Solvothermal Synthesis and Electrochemical Characterization of

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Solvothermal Synthesis and Electrochemical Characterization of Shape-Controlled Pt Nanocrystals Cenk Gumeci, Archis Marathe, Rachel Lynn Behrens, Jharna Chaudhuri, and Carol Korzeniewski J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2014 Downloaded from http://pubs.acs.org on June 16, 2014

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The Journal of Physical Chemistry

Solvothermal Synthesis and Electrochemical Characterization of Shape-Controlled Pt Nanocrystals Cenk Gumeci, †,¶ Archis Marathe,┴ ,¶ Rachel L. Behrens,† Jharna Chaudhuri,┴ and Carol Korzeniewski*,† † Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States ┴ Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, United States * Corresponding Author ([email protected]; +1-806-834-1228) ABSTRACT

A simple, surfactant-free solvothermal method is reported for the preparation of < 10 nm shapecontrolled platinum crystallites. Reactions were carried out in N,N-dimethyformamide (DMF) and DMF-water mixtures. Effects of reaction time and temperature, DMF-water ratio and metal precursor salt were examined. By tuning the reaction conditions, ensembles of Pt particles with dominant truncated octahedral / cuboctahedral, or cubic shapes could be formed from the metal acetylacetonate (acac) precursor salt. Metal nanocrystal development was monitored through the

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use of high resolution transmission electron microscope (HR-TEM), X-ray and electrochemical analysis methods. Voltammograms probing CO and formic acid oxidation over shape-controlled nanocrystals adsorbed to a glassy carbon electrode displayed expected features characteristic of extended (111) and (100) facets, confirming the stability and surface cleanliness of particles taken directly from the reaction mixture. A mechanism for Pt reduction and the growth and stabilization of preferentially shaped Pt nanocrystals in the DMF-water solvent system is proposed.

The involvement of DMF as a reducing agent and carboxylate ions as weakly

coordinating, and hence easily displaced, nanoparticle capping ligands is discussed.

KEYWORDS: Surfactant free synthesis, cubic Pt nanoparticles, electrocatalysis, carbon monooxide oxidation, formic acid oxidation. 1. Introduction Approaches for the preparation of metal nanocrystals that have uniform and well-defined shapes have been intensively studied in recent years due to the importance of surface structure on the kinetics of heterogeneous catalytic reactions.1-16 Pt and Pt bimetallic nanocrystals, in particular, have attracted a great deal of interest to support investigations in electrocatalysis.8-23 The surfaces of shape-controlled noble metal nanocrystals typically contain a mix of the low-index (111) and (100) facets in different proportions along with their connecting edge structures. The ability to prepare nanometer-scale metal particles with a regular arrangement of surface planes and a narrow size distribution has been an important step forward in the development of supported metal particle catalysts for practical applications.9-16 Challenges remain, however, particularly in the ability to control size and shape in the regime below 10 nm and to retain particle shape and structure without the need for strongly bound capping ligands.1,3


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We report herein a relatively simple, surfactant-free, solvothermal approach for the preparation of shape-controlled Pt nanocrystals. There has been increasing interest in solvothermal methods,11-14, 23-29 particularly in the development of bimetallic electrocatalyst materials.11-14, 29 The above ambient temperatures employed promote thermal mixing of reactants and provide some control over particle formation kinetics, leading to particle ensembles that have a targeted shape, composition and size distribution. The high boiling point solvent N,N-dimethyformamide (DMF) has been useful in supporting solvothermal syntheses of metal nanoparticles,11-14, 29-31 in some cases serving as both the reaction medium and a reducing agent.11-14 Although successfully employed to prepare shape-controlled Pt alloy particles from acetylacetonate (acac) precursor salts with optimal structures for O2 reduction electrocatalysis,11-14 the approach has not yet been demonstrated in reactions of Pt salts alone. The development of Pt nanoclusters from H2PtCl2 precursor in a DMF-water system at 140 °C has been described, but the resulting particles lacked a defined shape.32 In the work presented below, Pt ion reduction was carried out in a DMF-water solvent mixture under conditions that give < 10 nm particles with primarily cubic, or truncated octahedral (TO) (including the general class of cuboctahedral3) shapes. Effects of reaction time, DMF-water ratio, reaction temperature and metal precursor salt were examined. The transformation from irregular nanoparticles to well-faceted nanocrystals was monitored using high resolution transmission electron microscopy (HR-TEM) and electrochemical analysis methods. A mechanism for Pt reduction and the growth of preferentially shaped Pt nanocrystals in the DMF-water solvent system is discussed.


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2. Experimental Reagents: Platinum (II) acetylacetonate (Pt(acac)2), potassium (II) tetrachloroplatinate (K2PtCl4), potassium (IV) hexachloroplatinate (K2PtCl6), and Pt-black fuel cell catalyst were purchased from Alfa Aesar (Ward Hill, Massachusetts). Platinum (II) bromide (PtBr2), sulfuric acid (99.999 % purity) and formic acid (98.0 - 100 % purity) were obtained from Sigma-Aldrich (St. Louis, Missouri). DMF was purchased from Merck (Billerica, Massachusetts). All aqueous solutions were prepared with deionized water (18.2 MΩ-cm) from a four-cartridge Nanopure Infinity System (Barnstead, Dubuque, Iowa). Argon and carbon monoxide (Air Gas Southwest, The Woodlands, Texas) gases used were of ultrahigh purity. All the chemicals were utilized without further purification. Synthesis: In a typical preparation, approximately 16 mg (41 µmol) of Pt(acac)2 was dissolved in 40 mL of solvent. The mixture was sealed inside a poly(tetrafluoroethylene) lined autoclave and brought from ambient (22 ± 1°C) to the reaction temperature by heating at a rate of 10 °C min-1. The heating rate and reaction temperatures were selected to promote rapid nucleation of seed crystals followed by a period of slow growth, aiming to achieve shape-controlled nanoparticles with average size < 10 nm.12 After each reaction period, the solution was first cooled to ambient temperature, the particles were then recovered by centrifugation (45 min at 2000 rpm) and washed with aliquots of ethanol (twice) and ethanol-water (once) solutions. After the final rinse, the nanoparticles were resuspended in 5 mL of water for further characterizations. Structural Characterization: X-ray diffraction (XRD) patterns were collected at 40 kV and 44 mA using a Rigaku Ultima 3 X-ray diffractometer (Rigaku MSC, Woodlands, TX) with Ni filtered Cu-Kα-radiation. Samples were mounted on a silicon zero background sample holder. Scans were recorded in the 2θ range of 30°- 90° with a 0.03°/s scan rate. The identification of


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phases was made by referring to the Joint Committee on Powder Diffraction Standards International Center for Diffraction Data (JCPDS-ICDD) database. A JEOL JEM-2100 electron microscope equipped with an energy dispersive X-ray (EDX) spectrometer operated at 200 kV was used for transmission electron microscope (TEM) and high resolution TEM (HR-TEM) imaging. Samples for TEM were prepared by suspending a small amount of the nanoparticle sample in ethanol containing 20% added water by volume, dispersing in a low-power ultrasonic bath for 10 minutes and then placing 1-2 drops onto an ultrathin carbon, 400 mesh grid and drying in air. Electrochemical Characterization: Electrochemical studies were conducted in a conventional three electrode glass cell using a potentiostat (PC4/300, Gamry Instruments, Warminster, PA). A glassy carbon disc electrode (Pine Research, Raleigh, NC, 5 mm diameter) or bulk gold electrode was used as the working electrode. A Pt wire served as the counter electrode, and a reversible hydrogen electrode (RHE) was employed as the reference electrode.33 All potentials are reported in volts versus the RHE. The electrochemical measurements were conducted at ambient temperature. Prior to each experiment, the glassy carbon electrode was polished to a mirror finish using 0.05 µm alumina, followed by rinsing in an ultrapure water filled ultrasound cleaning bath to remove alumina particles from the surface. A thin catalyst layer was formed on the glassy carbon electrode by dispensing a measured volume of the catalyst suspension and allowing it to dry. The catalyst loaded electrode was then immersed in Ar saturated 0.5 M H2SO4. Cyclic voltammograms (CVs) were recorded at a scan rate of 50 mVs-1. The electrochemically active Pt surface area (ECSA) values were determined from the relation ECSA = QH / 230 µC ⋅ cm-2 where QH is the measured charge in µC for hydrogen adsorption, or


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desorption, and 230 µC ⋅ cm-2 is the charge required for adsorption (or desorption) of a full monolayer of hydrogen from preferentially oriented Pt nanoparticles in H2SO4 electrolyte.22 Values for QH were determined by integration of voltammograms across the range of 0.06 V to 0.60 V. The current in CVs was normalized to the ECSA value for a sample to determine the values of current density. In studies of electrocatalytic reactions, CVs were recorded in 0.5 M H2SO4 at a scan rate of 50 mVs-1. CO was adsorbed by exposing the electrode to CO saturated electrolyte for 15 minutes while the electrode was held at 0.05 V. Subsequently, the electrolyte was purged with Ar for 1 hour to remove residual CO from the solution. Formic acid voltammetry was carried out in 0.5 M H2SO4 containing 0.5 M HCOOH. The solution was purged with Ar for 30 min just prior to measurements.

3. Results and Discussion

Initially, the solvothermal synthesis was attempted in nominally dry DMF using Pt(acac)2 as the precursor. Although Pt particles formed in these experiments, the particles had irregular shapes and were largely agglomerated (Figure S1). Subsequently, reactions were carried out in DMF-water mixtures. Figure 1 shows TEM images of Pt nanocrystals that developed following 32 hours of reaction at 120 °C for water volume fractions in DMF varying from 1 % to 30 %. Nanoparticles prepared in DMF containing 1 % added water show a mix of small (< 10 nm) particles with TO and irregular shapes (Figure 1a). Increasing the water content to 3 % (Figure 1b) gave fairly uniform TO-shaped particles. A size distribution for particles prepared under this condition is reported in Figure 2c. Progressing from 5 % to 20 % water (Figure 1c-


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e), the crystallites transitioned from TO to cubic, with sizes > 10 nm in the latter case. However, increasing the solvent water content further resulted in agglomeration and a loss of shape control (Figure 1f).

Figure 1. TEM images of Pt nanoparticles prepared from Pt(acac)2 precursor in DMF containing the following amounts of added water (by volume): a) 1 %, b) 3 %, c) 5 %, d) 10 %, e) 20 % and f) 30 %. Reactions were carried out at 120 °C for 32 hours.

The trends displayed in Figure 1 can be explained by considering a few reactions that have potential to take place at the surface of a growing Pt particle (Scheme 1). It has been shown that DMF can serve as a reducing agent for metal ions.30 The aldehyde functionality in DMF can be transformed through coupled electron-proton transfer steps, much like in reactions of formaldehyde or acetaldehyde.34 These processes are summarized in Scheme 1. The competition among the steps depicted (1-4), influenced by the solution composition and Pt ion content, likely affect the development of the Pt nanoparticles. For example, DMF dissociative chemisorption at the surface of a Pt particle can provide a source of electrons needed for the reduction of Pt ions


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(1a, Scheme 1).30 The activation of water (1b, Scheme 1) at nearby sites will supply adsorbed oxygen species necessary for transforming the aldehyde to a carboxylate (2, Scheme 1).

particle

Ptn

O 1a. H

OC N(CH3)2 ads

C N(CH3)2

+ H+ + e—

particle

1b.

H2 O

2.

OC N(CH3)2 ads

Ptn

OHads + H+ + e— O

+ OHads

O particle

Ptn

O 3.

4.

O

C N(CH3)2

Pt2+ + 2 e— +

C N(CH3)2 + H+

particle

HO

C N(CH3)2

O O

Ptn

O

C N(CH3)2 ads particle

Ptn+1

Scheme 1. Proposed reaction mechanism for Pt ion reduction and the growth of preferentially shaped Pt nanocrystals. While the stable carboxylate can desorb from the particle, freeing sites for further reaction, small molecule carboxylates are known to adsorb weakly to Pt35-37 with different binding characteristics depending upon the structure of the surface plane.37-39 It is possible the carbamic acid formed following DMF oxidation can serve as a capping agent for Pt particles (3, Scheme 1) and affect the rate of Pt ion reduction at different crystallographic planes. Greater blocking of (100) planes by carbamic acid species may inhibit Pt ion reduction (4, Scheme 1) and enable a relatively faster rate of Pt addition to (111) planes, leading to the preferential growth of cubes over long reaction periods (vide infra).1-3 Deeper understanding of the factors that control carboxylate adsorption at different types of structural sites on Pt surface plans is needed to


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develop a more detailed model for the role carbamic acid may play in the shape-selective Pt particle growth processes. For reactions carried out in nominally dry DMF, the low activity of water will limit carboxylate formation, and hence, the availability of the capping agent in the solution. The small amount of capping agent expected can explain the agglomerated and irregular shaped particles discussed in Figure S1. Furthermore, with increasing water fraction in DMF, dilution of the carboxylic acid will tend to raise the pH, thereby creating a competition between the carbamate anion and OH- for adsorption/capping sites on the Pt particles. These effects likely contribute to the loss of shape selectivity in syntheses carried out in DMF solution containing 30 % water (Figure 1f). In exploring the solvothermal approach further, studies focused on the properties of nanoparticles formed from Pt(acac)2 precursor in DMF containing 3 % water. Figure 2 shows TEM images taken from a series that examined the time evolution of shape selectivity. Reactions were conducted at 120 °C for periods of 6, 12, 24, 32, 48 and 55 hours. After 6 hours, the yield of particles was low and the shapes were not well defined. The yields were higher for reaction periods of 12 hrs and 24 hrs, and particles with a TO shape started to emerge by the 24 hr point (Figure S2). After 32 hrs, most of the particles had developed a uniform TO shape with an average size of 6.4 (± 0.6) nm (Figure 2a-c and S3a). The high resolution TEM image in Figure 2b indicates a lattice spacing between (111) planes within a TO-shaped particle of ~ 0.21 nm. This value is close to literature reports (0.226 nm) for octahedral shaped Pt nanoparticles25 and is consistent with the d-spacing separating (111) planes derived from XRD spectra (Figure S4) of the samples. Pt cubes developed during reaction periods of greater than 32 hrs. Cubes with average size 7.4 (± 0.5) nm dominated the reaction mixture after 48 hrs (Figure 2d-f and S3b).


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These Pt cube characteristics persisted for reaction times up to 55 hrs, the longest period investigated. Based on the transition in color of the solution throughout the various times sampled, the precursor appears to be largely depleted after 48 hrs. Therefore, the particle growth rate is expected to greatly diminish after this point. The lattice fringes on the high resolution TEM image of the Pt cube in Figure 2e, 0.19 nm, matches well values reported in the literature for Pt cubic nanoparticles (0.19 nm16 and 0.196 nm40) and the d-spacing separating (200) planes derived from the associated peak in XRD measurements (Figure S4) of the samples.

Figure 2. TEM images and size distribution histograms for Pt nanoparticles prepared from Pt(acac)2 precursor in DMF containing 3% by volume of added water. The reactions were carried out at 120 °C for 32 hrs (a,b,c) or 48 hrs (d,e,f). The histograms show the size distribution for TO-shaped (c) or cube-shaped (f) particles. Each was determined from analysis of at least 200 particles.

Subsequently, Pt nanoparticle formation was investigated from the precursor salts K2PtCl6, K2PtCl4 and PtBr2 as described above for the DMF-water mixture (3 vol. % water) at 120 °C. For the halide salts, the nanoparticle yield was lower and the average particle size smaller


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relative to Pt(acac)2 for each reaction period. After 32 hrs, nanoparticle crystallites (∼5 nm) formed from K2PtCl4, but the shapes were not uniform. Considerably smaller crystallites (∼2-3 nm) formed from PtBr2. The particles generated from the Pt (IV) precursor were similarly small, but highly agglomerated and lacked facets. The results likely can be traced to the stronger interaction of the halides than the acac ligands with the Pt species. In the case of the halide salts, coordination with Pt ions will shift the reduction potential of the metal negative, requiring more reducing power to form the metal nanoparticles.41 In addition, halides readily adsorb to Pt surfaces42 and can strongly inhibit Pt deposition on (111) and (100) planes leading to slow particle growth. Consistent with these observations, Pt(acac)2 salts have been reported to give rise to more monodisperse nanoparticles than Pt (II) chloride salts.43 We also note that metal nanoparticles did not develop from Ni(acac)2, Co(acac)2 and Cu(acac)2 precursors under the solvothermal conditions described. This result can be expected within the scope of Scheme 1, given the more negative reduction potential of the metals relative to Pt ions. Preliminary studies also were carried out at temperatures above 120 °C. Effects of increasing reaction kinetics were evident. In a sequence of reactions performed from Pt(acac)2 precursor in DMF containing 3 % water at 200 °C, TO-shaped particles became abundant within 18 hrs. After 28 hrs of reaction, TO-shaped particles not only dominated, but were present in a narrow size distribution averaging 7.5 (± 0.6) nm (Figure S5). Interestingly, the TO shape persisted, and cubes were not evident, for reaction times as long as 55 hr, the longest period studied. At 200 °C, cube formation required higher fractions of water in DMF. Additionally, a mixture of cube and TO particles resulted at this reaction temperature for Pt(acac)2 precursor in DMF containing 20 % water (Figure S5). The persistence of TO-shaped particles at 200 °C may be the result of a faster nucleation rate leading to a greater number of particles in the reaction mixture and limited


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growth due to Pt ion depletion. Temperature effects are being investigated in greater detail in continuing work. Aliquots of the samples prepared as described in Figure 2 were subjected to further characterization by electrochemistry methods. CVs of Pt electrodes recorded in aqueous acid electrolytes are known to display features characteristic of dominant crystallographic surface planes.45 Responses in Figure 3 follow trends expected for shape-controlled Pt nanoparticles in H2SO4.9,10,21, 22,44-46 The peaks below 0.3 V are associated with surface processes that accompany the reductive adsorption of proton (negative scan) or oxidative desorption of hydrogen chemisorbed to Pt atoms (positive scan). The voltammogram in Figure 3a has features characteristic of developing TO particles.9,10,21,22,44-46

For example, the peaks near 0.25 V are

associated with hydrogen adsorption/desorption processes at (100) planes of Pt. Furthermore, the broad waves near 0.48 V are characteristic of changes in the double layer properties of extended (111) oriented facets on Pt, while the waves near 0.1 V arise from hydrogen adsorption/desorption from sites near the junction of (100) and (111) Pt planes. Figure 3b is similar, however the development of features near 0.35 V and the sharpening of the 0.1 V peaks is consistent with maturing (100) planes on the TO particles. Finally, the sharpening of the 0.25 V peaks and flat response followed by a sudden drop in current through the 0.3 V – 0.4 V region in Figure 3c is a signature of Pt cubes. There likely is some roughening, or disorder on the extended (100) planes, as contributions from (100)-(111) junctions also are evident in Figure 3c. Additionally, the particles as examined in this study, without electrochemical pre-treatment or extensive washing, may bring trace organic impurities to the electrochemical analysis step, since studies have shown it is possible to achieve sharper and slightly more symmetric features in voltammograms of shape-controlled Pt nanoparticles in sulfuric acid electrolytes.9,10,21,22,44-46 The


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TEM images included as insets in Figure 3 show the population of Pt nanoparticle crystallites in the samples display surface planes consistent with the features in the cyclic voltammograms. The results are in good agreement with those reported for shape-controlled Pt nanoparticles prepared by other research groups.9,10,21,22,44,46 Additionally, the voltammograms indicate the Pt particles resulting from the solvothermal treatment have a high degree of surface cleanliness and any capping residues produced are easily displaced by ions in the sulfuric acid electrolyte solution.


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Figure 3. CVs of Pt nanoparticles (~20 µg/cm2), following adsorption to a glassy carbon electrode, recorded at 50 mVs-1 in Ar saturated 0.5 M H2SO4. The Pt samples were prepared at 120 °C from Pt(acac)2 precursor in DMF containing 3 % (v/v) water for reaction periods of 32 hrs (a) 38 hrs (b) and 48 hrs (c). TEM images of the samples are shown in the insets. The current densities reported are expressed relative to the active Pt surface area of the immobilized nanoparticles. Figure 4 includes CVs that show responses for CO (Fig 4a and b) and formic acid (Fig. 4d and e) oxidation over the shape-controlled Pt particles. For CO, the current on the positive scan is


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low for all samples until about 0.4 V, at which point the pre-wave45-47 begins to appear just ahead of the main peak for CO oxidation (0.65 V – 0.85 V). The pre-wave region has been of longstanding interest, since it is associated with oxidative processes that lower the coverage of CO, and hence reduce its effect on Pt surface poisoning.45,47 The pre-wave appears in connection with restructuring of CO on terrace planes and is prominent in the voltammograms of CO stripping from the assemblies of TO and cube-shaped particles (Figure 4a and b). The main CO stripping peaks occur at potentials positive of the pre-wave. For the shape-controlled particles, the dominant wave contains two contributions. Following the initial rapid rise, the current peaks near 0.75 V before reaching a maximum at about 0.8 V. The peak near 0.75 V is sharper for CO oxidation over the cubic particles relative to the sample containing dominant TO-shaped particles, in very good agreement with earlier reports.46,48 Splitting of the main CO oxidation peak has often been observed in linear scan voltammetry measurements that probe the properties of adsorbed CO on Pt nanoparticles.46,48 The splitting reflects factors that include the variation in CO reaction rate at different types of structural sites on the particles, the distribution of particle sizes in a sample and the rates of CO mobility over confined regions on the surfaces.45,46,48 CO oxidation peak splitting is less evident for polycrystalline particles (Fig. 4c), likely due to the smaller ordered terrace domains. 45,46,48 For formic acid oxidation over shape-controlled Pt nanoparticles (Fig. 4d and e), the CV responses are in good agreement with those reported earlier for samples displaying dominant (100) or (111) planes.9,21 On the forward scan, the low currents are primarily the result of surface poisoning by reaction intermediates, particularly adsorbed CO. The waves near 0.5 V and 0.9 V are associated with the oxidation of these intermediates, and their removal from the surface frees sites for further reaction of formic acid through the reverse scan.21 The drop in current just


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positive of 0.9 V is typically ascribed to the formation of surface oxides, or strongly bound water structures that inhibit formic acid dissociation.21,45 Consistent with formic acid oxidation over extended surfaces of bulk Pt single crystal electrodes, the cube-shaped particles dominated by (100) planes support fast formic acid oxidation kinetics following removal of the surface poisons (Fig 4e).9,45,49 Additionally, similar to Pt(111) the TO-shaped particles display higher current densities on the forward scan in comparison to the reverse scan, reflecting the greater resistance of (111) planes to poisoning during the oxidation of small organic molecules.21,45,49 For the samples containing primarily cubic or polycrystalline (Fig. 4f) particles, the two waves that appear on the reverse scans have been attributed to formic acid oxidation on low coordination Pt sites at the boundary of crystalline planes (0.8 V), or on (100) oriented planes (0.45 V). The current densities in these two waves for the samples agree well with responses in the literature for cubic and polyoriented Pt nanoparticles.9,21,29


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0.05 mA/cm2Pt

0.2 mA/cm2Pt d

a

TO particles

TO particles

0

Current Density, j (mA/cm2Pt)

Current Density, j (mA/cm2Pt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

Cubic particles 0

0

0.5 mA/cm2Pt e Cubic particles 0

c

Pt-black

0.2 mA/cm2Pt

f

0 Pt-black 0 0.00

0.20

0.40

0.60

0.80

1.00

0.00

0.20

Potential (V vs RHE)

0.40

0.60

0.80

1.00

Potential (V vs RHE)

Figure 4. CVs of Pt nanoparticles (∼20 µg/cm2) recorded at 50 mVs-1 in Ar saturated 0.5 M H2SO4 in the presence of a CO monolayer (a-c) or 0.5 M HCOOH (d-f) for TO and cube-shaped particles and Pt-black, as indicated. The samples containing TO and cube-shaped particles were prepared at 120 °C from Pt(acac)2 precursor in DMF containing 3 % (v/v) water for reaction periods of 38 hrs (a,d) and 48 hrs (b,e). Arrows show the scan direction. The current densities reported are expressed relative to the active Pt surface area of the immobilized nanoparticles.

Conclusion The surfactant free, one-pot solvothermal method reported allows for the preparation of crystalline Pt nanoparticles that have a high degree of surface cleanliness and can be tuned to produce dominant TO, or cubic shapes. DMF appears to serve as both the solvent medium and a reducing agent. Well-defined TO and cubic crystallites were readily formed in DMF-water mixtures containing up to 30 vol. % water. A mechanism is proposed in which the carbamic acid by-product of DMF oxidation can function as a capping ligand capable of influencing particle


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growth preferentially along (111) or (100) crystal planes, depending upon the reaction temperature and time and DMF-water ratio. Electrochemical studies indicate the particle capping ligands are easily removed in 0.5 M H2SO4 electrolyte and the Pt crystallites retain their shape after immobilization at the surface of a glassy carbon electrode. In studies of CO and formic acid oxidation, CVs display features characteristic of reactivity at (111) and (100) facets of Pt TO and cubic-shaped nanocrystals. TO nanoparticles were found to be the least susceptible to poisoning by formic acid at low potentials, characteristic of dominant (111) facets. However, Pt cubes were more active toward formic acid oxidation than TO-shaped particles consistent with the high coverage of (100) facets. The ease of preparation and cleanliness of the nanoparticles generated suggests the solvothermal approach described has potential for use in the preparation of shapeselective Pt particles to support a wide range of applications. Extension to other types of metals and bimetallic materials is under study.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ¶These authors contributed equally. ACKNOWLEDGMENT The authors are grateful to the National Science Foundation (CHE-0909736 and MRI-0922898) for financial support. Special thanks are extended to Dr. Dimitri Pappas (Department of Chemistry and Biochemistry, Texas Tech University) for his scientific insight and proof reading of the manuscript and Dr. David Birney for assistance with ChemDraw software. ASSOCIATED CONTENT Supporting Information Experimental descriptions and results of TEM and X-Ray analyses. This material is available free of charge via the Internet at http://pubs.acs.org


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Solvothermal Synthesis and Electrochemical Characterization of

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