General Equation for Size Nanocharacterization of the Core−Shell Nanoparticles by X-ray Photoelectron Spectroscopy

A simple equation to determine the shell thicknesses of core-shell nanoparticles based on XPS data of their elemental composition

A simple analytical expression is obtained relating the radius of the core, the thickness of the shell of nanoparticles, and the intensities of X-ray photoelectron lines from the core and shell, recorded during one experiment. The effective evaluation of the proposed equation was verified by comparison with the results of calculations of the parameters of core-shell nanoparticles (NPs) using known methods, as well as by comparing the results of ratios between the radius and thickness of the shell of specific NPs and their evaluations using the transmission electron microscopy method. The formula proposed in this work also allows using EDS data to estimate the core-shell parameters of nanoparticles. It is shown that the equation obtained in this work is not inferior to the solutions of the already existing approximate equations in terms of the accuracy of the determined parameters, but it is more convenient to use, since the data of one experiment are sufficient for its application. A simple approach to determine the thickness of a shell of NPs based on information about the elemental composition of the core-shell of NPs measured by X-ray photoelectron spectroscopy is developed.

Strategies for extending charge separation in colloidal nanostructured quantum dot materials

Semiconductor colloidal metal chalcogenides (II-VI) in the form of quantum dots (QDs) and different heterostructures (core/shell, alloys, etc.) are of extensive interest in scientific research for both a fundamental understanding and technological applications because of their quantized size and different optical properties; however, due to their small size, the exciton (bound electron and hole) experiences a strong Coulombic attraction, which has a remarkable impact on the charge separation and photophysical properties of QDs. Thus, to achieve an efficient charge separation, numerous attempts have been made via the formation of different heterostructures, QD/molecular adsorbate (either organic or inorganic) assemblies, etc. These hybrid materials ameliorated the absorption of the incident light as well as charge separation. This article reviews the strategies for extending charge separation in these colloidal nanocrystals (NCs), which is one of the crucial steps to elevate the solar to electrical energy conversion efficiency in a quantum dot-sensitized solar cell (QDSC). The article summarizes the benefits of co-sensitization and experimental shreds of evidence for the multiple charge transfer processes involved in a QDSC. Studies have shown that in the co-sensitization process, prolonged charge separation occurs via the dual behavior of the molecular adsorbate, sensitization (electron injection) and capture of holes from photoexcited QDs. This perspective emphases band edge engineering and control of charge carrier dynamics in various core/shell structures. The impact of colloidal alloy NCs on charge separation and interesting photophysical properties was recapitulated via the steady-state and time-resolved photoluminescence (PL) and femtosecond transient absorption spectroscopic techniques. Finally, the prolonged lifetime and extent of charge separation for these hybrid NCs (or the composites) assisted in the development of a better light harvester as compared to the case of their pure counterparts.

Quantifying the Impact of Nanoparticle Coatings and Nonuniformities on XPS Analysis: Gold/Silver Core-Shell Nanoparticles

Spectral modeling of photoelectrons can serve as a valuable tool when combined with X-ray photoelectron spectroscopy (XPS) analysis. Herein, a new version of the NIST Simulation of Electron Spectra for Surface Analysis (SESSA 2.0) software, capable of directly simulating spherical multilayer NPs, was applied to model citrate stabilized Au/Ag-core/shell nanoparticles (NPs). The NPs were characterized using XPS and scanning transmission electron microscopy (STEM) to determine the composition and morphology of the NPs. The Au/Ag-core/shell NPs were observed to be polydispersed in size, nonspherical, and contain off-centered Au-cores. Using the average NP dimensions determined from STEM analysis, SESSA spectral modeling indicated that washed Au/Ag-core-shell NPs were stabilized with a 0.8 nm layer of sodium citrate and a 0.05 nm (one wash) or 0.025 nm (two wash) layer of adventitious hydrocarbon, but did not fully account for the observed XPS signal from the Au-core. This was addressed by a series of simulations and normalizations to account for contributions of NP nonsphericity and off-centered Au-cores. Both of these nonuniformities reduce the effective Ag-shell thickness, which effect the Au-core photoelectron intensity. The off-centered cores had the greatest impact for the particles in this study. When the contributions from the geometrical nonuniformities are included in the simulations, the SESSA generated elemental compositions that matched the XPS elemental compositions. This work demonstrates how spectral modeling software such as SESSA, when combined with experimental XPS and STEM measurements, advances the ability to quantitatively assess overlayer thicknesses for multilayer core-shell NPs and deal with complex, nonideal geometrical properties.

A Technique for Calculation of Shell Thicknesses for Core-Shell-Shell Nanoparticles from XPS Data

This paper extends a straightforward technique for the calculation of shell thicknesses in core-shell nanoparticles to the case of core-shell-shell nanoparticles using X-ray Photoelectron Spectroscopy (XPS) data. This method can be applied by XPS analysts and does not require any numerical simulation or advanced knowledge, although iteration is required in the case where both shell thicknesses are unknown. The standard deviation in the calculated thicknesses vs simulated values is typically less than 10%, which is the uncertainty of the electron attenuation lengths used in XPS analysis.

Facile method to synthesize magnetic iron oxides/TiO2 hybrid nanoparticles and their photodegradation application of methylene blue

Many methods have been reported to improving the photocatalytic efficiency of organic pollutant and their reliable applications. In this work, we propose a facile pathway to prepare three different types of magnetic iron oxides/TiO2 hybrid nanoparticles (NPs) by seed-mediated method. The hybrid NPs are composed of spindle, hollow, and ultrafine iron oxide NPs as seeds and 3-aminopropyltriethyloxysilane as linker between the magnetic cores and TiO2 layers, respectively. The composite structure and the presence of the iron oxide and titania phase have been confirmed by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectra. The hybrid NPs show good magnetic response, which can get together under an external applied magnetic field and hence they should become promising magnetic recovery catalysts (MRCs). Photocatalytic ability examination of the magnetic hybrid NPs was carried out in methylene blue (MB) solutions illuminated under Hg light in a photochemical reactor. About 50% to 60% of MB was decomposed in 90 min in the presence of magnetic hybrid NPs. The synthesized magnetic hybrid NPs display high photocatalytic efficiency and will find recoverable potential applications in cleaning polluted water with the help of magnetic separation.

One-step substrate nanofabrication and patterning of nanoparticles by lithographically controlled etching

We propose an integrated top-down and bottom-up approach to single-step nanofabrication of complex nanostructures made of different materials. The process, termed lithographically controlled etching (LCE), starts with a drop of an etching solution cast on the surface to be patterned. By placing a polymeric mold on the substrate, the stamp protrusions come into contact with the surface, thus protecting it, whereas the surface beneath the mold recesses is exposed to a thin layer of etching solution, allowing the surface to be etched. By dispersing nanoparticles into the etching solution, these can be deposited and self-organize in the recesses on the substrate as these are excavated. We demonstrate here the fabrication of complex structures and nanowires 30 nm wide. Moreover, by exploiting capillary forces, it is possible to deposit nanoparticles at precise positions with respect to optically addressable microstructures, thus realizing a multiscale functional pattern.

Incorporation of tripolyphosphate nanoparticles into fibrous poly(lactide-co-glycolide) scaffolds for tissue engineering

Poly(lactide-co-glycolide) (PLGA) has been widely used for scaffolding materials in tissue engineering. It degrades mainly via hydrolysis of the ester bonds into lactic acid and glycolic acid leading to the decrease in pH of the surrounding microenvironment. The current study was designed to quickly neutralize the acidic degradation products of PLGA fibrous scaffolds by incorporating tripolyphosphate (TPP) nanoparticles into PLGA fibers. A homogeneous mixture of PLGA and TPP was first obtained by water-in-oil emulsion-dispersion followed by freeze-drying. The dried blend was melt-spun to yield fibers which were processed into scaffolds and subsequently immersed into phosphate-buffered saline (PBS) to verify the degradation properties. The pH of the saline was monitored for a duration of 80 days. The amount of TPP was optimized to obtain a PLGA based scaffolds without acidic degradation problems. Cellular compatibility of the modified and pristine scaffolds was evaluated using rabbit adipose-derived stem cells (rASCs). It was shown that TPP particles within the fibers were roughly 100nm in diameter and mainly located inside fibers instead of on the superficial layer. The acidic degradation of PT-16 and PT-64 (PT-X is termed when the monomer molar ratio of TPP to PLGA was 1:X) was significantly improved as the pH values of their respective solutions were maintained in a well neutralized state during the degradation. PT-64 and PT-16 scaffolds could well support the attachment and proliferation of rASCs. Hence, the incorporation of TPP nanoparticles via an emulsion-dispersion method could be an effective strategy to improve/adjust the acidic degradation of PLGA and further pave the way for clinical applications of such polyesters.

Core/Shell semiconductor nanocrystals

Colloidal core/shell nanocrystals contain at least two semiconductor materials in an onionlike structure. The possibility to tune the basic optical properties of the core nanocrystals, for example, their fluorescence wavelength, quantum yield, and lifetime, by growing an epitaxial-type shell of another semiconductor has fueled significant progress on the chemical synthesis of these systems. In such core/shell nanocrystals, the shell provides a physical barrier between the optically active core and the surrounding medium, thus making the nanocrystals less sensitive to environmental changes, surface chemistry, and photo-oxidation. The shell further provides an efficient passivation of the surface trap states, giving rise to a strongly enhanced fluorescence quantum yield. This effect is a fundamental prerequisite for the use of nanocrystals in applications such as biological labeling and light-emitting devices, which rely on their emission properties. Focusing on recent advances, this Review discusses the fundamental properties and synthesis methods of core/shell and core/multiple shell structures of II-VI, IV-VI, and III-V semiconductors.

Core and shell sizing of small silver-coated nanospheres by optical extinction spectroscopy

Silver metal nanoparticles (Nps) are extensively used in different areas of research and technology due to their interesting optical, thermal and electric properties, especially for bare core and core-shell nanostructures with sizes smaller than 10 nm. Since these properties are core-shell size-dependent, size measurement is important in manipulating their potential functionalization and applications. Bare and coated small silver Nps fabricated by physical and chemical methods present specific characteristics in their extinction spectra that are potentially useful for sizing purposes. This work presents a novel procedure to size mean core radius smaller than 10 nm and mean shell thickness of silver core-shell Nps based on a comparative study of the characteristics in their optical extinction spectra in different media as a function of core radii, shell thickness and coating refractive index. From the regularities derived from these relationships, it can be concluded that plasmon full width at half-maximum (FWHM) is sensitive to core size but not to coating thickness, while plasmon resonance wavelength (PRW) is related to shell thickness and mostly independent of core radius. These facts, which allow sizing simultaneously both mean core radius and shell thickness, can also be used to size bare silver Nps as a special case of core-shell Nps with zero shell thickness. The proposed method was applied to size experimental samples and the results show good agreement with conventional TEM microscopy.

Determination of the oxide layer thickness in core-shell zerovalent iron nanoparticles

Zerovalent iron (nZVI) nanoparticles have long been used in the electronic and chemical industries due to their magnetic and catalytic properties. Increasingly, applications of nZVI have also been reported in environmental engineering because of their ability to degrade a wide variety of toxic pollutants in soil and water. It is generally assumed that nZVI has a core-shell morphology with zerovalent iron as the core and iron oxide/hydroxide in the shell. This study presents a detailed characterization of the nZVI shell thickness using three independent methods. High-resolution transmission electron microscopy analysis provides direct evidence of the core-shell structure and indicates that the shell thickness of fresh nZVI was predominantly in the range of 2-4 nm. The shell thickness was also determined from high-resolution X-ray photoelectron spectroscopy (HR-XPS) analysis through comparison of the relative integrated intensities of metallic and oxidized iron with a geometric correction applied to account for the curved overlayer. The XPS analysis yielded an average shell thickness in the range of 2.3-2.8 nm. Finally, complete oxidation reaction of the nZVI particles by Cu(II) was used as an indication of the zerovalent iron content of the particles, and these observations further correlate the chemical reactivity of the particles and their shell thicknesses. The three methods yielded remarkably similar results, providing a reliable determination of the shell thickness, which fills an essential gap in our knowledge about the nZVI structure. The methods presented in this work can also be applied to the study of the aging process of nZVI and may also prove useful for the measurement and characterization of other metallic nanoparticles.