Article pubs.acs.org/JPCC
Analyzing Imidazolium Bridging in Nanoparticle Networks Covalently Linked to Silicon Substrates Bernhard Basnar,† Marco Litschauer,‡ Gottfried Strasser,† and Marie-Alexandra Neouze*,‡ †
Center for Micro- and Nanostructures and ‡Institute of Materials Chemistry, Vienna University of Technology, Austria
ABSTRACT: We report the characterization of the formation of an imidazolium bridging unit in the synthesis of titania-based nanoparticle networks. By using a combination of different analytical techniques, such as ellipsometry or atomic force microscopy, we are able to show that the nanoparticles are indeed covalently bound to the substrate. By oxidation of the organic linker after monolayer formation, covalently bound, fully inorganic nanoparticle monolayer structures are obtained. Furthermore, the layer-by-layer deposition method is extended to the synthesis of thin films containing, in a controlled manner and at the same time, silica and titania nanoparticles.
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INTRODUCTION The exploration of nanoparticles as building blocks for functional materials led to a better understanding of how to control the size, shape, and composition of these nanoobjects. The newly arising challenge in the field of nanoparticle research concerns the design of nanoparticle assemblies and nanoparticle networks, with or without bridging units. Such materials represent a great perspective to exploit nanoparticle collective properties.1,2 In this context, a large amount of work was devoted to metal nanoparticle assemblies3 for various applications such as plasmonic or biosensing.4−10 Some works also focused on quantum dots, like CdSe or CdTe, for photoswitching features.11,12 Nevertheless, the works dedicated to metal oxide nanoparticle networks are much rarer. Among those, next to magnetite (Fe3O4) networks, investigated for magnetic applications,13,14 Buso and co-workers reported the assembly of silica nanoparticles as highly porous material.15 Some other metal oxide based systems were also reported, such as with SiO2,16 ZnO,17 or CeO2.18 To address this issue of synthesizing metal oxide nanoparticle networks, we recently published the synthesis of ionic nanoparticle networks (INNs), where metal oxide nanoparticles are covalently bridged by imidazolium units.19−21 To form this hybrid organic−inorganic material, the formation of the networks was driven by a nucleophilic substitution reaction occurring between an imidazole functional group and a chloroalkyl group, present on the nanoparticle surface. In most of the works concerning nanoparticle networks, the attention was focused on the final features, like UV−visible © 2012 American Chemical Society
absorption, surface-enhanced Raman scattering, or magnetization curves. However, although some highly interesting work has already been published concerning the detailed multitechnique characterization of nanoparticles,22−24 few efforts concerned the full characterization of nanoparticle linking units. One of the reasons for such a lack is probably the difficulty of such analyses. Examples demonstrating successful attempts toward characterization include the extensive investigations performed by Guerrero et al. for the characterization of the grafting of phosphonic acid units on titania by means of 17O nuclear magnetic resonance25 and X-ray photoelectron spectroscopy of thiols deposited on a gold surface.26 In this article, we report the detailed characterization of the formation of an imidazolium bridging unit in the synthesis of titania-based nanoparticle networks, using a mix of different analytical tools to obtain specific information on the bridge bond between the nanoparticles and the surface. In the last part, more complex systems containing titania nanoparticles as well as silica nanoparticles are presented.
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EXPERIMENTAL SECTION Synthesis of Titania Nanoparticles. An amount of 10 mL (33.96 mmol) of Ti(OiPr)4 was dissolved in 25 mL of dry ethanol. This mixture was added dropwise under vigorous stirring to 250 mL of water and adjusted to a pH of 1.7 with 1 Received: February 8, 2012 Revised: March 27, 2012 Published: April 2, 2012 9343
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phosphonic acid in ethanol for 21 h, followed by the same cleaning procedure used for the first step (sonication in isopropanol and blow-drying). Synthesis and Modification of SiO2 Nanoparticles with 3-Chloropropyltrimethoxysilane (SiO2_Im). Syntheses of silica nanoparticles and the surface functionalization of the silica nanoparticles with N-(3-propyltrimethoxysilane)imidazole were described in a previous work.19 Deposition of the Trilayer Material TiO2_SiO2_TiO2. After the deposition of chloro-modified titania nanoparticles on a silicon wafer, as described above, the substrates were immersed in a 0.25 M solution of imidazole-modified silica nanoparticle suspension in methanol for 24 h. For the construction of the third layer of nanoparticles, the substrates were, prior to sonication in isopropanol and blow-drying, immersed into the previously prepared solution of titania nanoparticles capped with 3-chloropropylphosphonic acid in ethanol for 24 h once more. (Photo)oxidative Degredation of the Organic Capping Material. UV irradiation was performed using a Memorase DE 50 EPROM Erase form UVP, LLC. The samples were placed inside the chamber and were irradiated with a mercury vapor discharge lamp (nominal intensity 12 mW/cm2) at 253.7 nm wavelength for up to 100 min. Oxidative cleaning using an oxygen plasma was performed using PLASMA PROZESSOR 100-E system from Technics Plasma GmbH. The sample is placed inside the instrument chamber and subjected to oxygen plasma for 10 min. The plasma is generated at an oxygen pressure of 0.7 Torr by a 300 W microwave (2.45 GHz) irradiation Nanopatterning. Nanopattern formation was achieved by field-induced oxidation using atomic force microscopy (AFM) (for details regarding the AFM instrument, see analytical techniques). To this end, a silicon wafer was modified with an octadecylsilane monolayer27 by immersion for 10 min into a 0.05 M solution of octadecyltrichlorosilane in toluene. After washing and drying of the sample, it was transferred into the AFM. Oxidative nanolithography was performed using a cantilever coated with a conducting diamond coating. The oxidation occurred at a tip bias of 5 V with a tip velocity of 100 nm/s and a deflection set point of 0.2 V in contact mode. Analytical Techniques. X-ray Powder Diffraction (XRD). Measurements were performed on a Philips X’Pert diffractometer using the Cu−Kα radiation (λ = 1.5406 Å). Dynamic Light Scattering (DLS). Measurements were carried out on an ALV/CGS-3 compact goniometer system, equipped with an ALV/LSE-5003 light scattering electronics and multiple τ digital correlator and a 632.8 nm JDSU laser 1145P. For the measurement, the solid was dissolved in water. The DLS experiments were carried out without previous sonication of the samples. The run time of one measurement cycle is 10 s. Every size distribution curve is obtained by averaging 10 measurements. Thermogravimetric Analyses (TGA). Analyses were performed on a Netzsch Iris TG 209 C with a 414 TASC controller in a platinum crucible with a heating rate of 10 K/ min under synthetic air. Ellipsometric Measurements. Measurements were performed with the alpha-SE system from J.A.Woolam Co., Inc., under an angle of 70°. Ellipsometric spectra between 380 and 900 nm wavelengths were recorded and fitted using a simple two-layer model with silicon substrate and a layer with a
mL of nitric acid (53%). During the addition, the reaction mixture was cooled to 4 °C using an ice bath. After complete addition, the ice bath was removed, and the mixture was stirred for 3 days at room temperature. Then, the solvent was removed under reduced pressure, and the white crystalline product was dried in a desiccator over P2O5 under vacuum. Synthesis of 3-Chloropropylphosphonic Acid. Synthesis of Dimethyl-3-chloropropylphosphonate. An amount of 18 g (160.41 mmol) of potassium tert-butoxide was suspended in 150 mL of THF. Afterward, 22.01 g (200 mmol) of dimethylphosphite was slowly added under vigorous stirring. After 2 h of stirring, the whole suspension was slowly added to a stirred suspension of 47.23 g (300 mmol) of 1bromo-3-chloropropane in 120 mL of THF in a 500 mL roundbottom flask. A white suspension was formed immediately. The mixture was heated to reflux for 20 min. After cooling to room temperature, the formed precipitate, potassium bromide, was filtered off and washed twice with 100 mL of diethyl ether. Then the solvents and byproduct were removed under vacuum (20 mbar at 170 °C). A slightly colored liquid was obtained. Yield: 17.9 g (60%, 96.25 mmol). 1 H NMR (250 MHz, CDCl3): 1.82−1.92 (m, 2H, P−CH2− CH2), 1.93−2.10 (m, 2H, P--CH2), 3.58 (t, 2H, Cl−CH2), 3.72 (d, 6H, P−O−CH3) ppm. 31 P NMR (250 MHz, CDCl3): 45.99 ppm. Synthesis of 3-Chloropropylphosphonic Acid. An amount of 6.169 g (33.07 mmol) of dimethyl-3-chloropropylphosphonate was mixed with 40 mL of hydrochloric acid (37%) and heated to reflux for 24 h. Afterward, the solvent was removed under reduced pressure, and residues of water were removed through azeotropic distillation by adding 20 mL of toluene. The yellowish liquid residue was crystallized from 50 mL of chloroform and filtered. The colorless crystalline product was dried in a desiccator over P2O5 under vacuum. Yield: 3.73 g (70%, 23.53 mmol). 1 H NMR (250 MHz, DMSO-d6): 1.63−1.69 (m, 2H, P− CH2−CH2), 1.83−1.90 (m, 2H, P−CH2), 3.67 (t, 2H, Cl− CH2), 7.29 (s, 2H, P−OH) ppm. 31 P NMR (250 MHz, DMSO-d6): 37.85 ppm. 13 C NMR (DMSO-d6): 24.2 (P−CH2−CH2), 26.4 (P− CH2), 46.1 (Cl−CH2) ppm. Modification of TiO2 Nanoparticles with 3-Chloropropylphosphonic Acid (TiO2_Cl). This was obtained by dissolving 0.466 g (2.94 mmol) of 3-chloropropylphosphonic acid in 200 mL of water and then adding 1 g of TiO2 nanoparticles, dissolved in 100 mL of water. A white suspension was formed immediately. The suspension was stirred overnight. Afterward, the modified particles were isolated by centrifugation, washed several times with ethanol and water, and finally dried in a desiccator over P2O5 under vacuum. Activation of a Silicon Surface. Silicon wafers with a native oxide layer are cut into small pieces (about 0.5 × 1 cm2), washed, and activated using UV/ozone treatment. These substrates are immersed into a 100 mM solution of N-(3propyltrimethoxysilane) imidazole in ethanol for varying times. Cleaning of the samples is performed by sonicating at low power for 1 min in dry isopropanol and subsequent blowdrying of the substrates. Deposition of TiO2_Cl on an Activated Silicon Surface. Deposition of the titania nanoparticles onto the N-(3propyltrimethoxysilane) imidazole-modified silicon wafers is undertaken by immersion of the slides into a 0.01 mg/mL solution of titania nanoparticles capped with 3-chloropropyl9344
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refractive index of 1.45, which has been shown to yield suitable results for thin films.1 Atomic Force Microscopy (AFM). Measurements were performed using a NanoMan V system from Veeco. For topographic measurements, PPP-NCHR cantilevers (Nano and More) were used in tapping mode. Tune parameters for the system were 0.5 V target amplitude and the measurement frequency set to 5% off-resonance. The measurement was performed at a damping of approximately 25% and scan rates of 1 Hz for 1 × 1 μm2 to 5 × 5 μm2 images. Lateral Force Microscopy (LFM). Measurements were undertaken using a PPP-CONTR cantilever (Nano and More) with a spring constant of 0.179 N/m. The approach was performed with a deflection set point of 1 V to ensure engaging of the system. After contact, the set point was changed to the value of interest, covering a range between 0.2 and 5 V (12−300 nN tip force). In situ control of LFM induced surface modification was undertaken with the same tip at a deflection set point of 0.1 V (6 nN tip force). High-resolution images of the modified surfaces were obtained in tapping mode as described above. X-ray Photoelectron Spectroscopy (XPS). Measurements were carried out with a system from SPECS Surface Nano Analysis GmbH with an XR-1000 X-ray source and a PHOIBOS-3500 hemispherical analyzer. The spectra were recorded using the Mg source. For overview scans (0−600 eV), a resolution of 0.1 eV and a dwell time of 0.1s were chosen, and the average of three scans was recorded. Peaks for the individual elements were recorded at 0.02 eV resolution, averaging five scans. Nuclear Magnetic Resonance (NMR). Spectra in the liquid state for 1H, 13C, and 31P nucleus were recorded on a Bruker AVANCE 250 (1H at 250.13 MHz, 13C at 62.86 MHz, 31P at 101.26 MHz) equipped with a 5 mm inverse-broadband probe head with a z-gradient unit. All analytical techniques, except XPS and LFM, were performed on multiple different batches. In the case of XPS measurements, the individual systems were measured thrice, with each time three sampling spots. The LFM measurements were performed twice for the investigated systems.
Figure 1. (a) Size distribution of the TiO2 nanoparticles obtained by DLS and (b) XRD diffractogram of the native titania nanoparticle powder.
will be referred to as TiO2_Cl. After modification, the TiO2_Cl particles were centrifuged and washed to eliminate unreacted phosphonic acid ligands. The amount of unreacted ligand, determined by thermogravimetric analysis of the supernatant water and the washing fractions, was 1.40 mmol. This result indicates that 1.54 mmol of P−Cl ligand was bound to the particles. Considering that the titanium dioxide nanoparticles have an average diameter of 4 nm and a density of 4.24 g.cm−3, we can estimate to have 7.0 × 1018 nanoparticles in 1 g of TiO2, each individual nanoparticle possessing a surface area of around 50 nm2. Thus, an individual particle, on average, has 132 ligand molecules attached, which is equivalent to 2.6 ligand molecules/nm2. This is a reasonable result considering that octadecylphosphonic acid exhibits about twice the density due to its highly crystalline packing.28 Confirmation of the amount of bound ligand was obtained through thermogravimetric analysis (TGA) on the modified nanoparticles. The weight over time graph of the TiO2_Cl (Figure 2) exhibited a total weight loss of 16.35 wt %. This means that a sample of 1 g of TiO2_Cl contains 0.837 g of titanium dioxide and 0.164 g of volatiles. These volatiles consist
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RESULTS AND DISCUSSION Characterization of the Nanoparticle Monolayer. The particles were synthesized by sol−gel reaction in an acidic medium. The particle diameter was estimated using dynamic light scattering (DLS). Figure 1a shows the results from the measurement with the average diameter being 5.6 ± 2 nm. To verify these findings, the nanoparticle diameter was investigated also with conventional powder X-ray diffraction (XRD). The Xray diffractogram obtained for the TiO2 nanoparticles (Figure 1b) presents the characteristic reflexions of anatase titania nanoparticles at 2θ = 25°, 38°, 48°, 54°, 62°, 69°, 74°, and 82° (marked by stars in Figure 1b). The Scherrer’s equation, applied on the titania (101) reflection at 25°, affords an average diameter of titania crystallites in the 4.0 ± 0.7 nm range. This result is slightly smaller than observed in DLS. However, DLS does not estimate the diameter for the particle alone; rather it detects the hydrodynamic radius of particles in solution. Due to interaction of the particles with the solvent molecules, the measured size is larger than the radius of the particles themselves. The native particles were modified in water with 3chloropropylphosphonic acid (P−Cl). These modified particles
Figure 2. Thermogravimetric analysis of the titania nanoparticles modified with 3-chloropropylphosphonic acid groups (TiO2_Cl). 9345
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of absorbed solvent molecules, which desorb between 25 and 300 °C (9.35%), and the decomposed material of the ligand, starting to degrade at 300 °C (7%). Keeping in mind the number of TiO2 nanoparticles calculated above (7.0 × 1018 g) and a molecular mass of 61.5 g·mol−1 for the degraded material of the P−Cl ligand (C3H6Cl are removed, and the remaining phosphor binds an additional oxygen), the sample weight loss indicates a coverage of 118 ligands per nanoparticle, which is equivalent to 2.4 ligands/nm2 of titanium dioxide nanoparticle (for detailed calculation, see ref 29). This value is slightly smaller than the one estimated above, but it nevertheless confirms a relatively dense packing of the organic molecules. The TiO2_Cl nanoparticles are nominally about 1.3 nm greater in diameter due to their organic ligand. However, as Xray diffraction is insensitive to the organic material, the thickness measured by XRD remained basically unchanged. In DLS, the value increased dramatically (diameter of about 200 nm) due to aggregation of the neutral nanoparticles in solution. The chemical nature of the modified nanoparticles was investigated using X-ray photoelectron spectroscopy (XPS). A sample of dried particles was measured using XPS (see Figure
Figure 4. Determination of the monolayer formation of a TiO2_Cl nanoparticle layer on a silicon substrate. (a) Change of the ellipsometrically determined thickness of the nanoparticle deposition over time. The inset shows the corresponding change in thickness for the deposition of the linker molecule to the silicon substrate. (b) AFM image of the nanoparticle monolayer after 21 h of deposition.
of the imidazole linker, which was completed in 30 min (Figure 4A, inset). This is to be expected as both the concentration of the nanoparticles in solution is much smaller and the reaction of the imidazole with the chlorine is kinetically much slower. The nanoparticle deposition saturates after approximately ten hours. The final value of the particle layer is 2.63 ± 0.25 nm. Keeping in mind that our nanoparticles are 4 nm in diameter with a thin organic cladding, the ellipsometric thickness obtained would correspond to a closely packed monolayer of these particles. However, as ellipsometry only yields an average thickness value, the result could also stem from a partially covered surface with particle agglomerates. To establish the type of particle deposition, atomic force microscopy (AFM) was employed. After 21 h of deposition, a homogeneously covered surface with particles of narrow size distribution was obtained (Figure 4B). The measured roughness for the titania nanoparticle monolayer is 1.3 nm (which is significantly higher than for the bare silicon wafer, rmsSi = 0.3 nm). Only very rarely were higher features, connected either to individual larger particles or to small agglomerates, observed. This result confirms the accuracy of the ellipsometry values reported above, corresponding to a densely packed particle monolayer. The fact that the monolayer is made up of individual particles also accounts for a significantly higher roughness as compared to the native silicon substrate. Establishing the size of the nanoparticles by AFM is not possible in this way, as the image of the particles is a convolution of the nanoparticle shape and the tip shape (nominally around 10 nm tip radius). However, by creating patterned monolayer structures it is possible to evaluate the height and, in this way, to get information about the particle diameter. To this end, we modified a silicon substrate with a hydrophobic coating of octadecylsilane (ODS) by immersion into a solution of octadecyltrichlorosilane in toluene. After
Figure 3. XPS spectrum of a TiO2_Cl nanoparticle powder.
3). As can be seen, all the relevant peaks for the nanoparticles (Ti, O) and the linker (P, C, Cl, O) can be distinguished. After characterization of the nanoparticles, monolayers of these particles were deposited. To this end, silicon-100 substrates with a native oxide layer were cleaned in oxygen plasma and, subsequently, immersed into a solution of N-(3trimethoxysilylpropyl)imidazole in ethanol. After 30 min of immersion, the deposition process was completed. The completion of the deposition was verified by means of ellipsometry. The surface coverage, as determined by spectroscopic ellipsometry, showed a thickness of 0.29 ± 0.05 nm. This value indicates the formation of a monolayer with a relatively low degree of order. This low degree of order is caused by the relatively short alkyl chain length of the ligand, which cannot self-assemble on the surface in a crystal-like fashion, leading to a disordered monolayer. The imidazole-functionalized surface is capable of undergoing a pseudoclick chemistry reaction with the chlorine molecule of the TiO2_Cl nanoparticles. The principle of this reaction was recently presented for the formation of silica or titania nanoparticle networks.19 To obtain such a nanoparticle monolayer, the imidazole-modified silicon substrate was immersed into a dilute solution of the TiO2_Cl nanoparticles. Figure 4 shows the evolution of the film thickness as a function of immersion time determined by ellipsometry. It can be seen that the deposition occurs much slower than for the deposition 9346
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rinsing and drying, oxidative nanopatterning was performed using AFM with an applied potential of 5 V. After the oxidation, the substrate was immersed into the trimethoxysilylpropyl imidazole linker solution which led to specific binding of the organic linker to the oxidized patterns. Subsequent immersion into the nanoparticle solution led to densely packed nanoparticle monolayer structures corresponding to the initially oxidized areas. Figure 5 shows an image with a corresponding cross-section through the image.
Figure 6. (a) XPS spectra of imidazole-modified silicon oxide nanoparticles before (blue) and after (red) reaction with a chloroalkane. (b, left) Zoom on the nitrogen peak of the XPS spectra of imidazole-modified silicon oxide nanoparticles before (blue) and after (red) reaction with a chloroalkane. (b, right) Zoom on the chlorine peak of the XPS spectra of imidazole-modified silicon oxide nanoparticles before (blue) and after (red) reaction with chloroalkanemodified titania nanoparticles. (c) XPS spectra of a titania nanoparticle monolayer (blue) and triple layer (red) on a Si/SiO2 substrate.
converted to a chloride, causing significant changes in the electronic environment of this element. Figure 6c shows the spectrum for a dense titania nanoparticle monolayer (blue) and of a trilayer (red) for higher XPS signal intensities. The spectra exhibit the peaks expected for the nanoparticles, the imidazole and chlorine linkers, as well as the silicon substrate with native oxide. Differences in the Si signals are caused by the oxide thickness of the silicon substrate. However, no change in the nitrogen or chlorine peaks (as compared to the changes in Figure 6b) is observed as a result of the chemical bond formation. This can be attributed to several factors. First, the overall intensity is very low due to the monolayer coverage, providing poor signal-to-noise ratio. Second, due to steric effects, only a small number of molecules directly beneath the nanoparticles are capable of forming the imidazolium bond. Thus, only a few percent of all the ligand molecules undergo the binding reaction, and these are, in addition, partially shielded by the titania nanoparticles, reducing their photoelectron signal even further. Stability of the Nanoparticle Trilayer Deposit. Having confirmed that the reaction itself basically occurs and can be observed by spectral changes, still confirmation of the bond formation in the case of the nanoparticles was missing. To overcome this problem, we investigated the mechanical stability of these monolayers by performing lateral force microscopy (LFM). LFM is a variant of AFM where the cantilever is in constant contact with the sample and, due to the tip pressing on the sample during the scanning process, shear forces are being exerted. These shear forces can lead to a displacement of the nanoparticles once the binding strength of the imidazolium bridge is overcome. Figure 7 shows an AFM image of an area,
Figure 5. AFM image of a patterned nanoparticle substrate with corresponding cross-section through the image along the dashed line in the image.
The measured height of the nanoparticles is between 4 and 6 nm. Keeping in mind that the particles (4 ± 0.7 nm diameter, 1.3 nm organic coating, and 0.6 nm linker on the surface) are embedded in a 2.6 nm thick ODS monolayer, the expected measurable height for the nanoparticles in this conformation is 3.3 ± 0.7 nm, which is slightly smaller than the measured value. However, differences in surface chemistry and elasticity are known to be responsible for deviations of up to a few nanometers in AFM height measurements.30 Therefore, the deviation between the expected and the measured height can be explained by the differences in the surface properties between the nanoparticles (short, hydrophilic linker) and the ODS (long, hydrophobic chain). The imidazolium formation between the imidazole-modified substrate and the TiO2_Cl nanoparticles was investigated through XPS measurements. Figure 6a shows the spectra for imidazole-modified silica nanoparticles before (blue) and after reaction with a chloroalkane. The nitrogen peak at 400 eV exhibits a change from a bimodal to a monomodal shape (see inset in Figure 6a and 6b), which is indicative of the imidazolium bond formation. Concurrently, the chlorine peaks exhibit both a broadening, from 2.7 to 3.5 eV full width at half-maximum, and a shift of 1 eV toward lower energies (see inset in Figure 6a and 6b) as a consequence of the bond formation (as compared to the free chlorine linker depicted in Figure 3). This is not surprising as the chlorine is 9347
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line where the tip is suddenly pushed into the sample at a different angle. Due to tip geometry and standard cantilever angles, this effect is more pronounced at the top turning point, causing the selective removal in this part of the trench. As the titania nanoparticles exhibit anatase crystal structure (Figure 1b), they should possess photocatalytic activity. To investigate this, a trilayer of nanoparticles (as used above) was subjected to irradiation with ultraviolet light from a mercury lamp. As expected, the ellipsometer shows a reduction in the height of the nanolayer structure, whereas AFM did not show any changes in the surface morphology. XPS, however, provided a clearer picture of the reactions occurring (Figure 8). No chlorine signal can be detected after 100 min of UV
Figure 7. AFM image of a titania nanoparticle trilayer after scratching in LFM mode with varying contact forces together with a cross section (bottom diagram) through the scratched area (dashed line in the top image).
where LFM scans with varying contact forces have been undertaken on a sample containing three layers of nanoparticles. These have been obtained by successive deposition of nanoparticles and linker molecules. It is immediately visible that already at very small forces some topographic contrast is visible. However, the change in height is marginal (0.2 nm) and might be caused by mechanical compression of the multiple layers. This can be due to rearrangement of the individual nanoparticles within the layers as there should be no cross-linking within each of the layers, providing a certain degree of flexibility. At higher forces (36 nN or above), mechanical removal of material is observed by the deposition of debris at the edge of the scratched area. For the height analysis of the corresponding trench, a cross-section through the trench was performed, and the zero value on the y-axis (height) was chosen to be the top nanoparticle layer, as this is the reference plane for the AFM measurement. In the graph, three distinct levels are visible, separated by approximately 3.4 nm each. This would correspond to the removal of the two top nanoparticle layers, with the bottom layer still remaining on the surface. The height value is slightly smaller than the actual particle diameter determined by XRD including the organic linker layer. However, as the particles are not positioned on a flat surface but rather in the pits created by the underlying nanoparticle layer, the expected height for a nanoparticle layer is 4.5 nm. The difference to the measured height can probably be attributed to height artifacts in the AFM measurement. Thus, the layer height corresponds to the thickness of individual nanoparticle layers, confirming the displacement of the nanoparticles. The fact that only at such high contact forces the displacement occurs indicates that the particles are, indeed, covalently bound to the substrate. Another interesting observation is that nanoparticles from the second layer have only been removed at the top end of the trench. This is caused by the higher forces exerted at the turning point of each scan
Figure 8. XPS spectra of titania trilayers in the original state, after illumination with ultraviolet light and after oxidation by oxygen plasma.
irradiation. The nitrogen peak is greatly reduced, albeit not completely eliminated. The phosphorus peak, however, did not change in intensity; rather it experienced a slight shift to higher energies. This confirms that the organic linkers connected to the nanoparticles are photocatalytically degraded leading to volatile nitrogen and chlorine compounds. The phosphor is oxidized to a phosphate which is not volatile and, thus, remains bound to the titania nanoparticles.31,32 The fact that the nitrogen peaks partially remain can be explained by the fact that the silicon substrate contains organic ligands which are not directly bound to titania particles. Thus, these are not degraded by the photocatalytic activity of the titania. Nevertheless, exposure to oxygen plasma leads to a further reduction in the ellipsometric thickness accompanied by a complete removal of the nitrogen peaks in the XPS. The peak for phosphor has a lower intensity which is possibly caused by a partial removal of the phosphor during the oxidation step. This poses the question whether the particles are still covalently linked to the surface if the phosphates are the only remaining parts of the linker. To elucidate this, LFM was again employed. As can be seen in Figure 9, the oxidized layer shows a higher stability at lower forces (7 nN or below), indicating either the removal of weakly bound organic moieties by the oxidation process or a more stable rearrangement of the nanoparticles due to the elimination of the compressible, organic bridge between the particles. Raising the contact force to 36 nN induced the formation of a trench which is associated to the mechanical removal of the nanoparticles as evidenced by the debris at the edges of the trench. This suggests that, in spite of 9348
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Figure 10. Scheme and corresponding AFM images of the successive deposition of (bottom) TiO2_Cl nanoparticles, (middle) SiO2_Im nanoparticles, and (top) TiO2_Cl nanoparticles together with respective topographic images and corresponding surface roughness values (rms = root-mean-square).
Figure 9. AFM image of an oxidized titania nanoparticle trilayer after scratching in LFM mode with varying contact forces together with a cross section (bottom diagram) through the scratched area (dashed line in the top image).
Deposition of an additional titania nanoparticle layer above the silica nanoparticles was performed. This third layer smoothened the surface (Figure 10, top).
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the removal of the organic linker, the particles are covalently connected to the surface and between one another through O− Si−O or O−P−O links, respectively. This is in agreement with findings that, e.g., oxidation of organosilane multilayers leads to the formation of mechanically stable SiO2 multilayers covalently linked to the surface via O−Si−O bonds.27 The height difference of 2.4 nm is about 1 nm less than in the previous experiment. This is in good agreement with the fact that the organic link between the particles, which was removed oxidatively, has a thickness of about 1.3 nm. Formation of the Bis-Nanoparticle Trilayer. The layerby-layer deposition method developed for obtaining thin films of titania nanoparticle networks, bound by means of imidazolium units, was extended to bis-nanoparticle networks. In such bis-nanoparticle networks, two different types of nanoparticles are present and deposited in a controlled manner. The two nanoparticles chosen as proof of principle in this article are titania and silica nanoparticles. The 4 nm diameter titania nanoparticles are functionalized with chloroalkyl groups as described before, while the 15 nm diameter silica nanoparticles are modified with alkylimidazole units. The activated silicon wafer is then successively dipped into TiO2− Cl, SiO2−Im, and again TiO2−Cl suspensions. After each deposition step, the modified wafer was washed and sonicated to eliminate unreacted nanoparticles. The obtained layers were investigated by atomic force microscopy. The deposition of the silica nanoparticles on top of the titania nanoparticle's first layer can clearly be observed in AFM (Figure 10, middle), were the single nanoparticles can be distinguished. The roughness of the obtained surface is much higher than for the titania monolayer (Figure 10, bottom). This increase of the roughness is explained by the size difference of the two nanoparticles used for the deposition, as represented in the middle scheme in Figure 10.
CONCLUSION In conclusion, we have reported on the investigation of titania nanoparticle monolayers. We have found that only through the combination of various spectroscopic and topographic techniques an assessment of the sample properties, especially the chemical bond formation, is possible. We have established that the titania nanoparticles can be deposited as discrete monolayers with high packing density. The particles are linked to the surface via covalent bonds containing imidazolium moieties. In a second part, we have shown that these hybrid organic− inorganic multilayers can be converted to purely inorganic layers via photocatalytic oxidation utilizing the optical properties of the titania nanoparticles or via oxygen plasma cleaning. The structures obtained in this fashion consist of nanoparticle layers which are covalently linked through O−P−O or O−Si− O links. The presented material system is an interesting candidate for the fabrication of highly defined (in all three dimensions), catalytically active surfaces. In a third, part we demonstrated that the paradigm of sequential deposition of nanoparticle monolayers is a promising route toward the fabrication of novel metamaterials. To this purpose, we presented trilayered systems where titania nanoparticles as well as silica nanoparticles were deposited in a controlled manner, leading to bis-nanoparticle hybrid material. The bis-nanoparticle trilayered hybrid thin film is a consequence of the tailorability of this layer-by-layer deposition method for nanoparticle networks.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 9349
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Notes
(30) Basnar, B.; Friedbacher, G.; Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. Appl. Surf. Sci. 2001, 171, 213−225. (31) Feichtenschlager, B.; Lomoschitz, C. J.; Kickelbick, G. J. Colloid Interface Sci. 2011, 360, 15−25. (32) Lomoschitz, C. J.; Feichtenschlager, B.; Moszner, N.; Puchberger, M.; Mueller, K.; Abele, M.; Kickelbick, G. Langmuir 2011, 27, 3534−3540.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Austrian Fonds zur Förderung der Wissenschaftlichen Forschung (FWF, Project P21190−N17).
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REFERENCES
(1) Suvakov, M.; Tadic, B. J. Phys.: Condens. Matter 2010, 22, 163201−163223. (2) Zabet-Khosousi, A.; Dhirani, A.-A. Chem. Rev. 2008, 108, 4072− 4124. (3) Christopher, J. K. Faraday Discuss 2004, 125, 409−414. (4) Chen, H. M.; Liu, R.-S. J. Phys. Chem. C 2011, 115, 3513−3527. (5) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. Chem. Rev. 2011, 111, 3669−3712. (6) Siavoshi, S.; Yilmaz, C.; Somu, S.; Musacchio, T.; Upponi, J. R.; Torchilin, V. P.; Busnaina, A. Langmuir 2011, 27, 7301−7306. (7) Yajima, T.; Yu, Y.; Futamata, M. Phys. Chem. Chem. Phys. 2011, 13, 12454−12462. (8) Lim, S. I.; Zhong, C.-J. Acc. Chem. Res. 2009, 42, 798−808. (9) Wang, X.; Mitchell, D. R. G.; Prince, K.; Atanacio, A. J.; Caruso, R. A. Chem. Mater. 2008, 20, 3917−3926. (10) Wen, F.; Waldoefner, N.; Schmidt, W.; Angermund, K.; Boennemann, H.; Modrow, S.; Zinoveva, S.; Modrow, H.; Hormes, J.; Beuermann, L.; Rudenkiy, S.; Maus-Friedrichs, W.; Kempter, V.; Vad, T.; Haubold, H.-G. Eur. J. Inorg. Chem. 2005, 3625−3640. (11) Jeong, S.-H.; Lee, J. W.; Ge, D.; Sun, K.; Nakashima, T.; Yoo, S. I.; Agarwal, A.; Li, Y.; Kotov, N. A. J. Mater. Chem. 2011, 21, 11639− 11643. (12) Lilly, G. D.; Whalley, A. C.; Grunder, S.; Valente, C.; Frederick, M. T.; Stoddart, J. F.; Weiss, E. A. J. Mater. Chem. 2011, 21, 11492− 11497. (13) He, L.; Hu, Y.; Kim, H.; Ge, J.; Kwon, S.; Yin, Y. Nano Lett. 2010, 10, 4708−4714. (14) Wacker, J. B.; Parashar, V. K.; Gijs, M. A. M. Langmuir 2011, 27, 4380−4385. (15) Buso, D.; Nairn, K. M.; Gimona, M.; Hill, A. J.; Falcaro, P. Chem. Mater. 2011, 23, 929−934. (16) Zhang, X.; Tang, G.; Yang, S.; Benattar, J.-J. Langmuir 2011, 26, 16828−16832. (17) Li, N.; Gao, Y.; Hou, L.; Gao, F. J. Phys. Chem. C 2011, 115, 25266−25272. (18) Tan, H. R.; Tan, J. P. Y; Boothroyd, C.; Hansen, T. W.; Foo, Y. L.; Lin, M. J. Phys. Chem. C 2012, 116, 242−247. (19) Litschauer, M.; Neouze, M.-A. J. Mater. Chem. 2008, 18, 640− 646. (20) Neouze, M.-A.; Litschauer, M.; Puchberger, M.; Peterlik, H. Langmuir 2011, 4110−4116. (21) Basnar, B.; Litschauer, M.; Abermann, S.; Bertagnolli, E.; Strasser, G.; Neouze, M.-A. Chem. Commun. 2011, 47, 361−363. (22) Baer, D. R.; Gaspar, D. J.; Nachimuthu, P.; Techane, S. D.; Castner, D. G. Anal. Bioanal. Chem. 2010, 396, 983−1002. (23) Grainger, D. W.; Castner, D. G. Adv. Mater. 2008, 20, 867−877. (24) Ott, L. S.; Finke, R. G. Coord. Chem. Rev. 2007, 251, 1075− 1100. (25) Brodard-Severac, F.; Guerrero, G.; Maquet, J.; Florian, P.; Gervais, C.; Mutin, P. H. Chem. Mater. 2008, 20, 5191−5196. (26) Techane, S. D.; Gamble, L. J.; Castner, D. G. J. Phys. Chem. C 2011, 115, 9432−9441. (27) Basnar, B.; Madera, M.; Friedbacher, G.; Vallant, T.; Mayer, U.; Hoffmann, H. Mikrochim. Acta 2000, 133, 325−329. (28) McDermott, J. E.; McDowell, M.; Hill, I. G.; Hwang, J.; Kahn, A.; Bernasek, S. L.; Schwartz, J. J. Phys. Chem. A 2007, 111, 12333− 12338. (29) Zhang, F.; Jin, J.; Zhong, X.; Li, S.; Niu, J.; Li, R.; Ma, J. Green Chem. 2011, 13, 1238−1243. 9350
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