Synthesis of Shaped Pt Nanoparticles Using Common Anions or Small Molecules as Shape-Directing Agents: Observation of a Strong Halide or Pseudo-halide Effect

To prepare tetrabutylammonium glycolate, 1.465 g glycolic acid was added directly to 12.63 mL of a 40% w/w aqueous solution of tetrabutylammonium hydroxide. The resulting salt was extracted with 40mL methylene chloride 8 times, dried over magnesium sulfate, and isolated by removal of the solvent under vacuum. In a typical preparation of non-aqueous glycolateprotected nanoparticles, 7.35 mg (1.25 x 10-6 mol) di-μ-chlorodichlorobis(ethylene)diplatinum(II) (Zeise’s dimer) was dissolved in 25 mL THF along with 8.0 mg tetrabutylammonium glycolate (4 eq.) The solution was then heated at 60 oC for approximately 5 min until the golden color of nanoparticulate platinum indicated completion of the reaction. See Figure S1.


Introduction
Utilizing platinum nanoparticle shape selectivity in chemical catalysis and electrochemical catalysis gives more efficient chemical processing at lower energy cost and is the subject of two recent reviews. 1,2To achieve such benefits, convenient methods of preparing catalytically active Pt metal nanocrystals of desired shape are needed.
A vast literature is available on the synthesis and catalytic reactivity of Pt metal catalysts, and synthesis strategies for preparing shape-and size-controlled Pt metal colloids have been reviewed. 3More recent methods of preparing shaped Pt metal nanoparticles include: Pt-ion reduction by H 2 in the presence of carboxylic acid [4][5][6] or amine 7 capping agents; Pt-ion reduction by citrate ion 4 or formic acid; 8 polyol reduction of Pt-ion in the presence of polyvinylpyrrolidone (PVP), 9 PVP/AgNO 3 , 10 PVP/NaNO 3 , 11 or oleylamine; 12 thermal decomposition of organometallic Pt precursors; [13][14][15] Pt(acetylacetonate) 2 reduction by oleylamine, 16 alkenes, 17 or metal carbonyls in the presence of oleylamine/oleic acid; 18 photo-induced reduction of Pt(IV) ion within micelles; 19 reduction of Pt-ion with wood nanofibers; 20 and supported Pt nanoparticles formed by borohydride reduction, 21 electrochemical reduction, 8 or electron-beam deposition. 22ntrol of Pt nanoparticle shape is usually achieved by the choice of surfactant or shapedirecting agent used and by careful regulation of experimental conditions, such as surfactant or shape-directing agent concentration, pH, temperature, ambient light intensity, and reaction time.
A wide variety of Pt particle shapes have been prepared through experimental control of metal particle nucleation and growth events.
In this study, common anions or small molecules are surveyed as possible shaped-directing agents with the goal of finding polymer-free methods for convenient preparation of shaped Pt nanoparticles.Given the known catalytic reactivity differences of Pt(100) and Pt(111) surfaces, formation of Pt metal nanocubes or nanotetrahedra is of primary interest.The outcome of this study was unexpected.Carboxylate, polycarboxylate, hydroxy-, amino-, phosphino-, or mercaptocarboxylate anions all showed poor shape-directing influence (even though such functional groups are present in shape-directing polymeric surfactants) as did chloride, bromide, carbonate, nitrate, perchlorate, phosphate, sulfate, or triflate ions.However, hydroxide and iodide anions serve as endogenous shape-directing agents in the H 2 reduction of Pt(IV) complexes to form, respectively, Pt nanocubes and nanotetrahedra.This strong "halide (or pseudo-halide) effect" is surprising, though not unprecedented.A recent review of the complex role that halide ions play in controlling anisotropic noble metal nanocrystal growth emphasizes the need for additional investigation of this phenomenon. 43
All routine transmission electron microscopy (TEM) was carried out on a Philips CM-20 TEM equipped with a LaB 6 thermionic electron source and an energy dispersive spectrometer (EDS) and operating at 200 kV accelerating voltage.High-resolution transmission electron microscopy (HR-TEM) was performed on a Philips CM-30 TEM equipped with a LaB 6 thermionic emitter and operating at 300 kV.Samples for TEM were dropped onto carbon-coated grids and allowed to dry in air.All powder X-ray diffraction (XRD) was carried out on a Scintag X-1 XRD in the θ-θ configuration and equipped with a Peltier-cooled solid-state detector.Samples for XRD were deposited onto a Si(510) plate and scanned in air.Ultraviolet-visible spectroscopy (UV-Vis) was carried out using a Hewlett-Packard 8452A photodiode array spectrophotometer.

Pt colloid syntheses
Formation of Pt metal colloids were surveyed using: Pt Cl‾, Br‾, or I‾) as shape-directing agents.A listing of syntheses performed along with nanoparticle shape outcomes is provided in Table 1.Detailed synthesis procedures are available as supplementary information (see ESI †).In a typical synthesis, Pt reagent is dissolved in water at a desired temperature and pH in the presence of a surface-directing agent under subdued lighting.Reducing agent is introduced, and the reaction is monitored until a golden yellow or brown color develops.Various control syntheses were conducted, as discussed below.

Pt colloid syntheses
Formation of shaped Pt nanoparticles by El-Sayed, et.al., 44 and shaped Ni and Pd nanoparticles by Reetz, et.al., 45 using polycarboxylate or α-hydroxycarboxylate shape-directing agents prompted a survey of Pt-ion reductions in the presence of simple bi-functional or poly-functional carboxylate ions.Reaction of Pt 2 (C 2 H 4 ) 2 Cl 4 and [Bu 4 N]glycolate in THF solution, using glycolate ion as both reducing agent and shape-directing agent, produces Pt nanoparticles of heterogeneous shapes (see ESI †, Fig. S1), suggesting that the Reetz method does not apply to Ption reduction.Reduction of [PtCl 4 ] 2 ‾ in water by either glycolate or tartrate ion requires elevated temperatures and gives poorly faceted, highly agglomerated Pt nanoparticles (see ESI †, Fig. S2).
Reduction of aqueous [PtCl 4 ] 2 ‾ solutions at room temperature using H 2 as reducing agent and either glycolate or tartrate ion as shape-directing agent gives less agglomerated Pt nanoparticles exhibiting heterogeneous shapes but includes some identifiable cubic and tetrahedral particles (see ESI †, Fig. S3).Hydrazine reduction of [PtCl 4 ] 2 ‾ within a stearic acid/water-in-oil emulsion at room temperature forms relatively large (ave.dia. 20 nm), spheroidal Pt nanoparticles that apparently form within a poly-carboxylate, spherical micelle (see ESI †, Fig. S4).Amino acid conjugate bases represent another class of potentially bifunctional carboxylate shape-directing agents.Basic pH is required to ensure a high degree of deprotonation (glycine; pK a = 9.60).When using glycine as a possible shape-directing agent, Pt colloid formation is accelerated by in situ displacement of chloro ligands of K 2 [PtCl 4 ] by iodide ion (KI) giving to form (glycinato) 2 PtI 2 .H 2 reduction of (glycinato) 2 PtI 2 at pH 10.5 yields highly faceted Pt nanoparticles exhibiting a variety of shapes with sizes ranging from 5 nm -20 nm (see ESI †, Fig. S5).Unfortunately, this method is not very shape selective.Control reactions indicate that while varying free glycine concentration has little effect on Pt nanoparticle shape, variation in reaction pH has a dramatic effect.Pt particles formed at pH 6 are sufficiently small (1nm -3 nm) that particle shape is not easily discerned (see ESI †, Fig. S6).Pt particles formed at pH 3.5 exhibit a variety of shapes, though the most common are multi-tetrahedral, star-like shapes and Pt multi-pods (see ESI †, Fig. S7). 46,47While such complex nanoparticle shapes are interesting, they possess a mixture of surface facets and are not desirable for shape-selective catalysis.
Diphenylphosphinoacetic acid (DPPA) and mercaptoacetic acid proved to have only limited usefulness as shape-directing agents.DPPA is soluble only at very high pH, which destabilizes Pt colloids, leading to particle precipitation.Mercaptoacetic acid apparently binds sufficiently strongly to Pt ion that NaBH 4 is required as reducing agent.The presence of DPPA produces faceted Pt nanoparticles as a mixture of truncated tetrahedral and cubo-octahedral shapes, while mercaptoacetic acid gives very small Pt particles (1 nm -2 nm) of irregular shape (see ESI †, Fig. S8).
Proline, serine, and phenylalanine are also poor shape-directing agents.Each of these amino acids produces 2 -5 nm Pt particles having a variety of spheroidal and oblong shapes (see ESI †, HR-TEM images reveal lattice fringe spacings of 0.20 nm for cubic Pt nanoparticles and of 0.23 nm for tetrahedral Pt nanoparticles consistent with {100} and {111} d-spacings reported by others for similarly shaped Pt nanoparticles. 48The expected {111} cross fringe pattern is also observed for tetrahedral Pt nanoparticles.Given that hydroxo and iodo ligands serve as highly selective shape-directing agents in H 2 reduction of Pt(IV) ion, knowing the kinetic stability of [Pt(OH) 6 ] 2 ‾ and [PtI 6 ] 2 ‾ precursor complexes in basic aqueous solutions and in the presence of H 2 becomes important.   These bands disappear with a first-order rate constant of ca. 15 x 10 -3 s -1 ( 1/2 = ca.46 s; determined from the rate of disappearance of the 490 nm band) with concomitant appearance of a single band centered at ca. 390 nm and a recognizable isosbestic point at ca. 375 nm.A previous kinetics study of [PtI 6 ] 2‾ ion hydrolysis over a pH range of 6-10.5 identified a first-stage iodide/water ligand exchange reaction forming [PtI 5 (H 2 O)] ‾ showing nearly identical spectral changes and a similar first-order rate constant of 16.5 x 10 -3 s -1 at pH 10 and 21.0 C. 50Given that [PtI 5 (H 2 O)] ‾ has a first-ionization pK a value of 8.55 at 25.5 C, 50 this complex would be 96% ionized at pH 10 giving [PtI 5 (OH)] 2‾ ion as the solution species assigned to the product absorption band centered at ca. 390 nm.

Roles of hydroxide and iodide ions in influencing Pt nanoparticle shape
Given that the adsorption enthalpy of hydroxide ion on a Pt(100) surface (ca.-280 kJ/mol), 51 is greater than that on a Pt(111) surface (ca.-255 kJ/mol), 52

Conclusions
Efficient methods of preparing cubic or tetrahedral Pt nanoparticles via solution reduction of common Pt(II) or Pt(IV) salts in the absence of polymeric surfactants have been found.
Hydrogen reduction of [Pt(OH) 6 ] 2 ‾ ion at basic pH gives high yields of cubic Pt nanoparticles, while hydrogen reduction of [PtI 6 ] 2 ‾ ion at basic pH gives high yields of tetrahedral Pt nanoparticles.Although minor shape effects are observed when Pt salts are reduced in the presence of simple carboxylate species, amino acids, or common inorganic ions, the exceptional shape effect of hydroxide ion and iodide ion is noteworthy.The presence of surface-bound hydroxide or iodide ions on Pt nanoparticle surface sites can be mitigated to promote chemical or electrochemical catalysis.Hydroxylated Pt metal surfaces are catalytically active toward methanol oxidation, 54 and CO displacement of chemisorbed iodine on pristine Pt metal surfaces is used in protocols for cleaning Pt metal surfaces. 55,56Although atomic-level understanding of how hydroxide and iodide direct Pt nanoparticle growth toward cubic or tetrahedral shapes, respectively, at basic pH has not been determined, it is hoped that shaped Pt metal nanoparticles prepared by this method prove to be effective catalysts under convenient operating conditions.

Fig. 1
Fig. 1 As-prepared Pt nanoparticles deposited from aqueous colloids.(Left) Cubic Pt nanoparticles prepared by H 2 reduction of [Pt(OH) 6 ] 2-at pH 9.7.Note the presence of a minority of tetrahedra and cubo-octahedra.(Right) Tetrahedral Pt nanoparticles prepared by H 2 reduction of [PtI 6 ] 2-at pH 10.Many truncated tetrahedra are present, but cubes are rare.

Fig. 2
Fig. 2 High-resolution TEM images of a typical cubic Pt nanoparticle (left image) and of a typical tetrahedral Pt nanoparticle (right image).Scale bars = 5 nm.

Fig. 4
Fig. 4 UV-Vis spectra collected over various reaction time intervals showing overall kinetic displacement of one iodo ligand of [PtI 6 ] 2 ‾ by hydroxide ion at pH 10 and 22 C giving [PtI 5 (OH)] 2‾ as product.

Fig. 5
Fig. 5 Three views, at different angles of TEM stage tilt, of a particularly large decahedron formed by reduction of [Pt(OH) 6 ] 2 ‾ with borohydride ion.
52rmation of cubic Pt nanoparticles might be expected from H 2 reduction of [Pt(OH) 6 ] 2 ‾ at basic pH.While hydroxide adsorption enthalpy on Pt(111) surfaces decreases with increasing surface coverage, this effect is not observed on Pt(100) surfaces, further supporting formation of Pt nanocubes at basic pH.51This surface coverage effect is attributed to differences in hydrogen bonding networks established among OH groups on these two surfaces and to the fact that a OH-H 2 O hydrogen bond is much stronger than a OH-OH hydrogen bond.Species that disrupt hydrogen bonding networks at Pt(100) surfaces could inhibit selective growth of that surface, possibly explaining the formation of P t cubo-octahedral and tetrahedral nanoparticle shapes when Pt-ion reduction occurs in the presence of simple anions or amino acid additives.Iodide ion binds more strongly to top and bridge sites of Pt(111) surfaces (bonding enthalpies of ca.-245 kJ/mol and -286 kJ/mol, respectively)53than to similar sites of Pt(100) surfaces (bonding enthalpies of ca.-225 kJ/mol and -265 kJ/mol, respectively)52.During H 2 reduction of [PtI 6 ] 2 ‾, iodide ions would presumably establish monolayer coverage on the surface of a growing nanoparticle, thus preventing adsorption of other species and enforcing retention of (111) surfaces to give Pt nanotetrahedra.