UC Santa Cruz

The extensive research in the development of nanomaterials for applications diverse as energy and medicine is a result of the unique properties of nanomaterials, especially given the countless reports of facile and safe techniques for controlled modification of nanomaterial properties. What is more interesting and is at the heart of all research is the connection between morphological, compositional, or other physical characteristic and the exhibited performance or function for a given application. To further the effectiveness of nanomaterial-based technologies we must additionally understand the relationship between our processing and treatment of the material and the resulting structural features, hence providing a route to control and exploit the material’s properties. This dissertation accounts my search for such a level of understanding of the behavior of materials on the nanoscale where manipulation of the nanoparticle interface has provided fine tuning of particle properties for selective sensing of chemical vapors as well optimization of conditions for electrocatalytic oxygen reduction.

Films of organically capped metal nanoparticles are a common method for sensing organic vapors by measuring conductivity loss upon film solvation. As this mechanism provides very sensitive detection, selectivity on the basis of solvation is not specific enough for a practical sensing. Instead, selectivity based on specific structural groups is required. Recently it was found that nitrene functionalized ruthenium nanoparticles selectively react with vinyl groups through imido transfer. This selectivity was exploited for a chemical sensor where the electronic conductivity of a solid film of ruthenium functionalized ruthenium nanoparticles was measured and compared in the presence of vapors of relative polarity including ethanol, acetone, n-hexane, 1-hexene, toluene and styrene. The ensemble conductivity was found to decrease upon exposure to each vapor with the largest drop resulting from toluene and styrene vapors. Interestingly, the decrease in conductivity was 2.5 times larger for styrene than toluene, despite the similarity in structure and hence solvation properties. This discrepancy is attributed to a chemical interaction between the ruthenium nitrene bond and vinyl groups that alter the nature of the conductive media between the metal cores and diminish conductivity based on specific functional groups rather than solvation alone.

Current fuel cell technologies are severely limited in part by a large amount of precious metal catalyst required for energy extraction. To remedy this issue, alloy nanoparticles have allowed for not only the reduction of precious metal required but have displayed actual enhancement in activity over either pure constituent metal. This enhancement results from the mixing of metal d bands upon alloying and thus an electronic environment in between that of the pure metals. Gold palladium nanoparticle alloys offer a promising route for an efficient electrocatalyst since an optimal electrocatalyst would exhibit properties in between than of gold and palladium. Dodecyne-capped AuPd alloy nanoparticles of varying compositions were therefore prepared through the co-reduction of metal-salt precursors with NaBH4. TEM measurements showed that the particles were largely in the range of 2–6 nm in diameter. XPS studies showed that the atomic Pd concentration varied from 65 to 100 %. Infrared spectroscopic measurements confirmed the bonding attachment of the dodecyne ligands on the nanoparticle surfaces, which rendered the nanoparticles readily dispersible in common organic media. Electrochemically, the resulting nanoparticles exhibited apparent catalytic activity in oxygen reduction with a volcano-shaped variation with the metal composition. The best performance was identified with the sample composed of 91.2 at% Pd that exhibited a mass activity over eight times better than that of commercial palladium black, and almost twice as good in terms of specific activity. This remarkable performance was accounted for by the manipulation of the electronic interactions between palladium and oxygen resulting from alloying with gold.

Of the many methods to enhance the electrocatalytic oxygen reduction activity of a metal nanoparticle, the incorporation of an active support, such as graphene quantum dots (GQD), has proven very effective. The strong electron withdrawing effects of the oxygen groups of GQDs as well as the ease of manipulating the level of these groups through thermal or chemical treatment may be exploited to find tune metal nanoparticles for electrocatalysis. GQD-supported palladium nanoparticles were synthesized by thermolytic reduction of PdCl2 in 1,2-propanediol at 80 °C in the presence of GQDs and then were subject to hydrothermal treatment at an elevated temperature within the range of 140 to 200 °C. Transmission electron microscopic measurements showed a raspberry-like morphology for the samples before and after hydrothermal treatment at temperatures ≤ 160 °C, where nanoparticles of ca. 8 nm in diameter formed large aggregates in the range of 50 to 100 nm in diameter, and at higher hydrothermal temperatures (180 and 200 °C), chain-like nanostructures were formed instead. X-ray photoelectron and Raman spectroscopic measurements revealed that the GQD structural defects were readily removed by hydrothermal treatments, and the defect concentrations exhibited a clear diminishment with increasing hydrothermal temperature, as indicated by the loss of oxygenated carbons in XPS and a drop in the D to G band ratio in Raman measurements. Voltammetric studies showed apparent electrocatalytic activity toward oxygen reduction, with a volcano-shaped variation of the activity with GQD defect concentration, and the peak activity was observed for the sample prepared at 180 °C with a mass activity of 23.9 A/g Pd and specific activity of 1.08 A/m2 at +0.9 V vs RHE. This peak activity is attributed to optimal interactions between Pd and GQD where the GQD defects promoted charge transfer from metal to GQDs and hence weakened interactions with oxygenated intermediates, leading to enhanced ORR activity. The corresponding defect concentration was higher than that identified with the platinum counterparts due to the stronger affinity of oxygen to palladium.

While the oxygen groups of non doped GQDs provide a facile source of electronic modulation for neighboring particles, elevated levels may result in loss of conductivity, stability, and performance. To overcome this obstacle, nitrogen may be incorporated into the graphitic structure. Similar to level of oxygenated groups on GQDs, nitrogen exists in pyrrolic, pyridinic, and quaternary forms depending on synthetic conditions. To determine the effect of each center on neighbor metal nanoparticle activity, nanocomposites (PdNGQD) based on palladium nanoparticles supported on nitrogen-doped graphene quantum dots were synthesized through a hydrothermal co-reduction method at 160 °C for various periods of time. Transmission electron microscopic studies revealed that all samples exhibited similar morphology with small nanoparticles clustered into larger superstructures of 100 nm and larger. X-ray photoelectron spectroscopic studies showed the NGQDs contained only pyridinic and pyrrolic nitrogen centers, and that the relative abundance of pyrrolic nitrogen increased with prolonging reaction duration whereas a concurrent decrease was observed with the pyridinic nitrogen. The binding energy of the Pd 3d electrons was also found to increase with increasing reaction time. Such a correlation with the pyrrolic nitrogen concentration suggests apparent interactions between palladium and the nitrogen moiety. Accordingly, Raman spectroscopic measurements exhibited an increase of the ID/IG ratio, indicative of an increasingly defective structure of the NGQD probably due to the increasing abundance pyrrolic centers which will provide more structural strain than the 6 membered pyridinic heterocycles within the graphitic backbone. Voltammetric studies revealed significant electrocatalytic activity towards oxygen reduction in alkaline media, with the best mass activity of 77.0 A/g at +0.84 V from the sample prepared by 1 h of thermal treatment, whereas the top specific activity of 31.6 A/m2 at +0.84 V from the sample prepared by 8 h‘s heating. These samples represent the optimal combination of electron withdrawing effects form the nitrogen centers and the oxygen groups and it was found that the palladium environment and thus electrocatalytic activity was much more correlated with the pyrrolic concentration rather than the abundance of oxygenate moieties.