Partial Cation Exchange Research Articles

Screening of solid acid catalysts for etherification of glycerol with isobutene under identical conditions

We compared the performance of more than 70 solid acid catalysts with Brønsted acid function for the etherification of glycerol with isobutene under identical reaction conditions of 75 °C, 10 bar, 6 h and with an isobutene/glycerol molar ratio of 3/1. The catalysts were selected from a wide range of solid acid catalysts including ion exchange resins, zeolites, silica, and heteropolyacids, and their counterparts modified by simple treatments, with more than half of them being investigated for the first time for this reaction. For the modified commercial acid catalysts, the desired glycerol tertiary butyl ether (DTBGE and TTBGE) selectivity improved from 75 to 87 wt% upon partial cation exchange of the sulfonic acid protons in Amberlyst™ type ion exchange resins with Na+ cations; from 66 to 85 wt% by hydrothermal steam treatment of zeolite H-Y (SiO2/Al2O3 = 80); and from 75 to 80 wt% with partial La+-exchange of zeolite H-Beta (SiO2/Al2O3 = 300), all at high (90–100%) glycerol conversion. Impregnation of the heteropolyacids, tungstosilicic acid (TSA) and tungstophosphoric acid (TPA), on silica at a loading of 50 wt% provided a glycerol conversion of higher than 65 wt% and with a desired ether selectivity in the range of 62 to 76 wt%. In general, total ether selectivity increased with decreasing acid capacity for ion exchange resins, while the desired ether selectivity was enhanced with increasing acid strength for zeolites and supported heteropolyacids on mesoporous silica. Data present a detailed guideline for the selection of solid acid catalysts for the etherification of glycerol with isobutene.

Colloidal Cu–Zn–In–S-Based Disk-Shaped Nanocookies

We present a colloidal synthesis of quaternary Cu–Zn–In–S (CZIS) nanoplatelets (NPLs) by means of partial cation exchange. Starting with the synthesis of highly monodisperse binary CuS NPLs with lateral dimensions of ∼64 nm and thickness of ∼5 nm, we further performed a cation exchange reaction in which copper was partly replaced by indium, leading to Cu–In–S NPLs. To enhance the stability of the resulting NPLs and to improve their optical properties, we carried out the ZnS shell growth via both the heterogeneous nucleation of ZnS on the NPLs and via partial cation exchange on the surface of the particles. The latter reaction resulted, however, in rather an alloyed than the core/shell structure, whereas the reaction between zinc and sulfur precursors yielded unusual cookie-like hexagonal shaped structure, in which ZnS trigonal extensions grew only on one of the basal planes of the plates along the thickness direction. Upon ZnS growth, the lateral dimensions of the resulting core/shell CZIS/ZnS and alloyed...

Template deposition of Sb2S3 for solid-state sensitized solar cells

Unavoidable impurities form in Sb2S3 absorber fabricated by conventional chemical bath deposition method which is harmful to the photovoltaic performance of Sb2S3-sensitized solar cells. To obtain high quality Sb2S3 absorber, a two-step template method is introduced to deposit Sb2S3 on mesoporous TiO2 film. This template method is performed by dipping ZnS-coated TiO2 films in a Sb source solution for several minutes. This low temperature method with short operation time allows large area deposition of Sb2S3. Properties of ZnS and Sb2S3 are systematically studied. X-ray photoelectron spectroscopy confirms the absence of impurities. The optimized Sb2S3 device exhibits a power conversion efficiency (PCE) of 3.69%, with an open-circuit voltage of 0.514 V, a short-circuit current-density of 12.14 mA/cm2, and a fill factor of 59.1% under AM 1.5G illumination which is better than the device fabricated by chemical bath deposition. The PCE of our Sb2S3 devices showed an obvious increase after 1–2 months storage which may be caused by reconstruction of the ZnS/Sb2S3 coating fabricated by partial cation exchange.

Copper's Role in the Photoluminescence of Ag1- xCu xInS2 Nanocrystals, from Copper-Doped AgInS2 ( x ∼ 0) to CuInS2 ( x = 1).

A series of Ag1- xCu xInS2 nanocrystals (NCs) spanning from 0 ≤ x ≤ ∼1 was synthesized by partial cation exchange to identify copper's contributions to the electronic structure and spectroscopic properties of these NCs. Discrete midgap states appear above the valence band upon doping AgInS2 NCs with Cu+ (small x). Density functional theory calculations confirm that these midgap states are associated with the 3d valence orbitals of the Cu+ impurities. With increasing x, these impurity d levels gradually evolve to become the valence-band edge of CuInS2 NCs, but the highest-occupied orbital's description does not change significantly across the entire range of x. In contrast with this gradual evolution, Ag1- xCu xInS2 NC photoluminescence shifts rapidly with initial additions of Cu+ (small x) but then becomes independent of x beyond x > ∼0.20, all the way to CuInS2 ( x = 1.00). Data analysis suggests small but detectable hole delocalization in the luminescent excited state of CuInS2 NCs, estimated by Monte Carlo simulations to involve at most about four copper ions. These results provide unique insights into the luminescent excited states of these materials and they reinforce the description of CuInS2 NCs as "heavily copper-doped NCs" in which photogenerated holes are rapidly localized in copper 3d-based orbitals.

Highly Crystalline Pd13Cu3S7 Nanoplates Prepared via Partial Cation Exchange of Cu1.81S Templates as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Chemical transformations via postsynthetic modification of colloidal nanocrystals have received great attention as a rational synthetic route to unprecedented nanostructures. In particular, the cation exchange reaction is considered as an effective method to alter the composition of the starting nanostructures while maintaining the initial structural characteristics. Herein, we report the synthesis of highly crystalline Pd13Cu3S7 nanoplates (NPs) via partial cation exchange of the Cu1.81S phase by Pd cations, with Cu1.94S NPs and Pd13Cu3S7/Cu2–xS janus heterostructure as the intermediate phases. The highly crystalline Pd13Cu3S7 ternary NPs exhibit excellent electrocatalytic performance toward the hydrogen evolution reaction (HER) under acidic conditions. The HER activity of Pd13Cu3S7 NPs with its overpotential as low as 64 mV at −10 mA cm–2 is superior to those of amorphous PdCuS and commercial Pd/C catalysts, demonstrating the importance of nanocrystal crystallinity in boosting the HER activity. They als...

Colloidal Gradated Alloyed (Cu)ZnInS/ZnS Core/Shell Nanocrystals with Tunable Optical Properties for Live Cell Optical Imaging

AbstractIn this work, Cd‐free, oleophilic (Cu)ZnInS(core)−ZnS(shell) core/shell quantum dots (QDs) with varying Cu content in the core and shell thickness were synthesized by heating‐up method (core) followed by hot‐injection route (shell). Effect of Cu ion doping (in core) and shell growth on the structural and optical properties of (Cu)ZnInS−ZnS core/shell QDs is reported. Incorporation of Cu ions causes the crystalline phase transformation of zinc blende ZnInS ternary alloy to wurtzite Cu−Zn−In−S quaternary alloy. The (Cu)ZnInS alloyed core QDs are encapsulated with a wider bandgap ZnS shell with gradient interface. The photoluminescence of (Cu)ZnInS core QDs exhibits wide tunable multicolor emissions from green (535 nm) to deep red (683 nm) with large Stokes shift. After the growth of ZnS layer, the PL peak position and absorption edge undergo a hypsochromic shift with enhanced photoluminescence quantum yield up to 47.31%, which is due to the partial cation exchange of Cu+ and In3+ with Zn2+, yielding a graded core/shell structure with smooth potential interface. The radiative excited state lifetime is lengthened from 22.91 ns to 291.11 ns. The mercaptoundecanoic acid (MUA) functionalized QDs have been successfully used for in vitro fluorescence cellular imaging of HEK‐293 and HeLa cells. These QDs are cell permeable and suitable for visible light induced multicolor fluorescence‐based cellular imaging.

Exploiting Crystallographic Regioselectivity To Engineer Asymmetric Three-Component Colloidal Nanoparticle Isomers Using Partial Cation Exchange Reactions.

The precise placement of different materials in specific regions of a nanocrystal is important for many applications, but this remains difficult to achieve synthetically. Here we show that regioselectivity during partial cation exchange reactions of metal chalcogenide nanocrystals emerges from crystallographic relationships between the precursor and product phases. By maximizing the formation of low-strain interfaces, it is possible to rationally integrate three distinct materials within uniform spherical and rod-shaped colloidal nanoparticles to produce complex asymmetric heterostructured isomers. Through sequential partial exchange of Cu+ in Cu1.8S nanocrystals with Zn2+ and Cd2+, five distinct ZnS/CdS/Cu1.8S nanosphere and nanorod isomers are accessible.

Understanding the Journey of Dopant Copper Ions in Atomically Flat Colloidal Nanocrystals of CdSe Nanoplatelets Using Partial Cation Exchange Reactions

Unique electronic and optical properties of doped semiconductor nanocrystals (NCs) have widely stimulated a great deal of interest to explore new effective synthesis routes to achieve controlled doping for highly efficient materials. In this work, we show copper doping via postsynthesis partial cation exchange (CE) in atomically flat colloidal semiconductor nanoplatelets (NPLs). Here chemical reactivity of different dopant precursors, reaction kinetics, and shape of seed NPLs were extensively elaborated for successful doping and efficient emission. Dopant-induced Stokes-shifted and tunable photoluminescence emission (640 to 830 nm) was observed in these Cu-doped CdSe NPLs using different thicknesses and heterostructures. High quantum yields (reaching 63%) accompanied by high absorption cross sections (>2.5 times) were obtained in such NPLs compared to those of Cu-doped CdSe colloidal quantum dots (CQDs). Systematic tuning of the doping level in these two-dimensional NPLs provides an insightful understandi...

Near-Infrared Cu–In–Se-Based Colloidal Nanocrystals via Cation Exchange

We developed a three-step colloidal synthesis of near-infrared (NIR) active Cu–In–Se (CISe)-based nanocrystals (NCs) via a sequential partial cation exchange realized in one pot. In the first step, binary highly copper deficient Cu2–xSe NCs were synthesized, followed by a partial cation exchange of copper to indium ions, yielding CISe NCs. This reaction allows for a precise control of the composition of the resulting NCs through a simple variation of the ratio between guest-cation precursors and parent NCs. To enhance the stability and the photoluminescence (PL) properties of the NCs, a subsequent ZnS shell was grown in the third step, resulting in CISeS/ZnS core/shell particles. These core/shell hetero-NCs exhibited a dramatic increase in size and a restructuring to trigonal pyramidal shape. The shell growth performed at a relatively high temperature (250 °C) also led to anion exchange, in which sulfur replaced part of selenium atoms close to the surface of the NCs, forming alloyed CISeS core structure. ...

Mesoporous Hollow Sb/ZnS@C Core–Shell Heterostructures as Anodes for High‐Performance Sodium‐Ion Batteries

Combining the advantage of metal, metal sulfide, and carbon, mesoporous hollow core-shell Sb/ZnS@C hybrid heterostructures composed of Sb/ZnS inner core and carbon outer shell are rationally designed based on a robust template of ZnS nanosphere, as anodes for high-performance sodium-ion batteries (SIBs). A partial cation exchange reaction based on the solubility difference between Sb2 S3 and ZnS can transform mesoporous ZnS to Sb2 S3 /ZnS heterostructure. To get a stable structure, a thin contiguous resorcinol-formaldehyde (RF) layer is introduced on the surface of Sb2 S3 /ZnS heterostructure. The effectively protective carbon layer from RF can be designed as the reducing agent to convert Sb2 S3 to metallic Sb to obtain core-shell Sb/ZnS@C hybrid heterostructures. Simultaneously, the carbon outer shell is beneficial to the charge transfer kinetics, and can maintain the structure stability during the repeated sodiation/desodiation process. Owing to its unique stable architecture and synergistic effects between the components, the core-shell porous Sb/ZnS@C hybrid heterostructure SIB anode shows a high reversible capacity, good rate capability, and excellent cycling stability by turning the optimized voltage range. This novel strategy to prepare carbon-layer-protected metal/metal sulfide core-shell heterostructure can be further extended to design other novel nanostructured systems for high-performance energy storage devices.

Boosting the Efficiency of Quantum Dot-Sensitized Solar Cells through Formation of the Cation-Exchanged Hole Transporting Layer.

In search of a viable way to enhance the power conversion efficiency (PCE) of quantum dot-sensitized solar cells, we have designed a method by introducing a hole transporting layer (HTL) of p-type CuS through partial cation exchange process in a postsynthetic ligand-assisted assembly of nanocrystals (NCs). High-quality CdSe and CdSSe gradient alloy NCs were synthesized through colloidal method, and the charge carrier dynamics was monitored through ultrafast transient absorption measurements. A notable increase in the short-circuit current concomitant with the increase in open-circuit voltage and the fill factor led to 45% increment in PCE for CdSe-based solar cells upon formation of the CuS HTL. Electrochemical impedance spectroscopy further revealed that the CuS layer formation increases recombination resistance at the TiO2/NC/electrolyte interface, implying that interfacial recombination gets drastically reduced because of smooth hole transfer to the redox electrolyte. Utilizing the same approach for CdSSe alloy NCs, the highest PCE (4.03%) was obtained upon CuS layer formation compared to 3.26% PCE for the untreated one and 3.61% PCE with the conventional ZnS coating. Therefore, such strategies will help to overcome the kinetic barriers of hole transfer to electrolytes, which is one of the major obstacles of high-performance devices.

Perovskite Quantum Dots and Their Application in Light-Emitting Diodes.

Perovskite quantum dots (PQDs) attract significant interest in recent years because of their unique optical properties, such as tunable wavelength, narrow emission, and high photoluminescence quantum efficiency (PLQY). Recent studies report new types of formamidinium (FA) PbBr3 PQDs, PQDs with organic-inorganic mixed cations, divalent cation doped colloidal CsPb1-x Mx Br3 PQDs (M = Sn2+ , Cd2+ , Zn2+ , Mn2+ ) featuring partial cation exchange, and heterovalent cation doped into PQDs (Bi3+ ). These PQD analogs open new possibilities for optoelectronic devices. For commercial applications in lighting and backlight displays, stability of PQDs requires further improvement to prevent their degradation by temperature, oxygen, moisture, and light. Oxygen and moisture-facilitated ion migration may easily etch unstable PQDs. Easy ion migration may result in crystal growth, which lowers PLQY of PQDs. Surface coating and treatment are important procedures for overcoming such factors. In this study, new types of PQDs and a strategy of improving their stabilities are introduced. Finally, this paper discusses future applications of PQDs in light-emitting diodes.

On the use of metal cation-exchanged zeolites in sorption thermochemical storage: Some practical aspects in reference to the mechanism of water vapor adsorption

The sodium form of industrial X-zeolite (13X) was compared with two other samples obtained by a partial (about 70%) cation exchange with Mg2+ (Mg-X) or a complete exchange with Ca2+ (Ca-X) in view of their use as adsorbents for water vapor in energy storage systems by sorption. Various sorption methods based on the specific interactions between two probing molecules (N2 and NH3) and specific surface sites were employed to monitor the hydration state of the zeolite surface as a function of the temperature of sample drying performed under vacuum degassing or gas-flow conditions. X-ray diffraction, volumetric sorption technique, temperature-programmed desorption, and gas flow calorimetry were combined with Monte Carlo simulations of the mechanism of water vapor adsorption on model zeolites to demonstrate that the general tendency to operate under mild drying conditions would always result in a state of partial hydration of the zeolite surface. To increase the amount of heat released upon subsequent adsorption from a flow of helium at a relative pressure of water vapor of about 0.03, the use of Mg-X sample was demonstrated the best alternative if the activation procedure was to be performed at 200°C. Here, the heat effect was 4–5 times greater than that measured in the case of 13X. This result is of high importance for the industrial-scale implementation of the energy storage by sorption.

Postsynthetic, Reversible Cation Exchange between Pb2+ and Mn2+ in Cesium Lead Chloride Perovskite Nanocrystals

CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) are promising materials due to their excellent optoelectronic properties. In this work, we show a successful partial and reversible cation exchange reaction between Pb and Mn in both CsPbCl3 NCs and CsMnCl3 NCs systems to yield luminescent CsPb1–xMnxCl3 NCs. By adjusting the reaction time, the photoluminescence from the exciton emission of CsPbCl3 and the electron transition of Mn2+ can be tuned gradually. This work highlights the feasibility of a postsynthetic interconversion of Pb2+ and Mn2+ in cesium lead chloride perovskite nanocrystals, which enables a new strategy to reduce the toxicity and adjust the emissions of CsPbCl3 NCs. In the end, we also discuss the plausible mechanisms for cation exchange in perovskite materials.

Lattice Strain and Ligand Effects on the Formation of Cu2–xS/I-III-VI2 Nanorod Heterostructures through Partial Cation Exchange

In the synthesis of anisotropic colloidal nanocrystal heterostructures, the interplay between many complicating factors such as interfacial chemistry, lattice strain, and coordinating ligands can make precise control over spatial distribution of composition extremely challenging. However, understanding how each complicating factor contributes to the growth mechanism can lead to otherwise difficult-to-achieve or unique structures and the means to tune their electronic/optical properties. Here, we report on the effects of lattice strain and the choice of ligands on the formation of Cu2–xS/I-III-VI2 colloidal nanorod heterostructures through partial cation exchange starting from Cu2–xS nanorods. Lattice strain can induce alternating Cu2–xS/CuGaS2 segments along a colloidal nanorod if CuGaS2 can nucleate easily from the sides of the nanorods. The choice in coordinating ligands can alter this preference to favor tip nucleation, in which case the resulting heterostructure has CuGaS2/Cu2–xS/CuGaS2 rod/rod/rod ge...

Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal Cu2Se Nanocrystals.

Stoichiometric Cu2Se nanocrystals were synthesized in either cubic or hexagonal (metastable) crystal structures and used as the host material in cation exchange reactions with Pb2+ ions. Even if the final product of the exchange, in both cases, was rock-salt PbSe nanocrystals, we show here that the crystal structure of the starting nanocrystals has a strong influence on the exchange pathway. The exposure of cubic Cu2Se nanocrystals to Pb2+ cations led to the initial formation of PbSe unselectively on the overall surface of the host nanocrystals, generating Cu2Se@PbSe core@shell nanoheterostructures. The formation of such intermediates was attributed to the low diffusivity of Pb2+ ions inside the host lattice and to the absence of preferred entry points in cubic Cu2Se. On the other hand, in hexagonal Cu2Se nanocrystals, the entrance of Pb2+ ions generated PbSe stripes “sandwiched” in between hexagonal Cu2Se domains. These peculiar heterostructures formed as a consequence of the preferential diffusion of Pb2+ ions through specific (a, b) planes of the hexagonal Cu2Se structure, which are characterized by almost empty octahedral sites. Our findings suggest that the morphology of the nanoheterostructures, formed upon partial cation exchange reactions, is intimately connected not only to the NC host material, but also to its crystal structure.

Postsynthetic Doping of MnCl2 Molecules into Preformed CsPbBr3 Perovskite Nanocrystals via a Halide Exchange-Driven Cation Exchange.

Unlike widely used postsynthetic halide exchange for CsPbX3 (X is halide) perovskite nanocrystals (NCs), cation exchange of Pb is of a great challenge due to the rigid nature of the Pb cationic sublattice. Actually, cation exchange has more potential for rendering NCs with peculiar properties. Herein, a novel halide exchange-driven cation exchange (HEDCE) strategy is developed to prepare dually emitting Mn-doped CsPb(Cl/Br)3 NCs via postsynthetic replacement of partial Pb in preformed perovskite NCs. The basic idea for HEDCE is that the partial cation exchange of Pb by Mn has a large probability to occur as a concomitant result for opening the rigid halide octahedron structure around Pb during halide exchange. Compared to traditional ionic exchange, HEDCE is featured by proceeding of halide exchange and cation exchange at the same time and lattice site. The time and space requirements make only MnCl2 molecules (rather than mixture of Mn and Cl ions) capable of doping into perovskite NCs. This special molecular doping nature results in a series of unusual phenomenon, including long reaction time, core-shell structured mid states with triple emission bands, and dopant molecules composition-dependent doping process. As-prepared dual-emitting Mn-doped CsPb(Cl/Br)3 NCs are available for ratiometric temperature sensing.

Efficient electrocatalysis of overall water splitting by ultrasmall NixCo3−xS4 coupled Ni3S2 nanosheet arrays

The overall water splitting into hydrogen and oxygen is one of the most promising ways to store intermittent solar and wind energy in the form of chemical fuels. However, this process is quite thermodynamically uphill, and thus needs to be mediated simultaneously by efficient hydrogen evolving and oxygen evolving catalysts to get any feasible output from it. Herein, we report the synthesis of such a catalyst comprising ultrasmall NixCo3−xS4-decorated Ni3S2 nanosheet arrays supported on nickel foam (NF) via a partial cation exchange reaction between NF-supported Ni3S2 nanosheet arrays and cobalt(II) ions. We show that the as-prepared material, denoted as NixCo3−xS4/Ni3S2/NF, can serve as a self-standing, noble metal-free, highly active and stable, bifunctional electrocatalyst for the two half reactions involved in the overall water splitting: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Furthermore, we demonstrate that a high-performance electrolyzer for the overall water splitting reaction can be assembled by using NixCo3−xS4/Ni3S2/NF as the electrocatalyst at both the cathode and the anode sides of the electrolyzer. This electrolyzer delivers water-splitting current densities of 10 and 100mA/cm2 at applied potentials of 1.53 and 1.80V, respectively, with remarkable stability for >200h in both cases. The electrolyzer's performance is much better than the performances of electrolyzers assembled from many types of other bifunctional electrocatalysts as catalyst couple. Moreover, the overall performance of the electrolyzer is comparable with the performances of electrolyzers containing two different, benchmark, monofunctional HER and OER electrocatalyst couple (i.e., Pt/C-IrO2).

Highly Emissive Divalent-Ion-Doped Colloidal CsPb1-xMxBr3 Perovskite Nanocrystals through Cation Exchange.

Colloidal CsPbX3 (X = Br, Cl, and I) perovskite nanocrystals (NCs) have emerged as promising phosphors and solar cell materials due to their remarkable optoelectronic properties. These properties can be tailored by not only controlling the size and shape of the NCs but also postsynthetic composition tuning through topotactic anion exchange. In contrast, property control by cation exchange is still underdeveloped for colloidal CsPbX3 NCs. Here, we present a method that allows partial cation exchange in colloidal CsPbBr3 NCs, whereby Pb2+ is exchanged for several isovalent cations, resulting in doped CsPb1–xMxBr3 NCs (M= Sn2+, Cd2+, and Zn2+; 0 < x ≤ 0.1), with preservation of the original NC shape. The size of the parent NCs is also preserved in the product NCs, apart from a small (few %) contraction of the unit cells upon incorporation of the guest cations. The partial Pb2+ for M2+ exchange leads to a blue-shift of the optical spectra, while maintaining the high photoluminescence quantum yields (>50%), sharp absorption features, and narrow emission of the parent CsPbBr3 NCs. The blue-shift in the optical spectra is attributed to the lattice contraction that accompanies the Pb2+ for M2+ cation exchange and is observed to scale linearly with the lattice contraction. This work opens up new possibilities to engineer the properties of halide perovskite NCs, which to date are demonstrated to be the only known system where cation and anion exchange reactions can be sequentially combined while preserving the original NC shape, resulting in compositionally diverse perovskite NCs.

Engineering the Electrical Conductivity of Lamellar Silver-Doped Cobalt(II) Selenide Nanobelts for Enhanced Oxygen Evolution.

Precisely engineering the electrical conductivity represents a promising strategy to design efficient catalysts towards oxygen evolution reaction (OER). Here, we demonstrate a versatile partial cation exchange method to fabricate lamellar Ag-CoSe2 nanobelts with controllable conductivity. The electrical conductivity of the materials was significantly enhanced by the addition of Ag+ cations of less than 1.0 %. Moreover, such a trace amount of Ag induced a negligible loss of active sites which was compensated through the effective generation of active sites as shown by the excellent conductivity. Both the enhanced conductivity and the retained active sites contributed to the remarkable electrocatalytic performance of the Ag-CoSe2 nanobelts. Relative to the CoSe2 nanobelts, the as-prepared Ag-CoSe2 nanobelts exhibited a higher current density and a lower Tafel slope towards OER. This strategy represents a rational design of efficient electrocatalysts through finely tuning their electrical conductivities.