerrum length given by e /εκBT, where e is the electronic charge, κB is the Boltzmann constant, T is the absolute temperature, ε is the dielectric constant of the solvent, and n is the concentration of free ions in solution. This effect has been shown to increase interparticle distances between polystyrene spheres when the dielectric constant is lowered by adding organic solvents to deionized water.29 Second, free oligomers may be absorbed onto the brush layer, further increasing the thickness of the nanoplatelet and thus increasing d-spacing. α-Zirconium phosphate (α-ZrP, Zr(HPO4)2·H2O) nanoplatelets synthesized through a hydrothermal method30 yielded nanoplatelets with a majority of irregular hexagonal geometry with an average size of 625 ± 280 nm (Figure 1a,b). α-ZrP is a convenient model for other layered materials which can be manipulated by colloidal assembly.18,20,21,30−33 Exfoliation was carried out using a polyoxyalkyleneamine, Jeffamine M1000 (chemical structure: H3C(OCH2CH2)19(OCH2CHCH3)3NH2, Huntsman Chemicals, MW ∼ 1000 g mol−1, hereafter referred to as M1000). Polyoxyalkyleneamines have been shown to be effective in exfoliating α-ZrP nanoplatelets30,34 in highly polar, aprotic solvents such as acetone. Tethering proceeds via cationic exchange of the primary amine group to the phosphate group on the α-ZrP nanoplatelets.35 The successful function1529

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Figure 2. Photonic crystals of nanoplatelets in organic solution. a, Reflection spectra of 625 nm α-ZrP nanoplatelets in butyronitrile at various concentrations. Reflectance peak shifts from red to ultraviolet as concentration of nanoplatelets increases. b, Photographic images of butyronitrile solutions containing α-ZrP nanoplatelets demonstrating colors shifting from violet to red as concentration decreases from left to right.

Table 1. d-Spacings of Iridescent Solutions Calculated from the Platz Equation peak wavelength, nm

332

392

431

488

526

556

592

658

d-spacing, nm

119.9

141.6

155.7

176.3

190.0

200.9

213.9

237.7

then redispersed at different concentrations in various organic liquids. A series of α-ZrP/butyronitrile (dielectric constant = 24.8) suspensions was prepared at different concentrations. The reflectance spectra of these suspensions display shifting sharp peaks representing colors changing from violet to red as the concentration of nanoplatelets was reduced (Figure 2a). UV− vis absorption and reflectance measurements of only M1000 dissolved in butyronite did not detect any peaks in the visible range, therefore confirming that M1000 and butyronitrile do not contribute to the iridescence spectra. A similar series of spectra was reproduced in acetonitrile (Supporting Information Figure S1a). Photographic images (Figure 2b) of the α-ZrP suspensions that were transferred into a custom-made cuvette (2.0 mm optical path length) made from two thin rectangular glass plates (25.4 mm × 15.0 mm × 1.3 mm, Fisher-Scientific) that were double-sealed by solvent-resistant epoxy (Permapoxy 5 min) and silicone (Loctite RTV587 Blue) demonstrate the brilliant colors displayed by the suspended photonic crystals. When enclosed within flat glass cuvettes, the lamellae suspended in solution tend to align parallel to the flat glass surface. In this configuration, most of the iridescence observed corresponds to the Bragg reflection at a scattering angle of 180°. The d-spacing of the lamellae can thus be estimated using the reflectance spectra with the Platz equation14,22 md = λmax/ 2n, where m is the Bragg order number, n is the solvent refractive index (butyronitrile n = 1.384), and λmax is the peak

alization by proton donors such as amines, even in solutions of low basic strength, is due to the relatively strong P−OH Brønsted acid groups in α-ZrP monolayers.34 The intercalation of M1000 into the interlayers weakens the interlayer binding and promotes exfoliation of the monolayers. The dielectric constant of M1000 is not known. Nevertheless, since it consists of mostly polyethylene oxide (PEO) segments (83 wt %) with a minority of polypropylene oxide (PPO) segments (17 wt %), a reasonably close assumption can be made by comparing an analogous PEO oligomer, which has a dielectric constant around 10.36 This value is lower than the dielectric constants of the solvents used in this study which range from 20 to 37. A typical exfoliation procedure is described here. A sample containing 0.5 g of α-ZrP powder was weighed and dispersed in 25 mL of acetone by sonication for 10 min. A 0.6 g mL−1 solution of M1000 in acetone was prepared. A volume of 17.7 mL of M1000 solution was added dropwise to the stirring αZrP dispersion. This dispersion was allowed to stir for 4 h. The dispersion was sonicated for 10 min followed by centrifugation at 10 000 rpm for 15 min. The sediment was removed leaving a clear suspension containing only exfoliated α-ZrP and excess polyoxyalkyleneamine. The clear suspension was centrifuged at 20 000 rpm for 3 h, and the exfoliated α-ZrP was collected as a gel while the excess polyoxyalkyleneamine remained in the supernatant. Unexfoliated nanoplatelets and excess M1000 were removed through this process. The exfoliated α-ZrP was 1530

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Figure 3. USAXS measurements. a, USAXS 2D diffractograms of α-ZrP/butyronitrile suspensions confirms the lamellar structure. Sample numbers given on top left corner of each pattern. b, Lorentz-corrected intensity profiles (I(q) − Ib)q2 (hollow circles) of the patterns in (a) fitted with calculated profiles (red lines). The calculated intensity gives a good fit to the observed data, validating the lyotropic lamellar model.

intensity of a periodic structure I(q) is generally represented by

wavelength. Table 1 summarizes the calculated d-spacings, showing that d-spacing can range from 120 to 238 nm, with the narrower range from 142 to 238 nm responsible for iridescence in the visible spectrum. In order to clarify the detailed structure, we performed ultrasmall-angle X-ray scattering (USAXS) measurements for six suspensions with different concentrations (see Experimental Methods). All USAXS patterns revealed partially oriented sharp, distinct diffraction at the same interval, indicating formation of lamellar mesophases (Figure 3a). The partial orientation is attributed to the shear flow that occurred when the suspension was injected into the X-ray glass tube. The Lorentz-corrected observed intensity profiles (I(q) − Ib)q2 (represented by hollow circles; I(q) and Ib, respectively, are the sector-averaged intensity and thermal diffuse scattering that is independent of q) are well simulated by the calculated intensity profile (red line) (Figure 3b) based on the lyotropic lamellar model proposed by Nallet et al.37 The scattering

eq 2; F(q) is the form factor of the platelet in eq 3 and S(q) is the structure factor in eq 4. I(q) = ⟨F(q)2 ⟩ − ⟨F(q)2 ⟩S(q)

F (q , L ) = L

sin(qL /2) , d (L ) = (qL /2)

(2)

⎛ (L − t )2 ⎞ 1 exp⎜ − ⎟ 2πσ 2σ 2 ⎠ ⎝

∞

∫−∞ d(L)·F(q , L) dL , ∞ ⟨F(q)2 ⟩ = ∫ d(L) ·F(q , L)2 dL −∞

⟨F(q)⟩ =

(3) 1531

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⎞ qd ̅k k⎞ ⎛ ⎟ cos⎜ ⎟ 2 2 N ⎠ ⎝ 1 − 2Δq d ̅ α(k) ⎠ k−1 ⎛ 2q2d ̅ 2α(k) + Δq2d ̅ 2k 2 ⎞ 1 ⎟ exp⎜ − 2 2 ⎝ 2(1 + 2Δq d ̅ α(k)) ⎠ 1 + 2Δq2d ̅ 2α(k) N−1

S(q) = 1 + 2

α(k) =

plane. The SAXS region reveals two groups of lamellar structures with the d-spacings of 5.6 and 7.3 nm (Figure 4c). The absence of the (002) peak shows that, even after drying, the α-ZrP layers do not restack into the pristine configuration but form a lamellar structure supported by M1000 oligomers. Iridescence was also observed in solutions of acetone (dielectric constant = 21.0), acetonitrile (dielectric constant = 36.6), and propionitrile (dielectric constant = 29.7). The α-ZrP concentration in butyronitrile ranges from 0.40 to 1.46 wt %, showing an inverse relation to the shift of reflectance peak λmax (Figure 4d). The Platz equation derived d-spacing was found to exhibit a linear dependence to the reciprocal of nanoplatelet volume fraction, 1/ϕ (Figure 4e). This result is typical of lamellar phases as d = L/ϕ, where L is the thickness of nanoplatelets.44 Identical trends in λmax against α-ZrP wt % and d-spacing against 1/ϕ for acetonitrile solutions are observed (Supporting Information Figure 1b,c). The occurrence of lyotropic mesomorphism is further confirmed by observing the solution under cross-polarized light. Under white light, an acetone solution of α-ZrP nanoplatelets displays a brilliant blue color (Figure 5a). Under cross-polarized light, the same solution displays birefringence fringes (Figure 5b) that coincide with the features seen under white light. Reflectance optical microscopy (Olympus BX 60) of a green α-ZrP/acetone solution reveals the presence of fine strands and globules which are the origin of iridescence (Figure 5c,d). These structures can range from 10 to 70 μm in length. The widths of these structures are fairly constant at about 5 μm suggesting the stacking of lamellae. Assuming that the d-spacing is 200 nm, we estimate that 25 layers of lamellae are stacked in each structure. The affinity of the solvent to the oligomeric brush is crucial to the manifestation of iridescence. α-ZrP nanoplatelets were dispersed in the solvents listed in Table 3 to study the solvent effect on iridescence. α-ZrP nanoplatelets dispersed easily in acetone and nitriles to form iridescent solutions. A transparent solution was formed in toluene while large aggregates remained in an aqueous dispersion at room temperature; iridescence was not observed in either solution. In comparison, M1000 is fully soluble in acetone, nitriles, and toluene, while only partially soluble in water. The Hansen solubility parameters45,46 (i.e., δd, δp, and δh, the cohesive energy contributions from dispersion forces, polarization, and hydrogen-bonding, respectively) of the solvents and PEO in Table 3 explain the solubility behavior. PEO is used to approximate the solubility parameters of M1000, since it has been reported that PEO and PPO are largely similar in terms of solubility parameters.47 It is apparent from Table 3 that the solvents of solutions that displayed iridescence have Hansen solubility parameters very similar to the oligomeric brush, in particular with respect to δd and δp, therefore indicating excellent solvent−brush affinity. Nevertheless, even though toluene is a good solvent for M1000, iridescence did not appear, which can be explained by the dielectric constants of the solvents.48 As mentioned before, a solvent should possess a high enough dielectric constant to allow lamellar phases to form. In this regard, the dielectric constant of toluene is essentially negligible. A thick coat of oligomer was observed on a hexagonal nanoplatelet of 760 nm in size (Figure 6a). The AFM phase image (Figure 6b) reveals a stiff phase at the base of the structure which is attributed to the α-ZrP monolayer and dense brush, with a soft phase forming a thick coat (thickness ∼ 70 nm) (Figure 6c,d). The AFM results appear to be consistent with the hypothesis that each nanoplatelet is covered by a dense

⎛

∑ ⎜⎝1 −

⟨(uk − u0)2 ⟩ 2d ̅ 2

≈

ηk 2d ̅ 2 8 (4)

The theoretical model proposed by Nallet et al. assumes that the lamellar layers, with a mean distance d̅, are subjected to thermal fluctuations such that the kth layer may fluctuate about its equilibrium position k·d̅ by a displacement of uk. The mean square average of the fluctuation, ⟨(uk−u0)2⟩, is proportional to the constant η in eq 4. Nanoplatelets with significantly large D/ L ratio can be easily undulated,38,39 leading to fluctuations even for very stiff layers. This analysis yields the structure factor S(q) given by eq 4. The calculated profile is obtained by plotting out the scattering intensity I(q), which is given as a function of the form factor F(q) (eq 3)40 and structure factor S(q)37 using eq 2. The term σ is the standard deviation of lamellar thickness, L, and is related to the smoothness of the layer and the characteristic interfacial thickness t. The parameters used for simulating the curves are given in Table 2. The weaker intensity Table 2. Model Parameters for Fitting USAXS Measured Intensity Profiles sample

N

d, nm

η, nm

L, nm

σ, nm

1 2 3 4 5 6

5 11 11 3 6 6

126 107.5 73.5 143 131 116.6

0.110 0.060 0.069 0.145 0.105 0.074

3.30 3.40 3.30 3.10 3.30 3.35

1.70 1.50 1.60 1.60 1.50 1.65

of the diffracted beam at higher q and the presence of the thick brush layers are probable causes of error that contributed to the deviation of measured data from the simulated curve in the higher q region observed in some diffractograms. Although the d values varied with the sample concentration, L and σ are almost identical for all samples. The values L ≈ 3.3 nm and σ ≈ 1.6 nm are appreciably large compared with the monolayer thickness of α-ZrP (0.68 nm).30,41,42 It was confirmed through WAXS measurements (see following paragraph) that the nanoplatelets were fully exfoliated. This leaves the dense layer of oligomer brush tethered to the surface of the nanoplatelets as a possible explanation for the large L and σ observed. WAXS of the suspensions detected rings at 0.450 nm, 0.262 nm, 0.172 nm, and 0.152 nm (Figure 4a), corresponding to the (hkl) planes of (100), (110), (210), and (300), respectively, for a pseudohexagonal lattice of α-ZrP layers with lattice parameters a = b = 5.24 Å.43 The absence of the (002) peak suggests that the nanoplatelets are fully exfoliated. Dried suspensions formed a white gelatinous solid that were subjected to simultaneous SAXS/WAXS analysis. Distinct rings at 0.462 nm, 0.382 nm, and 0.264 nm were detected in the WAXS pattern (Figure 4b). WAXS measurement of pure M-1000 crystals indicates peaks at the 0.460 and 0.384 nm positions; thus, the second ring is due to the formation of M1000 crystals and the first ring is likely a combination of the peaks from M1000 crystals and the (100) plane of the pseudohexagonal lattice of α-ZrP, while the third ring is assigned to the (110) 1532

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Figure 4. a, WAXS measurement of an iridescent α-ZrP suspension shows peaks at the 0.450 nm, 0.262 nm, 0.172 nm, and 0.152 nm positions. b and c, Simultaneous SAXS/WAXS measurement of dried iridescent α-ZrP suspension. b, WAXS 2D diffractograms show rings at the 0.462 nm, 0.382 nm, and 0.264 nm positions.Any ring that can be assigned to the (002) plane in unexfoliated α-ZrP layers is notably absent. c, SAXS region displays two groups of rings assigned to two distinct lamellar structures with d-spacings of 5.6 and 7.3 nm, respectively. Any ring that can be assigned to (002) plane in unexfoliated α-ZrP layers is notably absent. d, Peak positions (λmax) of reflectance spectra against α-ZrP wt %, demonstrating the inverse dependence of iridescence color on α-ZrP concentration. e, d-spacing shows a linear dependence on 1/ϕ, as expected from a lamellar phase in solution. d-spacing was calculated using the Platz equation d = λmax/2n.

the height of each nanoplatelet was dramatically reduced from 70 nm to 10 nm. This is likely due to several reasons. The absorbed oligomer was swollen with solvent that collapsed after heating as the solvent was driven off. The heating temperature was above the melting temperature of M1000 (33 °C, verified by a Q20 differential scanning calorimeter, TA Instruments), therefore freeing the oligomers to form a more tightly packed configuration. The disappearance of a soft phase after heating

brush layer followed by a loosely packed absorbed oligomer coat (see discussion below). Analysis of several nanoplatelets shows that all of them are covered by a soft oligomer coat. To verify the existence of a soft oligomer layer that is absorbed onto the nanoplatelets, a silicon wafer with nanoplatelets deposited from a dilute solution was heated to 70 °C overnight before measurement by AFM. In contrast to the nanoplatelets that were dried at room temperature (Figure 6), 1533

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Figure 5. Lyotropic mesomorphism of nanoplatelets. a, Photograph of an acetone suspension containing α-ZrP nanoplatelets enclosed in a glass cell under white light displays brilliant blue color. b, Photograph of same suspension under cross-polarized light revealing birefringent fringes that coincide with the features observed under normal light. The observation of birefringence is due to the presence smectic phases of nanoplatelets. c, Photograph of an acetone suspension containing α-ZrP nanoplatelets enclosed in a glass cell under white light displays a brilliant green color. d, Reflectance optical microscopy reveals the origin of iridescence to be in the fine strands suspended in the liquid. The iridescent strands range from 10 μm to about 70 μm in length and about 5 μm in width.

phase transition sequence from isotropic to nematic to columnar phase,26 resulting in smectic phase formation at very low concentrations.42 This phenomenon poses an interesting problem that warrants further investigation by modeling and simulation studies of their phase behavior. As a concluding remark, our study demonstrates a new route to manipulating mesomorphic structures of 2D crystals to form photonic structures in a nonaqueous environment. With the wide range of 2D crystals available, each possessing unique properties,18,20,21,30,32,33 this technology is expected to be of interest to many different fields of science and engineering.

Table 3. Comparison of Hansen Solubility Parameters and Dielectric Constant of Solvents and Polyethylene Oxide (PEO) Hansen solubility parametersa (MPa1/2 at 25 °C)

a

solvents

δd

δp

δh

dielectric constantb

acetone acetonitrile propionitrile butyronitrile toluene water PEO

15.5 15.3 15.3 15.3 18 12.3 15

10.4 18 12.3 12.5 1.4 31.3 12

7 6.1 8.2 5.1 2 34.2 24

21 36.6 29.7 24.8 2.4 80.1 10

■

EXPERIMENTAL METHODS

Materials. Zirconyl chloride (ZrOCl2 8H2O, 98%, Aldrich), phosphoric acid (85%, EM Science), and acetone (ACS grade, EMD) were used as received. A commercial polyoxyalkyleneamine, H3C(OCH2CH2)19(OCH2CHCH3)3NH2 Jeffamine M1000, with a reported average molecular weight of 1000 g mol−1 (Huntsman Chemical) was used to exfoliate α-ZrP. Synthesis of α-ZrP. A sample of 4.0 g of ZrOCl2·8H2O was mixed with 40.0 mL of 12.0 M H3PO4 and sealed into a polytetrafluoroethylene-lined pressure vessel and heated to 200 °C for 24 h. After the reaction, the product was collected by centrifugation, followed by washing by deionized water and centrifugation two times. The product was collected and dried at 65 °C for 24 h. The dried product was ground into fine powder with a mortar and pestle. UV−Vis Spectroscopy. Reflectance spectroscopy was performed using a Shimadzu 3600 UV−vis−NIR spectrophotometer. The α-ZrP suspensions were transferred into a custom-made cuvette (2.0 mm

45,46

Hansen solubility parameters are based on data from Barton. Dielectric constants of solvents are based on data from Lide48 and PEO from Koizumi and Hanai.36

b

(Figure 7) is consistent with this explanation as a tightly packed layer is harder than a loosely packed one. In summary, our experiments show that it is possible to induce the formation of photonic crystals in polar, aprotic solvents by making use of the lyotropic behavior of nanoplatelets. The tethering of charged oligomeric species on the nanoplatelet proves to be the key modification that allows for such fine control of d-spacings in nonaqueous solvents. The combination of charge- and sterically stabilized nanoplatelets (Figure 8) appears to play a role in circumventing the typical 1534

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Figure 6. Atomic force micrographs of single α-ZrP nanoplatelet (∼760 nm) on silicon wafer. a, Height image shows a layer of oligomer covering the nanoplatelet. b, Phase image clearly shows that the softer oligomer lies on top of a stiffer hexagonal nanoplatelet which forms the base. c, Profile of the nanoplatelet indicates the thickness of the oligomer and nanoplatelet is approximately 70 nm. d, 3D visualization of the oligomer-coated nanoplatelet.

Figure 7. Atomic force micrographs of a single α-ZrP nanoplatelet (∼1000 nm) on silicon wafer after heating at 70 °C. a, Height image shows a layer of oligomer covering the nanoplatelet. b, Phase image shows little phase contrast between silicon background and the nanoplatelet, suggesting the absence of a soft coat; furthermore, the basal nanoplatelet is no longer distinguishable (see Figure 4b). c, Profile of the nanoplatelet indicates the total height of oligomer coat and nanoplatelet is 10 nm. d, 3D visualization of the oligomer-coated nanoplatelet.

optical path length) made from two thin rectangular glass plates (25.4 mm × 15.0 mm × 1.3 mm, Fisher-Scientific) that were double-sealed 1535

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Figure 8. Proposed (I) charge- and (II) steric-stabilization mechanisms to explain the increase in interlamellar spacing. by solvent-resistant epoxy (Permapoxy 5 min) and silicone (Loctite RTV587 Blue). White BaSO4 powder was used as a standard for analyzing the reflectance of the samples. Atomic Force Microscopy. Tapping-mode atomic force microscopy (AFM) was carried out by a Bruker Dimension Icon AFM. The image was acquired in air using a MPP-21120-10 probe (Bruker; tip radius, nominal force constant and resonance frequency are 8 nm, 15°, 3 N m−1 and 75 kHz, respectively). Highly diluted solutions of nanoplatelets were dropped onto a Si wafer cleaned previously by piranha solution (3:1 37% sulfuric acid and 30% hydrogen peroxide mixture; note: piranha solution is highly corrosive and exothermic when mixed with organic matter; proper safety precautions should be taken during handling). The samples were dried at room temperature before scanning. AFM was performed on α-ZrP nanoplatelets that were deposited on a silicon wafer by slow evaporation at room temperature of a highly diluted α-ZrP/butyronitrile solution (∼1 ppm). Electron Microscopy. Transmission electron micrographs (TEM) were obtained on a JEOL JEM-1200Ex. Diluted solutions were placed on a TEM grid with carbon film and dried. Field emission electron microscopy (FESEM) was performed by a JEOL JSM-7500F unit using the Gentle Beam-Lo mode on samples prepared for AFM. Thermogravimetric Analysis. The mass fractions of oligomer and α-ZrP were determined by thermogravimetric analysis (TGA) using a TA Instruments Q500. The solutions were dried at 100 °C and subsequently heated to 900 °C in air at a rate of 20 °C per min and held at 900 °C for 60 min. The solid residue that remains is zirconium pyrophosphate (ZrP2O7), which can be converted to the equivalent αZrP mass fraction via stoichiometric calculations. Ultrasmall-Angle X-ray Scattering. The liquid crystalline structure of the samples was determined by ultrasmall-angle X-ray scattering (USAXS), which was performed at the Japan Synchroton Radiation Research Institute (JASRI) SPring-8 facility located in Hyogo, Japan. Measurement was taken at the BL03XU beamline49 using an incident X-ray with a wavelength of 0.100 nm. Scattered Xrays were detected using an imaging plate and a 4152 mm sample-todetector distance calibrated by the 001 diffraction peaks of silver behenate. The 2D scattering patterns were recorded on imaging plates (Fuji Film Co.), and exposure time was 1.0 or 2.0 s. The scattering vector is defined by q = (4π/λ)sin θ, where λ is the wavelength of the X-rays and 2θ is the angle between the incident X-ray beam and the scattered X-rays. Small-Angle and Wide-Angle X-ray Scattering. The smallangle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) measurements were conducted at the BL40B2 beamline in SPring-8, Hyogo, Japan. The distance between the sample and the detector was 2222 mm calibrated with silver behenate diffractions for

SAXS and 61.7 mm calibrated with CeO2 diffractions for WAXS. The scattering pattern was exposed on an imaging plate (FUJIFILM Co., Tokyo, Japan) with 3000 × 3000 pixels and the pixel size was 100 × 100 μm2 for SAXS and on a CMOS flat panel detector (C9728DK, Hamamatsu Photonics K. K., Shizuoka-shi, Japan) with 3000 × 3000 pixels, and the pixel size was 100 × 100 μm2 for WAXS.

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ASSOCIATED CONTENT

S Supporting Information *

Additional spectra and plots are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.-J.S.). Author Contributions ‡

M.W. and R.I. contributed equally.

Author Contributions

The manuscript was written through contributions of all authors. M.W. and H.-J.S. developed the concept and planned the experiments. M.W. prepared the photonic crystals and performed the reflectance spectroscopy, TEM, and FESEM experiments. S.H. and P.L. synthesized the nanoplatelets. R.I., T.H., and A.T. performed the USAXS experiments and data interpretation. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.W. thanks Jerry Ball for help with AFM, Prof. Luyi Sun for some helpful discussions, and Haiqing Yao for assistance with UV−vis spectroscopy. Partial financial support from Kaneka Corp. and Japan Polypropylene Corp. is highly appreciated. The authors would like to gratefully thank Dr. Junichiro Koike and Masahiko Asada (DIC Corporation) for kindly providing the opportunity to perform the USAXS measurements and Dr. Hiroyasu Masunaga for his assistance in the experiments on the BL03XU beamline. The use of Proposal No. 2013A1470 for providing beam time on the BL40B2 beamline in SPring-8, Hyogo, Japan, is acknowledged. 1536

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on January 31, 2014, with minor text errors. The corrected version was published on February 3, 2014.

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