In Situ Observation of Nucleation and Growth Process of Gold Nanoparticles by Quick XAFS Spectroscopy

In Situ Observations Reveal the Five-fold Twin-Involved Growth of Gold Nanorods by Particle Attachment

Crystallization plays a critical role in determining crystal size, purity and morphology. Therefore, uncovering the growth dynamics of nanoparticles (NPs) atomically is important for the controllable fabrication of nanocrystals with desired geometry and properties. Herein, we conducted in situ atomic-scale observations on the growth of Au nanorods (NRs) by particle attachment within an aberration-corrected transmission electron microscope (AC-TEM). The results show that the attachment of spherical colloidal Au NPs with a size of about 10 nm involves the formation and growth of neck-like (NL) structures, followed by five-fold twin intermediate states and total atomic rearrangement. The statistical analyses show that the length and diameter of Au NRs can be well regulated by the number of tip-to-tip Au NPs and the size of colloidal Au NPs, respectively. The results highlight five-fold twin-involved particle attachment in spherical Au NPs with a size of 3-14 nm, and provide insights into the fabrication of Au NRs using irradiation chemistry.

In Situ Kinetic Observations on Crystal Nucleation and Growth

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

Single plasmonic nanostructures for biomedical diagnosis

The field of single nanoparticle plasmonics has grown enormously. There is no doubt that a wide diversity of the nanoplasmonic techniques and nanostructures represents a tremendous opportunity for fundamental biomedical studies as well as sensing and imaging applications. Single nanoparticle plasmonic biosensors are efficient in label-free single-molecule detection, as well as in monitoring real-time binding events of even several biomolecules. In the present review, we have discussed the prominent advantages and advances in single particle characterization and synthesis as well as new insight into and information on biomedical diagnosis uniquely obtained using single particle approaches. The approaches include the fundamental studies of nanoplasmonic behavior, two typical methods based on refractive index change and characteristic light intensity change, exciting innovations of synthetic strategies for new plasmonic nanostructures, and practical applications using single particle sensing, imaging, and tracking. The basic sphere and rod nanostructures are the focus of extensive investigations in biomedicine, while they can be programmed into algorithmic assemblies for novel plasmonic diagnosis. Design of single nanoparticles for the detection of single biomolecules will have far-reaching consequences in biomedical diagnosis.

Extracting structural information of Au colloids at ultra-dilute concentrations: identification of growth during nanoparticle immobilization

Sol-immobilization is increasingly used to achieve supported metal nanoparticles (NPs) with controllable size and shape; it affords a high degree of control of the metal particle size and yields a narrow particle size distribution. Using state-of-the-art beamlines, we demonstrate how X-ray absorption fine structure (XAFS) techniques are now able to provide accurate structural information on nano-sized colloidal Au solutions at μM concentrations. This study demonstrates: (i) the size of Au colloids can be accurately tuned by adjusting the temperature of reduction, (ii) Au concentration, from 50 μM to 1000 μM, has little influence on the average size of colloidal Au NPs in solution and (iii) the immobilization step is responsible for significant growth in Au particle size, which is further exacerbated at increased Au concentrations. The work presented demonstrates that an increased understanding of the primary steps in sol-immobilization allows improved optimization of materials for catalytic applications.

Insights into the Synthesis Mechanism of Ag29 Nanoclusters

The current understanding of the synthesis mechanisms of noble metal clusters is limited, in particular for Ag clusters. Here, we present a detailed investigation into the synthesis process of atomically monodisperse Ag₂₉ clusters, prepared via reduction of AgNO₃ in the presence of dithiolate ligands. Using optical spectroscopy, mass spectrometry, and X-ray spectroscopy, it was determined that the synthesis involves a rapid nucleation and growth to species with up to a few hundred Ag atoms. From these larger species, Ag₂₉ clusters are formed and their concentration increases steadily over time. Oxygen plays an important role in the etching of large particles to Ag₂₉. No other stable Ag cluster species are observed at any point during the synthesis.

Insight into growth of Au-Pt bimetallic nanoparticles: an in situ XAS study

Au-Pt bimetallic nanoparticles have been synthesized through a one-pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous in situ measurement of X-ray absorption spectroscopy (XAS) and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at Indus-2 SRS at RRCAT, Indore, India. In situ XAS spectra, comprising both X-ray near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L₃-edges. While the XANES spectra of the precursors provide real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core-shell-type configuration of a bimetallic alloy in a one-pot synthesis method. The core-shell-type structure of the nanoparticles has also been confirmed by ex situ energy-dispersive spectroscopy line-scan and X-ray photoelectron spectroscopy measurements with in situ ion etching on fully formed nanoparticles.

Quick-EXAFS setup at the SuperXAS beamline for in situ X-ray absorption spectroscopy with 10 ms time resolution

The quick-EXAFS (QEXAFS) method adds time resolution to X-ray absorption spectroscopy (XAS) and allows dynamic structural changes to be followed. A completely new QEXAFS setup consisting of monochromator, detectors and data acquisition system is presented, as installed at the SuperXAS bending-magnet beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland). The monochromator uses Si(111) and Si(311) channel-cut crystals mounted on one crystal stage, and remote exchange allows an energy range from 4.0 keV to 32 keV to be covered. The spectral scan range can be electronically adjusted up to several keV to cover multiple absorption edges in one scan. The determination of the Bragg angle close to the position of the crystals allows high-accuracy measurements. Absorption spectra can be acquired with fast gridded ionization chambers at oscillation frequencies of up to 50 Hz resulting in a time resolution of 10 ms, using both scan directions of each oscillation period. The carefully developed low-noise detector system yields high-quality absorption data. The unique setup allows both state-of-the-art QEXAFS and stable step-scan operation without the need to exchange whole monochromators. The long-term stability of the Bragg angle was investigated and absorption spectra of reference materials as well as of a fast chemical reaction demonstrate the overall capabilities of the new setup.

Growth of block copolymer stabilized metal nanoparticles probed simultaneously by in situ XAS and UV-Vis spectroscopy

The growth of Au and Pt nanoparticles from their respective chloride precursors using block copolymer-based reducers has been studied by simultaneous in situ measurement of XAS and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at INDUS-2 SRS at RRCAT, Indore, India. While the XANES spectra of the precursor give real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed at the intermediate stages of growth. The growth kinetics of both types of nanoparticles are found to be almost similar and are found to follow three stages, though the first stage of nucleation takes place earlier in the case of Au than in the case of Pt nanoparticles due to the difference in the reduction potential of the respective precursors. The first two stages of the growth of Au and Pt nanoparticles as obtained by in situ XAS measurements could be corroborated by simultaneous in situ measurement of UV-Vis spectroscopy also.

Description and Role of Bimetallic Prenucleation Species in the Formation of Small Nanoparticle Alloys

We report the identification, description, and role of multinuclear metal-thiolate complexes in aqueous Au-Cu nanoparticle syntheses. The structure of these species was characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy techniques. The observed structures were found to be in good agreement with thermodynamic growth trends predicted by first-principles calculations. The presence of metal-thiolate complexes is then shown to be critical for the formation of alloyed Au-Cu architectures in the small nanoparticle regime (diameter ∼2 nm). In the absence of mixed metal-thiolate precursors, nanoparticles form with a Cu-S shell and a Au-rich interior. Taken together, these results demonstrate that prenucleation species, which are discrete molecular precursors distinct from both initial reagents and final particle products, may provide an important new synthetic route to control final metal nanoparticle composition and composition architectures.

In situ studies on controlling an atomically-accurate formation process of gold nanoclusters

Knowledge of the molecular formation mechanism of metal nanoclusters is essential for developing chemistry for accurate control over their synthesis. Herein, the "top-down" synthetic process of monodisperse Au13 nanoclusters via HCl etching of polydisperse Aun clusters (15 ≤ n ≤ 65) is traced by a combination of in situ X-ray/UV-vis absorption spectroscopy and time-dependent mass spectrometry. It is revealed experimentally that the HCl-induced synthesis of Au13 is achieved by accurately controlling the etching process with two distinctive steps, in sharp contrast to the traditional thiol-etching mechanism through release of the Au(i) complex. The first step involves the direct fragmentation of the initial larger Aun clusters into metastable intermediate Au8-Au13 smaller clusters. This is a critical step, which allows for the secondary size-growth step of the intermediates toward the atomically monodisperse Au13 clusters via incorporating the reactive Au(i)-Cl species in the solution. Such a secondary-growth pathway is further confirmed by the successful growth of Au13 through reaction of isolated Au11 clusters with AuClPPh3 in the HCl environment. This work addresses the importance of reaction intermediates in guiding the way towards controllable synthesis of metal nanoclusters.

Microwave-assisted synthesis of water-soluble, fluorescent gold nanoclusters capped with small organic molecules and a revealing fluorescence and X-ray absorption study

Colourless solutions of blue light-emitting, water-soluble gold nanoclusters (AuNC) were synthesized from gold colloids under microwave irradiation using small organic molecules as ligands. Stabilized by 1,3,5-triaza-7-phosphaadamantane (TPA) or L-glutamine (GLU), fluorescence quantum yields up to 5% were obtained. AuNC are considered to be very promising for biological labelling, optoelectronic devices and light-emitting materials but the structure-property relationships have still not been fully clarified. To expand the knowledge about the AuNC apart from their fluorescent properties they were studied by X-ray absorption spectroscopy elucidating the oxidation state of the nanoclusters' gold atoms. Based on curve fitting of the XANES spectra in comparison to several gold references, optically transparent fluorescent AuNC are predicted to be ligand-stabilized Au5(+) species. Additionally, their near edge structure compared with analogous results of polynuclear clusters known from the literature discloses an increasing intensity of the feature close to the absorption edge with decreasing cluster size. As a result, a linear relationship between the cluster size and the X-ray absorption coefficient can be established for the first time.

A laboratory-based hard x-ray monochromator for high-resolution x-ray emission spectroscopy and x-ray absorption near edge structure measurements

We report the development of a laboratory-based Rowland-circle monochromator that incorporates a low power x-ray (bremsstrahlung) tube source, a spherically bent crystal analyzer, and an energy-resolving solid-state detector. This relatively inexpensive, introductory level instrument achieves 1-eV energy resolution for photon energies of ∼5 keV to ∼10 keV while also demonstrating a net efficiency previously seen only in laboratory monochromators having much coarser energy resolution. Despite the use of only a compact, air-cooled 10 W x-ray tube, we find count rates for nonresonant x-ray emission spectroscopy comparable to those achieved at monochromatized spectroscopy beamlines at synchrotron light sources. For x-ray absorption near edge structure, the monochromatized flux is small (due to the use of a low-powered x-ray generator) but still useful for routine transmission-mode studies of concentrated samples. These results indicate that upgrading to a standard commercial high-power line-focused x-ray tube or rotating anode x-ray generator would result in monochromatized fluxes of order 10(6)-10(7) photons/s with no loss in energy resolution. This work establishes core technical capabilities for a rejuvenation of laboratory-based hard x-ray spectroscopies that could have special relevance for contemporary research on catalytic or electrical energy storage systems using transition-metal, lanthanide, or noble-metal active species.

A comprehensive study of the crystallization mechanism involved in the nonaqueous formation of tungstite

We present a detailed study on the nonaqueous synthesis of tungstite nanostructures with the focus on crystallization processes and the evolution of particle morphology. Time-dependent transmission electron microscopy (TEM) revealed a complex, particle-based crystallization mechanism involving first the formation of spherical and single-crystalline primary particles of 2-8 nm, which are cross-linked to large and unordered agglomerates, followed by their organization into rod-like structures of 40 × 200-400 nm. These rods undergo an internal ordering process, during which crystallographically oriented stacks of platelets develop. In situ small angle X-ray scattering (SAXS) experiments confirm this pathway of particle formation. The scattering intensity is dominated by the fast formation of rod-like particles, which cause an inter-platelet peak in the SAXS pattern with ongoing internal ordering. With continuous reaction time, the platelet stacks start to fall apart forming shorter assemblies of just a few platelets or even single platelets.

New findings for the composition and structure of Ni nanoparticles protected with organomercaptan molecules

Here we explore the synthesis of alkanethiol-coated Ni NPs following the one-phase reaction method by Brust et al. The reduction of NiCl2 with NaBH4 in the presence of dodecanethiol (C12SH) yields a complex product that is difficult to identify as illustrated in the figure of merit. We synthesized Ni(II) dodecanethiolate (C12S) (without the addition of NaBH4) for comparison and performed an exhaustive characterization with TEM, HR-TEM, AFM, MFM, XPS, XRD, UV-vis, magnetism, and FT-IR. It is found that the organic coating is not quite a well-organized self-assembled monolayer (SAM) surrounding the Ni cluster as previously reported. XPS and XRD data show slight differences between both syntheses; however, Ni(II) thiolate appears to be more stable than reduced Ni when exposed to ambient air, indicating the propensity of metallic Ni to oxidize. It has been shown that irradiating with TEM electrons over various metal thiolates leads to nanoparticle formation. We irradiated over Ni(II) thiolate and observed no evidence of NP formation whereas irradiating a reduced Ni sample exhibited an ~3.0 nm nanoparticle diameter. Magnetism studies showed a difference between both samples, indicating ferromagnetic character for the reduced Ni sample. According to our results, the product of the synthesis is comprised of ultrasmall metallic clusters embedded in some form of Ni(II) C12S. In this work, we open a discussion of the chemical nature of the core and the shell in the synthesis of Ni NPs protected with organomercaptan molecules.

Electronic transfer as a route to increase the chemical stability in gold and silver core-shell nanoparticles

This review article presents the collected recent findings and advancements in understanding and manipulating the electronic properties of the Au/Ag NP system from the standpoint of controlling the characteristics of heterostructured core-shell NPs. The discovery of the electronic transfer effect through analysis of both Ag-Au and Au-Ag type NPs inspired the analysis of the resulting enhanced properties. First, the background on the synthesis and characterization of Ag, Au, Ag-Au, Au-Ag and Au-Ag-Au NPs, which will be used as a basis for studying the electronic transfer and stability properties is presented. Next, Mie Theory is used to inspect the optical properties of the Ag-Au NPs, revealing subtle structural characteristics in these probes, which has implications to the plasmonic properties. This is followed by the inspection of the electronic properties of the Au-Ag NPs primarily through XPS and XANES analysis, revealing the origins of the electronic transfer phenomenon. The unique electronic properties are then revealed to result in improved particle stability in terms of susceptibility to oxidation. Finally, an assessment of the resulting enhanced plasmonic sensing properties is discussed. The results are presented in terms of synthesis technique, material characterization, understanding of the electronic properties and manipulation of those properties to create Au-Ag NPs with enhanced resistance to oxidation and galvanic replacement.

Nucleation and aggregative growth process of platinum nanoparticles studied by in situ quick XAFS spectroscopy

The early stage in the nucleation and subsequent aggregative particle growth of the colloidal platinum (Pt) dispersions produced by photoreduction in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) was quantitatively investigated by means of in situ quick XAFS (QXAFS) measurements. The stages of the reduction-nucleation and the association process (aggregative particle growth and Ostwald ripening) of Pt atoms to produce Pt nanoparticles was successfully discriminated in course of the photoreduction time. The present QXAFS analysis indicated that Pt nuclei (i.e., (Pt(0))(m) nucleates approximately m = 4) were continuously produced in the reduction-nucleation process at the early time, followed by the aggregative particle growth with the autocatalytic reduction of Pt ionic species on the surface of Pt nuclei to produce Pt nanoparticles. Subsequently the particle growth proceeded via Ostwald ripening, resulting in the production of larger Pt nanoparticles at a later time. It was also found that the aggregative particle growth follows a sigmoidal profile well described either by the solid-state kinetic model or by the chemical-mechanism-based kinetic model, specifically the Avrami-Erofe'ev or Finke-Watzky models. The difference in terms of the formation mechanism was observed between the reduction of Pt(IV)Cl(6)(2-) and Pt(II)Cl(4)(2-) as a source material. Also presented is that the addition of the photoactivator such as benzoin, benzophenone, and acetophenone in the system is very effective to enhance the rate for the formation of Pt nanoparticles.