Supplementary Material

Ligand-free Gold Nanoparticles as a Reference Material for Kinetic Modelling of Catalytic Reduction of 4-Nitrophenol

Sasa Guc, Julian Kaiserc, Galina Marzuna,b, Andreas Ottc, Yan Luc, Matthias Ballauffc,*, Alessio Zacconed, Stephan Barcikowskia,b, Philipp Wagenera,b*

aTechnical Chemistry I and Center for Nanointegration
Duisburg-Essen (CENIDE), University of Duisburg-Essen

bNanoEnergieTechnikZentrum (NETZ), University of Duisburg-Essen

cSoft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

dPhysik-Department, Technische Universität München, 85748 Garching, Germany

* To whom correspondence should be addressed:

Characterization of colloidal gold nanoparticles

The laser-generated gold nanoparticles were characterized for their UV/Vis spectrum (Fig. S1), isoelectric point (Fig. S1) and particle size distribution (Fig. S2).

Figure S1: UV/VIS absorption spectrum (left) and streaming potential (right) of laser-generated gold nanoparticles

The change in nanoparticle charge as function of pH shown in figure S1 (right) is related to their surface properties. In colloidal solution, the positively charged nanoparticle surface (see figure S3) is covered by anions or other negatively charged species that can adsorb at the nanoparticle/water interface usually leading to a negative zeta potential [1]. In pure water, hydroxyl ions are the most common species [2]. Consequently, the gold nanoparticle surface is covered by a certain amount of amphiphillic hydroxyl groups that result in different net charges as a function of pH. In Figure S1, the gold colloid was titrated with an acid, which then form a pH dependent equilibrium of Au-OH/Au-O- and Au-OH/Au-OH2+ groups on the particle surface. When approaching the isoelectric point (IEP) the nanoparticles become more neutral until the charge is finally overcompensated. If the pH exceeds the IEP, the charge of the particle is reversed from negative to positive due to the shift of the equilibrium to Au-OH2+ resulting in positive charge.

Figure S2 shows the particle size distribution measured by TEM (see experimental section) in a cumulative diagram. Normalization was done due to particle number, surface and volume/mass.

Figure S2: Particle-size distribution (n=370) due to TEM analysis normalized by number (blue dots), surface (red squares) and volume (black triangles). Arithmetic means of particle diameter due to all counted particles are shown by dashed lines at 7.2 nm (weighted by number, blue), 8.4 nm (surface, red) and 11.1 nm (volume/weight, black).

The electronic structure of the gold nanoparticles produced by pulsed laser ablation in water was measured by X-ray photoelectron spectroscopy (XPS). The measurements were performed using non-chromated Al-Kα (1486.61 eV) radiation as an excitation source and a hemispherical electron energy analyzer (Phoibos 100, SPECS) operating in the constant-pass-energy mode at 20 eV. The sample was prepared by dropping the colloidal solution on a silicon substrate and letting the solvent to evaporate. Results are shown in figure S3.

In accordance to the literature, characteristic peaks of gold were observed about 83.7 eV and 87.3 eV for Au (4f 5/2) and (4f 7/2) respectively [1, 3]. Due to the use of a silicon(100)-wafer, Si-satellites appear in the spectrum. The data show, that the surface of the laser-generated nanoparticles is partially oxidized. In the spectrum the main peaks can be attributed to Au0. The peak broadenings appears due to the combination of different oxidation states. In the present study, the peaks of Au+ overlap with the main peak and contribute a surface charge density with a proportion of 5 area% which is in good agreement with literature [1, 3].

Figure S3: XPS spectra of colloidal gold nanoparticles deposited on a silicon wafer.

A further structural analysis was performed by X-ray diffraction (XRD). For this purpose, 2 wt% of the gold nanoparticles produced by PLAL were adsorbed on graphene by colloidal deposition [9, 10]. Diffraction patterns were measured using a X’Pert Pro from Panalytical in a Q to 2Q scan.

In figure S4 the diffractogram of the powder Au/graphene is shown. The diffraction pattern reveal the diffraction peaks for gold cubic at 38.2°, 44.4°, 64.6° and 77.6° 2Q. Consequently, the laser ablation experiments results in crystalline and cubic-structured gold nanoparticles that show a significant amount of surface defects in form of oxidized surface atoms. The share of cationic gold sites is in the regime of few percent.

Figure S4: XRD patterns for laser-generated gold nanoparticles supported on graphene.

A deeper look into the ultrastructure of laser-generated gold nanoparticle was done by high-resolution transmission electron microscopy (Jeol JEM-2100, (NIMS, Japan)). Results are shown in figure S5 and reveals spherical particles with some grain boundaries. Especially for bigger particles there is a tendency for multi-grained particles.


Figure S5: HR-TEM image of laser-generated gold nanoparticles.

Synthesis and characterization of polymer-stabilized gold nanoparticles

The spherical polyelectrolyte brushes (SPB) were obtained and characterized as described previously [4]. In brief, for the generation of gold nanoparticles, 10 mL of an aqueous solution of 2.55 mM metal salt was added dropwise to 0.1 g of latex in 100 mL [5]. After the solution was purged with N2 to remove the oxygen for 30 min, a 3-fold excess of sodium borohydride was slowly added to reduce the metal ions. After the reduction, the sample was cleaned by ultrafiltration. TEM image of the SPB-gold nanoparticles is shown in Fig. S6.

Figure S6: TEM image of the SPB-gold nanoparticles.

Cited Literature:

[1] Muto, H.; Yamada, K.; Miyajima, K.; Mafune, F. J. Phys. Chem. C 2007, 111, 17221–17226.

[2] Sylvestre, J.P.; Poulin, S.; Kabashin, A.V.; Sacher, E.; Meunier, M.; Luong, J. H.T. J. Phys. Chem. B 2004, 108(43), 16864–16869.

[3] Merk, V.; Rehbock, C.; Becker, F.; Hagemann, U.; Nienhaus, H.; Barcikowski, S. Langmuir 2014, 30, 4213–4222.

[4] Schrinner, M.; Polzer, F.; Mei, Y.; Lu, Y.; Haupt, B.; Ballauff, M.; Goldel, A.; Drechsler, M.; Preussner, J.; Glatzel, U. Macromol. Chem. Phys. 2007, 208, 1542–1547.

[5] Wunder, S.; Lu, Y.; Albrecht, M.; Ballauff, M. ACS Catal. 2011, 1, 908–916.

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