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Revealing Complexity of Nanoparticle Synthesis in Solution by in Situ Hard X‑ray SpectroscopyToday and Beyond† Dorota Koziej* Laboratory for Multifunctional Materials, Department of Materials, ETH Zürich, HCI F505, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland ABSTRACT: Over the past few decades, material scientists have developed a wide variety of approaches to deliberately introduce chemical and structural inhomogeneities into nanoparticles, achieving for example intentional doping, concentration gradients, and multicomponent structures. This perspective presents a sense of how modern hard X-ray spectroscopic methods, far from merely providing new tools, are extending the way we study and understand such complex nanoparticles. Particularly, the possibility to select with high resolution the incident and emission hard X-ray energies offers unprecedented site selectivity, and rapid data acquisition helps to uncover the complex chemical world behind the state-of-the-art synthetic methods like hot-injection, Brust-Schiffrin, supercritical-flow, and nonaqueous synthesis. Therefore, today the in situ spectroscopic studies provide a new view into the genesis of heterogeneity in colloidal nanoparticles.
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milliseconds.11−20 Particularly, the total X-ray scattering technique in combination with the pair distribution function analysis method, which is sensitive at the same time to shortand long-range structural correlations, is playing an increasingly important role in studying the structural complexity of nanoparticles.19,21−25 The theory, experimental, and data analysis procedures for nanoparticles are reviewed in numbers of excellent contributions.26−28 The in situ studies of nucleation and growth of nanoparticles by means of hard X-ray scattering techniques have been covered in two recent reviews and thus are not further discussed here.11,29 In parallel, we, however, have observed genuine advances in hard X-ray spectroscopic techniques, which are element-specific and allow to be revealed the changes of chemical state and electronic structure of species in solution independent of their state of aggregations and minute concentration and with the temporal resolution of up to milliseconds. The following perspective puts into the spotlight how the unique characteristics of colloidal nanoparticles can be studied in the field of rapidly developing X-ray synchrotron spectroscopic techniques. First, the mechanism of nucleation and growth of nanoparticles in solution and the most important open questions are briefly described. Then, recent advances in X-ray spectroscopic methods, which have the potential to substantially contribute to the above challenges, are highlighted. Only the essential experimental details are given, but readers are directed to reports where comprehensive descriptions of the techniques and newly developed in situ reactors can be found. Finally, an
INTRODUCTION In the past, the word “inhomogeneous” has had a negative connotation, implying imperfection. However, the modern nanomaterials scientist has learned to take advantage of certain structural, compositional, and morphological inhomogeneities. Nowadays, the word “inhomogeneity” can be replaced by terms with a more positive meaning: intentional doping, concentration gradients, core−shell structure, Janus structure, multicomponent structures. This semantic change could not be possible without recent advances in fabrication strategies, which allow for the precise control of nanomaterial composition and properties.1−10 This increase in chemical and structural complexity of nanostructured materials represents a substantial challenge for in situ studies of their nucleation and growth. Today’s understanding of crystallization and the growth mechanism of nanoparticles in solution has greatly profited from the latest development in synchrotron based X-ray absorption and scattering techniques, which provide complementary chemical, electronic and structural information. The driving forces for these developments are brilliance of synchrotron source and fast detectors but also theory and analytical tools allowing analysis of experimental data. The highly energetic incident X-rays (>60 keV) with high flux penetrate through the walls of even conventional reactors and enable in situ X-ray scattering studies of reaction in solution. Xray diffraction (PXRD), total X-rays scattering, and small-angle X-ray scattering (SAXS) are commonly chosen to study the crystallization of nanomaterials in solution, because they allow for the recovery of morphological and structural changes of nanoparticles with temporal resolution up to hundreds of
Received: February 2, 2016 Revised: March 10, 2016
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This Perspective is part of the Up-and-Coming series. © XXXX American Chemical Society
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DOI: 10.1021/acs.chemmater.6b00486 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials emphasis is given to the examples particularly where in situ Xray spectroscopy has already answered specific questions that are inaccessible or equivocal with methods exclusively based on X-ray scattering.
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HOW DO NANOPARTICLES FORM IN SOLUTION? Today we know that crystallization in solution is far more complex than initially anticipated by the theory put forth by Gibbs in 1878 and the early experiments of LaMer and Dinegar.30 In general, we can divide the crystallization in solution into three stages: Formation of the “monomer” species. This is a chemical transformation of precursor(s) in solution preceding nucleation. The common chemical reactions are ligand exchange and condensation, reduction or oxidation of precursor by the thermal decomposition or by reaction with solvent or surfactant. Here, the name monomer is misleading and has to be understood as a species, which will be directly consumed to form solid nuclei. Depending on the reaction, the monomer can be ions, dimers, oligomers, prenucleation clusters, or even amorphous species. Formation of nuclei. The transformation from liquid to stable solid phase takes place when the concentration of monomers reaches the so-called supercritical level. Growth of nanoparticles. Chemical and structural transformation, which follows the formation of nuclei. Among different growth mechanisms we differentiate between dissolution and growth, Ostwald ripening, digestive ripening, recrystallization, ion diffusion, oriented-attachments, and mis-oriented-attachments. However, the scientific dispute on providing a unified description of crystallization and growth in solution is still ongoing.31−38 This is related to the fact that in solution diverse chemical reactions may take place parallel to the nucleation and growth. Some organic species in the reaction solution may be directly related to the formation of the monomer, and some are only side products, while others may act as surfactants and have an impact on the kinetics of nucleation and growth and the final composition of nanoparticles. Finally, newly formed nanoparticles are highly reactive and can catalyze additional reactions in solution. Thus, in many real cases the changes of size and size distribution of nanoparticles cannot be treated without taking into consideration intrinsic changes of the chemical environment.39−42 Modern in situ X-ray spectroscopic methods give us an unprecedented opportunity to reveal the diversity of chemical processes in solution and study their kinetics and the final composition and structure of nanoparticles.
Figure 1. (a) Absorption of incident X-rays of energy Ω promotes an electron to unoccupied states of an atom (XANES) or excites it above the vacuum level into continuum (EXAFS). (b) The outgoing photoelectron (gray) is scattered by the nearest neighbor atom (blue), which causes an oscillatory structure approximately 50 eV above the absorption edge. (c) The whole spectrum is measured in the same experiment, but it is conventionally divided between the XANES and the EXAFS regions. The EXAFS spectrum contains information about the local atomic environment in the proximity of the absorbing atom. (d) The region close to the absorption edge (XANES) comprises the intense absorption peak called whiteline and pre-edge features and contains information about the oxidation state of chemical species.
the excitation of core-level electrons, a portion of X-rays energy is adsorbed, which gives rise to the decrease of transmission through the sample and steep rise of the adsorption coefficient (μ), called the X-ray adsorption edge as shown in Figure 1. The absorption coefficient above the absorption edge displays a fine oscillatory structure due to interference between the outgoing and scattered photoelectron waves as shown in Figure 1b,c. Usually, we divide X-ray absorption spectroscopy into X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Speeding up EXAFSStructural Information. In general, EXAFS can provide quantitative information about the local atomic environment such as coordination number, type, and length of the metal−ligand bond independent of the state of aggregation. In situ EXAFS studies provide valuable insights into the mechanism and kinetics of transformation from precursor to monomer. Conventionally, the time necessary to collect a full EXAFS spectrum with sufficient statistics spans from minutes to an hour, limiting the application of the method to study slow reaction kinetics. The growing demand to study the reaction at time scales relevant for modern synthesis methods like hot-injections,7,44,45 microwave-assisted,46−48 microfluidics,49−51 and supercriticalflow52−54 has triggered an evolution toward quicker acquisition methods. A substantial increase in the time resolution to tens of milliseconds provides a piezo- or cam-driven continuous scanning of the angle of the crystal monochromator during acquisition, leading to the method name quick-EXAFS (or QEXAFS).55 The Fast-Readout Low-Noise (FReLoN) highframe-rate detector, which has been adopted for fast spectra collection in energy-dispersive mode, allows for further slimming down the collection time to even submilliseconds.56−58 The intensified utilization of EXAFS in nanoscience driven by the increasing availability of fast acquisition techniques
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HARD X-RAY SPECTROSCOPIC METHODS TO STUDY IN SITU NANOPARTICLES’ SYNTHESIS IN SOLUTION When hard X-ray light is projected onto matter, multiple photon−atom interactions may occur, where their probability is defined by the cross-section of a given interaction and depends on the element under investigation and the energy of incident X-rays. Hard X-ray spectroscopy involves promoting a core electron of an atom to higher unoccupied levels by incident Xrays (photon in) as shown in Figure 1a, followed by monitoring the transmission or the emission of photons during electron decay to fill the hole (photon out). At energies characteristic for B
DOI: 10.1021/acs.chemmater.6b00486 Chem. Mater. XXXX, XXX, XXX−XXX
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as of solid materials. With an introduction of high brilliance Xray sources, allowing for in situ XANES studies in solution at high temperature and pressure, the method has become almost a routine tool for following the transformation from precursor molecules in solution to the final nanoparticles. However, analysis of the output of these studies is the opposite of being routine. Even for the most investigated nanomaterials, for example, titania and gold, in situ studies continue to reveal new aspects of the reaction, adding to the fundamental understanding of the sophisticated and complex mechanism of nanoparticle synthesis. Photon in/Photon out SpectroscopyGaining a Site Selectivity. The implementation of high-energy resolution spectrometers,67,68 with the instrumental energy broadening smaller than the core-hole lifetime, has triggered a paradigm shift; the high resolution absorption and emission techniques have evolved from being principally used by condensed matter physicists to the wider community, recently attracting also colloidal nanoscientists. The high-energy resolution fluorescence detected (HERFD)-XANES and X-ray emission spectroscopy (XES) can be measured with the same experimental setup and provide in one experiment complementary information about the occupied and unoccupied electronic states of solid materials65,69−73 as shown in Figure 3. The electronic states in molecules are conventionally described in the framework of molecular orbitals theory, and thus HERFDXANES and vtc-XES probe the lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO), respectively. The secondary monochromator can be Johann-type,74 with spherically aligned analyzer crystals as shown in Figure 3a, or von Hamos-type,75 with a single cylindrically bent crystal. The advantage of the Johann-type spectrometer is very low peak-to-background ratio that however requires some time for the data acquisition; example XAS-XES spectra are shown in Figure 3d.65 In the von Hamostype spectrometer, the single cylindrically bent crystal is used together with the position sensitive detector, which allows for the fast data acquisition but compromises the peak-tobackground ratio and the efficiency; an example of the RXES map, which consists of RXES spectra recorded at different incident energies, is shown in Figure 3e.66 Moreover, recent advances in the simulation of theoretical spectra enable the comprehensive analysis and interpretation of the spectra measured with high-energy resolution. For the comprehensive description of the XAS and XES theory, codes used for data modeling and analysis and experimental setups readers are refereed to the dedicated reports.64,67−69,76−84 HERFD XAS. The HERFD-XAS is based on recording the intensity of the emitted X-rays (ω) with high-energy resolution, while scanning the incoming X-ray (Ω) across the absorption edge of an element as shown in Figure 3d. Therefore, the HERFD-XANES technique in comparison to the conventional XANES (a) gives access to more detailed information about the arrangement of molecular orbitals and their occupancies, (b) serves as a direct probe of unoccupied states, (c) facilitate study of the minute concentration of an atom in a strongly absorbing matrix, and (d) discriminates between the metallic core and oxidic shell of nanoparticles.85−87 Such an enhancement of site selectivity is essential for studying nanomaterials with complex chemical and structural composition. XES. XES probes the occupied states. The valence electrons can be probed indirectly or directly by scanning the intensity of the emitted X-rays due to the transition of the outer core (core-
coincided with the ability to reach exceptional control over shape, size, and size distribution of nanoparticle synthesis in colloidal solution. The amplitude and the phase of scattered photoelectrons is sensitive to the number and type of the nearest neighbor, which gives access to the reliable analysis of the subtle changes in the EXAFS spectrum of nanoparticles. The accuracy of the determination of interatomic distances is between 0.01 and 0.001 Å, but in certain cases it can be improved even by a factor of 100.58 The unprecedented sensitivity to structural details is beyond the reach of the advanced electron imaging resolution and coherent X-ray scattering techniques.59 The multiple-scattering analysis of the EXAFS spectrum allows for determining the coordination number, bond length, and disorder factor, which for the nanoparticle relates with its size and shape.59−63 The coordination number is basically a measure of the average number of nearest neighbors within a given shell. Moreover, to determine the total coordination number for multimetallic nanoparticles, EXAFS spectra are measured at multiple absorption edges. The recently developed analytical approaches provide a means to even obtain quantitative information about atomic distribution and, thus, enable discrimination between alloy, mixtures, and core−shell structures as shown in Figure 2.59,64 This information was not previously accessible because,
Figure 2. Example of Fourier transform of EXAFS spectra recorded at Pt (b) and Ru (c) edges in the alloy and core−shell bimetallic nanocatalysts. The data show distinct differences: in the Ru-core/Ptshell system, Pt appears to have a low Z neighbor; Ru EXAFS appears to be metallic in nature. That indicates that Pt segregates to the shell and Ru to the core. The fact that the Ru signal is much lower in the multiple-scattering region in the core−shell system compared to the alloy system points to a high degree of disorder around Ru atoms. Adapted with permission from ref 43. Copyright 2009 American Chemical Society.
in the case of a broad distribution of particle sizes, the larger particles dominate the experimentally determined values. XANESFingerprint of Chemical Information. The region close to the absorption edge (XANES) probed unoccupied electronic states above the Fermi level. It comprises the intense absorption peak called whiteline, post-edge and preedge features and is commonly perceived as a fingerprint of a chemical species as shown in Figure 1d. The whiteline is sensitive to chemical bonding and the oxidation state of the absorbing atom. Postedge features are dominated by multiplescattering resonances of the photoelectrons ejected at low kinetic energy and thus give information about the atomic position of neighbors, like interatomic distances and bond angles. Pre-edge features correspond to the transition of the photoelectron to unoccupied localized states and are evidence for a broken inversion symmetry that gives additional structural information. For those reasons conventional XANES has been for many years the favorite method for scientists to qualitatively study the oxidation state of both molecules in solution as well C
DOI: 10.1021/acs.chemmater.6b00486 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. (a) Scheme of the setup used for photon in (Ω)/photon out (ω) experiments. Absorption of X-rays with the incident photon energy Ω is creating a hole in the core level. In the secondary process, a core hole is filled by an electron from a higher level, and a photon with energy ω is emitted. The spectrometer is equipped with the secondary monochromator, which allows scanning the energy of emitted photon with high-energy resolution. (b, c) Schematics showing different electron-energy levels in a solid semiconducting material involved in the photon in/photon out process: HERFD-XANES (pink), vtc-XES (blue), and ctc-XES (green). (d) XANES spectra of La2O2CO3 nanoparticles recoded in HERFD mode with Johann-type spectrometer and fluorescence mode with conventional diode at ID26 beamline, ESRF in France.65 (e) Resonant XES maps of titania nanoparticles doped 2% with nitrogen recorded with von Hamos spectrometer at SuperXAS beamline at SLS in Switzerland. Reproduced from ref 66 with permission. Copyright 2013 The Royal Society of Chemistry.
to-core, ctc) or the valence electrons (valence-to-core, vtc) to the core holes. For molecules, the vtc-XES features represent transitions from filled orbitals that are dominantly ligand in nature and thus in contrast to EXAFS, and so vtc-XES is able to discriminate between ligands with similar atomic numbers such as O, N, C, etc., in the first coordination shell as shown in the example of Fe-species in Figure 4. Note that iron acetylacetonate is a commonly used precursor in the solutionbased synthesis of ferrite nanoparticles. Over the past few years the systematic efforts, foremost in the field of bioinorganic chemistry and catalysis, have paved the way to assess the identity of ligands bound to a metal center, quantify the degree of bond activation, and establish the protonation state of donor atoms.73,88−92 Particularly, for in situ colloidal synthesis, vtcXES promises to shed light on questions about metal precursors and their ligand interaction with solvents, especially with respect to intermediates before formation of monomer species. But due to the intrinsically low intensity of vtc transitions, the in situ experiments in solution, at high temperatures, high pressure, and at the relevant time scales
Figure 4. An example illustrating the ligand-sensitivity of XES: (a) The schematic of simplified molecular orbital diagram of Fe-species, with corresponding emission channels. (b) XES spectrum of Fe2O3 with inset showing the vtc region enlarged roughly 100-fold. (c) vtc-XES spectrum of Fe species with different ligands Fe(acac)3, Fe(TACN)23+, and Fe(CN)63−, where TACN is 1,4,7- triazacyclononane and acac is acetylacetonate. Adapted with permission from ref 88. Copyright 2011 American Chemical Society.
still remain challenging. Now, all eyes are on the X-ray free electron laser (XFEL), which with the brilliance massively higher than any conventional X-ray source may allow in the D
DOI: 10.1021/acs.chemmater.6b00486 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 5. Examples of in situ studies of sol−gel synthesis of titania in aqueous (a−c) and nonaqueous solution (d−f). In situ QEXAFS studies of mechanism of amorphous titania formation in aqueous sol−gel reaction: (a) Time resolved data. (b) An example of an LC-fit: Black line is the original spectrum after 775 s; dark gray line is the fit considering only the spectra of precursor (Ti(OPri)4) and final nanoparticles (amorphous titania); light gray is the fit considering the spectra of precursor (Ti(OPri)4), intermediate olligomeric species (dodecatitanate), and final nanoparticles (amorphous titania). Bottom graph shows the deviation of the fits from the measured spectrum. It is evident that two components are not sufficient to describe the measured spectrum and the LC-fit is only acceptable if reference spectra of three compounds are used. Adapted with permission from ref 97. Copyright 2010 American Chemical Society. (c) Changes of the relative fraction of individual species in solution. In situ XANES studies of mechanism of crystalline titania formation in aqueous sol−gel reaction. (d) Time resolved XANES data; (e) Spectra of three individual components obtained from MCR-ALS analysis. The MCR-ALS method allows the data analysis without prior knowledge of number of components and does not requires the spectra of individual components. (f) Changes of the relative fraction of individual species in solution and in the inset schematic of nucleation mechanism. Adapted by permission from ref 98. Copyright 2014 The Royal Society of Chemistry.
future monitoring of changes of the vtc-transition during chemical reactions on the femtosecond time scale.93,94 Note that XES was recently utilized for the in situ monitoring of transformation of Au2O3 nanoparticles to Au,95 NiO nanoparticles to Ni,71 and Prussian blue analogue to Co3O4 nanoparticles,96 but since they focus on solid-state transformation during calcination in gases they go beyond the scope of this discussion. Reactors for in Situ X-ray Spectroscopy Studies. The hard X-ray spectroscopy is measured at the energy specific for the element of interest (
