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Chemical Reactivity of Naphthalenecarboxylate-Protected Article pubs.acs.org/JPCC Chemical Reactivity of Naphthalenecarboxylate-Protected Ruthenium Nanoparticles: Intraparticle Charge Delocalization Derived from Interfacial Decarboxylation † † † ‡ ‡ † ‡ Limei Chen, Peiguang Hu, Christopher P. Deming, Wei Li, Ligui Li, and Shaowei Chen*, , † Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California 95064, United States ‡ New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou 510006, China ABSTRACT: Ruthenium nanoparticles were prepared by thermolytic reduction of RuCl3 in 1,2-propanediol containing sodium 2-naphthalenecarboxylate. Transmission electron microscopic measurements showed that the average diameter of the resulting 2- naphthalenecarboxylate-protected ruthenium nanoparticles (RuCOONA) was 1.30 ± 0.27 nm. Interestingly, hydrothermal treatment of the nanoparticles at controlled temperatures led to decarboxylation at the metal−ligand interface, and the naphthalenyl moieties became directly bonded to the metal cores, which was confirmed by infrared and X-ray photoelectron spectroscopic measurements. In comparison with the as-produced RuCOONA nanoparticles, the decarboxylated nanoparticles (RuNA) exhibited markedly different optical and electronic properties, as manifested by an apparent red shift of the photoluminescence profiles, which was ascribed to electronic coupling between the particle-bound naphthalene groups. Electrochemical measurements exhibited consistent results where a negative shift was observed of the formal potential of the particle-bound naphthalene moieties. This was attributed to intraparticle charge delocalization that led to extended spilling of nanoparticle core electrons to the naphthalene moieties. ■ INTRODUCTION Conjugated metal−ligand interfacial bonds may also be produced by exploiting the unique interfacial reactivity of Organically capped metal nanoparticles have been attracting organic ligands on nanoparticle surfaces. For instance, we extensive interest because the material properties may be recently observed that alkene derivatives might self-assemble readily controlled by the chemical nature of the metal cores and onto platinum nanoparticle surfaces, forming platinum−vinyl- the organic protecting ligands as well as their interfacial − idene or −acetylide bonds as a consequence of platinum- bonding interactions.1 5 In fact, the bonding linkages at the fi − catalyzed dehydrogenation and transformation of the ole n metal ligand interface have been observed to impact the groups.14 In another study,15 ruthenium nanoparticles were nanoparticle dimensions, morphology, and stability; inversely, protected by ferrocenecarboxylates, and galvanic exchange fl metal nanoparticles also have in uence on the ligand physical reactions with Pd(II) led to the deposition of a small amount 4,6,7 and chemical property, reactivity, and configuration. Of of Pd onto the nanoparticle surface, which catalyzed the these, ruthenium nanoparticles have been intensively studied decarboxylation of the organic capping ligands under hydro- due to its stability and affinity to many organic capping ligands, thermal conditions, such that the electrochemical profiles of the such as mercapto derivatives, alkylamines, diazo, acetylene, resulting nanoparticles were similar to those of biferrocene − nitrene, and carboxylate moieties.4,7 11 For instance, for derivatives. ruthenium nanoparticles functionalized by acetylene derivatives, Results from these studies offer a new, effective protocol for the formation of conjugated metal−ligand π-bonds leads to the interfacial functionalization and engineering of transition- intraparticle charge delocalization, and hence, the nanoparticles metal nanoparticles. In the present study, sodium naphthale- exhibit new optical/electronic characteristics that are analogous 11−13 to those of diacetylene derivatives. These results highlight Received: May 5, 2015 the significance of interfacial engineering in the manipulation of Revised: June 3, 2015 nanoparticle materials properties. Published: June 11, 2015 © 2015 American Chemical Society 15449 DOI: 10.1021/acs.jpcc.5b04312 J. Phys. Chem. C 2015, 119, 15449−15454 The Journal of Physical Chemistry C Article necarboxylate was used as a new protecting ligand to resolution 4 cm−1), with the nanoparticles deposited onto a functionalize ruthenium nanoparticles where Ru−O bonds ZnSe disk. PL measurements were performed with a PTI were formed at the metal−ligand interface;7 subsequent fluorospectrometer. UV−vis absorption spectra were acquired hydrothermal treatments at elevated temperatures effectively with an ATI Unicam UV4 spectrometer using a 10 mm quartz removed the carboxylate moieties even without the incorpo- cuvette at a resolution of 2 nm. X-ray photoelectron spectra ration of Pd catalysts, such that the naphthalene (NA) moieties (XPS) were recorded with a PHI 5400 XPS instrument −9 were now attached to the metal core by the Ru−C bonds. Note equipped with an Al Kα source operated at 350 W and at 10 that the bond strengths of Ru−O and Ru−C are actually quite Torr. Silicon wafers were sputtered by argon ions to remove similar, with the former16 around 460 cm−1 and the latter17 carbon from the background and used as substrates. The between 470 and 500 cm−1 as determined by Raman spectra were charge-referenced to the Si 2p peak (99.03 eV). spectroscopic measurements. In fact, experimentally, it was Electrochemistry. Electrochemical measurements were found that hydrothermal treatment did not compromise the carried out with a CHI 440 electrochemical workstation. A nanoparticle stability. Nevertheless, a drastic deviation was polycrystalline gold disk electrode (sealed in glass tubing) was observed of the optical and electronic properties of the used as the working electrode (surface area 0.40 mm2). A Ag/ resulting nanoparticles as compared to those of the as-produced AgCl wire and a Pt coil were used as the (quasi)reference and nanoparticles, as evidenced by a red shift of the photo- counter electrodes, respectively. Prior to use, the gold electrode luminescence (PL) emission and a negative shift of the formal was polished with 0.05 μm alumina slurries and then cleansed potential of NA groups on the nanoparticle surface. This was by sonication in Nanopure water. Note that the potentials were ascribed to effective electronic coupling between the particle- all calibrated against the formal potential of ferrocene bound NA groups where nanoparticle core electrons might spill monomers (Fc+/Fc) in the same electrolyte solution. into the organic capping ligands. ■ RESULTS AND DISCUSSION ■ EXPERIMENTAL SECTION The size and morphology of the RuCOONA nanoparticles Chemicals. Sodium hydroxide (NaOH, extra pure, Acros), were first examined by TEM measurements. From the fi sodium bicarbonate (NaHCO3, +99%, Fisher Scienti c), representative TEM image in Figure 1, one can see good − ruthenium chloride (RuCl3,35 40%Ru, Acros), naphthalene (NA, ultrapure, National Diagnostics), 2-naphthoic acid (NAA, 99%, Acros), and 1,2-propanediol (ACROS) were used as received. Solvents were purchased at their highest purity and used without further treatment. Water was supplied by a Barnstead Nanopure water system (18.3 MΩ·cm). Synthesis of Naphthalenecarboxylate-Protected Ruthenium (RuCOONA) Nanoparticles. The synthetic procedure was similar to that used previously for the preparation of acetate-stabilized Ru particles.15 In brief, 0.6 mmol of NAA, 0.6 mmol of NaOH, and 0.1 mmol of RuCl3 were dissolved in 1,2-propanediol (100 mL), and the solution was heated at 175 °C for 2 h under magnetic stirring. A rapid change of solution color was observed from dark orange to dark brown, signifying the formation of Ru nanoparticles. The solution was then cooled down to room temperature and purified by dialysis in Nanopure water for 3 days. Rotary evaporation of the resulting solution produced a solid, which was then rinsed with a copious amount of methanol to remove excessive free ligands and impurities, affording RuCOONA nanoparticles. Decarboxylation of RuCOONA Nanoparticles. A Figure 1. Representative TEM micrograph of RuCOONA nano- calculated amount of the RuCOONA nanoparticles obtained particles. Scale bar = 20 nm. The lower inset is a high-resolution image above was then dispersed in a mixed solvent of DMF/water of a nanoparticle (scale bar = 1 nm), and the upper inset is the particle (7:1 v:v) and transferred into a polyphenyl (PPL)-lined core size histogram. autoclave. The sealed autoclave was put in an oven and heated at 250 °C for 14 h. A precipitate was produced at the bottom of the autoclave that was collected and rinsed with methanol five times. The purified nanoparticles were denoted as RuNA, dispersion of the nanoparticles with no obvious agglomeration, which now became readily dispersible in common organic signifying effective stabilization of the nanoparticles by the − media, for example, CH2Cl2, N,N-dimethylformamide (DMF), naphthalenecarboxylate ligands due to the formation of Ru O and tetrahydrofuran (THF). bonds at the metal−ligand interface.7 High-resolution TEM Characterizations. TEM measurements were carried out measurements (lower inset) show well-defined lattice fringes of with a JEOL-F 200 kV field-emission analytical transmission the nanoparticle cores at an interplanar spacing of 0.23 nm, in electron microscope. In sample preparation, a drop of the good agreement with that of Ru(100) crystalline
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