Nanoparticle Assemblies as Probes for Self-Assembled Monolayer

ARTICLE pubs.acs.org/Langmuir

Nanoparticle Assemblies as Probes for Self-Assembled Monolayer Characterization: Correlation between Surface Functionalization and Agglomeration Behavior Bernhard Feichtenschlager,† Silvia Pabisch,†,‡ Herwig Peterlik,‡ and Guido Kickelbick*,§ †

Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9/165, 1060 Vienna, Austria Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria § Inorganic Solid State Chemistry, Saarland University, Am Markt Zeile 3, 66125 Saarbr€ucken, Germany ‡

bS Supporting Information ABSTRACT: The ordering of dodecyl chains has been investigated in mixed monolayers of phosphonic acid capping agents on the surface of hydrothermally prepared zirconia nanocrystals. Methyl-, phenyl-, pyryl-, and tert-butylphosphonic acids have been used to investigate series with different mixing ratios with dodecylphosphonic acid as the cocapping agent for the mixed monolayer formation. Fourier transform infrared (FTIR) studies revealed that an increasing amount (different for each type) of coadsorbed capping agent reduces the ordering of the dodecyl chains significantly. Small-angle X-ray scattering (SAXS) verified that with increasing amount of cocapping agent the agglomeration of the particles decreases. The strong correlation of the agglomeration behavior with the ordering of the surface-bound alkyl chains leads to the conclusion that interparticle bilayers, formed via long alkyl chain packing, are responsible and can be controlled on a molecular level by coadsorbing various molecules. On the basis of this correlation, nanoparticles can be used as probes for self-assembled monolayer investigation by an indirect structural method (SAXS) and correlated with the routine spectroscopical method for the chemical analysis of surface groups (FTIR).

’ INTRODUCTION Surface modification of nanoparticles with capping agents is the major tool to increase the compatibility between inorganic nano building blocks and their environment.1 These molecular compounds consist of anchor groups that allow for a stable attachment to the nanoparticle surface, a spacer to tune physical parameters such as wettability, and optionally a functional group that permits further chemical interactions with the environment, e.g., by copolymerization during the formation of nanocomposites.2 In many cases the spacer is a long alkyl chain, which increases the compatibility of the often hydrophilic inorganic surface with nonpolar media such as common solvents, monomers, or polymers. It is well-known that such long alkyl chains can form selfassembled monolayers (SAMs) on planar substrates3,4 on the crystal facets of nanoparticles, and even on small facets of clusters.5
8 Such ordered surface structures are expected to be responsible for the formation of alkyl chain bilayer structures between different nanoparticles,9
11 leading to the formation of nanoparticle assemblies and thermodynamically very stable agglomerates.7,12,13 This agglomeration is unfavorable in the development of homogeneous nanocomposites. One major question in this scope is how the thermodynamic driving force for such surface aggregates can be reduced and how their formation affects the nanoparticle agglomeration behavior. A possible method to investigate the influence of self-assembled r 2011 American Chemical Society

monolayer formation is to disorder the surface-bound long alkyl chains by coadsorbing a disturbing molecule to form a mixed monolayer. Such an approach, e.g., to tune the surface wettability, was already investigated for macroscopic substrates by using mixed monolayers of different alkanethiols on gold surfaces to decrease the overall alkyl chain ordering in the surface layer.14
20 Only little is known for the functionalization of metal oxide surfaces, e.g., by mixed phosphonic acid monolayers.21 The idea is to explore whether such a method is transferable to nanoscopic facets of transition-metal oxide nanoparticles and how the mixed monolayer formation affects the SAM ordering structure. A further intention is to evaluate how the alkyl chain modified surface controls the particle agglomeration behavior. Until now the mixed surface modification of nanoparticles has been investigated mostly for mixed alkanethiols on gold nanoparticles, e.g., as an emulsion stabilizing agent,22,23 to control the spacing between deposited particles,24 to fine-tune a certain physical surface property,25 or to dilute a surface-bound functionality for biological applications.26 For technologically highly relevant transition-metal oxide nanoparticles, such as ZrO2 (ceramics, nanocomposites), precise knowledge of mixed surface modification Received: June 20, 2011 Revised: September 2, 2011 Published: November 15, 2011 741

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and the underlying principles is still lacking. However, SAM@ transition-metal oxide nanoparticles play a significant role in tuning their dispersion behavior.27 Therefore, a goal of this study is the production of surface-modified nanoparticles which reveal reduced agglomeration through weak interparticle bilayers and are thus easily (re)dispersed in commonly used solvents. The literature reports mostly short chain n-alkyl capping agents as disordering cocapping agents.14,19 We will present here a systematic study of different organic moieties, such as aromatic systems, linear and branched alkanes, and their influence on the ordering of the particle surface SAM. As model systems hydrothermally prepared ZrO2 nanocrystals were used in combination with organophosphonate capping agents. Dodecylphosphonic acid was selected as the self-assembly system because it is noncrystalline at room temperature.28,29 Contrary to the usually applied octadecyl capping agents, the tendency of these molecules to form very strong interparticle bilayers is reduced, but the molecules still provide a certain hydrophobicity. In a previous work on the formation of SAMs on nanoparticles, we were able to show that crystalline octadecylphosphonic acid systems were qualitatively similar to the noncrystalline dodecyl system with respect to a possible SAM disordering by a mixed modification with a phenyl cocapping agent.30 Therefore, transferability of the results obtained in this study is expected for other long alkyl chain systems. For coadsorbing molecules we used methyl-, phenyl-, pyryl-, and tertbutylphosphonic acid in different mixing ratios with dodecylphosphonic acid to vary the sterical (molecular) properties of the cocapping agent. The monolayer formation and the agglomeration tendency were investigated by applying the Fourier transform infrared (FTIR) spectroscopic method and the small-angle X-rayscattering (SAXS) structural method.

62.80 MHz). Solid-state NMR spectra were recorded on a Bruker Avance DPX 300 instrument equipped with a 4 mm broad-band magic-angle spinning (MAS) probe head operating at 75.40 MHz for 13 C and at 121.39 MHz for 31P. The 13C spectra were recorded with ramped cross-polarization (CP)/MAS and 31P spectra with high-power decoupling (HPDEC) at a rotor frequency of 8 kHz. Elemental analysis was carried out at the Microanalytical Laboratory of the University of Vienna. Powder X-ray diffraction (XRD) measurements were carried out on a Philips X’Pert Pro instrument with Cu Kα radiation with a Bragg
Brentano arrangement and an angle speed of 6°/min where the sample was carried on Si single crystals under ambient conditions. Transmission electron microscopy (TEM) images were recorded on an FEI TECNAI G20 transmission electron microscope. High-resolution TEM (HRTEM) images were recorded on a FEI TECNAI F20 microscope. The samples were attached to Formvar copper grids by dispersing them in ethanol using an ultrasound cleaning bath, adding one drop onto the copper grid, and evaporating the solvent. Dynamic light scattering (DLS) measurements were carried out by noninvasive backscattering on an ALV/CGS-3 compact goniometer system with an ALV/LSE-5003 correlator and multiple τ correlator at a wavelength of 632.8 nm (He
Ne Laser) at a 90° goniometer angle. The dispersing media were purified before use with a syringe filter (200 nm mesh). The determination of the particle radius was carried out by the analysis of the correlation function via the g2(t) method followed by a linearized mass-weighting (mw) of the distribution function. SAXS measurements were performed under vacuum in transmission geometry using a rotating anode X-ray generator equipped with a pinhole camera (Nanostar, Bruker-AXS). Cu Kα radiation was monochromatized and collimated from crossed Goebel mirrors and detected with a 2D position-sensitive detector (Vantec 2000). Measurements were carried out at two different distances (13 and 108 cm) to cover a wide q range. All SAXS patterns were radially averaged and corrected from background scattering to obtain the scattering intensities dependent on the scattering vector q = (4π/λ) sin θ, where 2θ is the scattering angle and λ = 0.1542 nm the X-ray wavelength. A centrifuge of the type Hettich EBA 20 S with an 86 mm rotor radius was used for the nanoparticle separation and purification. Synthesis of Dodecylphosphonic Acid. The capping agent dodecylphosphonic acid was synthesized as described in a previous publication30 via an Arbuzov reaction and hydrolysis of the obtained phosphonate via application of literature procedures.33,34 15.6 mL (64 mmol) of 1-bromododecane and 12.4 mL (74 mmol) of triethyl phosphite were stirred for 3.5 h under reflux. The excess triethyl phosphite was removed at 100 °C and 12 mbar, and the remaining intermediate product was afterward refluxed with 70 mL of concentrated HCl for 22 h. The crude product was obtained as a white solid after concentration of the reaction mixture to 20 mL. The product was washed several times with acetonitrile and dried in an oil pump vacuum. It was further recrystallized from hexane and again dried in an oil pump vacuum. Yield: 9.5 g (38 mmol, 59% yield) of colorless crystals. 1H NMR (CDCl3): δ (ppm) = 0.84 (t, 3H, CH3), 1.26 (m, 18H, CH2), 1.55
1.82 (m, 4H, CH2CH2P), 8.56 (s, 2H, POH). 13C NMR (DMSO-d6): δ (ppm) = 31.83, 30.52, 29.68
29.62 (6C), 26.63, 22.99, 22.58 (CH2), 14.22 (CH3). 31P NMR (CDCl3): δ (ppm) = 39.5. IR (ATR): ν (cm
1) = 2954 (CH3), 2917 (νas, C
H, CH2), 2871 (CH3), 2849 (νs, C
H, CH2), 1469 (C
H), 1212 (PdO), 1003 (P
O), 942 (P
OH). Synthesis of 1-Pyrylphosphonic Acid. The synthesis of 1-pyrylphosphonic acid was carried out using a metal catalyst as described in the literature for the coupling of aryl halides with triethyl phosphite.35 A 1.5 g (5.3 mmol) portion of 1-bromopyrene was mixed with 50 mg (0.53 mmol) of anhydrous NiCl2 and 3.4 g (15.9 mmol) of triisopropyl phosphite. The excess triisopropyl phosphite was then removed in vacuum, and the

’ EXPERIMENTAL SECTION Materials. All solvents (HPLC grade) and chemicals were purchased from Sigma-Aldrich. Methanol was purified using a PureSolv (Innovative Technology Inc.) solvent purification system. Phenylphosphonic acid was recrystallized from acetonitrile. All other chemicals were used as received. Instrumental Analysis. Nitrogen sorption measurements were performed on a Micromeritics ASAP 2020 instrument. The samples were degassed under vacuum at 60 °C for at least 8 h prior to measurement. The surface area was calculated according to Brunauer
Emmett
Teller (BET) assuming a demand of 0.162 nm2 per N2 molecule.31 Thermogravimetric analyses (TGA) were performed on a Netzsch Iris TG 209 C in a platinum crucible heated from room temperature to 900 °C with a heating rate of 10 °C/min under synthetic air. The grafting densities F (molecules/nm2) were calculated using the following formula:32 F¼

Δm 1 NA  10
18 MR SBET

ð1Þ

where Δm is the mass loss from TGA between 200 and 800 °C (g/g). This is feasible because the onset of the thermal desorption for all used capping agents was higher than 250 °C. MR is the molecular mass of the organic moiety without a phosphonate group (g/mol), SBET is the N2 surface area of the bare metal oxide sample (m2/g), and NA is the Avogadro constant. FTIR spectroscopy measurements were performed on a Bruker Tensor 27 spectrometer under ambient air (64 scans at a resolution of 1 cm
1) in transmission mode using KBr (Aldrich) disks as a sample matrix. Liquid-state NMR spectra were recorded on a Bruker Avance 250 spectrometer (1H at 250.13 MHz, 31P at 101.26 MHz, 13C at 742

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remaining solid was dissolved in toluene, washed twice with water and with concentrated NaCl solution, and dried over Na2SO4. After removal of the solvent, the remaining solid was washed with cold toluene and dried to give 1.8 g (4.9 mmol, 92% yield) of brown crystals of 1-pyrylphosphonic acid isopropyl ester. 1H NMR (CDCl3): δ (ppm) = 1.45 (d, JHH = 6.2 Hz, 12H, CH3), 4.73
4.81 (m, 2H, CH), 8.11
8.25 (m, 11H, CH aromatic). 13C NMR (CDCl3): δ (ppm) = 128.7 (Cquart), 127.3, 126.3, 126.1 (15C, aromatic CH), 70.3 (isopropyl CH), 24.7 (CH3). 31P NMR (CDCl3): δ (ppm) = 29.4. IR (ATR): ν (cm
1) = 3046, 2973, 2931 (CH), 1247, 1237, 1041 (PdO), 992, 964 (P
O). Anal. Found: C, 71.8; H, 6.2; P, 8.5. Calcd: C, 71.72; H, 6.84; P, 8.41. For the hydrolysis of the phosphonate, 1.8 g (4.9 mmol) of the 1-pyrylphosphonic acid isopropyl ester was refluxed in 15 mL (180 mmol) of concentrated HCl and 15 mL of acetone for 1.5 days. Then all liquid components were removed in vacuum. The brown residue was then washed with H2O and dissolved in ethyl acetate and THF (1:1). This mixture was then again washed three times with water and with boiling cyclohexane. Recrystallization from ethanol gave 730 mg (1.9 mmol, 39% yield) of pure product. 1H NMR (MeOH-d4): δ (ppm) = 8.03
8.40 (m, 11H, CH aromatic). 13C NMR (MeOH-d4): δ (ppm) = 130.4 (Cquart), 129.9, 129.8, 129.1, 126.9, 126.5, 126.2, 126.0, 125.8 (15C, CH aromatic). 31P NMR (MeOH-d4): δ (ppm) = 27.9. IR (ATR): ν (cm
1) = 3041 (CH), 1215, 1187, 1185, 1088, 1035, 970, 953, 919, 848. Anal. Found: C, 66.5; H, 4.9; P, 11.2. Calcd: C, 67.61; H, 4.61; P, 10.90. Preparation of Zirconia Nanocrystals. The preparation and characterization of ZrO2 nanocrystals with a 22 nm spherical equivalent diameter, from mass-weighted distribution (18 nm, number-weighted), according to a literature procedure36 is described elsewhere.30 Nitrogen sorption plots of this system are presented in our previous work, justifying the use of eq 1 for grafting density estimation from TGA and surface area data.30 Zirconia nanoparticle surface modification reactions were performed as described in a previous publication,30 analogous to the wellknown process for titania surface functionalization.37 First, 5 mL of a 10 g/L aqueous nanoparticle dispersion was prepared by dispersing the zirconia nanoparticle powder for 30 min in an ultrasonic bath. Afterward, concentrated HCl was added dropwise to the dispersion to adjust the pH to 2. The phosphonic acid, e.g., 18.7 mg of dodecylphosphonic acid for a 7.5 mM total capping agent concentration, was dissolved in 5 mL of methanol and added to the particle dispersion, and the resulting mixture was stirred for 2 days. The particles were then isolated via centrifugation at 6000 rpm, washed three times with ethanol (centrifugation at 6000 rpm), and dried over P2O5 at 5 mbar for 24 h. The following are data for dodecylphosphonic acid@ZrO2. IR (ATR): ν (cm
1) = 2921 (νas, C
H, CH2), 2851 (νs, C
H, CH2), 1464 (C
H), 1034 (P
O, br). 13 C solid-state NMR: δ (ppm) = 30.0
19.5 (CH2), 15.0 (CH3). 31P solid-state NMR: δ (ppm) = 27.0 (br s). TGA: Δm(200
800 °C) = 14.5%, onset 268 °C. Anal.: C, 12.95; H, 2.45; P, 2.3.

Here G and B are numerical prefactors, p is referred to as the Porod exponent, Rg is the gyration radius, and erf(x) is the error function. The equivalent radius of spherical particles Rs is related to the radius of gyration by Rs = Rg(5/3)1/2. For weakly agglomerated systems, the structure function S(q)41,42 SðqÞ ¼

’ RESULTS Zirconia Nanocrystals. Hydrothermally grown ZrO2 nanocrystals have been used as model systems for SAM formation on nanoscopic crystal facets. A detailed characterization of the nanoparticles is given in previous papers.30,45 The particle powders consist of 100% monoclinic ZrO2 (baddeleyite) and exhibit an equivalent spherical diameter of 18 nm (numberweighted, peak value; 22 nm from mass-weighted distribution) in aqueous dispersion with the size distribution function showing a unimodal, narrow shape with a 20% relative peak width at halfmaximum. The particle powders are highly redispersible in water. After ultrasonication for 5 min and an equilibration of the suspension for 30 min, no second fraction in the particle size distribution could be detected by DLS, which would indicate stronger agglomeration. Equivalent spherical particle diameters from TEM (6 nm) and SAXS (6 nm) are significantly smaller than that from DLS because of the different methods and mathematical models used. One reason is the nonspherical nature of the particles displaying an aspect ratio of ∼2 but also the hydrodynamic shell of the particles detected by DLS from aqueous dispersions.45 Another reason for this significant deviation can be that a necessary prerequisite for DLS is to measure a dispersion which is free from agglomerates. As the dry nanopowders have been redispersed in water for DLS measurement, a low quanitity of agglomerates or aggregates or a small fraction of slightly larger particles can be present during the DLS measurements, which is not detectable in the form of another fraction but falsifies the total particle size. Characterization by TEM and HRTEM (Figure 1) reveals the single-crystal nature of the particles, which was also proven by applying other techniques.30,45 Self-Assembled Monolayer Formation on the Nanoparticle Surface. In a previous study we were able to prove that the disorder in dodecylphosphonic acid SAMs increases with an increasing amount of phenylphosphonic acid.30 This can be observed by a typical shift of the methylene C
H vibration of the C12 chain segments to higher wavenumbers with the

Small-Angle X-ray Scattering. The agglomeration behavior of the mixed monolayer end-capped nanoparticle powders was studied via SAXS using the following model to describe the agglomerated particle system. A unified equation for the scattering intensity I(q) has been proposed by Beaucage38
40 and consists of two functions, one based on Guinier’s law and the other on the structurally limited power law:


q2 Rg 2 IðqÞ ¼ @G exp 3

!

ð3Þ

describes the interference of the scattering of particles, containing the function G(2qRHS) being defined by Kinning and Thomas.41 Two additional parameters are used, the hard-sphere radius RHS, which gives the correlation distance of particles within a cluster, and a mean hard-sphere volume fraction η, which gives the probability of finding neighboring particles. The expression for S(q) was derived from the Percus
Yevick approximation.43 Fitting was performed by the software Mathematica. The hard-sphere volume fraction η is a numerical value describing quantitatively the degree of agglomeration of the modified zirconia nanoparticles: the higher the η, the stronger the tendency of the particles to agglomerate. From the hardsphere volume fraction, the actual volume fraction ηs is calculated by ηs = η/(RHS/Rs)3.44 The Beaucage model with a Porod exponent of 4 describes a particle with a specific radius of gyration. It is combined in this work with a structure factor obtained from a hard-sphere model to determine the interaction of particles.

’ THEORY

0

1 1 þ 24ηGð2qRHS Þ=ð2qRHS Þ

pffiffiffi #p 1 ðerf ðqRg = 6ÞÞ3 A SðqÞ þ B q "

ð2Þ 743

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Figure 1. Representative TEM image of ZrO2 nanocrystals (baddeleyite) (a) and HRTEM detailed image showing the singlecrystalline nature of two exemplary nanoparticles.

Table 1. List of the Studied Surface-Modifying Agents Figure 2. Thermogravimetric analysis curves of mixed modified nanoparticle powders of DPPA/PyPPA in different molar ratios.

DPPA/PhPPA, 2.4 ( 0.2 molecules/nm2 for DPPA/PyPPA, and 2.7 ( 0.6 molecules/nm2 for DPPA/tBuPPA, with a slight tendency to lower grafting densities for higher cocapping agent concentration within one series. The maximum grafting density for phosphonates at a planar metal oxide surface was reported to be 4.2 molecules/ nm2.46,47 One major reason for this difference between our found values and literature values is that Fadeev and Helmy, who report grafting desities from 4.2 to 4.8 molecules/nm2, assumed a demand of a N2 molecule during the nitrogen sorption experiments of 0.135 nm2, where we assume a 0.162 nm2 demand in our model. This results in a higher specific surface area and thus in lower grafting densities, by a factor of 0.83, in our work compared to the work of Fadeev and Helmy.46 Including this factor, the maximum coverage values for our systems still differ from this theoretical value for chemical reasons (OH surface groups, available binding sites for the anchor groups) and/or physical reasons (morphology effects; e.g., corners, edges, and other defects do not allow a uniform coverage of the particles) as already discussed in our previous publication.30 Furthermore, a systematic error for all series has to be taken into account, i.e., how precisely the BET surface value describes the surface area of the nanoparticle which is accessible for functionalization. This could be due to the fact that during the sorption measurement some parts on the surface are not accessible to nitrogen in the dried powder, but for the capping agent in the reaction dispersion. The formation of solid
solid interfaces is very likely when a colloidal system is isolated into air and additionally when it is put under vacuum as during the N2 sorption measurement.48 The generation of such interparticle interfaces can decrease the area accessible to N2. Thus, the coverage values could be underestimated by the BET method. However, this error is considered to be low, in the range of the measurement accuracy, because the nitrogen adsorption/desorption isotherm together with the developed pore size distribution shows only the typical mesopores resulting from the space between the packed particles.30 All mixed monolayers reached a slightly higher surface coverage than a pure DPPA monolayer. A possible explanation for this behavior is that the cocapping molecules bind to sites where DPPA is not able to attach, such as sterically not approachable locations on the surface. For PyPPA mixed with DPPA the lowest coverage compared to that of other mixed series was obtained, most likely also due to sterical reasons. The surface coverage for all mixed series is in a comparable range, which is assumed to be the maximum reachable coverage for each of

addition of a disturbing cocapping agent in IR spectroscopy. In the present study we extend the concept of nanoscopic surface disorder to other organophosphonates (Table 1) and show systematic trends depending on the molecular structure of the disturbing molecules. The influence of the sterical demand of the coadsorbing molecules was investigated by applying methyl- and tert-butylphosphonic acid as coadsorbing capping agents. Aromatic capping molecules were used to study the effect of potential π
π stacking phenomena (pyryl- and phenylphosphonic acid) on the C12 chain ordering. The functionalization reaction of the ZrO2 nanoparticles with a mixture of DPPA and order-disturbing capping agent to form a mixed monolayer on the nanoparticle surface was carried out by mixing the capping agents in a certain molar ratio (10, 30, 50, 70, and 90 mol % DPPA) to obtain a total concentration of 7.5 mM, which is the threshold concentration for a monolayer coverage within this specific system.30 Due to the fact that the reactivities of both capping agents are similar, a preferential addition on the nanoparticle surface was excluded. The surface coverage (molecules/nm2) of all samples was calculated from the mass loss from TGA due to the degradation of organic species in combination with the surface area of the particles obtained by BET measurements (140 m2/g). For mixed modified samples the coverage degrees were estimated by applying eq 1 by using an average molecular weight of the organic residue MR, resulting from the molecular weights of the two different capping agents and the mixing ratio. The coverage values were 2.1 ( 0.2 molecules/nm2 for pure DPPA, 2.7 ( 0.9 molecules/nm2 for DPPA/MPPA, 2.8 ( 0.6 molecules/nm2 for 744

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Table 2. Methylene C
H Vibration νas of the Dodecyl Chain for Different Percentages of Cocapping Agent on the Zirconia Nanoparticle Surface

a

[DPPA] (mol %)

rest MPPA νas(cm
1)

rest PhPPAa νas (cm
1)

rest PyPPA νas (cm
1)

rest tBuPPA νas (cm
1)

100

2921.9

2921.9

2921.9

2921.9

90 70

2922.6 2923.4

2922.0 2922.0

2922.7 2923.1

2922.6 2922.6

50

2923.1

2922.6

2923.7

2923.7

30

2923.0

2923.6

2923.8

2924.6

10

2924.4

2924.9

2924.1

2926.3

This series has been successfully reproduced with an average random error (standard deviation) of 0.3 cm
1. The deviation was always < 0.5 cm
1.

Figure 3. Scattering intensities from SAXS measurements for (a) MPPA@ZrO2, (b) PhPPA@ZrO2, (c) PyPPA@ZrO2, and (d) tBuPPA@ZrO2.

the mixed systems. The thermogravimetric analysis curves of the DPPA/PyPPA system (Figure 2) confirm the assumption that the molar ratio of capping agents in the reaction mixture controls the amounts of molecules on the nanoparticle surface after functionalization. This is feasible because the mass loss due to thermal degradation of the different bound capping molecules occurs at sufficiently different temperatures (onset of decomposition step: DPPA@ZrO2, 273 °C; PyPPA@ZrO2, 470 °C) so that the mass losses of the two different molecules can be distinguished. The composition can be estimated from the diagram in Figure 2 using 198 g/mol for the pyryl residue and 168 g/mol for the dodecyl residue to calculate mole percent from the weight percent composition obtained by the TGA measurements. For the sample containing 10 mol % DPPA and 90 mol % PyPPA in the reaction mixture, 11 mol % DPPA and 89 mol % PyPPA could be found on the particle surface by this

thermogravimetric method. For the other mixing ratios, a similar trend was observed (Figure 2), but the composition could not be determined as the DPPA degradation was not finished when the PyPPA degradation started. TGA cannot be applied to study the composition for mixed monolayers with other cocapping molecules because the decomposition steps for the different residues occur in the same temperature range. However, it is assumed that the recovery of the mixing ratio is similar for the other different cocapping agent series as our previous experiments with mixed monolayers of PhPPA and DPPA on this system indicate.30 FTIR Investigation of the Mixed Self-Assembled Monolayer Structure. The degree of ordering of the surface DPPA molecules has been investigated by applying FTIR spectroscopy. Self-assembled monolayer formation was detected via the shift of the long alkyl chain methylene group C
H vibrations to lower 745

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Table 3. Hard-Sphere Volume Fraction η of Zirconia Nanoparticle Powders with Different Percentages of Cocapping Agent at the Surface [DPPA]

rest

rest

rest

rest

(mol %)

MPPA η

PhPPA η

PyPPA η

tBuPPA η

100

0.128

0.128

0.128

0.128

90

0.116

0.105

0.112

0.117

70

0.096

0.107

0.082

0.103

50 30

0.097 0.078

0.079 0.055

0.104 0.100

0.050 0.049

10

0.036

0.035

0.050

0.040

0

0.025

0.025

0.025

0.026

wavenumbers with decreasing number of gauche defects in the alkyl chain. This indicates a higher ordering and thus a more dense chain packing in the SAM,49,50 with the asymmetrical CH stretching mode being the most sensitive and significant.46 Four sample series were obtained with DPPA and MPPA, PhPPA, PyPPA, or tBuPPA as the cocapping agent in different ratios. The FTIR spectra of the C
H stretching region are included in the Supporting Information (Figure SI1). The frequencies of the asymmetric methylene C
H stretching vibration for the dodecyl chain in the SAM for every molar percentage of a certain disturbing capping agent are listed in Table 2 and summarized in Figure 4. SAXS Investigations of the Agglomeration Behavior of Mixed Modified Nanoparticles. Parts a
d of Figure 3 show the scattering curves of the four different mixed surface layer nanoparticle powders, one diagram for each disturbing cocapping molecule. Qualitatively visible from these scattering curves is the trend that the shoulder around q = 1 nm
1, representing the nanoparticle dimension, is more pronounced with an increased packing of particles per volume fraction. The peak at q = 12.3 nm
1 originates from the diffraction at the ZrO2 (100) crystal planes. We could not use XRD to determine the degree of order, as one would expect a short-range order signal from the alkyl chains in the range from about q = 11 nm
1 to q = 13 nm
1 (∼0.47 nm distances for crystalline paraffins and ∼0.58 nm for typical alkyl SAMs4) which is unfortunately masked by the much stronger crystalline zirconia peak. The hard-sphere volume fraction η as a numerical parameter calculated from the scattering curves for the degree of agglomeration is listed in Table 3 for all series. All fit parameters as well as the results for the corresponding actual volume fraction ηs are found in the Supporting Information.

Figure 4. Methylene C
H vibration νas of DPPA@ZrO2 representing the alkyl chain ordering degree at different percentages of DPPA in the mixed monolayer on the nanoparticle surface. The rest to 100% coverage is the capping agent mentioned in the diagram. Lines are drawn to guide the eye.

different so that the whole array of curves appears sigmoidally shaped. This results from the observation that the molecules tBuPPA and PyPPA lead to a relatively high disordering already at low comolecule percentages, whereas molecules such as MPPA and PhPPA show (compared to a linear trend) a relatively higher increase in disordering in the higher comolecule percentage region. The first conclusion from the maximum shift from the FTIR experiments (Figure 4) is that bulky molecules such as tBuPPA have a stronger effect on disturbing the alkyl monolayer ordering in comparison to smaller molecules such as MPPA. However, the shape of the curve in Figure 4 can only be explained by identifying the structure of each mixed monolayer. Thereby, two structure types are assumed to be present within these mixed monolayers, already reported in the literature for nonfully covered long alkyl chain monolayers.32,51,52 In one type, different molecules are randomly mixed to form collective monolayers. This is analogous to incomplete coverage of substrates, which was already observed for C18 phosphonates46 and organosilane capping agents.32 In the second type, the molecules form island structures. This was observed, e.g., for octadecylphosphonic acids on mica52 and has been intensely investigated for mixed monolayers of thiols on gold surfaces: Whitesides et al. discuss completely mixed regimes and separate island-type regimes additionally into micro- and macro-phase-separated systems.20 A formation of preordered structures in solutions, which then adsorb to the nanoparticle surface, also might be responsible for the separation and can be observed for organosilanes but also for organophosphonates.46,53 Of course, a gradual mixture of these extreme cases is more probable for the systems investigated in this work. Furthermore, a general lower tendency to strong island formation is expected within the investigated systems because the C12 chains used in our studies do not show the same pronounced high ordering tendency as C18 chains.54 Island growth seems to be preferred if the interactions between two identical molecules are more favored compared to interactions between two different molecules: Examples for such higher interaction forces can be the sum of van der Waals interactions over a long chain, π
π interactions of extended aromatic systems, strong polar or ionic interaction, or hydrogen bonds.3,4

’ DISCUSSION Investigation of the Mixed Self-Assembled Monolayer Structure. The FTIR investigations (Figure 4) reveal that, for

each type of disturbing molecule (MPPA, PhPPA, PyPPA, tBuPPA), the ordering of dodecyl chains decreases with increasing percentage of these cocapping agents. This is concluded from the shift of its methylene vibrational band to higher wavenumbers.30 Though each of the various molecules lead to a decrease of the alkyl chain ordering with increasing molar percentage, the degree varies from molecule to molecule. All four curves consist of an approximately linear area in the middle mixing ratio, which means a content of cocapping agent of 30
70%. At lower and higher percentages, the behavior is 746

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Figure 6. TEM image (a) and detailed TEM image (b) of agglomerates of 100% DPPA@ZrO2 nanocrystals.

Figure 5. Hard-sphere volume fraction η representing the agglomeration of the mixed modified zirconia nanoparticles using different capping agents for mixed monolayer formation, applied in various mixing ratios. The residual percentage to 100% coverage is the capping agent mentioned in the diagram legend. Lines are drawn to guide the eye.

Using these structural models, the difference in the ordering behavior depending on the mixing ratio can be explained in the following way: Small and weak interacting molecules such as MPPA show a linear-like disordering behavior with increasing molar percentage because of the presence of a randomly mixed SAM for all mixing ratios. Additionally, as Offord and Griffin have suggested for mixed organosilane surface layers, the longer alkyl chains can form ordered structures around these small molecules with the chain packing not being dramatically affected at lower disturbing molecule surface concentration.55 Another example for a system showing such a behavior is the mixture of short and long alkanethiol molecules on gold.14
16,19 The described behavior would not be the case for the sterically far more demanding molecule tBuPPA. Thus, it can be observed that this molecule leads to a sharp increase in disordering at higher molar ratios. The larger but planar molecule PyPPA does not cause a very strong increase of disordering of alkyl chains with increasing mixing ratio of PyPPA to DPPA. However, the slope of the PyPPA trend curve at lower concentrations is the highest compared to those of all other molecules. A probable explanation is the existence of randomly mixed monolayers at low concentrations. The bulky molecules can effectively disturb the C12 SAM formation to change to an islandlike regime for higher concentrations of PyPPA, while the C12 chains can still be moderately ordered in DPPA@ZrO2 island regions. A similar result was observed and discussed by Whitesides et al. for mixed monolayers of alkanethiols with different chain lengths on gold. They showed that the ordering disturbance is most effective for randomly mixed systems.20 Similarly, Prado and Neves showed for mixed C8 and C18 chain phosphonates that a complete molecular homogeneous layer is formed, and the chains above a length of 8 carbon atoms are expected to be disordered.21 In another example, Vercelli et al. have proven a close-packed island structure via cyclic voltammetry for a system where (hexylferrocenyl)phosphonic acid and dodecylphosphonic acid were adsorbed to indium tin oxide (ITO).56 The latter is very likely to happen also in the investigated DPPA/PyPPA mixed layers at higher PyPPA concentrations because strong π
π interactions are energetically beneficial between neighboring PyPPA

Figure 7. Schematic drawing of two particle facets in particle agglomerates of 100% DPPA@ZrO2 (left) and mixed modified particles (right).

molecules. Finally, the PhPPA molecule represents a mixture of all these effects. Until now, mostly the effect of mixing alkyl chain capping agents of different chain lengths on the long alkyl chain ordering has been investigated.14
20 We have demonstrated that a variety of non-alkyl chain moieties can also be applied for this disordering approach, even on the facets of small nanoparticles, where in most of our investigated cases a random mixture is expected because of the high effectiveness of disturbing the ordering by the cocapping agent. Agglomeration Behavior of Mixed Monolayer Modified Nanoparticles. The hard-sphere volume factor obtained from SAXS describes the agglomeration behavior of the nanocrystals. It decreases by nearly 1 order of magnitude with increasing concentration of cocapping agent, regardless of which capping agent was used (Figure 5). The trend appears to be in general linear for the various molecules, but differs for the respective mixed SAM system. The behaviors at very low and very high mixing ratios are rather similar for each comolecule, but considerable deviations are observed at a 1:1 mixing ratio. At 50% cocapping agent addition, the tendency to avoid agglomeration in descending order is tBuPPA, followed by PhPPA, PyPPA, and MPPA. The TEM images of nanopowders capped with 100% DPPA@ZrO2 powder (Figure 6) give an impression of the degree of agglomeration for an η value of 0.13. The crystals are closely packed, and a spacing of approximately 1
2 nm between the particles is visible, which may originate from the interparticle bilayers (Figure 7). The interaction between the decorated facets of the ZrO2 nanocrystals can lead to ordered bilayers, more or less ordered interdigitation, or less ordered arrangements. The mixed modified particles presented here are less agglomerated in the macroscopic powder assembly and thus are considered to be 747

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easier to deagglomerate using common approaches such as soft ultrasound. This redispersibility enhancement is from high technological relevance and has been shown in our previous work.30 The surface properties, such as wettability with different solvents and hydrophobicity, also change with coadsorption of other molecules. Therefore, an optimum composition has to be found for the dispersion of particles in a specific environment. The fact that nanoparticles with a higher content of long alkyl chains on the surface are more strongly agglomerated can be explained by the so-called “zipper effect” of the surface-bound long alkyl chains. As shown in the scheme of Figure 7, a bilayer structure is formed between two particles. The system is nearly close packed, and thus, the agglomerates are difficult to break. This is a well-known phenomenon for C18 capping agents, but it also occurs for C12 chains.29 The alkyl chain packing significantly affects the dispersibility of such particle powders and dispersion stability.30 The hypothesis that a decreasing alkyl chain zipper effect is responsible for a decrease in agglomeration with the formation of a mixed monolayer is also supported by the observation from SAXS that the particles exclusively modified with 100% cocapping agent all show nearly the same agglomeration behavior. Regardless of the sterical demand of the used capping agent (MPPA or PyPPA), the η values are nearly

identical at this point. Therefore, it can be excluded that the observed hard-sphere volume fraction change in the mixed samples results just from different distances of the particles in the agglomerates. It seems that a general property is changed, which determines the strength of the interaction between two crystal facets of two different particles “zipped” together. This effect has been investigated in detail for (nonmixed) alkanethiols of different chain lengths on Au clusters by Terrill et al.11 They show a correlation between the degree of chain packing and particle agglomeration behavior using differential scanning calorimetry (DSC), SAXS, and atomic force microscopy (AFM) data, concluding that a zipper effect can also be strong for smaller particles because of their tendency for surface energy minimization. This effect is also described for gold nanoparticles with a surface area of approximately 100 m2/g by Lennox et al.,5,57 where they describe an agglomeration structure where the interparticle alkyl chains are highly ordered compared to the dangling chains which are not participating in the “zipper”. For larger facets we consider this effect, as shown in this work, also to be very strong, because over larger facets, the number of interacting alkyl chains is higher. The next point to discuss is why the trend of a decreasing zipper effect is different for the different used molecules. The difference in the hard-sphere volume fraction between the different series is the highest at a 50% mixing ratio. Furthermore, a linear-like trend for disordering with the molar percentage of cocapping agent would be expected for a total randomly mixed surface layer, but deviations can be observed. The explanation for these differences can again be found by taking two mixing regimes (islandlike and randomly mixed) into account, where molecules such as PyPPA tend more to an islandlike SAM and molecules such as tBuPPA tend more to a randomly mixed layer. Until now, this zipper effect has been investigated in the literature mostly for thiol@gold nanoparticles,5,11,57 and less is known for oxidic nanopowders. One example was published by Sahoo et al.,27 who attached hexadecylphosphonic acid to magnetite nanoparticles. For inorganic nanoparticle systems, a mixed modification approach is mostly used to tune surface properties such as polarity.22 In our work we, using this facile mixing approach, fine-tune the surface SAM ordering and thus the agglomeration behavior, not only via different mixing ratios, but also by using different coadsorbents, physically all of nonpolar nature. Correlation of Surface SAM Ordering and Particle Agglomeration Behavior. Can the molecular parameter, namely,

Figure 8. Correlation of hard-sphere volume fraction η from SAXS and attached DPPA methylene chain asymmetric vibration from FTIR of mixed modified ZrO2 nanoparticle powders. Lines are drawn to guide the eye.

Scheme 1. Surface SAM Disordering and Mixed Modified Particle Agglomeration Behavior Depends on the Nature of the Organic Moiety of the Used Cocapping Agent

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to the natural π-electron-rich molecular structure. For this reason, this bulky molecule has less influence on the ordering degree of the alkyl chains, e.g., as compared to the smaller phenyl capping agent. The agglomeration behavior of the prepared, mixed modified, nanoparticle powder series has been studied with SAXS, calculating a hard-sphere volume fraction number from the scattering curves as a value for the density of nanoparticle packing. From this study, we could deduce that with increasing degree of cocapping agent within one series the particle agglomeration decreased over nearly 1 order of magnitude, and thus, the redispersibility properties of the nanoparticle powders are improved (Scheme 1). For all different cocapping agent molecule series, a good correlation between the agglomeration behavior of the nanoparticles and the degree of ordering of the surfacebound long alkyl chains could be found. From this observation we conclude that for the investigated system a stronger alkyl chain ordering causes higher agglomeration of the particles. This can be explained by a zipper effect where the nanoparticles agglomerate via alkyl bilayers between their crystal facets (Figure 7). Pyrene moieties on the surface show a weaker correlation of alkyl chain ordering and particle agglomeration, which can be due to a competing pyrene
pyrene interparticle bilayer formation at the pyrene island regions, leading to more agglomeration. These results point out that the well-known chemistry of mixed monolayer formation on macroscopic substrates can be expanded to nanoscopic (metal oxide) crystal facets, resulting in tunable physical surface properties. Until now there has been a lack of systematic studies on the zipper effect connected to surface functionalization of technologically highly relevant metal oxide nanoparticles (nanocomposites, ceramics). With this study we have shown how surface tuning on the molecular level impacts nanoscopic surface ordering properties and thus the macroscopic phenomenon of particle agglomeration behavior. The next step would be to apply this knowledge to inorganic
organic nanocomposite technology with the additional parameter of a polymer matrix. Additionally, from an analytical point of view, taking the correlation between SAM ordering and particle agglomeration into account, we were able to characterize SAMs on nanoparticles by two fundamentally different methods: Nanoparticles can be used as probes for SAM investigations by applying an indirect method (SAXS), which gives indications on the SAM structure from the agglomeration behavior of the nanoparticles. The scattering results are coincident with the results of the direct method of FTIR spectroscopy, which directly probes the molecular surface structure. It is surprising that a structural approach (SAXS) shows a sensitivity comparable to that of a spectroscopic approach (FTIR).

the gauche defects observed by IR, and the macroscopic parameter, namely, particles per volume unit as investigated by SAXS, be correlated? Their correlation is plotted in Figure 8. The different series all follow a general trend: With decreasing wavenumber, which means less gauche defects within the alkyl chains due to higher ordering in the surface SAM, the agglomeration of the particles increases. This indicates that these phenomena are linked together, underlining the zipper effect theory of an interparticle bilayer formation. However, some data points do not show a strong correlation, and in the PyPPA series correlation even a stronger agglomeration is observed at higher alkyl disordering. Reproduction of these outlying points confirmed their significance. This may originate from strong interaction of pyrene molecules within the bilayer in these regions of the diagram; e.g., pyrene moieties from one particle strongly interact with others on the neighboring particle, which is very likely to occur due to the high tendency for strong π
π interaction in such systems. Also the tBuPPA series does not fit into the scheme at the point of very high disordering (2926.3 cm
1). The agglomeration there is nearly identical to that of other molecules with the same mixing ratio. This would mean that, at a certain point of disordering or alkyl chain dilution, the agglomeration behavior does not change and no zipper effect is present, and thus, further disordering of the alkyl chains does not further reduce the particle packing. From the correlation of FTIR and SAXS, we conclude that the structural phenomenon of agglomeration of alkyl chain modified nanoparticles is strongly related to the nanoscopic phenomenon of surface ordering, which can be controlled on a molecular level.

’ CONCLUSIONS We have investigated the ordering of dodecyl chains in mixed monolayers of phosphonic acid capping agents on zirconia nanocrystals (facets), prepared by a coadsorbing approach. Different phosphonic acid cocapping molecules to disturb the long alkyl chain ordering have been studied, namely, the small methyl, the bulky and planar pyryl, the sterically demanding tertbutyl, and the intermediate phenyl moieties. In FTIR studies, the C
H vibrational modes of the dodecyl chain methylene units were used to detect the degree of alkyl chain packing in the SAM via a blue shift with increasing gauche defects. The SAM, consisting of 100% dodecyl chains, showed moderate ordering with no full crystallinity of the layer as would be the case for C18,30 but is far more ordered than in a liquid state. This moderate ordering is expected and desired in terms of the dispersion behavior of the nanoparticles. With increasing amount of adsorbed cocapping agent, the ordering of the dodecyl chains decreased significantly. This effect was quantitatively different for different coadsorbed molecules (Scheme 1). Some moieties such as as the small methyl group showed less tendency to disorder the C12 chains, and bulky moieties such as tert-butyl resulted in the highest chain disordering. The fact that the alkyl chain disordering changes with the mixing ratio with the same trend for all different cocapping molecules can be explained by the presence of different regimes of ordering and disordering: namely, SAM structure types between the two extremes of a homogeneously mixed monolayer and an islandlike layer, where the two molecules are phase separated. Phase separation within the SAM is considered to be very strong for molecules such as pyrylphosphonic acid, having the tendency to homointeract due

’ ASSOCIATED CONTENT

bS

Supporting Information. Infrared spectra of DPPA mixed with MPPA@ZrO2, PhPPA@ZrO2, PyPPA@ZrO2, and tBuPPA@ZrO2 and fit parameters of the Beaucage model. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 749

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’ ACKNOWLEDGMENT The Austrian Science Fund (FWF, Project P20693) is gratefully acknowledged for the financial support of this work. We thank Dr. Berthold St€oger and Dr. Robert Haberkorn for XRD measurements and the University Service Center for Transmission Electron Microscopy, Vienna University of Technology, for their help in recording the TEM images. We also thank Dr. Michael Puchberger for solid-state NMR measurements and Christoph Rill for his help with organophosphorus chemistry.

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