Cation-exchange Procedure Research Articles

On the Suitable Choice of Metal for HgTe Nanocrystal-Based Photodiode: To Amalgam or Not to Amalgam

HgTe, thanks to its unique spectral tunability in the infrared, is the only material able to cover near-, short-, and mid-wave infrared. Current best devices rely on electrodes made from transparent conductive oxides and gold, but so far, none of these completely fit for the intended purpose. Gold is not compatible with Si foundries, and transparent conductive oxides are highly lossy in this spectral range, limiting electrode transparency. Metal-based electrodes appear as good alternative candidates but require further investigations. While obvious constraints of work function get raised, chemical stability appears equally important. Here, we screen the use of Au, Al, Ag, and Zn as possible metals and reveal that in the case of Ag, dramatic transformations of Ag and HgTe are observed. Especially, a cation exchange procedure can occur over a solid-state film without intentional heating of the sample. This process has then been studied by combining both structural and electronic probes. This work points out the importance of the careful choice of surrounding electrodes in the case of HgTe since the observed mechanism is likely not limited to Ag. On the other hand, both Au and Al appear stable toward this transformation.

Influencing the coupling between network building blocks in CdSe/CdS dot/rod aerogels by partial cation exchange.

The assembly of CdSe/CdS dot/rod nanocrystals (NCs) with variable length of ZnS tips into aerogel networks is presented. To this end, a partial region selective cation exchange procedure is performed replacing Cd by Zn starting at the NC tip. The produced aerogel networks are investigated structurally and optically. The networks of tip-to-tip connected NCs have an intricate band structure with holes confined to the CdSe cores while electrons are delocalized within the CdS also within connected building blocks. However, the ZnS tips act as a barrier of variable length and strength between the NC building blocks partly confining the electrons. This results in NC based aerogel networks with tunable strength of coupling between building blocks.

Optimized Cation Exchange for Mercury Chalcogenide 2D Nanoplatelets and Its Application for Alloys

II–VI two-dimensional (2D) nanoplatelets (NPLs) exhibit the narrowest optical features among nanocrystals (NCs). This property remains true for Hg-based NPLs, despite a cation exchange procedure to...

Strong Bidentate Coordination for Surface Passivation and Ligand-Shell Engineering of Lead Halide Perovskite Nanocrystals in the Strongly Quantum-Confined Regime

The surface of lead halide perovskite nanocrystals (PNCs) is unique compared to conventional metal chalcogenide or pnictogenide semiconductor nanoparticles for its ionic character and the dynamic ligand layer, which makes them unstable in stock solutions and hinders the development of surface engineering strategies. This work employs a chelating strategy to form stable coordination on the PNC surface. Through screening a series of heterocyclic aromatic carboxylates, we found the best ligand, picolinate (PIC), with exceptional passivation effect to the surface traps of CsPbBr3 PNCs in the strongly quantum confined regime, resulting in > 0.8 photoluminescence quantum yields. The exciton lifetime in the passivated PNC approaches the radiative decay limit in various solvents. From an NMR titration experiment, the binding affinity of PIC is estimated to be at least 15 to 30 folds stronger than the original ligand from synthesis. The NMR and FTIR spectroscopic data and first-principles calculations elucidate the bidentate nature of the PIC coordination at the surface Pb site and the coadsorption of the ammonium-PIC ion pair. In apolar solvents, such as cyclohexane, the binding of PIC is stoichiometric to the available surface sites, suggesting the structure as a potent candidate for anchoring functional molecular structures to the PNC surface. In polar solvents, the strong affinity of PIC on the PNC surface provides protection for carrying out the precipitation-redissolution purification procedure that removes synthetic residual from the as-synthetic PNC samples. By modifying the purification procedure, we also develop a cation exchange procedure to replace the original oleylammonium cation with desired structures that consist of an ammonium anchoring group. Our results provide a direction for constructing strong interactions to protect the vulnerable surface of PNCs and pave the road for developing surface engineering strategies to functionalize these nanoparticles.

“Green”, gradient multi-shell CuInSe2/(CuInSexS1-x)5/CuInS2 quantum dots for photo-electrochemical hydrogen generation

Colloidal quantum dots (QDs) are promising building blocks for the realization of future optoelectronic technologies. In particular, they are widely studied for photoelectric conversion, thanks to their low cost and high solar energy conversion efficiency. The majority of the QDs used in devices for photoelectrochemical (PEC) hydrogen generation is based on heavy-metal containing materials, such as Pb and Cd. Unfortunately, the use of these materials hamper their prospective to be commercially developed due to health and environmental concerns. In this work, by using a template assisted cation exchange procedure, we developed eco-friendly CuInSe2/(CuInSexS1-x)5/CuInS2 QDs based on a gradient multi shell architecture, exhibiting tunable Near Infra-Red (NIR) optical absorption and photoluminescence (PL) properties. The NIR QDs are used as sensitizer for mesoporous TiO2 and employed for PEC hydrogen production. The PEC device based on the gradient architecture showed a 200 % improvement compared to the simple core CuInSe2 QDs and 73 % compared to the core/shell structure. The enhancement is ascribed to favorable stepwise electronic band alignment and improved electron transfer rate with the incorporation of interfacial gradient layers. These results could pave the way for the synthesis of heavy-metal-free QDs, exhibiting broad application in emerging optoelectronic technologies.

Structural curiosities of lanthanide (Ln)-modified bentonites analyzed by radioanalytical methods

The effects of pH and lanthanide (La, Y) concentration were investigated on the release of iron from Ca-bentonite crystal structure. XRF results revealed that during the Ca–H cation exchange procedure iron loss was not observed. In the case of lanthanide modifications, the pH has low influence, meanwhile the concentration of lanthanide has high influence on iron loss. Thus, high amount of trivalent lanthanides cause the structural iron release.

Hydrogen (H2) adsorption on natural and cation-exchanged clinoptilolite, mordenite and chabazite

In this study, clinoptilolite (CLN), mordenite (MOR) and chabazite (CHA) from Turkey and those of H+-, Na+-, K+-, Li+-, Ca2+- and Mg2+- exchanged forms were investigated for hydrogen storage at 77 K using volumetric apparatus up to 100 kPa. Cation exchange procedures were carried out using 1.0 M HCI, KNO3, LiNO3, NaNO3, Mg(NO3)2 and Ca(NO3)2 solutions at 90 °C for 5 h. Structural characteristics of these zeolite samples were investigated using XRD, XRF and N2 adsorption techniques. Experimental results showed that the chabazite samples (0.474–1.082 wt %) had a higher hydrogen adsorption capacity than mordenite (0.224–0.337 wt %) and clinoptilolite (0.065–0.555 wt %) samples. The variations in the level of hydrogen adsorption of original and cation exchanged natural zeolites were discussed in terms of the induced differences in their structural properties.

Two-dimensional solid-phase extraction strategy for the selective enrichment of aminoglycosides in milk.

An orthogonal two-dimensional solid-phase extraction strategy was established for the selective enrichment of three aminoglycosides including spectinomycin, streptomycin, and dihydrostreptomycin in milk. A reversed-phase liquid chromatography material (C18 ) and a weak cation-exchange material (TGA) were integrated in a single solid-phase extraction cartridge. The feasibility of two-dimensional clean-up procedure that experienced two-step adsorption, two-step rinsing, and two-step elution was systematically investigated. Based on the orthogonality of reversed-phase and weak cation-exchange procedures, the two-dimensional solid-phase extraction strategy could minimize the interference from the hydrophobic matrix existing in traditional reversed-phase solid-phase extraction. In addition, high ionic strength in the extracts could be effectively removed before the second dimension of weak cation-exchange solid-phase extraction. Combined with liquid chromatography and tandem mass spectrometry, the optimized procedure was validated according to the European Union Commission directive 2002/657/EC. A good performance was achieved in terms of linearity, recovery, precision, decision limit, and detection capability in milk. Finally, the optimized two-dimensional clean-up procedure incorporated with liquid chromatography and tandem mass spectrometry was successfully applied to the rapid monitoring of aminoglycoside residues in milk.

Revisiting the Anion Framework Conservation in Cation Exchange Processes

We investigated the effect of cation exchange on the anionic framework of lightly doped CdSe:Te/CdS nanorods. In contrast with previously studied core/shell systems, the Te dopant, located in the center of the CdSe core, provides an extremely sensitive indicator for any structural changes of the anionic framework that may occur as a result of the cation exchange process. We first optimized the cation exchange procedure in order to retain the fluorescence properties of the CdSe:Te/CdS nanorods after exchange of Cd2+ for Cu+ and back to Cd2+. Next, using multiexciton spectroscopy, we were able to probe the magnitude of the exciton–exciton repulsion interaction and use that to assess the degree of crystal structure conservation. Our findings provide a much stronger proof that the anion framework is indeed rigid, showing no evidence of significant migration of the anionic dopant.

Comparison of sample preparation methods for the GC⿿MS analysis of monoethanolamine (MEA) degradation products generated during post-combustion capture of CO2

As the development of chemical absorption technology for post-combustion capture (PCC) of CO2 from coal-fired power station flue gases proceeds towards commercial deployment, the focus on establishing a thorough understanding of the degradation of the aqueous amine absorbents is increasing. However, there is a need to develop and demonstrate robust analytical methods that are capable of measuring the concentrations of amine degradation products in aqueous monoethanolamine (MEA) matrix during pilot-scale PCC. In this study, sample cleanup and derivatisation methods that enable reliable and robust analysis of MEA degradation products by GC⿿MS are described. Two sample cleanup methods were evaluated: dehydration (by rotary evaporation and molecular sieves) and cation exchange. The cation exchange sample preparation method was preferred for the analysis of organic degradation products in these samples because it achieved higher recovery and repeatability of GC⿿MS measurements than those obtained with the dehydration method. Furthermore, the cation exchange method resulted in less continued amine degradation during subsequent analysis steps because of its ability to separate acidic analytes from basic analytes, as well as to remove some inorganic interferences. Further improvement of the sensitivity, repeatability and accuracy of this GC⿿MS analytical method can be accomplished by: (a) increasing the scale of the cation exchange and/or derivatisation procedures; (b) optimizing the derivatisation reaction conditions; and (c) using a narrower bore (e.g. 0.25mm ID) GC⿿MS column. The proposed cation exchange and derivatisation procedures can be readily adopted for the quantification of organic degradation products in other aqueous amine absorbents to provide important insights into the degradation of amine absorbents during PCC of CO2.

PbS/CdS/ZnS Quantum Dots: A Multifunctional Platform for In Vivo Near‐Infrared Low‐Dose Fluorescence Imaging

Over the past decade, near‐infrared (NIR)‐emitting nanoparticles have increasingly been investigated in biomedical research for use as fluorescent imaging probes. Here, high‐quality water‐dispersible core/shell/shell PbS/CdS/ZnS quantum dots (hereafter QDs) as NIR imaging probes fabricated through a rapid, cost‐effective microwave‐assisted cation exchange procedure are reported. These QDs have proven to be water dispersible, stable, and are expected to be nontoxic, resulting from the growth of an outer ZnS shell and the simultaneous surface functionalization with mercaptopropionic acid ligands. Care is taken to design the emission wavelength of the QDs probe lying within the second biological window (1000–1350 nm), which leads to higher penetration depths because of the low extinction coefficient of biological tissues in this spectral range. Furthermore, their intense fluorescence emission enables to follow the real‐time evolution of QD biodistribution among different organs of living mice, after low‐dose intravenous administration. In this paper, QD platform has proven to be capable (ex vivo and in vitro) of high‐resolution thermal sensing in the physiological temperature range. The investigation, together with the lack of noticeable toxicity from these PbS/CdS/ZnS QDs after preliminary studies, paves the way for their use as outstanding multifunctional probes both for in vitro and in vivo applications in biomedicine.

Cation exchange dynamics confined in a synthetic clay mineral

In this work we report X-Ray Diffraction (XRD) and Energy Dispersive X-Ray Spectroscopy (EDS) measurements to investigate the confined cation exchange process in saline aqueous suspensions of a synthetic clay mineral from Lithium-Fluorohectorite to Nickel-Fluorohectorite, as well as the reverse process from Nickel-Fluorohectorite to Lithium-Fluorohectorite and also from Lithium-Fluorohectorite to Sodium-Fluorohectorite. The dynamics of these cation exchanges was followed and it was observed that these processes can be faster than 1 minute. The results are compared to the observations on samples prepared by cation exchange procedures for which the exchange process was performed on the time-scale of months.

Tuning the magnetic properties of metal oxide nanocrystal heterostructures by cation exchange.

For three types of colloidal magnetic nanocrystals, we demonstrate that postsynthetic cation exchange enables tuning of the nanocrystal’s magnetic properties and achieving characteristics not obtainable by conventional synthetic routes. While the cation exchange procedure, performed in solution phase approach, was restricted so far to chalcogenide based semiconductor nanocrystals, here ferrite-based nanocrystals were subjected to a Fe2+ to Co2+ cation exchange procedure. This allows tracing of the compositional modifications by systematic and detailed magnetic characterization. In homogeneous magnetite nanocrystals and in gold/magnetite core shell nanocrystals the cation exchange increases the coercivity field, the remanence magnetization, as well as the superparamagnetic blocking temperature. For core/shell nanoheterostructures a selective doping of either the shell or predominantly of the core with Co2+ is demonstrated. By applying the cation exchange to FeO/CoFe2O4 core/shell nanocrystals the Neél temperature of the core material is increased and exchange-bias effects are enhanced so that vertical shifts of the hysteresis loops are obtained which are superior to those in any other system.

ETS-10 Supported Au Nanoparticles for Solvent-Free Oxidation of 1-Phenylethanol with Oxygen

Au nanoparticles (NPs) were uniformly dispersed on ETS-10 titanosilicate using the cation exchange procedure and [Au(NH3)4](NO3)3 complex as the gold source. [Au(NH3)4]3+ cations were first introduced inside ETS-10 micropores, ligands were then released, Au3+ was reduced to Au+ forming electron-deficient Au clusters, and finally aggregation to Au NPs occurred. In comparison to the incipient-wetness and deposition–precipitation methods, the ion-exchange led to greater activity of the Au NPs in the oxidation of 1-phenylethanol by oxygen.

Assembled Monolayer Nanorod Heterojunctions

Compositional and interfacial control in heterojunction thin films is critical to the performance of complex devices that separate or combine charges. For high performance, these applications require epitaxially matched interfaces, which are difficult to produce. Here, we present a new architecture for producing low-strain, single-crystalline heterojunctions using self-assembly and in-film cation exchange of colloidal nanorods. A systematic set of experiments demonstrates a cation exchange procedure that lends precise control over compositional depths in a monolayer film of vertically aligned nanorods. Compositional changes are reflected by electrical performance as rectification is induced, quenched, and reversed during cation exchange from CdS to Cu(2)S to PbS. As an additional benefit, we achieve this single-crystal architecture via an inherently simple and low-temperature wet chemical process, which is general to a variety of chemistries. This permits ensemble measurement of transport through a colloidal nanoparticle film with no interparticle charge hopping.

Towards controlled synthesis and better understanding of highly luminescent PbS/CdS core/shell quantum dots

A two-step cation exchange procedure has been developed for synthesizing PbS/CdS core/shell quantum dots (QDs) with a much thicker shell than previously reported, which expands the flexibility of the current cation exchange approach. The thick-shell QDs allow relatively easy observation of the core/shell morphology by transmission electron microscopy as well as exhibiting characteristic absorption and emission of CdS when the shell thickness reaches 1.8 nm. X-ray diffraction patterns show gradual transformation from a rock salt PbS pattern to a zinc blend CdS pattern with increasing shell thickness and the overall diffraction pattern is indeed the same as that of the CdS standard when the shell is thicker than 3.6 nm. The thick-shell QDs were further analyzed by energy dispersive X-ray spectrometry. It is found that Pb only exists in the core region and is essentially absent in the shell region. All of these results consistently suggest that the shell is made of CdS, instead of ternary PbxCd1−xS alloy in thick-shell QDs. As direct experimental identification of the shell composition in thin-shell QDs is difficult, experimental data and calculations are combined to indirectly probe this issue. The comparison of band gap versus core size plots of different compositional models indicates that it is highly likely that the thin shell is also made of CdS. Importantly, these core/shell PbS/CdS QDs not only show significantly increased quantum yield up to 67% at the optimal shell thickness of about 0.7 nm, they are also much more photostable and thermally stable than the shell-free PbS QDs.

Novel Poly-Dopamine Adhesive for a Halloysite Nanotube-Ru(bpy)32+ Electrochemiluminescent Sensor

Herein, for the first time, the electrochemiluminescent sensor based on Ru(bpy)3 2+-modified electrode using dopamine as an adhesive was successfully developed. After halloysite nanotube slurry was cast on a glassy carbon electrode and dried, an alkaline dopamine solution was added on the electrode surface. Initially, polydopamine belts with dimensions of tens to hundreds of nanometers formed via oxidization of the dopamine by ambient oxygen. As the incubation time increased, the nanobelts became broader and then united with each other to form a polydopamine film. The halloysite nanotubes were embedded within the polydopamine film. The above electrode was soaked in Ru(bpy)3 2+ aqueous solution to adsorb Ru(bpy)3 2+ into the active sites of the halloysite nanotubes via cation-exchange procedure. Through this simple procedure, a Ru(bpy)3 2+-modified electrode was obtained using only 6.25 µg Ru(bpy)3 2+, 15.0 µg dopamine, and 9.0 µg halloysite nanotubes. The electrochemistry and electrochemiluminescence (ECL) of the modified electrode was investigated using tripropylamine (TPA) and nitrilotriacetic acid (NTA) as co-reactants. The different ECL behaviors of the modified electrode using NTA and TPA as well as the contact angle measurements reflected the hydrophilic character of the electrode. The results indicate that halloysite nanotubes have a high loading capacity for Ru(bpy)3 2+ and that dopamine is suitable for the preparation of modified electrodes.

Cation exchange in forest soils: the need for a new perspective

SummaryThe long‐term sustainability of forest soils may be affected by the retention of exchangeable nutrient cations such as Ca2+ and the availability of potentially toxic cations such as Al3+. Many of our current concepts of cation exchange and base cation saturation are largely unchanged since the beginnings of soil chemistry over a century ago. Many of the same methods are still in use even though they were developed in a period when exchangeable aluminium (Al) and variable charge were not generally recognized. These concepts and methods are not easily applicable to acid, highly organic forest soils. The source of charge in these soils is primarily derived from organic matter (OM) but the retention of cations, especially Al species, cannot be described by simple exchange phenomena. In this review, we trace the development of modern cation exchange definitions and procedures, and focus on how these are challenged by recent research on the behaviour of acid forest soils. Although the effective cation exchange capacity (CECe) in an individual forest soil sample can be easily shown to vary with the addition of strong base or acid, it is difficult to find a pH effect in a population of different acid forest soil samples. In the very acidic pH range below ca 4.5, soils will generally have smaller concentrations of adsorbed Al3+. This can be ascribed to a reduced availability of weatherable Al‐containing minerals and a large amount of weak, organic acidity. Base cation saturation calculations in this pH range do not provide a useful metric and, in fact, pH is modelled better if Al3+ is considered to be a base cation. Measurement of exchangeable Al3+ with a neutral salt represents an ill‐defined but repeatable portion of organically complexed Al, affected by the pH of the extractant. Cation exchange in these soils can be modelled if assumptions are made as to the proportion of individual cations that are non‐specifically bound by soil OM. Future research should recognize these challenges and focus on redefining our concepts of cation retention in these important soils.

Cation-exchange separation of Group V elements: Model experiments on isolation and chemical identification of Db

A cation-exchange procedure was developed for separating Nb from Ta, Pa from Ta, and Nb, Pa, and Ta from Zr, Hf, and lanthanides in dilute HC1/HF solutions. The stability of the fluoride complexes of Group IV and V elements decreases in the following order: Nb ∼ Pa > Zr > Hf > Ta. This procedure can be used in experiments on synthesis of superheavy nuclei for isolation of Db from the reaction products and for its chemical identification.

A Novel Technique to Determine Cobalt Exchangeability in Soils Using Isotope Dilution

The environmental risk posed by Co contamination is largely a function of its oxidation state. Our objective was to assess the potential biological availability of Co and the reactions and fate of soluble Co(II) after addition to soils with varying physical and chemical characteristics. A potential risk in quantifying exchangeable Co in soils using isotope dilution techniques is the possible presence of two species of Co in soil solution and adsorbed on soil solid phases [Co(II) and Co(III)], coupled with the possibility that when an isotope of Co is added it may undergo a change in oxidation state during the measurement phase. In this study, we have utilized an isotope dilution technique with cation exchange and high-performance liquid chromatography-inductively coupled plasma-mass spectrometry to determine the isotopically exchangeable Co fraction in several soils with varying characteristics such as differing Al, Fe, and Mn oxide content; pH; and organic carbon content. The application of the cation exchange procedure adjusts measurements of isotopically exchangeable Co to correct for the presence of non-exchangeable 57Co not in equilibrium with the solution phase. Results indicated that oxidation of added 57Co(II) to 57Co(III) or precipitation of 57Co(II) may occur on the surfaces of some soils, particularly those with a high pH or substantial quantities of Mn oxide minerals. No detectable Co(III)(aq) was found in the aqueous extracts of the soils examined.