Xiongwu Kang

Butylphenyl-functionalized palladium nanoparticles as effective catalysts for the electrooxidation of formic acid

Monodisperse butylphenyl-functionalized palladium (Pd-BP, dia. 2.24 nm) nanoparticles were synthesized through co-reduction of butylphenyldiazonium and H(2)PdCl(4) by NaBH(4). Because of this unique surface functionalization and a high specific electrochemical surface area (122 m(2) g(-1)), the Pd-BP nanoparticles exhibited a mass activity ∼4.5 times that of commercial Pd black for HCOOH electrooxidation.

Alkyne-Stabilized Ruthenium Nanoparticles: Manipulation of Intraparticle Charge Delocalization by Nanoparticle Charge States†

Monolayer-protected transition metal nanoparticles are a unique family of functional nanomaterials in which the properties of the materials can be readily manipulated not only by the chemical nature of the metal cores and the organic protecting ligands, but also the metal–ligand interfacial bonding interactions. The latter is largely motivated by recent progress in nanoparticle passivation by metal–carbon covalent bonds, where intraparticle charge delocalization may occur as a result of the strong metal–carbon interfacial bonding interactions, in sharp contrast to nanoparticles that are functionalized by mercapto derivatives. For instance, when ferrocene moieties are bound onto a ruthenium nanoparticle surface by ruthenium–carbene p bonds, effective intervalence transfer occurs between the ferrocenyl metal centers at mixed valence, as manifested in electrochemical and near-infrared (NIR) spectroscopic measurements and density functional calculations. Furthermore, when fluorophores are attached onto the nanoparticle surface by the same conjugated linkage, novel emission characteristics emerge that are consistent with those of dimeric derivatives with a conjugated spacer. In a more recent study, effective intraparticle charge delocalization was also observed with ruthenium nanoparticles passivated by alkynyl fragments. This result was ascribed to the unique interfacial bonding interactions (Ru C ) formed by ruthenium and sphybridized carbon atoms of the ligands. In these studies, the nanoparticle metal cores serve as the conducting media to facilitate charge transfer between the functional moieties covalently bound onto the nanoparticle surface. Therefore it is anticipated that the extent of intraparticle conjugation may be readily controlled by the nanoparticle charge state, which is the primary motivation of the present study. Experimentally, by exploiting the molecular capacitor characters of Ru C nanoparticles, the charge states of the nanoparticles were varied by simple chemical reduction or oxidation. The impacts of the nanoparticle charge states on the particle optical and electronic properties were then carefully examined by FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence measurements, and compared to those of the as-prepared nanoparticles. The synthetic procedure for the preparation of ruthenium nanoparticles passivated by 1-octynyl fragments (Ru-OC) has been detailed previously. TEM measurements showed that the nanoparticles exhibited an average core diameter of (2.55 0.15) nm. The nanoparticle charge states were then varied by chemical redox reactions. Specifically, to render the nanoparticles negatively charged, in a typical reaction, 5 mg of Ru-OC nanoparticles were dissolved in dichloromethane (1 mL); a freshly prepared water solution of NaBH4 (1 mL, 5 mgmL ) was then added. The mixture was stirred for 30 min and then water was removed. The resulting nanoparticles exhibited negative net charges and were denoted as Ru-OCRed. Positively charged nanoparticles were prepared in a similar fashion by mixing the nanoparticle solution with an aqueous solution of saturated Ce(SO4)2 for 30 min. The resulting nanoparticles were denoted as Ru-OCOx. Transition metal nanoparticles passivated with a lowdielectric organic protecting layer have long been known to act as nanoscale molecular capacitors. In fact, based on a concentric structural model, the nanoparticle capacitance (CMPC) can be estimated by CMPC= 4pe0e(r+d) r d, where e0 is the vacuum permittivity, e is the effective dielectric constant of the organic protecting layer, r is the radius of the metal core, and d is the length of the organic protecting ligand. For the octyne-passivated ruthenium (Ru-OC) nanoparticles, r= 1.275 nm, d= 0.848 nm (estimated by Hyperchem), and e= 2.6. Thus, the nanoparticle capacitance can be estimated to be about 0.92 aF. To quantify the change of the nanoparticle charge state after reduction or oxidation, we measured the open circuit potentials of the nanoparticles electrochemically. It was found that the as-prepared Ru-OC nanoparticles exhibited an open circuit potential of + 0.140 V (versus Ag/AgCl). After reduction by NaBH4, it decreased to + 0.024 V, whereas after oxidation by ceric sulfate, it increased to + 0.250 V. This result indicated that the reduced nanoparticles (Ru-OCRed) exhibited an average charging of 0.67 electrons per nanoparticle, whereas the oxidized nanoparticles (Ru-OCOx) were formed by an average discharging of 0.63 electrons per nanoparticle. Interestingly, despite these subtle changes of nanoparticle charge states, rather drastic impacts were observed on the nanoparticle optoelectronic properties. Figure 1 depicts the FTIR spectra of the nanoparticles before and after reduction or oxidation. For the as-prepared Ru-OC nanoparticles, the C C stretching band appeared at 1965 cm 1 (inset). In comparison to octyne monomers, for which the C C stretch[*] X. W. Kang, N. B. Zuckerman, Prof. J. P. Konopelski, Prof. S. W. Chen Department of Chemistry and Biochemistry, University of California 1156 High Street, Santa Cruz, CA 95064 (USA) Fax: (+1)831-459-2935 E-mail: shaowei@ucsc.edu Homepage: http://chemistry.ucsc.edu/~ schen

Controlled Assembly of Janus Nanoparticles

Janus nanoparticles were prepared by interfacial ligand exchange reactions of octanethiolate-protected gold (AuC8) nanoparticles with 3-mercapto-1,2-propanediol (MPD) at the air/water interface. AFM and TEM measurements showed that the resulting particles formed stable aggregates in water with dimensions up to a few hundred nanometers, in sharp contrast to the original AuC8 particles and bulk-exchange counterparts where the aggregates were markedly smaller. Consistent behaviors were observed in dynamic light scattering measurements. FTIR measurements of solid films of the nanoparticles suggested that the octanethiolate ligands were mostly of trans conformation, whereas the MPD ligands exhibited gauche defects as a consequence of the hydrogen-bonding interactions between the hydroxyl moieties of adjacent ligands. Raman spectroscopic measurements in an aqueous solution of pyridine showed that the pyridine ring breathing modes remained practically unchanged and the intensity profiles indicated minimal interactions between pyridine and the gold cores within the three nanoparticle ensembles. However, water bending vibrational features were found to be enhanced substantially with the addition of Janus nanoparticles, which was ascribed to the formation of clusters of water molecules that were trapped within the nanoparticle ensembles. No apparent enhancement was observed with the AuC8 or bulk-exchange particles.

Electronic conductivity of alkyne-capped ruthenium nanoparticles

Ruthenium nanoparticles (2.12 ± 0.72 nm in diameter) were stabilized by the self-assembly of alkyne molecules (from 1-hexyne to 1-hexadecyne) onto the Ru surface by virtue of the formation of Ru-vinylidene interfacial linkages. Infrared measurements depicted three vibrational bands at 2050 cm(-1), 1980 cm(-1) and 1950 cm(-1), which were ascribed to the vibrational stretches of the terminal triple bonds that were bound onto the nanoparticle surface. Thermogravimetric analysis showed that there were about 65 to 96 alkyne ligands per nanoparticle (depending on the ligand chainlength), corresponding to a molecular footprint of 20 to 15 Å(2). This suggests that the ligands likely adopted a head-on configuration on the nanoparticle surface, consistent with a vinylidene bonding linkage due to interfacial tautomeric rearrangements. With this conjugated interfacial bonding interaction, electronic conductivity measurements of the corresponding nanoparticle solid films showed that the nanoparticles all exhibited linear current-potential curves within the potential range of -0.8 V to +0.8 V at varied temperatures (200 to 300 K). The ohmic characters were partly ascribed to the spilling of core electrons into the organic capping layer that facilitated interparticle charge transfer. Furthermore, based on the temperature dependence of the nanoparticle electronic conductivity, the activation energy for interparticle charge transfer was estimated to be in the range of 70 to 90 meV and significantly, the coupling coefficient (β) was found to be 0.31 Å(-1) for nanoparticles stabilized by short-chain alkynes (1-hexyne, 1-octyne, and 1-decyne), and 1.44 Å(-1) for those with long alkynes such as 1-dodecyne, 1-tetradecyne, and 1-hexadecyne. This may be accounted for by the relative contributions of the conjugated metal-ligand interfacial bonding interactions versus the saturated aliphatic backbones of the alkyne ligands to the control of interparticle charge transfer.

Nitrene-functionalized ruthenium nanoparticles

Ruthenium nanoparticles protected by ruthenium–nitrene π bonds were prepared by refluxing “bare” ruthenium colloids (2.12 ± 0.72 nm in diameter) and 4-dodecylbenezenesulfonyl azide in sec-butylbenzene. Thermogravimetric analysis (TGA) of the resulting nanoparticles showed that on average there were about 84.1 ligands on the nanoparticle surface. XPS studies showed a 1:1 atomic ratio between nitrogen and sulfur, consistent with the formation of nitrene fragments by the thermal decomposition of the azide precursors. In addition, the binding energies of Ru3d and N1s electrons suggested a covalent nature of the RuN interfacial linkage which appeared in FTIR measurements with a vibrational band at 1246 cm−1. Because of such conjugated bonding interactions, extensive intraparticle charge delocalization occurred, and the nanoparticle-bound nitrene moieties behaved analogously to azo derivatives, as manifested in UV-vis and fluorescence measurements. Further testimony of the formation of RuN interfacial linkages was highlighted in the unique reactivity of the nanoparticles with alkenes by imido transfer, which was evidenced in spectroscopic and electrochemical studies.

Intraparticle charge delocalization of carbene-functionalized ruthenium nanoparticles manipulated by selective ion binding.

Olefin metathesis reactions of carbene-stabilized ruthenium nanoparticles were exploited for the incorporation of multiple functional moieties onto the nanoparticle surface. When the nanoparticles were cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-vinylpyrene, the resulting particles exhibited fluorescence characteristics that were consistent with dimeric pyrene with a conjugated chemical bridge, with three peaks observed in the emission spectra at 391, 410, and 485 nm. The behaviors were ascribed to intraparticle charge delocalization between the pyrene moieties afforded by the conjugated Ru═carbene interfacial linkages. Notably, upon the binding of metal ions in the crown ether cavity, the emission intensity of the nanoparticle fluorescence was found to diminish at 485 nm and concurrently increase at 391 and 410 nm rather markedly, with the most significant effects observed with K(+). This was accounted for by the selective binding of 18-crown-6 to potassium ions, where the positively charged ions led to the polarization of the nanoparticle core electrons that was facililated by the conjugated linkage to the metal surface and hence impeded intraparticle charge delocalization. Control experiments with a pyrene-crown ether conjugate (2) and with ruthenium nanoparticles cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-allylpyrene suggested that the through-bond pathway played a predominant role in the manipulation of intraparticle electronic communication whereas the contributions from simple electrostatic interactions (i.e., through-space pathway) were minimal.

Graphene Composites with Cobalt Sulfide: Efficient Trifunctional Electrocatalysts for Oxygen Reversible Catalysis and Hydrogen Production in the Same Electrolyte

Nitrogen and sulfur-codoped graphene composites with Co9 S8 (NS/rGO-Co) are synthesized by facile thermal annealing of graphene oxides with cobalt nitrate and thiourea in an ammonium atmosphere. Significantly, in 0.1 m KOH aqueous solution the best sample exhibits an oxygen evolution reaction (OER) activity that is superior to that of benchmark RuO2 catalysts, an oxygen reduction reaction (ORR) activity that is comparable to that of commercial Pt/C, and an overpotential of only -0.193 V to reach 10 mA cm-2 for hydrogen evolution reaction (HER). With this single catalyst for oxygen reversible electrocatalysis, a potential difference of only 0.700 V is observed in 0.1 m KOH solution between the half-wave potential in ORR and the potential to reach 10 mA cm-2 in OER; in addition, an overpotential of only 450 mV is needed to reach 10 mA cm-2 for full water splitting in the same electrolyte. The present trifunctional catalytic activities are markedly better than leading results reported in recent literature, where the remarkable trifunctional activity is attributed to the synergetic effects between N,S-codoped rGO, and Co9 S8 nanoparticles. These results highlight the significance of deliberate structural engineering in the preparation of multifunctional electrocatalysts for versatile electrochemical reactions.

Manipulation of intraparticle charge delocalization by selective complexation of transition-metal ions with histidine moieties.

Ruthenium nanoparticles were cofunctionalized with pyrene and histidine moieties through Ru═carbene π bonds. The selective complexation of the histidine moiety with transition-metal ions led to a marked diminishment of the emission peak at 490 nm which arose from the nanoparticle-bridged pyrene moieties that behaved analogously to pyrene dimers with a conjugated spacer. This is accounted for by the polarization of the core electrons by the added positive charge that impacted the intraparticle charge delocalization between the particle-bound pyrene moieties. This electronic interaction was likely facilitated by the π interactions between the metal ions and the imidazole ring as well as by the conjugated molecular backbone that linked the imidazole ring to the nanoparticle cores. Within the present experimental context, of all the metal ions tested, the impacts were much more drastic with Pb(2+), Co(2+), and Hg(2+) than with Li(+), K(+), Rb(+), Mg(2+), Ca(2+), and Zn(2+) ions, with the most sensitive variation observed with Pb(2+). This is ascribed to the enhanced π interactions of the histidine moiety with the Pb(2+), Co(2+), and Hg(2+) ions because of their capability of donating d electrons, a behavior consistent with prior studies based on conventional histidine-metal ion complexes.

Nanoparticle-Mediated Intervalence Charge Transfer: Core-Size Effects

Two types of platinum nanoparticles (NPs) functionalized with ethynylferrocene were prepared. The subnanometer-sized NPs (Pt10eFc) showed semiconductor-like characteristics with a bandgap of about 1.0 eV, and the other was metal-like with a core size of about 2 nm (Pt314eFc) and no significant bandgap. IR spectroscopic measurements showed a clear red-shift of the C≡C and ferrocenyl ring =C-H vibrational energies with increasing particle core size owing to enhanced intraparticle charge delocalization between the particle-bound ferrocenyl moieties. Electrochemical measurements showed two pairs of voltammetric peaks owing to intervalence charge transfer between the ferrocenyl groups on the nanoparticle surface, which was apparently weaker with Pt10 eFc than with Pt314 eFc. Significantly, the former might be markedly enhanced with UV photoirradiation owing to enhanced nanoparticle electronic conductivity, whereas no apparent effects were observed with the latter.

Chemical analysis of surface oxygenated moieties of fluorescent carbon nanoparticles

Water-soluble carbon nanoparticles were prepared by refluxing natural gas soot in concentrated nitric acid. The surface of the resulting nanoparticles was found to be decorated with a variety of oxygenated species, as suggested by spectroscopic measurements. Back potentiometric titration of the nanoparticles was employed to quantify the coverage of carboxylic, lactonic, and phenolic moieties on the particle surface by taking advantage of their vast difference of acidity (pK(a)). The results were largely consistent with those reported in previous studies with other carbonaceous (nano)materials. Additionally, the presence of ortho- and para-quinone moieties on the nanoparticle surface was confirmed by selective labelling with o-phenylenediamine, as manifested in X-ray photoelectron spectroscopy, photoluminescence, and electrochemical measurements. The results further supported the arguments that the surface functional moieties that were analogous to 9,10-phenanthrenequinone were responsible for the unique photoluminescence of the nanoparticles and the emission might be regulated by surface charge state, as facilitated by the conjugated graphitic core matrix.