Straightforward Synthesis of Metal Nanoparticles and Hierarchical

Jul 17, 2015 - The development of an eco-friendly and economical synthetic pathway of nanomaterials such as metal nanoparticles and nanoporous metals ...
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Straightforward Synthesis of Metal Nanoparticles and Hierarchical Porous Metals Assisted by Partial Film Boiling Phenomena Dong-Wook Lee,*,† Min-Ho Jin,† Chun-Boo Lee,† Sung-Wook Lee,† Jin-Woo Park,† Duckkyu Oh,† Ji Chan Park,‡ and Jong-Soo Park† †

Advanced Materials and Devices Laboratory and ‡Clean Fuel Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeongro, Yuseong, Daejeon 305-343, Republic of Korea S Supporting Information *

ABSTRACT: The development of an eco-friendly and economical synthetic pathway of nanomaterials such as metal nanoparticles and nanoporous metals remains a challenging topic in the field of nanomaterials. Here we report a novel and eco-friendly series of synthetic methods from the preparation of metal nanoparticles to the fabrication of hierarchical porous metals by using partial film boiling phenomena established by a reactor with the inverse configuration of conventional Leidenfrost drop reactors, which is much more favorable to large-scale production than conventional Leidenfrost drop reactors. As a result, we have revealed two important facts, which can offer fresh vision to the field of metal nanoparticle and nanoporous metal synthesis. The first one is that Pd and Pt precursor can be reduced to neutral Pd and Pt nanoparticles under basic condition established by partial film boiling phenomena without reducing agents such as ethanol and citric acid. The second one is that when citric acid and ethanol as a mild reducing agent were added into the precursor aqueous solution, citric acid also plays an important role as a pore-forming agent, and ethanol facilitates the aggregation of as-prepared metal nanoparticles, resulting in the formation of nanoporous Pd and Pt. Our work is believed to suggest more eco-friendly and straightforward approaches for fabrication of metal nanoparticles and nanoporous metals. However, further research is required to explore whether our novel synthetic pathway is available to other metal systems.

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INTRODUCTION Nanoporous metals are of considerable interest to scientists in the field of materials, chemistry, and chemical engineering, because they provide unique intriguing properties such as good catalytic activity, thermal and electrical conductivity, plasmonic behavior, high surface area, and low density.1−4 As a result, nanoporous metals currently find immense applications such as catalysis, electrocatalysis, fuel cells, sensors, hydrogen storage, and membranes.5−8 In addition, various strategies have been reported for synthesis of nanoporous metals, such as hard exotemplating,9−13 soft endotemplating,5,6,14−19 dealloying in appropriate corrosion conditions,20,21 pH-controlled aggregation,22,23 coalescence at oil−water interface,24 and template-free method.25,26 Among them, a templating method is a more popular approach because pore properties such as pore size and pore structure can be more readily tailored by changing the size and shape of hard templates or the length of alkyl chains of surfactants. However, a hard exotemplating method is well-known whose fabrication procedures are complicated and time-consuming, so it is not suitable for commercialization. In the case of the soft endotemplating reported until now, there have been perennial issues that must be addressed. Ionic and nonionic surfactants as a soft template are expensive and not eco-friendly because of © XXXX American Chemical Society

their low biodegradability derived from their long hydrophobic chains. Moreover, replacement of surfactants by ones with different chain length and molecular weight is required, or toxic solvents as a swelling agent are needed to expand the pore size of nanoporous metals templated by ionic surfactants or nonionic block copolymers. Thus, straightforward control of pore size in a small mesopore range, searching eco-friendly templating agents, and simple synthetic routes remains a challenge in the field of nanoporous metals.27 Meanwhile, reduction of metal precursors for preparation of nanoporous metals is generally carried out through thermal decomposition28 and electrochemical11,13−15 and chemical pathway5,6,16−19,22−24 using reducing agents such as sodium borohydride, hydrazine, ethylene glycol, ascorbic acid, glucose, etc. Several of the reducing agents are toxic and may cause injury to human organs, which restricts their widespread utilization,29 and electrochemical reduction needs additional electrochemical machines. Therefore, the development of straightforward and eco-friendly reduction methods of metal precursors continues to attract great attention.29 More recently, Elbahri and co-workers Received: January 13, 2015 Revised: July 14, 2015

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DOI: 10.1021/acs.chemmater.5b00143 Chem. Mater. XXXX, XXX, XXX−XXX

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Preparation of Nanoporous Pt Using Rod-Type Heaters. The fabrication of nanoporous Pt using rod-type heaters is carried out by the same method as KIE-4 except for employing H2PtCl6 aqueous solution (10 wt % Pt basis, PMRESEARCH Co.) instead of Pd(NO3)2 solution. The prepared nanoporous Pt samples are denoted as KIE-5 (Korea Institute of Energy research-5). Large-Scale Production of Nanoporous Pd and Pt Using RodType Heaters. A rod-type heater was immersed in the mixture of citric acid, 40 g of Pd(NO3)2 or H2PtCl6 aqueous solution (10 wt % Pd or Pt basis), and 150 mL of distilled water, and the mixture solution was superheated by raising the temperature of a rod-type heater to 240 °C rapidly. The weight ratio of citric acid and Pd precursor is shown in Table S1. After maintaining the superheated state for 10 min, 50 mL of ethanol was added into the solution. After additional 10 min, the solution was boiled down to solid residues on a hot plate, and the solid residues were calcined at 500 °C in air. Synthesis of Nanoporous Pd by a Conventional Thermal Decomposition Method. To compare our synthetic method with a conventional thermal decomposition method, we also prepared nanoporous Pd by a conventional thermal decomposition method. In a typical synthesis, 0.5 g of citric acid and 1 g of Pd nitrate hydrate powder (Aldrich) were mixed, and the well-mixed powder sample was calcined at 500 °C in air to prepare nanoporous Pd. Characterization. The mesopore and macropore properties of KIE3, KIE-4, and KIE-5 were taken by nitrogen sorption and mercury intrusion tests with a Micromeritics ASAP 2420 and an Autopore IV Micromeritics instrument, respectively. Their X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX-2200 V instrument operated at 1.6 kW. The X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos 165XP spectrometer. Transmission electron microscopy (TEM) analysis was conducted by using a FEI/TECNAI G2 instrument.

reported that Au precursors could be reduced without reducing agents in a levitated Leidenfrost droplet, and synthesis and coating of metal oxide on three-dimensional objects such as TEM grids and flexible polymeric substrates could be achieved in the same manner.30,31 In addition, they demonstrated that although specific surface area were low, nanoporous texture could be obtained by aggregation of as-prepared metal nanoparticles.30 They envisioned the reductant-free preparation of metal nanoparticles in a small levitated droplet established by the Leidenfrost effect (or a film boiling phenomenon). Thus, the development of film-boiling-assisted synthetic pathway, being more favorable to the mass production, is also a challenging topic. Here we first report two intriguing facts that Pd and Pt precursors can be reduced to Pd and Pt nanoparticles without any reducing agents by using partial film boiling phenomena, established by a reactor with the inverse configuration of conventional Leidenfrost drop reactors, which is much more favorable to large-scale production than conventional Leidenfrost drop reactors. In addition, when citric acid and ethanol as a reducing agent was added into our reactor system, nanoporous Pd and Pt can be easily synthesized from nanocomposites of citric acid and metal nanoparticles. Citric acid plays an important role as a nonsurfactant pore forming agent as well as a reducing agent, and the pore size of nanoporous Pd and Pt can be readily tailored by simply changing citric acid concentration of the nanocomposites. Ethanol facilitates the agglomeration of Pd and Pt nanoparticle. Our reactor system consists of a rod-type heater as a moving phase and bulk metal precursor solution as a stationary phase, whereas the conventional Leidenfrost drop reactors comprise a plate-type heater as a stationary phase and a levitated solution droplet as a moving phase. Our reactor system was inspired by previously reported inverse Leidenfrost effect.32−34 However, considering that the inverse Leidenfrost effect occurs when suddenly introducing superheated objects into water, our reactor system has a few differences with the inverse Leidenfrost effect, because in the case of our system, the temperature of the rod-type heater gradually increases near the Leidenfrost point after we introduce the rod-type heater into precursor aqueous solution at room temperature.

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RESULTS AND DISCUSSION

Nanoporous Pd Synthesis in a Leidenfrost Droplet Reactor. The Leidenfrost effect is a phenomenon in which a liquid droplet floats on superheated surface at temperature much higher than liquid boiling temperature. The levitation of liquid droplets arises from the formation of vapor layer between hot surface and the bottom of droplets. As reported by Elbahri et al., a temperature gradient between the droplet and the droplet-vapor interface induces the self-ionization of water to hydroxyl (OH−) and hydronium (H3O+) ions.30 Subsequently, basic condition is established by elimination of hydronium ions to the vapor layer. Metal cations can be reduced to neutral metal atoms under such basic conditions. Eventually the coalescence of the metal atoms leads to metal nanoclusters.30 As shown in Figure S1, inspired by the previous publication by Elbahri et al.,30,31 we demonstrated the synthesis of Pdnanoparticle aggregates in a Leidenfrost droplet reactor. After dripping a droplet of Pd nitrate solution on a hot plate superheated above 270 °C, the droplet stably floated on the hot plate maintaining its sphere shape. As the droplet slowly shrank and was dried during levitation on the hot plate, its color changed from orange through dark brown to black (Movie S1), indicating that Pd nanoparticles were formed by reduction of Pd cations into neutral Pd nanoclusters. After drying of the droplet, a black Pd flake, denoted as KIE-3 (Korea Institute of Energy research-3), remained on the hot plate, and microscopic morphology and pore properties of the product KIE-3 were characterized by means of transmission electron microscopy (TEM) and nitrogen sorption tests. The primary Pd nanoparticles show diameter of 2−5 nm and their coalescence gives wormlike secondary morphology (Figure S2a). A lattice distance of 0.225 nm in Figure S2b demonstrates that KIE-3 is predominated by the (111) planes of Pd, and the coherent lattice structure indicates

EXPERIMENTAL SECTION

Preparation of Nanoporous Pd in a Leidenfrost Droplet Reactor. A droplet of Pd(NO3)2 aqueous solution (10 wt % Pd basis, PMRESEARCH Co.) was dripped on a hot plate superheated above 270 °C. While the droplet stably floated on the hot plate, the droplet was dried, and its color changed from orange through dark brown to black. After drying of the droplet, a black Pd flake remained on the hot plate. Collecting these black Pd flakes, we obtained nanoporous Pd samples denoted as KIE-3 (Korea Institute of Energy research-3). Preparation of Nanoporous Pd Using Rod-Type Heaters. A rod-type heater was immersed in the mixture of citric acid, 4 g of Pd(NO3)2 aqueous solution (10 wt % Pd basis, PMRESEARCH Co.), and 200 mL of distilled water, and the mixture solution was superheated by raising the surface temperature of a rod-type heater to 240 °C rapidly. The weight ratio of citric acid and Pd precursor is shown in Table S1. After maintaining the superheated state for 10 min, 5 mL of ethanol as an antisolvent was added into the boiling solution. After additional 10 min, the rod-type heater was removed from the solution, and the solution was boiled down to solid residues on a hot plate. (Caution: After the solvent is almost eliminated, the temperature of the hot plate should be reduced to below 80 °C. Otherwise, solid product will burn out immediately after solvent is removed completely.) After the solid residues were calcined at 500 °C in air, we successfully prepared nanoporous Pd samples designated as KIE-4 (Korea Institute of Energy research-4). B

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Chemistry of Materials the connection and coalescence of primary Pd nanoparticles.35,15 The specific surface area of KIE-3 was measured to be 54 m2/g by a BET method using the lower P/Po part of adsorption isotherms, and its micropore surface area calculated from a t-plot was found to be 54 m2/g, which means that KIE-3 mainly consists of micropores. Its average pore diameter, calculated by Barrett− Joyner−Halenda (BJH) desorption pore size distribution, is 2.6 nm. The specific surface area of KIE-3 is about 2.7 times higher than commercial Pd black with that of 20 m2/g.35 Synthesis of Pd Nanoparticles and Nanoporous Pd Using Rod-Type Heaters. As shown in Figures S1 and S2, nanoporous Pd was successfully prepared in a Leidenfrost reactor without any reducing agents. However, in spite of the easy synthetic process and good pore properties of KIE-3, when mass production of KIE-3 is required, the dripping-drying process should be repeated dozens of times. Thus, inspired by previous publications reporting inverse Leidenfrost phenomena,32−34 we conceived a straightforward synthetic pathway being more favorable to mass production. To increase the capacity of metal nanoparticle production, we placed a rod-type immersion heater into a beaker containing metal precursor solution (Figure 1).

During maintaining the superheated state, the Pd precursor is reduced to Pd nanoparticles without any reducing agents. After that, ethanol as an antisolvent as well as a mild reducing agent was added into the boiling solution, leading to aggregation of the as-prepared Pd nanoparticles and further reduction. After eliminating the rod-type heater, we boiled the solution down to solid residues on a hot plate. Finally, the solid residues were calcined at 500 °C so that organic residues could burn out. As shown in Figure 3a, b, Pd nanoparticles with diameter of 2−5 nm were successfully prepared without reducing agents just by maintaining the superheated state at the interface between rod-type heater surface and solution for 10 min. As a result of the formation of Pd nanoparticles, the color of solution changed from orange to brown. Pd ions dissolved in the solution was reduced to Pd nanoparticles due to hydroxyl ions produced by a temperature gradient between heater surface and solution.30 A lattice distance of 0.225 nm indicates that Pd nanoparticles preferentially grew along the (111) directions (Figure 3b). Produced Pd nanoparticles formed aggregates because of the absence of stabilizers (Figure 3a). After the addition of ethanol as a mild reducing agent as well as antisolvent, the Pd particle aggregates became denser and unreduced precursor was further reduced (Figure 3c), and the solution color changed from brown to black accordingly (Movie S2). The aggregation of Pd nanoparticles is attributed to the weak polarity of ethanol, which weakens the electrostatic repulsion between Pd nanoparticles and facilitates the collision and fusion of Pd nanoparticles.36 Elbahri and co-workers previously added sodium hydroxide as a reducing agent into the Leidenfrost droplet to obtain the higher yield of metal nanomaterials.30 Finally, after boiling down and calcination, the grain size of Pd significantly increased due to sintering of Pd nanoparticles (Figure 3d). Lattice distances of 0.225 and 0.264 nm in Figure 3e correspond to (111) planes of Pd and (101) planes of PdO, respectively, and the mixture of Pd and PdO was formed due to oxidation during calcination at 500 °C in air. As shown in Table S2, the specific surface area and total pore volume of KIE-4-a are 2.6 m2/g and 0.005 cm3/g, respectively. Such low pore properties in the micropore and mesopore range are attributed to severe sintering and agglomeration of Pd nanoparticles during calcination at high temperature. Consequently, it was revealed that KIE-4-a did not give nanoporous texture with high surface area after thermal treatment at 500 °C, which provokes us into the use of pore forming agents. We exclude surfactant-based pore-forming agents, mainly used for preparation of mesoporous metals, because they are expensive and have nonbiodegradable hydrophobic organic segments so that synthesis processes of nanoporous metals with surfactants are considered to be less eco-friendly. Thus, we employ citric acid as a nonsurfactant pore forming agent that is generally used for preparation of disordered mesoporous silica. However, it should be noted that citric acid, used as a poreforming agent in this study, plays a significant role as a reducing agent. As shown in Figure 4, KIE-4-b and KIE-4-c were synthesized through the same synthetic pathway as KIE-4-a except for addition of citric acid into the Pd nitrate solution at a fixed citric-acid/Pd-precursor weight ratio (Table S1). The fixed amount of citric acid was dissolved in the diluted Pd nitrate solution, and the final mixture was superheated quickly by using the rod-type heater. After 10 min, Pd nanoclusters with particle diameter of 1.4−2.9 nm were successfully formed, and the aggregation of about 20 Pd nanoparticles formed Pd supraparticles with diameter of 12−19 nm (Figure 5a, b).

Figure 1. Photographs of (a) a rod-type heater and (b) partial film boiling reactor system.

The mesoporous Pd synthesized by using the rod-type heater is denoted as KIE-4 (Korea Institute of Energy research-4). The synthesis procedures and conditions for KIE-4 are summarized in Figures 2 and 4 and Table S1. As for KIE-4-a, the diluted Pd nitrate solution was superheated by raising the surface temperature of the rod-type heater to 240 °C rapidly.

Figure 2. Synthesis procedures of KIE-4-a with pictures for solution color change. C

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Figure 4. Synthesis procedures of KIE-4-b and KIE-4-c with pictures for solution color change.

Figure 5. TEM images of KIE-4-b. (a, b) KIE-4-b at step I. Scale bar: 100 and 5 nm. (c−e) KIE-4-b at step II. Scale bar: 20, 5, and 50 nm. (e, inset) X-ray maps for selected area. (f, g) KIE-4-b at step III. Scale bar: 20 and 5 nm.

that citric acid also plays an important role as a capping agent in the current synthesis system.4 After ethanol was added into the boiling solution (step II in Figure 4), coalescence of Pd nanoparticles and the nanocomposite formation between Pd nanoparticles and citric acid occurred as shown in Figure 5c−e. The coalescence of Pd nanoparticles in Figure 5c, d led to wormlike secondary morphology, which is almost consistent with that of KIE-3 shown in Figure S2a, b. The insets of Figure 5e exhibit X-ray mapping results for selected area of Figure 5e. Green and red spots correspond to Pd and C atoms dispersed on the selected area. The even distribution of green and red spots indicates that Pd nanoparticles and citric acid molecules formed nanocomposites.

Figure 3. TEM images of KIE-4-a. (a, b) KIE-4-a at step I. Scale bar: 20 and 5 nm. (c) KIE-4-a at step II. Scale bar: 20 nm. (d, e) KIE-4-a at step III. Scale bar: 20 and 5 nm.

Compared with Figure 3a, b, Pd nanoparticle size was smaller, and the aggregation of Pd nanoparticles was considerably suppressed because of the addition of citric acid, demonstrating D

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Chemistry of Materials Figure S3a, b show XRD patterns of pure citric acid and KIE-4-b before calcination. The first broad peak located at 2θ of about 21° in Figure S3b corresponds to amorphous carbon, which may be formed by partial carbonization of citric acid in superheated solution.37,38 The second broad peak at 2 theta of about 40.1° is attributed to Pd nanoparticles, which is in good agreement with the lattice distance of 0.225 nm in Figure 5d. The Pd nanoparticle size was estimated to be about 1.7 nm by calculation using Scherrer equation, which is smaller than that measured from the TEM images. In addition, the X-ray diffraction patterns in Figure S3a, b also support the idea of nanocomposite formation. The diffraction peaks of citric acid crystals were not observed in KIE-4-b samples before calcination, which demonstrates that nanocomposites were formed between amorphous citric acid and Pd nanoparticles without phase separation and crystallization of citric acid. Dissolved Pd precursor ions are coordinated by water molecules, resulting in formation of metal aqua complexes. Subsequently, Pd aqua complexes and citric acid form nanocomposites by the hydrogen bonding interaction between them.16,39 To synthesize mesoporous Pd, we eliminated the citric acid as a pore forming agent from the nanocomposites of Pd nanoparticles and citric acid by thermal treatment at 500 °C in air. We conducted nitrogen sorption tests for KIE-4-b and KIE-4-c samples after calcination to demonstrate whether citric acid played a significant role in forming a mesoporous structure. KIE-4-a prepared without citric acid showed a typical type II isotherm corresponding to nonporous matter, whereas KIE-4-b and KIE-4-c gave type IV isotherms with H2 hysteresis loops, indicating that mesoporous texture was successfully established by using citric acid as a pore forming agent (Figure 6a). Moreover, as the concentration of citric acid increased, the adsorbed nitrogen volume of the isotherm substantially increased (Figure 6a and Table S2) and the peak pore diameter of mesoporous Pd shifted from 3.8 to 5.3 nm (Figure 6b). On the basis of the nitrogen sorption results, it can be concluded that citric acid acts as a pore forming agent and the pore size of mesoporous Pd can be readily tailored by simply changing the citric acid concentration. For further investigation on the macropore structure of KIE-4-b, mercury intrusion tests were conducted. KIE-4-b showed trimodal pore size distribution comprising the mesopore range below 20 nm, the small macropore range of 0.05−3 μm, and the large macropore range of 6−60 μm (Figure S4). Its total pore volume and porosity was 2.0 cm3/g and 84.5%, respectively. In addition, the density of KIE-4-b was estimated to be just 3.49% of bulk solid Pd (Table S3). Taking the nitrogen sorption and mercury intrusion results into account, KIE-4-b provides hierarchical porous structures composed of micropores, mesopores, and small and large macropores, and its relative density is comparable to porous metals prepared by existing synthetic methods (Figure 7). In the same manner as Figure 3e of KIE-3, KIE-4-b also formed the mixture of Pd and PdO crystalline after calcination at 500 °C in air (Figure 5g and Figure S3c). To investigate the variation of pore structure and properties after reduction of PdO, we prepared KIE-4-d by conducting the reduction of KIE-4-b with 25 mM sodium borohydride aqueous solution. As shown in Figure S5, after reduction, the PdO peak at 336.9 eV shifted 1.7 eV toward lower binding energy of 335.2 eV, which corresponds to Pd 3d5/2 in metallic Pd, and broad XRD peaks in Figure S3d also indicates that the framework of KIE-4-d is composed of Pd nanoparticles.40 After reduction, the specific surface area decreased from 21.7 to 13.7 m2/g, whereas the pore volume increased from 0.032 to 0.063 cm3/g (Table S2).

Figure 6. Nitrogen sorption results for KIE-4-a, KIE-4-b, and KIE-4-c. (a) Isotherm plots. (b) BJH desorption pore size distributions.

Figure 7. Diagram of porous metals plotted on relative density vs pore diameter space.4

This is because the peak pore size increased from 3.8 to 24.5 nm by coalescence among adjacent Pd nanoparticles during the reduction process (Figure S6d and Table S2). Compared to Figure 5f, Figure S6b reveals that the primary Pd particle size slightly increased because of their coalescence during reduction. E

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Chemistry of Materials Synthesis of Pt Nanoparticles and Nanoporous Pt Using Rod-Type Heaters. We also confirmed whether it is feasible to synthesize Pt nanoparticles and mesoporous Pt structures using rod-type heaters in the same manner as KIE-4. The mesoporous Pt synthesized by using rod-type heaters is denoted as KIE-5 (Korea Institute of Energy research-5). The synthesis procedures and conditions for KIE-5 are summarized in Figures 8 and 10 and Table S1. As for KIE-5-a (Figure 8), diluted

Figure 8. Synthesis procedures of KIE-5-a with pictures for solution color change.

chloroplatinic acid aqueous solution was superheated by using a rod-type heater. As a result, the color of the solution changed from yellow to brownish red, indicating the formation of Pt nanoclusters without the addition of reducing agent such as ethanol and citric acid. After that, ethanol as a mild reducing agent as well as antisolvent was added into the boiling solution for the network formation of Pt nanoparticles and further reduction of unreduced precursor. When the solution color turned into dark gray, the rod-type heater was removed from the solution. Subsequently, the solution was boiled down to solid residues on a hot plate, and then the solid residues were calcined at 500 °C. As shown in Figure 9a, Pt nanoparticles with a lattice distance of 0.226 nm, corresponding to (111) planes of Pt, were successfully prepared without any reducing agents such as ethanol and citric acid just by maintaining the superheated state at the interface between rodtype heater surface and solution for 10 min. Pt ions dissolved in the solution was reduced to Pt nanoparticles due to hydroxyl ions produced by a temperature gradient between heater surface and solution.30 After the addition of ethanol and calcination at 500 °C in air, nanoporous Pt was successfully prepared. Even after calcination at 500 °C in air, the primary particle size of Pt scarcely increased, and (111) planes of Pt were maintained without oxidation of Pt (Figure 9d, e). Figure 10 shows the synthesis procedures of KIE-5-b by using citric acid as a poreforming agent. KIE-5-b was fabricated by the same synthetic pathway as KIE-5-a except for addition of citric acid into the Pt precursor solution. Although it is well-known that citric acid acts as a reducing agent, in this study, we employed citric acid as a templating agent for the formation and control of mesopores. Before the addition of ethanol (step I in Figure 10), Pt supraparticles of submicrometer consisting of Pt primary nanoparticles with the particle diameter of 2−4 nm were synthesized (Figures 11a−c). X-ray mapping results for Pt and C atoms in the selected area of Figure 11b demonstrate that Pt nanoparticles and citric acid molecules formed nanocomposites. In addition, on the basis of Figure S7, we further verified the formation of nanocomposites between Pt nanoparticles and citric acid molecules. The sharp diffraction peaks of citric acid disappeared because of the formation of nanocomposites. The broad peaks at 2θ of 39.8, 46.3, 67.4, and 81.3° are attributed to Pt nanocrystals. The Pt nanoparticle size calculated by Scherrer equation is about 6.1 nm, being larger than that measured from the TEM images. After calcination, even though the Pt primary particle size slightly

Figure 9. TEM images of KIE-5-a. (a) KIE-5-a at step I. Scale bar, 2 nm. (b, c) KIE-5-a at step II. Scale bar: 20 and 2 nm. (d, e) KIE-5-a at step III. Scale bar: 20 and 2 nm.

increased (Figure 11g), wormholelike mesoporous structures were successfully formed (Figure 11f). Figure S8 exhibits the XPS result of KIE-5-b after calcination. Intense peaks at 71.5 and 74.8 eV are F

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Figure 10. Synthesis procedures of KIE-5-b with pictures for solution color change.

Figure 12. Nitrogen sorption results for KIE-5-a and KIE-5-b. (a) Isotherm plots. (b) BJH desorption pore size distributions.

role as a pore-forming agent. In addition, Figure S9 and Table S3 provide information on macroporous structures of KIE-5-b. KIE-5-b showed a trimodal pore size distribution being composed of the mesopore range below 15 nm, the small macropore range of 0.3−5 μm, and the large macropore range of 5−60 μm (Figure S9). The total pore volume and porosity of KIE-5-b are 2.0 cm3/g and 80.4%. Its density was estimated to be just 1.86% of solid Pt (Table S3). In common with KIE-4-b, KIE-5-b also gives hierarchical porous structures and the relative density of KIE-5-b is comparable to existing porous metals (Figure 7). Partial Film Boiling Phenomena Established by Superheated Rod-Type Heaters. As the temperature difference between hot surface and boiling liquid increases, a type of boiling changes from nucleate boiling through partial film boiling to film boiling (or Leidenfrost effect). In the case of water, partial film boiling occurs when the temperature difference is about 30 °C ∼ 120 °C, and partial film boiling is converted into film boiling at Leidenfrost point (about 120 °C for water). Bubbles are quickly and vigorously formed on hot surface during partial film boiling, whereas a vapor film is formed due to higher formation rate of bubbles during film boiling.42 In our case, as shown in Movie S3 relating to synthesis of KIE-5-a, lots of bubbles formed on the surface of the rod-type heater 3 min after superheating was started. After 6 min, bubbles were much more vigorously formed,

Figure 11. TEM images of KIE-5-b. (a−c) KIE-5-b at step I. Scale bar: 200, 20, and 5 nm. (b, inset) X-ray maps for selected area. (d, e) KIE-5-b at step II. Scale bar: 20 and 2 nm. (f, g) KIE-5-b at step III. Scale bar: 100 and 20 nm.

ascribed to Pt 4f7/2 and Pt 4f5/2 in metallic Pt, respectively.41 Even after thermal treatment at high temperature in air, KIE-5-b provided Pt mesostructures in the zero-valence metallic state. Figure 12 and Table S2 exhibit nitrogen sorption results for KIE-5 samples. Both of the KIE-5-a and KIE-5-b showed typical type IV isotherms with H2 hysteresis loops, which is commonly associated with ink-bottle pores or voids among close-packed spherical particles (Figure 12a), and gave bimodal pore size distributions in a mesopore range (Figure 12b). Compared to KIE-5-a prepared without addition of citric acid, the primary peak pore diameter of KIE-5-b increased from 3.3 to 6.3 nm, and specific surface area decreased because of the pore size expansion. On the basis of these nitrogen sorption results, it was revealed that, in the case of Pt, citric acid also plays a significant G

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Chemistry of Materials however, the bubbles did not seem to form a stable vapor layer on the surface of the rod-type heater. In addition, as shown in Figure 1b, although the temperature of the rod type heater was set at 430 °C, real temperature of heater surface was 240 °C, indicating that temperature difference between hot surface and boiling liquid was more than 100 °C. Accordingly, it can be concluded that synthesis of metal nanoparticles and nanoporous metals in our reactor system was achieved by partial film boiling phenomena rather than film boiling. In a similar way, Elbahri and co-workers previously synthesized oxide nanoparticles by using film boiling phenomena, and skimmed off oxide nanoparticles floating on the surface of boiling solution.43 On the basis of such results, it was revealed that partial film boiling or film boiling of bulky aqueous precursor solution is much more favorable to continuous and large-scale production in comparison with Leidenfrost drop reactors. Large-Scale Production of Nanoporous Pd and Pt Using Rod-Type Heaters. To verify whether large-scale production of mesoporous metal is feasible by using rod-type heaters, we also synthesized mesoporous Pd (KIE-4-e) and Pt (KIE-5-c) by raising Pd or Pt precursor concentration 10 times. The detailed reactant composition is shown in Table S1. As a result, 3.9 and 3.8 g of nanoporous Pd and Pt were successfully obtained from 40 g of 10 wt % Pd or Pt precursor solution (including 4 g of Pd or Pt in precursor solution, respectively), which demonstrates that our reactor system is suitable for large-scale production of nanoporous metals with high yield. Figure 13 and Table S2 show nitrogen sorption isotherms and pore size distributions for KIE-4-e and KIE-5-c. They gave type IV isotherms with H2 hysteresis loops, being ascribed to ink-bottle pores among closepacked spherical particles. Compared to small-scale synthesis of KIE-4 and KIE-5, the peak pore diameter of KIE-4-e and KIE-5-c substantially increased to 26.8−27.4 nm, and the pore size expansion gave rise to a decrease in surface area to 14.1− 19.2 m2/g. Thus, it was demonstrated that our reactor system with the rod-type heater is available for large-scale production of nanoporous metals. Comparison with Nanoporous Pd Prepared by a Conventional Thermal Decomposition Method. To compare with a conventional thermal decomposition method, we synthesized nanoporous Pd (KIE-4-f) by simple calcination of Pd nitratecitric acid mixture powder. As shown in Table S2, compared with KIE-4-b, surface area of KIE-4-f significantly decreased by 67.7%, and pore size increased from 3.8 to 35.0 nm, which indicates that our method assisted by partial film boiling phenomena facilitates the formation of nanoporous metals with higher surface area. Comparison with Rod Type Heaters Inserted in a Glass Tube. To further confirm whether the partial film boiling led to reduction of metal precursors to metal nanoparticles, we fabricated a rod type heater with lower heat flux by inserting a rod type heater into a glass tube, and tried to synthesize Pt nanoparticles and nanoporous Pt (KIE-5-b) in a reactor with a rod type heater inserted in a glass tube. Figure S11 shows a photograph of Pt precursor solution after boiling for 10 min with a rod-type heater inserted in a glass tube, addition of ethanol, and additional boiling for 20 min. In the case of a rod-type heater without glass tube wall shown in Figure 10, after ethanol was added into the boiling solution (step II), the solution color turned dark gray because of Pt nanoparticle formation and aggregation. However, in the case of a rod-type heater with glass tube wall, even though additional boiling was conducted after addition of ethanol, the Pt precursor solution was not changed and still showed its original color. Therefore, it was revealed that

Figure 13. Nitrogen sorption results for KIE-4-e and KIE-5-c. (a) Isotherm plots. (b) BJH desorption pore size distributions.

partial film boiling in our reactor system is a crucial factor for reduction of metal precursors.

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CONCLUSIONS We have revealed two important facts that can offer fresh vision to the field of metal nanoparticles and nanoporous metals. The first one is that Pd and Pt precursors can be reduced to neutral Pd and Pt nanoclusters without reducing agents such as ethanol and citric acid just under the basic condition established by a partial film boiling reactor system. Employing such a system, reducing agents can be excluded from synthesis processes of metal nanoparticles, and its scale-up can be easily accomplished by increasing the concentration of metal precursor. The second one is that when cheap and eco-friendly citric acid as a reducing agent was added, it also plays an important role as a pore-forming agent. After two decades since the discovery of surfactanttemplated mesoporous materials, it has been recognized that the development of these materials on an industrial scale is chiefly limited by economical and environmental concern regarding ionic and nonionic surfactants as a structure-directing agent. More eco-friendly and economical synthetic approaches were found to be necessary.27 Thus, our work could promote the development of nanoporous metals at an industrial level. However, further research is required to fully understand the mechanism and explore whether our synthetic pathway is available to other metal systems. H

DOI: 10.1021/acs.chemmater.5b00143 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00143. Tables S1−S3, Figures S1−S11, and captions for Movies S1−S3 (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a research program (B5-2515) of the Korea Institute of Energy Research (KIER).

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DOI: 10.1021/acs.chemmater.5b00143 Chem. Mater. XXXX, XXX, XXX−XXX