A synthesis method to obtain porous platinum-based macrotubes and macrobeams with a square cross section through chemical reduction of insoluble salt-needle templates is presented.
The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed by rinsing and supercritical drying steps, often resulting in mechanically fragile materials. Here, a method to synthesize nanostructured porous platinum-based macrotubes and macrobeams with a square cross section from insoluble salt needle templates is presented. The combination of oppositely charged platinum, palladium, and copper square planar ions results in the rapid formation of insoluble salt needles. Depending on the stoichiometric ratio of metal ions present in the salt-template and the choice of chemical reducing agent, either macrotubes or macrobeams form with a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Elemental composition of the macrotubes and macrobeams, determined with x-ray diffractometry and x-ray photoelectron spectroscopy, is controlled by the stoichiometric ratio of metal ions present in the salt-template. Macrotubes and macrobeams may be pressed into free standing films, and the electrochemically active surface area is determined with electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials.
Numerous synthesis methods have been developed to obtain high surface area, porous platinum-based materials primarily for catalysis applications including fuel cells1. One strategy to achieve such materials is to synthesize monodisperse nanoparticles in the form of spheres, cubes, wires, and tubes2,3,4,5. To integrate the discrete nanoparticles into a porous structure for a functional device, polymeric binders and carbon additives are often required6,7. This strategy requires extra processing steps, time, and can lead to a decrease in mass specific performance, as well as agglomeration of nanoparticles during extended device use8. Another strategy is to drive the coalescence of synthesized nanoparticles into a metal gel with subsequent supercritical drying9,10,11. While advancements in the sol-gel synthesis approach for noble metals has reduced gelation time from weeks to as fast as hours or minutes, the resulting monoliths tend to be mechanically fragile impeding their practical use in devices12.
Platinum-alloy and multi-metallic 3-dimensional porous nanostructures offer tunability for catalytic specificity, as well as address the high cost and relative scarcity of platinum13,14. While there have been numerous reports of platinum-palladium15,16 and platinum-copper17,18,19 discrete nanostructures, as well as other alloy combinations20, there have been few synthesis strategies to achieve a solution-based technique for 3-dimensional platinum alloy and multi-metallic structures.
Recently we demonstrated the use of high concentration salt solutions and reducing agents to rapidly yield gold, palladium, and platinum metal gels21,22. The high concentration salt solutions and reducing agents were also used in synthesizing biopolymer noble metal composites using gelatin, cellulose, and silk23,24,25,26. Insoluble salts represent the highest concentrations of ions available to be reduced and were used by Xiao and colleagues to demonstrate the synthesis of 2-dimensional metal oxides27,28. Extending on the demonstration of porous noble metal aerogels and composites from high concentration salt solutions, and leveraging the high density of available ions of insoluble salts, we used Magnus’ salts and derivatives as shape templates to synthesize porous noble metal macrotubes and macrobeams29,30,31,32.
Magnus’ salts assemble from the addition of oppositely charged square planar platinum ions [PtCl4]2- and [Pt(NH3)4]2+ 33. In a similar manner, Vauquelin’s salts form from the combination of oppositely charged palladium ions, [PdCl4]2- and [Pd(NH3)4]2+ 34. With precursor salt concentrations of 100 mM, the resulting salt crystals form needles 10s to 100s of micrometers long, with square widths approximately 100 nm to 3 μm. While the salt-templates are charge neutral, varying the Magnus’ salt derivatives stoichiometry between ion species, to include [Cu(NH3)4]2+, allows control over the resulting reduced metal ratios. The combination of ions, and the choice of chemical reducing agent, result in either macrotubes or macrobeams with a square cross section and a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Macrotubes and macrobeams were also pressed into free standing films, and electrochemically active surface area was determined with electrochemical impedance spectroscopy and cyclic voltammetry. The salt-template approach was used to synthesize platinum macrotubes29, platinum-palladium macrobeams31, and in an effort to lower material costs and tune catalytic activity by incorporating copper, copper-platinum macrotubes32. The salt-templating method was also demonstrated for Au-Pd and Au-Pd-Cu binary and ternary metal macrotubes and nanofoams30.
Here, we present a method to synthesize platinum, platinum-palladium, and copper-platinum bi-metallic porous macrotubes and macrobeams from insoluble Magnus’ salt needle templates29,31,32. Control of the ion stoichiometry in the salt needle templates provides control over resulting metal ratios after chemical reduction and can be verified with x-ray diffractometry and x-ray photoelectron spectroscopy. The resulting macrotubes and macrobeams may be assembled and formed into a free-standing film with hand pressure. The resulting films exhibit high electrochemically active surface areas (ECSA) determined by electrochemical impedance spectroscopy and cyclic voltammetry in H2SO4 and KCl electrolyte. This method provides a synthesis route to control platinum-based metal composition, porosity, and nanostructure in a rapid and scalable manner that may be generalizable to a wider range of salt-templates.
CAUTION: Consult all relevant chemical safety data sheets (SDS) before use. Use appropriate safety practices when performing chemical reactions, to include the use of a fume hood and personal protective equipment. Rapid hydrogen gas evolution during electrochemical reduction can cause high pressure in reaction tubes causing caps to pop and solutions to spray out. Ensure that reaction tube caps remain open as specified in the protocol. Conduct all electrochemical reductions in a fume hood.
1. Magnus’ salt derivatives template preparation
NOTE: All salt templates should be chemically reduced within a few hours after preparation as prolonged storage results in a degradation of salt structure. This method describes each platinum-based macrotube and macrobeam product. To obtain additional specific product yield, conduct the method with replicate sets of salt template and reducing agent solutions.
2. Salt-template chemical reduction
NOTE: DMAB is toxic. Avoid breathing dust and skin contact by wearing PPE and conduct all associated tasks in a fume hood.
3. Prepare macrotube and macrobeam films
4. Material and electrochemical characterization
The addition of oppositely charged square planar noble metal ions results in near instantaneous formation of high aspect ratio salt crystals. The linear stacking of square planar ions is shown schematically in Figure 1, with the polarized optical microscopy images revealing salt needles that are 10’s to 100’s of micrometers long. A concentration of 100 mM was used for all platinum, palladium, and copper salt solutions. While the salt needle templates are charge neutral in that the total cation and anion charges are equal, the stoichiometry of the resulting salt needles can be varied with a tertiary combination of ions. For instance, platinum palladium salt template stoichiometry was varied with Pt2+:Pd2-:Pt2- ratios of 1:1:0, 2:1:1, 3:1:2 for a relative platinum-to-palladium ratio of 1:1, 3:1, and 5:1, respectively. In a similar manner, Pt2-:Pt2+:Cu2+ ratios of 1:0:1, 3:1:2, 2:1:1, and 1:1:0 resulted in Pt:Cu ratios of 1:1, 2:1, 3:1, and 1:0, respectively. The average length of the salt needles varied depending on the ratio of dissimilar ions.
The chemical reduction of Magnus’ salts, formed with a 1:1 ratio of Pt2+:Pt2- ions, with NaBH4 results in macrotubes with a generally hollow inner cavity and porous side walls shown schematically in Figure 1A and seen in the scanning electron micrographs in Figure 2. In Figure 2A-B, the macrotubes are seen to generally conform to the geometry of the salt needle templates with flat sidewalls and a square cross section. The macrotube sidewalls shown in Figure 2C appear to consist of fused nanoparticles on the order of 100 nm, but at higher magnification in Figure 2D, these nanoparticles appear to be exhibit fused nanofibrils approximately 4-5 nm in diameter.
Reduction of salts formed with different ratios of Pt2+:Pd2-:Pt2- with sodium borohydride (NaBH4) results in macrobeams with no hollow cavity, but rather a porous nanostructure throughout the square cross sectional area shown schematically in Figure 1B and seen in the electron micrographs in Figure 3. With a Pt2+:Pd2-:Pt2- ratio of 1:1:0, the macrobeams exhibit a nanostructure of fused nanofibrils 4-7 nm diameter seen in Figure 3A-B similar to the sidewall features seen in platinum macrotubes in Figure 2D. A Pt2+:Pd2-:Pt2- ratio of 2:1:1 presents compact nanoparticles 8-16 nm both on the macrobeam surface, as well as throughout the square cross section seen in Figure 3C-D. The chemically reduced 3:1:2 Pt2+:Pd2-:Pt2- salt ratio seen in Figure 3E-F exhibits macrobeams with nanoparticles similar to the 2:1:1 ratio though with a lesser density and higher porosity throughout the square cross section.
Reduction of Pt2-:Pt2+:Cu2+ salts with DMAB results in macrotubes with a hollow cavity, whereas the use of NaBH4 as the reducing agent results in macrobeams with a porous cross section shown schematically in Figure 1C. The DMAB reduced Pt2-:Pt2+:Cu2+ salts are shown in Figure 4. The macrotubes seen in Figure 4A-C reduced from 1:0:1 Pt2-:Pt2+:Cu2+ salt needles present the most distinct and largest square cross section with approximately 3 μm sides. Macrotube sidewalls present a highly textured surface, though unlike the platinum and platinum-palladium macrotube and macrobeam sidewalls seen in Figure 2 and Figure 3, without significant porosity. Macrotubes formed from 3:1:2 and 2:1:1 salt templates in Figure 4D-F and Figure 4G-I, respectively, reveal hollow cores with a cross section approximately 200 nm square and interconnected nanoparticle porous sidewalls from the exterior of the macrotubes to the inner cavity. A Pt2-:Pt2+:Cu2+ salt template with 1:1:0 ratio (which is the same template used for platinum macrotubes reduced with NaBH4) reduced with DMAB results in linear aggregations of nanoparticles generally conforming to the high aspect ratio salt-template, though with no hollow cavity as seen in Figure 4J-L.
Macrotube and macrobeam chemical composition was initially characterized with XRD shown in Figure 5, where salt template stoichiometry ratios are shown in Figure 5B-D. Platinum macrotubes in Figure 5A indexed to Joint Committee on Powder Diffraction Standards (JCPDS) reference number 01–087-0640. Platinum-palladium macrobeams indexed to JCPDS reference numbers 03-065-6418 for platinum–palladium alloy, 00-004-0802 for platinum, and 01-087-0643 for palladium in Figure 5B. Copper-platinum peaks indexed to JCPDS reference number 01-087-0640 for platinum and 03-065-9026 for copper, however, DMAB reduction to macrotubes indicates XRD superimposed peaks that shift toward either platinum or copper depending on the relative salt template stoichiometry as shown in Figure 5C suggesting alloy composition. NaBH4 reduced copper-platinum macrobeams exhibit distinct copper and platinum XRD peaks suggesting a bi-metallic composition seen in Figure 5D.
X-ray photoelectron spectra are shown for platinum, platinum-palladium, and copper-platinum macrotubes and macrobeams in Figure 6. Platinum macrotubes indicate little evidence of oxide species in Figure 6A suggesting a catalytically active surface. The XPS spectra for platinum-palladium macrobeams in Figure 6B-C also presents no indication of metal oxide context. Figure 6D-E shows the XPS spectra for DMAB reduced copper-platinum macrotubes suggesting predominantly metallic copper and platinum, with the presence of Cu2O only in the 1:0:1 Pt2-:Pt2+:Cu2+ salt-template sample. Bulk metal compositions were also determined using energy dispersive x-ray spectroscopy (EDS). The tabulated results comparing salt stoichiometry, EDS and XPS compositions for platinum-palladium and copper-platinum macrotubes and macrobeams are shown in Table 1 and Table 2, respectively. In general, salt stoichiometry correlates with the bulk metal composition indicated with EDS, though XPS reveals a surface enrichment for platinum for both platinum-palladium and copper-platinum structures likely due to a reduction-dissolution mechanism described in the Discussion section. For platinum-palladium macrobeams EDS determined Pt:Pd composition indicates 6.35:1, 3.50:1, 1.12:1 for the 3:1:2, 2:1:1, 1:1:0 salt-template ratios, respectively. XPS Pt:Pd ratios show the same general trend with 11.7:1, 6.45:1, and 1.89:1 for the 3:1:2, 2:1:1, 1:1:0 salt ratios, respectively. Copper-platinum macrotubes and macrobeams reduced with DMAB and NaBH4, respectively, show the same general trend between EDS and XPS determined metal compositions as seen in Table 2.
As an example of electrochemical characterization of pressed macrotube and macrobeam films, Figure 7A shows platinum macrotubes pressed into a free-standing film. Electrochemical impedance spectroscopy in 0.5 M KCl electrolyte is shown in Figure 7B across a frequency range of 100 kHz to 1 mHz, with the high frequency range shown in the inset. The specific capacitance of the platinum macrotube film is estimated from the lowest frequency in the specific capacitance (Csp) versus log (frequency) plot in Figure 7C. The estimated Csp is 18.5 Fg-1 with a corresponding solvent accessible specific surface area of 61.7 m2g-1. Figure 7D shows the cyclic voltammetry curves in H2SO4 electrolyte at scan rates of 0.5, 1, 5, and 10 mVs-1. The 1 mVs-1 scan is highlighted in Figure 7E exhibiting characteristic hydrogen adsorption and desorption peaks at potentials less than 0 V (vs Ag/AgCl) and an oxidative toe region and reduction peaks greater than 0.5 V (vs (Ag/AgCl).
Figure 1: Macrotube and macrobeam synthesis scheme. (A) Addition of [PtCl4]2- and [Pt(NH3)4]2+ (B) [Pt(NH3)4]2+ with [PdCl4]2− and/or [PtCl4]2−, or (C) [Cu(NH3)4]2+ with [PtCl4]2− and [Pt(NH3)4]2+ results in the formation of insoluble salt needles through linear stacking of oppositely charged square planar ions. Electrochemical reduction of salt needle templates forms either a porous macrotube or macrobeam with a square cross section. Representative polarized optical microscopy images of salt crystal templates are shown for each salt template type. Adapted from references 29, 31, and 32 with permission. Please click here to view a larger version of this figure.
Figure 2: Scanning electron micrographs of platinum macrotubes. Adapted from reference 29 with permission. Please click here to view a larger version of this figure.
Figure 3: Scanning electron micrographs of platinum-palladium macrobeams. Macrobeams formed from Pt2+:Pd2−:Pt2− salt-template ratios of (A)–(B) 1:1:0 (C)–(D) 2:1:1, and (E)–(F) 3:1:2, with 100 mM salt solutions and reduced in 100 mM NaBH4. Adapted from reference 31 with permission. Please click here to view a larger version of this figure.
Figure 4: SEM images of copper-platinum macrotubes reduced with DMAB. Macrotubes formed from Pt2-:Pt2+:Cu2+ salt-template ratios of (A)–(C) 1:0:1 (D)–(F) 3:1:2 (G)–(I) 2:1:1, and (J)–(L) 1:1:0. Adapted from reference 32 with permission. Please click here to view a larger version of this figure.
Figure 5: X-ray diffraction spectra for (A) platinum macrotubes (B) platinum-palladium macrobeams (C) copper-platinum macrotubes reduced with DMAB, and (D) copper-platinum macrotubes reduced with NaBH4. (B) Pt2+:Pd2−:Pt2− and (C)-(D) Pt2-:Pt2+:Cu2+ salt-template ratios are indicated on the spectra. Adapted from references 29, 31, 32 with permission. Please click here to view a larger version of this figure.
Figure 6: X-ray photoelectron spectra for (A) platinum macrotubes (B)–(C) platinum-palladium macrobeams; (B) Pt 4d5/2, Pt4d3/2, Pd 3d3/2, and Pd 3d5/2 peaks; (C) normalized Pt4f7/2 and Pt 4f5/2 peaks. (D)–(E) copper-platinum macrotubes reduced with DMAB; (D) normalized Pt 4f5/2 and Pt 4f7/2; (E) Cu 2p1/2 and Cu 2p3/2 peaks. Adapted from references 29, 31, 32 with permission. Please click here to view a larger version of this figure.
Figure 7: Electrochemical characterization of platinum macrotubes synthesized from 100 mM Magnus’ salts. (A) Platinum macrotube pressed film. (B) Electrochemical impedance spectroscopy (EIS) in 0.5 M KCl electrolyte at frequency range of 100 kHz to 1 mHz; (inset) high frequency EIS spectrum (C) Specific capacitance (Csp) in 0.5 M KCl electrolyte determined from EIS in (b). (D) CV in 0.5 M H2SO4 at scan rates of 10, 5, 1, and 0.5 mVs-1. (E) CV in 0.5 M H2SO4 from (D) at a scan rate of 1 mVs-1. Adapted from reference 29 with permission. Please click here to view a larger version of this figure.
Pt2+ : Pd2- : Pt2- | Stoic. Pt:Pd | EDS Pt:Pd | XPS Pt:Pd |
1:1:0 | 1:1 | 1.12:1 | 1.89:1 |
2:1:1 | 3:1 | 3.50:1 | 6.45:1 |
3:1:2 | 5:1 | 6.35:1 | 11.7:1 |
Table 1: Atomic ratio composition of Pt–Pd macrobeams synthesized from Pt2+:Pd2−:Pt2− salt ratios of 1:1:0, 2:1:1, and 3:1:2 determined from salt stoichiometry, energy-dispersive x-ray spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). Adapted from reference 31 with permission.
Pt2-:Pt2+:Cu2+ | Stoic. Pt:Cu | EDS Pt:Cu | XPS Pt:Cu | |
NaBH4 | 1:0:1 | 1:1 | 0.5:1 | 0.92:1 |
3:1:2 | 2:1 | 1.3:1 | 3.1:1 | |
2:1:1 | 3:1 | 2.5:1 | 4.0:1 | |
1:1:0 | 1:0 | 01:00 | 1.0:0 | |
DMAB | 1:0:1 | 1:1 | 0.7:1 | 2.2:1 |
3:1:2 | 2:1 | 1.5:1 | 5.8:1 | |
2:1:1 | 3:1 | 2.1:1 | 7.9:1 | |
1:1:0 | 1:0 | 01:00 | 1.0:0 |
Table 2: Atomic composition of Pt–Cu macrotubes and macrobeams reduced with NaBH4 and DMAB, respectively. Adapted from reference 32 with permission.
This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials. The use of Magnus’ salt derivatives as high aspect ratio needle shaped templates provides the means to control resulting metal composition through salt-template stoichiometry, and when combined with choice of reducing agent, control over the nanostructure of the macrotube and macrobeam porous sidewalls and cross sectional structure. The synthesis method may be varied by changing the salt ratios used to form the templates: Pt2+:Pt2-, Pt2+:Pd2-:Pt2-, and Pt2-:Pt2+:Cu2+. Critical to this method is the formation of salt needle templates resulting from the addition of noble metal square planar cation and anions. Salt formation is found to be sensitive to water impurities and pH requiring the use of deionized water. It is also critical to ensure that electrochemical reduction is conducted within a fume hood with reaction tubes uncapped to prevent overpressure from the vigorous hydrogen evolution that results.
With this method, the reduction of [MCl4]2- to M0, shown in Equation 1, releases four Cl– ions into solution near the salt template surface where M is either Pt or Pd:
The charge balance for each [MCl4]2- ion is reduced; it is thought to be maintained by two [M(NH3)4]2+ ions dissolving into solution. Four neutral ammonia molecules are released through the reduction of [M(NH3)4]2+ to M0 as shown in Equation 2:
The interaction of weakly basic ammonia with water is charge neutral to form NH4+ and OH– ions. The proposed reduction-dissolution action and nanoparticle surface free energy minimization likely contributes to the porous macrotube and macrobeam structures observed in Figure 2, Figure 3, and Figure 429,31,32. Given this proposed mechanism, the salt-templates are in part self-sacrificial given the conversion of some of the salt to the metal phase with the remainder of the salt leaving the template with open pores remaining in its place.
One obvious limitation to the generalizability of this approach is the small number of oppositely charged square planar metal ion combinations available. These are generally limited to coordination complexes of platinum, palladium, copper, gold, and nickel, for example: [PtCl4]2-, [Pt(NH3)4]2+, [Cu(NH3)4]2+, [AuCl4]–, and [Ni(CN)4]2-. The use of [Ni(CN)4]2-, while compelling as a low-cost transition metal that might be used in combination with platinum, palladium, and copper square planar cations, presents a significant safety issue with the liberation of CN– ions during electrochemical reduction in combination with hydrogen gas evolution. Other platinum and palladium coordination complexes have been demonstrated to precipitate insoluble salts35,36,37. The formation of high aspect ratio salt needles is believed to depend on the relative matching of cation and anion size, with greater mismatch leading to less product yield.
The hand-pressing of free-standing films works best with platinum macrotubes likely due to the entanglement of the high aspect ratio structures conforming to the salt-templates. These films are robust to mechanical manipulation with tweezers remaining intact between transfer steps from pressing to placement in an electrochemical vial; however, films will fracture with severe bending. Platinum-palladium microbeam pressed films are not as mechanically robust as platinum macrotubes, likely due to the smaller feature size of the macrobeams. Copper-platinum pressed films are the least mechanically durable of the metal combinations described in this method, though they are stable enough to transfer to electrochemical vials for impedance spectroscopy and cyclic voltammetry. Depending on practical device applications, a minimum of polymeric binder may be used to enhance the structural integrity of the copper-platinum films.
The primary advantage of this method is the simplicity, relative speed, metal composition control, and nanostructure of the macrotube and macrobeam synthesis, as well as the ability to press the synthesis products into free-standing films. With nanoscale feature sizes as small as 4-5 nm for platinum macrotubes, this synthesis method is comparable to preformed nanoparticle sol-gel methods to form noble metal aerogels but without the need for supercritical drying. Platinum-palladium and copper-platinum macrobeams and macrotubes, though, have a slightly larger nanostrucuture feature size ranging up to 50 nm. The larger feature size is partially offset by the ability to incorporate low-cost copper into the nanostructure and tune elemental composition. This method is envisioned to be scalable to any reaction volume from low milliliter to 10s of liters if required.
While the available square planar metal ions are limited for the formation of salt needles comprised of metallic cations and anions, the use of insoluble metal salts may be generalizable to salts where only one ion is metallic. This salt-templating synthesis method might create a much larger range of achievable metal, metal oxide, alloy, and multi-metallic nanostructures.
The authors have nothing to disclose.
This work was funded by a United States Military Academy Faculty Development Research Fund grant. The authors are grateful for the assistance of Dr. Christopher Haines at the U.S Army Combat Capabilities Development Command. The authors would also like to thank Dr. Joshua Maurer for the use of the FIB-SEM at the U.S. Army CCDC-Armaments Center at Watervliet, New York.
50 mL Conical Tubes | Corning Costar Corp. | 430290 | |
Ag/AgCl Reference Electrode | BASi | MF-2052 | |
Cu(NH3)4SO4•H2O | Sigma-Aldrich | 10380-29-7 | |
dimethylamine borane (DMAB) | Sigma-Aldrich | 74-94-2 | |
K2PtCl4 | Sigma-Aldrich | 10025-99-7 | |
Miccrostop Lacquer | Tober Chemical Division | NA | |
Na2PdCl4 | Sigma-Aldrich | 13820-40-1 | |
NaBH4 | Sigma-Aldrich | 16940-66-2 | |
Polarized Optical Microscope | AmScope | PZ300JC | |
Potentiostat | Biologic-USA | VMP-3 | Electrochemical analysis-EIS, CV |
Pt wire electrode | BASi | MF-4130 | |
Pt(NH3)4Cl2•H2O | Sigma-Aldrich | 13933-31-8 | |
Scanning Electron Microscope | FEI | Helios 600 | EDS performed with this SEM |
Shelf Rocker | Thermo Scientific | Vari-Mix™ Platform Rocker | |
Snap Cap Microcentrifuge Tubes, 1.7 mL | Cole Parmer | UX-06333-60 | |
X-ray diffractometer | PanAlytical | Empyrean | X-ray diffractometry |
X-ray photoelectron spectrometer | ULVAC PHI – Physical Electronics | VersaProbe III |