Materials Today Advances 5 (2020) 100037 Contents lists available at ScienceDirect Materials Today Advances journal homepage: www.journals.elsevier.com/materials-today-advances/ Understanding galvanic replacement reactions: the case of Pt and Ag F. Merkoçi a, J. Patarroyo a, b, L. Russo a, b, J. Piella a, b, A. Genç a, c, J. Arbiol a, d, N.G. Bastús a, **, V. Puntes a, d, e, * a  de Nanoci Institut Catala encia i Nanotecnologia (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193, Barcelona, Catalonia, Spain b Universitat Autonoma de Barcelona (UAB), Campus UAB, 08193, Bellaterra, Barcelona, Catalonia, Spain c Department of Metallurgy and Materials Engineering, Faculty of Engineering, Bartin University, 74100, Bartin, Turkey d ICREA, Pg. Lluís Companys 23, 08010, Barcelona, Catalonia, Spain e Vall d'Hebron Institut de Recerca (VHIR), 08035, Barcelona, Catalonia, Spain a r t i c l e i n f o a b s t r a c t Article history: Synthesis of nanocrystals (NCs), where material science elements are addressed with organic chemistry Received 15 September 2019 precision techniques, is especially challenging and difficult to control. This difficulty arises from the Received in revised form increased complexity of the mineralization processes and the generation of a liquid-solid interface. These 19 November 2019 aspects, along with a strong susceptibility to reaction kinetics, ultimately translate into serious chal- Accepted 19 November 2019 lenges for reproducibility and morphological control. By systematically varying the different parameters Available online 8 January 2020 used to control the morphology of NCs, including complexing agents, coreducers, and cooxidants, the general reaction landscape can be mapped and the most stable and reproducible recipes can be iden- Keywords: Pt-based hollow nanocrystals tified. We apply this concept to the model transmetallation reaction between immiscible Pt and Ag Synthesis forming hollow Pt NCs by galvanic replacement reactions. In this work, 648 synthetic recipes were Transmetallation performed and characterized per duplicate, from which a subset of 307 recipes leading to the controlled Alloys formation of hollow NCs were further analyzed to correlate reaction conditions with the final obtained Catalysis structure and stability (reproducibility). As a result, we present robust general synthetic protocols leading to the ad hoc production of Pt-based hollow NCs with independent control of shell thickness, void size, surface roughness, and degree of porosity. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction reaction proceeds, further corrosion processes promote the deal- loying of the formed shell, generating multiple pores, leading to the The process of carving voids into nanocrystals (NCs) as the basis formation of nanocages and nanoframes, which ultimately frag- of a new generation of advanced functional materials [1,2] with ment and collapse into small (<5 nm) metal NC debris [1,8]. outstanding properties [3e5] was initially described as a combi- This picture, however, fails to fully describe the ensemble of nation of galvanic replacement and nano-Kirkendall effects [1,6,7]. experimental results that have been puzzling scientists since the This approach includes a transmetallation process driven by the seminal work of Xia et al [6]. Observations suggest that a hollowing redox reaction between a colloidal suspension of preformed NCs e process cannot be fully explained by a simple combination of templates e and the cationic precursor of a more noble metal. The galvanic replacement reactions [6] and vacancies coalescence e reduction of the noble metal cation is confined to the vicinity of the kirkendall effect e [7] because mass and charge balance between template surface, nucleating onto it, and then growing and forming metal atoms and cations cannot fully explain the formed nano- an alloyed shell around it. During the more noble metal deposition, structures [9]. Consequently, the mechanistic understanding of the original NC is simultaneously oxidized and dissolved through galvanic replacement in fabricating hollow bimetallic nano- formed pinholes, resulting in hollow bimetallic structures. As the structures has been discussed for years [8]. It is accepted that the reaction product is determined by a set of thermodynamic and kinetic parameters that are intimately and intricately entangled to each other. While the essence of thermodynamic control is to * Corresponding author. minimize the total free energy of a system, the manipulation of the ** Corresponding author. E-mail addresses: neus.bastus@icn2.cat (N.G. Bastús), victor.puntes@icn2.cat deposition/dissolution rates allows trapping the system in ther- (V. Puntes). modynamically unstable, but kinetically enabled state [10]. https://doi.org/10.1016/j.mtadv.2019.100037 2590-0498/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2 F. Merkoçi et al. / Materials Today Advances 5 (2020) 100037 Advanced electron microscopy characterization [11,12] and quan- substoichiometric (0.25:1) to stoichiometric (1:1) and supra- titative microanalysis [13] have been fundamental to thoroughly stoichiometric (10:1) molar concentrations. In addition, for each investigate intermediate and final NC states. Thus, researchers in sample, a study on the effect of the type of etcher present in the re- the field have studied the effect of thermodynamic factors such as action media (HCl, HNO3, NaCl) and its relative molar concentration the surface free energies of template facets [14] and kinetic (0, 1, 10, and 100) with respect to that of Ag0 was carried out. Finally, factors such as precursor reduction rate, atom diffusion coefficients, the reaction was performed at two different temperatures (room degree of alloying, and surface passivation, all strongly affecting the temperature [RT] and 90  C) and characterized at three different re- morphology of the resulting shells [15e17]. action times (0.5, 4, and 24 h) as shown in Fig. 2B. A detailed Despite the effort, the actual control of the different hollow mor- description of the experimental procedure is provided in the sup- phologies has proved to be difficult. Owing to the fact that these plementary information section. colloidal systems are highly degenerated and that the energy barrier The systematic variation of these synthetic parameters gener- separating one morphology from another is relatively low [18], similar ated a collection of 648 experimental conditions, which were per- recipes may direct NC growth toward very different morphologies, formed and characterized per duplicate. Among the explored whereas disparate synthetic recipes may lead to similar NCs [1,18,19]. conditions, not all of them were successful in terms of well-defined These difficulties also account for the common problems of repro- morphologies, monodispersity, and high product yield. As a result, ducibility systematically found in the scientific literature [20e22]. these NC recipes were not further considered. Besides, we were Herein, we systematically study the influence of the different able to identify a subset of 307 recipes that consistently led to a synthetic parameters identified in the literature, including reaction reduced number of Pt-based hollow NC morphology types (Fig. 2C), temperature and time [11], metal template to precursor ratio [23], some of which have never been previously reported. presence of cosurfactants [1,24], coreducing agents [25], ionic Representative transmission electron microscopy (TEM) images species [24], and order of reagent addition [24], on the final NC of the different morphologies are presented in Fig. 3. Their corre- morphology. The study focuses on the particular case of PtAg lation with the synthetic parameters involved in their synthesis because of the well-described limitations in the control of NC final (Fig. 2A) allows concluding that rough and rather thick are the most morphology attributed to the poor miscibility between Ag and Pt at common structures, whereas thin and smooth surfaces are rarer. It low temperatures (3.5% lattice mismatch) [24], which results in is important to note that among the different morphologies, those rough polycrystalline hollow NCs composed of small Pt domains as presenting smooth, thin, and porous shells are particularly attrac- witness of a Volmer-Weber type of growth [8,26], which would not tive for catalysis because rough surfaces are more atomically be the most desired for catalysis [27]. This study allows observing relaxed than smooth ones [27]. In addition, thin shells imply less the role of the different constituents of the reaction. As a result, usage of Pt and porous structures maximize available surface, robust and reproducible general synthetic protocols leading to the which account in part for their reported higher catalytic perfor- ad hoc production of Pt-based hollow NCs with controlled shell mance per gram [27e29]. Details of the synthesis procedure and an thickness, void size, porosity, and surface roughness are presented. extended characterization of each individual Fig. 2 recipe and Indeed, this approach not only delivers rational synthetic pathways morphology are reported in the SI, section 2.2. leading to particularly desired morphologies but also allows un- Regarding the smooth and thin NCs, they are obtained when (i) a successful preparations to be explained, providing thus concepts high concentration of PVP promotes the protection (passivation) of and strategies to address ‘failed’ recipes. the surface of the Ag NC templates along with the stabilization of the Pt ions by forming Pt-PVP complexes [30]; (ii) a progressive slow 2. Results and discussion delivery of Pt precursor allows for the progressive reduction and deposition of Pt ions at low stoichiometric ratios; and (iii) the In our process of tailoring the hollowing process of PtAg bime- presence of a secondary etcher promotes the dissolution of the Ag tallic NCs, several chemical players crucially involved coexist: the Pt atoms not corroded by the small amount of deposited Pt. Note that precursor e totally or partially reduced e the Ag template and acids are also used to chemically cure the template surface by dis- oxidized Agþ, the surfactant polyvinylpyrrolidone (PVP), acids in solving the less reactive Ag2O domains present at the working pH the form of HCl or HNO3, halides e normally coming from the [31]. Furthermore, the presence of halides, as provided by K2PtCl4, precursor e such as Cl or Br, and dissolved oxygen (DO) present in HCl, or NaCl, favors the formation of AgCl domains onto the tem- the reaction media. These different reagents are involved in two plates surface, acting as a physical protective barrier which is later main tasks: (i) the reduction and deposition of a Pt shell onto the Ag exfoliated and solubilized by PVP (see SI, section 2.3) [1]. Halide ions, surface and (ii) the oxidation and dissolution of the Ag template, as nucleophilic species, also promote the oxidative corrosion of Ag in which drives the creation of the hollow structure. Reducing species the presence of dissolved oxygen which, unless removed, is present include metallic Ag0 and PVP, while oxidizing species comprise the in the reaction solution at significant concentrations at RT (3e9 mg/ Pt precursor, H3Oþ, and the DO present in the media together with L) [32], as control experiments show (see SI, section 2.4). Performing nucleophilic species, such as halides, which assist the oxidative the reaction at room temperature, instead of the original 100  C [6], corrosion of the Ag template (Fig. 1). allows adjusting the reaction rate and hence promotes a relatively To address this issue, we systematically varied the aforementioned slow corrosion process favoring the formation of thin smooth sur- different synthetic parameters thereby generating multiple synthetic faces, indicating again the importance of kinetic control. Finally, conditions schematically described in Fig. 2A. Accordingly, the hol- relatively short reaction times -few hours-are preferred to prevent lowing process is not only controlled by an exchange of electrons PVP acting as a Pt reducer and inducing Pt shell overgrowth (even between Ag and Pt but also affected by other agents that interfere in without the Ag NC template consumption), as shown in control the PtAg redox process, creating more or less stable intermediates and experiments (see SI, section 2.5). driving equilibrium toward different products. In detail, 50 nm silver Thus, by adjusting these synthetic parameters, the controlled for- nanocrystals (Ag NC) sacrificial templates ([Ag0] ¼ 0.25 mM) were mation of thinner or thicker shells, with more or fewer pores, and incubated with PVP at different Ag0 to PVP molar ratios (0, 0.1 and 10), bigger or smaller voids, can be independently achieved. It is worth with the aim of varying their surface state (passivation) degree. For pointing out that porous morphologies are generally not the pre- each condition described previously, the amount of Pt2þ precursor dominant ones because the thickness of the shell limits the extent of was systematically adjusted with respect to that of Ag0, from the dealloying processes needed for the formation of pores [11,21]. F. Merkoçi et al. / Materials Today Advances 5 (2020) 100037 3 Fig. 1. Map of competitive reactions. Representation of the chemical processes running in parallel during a typical hollowing process. In detail, speciation of the Pt precursor (A) and HCl (B), formation of Ag2O by the dissolved oxygen in the media (C), oxidation of Ag and dissolution of Ag2O by Hþ (D), formation of AgCl (E), galvanic replacement reaction between Ag and several different Pt species (F), and dissolution of AgCl (G). Fig. 2. A systemic approach to NC synthesis. A) Explored experimental conditions for the performed syntheses including 3 PVP concentrations, 3 Pt precursor concentrations, 4 different types of coetching agents each one at 3 different concentrations, 2 reaction temperatures, and 3 reaction times. The particular synthetic routes for the Pt-based hollow NCs displayed in Fig. 2 are explicitly indicated. (B) Analysis of synthetic conditions, for a total number of 648 different experimental conditions, which, tested per duplicate, leads to a total of 1296 synthesis. From these syntheses, only 307 recipes lead to the successful production of NCs in terms of well-defined morphologies, monodispersity, and high yield while the other 341 unsuccessful were discarded. (C) Morphological heatmap correlating the effect of PVP concentration with the degree of roughness, thickness, and porosity of 307 syntheses. (Every single line corresponds to a unique synthesis.) The color scale varies from yellow (low values) to deep red (high values). PVP ¼ polyvinylpyrrolidone, NCs ¼ nanocrystals. 4 F. Merkoçi et al. / Materials Today Advances 5 (2020) 100037 Fig. 3. Representative TEM images of Pt-based hollow NCs obtained. Collection of TEM images of Pt-based hollow NCs with different degrees of shell morphology and thickness showing (AeD) smooth, (EeH) rough, (IeL) popcorn-like rough surfaces, and more complex morphologies such as (M) PtHollow@PtCl2 Core@Shell NCs, (N) PtHollow@AgCl Core@Shell NCs, (O) urchin-like Pt hollow NCs, (P) Pt heterostructures with hollow domains, (Q) highly porous Pt NCs, (R) Pt nanoshells, (S) Ag@Pt Core@Shell NCs, and (T) Pt nanostars. For clarity, upper right panels show a representation of an individual particle. TEM ¼ transmission electron microscopy, NC ¼ nanocrystals. F. Merkoçi et al. / Materials Today Advances 5 (2020) 100037 5 This is shown schematically in Fig. 4, where a simplified representa- porosity e considering the experimental conditions of each experi- tion of the two major synthetic pathways governing this process is ment. Morphological features were extracted from the characteriza- presented (Fig. 4A). The first case, the ‘high reactivity route’, is the tion data of all the experiments, further analyzed using Image J. synthesis with non-passivated templates and/or non-complexed Pt Standard distribution and the respective standard deviations were species. The addition of either the Pt precursor or a combination of determined by measuring 150 to 900 particles of each condition. The both Pt precursor and acids yields thick and rough shells, character- relative roughness, the shell thickness, and the relative porosity were istic of the island-mode of growth of Pt onto Ag. When exposed to geometrically estimated (materials and methods and S.I.). The results further etching processes -for instance, HCl, HNO3, or NaCl- and were grouped in the diagrams shown in Fig. 5. The dominance of depending on the thickness of the shell, the dissolution of the warmer lines in the heatmaps indicates a natural tendency of the remaining (unreacted) Ag atoms leads to broken Pt shells before the system to form structures with rough surfaces and thick shells dealloying can be controlled. In the second case, the ‘low reactivity (>5 nm) with a low degree of porosity. This is as a consequence of the route’ performing the synthesis with passivated Ag NCs and/or com- Pt-Ag immiscibility [26], and the preference for the Volmer-Weber plexed Pt species leads to smooth-surface structures that can be type of growth, which is translated in the well-known synthetic controllably thinned and dealloyed yielding nanocages. If this process limitations in the production of ultrathin Pt-based hollow NCs with Ag is conducted in the absence of acids or deoxygenated water, higher templates [28]. In contrast, less abundant lighter lines of thinner and quantities of the Pt precursor are needed to completely oxidize the Ag smoother shells generally appear in conditions where chemical po- templates, promoting the formation of thicker Pt shells and, therefore, tentials are reduced [1]. fewer pores. Conversely, when performing the synthesis in presence In these diagrams (Fig. 5A), one can observe different features. of etchers, owing to the cooxidation process, a lesser amount of Pt First, a large orange area is always present rather centered e except precursor is needed to consume the Ag templates, allowing the for temperature e indicating that at these middle conditions the template's voiding stoichiometry to change. This is ultimately trans- system is less degenerated and hence reproducibility is easier to lated into the formation of thinner Pt shells that are easier to dealloy achieve, and monodispersity high, yielding NCs which are not very and, therefore, multiple pores can be generated, as shown in Fig. 4B. thin, not very smooth, and not very porous. As soon as one goes to Summarizing, these results clearly indicate that Pt shell thick- more extreme conditions, excess or lack of the reagent and/or re- ness and its morphology are two characteristics that can be action time, the system becomes more unstable and thin red and controlled independently. Thinner shells are obtained by the sum of light lines intercalate, allowing for different structures at the two different conditions:( i) minimum Pt layer growth using small expense of less reproducibility and morphological control. The only amounts of Pt precursors and ii) maximum voiding (etching) of the diverging graph is with temperature (T), where stability and core using cooxidants. Moreover, surface roughness can be reproducibility are achieved at higher T, yielding thick rough NCs. In controlled by lowering the reactivity of both the Pt precursors and this situation, the morphological possibilities are further limited. the Ag templates, using surfactants to complex precursors and to Note that initially, the hollowing process was described at high T, absorb onto NC surfaces. Porous structures are easier to obtain for the evident reasons shown here. However, when we decreased when the thickness of the shell is relatively thin and the etchant can T, we acceded to more kinetic control and consequently, morpho- easily reach the unreacted Ag atoms bound to Pt. logical control [1]. To facilitate the comparison of event probability, morphological By comparing individual morphological heatmaps, the stability heatmaps were generated by classifying the samples according to zones of the different synthetic parameters can be correlated. For their morphological characteristics e roughness, thickness, and example, by comparing the heatmaps of PVP passivation and Pt:Ag Fig. 4. Schematic representation of principal synthetic pathways governing the synthesis of Pt-based hollow NCs. (A) Representation of the synthetic pathways to produce distinct types of Pt-based hollow NCs. A detailed characterization of the thinnest shell case is shown in (B), where upper panels show low magnification HAADF-(S)TEM and bright- field TEM micrographs of individual particles and the lower panel shows an HRTEM image of a representative individual particle with details of the orange squared region and its corresponding power spectrum. TEM ¼ transmission electron microscopy, NCs ¼ nanocrystals, HAADF ¼ high-angle annular dark-field imaging, STEM ¼ scanning transmission electron microscopy, HRTEM ¼ high-resolution transmission electron microscopy 6 F. Merkoçi et al. / Materials Today Advances 5 (2020) 100037 Fig. 5. Morphological Susceptibility heatmaps. (A) Morphological heatmaps correlating the effect of the synthetic parameters explored with the degree of roughness, thickness, and porosity of more than 300 syntheses. (B) Comparison of two individual morphological heatmaps shown in Fig. 4, where the stability zones of the different synthetic parameters can be correlated and the overall reproducibility of the synthesis can be identified. Colors represent the conditions tested as shown in Fig. 1. Every single line corresponds to a unique synthesis. F. Merkoçi et al. / Materials Today Advances 5 (2020) 100037 7 stoichiometry, a central crossing area between the two diagrams is observed (Fig. 5B). This indicates that similar results can be ob- Synthesis of Pt-based hollow nanocrystals tained either by increasing the PVP concentration or decreasing the Pt2þ amount, two different manners of decreasing chemical po- To standardize all of our experiments, the synthesis consisted tential. This relationship is lost in the extremes of the graphs where of firstly preparing a 1.5 mL aqueous solution containing all the recipes become less robust. the explored reagents at different concentrations into which In summary, by systematically modifying reagents and core- 0.5 mL of the Ag NCs (1 mM) were rapidly added (for a total agents, great morphological control is achieved in the challenging 2 mL volume) to let the reactions begin. Each reaction was PtAg system. Six hundred forty-eight synthetic recipes were conducted for a specific time, at a specific temperature under explored from which a subset of more than 307 where further vigorous stirring. In detail, the standard volumes used analyzed to correlate the presence of different ingredients at were 1 mL of PVP (either 0, 0.05, or 5 mM corresponding to a different temperatures and times with the final obtained PVP to Ag ratio of 0, 0.1, and 20, respectively); 250 mL of K2PtCl4 morphology. Our data demonstrate that the Ag NC voiding and Pt (either 0.25, 1, or 10 mM, which, according to the Pt2þ vs Ag shell growing processes can be independently controlled, which redox stoichiometry correspond to a Pt to Ag ratios of 0.25, 1, allows for the design of ad hoc recipes for future intended mor- or 10 mM, respectively); and 250 mL of etchant solution which phologies and, at the same time, an understanding of the mecha- was either HCl, HNO3, NaCl or the correspondent controls nism behind synthetic failures. The presented systemic approach to with water (each one of them at three different concentra- NC syntheses may be extended to other general strategies for the tions: 2, 20, and 200 mM corresponding to an etchant to Ag mineralization of ions in solution controlled by organic reactants ratio of 1, 10, and 100, respectively. and surfactants for the production of advanced NCs [2]. Electron microscopy characterization Before the inspection, samples were precipitated and redis- Materials and methods persed in water several times for their purification. After sonicating the samples for 5 min, the solution was dropcasted Chemicals onto a Formvar-coated 200-mesh copper grid (Ted.Pella, Inc.) and left drying at room temperature overnight. TEM images were obtained using an 80 keV TEM (Jeol 1010, Japan) and Silver nitrate (SN, AgNO3) ReagentPlus, trisodium citrate high-resolution transmission electron microscopy (HRTEM) (SC, Na3C6H5O7), tannic acid (TA, C76H52O46), poly- and Scanning transmission electron microscopy (STEM)/ vinylpyrrolidone (PVP, molecular weight ¼ 55,000 g/mol), high-angle annular dark-field imaging (HAADF) images were and Potassium tetrachloroplatinate (II) (K2PtCl4) were pur- obtained in an FEI Tecnai G2 F20 S-TWIN HR(S) TEM, oper- chased from Sigma-Aldrich. All reagents were used as ated at an accelerated voltage of 200 kV. received without further purification. Distilled water passed through a Millipore system (r ¼ 18.2 MU) was used in all Morphological heatmaps experiments. Synthesis of silver nanocrystals Morphological heatmaps were generated using the charac- terization data of all the experiments, which were analyzed with Image J. The statistical distribution and the respective Silver nanocrystals of ~50 nm in diameter were prepared by standard deviations were determined by measuring 150 to the seeded growth method recently reported by Bastús et al. 900 particles. In detail, the RR, the ST, and the RP of the Pt- [33]. In a typical experiment, silver seeds were obtained by based hollow nanocrystals (shown in Fig. 3) was geometri- filling a three-neck round-bottomed flask with a 100 mL cally estimated as follows: For the RR, the cross-section of aqueous solution containing 5 mM sodium citrate (SC) and more than 150 particles was integrated in comparison with 0.1 mM of tannic acid (TA) and heated up with a heating their idealistic cross-section (which corresponds to the mini- mantle up to 100  C. After boiling had commenced, 1 mL of mum radius described inside the real cross-section), as 25 mM AgNO3 was injected into this solution under schematically shown in Scheme S1. The values of the ratios vigorous stir. The as-obtained silver seeds were grown by between the real and the ideal cross-section ranged from 1.0 cooling down the solution to 90  C and sequentially inject- to 2.2 in all the cases. Therefore the ‘perfectly smooth’ parti- ing 100 mL of 25 mM SC, 250 mL of 2.5 mM TA, and 250 mL of cles were assigned to the minimum ratio (1.0) and the ‘highly 25 mM AgNO3. This process was repeated up to four times. rough’ ones to the maximum one (2.2). The obtained values In the same vessel, the seed solution was diluted by were further normalized between [0, 1]. The RP was similarly extracting 20 mL of sample and adding 17 mL of Milli-Q estimated by counting the number of pores in the perimeter water, then 500 mL of 25 mM SC, 1.5 mL of 2.5 mM TA, and of the cross-section. The values obtained ranged from 0.0 1 mL of 25 mM AgNO3 were sequentially injected, again. (non-porous particles) to 23 (highly porous particles) and This process was repeated up to 5 times, progressively were also normalized between [0, 1]. Finally, the ST was growing the size of the Ag NCs until reaching the desired conventionally measured by using Image J and the values size (~50 nm). The obtained Ag NCs were purified by obtained ranged from 1.5 nm (minimum thickness) to 14.4 nm centrifugation and stored in either water, PVP, or in a solu- (maximum thickness) and, as in the previous cases, were also tion containing both TA and SC. See detailed characteriza- normalized between [0, 1]. tion in SI, section 2.1. 8 F. Merkoçi et al. / Materials Today Advances 5 (2020) 100037 Declaration of competing interest [13] H. Jing, H. Wang, Structural evolution of AgePd bimetallic nanoparticles through controlled galvanic replacement: effects of mild reducing agents, Chem. Mater. 27 (6) (2015) 2172e2180. The authors declare that they have no known competing [14] C. Zhu, D. Du, A. 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