60th Annual Report on Research 2015

Initially, we worked on exploring the effect of nanoenvironment on the catalytic properties of gold nanoparticles (AuNPs) immobilized on larger silica nanoparticles (SiNPs). We prepared aminated SiNPs and deposited ~10 nm AuNPs on the surface. Next, we alkylated the surface primary amines using 2-bromoisobutyryl bromide, and used this moiety to initiate atom transfer radical polymerization of 2-hydroxyethyl methacrylate (Figure 2). This provided polymer brush-grafted nanoparticles whose polymer brush length was controlled in the range of 5-30 nm by the polymerization time. We examined the catalytic activity of these nanoparticles using a model reaction, reduction of 4-nitrophenol (4-NP) to 4 aminophenol (4-AP) with NaBH₄ (see below). While we noted clear indications that the polymer brush length and the presence of hydroxyl groups in the polymer side chain affect the catalytic activity of AuNPs, we also observed loss of catalytic activity due to AuNP aggregation because of rapid dissolution of SiNP supports under the highly basic reaction conditions (Figure 3). As the result, we shifted our focus to ND as a more stable catalyst support material.

We used thiol-ene chemistry for both attachment of a polymer film to ND and subsequent immobilization of noble metal nanoparticles on the ND surface. We used a photo-initiator 2,2-dimethoxy-2-phenylacetophenone (DMPA) that was exposed to UV light at 254 nm, generating free radicals that cleave the S-H bonds in entaerythritol tetra(3-mercaptopropionate), PETMP, which then combine with vinyl monomers present in solution and with the double bonds present on the ND surface (Figure 4). Upon treatment with sodium borohydride (NaBH₄), Au, Pt, and Pd nanoparticles tethered to the surface of the polymer-coated NDs were formed, depending on the metal salt present. Figure 5 presents TEM images of Au, Pt and Pd nanoparticles on polymer-coated ND supports. A closer inspection of these NDs (Figure 5A) shows AuNPs with a diameter of ~3 nm on the polymer/diamond surface. The AuNPs follow the contour of the diamond support closely, suggesting that the polymer adhesion layer is thin. In addition to gold, in situ particle growth worked for the formation of Pt and Pd NPs on the ND particles, as can be seen in Figures 5D,G.

High resolution energy dispersive spectroscopy (EDS) obtained with scanning TEM (S/TEM) provided nanoscale chemical mapping of the polymer and Au, Pt, or Pd NPs attached to the polymer/ND surface. As evident from Figure 5, the polymer coating appears to cover the entire ND surface. The BF-S/TEM images in Figure 5A,D,G are clearly correlated to the maps of the characteristic sulfur-K X-rays at 2.3 keV shown in Figure 5B,E,H. One of the monomers in the polymer coating is PETMP which is a tetra-thiol molecule, the only sulfur-containing molecule. Therefore, the sulfur EDS map indicates that the polymer coating is confined to the surface of the ND shown in the BF-S/TEM images. Figure 5C,F,I correspond to the Au-L, Pt-M, and Pd-L x-ray lines at 9.7, 2.0, and 2.8 keV, respectively. These EDS maps demonstrate that metallic NPs are adhered to the surface of the thiol-ene polymer which coats the entire surface of the ND and follows the topography of the diamond surface with high fidelity. These maps complement the XPS data and show that the sulfur bonding environment corresponds to the local attachment of the metallic NPs.

We carried out preliminary studies of the catalytic efficiencies of the Au, Pt, and Pd NPs supported on the polymer/ND particles without addition of polymer brushes. We probed the reduction of 4-NP to 4-AP with NaBH₄ (Figure 6), which is easily monitored using UV-Vis absorbance spectroscopy when this reaction is catalyzed by metal nanoparticles. Instead of turnover frequencies (TOFs), we quantify the catalytic activity in units of catalytic cycles per active site per second. When used as the catalyst for the reduction of 4-NP the STYs for Au, Pt and Pd NPs immobilized on polymer/NDs were observed to be 0.003 ± 0.001, 0.021 ± 0.003, and 0.018 ± 0.04 s^-1, respectively. Reported values for Au and Pd nanoparticles in solution follow a similar trend, with AuNPs being less catalytically active than Pd when used in the reduction of 4-NP. However, the activity of the PtNP/polymer/NDs is higher than expected and it is possible that the attachment of the PtNPs to the polymer may influence the adsorption energy of 4-NP leading to a more favorable reaction environment than that of unsupported PtNPs.

Figure 1.  Schematic representation of the proposed idea.

Figure 2. Preparation of poly(2-hydroxyethyl methacrylate) brushes on silica nanoparticle surface.

Figure 3. (A) TEM image of AuNP decorated silica spheres with (B) a higher magnification HAADF-S/TEM image. (C) Silica dissolution and AuNP aggregation after brief exposure to basic solution. All scale bars are 50 nm.

Figure 4. Photoinitiator and monomers used in polymer coating of NDs. The latter were then decorated with metal nanoparticles via in situ reduction of metal salts.

Figure 5. Bright field (BF-S/TEM) images of polymer/NDs decorated with (a) Au, (d) Pt, (g) Pd. EDS images extracted from the sulfur K-line of the same polymer/NDs as seen in the BF-S/TEM images decorated with (b) Au, (e) Pt, and (h) Pd NPs, respectively. EDS images extracted from the (c) Au M-line, (f) Pt M-line, (i) Pd L-line of NP decorated polymer/NDs are of the same regions as seen in the BF-S/TEM and sulfur K-line EDS images.

Figure 6. Reaction scheme of conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) catalyzed by AuNPs (top). UV-visible absorption spectra showing conversion of 4-nitrophenol to 4-aminophenol in the presence of ND-supported AuNPs. Red arrows indicate isosbestic points (bottom).