Controlling Ion-Exchange Balance and Morphology in Cation

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Controlling Ion Exchange Balance and Morphology in Cation Exchange from Cu P Nanoplatelets into InP Crystals 3-x

Sungjun Koh, Whi Dong Kim, Wan Ki Bae, Young Kuk Lee, and Doh C. Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04859 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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Chemistry of Materials

Controlling Ion Exchange Balance and Morphology in Cation Exchange from Cu3-xP Nanoplatelets into InP Crystals Sungjun Koh†, Whi Dong Kim†, Wan Ki Bae§, Young Kuk Lee*,∥and Doh C. Lee*,† †Department

of Chemical and Biomolecular Engineering, KAIST Institute for the

Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea § SKKU

Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon-si, Gyeonggi-do 16419, Republic of Korea

∥Advanced

Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea

Abstract Synthesis of colloidal nanocrystals (NCs) which are not readily available via wet-chemical approach based on arrested precipitation has often relied on templated growth.

Cation

exchange, in which guest cations in bulk solution replace host cations in template NCs, has evolved as one of the most powerful examples.

Despite its versatility and facileness, there

are caveats, because most of the cation exchange processes presuppose the formation of crystalline defects which are more or less uncontrolled in terms of population and locations. The defect formation is a consequence of the imbalance between extraction and incorporation of host and guest cations.

Here, we demonstrate the controlling of ion exchange balance in 1 ACS Paragon Plus Environment

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Cu+-to-In3+ cation exchange reaction of Cu3-xP nanoplatelets (NPLs), which triggers nanoscale Kirkendall effect, a representative phenomenon of crystal defect generation, clearly shown by morphology change of NPLs.

Cation-exchanged NPLs exhibit various morphologies

depending on the ligand composition introduced in cation exchange reaction.

For example,

when Cu+ is expected to undergo solvation more than In3+ is dissolved, NPLs show cracks, i.e., Kirkendall voids.

High-resolution transmission electron microscopy images of partially

cation-exchanged NPLs show separate steps of Kirkendall void nucleation and growth at the Cu3-xP/InP interface.

Elemental analysis combined with a mathematical description of

stoichiometry of Cu3-xP segment enables the quantification of Cu+ vacancy concentration, which has to do with Kirkendall void nucleation.

Estimation of ion solvation energetics

reveals that the composition of solvating ligands is responsible for imbalance between cation solvation and desolvation resulting in increase in Cu+ vacancy.

In addition to defect

generation inside a crystal, defect removal is visualized via the transformation of cracked InP NPLs into hollow NPLs after annealing at high temperature.

Our in-depth study on ion

exchange balance and morphology change in cation exchange reaction not only provides mechanistic insights in internal defect formation and removal process accompanied with cation exchange, but also opens up the possibility of diverse morphology control of NCs.

Introduction Chemical synthesis of colloidal nanocrystals (NCs) has shown remarkable progress for the past three decades.1-4 With growing demands for application of NCs in a wide range of field, advanced techniques such as templated growth have evolved and enabled tailoring of size, shape, composition, structure, and morphology of NCs.5-10 One of the powerful techniques is utilization of cation exchange, a phenomenon describing replacement of the cations in an ionic 2 ACS Paragon Plus Environment

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compound by externally introduced cations.11 By virtue of high surface reactivity and low solid-state ionic diffusion barrier of NCs compared to bulk, it is facile to induce cation exchange reaction as post-synthetic modification of NCs.12, 13 In most cases, template NCs usually retain the overall shape after conversion.14, 15 In this regard, cation exchange could allow for the growth of NCs with composition or shape that are not likely to be achieved by typical bottom-up strategy.16-20 Although the cation exchange of colloidal semiconductor NCs has been in the spotlight as a versatile synthetic method, lack of understanding in dynamics and crystallographic effect impedes the realization of technological application. It is noteworthy that cation exchange reaction often results in crystalline defects in product NCs of which population and locations are not controlled.14 Indeed, cation-exchanged NCs contain high density of defects compared to bottom-up synthesized NCs. Low photoluminescence and optical stability of cationexchanged Cd- or Zn- chalcogenide core/shell NCs are attributed to the poor crystallographic quality of cation-exchanged products.16, 21 Occasionally, such defects lead to collapse of anion framework of product NCs. Son et al. observed complete disruption of anion sublattice of CdSe NRs after cation exchange to Ag2Se.12 In our previous work on direct Cd-to-Pb cation exchange of CdSe NRs, we also observed the transformation of CdSe NRs to spherical PbSe NCs in a certain reaction condition.22 We have raised the possibility of defect formation within NRs during cation exchange due to the diffusion difference between Cd2+ and Pb2+. However, mechanism of defect formation at atomic scale remains unclear. For better design of cation exchange reaction and practical use of cation-exchanged NCs, it is necessary to understand the defect formation pathway in the context of the imbalance between extraction and insertion of host and guest cations.

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Nanoscale Kirkendall effect entails a process in which the imbalance in ion diffusion rate causes formation of defects or voids within colloidal NCs.23,

24

In bulk, Kirkendall effect

normally takes place in intermetallic diffusion processes.25 Kirkendall voids evolve at the interface such as solder joints in microelectronic devices.26, 27 In nanoscale, the Kirkendall effect takes place in processes involving solid-state inter-diffusion such as cation exchange,28 galvanic replacement,29, 30 and oxidation, sulfidation or phosphidization of metal NCs.23, 31, 32 Similar to the case of bulk, the nanoscale Kirkendall effect results in defective sites inside NCs. While defect formation is considered detrimental for optoelectronic applications of product NCs, it could often be beneficial for catalytic or electrochemical properties of NCs.33, 34 A bigger issue is that no chemical protocol for cation exchange of NCs has come to grips with controlled ion exchange balance. In this sense, it is difficult to predict the condition that prevents or induces the nanoscale Kirkendall effect. Considering the nature of wet chemical reaction, organic ligands are expected to have a profound bearing on the ion exchange balance. Utilized in cation exchange to form complex with guest and host cations in bulk solution, organic ligands with different functional groups have different binding affinity for various cations based on hard and soft acids and bases (HSAB) theory, a qualitative concept that covers stability of metal-ligand complex.35 In this regard, the ion solvation and desolvation balance would depend on the selection or composition of ligand mixture utilized in the cation exchange. Previous studies focusing on cation exchange kinetics and dynamics with respect to organic ligands imply that controlling the type or composition of ligands would be key to manipulate the ion exchange balance.18, 36


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Scheme 1. Schematic description of morphology and structure changes of NPLs resulted from (a) ion exchange balance in Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs and (b) annealing at high temperature

Here, we report controlling of ion exchange balance and the following morphology change during Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs. Controlling the composition of organic ligands, trioctylphosphine (TOP) and oleylamine (OAm), introduced in cation exchange enables manipulating the extraction and incorporation balance between Cu+ and In3+, i.e., solvation of Cu+ and desolvation of In3+. Depending on the ligand composition, cationexchanged NPLs show clearly different morphologies as depicted in Scheme 1a. Especially, nanoscale Kirkendall effect takes place when the solvation of Cu+ outweighs the stoichiometric quantity of In3+ desolvation resulting in cracked morphology. High-resolution transmission electron microscopy (HR-TEM) analysis visualizes the nucleation and growth of Kirkendall voids at Cu3-xP/InP heterointerface during cation exchange. In addition, energy dispersive xray spectroscopy (EDX) analysis on partially cation-exchanged Cu3-xP/InP NPLs show that the elemental composition of the hetero-NPLs also depends on the ligands composition. By relating composition of Cu3-xP segment to reaction coordinate, we quantify the concentration 5 ACS Paragon Plus Environment


of Cu+ vacancy, which is the source of Kirkendall void nucleation. The prediction of ion exchange balance during the cation exchange has heretofore relied on HSAB theory, a concept that describes stability of metal-ligand complex in a qualitative fashion.35 We provide quantitative insights in solvation energetics that governs ion exchange balance. Moreover, in addition to the defect generation by diffusion imbalance during cation exchange, we present defect removal pathway by proposing a hollow NCs formation mechanism based on the observation of void coalescence inside cracked NPLs (Scheme 1b).


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Experimental Section Chemicals. Copper (II) chloride dihydrate (≥ 99%), Copper(I) iodide (≥ 99.999%), ntrioctylphosphine (≥ 97%), n-trioctylphosphine oxide (≥ 99%), oleylamine (≥ 70%), and 1octadecene (≥ 90%) were purchased from Sigma-Aldrich Co. Indium(III) iodide (≥ 99.999%) was purchased from Alfa Co. Tris(trimethylsilyl)phosphine (≥ 95%) was purchased from JSI Silicone Co. All the purchased chemicals were used without further purifications. Synthesis of Cu3-xP NPLs. 68.2 mg of copper(II) chloride dihydrate (0.4 mmol), 4 g of ntrioctylphosphine oxide and 20 mL of oleylamine were added into a round-bottom three-neck flask (100 mL, 14/20) equipped with a reflux condenser and a thermocouple. The flask was degassed under vacuum (< 150 mTorr) for 30 min at 120 °C with vigorous stirring. After the flask was filled with argon, a desired amount of n-trioctylphosphine was added to the flask. When the color of the mixture turned into yellow, the flask was heated up to 230 °C. In a glove box filled with argon, 25.1 mg of tris(trimethylsilyl)phosphine (0.1 mmol) diluted in 0.2 mL of 1-octadecene was taken out with a syringe, and quickly injected into the reaction flask. After 10 min of reaction, the flask was cooled down to room temperature. The blackish solution was mixed with toluene and ethanol, and the NPLs were precipitated by centrifugation for 10 min at 4000 rpm, and redispersed in toluene. The washing cycle was repeated for three times, and the precipitated Cu3-xP NPLs were finally dissolved in oleylamine and stored in a glove box for characterization or cation exchange reaction. Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs. For a typical cation exchange reaction with ϕTOP = 0.5, 495 mg of indium(III) iodide (1 mmol), 3.5 mL of ntrioctylphosphine, and 2.5 mL of oleylamine were added into a round-bottom three-nech flask (50 mL, 14/20) equipped with a reflux condenser and a thermocouple. The flask was degassed under vacuum (< 150 mTorr) for 30 min at 120 °C with vigorous stirring. After filled with 7 ACS Paragon Plus Environment


argon, the flask was heated up to desired reaction temperature. In a glove box filled with argon, Cu3-xP NPLs (In/P = 100) dissolved in 1 mL of oleylamine was taken out with a syringe, and quickly injected into the reaction flask. After 10 min of reaction, the flask was cooled down to room temperature. The brownish solution containing cation-exchanged NPLs was mixed with toluene and ethanol, precipitated by centrifugation at 4000 rpm, and redispersed in toluene. The washing cycle was repeated for three times, and the precipitated NPLs were finally dissolved in chloroform, toluene or tetrachloroethylene for characterization. Solvation of metal iodides in ligand mixtures. All procedures for solvation of metal iodides are conducted in a glove box filled with argon. 2 g of copper(I) iodide or indium(III) iodide and 1 mL of ligand mixture consists of n-trioctylphosphine and oleylamine with a desired ratio were added into a 10-mL scintillation vial. After the vial was put on a hot-plate set to a desired temperature and equipped with a thermocouple, the mixture was gently stirred until the solvation is in equilibrium. After remaining salts were settled, 0.1 mL of supernatant was taken with syringe and diluted in 1 mL of hexane to prevent solidification at room temperature. The process was repeated with several different temperatures and diluted metal-ligand complexes were stored in a glove box for analysis. Formation of hollow InP NPLs by annealing. After 10 min of cation exchange reaction, the flask was heated to 300 °C. Annealed for 30 min, the flask was cooled down to room temperature. The brownish solution containing cation-exchange hollow InP NPLs was mixed with toluene and ethanol, precipitated by centrifugation at 4000 rpm, and redispersed in toluene. The washing cycle was repeated three times, and the precipitated InP NPLs were finally dissolved in chloroform, toluene or tetrachloroethylene for characterization. UV-vis-NIR absorption spectroscopy. Samples diluted in tetrachloroethylene were transferred in a 1 cm quartz cuvette. The cuvette was installed in a Shimadzu UV-3600, and 8 ACS Paragon Plus Environment

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the absorption spectra were recorded, of which baseline was measured with tetrachloroethylene. Powder X-ray diffraction (XRD). Samples were prepared by drop-casting NPLs diluted in chloroform onto Si substrates. XRD patterns were recorded using a RIGAKU SmartLab equipped with a D/teX Ultra 250 detector with Cu Kα radiation (λ = 1.5406 Å) Transmission electron microscopy (TEM). Samples for imaging were prepared by dropcasting NPLs diluted in chloroform on a 300-mesh Ni grid with carbon support film. TEM images were obtained with a Tecnai F20 transmission electron microscope at 200 kV acceleration voltage. High-resolution TEM (HR-TEM) images were obtained with a Tecnai F30 ST transmission electron microscope at 300 kV acceleration voltage. Chemical compositions of samples were measured by energy dispersive x-ray spectroscopy (EDX) with the HR-TEM images. Fast-Fourier transform (FFT) patterns of HR-TEM images were analyzed using DigitalMicrograph software from Gatan Inc. For the void fraction measurement of cracked and hollow NPLs, all the boundaries between crystalline segments and backgrounds in TEM images were marked using Adobe Photoshop software. Using ImageJ software, the ratio between lateral area of voids and entire hexagon of particles were measured on ~80 particles in ~10 of low-resolution TEM and ~25 HR-TEM images for each experimental condition. Inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Samples for measurement were prepared by adding an internal standard such as zinc acetate for quantitative analysis and digesting in aqua regia with heating at 60 °C for more than 12 h. After the samples were further diluted with deionized water, elemental analysis was carried out with a ELAN DRC II for ICP-MS and a OPTIMA 7300 DV for ICP-AES. Using the elemental analysis, fraction of InP domain in NPLs was estimated using the equation as 9 ACS Paragon Plus Environment


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iIn fInP =

(iIn/iP)complete iCu (iCu/iP)pristine

+

iIn (iIn/iP)complete

where fInP is fraction of InP domain in NPLs, iIn and iCu are atomic fractions of indium and copper in NPLs, (iCu/iP)pristine is copper to phosphorus atomic ratio of pristine Cu3-xP NPLs, and (iIn/iP)complete is indium to phosphorus atomic ratio of fully cation-exchanged InP NPLs.


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Results and Discussion

Figure 1. TEM images of (a) Cu3-xP NPLs and (c) InP NPLs. HR-TEM images and corresponding FFT patterns of (b) Cu3-xP NPL and (d) InP NPL. (e) Description of phosphorus framework and unit cells of hexagonal Cu3P and InP. (f) Absorption spectra of Cu3-xP NPLs and InP NPLs. (g) XRD patterns of Cu3P NPLs and InP NPLs.

Cu+-to-In3+ Cation Exchange Reaction of Cu3-xP NPLs. Hexagonal Cu3-xP NPLs for cation exchange reaction were synthesized based on a literature procedure with modification.37 TEM images in Figure 1a show that Cu3-xP NPLs are hexagonal-shaped with lateral dimensions within the range of 100 nm. Two edges of Cu3-xP NPL facing each other is in parallel with (110) plane, of which d-spacing is 3.550 Å (Figure 1b). The intrinsic substoichiometry of Cu3xP

NPLs is confirmed by estimating the composition as NCu/NP = 2.89 using EDX

measurements, which is similar compared to values in literature.38 Utilizing this template, we attempted cation exchange reaction to obtain hexagonal InP NPLs. Referring a method by Trizio et al.,38 we used excess of indium iodide (NInI3/NP = 100) as the indium precursor and 11 ACS Paragon Plus Environment


a ligand mixture that consists of TOP and OAm as the reaction solvent. Figure 1c shows InP NPLs resulted from the cation exchange reaction in 1 mL of TOP and 6 mL of OAm (ϕTOP = 0.14), at 200 °C for 10 min. The shape and dimension of product InP NPLs are consistent with the template Cu3-xP NPLs. By analyzing FFT patterns of HR-TEM images (Figure 1d), we estimated the d-spacing in (100) planes of InP NPLs to 3.582 Å. There is a slight lateral lattice expansion after cation exchange reaction by 0.9% without transformation of anion framework, confirmed by comparing d-spacing values of parallel (100)InP and (110)Cu3 ― xP planes as depicted in Figure 1e. Figure 1g shows absorption spectra of Cu3-xP NPLs and InP NPLs. The broad signal across near-IR over 1000 nm in Cu3-xP NPLs is known to originate from localized surface plasmon resonance resulted from high density of free charge carrier (hole) owing to plenty of Cu+ vacancies.37 After cation exchange to InP NPLs, the plasmonic absorption completely disappeared, while a distinguishable edge evolves at ~750 nm. This edge is attributed to band-gap excitonic transition of along (001) direction considering bulk band gap of wurtzite InP (1.49 eV),39 exciton Bohr radius of zincblende InP (~11 nm),38, 40, 41 and the thickness of InP NPLs measured in TEM images (Figure S2). The disappearance of plasmonic absorption indicate that InP NPLs do not possess a comparable amount of cation vacancy, in contrast with Cu3-xP NPLs. In addition, the composition of InP NPLs (NIn/NP = 1.24) measured by EDX analysis is in agreement with a reported value.38 XRD patterns of Cu3-xP and InP NPLs verify the complete crystal conversion by cation exchange reaction (Figure 1g). When employed in cation exchange reaction of Cu-based NCs, tertiary alkylphosphines are known to effectively extract Cu+ in host NCs.14, 18, 42 Based on Pearson’s HSAB theory, it is reasonable to predict that Cu+ would have high affinity to form complex with R3P. In fact, R3P has been considered a prerequisite to initiate cation exchange of Cu-based NCs.18 We also found that Cu+-to-In3+ cation exchange reaction in our case does not proceed in the absence of


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TOP (Figure S3). Considering the ability of TOP to extract Cu+, we expect that controlling the amount of TOP in cation exchange would significantly influence the diffusion balance and reaction kinetics, i.e., solvation or desolvation of guest or host cations.14 In particular, if there is significant imbalance in diffusion rates between guest and host cations, crystalline defects inside NCs or their morphology change can be involved.28 Since generating or preventing defects and controlling morphology are often key to tailor optoelectronic or catalytic properties of NCs, it is important to elucidate the process caused by diffusion imbalance in detail. Thus, we attempted to control the diffusion balance between Cu+ and In3+ in the cation exchange reaction of Cu3-xP NPLs by controlling the ratio of TOP in mixture with OAm.



Figure 2. HR-TEM images of (a) partially cation-exchanged Cu3-xP/InP NPL and (b-f) fully cationexchanged InP NPL (scale bar = 10 nm). Cation exchange reactions were proceeded in ligand mixtures consist of TOP and OAm with (a, d) ϕTOP = 0.07, (b, e) ϕTOP = 0.14, and (c, f) ϕTOP = 0.5 at 200 °C and 250 °C. ϕvoid represents average fractions void of cation-exchanged NPLs. (g) Elemental composition measured by ICP-AES exhibiting Cu3-xP-to-InP conversion rates. Fraction of InP domain in NPLs is estimated using the equation noted in experimental section.

Nanoscale Kirkendall Effect by Ion Exchange Imbalance. Cu+-to-In3+ cation exchange reactions of Cu3-xP NPLs were performed in various ratio of TOP in ligand mixtures and reaction temperatures, and we monitored the morphology changes and reaction kinetics. TEM images in Figure 2 show morphologies of product NPLs resulted from Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs at 200 °C and 250 °C for 10 min in ligand mixtures of (i) 0.5 mL of TOP and 6.5 mL OAm (ϕTOP = 0.07), (ii) 1 mL of TOP and 6 mL OAm (ϕTOP = 0.14), and (iii) 3.5 mL of TOP and 3.5 mL of OAm (ϕTOP = 0.5). In the case of 200 °C and ϕTOP = 0.07, edges of Cu3-xP NPLs are converted to InP segments resulting in laterally


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heterostructured Cu3-xP/InP NPLs with ϕInP = 0.4 (Figure 2a). Similar to that typically observed in cation exchange reaction in two-dimensional nanocrystals, the reaction semms to proceed from edge to center. Since top and bottom planes are passivated by densely packed ligands, host cations would easily attack low-coordinated edges.28, 38, 43 At partially cationexchanged NPLs, most of InP segments form heterointerfaces with residual Cu3-xP segment along (100)InP plane, which is parallel to (110)Cu3 ― xP plane. We ascribe the predominance of (110)Cu3 ― xP/(100)InP interface to the tendency to lower the interfacial energy by minimizing linear density of interfacial bonding (linear interfacial bonding density of (110)Cu3 ― xP/(100)InP is the lowest among all possible Cu3-xP/InP interfaces perpendicular to lateral plane as shown in Figure S10.). In the case of ϕTOP = 0.14 at 200 °C, intact InP NPLs are obtained (ϕInP = 1) (Figure 2b), which are shown in Figure 1. When ϕTOP is increased to 0.5, interestingly, voids appear inside InP NPLs dividing a NPL into several segments while the entire hexagonal shape remains unchanged (Figure 2c). Voids occupy 8.2% of the volume of InP NPLs in average. In addition, most of inner edges of InP NPLs contacting voids seem to be terminated with (100)InP facets that originally form stable interfaces with (110)Cu3 ― xP facets. In the case of cation exchange reaction at 250 °C, Cu3-xP NPLs are fully converted to InP NPLs with voids in all applied conditions (ϕTOP = 0.07, 0.14 and 0.5) (Figure 2d, 2e and 2f). It is notable that void fraction increases with increasing ϕTOP while most of internal InP edges are also terminated with (100)InP facets. Not only the unprecedented observation of cracked morphology, it is interesting that the void fraction depends on ϕTOP and reaction temperature. Based on the trend that void fraction increases with increasing ϕTOP, three possible pathways for void formation can be hypothesized depending on the time of occurrence: (i) chemical etching of InP NPLs by TOP after cation exchange reaction is completed; (ii) chemical etching of Cu3-xP segment at Cu315 ACS Paragon Plus Environment


xP/InP

interface by TOP before cation exchange reaction is completed; or (iii) nanoscale

Kirkendall effect due to imbalance in rates of out-diffusion of Cu+ and in-diffusion of In3+ during the cation exchange. To verify scenario (i), we annealed intact InP NPLs obtained with ϕTOP = 0.14 in TOP at 200 °C for 1 h. However, we did not observe voids in the intact InP NPLs (Figure S13). For scenario (ii), we annealed pristine Cu3-xP NPLs at 200 °C in TOP for 1 h. As a result, optical density of Cu3-xP NPLs gradually decreased in absorption spectra with decreased lateral dimension (Figure S11 and S12), and voids did not appear inside Cu3-xP NPLs. It might be possible that relatively vulnerable edges of Cu3-xP NPLs are chemically etched by TOP at high temperatures, similar to etching of Cu2-xS segment in the case of spherical Cu2xS/ZnS

heterostructure.44 However, etching of Cu3-xP edges during cation exchange reaction is

not likely to result in the perfect preservation of hexagonal shape in Figure 2c, in contrast to that the spherical shape collapsed in the case of Cu2-xS/ZnS after etching of Cu2-xS segment.44 We regard that scenario (iii) is most plausible to explain the void formation during the cation exchange reaction. The accelerated exchange reaction with increased ϕTOP also supports the scenario implying that out-diffusion of Cu+ is facilitated (Figure 2g). As mentioned above, internal defects or slight change of morphology by nanoscale Kirkendall effect is difficult to capture in spherical or rod-shaped NCs, and would have substantial impact on NC properties that are critical for application of cation-exchanged NCs. Moreover, effective control over nanoscale Kirkendall effect would afford accessing variety of morphologies and expanded applicability of cation-exchanged NCs. Therefore, it is essential to understand the underlying mechanism of nanoscale Kirkendall effect and investigate the driving force in a quantitative fashion.


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Figure 3. HR-TEM images of (a-g) partially cation-exchanged Cu3-xP/InP NPLs and (h) fully cationexchanged InP NPL. Aliquots for TEM imaging are taken at reaction times of (a, g) 1 min, (b) 1.5 min, (c) 2 min, (d) 2.5 min, (e, f) 3 min, and (h) 5 min. Cu3-xP/InP, void/Cu3-xP and void/InP interfaces are marked with red, yellow and orange lines.

We examined, step by step, the proceeding of nanoscale Kirkendall effect in atomic scale using HR-TEM analysis. HR-TEM images of samples taken during the cation exchange with ϕTOP = 0.5 at 200 °C let us monitor Kirkendall void nucleation and growth at Cu3-xP/InP heterointerface. At the very early stage of reaction (1 min), InP segment with ~10 nm of lateral dimensions nucleated at the edge of Cu3-xP NPL building stable (110)Cu3 ― xP/(100)InP heterointerface (Figure 3a). Maintaining the interface, InP segment extends its domain toward center of NPL as reaction proceeds (Figure 3b). When the reaction time reaches 2 min, nucleation of voids starts to appear at the heterointerface of which width corresponds to ~3 monolayers of InP (Figure 3c). Figure 3d shows traces of void growth along Cu3-xP/InP interface. Even though most of Cu3-xP/InP interfaces consist of (110)Cu3 ― xP/(100)InP along



which voids are located eventually (Figure 3d and S4), we could also observe voids at (100)Cu3 ― xP/(110)InP (Figure 3c). At 3 min of reaction time, InP segment with ~10 nm of lateral size nucleated independently in Cu3-xP segment that faces already-grown InP segment across void (Figure 3e, 3f and S5). The separate nucleation of InP segment indicates that voids at the Cu3-xP/InP interface allow In3+ in solution to adsorb at Cu3-xP/void interface, resulting in beginning of another independent cation exchange reaction. Moreover, along with NPLs showing Cu3-xP/InP/void/InP structure as shown in Figure 3e and 3f, we observed fully cationexchange InP NPLs at 3 min of reaction time (Figure S9). The simultaneous observation implies that the reaction time necessary to convert Cu3-xP/InP/void/InP NPLs to InP NPLs is short enough to hinder the observation of intermediate states in between. It indicates that asformed voids accelerate overall cation exchange reaction by enabling additional fast edge In3+ adsorption and subsequent cation exchange. Furthermore, since voids are located across the entire lateral planes of a NPL, as shown in Figure 2e and 2f, a set of void formation and edge reaction is likely to take place repetitively with markedly accelerated total conversion rate. Atomic-resolution TEM images in Figure 3g, 3h and S6 show Cu3-xP/InP and InP/void/InP interface, each of which corresponds to before and after one cycle of reaction proposed above. In the light of bulk cases,23, 24 it is likely that Cu+ vacancies in Cu3-xP segment condense to nucleate voids at Cu3-xP/InP interface. High ϕTOP would result in high Cu+ vacancy concentration because of fast out-diffusion of Cu+. To verify the hypothesis, we quantified and compared Cu+ vacancy concentration in partially cation-exchanged Cu3-xP/InP NPLs with ϕTOP = 0.07 and 0.5 at 200 °C. In spite of Cu+-deficient nature, Cu3-xP NPLs do not transform to other phases such as Cu3P2. Thus, we assume that Cu/P ratio in Cu3-xP segment would only depend on the out-diffusion of Cu+ during cation exchange. We also assume that InP segments are vacancy-free. Based on the set of assumptions, we estimated the composition in Cu3-xP


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segment of Cu3-xP/InP NPLs using EDX measurement. We introduce two dimensionless terms: ξCE, reaction coordinate of cation exchange; and υCu3 ― xP, stoichiometric parameter of Cu3-xP segment. These dimensionless terms are the functions of atomic ratio of Cu, In and P in Cu3xP/InP

NPLs (see Supporting Information for definitions and details). Simply, ξCE represents

the atomic fraction of P in InP domain out of entire Cu3-xP/InP NPLs. ξCE increases from zero to unity as the cation exchange proceeds. υCu3 ― xP indicates the composition of Cu3-xP segment of Cu3-xP/InP NPLs compared to pristine Cu3-xP NPLs. υCu3 ― xP would pivot around unity depending on disparity between out-diffusion of Cu+ and in-diffusion of In3+ during the cation exchange. For example, if out-diffusion of Cu+ is balanced by stoichiometric in-diffusion of In3+, ξCE retains unity. When out-diffusion of Cu+ outweighs in-diffusion of In+, ξCE falls below unity. Plotting ξCE and υCu3 ― xP would visualize the Cu-vacancy concentration changes in terms of the extent of cation exchange and the ligand condition.



Figure 4. (a) Stoichiometric parameter and (b) lattice constant of Cu3-xP segments, depending on cation exchange reaction coordinate, of partially cation-exchanged Cu3-xP/InP NPLs reacted in mixtures of TOP and OAm with ϕTOP = 0.07 and ϕTOP = 0.5 at 200 °C. Reaction times for analyzed samples are marked in (a). Data points for each condition are indicated by blurred circles that form averages and error bars. Grey and green lines in (b) denote lattice constant of pristine Cu3-xP NPLs and lattice constant of InP times

3 for estimating lattice mismatch at the Cu3-xP/InP heterointerface considering 30 °

rotation relationship of Cu3-xP and InP unit cells.

Figure 4a shows changes of υCu3 ― xP depending on ξCE at 200 °C with ϕTOP = 0.07 and ϕTOP = 0.5. At early stages of reaction, υCu3 ― xP increases over unity in both cases. Since Cu3-


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xP

NPLs already possess a significant amount of Cu+ vacancies, the increase of υCu3 ― xP is

attributed to fast edge adsorption and vacancy filling by In3+. In the case of ϕTOP = 0.5, υCu3 ― xP starts to decrease below unity, as ξCE increases over 0.18. υCu3 ― xP keeps decrease as reaction proceeds until it reaches to ~0.8, and would eventually become unity with ξCE = 1. On the other hand, as ξCE increases over 0.3 in the case of ϕTOP = 0.07, υCu3 ― xP decreases below 1 but retains over 0.9. At similar ξCE, the difference in υCu3 ― xP in the cases of ϕTOP = 0.07 and 0.5 is significant. In other words, large amount of TOP leads to fast out-diffusion of Cu+. The increased Cu+ vacancies in the case of ϕTOP = 0.5 result in void nucleation. In addition, Cu3-xP/InP NPLs with 0.53 < ξCE < 1 did not appear in the case of ϕTOP = 0.5. Instead, both NPLs with ξCE = 0.53 and 1 were observed at 3 min. As we mentioned above, cation exchange accelerates after voids have been formed. The change in stoichiometry of Cu3xP

segment also gives rise to lattice mismatch at Cu3-xP/InP heterointerface. It is known that

the change in stoichiometry of Cu-based compound NCs such as Cu2-xSe NCs induces expansion or contraction of lattice.45 In addition, a recent work by Wolff et al. shows that lattice constant of bulk Cu3-xP changes in the range of a few picometers with 0.1 < x < 0.7.46 In our case, it is expected that lattice constant of Cu3-xP segment decreases with decreasing υCu3 ― xP. We estimated lattice constants of Cu3-xP segments by measuring (100)Cu3 ― xP plane spacing in FFT patterns of HR-TEM images of the partially cation-exchanged NPLs (see Supporting Information for details). As ξCE increases in the case of ϕTOP = 0.07, lattice constant of Cu3xP

segment remains to ~7.1 Å, the lattice constant of pristine Cu3-xP NPLs, within the error

range. Meanwhile, lattice constant notably decreases as ξCE increases in the case of ϕTOP = 0.5, reaching 7.035 Å at ξCE = 0.53. Consequently, lattice mismatch at Cu3-xP/InP heterointerface increases upto 1.8% at ξCE = 0.53, 0.9%p larger than in the case of ϕTOP = 0.07. The change in lattice constant of Cu3-xP segment is consistent in XRD patterns of samples 21 ACS Paragon Plus Environment


in the case of ϕTOP = 0.5 (Figure S14). In addition, XRD patterns of samples in the case of ϕTOP = 0.5 show further lattice mismatch at the heterointerface along the c-axis direction (Figure S14). We speculate that the instability at the Cu3-xP/InP heterointerface induced by increased lattice mismatch would assist the growth of as-nucleated voids along the interface.

Cation Solvation Energetics. It is expected that large amount of TOP induces fast solvation of Cu+ in Cu3-xP NPLs by forming stable complex with Cu+. Likewise to TOP that extracts Cu+, OAm is expected to bind favorably to In3+, each of which are classified as hard base and hard acid in Pearson’s HSAB theory. Namely, decrease of ϕOAm with increasing ϕTOP would result in fast desolvation of In3+, i.e., rapid in-diffusion of In3+ into NPLs. In the case of cation exchange reaction with ϕTOP = 0.5, it is unclear whether the solvation of Cu+ overwhelms desolvation of In3+, or, instead, desolvation of In3+ is indeed suppressed with high ϕTOP. In addition, since recursive experiments would be inevitable to control the ion exchange balance and the morphology of NCs by relying on a qualitative theory, it is desired to establish a quantitative guideline on the ion solvation energetics. Therefore, we attempted to quantify the solvation energetics in Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs. We employed an analogy in chemical equations between Cu+-to-In3+ cation exchange reaction in ligand mixtures and solvation reaction of CuI and InI3. Accompanied by thermodynamic relations including Van’t Hoff equation, measurement of equilibrium solubilities of CuI and InI3 in various composition of ligand mixture at several temperatures enabled reasonable estimation of free energy changes of Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs and solubility of CuI and InI3 in actual reaction temperatures in various ϕTOP (see Supporting Information for more details).


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Figure 5. (a) Estimated free energy change of Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs and (b) equilibrium solubility of CuI and InI3 at 200 °C and 250 °C in ligand mixtures of TOP and OAm.

It turns out that the increase in the content of TOP in ligand mixture greatly lowers the free energy of Cu+-to-In3+ cation exchange reaction at 200 °C and 250 °C, at which cation exchange reactions proceeded for samples in Figure 2. With ϕTOP = 0, cation exchange reaction does not begin since ∆GCE is over 0 at both temperatures, as we verified experimentally (Figure S3). InP segments appear at the very edges of Cu3-xP NPLs with ϕTOP = 0.007 at 200 °C (Figure S7), which indicates initiation of the spontaneous cation exchange with negative ∆GCE. 23 ACS Paragon Plus Environment


With increasing ϕTOP, ∆GCE continues to decrease and saturates after ϕTOP = 0.5. The drop of ∆GCE with increasing ϕTOP is more pronounced at 250 °C. In addition, the increment in ϕTOP leads to high solvation entropy of CuI, which justifies the temperature dependency of ∆GCE (Table S1). It is expected from HSAB theory that the increase in ϕTOP and reaction temperature facilitates the formation of stable Cu+-TOP complex and induces fast out-diffusion of Cu+ during the cation exchange reaction, resulting in high concentration of Cu+ vacancies left in Cu3-xP segment. However, it is unexpected that increase of ϕTOP indeed enhances the solvation of In3+ as well (Figure 5b). The solvation entropy of InI3 increases in addition to that of CuI, as ϕTOP increases (Table S1), resulting in increase in solubility at 200 °C and 250 °C. When ϕTOP increases from 0 to 0.07, equilibrium solubility of CuI increases by ~4 fold and that of InI3 also increases by ~2 fold. In addition, while the solubility of InI3 continues to increase as ϕTOP increases over 0.14, the solubility of CuI seems to saturate after ϕTOP = 0.14, even decreases after ϕTOP = 0.5. We believe that the continuous increase in solubility of InI3 with increasing ϕTOP breaks the balance between solvation of Cu+ and desolvation of In3+ during the cation exchange reaction, resulting in high density of Cu+ vacancy in Cu3-xP segment of Cu3-xP/InP intermediates.


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Figure 6. Schematic description of morphology change of InP NPLs during Cu+-to-In3+ cation exchange reaction with (a) moderate and (b) high ϕTOP, and (c) repetitive Kirkendall void nucleation and growth, and cation exchange in the case of high ϕTOP including (i) Kirkendall void nucleation at Cu3-xP/InP interface by vacancy coalescence, (ii) Kirkendall void growth along the unstable Cu3-xP/InP interface, (iii) Cu3-xP/void/InP interfaces after Kirkendall void growth, (iv) fast adsorption and cation exchange by In3+ at the edge of Cu3-xP segment through void, (v) Kirkendall void nucleation at Cu3xP/InP

interface with newly nucleated InP segment, (vi) Kirkendall void growth along the Cu3-xP/InP

interface, and (vii) formation of cracked morphology by sequential nanoscale Kirkendall effect.



Figure 6 summarizes the morphology change of InP NPLs during Cu+-to-In3+ cation exchange reaction depending on ϕTOP. In the case of moderate ϕTOP, rates of solvation of Cu+ and desolvation of In3+ are well balanced during the cation exchange reaction resulting in intact InP NPLs (Figure 6a). On the other hand, as increased ϕTOP enhances solvation of both Cu+ and In3+, out-diffusion of Cu+ overwhelms in-diffusion of In3+ triggering multiple nanoscale Kirkendall effect, which eventually leads to cracked morphology (Figure 6b). Figure 6c describes detailed repetitive reaction that consists of Kirkendall void nucleation and growth, and cation exchange reaction in the case of high ϕTOP, as follows: (i) large amount of TOP induces fast out-diffusion of Cu+ leaving high density of Cu+ vacancies in Cu3-xP segment, and the vacancies condense to nucleate Kirkendall void at the Cu3-xP/InP heterointerface. (ii) The void grows along the interface assisted by increased strain due to increased lattice mismatch. (iii) As-grown void divides Cu3-xP/InP NPLs into separate segments. (iv) In3+ ions diffusing through the void adsorb at the edge of Cu3-xP segment, and an independent cation exchange reaction begins. (v, vi and vii) Repetition of Kirkendall void nucleation and growth. Several cycles of the reaction cycle would result in cracked InP NPLs with voids placed through entire lateral planes as shown in Figure 2e and 2f.


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Figure 7. TEM images and HR-TEM images of cation-exchanged InP NPLs reacted at 300 °C for 10 min with (a, b) ϕTOP = 0.07, (c, d) ϕTOP = 0.14, and (e, f) ϕTOP = 0.5. ϕvoid represents average fractions of void of cation-exchanged NPLs.

Formation of Hollow InP NPLs. Multiple occurrence of nanoscale Kirkendall effect during Cu+-to-In3+ cation exchange is responsible for the cracked morphology of InP NPLs. However, it is interesting that most of reports regarding nanoscale Kirkendall effect demonstrate hollow NCs as a result, rather than cracked morphology with sharp inner edges in our case. For better understanding on formation and transition of internal defect of NCs during cation exchange and accessibility of expanded NC morphologies, the origin of NC morphological difference between previous reports and this work needs to be clarified. We noted that NC morphologies often change during cation exchange reaction simply by increasing or decreasing reaction



temperature.28,

47

In this context, we attempted the cation exchange reaction at elevated

temperature. The Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs proceeded at 300 °C for 10 min resulted in intact or hollow InP NPLs, of which morphologies are clearly distinguished from the cases of 200 °C and 250 °C. As shown in Figure 7, the hollowness of NPLs depends on ϕTOP of ligand mixture solvent, while the products for all three cases are fully cationexchanged InP NPLs. In the case of ϕTOP = 0.07, products are intact InP NPLs that preserve the original morphology of Cu3-xP NPLs (Figure 7a and 7b). Meanwhile, voids appear at the center of InP NPLs with ϕTOP = 0.14 and 0.5 resulting in hollow morphologies, and the sizes of voids are bigger with ϕTOP = 0.5 (Figure 7c, 7d, 7e and 7f). Based on the cation solvation energetics discussed above, it is reasonable to expect that nanoscale Kirkendall effect would intensify with increasing ϕTOP resulting in higher void fraction in InP NPLs. However, voids concentrated at the center of NPLs would not be the direct result from repetitive void formation and cation exchange reaction (Scheme 2). Thus, we regard that the formation of hollow morphology takes place after the cation-exchange reaction is completed. In nanoscale, the transformation of crystal including recrystallization or fusion is known to facile at relatively low temperatures compared to the case of bulk.13 For example, morphology or structure changes have been realized in metal chalcogenide NCs by annealing at their typical colloidal synthetic temperatures, e.g., oriented attachment of PbSe NCs by annealing at 190 – 250 °C.48 In this regard, it is likely that crystal ripening takes place in cracked InP NPLs after fast cation exchange reaction at 300 °C, a typical reaction temperature to synthesize cubic InP NCs.49


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Figure 8. TEM images and HR-TEM images of cation-exchanged InP NPLs reacted at 250 °C with (a, b) ϕTOP = 0.07 and (c, d) ϕTOP = 0.5, and InP NPLs after additional annealing at 300 °C for 30 min derived from NPLs with (e, f) low void fraction and (g, h) high void fraction. ϕvoid represents average fractions of void of cation-exchanged and annealed NPLs. (i) Schematic illustration of hollow structure formation process by void coalescence.



We attempted to decouple the high temperature annealing effect from cation exchange reaction on crystal ripening of cracked InP NPLs to hollow morphology. First, we prepared two kinds of cracked InP NPLs, which have low and high void fraction (ϕvoid = 0.019 and 0.093) resulted from cation exchange reaction at 250 °C with ϕTOP = 0.07 and 0.5 (Figure 8a,b for ϕvoid = 0.019, and 8c,d b for ϕvoid = 0.093, respectively). After 10 min of cation exchange reaction, the temperature was directly raised to 300 °C, and cracked InP NPLs are annealed for 30 min. Interestingly, cracked InP NPLs with voids through entire lateral planes converted to hollow InP NPLs with a void at the center (Figure 8e, 8f, 8g and 8h). In the case of InP NPLs with low void fraction, ϕvoid decreased from 0.019 to 0.008, indicating that crystal coalescence took place during the annealing process. It seems that voids originally positioning at the center coalesce to a bigger void, while voids apart from center are removed by fusion of adjacent InP domains. On the other hand, in the case of InP NPLs with high void fraction, ϕvoid increases from 0.093 to 0.32. In addition to crystal and void coalescence, weakly linked segments in the cracked InP NPLs would fall apart into the solution, resulting in wide hole at the center of NPLs. We believe that the crystal and void coalescence process, as depicted in Figure 8i, is driven by the propensity to minimize the surface energy of unstable edges, similar to that proposed for single large void formation in hollow NiO nanospheres.50 In other words, when sufficient thermal energy is supplied, voids would be removed or coalesce resulting in low edge surface energy, analogous to Ostwald ripening of spherical particles.51 Considering that III-V crystals have more covalent bond nature and higher lattice energies than ionic crystals with lower charge numbers, the reconstruction process of hexagonal InP NPLs would necessitate higher temperature. This is in line with the fact that colloidal synthesis of III-V NCs usually requires high temperatures (~300 °C) compared to Cu+-based NCs or chalcogenide NCs (~200 °C), where the defect formation and removal process might not be distinguished. The observation of crystal ripening at high temperature implies that clear 30 ACS Paragon Plus Environment

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distinction of reaction temperatures for post-synthetic ionic diffusion process and recrystallization in our case was the key for the separate observation of defect generation and removal.



Conclusion In summary, we demonstrated controlling of ion exchange balance in Cu+-to-In3+ cation exchange reaction of Cu3-xP NPLs and morphology change by defect formation. Increased ϕTOP in ligand mixture solvent resulted in voids through the entire lateral planes of InP NPLs. HR-TEM analysis revealed that Kirkendall voids nucleate at predominant (110)Cu3 ― xP/ (100)InP interface and grow along the interface as the ion exchange proceeds. Subsequently, InP segments newly nucleate at the edge of Cu3-xP segments through the voids. Multiple reactions comprising Kirkendall void generation and cation exchange result in cracked InP NPLs. Elemental analysis focused on Cu3-xP segment in partially cation-exchanged Cu3-xP/InP NPLs revealed that large amount of TOP leads to high Cu+ vacancy density in Cu3-xP segment that would condense to nucleate Kirkendall voids. Quantitative analysis on solvation energetics showed that increase of TOP not only enhances the solvation of Cu+ but also In3+ during cation exchange reaction, which finally accounts for the imbalance between solvation of Cu+ and desolvation of In3+ and high Cu+ vacancy density in Cu3-xP segment. In addition, annealing of cation-exchanged cracked InP NPLs at high temperature resulted in crystal ripening with void coalescence leading to hole size-controlled hollow NPLs. High temperature recrystallization implies that cation exchange at low temperatures insufficient for crystallization effectively decoupled ionic inter-diffusion and ripening processes, which have not been separated in metal chalcogenide NCs. The systematic study of this work would serve as a guideline for controlling the ion exchange balance in cation exchange reactions based on solvation energetics. In addition, we believe that our results would cast light on defect formation pathways and removal strategies during and after cation exchange reaction of NCs, or other nanoscale post-synthetic processes involving ionic inter-diffusion.


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Author Information Corresponding Authors *E-mail (Doh C. Lee): [email protected] *E-mail (Young Kuk Lee): [email protected]

Acknowledgments This work is supported by the National Research Foundation (NRF) grants funded by the Korean Government (NRF-2016M3A7B4910618 and NRF-2017R1A2B2011066), and by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Strategic Technology Development Program No. 10077471, ‘Development of core technology for highly efficient and stable, non-cadmium QLED materials’.

Supporting Information Detailed procedures for defining parameters for quantification of vacancy concentration and estimating thermodynamic properties of solvation energetics, and supplementary data including TEM images, EDX spectra, schematic description of Cu3-xP/InP interfaces, Van’t Hoff plots, absorbance change of Cu3-xP NPLs by annealing in TOP, and thermodynamic properties of solvation energetics

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Controlling Ion-Exchange Balance and Morphology in Cation

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