Electronic Structure of Individual Hybrid Colloid Particles Studied by

Jan 29, 2013 - Institute for Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH,. Hahn-Meitner-Platz 1, ...
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Electronic Structure of Individual Hybrid Colloid Particles Studied by Near-Edge X‑ray Absorption Fine Structure (NEXAFS) Spectroscopy in the X‑ray Microscope Katja Henzler,*,†,‡ Peter Guttmann,† Yan Lu,‡ Frank Polzer,§ Gerd Schneider,† and Matthias Ballauff‡,§ †

Institute for Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, D-12489 Berlin, Germany ‡ Institute for Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany § Department of Physics, Humboldt University Berlin, Newtonstr. 15, D-12489 Berlin, Germany S Supporting Information *

ABSTRACT: The electronic structure of individual hybrid particles was studied by nanoscale near-edge X-ray absorption spectromicroscopy. The colloidal particles consist of a solid polystyrene core and a cross-linked poly-N-(isopropylacrylamide) shell with embedded crystalline titanium dioxide (TiO2) nanoparticles (d = 6 ± 3 nm). The TiO2 particles are generated in the carrier network by a sol−gel process at room temperature. The hybrid particles were imaged with photon energy steps of 0.1 eV in their hydrated environment with a cryo transmission X-ray microscope (TXM) at the Ti L2,3-edge. By analyzing the image stacks, the obtained near-edge X-ray absorption fine structure (NEXAFS) spectra of our individual hybrid particles show clearly that our synthesis generates TiO2 in the anastase phase. Additionally, our spectromicroscopy method permits the determination of the density distribution of TiO2 in single carrier particles. Therefore, NEXAFS spectroscopy combined with TXM presents a unique method to get in-depth insight into the electronic structure of hybrid materials. KEYWORDS: Titania nanoparticles, microgel, nanocomposite, NEXAFS spectroscopy, X-ray microcopy, spectromicroscopy

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with high-resolution full-field transmission X-ray microscopy (TXM). This method delivers electronic information on the nanoscale.11,12 The TXM operates with high spectral resolution of E/ΔE = 1 × 104, enabling X-ray imaging with small photon energy steps of 0.1 eV. Additionally, the colloidal hybrid material can be investigated in their aqueous environment at cryogenic temperatures to avoid drying artifacts.13 Due to the large image field (∼15 μm), a TXM image shows already a large number of particles and thus leads to a secure evaluation.11 We demonstrate that NEXAFS spectromicroscopy in a TXM leads to an in-depth understanding of the structure of these hybrids on the level of a single carrier particle. A schematic representation of the microgel particles is shown in Figure 1. The microgel consists of a polystyrene (PS) core and a poly-N-(isopropylacrylamide) (PNIPAM) shell crosslinked by N,N′-methylenebisacrylamide. The synthesis of these particles and their properties in aqueous media has been reviewed recently.14 The microgel particles are transferred from

itanium dioxide (TiO2) nanoparticles are of great interest because of their unique combination of properties as for example the photocatalytic activity, high chemical stability, and low toxicity.1−5 Well-defined particles are available with a sol− gel process through the hydrolysis of titanium alkoxides.1,6,7 To prevent aggregation of the nanoparticles and concomitant loss of catalytic activity, TiO2-particles can be trapped within colloidal polymer carrier particles. These hybrid particles have become the subject of intense research recently inasmuch as they allow an efficient and secure handling of the nanoparticles in various applications.8 Furthermore, suitably designed polymeric carrier particles can serve as nanoreactors allowing us the generation of well-defined nanoparticles which would not be available by conventional solution chemistry.9,10 To understand and optimize the functionality of these colloidal hybrids, the structural and electronic properties must be known throughout all pertinent length scales. Moreover, this information should be available on the scale of a single carrier particle to avoid an averaging over a large ensemble. Here we report on the synthesis of TiO2 nanoparticles in a colloidal nanoreactor and the analysis by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined © 2013 American Chemical Society

Received: December 19, 2012 Revised: January 22, 2013 Published: January 29, 2013 824

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Figure 1. Schematic representation of the in situ formation of TiO2 nanoparticles in the microgel network. The microgels consist of a solid polystyrene core carrying a network of poly-N-(isopropylacrylamide) (PNIPAM). The microgel particles are dispersed in ethanol. A hydrolysis process of tetraethylorthotitanate (Ti(OEt)4) at room temperature forms TiO2 nanoparticles in the PNIPAM shell.

Figure 2. (a) Optical setup of the TXM for NEXAFS studies. The monochromatized X-ray beam from the undulator is focused by a glass capillary condenser into the object field. The zone plate objective forms a magnified image which is recorded by the CCD camera. (b) Xray microscopy images recorded at different photon energies (white scale bar 500 nm). Selection of a region with a single hybrid particle (red circle) to record the transmitted photon flux within the particle (I) and the incident photon flux (I0, blue circle). (c) NEXAFS spectrum of the marked region of interest (ROI) in the TXM image shown in part b.

the aqueous media to an ethanol−water-mixture in which the PNIPAM shell remains in the swollen state.15−19 Under these conditions the microgel dispersion is stable, and the titanium precursor tetraethylorthotitanate (Ti(OEt)4) can reach the whole volume of the PNIPAM layer. Small TiO2 nanoparticles are generated by slow and controlled hydrolysis of the titanium precursor.20−24 It was shown by Lu et al. that the slow addition of Ti(OEt)4 to spherical polyelectrolyte brushes (SPB) in a defined ethanol−water mixture permits to control the size of the generated TiO2 nanoparticles and their size distribution.25 Here the PNIPAM network acts as a nanoreactor during the hydrolysis process which traps the growing TiO2 nanoparticles. The hybrid particles consisting of the colloidal carrier and the embedded TiO2 nanoparticles form a stable colloidal suspension, and no precipitation takes place after storing the suspension for several months. The results from thermal gravimetric analysis indicate that 26.5 wt. % TiO2 is embedded into the PNIPAM shell of the carrier particles. A cryo transmission electron microscopy (TEM) micrograph of a hybrid particle is shown in Figure S1 in the Supporting Information. The TiO2 nanoparticles are nearly spherical with a diameter of (6 ± 3) nm (detailed analysis of the TiO2 particle size is shown in the Supporting Information, Figure S2). The strong inelastic scattering of electrons and the required high electron dose prevents a detailed structural analysis of the hydrated TiO2 nanoparticles in the cryo-TEM.26,27 X-rays provide a much larger penetration depth and deliver elementspecific information about electronic states. As we demonstrate here, TXM coupled with spectroscopy and cryogenic sample environment overcomes the limitations of TEM for hydrated samples. Here we used the TXM of the Helmholtz-Zentrum Berlin which is installed at the U41-SGM undulator beamline (electron storage ring BESSY II; Berlin, Germany). Figure 2 illustrates schematically the TXM spectroscopy measurements. The monochromatic X-ray beam is focused by an elliptically shaped condenser mirror (glass capillary) into the object field. The spectroscopic analysis of the image stack requires a constant magnification for all photon energies. Therefore, the zone plate objective and the charge-coupled device (CCD) camera have to be repositioned for each new photon energy step. The spatial resolution limit of the TXM is close to 10 nm and depends on the outermost zone width of the zone plate and the diffraction order used for imaging.28 For the NEXAFS

measurements, we used a zone plate with 25 nm outermost zone width in the first order of diffraction which results in a resolution of 20 nm (half pitch). Further information about the allocation for beamtime access is given in ref 29. NEXAFS spectromicroscopy of the TiO2 nanoparticles on the microgel particles is ideally performed at the titanium L2,3edge. Here the NEXAFS spectra provide information about the electronic configuration of the outer Ti 3d-orbitals, which reflects the atomic arrangement around the Ti atom. In the case of the titanium L2,3-edge the NEXAFS spectroscopy corresponds to the excitation of the Ti 2p states into the empty 3d states. Therefore, the NEXAFS spectrum of the Ti L2,3-edge is sensitive to the local bonding environment as well as the oxidation state and the respective crystal structure.30 A comparison of the measured TiO2 spectra with those reported in the literature11,31,32 indicates that anatase nanoparticles have been synthesized at room temperature. However, small photon energy shifts between our measured spectra and those reported in the literature cannot be excluded. Therefore, additional NEXAFS-TXM measurements have been conducted on a mixture of colloidal hybrid particles and an anatase reference material to verify the experimental results. Under these conditions, the spectra are recorded simultaneously which excludes an energy shift from monochromator adjustments during consecutive measurements. Figure 3a shows a cryoTXM image of the TiO2 nanoparticles embedded into microgels and anatase reference particles at a photon energy of 457.7 eV. The larger contrast of the reference material compared to the colloidal hybrid particles can be attributed to the significantly smaller size of the TiO2 nanoparticles in the hybrid particles. Figure 3a shows that an X-ray image contains many hybrid particles. Therefore, the analysis of individual particles as well as statistical information from many particles can be obtained. 825

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Figure 3. (a) Cryo-TXM image of TiO2 composite particles and anatase reference material at photon energy of 457.7 eV. (b) From the respective colored ROIs of the TXM image, the NEXAFS spectra of single TiO2 hybrid particles (blue) and the anatase reference material (orange; measured intensity divided by a factor of 3 to clarify the illustration) can be derived. Additionally, the averaged spectra over many hybrid particles shown in a is plotted (purple).

Figure 4. (a) NEXAFS spectra of three individual hybrid particles which are highlighted in Figure 3a. The intensity signal is normalized to the signal from the carrier particle background. (b) Intensity distribution of the normalized absorption at 457.7 eV of 125 hybrid particles from Figure 3a. The determined intensity distribution (blue bars) can be well-described by a Gaussian distribution (red line).

To validate our X-ray spectromicroscopy results, we used high-resolution transmission electron microscopy (HR-TEM) and selected-area electron diffraction (SAED) to study dried hybrid particles (see Figure S3 of the Supporting Information). The HR-TEM visualizes the lattice fringes with a spacing of 0.352 nm which corresponds to the (011) plane of the anatase crystal. The diffraction rings of the SAED can be assigned to the anatase crystal structure. Note that the TEM-SAED results arise from a small number of dried hybrid particles without statistical information. The generation of well-defined and highly crystalline TiO2 particles points to a brief nucleation step followed by a much longer growth period. An explanation for the two well-defined reaction steps can be ascribed to the potential of polar groups, like sulfonate, amid or alkoxide groups, to stabilize the transition states of the hydrolysis reaction.20−24 Thus, Han et al.21 report that the triblock copolymer Pluronic P-123 (PEO20PPO70PEO20) controls the hydrolysis and condensation reaction of titanium tetraisopropoxide in different salt or acid solutions and TiO2 nanocrystals with unique shape and crystal structure are obtained under these conditions. Additionally, Zhang et al.23 describe that the addition of urea to a hydrochloric acid−ethanol−water solution leads to the

Figure 3b shows the NEXAFS spectra of the anatase reference and a single hybrid particle (particle 1 in Figure 3a) and the averaged spectrum over all hybrid particles shown in Figure 3a. The spectra in Figure 3b show the typical fingerprint assigned to a TiO6 octahedral environment. The two main groups of peaks result from the 2-fold spin−orbit splitting of the titanium 2p level into the 2p1/2 (L2-edge) and 2p3/2 (L3edge) levels and the 2-fold splitting of the 3d level by the octahedral ligand field into t2g and eg states.30 Note that the TiO2 nanoparticles embedded into the microgel and the anatase reference particles show the same spectral features. The anatase reference material shows two additional peaks at photon energies below the L3-edge which are ascribed to the particle−hole coupling.30 These prepeaks are in the noise level for the single hybrid particles due to their small amount of TiO2. Averaging over many particles improves the signal-tonoise ratio sufficiently to resolve also these prepeaks. The asymmetry of the L3−eg doublet is the specific fingerprint for the crystallographic phase of TiO2.30 The direct comparison of the NEXAFS spectra from hybrid particles and the anatase reference material shows clearly that anastase nanoparticles were generated in the PNIPAM shell under room temperature conditions. 826

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carrier particles. For this purpose the normalized absorbance of the individual hybrid particles is analyzed at a photon energy of 457.7 eV. This photon energy of the L3−t2g edge is well-suited because of its narrow and intensive absorption peak. Afterward the intensity distribution is calculated from the obtained absorbance values. Figure 4b shows the distribution of the normalized absorbance versus the relative frequency of the respective absorption value derived from the analysis of 125 individual hybrid particles. The obtained values can be well-described by a Gaussian distribution function which implies that the synthesized TiO2 is randomly distributed over all carrier particles. In conclusion, the combination of NEXAFS spectroscopy with X-ray microscopy is a universal method to get an in-depth understanding of the electronic structure of inorganic nanoparticles on colloidal carrier particles in an aqueous environment. We have demonstrated that crystalline TiO2 nanoparticles with narrow size distribution (d = 6 ± 3 nm) can be synthesized at room temperature in the presence of the microgel particles. The slow addition of the titanium precursor and the dense PNIPAM-network permits to control the kinetics of the TiO2 nanoparticle formation. The analysis of the NEXAFS spectra indicates clearly that the generated TiO2 nanoparticles have an anatase crystal structure. Thus, the PNIPAM microgel acts as a nanoreactor which traps the growing TiO2 nanoparticles and controls the hydrolysis of the titanium precursor under room temperature conditions to generate small crystalline nanoparticles. Moreover, the NEXAFS-TXM spectromicroscopy provides an access to examine the structural homogeneity of the hybrid particles. We believe that this method helps to improve the development of suitable TiO2 nanostructures for applications in renewable energies, drug delivery or environmental pollution protection.

formation of TiO2 rodlike nanocrystals by the titanium precursor tetrabutyl titanate. These particles show a broad size distribution, and they have a rutile crystal structure. In our system, the polar amide groups of the PNIPAM network and the slow addition of Ti(OEt)4 controls the hydrolysis and condensation reaction which leads to small TiO2 nanoparticles with a narrow size distribution and an anatase crystalline phase. Additionally, NEXAFS spectroscopy combined with the high-resolution TXM delivers quantitative information about the homogeneity of the synthesized hybrid particles in their hydrated environment which is not possible with TEM. The uniformity of the crystalline phase and the distribution of the TiO2 particles are the key parameters for potential applications of the hybrid particles as a photocatalyst. These parameters can be excellently studied by NEXAFS-TXM spectromicroscopy which is demonstrated in the following. The uniformity of the TiO2 crystalline phase can be analyzed by the specific fingerprint of the Ti L3−eg edge. Generally, the shape and splitting behavior of the Ti L3−eg features was attributed by Krüger to a long-range band-structure effect which reflects the crystal structure of TiO2 on a length scale of about 1 nm.30 Figure 4a shows NEXAFS spectra of three hybrid particles highlighted in Figure 3a normalized to the intensity of the background signal occurring from the polymeric carrier particles. The spectra of particles 1 and 2 show the typical asymmetrical shape of the Ti L3−eg edge which is assigned to an anatase crystal phase of the TiO2 nanoparticles. However, the spectra of particle 3 in Figure 4a display slight differences in the shape of the Ti L3−eg edge. A large number of individual hybrid particles can be analyzed due to the large image field and the high spatial resolution of the TXM. In the absorbance range of particle 3 half of the NEXAFS spectra do not show the expected splitting of the L3− eg edge for anatase crystals. This means that 14% of all examined hybrid particles show a significant difference in the shape of the L3−eg edge feature. However, the shape of the Ti L3−eg edge cannot be assigned to another well-known crystal structure of TiO2, like rutile or brookite.11,31,32 The intensity difference of the spectra of individual hybrid particles can be attributed to the total mass of the TiO2 within the PNIPAM shell. A possible explanation for the experimental finding can be derived back to the lower signal-to-noise ratio of the recorded NEXAFS spectra. Therefore, the splitting of the L3−eg edge vanishes for hybrid particles with low TiO2 population. Hybrid particles containing significantly more TiO2 exhibit always the expected splitting of the L3−eg edge. As mentioned above, the differences in the absorption intensity between the NEXAFS spectra of individual hybrid particles can be attributed to the total mass of TiO2 within the PNIPAM-shell. Therefore, NEXAFS-TXM spectromicroscopy permits to analyze the mass distribution of embedded TiO2 in the microgel layer. We found by TEM that the size distribution of the generated TiO2 nanoparticles on different single carrier particles is comparable (see Supporting Information, Figure S2). Therefore, the measured intensity distribution in the NEXAFS spectra is proportional to the number of TiO2 nanoparticles for each carrier particle. However, the direct calculation of the total number of TiO2 nanoparticles on each carrier particle from the measured NEXAFS intensity is not possible, because the mass absorption coefficient of the hybrid material is not precisely known as a function of the photon energy. Therefore, we used directly the measured NEXAFS intensity to analyze the TiO2 distribution from many single

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of the used methods, cryo-TEM images of the hybrid particles, HR-TEM image of the dried hybrid particles, and the SAED pattern of and the size distribution anaylzes of the TiO2 nanoparticles on the microgel particles. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft, SPP “Intelligente Hydrogele” is gratefully acknowledged. The manuscript was written by K.H., G.S., and M.B. The TiO2 nanoparticles embedded into microgels are synthesized by Y.L. K.H. and P.G. prepared the samples for cryo-X-ray imaging and collected the NEXAFS-TXM data shown in the context of this work. The analysis of the NEXAFS spectra was done by K.H. F.P. performed all TEM measurements. 827

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(28) Rehbein, S.; Guttmann, P.; Werner, S.; Schneider, G. Opt. Express 2012, 20, 5830−5839. (29) http://www.helmholtz-berlin.de/user/index_en.html (accessed Jan 20, 2013). (30) Krüger, P. Phys. Rev. B 2010, 81. (31) Degroot, F. M. F.; Figueiredo, M. O.; Basto, M. J.; Abbate, M.; Petersen, H.; Fuggle, J. C. Phys. Chem. Miner. 1992, 19, 140−147. (32) Ruus, R.; Kikas, A.; Saar, A.; Ausmees, A.; Nommiste, E.; Aarik, J.; Aidla, A.; Uustare, T.; Martinson, I. Solid State Commun. 1997, 104, 199−203.

ABBREVIATIONS BIS, N,N′-methylenbis(acrylamide); HR-TEM, high-resolution transmission electron microscopy; KPS, potassium persulfate; NEXAFS, near-edge X-ray absorption fine structure; NIPAM, N-isopropylacrylamide; PNIPAM, poly-N-(isopropylacrylamide); PS, polystyrene; SAED, selected-area electron diffraction; SDS, sodium dodecyl sulfate; SPB, spherical polyelectrolyte brush; TEM, transmission electron microscopy; Ti(OEt)4, tetraethylorthotitanate; TXM, transmission X-ray microscopy

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