- Open Access
Ion exchange: an advanced synthetic method for complex nanoparticles
© The Author(s) 2019
- Received: 7 February 2019
- Accepted: 29 April 2019
- Published: 3 June 2019
There have been tremendous efforts to develop new synthetic methods for creating novel nanoparticles (NPs) with enhanced and desired properties. Among the many synthetic approaches, NP synthesis through ion exchange is a versatile and powerful technique providing a new pathway to design complex structures as well as metastable NPs, which are not accessible by conventional syntheses. Herein, we introduce kinetic and thermodynamic factors controlling the ion exchange reactions in NPs to fully understand the fundamental mechanisms of the reactions. Additionally, many representative examples are summarized to find related advanced techniques and unique NPs constructed by ion exchange reactions. Cation exchange reactions mainly occur in chalcogenide compounds, while anion exchange reactions are mainly involved in halogen (e.g. perovskite) and metal-chalcogenide compounds. It is expected that NP syntheses through ion exchange reactions can be utilized to create new devices with the required properties by virtue of their versatility and ability to tune fine structures.
- Nanocrystal synthesis
- Chemical transformation
- Shape control
- Composition control
- Phase control
Unlike a bulk material system, the advantage of nanomaterials is the ability to control their physical and chemical properties depending on their size and shape. These unique properties provide great opportunities for a wide range of applications, such as electrocatalysts [1–7], photovoltaic cells [8–11], batteries [12–15], sensors [16–19], biomedical [20–22] and electronic devices [23–25]. Thus, many studies have been focused on methods to tune the physical size, phase, and nano-dimensional shapes such as nanospheres, nanorods (NRs), and nanoplates [26–33].
Among many chemical and physical methods, solution-based colloidal synthesis has been widely studied because the size and shape of the nanoparticles (NPs) can be readily adjusted and uniform NPs can be achieved [34–40]. This method is a bottom-up approach in which the NPs are formed on the atomic or molecular scale as building blocks. In general, colloidal synthesis is carried out at a high temperature using inorganic or organometallic precursors and ligands. The synthetic process follows the La Mer model in which the growth of NPs to a certain size occurs after the nucleation step . Nucleation of colloidal-based NPs can be initiated by the hot-injection technique [42, 43], which comprises the rapid injection of precursors onto hot mixtures. The morphology and composition of NPs are controlled through a combination of the precursors, ligands, reaction temperatures, reaction times, etc. These protocols have been well established and various classes of material with narrow size distributions can be obtained [35, 38, 44–51]. However, the development of new synthesis strategies is still required since new functionality and enhanced properties of NPs are always in demand. As an alternative to the conventional colloidal synthesis method, the ion exchange of NPs through a secondary chemical transformation has recently been developed [52, 53]. The ion exchange reaction is initiated by providing new ions to a template NP compound. This technique has received considerable attention because it can overcome the limited structures and compositions fabricated by traditional synthesis methods. Most of the NPs formed by directly colloidal synthesis have the stable phase, composition, and morphology to satisfy the thermodynamic conditions . Contrarily, the ion exchange synthesis can lead to NPs in a non-equilibrium state by the rapid replacement of ions by solid-state diffusion, even at room temperature [52, 55, 56]. This secondary transformation synthesis allows kinetically controlled NPs which are not accessible by traditional direct synthesis routes. Thus, the NPs transformed by an ion exchange method often yield heterostructures , core–shell structures [58–63], and metastable phases [55, 64] that are difficult to achieve via conventional syntheses. Numerous applications were reported by utilizing ion exchanged-NPs in various devices such as solar cells, photocatalysts, and lithium batteries. In such applications, their performances were enhanced when the NPs were synthesized via ion exchange method [59, 60, 65–68].
In ion exchange reactions, the ions of the parent NP compound diffuse out from the lattice and become solvated by solvents. At the same time, the substituted ions are introduced into the lattice through inward diffusion. Especially, NPs can adopt higher diffusion rates than those occurring in bulk syntheses because they have a large volume-to-surface area ratio. The ion diffusion process in the lattice structure of NPs can be influenced by kinetic factors such as the reaction zone , lattice framework , density of vacancies , and interstitial sites . The reason why NPs in metastable phases and shapes can be formed is due to the unique kinetic mechanism of the ion exchange reaction. Additionally, a thermodynamic driving force can be introduced as a tool for predicting the spontaneity of ion exchange reactions such as lattice energy , the solubility of the ions [73, 74], and hardness [75, 76]. Ion exchange reactions can be represented by the complex action of kinetic and thermodynamic concepts. This review covers the mechanism of ion exchange reactions influenced by kinetic/thermodynamic factors and introduces examples of NP synthesis through ion exchange reactions.
Generally, high mobility of ions in a lattice is required for facilitating ion exchange by solid-state diffusion. The diffusion of cations can easily occur since their ionic radii are generally smaller than those of anions. Due to this main advantage, a cation exchange reaction is more likely to occur than an anion exchange one. The compositions of NPs can be efficiently tuned by cation exchange that maintains the original NP morphology and in some cases, changes it via structural reorganization. Cation exchange reactions often lead to new products with complex structures such as heterostructures and metastable phases that are not accessible through conventional synthetic methods.
We start this review by introducing the kinetic and thermodynamic factors to understand the mechanism of cation exchange reactions. Next, we summarize examples of cation exchange reactions in group I, II, and IV metal-chalcogenide compounds, which are the most studied cases among the cation exchange reactions in NPs. We also cover cation exchange reactions that occur within other transition metal compounds. In addition, we show the evolution in the characteristics of the NPs through cation reactions and their device applications.
2.1 Kinetic factors
Cation exchange reactions proceed through the inward diffusion of new ions and the outward diffusion of host ions. The exchange of cations often occurs in a non-equilibrium state dominated by kinetics rather than thermodynamics. To fully understand the mechanism, it is important to identify the kinetic factors that influence ion diffusion. The focus in this section is cation exchange reactions by introducing the factors related to dynamics, such as the reaction zone, lattice structure, and defects in the crystal structure.
2.1.1 Reaction zone
The reaction zone is the region where substitution reactions by solid-state diffusion take place between the parent and product NPs . This concept can be used to predict the morphology and phase of the NPs after the cation exchange reactions. The difference in the diffusion rates between two cations controls the overall reaction rate of the ion exchange reaction and determines whether the whole surface or only a specific zone of the NPs behaves as the reaction region. The reaction zone is the region where inward diffusion of host ion and outward diffusion of newly ion occur simultaneously and ion exchange reaction occurs. If this region is the whole surface of template NPs, inter-diffusion occurs at the fully interface of NPs and a core shell structure can be formed. In addition, the reaction zone directly affects the morphology and the crystal structure of the product NPs. For example, the CdSe NR template can preserve the NR shape or transform it into a spherical shape of Ag2Se controlled by the reaction zone during cation exchange reactions . If the reaction zone is the entire surface of the CdSe NR template, the anion sublattice can be deformed at the intermediate step of whole cation exchange reaction. At the same time, inter-diffusion of ions and anion/cation rearrangements occur at a non-equilibrium state. Finally, when the equilibrium state is reached, the shape of product NPs transforms to sphere because it is thermodynamically preferred. On the other hand, if the specific part of the NP crystal structure is the reaction zone, the local lattice distortion of the template NPs occurs at a level that is sufficient to maintain the structure of anion sublattice. In this case, the shape of the NR template shape can be maintained during the conversion of the chemical composition. The reaction zone is one factor influencing the morphological changes during cation exchange reaction. Since the driving force for the shape change is very complicated, many other factors such as the crystal structure should be considered to fully understand the morphology evolution during cation exchange reaction in NPs.
2.1.2 Lattice structure
The stability of the anion sublattice structure determines the overall morphology of the NPs after the cation exchange reaction. A volume change in the lattice structure of the template NPs can be caused by a distinctly different size between the host ions and the newly introduced ones. Morphological changes of template NPs can be determined by the degree of strain tolerance that the lattice structure can relieve when the new ions are introduced. When the critical point is reached by the stress applied to the lattice structure due to replacement by the new ions, void formation or shape changes from spherical to rod may be induced to relieve the stress. In the case of CdX (X=S, Se, or Te) NP templates, the morphological transformation by volume change can be developed through cation exchanges by Pd2+ or Pt2+ ions . As the fractional volume change (∆V/V) from 25 to 46% occurs, the shape of the parent NPs is transformed to fragmented NPs or else voids are formed.
One of the most important factors for ion diffusion during a cation exchange reaction is defects such as vacancies and interstitial sites in a crystal structure [71, 77, 80–82]. When defects exist in the template NPs, the ion diffusion pathway having a low activation energy is formed, and the inter-diffusion of ions can be promoted along this pathway during cation exchange reaction. Groeneveld et al.  introduced the concept of a Frenkel pair for modeling a diffusion process in the presence of defects. When the cation exchange of Cd2+ into a ZnSe template occurs, a Frenkel pair acts to promote the external diffusion of Zn2+ ions. In this case, the activation energy required for the formation of the Frenkel pair can be overcome at high temperature so that the cation exchange reaction is spontaneously performed.
Vacancies in a crystal structure of template NPs can accelerate cation exchange reactions. For example, the ion exchange reaction of copper selenide is promoted by Cu vacancies . When Cu+ is exchanged for Zn2+ and Cd2+, Cu2−xSe with a high vacancy density is more active than Cu2Se for the cation exchange reaction. Because the vacancy provides a pathway for ion diffusion, the activation energy for cation exchange is lower, which causes the more active cation exchange reaction from Cu2−xSe to ZnSe or CdSe than occurs in Cu2Se.
2.2 Thermodynamic factors
Not only can the spontaneity of the reaction be determined through the introduction of thermodynamic factors involved in cation exchange reactions, it also helps to select suitable ligands and solvents for each NP template.
2.2.1 The energy concept of cation exchange
C − X → C + X (dissociation)
Mn+ → M (desolvation)
M + X → M − X (association)
C → Cn+ (solvation)
In the process of dissociation and association, the lattice energies of the crystal structure influence the reaction process. The lattice energy of ionic crystals is the energy required to break bonds in the crystal structure at 0 K and separate it into individual ions. The larger the lattice energy, the more stable the crystal structure. A spontaneous exchange reaction can be predicted if the lattice energy of the product NP is larger than the parent NP.
The spontaneity of the reaction can be predicted in the desolvation and solvation steps through the solubility between the cation and the solvent [52, 73]. The solubility of the host cation in a solvent should be high for the ion exchange reaction to proceed well .
Since the volume-to-surface ratio of NPs is high, the surface energy affects solid-state exchange reactions . However, it is not easy to calculate the surface energy because of the complexity of the ligand and/or surface lattice structure.
2.2.2 Parson’s hard and soft acids and bases (HSAB) theory
Colloidal NPs are usually surrounded by ligands which serve to stabilize the NP surface. Therefore, it is important to understand the interaction between the ligand and surface ions of NPs. The HSAB theory is used as a tool to predict the affinity among solvents, ligands, and ions. This theory infers that hard acids are preferred over hard bases and weak acids are adopted by weak bases . If the host cation can form a more stable acid–base pair with a solvent/ligand than the pair of ingoing cation-solvent/ligand combination, the affinity between the host cation and the solvent/ligand is high. This high affinity can lead the cations to be removed from the template NPs. For example, when the soft acid cation (e.g. Cu+, Pb2+, Ag+) is the host cation and the hard acid cation (e.g. Zn2+, Cd2+, In3+) is the ingoing cation, the affinity between the host cation and soft base ligand is higher than the affinity between ingoing cation and ligand. Therefore, using soft base ligands (e.g. tri-n-octylphosphine) for soft acid cations in metal-chalcogenide NP templates would be thermodynamically favorable to facilitate an ion exchange reaction. Oleic acid and oleylamine (OA) are widely used as hard base ligands in cation exchange reactions forming stable metal ion-ligand complexes, which can support the solvation of the hard acid metal cation of the parent NP. In this case, hard acid cation (Zn2+, Cd2+, In3+) can be easily replaced by soft acid cations due to high affinity between hard base ligand and host cation. In addition, the affinity of the host cation and the solvent (e.g. ethanol, methanol, hexane) to promote the cation exchange reaction should be considered .
The HSAB theory suggests that a suitable solvent or ligand for a host cation will play a positive role in the exchange reaction. However, it is difficult to predict this exactly because the ligand and solvent are simultaneously involved in the ion exchange reaction and are influenced by many other variables, such as surface defects, decomposition of the ligand, and other chemical reactions. In addition, it is important to consider the kinetics since most ion exchange reactions occur in a non-equilibrium state.
Many studies have been progressed for the cation exchanges between I–VI compounds and II–VI compounds. The first mechanistic study of cation exchange in NP compounds is the transition from CdSe NPs to Ag2Se NPs demonstrated by Son et al. . The morphology of the NPs when the cation exchange reaction from CdSe to Ag2Se occurs can be maintained or changed due to the reaction zone depending on the NP size. It has been confirmed that NP phases can easily be controlled in a short time through cation exchange because the kinetics of the reaction in NPs are much faster than in bulk reactions. Ag2Se NPs react with Cd2+ to transform back to CdSe, thus showing complete reversibility between CdSe and Ag2Se through cation exchange. Since this initial study, others on cation exchange reactions in group I and II chalcogenide NPs have been actively progressed to precisely control their morphology and/or phase.
Cation exchange reactions also have the advantage of precise control of the NP phase. Zhang et al.  investigated the cation exchange reaction between CdS and Cu2−xS in NRs. Cu2−xS has several phases depending on the ratio of Cu to S (yarrowite: Cu1.12S, spionkopite: Cu1.39S, geerite: Cu1.6S, anilite: Cu1.75S, digenite: Cu1.8S, roxbyite: Cu1.81S, djurleite: Cu1.96S, and chalcocite: Cu2S). Since each Cu2−xS phase has different characteristics, it is necessary to fine-tune the phase to obtain the desired properties. When the cation exchange reaction occurs from CdS to Cu2−xS, the reaction proceeds in three steps: (1) Cu2−xS island formation, (2) core–shell CdS@Cu2−xS heterostructure formation, and (3) complete conversion to Cu2−xS. The phase of the resulting Cu2−xS NRs can be controlled by the reaction time and the amount of the copper precursor. When the reaction time is short and the amount of precursor is small, the Cu2−xS phase is roxbyite. However, when the reaction time and quantity of precursor are increased, the Cu2−xS phase exhibits low chalcocite after the djurleite phase has formed.
The NPs synthesized by cation exchange can be applied to various devices with enhanced properties and performance. Tang et al.  investigated a cation exchange reaction to synthesize CdS@Cu2S core–shell NRs from CdS NRs, which exhibited high performance as an absorbing layer in a solar cell. The device showed 5.4% energy conversion efficiency, which is higher than other NRs of similar materials that have been studied previously. Feng et al.  demonstrated the cation exchange reaction of a shell to ZnS in Au@AgAuS core–shell NPs, revealing a yolk-shell hybrid structure. These uniquely structured Au@ZnS NPs showed high photocatalytic activity due to optimization of the plasmon-exciton coupling and plasmon-enhanced electron–hole separation. Huang et al.  synthesized ZnSe NPs having a hollow structure by cation exchange and etching from Cu2−xSe NPs. The ZnSe NPs showed high photocatalytic activity because they have a hollow structure with a proper bandgap. Dogan et al.  demonstrated the formation of heterojunctions in NRs using the cation exchange from CdSe to Cu2Se. In a single nanowire CdSe, CdSe-Cu2Se heterojunctions were formed by a masked cation exchange reaction through electron-beam radiation. The masked cation-exchanged NRs can be utilized on-chip and to tune the properties of a device, and so it is expected that they will become useful in this respect.
Studies on cation exchange reactions have progressed not only in chalcogenide compounds but also in pnictide compound. De Trizio et al.  demonstrated the cation exchange reaction from hexagonal Cu3−xP to wurtzite InP. The copper vacancy in Cu3−xP promoted the cation exchange reaction to InP and the optical absorption property changed as the cation exchange reaction proceeded.
Anion exchange is another case of conversion chemistry studied that has developed into a sub-area of ion exchange. Generally, anion exchange reactions are slower than cation exchange due to the low mobility and large ionic radii of anions . Therefore, sluggish anion exchange often requires a longer reaction time and higher reaction temperature than for cation exchange. The benefits of the sluggishness can be utilized in partial anion exchange by controlling the slow reaction kinetics . Cations can easily spread in template NPs while the anion sublattice of NPs is preserved due to the smaller ionic radii of the cations in cation exchange [98, 99]. Thus, cation exchange is usually arranged by an anion sublattice and the morphology of the NPs is not transformed. However, cation diffusion in anion exchange is often advanced and the cation sublattice is disrupted. Consequently, the morphology of NPs synthesized by anion exchange is transformed to hollow structures because of the ‘Kirkendall effect’ in most cases of the anion exchange of, for instance, metal-chalcogenide NPs .
The mechanism of anion exchange can be explained by the theories of mass action and thermodynamic energy. First, in anion exchange reactions explained by the theory of mass action , increasing the concentration of reagents in a solution can promote the kinetics to become similar to cation exchange. For example, metal sulfides have more positive Gibbs free energy of formation than metal oxides, but the anion concentration can induce an energy imbalance and facilitate the sulfidation reaction of metal oxides. However, the theory of mass action is not always applicable to the actual anion exchange reactions in NPs since there are many other factors controlling the overall reaction [100–102]. Second, the thermodynamic theory established for anion exchange is analogous to cation exchange . The Gibbs free energy for a cation exchange reaction is affected by that of the compounds and the reduction potential of the cations. However, it is difficult to adapt the thermodynamic theory of cation exchange reactions to anion exchange because the anion source often requires decomposing or activating processes to become an active reagent. The emitted anions after the reaction can also cause uncontrolled additional reactions, such as O2− ions into OH–. Thus, the thermodynamic spontaneity of anion exchange is determined by the precursors of the incoming anions and further reactions of the outgoing ions .
Research on anion exchange has been widely conducted on perovskites and metal oxides [99, 103–115]. The reaction temperature and time are generally different between the anion exchange reactions using halides with perovskites and the reactions of chalcogen groups with metal oxides. When anion exchange is performed in perovskites synthesized with halogen groups, the reaction requires much lower energy than for reactions from metal oxides to metal chalcogenides (a lower reaction temperature and a shorter reaction time) [98, 104].
NP synthesis through ion exchange is noted as one of the most advanced synthetic methods due to its versatility leading to novel NPs with complex heterostructures (e.g. core–shell, segmented, etc.) and thermodynamically metastable phases, which are considered as limitations in conventional synthesis methods. The main factors controlling ion exchange are the thermodynamic and kinetic properties of the ions. As demonstrated by many examples of ion exchange reactions yielding new NPs, this is a promising state-of-the-art technique to explore new nanostructured materials and provide new design principles for nanosynthetic approaches.
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03936427). This work was also supported by the Chung-Ang University Graduate Research Scholarship in 2017.
All the authors contributed to the writing of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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