From Single Atoms to Nanocrystals: Photoreduction of [PtCl6]2– in

Nov 26, 2013 - Structured platinum nanoclusters Ptn (n = 5–30) capped by poly(N-vinylpyrrolidone) (PVP) have unique and highly attractive properties...
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From Single Atoms to Nanocrystals: Photoreduction of [PtCl6]2− in Aqueous and Tetrahydrofuran Solutions of PVP Yuri Borodko,† Peter Ercius,§ Danylo Zherebetskyy,† Yihai Wang,‡ Yintao Sun,‡ and Gabor Somorjai*,†,‡ †

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Chemistry, University of California, Berkeley, California 94720, United States § National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: Structured platinum nanoclusters Ptn (n = 5−30) capped by poly(N-vinylpyrrolidone) (PVP) have unique and highly attractive properties as potential selective catalysts. We show that the assembly of Pt mononuclear compounds in aqueous and tetrahydrofuran (THF) solutions under UV irradiation proceed via several steps: formation of linear Ptn clusters (n = 2− 8), coalescence into mesocrystals, and transformation into Pt nanocrystals. The “quantum” size range of Ptn (n = 5−100) clusters is intermediate between those clusters with molecular properties and those with metallic properties. The PVP “cage” acts as a nano reactor and can hinder diffusion of photoexcited Pt atoms. The diffusion of the Pt from the polymer cage is strongly affected by the hydrophobic or hydrophilic property of the solution. An aqueous solution of [PtCl6]2− + PVP transforms into noncrystalline aggregates of molecules of less than 1.5−2 nm in diameter, whereas in THF solution Pt nanocrystals increase proportional to the UV irradiation time up to 10 nm in diameter. Dynamic imaging by high-resolution transmission electron microscopy and low-frequency UV Raman spectra show the initial stages of Pt atoms assembled into Ptn clusters. The assignment of the Raman bands is supported by density functional theory calculations. The proposed scheme of photoinduced reactions suggests the coupling of coordinatively unsaturated Pt ions inside the amidate-rich polymeric stabilizer.

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INTRODUCTION Pt nanoparticles are promising materials for many practical applications such as potential new types of selective catalysts1 or anticancer drugs.2 The mechanism of growth of metal nanoparticles is an important issue. Pt clusters with a variety of structures and oxidation states have been synthesized, and the formation of covalent metal−metal bonds due to the overlap of metal d-orbitals has attracted considerable interest.3 Currently, great efforts are focused on development of synthetic methods and studies of chemical and physical properties of small metal clusters with 5−100 atoms. This “quantum size” range is intermediate between those clusters with molecular structure and those with metallic structure.4 A variety of methods have been used for the preparation of small Pt clusters, e.g., liquidphase synthesis, evaporation in inert gas or in vacuum, and nucleation of metal clusters by irradiation. Many questions about the photoinduced transformation of mononuclear Pt compounds into Pt nanocrystals in liquid phase remain unanswered; in particular, the structure and properties of intermediates on the way from single metal atoms to metallic nanocrystals. We show in this paper that the growth mechanism and resulting structures of Pt clusters in a liquid-phase photoinduced reaction strongly depends on conditions of the photoconversion of [PtCl6]2− inside macroligands, PVP, and PAMAM (poly amidoamine) dendrimers. The structural transformation of the type “disorder-to-order” from Pt © 2013 American Chemical Society

molecular aggregates to metallic nanocrystals occurs between 1.5 and 2 nm.5 PVP is a polydentate ligand with the amidate functional groups, which can stabilize coordinatively unsaturated photoreduced Pt compounds. Here we show by resonance Raman spectroscopy and atomic resolution TEM that UV light-induced conversion of platinum hexachloride in aqueous solutions of PVP leads to the formation of Pt clusters with molecular structure; however, in THF solutions of PVP it leads to metallic Pt nanocrystals. UV irradiation of [PtCl6]2− induces the reduction of Pt hexachloride and dehydrogenation of PVP. The change in balance of the hydrophobic/hydrophilic interaction between the solution and PVP is responsible for the change in mechanism of Pt nanoparticle growth. This is very different from phototransformation of [PtCl6]2− in pure aqueous solution shown in previous work.6 The proposed scheme of the photoinduced reaction suggests a coupling of coordinatively unsaturated Pt ions inside a “cage” of amidaterich polymeric stabilizer.

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EXPERIMENTAL SECTION Reactants and Chemicals. H2PtCl6·6H2O, Sigma-Aldrich; poly(N-vinylpyrrolidone) (PVP), M w ∼29 000, Aldrich; Received: October 7, 2013 Revised: November 25, 2013 Published: November 26, 2013 26667

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PAMAM, Mw = 14 215, 10.2 wt % methanol solution, Dendritech Inc.; tetrahydrofuran, Sigma-Aldrich; deuterium oxide (D2O), 99.9 atom % D, Aldrich; deionized water, 18.2 MΩ cm. Synthesis of Nanoparticles. PVP-stabilized Pt nanoparticles were synthesized in aqueous solution via UVirradiation at room temperature in an Ar atmosphere. In a typical synthesis, the Pt precursor H2PtCl6·6H2O and PVP were dissolved in pure water with concentrations of 10 mM and 100 mM, respectively, and mixed in the volume ratio of 1:1. The yellow solution mixture was stored in darkness for 24 h to allow for complexation and then was transferred into a 1 cm diameter quartz tube tightly sealed with a rubber stopper. The tube was purged with Ar and inserted into a Rayonet RPR-200 UV reactor. The reaction took place under 254 nm UV irradiation (∼18mW cm−2). Changes in the concentration ratio of [H2PtCl6]:[PVP] showed little effect on Pt particle growth; in contrast, changes in the UV intensity and solvents (H2O and THF) showed noticeable effects. To dissolve H2PtCl6 and PVP in THF solution, 5 vol % deionized H2O was added. The synthesis of Pt-PAMAM (G4OH) nanoparticles has been described recently elsewhere.5 Spectroscopy. UV−vis spectra were measured in transmission mode with a Perkin-Elmer LAMDA 650 spectrophotometer (190−900 nm). The Raman spectrometer was used with a continuous wave intracavity-doubled Ar ion laser operating at 244 nm and diode-pumped solid-state (DPSS) 532 nm lasers. Backscattered light was collected and directed into a fully automated spectrometer (iHR550, Horiba) and was optimized with bandpass and edge filters. Spectra were recorded with an open electrode CCD detector Synapse Horiba (1024 × 256 pixels) that was thermoelectrically cooled to −70 °C. The samples were studied as solid drop-cast films on Al foil and as a liquid in porcelain and Teflon containers. To eliminate decomposition of Pt-PVP samples under UV irradiation, we used a rotating sample holder.7 X-ray photoelectron spectra (XPS) were taken on a PHI 5400 ESCA/XPS system equipped with an Al anode X-ray source. TEM Characterization. Low-resolution transmission electron microscopy (TEM) images were obtained with a JEOL JEM-2100 operated at 200 kV accelerating voltage. The samples were drop-cast from an aqueous solution on Cu grids with an ultrathin (3−5 nm thick) carbon film. TEM and scanning TEM (STEM) images of Pt-G4OH and Pt-PVP nanoparticles with atomic resolution were acquired at the National Center for Electron Microscopy (NCEM) using TEAM I, an FEI Titan 80−300 S/TEM aberration corrected for spherical and chromatic aberrations. In TEM mode, TEAM I was operated with an 80 kV accelerating voltage to minimize beam damage and with third-order spherical aberration C3 = −10 μm for “negative Cs” imaging conditions. Thus, atoms appear bright in positive defocus (overfocus) in each highresolution TEM (HR-TEM) image shown. TEM images and time series were acquired with a Gatan US1000 CCD camera with a readout rate of approximately 0.5 s for 1024 × 1024 image pixels5 and 0.21 s for 512 × 512 image pixels. Low-angle annular dark field (LAADF) STEM images using TEAM I were acquired with a convergence semiangle of 30 mrad (to increase signal for single atoms), 70 pA of beam current, 80 kV accelerating voltage, and an inner detector semiangle of 30 mrad. The full width at half-maximum (fwhm) of the focused electron probe is ∼0.1 nm and is capable of imaging single Pt atoms. Higher intensity in STEM images indicates regions of

higher density, such as Pt atoms. HR-STEM is used to assess particle size and the existence of single Pt atoms, and HR-TEM is used to observe the dynamics and structure of Pt nanoparticles. To minimize background noise in the aberration-corrected TEM and STEM images, the samples were dropcast on suspended graphene membranes prepared by direct polymer-free transfer onto Quantifoil TEM grids. Computational Method. DFT computations were carried out with the Gaussian 09 computational package.8 Geometry optimizations of neutral molecules without symmetry constrains were performed at the DFT level employing the Perdew−Burke−Ernzerhof exchange−correlation functional9 and the LANL2DZ basis sets10 (more details in Supporting Information).

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RESULTS AND DISCUSSION Small Ptn Clusters in Aqueous and THF Solution. TEM and STEM were used to measure the size and structural evolution of Pt aggregates stabilized by PVP in aqueous and THF solutions. The samples were extracted from a larger batch after a specified time of exposure to UV irradiation. Figure 1A− E shows TEM images of typical PVP-capped Pt nanoparticles produced in aqueous solution after UV irradiation for 0, 30, 60, 90, and 240 min, respectively.

Figure 1. TEM images of PVP-capped Pt nanoparticles A−E synthesized under UV irradiation (0, 30, 60, 90, and 240 min) in aqueous solution. All scale bars are 20 nm. (B*) Low-angle annular dark field (LAADF) STEM images of Pt-PVP structures on graphene support. Sample was irradiated for 30 min in aqueous solution. The white dots are single Pt atoms, and some aggregates are visible. (B**) Selected region of a HR-TEM image of Pt-PVP aggregate on a graphene support. The Pt−Pt group interatomic distance is about 0.32 nm.

No noticeable features were observed of the initial material (labeled 0 min) with the JEOL JEM-2100 microscope (Figure 1A). The sample is expected to contain only dispersed single Pt ions, and the JEM-2100 lacks the resolution and contrast to resolve these features. After 30 min of UV exposure, colloidal particles with an average diameter of 1.2 nm were observed (Figure 1B). During 60 min of UV irradiation, the Pt nanoparticles increase to ∼1.4 nm in diameter, and some quasi-linear aggregates form (Figure 1C). Particles ∼2.5 nm in 26668

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material irradiated for 30 min shows strong diffraction contrast from several particles in the frame as indicated by their very dark contrast. This type of contrast occurs when the crystalline planes are aligned with the electron beam. HR-TEM images in Figure 2B*,C* and HR-STEM images in Figure 2B**,C** show material irradiated for the same time as the corresponding low-resolution TEM images. They clearly show that in a THF/ H20 (20:1) solution Pt-PVP under UV irradiation grew to Pt nanocrystals with well-ordered crystalline structure. The kinetics of Pt nanoparticle growth in aqueous and THF solution differ substantially. In aqueous solution, after 10 min irradiation Pt-PVP forms aggregates near 1.5 ± 0.3 nm in diameter and this changes little for 60 min (Figure 3A). After 60 min of irradiation these aggregates spontaneously form big spherical-like particles (Figure 1E). Such behavior is probable for micelle-like aggregates when the interaction between the hydrophobic part of the capping PVP and H2O changes because of dehydrogenation of PVP under UV light. The hydrophobic effect is the driving force for micelle size increase.11 In a mixture of THF/H2O (20:1), the nanoparticle size increases proportional to the irradiation time (Figure 3A), and the resulting Pt nanoparticles are well-formed crystals (Figure 2). We speculate that in THF/H2O solution the amphiphilic PVP, where pyrrolidone rings form hydrophilic domains and the polyvinyl backbone hydrophobic domain, is an inverse cationic micelle with Pt cations solvated by H2O molecules located in the center of an inverse micelle and hydrocarbon chain extending out. Such a “nano reactor” can hinder the diffusion of photoreduced Pt atoms out of the cage. The UV− vis spectroscopic studies of nanoparticles grown in an aqueous solution of H2PtCl6−PVP show that the strong absorption in the UV region (charge-transfer bands) decreased, whereas new weak d−d bands appeared in the near visible region at 390 and 480 nm (Figure 4A,D). Two isosbestic points at 290 and 276 nm are clearly visible and demonstrate consecutive transformation of [PtCl6]2− in aqueous solution (Figure 4A,B). The spectrum at 120 min irradiation shows a strong Raleigh scattering due to the formation of nanoparticles (Figure 4C). Dynamic Structural Transformation of Ptn Clusters. In Figure 5, select images of time series of HR-TEM images acquired with TEAM I show the behavior of Pt-PAMAM (Figure 5A) and Pt-PVP (Figure 5B) clusters under electron beam irradiation (all frames are available in movies provided as Supporting Information). Although electrons interact more strongly with the clusters, this radiation is thought to create “hot atom” states similar to UV light. Both PVP (55 000 amu) and PAMAM (14 200 amu) stabilize small Ptn (n = 2−8) clusters with linear −Pt−Pt− chains. These small Pt clusters are the building blocks for mesocrystals formed during photoinduced reactions of Pt-PVP and Pt-PAMAM in solution. Recently it was shown that PtPAMAM nanocrystals can form when the size of Pt aggregates is approximately 1.5 nm.5 Our tentative estimate is that the Pt− Pt distances in Pt-PVP aggregates in aqueous solution are about 0.32 nm, which is larger than the typical covalent Pt−Pt interatomic distances 0.25−0.28 nm and close to the noncovalent Pt−Pt distance in hydroxo-bridged-bis platinum Pt(OH)2Pt.12 Carbon monoxide was utilized to probe the electronic state of Pt ions. After 10 min of irradiation of [PtCl6]2− + PVP in an aqueous solution, CO was bubbled through the solution. Then the solution was cast on Al foil and

diameter appeared after 90 min of UV irradiation as well as some linear aggregates (Figure 1D). Under continuous UV irradiation in aqueous solution for 240 min, the smaller Pt nanoparticles coalesce into large 60−80 nm spherical-like aggregates (Figure 1E), which disintegrate into 1.6 nm clusters after sonication (Figure 1F). This demonstrates that the large aggregates are composed of the smaller nanoparticles loosely bonded together. Aberration-corrected imaging in TEAM I provided information about the initial stages of the growth of Pt-PVP clusters. This instrument provides sub-angstrom pointto-point resolution with sufficient contrast to resolve single Pt atoms in both TEM and STEM modes. The solution was dropcast on a graphene film for maximum contrast. Figure 1B* shows a high-resolution STEM image of the unirradiated PtPVP from aqueous solution. The smallest white dots scattered across the field-of-view are single Pt atoms, and some agglomerates of Pt atoms are also evident. Small mesocrystals (C O−Pt stretch of PVP + [PtCl6]2− in the region of 500−5500 cm−1. (C) Resonance Raman overtones of Pt−Cl of [PtCl6]2−+ PVP in D2O solution exposed to UV irradiation of 254 nm, 20 mW cm−2. (D) Effect of irradiation time on band intensities of Pt−Cl at 334 cm−1 (1) and on OH band at 3430 cm−1 (2).

Supporting Information). An overview of the predicted Raman spectra of Pt2Cln clusters is depicted in Figure 9A. Linear configurations of Pt clusters are energetically less favorable; however, polymeric bridging ligands can stabilize linear Ptn structures.21 An increase in the number of Cl atoms decreases the electron density of Pt−Pt bonds, making Pt−Pt bonds longer and lowering the frequency of the Pt−Pt vibrations (Figure 9A). Raman intensities of Pt−Pt bands are weaker than those of Pt−Cl bands which are mixed with Pt−Pt modes (Supporting Information). To analyze Raman spectrum of Pt-PVP in aqueous solution (Figure 8B) we calculated the spectrum of the model compound: bridging [di-μ-hydroxo-bis platinum] (Figure 9B). The calculated spectrum depicted in Figure 9B shows ν(OH) at 3601, δ(PtOH) at 981, ν(Pt−Cl) at 366 and 342 cm−1, and characteristic bands of Pt(OH)2Pt bridging dimer at 173, 453, and 557 cm−1 (Supporting

Scheme 3

Scheme 4

assign these new bands to specific vibrational modes. Raman spectra were evaluated for naked Ptn clusters (n = 2−6) and binuclear Pt clusters in different oxidation states (see

Figure 8. (A) UV resonance Raman (244 nm) spectra of [PtCl6]2−-PAMAM after 60 min irradiation. (B) Raman (532 nm) spectrum of [PtCl6]2−PVP in aqueous solution after irradiation for 90 min by 254 nm light. 26672

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Scheme 5

platinum disorder aggregates formed with less than 2 nm diameter.

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ASSOCIATED CONTENT

S Supporting Information *

Movies showing the dynamics of Pt cluster transformations at atomic resolution (Movie S1-A (Pt-PAMAM) was acquired with 1 s of acquisition time and 0.5 s of read-out time for a total of 1.5 s between frames; Movie S1-B (Pt-PVP) was acquired with 0.3 s of acquisition time and 0.21 s of read-out time for a total of 0.51 s per frame; these movies are the same data used in Figure 5); animations of the DFT results; vibrational modes of Pt(OH)2Pt. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. (A) Overview of Raman spectra of Pt2Cln clusters. (B) Computed Raman spectrum of binuclear PtCl2(OH)2PtCl2 complex and the shape of vibrational modes of tetranuclear ring.

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Information). The 173 cm−1 band is assigned to symmetrical vibration of a Pt···Pt group with an interatomic distance of 0.3227 nm, whereas the bands at 453 and 557 cm−1 are attributed to O−O vibrations (all shapes of vibrational modes are available in movies in Supporting Information). These unscaled predicted vibrational frequenciess of Pt(OH)2Pt are close to those found experimentally for [PtCl6]2−-PVP in aqueous solution after irradiation (Figure 8B), and this supports the formation of bridging hydroxo platinum compounds. UV-Light-Induced Pt Ion Aggregation. From the spectroscopic results and dynamic observations of Pt cluster formation by HR-TEM, we can speculate about the likely steps of Pt cluster formation under UV irradiation. As shown in Scheme 1, three coupling reactions could occur during UV irradiation of [PtCl6]2PVP. Our spectroscopic data show the formation of Pt cations with ligated carbonyl, hydroxyl, and chloride ligands in the first coordination sphere of Pt. In the first step, UV light induces the electron transfer of the n → d* type from carbonyl ligands to Pt which lead to reduction of Pt(IV) cations to Pt(III). The short-lived, reduced Pt atoms are loosely bound to PVP and are very mobile22,23 (Supporting Information). Then, the Pt atoms can either reoxidize and return to their background state or alternatively couple with neighbors to form Pt dimers before reoxidizing via back metalto-ligand electron transfer (Scheme 5). Several of the PVP carbonyl groups can act as bridging ligands in polynuclear Pt compounds, providing strong chelate stabilization. The formation of small Ptn (n = 2−8) clusters with a linear structure is clearly visible in HR-TEM images in Figures 1B** and 5 and in Supporting Information. Photoinduced conversion of [PtCl6]2− + PVP strongly depends on the hydrophilic/hydrophobic properties of the solvent. In an aprotic medium such as THF, well-shaped Pt nanocrystals (1.5−10 nm) formed, while in an aqueous solution hydroxo

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/fax: (510) 642-4053. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, Material and Engineering Divisions of the U.S. Department of Energy under Contract DE-AC02-05CH11231. The National Center for Electron Microscopy at Lawrence Berkeley National Laboratory is supported by the U.S. Department of Energy under Contract DE-AC02-05CH11231. This research used resources of the National Energy Research Scientific Computing Center (NERSC) supported by the Office of Science of the U.S. Department of Energy. Y.W. appreciates support from the Basic Research Program of Young Scientists by the National Natural Science Foundation of China and Chinese University of Hong Kong.

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